Boron segregation and effect on hydrogen energetics near tungsten surfaces: A first-principles study

To understand the effect of boronization coating on the tungsten (W) divertor in fusion reactors, the segregation of boron (B) and the resulting effect on the behavior of atomic hydrogen (H) and molecular H-2 near the W(100), W(110) and W(111) surfaces have been studied using density functional theory calculations. The segregation mechanisms of substitutional and interstitial B were considered. The segregation energies indicate B atoms below W surfaces will segregate to the surface as B clusters on the W(110) and W(111) surfaces, or as individual atoms on the W(100) surface. The B segregation near the W surfaces causes deformation or reconstruction of the surface lattice, and the B atoms on the W surfaces impact the surface behavior of H and H-2. The minimum adsorption energy of H on the W(100) surface increases with increasing number of B atoms, although the migration activation energy of H does not change monotonically with B coverage. The addition of B atoms on the W(110) surface increases the achievable H coverage but decreases the H migration on the surface. Excluding the clean surface, the minimum adsorption energy of H on the W(111) surface increases with increasing B coverage, but the activation energy for the H migration on the surface is essentially unaffected. The effect of B on H adsorption on the W surfaces is attributed to the B-H interaction, in addition to the influence of B on local surface configuration. H-2 adsorption on the W surfaces depends on the surface orientation and to a lesser extent the B coverage. For all considered B coverages, H-2 can be chemically adsorbed on the W(100) surface with an adsorption energy of similar to -0.5 eV and the B atoms increase the dissociation energy barrier of H-2. With increasing B coverage, the adsorption energy of H-2 on the W(110) surface increases and the dissociation energy barrier is only slightly modified. No chemisorption of H-2 is found for the W(111) surface with B coverage equal to or less than 3.0 ML.