Hydrogenation properties of Ti-V-Mn alloys with a BCC structure containing high and low oxygen concentrations

The hydrogenation properties of Ti_1.0V_1.1Mn _0.9 alloys with a BCC structure, prepared using two V materials with different oxygen concentrations, were studied. The sample prepared using a conventional-grade V material contained 0.530 mass% of oxygen, and the other sample prepared from a low-oxygen V material contained 0.051 mass% of oxygen. Both samples showed P-C isotherms with two plateaus corresponding to formation of monohydrides and dihydrides. The sample with the higher oxygen concentration had an equilibrium pressure for dihydride formation 6.7 times higher than that of the sample with the lower oxygen concentration at 0 °C. There was no significant difference between the two samples with regard to the equilibrium pressure for formation of monohydride or the hydrogen capacity. The formation enthalpies were calculated from the van't Hoff plots: -30.2 kJ/molH₂ and -35.5 kJ/molH₂ for the dihydrides, and -39.7 kJ/molH₂ and -39.8 kJ/molH₂ for the monohydrides. Scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM-EDX) and X-ray diffraction (XRD) indicated that the sample with the higher oxygen concentration contained a Ti-rich secondary phase, which caused changes in the composition and in the lattice parameter of the main phase. These results indicated that increasing the oxygen concentration altered the hydrogenation properties as a result of (1) the effect of oxygen solved in the main phase on hydrogen occupation and (2) a composition change resulting from secondary phase formation.