H. Couderc

Dielectric and thermal properties of boron nitride and Silica epoxy composites

Samples containing only submicrometric filler (BN), only nanometric filler (Silica) and both fillers were prepared by gravity moulding. A pure epoxy sample was also prepared to quantify the enhanced properties. Glass transition temperatures were measured by Differential Scanning Calorimetry. Dielectric Breakdown data were analysed using the Weibull statistical method and Dielectric Spectroscopy was used at low and high voltage to measure the samples permittivity. The glass transition temperature is decreased by BN presence but not by nanosilica. At ambient temperature, the β relaxation peak is not affected by nanoparticles nor by applied voltage. Strangely, the sample containing the two nanoparticles type is not affected compared to pure epoxy. On dielectric spectra, a supplementary relaxation peak for samples containing nanosilica is present. This peak is associated with the interphase between nanosilica and polymer.

Dielectric response of various partially cured epoxy nanocomposites

Insufficient crosslinking and water uptake during fabrication or manipulation are known to affect the dielectric response of epoxies. Post thermal treatment may result in the completion of cross-linking, partial removal of water, and aging. In order to study the effect of manufacturing imprecision on dielectric response, several under-cured epoxybased nanocomposite samples with modified nanoclay fillers were investigated. In addition, the influence of silane coupling agents and the use of ultrasonic waves on the nanoclay intercalation were also studied. The structure of the samples and the extent of cross linking were characterized using X-ray Diffraction (XRD) and Differential Scanning Calorimetry (DSC) respectively. It was found that surface silanization lead to improved clay intercalation and higher extent of intercalation/exfoliation. The influence of post thermal treatment on the dielectric response of the materials was investigated using Broadband Dielectric Spectroscopy (BDS). Once the samples were in a stable dielectric state, relaxation maps were performed. It was found that the samples with silanized nanoclay have the lowest activation energy and they also proved to be the “strongest” vitreous materials.

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-based composites to electrical discharge

In this work, differences in surface erosion conditions stemming from the use of different discharge contexts were illustrated. It was shown that the end results may affect easily the conclusions of a comparative study. Epoxy and its composites containing silica were used for the tests. Despite accepted variants in the material processibility and affected properties, in some conditions, it was concluded that epoxy composites containing nanosilica exhibited a superior surface resistance to erosion. Also, reducing the size of silica particles in the epoxy composite was found to result into less accumulated space charge as measured using the thermal step method.

The dielectric permittivity of ceramic powders used in composite polymers

The aim of this study is to measure the complex dielectric constant over a wide range of frequencies for ceramic powders often used as additives in polymer composites. The dielectric responses of alumina and boron nitride powders are presented. The variation of the dielectric constant in the studied frequency range (10-1 Hz to 10 kHz) showed that there is absorption of the water on the surface of the powder. Therefore, different treatments were applied: heating at 100°C for 48 hours and under vacuum at room temperature. A significant change on the value of the permittivity at low frequency has been observed after treatment. By comparing the results obtained before and after treatment, the dielectric response of the powder measured under vacuum showed stability over the studied frequency range at room temperature. The measured values of the permittivity of the powder were used to estimate those of the bulk material using a mixing law. This situation is discussed.

Dielectric properties of Boron Nitride and silica epoxy composites

Epoxies are widely used materials, especially in high voltage insulation for its mechanical and dielectric properties. Boron Nitride is an attractive filler because of its high insulating properties associated with a high thermal conductivity. Nanometric Silica used as filler is actually widely studied in the literature and enhancement of dielectric breakdown and insulating properties has been shown. Using a combination of these two fillers in an epoxy matrix may produce a new material with improved properties. Samples containing only micrometric filler (Boron Nitride), only nanometric filler (Silica) and both fillers were prepared by gravity moulding. A pure epoxy sample was also prepared to quantify the enhancement properties. Broadband Dielectric Spectroscopy was used to study the influence of nanoparticles on the relaxation mechanisms in the composites. An additional relaxation peak was highlighted for samples containing nanosilica between the local β relaxation, associated with crankshaft motions of the hydroxylether groups, and the main α relaxation, corresponding to glass transition cooperative movements. This relaxation phenomenon was found to be associated with the water bounded at the surface of the silica particles. Further relaxation investigation will be performed to clarify the influence of water, its state and its role.

Fabrication and dielectric properties of polypropylene / silica nano-composites

Polymer nanocomposites have been shown to exhibit enhanced properties compared to those of the matrix. However the main problem in their fabrication is to obtain an efficient dispersion of nanoparticles in the matrix. In this work, polypropylene powder and nanosilica colloidal suspensions were mixed using planetary milling at 1 wt%. Two sizes of nanosilica particle were used (10–20 nm and 140–150 nm). Different rotation speeds and milling durations were tested for the fabrication process. Thermo Gravimetric Analyses were conducted to ensure that there was no contamination during fabrication. The resulting samples were studied by Dielectric Breakdown, Dielectric Spectroscopy and Thermo Gravimetric Analysis.

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.

Dielectric response of filled and flexibilized epoxy resin

In this work, several nanostructured microcomposite epoxy resin compounds, with or without flexibilized network, are analyzed. Scanning Electron Microscopy is used for the investigation of the phase separation between the flexibilizer and the polymer matrix. Differential Scanning Calorimetry results showed an enhanced molecular mobility for the polymer chains confirmed the expected influence of the flexibilizer. Also, the addition of micro and nanoparticles into epoxy resin increased the glass transition temperature since the fillers prevent the molecular movement of the polymeric chains. Broadband Dielectric Spectroscopy allowed us to determine the influence of the flexibilizer and the influence of the nano and micro filler ratio on the relaxation time versus temperature. After fitting the relaxation peaks using Havriliak-Negami functions, noticeable differences were observed for the β relaxation processes.

Electrical resistivity characterization of silicon carbide by various methods

In this paper, the variation of the resistivity of SiC powder at low and high voltage as a function of milling time was investigated by controlling the particle size, morphology and compactness (applied pressure). The particle shape and size of commercially available SiC powders were altered by ball milling. The electrical properties of the various SiC powders were characterized at low voltage by using a pressure controlled ohmmeter and frequency-domain spectroscopy. A resistivity increase from 7 to 54 MΩ.cm was observed when the grinding time increased from 5 to 30 minutes, which corresponded to a decrease of the particle size from 54 μm to ~4.5 μm. The nonlinear characterization I-V for high electrical field was investigated. X-ray diffraction and scanning electron microscopy have been carried out to highlight the size decreasing and crystalline structure of the milled powder.