Enis Tuncer

Dielectric mixtures: electrical properties and modeling

A review of the current state of understanding of dielectric mixture properties, and approaches to use numerical calculations for their modeling are presented. It is shown that interfacial polarization can yield different non-Debye dielectric responses depending on the properties of the constituents, their concentrations and geometrical arrangements. Future challenges on the subject are also discussed.

Electrical properties of epoxy resin based nano-composites

We investigate the electrical properties of composite materials prepared as nano- and sub-micron-scale metal-oxide particles embedded in a commercial resin. The filler particles are barium titanate and calcium copper titanate. The physical and structural characteristics of the constituents and the fabricated composites are reported. Electrical characterization of the composite samples is performed using time- and frequency-domain dielectric spectroscopy techniques. The electrical breakdown strength of samples with nano- and sub-micron-sized particles have better electrical insulation properties than the unfilled resin.

Dielectric relaxation in dielectric mixtures: Application of the finite element method and its comparison with dielectric mixture formulas

In this article, the frequency dependent dielectric properties, e(ω), of an “ideal” binary composite structure were investigated by using the finite element method in the frequency domain. The material properties of the phases, i.e., dielectric permittivity, e, and direct-current conductivity, σ, were assumed to be frequency independent. Moreover, the inclusion phase was more conductive than the matrix phase. The inclusions were infinitely long unidirectional cylinders which could be assumed to be hard disks in two dimensions in the direction perpendicular to the cylinder direction. Three different inclusion concentration levels were considered, e.g., low, intermediate, and high. The calculated dielectric relaxations were compared with those of the dielectric mixture formulas in the literature and it was found that there were no significant differences between the formulas and the numerical solutions at low inclusion concentration. Furthermore, the obtained responses were curve fitted by the addition of t...

Enhancement of dielectric strength in nanocomposites

In this paper, we report the dielectric breakdown properties of a nanocomposite, a potential electrical insulation material for cryogenic high voltage applications. The material is composed of a high molecular weight polyvinyl alcohol and nanosized in situ synthesized titanate particles. The dielectric breakdown strengths of the filled material samples, measured in liquid nitrogen, indicate a significant increase in their strengths as compared to unfilled polyvinyl alcohol. We conclude that nanometre-sized particles can be adopted as a voltage stabilization additive.

Non-Debye dielectric relaxation in binary dielectric mixtures (50-50): Randomness and regularity in mixture topology

In this article, the frequency dependent dielectric properties e(ω) of ordered and disordered two-dimensional binary composite structures were investigated and compared. The ordered structures were composed of hard disk inclusions in a matrix phase, and the inclusions were distributed on lattice sites. The disordered structures were, on the other hand, composed of 16 × 16 square networks (crossword puzzle-like structures), and the phases were assigned randomly to each square. The material parameters of the phases were assumed to be frequency independent (e and σ being constant). Numerical calculations were performed using the finite element method in the frequency domain. We have found that the dielectric relaxation character of the structures, which were due to the interfacial (or Maxwell–Wagner–Sillars) polarization, changed drastically depending on the conductivity ratio of the phases and topology of the structures. Application of a recently developed dielectric data analysis method have resulted addit...

On dielectric breakdown statistics

In this paper, we investigate the dielectric breakdown data of some insulating materials and focus on the applicability of the two- and three-parameter Weibull distributions. A new distribution function is also proposed. In order to assess the model distributions trustworthiness, we employ the Monte Carlo technique and, randomly selecting data-subsets from the whole dielectric breakdown data, determine whether the selected probability functions accurately describe the breakdown data. The utility and strength of the proposed expression are illustrated distinctly by the numerical procedure. The proposed expression is shown to be a valuable alternative to the Weibull ones.

Electrical properties of filled silicone rubber

Rubber materials for high-voltage outdoor applications have been studied. They were prepared as mixtures of silicone polymer with different concentrations of powdered aluminium trihydrate as a filler. The dielectric properties, *( ) = ´-i ´´, were measured at different temperatures using a low-frequency dielectric spectroscopy technique. Without any filler a classical interfacial relaxation was pronounced with dominating direct-current (dc) conduction in the losses, ´´. On increasing the concentration of filler, a low-frequency dispersion mechanism started to dominate the relaxation behaviour. On adding the filler, the dc conduction first decreased and then increased again after a certain concentration level was passed. This behaviour showed that the filler particles acted like scattering centres or traps for conduction when the concentration was low. As the concentration increased, the conductivity of the filler particles and of the interface (between the filler particles and the polymer matrix) started to dominate the conduction. Master curve shifts of the data showed an Arrhenius type of activation. The data were modelled by superposition of three different processes, Havriliak-Negami dipolar relaxation, dc conduction and low-frequency dispersion (hopping conduction) contributions, using a nonlinear least-squares fitting method.

Comparison of methods for estimating continuous distributions of relaxation times

The nonparametric estimation of the distribution of relaxation-time approach is not as frequently used in the analysis of dispersed response of dielectric or conductive materials as are other immittance data analysis methods based on parametric curve fitting techniques. Nevertheless, such distributions can yield important information about the physical processes present in measured material. In this paper, we apply two quite different numerical inversion methods to estimate the distribution of relaxation times for glassy Li0.5La0.5TiO3 dielectric frequency-response data at 225K. Both methods yield unique distributions that agree very closely with the actual exact one accurately calculated from the corrected bulk-dispersion Kohlrausch model established independently by means of parametric data fit using the corrected modulus formalism method. The obtained distributions are also greatly superior to those estimated using approximate function equations given in the literature.

Structure–property relationship in dielectric mixtures: application of the spectral density theory

This paper presents numerical simulations performed on dielectric properties of two-dimensional binary composites. The influence of structural differences and intrinsic electrical properties of constituents on the composites overall electrical properties is investigated. The structural differences are resolved by fitting the dielectric data with an empirical formula and by the spectral density representation approach. At low concentrations of inclusions (concentrations lower than the percolation threshold), the spectral density functions are delta-sequences, which corresponds to the predictions of the general Maxwell–Garnett (MG) mixture formula. At high concentrations of inclusions (close to the percolation threshold) systems exhibit non-Debye-type dielectric dispersions, and the spectral density functions differ from each other and that predicted by the MG expression. The analysis of the dielectric dispersions with an empirical formula also brings out the structural differences between the considered geometries, however, the information is not qualitative. The empirical formula can only be used to compare structures. The spectral representation method on the other hand is a concrete way of characterizing the structures of the dielectric mixtures. Therefore, as in other spectroscopic techniques, a look-up table might be useful to classify/characterize structures of composite materials. This can be achieved by generating dielectric data for known structures by using ab initio calculations, as presented and emphasized in this study. The numerical technique presented here is not based on any a priori assumption methods.The paper presents numerical simulations performed on dielectric properties of two-dimensional binary composites on eleven regular space filling tessellations. First, significant contributions of different parameters, which play an important role in the electrical properties of the composite, are introduced both for designing and analyzing material mixtures. Later, influence of structural differences and intrinsic electrical properties of constituents on the composite’s over all electrical properties are investigated. The structural differences are resolved by the spectral density representation approach. At low concentrations of inclusions (concentrations lower than the percolation threshold), the spectral density functions are delta-sequences, which corresponds to the predictions of the general MaxwellGarnett mixture formula. At high concentrations of inclusions (close to the percolation threshold) systems exhibit non-Debye type dielectric dispersions, and the spectral density function differ from each other and that predicted by the Maxwell-Garnett expression. The analysis of the dielectric dispersions with an empirical formula also illustrate structural differences between the considered geometries, however, the information is not qualitative. The empirical formula can only be used to compare structures. The spectral representation on the other hand is a concrete method to characterize the structures of dielectric mixtures. Therefore, like in other spectroscopic techniques, a look-up table might be usefull to classify/characterize composite materials structure. This can be achieved by generating dielectric data for known structures as presented and emphasized in this study. Finally, the numerical technique, without any a-priori assumptions, for extracting the spectral density function is also presented.

Electrical Insulation Characteristics of Glass Fiber Reinforced Resins

Non-metallic structural materials that act as an electrical insulation are needed for cryogenic power applications. One of the extensively utilized materials is glass fiber reinforced resins (GFRR) and may also be known as GFRP and FRP. They are created from glass fiber cloth that are impregnated with an epoxy resin under pressure and heat. Although the materials based on GFRR have been employed extensively, reports about their dielectric properties at cryogenic temperatures and larger thicknesses are generally lacking in the literature. Therefore to guide electrical apparatus designers for cryogenic applications, GFRR samples with different thicknesses are tested in a liquid nitrogen bath. Scaling relation between the dielectric breakdown strength and the GFFR thickness is established. Their loss tangents are also reported at various frequencies.