Dielectric response and thermal conductivity of epoxy resin filled with nanoalumina particles of different size in α, γ and δ phase

Specific properties of insulation systems based on thermosetting and thermoplastic polymers can be enhanced by adding different types of inorganic fillers. The focus of this paper is on the effect of the filler size and modifications of the crystallinity of aluminum oxide (Al₂O₃) nanoparticles on the dielectric and thermal properties of epoxy-based composites. Nanoalumina in the α, γ and δ phases with different average particle size were introduced into epoxy resin, using a conventional mechanical mixing method and ultrasonication processing. Surface treatment of the particles was realized by silanization, in order to improve the compatibility of the host polymer and the ceramic filler, by organizing physical and chemical bonding between the dissimilar materials. This was done to obtain a uniform dispersion of nanoparticles in the polymer matrix. Five types of alumina-epoxy composites were fabricated with an average particle size ranging between 20 and 80 nm. The filler loading was 2 wt.% for all composites. Neat epoxy samples were used as reference. The morphology characterization of the particles and synthesized specimens was carried out by transmission electron microscopy and X-ray diffraction techniques. Dielectric spectroscopy measurement was realized in a broad frequency range between 0.01 and 10 MHz, for temperatures between -20°C and 120°C. Tentative explanations are given for the observed differences in the relative permittivity and dielectric loss factor between nanocomposites and the reference polymer.