Jiri Chvojka

A study on the usage of nonwoven nanofibers in electrical insulating materials

Nonwoven nanofibers as a new material for possible improving properties of electrical insulation are considered in this paper. Polyamide nanofibers are used for modification of commonly used three-component mica-based high voltage electrical insulating material (epoxy resin, glass cloth and mica). Influence of different volume ratio and density of the nanofibers on the properties of resulting composites was analyzed. Melting behavior of nonwoven nanofibers and structure of resulting composites were examined by differential scanning calorimetry (DSC) and by scanning electron microscopy (SEM). Dielectric strength and voltage dependence of dissipation factor were subsequently tested. Mechanical properties were analyzed via Charpy impact test. The obtained results demonstrated that voltage dependence of dissipation factor showed interesting behavior especially at electric field intensity greater than 5kV/mm when the partial discharge activity starts to occur. Under these circumstances some of the modified composites are characterized by notable lower values of dissipation factor. Moreover, while the impact strength of some of the modified composite increased (of about 17%), dielectric strength stayed the same or better than expected.

The usage of nonwoven nanofibers for improving properties of electrical insulation

The nonwovens and nanofibers as a new material for possible improving properties of electrical insulation are introduced in this paper. Nanofibers were used to modify a commonly used two-component like mica-based high voltage electrical insulating material. The tested material in the form of resin-rich sheets was modified using a various number of layers (1, 2 and 3) of the nanofibers made from Polyamide 6 (PA6) in the various surface densities (1, 2 and 3 g·m-2). Specimens were manufactured at increased temperature and pressure. The volume resistivity, voltage dependence of dissipation factor and mechanical properties were tested. The structure of the nanofibers was examined by scanning electron microscopy (SEM), the manufactured composites were analyzed under optical microscopy. Obtained results showed that composites with layers of nonwoven nanofibers are characterized by lower values of dissipation factor at electric field intensity greater than 5 kV/mm when the partial discharge activity starts to occur. Mechanical tests showed that while the Youngs modulus of the modified composites notably increased (of about 27 - 43.5 %), flexural strength stayed on the same values as unmodified composites.

Using of statistical tools within optimalization of design of material for high-voltage applications

Paper deals with evaluation of chosen properties of newly developed modification of insulation material, composite based on glass-mica-epoxy material. The main reason for this analysis was to optimize the specific design of material. For improving some of mechanical and electrical properties the currently used material was modified by incorporation of nonvowen nanolayers into its structure before the curing. Basic material (without any nanolayer) and two variants of material with incorporated nanolayer (with one and two layers) of the same size were prepared in two different generations. Several nanolayers with unknown and not identical density were obtained and used for the production of the first generation of samples. The second generation was prepared from homogeneous nanomaterial with known density. Process of manufacturing was identical for samples of both generations (with the same glass-mica-epoxy composite). Tools, which can be used to evaluate material design and to determine material wearout failure, are investigated in this paper. Graphical and computation statistical tests were used for evaluation of measured data. Box plot, probability plot were used from the group of graphical methods and t-test for mean value (factor analysis of variance) and F-test for homogeneity of variances (Bartletts test) from the group of computation methods. Data from the measurement of dielectric strength, impact strength and resistivity were used for the statistical evaluation. On the basis of statistical analyses, risky samples were identified and will be additionally analysed. These information can help with production of the next generation of samples.

Preparation of composite scaffolds from micro / nano fibers and biocompatible hydrogels

S of the Problem: In order to understand the formation of strain-induced martensite (SIM) of austenitic stainless steels, phase textures were investigated both before and after static and cyclic loading, namely plastic deformation is made intentionally. Moreover, in-situ measurements of the strain-induced martensitic transformation that takes place during tensile loading at room temperature were performed. Even in the low plastic strain regime, with loading to yield stress, the SIM transformation occurred. However, the area fraction of the martensite formation did not increase significantly even when the sample was loaded to the ultimate tensile strength. On the other hand, by the cyclic loading, the area fraction of the martensite formation increases significantly when the maximum cyclic load is more than 80%UTS. In other word, the SIM formation is apparently absent when the samples are loaded with less than 70%UTS, although those samples are fractured completely. No clear frequency effect (1Hz vs. 30Hz) is detected. With the analysis, two different SIM characteristics were clarified following plastic deformation. The martensitic structures were obtained in the twin deformation and slip bands. The severity of martensite formation increased with increasing C content. It was found that martensite was formed mainly in austenitic stainless steel lacking Mo, whereas a high Mo content led to a strong martensite structure, i.e., a weak martensite. The formation of martensite occurred from austenite viamartensite, and was related to the slip deformation. The Mo element in austenitic stainless steel had high slip resistance (or stress-induced martensite transformation), due to the large size of the Mo atom. This resulted in the creation of weak martensite. The phase structures of the strained austenitic stainless steels were interpreted using a proposed, i.e., the martensitic transformations.We present various types of group III-nitride microand nano-structures for novel classical and quantum photonic applications. We demonstrate phosphor-less white-color light generation, unidirectional light propagation, ultrafast single photon generation, and room temperature exciton-polariton generation using these group III-nitride based photonic structures. First, multicolor and broadband visible light emitting diodes based on GaN hexagonal truncated pyramid and columnar structures were demonstrated [1, 2]. Second, by using GaN/InGaN core−shell QW semiconductors grown on tapered GaN rods, which have a large gradient in their bandgap energy along their growth direction, highly asymmetric photonic diode behavior was observed [3]. Third, we utilized a novel approach of the self-aligned deterministic coupling of single quantum dots (QDs) to nanofocused plasmonic modes, which enhances spontaneous emission rate of QDs as high as ~ 22 over a wide spectral range [4]. We also discuss about effective method for enhancing collection efficiency of the QDs formed in these photonic structures [5]. Finally, we developed a novel excitonpolariton system working at room temperature resulting from strong coupling between a two-dimensional exciton and whispering gallery mode photon using a core−shell hexagonal wire with GaN/InGaN multiple quantum wells [6]. An overview and comparison of the characteristics of the above nanostructures will be given.

Nanofibrous Composite Materials Integrating Nano/Micro Particlesbetween the Fibres

This article deals with the continual incorporation of particles by the ultrasonic dispersion in situ into a nanofibrous matrix produced by the electrospinning process. The new technology is based on the use of the needleless electrospinning method in combination with the ultrasound-enhanced dispersion of sub-micro or micro particles, which are deposited between nanofibres onto the support material. The main advantage of this technology is the independence of particle-incorporation of the electrospinning process. The particles are trapped between the fibres and they remain uncovered by polymer, thus maintaining all their active properties. Such materials can be cut with scissors without the particles being released. In this paper the authors present figures from scans of the electron microscopy of the newly-designed nanocomposite material and its morphological analysis, such as the particle distribution. The material was used as a sorbent of bis(2-chlorethyl) sulfide (mustard gas) with a sorption time greater than 240 minutes. Such material has been developed to be used for protection against chemical warfare agents; yet, it can be employed for several other applications depending on the powder material dispersed onto the nanofibrous layer.