Applications of Surface Science 14 (1982-83) 173-182 173 North-Holland Publishing Company

S U R F A C E S E G R E G A T I O N O F O X Y G E N IN V A N A D I U M

J.-M. W E L T E R and H.-N. W A C H E N D O R F

lnstitut fiir FestkOrperforschung, Kernforschungsanlage Jiilich, D - 5170 Jiilich, Fed. Rep. of Germany

Received 21 July 1982; accepted for publication 30 September 1982

The surface-volume segregation behaviour of oxygen in vanadium was analyzed by measuring the oxygen surface concentration of a thin strip in an ultra-high vacuum environment up to a temperature of 1400 K. The temperature of the specimen was deduced from its ideal resistivity for which the characteristic was first established: PT = 0.0115 + 0.6574 × 10-3T- 0.0943 x 10 6T2 ~J2 m. The surface concentration of the oxygen was determined by secondary ion mass spectrometry using the O- ion. Heating the specimen above about 800 K leads to a dissolution of the surface oxygen into the bulk, cooling below 800 K regenerates an oxygen layer at the surface. The temperature dependence of the oxygen surface concentration could be described with an ideal solution model. For the surface-volume segregation enthalpy the value AH ° = 117 + 4 kJ/mol was obtained.

I. Introduction

Hydrogen is dissolved in large amounts in the Va metals (vanadium,

n iobium and tantalum) and shows a high bulk diffusivity [1,2]. Therefore the

Va metals can be considered as interesting materials for the hydrogen technol-

ogy. For instance, the use of vanadium as a permeat ion membrane or for

storage purposes has been suggested [3,4]. An impor tant handicap of these

metals for such applicat ions is that the surface state must be control led very

carefully in order to obtain high hydrogen uptake or release rates. The reason

is the very effective sealing of the bulk against hydrogenat ion by oxygen-rich

surface layers up to a temperature of approximate ly 750 K. On the other hand

this sealing represents an advantage in many me ta l -hyd rogen experiments

because the hydrided specimen can be heated up to 500 K in vacuum or in air

without loosing hydrogen.

Various arguments, based on experiments per formed on n iobium and

tantalum, have been put forward to explain the rupture of the surface barrier

at higher temperatures. Al though in a very pure hydrogen gas containing less

than 1 ppm H 2 0 the reduct ion of n iobium oxides appears to be possible in the tempera ture range around 700 K [5], this will not work when a gas of

commerc ia l high-puri ty is used. A better explanat ion for the disappearance of

0 3 7 8 - 5 9 6 3 / 8 3 / 0 0 0 0 - 0 0 0 0 / $ 0 3 . 0 0 © 1983 Nor th -Ho l l and

174 J.- M. Welter, H.- N. Wachendorf / Surface segregation of o.\vgen in 1 /

surface oxygen above 700 K, which has been observed by Auger and LEED spectroscopy [6-9] and more indirectly through the kinetics of the meta l -oxygen reactions [10,11], is based on the existence of a surface segrega- tion layer of oxygen which is redissolved by the bulk at high temperatures.

The aim of the present experiment was to check if the surface segregation model holds also for vanadium by following the evolution of oxygen surface layers on polycrystalline strips by secondary ion mass spectrometry (SIMS) in the temperature range of 300 to 1400 K. As electrical resistivity measurements represent a simple method to record the temperature of strip-like specimens

1

l u u

F? Fig. 1. UHV apparatus with manipulator (1), ion gun (2), mass spectrometer (3), electron gun evaporator (4), thickness monitor (5), hydrogen storage container (6).

J.-M. Welter, H.-N. Wachendorf / Surface segregation of o.~vgen in V 175

and to follow in subsequent experiments the uptake of gaseous elements, the characteristic of the ideal resistivity for this temperature range was first determined. Finally, the effectiveness of a palladium coating for the adsorption of hydrogen below 700 K will be pointed out.

2. E x p e r i m e n t a l

A schematic of the ultra-high vacuum apparatus used for this experiment is shown in fig. 1. It contains:

a manipulator (1) with an electrically isolated specimen attachment for long strips; - a SIMS unit consisting of a differentially pumped scanning ion gun (2) and a quadrupole mass spectrometer (3) equipped with an energy filter, a peak selector and an ion counting chain; - an electron gun evaporator (4) and a thickness monitor (5) placed below a cooled hood; - a storage container for super-pure hydrogen (6) based on FeTiH [3].

The tilt movement of the manipulator permits the vanadium strip to be placed either in front of the ion gun or above a conveniently designed aperture in the hood. The dimensions of the specimens were 70 × 3 x 0.025 mms. They were cut from an ultra-high vacuum degassed vanadium foil made from float-zone refined material (MRC, Orangeburg, NY); the major impurities were silicon and oxygen. The ratio of the resistivity at room temperature to that at liquid helium temperature was approximately 20. The temperature of the sample was raised by Joule heating.

Two potential leads and occasionally a P t /P tRh thermocouple were spot- welded on the back-side of the central part of the strip. Further experimental details are given in ref. [13].

3. R e s u l t s and d i s c u s s i o n

3.1. Resistivity characteristic

Only the central 10 mm of the strip were used to establish the resistivity characteristic. The inhomogeneity of the temperature was controlled with a disappearing filament pyrometer. It was less than 4 K at 1365 K and can be neglected. In a first run the thermocouple consisted of wires with a diameter of 0.1 mm. They led to a temperature value which at the highest temperature was 150 K below the reading of the pyrometer. To minimize the perturbation introduced by the thermocouple, wires with a diameter of 0.015 mm were eventually used. The deviations at 1150 and 1400 K were reduced to 25 and 10

176 J. - M. Welter, H.- N. Wachendorf / Surface segregation of oxygen in V

T [°C ] D 0 200 40 600 800 1000 1200

/ of . . . . . . . . . . . . . . . . . . . . . . . .

3od ' ' s ~ ' '78d ' 9 ; 6 ' i1~o ' i~o6 ' isoo T I K ] "

]Fig. 2. Ideal resistivity-temperature characteristic of vanadium.

K, which especially at the lower temperature is within the error range of the pyrometer.

The resistivity characteristic is shown in fig. 2. It was calculated from the measured resistance by using the value of 0.1962 kt~2 m for the ideal resistivity at 293.2 K [12]. An estimate of the geometrical factor from the dimensions of the specimen gave the slightly higher value of 0.2193/~2 m. The characteristic agrees well with the data given by HOrz [14] for the temperature ranges of 283 to 338 K and 1223 to 1773 K, although his claim for a linear relationship at high temperature is only partially acceptable. Indeed, the characteristic can be described extremely well in the considered temperature range by a second-order polynomial with a confidence coefficient of 1 . 00 :Or=0 . 0115 + 0 . 6 5 7 4 x 1 0 - 3 T - 0 . 0 9 4 3 × 10 6T2 in /x~'2 m for 2 8 0 < T < 1400 K. p~ is the ideal resistance as defined by p(measured)= P4.2 + Pr. Like in niobium, the devia- tion from linearity of p~ is here probably also due to multiple scattering of the conduction electrons [15]. The determination of the temperature from resisitiv- ity can be falsified, e.g., by the pick-up of oxygen during the experiment,

I

0.~

0 ~01 i , , , , L , , , . i . . . . i i , , ,

. . . . 5 lo 15 20 £ t [rain]

t "c

~0 O E

? C,

5 ~ o I

1

Fig. 3. Decrease of the oxygen surface concentration on vanadium ( ~ ) after a sudden drop of the

water vapour pressure in the UHV apparatus (©).

J. - M. Welter, H. - N. Wachendorf / Surface segregation of oxygen in V 177

T I.0 F'- 0

0.5

T [°C] 200 L.O0 600 800

• I ' ] ' I ' I , I ' I ' I ' I ' I

• - O - - " O L o ~ 0~0~0~0 ~0

0 I • I , I i I , I , I , I , I , I , I 300 500 700 900 1100

T [ K ] I'-

Fig. 4. Decrease of the oxygen surface concentration on vanadium during heating (static SIMS).

because this would increase P4.2 by app rox ima te ly 0.05 #~2 m per at% oxygen [17]. Therefore P42 must be checked before and after each exper iment .

3,2• S u r f a c e segrega t ion o f o x y g e n

As a p robe for the oxygen in the surface layers the O ion was mainly used [16]. It was assumed that its peak intensi ty is p ropor t iona l to the oxygen concent ra t ion . Its var ia t ions with argon ion b o m b a r d m e n t , changes of the residual a tmosphere in the appa ra tus and heat ing of the sample are p lo t ted in figs. 3 to 5.

At room temperature , the bulk oxygen has a di f fus ion coefficient of the o rde r of 10 27 m2 / s and is a lmost immobi le [17]. In the ul tra-high vacuum chamber the oxygen on the surface is main ly cont ro l led by the energy and the current densi ty of the argon ions and by the par t ia l pressure of the water vapour : its concent ra t ion saturates when the dynamica l equi l ibr ium between

T 1.0

0

~" 0.5 b

T [°C ] 200 /-.00 600 800 1000

O ~ ,~-o,-o :_o _ / . . . . . . . . . . . . . 300 500 700 900 1100 1300

T [ K ] Fig. 5• Evolution of the oxygen surface concentration on vanadium during heating (dynamic SIMS)• The solid line represents the fit of eq• (1) to the experimental data (C)) for T> 950 K.

178 d.- M, Welter. H.- N. Wachendorf / Surface segregation of oAT¢et in V

the sputtering and the sorption processes is reached. This is demonstrated in fig, 3: the oxygen concentration dropped when the pressure of the water vapour was decreased from 10 -~ to 10 m mbar by filling the main baffle with liquid nitrogen and switching on the titanium sublimator. The energy of the argon ions was 3 keV and the current density 0.2 A / m 2. A static SIMS mode (energy of the argon ions 2 keV, current density 10 4 A / m 2) was used to measure under conditions of minimal perturbation of the surface population the evolution of an uncleaned surface during the heating of the sample at a rate of 5 K/rain. An important desorption process occurs in the temperature range of 400 to 500 K (fig. 4). The intensity of the signal was too small to distinguish any structure in the high temperature part of the curve. To study this part, the primary ion current was increased by approximately three orders of magni- tude. A consequence of this heavy bombardment is a complete cleaning of the surface at room temperature. Fig. 5 shows that the oxygen surface concentra- tion remains low until 800 K, increases abruptly within a temperature range of 150 K and decreases slowly again. The same evolution was followed roughly by the intensities of the peaks of the VO + and V2 O+ ions. The measurements were made under the following conditions: energy of the argon ions 2 keV, current density 0.5 A / m 2 and heating rate of the specimen 5 K/min.

These observations can be described by a model which is based both on the tendency of the oxygen atoms dissolved in the bulk to segregate on the surface at low temperatures and on their temperature dependent mobility. Once the segregation layer has been destroyed by sputtering, it will be renewed on'ly at those temperatures where the mobility of the bulk oxygen toward the surface is high enough to compensate for its erosion by the argon ions. The regeneration of the segregation layer by oxygen coming from the bulk appears to start at 800 K where the oxygen exhibits a diffusion coefficient of 2 x 10- t4 m2/s [17]. The equilibrium situation is reached at 950 K and the diffusion coefficient of 4 x 10 ~3 m2,/s is now high enough, so that the removal of surface oxygen by sputtering can be neglected. The thermodynamical equilibrium can be de- scribed by equalizing the chemical potentials/~h and /~ of the oxygen in the bulk and in the surface layer, respectively. The simplest approximation as- sumes an ideal behaviour within the surface population. The chemical potential reads

t~ = ~I ° - T S , ° + R T I n ( O / O o - 0 ) ,

where R is the gas constant. The partial molar enthalpy H ° and excess entropy ~o describe the interaction between an oxygen atom and the substrate which is large in comparison to the interaction between two oxygen atoms. As the surface concentration 0 may be high with respect to the concentration of available sites 00, a blocking term must be considered in the configurational partial molar entropy. This is not necessary for the bulk oxygen because of its low concentration C. It can be estimated to be approximately 1 at%~ which is

J. - M. Welter, H. - N. Wachendorf / Surface segregation of o.!ygen in V 179

well below the concent ra t ion of avai lable sites C 0. This low concent ra t ion al lows also a neglect of the interact ions within the bulk oxygen popula t ion , and the par t ia l molar en tha lpy and excess en t ropy are independen t of C:

--o T~Ob + R T I n ( C / C o ) " /X b = H b -

The re la t ionship between the two concent ra t ions 0 and C reads:

0 o - 0 - Co exp ~ e x p [ ~ T - ) . (1)

--o -- -- - - o _ _ S O A H ° = H b - H ° and J S ° = S b ~ are the par t ia l molar en tha lpy and excess en t ropy of segregation. In the most general case they may depend on C. The quan t i ty A H ° can be deduced from a plot of l n [ 0 / ( 0 o - 0)] versus I / T . This has been done in fig. 6 by using for 0/00 the rat io of the peak intensit ies of the O - ion at t empera ture T and at 970 K, where the secondary ion intensi ty has its max imum value. The da ta show indeed an Arrhen ius behaviour which suppor t s the model of an ideal surface oxygen popula t ion . The slope gives for the par t ia l molar en tha lpy of segregat ion the value A H ° = 116.8 k J / t o o l . The error has been es t imated to be approx ima te ly + 4 k J / t o o l .

Table 1 shows a compi la t ion of exper imenta l values of AH ° for the Va metals when they conta in small amounts of oxygen. The par t ia l molar en tha lpy of segregat ion for vanad ium is larger by 50 to 100% than the ones for t an ta lum and niobium. It should be noted that for these two metals AH ° seems to decrease with increasing oxygen concentra t ion . A s t rong tendency of oxygen

30

20 ~ 1 5 13:)

' 10 .. 8

~ 6

~ 4

T [K] 1400 1300 1200 1100 1000

/ o/O

o

o/ 3 / 2 / o

o o/ 1 , , , i . . . . i . . . . i , , , , i , , , , i , , t i I i i j ,

7 8 9 10 1/T [IO<'K "1]

Fig. 6. Arrhenius plot of the oxygen surface concentration on vanadium versus temperature according to eq. (I).

180 J.-M. Welter, H.-N. Wachendorf / Surface segregation of oxygen #l V

Table 1 Selected values of the partial molar enthalpy of segregation AH ° of oxygen dissolved at low concentration C in the Va metals

Metal AH ° C Method Ref. (k J / tool) (at%)

V 117 0.1 SIMS This work Nb 71 0.1 Auger [6]

68 0.5 Auger [6] 75 0.5 Auger [7] 53 0.5 Auger I7] 60 0.1 Auger [8] 63 0.7 Auger [8] 55 0.5 Kinetics [ 11]

Ta 80 0.1 Auger [7] 62 < 0.1 Auger [9]

100 Kinetics [ 10] 39 0.5 Kinetics [11 ]

dissolved in the Va metals toward surface segregation is to be expected by considering the two principal contributions to the segregation driving force. These are the reduction of the surface free energy and the relief of lattice strain when a solute atom is transferred from the bulk to the surface [18,19]. A feeling for the decrease of the surface free energy can be obtained from a comparison of the values 1.90 and 0.09 J / m 2 for vanadium metal and for vanadium pentoxide, respectively [20,18]. Furthermore, when oxygen is ex- pelled from the metal, the lattice parameter shrinks by 15% per unit concentra- tion. Although various formulae exist which relate for binary metallic alloys the partial molar enthalpy of segregation to more elemental quantities like surface free energies or misfit parameter [18,19], they are of little use in the present situation, because some important parameters are still unknown for systems involving metallic and non-metallic elements.

3.2. Hydrogen uptake

Vanadium dissolves hydrogen exothermically [2]. Doping levels at a given pressure of the hydrogen gas increase with decreasing temperature. But the previous paragraph has shown that up to 750 K the natural surface of vanadium is covered with oxygen. This layer works as a barrier for the uptake of hydrogen, although it is not yet clear whether it influences the adsorption of the hydrogen molecule or its dissociative chemisorption. The segregated layer can be sputtered away at tow temperatures and the kinetics will prevent its reconstruction. To avoid the adsorption of new oxygen rich layers from the gas before or during hydriding, the surface is usually coated with a thin film of a

J. - M. Welter, H. - N. Wachendorf / Surface segregation o f oxygen in V 181

metal which has a high permeabi l i ty for hydrogen, e.g. pal ladium[4,21-23,26] . Then the dissolut ion of hydrogen is governed by its d i f fus ion in the bulk mater ia l as our exper iment shows.

The pressure in the chamber was lowered below 10-~0 mbar. The sample was heated dur ing l h at 1400 K and then sput ter etched. High-pur i ty hydrogen from the hydr ide s torage conta iner was in t roduced in the chamber up to a pressure of 1 bar. N o not iceable up take of hydrogen (less than 1%) was observed in the t empera tu re range up to 750 K. In a second run, after the c leaning procedure , the sample was coated with predegassed pa l lad ium. Dur- ing the depos i t ion of a film with a thickness of 0 .12/zm pa l lad ium, the pressure increased in the chamber up to 6 × 10 10 mbar. A hydr id ing run was made at 410 K in an hydrogen a tmosphere of 1 bar. The up take of hydrogen was fol lowed in situ by the increase of resistivity. The convers ion factor which was used is 0.98/zI2 cm per 1% H [24]. With in seconds, the hydrogen concent ra t ion rose to 23% H and then saturated. The equi l ibr ium concent ra t ion should be 45% H, but the phase d iagram [2] shows that at 410 K the solubi l i ty l imit of the solid solut ion is ob ta ined at 18%. Below the surface, the hydr ide VzH starts to grow and to shield the inside of the specimen, because the mobi l i ty of hydrogen decreases dras t ica l ly when going from the di lute solid solut ion to the hydr ide [25].

Acknowledgements

The suppor t of Professor H. Wenzl for this work and the technical assis- tance of H.J. Bierfeld, K.H. Kla t t and J. Wit t are grateful ly acknowledged. The authors are indeb ted to Professor G. Comsa for cri t ical ly reading the manuscr ip t .

References

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[3] H. Wenzl, J. Less-Common Metals 74 (1980) 351. [4] H. Wenzl, K.-H. Klatt, P. Meuffels and K. Papathanassopoulos, in: Proc. Intern. Symp. on

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182 J. - M. Welter, H. - N. Wachendorf / Surface segregation of o.~vgen in V

[10] D.E. Rosner, H.M. Chung and H.H. Feng, JCN Faraday I, 72 (1976) 859. [11] G. Hgrz, H. Kanbach, R. Klaiss and H. Vetter, in: Proc. 7th Intern. Vacuum Congr. and 3rd

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