POISONING BY AIR OF AB(5) TYPE RARE-EARTH NICKEL HYDROGEN-ABSORBING ALLOYS

Poisoning by air of Mm(0.9)Y(0.1)Ni(4.9)Mn(0.1) and LaNi4.55Al0.45 alloys with CaCu5 structure was studied to clarify their applicability to refrigeration systems. The decrease in hydrogen capacity on hydrogen absorption-desorption cycling with air contamination was limited to a certain degree under an extended air concentration up to 1.0 vol.%. The poisoned alloys recovered their hydrogen-absorbing capacity on reactivation treatment. These results and X-ray powder diffraction analysis suggest that the degradation was not caused by complete oxidation. Thus, we propose that the degradation is caused by adsorption of oxygen in air, and that the poisoned state is an intermediate state which is more stable than the adsorption equilibrium state but less stable than the oxidation state. Poisoning by adsorption was analyzed using Langmuir's adsorption theory to explain the relations between the degradation rate and concentration of air.