| Co-Investigator(Kenkyū-buntansha) |
YUKAWA Hiroshi Nagoya University, Graduate School of Eng., Research Associate, 工学研究科, 助手 (50293676)
EZAKI Hisakazu Suzuka College of Technology, Dept.of Mat.Sci.and Eng., Associate Professor, 材料工学科, 助教授 (80160357)
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| Research Abstract |
The electronic structures of LaNi_5 and Mg_2Ni both containing variety of alloying elements, M, are investigated by the DV-X_<alpha> cluster method. It is found that hydrogen atoms make a strong chemical bond with Ni atoms rather than La or Mg atoms in pure LaNi_5 or in pure Mg_2 or in pure Mg_2Ni, despite the larger affinity of La or Mg atoms for hydrogen than Ni atoms in the binary metal-hydrogen system. The presence of such metal-hydrogen interactions is characteristic of hydrogen storage alloys.
It is also shown that the stability of hydrides formed in the hydrogen storage alloys is well understood in terms of the nature of the chemical bond between atoms in small metal polyhedra where hydrogen is stored and also of the crystal structural evolution during hydrogenation. For example, for the LaNi_5 system, the ratio of the La (M) -Ni bond order to the Ni (M) -Ni bond order correlates well with the experimental data of the equilibrium plateau pressure of hydrogen. On the other hand, for the Mg_2Ni system, both the Ni (M) -Mg bond strength and the Ni (M) -H bond strength correlate well with the measured enthalpy of formation for the hydride.
Further, the roles of the hydride forming elements, A, (e.g., La, Zr, Ti, Mg) and non-forming elements, B, (e.g., Ni, Mn, Fe) are elucidated. Also, it is shown that the optimum A/B compositional ratio of hydrogen storage alloys is predictable in terms of a simple parameter, 2 Bo (A-B) / [Bo (A-A) + Bo (B-B) ] , where the Bo (A-B) , the Bo (A-A) and the Bo (B-B) are the bond orders between atoms given in the parentheses. This bond order ratio is highest in Mg_2Ni among the hydrogen storage alloys, because Bo (Mg-Ni) is larger than Bo (Mg-Mg) . Thus, some criteria for alloy design were obtained from this study.
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