Página 2 dos resultados de 2573 itens digitais encontrados em 0.093 segundos

‣ 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...

‣ The design, synthesis and properties of pressure-processable biodegradable block copolymers

Lovell, Nathan Gary
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 95 p.; 4539611 bytes; 4543521 bytes; application/pdf; application/pdf
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In this thesis, biodegradable block copolyesters were specifically designed and synthesized for their susceptibility to pressure-induced mixing. These baroplastic materials are capable of being processed and molded through the application of pressure at temperatures far below those needed in traditional melt-processing of biodegradable polyesters. Pressure molding at low temperatures reduces the danger of chain degradation during processing that is ubiquitous in biodegradable plastics. The compressible regular solution model (CRS), in combination with group contribution (GC) methods and pressure-volume-temperature (PVT) data, was used to predict the phase behavior and pressure-induced miscibility of several block copolymer systems. Small-angle neutron scattering (SANS) experiments confirmed that amorphous forms of the poly([epsilon]-caprolactone-ran-5 ethylene ketal [epsilon]-caprolactone)-block-poly(lactic acid) (PmCL-b-PLA) system exhibit pressure- induced miscibility, and small angle x-ray scattering (SAXS) revealed that the position of the upper disorder-to-order temperature (UDOT) in those systems is near the values predicted by the CRS model.; (cont.) Differential scanning calorimetry measurements suggest that poly(1,5-dioxepan-2-one)-block-poly(lactic acid) (PDXO-b-PLA)...

‣ Micro and nano mechanics of materials response during instrumented frictional sliding

Bellemare, Simon C. (Simon Claude)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 109 p.
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Over the past decade, many computational studies have explored the mechanics of instrumented normal indentation. In contrast, very few studies have investigated quantitative aspects of frictional sliding contact in the elasto-plastic regime. In this thesis, a new framework was developed to establish relationships between the frictional sliding response, material properties and contact parameters. Dimensional analysis enabled to define scaling variables and dimensionless functions. Finite element methods were used to simulate the process of steady-state frictional sliding and evaluate the dimensionless functions. In frictional sliding, the representative plastic strain was found to be more than four times as large as in normal indentation. Further comparison with indentation indicated a three fold increase in the maximum pile-up height and an increased influence of the strain hardening on hardness. Experimental studies were conducted with and without a liquid lubricant in selected material systems. Quantitative agreements with numerical predictions were observed in all cases. The strong influence of the strain hardening exponent on the pile-up height was illustrated from frictional sliding results obtained in copper and copper-zinc specimens of different grain sizes.; (cont.) Also...

‣ Electronic structure and quantum conductance of nanostructures

Lee, Young-Su, Ph. D. Massachusetts Institute of Technology
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 158 p.
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This thesis is dedicated to development and application of a novel large-scale first-principles approach to study the electronic structure and quantum conductance of realistic nanoscale materials. Electron transport at the nanometer scale involves phenomena which are beyond the realm of classical transport theory: the wave character of the electrons becomes central, and the Schrddinger equation needs to be solved explicitly. First-principles calculations can nowadays deal with systems containing hundreds of electrons, but simulations for nanostructures that contain thousands of atoms or more need to rely on parametrized Hamiltonians. The core of our approach lies in the derivation of exact and chemically-specific Hamiltonians from first-principles calculations, in a basis of maximally-localized Wannier functions, that become explicit tight-binding orbitals. Once this optimal basis is determined, the Hamiltonian matrix becomes short-ranged, diagonally-dominant, and transferable - i.e. a large nanostructure can be constructed by assembling together the Hamiltonians of its constitutive building block. This approach is first demonstrated for pristine semiconducting and metallic nanotubes, demonstrating perfect agreement with full first-principles calculations in a complete planewave basis.; (cont.) Then...

‣ Synthesis and characterization of conducting polymer actuators

Vandesteeg, Nathan A. (Nathan Alan)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 126 leaves
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Conducting polymers are known to mechanically respond to electrochemical stimuli and have been utilized as linear actuators. To date, the most successful mechanism for actuation is ionic ingress and egress, though mechanisms based on conformational changes and molecular interactions have also been proposed. In the pursuit of new conducting polymer actuators it is necessary to design, synthesize, and characterize new materials, spanning scientific disciplines from synthetic chemistry to materials and mechanical engineering. As such, the topics of synthesis and characterization of new conducting polymers are discussed, highlighting developments in techniques and instrumentation. Actuation in poly(3,4-ethylenedioxythiophene), or PEDOT, and composites of PEDOT and carbon nanotubes is presented, demonstrating strains of 4.5% and strain rates of 0.2% per second with faster responses generated in carbon nanotube composites. Actuation in poly(3-hexylthiophene) is presented, demonstrating the observation of a novel actuation mechanism relating the potential of the polymer to the mechanical response. Further study of the actuation of polypyrrole at temperatures above 25°C is also discussed, in which response times decrease and magnitudes increase with temperature. Discrete time models of equivalent circuits and diffusion are utilized to predict conducting polymer actuator performance.; by Nathan A. Vandesteeg.; Thesis (Ph. D.)--Massachusetts Institute of Technology...

‣ Evaluation of potential applications for templated arrays of heterostructural semiconductor nanowires as light emitting devices

Zou, Ting, M. Eng. Massachusetts Institute of Technology
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 85 leaves
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While light emitting devices, such as laser diodes (LDs) and light emitting diodes (LEDs), were first introduced decades ago, they have been the subject of continuing research and improvements due to their relatively poor performance. Evolution has occurred in both the design of light emitting devices and in the materials from which they are made. This thesis examines new proposals for use of templated arrays of heterostructural semiconductor nanowire light emitting devices, from both engineering and business points of view. The effects of wire spacing and diameter on reliability and performance (both the internal quantum efficiency and the extraction efficiency) are evaluated. Business models for development of nanowire arrays for light emitting devices are discussed.; by Ting Zou.; Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2006.; Includes bibliographical references (leaves 82-85).

‣ Modeling of pattern dependencies in the fabrication of multilevel copper metallization

Cai, Hong, Ph. D. Massachusetts Institute of Technology
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 303 p.
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Multilevel copper metallization for Ultra-Large-Scale-Integrated (ULSI) circuits is a critical technology needed to meet performance requirements for advanced interconnect technologies with sub-micron dimensions. It is well known that multilevel topography resulting from pattern dependencies in various processes, especially copper Electrochemical Deposition (ECD) and Chemical-Mechanical Planarization (CMP), is a major problem in interconnects. An integrated pattern dependent chip-scale model for multilevel copper metallization is contributed to help understand and meet dishing and erosion requirements, to optimize the combined plating and polishing process to achieve minimal environmental impact, higher yield and performance, and to enable optimization of layout and dummy fill designs. First, a physics-based chip-scale copper ECD model is developed. By considering copper ion depletion effects, and surface additive adsorption and desorption, the plating model is able to predict the initial topography for subsequent CMP modeling with sufficient accuracy and computational efficiency. Second, a compatible chip-scale CMP modeling is developed.; (cont.) The CMP model integrates contact wear and density-step-height approaches, so that a consistent and coherent chip-scale model framework can be used for copper bulk polishing...

‣ Adsorption and multilayer assembly of charged macromolecules on neutral hydrophobic surfaces and applications to surface patterning

Park, Juhyun, Ph. D. Massachusetts Institute of Technology
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 148 leaves
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Micrometer- and nanometer-scale chemical patterns are indispensable and ubiquitous in a range of applications, such as optoelectronic devices and (bio) chemical sensors. This thesis studies chemical surface patterning utilizing polyelectrolyte multilayers for electronic and biological applications. It focuses on both fundamental study and application development in the field of layer-by-layer self-assembled composite thin films, with the goal of defining new concepts allowing for technological breakthrough. In the process of completing it, a multicomponent patterning technology that has been a bottleneck in realizing practical devices utilizing the multilayers has been developed. To achieve this goal, a multilayer transfer printing concept was applied to serial printing of individual device components. The main achievements include fundamental studies about uniform multilayer assembly of charged macromolecules on neutral hydrophobic surfaces as the principle of the technique, and the demonstration of multicomponent patterning of polyelectrolyte/nanoparticle composite thin films on a flexible substrate.; (cont.) Extending the technique toward nanometer-scale patterning, a new polymeric mold material that was suitable for sub-100 nm structuring was studied and used for chemical patterning for flow control in microfuidic devices and nanoparticle assembly for potential biological applications...

‣ Nanomechanics of cartilage extracellular matrix macromolecules

Han, Lin, Ph. D. Massachusetts Institute of Technology
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 201 p.
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In this thesis, the shear and self-adhesion nanomechanical properties between opposing cartilage aggrecan macromolecules were probed. In addition, nanoscale dynamic oscillatory mechanical properties of cartilage and its type II collagen network was measured. Aggrecan shear nanomechanics was assessed via microcontact printing and lateral force microscopy. Lateral force between aggrecan and the probe tip, and compression of aggrecan was simultaneously measured in 0.001 - 1.0 M NaCl aqueous solutions. Using the microsized tip (Rtip ~ 2.5 [mu]m) enabled a large assembly of ~ 103 aggrecan molecules to interact simultaneously, closely mimicking the in vivo conditions.Both electrostatic and nonelectrostatic components were identified to importantly contribute to aggrecan shear. At near physiological IS (0.1 M), significant rate dependence was observed, suggestive of visco/poroelastic interactions within the aggrecan layer. By using an aggrecan end-functionalized colloidal tip, shear of two opposing aggrecan layers was assessed in a similar fashion. Lower lateral force and a more marked rate dependence were measured compared to the shear of a single layer, due to the aggrecan inter-layer molecular interpenetration and the different local z-dependent charge density distribution. The addition of Ca2+...

‣ Exploring kinetics and thermodynamics in fast-ion conductors and hydrogen-storage materials using ab-initio molecular dynamics

Wood, Brandon C. (Brandon Christopher)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 190 p.
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We investigate the interplay between various kinetic processes and thermodynamic factors in three materials--silver iodide (AgI), cesium hydrogen sulfate (CsHSO4), and sodium alanate (NaAlH4)-using ab-initio molecular dynamics simulations. The time-averaged and instantaneous silver substructure in the fast-ion conductor AgI is analyzed, resulting in a set of ordering rules that govern the distribution of the mobile silvers in the first coordination shell surrounding an iodine. We find evidence of an independent phase transition of the silver ions which drives the structural transformation to the high-mobility phase. A thermodynamic motivation for the existence of fast-ion conduction is suggested in terms of an entropic stabilization associated with the decrease in silver mobility upon melting. We also find a unique chemical signature for the fourth nearest-neighbor silver to an iodine. This fourth silver is weakly bound and relatively unconstrained, and we isolate it as the predominant agent in the diffusion process. Next, a detailed statistical analysis is performed on simulations of the fuel-cell electrolyte CsHSO4 to isolate the interplay between the dynamics of the O-H chemical bonds, the ... H hydrogen bonds, and the SO4 tetrahedra in promoting proton conduction. A high reversal rate limits the apparent success rate of the otherwise rapid chemical-bond dynamics...

‣ Evaluation of on-chip integration of magneto-optic isolators

Chen, Xiaoyan, M. Eng. Massachusetts Institute of Technology
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 62 leaves
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The recent development of on chip integration of magneto-optic isolators is reviewed. Two major categories of structural designs for waveguide isolator (nonreciprocal mode conversion and nonreciprocal phase shift) have both merits and short comings for integration. The advances in integration techniques and materials related developments in the last decade are a big step towards monolithic integration of isolator with other optical devices. But whether or not the integrated magneto-optic isolators will penetrate the market eventually would also depend not its competing technologies.; by Xiaoyan Chen.; Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2007.; Includes bibliographical references (leaves 57-62).

‣ Economic and environmental evaluation of end-of-life aerospace aluminum options using optimization methods

Chen, Emily, S.B. Massachusetts Institute of Technology
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 59 leaves
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The benefits of recycling have long been understood and the conspicuous energy savings of secondary aluminum production have caused aluminum recycling to increase. Obsolete aircraft are a valuable source of aluminum scrap and recent efforts to fortify the aerospace aluminum recycling infrastructure have drawn attention to the potential of sophisticated sorting methods to maximize the economic gain of using aerospace scrap in secondary production. The aim of this research was to use linear optimization to assess the economic viability of sorting technologies for enabling wrought products in general and aerospace alloys in particular to be recycled back to high value applications. A chance-constrained model was used to select the alloys that consumed the largest quantity of aerospace alloys in their production, thereby establishing a strategic portfolio of finished goods. Ten of the fifteen alloys in the portfolio were of the 2xxx and 7xxx alloy series that are standard in the production of aerospace components. An aerospace end-of-life case study was performed in which cases varied by their input scrap streams, each having a compositional uncertainty associated with the different degrees of sorting that methods currently in use and technologies in development can achieve. The chance-constrained model calculated the production cost for each case and determined that when aerospace components were identified to the precision of individual alloys...

‣ GeSi photodetectors and electro-absorption modulators for Si electronic-photonic integrated circuits; GeSi photodetectors and EA modulators for silicon EPIC

Liu, Jifeng, Ph. D. Massachusetts Institute of Technology
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 190 p.
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The silicon electronic-photonic integrated circuit (EPIC) has emerged as a promising technology to break through the interconnect bottlenecks in telecommunications and on-chip interconnects. High performance photonic modulators and photodetectors compatible with Si complimentary metal oxide semiconductor (CMOS) devices are indispensable to achieve this goal. A photonic modulator generates optical "1" and "0" signals by switching the light on and off, while a photodetector converts the optical signals to electrical ones so that they can be processed by a CMOS circuit. Due to its compatibility with Si CMOS processing and adequate optoelectric properties, epitaxial GeSi material has been considered as a promising candidate to achieve this goal. This thesis investigates epitaxial GeSi photodetectors and electro-absorption (EA) modulators integrated with high index contrast Si(core)/Si02(cladding) waveguides to form an EPIC circuit on a Si platform with CMOS compatibility. Tensile strain is introduced into the GeSi material to enhance its optoelectronic properties. The effect of tensile strain on the band structure of Ge is systematically studied, and the deformation potential constants of Ge are derived from the experimental results with relatively high accuracy.; (cont.) Methods to engineer the tensile strain in Ge are demonstrated. Tensile strain in small...

‣ 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...

‣ Design of a CMOS compatible, athermal, optical waveguide; Design of a complementary metal oxide semiconductor compatible, athermal, optical waveguide

Fernandez, Luis Enrique, S.B. Massachusetts Institute of Technology
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 49 p.
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This paper explores a possible design for a CMOS compatible, athermal, optical waveguide. The design explored is a slot waveguide with light guided in the low index material. A design paradigm is proposed which shows the relationship between cross-sectional parameters and their impact on both the effective index of refraction and the thermo-optic coefficient of the device. Two materials choices were explored to serve as the low index material, poly(ether imide) (PI) and poly(methyl methacrylate) (PMMA). The slot waveguide with PI as the low index material had a simulated, device thermo-optic coefficient of -8.5 x 10-4K-1, and the slot waveguide with PMMA as the low index material had a simulated, device thermo-optic coefficient of 1.7 x 10-5K-1.; by Luis Enrique Fernandez.; Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2007.; Includes bibliographical references (p. 49).

‣ A technical and economic evaluation of novel pH-responsive core-shell nanoparticles : delivering innovation from laboratory to market

Cho, Eun Chol, M. Eng. Massachusetts Institute of Technology
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 71 leaves
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Many potentially powerful therapeutic strategies for the treatment of disease require the delivery of drugs into the cytosolic or nuclear compartments of cells. Members of the Irvine laboratory have developed a novel pH-responsive core-shell nanoparticle system that can achieve efficient and non-cytotoxic drug delivery into the cytosol. Another advantage is that the shell can be easily modified to bind to different types of drug agents and incorporate ligands for specific cell targeting. Experimental analysis of the newly synthesized nanoparticles with various shell structures has demonstrated that modification of the shell does not compromise their cytosolic delivery. These nanoparticles, if successful, will improve the therapeutic potential of a wide range of drugs. However, critical issues on the research side need to be resolved, and an appropriate intellectual property strategy should be initiated in the near future. Applications to siRNA delivery and vaccines have been examined in depth, as cytosolic delivery is one of the main challenges in these fields. Partnerships with large pharmaceutical companies are critical in order to acquire key patents on siRNA/antigen. Even though the market is competitive, there is a strong demand for innovative delivery platforms; provided that the overall profile of the core-shell nanoparticles is comparable to that of emerging drug delivery systems...

‣ Substrate and channel engineering for improving performance of strained-SiGe MOSFETs

Gupta, Saurabh, Ph. D. Massachusetts Institute of Technology
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 198 leaves
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With VLSI technology moving closer towards fundamental physical limits, a way to further improve the transistor drive current for superior circuit performance is enhancing the average velocity of carriers in the channel. In order to enable technology boosters like strained channels on the commercially developed Si platform, a low threading dislocation density SiGe platform is required. A relaxed SilGe. graded buffer creates a larger lattice constant on a Si substrate while providing low threading dislocation densities (TDD) on the order of 105 cm-2. For many III-V devices on Si, such as high-efficiency multi-junction solar cells, the SiGe graded buffer approach is hindered somewhat by the fact that relatively thick buffers must be grown to relieve the strain effectively. A way to reduce the thickness of SiGe graded buffer is to use larger mismatch (10% Ge difference) at each step. Growing a larger mismatch layer at high temperature leads to a higher threading dislocation density (TDD). In this work, the rate of plastic strain relaxation in a mismatched layer and misfit dislocation distribution in the layer are identified to be important parameters which affect the TDD in a mismatched SiGe layer.; (cont.) Reducing the TDD in a mismatched SiGe layer on Si requires slow plastic strain relaxation which can be achieved by growing layers at slower growth rates. Using lower growth temperatures to reduce the growth rates...

‣ 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...

‣ Understanding and development of combined acoustic and magnetic actuation of Ni₂MnGa single crystals

Techapiesancharoenkij, Ratchatee, 1979-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 148 p.
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Ni-Mn-Ga based ferromagnetic shape memory alloys (FSMAs) have emerged as a promising new class of active materials capable of producing a large (several %) magnetic-field-induced strain (MFIS). FSMAs still have several characteristic shortcomings that may limit their potential applications. A threshold field of 2 to 4 kOe must be overcome to initiate twin boundary motion and a larger field is required to achieve full actuation. The operating window of the stress output from FSMA actuators is narrow and limited to the range between 0.5 and 2 MPa. Outside the operating range, the strain output diminishes significantly. This thesis addresses these limitations and reports potential techniques to decrease the required threshold field and increase the stress and strain output of FSMA actuation. The demagnetizing field due to magnetic poles on the surface of the sample is found to significantly influence the maximum field needed for full MFIS. The demagnetizing field decreases the effective internal field inside the FSMA sample; as a result, for a given external field, the magnetic driving force is reduced by the demagnetizing field. For a small demagnetization factor, full MFIS can be achieved at a field as low as 0.5 kOe. However, for a high demagnetization factor...

‣ Assembly and detection of viruses and biological molecules on inorganic surfaces

Sinensky, Asher Keeling
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 175 p.
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This work is composed of three distinct, albeit related, projects. Each project is an exploration of the ways in which interactions between inorganic surfaces and biological molecules can be advantageously exploited. The first project entitled, Biomolecular Recognition of Crystal Defects extended the phage display technique to the detection of crystal defects. The system used is based on the M13 bacteriophage with 7-residue constrained random sequence on protein III. After considerable experimentation a procedure described as 'Diffuse Selection' was developed for selecting defects on crystal surfaces. Challenges occur because it is difficult to drive phage display towards the selection of particular surface features as opposed to whole surfaces. After multiple iterations, diffuse selection was optimized and consensus sequences were achieved. Virus binding was characterized using Atomic Force Microscopy, Fluorescene Microscopy and Titration. Using a simple bimolecular model, the binding sequence identified through this work is shown to have a binding constant 100,000 times better than a random peptide sequence. The second project entitled, Surface Patterning of Genetically Programmed Viruses, developed a generalizable approach to patterning viruses regardless of the genetic modification made to the virus. Genetic modifications are made in order to create viruses which will construct inorganic materials on their bodies in the appropriate chemical environment. Three generalizable virus patterning approaches were developed based on hydrophobic...