8 Jun 1992 - Zeitschrift fur Physikalische Chemie, Bd. 183, S. 485-489 (1994). © by R. Oldenbourg Verlag, Miinchen 1994. -. 0942-9352/94 $ 3.00 + 0.00.
Zeitschrift fur Physikalische Chemie, Bd. 183, S. 485-489 (1994) © by R. Oldenbourg Verlag, Miinchen 1994 0942-9352/94 $ 3.00 + 0.00 -
Oxygen-, Boron- and Nitrogen-Containing Zirconium-Vanadium Alloys * as Hydrogen Getters with Enhanced Properties By V. A. Yartys', I. Yu. and Yu. F. Shmal'ko
Zavaliy, M. V. Lototsky, A.
B. Riabov
Karpenko Physico-Mechanical Institute, Academy of Sciences of Ukraine,
290601 Lviv, Ukraine
Doped Zr-V alloys / Hydrogen getters Zr-V and Zr-V-Fe alloys doped by additions of the oxides of R (Rare-Earth Metals), B, Al and Si, as well as BN and KN03, were investigated as hydrogen gettering materials. Hydrogen absorption properties were shown to depend upon phase, structural and thermodynamic characteristics of these alloys.
1. Introduction
alloys are among the most promising hydrogen getter materials [1 3], Hydrogen absorption characteristics may be essentially enhanced by introducing some metal oxides into their composition [4, 5]. However, the reason for such an improvement is not yet clear. The aim of this work is to elucidate the relationships between phase, structural and hydrogen absorption properties of Zr-V and Zr-V-Fe alloys doped by R (Rare-Earth metals) oxides, A1203, B203 and Si02 as well as by N-containing additions (BN and KN03). Zr-V-Fe —
2.
Experimental part
The samples were prepared from pure Zr, V, Fe and the corresponding oxide or N-containing compound (1—20 wt.%) by arc melting in argon atmosphere. The phase-structural investigation of the original alloys and *
at the International Symposium on Metal —Hydrogen Systems, Applications, Uppsala, Sweden, June 8 12, 1992.
Presented
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their hydrides was carried out by the use of an X-ray powder diffractometer DRON-2.0 (Cu-Kz radiation). Hydrogen absorption characteristics of the alloys were determined by the application of a standard volumetric method of constant volume. The samples were preliminarily activated in 10 Pa vaccum at 600 K in a steel reactor. Then 0.05 0.1 MPa hydrogen was injected into the reactor and the pressure changes were measured. Hydrogen getter characteristics of the alloys were studied by a capillary method at constant hydrogen pressure of 5 10~4 Pa at room temperature (activation at 1100 K). —
3. Results and discussion Phase-structural characteristics of Zr-V-Fe
alloys (0 33 at.% Zr) as well their hydrogen absorption properties were reviewed in [6, 7]. These alloys consist of HCP-phase a-Zr and Laves phase ^zro (Zr-V-O), then the oxide is reduced with //-phase formation (region III). In the case of PZrQ (Zr-V-R203) « PZlQ (Zr-V-O) (region II), redox interaction between R203 and molten Zr-V proceeds without /7-phase formation. An analysis of the crystallographic data for the original and the hydrogenated alloys resulted in elucidation of the following regularities: 1. The increase of an oxide content is followed by a reduction of the //-phase lattice parameters in both original and hydrogenated samples. Such a behaviour can be explained by a possible growth of oxygen content in the //-phase, leading to some compression of the crystal lattice. 2. The dependence of a-Zr lattice parameters on oxide content in the original alloys is characterized by an increase of hexagonal unit cell parameters with a rise in the oxide addition. This is obviously caused by oxygen dissolution in a-Zr. Comparison of the results obtained with published data on the Zr-O system [9] suggests that the quantity of oxygen dissolved in a-Zr can reach 25 at.%. It should be noted that, in all cases, an FCC zirconium suboxide is present in the original alloy. Hydrogenated alloys contain an oxygen-stabilized cubic ^-modification of zirconium oxyhydride. and oxide.
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3. Modification of the original alloy by oxides, as a rule, does not lead changes in the Zr(V,Fe)2-phase crystallographic characteristics. The case of the Al203-doped alloy is the only exception, where aluminium, formed to
by reduction of the oxide, partially substitutes for vanadium and iron, leading to the hexagonal A,-Laves phase formation instead of the cubic A2-phase. Hydrogen absorption characteristics of the modified alloys differ noticeably from those of the Zr-V-Fe alloys. The best improvements were observed in the cases where >i-phase formation occurs. These alloys have minimum induction periods (5 80 s) and maximum hydrogen absorption rates (saturated hydrides were formed within 1 5 min). In the cases of B203 and R203 (R Sm, Nd and Yb) additions the hydrogen absorption capacity increased from 2.0 to 2.6 2.8 wt.% H. The increase of the hydrogen absorption capacity up to 2.8 wt.% H correlates with an anomalous increase of the ^-phase lattice parameters (from 1.217 nm for pure Zr3V30 to 1.253nm for Zr3V3OD4.93 [10] and 1.296—1.306 nm for oxide-modified hydrides). The linear dependence of the FCC-lattice parameters on hydrogen contents [10] shows that the highest hydrides formed correspond to the Zr3V3OH12 stoichiometry. Taking into account the neutron diffraction data for the Zr3V3OD4 93 structure, the maximum hydrogen content was calculated to be 8.5 H atoms per formula unit [11]. Oxide modification of the alloys leads to essentially greater expansion of the original ^-structure during hydride formation, the highest value of the lattice parameter being 1.306 nm for the saturated —
—
=
—
of the ZrS0W50 + 20 wt.% Sm203 alloy. Due to such expansion, the maximum hydrogen absorption capacity of intermetallic compounds increases, too. Two models of the hydride structure appear from the crystal chemical analysis to be the most likely model I (12 at.H/form. unit): 32 H in Zr3V2 + 96 H in Zr2VlV2 + 48 H in ZrVlV22 + 8 H in V24 + 8 H in Zr6 (these sites in Ref. [11] were designated as Dl + D2 + D4 + D5 + D7). Modell II: The only difference from models I is the suggestion that 96 sites in Zr3V2 (D3) are filled instead of 32 sites in Zr3V2 (Dl). This is followed by an increase of H/form. unit up to 16. To satisfy the rfH-H ^ 0.2 nm criterion, the shift of H atoms from centres of D2 and D4 interstices into the common triangular faces D2 —D2 and D4 —D4 was sug-
hydride
gested. Fig. 2 shows the dependence of the rate of low pressure hydrogen absorption by Zr50V5o + 3.26 wt.% B203 alloy on the quantity of hydrogen already dissolved. For comparison, the same characteristics of a plasma sprayed titanium standard getter are shown. The Zr-based sample provides a four-fold increase in the rate of hydrogen absorption and a three-fold increase in the hydrogen absorption capacity. Thus, application of the investigated materials as hydrogen getters will allow significant improvements of the main
characteristics of gas absorbers. 1544
O-, B- and N-Containing Zr-V Alloys as H2 Getters with Enhanced Properties
489
1.6
o.o
H-1-1-,-1-,-1-,-1
0.0
0.4
1.2
0.8
1.6
Q, jttmol 4 Fig. 2. The dependence of sorption rate (S) at 5 10 Pa hydrogen pressure vs the quantity (Q) of hydrogen absorbed: Zr5oV5o + 3.26% wt. B203 (1), titanium getter (2). -
References 1. C. Boffino, A. Barosi and A. Figini, Fr. Pat. 2447745 (29. 8. 82). 2. G. L. Saksagansky, Electrophysical Vacuum Pumps. Energoatomizdat, Leningrad, 1985. 3. V. S. Kogan and V. M. Shulayev, Adsorption-diffusion Vacuum Pumps (Vacuum Pumps with Nonevapourable Getters). Review, TsNIIatominform, Moscow, 1990. 4. J. Suzuki, K. Hirosava, T. Yamaguchi, T. Saito and S. Terazava, U.S.Pat. 4721697 (26. 1. 1987). 5. M. H. Mendelsohn and D. M. Gruen, U.S.Pat. 4468424 (23. 11. 1982). 6. V. A. Yartys', I. Yu. Zavaliy, M. V. Lototsky, I. I. Bulyk, P. B. Novosad and Yu. F. Shmal'ko, Fiz.-Khim. Mekhanika Materialov 27 (1991) 26. 7. V. A. Yartys', I. Yu. Zavaliy and M. V. Lototsky, Koord. Khim. 18 (1992) 409. 8. I. S. Kulikov, Thermodynamics of Oxides. Reference Book, Metallurgiya, Moscow, 1986. 9. L. M. Kovba, I. I. Kornilov, E. M. Kenina and V. V. Glazova, Dokl. Akad. Nauk SSSR, 180(1968) 3. 10. F. J. Rotella, H. E. Flotow, D. M. Gruen and J. D. Jorgensen, J. Chem. Phys. 79 (1983) 4522. 11. D. G. Westlake, J. Chem. Phys. 79 (1983) 4532.
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