Research On Mechanism Of Combustion Synthesis Of Mg₂Ni And The Hydrogen Storage Property Of Mg-Al-Ni

Combustion synthesis method shows its unique advantage in preparation for Mg₂Ni hydrogen storage alloy. However, the knowledge of the reaction process and mechanism during combustion is extremely limited because of the unique Mg-Ni system and the rapid reaction process and high synthesis temperature. So the research on the mechanism of self-propagating combustion synthesis Mg₂Ni has its important significance in both the perfection of the combustion method itself and extensive application of Mg₂Ni.In this paper, a relatively general research on the weak exothermic reaction system of Mg₂Ni has been made from thermodynamics and structure dynamics. The relationships among the preheat temperature, adiabatic temperature and the melt rate of Mg₂Ni are definite through the calculation of adiabatic temperature under ideal state.The pure intermetallic Mg₂Ni are synthesized by self-propagating combustion mode and thermal explosion mode respectively, between which a comparison of the reaction process has been made, and the different technical demands are definite. The effect of different technical parameters in preparation for Mg₂Ni during thermal explosion reaction mode on the phase compose and morphology of product has been studied, these parameters include preheat temperature, preheating velocity, temperature holding time, pressure, Mg granularity and diluent. On this basis, an optimal technical parameters scheme of combustion synthesis Mg₂Ni has been summed up. Especially, a solid combustion at lower temperatures has been observed, and the reaction extent increased with the preheat rate declined.A further experiment of Combustion Front Quenching (CFQ) was brought to success in cylindrical steel mould by adding some Al-Ti-C powder, which is accepted high exothermic system on the top of Mg-Ni samples. The physical and chemical changes of all region in the quenched samples during the combustion synthesis were followed by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS) and x-ray diffraction (XRD). The results show that the synthesis of Mg₂Ni was directly rather than through other intermediates and the process of synthesis Mg₂Ni can be expressed as: ( i ) solid combustion in the preheat phase; (ii )liquid combustion in the combustion process; (iii)cooling and crystal of the product after the reaction; (iv)uniformity of the compositionin the heat preservation process.The mechanism of synthesis Mg₂Ni could be described with a solid-state diffusion and dissolution-precipitation model. The combustion reaction between Mg and Ni particles started with the solid-state diffusion reaction firstly, which resulted in the partial melt of Mg and then proceeded by a dissolution-precipitation mechanism. Namely, the solid-state diffusion reaction between Mg and Ni particles at lower temperature led to the partial melt of Mg particles on the Mg /Ni boundary, which resulted in the dissolving of Ni particle into the Mg liquid, at the same time the eutectic reaction occurred, as the Ni content increased, the Mg₂Ni grains then precipitated when the Ni content in the Mg-Mg2Ni eutectic was saturated. It should be noted that the model is applicable for both thermal explosion mode and self-propagating mode.A new phase Mg₃AlNi₄ has been obtained by combustion synthesis through partial substitute for Mg with Al in Mg₂Ni. The discharge capacity and cycle life of the new alloy have been improved significantly. The effect of mechanical grinding (MG) on the properties of alloy has also been investigated; the results show that MG is effective to improve electrochemical capacity and cycle life of alloys.