The absorption of deuterium by cerium-containing binary alloys

Journal of Nuclear Materials 73 (1978) SO-57 0 North-Holland Publishing Company

THE ABSORPTION OF DEUTERIUM BY CERIUM-CONTAINING BINARY ALLOYS * Dean H.W. CARSTENS Los Alamos Scientific Laboratory, University of California, Los Alamos, New Mexico 87544, USA Received 29 June 1977

A study of the absorption of deuterium gas by a series of binary alloys of cerium, CeJCo, Ceg.3Co, Cee.sNi, and Ce8.5Fe, has been made. Measurements of equilibrium pressures versus concentration were taken at 400 and 500°C for each and in addition the variation of pressure over the range 400-650°C was determined at concentration of D/Cc = 1 and D/Cc = 2 for all systems. The results can be semiquantitatively explained with a model based on compound formation. Heats of reaction of deuterium with the various alloys are: -51.9, -54.2, -55.9, and -64.3 kcal/mole D2 for Ceg,$o, Cea_SFe, Ce$o, and Ce,+sNi, respectively. The heat of formation of CeD2 was measured: -45.6 kcal/mole D2. The alloys are evaluated as tritium-getters for possible applications to CTR technology. Une Etude de I’absorption du deuterium gazeux par une &rie d’alliages de cerium; Ce$o, Ce&o, Ce4,sNi et CeB,SFe a BtBeffectuke. Des mesures des pressions d’bquilibre en fonction de la concentration ont CtBr6alisBes i 400 et 500°C pour chaque alliage et en outre la variation de la pression sur I’intervalle de 400 B 650°C a BtBdBtermi&e aux concentrations de D/Cc = 1 et D/Cc = 2 pour tous les syst&mes. Les resultats peuvent dtre expliqu6s semiquantitativement par un modile bask sur la formation de compos8s. Les chaleurs de reaction du deuterium avec les diffhrents alliages sont -51,9, -54,2, -55,9 et -64,3 kcal/mole D2 pour Ceg,3Co, Ce8,sFe, Ce3Co et Ceq,:Ni respectivement. La chaleur de formation de CeD2 a BtB mesuree 6gale B -45,6 kcal/mole D2. Les alliages ont BtBBvalues comme getters du tritium en vue des applications possibles i la technologie dcs reactcurs CTR. Die AbsorptionvonDeuteriumgas durch mehrere binire Legierungen, wie Ce$o, Ceg,$o, Ce4,sNi und Ceg,gFe, wurde untersucht. Der Gleichgewichtsdruck wurde in AbhCngigkeit von der Konzentration bei 400 und 500°C gemessen, ferner wurde die Anderung des Druckes zwischen 400 und 650°C bei einer Konzentration von D/Cc = 1 und D/Cc = 2 fiir alle Systeme bestimmt. Die Ergebnisse kiinnen halbquantitativ mit einem auf einer Verbindungsbitdung beruhenden Model1 erkltit werden. Die Enthalpien der Reaktion des Deuteriums mit den Legierungen Ceg,3Co, Ceg,gFe, Ce3Co und Ceq+Ni betragen -51,9, -54,2, -55,9 bzw. -&,3 kcal/mols. Die Bildungsenthalpie von CeD2 wurde zu -45,6 kcal/mol bestimmt. Die Legierungen werden als Tritiumgetter fiir mijgliche Anwendungen ftir die Fusionsreaktortechnologie bewertet.

[ 1] and several possible binary melts have been found [2]. Most of these contain either yttrium or a lanthanide metal as the major, hydrogen absorbing component coupled with about 10% by weight iron, nickel, or cobalt. Because of their relatively low melting points and the low equilibrium pressure of hydrogen over cerium, eutectic melts containing this metal seemed promising and a study of the hydrogen-absorption capabilities of a number of such species was undertaken. Measurements were made on four cerium-containing species, Ce3Co, Ce,.,Co, Ce4.5Ni, and Cea.5Fe, having approximate melting points [3] of 470,435,470, and 68O”C, respectively. Deuterium rather than hydrogen

1. Introduction In many of the conceptual plans for controlled thermonuclear reactors (CTR) a liquid-lithium blanket has been proposed as a means of cooling the reactor and as a breeder of the required tritium fuel. The problem of removing tritium from lithium and keeping it at the necessary low levels is difficult and a number of techniques have been proposed. One of the more promising proposals is the use of eutectic melts containing hydrogen-getter metals as extractive liquids l

Work completed under the auspices of the US Energy Research and Development Administration. 50

D.H. W. Gzrstens/Absorption of deuterium by cerium-containing binary alloys

was used for these initial experiments because of its closer similarity to tritium. For comparison and consistent data between the metal and the alloys, experiments were also done with cerium itself. Two types of experiments were done on all five systems. First the measurements of equilibrium pressure versus composition (Sieverts’-type experiments) were made at temperatures of 400 and 500°C for all. Note that, except for the Fe case, the former temperature is below the melting point of the alloy while the latter is above. Secondly, measurements of equilibrium pressure versus temperature over the range 400-650°C were made at two compositions, D/Cc = 1 and D/Cc = 2, for all five.

2. Experimental details The samples were prepared by arc melting weighed mixtures of the required metals. The cerium used had a stated purity of 99.9%, the other metals, 98-99%. The deuterium was 99.5% pure (impurities 1 ppm Oa, 24 ppm Na, balance H,) and was used without further purification. The alloys were fairly readily oxidized and over several weeks completely reacted when exposed to air. Therefore, they were stored in an inert-

/I

__

Cu- SEALED

SAhfPLE

FLANGE

FURNACE

RESISTANCE

TC.

Tb

Fig. 1. Experimental apparatus.

HEATER

51

gas dry box or under vacuum as much as possible. Furthermore, small particles or powders were not used in the experiments; instead large chunks (3-5 mm dia.) were used since these appeared to maintain their purity longer. The fused mixtures were kept in the massive state as long as possible and only broken up as needed. In all samples used for measurements the contaminant layer was judged to be very thin (<0.05 mm). The gas-measuring apparatus, depicted in fig. 1, consists of a main manifold (volume 25 cc) and two vessels having volumes of 0.1 and 1.O1. The pressures are measured using either a 1-torr or lOOO-torr Barroccl capacitance manometer (Datametrics Inc., Wilmington, Mass.). The latter head is calibrated against a Texas Instrument (Digital Systems Division, Houston, Texas) precision quartz-spiral manometer at several points. The volumes of the various parts of the apparatus are determined using PV measurements and a calibrated standard volume. Lines from the main manifold lead to a vacuum system and the deuterium cylinder. The entire measuring apparatus is enclosed in a thermostated cabinet maintained at 40 + 1°C. The temperature of the apparatus is determined using a thermocouple (TC in fig. 1) connected to the small volume. Except for Kel-F elastomers in the valves, the entire apparatus is constructed of metal. The samples are heated inside a furnace constructed of type 304 stainless steel which connects to the manifold via a copper-gasketed high vacuum flange. Two thermocouples, one leading to a controller, the other to a digital thermocouple indicator (Doric Scientific, San Diego, Calif.) sit in holes in the furnace body. The furnace is heated with a resistance heater and, to reduce deuterium absorption, its inside surfaces up to the sealing knife edge are gold coated. The samples (2-4 g each) are contained in small stainless steel cups inside the furnace. The blank absorption arising from these cups was negligible. Sieverts’-type experiments were carried out in the usual way [4] : samples were weighed out, placed in the apparatus then outgassed under vacuum at the experimental temperature. Measured amounts of deuterium gas were then successively added and following each addition the system was allowed to equilibrate and the equilibrium pressure measured. From this data plots of pressure versus deuterium concentration in the metal were constructed. The pressure was recorded

52

D.H. W. Chrstens /Absorption

of deuterium by cerium-containing

against time as an aid in judging the approach to equilibrium. Except for one run with cerium, the Sieverts’ experiments were not carried out for decreasing concentrations. For the measurements of pressure versus temperature at constant composition, samples of the desired concentration were prepared by reacting the metals with measured amounts of gas then allowing them to equilibrate, generally by cycling the temperature. The equilibrium pressures at a number of temperatures were then measured. Data points were taken for both increasing and decreasing temperatures to check for hysteresis effects - none was observed. Once the samples had initially equilibrated, the pressure rapidly became constant following temperature changes and these experiments proceeded rapidly. Because of the finite volumes of the manifold and reactor, at pressures above about 100 torr the composition in the melt changed significantly as the deuterium was expelled. These higher data points were corrected to constant composition using interpolations based on the plots from the previous experiments. As the temperature was decreased and the equilibrium pressures became smaller a point would be reached at which the measured pressures would go negative. This was believed to be caused by the gettering by the metal mixtures; in effect they became better pumps than the pumping system employed to produce the reference vacuum. Thus, using this apparatus reliable measurements could only be made down to about 10V4 torr.

binary alloys

to equilibrium, the third phase, was then very slow and several hours were needed before the pressure stopped changing. This final approach could be speeded up by cycling the sample, for example between 400 and 600°C two or three times. Once equilibrium had been attained, the pressure responded immediately to changes in temperature. In these respects the cerium materials behaved similarly to most other lanthanide metals on hydriding. The melts did not appear to react with the stainless steel cups used to contain them at the temperatures of this study. Occasionally the cups showed slight, colored films presumably due to reactions with impurities in the gas but there was never evidence of attack by the metals themselves. Surprisingly, the samples tended to keep their shapes even after melting and hydriding. This was believed to be due to the oxide films present tending to increase the surface tension preventing the coalescence of the various pieces. Undoubtedly this fiim helped prevent the reaction between the melts and the cups also. The absorption data for the reaction of deuterium with cerium has been previously reported [5] and the absorption curves are similar to those for other rare earth metals [4]. As the concentration of D2 in the metal increases on initial absorption, the pressure rises onto a plateau region of constant pressure which occurs because of the presence of the binary Ce-CeDa

800

t

400°C ABSORBING DESORBING

* A

Ce

i

I

r

i

Ii

3. Results Although equilibrium absorption rather than kinetic data was the objective of this study, some discussion of reaction rates is appropriate. The behavior of all the materials studied was similar, each generally going through three distinct phases upon reacting with the deuterium gas at 400-500°C. The first phase was an incubation period of very slow absorption occurring on the initial application of the gas. This phase never lasted for more than a few minutes and for fresh clean samples was absent. This was followed by an extremely rapid reaction; generally without about ten minutes 99% (or for the samples used here about 0.005 moles D2) of the applied gas was absorbed. The final approach

D/Cc-

&

Fig. 2. Equilibrium pressure vs. D/Cc ratio for Ce and ceriumcontaining alloys at 400 and 500°C.

D.H. W. Carstens /Absorption

53

of deuterium by cerium-containing binary alloys

1000 (Ce D,),Co 100

100

~ D/ce=2.0

IO

IO

I

I

-c 8 5 a

T 8 a

.

0.1

0.1

. 0.01

0.01

‘. . D/Cc= 1.0

0.00

0.001

2 0 000

I

2.0

I

1.5

/

I

1.0

I/T x IO3

0.000

2.0 I.

I.5

IO

Fig. 3. Variation with temperature of deuterium equilibrium pressure over Ce metal at different D/Cc ratios.

I/T x IO3 Fig. 4. Variation with temperature of deuterium equilibrium pressure over Ce&o at different D/Cc ratios.

system. Following this, on passing the composition CeDa the equilibrium pressure rapidly begins to rise again because of the return to a one-phase system and the decreasing solubility of Dz in the dihydride. For cerium metal the plateau pressure is relatively low, being below 1 torr at 7OO”C, for example [5]. The measurements of the variation of equilibrium pressure for the cerium-deuterium system were repeated and the plots of pressure versus D/Cc at 400 and 500°C are shown in fig. 2. These plots are in good agreement with the previous work considering the increased absorption at the lower temperatures used here. The absorption plots for the four cerium mixtures are also depicted in fig. 2. Each is similar to the analogous curve for the pure metal although the alloys absorb smaller amounts of deuterium. This absorption decreases down the series Ce, Cea.sFe, Ces.aCo, CesCo, Cede5Ni. At these temperatures the equilibrium pressures in the plateau regions in all the five systems were below lo-* torr. In one experiment, Ce at 4OO”C, pressure measurements were made for the metal while both absorbing and desorbing the gas. The two sets of data, as can be

seen in fig. 2, fall on the same curve; thus, there is no discernible hysteresis for this system and, presumably, for the others. The results of the measurements of equilibrium pressure versus temperature at various constant concentrations with Ce metal are shown in fig. 3, where the natural logarithm of the equilibrium pressure versus the reciprocal of the absolute temperature (ln P vs. l/T) is plotted for values of the D/Cc ratio = 1,2, 2.2, and 2.3. Similar curves for CesCo for D/Cc = 1 and 2 are given in fig. 4. This set of curves is typical of those found for the remaining four systems. For these experiments reasonably good data could be obtained with this apparatus down to about 10e4 torr, but considerable scatter is observed above 100 torr in all cases. This arises from the relatively inaccurate technique used in the interpolative method needed to return all the data to constant concentration throughout a given experiment. As the pressure rose this necessitated a correction of almost a factor of 2 in the pressure, limiting the accuracy of the interpolation, For all pressure vs. temperature experiments the general technique was to begin at the highest tempera-

D.H. W. Carstens / Absorption ofdeuterium

54 Table 1 lnP= -A/T+

by cerium-containing

binary alloys

B

Species

A (X 1O-3)

r2

B

AHF (kcal/mole D2) Calc.

Exp.

CeDI .O (Ref. 4)

(CeDI.l)$o (CeDI.ddo

(CeDr.cLr.sNi (CeDr.c)s.sFe CeD2.o CeD2.2 CeDz.3 (CeD2.c)@ (CeDz.c)s.@ tCeDa.eL+.sNi (CeDa.o)s.sFe

23.0 t 23.309 28.1 + 26.1 f 32.3 f 21.3 r 10.3 6.4 5.1 8.8 9.7 8.4 10.4

1.1 f 0.014 1.5 1.9 1.8 1.9

22.5 t 23.498 29.3 f 26.7 f 34.3 + 27.3 t 15.5 13.8 12.9 16.5 17.5 17.4 14.9

1.3 f 0.014 1.8 2.2 2.1 2.2

ture and take measurements every SO’C as the temperature was dropped over the range covered. Then additional measurements were taken on increasing the temperature, these points lying between the original decreasing set. Thus, roughly half the data points fall in either set. In all six systems the two sets of points appeared to fall on the same line, and again no evidence of hysteresis was observed. In table 1 the data is summarized for all five systems. Here the least-squares values of the two constantsA and B, obtained from fits to the equation In P(torr) = A/T +B, are tabulated. As a means of judging the data for the experimental curves not presented here, values of Y’ the coefficient of determination (a leastsquares goodness-of-fit parameter with 0 Q r2 G 1, r2 = 1 for a perfect fit) are also tabulated for each case. In general these values indicate that the fits were much better for the D/Cc = 1 experiments and r2 decreases as the concentration increases. Again this is apparently a result of the interpolative treatment of the data. The errors shown in table 1 are the probable errors. In two cases (Ces.sFe and Ces.eCo) X-ray powder patterns were run on the deuterided samples. The major phase in all cases was CeD, . Very prolonged Xray patterns run on the latter showed additional weak lines but these could not be correlated with patterns of known Ce-Co alloys or any reasonable impurity.

0.992 0.998 0.983 0.996 0.999 0.990 0.986 0.900 0.991 0.998 0.993 0.998 -___

45.6 + 46.32 55.9 f 51.9 t 64.3 t 54.2 f

2.6

45.6

3.0 2.8 3.6 3.8

50.6 48.4 52.9 47.1

4. Discussion The phase diagrams of the Ce-Co, Ce-Ni, and Ce-Fe systems have all been previously reported ([3, 71 and references, therein) although some of the data is of dubious accuracy because of the use of impure Ce [3]. The Ce-Co system has recently been restudied and the position and melting points of the Ce-Ce3Co (MP = 435°C) and CesCo-CeCo (MP = 445°C) eutectics seem correct; however, the compound reported to be CesCo (MP = 470°C) may be actually CegCo4. Based on X-ray analysis the next compound in this system is CeCo2 [6]. The compound previously identified as Ce3Ni (MP = 485°C) is known to be Ce,Ni3 [9] but the reported [7] positions of the two associated eutectics have not been substantiated. The existence of CeNi has been established [lo]. The Ce-Fe system differs from the others in that it does not show an analogous compound near 70-75% Ce and in addition the phase diagram is moved to high temperatures. A eutectic (MP = 640°C) is seen at 89.5% Fe. CeFea appears to be the first confirmed compound of the Ce-rich side [3]. The decreased absorption of deuterium by the alloys compared to the pure Ce metal will now be discussed in terms of a semi-empirical, semi-quantitative model. Within an alloy at a temperature near the

55

D.H. W. Carstens /Absorption of deuterium by cerium-containing binary alloys

melting point at which diffusion and rearrangement of the crystal lattice could occur, one would expect the reaction Ce,M +xDa =xCeDa + M ,

(1)

to go to completion if the deuteride were more stable than the intermetallic. In this case one would expect an absorption capacity of the alloy equal to that of the metal alone. As the temperature of the solid were lowered, however, the stability of the intermetallic would increase and the rearrangement could not occur tending to tie up some of the Ce metal and prevent it from reacting; the net result being the reaction would be represented by the equation (written for 1 mole of D2)

5

Ce,M t D2 = CeD2 t &

CeM, 9

(2)

where in the cases discussed here n = 1 for Ce-Fe and Ce-Co and n = 2 for Ce-Ni. It is of course possible that the resulting alloy would then react further to form a ternary deuteride but this would undoubtedly have an equilibrium pressure far higher than 1 atm at these tem~ratures. As an example of this equ~ibrium pressures of several atmospheres at room temperature have been noted for ternary hydrides of ThNi, ThNia , Th,Nia, ThCo, and Th&os [l 11. The equilibrium pressure of thorium dihydride is far less than 1 atm indicating these ternary hydrides are less stable with respect to the hydride and are prevented from decomposing only by the lack of diffusion and the stability of the intermetallic lattice. Heating of any of these ternary alloys to higher tem~rature should lead to a reaction analogous to eq. (I), unless the intermetallic alloy was exceptionally stable. To illustrate this model we will discuss the process of deuteriding a specific example, Ce4.sNi, at a point above the melting point of the melt. As deuterium is absorbed by the melt in the initial stages CeDa begins to precipitate out. Adding more deuterium continually removes Ce from the melt and its concentration moves to the left of the phase diagram until the liquidus is reached (about 63% Ce) and CeNi beings to precipitate also. At this point the system should become invariant due to the presence of three phases Ce-Ni(l), CeNi(s), and CeDz(s) and a pressure plateau should result. When all the metal had been converted to CeNi or CeDa , if rearrangement were not possible the pressure

would rapidly begin to rise as in a typical hydride. If partial diffusion were possible the CeNi would begin to decompose to more CeD2 (increasing the absorption capacity) and lower binary Ce-Ni alloys would form. The process would stop when a stable (or metastable) Ce-Ni alloy was attained by the system. Because at 4OO’Cthe Ce-Ni system (and Ce-Co likewise) are close to the melting point and relatively “soft” we would expect similar behavior at this lower temperature. Indeed, this was seen in the experiments which indicated little difference between either system at 400 or 500°C. Within the framework of this model the fraction of Ce metal available for absorbing deuterium is (x - l/n)/x where n = 1 for Ni and 2 for the Fe and Co systems. In table 2 the calculated values are compared with the experimental values, where the latter were determined by comparing the position of the pressure rise in the alloys with that of pure Ce. The data at each temperature was used to calculate the fraction. In this table the division between the absorbing Ce and the remaining stable compound is also given for all cases. The agreement between the two sets of data is probably as good as can be expected for this model and qualitatively it does predict the right direction in the trend of the data. In all cases the alloys absorb more deuterium than predicted by this division suggesting for most of the examples that partial diffusion occurred following the formation of the first stable alloy. For the Ce-Fe case another mechanism seems probable. Here the measurements were taken considerably below the melting point and it is conceivable that rearrangement could not take place during any part of the experimental deuteriding. Also, the samples were not given an annealing step. Thus, most of the Ce in the sample could have been available for deuteriding

Table 2 Alloy

Division

Fraction of absorbing Ce Predicted

Experimental

CejCo

2.5 Ce + ?$eCoz

0.83

0.89

Ces.3Co

4.8 Ce + $eCoz

0.91

0.92

Ceq.sNi

3.5 Ce + CeNi

0.78

0.86

Ceg.sFe

8 Ce + iCeFez

0.94

0.99

56

D.H. W. Carstens /Absorption

of deuterium

during the experiments as the solid could not rearrange itself by diffusion, In table 1 we have tabulated the experimental heat of reaction 2, AH = AR (where A is the least squares constant and R is the gas constant). Eq. (2) can be rewritten as the sum of three equations: Cc(s) + D*(g) = CeDa(s) ,

--&

Ce,M(I)

=s --&

Cc(s)+ 2

M(s)=A

M(s), (3b) CeM,(s) .

(3~)

by cerium-containing

binary alloys

value should influence AH to a greater extent. (4) Finally, it is empirically known that heats of formation of a series of binary alloys having a common metal vary approximately as the differences of the oxidation potentials. Since the oxidation potentials of Fe, Co, and Ni are similar one would predict AHF to be roughly the same in all cases considered here. The value could differ in the Ni case because of the different stoichiometry and binding in the solid, but to a first approximation it should be equal to the others. Thus, to sum up, since AHM and the contribution due to the heat of fusion should be small, we can approximate eq. (4) by

It follows then that the heat of reaction can be expressed as the sum

where ALit-, is the approximate heat of formation of CeDz from a mole of deuterium gas and a mole of Ce metal, AH~ is the heat of mixing of x moles of Ce with one mole of the metal M, and LVIF is the heat of formation of one mole of solid CeM,. Except for the heat of formation of the dideuteride, none of the heats has been measured experimentally; however, a few generalities can be drawn, all gathered from considerations of similar alloys [ 121. (1) In the case where the deuteride is being formed from the alloy in the solid rather than the liquid state, the heat of mixing would be that for the solid mixture rather than the liquid. The two should be both small; typical values of heats of mixing are less than 5 kcal/ mole for 50 : 50 alloys and approach zero for pure metals and the small change on going from the solid to the liquid mix and the concurrent change in the heat of mixing should have little effect on the total heat of reaction 2. (2) The heat of fusion of Ce,M should be roughly equal to that of Ce itself which in turn is about equal to 1 kcal/mole. This is much smaller than the values we are considering and its influence on AH should be small. Therefore the change in slope of the In P vs. l/T curve should be negligible on passing through the melting point, as was noted in the experiments. (3) Contrary to these values, the heat of formation of the CeM, should be relatively large because of the ordered nature of the solid intermetallic. Thus, this

where AHF has the same value for all alloys studied in either (or both) the solid and the liquid state. An upper limit to the heat of formation of the inter. metallic is probably -25 kcal/mole. Using this value and eq. (5) we have calculated the predicted values of AH and these are shown in the last column of table 1. As can be seen this approximation nicely reproduces the trends in the experimental values but it fails quantitatively. To adequately reproduce the data with eq. (5) we would have to assume AHF = -60 kcal/mole, a value which appears unreasonable. A more realistic approach would be to allow AHI, to vary from case to case. It is known that impurities present in a metal can affect the dissociation pressures and the heats of formations of hydrides of transition metals. VH, ,for example, is very sensitive to this [8]. It would not be too surprising then if AHF varied from alloy to alloy and this effect could account for many of the discrepancies in the comparison between the experimental values and those calculated from eq. (5).

5. Evaluation

of the alloys as getters

In considering the various alloys as getters in practical fusion reactors a number of criteria must be considered. (1) Since the getters would be separating tritium from the blanket at its operating temperature, the equilibrium pressure over the getter should be the

D.H. W. Carstens /Absorption of deuterium by cerium-containing binary alloys Table 3 Species

Ce

Ce

Ce$o %..3Co

Ce4.SNi Ceg.sFe

P (torr) at PC 500 (X 10-4)

750

1000

I

1.02 6.3 3.3 15 1.9

a4 1400 490 7600 350

9 9 6 3

lowest attainable. In most blanket designs this temperature is in the SOO-700°C range. (2) A higher temperature would be needed to decompose the resulting tritide in the regeneration step, and a high value of AH is desirable. (3) The alloy must be chemically compatible with lithium and structural materials. In table 3 we have calculated equilibrium pressures at three temperatures of practical interest, using the data in table 1. Note that since the getters would be operating at lower concentrations than those used in this study, these tabulated values (the predicetd plateau equilibrium pressures) represent upper limits only. Because of the alloys’ similarities one would expect the pressures to follow these directions at lower concentrations also. For a blanket operating at 500°C or lower there is little to choose between the various getters (excluding Ces.sFe because of its higher melting point) since the three alloys all have similar equilibrium pressures. On the basis of its high AH, Ce4.5Ni would be the best choice. For blankets operating above about 65O”C, Cea.s Fe would become the obvious choice because of

51

its lowest equilibrium pressure. As the temperatures increase and pressures rise these criteria becomes more important. In all cases chemical compatibility could make the final decision.

Acknowledgement The author would like to thank C. Radosevich for preparing the arc-melted samples.

References R.A. Krakowski, R.L. Ribe, T.A. Coultas and A.J. Hatch, eds., US-AEC (ERDA) Joint Report, LA-5336, ANL8019 (1974) p.84. J.L. Anderson, D.H.W. Carstens and R.M. Alire, Proceedings of the International Conference on Radiation Effects and Tritium Technology for Fusion Reactors, Gatlinburg, Tenn. (1975). ‘I R.P. Elliott, Constitution of Binary Alloys, First Supplement, (McGraw-Hill Book Co., Inc., New York, 1965). ,I W.M. Mueller et al., Metal Hydrides (Academic Press, New York, 1968). W.L. Korst and J.C. Warf, lnorg. Chem. 5 (1966) 1719. i K. Nassau, L.V. Cherry and WE. Wallace, J. Phys. Chem. Solids 16 (1960) 123. [7] M. Hansen, Constitution of Binary Alloys (McGraw-Hill, Book Co., Inc., New York, 1958). [8] J.J. Reilly and R.H. Wiswall, Inorg. Chem. 11 (1972) 1691; 9 (1970) 1678. [9] R.B. Roof, Jr. et al., Acta Cryst. 14 (1961) 1084. [lo] J.J. Finney and A. Rosenzweig, Acta Cryst. 14 (1961) 69. [ 111 K.H.J. Buschow, H.H. Van Ma1 and A-A. Miedema, J. Less-Common Metals 42 (1975) 163. [ 121 C. Wagner, Thermodynamics of Alloys (Addison-Wesley Publishing Co., Inc. Reading, Mass., 1952).