The thermodynamic parameters for solution of hydrogen at infinite dilution and the (a+B) plateau thermodynamic properties for hydrogen absorption at moderate temperatures for a series of Pd‐based f.c.c binary alloys with relatively low solute metal content have been correlated from the viewpoint of whether the alloy lattice is expanded or contracted to parent Pd lattice. Newly determined data for Pd‐Zn, Pd‐Sb, Pd‐Bi, Pd‐Cr, Pd‐Mo and Pd‐Mn alloys are included. When alloys with solute metals in the same groups are compared, the lattice parameters of Pd binary alloys with the 4 d and 5 d transition elements are, in general, larger than those with the 3 d elements. The relative partial molar enthalpy, ΔH0H, at infinite dilution becomes more exothermic with an increase of lattice parameter for the lattice‐expanded alloys with the exception of Pd‐Pt alloys. Conversely, for contracted alloys, with the exceptions of Pd‐Li, Pd‐Zn, Pd‐Al and low Ti content Pd‐Ti alloys, the enthalpy becomes less exothermic with increasing lattice contraction. The dilute phase solubility at the same temperature as reflected by the relative chemical potential of dissolved hydrogen at infinite dilution, Δμ0H, increases with increasing lattice expansion for the expanded alloys, with the exceptions of Pd‐Pt and Pd‐Nb (Ta), whereas for the contracted alloys, the solubility decreases with increasing lattice contraction, with the exception of the Pd‐Li alloys. The standard free energy change, ΔG0plat' for B‐hydride formation in the expanded alloys decreases with increase of lattice expansion with the exceptions of the Pd‐Pt, Pd‐Zr, Pd‐Sb, Pd‐Nb(Ta) and Pd‐Mn alloys. For the contracted alloys, the B‐hydride becomes less stable with increasing lattice contraction except for the Pd‐Li alloys. The isobaric hydrogen solubilities in Pd alloys at relatively high pressures may be attributed to the influence of the solute metal atoms on the Pd band structure and to the exclusion of H from site occupation due to solute atoms in the nearest neighbor shells; both of these effects are independent of the lattice expansion and contraction.

The hydrogen absorption characteristics of fully annealed Pd‐Rh alloys containing up to 10.0 at.% Rh have been investigated at temperatures between 273 K and 433 K and hydrogen pressures up to 1000 Torr by means of pressure‐composition isotherm measurements. For the p‐c isotherms of Pd‐7.5 at.% Rh alloy, in addition to the normal (α+β) two‐phase plateaus, “anomalous plateaus” are observed near the normal βmin phase boundaries. The formation of this anomalous plateau implies that in the fully annealed alloys some precursory phase separation exists, together with the metastable, homogeneous solid solution phase. The low pressure hydrogen solubilities decrease with an increase of Rh content and the main plateau pressures increase with the solute metal content. The hydrogen solubilities exhibit a maximum at about 7.5 at.% Rh at 1000 Torr. The derived relative partial molar thermodynamic quantities at infinite dilution, and also the plateau thermodynamic parameters are in agreement with the previously reported results. These thermodynamic quantities are not affected by the formation of the anomalous hydride phase, because the hydride does not form in the dilute phase region, and because the amount of hydrogen involved in the anomalous hydride phase is small compared to the main hydride phase.

The solubility of hydrogen in Pd – Ti and Pd – Zr solid solution alloys containing up to 6.0 at.% Ti and Zr has been measured at temperatures between 273 K and 433 K and hydrogen pressures up to 1000 Torr. The relative partial molar enthalpy, ΔH0H, at infinite dilution for Pd – Ti alloys shows a minor minimum around 4 at.% Ti, in spite of the Pd lattice contraction on Ti substitution. For Pd – Zr alloys the ΔH0H values increase in exothermicity regularly with Zr content. The relative partial molar entropy, ΔS0H, at infinite dilution in both alloy series decreases with the solute contents. The apparent H‐H attractive interactions, g1, in Pd – Ti(Zr) alloys decrease with alloying metal content, and the trend of the g1 value with Ti content is dissimilar to those found for other lattice contracted alloys. The stability of the β‐phase hydride in Pd – Ti alloys decreases with Ti content and the β‐phase hydride in Pd – Zr alloys also becomes less stable with Zr content, albeit by a small amount, despite being a lattice expanded alloy system. Solvus compositions at the αmax phase boundary in both Pd – Ti and Pd – Zr alloys are almost equal at the same alloying compositions of each alloy series, although increasing with alloying metal content in both cases.

Hydrogen in Metals J Metals 1 Pd -Gd Solid Solution Alloys j Phase Transitions 1 Pressure-composition Isotherms J ThermodynamicsThe thermodynamic quantities for absorption of hydrogen by disordered Pd-Gd solid solution alloys containing up to 8.0 at.% Gd have been determined at temperatures between 273 and 473 K and hydrogen pressures up to 100 Torr by measuring the pressurecomposition isotherms. The standard relative partial molar enthalpy of solution of hydrogen, A Hi?,, at infinite dilution becomes more exothermic with increasing Gd content. Whereas the standard relative partial entropy, AS:, at infinite dilution decreases with Gd content; this implies that the octahedral interstices are excluded for hydrogen occupation by Gd neighbors. Substitution of Gd in the Pd lattice decreases the apparent H-H attractive interaction and for the Pd-4.5 and 6.0 at.% Gd alloys the H-H interaction becomes repulsive over a certain range of H contents. The results are discussed in terms of the quasi-chemical model by taking into the constant volume correction. The stability of P-hydride phase increases with Gd content, and the extent of the phase boundary, i.e., the hydrogen solvus composition at a given temperature, is more favourable by Pd alloying with Gd.

diffusion in solids diffusion in solids D 4000 -009Permeability and Diffusivity of Hydrogen in Palladium-Rich Pd-Y(Gd)-Ag Ternary Alloys.-The permeabilities and diffusivities of hydrogen in Pd100-x-yLnxAgy ( Ln: Y, Gd; y = 0-24; y+3x = 24) are measured as a function of composition in order to study the applicability of these alloys as diffusion membranes for hydrogen purification. In the pressure and temp. ranges studied (267-667 kPa, 523-673 K) the permeability and solubility are not significantly dependent on the hydrogen input pressure. The best materials for hydrogen diffusion membranes are the Pd-Ln-Ag alloys containing 15-20 at.% Ag and 1.3-3.0 at.% Ln. -(SAKAMOTO, Y.; CHEN, F. L.; FURUKAWA, M.; NOGUCHI, M.; J.