Computer Simulation of Structural Stability by Hydrogenation in Hydrogen Storage Materials
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1127-T05-10
Computer Simulation of Structural Stability by Hydrogenation in Hydrogen Storage Materials Masahiko Katagiri,1 Hidehiro Onodera1 and Hiroshi Ogawa2 1 National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan 2 National Institute of Advanced Industrial Science and Technology, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan
ABSTRACT Microscopic mechanism of Hydrogen-Induced Amorphization (HIA) in AB2 C15 Laves phase compound is studied. Experimentally, compounds in which A atoms are contracted and B atoms are expanded in Laves phase show HIA. It suggests that the relative atomic size is the controlling factor. We investigate the role of the size effect by molecular Dynamics (MD) methods using Lennard-Jones pair-wise potentials. Our simulations show that in such a compound, the bulk modulus is remarkably reduced by hydrogenation compared to the isotropic tensile load, so that elastic instability is facilitated. This situation is caused by the negative increase of the pressure-fluctuation contribution in the elastic constant. The relaxation of B-atom positions by hydrogenation gives the fluctuation contribution. The fluctuation effect is essential in the elastic instability, although the energy change is small. INTRODUCTION Hydrogen is attractive as a fuel because its oxidation product (water) is environmentally benign, it is lightweight and it is highly abundant. It has been suggested that metal hydrides are the most promising materials for hydrogen storage, especially the lightweight metal alloy hydrides. Getting high hydrogen uptake is only a part of the problem. Hydrogen-Induced Amorphization declines the performance characteristic and it is related to the life as the hydrogen storage materials. The finding of the mechanism of HIA gives the insight of prevention of amorphization and may lead to the guide into the development of the new hydrogen-storage materials. Hydrogen-Induced Amorphization (HIA) is a phase transformation from crystalline to amorphous induced by hydrogenation. HIA is a potential method for preparing amorphous alloys since hydrogenation and dehydrogenation can be done easily and rapidly. In addition, HIA has a close relation with the structural change of hydrogen-storage alloys which determines their performance [1]. For effective materials design using HIA, it is important to understand the atomistic mechanism of this process. For this purpose, computer simulation is a powerful tool. Aoki et al. [2] studied HIA of C15 Laves phase AB2 compounds experimentally. They reported that the relative atomic size is the controlling factor of the occurrence of HIA. HIA does not occur when the ratio of the Goldschmidt radii of A and B atoms, RA / RB, is less than 1.37 (A = rare earth, B = Al). In this ratio, both A and B atoms contract compared to those in the pure crystals [2]. On the other hand, HIA occurs when the ratio is larger than 1.37 (A = rare earth, B =
Fe, Co, Ni). In this ratio, A atoms contract and B atoms expand. Thus it is known that the size effec
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