Validation of mixture equations for dielectric-magnetic composites

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Abstract
Mixture equations relate the effective macroscopic electromagnetic properties (dielectric constant and magnetic permeability) of a composite material to the intrinsic microscopic electromagnetic properties of its constituents. The predictions of mixture equations in popular use generally agree with each other when particles are widely separated (small volume fraction), but begin to diverge when the volume fraction of particles in the composite is greater than 10%. This paper reviews the assumptions and characteristics of different representative mixture equations. The validity of these mixture equations for application to magnetic composites was tested by comparing their predictions of macroscopic electromagnetic properties to VHF and UHF measurements of these properties for a series of well-characterized dielectric-magnetic composite samples comprised of ferrite particles in an epoxy binder. The investigation emphasized the parameter regimes where large disparities between the predictions of the various equations exist, namely, large volume fraction of particles, complex dielectric constant and magnetic permeability (lossy material), and large contrast ratio between particle and binder properties. Correlation and lack of correlation between the mixture equation predictions and experimental data are shown.