Effect of a magnesium depletion on the Mg–Ni–Y alloy hydrogen absorption properties

To improve the hydrogen absorption properties of Mg2Ni, three kinds of specimens were prepared. They were denoted as MGNIY, DeMg–MGNIY and DeMg–MIX. The first one was a cast Mg–Ni–Y alloy, the second was prepared by vacuum heating of MGNIY to remove some part of Mg and the third was prepared by vacuum heating of a mixture of pulverized Mg–Y and Mg2Ni. The term DeMg denotes removal of Mg by vacuum heating. These samples were characterized by XRD, SEM and EPMA and found to be composed of Mg, Mg2Ni and crystallographically unknown phases. Each material was found to have its own distribution of yttrium. The composition of the crystallographically unknown phase was determined by EPMA to be MgYNi3. Yttrium-alloying and subsequent DeMg treatment caused a significant improvement in the hydrogen absorption rate. It showed hydrogen uptake amounting to about 3 mass% for DeMg–MGNIY at 473 K in 80 ks, although the improvement at room temperature could not be distinguished from that of Mg2Ni. MGNIY gave similar absorption properties with those of DeMg–MGNIY at 473 K, but showed significantly slower absorption rate than Mg2Ni at room temperature. The properties of DeMg–MIX were found to be similar to those of Mg2Ni at both temperatures. On account of the sample characterization, it was concluded that the improvement in hydrogen uptake by DeMg–MGNIY and MGNIY at 473 K was due to their specific yttrium-distribution and catalytic activity.