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.