Vanadium Carbide Coating As Tritium Permeation Barrier:A DFT Study

Nuclear fusion energy is one of the advanced energy in the future due to its advantages of high energy generation efficiency and pollution-free.However,in the process of usage high-pressure hydrogen,at any of stages,including production,storage,transportation and separation in fusion reactors,hydrogen will rapidly diffuses and permeates into structural materials such as stainless steel,resulting in the loss of hydrogen and the hydrogen embrittlement in structural materials.Hydrogen permeation barrier?HPB?layers deposited on the structural materials can significantly reduce the hydrogen and its isotopes penetration,which is also the solution that's used a lot.Density functional calculations are used to investigate hydrogen?H?behaviors in different bulk vanadium-carbide?V_xC_y?phases and VC?001?surface.Molecular H₂dissociation,atomic H diffusion and penetration are analyzed using the transition state theory.Carbon?C?vacancies and structural symmetry both influence the stability of V_xC_y phases.The appearance of vacancy peaks are emanated from unscreened V-V bonds through the C vacancy site.The stability of H in V_xC_y phases dependents on the number and strength of bonding and anti-bonding peaks.C vacancies distinctly improve H diffusion barrier due to a significantly endothermic energy of detrapping from C vacancy at first,then H diffuses like an interstitial atom until be trapped by another C vacancy.But too many C vacancies?eg.phases transfer from V₈C₇ to V₄C₃?may result in lower energy barriers of detrapping from one C vacancy.The significantly negative H formation energy and low solution numbers in C vacancies together with the vacancy-induced high diffusion barriers all indicate that vacancy-ordered V₄C₃ can be the most promising phase of V_xC_y hydrogen permeation barrier.Energetic,vibrational,electronic consequences,and quantum effects on the H behaviors are discussed thoroughly.In addition,H₂ prefers to be close to the VC?001?surface as physical adsorption,providing an environment conducive for further dissociation,and dissociates into atomic H adsorbed at the top C atom sites with co-adsorption state.The dissociation rate on the surface is mainly limited by the temperature-controlled activation energy barrier.The adsorptivity of atomic H by the surface tends to decrease as increasing of H coverage.For atomic H penetration through the VC?001?surface,a significantly endothermic energy barrier and the low diffusion prefactor suggest that the main resistant effect of H permeation takes place at the surface in VC.Our theoretical investigation indicates V_xC_y is a promising hydrogen permeation barrier.