First-principles study of hydrogen diffusion inα−Al2O3and liquid alumina

Abstract

We have studied the energetics and mobility of neutral hydrogen in alumina ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ using ab initio density-functional calculations. The mobility of hydrogen was studied in corundum $(\ensuremath{\alpha}\ensuremath{-}{\mathrm{Al}}_{2}{\mathrm{O}}_{3})$ as well as in liquid alumina. Using both static as well as molecular-dynamics calculations, and applying classical transition state theory, we derive the temperature-dependent diffusivity of hydrogen in $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ as $D(T)=(21.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}{\mathrm{m}}^{2}/\mathrm{s})\mathrm{exp}(\ensuremath{-}1.24\mathrm{eV}/kT).$ The corresponding diffusivity of hydrogen in liquid/amorphous alumina, derived directly from ab initio molecular dynamics calculations, is $D(T)=(8.71\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}{\mathrm{m}}^{2}/\mathrm{s})\mathrm{exp}(\ensuremath{-}0.91\mathrm{eV}/kT).$ The computed diffusivity compares very well to experimental data. We conclude that diffusion of neutral hydrogen through the bulk of alumina is a good approximation of the mechanism for hydrogen mobility in corrosion scales. The representation of grain-boundary structures by amorphous alumina is, probably, realistic at higher temperatures.