Prolonging electrical lifetime of mesogenic epoxy based alumina-mica composite sheet: Optimization of mica content by electrical tree progress simulation

Electrical lifetime of alumina-mesogenic epoxy composite sheet is improved by the addition of mica without reducing thermal conductivity. We developed an electrical tree progression simulation to optimize the composition ratio of four species of fillers (mica and three types of alumina fillers). This simulation is based on the electric field analysis with the use of a two-dimensional finite-difference method. The simulation revealed that the electrical lifetime of the composite sheet increases depending on the amount of mica filler although it is saturated at 1.3 vol. %. The calculated electrical lifetime of the sheet with mica filler is 1.5-2.0 times longer than that of the sheet without mica. This result agrees with the experimental at high applied voltage of 7 kVrms. A low mica ratio is suitable in the view of the thermal conductivity of the composite sheet because thermal conductivity of mica filler is much lower than that of alumina filler. The effective amount of mica filler was determined at 1.3 vol. % to possess high thermal conductivity and long electrical lifetime. As a result, a composite sheet with thermal conductivity of 8.7 W m-1 K-1 and the electrical lifetime 13 times longer than the sheet without mica at 4 kVrms was obtained.