Effects of elastic strain energies on a hydride precipitation in LaNi 5 -based compounds
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Effects of elastic strain energies on a hydride precipitation in LaNi5-based compounds K. Tanaka1, H. Inui2, M. Yamaguchi2 and M. Koiwa2 1 Department of Advanced Materials Science, Kagawa University, 2217-20 Hayashi-cho, Takamatsu 761-0396, Japan 2 Department of Materials Science and Engineering, Kyoto University, Yoshidahon-machi, Sakyo-ku, Kyoto 606-8501, Japan ABSTRACT The elastic energies associated with hydride formation have been calculated using coherent elasticity theory. The energies modify the required condition for hydride nucleation to lower temperatures or higher hydrogen gas pressures. Since the elastic energies are quite large, hydride cannot form in a crystal or on a planer surface without assistance of a lattice defect. Hydride can form with reasonable excess hydrogen gas pressure only at a corner of specimen at which the most part of elastic energy is released. INTRODUCTION Alloys based on the intermetallic phase, LaNi5 have been used as negative electrode materials of rechargeable nickel-metal hydride batteries due to their fast activation, high storage-capacity, long cycle-life and excellent electrochemical charge/discharge kinetics [1-4]. The full hydride phase formed when hydrided at ambient temperature is known to be LaNi5H6. It is well known that a high density of dislocations is introduced and the material is severely cracked at hydriding. The phenomena are closely related to a large elastic energy rising in the materials by the large volume expansion of about 20% at hydriding. The elastic energies are expected to modify not only the internal structure or size of the particles, but also the kinetics of hydriding. The most well known effect of elastic energy on phase transformations is the difference in chemical and coherent spinodal temperatures in phase decomposition. The difference comes from that the additional driving force to overcome the elastic energies is required for the beginning of coherent spinodal decomposition. The same analogy can be applied for hydriding processes; the change in required hydrogen gas pressure or temperature for hydride nucleation is expected. In order to evaluate the changes, the magnitude of elastic energies at hydriding has to know. In this paper, we report calculated elastic energies using coherent elasticity theory and present required excess hydrogen gas pressures for hydriding when the elastic energies are taken into account. CALCULATION OF ELASTIC ENERGIES AND EXCESS HYDROGEN GAS PRESSURE Elastic strain energy For calculating elastic energies at hydriding, information of shape change (unconstrained strain) and elastic constants are required. The shape change at hydriding is calculated from the reported lattice constants of both LaNi5 [5] and LaNi5H6 [6] as tabulated in table I. The present authors have reported the single crystal elastic constants of LaNi5 as listed in table II [7]. For a
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rigorous calculation, the constants of LaNi5H6 are also required. However, since severe cracking occurs at hydriding, the values are hard to obtai
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