Microstructural Dependence of the Voltage Sensitivity of the PTCR Effect in Donor Doped Barium Titanate

Abstract

Barium titanate positive temperature coefficient of resistivity (PTCR) ceramics are widely used in the electrical device industry. It is known that the PTCR effect shows a voltage sensitivity similar to voltage-dependent resistors. However, the microstructural aspects controlling this phenomenon have not been studied in detail. In this study, the voltage sensitivity of PTCR materials displaying a wide range of microstructures and properties was measured. Results show that though all materials tested showed considerable voltage sensitivity, the extent of the effect is strongly controlled by microstructural features. The presence of a grain boundary insulating second phase has the most significant effect. In these materials the room temperature resistivity is relatively insensitive to the voltage but the high temperature resistivity is significantly decreased. In the absence of such a second phase, resistivities are lowered by similar amounts at all temperatures, including a large decrease in the room temperature value. The difference in behavior between these two types of microstructures has been attributed to the difference in the number of available conducting pathways, and the resultant differences in local electric fields generated in the near-grain boundary regions. Thus, microstructural aspects play a pivotal role in determining the voltage sensitivity of these materials. An understanding of these relationships will enable engineering of better devices.