Abstract
Non-equilibration of Mg2Ni by nickel enrichment was investigated by using the bulk mechanical alloying (BMA). Although little or no solubility of nickel in Mg2Ni was present in the phase diagram of Mg-Ni system, BMA enables us to make solid-state synthesis of Mg2-xNi for 0<x<0.5 with success. The synthesized Mg2-xNi has a common crystalline, nano-sized structure of Mg2Ni together with a small amount of amorphous phase. Through precise analysis of the measured pressure-composition isotherms for these Mg2-xNi samples, it was found that two hydride phases should exist for Mg2NiH4. This formation of low-temperature phase Mg2NiH4 is common to hydride formation from the nano-structured Mg2Ni. With nickel enrichment, the maximum storage capacity decreased irrespectively of the holding temperature. This might be because the hydrogen-site energy distribution is broadened by the intrinsic strains constraining the nano-grain of Mg2Ni. This is just corresponding to the hydrogen ordering especially in low temperature. Effect of non-equilibration on the hydrogen absorption can be seen in the monotonic reduction of the transformation temperature from high-temperature to low-temperature phases of Mg2NiH4. The amorphous phase became never negligible in Mg1.5Ni. Its hydrogen behavior was a little different from those for Mg2Ni and Mg1.75Ni, especially in low temperature.
