The diffusion and solubility of He in palladium with a sub-microcrystalline structure were investigated by thermal desorption of He from He-saturated specimens at 293 to 508K and saturation pressures of 0.1 to 35MPa. As the saturation pressure rose, the effective diffusion coefficient increased, exhibited a plateau, and then decreased to its initial value. Along with the four plateaus discovered earlier, the solubility versus saturation pressure dependence in the range of 25.5 to 35.0MPa demonstrated a fifth plateau, where the solubility was as high as 3.0 x 1017/cm3. It was shown that the He diffused along grain boundaries, at which clusters (traps) consisting of eight to ten vacancies were localized, and dissolves in these clusters. The high value of Ceff in the fifth plateau was explained by pair-wise merging of adjacent vacancy clusters. From the Deff(P) dependences, the vacancy cluster concentration was estimated to be 2.32 x 1016/cm3. Within the experimental error, this value coincided with that obtained from the solubility data. Calculations of the energy of He-defect interaction in sub-microcrystalline Pd that were made using the molecular dynamics method supported the experimental results.

Grain-Boundary Diffusion and Solubility of Helium in Submicrocrystalline Palladium. A.N.Zhiganov, A.Y.Kupryazhkin: Technical Physics, 2005, 50[8], 1026-33