Notable hydriding properties and their origin in a nanostructured Mg₂Ni-H system

Hydriding and structural properties of a nanostructured Mg₂Ni-H system synthesized by reactive mechanical grinding were investigated. The dissolved hydrogen content of the system reached up to 1.6 mass% (Mg₂NiH_1.8) without changing the crystal structure of an intra-grain region composed of the Mg₂NiH_0.3 crystallites, and the dehydriding reaction occurred even at 413 K. To clarify an origin of the notable hydriding properties, structural characterizations of the system were carried out. In Mg₂Ni-H mechanically ground for 60 min under a hydrogen atmosphere, the HRTEM observation revealed that the sample consists of the equiaxed crystallites with an average size of 15 nm. Moreover, each crystallite is found to be surrounded by the heavily deformed and disordered region nearly 1.5 nm in width. As was evident from the thermal and magnetical analyses, a volume fraction and its hydrogen content of the inter-grain region were around 0.30 and 4.0 mass%, respectively, by the grinding only for 60 min. These results were also supported by the small angle neutron scattering measurements of the nanostructured Mg₂Ni-D system. It was concluded that a large amount of hydrogen was dissolved into the disordered inter-grain region. In addition, a cooperative dehydriding reaction between both the intra- and inter-grain regions were discussed as one of the origin for the low-temperature dehydriding around 413 K.