Conductivity relaxation experiments on donor doped barium titanate ceramics: Effect of microstructure

The re-equilibration kinetics of n-conducting BaTiO3 ceramics upon a sudden change of the oxygen partial pressure of the surrounding gas phase has been investigated by application of conductivity relaxation experiments at 900 °C using the van der Pauw method. The relaxation curves show two distinct equilibration processes. The first (fast) relaxation refers to diffusion of oxygen most probably via oxygen vacancies along the core regions of grain boundaries. The second (sluggish) relaxation process is related to slow diffusion of cation vacancies from the grain boundaries into the bulk. Even after 1000 h, disk-shaped samples with an average grain size of 4 μm and thicknesses between 0.5 and 1.0 mm are not fully equilibrated with the gas phase. A modified Schottky barrier model, taking account of diffusion profiles of cation vacancies at the grain boundaries, enables a phenomenological interpretation of the conductivity relaxation curves. In addition, the defect chemistry of core regions of grain boundaries in n-type BaTiO3 ceramics has been studied by conductivity measurements as a function of oxygen partial pressure [0.5 > p(O2)/bar > 0.001].