Diffusion of helium in nickel

Abstract

Helium has been implanted into single and polycrystalline nickel samples at temperatures between 800 and 1250°C with implantation depths between 18 and 80 μm. Simultaneously the helium re-emission from the sample was measured by a mass spectrometer. It has been shown that the time dependence of the observed re-emission rate is governed by volume diffusion of the helium. From measurement of this time dependence as a function of temperature, the helium diffusion constant has been determined to be DHe = 10−2.2 ± 0.3 exp(−0.81 ± 0.04 eV/kT cm2 s−1.The He diffusion data are interpreted in terms of an interstitial diffusion mechanism hindered by thermal vacancies. Within this model the measured activation energy can be identified with the difference of the energy for dissociation of a helium atom from its substitutional lattice site, EHe(s)diss, and the vacancy formation energy, EVF. The extracted dissociation energy, EHe(s)diss≈2.4 eV, is in good agreement with release measurements.