Ce travail de thèse porte sur l'étude de l'intégration des fonctions d'amortissement vibroacoustique dans des plaques en matériau composite. Dans un premier temps, pour une méta-plaque non composite, les caractéristiques d'atténuation des vibrations ont été étudiées avec un réseau périodique de résonateurs (poutre-masse) attachés à une plaque.L'effet de périodicité et des résonances locales permet de bloquer la propagation d'ondes sur des plages de fréquences données, appelée bandes interdites. Des simulations numériques basées sur la MEF ont permis de calculer les diagrammes de bandes des plaques périodiques et d'analyser la sensibilité de ces bandes interdites aux différents paramètres de conception de méta-plaque. Les calculs de coefficient de transmission d'une onde plane selon la direction perpendiculaire à la zone traitée, mettent en évidence la présence des bandes interdites prédites par le diagramme de bande. Les réponses fréquentielles de plusieurs modèles sont comparées, pour mettre en évidence les écarts entre une structure infinie et une structure finie. Une méthode automatique originale est mise au point pour détecter les gammes de fréquences des bandes interdites pour une méta-plaque finie.Ces méthodes théorique et expérimentale sont alors appliquées à des plaques composites SMC avec des perforations périodiques. Des essais avec une excitation unidirectionnelle et une excitation ponctuelle sont réalisés. L'influence de chaque type de sollicitation sur la réponse vibratoire des plaques est analysée pour mettre en évidence la détectabilité des bandes interdites. Enfin, la robustesse des bandes interdites aux variations du réseau périodique est validée. / The vibration attenuation characteristics of a metamaterial plate were investigated theoretically and experimentally with a 2-dimensional periodic array of resonators (mass-beam) attached to a thin homogeneous plate.The sensitivity analysis of the band gap frequency range took into account the uncertainties of all the design parameters of the metamaterial plate. The theoretical approach used the finite element method (FEM) to compare the predicted band gaps with those derived from infinite and finite models of the metamaterial.An original automatic method is proposed to detect the frequency ranges of band gaps in finite metamaterial based on the behavior of the corresponding bare plate. Directional plane wave excitation and point force excitation were applied to evaluate the efficiency of the detection method. The results of these analyses were compared with experimental measurements. Frequency ranges of experimental vibration attenuation are in good agreement with the theoretically predicted complete and directional band gaps.These theoretical and experimental methods are then applied to SMC (Sheet Molding Compound) composite plates with periodic perforations. Tests with unidirectional excitation and point force excitation are performed. The influence of each type of excitation on the vibratory response of the plates is analyzed in order to demonstrate the detectability of the bands gaps.Finally, the robustness of the band gap to the variations of the periodic lattice is validated following an integration of perturbations: addition of local mass on half of the unit cells according to a periodic or random positions.

For the past few decades, a frenzy of attention has been given towards the development of an assortment of photocatalysts as a solution for various environmental problems. TiO2 is the most widely used photocatalyst. TiO2 is biocompatible, chemically and thermally stable but TiO2 and a vast majority other photocatalysts have large band gaps, and hence they find applicability only in the UV region of the solar spectrum. These large band gap photocatalysts suffer a severe limitation with regard to their overall process efficiency as the UV region contributes to about 3 to 4 % of the solar spectrum in terms of energy. This thesis concentrates on the progress towards the generation of visible light active photocatalysts. Lanthanum transition metal perovskites were synthesized to incorporate B site doping in the following fashion; LaCrxFe1-xO3, LaMnxFe1-xO3 and LaCrxMn1-xO3 (x= {1, 0.25, 0.5, 0.75}). These perovskites configurations were selected as LaCrO3 has a conduction band edge suitable to activate most photocatalytic reactions, but LaCrO3’s large band gap energetically hinders the photocatalysis. Doping LaCrO3 with Fe and Mn allowed for tuning their band gaps and made various photocatalytic reactions feasible, namely CO2 reduction and photoassisted dye degradation. Validation of the perovskite's crystal structure was established through the study of their XRD patterns. The perovskite exhibited crystallinity throughout all doping concentrations. At some doping concentrations, due to low or high degree of tolerance factor, the presence of hexagonal and rhombohedral crystal phases was seen. Analysis of the electronic structure of these perovskites was conducted through diffuse reflectance spectroscopy measurements and electrochemical impedance spectroscopy. Doping transition metals in B site of the perovskite led to the narrowing of band gap energy with the increase in the concentration of the higher atomic number transition metal. About 38% reduction in band gap was achieved in LaCrxFe1-xO3. The band gap constituted of Mott- Hubbard gap and charge transfer gap. For the species LaCrxFe1-xO3, interband states exist with an energy gap as large as 1.3 eV for X=1 and 0.75. These states manifested as Urbach tails and are clearly documented in the absorption spectrum data. At x=0.5 and below, evidence of mixing is seen in LaCrxFe1-xO3, leading to the diminution of these interband states, although not to full extent, their energy was reduced by about 0.5 eV. In LaCrxMn1-xO3 and LaMnxFe1-XO3, the absence of Urbach tail and absorption edge is observed. The band edge positions of most of these perovskites provided a large enough over potential to cause the reduction of CO2. Future efforts on the photocatalytic activity study of these perovskites through dye degradation and CO2 reduction are in progress. Preliminary results of photoassisted dye degradation are shared in this thesis.

abstract: Nanoscale semiconductors with their unique properties and potential applications have been a focus of extensive research in recent years. There are many ways in which semiconductors change the world with computers, cell phones, and solar panels, and nanoscale semiconductors having a promising potential to expand the efficiency, reduce the cost, and improve the flexibility and durability of their design. In this study, theoretical quantum mechanical simulations were performed on several different nanoscale semiconductor materials, including graphene/phosphorene nanoribbons and group III-V nanowires. First principles density functional theory (DFT) was used to study the electronic and structural properties of these nanomaterials in their fully relaxed and strained states. The electronic band gap, effective masses of charge carriers, electronic orbitals, and density of states were most commonly examined with strain, both from intrinsic and external sources. For example, armchair graphene nanoribbons (AGNR) were found to have unprecedented band gap-strain dependence. Phosphorene nanoribbons (PNRs) demonstrate a different behavior, including a chemical scissors effect, and studies revealed a strong relationship between passivation species and band gap tunability. Unlike the super mechanical flexibility of AGNRs and PNRs which can sustain incredible strain, modest yet large strain was applied to group III-V nanowires such as GaAs/InAs. The calculations showed that a direct and indirect band gap transition occurs at some critical strains and the origination of these gap transitions were explored in detail. In addition to the pure nanowires, GaAs/InAs core/shell heterostructure nanowires were also studied. Due to the lattice mismatch between GaAs and InAs, the intrinsic strain in the core/shell nanowires demonstrates an interesting behavior on tuning the electronic properties. This interesting behavior suggests a mechanical way to exert compressive strain on nanowires experimentally, and can create a finite quantum confinement effect on the core. / Dissertation/Thesis / Doctoral Dissertation Physics 2016

Submitted by Automa??o e Estat?stica (sst@bczm.ufrn.br) on 2017-03-28T19:29:06Z No. of bitstreams: 1 AdllerDeOliveiraGuimaraes_TESE.pdf: 3828999 bytes, checksum: 3e7dfbe3050d7b59dfd94da65f4fdbe5 (MD5) / Approved for entry into archive by Arlan Eloi Leite Silva (eloihistoriador@yahoo.com.br) on 2017-03-29T18:27:22Z (GMT) No. of bitstreams: 1 AdllerDeOliveiraGuimaraes_TESE.pdf: 3828999 bytes, checksum: 3e7dfbe3050d7b59dfd94da65f4fdbe5 (MD5) / Made available in DSpace on 2017-03-29T18:27:22Z (GMT). No. of bitstreams: 1 AdllerDeOliveiraGuimaraes_TESE.pdf: 3828999 bytes, checksum: 3e7dfbe3050d7b59dfd94da65f4fdbe5 (MD5) Previous issue date: 2016-12-02 / A crescente demanda por tecnologia de comunica??o sem fio na sociedade moderna exige a constru??o de circuitos integrados de micro-ondas cada vez mais sofisticados. Neste sentido, as antenas planares de microfita se destacam devido seus tamanhos reduzidos e por serem utilizadas em diversas aplica??es, com ?nfase em aeronaves, sat?lites e sistemas de comunica??es m?veis. Em adi??o, pesquisas recentes mostram o uso da intelig?ncia computacional na ?rea de telecomunica??es para an?lise de novas aplica??es em antenas de microfita, como tamb?m na otimiza??o das aplica??es existentes, buscando um melhor desempenho na recep??o e/ou transmiss?o de sinais. Nesse contexto, o presente trabalho tem como objetivo obter as propriedades de radia??o de antenas de microfita com uso de novos modelos de estruturas do tipo Photonic Band Gap (PBG), baseadas em malhas com distribui??es peri?dicas e quase peri?dicas, aplicadas no substrato. Nos procedimentos pr?ticos construiu-se prot?tipos de antenas que foram utilizados para obter um banco de dados para alimentar o processo de treinamento de uma Rede Neural Artificial (RNA), visando a obten??o da frequ?ncia de opera??o das antenas. Por fim, foi otimizado o procedimento de obten??o da frequ?ncia de projeto e estrutura PBG necess?rios para a antena ressoar em uma dada frequ?ncia de opera??o. De acordo com os resultados obtidos, pode-se concluir que ? poss?vel ? utiliza??o de uma RNA na otimiza??o de um projeto de antenas de microfita com substrato PBG. A valida??o dos resultados fornecidos pela RNA foi realizada com base na constru??o de prot?tipos das antenas, os quais apresentaram uma boa concord?ncia entre os valores simulados e medidos. / The increasing demand for wireless technology in contemporary society requires the construction of integrated microwave circuits each time more sophisticated. In this sense, the planar microstrip antenna stands out because their small sizes and used in various applications, with emphasis on airplanes, satellites and mobile communication systems. In addition, recent researches shows the use of computing intelligence in telecommunications for analysis of new applications at microstrip antennas, as also the optimization of existing applications, searching for a better performance at the reception and / or signal transmission. In this context, this work has as objective get the radiation properties of microstrip antennas using the new models of Photonic Band Gap (PBG) structures based in meshes with periodic distributions and quasi-periodic printed on the substrate. In the practical procedures, prototypes of antennas were constructed that were used to obtain a database to feed the training process of an Artificial Neural Network (ANN), in order to obtain the resonance frequency of antennas. Finally, it was optimized the procedure for obtaining the project frequency and PBG structure required for a given operating frequency. According to the results, it can be concluded that it is possible to use an ANN in the optimization of a project of microstrip antenna with PBG substrate. The validation of the results provided by the ANN was based on building prototypes of the antennas, which showed good agreement between the simulated and measured values.

Submitted by Automa??o e Estat?stica (sst@bczm.ufrn.br) on 2017-05-31T20:37:34Z No. of bitstreams: 1 EdiRozemberghBrasileiroDaSilvaBrandao_TESE.pdf: 21206961 bytes, checksum: 7b22e84f9eeadd1446a3596114b8ad22 (MD5) / Approved for entry into archive by Arlan Eloi Leite Silva (eloihistoriador@yahoo.com.br) on 2017-06-01T19:35:34Z (GMT) No. of bitstreams: 1 EdiRozemberghBrasileiroDaSilvaBrandao_TESE.pdf: 21206961 bytes, checksum: 7b22e84f9eeadd1446a3596114b8ad22 (MD5) / Made available in DSpace on 2017-06-01T19:35:34Z (GMT). No. of bitstreams: 1 EdiRozemberghBrasileiroDaSilvaBrandao_TESE.pdf: 21206961 bytes, checksum: 7b22e84f9eeadd1446a3596114b8ad22 (MD5) Previous issue date: 2016-10-20 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior (CAPES) / Neste trabalho, apresentamos um estudo te?rico da propaga??o das ondas eletromagn?ticas em estruturas de multicamadas denominadas de Cristais Fot?nicos. Dentro deste escopo, este trabalho possui tr?s vertentes. Na primeira parte, estudamos teoricamente os espectros de transmiss?o em quasi-cristais fot?nicos unidimensionais, composta de materiais SiO2 (A) e TiO2 (B), organizada seguindo a sequ?ncia de Octonacci, onde o en?simo est?gio da multicamada Sn ? dado pela regra Sn=Sn-1Sn-2Sn-1, para n ? 3 e com S1=A e S2=B. A express?o de transmit?ncia foi obtido empregando um c?lculo te?rico baseado no m?todo da matriz de transfer?ncia. Para ondas incidentes normalmente, observamos que, para uma mesma gera??o, os espectros de transmiss?o para ondas transversais el?ctricas (TE) e ondas transversais magn?ticas (TM) s?o iguais ao menos de forma qualitativa, e eles apresentam uma propriedade de escala em que um comportamento auto-similar ? obtido, como uma evid?ncia de que esses espectros s?o fractais. Os espectros mostram regi?es onde os band gaps unidirecionais emergem para as gera??es espec?ficas da estrutura fot?nica de Octonacci, exceto para ondas TM. Para ondas TE, notamos que todos eles t?m quase a mesma largura, por diferentes gera??es. N?s tamb?m relatamos a localiza??o de modos como consequ?ncia da quase-periodicidade da heteroestrutura.Na segunda parte, investigamos os espectros de transmiss?o ?ptica para as ondas de polariza??o s (TE) e p (TM) em quasi-cristais fot?nicos unidimensionais em uma estrutura de multicamadas quase-peri?dicas composta por camadas alternadas de SiO2 e metamateriais, organizada seguindo a sequ?ncia de Octonacci. As equa??es de Maxwell e t?cnica da matriz de transfer?ncia s?o usadas para obter o espectro de transmiss?o para propaga??o de campos eletromagn?ticos incidentes normalmente e obliquamente. Assumimos a resposta dispersiva do tipo Drude-Lorentz para a permissividade diel?trica e permeabilidade magn?tica dos metamateriais. Para ondas incidentes normalmente, observamos que o espectro n?o tem comportamento auto-similar ou simetria espelho e tamb?m possui a aus?ncia de band gaps ?ptico. Tamb?m mostramos o surgimento de band gaps completos e pseudos refletores de Bragg (ou espelhos de Bragg).Na terceira e ?ltima parte, estudamos a propaga??o dos polaritons de plasmon em sistemas formados por multicamadas peri?dicas e quase-peri?dicas organizadas de acordo com a sequ?ncia de Octonacci, a partir da descri??o do comportamento dos seus modos de volume e de superf?cie. Atrav?s de c?lculo anal?tico e num?rico computacional, obtemos inicialmente os espectros de frequ?ncia dos polaritons de plasmon nestas superestruturas. Posteriormente, investigamos como a quase-periodicidade modifica a sua estrutura de bandas em rela??o ao caso peri?dico, induzindo os seus espectros a uma forma auto-similar, caracterizando a sua fractalidade/multifractalidade.

abstract: High Impedance Surfaces (HISs), which have been investigated extensively, have proven to be very efficient ground planes for low profile antenna applications due to their unique reflection phase characteristics. Another emerging research field among the microwave and antenna technologies is the design of flexible antennas and microwave circuits to be utilized in conformal applications. The combination of those two research topics gives birth to a third one, namely the design of Conformal or Flexible HISs (FHISs), which is the main subject of this dissertation. The problems associated with the FHISs are twofold: characterization and physical realization. The characterization involves the analysis of scattering properties of FHISs in the presence of plane wave and localized sources. For this purpose, an approximate analytical method is developed to characterize the reflection properties of a cylindrically curved FHIS. The effects of curvature on the reflection phase of the curved FHISs are examined. Furthermore, the effects of different types of currents, specifically the ones inherent to finite sized periodic structures, on the reflection phase characteristics are observed. After the reflection phase characterization of curved HISs, the performance of dipole antennas located in close proximity to a curved HIS are investigated, and the results are compared with the flat case. Different types of resonances that may occur for such a low-profile antenna application are discussed. The effects of curvature on the radiation performance of antennas are examined. Commercially available flexible materials are relatively thin which degrades the bandwidth of HISs. Another practical aspect, which is related to the substrate thickness, is the compactness of the surface. Because of the design limitations of conventional HISs, it is not possible to miniaturize the HIS and increase the bandwidth, simultaneously. To overcome this drawback, a novel HIS is proposed with a periodically perforated ground plane. Copper plated through holes are extremely vulnerable to bending and should be avoided at the bending parts of flexible circuits. Fortunately, if designed properly, the perforations on the ground plane may result in suppression of surface waves. Hence, metallic posts can be eliminated without hindering the surface wave suppression properties of HISs. / Dissertation/Thesis / Ph.D. Electrical Engineering 2013

Doctor of Philosophy / Department of Mechanical and Nuclear Engineering / Liang-Wu Cai / A computational system is built for conducting deterministic simulations of three-dimensional multiple scattering of elastic waves by spherical inclusions. Based on expansion expression of elastic wave fields in terms of scalar and vector spherical harmonics, analytically exact solutions of single scattering and multiple scattering are obtained, implemented and verified. The verification is done by using continuities of displacement and surface traction at the interface between an inclusion and host medium, energy conservation and published results. The scatterer polymerization methodology is extended to three-dimensional multiple scattering solution. By using this methodology, an assemblage of actual scatterers can be treated as an abstract scatterer. This methodology is verified by using different approaches, with or without scatterer polymerization, to solve a physically the same multiple scattering problem. As an application example, band gap formation process for elastic wave propagation in cubic lattice arrangements of spherical scatterers is observed through a series of numerical simulations. Along the direction of the incident wave, scatterer arrangements are viewed as comprising layers of scatterers, within which scatterers form a square grid. Starting from one layer and by increasing the number of layers, near-field forward wave propagation spectra are computed as the number of layers increases. These simulations also demonstrates that the computational system has the capability to simulate multiple scattering solutions of elastic waves in three-dimension.

Philosophiae Doctor - PhD / In this thesis we report on the metal-mediated-thermally induced changes of the structural and optical properties of hydrogenated amorphous silicon deposited by hot-wire CVD, where aluminium and nickel were used to induce crystallization. The metal-coated amorphous silicon was subjected to a thermal annealing regime of between 150 and 520°C. The structural measurements, obtained by Raman spectroscopy, show partial crystallization occurring at 350 °C. At the higher annealing temperatures of 450°C and 520°C complete crystallization occurs. Reflection and transmission measurements in the UV-visible range were then used to extract the optical properties. By adopting the effective medium approximation a single optical model could be constructed that could successfully model material that was in different structural phases, irrespective of metal contamination. Changes in the absorption of the material in various stages of transition were confirmed with a directly measured absorption technique, and the modelled absorption closely followed the same trends This study forms part of the larger overall solar cell research project, of which the primary aim is to eventually develop a silicon solar panel that optimises the characteristics for best performance. / South Africa

Ce manuscrit porte sur la conception d’un transistor à effet de champ destiné à l’observation de la photoluminescence de l’exciton et des complexes excitoniques chargés du ZnO sous l’influence d’un champ électrique. Pour cela, des simulations ont permis de définir un cahier des charges de la structure du transistor afin de bloquer la conductivité dans le canal de ZnO et d’appliquer un champ électrique intense. La seconde partie concerne le choix du matériau de grille et de l’électrode transparente de surface pour l’observation de la photoluminescence dans le canal. L’oxyde de gallium (-Ga2O3) a été choisi car il présente un grand gap, des propriétés d’isolant et de semi-conducteur avec dopage. Cependant les films de Ga2O3 dopés avec Ti, Sn, Zn et Mg élaborés par MOCVD n’ont pas révélé de conductivité. Les films d’alliages (Ga,Sn)2O3 n’ont pas non plus montré de conductivité et leur structure est étudiée intensivement. Des traitements plasma radiofréquence sous flux d’argon, d’oxygène ou d’hydrogène ont permis de montrer que l’implantation de l’hydrogène donne lieu à un niveau donneur avec une énergie d’activation de 7 meV. La conductivité est toutefois modulée par le dopage en Sn et les traitements s’accompagnent d’un changement de la sous-stœchiométrie en oxygène qui diminue la transparence à cause de la formation de niveau profond de lacune d’oxygène. La structure finale de la grille transparente dans l’ultraviolet pour l’observation de la photoluminescence du ZnO peut donc être élaborée par une grille diélectrique de -Ga2O3 puis une électrode conductrice transparente de (Ga,Sn)2O3 traitée superficiellement par un plasma d’hydrogène. / This manuscript covers the design of a field transistor for the observation of photoluminescence of the exciton and the charged excitonic complex of ZnO under the influence of an electric field. For this, simulations have helped to define the specifications of the transistor structure to block the conductivity in the ZnO channel and applying a strong electric field. The second part concerns the choice of gate material and the surface transparent electrode for the observation of photoluminescence in the channel. The gallium oxide (-Ga2O3) was chosen because it has a large gap, insulating properties and semiconductor properties with doping. However, Ga2O3 films doped with Ti, Sn, Zn and Mg MOCVD did not show conductivity. Films of alloys (Ga,Sn)2O3 have not shown either conductivity and their structure is studied intensively. Radio frequency plasma treatment under a flux of argon, oxygen or hydrogen have shown that implantation of hydrogen gives rise to a donor level with 7 meV activation energy. However, the conductivity is modulated by doping Sn and treatments are accompanied by a change of sub-stoichiometry in oxygen, which reduces the transparency due to the formation of deep level of oxygen vacancy. The final structure of the transparent gate in the ultraviolet for the observation of photoluminescence of ZnO can be prepared by a dielectric gate -Ga2O3 and a transparent conductive electrode of (Ga,Sn)2O3 surface treated by a plasma of hydrogen.

No description available.