PTCR PROPERTY IN CARBON NACL COMPOSITES

The composition dependence of resistivity and the positive temperature coefficient of resistivity (PTCR) were investigated in composites of electrically conducting carbon and insulating NaCl. Resistivity at 20-degrees-C of the composites decreased rapidly with increasing carbon volume fraction above a critical volume fraction, V(c), which was well expressed by an equation of the percolation theory. The value of V(c) decreased from 25 vol % to 4 vol % when the particle size ratio of NaCl/carbon increased from 0.5 to 50. When the carbon volume fraction was close to the value of V(c), the composites showed pronounced PTCR properties with a resistivity increase of five orders of magnitude and a temperature coefficient of 15%-17% K-1. A composite of all inorganic materials with a controllable PTCR property is suggested to be possible.