Hydrogen diffusion and vacancies formation in tungsten: Density Functional Theory calculations and statistical models

The interaction of hydrogen with tungsten is investigated by means of the Density Functional Theory (DFT) and statistical methods based on the transition-state theory and thermodynamics. This model yields temperature-dependent data that can help understanding macro-scale experimental results. Within this model, the concentrations of trapped hydrogen atoms at thermodynamic equilibrium are established. Taking into account the configurational entropy, hydrogen is shown to induce vacancy formation below 1000 K. Based on this model, TDS spectra are simulated with a basic kinetic model to provide some better insight into the desorption process of hydrogen. Finally, revised mechanisms for hydrogen diffusion in tungsten are proposed; we conclude that the discrepancy existing between the experimental diffusion coefficient measured by Frauenfelder (1969) and the one calculated by DFT would be reconciled provided one uses two different diffusion regimes that would depend on temperature and vacancies concentration.