Nuclear Fusion (Jan 2023)

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

  • L. Yang,
  • B.D. Wirth

DOI
https://doi.org/10.1088/1741-4326/acc6de
Journal volume & issue
Vol. 63, no. 6
p. 066002

Abstract

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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 ( $2\bar 1\bar 10$ ) 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 ( $2\bar 1\bar 10$ ) surfaces will have relatively higher H retention than the other W _x B _y surfaces evaluated in this work.

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