Electrochemical machining (ECM) is a non-conventional manufacturing process, which uses electrochemical dissolution to shape any conductive metal regardless of its mechanical properties and without leaving behind residual stresses or tool wear. Therefore, ECM can be an alternative for machining difficult-to-cut materials, complex geometries, and materials with improved characteristics, such as strength, heat-resistance or corrosion-resistance. Notwithstanding its great potential as a shaping tool, the ECM process is still not fully characterised and its research is an on-going process. Various phenomena are involved in ECM, e.g. electrodynamics, mass transfer, heat transfer, fluid dynamics and electrochemistry, which occur in parallel and this can lead to a different material dissolution rate at each point of the workpiece surface. This makes difficult an accurate prediction of the final workpiece geometry. This problem was addressed in the first part of the present thesis by developing a simulation model of the ECM process in a two-dimensional (2D) environment. A finite element analysis (FEA) package, COMSOL multiphysics® was used for this purpose due to its capacity to handle the diverse phenomena involved in ECM and couple them into a single solution. Experimental tests were carried out by applying ECM on stainless steel 316 (SS316) samples. This work was done in collaboration with pECM Systems Ltd® from Barnsley, UK. The interest of studying ECM on stainless steels (SS) resides on the fact that the application of ECM on SS typically results in various different surface finishes. The chromium in SS alloys usually induces the formation of a protective oxide film that prevents further corrosion of the alloy, giving the metal the special characteristic of corrosion resistance. This oxide film has low electrical conductivity; hence normal anodic dissolution often cannot proceed without oxide breakdown. Partial breakdown of the oxide film often occurs, which causes pits on the surface or a non-uniform surface finish. Therefore the role of the ECM machining parameters, such as interelectrode gap, voltage, electrolyte flow rate, and electrolyte inlet temperature, on the achievement of a uniform oxide film breakdown was evaluated in this work. Experimental results show that the resulting surface finish is highly influenced by the over-potential and current density, and by the characteristics of the electrolyte, flow rate and conductivity. The complexity of experimentally controlling these parameters emphasised the need for the development of a computational model that allows the simulation of the ECM process in full. The simulation of ECM in a three-dimensional (3D) environment is crucial to understand the behaviour of the ECM process in the real world. In a 3D model, information that was not visible before can be observed and a more detailed realistic solution can be achieved. Hence, in this work a computer aided design (CAD) software was used to construct a 3D geometry, which was imported to COMSOL Multiphysics® to simulate the ECM process, but this time in a 3D environment. This enhanced simulation model includes fluid dynamics, heat transfer, mass transfer, electrodynamics and electrochemistry, and has the novelty that an accurate computational simulation of the ECM process can be carry out a priori the experimental tests and allows the extraction of enough information from the ECM process in order to predict the workpiece final shape and surface finish. Moreover, this simulation model can be applied to diverse materials and electrolytes by modifying the input ECM parameters.

abstract: Fission products in nuclear fuel pellets can affect fuel performance as they change the fuel chemistry and structure. The behavior of the fission products and their release mechanisms are important to the operation of a power reactor. Research has shown that fission product release can occur through grain boundary (GB) at low burnups. Early fission gas release models, which assumed spherical grains with no effect of GB diffusion, did not capture the early stage of the release behavior well. In order to understand the phenomenon at low burnup and how it leads to the later release mechanism, a microstructurally explicit model is needed. This dissertation conducted finite element simulations of the transport behavior using 3-D microstructurally explicit models. It looks into the effects of GB character, with emphases on conditions that can lead to enhanced effective diffusion. Moreover, the relationship between temperature and fission product transport is coupled to reflect the high temperature environment. The modeling work began with 3-D microstructure reconstruction for three uranium oxide samples with different oxygen stoichiometry: UO2.00 UO2.06 and UO2.14. The 3-D models were created based on the real microstructure of depleted UO2 samples characterized by Electron Backscattering Diffraction (EBSD) combined with serial sectioning. Mathematical equations on fission gas diffusion and heat conduction were studied and derived to simulate the fission gas transport under GB effect. Verification models showed that 2-D elements can be used to model GBs to reduce the number of elements. The effect of each variable, including fuel stoichiometry, temperature, GB diffusion, triple junction diffusion and GB thermal resistance, is verified, and they are coupled in multi-physics simulations to study the transport of fission gas at different radial location of a fuel pellet. It was demonstrated that the microstructural model can be used to incorporate the effect of different physics to study fission gas transport. The results suggested that the GB effect is the most significant at the edge of fuel pellet where the temperature is the lowest. In the high temperature region, the increase in bulk diffusivity due to excess oxygen diminished the effect of GB diffusion. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2014

No description available.

Scanning electrochemical microscopy (SECM) is used to deposit gold nanoparticles on a glassy carbon electrode (GCE). Deposition conditions, including the tip-substrate distance, current density, substrate potential, and addition of Ag ions in the electrolyte are changed to study the effects on gold spot size and particle morphology. Atomic force microscopy (AFM) is used to analyze the gold nanoparticles. The size and shape of the nanoparticle can be controlled by different SECM experimental conditions. OMSOL Multiphysics software is used to simulate the results of SECM deposition. By comparing the simulation results and experimental results, the deposition process can be understood better. Heterogeneous irreversible reaction rate constant of the reaction happened on GCE can be estimated.

The search for more sustainable energy to match the growing energy demand begins with finding more dispatchable resources such as solar energy. As one of the promising solar technologies, concentrated solar power (CSP) has a full capacity to store thermal energy for extended operation. Nevertheless, some key components in CSP systems usually face extreme environment, such as uneven solar flux, cyclic thermal expansion, structural degradation on the solar absorber tubes in a Concentrated Solar Power Tower (CSPT) Plant. In this study, we applied Multiphysics simulation to explore the benefits of introducing optimized fins for heat transfer enhancement and uniform temperature distribution, the goal is to improve the thermal efficiency of such advanced solar absorber tubes. The results of this study can supply design guidance for the manufacturing process of absorber tubes, and eventually can benefit the solar energy community for the next generation of molten salt based CSP system.

No description available.

Ce projet de thèse aborde la question de l’intégration hétérogène de nanofils interconnectés sur des puces microélectroniques à destination de dispositifs de type MEMS et NEMS. Ces dispositifs visent à adresser la problématique globale qu’est le « More than Moore », c’est-à-dire la transformation des filières CMOS classiques pour permettre le développement de nouveaux micro et nano-composants intégrés.En particulier, ces dernières années, une variété de dispositifs à base de nanomatériaux ont vu le jour, conférant à des dispositifs de type micro-actionneurs et micro-capteurs de nouvelles fonctionnalités et/ou des performances accrues, e.g. en termes de résolution, sensibilité, sélectivité. Nous nous intéresserons ici à un certain type de nanostructures, les nanofils d’oxyde de zinc (ZnO), qui ont surtout été utilisés pour concevoir des dispositifs dont le principe de fonctionnement exploite l’effet piézoélectrique, souvent astucieusement combiné avec leurs propriétés semiconductrices. En effet, sous l’effet d’une contrainte mécanique ou d’un déplacement, les nanofils piézoélectriques génèrent un potentiel électrique (piézopotentiel). Si, en outre, les nanofils sont semiconducteurs, le piézopotentiel peut être utilisé pour contrôler un courant externe en fonction de la contrainte mécanique imposée au nanofil (effet piézotronique). L’avantage d’utiliser des nanostructures unidimensionnelles réside dans la modularité de leurs propriétés mécaniques et piézoélectriques en comparaison avec le matériau massif. Par ailleurs, leur intégration est aujourd’hui possible par des voies de croissance compatibles avec les procédés microélectroniques (CMOS/MEMS). Toutes ces considérations rendent possibles la conception de dispositifs très haute performance combinant la faible dimension des éléments fonctionnels (et donc une forte densité d’intégration synonyme de haute résolution spatiale) et leur sensibilité à des phénomènes d’échelle nanoscopique.Dans ce projet de thèse, on adoptera une vision très technologique de la conception de capteurs matriciels à base de nanofils piézoélectriques verticaux en ZnO. S’appuyant sur la prédiction des performances théoriques et la levée des verrous technologiques associés à la conception et la fabrication du capteur, cette étude s’attache à fournir des prototypes faisant la preuve de concept de ces dispositifs haute performance. Dans un premier temps, la réflexion s’articule autour de modèles multi-physiques par éléments finis (FEM) de la réponse piézoélectrique d’un seul nanofil en flexion, modèle que nous avons fait évoluer vers des pixels complets représentatifs d’un nanofil interconnecté dans une matrice. Sur la base de ces considérations, nous avons imaginé des moyens de caractérisation de la réponse piézoélectrique d’un fil, puis d’un pixel. Le banc de caractérisation mis en place a mis en évidence la complexité d’une mesure piézoélectrique systématique, calibrée et décorrélée des éléments environnants du pixel. Des solutions technologiques adéquates ont pu être imaginées et mises en œuvre à travers la réalisation de pixels élémentaires caractérisables et dont la réponse piézoélectrique peut être prédite théoriquement.Cette réalisation a fait appel à un développement en plusieurs étapes, incluant la croissance par voie chimique des nanofils en ZnO, puis la conception de la matrice d’électrodes contactant individuellement les nanofils. La première se découpe en deux étapes : d’abord le choix d’une couche de germination favorisant la croissance sur puce silicium et compatible avec les procédés de salle blanche ; ensuite le développement d’un procédé de croissance permettant la localisation des nanofils au sein d’une matrice d’électrodes. La seconde moitié du travail de fabrication a consisté à définir et à optimiser l’empilement technologique respectant toutes les considérations abordées jusqu’alors, et à définir les procédés technologiques aboutissant à la fabrication de la matrice finale. / This thesis project deals with the question of heterogeneous integration of interconnected nanowires on microelectronics chips in a view to MEMS and NEMS type devices. These devices aim to address the global problematic of “More than Moore”, that is the transformation of classical CMOS microelectronics processes to enable the development of new integrated micro and nanocomponents.In particular, over the past few years, a variety of nanomaterial-based devices have arisen, revealing micro-actuators and micro-sensors with new functionalities and/or improved performances, e.g. in terms of resolution, sensitivity, selectivity. Here we will focus on a certain type of nanostructures, Zinc Oxide (ZnO) nanowires, which have mostly been used so far to design devices whose working principle exploits the piezoelectric effect, often judiciously combined with their semiconducting properties. Indeed, when submitted to a mechanical constraint or displacement, piezoelectric nanowires generate an electrical potential (piezopotential). If, in addition to this, nanowires are also semiconducting, the piezopotential can be exploited to control an external current as a function of the mechanical constraint imposed to the nanowire (piezotronic effect). The advantage of using one-dimensional nanostructures lies into the modularity of both their mechanical and piezoelectric properties, in comparison with the bulk material. Moreover, their integration is now possible thanks to growth processes compatible with microelectronic processes (CMOS/MEMS). All these considerations make it possible to design very high performance devices combining the very small dimension of their functional unit elements (hence a high integration density which implies a high spatial resolution) and their sensitivity to nanoscale phenomena.In this project, we will adopt a very technology-oriented vision of the design of vertically-aligned ZnO-piezoelectric-nanowire matrix-type sensors. Relying on theoretical performance predictions and technological choices to solve device design and fabrication issues, this study aims to produce proof-of-concept prototypes of these high performance devices. First of all, the design process is elaborated based on finite element multiphysics models (FEM) of the piezoelectric response of a single bent nanowire, which we upgraded towards complete pixels, representative of an interconnected nanowire within a matrix. Following these considerations, we have imagined means of characterization of the piezoelectric response of a wire, then of a pixel. The implemented characterization experiment highlighted the complexity of carrying out a systematic, calibrated piezoelectric measurement, decorrelated from the environment of the pixel. Adequate technological solutions could then be implemented through the fabrication of elementary pixels suitable for characterization and whose piezoelectric response could be predictively modeled.This technological part of the work encompassed several development stages, including the chemical growth of ZnO nanowires and the design of the electrode matrix contacting the nanowires individually. The former splits into two steps: first choosing a clean-room compatible seed layer which will favor growth on a Silicon chip; secondly developing a selective growth process enabling the localization of nanowires within a predefined matrix of electrodes. The second part of the fabrication work focused on defining and optimizing the technological stack with respect to all the above mentioned considerations, and implementing the technological processes yielding the final targeted matrix.

Orientador: Philippe Remy Bernard Devloo / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Matemática Estatística e Computação Científica / Made available in DSpace on 2018-08-26T08:30:23Z (GMT). No. of bitstreams: 1 Farias_AgnaldoMonteiro_D.pdf: 4236142 bytes, checksum: 173a1f60f06933f3f12600b1c0be3c9b (MD5) Previous issue date: 2014 / Resumo: Os assuntos de interesse nesta tese dizem respeito a formulações não clássicas do método dos elementos finitos (MEF). Neste sentido, o foco principal está no desenvolvimento de ferramentas computacionais para o MEF visando simulações de problemas multifísicos. Este tipo de problema ocorre, frequentemente, nas aplicações de Engenharia modeladas pelo acoplamento de diversos fenômenos físicos, os quais podem ser resolvidos numa única simulação numérica. A esta modelagem dá-se o nome de simulação multifísica. Neste contexto, para se obter uma simulação otimizada, é conveniente dar um tratamento diferenciado para cada fenômeno físico envolvido. No MEF, por exemplo, tal abordagem multifísica pode ser realizada pela escolha de diferentes subespaços de aproximação, em conformidade com os fenômenos considerados. Para efeito de verificação do código desenvolvido, resolvem-se dois problemas no contexto de simulação multifísica. Um problema de acoplamento fluido-estrutura em poroelasticidade linear e um problema de escoamento em meios porosos com injeção de traçador. Utilizam-se subespaços de aproximação H1-conformes para o deslocamento da matriz porosa, Hdiv-conformes para o fluxo de fluido e funções descontínuas para a aproximação da pressão do fluido e da saturação. Outro desenvolvimento diz respeito a formulações combinadas de Galerkin contínuo-descontínuo para problemas de elasticidade linear. Na abordagem proposta, os espaços de elementos finitos são formados por funções contínuas ou descontínuas, em diferentes regiões do domínio. Estuda-se também o esquema de Petrov-Galerkin com funções teste otimizadas via simetrização. Aplica-se uma nova abordagem para este método através de duas aproximações pelo método de Galerkin contínuo. A implementação e avaliação do desempenho de todas as formulações propostas são feitas no ambiente de programação orientada a objetos chamado NeoPZ / Abstract: The issues of interest in this thesis are related to non classical formulations of the finite element method (FEM). In this sense, the main focus is on developing computational tools for the FEM targeting the simulation of multiphysics problems. This type of problem often occurs in engineering applications modeled by coupling several physical phenomena, which can be resolved in a single numerical simulation. This kind of modeling is called multiphysics simulation. In this context, to obtain an optimized simulation, it is convenient to give a different treatment for each physical phenomenon involved. For instance, in the FEM context, such multiphysics approach can be accomplished by choosing different approximation subspaces in accordance with the phenomena considered. For the verification of the developed code, a problem of fluid-structure interaction in linear poroelasticity and a problem of flow in porous media with tracer injection are solved in the context of multiphysics simulation. H1-conforming approximation is used for the displacement of the porous matrix, Hdiv-conforming for the fluid flow and discontinuous functions for the approximation of fluid pressure and saturation. In another development, the discontinuos-continuous Galerkin formulation is considered for problems of linear elasticity. In such formulation, the spaces finite elements are formed by continuous or discontinuous functions in different regions of the domain. Another study refers to the Petrov-Galerkin scheme with test functions optimized by symmetrization. A new approach to the method by means of two approximations by continuous Galerkin method is proposed. The implementation and verification of all considered formulations are made on the object-oriented programming environment called NeoPZ / Doutorado / Matematica Aplicada / Doutor em Matemática Aplicada

No description available.

The research contained herein yielded an open source interpolation library implemented in and designed for use with the Python programming language. This library, named smbinterp, provides an interpolation to an arbitrary degree of accuracy. The library is parametric in that is can take input from the user to adjust the underlying interpolation mechanism. The characteristics and behavior of the library according to the adjustment of these parameters is presented herein, as well as the results of a mesh resolution study depicting the accuracy obtained by the library. The smbinterp library was designed with parallel computing environments in mind. The library includes modules that allow for its use in high-performance computing environments. These modules were implemented using built-in Python modules to simplify deployment. This implementation was found to scale linearly approximately 180 participating compute processes. The smbinterp library was designed to be mesh agnostic. A plugin system was implemented that allows end users to conveniently and consistently present their numerical results to the library for rapid prototyping and integration. Two plugins are provided as examples and for documentation of the plugin mechanism.