Página 1 dos resultados de 50006 itens digitais encontrados em 0.038 segundos

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
ENG
Relevância na Pesquisa
76.735933%
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
ENG
Relevância na Pesquisa
76.78402%
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
ENG
Relevância na Pesquisa
76.76291%
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...

The impact of improved materials in poly(vinyl chloride)-based endotracheal tubes

Domike, Kristin Rebecca, 1981-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 71 leaves; 4987553 bytes; 4994618 bytes; application/pdf; application/pdf
EN_US
Relevância na Pesquisa
76.735933%
Endotracheal tubes (ETs) are used to aid artificial ventilation in millions of medical patients every year and are known to invoke the proliferative phase in the cell linings. The technical objective of this work was to investigate in vitro the interaction between epithelial cells and current poly(vinyl chloride)-based ET materials, as well as some ET samples embedded with materials intended to improve biocompatibility properties of the tubes. Cells were grown in wells with small samples of ETs and proliferation and migration were observed using phase microscopy. ETs appeared to increase cell growth wherever cells came into contact with the material. The cell morphology altered once in contact with the ET sample. Cell growth on and around the ETs with embedded material appeared to slow, but had significant visible changes in cell morphology. The need for continued research in this area of research and development and future steps are addressed. A proposal for starting a company around a safer material for use in endotracheal tubes was developed and showed significant barriers to entry for a small medical device company with a single product. Subsequently, the most appropriate approach for bringing a new ET to the market would be by way of a licensing with an existing manufacturer.; by Kristin Rebecca Domike.; Thesis (M. Eng.)--Massachusetts Institute of Technology...

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
76.76291%
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
76.82658%
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
76.76291%
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
ENG
Relevância na Pesquisa
76.78402%
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
ENG
Relevância na Pesquisa
76.76291%
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
ENG
Relevância na Pesquisa
76.76291%
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...

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
76.800986%
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
76.800986%
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
76.78402%
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
76.814927%
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...

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
76.800986%
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
76.83248%
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...

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
76.800986%
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...

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
ENG
Relevância na Pesquisa
76.78402%
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...

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
76.800986%
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
76.814927%
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'...