Hydrogen interactions with intrinsic point defects in hydrogen permeation barrier of α-Al2O3: a first-principles study

Abstract

It is crucial to understand hydrogen interactions with intrinsic point defects in the hydrogen permeation barrier (HPB) of α-Al₂O₃, finding underlying reasons for the not-so-low H-permeability of the barrier, and thereby produce samples with tailored defects for optimal performance. Using density functional theory (DFT), the formation energies of intrinsic point defects in an α-Al₂O₃ lattice, including extrinsic H-related defects (H_i, V_Al–H complex, HO_i and H_O), in all possible charged states, are first calculated under HPB working conditions, to determine the dominant basic defect species for hydrogen. We find that the stable forms of H-related defects in α-Al₂O₃ are charged H interstitials (H^q_i, where q is the charge state of the defect) and hydrogenation of the bulk V_Al³⁻ ([V_Al³⁻–H⁺]^q), under hydrogen-rich conditions. As the system reaches equilibrium, H in α-Al₂O₃ is mainly present in the H_i⁺ state, and preferentially exists in the form of [V_Al³⁻–H⁺] and H_O⁺. Migration processes of the dominant defects are further investigated, predicting that H_i⁺ is the predominant diffusion species in α-Al₂O₃. [V_Al³⁻–H⁺]²⁻ and H_O⁺ can release trapped hydrogen during high temperature annealing, contributing to the H-transport in α-Al₂O₃. The formation energy is much higher than the migration energy for H_i⁺, suggesting that the migration of H_i⁺ is the bottleneck for creating low enough H-permeation in α-Al₂O₃, and corresponding strategies for optimum H-suppressing performance for an α-Al₂O₃ HPB are proposed.