A unified model for the permittivity and thermal conductivity of epoxy based composites

In this paper, three-dimensional models are presented that were developed for both the relative permittivity and the thermal conductivity of composites based on epoxy in an attempt to understand the way in which modified fillers interact with epoxy. Various ceramics were used as fillers, namely magnesium oxide, aluminium nitride, aluminium oxide, hexagonal boron nitride and cubic boron nitride with an average particle size between 22 and 150 nm. The concept of the models is mainly based on two competing mechanisms, i.e., polymer re-organization and water uptake. These were realized in the models by assuming two interfacial layers around the particles, an inner and an outer layer. The former refers to water uptake and the latter demonstrates the influence of the matrix by the presence of surface modified particles and exhibits different properties than both the filler and polymer matrix. The thickness of the inner layer is constant while the outer layer thickness has a dynamic behaviour which strongly depends on the interparticle distance and thus, on the filler content. The same assumptions were made for both models and an accurate fit was achieved between the experimental data and model results.