Electromagnetic shielding of epoxy resin composites containing carbon fibers coated with polyaniline base

Abstract

Polymers filled with carbon fibers have recently received attention due to their remarkable conducting and dielectric properties. The fibrous character of the filler causes that the percolation threshold of these systems is reached at 1 - 2 vol. % of conducting component. Coating the fibers with a non-conducting layer can substantially increase the percolation threshold, thus enabling to broaden the range of concentrations where the DC conductivity of material is low and its behavior is not affected by the instabilities in the vicinity of the percolation threshold. As far as dielectric properties are concerned, at high frequencies they are mainly controlled by the polarization of induced dipoles of the fibers or their clusters. Thus, by increasing filler loading, i. e. with higher number of induced dipoles, an improvement of dielectric properties can be expected. The present study has been aimed at electromagnetic interference shielding properties of epoxy resin composites containing short carbon fibers coated with a layer of non-conducting polyaniline base. Due to the coating, the percolation threshold shifted to 16-20 vol. % of the filler. Such high concentration caused a considerable increase in complex permittivity and AC conductivity of investigated material below the percolation threshold. The evaluation of shielding effectiveness and the skin depth the radiation can penetrate, however, have revealed that the material is still not suitable for commercial applications. Nevertheless, the composites of short carbon fibers coated with non-conducting polyaniline base show a high AC conductivity in high frequencies (10 MHz -1300 MHz) and low DC conductivity at the same time. They can thus be used for transmitting high-frequency signals, and for shielding of low-frequency ones. Moreover, they do not short-circuit the surface of electronic systems.