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(ε). Adding high ε filler in polymer is a major strategy, while it decreases much Eb and increase Dl. Fillers with lower ε 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 ε, nanoparticles (silica, alumina, barium titanate) that have different ε are selected and are similarly surface treated to prepare polypropylene composites. Similar filler morphology, filler surface chemistry, dispersion and polymer crystallization are controlled. The ε of composites increases with filler ε, since high ε filler contributes more filler polarization. Composites filled with high ε filler and middle ε filler can be described by Jaysundere-Smith model, while composites filled with low ε filler are higher than models, because influence of interfacial polarization is obvious for low ε filler. The Dl of composites shows no relation to filler ε, since the intrinsic DI of filler is irrelevant to filler ε, meanwhile interfacial defects activate similar degree of loss at interface regardless of change of filler ε. The Eb of composites displays little relevance to filler ε. Only when filler ε 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.