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‣ Grain size effects on the fatigue response of nanocrystalline materials

Hanlon, Timothy, 1977-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 158 p.; 5653368 bytes; 11346872 bytes; application/pdf; application/pdf
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The resistance of metals and alloys to fatigue crack initiation and propagation is known to be influenced significantly by grain size. Based on a wealth of experimental results obtained from microcrystalline metals, where the grain size is typically greater than 1 um, it is widely recognized that an increase in grain size generally results in a reduction in the fatigue endurance limit. On the other hand, a coarser grain structure can lead to an increased fatigue threshold stress intensity factor range, as well as a decrease in the rate of fatigue crack propagation. The relevance of these trends to ultra-fine-crystalline metals (grain size between 100 nm and 1000 nm) and nanocrystalline metals (grain size less than 100 nm) is relatively unknown. Such lack of understanding is primarily a consequence of the paucity of experimental data on the fatigue response of metals with very fine grains. In this work, the fatigue behavior of electrodeposited, fully dense, nanocrystalline pure Ni, with average and total range of grain sizes well below 100 nm, was examined. The fatigue response of nanocrystalline Ni was also compared with that of ultra-fine-crystalline and microcrystalline Ni wherever appropriate. It was found that grain refinement to the nanocrystalline regime generally leads to an increase in resistance to failure under stress-controlled fatigue whereas a deleterious effect was seen on the resistance to fatigue crack growth. To explore the generality of the above trends...

‣ BaTiO₃ based materials for piezoelectric and electro-optic applications

Avrahami, Ytshak, 1969-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 95 p.; 1742528 bytes; 1749399 bytes; application/pdf; application/pdf
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Ferroelectric materials are key to many modem technologies, in particular piezoelectric actuators and electro-optic modulators. BaTiO₃ is one of the most extensively studied ferroelectric materials. The use of BaTiO₃ for piezoelectric applications is, however, limited due to the small piezoelectric coefficient of the room temperature-stable tetragonal phase. Furthermore, research on BaTiO₃ for integrated optics applications remains sparse. In this work Zr-, Hf-, and KNb- doped BaTiO₃ materials were prepared in a composition range that stabilizes the rhombohedral phase. These materials were prepared as bulk polycrystals using a standard solid-state reaction technique in order to test the piezoelectric and dielectric properties. Some compositions were then chosen for thin film deposition. The films were deposited using pulsed laser deposition on MgO and SOI substrates. Growth orientation, remnant strain and optical properties were then measured. X-ray diffraction was used to confirm the existence of a stable rhombohedral phase. Dielectric measurements confirmed the expected phase transition temperatures. A piezoelectric coefficient of d₃₃=290-470pc/N was measured for Zr- and Hf- doped BaTiO₃, compared with d₃₃=75pC/N for pure BaTiO₃. The electrostrictive coefficient of the KNb-doped material...

‣ Heat flow and material degradation during laser metal forming

Haidemenopoulos, Gregory N
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 146 leaves; 6469560 bytes; 6469318 bytes; application/pdf; application/pdf
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by Gregory N. Haidemenopoulos.; Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Ocean Engineering and Dept. of Materials Science and Engineering, 1985.; MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING; Bibliography: leaves 144-146.

‣ Semiconductor nanocrystal composite materials and devices

Lee, Jinwook, 1966-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 126 leaves; 9038633 bytes; 9038391 bytes; application/pdf; application/pdf
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This thesis describes the synthesis and characterization of semiconductor nanocrystal (quantum dot, QD) embedded composite materials and possible device applications of the resulting luminescent materials. Chemically synthesized ZnS overcoated CdSe, (CdSe)ZnS, QDs are incorporated into a polymer host material. The main challenge in the preparation of QD-polymer composites is the prevention of both phase separation and aggregation of the QDs within the polymer host material, while sustaining the original quantum efficiency of the QDs in their growth solution. Possible ways to incorporate QDs into an optically clear polymer matrix are considered. A guideline for a successful QD-polymer composite is discussed for various polymer systems: ligand polymers, ligand monomer and covalent bonding to a polymer matrix, and in-situ polymerization. The best composite system is based on incorporation of QDs into a poly(laurylmethacrylate) matrix during in-situ polymerization in the presence of TOP ligands. The successful incorporation of QDs into a polymer host material demonstrates the ability to form QD-polymer composite light emitting materials. The emission spans nearly the entire region of saturated and mixed colors with narrow emission profiles. The light emission spectra of QD-polymer composites excited by a blue diode light are also simulated by Monte Carlo methods and compared to the measured spectra from actual devices. The synthesis and characterization of QD-microspheres...

‣ Commercial application of aluminum honeycomb and foam in load bearing tubular structures

Bartolucci, Stefano, 1976-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 156 leaves; 8319626 bytes; 8340736 bytes; application/pdf; application/pdf
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Small dimension engineering tubular structures subjected to a complex load system are designed like hollow circular shells. For minimum weight design, the ratio between the shell radius and the thickness has to be as large as possible, but its maximum value is limited by the onset of local buckling. Tubular natural structures subjected to a complex load system have often an outer shell of solid material supported by a low density, compliant core, which makes them more resistant to local buckling. Biomimicking of natural constructions offer the potential to improve the design of small diameter tubular engineering structures. Here, the fabrication technology of biomimicked engineering tubular structures integrating aluminum foam or honeycomb as core material is discussed. A viability analysis is presented including technical performance, cost, utility, and risk assessments. Aluminum compliant core shells have potential for substituting CFRP and aluminum tubular structures in aerospace and high-level sport applications. The case of sailboat masts was considered in detail. Results of our analysis proved that use of honeycomb as core material can lead to a significant reduction of the mast weight. Business opportunities based on this application are discussed.; by Stefano Bartolucci.; Thesis (M. Eng.)--Massachusetts Institute of Technology...

‣ Structure and properties of hydrogen and covalently bonded side group liquid crystalline block copolymers

Osuji, Chinedum, 1976-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 226 leaves; 11367870 bytes; 22199544 bytes; application/pdf; application/pdf
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Diblock copolymers incorporating liquid crystallinity via a lateral attachment of mesogenic species to a flexible backbone were studied for their microstructure-property relationships. Two families of materials were studied, distinguished primarily by the method of attachment of the mesogen to the polymer. In the first, azobenzene based mesogens were covalently tethered to the isoprene blocks of a poly(styrene)-block-poly(isoprene), P(S-b-ILC), series of diblock copolymers. In the second family of polymers, hydrogen bonding was used to complex bi-phenyl based mesogens to the acrylic acid units of a poly(styrene)-block-poly(acrylic acid), P(S-b-AA) diblock copolymer. The morphology of the P(S-b-ILC) system was characterized as a function of composition, and clear correlations between the microphase separated structure and the thermal properties of the liquid crystalline mesophase were observed and accounted for. Control of the hierarchical structure in these materials was pursued and achieved independently via surface epitaxy, oscillatory shear and, for the first time in this class of materials, via magnetic fields. It was found that the morphology adopted by the material under the external fields is strongly dependent on the orientation of the liquid crystalline mesophase with respect to the inter-material dividing surfaces present due to the microstructure. Oscillatory shear of a P(S-b-ILC) resulted in the first observation of a novel transverse cylindrical microdomain morphology in one case. The investigation of hydrogen-bonded side-group liquid crystalline block copolymers based on P(S-b-AA) followed a screening of several other candidate host diblock-mesogen pairs. The structure and thermal properties of a model P(S-b-AA) diblock and a homopolymer acrylic acid complexed with mesogens were studied and characterized as a function of composition. A high molecular weight analogue...

‣ Rehardenable materials system with diffusion barrier for three-dimensional printing

Yuen, Cheong Wing, 1972-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 197 p.; 16760335 bytes; 16760143 bytes; application/pdf; application/pdf
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Three-Dimensional Printing (3DP) is a solid freeform fabrication process being developed for the direct manufacture of functional tooling and prototypes from a computer solid model. One of its many important applications is the fabrication of metal tooling for plastic injection molding. In order to achieve a fully dense 3DP metal tool, the sintered powder skeleton is infiltrated with a molten alloy, which has a melting point lower than the skeleton material. However, the choices of materials systems are limited by the interactions of the metal powders and infiltrants during the infiltration process. Currently, the materials system with the best wear resistance for 3DP metal tooling consists of 420 stainless steel powder and bronze infiltrant. However, it only has an overall hardness of 25 HRC because the bronze infiltrant is soft and not hardenable. A hardenable 3DP metal system is desirable. The main goals of this thesis research are: 1) to improve the flexibility of choice of metal powders and infiltrants by using a diffusion barrier to isolate them; and 2) to demonstrate the diffusion-barrier approach with steel and hardenable copper-alloy infiltrant. The model materials systems in this study consist of stainless steel and tool steel powder skeletons with Cu-20Ni-20Mn infiltrant. It was demonstrated that TiN coating deposited on steel substrates by CVD successfully prevented the reaction between the steel and molten Cu-20Ni-20Mn at 1200° C. In general...

‣ Alloy design or three-dimensional printing of hardenable tool materials

Guo, Honglin, 1965-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 174 leaves; 8524668 bytes; 8524474 bytes; application/pdf; application/pdf
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Three-dimensional printing (3DP) is a state-of-the-art manufacturing technology, One of its many important applications is to fabricate the metal tooling for plastic injection molding. In order to achieve a fully dense 3DP metal tool, the current 3DP process involves five steps: printing, debinding, sintering, liquid metal infiltration and heat treatment. Due to the infiltration requirement, all 3DP tools made to date are composed of a high-strength skeleton material and a low-strength infiltrant. The search for a hardenable/hard 3DP system is the mission of this dissertation. Five major accomplishments can be found in the dissertation: (1) development of the 420/bronze material system for 3DP tooling; (2) development and optimization of the post-processing of the system; (3) development of a computer model simulating the interaction of powder/liquid infiltrant; (4) computer-aided material system design and (5) methodology exploration of the material system development.The 420/bronze material system with a minimum reaction was developed experimentally, by screening 30 potential material systems. Compared to the initial 3DP and reactive system, the system strength was significantly improved. More than 50 3DP injection tools have been fabricated using this material system. A 3DP tool made of the 420/bronze system has been used to mold more than one hundred thousand plastic products without major repairing of the tool. It was the first time that an injection mold was made of 60 vol% 420 and 40 vol% bronze. The system was a milestone in the 3DP material system development. In addition...

‣ Sputtered silicon oxynitride for microphotonics : a materials study

Sandland, Jessica Gene, 1977-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 134 leaves; 5506996 bytes; 5524537 bytes; application/pdf; application/pdf
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Silicon oxynitride (SiON) is an ideal waveguide material because the SiON materials system provides substantial flexibility in composition and refractive index. SiON can be varied in index from that of silicon dioxide (n=1.46) to that of silicon-rich silicon nitride (n-2.3). This flexibility in refractive index allows for the optimization of device performance by allowing trade-offs between the advantages of low-index contrast systems (low scattering losses and easy fiber-to-waveguide coupling) and the benefits of high-index-contrast systems (small waveguide size and tight bending radii). This work presents sputter processing as an alternative to traditional CVD processing. Two room-temperature SiON sputter processes are explored. The first process is a co- sputtered deposition from a silicon oxide and a silicon nitride target. The second is a reactive sputtering process from a silicon nitride target in an oxygen ambient. Silicon nitride sputtered from a silicon nitride target is also investigated. Models are provided that predict the index and composition in both the reactive and co- sputtered depositions. The cosputtered deposition is shown to follow a mixture model, while the reactive sputter deposition is shown to be either Si-flux limited or O-flux limited...

‣ Low temperature processing of baroplastic core-shell nanoparticles and block copolymers

González-León, Juan A. (Juan Antonio)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 144 p.
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Baroplastics are nanophase polymeric materials comprised of two components that can miscibilize under pressure thereby facilitating flow. The possibility of processing these materials at low temperatures was the main focus of this work. Block copolymer baroplastics comprised of a low Tg and a high Tg component that microphase separate, such as polystyrene-block-poly(butyl acrylate) (PS-b-PBA) and polystyrene-b-poly(2-ethyl hexylacrylate) (PS-b-PEHA), were synthesized by ATRP and processed at reduced temperatures by compression molding. The resulting processed specimens were clear and well-defined solid objects. Structural characterization studies on the processed baroplastics showed that the mixing between components during processing is incomplete and distinct hard and soft domains are present even after multiple processing cycles. This suggests that the processing is of a semi-solid nature, where the rigid PS domains are mobilized by the low Tg component. Processing of a control sample exhibiting pressure-induced demixing, polystyrene-block-poly(lauryl methacrylate) (PS-b-PLMA), yielded incompletely processed objects under the same processing conditions and inferior mechanical properties to its acrylate counterparts.; (cont.) Low temperature processing of baroplastics and the proposed semi-solid processing mechanism were further demonstrated with the study of core-shell nanoparticles...

‣ Gene-supplemented collagen-glycosaminoglycan scaffolds for nonviral gene delivery in articular cartilage tissue engineering

Capito, Ramille M. (Ramille Marie)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 260 p.
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Three-dimensional scaffolds and growth factors have been shown to be important for articular cartilage tissue engineering. A major problem in using recombinant proteins in vivo, however, is the inability to maintain therapeutic levels over prolonged times due to degradation or diffusion from the defect site. The goal of this thesis was to develop a method to employ type II collagen-glycosaminoglycan (CG) scaffolds for the nonviral delivery of the gene encoding for insulin-like growth factor (IGF)-I, as a novel means to provide a local, elevated, and prolonged release of a therapeutic growth factor via transfection of cells seeded or migrating within the scaffold. In vitro studies were performed to evaluate gene-supplemented CG (GSCG) scaffolds, including: 1) the type of expansion medium to use for growing chondrocytes prior to transfection, 2) methods of incorporating genes within scaffolds, 3) additional incorporation of transfection enhancers, and 4) the use of mesenchymal stem cells (MSCs) as an alternative cell source for articular cartilage tissue engineering.; (cont.) The medium used during monolayer expansion not only had a significant effect on subsequent biosynthesis and chondrogenesis in CG scaffolds, but also on gene transfer to chondrocyte monolayers. The expansion medium that resulted in enhanced 3-D biosynthesis and gene transfer to cells in monolayer was used throughout the rest of the studies. Greater plasmid retention within GSCG scaffolds was achieved by chemically cross-linking the plasmid IGF-1 (pIGF-1) to the scaffold (compared to simple plasmid absorption)...

‣ Synthesis and characterization of conducting polymer actuators

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

‣ Gene-supplemented collagen scaffolds for non-viral gene delivery for brain tissue engineering

Bolliet, Catherine
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 129 p.
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Recent advances in tissue engineering, combining an extracellular matrix (ECM)-like vehicle with therapeutic molecules, cells and/or genes has yielded promising results for brain injury repair. The purpose of this thesis was to develop a collagen scaffold for the non-viral delivery of the gene encoding for Glial Cell-Derived Neurotrophic Factor (GDNF); hence to provide a local, long-term release and overexpression of GDNF via transfection of cells seeded into the scaffold or endogenous cells. The first part of the thesis aimed to investigate the in vitro transfection of marrow stromal stem cells (also referred to as mesenchymal stem cells, MSCs) in monolayer with plasmid GDNF ([mu]gDNF). Several parameters were evaluated: the choice of a transfer reagent (GenePorter2 versus Lipofectamine 2000), the doses of plasmid incorporated in the liposomes (ranging from 0.2[mu]g to 2[mu]g), the post-transfection medium (Medium 1: DMEM low glucose, 20% FBS and 1% antibiotic versus Medium 2: DMEM low glucose, 20% FBS, 1% antibiotic and 10ng/ml FGF-2) and the culture environment during transfection (static versus dynamic). The objective of the second part was to determine the conditions, including the design of the scaffold and the method of seeding...

‣ Grain size control and microstructural evolution in nanocrystalline Ni-W alloys

Detor, Andrew J. (Andrew Joseph)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 131 leaves
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Nanocrystalline materials have been studied extensively over the past two decades because of their useful and interesting physical properties. In most cases, these properties derive from the fine characteristic length scale of nanocrystalline structures and are superior to those realized in traditional coarse-grained materials. A fundamental challenge, however, involves the synthesis of high-quality specimens, which represent a classical far-from-equilibrium state due to the large presence of high-energy interfaces. Alloying presents a possibility to reduce this energy penalty through solute segregation and thermodynamic stabilization of the grain boundaries. The present work exploits this concept in the nanocrystalline Ni-W system. Atomistic computer simulations are used to evaluate the potential for stabilization based on the equilibrium solute distribution and energetics of nanocrystalline structures. Following this, a synthesis technique based on electrodeposition is developed where precise control over the alloying addition correlates with precise control over grain size.; (cont.) Investigations of the microstructure involving techniques such as transmission electron microscopy, x-ray diffraction, and atom probe tomography provide a detailed view of the structure and solute distribution in these materials...

‣ Assessment of in vitro engineered microvascular networks and their application in the treatment of chronic wounds

King, Connie Hong-Yee
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 72 p.
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As the number of individuals suffering from tissue loss and end-stage organ failure continues to grow, researchers are turning to tissue engineering to provide better methods of treatment. The field, however, still faces many technical challenges that are limiting its applications. One challenge faced in engineering more complex tissues and organs is the need for inherent microvasculature to supply the tissue with nutrients and oxygen. Researchers at The Charles Stark Draper Laboratory have developed a method for engineering microvascular networks in vitro using various microfabrication techniques. This paper discusses the current state of the research and technical challenges to overcome before commercializing the technology. The feasibility of using the networks in the nearer term application of treating chronic wounds will also be assessed, and a potential business strategy will be laid out.; by Connie Hong-Yee King.; Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2007.; "September 2007."; Includes bibliographical references (p. 67-72).

‣ Analytical study and cost modeling of secondary aluminum consumption for alloy producers under uncertain demands

Li, Yaoqi
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 96 p.
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A series of case studies on raw materials inventory strategy for both wrought and cast aluminum alloy productions were conducted under recourse-based modeling framework with the explicit considerations of the demand uncertainty compared to the traditional strategy based on point forecast of future demand. The result shows significant economic and environmental benefits by pre-purchasing excess amount of cheaper but dirtier secondary raw materials to hedge the riskier higher-than-expected demand scenario. Further observations demonstrate that factors such as salvage value of residual scraps, cost advantage of secondary materials over primary materials, the degree of the demand uncertainty, etc. all have direct impacts on the hedging behavior. An analytical study on a simplified case scenario suggested a close form expression to well explain the hedging behavior and the impacts of various factors observed in case studies. The thesis then explored the effects of commonality shared by secondary materials in their application in multiple final products. Four propositions were reached.; by Yaoqi Li.; Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2008.; Includes bibliographical references (p. 93-96).

‣ Novel thermoelectric materials development, existing and potential applications, and commercialization routes

Bertreau, Philippe
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 86 leaves
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Thermoelectrics (TE) are devices which can convert heat in the form of a temperature gradient into electricity, or alternatively generate and absorb heat when an electrical current is run through them. It was established in the 1950's that the effectiveness of a thermoelectric could approximately be described in terms of a dimensionless figure of merit ... being respectively the Seebeck coefficient, the electrical resistivity and the thermal conductivity of the material. Until recently, ZT1 was the best performance these materials could achieve. However, the field of thermoelectrics advanced rapidly in the five last years, leading to the first significant breakthroughs in this area in the past fifty years, with materials with ZT up to 3 being reported. It is therefore interesting to wonder what new applications and markets these improvements at the material level could lead to. The first section of this thesis is a review of the principles of TE technology, the current materials and their level of performance. The recent materials developments are also described.; (cont.) The commercialization of TE is then discussed, along with the requirements in terms of performance and costs which would have to be achieved to make TE a further commercial success. Eventually...

‣ Investigation of bone response to implant materials by electron microscopy and computer simulation

Wang, Hao, 1974-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 148 p.; 8217464 bytes; 8237183 bytes; application/pdf; application/pdf
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(cont.) implementation of this scintigraphic method for quantitative studies of osteoblast-mediated mineralization in vitro. A 2-D truss finite element model is used to study the remodeling of trabecular bone. Using strain energy density (SED) as the optimization object and the trabecular width as the optimization variable, an optimal structure with minimum SED was achieved. This structure is similar to real bone in the dense outside, porous inside, and orientation of the trabeculae. The bone density distribution pattern also matched with previous result by other people. Different implants were introduced to simulate the replacement for the femoral head. It has been proved that the difference in Young's modulus between bone and implant materials is the main reason for the long-term bone loss (stress screening). This problem can be alleviated by proper implant design and resurfacing instead of replacing the whole femoral head.; Initial fixation with bone and the long term bone loss are two main problems associated with total hip replacement (THR), which are studied by electron microscope and computer simulation in this thesis. Bare Titanium-6 wt% Aluminum-4 wt% Vanadium (Ti64) implants, Ti64 implants with plasma-sprayed hydroxyapatite (PSHA)...

‣ Technology assessment and market analysis of solid state ultracapacitors

Jiang, Zibo
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 98 p.
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This report provides quantitative analysis of Solid State Ultracapacitors (SSUs) from technological and financial perspectives. SSUs are Ultracapacitors with solid electrolytes predicted to have huge application potential as the electrical energy storage device in Hybrid Electrical Vehicles (HEVs) due to the projected high energy density. The potential high energy density of SSUs is achieved through engineering dielectric materials to possess high breakdown voltage and/or DC permittivity. Among the available SSU models, Electrical Energy Storage Units (EESUs) have been reported to possess energy density as high as 280 Wh/kg with the permittivity and breakdown voltage enhancements achieved through engineering composition modified barium titanate powders. Organic Solid State Ultracapacitors (OSSUs) is a proposed concept of SSUs with conductive particle filled polymer systems as the dielectric material to take advantage of the systems' giant permittivity phenomenon reported under AC. However, through experiments and modeling, such giant permittivity is not found under DC and it is thought that the reported AC giant permittivity may be strongly distorted by the eddy current loss in the commonly used equivalent circuit characterization model and therefore does not contribute to the energy density enhancement. It is also found that the geometric dispersion of conductive particles does not contribute to the energy storage capability. Hence...

‣ Integration of III-V optical devices and interconnects on Si using SiGe virtual substrates

Yang, Vicky Kung-Fan, 1975-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 160 p.; 14074873 bytes; 14074623 bytes; application/pdf; application/pdf
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Because of the limitations to the functionality that Si can provide, integration of light emitting materials such as GaAs and other III-V materials provides the promise for the combination of electrical and optical devices onto a Si platform. Monolithic integration, compare to hybrid integration, is the more cost effective and reliable way to combine these dissimilar materials. However, materials compatibility issues such as lattice mismatch, polar-on-non-polar epitaxy, and thermal mismatch make the direct growth of high quality GaAs-based material on Si difficult. However, with the use of SiGe virtual substrates, Si substrates that have the Ge lattice constant, this goal can be achieved since Ge and GaAs have a small lattice mismatch of only 0.07%. High quality GaAs on Si having a threading dislocation density lower than 3x 106 cm'2 on offcut SiGe virtual substrates has been demonstrated. While utilizing SiGe virtual substrate technology to enable high quality GaAs on Si, we have studied the crack formation in GaAs layers on SiGe resulting from the thermal mismatch between the film and the substrate. GaAs has a thermal expansion coefficient of 5.8x10-6 /ʻC while the value for Si and the SiGe substrate is 2.6x10-6 /C. The thermal expansion coefficient of the SiGe virtual substrate is mainly dictated by the underlying Si substrate. As a result of the thermal mismatch...