Electrical properties of composites in the vicinity of the percolation threshold

The electrical response of thermoplastic composites composed of carbon blac k and high-density polyethylene near the electrical percolation threshold ( p(c)) has been investigated through the study of the volume resistivity and complex permittivity. The change in conductivity beyond p(c) exhibited a c ritical exponent that was greater than predicted from percolation theory. C omposites with carbon black contents slightly larger than p(c) exhibited th e greatest sensitivity in volume resistivity with temperature variations un der the melting point of polyethylene. In addition, percolating composites with low carbon black contents exhibited significant "negative temperature coefficient" (NTC) effects and improvements in conductivity with annealing. Maxwell-Wagner interfacial polarization resulted in moderate increases in both the permittivity (epsilon') and dielectric loss factor (epsilon ") bel ow p(c), while at percolation, an abrupt and dramatic increase was observed for both components of the complex permittivity. (C) 1999 John Wiley & Son s, Inc.