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 v
olume 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 NaC
l/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 pro
perties with a resistivity increase of five orders of magnitude and a
temperature coefficient of 15%-17% K-1. A composite of all inorganic m
aterials with a controllable PTCR property is suggested to be possible
.