Positron lifetime study on the degradation of LaNi5 and LaNi4.8Sn0.2 during hydrogen absorption-desorption cycling

LaNi5 and LaNi4.8Sn0.2 alloys have been subjected to pressure-induced cycling tests at 248 K under a hydriding condition at a hydrogen pressure of 3 MPa and under a dehydriding condition of evacuating by a rotary pump. At the beginning of cycling the hydrogen absorbing capacity of LaNi5 was larger than that of LaNi4.8Sn0.2. However, the capacity of LaNi5 Was markedly decreased by the cycling, whereas the capacity of LaNi4.8Sn0.2 was hardly changed. After 100 cycles, the capacity of LaNi5 became smaller than that of LaNi4.8Sn0.2. On the other hand, it was also found that the increase in the positron mean lifetime by the cycling for LaNi4.8Sn0.2 was smaller than that for LaNi5, revealing that substitution of Sn suppresses the formation of vacancy during the cycling. This result suggests that the degradation in hydrogen absorbing capacity should be attributed at least partly to trapping of hydrogen at excess vacancy formed by hydriding. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.