Investigations on Microstructures of Ball–milled MmNi5 Hydrogen Storage Alloy

• Ball–milling of MmNi5 improved the hydrogenation behaviour.
• Best result was observed for alloy ball–milled at 100 rpm for 1 h duration.
• Hydrogenation properties were correlated to microstructure.
• The optimum values of particle and grain size were 2 μm and 18 nm respectively.

Five samples of MmNi5 hydrogen storage alloy are prepared by ball–milling the MmNi5 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 MmNi5 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 Å3.

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