Effects of addition of MgO fillers with various sizes and co-addition of nano-sized SiO₂ fillers on the dielectric properties of epoxy resin

Complex permittivity (ϵ_r' and ϵ_r"), conductivity (σ), and glass transition temperature (T_g) were measured for composites of epoxy resin and MgO fillers with various sizes in a wide temperature and frequency range. The effect of coaddition of nano-sized SiO₂ was also examined. It has become clean that T_g decreases by the addition of MgO fillers, and its decrement becomes more with a decrease in filler size. Moreover, T_g decreases if the silica nanofillers are co-added. However, all the three important parameters for electrical insulation ability of polymers, namely ϵ_r', ϵ_r", and σ, decrease with the decrease in size of MgO fillers or by the addition of silica nanofillers at high temperatures and low frequencies. This is a very astonishing result, since it seems reasonable that a polymer with a high T_g should have a better insulating ability. Probably, the above-mentioned results indicate that the charge transport becomes more difficult, presumably resulting from the suppression of molecular motion with the decrease in size of MgO fillers and by the co-addition of SiO₂ nanofillers. This indicates that the suppression of molecular motion becomes very strong if the filler size is small, making the carrier transport difficult even in polymers with low T_g.