Combustion synthesis of Mg2Ni and Mg2NiH4

Combustion synthesis of Mg2Ni/Mg2NiH4, a typical hydrogen storage alloy, has not been reported in the literature. In this paper, the possibility of combustion synthesis of Mg2Ni/Mg2NiH4 is experimentally studied, in which the influence of particle size, mixing method, compacting pressure and pressurized hydrogen atmosphere on the ignition of Mg-Ni(-H) compact was mainly examined. One end of six kinds of cylindrical compacts was ignited and then the obtained meterials were identified by X-ray diffraction analysis. For obtaining information of reaction mechanism, these samples were also analyzed by high-pressure thermogravimetry, differential thermal analysis, and high-temperature microscope. As a result, combustion synthesis of Mg2Ni was observed and the ignition temperature of the Mg-Ni compact was seen to be close to the eutectic temperature of the Mg-Ni system. The highest purity of Mg2Ni was obtained from the compact prepared with very fine nickel powder (2-3 micron), wet mixing and high loading pressure of 1 GPa. Most significantly, Mg2NiH4 was directly synthesized by heating the compact of magnesium and nickel under a hydrogen pressure as low as 2.0 MPa, although commercially-available pure Mg2Ni did not absorb hydrogen at all. These results arise the possibility of a new production process of hydrogen storage alloy without an activation treatment.