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MOSFET channel engineering using strained Si and strained Ge grown on SiGe virtual substrates

Lee, Minjoo Lawrence, 1976-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 161 p.; 10952694 bytes; 10952453 bytes; application/pdf; application/pdf
ENG
Relevância na Pesquisa
137.98404%
As the economic and technological benefits of scaling in very-large-scale-integrated (VLSI) circuits decreases, the use of alternative channel materials such as germanium and strained silicon ([epsilon]-Si) is increasingly being considered as a method for improving the performance of MOSFETs. While [epsilon]-Si grown on relaxed Si[sub]l-x Ge[sub]x (i.e. single-channel heterostructure) is drawing closer to widespread commercialization, it is currently believed that almost all of the performance benefit in CMOS implementations will derive from the n-MOSFET. [epsilon]-Si p-MOSFETs demonstrate enhanced hole mobility, but the enhancement has been shown to degrade at high vertical fields for reasons that are still poorly understood. Dual-channel heterostructures, where a compressively-strained, Ge-rich layer is grown between the [epsilon]-Si cap and relaxed Si[sub]l-x Ge[sub]x virtual substrate have been shown to offer much larger hole mobility enhancements. One of the primary goals of this thesis is to understand and improve the performance of both single- and dual-channel heterostructure p-MOSFETs. The approach taken was to grow novel heterostructures and then fabricate MOSFETs using a short process flow. Cross-sectional TEM was constantly employed as a way to connect microstructure with mobility characteristics. In this way...

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
EN_US
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138.03982%
(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
ENG
Relevância na Pesquisa
148.04314%
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...

Size dependence of mechanical responses of materials in small-volume structures

Choi, Yoonjoon, 1967-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 144 p.; 5514870 bytes; 9747528 bytes; application/pdf; application/pdf
ENG
Relevância na Pesquisa
138.02246%
The effect of length scales on mechanical responses of materials in small-volume structures, such as thin films and patterned lines on substrates, is examined. The understanding of stress evolution and deformation behavior of those materials is one of major factors in the fabrication, performance, and reliability of microelectronic devices. The length scales involved in such materials are on the order of 1 min., and the deformation characteristics of the materials exhibit a strong dependence on their physical size such as film thickness and line width, as well as their microstructural length scale such as grain size. In this thesis, analytical, experimental, and computational approaches are utilized to elucidate the effect of length scales on the time-independent inelastic deformation of materials in small-volume structures. Experiments focus on continuous thin films and patterned lines of Al on Si substrates. The inelastic deformation of a material can be described by parameters which range from global constitutive (stress-strain) relations to atomistic descriptions of defect generation. An analytical model based on the generation of an array of dislocation loops is used to rationalize the film-thickness and grain-size effect on the stress evolution of thin metallic films on thick substrates. The consequence of the dislocation generation restricted by the film thickness and grain size leads to the full description of stress-temperature histories in an Al film on a Si substrate...

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
ENG
Relevância na Pesquisa
138.03982%
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...

An evaluation of grain boundary engineering technology and processing scale-up

Zelinski, Jeffrey A
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 74 p.; 4340616 bytes; 4343633 bytes; application/pdf; application/pdf
ENG
Relevância na Pesquisa
138.04556%
Grain boundary engineering is the manipulation of low stacking-fault energy, face- centered cubic material microstructures to break the connectivity of the general grain boundary network through the addition of special grain boundaries. Grain boundary engineering processing consists of thermomechanical cycling, i.e. repeated strain and annealing sequences and provides a method of producing more robust polycrystalline materials. This evaluation presents an introduction to the fundamental principles of grain boundary engineering, reviews the processing techniques and relevant intellectual property, analyzes the processing variables and their effect on a manufacturing line, surveys the current market and competition, and provides a preliminary cost analysis.; by Jeffrey A. Zelinski.; Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2005.; Includes bibliographical references (p. 50-52).

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
ENG
Relevância na Pesquisa
147.98832%
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).

Breaking symmetries in ordered materials : spin polarized light transport in magnetized noncentrosymmetric 1D photonic crystals, and photonic gaps and fabrication of quasiperiodic structured materials from interference lithography

Bita, Ion
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 219 leaves
ENG
Relevância na Pesquisa
137.99488%
Effects of breaking various symmetries on optical properties in ordered materials have been studied. Photonic crystals lacking space-inversion and time-reversal symmetries were shown to display nonreciprocal dispersion relations, and to exhibit a remarkable set of symmetry-related properties. Even in 1D, these materials are found to display indirect photonic band gaps, backward wave propagating modes (antiparallel phase and group velocities) which enable negative refraction at the air-crystal interface, ability to allow bending light with perpendicular magnetic fields, unidirectional superprism effects, etc. By calculating the complex photonic band structure, we show that the gap modes differ fundamentally from the commonly assumed evanescent modes with purely imaginary wave vectors - solely due to symmetry, we show that the gaps of nonreciprocal photonic crystals have complex wave vectors with both imaginary components and non-zero, frequency dependent real components. This basic finding is further studied in the context of tunneling dynamics, by considering the problem of tunneling time for nonreciprocal photonic band gap barriers (the tunneling wave packet has an energy in the middle of the gap).; (cont.) It was found that the classical Hartman effect (independence of tunneling time on barrier length...

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.
ENG
Relevância na Pesquisa
138.05813%
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...

Micromachined printheads for the direct evaporative patterning of organic materials

Leblanc, Valérie, Ph. D. Massachusetts Institute of Technology
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 139 p.
ENG
Relevância na Pesquisa
138.01233%
Organic optoelectronic devices are appealing for low-performance applications on very low cost and flexible substrates, due to their low-temperature processing. However, it still remains a challenge to develop suitable fabrication techniques to pattern organic thin films on low-cost, large-area substrates. The two techniques used commercially are inkjet printing of polymers, which limits the morphology and performance of devices, and shadow-masking of vacuum sublimation for small molecule materials, which is not scalable to large-area substrates. In this thesis, we investigate the use of MicroElectroMechanical Systems (MEMS) to provide new ways of patterning organic materials deposited by an evaporative process. We present the design, fabrication, modeling and characterization of two generations of micromachined printheads developed to expand the possibilities of printing of organic optoelectronics. The design and fabrication of a compact electrostatic actuator enabling the first generation of printhead is first presented. It is then used to actuate a microshutter, and modulate the flux of evaporated organic materials in a vacuum chamber. We prove the feasibility of evaporative printing of small molecular organic materials at resolutions of the order of 800 dpi with high-throughput on large areas.; (cont.) We demonstrate that MicroElectroMechanical Systems can be used to pattern organic thin films in a way that combines the advantages of ink-jet printing and thermal evaporation. We also present the design and fabrication of a microevaporator for molecular organics...

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
ENG
Relevância na Pesquisa
138.04612%
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...

Indentation of plastically graded materials

Choi, In-Suk, Ph. D. Massachusetts Institute of Technology
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 126 p.
ENG
Relevância na Pesquisa
138.05806%
The introduction of certain controlled gradients in plastic properties is known to promote resistance to the onset of damage at contact surfaces during some tribological applications. Gradients in composition, microstructure and plastic properties can also be deleterious to contact-damage resistance in some situations. In order to realize such potentially beneficial or deleterious effects of plastic property gradients in tribological applications, it is essential first to develop a comprehensive understanding of the effects of yield strength and strain hardening exponent on frictionless normal indentation. To date, however, systematic studies of plasticity gradient effects on indentation response have not been completed. A comprehensive parametric study of the mechanics of indentation of plastically graded materials is completed in this work by recourse to finite element (FE) computations. On the basis of a large number of detailed computational simulations, a general methodology for assessing instrumented indentation response of plastically graded materials is formulated so that quantitative interpretations of depth-sensing indentation experiments could be performed. The specific case of linear gradient in yield strength is explored in detail.; (cont.) The FE analysis leads to a universal dimensionless function to predict load displacement curves for plastically graded engineering materials. Experimental validation of the analysis is performed by choosing the model system of an electrodeposited nanostructured Ni-W alloy...

Percolation and homogenization theories for heterogeneous materials

Chen, Ying, Ph. D. Massachusetts Institute of Technology
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 145 p.
ENG
Relevância na Pesquisa
138.03052%
Most materials produced by Nature and by human beings are heterogeneous. They contain domains of different states, structures, compositions, or material phases. How these different domains are distributed in space, or in other words, how they connect to one another, determines their macroscopic properties to a large degree, making the simple rule-of-mixtures ineffective in most cases. This thesis studies the macroscopic effective diffusion, diffusional creep, and elastic properties of heterogeneous grain boundary networks and composite solids, both theoretically and numerically, and explores the microstructure-property correlations focusing on the effects of microstructural connectivity (topology). We have found that the effects of connectivity can be effectively captured by a percolation threshold, a case-specific volume fraction at which the macroscopic effective property undergoes a critical transition, and a set of critical scaling exponents, which also reflect the universality class that the property belongs to. Using these percolation quantities together with the generalized effective medium theory, we are able to directly predict the effective diffusivity and effective diffusional creep viscosity of heterogeneous grain boundary networks to a fairly accurate degree. Diffusion in composite solids exhibits different percolation threshold and scaling behaviors due to interconnectivity at both edges and corners. Continuum elasticity suffers from this complexity as well...

Synthesis and microfabrication of elastomeric biomaterials for advanced tissue engineering scaffolds

Bettinger, Christopher John, 1981-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 252 leaves
ENG
Relevância na Pesquisa
138.10568%
The subject of this thesis lies at the interface of microfabrication technology and advanced biomaterials synthesis and processing for use in designing and fabricating novel tissue engineered constructs. The unifying theme is to use micron and sub-micron fabrication strategies to form advanced tissue engineering scaffolds which are able to precisely control the microenvironment of cells. These efforts are organized into two thrusts; (1) materials synthesis and process development for microfluidic scaffold fabrication and (2) micro- and nanofabricated synthetic substratum for controlling cell function. In the first thrust, materials-specific processes for the fabrication of poly(glycerol-co-sebacate), a synthetic elastomeric biodegradable polyester, into three-dimensional, hepatocyte-seeded microfluidic constructs is discussed. Material advantages of natural proteins motivated the fabrication of next-generation microfluidic scaffolds using silk fibroin from the Bombyx mori as a bulk material. The need to combine the advantages of both natural proteins and synthetic polyesters motivated the synthesis and characterization of a new class of biodegradable elastomers termed poly(1,3-diamino-2-hydroxypropane-co-polyol sebacate) (APS). APS polymers are tunable and possess the advantages of both natural and synthetic polymers. APS polymers induce a favorable biomaterial-tissue response including reduced fibrous capsule formation and macrophage recruitment compared to PLGA. In vivo degradation half lives could be controlled to between approximately 6 and 100 wks by adjusting polymer composition and processing. The second thrust focuses on the interaction with cells and synthetically fabricated nanotopographic substrates for potential in vascularized tissue engineering applications.; (cont.) The contact guidance response of human embryonic stem cells to poly(dimethylsiloxane) (PDMS) substrates with 600 nm ridge-groove geometry and 600 ± 150 nm feature height was characterized. This motivated the study of endothelial progenitor cell function and morphology on nanofabricated PDMS substrates. Endothelial progenitor cells (EPCs) were found to exhibit increased doubling time from 16.2 ± 0.8 to 20.9 ± 1.9 h for cells grown on flat and nanotopographic substrates...

Assembly of biological building blocks for nano- and micro-fabrication of materials

Chiang, Chung-Yi
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 137 p.
ENG
Relevância na Pesquisa
137.99595%
Experimental studies were performed to fabricate various material structures using genetically engineered M13 bacteriophage. This virus template showed superior controls of material syntheses from nanoscale to microscale. Structures including nanowires, nanoparticle arrays, hetero-particle arrays, and micro-fibers were fabricated using the engineered MI3 virus as the building block and mineralization platform. The mineralization mechanisms were revealed by alternating the types and amounts of peptide motifs displayed on the viral templates. The results showed the importance of a fused peptide motif to mediate the mineralization process of a material, which was dominated by either physical absorption or chemical nucleation. The potential applications of the materials synthesized using the viral template, including energy generation and biosensors, were also demonstrated. For the first time, several types of highly engineered MI 3 virus were used to fabricate nanostructures such as nanowires, nano-arrays, hetero-particle arrays, and complex nanowires. A type 8 phage library was reported to screen peptide motifs for making nanowires. A multi-functionalized viral template, type 8-3 virus, was engineered and demonstrated to create a variety of nano-archietetures. A type 8+8 virus was used to create complex nanowires embedded with different materials. In addition...

Pottery engineering in ancient Guerrero, Mexico : the site of Las Fundiciones

Reitzel, Hannah Lynn
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 66 leaves
ENG
Relevância na Pesquisa
138.07595%
This thesis examines ancient pottery from the site of Las Fundiciones, Guerrero, Mexico. Because this site provides the only documented evidence of prehispanic metal smelting in Mesoamerica it is critical to examine artifacts found there, in this case pottery, using the tools of materials science. These tools allow archaeologists to see past "stylistic preferences" to the design, functional, and compositional differences in artifacts that result from choices among a range of processing alternatives. Thus, this thesis seeks to show that potters at Las Fundiciones were engineering their pottery in their choices of materials and design processes. A selection of pottery sherds (representing different functional vessel types) from the site were petrographically analyzed, and nine clays collected in and around the site were petrographically analyzed and subjected to a variety of chemical, mechanical, and thermal tests to determine their material properties. Clays are matched with petrographic groupings of potsherds, showing that potters were using local clays. The clays are ranked according to their ability to withstand thermal shock, since this is an easily quantifiable measure of the viability of ancient pots for use in food preparation. Lastly...

Oxide based thermoelectric materials for large scale power generation

Song, Yang, M. Eng. Massachusetts Institute of Technology
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 69 leaves
ENG
Relevância na Pesquisa
138.00882%
The thermoelectric (TE) devices are based on the Seebeck and Peltier effects, which describe the conversion between temperature gradient and electricity. The effectiveness of the material performance can be described by its figure of merit, ZT, which is defined as ZT = [alpha]²[sigma]T / [kappa] , where a is the Seebeck coefficient of the material, a is the electrical conductivity and [kappa] is the total thermal conductivity, and T is the temperature. In the past, TE power generation has been confined to niche applications. It has been technically and economically more efficient to produce electricity using traditional generators rather than a thermoelectric generator. However, recent significant advances in the scientific understanding of quantum well and nanostructure effects on TE materials properties and modem thin layer and nanoscale manufacturing technologies have combined to create advanced TE materials with high figure of merit (>3). An engineering analysis performed in this study identified large scale waste heat recovery opportunities that are suitable for advanced TE power generation systems.; by Yang Song.; Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2008.; Includes bibliographical references (leaves 63-65).

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.
ENG
Relevância na Pesquisa
137.99595%
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...

Multilayer composite photonic bandgap fibers; Composite photonic bandgap fiber materials and fabrication

Hart, Shandon D. (Shandon Dee), 1978-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 126 leaves; 6020400 bytes; 6018127 bytes; application/pdf; application/pdf
ENG
Relevância na Pesquisa
138.01297%
Materials and fabrication techniques are developed that lead to the successful fabrication of multilayer composite photonic bandgap fibers. The pertinent background in electromagnetic theory of multilayer dielectric mirrors and optical fibers is surveyed. Materials properties constraints are outlined, with emphasis on those constraints related to processing strategy and ultimate target length scale. Interfacial energy is measured in a chalcogenide glass / organic polymer composite system selected for fiber fabrication. A classical capillary instability model is employed to predict the feasibility of fiber fabrication based on material properties; from this model, quantitative materials selection criteria related to ultimate length scale are derived. Good agreement is found between the calculated materials selection criteria and controlled fiber experiments. The fiber fabrication techniques are described and analyzed; chalcogenide film deposition is characterized using Raman and electron microprobe spectroscopy, and heat transfer during fiber drawing is modeled using a commercial finite-element software package. The developed materials and fabrication processes are used to perform two case studies in novel photonic bandgap fiber fabrication; the first case study deals with externally reflecting omnidirectional 'mirror-fibers'...

Phase transformations and microstructural design of lithiated metal anodes for lithium-ion rechargeable batteries

Limthongkul, Pimpa, 1975-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 166 p.; 11970072 bytes; 11969828 bytes; application/pdf; application/pdf
ENG
Relevância na Pesquisa
138.17278%
There has been great recent interest in lithium storage at the anode of Li-ion rechargeable battery by alloying with metals such as Al, Sn, and Sb, or metalloids such as Si, as an alternative to the intercalation of graphite. This is due to the intrinsically high gravimetric and volumetric energy densities of this type of anodes (can be over an order of magnitude of that of graphite). However, the Achilles' heel of these Li-Me alloys has been the poor cyclability, attributed to mechanical failure resulting from the large volume changes accompanying alloying. Me-oxides, explored as candidates for anode materials because of their higher cyclability relative to pure Me, suffer from the problem of first cycle irreversibility. In both these types of systems, much experimental and empirical data have been provided in the literature on a largely comparative basis (i.e. investigations comparing the anode behavior of some new material with older candidates). It is the belief of the author that, in order to successfully proceed with the development of better anode materials, and the subsequent design and production of batteries with better intrinsic energy densities, a fundamental understanding of the relationship between the science and engineering of anode materials must be achieved...