AC conductivity of carbon fiber–polymer matrix composites at the percolation threshold

This paper reports results on experimental investigation of the conductivity behavior of carbon fiber filled polymer composites at the percolation threshold. Two types of carbon fiber‐epoxy matrix composites have been studied and comparison of the measured data has been made. These two types of composites differ in the surface modification of carbon fibers (in one case the surface of carbon fibers is covered with polymer beads using the microencapsulation technology, in the other their surface stayed unmodified). Experimental data reveal that surface modification of carbon fibers influences greatly the DC conductivity (percolation threshold moves to higher concentrations) but does not influence the AC electrical properties. From the frequency dependence of conductivity upon fiber concentration it becomes clear that it is not possible to predict the high frequency conductivity (electromagnetic interference shielding properties) based on the DC conductivity. Percolation behavior of conductivity as a function of conductive filler concentration is typical only for DC or low frequency AC conductivity. The percolation threshold gradually vanishes for high frequencies of electromagnetic field. The temperature dependence of electrical properties has also been studied. Composites with concentration near the percolation threshold show the switch‐off effect (at the specific temperature the DC conductivity drops by several orders of magnitude). This switch‐off effect does not occur for high frequency AC conductivity.