Microstructure of ball milled MgH2 powders upon hydrogen cycling: An electron microscopy study

Microstructure of MgH2 powder particles milled for 5–60 h was studied after the powders were hydrogen cycled. Using electron microscopy we estimated individual grain size and particle size of cycled powders to compare with those before H-cycling. The nano-size grains were found to grow (3–10 times) whereas the particles undergo a mild refinement upon H-cycling. The most prominent grain growth was within the 1st dehydrogenation. The smaller grains in the milled powders demonstrated faster growth upon cycling. The average grain size estimated in this study for the cycled powders were >200 nm. The grain and particle size approached a regular or log-normal distribution with cycling. The lattice strain introduced in the milled MgH2 powders was significantly removed by cycling for less milled powders (milled for <15 h). Longer milled (30–60 h) MgH2 powders could not fully recover from lattice strain by H-cycles (six cycles) as observed in the present study.