PTCR characteristics in undoped barium titanate ceramics with core-shell type duplex microstructures

Undoped barium titanate ceramics exhibiting a distinct positive temperature coefficient of resistivity (PTCR) effect have been produced by sintering high-purity barium titanate powder compacts in the temperature range 1300-1400/spl deg/C in air. The formation of core-shell type duplex microstructures in the materials covered with a nearly full dense layer is responsible for this phenomenon, where undoped barium titanate ceramics (which are normally insulating) were converted to semiconductors and, moreover, exhibited distinct PTCR effects. The core-shell grain structure is described as a duplex microstructure consisting of a large-grained (/spl ap/50 /spl mu/m) semiconducting phase inside and a small-grained (<10 /spl mu/m) insulating phase outside.