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Grain-boundary diffusion and solubility of helium in submicrocrystalline palladium
Authors:A N Zhiganov  A Ya Kupryazhkin
Institution:(1) Ural State Technical University (UPI), ul. Mira 19, Yekaterinburg, 620062, Russia
Abstract:The diffusion and solubility of helium in palladium with a submicrocrystalline structure are investigated by thermal desorption of helium from He-saturated specimens at temperatures T = 293–508 K and saturation pressures P = 0.1–35 MPa. As the saturation pressure rises, the effective diffusion coefficient increases, exhibits a plateau, and then decreases to its initial value. Along with the four plateaus discovered earlier, the solubility versus saturation pressure dependence in the range 25.5–35.0 MPa demonstrates a fifth plateau, where the solubility is as high as (3.0 ± 0.4) × 1017 cm−3. It is shown that the helium diffuses along grain boundaries, at which clusters (traps) consisting of eight to ten vacancies are localized, and dissolves in these clusters. The high value of C eff in the fifth plateau is explained by pairwise merging of adjacent vacancy clusters. From the D eff(P) dependences, the vacancy clusters concentration is estimated as C* = 2.32 × 1016 cm−3. Within the experimental error, this value coincides with that obtained from the solubility data. Calculations of the energy of helium-defect interaction in submicrocrystalline Pd that are made using the molecular dynamics method support the experimental results.
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