Chem. Mater. 1992,4,631-639 the Foundation for Fundamental Material Research (FOM) with financial aid from the Netherlands Organization for Scientific Research (NWO). Registry No. la, 117677-86-8; lb, 139607-59-3;IC,139607-60-6;
631
Id, 139607-61-7; le, 139607-62-8. Supplementary Material Available: Structures of lb and ICand tables of positional and thermal parameters, bond distances and angles (10pages). Ordering information is given on any current masthead page.
Hydrogen-Induced Disproportionation of the Intermetallic Compound ZrCo M. Devillers, M. Sirch, and R.-D. Penzhorn* Kernforschungszentrum Karlsruhe, Institut fur Radiochemie, Postfach 36 40, 7500 Karlsruhe, Germany Received November 14, 1991. Revised Manuscript Received February 7, 1992 Disproportionation of the intermetallic compound ZrCo in the presence of hydrogen into ZrHz and ZrCoz has been observed in the temperature range 350-500 "C during both the absorption and the desorption mode. Quantitative reproportionation into ZrCo can be achieved by vacuum annealing at high temperatures. The resistance of a sample toward degradation was found to be determined by its previous history.
Introduction Z~CO'-~ can be considered as one of the most promising getters to replace uranium (present reference material) for the handling, transport, and storage of tritium in the fuel cycle of future fusion devices! At a typical storage temperature of 25 "C, ZrCo hydride has an equilibrium Pa in the absorption mode, provided pressure of about the amount of stored gas is limited to a plateau region of 0 < %/nm < 2.7 (Q = H, D, or T). The immobilized gas can be recovered at moderate temperatures, e.g., as deduced from isopleth experiments, a Hz equilibrium pressure of 1 bar is attained at only 375 O c a 2 In addition, from the point of view of safety, ZrCo and its hydride have proven to be much less pyrophoric than uranium and its hydride. To complete the data base needed for the evaluation of ZrCo as a getter material for use in tritium technology, work is presently in progress in our laboratories on (i) the solubility of hydrogen isotopes in ZrCo,5 (ii) the aging of Zrco tritide! and (iii) the definition of the stability range of the ternary hydride ZrCoH,. Previously, we reported that ZrCo samples can be submitted to 35 loading/deloading cycles at room temperature employing 100 kPa of Hz to load and 1.5 h at 450 "C under Pa to deload without loss in storage a vacuum of (1)Penzhom, R.-D.;Devillers, M.; Such, M. J.Nucl. Mater. 1990,170, 217. (2)Devillers, M.; Such, M.; Bredendiek-Khper, S.; Penzhorn, R.-D. Chem. Mater. 1990,2,255. (3)Konishi, S.;Nagasaki, T.; Yokokawa, N.; Naruse, Y. Fusion Eng. Design 1989,10, 355. (4) Leger, D.; Dinner, P.; Yoshida, H.; Fleming, R.; Anderson, J.; Andreev, A.; Asahara, M.; Boissin, J. C.; Finn, P.; Gouge, M.; Iseli, M.; KaDishev. V.: Kabaashi. S.:Kuteev. B.: Kveton. 0.: Muller. M.: Murdoch. D.; Nagbhima, K.; Nasise, J.; Penzhorn, R.-D.; Sebennikov, D. V.; Shantalov, G.; Sherman, R.; Suzukii, T.; Sze, D.; Tanemori, N.; Vasil'ev, V.; Willms, S. ITER Fuel Cycle; ITER Documentation series No 31, IAEA, Vienna, 1991. (5)Devillers, M.;Sirch, M.; Penzhorn, R.-D., to be published. (S! Ache, H. J.; Glugla, M.; Hutter, E.; Jourdan, G.; Penzhorn, R.-D., Rahng, D.; Schubert, K.; Sebenmg, H.; Vetter, J. E. Fusion Eng. Design 1990,12, 331.
capacity or change in the hydriding kinetics.' Using relatively large amounts of ZrCo (25 g) Shmayda et al.' recently observed a deterioration in loading kinetics after the sample had undergone a steep temperature rise to 400 "C in the deloading mode. To clarify this discrepancy, we investigated systematically the effect of high temperatures (>400"C) and high pressures (>lo0kPa) on the stability of the intermetallic hydride ZrCoH,.
Experimental Section Reagents. Ingots of the intermetallic compound ZrCo were purchased from SAES Getters, Milano, Italy. The activation procedure, resulting in the formation of a homogeneous powder, and the characterization by X-ray diffraction,microprobe analysis, and conventional surface analytical techniques of the starting and the activated materials were as reported in a previous publication.2 Prior to an experiment, the activated ZrCo powder was always subjected to two additional loading/deloading cycles, i.e., (i) loading with 10 kPa of Hz at room temperature up to the stoichiometrical composition ZrCoHs and (ii) deloading while heating for 1.5 h at 450 "C under a vacuum of lo4 Pa. The purity and purification method of gaseous hydrogen and deuterium are as described in ref 2. Apparatus. Loading/deloading experiments at pressures up to 1.2 x 106 Pa were carried out in an apparatus described in detail elsewhere.2 For the investigations at pressures up to 4 MPa, a Nimonic autoclave with a volume of 0.01 L manufactured by NOVA Swiss, Effretikon, Switzerland,was used. The autoclave is equipped with t w o pressure sensors type 8210 from Burster Priizisionsmesstechnik, Gernsbach, Germany, operating in the pressure ranges (1-5)X lo6or (1-50)X 106 Pa, respectively. The autoclaves were externally heated in an oven of large heat capacity, whose temperature could be held constant to A1 "C with Eurotherm, Limburg, Germany, controllers. To avoid direct contact of the ZrCo powder with the metallic surface of the autoclave the samples (approximately1 g) were placed inside of a quartz tube. Three axially positioned Ni/NiCr thermocouples located at different heights within the quartz tube served to register the (7)Shmayda, W.T.; Heics, A. G.; Kherani, N. P. J. Less Comm. Met. 1990,162,117.
0S97-4756/92/2804-0631$03.00/00 1992 American Chemical Society
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632 Chem. Mater., Vol. 4, No. 3, 1992
Ib IC
Table I. Disproportionation of ZrCoQ, (Q = H,D) in the Absorption Mode: Experimental Parameters final parameters inductiond Deriod X' time, h eauilibrium Po,*kPa N' 8, "C 0.98 no 16 43.5 0 500 1 0.98 no 1 97.3 500 1.02 1 no 2 100.0 500 H2 0.96 3 l h 3 117.8 450 H2 16 0.66 4 no 121.7 400 H2 0.95 18 l h 0 115.5 450 HZ 0.64 64 0 5h 113.3 400 H2 0.76 114 1 50 h 110.7 400 H2 8 1.03 2 0.5 h 102.7 500 D2 1.12 If 4.9 no 3 250 H2 63 0.78 24.9 no 0 450 D2 4 0.98 1 100.4 20 9 500 D2
109 PIPal' 4.43 4.91 4.92 5.03 5.05 5.01 5.04 5.03 4.93 5.10 4.04 4.93
Id le 2a 3a 3b 3c 3d 4a 4b Figures refer to different ZrCo samples; letters refer to successive experiments carried out with the same sample. Initial gas pressure at the end of the induction period (see text). Number of disproportionation/reproportionationcycles (see text). dDuration of the constant pressure period during which the x vs log P data fit well onto the corresponding isotherm. 'Overall hydrogen content in ZrCoQ, ( x = ng/nzrco)and logarithm of the equilibrium pressure at the time given in column 7. fNo further pressure change during 80 h. temperature. Typically, the temperature gradient between the bottom of the quartz tube and a height of 7.5 cm (approximately 6 cm above the sample) was found to be 12 "C a t an average temperature of 400 OC. Prior to the loading/deloading runs the autoclave was leak tested with u p to 4 MPa He and/or HD X-ray Debye-Scherrer diffraction patterns were obtained with a Siemens diffractometer U 13-007, Karlsruhe, Germany, using the Cu Ka radiation.
Results and Discussion 1. Stability of the ZrCo Intermetallic. In agreement with the phase diagram of the binary system Zr/Co? the intermetallic ZrCo phase was found to be stable up to 1100 "C under vacuum heating. Only small amounts of the ZrCo2 Laves phase were detected by X-ray diffractometry, when samples of the alloy were heated at 900 OC or above and subsequently quenched or rapidly cooled to room temperatures2 2. Stability of the ZrCo Hydride. The intrinsic stability of the ZrCoH, phase is reflected in its very low dissociation pressure at room temperature. It is also in line with the fact that the intermetallic compounds Zr2C0 and Zr3Co, which do not form stable ternary hydrides, are reported to absorb hydrogen according to the following
reaction^:^
Zr2Co + 5/H2 = ZrCoHB+ ZrH, Zr3Co + Y2H2= ZrCoH,
+ 2 ZrH,
(1) (2)
The overall composition corresponding to these reaction products, Le., "Zr2CoH5"and "Zr3CoH,", is comparable to the values of ZrCo0,5H2.42 and ZrCo0.33H2.28, found by Padurets et al.l0 They carried out a systematic investigation of the storage capacity of Zr,,Co, alloys without identification of the involved phases. Assuming the formation of the stoichiometric hydride ZrH2, a substoichiometric ternary hydride of composition ZrCoHh with x = 0.16can be calculated from their results. 3. Disproportionation of ZrCoH,. In view of the potential use of ZrCo as a getter for the reversible storage of tritium, the phase stability of the material was analyzed in both the absorption and the desorption mode. (8) (a) Pechin, W. H.; Williams, D. E.; Lareen, W. L. ASM Trans. 1964, 57,464. (b)Bataleva, S. K.; Kuprina, V. V.; Burnasheva,V. V.; Markiv, V. Y.;Ronami, G. N.; Kuznetaova, S.M. Bull. Moscow Uniu. Chem. Ser.
1970,25,33. (9) Van Essan, R. M.; Buschow, K. H. J. J.Less-Comm. Met. 1979,64, 277. (10) Padureta, L.N.;Chertkov, A. A,; Mikheeva, V. I. Russ. J.Znorg. Chem. 1977,22, 1750.
U
I
nH'nZrCo
Figure 1. Several absorption isotherms of the ZrCo/H2 system a t temperatures between 200 and 450 "C. The isotherm at 500 OC illustrates the occurrence of degradation up to nH = 0.98, above which no further hydrogen uptake is observ (experiment l a in Table I).
d
3.1. Absorption Mode. In a series of runs the effect of temperature, pressure, exposure time, and previous history of the sample on the stability of ZrCoH, phases was investigated. A typical experiment giving evidence for the degradation of the intermetallic compound ZrCo during hydrogen absorption at 500 "C is described in Figure 1. During eight successive additions of hydrogen up to an equilibrium pressure of 30.66 kPa, corresponding to a loading of ZrCoHo.lo, a "normal" absorption isotherm was obtained. Upon further addition of hydrogen up to a total initial pressure of 43.5 kPa, a slow and regular decrease in pressure was noticed. After a period of about 16 h equilibrium was attained at a final pressure of 26.9 kPa and a solid composition given by nH/nwo = 0.98. Successive isothermal additions of more hydrogen up to a pressure of 120 kPa led to no further gas uptake. When the sample was allowed to cool to room temperature in the presence of this excess of gaseous hydrogen, a loading corresponding to the solid composition ZrCoHl.oswas obtained. Knowing that under these conditions the activated ZrCo powder converts to the stoichiometric hydride ZrCoH3, it is concluded that ita total storage capacity has decreased by a factor of nearly 3. The observed loss of storage capacity is assigned to hydrogen-induced composition changes of the starting material. In fact, in spite of the poor crystallinity of the hydrided materials, powder
Chem. Mater., Vol. 4, No. 3, 1992 633
Disproportionation of the Intermetallic Compound ZrCo
ZrCo p'po
+
H,
~
0.90 l
'
O
°
C
nz,cO = 0.01 mol (fresh material) 3 [ O C ] Po[kPa]
I
I
1. 500 2. 450 3. 400
0.70
0.60 0
5
20
15
10
43.5 115.5 113.3
time [h]
Figure 3. Disproportionation of fresh ZrCoH, in the absorption
mode.
ZrCo
+
H,
1
d
nm
0.80-
0.70 -
0.01 mol N 0 1
a b
500 500
c d
500 2 450 0 450 3 number of dlsproportionation -
e N
0.60
=
3[%1 Po [kPal
.
43.5 97.3 100.0 115.5 117.8
1
5
10
15
20 time [h]
Figure 4. Influence of the previous history of a ZrCoH, sample
on its disproportionation fate in the absorption mode.
of nearly 5 h is observed followed by a very slow pressure decrease, which was still far from equilibrium even after 64 h. The hydride obtained at room temperature was found to have the composition Z ~ C O H ~which . , ~ , indicates a loss of storage capacity of only about 10%. Experimentally it was observed that disproportionated samples reproportionate by vacuum annealing under conditions that will be described in detail below. To investigate the effect of the previous history of a sample on ita resistance toward degradation, several consecutive runs were carried out with previously activated samples. The number N of disproportionation/reproportionation cycles to which each sample was subjected prior to each experiment is given in column 5 of Table I (fresh material is denoted by N = 0). To identify critical loading conditions the effect of N on the disproportionation rate was investigated a t two different temperatures (Figure 4). For example, after reactivating the sample from experiment l a by heating for 2 h at 500 "C under a vacuum of lo4 Pa, the sample was reloaded in one step at 500 OC with hydrogen a t a total pressure of 93.7 kPa (experiment l b in Table I). Curve b in Figure 4, which describes this ex-
Devillers et al.
634 Chem. Mater., Vol. 4, No. 3, 1992
exptD 5 6a 6b
%ob 2.15 2.29 2.70 2.92 2.29
6c
6d
Table 11. Disproportionation of ZrCoH. in the Desorption Mode: Experimental Parameters deloading conditions final parameters 8, o c v,L Po,ckPa induction period, h time, h equilibrium x* 100.0 0.67 450 0.226 16 Yes 1.06 100.0d 0.83 114 Yes 400 0.228 1.32 8.3 375 0.641 101.3d 22 no 1.20 6.0 350 0.122 101.2 121 no 2.18 5.5 400 0.228 90.7 69 no 1.05 129 yes 1.19
log P[PaIe 4.77 4.76 4.98 4.94 4.83 4.78
a See footnote a in Table I. *Initial hydrogen content in ZrCoH, ( x = nH/nZrCo).Initial desorption pressure at the deloading temperature. dAfter addition of H, to equalize all initial pressures. e See footnote e in Table I.
Table 111. Disproportionation of ZrCoH, in the Desorption expta %Ob Po,Pa 8, OC t,e h 7 2.0
