Candidate mechanisms controlling the electrical characteristics of silica/XLPE nanodielectrics

The incorporation of silica nanoparticles into polyethylene has been shown to increase the breakdown strength significantly compared to composites with micron scale fillers. Additionally, the voltage endurance of the nanocomposites is two orders of magnitude higher than that of the base polymer. The most significant difference between micron-scale and nano-scale fillers is the large interfacial area in nanocomposites. Because the interfacial region (interaction zone) is likely to be pivotal in controlling properties, this paper compares the behavior of nanoscale silica/ cross-linked low density polyethylene nanocomposites with several silica surface treatments. In addition to breakdown strength and voltage endurance, dielectric spectroscopy, absorption current measurements, and thermally stimulated current determinations (TSC) were performed to elucidate the role of the interface. It was found that a reduction in the mobility in nanocomposites as well as a change in the defect size may be key to explaining the improvement in the properties. Introduction Nanocomposite-dielectrics, or nanodielectrics, can exhibit dielectric properties that are better than the base polymer and significantly better than conventional composites with micron scale filler. For example, an increase in dielectric strength and an accompanying reduction in space charge has been documented for the case of nano-TiO2 filled epoxy resin compared to micron scale TiO2 filled epoxy composites [1] and titania filled low density polyethylene (LDPE) composites compared to micron scale TiO2 filled LDPE composites [2]. In the nano-TiO2 filled epoxy composites, the electrical voltage endurance was three orders of magnitude higher than that of the base resin [3]. The improvements in nanocomposite electrical behavior have been attributed to (i) changes in the space charge distribution [4, 5], (ii) a reduction in the internal field caused by the decrease in particle size, and (iii) changes in the polymer morphology [2]. The particle–polymer interface is critical in determining the properties of nano-filled materials because at the same particle loading, nanocomposites have a much greater interfacial area than microcomposites. Since a significant percentage of polymer will be in these interfacial areas (zones), the properties of the overall composite will be dominated by the interfacial properties [6] even at low volume fractions of filler [7, 8]. Some authors have emphasized that the interaction zone around the particles is a ‘‘quasi-conductive’’ region that partially overlaps in the nanocomposites [9]. These overlapped interface regions may allow charge dissipation, which, in turn, could be expected to improve the dielectric breakdown strength and voltage endurance characteristics by improving the internal M. Roy R. K. MacCrone L. S. Schadler (&) Materials Science and Engineering Department, Rensselaer Polytechnic Institute, Troy, NY 12180, USA e-mail: schadl@rpi.edu J. K. Nelson Electrical, Computer, and Systems Engineering Department, Rensselaer Polytechnic Institute, Troy, NY 12180, USA J Mater Sci DOI 10.1007/s10853-006-0413-0