Submitted by Márcio Maia (marciokjmaia@gmail.com) on 2016-08-08T11:50:47Z No. of bitstreams: 1 arquivototal.pdf: 4720590 bytes, checksum: c0e901cb90be3c102d0bd3f7a4bce0bd (MD5) / Made available in DSpace on 2016-08-08T11:50:47Z (GMT). No. of bitstreams: 1 arquivototal.pdf: 4720590 bytes, checksum: c0e901cb90be3c102d0bd3f7a4bce0bd (MD5) Previous issue date: 2016-02-04 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / Conselho Nacional de Pesquisa e Desenvolvimento Científico e Tecnológico - CNPq / The development of new materials and technologies have been shown to be very important when the goal is to make solar energy will be more economically attractive. One way to promote this attractiveness is increasing the efficiency of solar collectors, which can be done by use of so-called selective surfaces, or deposited coatings on absorbing plate solar radiation, which allow a maximum of absorbance in the solar spectrum while maintaining a minimum emittance in the infrared. Absorbers for producing films, it is necessary to use materials technology, energy and environmental efficiency, such as inorganic polymers (geopolymers). In the Earth's tropical regions, such as the state of Paraiba, there are materials with potential use for the synthesis of inorganic polymers that have not been evaluated, the Sugar Cane Bagasse Ash (SCBA) is one of them. So that this study sought to assess the SCBA's potential use as raw material for production of solar absorbing films. Through the use of two deposition techniques in three types of substrates, it was concluded that the SCBA has intrinsic selectivity, having potential use for the production of solar absorbing films. So that the best result set (film + substrate) exhibited peak and average absorption of respectively 88% and 65%. This surface was obtained from the SCBA.5.250 diluted in 10 g of H2O deposited by painting techniques on the glass substrate. / O desenvolvimento de novos materiais e tecnologias tem se mostrado de extrema importância quando o objetivo é fazer com que a energia solar venha a ser mais atrativa economicamente. Um dos modos de promover essa atratividade é aumentando a eficiência dos coletores solares, o que pode ser feito por meio da utilização das chamadas superfícies seletivas, ou seja, revestimentos depositados sobre placas absorvedoras de radiação solar, que permitem um máximo de absortância no espectro solar enquanto mantêm um mínimo de emitância no infravermelho. Para a produção de filmes absorvedores, faz-se necessário a utilização de materiais com eficiência tecnológica, energética e ambiental, tais como os polímeros inorgânicos (geopolímeros). Nas regiões tropicais da Terra, como é o caso do Estado da Paraíba, há materiais com potencial de utilização para a síntese de polímeros inorgânicos que ainda não foram avaliados, a Cinza do Bagaço da Cana-de-açúcar (CBC) é um deles. De forma que o presente trabalho procurou avaliar o potencial de uso da CBC como matéria-prima para produção de filmes absorvedores solares. Através da utilização de duas técnicas de deposição em três tipos de substratos, concluiu-se que a CBC apresenta seletividade intrínseca, tendo potencial de uso para produção de filmes absorvedores solares. De maneira que o melhor resultado do conjunto (filme+substrato) exibiu pico e média de absorção de, respectivamente, 88% e 65%. Essa superfície foi obtida a partir da CBC.5.250 diluída em 10 g de H2O depositada pela técnica de Pintura sobre o substrato de vidro.

La tomographie par rayons X multi-énergétiques (MECT) permet d'obtenir plus d'information concernant la structure interne de l'objet par rapport au scanner CT classique. Un de ses intérêts dans l’imagerie médicale est d'obtenir les images de fractions d’eau et d’os. Dans l'état de l'art, les intérêts de MECT n'est pas encore découvert largement. Les approches de reconstruction existantes sont très limitées dans leurs performances. L'objectif principal de ce travail est de proposer des approches de reconstruction de haute qualité qui pourront être utilisés dans la MECT afin d’améliorer la qualité d’imagerie.Ce travail propose deux approches de reconstruction bayésiennes. La première est adaptée au système avec un détecteur discriminant en énergie. Dans cette approche, nous considérons que les polychromaticités de faisceaux sont négligeables. En utilisant le modèle linéaire de la variance et la méthode d'estimation maximum à postériori (MAP), l'approche que nous avons proposé permets de prendre en compte les différents niveaux de bruit présentés sur les mesures multi-énergétiques. Les résultats des simulations montrent que, dans l'imagerie médicale, les mesures biénergies sont suffisantes pour obtenir les fractions de l'eau et de l'os en utilisant l'approche proposée. Des mesures à la troisième énergie est nécessaire uniquement lorsque l'objet contient des matériaux lourdes. Par exemple, l’acier et l'iode. La deuxième approche est proposée pour les systèmes où les mesures multi-énergétiques sont obtenues avec des faisceaux polychromatiques. C'est effectivement la plupart des cas dans l'état actuel du practice. Cette approche est basée sur un modèle direct non-linéaire et un modèle bruit gaussien où la variance est inconnue. En utilisant l’inférence bayésienne, les fractions de matériaux de base et de la variance d'observation pourraient être estimées à l'aide de l'estimateur conjoint de MAP. Sous réserve à un modèle a priori Dirac attribué à la variance, le problème d'estimation conjointe est transformé en un problème d'optimisation avec une fonction du coût non-quadratique. Pour le résoudre, l'utilisation d'un algorithme de gradient conjugué non-linéaire avec le pas de descente quasi-optimale est proposée.La performance de l'approche proposée est analysée avec des données simulées et expérimentales. Les résultats montrent que l'approche proposée est robuste au bruit et aux matériaux. Par rapport aux approches existantes, l'approche proposée présente des avantages sur la qualité de reconstruction. / Multi-Energy Computed Tomography (MECT) makes it possible to get multiple fractions of basis materials without segmentation. In medical application, one is the soft-tissue equivalent water fraction and the other is the hard-matter equivalent bone fraction. Practical MECT measurements are usually obtained with polychromatic X-ray beams. Existing reconstruction approaches based on linear forward models without counting the beam polychromaticity fail to estimate the correct decomposition fractions and result in Beam-Hardening Artifacts (BHA). The existing BHA correction approaches either need to refer to calibration measurements or suffer from the noise amplification caused by the negative-log pre-processing and the water and bone separation problem. To overcome these problems, statistical DECT reconstruction approaches based on non-linear forward models counting the beam polychromaticity show great potential for giving accurate fraction images.This work proposes a full-spectral Bayesian reconstruction approach which allows the reconstruction of high quality fraction images from ordinary polychromatic measurements. This approach is based on a Gaussian noise model with unknown variance assigned directly to the projections without taking negative-log. Referring to Bayesian inferences, the decomposition fractions and observation variance are estimated by using the joint Maximum A Posteriori (MAP) estimation method. Subject to an adaptive prior model assigned to the variance, the joint estimation problem is then simplified into a single estimation problem. It transforms the joint MAP estimation problem into a minimization problem with a non-quadratic cost function. To solve it, the use of a monotone Conjugate Gradient (CG) algorithm with suboptimal descent steps is proposed.The performances of the proposed approach are analyzed with both simulated and experimental data. The results show that the proposed Bayesian approach is robust to noise and materials. It is also necessary to have the accurate spectrum information about the source-detector system. When dealing with experimental data, the spectrum can be predicted by a Monte Carlo simulator. For a variety of materials, less than 5% estimation errors are observed on their average decomposition fractions.The proposed approach is a statistical reconstruction approach based on a non-linear forward model counting the full beam polychromaticity and applied directly to the projections without taking negative-log. Compared to the approaches based on linear forward models and the BHA correction approaches, it has advantages in noise robustness and reconstruction accuracy.

A ratchet is an asymmetric, non-equilibrated system that can produce a directed current of particles without the need for macroscopic potential gradients. In rocked quantum electron ratchets, tunnelling and wave-reflection can induce reversals in the direction of the net current as a function of system parameters. An asymmetric quantum point contact in a GaAs/GaAlAs heterostructure has been studied experimentally as a realisation of a quantum electron ratchet. A Landauer model predicts reversals in the direction of the net current as a function of temperature, amplitude of the rocking voltage, and Fermi energy. Artifacts such as circuit-induced asymmetry, also known as self-gating, were carefully removed from the experimental data, which showed net current and net differential conductance reversals, as predicted by the model. The model also predicts the existence of a heat current where the net electron current changes sign, as equal numbers of high and low energy electrons are pumped in opposite directions. An idealised quantum electron ratchet is studied analytically as an energy selective electron heat engine and refrigerator. The hypothetical device considered consists of two electron reservoirs with different temperatures and Fermi energies. The reservoirs are linked via a resonant state in a quantum dot, which functions as an idealised energy filter for electrons. The efficiency of the device approaches the Carnot value when the energy transmitted by the filter is tuned to that where the Fermi distributions in the reservoirs are equal. The maximum power regime, where the filter transmits all electrons that contribute positively to the power, is also examined. Analytic expressions are obtained for the power and efficiency of the idealised device as both a heat engine and as a refrigerator in this regime of operation. The expressions depend on the ratio of the voltage to the difference in temperature of the reservoirs, and on the ratio of the reservoir temperatures. The energy selective electron heat engine is shown to be non-endoreversible, and to operate in an analogous manner to the three-level amplifier, a laser based quantum heat engine. Implications for improving the efficiency of thermionic refrigerators and power generators are discussed.