Página 6 dos resultados de 3096 itens digitais encontrados em 0.069 segundos

‣ Contact fatigue : life prediction and palliatives

Conner, Brett P. (Brett Page), 1975-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 135 p.; 5836981 bytes; 5836737 bytes; application/pdf; application/pdf
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Fretting fatigue is defined as damage resulting from small magnitude (0.5-50 microns) displacement between contacting bodies where at least one of the bodies has an applied bulk stress. The applicability and limits of a fracture mechanics based life prediction is explored. Comparisons are made against highly controlled experiments and less controlled but more realistic experiments using a novel dovetail attachment fixture. Surface engineering approaches are examined from a mechanics perspective. Using a new tool, depth sensing indentation, the mechanical properties of an aluminum bronze coating are determined. Fretting fatigue experiments are performed on specimens coated with aluminum bronze and on specimens treated with low plasticity burnishing. Low plasticity burnishing is a new method of introducing beneficial compressive residual stresses without significant cold work at the surface. A mechanics based approach to the selection of palliatives is addressed.; by Brett P. Conner.; Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2002.; Includes bibliographical references (p. 125-135).; This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.

‣ In vivo activation and biocompatibility of a MEMS microreservoir drug delivery device

Shawgo, Rebecca Scheidt, 1976-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 129 p.; 5765301 bytes; 5765108 bytes; application/pdf; application/pdf
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Temporal and spatial control over the delivery of therapeutic compounds is an important, fertile, and rapidly advancing field of study in medicine. This work describes the advancement of a new technology of drug delivery from a benchtop prototype releasing tracer molecules to an implantable device for initial animal studies. The improved MEMS (micro-electro-mechanical systems) device was used for the subcutaneous delivery of both tracer molecules (fluorescein and mannitol) and a chemotherapeutic agent (carmustine) in rats. Both temporal and spatial profiles of the tracer molecules were established; only the temporal kinetics of the carmustine were studied. The MEMS drug delivery device is based on a silicon substrate into which microreservoirs are etched. Each reservoir contains an individual dosage of drug and is independently addressable. The microreservoirs are covered with gold membranes which act as anodes. The application of an anodic voltage, in an aqueous solution containing chloride ions, electrochemically transforms gold into gold chloride which is readily soluble in water. This device allows the delivery of both solid and liquid drugs of a wide variety of compositions.; (cont.) It is important to study the biocompatibility of the device activation process as well as that of the component materials since the activation of the MEMS drug delivery device depends on an electrochemical reaction. Other researchers have studied the biological response to gold...

‣ Enhanced performance of optical sources in III-V materials using photonic crystals

Erchak, Alexei A. (Alexei Andrew), 1976-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 216 p.; 25911085 bytes; 25910844 bytes; application/pdf; application/pdf
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This thesis applies the unique properties of photonic crystals to enhance the performance of several III-V optical sources. Emphasis is placed on the primary limitation of using photonic crystals for III-V optical sources: nonradiative recombination pathways introduced at the high dielectric contrast interfaces of the photonic crystal. Chapter 1 begins the thesis by providing a theoretical and historical framework for photonic crystals, and lays the foundation for the design of the novel optical sources presented in the later chapters. Chapter 2 develops a framework for improving the external efficiency of a semiconductor LED using photonic crystals. The design, fabrication, and testing of a novel high-efficiency semiconductor LED is described in detail. To improve coupling to radiation modes without introducing nonradiative surface recombination pathways, a triangular lattice photonic crystal in two-dimensions is etched into the top layer of an asymmetric InGaP/InGaAs quantum well structure emitting at 980 nm. Enhanced injection and external efficiency of the LED is demonstrated. Chapter 3 develops methods of laterally steam oxidizing AlAs into A1203 to form large-area wide stop-band one-dimensional Al203/GaAs photonic crystals. The mechanical stability of the high-dielectric contrast interface determines the quality of the photonic crystal and is therefore explored in detail by examining the AlAs steam oxidation process. The A1203/GaAs photonic crystal is integrated with an InP/InGaAs absorber region and used to self-start ultra-short 35 femtosecond pulses in a Cr4+:YAG laser. Chapter 4 develops the design...

‣ Using narrowband pulse-shaping to characterize polymer structure and dynamics : Deathstar GHz spectroscopy

Vachhani, Neal Arvind, 1981-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 122 leaves; 2950805 bytes; 2946460 bytes; application/pdf; application/pdf
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(cont.) The validation of this technique for probing amorphous polymer structure and dynamics lays the ground for further study of heterogeneous materials, such as nanocomposites and block copolymers.; A narrowband pulse-shaper called the Deathstar has been used along with a picosecond acoustic technique to study amorphous polymers. The temperature dependence of the longitudinal acoustic velocity and the frequency dependence of the acoustic attenuation have been measured. The frequency range of longitudinal phonons studied is not directly accessible by other spectroscopies. Probing material response in this intermediate regime is valuable because it helps characterize secondary transitions and energy dissipation mechanisms in polymers. Broadband experiments have been done to study the temperature dependence of the acoustic velocity for polystyrene and poly(methyl methacrylate) from 10 K to 300 K. The results are in line with literature values and the predictions of a model based on acoustic impedance mismatch theory. Narrowband studies with the technique used were previously limited to amorphous silica. They are extended for the first time to amorphous polymers. The Deathstar GHz spectroscopy is used to determine the absolute acoustic attenuation coefficient as a function of frequency for PMMA. The values obtained are similar to those found in literature. However...

‣ Next generation optical receivers : integration and new materials platform

Zhang, Yiwen, 1978-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 81 leaves; 4643551 bytes; 4652424 bytes; application/pdf; application/pdf
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Future growth of optical communication into new application and market space is highly dependent on the ability of optical receivers to increase functionality while reducing price and physical size. Current hybrid receiver technology is inadequate in meeting the cost and performance demands of future market. Monolithic integration and new material systems are potential solutions and have been the focus of research investigation. This thesis summarizes the research progress of monolithic integration on InP, and the achievements in realizing 1.55[micro]m photodetector on GaAs and Si and their potentials for monolithic opto-electronic integrated circuits. The overall trend for next generation receivers is to move towards higher levels of integration, with investigation in new material systems that have the potentials for lower cost and larger scale integration. The impact of monolithic integration optical receiver components is analyzed in a cost analysis model.; by Yiwen Zhang.; Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2004.; Includes bibliographical references (leaves 77-81).

‣ Experimental and finite element analysis of high pressure packer elements

Berger, Stephanie, 1981-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 30 leaves; 1846862 bytes; 1847692 bytes; application/pdf; application/pdf
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Packer elements are traditionally rubber seals that can operate under specified downhole conditions and provide a seal for either a short-term, retrievable, or a long-term, permanent, completion. In this case a retrievable 19.7cm (7-3/4") packer element for a high-pressure high-temperature (HPHT) environment was designed and tested. The element created a seal between the mandrel, or tubing, and the casing. At high temperature and pressure rubber needs to be contained so that it will create and maintain an energized seal. In this study only Aflas rubber was tested. Various backup systems were examined; some more traditional designs such as the carbon steel foldback ring were compared to more experimental ideas. Results of theoretical simulations were compared to actual test results in order to gain a greater understanding of element behavior. Experiments were also performed to study the process of element setting, which is difficult to observe due to the high pressures and temperatures required. In a related study alternative materials to rubber such as annealed high-conductivity oxygen-free copper were tested to determine if the properties could be applied for packer element applications. The most successful design was the foldback ring with an anti-extrusion PEEK ring under the gage ring. This design passed a liquid test at 134 MPa (19.5k psi) differential pressure and a gas test at 87.6 MPa (12.7k psi) differential pressure. New designs such as the split ring with mesh and the garter spring with mesh did not pass fixture tests but could be successful with further modifications. FEA was used as an analytical tool to create simulations of the element after a setting force is applied. The modeling was shown to correlate to the actual test results and therefore it would be a good tool to use in future studies.; by Stephanie Berger.; Thesis (S.M.)--Massachusetts Institute of Technology...

‣ Design of chemistry and morphology of polymer filtration membranes

Akthakul, Ariya, 1973-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 174 p.; 10741204 bytes; 10741010 bytes; application/pdf; application/pdf
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To improve membrane materials in water filtration, which currently display broad pore size distribution, hydrophobic chemistry, and fouling behavior, a novel design of chemistry and morphology of membranes is employed. First, through fundamental studies of morphological formation both in bulk by lattice-Boltzmann (LB) simulation methods and at the surface by observation of electron micrographs, it is illustrated that phase separation via spinodal decomposition is responsible for pore development. This understanding suggests the possibility to tailor a uniform and interconnected porous membrane by using the spinodal structure. Considering that the control of spinodal porous structure on a nanoscale can be challenging, an alternative approach to achieve a similar interconnected morphology by utilizing the self-assembled structure of a graft copolymer is presented. This graft copolymer permits not only the design of morphology through its architecture, but also the design of chemistrythrough its chemical components. Here, a comb-type structure of a copolymer is applied; this structure contains a hydrophobic poly(vinylidene fluoride) (PVDF) backbone for structural integrity and hydrophilic poly(ethylene oxide) (PEO) side chains for preferential water transport. A membrane with the microphase-separated structure of this copolymer at the surface is then utilized to clean oily water wastes where the membrane rejects more than 99.9% of the oil without fouling. This membrane can also perform molecular sieving...

‣ Continuum models for intergranular films in silicon nitride and comparison to atomistic simulations

Bishop, Catherine M. (Catherine Mary), 1975-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 155 leaves; 5623325 bytes; 9943528 bytes; application/pdf; application/pdf
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A continuum thermodynamic model is developed for the treatment of interfaces in materials. This phase-field model includes energetic contributions from chemical, structural and electrostatic effects. A small parameter set is introduced in the model. These parameters should be adjusted based on the relation of equilibrated phase-field structures to atomistic simulations and experimental observations. To compare the continuum fields to discrete data sets, coarse-graining methods are proposed. Two methods of measuring local atomic structure are developed, one based on Voronoi tessellations and the other based on bond-angle distribution functions. A coarse-graining method based on volume averaging over Voronoi tessellations is employed to continuize both structure and composition information. These fields give insight into the local atomic environment that is necessary for devising continuum models. The stability of intergranular films in CaO-doped Si₃N₄ is examined. A modified associate model that incorporates bulk-thermodynamic Gibbs free energy functions and treats non-bulk equilibrium compositions is developed. Kinetic equations are derived to solve for the equilibrium fields. While the simulations do not reach equilibrium, some trends can be observed. Locally charged domains are kinetically stable near the film. Non-uniform calcium distributions in the doped films indicate that calcium is preferentially located near the film-grain interface. The structural width of the film can be tuned by altering appropriate model parameters.; (cont.) Future avenues of research suggested by this thesis include: developing algorithms for measuring local structure and orientation in multi-component systems and devising atomistic simulations of intergranular pockets that couple experimental information with interatomic potentials.; by Catherine M. Bishop.; Thesis (Ph. D.)--Massachusetts Institute of Technology...

‣ An analytic solution for magnetization distribution in multigrain ferromagnetic materials in an applied magnetic field

Sunter, Kristen A. (Kristen Ann), 1982-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 22, [20] leaves; 1987029 bytes; 1985521 bytes; application/pdf; application/pdf
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The magnetic behavior of a material is governed by the variation in anisotropy direction from grain to grain as well as the changes in ferromagnetic parameters at grain boundaries and other defect regions. For example, transmission electron microscopy results show that chromium segregation occurs at the grand boundaries in CoCrTa films, which are used in hard disk drives. In this paper, we model the case of two adjacent semi-infinite grains with arbitrary crystalline orientations with respect to each other. A Gaussian distribution is used to model the change in magnetic properties at the interface, and boundary conditions are imposed on the direction of magnetization deep within the grains and at the interface. The effects due to the diffuse interface are included using perturbation theory. The sum of the exchange, anisotropy and Zeeman energies is minimized, and the resulting Euler equation is solved analytically. A profile of the magnetization orientation in an inhomogeneous medium in an applied field is obtained to show the extent of the effects of grain boundary segregation. These results can direct future large-scale computer calculations and media improvement.; by Kristen A. Sunter.; Thesis (S.B.)--Massachusetts Institute of Technology...

‣ A new model for electric force microscopy and its application for electrostatically generated phase difference in tapping mode AFM; New model for EFM microscopy and its application for electrostatically generated phase difference in tapping mode Atomic Force Microscope

Stone, Peter (Peter Robert)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 37 leaves; 1632936 bytes; 1632286 bytes; application/pdf; application/pdf
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The harmonic force balance method was used to model and simulate electric force microscopy (EFM) and electrostatically generated phase difference in tapping mode AFM (EPTA) measurements. Simulations show that the harmonic force balance approach matches and explains EFM and EPTA experimental results well. Simulations also show that the model depended on both geometric and materials parameters. The harmonic force balance model was subsequently used to directly simulate a previously performed EPTA experiment. Data obtained from the model showed a remarkable similarity to the experimentally obtained data, thus validating the use of the harmonic force balance model to simulate EPTA data.; by Peter Stone.; Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2005.; Includes bibliographical references (leaf 37).

‣ Multifaced stone and ceramic moulds from Bronze Age Anatolia : building an analytical protocol of mould properties and behavior during the process of metal casting

Biçer, Katherine K. (Katherine Kershen)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 193 leaves; 15895523 bytes; 15903739 bytes; application/pdf; application/pdf
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A three phase analytical protocol is developed to systematize the study of multifaceted serpentinite bronze-casting moulds from Bronze Age Anatolia (ca. 3500-1700 B.C.). These moulds represent a class of metal processing tools that reflect material properties and perhaps also social concerns in their development and use. In phase I of the protocol, standard serpentinite tiles are heated to known temperatures and the crack density and ineral changes at each temperature are determined using scanning electron microscopy (SEM) and energy dispersive x-ray (EDX) analysis. These results then serve to calibrate the cracking and mineralogical behaviors of serpentinite replica moulds used in controlled casting experiments during phase II. Metallography on several phase II cast objects provides additional information on heat flow through the stone. In phase III, the protocol is modified appropriately for the non-destructive study of four archaeological moulds using x-ray diffraction (XRD), x-ray fluorescence (XRF), and SEM. The applicability of the protocol to other stone and ceramic materials is discussed.; by Katherine K. Biçer.; Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2005.; Includes bibliographical references (leaves 91-96).

‣ Genesis : the search for origins : the curation and contamination control of returned solar wind samples

Jackson, Benjamin K
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 62 leaves; 4953582 bytes; 4956146 bytes; application/pdf; application/pdf
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The purpose of the studies carried out in this thesis was to aid in the curation of samples of solar wind returned to earth on the Genesis spacecraft. An experimental study was carried out to aid development of a set of protocols for the laser scribing and subdivision of the Genesis silicon collector array materials. Optimisation of the scribing speed and the positioning of the focal point of the laser were carried out. It was found that scribe width was independent of both factors. Slower scribing speeds were found to produce deeper scribes, while heating effects were minimised with faster speeds. Vertical movement of the stage of 5 pm/pass was found to optimise the focal point of the laser, and minimise heating effects. A procedure to measure the flexural modulus of samples was proposed to quantify the success of the optimisation of the scribing parameters. A theoretical study was carried out to develop a predictive kinetic model for the oxidation of the silicon collector arrays during flight. The mechanism proposed for the increase in oxide thickness over that present pre-flight was the formation of a less-dense suboxide at the SiO₂/Si interface. The driving force is the elevated temperature of the collectors during collection...

‣ Substrate engineering for monolithic integration of III-V semiconductors with Si CMOS technology

Dohrman, Carl Lawrence
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 172 p.
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Ge virtual substrates, fabricated using Si1-xGex-.Ge, compositionally graded buffers, enable the epitaxial growth of device-quality GaAs on Si substrates, but monolithic integration of III-V semiconductors with Si CMOS using this platform is hampered by the large thickness of the Si1-xGex graded region. To address this issue, the Silicon on Lattice-engineered Silicon (SOLES) was developed, consisting of a silicon-on-insulator (SOI) structure fabricated on a Ge virtual substrate. Placement of the Si device layer at the surface makes it possible to process this platform similarly to typical SOI wafers, with the added functionality of a buried III-V template which can be used for GaAs device fabrication. This platform was fabricated using a scalable layer transfer technique. AlInGaP LEDs were also demonstrated on a SOLES substrate. In addition, an alternative growth process was investigated for Si1-xGex virtual substrates with lower threading dislocation density (TDD) and thickness. This process, the thermally relaxed ultra-thin (TRUT) buffer process, consists of coherent growth of lattice-mismatched Si1.xGex layers, followed by post-growth annealing. Growth of TRUT buffers over the Si0.5Ge0.5 to Si0.3Ge0.7 alloy range with high strain levels resulted in the nucleation of surface defects which appear to limit the maximum strain rate of compositionally graded buffers. However...

‣ High efficiency thin film silicon solar cells with novel light trapping : principle, design and processing

Zeng, Lirong, Ph. D. Massachusetts Institute of Technology
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 237 p.
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One major efficiency limiting factor in thin film solar cells is weak absorption of long wavelength photons due to the limited optical path length imposed by the thin film thickness. This is especially severe in Si because of its indirect bandgap. This thesis invents a novel light trapping scheme, the textured photonic crystal (TPC) backside reflector, which can enhance path length by at least several hundred times the film thickness for sufficient absorption. Physical principles and design optimization of TPC are discussed in detail. Thin film Si solar cells integrated with the new back reflector are successfully fabricated and significant efficiency enhancement is demonstrated.The new back reflector combines a one-dimensional photonic crystal as a distributed Bragg reflector (DBR) and reflection grating. The DBR achieves near unity reflectivity in a wide omnidirectional bandgap completely covering the wavelengths needing light trapping, and the grating can diffract light into large oblique angles and form total internal reflection against the front surface of the cell. The unique combination of DBR and grating tightly confines light inside the cell, effectively changing the path length from the thickness of the cell to its width.The back reflector parameters and the antireflection coating are systematically optimized for thin film Si solar cells through simulation and experiments. A 2 [mu]m thick cell can achieve 54% efficiency enhancement using the optimized design.For proof of concept...

‣ Structural, magnetic, and optical properties of orthoferrite thin films

Supplee, William Wagner
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 43 leaves
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Pulsed laser deposition was used to create thin films of Ce-Fe-O and Y-Fe-O systems. Deposition temperature and ambient oxygen pressure were varied systematically between samples to determine which deposition conditions were most favorable to the formation of cerium/yttrium orthoferrite. The structure and composition of each film were then determined using X-ray diffraction and wavelength dispersive spectroscopy respectively. In addition, the magnetic and optical properties of the yttrium films were characterized to determine the suitability of these materials as Faraday isolators at A=1550 nm. Results show that orthoferrite crystal structures in these systems are not stable in the temperature and oxygen ranges tested. It was also found that increasing oxygen pressure caused exponential decay in the deposition rate. Most films were amorphous, exhibiting a paramagnetic M-H plot and a Verdet coefficient between 0.37 and 0.89 deg cm-1 Gauss-1; by William Wagner Supplee, Jr.; Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2007.; Includes bibliographical references (leaves 41-43).

‣ Tunable micro-cavities in photonic band-gap yarns and optical fibers

Benoit, Gilles, Ph. D. Massachusetts Institute of Technology
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 140 leaves
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The vision behind this work is the fabrication of high performance innovative fiber-based optical components over kilometer length-scales. The optical properties of these fibers derive from their multilayer dielectric photonic band-gap structure that exhibits omnidirectional reflectivity. The theoretical tools needed to design, analyze and optimize such structures are introduced. We show that defect layers in these otherwise periodic structures act as optical micro-cavities that enable precise design of the fibers' spectral response. Fabrication of these composite fibers by thermal drawing of a macroscopic preform in the viscous state requires solving material selection challenges in order to identify pairs of materials with high refractive index contrast and similar thermo-mechanical properties. Operational wavelengths ranging from the UV to the IR are demonstrated and made possible by the wavelength scalability of the photonic band-gap structure and accurate knowledge of the materials' dispersion relation afforded by broadband spectroscopic ellipsometry. The fundamentals of this technique, which is used to characterize a number of dielectrics, semi-conductors and metals, are surveyed. Two fiber structures are then explored: fibers for external reflection and hollow-core transmission fibers.; (cont.) We demonstrate that the resonance wavelength of Fabry-Perot cavities embedded in reflecting fibers can be tuned reversibly under applied elastic strain or external illumination at 514 nm. A simple opto-mechanical model is developed to assess the mechanical tuning efficiency while a review of the photodarkening effect in chalcogenide glasses and accurate measurements of the amplitude and response time associated with its transient component are presented to explain and optimize the optical tuning scheme. Modulation of the fibers' reflectivity near their cavity resonant wavelengths is demonstrated at various frequencies. Based on these results...

‣ Field induced orientation of semicrystalline and non-crystallne block copolymer microdomain patterns

Park, Cheolmin, 1970-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 225 leaves; 25606277 bytes; 25606036 bytes; application/pdf; application/pdf
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Various block copolymer microdomain structures are controlled in bulk as well as in thin film by employing flow fields, directional solidification, and substrates. In bulk systems, flow fields generated by the 'roll casting' process orient amorphous cylindrical microdomains along the flow direction in a semicrystalline block copolymer. Subsequent crystallization of the crystalline block is significantly influenced by the pre-existing oriented amorphous cylindrical microdomains. The orientation of crystalline lamellae is achieved parallel to the cylinder axis, completely suppressing spherulite formation. Microdomain structures of block copolymers are also controlled in thin films by directional solidification of a crystallizable solvent. This new method is based on the use of crystalline organic materials, which are solvents for the block copolymers above their melting temperatures. The directional crystallization of the solvent induces the directional microphase separation of the block copolymer. Furthermore, the flat (001) crystal face of benzoic acid or anthracene provides both a surface for epitaxy of a semicrystalline polyethylene block as well as a confining surface for the thin polymer film which forms between the crystallizing solvent and the glass or silicon wafer substrate. Several semicrystalline and non-crystalline block copolymers were directionally solidified using a crystallizable solvent. A bi-axially ordered edge-on crystalline lamellar structure is obtained due to the epitaxy between a melt-compatible semicrystalline block copolymer and benzoic acid single crystal. Directional solidification generates vertically aligned lamellar and cylindrical microdomain structures of conventional non-crystalline block copolymers such as PS/PMMA and PS/PI.; (cont.) The combination of epitaxy and directional solidification with a strongly segregated cylinder forming semicrystalline block copolymer produces a perpendicularly aligned cylindrical microstructure with each cylinder containing precisely one crystalline lamella. In addition...

‣ Effect of radiation on silicon and borosilicate glass

Allred, Clark L. (Clark Lane), 1972-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 255 p.; 9567443 bytes; 9567245 bytes; application/pdf; application/pdf
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A study was made that is logically divided into two parts, both involving radiation damage effects. The first is a study of the effects of neutron and gamma radiation on the dimensions of two borosilicate glasses, Pyrex® and Hova SD-2®. These two glasses are commonly used as substrates for silicon microelectromechanical (MEMS) devices, and radiation-induced compaction in a substra.te can have deleterious effects on device performance. Results are presented for density changes induced in both glasses by neutron irradiation. Pyrex was shown to compact at a rate of (in [delta]p[rho]/p[rho] per n/cm2̂) 8.14 x 10-̂20 (thermal) and 1.79 x 10-̂20 (fast). The corresponding results for Hoya SD-2 were 2.21 x 10-̂21 and 1.71 x 10-̂21, respectively. On a displacement per atom (dpa) basis, the compaction of the Pyrex was an order of magnitude greater than that of the Hoya SD-2. Our results are the first reported measurement of irridiation-induced densification in Hoya SD-2. The compaction of Pyrex agreed with a previous study. Our results for gamma irradiations were unexpected. Silicon MEMS strain gauges mounted on glass wafers were gamma-irradiated to hundreds of Mrad. Based on expectations from the literature, the Pyrex was supposed to compact to a level easily measurable by the MEMS strain gauges. Almost no substrate compaction registered in the strain gauges...

‣ Attractive electrostatic self-assembly of ordered and disordered heterogeneous colloids

Maskaly, Garry R. (Garry Russell), 1978-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 193 p.; 7064851 bytes; 7064543 bytes; application/pdf; application/pdf
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Ionic colloidal crystals are here defined as multicomponent ordered colloidal structures stabilized by attractive electrostatic interactions. These crystals are colloidal analogues to ionic materials including zincblende, rocksalt, cesium chloride, and fluorite. A thermodynamic study revealed that the screening ratio, charge ratio, and monodispersity are critical parameters in ionic colloidal crystal (ICC) formation. Experimentally, small ordered regions were observed under ideal thermodynamic conditions. However, no larger crystalline regions were found in these samples. The kinetics of ICC formation was studied using a variety of computational techniques, including Brownian dynamics, Monte Carlo, and a Newton's method solver. These techniques have each elucidated properties and processing conditions that are important to crystallization. The Brownian dynamics and Monte Carlo simulations showed that the previous experiments were highly undercooled. Furthermore, a narrow crystallization window was found, demonstrating the need to create particle systems that meet the narrow parameter space where ICCs should be stable. Pair interaction potentials were evaluated for their accuracy using a Poisson-Boltzmann (PB) equation solver. The PB solver was also used to further refine crystalline formation energies so that systems can be more accurately tailored. A surprising result from the PB solver showed that the lowest formation energy occurs when the quantity of surface charges on both particles are equal. Although this result is not predicted by any colloidal pair potentials...

‣ Growth and characterization of bismuth perovskite thin films for integrated magneto-optical isolator applications

Taussig, Alexander R
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 156 leaves
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In this thesis, we discuss the motivation for integrated magneto-optical isolators and explain why the orthoferrite is such an attractive materials class for this purpose. We then derive from first physical principles the dependence of Faraday rotation, absorption, and certain figures of merit on the material's dielectric tensor elements. Next, we use pulsed laser deposition to grow thin films of BiFeO3 on MgO (001) and SrTiO3 (001) substrates. After optimizing growth conditions to obtain high quality films, we characterize the films' crystal structure with two-dimensional x-ray diffraction. We then examine the magnetic, optical, and magneto-optical properties of these films. We find that the highly textured films grown on SrTiO3 are monoclinic with an out-of-plane c-axis aligned with the (001) direction of the substrate and approximate pseudocubic lattice parameters of a = b = 4.04 A, c = 3.95 A, and 90° - [beta] = -0.88°. These films are weakly magnetic, with a magnetization of 1.2 emu/cm3 at an applied field of 10 kOe; highly absorptive, with an average absorption coefficient of 910 cm-1; and possess a low specific Faraday rotation of 320/cm at 1.8 kOe of applied field. As expected, we find that the magneto-optical figure of merit is negligible for this material due to its high absorption...