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 co-addition of nano-sized SiO
₂
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 SiO
₂
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
.