Electric field driven solid state reactions—microscopic investigation of moving phase boundaries in the system MgO/MgIn2O4/In2O3

Abstract

An external electric field acts on the mass transport in ionic solids as a second driving force in addition to the chemical potential gradient. The acceleration of a spinel-forming reaction between two binary oxides under the influence of an external electric field is studied in the model system MgO/MgIn₂O₄/In₂O₃. A simple kinetic model based on defect thermodynamics and linear transport theory is presented. The results indicate that the morphology during the early stages of growth of MgIn₂O₄ in the electric field is determined mainly by the initial grain structure of the In₂O₃ layer. The reaction front advances faster in the vicinity of grain boundaries. Morphological instabilities predicted by a continuum model of the different transport properties of MgO, MgIn₂O₄ and In₂O₃ are less important. The grain boundaries act as fast diffusion paths, resulting in large MgIn₂O₄ thickness variations along the interface. Accordingly the MgIn₂O₄/MgO interface adopts a highly curved morphology. Depending on the curvature of the MgIn₂O₄/MgO interface, the advancement of the interface proceeds either via a dislocation mechanism or via a ledge mechanism. Most probably, these two mechanisms result again in locally different growth rates, so that a feedback mechanism is established via the local micromorphology of the interface.