Enhanced surface insulation and depressed dielectric constant for Al2O3/epoxy composites through plasma fluorination of filler

Al2O3, an insulating ceramic with excellent mechanical and thermal properties, has been a kind of ideal fillers for composites. However, due to its poor charge transport capacity, composites doped with Al2O3 have a bad surface insulation property. In this study, nano-Al2O3 particles were modified by plasma fluorination using dielectric barrier discharge, and then doped into epoxy resin to prepare composites. In the preparation stage, the plasma treatment with and without fluorine were differentiated to determine whether fluorine or plasma itself had an influence on the microstructure of Al2O3. The changes of dielectric and surface insulation properties of composite materials before and after filler fluorination were studied. The variation of dielectric constant as a function of filler content has presented a Z-shape curve. Both larger particle diameter and fluorination treatment can reduce the value of dielectric constant, and lead the curve to a monotonous increasing trend. A model of dielectric distribution by the finite element method was constructed to explain the variation. Before fluorination treatment, the flashover voltage as a function of filler content presents a multi-inflection variation. After fluorination, the surface charge dissipation capacity together with flashover voltage were enhanced, and the fluorination treatment could increase the flashover voltage by up to 34.7% of the pure epoxy. The modification process and test results are of guiding significance for improving the insulation properties of epoxy composites.