Chemical diffusion in metal oxides. Example of TiO₂

The present work considers the gas/solid equilibration kinetics for nonstoichiometric oxides in general and TiO₂ in particular. Several criteria that must be addressed in the determination of the chemical diffusion coefficient are outlined. The equilibration kinetics during oxidation and reduction are considered in terms of rate controlling reactions, including (1) bulk diffusion, (2) surface reaction, (3) transport in the segregation-induced electric field, (4) multispecies diffusion of different rates, and (5) multispecies diffusion of comparable rates. The effect of nonstoichiometry on chemical diffusion is considered in terms of the microdiffusion coefficient related to the transport kinetics within very narrow ranges of nonstoichiometry, in which the specific diffusion data may be considered in terms of the specific defect disorder. The available data on the equilibration kinetic for the O ₂/TiO₂ system are discussed.