THERMODYNAMICS OF SOLUTION OF HYDROGEN IN PD-CU AND PD-CU-AU SOLID-SOLUTION ALLOYS

Thermodynamic quantities for absorption of hydrogen by Pd-Cu alloys with up to atom fraction of X(Cu) = 0.2, and by Pd-Cu-Au ternary alloys with up to (X(Cu) + X(Au)) = 0.2, where X(Cu) = X(Au), have been determined at temperatures between 273 K and 423 K and hydrogen pressures up to 1000 Torr by measurements of the pressure-composition isotherms. The hydrogen absorption characteristics of these alloys have been compared with that of the previously reported Pd-Au binary alloys. The room temperature lattice parameters of hydrogen-free Pd-Cu-Au alloys increases with the solute content, albeit by a small amount. The relative partial molar enthalpy, DELTA-H(H)0, at infinite dilution in Pd-Cu-Au alloys increases in exothermicity with increasing (Cu + Au) content, and the alloy composition dependence of the DELTA-H(H)0 value falls near the mid-points between those of each Pd-Cu and Pd-Au binary alloy. The relative partial molar entropy, DELTA-S(H)0, at infinite dilution in Pd-Cu-Au alloys decreases with the solute content, and the DELTA-S(H)0 values at the same alloying content are larger than that of Pd-Au alloys, but are smaller than that of Pd-Cu alloys. The [H-Cu + H-Au] pair interaction energies between the dissolved hydrogen and the substitutional solute atoms in the ternary alloys have been evaluated according to a quasi-chemical model with zeroth order approximation by taking into account the "constant pressure" to "constant volume" correction. The hydrogen solubility behaviour at low hydrogen concentration in Pd-Cu-Au ternary alloys can be described as due to the counteraction of the repulsive H-Cu pair interaction and the attractive H-Au pair one in the Pd host lattice. The stability of the beta-phase hydride at a given temperature in Pd-Cu-Au alloys decreases with the solute content, and the alloy composition dependence of the stability is also placed near the mid-points between those for each Pd-Cu and Pd-Au binary alloy.