Karel Liedermann

Study of electrical and rheological properties of natural and other oils

This paper deals with the analysis of promising environmentally compatible electrical insulating fluids for electrical engineering. It starts with the overview of the current state of the art and of the latest trends in the use of biodegradable natural oils. In the experimental part, castor oil, linseed oil, sunflower oil and rapeseed oil as well as their components, fatty acid esters, were studied. Dielectric properties were measured using precision LRC meters Agilent 4284A and 4980A including dielectric liquid test fixture Agilent 16452A. Also, Agilent graphical language environment VEE Pro was used for the communication between a PC and Agilent analyzers. Electrical properties of the above-mentioned oils are presented in the frequency domain in the 20 Hz - 1 MHz in the temperature interval 283 K - 363 K; also shown are rheological properties in the 293 K - 363 K temperature range.

Dielectric relaxation in Relanex insulation during ageing

Relanex is a commercial insulating composite which is widely used in the slot insulation of the hv motors. The aim of the research was to find out if dielectric relaxation spectroscopy reveals any changes in the dielectric spectrum in the course of the ageing. Another question was whether the dielectric spectrum changes in any way with the applied voltage.

Electrical properties of natural oils and methyl esters of their constitutive fatty acids

This paper deals with the analysis of prospective environmentally compatible electrical insulating fluids for electrical engineering in relation to their chemical structure. The paper starts with the overview of the current state of the art and of the latest trends in the use of synthetic and biodegradable natural oils. In the experimental part, rapeseed oil, sunflower oil, soybean oil, methyl palmitate, methyl laurate, methyl oleate and methyl stearate were studied. Dielectric properties were measured using precision LRC meters Agilent 4284A and 4980A including dielectric liquid test fixture Agilent 16452A and also by the Novocontrol Alpha-A analyzer. Electrical properties are presented in the frequency range 1 × 10-2 - 1 × 107 Hz range in the temperature interval -20°C to 80°C. The paper goes on with the study of the suitability of individual oils for lower temperature, including the impact of the chemical structure and formulation on electrical properties.

Study of low-frequency dielectric relaxations in polysaccharides for medical applications

Dielectric relaxation spectra of four polysaccharides (hyaluronic acid in a form of sodium salt, chondroitin sulfate C, hydroxyethylcellulose Natrosol H and carboxymethylcellulose Blanose D) were studied by time-domain dielectric relaxation spectroscopy in the frequency range 10/sup -5/ to 10/sup -1/ Hz. Observed spectra were interpreted as a sum of one conductivity process and one or two relaxation processes using the Havriliak-Negami formula. Numerical values of the parameters of conductivity and relaxation processes were calculated and the physical interpretation of the observed data is discussed.

Dielectric properties of epoxy resins with oxide nanofillers and their accelerated ageing

The aim of this paper is to discuss the impact of nanoparticles in epoxy matrix on dielectric parameters in the course of ageing. One of the directions pursued in the development of epoxy resins is the addition of nanofillers which act as efficient barriers for the propagation of electrical trees. In view of the prospects for the potential replacement of currently-used epoxy-based electric insulations with epoxy nanocomposites, it is necessary to know the behavior of such epoxy resins with nanofillers during aging. For this purpose, epoxy nanocomposites were exposed to accelerated aging and changes in dielectric properties were observed. Experimental samples were manufactured from bisphenol-A based epoxy resin for high voltage applications and oxide nanofillers. The volume of nanofiller added into epoxy resin was 1.5 wt %. Sample thickness was of the order of a few hundreds of μm. Complex permittivity (ε), inner resistivity (ρi) and loss factor (tan δ) were measured at temperatures from -153 °C to +167 °C and in the frequency range 10-2 to 106 Hz.

Dielectric relaxation spectroscopy of epoxy resins with TiO 2 , Al 2 O 3 , WO 3 and SiO 2 nanofillers

The subject of this study is dielectric properties of a four-component casting epoxy resin containing TiO2, Al2O3, WO3 and SiO2 nanofillers over the frequency range 20 Hz - 1 MHz with the aim to investigate the possibilities of using dielectric spectroscopy for diagnostics of such nanocomposites or structures constructed of them. Both positive and negative features of nanofillers in the epoxy resin are examined. The paper explains how to prepare samples for dielectric measurements, then proceeds with the presentation of results themselves before arriving to interpretations.

New mixed rules on composite materials

The paper studies dielectric properties of elastomer filled with Ni spherical particles of different sizes within the range 35 - 115 mum. The experimental analysis is followed by the examination permittivity/conductivity mixing rules. A new mixing formula for the composite system consisting of spherical metallic particles and polymer matrix is presented. This formula includes both Ni particle content and Ni particle size as two independent variables.

Dielectric spectroscopy of hyaluronic acid and its salts

Hyaluronic acid is a high-molecular-weight polysaccharide found mostly in a human body and widely used in biomedical applications. Our study was focused on its electrical properties in the temperature range 25 K - 325 K and in the frequency range 100 Hz - 1 MHz. Three conductivity regions were found: DC conductivity, universal dielectric response and the super linear power law with the observed frequency dependence of conductivity being sigma ~ omega1.06. Activation energy of the DC conductivity process was established to be 5.4 MJ/kmol.

Study of the Molecular Mobility of Polysaccharide Solid Thin Layers by Dielectric Relaxation Spectroscopy

Temperature dependence of measured dielectric relaxation spectra (DRS) in the frequency range 20 Hz - 1 MHz of hydroxyethylcellulose (HEC) are in the temperature range 100 – 350 K. of Arrhenius character with one relaxation process at 150 – 250 K. This process reflects most probably β-relaxation of the side chain groups. Calculated activation energy of this process was 5730 kJ/mole. Four types of polysaccharides were studied at 293 K temperature: hyaluronic acid (HA), chondroitin sulfate (CHS), HEC and carboxymethylcellulose (CMC), in the low-frequency range 10 −5 - 10 0 Hz. Measured dielectric spectra were interpreted as sum of one A.C. conductivity process and of up to two relaxation processes. The relaxation processes were described by means of the Havriliak-Negami formula and their parameters were related to the molecular structure of the polymers. The low value of a in CHS is related to its strong coupling due to the presence of two polar groups in its monomeric unit, whereas low values of α × β are interpreted as being due to the strong steric hindrances caused by long pendants present in HEC.