Formation of thin foil of the ordered Pd-Cu solid solution with a CsCl-type lattice during magnetron sputtering

Abstract

Among Pdbased membrane alloys (1), the Pd-Cu system has long attracted attention for deep hydrogen purification. This system is distinguished by the for� mation of a CsCltype ordered solid solution ( β phase) (2) in a rather narrow composition range close to Pd- 40 wt % Cu. It is accepted (3) that the CsCltype lattice, as compared to the less dense fcc lattice of the disordered solid solution ( α) phase), and, respectively, the shorter distance between octahedral voids (involved in hydro� gen diffusion) are responsible for a lower barrier for diffusion and a manyfold enhancement of hydrogen permeability. According to the results of numerous experimental studies summarized in (3), the hydrogen diffusion activation energy is 0.035 eV in the ordered solid solution, 0.325 eV in the disordered solid solu� tion, and 0.23 eV in palladium. The hydrogen diffu� sion coefficient at 300 K in the β phase is almost four orders of magnitude higher than in the α phase and two orders of magnitude higher than in Pd (3). Thus, the first, major, way of improving the perfor� mance of a Pd-Cu alloy membrane is to increase hydrogen permeability through ordering of the solid solution. The kinetics and mechanism of solid solu� tion ordering have been studied for more than three