Further studies of the isotherms of LaNi5-xSnx-H for x=0-0.5

Complete hydrogen isotherms have been measured using highly stoichiometric, activated LaNi5 over a wide temperature range, 373-228 K. Van't Hoff plots for hydride formation and decomposition have been constructed using the isotherms in the low temperature range from 228 to 298 K. The following thermodynamic results are obtained: \Delta H-plat\=15.4 kJ/mol 1/2 H-2 and \Delta S-plat\=54.4 J/K mol 1/2 H-2. These should be regarded as reliable in view of the nearly stoichiometric LaNi5 employed and because utilizing data over a wide, low temperature range avoids problems introduced by degradation of the alloy which can be a factor at higher temperatures due to the inherent instability of the H-containing alloy. It has been observed previously in the LaNi5-H system that the absorption plateau measured with virgin alloys, i.e., before activation, is significantly higher than the subsequent values. This phenomenon was investigated in this research using the LaNi5-xSnx alloys, with x=0, 0.05, 0.10, 0.20, 0.25, 0.32, 0.40 and 0.50. Isotherms for the virgin and activated alloys were measured at 373 K. The difference between the initial plateau pressures for hydride formation and values for activated material decrease with increase of x. An important result is that the initially high plateau pressure for hydride formation for alloys is related to hysteresis because the difference between the plateau pressures before and after activation almost vanishes at a Sn content of x=0.32, where hysteresis itself nearly vanishes. (C) 1998 Elsevier Science S.A.