Uptake Rates for Hydrogen by Nb and Ta; Effect of Thin Metallic Overlayers

摘要

The hydrogen uptake rates of clean Nb and Ta foils were measured as a function of temperature and pressure. The sticking coefficient (number of hydrogen atoms entering the bulk / number of hydrogen atoms hitting the surface) of hydrogen on clean Nb and Ta, at approx. = 2.2 x 10 exp -5 Torr hydrogen and room temperature, was found to be surprisingly low, approx. = 0.005 and approx. = 0.05 respectively, resulting in very low uptake rates under these conditions. This result is explained on the basis of a model recently proposed. It was found that these uptake rates can be significantly enhanced or decreased by the surface deposition of a thin layer of Pd, Pt, or Ni. The hydrogen uptake rates, i.e., sticking coefficients, were measured as a function of the overlayer thickness. In the case of Pd it was found that a layer less than one monolayer thick will either not change or even inhibit the hydrogen uptake, whereas a layer over one monolayer thick will dramatically enhance the sticking coefficient to a value of 1. It was found that the change at one monolayer is related to a phase transition of the Pd surface layer at that coverage. Both Pt and Ni do not exhibit this effect but do influence the uptake rate by an amount depending on their thickness. Thick overlayers inhibit uptake due to a limited permeation rate through the layer. (ERA citation 05:017851)