Depth-concentration profile of hydrogen in niobium

Hydrogen concentration profiles near the surface of hydrogen-charged Nb samples have been measured by utilizing the ${}_{}{}^1{}_{}{}^{}\mathrm{H}({}_{}{}^{15}{}_{}{}^{}\mathrm{N}, \alpha \gamma ){}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ reaction. The Nb foils were charged under various ambient conditions to concentrations corresponding to the single-phase ${\alpha }^{\prime }$ region in the phase diagram. Each sample exhibited a strong H surface peak followed by a sharp dip and a gradual increase in H concentration to a plateau. The rate at which the hydrogen concentration increases towards the bulk as well as the height of the plateau is related to the method of sample preparation, the higher the oxygen content, the lower the hydrogen concentration near the surface. These observations suggest that the H concentration near the surface of ${\alpha }^{\prime }$-Nb-55 at.% H is lower than that in the bulk because of the presence of oxides and dissolved oxygen. They also indicate that samples charged with hydrogen after heating to over ∼250°C in a pressure of >1×${10}^{-6\mathrm{}}$ Torr will develop an oxygen-enriched and hydrogen-depleted zone near the surface.