Internal and External Oxidation of Nonmetallic Compounds and Solid Solutions (I)

A formal analysis is given treating the phenomenon of internal and external oxidation of oxide solid solutions. Theoretically, two situations can arise if those oxides are brought in an environment of higher oxygen potential. (1) Demixing and subsequently the formation of an outer layer of the reaction product occurs. (2) The reaction product forms internally; the two‐phase front advances into the interior of the solid solution. Some analogies with the phenomenon of internal alloy oxidation are discussed. Normally, a parabolic reaction kinetics prevails. Depending on the composition of, and the transport coefficients in the solid solution, a change from an internal to an external reaction may occur, and a criterion for this change is deduced. Internal oxidation (and reduction) is a means to disperse a second phase into an oxide matrix. This process may alter electrical, chemical and mechanical properties of the solid in question and thus could be of interest for materials science as well as geochemistry.