The addition of fibers to an otherwise brittle matrix has a profound influence on the mechanical properties of the composite. By "fiber" is meant an object that is approximately axisymmetric, and much longer than it is wide. The aspect ratio, the ratio of length to width, is usually on the order of 100 or more for typical fibers.
The addition of fibers can also have a strong effect on the electrical properties (DC conductivity and AC impedance) of the composite, but only when the added fibers are highly conductive compared to the matrix [1]. Long, thin inclusions like fibers, which are insulating with respect to the matrix, have little effect on overall electrical properties [1], although of course they can still affect mechanical properties. But if the fibers, which are usually added for mechanical purposes, are highly conducting with respect to the matrix, electrical measurements, which are indirect and non-destructive, are sensitive to the presence of fibers. If the properties of the fibers with respect to the matrix varies with frequency, then effects of the fibers on the overall electrical properties will be seen in the complex impedance plane.
The present study addresses the microstructure-electrical property relationships of such a composite material, made up of an electrically conductive matrix that contains small amounts of short conductive fibers. In addition to measurements made on actual composites, laboratory simulations involving single wires in dielectric media were carried out to show how the presence of fibers affects the electrical properties in well-controlled situations in which the fiber size and position were precisely known. These laboratory simulations were checked by pixel-based theoretical computations, to verify the phenomena observed and the mechanisms inferred. The results are applicable to relevant conductive fiber/poorly conductive matrix situations, including ceramic, cement/concrete, and polymer matrices. We next briefly review the relevant literature before going on to our new results and frequency-switchable fiber coating model.