We study the processes of thermal desorption of hydrogen and hydride disproportionation for the. Zr3 FeOx (x = 0â1.0 ) intermetallic compounds depending on ...
Materials Science, Vol. 43, No. 5, 2007
SPECIFIC FEATURES OF THE PROCESSES OF THERMAL DESORPTION AND HDDR IN THE Zr3 Fe Ox Hy SYSTEM I. V. Koval’chuk, R. V. Denys, and I. Yu. Zavalii
UDC 546.112′ 831
We study the processes of thermal desorption of hydrogen and hydride disproportionation for the Zr3 Fe O x (x = 0–1.0 ) intermetallic compounds depending on the amount of oxygen. As the oxygen content of the Zr3 FeOx compounds increases, their susceptibility to disproportionation at room temperature decreases. It is shown that, for the Zr3 Fe Ox compounds, the character of thermal desorption of hydrogen in a vacuum and the character of disproportionation in hydrogen at high temperatures strongly depend on the oxygen content.
Intermetallic compounds of the form A3 B, where A is Zr and B is Fe or Co, are efficient hydrogen getters and absorbers with a maximum hydrogen capacity of 6.9 H / f.u. [1]. The crystal structure of these compounds belongs to the Re3 B type and the structure of the metal matrix remains unchanged in the process of hydrogenation [2]. The number of investigations devoted to the analysis of the specific features of the hydrogen sublattice of hydrides (deuterides) of these compounds is quite large [3, 4]. Since the indicated compounds can dissolve oxygen up to the stoichiometry A3 B O, their properties can be modified. In [5], the region of existence of a solid solution is found (depending on the oxygen content) for the phase Zr3 Ni Ox (x = 0.6–1.0 ). Note that this phase is not formed in the Zr– Ni binary system. It is shown that even small changes in the amount of oxygen in the A3 BOx compounds lead to significant changes in their hydrogen-sorption capacity [6]. The structure of the hydrogen sublattice of Zr3 Ni Ox Dy deuterides and the positions of oxygen atoms in these compounds were determined according to the data of neutron-diffraction analysis in [3, 4]. The magnetic properties of the Zr3 Fe Ox compounds (depending the oxygen content at different temperatures) and the structure of the Zr3 Fe O0.4 Dx deuteride were investigated in [7]. In what follows, we study the influence of oxygen on the stability of Zr 3 Fe Ox Hy hydrides and the hydrogenation–disproportionation–desorption–recombination (HDDR) processes. Experimental Procedure The alloys were synthesized by electric-arc welding on a copper water-cooled hearth in an atmosphere of pure argon. We used compact metals (Zr and Fe) with contents of the main component not lower than 0.998 atomic fractions. Oxygen was added in the form of Zr O2 oxide preliminarily pressed under a pressure of 5 tons. The phase-structural analyses of alloys and their hydrides were carried out in a DRON-3.0 X-ray diffractometer in the Cu Kα-radiation. The lattice parameters of all original alloys and their hydrides were computed by using the CSD program package [8]. The crystal structures of the chosen hydrides were determined by the Rietveld method [9] with the help of the FullProf program [10]. The alloys were hydrogenated at room temperature under a pressure of hydrogen of 0.1 MPa. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, Lviv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 5, pp. 81–84, September–October, 2007. Original article submitted June 21, 2007. 1068–820X/07/4305–0689
© 2007
Springer Science+Business Media, Inc.
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Fig. 1. Hydrogenation curves of Zr3 Fe Ox alloys at room temperature under a pressure of hydrogen of 0.1MPa.
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(c) Fig. 2. Curves of thermal desorption of hydrogen from the Zr3 Fe O x H y hydrides (at a heating rate of 5°C / min in a vacuum of 10– 100 mPa).
The amount of absorbed hydrogen was found according to the changes in its pressure in a known volume. The thermal stability of hydrides was studied by the method of thermal desorption spectroscopy (TDS) in a dynamic vacuum at a heating rate of 5°C / min.
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Fig. 3. Curves of hydrogen pressure over the saturated hydrides (disproportionation) (a–c) and the TDS curves of disproportionated alloys (recombination) (d–f).
Results and Discussion For the investigation of hydrogenation and thermal desorption characteristics, we choose samples of the Zr3 Fe, Zr3 Fe O0.4 , Zr3 Fe O0.6 , and Zr3Fe O1.0 compounds based on a Zr3 Fe Ox ( 0 ≤ x ≤ 1.0 ) solid solution. The hydrogen sorption capacities of these alloys were found and confirmed by the structural investigations in [7]. The curves of hydrogen absorption by the activated samples are plotted in Fig. 1. It is discovered that, as the oxygen content of the Zr3 Fe Ox compounds increases, the rate of hydrogen absorption and the hydrogen content of the compounds (the number of hydrogen atoms per formula unit of Zr3 Fe Ox ) decrease. However, for the oxygen-free samples, the hydrogen content is much (by about 20%) lower than expected for the saturated hydride (6.7 H / Zr3 Fe) [1]. This is explained by a partial disproportionation of the sample as a result of significant heat release in the process of absorption. In the process of absorption, the mean temperature of the sample becomes as high as 300°C. Despite the substantial heating in the process of hydrogenation, the oxygen-containing samples do not disproportionate under the same conditions. The partial decomposition of Zr3 Fe is confirmed by the high-temperature peak of desorption at 650°C in the curve of thermal desorption (Fig. 2a) corresponding to the desorption from zirconium hydride. In Fig. 2a, we also plot the TDS curve for the Zr 3 Fe H6.7 hydride obtained under “mild” conditions when hydrogen is delivered into the autoclave in small portions in order to guarantee that the sample is not heated to temperatures higher than 50°C. It is easy to see that the oxygen atoms in the structure of the compounds strongly affect the thermal-desorption properties of their hydrides. For the oxygen-containing samples (Fig. 2), the major part of hydrogen is released at temperatures below 300°C. This change in the thermal stability of hydrides is explained, on the one hand, by the decrease in the energy of bonding of hydrogen atoms with the metallic matrix in the presence of oxygen atoms and, on the other hand, by the redistribution of oxygen atoms over the voids of the metallic matrix in the processes of adsorption and desorption of hydrogen by the Zr3 MOx compounds (where M is Fe, Co, or Ni) [7].
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(c) Fig. 4. X-ray diffraction patterns of the Zr3 Fe O0.6 samples: (a) disproportionated in hydrogen at 400°C, (b) disproportionated in hydrogen at 800°C, (c) recombined.
The process of disproportionation was realized by heating the saturated hydrides in hydrogen (the initial pressure was equal to 0.1 MPa) up to temperatures of 750–800°C at a rate of 5°C / min. It is shown (Figs. 3a–c) that the Zr3 Fe and Zr3 Fe O0.4–0.6 samples disproportionate at fairly low temperatures of about 330°C, which is confirmed by the additional hydrogen absorption (after its partial desorption at temperatures higher than 100°C). For the Zr3 Fe O1.0 sample, a significant decrease in the pressure of hydrogen is observed only at temperatures above 700°C. The spectra of thermal desorption of hydrogen (Figs. 3d – f) also show that, in all cases, the process of hydrogen absorption is connected with disproportionation. The peaks of hydrogen release within the range 610 – 670°C correspond to zirconium hydride or zirconium suboxides. The changes in the temperature of the main peak of desorption for samples with different contents of oxygen and for the Zr3 Fe O0.6 sample disproportionated at different temperatures are of significant interest. These changes are connected with the formation of the Zr H2 hydride or less stable zirconium oxyhydride (Zr Ox Hy ). After the disproportionation of Zr3 Fe Ox at 400°C, the low-temperatures peaks are also observed in the TDS curve (Fig. 3e). The X-ray phase diffraction analysis of the samples heated in hydrogen to 400°C reveals the complete decomposition of the Zr3 Fe intermetallic hydride according to the following scheme: 2 Zr3 Fe Hx + H2 → 5 Zr H2 + Zr Fe2 . For the oxygen-containing samples, the corresponding scheme takes the form 2 Zr3 Fe Ox Hy + H2 → Zr4 Fe2 Ox Hy + 2 Zr Ox H2 – y (see the example presented for Zr3 Fe O0.6 in Fig. 4).
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The data of X-ray phase diffraction analysis completely agree with the TDS results. Indeed, the low-temperature peaks of hydrogen release in the temperature range 70–160°C (Fig. 3e) correspond to its release from the Zr4 Fe2 Ox Hy hydride and the peaks at a temperature of 535°C correspond to its release from Zr Ox H2 – y . For the oxygen-containing samples, the absorption of hydrogen at temperatures higher than 700°C corresponds to the reactions of disproportionation of the Zr4 Fe2 Ox Hy intermetallic hydride accompanied by the formation of Zr O2 , Zr H2 , and Zr Fe2 (Fig. 4b) and the decomposition of Zr Ox H2 – y oxynitrides into Zr O2 and Zr H2 . The desorption of hydrogen in a vacuum from the disproportionated samples leads to the complete recombination of the original Zr3 Fe Ox compound (Fig. 4c). In this case, we observe the homogenization of the alloy (dissolution of the impurity phases) confirmed by the X-ray diffraction patterns. CONCLUSIONS As the oxygen content increases in the process of hydrogenation of Zr3 FeOx samples, both the hydrogen content and the rate of hydrogen absorption decrease. Moreover, the susceptibility of the Zr3 Fe Ox samples to disproportionation at room temperature also decreases as their oxygen content increases. The character of thermal desorption of hydrogen from Zr3 FeOx samples strongly depends on the oxygen content. The hydrides of the oxygen-containing samples are characterized by lower thermal stability and the major part of hydrogen is released at temperatures lower than 300°C. As a result of hydrogenation at high temperatures, we observe the disproportionation of the Zr3 Fe Ox compounds running, in the presence of oxygen, in two stages. In the first stage, parallel with the hydride of zirconium suboxide (Zr Ox H2 – y ), we observe the formation of Zr4 Fe2 Ox Hy (330°C, 0.1 MPa). In the second stage, Zr4 Fe2 Ox Hy and ZrOx H2 – y disproportionate into Zr O2 , Zr H2 , and Zr Fe2 (> 700°C). The present work was performed under the financial support of the INTAS Grant No. 05-1000005-7671. REFERENCES 1. V. A. Yartys, H. Fjellvag, B. C. Hauback, et al., “Neutron diffraction studies of Zr-containing intermetallic hydrides with ordered hydrogen sublattice. II. Orthorhombic Zr3 Fe D6.7 with filled Re3 B-type structure,” J. Alloys Comp., 278, 252–259 (1998). 2. V. A. Yartys, H. Fjellvag, B. C. Hauback, et al., “Neutron diffraction studies of Zr-containing intermetallic hydrides with ordered hydrogen sublattice. V. Orthorhombic Zr3 Co D6.9 with filled Re3 B-type structure,” J. Alloys Comp., 360, 173–182 (2003). 3. I. Yu. Zavaliy, R. Cerny, I. V. Koval’chuk, and I. V. Saldan, “Hydrogenation of oxygen-stabilized Zr 3 Ni O x compounds,” J. Alloys Comp., 296, 312–316 (2000). 4. I. Zavaliy, R. Cerny, I. Koval’chuk, and A. Riabov, “Crystal structure of Zr3 Ni O0.8 D6.05 and Zr3 Ni O1.0 D5.72 deuterides,” Visn. Lviv. Univ., Ser. Khim., 43, 48–52 (2003). 5. R. MacCay and H. F. Franzen, “Single crystal X-ray study of Zr3Ni O,” J. Alloys Comp., 186, 7–10 (1992). 6. I. Yu. Zavaliy and I. V. Koval’chuk, “Hydrogenation of oxygen-stabilized Zr3 Fe O x phases with Re3 B-type of structure,” in: Abstr. of the 8th Internat. Conf. on the Crystal Chemistry of Intermetallic Compounds (Lviv, September 2002), Lviv (2002), p. 133. 7. I. Yu. Zavaliy, R. V. Denys, R. Cerny, et al., “Hydrogen-induced changes in the crystal structure and magnetic properties of the Zr3 MOx (M = Fe, Co) phases,” J. Alloys Comp., 386, 26–34 (2005). 8. L. G. Akselrud, Yu. N. Grin, P. Yu. Zavalij, V. K. Pecharsky, et al., “Use of the CSD program package for structure determination,” in: Abstr. of the 2nd Europ. Powder Diffraction Conf., Enschede, Netherlands (1992), p. 41. 9. R. Young (editor), The Rietveld Method, IUCr.–Oxford Univ. Press, Oxford (2000). 10. J. Rodriguez-Carvajal, FULLPROF Version 0.2, LLB, Saclay (1998).
