An investigation of the effect of temperature and interstitial composition on the yielding characteristics of high purity niobium single crystals has been carried out. Single crystals of [110] axial orientation were purified and grown using an electron beam, floating-zone, zone-refiner. Tensile specimens prepared from the single crystals were tested in tension at temperatures from +25° to -197°C. at a strain rate of 0.057 per minute. The variation of yield stress with testing temperature was found to be discontinuous and consisted of two curves which intersected at approximately -100°C. The tensile results can be correlated to both Fisher's interpretation of the Cottrell-Bilby theory of yielding and to the theory of Zener and Holloman, which involves a thermally activated process. The effect of additions of hydrogen, oxygen and nitrogen to niobium single crystals over the temperature range +25° to -197°C was to increase the yield stress. The unusual temperature dependence of yield stress has been explained in two possible ways: 1. Ordering or superlattice formation at the temperature at which the anomalous behaviour occurs. 2. The preferential release of screw dislocations at stress levels lower than that required for edge dislocations. The result is a decrease in the temperature dependence of yield stress at the temperature when edge dislocations begin to take part in yielding. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

This thesis reports the spectroscopic studies of two gaseous molecules, niobium nitride (NbN) and vanadium oxide (VO). The ³∏ — ³∆ electronic transition of NbN was recorded by laser-induced fluorescence at Doppler-limited resolution as well as at sub-Doppler resolution. Two almost identical branch features are observed in the ³∏₀ — ³∆₁ sub-band because the A-doubling in the ³∏₀ sub-state is large and almost J-independent. The ³∏₁ — ³∆₂ transition is shifted 600 cm⁻¹ to the red of its central first-order position as a result of very large second-order spin-orbit interaction effects. The shift is believed to be caused principally by the coupling of the ³∏₁ component with the ¹∏ state from the same electron configuration, with a smaller contribution from coupling of the ³∆₂ component of the ground state with the low-lying ¹∆ state. The ³∏ and ¹∏ states are unusual in that their zero-order energies are calculated to be within 100 cm⁻¹, based on the newly observed ¹∏₁ - ³∆₂ (1,0) transition; this means that they form a very fine example of a "super-multiplet", where the spin-orbit effects within and between the states of a particular electron configuration are larger than their separations. The spin-orbit interactions are so extensive that the fine structure can only be fitted using effective rotational and hyperfine Hamiltonians for the individual sub-states, as in case (c) coupling. From the determined hyperfine constants h and C⍳ for the three ³∏(v — 0) components, the Fermi contact constant b was found to be negative, which is consistent with the configuration πδ. Rotational analysis gave the ³∏ and ³∆ bond lengths as 1.6705 Å and 1.6622 Å, respectively. The near-infrared electronic system of VO has been recorded in emission at Doppler-limited resolution with the 1-m FT spectrometer at Kitt Peak National Observatory. The spectrum in the 4000-14000 cm⁻¹ region consists of numerous transitions with most of them extensively analyzed. Two isolated sub-bands at 7200 cm⁻¹ have been assigned as the two spin components of a ²∏ — ²∆ transition and rotationally analyzed. The rotational constant for the lower state is found to be larger than that for the σδ² X ⁴∑⁻ ground state, indicating that the ²∆ state arises from the electron configuration σ²δ. The configuration assignment was confirmed by the derived spin-orbit coupling constant for the ²∆ state. Similar reasoning applied to the ²∏ upper state suggests that it may arise from the configuration σ²π, though the preliminary study of the hyperfine structure argues against this assignment. / Science, Faculty of / Chemistry, Department of / Graduate

A sensitive nuclear magnetic resonance spectrometer has been constructed using Field Effect Transistors in a Robinson configuration. The spectrometer has been used to study the anomalous nuclear magnetic resonance spectra of single crystals of NbSe₂. An analysis of the field dependence of the line width in the low temperature phase has demonstrated that this results from a distribution of Knight Shifts. Such a distribution is not consistent with a structural transformation involving only two nonequivalent sites as proposed by Ehrenfreund et al. In addition accurate measurements of the Knight Shift and electric field gradient tensor have been made in the high temperature phase at 77K and 300K. The Knight Shift has a very large anisotropic component but an almost zero isotropic component which is indicative of negligible s-electron character at the: Fermi surface. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

No description available.

No description available.

A construção de sensor potenciométrico, utilizando Nióbio acoplado a eletrodo de referência de prata/cloreto de prata num mesmo instrumento, possibilita o acompanhamento da variação de potencial que ocorre durante uma reação ácido-base em meio aquoso. Foi utilizado eletrodo de vidro combinado, cuja membrana de vidro foi danificada fisicamente, porém com sua referência interna em pleno funcionamento. O eletrodo de Nióbio foi confeccionado com metal polido ou então com a superfície modificada com óxido eletrodepositado. O crescimento de óxido sobre o Nióbio foi obtido por eletrólise em meio a H3PO4 diluído. O metal ou o par metal/óxido foi fixado no eletrodo de vidro contendo uma solução eletrolítica. O eletrodo de Nióbio não apresenta boa sensibilidade quando utilizado como indicador em potenciometria direta, mas é sensível às variações de potenciais que ocorrem em titulações ácido-base. O eletrodo de Nióbio metálico pode substituir o eletrodo de vidro combinado na determinação do teor de acidez de vinagres. / The construction of potentiometric sensors, using Niobium combined with the reference electrode silver / silver chloride in a single instrument, allows the variation of potential monitoring that occurs during an acid-base reaction in aqueous media. It was used a combined electrode glass, whose glass membrane was damaged physically, but with the internal reference in full operation. The Niobium electrode was made with polished metal or with the surface modified by electrodeposited oxide. The oxide growth was obtained by electrolysis in the H3PO4 diluted media. The metal or metal/oxide was fixed in glass electrode containing a electrolyte solution. The Niobium electrode doesn\'t show sensitivity when used as an indicator in direct potentiometry, but is sensitive to changes of potential that occur in acid-base titrations. The electrode of Niobium metal can replace the glass electrode combined in determining the level of acidity of vinegar.

A construção de sensor potenciométrico, utilizando Nióbio acoplado a eletrodo de referência de prata/cloreto de prata num mesmo instrumento, possibilita o acompanhamento da variação de potencial que ocorre durante uma reação ácido-base em meio aquoso. Foi utilizado eletrodo de vidro combinado, cuja membrana de vidro foi danificada fisicamente, porém com sua referência interna em pleno funcionamento. O eletrodo de Nióbio foi confeccionado com metal polido ou então com a superfície modificada com óxido eletrodepositado. O crescimento de óxido sobre o Nióbio foi obtido por eletrólise em meio a H3PO4 diluído. O metal ou o par metal/óxido foi fixado no eletrodo de vidro contendo uma solução eletrolítica. O eletrodo de Nióbio não apresenta boa sensibilidade quando utilizado como indicador em potenciometria direta, mas é sensível às variações de potenciais que ocorrem em titulações ácido-base. O eletrodo de Nióbio metálico pode substituir o eletrodo de vidro combinado na determinação do teor de acidez de vinagres. / The construction of potentiometric sensors, using Niobium combined with the reference electrode silver / silver chloride in a single instrument, allows the variation of potential monitoring that occurs during an acid-base reaction in aqueous media. It was used a combined electrode glass, whose glass membrane was damaged physically, but with the internal reference in full operation. The Niobium electrode was made with polished metal or with the surface modified by electrodeposited oxide. The oxide growth was obtained by electrolysis in the H3PO4 diluted media. The metal or metal/oxide was fixed in glass electrode containing a electrolyte solution. The Niobium electrode doesn\'t show sensitivity when used as an indicator in direct potentiometry, but is sensitive to changes of potential that occur in acid-base titrations. The electrode of Niobium metal can replace the glass electrode combined in determining the level of acidity of vinegar.

Ce travail de thèse est consacré au développement de nouvelles méthodologies en catalyse mettant en jeu des complexes métalliques à base de métaux non nobles, en l’occurrence le cobalt et le niobium. Ces deux métaux ont été utilisés pour catalyser des réactions d’hydrofonctionnalisation de liaisons multiples carbone‒carbone. L’objectif était de proposer des systèmes catalytiques compétitifs à base de métaux non précieux et de promouvoir leur utilisation par rapport à d’autres métaux peu abondants et onéreux. Un complexe de cobalt bien défini, HCo(PMe3)4, a été employé pour catalyser des réactions d’hydroboration régio- et stéréosélectives d’alcynes internes, ainsi que des réactions de diboration d’alcynes. D’autre part, un système catalytique au niobium(V) cationique a été mis au point et utilisé pour diverses réactions d’hydrofonctionnalisation intramoléculaires, permettant la synthèse d’une large famille d’hétérocycles. De manière à mettre encore davantage en lumière ce métal et ces bienfaits, des travaux ont été réalisés en catalyse énantiosélective. Un système catalytique chiral de niobium(V) a été utilisé pour l’hydroalkoxylation d’alcènes énantiosélective. Des résultats encourageants ont été obtenus même si ce système doit encore être optimisé et testé en présence de différents hydroxyalcènes. / This PhD work has focused on the development of new methodologies in catalysis based on non-noble metals: cobalt and niobium. These two metals have been used to catalyze hydrofunctionalization reactions of carbon‒carbon multiple bonds. The aim of those projects was to propose competitive catalytic systems based on non-precious metals and to promote their use compared to other rare and expensive metals. To this end, we successfully demonstrated that a well-defined cobalt complexe HCo(PMe3)4 is an efficient catalyst for regio- and stereoselective hydroboration reactions of internal alkynes, as well as diboration reactions. Also, a new catalytic system based on cationic niobium(V) has been developed and used to catalyze intramolecular hydrofunctionalization reactions leading to the synthesis of a large family of heterocycles. In order to reveal even more the potential of niobium in catalysis, we aimed to propose a chiral system able to catalyze enantioselective hydroalkoxylation of alkenes. Despite the promising results, some more efforts on the optimization of this system still need to be done.

Orientador: Luiz Carlos da Silva Filho / Banca: Daniel Rinaldo / Banca: Antonio Eduardo Miller Crotti / O Programa de Pós Graduação em Ciência e Tecnologia de Materiais, PosMat, tem caráter institucional e integra as atividades de pesquisa em materiais de diversos campi / Resumo: Os derivados de 4H-cromemos constituem uma importante classe de produtos naturais, e o interesse em sua química se dá por conta dos inúmeros efeitos biológicos e farmacêuticos, assim como sua utilização como intermediários nas sínteses de inúmeros produtos naturais e reagentes medicinais. Esses derivados podem ser obtidos através das reações multicomponentes (RMC). As RMC's ocorrem entre três ou mais reagentes em uma única etapa reacional, ao mesmo tempo frasco. São reações com economia de tempo e energia. O pentacloreto de nióbio é um ácido de Lewis forte, que apresenta grande aplicabilidade em reações orgânicas, incluindo as reações multicomponentes. Dentre os diversos métodos de síntese de derivados de 4H-cromenos, desenvolveu-se neste trabalho a síntese através da reação multicomponente entre derivados de benzaldeído, 3,5-dimetóxifenol e acetoacetato de metila, utilizando pentacloreto de nióbio como ácido de Lewis. As reações foram realizadas a temperatura ambiente, sob atmosfera inerte (n2) e utilizando diclorometano anidro (CH2Cl2) como solvente, com um tempo reacional de 72 horas. Os produtos obtidos foram isolados por coluna cromatográfica em sílica gel e submetidos a análises de caracterização estrutural, fotoquímica e fotofísica para um estudo de sua potencial aplicação como corantes sensibilizadores de dispositivos fotoeletroquímicos de Gratzel. A análise dos resultados obticos demonstram que o pentacloreto de nióbio é um bom promotor da reação multicomponente, obtendo-se bons rendimentos (62 a 91%) / Abstract: 4H-chromete derivatives constitute an important class of natural products, and the the interest in its chemical occurs because of the numerous biological and pharmaceutical effects, as well their use a intermediates in the synthesis of numerous natural products and medicinal reagents. These derivatives can be obtained thorugh the multicomponent reactions (MCR). The MCR's occur between three or more reagents in one reaction step only, in the same pot. These reactions have time and energy saving. The niobium pentachloride is strong Lewis acid, which has wide applicabiblity in organic reactions, including multicomponent reactions. Among, the various methods of synthesis of 4H-chromene derivatives, developed in this work, the synthesis by multicomponent reactions between benzaldehyde derivatives, 3,5-dimethoxyphenol and methyl acetoacetate, using niobium pentachloride as the Lewis acid, is described. The reactions were carried out at room temperature, under inert atmosphere (N2), using dichloromethane anhydrous (CH2Cl2) as solvent with a reaction time of 72 hours. The obtained products were isolated by column chromatography on silica gel and subject to analysis of structural characterization, photophysics and photochemistry for a study of their potential use as sensitizing dyes of Gratzel photoelectrochemical devices. Our results show that niobium pentachloride is a good promoter to multicomponent reaction studied, obtaining high yields (62-91%) / Mestre

TiO2 is a fascinating material proving its usefulness in a wide range of applications. In photocatalytic and gas sensitization applications, it has been utilized as a stable semiconductor for the organic matter of waste water cleaning and impetus on environmental consciousness the need for highly efficient combustion engines, controlled industrial processes and monitoring the hazardous gas level in the environment. Nanocrystalline TiO2 modified with Nb incorporation has been produced by LPD technique that deposited at 40 oC with (NH4)2TiF6 for 0.2 M, and 0.6 M for boric acid and a few mount of Nb solution. The deposition rate can be controlled perfectly about 40-50 nm/h. The addition of Nb stabilizes the anatase phase and retards the grain growth during high temperature annealing. XRD and RAMAN spectra of undoped and Nb doped samples show that the undoped one has converted to rutile at 900 oC, meanwhile the doped one remains anatase phase. Nanocrystalline films stabilize at 800 oC with grain size of about 7 nm have successfully been synthesized by the additives of Nb, which appear to be an adequate dopant to improve the photocatalytic and gas sensor performances. The ability of niobium introduction is of retarding the anatase to rutile transformation and hindering its grain growth.