Microstructures and Electrical Properties of BaTiO3 PTC Ceramics

Abstract

The microstructure, especially porosity, of PTC (positive temperature coefficient) thermistor based on BaTiO3 was controlled with a forming pressure. The relationship between theirPTCR properties and microstructureswas investigated with an optical and SEM (Scanning Electron Microscope) images and digital multimeter. Disk samples were fabricated by pressinguniaxially at various pressures of 100~15000kg/cm2 and sintering at 1265°C in reducing atmosphere and finally re-oxidizing at 700°C in air. The porosity of the samples decreased rapidly from 45% to 8% with increasing the forming pressure from 100 to 1000kg/cm2andbecame 4% at 15000kg/cm2with slowdecreasing of porosity in the pressure range of 1000~15000kg/cm2.With increasing the forming pressure, the resistivity jump of samplesdecreased rapidlyfrom 0.5 to 2.9 at about1000kg/cm2that corresponds tothe porosity of 15% and was saturated above this pressure. It is considered that there is a critical amount of porosity for having PTCR effect, which was about 15% in our samples. In addition, the porosity of the sample has a greater influence on the resistivity jump than on theresistivity at room temperature, which is due to the oxidation of grain boundary through a favorable channel of oxygen such as a pore.