There is a need in electronic systems and pulsed power applications for capacitors with high energy density. From a material standpoint, capacitive energy density improves with increasing dielectric constant and/or breakdown strength. Current state-of-the-art polymeric capacitors are, however, limited in that their dielectric constant is low (2–4). Our approach to improve polymer film capacitors is to combine, through microlayer coextrusion, two polymers with complementary properties: one with a high breakdown strength (polycarbonate) and one with a high dielectric constant (polyvinylidene fluoride-hexafluoropropylene). As opposed to the monolith controls, multilayered films with various numbers of layers and compositions subjected to a pulsed voltage exhibit treeing patterns that hinder the breakdown process. Consequently, substantially enhanced breakdown strengths are measured in the mutilayered films. It is further shown, by varying the overall film thickness, that the charge at the tip of the needle electrode is a key parameter that controls treeing. Based on the acquired data, a breakdown mechanism is formulated to explain the increased dielectric strengths. Using the understanding gained from these systems, selection and optimization of future layered structures can be carried out to obtain additional property enhancements.
The effect of introducing a multilayer microstructure on the dielectric properties of polymer materials is evaluated in 32- and 256-layer films with alternating polycarbonate (PC) and polyvinylidene-hexafluoropropylene (coPVDF) layers. The permittivity, dielectric loss, dielectric strength, and energy density were measured as a function of the relative PC/coPVDF volume concentrations. The permittivity follows an effective medium model while the dielectric strength was typically higher than that predicted by a volume fraction based weighted average of the components. Energy densities as high as ∼14J∕cm3, about 60% greater than that of the component polymers, are measured for 50% PC/50% coPVDF films.
Layer‐multiplying coextrusion was used in conjunction with isothermal recrystallization to study the confined crystallization of polyvinylidene fluoride (PVDF) and polyvinylidene fluoride‐tetrafluoroethylene (PVDF‐TFE) using polycarbonate (PC) and polysulfone (PSF) as confining materials. Three layered systems were produced (PC/PVDF, PSF/PVDF, and PC/PVDF‐TFE) with layer thicknesses ranging from 525 to 28 nm. The crystal morphology was affected by both layer thickness and recrystallization temperature. Specifically, increased recrystallization temperature and decreased layer thickness facilitated the formation of high aspect ratio in‐plane crystals in both PVDF based polymers. On the other side of the spectrum, thicker layers and lower recrystallization temperatures produced on‐edge PVDF crystals and isotropic PVDF‐TFE crystals. The morphology was correlated with oxygen permeability, which decreased by almost two orders of magnitude compared with the bulk. A variety of crystal structures were obtained and explained with nucleation and diffusion theory. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011
ABSTRACT:The dielectric strength and energy storage capability of poly(vinylidene fluoride-hexafluoropropylene) copolymer (P[VDF-HFP]) films are enhanced by interleaving layers of PVDF copolymer with thin layers of polycarbonate (PC). To gain insight into the breakdown processes in such materials, focused ion beam (FIB) milling in conjunction with scanning electron microscopy (SEM) was used to study the effect of a breakdown on the film. FIB can sequentially mill cross sections that are each imaged by SEM. The technique can provide quasi-3D images across the film and give a detailed view of the damage caused by an electrical breakdown. Here, breakdowns initiated using a needle-plane electrode configuration were imaged. In homogeneous films, the damage was confined to the small volume at the pinhole site.In 32-layer 50/50 PC/P[VDF-HFP] multilayer films, damage extending laterally up to $ 15 lm into the film along the layer interfaces was seen. In addition to the delamination, layer buckling and distortion were apparent. The damage varied with the sample orientation, but the images indicate that the interfaces play an important role in the breakdown. They suggest that modifying the interface properties can be a strategy to further improve the dielectric strength of multilayer polymer dielectric materials.
The dielectric lifetime and corresponding damage morphology of polycarbonate/poly(vinylidene fluoride-co-hexafluoropropylene) (PC/P(VDF-HFP)) layered systems are studied under constant direct current (DC) field. Melt blends of the two polymers are also considered for comparison. The dielectric lifetimes of the latter are systematically much shorter than the layered systems. The interfaces between the polymers act as flaws that induce up to two orders of magnitude difference between the layered and blend systems. The capacitance values versus time during breakdown progression exhibit an inversed S-shape pattern. The three regimes in the S-shape pattern are consecutively attributed to randomly isolated break-downs, interconnecting breakdowns, and wearing-out of the capacitor film. The film breakdown images during dielectric lifetime test confirmed the transition from randomly isolated breakdowns to interconnecting breakdowns. This transition was further evidenced by a bimodal distribution in the Weibull analysis.
The long-term dielectric lifetime properties of multilayered polycarbonate/poly(vinylidene fluoride-co-hexafluoropropylene) [PC/P(VDF-HFP)] films were measured as a function of the layer thickness. An optimum layer thickness of 160 nm was determined with the longest dielectric lifetime. The morphology of the damaged sites after dielectric breakdown was examined using scanning electron microscope. Acoustic emission detection system was coupled with the dielectric setup to correlate fracture events and dielectric breakdown to thereby elucidate the mechanisms of the enhancements in dielectric lifetime properties. Two types of acoustic signals were always observed during the breakdown process for multilayered films. The high-amplitude signals were attributed to the formation of breakdown pinholes caused by the primary discharge from top to bottom electrode. The subsequent lowamplitude signals were attributed to internal discharges that could further damage the film. The total number of acoustic hits, in particular, low-amplitude hits, increased with decreasing layer thickness, indicating more internal discharges occurred along the layered interface. It was concluded that the breakdown event initiated at a defect initiated "hotspot" formed because of internal pressure buildup. The film was punctured when the pressure buildup inside the film overcame the mechanical strength of the film. More number of PC layers and layer interfaces were desirable to slow down and divert the damage propagation through the film thickness direction. The crazes in P(VDF-HFP) can, however, easily propagate across PC layers with less than 160 nm layer thickness.Recent studies on the layered polymer capacitor films have focused on dielectric properties, including dielectric lifetime, 3 breakdown strength, 1,2 and hysteresis. 14 Significant enhancements were observed for dielectric lifetime and breakdown strength, and hysteresis was reduced by decreasing poly(vinylidene fluoride)(PVDF) layer thickness. Wolak 15 and Mackey 2
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.
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