The role of point defects and defect gradients in flash sintering of perovskite oxides

Abstract The present study investigates the impact of point defects and their redistribution on the flash sintering process. Strontium titanate was chosen as a model system for the group of perovskite ceramics. The characteristics of flash sintering of strontium titanate were analyzed with different acceptor dopant concentrations. The onset of flash sintering was found to be dependent on the acceptor dopant concentration, as expected by the increasing conductivity. A gradient in the microstructure was found after flash sintering with larger grain sizes at the negative electrode. TEM-EDS measurements indicated Ti enrichment at the positive electrode for undoped strontium titanate and strong acceptor segregation for doped strontium titanate. In contrast, grain boundaries at the negative electrode were found to be stoichiometric for the undoped case and the acceptor segregation was less obvious for the doped case. Based on these results and the space charge behavior of strontium titanate, we infer that a gradient of the oxygen vacancy concentration is induced by the electric field during flash sintering: at the positive electrode the oxygen vacancy concentration is higher than at the negative electrode. For strontium titanate it is well known that a high oxygen vacancy concentration reduces the space charge and, hence, acceptor segregation, which agrees well with the experimental findings. Overall, the present study highlights the importance of point defect gradients and space charge for flash sintering of strontium titanate, which may be applicable for many other functional ceramics.