There is a need in electronic systems and pulsed power applications for capacitors with high energy density. Current state-of-the-art polymeric capacitors (BOPP, PET) only have a maximum energy density of 5-6 J/cc. From a material standpoint, the energy density improves with increasing dielectric constant and/or breakdown strength, and the loss is diminished by reducing dissipation factor and high field polarization hysteresis. Our approach to improve polymer film capacitors is to combine, through microlayer coextrusion, two polymers with complimentary properties: one with a high dielectric constant (polyvinylidene fluoride type polymers - PVDF) and one with a high breakdown strength (polycarbonate). Multilayered films with many alternating layers of polymers exhibited improved breakdown characteristics due to the development of a “treeing” type failure mechanism. In addition, a reduction of polarization hysteresis was observed due to layer confinement effects on charge migration in the PVDF based layers. This charge migration, either from surfactant or catalyst residue, was studied in detail using broadband dielectric spectroscopy, which revealed an ion concentration and diffusion coefficient of 2E21 ions/m3 and 2E-13 m2/s, respectively, for films with layer thicknesses of 430 to 50 nm.
Using the understanding gained from these systems, films with energy densities as high as 16 J/cc while maintaining a dissipation factor of 0.009 and low hysteresis were obtained. Selection and optimization of future layered structures can be carried out to obtain even higher energy densities and lower dielectric losses. In particular, the morphological structure within the semi-crystalline polymeric layers may be used to further enhance the dielectric properties. Microlayering in combination with heat treatment were used to control the morphology of several layered systems (i.e. PC/PVDF, PSF/PVDF, and PC/P[VDF-TFE]). A range of crystal orientations were produced that included isotropic, on-edge, and in-plane.