Investigations on Microstructures of Ball–milled MmNi5 Hydrogen Storage Alloy

Abstract Five samples of MmNi 5 hydrogen storage alloy are prepared by ball–milling the MmNi 5 alloy at various speeds for different periods. For the first time, it has been shown that an optimum microstructure in terms of homogeneity, particle size, crystallite size and unit cell volume is necessary to give best hydrogenation properties. The alloy ball–milled at the speed of 100 rpm for 1 h duration shows optimum hydrogenation behaviour. This alloy is activated completely in the 2nd hydrogenation–dehydrogenation cycle with hydrogen storage capacity of 1.5 wt%, and 90% hydrogen absorption/desorption is recorded in 2 min; whereas the same for un–milled MmNi 5 alloy is 5th cycle, 1.4 wt% capacity and 5 min respectively. Optimum microstructure for the alloy ball–milled at 100 rpm for 1 h duration corresponds to homogeneous size distribution of particles with average value of 2 μm, crystallite size as 18 nm and unit cell volume as 87.7075 A 3 .