HYDRIDING INFLUENCE OF INDIUM ON Pd-In ALLOYS

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Hydrogen absorption and desorption isotherms were measured in a microbalance in the pressure range between 0.001 MPa and 3.5 MPa and at seven temperatures between 381 K and 621 K on four Pd-In alloys between 3.6% and 15% (in atom) In as well as on pure Pd. The results are similar to those obtained on the analogous alloys Pd-Ag and Pd-Sn and are interpreted in the same manner. Indium has two opposing effects upon the solubility of H in the Pd alloys, increase of the Fermi energy upon the donation of valence electrons and a lattice dilatation. Al valence electron concentrations below 0.5, the lattice dilatation predominates and the H solubility is increased with addition of In. At valence electron concentrations above 0.5, the influence of elevated Fermi energy predominates and the H solubility is lowered by addition of In. The molar enthalpy of Pd hydriding al infinite dilution is strongly increased with increasing In content, bur the molar entropy is not much affected by In. The excess chemical potential of hydrogen at small hydrogen concentrations and at a given temperature increases with increasing In. The apparent H-H attractive interaction energy, WH-H, is decreased by the presence of In.