Microwave properties of carbon black-epoxy resin composites and their simulation by means of mixture laws

This article reports on a study of the dielectric properties of carbon black dispersions in an insulating epoxy matrix at microwave frequencies. Measurements showed that the complex permittivity of the composites depends strongly on the nature and concentrations of the conducting medium. The experimental values of the complex permittivity were compared to those obtained by using different mixing laws. We show that effective medium theories correctly account for the experimental results at low conducting particle concentrations. At concentrations higher than a few percent, these laws fail to interpret experimental results and all tentative results must take into account parameters such as the particle size, their distribution, and the existence of agglomerates.