The Utilization of Thermogalvanic Cells for Studying the Freezing of Electrolytic Solutions and Ion Migration in Ice

VoL 112, No. 6 C O M P O S I T I O N O F C O M P L E X E S B Y C A L O R I M E T R Y 621

Penneman and Jones (6) obtained similar values, kl = 1.5 X 10 -5 and k2 -- 5.7 X 10 -3 , for 25~ by ob- serving the inf rared absorption spectra of the compounds in order to establish their concentrat ion in solution. Rothbaum (7) obtained the fo l lowing values for 20~ by measurement of the electrode potentials of copper in cyanide solutions

kl = 1 X 10 -~ to 5 X 10 -5 k2 ~ 5 X 10 -2

Of these methods, the measurement of electrode potentials is the least satisfactory, as there is doubt that the potential is revers ib le and that it actual ly repre - sents the assumed reaction. The method of Permeman and Jones requi red that various complex compounds be prepared so that their absorption spectra could be obtained. The detection of the C u ( C N ) 2 - ion was not feasible because of its low re la t ive absorption co- efficient, and the concentrat ion of this ion and also of the cyanide ion had to be obtained by difference. In comparison, the method of increments did not requi re the isolation of any of the complexes.

Acknowledgment The author wishes to acknowledge the assistance of

J. M. Sherfey who did the labora tory work on the calor imetr ic measurements of the cuprocyanide system, and the interest and support of Fielding Ogburn in the calculations. The author also wishes to thank the Division of Research, Chemist ry Branch, Atomic Energy Commission, for their continuing support of the

project on electrochemical ca lor imetry of which project this research is a byproduct.

Manuscript received Aug. 17, 1964; revised manuscript received Nov. 9, 1964.

Any discussion of this paper wil l appear in a Dis- cussion Section to be published in the December 1965 JOURNAL.

REFERENCES 1. F. J. C. Rossotti and H. Rossotti, "The Determina-

t ion of Stabi l i ty Constants and other Equi l ibr ium Constants in Solution," McGraw-Hi l l Book Co., New York (1961); H. L. Schl~ifer, "Complex Format ion in Solution. Methods for the Deter - minat ion of the Composition and Stabil i ty Con- stants of Dissolved Complex Compounds," Springer, Ber l in (1961).

2. K. B. Yatsimirskii and V. P. Vasil 'ev, "Instabi l i ty Constants of Complex Compounds," Pergamon Press, Oxford, New York (1960); Stabil i ty Con- stants. Par t II, Inorganic Ligands, J. Bjer rum, G. Schwarzenbach, L. G. Sillen. Special Publ ica- tion No. 7, The Chemical Society, Burl ington House, London W.1.

3a. P. Job, Ann. Ch~m., [10] 9, 113 (1928) ; Compt. rend., 180, 928 (1925).

3b. Ann Chim., [11] 6, 97 (1936). 4. J. M. Sher fey and A. Brenner , This Journal, 105,

660 (1958). 5. W. C. Vosburgh and G. R. Cooper, J. Am. Chem.

Soc., 63, 437 (1941). 6. R. A. Penneman and L. H. Jones, J. Chem. Phys.,

24, 293 (1956). 7. H. P. Rothbaum, This Journal, 104, 682 (1957).

The Utilization of Thermogalvanic Cells for Studying the Freezing of Electrolytic Solutions and Ion Migration in Ice

Minas Ensanian Physical Chemistry Group, Bell Aerosystems Company, Bufjalo, New York

ABSTRACT

Ion t ransport during the freezing of electrolytic solutions to dry ice temperatures has been fol lowed by means of a thermogalvanic cell. The low- t empera tu re l imit of an operat ing cell depends p r imar i ly on ion size and polarizabili ty. Unexpla ined low- tempera tu re changes in electrode polar i ty as wel l as spontaneous outbursts of energy have been observed. The technique offers a method for s tudying the ion-reject ion process at an advancing ice- l iquid interface as wel l as ion migrat ion through ice.

With respect to the demineral izat ion of saline wate r by the freezing method it is known that an ion- re jec - tion process (1, 2) is in operat ion at the advancing sol id/ l iquid interface. Al though a number of studies have appeared in re la ted areas such as the electr ical conduct ivi ty of ice, etc. (3-20) the l i te ra ture wi th re - spect to this problem is severe ly l imited and the mech- anisms and phenomenon poorly understood.

This communicat ion summarizes an explora tory ex- aminat ion of beryl l ium, aluminum, iron, copper, magnesium, zinc, tin, and lead thermogalvanic cells in which the electrolyte surrounding one of the electrodes is gradual ly frozen. The principle object ive was to as- certain the feasibi l i ty of using a thermoeel l as a tool for the invest igat ion of ion migra t ion dur ing the f reezing oI elecrro~yrlc solutions and subsequen~iy in the resul t ing doped ice.

In spite of difficulties that are inherent in the study of any nonequi l ibr ium system the exper imenta l results, a l though prel iminary, are never theless of such a nature as to strongly demonstra te the potent ia l of the the rmo- cell approach.

Experimental Procedure The thermogalvanic cells consisted simply of a ve ry

large Pyrex U- tube which contained about 400 ml of

electrolyte and at whose ends were two identical electrodes par t ia l ly immersed in the electrolyte. The metal electrodes approximate ly (1/64 x 1/2 x 3 in.) were made f rom avai lable sheet mater ia ls and cleaned by s tandard procedures. In one case the electrode was placed against a thin t ipped glass tube containing a ch romel -a lumel thermocouple and posit ioned in the center of the arm with a min imum distance sl ightly more than ~/4 in. f rom the glass wall.

The other electrode which remained at ambient t empera ture was at tached in a s imilar m a n n e r to a m e r - cury thermometer . Tempera tu re measurements were made at the tips of the electrodes which were 34 in. apart.

Al l emf's were measured by L&N null and the rmo- couple po~en~iomemrs and precuu~iuzL~ w ~ ~ak~L~ ~, insure against metal l ic thermocouple effects.

The cold arm of the U- tube was surrounded by a polyethylene jacket which extended about V2 in. above and 2�89 in. below the level of the electrode and which was kept f irmly packed with powdered dry ice. The electrolytes were 0.1M solutions of reagent grade be ry l - lium, aluminum, i ron (ic), copper (ic), magnes ium and zinc sulfates except for tin (ous) chloride and lead nitrate. In the case of the t in salt a wel l mixed 400 ml suspension of an equivalent amount of mater ia l was

) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 134.129.182.74Downloaded on 2014-11-19 to IP

http://ecsdl.org/site/terms_use

622 J O U R N A L O F T H E E L E C T R O C H E M I C A L S O C I E T Y J u n e 1965

used al though a large par t of the suspension sett led at the bottom of the U-tube, no such set t l ing was observed with the nonacidified iron salt which appeared to be in solution. No acids were added to these solutions which along with the electrodes were newly made for each experiment .

An exper iment would commence on packing the jacket (about 30 rain after insert ion of the electrodes) wi th about 2500g of powdered dry ice. Due to factors such as dry ice evaporat ion and recrystal l izat ion r e - packing was necessary during the course of an exper i - ment, and since it was impor tant to s tandardize the freezing rate the problem was minimized by a slow continuous packing and it was found that the v ib ra - t ional effects in so doing had no effect on the potentials. Minor tempera ture variat ions as a resul t of this pure ly mechanical (packing) factor are not serious unless they occur at wha t may be t e rmed "cut-off" (To) or " transi t ion tempera tures" (TD and al though difficult to reproduce may be defined as follows. (To) denotes that t empera tu re at which the output potential and cur ren t become zero, i.e., the thermocel l is no longer operat ive so long as the cold electrode is at that t e m - pera ture or lower. The la t ter te rm (TD refers to that t empera tu re at which a polari ty change is observed, i.e., when both the potential and current drop to zero and then rise again; however , in so doing the polar i ty of the cold and hot electrodes of the cell have changed with respect to their original connections wi th the posit ive and negat ive posts of the potent iometer .

Experimental Results Some exper imenta l results are shown, and in Tables

I and II the values represent the average of th ree experiments .

Dur ing a 4-hr period the ambient tempera tures (T2) never var ied more than --+_2~ The polar i ty for the cold and hot arms of the cell refers to the posit ive and

Table I. Observed polarity, cut-off, and transition temperatures for the systems studied

E l e c t r o d e R a d i i o f ( T o ) ( T t ) P o l a r i t y F u n c t i o n s y s t e m i o n , A ~ ~ C o l d H o t o f t h e c o l d

e l e c t , e l e c t , e I e e t r o d e

B e r y l l i u m 0 . 3 4 -- 7 0 -- 3 8 + -- C a t h o d e - - 6 0

( 2 + ) A l u m i n u m 0,57 -- 63 + -- Ca thode

( 3 + ) I r o n 0 . 6 7 -- 6 0 - - 41 + -- C a t h o d e

0 . 8 3 - - 6 0 ( 3 + ) ( 2 + )

C o p p e r 0 . 7 0 -- 4 5 -- 3 3 - - + A n o d e - - 4 5

( 2 + ) M a g n e s i u m 0.78 -- 22 -- + A n o d e

( 2 + ) Zinc 0.83 -- 9 -- + Anode

(2+) t i n 1 . 12 - - 70 -- + A n o d e

( 2 + ) L e a d 1 .20 -- 2 9 - - 7 - - + A n o d e

( 2 + )

I o n i c r a d i i a r e t a k e n f r o m G o l d s c b m i d t ( 2 1 ) . A m b i e n t t e m p e r - a t u r e s w e r e g e n e r a l l y 2 5 ~ B e , F e , a n d C u s y s t e m s d i s p l a y e d t w o l o w - t e m p e r a t u r e p o l a r i t y t r a n s i t i o n s .

Table II. Magnitudes of potentials before and after freezing

M a x i m u m H i g h e s t o b s e r v e d p a t e n - o b s e r v e d paten-

Electrode t i a l b e f o r e t i a l a f t e r s y s t e m f r e e z i n g , m v T e m p , ~ f r e e z i n g , m v T e m p , ~

negat ive terminals of the potent iometer pr ior to any low tempera tu re transitions where such occurred. At times, as the electrodes were placed in the solution, one or two polar i ty changes would occur, and al though the solutions were not made acidic, these were probably due to invisible gas films or local cell effects on the meta l surfaces, since exper iments conducted, e.g., with magnes ium in very weak acid solutions, displayed polar i ty changes (as a resul t of some gas evolution) which were almost oscil latory in na ture while the t empera ture differential mainta ined between the electrodes was negligible.

However , the polar i ty would immedia te ly stabilize after the jacke t was packed with dry ice. For the systems studied the average t ime to f reeze was 19 min and the average freezing t empera tu re was 7 ~

At tempera tures below --30~ both beryl l ium and tin were found to be qui te sensi t ive to minor tempera ture variat ions ( _ 2~ Tin exhibits numerous outbursts of energy if the f reezing operat ion is in ter - rupted, i.e., if the t empera tu re is held constant for several minutes then fol lowed by a 1 ~ or 2 ~ rise. Plots of output vol tage vs. t empera tu re for both metals under these circumstances are almost periodic with pro- nounced dampening in the bery l l ium curve.

For example, if the emf of the tin thermocel l at --34~ is 90 mv, then t empera tu re stabilization will produce a rapid decay and reduce the vol tage to zero. When this is fol lowed by a slight rise in t empera ture several minutes later, the emf suddenly exceeds 0.1v and again decays to zero on t empera tu re stabilization. This has been tested at lower tempera tures and con- firmed in v iew of profound exper imenta l difficulties associated with tempera ture control. However , if f reezing proceeds un in te r rupted (continuous packing of the dry ice is necessary) the cell vol tage is reasonably stabilized approximate ly up to the apparent cut-off temperature .

Under the exper imenta l conditions described here great difficulty was encountered in a t tempt ing to cool the tin cell be low --70~ and it is quite possible that the system wil l continue to operate at considerably lower tempera tures ; the same may be t rue of beryl l ium.

Another in teres t ing factor is that when a system is " thermal ly shocked," i.e. by a sudden removal of the dry ice when the thermocel l is at a low tempera tu re (see Shock Tempera tu re in Table I I I ) , the established zero potent ia l state or the absence of any vol tage may persist f rom 2-25 min, af ter which the voltage suddenly switches back on. Al though there is no strong evidence as yet, there seems an indication that except perhaps for iron and tin (whose durat ion is short) the durat ion of the zero potent ial state is r emarked ly uniform, viz. , around 15 rain. However , a longer period is general ly requi red to at tain the voltages and respect ive t empera - tures shown in Table III. As a result of thermal shock- ing one of two things may occur, viz., the high voItage may persist through the phase transit ion or it may begin to decay wi th in 10 '0 or 20 ~ of the freezing point and finally cut-off near 0~ fol lowed by a polari ty change, a l though producing negl igible voltage.

In this respect the iron thermocel l has on several oc- casions exhibi ted an ex t raord inary behavior in that,

Table III. Thermal shock temperatures and potentials

E l e c t r o d e V o l t a g e , m v T e m p , ~ s y s t e m S h o c k t e m p , ~

Be I00 12 I00 12, A1 275 9.5 375 -- 12 F e 7 12 50 --28 Cu 9 7.5 21 --27 M g 4 0 9 4 0 9 Z n 9 12 13 6 .3 S n 9 8 .5 9 0 - - 3 4

P b 2 9 2 7 -- 23

Be --69 >100 - - I0 A1 -- 78 500 -- 8.5 Fe --74 100 --7 Cu -- 37 13 -- 8 M g -- 30 60 0 Z n --30 14 0 . 5 S n -- 70 14 -- 8.8 P b -- 50 14 -- 1.2



Vol. 112, No. 6 UTILIZATION OF THERMOGALVANIC CELLS 623

under conditions of thermal shock (which induces de- fects within the crystal latt ice and thus facili tates dif- fusion), the polar i ty t ransi t ion has occurred almost at the same tempera tu re as when the heat ing rate was considerably slower, i.e., dur ing the ini t ial cooling. Dur ing thermal -shock exper iments no unusual colora- tions were noticed in ice in the region of the electrode at the t ime the dry ice was removed, except for a dark blue color of short durat ion in an isolated case wi th copper; however , the possibility exists. Likewise it was not possible to de termine the effects of the cracking (noise resul t ing f rom expansion) on the potent ial in the absence of an automatic recording device.

Discussion Neglect ing for the moment any chemical factors as-

sociated with the phenomena of impur i ty accommoda- tion within the ice lattice, one may expect some cor re - lat ion between (To) and the ionic radii as is shown in Table I. The behavior of bery l l ium and tin are to be expected in v iew of beryl l ium's small size and tin's ve ry high polarizabili ty.

The re la t ive ly low cut-off t empera tu re for lead (as compared with magnesium and zinc) is also a reflection of polarization phenomena.

On the assumption that ca t ion-migra t ion through ice is somehow related to the ionic-potent ia l (after Cart- ledge) (22) then (To) should decrease in the same order as the electrode metals are listed, wi th bery l l ium having the lowest cut-off t empera tu re and lead the highest. This would be t rue if migra t ion depended solely on the abili ty of the cation to polarize the sus- ceptible 0 2 - ion and might help to explain the ra ther close proximi ty of beryl l ium, aluminum, and iron in Table I.

One may draw an analogy be tween ice and quartz on the assumption that ice has a quar tz - l ike structure, i.e., by substi tut ing the oxygen atoms for the silicon in quartz al though the hydrogens do not bond the oxy- gens in the same rigid manner. The compara t ive role of ionic potential for r igid and nonrigid oxygen bond- ing may be shown by comparing this work with that of Gibson and Vogel (23) who found that Li + moves much more easily th rough crystal l ine quartz than do Na + and Cu +, whi le Mg 2+, a l though smaller than Na +, scarcely moves at all.

The fact that the cells are opera t ive at low t emper - a ture would seem to indicate the existence of discrete wa te r molecules (24) in the ice.

Again on the basis of ionic potent ia l in this invest iga- tion, copper, magnesium, and zinc fall into the proper order.

When negligible current is d rawn the total vol tage (ED produced by a thermogalvanic cell is given by (d,E/dt) (T2-T1) and therefore, the output should increase with an increase in the t empera tu re differential. However , on the assumption that the freezing process wil l in terfere with ion migra t ion and the format ion of ice (via the ion-re jec t ion mechanism) wil l deplete ion concentration, one would expect a potent ial drop (during the freezing cycle) just pr ior to solidification, but this is not always the case. In fact, the stabil i ty of the vol tage during the phase t ransi t ion (l iquid to solid) is at t imes remarkab le and par t icular ly in the case of bery l l ium and tin the exper imenta l voltages are greater at thc frcczing pcint than thc thcoret ical based 6n the above expression. The magni tudes of the potentials before and after f reezing are shown in Table II.

Wi th respect to hysteresis ( revers ibiI i ty of emf, t empera tu re plots) the reproducibi l i ty of a t ransi t ion t empera tu re (polar i ty change) ordinar i ly could only be expected (during a w a r m - u p period) if the heat ing and freezing rates were equal in v iew of the residual entropy of ice, since the crystal can exist in any one of a large number of configurations, each correspond- ing to certain orientat ions of the water molecules.

In v iew of the scarcity of data t ru ly characterist ic of the systems studied, the results are too complicated for a ready in terpre ta t ion of the mechanism of the observed low- t empera tu re polar i ty transitions.

A polar i ty t ransi t ion denotes a change in the sign of the cur ren t t ranspor t ing species (analogous to the sign of the Hall constant of a semiconductor) and the phenomena may be a reflection of elastic re laxat ion in the crystal l ine mater ia l resul t ing in a molecular re - orientat ion and therefore, in spite of steric factors (ionic radii) the possibility of "anionic conduction" exists.

Quartz and lead iodide, for example, display aniso- t ropy in their conductivity, and in the la t ter the aniso- t ropy is several powers of ten at low tempera tures and is t empera ture dependent.

If the observed transit ions are due to processes at the electrode surface or are interfacial in nature, then they might be associated with the burst ing of surface films or some type of semiconductor phenomena at the in te r - face (meta l /doped ice) i r respect ive of the mul t ip l ic i ty of the low tempera tu re polar i ty transitions, e.g., at --38 ~ and --60~ for beryl l ium.

In an isolated incident a copper thermocell , af ter being at (To) for about 5 rain displayed an outburst of energy exceeding by a factor of 10 any previously re- corded vol tage wi th this system in this investigation.

The origin of the factor responsible for the observed transit ions may possibly be ascertained by the sym- met ry of vo l t age - t empera tu re plots in the transi t ion region, i.e., whether the curves are parabolic. This would entail refined automatic recording equipment as wel l as means for greater t empera tu re control.

The iron and copper curves in the t ransi t ion region, although plotted from limited and preliminary data, appear to exhibi t symmetry, and on this basis one may conclude that the phenomena resides wi thin the ice.

Numerous questions however remain to be answered, viz., the effects of degassing the electrolyte and the use of an iner t gas atmosphere, character izat ion of the metals and their surfaces, changes as a resul t of va ry - ing the anion of the salt, pH, the effects of var ia t ions in the length of the ice below the electrode and the r - mal gradients and concentrat ion gradients.

With respect to these considerations and the fact that the exper imenta l work was carr ied out under conditions where tempera ture regulat ion and the meas- u rement of potent ial and tempera tu re was done man- ually, the exper imenta l er ror may be considerable. Nevertheless, in terms of the desired objective, the value of the approach has been demonstrated. It is possible that, if the f reezing and heat ing rates are extended to periods of several days or longer and wi th a fair degree of t empera tu re control, one may reasonably expect data that wil l lead not only to a be t ter unders tanding of the phenomena involved dur ing the freezing of electrolyt ic solutions, but perhaps to the nature of the ion reject ion as well. There is no reason to bel ieve that wi th some ingenuity, perhaps entai l ing a h igh-pressure electrode, even hydrogen ion migra - tion through ice cannot be invest igated by this technique.

Re turn ing to the problem of the desalination of sea wate r by the freezing process, if the observed outbursts of energy reside in the bulk of the ice, then with regard to the large-scale demineral izat ion of salt w a t e r the remote possibility arises of recover ing some of the spent energy by taking advantage of the the rmoelec- tric propert ies of ice. Af te r all, thunders torm electr ic- i ty (25) is somehow associated with the freezing of water droplets, the rmal gradients, and charge separa- tion.

Conclusion 1. These exper iments indicate that the thermocel l

approach can be developed into a method for s tudying the f reezing of electrolytes and ion migra t ion in ice.



624 JOURNAL OF THE ELECTROCHEMICAL SOCIETY J u n e 1965

2. Thermogalvanic action in dilute electrolytes has been shown to persist near dry ice temperatures, ion size and polarizabil i ty being important .

3. Although the evidence is l imited it appears that low- tempera ture polari ty t ransi t ions are a bu lk ra ther than a surface phenomena.

4. Outbursts of energy at low temperatures may be indicative of cooperative re laxat ion processes in ice.

5. The observed increase in ion migrat ion as the result of a slight temperature rise in the doped ice seems reconcilable with the known fact that water from aged arctic ice is potable, the older ice having experienced two or more w a r m - u p periods.

6. Fu ture refinements of this work must consider the na ture of the electrode surface with regard to nucleat ion and growth of ice crystals.

Acknowledgment The author is grateful to Robert T. Foley for his in -

t roduction to the subject of thermogalvanic potentials. This study has proceeded only because of his complete cooperation and that of Robert E. Salomon and Wil l iam Rogers, Jr. Suggestions from Alfonso R. Gennaro and Andr6 J. de Bethune are grateful ly acknowledged. The author is indebted to C. Roland Eddy for an in - teresting discussion concerning the origin and na tu re of the observed polarity changes and to the Leeds & Northrup Company for the loan of a potentiometer, and lastly, Charles Counts for assistance in the lab- oratory.

The init ial phase of this work was done at Melpar, Incorporated, and Temple Universi ty.

Manuscript received June 2, 1964; revised m a n u - script received Nov. 23, 1964.

Any discussion of this paper will appear in a Dis- cussion Section to be published in the December 1965 JOURNAL.

REFERENCES 1. T. G. Thompson and K. H. Nelson, Sears Founda-

tion J. Marine Research, 13, 2 (1954). 2. J. P. Lodge, M. L. Baker, and J. M. Pierrard, J.

Chem. Phys., 24, 716 (1956). 3. E. J. Workman and S. E. Reynolds, Phys. Rev , 78,

3 (1950). 4. R. S. Bradley, Trans. Faraday Soc., 53, 687 (1957). 5. F. Heinmets, Nature, 188, 925 (1960). 6. E. J. Murphy, Phys. Rev., 79, 203 (1950). 7. N. V. Cohan, M. Cotti, J. V. Ir ibarne, and M. Weiss-

mann, Trans. Faraday Soc., 58, 490 (1962). 8. A. Spernol, Z. Elektrochem., 59, 31 (1955). 9. E. W. B. Gill and G. F. Alfrey, Brit. J. AppI. Phys.

Suppl., 2, S. 16 (1953). 10. L. Onsager and M. Dupuis, "Thermodynamics of

I r revers ible Processes," p. 294, "The Electrical Propert ies of Ice," Rendiconti della Scuola In te r - nazionale di Fisica Enrico Fermi, Corso X. Bologna: Zanichelli (1960).

11. Ad. Ste inmann, Helv. Phys. Acta, 30, 581 (1957). 12. F. Humbel, F. Jona, and P. Scherrer, ibid., 26, 17

(1953). 13. J. C. Decroly, H. Gr~inicher, and C. Jaccard, ibid.,

30, 465 (1957). 14. C. Jaccard, ibid., 32, 89 (1959). 15. M. Eigen, L. De Maeyer, and H. Ch. Spatz, Ber.,

68, 19 (1964). 16. J. D. Dunitz, Nature, 197, 860 (1963). 17. B. Cleaver, E. Rhodes, and A. R. Ubbelohde, Dis-

cussion Faraday Soc., 32, 210 (1961). 18. S. A. Rice, ibid., 32, 181 (1961). 19. A. Klemm, ibid., 32, 203 (1961). 20. A. S. Dworkin, H. R. Bronstein, and M. A. Bredig,

ibid., 32, 188 (1961). 21. V. M. Goldschmidt, Chem. Bet., 6@, 1263 (1927). 22. G. H. Cartledge, J. Am. Chem. Soc., 52, 3076 (1930). 23. G. Gibson and R. C. Vogel, J. Chem. Phys., 18, 1094

(1950). 24. L. Pauling, J. Am. Chem. Soc., 57, 2680 (1935). 25. J. Latham, Nature, 200, 4911 (1963).

Technical Notes @ Improved Techniques for Acid Polishing and Forming Etch Pits

at Dislocations on the (100) Face of Copper Crystals Lawrence D. Dyer

Research Laboratories, General Motors Corporation, Warren, Michigan

The purpose of this note is to report two useful im- provements in the methods of surface preparat ion and characterization of copper single crystals. The first is an addit ion of phosphoric acid to the cupric chloride polishing solution of Young (1). This improvement gives a surface sufficiently smooth to reduce the electropolishing t ime to as little as �89 min. The second is an electrochemical technique for forming excellent dislocation etch pits on the (100) face. The etching is carried out in the same phosphoric acid solution in which the crystal is electropolished, thereby avoiding the possibility of contaminat ion in t ransferr ing the crystal from one vessel to another.

Sample Preparation The test samples were single crystals of copper in

the shape of bars 1/~ x a/4 x 21/~ in. cemented to % in. thick glass slides with a commercial cyanoacrylate

resin. They contained 1.5 to 3 x 104 dislocation si tes/cm 2 on their (100) working surfaces (2). Prior to electropolishing, the orientat ion of the (100) planes of each crystal was determined to wi th in 0.01 ~ relat ive to the actual surface of the sample (3). Then each crystal was polished to this (100) orientat ion in an acid polishing machine (4), and the final smoothing was done electrolytically in a bath of ,60% orthophos- phoric acid.

Acid Polishing Improvement During the ini t ial polishing, it was sometimes nec-

essary to remove a wedge of mater ia l up to 0.10 in. thick in order to obtain a surface of the desired or ien- tation, To accomplish this in a reasonable time, con- centrated hydrochloric acid saturated with CuC12 �9 2I-I20 w'as used. At this stage the surfaces of the crystals were ra ther rough and would require 20-40 min elec-



Documents

The Utilization of Thermogalvanic Cells for Studying the Freezing of Electrolytic Solutions and Ion Migration in Ice