NSTI Nanotech 2009

Role of Pressure Fluctuation in Hydrogen-Induced Amorphization for Hydrogen Storage Materials

M. Katagiri, H. Onodera
National Institute for Materials Science, JP

Keywords: hydrogen storage, hydrogen induced amorphization, elastic instbility, molecular dynamcis

Abstract:

Microscopic mechanism of HIA in AB2 C15 Laves phase compound is proposed by classical MD. Atomic ratio of RA/RB is greater than 1.37 (A = rare earth, B = Fe, Co, Ni), HIA occurs. In such a HIA compound, A atoms contract and B atoms expand compared to pure crystals. It suggests that the relative atomic size is the controlling factor. Therefore, we simply performed MD using Lennard-Jones pair-wise potentials and investigated the role of the size effect. Our simulations show that in such a HIA compound, the bulk modulus is remarkably reduced by hydrogenation compared to the isotropic tensile loading, and elastic instability is facilitated. It is found that the instability is caused by the negative increase of the pressure-fluctuation contribution in the elastic constant. It is shown that the expanded B atoms can be relaxed by hydrogenation and the relaxation gives the negative increase in the elastic constant. As a result, an elastic instability occurs and the structure becomes unstable. It is concluded that the role of the size effect in HIA is to allow the atomic relaxation on hydrogenation and to facilitate the elastic instability by the increase of pressure fluctuations. It is necessary to consider the dynamical feature to understand the role of the atomic size ratio in the amorphization.
 
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