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Orientador: Luiz Carlos da Silva Filho / Banca: Kleber Thiago de Oliveira / Banca: Fabio Gorzoni Doro / Banca: Valdecir Farias Ximenes / Banca: Manoel Lima de Menezes / Resumo: A demanda por fontes de energias renováveis já é uma necessidade mundial, pois, o crescimento populacional e industrial trouxe concomitantemente um aumento no consumo de energia. Infelizmente, a matriz energética dos países que estão em pleno desenvolvimento se baseia na queima de combustíveis fósseis, o que traz sérios problemas ambientais e para a saúde da população. Tendo em vista esta necessidade, este trabalho retrata o estudo sobre o uso do pentacloreto de nióbio (NbCl5) como ácido de Lewis em reações multicomponentes para a síntese de compostos derivados de cumarinas e xantenos, compostos com potencial aplicação como corantes sensibilizadores em dispositivos fotovoltaicos. Os compostos de interesse foram obtidos através dos reagentes de partida: 4-hidróxicumarina, derivados fenólicos 3,5-dimetóxifenol e 3,4,5-trimetóxifenol, aldeídos aromáticos e 1,3-ciclo-hexanodiona. Foram estudadas as condições reacionais (% NbCl5, solvente utilizado e tempo reacional) para se estabelecer protocólos de síntese para a formação dos produtos de interesse na presença do pentacloreto de nióbio. O pentacloreto de nióbio demonstrou ser capaz de promover as reações estudadas neste trabalho. Esta nova possibilidade de obtenção destes compostos é de fundamental importância, pois, estes compostos possuem um leque muito amplo de aplicações, das quais algumas foram citadas neste trabalho. / Abstract: The demand for renewable energy sources is already a worldwide need, because, the population and industrial growth concurrently brought an increase in energy consumption. Unfortunately, the energy matrix of the countries that are in full development is based on burning fossil fuels, which causes serious environmental problems and the health of the population. In view of this need, this work shows that the study about the use of niobium pentachloride (NbCl5) as Lewis acid in multicomponent reactions for the synthesis of coumarin and xanthene derivatives of compounds with potential application as sensitizing dyes in photovoltaic devices. These compounds were obtained from the starting materials: 4-hydroxycoumarin, phenolic derivatives (3,5-dimethoxyphenol and 3,4,5-trimethoxyfenol, aromatic aldehydes and 1,3-cyclohexanedione. The reaction conditions (% NbCl5, solvent and reaction time) to establish protocols synthesis for the formation of the products in the presence of niobium pentachloride were studied.Niobium pentachloride has been shown to be capable of promoting the reactions studied in this work. This new method of obtaining these compounds is of fundamental importance, considering that these compounds have a wide range of applications, some of which have been cited in this work / Doutor

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Th. doct.--Sciences de la terre et de l'eau--Montpellier 2, 2001. / Textes en français et en anglais. Bibliogr. p. 222-238. Résumé en anglais.

Reproduction de : Thèse doctorat : Physique : Metz : 2004. / Titre provenant de l'écran-titre. Notes bibliographiques.

Thesis (Ph. D.)--Florida State University, 2004. / Advisor: Dr. Samuel L. Tabor, Florida State University, College of Arts and Science, Department of Physics. Title and description from dissertation home page (viewed June 16, 2004). Includes bibliographical references.

Thesis (Ph. D.)--School of Physics, Georgia Institute of Technology, 2004. Directed by Walter A. de Heer. / Vita. Includes bibliographical references (leaves 84-90).

For this thesis niobium oxides (NbO [subscript x]) with the goal of obtaining phase pure epitaxial NbO₂ were grown under variable conditions by molecular beam epitaxy (MBE) on different substrates, mostly (111) oriented strontium titanate. The films were characterized by X-ray diffraction (XRD) and reflection high-energy electron diffraction (RHEED) and from that their epitaxial relationship to the substrate was determined. With X-ray photoelectron spectroscopy (XPS) the oxidation states and stoichiometry of the films were determined. Asymmetric NbO₂ Nb 3d core-level spectra are clearly seen for the first time in XPS. Additionally, the film thickness was measured by X-ray reflectivity (XRR). Judging from the XPS spectra and diffraction data the goal to grow epitaxial NbO₂ of high phase-purity was achieved. / text

The dissolution rates of niobium and zirconium cylinders in liquid steel have been measured in a 'dynamic way', with the help of a data acquisition and process control facility. Two distinct periods were identified; the steel shell period and the free dissolution period. / Lower bath superheats allowed a reaction to take place at the steel shell/niobium interface while higher superheats didn't; the intermetallic compounds Fe$ sb 2$Nb and Fe$ sb 2$Nb$ sb 3$ were identified as the reaction products. Niobium dissolved relatively slowly in liquid steel and its dissolution speed was increased under dynamic conditions (i.e., inductively stirred baths). / In the case of zirconium, an exothermic reaction occurred at the steel shell/zirconium interface and the intermetallics Fe$ sb 2$Zr and FeZr$ sb 2$ were identified as the reaction products. This reaction was triggered at 1220 K by the formation of a liquid Fe-Zr (76 at% Zr) eutectic. For the experimental conditions under which this study was performed, the hydrodynamic conditions of the steel baths did not seem to influence the dissolution rates of zirconium. / A simplified mathematical model was used in order to simulate the coupled heat and mass transfer phenomena which take place during the two periods.

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