Space charge segregation during the cooling process and its effect on the grain boundary impedance in Nb-doped BaTiO3

Highly donor-doped and large grained BaTiO3 ceramics can be prepared by sintering in a reducing atmosphere. If these specimens are again heat treated at a certain sintering temperature and cooled in air, PTCR (positive temperature coefficient resistor) characteristics appear. When the cooling rate is low, another third RC (resistance-capacitance) component appears besides that of the grains and grain boundaries in impedance analysis. This RC component is associated with donor segregation. It is not observed when the cooling rate is high. In the slightly donor-doped specimens where the cooling rate, microstructure and R (resistivity)–T (temperature) properties are similar to the former, this third RC component also does not appear. The segregation related phenomena were confirmed by STEM/EDS analysis. These trends correspond to the space charge segregation theory that explains that the space charge potential, which acts as the driving force for donor segregation, becomes large during the cooling process in air.