Effect of ï?-ï?-ï?cyclic phase transformations on the hydrogenation/dehydrogenation kinetics of MgH2 binary system

Reactive ball milling (RBM) approach, using a high-energy ball mill operated at room temperature under 50 bar hydrogen gas atmosphere was employed to synthesize MgH2 powders. A single low-pressure phase (tetragonal structure) of ï-MgH2 phase with tetragonal structure was obtained after 6 h RBM time. This phase tended to transformed into less stable high-pressure ï-phase (orthorhombic structure) upon milling for 25 h. However, increasing the RBM time to 75 h led to ï-to-ïï phase transformations. Cyclic ï-ï-ïï phase transformations were observed several times with changing the RBM time between 25-200 h. The results of the present study showed that existence of metastableï ïï-phase in the milled powders led to a significant improvement on the hydrogenation/dehydrogenation kinetics. The decomposition temperature and the activation energy of the powders obtained after 200 h of RBM time, which containing a large volume fraction of ï-phase, were 399 oC and 131 kJ/mol, respectively. In addition, at 300 oC, MgH2 nanocrystalline obtained upon 200 h of RBM possessed excellent hydrogenation properties for any pure MgH2 system, indexed by high hydrogen storage capacity (7.54 wt.%) with complete 600 absorption/desorption cycles.