Topological description of composite microwave-absorbing materials by application of the spectral density function method

摘要

Extraction of topological information describing two-phase composite systems has previously been achieved by representing the effective permittivity of such a system in terms of spectral density functions. Recently, it has been shown that the unknown spectral density function in binary mixtures can be determined using a novel numerical technique in which constrained least-squares and Monte Carlo techniques are applied simultaneously to solve the ill-posed inversion problem of the spectral density representation. In this article, experimental data obtained in the frequency range 0.5 to 18 GHz is analyzed. Both non-magnetic and magnetic microwave-absorbing materials are studied. The first material consists of multi-layered spherical particles dispersed in a matrix of paraffin wax. The filler particles are tungsten-coated glass micro-bubbles, with an outer insulating coating of alumina. The second material is formed with iron powder filler, yielding a material which exhibits bulk magnetism. In principle, the spectral density function method can be applied to measurements of bulk magnetic permeability, rather than dielectric constant, although this has not previously been attempted. The potential of this approach is assessed here.