Stability and kinetics of helium interstitials in boron carbide from first principles

When boron carbide is used in nuclear reactors as a neutron absorber, helium concentrations on the order of a few atomic percent can be attained. It is thus of primary importance to know the distribution and kinetics of helium atoms in boron carbide. In spite of a variety is of experimental works devoted to the characterization of the microstructure and He bubbles in boron carbide irradiated in reactor, there is a serious lack of knowledge concerning the basic mechanisms governing helium kinetics. This study is devoted to the stability and mobility of helium interstitial atoms in carbon rich boron carbide. The lowest energy He insertion sites were screened through density functional theory and the most probable migration paths and energy barriers were investigated using the nudged elastic bands (NEB) approach. The results suggest that in a wide range of temperatures He interstitials undergo 2D diffusion confined between two 〈111〉 planes. The onset of 3D diffusion is expected, according to our calculations, with an activation energy close to 2 eV.Our result is in qualitative agreement with the observation of flat bubbles with 〈111〉 orientation, although a quantitative comparison with He diffusion data is hindered by discrepancies and microstructure issues in available experimental results.