First principles study of hydrogen behaviors in hexagonal tungsten carbide

Abstract

Understanding the behavior of hydrogen in hexagonal tungsten carbide (WC) is of particular interest for fusion reactor design due to the presence of WC in the divertor of fusion reactors. Therefore, we use first-principles calculations to study the hydrogen behavior in WC. The most stable interstitial site for the hydrogen atom is the projection of the octahedral interstitial site on tungsten basal plane, followed by the site near the projection of the octahedral interstitial site on carbon basal plane. The binding energy between two interstitial hydrogen atoms is negative, suggesting that hydrogen itself is not capable of trapping other hydrogen atoms to form a hydrogen molecule. The calculated results on the interaction between hydrogen and vacancy indicate that the hydrogen atom is energetically trapped by vacancy and the hydrogen molecule can not be formed in mono-vacancy. In addition, the hydrogen atom bound to carbon is only found in tungsten vacancy. We also study the migrations of hydrogen in WC and find that the interstitial hydrogen atom 0 *Author to whom correspondence should be addressed.Email address: csliu@issp.ac.cn Tel: 0086-551-5591062Preprint submitted to Journal of Nuclear Materials February 26, 2018 prefers to diffusion along the c axis. Our studies on the hydrogen behavior in WC provide some explanations for the experimental results of the thermal desorption process of energetic hydrogen ion implanted into WC.

Effects of nitrogen on hydrogen retention in tungsten: First-principles calculations

“…In previous theoretical studies, impurities have been found to interact strongly with the intrinsic defects and hydrogen in tungsten, which sheds light on the influence mechanism of impurities on the hydrogen retention, especially for carbon [16][17][18][19][20]. However, little theoretical work has been reported about the influences of nitrogen on hydrogen dissolution and diffusion in tungsten.…”

Shielding materials in the compact spherical tokamak

“…This fourth and final stage is related to deuterium bonded to carbon and accounts for only a small fraction of the total trapped inventory. It should be noted that the binding energy between deuterium atoms in the above sites is calculated to be negative [62], suggesting molecule formation is unfavourable.…”

“…These studies can provide some design tools toward minimizing hydrogen isotope retention. First of all, diffusion is faster in the c-axis of WC [62,63], suggesting that strongly textured materials would enable a directional dependence to the inventory build-up. Secondly, because trapping is dominated by the presence of interstitials and vacancies, both of which are intrinsic defects caused by deviations from stoichiometry, the carbon content can and should be properly controlled.…”

Radiation-Induced Evolution of Tungsten Carbide in Fusion Reactors: Accommodation of Defect Clusters and Transmutation Elements

“…On the other hand, WC has been the subject of many atomic-scale computational studies, including investigation of bulk properties, point defects, ,, self-diffusivity, radiation damage, , and gas retention. ,− Previous DFT work ,, has shown that in dilute conditions, C vacancies are the most easily accommodated intrinsic point defects, followed by C interstitials (trigonal site on the C sublattice) and W vacancies, while W interstitials are highly unfavorable. This is evident from the formation energies of the respective defects, reproduced in Table .…”