Broadband Characterization of Flexible Conductor-Dielectric Composites

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Abstract

Broadband measurements are important for characterizing a wide range of materials for communications applications at microwave and mm-wave frequencies. Here we report on broadband measurements of the effective permittivity of conductor-dielectric flexible composite materials over the frequency range 0.05 - 67 GHz. The materials under test are comprised of silver nanoparticles dispersed in an elastomer host material (polydimethylsiloxane) with variable volume fraction from 0.75% - 5.8%. Broadband calibration and de-embedding techniques are applied, along with finite-element simulations, to determine the complex effective permittivity for four different silver nanoparticle volume fractions from the measured scattering parameters. The resulting effective permittivity is well described by Maxwell-Garnett mixing formula, and the material coefficients extracted from fits include the conductivity, permittivity, and shape factor of the silver nanoparticle inclusions. Our analysis shows that as the filling fraction increases, the permittivity and conductivity of the inclusions increase dramatically, while the shape factor decreases. This results in much higher-than-expected enhancement of the effective permittivity for a range of volume fractions, which is likely due to aggregation or agglomeration of conducting particles in clusters with high interfacial polarizations.

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