La-mg-ni System Ab_n (n = 2,3)-type Hydrogen Storage Alloy And Hydride Crystal Structure And The Mh Electrode Performance

In this thesis, previous research works on non-AB₅ type RE-based hydrogen storage alloys have been extensively reviewed. On this basis, the microstructural characteristic and hydrogen storage characteristic of La-Mg-Ni based AB₂-type and AB₃-ype hydrogen storage alloys were selected as the subject of this study. At the same time, the thesis also investigated the AB3-type intermetallic hydride's crystallographic property.The structural characterization of La_2-xMgNi_9-5X(x=0¹.0)were investigated by means of XRD and Rietvled method. It is found that all these alloys consist of LaMgNi₄ phase and (La,Mg) Ni₃ phase except LaMgNi4 alloy which isa single phase. In the LaMgNi4 phase, La atom occupy at 4a position, Mg atom occupy at 4c position .Mg atom is easy into La_1-xMgxNi₂ phase, with Mg/La ratio increase to 1:1, the crystal structure transform from pseudo-binary MgCu₂ type to fully ordered ternary MgCu₄Sn type superstructure. The electrochemical and P-C-T test indicated the single LaMgNi4 alloy absorbs 3.8H/f.u at 50℃ and 18atm and the discharge capacity is only 170 mAh.g^-1 because the P-C-T curve platform area is narrow and reversibility is poor. In the (La,Mg) Ni₃ phase, Mg atom occupy at 6c position and occupation factor is 0.4. The maximum discharge capacity is 392mAh.g^-1 of the La₂MgNi₉ alloy.The structural and electrochemical properties of LaMgNi_3.7M_0.3 (M= Ni, Al, Mn, Co, Sn,Cu) were investigated in detail. It can be found that,by XRD and EPMA, the alloys consist mainly of LaMgNi4 phase with the cubic MgCu₄Sn structure and the Sn element in the LaMgNi_3.7Sn_0.3 alloy emerged mainly with the LaNiSn phase. Rietveld analysis for LaMgNi_3.7Al_0.3 alloy indicate that substituted Al element primarily occupies 3g position in the LaNi₅ phase. The solid solubility of the substituted elements in the LaMgNi₄ phase are in sequence of Mn> Cu> Co> Al> Sn. The electrochemical measurements show that the maximum discharge capacity increases from 245.2mAh/g(M=Sn) to 293.2mAh/g (M=Co), and the cycle stability are bad except LaMgNi_3.7Sn_0.3 alloy discharge capacity retention rate reach at 70% above after 50 cycle numbers. Meanwhile, the high rate discharge abilities of the alloy electrodes are in sequence of Al> Sn> Cu> Mn> Ni> Co(Id= 900mA/g). The hydrogen diffusion velocity is controlling factor for the dynamics ability.The composition of La_0.67Mg_0.33Ni_3.0 alloy synthesized with induction melting preparation methods and the effects of annealing treatment time on their crystal structure have been studied in this part. When the burning-loss quantity of La chemical element and Mg chemical element control at 4% and 20% respectively, 850℃ and 75h annealing treatment can guarantee the PuNi₃ type phase abundance at about 82wt%. It is show that increasing the annealing treatment time can improve the phase abundance of the PuNi₃ type. The structure of La(1-x)Mg_xNi₃(x=0,0.33,0.67) alloys were investigated by means of Rietveld method. It can be found that (La,Mg)-Ni and Ni-Ni distances in (La,Mg)Ni₃ phase decease with the increase of the Mg element and the change of (La,Mg)-Ni distance in AB₂ unit have a process of acceleration. It indicated the hydrogen storage capacity(H/M) decease with the increase of the Mg element and the variety of the hydrogen storagecapacity(H/M) in AB2 unit have a process of acceleration.The structure of Lao.67Mgo.33Ni2^Coo^ alloy and Lao.75Mgo.25Ni3.oCoo.5 alloy were also investigated in detail. It can be found that, by XRD and NRD, the Lao.67Mgo.33Ni2jCooj alloy consist mainly of (La,Mg)Ni3 phase with the rhombohedral PuNi3 type structure and the others are LaMgNi4 phase (MgCi^Sn type structure) and La2Ni7 phase (Ce2Ni7 type structure). In the (La,Mg)Ni3 phase ,the Mg atom position only occupy at 6c sites and the Co atom position occupy at 18h and 6c positions. In the Lao.75Mgo.25Ni3.oCoo.5 alloy , Mg atom occupy at 4f position and Co atom occupy at 4e> 4f % 6h and 121 positions of the Ce2Ni7 type structure .The distributing of Co element mainly located...