Effects of lossy, layered filler particles on the bulk permittivity of a composite material

The ability to control the frequency at which a dielectric material exhibits maximum loss (the relaxation frequency) is of interest in telecommunications and radar absorption. A theoretical investigation of the behaviour of the complex bulk permittivity of a composite material with coated, spheroidal filler particles is presented. In the model, the filler particles are replaced mathematically by electric multipole sources located at their centres (Harfield N 2000 J. Mater. Sci. 35 5809-16). It is shown how factors such as particle shape, orientation with respect to the applied electric field, thickness of coating and permittivity value of the individual phases influences the bulk permittivity of the composite material. For a composite with coated filler particles, one or two relaxation frequencies may be observed. Employing the theory of Pauly and Schwan (Hanai T 1968 Electrical properties of emulsions Emulsion Science ed P Sherman (London: Academic)), particular attention is paid to the way in which the relaxation frequencies are affected by the material parameters.