Annealing conditions with Mg vapor-pressure control and hydrogen storage characteristic of la ₄MgNi ₁₉ hydrogen storage alloy

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 La ₄MgNi ₁₉ alloy using a temperature-gradient furnace. By controlling the Mg vapor pressure under the sealing atmosphere, we successfully obtained homogeneous La ₄MgNi ₁₉. We prepared a P _Mg-T diagram of the phases formed for La ₄MgNi ₁₉ composition, and found the optimum annealing conditions for the La ₄MgNi ₁₉ phase. The optimum annealing conditions obtained were between T= 1123 K, P _Mg = 0.4 kPa and T = 1223 K, P _Mg = 2 kPa. However, we found the coexistence of 3R-type and 2H-type structures, which are polytypes of La ₄MgNi ₁₉. 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, P _H2 = 0.1 MPa, H/M= 1.1, 1.5 mass%H ₂, 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.