Página 16 dos resultados de 3096 itens digitais encontrados em 0.065 segundos

‣ Bimetallic bars with local control of composition by three-dimensional printing

Techapiesancharoenkij, Ratchatee, 1979-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 112 p.; 1551479 bytes; 1998869 bytes; application/pdf; application/pdf
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Three Dimensional Printing (3DP) is a process that enables the fabrication of geometrically complex parts directly from computer-aided design (CAD) models. The success of 3DP as an alternative manufacturing technology to bulk machining of materials for complex parts has been demonstrated. By proof of concept, 3DP has demonstrated the ability to create parts with Local Control of the Composition (LCC). LCC allows tailoring the material properties in regions of a part for functional purposes. In this work, LCC was studied and demonstrated by fabricating bimetallic bars consisting of two layers of Fe-Ni alloys with different composition and, hence, different thermal expansion properties; the coefficient of thermal expansion (CTE) of Fe-Ni system is sensitive to its composition. Two types of the binder/dopant slurries were made for making the LCC bars. One type consisted of dispersions of Fe₂O₃ particles in water, and the other consisted of dispersion of NiO in water. The LCC bars were successfully made by printing the Fe₂O₃/NiO slurries into Fe-30Ni base powders. After heat treatment to impart strength to the printed bars, the bars were successfully retrieved from unbound powders. The bars, then, were annealed at 1400 ⁰C for 2 hours for sintering and homogenization. The final composition of the base powders were changed accordingly. In the layers on which an Fe₂O₃ slurry was printed...

‣ Selective SiGe nanostructures

Langdo, Thomas Andrew, 1974-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 215 p.; 21370706 bytes; 21370460 bytes; application/pdf; application/pdf
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Selective epitaxial growth (SEG) of SiGe on patterned SiO2/Si substrates by ultra-high vacuum chemical vapor deposition (UHVCVD) shows promise for the fabrication of novel SiGe microelectronic structures. This work explores selective growth conditions in the SiH2Cl2/SiH4/GeH4/H2 system between 650-850⁰C, without the addition of C12 or HC1, on substrates patterned by both conventional and interferometric lithography. We have achieved several important milestones for the fabrication of vertical MOSFETs by selective growth in 100 nm SiO2 features patterned by interferometric lithography. We have observed excellent selectivity to SiO2 masks with SiH2C12 at 750⁰C, perfect epitaxial Si filling of SiO2 features, the facet morphology during growth, and the effects of n-type doping on selective growth. We have also fabricated extremely sharp p-n diode doping profiles. With the above accomplishments we have demonstrated the feasibility of vertical MOSFET fabrication through selective epitaxial growth. To realize the advantages of advanced MOSFET designs on silicon-on-insulator (SOI) substrates, we have developed a facet-free raised source/drain process utilizing moderate n-type doping of Si selective growth and <110>-oriented vertical SiO2 sidewalls. However...

‣ Langasite bulk acoustic wave resonant sensor for high temperature applications

Seh, Huankiat, 1974-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 180 p.; 3581022 bytes; 3593155 bytes; application/pdf; application/pdf
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(cont.) The self consistent defect model established the defect chemistry of langasite, enabling important parameters describing reduction (Er = 5.70± -0.06eV and 6.57±-0.24eV for acceptor and donor doped langasite respectively) and oxidation (Eo = 2.18±0.08eV), intrinsic electron-hole generation (Eg [approx. equals] 4.0-4.4eV) and defect ionization (ED-ion = 52±0.06eV for Nb ionization), to be extracted. The predictive defect model was used to calculate the dependence of the partial ionic and electronic conductivities and mass change as functions of temperature, dopant level and pO₂. Given that the magnitudes of conductivity and mass change directly affect the resolution and sensitivity limits of langasite resonators, their predictions allowed for the definition of acceptable operating limits and/or the design of properties for optimum resolution and sensitivity. Two high temperature applications of resonant sensors were studied. Praseodymiumcerium oxide was selected for oxygen partial pressure monitoring and is representative of films which change mass upon absorption or desorption of gaseous species. Barium carbonate film was selected for NO₂ sensing and is representative of films which change mass upon reaction with the gas phase to form a new product phase. Both sensors showed sensitivity to their respective target chemicals and demonstrated the feasibility of high temperature sensor applications. The performance of each sensor was discussed and suggestions for improving sensor performance were presented.; The high temperature transport properties of langasite...

‣ Operational sustainability metrics : a case of electronics recycling

Atlee, Jennifer Robinson
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 127 p.; 10729810 bytes; 10736001 bytes; application/pdf; application/pdf
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In the past 15 years corporations and governments have developed a growing appreciation of the need for "sustainability" and have worked the term into their goals, strategy and mission statements. Despite extensive efforts to define the term, there is still little clarity on how to move toward sustainability or measure improvements. Further advances toward sustainability will require system specific metrics to assess both current performance and the impact of operational, technological or regulatory changes on that performance. Not only are there currently few operational metrics by which to practically assess progress toward sustainability, there is also very little understanding of how to judge the effectiveness of such metrics. Electronics recycling is used in this thesis as a case problem in developing and evaluating system specific performance metrics for sustainability. Electronics recycling is a growing national and international concern due to the increasing volume of waste, the potential toxicity of the scrap, and reports of improper handling and disposal. Despite this concern, there is limited understanding about the electronics recycling system. There is a need for systematic ways to describe system functioning and quantitative methods to assess system performance. Existing evaluations of eco-efficiency or sustainability are either too aggregated to guide operational decisions or too complex and data intensive to be performed in the context of a low-margin system. A range of performance metrics were developed and assessed for several electronics recycling operators. These included measures of resource recovery and environmental performance.; (cont.) These metrics were assessed for their ability to provide insights on resource efficiency comparable to more complex indicators...

‣ 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...

‣ Determination of special boundary coordination at quadruple nodes using EBSD

Ng, Christopher, 1983-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 35 leaves; 1912222 bytes; 1911566 bytes; application/pdf; application/pdf
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Grain boundaries are known to play an important role in materials properties including corrosion and cracking resistance. Some grain boundaries are resistant to corrosion and cracking and are known as "special" boundaries. While the structure of individual grain boundaries is important, the connectivity of the grain boundaries largely determines the properties of a bulk material. The coordination and connectivity of special grain boundaries have previously been studied in two dimensional grain boundary networks and are quantified by the triple junction distribution (TJD), which has been found to be non-random. The study of connectivity has been extended to three dimensions and simulations have previously been done to obtain a quadruple node distribution (QND) which was also non-random. Using Electron Back-Scattered Diffraction to characterize grain boundaries in copper and aluminum, this project obtains an experimental quadruple node distribution and verifies that it too is non-random.; by Christopher Ng.; Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2005.; Includes bibliographical references (leaf 21).

‣ Fabrication of water-soluble gold nanoparticle aggregates

Bennett, Samantha E
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 24 leaves; 1763383 bytes; 1761613 bytes; application/pdf; application/pdf
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Mixed monolayer protected gold nanoparticles were linked using octanedithiol to form aggregates containing hundreds of nanoparticles. These aggregates are an interesting material, posing potential applications in the fields of chemistry, biology and materials science. This study examined the dependence of aggregate size and morphology on temperature of formation, using AFM and TEM imaging. The aggregates formed at 70°C averaged 105nm in width, as compared to 70nm for the room temperature aggregates. The TEM images showed increased density for the 70°C aggregates. In a further study, the room temperature aggregates were functionalized through a place exchange reaction with 1 -mercapto-undecane- l-sodiumsulfonate (MUS), a thiolated ligand with a polar head group. A two-phase test of the water-solubility indicated that the aggregates were fully soluble. TEM images showed a slight increase in size, though similar morphology to the insoluble aggregates. The ability to induce water solubility in the aggregates opens up many potential applications in the field of bionanomaterials.; by Samantha E. Bennett.; Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2006.; Includes bibliographical references (leaves 23-24).

‣ Commercial assessment of roll to roll manufacturing of electronic displays

Randolph, Michael Aaron
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 81 leaves
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The cost of manufacturing electronic displays currently limits the range of applications and markets into which it is currently economically feasible to adopt displays. Roll-to-roll manufacturing has been identified by the display industry as a new and fundamentally different manufacturing paradigm that has the potential to significantly reduce the manufacturing cost of a display relative to the conventional approaches used in the industry. This manufacturing cost reduction could have a profound impact on the display industry by not only transforming the display manufacturing infrastructure, but also by permitting electronic displays to penetrate new markets. The purpose of this thesis is to determine how roll-to-roll manufacturing technology could develop and to assess what impact the technology could have on the electronic display manufacturing industry. This work first identifies the material, patterning, and equipment technologies that need to come together in order for roll-to-roll manufacturing to be industrially feasible, and then determines how and if the technology will offer a cost reduction over conventional manufacturing techniques.; (cont.) Next, the markets for displays are segmented and analyzed to discern whether niche initial markets exist where roll-to-roll could have a distinctive advantage and gain traction. Competitive technologies such as LCD and modular LED are discussed and it is determined that roll-to-roll displays must compete with LCD technology on the basis of price in the markets in which LCD has incumbency in order to achieve widespread adoption. The display industry structure is analyzed by means of an assessment of the supply chain...

‣ Potential commercial application of a bi-layer bone-ligament regeneration scaffold to anterior cruciate ligament replacement

Li, Jessica C. (Jessica Ching-Yi)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 94 p.
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A business model was created in order to explore the commercial application of a bi-layer bone-ligament scaffold to the treatment of torn anterior cruciate ligaments (ACL) requiring replacement. The two main keys in producing the bone scaffold are triple co-precipitation of type-I collagen, chodroitin-6-sulphate, and calcium phosphate minerals and the use of lyophilization to create a network where all the materials are homogeneously dispersed and present in significant amounts. This process allows the creation of a porous network whose physical characteristics, mechanical properties, and material content can all be changed to create a scaffold that closely mimics natural bone. A collagen and chondroitin-6-sulphate scaffold is used for ligament regeneration. The ACL replacement market was chosen because it is one of the most commonly surgically repaired ligaments in the body and because all of the current treatments have drawbacks.; (cont.) The exercise of creating a business model made it clear that the commercial potential of starting a company that focused on marketing a direct ACL replacement scaffold would most likely not be successful mainly because surgeons would hesitate to use this product over current methods that are satisfactory and it would be difficult to separate our product from other newer methods which all boast similar advantages over current treatment options. However...

‣ Intrachip clock signal distribution via si-based optical interconnect

Ahn, Donghwan
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 182 leaves
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The Optical clocking has emerged as an innovative alternative approach to the electrical clocking, in order to overcome the difficulties associated with electrical interconnects in the synchronization of high-performance multi-GHz microprocessors. Because the on-chip optical interconnect must be embedded in current Si microprocessors, the optical clocking requires that the electronic-photonic integrated circuits (EPIC) consisting of Si-based photonic devices be developed on Si CMOS platform. We have identified the H-tree waveguide network and waveguide-integrated photodetectors as key photonic devices required for intrachip optical clocking for microprocessors. We have demonstrated successful optical signal distribution through SiON, waveguide H-tree network with 64 fanouts. A variable bending radius approach in H-tree design was used to optimize the performance of the optical clock signal distribution. The conventional y-splitter showed significant optical loss and unequal power-splitting ratio, which becomes increasingly problematic as the number of levels of a H-tree network increases and can result in increased skew. We devised a novel extended offset splitter, which reduced the splitting loss to < 3% and demonstrated 49:51 power split ratio. We have fabricated Si vertical p-i-n photodetectors that are monolithically integrated with compact silicon-oxynitride channel waveguides.; (cont.) 830nm light guided by the waveguide was coupled to the photodetector through evanescent-wave coupling. We measured over 90% coupling efficiency. A two-step process that consists of 1) mode-coupling from a guided mode in the input waveguide to a leaky mode in the waveguide in contact with photodetector and 2) gradual evanescent wave coupling from waveguide towards photodetector...

‣ 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...

‣ Shape morphing structures via intercalation compounds

Wong, John T., M. Eng. Massachusetts Institute of Technology
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 40 leaves
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Recent research has allowed the use of electrode-active materials as actuators by harnessing reversible volumetric change due to intercalation during charging and discharging. These actuators provide a relatively large strain, 2-3%, while requiring less than ten volts to operate with a cycle time on the order of one hundred seconds. This technology can be used in any moderate bandwidth, high-force, high-strain application, including shape-morphing helicopter blades, boat hulls, satellites, and any other structure that benefits from shape change. An analysis is performed on the state of the technology, the intellectual property held, and the potential markets that exist. A recommendation is made to pursue the technology, while cognizant of the fact that it is still in a seed stage and requires significant time and financial investment before entering production. Two business models are proposed and rough market calculations are also presented. The basis of this project is work done at the Massachusetts Institute of Technology by Professor Yet-Ming Chiang and Professor Steven Hall. Industrialists, including but not limited to, William Fallon and Dan Ursenbach of Sikorsky Aircraft in Stratford, Connecticut are also involved in the project.; by John T. Wong.; Thesis (M. Eng.)--Massachusetts Institute of Technology...

‣ Ultra-lightweight nanorelief networks : photopatterned microframes

Choi, Taeyi
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 153 p.
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Lightweight nano-network structures in polymers have been fabricated and investigated for their mechanical properties. Fabrication techniques via holographic interference lithography and phase mask lithography were implemented for periodic and quasiperiodic bicontinuous polymer-air structures on the submicrometer length scale. For 3D quasiperiodically nanostructured materials, quasicrystalline phase mask lithography utilizing 2D quasiperiodic phase mask was successfully employed. 2D hexagonal arrays of air cylinders in SU8 polymer films and 3D four-beam connected (3- R3m ) and octagonal quasicrystalline SU8 films were fabricated and analyzed in this thesis. For investigating the mechanical properties of various nano-network structures, three different methods of mechanical characterization were applied. Atomic force microscopy with its nanometer scale resolution was adopted to conduct force measurements to probe local elastic properties of the sample. Templated by the light intensity distribution from three-beam interference, the spatial distribution of elastic modulus was observed in the pattern of 2D hexagonal air-cylinder and a uniform SU8 polymer film by AFM nanoindentation. A second method for mechanical characterization, the microtensile tester enabled us to evaluate a symmetry effect on the elastic and plastic properties of the polymer fibers and thin films. Large plastic deformation of 200nm-diameter struts comprising the 3D periodic and quasiperiodic microframes of the normal brittle bulk polymer was discovered and is an example of length-scale dependent mechanical behavior. Crack propagation and energy absorption were guided along the symmetry directions in the periodic structures. However...

‣ Composite cathodes for lithium rechargeable batteries

Olivetti, Elsa A
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 155 p.
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The utility of incorporating continuous, nanoscale vanadium oxide phases within preferred domains of self-organizing copolymers was investigated towards the fabrication of composite, nanoarchitectured electrode materials for solid-state rechargeable batteries. In situ growth of cathodic phases within ion-conducting copolymer domains was explored as a means to control morphology and to increase the surface-area-to-volume ratio, thereby increasing the specific electrode area for faradaic reactions and decreasing ion diffusion distances within the electrode-active material. Copolymers of microphase-separating rubbery block and graft copolymers, previously developed as solid electrolytes, provide a matrix for directing the synthesis of an inorganic battery-active phase. The copolymers include poly[(oxyethylene)9 methacrylate]-block-poly(butyl methacrylate) (POEM-b-PBMA) with a domain periodicity of -35 nm made by atom transfer radical polymerization, and poly[(oxyethylene)9 methacrylate]-graft-poly(dimethyl siloxane) (POEM-g-PDMS) with a domain periodicity of-17 nm made by free radical polymerization. The resulting microphase-separated polymer is a structure of alternating hydrophilic (Li-ion conducting) and hydrophobic regions.; (cont.) Sol-gel chemistry involving a vanadium alkoxide precursor enabled the in situ growth of cathode-active vanadium oxide within the continuous ion-conducting POEM domains of the microphase-separated copolymers. Resulting films...

‣ Anisotropic dewetting in ultra-thin single-crystal silicon-on-insulator films

Danielson, David T. (David Thomas)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 156 p.
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The single crystal silicon-on-insulator thin film materials system represents both an ideal model system for the study of anisotropic thin film dewetting as well as a technologically important system for the development of the next generation of MOSFET devices. The scientific community has misinterpreted the mechanism behind SOI dewetting, attributing it to a stress-driven instability. In this work, we characterize and measure the kinetics and geometry of thermally-activated dewetting in ultra-thin SOI films as a function of SOI thickness, crystallographic patterned-edge orientation, and temperature. Using experimental data and physical reasoning, we strongly argue against the previously existing scientific consensus that SOI dewetting is a stress-driven instability and present a 5-step isotropic surface-energy-driven dewetting model based upon the capillary film edge instability and the generalized Rayleigh instability, in which we integrate existing theories and in addition develop an expression for the edge retraction distance during the initial uniform edge thickening and retraction phase of dewetting at a patterned mesa edge for the first time, predicting a square root dependence with time and an inverse power dependence on film thickness. In our study...

‣ Lattice mismatched epitaxy of heterostructures for non-nitride green light emitting devices

Mori, Michael James
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 165 p.
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In this project, we implement modern metal organic chemical vapor deposition (MOCVD) technology to fabricate monolithic platforms which integrate traditionally incompatible materials with the ultimate goal of achieving high brightness green to amber light emitting diodes (LEDs) and laser diodes (LDs). Unconventional compositions of aluminum indium gallium phosphide (AlInGaP), with lattice constants less than that of GaAs, offer improved electrical and optical characteristics over commonly used GaAsmatched material. Also, integration of optical III-V material on the CMOS platform has long been a technological goal, and these compositions of AlInGaP are amenable to monolithic integration on the (100) Si platform. In this thesis, we pioneer technology to integrate high quality, novel AlInGaP alloys on III-V substrates (and elsewhere, this technology is successfully extended for III-V integration on (100) Si). We first focus on creating a virtual substrate upon which any lattice constant intermediate to GaAs and GaP is available. Large amounts of lattice mismatch are ultimately relaxed through incremental introduction of strain in compositionally graded epitaxial layers, thus breaking the typical lattice-matched constraint of semiconductor systems. Tensile relaxed gallium arsenide phosphide (GaAsP) graded layers yield virtual substrates with extremely low threading defect densities ([rho]t=104cm-2)...

‣ Towards first-principles electrochemistry

Dabo, Ismaila
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 151 p.
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This doctoral dissertation presents a comprehensive computational approach to describe quantum mechanical systems embedded in complex ionic media, primarily focusing on the first-principles representation of catalytic electrodes under electrochemical conditions. The accurate electrostatic description of electrified metal-solution interfaces represents a persistent challenge for ab-initio simulations and an essential requisite for predicting the electrical response of electrochemical convertors-i.e., the correspondence between the macroscopic voltage and the microscopic interfacial charge distribution. The approach consists of controlling the electrode voltage via its conjugate extensive variable, namely, the charge of the system. As a preliminary to the study of electrified interfaces in ionic media, we analyze charged slabs in vacuum subject to periodic boundary conditions. We show that the corrective potential (defined as the difference between the exact open-boundary potential and the periodic potential obtained from a Fourier transform) varies smoothly over space, allowing for its determination on a coarse mesh using optimized electrostatic solvers. Because this scheme takes into account exact open boundary conditions, its performance is considerably superior to that of conventional corrective methods. Extending this computational scheme...

‣ Apatite-polymer composites for the controlled delivery of bone morphogenetic proteins

Yong, Tseh-Hwan
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 140 leaves; 9942234 bytes; 9948145 bytes; application/pdf; application/pdf
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Current treatment of bone defects due to trauma, cancer, or degenerative spine diseases involves the implantation of a bone graft. Autografts, which are harvested from the patient's own body, are associated with problems of limited availability and surgical morbidity. The use of allografts obtained from donors is also not desirable due to the risks of disease transmission and the costs of maintaining bone banks. The ideal solution would be to regenerate native bone to fill the defects. A group of potent growth factors known as bone morphogenetic proteins (BMPs) have been hailed as alternatives to bone grafts due to their ability to elicit new bone formation. Clinical use of BMPs involves loading the protein solution onto collagen sponges and subsequent implantation. However, these conventional collagen carriers show rapid clearance of BMPs within [approx.] 2 weeks, whereas bone healing is a longer process, especially in higher mammals. The poor BMP retention in collagen sponges may explain the greater response variability in higher mammals, ranging from full bone bridging within weeks to no bone union. These sponges are also not capable of tunable or multifactor release that could benefit healing in certain anatomic sites, e.g. avascular sites and prolonged non-unions. Hence...

‣ Energy absorption in Ni-Mn-Ga/ polymer composites

Feuchtwanger, Jorge
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 143 p.; 3538896 bytes; 3538683 bytes; application/pdf; application/pdf
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In recent years Ni-Mn-Ga has attracted considerable attention as a new kind of actuator material. Off-stoichiometric single crystals of Ni2MnGa can regularly exhibit 6% strain in tetragonal martensites and orthorhombic martensites have shown up to 10% strain when subjected to a magnetic field. These crystals are brittle and the production of single crystals can be quite costly. Terfenol-D, a commercially available giant-magnetostrictive material, suffers from some of the same drawbacks. It was found that composite materials made from Terfenol-D particles in a polymeric matrix could solve the issue of the brittleness while retaining a large fraction of the strain output of the alloy. At first glance a similar approach could be used to solve the brittleness issue of Ni-Mn-Ga, but the low blocking force of these alloys reduces the chances of achieving a Ni-Mn-Ga/polymer composite actuator. However, the stress-strain loops for Ni-Mn-Ga show a large mechanical hysteresis. This ability to dissipate energy makes this alloys very desirable for damping applications, and by putting particles of Ni-Mn-Ga in a composite, their brittleness becomes less of an issue.; (cont.) It is shown that by curing Ni-Mn-Ga/polymer composites under a magnetic field it is possible to align the particles in chains and to orient them so they will respond to a uniaxial load. The magnetic measurements show that there are twin boundaries in the particles that can be moved by an external stress. Stress-induced twin boundary motion in the particles is confirmed more directly by x-ray diffraction measurements...

‣ 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...