Variation with Processing Conditions of Bulk and Grain Boundary PTCR Phenomena in Doped BaTiO3

The use of combined impedance and modulus spectroscopy to analyse a.c. impedance data for doped polycrystalline BaTiO3 shows the presence of two component impedances which are attributed to bulk and grain boundary effects. This assignment is based on the temperature dependence of their associated capacitances: the grain boundary capacitance does not change significantly with temperature, whereas the bulk capacitance passes through a maximum at the Curie point and shows Curie-Weiss behaviour at higher temperatures. The PTCR effect, characterised by a large increase in resistance as the temperature is raised through the Curie point, is shown by both components. The influence of processing conditions on the PTCR effect, especially the rate at which the sample is cooled to room temperature after sintering, is discussed. At the shorter rates, the PTCR response is dominated by the bulk impedance, whereas at longer times, the grain boundary impedance dominates.