Dielectric measurements and analysis for the design of conductor/insulator artificial dielectrics

Abstract

The complex permittivity and magnetic permeability of an artificial dielectric formed from metallised microspheres dispersed in paraffin wax are presented for wide ranges of filler volume fractions and of frequency. The measurement data are used to test the suitability of a range of simple analytical models (including effective media and percolation formalisms) for the design of 'electromagnetically' functional materials. The data are discussed in terms of the challenges encountered in producing reliable electromagnetic measurements on this type of material and the influence of sample preparation on the measured properties. The dilemma faced by the design engineer in selecting the appropriate blend of theory and experiment when tailoring the electromagnetic response from such a composite material is highlighted. A clear percolation transition is observed in the materials dielectric properties, and it is demonstrated that direct measurement of real permittivity near and above the percolation threshold remains a significant measurement challenge. Statistical percolation models are shown to enable an accurate fit to the measured data although these models cannot be used to determine the percolation threshold which is simply a fitting parameter. Further evidence is provided to demonstrate the applicability of percolation theory to the material system investigated.