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

Alpha-alumina (α-Al2O3) 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-Al2O3. [H+− V−3Al]and [H+ − V0O] defects have much lower formation energies at EG/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-Al2O3 TPB. However, the minimum diffusion barrier for OiH− migration to the first nearest neighbor O interstitial site is 0.44 eV, which is so low that OiH− 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-Al2O3. Our results provide a sound theoretical explanation for recent experimental results of H permeation in α-Al2O3 under irradiation environment.

α-氧化铝（α-Al₂O₃）被视为氚渗透阻挡层（tritium permeation barrier, TPB）的理想候选材料，具备优异的抗氚性能。然而在聚变反应堆中，辐照诱导缺陷会与制备引入缺陷发生累积，进而大幅降低该阻挡层的防护性能，其内在机制尚未明晰。本文采用第一性原理密度泛函理论（first-principles density functional theory, DFT）方法，探究辐照诱导点缺陷对氢（H）在α-Al₂O₃中的溶解与扩散特性的影响。研究发现，[H⁺−V³⁻_Al]与[H⁺−V⁰_O]缺陷在富铝与富氧生长环境中于EG/2处的形成能极低，表明氢原子易被空位型辐照诱导点缺陷捕获，由此可预期更高的氢滞留量，这与实验结果相符。此外，通过计算氢-缺陷复合体的多种不同扩散路径及其对应的扩散系数，可以推断氢原子与空位型点缺陷难以作为束缚实体共同扩散。因此，孤立的空位型辐照诱导点缺陷可捕获多个氢原子形成氢-缺陷复合体，阻碍氢的扩散过程，从而提升α-Al₂O₃基TPB抗氢渗透的防护效率。但氧间隙氢（OiH⁻）迁移至最近邻氧间隙位的最低扩散势垒仅为0.44 eV，该势垒极低，使得OiH⁻在室温下可快速迁移。这种快速的氢扩散路径，可能是辐照环境下α-Al₂O₃抗氢渗透效率低下的内在机制。本研究结果为近期辐照条件下α-Al₂O₃中氢渗透的实验结果提供了可靠的理论解释。