Modeling of conductivity relaxation in oxide ceramics taking account of concentration dependent chemical diffusivities and surface exchange coefficients

Abstract Conductivity relaxation of disk-shaped ceramic samples upon a sudden change of the oxygen partial pressure of the surrounding gas phase has been simulated by application of a three-dimensional finite element model. The geometry is based on a homogeneous cylindrical specimen with a diameter of 10 mm and a thickness of 0.1 mm, where four peripheral line contacts are attached to the sample in accordance with the van der Pauw method. Both the chemical diffusion coefficient and the chemical surface exchange coefficient are assumed to vary with the oxygen non-stoichiometry during the oxygen exchange reaction (oxidation or reduction). The alteration of the oxygen non-stoichiometry as well as the relaxation of the electrical conductivity are calculated simultaneously. The modeling of conductivity relaxation curves is applied to the SOFC cathode material La 0.4 Sr 0.6 FeO 3 −  δ (LSF) at 900 °C. The simulation of conductivity relaxation curves can be performed for arbitrary oxygen partial pressure changes. The present modeling approach allows the interpretation of significant differences between oxidation and reduction steps owing to the concentration (non-stoichiometry) dependence of the kinetic parameters.