A microcalorimetric investigation of the thermodynamics and kinetics of hydriding-dehydriding reactions

Abstract

A microcalorimetric technique was used to study the thermodynamics and kinetics of the hydriding-dehydriding reactions of some AB 5-type alloys. The effects of Ce, Co and Sn substitution were analyzed. Co and Sn substitution increase the stability of the hydride state, while Ce substitution has the opposite effect. These observations are rationalized on the basis of the rule of reversed stability and by the unit cell volume changes upon substitution. Co substitution decreases the hydrogen diffusion coefficient due to the smaller volume expansion of the alloy during hydrogen absorption. Ce substitution for La results in a show hydrogen chemisorption rate on the surface of the alloy. In order to use the microcalorimeter to study the kinetics of the hydriding reaction. A correlation due to a smeared function of time-delay effect of equipment through a convolution integration was necessary. The rate-determining step changes during the course of the hydriding reaction. The validity of the shrinking-core model was tested. The diffusion coefficients of hydrogen in the β-hydride phases of the tested alloy phases are in the range of 10 0 to 10 10 cm 2 s 1.