Development of novel Mg-based ternary alloys with high hydrogen storage capacity

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Mg-based hydrogen storage alloys are promising in view of high storage capacity. In this study, novel Mg-based hydrides were explored in 1) MgH_2-TM (TM = Mn, Fe, Co, Ni, Cu), 2) MgH_2-CaH_2-Ni, and 3) MgH_2-REH_3 (RE = Y, La) systems by using a cubic anvil-type high-pressure apparatus. The high-pressure apparatus used was capable of generating pseudo-isostatic pressure as high as 6 GPa. As a result, novel hydrides of Mg_2Ni_3H_<3.4> (orthorhombic), Mg_3MnH_<5.6> (monoclinic), (Ca_<0.6>Mg<0.4>)_2NiH_x (BCC), MgCaH_4 (BCC), MgY_2H_<7.8> (FCC), Mg_3LaH_9 (tetragonal) were prepared under high pressure range of 3〜5 GPa. With respect to a hydrogen content, Mg_3MnH_<5.6> was found to have 4.2 mass% of hydrogen. These novel high-pressure hydrides decomposed along with hydrogen desorption in the temperature range of 460〜650 K except for MgY_2H_<7.8>. MgY_2H_<7.8> was able to hold its crystal structure even after partial hydrogen desorption at 600 K. From first principle calculations, it was found that, for MgY_2H_<7.8>, emitted hydrogen originated from O-sites, and remaining hydrogen on T-sites played an important role for holding the crystal structure. The first principle calculation also supported the lattice expansion after the partial hydrogen desorption. In addition to exploring novel hydrides, the effects of hydrogen treatments on the grain size of AZ31 alloys were investigated. As a result of optimization of hydrogenation and dehydrogenation conditions, it was found that MgH_2, Al, Mg_<0.42>Al_<0.58> phases appeared after hydrogenation (disproportionation process), and an original solid solution phase was reformed after dehydrogenation (recombination process). The AZ31 alloys subjected to the HDDR phenomena was found to have fine grain size of around 100 nm. The hydrogen treatments were also applied to plate-shape AZ31 samples. In such case, the surface area of 20 μm in thickness was HDDR-treated.