Imaging the effect of dielectric breakdown in a multilayered polymer film

The dielectric strength and energy storage capability of poly(vinylidene fluoride-hexafluoropropy- lene) copolymer (P(VDF-HFP)) films are enhanced by interleaving layers of PVDF copolymer with thin layers of polycarbonate (PC). To gain insight into the break- down processes in such materials, focused ion beam (FIB) milling in conjunction with scanning electron mi- croscopy (SEM) was used to study the effect of a break- down 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 con- figuration were imaged. In homogeneous films, the dam- age was confined to the small volume at the pinhole site. In 32-layer 50/50 PC/P(VDF-HFP) multilayer films, dam- age extending laterally up to � 15 lm into the film along the layer interfaces was seen. In addition to the delami- nation, 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 inter- face properties can be a strategy to further improve the dielectric strength of multilayer polymer dielectric materi- als. V C 2011 Wiley Periodicals, Inc † . J Appl Polym Sci 123: 2548- 2557, 2012