Study on the Combustion Synthesis for Producing High Performance Hydrogen Absorbing Alloy

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The intermetallic compound of Mg_2Ni has been regarded as one of the most attractive hydrogen storage alloys because of very high hydrogen absorbing quantity of 3.6 mass %. However, conventional production system of Mg_2Ni has two less productive processes which are several times remeltings and activation process. To overcome these problems, combustion synthesis has been studied for producing directly Mg_2NiH_4 in this study. The result shows the successful synthesis of Mg_2zNiH_4, proposing combustion synthesis as one of the promising process. For realizing practical production of Mg_2NiH_4 in hydriding combustion synthesis, rates of hydriding and dehydriding must be studied. For the first attempt for this purpose, reaction mechanism has been studied in hydriding and dehydriding combustion synthesis of Mg_2NiH_4, by using PTC apparatus, DSC and in situ X-ray diffractometer. Nine reactions in this process were obtained for two cycles of heating and cooling in hydrogen pressure of 1.0 MPa as follows.In the first cycle ; (1) Mg+H_2*MgH_2(partially), (2) MgH_2*Mg+H_2, (3) 2Mg+Ni-Mg_2Ni, in the heating period. and (4) Mg_2Ni+2H_2*Mg_2NiH_4(HT), partially (5) Mg_2NiH_4(HT)*Mg_2NiH_4(LT), in the cooling period. In the second cycle ; (6) Mg_2NiH_4(LT)*MMg_2NiH_4(HT), (7) Mg_2NiH_4(HT)*Mg_2Ni+2H_2, in the heating period. and (8) Mg_2Ni+2H_2*Mg_2NiH_4 (HT) partially, (9) Mg2NiH4 T)*Mg_2NiH_4(LT), in the cooling period.The X-ray diffraction intensity of Mg_2NiH, after the second cycle of hydriding is 6 times of that after the first cycle of hydriding, although only one single phase of Mg_2Ni exists at 823K before the hydriding either the first cycle or the second cycle.