Influence of irradiation-induced point defects on the dissolution and diffusion properties of hydrogen in α-Al₂O₃: a first-principles study

Abstract

Abstract Alpha-alumina (α-Al 2 O 3 ) is considered to be an ideal candidate material for the tritium permeation barrier (TPB) with excellent tritium resistance properties. However, in a fusion reactor, the irradiation-induced defects could sum up on fabrication-induced defects so to reduce drastically the barrier performance. The underlying mechanism is still not settled. In this paper, the first-principles density functional theory (DFT) approach is used to explore the influence of irradiation-induced point defects on the dissolution and diffusion properties of hydrogen (H) in α-Al 2 O 3 . <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mfenced close="]" open="["> <mml:mrow> <mml:msup> <mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> </mml:mrow> </mml:msup> <mml:mo>−</mml:mo> <mml:msubsup> <mml:mrow> <mml:mi>V</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">A</mml:mi> <mml:mi mathvariant="normal">l</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>3</mml:mn> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:mfenced> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mfenced close="]" open="["> <mml:mrow> <mml:msup> <mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> </mml:mrow> </mml:msup> <mml:mo>−</mml:mo> <mml:msubsup> <mml:mrow> <mml:mi>V</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">O</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>0</mml:mn> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:mfenced> </mml:math> defects have much lower formation energies at E G/2 in both Al-rich and O-rich growth environments that H atoms are easily captured by vacancy-type irradiation-induced point defects. As a result, higher H retention can be expected, which is consistent with the experimental results. Moreover, by calculating several different diffusion pathways of H-defect complexes and the corresponding diffusion coefficient, it can be inferred that H atoms and vacancy-type point defects can hardly diffuse as a bound entity. Therefore, isolated vacancy-type irradiation-induced point defects can trap multiple H atoms to form H-defect complexes and impede the diffusion process of H, which can enhance the efficiency of protection against H permeation through α-Al 2 O 3 TPB. However, the minimum diffusion barrier for O i H − migration to the first nearest neighbor O interstitial site is 0.44 eV, which is so low that O i H − can migrate quickly at room temperature. This fast diffusion pathway for H could be the underlying mechanism for the low efficiency in preventing H permeation through irradiated α-Al 2 O 3 . Our results provide a sound theoretical explanation for recent experimental results of H permeation in α-Al 2 O 3 under irradiation environment.

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