Of Lani <sub> 5-x </ Sub> Al <sub> The X </ Sub> Alloy Absorption Desorption Properties Study

The thermodynamic properties of the hydriding-dehydriding process for the LaNi4.25Alo.75 and LaNi4.15Alo.85 alloys, and the dynamics properties of the LaNi4.25Al_0.75 hydrogen absorption, have been studied in this paper, as well as their activation behaviors, the influence of the annealing treatment on their microstructures and hydriding-dehydriding properties.The LaNi_5-xAl_x alloy may be well activated by hydriding at room temperature with the hydrogen purity of 99.999% and the pressure above one MPa, and then dehydriding by pumping at higher temperature for at least five cycles. There is a gestation period in their first activation hydriding process, which is influenced by the surface status of the alloys specimen. Because of the cracking of the oxidation layer over the surface of alloys and the powdering of their particles, the hydrogen adsorption-rate increases remarkably in the second hydriding cycle, and reaches constant after several hydriding and dehydriding cycles.The pressure-composition isothermal has a shaper plateau for the untreated LaNi_5-xAl_x alloys prepared by high frequency induction melting, and much of hydrogen remained in the specimen after dehydriding. The annealing treatment decreases the lattice strain and induces the crystal cell volume expansion. According to the XRD analysis, the annealing treatment conditions for these alloys is to heat them in vacuum at 1273K for more than 6 hours.At the temperatures of 313K, 333K and 353K, the plateau pressures of LaNi_4.25Al_0.75 are 5.5 kPa, 14.7 kPa and 37.2 kPa respectively for the hydrogen absorption, 4.4 kPa, 13.4 kPa and 32.5 kPa respectively for the hydrogen adsorption. And for the LaNi_4.15Al_0.85 alloy, the plateau pressures are 2.7 kPa,8.2 kPa and 21.2 kPa for hydriding, and 2.4 kPa, 7.3 kPa and 18.5 kPa for dehydriding at the above temperatures. At these temperatures the hysteresis coefficients between the hydrogen absorption and adsorption reactions are 0.22 ,0.09 and 0.15 respectively for LaNi_4.25Al_0.75, and 0.118, 0.116 and 0.136 respectively for the LaNi_4.15Al_0.85 alloy.The enthalpy and entropy are -43.9 kJ/(mol H₂) and -115.8 J/(mol H₂) respectively for the LaNi_4.25Al_0.75 hydrogenation reaction, and 45.9 kJ/(mol H₂) and 120 J/(mol H₂) respectively for its dehydrogenation reaction. And for the LaNi_4.15Al_0.85 alloy, they are -47.3 kJ/(mol H₂) and -121.1 J/(mol H₂), 46.9 kJ/(mol H₂) and 118.9 J/(mol H₂) respectively for the hydrogenation and dehydrogenation reactionThe hydriding rate of LaNi_4.25Al_0.75 in a phase is faster than that inα+βphase region. The rate constants are different in a phase and in a+P phase region, which are linear with (P/Pe)^1.5[1-(P_f/P)^1.5(C/C_f)] and (P/Pe)²[l-(P_f/P)²(C/C_f)] respectively in these phase at different hydrogen concentrations. The activation energy of the hydriding reaction inα+βphase region is 35.6kJ/mol H₂.