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Sheets of silicone rubber (SiR) were aged under various aging conditions to examine its effects on the mechanical and dielectric properties of SiR. Mainly by analyzing the results obtained by mid-infrared absorption spectroscopy, it has become clear that cross-linked structures with siloxane bonds are formed under all the conditions. This makes SiR hard and then brittle, but it also makes better the dielectric properties apparently. An early stage of degradation in SiR can be detected by EAB and complex electric modulus, which is defined as the reciprocal of complex permittivity. Therefore, EAB and complex electric modulus are useful to monitor the aging status of SiR. [ABSTRACT FROM AUTHOR]

Epoxy resin and its microcomposite are widely used as electrical insulation in electrical equipment. Surface flashover and breakdown properties of these materials can be influenced by dielectric relaxation process and charge carrier behavior. The charge transport characteristics and dielectric relaxation process for neat EP and its microcomposites were studied by means of dielectric relaxation spectroscopy. The glass transition temperatures Tg of the two kinds of samples were investigated with differential scanning calorimetry measurement, which were approximately 105 ?C and 120 ?C, respectively. Both the two kinds of samples were sputtered with gold electrodes on two sides, and electrodes' diameter was 30 mm. The properties of dielectric relaxation was measured by Concept 80 Novocontrol broadband dielectric spectrometer at various temperatures from 100 ?C to 180 ?C. There is a relaxation peak which labeled ? in the range of high frequencies on account of the molecular chain movement or motion of segmental chains when the temperature is above Tg. At the same time, the dc conductivity attributed by the charge carrier transport occurs in the range of low frequencies. Besides, there are different relaxation times for molecular chains with different length scales. Moreover, there is a broad distribution of relaxation time for both neat EP and its microcomposites and the relaxation time distribution at different temperatures was calculated. In addition, it was found that the relation between peak frequency of relaxation process and temperature follow the Vogel-Fulcher-Tammann (VFT) law, and temperature dependences of dc conductivity also obey the VFT equation. The Vogel temperatures of dielectric relaxation process and charge carrier transport were calculated by fitting the results of permittivity and conductivity. The glass transition temperatures for neat EP and its microcomposite were estimated to be 102 ?C and 117 ?C through Vogel temperatures, which are in consistence with DSC measurement results. It represents that free volume in the two kinds of samples increases with the increase of temperature, thus enhancing molecular chain movement and charge carrier transport process. [ABSTRACT FROM PUBLISHER]

Complex permittivity ( ϵr' and ϵr") and conductivity were measured in a wide temperature range for dicyclopentadiene (DCP) resin and epoxy resin, which show glass transition at a similar temperature around 150 °C. Furthermore, space charge distributions remaining in the two resins that had been polarized in the same wide temperature range were measured at room temperature. As a result, it was found that ϵr ?, ϵ r?, and conductivity are much lower in DCP resin than in epoxy resin at almost all the temperatures and frequencies. A further analysis using complex electric modulus, which is the inverse of the complex permittivity, indicates that charge transport is much more difficult in DCP resin. Furthermore, while similar small amounts of homocharges appear in DCP resin at any polarization temperatures, a significant accumulation of heterocharges, most likely due to ions, is induced in epoxy resin in the vicinity of the cathode/sample interface at polarization temperatures above the glass transition temperature. These results indicate that DCP resin possesses superior stable dielectric behavior, especially at high temperatures. [ABSTRACT FROM PUBLISHER]

Dielectric behavior was compared experimentally between polyethylene terephthalate (PET) and poly(ethylene 2,6-naphthalate) (PEN). Due to their similar chemical structures, the two polymers exhibit many parallel dielectric properties. While the two polymers exhibit fairly similar thermally stimulated polarization and depolarization currents (TSPC and TSDC), the temperature at which TSPC or TSDC starts to increase rapidly is about 20 ?C higher in PEN than in PET, mostly likely reflecting the difference in their glass transition temperatures (Tg's). At temperatures about 30 ?C lower than Tg, the two polymers show a hump in their first-run TSPC spectra, probably originating from impurity or moisture. Both the real and imaginary parts of complex permittivity, ϵr' and ϵr", increase significantly at temperatures above their Tg's for both PEN and PET, since molecular motion becomes active. Shoulders and plateaux clearly appear in ϵr' and ϵr" spectra of PEN, which move toward higher frequencies with an increase in temperature. To further analyze them, complex electric modulus M* was introduced. As a result, it has become clear that electric conduction dominates the dielectric behavior of PET and PEN at temperatures above Tg, especially at low frequencies. [ABSTRACT FROM AUTHOR]

Effects of nanofiller addition on four typical dielectric properties, namely permittivity εr′, dielectric loss factor εr″, space charge accumulation, and partial discharge (PD) resistance were evaluated for polypropylene (PP) and its nanocomposites (NCs) with nanoclay. While εr′ and εr″ are almost independent of temperature and frequency in the base unfilled PP, they are highly dependent on the two parameters in the two NCs. Namely, εr′ increases significantly at temperatures above 20 °C and the frequency spectrum of εr″ shows at least one temperature-dependent peak. Furthermore, space charge appears abundantly in the two NCs compared to the base PP. These results indicate that plenty of mobile carriers and/or dipoles, probably resulted from the manufacturing process, remain in the two NCs. Notwithstanding the above-mentioned 'inferior' insulating properties, the two NCs have an improved PD resistance compared with the base PP. Namely, the erosion depth on the surface induced by PDs is the smallest in the NC with the largest filler content, while it is the largest in the base PP. Such differences in the effects of nanofillers on different insulating properties are attributable to the fact that nanofillers can improve the PD resistance simply by their presence, while the chemicals needed for uniform dispersion of nanofillers may sometimes increase the permittivity and abundance of charge carriers. [ABSTRACT FROM AUTHOR]

Dielectric properties were compared between two kinds of low-density polyethylene (LDPE) composites prepared by adding near spherical MgO fillers with different diameters of around several ten nm and several μm. In the whole range of temperature from 0 °C to 90 °C, the conductivity is decreased and the permittivity is increased by the addition of fillers, irrespective of their sizes. However, the decrement in the conductivity is more significant and permittivity increment is suppressed more in the case of the nm fillers. In addition, a drastic increase in dielectric loss factor, observed at low frequencies, is suppressed more significantly by the addition of the nm fillers. While the formation of packet-like charge, observed in LOPE, is suppressed by both fillers, the effect is far more significant in the nanocomposites. This means that the nano-fillers suppress the conductivity enhancement by the voltage application more effectively. [ABSTRACT FROM AUTHOR]

Complex electric modulus, defined as the inverse of complex relative permittivity, can be a significantly powerful tool for analyzing dielectric behavior of a polymeric insulating material, especially at relatively high temperatures, where complex permittivity usually becomes very high due to electrode polarization and carrier transport. In this paper, a typical example of the above is clearly shown by referring to an experimental result obtained for epoxy resin. [ABSTRACT FROM AUTHOR]