Thermodynamic Behavior of Hydrogen in Palladium‐Gadolinium Solid Solution Alloys

Abstract

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.

Thermodynamics of Hydrogen Dissolved in Pd‐Tb and Pd‐Dy Alloys

“…The AH: values in both alloys become more exothermic with increasing Tb and Dy contents; the exothermicity of hydrogen solution in Pd-Dy alloys is a little larger than that in Pd-Tb alloys at the same composition, and the AH: values in these alloys are less exothermic compared to the previously determined values for Pd-Gd alloys [38] which expand the Pd lattice more, conversely they are more exothermic than those for Pd-Sc [23] and Pd-Yb [41] alloys which have a smaller lattice expansion. The AH: value for 8.0 at.% Tb alloy tends to become again less exothermic.…”

“…On the other hand, the gl values, as shown in Table 1, become less negative with Tb and Dy contents, i.e., the apparent H -H attractive interaction free energies decrease in the presence of Tb(Dy). It can be seen that the stability of dissolved hydrogen in Pd-Tb and Pd-Dy alloys increases with increasing the solute metal content, and it is nearly the same for both alloys at the same content up to about 6at.Vo Tb(Dy), but the dissolved hydrogen in both alloys is more unstable compared to that in high Gd content Pd-Gd alloys [38], while the dissolved hydrogen is more stable than those in Pd-Sc . z is the number of neighboring lattice sites around a given interstices; z = 6 for the first nearestneighbour shell, and z = 14 for the first and second nearestneighbour shells for octahedral interstices in f.c.c.…”

Towards Designing Stable Pd‐Based Membranes for Hydrogen Gas Separation: A Statistical Thermodynamics Approach

“…The calculated phase boundaries are in good agreement with the experimental results reported in ref. [23][24][25][26][27] .…”