Solubility of Hydrogen in Palladium-Yttrium-Silver Alloys

Thermodynamic quantities for absorption of hydrogen by Pd_95–xY_xAg₅ solid solution alloys with up to x = 6.3 at.% Y have been determined from measurements of pressure-composition isotherms at temperatures between 273 K and 473 K and hydrogen pressures up to 1000 Torr. The hydrogen absorption characteristics have been compared with those of the previously reported Pd–Ag and Pd–Y binary alloys. The relative partial molar enthalpy, ΔH_H⁰, at infinite dilution of hydrogen in Pd_95–xY_xAg₅ alloys increases in exothermicity with increasing Y content from that of Pd–5.0 at.% Ag, almost parallel to that in Pd–Y binary alloys. The relative partial molar entropy, ΔS_H⁰, at infinite dilution in the ternary alloys decreases with Y content from that of Pd–5.0 at.% Ag alloy. The hydrogen solubility behavior at low hydrogen concentration in the Pd–Y–Ag alloys under a “constant volume” condition can be described as due to mainly the strong H–Y attractive pair interaction in the Pd host lattice. The stability of β-hydride and hydrogen concentration at α_max phase boundary increase following alloying with Y from that of Pd–5.0 at.% Ag alloy, respectively, and the changes in the properties with Y content are similar to that in Pd–Y binary alloys.