• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 78
  • 40
  • 25
  • 12
  • 10
  • 8
  • 3
  • 3
  • 2
  • 1
  • 1
  • 1
  • 1
  • 1
  • 1
  • Tagged with
  • 200
  • 65
  • 32
  • 29
  • 23
  • 18
  • 17
  • 17
  • 14
  • 14
  • 13
  • 11
  • 11
  • 11
  • 11
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
131

Intégration et mesures de magnéto-transport de nano-objets magnétiques obtenus par voie chimique

Dugay, Julien 13 December 2012 (has links) (PDF)
L'étude du transport électronique dans des nano-objets métalliques et magnétiques issus de la chimie est un challenge en spintronique. En particulier, le manque de résultats expérimentaux révèle la difficulté à positionner ces nano-objets entre des électrodes de mesures tout en préservant leurs propriétés (magnétisme, intégrité des barrières tunnel organiques...). Ce travail de thèse vise à contourner ces difficultés et à étudier le magnétotransport dans ces systèmes. Pour cela, nous avons conçu et développé à l'intérieur d'une boîte à gants couplée à un bâti de pulvérisation cathodique des systèmes expérimentaux d'assemblages de nano-objets. Nous avons étudié les mécanismes mis en jeu lors de l'assemblage par la technique de dip coating, et réussi à déposer des monocouches de nanoparticules (NPs) de natures différentes (FeCo, Fe, Co) sur des surfaces d'Au, de SiO2 et de résine fine (40 nm). Ces résultats, couplés à une technique de nanoindentation, ont permis de mesurer quelques - voire une- NP(s). Une autre technique, la diélectrophorèse, s'est révélée simple et efficace pour piéger et orienter des nano-objets de taille, de nature, et de forme différentes entre des électrodes. Grâce à cette technique et au dépôt d'une couche protectrice d'alumine, nous avons étudié les propriétés de magnétotransport de plusieurs types de nano-objets sensibles à l'oxydation ou à la vapeur d'eau: NPs de Fe, de Co, FeCo et [Fe(H-trz)2(trz)](BF4)] (composés à transition de spin). Trois jeux de barrières tunnel organiques greffés sur des NPs de fer ont présenté de la magnétorésistance tunnel jusqu'à température ambiante. De plus, des nano-objets de [Fe(H-trz)2(trz)](BF4)] de facteurs de forme variable, ont montré une variation de la conductance liée à la transition de spin. Enfin, nous avons étudié l'influence de la longueur des ligands sur les propriétés de conductions de NPs de Cobalt, qui a validé nos méthodes d'échange de ligands et ont pu être analysées quantitativement. Nos travaux rendent désormais envisageable l'utilisation de NPs issues de la chimie dans différents domaines de la spintronique.
132

Environmental influences on the physiological and behavioural growth responses in salmonids : with reference to the growth-dip phenomenon

Sprague, Matthew January 2006 (has links)
Photoperiod manipulations are widely used throughout the Atlantic salmon (Salmo salar) farming industry as a means of producing a product of uniform quality all-year round. However, farmers still remain sceptical over their effectiveness to regulate growth and maturation during the on-growing stage. Furthermore, reports of a characteristic growth-dip following light exposure suggest that light may negatively affect the physiological performance of fish in the short-term. Thus, this thesis investigates the effects of light characteristics (spectral quality, intensity and photoperiod) on growth and maturation of salmonid fish and addresses some of the uncertainties surrounding photoperiod use currently reported within the industry. Rainbow trout (Oncorhynchus mykiss) are seemingly an ideal model species for examining photoperiod effects on growth. Consequently, the application of constant light exposure (LL) at two different intensities (28W and 16W) during two different thermal conditions (summer and winter) was examined on individually tagged fish. Feed intake and growth appeared to be related to the ambient water temperature and did not appear to be affected by intensity or photoperiod, although the onset of constant light did appear to initially affect growth rate. This may indicate that LL has a limiting effect on the growth of trout or that the prevailing water temperature at which light is applied may override the photoperiodic effect. Furthermore, the lack of enhanced growth in trout exposed to LL, unlike that demonstrated for other salmonids, suggest that there may be a species-specific response to environmental variables. Thus, questions regarding photoperiod effects should be limited to the species in question. The main source of variation in results observed under photoperiod manipulations stems from the salmon industry. Atlantic salmon post-smolts were reared in seawater tanks and either maintained under a natural photoperiod (NP) or exposed to a simulated natural photoperiod (SNP), constant light superimposed on the natural light (NPLL) or constant light only (LL). Artificial light onset, irrespective of photoperiod, resulted in an apparent trend for a reduced appetite lasting up to 60 days. Furthermore, the onset of constant light resulted in a significant chronic elevation of plasma cortisol levels and changes to growth and thyroid hormone levels, providing direct evidence that constant light exposure induces stress. In addition, fish exposed to SNP failed to exhibit a stress response despite a low feed intake. However, differences in the plasma melatonin levels during twilight times, as compared to NP, suggest that gradual changes in the natural light intensity throughout the day, particularly around dawn and dusk, may be important for synchronizing daily events. No differences in growth were observed between the NP and NPLL regimes, although fish reared in an enclosed regime (SNP and LL) exhibited a significantly lower weight gain than fish in an open environment (NP and NPLL). This further highlights the impact that the rearing environment has on the growth performances of fish and the need for commercially run trials. Advances in lighting technologies and a greater understanding of how light is transformed through the water column have focussed research on the spectral sensitivity of fish. Therefore the lighting efficiency of novel blue narrow bandwidth LED lighting units through the water column and their effects on growth and maturation performances of salmon reared in commercial production cages were compared against the standard metal halide units currently utilized throughout the industry. LL application, irrespective of intensity or spectrum, reduced the numbers of fish maturing as compared to fish reared under a natural photoperiod. However, this was greatest under the standard metal halide units reflecting a greater light penetration and perception as determined by plasma melatonin levels. The metal halide groups exhibited the greatest relative weight gain over the trial period as compared to control fish. No evidence was observed for a growth-dip under metal halide light, although blue lit treatments exhibited an initial significant reduction in food consumption, suggesting a possible welfare issue. Nevertheless, the prototype blue LED units showed possible potential for commercial application by penetrating the water depth at half the distance of the metal halide units for only one eighth the power and one fifth the brightness. However, further tests of these prototype spectral units are required to examine the potential welfare and physiological growth and reproductive effects. These studies have shown that the efficacy of artificial light regimes is largely dependent upon the effectiveness of the light source through the underwater environment and its perception by fish, providing a sufficient intensity is emitted exceeding the physiological threshold level for the species cultured. Moreover, whilst the onset of artificial light may elicit a stress response and demonstrate a trend for a suppression of appetite for salmon reared in experimental tanks, no compelling evidence for a suppression of appetite or growth was found under normal commercial cage conditions. This suggests that the growth-dip observed within the industry may in part be a combination of a physiological response to the onset of light further exaggerated by the farmer’s perception and altered judgement in feeding. In addition, the results obtained from this study have helped to standardize the use of light regimes within the industry. Nevertheless, further studies are necessary to fully elucidate the underlying mechanisms which may govern growth and maturation in fish following the onset of light exposure.
133

Uma Abordagem para Detecção Automática de Planos em Modelos Digitais de Afloramentos Baseada em PCA

Gomes, Róbson Koszeniewski 19 September 2014 (has links)
Submitted by Nara Lays Domingues Viana Oliveira (naradv) on 2015-07-15T18:16:55Z No. of bitstreams: 1 ROBSON.pdf: 2705073 bytes, checksum: d6eefccd3ecb288572c9262f7dc4079a (MD5) / Made available in DSpace on 2015-07-15T18:16:55Z (GMT). No. of bitstreams: 1 ROBSON.pdf: 2705073 bytes, checksum: d6eefccd3ecb288572c9262f7dc4079a (MD5) Previous issue date: 2014 / PROCERGS - Companhia de Processamento Dados do Estado Rio Grande Sul / A coleta de dados espaciais tem sido intensamente empregada na área geológica, através da técnica de LIDAR (Light Detection and Ranging). Este tipo de sensoriamento digital remoto de alta resolução e precisão, resulta em modelos digitais 3D que permitem uma análise mais detalhada e quantitativa de estruturas heterôgeneas, como afloramentos. Um dos estudos realizados pelos geólogos são análises sobre a geometria da formação de rochas, onde a informação de orientação de um plano inclinado é um indicativo para a compreensão global da estrutura. Este trabalho propõe a utilização da técnica de Análise de Componentes Principais (PCA) para calcular e detectar automaticamente todos os planos em uma nuvem de pontos. Uma ferramenta foi construída para implementar a visualização do modelo digital e apurar os melhores planos. Um estudo foi realizado a fim de validar as informações encontradas pelo método proposto e dados medidos em campo. / The use of LIDAR (Light Detection and Ranging) systems for gathering spatial data has been extensively used in geological studies. This type of digital remote sensing delivers high resolution and accuracy, resulting in 3D digital models which allow a more detailed and quantitative analysis of heterogeneous structures, as outcrops. One of the studies is based on analysis of the forming rocks geometry. The orientation of a slope plane is an indication for the overall undestanding of the structure. This work proposes a new method to automatically compute and detect all possible planes in a point cloud, based on Principal Component Analysis (PCA) technique. A software tool was constructed to implement the digital model visualization and compute the best planes. A study was conducted to compare and validate the results of the method and the field data collected.
134

Textil fotokatalytisk vattenreningsreaktor : Optimering av beredningsparametrar för titandioxidbeläggning på polyestersubstrat ämnad för vattenrening / Textile photocatalytic water purifying system : Optimization of the processing parameters of the titanium dioxide coating on the polyester substrates designed for water purification

Helenius, Emma January 2012 (has links)
I denna kandidatuppsats avlägsnas organiska föroreningar från vatten genom användning av textilier, vars yta belagts med nanopartiklar utav ämnet titandioxid. När titandioxid belyses med UV-strålning sker en fotokatalytisk reaktion, vilket i sin tur bryter ned bland annat bakterier och organiska färgämnen. I hopp om att finna ultimat vidhäftning mot textil passande titandioxid har olika typer av beläggningar studerats; akrylbaserade, sol-gel och slutligen en hybrid. Dessutom har fem olika konstruktioner av textila bindningar jämförts i syfte att undersöka dess påverkan på den fotokatalytiska effekten. Förorenade vatten är ett stort problem, både i utvecklingsländer och i industriella sammanhang. Att använda en textil fr att rena nedsmutsat vatten är en lockande tanke, speciellt attraktivt är förmågan att dra nytta av endast solenergi. En av fördelarna med textilier inom området är möjligheten att tillverka stora tygareor på ett enkelt och kostnadseffektivt vis.Elementär analys visade att TiO2-partikellager på textilierna fastande i olika grad beroende på beläggningsteknik. Avfärgning genom nedbrytning av metylenblått studerades under sollampa. Proverna utvärderades genom att vid bestämda tidpunkter avläsa kromavärdet på färglösningen. För att erhålla mer information om textilernas effekt gjordes även en bakterieanalys, samt bildtagning i ytterlig förstorning med SEM. Resultaten visade goda fotokatalytiska egenskaper för samtliga beläggningar, däribland är akryl- och hybridbeläggningarna mest lovande. Utöver det visade de textila bindningarna på substratens konstruktioner ha en påverkan på den vattenrenande effekten.In this bachelor essay work organic contaminants are removed from water using textiles, whose surface is coated with nanoparticles of titanium dioxide. When titanium dioxide is illuminated with UV radiation a photocatalytic reaction occurs, which in turn degrades substances such as bacteria and organic dyes. In hope of finding the ultimate adhesion for a textile coating suitable with titanium dioxide, various types of coatings are studies; acrylicbased, sol-gel and finally a hybrid coating. In addition, five different structures of textiles are compared in order to examine their different outcome in photocatalytic effect.Contaminated water is a major problem in both developing countries and industrial context. The use of a fabric to purify polluted water is an attractive idea, especially because the ability to exploit only solar energy. One of the benefits with use of fabric is the ability to producelarge areas of fabric, a process which is both simple and costeffective.Elemental analyses showed varying degree of fasten of the particle layer of TiO2 depending on the coating technology used. Decolorization by degradation of methylene blue was studied under a solar energy AM 1,5 lamp. The samples were evaluated by reading the chroma value of the color solution at specified moments. To obtain more information the coated textiles went through bacterial analysis, as well as picture shooting in extreme magnification with SEM. The results showed good photocatalytic properties for all coatings, among them were the acrylic coating and the hybrid most promising. In addition, results showed that depending on textile structures different effects in water purification was achieved. / Program: Textilingenjörsutbildningen
135

MARTE based model driven design methodology for targeting dynamically reconfigurable FPGA based SoCs

Quadri, Imran Rafiq 20 April 2010 (has links) (PDF)
Les travaux présentés dans cette thèse sont effectuées dans le cadre des Systèmes sur puce (SoC, Systemon Chip) et la conception de systèmes embarqués en temps réel, notamment dédiés au domaine de la reconfiguration dynamique, liés à ces systèmes complexes. Dans ce travail, nous présentons un nouveau flot de conception basé sur l'Ingénierie Dirigée par les Modèles (IDM/MDE) et le profilMARTE pour la conception conjointe du SoC, la spécification et la mise en oeuvre de ces systèmes sur puce reconfigurables, afin d'élever les niveaux d'abstraction et de réduire la complexité du système. La première contribution relative à cette thèse est l'identification des parties de systèmes sur puce reconfigurable dynamiquement qui peuvent être modélisées au niveau d'abstraction élevé. Cette thèse adapte une approche dirigée par l'application et cible les modèles d'application de haut niveau pour être traités comme des régions dynamiques des SoCs reconfigurables. Nous proposons aussi des modèles de contrôle générique pour la gestion de ces régions au cours de l'exécution en temps réel. Bien que cette sémantique puisse être introduite à différents niveaux d'abstraction d'un environnent pour la conception conjointe du SoC, nous insistons tout particulièrement sur sa fusion au niveau du déploiement, qui relie la propriété intellectuelle avec les éléments modélisés à haut niveau de conception. En outre, ces concepts ont été intégrés dans le méta-modèleMARTE et le profil correspondant afin de fournir une extension adéquate pour exprimer les caractéristiques de reconfiguration à la modélisation de haut niveau. La seconde contribution est la proposition d'un méta-modèle intermédiaire, qui isole les concepts présents au niveau transfert de registre (RTL-Register Transfer Level). Ce méta-modèle intègre les concepts chargés de l'exécution matérielle des applications modélisées, tout en enrichissant la sémantique de contrôle, provoquant la création d'un accélérateur matériel reconfigurable dynamiquement avec plusieurs implémentations disponibles. Enfin, en utilisant les transformations de modèlesMDE et les principes correspondants, nous sommes en mesure de générer des codeHDL équivalents à différentes implémentations de l'accélérateur reconfigurable ainsi que différents codes source en langage C/C++ liés au contrôleur de reconfiguration, qui est finalement responsable de la commutation entre les différentes mplémentations. Enfin, notre flot de conception a été vérifié avec succès dans une étude de cas liée à un système anti-radar de détection de collision. Une composante clé intégrante de ce système a été modélisée en utilisant les spécifications MARTE étendu et le code généré a été utilisé dans la conception et la mise en oeuvre d'un SoC sur un FPGA reconfigurable dynamiquement.
136

Seismic-Reflection and Seismic-Refraction Imaging of the South Portuguese Zone Fold-and-Thrust Belt

Schmelzbach, Cedric January 2007 (has links)
The South Portuguese Zone (SPZ), which host world-class massive sulphide deposits, forms the southern fold-and-thrust belt of the Iberian Variscan orogeny. This thesis focuses on seismic-reflection and seismic-refraction processing efforts on a subset of the IBERSEIS deep seismic-reflection data set aiming at resolving the SPZ upper crust in high resolution. A comparison of different crooked-line seismic-reflection imaging schemes showed that a processing sequence involving dip-moveout corrections, a common-midpoint projection, and poststack time migration of common-offset gathers provided the most coherent images considering the crooked acquisition geometry. Correlation with surface-geological data allows four units of different reflection character to be identified: the ~0–2 km deep Upper Carboniferous Flysch group, the highly reflective ~2–4 km thick and up to ~5 km deep Volcano-Sedimentary Complex (VSC) group, and two deep Paleozoic metasedimentary units, with the shallower Phyllite-Quartzite group exposed in an antiform. Prominent diffracted energy was enhanced using a modified Kirchhoff imaging routine. High reflectivity and distinct diffractions mark extensive dike bands at 6–12 km depth, possibly related to the intense hydrothermal activity that led to the formation of the ore-bearing VSC group. Source-generated noise obscures potential signals from depths shallower than ~500m depth on the seismic-reflection sections. P- and SV-wave first-arrival traveltimes were inverted for velocity models imaging the shallowest crust. Overall, the velocity models correlate well with surface-geological data marking high (>5.25 km/s) and uniform P-velocities for the Flysch unit in the southern SPZ. A prominent P-wave low-velocity body (~4.5 km/s) is resolved where the Phyllite-Quartzite unit forms the core of an antiform. P-velocities fluctuate the most in the northern SPZ with Flysch group units exhibiting high velocities (>5.25 km/s) and VSC group bodies showing intermediate velocities (~5 km/s). Low VP/VS-ratios (~1.8) computed for the southern profile part are interpreted as less deformed Flysch-group units, whereas high VP/VS-ratios (~1.9) indicate fractured units.
137

Optical Characterization and Energy Simulation of Glazing for High-Performance Windows / Optisk karakterisering och energisimulering av smarta fönster

Jonsson, Andreas January 2009 (has links)
This thesis focuses on one important component of the energy system - the window. Windows are installed in buildings mainly to create visual contact with the surroundings and to let in daylight, and should also be heat and sound insulating. This thesis covers four important aspects of windows: antireflection and switchable coatings, energy simulations and optical measurements. Energy simulations have been used to compare different windows and also to estimate the performance of smart or switchable windows, whose transmittance can be regulated. The results from this thesis show the potential of the emerging technology of smart windows, not only from a daylight and an energy perspective, but also for comfort and well-being. The importance of a well functioning control system for such windows, is pointed out. To fulfill all requirements of modern windows, they often have two or more panes. Each glass surface leads to reflection of light and therefore less daylight is transmitted. It is therefore of interest to find ways to increase the transmittance. In this thesis antireflection coatings, similar to those found on eye-glasses and LCD screens, have been investigated. For large area applications such as windows, it is necessary to use techniques which can easily be adapted to large scale manufacturing at low cost. Such a technique is dip-coating in a sol-gel of porous silica. Antireflection coatings have been deposited on glass and plastic materials to study both visual and energy performance and it has been shown that antireflection coatings increase the transmittance of windows without negatively affecting the thermal insulation and the energy efficiency. Optical measurements are important for quantifying product properties for comparisons and evaluations. It is important that new measurement routines are simple and applicable to standard commercial instruments. Different systematic error sources for optical measurements of patterned light diffusing samples using spectrophotometers with integrating spheres have been investigated and some suggestions are made for how to avoid such errors.
138

Voltage Source Converters with Energy Storage Capability

Xie, Hailian January 2006 (has links)
<p>This project deals with voltage source converters with energy storage capability. The main objective is to study the possible benefits of energy storage to a power system with a VSC as the interface between them.</p><p>First of all, a converter control system is proposed for a two level VSC. In the conventional converter control, the control system usually takes the voltage measured at the point where the converter is connected and calculates the reference voltage for the converter; with a modulation system the converter then produces the required 'average voltage'. In this project, a novel flux modulation scheme, combined with the deadbeat current control strategy, is proposed. The current controller is capable of controlling both positive and negative sequence current components. With flux modulation, the control system measures the bus flux and commands the converter to generate the required flux.</p><p>Based on the proposed control strategies, several application studies have been carried out.</p><p>The first application study investigates the effect of energy storage on the power quality at the point of common coupling when a system is subject to load disturbances. The voltage at PCC in a weak network is very sensitive to load changes. A sudden change in active load will cause both a phase jump and a magnitude fluctuation in the bus voltage, whereas reactive load changes mainly affect the voltage magnitude. With the addition of energy storage to a StatCom, it is possible to compensate for the active power change as well as providing reactive power support. In this thesis, some effective active power compensation schemes are proposed. Simulations and experiments have been performed to verify the compensation schemes. The results show that a StatCom with energy storage can significantly reduce phase jumps and magnitude deviations of the bus voltage.</p><p>pact of the energy storage on the performance of weak systems under fault conditions has been investigated. The investigation was done by studying an example system. The system model was established based on a real system, in which some induction motors driving pumps along a pipeline are fed from a radial transmission line. Studies show that for a weak system with induction motor loads, a StatCom with certain energy storage capacity will effectively improve the system recovery after faults. Although this incurs extra cost for the increasing dc voltage rating and size of the dc side capacitor, the overall rating of the converter can be reduced by utilization of the proposed active power compensation scheme.</p><p>The last case study investigates the possible use of a StatCom with energy storage to improve the power quality at the point of common coupling where a cyclic load is connected. Studies show that by providing both fast reactive and fast active power support to the network, not only the voltage magnitude can be well controlled, but also the voltage phase jump can be reduced significantly.</p>
139

Utilisation des résonateurs piézoélectriques fonctionnant en mode d'épaisseur pour la réalisation de capteurs

Ivan, Ioan Alexandru 28 November 2006 (has links) (PDF)
Les capteurs participent à la qualité de la vie quotidienne, beaucoup de domaines d'activité modernes demandant des capteurs performants. Les résonateurs piézoélectriques sont bien connus pour leurs applications pour la génération de fréquences stables, mais leur potentiel d'utilisation pour la réalisation de capteurs est moins exploité.Le mémoire présente deux applications des résonateurs pour la réalisation de capteurs de force et de température.La particularité du capteur de force tient au fait que le résonateur lui-même sert aussi comme sonde de température et que les signaux servant aux mesures de la force et de la température sont isolés électriquement, ce qui simplifie la mise en œuvre. Cela est possible par l'utilisation de modes anharmoniques des résonateurs et par le choix de la direction d'application de la force à mesurer, chaque mode étant essentiellement sensible soit à la force soit à la température. Le mémoire présente en particulier une méthode de dessin de jeux d'électrodes pour un résonateur plan-convexe opérant sur deux modes de cisaillement d'épaisseur (modes B et C).La seconde partie du mémoire est consacrée à la caractérisation expérimentale (topographies par rayon X, réponses électriques et comportements thermiques) de capteurs résonants thermosensibles en quartz et en GaPO4 miniaturisés pour la mesure précise de la température sur une très large gamme jusqu'aux températures cryogéniques. L'étude a en particulier porté sur des investigations des couplages entre modes, responsables du phénomène «activity-dip».Un système électronique complet et un logiciel de calcul de la force et de la température à partir des fréquences délivrées est développé
140

Síntese de manganita dopada com estrôncio e cobaltita dopada com antimônio e caracterização microestrutural

Tarragó, Diego Pereira January 2012 (has links)
A grande variedade de propriedades dos óxidos com estrutura perovskita permite sua aplicação em diversas áreas da tecnologia. Em especial, as propriedades eletroquímicas e a estabilidade térmica de alguns destes compostos faz destes materiais os mais usados em cátodos de dispositivos para geração de energia como as células a combustível de óxido sólido (SOFC). As SOFC são dispositivos que podem atingir altas eficiências na conversão de energia elétrica e são passíveis de utilização na geração de energia sustentável e distribuída. Para estas células, as perovskitas compostas de manganita de lantânio dopada com estrôncio (LSM) preenche a maioria dos requisitos para aplicação como cátodos da SOFC. Nas células a combustível de óxido sólido de temperatura intermediária (IT-SOFC) há a possibilidade de construir a célula com filmes finos dos materiais cerâmicos suportados em um interconector metálico. No entanto, para os cátodos das IT-SOFC as manganitas não possuem bom desempenho eletroquímico devido à menor temperatura de operação do dispositivo, sendo necessário o desenvolvimento de condutores mistos para aumentar os sítios reativos para reação de redução do O2. Assim, o desenvolvimento de cátodos utilizando a perovskita cobaltita de estrôncio dopada com antimônio (SCS) tem sido proposto para esta aplicação. Portanto, a execução deste trabalho visou à obtenção tanto da LSM quanto da SCS que são materiais para SOFC e IT-SOFC, respectivamente. A LSM também foi avaliada frente a condições de processamento e sinterização. Os pós de LSM foram obtidos através da síntese por combustão, variando a quantidade e tipo de combustível atentando aos aspectos morfológicos resultantes dos parâmetros selecionados. O método de sol-gel foi testado para obtenção da LSM, para fins comparativos. O método de combustão também foi utilizado para obter a SCS, observando a influência do tempo de calcinação sobre a formação das fases. Os pós das manganitas que apresentaram as características mais promissoras foram depositados sobre substratos densos de zircônia estabilizada com ítria (YSZ), na forma de filmes finos pela técnica de dip coating. A perovskita LSM pôde ser obtida de forma monofásica após calcinação e o resultado final da formação da estrutura romboédrica não foi alterado em função do excesso e do tipo de combustível ou do método de síntese utilizado. Agregados com morfologia distinta, formados por partículas nanocristalinas com tamanho médio de aproximadamente 30 nm, foram observados quando a sacarose foi utilizada como combustível e também quando misturada na solução precursora com ureia, formando cristalitos com tamanho médio próximo a 20 nm. A sacarose também promoveu a obtenção do pó de maior área superficial específica (34,9 m²/g) que apresentou taxas de sinterização mais elevadas que os outros pós. As deposições da LSM sobre a YSZ formou filmes contínuos quando os substratos utilizados possuíam rugosidade intermediária e também quando as deposições iniciais foram feitas utilizando baixas velocidades de retirada do banho. O uso de uma solução à base de água destilada com alto teor de ligantes promoveu a obtenção de um filme fino de 3 μm e com uma microestrutura porosa. A síntese por combustão também possibilitou a obtenção da perovskita SCS, no entanto após 6 horas de calcinação o pó ainda apresentou fases secundárias e maiores tempos podem ser necessários para a obtenção de um material monofásico. O alto calor da síntese do pó de SCS formou agregados densos, porém nanocristalinos. / The great variety of properties of oxides with the perovskite structure allows their application in several fields of technology. Specially, the electrochemical properties and the thermal stability of some of these compounds make these materials the most used in devices for energy generation such as the solid oxide fuel cells (SOFC). SOFC’s are devices the reach high efficiencies in the conversion of electric energy and are likely to be used in sustainable and distributed energy generation. For this cells, perovskitas composed by strontium doped lanthanum manganites (LSM) is the material that fits most of requirements for the application in SOFC’s cathodes. In the intermediate temperature solid oxide fuel cells (IT-SOFC) there is the possibility to build a cell with thin ceramic films supported in a metallic interconnector. However, for IT-SOFC’s cathodes, LSM no longer has a good electrochemical performance due to the device’s lower operating temperatures, being necessary the development of mixed conductors to increase the amount of reactive sites for the reduction of O2. In this context, the development of cathodes based on antimony doped strontium cobaltites (SCS) perovskites is being proposed to this application. This work was executed looking for the obtaining of LSM, a material for SOFC cathodes, and to evaluate its behavior against processing conditions; and for the obtaining of SCS, a material for IT-SOFC cathodes. LSM powders were obtained by combustion synthesis, varying the amount and type of fuel, and observing the resulting morphological aspect conferred by the selected parameters. For comparison, the obtaining of LSM powders was carried out via the sol-gel method. The combustion method was also used for the obtaining of SCS, watching for the calcination time. LSM powders were processed and powder with most promising characteristics was used in dispersions for dip coating of dense YSZ substrates. The LSM perovskite was obtained as a single phase powder after calcination without influence of the fuel excess, fuel type or synthesis method, in the final formation of the rhombohedral structure. Aggregates with distinct morphology, formed by nanocrystalline particles with average size around 30 nm, were observed when sucrose was used as fuel and also when mixed with urea in the precursor solution, which formed crystallites with average size near 20 nm. The use of sucrose also promoted the obtaining of the powder with the higher specific surface area (34,9 m²/g) which presented sintering rates higher than the other powders. When not too rough, the YSZ substrates had more continuous films formed in its surface and also when the first depositions were made with the substrates slowly emerging from the bath. The use of a solution based on distilled water with higher amount of binder, promoted the obtaining of films as thin as 3 μm and with a porous microstructure. Combustion synthesis also led to the obtaining of the SCS perovskite, however after 6 hours calcination the powder still presented secondary phases and longer calcination periods may be necessary for the obtaining of a single phase material. The high heat of the SCS synthesis reaction formed dense, yet nanocrystalline aggregates.

Page generated in 0.0343 seconds