| With the natural energy depleting and the awareness of environmental protection enhancing, it is urgent to develop the clean new energy sources. As a clean secondary energy, hydrogen arouses extensive attention.The problem of hydrogen storage and transport is solved effectively through studying hydrogen storage materials.The AB3-type La0.67Mg0.33Ni2.5M0.5(M=Co,Cu) hydrogen storage alloys in which ferromagnetism Ni was partly substituted with ferromagnetism Co and diamagnetism Cu were studied in this thesis. La0.67Mg0.33Ni3.0, La0.67Mg0.33Ni2.5Co0.5 and La0.67Mg0.33Ni2.5 Cu0.5 were firstly prepared by vacuum induction furnace, and then treated using the conventional heat treatment and heat treatment under an external magnetic field, respectively. In this paper, the alloys performances were analysised by the XRD, PCT and DSC. The influence of element substitution and heat treatment to the performance of thermodynamics and kinetics were studied systematically. And the magnetic-heat treatment process of alloys was optimized. At the same time, the hydrogen absorption/desorption kinetic mechanism of the La0.67Mg0.33Ni2.5M0.5(M=Co,Cu) alloys was studied by Chou model.It is found that the La0.67Mg0.33Ni3-xMx(M=Co,Cu)(x=0,0.5) alloys are consist of two major phases of (La,Mg)2Ni7 and LaNi5 and minor phase of (La,Mg)Ni3. In the as-cast alloys, when the Ni element was partly substituted by Co and Cu, the hydrogen storage capacity increased, the temperature of hydrogen desorption decreased, and the absorption/desorption tc reduced too. Thereinto, the effect of Co element substituting was best. Those results indicated that elements substitution could modify the thermodynamics and kinetics performance of alloys. After the as-cast alloys heat treamment, the minor phase transformed into main phase and the the hydrogen storage capacity increased obviously. However, the performance of hydrogen desorption was not steady and the temperature of hydrogen desorption raised.The magnetic-heat treatment process of La0.67Mg0.33 Ni2.5M0.5(M=Co,Cu) alloys was optimized by orthogonal trial. The influence of magnetic-heat treatment to the performance of thermodynamics and kinetics were explored preliminarily. The results indicated that the optimum scheme was 850℃×1h×4T for La0.67Mg0.33Ni2.5Cu0.5 alloys and it was 800℃×2h×1T for La0.67Mg0.33Ni2.5Co0.5 alloys whose hydrogen storage characteristics were better, it could absorb 1.40 wt% hydrogen and desorb1.37wt% hydrogen. The DSC measurement showed that the enthalpy was -44.1kJ/mol and the endothermic peak was 77.4℃. the characteristic times(tξ=0.875)of hydriding/dehydriding reaction were 10.3s and 166.5s, respectively.The influence of magnetic-heat treatment to hydrogen absorption kinetics curves of La0.67Mg0.33Ni2.5M0.5(M=Co,Cu) alloys was studied by Chou model. At the same time, the action mechanism of temperature and pressure effecting to the hydriding/dehydriding kinetics was analysised. The results showed that rate determining step was the hydrogen diffusion among the hydrid layer in the hydrogen absorption and desorption process. Compare to the characteristic time (tc) of hydrogen absorption and desorption, magnetic-heat treatment modified the performance of hydrogen absortion knetics, while the one of hydrogen desorption knetics was not steady; The temperature could not influence the performance of hydrogen absorption and desorption obviously while the pressure had great influence to the hydrogen absorption knetics, the higher pressure, the quicker speed of hydrogen absorption. In a word, La0.67Mg0.33Ni2.5Co0.5 alloys had a better kinetic performance. The characteristic time(tc)of hydriding/ dehydriding reaction were 91.4s and 379.3s, respectively. The calculated adsorption/desorption activation energies were 16.3 kJ/mol H2 and 23.3kJ/mol H2, respectively.
|
PDF Full Text Request