Thermodynamics of hydrogen solution in Pd–Y alloys

Abstract

Abstract Equilibrium hydrogen isotherms (343–523 K) have been determined for some low Y content fcc Pd –Y alloys. At a given p H 2 ≤ 1 bar, the hydrogen solubilities increase with atom fraction Y, X Y . Partial thermodynamic parameters, ΔH H and ΔS H , have been determined from these isotherms over a range of H contents. Extrapolation of ΔH H and ΔS H to infinite dilution of H, give ΔH H ∘ and ΔS H ∘ for 1 2 H 2 (g, 1 bar) absorption where the superscript zero indicates infinite dilution of H in the solid phase and 1 bar for H 2 in the gaseous phase. These parameters become increasingly negative with atom fraction Y. Thermodynamic factors, ( ∂ lnp H 2 1 / 2 / ∂ ln r ) T where r = H-to-metal, mol ratio, are needed for the interpretation of diffusion parameters. From equilibrium isotherms, lnp H 2 1 / 2 vs ln r , the slopes give the thermodynamic factors over a range of H contents and temperatures. Plots of the thermodynamic factors as a function of r exhibit minima which suggests that Fick's diffusion constants should also exhibit minima.