Morphological Stability in the Course of Solid State Reactions: The System Fe3O4 ‐ Mn3O4 ‐ Cr2O3

We have investigated experimentally and theoretically the problem of morphological stability of interfaces in a nonequilibrium two‐phase ternary system while it equilibrates. The system chosen for this study was Fe3O4 ‐ Mn3O4 ‐ Cr2O3 at T = 1400 °C. Depending on the spinel reactant and the sesquioxide reactant, the initially planar interface is either unchanged, or develops nonplanar morphologies of different kind. The theoretical analysis starts with the independent flux equations and the appropriate boundary conditions and investigates whether or not small perturbations of the planar interface will grow or decay in the course of time. A critical parameter can be defined which is negative for stable and positive for unstable interfaces. This parameter is a complicated function of all the transport coefficients, the starting compositions and the chemical potentials of the components in both phases and can in principle be calculated, as long as local equilibrium prevails during the reaction.