| In this thesis, the publications on the effect of chemical composition and preparation methods on the phase structure, hydrogen storage properties and electrochemical properties of AB5 and AB3 type hydrogen storage alloys were reviewed. On this basis, Co-free La(NiMMn)6.6 ,La(NiMn)6 and AB3 type La-Mg-Ni alloy ,were chosen as objects of study. By means of XRD analysis, PCT and electrochemical measurements, the effects of chemical composition, the preparation methods such as annealing and rapidly quenching on the phase structures, hydrogenation-dehydrogenation characteristics and electrochemical properties were studied systemically.Based on the aforesaid investigation, the La(NiMn)5.6 alloy was chosen and prepared in different ways, i.e. as the conventional casting(as-cast), high-temperature annealing(100Cx168h), the melt-spinning(35m/s), the melt-spinning (35m/s) +low-temperature annealing (400CX1h) .It is found that the alloy prepared with all these methods were all of the CaCu5 phase with different microstructures and hydrogen storage properties. There was second phase in the as-cast alloy. Compared to the as-cast alloy, the annealing and melt-spinning alloys were single phase. The hydrogen capacity per metal atom of spinning +low-temperature annealing was 1. 24, but after spinning was 1.09. In the meantime, the hysteresis was very small and plateau was flatness after high annealing. The electrochemical performance measurement shows that the high heat treatment and spinning +low-temperature annealing can effectively improve the electrochemical activation property, but spinning decrease.For the a(NiMMn)5.6 alloys, the effect of La partial replaced by Ce, nickel partially replaced by other elements such as Cu, Al, Sn on the structure and hydrogen storage properties of melt-spun alloys were studied. It is found that all melt-spun alloys are all of single phase. The partial substitution of Ce in La(NiMn)5.6 results in improve active rate, high equibilibrium , low decrease of the hydrogen capacity , but low hysteresis. It was found that Cu-, Al- and Sn-substitutions could lower the plateau pressure and decrease the hysteresis of La(NiMMn)5.6 The hydrogen capacity of partially replaced by Cu, Al decrease and replaced by Sn increase. The electrochemical capacity of partially replaced by Cu, Al can be improved, but replaced by Sn worse.For the LaNi4.75Mn1.25 alloys, the effect of high-temperature annealing and the melt-spinning on the structure and hydrogen storage properties. There was second phase in the as-cast alloy. Compared to the as-cast alloy, the annealing and melt-spinning alloys were single phase. Theas-casting+high-temperature annealing alloy has more a homogeneous equiaxed microstructure with large grain size. Hydrogen absorption - desorption studies at 303K showed: LaNi4.75Mn1.25 alloys have an increase of the plateau pressures and decrease in both enthalpy and entropy. The annealing is a 'flat' structure of the pressure - composition isotherms, a rather broad range of working H storage capacities, a high hydrogen capacity. Plateau pressure increase and hydrogen content of melt-spinning alloy decrease. The melt-spinning alloy is a 'sloper' structure of the pressure - composition isotherms. Compared to the as-cast alloy, the high-temperature annealing has worse activation properties and lower discharge capacity; the melt-spinning alloy has a poorer activation property and lower discharge capacity.For La0.67Mg0.33Ni3N La0.67Mg0.33Ni2.5Co0.5 La0.67Mg0.33Ni2.8Co0.5 alloy, muliti-phase strucyures were abserved in all of these three alloys. All the alloys the main phase are composed of the (La, Mg) Ni3 phase (PuNi3 -type structure) , but La0.67Mg0.33Ni2.8Co0.5 composed of the (La, Mg)2Ni7 phase. PCT showed, plateau pressure decrease and hydrogen content increase with the order of La0.67Mg0.33Ni3, La0.67Mg0.33Ni2.5Co0.5, La0.67Mg0.33Ni2.8Co0.5 alloys. The maximum discharge capacities of these alloys obtained were: 389mAh/g 404 mAh/g 387 mAh/g. |
PDF Full Text Request