Abstract
The pressure-composition (P-C) isotherm of hydrogen in alloys is important for the practical use of hydrogen-absorbing alloys. A numerical expression method was developed for the plateau region of the P-C isotherm in LaNi5, LaNi4.8Mn0.2 and La0.8Y0.2Ni5 alloys with the CaCu5 structure.Samples were prepared by using argon arc melting and annealing. The desorption isotherms at 298 K were used for testing the numerical expression. The regular solution model with a term proportional to hydrogen composition as an excess chemical potential was applied to the solid solution and hydride phase region. A normal cummulative distribution function (ncdf) model with composition as frequency was applied to the plateau region.All the isotherms were expressed accurately with the models employed and characteristic parameters of the alloys were obtained. The changes in the micro-structural heterogeneity in the alloys by annealing and the Mn or Y substitution were in accordance with those in the standard deviation (σ) of the ncdf which presents the plateau slopes.Consequently, the normal cummulative distribution function model for the plateau region was found to be helpful for the development of the alloys because σ presents both the plateau slope and the micro-structural heterogeneity in the alloys.
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