M.F. Frechette

Dielectric properties of XLPE/Sio2 nanocomposites based on CIGRE WG D1.24 cooperative test results

A comprehensive experimental investigation of XLPE and its nanocomposite with fumed silica (SiO2) has been performed by CIGRE Working Group D1.24, in cooperative tests conducted by a number of members; covering materials characterization, real and imaginary permittivity, dc conductivity, space charge formation, dielectric breakdown strength, and partial discharge resistance. The research is unique, since all test samples were prepared by one source, and then evaluated by several expert members and their research organizations. The XLPE used for preparation of the nanocomposites was a standard commercial material used for extruded power cables. The improved XLPE samples, based on nanocomposite formulations with fumed silica, were prepared specifically for this study. Results of the different investigations are summarized in each section; conclusions are given. Overall, several important improvements over unfilled XLPE are confirmed, which augur well for future potential application in the field of extruded HV and EHV cables. Some differences/discrepancies in the data of participants are thought to be the result of instrumental and individual experimental technique differences.

Characterization of epoxy microcomposite and nanocomposite materials for power engineering applications

This article presents the results from round-robin tests performed on epoxy composite materials. These results show the potential of these materials for use as electrical insulation in some specific applications. A small section of the article addresses the health and safety issues related to the use of nanoparticles in the electrical power engineering industry. We define epoxy nanocomposites as epoxy-based materials containing exclusively nanosized filler particles. Epoxy microcomposites are defined as epoxy materials containing exclusively microsized filler particles, and epoxy micro+nano composites are materials containing both microsized and nanosized particles.

Nanostructured polymer microcomposites: A distinct class of insulating materials

Experimental evidence was produced and gathered to demonstrate the distinct nature of nanostructured polymer microcomposites. The case of a polymer composite consisting of a high-content of micrometric quartz with a small adjunct of nanoclay is discussed. Emphasis is put on dielectric behavior studies while some results on thermal characteristics are presented. Overall results strongly support the potential of this class of insulating material for electrotechnical applications.

Electrical properties analysis of micro and nano composite epoxy resin materials

This work deals with the study of micro and nanosilica filled epoxy resin samples carried out in the framework of CIGRE WG D1.24 cooperative test program. This program focused on chemical, electrical and electrostatic properties of epoxy based nanodielectrics for electrical engineering applications. Epoxy based samples filled with micro and/or nanoparticles of silica were characterized by transmission electron microscopy, dielectric spectroscopy, conduction current and space charge measurements. These mutually complementary techniques were used to examine the effect of the size and quantity of silica particles on the electrical properties of the analyzed materials. The analysis of charge injection, polarization, trapping and conduction phenomena has allowed the modeling of dielectric behavior of the studied materials under multiple stresses. The Schottky Injection and Space Charge Limited Current models were studied to explain conduction phenomena. A composition of micro and nano-sized silica particles accumulating the smallest amount of space charge is also proposed.

Polymer nanocomposites-major conclusions and achievements reached so far

The dielectric properties of a number of polymeric nanocomposites (PNC) have been investigated and reported, and there are very good reviews available, for example, see [1]-[3]. CIGRE Working group D1.24 has also performed several collaborative investigations on mostly epoxy- and polyethylene-based nanocomposites, which are reported in CIGRE publications [4], [5] as well as in archived papers [6], [7]. Dielectric nanocomposites investigated in the literature include various polyolefins such as polyethylene (PE; and PE blends) and polypropylene, ethylene vinyl acetate, polyamine, epoxy, and elastomers such as silicone rubber, containing various nanofillers such as metallic oxides, silica, alumina, titanium oxide, zinc oxide, and layered silicates (clays). Due to the very high specific surface area of nano-sized fillers, a few percent addition can significantly affect the dielectric properties of a polymeric material. The most common and practical processing methods suitable for thermoplastic nanocomposites are melt compounding, using a mixer, extruder, or both, and mixing in the liquid phase prior to polymerization for thermosetting resins, a process commonly called the in situ polymerization process [8]. Figure 1 gives examples of typical microstructures of polyolefin-based nanocomposites processed by melt compounding. The striking similarity of the microstructure shown in Figures 1(b) and 1(c) should be noted as both were obtained in two different labs from the melt compounding of fumed silica and a thermoplastic resin using a twin screw extruder. Similar microstructures are also reported for isotactic polypropylene/SiO2 nanocomposites melt blended by extrusion [9].

Nanodielectric surface performance when submitted to partial discharges in compressed air

Bulk samples of a nanodielectric material were synthesized. This insulating material consisted of a mix of epoxy resin, a SiO/sub 2/ load and a percent fraction of inorganic nanoparticles. The aim of the present experiment was to determine the performance of the nanodielectric surface when exposed to partial discharges as compared to that of an epoxy containing only the micrometric SiO/sub 2/ load. A discharge situation featuring a triple-junction condition was used. Low-intensity discharges were produced along a gap formed by the interface between compressed air and the bulk sample. The material containing a small amount of nanoparticles was found to resist much more the present discharge conditions, showing an improved performance as compared to that observed in the case of the epoxy without nanoclay.

Epoxy/BN micro- and submicro-composites: dielectric and thermal properties of enhanced materials for high voltage insulation systems

Hexagonal boron nitride (h-BN) is a very promising material for application in high voltage insulation engineering due to its high thermal conductivity and good electrical insulating properties. In order to study the effect of incorporating BN particles in epoxy resin, composites with different filler sizes and several BN loadings have been fabricated. Two different filler sizes, one micrometric with an average grain size of 9 μm and a submicrometric one with 0.5 μm, have been used to form composites. The amount of either type of BN in the matrix has been varied from 1 to 5 wt%. Dielectric and thermal performances of the test specimens have been assessed by means of Dielectric Spectroscopy, Differential Scanning Calorimetry, surface erosion, AC breakdown tests and thermal conductivity measurements. It has been found that incorporation of BN particles in the epoxy resin resulted in significant improvements of parameters such as resistance to electrical discharge, as well as diminished dielectric losses for the composites at higher temperatures. Furthermore, BN composites with 5 wt% filler loadings have shown a noteworthy enhancement of thermal conductivities, which was more distinct for the submicrometric BN composite.

The emerging field of nanodielectrics an annotated appreciation

The turmoil of activities that has surrounded nanotechnology in recent years continued to grow and outburst in several directions. This has reflected as well in the field of nanodielectrics where a great number of contributions resulted, a progress that the authors will comment. This annotated appreciation will be developed with a concentration on information, known facts and uncertainties with a zest of critique. Force is to conclude that the felt potentials remain to be demonstrated in the context of an improved profitability

Nanostructured epoxy/POSS composites: enhanced materials for high voltage insulation applications

In this study, the dielectric and thermal properties of nanostructured epoxy/POSS (Polyhedral Oligomeric Silsesquioxanes) composites were investigated, using a reactive Triglycidylisobutyl-POSS (TGIB-POSS) additive from 1 up to 10 wt%. POSS has been successfully dispersed at a molecular level for low content composites, which show a remarkably improved resistance to corona discharges, with up to 60% less eroded sample volume, along with significantly increased dielectric breakdown strengths and thermal conductivities. Epoxy/POSS composites containing 5 wt% and more of the POSS additive exhibit agglomerations, which have been observed by SEM. Furthermore, dielectric spectroscopy revealed additional interfacial loss peaks for such composites containing 5 wt% POSS and more, in addition to the α- and β-peaks known for epoxy.

Post-heat treatment effect on the dielectric response of epoxy samples

In the present experiment, the dielectric response of various types of epoxies was investigated in the frequency and time domain. The samples consisted of nanostructured epoxy microcomposites and reference microcomposites. Water ingression at the fabrication step and/or at measuring time is known to affect the dielectric response of materials such as epoxy. Prior to measurement, some samples were submitted to 160°C during 48 hours under vacuum. Heating the epoxy sample under vacuum was found to have a substantial effect on dielectric properties specially when the microcomposite was nanostructured. In the nanodielectric case, pre-treatment conditions could produce a drop in the dielectric constant, about 5% at around 10-2 Hz. This observation would be consistent with the removal of the polarisability associated with water molecules.