Nuclear Materials and Energy (Mar 2022)
Influence of thin surface oxide films on hydrogen isotope release from ion-irradiated tungsten
Abstract
We studied the influence of thin, electro-chemically grown tungsten (W) surface oxide films on hydrogen isotope release from W. As deuterium (D) reservoir underneath the oxide, we used a defect-rich, ion-irradiated W layer that was filled with D prior to oxidation. Several oxide films with thicknesses between 5 and 100 nm were studied and compared with tungsten with a natural oxide film. The release of D through the oxide film was analyzed with thermal desorption spectroscopy (TDS). The depth-resolved concentration profiles of D in the sample were measured with nuclear reaction analysis at all experimental steps. Changes of the morphology of the oxide film due to the release of D were investigated with scanning electron microscopy (SEM).In TDS studies, we found that the thin oxide films significantly influence the release behavior of D from W. The first D release peak (at 560 K) is shifted towards higher temperature (or later times) with increasing oxide thickness. This indicates that the oxide film acts as both a D reservoir and a transport barrier that delays D release at temperatures above 475 K. At this temperature, D also starts to interact chemically with the oxide film and is released not only as HD or D2 but also in the form of heavy water (HDO and D2O). Above 700 K, D is released only in form of heavy water as long as enough oxide is available. Accordingly, SEM images after TDS show a strong modification of the oxide film. For film thicknesses of 5–10 nm, all oxide is removed from the surface and smooth metallic W remains. For 15 nm, the surface is still partially covered by oxide islands with several micrometer of metallic W between them. From the fact that D is still only released as heavy water at high temperatures, we conclude that the mobility of D atoms at the surface is very high. Even D atoms that surface far from an oxide island apparently travel along the surface to form an O-D group at the W oxide before they recombine with another D atom to form water.Our results indicate that the oxide film becomes relevant for the D release during TDS if the ratio of O atoms on the surface to D atoms in the sample is larger than 5–10 %. Consequently, even the natural oxide film (1–2 nm) that forms on W upon contact with air may significantly influence the D release spectra from TDS for experiments with low D retention.
