Analysis of hydrogen diffusion coefficient during hydrogen permeation through niobium and its alloys

The hydrogen solubility and the hydrogen permeability of Nb-based hydrogen permeable membranes are investigated in order to examine the hydrogen diffusion coefficient during the hydrogen permeation at high temperatures. It is shown that the hydrogen dissolution reaction into niobium and its alloys does not follow the Sieverts' law at the pressure conditions of the practical hydrogen permeation, in contrast to the case of Pd-based alloys. The hydrogen diffusion coefficients during the hydrogen permeation are evaluated from a linear relationship between the normalized hydrogen flux, J.d, and the hydrogen concentration difference, ΔC between the inlet and outlet sides of the membrane with the thickness of d. It is found that the hydrogen diffusion coefficient for pure niobium is much lower than the reported values measured for dilute hydrogen solid solutions. Surprisingly, the hydrogen diffusion is found to be faster in Pd-Ag alloy with face-centered cubic (fcc) crystal structure than in pure niobium with body-centered cubic (bcc) crystal structure at 773 K during the hydrogen permeation. It is also found that the addition of Ru or W into niobium increases the hydrogen diffusion coefficient under the practical conditions.