The diffusion of H and D in Nb and Ta at low temperatures

The mobility of hydrogen and deuterium in niobium and tantalum has been studied by quenching to 4.2 K and annealing between 10 K and 100 K. The concentration of quenched hydrogen has been computed from the resistivity increase after quenching. The decrease of the residual resistivity during annealing is interpreted by the trapping of hydrogen and by the formation of precipitations. By the variation of the hydrogen concentration and the impurity content, the annealing stage corresponding to the intrinsic diffusion of the hydrogen has been identified. A diffusion coefficient of 1 x 10-14 cm2s-1 at 34 K and an activation energy for diffusion of 0.069 eV have been determined for hydrogen in niobium. These values agree well with the extrapolation of measurements above 100 K. For deuterium in niobium a diffusion coefficient of 2 x 10-14cm2s-1 at 50 K and an activation energy of 0.086 eV have been found. For hydrogen and deuterium in tantalum the measurements give 3 x 10-14cm2s-1 at 42 K and 0.064 eV and 1 x 10-14cm2s-1 at 40 K and 0.065 eV. (author)