PTCR effect in donor doped barium titanate: review of compositions, microstructures, processing and properties

Barium titanate is widely used in the fabrication of thermistors with a positive temperature coefficient of resistivity (PTCR). The resistivity can increase by several orders of magnitude near the phase transition temperature T-c for the ferroelectric tetragonal to the paraelectric cubic phase transformation. There is general agreement that the anomaly in the change of electrical resistivity of donor doped BaTiO3 around T-c is due to the grain boundary effect. The Heywang-Jonker model and other mechanisms involving the nature of the electrical barrier formed across the grain boundaries of polycrystalline BaTiO3 are reviewed. The compositional effect on BaTiO3 based PTCR properties is listed and discussed. The influences of manufacturing methods under different stages including the initial doping methods, sample forming methods and final heat treatments on PTCR properties are compared. The complex interrelationships between compositions, microstructures, processing and PTCR characteristics are well discussed.