Santanu Singha

Dielectric properties of epoxy nanocomposites

The dielectric properties of epoxy nanocomposites with insulating nano-fillers, viz., TiO2, ZnO and AI2O3 were investigated at low filler concentrations by weight. Epoxy nanocomposite samples with a good dispersion of nanoparticles in the epoxy matrix were prepared and experiments were performed to measure the dielectric permittivity and tan delta (400 Hz-1 MHz), dc volume resistivity and ac dielectric strength. At very low nanoparticle loadings, results demonstrate some interesting dielectric behaviors for nanocomposites and some of the electrical properties are found to be unique and advantageous for use in several existing and potential electrical systems. The nanocomposite dielectric properties are analyzed in detail with respect to different experimental parameters like frequency (for permittivity/tan delta), filler size, filler concentration and filler permittivity. In addition, epoxy microcomposites for the same systems were synthesized and their dielectric properties were compared to the results already obtained for nanocomposites. The interesting dielectric characteristics for epoxy based nanodielectric systems are attributed to the large volume fraction of interfaces in the bulk of the material and the ensuing interactions between the charged nanoparticle surface and the epoxy chains.

Permittivity and tan delta characteristics of epoxy nanocomposites in the frequency range of 1 MHz-1 GHz

To achieve a compact and reliable design of electrical equipment for the present day requirements, there is an urgent need for better and smart insulating materials and in this respect, the reported enhancements in dielectric properties obtained for polymer nanocomposites seems to be very encouraging. To further understand the dielectric behavior of polymer nanocomposites, this experimental work reports the trends of dielectric permittivities and tan delta (loss tangent) of epoxy nanocomposites with single nano-fillers of Al2O3 and TiO2at low filler concentrations (0.1%, 0.5%, 1% & 5%) over a frequency range of 1 MHz-1 GHz. Results show that the nanocomposites demonstrate some very different dielectric characteristics when compared to those for polymer microcomposites. Unlike the usual expectations of increasing permittivity with increasing filler concentration in polymer microcomposites, it has been seen that up to a certain nano-filler concentration and depending on the permittivity of the nano-filler, the permittivities of the epoxy nanocomposites are less than that of the unfilled epoxy at all the measured frequencies. This suggests that there is a very strong dependence of the filler concentration and nano-filler permittivity on the final permittivity of the nanocomposites at all these frequencies. But, in the case of tan delta behavior in nanocomposites, significant effects of filler concentrations were not observed with both Al2O3 and TiO2 fillers. Tan delta values in nanocomposites with Al2O3 fillers are found to be marginally lower at all filler concentrations when compared with the value for unfilled epoxy. But, in TiO2Oepoxy nanocomposites, although the variations in tan delta are not significant with respect to unfilled epoxy, some interesting trends are observed with respect to the frequencies of measurement.

Influence of filler loading on dielectric properties of epoxy-ZnO nanocomposites

Experimental investigations into the dielectric properties of epoxy-ZnO nanocomposites at different filler loadings reveal few unique behaviors (at certain filler loadings) and also advantageous characteristics in contrast to the properties obtained for the corresponding microcomposites. Results demonstrate that in nanocomposites, it is possible to achieve lower values of permittivity and tandelta with respect to unfilled epoxy over a wide frequency range. Analysis of the results attributes this interesting observation to the interaction dynamics between the epoxy chains and the ZnO nanoparticles at the interfacial area. The dc volume resistivities and ac dielectric strengths of nanocomposites were also experimentally determined in the present study and the obtained characteristics are found to be different as compared to the results obtained for microcomposites. The volume fraction and nature of the interfaces in the bulk of the composites seem to influence this difference in the examined dielectric properties of the nanocomposites.

Complex permittivity characteristics of epoxy nanocomposites at low frequencies

The complex permittivity characteristics of epoxy nanocomposite systems were examined and an attempt has been made to understand the underlying physics governing some of the unique macroscopic dielectric behaviors. The experimental investigations were performed using two different nanocomposite systems with low filler concentrations over the frequency range of 10-2-400 Hz, but for some cases, the data has been reported upto 106 Hz for a better understanding of the behaviors. Results demonstrate that nanocomposites do possess unique permittivity behaviors as compared to those already known for unfilled polymer and microcomposite systems. The nanocomposite real permittivity and tanδ values are found to be lower than that of unfilled epoxy. In addition, results show that interfacial polarization and charge carrier mobilities are suppressed in epoxy nanocomposite systems. The complex permittivity spectra coupled with the ac conductivity characteristics with respect to frequency was found to be sufficient to identify several of the nanocomposite characteristics like the reduction in permittivity values, reduction in the interfacial polarization mechanisms and the electrical conduction behaviors. Analysis of the results are also performed using electric modulus formalisms and it has been seen that the nanocomposite dielectric behaviors at low frequencies can also be explained clearly using this formalism.

Polymer composite/nanocomposite processing and its effect on the electrical properties

One of the biggest challenges when considering polymer nanocomposites for electrical insulation applications lies in determining their electrical properties accurately, which in turn depend on several factors, primary being dispersion of particles in the polymer matrix. With this background, this paper reports an experimental study to understand the effects of different processing techniques on the dispersion of filler particles in the polymer matrix and their related effect on the dielectric properties of the composites. Polymer composite and nanocomposite samples for the study were prepared by mixing 10% by weight of commercially available TiO2 particles of two different sizes in epoxy using different processing methods. A considerable effect of the composite processing method could be seen in the dielectric properties of nanocomposites

Comparative aging characteristics between a high oleic natural ester dielectric liquid and mineral oil

This paper presents a comparative evaluation of the aging characteristics between a high oleic natural ester dielectric liquid (HONE) and mineral oil based on an accelerated single-temperature thermal aging experiment at 150°C for a period of 3000 hours. The important physical (e.g., viscosity, moisture content, acidity) and dielectric (e.g., power factor, alternating current (AC) dielectric breakdown) properties of both the oils were measured and analyzed at different aging intervals. As expected, results show differences in aging behaviors between mineral oil and the natural ester liquid. The relative moisture content in HONE had stable and lower values during the aging duration as compared to the case of mineral oil, where the relative moisture content increased with the aging duration. Further, unlike the observations in mineral oil, the power factor (PF) and acidity of HONE increased drastically with increasing aging times. The most relevant observation made from this study was that in spite of this significant increase in the PF and acidity of HONE with aging, its AC dielectric breakdown voltage was quite stable, which is not observed in the case of mineral oil. This indicates that for natural ester liquids, higher values of PF (or dielectric dissipation factor) and/or acidity due to aging may not necessarily give a true picture on the dielectric performance of the ester. It is therefore important to note here that the threshold limits of some of the diagnostic parameters which are already established for mineral oil do not apply for ester liquids.

Very fast transient overvoltages in GIS with compressed SF/sub 6/-N/sub 2/ gas mixtures

This paper discusses the characteristics of very fast transient overvoltages (VFTO) in SF/sub 6/-N/sub 2/ gas mixtures at different percentages of SF/sub 6/. A comparison of the VFTO characteristics of pure SF/sub 6/ with those of pure N/sub 2/ is also presented. The investigations are performed using a laboratory model GIS bus duct having a test gap used for simulating a switching event leading to the generation of VFTO. A capacitive voltage sensor is used to measure the VFTO peak magnitude and temporal characteristics. Measurements were carried out at two different gap spacings (0.20 and 0.61 mm) over a pressure range of 100 to 500 kPa. VFTO characteristics for N/sub 2/, SF/sub 6/ and SF/sub 6/-N/sub 2/ mixtures obtained from the experiments show similar trends. The level of surge peak magnitude is <2.0 pu for all cases when the gap was 0.20 mm, but it reaches a maximum of 2.41 pu at 0.61 mm gap. At 0.20 mm gap, in SF/sub 6/-N/sub 2/ mixtures, the difference in peak magnitudes is not significant for 10% and 20% SF/sub 6/ mixtures (between 200 and 400 kPa) and also for pure SF/sub 6/ and 40% SF/sub 6/ (between 200 and 300 kPa). The occurrence of corona stabilization during breakdown of the gap may be the cause for such a behavior. Unlike the above observations at 0.20 mm gap, at 0.61 mm gap, the peak magnitudes strictly increase with pressure for the pure gases and gas mixtures. At 0.20 mm gap, the time to breakdown of the gap is found to be almost constant in all cases. But at 0.61 mm gap, the time to breakdown is seen to be dependent on the mixture, pressure, and breakdown voltage, and this observation is in accordance with Toeplers spark law.

Electrical Discharge Resistant Characteristics of Epoxy Nanocomposites

This paper reports the electrical discharge resistant characteristics of epoxy nanocomposite systems with SiO2 and Al2O3 nano-fillers. A comparative study is performed between unfilled epoxy systems, nanoparticle filled epoxy systems and a bimodal system containing both micrometer and nanometer sized fillers of the same material. The samples are exposed to surface discharges and the levels of surface degradation are analyzed through SEM and surface roughness measurements. Significant variations were observed in the electrical discharge resistant characteristics between the different composite systems and it is seen that the introduction of nano-fillers to epoxy is advantageous in improving the electrical discharge resistance of epoxy.

Toepler's spark law in a GIS with compressed SF/sub 6/-N/sub 2/ mixture

This paper deals with the experimental measurements and analysis of the formative time lags to breakdown and an estimation of the Toeplers constant for gas gaps, under the application of 50 Hz AC voltage. The experiments were carried out in a 145 kV gas insulated system (GIS) bus duct with pure N/sub 2/, pure SF/sub 6/ and SF/sub 6/-N/sub 2/ mixture as insulating media. The formative time lags to breakdown in the gas gaps were measured using a fast response capacitive sensor. Toeplers spark law has been used to explain the breakdown phenomenon in the GIS and the values of Toeplers constant (k/sub t/), which gives an estimation of the formative time lags, were determined. Results show that the formative time lags vary inversely with gas pressure and the gas mixture concentrations for two gaps studied (0.46 mm and 0.61 mm). In the case of another gap (0.20 mm), the variation in the formative time lags with pressure as well as SE, concentration in the mixture has been found to be negligibly small between gas mixtures, although significant variation can be seen between pure SF/sub 6/ and pure N/sub 2/. Toeplers constant, k/sub t/, increases with gas pressure as well as SF/sub 6/ concentration in the mixture for the gaps studied. Hence, k/sub t/ is a function of the gas pressure and the concentrations of SF/sub 6/ in the gas mixture for the above-mentioned gaps.

Reduction of Permittivity in Epoxy Nanocomposites at Low Nano-filler Loadings

Experimental studies reveal a reduction in the values of permittivity for epoxy nanocomposites at low filler loadings as compared to neat epoxy over a wide frequency range. This permittivity reduction is attributed to the interaction dynamics between nanoparticles and epoxy chains at the interface region and interestingly, this interaction has also been found to influence the glass transition temperatures (Tg) of the examined nanocomposite systems. Accordingly, a dual nanolayer interface model for an epoxy based nanocomposite system is analyzed to explain the obtained permittivity characteristics.