Hydrogenation Properties of TiFe Doped with Zirconium
The goal of this study was to optimize the activation behaviour of hydrogen storage alloy TiFe. We found that the addition of a small amount of Zr in TiFe alloy greatly reduces the hydrogenation activation time. Two different procedural synthesis methods were applied: co-melt, where the TiFe was melted and afterward re-melted with the addition of Zr, and single-melt, where Ti, Fe and Zr were melted together in one single operation. The co-melted sample absorbed hydrogen at its maximum capacity in less than three hours without any pre-treatment. The single-melted alloy absorbed its maximum capacity in less than seven hours, also without pre-treatment. The reason for discrepancies between co-melt and single-melt alloys was found to be the different microstructure. The effect of air exposure was also investigated. We found that the air-exposed samples had the same maximum capacity as the argon protected samples but with a slightly longer incubation time, which is probably due to the presence of a dense surface oxide layer. Scanning electron microscopy revealed the presence of a rich Zr intergranular phase in the TiFe matrix, which is responsible for the enhanced hydrogenation properties of these Zr-doped TiFe alloys.
HYDROGEN ABSORPTION IN RARE EARTH INTERMETALLIC COMPOUNDS
ABSTRACT A review is given of the various rare earth intermetallic compounds in which the possibility of absorption of large quantities of hydrogen gas has been reported in literature hitherto. The experimental material is compared with expectations of a model that predicts the tendency of hydrogen absorption from the stabilities of the uncharged intermetallic compounds and the corresponding binary rare earth hydrides. A brief evaluation is given of the applicability of other models describing the hydrogen absorption in these materials. Experimental evidence has been obtained that in some cases the absorption of hydrogen gas leads to phase separation or microprecipitation of the binary hydride. A discussion is given of the origin of this effect and its importance in applications involving continuous H2 absorption and desorption cycles.
Kinetic measurements of LaNi5Hx and MmNi4.15 Fe0.85Hx with constant pressure differentials
Abstract Reaction rate measurements were carried out for the decomposition of LaNi5Hx and MmNi4.15Fe0.85Hx, in the two-phase region, in the temperature range 20–60°C. Isothermal conditions were maintained by using the “thermal ballasting” method and high heat transfer reactors. The desorption kinetics were determined from isothermal experiments with constant pressure differentials. Under these conditions, it was possible to obtain reaction rates depending on temperature only. The results for LaNi5 show a linear dependence of ln( C 0 C ) on time t, indicating first-order kinetics and yielding reaction rate constants in the range 0.018–0.075 s−1. An Arrhenius plot yields an activation energy Ea of 6.9 kcal mol−1. The dependence of the reaction rate on pressure for LaNi5 is expressed in the rate equation by a factor ΔP = Peq − Pdwn, where Pdwn is the downstream pressure. For MmNi4.15Fe0.85Hx, the slope of the equilibrium plateau pressure makes the separation of the pressure dependence of the rate constant k from the temperature dependence more complex than for LaNi5.