Study On The Synthetic Technology, Properties And Mechanism Of The Magnesium-Nickel Based Hydrogen Storage Materials

The effects of the technologic factors of ball milling and heat treating, rare earth replacement elements and LaNi₅ on the structure, microstructure, thermal stability and hydrogen storage properties of magnesium based hydrogen storage alloys were investigated by using the X-ray diffraction (XRD), scanning electron microscope (SEM), energy spectrum analysis, differential thermal analysis (DTA) and the battery test apparatus according to the development and shortcoming of the alloys. And the mechanism of amorphous formation, phase transition, self-discharge and capacity degradation of the magnesium based hydrogen storage materials during the ball milling, heat treating and electrochemical charging/discharging process were also studied respectively this paper and the results were showed as the following:The relationship between the technologic parameters of ball milling and the structure, thermal stability, electrochemical performances of the MgNi binary alloy were investigated for the first time and the results showed that the size of the particles, thermal stability and the activation times would decrease, the rate of self-discharge would increase, the discharge capacity and the rates of capacity degradation would increase and then decrease with increasing in the velocity and time of ball milling and in ball to powder weight ratio. The maximum discharge capacities are 509.6mAh/g at the discharge current of 20mA/g and 236.9mAh/g after 30 cycles, the rate of self-discharge is 3.86%.d^-1 after open-circuit for 30d with the best technology of ball milling of MgNi alloy is as the following: the velocity and time of ball milling are 400r/min and 35h, respectively, the rate of ball weight (M_Φ20: M_Φ10: M_Φ5) is 1: 2: 3 and the ball to powder weight ratio is 25:1.The structure, the thermal stability and electrochemical characteristics of the Mg₁-xM_xNi(M=La, Pr, Nd, Y, x=0.01⁰.10), Mg₀.95La₀.05-xM_xNi(M=Pr, Nd, Y, x=0.01⁰.04) magnesium based hydrogen storage materials were studied for the first time and the resultsshowed that the thermal stability and the properties of self-discharge would increase, the voltage of discharge plateau would increase and then decrease with increasing in the content of replacement elements. The quaternary Mg0.95La0.03Y0.02Ni specimen possess better activation characteristics, high-rate discharge properties, self-discharge performance, cyclic stability and reach the better cooperate function with magnesium partial replaced by lanthanum together with yttrium, the capacities of the sample are 475mAh/g at the discharge current of 20mA/g and 410mAh/g after 30 cycles, the rate of self-discharge is 1.6%-d'1 which is 58.5% lower than that of the MgNi sample.The effects of the factors of ball milling and heat treating on the structure, microstructure, grain size, thermal stability and hydrogen storage properties of the Mg-Xwt%LaNi5(X=59⁸5) composites were investigated for the first time. The specimen has good high-rate discharge and self-discharge properties but bad cyclic stability. The maximum electrochemical discharge capacity of the samples is 430mAh/g which is 1.3 times than that of AB5 type hydrogen storage material, the least rate of self-discharge of the sample is 2.65%-d'1 which is 31.3% lower than that of the MgNi sample. After heat treatment at 763K for 35d, the composites are stable and consist of Mg2NiLa, Mg2Ni and Mg17La2(X=59); Mg2NiLa, Mg2Ni and MgNi2 (X=62⁸0); MgNi2, LaMg2Ni9 and Mg2NiLa (X=85) phases.The local melt-interdiffusion-rapid solidification mechanism of the as-milled MgNi amorphous formation is put forward for the first time: Part of the specimen could be melted and nickel would be studded into the melted sample, and the crystal structure which its composition is asymmetrical could be obtained after rapid solidification by the impact of balls. The amorphous film on the interface of the sample would be formed during the process of melt-interdiffusion-rapid solidification by means of ball collision and the thickness would increase with the ball milling time prolonging, thus, the composition trend towards uniformityand the atom rank become out-of-order to form amorphous.The phase transformation of the milled MgNi amorphous during the heat-treatment is proposed by means of high temperature X-ray diffraction: When the energy of the milled MgNi sample is not high enough, the DTA curves of the amorphous specimens consists of only one exothermic peak corresponds to crystallization to Mg2Ni and MgNi2 phases. On the other hand, when the energy is high enough, the DTA curves of the samples consists of two exothermic peaks, and the first one corresponds to crystallization into Mg2Ni phase, and the second exothermic peak corresponds to MgNi2 phase.The self-discharge mechanism of the milled magnesium hydrogen storage materials is put forward for the first time by investigating the transformation rule of the structure and microstructure of the as-milled and as-self-discharged specimens: The reversible capacity loss of the milled specimens would be caused by the H2 formed during the open-circuit specimens due to the hydrogen atom which decomposed from the hydrides in the surface layer of the particles can easily diffuse to the surface of the particles because of the high concentration of hydrogen in the samples. The Mg(OH)2 phase would formed on the surface of the particles during the open-circuit state which could reduce the content of hydrogen storage element magnesium and electric conductivity and thus cause the irreversible capacity loss.The mechanism of capacity degradation of the milled magnesium hydrogen storage materials is proposed by studying the transformation rule of the structure and microstructure of the specimens after different cycles: The partially amorphous phase of the milled samples would decompose to Mg2NiH4 and Ni phases and magnesium on the surface of the particles would be oxidized to Mg(OH)2 phase during the electrochemical charge/discharge in alkalescence solution. Therefore, the discharge capacity of the milled magnesium based hydrogen storage materials would reduce with the decrease in the content of amorphousphase and hydrogen storage element magnesium during the charge/discharge cycles. And the crystallization of the amorphous phase is the main reason for the discharge capacity loss of the specimens.