Positive temperature coefficient of resistivity effect in undoped barium titanate ceramics

A distinct positive temperature coefficient of resistivity (PTCR) effect has been observed in undoped barium titanate ceramics sintered in the temperature range 1300–1400 °C in air. Although undoped barium titanate is normally insulating and ferroelectric at room temperature, it has been found that semiconducting barium titanate ceramics with a distinct PTCR effect could be produced by sintering high-purity, highly sinterable barium titanate powders in air. The resulting semiconducting barium titanate ceramics, with a relative sintered density of about 98%, had a core-shell structure with a semiconducting phase inside and an insulating phase outside. The outer insulating part had a microstructure consisting of rather small grains with an average grain size of less than 10 μm, while the inner semiconducting part had a large-grained microstructure with an average grain size of around 50 μm.