NOTABLE HYDRIDING PROPERTIES AND THEIR OR IGIN IN A NANOSTRUCTURED MG2NI-H SYSTEM

Hydriding and structural properties of a nanostructured Mg2Ni-H system synthesized by reactive mechanical grinding were investigated. The di ssolved hydrogen content of the system reached up to 1.6 mass% (Mg2NiH 1.8) without changing the crystal structure of an intra-grain region c omposed of the Mg2NiH0.3 crystallites, and the dehydriding reaction oc curred even at 413 K. To clarify an origin of the notable hydriding pr operties, 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 equia xed crystallites with an average size of 15 nm. Moreover, each crystal lite is found to be surrounded by the heavily deformed and disordered region nearly 1.5 nn in width. As was evident from the thermal and mag netical analyses, a volume fraction and its hydrogen content of the in ter-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 sma ll angle neutron scattering measurements of the nanostructured Mg2Ni-D system. It was concluded that a large amount of hydrogen was dissolve d into the disordered inter-grain region. In addition, a cooperative d ehydriding reaction between both the intra- and inter-grain regions we re discussed as one of the origin for the low-temperature dehydriding around 413 K.