Página 10 dos resultados de 2573 itens digitais encontrados em 0.073 segundos

‣ Electrical properties and defect structures of praseodymium-cerium oxide solid solutions

Stefanik, Todd Stanley, 1973-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 135 p.; 6121255 bytes; 6107117 bytes; application/pdf; application/pdf
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A defect chemistry model consistent with observed trends in the pO2 and temperature dependence of electrical conductivity in praseodymium cerium oxide (PCO) was developed. Four point DC conductivity measurements were made from 1 atm to 1018 atm p02 over isotherms ranging from 600-1 000ʻC in materials containing 0-20% Pr. A pO02-dependent ionic conductivity was observed at high pO2 values in compositions containing 0.5% and 1% Pr. This behavior was attributed to oxidation of Pr3+ to Pr4+ under oxidizing conditions, thereby decreasing the concentration of acceptor dopants in the PCO material. In compositions containing 10% and 20% Pr, an electron hopping conductivity was observed at high pO02 values. This contribution was strongest at low temperatures and was attributed to the formation of a praseodymium impurity band within the CeO2 band gap. Defect association significantly altered the predicted pO2 dependence of the impurity band conductivity, especially at low temperatures. The temperature dependences of the thermodynamic parameters governing defect formation and transport in PCO were determined. The reduction enthalpy of cerium was significantly decreased with additions of Pr from approximately 4.7 eV (the value in pure CeO2) to 3.4 eV in 20% PCO. The energy between the Pr impurity band and the CeO2 conduction band was approximately 0.95 eV for 10% and 20% PCO samples. The measured trap depth was significantly higher (approximately 1.6 eV) in 0.5% and 1% PCO. The migration enthalpy for impurity band hopping conductivity was approximately 0.55 eV...

‣ An investigation of corrosion mechanisms of constructional alloys in supercritical water oxidation (SCWO) systems

Kim, Hojong, 1974-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 186 leaves; 13676939 bytes; 14760905 bytes; application/pdf; application/pdf
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Supercritical water oxidation (SCWO) is a technology that can effectively destroy aqueous organic waste above the critical point of pure water. These waste feed streams are very aggressive and pose material performance issues. As potential alloys in construction of SCWO systems, nickel-base alloys are tested. Corrosion in aqueous feed streams of ambient pH values of 2, 1 and 7 is studied both at supercritical (-425⁰C) and subcritical (-300-360⁰C) temperatures with a constant pressure of 24.1MPa. Dealloying of Ni and Fe, and oxidation of Cr and Mo are observed at subcritical temperatures at a pH value of 2. At a pH value of 1, even chromium is selectively dissolved and only molybdenum forms a stable oxide at the subcritical temperature. At supercritical temperatures, normal thin oxidation occurs at both pH values of pH 2 and 7. In contrast, in the neutral pH solution, dealloying is not observed at any temperature. Stress corrosion cracking (SCC) in acidic feed streams is observed both at the supercritical and subcritical temperatures. In order to understand the corrosion mechanisms, the chemistry of a feed stream, the formation of the dealloyed oxide layer, and the level of stress are investigated.; (cont.) The suppression of dealloying at supercritical temperatures comes from the low proton concentration associated with the low dissociation constant of HCl and water. However...

‣ Shear-induced homogeneous deformation twinning in FCC aluminum and copper via automistic simulation; Shear-induced homogeneous deformation twinning in face-centered cubic aluminum and copper via automistic simulation

Boyer, Robert D. (Robert Damian), 1978-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 80 leaves; 1113038 bytes; 1454882 bytes; application/pdf; application/pdf
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The {111}<112̄> shear stress-displacement behavior for face-centered cubic (fcc) metals, aluminum and copper, is calculated using empirical potentials proposed by Mishin and by Ercolessi, based on the embedded atom method (EAM), and compared with published ab initio calculations. In copper close agreement is observed in the results given by the Mishin potential for both the ideal shear strength and local atomic relaxation during shear, although the extent of plastic deformation before failure is over-predicted. In aluminum, both the Mishin and Ercolessi potentials are used, with only the former able to capture the majority of the behavior exhibited in first principle calculations. Both potentials are shown to have difficulties modeling the effects of directional bonding. Calculations of the multiplane generalized stacking fault energy in both materials reveal that aluminum has a longer range of atomic interaction than copper. Using molecular dynamics and static energy calculations, deformation twins are shown to form by homogeneous nucleation, slip and subsequent coalescence of partial dislocations in both copper and aluminum. The minimum energy path for formation of a two-layer microtwin, and the energy barriers to its further growth are analyzed for the two metals. The mechanism observed is interpreted with reference to existing work on the nucleation of microtwins in body-centered cubic metals.; by Robert D. Boyer.; Thesis (S.M.)--Massachusetts Institute of Technology...

‣ 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...

‣ Polyelectrolyte multilayers : nanofabricated architectures for bio-interface materials

Mendelsohn, Jonas Daniel, 1975-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 135 p.; 14268994 bytes; 14268753 bytes; application/pdf; application/pdf
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The layer-by-layer process, whereby aqueous solutions of oppositely charged polymers are alternately and repeatedly deposited onto a substrate, has emerged in recent years as a promising approach for creating thin films with nanoscale control of structure, composition, and surface properties. Applications ranging from surface modification to optical and electronic devices have arisen from the versatility of this nanocomposite fabrication technique. The additional ability to assemble into films a wide variety of biological entities, such as enzymes and DNA, has expanded the use of polyelectrolyte multilayers for biosensor and other biomaterials applications. This thesis further explores the rationale of using multilayers as biomaterials, with particularly emphasis on the importance of the underlying molecular architecture. Many of the results presented here concern films assembled from weak polyions, i.e., ones with pH-dependent charge densities, including poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH). Using weak polyions enables the creation of thin films with chemical and structural properties controlled with nanoscale precision by simply adjusting the pH of the polymer solutions. Under certain assembly conditions...

‣ Development of an advanced materials system for tooling produced by three-dimensional printing; Development of an advanced materials system for tooling produced by 3DP

Irani, Zubin Jamshed, 1972-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 118 leaves; 8271192 bytes; 8270954 bytes; application/pdf; application/pdf
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by Zubin Jamshed Irani.; Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1999.; Includes bibliographical references (leaves 117-118).

‣ 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...

‣ The electromigration drift velocity and the reliability of dual-damascene copper interconnect trees

Wei, Frank L. (Frank Lili), 1977-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 98 p.; 3325265 bytes; 3325072 bytes; application/pdf; application/pdf
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Cu has replaced Al as the interconnect metal of choice for high performance Si-based integrated circuits. Its electromigration behavior must be quantified and an experimental basis for a circuit-level reliability assessment is needed. Experiments on straight two-terminal via-to-via Cu dual-damascene segments with different line lengths, both with via-above and via-below geometries, have been carried out. By contrasting the failure characteristics of via-above and via-below structures, the Cu/Si3N4 interface has been identified as the site for void nucleation and the most dominant diffusion path. Consequently, an asymmetry in lifetime exists between via-above and via-below interconnect lines. It has also been found that at short line lengths, true Blech immortality occurs only for very short lines, at best, due to the ease of void nucleation. Immortality due to void growth saturation is also limited, because, in the absence of the conducting refractory-metal current-shunting overlayers characteristic of Al technology, very small voids at vias can cause failures. We find that at long lengths a sub-population of Cu lines is immortal. We propose that this is a result of non-blocking liners at the base of the vias associated with the high stresses developed at the ends of the lines. In order to quantify the fundamental Cu electromigration kinetics which precedes all failure modes...

‣ Characterization of nano-arrays fabricated via self-assembly of block copolymers

Shnayderman, Marianna, 1982-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 29 leaves; 2073814 bytes; 2072468 bytes; application/pdf; application/pdf
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This research focused on methods for regulating arrangement of self-assembled block copolymers by understanding fabrication conditions and their effects on the polymers on flat and patterned substrates. Block copolymer self-assembly is a simple and low cost process for creating lithographic masks with features under 100nm in dimension. These patterns can be transferred to more permanent materials for applications in electronics, magnetic devices, as well as sensors and filters. Polystyrene-poly(ferrocenyldimethylsilane) block copolymer thin films were characterized in terms of their spin curves, PSF spherical domain cross sectional area distributions, and correlation distances. Optimal fabrication conditions were selected from studying polymer behavior on flat substrates and then used for templated substrate studies. Substrates that were templated with grooves produced quantized numbers of rows of spherical domains ranging from 4 to 7. Behavior in these grooves was characterized in terms of groove width constraints, cross sectional domain area distributions, and row ordering. For all templated arrays, the lengths of ordered regions were more than 2 fold higher than the diameters of ordered regions of arrays on flat substrates. The characterization accomplished in this work will be used to compare block copolymers with similar volume fractions of the blocks that allow sphere microdomain formation but of different molecular weights. The ultimate goals are to establish how the molecular weight of this block copolymer affects its self assembly on templated and on flat substrates and to use this factor as well as fabrication conditions and template geometries to engineer arrays with desirable properties.; by Marianna Shnayderman.; Thesis (S.B.)--Massachusetts Institute of Technology...

‣ Fabrication process changes for performance improvement of an RF MEMS resonator : conformable contact lithography, Moiré alignment, and chlorine dry etching

Sakai, Mark
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 106 p.; 5996459 bytes; 6000901 bytes; application/pdf; application/pdf
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This thesis presents fabrication process improvements for a RF MEMS resonator for the purpose of improving the quality factor (Q) and extending the frequency range. The process changes include the use of conformable contact lithography (CCL) and chlorine-based dry etching for improved fine-feature patterning and Moiré -based alignment techniques to allow for a non-self-aligned process. The resulting control over feature size and structure are expected to improve Q and enable higher frequency resonators. A CCL process utilizing moir6 alignment marks is described. An automated Moiré -based alignment system using Labview software is presented which demonstrates sub-100 nm alignment accuracy for a single alignment mark. A full-wafer alignment experiment is described that demonstrates average pattern placement errors of ... for the x- and y-directions respectively. The experimental limitations are analyzed and suggested improvements to the system are detailed. Chlorine dry etching experiments are conducted in order to produce a straight sidewall etch through the "stack" of resonator materials (chrome, aluminum nitride, and molybdenum). A combination of Cl₂/0₂, Cl₂/Ar, and CF₄0₂ plasmas at low pressure (2 mTorr), high microwave/source power (500W)...

‣ 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...

‣ Stress corrosion cracking and hydrogen embrittlement of thick section high strength low alloy steel

Needham, William Donald
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 277 leaves
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An experimental study was conducted to evaluate the corrosion performance of weldments of a high strength low alloy(HSLA) steel in a simulated seawater environment. This steel, designated HSLA80, was developed by the United States Navy for use in ship structural applications. Stress corrosion CRACKING(SCC) and hydrogen embrittlement(HEM) were investigated by conducting 42 Wedge-Opening load(WOL) tests as a function of stress intensity and corrosion potential and 33 Slow Strain Rate(SSR) tests as a function of strain rate and corrosion potential. The corrosion potentials were chosen to simulate the environmental conditions of free corrosion, cathodic protection and hydrogen generation. The results from this investigation indicated that HSLA 80 base metal and weldments were susceptible to hydrogen assisted cracking(HAC) in a seawater environment under conditions of continuous plastic deformation and triaxial stress in the presence of hydrogen. The heat-affected zone of the weldment was found to be the most susceptible portion of the weld joint. A lower bound was established for the critical stress intensity for stress corrosion cracking for HSLA 80 base metal and weldments.; by William Donald Needham.; Thesis (Ocean E.)--Massachusetts Institute of Technology...

‣ Property determinants of dextran:polyethylene glycol adhesive sealants

Shazly, Tarek (Tarek Michael)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 100 leaves
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Internal surgical intervention necessitates the intentional wounding of tissue. In certain clinical procedures, the desired wound healing response requires the use of closure techniques, such as suturing or stapling of disjoined tissues. Risk factors associated with these techniques are largely attributed to the discrete nature of the mechanical forces arising in the tissues. Adhesive sealants can mitigate risk by imparting a continuous stress distribution to tissues upon closure, as opposed to destructive stress concentrations. A novel class of dextran:polyethylene glycol hydrogels are a potential alternative to the limited selection of available adhesive sealants. Multiple compositional variations are available for both the dextran and polyethylene glycol components, making a wide range of clinically relevant material properties achievable. Key material properties determining sealant efficacy include hydration and degradation in an aqueous medium, elastic modulus, adhesion strength to tissue, and biocompatibility. Relationships between these pertinent properties and available compositional variations are determined for dextran:polyethylene glycol materials.; (cont.) Gravimetric, mechanical and biological testing reveal the following compositional determinants of material properties in dextran:polyethylene glycol copolymers: constituent molecular complexity dictates material hydration and degradation...

‣ Inverse opal hydrogel scaffolds as lymphoid microenvironments for the study of immune cell migration and immunotherapy

Stachowiak, Agnieszka (Agnieszka N.)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 147 p.
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Immunotherapies harness the inherent potential of the body to destroy foreign or infected cells, by stimulating new or enhancing existing immune responses. One way to boost insufficient native immunity might be to engineer lymphoid tissue at disease sites (e.g., tumors). Such neotissues could be formed by attracting or transferring immune cells into a defined microenvironment supportive of their activation. In addition to having therapeutic potential, such artificial lymphoid scaffolds could be used as in vitro models to investigate how immune cells respond to changing properties of their environment (e.g., architecture or biochemistry). We developed and tested macroporous poly(ethylene glycol) (PEG) hydrogel scaffolds as a platform for lymphoid tissue models and immunotherapy. By applying the method of colloidal crystal templating to materials and length scales appropriate for soft tissues, PEG hydrogels with arrays of ordered, interconnected pores were prepared. These combined the attractive properties of hydrogels, including biochemical versatility and tissue-mimetic mechanical properties, with the benefits of a macroporous structure, including improved nutrient transport and a defined space for cell interactions.; (cont.) Moreover...

‣ Ordered arrays of nanocrystals : synthesis, properties and applications

Bhaviripudi, Sreekar
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 166 p.
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Nanoscale materials, including nanocrystals and carbon nanotubes, exhibit an appealing array of physical properties, and provide an interesting prospect for research both from a fundamental as well as a technological perspective. The current emerging themes in nanoscale research are: controlled synthesis with well defined sizes and geometries; unraveling their fundamental physical properties; and assembly of these nanoscale building blocks into functional devices. Although several approaches for producing the nanoparticles have been reported in the past decade, a general, large scale method for controlled synthesis of well-defined nanoparticles in the 1-5 nm size regimes is yet to be found. A general method that enables both syntheses of nanoparticles and their assembly on substrates is critical towards furthering technological applications. The work described here involved developing a method that utilized principles of self assembly in conjunction with inorganic and organic synthetic chemistry for the controlled synthesis of ordered arrays of nanocrystals. A unique attribute of this technique is it combined themes one and three, aforementioned, into a single step. First, uniform arrays of various mono- and hetero-bimetallic nanoparticles with sizes in the range of 1-5 nm were synthesized on various substrates using PS-P4VP block copolymer (BCP) templates. These arrays of monodisperse nanoparticles were employed as catalysts for the diameter-controlled growth of SWNTs.; (cont.) Comparisons on their catalytic activities provided valuable insight on the catalyst-assisted growth of SWNTs. Alternate ways to improve the catalytic yield of nanotubes employing bi-metallic nanoparticles as well as novel catalysts for nanotube growth are also being reported for the first time. Importantly...

‣ Micromechanical actuators for insect flight mechanics

Zhou, Hui, M.S. Massachusetts Institute of Technology
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 97 p.
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This project aims to develop MEMS actuators to aid in the study of insect flight mechanics. Specifically, we are developing actuators that can stimulate the antennae of the crepuscular hawk moth Manduca Sexta. The possible mechanosensory function of antennae as airflow sensors has been suggested, and recent discoveries of our collaborators reveal that mechanosensory input from the antennae of flying moths serves a similar role to that of the hind wings of two-winged insects, detecting Coriolis forces and thereby mediating flight stability during maneuvers. Early evidence suggests that mechanical stimulus of the antennae may enable flight control. In addition, the crepuscular hawk moth Manduca Sexta has a wide wingspan (~110 mm) and is capable of carrying at least one quarter of its own weight. Thus, studying the flight of Manduca Sexta by attachment of microsystems seems plausible. The goal of our project is to design and fabricate micromechanical actuators, which will be mounted onto the moth antennae. Our collaborators will study the flight control mechanism by mechanical stimulation. Our first step was to fabricate "dummy" silicon rings for our biologist collaborators for implant experiment. A series of mounting kits were developed...

‣ High fracture toughness and high modules silicone resins

Li, Zhongtao, 1971-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 163 leaves; 12856042 bytes; 12855801 bytes; application/pdf; application/pdf
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Rigid silicone resins, generally referred to as polyalkylsilsesquioxanes, are an important class of hybrid thermosetting polymers with both inorganic and organic characteristics. They have superior thermal stability, heat resistance, fire resistance, and still can be easily processed. Silicone resins recently have attracted much interest as low dielectric constant materials replacing silicon dioxide as interlevel dielectrics. Unfortunately, poor mechanical properties, such as brittleness and low strength, limit their broader acceptance and applications. Efforts to toughen them date back to the 1970's, but little success has been achieved until now. Conventional polymer toughening techniques, such as incorporating second phase particles directly into the resins, typically do not work. Neither does decreasing the crosslink density of the resin network, which can compromise modulus and other properties. High fracture toughness and modulus addition cure rigid silicone resins are obtained in this study by a combination of intrinsic toughening and extrinsic toughening techniques. The addition cure silicone resin comprises two components: polyphenylsilsesquioxane oligomers containing silicon vinyls and low molecular weight silane crosslinkers containing silicone hydrides. The resin is cured by hydrosilylation between the two. Intrinsic toughening improves the plasticity and rigidity of the resin by choices of crosslinkers and polyphenylsilsesquioxane oligomers. The characteristics of molecular structure of the crosslinker and polyphenylsilsesquioxane oligomers that contribute to high fracture toughness and modulus are identified.; (cont.) Extrinsic toughening with rubber particles and rigid inorganic fillers further utilized the improved plastic deformation capability from intrinsic toughening This approach not only increases the fracture toughness from 0.3MPam⁰ç⁵ to as high as 1.44 MPam⁰ç⁵...

‣ Dynamic actuation properties of Ni-Mn-Ga ferromagnetic shape memory alloys; Dynamic actuation response of Ni-Mn-Ga ferromagnetic shape memory alloys

Henry, Christopher P. (Christopher Paul), 1974-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 201 leaves; 12350462 bytes; 12350213 bytes; application/pdf; application/pdf
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Dynamic magnetic-field-induced strain actuation of up to 3% with a frequency bandwidth of least 500 Hz in Ni48.5Mn29.5Ga21 ferromagnetic shape memory alloys (FMSAs) is achieved. Hardware was designed and constructed to measure frequency bandwidth, magnetic-field-induced strain, stress and magnetization driven from an applied magnetic field. The bandwidth in this investigation was only limited by inductive reactance of the hardware, not by fundamental limitations of Ni-Mn-Ga. Degradation of the peak dynamic actuation strain occurred from 3.0% to 2.6% with increasing number of cycles from Nz1,000 to N 100,000. Measurement of strain, stress, and magnetization driven by a magnetic field permitted the comparison of measured properties versus properly defined thermodynamic properties. The peak thermodynamic piezomagnetic coefficient is d3, 1,= 2.5 x 10-7m / A compared to the experimental slope, dE/dH, of 1.0 x 10-7 m / A at N-1,000 cycles and 1.4 x 10-7 m / A at N-100,000 cycles, respectively. The thermodynamic piezomagnetic coefficient is five times greater than Terfenol-D with d31 = 5.0 x 10-m / A. The magnetic susceptibility varies between 3-10, while the twinning stiffness varies between 30-40 MPa within the average bias stress range of 0.3 to 2.8 MPa. At optimum fields and bias stresses...

‣ 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...