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Abstract

Introduction of polymers into the cement composites improves same of the properties of concretes and mortars. Therefore, the polymer-cement composites are successfully used in construction. The model of microstructure formation in cement composites modified with thermoplastic polymer (pre-mix modifiers) has already been developed and successfully implemented. However, the formation of microstructure in the case of epoxy-cement composites (containing post-mix modifier) demonstrates same peculiarities which should be taken into account when modelling the process. The microstructure of epoxy-cement composites and its formation is discussed in the paper. The model is offered, formulated on the basis of the microscopic observations and results of testing.
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Abstract

Transient heat transfer is studied and compared in two plane-parallel composite walls and one EPIDIAN 53 epoxy resin wall acting as a matrix for both composites. The first of the two walls is made of carbon-epoxy composite; the other wall is made of glass-epoxy composite, both with comparable thickness of about 1 mm and the same number of carbon and glass fabric layers (four layers). The study was conducted for temperatures in the range of 20-120 °C. The results of the study of thermal diffusivity which characterizes the material as a heat conductor under transient conditions have a preliminary character. Three series of tests were conducted for each wall. Each series took about 24 h. The results from the three series were approximated using linear functions and were found between (0.7-1.35) x 10-7m2/s. In the whole range of temperature variation, the thermal diffusivity values for carbon-epoxy composite are from 1.2 to 1.5 times higher than those for the other two materials with nearly the same thermal diffusivity characteristics.
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Abstract

The paper presents the application of similarity theory to investigations of transient heat transfer in materials with complex structure. It describes the theoretical-experimental method for identification and design of the structure of two-component composite walls based on the research of the thermal diffusivity for the composite and its matrix separately. The thermal diffusivity was measured by means of the modified flash method. The method was tested on two samples of double-layer ‘epoxy resin – polyamide’. All the investigated samples had the same diameter of 12 mm and thickness ranging from 1.39–2.60 mm and their equivalent value of thermal diffusivity ranging from (1.21–1.98)×10-7m2/s. Testing the method and research on carbon/epoxy composites was carried out at temperatures close to room temperature.
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Abstract

Noise diagnostics has been performed on the cold field-emission cathode in high-vacuum. The tested cold field-emission cathode, based on tungsten wire with ultra-sharp tip coated by epoxy was designed to meet the requirements of transmission electron microscopy, which uses a small and stable source of electrons. Current fluctuations are reduced by improving the structure and fabrication technology. Noise was measured both in time and frequency domains, which gives information about current fluctuations and also about charge transport. Mutual correlation between the noise spectral density, extractor voltage and beam brightness was analyzed.
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Abstract

This article describes the influence of thermal and dielectric properties of materials to properties of electrical insulating systems in high voltage electrical equipment. The aim of this experiment is to improve the thermal and dielectric properties of electrical insulating (composite) materials using micro fillers of aluminium oxide Al2O3. Supplement of fillers of aluminium oxide with better thermal conductivity to the electrical insulating systems can be modified to increase their thermal conductivity. Improving the thermal conductivity of electric insulation by addition of micro- or nanofillers and in the same time not adversely affecting the dielectric properties is the objective of the study. Paper is presenting the results measured on prepared samples. Improved thermal conductivity is compared with other dielectric properties as: dissipation factor temperature dependences, resistivity and dielectric spectroscopy. To determine the dielectric insulating properties the following characteristics were measured: tanδ versus temperature from 110°C to 150°C, absorption and resorption currents, volume resistivity. Furthermore, this article describes analysis of moisture and conductivity the material by dielectric spectroscopy.
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Abstract

This article presents the results of an examination performed on a set of samples of glass-epoxy core rods used in composite insulators with silicone rubber housings. The goal of the examination was to test the aging resistance of the core material when exposed to Direct Current (DC) high voltage. Long term exposure of a glass-epoxy core rod to DC high voltage may lead to the gradual degradation of its mechanical properties due to the ion migrations. Electrolysis of the core material (glass fiber) may cause electrical breakdown of the insulators and consequently lead to a major failure. After being aged for 6000 hours under DC high voltage, the samples were subjected to microscopic analysis. Their chemical composition was also examined using Raman spectroscopy and their dielectric losses and conductance in the broad range of frequencies were tested using dielectric spectroscopy.
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