The Ceto getter—its chemical structure and hydrogen gettering properties

The Ceto Getter--its Chemical Structure and Hydrogen Gettering Properties J. H. N. VAN V U C H T

Phihps Research Laboratories, N. V. Phdips' Gloeilampenfabrteken, Eindhoven, Netherlands Ceto, which is an alloy o f thorium, mischmetall and aluminium & used as a non-evaporating getter. In the course o f an elaborate investigation o f the ternary system Th-Ce-AI, it has been proved that its crystallographic structure is o f the CuAI2 type, just like the closely related binary compound Th2AI. The properttes o f the ThzAI, Ceto and intermediate compounds with regard to hydrogen were studied. X-ray and neutron diffraction, supplemented by nuclear magnetic resonance measurements, each at liquid nitrogen temperature, and room temperature, gave information about the structure o f the reaction products. Hydrogen equdibrium-pressure measurements at temperatures from 250°C up to 700°C gave thermodynamic information and enabled to calculate the equilibrium pressures at lower temperatures. A kinetic study, based on a simple model, o f an autocatalyttc sorption is reported.

Introduction

played by each constituent, could be u n d e r s t o o d o n the g r o u n d s of Its chemical structure.

F o r some time the n o n - e v a p o r a t i n g getter Ceto 1 has been the subject of a n e l a b o r a t e f u n d a m e n t a l investigation in o u r laboratories. This getter h a s excellent properties a n d it was h o p e d t h a t some of them, as well as the role which was

The structure of Ceto T h e getter consists of a q u a t e r n a r y alloy. It Is m a d e by

.*fce~O

ThA~

"-----e~---2~~"/- 7 Th2AI+CeAI +THAI

.

CeAt

!l,t.,. I

Th;AI ~ _ Th2ll.t.ctCe 3 1 7 ~

~ pCe3AI

5~

(~21 ThuA?+OtCe3Al+fc c (Th.Ce]

Th Ce FIG. I. Ternary system Th-Ce-AI Tentative diagram at 500°C. For the range of stab]hty of and flCe3Al see van Vucht 1960 (to be pubhshed). 170

171

The Ceto Getter--its Chemical Structure and Hydrogen Gettering Properties sintering a mixture of thorium powder and a powdered alloy called Ceral at 900°C in vacuum. This Ceral consists of CermischmetaU (80 per cent cerium, 20 per cent lanthanum) and aluminium, and has the chemical composition (Ce,La)AI2. The total getter composition (in atoms) is about

./"

!

.....

r-

10 Th, 2.5 Ce, 0.5 La, 6 A1. A possible determination of the chemical structure meant the examination of the quaternary system Th--Ce-La-AI. It proved, however, a great simplification that a getter with exactly the same properties could be obtained if the mischmetall was replaced by pure cerium. So the quaternary system could be reduced to a ternary one : Th--Ce-A1.

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With the help of X-ray analysis and metallographical and thermo-analytical methods it was possible to determine most of the phases in this system. The results with respect to the binary systems have been published before and were confirmed by the work of others 2-7. The total result is, briefly, given by Fig. 1. Here the Ceto composition has got the number 1. It is seen that its structure, practically for 95 per cent, is isomorphous with that of Th2A1, the neighbouring binary compound. Th2AI is tetragonal with a = 7.616A and c = 5.861/~. So is Ceto with a = 7.606 .~ and c = 5.860 ~ . The structure of both compounds is of the CuA12-type. The only difference between the two atomic structures is that the thorium atoms in Th2A1 are replaced by a very probably random mixture of thorium and cerium atoms (in a ratio of about 10 : 3).

~

0 0

~ The system T h ~ A I - H 2 ; introduction Most of the work in connexion with the hydrogen gettering properties was done on the binary compound ThzA1, as it is the simpler and better defined one. A t several points, however, the influence of substituting thorium by increasing amounts of cerium has been checked. It was found that the behaviour of the getter did not change essentially by this substitution. Therefore in this report our communications will be confined to results of the system Th2A1-H2. They concern the reaction products of hydrogen with the metal, the conduct of hydrogen in the metal and finally the reaction from the kinetic point of view. The techniques used in our work shall not be described extensively here. A fully detailed report will appear elsewhere m the near future. The atomic structure of Th~AI As is commonly known hydrogen often forms interstitial solutions or compounds with metals. Therefore it will be worthwhile to study the packing of atoms in the Th2A1 structure. Fig. 2 shows, next to a basal plane projection, the configuration of the thorium atoms in perspective. F r o m this picture it IS easily seen that the places where the hydrogen atoms are most likely to be located can be divided in two kinds of tetrahedral holes. Per unit cell, that is per ThsAl4 there are sixteen sites of one kind (crystallographicaUy equivalent). They are located in tetrahedra that are coupled two and two together by joining their base (" double holes ").

j)

Thonz=O

,41 on z=+I/~

Basalplane

FIG. 2. Crystal structure of Th2AI

Their position is (x, 3 + x , 4-z), ( 3 + x , x, 4-z), (x, 3 -- x, 4- z) (3 -- x, x, 4- z) (3 + x, x, 3 4- z), (x, 3 - - x, ½ 4- z), ( 3 - x,~c, ½ ! z ) , ( ~ , 3 + x, ½ 4- z). Furthermore we notice 4 tetrahedral holes 4- (3, 0, ¼) and 4- (0, 3, ¼)With the thoriums in their proper positions (Fig. 2 is somewhat idealized), they are rather narrow, compared to the sixteen holes first mentioned.

Hydrogen equilibrium pressure isotherms One of the techniques used for determining which compounds are formed and how stable the reaction products are is the technique, already applied by Sieverts, of measuring equilibrium pressure isotherms. In our case a very simple apparatus was used with a volume of ca 1.5 1. In this volume the hydrogen equilibrium pressure was measured with the help of a Pirani manometer above 200 mg of finely crushed (grain size < 35/~)ThEAI powder. The isotherms obtained are shown m Fig. 3. There are several remarkable points in this figure. First of all we find as an ultimate H content of the metal a number very near to sixteen H atoms per unit cell. Near to this value the equilibrium pressures rise very rapidly to values that exceed one atmosphere. Secondly the isotherms have no plateau. This is an indication t h a t - at these temperatures--no two phase region exists. On the

172

J.H.N.

VAN VUCHT

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FIG.

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3.

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Isotherms~oftheequthbriumpressureofhydrogenaboveTh2AI.

contrary, up to an H content of ca. 4 per unit cell the behaviour of hydrogen in the metal must be very ideal for the pressure isotherms follow the law CH = K ~/PH2

where C H is the hydrogen concentration in the getter metal. For the 400°C isotherm this is shown in Fig. 4. The existence, however, of the point of inflexion in the isotherms at the concentration of about 4H/unit cell might be an indication that at lower temperatures a two phase region occurs. A similar point ofinflexion is seen at the composition ThsAI4H12. The set of isotherms can give us thermodynamical data as well. In Fig. 5 In p has been plotted versus 1 / T for three different H concentrations. The slopes of the lines give the heat of solution of one mole H2 into the metal (at constant H content). Apparently the heat of solution increases with hydrogen content. Again this is an indication that at lower temperatures a two-phase region may appear. Furthermore it is possible from these lines to determine the equilibrium hydrogen pressure (in the thermodynamical sense). Thus an extrapolation to room temperature of the line for Th8AI4H4 leads to an equilibrium pressure of 10-13 Torr. This pressure

will never be reached however, mainly because of inhibiting effects of preferentially adsorbed surface-deteriorating gases. 9~

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H o~cell FIG. 4. The 400°C isotherm on a log-log scale.

The Ceto Getter--its Chemical Structure and Hydrogen Gettering Properties

P~IH~III

kgcoVmol Q=28kgcal/mol 30kgcal/mol

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Q=

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--/-/

,

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25

/

9,0

I

1.5

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05

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FIG. 5. Eqmhbrium pressures (logarithmic) versus 1000/T m the system Th2AI-H2

X-ray investigations Powder specimens of Th2AI, loaded with increasing amounts of hydrogen were investigated with X-rays. The specimens weighed 500 mg and were carefully homogenized after the hydrogen was sorbed. Then they were protected

173

by coating them with a solution of acetyl cellulose in dried acetone, without exposing them to air. After that the powder was taken out of the sorption apparatus and prepared in the normal way for room temperature X-ray diffraction on a Philips Diffractometer. The diagrams obtained were quite satisfactory and could easily be indexed. Most of them showed one phase. Only between the concentrations H0 and H6 two phases were found, isostructural (with respect to Th and A1) but differing in lattice parameters. In this two-phase region an increase of the H content of the specimen resulted only in a diminution of the H0-phase lines and an amplification of the H6-phase lines. The results of the X-ray work are compiled in Fig. 6. Obviously one of the two-phase regions, which were presumed on the grounds of the isotherm measurements, does exist at room temp. The critical temperature must lie between room temperature and 250°C. The point of inflexion in the curves at 1 t 4 (Fig. 3) above this temperature thus proves to be a reliable indication. The second remarkable point in Fig. 6 is the anomality in the distortion of the tetragonal unit cell in the a direction, occurring exactly at the composition Hs. It is intriguing that exactly the half-filled cell should have the longest a-axis. One might think of an ordering of H atoms. Certainly it is a new valuable indication for the existence of a second phase, having the ideal composition ThsA14H8 and showing a widening region of homogeneity at rising temperatures.

Neutron diffraction experiments

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x (H at/eeP, I Fro. 6. Cell dimensions as a function o f hydrogen content o f Th2AI at

room temperature.

Since with X-ray diffraction it is impossible to locate exactly the hydrogen atoms in our metal powders, neutron diffraction w~,s used in an attempt to solve this problem. Because hydrogen scatters neutrons incoherently we made specimens containing deuterium. The diffraction measurements were carried out in Kjeller (Norway) at the J.E.E.P. reactor in co-operation with J. A. Goedkoop, B. Loopstra and J. Bergsma. Before long a detailed report of this work will be published, but in the meantime a short survey of the results is given here. The powder specimens used weighed about 25 grams and were of the compositions ThsA14Ds, Th8AI4D12 and ThsA14D16, lying each in the single-phase region at room terr,'erature. The diagrams obtained are shown in Fig. 8. For comparison the X-ray diagrams for the corresponding H compositions are given in Fig. 7 It is seen clearly that in the X-ray diagrams the increase in hydrogen only effects a line shift due to the lattice expansion. The intensities of the reflections are unchanged. In the neutron diffraction diagrams, however, there is not only a line shift but (more important) a big change in intensity, due to the scattering power of the deuterium atoms. These observed intensities agreed fairly well with the calculated ones for a random distribution of the deuterons over the sixteen positions in the "coupled tetrah e d r a " (m ThsA14D16 all positions are occupied). On the one hand this result concurred with our experience that sixteen was the maximum number of hydrogen atoms sorbed per cell. On the other hand a random aistribution of protons in the half-filled interstitial lattice could not explain the behaviour of the a dimension of the unit cell.

J. H. N. VAN VUCHr

174

However there are a few types of partial order that are not distinguishable from total randomness by diffraction, e.g. the situation that each double hole contains one proton, but this proton has equal chances to be in the upper or the lower part, or the situation that doubly filled " d o u b l e holes " and totally

diluted with fine dry quartz powder, thus isolating the metal grains from each other. At room temperature the resonance lines of the compositions ThsAlaH8 and ThsAI4Hi2 were very narrow. This may be explained by a " m o t i o n a l narrowing" effect. 2500

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FIG. 7. X-ray diagrams of Th2AI and three different hydrogen containmg compositions. empty " double h o l e s " are mixed at random. These partial order types give only rise to weak effects in the diffuse background. Moreover a short range order is not detectable in this way as well. In order to make a possible order grow more distract a diffraction diagram of ThsA14D8 at --190°C was taken. This diagram differed only in minor details from the one taken at room temperature, and left no room for configurations otherwise than random with the restrictions mentioned before.

Nuclear magnetic resonance More information we tried to get from nuclear magnetic resonance measurements. This work was done in co-operation with D. Kroon and C. v. d. StolpeS. Because of the expected skin effect our powder specimens of about 10 grams were

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=- 20P) FIG. 8. Neutron-diffraction dmgrams of Th2AI and the corresponding deuterium containing compositions of Fig. 7. The protons are moving from interstice to interstice in a time short compared to the characteristic time for the spin-spin magnetic interaction in this substance. By reducing the number of " interstitial vacancies ", as is done by saturation of the metal with hydrogen in Th8AI4HI6, or with a mixture of equal amounts of hydrogen and deuterium in ThsAI4H8Ds, we are able to restrain the translations. Consequently a broadened resonance line is observed at room temperature for these compositions The measured second moments $2 (the mean square widths) of these lines agreed well with the calculated ones, if we used the positions found by neutron diffraction for ThsAI4D16. The second moment of ThaA14HsD8 agreed with the assumption of a random mixture of protons and deuterons. Going to low temperatures the mobility of protons can be reduced even in partially filled lattices. This is shown in Fig. 9 for ThsAI4H8. A similar behaviour was found for

The Ceto Getter--its Chemical Structure and Hydrogen Gettering Properties 70

9

~ A~ HO 9

T'

AH{G) 8

6 5 1 3 2 ! I

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150

200

250

300

.~ TPK)

I!

FIG. 9. Nuclear magnetic resonance. Line width zJH and second moment $2 versus temperature for ThsAI4H 8.

However, there was a difference. In the case of ThaAI4Hs the high-temperature narrow line gradually broadened when the temperature was lowered, whereas in the case of ThsAI4HI 2 the intensity of the narrow line decreased, while at the same time a broad line appeared (see Fig. 10).

175

diffraction, but we did not find a new two-phase region. The resonance line of ThsA14Hs started to broaden at 250°K (Fig. 9). Below 230°K the second moment of the line changed slowly from 5 Gauss2 to about 8 Gauss2 at 170°K. Below 170°K the lines showed a remarkable flat top. This line shape is characteristic for resonance of a twoproton system or may be for a four-proton system. Anyhow this should mean that, at HO°K and lower some kind of order is present. The value of $2 does not agree with a configuration where " d o u b l e h o l e s " are doubly filled. So all what can be said is that the nuclear magnetic resonance results for ThsA14H8 and ThaAI4H12 at lower temperatures are not in accordance with the conclusions drawn from low temperature diffraction work on ThsAI~I)8 with neutrons and on Th8A14H12 with X-rays. There is, however, not necessarily a contradiction between them, for we have to bear in mind that the proton resonance method only gwes short-range information, whereas diffraction work typically gives longrange information. In fact we might see this as a demonstration of how the two methods are supplementary to each other.

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Besides the work, which substantially concerned static problems, kinetic experiments were done. An interesting phenomenon was the autocatalytic sorption of hydrogen. The crushed Th2A1 as such generally is inactive at room temperature. This is quite naturally, for, unless very strong precautions have been taken, the surface of the getter particles will be covered by a protecting layer of e.g. the very stable oxides. The getter can be activated by heating it in vacuo at about 900°C. After subsequent cooling, a 200 mg powder specimen sorbs more than 20 Torr litre hydrogen within half a minute at room temperature. Th~s activity can be destroyed very easily by deteriorating the surface with " poisoning" gases, as there are oxygen, water vapour, CO2, CO and nitrogen. If an activated specimen of Th2A1 is deliberately de-

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Kinetics of the reaction T h z A I - H 2 ; autocatalysis

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I 250

T 300

350

O8

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= 7(o/<) 1. 2.

FIG. 10. Nuclear magnetic resonance for ThsAI4H12. Line w~dth zJH versus temperature. Ratio of the integrated areas of the narrow (An) and broad (Ab) hnes versus temperature.

Apparently ThaA14H12 was segregating at this temperature, splitting up in two phases, one with the broad line and one with the slowly widening line. These might be the phases ThaA14H8 and ThaAI4H16 , which is a reasonable supposition in view of the observed points of inflexion in the isotherms. We tried to confirm this by low-temperature (--190 °C) X-ray

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60

80

I~

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t fmm) FIG. 11. Autocatalytic sorptzon of hydrogen by Th2A1. in a constant volume versus time.

H2-pressure

176

J.H.N.

activated by exposing it to 5 mm of oxygen at room temperature during 10 rain, the sorption rate of hydrogen (after removing the oxygen) has become immeasurably small. After about 30 min, however, a slow sorption sets in, its rate growing until about half of the gas is sorbed. Then it slows down again. This behaviour is shown in Fig. 11. Here Pt/2 On a constant volume) is plotted against time. A new amount of hydrogen admitted is sorbed much quicker. This can be repeated several times. This phenomenon might be explained in the following way. The deliberately caused inactwity is due to a surface layer of oxide. The layer is very thin (of the order of a few monolayers). The hydrogen can only pass through it very slowly (whether because of imperfections as microcracks, or because of the atomic structure). If some hydrogen gets through, however, the underlying metal expands (in our case anisotropically). The protecting skin is stretched until cracks arise. Then the hydrogen is able to pour through these cracks, causing the metal to expand at a greater rate and thus the skin to produce more failures. In this way a self-accelerating sorption results. The reaction has to slow down m the end by lack of hydrogen, which indicates a certain pressure dependence. On the basis of this model we tried to analyse the series of sorption curves of Fig. 11. In our picture

VAN VUCHT

dp --

dt

=

f((o).f'(p)

where co is the active surface area, which is a function of the amount of hydrogen already sorbed at time t. We can take -dp/dt for constant pressure values in a series of sorption curves such as in Fig. 11 and plot them against the amount of hydrogen taken up already. Thus we get a bundle o f lines, coming from one point on the --alp~dr axis (Fig. 12). By looking in this figure at a constant hydrogen concentration in the metal and plotting the -alp~dr values versus the corresponding p values on a log-log scale (Fig. 13) we find that the hydrogen sorption rate is linearly dependent on the pressure above the getter. This result is not surprising and makes the model used more acceptable. I00

5o

,1o

~ I 2

I 5

power I

355 ° power 07

r tO --

T

1,5 °

7 90 .~

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I

I00

7og p

FIG. 13. Autocatalytlc sorptJon of hydrogen by Th2AI. --dp/dt

versus pressure at c/co~ ~ 3.6 per cent.

Conclusion

o

l

2

3

FlG. 12. Autocatalytic sorpt~on of hydrogen by Th2AI. --dp/dt

versus c/c~, at various pressure values.

5

Summarizing our results concerning the behaviour o f hydrogen m Th2A1 we can state the following. A t temperatures of 300 °C and higher and at concentrations up to 4H/unit cell the dissolved hydrogen atoms behave ideally. Yet there exists a slight attraction between them, which results in a two phase region at room temperature between ThaA14Ho and ThsAI4Hs. There are indications that ThsAI4Hs is a separate compound next to ThsA14H16, the maximum hydrogen containing composition. At room temperature where protons move very rapidly in the lattice they form an uninterrupted series of solid solutions. At lower temperatures nuclear magnetic resonance experiments give reason to believe that on a very small scale two phases are mixed. Moreover they give arguments for an ordering of H atoms in the interstitial lattice to pairs or quartets at lower temperature. This could not be confirmed with neutron diffraction on deuterium compounds. We are trying now to complete this picture by measuring the specific heats of the various compositions, going from

The Ceto G e t t e r - - i t s Chemical Structure and H y d r o g e n Gettering Properties liquid nitrogen temperature to room temperature, and by m e a s u r i n g t h e i r p a r a m a g n e t i c susceptibility. T h e results o f t h e s e i n v e s t i g a t i o n s will b e r e p o r t e d m d u e t i m e .

Acknowledgement I w a n t t o e x p r e s s m y t h a n k s t o all w h o h a v e c o - o p e r a t e d in this w o r k a n d especially t o m y a s s i s t a n t s P. Colijn, J. Pegels a n d J. M . B o g a a r d .

177

References 1 W. Espe, M. ICJloll, and M. P. Wilder ; Electronics 23, 80 (1950). 2 p. B. Braun and J. H. N. van Vucht ; Acta Cryst. 8, 117 (1955). 3 p. B. Braun and J. H. N. van Vucht ; Acta Cryst. 8, 246 (1955). 4 j. R. Murray ; J. Inst. Metals 84, 92-96 (1955). s j. H. N. van Vucht ; Z. Metallk. 48, 253-258 (1957). 6 j. H. N. van Vucht ; Philips Res. Rep. 12, 351-354 (1957). 7 R. T. Wemer, W. E. Freeth, and G. V. Raynor ; J. Inst. Metals 86, 185-188 (1957). 8 D. J. Kroon, C. v. d. Stolpe, and J. H. N. van Vucht ; Archives des Sciences, Gen~ve 12, fasc. sp6c., 156-160 (19S9).