The effect of filler permittivity on the dielectric properties of polymer-based composites

Polymer dielectrics are promising for high breakdown strengthen (Eb) and low dielectric loss (Dl) but suffer from low permittivity(epsilon). Adding high epsilon filler in polymer is a major strategy, while it decreases much Eb and increase Dl. Fillers with lower epsilon might enable higher Eb and lower Dl by removing high electric field. However, this conclusion remains controversial because of many experimental interferences, such as interface, filler morphology and polymer crystallization. Here, to systematically investigate the impact of filler epsilon, nanoparticles (silica, alumina, barium titanate) that have different epsilon are selected and are similarly surface treated to prepare polypropylene composites. Similar filler morphology, filler surface chemistry, dispersion and polymer crystallization are controlled. The epsilon of composites increases with filler epsilon, since high epsilon filler contributes more filler polarization. Composites filled with high epsilon filler and middle epsilon filler can be described by Jaysundere-Smith model, while composites filled with low epsilon filler are higher than models, because influence of interfacial polarization is obvious for low epsilon filler. The Dl of composites shows no relation to filler epsilon, since the intrinsic DI of filler is irrelevant to filler epsilon, meanwhile interfacial defects activate similar degree of loss at interface regardless of change of filler epsilon. The Eb of composites displays little relevance to filler epsilon. Only when filler epsilon is very low, Eb can be partly improved, owning to mitigated electric field. This work presents a fundamental insight on the mechanism of filler permittivity and provide a guidance for selection of dielectric filler.