N. A. Freebody

Dielectric properties of epoxy/BN micro- and meso-composites: Dielectric response at low and high electrical field strengths

By incorporating boron nitride (BN) in epoxy resin, we aimed to affect the morphology of the underlying composites. Two different filler sizes, one micrometric with an average grain size of 9 μm and a sub-micrometric one with 0.5 μm, have been used to form composites. The amount of each type of BN in the matrix has been varied from 1 to 5 wt%. Dielectric response measurements at ambient temperature revealed slightly lower real permittivity values of BN composites over neat epoxy and also an increase in imaginary permittivity. In addition, the BN composites had improved thermal stability of properties as compared to neat epoxy, due to lower values in real permittivity as well as in dielectric loss, for temperatures above glass transition temperature. Further dielectric spectroscopy showed significantly decreased values in both real and imaginary permittivity of BN composites, when subjected to high field strengths and very high temperatures compared to measurements at low field strengths. Differential Scanning Calorimetry (DSC) measurements have shown that reticulation was likely to be hindered due to numerous agglomerations in the matrix for the case involving the sub-micrometric BN composite with 5 wt% filler content.

Surface resistance of epoxy/BN micro- and Meso-composites exposed to electrical discharges

Boron nitride (BN) has shown in the past to improve DC breakdown strength as well as augmenting thermal conductivity. Two different filler sizes, one micrometric with an average particle size of 9 μm and a sub-micrometric one, 0.5 μm in size, have been used to form composites. The amount of each type of BN in the matrix has been varied from 1 to 5 wt%. The samples have been exposed to electrical discharges using a point-to-plane geometry operated in open air. The experiment was conducted at an enhanced frequency of 300 Hz to accelerate the erosion process. The surface erosion has been evaluated after several distinct time steps using a mechanical profilometer. It has been found that introducing BN into epoxy decreases the amount of eroded volume of the organic matrix with decreasing filler amount. In addition, micrometric BN composites performed better than their sub-micrometric counterparts. The dielectric response has been evaluated using broadband dielectric spectroscopy (BDS) at 20°C before the erosion, as well as after each time step. An increase of dielectric losses at very low frequencies for all samples after erosion has been observed, accompanied by a slight increase in real permittivity. Besides the β-peak, the formation of an additional loss peak, located around 1 Hz, has been found.

Balanced nanocomposite thermosetting materials for HVDC and AC applications

There is a need to develop materials with controlled electrical resistivity, reduced space charge accumulation, higher thermal conductivity, higher dielectric strength and enhanced voltage endurance to cope with DC stresses in High Voltage Direct Current (HVDC) transmission systems in addition to HVAC requirements. If the balance of properties, performance and process requirements are achieved this may lead to HVDC insulation systems and equipment having a reduced footprint, larger power densities, and greater multi-stress resilience with longer service lifetimes. It reports findings of a project that is engaging this challenge and investigates the development and scaling of new thermoset based nanocomposite electrical insulation materials for HVDC power transmission applications. Some of the results such as increased electrical breakdown strength and reduced electrical conductivity for reactively surface functionalised nanosilica, and increased thermal conductivity for nano boron nitride and their significance in regard to the wider application of these electrical insulation materials are also discussed. With sufficient understanding of these properties, their trade-offs and process requirements it is possible to tailor balanced materials for specific use in HVAC or HVDC components.

Conduction currents and time to frequency domain transformation for epoxy resin nanocomposites

This paper concerns several epoxy resin nanocomposites. A DER 332 epoxy resin was chosen as matrix and nanosilica and/or Boron Nitride were chosen as fillers. Conduction currents results obtained using Polarization and Depolarization Current (PDC) tests at room temperature are presented and discussed. Different conduction phenomena were observed following the analysis of the variation of the current density versus the applied electric field. Using the depolarization currents obtained at room temperature under several applied electric fields, time to frequency domain transformation was performed. For this purpose, the currents were fitted using a general time response function based on Curie-von Schweidler law and on the transform proposed by Hamon. The empiric law proposed by Helegeson was also investigated. The time to frequency domain transformed spectra were compared with those obtained by Dielectric Spectroscopy.

Surface ageing of dielectric materials in a controlled atmosphere: A Raman microprobe study

This paper takes a novel approach to the problem of characterising electrical ageing in solid dielectrics through ex-situ experiments that seek to reproduce the chemistry of electrical treeing in bulk. Plaque specimens of a range of polymers, including polyethylene, polystyrene, polyether ether ketone and silicone rubber, were subjected to surface ageing via corona discharge in air or in a closed cell where the atmosphere can be controlled and adjusted. The residual products on both the sample surface and the high voltage electrode were characterised by Raman microprobe spectroscopy and the resulting fingerprints were compared with those previously identified within electrical trees. Analysis of the electrodes aged in air and nitrogen revealed varying evidence of sp2 hybridized carbon, and fluorescence, both of which are products previously associated with the processes involved in electrical treeing. The significance of the results is discussed with respect to electrical treeing.

Molecular indications of structure property relationships in enhanced nanocomposites

To obtain good dispersion of nanofillers in nanodielectrics, the surface of nanopowders or colloidal particles can be chemically treated to make them more compatible with the matrix. Epoxy nanocomposites that contain surface functionalized nanosilica, either colloidal or powdered, show higher breakdown strengths than the reference epoxy material. For nanodielectric composites containing 2 wt % of nanosilica, a breakdown strength some 25% higher than that of the unfilled epoxy was obtained. Spectroscopic measurements were obtained from all samples and multi variate statistical analysis was applied to enable selected material properties to be regressed allowing molecular associations of these properties to be identified. Electrical breakdown strength was found to be related to the presence of silica and the degree of curing whereas space charge accumulation was found to be related to a reduction in polar ester formation. In contrast we find clear evidence of space charge accumulation being related to more shallow charge trapping centers.

Thermal and dielectric properties of clay/epoxy nanocomposites with low percentage of graphite oxide

This work is concerned with hybrid nanocomposite materials that contain both clay and graphite oxide incorporated in an epoxy matrix. In order to assess the challenge of improving the thermal conductivity while not changing or even improving the electrical properties, a new composite system was designed by applying an additional thermal conductive phase represented by the graphite oxide. In this paper, extremely low quantities of graphite oxide were used (going from 0.0012 to 0.0025 wt%) in order to avoid the forming of an electrical conduction percolation network. The impact of the additional filler on the thermal and electrical properties of the clay/graphite oxide/epoxy nanocomposites were investigated. Using thermal conductivity measurements, it was found that even for low quantities of GO filler added to the clay/epoxy nanocomposite material, the thermal conductivity is improved significantly. Moreover, using dielectric characterization techniques (Dielectric Spectroscopy, Space Charge or Dielectric Breakdown measurements), it was found that the electrical properties of the material remain unchanged or are slightly improved by the extra graphite oxide filler.

A dielectric study on colloidal silica nanoparticle Layer-by-Layer assemblies on polycarbonate.

This study focuses on the dielectrical characterization of polycarbonate films coated with silica nanoparticle bilayers assembled through the Layer-by-Layer (LbL) technique. This is the first attempt of dielectric characterization performed on LbL-treated plastic substrates. To this aim, LbL coatings consisting of oppositely charged colloidal silica have been built on a polymeric substrate (polycarbonate). Then, dielectric features such as space charge accumulation, electrical voltage breakdown, and resistance to corona discharge (through laser ablation) have demonstrated that the colloidal silica nanoparticle assemblies can influence the nature of the trapped space charges and affect the resistance of polycarbonate to corona discharge, changing the distribution of the laser energy on impact.

PD-induced surface degradation of insulation-embedded cavities: Microscopic investigation

With the aim of developing defect-based life models (i.e. life models in which breakdown is explicitly associated with a partial discharge-induced damage growth from a defect beyond a critical level), ageing tests were carried out in the lab on XLPE sandwich specimens containing artificial cavities. PD activity taking place in the cavities was continuously monitored during ageing. The behavior of discharge repetition rate and amplitude were extracted from the monitoring records. The modification of the cavity surface at various ageing times was investigated using microscope observation. Brownish solid byproducts, having oxygen containing moieties, were found covering uniformly the entire area of the cavity surface. After the byproduct was wiped off, degradation pits were observed. The variation of PD repetition rate and amplitude, as well as the relevant PD patterns, could be related to the modification of the cavity surface.

Raman microprobe analysis and ageing in dielectrics

Subsurface voids in samples of electrically stressed low density polyethylene (LDPE) were analysed using confocal Raman microprobe spectroscopy (CRMS). An optical depth profiling technique was used to probe a void along the optic axis whilst a burst void was analysed at various lateral positions. Spectra from the voided samples showed signatures with similar features to those found in previous studies of electrical trees in polyethylene, including the presence of the D and G bands of disordered sp2 carbon. Results and spectra were then compared to the depth profiles and spectra from block and thin film samples of polyethylene and polystyrene which indicated that the established theory behind CRMS is oversimplified and that the detected signal is largely influenced by the optical properties of the material in question. Overall the study showed that despite some spatial resolution limitations of the technique, depth profiling is a useful tool in the analysis of aged polymers and dielectrics as it can show the variations in chemical composition with respect to position along the lateral and optic axis, a property especially relevant to electrical trees.