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

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 x 10(19) atoms/m(2) (approximate to 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) x 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.

Automated summary

In this study, the uptake of deuterium into tungsten through thin films of tungsten oxide was investigated. It was found that oxygen in the oxide film was partially removed by deuterium plasma, leading to tungsten enrichment near the surface. Deuterium accumulated in the oxide film but did not penetrate into the underlying self-damaged tungsten. After storage, deuterium content in the oxide layer decreased significantly, indicating that surface oxide films effectively block deuterium uptake into metallic tungsten.