MOLECULAR-DYNAMICS SIMULATIONS OF HYDROGEN DIFFUSION IN NIOBIUM : INFLUENCE OF IMPERFECTIONS

The trapping mechanism of hydrogen in niobium has been investigated within the molecular-dynamics approach. Simulations of the diffusion process of this impurity were performed in which Nb-Nb interaction was described by an N-body potential. The potential parameters were adjusted with respect to static and dynamic properties of a Nb crystal. Nb-H interaction was represented by a two-body potential. The Arrhenius diagram of the H diffusion coefficient obtained by molecular dynamics in the single-crystal case provides too small values of activation energy in comparison with experimental data. However, molecular-dynamics simulations indicate a large increase of these values in the presence of defects. The influence of imperfections on the diffusion of hydrogen becomes less important at around 1000 K. This conclusion confirms the experimental results revealing a good characteristic (Q valueg${10}^{10}$) of superconducting cavities after thermal treatments.