Contents I. Introduction
Nanodielectrics, defined as composites with filler particles less than 100 nm in one dimension embedded in a polymer matrix, exhibit significant improvements in dielectric strength [1] as well as order of magnitude improvements in voltage endurance [2]. These improvements are exciting both in terms of commercial application and for the fundamental insight that can be gained by studying the mechanisms controlling the enhanced dielectric response. In a 1994 theoretical paper, Lewis [3] anticipated these property changes and suggested that the large interfacial area created by the nanofillers was critical to the changes that would be observed. For example, as the filler size decreases from to 100 nm, the polymer/filler particle surface area per unit volume of material at a given loading level increases tenfold (assuming spherical particles and neglecting particle agglomeration). If, as proposed by Lewis [4], and observed directly from thermalmechanical property measurements [5], [6], these internal surfaces create interfacial polymer as thick as 20 nm with properties different from the bulk polymer, then it is not surprising that unique dielectric properties are observed. There is, however, no description of polymer nanodielectrics that hypothesizes the mechanism from the molecular scale up to the bulk level and there is a dearth of understanding of the mechanisms at each length scale. This paper brings together the literature in this area and presents a working hypothesis for the multiscale phenomena in polymer nanodielectrics. The hypothesis is then supported from recent results in nanoscale silica/XLPE composites using electroluminescence, pulsed electroacoustic analysis, absorption current studies, thermally stimulated current measurements, a comparison of impulse, ac, and dc breakdown results, and voltage endurance data.
