Phase abundances in AB 2 metal hydride alloys and their correlations to various properties

Abstract

The phase abundances of 16 alloys with various contents of Sn, Co, Al, and Fe additives were studied by X-ray diffraction analysis. Due to relative small percentages, the relative abundances of these minor phases were obtained through smoothing, curve fitting, and area integration and correlated qualitatively to the amount of additives in the alloys. In the Laves phases, Sn, Co, and Fe with more outer-shell electrons than that of the host alloy promote C15 phase. Al, with less outer-shell electrons, on the other hand, promotes C14 phase. In the AB phases, Sn promotes the formation of ZrNi phase and suppresses the formation of TiNi phase. In the competition between Zr 7Ni 10 and Zr 9Ni 11 (both are the products from a solid-state transformation), Sn promotes Zr 7Ni 10 while both Fe and Al promote Zr 9Ni 11. The phase abundances were further correlated to the gas phase and electrochemical properties of the alloys. C14, Zr 9Ni 11, and TiNi phases were found to increase both the gas phase and electrochemical storage capacities, lower the hydrogen equilibrium pressure, decrease the half-cell high-rate dischargeability, improve both charge retention and cycle life, and slightly decrease the capacity recovery after storing at 45 °C for 60 days. C15, ZrNi, and Zr 7Ni 10 phases work in the opposite manner.