Research that has been recently carried out on t h e dielectric behaviour of conductor-insulator composite systems in the microwave domain has shown that the complex permivitty e* = e ' j e " depends strongly on the nature and the concentration of the conducting medium [1-4]. The dielectric permittivity of the samples has been calculated from the experimental determination of reflection and transmission coefficients of electromagnetic waves. The experimental values of complex permittivity in these media are in disagreement with the provisions of effective medium theory [5]. This is due to the fact that this theory does not take into account the formation of clusters of particles in contact which is revealed by d.c. electrical conductivity measurements [6-8]. This disagreement has also been observed for high volumetric concentrations of conducting medium in the visible and infrared regions [9-11]. Measurements performed as a function of sample thickness of carbon black (Monarch 700)--epoxy for different concentrations of conducting medium show that the measured reflection coefficient agrees with that calculated for an equivalent homogeneous medium. However, the calculated transmission coefficient differs significantly from the calculated value [12]. Such anomalies lead us to wonder about the validity of the concept of effective permittivity, which is supposed to represent, as is generally admitted, heterogeneous medium behaviour when particle size is much smaller than sample size and wavelength. The electrical properties of heterogeneous media, which are constituted of pure dielectrics or conductors, are perfectly defined by mixture formulae [13-16]. In the case of conductor-insulator composite systems, these properties are strongly related to the inclusion size and to their shape [17, 18]. Accordingly, propagation laws which intervene in diffraction of the incident wave by the medium, answer some of the questions relating to mixtures by defining a complex propagation constant but not an equivalent permittivity [19-21]. This work is particularly interested in the different

Dielectric Properties of Carbon Black-Epoxy Resin Composites in the Frequency Range of 100Hz-15MHz

Microwave propagation through carbon black-epoxy resin composites

The complex permittivity of carbon black/epoxy composites containing different amounts of conductive carbon black particles was reported in the frequency range 100 Hz-15 MHz and over the temperature range 23-80◦C. The shape of experimental spectra of these heterogeneous media gives evidence of a typical dielectric relaxation process below the threshold percolation. The temperature dependence of the dielectric response has been analysed below and at near the epoxy glass transition temperature (Tg∼=80◦C), for various carbon concentrations. Jonscher's phenomenological model has been used for modelling this relaxation process. The activation energy, deduced from Arrhenius equation, is found to be insensitive to the carbon black concentration. This behavior is discussed in term of the interfacial interaction between the carbon particles and the polymer. Besides, the increase of both parts of the complex permittivity with increasing the carbon black concentration and the temperature is compared to the recently proposed Shin's mixture law.

Temperature effect on dielectric properties of carbon black filled epoxy polymer composites

Dielectric properties of carbon black-epoxy resin composites were studied in the frequency range between 100 Hz and 15 MHz and over a temperature range 23-98°C. The results show that the composites exhibit, below the critical concentration, a dielectric relaxation phenomenon due to dipolar groups of expoxy resin and to interfacial polarization. This relaxation phenomenon was modeled by the Cole-Cole model.

Dielectric Properties of Carbon Black-Epoxy Resin Composites in the Frequency Range of 100Hz-15MHz

Citations

Microwave propagation through carbon black-epoxy resin composites

Temperature effect on dielectric properties of carbon black filled epoxy polymer composites

Dielectric Properties of Carbon Black–Epoxy Resin Composites Studied with Impedance Spectroscopy*

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