(The) changes of the hydrogenatuin properties of AB5type intermetallic compounds upon the pressure induced hydriding-dehydriding cycling

Abstract

In order to understand the intrinsic and extrinsic degradation mechanism, the degradation behaviors of the hydrogen absorption capacity and the hydriding kinetics of $LaNi_5$, $LaNi_{4.7}Al_{0.3}$ and $MmNi_{4.5}Al_{0.5}$ are investigated during the hydriding-dehydriding cycling in pure hydrogen and mixed gases ($H_2$-Co and $H_2$-$O_2$). And then, the degradation behaviors of the hydrogenation properties of $LaNi_5$ during the cycling are simulated. These theoretical results are compared with the experimental ones. Also, the kinetic studies on the hydriding reaction are carried out, because informations on the rate controlling step for the hydriding reaction are required for the computer simulation. In Chapter II, the intrinsic degradation behavior of $LaNi_5$ is investigated by the changes of Pressure-Composition-Temperature (P-C-T) curves of $LaNi_5-H_2$ system and the hydrogen desorption technique after the pressure induced hydrogen absorption-desorption cycling. Thermal desorption experiment of $LaNi_5$ hydrides shows only one peak in the hydrogen evolution rate vs. temperature plot for the activated samples but two peaks for the degraded samples. Low temperature peak of the degraded samples and a peak of the activated samples are related to the hydrogen evolved from the sites which can absorb and desorb reversibly. The high temperature peak of the degraded samples seems to be associated with the hydrogen from the stable hydrides such as several series of La-Ni stoichiometric compound hydrides and $LaH_2$. Such stoichiometric compound hydrides and $LaH_2$ will be formed by the disordering of the lattice and the changes of local composition of the degraded $LaNi_5$ hydrides. Ni clusters formed at the surface by the cycling are confirmed by the magnetization measurement, which indicates the formation of a series of stable hydrides. Thus it is concluded that the formation of the various stoichiometric compound hydrides and $LaH_2$ were responsible for the i...