Fracture phenomena in micro‐ and nano‐layered polycarbonate/poly(vinylidene fluoride‐co‐hexafluoropropylene) films under electric field for high energy density capacitors

Abstract

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

Electromechanical deformation and failure of multilayered films

“…The forced assembly multilayer coextrusion process enables many polymeric systems having unique architectures to be produced including multilayer films, 10 cellular film/foam materials, 11–14 and fibrous membrane systems 9,15–18 . In addition, there has been numerous studies on the multilayered coextruded polymer systems directed toward applications including gas barrier films, 8,19 optical films, 20 shape memory films, 21–24 packaging film/foams, 25 fibrous membranes for filtration, 18,26 antibacterial membranes, 27,28 cellular membranes having through pores 12 and multilayer films with unique dielectric properties 29–36 . These investigations have emphasized the importance of scale within the multilayer films relative to enhanced performance characteristics.…”

Enhanced dielectric properties of immiscible poly (vinylidene fluoride)/low density polyethylene blends by inducing multilayered and orientated structures

“…However, a relatively increased breakthrough could be probably obtained according to references [18,19] which are attributed to the results from the barrier effect of the layers. Even such barrier effect can prolong the dielectric lifetime during breakdown progress [17,30].…”

“…For miscible polymer blend systems, researchers found that the micro-layered structure could effectively enhance the dielectric performance, including dielectric constant, loss, strength and energy density. Dielectric performance of polymeric blends, such as PVDF/polycarbonate (PC) [15], linear low density polyethylene (LLDPE)/ethylene vinyl alcohol (EVOH) [16] and PC/Poly(vinylidene fluoride-cohexafluoropropylene) [17][18][19] were thoroughly studied by co-extruding multilayer structure, aiming to enhance the energy density for electronic and electrical power applications. Due to the well regular arrangement of the nano-inclusions and the amazing increase on the number of the layer, micro-layer co-extrusion obtained by layer multiplication technology becomes one of the popularly adopted methods in processing functional polymeric film or in molding containers [20].…”

Microlayer and nanolayer tubing and piping via layer multiplication coextrusion. I. Validation

“…The attractiveness of this technology has even led Dow to license it to the company EDI for commercial production and sale . It has been effectively demonstrated over the years that this technology has the capability to provide enhanced material properties in many polymer systems with wide ranges of application …”