ABSTRACT
The positive temperature coefficient (PTC) dopants typically have a higher ionic valence than either barium (Ba) or titanium (Ti) of the host structure. This chapter considers the PTC effect in terms of two mechanisms: the semiconducting properties of the doped barium titanate (BT) and the grain boundary barrier effect. PTC thermistors are applicable not only for temperature-change detection, but also for active current controllers. The thermistor, when self-heated, exhibits a decrease in the current owing to a large increase in resistivity. When a semiconductive BaTiO3 ceramic is oxidized to make a resistive surface layer, it can be used as a high capacitance condenser. A new type of grain boundary layer (GBL) capacitor has been developed using electrically resistive grain boundaries on the semiconductive grains. This type of capacitor exhibits excellent frequency characteristics, and can be used as a wide bandpass capacitor up to several GHz.
