Electric and Dielectric Properties of Composites of Polyaniline Coated Short Carbon Fibers in Epoxy Resin

Conductivity and dielectric properties of composites of short carbon fibers coated with a layer of non-conducting form of polyaniline, polyaniline base, in epoxy matrix in the range of 100 Hz to 1300 MHz has been investigated. Unlike the composites with uncoated filler, the properties of the coated material were quite different and significantly depending on the filler content. The measurement of direct current (DC) conductivity revealed that while the percolation threshold of the uncoated fibers lays between 1 - 2 wt. % of the filler, for coated particles their non-conducting layer increased this value to ca 16-20 vol. %. Consequently, the conductivity of the composite with 7 vol. % of uncoated particles, i.e. far above the percolation threshold, reached a high value of 2×10^-3 S m^-1, while at the same filler concentration conductivity of the composite material with coated fibers still occurs in the range below percolation limit with a low conductivity similar to matrix value (≈ 10^-11 S m^-1). The frequency spectra of complex permittivity indicated that, in spite of different fiber character, the difference in polarization of induced dipoles of coated and uncoated fibers or their clusters is not large. The comparison of composites with coated and uncoated fibers below the percolation limit (7 vol. % for the former and 1 vol. % for the latter), both with high DC resistivity, showed that the AC conductivity of the sample filled with coated fibers rose with frequency more quickly than that containing uncoated fibers. This suggests the composite with coated fibers could be a suitable isolating material in low-frequency range and conducting for high-frequency signals.