Dielectric effects on positive temperature coefficient composites of polyethylene and short carbon fibers

By adding the short carbon fibers to the polyethylene matrix, excellent positive temperature coefficient (PTC) effect was achieved. Alternating current (AC) electrical properties of this PTC composite were studied as a function of frequency. The analysis of AC electrical conductivity and dielectric permittivity was done by using a micro-morphology model, which included conductive carbon fiberaggregates in series with an equivalent circuit of resistor-capacitor parallel that represent the blends at these contact regions. The observed electrical properties of PTC composites were due to the breakage of the conduction networks caused by thermal expansion. The dielectric behaviors of the interfacial polarization between polyethylene matrix and carbon fibers could be described by Maxwell-Wagner-Sillars relaxation when the composite was heated above 116 degrees C. The analysis of the electric modulus in the frequency range from 100 Hz to 10 MHz revealed that the interfacial relaxation followed the Cole-Davidson distribution of relaxation time. (C) 2007 Elsevier Ltd. All rights reserved.