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

Mg_2Ni金属间化合物的吸氢量高达3.6质量%，被认为是最具吸引力的贮氢合金之一。而传统的Mg_2Ni生产体系存在多次重熔和活化两种生产效率较低的工艺。为了克服这些问题，本研究采用燃烧合成法直接生产Mg_2NiH_4。结果表明，Mg_2zNiH_4的合成是成功的，燃烧合成是一种很有前途的合成方法。为了实现氢化燃烧合成Mg_2NiH_4的实际生产，必须对氢化和脱氢速率进行研究。本文采用PTC、DSC和x射线衍射仪对氢化和脱氢燃烧合成Mg_2NiH_4的反应机理进行了初步研究。在1.0 MPa氢气压力下加热和冷却两个循环，得到了9个反应。在第一个循环中；(1) Mg+H_2*MgH_2（部分），(2)MgH_2*Mg+H_2, (3) 2Mg+Ni-Mg_2Ni。(4) Mg_2Ni+2H_2*Mg_2NiH_4(HT)，部分(5)Mg_2NiH_4(HT)*Mg_2NiH_4（LT）。在第二个循环中；(6) Mg_2NiH_4(LT)*MMg_2NiH_4(HT), (7) Mg_2NiH_4(HT)*Mg_2Ni+2H_2，在加热期。(8)部分Mg_2Ni+2H_2*Mg_2NiH_4 (HT), (9) Mg2NiH4 T)*Mg_2NiH_4（LT）。第二次氢化后的Mg_2Ni的x射线衍射强度是第一次氢化后的6倍，尽管在第一次和第二次氢化前的823K处Mg_2Ni只存在一个单相。

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.