Electromechanical Response of Multilayered Polymer Films for High Energy Density Capacitors
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Electromechanical Response of Multilayered Polymer Films for High Energy Density Capacitors. Mason A. Wolak,1 James S. Shirk,1 Matt Mackey,2 Joel Carr,2 Ann Hiltner,2 and Eric Baer2 1
Optical Sciences Division, U.S. Naval Research Laboratory, Washington, DC 20375. USA. Department of Macromolecular Science and Center for Applied Polymer Research, Case Western Reserve University, Cleveland, OH 44106-7202. USA. 2
ABSTRACT Multilayered films comprising alternating layers of polycarbonate (PC) and poly(vinylidene fluoride-hexafluoropropylene) (P[VDF-HFP]) show an enhanced dielectric strength (EB> 750 kV/mm) and an increased energy storage density (Ud ~ 13.5 J/cm3) compared to monolithic PC and P[VDF-HFP] films. Here the role of electromechanical effects in the breakdown of multilayer films is explored both by imaging the changes in the layer structure caused by electrical fields below the breakdown field and by a direct measurement of the strain in multilayer PC/ P[VDF-HFP] films subjected to similar fields. Focused Ion Beam (FIB)/ Scanning Electron Microscopy (SEM) images of the layer structure in films subjected to repeated cycles at near-breakdown fields showed local changes in the thickness of individual layers, suggesting that mechanical forces arising from field-induced compression may play a role in the steps preceding the breakdown. The directly measured field induced strain showed evidence for both an elastic and a flow component to the strain. The mechanical responses of films with 50 vol% P[VDF-HFP] were modeled as simply the sum of an elastic and viscous flow. The observed electromechanical properties vary with the layer structure. This suggests that multilayering polymers may provide a means to mitigate deleterious electromechanical effects in low modulus, high dielectric materials. INTRODUCTION It was recently reported that multilayer films comprising alternating layers of polycarbonate (PC) and poly[vinylidene fluoride-hexafluoropropylene] (P[VDF-HFP]) exhibit superior dielectric properties including high dielectric strength, low dielectric loss, and high deliverable energy density.1 Films containing 256 layers of 50 vol% PC and 50 vol% P[VDFHFP] have a dielectric strength, EB, of > 750 kV/mm and an average maximum energy density, Ud, of 13.5 J/cm3. The measured energy density represents a 100% increase relative to parallel plate capacitors fabricated from PC control films, and a 60% increase relative to similar devices fabricated from monolithic P[VDF-HFP] control films.2 The enhanced dielectric strength (relative to single component films of PC and P[VDF-HFP]) appears to originate from a barrier effect,3 whereby the interfaces between layers serve to impede the propagation of a breakdown tree through the film in the direction of an applied field. We have reported on the use of a Focused Ion Beam (FIB)/ Scanning Electron Microscopy (SEM) technique to image the layer structure both before and after the application of a field resulting in electrical breakdown.4 The FIB/SEM technique revealed that the br
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