La4MgNi19水素吸蔵合金のMg蒸気圧制御におけるアニール条件と水素吸蔵特性

Abstract

In La-Mg-Ni system hydrogen storage alloys, which have been attracting attention as a promising negative electrode material for Ni-MH secondary batteries, there are many phases with a variously layered stacking structure. When subjected to annealing at high temperature, the composition of these alloys becomes inhomogeneous due to the volatilization of Mg. It is therefore difficult to prepare a single phase by the normal annealing procedure. We carried out high-temperature annealing processing in which we controlled the Mg vapor pressure of La4MgNi19 alloy using a temperature-gradient furnace. By controlling the Mg vapor pressure under the sealing atmosphere, we successfully obtained homogeneous La4MgNi19. We prepared a PMg-T diagram of the phases formed for La4MgNi19 composition, and found the optimum annealing conditions for the La4MgNi19 phase. The optimum annealing conditions obtained were between T=1123 K, PMg=0.4 kPa and T=1223 K, PMg=2 kPa. However, we found the coexistence of 3R-type and 2H-type structures, which are polytypes of La4MgNi19. The lattice constants of the 3R type were a=0.5031 nm, c=4.826 nm, and those of the 2H type were a=0.5032 nm, c=3.216 nm. The hydrogen storage characteristics of the annealed alloy were obtained by measuring the PCT curves at 333 K. The PCT curves showed an almost flat plateau, PH2=0.1 MPa, H/M=1.1, 1.5 mass%H2, and good cycle characteristics. It was confirmed that the dehydrogenated alloy retained the same structure but an expansion of 1 to 2% was observed in the c-axis.