Multiphase or multilayered ceramics are technologically relevant materials. Due to the ongoing miniaturization, surfaces and phase boundaries are in particular in the focus of interest. Solid state reactions, negligible on macroscopic length scales, become important, because of the short distances for diffusional transport. In addition to high temperatures, ceramic materials are often exposed to high electrical fields. Solid state reactions on macroscopic and microscopic level are already described extensively in the literature [1, 2]. Despite the importance for thin film applications, there are only a small number of studies on solid state reactions in external electrical fields [3, 4].
In addition to the chemical potential gradient an external electrical field acts as a second driving force for mobile components. A model, based on linear transport theory and defect thermodynamics, predicts a time independent growth rate for the product layer, when attaching the trivalent oxide to the cathode side. The growth rate is strongly enhanced compared to the reaction without electric field. The role of grain boundaries as fast diffusion paths for field driven solid state thin film reactions is emphasized. The morphology of the product layer is significantly different compared to reactions not driven by an external electric field. The electrochemical cells for our experiments were made in thin film technique by means of PLD. The morphological evolution of the boundaries, the microscopic structure and the crystallographic orientation of the adjoining phases are investigated by FESEM (EDX) and TEM.