Electrical properties of carbon/epoxy composites in wide frequency band

Abstract

The self-consistent theory for two-phase composites describes the dependence of electrical properties on frequency and carbon filler volume fraction in wide frequency band. In carbon -black/epoxy -resin composites, the conductivity increases abruptly and the real part of permittivity diverges around the carbon volume fraction of 0.18. The variation of conductivity on frequency which shows the three stages of variation becomes less pronounced as the carbon volume fraction increased, and the variation of real permittivity on frequency which also has the three stages of variation becomes more pronounced as the percolation threshold is approached from either side. In unidirectional carbon-fiber/epoxy-resin composites, the conductivity and the real part of permittivity have anisotropy. The longitudinal (E vector parallel to fibre axes) conductivity which satisfies the rule of mixture, is shown to be constant from DC to several MHz and decreases abruptly with frequency above several 10 MHz. The transverse (E vector transverse too fibre axes) conductivity which has three stages of variation with frequency, increases with increasing fibre volume fraction, while the real permittivity which also has the three stages of variation with frequency, decreases with increasing fibre volume fraction.