Doping α-Al2O3 to reduce its hydrogen permeability: Thermodynamic assessment of hydrogen defects and solubility from first principles

This paper assesses the role of doping on the hydrogen permeability and electronic properties of α-Al₂O₃. Formation energies of intrinsic and extrinsic defects in α-Al₂O₃ were calculated using density functional theory. Using these energies as input, a thermodynamic model was utilized to identify the equilibrium defect concentrations (barring hydrogen defects) in undoped and doped α-Al₂O₃ under aluminization conditions of 1100 K, over a range of p_O2 at a fixed doping level of 1 ppm. Defect concentrations calculated at 1100 K under p_O2-rich conditions were used as input to establish hydrogen and electronic defect concentrations under functional conditions of 300 K, over a range of p_H2. The effect of dopants on the fraction of free hydrogen interstitials, which has implications on diffusivity, and the overall hydrogen solubility, was found to be substantial and distinct. Relative to the undoped case, Mg-doping increased the concentration of free hydrogen interstitials, the primary diffusing species, by 10⁷ times, whereas Ti-, Si-, Fe-, Cr-doping eliminated it to negligible amounts. Comparing the impact on total hydrogen solubility, Mg-doping increased it by 10⁴ times; Fe- and Cr-doping increased it negligibly by ∼1.5 times. In contrast, Ti- and Si- doping decreased it to nearly 1/3 that of the undoped case. Analyzing the role of isolated defect concentrations and binding energies of defect complexes helps elucidate these effects. Effect of dopant concentrations of 10 and 100 ppm was also investigated, with the conclusion that doping with Si and Ti at 1 ppm is the best strategy to reduce hydrogen diffusivity and solubility by the greatest amount. The findings aid in the design of effective hydrogen permeation barrier layers for use in hydrogen storage and transport infrastructure as well as in the understanding of defect states in Al₂O₃ used in electronic devices, such as resistive switching.