Página 17 dos resultados de 2573 itens digitais encontrados em 0.141 segundos

‣ Monolithic heteroepitaxial integration of III-V semiconductor lasers on Si substrates

Groenert, Michael
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 188 p.; 2629996 bytes; 2629752 bytes; application/pdf; application/pdf
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Monolithic optoelectronic integration on silicon-based integrated circuits has to date been limited to date by the large material differences between silicon (Si) and the direct-bandgap GaAs compounds from which optoelectronic components are fabricated. Graded Ge/GeSi buffer layers grown on standard Si substrates have been shown to produce near-lattice matched virtual substrates for GaAs integration on Si. This study investigated the crystal growth conditions and device fabrication techniques necessary for successful GaAs-based laser integration on Ge/GeSi buffer layers on Si substrates. The nucleation conditions for GaAs on Ge/GeSi/Si substrates have been comprehensively examined. High-temperature ( 2 700⁰ C) initiation with properly chosen V/III gas flow ratio yields high-quality, stacking fault-free GaAs films on Ge/GeSi/Si substrates, but also encourages the vapor-phase transport of Ge from the substrate into the active regions of integrated GaAs devices. A new two-step GaAs nucleation process was developed that enabled the first demonstration of high-quality Ge-free GaAs light-emitting diodes on Ge/GeSi/Si substrates. The large thermal expansion mismatch between Si, Ge, and GaAs introduces additional strain to integrated device layers on Ge/GeSi/Si substrates grown at high temperatures. This study conclusively demonstrated the link between thermal mismatch strain and increased misfit dislocation formation in InxGa(lx)As/GaAs quantum well structures integrated on Ge/GeSi/Si substrates.; (cont.) The thermal mismatch strain was successfully countered by the introduction of compressive InGaAs graded buffer layers above the Ge/GeSi/Si substrate surface...

‣ Properties and sensor performance of zinc oxide thin films

Min, Yongki, 1965-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 152 p.; 3587906 bytes; 5309302 bytes; application/pdf; application/pdf
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Reactively sputtered ZnO thin film gas sensors were fabricated onto Si wafers. The atmosphere dependent electrical response of the ZnO micro arrays was examined. The effects of processing conditions on the properties and sensor performance of ZnO films were investigated. Using AFM, SEM, XRD and WDS, the 02/Ar ratios during sputtering and Al dopant were found to control the property of ZnO films. Subsequent annealing at 700 C improved the sensor response of the films considerably although it had only minor effects on the microstructure. DC resistance, I-V curves and AC impedance were utilized to investigate the gas response of ZnO sensors. ZnO films prepared with high O2/Ar ratios showed better sensitivity to various gases, a feature believed to be related to their lower carrier density. Al doped ZnO showed measurable sensitivity even with lower resistance attributable to their porous microstructure. AC impedance identified two major components of the total resistance including Schottky barriers at the Pt-ZnO interfaces and a DC bias induced constriction resistance within the ZnO films. Time dependent drift in resistance of ZnO films has been observed. Without applied bias, the ZnO films showed a fast and a slow resistance change response when exposed to gases with varying oxygen partial pressure with both response components dependent on operating temperature. Even at the relatively low operating temperatures of these thin film sensors...

‣ Development of an in vitro model of contraction by fibroblasts

Freyman, Toby M., 1974-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 204 p.; 17663660 bytes; 17663414 bytes; application/pdf; application/pdf
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Dermal scars in adult humans are mechanically and functionally inferior to normal skin and can be physically disfiguring. The contraction of the wound by fibroblasts has been linked to the formation of scar. The mechanical and chemical signals, which control the contraction, are being investigated through the use of models of fibroblast contraction to understand the conditions which promote tissue regeneration. A cell force monitor (CFM) was designed and constructed to measure quantitatively the contraction of a highly-porous, collagen-GAG matrix by fibroblasts. Using this device, contractile force, displacement, and kinetics were compared for different values of cell density and total stiffness resisting fibroblast contraction. In addition, observation of live cells contracting individual matrix struts established the cellular mechanisms responsible for the matrix contraction measured in the CFM. Observation of live cells revealed that macroscopic contraction of the collagen-GAG matrix was the result of forces generated during cell elongation.; (cont.) Contractile force normalized by the number of attached cells (1 nN per cell) was independent of cell density (400 - 2,000 cells/mm3) and total stiffness resisting contraction (0.7 - 10.7 N/m). Total contractile force was dependent on the cell density. These results indicated that the contractile force developed during fibroblast elongation was determined at the level of individual cells (not cooperatively) and was limited by force per cell (not displacement per cell). The kinetics of macroscopic matrix contraction were also independent of cell density and system stiffness; contractile force reached an asymptotic value in 15 h. Observation of live cells found the macroscopic time dependence likely resulted from the stochastic nature of cell elongation initiation and the time required for the fibroblasts to elongate completely (-2-4 h). Therefore...

‣ Dielectric and electromechanical properties of barium and zirconium co-doped sodium bismuth titanate

Sheets, Sossity A. (Sossity Amber), 1973-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 179 p.; 7330937 bytes; 7330733 bytes; application/pdf; application/pdf
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Compositional exploration was conducted within the alkaline bismuth titanate system by doping on the A- and B- sites with Ba⁺² and Zr⁺⁴, respectively. Results on the phase, dielectric and electromechanical properties of single crystals and polycrystals for this new family of relaxor perovskite ferroelectrics are presented. The actuation and polarization characteristics in this system were found to be highly sensitive (within 2 mol%) to cation doping levels, and tailored compositions successfully isolated predominantly electrostrictive actuation at room temperature. Ultra-high room temperature electrostriction was observed in co-doped (Ba + Zr) NBT polycrystals (NBT-14BT-4NBZ) and <100> single crystals (NBT-12BT-4NBZ), up to 0.24% and 0.45% strain, respectively, with negligible hysteresis at 0.05 Hz. Polycrystals with d₃₃ of up to 780 pC/N and single crystals with d₃₃ up to 2000 pC/N were measured. The low frequency actuation properties in the NBT-BT-NBZ compositions surpass highest reported values of strain and d₃₃ for polycrystalline PMN and PLZT and single crystal PMN conventional lead electrostrictors. Predominantly ferroelectric room temperature unipolar actuation in polycrystalline NBT-14BT-3NBZ at 0.05 Hz was observed to be linear and non-hysteretic...

‣ Process-based cost modeling of tool-steels parts by transient liquid-phase infiltration of powder-metal preforms

Barradas Martinez, Juan Alfredo, 1974-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 75 leaves; 5560243 bytes; 5567971 bytes; application/pdf; application/pdf
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(cont.) cost between these two processes was related mainly to their powder scrap rates, 15 % for the Pressing-TLI and 80% for the 3DP-TLI. The high scrap rate value of the 3DP process originates from the fact that powder is sieved before printing, eliminating the coarse and very fine particles. A possible option to decrease this value is to recycle or sell the extra powder, which will reduce the fabrication cost significantly. The model also shows that the main cost for both processes is the powder cost. TLI technical parameters such as heating and cooling rates were included in the model in order to predict the cost behavior when those are manipulated. Because the powder cost dominates the total fabrication cost, variations in the heating and cooling rates do not significantly affect the cost.; Tool steels are iron-based alloys that are melted and processed to develop characteristics useful in the working and shaping of other metals. Tools for such processes must withstand high loads without breaking and without undergoing excessive wear or deformation. Fabrication of direct tool steel parts with complex geometry is possible using Transient Liquid-Phase Infiltration (TLI) in conjunction with Three-Dimensional Printing (3DP). Tool steel parts can also be manufactured using TLI in combination with Cold Powder Methods such as Uniaxial Pressing. Both approaches produce a final part of homogenous composition without significant dimensional change...

‣ Silicone resins and their composites

Wu, Yuhong, 1972-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 250 leaves; 28259628 bytes; 28259425 bytes; application/pdf; application/pdf
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Addition cure (X1-2672) and condensation cure (4-3136) silicone resins have been studied for their mechanical property change with temperature. Properties include maximum flexural stress, flexural modulus and fracture toughness K[sub]IC. Temperature effect on mechanical properties of addition cure resin is substantial and also depends on the crosslinkers used. Generally the maximum stress and flexural modulus decrease with temperature, and the dependence upon crosslinkers in addition cure resin is obvious. Fracture toughness data of addition cure silicone resins have a peaking behavior with the peak appearing [approximately] 58-101C̊ (depending on the crosslinker) below their glass transition temperatures. This can be explained by the competing effect between network mobility and rigidity of the silicone polymer. Rate effect on fracture toughness of silylphenylene crosslinked 2672 has also been studied. It is concluded that the temperature effect on such a system is more dominant compared to the rate effect. The condensation resins also experience decrease in modulus and strength but the toughness changes little with temperature. This is due to its tight network structure. Silylphenylene crosslinked addition cure resin (2672B) and the toughened condensation cure resin (3136T) were used to make silicone fiberglass laminates. They have been successfully processed with a vacuum bagging technique. Silicone resin composites are proved to be thermally stable...

‣ Surface forces during electrophoretic assembly of micron scale silica particles

Kershner, Ryan J. (Ryan Joseph), 1976-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 124 p.; 4307375 bytes; 4307182 bytes; application/pdf; application/pdf
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A system of platinum microelectrodes was fabricated on a sapphire substrate by lithographic patterning and used to manipulate 1.58 [mu]m silica particles in-plane. A digital video system was used to image the motion of particles far from the electrodes and their deposition onto the working electrode during application of a DC potential. The role of electrode reversibility was investigated and the performance of the as-deposited electrodes was improved by electrolytic plating of platinum. Particles were also seen to adhere to the substrate before reaching the electrode. Force distance curves were recorded using a colloid probe atomic force microscopy technique to directly measure the interaction of the silica particles with the sapphire substrate. This data validated the observed adhesion at the electrode and provided further support for the temporal and spatial reduction in pH. The role of Faradaic processes and the diffusion of potential determining ions in electrophoretic deposition was also considered. The zeta potential of planar sapphire substrates for three different crystallographic orientations was measured by a streaming potential technique in the presence of KCl and (CH3)4NCl electrolytes. The streaming potential was measured for large single crystalline C-plane (0001)...

‣ Structural, vibrational and thermodynamic properties of carbon allotropes from first-principles : diamond, graphite, and nanotubes

Mounet, Nicolas (Nicolas Frank)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 104 p.; 4519769 bytes; 4524067 bytes; application/pdf; application/pdf
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The structural, dynamical, and thermodynamic properties of different carbon allotropes are computed using a combination of ab-initio methods: density-functional theory for total-energy calculations and density-functional perturbation theory for lattice dynamics. For diamond, graphite, graphene, and armchair or zigzag single-walled nanotubes we first calculate the ground-state properties: lattice parameters, elastic constants and phonon dispersions and density of states. Very good agreement with available experimental data is found for all these, with the exception of the c/a ratio in graphite and the associated elastic constants and phonon dispersions. Agreement with experiments is recovered once the experimental c/a is chosen for the calculations. Results for carbon nanotubes confirm and expand available, but scarce, experimental data. The vibrational free energy and the thermal expansion, the temperature dependence of the elastic moduli and the specific heat are calculated using the quasi-harmonic approximation. Graphite shows a distinctive in-plane negative thermal-expansion coefficient that reaches its lowest value around room temperature, in very good agreement with experiments. The predicted value for the thermal-contraction coefficient of narrow single-walled nanotubes is half that of graphite...

‣ 2D and 3D periodic templates through holographic interference lithography : photonic and phononic crystals and biomimetic microlens arrays; Two-dimensional and three-dimensional periodic templates through holographic interference lithography

Ullal, Chaitanya K. (Chaitanya Kishore)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 133 leaves; 6752048 bytes; 6757595 bytes; application/pdf; application/pdf
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In this thesis a simple technique for controlling structure via holographic interference lithography was established and implemented. Access to various space groups including such important structures as the level set approximations to the Diamond, the Schwartz P structure, the FCC, and the non centrosymmetric Gyroid structures were demonstrated. The ability to make 3D structures over a large area, with low defect densities and periodicities on the sub/i scale opens a whole range of opportunities including such diverse areas as photonic crystals, phononic crystals, drug delivery, microtrusses, tissue scaffolds, microfluidics and colloidal crystallization. A correlation between structure and photonic band gap properties was established by systematically exploring the 11 FCC space groups. This resulted in a technique to search for photonic band gap structures. It was found that a fundamental connectivity caused by simple Fourier elements tended to support gaps. 2-3, 5-6 and 8-9 gaps were opened in the f.c.c lattices. The F-RD and 216 structures were newly shown to have complete band gaps. Two of the three previously established champion photonic crystal structures, viz. the Diamond and the Gyroid presented practical fabrication challenges...

‣ The role of hydrogen in the growth of carbon nanotubes : a study of the catalyst state and morphology

Kim, Jin Suk Calvin
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 52 leaves; 2308459 bytes; 2309295 bytes; application/pdf; application/pdf
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The role of hydrogen in chemical vapor decomposition (CVD) of C2H4 for growth of carbon nanotubes (CNTs) was investigated. Fe/A1203 (1/10 nm) catalyst layers were used for growth on Si substrates and the times at which H2 was introduced during the 40 minute temperature ramp, 15 minute annealing (without C2H4), and 15 minute growth (during which C2H4 was flowing) stages was varied. When H2 was introduced before heating, CNTs grew to a length of [approx.] 0.3 mm. However, CNT growth was severely suppressed when H2 was introduced at different points during temperature ramp. Recovery of CNT growth was observed when H2 was introduced during the annealing and growth stages. Under optimum conditions, an [approx.] 1 mm thick carpet of CNTs could be obtained. The chemical state and morphology of the catalysts as a function of the time of H2 introduction were examined using XPS and AFM, respectively. We found that the as-deposited state of Fe was an iron oxide, due to reaction with 02 in the atmosphere, and that the H2 reduced the iron oxide to different oxidation states, depending on the time of H2 introduction. AFM inspection showed that surface roughness could also be correlated with areas of vertical CNT growth.; (cont.) A preliminary model for CNT growth in which the oxidation state of iron determines its catalytic activity is proposed...

‣ Mechanical behavior of dip-brazed aluminum sandwich panels

Hohmann, Brian P. (Brian Patrick)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 161 leaves
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An experimental study was carried out to determine the mechanical behavior of sandwich panels containing cellular cores of varying shape. Compression and four point bend tests were performed on sandwich panels with square and triangular honeycomb cores. These honeycombs were made of perforated aluminum sheet of repeating diamond and hexagonal patterns. The sandwich panel assemblies were joined via dip brazing. Defects were introduced into some panels to quantify the effect on strength and stiffness. Hybrid sandwich panels, consisting of foam material in the void spaces of the square and triangular cells were evaluated for the effect on the defect tolerance of the structures. The results showed that sandwich panels with diamond shaped cores had compressive strengths approximately four times greater than hexagonal shaped cores. In four point bending the diamond cores were approximately twice as stiff as cores made from hexagonal patterned sheet. The introduction of defects lowered strength by about 30% for diamond cores in compression, and about 15% for hexagonal cores. In four point bending this strength reduction was not as significant due to shear stresses damaging periodicity at a faster rate than in compression.; (cont.) The use of foam within the cells resulted in higher absolute peak compression and flexure loads...

‣ Influence on molecular geometry and chain conformation on properties of polyelectrolyte multilayers

Zacharia, Nicole Suzan
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 136 leaves
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The layer-by-layer (LbL) method of self-assembly is a versatile technique for fabricating thin polymer films. This thesis compares the properties of LbLfilms composed of different weak polycations. Slight perturbations in film assembly conditions can lead to large differences in film properties. Polyion bacisity and architecture are less understood variables. The polycations used are of similar chemical composition but different molecular geometries and basicity. Films studied were composed of poly(acrylic acid) (PAA) in combination with linear poly(ethylene imine) (LPEI), poly(allyl amine hydrochloride) (PAH), branched poly(ethylene imine) (BPEI), or poly(amidoamine) (PAMAM) dendrimer, generation four, amine surface. Various properties of these films are compared; including film thickness, chemical functional group availability, and film composition. Carboxylic acid group ionization is found to increase with assembly pH, and PAA content is found to decrease. PAH, the most basic of the polycations, forms films that are the thinnest and the most ionized while PAMAM films have the most free free acid groups and form the thickest films.; (cont.) Permeability to chloroethyl ethyl sulfide vapor was seen to correlate with film ionization and therefore ionic crosslink density; diffusivity was highest in films that deposit in "loopy" layers and solubility was highest in films with the highest degree of ionization. The diffusion of these polycations (specifically in the direction of film growth) is shown to be able to disrupt LbL heterostructures. A model system of a strong polycation (here poly(hexylviologen)) and PAA was used to show rejection of PXV in favor of a weak polycation...

‣ Investigation of long-term cyclic loading effects on initially intact cartilage

Wu, Bruce Y. C., 1980-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 173 leaves
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Cartilage, particularly its collagen network, has been suggested by many to be fatigue prone, but the mechanism via which fatigue damage occurs is not yet well-understood. This work seeks to provide a simple framework to further our understanding of the degeneration of this tissue a result of fatigue damage via experimentation and analytical modeling. It was as observed that subjecting initially intact bovine tibial plateau cartilage to repeated long-term applications of an indentation loading protocol, which does not induce any observable damage over a single short-term application, can result in surface damage. The damage can be associated with an increase in the GAG release and decrease in the tissue nominal equilibrium modulus. The ability of the tissue to regain its original configuration also deteriorated with increasing number of loading cycles, but the change was generally gradual. Changes in the optimized parameters obtained from fitting a 1-D rheological model to both the relaxation and cyclic loading responses of cartilage in the undamaged and damaged states suggested that both GAG and collagen have been weakened as a result of damage. A portfolio of data describing the 3-D mechanical behavior of calf tibial plateau cartilage was obtained in this work. It was found that both the axial and lateral responses of cartilage were nonlinear during stress relaxation. Also...

‣ Computational studies of stress and structure development resulting from the coalescence of metallic islands

Takahashi, Andrew Rikio
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 65 leaves
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Thin film component properties are critical design elements in almost all industries. These films are particularly important in the performance of micro- and nano-electromechanical systems (MEMS and NEMS). Residual stress in thin film components is often treated as an unavoidable side effect of processing steps and the degree of residual stress can drastically affect the performance and properties of the final product. While high levels of residual stress are often detrimental to performance, control of the stress and stress gradients can also be used to enhance performance and even generate new capabilities. The work presented in this thesis examines the role of island coalescence in the development of structure and stress in thin films. The primary methods of investigation are molecular dynamics (MD) and finite element analysis (FEA). The semi-empirical MD calculations show that coalescence is a very rapid process for unconstrained spheres and for hemispheres allowed to slide on a frictionless substrate. Particle rotations are commonly observed during the coalescence calculations. The extent of neck formation between 2 particles is consistent with continuum models even down to length scales which would normally be outside the range in which the models might be expected to be applicable. The MD calculations also show that internal island defects may be induced by the island coalescence process...

‣ Block copolymer-templated iron oxide nanoparticles for bimodal growth of multi-walled carbon nanotubes

Yazzie, Kyle E
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 74 leaves
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Since their discovery carbon nanotubes (CNTs) have sparked great interest due to their exceptional mechanical, electrical, and thermal properties. These properties make carbon nanotubes desirable for numerous applications including: nanoelectronics, high-strength composites, energy storage, superhydrophobic surfaces, sensors, and biomaterial interfaces. Bulk synthesis of carbon nanotubes with controlled physical features, i.e. length, diameter, multiwalled vs. single walled, carbon nanotube chirality, etc. is necessary to make full use of carbon nanotubes exceptional properties in commercial aspects. Typical carbon nanotube synthesis processes use chemical vapor deposition (CVD), arc-discharge, and laser ablation. Synthesizing carbon nanotubes via CVD typically involves depositing a thin metal film on a silicon substrate, and heating the substrate so that the thin metal film dewets and forms metallic nanoparticles. A hydrocarbon gas is then flowed over the nanoparticles to initiate carbon nanotube growth. Though these thin metal film catalysts are easy to prepare, they offer poor control over nanoparticle diameters and areal density. It has been shown that physical properties of carbon nanotubes, such as diameter and uniformity of growth...

‣ Potential commercialization of a collagen-GAG scaffold for liver regeneration

Southworth, Adam R
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 65 p.
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The potential for commercializing a scaffold made of collagen and glycosaminoglycan to help regenerate cirrhotic liver was analyzed and a business plan and model were created. Using a lypholization technique, a bulk-sized and highly porous scaffold is created. It is then inserted into the hole created by the excised liver scar tissue. By blocking contraction of the wound and mimicking the natural extracellular matrix, the scaffold induces regeneration of normal liver tissue. The in vivo approach is compared to several other experimental treatments of cirrhosis found in the literature. The difficulties that need to be addressed are explained and potential solutions are given. A cost model was created, incorporating equipment, labor, FDA, and raw material costs. This model was combined with information regarding the cost of current liver transplant procedures to create a profitable business plan based on the collagen-GAG scaffolds. A manufacturing and product sales business model was chosen due to the fairly low level of market competition and moderate barrier to entry. The intellectual property landscape is described and analyzed in terms of problematic existing patents and the potential for protecting the proposed scaffolds. A timeline for future research and development was created...

‣ Effect of volume fraction of solids on the compressive stress-strain behavior of collagen-glycosaminoglycan scaffolds

Leung, Janet (Janet H.)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 39 leaves; 1743551 bytes; 1743152 bytes; application/pdf; application/pdf
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This thesis aims to examine the effect of volume fraction of solids in collagen-glycosaminoglycan (GAG) scaffolds on the compressive-strain behavior of the structure and compare these results to the open-cell foam model. Collagen-GAG (CG) scaffolds have been used for regenerating skin, conjunctiva, and peripheral nerves with varying levels of success. In these uses, the temporary scaffolds are often deployed with a non-degradable support structure such as a waterproof film or a silicone neural tube which are removed after healing is complete if it is outside the body (for skin regeneration) or are expected to remain permanently in the body (for nerve regeneration). Unfortunately, leaving non-degradable implants in the body could provoke immune responses. At the same time, to remove supports that have been implanted in the body after healing has been completed would result in more injury to the site and other medical complications. For a truly temporary implant, the scaffold must in its entirety be degradable. Thus, the bulk mechanical properties of the scaffold are important to study. Previous research has concentrated on the effects of cells on the scaffolds on a microlevel. However, the scaffold must also be able to bear mechanical stress from surrounding tissues to keep the wound open and provide mechanical support for the body...

‣ The mechanism of thin film Si nanomachining using femtosecond laser pulses

Jia, Jimmy Yi-Jie, 1980-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 85 p.
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Femtosecond (fs) laser ablation has been the subject of intense recent research. The pulse time ('width') is shorter than the electronic relaxation time, resulting in a decoupling of the period of laser illumination and the melting of the substrate. Since the laser does not directly heat the substrate, and there has been no directly observed heat affected zone (HAZ) nor vaporization, fs laser ablation is sometimes considered to be a direct solid-vapor phenomenon. Recent research indicates that the phenomenon is not as straightforward as assumed. Time-of-flight spectroscopy used to measure the reflectivity indicates that molten material is present for a few hundred picoseconds, well after the laser pulse. A material-modification threshold has been observed that is below the ablation threshold. This indicates that the laser can affect the substrate without ablation occurring. However, many scanning electron microscopy (SEM) studies have been performed, but material change in the substrate have not been observed. Transmission electron microscope (TEM) studies have also been done, but they have all been carried out in plane-view, so that it is difficult to separately observe bulk and surface effects. In this study, cross-sectional TEM analysis of holes drilled in single crystal silicon films in silicon-on-insulator (SOI) structures have been carried out...

‣ Characterization of OMVPE-grown GaSb-based epilayers using in situ reflectance and ex situ TEM; Characterization of organometallic vapor phase epitaxy-grown GaSb-based epilayers using in situ reflectance and ex situ transmission electron microscopy

Vineis, Christopher J. (Christopher Joseph), 1974-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 238 p.; 23877714 bytes; 23877469 bytes; application/pdf; application/pdf
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The focus of this thesis was to investigate and characterize GaSb, GaInAsSb, and AlGaAsSb epilayers grown by organometallic vapor phase epitaxy (OMVPE). These epilayers were principally characterized using in situ spectral reflectance and ex situ transmission electron microscopy (TEM). An in situ spectral (380-1100 nm) reflectance monitoring system was designed and fitted to the OMVPE reactor. It was determined that longer wavelengths are more useful for quantitative growth rate analysis, while shorter wavelengths are more sensitive to the GaSb substrate oxide desorption process. It was also determined that the GaInAsSb and AlGaAsSb alloy compositions could be determined accurately using in situ reflectance ratios. Use of the in situ reflectance monitor to efficiently perform necessary reactor/growth calibrations was also demonstrated. Analytic functions were used to model the refractive indices of GaSb, AlGaAsSb, and GaInAsSb. Specifically, Adachi's Model Dielectric Function [1, 2] was curve-fit to data for GaSb between 400 and 1000 nm, and fourth-order polynomials were fit to data for GaSb and GaInAsSb between 1 and 3 gnm. A linear interpolation of binary functions was used to generate a refractive index model for AlGaAsSb between 1 and 3 m as a function of Al fraction. These models were helpful in interpreting in situ reflectance data...

‣ PH-gated porosity and reversible swelling transitions in polyelectrolyte multilayers

Hiller, Jeri' Ann, 1976-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 149 p.; 12589486 bytes; 12589294 bytes; application/pdf; application/pdf
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This thesis explores the design of novel pH-responsive polyelectrolyte multilayer (PEM) films. The discoveries of discontinuous pH-induced swelling transitions and concomitant conformational memory effects as well as pH-gated nano- and microporosity in PEMs, are described. The PEMs in this thesis are created by an aqueous processing technique, which enables the straightforward manipulation of processing variables to create molecular-level designed polymeric coatings. The first part of this thesis examines discontinuous pH-induced swelling transitions and reversible memory effects in certain PEMs of poly(allylamine hydrochloride) and (poly(styrene sulfonic acid) (PAH/SPS). It is demonstrated that the systematic design of molecular organization, hydrophobic character, and a variable electrostatic nature, enables these phenomena in PAH/SPS films. Consequently, co-existing conformational states result, which allow the regulation of the affinity of molecular species to the PEM as well as their sustained release from the PEM. The second part of this thesis describes phase-separation phenomena in PEMs comprised of PAH and poly(acrylic acid) (PAA). It is shown that specifically designed PAH/PAA films exhibit pH-gated nano- and microporosity. Further...