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

Mg_2Ni金属间化合物因其高达3.6mass%的吸氢量而被认为是最有吸引力的储氢合金之一。而传统的Mg_2Ni生产系统存在两个生产效率较低的工序，即多次重熔和活化工序。为了克服这些问题，本研究采用燃烧合成法直接制备Mg_2NiH_4。结果表明，Mg_2zNiH_4的合成是成功的，燃烧合成法是一种很有前途的合成方法。为了实现Mg_2NiH_4的实际生产，必须研究Mg_2NiH_4的分解率和脱氢率。本文首次用PTC装置、DSC和原位X射线衍射仪研究了Mg_2NiH_4的燃烧合成反应机理。在1.0MPa氢气压力下，经过两个循环的加热和冷却，共得到9个反应，第一个循环为：（1）Mg+H_2*MgH_2（部分），（2）MgH_2*Mg+H_2，（3）2 Mg +Ni-Mg_2Ni。（4）Mg_2Ni+2H_2·Mg_2NiH_4（HT），部分（5）Mg_2NiH_4（HT）·Mg_2NiH_4（LT）。在第二次循环中，（6）Mg_2NiH_4（LT）*MMg_2NiH_4（HT），（7）Mg_2NiH_4（HT）*Mg_2Ni+2H_2，在加热阶段。（8）Mg_2Ni+2H_2·Mg_2NiH_4（HT），（9）Mg_2NiH_4（T）·Mg_2NiH_4（LT），在第一次和第二次循环的热处理前，Mg_2NiH的X射线衍射强度在823 K时都只有单相Mg_2Ni，但在第二次循环的热处理后，Mg_2NiH的X射线衍射强度是第一次循环的6倍。

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.