Absorption of hydrogen by Pd–Nb(Ta) solid solution alloys

Abstract

Thermodynamic quantities for absorption of hydrogen by Pd–Nb and Pd–Ta alloys in the composition range 2.0–8.0 atom % Nb and 2.0–6.0 atom % Ta have been determined from measurements of pressure–composition isotherms at temperatures between 273 and 433 K and hydrogen pressures up to 1000 Torr. The plateau pressure of hydrogen corresponding to the α→β phase transition increases with Nb(Ta) content, despite the lattice expansion upon alloying with Nb(Ta), and the solubility of hydrogen decreases with increasing the solute metal content. The relative partial molar enthalpy, ΔH⁰_H, at infinite dilution increases in exothermicity up to ca. 4 atom % Nb(Ta), and then in the alloys at greater than 4 atom % Nb the ΔH⁰_H values decrease in exothermicity. The ΔH⁰_H values for the Pd–Ta alloys are nearly constant up to 6 atom % Ta. The behaviour of the ΔH⁰_H values for these alloys conforms to that expected on the basis of the ‘expanded’–‘contracted’ classification of Pd alloys. The relative partial molar entropy, ΔS⁰_H, at infinite dilution decreases more or less regularly with Nb(Ta) content. This implies that the octahedral interstices are excluded from hydrogen occupation by Nb and Ta nearest neighbours. The apparent H–H attractive interaction associated with the excess chemical potential decreases in the presence of Nb(Ta). The stability of the β-hydride phase decreases with Nb(Ta) content. The enlargement of the solvus composition at the α_max phase boundary at a given temperature is more favourable after alloying with Nb(Ta). The fact that alloying palladium with Nb(Ta) decreases its hydrogen solubility at, e.g. 1 atm, is noteworthy since pure Nb(Ta) absorbs hydrogen to form NbH_∼2.