Microstructural and electrical property evolution in an acceptor-dopant free positive temperature coefficient thermistor

The effect of varying sintering temperature in the range 1270–1430 °C on the resistivity–temperature characteristics of semiconducting BaTiO3 based positive temperature coefficient of resistance thermistors containing a donor-dopant, but without acceptor doping, was investigated by impedance spectroscopy. As the sintering temperature was increased the specimen resistivity around the Curie temperature decreased, while the peak resistivity, obtained above the Curie temperature, remained approximately constant. The change in PTC behaviour with increasing sintering temperature is inconsistent with the standard double Schottky barrier model, but is explained in terms of grain size variations coupled with a, sintering temperature independent, grain boundary barrier layer thickness of 0.50±0.04 μm.