ALUMINIUM NITRIDE AND RELATED MATERIALS A Thesis …aluminium nitride from ammonium hexafluoroaluminatCi, The x-ray characteristics and the thermal stability of a new compound, ^-AlP^

ALUMINIUM NITRIDE AND RELATED MATERIALS

A Thes i s presented f o r the degree of

DOCTOR OP PHILOSOPHY

of the

COUNCIL FOR NATIONAL ACADJMIC AWARDS

London

by

No G„ COLES, MoSCo

Depajrtment of Chemistry^ The F b l y t e c h n i c , Plymouth,

August. 1970

" ^.500214

CLOSS T Fk^. l-U CoL No.

Abstrac t

The preparat ion and propert i e s of aluminiuin n i t r i d e are reviewed

v/ith s p e c i a l reference to the nev/er production methods and f a b r i c a t i o n

techniqueSo

Information so f a r a v a i l a b l e on the s i n t e r i n g of m a t e r i a l s i s

summarised. For n i t r i d e s the s i n t e r i n g i s inf luenced by a d d i t i v e s

or impur i t i e s such as oxides formed by p a r t i a l h y d r o l y s i s and oxidationo

Res i s tance to oxidation i s increased by s i n t e r i n g and ho t -pres s ing

the r e f r a c t o r i e s .

The k i n e t i c s and products of ox idat ion of n i t r i d e s so f a r studied

depend mainly on the i n t r i n s i c r e a c t i v i t y of the mater ia l and a v a i l a b l e

surface at whi.ch oxidation can occur .

In the present research condit ions are inves t i ga ted f o r preparing

aluminium n i t r i d e from ammonium hexafluoroaluminatCi, The x-ray

c h a r a c t e r i s t i c s and the thermal s t a b i l i t y of a new compound, ^-AlP^ a r e

e s tab l i shed . Attempts to study the k i n e t i c s o f the commercial methods

of aluminium n i t r i d e formation are summarised. The r e a c t i v i t y o f aluminium

n i t r i d e i s examined by c o r r e l a t i n g changes i n phase composition, s u r f a c e

a r e a , c r y s t a l l i t e and aggregates s i z e s with h y d r o l y s i s and oxidat ion time

and temperature. The k i n e t i c s and ra tes of reac t ion are in f luenced by

the c r y s t a l l i t e and aggregate s i z e s of the aluminium n i t r i d e , by

d i f f e r e n c e s i n type of c r y s t a l s t r u c t u r e , by the molecular volume of the

oxide products , and by f luor ide a d d i t i v e s . Ageing and s i n t e r i n g of the

aluminas are addi t iona l factors* The a c t i v a t i o n energy of the oxidat ion

of aluminium n i t r i d e i n a i r i s determined as 53 k>cals/molec

Acknowledgements

I wish to thank Dr« A« Bo Meggy f o r the helpp comment and general

supervis ion of t h i s worko I a l so yrish to acknowledge the h e l p f u l

d i scuss ions I have had with Dr* D© Ro G l a s s o n , Thanks are a l s o due to

Miss lo Gratton^ the Polytechnic Ldbrariarip and her s t a f f , f o r t h e i r

pat ient and w i l l i n g cooperationo The help of Mr, Tapper with the

computation has been g r a t e f u l l y appreciated. , I would also l i k e to

thank Mrso E o Adams, J o A o C a ^ A , Down^ Dro So A. A, Jayaweera and

Mai^garet Sheppard f o r t h e i r a s s i s t a n c e and cooperat ion. I am indebted

to the Governors of the Polytechnic f o r the research award and the

f a c i l i t i e s o

F i n a l l y p I would l i k e to express my thanks to Anne f o r her

understanding and encouragement throughout t h i s work*

i i

Ta.bles

1. The e f f e c t of vajrlous media on aiijjniniujn n i t r i d e ^ 9

2. The l a t t i c e parameters of alujiiira.\iin n i t r i d e o 11

3« The heat of forniation of aliJininium n i t r i d e . 13

The decomposition temperature o f aluminium n i t r i d e „ 1 5

5» A canparison of the mechanical p.roperties of al^jminium

n i t r i d e with c e r t a i n c e r a m c s , 1 7

6 . The tiierraal expansri.on of c e r t a i n m a t e r i a l s - 1 9

7 o A compari.son of the cr^'stal s t r u c t u r e s of c e r t a i n

m a t e r i a l s . 20

8« Ammonium hexafluoroaluminate heated a t 220°G i n dry a i r . 63

9« Ammonium hexaflucroaluminate heated a t 3hO^C i n dry a i r , 64

lOo Comparison of the observed and c a l c u l a t e d d-spacings f o r ^ -AlPô 66

11. Comparison of d-spacings of ammonium hexaifluoroaluminate heated i n moist a i r for various times with Y - A l P j j , some bas ic f l u o r i d e s and aluminium h y d r o x i d e » • ^ 68

12- Ammonium hexafluorcalumi.nate heated at "lO'^C f o r 18 hours i n d r i e d a i r . 71

13*> AmmoQium hexaiT-uoroaluminate heatedîn d r i e d ammonia a t 960"C f o r 1 hour and cooled to h-00 C i n ammonia. 75

14. Compari.son o f the observed x - r a y i n t e n s i t i e s of ammonium he.xafluoicaliiminate heated at 960 C i n variou;3 flow ra tes of ammonia. 79

15- X-ray data of bas ic f l u o r i d e s and alumir^Lum f l u o r i d e hvurates . 83

16. Comparison of the d-spacings of tiie reported S - A l F ,

and I - A I P ^ . I du IJo L a t t i c e parameters of some complex hexaf luorides* 86

XIX

18. (a ) I n f r a - r e d a n a l y s i s of the thernial decomposition of

ammonium hexaLfiuoroaluminatea 58

(b) Density of some aluminium compounds • 8 9

19. Intsratomic d i s tances i n (j^..-aluminium f l u o r i d e and r e l a t e d compounds. 9 3

2 0 o Comparison o f the c r y s t a l propert i e s o f AIN v/ith

poss ib le impur-ities i n the Serpek pnDcesSe 9 9

21* X -ray a n a l y s i s of hydrolysed aluminium n i t r i d e . 118

2 2 . I n f r a - r e d a n a l y s i s of hydrolysed aluminium n i t r i d e . 119

23o Act iva t ion energies of aluminium ni tr i .de and r e l a t e d compoiinds^ IZ4.3

24- A comparison of the rate constants of m i l l e d samples

at various temperatures. 149

2 ^ . Aluminium n i t r i d e heated at 900°C f o r 9 6 ho^jx-s« I 5 6

2 6 . S i l i c o n n i t r i d e heated a t 1000°G f o r 9 6 hoi:rs. 1 5 7

2 7 o Var ia t ions i n aggregate s i z e s during the conversion of AIN to iX -^-^2*^3 a t 1000*^0 i n a i r . l 6 2

2 8 0 Free energy of formation of aluminium carb ide from the r e a c t i o n of alumi.n-a and car^bon. 1 3 0

1 . The change i n conduct iv i ty of AlP^ so lut ion when adding NH^F s o l u t i o n ,

2 , (a) The Van-Arkel arrangement

(b) Determination of the kn i f e edge angle of a 9 cm povrcLer camerao

3« The percentage v.-eight l o s s of aruj:onium h'.y r-Quoroaluminate h-?att^ "in dr i ed a i r for various tefnoeratures and times.

1 9 2

a

3 9

I it 13 \J. t - .0

6 o

(b) D i f f e r e j i t i a l -uheraal ar-.âiy:^!of V|'-..i^'\..

The -ihtinnal decomposition of amironiv-^ hev'^fX>Jorcgallate,

.-,mi:".c

7-

1 3 .

(a)- (d) The fijTimoni'jir. rc tra f luororo iuminîre / -a lumin ium flucjri.oe o-.y'.t-ll-:^-j--:>iic - j . . ' ^ t i ' . : . .

(b)

I G . (a)

(b)

(b)

(a)-

atcr.-dc arror:gan'cnt ino<^-/'l?ô

The atomic arr.-»xif;cr-ent in . ' J l . o

Elec tron d i f f r a c t i o n pattern of the aluminium f i i r r ,

:ile:;tron d j . f f r a c t i o n pattern of th-: ' r.7z*-] . dr-c' aluninium .filrPo

(a) Electrrjr^ mi'::r:.

1'h-: ci-r.r;- ^r. , -^.r.j '-'U- 3'^riace during trie nrdxiing of alun^^ r.iMi:; --iit :d d^c

m±il-'.ng of aluminj'^m r . i tr ldec

(d) Comparison of the -specifa.c surfac- i . surface are; and average C i y s t a J l i t s si^'.r. \7i:;h perr:^r.-âg^: h> 'droIys i£ .

^1,

9 b

lev

1 1 5

lit-. The percentage conversion of aluminium n i t r i d e to oC-a lumir^ a t var ious temper^tursso 132

15. The percentage conversion o f alumini-am n i t r i d e heated at 930 C f o r 3 hours. I 3 6

1 6 • (a) A plot of f i . r s t - o r d e r k i n e t i c s v/ith time f o r vajrious temperatureso 138

(b) A plot of log t (time) wi.th l o g x (x i s % conversion)^

17 . A p lot of y against timeg t , a t 905*^C. 139

18 . A p lot of 2^ and y against timej, t , f o r periods up to 30 hours, 141

2

19o A p lot of Z against timer t , f o r v a r i o u s teinperatures. 1A4

20» Arrhenius plot f o r the oxidat ion of alumini--^ n i t r i d e . 145

2 1 . The e f f e c t of m i l l i n g on the oxida.tion of aluminium n i t r i d e at 872 C . I 5 I

2 2 . The e f f e c t of m i l l i n g on the oxidation of aluminium n i t r i d e a t 905 C . 152

25o The e f f ec t of aluminium f l u o r i d e on the ox idat ion of aluminium n i t r i d e . l^h-

?.L*~ Srrnie e lec tron micrographs o f alumijiium and s i l i c o n 158-2 7 , n i t r i d e s and oxidised samples. I 6 I

28* Comparison of the rate of oxidat ion of AlN a t lOOO^^C with the change i n ( i ) s p e c i f i c s u r f a c e , ( i i ) the number of c r y s t a l l i t e s ^ (^' /g) p ( i i i ) the average c r ^ ' s t a l l i t e s i z e . I 6 3

29. Comparison of the oxidation of aluminium and s i l i c o n

n i t r ides , . I66

30 . The free energy of formation of some n i t r i d e s . 173

3 1 . A comparison of the free energy of formation of AlN from i t s e lonents . 175

3 2 . Free energy of formation of AlN from var ious methods o f

preparat ion. 177

33. Plot of aga ins t oC (p^^ or V-^) * > 178

34 . Comparison of the f ree energy of the formation of oC-alumina and s i l i c a from the n i t r i d e s . 184

vx

OONTmTS

Abstract mo oo o <> . o o o o <> o o o o o o o . i

Aclaiowledgements •<> o* c o oo r . .<> c o «. •<> i i

Tables o e o u o o o o o o . o OD oo o > o o o a o i H i

F i g u r e s o . o « o* « . , <, . o v

CHAPTER 1 L i t e r a t u r e survey of alijminium n i t r i d e and r e l a t e d rna-terials

1 , 1 Tj}^_formation and chemical p r o p e r t i e s of aluminium

n i t r i d e » c . e « • e n oo o o o e

I ^ l e i The formation of aluminium n i t r i d e . o o - * 1

I » l c : i ( a ) The formation of aluminium n i t r i d e from i t s elements 1 I , l . i ( b ) The formation of the n i t r i d e from the decomposition

of alumina v/ith caxbon i n the presence o f ammonia or nj.tmgen l o . o o . o . « « » » 2

I . l . i ( c ) The thermal decomposition of ammonium h e x a f l u o r c -

aluminate in ammonia o o . o c o c c 4

I „ l o i ( d ) l . i iscellaneous methods of preparat ion <>• « o » . 3

1 * 1 . i i . The chemical p r o p e r l l e s of aluminium n i t r i d e . * 7

I ^ 2 The cryata l loaraphy and the heat of formation and thermal, stabi3.ity_of^ aluminium n i t r i d e

lB2oi The crys ta l lography of aluminiiom n i t r i d e « « 1 0

I g 2 o i i The heat of formation and thermal s t a b i l i t y of aluminiiim n i t r i d e o o . » o o . o . o . o 1 2

I « 3 The phys ica l and mecbânlcal proper-bies of aluminium nitri .de and r e l a t e d materj.als

I o 3 o i The p h y s i c a l proper t i e s of aluminium n i t r i d e oo lit-

1 * 3 * 3 1 The mechanical proper t i e s of aluminium n i t r i d e and r e l a t e d m a t e r i a l s . o . . . , o . o o . 1 6

I « 4 The f a b r i c a t i o n and uses of aluminium n i t r i d e and r e l a t e d mater ia l s

I o 4 o i The f a b r i c a t i o n of aluminium n i t r i d e . o . . 2 1

I . 4 o i i The uses o f aluminitim n i t r i d e and r e l a t e d m a t e r i a l s 2 3

Iêferences to Chapter l o o oo -<i . o 2 5 -3 3

CHAFT'ER I I Experimental Techniques

I I . 1 The preparation o f ammonium hexafluoroaluminate 3 4

I I . 2 Chemical A n a l y s i s ... « o oo 3 5

I I o3 Thermal Ana lys i s

I I . 3 « i Thermogravimetric a n a l y s i s . o • « . o . » 3 5

I I . 3 . i i D i f f e r e n t i a l thermal a n a l y s i s . „ . „ . « <,* 3 6

I I . 4 I n f r a - r e d A n a l y s i s o o o . o . o o . . • • 3 7

I I o 5 X-ra,y A n a l y s i s

I I . 5 « i Powder d i f f rac tometry c- o. o. 3 8

I I . 5 c i i Powder camera . . o - <.« . <. • » <. 4 0

I I . 5 » i i i E r r o r s in powder photographs •„ .<> o . . o « c 4 0

I I . 6 E lec tron micrxjscopy o o o o . « c o . o » o 4 3

I I . 7 Est imat ion of p a r t i c l e s i z e

I I * 7 « i E lec tron microscop ica l technique o^ 4 4

I I . 7 » i i 'Optical microscopica l techni.que .«. o o e. . o 4 5

1 1 . 8 Surface area measurgnentsa. o * o o o o o o « • 4 5

11 .9 Ba31 m i l l i n R o o o . 4 7

11 ^ 10 Experimental techniques used in prepaiinp^ aluminium n i t r i d e

I I . l O o i Preparation from anmcnium h e x a f l u o r o a l u m i n a t e o » * . 4 8

I l o l O . i i Preparat ion by the Serpek react ion •« . c 5 0

n . l O o i i i Preparation o f the n i t r i d e from i t s elements o o . o 5 4

References to Chapter I I . - . » 5 6 -

5 7

CHAPT-ER I I I The formation of al.umin.ium n i t r i d e

I I I . l The thermal decomposition of ammonium hexaf luoro-aluminate in_ var ious atmospheres

I l l o l . i The thermal decomposition of ammonium hexaf luoroaluminate i n a i r o o o o oo « o o . • « 5 8

I l l o l . i i The thermal decomposition of ammonium hexaf luoro-aluminate in aanmonia .<> o o . . . • « . 7 2

I l l . l o i i i X-ray a n a l y s i s of ammonium hexafl-uoroaluminate 85

I l l o l e i v The s t r u c t u r a l rearr-angonents during the formation of aluminium n i t r i d e by the thermal decomposition of ammoniimi hexafluoroaluminate . o o . « « 85

1 1 1 , 2 Th.e formation o f âluminium ni tr ide^by the Serpek process 96

I I I o 3 Th_e fbrmatLon of aluminium n i t r i d e from i t s elements 1 0 1

I I I . 3 . i K i n e t i c s of metal n i tr i .dat ion .» o . .<> 1 0 1

I I I ^ 3 - i i The formation of the ni tr i .de from i t s elements .„ 1 0 2

References to Chapter l l l o » 1 0 8 -

1 1 0

CHAPTER I V The r e a c t i v i t y of aluminium n i t r i d e

Ij^-troduction . o . o . o I l l

I V c l * i The h y d r o l y s i s and oxidat ion o f n i t r i d e s . . 1 1 3

I V o l . i i Mater ia l s e . . o on o c 1 1 ^

I V o 2 The hydrol.ysis of alumini.um n i t r i d e

I V . 2 . i The h y d r o l y s i s i n water © . . o 1 1 6

I V . 2 o i i The h y d r o l y s i s i n steam .« .„ 1 2 2

I \ r ^ 3 The oxidation of al.uminium n i t r i d e

I V . 3 ^ i The oxidat ion reac t ion .» 1 2 4

I V « 3 o i i The r a t e laws of oxidat ion •<> 1 2 5

r / . 3 . : _ i i The k i n e t i c s o f the oxidat ion of aluminium n i t r i d e . . 1 2 6

I V . 3 * i v The e f f e c t of p a r t i c l e s i z e on the r a t e of r e a c t i o n IZ^B

I V . 3 . V The e f f e c t c f impur i t i e s on the oxidat ion 1 5 0

I V o 3 , v i The change i n surface proper t i e s during the oxidation of SLluminium and s i l i c o n n i t r i d e . . « . 1 5 3

References to Chapter I V , « , . o . o ... 1 6 7 -1 6 9

CHAPTER V Therroodvnamical aspects o f the formation and r e a c t i v i t y o f aluminium n i t r i d e

The formation of alujniniujn n i t r i d e o o o o »<, « « 170

The formation from the elements o » o o 172

The formation from the Serpek reac t ion . „ <>• 17^

The formation from the aluminium h a l i d e d e r i v a t i v e s 179

The r e a c t i v i t y of alumi.nium n i t r i d e

The o x ± d a t i o n of aluminium and s i l i c o n n i t r i d e 183

References to Chapter V c o . o - o « o 186

CHAPTER V I Concluding summajry and sufiêstions f o r f u r ^ e r v/ork 187

V . l

v . l . i

V . l . i i

V o l c i i i

V . 2

V « 2 . i

Appendix I

Appendj.x I I

Appendix I I I

Appendix I V

Appendix: V

Appendix V I

APPEN-PICES

Preparation o f ammonium hexafluoroaluminate 189

Methods o f Chemical A n a l y s i s 1 9 ^

Computer program (NGITO) i n Fortran I V f o r the lEM 1 1 3 0 - Est imat ion of cry s t a l l ographic parajTieters 2 0 5

Computer program (FODH) i n For tran I V f o r the imA 1 1 3 0 = Es t imat ion of sur face areas 208

Computer Program ( N I C K I T ^ E T I C ) in F o r t r a n I V f o r the im JI30 - Es t imat ion of k i n e t i c paj?ameter3 2 1 2

Repr in t - The formation and r e a c t i v i t y of n i t r i d e s pELTt H I , BoroHj aluminium and s i l i c o n n i tr ides j^ J .Appl .Chem. , 1 9 6 9 , 1 9 , 1 7 8 2124-

Appendix V I I Thermochemicai Data 219

CHAPTER I L i t e r a t u r e survey o f aluminium n i t r i d e and r e l a t e d minerals

The survey i s d iv ided in to four s ec t ions as l i s t e d belov/:

1 . The formation and chonica l proper t i e s o f aluminium n i t r i d e .

2 . The c i y s t a l l o g r a p h y and the heat of formation and thermal

s t a b i l i t y of aluminium n i t r i d e .

3 o The p h y s i c a l propert i e s o f aluminium n i t r i d e .

4 . The f a b r i c a t i o n and uses of aluminium n i t r i d e ,

l o l The formation and chemical proper t i e s of aluminium n i t r i d e

I . l o i The formation of aluminium n i t r i d e

The preparat ion of aluminium n i t r i d e i s confined to four main

processes;

(a) The formation of the n i t r i d e from i t s elements.

(b) The fonnation of the n i t r i d e from the decomposition of alumina

wi th carbon i n the presence of n i t rogen , i . e . the Serpek process .

( c ) The thermal decomposition of ammonium hexafluoroaluminate i n the

presence of ammonia.

(d) The thermal decomposition of the aluminium halides^ hydride and

sulphide with ammonia.

I . l . i ( a ) The formation of aluminium n i t r i d e Vvom i t s elements

The s o l u b i l i t y of aluminium f o r nitrogen has been d i s cus sed by many

authorsj, and i t i s the key f a c t o r to t h i s method of fomat ion. . P i t c h e r

( l ) and others (2-43: 139 1 5 7 ) have reported that at temperatures

between 5 3 0 and 8 2 0 ^ 0 aluminium absorbs nitrogen to give the n i t r i d e ,

although the react ion appeared to be more vigorous a t 1 3 0 0 - 1 4 0 0 ° C . These

statements were contradicted by C l a u s ( 6 ) and L a f f i t e et a l ( 8 ) who concluded

that nitrogen was almost inso luble i n aluminium both i n i t s l i q u i d and s o l i d

state* Rontgsn and B.rauî (7)..j hov/ever,^ s tated that although nitrogen

i.s almost inso lub le i n aluminium i n the c r y s t a l l i n e state'^ the absorption

increases v/ith temperature* The preparat ion of the n i t r i d e us ing ai/imonia

inste.ad of nitrogen has a l so been studied ( l 5 ) - The use of ammonia to

n i t r i d e s t e e l s i s we l l knowr^ and th* r e s u l t a n t formation of aluminium

n i t r i d e i n f u l l y - k i l l e d s t e e l s has been e s t a b l i s h e d .

The e a r l i e s t reference to formafion of the nitr i .de from the elements

i s the v/ork of Briegleb and Gerjther O-O) i-i"' 186^ v/ho stated t h a t EtLuminium

n i t r i d e i s formed when "aluminium tu.mings are heated i n an atmosphere

c f nitrogen"- Arous ( l l ) s tated that i f aluminium i s heated by e l e c t r i c a l

current i n the presence of nitrogen^ aluminium n i t r i d e i s formed,

Ali.jninium n i t r i d e was formed ( l 2 ) when ali^minium was burnt i n oxygen^

and nJ.trogen sabs t l tu ted f o r oxygen v;hile the aluminium i s s t i l l burning.

The pu.ri.ty o f any n i t r i d e formed by t h i s method i s q u e s t i o n e d „ as the

presence of al'jmina i s anticipated. , The formation of c r y s t a l s of the

n i t r i d e from i t s elements at e levated temperatures i s reported ( l 2 0 ) .

The use of a c a t a l y s t v/as introduced i n 1953 ( l 6 ) , F i n e l y - d i v i d e d

alumijiium was heated v/ith 1- 3 wtopercent of a f l u o r i d e c a t a l y s t i n the

presence of nitrc-gen to give a l i r n i i n i n i . t r i d e * Long and F o s t e r ( l 7 )

have stated that high p u r i t y n i t r i d e can be made by s t r i k i n g an arc

bet\veen tv/o pure aluminium electrodes i n a nitrogen atmosphere^ vdth a

f i n a l treatment i n argon to increase i t s s t a b i l i t y towards water*

1 .1 , i (b) The formation of the n i t r i d e from the decomposition of alumina

vrith carbon i n the presence of ammonia o r ni trogen

T h i s method v/as extens ive ly inves t iga ted by Serpek (45) i n 1914^

and was l a t e r to be known as the Serpek process . P r i o r to Serpek^s v/ork^

Caro ( 3 0 ) u s ing aluminium carbide prepared the n i t r i d e ^ Although t h i s

process proceeds e f f i c i e n t l y a t high temperatures^ i t was found that

very l i t t l e n i t r i d e was formed a t l 6 0 0 * ^ C and su lphur dioxide was

detrimental to the process ( 3 l ) » The Serpek process has been discussed

by a number of authors ( 3 2 - 4 3 ) 5 Prankel ( 4 4 ) s ta ted that when us ing

producer ga^ i n s t e a d of nitrogen^ a higher temperature i s requ ired .

The n i t r i d e , according to the Serpek r e a c t i o n , i s best formed by a

mixture of n i trogen and hydrogen ( 4 5 ) . C a r b o - n i t r i d e s have been formed

during t h i s process ( 4 6 ) 0 The e a r l i e s t re ference which i s r e l a t e d to

t h i s method of preparat ion i s reported i n I 8 7 6 by M a l l e t ( 4 7 ) o Aluminium

was heated i n a carbon c r u c i b l e and impure aluminium n i t r i d e was formed.

Various addi t ions to alumina i n the process such as z inc c h l o r i d e ,

f e r r i c oxide ( 4 3 ) 5 and i r o n ( 4 9 ) used as c a t a l y s t s , have been reported

as b e n e f i t i n g the formation of the n i t r i d e . A v i t r i f i e d r e f r a c t o r y

c o n s i s t i n g o f alumojia and aluminium n i t r i d e has been reported v/here the

method used was to heat the mater ia l s i n a graphi te c r u c i b l e at about

one hundred degrees above the sof tening point of the r e f r a c t o r y ( 5 0 ) <,

C r y s t a l s of aluminium n i t r i d e have been prepared according to the Serpek

pzx)cess ( 5 1 ) - The r e v i v a l of the process by Pechiney (52 , I 6 4 ) i n 1 9 5 6 - 7 3

l e d other people ( 5 6 ) to r e - i n v e s t i g a t e the processo The n i t r i d e has been

prepared by t h i s method using calc ium aluminate as a binder ( 5 3 - 5 5 ) . V/hen

alijmini'om n i t r i d e i s prepared by zhe Serpek process „ the product i s impure

and has been known to contain up to 1 2 ^ carbon ( 1 0 2 ) « Methods have been

described f o r removal of such impur i t i e s ( 1 0 1 - 5 ) c

More r e c e n t l y the Serpek process has been adapted us ing a nitrogen

plasirasourcs ( 5 9 ; , 9 S c , 9 9 ) c

I . l . i ( c ) The thermal decomposition of ammonium hexafluoroaluminate i n ammonia

AmmorLium hexailuoroaluminats ( (Ml, )-,AltV) can be oreoared from molar

scV;.tions of aluminium ch lor ide and ammonium f l u o r i d e (78).

6NK,^P + AlCi^_ {m^)ÂJI'^ + 3NH^C.l.

Th_is ariimonlixn complex can a lso prepared from ammonium f l u o r i d e and

f r e s h j y p r e c l p i t a t t d aluminium hydroxide ( 7 9 ^ 9 6 . 9 7 ) = The s a l t has

been mentioned as a by-product of va^îous other r e a c t i o n s , such as the

anrnionation c f phosphoric a c i d (Bl) ^ anion exchange i n c l a y s ( 8 2 ) 5 the

formation from alumina and ammonium f l u o r i d e (89 , 83)? the formation

from aluminium n i t r a t e and ammonium f l u o r i d e (79).'! and the formation

from ammoni.um al-am and hydrof luor ic a c i d ( 5 ) , The complex i s the

ammonium s a l t of flucrcai^-iminic a c i d c r hydro-f l i^-roaluminic ac id^ as i t

was prev ious ly known (83)o Desnite i t s obvious a s s o c i a t i o n with

c r y o l i t e (NaÂlF^), ammoniijm hexafluoroaluminats i s r e l a t e d to perovski te

and the potassi>m:i hexa f lucrop la t ina te (K^PtCI^) i n i t s s t ruc ture (8if).

The exi.st'ence o f ammoni.um hexafluoroalumina-*-* has been v e r i f i e d by

a r;.^ber of authors (85-39ôWien heatc-d i n amn* a i t decomposes i n stages

to alumi.nium rd-trideo The stages are summarised as f o l l o w s : -

( N K ^ ) ^ A I F ^ ^!}bs> ^îÂlF^ (1)

N H ^ A I P ^ ] ^ AJF^ ( 2 )

A l F , AUJ (3)

In Lhe pr-asent work« th*? temperatures at- v/hich these reac t ions occur

i.3 found to d i f f e r from the r e s u l t s of Shinn et a l ( 9 0 ) and Maak ( 9 l ) -

Rear-tlon temperatures according to Shi.nn et a l :

( 1 ) 1 - 0 ° C ( 2 ) 3 0 0 ° C ( 3 ) 7 2 0 ° C

Reac t ion tempera tures a c c o r d i n g t o Maak:

( 1 ) 300°C ( 2 ) 600-700°C ( 3 ) 800°C

Shinn et a l a l s o r e p o r t s the ex i s t ence o f *|^-A1F,.

The d e c o m p o s i t i o n c f t h e f l u o r o a l u m i n a t e t o f o r m a l u m i n i u m n i t r i d e

has a l s o been s ud ied by i^'unk and Boehland ( 9 3 ) .

I . l , i ( d ) Mi sce l l aneous methods o f p r e p a r a t i o n

There have been r e p o r t e d v a r i o u s methods o f p r e p a r i n g a lumin ium n i t r i d e

wh ich do not appear to have a wide commercial a p p l i c a t i o n , but a r e r e p o r t e d

here .s n o s s i b l e a l t e r n a t i v e s t he more common methods. Perhaps the most

noted o f these methods i s the r e a c t i o n o f a i ihydrous a l u m i n i u m c h l o r i d e v ^ t h

ammoniaj knovm as the Grov/i. . î-ocess. T h i s method i s p r i n c i p . a l l y used

t o p repare s i n ^ e c r y s t a l s o f a lumin ium n i t r i d e wh ich a re t o be used f o r

e l e c t r o n i c dev ices ,

Tiede et a l (57) r e p o r t e d t h a t when animonia i s passed o v e r pure

a lumin ium c h l o r i d e the monoammoniate i s f o r m e d , and when t h e s a l t i s

decomposed onatungsten s p i r a l a t 1000°C a l u m i n i u m n i t r i d e i s produced.

Th i s process has been s t u d i e d more r e c e n t l y by Renner ( 5 8 ) , v;hose r e s u l t s

c o n f i r m the work o f T iede and o t h e r s . T h i s method can a l s o be used t o

prepare t he n i t r i d e s o f s i l i c o n ( 6 3 * 6 6 ) , b o r o n , g a l l i u m and i n d i u m ( ! 8 ' ,

and i t has been adapted by Bopper (60) , i n h i s s tudy o f t he f o r m a t i o n

o f non-ox ide c o a t i n g s by p y r o l y s i s . The method has a l s o been used t o

coa t v a r i o u s ceramic m a t e r i a l s w i t h a lumin ium n i t r i d e . I f t he t empera ture

o

i s r a i s e d t o 2500 C, i t i s r e p o r t e d t h a t al-omdnium n i t r i d e o f h igh p u r i t y

and d e s i r e d p a r t i c l e s i z e can be p repared f r o m the c h l o r i d e and ammonia

(62 ) .

I t has been observed (6k-) t h a t i f a lumin ium i s r e a c t e d w i t h t he

a lumin ium c h l o r i d e mono-ammoniateg an a lumin ium c h l o r i d e - a l u m i n i u m

n i t r i d e complex i s foimeds

AlC.lÎ'JH^ + A l A i C l ^ . AU'I +

which i s a p p a r e n t l y so s t a b l e t h a t i t does n o t decompose when s t r o n g l y

heatedo I t t h e r e f o r e may v / e l l be t h a t t h i s compound i s more r e s i s t a n t

t o o x i d a t i o n than the pure n i t r l d e o

V/iberg and May (65) have r e p o r t e d t h a t a lumin ium n i t r i d e can be

produced f rom the a d d i t i o n compound o f a lumin ium h y d r i d e and amnonia.

The r e d u c t i o n o f h y d r i d e s u s i n g ammonia has r e c e n t l y been s t u d i e d by

Berg e t al- (66) u s i n g s i l i c o n hydrLde^ Components v^hich sLre c o r r o s i o n -

r e s i s t a n t and v/hich possess mechanical s t r e n g t h have been p repa red f r o m

alumir_ium h y d r i d e and amnonia o r n i t r o g e n p u s i n g such processes as s l i p

c a s t i n g ^ e x t n i d i n g , o r ho t p r e s s i n g (67)0

Aluminium n i t r i d e has been prepared i n a non c r y s t a l l i n e f o r m by

pass ing d r y ammonia ove r a l u m i n i u m phosphide a t 1000-1100 C ( 6 8 ) .

A l P -f. m-^ AITJ + +

The phosphorus produced by t h i s r eac t ion-condenses i n the system. I t i s

removed by pass ing an excess o f n i t r o g e n t h r o u g h the system a t 1375-

1600°C , The phosphorous i s c a r r i e d away., separa ted f ixan t h e n i t r o g e n ,

and r e a c t e d vn. lh a lumin ium powder a t 1250°C t o g i v e a l u m i n i u m phosph ide .

The phosphide can be r e t u r n e d t o t h e system^ t h u s making t h e process

c o n t i n u o u s (69)*

I f ammonia i s passed over aluminas heated a t a lOOO^Cj a lumin ium

n i t r i d e i s formed ( 7 0 ) « However^ t h e p r o d u c t i s in5)ure due t o f o r m a t i o n

o f o x y - n i t r i d e s ( 7 l ) o T h i s p r e p a r a t i o n i s ques t i oned as i t i s

the rmodynamica l ly u n f a v o u r a b l e a t a 1000°C«

There have been e a r l y r e p o r t s t h a t i f a lumin ium s u l p h i d e i s heated

i n n i t r o g e n ^ a l u m i n i u m ^ n i t r i d e i s formed ( 7 2 ^ 7 3 ) o Borchers aJid Beck

( 7 ^ ) have r e p o r t e d a process i n v o l v i n g e l e c t r o l y s i s . Impure n i t r i d e

i s formed when m i x t u r e s u s i n g pov^dered a lumin ium are n i t r i d e d ( l 8 ) ;

when l i q u i d suLuminium i s n i t r i d e d (iS) g and a l s o by t h e t h e m i a l

evapo ra t i on o f a luminium i n vacuo ( 2 ) , presumably due t o t h e presence

o f r e s i d u a l n i t r o g e n * Alumin ium n i t r i d e can be p repared i n d u s t r i a l l y

by n i t r i d i n g a m i x t u r e o f a lumin ium powder w i t h Ôfo a lumin ium n i t r i d e

(20) . H i g h p u r i t y n i t r i d e has been prepared by n i t r i d i n g a l u m i n i u m ,

a luminium n i t r i d e and a lumin ium f l u o r i d e m i x t u r e s ( 22 -24 ) . The f l u o r i d e

o

i s used as a c a r r i e r o r c a t a l y s t and v o l a t i l i s e s a t 12 00 C. Aluminium

n i t r i d e i s formed by n i t r i d i n g a lumin ium a l l o y s , p r i n c i p a l l y i r o n -

a lumini \ im (Zk) p s i l i c o n - a l u m i n i u m ( 2 6 ) , a l u m i n i u m - z i n c (27)5 and l i t h i u m -

aluminium (91) a l l o y s . The combust ion o f a lumin ium w i t h n i t r o u s ox ide

(28) and a n i t rogeneous o r g a n i c chemical^, such as t he t h ioca rbamide ( 2 9 )

t o f o r m the n i t r i d e have been r e p o r t e d . Alumin i i im n i t r i d e has been

rormed f r o m aJLumina u s i n g a n i t r o g e n plapma j e t (lOO) o r by h e a t i n g

a luminium phosphate and carbon (75)2 o r o r t h o c l a s s (76) i n an atmosphere

o f n i . t r o g e n . Jander and Weiss (77) have r e p o r t e d t h a t stannous n i t r i d e

and alumi nium bromide r e a c t t o g e t h e r t o f o r m a l u m i n i u m n i t r i d e .

I . l . i i The chemica l p r o p e r t i e s o f eiluminium n i t r i d e

I t has been r e p o r t e d t h a t when a lumin ium n i t r i d e i s heated i n a i r

i . t t u r n s g r e y . I t was a l s o r e p o r t e d t h a t a lumin ium n i t r i d e h y d r o l y a e s

t o g i v e t he hydrox ide ( 5 6 1 1 4 ) s and the a c t i o n o f d i l u t e s u l p h u r i c

a c i d r e s u l t s i n the f o r m a t i o n o f a lumin ium h y d r o x i d e . Hardtung ( IO5)

r e p o r t e d t h a t hydrogen does no t r e a c t w i t h a lumin ium n i t r i d e . The

origi .naJ. o b s e r v a t i o n o f M a l l e t t h a t when a lumini inn n i t r i d e i s hea ted t o

rridnizss i t deconiposes to g i v e a l u m i n a j was c o n f i r m e d by P i t c h e r ( l 0 6 ) c

P i t c h e r and SpengeL ( IO7) i n 1925 r e p o r t e d t h a t d r y halogens r e a c t e d

s l o w l y th aJ-uminiiim n i t r i d e - and in f a c t a t 760°C a lumin ium n i t r i d e

forms ^he c h l o r i d e i n the presence o f c h l o r i n e . Bromine i s observed t o

reacb s l o w l y wi t h Ihe n i t r i d e o n l y a t h i g h t empera tures ( 5 6 ) , C o n t r a r y

to these r e p o r t s 3 Flament ( IO9) has observed t h a t t he halogens do n o t

râc*. vri.th aluminixjJD n i t r i d e , but no phys i . ca l c o n d i t i o n s were g i v e n ^

Bradshsw and Matthew (110) r e p o r t e d i n 1958 t h a t the n i t r i d e

o?ddize3 i r . oxygen a t temperatures g r e a t e r than a thousand degrees,

T a y l o r and Len ie ( i l l ) have s t u d i e d the p r o p e r t i e s o f a lumin ium n i t r i d e

e x t e n s i v e l y (see Table l ) ^ and have r e p o r t e d t h a t i t i s n o t h i g h l y

r e s i . s t an t t o c o r r o s i o n by m i n e r a l a c i d s , and i t s t a r t s to o x i d i s e i n

c i i r a t 600°Ct A r e v i e w o f the o x i d a t i o n o f a lumin ium n i t r i d e i n d i f f e r e n t

atnespheras and teraperatureSjj and f o r v a r y i n g i n t e r v a l s o f t ime^ has

been d s s c r i b e d ( 1 1 2 ) , and w i l l be compared \ 7 i t h the p r e s e n t \Jork. l a t e r *

A l J i n i i i i j j a n i t r i d e has been produced v/hich i s i n f a c t s t a b l e t o m o i s t

a i r ( 1 1 5 ) 0 The o x i d a t i o n o f the n i t r i d e was i n v e s t i g a t e d i n 1962 by

Cooper . t , a l (1^5)5 'our the act^ v a r i o n ene rg ie s f o r o x i d a t i o n v a r y

ccni^di=:T-ab';y even f o r t he sane s p e c i f i c su r face^ The o x i d a t i o n i s s a i d

-.0 fo l low a p a r a b c l i c l a w .

8 .

Table 1

The e f f e c t o f v a r i o u s media on a l u m i n i u m n i t r i d e

' "a ter A c i d A l k a l i M i s c e l l a n e o u s !\ef erence

rfecomposes At tacked by D'rcomposes Gaseous c h l o i i n e (177)

c o n c e n t r a t e d c a i t p l e t e l y . a t n o m a l pressure

su lphur r l c . decomposes i t a c i d »

Hydrolyses

e a s i l y w i t h

NaOH.

c o m p l e t e l y .

( 7 8 , l l O

R e s i s t a n t t o Decomposed ( : 8 2 '

comjnon a c i d s . by NaOH.

P a r t i a l l y R e s i s t a n t t o BÔ^ (111)

r e s i s t s mol ten a lumin ium

l i n e r a l ac i ' 3 and c r y o l i t e T-Ia-ÂIF^,

Deccmposes Ko e f f e c t v / i t h (ISO ^ 18^)

P --^'imposes i n hot c h l o r i n e , bromine

i n ho t coi c ' JaOK o r i o d i n e .

l iNO^, ^ 3 ^ 4 U n a f f e c t e d by

m o l t e n a l u m i n i u m ,

I r a n o r s i l i c o n .

( 1 1 7 ) "

9.

To2 T i i e^ j ^ r v s t ^ l J -O^r^ th^_ beat^ o f fonp-at iof . and thenna l s t a b i l i t y o f

a ju ja in lun : n i t r i d e

I • 2 o i The :^'^".^^^\p_fi^"^P^ aVjmi.niAun n i t r i . d e

The c } " y s t a l sfcr ' . ictiire o f aJvjn.Lnluin n i t r i d e was f i r s t s t u d i e s by O t t

ilPy) ir, '}3?.U.o Ha r e p o r t s t h a r i t c r y s t a l l i z e s i n the hexagonal system

w i * } \ a c / a : : ^ t i o o f 1 . 6 0 1 anr5 an aLvimin:3.iun''nitrogen i n t e r - a t o m i c d i s t a n c e

o f IQS'P^. j i . The i o n i o charges I n s i n g l e c r / s t a l a o f aj.uminiura n i t r i d e

he.vt b&e-ii s';-j.di*id ( 1 3 0 ) ^ and *.he e l e . ? i r o n i c s t r u c t u r e has been d iscussed

{'LM)O The uii;:.t c e l l dimensions were l a t e r repoi - ted by a number o f

wo-yer5 (733 l i l ^ 122 , 1 2 8 y 132-6) (sse Tab le 2 ) . The term^ i d e a l w u r t z i t e

r-;oi;jr^^^ ( l , »e . hexagonal s t m c t u r e o f z i n c s u l p h i d e ) , }ias been used t o

desc.r^.be a luminium n i t r i d e by s e v e r a l a u t h o r s ( l l 7 j , 123 )^ but I ^ i r r y e t a l

(157) f rcm t h e i . r s t r u c t u r a l invesTii.gatic 'ns o f a2.uminium n i t r i d e , c o n f i r m e d

trie e a r l y p roposa l s (129) and revea led a d i s t o r t i o n a l o n g the c - a x i s and

the s:'--ro.cr,ora o f t h e rJ . t r i .de has l a t t e r l y been compar*ed w i t h the ZnO

sr.r^^Û-ii^. The i n f l u e n c e o f the bonding i'oixies on t h i s d e v i a t i o n f r o m an

i d e a l a t r j o t a r ^ r^s been 3tudi.ed by Zndanov and Brysneva ( 1 3 6 ) .

Daâ ^las been p u b l i s h e d on the ex i s t ence o f cub i c aJ.uminium n i t r i d e

l a - UlOu X) form-^d by n i t r i d i n g a s t e e l c o n t a i n i n g a lumin ium (94)9 but

the f o r m a t i o n c f a cable n i t r i d e i s ques t ioned by Kohn e t a l ( i J . S ) .

HCT /evsv-j when the n i t r i d e i s sub jec ted t o h i g h s t a t i c and dynamic pressures

a m o d i f i c a t ; l o n o f the s t r . i c b j T ^ i s observedo b u t no c o n c l u s i v e evidence o f

a p o s s i b l e w i r t - z i t e - s p h a l . e r i t e t r a n s f c m a t i o n i s r e p o r t e d ( 9 ) .

Table 2

The l a t t i c e parameters o f a lumin ium n i t r i d e

a CX) c a ) c / a r a t i o Reference . - .

3 . 1 1 3 1 . 6 0 1 (132)

4 . 9 6 5 1.599 (132)

1 . 6 0 0 (133)

^;.\0 * 0 . 1 4 . 9 6 3 0 . 0 1 1.601 (134)

3 o l O + 0 . 0 1 4 o 9 6 8 + 0 . 0 1 1 . 6 0 2 (122)

z , „ 9 8 0 1.601 (111)

% 1 1 0 + 0 . 0 0 2 4 . 9 8 0 ^ 0 . 0 0 2 1,601 (135)

3 . 0 5 + 0 . 0 2 ^ - 4 o 9 3 ± O i 0 6 1.601 (128)

/ , . 9 6 8 1.595 (78)

11

I « 2 « i X J^^:^£x^^.L j lpJ 'mation and the rmal s t a b i l i t y o f a l u m i n i u m n i t r i d e

The hea t o f f o r m a t i o n o f a l innini ian n i . t r i d e has been s t u d i e d by a number

o f v/orke.T-s e x t e i d i n g over a p e r i o d o f some T i f t y y e a r s , M a t i g n o n ( 1 3 9 )

in was t h e f i r s t to l o o k a t t b i s f ea t i t r e . , S i x yea r s l a t e r , Moldenhawe

(lAO) again l o c k e d a t the heat o f f o r m a t i o n , b u t no r e s u l t s a re g i v e n i n

the l i t e r a t u r f i o P re sco t t and Hencke ( l i + l ) quote a f i g u r e o f " 8 0 » 4 3 k . c a l / m o l e

(see Table 3 ) . M e l l o r ( 9 7 ) g i v e s a va lue f o r the s p e c i f i c heat o f a lumin ium

n i t rn.dc a=>

0 0.1803 + 2.750 X 10 ^ T + l o 9 3 / x 10 ^ T^

TJV5.3 t s r m i .nvc lves a power c f 10 ^ and l a t e r express ions do n o t i n v o l v e

any Verms s m a l l e r than 10 Prom Tab le 3 t h e r e i s a w ide range o f va lues

f o r the hea t o f foimat i .ono T h i s i s p r o b a b l y due t o t he e x p e r i m e n t a l methods

used by each av:thor. F o r example Weugebauer e t a l (147)s u s i n g a bomb

o a l o r i m e t e r , g i v e s a v a l u e w h i c h i s i n agreement w i t h Sato (143)5 whereas

Ne:.'iiimamm ^ t _ a l (1^2) vised sodium f l u o r i . d e as a " c a t a l y s t " . A p i n e t a l ( I 4 8 )

usftd a l i j m i n i ' i m and l e a d n i t r r i .de i n an e x p l o s i v e method t o de t e rmine the heat

o f f o r m a t i o n * S c h i s s e l and V/ i l l i a r a s (149) deduced t h e i r va lue u s i n g the

mass"specti:i::metry t h i r d lawo Fran T a b l e 3 a s t a t i s t i . c a l a n a l y s i s suggests t h a t

the hea t o f foTTnation i s in t he r e g i o n o f -7b koCa l s /mo le .

Y/smer (ihB) f o r the r e a c t i o n o f t he elements t o g i v e a l u m i n i u m

n i t r i . d e a t 1800°G r e p o r t s a va lue o f the e q u i l i b r i u m gas c o n s t a n t (Kp) as

l o g K.p : l^hBo Kubaschewski and Evans ( I 5 0 ) equated the f r e e e n e r ^ o f

for .mat ion o f Al l^ ass

G -^154,000 ^ 44.5T

f o r t he eq i i a t i cn 2A1 + -- 2A1N.

12

Tabj.e 3

The heat o f f o n n a t i o n o f a lumin ium n i t r i d e

AHp^g kcal/mvol« Referen.-ê

8 0 . ^ , 3 ( l U )

- 3 ' . ^ (142)

- ? . . 7 (143)

^ 6 L , 0 (144)

- 6 4 o 0 (145)

-:-fS,i.7 C . 2 0 (147)

- 5 " o 6 ( l i f B )

- 6 3 . 0 (149)

^ 7 6 . 5 1 1 -0 (150)

7 i . O (180)

13.

O l e t t e and ^^me.Ancey-^ôret ( l 5 l ) i n a r e c e n t a r t i c l e have

s t u d i - d the v a r i a t i o n o f f r e e energy o f f o r m a t i o n o f t he some oxides

and n i t r i d e s , among them a lumin ium n i t r i d e .

I t has been r e p o r t e d t h a t a lumin ium n i t r i d e has no m e l t i n g p o i n t

and d i s s o c i a t e s i n t h e range o f 2 2 0 0 - 2 4 0 0 ° C i n agreement v / i t h the e q u a t i o n :

I n f a c t , no m e l t i n g has been observed as h i g h as 2700^0 ( 1 5 8 ) . The

v a p o u r i s a t i o n behaviour o f a lumin ium and boron n i t r i d e has been s t u d i e d

u s i n g an e f f u s i o n method (132)^ T h i s method appears t o be complex and

no t t o t a l l y r e l e v a n t t o t h e p resen t s t u d i e s , l lah e t a l (153) have shown

f r o m t h e i r thenriodynamic c a l c u l a t i o n s , t h a t a luminium n i t r i d e i s s t a b l e

a t 1727^0 ( 2 0 0 0 \ ) which agrees vrith e s t a b l i s h e d f a c t s . The v a p o u r i s a t i o n

pressure ( l 5 4 ) and the decompos i t ion o f a lumin ium n i t r i d e - ( 1 5 4 » 160, l 6 l )

have been s t u d i e d . The a c t i v a t i o n energy o f s u b l i m a t i o n f o r g a l l i u m

n i t r i d e has been s t u d i e d u s i n g mass spec t rome t ry methods ( l 5 5 ) . The

v a p o u r i s a t i o n behaviour (155, 159) and t h e k i n e t i c s o f v a p o u r i s a t i o n o f

a lumin ium n i t r i d e have a l s o r e c e n t l y been d i scussed ( 1 5 6 ) .

1.3 The p h y s i c a l and mechanical p r o p e r t i e s o f a lumin ium n i t r i d e and r e l a t e d m a t e r i a l s

1 .3." The p h y s i c a l p r o p e r t i e s o f a lumin ium n i t r i d e

Severa l n i t r i d e s i n c l u d i n g a lumin ium n i t r i d e have been p l a c e d i n a

diamond shape p a t t e r n and t h e o r i e s p u t f o r w a r d t o r e l a t e t h e i r p r o p e r t i e s

v / i t h r e f e r e n c e t o t h e i r p o s i t i o n s i n t h i s t a b l e ( l 2 4 ) .

l l a t i g n o n (37) s t a t e d t h a t a lumin ium n i t r i d e does n o t m e l t , b u t i n

f a c t d i s s o c i a t e s a t 2200°C. He rze r ( I I 5 ) i n 1927 r e p o r t e d t h a t i t

decomposes a t g r e a t e r than 1 4 0 0 ° C , and s ince t h e n v a r i o u s r e sea rch

vôrkers (106 , 111, 117, 14^ have r e p o r t e d t he decompos i t ion tempera ture

14.

Table 4

The decompos i t ion temperature o f a lumin ium n i t r i d e

Decomposi^n-on Temp. Preference

2i30 "c © 4 almoanheres (108)

2230-2240^'c (138)

2i.C0''c (111)

2200'^C © 4 abnospher-es ( l l 6 )

223G°C (117)

15.

o f a lumin ium n i t r i d e . The d e n s i t y o f a lumin ium n i t r i d e has been r e p o r t e d

as 3.0Z,9, 3.004 (115), 3.25 ( l l 7 ) , 3.30 ( 1 2 3 ) , and 3.26 ( i l l ) .

CrystaJ-S o f a luminium n i t r i d e have been r e p o r t e d as w h i t e ( 1 0 8 , 1 2 2 ) ,

pa l e y e l l o w ( 9 7 ) , and b lue ( 9 7 , 108 , 122 , 1 3 4 ) , and i n f a c t t he g iovr th

and p r o p e r t i e s o f s i n g l e c r y s t a l s o f a l u m i n i u m n i t r i d e has been s t u d i e d

by V/itzke (119) and Matsuraara and Tanake ( l 2 0 ) . The occurence o f b lue

c r y s t a l s has been a t t r i b u t e d t o c o b a l t i m p u r i t i e s ( 1 2 1 ) .

The e l e c t r i c a l p r o p e r t i e s o f a lumin ium n i t r i d e have been s t u d i e d

( l 2 4 r 126-128). I t has been r e p o r t e d t h a t a lumin ium n i t r i d e i s a

" t y p i c a l d i e l e c t r i c " ( 1 2 5 ) , and t he e p i t a x i a l g rowth on s e v e r a l s i n g l e

c r y s t a l s s u b t r a t e s o f a lumin ium n i t r i d e has been s t u d i e d (l28).

I . 3 . 1 i The mechanical p r o p e r t i e s o f euLuminium n i t r i d e and r e l a t e d m a t e r i a l s

Ceramic m a t e r i a l s have such d e s i r a b l e p r o p e r t i e s as r e s i s t a n c e to wear ,

m e c h a n i c a l l y h a r d , good e l e c t r i c a l i n s u l a t i o n , and r e s i s t a n c e t o c o r r o s i v e

envi.ronmentp b u t the m a c h i n a b i l i t y o f t h e ceramic i s such t h a t t o produce

a r t i c l e s vri.th c l o se d i m e n s i o n a l t o l e r a n c e s r e q u i r e s an expensive o p e r a t i o n .

However, a m a t e r i a l has been e s t a b l i s h e d w h i c h n o t o n l y possesses t h e

reputed ceramic p r o p e r t i e s , bu t a l s o has r e s i s t a n c e t o t h e r m a l shock and

i s capable o f b e i n g produced vd.th an improved degree o f d i m e n s i o n a l

accuracy . T h i s new m a t e r i a l i s s i l i c o n n i t r i . d e . A con^jar ison o f t h e

foimabili-t^y^ w i t h r e spec t t o mechanical c h a r a c t e r i s t i c s o f a l u m i n i u m n i t r i d e ,

w i t h s i l i c o n n i t r i d e and o t h e r ceramic m a t e r i a l s (see Tab le 5) suggests

t h e p o s s i b l e use o f a lumin ium n i t r i d e i n a s i m i l a r f i e l d .

16,

Table 5

A comparison o f the mechanical p r o p e r t i e s o f a l u m i n i u m n i t r i d e w i t h c e r t a i n ceramics

T t i n s i i Strer.f^:

e .h

Shear Str^MPi.h

Knoop fiaxdness

ftiodulus o f .Rl o.-i f.v

: o e f f . o f Temp.

M a t e r i a l Temp. kg/rmn^ Temp.

% kg/nTrn^ Temp.

°C

Phermal. E-xponsion X 10-^

o f S t ab -

a l 'niiitivjn n . t r i f l e

27

16.93

12.7

25

] 000

1230 (lOOg)

33030

132300

28100

25

ICOC

140c

3.64

4.03

1

2400

1 s i l i c o n [ n i t r i d -

1 .3-2.73

20 16

14.7 20

1200 3337

(I20g) 2+360

• 4600

30c

HOC

2

2.3

1900

tanteT3.um ca " b i

2-3 29100 6

t i oan i i j in b s.i.Oe

24o3 20 34000 6 2980

molybdenum d i l . l i c i d e

28

29.4

980

1200

21

6

980

.100

733 (50g)

43000 8

9.2

2030

17 .

I t i s r e p o r t e d ( i l l ) t h a t a lumin i iun n i t r i d e has h i g h t he rma l

c o n d u c t i v i t y ^ lov/ thexroal e x p a n s i o n j and good t h e n n a l shock r e s i s t a n c e .

I t i s observed t h a t t he mechanical p r o p e r t i e s o f a lumin ium n i t r i d e and

s i l i c o n n i t i i d e decrease w i t h t empera tu re , bu t when compared vrlth

molybdenum d i s i l i c i d e , the d rop i n shear s t r e n g t h i s no t so d r a s t i c .

Hcwever-, the t e n s i l e s t r e n g t h o f molybdenum d i s i l i c i d e i nc r ea se s s l i ^ t l y

w i t h t s n p e r a t u r e . The d i f f e r e n c e i n c r y s t a l s t r u c t u r e i s n o t e d as a

p o s s i b i l i t y f o r the e x p l a n a t i o n o f these o b s e r v a t i o n s (see Tab le 7 ) .

The knoop hardness f i g u r e s suggest t h a t a l u m i n i u m n i t r i d e i s no t so

rêsis tant t o mechanical d e f o r m a t i o n as s i l i c o n n i t r i d e . A lumin ium n i t r i d e

has a l o w e r modulus o f e l a s t i c i t y t h a n most o f t h e ceramics c o n s i d e r e d ,

but i s neve r the le s s compa t ib l e w i t h the m a t e r i a l s i n te rms o f i t s e l a s t i c

d e f o r m a t i o n . The bending s t r e n g t h 5 however^ reduces r a p i d l y w i t h

r e spec t t o tempera ture f o r a lumin ium n i t r i d e as v a l u e s o f 38^500 p . s , i .

a t 25* 0 f a l l i n g t o 18,000 p . s ^ i . a t 1400°C are r e p o r t e d . The t h e r m a l

s t a b i l i t y range o f a lumin ium n i t r i d e , however, i s r e p o r t e d t o be h i g h e r

t h a n t h a t o f s i l i c o n n i t r i d e , bu t no t so h i g h as t i t a n i u m b o r i d e .

A comparison o f t he c o e f f i c i e n t o f t he rma l expans ion o f a lumin ium

arid s i l i c o n n i t r i d e s shows t h a t w i t h r e spec t t o t e m p e r a t u r e , a lumin ium

n i t r i d e w i J l e:xpand more t h a n s i l i c o n n i t r i d e . However, t h i s i s s t i l l

l e s s than t h a t o f t he o t h e r ceramics c o n s i d e r e d (see T a b l e 6 ) .

The t he rma l shock c h a r a c t e r i s t i c s o f a l u m i n i u m n i t r i d e a re b e t t e r

than a l l the ceramics c o n s i d e r e d 5 except s i l i c o n n i t r i d e , and i t i s

concluded t h a t the use o f a lumin ium n i t r i d e as an e n g i n e e r i n g m a t e r i a l

has d i s t i n c t p o s s i b i l i t i e s , e s p e c i a l l y as t h e p r o p e r t i e s c o u l d be enhanced

by hot p r e s s i n g the m a t e r i a l as observed w i t h boron n i t r i d e . I n f a c t .

18,

Table 6

The theimal expansion of c e r t a i n m a t e r i a l s (92)

M a t e r i a l Percentage expansion from 250^0 to

l ) 500 2) 1000 3) 1500^0

a-uminium n i t r i d e 0.23 0.54

- s i l i c o n n i t i i d e 0.10 • 0 .28 0 .54 i.

^ ~ s i l i c o n n i t r i d e 0.07 0 .22 0.46

molybdenxim s i l i c i d e 0.37 0.83

o<.--quartz . 0.92

alumina 0»36 0.83 1.37

19 .

Table 7

A comparison o f the c r y s t a l s t r u c t u r e s o f c e r t a i n m a t e r i a l s

M a t e r i a l C r y s t a l S t r u c t u r e

Type L a t t i c e Parameters a ( g ) c ( g ) ^â

al.urainium n i t r i d e hexagDnal Z nS

(XnO)

3 . 1 0 4 4 * 9 6 5 1 . 6 0 0

o C"Silicon n i t r i d e

^ - s i l i c o n n i t r i d e

hexagonal

7 . 7 6

7 - 5 9

5 c 6 4

a . 9 2

0 . 7 2 5

0 . 3 8 5

t a n t a l u m crabide cub i c NaCl 4 . 4 5 6

titainium boride hexagonal 3 . 0 2 6 3 . 2 1 3 I0O62

molybdenimi d i s i l i c i d e tetragonal MoSi^ 3 . 2 0 3 7-887 2 . 4 6 3

20.

i t has been showm t h a t the t h e r m a l shock parameters o f a lumin i imi n i t r i d e

a re improved by t h i s t e c h n i q u e .

I . i * . The f a b r i c a t i o n and uses o f a l u m i n i u m n i t r i d e and r e l a t e d m a t e r i a l s

I . i i . » i The f a b r i c a t i o n o f a lumin ium n i t r i d e

Rey (117) suggested the p o s s i b i l i t i e s o f a lumin ium n i t r i d e as a

r e f r a c t o r y m a t e r i a l , and i t was e s t a b l i s h e d t h a t a lumin ium n i t r i d e c o u l d

i n f a c t be used as a c o n t a i n e r f o r a l u m i n i u m up t o 2000°C (=-?). S i n t e r e d

a lumin ium n i t r i d e o b j e c t s ( I 6 2 ) were produced by p l a c i n g pov;der around a

g r a p h i t e co re and h e a t i n g t h e assembly t o 2 0 0 0 - 2 5 0 0 ° C . The shape o f the

a r t i c l e was determined by the des ign o f t h e c o r e . Porous r e f r a c t o r y b r i c k e .

and n i t r i d e d a r t i c l e s were a l s o o b t a i n e d by h e a t i n g a m i x t u r e o f corundum,

coke , c a l c i u m alurainate cement, and w a t e r o r b a u x i t e a t l60O°-1800*^C i n

nltrogenm The p r o d u c t s were f o u n d t o have d e n s i t y o f g r e a t e r than 1 and

c o n t a i n e d more than 30^ n i t i o g e n . Compacted a lumin ium n i t r i d e c r y s t a l s

bonded by a l umin ium n i t r i d e and c o n t a i n i n g s m a l l i m p u r i t i e s v/ere heated

to 1500°C i n a pure n i t r i d i n g atmosphere, r e s u l t i n g i n s i n t e r e d o b j e c t s ,

d e t a i l s o f con?>osit ion and c o n d i t i o n s b e i n g g iven ( I 6 3 ) . Aluminium n i t r i d e

powders were heated as a compacted mass t o 200O2500°C i n a n i t r o g e n

atmosphere. The vapour phase o f a lumin ium n i t i ^ d e i s formed ar.d on c o o l i n g

AlN r e c i y s t a l l i z e s Eiround the g r a i n s of• the compacted mass. Ano the r method

of producing a moulded a r t i c l e , was t c adc e s u i t a b l e or^ganic b i n d e r t o the

powderec mixture of aluminium n i t r i d e and a sma l l amount . o f m i n e r a l i s e r . . The

m i x t u r e was heated i n an o x i d i s i n g ^.^ -êre below :;00^C t o e l i m i n a t e

t he b i n d e r and ma in t a ined i n ammonia t o remove the oxygen, ,and t h e

t s n p e r a t u r e r a i s e d .

a .

Coni i ' a ry t o t he r e p o r t o f Rey ( l l ? ) j t h a t the c o n d u c t i v i t y o f

al-ji2.1rii-jin n i t r i d e up t o lÔO^C i s so l o w t h a t i t c o u l d be used as an

- i j i s u l a i i n g m a t e r i a l , e l e c t r o d e s o f a lumin ium n i t i d d e nave been prepared

( l 6 5 ) y and c o u l d be s u b s t i t u t e d f o r g r a p h i t e i n t r i e H a l l process ( l ? 5

1 6 6 ) . A r e f r a c t o r y m a t e r i a l s i m i l a r i n sompos i t i on t o these e l e c t r o d e s

was produced by hot p r e s s i n g a t i t a n i u m c a r b o - n i t r i d e - a lumin ium

n:.*:-ri'Ie m i x t u r e t o g i v e a b t ' l k d e n s i t y o f 2 . 6 gram.cc and an e l e c t r i c a l

r e s i s t a n c e o f 2 5 0 - 3 0 0 0 x l O " ^ ohmîns. ( 1 6 ? ) .

Objec ts o f al^jminium n i t r i d e v/ere prciduced by c a l c i n i n g a lumin ium

a luminium n i t r i d e powders t o g e t h e r v / i t h a c a t ^ j l y s t i n n i t r o g e i :

atmosphere ( l 6 8 ) c Aluminium n i t r i d e a r t i c l e s were p repared by m i x i n g

a lummium vn.th 7-50~A cyanam-ide^ dicyanamide o r melamine and h e a t i n g f o r

3 0 - 9 0 minu tes i n n i t r o g e n a t 1200-1400*^0 ( l b 9 ) o To s t a b i l i s e the n i t i - i d e

aga ins t water^ the p r o d u c t v/as heated i n oxygen b e f o r e o r aVter p r e s s i n g

and s i n t e r i n g a t 2 0 3 0 - 2 1 5 0 ' ^ C .

S i r t e r s f d r e f r a c t o r y m i x t u r e s o f a lumin ium n i t r i d e and t i t a n i - u m b o r l d e

ha c: been prepared ( l 7 0 ) o r i e f r a c t o r i e s were o b t a i n e d f rom a mi ,x t \ j re o f

alum^n.ium ajid . ' s i l i con powderi? v.d ch ;:'-10;-c p a r a f f i n b i n d e r and a f l u o r i d e

\'i^J:^3^y^t i n a n i t r o g e n atmosphere and hca t i .ng i n stages t o irV20°G ( 1 8 4 ) 0

/_l^^:iiii«jm n i t r i d e and a s o l i d s o l u t i o n o f a lumln i - im n i t r i d e and s i l i c o n

o a r b i d e j t o g e t h e r v / i t h a p p r o x i m a t e l y 2% vra.ter v/ere h i ' d r o s t a t i c a l l y pressed

t o give- a ha rd danse and s t r o n g l y bonded r e f r - a c t o r y ( l 7 l ) » S i l i c o n

ca rb ide i s a l s o r apo r t ed t o b e n e f i t the p r o p e r t i e s o f a lumin ium r i i t r i d e

( 1 7 4 5 lS'0„ 1 9 1 ) * C r u c i b l e s were formed by e x t r u d i n g thie pradu-ct a T t e r i t

b^d been sJ ov/iy heated to remove the v/a ter« The s i n t e r i n g o f t he a lumin ium

r i l t v i l e - a lu i .L i rL iu in ryr. iem \v.\s rv^cer i t jy been s t u d i e d ( 1 . . ' / ' ) - '^'^"^^ r e 3 \ 3 l t s W C T T ^

2 2 .

a p p l i e d i n the manufacture o f a lumin ium n i t r i d e c r u c i b l e s . The mechanism

o f s i n t e r i n g i n general and f a c t o r s i n f l u e n c i n g s i n t e r i n g has r e c e n t l y

been rev iewed ( 1 7 3 ) . A v i t r i f i e d r e f r a c t o r y composed o f a lumina and

a luminium n i t r i d e was made ' ly f u s i n g t he two compounds i n a g r a p h i t e

c r u c i b l e a t 2000-2300*^0 ( I S 3 . Care was t aken n o t t o exceed 2300°C5

because the n i t r i - d e may s t a i c to decompose i n t h i s t e r rpe ra tu re r e g i o n ,

I . 4 « i i The uses o f a lumin ium n i t r i d e and r e l a t e d m a t e r i a l s

Since the d i s c o v e r y t i u i t s i l i c o n n i t r i d e has p r o p e r t i e s s i m i l c i r t o

bo-th a c o n v e n t i o n a l cerajnic m a t e r i a l and a semi -conduc t ing m a t e r i a l ^ the

i n t e r e s t i n t he p o s s i b l e a p p l i c a t i o n s o f a lumin ium n i t r i d e has grcvric

S i l i c o n n i t r i d e f r o m the v i i - t u e o f i t s p h y s i c a l , chemica l and mechanica l

p r o p e r t i e s , has been used as h i g h tempera ture e l e c t r i c a l i n s u l a t o r s ,

r e f r a c t o r y f o r a l u m i n i u m , e n g i n e e r i n g components, A j m a c e l i n i n g s and

b a r i n g s , vrfiere t h e c o e f f i c ent o f f r i c t i o n compares w i t h such m a t e r i a l s

as P^T.F .E. and g lazes ( l " / ^ ,

Aluminium n i t r i d e i s - e s i s t a n t t o most common a c i d s and a l k a l i s and

i n f a c t a r t i c " ^s s p e c i f i c a l l y to be used i n c o r r o s i v e environment have

been made ( 1 7 6

The appi j-c a t ions o f a luminium n i t r i d e have g e n e r a l l y been d i r e c t e d

1" p roduc ing r e f r a c t o r y a r t i c l e s o f a l u m i n i u m n i t r i d e ( . V 1 1 8 , 1 8 3 , 1 9 4 #

1 8 6 such as c r u c i b l e s , o r combining a lumin ium n i t r i d e — 1 ; . - .,.M^_...ias

Acanium cr • - b o n - n i t r i d e ( 1 8 2 - , s i l i c o n n i t r i d e ( l 8 5 18?) and s i l i c o n

ca rb ide ( 2 O 4 t o f o r m such . . r t i c l e s . Alumin ium i i ^ - i d c .as been used t o

coat graphic*- ( l 3 ) , quar tz ( 6 1 ) , and a lumina ( 1 8 8 ) . and o t h e r r e f r a c t o r y

m a t e r i a l s ( 1 3 3 ) , t o improve t h e s u r f a c e p rope r t . . ô o f the ceramic m a t r i x .

S i l i c o n n i t r i d e has been used as a b i n d e r f o r r e f r a c t o r y ox ides ( I 8 9 )

and t h e r e i s p o s s i b l e use o f .aluminium n i t r i d e i n t h i s f i e l d .

Aluminium n i t r i d e has been used as a c a r r i e r f o r mol ten a lumin ium ( l ? )

and i n t h i s r e spec t i s more r e s i s t a n t t o c r y o l i t e - a l u m i n a m e l t s than

s i l i c o n n i t r i d e o r some me ta l b o r i d e s and c a r b i d e s ( l 9 0 ) ,

- 23 -

As a luminium n i t r i d e i n compact f o r m i s n o n - r e a c t i v e up t o l 6 0 0 ° C j , i t

has been developed f o r use i n connec t i on w i t h n u c l e a r r e a c t o r s ( l 9 2 p 1 9 3 )

and has a h i g h temperature i n s u l a t i o n m a t e r i a l ( 1 9 6 ) « Aluminium n i t r i d e

has been used i n the p r o d u c t i o n o f r a d i o a c t i v e i s o t o p e s ( 1 9 4 , 1 9 5 ) .

T i t a n i u m s a l t s have been added t o improve the conductance o f a lumin ium

n i t r l d C o The r e s u l t i n g cermet has been used as e l e c t r o d e s t o produce

aluminium by t he H a l l process ( l 9 7 r 1 9 B ) . Semi-conductors have been

produced u s i n g c o - a x i a l f i l m s o f a lumin ium n i t r i d e w i t h the elements o f

compounds o f germanium and s i l i c o n when the d i f f e r e n c e i n l a t t i c e parameters

i s l e s s than iÔfo ( 1 9 9 ) » The p r e c i p i t a t i o n o f a lumin ium n i t r i d e i n l o w

carbon s t e e l b e n e f i t s t he d e f o r m a t i o n c h a r a c t e r i s t i c s o f t h a t p a r t i c u l a r

s t e e l ( 2 0 0 ) , A lumin ium n i t r i d e has been used as a s l o w - w o r k i n g f e r t i l i s e r

( 2 0 3 ) 3 as a secondary a c c e l e r a t o r i n t he v u l c a n i z a t i o n o f a co -po lymer

( 2 0 0 ) 3 and i n the p r o d u c t i o n o f a roma t i c hydro-ca rbons ( 2 O I ) . T h i n

r e f r a c t o r y l a y e r s o f a l u m i n a j a lumin ium n i t r i d e and s i l i c o n n i t r i d e have

been used as c o v e r i n g l a y e r s f o r e l e c t r o n i c d e v i c e s ( 1 7 3 ) « Aluminium

n i t r i d e has been used i n connec t ion v / i t h r e i n f o r c e d f i l a m e n t s ( 5 , 9 5 ) „

I n t h i s t h e s i s the a u t h o r has s t u d i e d a j id compared e x i s t i n g and nev/er

processes f o r t he p r o d u c t i o n o f a lumin ium n i t r i d e , v i z .

( a ) Seipek p rocess ; the p r e p a r a t i o n o f the n i t r i d e f r o m i t s elements; the r e a c t i o n o f ammonium h e x a f l u o r o a l u m i n a t e i n aimionia.

The f o r m a t i o n o f a lumin ium n i t r i d e has been i n v e s t i g a t e d the rmodynamica l ly

u s i n g t he a v a i l a b l e the rmal data* The r e a c t i v i t y o f a lumin i imi n i t r i d e has

been examined w i t h r ega rd t o a tmospher ic o x i d a t i o n and p o s s i b l e h y d r o l y s i s by

steam o r l i q u i d w a t e r , p a r t i c u l a r l y f o r samples o f s i z e ranges p o s s i b l y s u i t a b l e

f o r s i n t e r i n g o r h o t - p r e s s i n g . Changes i n phase compos i t i on and c r y s t a l

s t r u c t u r e on m i l l i n g and o x i d a t i o n o r h y d r o l y s i s o f a lumin ium n i t r i d e have been

i n v e s t i g a t e d by X - r a y , o p t i c a l and e l e c t r o n - m i c r o g r a p h i c methods. Sur face area

de te r rn ina t ions have been made by gas s o r p t i o n and c o r r e l a t e d w i t h changes i n

c r y s t a l s t r u c t u r e and c r y s t a l l i t e s i z e on o x i d a t i o n o r h y d r o l y s i s . Some

compariscns have been made w i t h the r e a c t i v i t y o f s i l i c o n n i t r i d e .

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1 4 5 . Brev/er, L . , Bromley, L . A . , G i l l e s , P.V.'., S: L o f g r o n , N o L o , "The Chemis t ry and M e t a l l u r g y o f r.ii see l I an ecus feterials" . (McGraw H i l l ) , 1 9 5 0 .

1 4 6 . V/emer , G . , M e t a l u r g i e . V . G u s s e r e i r e c h , 1 9 5 3 ? 3 , 324-

1 4 7 . Neugebauer, C . A . , & Margrave , J o L , , Z .anor^f'. Chem. , 1 9 5 7 s 2 9 0 , 8 2 .

I i i 8 . A p i n , A .Ya . , Lebedev, Y u . A . , & Nefedova, O . I . , Z h u r . P i z . K h i m . , 1 9 5 8 . 12, 8 1 9 .

1 4 9 . S c h i s s e l , P . O . , & V / i l l i a r a s , V / . S . , B u l l . A m . P h y s . S o c . , 1 9 5 9 ^ 4 , ( 2 ) , 139.

1 5 0 . Kubaschevrski, 0 , , Evans, E . L . L . , cS: A l c o c k , C . B . , " J . I e t a l l u r g i c a l Thermoc h e m i s t r y " , (Pergamon P r e s s ) , 1 9 6 7 -

1 5 1 . O l e t t e , M . , cS: Ancey-Moret , M . F . , Revue de M e t t a l l u r g i e , I 9 6 3 , 5 7 0 .

1 5 2 . Hoch, M . , Sz V /h i t e , D . , Ohio S t a t e U n i v e r s i t y Research F o u n d a t i o n , O c t . 2 9 t h , 1 9 5 6 .

1 5 3 . Mah, A . D . , K i n g , E . G . , V / e l l e r , V / o V / . , t5c C h r i s t en sen, A . O . ^ U o S o B u r . Mines R e p . I n v e s t . , No. 5 7 1 6 . 1 9 6 1 .

1 5 4 - Dredger , L . H . , Dadape, V . V . ^ & Marg rave , J . L . , JoPhys.Chem,, 1 9 6 2 , 66 , 1 5 5 6 .

1 5 5 . Mun-.r, Z . A . , N . S . A . , I 9 6 3 . 1 7 ^ 2 3 3 0 7 .

1 5 6 . H i l d e b r a n d , D . L . , & H a l l , V / . F , , J .Phys.Chera. , 1 9 6 3 , 67 , 8 8 8 .

1 5 7 - S thap i t anonda , P . , & Margrave , J . L . , J.Phys.Chem. , 1 9 6 3 s 60 , I 6 2 6 .

1 5 8 . F i s h e r , A . , Phys V e r b . , 1957, 8 , 2 0 4 -

1 5 9 . Kn ippenbe rg , K . F . , u n p u b l i s h e d .

1 6 0 . G u i o c h o n . , G. , cS: Jacque, L , , B u l l . -Soc .Chemiciue de F rance , 1 9 6 3 , 8 6 3 .

1 6 1 . Bha t , I . R . , & Khan, I , A ' . , I n d i £ J.Chem., 1 9 6 7 , 5, ( l O ) , 499 .

1 6 2 . Pechiney, C o . , Ger . f ^ t e n t No, , ± , 0 2 3 , 7 1 7 , 1 9 5 8 .

163. B o l l a c k , R . , & Rey, M . , French Patent N o . , 1 ,149 ,539, 1957-

l 6 4 o Pechiney, C o . , French Intent N o . , 1,123 ,3035 1 9 5 6 c

3 1 .

165. Ceramic A b s t r . , 1960, 193a.

166. R i c k e r , R.Wo, & V/al lace , P . P . , U . S . Patent No., 3,0115983, 1959.

167.. Madek, E o A . , & R i c k e r , R.V: . , U.So Patent No., 3 , 0 1 1 , 982.

168. Morganite Research & Development L t d . , Be lg . Patent No., 620,323, 1962.

169. B a r t l e t t , H . J . , Laipp, D . T . , & Watson, G . R . , U . S . P a t m t No., 3 ,141 ,737 , 1964.

170 Mandorf, V . , U . S . Patent No., 3 ,251 ,700, I 9 6 6 0

171. Morganite Research & Development L t d . , French Ratent No., 1 ,422,049, 1965.

172o Matsuo, S . , Komeya, K . , & Matsuki , Y . , Yogyo Kyokai S h i . , I 9 6 5 , 73y ( 2 ) , 34.

173. ThumniLer, P . , & Thomm.a, W. , Met .Mat . , ( M e t . R e v « ) , 1967, 69.

: -Tli^SêfVJ^^' & Hx^tedVJ .H. ' , BDv/derMetSTlurgy, 19 61, 8, 196.

175. P l e t y u s h k i n . A . A . , & S l a v i n a , N . G . , Izr .AkadcNauk. S . S . R » N . M a t e r . , 1968, i , ( 6 ) , 893-

176. S c h r a i g e r , M . T . , Zaôlskaya L a b . , 1960, 26, 1223.

177. Beeghly, H ^ , , A n a l . C h e m . , 1949, 21 , ( l 2 ) , 1515.

178. Beeghly, H . P . , AnaloChem., 1952, 24, ( 7 ) , 1-095.

179. Duboyik, T.V;. Kaaakov,. V.K'i , R e f r a c t o n e s , ( U . S . S . R . ) » 1967, 32 , 3 , 1311. • •

180« Hi ldenbrand, D . L , , & Thea id , L . P » , U,S,Dept .Comm,,Off ice T e c h . S e r v . , - AD-258, ,410 , 1961.

181. Liaduk, J . A . , & i i i c h e r , R.W. , U.o . - a t e n t No., 3 ,011 ,982 , 1959.

182. B o l l a c k , R . , & Rey, M . , French Patent No., 1 ,149,339, 3 '57.

183. Matsuo. S . , Konseya, K . , & Matsukin , Y . , Yoyyo. Kirokai S h i , 1965, 73 , ( 2 ) , 34.

184. Chemical A b s t r a c t s , 1966, 6^, 15043c.

185. Moi^ganite Research & Develop. Co . Ltd« , B e l g . In ten t No., 620,323, 1962.

186. Buryk ina , A . L . , & Evtushok, T . M . , Vysokotemp P o k i y t i y a T r . S e n i n a r a , ( L e n i n g r a d ) , 1965, 6 , 145-

187. Tokyo. Shibara E l e c t r i c C o . , B r i t . I n t e n t No., 1,098,867^ 1968.

32.

188* B r L t . Patent No«, 717,463p 1955.

189« Matkovichp V . I . , Cot ton , E , ^ & Peretp J o L » . U»S« I n t e n t No.^ 3p259,509p 1966.

190^ Naumchik, NoG,, Samsonovj G . V „ 5 Naumchik.) A . N . , & l a p i n ^ A.A. I z v . Vyssh , Ucheb^ Zaved. T s v e t . Met* 5 1966^ 9^ (6)^ 57c

191. Mutech ChCTi. Eng. 1964j, 9* 45.

192. A u s t r i a PSit&it No.p 242^3775 19^5^

193. Bei:gp D . , L e w i s , D.Y/c 5 Dakin , ToY/,, S e s t r i c k , D . E , ^ & E s p o s i t o , J C N ^ P UcSo A i r Force S y s t . Command Tech . Rep« p APJa>-TR-66,(l)s I 9 6 6 .

194. Charltonp J . C . , & Evans , C p C . , B r i t . I n t e n t No, , 84^25* 1954.

195. Kihryp I . G . , & Adolf ^ J<,P,, B a l l , S o c . C h i m . P r . , 1967^^ 9 , 3414.

196. I t l c h e r , R.'A^, & Wal lace , P „ F . , U,So Patent No. , 3,011j,983,» 1959o

197. U^Sc Patent No. , 3,328^280, I967 .

198. Ruchravein . R e A . , French Intent No. , I , 371s868 , I 9 6 4 .

199. L e s l i e , V/ .C. ^ et a l . T r a n s . A . S . M . , 1954, ^ 6 , 1470.

200. B y m s , A . C . , & Von Stroh^ G . , U . S . Patent No. , 2A59,915p 1949.

201. Coupard, C . I . , & D . P . , French Patent No. , 870,636^ 1942. .

202. Sato , S . , & Trokava , H . , Repts . S c i . Research . I n s t . , ( J a p a n ) , 1953, 29 , 439.

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33.

CHAPTER I I . =bcperimental Techniques

T h i s c i iapter i s devoted to the experimental techniques used i n

the present, worko Reference w i l l be made to t h i s chapter when the

r e s u l t s of a p a r t i c u l a r s ec t ion are considered^

The methods observed werej

1- The foimation o f ammonium hexafluoioaliominate.

2 . Chemical a n a l y s i s .

3. Thermal a n a l y s i s ,

4» I n f r a - r e d a n a l y s i s «

5« X-ray a n a l y s i s .

6. E l e c t r o n microscopy techniques.

7 . P a r t i c l e s i z e a n a l y s i s .

8» Surface a r e a dete iminat ion.

9 i ' B a l l • m i l l i n g . .

lOg. Methods o f n i t i d d e p r e p a r a t i o n .

I I . l The preparat ion of ammoniijm hexai'luoroaluminate

Ammoni.um hexafluoroaluminate was prepsired by the fo l lowing methods,

( l ) Reaction o f aqueous ammonium f l u o r i d e v/ith an aqueous suspension of

Bayer hydrate**, «Alg0^.3H20. (2 ) By react ing aqueous ammonium f l u o r i d e

wi th aqueous so lut ions of aluminiiim f l u o r i d e , and aluminium c h l o r i d e . *

(3) By r e a c t i n g a l c o h o l i c amnonium f l u o r i d e w i th a l c o h o l i c aluminium

bromide or.aluminium n i t r a t e .

The method .se lected was to d i s s o l v e aluminium f o i l i n aqueous HP to

give a so lut ion of aluminium f luor ide^ and a f t e r f i l t r a t i o n to r e a c t the

so lut ion wi th aqueous ammonium f l u o r i d e . D e t a i l s are given i n Appendix I ,

3if

I I . 2 Chemical a n a l y s i s

The c h a n i c a l a n a l y s i s o f the ajnmonia c o n t e n t i n ammonium

h e x a f l u o r o a l u m i n a t e was c a r r i e d o u t u s i n g t he K j e l d a h l t echn ique ( 3 ) .

The n i t r o g e n c o n t e n t i n a lumin ium n i t r i d e v/as de te rmined u s i n g t he method

o f Passer e t a l ( 4 ) - The f l u o r i d e c o n t e n t o f ammonium h e x a f l u o r o a l u m i n a t e

was determined u s i n g the V / i l l a r d - ? / i n t e r t echn ique on a m i c r o - s c a l e . T h i s

method was found no t t o be s a t i s f a c t o r y f o r t h e f l u o r i d e a n a l y s i s o f

a luminium f l u o r i d e and a method based on t he r e p o r t o f Bognar and Negy ( 6 )

v/as adopted. The a lumin ium con ten t o f a l l the compoimds was de te rmined

by p r e c i p i t a t i n g the a luminium as a lumin ium o x i n a t e ( 2 , 7 ) .

D e t a i l s o f the a n a l y t i c a l methods are g i v e n i n Appendix I I .

I Io3 Thermal a n a l y s i s

The t h e r m o a n a l y t i c a l t echniques used i n the p r e sen t s tudy v/ere

( a ) Thennograv imet r ic A n a l y s i s , (b ) D i f f e r e n t i a l Thermal Analys iSo

I I . 3 . i T h e m o g r a v i j n e t r i c a n a l y s i s

Thermogravimetry i s t he con t inuous o r f r e q u e n t measurement o f we igh t

a t a p a r t i c u l a r t e m p e r a t u r e , o r as a f u n c t i o n o f tempera ture change. The

measurement o f w e i g h t and tempera ture as the specimen i s b e i n g heated i s

n o t 3in?)lep s e v e r a l e r r o r s t e n d i n g t o d i s t o r t t he system ( 2 l ) o

The balance used i n t h i s a n a l y s i s was based on a sugges t ion o f Gregg

and Winsor ( 2 2 ) p u s i n g a n u l l p o i n t thermo-halance d e s i g n . The balance

( S t a n t o n ' s A49) c o u l d r eco rd w e i g h t s t o an accuracy o f 0 . 2 mg and was

supported on a dex ion f rame above the fX imace . The f u r n a c e was c o n s t r u c t e d

i n the f o l l o w i n g manner. Nichrome w i n d i n g s was p l a c e d i n double t u r n s

round a grooved s i l i c a tube o f 1 i n c h I . D , The tube was suppor ted i n a

m i x t u r e o f v e r m i c u l i t e and asbestos c o n t a i n e d i n a c y l i n d r i c a l s t a i n l e s s

s t e e l case. One end o f the s i l i c a tube was sea led . The nichrome wind ings

35 -

were a t t a ched t o a temperature c o n t r o l l e r (Sunvic ( A J l ^ I ^ ) ) . , The

specimen to be heated was suspended i n the f u r n a c e by nichrome \v±rG

connec t i ng t he base o f a w e i g h t i n g pan t o t he nichrome c r u c i b l e holder^

Nichrome v/as used , as i t does no t beg in t o o x i d i s e a p p r e c i a b l y i n w i r e

fo rm u n t i l llOO^C ( 2 4 ) o The specimen was p l a c e d i n a 1 0 c . c „ p o r c e l a i n

c r u c i b l e ^ p l aced i n the c r u c i b l e h o l d e r and suspended i n the fu rnace^

A chromel-al .umel coup le5 capable o f r e c o r d i n g tempera tures up to a

llOO^C was p laced a few c e n t i m e t r e s f r o m the specimen.

The f u r n a c e was b rough t t o temperature aoid t h e w e i g h t changes were

observed a t a p a r t i c u l a r t empera tu re .

I I . 3 . i i D i f f e r e n t i a l t he rma l a n a l y s i s

I n t r o d u c t i o n / "

D i f f e r e n t i a l thermal a n a l y s i ^ i s a t echn ique by which t h e r m a l

e f f e c t s are measured, u s u a l l y as the sample i s hea ted o r c o o l e d . I n

bas ic technique i t i s r e l a t e d t o c a l o r i m e t r y . As heat i s added t o a

c a l o r i m e t e r o r i s o l a t e d s y s t a n , the tempera ture o f the system w i l l r i s e

a p p r o x i m a t e l y l i n e a r l y . V / i t h a specimen p r e s e n t , the specimen has t o be

heated as w e l l so t h e r e w i l l be a decrease i n the r a t e o f tenqêrature

i n c r e a s e . I f the specimen undergoes some t r ^ s f o r m a t i o n ^ a d d i t i o n a l

heat must be added t o the system t o change the specimen t o i t s new f o r m .

I f a r e f e r ence j m c t i o n i s p resen t w i t h i n t he sys tem, t h e t empera tu re

d i f f e r e n c e a f t e r t h e t r a n s i t i o n w i l l be z e r o . I f t h e t empera ture

d i f f e r e n c e was p l o t t e d aga in s t t empera ture i n s t e a d o f heat i n p u t , and i f

the thermocouple was i n s e r t e d i n the specimen and r e f e r e n c e thermocouple

i n an i n e r t m a t e r i a l , t h i s v/ould r e s u l t i n the e s s e n t i a l s o f d i f f e r e n t i a l

t he rma l a n a l y s i s . The the rmal t r a n s f o r m a t i o n t a k i n g p l a c e can be de t ec t ed

by such a system f r o m the p l o t o f temperature d i f f e r e n c e a g a i n s t temperature,

36 -

The sample shape and s i z e v / i U a f f e c t the shape o f t h e p l o t p o r

peak a a s i t i s known <,

An a c c o u n t o f t h e e3q)erLmental f a c t o r s w h i c h a f f e c t t h e shape o f a

d i f f e r e n t i a l c i i r v e h a s been g i v e n e l s e w h e r e ( 2 l ) .

A p p a r a t u s

The a p p a r a t u s u s e d was b a s e d on the d e s i g n o f Gr imshaw e t a l ( l 8 ) »

28 gauge v / i r e thermocoup le s (n ichrome and a l u m e l ) v/as p l a c e d i n t h e

doub le h o l d e r v a t h e a c h s e c t i o n c o n t a i n i n g a common l e a d v ^ i c h was p l a c e d

i n i d e n t i c a l p o s i t i o n to i t s n e i g h b o u r .

The sample u n d e r t e s t vias p a c k e d i n t o one o f t h e two c u b i c a l compart

ments ( l cm w i d e ) o f t h e t h i n w a l l e d r e f r a c t o r y holder^, t h e o t h e r b e i n g

f i l l e d w i t h i n e r t m a t e r i a l . T h i s h o l d e r was f i t t e d i n t o t h e l o w e r h a l f

o f a r e f r a c t o r y b l o c k p the upper h a l f f o r m i n g t h e c o v e r s V/hen assembledg

the c y l i n d r i c a l b l o c k v/as p l a c e d i n a tube f u r n a c e ( G r i f f i n & G e o i ^ e ) o f

s i m i l a r d i a m e t e r . The thermocoup le s were c o n n e c t e d t o a r e c o r d i n g

i n s t r \ u n s n t (Cambridge I n s t r u m e n t s L t d , ) , The t e m p e r a t u r e o f the spec imen

was measured by i n s e r t i n g a s i m i l a r r e f r a c t o r y b l o c k i n t h e o t h e r s i d e

o f t h e f u r n a c e o The b l o c k c o n t a i n e d t h e same i n e r t m a t e r i a l a s u s e d i n

t h e sample b l o c k and t h e t e m p e r a t u r e vreis r e a d by p l a c i n g a n i c h r o m e

thermocouple i n the b l o c k and c o n n e c t i n g i t to a d i r e c t l y c a l i b r a t e d

t e m p e r a t u r e r e c o r d e r ^ u s i n g s e l f c o m p e n s a t i n g thermocoup le l e a d s .

The r d s e i n t e m p e r a t u r e was r e c o r d e d f o r t h r e e t e m p e r a t u r e s e t t i n g s

and f o r a h e a t i n g r a t e o f 10-13°C p e r min a f u r n a c e s e t t i n g o f 180 V v/as

usedo

1 1 , 4 I n f r a - r e d a n a l y s i s

T h e i n s t r u m e n t u s e d f o r t h i s work was a Unicam S P 200 d o u b l e beam

spec trophotometer* The i n s t r u m e n t h a d an a a r c o o l e d N e m s t g l o w e r a s i t s

- 37 -

s o u r c e o f r a d i a t i o n . , and a monochromatcr o f the L x t t r c w a r r a n g e m e n t . The

d e t e c t o r v/as a G o l a y pneuma.tic t y p e . The c h a r t r e c o r d e r v/as o f the f l a t

bed. t y p e . The i n s t r u m e n t o p e r a t e d o v e r a r a n g e o f 65O cm to r-.m

v a t h a change i n s c a l e a t 2000 cm "~. Sodium c h l o r i d e p r i s m s were used

thrcôghcut t h i s work .

Sample p r e p a r a t i o n

S e v e r a l t e c h n i q u e s have been deve loped f o r o b t a i n i n g i n f r a red

a b s o r p t i o n s p e c t r a o f s o l i d s o T h e s e i n c l u d e m u l l i n g o r s u s p e n d i n g t h e

s o l i o :in a l i q u i d medium and t h e p r e s s i n g the sample i n a 3 . k a l i hÔ.idesc

The l i q u i d t e c h n i q u e v/as employed throughout t h i s v/ork - The comnicti

t e c h n i q u e i s to suspend a few m i l l i g r a m s i n a c o m m e r c i a l Nu.iol m u l l .

Ho'.vever; t h e r e p o r t e d v a l u e s o f the a b s o r p t i o n s p e c t r a o f t h e ammonium

f l u o r i . d e and i t s complexes a r e i n the range 1420 - 1490 cm " ^ and ,5000 -

3 3 0 0 cm o N u j o l gave s t r o n g a b s o r p t i o n peaks a t I 4 6 O cm and 2850 cm ^•

T e t r a c h l o r o e t h y l e n e g i v e s s t r o n g a b s o r p t i o n p e a k s a t 780 cm so i t was

d e c i d e d to u s e the N u j o l m u l l i n t h e range 65O - I'P.OO cm and

t e t r a c h l o r o e t h y l e n e i n the range 1200 - 5 0 0 0 cm .

A fe-,v m i l l i g r a m s o f each spec imen were ground i n an a g a t e and mortar^

A fe-.Y c r o p s o f the s o l v e n t was added and the m i x t u r e was reground and

placcjd between t h e sodium c h l o r i d e p l a t e s = The p l a t e s were mounted i n

the h o l d e r and t h e a s s e m b l y p l a c e d i n the i n s t r u m e n t u s i n g a r e f e r e n c e

beam o f a i r .

11*5= X - r a y a n a l y s i s

Two methods o f i d e n t i f i c a t i o n were u s e d ; po\'/der d i f f r a c t o m e t i y and

a 9 cm x - r a y powder camera ,

- '^'5 * - B;>v/der d i f f r a c t o m e t r y

I n the p r e s e n t work most o f the compounds could, be i n d e x e d on

o r t h o g o n a l a x e s .

- 38 "

From nX = 2 d s i n 0 ( l )

ana = hâ- . -^^b- . l / c - ( 2 )

where hj, k , 1 and the c i y s t a l i n d i c e s and a^ and c and t h e c r y s t a l

p a r a m e t e r s and by s u b s t i t u t i n g e q u a t i o n ( 2 ) i n e q u a t i o n ( l ) ^ i t i s

p o s s i b l e t o show a r e l a t i o n s h i p between t h e B r a g g a n g l e and t h e i n t r i n s i c

c r y s t a l l o g r a p h i c p r o p e r t i e s o f a p a r t i c u l a r s y s t e m , the l e n g t h o f the

' e q u a t i o n depending on t h e symmetry o f the c r y s t a l s y s t e m

i ^ e . s i n ^ Q = ^ 1 ^ ~ •** " 2 * ~

a b c 2

The agreement between t h e c a l c u l a t e d s i n 9 v a l u e s o r d - s p a c i n g s

and t h o s e observed w i l l i n d i c a t e t h e a c c u r a c y o f e s t i m a t i n g the c e l l

p a r a m e t e r s a , b , and c .

I n t e n s i t y o f r a d i a t i o n e m i t t e d by an e lement i s p r o p o r t i o n a l to

the atomic s c a t t e r i n g f a c t o r f and f = 3, the a t o m i c number o f t h e e l e m e n t ,

when 0 i s z e r o , but f a l l s o f f a s Q i n c r e a s e s * I n t h e p r e s e n t s t u d y

compounds such a s a lumin ium n i t r i d e , a l u m i n i u m f l u o r i d e and a l u m i n a were

x - r a y e d and as Z i'or a l i jmin ium i s 13, t h e o b s e r v e d i n t e n s i t y w i l l d e c r e a s e

w i t h i n c r e a s i n g ©• C o n s e q a e n t l y p i t was a n t i c i p a t e d t h a t the d - s p a c i n g s

a t h igh 0 a n g l e s would be too vfeak t o make a c c u r a t e m e a s u r e m e n t s » F o r low

0 a n g l e s g r e a t e r a c c u r a c y i s o b t a i n e d w i t h t h e r e c o r d i n g s o f a

d i f f r a c t o m e t e r o

Sytmple p r e p a r a t i o n and i n s t r u m e n t a l t e c h n i q u e

T e c h n i q u e s f o r sample p r e p a r a t i o n have beeri g i v e n ( 2 3 ) » but a s the

i d e n t i f i c a t i o n o f t h e sample o n l y i s r e q u i r e d , a f a i r l e s s c o m p l i c a t e d

t e c h n i q u e than t h o s e d e s c r i b e d was u s e d .

39

T o a sample o f powder p l a c e d on a v/atch g l a s s v/as added a s m a l l

amount o f " d u r o f i x " and a c e t o n e . T h e p r e s e n c e o f t h e s e two s u b s t a n c e s

d i d n o t a l t e r the x - r a y c h a r a c t e r i s t i c s o f the sample and s e r v e to

c o a l e s c e and m a i n t a i n the powder on a f l a t g l a s s s l i d e . T h i s g l a s s

s l i d e was s u p p o r t e d i n the x - r a y beam and r o t a t e d f rom to 2^0° . The

r e f r a c t e d beam was c o l l e c t e d by a g e i g e r o r p r o p o r t i o n a l c o u n t e r and

t r a n s l a t e d t o the c h a r t r e c o r d e r by means o f a panax r a t e m e t e r o r

B e r t h o l d r t i t e m e t e r / d i s c r i m i n a t o r j , d e p e n d i n g w h i c h d i f f r a c t o m e t e r was

u s e d . The c h a r t was c a l i b r a t e d i n i n t e r v a l s a c c o r d i n g t o the speed o f

r o t a t i o n o f the s a m p l e , f r o m v/hich a d i r e c t r e a d i n g o f 0 c o u l d be made.

I I . 5 . i i Powder camera

A 9 c m - r a d i u s Unicara powder camera was u s e d w i t h the Van A r k e l

arrangement ( s e e F i g . 2 a ) •

A powder f i l m ( K o d i r e x 3 . 5 cm x 2o8 cm) was c u t t o t h e r e q u i r e d

d i m e n s i o n u s i n g a g u i l l o t i n e (Unicam S o 6 l ) p l a c e d around t h e sample i n

the camera s u p p o r t and t h e l i d . s e t i n p o s i t i o n . The c a m e r a was mounted

a d j a c e n t t o the x - r a y tube and êvacuated f o r two t o t h r e e m i n u t e s b e f o r e

the x-ray g e n e r a t o r was s w i t c h e d o n . A w o r k i n g v o l t a g e o f i O kV and 6 mA •

w a s ' u s e d i n a l l c a s e s f o r e3q)Osures o f 2 to 4 h o u r s .

A f t e r e x p o s u r e , t h e powder f i l m s were d e v e l o p e d f o r 4 m i n u t e s , (Kodax

D 1 9 b ) , r i n s e d i n w a t e r and f i x e d f o r 4 m i n u t e s (Kodax PX40)p washed f o r

h a l f an h o u r , and l e f t h a n g i n g to d i y .

I I . 5 . i i i E r r o r s i n powder photographs

The p o s s i b l e e r r o r s i n powder photographs a r e ( a ) t h e n o n - c o i n c i d e n c e

o f t h e a x i s o f t h e camera and t h e r o t a t i o n a x i s o f the s p e c i m e n ; t h i s

e r r o r c a n be e a s i l y a c c o u n t e d f o r and w i l l be d i s c u s s e d l a t e r , ( b ) the

f i n i t e h e i g h t of the spec imen; i n a 9 cm camera o n l y a p p r o x i m a t e l y 2mm

o f spec imoi i s i r r a d i a t e d and h e n c e t h i s e r r o r was n e g l e c t e d ,

- 40 -

I o

FigiVan Arkel a r rang erne nt

I

( c ) a b s o r p t i o n and diver^gence o f t h e x - r a y beam? the e r r o r s p r o d u c e d by

t h e s e e f f e c t s a r e s i m i l a r , f o r w i t h a b s o r p t i o n o n l y a spec imen i n w h i c h

t h i s e r r o r i s n e g l i g i b l e can the c e n t r e s o f t h e pov/der l i n e s be t a k e n a s

the c o r r e c t p o s i t i o n s , and u s u a l l y the measured v a l u e o f 0 i s too l a r g e

( 9 1 ) « Hence t h e observed d - s p a c i n g w i l l be too s m a l l v/hen compared vri.th

the s t a n d a r d s . The o p p o s i t e o f t h i s e f f e c t i s t r u e w i t h d i v e r g e n c e o f

t h e x - r a y beam*,

From t h e e q u a t i o n

n X = 2 d s i n ©

when c o n s i d e r i n g f i r s t o r d e r v /ave lengths

d =: ( V 2 ) c o s e c 9

Ld = A / 2 c o s e c 0 c o t O o 0

A d = - d c o t Qo LQ

Now i n t h e Van A r k e l arrangement 0 = 9 0 - /l^R where S i s t h e

d i s t a n c e between c o r r e s p o n d i n g d i f f r a c t i o n a r c s and R i s t h e r a d i u s o f

the c a m e r a . So when S i s l a r g e A Q v / i l l be l a r g e . An e x t e n s i v e m a t h e m a t i c a l

s u r v e y f o r t h i s e r r o r has been c i t e d ( 9 ) o To compensate f o r the e r r o r n a

p l o t o f the u n i t c e l l p a r a m e t e r a g a i n s t the N e l s o n - R i l e y f u n c t i o n c o u l d be

(8)9 ( d ) f i l m s h r i n k a g e ; when t h e f i l m i s d e v e l o p e d d e p e n d i n g on3 among

o t h e r f a c t o r s o the t e m p e r a t u r e o f the d e v e l o p e r ^ the f i l m may s h r i n k c To

compensate f o r t h i s i t was assumed t h a t the s h r i n k a g e was u n i f o r m throughout

the f i l m and t h e e r r o r c o u l d be c a l c u l a t e d i f t h e k n i f e edge a n g l e (0^)

was known f o r the camerao

Measurements were made u s i n g m i c r o m e t e r c a l l i p e r s o f t h e k n i f e edge a t

the p o i n t o p p o s i t e to the emergent x - r a y beam termed a s C i n F .g* 2b, and o f

t h e camera r a d i u s .

h2

From Fi -g . 2b

= s i n 29j^

4 = 2 sin""^ ^ / 2 R

. = 36C5 - 2 s i n " ^ V2R

, , 0^ = 90 - i s i n " ^ ^/2R

/ Jfj^ = 86o40 + 0.01°

from which the f i l m c o n s t a n t w i l l be •

• V Measurement o f the f i l m

T h e powder f i l m s were measured u s i n g an i n s t r u m e n t c o n s t r u c t e d from

two metre m l e s s e p a r a t e d and r i b b e d t o g e t h e r by v a r n i s h e d t h r e e plywood*

Adequate a l l o v / a n c e was made f o r p l a c i n g t h e f i l m between the two metre

r u l e s and a p l a s t i c c u r s o r was p l a c e d o v e r the r u l e s * Any t o l e r a n c e i n

the c u r s o r was compensated by a s m a l l p i e c e o f watch s p r i n g between the

r u l e and t h e c u r s o r . A v e r n i e r s c a l e o f 1 mm i n t e r v a l s was made i n the

c u r s o r e n a b l i n g measurement t o be made to the second d e c i m a l p l a c e .

A c c u r a c y o f measurement was + 0,01 cm©

As a r e s u l t o f c a r e f u l measurement the e s t i m a t i o n o f t h e c r y s t a l l o -

g r a p h i c p a r a m e t e r s were made u s i n g a computer program based on I t o ' s method

( s e e Appendix I I I ) .

1 1 , 6 . E l e c t r o n m i c r o s c o p y t e c h n i q u e s The m i c r o s c o p e

T h e P h i l i p s Qd 100 B e l e c t r o n m i c r o s c o p e v / i th a r e s o l u t i o n o f 25 2

was u s e d i n t h i s s t u d y . The m i c r o s c o p e i s u s u a l l y o p e r a t e d a t 80 K V , The

pumping sys tem c o n s i s t s o f a prevacuum l o t a i y pump, a m e r c u r y d i f f u s i o n

pump and an o i l d i f f u s i o n pump. The m a g n i f i c a t i o n i s a l t e r e d by a d j u s t i n g

- 43

the c u r r e n t s t o t h e e l e c t r o m a g n e t i c l e n s e s * F o c u s s i n g o f the image i s

c o n t r o l l e d when v a r y i n g the s t r e n g t h s o f t h e m a g n e t i c f i e l d s ^ by

c h a n g i n g t h e c u r r e n t s g e n e r a t i n g them. The image o f the sample i s

p r o j e c t e d onto a f l u o r e s c e n t s c r e e n d i r e c t l y i n f r o n t o f t h e o b s e r v e r ^

T h e r e a r e f a c i l i t i e s p r o v i d e d f o r p h o t o g r a p h i n g t h i s image w i t h a 35 mm

Camera w h i c h c a n be lov /ered i n t o p o s i t i o n i m m e d i a t e l y i n f r o n t o f the

s c r e e n . G e n e r a l l y ^ a f o u r second e x p o s u r e i s u s e d and the m a g n i f i c a t i o n

o f t h e photomicrographed image o b t a i n e d f r o m compar ing t h e i n s t r u m e n t a l

and the i l l u m i n a t i o n s e t t i n g s w i t h c a l i b r a t e d s t a n d a r d t a b l e s o The

camera c o n s t a n t X L i s c a l c u l a t e d u s i n g a compound o f known u n i t c e l l

d imens ions^ b u t i f the same a c c e l e r a t i n g v o l t a g e i s maintet ined t h e n A L

r e m a i n s c o n s t a n t , i s t h e e f f e c t i v e camera l e n g t h .

Sample p r e p a r a t i o n

Spec imens w e r e p r e p a r e d by p l a c i n g a f i l m o f c a r b o n on a c o p p e r g r i d

i n the normal manner, ' he g r i d v/as s u p p o r t e d u n d e r an i n f r a red l a m p , A

few m i l l i g r a m s o f t h e sample were p l a c e d i n d i s t i l l e d w a t e r o r a c e t o n e i n

a t e s t tube and p l a c e d i n an u l t r a - s o n i c medivun to e n s u r e t h a t t h e sample

was t h o r o u g h l y m i x e d , A s i n g l e d r o p o f the s u s p e n s i o n was p l a c e d on t h e

g r i d p the h e a t f r o m t h e lamp e v a p o r a t e d the w a t e r l e a v i n g a d i s p e r s i o n o f

the sample on t h e c a r b o n f i l m .

F i n a l l y t h e g r i d was p l a c e d i n a spec imen h o l d e r t o be i n t r o d u c e d i n t o

the microscopeo

I I » 7 E s t i m a t i o n o f p a r t i c l e s i z e

The p a r t i c l e s i z e was e s t i m a t e d u s i n g e l e c t r o n and o p t i c a l m i c r o s c o p y ,

I I » 7 , i E l e c t r o n m i c r o s c o p i c a l t e c h n i q u e

E l e c t r o n m i c r o g r a p h s o f t h e r e l e v a n t samples were taken u s i n g a 35 mm

c a m e r a .

From the p r i n t o b t a i n e d and knD-/n.ng the t o i r d m a g n i f i c a t i o n ( i . e .

the m,--xgnification o f the m i c r o s c o p e and the m a g n i f i c a t i o n c h a r a c t e i i s t i c s

o f the e n l a r g e r ) , an e q u i v a l e n t d i s t a n c e to lyUm c o u l d be measured on the

photograph. The d i m e n s i o n s o f the i r r e g u l a r p a r t i c l e s were measured w i t h

r e s p e c t to t h e M'l m e q u i v a l e n t d i s t a n c e and the mean g i v e n to r e p r e s e n t

the p a r t i c l e d i m e n s i o n . As the p a r t i c l e s v/ere i l l - f o m e d ^ a range o f s i z e

i s quoledo

I I » 7 . i i O p t i c a l m i c r o s c o p i c a l t e c h n i q u e s

The method has been d e s c r i b e d i n g r e a t d e t a i l ( l O ) . The ins tT- iment

•js^'i was a V i c k e r s p o l a r i s i n g m i c r o s c o p e v / i th m a g n i f i c a t i o n s 400x and

l600,y. T'he samples were spread on a g l a s s s l i d e and p l a c e d on ti .e microscopCo

The s i z e s were e s t i m a t e d by corigâring the p a r t i c l e s v / i th s t a n d a n l d i m e n s i o n s

on an e y e p i e c e g r a t i c u l e ( l l ) . The g r a t i c u l e d i m e n s i o n s v/ere c a l i b r a t e d

to r e p r e s e n t d i f f e r e n t ykjn s i z e s f o r e a c h m a g n i f i c a t i o n . A g a i n , a s the

p a r t i c l e s a r e i r r e g u l a r , a range o f s i z e i s q u o t e d .

V/'i.th both t e c h n i q u e s o f measurement , c a r e must be taken v/hen measur i .ngj

that, the sample i s r e p r e s e n t a t i v e o f the t o t a l m a s s . T h i s i n v o l v e s p r e c i s e

and c a r e f u l o b s e r v a t i o n o f the sample c h a r a c t e r i s t i c s .

I I 0 8 S u r f a c e a r e a measurements

.Cias s o r p t i o n measurements g i v e i n f o r m a t i o n a s to the s p e c i f i c s u r f a c e

and the a v e r a g e c r y s t a l l i t e s i z e o f a powder* A g e n e r a l t r e a t i s e o f t h e

s u b j e c t has been g i v e n by Gregg and Si.ng ( l2 ) .

The method u s e d to d e t e r m i n e s u r f a c e a r e a s was due to B.runaer_n Kmmett

& T e l l e r and i s knovm a s the B . E c T c p r o c e d u r e ( l3 ) . The B.EoTo e q u a t i o n

g i v e s s

P ^ c - 1 ^ p ^ _ J _ / \ X c ** — x c

x (p^ - p ) m p^ m

4 5

where p i s the p r e s s u r e o f the a d s o r b a t e vapour i n e q u i l i b r i u m w i t h

a b s o r b e n t ! p^ the s t a n d a r d vapour p r e s s u r e o f a d s o r b a t e vapour ; x i s t h e

amount o f v a p o u r adsorbed s i s t h e c a p a c i t y o f f i l l e d monolayer^ c i s

a c o n s t a n t s

A d s o r p t i o n i s o t h e r m s a r e c l a s s i f i e d i n t o f i v e t y p e s of w h i c h t y p e

I I i s o t h e r m s g i v e the b e s t a g r e a n e n t w i t h t h e BoE«,To e q u a t i o n o v e r l i m i t e d

ranges o f vapour p r e s s u r e (14)- Thus a p l o t o f — a g a i n s t ^ / p ^

would g i v e a s t r a i g h t l i n e o f s l o p e — ~ - r ^ and i n t e r c e p t - ~ - . F r o m t h e X C X c

m m

q u a n t i t i e s a v a l u e o f x ^ c a n be founds The s p e c i f i c s u r f a c e 3 i s r e l a t e d

to x^ by the e q u a t i o n ;

S = ^m . N . A^ M

v^here M i s t h e m o l e c u l a r w e i g h t o f the a d s o r b a t e

N i s A v o g a d r o ' s number A i s the c r o s s s e c t i o n a l a r e a o f an a d s o r b a t e m o l e c u l e i n a m

comple ted monolayer*

The s p e c i f i c s u r f a c e i s r e l a t e d to the a v e r a g e p a r t i c l e s i z e 1 b y j

s = i -

vrfiere ^ i s the d e n s i t y o f adsorbent*.

Apparatus

The s o r p t i o n b a l a n c e i s based on the d e s i g n o f G r e g g (15s l 6 ) c The

b a l a n c e arms a r e made o f g l a s s and s u p p o r t e d on n e e d l e s . One arm o f t h e

b a l a n c e s u p p o r t s b u c k e t s f o r the sample and c o u n t e r w e i g h t s ^ t h e o t h e r arm

e i t h e r a s o l e n o i d o r magnet e n c l o s e d i n g l a s s and s u r r o u n d e d by a n e x t e r n a l

s o l e n o i d . The whole a s s e m b l y i s e n c l o s e d i n g l a s s and c o n n e c t e d t o a sys t em

o f e v a c u a t i o n pumps and gas r e s e r v o i r s . T h e p r e s s u r e r e a d i n g s were made

IS

w i t h a g r a d u a t e d mercury manometer. The c u r r e n t i n the e x t e r n a l s o l e n o i d i s

va.r ied to o b t a i n the b a l a n c e p o i n t , w h i c h i s n o t e d by compar ing the p o s i t i o n

o f a h o r i z o n t a l m e t a l p o i n t e r w i t h a r e f e r e n c e p o i n t e r - The i n s t r u m e n t i s

c a l i b r a t e d by m e a s u r i n g the c u r r e n t r e q u i r e d t o o b s e r v e the n u l l p o i n t f o r

knov/n weightSo Buoyancy c o r r e c t i o n s wer^ made to the r e a d i j i g s a g a i n by

compar i son vrxth a s t a n d a r d m a t e r i a l .

T e c h n i q u e

The sample was p l a c e d i n the specimen b u c k e t and o u t - g a s s e d to remove

p h y s i c a l l y adsorbed vapour* T h i s was c a r r i e d out a t 2 0 0 ° by s u r r o u n d i n g

the b a l a n c e l i inb w i t h a f u r n a c e {U)^

k Dewar f l a s k c o n t a i n i n g l i q u i d oxygen v/as p l a c e d around the b a l a n c e

l i m b . The a b s o r b a t e was n i t r o g e n g a s « The i s o t h e r m s were measured a t o

-183 C . The w e i g h t o f the sample v/as d e t e r m i n e d i n vacuo* Ni trogen was

i n t r o d u c e d i n t o the sys tem and e q u i l i b r i u m v/as a c h i e v e d b e f o r e s i .multaneous

r e a d i n g s o f sample w e i g h t s and gas p r e s s u r e v/ere t a k e n *

From the r e s u l t s the s u r f a c e a r e a measurements were found w i t h the

a i d o f a computer program d e v i s e d by F * OM^Jeil l and D e n i s e H a r r i s (5) i n

F o r t r a n I V u s i n g an loB^M* 1130 computer a s d e t a i l e d .in Appendix I V *

11,9 B a l l m i l l i n g

The al.uminium n i t r i d e v/as m i l l e d i n c y l i n d r i c a l p o r c e l a i n p o t s

c o n t a i n i n g a number o f b a l l s o f the same m a t e r i a l . T h e pot had r a d i u s e d

ends to p r e s e n t a smooth i n t e r i o r s u r f a c e f r e e from c o m e r s and c r e v i c e s *

H a v i n g p l a c e d the m a t e r i a l i n the pot^ t h e open end was s e c u r e d v / i th a l i d

and q u i c k - r e l e a s e bar* The pot was p l a c e d i n a j i g and l a y i n a h o r i z o n t a l

p o s i t i o n ^ and m e c h a n i c a l l y r o t a t e d so a s t o e n s u r e e f f e c t i v e c o n t a c t v / i t h

the p o r c e l a i n ba^ '

47

F o l l o w i n g the m i l l i n g p e r i o d , the p r o c e s s o f r e c o v e r y was a s f o l l o v / s :

The p o t was h e l d v e r t i c a l l y above a s i e v e p o s i t i o n e d on a l a r g e d i s h o The

p o r c e l a i n baJ.ls were c o l l e c t e d on the s i e v e and t h e p o w d e r e d n i t r i d e

c o l l e c t e d i n the dish. The b a l l s and t h e i n s i d e o f t h e p o t w e r e b r u s h e d

v i g o r o u s l y t o remove t h e n i t r i d e . Any r e m a i n i n g n i t r i d e was b r o u g h t o u t

by jDtating the c l o s e d pot w i t h a p o r t i o n o f a c e t o n e , v /hereupon the ni . tride

and a c e t o n e were removed f r o m t h e pot and t h e n i t r i d e was f r e e d f r o m

a c e t o n e by d r y i n g in an ove / i a t 70^C o v e r n i g h t s

8C^- r e c o v e r y o r b e t t e r vras a c h i e v e d \ v i t h t h i s o p e r a t i o n .

I I • 10 E x p e r i m e n t a l t e c h n i q u e s used i n p rc -pa r inp ( .• . l i - iminium r i i t r i d g

I l o l O o i P r e p a r a t i o n from ammonium h e x a f l u o r o a l t - i m i n a t e

The p r e p a r a t i o n o f a l u m i n i u m n i t r i d e v/as a t t e m p t e d b y the t h e n n a l

d e c o m p o s i t i o n o f ammonium h e x a f l u o r o a l u i n i n a t e i n g a s e o u s ammonia ,

Ttie : removaI o f gaseous i m p u r i t i e s

Vvhcn d e c o m p o s i n g the- h t x a f J u o r o a l u m i n - ^ i t e compound i n a i r i t v.-as n o t e d

t h a t a f t f - j r p e r i o d s o f an h o u r a t h i i g h e r t e m p e r a t u r e s ( > 500^C) t h e

f o r m a f . i o n o f compounds s i m i l a r i n x - r a y c l i a r a c t e r l s t i c s t o b a s i c f l u o r i d e s

( 2 4 ) were formeda The r e m o v a l o f t h e m o i s t u r e i n t h e s y s t e m v/as t h e r e f o r e

v i t a l . c

Tht: a i r v/as passed th rou r^h tvro r . l r e sche l b o t t l e s and a ho j* i c e n t a l g l a s s

t u b e c o n t a i n i n g potassiLim h y d r o x i d e p e l l e t s * The g l a s s lûbe v/as \i3ed t o i m p r o v e

c:ont,.T.ct be tween t;he h y d r o x i d e and t h e a i r * As a consequ eiice p t h e r e m o v a l

o f m o i s u j r c f r o m t h e s y s t e m a p p e a r e d t o be e f f e c t i v e ^ f o r i n .18 h o u r s a t

62^^C no b a s i c f l u o r i d e v/as I 'onned v/hen d e c o m p o s i n g t l i e h e x a f l u i ^ r o a J a m n a t e e

The p r e s e n c e o f a smÛi p e r c e n t a g e of ox^'gen v/as a n t i c i n a t o d an t h e

n i t r o g e n -' as used and to- remove i t the gas v/as b u b b l e d t h r o u g h p y r o g a l l o . l

s o l u t i o n . To p r e p a r e ' p y r o g a l l o l s o l u t i o n s 15 grams o f A.Ro p y r o g a l l o l v/ere

acJued t o a q u i c k f i t t e s t t u b e . A t ) j b e was i n s e r t e d v i a a s i d e a r m p i e c e

and 100 m i s o f 4Q/o aqueous p o t a s s i u m h y d r o x i d e was a d d e d i j n d e r n i t r o g e n .

The s o l u t i o n v/as p r e p a r e d i n t h i s m a n n e r because o f i t s a c t i i * e a f f i n i t y

f o r o x y g e n . The n i t r o g e n v/as b u b b l e d t h r o u g h t h e p y r o g a l L o l s o l u t i o n and

p a s s e d t h r o u g h a U - t u b e c o n t a i n i n g magnes i imi p e r c h l o r a t e , p r i o r t o p a s s i n g

i t t h r o u g h t h e d r y i n g s y s t e m .

F u r n a c e

The f u r n a c e v/as a h o r i z o n t a l t u b e t.^-pe u s i n g c h r o m e l - a l u m c l

t h c - r m o c o u p l e s v / i t h a maximum t e m p e r a t u r e o f 1000 C,

V/hen ajnmonium h e x a f l u o r o a l u m i n a t e i s t h e r m a l l y d e c c m p o s e d , amm.onium

f l u o r i d e i s p r e s e n t . A t t e m p e r a t u r e s above 520*^0 i t v r i l l e x i s t i n i t s

d i s s o c i a t e d s t a t e as ammonia g a s and h y d r o f l u o r i c a c i d gas^ Hov/ever , t h e

r e c o m b i n a t i o n o f t h e s e tv/o ga se s t a k e s p l a c e a t t e m p e r a t u r e s l e s s t h a n

p 2 0 ° C g i v i n g ammonium f l u o r i d e w h i c h c o n d e n s e s on t h e c o l d e r p a r t s o f t h e

p u r o x t u b e . I t a p p e a r e d t o c o n d e n s e i n one p a r t i c u l a r r e g i o n and t h e bui- ' id

up o f t h e compound vâs s t i f f i c i e n t t ( v i g o r o u s l y a t t a c k t h e p u r o x u n t i l t.he

t u b e f e l l a p a r t . Kence t h e t u b e l i . f e i s d r a s t i c a l l y s h o r t e n e d ^ T o overcome

t h i s p r o b l e m , , t h e t ube end v;as m a i n t a i n e d a b o v e t h e d e c o m p o s i t i o n t e m p e r a t u r e

o f ammonium f l u o r i d e i n t h e f o l l o v r i i i g manne r . The t u b e end v/as wound v / i t h

s e v e r a l t u . m i n g s o f n i . chrome w i r e on a l a y e r o f p a p e r a s b e s t o s , A t h i c k

l a y e r o f a l u m i n a cement v/as p l a c e d on t h e w i n d i n g s a.nd l e f t t o sc-t h a r d . The

ends o f t h e v / i n d i n g s were c o n n e c t e d t h r o u g h a j i a v o m e t e r t o a viiri.a.c v / i t h a

majtimum c u r r e n t o f s i x ampso The exces s l o a d on t h e t u b e vas c o m p e n s a t e d by

s u p p o r t i n g t h e t u b e b e l o w t h e a l u m i n a cemeni: ] a y e r .

The v a r i a c v/as s e t j u s t b e l o w i t s nELximam r a t i n g and a t e m p e r a t u r - e

o f 600^0 v/as o b s e r v e d i n t h e f u r n a c e e n d . T h i s i n i t i a l l y u p s e t t h e t h e r r r t i l

e q u i l i b r i u m and t h e d i s c r e p a n c i e s v/ere a c c o u n t e d f o r b y a l l a v / i . n g t h e gas t o

49

be u s e d5 t o p a s s t h r o u g h t h e s y s t e m f o i - a p e r i o d o f some 30 m i n u t e s .

The ammonium f l u o r i d e c o n d e n s a t e v. as t h e r e f o r e c o l l e c t e d i n a s i x

I n c h l e n g t h o f c o p p e r t u b i n g w h i c h v/as r i g i d l y set i n t h e p u r o x t u b e v / i l h

a.laTina c e m e n t . Les s t h a n h a l f o f t h e c o p p e r txibe vâs cooled by p l a c i n g

a f e w t u r n i n g s o f t h i n n e r c o p p e r t u b i n g a r o u n d i t and p a s s i n g ^va te r

t h r o u g h t h e m .

A f t e r each run, t h e c o p p e r t u b i n g w h e r e -Jie ammonium f l u o r i d e v/as

nc.v o b s e r v e d t o condense v/îs c l e a n e d v / i t h a small b r u s h .

The e x i t gases w e r e pa s sed t h r o u g h (a) a p l a s t i c b o t t l e c o n t a i n i n g

g l a s s w o o l , ( b ) a d r e s c h e l b c t . t l e c o n t a i n i n g p o t a s s i u m h y d r o x i d e p e l l f r t s ,

arid ( c ) a p l a s t i c b o t t l e c o n t a i n i n g d i s t i l l e d ^'.-ater^

The f i . r s t p l a s t i c b o t t l e was i n q u i r e d t o p r e v e n t a n y f l u o r i d e a t t a c k i n g

t h e d r e a c h e l b o t t l e w h i c h i n t u r n was r e q u i r e d t o p r e v e n t m o i s t u r e : d i f f u s i n g

back i n t o t h e s y s t e m . The a-nmonia e x i t gas was p a r t i a l l y d i s s o l v e d i n t he

d i s t i l l e d w a t e r ; a n y exces s was p a s s e d t o a furne c u p b o a r d .

The ammonium h e x a f l u o r o a l i o m l - n a t o a n d i t s d e r i . v a t i v e s v/ere p l a c e d i n

a pt:r\::x b o a t w h i c h v/as i n s e r t e d i n t o t h e h o r i z o n t a l t u b e f u r n a c e and p l a c e d

m t h e h o t zone u n d e r t h e g a s i n q u e s t i o n . The r a t e o f gas f l o w v/fis measured

u s i n g c a l j . b r a t e d f l o w m e t e r r / jbeso

I l . l O o i i P r e p a r a t i o n b y t h e Se rpek r e a c t i o n

T h e r e a r e e n u m e r a b l e p r o b l e m s ^ such as t e m p e r a t u r e c o n t r o l a n d

f l u c t u a t i c n s j t h e r m o c o u p l e m a t e r i a l s a n d s y s t o n d e s i g n s v/hen s t u d y i n g c h e m i c a l

r e a c t i o n s a t e l e v a t e d t e m p e r a t u reso i T h e p o s s j . b i M t y o f d e s i g n i n g a, h i g h

' •?mper?i ture f u r n a c e was c o n s i d e r e d - , b u t f o r t h e r e a s o n s s t a t e d a b o v e , an

a l t f . m a t i v e me thod v/as d e s i r a b l e .

50

The u s e o f o p t i c a l m i c r o s c o p y a p p e a r e d t o o f f e r a s u i t a b l e method

f o r e x a m i n i n g the r e a c t i o n *

Tht: nîcroscope u s e d in t h e i n v e s t i g a t i o n was a G r i f f i n - T e l i n ho t

s t a g e m i c r o s c o p e w h i c h h a s the f o l l o w i n g advantages^

( a ) E l a b o r a t e v / a t e r - c o o l i n g a r r a n g e m e n t s f o r t h e p r o t e c t i o n o f t h e

m i c r r s ^ j p e a r e u n n e c e s s a r y and t h e i n s t r u m a i t c a n be a d a p t e d t o examine

thf. b s h a v i o u r o f n a . t e r a a l s a t h i g h temperat 'ure by x-.r3,y d i f f r a c t i o n a s

W-T:] ! as by o p t i c a l microscopy*

( b ) The' method o f mounting t h e sample makes p r o v i s i o n -ôr e x a m i n a t i o n

o f the i n t e r n a l s tr - . io ture by t r a n s m i t t e d l i g h t - C r y s t a J . s p r e s e n t i n a

r^r;;^: oan be v i ewed i n normal and i n p o l a r i s e d l i g h t .

( c ) The i n s t r u m e n t p r o v i d e s s p e c i f i c a l l y f o r v e r y hi^h fcemperatrxre

c b s ' ^ r ' / a t i c n s and r e p r c - d u c i b i l i t y c f r e s u l t s ' r e p o r t e d to be w i t h i n

± ^=G.

The i n s t r a m e n t i s composed o f a pov/er s u p p l y , w h i c h i n c o r p o r a t e s a

3 i n c h scal .e m e t e r c a l i b r a t e d d i r e c t l y f o r u s e vr i th 20?5 Rh-Pt/595 Rh-Pt-

• h^nnocouplfc^ and a l e e k p e t r o l o g i c a l b i n o c t i l a r - t y p e m i c r o E c o p e .

The i n s t r o r a e n t i ^ p r o v i d e d wi.th a g a s t i g h t c e l l w h i c h h o u s e s t h e

thfjrmocouple e l e m e n t s and a t the same t i m e permi . t s t h e u s e o f a n y

d e s i r e d atmosphere* A t h i g h t e m p e r a t u r e s o x i d a t i o n o f t h e thermocouple

iTiay ccciT* and iJri^ v o l a t d l i s a t i o n o f the thermocoup le v / i l l i m p a i r v i s i o r .

o f th& sample* P a s s i n g o f n i t r o g e n r b ^ d ^ t e s t h i s e f f e c t and t h e u s e o f

t h i s gas i s a l s o desdjrable f o r t h e S e r p e k r e a o t i o n .

The i n l e t and o u t l e t p o s t s o f d e c e l l v/ere guarded. f.roiii m o i s t u r e

by U - t u b e s c o n t a i n i j i g o a l c l u m c h l o r i d e o

The thermccoupl s *^ni.t3 a r e p r o v i d e d w i t h necpr^ne g a s k e t s vrtiich

make ar. e f f i c i e n t s e a l v/hen the r e t a i n i r . g n u t s on t h e g a i d e r o d s a r e

t i g h t e n e d .

- 51 -

The ce l3 . was a t t a c h e d t o the m i c r o s c o p e s t a g e by means o f s l o t t e d

armsj; w h i c h i n the l o c k e d p o s i t i . o n e x e r t a s l i g h t p r e s s u r e on t h e c e l l

t*. e n s u r e good t h e r m a l c o n t a c t w i t h t;he r e v o l v i n g m i c r o s c o p e s tage^ and

s c any h e a t r a d i a t e d w h i c h i s s m a l l p v / i l l b e c c n d u c t e d away a n d d i s s i p a t e d

i n the body o f t h e i n s t r u m e n t *

The a l i g r j n e n t o f the instrumen*^ was c h e c k e d b e f o r e e a c h c p e r a t . c n

t-o s n s u r e optim^jni \ r i s l b i l i t y -

Wên u s i n g t h e i n s t r u m e n t t h e b - ^ a v a o u r o f a n y m a t e r i a l and the

aCT^racy o f tempezB-tuj-e mea3uxem?-nt i s dependent on two f a c t o r s , , ( a ) the

ccrTzrct f a b r l c a + i o r t o f t h e mlcrof'-tma'js assembly, , ( b ) t e m p e r a t u r e

g r a d i e n t w i t h i n the s a m p l e .

( a ) F a b r i c a t i o n o f the t h e m c c o u p l e e l e m e n t s

The cptimum p e r f o r m a r c e o f the m i c r o f u r n a c e a s s e m b l y i s dependent

upor> the thenjioîijinctions b e i n g formed i n en e r a c t and r e p r o d u c i b l e

manner.

A l e n g t h o f 5^ Rh~PL a l l o y ( a p p r o x i m a t e l y 3 cm) i s p l a c e d i n the

n e g a t i v e t e r m i n a l d e s i g n a t e d by a ? m a l l r e d dot and sec^jred b y t h e

c l a m p J j i g s c r e w . The o t h e r e x t r e m i t y o f the v/ire was s u c h t h a t when

p l a c i n g the thermocouple ^^nit i-nto the c e i l ^ i t ex tended j u s t t o the

edge o f the windovj a p e r t u r e . A e i m i l a r l e n g t h o f 0.5 mm d i a m e t e r

20fc R h - P t a l l o y wire- wa£ damped t c the p r * s i t i v e t e r m i n a l . T h e s e two

w i r ^ s f o r m the s u p p o r t s f o r the h e a t e r e lement p r o p e r , w h i c h i s composed

o f 0.2 nan d i a m e t e r w i r e s o f ths same a l l o y c o m p o s i t i o n . T o j o i n the two

r s s p a o t i v ^ v?lreB t o g e t h a r c the o p e r a t i o n r e q u i r e s a s m a l l o x y - c o a l g a s

f l a m s . The t h i n n e r d i a m e t e r w i r e i s f i . r s t p l a c e d in f l a m e a n d a s m a l l

bead concpar lb le i n d i jnens ion to the d i a m e t e r o f the t h i c k e r w i r e i s formed*

52

The "bead io placed adjacent to the head of the t h i c k e r wire and the ^virea

pl.aced in the flame u j i t i l f u s i o n i s complete. The operat ion , which

requires a cei-i:ain amovint of s k i l l , i s repeated with the o ther two v/ires

of d i f f e r i n g diameters. The f i n e w i r e s are then trimmed to approximately

lo5 cm and the supporting v/ires turned inv/ard u n t i l the f i n e w i r e s are

crossed. The f i n e wires are trimmed i n t h i s p o s i t i o n us ing a g u i l l o t i n e

and j i g and the v/elding of the juxtaposed f i n e w i r e s i s completed i n the

flame. Any f i n a l trimming may be done us ing the g u i l l o t i n e . F i n a l l y ,

the f i n e v/ires were squeezed together i n the heighbourhood of the junct ion

over a length of 5 mm, A r i g i d observat ion of the procedure enables

repi^ducible r e s u l t s to be obtained,

(b) Tgnperature gradient i n the sample

Dae to a f i n i t e v;-idth between the thermocouple w i r e s adjacent to the

point o f junc t ion of the 0.2 mm diameter w i r e s , and the presence o f the

sample i n t h i s region, there w i l l be a temperature gradient v / i th in the sample,

T h i s i s reported to be of the order of 4°G ( i . e . - 2^C from the j u n c t i o n

point at which the ac tua l temperature i s measured. I t i s there fore

necessajry not to overload the thermocouple w i th the m a t e r i a l to be

inves t i ga ted . The mater ia l s v;ere ground to l e s s than 200 mesh.

To c a l i b r a t e the thermocouple A . R potassium sulphate was used as

i t has a sharp and reproducible mel t ing point a t 1 0 7 2 ° C . I t a l s o under

goes a polymorphic t r a n s i t i o n a t 5 8 0 ° C ,

To moiu-it the standard and m a t e r i a l s to be examined, the thennocouple

was dipped i n so lut ion of an i n e r t solvent and placed i n p^JV/dered .

sample. I t was found that a coliamn of appro^c^r^tely 0*5 mm gave the

optimijm r e s u l t s with the standard, and t h i s dimension was adopted with

the mater ia l s to be examined.

53 -

The thermocouple and sample were p laced i n the gas t i g h t c e l l and

the assembly mounted on the microscope stage* Nitrogen at a moderate

flow ra te was passed through the system f o r a few minutes to remove any

a i r present . The ra te of gas f low was found as i n the previous technique

us ing a flowmeter. The connection betv/een the power u n i t and the thermo

couple was made and the heat input increased gradual ly u n t i l the temperature

required v/as obtained.

A f t e r the r u n , carefVil removal of the sample was made, A smaJ.1

amount of co l l od ian was added to the sample and i t was moiinted on a very

t h i n g l a s s f i b r e and l e f t to dry f o r 30 minutes. The f i b r e v/ith the sample

v;as then placed i n the 9 cm povider camera and a powder photograph was taken

as p r e v i o u s l y descr ibed.

The thermocouples were cleaned by i n s e r t i n g them f i r s t i n concentrated

hydrochlor ic a c i d and dry ing them and p l a c i n g them i n water , then acetone,

and a f i n a l dry ing i n an oven at 60*^C.

I l . l O . i i i Preparat ion of the n i t r i d e from i t s elements

Reports that aluminium can be n i t r i d e d belov/ i t s melt ing p o i n t , i . e .

above 400*^ were inves t iga ted . The m i c r o c r y s t a l l i n i t y of aluminium f i l m s

and t h e i r changes on attempted n i t r i d a t i o n were s tudied by e l e c t r o n

microscopy and d i f f r a c t i o n .

The aluminium f i l m was prepared by f i r s t depos i t ing a carbon f i l m on

mica. The mica p l a t e was then f i x e d i n s i d e the high vacuum un i t a t about

10 cm from a h e l i c a l tungsten f i lament . Aluminium powder was placed i n the

f i lament and the system evacuated. The f i l ement cu trent was increased

s u f f i c i e n t l y s lowly to avoid rapid temperature changes, thus avoiding any

unnecessary s t r e s s e s i n the f i l m . A f t e r the aluminium had evaporated, the

r e s u l t a n t f i l m and carbon base were s tr ipped from the mica p la te and placed

34

on a copper g r i d i n the normal mannero The f i l m was observed under the

microscope and a d i f f r a c t i o n pattern vjas taken .

The copper g r i d with the deposited aluminium f i l m was p laced i n a

g lass vesse l which was f lushed with nitrogen gas a t room temperature.

The nitrogen pressure was reduced to 0,2 atmospheres by pumping out the

excess gas from the vesse l s The whole assembly v/as laz/ered into a

furnace prese t a t 3 0 0 ° C . The f i l m s v/ere heated f o r 3 hours and

re-examined under the micro scope,»

55

! • V / i l l ard j , H.H, , & Winters^ O . B . , IndpEng.Chem.Arial .Ed. , 1935, 1 , 7o

2- ' Voge l , A,Iftj> "A Tes t Book of Q u a l i t a t i v e Inorganic A n a l y s i s " , 2nd E d . , 1931, ( L o n ^ a n s ) .

3 . L i n s t e a d , R . P . , E l v i d g e , J o A , , & V/al ley , M , , "A Course i n Modem Techniques of Organic Choni s try" , 1935* (Butter\M3rths, London).

4. P&sser, R«G. , et alo Ana lys t , 19^2, 87 , 301«

3 . O ' N e i l l ^ P.5 & Harris^, D^^ PODH, (Fortran ' I V ) p Plymouth PDlytechnic , 1969•

6. Bognar, J o , & Nagy, L . , K o l i a s z a t i Lapok, 1938» 91p 338.

7. Hejduk, J o , & Novak, J , , Z .Ana l .Chan . 1968, 23^, ( 5 ) , 327-

8. Henny, N o F . M , , L ipson , H.j , & V/ooster, V i ' o A « , "The I n t e r p r e t a t i o n o f X~ray D i f f r a c t i o n Hiotographs", (Macmillan & C o . ) ^ 1960»

9 . K l u g , HoP, , & Alexander^ U E . , "X-ray D i f f r a c t i o n Procedures f o r P o l y c r y s t a l l i n e and Amorphous M a t e r i a l s " , ( j . V / i l e y ) , 1934.

l O o B , S . 4306, F ^ r t A, 1963.

11. B . S o 3623, 1963.

12. Gregg, S . J p , & S ing , K . S . V J . ^ "Adsorption, Sur face Area and P o r o s i t y " , (Academic P r e s s ) , I967.

13. Brunaer, S . , Bnmett, P . H . , cS: 1 e l l e r , E . j, J o A m . C h e m . S o c , , 1938 , 60 , 309

14. Gregg, S . J o , "Siirface Chani s try of S o l i d s " , (Chapman* H a l l ) , 1951.

15o Gregg^ S . J . , J.ChQDoSoCo, 1946, 36I0

16. i b i d . , 1933, 1438.

17. G l a s s o n , D . R . , S o C I . Monographs, 1964, No. 18, 4OI.

18. Grimshav/, R .V/ . , Heaton, E . , & Robert s , A . L . , T r a n s . C e r a m . S o c . , 1945. 44. 76.

19« ShJ^-nd. B o , C r o c k e t t , D . S . , A Haendler , H .M. , Inorg.Chem., I966, 5, (11), 1967.

20^ Haendler, H^M. , Johnson, F . A . , & C r o c k e t t , D . S . , J . Am.Chem.Soc., 19383 80, 2620

21. G a m , P . D . , "Therrooanalytical Methods o f Inves t igat ion"^ (Acadanic P r e s s ) , 1963.

56

22o Gr«gg, S^J.p & V/insor, G.V/op Analyst^ 1945p JO^ 336.

23. I^raer j H.S<,5 Rooksby, H.P.p & V/ i l son , A . J . C , "X-ray Dif fract ion by Ib lyc iys ta l l ine Materials", (Chapman & H a l l ) , 1960<,

24. Cowley, J , & Scott, M o , J ^ A . C . S . , 1948, JO, 105.

57 -

CKAFTER I I I ' he formation of aluminium n i t r i d e

I n the present r e s e a r c h , three methods f o r the formation of

aliiminixim n i t i d d e were t r i e d . They were ( l ) t h e m a l decomposition of

ammonium hexafluoroaluminate i n ammonia^ ( 2 ) the formation o f the

n i t r i d e from an intimate mixture of oc -alumina and carbon i n the

presence of n i trogen , and (3 ) the n i t r i d i n g of pure aluminium.

I I I . l The thermal decomposition of ammonium hexafluoroaluminate i n

var ious atmospheres

The thermal deccmposition of the ammonium-aluminium-fluorine

complex was s tudied i n a i r , d r i e d a i r , and d r i e d ammonia.

m . l . i ' The theiToal decomposition of ammonium hexafluoroaluminate i n a i r

Approximately 0.25 grams of ammonium hexafluoroaluminate v/as weighed

out i n a purox ( r e c r y s t a l l i s e d alumina) boat ajid p laced i n the tube

furnace as described i n Chapter H . A i r was pumped through the furnace

a t a moderate r a t e to a l low s u f f i c i e n t ranova l of any v o l a t i l e matter,

V/hen the a i r was to be dr ied i t was passed through a potassium hydroxide

dxying system. The complex was heated i n a i r a t var ious tenperatures

- for set i n t e r v a l s o fîme and subsequent weight l o s s e s were determined.

A plot of percentage weight l o s s against time f o r d i f f e r e n t temperatures

was made (see F i g , 3 ) . At var ious i n t e r v a l s in the heat ing c y c l e ,

samples were removed f o r x - r a y a n a l y s i s to determine the phases present

(see F i g . 4 ) . The thermal and x - r a y a n a l y s i s gave the fo l lowing resul ts ' .

Ammonium hexafluoroaluminate i s s tab le in , a i r up to 130*^0, vên a

weight l o s s of' ^ was observed. T h i s decomposition i s probably due to

l o s s o f water present i n the system. From the r e s u l t s o f the d i f f e r e n t i a l

thermal a n a l y s i s a small peak i s recorded at 100*^C (see F i g . 3 a ) . At o

220 C the formation of ammonium te traf luoroa luminate begins to take

58

.3

F i c . 3 . ' f ' - i ^ vvvcei\t-^t:e v ; t . loG" o f an::.or.iu:^ hexfifl- 'V;rop3viuinate hei-ted i n

c.iîed r . i r f o r vc i icr . r - te:nr*e-; r tv.res : ncl tinov-.

-X.

G

Z2o"c

place . The weight l o s s a t t h i s teniperature i s therefore due to the

v o l a t i l i s a t i o n of annionium f l u o r i d e . The c h a r a c t e r i s t i c d-spacings f o r

(NH^)ÂIF^ are 5-2 and 4-A-7. From Table 8 the d-spacings are 5 .0? and o

if.A- r e s p e c t i v e l y a.fter 5 hours at 220 v/hich suggests that the number

of moles of ammonium f l u o r i d e present i n the sample i s s l i g h t l y l e s s

than t h r e e . A f t e r 4 hours at t h i s temperature, the t e tra f luoroa luminate

phase i s prevalent (see Table 8 ) . As the temperature i s increased the

l o s s of ammonium f l u o r i d e continues and a t 340^0 d-spacings intermediate

between those recorded f o r ammonium tetraf luoroalwmi nate and those of a

phase i d e n t i f i e d as >j'-aluminium f l u o r i d e v/ere observed (see Table 9 ,

F i g . 8 ) . T h i s intermediate compound e x i s t s even a f t e r 5 hours at 340°C.

The thermal a n a l y s i s curve at 420°C confirms the ex i s tence of t h i s

intermediate as the percentage v/eight changes f o r ammonium t e t r a f l u o r o -

aluminate and "j^-aluminium f l u o r i d e are 3 ^ and 5^° r e s p e c t i v e l y , and a

weight change of 51»^^ i s observed a f t e r 4 hours at 420*^C. To v e r i f y

the nature of the intermediate , i n f r a - r e d a n a l y s i s gave peaks a t 1430

and 3240 cm which are due to NH."*" ions and hence t h i s intermediate i s 4

of the type AIF.^ ( N H ^ F ) ^ . T O determine the stoichiometry of the compound,

a chemical a n a l y s i s o f the ammonia content was made, ^he perxientage

ammonia present was detemiined as 4 -32^ and the t h e o r e t i c a l aimipnia

content of ammonium tetr^f luoroaluminate i s 1 4 . 0 ^ . The formula o f the

intermediate observed was therefore A I F ^ . 0 . 3 ( N H ^ F ) v/hich i s i n

agreement with the 51.2?o weight l o s s . Furthermore., from the d i f f e r e n t i a l thermal a n a l y s i s of ammonium hexafluoroaluminate a peak over the

temperature range 200 to 420°C vdth a maximum a t 335*^0 i s observed.

Ammonium tetrafluorostluminate therefore does not appear to e x i s t a s a

- 62

Table 8

Ammonium hexafluoroaluminate heated a t 220*^0 i n dry a i r

3 hours 4 hours

d I d I

6o284 11 6o231 ' 4 i

3 . 0 7 4

4o398 2

3 . 377 13 3.349 2

3 . 1 2 2 11 3 . 0 8 0 2

2o523 4 2.496 2

2.343 4 2 . 3 2 8 2

2 . 2 1 8 4 2.197 2

2.104 3 2.090 2

1 .97. 2 1.958 1

I 0 8 I 3 4 I . 8 O 5 3

l o 7 8 8 8 1 . 7 7 8 5

lo723 2 l o 7 0 8 2

1.396 3 1.591 2

1.378 2 1 . 5 7 1 ]

1.549 3 1 . 5 4 2 3

d (NH^)3A1F^ - ^ ^ A l F ^

6.40

5 . 2

4.47

3 . 6 0

3 . 1 4 3.13

2 . 5 7 2 . 5 4

2 . 3 6

2.22

2.03 2 . 1 1

1 . 9 9 1 . 9 8

1 . 8 2 1 . 8 3

1 . 8 0

1.71 1 . 7 3

1 . 6 1

l o 5 9

1.38

1.56

6 3

Table 9

Aimnonium hexafluoroalumiriate heated at 340°C i n dry a i r

^ hoiir

d I

6 * 2 2 4 3

3 . 5 3 5 ras

3 . 0 8 3 m

5 hours

6 o l U 4

3 o 6 2 0 9

3 . 0 3 9 2

'NH. AlP, 4 4

6 . 4

3 . 6 0

3 o l 2

5'.A1F3

6 . 0 2 9

3 o 5 A l

3.042

3 = strong

• ins = moderately strong

m."=:-. moderate©

64

s t a b l e phap.*? i n t h e d e c o m p o s i t i o n , b u t m a y b e c o n s i d e r e d t h e e n d m e m b e r

o f t h t s e r i e s MF^.NH^F - ^-AlFy S h i n n e t a l ( 1 2 ) c o n c l u d e d f r o m x - r a y •

r e s u l t s t h a t t h e t e t r a f l u o r o a l u m i n a t e i s a d i s t i n c t p h a s e i n t h e

d e c c m p o s i t i o n e v e n t h o u g h d y n a m i c t h e n n o g r a v i m e t r i c a n g i l y s i s r e v e a J e d

a p o i n t o f i n f l e c t i o n c o r r e s p o n d i n g t o a w e i g h t l o s s o f 2 . 1 - 2 . 2 5

m o l e s o f a m m o n i u m f l u o r i d e o O t h e r vjorkers ( l 3 , I4 ) a l s o r e p o r t t h e

e x i s t e n c e o f s t a b l e t e t r a f l u o r o a l u m i n a t e p h a s e i n t h e t h e r m a l d e c o m p o

s i t i o n a n d i t s e e m s t h a t t h e i r c o n c l u s i o n s are w r o n g .

D i f f e r e n t i a l t h e r m a l a n a l y s i s o f t h e s t a r t i n g c o i r p l e x r e v e a l e d a

p e a k w i t h a m a x i m u m a t 4 7 0 ^ 0 . X - r a y a j i a l y s i s g a v e d - s p a c i n g s c o m p a r a b l e

t o t h o s e o f S h i n n e t a l ( 1 2 ) , H o v / e v e r j . S h i n n , e t a l ( 1 2 ) w e r e n o t a b l e

t o m a k e f u l l c h e n i c a l a n a l y s i s o f - A l P ^ d u e t o t h e r e s i s t a n c e o f t h e

c o m p o u n d t o d e c o m p o s i t i o n . T h e y m a n a g e d t o p r e p a r e a n i m p u r e f o r m o f

- a l u m i n i u m f l u o r i d e b y p a s s i n g g a s e o u s h y d r o g e n f l u o r i d e o v e r a l u m i n i u m

b r o m i d e o r c h l o r i d e a t t e m p e r a t u r e s f r o m I 5 0 t o 5 0 0 ° C , a s i n d i c a t e d b y

t h e x - r a > p a t t e r n . T h e x - r a y p a t t e r n h o w e v e r , v / a s g e n e r a l l y a m o r p h o u s ,

b u t t h e t r a n s i t i o n t o OC - a l u m i n i u m f l u o r i d e w a s o b s e r v e d w h e n t h e i m p u r e

f l u o r i d e w a s h e a t e d i n n i t r o g e n a t 7 3 0 ° C , I n t h e p r e s e n t w o r k i n f r a r e d

a n a l y s i s s h o w e d t h e a b s e n c e o f a n y a m m o n i u m i o n s i n t h e c o m p o u n d J j ' - A I F ^ .

A c h e m i c a l a n a l y s i s f o r e i l u m i n i u m a n d f l u o r i d e w a s m a d e ( s e e A p p e n d i x I I ) ,

a n d t h e c o m p o u n d w a s t h e r e f o r e u n e q u i v o c a l l y e s t a b l i s h e d a s ^ - a l u m i n i u m

f l u o r i d e . T h e x - r a y c h a r a c t e r i s t i c s o f t h e c o m p o u n d w i l l b e d i s c u s s e d

l a t e r .

V / h e n t h e t e m p e r a t u r e w a s r a i s e d t o 530*^0 t h e f o r m a t i o n o f ^ - a l u m i n i u m

f l u o r i d e v r a s e v i d e n t a f t e r a q u a r t e r o f a n h o u r . H o w e v e r , a t 6 2 0 ° C a f t e r

1 h o u r t h e f o r m a t i o n o f a p h a s e w h i c h g a v e a n u m b e r o f s p a c i n g s w h i c h

d i d n o t f i t w i t h t h e p r e v i o u s d a t a w a s o b s e r v e d ( s e e T a b l e 1 1 ) , T o

65 -

Table 1 0

Comparison of the observed and calculated d-spacings for ^ - A l P ^

(a = 3 .53 ± O o C l S ; = 6 o 0 2 + O o O l 2)„ ^

ôbs ^ a l c ^ 60O3 6 . 0 2 0 0 1

3 . 5 4 3 . 5 3 1 0 0

3 . 0 3 3 . 0 4 1 0 1

2 c 0 2 o 0 1 0 0 3

1 . 9 2 1 . 9 2 1 1 2

l o 7 6 l o 7 6 2 0 0

l c 7 0 I069 2 0 1

1 . 5 4 1 « 3 3 2 1 1

1 . 5 0 l o 5 0 0 0 4

1.38 1.38 1 0 4

1 . 3 3 1 . 3 2 2 0 3

1 . 2 4 1 . 2 4 2 1 3

6 6

e s t a b l i s h the nature of t h i s compound the sample was x-rayed a f t e r 4

hourSy 18 hours and 6 5 hours at t h i s temperature. The r e s u l t s are given

i n Table 1 1 . A comparison of the d-apacings observed wi th those reported

f o r bas ic f l u o r i d e s ( 1 5 ^ 16) revealed the p o s s i b i l i t y of a bas i c f l u o r i d e

being formed due to the moisture content i n the a i r . A f l u o r i d e a n a l y s i s

of the specimen heated f o r 18 hours a t 620*^0 gave a percentage f l u o r i d e

content of 4 1 . 7 ^ which corresponds to a formula AlP , q . Hence to the

balance the stoichiometry OH ions v / i l l s u b s t i t u t e i n the l a t t i c e to

give the emperlcal formula A l OH F. , v/here x = O . 8 I 5 and as the time X J — X

of heating i n increased the l o s s fluor-ine i n c r e a s e s and x i n the

emperlcal formula w i l l i n c r e a s e . A formula i n v/hich x = 2 ( i . e . A 1 ( 0 H ) 2 F )

f o r a bas ic f l u o r i d e has been reporLed ( 1 5 ) » Prom Table 1 1 as the time

of heating i s increased the ion ic f r a c t i o n of ( O H ) i n c r e a s e s and peaks

are observed a f t e r 6 5 hours which a r e not present i n the previous cases

of 1 , 4 and to a l e s s e r extent 18 hours. The A i - F bond lengths i n

aluminium f luor ide are of the order of 1 . 7 S and t h i s i s i n c r e a s e d to

1.84 ^ f o r the bas i c f l u o r i d e A l ( O H ^ ^ ^ F ^ ^^) l.OS.HÔ, so the

introduct ion of hydroxyl ions w i l l weaken the c r y s t a l l a t t i c e , and w i l l

f i n a l l y r e s u l t i n the t o t a l removal of f l u o r i n e . T h i s forms the b a s i s

f o r the a n a l y t i c a l , method to determine the f l u o r i d e content o f aluminium

f l u o r i d e by pyrohydrolys i s ( 1 7 ^ 1 8 ) . There are c o n f l i c t i n g reports as

to the temperature at which the thermal h y d r o l y s i s of aluminium f l u o r i d e

i n i t i a l l y takes p l a c e , but temperatures as low as 3 0 0 ° C have been

reported ( 1 9 ) . I t i s noted that as the time of heat ing i n moist a i r

i s increased the d-spacings r e s p e c t i v e l y descrease and there i s

s i g n i f i c a n t change a f t e r 6 3 hours (see Table l l ) . Furthermore, the

dens i ty o f the f l u o r i d e heated i n moist a i r f o r 18 hours was measured as

67

Table 1 1

Comparison of the d-spacinfis o f ammonium hexafluoroaluminate heated i n

moist a i r at 6 2 0 ° C f o r 1 , Us 1 8 and 6 5 hours v/ith Y - A l P ^ , a bas ic

f l u o r i d e , aluminium hydroxide and al.umina

1 hour 4 hours 1 8 hours 6 5 hours

^^-AlP^ obs obs d -obs I . • - .olis d X

d • ... ^

' d z

6 . 0 2 9 5 . 9 3 w

5 . 3 2 w 5 . 3 1 vs 5 o 2 9 s 4 . 9 0 s

3 . 8 / 4 - w

5 . 5 3 4 . 7 2

4 . 3 6

3 . 5 4 7 3 . 5 1 V S l 3 . 4 6 ms 3 . 4 6 mw 3 . 5 2 m 3 . 4 B

3 . 3 6 ms 3 c 3 7 s 3 . 2 8 m 3 . 1 9

2 . 9 9 9 3 - 0 5 vw

2 . 8 4 n i w

2 . 9 3

2 . 8 3

3 . 0 8

2 . 6 8 W W 2 . 6 7 w 2 . 6 7 m 2 . 7 0 m 2 . 6 9

2 . 3 0 0 2 . 5 3 vvw • 2 . 5 2

2 . 3 9

w

W W

2 . 5 8 mw 2 o 5 5

2 . 3 7

2 . 4 3 2 . 4 3

2 . 3 4

2 . 2 8

2 . 2 7 0 2 . 2 m 2 . 1 8 w 2 . 2 8 vw 2 . 2 5 2 . 2 1

2 . 1 8 w 2 . 1 9 vvw 2 . 1 6 2 . 1 4

2 . 1 2 9 2 . 1 1 m 2 o 0 9 w 2 . 0 9 VI 2 . 0 8 2 . 0 0 2 . 0 6

2 . 0 0 3

1 . 9 1 5 1 . 9 7 w 1 . 9 6 1 . 9 7

1 . 7 7 3 1 . 8 3 w 1 . 8 9 1 . 8 3

1 . 7 3 6 1 . 7 5 nT7/ 1 . 7 4 mv/ 1 . 7 4 1 . 7 3 1 . 7 6

1 . 7 0 7 1 . 6 0 1 . 6 5 1 . 6 8

1 . 3 8 2 1 . 5 8 w 1 . 5 5

I 0 5 3 2 1 . 5 1

1 . 4 0

1 . 5 0

1 . 3 7 9 1 . 3 7 1 . 3 8

l o 3 3 2 1 . 2 5 w / m

1 . 2 8 1 . 2 8

i 1 - 6 8 -

vs = veiy strong = - 2^3

s = strong

m - n>oderate • V = ^W^.HÔ

w = weak

w/ = very weak etc.

= Al(OH), (Bayerite)

- 69

2 .Vf + 0.06 g .cc vrfiich cccipares w e l l with the reported va lues f o r

bas i c aluminium f l u o r i d e s ( 1 5 ) 3 iê* 2.25 and 2.45 + 0 „ 2 ( 2 0 ) »

When the sample was heated i n c a r e f u l l y - d r i e d a i r (see Chapter I I )

at 620*^0^ the specimen was i d e n t i f i e d as ^ - A l F ^ . The d-spacings o f a

sample heated at 700*^0 no longer agreed with those f o r ^-aluminium

f l u o r i d e , A comparison v/ith the values reported f o r -aluminium

f l u o r i d e ( 2 l ) shov/ed that i n f a c t a c r y s t a l l o g r a p h i c transformation had

taken p l a c e . K - f f e r e n t i a l thermal a n a l y s i s a l s o showed t h i s change

and a small peak was es tabl i shed at 690°C (see F i g . 5 a ) . To check t h i s

f a c t a sample analysed as ^ - A I F ^ was heated under s i m i l a r condi t ions

(see Chapter I I ) and a peak v/as observed at JOO^C ( F i g , 5 b ) . Chemical

a n a l y s i s of the product concluded that ©(-aluminium f l u o r i d e was formed

at 695° i 5° c .

There are s evera l reports i n the l i t e r a t u r e that aluminium f l u o r i d e

undergoes a phase t r a n s i t i o n a t 445°C (22^ 23) and 46O + 4°C ( 2 4 ) .

Repeated experiments between room temperature and 700°C w i t h ammonium

hexafluoroaluminate and ^-aluminium f l u o r i d e gave no evidence of a

phase transformation f o r aluminium f l u o r i d e other than that a t 6 9 5 ° + 5°C

as prev ious ly descr ibed . I t should be noted that the ex is tence of

^-AJ.F-j over a v/ide region of temperature ( 4 2 0 - 6 9 5 ° ) <. The

alviminium f l u o r i d e transformation i s i r r e v e r s i b l e as a specimen of

oC- aluminium f l u o r i d e was heated a t 600°C and the d-spacings of QC, -aluminium

f l u o r i d e v/ere maintained. The presence of OC-aluminium f l u o r i d e i s

observed at temperatures up to 880°C, However, when (p(-aluminium f l u o r i d e

i s heated f o r 18 hours a t 710°C i n dry a i r ^ ' d - s p a c i n g s other than those

f o r OC-alurainitmi f l u o r i d e are observed (see Table 12). T h i s i s probably

due to removal o f f l u o r i n e from the f l u o r i d e and s u b s t i t u t i o n by oxygen.

70

Table 1 2

Ammonium hexafluoroaluminate heated a t 710°C f o r 18 hours i n d r i e d a i r

4 o 7 9

3 . 6 7

3 . 5 0 3 o S 2 3 . 4 8

2 . 6 5 2 o 5 5

2 o l 7 2 o l 2 2 . 1 6

l o 8 6

1 . 5 1 . 5 1

1 . 4 5 1 . 4 6

1 . 4 1 . 3 6 1 . 4 0

7 1 -

The t h e m a l decomposition o f ammonium hexaf luoroga l la te i s s a i d

to take two cour-ses (see Figo 6)0 P i j r s t l y ga l l ium flizoidde can be formed

as 13 observed -with the aliimini\im compound o r the second course i^ .sul t ing

i n galli'om n i t r i d e ^ Tirf?ich was proposed by Kannebcru-i & Klemm (46) to exp la in

the weight changes observed when heat ing the sample i n vacuo o I n the

/ p r e s e n t vfork when ammonium hexafT-uoroaluminate was heated i n vacuo a t o

4-50 a white x/^ray amorphous compound was fcimed f o r which no chemical

a n a l y s i s v/as madCo T h i s i n v e s t i g a t i o n was not continued as means of

q u a l i t a t i v e i d e n t i f i c a t i o n of the product was not p o s s i b l e .

Two d i f f e r e n t forms of galliijim t r i f l u o r i d e are reported to e x i s t s

orxe obtained as above and one by the f l u o r i n a t i o n of galliiam oxide as

600°C (25)« Recent ly a c r y s t a l s t r u c t u r e of ga l l i i jm f l u o r i d e has been

reported (5)»

I t was concluded that the decomposition of ammonium hexafluoroalimiinate

i a a i r took the follo;ving courses

(NH^)^ AlP^. V f ^ ^ y , o<-AlF3 i^20-330°c 695°

I l l o l c i i The theii3:ial decomposition of amnonium hexaflôroaluminate i n

ammonia

I t has been reported (13.) 26) that aluminiiim n i t r i d e i s formed when

annnonixim hexafluoroaluminate i s heated i n ammoniac Due to the ease of

formation of the f l u o r i d e complex and t h a t f a c t tha t aluminivnn f l u o r i d e

i s a waste product i n the phosphate industryg and could there fore be

subsequently converted to a u s e f u l source^ t h i s method o f fanning the

n i t r i d e was ex tens ive ly i n v e s t i g a t e d .

72 -

TheiTual decomposition cf ammonitDn hexafluorcgallate ( ^ . 3 6

WH,GaP. GaP,

•2EP

-HP •HP Ga(NH2)^2 Ga(NH)P GaN

- 73

As i n the decomposi t ion o f t he complex i n a i r t h e complete removal

o r emmcnl-jm f l u o r i d e was no t observed ^_J i t i l 3 3 0 ° C v/hen an x - r a y t r a c e o f

a sample heated f o r 2 hicars a t t h i s t empera tu re was p o s i t i v e l y i d s n t i f i e d

fis ^ - A l F ^ e ^''hen the t o n p e r a t u r e was increased^ the the rma l decompos i t i on

,in c a r t i f u l l y d r i e d ammonia v/as analogous to decompos i t ion i n a i r © A t

620*^0 1^-aluminii-un f l u o r i d e \va.s observed w h i c h conve r t ed t o - a lumin ium

f l u o r i d e a.t JOO^C i n aninonia ,, TheoC-phase was s t i l l observed at 8 4 0 and.

8 S 0 " C » The temperature was r a i s e d t o 5 ^ 0 C and the specimen v/as heated

Per on hour and cooled t o hOO^C under a flav? o f ajnmonia t o p reven t o x i d a t i o n

o f the sample when removing i t fj-om the f u n i a c s . An x - r a y t r a c e o f the

product, I'eveaJ.ed a luminium n i t r i d e i n t h e presence of a l u m i n i u m f l u o r i d e

(see l i a b l e 1 3 ) . To s tudy the e f f e c t o f the r a t e o f f l o v / o f ammonia on

t h e r e a c t i o n at t h i s tempera ture ^ h a l f gram samples were weighed out and

p laced i n the tube f u r n a c e under a curr-ent o f ammonia at 9 6 o ^ C .

A p l o t o f percentage w e i g h t l o s s a g a i n s t r a t e o f f l o w o f ammonia was

made (see F i g o ? ) • ^ v/eigJit los^ i o f 6 l e 5 8 ^ ^vas recorded v/hen t h e f l o v /

r a t e o f amjnonia v/as zero* The t h e o r e t i c a l v /e ig l i t l o s s f o r the t r ans fo r r rB . t ion

o f ammoni'jm h e x a f l u o r o a l u m i n a t a to a lumin ium f l u o r i d e i s 3"/ê Accord ing

t o the i-eacti.on

( N H ^ ) ^ kW^ + im^ AIN -r- 6NH^F

the t h e o r e t . i c a l v/eight l o s s f o r the f o r m a t i o n o f alumi.nium n i t r ^ i d e i s

7 8 « 9 f i ? o « An x - r a y t r a c e o f the produc t r s v e a l e d twc- peaks w h i c h gave

d-spaciJigs comparable w i t h those e s t a b l i s h e d f o r a luminium n i . t r i d e , , The

presence o f a luminium n i t r i d e i n a zero f l e w r a t e o f ammonia can be

exr)lai.ned as f o l l o w s ^

Table 1 3

Ammonium h e x a n u 0 roal.umiriat e heated i n d i - i ed ammorj-i

and c o o l ed t o 400°C i n ammonia.

d. . O D S o

I n t s n i i i t y * Â.1JI

5 . 4 8 2 5 o 5 2

2 . 6 8 3 2 o 7 0

? c 5 3 3 2 o 5 1

2 . 4 6 1 2 o 4 9

2 2 . 3 7

2 . 0 7 3 2 . 0 7

..8.1 1 1 . 8 3

^^"!'^ .1 1 . 7 6

1 . ^ 9 4 l v 5 9

1 1 . 5 6 1 . 5 6

1 1 . 4 6

* I n t e n s i t y measured as the peak h e i ^ t »

7 5

.1^, -

P i g . ? • The thermal deconposition of anmoniuia hexaf louroalunina

i n v a r i o u s f low r a t e s of ammonia.

i

1 8

3 - 76 -

o AramonrrAui f l u o r i d e sublimes a t 520 C t o g i v e ammonia and gaseous h y d r o f l u o r i c a c i d a c c o r d i n g t o the r e a c t i o n

NH.P NH^ + HP

Hence the f o m a t i o n o f a luminium n i t r i d e i n t h i s case will be a r e s u l t

o f t h e r e a c t i o n o f al.umim,um f l u o r i d e and ammonia a c c o r d i n g t o t h e

r e a c t i o n s

(NH, ) , A l P . —»^ A i P , -i- m^F L 3 ^ J

N H , + H P

A.1P, + N H , — ^ AIM + 3HF

To v e r i f y t h i s s ta tement an i n d e p e n d e n t l y prepared specimen o f X , - a lumin ium

f l u o r i d e was heated i n ammorj.a a t 9Sd^C and a lumin ium n i t r i d e was p o s i t i v e l y

i d e r . t l f i e d o

Prom the r e a c t i o n

A l F ^ +• NH^ ^ AIN 3H?

the e q u i l i b r i u m cons tan t v ; i t h respecr. t o p a r t i a l , p ressure o f the gases

presen t K - w.iJLL be g iven by

\ - P H F ^

^ N H ^

Reducing the p a r t i a l pressure c f gaseous h y d r o f l u o r i c a c i d i n the system

v r i l l t h e r e f o r e promote the f o r m a t i o n o f the n i t r i d c c I'he ammonia gas

flowplng t h rough the system yii.ll t h e r e f o r e remove the gaseous KP as t he

condensate ammonium f l u o r i d e and hence the inc rease i n r a t e o f f l o w o f

ammonia v d l l r e s u l t i n inc rease o f t h e n i t r i d e f o m a t i o n wh ich i s observed

as an i nc rease i n we igh t lo3s= A comparison o f the peak i n t e n s i t i e s o f

d i f f r a c t c m e t e r t races o f samples a l s o shov/ed the r e l a t i v e i n c r e a s e o f

- /(

n i t r i d e w i t h respect t c f l u o r i d e as t h e f l o w m t e o f ammonia i s i nc reased

(see Table I 4 ) - At a r a t e o f 5 0 cc/mi.n a \ve±g.ht l o s s o f 7 8 ^ i s observed*

T h i s corresponds t o 9 8 ^ 6 ^ c o n v e r s i o n t o aJ-i^mi.niiim n i . ^ r id su A n i t r o g e n

a r a l y s i s (see Chapter I I ) o f a sample heated in airmonia a t a r a t e o f

4 8 c c / m i n gave a n i t r o g e n con ten t o f 3 3 * 6 l % 5 v/hich corresponds t o 9 8 ^ 5 ? ^

a lumi .av jn n i t r i d e c As the r a t e i s I n c r e a s e d t he re i s a m a r g i n a l i nc rease

i n n i t r i d e con t en t as i s shown i n T'^g« 7o At a r a t e c f I 6 0 c c / m i n a

n i t r i d e con t en t o f 9 8 . 7 ? d was analysed., F u r t h e r i nc rease i_n r a t e d i d n o t

inc rease the percentage conver s ion o f the complex t o a lumin ium n i t r i d e .

A specimen heated f o r 4 hours a t 1 2 0 c c / m i n was ana lysed and gave a

n i t r i d e c o n t e n t o f 9 9 . 1 / ^ «

I t appears th-at the thermaa. decompos i t ion o f ammonium h e x a f l u o r o a l u m i n a t e

i n d r y ammonia g i v e s a n i t r i d e y i e l d o f the o r d e r o f 99/'"" Aluminium, f l u o r i d e j

pr t i sent he re as an i m p u r i t y , , i s r e p o r t e d t o be used as a b i n d e r when

s i n t e r i n g the n i t r i d e so t he p-resence o f the f l u o r i d e v/o'old no t be

d e t r i m e n t a l v/hen the n i . t r i d e i s t o be used i n t h i s marinero Ho//ever^ t he

presence o f the f l u o r i d e i.s shown t o i n c r e a s e r a p i d l y the r ^ t e o f o x i d a t i o n

o f a lumin ium n i t r i d e (see Chapter I V ) . The o n l y d isadvantage o f t he

p r e p a r a t i o n i s the r a p i d a t t a c k on the purox rube by ammonium f l u o r i d e

sericâ3."]y r e d u c i n g the f u r n a c e tube l i f e . I f t h i s problem c o u l d be o v e r

come the successfV'-l i n d u s t r i . a l a p p l i c a t i o n o f t h i s method may be p o s s i b l e .

Repor t s t h a t al.uminium n i t r i d e i s formed f r c m ammonium h e x a f l u o r o -

alôminate i n ammoni.a a f 5 0 C ° C ( 2 6 ) a re no t v e r i f i e d . These v/orkers r e p o r t

t h e presence o f brown and graven a lumin ium n i t r i d e w h i c h would suggest

UTidesir^ble i m p u r i t i e s p resen t i n the n i t r i d e ^ I n the p resen t s tudy the

n i t r i d e formed was a w h i t e c r ^ - s t a l l i n e po^jider o f e x c e l l e n t p u r i t y .

- 7 8

Table 1

Ccnt-parlson o f the observed x - ra j^ i n t en s i t±ea of_ anmoniujii hexaTiuoroaJ-uminate

heated a t S60*^C i n v a r i o u s f l o w r a t e s o f ammonia.

Rate ( c c / m i n ) 5 8 . 1 2 3 . 5 4 3 7 1 9 6 , 7

^ÂIN I I I I I I

3 . 4 8 1 2 8 8

2 . 7 0 2 o 6 9 1 2 1 2 2 8 3 6

2 . 5 1 2 . 4 9 2 o 5 i

2 c 4 6

1 2 1 2 1 2

4 1 6 1 6

2 . 3 ? 2 2 . 3 7 4 4 6 6 1 6 1 8

2 o l i 9 2 . 1 2 1 2 1 2

2 c 0 7 4 2 . 0 7 1 6 4 2 1 1 1

2 = 0 1 9

1 . S 2 9 I 0 8 I 2 6 8

1M7!)^J 1 . 7 3 * 4 4 4 1

1 . 6 C 6

l o 5 9 8 8 4- 2

1 « 5 6 0 l o 5 5 7 l c 3 6 4. 4- 6 1 2

1 . 4 8

1 . 4 1 4 1 . 4 0 2 4 8

1 . 3 5 3 l o 3 4 3

•70

I t hias been r e p o r t e d chat g a l l i u m and i n d i u m n i t r i d e (2? ) are

formed v/hen the co r r e spond ing h e x a f l u o r c m e t a l l a t e compound i s heated, i n

ammoni.a a t bj^gh t e m p e r a t u r e s » G a l l i u m n i t r i d e i s a l s o r-eported tc be.-

foiTned f rom ainmoni-.:un h e x a f l u o r o g a l l a u e a t l o w e r teinpoxatures Thr.

complex i s heated i n argon a t 175^^ i n i t i s J l y t;o ammonium t r r - t ra r iuo io- -

g a l l a f . . The t e t r a f l u c r - c g a l l a t e i s heated i n ammonia a t 1 3 0 ^ 0 to fo rm

the aad.:ct ammoni'wm t e t r a f l u o r o g a l l a t s :ro;:o-/•-in.T.o.nifvJ;--', As t he t empera f j . re

i s rai .sed , monc-.'jjrino ° a l l i ! . ^ : f l uo r i c Je (CaP-, .I'Jti.^) i s fo rne r i wtt ich l o s e ^

gas'.ôus h y d r o f l u o r i c a c i d i n the pxêsence o f ammonia a t 300*^0 to f o r m

g a l l i - . m n i L r i d e o As t h i s o f f e r s a lov/ r .anporatur? a i teLTiaf : ive to

f o r m a t i o n of ' n i t r i d e s fix^m t h e i r corr-esponding hexaflLiOi 'omet .a l ia tes^ a

s i j i d l a r pTX>gram v/as adopted f o r a^^.oni.um hexa i ' luoroa lumlna tec

Aminoni-jm t e t r a l l u o r c a l u m i n a t e was mride a c c c r c i n g tc ti^s r e a c t i o n

HBP; + Al(OH):; + NH, OH - -^ NH, Ali"-' + H . B O , + HÔ

d e t a i l s are g i v e n i n Appendix I c

Ajnmoniw. t e t r a f l - j o r o a l u m i n a t e was heated i n ammonia a t 1 1 0 C i 'o r

pe r iods up t c 3 0 hours^ but no d i f f e r e n c e i n s t r u c t u r e v/as d e t e c t e d by

i n f r a - r e d and x - r a y anal^^siso To c o n t i n u e t he method o f Mermûnt e t a l ^

(28) a t t empts were made to prepaj:*e aluraJnium f l u o r i d e monoammoni.atee

The ammoniates o f vhe c h l o r i d e s ( 2 . 9 ) r bromides ( 3 0 ) ^ and j c a id^^ -

( 3 9 ) cire known^ and i t i s r e p o r t e d t h a t t he mono-anrnoniate can be

prepared by adding a luminium f l u o r i d e v e r y s lowly to l i q ; j i d ammonia ( 3 i ) ' »

To pr-epare the a lu/ ianium f l u o r i d e mono-ammonia te„ l i q u i d ammonia

was prepared by pass ing d r i e d ammonia gas i n t o a q u i c k f i t t e s t tube

placed i n an i c e t r a p ( a dev/ar c o n t a i n i n g s o l i d GO^)» Tc the o t i - e r s ide

o f the t r a p was connected a dr>''ing tube c o n t a i i r i n g po tass ium h y d r o x i d e

8 0 -

pe?tlo.xF. to p r even t mo i s tu re f r o m e n t e r i n g the sye^tem*. <p«^-Aluminium fiu.ori.de

(p2^pai'ed by h e a t i n g ammonium hexafiuorâJ.imdnatrT i n p u r i f i e d nlti-ogen at.

7.U-0~G) was ve ry s lov / ly added zo the l i q u i d animonia anc the m l - x t v . ^ v a s

l e f t i n a f.-me clipboard t c reach e q u i l i b r i u m . The quicK: f i t test zxihe

v/as g m d u a l l y removed f r o m the i c e t r a p û rh t h a t indL05d tcinp-:rai^-^re

d i f f e r e n c e v/as s u f f i c i e n t t o remove any excess j i q u i d a'!!-ni:*r.ia „ '-h'-r whi*.o

cov/der v;as examined u s i n g x - r ^ i y and i n f : :^-red oji-al V5ji s anri no zt'r ^" '' "-...ra'L

d i f f e r - ences f rom oc -altomini-jm f l u o r i d e v/erd obser-ved. Henc:: t h - : wcrk o*

C l a r k (51) v/as no t conf i rmedo

At tempts v/ei^ made to prepare trie monc-- ammonia te by ])a35;inK a-Timo-.if.

g. .s overo^ -al.Jiniriium f l u c r i d e a*i; .1 lO'^ and l60^C f o r period--, up 1.0 .00

hours bu t no s v r u c t u r a l changes v/ere obse:rvod<, I t wai, er-tncii-atecj that,

^ - a l u m i r j . u m f l u o r . i d e liad a more open s t r u c t u i - ^ j , but no mor.::'-^;iPr;;no f ' l-r-:.-1

adduct v/as observed usi.ng the above methods.

Using the Sehomaker-Stevenson r e l a t i o n s l i i p {32} t h e bcn-'iiî^^j fo'r ' hf."

kr.ov/n OL--ali^minium f l u o r i d e structu.; :^ ( 3 3 ) v/as c a l c u l a t e d as 6b/-. i o n i c

i n charac te r^ whereas a l i m i n i ^ j m c h l o r i d e ^ i o d i d e and bromides ar--

p reva len t ly" ccvalenc i n cha_ractero Boron t r t f l u c r i d e ir.ono-îrriTior.iav i ^ ^

r e a d i l y formed f rom borcn t r i f l u c r i d e and ainmo.nia and the r^^sultr ixg

chemi.cal bonding i n t h e adduct i s c h a r a c t e r i s i . j . c a l l y c c v a l e n i c rViri:-h£r!ri:;r?r,

a iuminium f l u o r i d e i s repx^rted to have the rd.ghest l a t t i c e energy ot' the

gr-c:;p I l l b meta l t r i h a l i d e s ( 3 4 ) . The absence of i*orTP.at.i.on of a2iiiTj.ni:um

f l u c r i d e mj^no-amm.oniate i s t h e r e f o r e a t t r i b u t e d t o inl- ierBnt chcirdcaJ

na.tp^ re o f ai.'.^minium f l u c r i d e bonding*

Other routes f o r the f o r m a t i o n o f a lumin ium n i t r i d e from ammoraum

hexLafJuorc-aJ-uminate than those e s t a b l i s h e d ax-e not p o s s i b l e ^ and ihe

- 8 1

method o f f o r m a t i o n einalogous t o t h e v/ork o f iAermant e t a l (28 ) on the

g a l l i u m compo'ond i s n o t c o n f i n n e d .

IXje t o the s i m i l a r i t y between animonolysl?= and h y d r o l y s i s a stU'dy o f

the s o l u b i l i t y o f a lumin ium f l u o r i d e i n d i s t i J . l e d v/ater was inadec Also

t h e s t r u c t u r e o f ^ - a l u m i n i u m f l u o r i d e i s no t yet eva lua t ed and the

f o r m a t i o n o f any hydra tes v d t h the compound may g i v e i n f o r n f i a t i o n about

the l a t t i c e space v a c h i n t h e f l u o r i d e . The s o l u b i l i i y o f a lumin i i jm

f l u o r i d e I n v/ater i s r e p o r t e d as 0^35% ( 2 ) presumably as the (A - t r i h y d r a v e c

The hydra tes o f a luminium f l u o r i d e are numerous (see T£Lbl6 15) arid

Ecgachov and Myasmkov ( 3 6 ) shov/ed t h a t 0 -19ô v/ater o f c r y s t a l l i z a t i o n v/as

present i n a lumirdum f l u o r i d e v/hen l e f t i n a i r o f varâ.ng r e l a t i v e

humid i tyc There i s a r e p o r t t h a t v/hen alumi.nJLum I'lv-.oride t r i n y d i i i t e i s

dehydr-atedj a nevf c a t a l y t i c f o r m o f alumj.nium f l u o r i d e ( y ^ - A l F . ) i s

formed ( 3 7 ) . A comparison o f the d-spaclngs o f -al-.mnr:ium f l u o r i d e v/i.th

those r e p o r t e d f o r t h i s c a t a l y t i c f l u o r i d e (see Tab le 1 6 ) shov/s c e r t a i n

s i m i l a r i t i e s . . , but c o n t a i n s a number o f l i n e s no t p resen t i n ^-al--jro.ni.;)m

f l u o r i d e . The p o s s i b l e h y d r o l y s i s o f the a lumin ium f l u o r i d e to t'oijn a

basic f l u o r i d e may a l s o o c c u r .

I t has been proposed ( 1 2 ) t h a t t h i s -al iuni .nium f l u o r i d e i s i n f a c t

an open |^ s t r u c t u r e w i t h a r e s i d u a l amount o f w a t e r p t e s e n t .

To s t i jdy the s o l u b i l i t y o f the f l u o r i d e s v/eighed amounts were p laced

i-n two separate 1 0 0 ml q u i c k f i t c o n i c a l f l a s k s . 5 0 mis o f d i s t i l l e d v/ater

v/ere added t o each f l a s k and the f l a s k s sealed v / i t h f i r m l y f i x e d s t eppers .

The f l a s k s v/ere g e n t l y shiaken iji a wa te r b a t h set a t 25^0 I 'o r 1 7 days .

I n t e r m i t t e n t l y samples were e x t r a c t e d ^ c a r e f u l l y d r i e d by v/ashing i n

acetone and p l a c i n g i n an oven a t 5 0 ^ 0 and v/eighed. X - r a y and i r i f r a - r - e d

- 82

Table 13

X - r a y data o f bas ic aluminium f l u o r i d e s and aluminium f l u o r i d e Kydrates

Compound a (X) b (fi) c (2) ' Ref,

AIP^.SHÔ

AlP^.HÔ

AIP^1.33H20

AlP^ 3.H2O

A1P^.3H20

Al(F^^^3,OH^^25)Vl6H20

Al(^1.65'^"l.35^'/^^^2°

4A-

3.610 20

11»42 2 ia8 8.54 41

. 9o20 9.30 43

7.734 7.330 44

7.734 3.665 45

9.83 15

9.849 15

9o87 15

- 83 -

Tab le 3 6

Comparison o f the d spacings o f the reportedy^^-AlF^ ( 3 7 ) and ^-AlFô

^ j - A l F 3 ( c a l c ) ^ - A l P ^

6 o 0 2 6 . 0 0 2

3 . 5 3 3 . 5 6 3

3 . 0 4 3 - 0 6 3

3 . 0 0 1

2 . 4 S i t

2 c 2 9 5

2 . 1 6 1

2 . 0 1 2 . 0 0 1

1 . 9 2 i < , 9 1 4

1 . 7 6 1 . 7 8 2

1 . 7 3 2

1 . 6 9 1 . 708

1.665

1.585

1 . 5 3 l v 5 3 2

1 . 5 0 j . 5 0 0

1 . 3 8 1 „ 3 7 7

1 . 3 2 1.310

1 . 2 8 4

1 . 2 4 J . 2 4 2

1.2_16

1 . 2 0 0

1 . 188

l . ^ i48

3 .134 - 84 -

A n a l y s i s of the dr i ed sangjle were madee A f t e r the period of 17 days^

there was no o v e r a l l change i n weight and no s t r u c t u r a l d i f f e r e n c e s were

observedc I t appears that ^ and oi -alvnninium f l u o r i d e s a r e r e s i s t a n t

to h y d r o l y s i s under these condi t ions . However, b a s i c f l u o r i d e s are

formed as shown a t h igher temperatures*

I I I l , i i i X - r a y a n a l y s i s of ammonium hexafluoroalumlnate

To determine the l a t t i c e parameters o f the canplex a powder

specimen was ptrepared and placed i n the 9 can camera (see Chapter I I ) ^

A f t e r a 2 hotir exposure, the developed f i l m v/as measured as p r e v i c u s i y

descr ibed . The i n t e n s i t y o f the powder l i n e s are dependant on the

a r i t h m e t i c mean of the s c a t t e r i n g f a c t o r ( f ) , ( i . e . f i s the r a t i o of the

ajnplitude of s c a t t e r i n g of an atom to the amplitude of s c a t t e r i n g of a

s ing le e l ec t ron) and as t h ^ Bragg angle (0 ) increases j , f deoreases*

Cor^sequentlyp f o r an accurate va lue o f the l a t t i c e parameter^ Cohen s

l e a s t sqixares method was enployed (8)^ as t h i s method reduces the

s t a t i s t i c a l e r r o r involved when u s i n g a small number o f powder l ine s^

The l a t t i c e parameter of ammonium hexafluoroaluminate was determined

as 8.93 + 0,01 X, which agrees w e l l w i t h e s tab l i shed va lues ( see Tab le 17)

I l l . l . i v The s t r u c t u r a l rearrangements during the formation o f al^jminium

n i t r i d e from the thermal decomposition o f ammonium h e x a f l u o r c -

aliiminate-

Aramonium hexafluoroaluminate has a te tramolecular u n i t c e l l c f

dimension 8»93 + 0,01 2 . The space group has been ass igned as Fm3m-0^

with atoms i n the pos i t ions :

NH^ (1) : ikh) h> h • (2 ) : (8c) h id, h

85 -

Table 1 7

Compound ^ ^ ^ ^ , 0 , Ref ^ Dimensions (A) '

i ^ l J f o F ^ 9 - 1 7 I,

(NH^)2^gPe.P^Ô.i4HÔ 9 . 1 0 4

Na,PeP 9 . 2 6 4

K^FeP^ 9 o 9 3 4

Rb^PeP^ 1 0 . 2 3 4

(NH^)^SiP^ 8 . 3 9 4

Rb^SiP^ 8 o i t 4 1 0

(NH^)ÂIP^ 8 . 9 0 4 0

(NH^)yilPg 8 o 9 3 Present vrork

NaÂlP^ 7 . 8 0 5 . 6 1 5 . 4 6 9 0 ^ 1 1 ' 4 1

Na^ÂlPg. ( 5 0 0 ^ 6 ) 7 . 9 5 4 2

AlFg 8 . 4 6 8 5-944 4 1

KÂl P^^^ O.IHÔ 8 o 4 3 5 4 1

(NH^)3lnP^ 6 . 5 3 9 . 4 9 4 0

(NH^)^GaPg 9 . 0 4 9

8 6

A l s (Aa) 0, 0 , 0.

F ; (2ife) xÔÔ; O^x^C; 0 « 0 , x s xÔÔ; O^XpO; OpO^x^

v/here x -• 0,23 =

There i s an octahedral arrangement o f f l u o r i n s atoms aicund each aj.umirKlum

atom with face centred cubic d i s t r i b u t i o n o

The f i r s t thermal t r a n s i t i o n o f the above system i s the r ^ j o v a l of

ainnscnxvm f l i i o r i d e from the system^ I f i t i s considered bha:: at any i n s t a n t o

below 420 C i n the decomposition*, the phase ammoni jm tt^trafiuorcalT-jiiir.ate

e x i s t s then the fo l lowing observation can be made.

Ammonium te tra f luoroa luminats has been assigned to space group

VL/mm - ' A with atoms i n s p e c i a l p o s i t i o n s

AI t ( Id ) '

F ( 1 ) : (2e) 0,^,h i . O , ^ .

P (2 ) s (2h) ^î^jz; - ^ ^ i / Z , where z = 0 . 2 1 .

From s t r u c t u r a l cons iderat ions the removeil o f t wo moles o f ammonium

f l u o r i d e woiold be followed by c o n t r a c t i o n of the cubic l a t t i c e i n tv/o

d i r e c t i o n s to form l a y e r s of AlF^ cctahedra Joined by shared c o m e r s ( i f?)*

The major adsorption i n the i n f r a - r e d a n a l y s i s has been ass igned to

The shar ing of the f l u o r i d e ions does not d i s t o r t the AlF^ oczahedin. a s

the 2 band i s not apprec iably a f f e c t e d ( see Table 1 8 a ) . Th.e NH. grc-^jps

would l i e betv/een these layers^ The removal of the tvTO molea of amsaonium

f l u o r i d e d i s t o r t s the H-F and P - F octahedral in teratomic d i s t a n c e s by l e s s

than 2 » 5 ^ which suggests the AiP- octahedron i s not unaf fec ted i n the

i n i t i a l deconiposition. Thus i t wCTiild seem poss ible the amncniuin atcms

assoc ia ted with s p e c i a l p o s i t i o n (8c) i n ammcr.ium hexaf iuorcaluminate

87 -

Table 18a

I n f r a - r e d a n a l y s i s of the thermal decomposition of axninoniiim hexaTluoro-

alianinate

Compound ^ ^ \ ^ \ • 2 + \ " ^ Rs^**

(NH, ) ,A1F^ 3240 3050 lii.30 Present ^ ^ work.

Heated 2 h r .

a t 220°C

Heated 4 hr ,

at if20°C

3240 3050 1435

NH.P 1484 (4) 4

$ffi^)ÂlF£ 3250 3060 1428 (4)

NHÂIP^ 3230 3120 2905 1800 1435 (4)

88

Table 18b

D e n s i t i e s of some almniniuin compoxinds

Coinpoiind

A 1 P 3 . 3 ^ 2 °

AIN

Densi ty (gccc"''")

1.98

3.25

2.17

1.914

3. . ?

3.97

89 -

are i n i t i a l l y removed i n the deconpos i t iono Hov/every these supposi t - icns

are no t c o n c l u s i v e »

The t o t a l removal o f ammona'am f l u o r i . d e r e s u l t s i n the, y^/-.].?-. ^-t r..'..^ cv--

v/hich indexes s a t i s f a c t o r i . l y vrlth a t e t r a g o n a l c e l l oV u n i t c s l I . diicen.si.oi:-

a =: 3 . 5 3 + OoOl X C ^ 6 . 0 2 + 0 . 0 1 X (see Tab le l O ) . The bas ic i?i.rucv.i;rc

o f t e t r a f i u o r o a l u m i n a t e i s unchanged i n the decompos i t ion iz- ' ^ -A. lF ,o ^tr:.

c o n t r a c t i o n o f t h e l a t t i c e i s e s s e n t i a l l y concerTied w i t h the r>-ayis

shc\'m by F igs* 8a and 8 b o A g r e a t e r d i s t o r t i o n o f the ^qqj- spa:;ir.g v;:.tn

i n c r e a s i n g temperature i s observedc The removal o f ammcniiain f l u o r i d e f i x m

the t e t r a f l u o r o a l u m i n a t e phase by the ammoniimi atoms and one of* -L'he r.o-:-

b r i d g i n g f l u o r i n e atoms v/ould r e s u l t i j i a s t ruc tu jê composed pU-r a-

AlP, g.roups w i t h f o u r f o l d c o - o r d i n a t i o n , s h a r i n g c o m e r s wfi th each

a luminium atom be ing connected by an unsh-ared f l u o r i n e atom {s'-je Pigs* 3c

and 8 d ) c V/hite and Roy ( ^ 8 ) have proposed a f o r m u l a c o n n e c t i n g th ' : irJ*:-;a-

red spectrum v / i t h t h e m e t a l - - f l u o r i d e i n t e r a t o m i c d i s t a n c e 2

P where F =

wher^ ' ) = mad.n m e t a l - f l u o r i d e f r e q u e n c y

^ = reduced mass o f t he m e t a l - f l u o r i d e i o n p a i r

^^Z^ = p roduc t o f t he e f f e c t i v e cha i se

r = m e t a l - f l u o r i d e i n t e r a t o m i c d i s t a n c e .

By comparison w i t h i n t e r a t o m i c di .stances i n --aluminium f l u o r i d e

(see Tab le 1 9 ) and u s i n g the r e p o r t e d m e t a l - f l u o r i . d e v i b r a t i o n s ( l 2 ; f o r

1 and - a lumin ium f l u o r i d e s an A l - P i n t e r a t o m i c d i s t a n c e i n ^^-AlF-^ I s

c a l c u l a t e d as l o 7 7 2*

- SG

^1 i :

0

• L.

too I.V:S i-i

U4 3.0

loo 300 4 0 0 6co

Fig. 8. ( a ^ ) The aanonion tetrafloaroaluainate^«>alualiiiaB flourlde uystallographie tranaitloh^

r 91 -

Table^_19

Interatomic d i s tances i n -aluminium f l u o r i d e and r e l a t e d conqôunds

Compound Ref . A l - A l interatomic A l - F interatomic F - F interatomic d i s tance d i s tance d i s tance

(NH^)3 A l F ^

NH.AIF, 4 4

3 8

6 . 3 1

6 . 3 5 ^ 5 o 0 7 , 3 . 5 9

1 . 7 6

1 . 7 9

2 o 4 9

2 « 5 4

Al UH 1 . 3 5 1 . 6 5

3 3

present v/ork

1 5

3 c 5 0 8 1 . 7 9

1 = 7 7 , 3 . - 0 1

1 . 8 4

2 = 5 0 3

L . l.'.l

- 9 3 -

The c a l c u l a t e d x - r a y d e n s i t y o f ' j ^ -a l i jmin ium f l uo r i c3e I s 1*96 goco

\7hich ag.rees w e l l v / i t h the observed va lue o f 1 ^ 9 2 -i- O.O4 g.cc~'^. T h i s

agreement suggests t h a t t l i e proposed a l u m i n i u m - f l u o r i n e c o n i ' i g u r a t i o n (see

F i g . 8 d ) i s f o u r p l a n a r f l u o r i n e atoms a t a d i s t a n c e l o 7 7 ^ f r o m the

a lumin ium atom and two flu.orQ.ne atx>ms a t a d i s t a n c e o f 3 » 0 1 ?. f r o m the

a lumin ium atom. The A l - F c o n f i g u r a t i o n i n -ÂlF, i s a r e g i u a r octahedron

o f A l - F i n t e r a t o m i c d i s t a n c e 1 . 7 9 8 . The c o l l a p s e o f t he ^ Vaz-e-:jenlre6

t e t r a g o n a l s t r u c t u r e t o f o r m the rhomhedral < - A l F ^ s t n j c t u r s i s agfd.n

p r o b a b l y a s soc ia t ed v / i t h the c - a x i s and a c o n t r a c t i c n o f the A l - F spac ing

o f 3 o 0 1 2 t o 1 . 7 7 X v/ould r e s u l t i n a cube o f s ide 3 o 5 4 2 ar^d a c a l c u l a t e d

d e n s i t y i s 3 . 4 8 g.cc The r e p o r t e d d e n s i t y o f -A.IF^ i s 3 ^ 2 5 and *:ho

l a t t i c e parameters i s 5 - 0 2 9 X ( c< = 5 8 ' ~ ' 3 1 \ ) s o the t r a n s i t i o n t o c ^ - A l F ^

in 'A) lves a s l i g . h t d i s t o r t i o n o f the pseudc-cuV i o c e l l . T h i s rearrangeme-nr.

o f Al'-F c o n f i g u r a t i o n i s a l s o shown f r o m the r e s u l t s o f d i f f e r e n t i a l thermal,

a n a l y s i s . The r e s u l t a n t - A l F - 3: : ructure i s r ep re sen t ed i n F i g . 9ao 'I'he

s p l i t t i n g o f the band (see Table 1 8 ) i s a t t r i b u t e d to the presen^'.e o f

the tvvc oc tahedra . Moreover the d i f f e r e n c e i n d e n s i t i e s o f the ând. the

oC m o d i f i c a t i o n o f a lumin ium f l u o r i d e suggests t h a t ammonia o r w a t e r mighr;

be s u b s t i t u t e d i n the moi^ open ^ - A l F ^ s t r u c t u r e . T h i s v/as n o t e>::periment-

a l l y v e r i f i e d a t 2 5 and 6 0 ^ o However t h e ^ - A i F ^ i s seen t o take up

mcr-^tur-e.-ncre r e a d i l y than ^ - a l u m i n i u m f l u o r i d e a t e l e v a t e d tempera cures .

Moreover a t t empts t o f cxm alumini.um n i t r i d e f r o m ammonium hex*3.f.lucroaluminate

and gaseous ammonia a t 5 2 0 ° C \7ere u n s u c c e s s f u l , Thes-3 obser^rai icns would

suggest t h a t t he k i n e t i c s f o r t l ie r e a c t i o n

A l F ^ + NH^ — A I N -r 3 H P

as v / e l l as the themcdynamics (see Figo 3 3 ) a re u n f a v o u r a b l e ^

9 4

10)

© (b)

t . t t S A

a) MF^ - 95 i») AiN

I l l , 2 The fonnation o f aJ-uminxum n i t r i d e by t h e Serpek process

Serpek ( 4 9 ) pa ten ted a process o f making al-uminium n i t r i d e i'crmed f r-om

ths?. r e a c t i o n o f gaseous n i t r o g e n v / i t h an i n t i m a t e m i x t u r e o f al^-imina and

carbor.;t

AlÔ^ + 3 C - —> AIN + 3C0

T h i s method o f n i t r i d e f o r m a t i o n i s now known as t he Serpek prorje.'ts.

Other workers have s t u d i e d the react i .on and the re i s g r e a t c o n f l i c t i n

t h e l i t e r a t u r e as t o the exact process v a r i a b l e s r e q u i r e d to b r i n g about

the optimum aluminium n i t r i d e y i e l d e

Tucker and Read ( 5 0 ) r e p o r t e d the absence o f aluraini'-jm n i v r i d ' 5 I'rcm

the above r e a c t i o n a t iSOO^Cj bu t F r a n k e l ( 5 i ) £hov;ed t h a t 10)1 n i . t r i a e

f o r m a t i o n i s observed as low as lÔO^C.. and t h i 3 percentage n i t r o g e n

f i x a t i o n i n i t i a l l y i nc reases w i t h i n c r e a s i n g temperîti^re o

The presence o f i m p u r i t i e s i s r e p o r t e d t o c a t a l y s e t h e r sac t i .on

acco rd ing t o M e l l o r ( 3 2 ) such t h a t the n i t r i d e i s formed a t . 1 3 ^ 0 ° C

u s i n g baux i t e ±n p lace o f a l u m i n a , but Repenko e t _ a j o ( 5 3 ) ^ i d not

c o n f i r m t h i s r e p o r t s

Peacock and du Pont ( 5 4 ) concluded t h a t the r e a c t i o n a t 1 3 0 0 ^ 0 ana

5 0 0 mm Hg was

The compound ^^2^Jf^6 '' ' '- ' c o n t a i n 4 2 ^ n i t r o g e n arid I'^rankel u s i i i g

the same c o n d i t i o n s as Peacock and du Pont observed 1 9 - 4 ? ^ ni.t,rcgen

f i x a t i o n a t pressures as l o w as 1 0 0 mg Hg»

I t has been p r e v i o u s l y s t a t e d t h a t i n c r e a s i n g the r e a c t i o n tempera ture

inc reases the y i e l d but t o o h i g h a tempera ture appeajTs t o be d e t r i m e n t a l .

9 6

T h i s v/as observed by Pechiney (55) f r o m an a n a l y s i s o f t he r e a c t i o n

p r o d u c t . The r e s i d u a l a lumina con t en t decreased w i t h i n c r e a s i n g

temperature t o a minimum o f lo^'o a t 1725*^0 but i n c r e a s e d to h-^2yo a t

1825^0 i n th ree q u a r t e r s o f t h e o r i g i n a l t i m e . R e s i d u a l carbon was

a l s o p r e sen t i n t he charge which was removed by r o a s t i n g i n a i r a t

700 to 800°C o From the p resen t v/crk 1^% conve r s ion t o pC-aJumina i.s

observed i n 5 hours a t 800°C and t h i s suggests t h a t t h e r e s i d u e al*amir.a

c o n t e n t v/ould be increased by roas t ingo Fur thermore a t tempera tures i n

t he r e g i o n 1600-1800^0 the s i d e r e a c t i o n

AIN + C ^=dfc AlCN

i s r e p o r t e d (56)* 0o2;G a lumin ium sub-cyanide con ten t i s ana lysed a t

1600°C and t h i s increases t o 1% a t l 800°Co Never the less Tucker and Read

(51) s t a t e d t h a t the r e a c t i o n i s bes t completed i n t he tempera ture range

1800-2000^0.

The presence o f o t h e r i m p u r i t i e s than a lumin ium sub-cy6.nlde i s

expec ted . A c c o r d i n g l y F r a n k e l observed t h a t a lumin ium c a r b i d e (A l^C^)

i s formed as a sub l ima te when alumi.na and carbon a re heated t o g e t h e r i n

a i r f r o m t h e r e a c t i o n

2 A l ^ O , + 9C — A l . C , + 600 2 3 ^ 3

Thi.s vrould suggest t h a t t h e r e a c t i o n i s s t r o n g l y exo the rmic a.s the

s u b l i m a t i o n temperature i s r e p o r t e d as 2100''C, A l s o compounds o f the

gene ra l f o r m u l a ( A l N ) ^ . A l ^ C ^ v;here n - 1 t o 6 are formed when a lumir i i i im

n i t r i . d e i s heated i n a carbon c r u c i b l e i n the presence o f n i t r o g e n ( 5 7 ) »

Repenko e t a l . (53) r e p o r t e d t h a t a s p i n e l o f the typ^ AlO.ft- lÔ^ i s f o m e a

i n the r e a c t i o n . Yamaguchi & Yanagida (58) ho-vvevsr r e p o r t e d t h a t a spin^^l

o f the t y p e AlM.AlÔ^ i s formed i n a r e d u c i n g atmosphere above 1650*^0 (see

9 7 -

Table 2 0 ) , Oxycarbides are a l s o r e p o r t e d to be formed i n the reacbiono

Pechiney ( 5 9 ) have r e p o r t e d t h a t b e t t e r than a 39/o y i e l d o f alumiriiuj-n'

n i t r i . d e i s observed a f t e r a p u r i f i c a t i o n process w h i c h i n v o l v e s a f i n a l

c h l o r i n a t i n g stage a t 4 5 0 - 6 5 0 ° C o C h l o r i n e i s r e p o r t e d t o a t t a c k al-oTr-:isi±<:m

n i t r i d e a t 9 0 0 C f o r m i n g a lumin ium c h l o r i d e ana so unecessary xmpuri t ie . - ,

may be i n t r o d u c e d by t h i s p u r i f i c a t i o n stageo Hence the irH'.er'rnt

i m p u r i t i e s l i k e l y to be p:^fesent as a r e s u l t o f the r e a c t i o n ar=- d e t r i m e n t a l

t o the process^ However Repenko e t a lp ( 5 3 ) has shown, t h a t unr fe r the

same exper imen ta l c o n d i t i o n s a g r e a t e r percentage o f n i t r i d e :is I'orrnea i f

the i n i t i a l r e a c t i o n m i x t u r e c o n t a i n s the s p i n e l l i d e A l O c A l ^ ^ ^ . r a t h e r

than al ' jmana. They a l s o showed t h a t p ro longed h e a t i n g incrêased the

n i . t rade con ten t v/hich i s a d i r e c t c o n t r a d i . c t i on o f t he r e s u l t s o f Franl-iel ( f ; l )

I t wo^jld seem t h a t temperatures i n the rv^glon 1 7 0 0 - 1 ? 5 0 ' ^ C g i v e tr.i-,

optim'jm r e s u l t s , e s p e c i a l l y as i t i s r epo r t ed t h a t t h e r e a c t i o n v e l o c i t y o f

a lumina r e d u c t i o n by carbon i nc r ea se s a t a f a s t e r r a t e than r h e d i f . f u s i c n

v e l o c i t y o f n i t r o g e n i n t h e d i f f u s i o n mass above 1 7 5 0 ° C „

Franke l ( 5 l ) was a b l e t o show t h a t i n f a c t the Serpek prcces? i s

r e v e r s i b l e and thus the equa t i on can be v / r i t t e n

AlÔ^ + 3C + N ^ . * - ^ • 3 C 0

He assumed t h a t t he s o l i d s aluminar, carbon and alumini.um n i ' - . r i d e ex±3Z i n

the re s tandard s t a t e s and hence t h e e q u i l i b r i u m i s a f u n c t i o n o f the

p a r t . i ^ pressures o f the gases i n the system, such t h a i

\ However, due t o i m p u r i t i e s d iscussed above, t h i s assumption i s t o a i j r s L

app rox ima t ion o n l y v a l i d f o r l o w e r tempera tures I n the cunsider-sd

~ 9B -

Table 2 0

Comparison of the c r y s t a l proper t i e s of AJl^ with the poss ib l e i m p a r i t i e s i n the Serpek process

Compound R e f r a c t i v e Index U n i t C e l l Pararaetersj, S Densix:y R e f .

a o

AIN 2 .13 - 2.20 3 c l l 4c98 3 o 2 0 57

AlNoAlÔ^ 1«80 7 . 4 9 0 3 . 7 3 58

AlNoCAlÔ^)^ 64

Al^C^ 2 o 7 0 3 . 3 3 0 24o89 3 . 0 0 5 7

^•(Pf2 ^'^'"^ AlÔ^ 1 . 7 7 0 - 1.795 53

AlgO 53

AlCN 55

Al^CO 57

A I O . A I 2 O 3 1.99 - 2 . 0 3 53

99

t empera ture range . Moreover F r a n k e l ( 5 1 ) has s t a t e d t h e e q u i l i b r i u m

c o n s t a n t i n c o r r e c t l y as K = C /C^ where C i s c o n c e n t r a t i o n . Hence by 2

s t u d y i n g the e f f e c t o f a gaseous m i x t u r e o f carbon monoxide and n i t r o g e n

on an i n t i m a t e m i x t u r e o f a lumina and carbon as e v a l u a t i o n o f t he

chemica l e q u i l i b r i u m cou ld be made.

I t has been r e p o r t e d ( 5 3 ) t h a t t he e q u i l i b r i u m c o n s t a n t can be

c a l c u l a t e d f r o m the equa t ion

l o g Kp = 1 4 . 7 1 - 27743/T ( 2 )

Consequent ly , a t 1600°C and a tmospher ic p ressure t h e gaseous m i x t u r e v ^ i l l

c o n t a i n up to 3 ^ ° N ^ . A t I7OO the n i t r o g e n c o n c e n t r a t i o n would be

expected t o be 1^1 f r o m the equa t ions ( l ) and ( 2 ) assuming i d e a l c o n d i t i o n s .

These r e s u l t s agree w i t h those o f F r a n k e l ( 5 1 ) v;ho r e p o r t s a gaseous mix tu r s

o f 5 0 - 6 5 % carbon monoxide a t l 6 0 0 ° . F r a n k e l a l s o e s t a b l i s h e d a v/eight

r a t i o o f 2 : 1 ( A l ^ O ^ i C ) v/as s a t i s f a c t o r y t o f o r m the n i t r i d e . He also

concluded t h a t the r a t e o f r e a c t i o n i s dependent on the f o r m and r e a c t i v i t y

o f the carbon i n t he process .

From the above o b s e r v a t i o n s an a t t empt was made t o c a r r y ou t the

r e a c t i o n u s i n g the G r i f f i n T e l l i n ho t s tage microscope d e s c r i b e d i n

Chapter I I . A f i n e m i x t u r e o f a lumina and carbon i n a v /e ight r a t i o o f

2 : 1 was made. To a sma l l q u a n t i t y was added a d rop o f sodium s i l i c a t e ,

which moistened the m i x t u r e . The P t /Rh thermocouple was i n s e r t e d i n t o

t he m i x t u r e and ve ry c a r e f u l l y e x t r a c t e d t o ensure t h a t s u f f i c i e n t

m a t e r i a l was present i n the thermocouple t i p (see Chapter I I ) . Any

s p u i i o u s m a t e r i a l v/as c a r e f u l l y removed v / i t h a f i n e b r u s h .

The thermocouple v/as i n s e r t e d i n t o the gas t i g h t chamber and n i t r o g e n

was passed ove r a t a r a t e o f 50-60 cc per m i n u t e . The tempera ture was

100 -

grad 'oa l ly r a i s e d t o 1700*^0. Temperature f l u c t u a t i o n s vêre n o t e d . T h i s

i s p r o b a b l y due t o the s t r o n g endothermic r e a c t i o n when a l u m i n i u m n i t r i d e

i s formed i n t h i s manner. A f t e r 1 hour a t 1700 - 10*^0 the sample was

removed and submi t t ed t o x - r a y examinat ion*

Aluminium n i t r i d e was observed bu t due t o t h e v e r y smal.l q u a n t i t y

o f m a t e r i a l the povfder l i n e s were v e r y weako Some o t h e r weak pov/der l i n e s

were observed, b u t these c o u l d no t be p o s i t i v e l y i d e n t i f i e d . I t was hoped

t o s tudy the gaseous e q u i l i b r i u m i n the r e a c t i o n by pas s ing CO/n^ m i x t u r e s

o v e r the a lumina /ca rbon sampleSj but i n h e r e n t p.robleras ariose conce rn ing

the chemical a n a l y s i s o f the r e a c t a n t s and p r o d u c t s « Due t o t he smal l

q u a n t i t y o f sample ( l e s s than 10 mg) q u a n t i t a t i v e ana lys i . s o f s o l i d

components o f the r e a c t i o n proved imposs ib l e w i t h t h e t echn iques a v a i l a b l e .

I t v/as i n t ended t o use an o p t i c a l c r y s t a l l o g r a p h i c t o i d e n t i f y the amounts

and t he compos i t i on o f the phases present^ M t t h i s v/as no t successf ' iL

p a r t l y because o f poor crystaQ. f o m ^ t i o n and p a r t l y be-âuse the m a t e r i a l

was opaqueo A t o t a l a luminium d e t e r m i n a t i o n e^go s p e c t r o g r a p h i c a l l y p

v/ould no t have y i e l d e d any u s e f u l i n f o r m a t i o n . The most p r o f i t a b l e l i n e

would have been t o develop a micro-method f o r t he d e t e r m i n a t i o n o f

conbined n i t r o g e n bu t there was i n s u f f i c i e n t t i m e a v a i l a b l e a t t h i s stage

o f the w o r k . Hence;; due t o t h e pz-c-blems o f chemica l a n a l y s i s , , no f u r t h e r

o b s e r v a t i o n s o f t he Serpek process c o u l d be made.

I I I , 3 The f o r t r i a t i o n o f a lumin ium n i t r i d e from, i t s elements

I n t n r i d u c t i o n

I I I . 3 - 1 K i n e t i c s o f me ta l n i t r l d a t i o n

Jietal, r d . t r i d a t i o n i s expected t o conform t o t he p r i n c i p l e s a p p l y i n g

t o ox ide g r o w t h . As d iscussed i n Chapter IV^ the o x i d a t i o n o f meta l s

101

f o l l o v / l i n e a r , p a r a b o l i c 5 and l o g a r i t h m i c r e l a t i o n s h i p s and as i s

obs-brved f o r the o x i d a t i o n o f alum.inium ( 6 1 ) 3 any combina t ion o f these

laws may be used t o de sc r ibe a n i t r i d a t i c n curve. The k i n e t i c s o f t he

n i . t r i . d a t i o n o f a lumini \ im i.s :reported ( 6 2 ) t o be i n i . t i a l l y l i n e a r bu t

wi . th i n c r e a s i n g temperature becomes p a r a b o l i c «

Factcr-g^ a f f e c t i n g the r a t e o f reac t . ion

V.'hen a n i t r i d e i s formed f r o m a metal and i t s gaseous env i ronment5

the i ^ n i . t i a l n i t r i d e l a y e r may a c t as a ba i r ^ i e r and f o r f u r t h e r r e a c t i o n

to t ake p lace t he r eac t an t s mijst pass th roug i i t h i G b a r r i e r ^ There fo re^

f a c t o r s which e i f f e c t the o v e r a l l r a t e o f r e a c t i o n a re (a ) the supp ly o f

ga5ec;j3 r e a c t a n t t o the i n i t i a l n i t r i d e l a y e r and (b ) treLnsport o f the

r e a c t a n t s through the n i t r i d e l a y e r . Each o f these fa . c to r s a re governed

by the e f f e c t o f t e m p e r a t u r e , p ressure and any d e f e c t s t r u c t t i r e i n the

n i t r i . d e l a y e r .

The mechanical s t a b i l i . t y o f the n i t r i d e l a y e r i s a l s o i m p o r t a n t i n

d e t e r m i n i n g the exi:ent o f r eac t i onc The s t r e n g t h of t h e n i t r i d e l a y e r

depends on the d i f f e r e n c e i n mo lecu l a r volume ard t ype o f c r y s t a l l a t t i c e

o f the product l a y e r compared w i t h the o r i g i n a l m e t a l ( 6 3 ) «

S i n t e r i n g o f the n i t r i d e l a y e r can a l s o cause a c h ^ g e i n l a t t i c e

dimension e*g« Ca^N^p the l a t t i c e d imens ion f a l l s f r o m 1 1 ^ 4 2 t o 1 1 , 3 3 X

on s i n t e r i n g ( 3 3 ) -

For n i t r i d a t i o n o f al 'ominium t h e r e i s a f r a c t i o n a l volume change o f

+ 0 , 1 9 and hence a s t a b l e n i t r i d e f i l m i s a n t i c i p a t e d ,

I I I * 3 * i . i The f o n n a t i o n o f the n i t r i d e f r o n i t s elements

On h e a t i - g alumini.ijn f i l .ms prepared as d e s c r i b e d i n Chapte r I I i n

ra . t rogen :.inder t h e desc r ibed c o n d i t i o n s ^ no n i t r d d a t i o n appeared t o take

1 0 2

p l a c e a t temperatures up t o 5 0 0 ^ C = The raic:ro3ccpical examina t ion revea led

t h a t c r y s t a l l i t e gro\'irth had occured t o a l i m i t e d e x t e n t as can be seen by

comparison o f Big* ] l a and b . However-^ the r i n g s i n P i g , lOb index f o r

f „ o . c A l a c - i n I ' i g . 10a.

The growth i s nuc lea ted a t p o i n t s ?uch as a and b as shov/n i n F i g o l i b ,

The f i l : n appears t o have s i n t e r e d as can be cbser-ved friv-m the e lec t i ron

d i f f r a c t i o n p a t t e r n s o f t he " n i t r i d e d ' ' f i l m ( r i g . 1 0 b ) , The p a t t e r n i s

more s p o t t y and t h i s o b s e r v a t i o n i s c o n s i s t e n t v / i t h p r e v i cus v/ork on meta l

fi."lms (60-

Accord ing t o o t h e r v/orkers the f o r m a t l c n o f a lumin ium n i t r i d e i s

r e p o r t e d to commence a p p r e c i a b l y a t 530 (6z) and 600-650 (60) both a t a

p a r t i a l pressure o f 100 mm o f mercury ( 0 ^ 1 2 a tm- ) • ' "he k i n e t i c s o f

n i t r i d a t i o n a re i n i t i a l l y l i n e a r bji.t change t c p a r a b o l i c i n the t empera tu i^

range 580-590'^Co At tempts t o n i t j ^ i d e alumiri lum a t i^.50°C were made by the

same a u t h o i ^ b u t wi . th success^ v/hereas a j i ^ r i i n ium i s r e p o r t e d t o o x i d i s e a t

t h i s temperaturCe M e t a l f i l m s v/hen prepared^ have a l a r g e s u r f a c e area t o

v/eight r a t i o and are i n h e r e n t l y u n s t a b l e and h i g h l y d e f e c t i v e i n s t r u c t u r c c

As i s cbseived i n the case o f i r o n f i ! lms ( l l ) ^ t h e s i n t e r i n g o f the

aJumiXi-iiyn f i l m i s p r o b a b l y due t o the s t r u c t u i ^ x l d e f e c t s such as vacancies

d i f f u s i n g through the biil.k m a t e r i a l t o the meta l s 'arface and r e d u c i n g the

o v e r a l l c r y s t a l l i t e area^ T h i s mode o f d i f . f u s i o n i s p o s s i b l e as the

temperatures cons ide red are v / e l l above the Tamman tempera ture^

T h i s s i n t e r i n g minimi.ses p o s s i b l e i n i t i a t i o n o f t he r e a c t i o n v / i t h

ni t rc-gen on the alumini: ;m sur-face a t 5 0 0 ^ 0 . At '-4-75^0 and 5 1 5 * ^ ' ' - ; v ;e ight

cl-ange.3 o f Ool and 0 . 2 5 ^ g / o m ' f o r t he n i t r i d a t i o n o f a lumin ium have been

reportêd ( 6 2 ) « A comparison o f the r a t e s o f n i t r i d a t i o n and the t^ces o f

1 0 3

Pig 10a* Electron dif fract ion pattern of the

eaxixainAiun f i l m .

P i g . 10b. Electron di f fract ion pattern of

f i l m heated in nitrogen.

P i ^ l i b : ; i c ; c t r o n n u c r o - r a p h o f t h e f i l r ^ h e a t s u

i n n i t r o - e n .

P i : ; U u E l e c t r o n i - i i c r o - i - a p h o f t h e c i l - a . . u r . i u n f i l r . .

• ••:-. \ • ,.:-^^•'J•^•'•i^•>•:••::^^^: y •<•^^^

o x i d a t i o n ( 6 ) show t h a t a lumin ium r e a c t s more i : a p i d l y i n t h e presence o f

oxygen^ even at temperatures approach ing t h e m e l t i n g p o i n t o f a l u m i j i i u m .

The f o r m a t i o n o f a luminium n i t r i d e f r o m i t s elements has been s t u d i e d

i n the temperatxire range 600-1800°Co A t 600"C chemical a n a l y s i s revea led

t h a t t h e powdered lump was s a t u r a t e d w i t h n i t r o g e n aind y e t t h e f o r m a t i o n o f

a lumin ium n i t r i d e v/as not observedo C e r t a i n au tho r s ( 3 ) c o n s i d e r t h a t even

a t low n i t r o g e n c o n t e n t ( a t JOO^C 0«.0064 - 0.0070^^^, a t 850° 0.0058-0.Ol43?a^)

t h e compound AIN i s d i s t r i b u t e d on t he su r f ace o f t he mol ten m e t a l as a v e r y

tYiln f i l m . Repenko e t a l , (53) r e p o r t s s u b s t a n t i a l n i t r i d e f o r m a t i o n a t

t empera tures g r e a t e r than 1700°C where the h i g h c o n c e n t r a t i o n o f ELluminium

vapour and n i t r o g e n gas i s s u f f i c i e n t t o produce s i n g l e c r y s t a l s o f

a lumin ium n i t r i d e *

The f o r m a t i o n o f alumini-om n i t r i d e f r o m molten aJiuninium and n i t r o g e n

gas has been desc r ibed a l s o by Cooper e t a i , ( l ) . They emphasise the need

f o r p r e v e n t i n g the r e a c t i o n b e i n g i r l i i b i t e d by l a y e r s o f n i t r a d e fo rmed on

the s u r f a c e o f t h e mol ten metalc

An a t tempt to prepare a lumin ium n i t r i d e as p r e v i o u s l y d e s c r i b e d was

made (see Chapter I I ) . A p o r t i o n o f f i n e l y - d i v i d e d a lumin ium powder was

p laced i n a h o r i z o n t a l tube f u m a c e a t lOOO 'C under p u r i f i e d n i t r o g e n «

A f t e r tv/o hours the sample v/as removed and c o o l e d t o room t e m p e r a t u r e . The

s u r f a c e o f the sample v/as obser--/ed t o be s i n t e r e d . The sample was ground

and r ep l aced i n the f U m a c e under n i t r o g e n a t 1000°C f c r a f u r t h e r two

hou r s . The coo led samp].e v/as x - r a y amorphous and no a p p r e c i a b l e w e i g h t

change v/as recorded . Even w i t h repeated c y c l i n g o f the r o u t i n g no n i t r i d e

fo r r r i a t ion v/as observed.

106

I t has been r e p o r t e d t h a t alunnj-.vam n i t r i d e i s formed f r o m aJi jminium

pov/der and n i t r o g e n a t cemperat^jres as lov/ as c30°C [GOj^ '^he same au t i i o r ;

r e p o r t i n c r e a s i n g the p a r t i c l e s i z e o f the a l j m i n i ' j m pov/der decreases t i i e

r a t e o f r e a c t i o n t o form the n i t r i d c o However., o r i l y a t t empera tu res above

lOOO' C are y i e l d s o f b e t t e r than 90^^ obb-ervedo Thes^ obse: v a t i o n s are

no t c c n f i r m e d f r o m the presen t v/orkr. and i t woi: ld t h e r e f o r e appear thiat

the f o r m a t i o n o f the n i t r i d e f r o m the e.l.-men^s i s besc f a c i J . i t a t e d a t

ej .evated tempera ture v/hsre the r eacL ion t a k e £ p lace i n the vapour s t a t e

(fci.p. PA ~ 2 4 6 7 ° C ) , HcFweverr, a t temperatures i n the r e g i o n o f the b o i l i n g

p o i n t o f a luminium^ any n i r , r i d e v/ i . ] . l b i u n s t a b l e and d i s s o c i a t e i n t o i t s

e.iexents^ thus making the r e a c t i o n r e v e r s i b l e and tempera tures o f IJrOO-

iBOo'^C are recommended as i t ohse-n/ed in the worx o f l<epenko e t aj.c (53 ) s

a t v/hich tempera ture a luinira . -^ has an a p p r e c i a b l e vapour p ressu re .

- 107

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110

CHAPTER IV The r e a c t i v i t y of aluminium n i t i a d e

IV o l Introduct ion

The development of s i n t e r i n g and h o t - p r e s s i n g techniques has

enabled aluminium n i t r i d e , and the n i t r i d e s of s i l i c o n and boron,

to be used more ex tens ive ly as ceramics . The r e s i s t a n c e o f these

n i t r i d e s to h y d r o l y s i s and oxidation i s i n c r e a s e d cons iderably

when they are hot pressed up to temperatiires approaching 2000* C.

These condit ions may br ing about s t r u c t u r a l changes i n the n i t r i d e s

which may compact the c r y s t a l l a t t i c e thus a l t e r i n g the

c h a r a c t e r i s t i c s o f the mater ia l*

However, the r a t e s of h y d r o l y s i s and o x i d a t i o n would seem to

depend on the i n t r i n s i c r e a c t i v i t y and surface a v a i l a b l e f o r

chemical r e a c t i o n . Impedance o f the r e a c t i o n s by l a y e r s o f

products surrounding the remaining n i t r i d e p a r t i c l e s may become

s i g n i f i c a n t . I n the present chapter , production and s i n t e r i n g of

aluminium n i t r i d e are considered i n r e l a t i o n to changes i n , phase

compositionp s i ir face a r e a , and c r y s t a l l i t e , and a ^ r e g a t e s i z e s on

hydro lys i s . .and oxidat ion under var ious experimental cond i t i ons .

The chemical r e a c t i v i t y o f n i t r i d e s i s c o n t r o l l e d cons iderably

by the extent to which they have been s i n t e r e d during t h e i r

f o m a t i o n and any subsequent c a l c i n a t i o n © At present , there i s

more a v a i l a b l e l i t e r a t u r e on the s i n t e r i n g o f oxides which i s

expected to resemble that of n i t r i d e s . The theor i e s of s i n t e r i n g

are w e l l e s tab l i shed ( 1 - 8 ) , and the subjec t has r e c e n t l y been

. 1 1 1 -

reviewed (9). S i n t e r i n g i s enhanced by compacting the powdered

n i t r i d e before c a l c i n i n g i n vacuo to prevent poss ib l e h y d r o l y s i s

and oxidat ion (10)„ T h i s can be observed by comparison o f the

present work and the data reported f o r the oxidat ion o f hot pressed

aluminium n i t r i d e (l6)

A p r e r e q u i s i t e f o r s i n t e r i n g i s the production of f i n e l y -

divided mater ia l with a s u i t a b l e p a r t i c l e s i z e range. Development

of s i n t e r i n g i s l i m i t e d by impur i t i e s such as the oxide

impur i t i e s produced by p a r t i a l n i t r i d e h y d r o l y s i s and o x i d a t i o n ,

and p a r t i c x j l a r l y gas-producing containments such as hydroxides and

carbonateso When boron n i t r i d e i s p u r i f i e d at h igher temperatures

to reduce oxide content , the increased p a r t i c l e s i z e make subsequent

hot press ing more d i f f i c u l t (14)5 but the add i t ion of s i l i c a g l a s s

w i l l bond the m a t e r i a l xander these condi t ions (15).

S i n t e r i n g i s g e n e r a l l y acce l era ted by low melt ing a d d i t i v e s

( 1 3 ) , but such add i t ions may cause s er ious reduct ions i n o p t i c a l

and mechanical p r o p e r t i e s .

1 1 2

I V o l . i o Xbe hydro lys i s and c Y i ^ a t l o n of r t i t r l d e a .

The r e s i s t a n c e of powdered r e f r a c t o z y n i t r i d e s t c the a c t i o n of*

water and aqueous ac ids and a l k a l i s has been suminarieed by Samsonov ( l 6 ) a

I n n i t r i d e production u s u a l l y oxygen must be excluded; f o r i t prevents

nitrogen f r a n reac t ing vrith c l e a n metal surfaceSo Occaa iona l ly , formation

of an i n i t i a l n i t r i d e si:rface l a y e r protects aga ins t &vy subsequent

05cygen a t t a c k , and permits nitr idatiozx to proceede When the n i t r i d e

l a y e r i s d e s t a b i l i s e d by h y d r o l y s i s , the n i t r i d e ions ere replaced by

J^droaiyl ions and s ince one n i t r i d e i s r e p l a c e d by three hydroxyl i o n s ,

the f i l m becomes ve jy weak and ruptures w h i l s t very thlrt ( l ? ) o At

higher temperatures, decomposition of the j^ydroxides to oxides causes

f u r t h e r fragmentation ( l 8 ) o For example, Ir* the n l t r i d a t i o n of icegnesiam

between 4 0 0 ° and 6 5 0 ° , metal evaporaticT' i s promoted by v/aces of water

vapour but i n h i b i t e d by c^gen ( l 9 ) o

The f cnnat ion , hydro lys i s ana oxi-dation of the i o n i c c a l c i m n i t r i d e

(Ca^N^) and magnesium n i t r i d e (Mg^N^) nave been descr ibed ( 2 0 ) o Mere

recent ly i t has been fouod that z inc and cadr;iun f i l m s do net n i t r i d e i n

nitrogen or ammonia at temperatures beiow t h e i r melt ing po in t s , and are

formed only at appreciable ra tes at tonperatures above 6 0 0 ° {^9)o Tney

are i^ydrolysed rap id ly to ammotvia soluble complexes, lA{m^)^{OH)^ where

X 4 f o r M = Zn or C d ( i 7 ) o Comparison of the molecular s u s c e p t i b i l i t i e s

of magnesium, z inc and cadmium r d t r i d e s shows that the p c i a r i s i i a g a c t i o n

of tê metal i c n decreases from magnesium to z inc to cadmium^ N i t r i d e s i n

Group I I I of the Per iodic Table (boron, etluminium and g a l l i u m ) and Group

X V ( a i l i c o n and t i n ) show covalent c h a r a c t e r and are more r e s i s t a n t to

i ^ d r o l y s i s and oxidation trian the n i t r i d e s c f magnesiiim and b e i y l l i u m ( 2 6 ) ^

1 1 3

Boron nitride appears to be more chemically-reactive than

aluminium and silicon nitride. Finely divided boron nitride is hydrolysed

slowly by hot water and dissolves completely in boiling 20% sodium

hydroxide in thirty minutes. It is also observed to dissolve in acids

and alkalis al 20^C (22). By comparison, the reactions of the nitrides of

aluminium and silicon with hot water are inhibited by coatings of hydrous

alumina or silica respectively.

Hydrolysis is generally slow in mineral acids and alkalis (23, 24),

but there is complete dissolution of aluminium nitride in 30% sodium

carbonate solution at 80^, and of silicon nitride in boiling hydrofluoric

acid (25). All the nitrides oxidize at higher temperatures above 600* .

the most resistant to oxidation being silicon nitride with respect to

temperature.

I V . l i i Materials

The higher temperatures generally required for the production of

aluminium nitride (see Chapters III and V), cause sintering to the

extent that samples generally have specific surfaces of below 1 m^.g''.

and average crystallite sizes of over 2

More finely-divided aluminium nitride was obtained by ball-milling.

Thus, 5 gram batches of aluminium nitride were ball-milled for times up

to 10 hours. Changes in specific surface and average crystallite size

are shown in Fig. 12.

Electron micrographs showed that the original nitride consisted of

single crystals and aggregates of about I - 2 ^ m size, see Fig. 24a

The single crystals were fractured in the earlier stages of the milling

and the fragments were incorporated into the aggregates which remained

approximately the same size throughout the periods of millings, see Fig. 24.b.

114-

The ohango i n speolfio surface during the a i U i n e of A1N« I 4 — 4

The change s»erage < n y a t a l l i t e tjlee during the n l l l l n g of AlH,

2 4 6 8

F i ^ . 1 2 ,

-. 115 -

This the average crystallite size decreased rapidly at first and later slowly when

the crystallites became of sum micron size,

A closer x-ray examination of the milled materials showed that any

inherent line broadening was in fact mainly due to strain introduced by milling

rather than particle size (68. 69), since the average ciystaUite size was always above 0.4 ^ m.

IV. 2 The hydrolyfiifi of nhiminhim nitrirtf

The sample of aluminium nitride which had been milled for S hours

(S = 4.0 m V , average crystallite size 0.47 yvw m) was hydrolysed in water at

95ând in steam at 580-520°C and 680-640^C.

IV 2.1 The hydrolysis in water ("Wet" hydrolysis)

The sample was hydrolysed in water at 95^C for prespecified times,

after which the partially hydrolysed smaple was filtered off. The residue was

washed twice with SO ml portions of acetone to remove immediately most of

the water remaining in contact with the nitride. This would prevent further

hydrolysis and ageing (26). The last traces of acetone and water were removed

by out gassing the samples at 150^jn_vacuo.on an electrical sorption babnce (26), before measur-

ment of its surface area by determining the adsorption isotherm of nitrogen at

-183°C,

Portions of each sample were thermally analysed at (a) 200^C in air

when physically adsorbed water was completely removed, (b) 500*^C in air

when the bayerite ( c< - AJjOj SHjO) and boehmite (AI2O3H2O or ^ -AlO.OH)

were almost completely dehydrated (27), and (c) lOOOO^Cor 1200°Cinair

when the remaining aluminium nitride had been oxidised completely to

oC -alumina; the final weight also gave the total amount of alumina formed.

116-

TI .-s ;:ha3e5 pr&7.«n: ware i d e n t i f i e d t y x - r a y d i f r r a o t o m e t r y and

i r i r r a - r e d spec^roscopyo The presence o f re ta ined , n i t r i d e ; and bcehmite

.V..- - vovealîd by t he jc-'ray d i f f r a - ^ t o m j i f i v r t races* Ho\'rtrv'er, t h e b a y e r i i e

v;di jc-i'ay amorrphouo ard waF l e t a c t e d t o g e t h e r v ^ t h boehnji te by u s i n g thr;

'• ;•J.:•: .•Cul-l tcchni-nir: t h r SP 2C0 i r j f r a - r e d Jipe'^^lr-'-photoiaetftr. The

rv-.v.fLr^ o f th*: x-r^xy ar^d J_nf!*a-red ftiialysis a.r«- giv^r* i n Table? 21and22.

Vr.c: produGt*i c f ''-v'drolyHiJ= by v/3ie.r co f ixa ined approxi ina teay 1

rr,c »r;?'-lr; o f bay^^rit''^' r . j every ^ m lfcci..le--3 c f boehmiT-r and t h e i r f o n o a t i o n

"cnrL?* Tjnt r.-i-^h th-r pH c o n d i t i o n s i n the s u r r o v r . d l n g cEclutionSo I t

:-i - ' . p o r i t i d t h a t a t p f i val-.ifr53 gr<?ar-:r xhan / boshraxtc- i s formed f r o m

•jn'jrphcus >-a.(uniniUiT ydnoxidt^ whxrh w i l l r ran=foi-m a t p:-i 9 t o bayerit^ô

Th« t aye . r i r e -g ibbî lE : :ransfo-nnation takes p l a c e an pH 12 a t ^ 0 - 6 0 ° C (28) o

TTI^: pH o f the . surrouadi j ig s o l u t i o n vras es t i ina ted a~ b e i n g i r . the range

7-11 which i s ccn.<ji s t e m w i t h t h e f i . n d l n g s o f x - ray and i n f r a - r e d a n a l y s i s ,

I r i s repOT'ted a l so t h a i a h i g h l y p r o t e c t i v e f i l m o f boehmire i s fonned

vTheo â•a'T!iniuIrl metal i s d ipped i n t o b o i l i n g w a t e r ( 3 3 ) .

Tliv h o u r - m i l l e d 3ample o f a l u m i n ' m r ' " r i d e was K y d r o l y s e d fcy

sfcoot dCfj a t 9, ^C i n w a t e r ir. 5 hours •Fxi„.l3(z)), T h e r e a f t e r , h y d r o l y s i s

wrj^ oongjar-et ively S I O T . Th^ o o n s i d e r i i l J iri reases i n s p e c i f i c s^^rfaoe

Sj, i u n n g the f i r s t 5 bours c f l i y d r c l y s i s ( P i g . 1 3 ( a ) ) i n d i c a t e that

c i 7 / 5 c r i l j i t e s o f al--ualna h y d r a t e m;.st s p l i t o f f f r o m the n i t r i d e p a z i i i c l e s

bi-'fore t h e i r grOT-vt-h i s p r o m o t f j i by ageingo The n v i c l e e t i o n enei^gy o f

th^=!J^ o r y s t a l l i t e ? s i s a f u n c t i o n o f t h e i n i t i a l n i t r i d e s u r f a c e a v a i l a b l e

fcr* h y d r c l v s i e . . The h^^dto-^s-ali-iinin?. 3 1 3 ' s t a l l i t e s grew as t he h y d r c l y s i x

i 3 c o n r i r u e d and tht=re i s a subsequent decrease i n spec i f^ ic au r faoe a ^

thti 'Kydrous oxidr? c r y s t a l l i . t e s :5o-r:r?e on the n i t r i d e p a r t i - ^ l e s .

117

Table 21

X-ray analysis of hydrolysed aluminium nitride

aluminium-

nitride

milled 5 hours

hydiolysed 5 hrs.

hydrolysed 10 hrs

hydrolysed 20 hrs

boehmite gibbsite bayerite

2.71

2.47

2.37

Intensity (I )

180

120

320

6.01 40

3.13 30

2.69 60

2.47 3

2.36 50

6,01 80

3.13 60

2.68 30

2.36 70

6.01 210

3.13 60

2.68 30

2.37 80

6.11

3.16

2.35

3.11

2.45

3.19

2.69

2.45

-118-

Table 22

InPra-red anal.vsi.s of hvdroJys.d alxaminitim n i t r d d e

5 iir:f. 10 hr-».. hydr-:?lys.; ?

20 h»-=. bylr'.''jyEi ~ boe imlt-i ba erj te

cm cm U (30 (29

3500 shj Hi 3500 shp IP. 3260 .m ;»< 60 3660

3070 5h. w 3070 303y 330)

2130 vr 1250 w 1150 1149 3420

1070 s 1070 s 1070 w 1080 1063 1020

970 vT 980 w 720 w 975

720 rr 720 w 750 73^

694

770-720

d V - _ si^Duldar

strong

b broad

m -

w - v/salr

t l 9

••••'Vi

« o

AIM

i

if V. .•>

V

r l I O CB O

s

5 o m I

The ciystallites eventually fonn an impermeable layer of

hydrous oxide which prevents any further appreciable hydrolysis Fig 13 (c).

This behaviour is analogous to the wet hydrolysis of calcium and magnesium

nitrides and the hydration of lime (20).

The actual variations in surface area, when 1 gram of AIN is

hydrolysed is shown in Fig. 13 (b). Changes is siuface area of the unchanged nitride ^

(represented by the dashed curve in Fig. 13 (b) are calculated on the basis of the reaction

proceeding by inward advancement of the nitride-hydrous oxide interface (34). The

surface area of the hydrous alumina (by difference) accounts for practically aU of the/

total surface area {cS. dashed curve). The corresponding average crystallite sizes

of the hydrous alumina and the remaining nitride as shown in Fig. 13 (d) are

0.018 • 0.024 m and about 0.25 |fUn respectively.

The hydrolysis involves change in crystal structure from the hexagonal

structure of AlN to the orthorhombic structure of boshmite and the hexagonal

structure of bayerite and a considerable volume increase (0.724 of the

original volume). Since the nitride changes its volume 1.724 fold when

it forms hydrous alumina, hydration of x gram in a 1 gram sample of

aluminium nitride would cause the proportionate volume change

(I -x) + 1.724X = (1 + 0.724 x)

The proportionate change in surface = (I + 0.724 x)^^^.

In general, if S and S* are the surfaces of the aluminium nitride and

its partly hydrolysed product, then,

Sl S =(I+0.724x)2''

i.c. = (1 +0.724 x)

121

Gcxpl-:t^ hy'^n'ilysis v/c-ld give a ^ûrface chaiigs o f ( l ,72 i f ) ' ' l . i f J B j

i r thsre v/as no s p l i t t i n g c f c r y s t a l l i t e s product* Increaâ&o i n the S

ri^j2.'b:\r/ oP C2ystalJLite.5 car^ e=tlaiatcd 34 from the ratio .L^g

•,vhe:i*r: S-, id S are the values of the surl a'-e a r e a of the hydrous

almniria I • 1.438* and the s p e c i f i c s u r f a c e o f the ori .g inal alijminiura

i i i t rade reapectiv'elys allciTance btang n^de f o r the change i n l a t M c e

stf.'j;:tizj:*5:- T h a s , a f t e r 3 hours h y d r o l y s i s each o r i g i n a l AIN c r y s t a l . l i t e

i:j.-nducv::3 .TiH^500 C T y . s t a H i t e s o f hydrcus ozide whj.ch i n c r e a s e s to 15^200

^rr^,r 10 Kouxoj althcugl-: therv. i s l i t t l e fur-ther hydirolyais . Betv/een

;) rxnO 10 >;.Tars,, i n c r e a s e iri suxfa'.'-- I s caused by a d d i t i o n a l hydration

vji'j ••ôinpletior. of th^ r^-.o'otai l i . ? a t i o n c f the newly formed hydroas

ai.'-anina;' to t h e i r r.oMoal l a t l i c f ! otrL;ctur^5.; t h i s i s cc^nterbalanced by

ageing j f the hydrcM.c al^î.na v/hicr. becomes more Tcark'.-ii a f t e r 20 houT«

vrher. thfci ovt'r.all -r>'.^tall."j + r 5.r.cr* aK =- i s rrduced to 7*^00.

O p t i c a l and e l ec t ron i p i c r o g r a p h s h o w that the hydi-^Dlysed m a t e r i a l

r.or.rsletd of lar^ge aggr^gat^?- o f ever 100 s i z e corapared v/ith the

i»niaj-'.er aggregates .-P about l - ? r i s e o f the o r i g i n a l n d l l e d n i t r i d e .

Tlv3 e.slJmation of the p a r t i a l nize i s deBr.;ra.bed i n Chapter- ! ! •

AfT^r hydrolycii^ f o r 3 hc-rrs the p a r t i c l e size i s 100-130 ^ ir* and

tni3 i s increased i r .rnngy to 120-160 ^ m i n a f u r t h e r 3 hours . The

ni5.v^rial. s i n t e r s a.fter thds per iod to g i v t l arpe .aggr tga te s c f i*20-.^00y*\m

at 20 hours h y d r o l y s i s . Thsrss independent reau l t? . agree \vith s u r f a c e

ar+â :jh^»-:acteri3t ics of the mi-llcd mater ia l •

3 .V .2 .± i The h y d i o l y s i s i n steam ("Etcy" 'hydrolys i s )

The pyr':^l^vîôlys:i.o of al-Hnirii-Lir. n i t r i d e was ^tpjdied at temperatures

i n th" range 580-520^ and 630-640*^0 r+^^pcctive^y. The a r . a l y s i s of

al'-unini;t;in n i t r i d e f o r Viilrou.~.n can c a r r i e d cut under s i m i l a r condi t ions

.silr.i/, 5odrxTi or pota^- i ii:: hydrorîie p e l l e t s .

- 122 -

A v.-eighed airiouiit o f n i t r i d e i n a puroy. buat was p laced ijn a

hor izonta l fcabe f \ j m a c 9 a!id ateani suppl ied a t an approximately constant

r a t e , A blank ran was mada -tc detend.ne T.he rate o f steam pass ing t h r o u ^

the c-ystem. T h i s e s tab l i shed asi 2,L oc /min. As a r a s i i l t of the

3tâi3. a tTmperatu.rf- dr.:p of UX^GO^C was observed.

Any aiMonia given o f f by the p y r o - h y d r o l y s i a vâs oo l i e c t ed i n a

beairer i n r i i c h v a:f p laced 25 mis c f ^ b o r i c a c i d and 3 d.cops of a mixed,

methyl rec roicocresol green i n d i c a t o r * The amaznt c f ananonia evolved

waft detej i i - fid by t i t r a t i n g the condensate aga ins t O . i N hydrochlord.c

ac id*

O.if^ ammonia v/as formed i n 10 hours a t 560-320'^C and 2.ii?o i n 7 hours

a t 680^6iiD^C. T h i s corresponds 0.97% and Sofi^ h y d r o l y s i s r^spect ive lyp

The amount o f h y d r o l y s i s i s low compared wi th i i q u i d - s o l i d r e a c t i o n and

i t woiiia •'?>-iggest among o ther th ings that a d i f f e r e n t mechanism of

h y d r o l y s i s i s r a t e - c o n t r o l l i n g .

The r e s u l t s sogge5.t that the ateam promotes s i n t e r i n g o f the n i t r i d e

and presents such a r a p i d hydnniysi.s a s i n the l i q m d - s o i i . i r e a c t i o n .

The s i n t e r i n g o f the nj . tr lde i - a bsLrrier to f u r t h e r h y d r o l y s i s and the

r a t e o r t^ydrolysis i s v e i y much c u r t a i l e d a s a r e s i i l t .

IV^3 The oxidat ion o f alunniniâm n i t r i d e

As AIN has a p p l i c a t i o n as a hi.gh temperature mater ia l^ a study has

been undertaken c f ( i ) mechanism o f react ion a t these tenperatures v/ i th

parî'ûlar re ference cry^fcaXlite s ize,; ( i i ) the change i n s u r f a c e

p r o p e r t i e s a3 the r e a c t i o n takes p l a c e . Aluziinium n i t r i d e o x i d i s e s a t

temperatures above 600*^0 forming aluzninium oxide i n one o f i t s

c r y s t a l l c g r a p h i o forms depending on the ten^jerature a t which the r e a c t i o n

i s s tud ied .

'123

The kinetics of the oxidation of aluminium nitiide and the effect

of paiticle size on the rate of reaction was studied, using a theimograveimetric

balance in the temperature range of 700^C to lOOO^C. The experimental

technique is described in Chapter II

Although ihe initial mass of nitride does not appreciably affect

the reaction rate (35), one-gram samples of aluminium nitride were used

in the experiments for ihis weight is more convenient when interpreting

the results.

One-gram samples were placed in a 10 cc porcelam crucible and

suspended a lew centimetres from the thermocouple. The same conditions

were employed throughout the experiments

IV.3i The oxidation reaction

Although moisture is known to increase the rate of oxidation (36.37)

no attempts were made to dry the au in which the nitride was oxidised so that

oxidation kinetics experimentally derived could be applied to the material when

in general use.

Therefore as moistme is present in the system, ammonia will be

produced according to the reaction.

2 A I N + 3 H 2 O - ^ A l 2 0 3 + 2 N H 3 . .

As the temperature increases, the ammount of ammonia decreases and

the amount of nitrogen increases according to the reaction

2 N H 3 + 3 H 2 .

such thai in the temperature range considered, the amount of ammonia

present is very small compared with that of nitrogen. The activation

energies for reaction of aluminium nitride with moist oxygen and moist

nitrogen are 26.5 and 7.3 k. cal. mole'* respectively, and the rale

determining step for the low activation energy for the reaction of moist

nitrogen is attributed to absorption (38).

124.

it was therefoie assumed that the raaui reaction taking place was

2AIN + 2 © 2 A I 2 O 3 + N 2

IV.3.ii The rate laws of oxidation

The rate laws of oxidation are classified into three main groups:

(a) Logarithmic Rate Eqnations:

(b) Linear Rate Equation:

(c) Parabolic Rate Equation.

These rate equations are dependent on a number of factors (61)

among which are temperature, oxygen pressure, elapsed time of reaction,

surface prepaiaiion and pre-treatment of the system.

(a) Logarithmic Rate Equations

These equations can be mathematically expiessed as

a) x=k| log(t + lQ) A

b) a = B-k2togt X

where x is some measure of the oxide formed, A and B are constants and

k I and k2 are the respective rate constants and are known as a) direct

logarithmic growth and b) inverse logarithmic growth respectively.

A number of theories have been put forward to explain these laws

based on various rate determining mechanisms such as cavity formation in

the oxide film or chemisorption (62)

(b) Linear Rate Equations

Linear oxidation can be desciibed by

dt

i.e. X = kj t + c

If a relationship isgovemed by linear oxidation, the surface reaction or phase boundary reaction

is said to be rate determining.

125-

(o) parabol ic Rate Equations

Parabo l i c oxidat ion i s a d i f f u s i o n - c o n t r o l l e d process and the amount

of oxide formed i n an i n t e r v a l of time ( t ) i s proport ional to the square

root of t . I t can be expressed as

dx k, dt =

X

i . e . - k, t , X = k, t^

I V ^ 3 , i i i The k i i e t i c s of the ox idat ion of aluminium n i t r i d e

The f^ - tors which govern the k i n e t i c laws f o r gas s o l i d r e a c t i o n s a r e ;

the ra tes of nuc lea t ion and r e c r y s t a l l i z a t i o n , the chemical r e a c t i o n a t

the phase boundary, and the r a t e s of d i f f u s i o n . The g a s - s o l i d r e a c t i o n

i n i t i a l l y takes place a t the sur face of the s o l i d and the p a r t played

by a sur face i n a chemical reac t ion has been d i scussed by G-regg ( 4 3 ) .

G-regg g ives the c l a s s i c a l example of the decomposition o f ca lc ium

carbonate and cr ns iders the key to the c ^ntinuation of the r e a c t i o n

Ga ( 0 > Ca 0 + CO^

i s the sur face heterDgeneity- On the s u r f a c e of a s o l i d there

are regions of p a i - t i c u l a r l y high p o t e n t i a l energy where the energy

b a r r i e r opposing decomposition i s lower than elsewhere, and these areas

are the regions of nuc leat ion or a c t i v e s i t e s . I n other words, the

points at which the chemical reac t ion can beg in ,

VtTien d i s c u s s i n g the reac t ion o f powders, i t i s necessary to cons ider

the i n f l u m c e of reac tant p a r t i c l e s i z e and shape on the r e a c t i o n r a t e .

By cons ider ing the assumption that a gas s o l i d r e a c t i o n may be c o n t r o l l e d

by ra te o f gas d i f f u s i o n through a product l a y e r Jander ( M*) d e r i v e d

the e _tion k ; = 1 - ( 1 - F ) ^

v;here k :.s the ' p a r a b o l i c rate constant and t i s the time i n which a mas $.

o f reactant i s converted to a mass o f product P per u n i t mass.

- 1 2 6 -

Howeverj, Jander ' s equation did not cons ider that ( i ) the r a t i o of

surface area of p a r t i a l l y oxidised c r y s t a l l i t e to a r e a reac t ion i n t e r f a c e

progress ive ly changes during the r e a c t i o n (4^6), and ( i i ) the reactant

and product molar volumes are not equal . C a r t e r (45 ) modified J a n d e r ' s

equation to give

[ i . ( A - i ) # + ( A - i ) ( i - P ) * = z + 2 ( I - a : ^ r

v/here r i s the i n i t i a l radius of a p a r t i c l e and i s the r a t i o of product

and vf.p^nf->^r\t molecular volumes.

Severa l k i n e t i c equations have been der ived and the equation v/hich

connects the k i n e t i c parameter v/ith the experimental ly determined funct ion

i s dependent on v.-hether the o r i g i n a l powder p a r t i c l e s are considered to

be d i s c - l i k e ( 3 9 ) , c-ibic ( 4 0 ) , s p h e r i c a l ( 4 1 ) , o r needle shaped ( 4 2 ) .

From e lec tron microscopy, the p a r t i c l e s to a f i r s t approximation

co'jld be supposed to be c i r c u l a r . The geometry of s i t u a t i o n v/as l i m i t e d

to 2 dimensions by the instrLiment, and i t v/as assumed that the p a r t i c l e s

v/ere s p h e r i c a l i-ather than d i s c - l i k e . The r e s u l t s of the thermogravimetric

study o f the oxidation of aluminium n i t r i d e f i t t e d the equation der ived

f o r spher i ca l p a r t i c l e s .

K i n e t i c ecraation

The ra te of reac t ion v;as considered to be experimental ly expressed

as the amoi^nt of a l u r i r ' •: n i t r i d e converted to alumina i n time t as F

v/here F - m t - - 1 ^^r

v/here m^ = mass of the ScOiple at time t

= i n i t i a l mass

- Molecular weight of the product

= Molecular weight of the reac tant

and the value of F ^vd.ll vary from 0 ( t = O) to 1,

- 127 -

I t i s aseumed that the p a r t i c l e s ere sphericaJ. and 30

i f V i s the volums ot a p a r t i c l e and r i s the radtu£

Tf i s voivjEe of -^îildTial AIN it ar y t ine t and esauming that the

shape of the p a r t i c l e does not change during oxidat ion

then - V = VP where VF i s the roliime changed t c alumina i n

time

I f r . i s the radlug of the p a r t i c l e af r e a i d u a l AlW

r^ T ( I - P ) ^

The vr-lsê of a l jn i lna fo-naed V ^ i s eqiif.I to the d i f f e r e n c e I n the i n i t i a l

voluue and the volume c f the res-idual AIN and there i s a molecu lar volums

changa

wherr^ X i s the stolchlonietr iC r a t i o of the molecular ?o l ' j ces of alumina

and aluminliau n i t r i d e according to the r e a c t i o i ;

2 AIN - V 2 0^ AiÔ^ * N^.

The t o t a l volume (V) at any tlroe t i s therefore equal t c t h e f um of the

volume of aiiamizja formed and the volume of ti-ie r e s i d u a l n i t r i d e o

:r (V = ( V - V ? ) ) X + «• - VF

= V ( l =• F * X F )

Tnerefore r (1 - F

30 the tota3 -;h5.ckness x of r he l a y e r of alumina w i l l be

128

I f the rate of d i f f u s i o n i s very great i n r e l a t i o n , the r a t e of

react ion then i s - dN dt

v/here A i s the surface a r e a of the s p h e r i c a l p a r t i c l e

A = 47rr^

r^

ÂU I

v/here v -j ^ i s the molar volume of aluminium n i t r i d e ,

dt v ^ ^ , dt 1

dt - '^l ""aw

J '

1 - ( 1 - F F = v^^j t / r

and i f d i f f u s i o n i s not ra te c o n t r o l l i n g 1 - ( 1 - F ) ^ v ; i l l vary l i n e a r l y

vath time.

But i f ra te of d i f f u s i o n contro l s the r e a c t i o n , the der ived

parabol ic law i s expressed as:

dx _ k dt ~ —

X

v/hich, v/hen in tegra ted , g ives

x^ = ak^t

where x i s the th ickness of the product;

• 129

1 1 3 r ( 1 - F + F ) ^ - ( l - F )

r ^ ( 1 - F + F ) * - ( 1 - F ) ^ ^ =

( 1 - F + F ) 3 - ( 1 - F ) ^ ^ = 2k2t

2 so a p lot of Z v/ith time v / i l l vary l i n e a r l y i f d i f f u s i o n i s the r a t e

c o n t r o l l i n g process .

Est imation of k i n e t i c parameters

C a l c u l a t i o n of the mass converted i n time t , F ;

F = m. - mo/ m ^ - 1 t ' o Mr

Mr = 2M^^j v;here M ^ ^ i s the molecular weight of aluminium n i t r i d e . - \

Mp = M the molecular v/eight of alumina ^ •2 3

M j t = 82

Mp = 102

Mp/j^^ - 1 = 102/Q2 - 1 = 0 .244.

F = m - m _ A m •rT^J" ~ 0.24Mn u, 24Mn

C a l c u l a t i o n of

\ i s the s to ichiometr ic r a t i o ôf the molar volumes of aluminium

nitxi-de and alumina- From the x - r a y r e s u l t s of the product of ox idat ion ,

the aliamina was i d e n t i f i e d as -alumina by comparison o f observed d-spacings

and the d-spacings given by the A * S . T . M . index see Table 25. Aluminium

n i t r i d e i s reported to convert to alumina at temperatures as lov/ as

600 C (38) where the form of adumina i s p o s s i b l y ^ - a l u m i n a .

The densi ' \es of aluminium n i t r i d e and -alumina were taken to be

3 . 2 / and 3-97 g.- -m r e s p e c t i v e l y .

- 1 3 0 . -

These values gave the molar volumes of aluminium n i t r i d e and

oC -alumina as 27.55 and 12 ,57 cm^ r e s p e c t i v e l y ,

= 1 .19 .

2 Hence^ knowing / \and P^ the values of y and Z can be found.

2

The c a l c u l a t i o n of Z i s p a r t i c u l a r l y tedious and a computer

prDgramme i n Portr^m IV f o r IBM 11.30 Computer has been v /r i t ten to

c a l c u l a t e these funct ions (See Appendix V ) , Graphs of percentage

conversion ( P i g . 14)? y and ( F i g s . 17 and 19) f o r var ious times 2

at d i f f e r e n t temperatures v/ere dravm and from the slope of Z against

time the r e a c t i o n rate constants (k^) v/ers found.

The determination of the a c t i v a t i o n energy f o r the ox idat ion react ion

The rate of react ion i s proport ional to the r a t e constant k. The

r e l a t i o n s h i p vri.th the rate constant and the a c t i v a t i o n energy f o r the

process i s given by the Arr+ienius equation

k = A.e .

where A i s the frequency f a c t o r o r pre-exponential f a c t o r . — E / R T

e i s sometimes termed the Boltzman f a c t o r where E i s the

a c t i v a t i o n energy^ T i s the temperature i n degrees Kelvinp and R i s the

gas constant.

From t h i s equation

Ink = InA - E / R T

. logôk = log^Q A - 303RT

= logÔ A - E 4 ^ 5 6 T

Hence a p lo t of log-j Q k against V T w i l l give a s t r a i g h t l i n e of slope

E / 4 ' . 5 6 , from v.-hich the a c t i v a t i o n energy f o r the process can be c a l c u l a t e d « 131 »

S o * !

i 1 t

T 5 I

F i g . 14* The percentage conversion of aluminium n i t r i d e

to a-alumina at var ious tenperatures^

- 132 -

lAany f ac tors contr ibute the i n i t i a l shape of an oxidat ion i sotherm,

and these have been enumerated by Gulbransen and Andrew ( 4 8 ) . These

f a c t o r s are ( i ) the in f luence of the decrease of roughness or sur face

area as the react ion proceeds, ( i i ) the in f luence of the heat evolved

i n the react ion on the rate of r e a c t i o n , ( i i i ) the e f f e c t of

impur i t i e s i n the oxide as the react ion begins , ( i v ) the change in

oxide composition, (v) the in f luence of p o t e n t i a l f i e l d s at the gas

oxide i n t e r f a c e due to absorbed oxygen ions according to the

mechanism of Mott ( 4 9 ) .

I t has been shovm ( 4 ^ ) that as the reac t ion proceeds, that the

2 " l sample of i n i t i a l surface area of 1.1 m og increases to an

2 -1 estimated 1.6 m g i n a per iod of ifO hours .

In the per iod s tud ied , tha t i s 0 to 7 hours , there i s an 2 -1

expected increase i n surface a r e a of 0 ,1 m g so i t i s assumed

that the e f f e c t of increase i n surface area i s n e g l i g i b l e .

Prom a l i t e r a t u r e surveyp a value of A H = - I 3 6 k .ca l /n io le (tO)

f o r the heat of react ion i s obtained. As the temperature maintained

a t + 2* 0 throuôut the experiment, t h i s f a c t o r was not considered

to a f f e c t the s i t u a t i o n i n the temperature range cons idered. However,

t h i s e f f e c t i s apparently important a t h igher temperatures ( 3 5 ) .

The est imation of any impur i t i e s i n the i n i t i a l oxide was not

made i n t h i s study, and hence the e f f e c t of t h i s f a c t o r cannot be

considered.

I t has been reported that there i s a change i n dens i ty from the

sur face oxide and i n t e r n a l oxide when metal pov/ders are ox id i sed ( 3 l ) ,

- 133

but i n the present study the dens i ty of the oxide v/as assumed to be

constant although t h i s e f f e c t could i n f l u a i c e the i n i t i a l oxidat ion

isotherm,

r^pas and Kingery (52) have studied the e l e c t r i c a l conduct iv i ty

o f s ingle c r y s t a l and p o l y c i y s t a l l i n e alumina i n the temperature

range 1400-1800^0 under var ious oxygen p r e s s u r e s . At the high

pressure reg ion , the conduct iv i ty increased s l i g h t l y w i th p r e s s u r e .

T h i s v/as a t t r i b u t e d to absorption of oxygen on the sur face where

i t , according to the equation

I O2 (g) ^ 3/2 0^~ ( a d s ) . + V ^ ^ 3 ^) 3 ®

I f t h i s e f f e c t i s true of aluminium n i t r i d e at atmospheric pres sure ,

the production of voids would act as nuc leat ion s i t e s , i . e . regions

of higher p o t e n t i a l e n e r ^ from v;hich the react ion i s able to proceed.

From the d i s c u s s i o n above^ i t i s assumed the e f f e c t of the

absorption of oxygen and the assumed d i f f e r e n c e i n dens i ty betv/een the

surface oxide with respect to i n t e r n a l oxide p lay a major p a r t i n the

i n i t i a l ra te c u r v e .

The types of oxidation i s o t h e m have been descr ibed ( 4 4 ) , and

a comparison of the isotherms obtained i n t h i s study shov/ that the

isothemis are dece lar tory throughout^ that i s the ra te p r o g r e s s i v e l y

decreases as the reactant i s consumed- Hov/ever, there i s a s l i g h t

discrepancy i n the cont inu i ty of the isotherms. I t v/as thought

that perhaps t h i s may be due to an experimental e r r o r and a gram

of kl^-^ was heated at 960^0 under the same condi t ions and the

13if -

weigh t o f the sample remained cons t an t th roughoutp thus o b v i a t i n g

t h e i d e a o f exper imen ta l e r r o r . To f u r t h e r i n v e s t i g a t e t h i s e f f e c t

a d e t a i l e d p l o t o f the o x i d a t i o n o f a lumin ium n i t r i d e was t aken a t

3J>0^C a t t e n minute i n t e r v a l s over a p e r i o d o f 3 hourso The r e s u l t s

are graphed i n Pigo 15* Prora t h i s graph t he re i s change i n shape

i n the r eg ion A A ' and t h i s begins t o occur a f t e r a p p r o x i m a t e l y

1 hour .

The e f f e c t cou ld be e x p l a i n e d by t a k i n g the p o i n t s a^ b , c , d«

A t the s t a r ^ o f the r e a c t i o n p n u c l e i are formed a t a reas o f h i ^

p o t e n t i a l e n e i ^ as p r e v i o u s l y d e s c r i b e d . These n u c l e i are random

and the a rea o f r eac tan t pix)duct i n t e r f a c e i s s m a l l and hence the

r e a c t i o n r a t e i s slov/. At the p o i n t b t h e n u c l e i have s t a r t e d t o

grow and w i t h new n u c l e i be ing formed the r eac t an t p r o d u c t i n t e r f a c e

has grown,, and hence t h e r e a c t i o n i s f a s t e r .

A t p o i n t c the n u c l e i have grown t o s u f f i c i e n t s i z e such t h a t

they o v e r l a p and cover the s u r f a c e c o n p l e t e l y , so t h e r e a c t i o n begins

t o d e c e l e r a t e as f u r t h e r r e a c t i o n r e s u l t s i n the r e a c t a n t p r o d u c t

i n t e r f a c e dec reas ing i n a rea .

I n t he r e g i o n o f d t h e r e i s a s l i g h t i n c r e a s e i n the reaction.

T h i s may be due t o the f a c t t h a t t h e formation o f i n i t i a l o x i d e

l a y e r takes p lace a t f a s t e r r a t e than t he nitrDgen, formed i n the

r e a c t i o n , d i f f u s i n g th rough the o x i d e l a y e r o The r e s u l t i s a

b u i l d - u p o f pressure a t t he r e a c t i o n i n t e r f a c e which causes t h e

s u r f a c e t o break and hence the exposed n i t r i d e a c t s as a n u c l e a t i o n

p o i n t f o r f u r t h e r ox ide g r o w t h .

135

4-5

Time, Kf

•P

b

1 1 4 -8

^—1—4 lo

F i g . 15. The percentage converamon o f a lun in ix im

n i t r i d e heated a t 930* C f o r 3 h o u r s .

- 136 -

Thii-3 oonceot o f a breakdcv/n i n the o x i d e l^yer i s d e s c r i p t i v e oi"

l i j i e a r k-inetics^ and has been observed i n OLher gas s o l i d systems

(iVOy 4 i j 53)0 A p l o t o f f i . r s t oro.or kijietic-.= 3âlnst time sterns i n

Tact there i s a change i n the ord»';r o f r e a c t i c r j , TrJ.^^ can be seen in

: 'ag. 16 \7}iei' ' the slope :.^hsnge£ a t a., a* no b ' foj- the p l o t o f Q

VLôln-at time f o r .960°C and 910^C rcsTêc tavî-iy. v;here - .:og V i ' " .

A p l o t ot' l o g t ( t ime) against log x (x -. percentage conver-sicn

uC aluiri.i.na.) v/a^ made (see F i g - l '3b) r.rcm which. 'i:h<: slope g ives the

order o f r e a c t i o n . From the:^-e p lo t s i t v,?j.s ai&o observed tr:at th^::

ord.er oi' react ion changed frxr-m ^ to 1 i n rj\( 1?-//: '" tomper^tijj-e regicr .o

7:hi*?i i.s due to the fac t that although cj-adp.t.icn o r alujfitniujii n i f r i d ' :

begins above 600'"C. th.e-ve i r not an app-' -?:*I ar.". e amount o:' o:ri'J-e forced

and thei'a i s on-iy a suj^face react ion r r - » . ; i . - ) g c.a.'-t a t these temperatures.

A t h igher temperatures the order reac t ion i s i n i t i a l l y 1 changing t o a

i i : n t l i a'Dove 9^0"^!! ' the ov.-^ral i r-i-ao t ..on '•rirc'tios are paraboliCo

I'rorri the plot o f y against t ine ( t ) f o r var ious temperatures f o r

a period of approximat'^ly -p; hoL:r> the rclati',:nj=.rdp becv/een y and t i s

approximately l i n e a r (see r i g » -7 ')^ ad;nng f u r r h t t * e\d.drnec.- to the

i d e a of nucleat ion as a sorfaoe r-Hai'.tion br-ing th:. i n i t i a l r a t e

c o n t r c l l l n g mechanism. Th i s i s f u r t h e r exempl:i.?ied by thi e f a c t that 2

tihe r e l a t i o n s h i p Z to t i s no t i n i t i a l l y - Vineî::- i ^ the lov;

temperature region (see F i g o 1 9 ) s and h^-ice d i f f u s i o n play.-^ very

l i t t l e p a r t i n the I n i t i . a l stage o f The reaction^

137

T - 5 JL'C

P i g . 16 .

0 - 5 I

l e g s .

( a ) A p l o t o f f i r s t - o r d e r k i n e t i o s

iTith t l f io f o r var ious teiapetaturefl*

(b) A p l o t of l o g t ( t i n e ) w i th l o g x

( z i s ^ conversion)*

- 138 -

Ha.vever, as t h e r e a c t i o n proceeds , d i f f u s i o n becomes the c o n t r o l l i n g

mechanism as t he r e l a t i o n s h i p betv/een z and t becomes l i n e a r . T h i s i s

t r u e f o r va lues o f P bet^veen 0.003 and t h e ^ f i n a l r e ad ings taken i n the

k i n e t i c s tudy . So the mechanisms wh ich o c c u r a re l i n e a r k i n e t i c s

f o l l o v / e d by p a r a b o l i c k:^net ics fo l lovêd by a s h o r t r e g i o n o f l i n e a r

k i n e t i c s and t he r e t u m to p a r a b o l i c k i n e t i c s . T h i s t y p e o f l i n e a r

pcirir.lxCi i c r e l a t i o n s h i p hos been encoi jn tered by o t h e r au tho r s (59) and

'.i:- •*::li.:/:-.'.oy: of c-;.licon ni.i'.i'i.oe (54) t':\ri I c i n c t i c equa t ion used i n

l h i 3 type of mechon.i3rn X'a

v r •:- A A V/ rz k t C

•.•.•r-.rjTv: i s the v/eight incrd-r-3e

k i s l.i~:c o v e r a l l ra te cons-tant

i:;ec:u^:":i"^: v.ould conr^ccuently not l\ c l e t ^ i l e d p i c t u r e o f t h e

oxidation klnet icr: . Tl-.e convorEO ::>f t h i n r.v^ch.'r-tiiyiv: (knc-r. us p p . r a l i n e a r

k i n e t i c s ( 6 1 ) ) i s no t t o be confu.scd v - l t h t h e above e q u a t i o n

T:i!?.?.a k i n e t i c .';t;udie^: h .ve he-jn liin.itod t o 0. seven hour p e r i o d i n

which a m iximujn o f 30 p e r cen t convers ion t o .-.luirLina v/as observed . To

o b t a i n an o v e r a l l p i c t u r e , samples o f a lumin ium n i t r i d e o f s u r f a c e a rea

2 - 1

l . l n g v;ere heated f o r l o n g e r p e r i o d s o f t i " e r.nd as m^ .ch -"s 94 per

cent conve r s ion to ali-unina i s observed i n 130 hours a t 1000°C« Ra rabo l i c

k i n e t i c s ' re obeyed i n t he i n i t i a l p e r i o d o f 2 - 22 h o u r s , as can be seen 2

f r o m a p l o t o f Z a g a i n s t t ime ( t ) ( P i g . 1 8 ) . Dtuîng t h e f i r s t ho\ i r

i n t e r f a c e r e a c t i o n i s r a t e c o n t r o l l i n g as can be seen f r o m a p l o t o f

y aga in s t t ( F i g . 18 ) • A f t e r the 22 h o u r s , t h e p l o t o f Z a g a i n s t t

i s no l o n g e r l i n e a r f o r va lues o f F g r e a t e r than 0 , 6 .

- 140 -

1

'4

iO 1

So Fig. 18 .

A p l o t of ^ aga ins t t i n s * t «

f o r periods up to }0 hours .

4 0

- 141 -

This i s probably due to the l a y e r of oxide no longer growing i n regu lar

fash ion and the approach to l i n e a r k i n e t i c s from the p lot of y aga ins t

t j suggest the r e a c t i o n i s mainly tak ing p lace a t the r e a c t i o n in ter facSo

This i s a r e l a t i v e l y slow process from which the oxide l a y e r w i l l progiess

u n t i l i t f i n a l l y s i n t e r s and prevents f u r t h e r d i f l \ j s i o n of oxygen to the

i n t e r f a c e c

Prom p l o t s of Z agaicist time f o r d i f f e r e n t temperatures, (stse Fig<,l9)

the slopes of the s t r a i g h t l i n e portions were found and these values were

taken as the r e a c t i o n ra tes (k)o A f u r t h e r p lo t of log k aga ins t the

r e c i p r o c a l of temperature i n degrees K was made (see Pigo 30) and the

slope was equivalent to eAO56H where E i s t h e a c t i v a t i o n energy and a value

of 53 k c a l s / n o l e was c a l c u l a t e d and the equation can be expressed as:

This value ccanpares w e l l wi th the ox idat ion of aluminium n i t r i d e i n

p u r i f i e d osygenj to form alumina ( v i s : 50o8t3 k cal /mole f o r a sur face area

of OCM6 m'g (59)j and 49 k ca l /no lQ (35) i n the temperature range

650-1 iOO°p no surface area measurements being g iven) ( see Table 25)0 This

agreement emphasises that the main r e a c t i o n taking place when aluminium

n i t r i d e i s heated i n a i r i s the f o n n a t i c n of aluminao

I t i s concluded that the reac t ion fo l lows f i r s t the f o m a t i o n of

alumina a t n u c l s a t i o n points on the surfaceo While the regions grew

l i n e a r , k i n e t i c s are observed u n t i l a coherent l a y e r ferns when parabo l i c

k i n e t i c s are recorded, u n t i l there appears a s l i g h t break i n the coherent

f i l m a l lowing f u r t h e r nucleat ion u n t i l the surface i s recovered and

parabol ic k i n e t i c s are f u r t h e r observedo

142

Table 23

A c t i v a t i o n energies o f the ox idat ion of alumird.um n i t r i d e and r e l a t e d

mater ia l s

Fonnula Condit ions A c t i v a t i o n energy k ca l s /mo le R e f .

A l

AI2O3

A I N

AU I

AIN

Sic

S i c

660 - 850°C

s i n t e r i n g s t u d i e s 1300 - 1900

^2

^2

a i r

a x r

o x i d i s e s to amorphous SiO,

o x i d i s e s to c r l s t o b a l l i t e

99

130 - 170

51.8

49

53

61

68

15.6

20.2

23.9

12

59

59

35

present work

54

54

31

31

32-

143 =

I

A p l o t o f 2° againat tioB, t » f o r v a r i o u s teoperati iros*

- 144-^

To uj iderl lne t h i s breakdown i n the ooherent layer, the aotlvation

e n e x ^ f o r the reac t ion of moist n i trogen wi th a l ius lnluA n i t r i d e in a

temperature range o f 600-800^ i s reported (38) a s 7«3 koal/aole.

However, a value f o r the enez:gy o f a c t i v a t i o n f o r the reaction of

aliminixim n i t r i d e i n dzy oxygm U reported as 24«3 keal/oole (3S), but 2 -1

the sur face area semple used i n t l a t study was 1.7 a 6 * and hence by

oaq)ari8on the n i t r i d e powder used ^ \ the present wozk would be acre

r e a c t i v e above 700^0 and an a c t i v a t i o n enexgy o f l e s s than 7*3 koalAK>le'

could be expf ic tcd f o r the- rrsîction bcV-veun the sar.ple and ni trogen- The

i o n i c i t . d i u s o f ni t . rocr-n 1-71 -** -nd i i - i s doubtful v/hcther nitrogfin 2-

would d i f f u s e ;i3 T-' ao nit\Xintr\ nouli} d i f f j - j c a s atomii r a t i i c r thun i o n s .

The atoraic radius o f nacrogen i a 0„71 '^-9 and the i o n i c r a d i u s o f aluminium

i s 0.5 30 f o r pure ly geometric reasons the aluainlura i o n s would d i f f u s e

along the g r a i n boundaries r a t h e r than t}urougJi the c r y s t a l l i n e m a t e r i a l

(60)1 f a s t e r thriXi the ri itrogen atomse C m s o q u e n t l y , due to the assuasd

low a c t i v a t i o n energy o f the n i t r i d e i n the presence of nitrogen, and the

e a s i e r d i f f u s i o n o f aluminium i o n s , there i s a break in the coherent oxide

surface to al low the removal o f n i t r o g e n . As the continuation of a

coherent oxide l a y e r growth i s observed without a f u r t h e r breakdown in

oxide s u r f a c e , the f u r t h e r reraoval o f nitrogen from the system would seem

to be by some d i f f u s i o n p r o c e s s , perhaps through the f i b r o u s oxide layer

( a ) .

As oxidat ion w i l l be c o n t r o l l e d by the r a t e of arrival o f roactant

a t the r e a c t i o n i n t e r f a c e , d i f f u s i o n of the fastest moving spaoies i s

ra te c o n t r o l l i n g .

. 146 •

As p r e v i o u s l y discussedj , the d i f f u s i o n o f a lumin ium i o n s would be

f a s t e r than t h a t o f n i t r o g e n atomso I n a s tudy o f t h e o x i d a t i o n o f pure

a lumin ium i n the tempera ture range 400-630°Cp the r a t e c o n t r o l l i n g

species v/as i d e n t i f i e d as Al"^"*" and an a c t i v a t i o n energy o f 52 k . c a l / m o l e

(60) a j id 54 k . c a l / m o l e (64) was r ecorded i n accordance w i t h p a r a b o l i c

k i n e t i c s . These va lues a re i n e x c e l l e n t agreement w i t h the p resen t

r e su l t s^ , and a g r a i n boundary d i f f u s i o n o f Al*^^ th rough the o x i d e f i l m

t o r e a c t a t the gas o x i d e i n t e r f a c e i s compa t ib l e v / i t h the e x p e r i m e n t a l

r e s u l t s ob t a ined i n t h i s work .

The d i f f u s i o n o f oxygen i n p o l y c r y s t a l l i n e a lumina has l e d to

a c t i v a t i o n ener^gies o f I3O-I7O k . c a l / m o l e (591^65^66) and comparison

w i t h 53 k .Cal /mo le would suggest t h a t d i f f u s i o n o f oxygen i o n s i n

alur-if^-i l in n i t r i d e i s no t r a t e c o n t r o l l i n g * T h i s d i s a g r e e s w i t h the

i . c su l t s o f Guichon and Jacque (35 )» v/ho concluded oxygen i o n

d i f f u s i o n t o be r a t e c o n t r o l l i n g * T h i s hypo thes i s v/ould t h e r e f o r e

appear t o be i n c o r r e c t f r o m the present w o r k . Hov/ever^ t h e y showed

t h a t the re was a r e l a t i o n s h i p between the r a t e o f r e a c t i o n a j id p a r t i a l

pressure o f oxygen i n the systerrir, b u t l i m i t a t i o n s i n t echn ique d i d no t

a l lov / an e v a l u a t i o n o f t h e o r d e r o f r e l a t i o n s h i p .

The r e s u l t s o f Cooper e t a l (59) showed the r e l a t i o n s h i p betv/een

the o x i d a t i o n r a t e and p a r t i a l p ressure as

o x i d a t i o n ra te r t . ^

The mechanism o f absorbed oxygen on the s u r f a c e o f p o l y c r y s t a l l i n e a lumina

(52) which i o n i z e d acco rd ing t o

3 O2 (g) 3/2 0^" (ads) + V (A ) 3 e

has been adapted f o r the s u r f a c e r e a c t i o n s o f a lumin ium n i t r i d e .

147

V/hen the process a t t a ins equi l ibr ium,

a constant

:5

The p r o d u c t i o n o f the p o s i t i v e holes a t t he gas ox ide i n t e r f a c e w i l l

de tennine the ex t en t o f the d r i v i n g g r a d i e n t o f Al^"*" i o n s j v;hich i s

r a t e c o n t r o l l i n g f r o m a lumin ium n i t r i d e t h r o u g h the o x i d e l a y e r t o

t he ox ide-gas i n t e r f a c e .

I V o 3 o i v The e f f e c t o f p a i ^ c l e s i z e on the r a t e o f r e a c t i o n

5 gram samples o f a lumin ium n i t r i d e o f o r i g i n a l s u r f a c e a rea 2 - 1

Ook mf .go were m i l l e d i n a p o r c e l a i n vesse l^ w i t h a number o f a lumina

b a l l s o f d i f f e r e n t d iameters^ f o r p e r i o d s o f 2^ 5 and 10 h o u r s . The

o x i d a t i o n r a t e s o f 2 hour and 10 hour m i l l e d samples were e s t ima ted

as p r e v i o u s l y d e s c r i b e d a t 872^5 910^, and 9 6 0 ° C , and compared w i t h

o x i d a t i o n r a t e s o f the o r i g i n a l s a m p l C o

The 5 hour miJ.led sample was se t as ide f o r h y d r o l y s i s s t u d i e s .

A f t e r each m i l l i n g p e r i o d s t h e m a t e r i a l was recovered as d e s c r i b e d

i n Chapter I I . The "surfaces areas v/ere determined as p r e v i o u s l y , by

the BoEpT» p rocedure . From the deduced p a r t i c l e s i z e , a comparison o f

the r ^ t e cons tan t s f o r t h e p a r t i c l e s i z e s f o r a s p e c i f i c temperature

i s g i v e n i n Table 24*

14B

Table 24

c oTpaidson of the react ion r a t e s of constants of m i l l e d samples a t

v a i i o u s temperatures.

T . , . E g W , . - ^ ^ J ^ H . t . C o „ . t » t - 3 - 1 5

872 • 0 0,077 X 10"^ 0 .4

0.258 x 10"^

10 0.436 X 10"^

2 0.258 X 10"^ 2 .7

4 . 8

905 0 0.235 X l O ' ^ 0 .4

2 0 . 6 1 x 1 0 " - ^ 2.7

10 0.67 x 10"^ 4 .8

960 0 0 .47 X 10"^ 0 . 4

2 0.89 X 10"^ 2.7

10 0.79 X 10"^ 4 .8

- 149-

As expected^ when i n c r e a s i n g the s u r f a c e area o f t h e m a t e r i a l , the

sample becomes more r e a c t i v e as can be seen f r o m Tab le 24 wh ich compares

the r a t e o f r e a c t i o n w i t h s u r f a c e a r e a . A t v / e l v e - f o l d i n c r e a s e i n

s u r f a c e area as a r e s u l t o f m i l . l i n g t he m a t e r i a l f o r 10 hours i nc reased

the r e a c t i o n r a t e c o n s t a n t by the o r d e r o f 5 6 t imes a t 8 7 2 ° C ,

As the temperature is inc reased^ however, the inc rease i n r e a c t i o n

r a t e i s no t so g r e a t - For example^ a t 905^0 the m i l l i n g o f t h e sample

f o r 10 hours i n t r o d u c e d an i n c r e a s e i n r a t e c o n s t a n t o f 28 , y e t a t 910°C

the incrCELse i s o f the o rde r o f 2 1 .

V/hen comparing t he r e s u l t s o f a lumin ium n i t r i d e heated i n a i r h a v i n g

been m i . l l e d f o r 2 hours and 10 hour-s r e s p e c t i v e l y ; a t 81 2 t he re i s o n l y

a s l i g h t m o d i f i c a t i o n i n the r a t e c o n s t a n t (see F i g . 2 1 ) , and a t 905

(see F i g . 22) and 960 e s t ima ted r e a c t i o n r a t e v / i t h i n e x p e r i m e n t a l e r r o r

i s constantc T h i s e f f e c t has a l s o been observed i n the o x i d a t i o n o f

chromium n i t r i d e i n a i r ( 6 3 ) .

The r e s u l t s suggestr, t h e r e f o r e 5 t h a t i n c r e a s i n g the s u r f a c e

area by a f a c t o r o f 63 tends t o make t he sam.ple more r e a c t i v e b u t

any f \ j 3 r t h e r i nc rease i n the s u r f a c e a rea has no a p p r e c i a b l e e f f e c t

on the r e a c t i v i t y o f a lumin ium n i t r i d e .

I V . 3 . v The e f f e c t o f i m p u r i t i e s on the o x i d a t i o n

As one o f the methods o f p r e p a r i n g a lumin ium n i t r i d e s t u d i e d was

t he thermal decomposi t ion o f ammonium h e x a f l u o r o a l u m i n a t e i n gaseous

ammor-iaj, the e f f e c t o f alum.inium f l u o r i d e on the o x i d a t i o n o f

a luminium n i t r i d e v/as i n v e s t i g a t e d .

V/eighed amounts o f - a lumin ium f l u o r i d e ( f o m e d by decomposing

ammonium h e x a f l u o r c a l u m i n a t e i n p u r i f i e d n i t r o g e n a t 7 5 0 ° C ) were added

50

t o h f .

onm'Mi

The e f f e o t of m i n i n g on the ox idat ion

of a l u a l n l u a n i t r i d e a t 8 7 ^ 0 .

- 151b-

t o 1 gram weights o f a lumin ium n i t r d d e smd the f l u o r i d e p resen t

expressed as a we igh t percentage .

The e f f e c t o f adding the f l u o r i d e i s shov-Ti i n F i g . 2 3 . The r a t e o f

o x i d a t i o n i s remarkably increased* ' Uô2. wrt^% i nc reases t he r a t e o f

o x i d a t i o n - bu t the ' f i n a l ^ amount o f decompos i t ion observed i s l e s s

than t h a t observed v/ith no f l u o r i . d e p r e s e n t . 9 . 3 v j t o f o a g a i n inc reases

the o x i d a t i o n r a t e more than Uc2}^<, bu t a f t e r a p e r i o d o f a p p r o x i m a t e l y

1.5 hours^ the fluoiTi.de i t s e l f beg ins t o o x i d i s e and a w e i g h t l o s s i s

observed. A 1 9 . 8 vn^fo a d d i t i o n i n f a c t r e s u l t e d i n a cons t an t we igh t

l o s s and hence the f l u o r i d e p r e f e r e n t i a l l y o x i d i s e s and can no l o n g e r

be cons idered an impuri . t V o

F l u c r i d e s are a l so knov.Ti t o have a c a t a l y t i c e f f e c t on the

ox i . da t i on o f meta l s ( 6 ? ) .

I V o 3 . v i The chanRC i n s u r f a c e p r o p e r t i e s o f a lumin ium n i t r i d e and

s i l i c o n r . i . t r i d e

Separate h a l f gram p o r t i o n s o f a lumin ium and s i l i c o n n i t r i d e v/ere

oxiui-sed i n a i r a t QOO^ 9 0 0 ^ 1 0 0 0 * ^ 0 , As expected^ the c o n v e r s i o n t o

aluinlna takes p l a c e more r a p i d l y as t he temperature i s i n c r e a s e d .

Weight changes were measured when the sample was coo led t o room

temperature and percentage conve r s ion was found f o r each tempera ture

a t d i f f e i - ^ n t i n t e r v a l s o f t i m e . The p r o d u c t s f rom the o x i d a t i o n were

i d e n t i f i e d by x - r a y techniques as d e s c r i b e d i n Chapter I I and were

i d e n o i f i . e d as -a lumina ( o( -Al^*-*^)^ and s i l i c o n d i o x i d e (S iJ^p

t r i d y m i t e and c i a s t o b a l l i t e ) even i n the e a r l y stages o f t h e r e a c t i o n

(see Tab les 25 and 2 6 ) < , C e r t a i n samples ^vere f u r t h e r examined by

153

2au O ALN

QAj

t 4 F i g , 23. The effect of altminlum f luoride on

the oxidation of aloDiniua n i tr ide*

154 -

the use o f a P h i l i p s MdOO electron microscope (see Pigs . 2 4 - 2 7 ) «

A laj^ger amount of n i t r i d e was required t o measure the change

i n surface area and 5 gram samples were oxidised i n a i r f o r various

timesj, at a 1000*^C« The cooled samples were outgassed i n vacuo at

200°C before t h e i r spec i f i c surfaces were determined by the BcEoT,

procedure ( 4 7 ) *

The deduced average c r y s t a l l i t e sizes were compared w i t h the

p a r t i c l e sizes ranges determined by e lect ron microscopyo

No oxyni t r ides have been observed i n the present study and i t

appears that oxyni t r ides are fomed on ly at higher temperatures ( 5 5 )

where the respective n i t r i d e s and oxides are mutuaJ-ly so lub le , or

i n the case o f aluminium n i t r i d e by n i t r i d i n g o < -alumina i n ammonia

a t low tanperatures ( 5 6 ) . Fig* 28 shov/s the o v e r a l l va r i a t i ons

f o r aluminium n i t r i d e i n spec i f i c surface^ and average c r y s t a l l i t e

size dur ing the conversion to alumina. These are compared to the

ox ida t ion rates Fig« 28d3 changes i n the number o f c r y s t a l l i t e s

F i g . 28c5 and the average c r y s t a l l i t e sizes of the i n d i v i d u a l

unchanged aluminiiira n i t r i d e and the product oc -alumina F i g . 28f«

The aggregate siges,, estimated by e lect ron microscopy are given i n

Table 2 ? to be compared vn.th the c r y s t a l l i t e s izes .

1 5 5

Table 2=

Aluminium n i t r i d e heated at 9 Q 0 ° C f o r 9 6 hours

s strong

V/ = weak

vi,v = very weak

3 . 4 8 w

2 , 6 8 3 2 . 7 0

2 . 3 3 vw ^ 2o55

2 . 4 ^ V/ 2 . 4 9

2 . 3 6 s 2 . 3 ? 2 . 3 8

2.OR V/ 2 . 0 9

1 . 8 3 w 1 . 8 3

1 , 6 w 1 . 6 0

1 . 3 3 V7 1 . 5 6 1 . 5 3

1 . 4 0 -ij 1 . 4 1 1 . 4 3

1 . 3 7 w 1 . 3 7

1 . 3 1 w l o 3 2

l o 3 0

156 -

Table 26

S i l i c o n n i t r i d e heated at 1 0 0 0 ° C f o r 9 6 hours

d , obs I d a

d c

4 . 1 4 4 . 0 4 4 . . 3 0

3 . 7 7 w 3 . 8 8 4.08 3 . 3 7 3 . 8 1

3 . 2 3 s 3 . 1 4 3 . 2 5

2o98 s 2o89 2 . 9 2 2 . 9 6

2 . 8 2 2 . 8 4

2 . 4 6 s 2 . 5 3 2 o 4 7 2 A 7

2 . 3 vw 2 . 3 2 2 . 3 4 2 . 3 7

2 „ 2 9

2 , 1 7 V/ 2 . 1 6 2 . 1 7 2 c l 2 2 c 0? 2 . 0 ? 2 . 0 3

2.91 s 1 . 9 4 1 -99 1 . 9 3 1 . 9 7

lo?95 1 . 8 ? 1 . 8 7

1 . 7 4 3 1 . 7 5 1 . 7 6 1 . 7 6

1 . 6 9 1 . 6 4

l o 5 8 W V / lo60 1 . 6 4 1 . 6 1 1.58

l c 5 1 W W l o S l 1 . 4 6 1 . 5 0 1 . 5 2

1 . 3 3 w 1 . 3 2 1.38 1 . 4 1 . 3 3

d c r l s t o b a l l i t e a d^ c r i s t o b a l l i t e

d^ t r idyra i te

157

Pig . 2ifa, AluainlMi n i t r i d e .

24b. Alumin*to nitr ide Milled 5 houra. | ^ | ^ ^ * |

Pig , 25b. AluninivM nitride heated 24 homrm at 1000^0,

1

Pig 25a. AliMiiniwt nitride heated 5 hou»a at 100( 1 ^

F i g . 2 6 a . Aluminium n i t r i d e hea ted 9G hours I ' ^ o C

P i s . 26b. S i l i c o n n i t r i d e

4

I T

P i g .

7 b . S i l i c o n n i t r i d . rs a t 1000 C .

/

Table 27

Variations in aggregate sizes during the conversion of AIN to ^ " ^ 2 ^ at 1000* 0 i n a i r .

Time, hours % conversion aggregate ' s ize ( A mJ

0 0 1 - 2 .

5 51.3 3 - 5

24 67o7 3 - 6

96 82.7 3 - 7

163 85o9 4 - 8

162 -

Fig*^ 28.: Caparison of the rate of oxidation of AlK at alOOCPc v i t h the change i n i)8peolf io surfaoe i i ) the nsmber of exystaUites i i i ) the average o i y s t a l l i t e siEe^j

i

i

/

- 163 -

V/hen considering aluminium n i t r i d e , the v a r i a t i o n i n ox ida t ion

rate i s accompanied by a corresponding change i n s p e c i f i c surface. The

oxidat ion rate i s accelerated over the f i r s t 50 per cent ox ida t ion and

becomes progressively slov;er as the reac t ion proceeds, see p i g .

There i s an i n i t i a l increase i n s p e c i f i c surface accompanied by a l i n e a r

rate i n oxida t ion v/hich indicates tha t the alumina i n i t i a l l y formed on

the surface of the n i t r i d e s p l i t s o f f to give smaller c r y s t a l l i t e s .

This s p l i t t i n g i s due to the change i n molar volume when conver t ing t o

oC -alumina and the corresponding change i n c r y s t a l l a t t i c e . A measure o. 3

of the c r y s t a l l i t e s p l i t t i n g i s given i n P .g.28c. vriiere a p l o t o f / S

against time i s given where S* i s the ac tua l surface area. Molar volume

change i s not required (57) i n determining t h i s parameter. The extent

of c r y s t a l l i t e s p l i t t i n g i s comparable v / i th ox ida t ion o f t i t a n i u m

n i t r i d e (58) , but f a r less than tha t o f calcium n i t r i d e (4^0. The

Tammann temperature, v/hich i s h a l f the mel t ing poin t o f a compound i n

'^K, are approximately 1340' K and 1200°K f o r aluminiiim n i t r i d e and alumina

respect ively , i n d i c a t i n g that the s i n t e r i n g o f ot-alumina would be more

extensive at lOOO^C. This extensive s i n t e r i n g o f the £7<.-alumina tends t o

bind the un- .cLdised n i t r i d e pei r t ic les together. This i s confirmed by

the changes i n average c r y s t a l l i t e sizes deduced from the surface area

measurements. Accordingly, the aggregate sizes indica tes tha t the

o r i g i n a l n i t r i d e contains mainly s ingle c r y s t a l l i t e s , and the ox ida t ion

resul ts i n the fonnat ion o f aggregate s o f 3 - 8^m (see P i .5. 28) i n

which the i n d i v i d u a l c r y s t a l l i t e s appear to be over 0.5yt<m i n the l a t e r

stages of the ox ida t ion . This i s v e r i f i e d from P i g . 28pvêre the

c r y s t a l l i t e size of alumina a t 60 per cent conversion i s approximately

- 164

0 . 3 ^ m . Beyond 80 per cent conversion, the oxide layers s in t e r s and

prevents the gaseous d i f f u s i o n of oxygen through the oxide layer and

the process i s c o n t r o l l e d by a so l id - s t a t e d i f fV i s ion mechanism.

The mechanism now involved i s extremely complex and depends on

the area and defec t s t ructure o f the contact areas between t h so l id r e -

actante and the product". As the react ion rate i s r e l a t i v e l y slower, the

movement o f the reac t ion i n t e r f a c e i s very slov/. This may be due to the

fact tha t there i s a small contact area between the alumina and aluminium

n i t r i d e at the i n t e r f a c e brought about by the s i n t e r i n g of the alumina

layer .

As there i s a gradual reduction i n the average c r y s t a l l i t e s ize of

ail^-miinium n i t r i d e and a sudden increase i n the average c r y s t a l l i t e size

o f alumina a f t e r lOfo conversion, the i n t e r f a c i a l contact between the

0( -alumina and aluminium n i t r i d e wou^d not be expected to be grea t , and

hence the ra te of s o l i d state d i f f u s i o n and ra te o f react ion i s very slow.

By Gontr s t , s i l i c o n n i t r i d e i s more r e s i s t a n t t o ox ida t i on as can

be seen from F i g . 29. At a lOOO^C a f t e r 60 hours, 2&fo conversion to

c r i s t o b a l l i t f c i s observed and the corresponding conversion to oc -alumina

i s 93? . As ox ida t ion occurs, the s p e c i f i c surface o f s i l i c o n n i t r i d e

i n i t i a l l y decreases, as t h e o i . - c r i s t o b a l l i t e formed acts as a mine ra l i se r .

The Tamraan temperature f o r s i l i c a i s 720* and the s p e c i f i c surface i s 2 - 1

observed t o f a l l f rom 1.7 to below 0,2 m .g v^hen one t h i r d o f the

s i l i c o n n i t r i d e i s ox id i sed . The s i l i c o n n i t r i d e and s i l i c a thus tend

to bond together. Consequently, the ox ida t ion i s inc reas ing ly c o n t r o l l e d

by so l i d state d i f f u s i o n , and, as observed, the ra te of ox ida t ion i s

slower than i n the case of aluminium n i t r i d e .

165 -

Fig,29. Comparison of the oxidation of aluminium and silicon^n'itrldes.

t t

MbMta aoolIlB boa autatmalB

1 . Bxztig G.F. , K o l l o i d z e i t s c h r i f t , 1941, 97, 281.

2. Kingery.,V/.D., " In t roduc t ion to ceramics", I96O, (New York; W i l e y ) . r

5.,, Coble, R . L . , J . A p p l . R i y s . , I 9 6 I , ^ 2 , 787, 793.

Li.. Coble, R . L . , "Fundamental phenomena i n the M a t e r i a l Sciences", 196^5 V o l . 1 (New York; FLenum Press).

5. Kuczynski, G.C., "Theory o f s o l i d state s i n t e r i n g " , I 9 6 I (New Yoric; V/ i ley, In te rsc ience) .

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42o M a s s o t h , F . E . , & H e n s e l , W . E . , J . H i y S o C h e m . , I 9 6 I , 65s 6 3 6 .

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1 6 9

CHAPl'ER Vc 'Iêrnpdynaffiical_ a s p e c t s o f t h e f o r m a t i o n ar.d r e a c t i v i t y o f

a luminium n i t r i d e «

V . l The f o r m a t i o n o f sJ.uminiuin r . i t r i . d e

The r e a c t i o n c o n s i d e r e d v/ere;

A l + ^N^ — ^ AIN ( l )

A l - NH ^ — > A I N + ( 2 )

iklÔ^ * V 2 C ^ ^ N ^ ^ AIN + 3 / 2 Co ( 3 )

A l F ^ + NH^ — V AIN - 3HP (L)

A l C l ^ + ^ A I N 3HC1 ( 5 )

A I N ^ 3 / 2 0 2 ' - ^ ^-'^^2^3 ( 6 )

Thermodynamical d a t a f o r the s u b s t a n c e s i n t h e above r e a c t i o n s

were o b t a i n e d from t h e JAKAP T h e r m o - c h e m i c a l t a b l e s ( 7 ) 0 T h i s d a t a i s

g i v e n i n Appendix. V I I I „

Due t o the . lack o f thertnod;>mamical d a t a f o r 0 ^ ^ ) ^ A l i n t h e

rcactd-cn

( N H ^ ) ^ A l F ^ 4- > AIN + em^V

6 m-^ +• 6 H F

the r c u t e A i F ^ + NH^—» AIN -t- 3HP v/as chosen and t h e d i s c r e p a n c i e s

i n t i o d u c e d by d o i n g t h i s w i l l be d i s c u s s e d l a t e r * A l s o comparison i s

made vd.th the f o r m a t i o n o f t h e n i t r i d e from t h e c h l o r i d e ana logue*

The f r e e energy o f e l e m e n t s and t h e i r compounds cannot be measured

i n a b s o l u t e termSg b u t i t i s p o s s i b l e to o b s e r v e t h e change i n

tha imodynaraica l p r o p e r t i e s v;hen a c h e m i c a l r e a c t i o n i s i n d u c e d . The

change i n s t a n d a r d f r e e energy A ^ g i v e s an i n d i c a t i o n o f the e x t e n t

to v;hich a r e a c t i o n o c c u r s and i s r e l a t e d to the eqô i l ibr ium c o n s t a n t K

by the e q u a t i o n

A G ° =: ==RTlnK ( i )

« 170 "

F o r a c h o n i c a l r e a c t i o n ^ i t i s p o s s i b l e to p l o t t h e s t a n d a r d f r e e

energy change p e r u n i t mass a s a f u n c t i o n o f t e m p e r a t u r e i n d e g r e e s

K e l v i n , T h i s p l o t i s knovm a s E l l i n g h a m D i a g r a m . The main d i s a d v a n t a g e

o f t h e E l l i n g h a m Diagram i s t h a t the f r e e energy change a s a f u n c t i o n o f

t e m p e r a t u r e r e f e r to s t a n d a r d s t a t e s o f the s y s t e m , v /h ich a r e not

r e a l i s e d i n a c t u a l s y s t e m s . F o r i n s t a n c e ^ the m u t u a l s o l u b i l i t y o f the

s o l i d components may i n c r e a s e w i t h t e m p e r a t u r e , and the g a s e s a r e

g e n e r a l l y not i n t h e i d e a l s t a t e . As a r e s u l t a p r e s s u r e c o r r e c t i o n

must be a p p l i e d and t h i s c a n be c a l c u l a t e d from the Van H o f f e q u a t i o n

^ G _ =A&° + RT ( 3 ' l n a / , . \ - 5^1" a / ^ 4 . \ ( i i ) T T ^ ( p r o d u c t s ) X ( r e a c t a n t s ; ^ '

V/hen A G ° ( t h e s t a n d a r d f r e e energy c h a n g e ) i s z e r o t h e e q u i l i b r i i ,um

c o n s t a n t K i s u n i t y . Hence A G ] ^ ^ 0 would i n d i c a t e t h a t K - < ^ 1 and

the produc^ ^ a r e p r e s e n t i n lov^ c o n c e n t r a t i o n a t e q u i l i b r i u m . The

c o n d i t i o n A . G ^ - ^ ^ 0 s u g g e s t s t h a t the r e a c t i o n i s f e a s i b l e ^ and the more

n e g a t i v e the v a l u e o f l \ G^^ the more t h e x m o d y n a m i c a l l y s t a b l e t h e product .

T h e c o n d i t i o n A G ^ r e f e r s to a t h e r m o d y n a m i c a l l y . p o s s i b l e

p r o c e s s . T h i s p r o c e s s i s s u c h , t h a t i f t h e r e a r e no d i f f u s i o n b a r r i e r s

o r r e s t r i c t i v e f o r c e s the r e a c t i o n w i l l p r o c e e d o f i t s ovm a c c o r d . The

f r e e energy change i s t h e r e f o r e i m p o r t a n t because i f a p r o c e s s i s

t h e r m o d y n a m i c a l l y f e a s i b l e and i n r e a l i t y does not t a k e p l a c e p then i t

i s c l e a r some r e s t r i c t i n g f o r c e i s r e q u i r e d to be overcome.

" The f r e e energy change i s r e l a t e d t o t h e hea t o f f o r m a t i o n ( A H ) by

A G = A H - T A S ( i i i )

vrfiere A S i s the change i n e n t r o p y . I t i s n e c e s s a r y t o c a l c u l a t e the

hea t o f f o r m a t i o n a s t h i s i n d i c a t e s v /hether an e n d o t h e r m i c o r exo thermic

r e a c t i o n has taJcen p l a c e .

171 -

I n the p r e s e n t work A G ° and A H'~' were c a l c u l a t e d from t h e

e q u a t i o n s

A G ° = A G , , - A G , ^ ( i v ) p r o d u c t s r e a c t a n t s ^ ^

and A H° = A H . ^ - A H , ^ ( v ) " prod.ucts r e a c t a n t s

V , l The f o r m a t i o n o f a l u m i n i u m n i t r i d e

V„ 1 , i The f o r m a t i o n from the e l ements

Due to the advances i n the t e c h n i q u e s oi' p u r i f y i n g m e t a l S j the

f o r m a t i o n o f a compound from i t s e l e m e n t s has become i n c r e a s i n g l y

i m p o r t a n t . F i g . 3 0 shows the r e l a t i v e s t a b i l i t i e s o f v a r i o u s n i t r i d e s

formed from t h e e lement and n i t r o g e n . I t vn.ll be s e o i t h a t a lumin ium

n i t r i d e i s e x p e c t e d to be more s t a b l e a t e l e v a t e d t e m p e r a t u r e s than

c a l c i u m o r magnesium n i t r i d e s ^ but not a s s t a b l e a s t i t a n i u m n i t r i d e .

From F i g . 32 t h e f r e e energy change f o r the r e a c t i o n

A l A1.N ( 1 )

i s n e g a t i v e i n t h e t e m p e r a t u r e r e g i o n 0 - 2830°K and hence the r e a c t i o n

i s f e a s i b l e v / i t h i n t h i s temperat^jre . However , i t i s c o n c l u d e d from the

p r e s e n t work ( s e e C h a p t e r I I I ) , , t h a t a t 500°C (773*^K)p w h i c h i s v / e l l abov^

the Tamman temperature ( 3 3 0 ° C ) f o r a luminiump t h a t s i n t e r i . n g o f the m e t a l

f i j j n o c c u r s . To overcome t h i s b a r r i e r , t h e r e a c t i o n c a n be c a r r i e d out

i n the v a p o u r phase . The r e a c t i o n betv/een a lumin ium and n i t r o g e n i s

e x o t h e r m i c . The r e a c t i o n wn.ll o c c u r a t t e m p e r a t u r e s l e s s t h a n the

b o i l i n g p o i n t o f aluminii-un ( 2 7 2 3 ° K ) a s i s r e p o r t e d ( l ) . T h e s e w o r k e r s

o b s e r v e d alum.iaium n i t r i d e a t 1300°C ( 1 7 7 3 ° K ) where the c a l c u l a t e d h e a t

o f r e a c t i o n i s - 7 8 K c a l / m o l e .

Above 2850'^K, ( 2 5 7 7 ^ C ) A G ° i s p o s i t i v e and the d e c o m p o s i t i o n o f

. a l i jminium n i t r i d e to i t s e l e m e n t s i s e x p e c t e d to begi.n a t t h i s

1 7 2

Fig. 30. The free energy of foraation of soae nitrides i

loo 3 c o . -70D tcoo

t e m p e r a t u r e . Th i . s f i g u r e a g r e e s w i t h t h e r e p o r t e d v a l u e s f o r t h e

d e c o m p o s i t i o n ten^êrature o f a lumin ium n i t r i d e ( s e e T a b l e 4 ) *

The theimodynamic c a l c u l a t i o n s a g r e e v / e l l w i t h Mah e t a l (2),

but d i s a g r e e v / i t h t h o s " o f V/ icks and B l o c k ( 3 ) , ( s e e F i g . 3 1 ) .

F o r the r e a c t i o n M + i^2(g) ^"^^(s) o p e r a t i n g f r e e

energy change w i l l be g iven by

o RT = A G ^ - ~ I n P^^

The f o m a t i o n o f a lumin ium n i t r i d e by n i t r i d i n g a l u m i n i u m w i t h

ammonia i s a l s o c o n s i d e r e d s

A l N H ^ AIN + 1 H ^ ( l a )

The change i n e n t r o p y i s v e r y s m a l l a n d from F i g . 3 2 t h e r e i s . v e r y

l i t t l e change i n the s l o p e f o r t h i s r e a c t i o n . T h e r e f o r e , A G ^ i s

a p p r o x i m a t e l y e q u a l t o t h e heat o f f o r m a t i o n A H ^ , The r e a c t i o n to a

f i r s t a p p r o x i m a t i o n i s t h e r e f o r e i n d e p e n d e n t o f t e m p e r a t u r e . I n F i g , 3 2

the p l o t s f o r t h e n i t r i d a t i o n o f a l u m i n i u m u s i n g n i t r o g e n and ammonia

i n t e r s e c t a t 4 5 0 ° C . As the t e m p e r a t u r e i n c r e a s e s A G ^ f o r r e a c t i o n ( l )

becomes l e s s n e g a t i v e whereas A G° f o r r e a c t i o n ( l a ) r e m a i n s a p p r o x i m a t e l y

c o n s t a n t . I t would t h e r e f o r e sugges t t h a t i t i s t h e m o d y n a m i c a l l y e a s i e r

to form the n i t r i d e from ammonia a t t e m p e r a t u r e s above 450*^0. T h i s

p o i n t may a l l o w the n i t r i d e to be p r e p a r e d u n d e r the e x p e r i m e n t a l

c o n d i t i o n s d e s c r i b e d i n C h a p t e r I I , s u b s t i t u t i n g ammonia f o r n i t r o g e n

and n i t r i d i n g the a lumin ium f i l m .

V . l . i i The f o r m a t i o n f rom the S e r p e k r e a c t i o n

V/hen c o n s i d e r i n g the Serpek r e a c t i o n to form 1 mole o f a lumin ium

n i t r i d e

A l 0 + V2 C ( s ^ ^ ^ A U ^ , . + , ( 3 )

1 7 ^ "

- l O J .

- a o J .

1

- s o l

Wig, 31. A ecaiparlgon of the free energy of fonaatlon of AIN from i t s elaaents.

5 6 o

- 175 -

and a s s u m i n g t h e s o l i d ccmponents t o be I n the s t a n d a r d s t a t e , t h e

e q u i l i b r i . u m c o n s t a n t i s g i v e n by

From F x g , 3 2 A G ° = 0 i s a t 1980°K l y O O ^ C ) .

At atmosphere p r e s s u r e p^^ "*" ~ ^* suppose P Q Q oC.

3/2 Hence = QC,^^

I n K = ^ l o g _oc£ ^ 1-oi

Thus from the c a l c u l a t i o n s f o r A f o r t h i s r e a c t i o n ^ J n i s

d e r i v e d by e q u a t i o n ( l ) .

When I n K ^ O^^G^^ 0 and + o( " 1 - 0 .

Consequent lyp from F i g . 13 t h e r e q u i r e d r o o t o f CX i s 0 . 6 8 . Hence the

e q u i l i b r i u m c o n c e n t r a t i o n o f carbon monoxide i s GOfo a t 1 a t m s . t o t a l

p r e s s u r e . . From the p r e v i o u s work ( s e e C h a p t e r I I l ) j . r t h e r e s u l t s o f

F r a n k e l and Read ( 4 ) a r e i n good agreement w i t h t h i s v a l u e . A.t 1900°K o *

(1627 C ) t h e c a r b o n monoxide e q u i L i b r l u m c o n c e n t r a t i o n i s 51?^- T h e r e f o r e

f o r the t e m p e r a t u r e range 1 9 0 0 - 1 9 8 o \ a n i t r o g e n c o n c e n t r a t i o n o f 32-49^

i s r e q u i r e d i n the r e a c t i o n . From F i g , 32 A i s n e g a t i v e above 1980°K

and the r e a c t i o n i.s t h e r m o d y n a m i c a l l y p o s s i b l e above t h i s t e m p e r a t u r e a t

lOTrer n i t r o g e n c o n c e n t r a t i o n s . The r e a c t i o n i s e n d o t h e r m i c , and a t

2000°K the h e a t o f f o r m a t i o n i s o f the o r d e r o f 80 K c a l s / m o l e .

The w o r k i n g f r e e energy'- i s g iven by:

3 A G = ^ G ° V 2 R T I n P ^ Q + ^ . ^ T I n p ^2

176

Fig» 32* Free energy of formation of AIN fr<»i various methods of preparation.

- 177-

I t must be remembered t h a t the above c a l c u l a t i o n s v/ere m.ade u s i n g

i d e a l c o n d i t i o n s . Howrevery a s p r e v i o u s l y d i s c u s s e d ^ s i d e r e a c t i o n s

- s u c h a s

A I N + C ^ AlCN

may o c c u r auid i d e a l c o n d i t i o n s m a ^ n o t be r e a l i s e d a t e l e v a t e d

tempteratTJLreso An e x a m i n a t i o n o f the r e a c t i o n to form the n i t r i d e from

alumijri.um c a r b i d e and n i t r o g e n would t h u s sefem p t r o f i t a b l e . The r e a c t i o n

i s proposed a s '

. i - A J ^ C ^ + AIN -K 40 ( 7 )

From P i g . 32 the r e a c t i o n i s p o s s i b l e a t a l l t e m p e r a t u r e s i n the

c o n s i d e r e d temperature^, but above 2100* C a lumin ium c a r b i d e s u b l i m e s .

The r e a c t i o n i s weak ly ex o t herm ic a s A . H ? ^ - J Q i s -4o22 K c a l s / m o l e .

I t has been sugges ted ( 5 ) t h a t a lumin ium c a r b i d e i s i n i t i a l l y

formed i n the Serpek r e a c t i o n by the r e d u c t i o n o f a l u m i n a . The proposed

e q u a t i o n i s

i AlÔ^ + i - A l ^ C ^ + V2 CO ( 8 )

•and subsequent c o n v e r s i o n to a l u m i n i u m n i t r i d e i s s u g g e s t e d by

r e a c t i o n ( 7 ) . R e a c t i o n ( 8 ) i s t h e r m o d y n a m i c a l l y - ^ f a v o u r a b l e a s CiG^

i s p o s i t i v e i n t h e temperature range c o n s i d e r e d ( s e e T a b l e 2 8 ) * The

f o r m a t i o n o f the n i t r i d e v i a t h e c a r b i d e i s t h e r e f o r e not t h e r m o d y n a m i c a l l y

sound. The f o r m a t i o n o f a l u m i n i j m n i t r i d e by t h e S e r p e k r e a c t i o n must

be by the r e d u c t i o n o f a l u m i n a v / i t h c a r b o n i n the p r e s e n c e o f n i t r o g e n

w i t h o u t - a n y i n t e r m e d i a t e r e a c t i o n s o c c u r i n g .

V . l . i i i The f o r m a t i o n from the a l u m i n i u m h a l i d e d e r i v a t i v e s

I n t h e r e a c t i o n o f alumini.iom f l u o r i d e v a t h - g a s e o u s ammonia to form

the n i t r i d e

. T , 1 7 9

T a b l e 28

i . A l ^ O ^ + C ^ A l ^ C ^ + V2 Co

Ten?: ioK) A ( k c a l s /mo l e )

0 1Z,.5.996

300 - l l A - 7 3 3

1000 + 81 .366

1500 + 48o993

2000 + 17*438

180

t h e e q u i l i b r i u m c o n s t a n t i s g i v e n by

% " ( 4 a )

A G ° = 0 a t 1 5 0 o \ from F i g . 32; a s i m i l a r gaseous r e l a t i o n s h i p a s w i t h

the Sexpek r e a c t i o n i s shown b^ e q u a t i o n 4 a and an e q u i l i b r i u m

c o n c e n t r a t i o n o f Glô HF v.ould be e x p e c t e d a t 1 5 0 0 \ , However , the

r e a c t i o n was e x p e r i m e n t a l l y o b s e r v e d a t 9 6 0 ° C ( 1 2 3 3 ^ ) and t h e

c a l c u l a t e d e q u i l i b r i u m c o n s t a n t s a t .1200^K and 1300*^K v/ere 3 .814 x l O " ^

and 8.269 x lO'^-^ r e s p e c t i v e l y . The c o n c e n t r a t i o n o f HF .at 1 2 0 0 ^ and

1300'^K a s s u m i n g i d e a l c o n d i t i o n s v/ere 7«15^ and l8olf/o r e s p e c t i v e l y .

The r e a c t i o n . takes p l a c e a t 960*^C a l t h o u g h t h e f r e e energy change i s

+17 K c a l s / m o l e , aJid the c o n d i t i o n s t h e r m o d y n a m i c a l l y u n f a v o u r a b l e . But

t h e r e i s a n a p p r e c i a b l e p a r t i a l p r e s s u r e o f H F . The HF i s swept away

by e x c e s s ammonia d r i v i n g the reaction towards n i t r i d e f o r m a t i o n .

I n p r a c t i c e the r e a c t a n t v/as ammonium h e x a f l u o r o a l u m i n a t e and t h e

r e a c t i o n i s

(NH^)ÂIF^ -V 4^^H^ — ^ A I N + Sm^F

At 960*^0 the ammonium f l u o i u d e e x i s t s a s i t s gaseous conônents ammonia

and gaseous h y d r o f l u o r i c a c i d and the ammonium h e x a f l u o r o a l u m i n a t e i s

c o m p l e t e l y decomposed t o a l u m i n i u m f l u o r i d e , ammonia, and gaseous

h y d r o f l u o r i c a c i d . The c o n d i t i o n s a r e t h e r e f o r e the same a s i n e q u a t i o n

( 4 ) , excêpt t h a t a lumin ium f l u o r i d e may be p r e s e n t f o r r e a c t i o n i n the

meta s t a b l e f orm. I n t h i s c a s e , c h e m i c a l e q u i l i b r i u m w i l l be f u r t h e r

d i s t o r t e d due to the p r e s e n c e o f the h y d r o f l u o r i c a c i d g a s formed from

« 181 =.

ammonium f l u o r i d e and t h e r e v e r s e o f r e a c t i o n ( 4 ) i s f a v o u r e d ^ However

i n t h e p r e s e n c e o f f l o v j i n g ammonia t h e HF formed w i l l be c a r r i e d away

and i n r e a c t i o n ( 4 ) the forvvard c a s e i s v e r y much favoured . . T h i s i s

o b s e r v e d e x p e r i m e n t a l l y and t h e e f f e c t o f i n c r e a s i n g t h e ammonia f lov/

r a t e i s t o increase the amount o f n i t r i d e formed p e r u n i t t ime u n t i l a

r a t e of some 50 co s p e r min i s o b s e r v e d ( s e e F i g . ? ) a f t e r w h i c h the

amount o f n i t r i d e formed i s c o n s t a n t « One p o s s i b l e explSLnation f o r

t h i s i s t h a t aliominium f l u o r i d e h a s the h i g h e s t l a t t i c e e n e r g y o f the

Group I I I h s i l i d e s ( I 3 O O k . c a l / r a o l e ) ^ due to the n a t u r e o f i t s c h e m i c a l

bond ing . The l a t t i c e energy d e c r e a s e s to 1120 k c a l / m o l e f o r alxmiinium

i o d i d e . The r e a c t i o n i s endothermic and i s f a v o u r e d by an open s y s t e m .

B y compar i son the r e a c t i o n betv/een a l u m i n i u m c h l o r i d e a n d

ammonia to form t h e n i t r i d e i s t h e r m o d y n a m i c a l l y more f a v o u r a b l e a t

temperatxires g r e a t e r than 560°K where A G° f o r the r e a c t i o n i s z e r o

( s e e F i g . 3 2 ) . The n i t r i d e i s r e p o r t e d to be formed f r o m t h e c h l o r i d e

a t t e m p e r a t u r e s i n the r e g i o n o f a lOOO^C ( 6 ) , bu t t h e a d d u c t mono-

aramino a luminium c h l o r i d e i s f i r s t formed and decomposed by d i r e c t

h e a t i n g i n ammonia. The monoammoniates a r e a l s o formed f r o m t h e

iodidCj; bromide , and boron t r i f l u o r i d e but no adduct i s formed with

a lumin ium f l u o r i d e ( s e e C h a p t e r I I I ) .

The h e a t o f r e a c t i o n i s l e s s endotherraic f o r t h e f o r i n a t i o n o f

the n i t r i d e from the c h l o r i d e than the f l u o r i d e . A s m a l l e r h e a t i n p u t

i s r e q u i r e d and the r e a c t i o n c a n be c a r r i e d out b e s t a t l ov /er

t e m p e r a t u r e s .

V . 2 T h e r e a c t i v i t y o f alumi.nium n i t r i d e

The o x i d a t i o n o f a l u m i n i u m n i t r i d e i s t h e r m o d y n a m i c a l l y c o n s i d e r e d

and compared v/ith the o x i d a t i o n o f s i l i c o n n i t r i d e ^ Due to l a c k of

= 1 8 2 -

thennodynamic d a t a on the h y d r o x i d e s o f a lumin ium the r e a c t i o n

A I N + 3H2O A l ( O H ) ^ + NH^

c o u l d n o t be c o n s i d e r e d .

V ^ S o i The o x i d a t i o n o f altuniji ium and s i l i c o n n i t r i d e s

The f r e e ener^gy change f o r t h e r e a c t i o n

A I N + ^ 2 O2 ^ A] - +

i s n e g a t i v e i r r e s p e c t i v e o f t e m p e r a t u r e ( s e e F i g . 34)« The o x i d a t i o n

i s r e p o r t e d to b e g i n a t t o n p e r a t u r e s a s low a s 600^C ( 6 ) . I n t h e

p r e s e n t stucSy the n i t r i d e d i d not a p p r e c i a b l e o x i d i s e u n t i l 800*^C.

The e q u i l i b r i u m c o n s t a n t i s g i v e n by

\ - V -where ^ = p^^

A t 1100°K Kp i s 1 . 0 0 8 x 1 0 ^ ^ .

The c o n c e n t r a t i o n o f n i . trogen p r e s e n t i n the g a s phase a t 1 1 0 0 ° K a t

e q u i l i b r i u m i s t h e r e f o r e i n f e n i t e s i i n a i and the r e a c t i o n i s i r r e v e r s i b l e .

Al though the o x i d a t i o n i s t h e r m o d y n a m i c a l l y f a v o u r a b l e the boundary

c o n d i t j . o n s and the p e r c e n t a g e o x i d e formed i s dependant on the d i f f u s i o n o f

the Al'^"*'ions whereas f o r r e a c t i o n ( 4 ) the f o r m a t i o n o f t h e p r o d u c t s i s

de termined by the t h e m o d y n a m i e s o f the s y s t e m .

3 i l : i c o n n i t r i d e i s more r e s i s t a n t to o x i d a t i o n t h a n a l u m i n i u m

n i . t r l t l e ( s e e F i g . 3 4 ) ; C r i s t o b a l l . l i t e and t r i d y m i t e a r e formed v/hen

s i l i c o n n i t r i d e i s heated ( s e e T a b l e 2 6 ) 0 The f r e e energy of f o r m a t i o n

o f Cristobal l i t e , t r i d y m i t e and q u a r t z from the e l e m e n t s d i f f e r by some

50 c a l s i n the t emperature r a n g e 298 •- 2 0 0 0 ° K ( 3 ) . F o r t h e r e a c t i o n

S i^N^ + 3O2 2 S i O ^ + 2N2

183

•.V

-low-

-600 $00 +

looo {500

Pla* 3{», A o'oaparlabn of the free enevgyof

f oxnation of a<»alu)Dlna and s l l i o a froa the respective nitridea*

- 184 -

the equiJ-ittrium constant i s given by

• P

^0.

= ( I ôCf

At llOO^K Kp = 2<,28 x 10^^ and hence the amount o f nitrogen present

i n the gas at equil ibrj .um i s e r f e c t i v e l y zero,

A compari.son o f the thennodynamic s t a b i l i t i e s of these n i t r i d e s

vrl'ch o.xygen shov/ tha t per mole of n i t r i d C j the s i l i c o n n i t r i d e i s less

stable J, but t h i s i s not rea l i sed i n p r a c t i c e , although a react ion i s

ene rge t i ca l ly feasible^ the k i n e t i c s o f the proce.ss may not be favourable

as pointed out i n the above examples.

185

! • Repejiko, K . N , , et a l * NauchaoSb.Pr.Ukr. j, Naucha-Isaled. , In s t . Ogneuporov, 1962,

2o Mah A ,D. , et a l . U.SoBur.Mines R e p t . I n v e s t N o , 5716^ I 9 6 I ,

3 . V/icks, C . E , 5 & Block, F , E . ^ U.S.Bur .Mines., B u l l . , 6O5, I 9 6 3 .

4. Tucker, A . A . , & Read, HoL., Trans,Amer.Electrochem,Soc. , 1912, 22, 57.

5. Caro, N , , Z.angnew Chem., 1910, 42, 4D.

6. Renner, T . , Z.anorg.Chem,, 1959 p 298 ^ 22,

7 . JANAF Thennocheraical Tables, Dow Chemical C o . , (New Y o i k ) , JL9 60-65 •

« 186 -

GriAI 'EH, IV' C o.r-C ludi. ng suiam r i . e 5 an d suRge s - t l on 5 f o r f u r the r v.'O rk

Aluminium n i t r i d e i s belîg successfol iy prepajred from therrro

d,ecompo3Xîcn of ti";e amHioiLVum hexaf LuoroG.l-.imiriate i n aiTimor.ia* The

re'^cti.ori i s e s sen t i a l ly betv/-*:en o:-*-aiunu.nli:!m f l u o r i d e and aznmonia and

takîs place below 1000*^00 I'hougn tr-e r-âction i s îndotbennic and the

thermodynajni.c ccndl t icns are unfavoi/rg.bi.-e,, :he piioc^.sî cati be cpjrried

ou^. v/itho-J.t d i f f i c u l t y a: .d produces matGrdal of high pur i tyo Tha

râcîcn bet?,veen niT;rogen and al-.imin.v.;m i s 'SKothemiic and the condi t ions

ara therKiodynamically favourable i,n theory^ but i n pr-actice i t i s fcvaid

:;haT, the r-oa.ctlon doerr-: not taJce place :Satif?,fao.tor.d.l3'- even on

tfisse f i l n s at i-7-mper'at:uree up to tiie mel t ing point o f a.l-^Td.nj-.i:r.o I t

appea:- 3 that a p ro tec t ive l aye r i?- foirri-aci arid from the l l te .r-dture the

'^.ffectivc .refaction i s a gas phîsr r sae t ion betv-een aluminiuin vapo-j.r and o

ni,tjX)gen i*equiring temperaturej?- sjxDUjna 1600 C« The Serpek reac t ion i s

endothemdc and takes pla:ê readidy at about the same t e m p e r a t u i ^ b u t

XL 2 3 d i f f i c i i l t y to obtair i proiuL^-3 c f h igh p-^rlxyo

Trius i t seems triat :he preparat ion from f luo idde and ammonia has

cert ioin techni'?al adw/itagss ove,-r other- processes*

The work, has shown that even I n a fjz-:ely div.idsd formg aluminium

n i t r i c i a had reasor/able i-îsiF.tance zo ox ida t ion at high r-emperatures.

There do -s not seem to be any obstacles to making sinter-ed materlai.ep

althov^b t h i s v/as not attempted .m the prer^.^nt \vorko The presence o f

fLuorj .de i n small amo\int3 might ac 'ual- ly assi.c.t i n the process,

T"ne r eac t i v ' l t y of ihe n i t r i d e i s determined by the p a r x i c l e and

c ryE ta l l i . t e sizes and the ava i lab le sur-face* The nitrîde i s observed

to hydrolyse mor-e i n l i q u i d water than i.::- steam. This may be a ' / a l i a

7

po?Ln;-. when usi.ng i/he r'abricatîd ina'jei'i>jJL at. elevafced t;mperatiires i n

mci.s-: aunGspher-e3„ The ox ida t l cn k ine t i c r . of aliiiranioiTi n i t r i d e *ir«

i n i ^ i a l i y linea-r chejiging :o para.bcliCf, and are con t ro l l ed by aluminj.ura

ior. di,Cf>^3io2i thix-ugji the o.-ild-^ ].>? v'er'o i s coxnrraDn v/ith chrcmiîra

r:::,tride^ a br-eak dov/ri i r : the p?.rabolic k:Lried.c--^ i s obse.t"vedo This i s

possibly due to a pr^sssu.re b u i l d i;.p o f n;?--,rogen i n the void.- o f the

reacTion .inte r f - .oe ^

Aluitinium r-it'.ride.u hcwev5:j'y i s inorc; r eac t ivs than s^.iiccn n i t r i - d e j

bv, i t i s oi'gge^jtc-d j'- -rjTn a comparlsi-Gr. o f t.'ie mechanical prcpei ' t ies tt'.a.t

al\^T:ijvi.um rli^•:dde has def i .n i re po^^sible use rid a ceranile r i a i e r i a l c

Hov/ever. i t appear^s tnac addiLii.ons to Lhe r . i t r i d e mjsi be msde beforc-

the cptinroin sintex'l.ng condition?, ai:^ rtiali.sedc Tne e r fec t o f rhese

addi t ives on the r eao t i t r i t y o ' ^ l i t r^de rnigVtt, prcvo. a usefi j j . sti>'dy.

The ^C'v; densi ty or |^-ali.nnijii- '^. f l u o r i d e which has been

confinned by several density mea.^L-r-iinrnt^: Lr.. accordance vritti the

tetr'ajgonal c e l l suggi^strd xn the pre--s3nt worKj ponea a problem j n

.•î .r jolurti l chemistry v/ti.ich rtsoûires f i - i r ther inves-:Igationo Likewiseg

i,he i n a b i l i t y oi' e i the r the oCcr ^"alL::crd.rdjm . f luor ide to fc rm cornpo_jids

vri.tri iiirmonza. o r \vater_,, althoug}! the l a t t e r are kncv/n " .o e x i s t

as AJ-Pj^JHÔ requires i \ ; r^her inv!^s t i gB.tior-«

The fonnation of other n i t r i d e s by the f luordde route would seem t c

o f r e r 3i.Tdl.ar advarxtages as ir. the caee o f al'-jmini-iuit. In p.articijlar_„

anniori/ni f^ \ ;os i l ica teo which i s r e a d i l y obtai-ned i n a pure f'cnn and v/hich

vcla t ises ooffipleteiy on heating i n the forin o.:' s i l i c o n t e t r a f l t j c r i d e «

ammcinia and gaseous hydroi'j.uori.o acid rnight be v>:peoted tc form s i l i c o n

n i t r i d e at about the same temper-aturer ann t h i s vr;yjl6. offe::-- an a l t e rna t i ve

ix-.vte to t h i s mate r ia l -

135

APPENDIX I

The preparations of ammonium hexafluoroaluminate and ajnmonium

t e t raf luo ro a 1 umin a t e.

Condumetric study of the reaction betv/een aluminium fluoride and

ammonium fluoride.

189 «

The preparation of ammonium hexafluoroaluminate

Dissolve aluminium f o i l (99o999?2)(Ôg) in h-Cô hydrofluoiic acid

(250 mis) . F i l t e r the thick grey saturated solutions. Aglommerates

of aluminium fluoride trihydrate ( oC - AlF^ ^ 2* ^ separate out and

these were immediately redissolved in d i s t i l l e d water. IN ammonium

fluoride solution i s added unt i l a TA ite precipitate i s formedo The

precipitate was dried under vacuum over potassium hydroxide pel lets .

The product was analysed as amnonium hexafluoroalurainate v/ith no

evidence for the formation of ammonium tetrafluoroaluminate. The

aluminium fluoride-ammonium fluoride-water system v/as further studied

condumetrically.

The preparation of ammonium tetrafluoroaluminate

To prepare fluoboric acid 90 mis of 40 0 hydrofluoric acid were

added to 4O grams of boric acid AcR with s t i rr ing u n t i l the boric acid

crysta ls were completely dissolved. To the solution v;as added the

stoichiometric proportions of aluminium hydroxide and ammonia solution.

The mixture was heated at 75°C for 1 hour and l e f t to cool overnight.

Crystals were observed in the presence of a white residue. The crysta ls

were separated and the aluminium hydroxide impurities removed by

redissolving the crysta ls with the entrained hydroxide in v/atero The

hydroxide was separated by f i l t e r i n g and the f i l t r a t e v/as l e f t to

reoxystal l ize . The resulting crys ta l l ine paste was ident i f ied as

ammonium tetrafluoroalurainate,

Condumetric study of the reaction between aluminium f luoride and

ammonium fluoride

100 mis of U'i aluminium f luoride, prepared as previously described,

was placed in a 400 ml p las t i c beaker. The beaker v;as supported in a

" 190 -

water bath thermostatically maintained at 23*^C. The condumetric c e l l

consisted of two small platinum plates mounted on thin glass capi l lary

tubingj v;hich v/ere kept at a standard distance apart by f ix ing in

c i r c u l a r polythene blocko The connection between the bridge and the

p].atinum electn:>des v;as made across a mercury bri.dge* The dip c e l l

was placed in the aluminium fluoride solution and a conductivity

reading taken« 5 aliquots of l .N Iffl^P were added to the solution

from a burette ^ the solution being constantly s t i r r e d . Readings were

taken after each aliquot was added and a graph of amount of ammonium

fluoride added against the resultant conductance of the solution was

drawn (see F ig . l ) . Prom Figo 1 a plateau i n the curve corresponding

to a mole ratio of alumini'jm fluoride to ammonium fluoride of 1 to 1

i s noted (point A)^ and i s therefore probably due to the precipitation

of ammonium tetrafluoroaluminate although x-ray analysis did not show

any formation of this phase. At point ammonium hexafluoroaluminate

starijs to precipitatec No observation of a pentafluoaJ-uminate was

recorded r, as has been noted ij\ the corresponding potassium fluoride-

aluminium fluoride ( l ) sind gallium fluoride-ammonium fluoride ( 2 )

systemso

191

t

3 U,

iOO 3oO 400

1.92

1. Coxp Bo, & Shaipe, A.Gc^ JoChem.Soc., 195i*-p 179Sp

2. Brer/er, P.Ho, Carton, Goo & Goodgame, D . H . L , , J,Inorg«I^l*Cheip.. J-959s i s 560

'195'

APPETOIX I I

Ifethods of GhemlcfiuL AnalyaiSi

Accirrate weighing technique using a microbalance

A small nickel crucible -was heated over a mekker burner and was

adjusted at intervals so that a l l the crucible came into contact with

the hottest part of the flamep which was approximately 1000*^0* After

10 minutes the crucible v/as transferred with nickel tongs to a

dessicator. The dessicator v/as placed i n the balance room and allov/ed

to cool to room temperature i n 30 minutes o The crucible was placed on

the balance using plast ic tipped tv/eezers and l e f t for 10 minutes to

redioce buoyancy effects to a minimum. The weight was recorded and the

process repeated unt i l a constant weight was observed. At this juncture,

the specimen for analysis was introduced into the crucible and hence the

weight of specimen was found.

Deteiroination of ammonia in ammonium hexafluoroaluroinate

The estimation of ainmonia was completed using a Kjeldaii l apparatus.

The reagents used in the technique were 4C?S sodium hydroxide, 2?5 boric

acid5 O.IN sulphuric acido

An accurately weighed milligram amount of ammonium hexafluoro

aluminate (5Qing) was dissolved i n d i s t i l l e d water and made up to 100 ml

in a volumetric f l a sk .

5 ml of the solution was pipetted into the d i s t i l l a t i o n unit and

washed through with 5 mis (2 x 2 « 5 ) of d i s t i l l e d water. 5 nds of k-Ofo

sodium hydroxide was added and steam introduced into the d i s t i l l a t i o n

chamber. The d i s t i l l a t e was collected in 10 mis of 25b boric acid with

k drops of the mixed bromo-cresol green/methyl red indicator and

t i trated against /lOO sulphuric acido

195

The percentage ammonia was determined from the equation belows

5 X normality = T i t e r (T) X Normality ( 0 , 0 l ) ,

Noiroality = T x 0,01

5

Cone, = T X 0.01 X 17.01 g / l

5

fo =: T X 0.01 X 17 .01

5 X 20 Y/

where V/ = weight of ammonium s a l t dissolved in 50 ml.s,

Ammoriium sulphate v/as used as a standajnd in the technique.

Determination of the nitrogen content i n aluminiuin n i tr ide

The specimen to be analysed was placed in a 6 inch side-aim pyrex

test tube, A fev/ turnings of fine copper tubing with water passing

through were placed around the neck of the test tube to cool i t ,

Pctassi\im hydroxide pel lets were placed on top of the specimen.

The tube was surrounded by an open ended protective glass cyl inder.

When heated to avoid vigorous splashing of the molten hydroxide, a pyrex

glass stopper c i r c u l a r i n form, was mounted a few millimetres from the

top of the pel lets . A trap was placed before the test tube. P r i o r to

this was placed a dreschel bottle containing d i s t i l l e d water. Immediately

a f ter the test tube was placed a quickf i t trap. This trap was connected

to a sintered glass in le t which was lowered into a 500 ml conical f l a s k .

Preparation of mixed, indicator

0 ,1 grams of bromocresol green and methyl red were careful ly v/eighed

out and respectively dissolved in tvro 100 ml portions of 95? ethanol.

The solutions were mixed in a 5 to 1 rat io of broraocresol green to methyl

red.

196

Method

0 « 1 - 0 .4 grains of nitr ide were accurately weighed and placed i n the side-arm test tube, 10 grams of A . R sodium l^droxide pel lets were placed on top of the nitride* 100 mis of boric acid and 1 ml of mixed indicator were placed in the trap pr ior to the teat tube^ to ensure that any feed back of the ammonia woiild be accounted for . A s imi lar portion of reagents were added to the conical f l a sk . Argon (B*OoCo) was passed through the system at a rate of 60-70 bubbles a minute for 3-5 minutes to expel the a i r in the sys t^ i . The sol ids i n the test tube were gently heated with a mekker burner placed under the test tube. Any l iquid that condensed at the neck of the tube was ronoved with a small bunsen flame. The ammoniacal l iqu id v/as carr ied into the quickfit trapp where i t condensed. V/hen a l l the Hquid had condensedj the trap was gently v;armed and the ammonia collected i n the 2^ boric acid solution in the conical flasko The resulting solution was t i t ra ted against O.IN sulphuric acid and the nitrogen content deteimined i n the noimal manner. Dsteimnation of fluorine

The fact that aluminium interferes i n the recovery of fluoride i s

well knowno However^ when the aluminium ion to f luoride ion rat io i s

100, no interference i s observed using the V/illaixi-Winter d i s t i l l a t i o n

procedure, ( l ) V/hen this ratio i s increased to 500 there i s a 5 to 10??

negative bias ( 2 ) . On the basis of wt rat io a 4:1 rat io i s tolerable (3) .

The presence of these two elements together w i l l also interfere

with the recovery of aluminium. Even small amounts of fluoride intex^fere

in the detemdnation of aluminium colorimetrical ly and the formation

of the AlFg ion hinders the estimation of aluminium by flame photometry

(5 ) . A spectroscopic method has been reported for the determination

of aluminiiim in the presence of f luoride whereby the addition of boric

acid s tab i l i ses the fluoride by forming the BF^ ion. However, only 47

to 19fo recovery i s reported (6 ) ,

For the determination of the components i n the same compound,

methods have been suggested such as wide l i n e nuclear magnetic

resonance ( 7 ) , and electron spectroscopy ( 8 ) ,

Determination of fluoride in ammonium hexafluoroaluminate

From the above methods, for aluminium present in aramonitim hexa

fluoroaluminate the V/illard-V/inter method adopted for the analysis of

fluoride ions, using potassium cryol i te as a standard. The method i s

based on the removal of fluoride by steam d i s t i l l a t i o n , and subsequent

t i t ra t ion against thorium n i tra te solution using an alizeirln complex

as an indicator and a mono-chlo* oacetic acid buffer©

Standardisation of the thorium nitrate solution

2,211 grams of sodium fluoride v/ere dissolved in d i s t i l l e d water

and made up to a l i t r e in a volumetric f l a s k . The buffer solution v/as

adjusted to pH 3i> using ^/ lO sodium V\ydroxide. 10 ml of the sodium

fluoride solution was added to 25 mis of d i s t i l l e d water, 10 drops of

indicator were slowly added, 10 mis of absolute alcohol were added,

generating a pink colouration of the solution. This pink colour v/as

dischai^ged by adding one or two drops of deci-norTnal hydrochloric acid.

P i j ia l ly , 1 ml of buffer solution adjusted to pH 3 was added, and the

solution t i trated against thorium n i tra te solution using a blank

t i trat ion of d i s t i l l e d water.

Test Titrat ion ( T ) - Blank Ti trat ion ( B ) = 10 X 0.053

1 ml of thorium nitrate solution = 10 x 0.053

(T - B )

= 198 -

Method

The d i s t i l l a t i o n was carried out using a quickf i t micro-apparatus. An accurately weighed micro-quantity of the complex was placed i n a three-necked pear shaped f la sk . 10 ml and 5 ml of 70jS phosphoric acid and perchloric acid respectively were slowly pipetted into the flask^ excess glass wool was added and the f lask placed in a heating mantle. The temperature was raised u n t i l a 100*^0 5 at which the quantity of water present d i s t i l l e d over© After a few minutes, the tenperature began to r i s e again and was maintained at a 140-150*^0 by running i n d i s t i l l e d water from the dropping f lask as required. 50-75 mis of d i s t i l l a t e f o r 50 milligrams of sol id were collected in approximately 2 hours.

The d i s t i l l a t e was made up to a 100 mis in a volumetric f l a s k ,

25 mis were taken and the fluoride content of ammonxuin hexafluoroaluminate

found by t i t ra t ing against thorium n i trate colution.

The chemical analysis of aluminium fluoride

The determination of f luoride ions using the V/iHard-Winter

d i s t i l l a t i o n technique, was found to be unsuccessful v/ith and

aluminium f luoride. Aluminium has been determined i n the present study

by decomposition i n concentrated hydrochloric acid and subsequent

estimation as aluminium oxinate.

One method i s to Aise the sample with a l k a l i carbonate s i l i c a

mixture in fixed proportions, whereby fluoride i s converted to the a l k a l i

metal f luoride (74) •

The fluoride can be subsequently determined by passing the solution

through an ion exchange column to form hydrofluoric acid. As there w i l l

be a l k a l i carbonates and a l k a l i s i l i c a t e s present the fomation of carbon

dioxide and s i l i c i c acid i s expected, and hence the effluent i s l i k e l y

« 199 «

to contain hydrofluosi l ic ic ac id . Calcium chloride can be added to

convert this to hydrochloric acid which i s t i t r a t e d hot against sodium

hydroxide using a standard indicatoro The operation technique i s time

consuming^ varLedp and tediotiSo The results reported using th is

technique are lowo

A method which I s s imilar in principle but less varied in

technique, i s described by Bognor and Nagy ( 9 ) . This method was used

to deteimine fluoride i n aluminium fluorideo

Preparation of reagents used.

Potassium sodium carbonate

A.R grade sodium and potassium crabonate i n the stoichiometric

proportions were blended together i n a b a l l m i l l for several hours.

S i l i c a powder

Finely divided s i l i c a powder was roasted in a furnace to remove

any water and cooled to room temperatvire and kept in a dessicator.

1% lead chloride solution

5 grams of lead chloride were accurately weighed out and dissolved

by gently heating in d i s t i l l e d water in a 400 ml conical f l a s k . The

solution was cooledj transferred with washings to a 500 ml volumetric

flaskp and made up to the marie.

Saturated lead chloride solution

The solubi l i ty per 100 ml of water of lead chloride i s 1.49 grams.

An amount greater than th is figure was dissolved in a 100 ml of d i s t i l l e d

water to give a saturated lead chloxide solution.

/ l O mercuric nitrate

50 grams of mercury (99.98^ pure) were weighed out i n a conical

f l a s k . 125 mis of d i s t i l l e d water were added and the solution gently

- 200 -

heated over a mekker burner i n a fume cupboards A few drops of

concentrated n i t r i c ac id were i n t e m i t t e n t l y added c a r e f u l l y , u n t i l the

mercujry had d i s s o l v e d . The so lu t ion v/as boi led to dr ive of*f the n i t r o u s

fXimeSp cooled and made up to 500 mis i n a voliametric f l a s k , 50 mis o f

the mercuric n i t r a t e so lut ion were d i l u t e d to 500 mis to g ive an

approximate ^ A O solution*,

OmlSi e r iog lauc ine

0,1 grams of er ioglaucine were d i s s o l v e d i n d i s t i l l e d water and

made up to the mark in a 100 ml volumetric f l a s k o

25 mis of ^ / l O ammonium thiocyanate were d i l u t e d to 150-200 ml with

d i s t i l l e d water and 10 ml of the d i l u t e n i t r i c a c i d and 2 nil of the i r o n

alum so lut ion were added. The s o l u t i o n was t i t r a t e d aga ins t the meinuric

n i t r a t e solut ion^ u n t i l the end point of the red co lour o f f e r r i c

thiocyanate j u s t disappears^ l e a v i n g the so lu t ion water whi t e .

The no imal i ty of the mercuric n i t r a t e so lu t ion was determined a s

0,09275 N.

Method

0.1 grams of o< -aluminium f l u o r i d e v/ere a c c u r a t e l y weighed us ing the

micro-weighing technique. The f l u o r i d e v/as thoroughly mixed with 0.3

grams o f roasted s i l i c a powder and 1.5 grams o f potassium sodium carbonate

and s lowly fused i n a platinum c r u c i b l e a t 700*^0. A f t e r 15 minutes the

fus ion mixture was cooled u n t i l the d i spersed melt v/as n e a r l y s o l i d and

then the c r u c i b l e was placed i n c o l d water . The melt was trauisferred

by i n v e r t i n g the c r u c i b l e and gent ly tapping the bottom u n t i l the s o l i d

f e l l i n t o a 200 ml beaker containing 50 ml o f hot water . Any remaining

s o l i d lumps were removed with a rubber policeman and a few mis of hot

water were added to the c r u c i b l e to ensure ccmplete removal of the

melt . The slurry vras l e f t to stand f o r 2 hours©

201

The suspension was f i l t e r e d in to a 250 ml volumetric f l a s k and

the f i l t r a t e made up to the maric^ 50 ml o f the f i l t r a t e were n e u t r a l i s e d

i n a 200 ml beaker wi th hydrochlor ic a c i d u n t i l red wi th methyl orange

i n d i c a t o r . The n e u t r a l so lut ion was heated to 70°C on a water bathj,

and 50 ml of 1% l e a d ch lor ide so lu t ion were added wi th s t i r r i n g . The

so lut ion was re-heated to 70*^C. T h i s s o l u t i o n which now contained a

p r e c i p i t a t e was n e u t r a l i s e d with 0o2N sodium hydroxide so lu t ion u n t i l

a ye l low co lour j u s t appears. The so lut ion was al lowed to stand f o r

22*. hours o

The p r e c i p i t a t e was c o l l e c t e d on a G-3 s in tered g l a s s f i l t e r and

washed three times with 10 ml of sa turated l ead c h l o r i d e s o l u t i o n , and

twice w i t h 10 ml of 50^ ethanol . The p r e c i p i t a t e was then d i s so lved i n

5 ml of hot 4 .0 N sodium hydroxide s o l u t i o n . The s o l u t i o n was cooled and

10 ml; o f 1.84 s . g . su lphur ic a c i d were c a r e f V i l l y added. 1 drop of

potassium f e r r i c y a n i d e and 0.3 ml of 0.1% er iog lauc ine i n d i c a t o r were

added and the cold so lut ion was t i t r a t e d c a r e f u l l y wi th the 0.09275 N

mercuric n i t r a t e so lut ion u n t i l the end point p due to the co lour change

from green-yel low to carnat ion redp was e s t a b l i s h e d . The l a s t few drops

are added slowly as the co lour change i s s low. Throughout the operat ion

undue d i l u t i o n was avoided.

Determination o f aluminium content

A s o l u t i o n containing a known weight o f the p a r t i c u l a r compound

was made up to 250 mis. i n a volumetric f l a s k . 25 mis of d i s t i l l e d water

were added to 5 nils o f the above s o l u t i o n . The so lut ion was heated to

60* 0 over a bunsen flame and 36 mis o f a 2% so lu t ion of 8-hydroxy-

quirxoline was added. To coraplete the p r e c i p i t a t i o n of the aluminium

presentp approximately 4 grams of ammonium ace ta te d i s s o l v e d i n a l i t t l e

202

water was added. The so lu t ion was l e f t to coo l and a ye l low f l o c c u l a n t

p r e c i p i t a t e was observed^

The p r e c i p i t a t e was f i l t e r e d through a 50 ml Gl*. f i l t e r beaker and

dr ied a t 120°C overnight . As the beaker was weighed before and a f t e r

f i l t e i l n g p the weight of altuninium oxinate isknovm vid hence the weight

of aluminium can be found from the f a c t that 1 nig o f aluminixim oxalate

contains 0*0587 mg of ailuminiumo Hence the percentage aluminium i n

the hexaXluoroaluminate i s given bys

0.0587 X . IQOc

W

where x = -jveight of oxinate p r e c i p i t a t e

W = o r i g i n a l v;eight of ammonium hexafluoroaluminate,

The standard f o r the determination v/as potash alum.

203

Willardp HoHo , and Winters^ O^Boj IndoEng.Chem.Anal.Ed. , 1933p 1 P 7 .

2. Wade , MoAo p Yamamarap S . S . , , AnaloChem., 1965x, J Z P 1276,

3. Grimaldig F.Sog et a l . Anal .Chem., 1955s 918.

4. Murtonp F . R . C . p Inst^SupoSanit . ^ 19k9o 12p 728o

5o Eschelmanns HoC.p et a l . Anal.Chem.^ 1959p 31^ 183.

6. Kurtzp U T . j , e t a l . Appl.p Spectroscc^ 1968p 22^ (4)p 283.

7. Ferrenp W , ? . ^ and Shane^ Nop Anal.Chem.p 1967p 32P 196.

8o 0*Nei l lp P;p p r i v a t e coranrunication.

9<. Bognorp J . , and Nagyp L . ^ K o l i a s z a t i Lapokp 1958p 21? 338.

20k.

APPENDIX I I I

Computer program (NGITO) i n F o r t r ^ I V f o r the IBM 1130

Est imation o f c i y s t a l l o g r a p h i c parameters«

205

r ^ —. I I - <

— • - 11 - I I

' • • — — ^ X, -T „ . j • - r - _Ji;

• • - T ; - ' ] ' ^ - ;m-- t|'-^— -•

I 1

i I t

V- - • —. V ' -J ' -• • — * ; ( • . . .

C - ' •-. •'' ' ' ' r C : I I : . c • -•: , r* p

:t >;

1- o

1 1 . <: ' i

C f

- 205 ^

Computer program (POEH) i n F o r t r a n I V f o r I . B . M . 1130 - Es t imat ion

of sur face a r e a s .

208

L O G •:>oi\f\ conn

V? 05

/ / FOP

r-=WFlG'-T f ' : ITyOr:F' . ABSOPRFD. W = ' . ' /FIGH' 0 --^FSS'J'^f^, ' = pR"S£'wKr AD, CiG^WF IGHT OF '^c' C A V - > L c 00= A Ti ; D A T I ov VArifMiR D " , P 5 5 M C ' F '

S A ' P L " . A T = A T V C c p -H - R I C ^ ^ I ' ^ ^ O G F K ARSOt^°FD P F = ? G ' ^ A ^ '

y A \ - D Y A N D A L S O O F J - ' ^ ^ U T D A T A

T:-r - : -^LLÂi^G nAr^f-^, y v ; / A N : : Y V - I . \ I N ^ H A T C K . " : ^ ^ I I \ F C P ^ ^ A ^ 2 F 8 . C

I ^ f r> V A T THl^'iD C A T ; A - IV- . •,. \r p<\-*?vAT F I G , 2 Ar D rl2»^ T ; - - P , \ - V T A D G A ^ " r. " p A j "> CAÔ F?f . '» ' - 'AT F 1 2 . 5 A . ' . :D F i O , 2 -•-^'r-r C A P " ; S C " \ S ' " T t r r ' A SFT ' - r ^ I^ A L C A : : r - ' W S T y'- vL'>'<-<

o CM

APPENDIX V

Computer program ( N I D K I N E T I C ) i n F o r t r a n I V f o r I . B . M . 1130

Es t imat ion of k i n e t i c parameters.

213

PAGE 2 N I C K I N E T I C PROGRAM

C OXIDATION OF ALN AT l O l O C UNMILLED DIMENSION A N ( 5 0 ) NREAD=2 N W R I T = 3 W R I T E ( N W R I T o l 0 7 )

107 F 0 R M A T ( l H l , 3 X f 'X» t 8X , ' Y • tSX , • A S lOX , • D ' • 13X , < 2 • 113X» • R • • H X • • G S 12 I X t ' O ' • l ^ X f • / )

R E A D ( N R E A D » 9 5 ) N READ(NREAD»96)W R E A D ( N R E A D . 9 7 M A N ( I ) • I « l t M . . - - r . : . - . . - . : . -D O 10 I o l , N P C ( A N ( I ) ) / ( 0 , 2 4 3 9 * W ) F o l - P CALL CUBRT(F9X) Y o l - X A=SORT(F) D o 0 . 5 » ( ( 1 - A ) * * 2 ) S B P * 0 O 1 ^ EoF+S CALL CUBRT(E»B) Z ° ( B - X ) * ^ » 2 . T=SQRT(F+S) * R o O « 5 » ( ( T - A ) * # 2 ) ^ G = A L 0 G < 1 « / F ) Ji|

. .OoALOG(P/F) W V o ( 0 . 5 * ( 1 - ( F # * 2 ) / ( F * * 2 ) ) ) I W R I T E ( N W R I T t 9 f l ) X » Y t A f D » 2 » t ? t G » 0 » V '

10 CONTINUE 95 FORMAT*12) 96 F 0 R M A T ( F 6 « 4 ) 97 F 0 R M A T ( 1 3 F 6 t ^ )

98 F 0 R M A T ( 3 ( F 6 . 4 » 3 X ) t 3 ( ^ 9 . 7 t 5 X ) • F 6 . A • 5 X , F 9 • 7 » 5 X , F 1 2 . 1 0 1 CALL EXIT END

FEATURES SUPPORTED ONE WORD INTEGERS IOCS

CORE REQUIREMENTS FOR COMMON 0 VARIABLES U 2 PROGRAM 306

END OF COMPILATION

/ / XEO

Reprint - The formation and r e a c t i v i t y o f n i t r i d e s p P a r t I I I .

Boron, aluminiiim and s i l i c o n n i t r i d e s ^ JoAppl.Chem. p 1969p Ig.^ 178.

215

178 Coles et al . : Formation and Reactivity of Nitrides. III.

FORMATION AND REACTIVITY OF NITRIDES m.* BORON, ALUMINIUM AND SILICON NITRIDES

By N. G. C O L E S , D. R. GLASSON and S. A. A. JAVAWEERA

The reactivities of boron, aluminium and silicon nitrides have been compared. Samples of these nitrides have been converted to oxides by being calcined in air. Changes in phase composition, surface area, crystallite and aggregate sizes have been correlated with oxidation time and temperature.

Crystallites of alumina. a-AliOj, split off from the remaining aluminium nitride before they sinter and inhibit further oxidation. The diboron trioxide, BjOj , and silica, SiOa (a-cristobalite), immediately act as mincraliscrs for the remaining boron and silicon nitrides, and progressively retard the oxidations.

Introduction The formation, hydrolysis and o.xidation of the ionic

calcium and magnesium nitrides, CaaNj and M g i N j , have been described in Pari II .* More recently, the authors have found that zinc and cadmium films do not nitride in nitrogen or ammonia at temperatures below their m.p., 420° and 320" respectively. The nitrides, ZnsNi and C d s N i , are formed only at appreciable rates at temperatures above 600'. They arc hydrolysed rapidly to ammonia-soluble complexes, MCNHaMOH):, where < 4 for M = Zn or Cd.^ Comparison of the molecular susceptibilities of Mg, Zn and C d nitrides shows that the polarising action of the metal ion decreases from Mg to Zn to Cd. Nitrides in Group IH (B, Al , Ga) and Group IV (Si, Sn) show covalent character and are more resistant to hydrolysis and oxidation.^ In the present paper, the reactivities of the covalent nitrides of B, Al and Si are compared with one another.

Thermodynamics of nitride formation and the relation between bonding and crystal structure have been discussed in Pan 1. Boron nitride is more chemically reactive than nitrides of A l and Si.* Finely divided B N is hydrolysed slowly by hot water and dissolves completely in boiling 20% NaOH within 30 min. Dissolution is perceptible also at 20° in acids and alkalis.' Reactions between aluminium nitride or silicon nitride and hot water are inhibited by coatings of hydrated alumina or silica on the outside of the material. Hydrolysis is generally slow in mineral acids and alkalis,*'^ but there is complete dissolution of aluminium nitride in 30% N a j C O j at 80° and of silicon nitride in boiling HF.« Al l of these nitrides are oxidised in air at higher temperatures. Boron nitride powder oxidises appreciably above 800°,^-'^''° the rate der>ending substantially on the preliminary calcination temperature.*' Nitrides of aluminium and silicon oxidise at temperatures above 600° and 800" respectively, and again oxidation rates depend mainly on the intrinsic reactivity of the material and the available surface at which oxidation can occur.^ Changes in phase composition, surface area and crystallite and aggregate sizes are now correlated with sintering and oxidation conditions for the above nitrides.

Experimental Procedure

Separate portions of powdered nitrides of boron, aluminium and silicon (Alfa Inorganics Inc.) were calcined in air for various times at fixed temperatures. Oxidation rates were estimated from weight changes of the samples during calcination.^ The cooled products were outgassed at 200° in

vacuo before their specific surfaces were determined by B . E . T . procedure*^ from nitrogen isotherms recorded at - 1 8 3 ° on an electrical sorption balance.*^ '* The deduced average crystallite sizes (equivalent spherical diameters) were compared with particle size ranges determined by optical or electron microscopy.

Phase composition identification Samples were examined for phase composition and crys-

tallinity using an A^-ray powder camera and a Solus-Schall ^-ray diffractometer with Geiger counter and Panax rate-meter. Certain samples were further examined by optical and electron microscopes (Philips EM-100).

Results Fig. I (a), (b) and (e) shows the overall variations in

specific surface, 5, and average crystallite size during the conversion of aluminium nitride to alumina at 1000** in air. These are compared with oxidation rates Fig. 1 (d), changes in the number of crystallites Fig. 1 (c), and average crystallite sizes of the individual unchanged A I N and its oxidation product, a-AliOa Fig. 1 (f). Corresponding variations in the aggregate sizes arc summarised in Table I .

— ^

} 1 t

9 V y V I -

1 O J .

] - ^ ^

r \ -

... ' '

\J\ 1 1

1 I ,,, < lllfnl ' ' innJ

- / - ' \ r

CCfJ'/ERSION.% CON\.'ERSrOf4.%

Fig. I . Calcination of aluminium nitride in air at IOOO''c In (b), broken cur\'e represents actual surface area {$') for an

initial one-gramme sample of aluminium nitride

•Part II: / . appl. Chem., Lond., 1968, 18, 77

J . appl. Chem., 1969, Vol. 19, June

Coles et al.: Formation am/ Reactivity of Nitrides. III. 1 7 9

T A B L E 1

\'arialions in agt regate sizes during the conversion of AIN lo a-AIÔ, at 1000 t in air

Time, h Conversion, % Range of aggregate size,

0 0 1—2 5 51 3 3—5

24 67-7 3—6 96 82-7 3—7

168 85-9 4—8

The samples of boron nitride and silicon nitride had specific surfaces, S, = 11-5 and 1 -7 g '; average crystallite sizes, 0-23 and 11 pm respectively. They oxidised in air at 800" and 1000 1200 ; rates are shown in Fig. 2. where they are compared with those for aluminium nitride at 900—1100 . Electron micrographs of the nitrides and their oxidation products arc prcscnled in Figs 3 and 4.

Discussion

Oxidation of aluminium nitride Aluminium nitride, A I N . is converted to a-AljO^ at 1000

in air. A'-ray powder photographs and diffractometer traces give no indications of any oxynitrides being formed at [emperaturcs between 800 1100 . The oxidation at 1000 , Fig. 1 (d), accelerates somewhat during the conversion of the first 5 0 o f the nitride, and then becomes progressively

20 0 TiME.h

Fig. 2. Calcination of boron, silicon am! aluminium niirithw in air at tiijfereni lemperaiures

(a> tx>ron nitride at 800 c <b) ; : silicon nitride at 1000 c. at 1200°c (c) and (d) _j ^ aluminium nitride at 900 i\ O

at 1000 ( and , . . . at 1100 r (Only a representaii\c selection of points is shown for the sake of

clarity)

(a) ( b) ( c ):

(d) ( c ) ( f )

Fig. 3. Electron micrographs of boron nitride calcined in air at 800 c (a) and (b) after 24 h, (c) 48 h. (d) 96 h, and (e) and (f) 144 h

J . appl. ( hem.. 1969, \ ol. 19. June

IHO Coles et al.: formation ami Reactivity of Nitriiles. III.

( c )

f*ig. 4. Llectron micrographs of silicon nitride calcinetl in air at I200"r (a) original sample, (b) after 5 h and (c) after 35 h

slower especially after about SO"., con\ersion. These \aria-lions in rate are accompanied by corresponding increases and decreases in specific surface. 5, in Fig. 1 (a) and (b). and in actual surface area. S . for an initial I g-sample of AIN illustrated by the broken-lined cur\e in Fig. 1 (b». C onsequently, the average crystallite size of the material at first decreases and later mcreases, Fig. 1 (c).

Several factors may contribute to the detailed shape of the initial rate curve of an oxidation isotherm.' ^ e.g. decreases in surface heterogeneity as the reaction prtKeeds. changes in specific surface or in local surface temperature due to heat of reaction, solubility effects, impurity concentrations, possible changes in oxide composition and electrical double layer efiects. In accordance uilh the oxidation of coarser samples of aluminium nitride,"' a specific amount of oxide must be formed (depending on the sfêcific surface of the sample) before a coherent alumina layer can he produced. Meanwhile, the free nitride surfaces remain exposed to the gas phase, so that the kinetics approach linearity. When there is sutTicient oxide of rational crystallite-si/c composition, it sinters lo form surface films through which normal gaseous diffusion cannot easily occur. The reaction becomes controlled by solid-state diffusion, with the kinetics becoming parabolic and the surface area decreasing as observed after about 50",, conversion in Fig. I (a), (b) and (d). Parabolic kinetics are characteristic also when the oxidation of aluminium produces better crystallised aluminas.'"

The increases in S (and decreases in average crystallite size) during the acceleratory and approximately linear stages of the oxidation. Fig. I (a), (b) and (d), indicate that the alumina initially formed on the surface of the nitride splits off to give smaller crystallites. Any additional spalling at the nitride oxide interface when the samples were cooled for surface area determination was negligible by comparison, since the reheated samples proceeded to give oxidation rates similar to those of samples which had been continuously heated. Thus, the extent of crystallite splitting depends generally on differences in molecular volume and type of crystal lattice compared with the original nitride (cf. Pilling-Bedworth rule for oxidised metals'"), and also on the rate of oxide sintering as discussed in Part I.-* Although the conversion of AIN to a-Al .O . i involves practically no fractional volume cfiange,"' nevertheless the crystal lattice change is

apparently sullicient to cause a limited amount of crystallite splitting. In this instance. (.V 5)^ directly represents the increases in the numb>er of crystallites as oxidation proceeds. Fig. I (c). no allowance being required for molecular volume changes.''' The maximum increases of less than twenty-fold are comparable with thi>se found for the oxidation of T i N -dcscribed more fully in the next paper. They are much smaller than the increases during the oxidatitm and hydrolysis of C ajN>. vi/. 5 y 10"' and 10'^ respeciively.'-^

The m.p. of AIN (> 2400 ) and A l . O j (2050 ) give Tam-mann temperatures (half m.p. in K ) of > 13.16 K . and 1160 K , indicating that sintering of alumina should be much more extensive than that of aluminium nitride at 1000 . In the latter stages of the oxidation, the alumina appears to act as a mineraliser for the remaining aluminium nitride particles, inhibiting their further oxidation, cf Fig. 1 (d). and moulding together the material. Changes in the average crystallite si/e of the aluminium nitride (assuming no appreciable sintering) and the a-AI.O.^ are deduced from the surface-area data and shown in Fig. I (f). These confirm the ultimate sintering of the alumina, and the larger crystallite sizes given during the first half of the oxidation (above the broken-line) are caused probably by some of the newly-formed alumina not being detached from the nitride surface. .Accordingly, the aggregate sizes of the materials (Table I) indicate that the original nitride consists mainly of single crystallites. The alumina produced tends to promote formation of aggregates, mainly of 3 — 8 \im sizes, in which the individual crystallites appear to be over 0 5 ixm in the later stages of the oxidation as also indicated in Fig. 1 (f). Similarly, 86°„ oxidation of aluminium nitride in 14 h at 1100 . Fig. 2(d). gives alumina crystallites of 0-5—1 nm size, sintering al the higher temperature being restricted by the shorter oxidation time.

Oxidation of nitrides of boron and silicon The boron nitride and silicon nitride oxidise at 800 and

1200 respectively, giving diboron trioxide, B i O j , and silicon dioxide, S iO, (a-cristobalite). even in the early stages. In contrast to the behaviour of aluminium nitride on oxidation, the specific surfaces of the materials initially decrease rapidly as the boric oxide (m.p. ^ 450 ) and the silica (m.p. 1710 ) act as mineralisers. Although the silica does not melt, it is vêll

J . appl. C hem., 1969, Vol. 19, June

Coles et a l . : Formation and Reactivity of Nitrides. III. 1 8 1

above its Tammann temperature (720°) and the specific surface, 5, falls from 1-7 to below 0-3 m^g-' when one third of the silicon nitride has been oxidised. The products tend to bond together and shrink, cf. bonding and hot pressing of boron nitride with silica glass.^° Consequently, the rates decrease considerably for both nitrides as their oxidations become increasingly controlled by liquid- or solid-state diffusion, especially the latter, cf. Fig. 2 (a) and (b).

The original boron nitride has a flaky texture with rod-shaped and hexagonal plate-like particles. About 20% of the BN is converted to B 2 O 3 after 24 h calcination at 800" in air. Fig. 2 (a), while the hexagonal plates become rounded and tend to form aggregates as in the electron micrographs in Fig. 3 (a) and (b). Further calcination continues this aggregation, cf. Fig. 3 (c) at 48 h, and after 96 h the rod-shaped particles become distorted by the newly-formed B2O3, cf. Fig. 3 (d). When over 80% of the BN has been oxidised after 144 h, there is sufficient B2O3 to crystallise out and change the appearance of the aggregates as in Fig. 3 (e) and (f). The

silicon nitride particles. Fig. 4 (a), also form aggregates with rounded edges when only about 20% of the nitride has been converted 1 0 silica after 5 h calcination al 1200" in air as in Fig. 4(b). Further calcination (35 h) produces larger aggregates as in Fig. 4 (c).

Acknowledgment The authors thank Mrs. M. A. Sheppard for her assistance

in the analytical work; the University of London, Imperial Chemical Industries Ltd., and the Science Research Council for grants for apparatus and a S . R . C . Research Technician-ship (for M.A.S.); the College Governors for a Research Assistantship (for N . G . C . ) .

John Graymore Chemistry Laboratories, College of Technology,

Plymouth . -c f . a i s o S.. A. A. Jayaweera,

,Ph.D. t h e s i s (London).

References

' Glasson, D. R., & Jayavveera, S. A. A., J. appl. Cbem., Loud., 1968, 18, 77

2 Glasson, D. R., & Jayawecra, S. A. A., 'Surface phenomena of metals'. Soc. chem. Ind. Monogr., 1968, No. 28, p. 353 (London: The Society)

3 Glasson, D. R., & Jayaweera, S. A. A., J. appl. Chem., Lond., 1968, 18, 65

* Samsonov, G. V., 'High temperature materials", 1964, Handbook 2, pp. 235, 266 (New York: Plenum)

' Taylor, K., Ind. Etigng C/iem., 1955, 47, 2506 « Renner, T., Z. atiorg. Chem., 1959, 257, 127 ' Collins. J . , & Cerby, R., J. Metals, N. Y., 1955, 7, 612 « Billey, M., Annls Chim., 1959, 4, 795 ' Zagyanskiy, I. L.,&Samsonov.G.\.,Zh.prJkl.Khim., 1952,25,557

Samsonov. G. V., Markovskiy, L. Ya., Zhigach, A. F . , & Valyashko, G . M., 'Bor, ego socdinenya i splavy', 1960 (Kiev: Akad. Nauk Ukr. SSR.); USAEC translation, 1962, No. 5032,(0, 209

" Podszus, E . , Z. aiiorg. Chem., 1917, 30, 156 '2 Brunauer, S., Emmett, P. H., & Teller, E . , 7. Am. chem. Soc,

1938, 60. 309 Gregg, S. J . , J. chem. Soc, 1946, p. 561 Sartorius-Werke, 'Eleclrono-vacuum balances', 1963 (Got-

tingen: Sartorius-Werke, A.-G.) " Gulbranscn, E. A., & Andrew K . F. , J. electrochem. Sac, 1951,

98.241 Cooper. C. F . , George, C . M., & Hopkins, S. W. J . , 'Special

ceramics', Proc. Symp. Br. ceram. Res. Ass., 1962, p. 49 (London: Academic Press)

" Sleppy. W. C , J. electrochem. Soc, 1961, 108, 1097 '» P'l 'ne, N. B.. & Bedworth, R. E . , J. Inst. Metals, 1923, 29.

" Glasson, D. R., J. appl. Chem., Lond., 1958. 8, 798 " Carborundum Co., B.P. 1.052.295

J . appl. Chem., 1969, Vol. 19, June

c

Kiermoohemlcal Data

220 «

r

• r a l . m o i r ' dm-

T. ' K . f

S* - t K ' -

C . u u o .CUO - 1 0 0 . ? 9 9

100 11 .<û9 7 . s y i ft ! T i l - I ' f l . O T p 700 1 9 . 1 17 i e . ? 0 7 ?•> t<17 7 , 0 3 ^ - i i s p . o i : ; Q & ?• L i O . ( too

?oo 71 . 9 T ? 74.^0''. ?h i iÔ . C I - 16S.^73 - c o ? 3 . 1 11 7 1 . e x . 7,70' .

^dp * y * " 7 0 hi"? - * l ( j T . f t O s "

7 1 » . 1 9 0 7 , l i 9 - i r 7 . i 9 ? 700 7 i . a 7 0 4 & . 79;> ; ? 9*1 9 . 6 9 4 - 1 6 6 , * 3 7 600 I t 097 - I 6 6 . 0 f t 5 9 0 0 7 6 . T 1 7 ; ' i 7«(? U . 9 * . 9 - 1 ^ S , A 9 * .

1000 ? T . y o o i e 1 7 . 6 - ^ - 1 ^ 7 . 4 2 0

1 100 7 7 . 7 1 7 S B . 8 1 7 CO 31 1 ^ 0 . 3 1 ' i - l ^ ' ' . 7 5 5 i : c o 77 , 4 . 00 M . 1 9 1 '•I 9^)1. 2 3 . 0 8 7 - 1 5 6 . n 7 t I ' O O 7 7 . b 5 0 b > - l 6 ^ . ; 7 9 IfaCO 7 7 . 6 6 7 r s . t ) 9 4T> • o n ? e . 5 9 » i - 16*.,67*. 1^00 77.7t .C ' , 7 . 3 b l fct . 6 3 9 U . 3 6 7 - 1 6 3 . 9 I ! . ?

7 7 , 6 0 0 t 7 .3C3 - 1*1?.r*.* n o o ' • T . e a i • l o e J 6 . 9 ? Q - 1 6 7 . 5 ? e 1900 7 7 . 9 7 S 7 ? . ' . ? t SO • 3 ^ 9 1 9 . 7 1 0 - 1 6 1 , e : r :<ioo 7 7 . 9 6 9 T > . 9 1 f t 11 • I M ' • 7 . ? > n - 1^1.^.7 6 1000 78.UOO 1 7 b . 1 7 ? ^? . 7 i r . • •^ .312 - 1 6 0 . l i e

159 .JOM 1 6 7 . 7 8 ' . 1 1 6 . i 9 ? I S O . 6 1 6

1 1 0 . b O l

"IJB'.CO*-

1 3 7 . 9 3 6 1 7 7 . 7 7 ? 1 7 1 . 6 B 9 1 1 5 . 1 7 1 1 1 0 . 1 ' . ?

10A.BR& 9 9 . ? « ? 9 - ) . 7 M 8 8 . 7 7 ? e ? . a 3 9

7 7 . 6 1 ' . 7 7 . 1 1 3 B S . B U 6 1 . 1 1 6 1 6 . n e

IMF IN I Tt 31S.71 .P 1 7 1 . i y 7 110.TOO

1 C 0 , 6 ? 7 7 f l . 0 7 1

' " V i r . f t ' l p

4 8 . A 7 0 30.7 '» ? i . 7 « . 7 7 8 , 7 1 0 7 ' . . 1 1 B

:o.9ie J 8 . 0 f l 3 1 1 . 7 6 1 n . 7 7 9 1 7 . 0 6 0

10 170 o .TTO ft.II? 7 . 0 5 1 ' 6> 116

ALUMINim THICHLOHIDE ( A l C l ^ )

- 2 5 . 4 5 + O . K i c n l . d c g - ' ^ c c l o ' ^

( t o d i c e r ) - l 4 6 0 ) ' K .

C CRYSTAL)

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HOL- WT. - U ; . 3 S 1

1

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.44 k c o l . BOlC

a 2 9 8 . 1 5 ( t o d l o e r ) - 2 8 . 0 * O-S I c c a l - c o l e

H e a t o f P o r c J t l o n .

J . p . C o u g h l l n , P h y o . C h e n . 62,

a l u a l n u c c h l o r i d e o n d a l u o l n u n o t 3 0 5 . 1 5 K

- - 5 1 . 6 7 • 0 . 1 4 k c a l . a o l e " ^ f o r t h e r e a c t i o n

419 ( 1 9 S e ) c e n s u r e d t h e h e O l o f o o l u t l o n o f c r y o ' - a l l l n e a r . h y d r o ' J f l

l.T 4 . 3 6 0 a h y d r o c h l o r i c o d d n o l u t l o n . Daaed o n t J ic d o t e , £H r 2 5 8 . 1 5 A l ( c ) * 3 ( H C 1 - 1 2 . 7 3 1 H - 0 ) { 1 ) - A l C l , ( c )

, ' o U e o f £H' 293.1S A l C l j t c ) e o l c u l o t e d t o be - 1 6 6 . 5 6 + 0 - 2 0 k c a l - n o l e •nic h e a t a o f s o : - J t l o n

o f A l C l , ( c ) r e p o r t e d b y t h e p r e v l o u a I n v o H l s e i o r B a r c H a t e d as f o l l o w s i

Kane o f T n v o B t l p n t o r H e a t o f S o l u t i o n

k c a l . R O l o ' ^

C o n c e n t r a t i o n o f S o l - J t l a . T

o o l o o o f H j C / c o l e A l C l j U ;

T t i o a e e n ( l )

R o t h a n d B u c h n c r ( 2 )

R o c h a n d B o r g c r ( 3 )

- 7 6 . 5 + 0 . 3 ( l a ' C . ) 1 2 5 0

- 7 8 . 0 9 + 0 - 2 7 ( 2 0 ' C . ) 2 5 0 0 - 4 7 3 0

- 7 9 . 4 5 ( a o ' C ) 4 0 0 0 - 9 5 0 0

( 1 ) J . T h o a a e n , " T h e r a o e h e n l a c h e U n t c r o u c h u n e e n , " B o r t h , L e l p t l g , 1 8 8 2 - 1 6 8 6 .

( 2 ) W. A . R o t h a n d A . B u c h n e r . 2 - E l e ' r t r o c h c a . 4 0 , I f o . 2 . 8 ? ( 1 9 3 4 ) .

( 3 ) W. A . R o t h and E - S B r e e r , 2. E l e l t t r o c h e a . 4 4 , f l o . 8 , S40 ( 1 9 3 8 ) .

Due t o l a c k o f a p p r o p r i a t e a i K l l l a r y d a t a , t h e o e h e a t a o f a o l u t l o n w e r e n o t u o e d t o c B l c u l f l t e £ ^

T h e h e a t a o f f o r m a t i o n ( 2 0 " C . ) f o r A l C l j ( c ) w e r e r e p o r t e d t o be - 1 6 7 a n d - 1 0 7 . 5 j 0 . 4 k c o l . c o l e

B.^d E . T a n k c , Z. a n o r g . a l l g c a . C h c a . 2 0 7 , 166 ( 1 9 3 2 ) a n d H . S l e n o n o e n , 2 - E l c k t r o c h c o . 5 5 , H o .

r e s p e c t i v e l y . T h e y w e r e o b t a i n e d b y n o l u t l o n c e l o r l n c t r y a n d n o t u o e d f o r n l o l l e r r e a s o n -

• f o r A l C l j C c ) .

- * t y V . K l e c a

4 , 337 ( 1 9 5 1 ) ,

H e a t C a c a e l t y a n d E n t r o p y .

T h e l o v t e m p e r a m r c fteot c o p o c l t y , IS'-ZZQ'K-, »n<l t h e hlefi t e a p e r o i u r e h e a t c o p o r i t y , 2 e o ' - 4 6 5 . € * K . . w e r e

d e t o r n i n e d b y w . E- H a t t o n , 0 . C. S l n k e and D . H- S t u l l . a n d R . A . H c D o n o l d , T h e r o a l L o b o r o t o r y , The >3w C h e n i e o l

C o a p a n y , " p r i v a t e c o a a u n l e n t i o n , 1 9 8 0 , r e o p e c t l v e l y . The t w o o e t a o f d a t a w e r e j o i n e d o a o o t h l y o : 2 9 9 ' K . b y a

g r o p h l c o l a e t h o d . Above 4 6 5 . 6 ' K . t h e v o l u c a w e r e e a t l c B t e d b y ( r . r n p h l c s l e x t r n p o l a * l o r . . S j g o l e r e p o r t e d

b y p . I . O e t t l n g , p r l v o t c c o c c u n l c n t l o n , S e p t e c b e r 1 0 , 19G2, b a s e o o n t h e d a t a d c t e r o l n e d b y • ' . E- H a t t o n ,

a . C Sl r .v .e a n d D . R . S t u l l , l o c . c : t - , u n l n g S*g ( e x t r o p . ) - 0 - 0 9 4 c a i . d e g . ' * B o l e " ^ T h e h i g h : e = r e r a i u r e

n e a t c a t ^ b c l t l e o w e r e n l o o r e p o r t v l L y W. P l a t r . e r , Z- o n o r ^ . Cheo- 2 0 0 , 3 3 2 ( 1 9 3 1 ) w h i c h w e r e n o t - . i n e i .

D a o . 3 1 , I B e O l J u n * 3 0 , l O O l i J u n a 3 0 , l O Q S i K a r . 3 1 . 1984

W e l L l n K L a t a -

a n d Mere r e p o r t e d t o be 4 e s . G ' K . ar .d ft.5 I t c o l . o o l e "

v a l u e ' ^ f fiH* a d o p t e d waa d e t e n r . l n e d l y ft. A . K c l A n a l d , l o r - c l t .

H e a l and T e n p e r a t u r e o f S u b l _l c a t i o n .

r e a p e t t l v e l y . b y W. P l o c h c r , I m * . c ! : TTie

2 9 9 . 1 5 d v e r o e - ! o f alx aH

waa c a l c u l a t e d f r o o t h e v a p o r [ . r c B H u r e d a t a by t r i o aeeond l a w o e t h o d . The a d o p t e d v a l u e I n t h e

a 298 15 ^ " ^ " " o d e r i v e d f r o o o l n v o p o r p r e o o u r e d r t e r o i n a t l o n a . P o r d e t a l l a aee A l , C l g ( e ) t a b l e .

T woa o o l l n f l l e d ao t h e t e n p e r a t u r c a t w n l c n t h e f r e e e n e r g y rh(tn(i;e o f t h e r e a o l l o n 2 A l C I . ( r ) - A l 2 C l g ( g )

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f ^ • 1 . i n f t i ' ' ' i r , . '

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0 . 0 0 0 lOO l ' . ( I N J1 f / •

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n . p . Poi- ier . E. C. Z e l l e r , J . Cn*o. Pi.yoleB i i , 856 (1960) have r c r o r l e d 1 l o » o f A l ^ P j " t a too t lOOO'K.

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f o r o o c I o T i r e p o r t e f i t o b o - 7 6 . 5 t O - Z k c o l . n o l c ' ^ o n d - 7 5 . 5 t 0 . 4 k c a l . o o l c * ^ . THo f o i - m e r v a l u e - o s r e

p o r t e d b y C A . f J e u g e t a u e r e n d J . L . K a r e r o v o , 2. A n o r g . A l l g c n . C h e n . g;90, 8 ? ( 1 9 5 7 ) . The l a t e r v a ^ u e was r e

p o r t e d b y fi. 0 . K a h , E . 0 . S i n g . w . V e l l c r , a n d A . v . C h r l n t e n o e n , U - S . B u r e a u o f M l n c o f l e p o p t o f ; r . veo t ,* . -

e a t l o n o 5 7 1 6 ( 1 9 5 1 ) .

V a p o r p r e o f l u r e c e a a u r e s e n t o a g r e e w i t h t h e a e l e e t e d h e a t o f f o r o a t l o n . P a r I n n t a n c c 2 9 8 . l e "

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o f B u f c l l c i a t l o n , 7 8 . c ' - t ea ; , a o l e " ^ , f o r A i ( c ) . T h l a h e a t o f r e a c t i o n wae o b t a i n e d f r o a a t n l r d l a > c o L c u l a : i c r .

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. a n d L . P. T h c a r d , A e r o n u t r o . T i c T e c h n i c a l R e p o r t U - 1 4 9 7 ( 1 9 6 1 ) . V o p o r p r e o o u r c • c a o u r c n o n t f l w i t h a e l . - r c r a l f l n c e

i n a v n c u u n o y o t c n t y L . H - D r c £ e r , V . v . D a d a p e , a n d J . L . K f l r g r a v e , J . P h y a . C n e n . 6 6 , 15S6 t l S E Z ) a g r e e i - i : - - .

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p e r a t u r e ( 5 1 - 2 S 8 . 1 5 ' . ) a r . d ^ h l g h t e n p e r a t u r e ( 2 3 9 . I S . l f l O O ' K ) h e a t c a p a c U l e a a n d e x t r a p ; i a t e d t h e : c a -

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Lv A . S c n n e i d e r ana Q. Oot lo - .^ , Z- or.iVK- J - a l l t ^ c a . CJica- Z T ? . 41 ( 1 9 5 4 ) .

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U s ' to 1 2 0 0 ' K . J , J . fioaearcfj N o t l . Eur. S t a n d a r c a 5 7 , 6 7 ( 1 9 5 6 ) , w e r e e n p l o y o d I n t r t l o t a b i c . Low t c n p c r e t u r e

o e o s i i r c n e n t a w e r e a l s o a a d e b y ? . S l c o n a n d R. C- S w a i n (30 -2aO*K. ) , Z. P h y o l k . C h e n . 3 8 B . 189 t l 9 i S ) ; £ . C .

K e r r , H . L . J o h n s t o n , find H . C . H o l l e t l ( 2 0 * t o 2 S 5 ' K . ) , J . A o . C h e n . S o c 7 2 , 4 7 * 0 ( l 9 5 p ) ; J . w . E d w a r d s a n d

a. L . K l n s B t o n ( 5 3 - 2 S l ' K . ) , T r o n B . P a r a d a y S o c 5 3 , 1 3 1 J - 2 2 [ 1 9 6 2 ) ; E . H . R o d l s l n o o n d K . 2. G o r . c l ' a k l l , { 1 0 0 -

9 0 0 ' K . ) , Z h u r . P l z . K h l m . 5 2 . 1 6 5 3 - 6 2 ; 1 9 5 8 ) ; B . E . W a l k e r , J . A . G r a n d , a n d R. H . M i l l e r ( a O O - 9 0 0 ' K . ) , j . P h y a .

C h e n - oO, 2 5 1 - J ( 1 9 5 6 ) ; B . D f t h o o n , E . 3 - H r o c k e t t , a n d T . E . E r o c k e i t ( 7 O 0 - 1 4 0 0 ' K . ) . j . P n y a . C h e n . 6 7 . i S f i J

; : 9 6 3 ) , a n d L . T e r e b c B l ( 0 - l 3 0 0 ' j ( . ) , H e l v . C n i n . A c t a . i2> ( l W 4 ) . A l l o f t h e ftfcove l o w t c E p e r a t u p e a o t a

a r e i n ôod flgrococnt w l t n P u r u k o w a ' a w o r k -

The n e a t c o p a c l t l c a a b o v e : 2 C 0 * K . . 1 Z O O - . S O O ' K . ) w e r e t : i k e n f r o n t h e c n i n o l p y n o o n n r e n c n t o o f P- B . K o r . t o r ,

L . 3 . U : a r e v a , V . V . Ka 'ndyba , a n d E . H . P o a l c h o v , U k r . P l z . 2 n . 7, 2 0 5 - 1 0 ( 1 9 6 2 ) . T h n h e a t c a p o c l t y v a l u e s

a b o v e 2 5 0 0 * K . w o r e e x t r a p o l a t e d . H l j h : e n p e p o t u r e m e a a u r e n e n t o w e r e a l a o r .adc by t h e f o l l o w i n g i n v e a t l E y n o r a :

V . Y a . C h c W i o v a k o l ( 5 O O - 2 0 O 0 " K . ) . I n i h - P l z . Zn., A k o d . H o u k . B e l o r u a a k SSR 5 , 6 2 - 5 ( 1 9 6 2 ) ; A- P i . - r . - l e r and H .

O l c t v e C l l O O - 2 i O O * K . ) , C o n p t . R e n d - 2bi^, 4 2 5 5 - 9 5 ( 1 9 6 2 ) ; C . H . S n o x o t e and B . P . N a y l o r ( 5 0 0 - 1 8 0 0 ' h ' . ) , J . Arc.

C h e o . S o c . 6 7 , 72 ( 1 9 4 5 ) and H . L J o h n a t o n a n d M . H o c h ( 1 0 0 0 - 2 0 D O * K . ) , J . P h y o . C h c o . 6 5 . 1 1 8 4 - 5 ( l ? 6 1 ) ; onO

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3100 3 700 9300 3400 3500

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1 9 5 0 , J - B e l r e r , L . 0 . S f l v c d o f f a n d H . L - J o h n a t o n , O h i o S t a t e U t i i v , R a o . P o u n d - P r o ; . 5 1 6 , r e p o r t n o - 6 , 1 9 6 5 .

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Deferences

2 . i , * , 5 . 6 , 7 . 9 . 9 , 1 0

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Tiie d i r e c t c o e b l n a t l c n veluea o f Sot. i-Rle.- . ter ( l i ) and von V a r t e n t e r g - Kanlsch ( J 4 ) a re 0 -71 leaa n e g a t i v e i r j n t h a t Of R a e B l n l ; nowever, a l l o f the o t h e r eet.ioda f a v o r the aore n e g a t i v e v a l u e , "nie Roth - R l c h t e r v a l u t aay t e a f f e c t e d by e r r o r a I n tne • e a s u r e i e n t o f --ne ex t en t o f r e a c t i o n by HCl t l i r i t u n ( 9 , 2 2 ) . . i n c e a set o f c j i p e r l i e n t a taaeJ cn d e t e n t i i n a t i e n o f the arsDunt o f Hg gave a va lue o f -ZZ.Zl K E B I . « o l e ' ^ .

R o a e l n l ' e va lue la c o n f l m e d t > B v a r i e t y o f independent ne thoda. The a o l u t l c n c e l o r i a e t r y ( 3 ) . incup/ i i t l a r e l e t e d t t i rouEi i a coxplea r e a c t i o n acnese, la taaes cn e rec l ea wf i l c f i i r e n o - q u i t e w e l l k n o - n . E . « . f . e e a s u r e M n t s ( I S , 1(1,17.19, I J .CO) g i v e ap* 2gg_,2 - - 2 2 . J i i ; aea .c l r .c an u n . - e r t a ' n i y o f 1.0 BW. m E*. t h l o leads t o aH^ - -22 .02 • 0 . 0 1

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The e q u i l i b r i a o f Lewis ( G 2 5 - £ 9 r * K l g i v e nX, when ccablned w i t h the a t a t l a t l c a l en t r . -p les l aw) and -22 .10 * 0.04 ( m i n J l e k ) .

1 . P. D. R o a e l n l , J . Res. K . B . S . . 1 9 1 ? ) . 2 . D- 0- vagaan, p r i v a t e coanuTilca11.-.-,, x a t i a n a l r u r e e u o f Ctandarda. June 26 i . U. H. Jonnaon, J . B . Ant roae , J . Ses. . ' i .P.S. ^ : A , 42 / ( 1 6 6 3 ) .

F-eknian, P . D. Roaa in t , I M J . , J , S-JT i l a l T ) . i l f a n j . 17, 1917 ( i 9 f l i ) .

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J\Cr (Free energy of formation of reactions considered on Paget70)

Reaction 2 3 4 5 6

0 4 ^ 3 . 1 5 2 + 9 6 ^ 7 2 2 + 3 6 ^ 8 0 9 - 1 2 3 . 8 5 2

2 9 8 - ( ; ' 4 . 6 2 9 > 7 1 . 2 6 9 + 7 8 . 5 1 8 + 1 7 . 6 5 3 - 1 2 0 . 4 4 4

A D O + 6 6 0 6 9 9 + 7 1 . 3 6 6 + 1 0 o 9 3 6 - 1 1 9 . 1 7 1

6 0 0 + 5 7 . 7 6 6 + 5 8 . 1 2 5 - 2 . O 6 9 - 1 1 6 . 7 3 3

8 0 0 " 6 5 o l 7 6 + 4 8 » 9 3 6 + 4 4 . 8 1 1 » 1 4 . 3 4 5 - 1 1 4 . 3 5 4

:.ooo - 6 5 . 5 6 2 + A O o 2 2 6 + 3 1 - 6 9 6 - 2 7 . 2 9 9 - 1 1 2 . 0 1 7

1 2 0 0 - 6 5 o 6 8 2 + 3 I . 6 Z f 4 + I 8 . 7 0 7 - 3 9 . 5 6 0 - 1 0 9 . 7 1 7

moo - 6 5 0 8 6 2 + 2 3 . 2 5 9 + 5 . 9 8 0 - 5 1 . 6 1 5 - 1 0 7 . 4 5 2

1 6 0 0 - 6 6 . 0 7 7 + 1 4 - . 8 0 1 - 6 . 6 1 6 - 6 3 . 4 7 3 - 1 0 5 . 2 2 7

1 8 0 0 - 6 6 . 3 1 9 + 6 . 5 5 5 - 1 9 . 0 4 4 - 7 5 . 1 2 0

2 0 0 0 - 6 6 . 5 7 3 - 1 . 6 3 4 - 3 1 . 3 0 9 - 8 6 . 6 9 3

2 2 0 0 - 6 6 . 8 2 5 - 9 . 7 1 5 - 4 3 . 4 1 2

2 4 0 0 - 6 7 . 1 0 7 - 1 7 . 7 1 0 - 5 5 . 3 8 3

235.

AH (Heat of fonnation of reactions considered on fê lTO)

Reaction 2 3 4 5 6

0 - 6 5 o 4 3 3 +83.152 + 9 6 . 7 2 2 +36.739 - 1 2 3 . 9 5 2

2 9 8 - 6 4 » 9 6 0 ^ . 5 7 5 +98.640 + 3 7 . 4 3 1 - 1 2 4 . 2 0 0

4 0 0 - 6 4 . 5 7 7 + 8 4 . 6 5 2 + 9 8 0 7 1 5 + 3 7 o 0 8 7 - 1 2 4 . 1 2 9

6 0 C - 6 3 . 7 7 2 + 8 4 . 4 0 . 7 + 9 8 . 4 2 7 +36.547 - 1 2 3 . 9 4 5

8 0 0 - 6 3 . 1 1 0 + 3 4 . 0 0 8 + 9 7 ^ 5 9 2 + 3 5 . 6 3 5 - 1 2 . 7 7 9

1 0 0 0 - 6 5 0 1 7 4 + 8 3 . 4 4 9 +96.851 + 3 4 . 5 6 0 - 1 2 3 . 6 0 0

1 2 0 0 - 6 4 . 7 5 2 + 8 2 . 8 2 2 - 3 5 . 9 0 3 + 3 3 . 3 6 6 - 1 2 ; . 4 1 5

1 4 0 0 - 6 4 . 4 5 1 + 8 2 . 1 0 7 - r 9 4 « 7 6 8 + 3 2 . 0 6 9 - 1 2 3 . 1 7 1

1 6 0 0 - 6 4 . 2 3 6 + 8 1 . 3 2 4 + 9 3 . 4 7 3 + 3 0 . 7 0 3 - 1 2 3 . 3 7 4

1 8 0 0 - 6 4 . 0 8 6 + 8 0 . 4 8 6 + 9 2 . 0 7 1 + 2 9 . 2 8 9

2 0 0 0 - 6 3 . 9 8 3 + 7 9 . 5 9 9 + 9 0 o 5 7 1 + 2 7 0 8 3 8

2 2 0 0 - 6 3 . 9 0 8 +780673 + 8 3 . 9 7 6

2 4 0 0 - 6 3 c 8 5 3 - ^ 7 7 . 7 0 9 + 8 7 0 2 4 1

23.

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