Abstract
Hydrogen absorption alloys are functional materials that can be used for thermal and mechanical energy conversion by utilizing the heat of reaction in hydrogen absorption and desorption. On the other hand, they show greater or lesser degradation behavior in a hydriding capacity resulting from absorption and desorption cycling. The degradation by cycling is divided into two phenomena: intrinsic degradation attributed to the structural or phase changes of the alloy, and extrinsic degradation due to the surface contamination of the alloy by gaseous impurities in hydrogen gas. In LaNi5 having a CaCu5 structure, it has been recongnized that the intrinsic degradation is relatively large. In the present study, the intrinsic degradation of CaNi5 having the same structure as LaNi5 was examined with respect to the effects of temperature and Mm and/or Al substitution for Ca and Ni on the degradation properties of the CaNi5.
The intrinsic degradation of CaNi5 was slightly larger than that of LaNi5. The temperature dependence of the degradation, which becomes larger with rising temperature, was exhibited clearly and quantitatively. Al substitutions for Ni were shown to prevent the degradation of CaNi5. The number of cycles needed to achieve 50% degradation of the initial capacity of (CaMm) (NiAl)5 alloy at a temperature below 333 K was over 50,000. Consequently the (CaMm) (NiAl)5 alloy seemed to have an excellent cycle life for use in a practically absorption and desorption cycling system. From observation of the fully degraded CaNi5 sample using transmission electron microscopy, the grain sizes of the CaNi5 crystals were recognized to be about 10 nm in diameter. Accordingly, the mechanism of intrinsic degradation of CaCu5-type alloy by absorption and desorption cycling was inferred to be the disordering of crystal structure by reduction of the grain size of the crystal, in a manner different from disproportionation.
