Dissertations / Theses on the topic 'Microstructures'
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Jensen, Jens A. D. "Engineering of metal microstructures : process-microstructure-property relationships for electrodeposits /." Linköping : Univ, 2002. http://www.bibl.liu.se/liupubl/disp/disp2002/tek784s.pdf.
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Smith, Benjamin Daniel. "Microstructure-sensitive plasticity and fatigue of three titanium alloy microstructures." Thesis, Georgia Institute of Technology, 2013. http://hdl.handle.net/1853/49237.
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Titanium alloys are employed in many advanced engineering applications due to their exceptional properties, i.e., a high strength-to-weight ratio, corrosion resistance, and high temperature strength. The performance of titanium alloys is known to be strongly affected by its inherent microstructure, which forms as a result of its thermo-mechanical processing. These microstructures produce compromise relationships between beneficial and detrimental effects on the alloy's performance. To study these structure-property relationships, two distinct crystal plasticity algorithms have been calibrated to data acquired from cyclic deformation experiments performed on three different Ti microstructures: (1) Ti-6Al-4V beta-annealed , (2) Ti-18 solution-treated, age-hardened (STA), and (3) Ti-18 beta-annealed, slow-cooled, age-hardened (BASCA). The calibrated models have been utilized to simulate fatigue loading of variant microstructures to investigate the influence of mean grain size, crystallographic texture, and phase volume fraction. The driving force for fatigue crack nucleation and propagation is quantified through the calculation of relevant fatigue indicator parameters (FIPs) and radial correlation functions are employed to study the correlation between favorably oriented slip systems and the extreme value FIP locations. The computed results are utilized to observe fatigue performance trends associated with changes to key microstructural attributes.
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Trancik, Jessika. "Silk microstructures." Thesis, University of Oxford, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.249173.
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Ruddock, Guy James. "Martensitic microstructures." Thesis, Heriot-Watt University, 1994. http://hdl.handle.net/10399/1371.
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Chen, Kevin M. (Kevin Ming) 1974. "Ordered photonic microstructures." Thesis, Massachusetts Institute of Technology, 2001. http://hdl.handle.net/1721.1/8785.
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Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2001."February 2001."
Includes bibliographical references (p. 149-157).
This thesis examines novel photonic materials systems possessing order in the atomic, microscopic, and macroscopic dimensional regimes. In the atomic order regime, a structure-property investigation is done for Er203 in which the first report of room temperature photoluminescence (PL) is provided. Thin films of the rare earth oxide were deposited via reactive sputtering of Er metal in an Ar/02 ambient, and subsequently annealed to promote grain growth. Heat treatment consisting of a 650°C followed by 1000°C anneal produces maximum crystallinity as measured by glancing angle x-ray diffraction. These films show characteristic PL at [lambda]=1.54 [mu]m. In the microscopic order regime, omnidirectional reflectors and thin film microcavities are demonstrated using sol-gel and solid-state materials. A first demonstration of omnidirectional reflectivity in sol-gel structures was accomplished using a dielectric stack consisting of 12 spin-on Si02/Ti02 quarterwave sol-gel films. Similarly, solid-state dielectric stacks consisting of 6 Si/Si02 sputtered films were used to demonstrate the same principle. Microcavities were formed using sol-gel structures, producing a low quality factor Q=35 due to limitations in film thickness control and lossy interfaces from stress-induced cracks. The high index contrast Si/Si02 microcavities enabled Q ~1000 using 17 total layers following hydrogenation of dangling bonds within the amorphous Si films. Combining fabrication processes for the solid-state microcavity and Er20 3 films, a device was fabricated to demonstrate photoluminescence enhancement of an Er20 3 film embedded in a microcavity. The structure consisted of 3-bilayer mirrors on either side of an Si02/Er203/Si02 cavity. The Q~300 was near the theoretical value for such a structure. At room temperature, PL of Er20 3 was enhanced by a factor of 1000 in the microcavity compared to a single thin film. In the macroscopic order regime, self-assembly of micron-sized Si02 and polystyrene latex colloidal particles into 2D crystals is presented. The colloidal assemblies offer a relatively easy processing route for fabrication of photonic bandgap structures. Large (> 1 mm diameter) single crystal grains of colloids were formed using controlled evaporation and fluid flow techniques. A novel solution enabling postprocessing of the fragile ordered assemblies is presented in which polyelectrolyte multilayers serve as adsorption platforms that anchor the colloidal assemblies. Tailorability of the polyelectrolyte surface properties (charge density, morphology) enables tuning of the colloid adsorption behavior. The polyelectrolyte surface affects colloid adsorption by influencing its surface diffusion. Observations of colloid surface diffusion were made using optical microscopy. Use of polyelectrolytes patterned via rnicrocontact printing enables fabrication of colloid assemblies containing predesigned point and line defects. The patterned polyelectrolyte adsorption template allows placement of colloids in specific geometric arrangement, making possible the realization of sensors or functional photonic bandgap devices such as waveguides or photon traps. Three mechanisms were used to control· adsorption: (1) pH of the colloid suspension, which determines the ionization of the uppermost surface of the polyelectrolyte multilayer; (2) ionic strength of the suspension, which determines the extent of charge screening about the colloid and polyelectrolyte; and (3) concentration of added surfactant, which causes charge screening and introduces hydrophobic interactions between the surfactant and polyelectrolyte.
by Kevin Ming Chen.
Ph.D.
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Syed, Mujtaba [Verfasser], and Jürgen [Akademischer Betreuer] Wilde. "Fatigue analysis of microstructures." Freiburg : Universität, 2016. http://d-nb.info/1122743335/34.
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Hsu, Yi-Chu. "Damping treatments for microstructures /." Thesis, Connect to this title online; UW restricted, 2003. http://hdl.handle.net/1773/7054.
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Gurumurthy, Ashok. "Simulation methodologies for multiphase three-dimensional microstructures." Diss., Georgia Institute of Technology, 2014. http://hdl.handle.net/1853/52261.
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There is a need for simulation methodologies for multiphase three-dimensional microstructures that can be used in numerical simulations of material behavior or in exact computation of effective properties using microstructural correlation functions. Specifically, the methodology must be able to generate verifiably realistic microstructures, with complex morphology accurately represented.
Striving to address that need, the research presented here develops a general microstructure simulation toolbox for multiphase two- and three-dimensional microstructures consisting of one connected phase and one or more particulate phases. Previous work by other researchers has found successful solutions to a variety of special cases of the general problem, but most of them are intended for binary microstructures, and nearly all simulate only two-dimensional microstructures. The toolbox presented here attempts to exceed those limitations.
Its framework is a Metropolis stochastic-optimization routine running a simulated-anneal schedule, with particle position coordinates defining the configuration space and a range of forms available for the ﾓenergyﾔ? function. The toolbox allows several parameterizations of the microstructure, supplying all elementary properties (phase volume fractions, mean sizes, etc.) and some non-elementary properties (distributions of elementary properties, properties relating to inter-phase distances and morphology) of microstructures as possible parameters.
The toolbox is able, as one special case, to simulate realistic microstructures of uniaxially compacted mixtures of elemental Al-Ti-B powders and achieve basic microstructure-processing correlation. Statistical tests involving microstructural correlation functions bear out the realism. The toolbox is also able to generate virtual microstructures for the same system, for use in the design of experiments (which are in fact high-strain-rate impact simulations), and for evaluating hypotheses involving achievable material properties.
The Al-Ti-B powder compacts are potential advanced energetic materials that, when subjected to high-strain-rate impact (which may or may not constitute shock compression), explosively release heat by anaerobic reaction according as certain incompletely understood conditions are met or not. The study of those conditions and the mechanism of reaction initiation (carried out by a collaborator) is the specific application that the simulations in this work cater to.
To ensure realistic morphology in simulated Al-Ti-B microstructures, this work included reconstruction (carried out by montage serial sectioning) of large three-dimensional volumes of Al-Ti and Al-B binary compacts for two sets of powders that yielded actual 3 D Ti and B particle images. Accordingly, advancement of the experimental technique of montage serial sectioning and a quantitative characterization of the real powder microstructures also formed part of this research.
While only examples from Al-Ti-B powders are used throughout this work, it is clear that the methods will apply to other similar systems.
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Borhani, Ehsan. "Microstructure and Mechanical Property of Heavily Deformed Al-Sc Alloy Having Different Starting Microstructures." 京都大学 (Kyoto University), 2012. http://hdl.handle.net/2433/152522.
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Schiltges, Gilbert. "Continuum mechanical investigations on microstructures /." [S.l.] : [s.n.], 1999. http://e-collection.ethbib.ethz.ch/show?type=diss&nr=13265.
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Schiller, Claire H. "Modelling of microstructures in metals." Doctoral thesis, Universite Libre de Bruxelles, 1989. http://hdl.handle.net/2013/ULB-DIPOT:oai:dipot.ulb.ac.be:2013/213260.
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Darling, Kris Allen. "Thermally Stability of Nanocrystalline Microstructures." NCSU, 2009. http://www.lib.ncsu.edu/theses/available/etd-03182009-141051/.
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Fe based alloys were high energy ball milled to produce supersaturated solid solutions with a nominal grain size of ~10nm. Solutes such as Y, W, Ta, Ni and Zr were selected based on their propensity to grain boundary segregated in Fe. Based on preliminary heat treatments Zr was selected as the solute of choice. Upon further heat treating experiments and microstructural analysis it was found that Zr solute additions of < 4at% could stabilize a nanocrystalline microstructure of <100nm at temperatures in excess of 900oC. This is in stark comparison to pure nanocrystalline Fe which shows coarsening to the micron scale after annealing above 600oC. Reduction in grain boundary energy due to Zr segregation and solute drag are proposed as mechanism responsible for the observed thermal stability.
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Lee, Kim Wei. "Electromechanical modelling of trapezoidal microstructures." Thesis, Imperial College London, 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.406081.
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Titley, John Graham. "The microstructures of estuarine particles." Thesis, University of Plymouth, 1988. http://hdl.handle.net/10026.1/1793.
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Miller, Patricia. "Nonlinear propagation in periodic microstructures." Thesis, University of Glasgow, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.362950.
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Wharam, David Andrew. "Electronic transport in semiconductor microstructures." Thesis, University of Cambridge, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.315922.
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Prange, Sharon M. (Sharon Marie). "Mechanics of planar periodic microstructures." Thesis, Massachusetts Institute of Technology, 2007. http://hdl.handle.net/1721.1/40462.
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Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2007.Includes bibliographical references (leaf 31).
The deformation of two-dimensional periodically patterned elastomeric sheets has been shown to trigger interesting pattern changes that are both repeatable and predictable (Bertoldi et al., 2007). Here, both square and hexagonal lattices of these sheets under axial compression are investigated both with empty voids, and also with inclusions introduced into the voids in specified patterns. A local buckling instability in the square lattice and shear instability in the hexagonal lattice trigger the change in pattern in the structure upon reaching a critical stress during compression. Experimental and numerical results are obtained that show the ability to predict and control the pattern changes that are triggered. The shape of the pattern change, the areas of the lattice in which it is triggered, and the extent to which the pattern is accentuated can all be controlled in a predictable manner. While the results here are on the millimeter length scale, they should also be applicable at the micro- and nano-scales, leading to photonic and phononic applications.
by Sharon M. Prange.
S.B.
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Chiu, N. X. N. "Designing microstructures for sodium reduction." Thesis, University of Nottingham, 2016. http://eprints.nottingham.ac.uk/33595/.
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The aim of this project was to develop the tools and knowledge to reduce dietary sodium by mitigating restrictions to flavour delivery and enhancing saltiness perception through sodium contrast effects in the mouth. This is achieved by restructuring semi-solid and liquid model food systems to achieve maximum flavour delivery for enhanced perception. The project considered two model systems: stable foams and double emulsions. Stable foams were developed to evaluate air inclusions as a potential sodium reduction strategy. Saltiness perception was enhanced as the levels of air inclusion increased and the incorporation of air also increased the delivery of a congruent mushroom aroma, ultimately this resulted in an enhanced overall flavour perception. The release of volatile aroma compounds from the aerated matrix was dependent on the hydrophobicity (Log P) of the volatile. Double water-in-oil-in-water (w1/o/w2) emulsions were evaluated as vehicles to entrap (during storage) and then deliver sodium during oral processing, ultimately to enhance saltiness perception. The emulsions (w1/o/w2) stabilised with a commercially modified octenyl succinic anhydride (OSA) starch (NC46) were able to encapsulate sodium within the inner water phase (w1), retaining 97 % of this sodium for up to 90 d and partially releasing the sodium during oral processing. The release mechanism was the digestion of the stabilising starch by oral α-amylase. When compared to a protein stabilised emulsion, a 23.7 % decrease in overall salt was achieved using NC46 stabilised w1/o/w2 emulsions, without compromising perceived saltiness. To optimise the stability and delivery of sodium from the double emulsion, different levels of OSA modification were evaluated. High levels (3%) of OSA modification increased storage stability and low (0 % OSA) and intermediate levels offered enhanced saltiness. The optimised (1.5% and 2 % OSA) w1/o/w2 emulsion was stable and conferred a 15 % reduction in total sodium without compromising saltiness. These results provide new insights into using colloidal systems to efficiently deliver sodium and aroma volatiles for perception. The sodium redistribution and contrast effects demonstrated in this work may provide new avenues to achieve sodium reduction, particularly in semi-solid and liquid systems.
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Paudel, Bhim L. "Magnetotransport in GaMnAs Based Microstructures." Miami University / OhioLINK, 2011. http://rave.ohiolink.edu/etdc/view?acc_num=miami1331927548.
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Chen, Hsiou-Lien. "Microstructures of Mineralized Cellulosic Fibers." Connect to resource, 1995. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=osu1219416246.
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Nagarajan, Pratapkumar. "Rapid production of polymer microstructures." Diss., Atlanta, Ga. : Georgia Institute of Technology, 2008. http://hdl.handle.net/1853/26539.
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Thesis (Ph.D)--Polymer, Textile and Fiber Engineering, Georgia Institute of Technology, 2009.Committee Chair: Dr. Donggang Yao; Committee Member: Dr. John.Muzzy; Committee Member: Dr. Karl Jacob; Committee Member: Dr. Wallace W. Carr; Committee Member: Dr. Youjiang Wang. Part of the SMARTech Electronic Thesis and Dissertation Collection.
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Perrin, Camille. "Etude expérimentale et modélisation des microstructures de déformation plastique intragranulaires discrètes." Thesis, Metz, 2010. http://www.theses.fr/2010METZ030S/document.
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L’amélioration des techniques de caractérisation (EBSD, MET, AFM) permet actuellement une meilleure compréhension des mécanismes plastiques intra-granulaires pour des poly-cristallin déformé. Les observations expérimentales montrent que les processus de plastification sont fortement hétérogènes et intermittent à l’intérieur des grains. Les modèles micromécaniques à champs moyens développés ces dernières décennies ne considèrent pas proprement les hétérogénéités intra-granulaires du glissement plastique. Or il est aujourd’hui démontré (simulations de Dynamique des Dislocations Discrètes par exemple) que la prise en compte de l’auto-organisation des dislocations à l’intérieur des grains est fondamentale pour mieux comprendre et expliquer les effets de taille de grains sur le comportement mécanique des polycristaux. Dans cette étude, deux approches complémentaires ont été développées : Une approche théorique qui consiste aux calculs des champs élastiques (contrainte interne et rotation de réseau) dus à une distribution discrète de boucles de dislocations contraintes par le joint de grains, et une approche expérimentale dont le but est de caractériser quantitativement les longueurs caractéristiques (espacements inter-bandes, et niveau de plasticification dans les bandes) pour des polycristaux à plusieurs tailles de grains se déformant plastiquement et de mesurer les rotations de réseau locales associées (mesure EBSD de désorientation de réseau cristallin) en vue de les comparer au champs de rotations élastiques calculés par le modèle. Le modèle a également été étendu pour permettre l’étude à des microstructures plus complexes, comme par exemple, les cellules de dislocationsThe improvement of the materials characterization techniques in the last years has given access to new important information about the microstructure of polycrystalline metals. From experimental studies of deformed polycrystals, plastic strain within grains is known to be strongly heterogeneous and intermittent. As a consequence of the collective motion of dislocations, sample surfaces are indeed characterized by the presence of slip lines and slip bands (as slip traces). In the present study, a new micromechanical approach is developed to derive the mechanical fields (stresses, distortion, lattice curvature, elastic energy) arising from the presence of an inelastic strain field representing a typical internal "microstructure" as the one observed during the plastification of metallic polycrystals. This "microstructure" is due to the formation of discrete (spatial-temporal) intra-granular plastic slip heterogeneities which are modelled using discrete distributions of circular glide dislocation loops for a grain embedded in an infinite elastic matrix. Then, field equations have been solved using the method of Fourier Transforms. In contrast with the mean field approach based on the Eshelby formalism, it is then found that stress and lattice curvature fields are not more uniform inside the grain. A grain boundary layer actually appears where strong gradients occur and whose thickness depends on the introduced internal lengths. These results are compared with experimental measurements of local lattice rotation fields obtained by orientation imaging mapping (OIM). The model is able to capture different behaviours between near grain boundary regions and grain interior. The model was also develop to allow the study of more complex microstructures like the dislocation cells
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Rudaz, Nicolas. "Incrustation de microstructures par écarts chromatiques /." [S.l.] : [s.n.], 2003. http://library.epfl.ch/theses/?display=detail&nr=2757.
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Sun, Wenqing. "Fabrication and applications of zeolite microstructures /." View abstract or full-text, 2005. http://library.ust.hk/cgi/db/thesis.pl?CENG%202005%20SUN.
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Brown, Robert James. "Electronic transport in GaAs/AlGaAs microstructures." Thesis, University of Cambridge, 1990. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.335681.
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Field, Mark. "Single electron effects in semiconductor microstructures." Thesis, University of Cambridge, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.308187.
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Solomon, Matthew Daniel. "Laterally driven self-assembly of microstructures." Thesis, Imperial College London, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.272242.
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Halfpenny, Angela. "Recrystallization microstructures and mechanisms in quartzites." Thesis, University of Liverpool, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.485942.
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Electron backscatter diffraction (EBSD) has been used to analyse 25 quartz rich rocks. The rock samples represent a range ofcommon microstructures which exhibit variations in the defonnation conditions such as changes in temperature and the amount of strain accumulated. As natural sample's defonnation conditions are poorly constrained, five out of the 25 samples were experimentally defonned samples. EBSD has been used to measure the full crystallographic orientation of all the grains contained within the mapped area The mapped microstructures have been separated out in to original 'parent' grains and recrystallized 'daughter' grains. Neighbour-daughter grains are recrystallized grains which are still in contact with a parent grain (although not necessarily its own). The samples exhibit between 10% and 95% recrystallized microstructures. The samples can be separated into two main groups based upon their microstructural and statistical characteristics. The first group represents samples which have an average subgrain size which is similar in size to the neighbour-daughters. The parent grains show a systematic increase in misorientation from the centre ofthe grain to the edges. These data are consistent with subgrain rotation (SGR) as being the controlling nucleation and recrystallization mechanism The second group ofsamples show an average subgrain size which is much larger than the size of the neighbour-daughter grains. The internal defonnation of the parent grains is randomly arranged and does not gradually increase. These data are inconsistent with SGR The recrystallization was facilitated by bulging at low temperatures or during strain-induced grain boundary migration (SIGBM). All samples studied exhibited angles between the parent and neighbourdaughter grains which had increased after nucleation and recrystallization had taken place. Each sample analysed had at least 50% of the grain boundaries with misorientation angles ofgreater than 30°. Other processes have increased the misorientation angles. The distnbutions of the neighbour-daughter grains have also been redistnbuted from being in contact with the parent they are theorized to have recrystallized from to being located either next to another parent or in the matrix. The microstructures have been modified. Grain boundary sliding (GBS) is interpreted as the controlling modification mechanism which caused the neighbour-switching and further rotations of the recrystallized grains to cause the increased misorientation angles observed.
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Horrigan, Emma. "Disordered microstructures and anomalous mechanical properties." Thesis, University of Exeter, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.496775.
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This study concerns the effects of structural heterogeneity upon mechanical properties, particularly on negative Poisson's ratio. Two separate methods were used to generate two-dimensional honeycombs optimised for a series of material properties, in particular large negative Poisson's ratio and high stiffness. The properties and heterogeneity of crumpled and recycled materials were compared.
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Wasey, Jonathan Arthur Edward. "Spontaneous emission within wavelength-scale microstructures." Thesis, University of Exeter, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.364468.
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Basanta, David. "Using genetic algorithms to evolve microstructures." Thesis, King's College London (University of London), 2005. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.421869.
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Tonry, Catherine Elizabeth Henzell. "Computational electrohydrodynamics for fabricating polymer microstructures." Thesis, University of Greenwich, 2015. http://gala.gre.ac.uk/18149/.
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The aim of the work presented in this thesis is the development of two computational models of two processes that can be used to shape molten polymers on a micro-scale, namely Electrohydrodynamic Induced Patterning (EHDIP) and Electric Field Assisted Capillarity (EFAC). These related processes both use the dielectric forces at the interface between a polymer and another dielectric such as air. When the molten polymers are placed in a shaped electric field the imbalance in these dielectric forces causes the polymer to flow in a controlled way creating shapes in the polymer melt, this is the basis for the EHDIP process. The shaped electric field is controlled by the morphology of the top mask which acts as an electrode. This process is further extended by introducing a heavily wetted surface on the top mask which results in capillary forces that cause the polymer melt to coat the top mask creating a fully enclosed shape. This process can be used to create enclosed micro-channels or micro-capsules. Thus results and discussion presented herein highlight several possible application routes for industrial manufacturing. The process is discussed here for microstructures of 1 µm to 200 µm in size. The range at which the processes work is not fully understood, however the EHDIP process has been shown to work at a nanoscale producing structures around 100 nm in size. From a comprehensive literature review, the underlying theory and mechanisms of this process were identified and the governing equations derived. Computational models were developed based on the underlying physics. These models were initially developed in PHYSICA version 3g and later they were implemented into COMSOL Multiphysics as the latter proved to be more stable. The results from the computational models were compared to the limited experimental data available. The results from the computational models show that the mask shape was found to have the largest effect on the final structure of the shaped poly-mer. Due to capillary forces the shape of the microstructure at the top mask mimics the shape of the mask. In the lower section of the enclosed microstructure there is a force balance between surface tension, dielectric forces and internal pressure, giving a rounded morphology. Furthermore, by wetting the lower mask, flat bottomed structures can be produced. By both shaping and wetting the lower mask the shape of the microstructure can be even further modified. However, sharp cornered masks are unsuitable for this process. The effects of other key parameters such as air gap, contact angle, polymer permittivity and applied voltage were investigated through a sensitivity analysis. Changing the permittivity is shown to have an effect on the final microstructure. The change is small; however the permittivity does affect the speed of the process. The contact angle between the top mask and the polymer modifies the thickness of the polymer at the top of the structures. Increasing the contact angle causes a decrease in polymer thickness due to a reduction in the capillary force. The depth of the structures can be altered by changing the air gap; hence a larger air gap gives a deeper structure. The initial polymer thickness has no effect on the top of the structure but determines the thickness, shape and curvature of the lower part of the structure. The applied voltage controls the electrostatic forces and hence the speed of the process. For a low voltage the electrostatic forces are not strong enough to initiate the process and an enclosed microstructure does not form. If the voltage is too high, the structure forms quickly and bubbles can be entrapped at the top mask. With the correct mask shapes the processes can produce a wide variety of microstructures. These would have a wide range of applications either in the communications sector as fibre-optical wave-uides or in the biomedical sector as microstructures used in BioMEMS. Further development of the process is required to ensure that the process can be controlled. The models presented here are initial investigations of this but further experimental work is required along with the expansion of the model into three-dimensions.
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Martínez-Ayers, Raúl Andrés 1977. "Formation and processing of rheocast microstructures." Thesis, Massachusetts Institute of Technology, 2004. http://hdl.handle.net/1721.1/28883.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2004.Includes bibliographical references (leaves 108-114).
(cont.) given slurry was proposed. The fluidity of rheocast A357 alloy slurries was contrasted with the fluidity of superheated liquid. Rheocast slurries with 37% solid particles,were found to flow about half as far as fully liquid alloy superheated 20⁰C above the liquidus.
The importance of semi-solid metal processing derives primarily from its ability to form high integrity parts from lightweight alloys. Since the discovery of the semi-solid metal microstructure, most part production was by reheating of billets which possessed a suitable microstructure ("thixocasting"). However, it is now apparent that there are significant advantages of forming semi-solid slurry directly from liquid alloy ("rheocasting") and efficient rheocasting processes have been engineered. In this work, experimental and analytical approaches were taken to study how non-dendritic microstructures form and evolve in Al-4.5wt%Cu alloy during the earliest stages of solidification. Experimental results showed that particles in quenched rheocast alloy were already spheroidal, and free of entrapped eutectic, after 5 seconds of solidification time. Spheroidal particles were also formed by reheating equiaxed dendrites of approximately 10 [micro]m radius above the eutectic temperature for 5 seconds, but these spheroids contained entrapped eutectic. In both rheocasting and reheating experiments, the average particle radius was found to increase with solidification time at a rate that closely follows the classical dendrite arm ripening curve. Particle growth models developed were compared with the average particle radius measurements, and particle solute content measurements. The maximum cooling rate to maintain spheroidal interface stability at various solid fractions was studied experimentally. A modified stability model which considered particle interaction through solute field overlap was developed and found to be in good agreement with experimental data. A simple method for the foundry to determine the maximum cooling rate for a
by Raul A. Martinez-Ayers.
Ph.D.
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Dirckx, Matthew E. "Demolding of hot embossed polymer microstructures." Thesis, Massachusetts Institute of Technology, 2010. http://hdl.handle.net/1721.1/61520.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2010.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Cataloged from PDF version of thesis.
Includes bibliographical references (p. 219-229).
Polymer-based microfluidic "lab on a chip" technology promises to reduce cost and extend access to medical diagnostic tests that formerly required expensive and labor-intensive lab work. The predominant methods for manufacturing these devices are miniaturized molding processes including casting, injection molding, and hot embossing. These techniques have in common the use of a mold to define the shape of functional features (fluidic channels), the separation of the part from the mold as a process step (demolding), and the intended re-use of the mold to produce additional parts. The demolding step in particular poses significant challenges for mass production. Demolding affects several issues including production rate, part quality, and mold lifetime, and demolding-related defects are frequently observed. Despite its importance, there has been no comprehensive effort to analyze demolding theoretically or experimentally. This thesis aims to deepen the understanding of demolding of polymer microstructures in order to facilitate mass manufacturing of polymer-based devices with micro-scale functional features, such as microfluidic chips. A theory of demolding mechanics has been proposed that combines the effects of thermal stress, friction, and adhesion in a unified framework. A metric by which demolding can be characterized experimentally--the demolding work--has been proposed by analogy with interfacial fracture and has been related to underlying physical mechanisms. Finite element simulations based on this theory of demolding have been performed to investigate the effects of important parameters, including demolding temperature and feature geometry. A test method for characterizing demolding by directly measuring the demolding work for individual microstructures has been developed and applied to hot embossing to study the effects of process parameters such as demolding temperature, the effects of feature geometry and layout, and the impacts of mitigation strategies such as low-adhesion mold coatings. The results of these demolding experiments broadly agree with expected trends based on the theory of demolding mechanics proposed herein. A dimensionless parameter aggregating the effects of feature geometry and layout has been identified and related to the occurrence of demolding-related defects, the demolding process window, and the demolding temperature that minimizes the demolding work. These findings have been generalized to provide processing and design guidance for industrial application of polymer micro-molding.
by Matthew E. Dirckx.
Ph.D.
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Koucky, Michael Harten. "PIEZOELECTRIC POLYMER MICROSTRUCTURES FOR BIOMEDICAL APPLICATIONS." The Ohio State University, 2009. http://rave.ohiolink.edu/etdc/view?acc_num=osu1238080858.
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Guenther, Justin. "Magnetoresistance in Permalloy/GaMnAs Circular Microstructures." Miami University / OhioLINK, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=miami1407772238.
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Drahy, Franck. "Microstructures et comportements de matériaux heterogenes." Paris 6, 1995. http://www.theses.fr/1995PA066815.
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L'objet de cette présente étude est d'analyser l'intérêt d'incorporer un apport expérimental objectif dans un modèle numérique micro-macro, afin de décrire et de prédire au mieux le comportement des matériaux heterogenes sur différentes échelles. Dans le cas du béton, une analyse d'images sur différentes éprouvettes réalisées à densité et taille de granulats fixées, permet d'approcher le variogramme relatif à la fonction de distribution des heterogeneites initiales. L'incorporation de cette corrélation initiale dans la microstructure d'un milieu discrétise, réseau triangulaire de barres articulées, se fait par l'intermédiaire d'une distribution spatiale des caractéristiques élastiques et a rupture de ces barres. En chargement de traction simple, la variation spatiale de ces caractéristiques définit un paramètre de fragilité qui conditionne le passage d'un comportement de nature fragile à un comportement ductile de ces structures discrètes. Cette modélisation discrète des matériaux heterogenes quasi-fragiles se fait sur les mêmes bases que la mécanique de la rupture. Enfin, un modèle de fissuration, base sur un front de fissure jumelant la mécanique de la rupture a la microstructure, est alors propose pour obtenir la cohérence de la discrétisation. A partir d'une définition de la densité critique de défauts, l'application de ce modèle discret permet la représentation des effets d'échelle dans les structures et la prédiction de la localisation de l'endommagement d'une échelle de structure à l'autre.
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Leith, Steven D. "Electrodeposition of NiFe 3-D microstructures /." Thesis, Connect to this title online; UW restricted, 1998. http://hdl.handle.net/1773/9855.
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Fon, Warren Chung Wah Cross Michael Clifford. "Thermal properties of nano- and microstructures /." Diss., Pasadena, Calif. : California Institute of Technology, 2004. http://resolver.caltech.edu/CaltechETD:etd-05262004-123035.
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Drahy, Franck. "Microstructures et comportements de matériaux hétérogènes /." Cachan : Laboratoire de mécanique et technologie, 1995. http://catalogue.bnf.fr/ark:/12148/cb35846310v.
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Brick, Peter. "Coherent optical nonlinearities in semiconductor microstructures." Diss., The University of Arizona, 2001. http://hdl.handle.net/10150/280073.
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This dissertation presents investigations of fundamental optical nonlinearities in semi-conductor microstructures. Two distinct effects are studied. In the first part the excitonic optical Stark effect in InGaAs and GaAs multiple quantum-well structures is investigated by means of pump-probe spectroscopy. For nonresonant excitation below the excitonic transition the direction of the shift of the resonance depends on the polarization of the pump and probe pulses. In particular, for anti-circular polarization a surprising red-shift is observed. For resonant excitation, induced absorption energetically above and below the exciton transition and bleaching of the resonance is found. Experiments using both resonant and nonresonant excitation reveal the importance of bound and unbound two-exciton states in absorption changes of the 1s heavy-hole exciton resonance. It is found that higher-order Coulomb contributions determine the intensity as well as the time dependence of the differential excitonic absorption. In addition, the influence of light-hole excitons is analyzed. It is shown that the direction of the optical Stark shift for nonresonant excitation depends also sensitively on the heavy-hole to light-hole splitting and the detuning of the pump pulse. For very high pump intensities and nonresonant excitation the absorption is split when a circularly polarized pump and a linearly polarized probe beam are used. For co-circular excitation traces of hyper-Raman gain are observed. In the second part of this dissertation, the nonlinear optical response of semiconductor microcavities in the nonperturbative regime is studied in resonant single-beam transmission and pump-probe experiments. In both types of experiment, a pronounced third transmission peak lying spectrally between the two normal modes is observed. Its dependence on the probe intensity, pump intensity, pump-probe delay, exciton-cavity detuning and pump detuning is investigated. For single-beam transmission, the energy of the third peak parallels the position of the cavity resonance. It is more pronounced for circularly polarized excitation and lasts longer than the two normal modes. For pump-probe experiments, the third peak increases with decreasing probe intensity and increasing pump intensity. Its energy is close to the low-energy side of the pump spectrum and virtually unaffected by the cavity-exciton detuning. The appearance of the third peak requires temporal overlap of pump and probe pulses. The origin of this complex nonlinearity is the quantum nature of light, which induces intraband polarizations in the presence of a coherent driving field and a finite carrier density. It is found that the coupling of the intraband polarizations via guided modes to the polarization of the fundamental longitudinal mode is responsible for the third transmission peak. A fully quantized theory reproduces the experimental observations.
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Squire, E. K. "Light emitting microstructures in porous silicon." Thesis, University of Bath, 1999. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.285287.
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Mayes, Eric Leigh. "Nanoparticle-based routes to functional microstructures." Thesis, University of Bath, 2001. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.392011.
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Sosa, John Manuel. "Development of Tools for 2D and 3D Microstructural Characterization and Their Application to Titanium Alloy Microstructures." The Ohio State University, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=osu1420629389.
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Singh, Harpreet. "Computer simulations of realistic microstructures implications for simulation-based materials design/." Diss., Atlanta, Ga. : Georgia Institute of Technology, 2007. http://hdl.handle.net/1853/22564.
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Thesis (Ph. D.)--Materials Science and Engineering, Georgia Institute of Technology, 2008.Committee Chair: Dr. Arun Gokhale; Committee Member: Dr. Hamid Garmestani; Committee Member: Dr. Karl Jacob; Committee Member: Dr. Meilin Liu; Committee Member: Dr. Steve Johnson.
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Bonhomme, Céline. "Turbulences et ondes en milieu naturel stratifié : deux études de cas : étude du mélange turbulent et des ondes internes du lac Pavin (Auvergne, France) ; influence des ondes de Rossby sur la concentration en chlorophylle de surface dans l'upwelling du Pérou." Phd thesis, Université Paris-Est, 2008. http://tel.archives-ouvertes.fr/tel-00558713.
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Dans les milieux aquatiques, les facteurs liés à l'hydrodynamique et au mélange aux échelles de la turbulence sont déterminants pour le transport des substances chimiques et des micro-organismes. Ce travail s'intéresse à ces phénomènes physiques et se situe à la frontière entre la limnologie physique et l'océanographie physique, au travers de l'étude de deux cas : le lac Pavin (Auvergne, France) et la région d'upwelling au large du Pérou. Un des intérêts de ce travail réside dans le changement d'échelle de l'observation de milieux naturels aquatiques allant de l'échelle millimétrique (étude de la microstructure dans le Lac Pavin) à l'échelle de la centaine de kilomètres (étude des ondes de Rossby). Le Lac Pavin est le seul lac méromictique français, ce qui signifie que le compartiment inférieur du lac ne se mélange pas annuellement. La méromicticité concerne pourtant bien d'autres bassins, plus connus et plus vastes comme la Mer Noire, la Mer Caspienne, le lac Baïkal, d'où son importance écologique. Sa méromicticité lui confère des propriétés physiques particulières, étudiées dans le cadre de cette thèse. L'étude de la variabilité du mélange diapycnal à différentes échelles de temps et d'espace dans le lac Pavin au travers de mesures de microstructure et de surveillance en continu des principaux paramètres physiques a conduit à mettre en évidence la variabilité de la stabilité de la colonne d'eau à l'échelle interannuelle. Par ailleurs, les propagations d'ondes internes de gravité sont étudiées dans ce cadre particulier à l'aide d'un modèle linéaire bidimensionnel. Elles génèrent des mouvements advectifs verticaux importants dans toute la colonne d'eau dont il faut tenir compte dans les protocoles d'échantillonnage chimiques ou biologiques. Enfin, une intrusion sous lacustre semble avoir une grande importance dans la détermination de la profondeur du mélange hivernal et le maintien de la méromicticité. La zone de l'upwelling du Pérou est parmi les plus poissonneuses au monde. Elle est caractérisée par une forte productivité primaire, observable par satellite. L'observation des ondes de Rossby a permis de mettre en évidence qu'elles génèrent plus de 50% de la variabilité observée à l'échelle intra saisonnière sur la concentration en chlorophylle de surface, à distance des côtes. Les ondes transportent la chlorophylle vers le large et participent à la détermination de l'extension de la zone de forte productivité primaire. A travers des mesures variées allant de la microstructure à l'observation satellite et grâce à l'utilisation de modèles conceptuels, la thèse met en évidence l'influence certaine des facteurs hydrodynamiques sur l'organisation des paramètres biogéochimiques de ces deux écosystèmes
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Garcia, Maxime. "Etude de la formation des microstructures au cours du traitement de revenu dans les alliages de titane proche-β." Thesis, Paris Sciences et Lettres (ComUE), 2019. http://www.theses.fr/2019PSLEC006.
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Ce travail de thèse s’intègre dans un contexte d’optimisation des traitements thermomécaniques et thermiques des alliages de titane forgés, pour les applications aéronautiques. Il prend la suite d’un premier travail qui portait sur l’ effet des traitements thermiques sur le développement des microstructures dans l’alliage Ti18. Cette première étude, qui impliquait les mêmes partenaires, a donné lieu à des résultats extrêmement prometteurs pouvant avoir des retombées industrielles importantes, ce qui implique aujourd’hui la nécessité d’acquérir des connaissances nouvelles sur la genèse des microstructures dans la famille des alliages de titane proche-beta. Ce projet porte donc sur la compréhension des phénomènes liés aux séquences de transformations de phases dans la famille des alliages de titane proche-beta (alliages Ti555, NATIS, Ti18, en particulier), ainsi que leur influence sur les propriétés mécaniques finales des alliages, avec pour application principale les trains d’atterrissageThe aim of the study is to optimize heat traitements on forged titanium alloys for aerospace applications. This is the following of a first thesis working on the effect of heat treatement conditions on the development of microstructures in the Ti18 alloy. This first study shown very promising results and could have many benefits applications. That is why it is essential to acquire some informations on microstructure genesis in near-beta titane alloys. The aim is to understand phenomenons on phase transformation sequences in the family of near-beta titanium alloys (specificaly Ti555, NATIS, Ti18) and their influences on the final mecanical properties, with main application for landing gear
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Au, Yat-Yin. "Light scattering studies of metallic magnetic microstructures." Columbus, Ohio : Ohio State University, 2006. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=osu1141842225.
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Müller, Jens. "Accurate FE simulation of three-dimensional microstructures." [S.l. : s.n.], 2001. http://deposit.ddb.de/cgi-bin/dokserv?idn=963852728.
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Rowland, Harry Dwight. "Thermomechanical Manufacturing of Polymer Microstructures and Nanostructures." Diss., Georgia Institute of Technology, 2007. http://hdl.handle.net/1853/14642.
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Molding is a simple manufacturing process whereby fluid fills a master tool and then solidifies in the shape of the tool cavity. The precise nature of material flow during molding has long allowed fabrication of plastic components with sizes 1 mm 1 m. Polymer molding with precise critical dimension control could enable scalable, inexpensive production of micro- and nanostructures for functional or lithographic use.
This dissertation reports experiments and simulations on molding of polymer micro- and nanostructures at length scales 1 nm 1 mm. The research investigates two main areas: 1) mass transport during micromolding and 2) polymer mechanical properties during nanomolding at length scales 100 nm. Measurements and simulations of molding features of size 100 nm 1 mm show local mold geometry modulates location and rate of polymer shear and determines fill time. Dimensionless ratios of mold geometry, polymer thickness, and bulk material and process properties can predict flow by viscous or capillary forces, shape of polymer deformation, and mold fill time. Measurements and simulations of molding at length scales 100 nm show the importance of nanoscale physical processes distinct from bulk during mechanical processing. Continuum simulations of atomic force microscope nanoindentation accurately model sub-continuum polymer mechanical response but highlight the need for nanoscale material property measurements to accurately model deformation shape. The development of temperature-controlled nanoindentation enables characterization of nanoscale material properties. Nanoscale uniaxial compression and squeeze flow measurements of glassy and viscoelastic polymer show film thickness determines polymer entanglement with cooperative polymer motions distinct from those observed in bulk.
This research allows predictive design of molding processes and highlights the importance of nanoscale mechanical properties that could aid understanding of polymer physics.