Surface stability and H adsorption and diffusion near surfaces of W borides: a first-principles study

Journal Article · 10 April 2023 · Nuclear Fusion

DOI: https://doi.org/10.1088/1741-4326/acc6de · OSTI ID:1969435

Yang, L. ; Wirth, B. D.

Abstract Understanding the behavior of tungsten boride (W _x B _y ) surfaces in a fusion reactor environment is an important topic since boronization is a common wall conditioning method used in fusion Tokamaks. We report the results of density functional theory calculations that investigate the surface stability of W _x B _y (tetragonal I4 ₁ /amd -WB, hexagonal P 6 ₃ / mmc -WB ₂ and tetragonal I4/m- W ₂ B) with low-index orientations, as well as hydrogen (H) energetics near W _x B _y surfaces. For single element terminated W _x B _y surfaces, B terminated surfaces are more energetically stable than W terminated as a result of significant reconstruction of B. The H surface adsorption energy and activation energy of H diffusion penetration below W _x B _y surfaces are mainly related to the outer termination. Specifically, the WB(001) surface terminated with two B layers, referred to as WB(001)-T _BB , has higher H adsorption affinity and lower H diffusivity on this surface than other terminations, which is controlled by the significant charge transfer from B to H. However, B atoms on the WB ₂ (0001)-T _BB surface decrease both H adsorption and diffusivity on the surface, but enhance H diffusion below the surface in comparison to W terminated WB ₂ (0001) surface. H would be trapped and diffuse within atomic surface gaps on the WB ₂ ( $2\bar{1}\bar{1}0$ ) surface, while H below the surface layer would jump along the [0001] direction rather than diffuse into bulk. The surface diffusion activation energy of H on the W ₂ B(001) surface slightly varies with terminations. Once H crosses the surface layer of W ₂ B(001) with either termination, it prefers to diffuse into the bulk, or back towards the surface, rather than move parallel to the surface. Interestingly, WB ₂ (0001) and WB ₂ ( $2\bar{1}\bar{1}0$ ) surfaces will have relatively higher H retention than the other W _x B _y surfaces evaluated in this work.

• Sponsoring Organization: USDOE
• OSTI ID: 1969435
• Alternate ID(s): OSTI ID: 1963129
• Journal Information: Nuclear Fusion, Journal Name: Nuclear Fusion Journal Issue: 6 Vol. 63; ISSN 0029-5515
• Publisher: IOP PublishingCopyright Statement
• Country of Publication: IAEA
• Language: English