Hydrogen storage behavior of non-equilibrium, nano-structured Mg₂ via BMA

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 Mg_2-xNi for 0 < × < 0.5 with success. The synthesized Mg_2-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 Mg_2-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 Mg_1.5Ni. Its hydrogen behavior was a little different from those for Mg₂Ni and Mg_1.75Ni, especially in low temperature.