First-principles insight of hydrogen dissolution and diffusion properties in γ-Al2O3

Abstract

First-principles theory is applied to identify the structure of γ-Al2O3 first. Then, hydrogen dissolution and diffusion behavior was further studied to reveal the reason that permeation reduction factor (PRF) of γ-Al2O3 is lower than that of α-Al2O3. Our calculations show that the formation energy for Al vacancy at the octahedral interstitial site (VAl−OIS−3) is lower than that at the tetrahedral interstitial site (VAl−TIS−3), and are far lower than those of other defects in both defective spinel and non-spinel γ-Al2O3. Thus, the stability of VAl−OIS−3 is higher than that of VAl−TIS−3 in both Al-rich and O-rich environments. However, when hydrogen atoms are introduced into γ-Al2O3, the stability of [VAl-TIS-H]−2 is higher than that of [VAl-OIS-H]−2 in the defective spinel γ-Al2O3 structure, while the opposite is true for the non-spinel γ-Al2O3 structure. The vibrational frequencies for OH− in [VAl-TIS-H]−2 and [VAl-OIS-H]−2 for defective γ-Al2O3 are calculated to be 3608 cm−1 and 3374 cm−1, respectively, which is in excellent agreement with the infrared (IR) absorption peaks observed at ∼3500 and ∼3300 cm−1. The defective spinel model is more suitable for describing the γ-Al2O3 structure. In defective spinel γ-Al2O3, there are many native Al vacancies, which are arranged in a straight line along the [21¯1¯0] direction. The migration barrier for H diffusion along these native Al vacancies is so low that these Al vacancies can provide a rapid diffusion channel for H. Thus, the permeability of H in γ-Al2O3 is much higher than that in α-Al2O3 leading to a lower permeation reduction factor (PRF). Our results can provide not only a sound theoretical explanation for the low PRF of γ-Al2O3 but also a direction to improve the efficiency in preventing H permeation through FeAl/Al2O3 tritium permeation barriers (TPBs).