Influence of thin tungsten oxide films on hydrogen isotope uptake and retention in tungsten – Evidence for permeation barrier effect

Abstract

• Thin (33–55 nm) tungsten oxide films prevent deuterium uptake into tungsten (W). • Defect-rich, self-damaged W underneath the oxide is used to detect permeation. • Deuterium (D) enters the oxide film, thermalizes in it and diffuses through it. • Different enthalpy of solution of W oxide/metal acts as permeation barrier for D. • Oxygen is removed from the first 13.5 nm of the oxide independent of film thickness. We studied the uptake of deuterium (D) into tungsten (W) through thin films of W oxide. Two surface oxide films with thicknesses of 33 and 55 nm were thermally grown on W substrates. In the following, the oxidized samples were exposed to low-energy D (5 eV/D) from a D plasma at a sample temperature of 370 K. A defect-rich layer of self-damaged W underneath the oxide was used as a getter layer to enable the detection of D that penetrates the oxide film. Depth-resolved concentration profiles of D and oxygen (O) were obtained after the plasma exposure by nuclear reaction analysis and Rutherford backscattering spectrometry. We have found that oxygen is partially removed from the first 100 × 10 19 atoms/m 2 (≈ 13.5 nm) of the oxide film by the D plasma which leads to a W enrichment in the near surface region. Independent of the oxide thickness, an oxygen removal rate of (5.4 ± 0.7) × 10 −4 O atoms per incident D atom was observed. Furthermore, D accumulates in the oxide film to concentrations of up to 1.3 at. %, but does not penetrate into the underlying self-damaged W. After a storage period of ten months at room temperature in vacuum, the D content in the oxide layer has decreased substantially, but still no D has penetrated into the metallic W. It is evident that surface oxide films on W effectively block the D uptake into metallic W. However, the D uptake into metallic W is not limited by the transport in the oxide film itself. D diffuses fast throughout the oxide but is stopped at the interface to the metallic W. We attribute this behavior to the difference in the heat of solution for D in W oxide and metallic W. D cannot overcome this barrier once it is thermalized to 370 K within the W oxide film.