Energy storage in polymer films with high dielectric constant fillers [Feature Article]

A polymer filled with high dielectric constant particles to achieve greater dielectric constant and energy density has obvious appeal. It also is fraught with problems. At low-volume fractions of high dielectric contrast filler, little electric field resides in the filler, and most of the increase in effective dielectric constant results from an increase of electric field in the polymer matrix. For spherical filler and a dielectric contrast (ratio of dielectric constants) greater than 100 between the filler and polymer matrix, the energy density of the composite increases rapidly for volume fractions >35%. For the case of randomly oriented prolate spheroidal filler with an asperity of 3 (i.e., the major axis is three times larger than the minor axis), the energy density increases rapidly for volume fractions >18%. With the assumption that breakdown is determined by the average field in the matrix, the energy density increases with dielectric contrast.