First-principles study on the dissolution and diffusion properties of hydrogen in α-Al2O3

In fusion reactors, tritium permeation barrier (TPB) technology is one of the key scientific technologies. alpha-Al2O3 has been considered an ideal candidate material for TPBs. In this work, a series of first-principles calculations have been performed to investigate the dissolution, clustering and diffusion behavior of hydrogen (H) in bulk alpha-Al2O3, The calculation results show that the most energetically stable form of hydrogen in a perfect alpha-Al2O3 crystal under H-2 gas annealing treatment is the H-2 molecule. This can also be confirmed by the electron localization function results. The attraction between two H atoms located in first and second nearest octahedral interstitial sites (OISs) is so strong that if multiple H atoms are dissolved in alpha-Al2O3, these atoms can migrate toward their adjacent H atoms and form clusters, which will prevent further diffusion of H. The most stable configuration of H cluster in alpha-Al2O3 is 2H(i), with the smallest formation energy and the largest average binding energy. The formation energy and binding energy are similar to those of the gaseous H-2 molecule. We have derived the temperature-dependent diffusivity of the H-2 molecule in alpha-Al2O3 as D(T) = (3.65 x 10(-7)m(2)/s)exp(-2.27 eV /KT), which is in good agreement with the experimental values. In addition, both the dissolution energy and migration barrier of the H-2 molecule are so high that dissolution and diffusion of the H-2 molecule in alpha-Al2O3 are very difficult, resulting in low hydrogen permeability.