First Principle Calculation On Fundamental Properties Of PdCu Alloys

As a kind of promising material for hydrogen permeation, the palladium-copper alloy has raised considerable research interests due to its wonderful hydrogen selectivity and permeability, nice mechanical properties, and high thermal stability. Compared with pure palladium, Pd Cu membranes with a Cu composition of 47.5 at.% have the highest hydrogen permeability and possess better resistance to poisoning gases. Pd Cu membranes also show better mechanical properties, wider operating temperature range, and much cheaper prices than pure Pd membranes, which lead to a wide application of Pd-based membrane operation in the industry. Hydrogen separation process is conducted at high temperature, but little data is available in the literature with respect to the temperature-dependent properties of Pd Cu alloys. The existence of B2-L10 phase transition under high temperature greatly reduces the efficiency of hydrogen permeation in Pd Cu alloys. It is therefore necessary to have a systematic investigation on Pd Cu with 47.5 at.% Cu and the composition of Pd₅₀Cu₅₀ is purposely chosen since it is very near to the composition related above. By means of first principle calculation with the combination of quasi-harmonic approximation and thermal electronic excitation, the present work is dedicated to study the ground-state properties, as well as temperature-dependent thermodynamic and mechanical properties of Pd Cu with low-temperature B2, high-temperature disordered fcc, possible phases like L10 and disordered bcc structures. Moreover, this work illustrates the effects of Nb and Cr as doped atoms on phase stability, phase transition, and dehydrogenation of Pd₅₀Cu₅₀.Results show that the ordered（B2, L10） as well as bcc and fcc solid solutions of Pd₅₀Cu₅₀ are not only energetically favorable with negative heats of formation, but also mechanically stable. Calculations also reveal that heat capacities of the four structures of Pd₅₀Cu₅₀ at a certain temperature from 0 to 1200 K are very close to each other, and that the elastic constants of each structure have a descending sequence of C11â†'C12â†'C44 within the entire temperature range. Besides, the B2 structure has the smallest coefficient of thermal expansion as well as brittleness among the four structures, suggesting that the B2 structure of Pd₅₀Cu₅₀ should be more appropriate for hydrogen permeation in terms of thermodynamic and mechanical properties.As doped in Pd₅₀Cu₅₀ with the B2 structure, Nb and Cr both tend to substitute the Cu atoms with negative heat of formation, which does help to stabilize the B2 structure. The addition of Nb and Cr also impedes the transition from B2 to L10 under high pressure. According to the Bain transition, the energy differences between B2 and L10 decrease sharply after doping Nb and Cr, which makes it easy to form L10 structure from B2. The behavior of doping has nothing to do with the occupation of H atom in Pd Cu alloy, i.e. the tetrahedral site（T） in B2 and the octahedral site（O） in L10. However, with the addition of Nb and Cr, the heat of solution of H atom at interstitial sites of various structures becomes bigger, which would make contribution to dehydrogen in the alloys.