THERMODYNAMIC PROPERTIES FOR SOLUTION OF HYDROGEN IN PD-AG-NI TERNARYALLOYS

The hydrogen absorption characteristics of well-annealed Pd100-2xAgxNi x ternary solid solution alloys with x (at.%) =1.5, 3.0, 4.5, 6.0 and 7.5 have been investigated at temperatures between 273 and 433 K at hy drogen pressures up to 1000 Torr (133.32 kPa) by means of pressure-com position isotherm measurements and X-ray measurements of the lattice p arameters of hydrogen-free and hydrogenated alloys. The room temperatu re lattice parameters of hydrogen-free ternary alloys decrease slightl y with increasing solute metal content. The relative partial molar ent halpy Delta H-H degrees at infinite dilution for the Pd-Ag-Ni alloys b ecomes more exothermic with solute metal content, despite the lattice contraction of the alloys. The behaviour is near the mid-point between those of Pd-Ag and Pd-Ni binary alloys. The partial molar entropy Del ta S-H degrees(beta = 1) at infinite dilution decreases with solute me tal content. The exceptional behaviour of the Delta H-H degrees values in the ternary alloys for the hydrogen absorption of Pd alloys on the basis of the lattice 'expanded' or 'contracted' categories may be att ributed to the preferential occupation of Pd-rich interstices near Ag atoms rather than near Ni atoms in the alloy solid solutions. The beta -hydride in Pd-Ag-Ni ternary alloys becomes more unstable with increas ing solute metal content, and the standard free energy change Delta G( plat)degrees at 298 K for the hydride formation is nearly between that for each binary alloy.