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‣ 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...

‣ 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...

‣ Mechanical behavior of closed-cell and hollow-sphere metallic foams

Sanders, Wynn Steven, 1974-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 299 p.; 16699019 bytes; 16739343 bytes; application/pdf; application/pdf
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(cont.) The elastic anisotropy and yield surfaces are fully characterized, and numerical equations are developed to allow the simple evaluation of the effect of geometric and material properties on the mechanical behavior of hollow-sphere foams. The analysis indicates that at relative densities of 10%, hollow-sphere foams have theoretical moduli and strengths that are three times those of existing metallic foams, and this increases to a factor of ten at relative densities below 5%. Several concepts are presented to allow the incorporation of defects into the model, including random packing, variations in bond size, and variations in sphere relative thickness. Finally, the performance of hollow-sphere foams is compared to other low-density engineering materials on a structural basis; hollow-sphere foams offer a beneficial alternative.; Metal foams are low-density materials with multifunctional attributes that make them appealing for numerous uses, including thermal insulation, heat sinks, acoustic insulation, energy absorption devices (crash protection), lightweight structural sandwich panels (as the core material), and vibration damping devices. Metallic foams are commercially available as closed-cell and open-cell foams. Unfortunately...

‣ Nanomechnics of crystalline materials : experiments and computations

Van Vliet, Krystyn J. (Krystyn Joy), 1976-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 165 leaves; 8532263 bytes; 8532072 bytes; application/pdf; application/pdf
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In this thesis, experimental, computational and analytical approaches are employed to examine systematically the mechanisms of deformation in crystalline materials. Such insight can be used to exploit and avoid contact in actuator and sensor applications, to derive mechanical properties for engineering of materials, and to investigate the fundamental role of defects. Here, localized mechanical contact of material surfaces is utilized to elucidate the effects of length scales on the transition from elastic (reversible) to plastic (irreversible) deformation. As the mechanical response of a material can be described by parameters which range from empirical constitutive (stress-strain) relations to fundamental descriptions of atomic interactions, the deformation response can be related to global mechanical properties such as yield strength, as well as to local phenomena such as dislocation nucleation. The concurrent design and implementation of experiments including micro- and nanoindentation and uniaxial compression, in situ experiments on a model, two-dimensional crystalline analogue, and computational modeling at the continuum (finite element) and atomistic (molecular dynamics) levels presented herein provide a unique opportunity to develop and validate hypotheses and analytical algorithms. Indeed...

‣ Modeling effects of microstructure for electrically active materials

García Muñoz, Ramiro Edwin, 1972-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 150 leaves; 5894603 bytes; 10614555 bytes; application/pdf; application/pdf
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A theoretical framework is proposed for the description of multifunctional material properties. The focus of this theory is on deriving equilibrium and kinetic equations for electrically active materials, particularly for rechargeable lithium-ion batteries and piezoelectric and electrostrictive microstructures. In both cases, the finite element method is applied to account for the effects of microstructure. Other derived equations that result from this theory are the wave equation in the limit of chemically homogeneous solids, and transport equations of charged species in conductive, non-polarizable, magnetic solids, as well as in polarizable non-magnetizable solids. The effects of microstructure in cathode materials for the Li[sub]yC₆/Mn₂O₄ rechargeable battery system are modeled, and several two-dimensional arrangements of particles are proposed to increase its power and energy density. Four ways are suggested to improve battery performance: controlling the transport paths to the back of the cathode, maximizing the surface area for intercalating lithium ions, engineering the porosity of the electrolyte phase, and distributing the lithium-ions evenly at the front of the cathode. The effects of grain size and crystallographic texture of piezoelectric and electrostrictive materials is simulated for BaTiO₃ and PZN-PT. Results show that the high anisotropy of the underlying single-crystal properties enhances the macroscopic piezoelectric response with respect to a single-crystal. For BaTiO₃...

‣ 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...

‣ Formation of in-plane crystals of molecular organic semiconductors

Mascaro, Debra Jane Lightly, 1972-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 204; 11727974 bytes; 11724764 bytes; application/pdf; application/pdf
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Molecular organic semiconductors have recently been applied widely as active layers in electronic and optoelectronic devices such as organic field-effect transistors and light-emitting devices. The use of organic active layers derives both from materials considerations and from processing advantages. The goal is to enable novel applications and structures that are difficult to produce with conventional materials (e.g., large-area, flexible, or transparent devices). Because charge transport efficiency increases with improved molecular ordering, the formation of large-area single crystals of these materials is essential for achieving integrated active organic structures. This thesis presents (1) an investigation of substrate surface modification as a means of inducing order within polycrystalline organic thin films, and (2) a new method for oriented, in-plane growth of millimeter-scale single crystal needles of molecular organic semiconductors. Two types of modified surfaces are investigated: (a) self-assembled nanofilms (SANFs), and (b) submicron periodic surface-relief structures. Polycrystalline thin films of pentacene and tetracene are vacuum sublimed onto pre-modified substrates. This work initially evaluates the influence of organosilane SANFs on morphology and field-effect transistor performance. It then attempts to use lithographically-defined surface relief to orient pentacene and tetracene grains via graphoepitaxy during thin film deposition. The periodic surface relief selects several preferred grain orientations...

‣ Evaluation of monolayer protected metal nanoparticle technology

Wu, Diana J
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 77 p.; 4035365 bytes; 4038509 bytes; application/pdf; application/pdf
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Self assembling nanostructured nanoparticles represent a new class of synthesized materials with unique functionality. Such monolayer protected metal nanoparticles are capable of resisting protein adsorption, and if utilized as a coating could have broad application in a wide range of industries from consumer products to maritime shipping to medical instruments. The formation of proteic films can adversely affect the performance of materials and is often a limiting factor in device effectiveness. In many instances such as sensors or medical implants, regular cleaning or disposal of the instrument is not a viable option, thus there exists a demand for additional means to prevent nonspecific protein adsorption. Existing protein resistant coating options are still not completely effective, and monolayer protected metal nanoparticle coatings could be a superior means by which to prevent protein adsorption onto material surfaces.; (cont.) This paper explores the commercialization potential of monolayer protected metal nanoparticle coatings for protein resistance; identifying application potential, evaluating potential markets, exploring intellectual property, analyzing the economics of monolayer protected metal nanoparticle synthesis, examining existing technologies...

‣ Virus-enabled synthesis of titanium oxide nanowires

Liau, Forrest (Forrest W.)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 23 p.; 1120985 bytes; 1119089 bytes; application/pdf; application/pdf
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Bio-assisted materials fabrication methods allow for the production of high technology materials and devices at lower costs and with less environmental impact. To expand the biological toolkit for synthesizing materials, we demonstrated titanium oxide nanowire synthesis with use of engineered M13 virus at room temperature. In this virus-enabled synthesis process, negatively-charged titanium fluoro complexes nucleate at positive amine sites on the virus, and a subsequent anion-scavenging reaction drives the synthesis of titanium oxide on the virus. TEM imagery provided visual validation of the nanowire formation, and XRD analysis identified the crystalline structure as anatase.; by Forrest Liau.; Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2006.; Includes bibliographical references (p. 21-23).

‣ Materials production economics : an examination of the variables and relationships that drive materials production and recycling in the world economy

King, Yao-Chung
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 29 leaves; 2074677 bytes; 2073295 bytes; application/pdf; application/pdf
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Introduction: New materials are being developing each year that could revolutionize the world. However, while development of novel materials in the lab brings us one step closer to next latest-and-greatest innovation, the following and perhaps similarly difficult step requires bringing these materials to the world market. Indeed, "although U.S. firms have invested the majority of materials introduced over the past half-century, they have failed to commercialize many of these innovations" (Eagar, 98). For a material introduction to be successful, it will be useful to understand the trends involved within the market for such an introduction and for continuing survival.; by Yao-Chung King.; Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2006.; Includes bibliographical references (p. 29).

‣ 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...

‣ Designing new electrode materials for energy devices by integrating ab initio computations with experiments

Kang, Kisuk
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 161 p.
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Utilization of Ni2+/Ni4+ double redox couple in electrochemical reactions has been tested as a way to gauge useful properties such as high capacity in electrode materials. The feasibility of a Ni2+/Ni4+ active redox couple is confirmed in a new layered electrode material, Lio.gNi045Ti5502. First principles calculations combined with experiments show that the degree of cation disordering in the material arising from both synthesis conditions and the electrochemical reaction is critical in performance of this material as the electrode. In an attempt to fully utilize Ni2+/Ni4+ double redox couple, Li2NiO2 in the Immm structure was successfully synthesized and its electrochemical behavior upon delithiation was evaluated. The material shows a high specific charge capacity of about 320 mAh/g and discharge capacity of about 240 mAh/g at the first cycle. The stability of Li2NiO2 in the Immm structure is attributed to the more favorable Li arrangement possible as compared to a Li2NiO2 structure with octahedral Ni. The electrochemical data, first principles calculation and EXAFS analysis all indicate that the orthorhombic Immm structure is rather prone to phase transformation to a close-packed layered structure during the electrochemical cycling.; (cont.) The possibility of stabilizing the orthorhombic Immm structure during the electrochemical cycling by partial substitution of Ni is also evaluated. First principles computations of some chemically substituted materials identified Pt substitution as a way of stabilizing the Li2(Ni...

‣ Evaluation on the thin-film phase change material-based technologies

Guo, Qiang, M. Eng. Massachusetts Institute of Technology
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 69 leaves
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Two potential applications of thin film phase-change materials are considered, non-volatile electronic memories and MEMS (Micro-Electro-Mechanical Systems) actuators. The markets for those two applications are fast growing and rapidly changing, so new materials technologies with superior performance are of great interest. Devices made with thin film phase change materials are shown to offer significant performance improvements for memory applications and new triggering mechanisms for MEMS actuation. IP (Intellectual Property) analyses for both markets indicate significant new IP space in each of them. Rough estimations for cost and revenue are provided.; by Qiang Guo.; Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2006.; Includes bibliographical references (leaves 68-69).

‣ Biorubber (PGS) : evaluation of a novel biodegradable elastomer; Biorubber Poly(glycerol sebacate) : evaluation of a novel biodegradable elastomer

Manzanedo, Diana
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 51 p.
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Poly(glycerol sebacate) (PGS), or biorubber, is a tough, biodegradable elastomer made from biocompatible monomers. The material was designed, synthesized and characterized in the Department of Chemical Engineering at MIT. Its main features are good mechanical properties, rubberlike elasticity and surface erosion biodegradation. PGS was proved to have similar in vitro and in vivo biocompatibility to PLGA, poly(L-lactic-co-glycolic acid), a widely used biodegradable polymer. PGS has been tested for use as nerve guide material and to fabricate artificial capillary networks for tissue engineering applications, both yielding promising results. Currently, the PGS research group continues to develop the material and to seek applications to maximize market potential and impact in the medical field, i.e. stenting (cardiovascular and non-vascular) and tissue engineering (cardiovascular and musculoskeletal). These markets were estimated at $5 billion dollars [1] and potentially over $10 billion dollars [2], respectively in the U.S. for 2004. Another promising field involves drug delivery, particularly in combination devices like drug-eluting stents. The potential non-medical applications are biodegradable rubbish bags, the absorbent material used in sanitary napkins or diapers...

‣ An analysis of the Supramolecular NanoStamping technology for its market potential based upon a review of DNA microarray intellectual property

Li, Kathy H. (Kathy Hsinjung)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 38 leaves
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Supramolecular NanoStamping is a novel printing method for exploiting the supramolecular interactions between organic and biological molecules. This technology is advantageous because of the ability to transfer a massive amount of chemical and spatial information, its high resolution, the growth of masters used multiple times and the versatility of initial master fabrication. The technology may be used to make DNA microarrays which are an essential tool to genomic research assisting in gene expression and genotyping. This paper explores the potential of bring Supramolecular NanoStamping technology to the microarray market. An in depth analysis of the current patent landscape of DNA microarrays is conducted to recognize the various competitors and the coverage of their patents. In addition, a better understanding of the landscape was achieved by assessing the major litigation that has occurred in the field. By engaging in a thorough intellectual property analysis, the commercialization potential of Supramolecular NanoStamping technology was realized through a licensing model.; by Kathy H. Li.; Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2006.; Includes bibliographical references (leaf 38).

‣ 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...

‣ Nanomechanics and ultrastructural studies of cortical bone : fundamental insights regarding structure-function, mineral-organic force mechanics interactions, and heterogeneity

Tai, Kuangshin
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 256 leaves
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Although the mechanics of bone has been studied extensively at the micro- and macro-scale, the nano-scopic level is perhaps the most illuminating as this is the length scale at which the individual constituents interact. Bone is made primarily up of type I collagen, hydroxyapatite mineral, a variety of non-collagenous proteins, and water. A multitechnique experimental and modeling approach methodology was used break down several of the deformation mechanisms that exist in bone at the nanoscale including the effect of mineral content, cohesive-frictional plasticity, increased ductility through mechanical heterogeneity, and intermolecular forces. To show that mineral content had a significant effect on both nanomechanical properties and ultrastructural deformation mechanisms of bone, partial and complete demineralization was carried out to produce samples ranging from -0-58 wt.% mineral content. Nanoindentation experiments perpendicular to the osteonal axis found a 4-6x increase in stiffness for the -58 wt.% sample compared to the completely demineralized -~0 wt.% samples. These results are discussed in the context of in situ and post-mortem AFM imaging studies which shed light on nanoscale mechanisms of deformation including collagen fibril deformation and pressure induced structural transitions of the mineral component.; (cont.) A finite element elastic-plastic continuum model was able to predict the nanomechanical properties of the different samples on loading and unloading. In addition...

‣ 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)...

‣ 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).

‣ Time-resolved spectroscopic characterization of ballistic impact events in polymer and nanocomposite materials

Saini, Gagan
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 95 p.
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A detailed understanding of how materials respond to ballistic shock-loading is critical for the design and development of new protective materials. However, the nonlinear viscoelastic deformation present in polymers and nanocomposites during and immediately following a ballistic impact event is not currently well understood. The dynamic mechanical responses of materials experiencing ballistic shock-loading conditions are quite complex, with large amplitude compressions resulting in strain rates in excess of 106 s-1 and pressures exceeding several GPa. Historically, if one wants to study materials under ballistic shock loading conditions, a gas gun apparatus is necessary to generate appropriate high strain rate events. However, advances in high power ultra-fast laser amplifier systems have opened the possibility of optically generating ballistic shocks which are comparable to a shock wave generated by gas gun apparatus. Time-resolved mechanical property information, such as elastic modulus, bulk modulus, shear modulus, and Poisson's ratio are measured using impulsive stimulated thermal scattering, a laser-based photoacoustic technique.; (cont.) A series of polymeric and polymer based nanocomposite material systems are studied, including multilayered thin films of alternating layers of polymer and hard nanoparticles...