O objetivo deste estudo foi produzir, por electrospinning, fibras de pentóxido de nióbio(Nb2O5), usando como precursor o nióbio metálico. A solução utilizada para o electrospinning foi preparada pela dissolução do precursor em ácido fluorídrico (HF), seguida da adição de ácido acético e da solução polimérica de polivinilpirrolidona (PVP). A solução final foi submetida ao processo de electrospinning com tensão elétrica variando de 14 a 16 kV, distância entre coletor e capilar de 13 cm e fluxo de 1,5 mL/h. As fibras obtidas foram submetidas a tratamento térmico às temperaturas de 600, 700 e 800°C por um período de 1 hora, com taxa de aquecimento de 0,8°C/min. As fibras foram caracterizadas através de análises térmicas, espectroscopia de infravermelho por transformada de Fourier (FTIR), difração de raios X (DRX), microscopia eletrônica de varredura (MEV), microscopia eletrônica de transmissão (MET), espectroscopia de reflectância difusa (ERD). Também foram realizadas medidas de tamanho de cristalito pela fórmula de Scherrer, diâmetro médio com auxílio do aplicativo Image Tool e área superficial pelo modelo proposto por Brunnauer, Emmet e Teller (BET). Os resultados indicaram que foram formadas fibras da fase hexagonal (TT- Nb2O5), e que o aumento da tensão aplicada provocou uma redução no diâmetro das fibras, sendo 90 nm, o menor diâmetro médio obtido para as fibras produzidas aplicando uma tensão de 16 kV e sinterização a 700 ºC. O tamanho de cristalito médio aumentou de 18,48 para 36,08 nm, com o aumento da temperatura de tratamento térmico, resultando em queda da área superficial de 43,6 para 31,3 m2/g. Os valores de band gap medidos variaram de 3,32 a 3,57 eV, indicando que as nanofibras são semicondutores de gap largo. / This study aimed to produce by electrospinning, niobium pentoxide (Nb2O5) fibers, using metallic niobium as precursor. The solution used for electrospinning was prepared by dissolving the precursor in hydrofluoric acid (HF), followed by addition of acetic acid and solution of PVP polymer. The final solution was subjected to the process of electrospinning with voltage ranging from 14 to 16 kV, the distance between collector and capillary was 13 cm and flow of 1.5 mL / h. Fibers obtained were subjected to heat treatment at temperatures of 600, 700 and 800 ° C for a period of 1 hour, at a heating rate of 0.8 °C / min. Finally, Nb2O5 fibers were characterized by thermal analysis, Fourier Transform Infrared Spectroscopy(FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), diffuse reflectance spectroscopy (ERD). The crystallite size was measured by the Scherrer equation, the average diameter was obtained by Image Tool and the surface area by the model proposed by Brunnauer, Emmet and Teller (BET). The results showed that fibers were formed and presented hexagonal phase (TT-Nb2O5), and that the increase of the voltage caused a decrease in fiber diameter, with the smallest average diameter of 90 nm, obtained for fibers produced by applying a voltage of 16 kV and sintering at 700 °C. The average crystallite size increased from 18.48 to 36.08 nm with increasing calcination temperature, resulting in decreased surface area of 43.568 to 31.344 m2 / g. The band gap values measured ranged from 3.32 to 3.57 eV, indicating that the nanofibers are wide band gap semiconductors.

O objetivo deste estudo foi produzir, por electrospinning, fibras de pentóxido de nióbio(Nb2O5), usando como precursor o nióbio metálico. A solução utilizada para o electrospinning foi preparada pela dissolução do precursor em ácido fluorídrico (HF), seguida da adição de ácido acético e da solução polimérica de polivinilpirrolidona (PVP). A solução final foi submetida ao processo de electrospinning com tensão elétrica variando de 14 a 16 kV, distância entre coletor e capilar de 13 cm e fluxo de 1,5 mL/h. As fibras obtidas foram submetidas a tratamento térmico às temperaturas de 600, 700 e 800°C por um período de 1 hora, com taxa de aquecimento de 0,8°C/min. As fibras foram caracterizadas através de análises térmicas, espectroscopia de infravermelho por transformada de Fourier (FTIR), difração de raios X (DRX), microscopia eletrônica de varredura (MEV), microscopia eletrônica de transmissão (MET), espectroscopia de reflectância difusa (ERD). Também foram realizadas medidas de tamanho de cristalito pela fórmula de Scherrer, diâmetro médio com auxílio do aplicativo Image Tool e área superficial pelo modelo proposto por Brunnauer, Emmet e Teller (BET). Os resultados indicaram que foram formadas fibras da fase hexagonal (TT- Nb2O5), e que o aumento da tensão aplicada provocou uma redução no diâmetro das fibras, sendo 90 nm, o menor diâmetro médio obtido para as fibras produzidas aplicando uma tensão de 16 kV e sinterização a 700 ºC. O tamanho de cristalito médio aumentou de 18,48 para 36,08 nm, com o aumento da temperatura de tratamento térmico, resultando em queda da área superficial de 43,6 para 31,3 m2/g. Os valores de band gap medidos variaram de 3,32 a 3,57 eV, indicando que as nanofibras são semicondutores de gap largo. / This study aimed to produce by electrospinning, niobium pentoxide (Nb2O5) fibers, using metallic niobium as precursor. The solution used for electrospinning was prepared by dissolving the precursor in hydrofluoric acid (HF), followed by addition of acetic acid and solution of PVP polymer. The final solution was subjected to the process of electrospinning with voltage ranging from 14 to 16 kV, the distance between collector and capillary was 13 cm and flow of 1.5 mL / h. Fibers obtained were subjected to heat treatment at temperatures of 600, 700 and 800 ° C for a period of 1 hour, at a heating rate of 0.8 °C / min. Finally, Nb2O5 fibers were characterized by thermal analysis, Fourier Transform Infrared Spectroscopy(FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), diffuse reflectance spectroscopy (ERD). The crystallite size was measured by the Scherrer equation, the average diameter was obtained by Image Tool and the surface area by the model proposed by Brunnauer, Emmet and Teller (BET). The results showed that fibers were formed and presented hexagonal phase (TT-Nb2O5), and that the increase of the voltage caused a decrease in fiber diameter, with the smallest average diameter of 90 nm, obtained for fibers produced by applying a voltage of 16 kV and sintering at 700 °C. The average crystallite size increased from 18.48 to 36.08 nm with increasing calcination temperature, resulting in decreased surface area of 43.568 to 31.344 m2 / g. The band gap values measured ranged from 3.32 to 3.57 eV, indicating that the nanofibers are wide band gap semiconductors.

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Made available in DSpace on 2014-10-09T12:55:24Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:07:13Z (GMT). No. of bitstreams: 0 / Este trabalho trata da avaliação da compressibilidade de misturas de pós de nióbio e alumínio, previamente ativadas por moagem de bolas de alta energia. Foram variados o tempo de moagem e a quantidade de ácido esteárico, adicionado como agente controlador de processo (ACP), assim como o tamanho médio de partícula do pó de alumínio. Os pós de partida e as misturas moídas foram caracterizados por microscopia eletrônica de varredura (forma das partículas) e por difração de raios laser (distribuição granulométrica). Uma máquina universal de ensaios foi utilizada nos testes de compressibilidade mediante compactação das amostras numa matriz cilíndrica de aço-ferramenta. Os dados de compressibilidade foram comparados graficamente segundo curvas de densidade (relativa) versus pressão. As diferenças de comportamento na compactação das amostras foram correlacionadas com as diferentes condições de moagem. Houve um decréscimo da compressibilidade com o aumento do tempo de moagem e da quantidade de ACP. Menos compressíveis foram também as misturas provenientes do pó de alumínio com maior tamanho de partícula. Uma nova equação de compactação foi ainda proposta, cujo ajuste aos dados deste trabalho foi superior aos verificados nas equações de modelos anteriormente publicados. / Dissertação (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Dissimilar welding has been subject of several investigations due to its potential importance in various industrial fields such as transportation, energy generation and management. Dissimilar welding can increase the design efficiency, by the use of complementary alloys with different properties, cost cutting and light weighting structures. The use of different materials within a component or structure to best suit a particular task, requirement or increase its life and performance has always been an ambition of several designers and engineers. This project investigated the joining steel and aluminium for the automotive industry and also stainless steel and titanium to be applied in the civil nuclear energy generation industry. These dissimilar metallic combinations are metallurgically incompatible and the formation of brittle intermetallic phases (IMC) need to be controlled or eliminated. To join steel to Al, laser spot welding process was selected, to avoid the bulk melting of steel and Al at the joint interface that enhance the formation of brittle IMC. This part of the work was focused in controlling the joining process to control the IMC formation of galvanized and uncoated steel to Al and verify if it was possible to have a sound and reliable joint in the presence of an IMC layer. In the second part of this study, stainless steel to titanium joining, a different approach was taken with the application of weld metal engineering to modify or eliminate the IMC formation. Several metals were evaluated as potential interlayers to use and laser welding with a Ni interlayer was evaluated with moderate success, due to the modified IMC with improved mechanical properties and the good compatibility between Ni and the stainless steel. A further improvement was achieved when Cu was brazed between stainless steel and Ti using CMT (Cold Metal Transfer) a low heat input MIG process. The final attempt was to use a different interlayer that was 3D printed and deposited in several layers. This interlayer was composed Cu and Nb that were selected as candidates to avoid the IMC formation between the stainless steel and Ti. With this approach it was possible to build an IMC free component and possibly improve and avoid IMC formation in several other dissimilar metallic combinations.

No description available.

This thesis focuses on the development of method to the photochemical synthesis of niobium nanoparticles (NbNP) using Irgacure 907 (I-907) photoinitiator. This investigation is composed of two parts; whereas the mechanistic study of formation of particles was investigated first, and then followed by particles property characterization. By studying the mechanism of formation we were able to obtain knowledge on how to control the size of NbNP. This knowledge provided us with ability to generate a library of nanoparticles with the varying sizes. Furthermore, the study of I-907 photoproducts has given insight for an alternative method for the synthesis of NbNP using a milder reducing agent, 4-(methylthio)benzaldehyde (MSBA). Exposure of NbNP to air causes their oxidation leading to the formation of niobium oxide nanoparticles (NbONP). The oxidation of NbONP was characterized with variety of techniques (XPS, EDS and HRTEM) that demonstrated the core-shell structure of the nanoparticles. These methods indicated that the core is metallic Nb0 and the shell is the niobium oxide, Nb2O5. Since Nb2O5 is known be a strong Brønsted acid, we tested the Brønsted activity of NbONP with pH sensitive dye coumarin-6 (C6) (monitored using fluorescence and UV-vis). The results of these spectroscopic experiments indicated that NbONP can protonate C6, thus serving as confirmation for the acidity of NbONP. Furthermore, particles with varying sizes were tested with C6 to check if the difference in size affects the acidity. It was observed that the particles with the larger sizes have the strongest acidity and the particles of smaller sizes are less acidic.