Cerâmicas piezelétricas possibilitam posicionamento e sensoriamento de precisão ou captação de energia mecânica valendo-se do efeito piezelétrico, capaz de converter energia mecânica em elétrica ou o contrário. Para aprimorar ou estender as aplicações dessas cerâmicas, mecanismos flexíveis podem ser acoplados a elas, formando um Dispositivo Piezelétrico Flextensional (DPF). No projeto desse tipo de estrutura, o conceito de Material com Gradação Funcional (MGF) é interessante, já que esses materiais apresentam variações graduais de suas propriedades efetivas, permitindo a alternância entre um material mais flexível e um mais rígido de acordo com a intensidade de deslocamento desejada em cada região da estrutura. Assim, neste trabalho, implementa-se o Método de Otimização Topológica (MOT) no projeto de estruturas gradadas com o intuito de identificar as vantagens e desvantagens da utilização do conceito de MGF em DPF. Esse método combina algoritmos de otimização e o Métodos dos Elementos Finitos (MEF) para distribuir material dentro de um domínio fixo através de um modelo de material, que no presente caso é o de Material Isotrópico Sólido com Penalização (MISP) adaptado a MGF. Na fabricação desses dispositivos otimizados, utiliza-se a Sinterização por Jato de Plasma (SJP) para a obtenção de tarugos gradados que são submetidos a processos de eletro-erosão e de corte a laser. Por fim, para a verificação dos resultados numéricos, utiliza-se um vibrômetro para aferir os deslocamentos dos protótipos de atuadores fabricados. / Piezoelectric devices enable precision positioning and sensing or mechanical energy harvesting based on the piezoelectric effect. In flextensional piezoelectric devices, flexible coupling structures are attached to ceramics to improve or extend the application possibilities. On the design of this kind of structure, the concept of Functionally Graded Materials (FGM) can be interesting, since it allows gradual variations of its effective properties along some direction by mixing two or more materials. Thus, in order to identify the advantages and disadvantages of using FGM, graded flexible coupling structures that maximize the performance of piezoelectric devices are obtained by implementing the Topology Optimization Method (TOM). This method combines optimization algorithms and the Finite Element Method (FEM) to distribute material inside a fixed domain. In this work, the formulation is based on the Solid Isotropic Material with Penalization (SIMP) material model adapted for the FGM concept, which can represent continuous change in material properties along the domain. Resulting optimal graded topologies of coupling structures are presented and compared with homogeneous structures. Finally, graded devices are manufactured through Spark Plasma Sintering (SPS) technique in order to be characterized, validating numerical results. The numerical results demonstrate the TOM efficacy in designing functionally graded piezoelectric devices and show, by its implementation, significant gains in graded mechanisms performance when compared with analogous homogeneous. Furthermore, the feasibility of proposed manufacturing process is confirmed, allowing the fabrication of prototypes with expected behavior.

Dans cette étude de doctorat, de la poudre de Y3Al5O12 (YAG) a été synthétisé avec plusieurs méthodes: atomisation, synthèse de réaction et co-précipitation. Le procédé de synthèse le plus prometteur, la co-précipitation, a été optimisé pour obtenir une poudre hautement frittable, présentant une phase pure. Le traitement approprié avant frittage, comprenant calcination, efficace dispersion et séchage homogène, a été réalisé. La poudre de YAG a été dispersée avec plusieurs méthodes, avant frittage. Grâce à la technique Spark Plasma Sintering (SPS) une évaluation très rapide de l'influence des différents traitements a été réalisée. Il a été constaté que, pour une poudre obtenue avec une telle synthèse, la méthode de dispersion est cruciale afin de obtenir une transparence élevée du materiaux fritté. En raison de l'aptitude au frittage de la poudre, du traitement approprié avant frittage et de la basse température du cycle effectué par SPS, un matériau à grains fins a été obtenu. Des mesures de spectroscopie de photoélectrons (XPS) ont été effectuées sur la poudre de YAG et sur le matériaux fritté, et plusieurs differences ont été mises en évidence. Quelques hypothèses ont été faites pour expliquer les differences observées, et certains preuves supplémentaires pour les vérifier ont été mises en avant. Une poudre composite alumine-zircone-YAG a été synthétisé à partir d'une poudre d'alumine commerciale, qui a été fonctionnalisé avec chlorures de yttrium et de zirconium. Les traitements de pré-frittage appropriées ont été effectués, comprenant une calcination à basse température et une calcination "rapide", pour favoriser la germination des petits cristallites. Deux méthodes de mise en forme, coulage et pressage, ont été réalisées. Il a été constaté que le coulage permet la production de matériaux céramiques beaucoup plus homogènes. Une caractérisation mécanique préliminaire a été effectuée. Une caractérisation spectroscopique des poudres d'alumine-YAG, traités à basse et haute température, a été réalisée. Il a été mis en évidence que l'état d'hydratation des poudres varie avec la fonctionnalisation par les chlorures. La présence de sites Y sur la surface des poudres composites a été mise en évidence par XPS et à partir des spectres infrarouges (spectroscopie infrarouge par transformée de Fourier, FTIR) des échantillons soumis à des différentes pressions de monoxide de carbone (CO). / In this PhD study, Yttrium aluminum garnet (Y3Al5O12, YAG) powder was synthesised with several methods, i.e. spray drying, reaction synthesis and co-precipitation. The most promising synthesis method, i.e. co-precipitation, was optimized to obtain a pure phase, highly sinterable powder. The appropriate pre-sintering processing, i.e. calcination treatment, e fficient dispersion, homogeneous drying, were performed. YAG powder was dispersed with several methods, prior to sintering. Through Spark Plasma Sintering (SPS) a very fast assessment of the influence of the various treatments was performed. It was found that, for the synthesised powder, the dispersion method is crucial to obtain a high transparency. Due to the high sinterability of the powder and to the appropriate pre-sintering treatment, a low temperature SPS cycle was performed, and a fine-grained material was obtained. X-ray Photoelectron Spectroscopy (XPS) measurements were performed on YAG powder and on the sintered material, and several di fferences were evidenced. Some hypothesis were made to explain the observed di fferences, and some additional proofs to verify them were put forward. A composite Alumina-YAG-Zirconia powder was synthesized, starting from a commercial alumina powder, which was functionalised with Yttrium and Zirconium chlorides. The appropriate pre-sintering treatments were performed, comprising a low temperature calcination and a "fast" calcination, to favour the germination of small crystallites. Two shaping methods, i.e. slip casting and pressing, were performed. It was found that slip casting allows the production of much more homogeneous ceramic materials. A preliminary mechanical characterization was performed. A spectroscopic characterization of Alumina-YAG powders, heat-treated at low and high temperatures, was performed. It was evidenced that the hydration state of the powders changes with chlorides functionalization. The presence of Y sites on the surface of the composite powders was evidenced by X-ray Photoelectron Spectroscopy and from the di fference spectra of powdered samples subjected to various CO pressures.

Les météorites sont des témoins de planétésimaux formés dans les cinq premiers millions d'années du système solaire. Leur grande diversité est liée à des évolutions thermiques différentes de leur corps parent, allant d'un simple métamorphisme à une différenciation complète manteau-noyau. Cette thèse s'intéresse à la mobilité relative silicate-métal dans les objets ayant connu un métamorphisme thermique ou la fusion partielle. Deux approches sont considérées : une naturaliste basée sur l'étude texturale d'échantillons naturels, l'autre expérimentale dans le but d'identifier et comprendre les processus physiques susceptibles de s'être produits. L'étude texturale sur les métaux et sulfures des chondrites ordinaires de type H, montre que, lorsque le métamorphisme augmente ces deux phases se séparent, changent de forme et croissent. Cette évolution se fait dans un ordre bien précis, cohérent avec les données de géochimie et les modèles thermiques, permettant de distinguer deux échantillons d'un même type pétrologique. On propose donc une nouvelle classification permettant de préciser l'actuelle. L'étude expérimentale de croissance cristalline a été menée sur des analogues synthétiques de météorites : le système forstérite+nickel±silicate fondu (Fo:Ni±SiF). Cette étude a été précédée par la mise en place d'un protocole de frittage par une technique peu connue en géosciences : le Spark Plasma Sintering. Les résultats montrent des mécanismes très différents entre Fo et Ni selon la proportion et la nature de chaque phase. Ces résultats sont en accord avec les observations faites sur objets naturels et permettent de préciser l'histoire thermique des planétésimaux.

<p>The aim of this study is to synthesize a material with high thermal conductivity and a low coefficient of thermal expansion (CTE), useful as a heat sink. Carbon nanofibres (CNF) are first coated with copper by an electroless plating technique and then sintered to a solid sample by either spark plasma sintering (SPS) or hot pressing (HP). The final product is a carbon nanofibre reinforced copper composite. Two different fibre structures are considered: platlet (PL) and herringbone (HB). The influence of the amount of CNF reinforcement (6-24 %wt), on the thermal conductivity and CTE is studied. CNF has an excellent thermal conductivity in the direction along the fibre while it is poor in the transverse direction. The CTE is close to zero in the temperature range of interest. The adhesion of Cu to the CNF surface is in general poor and thus improving the the wetting of the copper by surface modifications of the fibres are of interest such that thermal gaps in the microstructure can be avoided. The poor wetting results in CNF agglomerates, resulting in an inhomogeneous microstructure. In this report a combination of three different types of surface modifications has been tested: (1) electroless deposition of copper was used to improve Cu impregnation of CNF; (2) heat treatment of CNF to improve wetting; and (3) introduction of a Cr buffer layer to further enhance wetting. The obtained composite microstructures are characterized in terms of chemical composition, grain size and degree of agglomeration. In addition their densities are also reported. The thermal properties were evaluated in terms of thermal diffusivity, thermal conductivity and CTE. Cr/Cu coated platelet fibres (6wt% of CNF reinforcement) sintered by SPS is the sample with the highest thermal conductivity, ~200 W/Km. The thermal conductivity is found to decrease with increasing content of CNFs.</p>

Ionic transport in metal oxides is crucial for the functioning of a broad range of different components, such as heat resistant alloys designed for high temperature applications and oxide electrolytes in solid oxide fuel cells. This thesis presents results from in situ electrochemical studies of properties related to ionic transport in metal oxides that are important for their applications as protective oxides and ionic conductors. Heat resistant alloys of alumina-former type are known to form an adherent, slowly growing and protective aluminium oxide (Al2O3) scale that protects metals from chemical degradation at high temperature. In situ impedance spectroscopy was used to study highly pure and dense samples of a-alumina in the temperature range 400 – 1000 °C. It was shown that surface conduction on the sample could severely distort the measurement below 700 °C. The magnitude of the distortions appeared to be sensitive to the type of electrodes used. The use of a so-called guard electrode was shown to effectively block the surface conduction in the measurements. By varying the grain size of the sintered alpha-alumina samples, the influence of grain size on the overall conductivity of the a-alumina was studied. It was shown that the activation energy for conductivity increased as the grain size decreased. Molecular dynamics calculations were performed in order to elucidate whether Al- or O ions are dominant in the ionic conductivity of the alpha-alumina. Comparing the calculation and experimental results, the dominating charge carrier was suggested to be oxygen ions. Moreover, the ionic transport in thermally grown alumina-like oxide scales formed on a FeCrAl alloy was studied in situ by impedance spectroscopy between 600 and 1000 °C. It was shown that the properties of these scales differ largely from those of pure and dense alpha-alumina. Furthermore, the conductivity is mainly electronic, due to the multiphase/multilayer microstructure and substantial incorporation of species from the base metal. However, the diffusivity obtained from the ionic conductivity was in line with diffusion data in literature obtained by other methods such as thermogravimetry. Besides, the initial stage of oxidation of a number of Fe-, Ni- and Co-based alloys at temperatures between 500 and 800 °C was studied in situ by high temperature cyclic voltammetry, in which the oxygen activity was changed over a wide range. From the resulting voltammograms the redox reactions occurring on the alloy surface could be identified. It was concluded that the base metal oxidized readily on these alloys before a protective chromia- or alumina-like scale is formed. The base metal oxide is most likely incorporated into the more protective oxide. Further, the oxygen ionic conductivity of highly pure and fully dense yttria-stabilized zirconia produced by spark plasma sintering was studied by impedance spectroscopy. The aim was to evaluate intrinsic blocking effects on the ionic conduction associated with the space charge layer in the grain boundary region. It was observed that the ionic conductivity of the spark plasma sintered oxides is equal or slightly higher than what has been achieved by conventional sintering methods. In addition, it was shown that the specific grain boundary conductivity increases with decreasing grain size, which can be explained by a decreasing Schottky barrier height (i.e., decreasing blocking effect). The quantitative results from this work verify the space charge model describing the influence of grain size on the ionic conductivity of yttria-stabilized zirconia through dopant segregation and oxygen vacancy depletion along the grain boundaries. / QC 20100825

University of Macau / Faculty of Science and Technology / Department of Computer and Information Science

Points-to analysis is a static program analysis that tries to predict the dynamic behavior of programs without running them. It computes reference information by approximating for each pointer in the program a set of possible objects to which it could point to at runtime. In order to justify new analysis techniques, they need to be compared to the state of the art regarding their accuracy and efficiency. One of the main parameters influencing precision in points-to analysis is context-sensitivity that provides the analysis of each method separately for different contexts it was called on. The problem raised due to providing such a property to points-to analysis is decreasing of analysis scalability along with increasing memory consumption used during analysis process. The goal of this thesis is to present a comparison of precision and scalability of context-sensitive and context-insensitive analysis using three different points-to analysis techniques (Spark, Paddle, P2SSA) produced by two research groups. This comparison provides basic trade-offs regarding scalability on the one hand and efficiency and accuracy on the other. This work was intended to involve previous research work in this field consequently to investigate and implement several specific metrics covering each type of analysis regardless context-sensitivity – Spark, Paddle and P2SSA. These three approaches for points-to analysis demonstrate the intended achievements of different research groups. Common output format enables to choose the most efficient type of analysis for particular purpose.

Pulsed Detonation Engines (PDEs) have the potential to revolutionize fight by better utilizing the chemical energy content of reactive fuel/air mixtures over conventional combustion processes. Combustion by a super-sonic detonation wave results in a significant increase in pressure in addition to an increase in temperature. In order to harness this pressure increase and achieve a high power density, it is desirable to operate PDEs at high frequency. The process of detonation initiation impacts operating frequency by dictating the length of the chamber and contributing to the overall cycle time. Therefore a key challenge in the development of a practical PDEs is the requirement to rapidly initiate a detonation in hydrocarbon-air mixtures. This thesis evaluates the influence of spark energy and airflow velocity on this challenging initiation process. The influence of spark energy, number of sparks and airflow velocity on Deflagration-to-Detonation Transition (DDT) was studied during cyclic operation of a small-scale PDE at the General Electric Global Research Center. Experiments were conducted in a 50 mm square transitioning to cylindrical channel PDE with optical access operating with stoichiometric ethylene-air mixture. Total spark energy was varied from 250 mJ to 4 J and was distributed between one and four spark plugs located in the same axial location. Initial flame acceleration was imaged using high-speed shadowgraph and was characterized by the time to reach 20 cm from the spark plug. Measurements of detonation wave velocity and emergence time, the time it takes the detonation wave to exit the tube, was measured using dynamic pressure transducers and ionization probes. It was found that the flame front spread was faster at higher spark energies and with more spark locations. Initial flame acceleration was 16% faster for the 4-spark, 4 J case when compared to the baseline 1-spark, 1 J case. When looking at the effect of airflow on the influence of spark energy, it was found that airflow had a larger effect on emergence time at high energies, versus energies less than 1 J. Finally, for a selected case of 0.25 J spark energy and 4 sparks, the velocity of the fuel-air mixture during fill was found to have a varying influence on detonation initiation and emergence time.

Die vorliegende Arbeit enthält neuartige Ergebnisse festkörperanalytischer Untersuchungen zu Synthese, Struktur und Eigenschaften von anodischen Konversionsschichten auf Vanadium und Zirkonium. Schwerpunkte sind die Darstellung anodischer Oxidschichten auf den Metallen Vanadium und Zirkonium sowie ihre Charakterisierung mit ausgewählten Methoden der Festkörperanalytik. Am Beispiel des Vanadiums wird die anodische Schichtbildung in Essigsäurebasiselektrolyten untersucht. Unter anderem wird anhand von Strom-Spannungs-Kennlinien das unterschiedliche anodische Verhalten der physikalisch ähnlichen Metalle Vanadium und Zirkonium diskutiert. Neben den Methoden der Röntgenbeugung, Infrarotspektroskopie, Photoelektronenspektroskopie und Elektronenmikroskopie wird die Ramanspektroskopie als ein geeignetes Mittel zur Unterscheidung der Oxidphasen verwendet. Die amorphen VOx-Schichten und kristallinen ZrO2- bzw. modifizierten Schichten wurden sowohl unter anodischen als auch unter elektrochemischen Plasmabedingungen erzeugt. Die Korrelation zwischen Schichtstruktur und Syntheseverfahren wird herausgearbeitet.

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