Effects Of Annealing Treatment On Microstructure And Electrochemical Properties Of R-Mg-Ni Based A₂ B₇ Type Hydrogen Storage Alloy

In this thesis, previous research works on La-Mg-Ni system alloys had been extensively reviewed. On this basis, the La-Mg-Ni based A2B7-type hydrogen storage alloys was selected as the subject of this study. By means of ICP, XRD, EPMA, Rietveld analysis and the electrochemical test methods, the hydrogen storage alloys were investigated systemically. Firstly, on the base of exact chemical composition, La_0.63Gd_0.2Mg_0.17Ni_3.1Co_0.3Al_0.1 alloys were annealed with different time and then we studied on their microstructure and electrochemical properties. It is found that prolonging time of annealing treatment could improve overall electrochemical properties of the alloy. Sencondly, we study the microstructure and electrochemical properties of the same alloy with different annealing temperature. It is found that it is harmful for the alloy if the annealing temperature was too high. Because of Mg content is very important for the A2B7-type hydrogen storage alloys, so we finally studied on the microstructure and electrochemical properties of La_0.8-xGd_0.2Mg_xNi_3.1Co_0.3Al_0.1(x=0⁰.5)La_0.63Gd_0.2Mg_0.17Ni_3.1Co_0.3Al_0.1 hydrogen storage alloys were annealed with different time (t=8h¹68h) at 1173K in Ar atmosphere. The results showed that prolong annealing time led to increase in Ce₂Ni₇- type phase abundance and the Ce₂Ni₇- type phase abundance was about 94.5% when t=168h. The electrochemical measurements showed that the electrochemical properties of annealed alloys have close contact with the phase abundance of Ce₂Ni₇- type, discharge capacity of the alloys increased with the increasing of annealing time, the discharge capacity of the alloys reached 386.8mAh/g when t=168h; prolonging annealing time could improve cyclic stability of the La_0.63Gd_0.2Mg_0.17Ni_3.1Co_0.3Al_0.1 alloy electrodes. After 100 charge–discharge cycles, the discharge capacity retention rate S100=90.3⁹1.5% when t=16¹68h. In addition, annealing treatment could improve dynamic performance effectively while had little effect on high rate discharge ability of the alloys.La_0.63Gd_0.2Mg_0.17Ni_3.1Co_0.3Al_0.1 hydrogen storage alloys were annealed with different temperature (T=1173K¹223K) in Ar atmosphere. The results showed that increase annealing temperature appropriately led to increase in Ce₂Ni₇- type phase abundance and the Ce₂Ni₇- type phase abundance was about 91.6% when T=1198K. But if annealing temperature was too high would reduce phase aboundence of Ce₂Ni₇- type while increase phase aboundence of CaCu₅-type. The appropriate annealing temperature would enhance the discharge capacity of the alloy electrodes and the discharge capacity of the alloys reached 382.7mAh/g when T=1198K; It would do harm of the discharge capacity and cyclic stability of the La_0.63Gd_0.2Mg_0.17Ni_3.1Co_0.3Al_0.1 alloy electrodes.when annealing temperature was to highOn the base of previous research works, according to the experiment results, we did the research about the effect of different Mg content on microstructure and electrochemical properties of the La_0.8-xGd_0.2Mg_xNi_3.1Co_0.3Al_0.1 (x=0～0.5)alloys. The appropriate content of Mg could increase phase aboundence of Ce₂Ni_7- type and phase aboundence of Ce₂Ni_7- type was 87.0% when x=0.15. The Mg content had close contact with surfactantproperty,discharge capacity and cyclic stability of the alloys. The discharge capacity was 391.4mAh/g and the discharge capacity retention rate S₁₀₀=92.0% when x=0.15. The appropriate content of Mg could improve dynamic performance effectively...