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In this study, large-area dielectric breakdown performances of various bi-axially oriented polypropylene (BOPP)-silica nanocomposite films are studied by utilizing the self-healing multi-breakdown method presented in the Part I of this publication. In particular, the effects of silica filler content, pre-mixing method, co-stabilizer content and film processing on the large-area breakdown performance are analyzed. Nanostructural and film cross-sectional analyses are correlated to the breakdown responses. The optimum silica filler content is found to reside at the low fill fraction level (~1 wt-%) and automatic pre-mixing of the raw materials and the optimization of the orientation temperature are found to be preferable. The co-stabilizer Irgafos 168 is found to have a significant effect on the breakdown distribution homogeneity of the reference BOPP films. The breakdown response of the silica nanocomposites is found to be not only dependent on the active measurement area but also on the voltage ramp rate, indicating that the silica nanocomposites exhibit altered internal charge behavior under DC electric field. The area-and ramp-rate-dependence results exemplify the importance of careful breakdown strength evaluation of dielectric polymer nanocomposites. Above all, the results emphasize the fact that a thorough understanding and the optimization of the film processing parameters are crucial for achieving improved breakdown response in dielectric polymer nanocomposite films.

The central goal of dielectric nanocomposite design is to create a large interfacial area between the matrix polymer and nanofillers and to use it to tailor the properties of the composite. The interface can create sites for trapping electrons leading to increased dielectric breakdown strength (DBS). Nanoparticles with a bimodal population of covalently anchored molecules were created using ligand engineering. Electrically active short molecules (oligothiophene or ferrocene) and matrix compatible long poly(glycidyl methacrylate) (PGMA) chains comprise the bimodal brush. The dielectric breakdown strength was evaluated from recessed samples and dielectric spectroscopy was used to study the dielectric constant and loss as a function of frequency. The dielectric breakdown strength and permittivity increased considerably with only 2 wt% filler loading while the dielectric loss remained comparable to the reference epoxy.

Graphene oxide (GO) was produced using acidic graphite oxidation and dispersed within an epoxy matrix using a solvent-based technique, to give nanocomposites containing up to 2 wt% of GO. Transmission and scanning electron microscopy revealed a fine dispersion of graphitic sheets which alters the nanocomposite's fractured surface morphology, while Fourier transform infrared spectroscopy revealed an excess of epoxide groups in the system, which are associated with the included GO. These additional moieties react with hardener amine groups and, consequently, displace the reaction stoichiometry away from the optimum. The result of this is a change in the network architecture and, in particular, the introduction of epoxy-terminated branches, which modify the dielectric c relaxation. During post-curing, hydroxyl groups on the GO surface react with residual epoxide groups through etherification reactions, to give a marked increase in the glass transition temperature. These reactions lead to increased interfacial interactions between the GO and the matrix, which contribute to an increase in tensile performance. In addition, post-curing also reduces the defect content within the GO lattice which, in turn, increases the electrical conductivity, dielectric permittivity and low frequency losses of the system. Associated chemical pathways are proposed.

Nanodielectrics, a 21st-century phenomena, is envisioned to be the answer for material challenges in progressive highvoltage technology. It is well known that the proper dispersion of nanoparticles plays a key role in improving the dielectric properties of a material, but to understand where changes in the properties of a material originate, it is also essential to reveal the multiscale structure of the material. In this study, the dielectric permittivity, breakdown strength, and structure of nano calcium carbonate (nano-CaCO 3 )/polypropylene composites with 1.8-8.1 wt % doping were characterized systematically. The combined results from transmission electron microscopy, Raman microscopy, and optical microscopy show that the quality of nanodispersion was similar in all of the filler concentrations studied. However, all of the samples also contained smoothly distributed microparticles. The density of the microparticles increased exponentially when the concentration of nano-CaCO 3 was increased in the manufacturing process. The dielectric direct-current breakdown of the composites had a maximum at 1.8 wt % concentration and then decreased as the filler concentration was increased. The differences could be explained by the existence of large microparticles rather than the quality of the nanoparticle dispersion; this indicated the importance of multiscale characterization. V C 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci.2013, 131, 39504.