Ilona Plesa

Properties of Polymer Composites Used in High-Voltage Applications

The present review article represents a comprehensive study on polymer micro/nanocomposites that are used in high-voltage applications. Particular focus is on the structure-property relationship of composite materials used in power engineering, by exploiting fundamental theory as well as numerical/analytical models and the influence of material design on electrical, mechanical and thermal properties. In addition to describing the scientific development of micro/nanocomposites electrical features desired in power engineering, the study is mainly focused on the electrical properties of insulating materials, particularly cross-linked polyethylene (XLPE) and epoxy resins, unfilled and filled with different types of filler. Polymer micro/nanocomposites based on XLPE and epoxy resins are usually used as insulating systems for high-voltage applications, such as: cables, generators, motors, cast resin dry-type transformers, etc. Furthermore, this paper includes ample discussions regarding the advantages and disadvantages resulting in the electrical, mechanical and thermal properties by the addition of micro- and nanofillers into the base polymer. The study goals are to determine the impact of filler size, type and distribution of the particles into the polymer matrix on the electrical, mechanical and thermal properties of the polymer micro/nanocomposites compared to the neat polymer and traditionally materials used as insulation systems in high-voltage engineering. Properties such as electrical conductivity, relative permittivity, dielectric losses, partial discharges, erosion resistance, space charge behavior, electric breakdown, tracking and electrical tree resistance, thermal conductivity, tensile strength and modulus, elongation at break of micro- and nanocomposites based on epoxy resin and XLPE are analyzed. Finally, it was concluded that the use of polymer micro/nanocomposites in electrical engineering is very promising and further research work must be accomplished in order to diversify the polymer composites matrices and to improve their properties.

Dielectric Properties of Nanodielectrics with Inorganic Fillers

One of the main targets of the research in the field of polymer nanocomposite dielectrics is to obtain new materials with improved dielectric properties (resistivity, dielectric strength, permittivity and dielectric losses). In this paper the variation of the real part of the permittivity and of the loss tangent with the frequency are investigated for three formulations of nanocomposites obtained from polyethylene filled with nanoparticles of SiO2, TiO2 and Al2O3, respectively. The influence of the filler concentration (between 2 and 10 wt%) on the dielectric behavior of the nanocomposite is analyzed as well. To simulate the electrical behavior of the polymer-filler interface which might explain the experimental results a 3D electrostatic model proposed on the basis of Tanakas multi-core model is discussed. This model allows one to study the influence of several parameters such as the nanoparticle diameter, thickness of the interface layers, concentration and permittivity of the nanoparticles or the permittivities of the interface layers, on the effective permittivity of a plane nanodielectric sample.

Dielectric properties of LDPE-SiO 2 nanocomposites

The dielectric behaviour of nano-SiO2 filled low density polyethylene is investigated over a frequency range of 10 mHz–10 MHz and for different temperatures from 250 K to 350 K. It is shown that the presence of nanoparticles change significantly the dielectric behaviour of the polymer system. The frequency variations of the permittivity and of the tan delta emphasize a α-relaxation process, for each of the nanocomposite samples. The relaxation is more important and occurs at higher frequencies with the increase of the filler content. The increase of the temperature leads to a shift of the relaxation frequency to higher values. Results from chemiluminescence measurements and from X-ray diffraction analysis are discussed in connection with the dielectric behaviour.

The influence of surface modification on the electrical properties of silicon carbide flakes

Field grading materials with nonlinear behaviour are used in many high voltage applications to avoid any stress concentration that can deteriorate the material performance. In the present work, the current-voltage characteristics of surface functionalized silicon carbide (SiC) flakes were studied and compared to untreated particles. The nonlinear behavior of SiC flakes can be described by transport mechanisms at the grain contact, which can be modeled by Schottky-like barriers. The influence of the organic surface layer on the conduction mechanisms and the corresponding electrical properties were evaluated. In addition, physico-chemical analysis, including X-ray photoelectron spectroscopy (XPS), optical microscopy and zeta-potential measurements were accomplished in order to analyze the surface properties of the SiC flakes, prior to and after surface modification.

Effects of gamma irradiation on resistivity and absorption currents in nanocomposites based on thermoplastic polymers

The functional characteristics depend strongly on the chemical structure of polymer and on the formulation of material, as well. In the present work, the effects of gamma irradiation at different doses on the resistivity and on the absorption currents in nanocomposites of thermoplastic polymers, such as low density polyethylene (LDPE), polypropylene (PP) and ethylene-propylene-diene monomer rubber (EPDM) are analysed. The influences of the polymer structure as host matrix (LDPE, PP, EPDM) and nano SiO2 concentration (between 2 and 5 wt.%) on electrical features are discussed. A correlation between electrical and thermal properties is illustrated for all formulations.

Electrostatic model of LDPE-SiO 2 nanodielectrics

An electrostatic model to explain and predict the dielectric properties of nanocomposites made of low density polyethylene (LDPE) filled with SiO2 nanoparticles is presented. In the present approach the modeled nanodielectric is a polymer matrix with uniformly distributed identical spherical nanoparticles embedded, each nanoparticle being surrounded by a three-layer interface. Assuming a possible structure of the interface, an estimation of the dipole types and concentrations is made and then the permittivity and charge distribution inside the interface regions are estimated and used in a numerical model based on the finite element method. The computational domain of the 3D numerical model developed for the LDPE-SiO2 nanodielectric is reduced to an elementary fraction of the whole geometry (a cube containing eight nanoparticles), by taking into account the existing physical symmetries imposed by appropriate boundary conditions. This model is implemented in the finite element method based software package COMSOL Multiphysics for three filler concentrations: 2, 5 and 10 wt%. The results show a good correlation between the effective permittivity calculated with our model and the experimentally measured permittivity and emphasize the influence of the space charge presence inside nanodielectric on the electric field repartition and on the effective permittivity. A comparison between our results and those obtained with other models is also discussed.