Thesis (Ph. D.)--University of Texas at Austin, 2004. / Supervisor: James W. McGinity. Vita. Includes bibliographical references.

Inaug.-diss.--Leipzig. / Lebenslauf. "Literaturverzeichnis": p. [87]-91.

A comparison of the wet-spinning and dry-jet wetspinning of acrylic fibres has been carried out using a commercial acrylic polymer (Courtelle). redissolved in dimethyl formamide. The higher speeds possible in dry-jet wet-spinning have been related to the higher free velocity and the higher draw ratios possible. It is believed that the presence of the air-gap in dry-jet wet-spinning allows the removal of the dies well effects as well as other viscoelastic behaviour before coagulation and that this leads to fibres with superior mechanical properties, especially in improvements in extensibility. Conditions are described which allow the production of high tenacity acrylic fibres with tenacity up to 5.8 g/d tex. In an attempt to produce fibres with better mechanical properties under hot-wet conditions, copolymershave been prepared using bicyclo [2,2,1] hepta-2,5-diene as a comonomer. Fibres from such copolymers have low extensibilities and satisfactory fibres could be made only by incorporating, in addition to the bicyclo [2,2,1] hepta-2,5-diene monomer, itaconic acid and by dry-jet wet-spinning. In one such case a fibre was obtained with a slightly higher hot-wet modulus and a considerably reduced hot-wet extensibility when compared with Courtelle fibre

Traditionally ultrafine fibers below 1 dpf are produced by extrusion followed by mechanical drawing. A modified melt spinning apparatus with high-speed air nozzle was designed and fabricated to produce continuous polypropylene filaments by cold air drawing only. With this setup, the fiber is quenched and simultaneously attenuated by a symmetric cold air jet. Since the formation of fiber structure is highly dependent on the processing conditions, the new process will provide a unique operation window to study fiber attenuation and structural formation under high-speed cold air drawing. Based on computational fluid dynamics simulation results, a parametric study was carried out under different process conditions which include processing temperature, air velocity and polymer volume flow rate. Effects of changes in processing variables on the fiber diameter, molecular orientation, crystallinity, tensile strength and other properties were studied. Furthermore, a theoretical model was developed to analyze the non-isothermal fiber attenuation mechanisms. The new knowledge obtained in this study would likely yield a new process for producing innovative fiber products.

Orientador: Rafael Zadorosny / Resumo: Os materiais supercondutores são extremamente promissores a novas aplicações. Podemos encontrar diversos estudos sobre o supercondutor YBCO, porém o tratamento térmico para a formação da fase YBa2Cu3O7-δ (Y123) na forma de nanofibras ainda não é bem definido na literatura. Para estudar de forma sistemática essa questão, e definir uma boa rota de tratamento térmico, fios com diâmetros em torno de 300 nm foram obtidos por Solution Blow Spinning (SBS) a partir de uma solução contendo acetatos metálicos de ítrio, bário e cobre, polímero PVP (polivinilpirrolidona), ácidos acético e propiônico, e metanol. Os fios poliméricos foram submetidos a um tratamento térmico para obtenção da fase Y123. Este tratamento foi dividido em duas partes, a primeira variando a temperatura máxima de sinterização (850°C, 875°C, 900°C e 925°C) por 1 hora. E a segunda variando o tempo em que as fibras ficaram submetidas à temperatura de 925°C (10 min, 30 min, 1h, 3h e 6h). Para analisar a influência da temperatura e do tempo de sinterização foram utilizados Microscopia Eletrônica de Varredura (MEV), Difração de raios-X (DRX) e Magnetometria AC. As análises de DRX indicaram a presença do YBa2Cu3O7-δ em todas as amostras, porém, com fases secundárias. As medidas de susceptibilidade magnética revelaram influência da temperatura de sinterização na blindagem magnética da amostra, indicando que as amostras tratadas a 925°C/1h obtiveram a melhor resposta magnética. Sendo assim, o melhor tratamento térmico foi o... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: Although the production of YBa2Cu3O7-δ (Y123) has been extensively reported in the literature, there is still a lack of information concerning the ideal heat-treatment routes applied to Y123 nanowires. Thus, wires with diameters around 300 nm were prepared by Solution Blow Spinning (SBS) from a solution containing yttrium, barium and copper metal acetates, PVP polymer (polyvinylpyrrolidone), acetic and propionic acids, and methanol. The resulting polymer wires were heat-treated to obtain the Y123 ceramic phase. Those treatments were divided into two parts: in the first one, which lasted 1 hour, the maximum sintering temperature was changed (850°C, 875°C, 900°C, and 925°C); in the second one, the time of the plateau at 925°C was altered (10 min, 30 min, 1h, 3h, and 6h). The structural characterization was carried out by Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), and AC Magnetometry. SEM images revealed that all samples yielded wires with an average diameter ranging from 241 nm to 376 nm. XRD analysis indicated the presence of the secondary phase BaCuO2 in all samples, as well as the phase Y123. Magnetic measurements corroborate the other characterizations, and indicate the sample sintered at 925 °C/1h as the one giving the highest magnetic response. / Mestre

No description available.

No description available.

No description available.

No description available.

No description available.