Helium stability and its interaction with H in α-Al2O3: a first-principles study

Area of Science:

• Materials Science
• Nuclear Engineering
• Computational Physics

Background:

• Helium (He) is an extrinsic defect in alpha-alumina (α-Al2O3) tritium breeding blankets (TPBs) arising from tritium decay and nuclear reactions.
• Understanding He-H interactions is critical for predicting material performance and lifetime in TPBs.

Purpose of the Study:

• To investigate the stability, structure, and diffusion of He-related defects and their interactions with Hydrogen (H) in α-Al2O3 under H2-rich conditions.
• To determine the impact of these complexes on hydrogen migration.

Main Methods:

• Density Functional Theory (DFT) calculations were employed to model defect structures and energetics.
• Simulations considered various defect sites including aluminum vacancies (VAl), oxygen vacancies (V), oxygen interstitials (Oi), and octahedral interstitial sites (OISs).

Main Results:

• Helium atoms preferentially occupy aluminum vacancies (forming HeAl(3-)) or interstitial sites in α-Al2O3.
• Stable He-H complexes, such as [HeAl(3-)-H(+)](2-) and [HO(+)-He](+), were identified.
• The presence of HeAl(3-) and [V-Hei](0) complexes significantly increases the activation energy for hydrogen migration.

Conclusions:

• Helium forms stable complexes with intrinsic α-Al2O3 defects, particularly at aluminum vacancies.
• These helium-containing defects act as barriers to hydrogen diffusion.
• The findings suggest that helium accumulation will hinder hydrogen transport in α-Al2O3 TPBs.