Página 16 dos resultados de 2573 itens digitais encontrados em 0.128 segundos

‣ Transition metal gettering studies and simulation for the optimization of silicon photovoltaic device processing

Smith, Aimée Louise, 1971-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 119 p.; 8688364 bytes; 8688123 bytes; application/pdf; application/pdf
Português
Relevância na Pesquisa
87.80247%
We use what is known about transition metal (TM) defect thermodynamic driving forces and kinetic responses to make predictive simulation of gettering during solar cell fabrication possible. We have developed a simulator to explore the impact of various device and process parameters on gettering effectiveness. The relevant heat treatments are ramps up in temperature, isothermal annealing, and cools from high temperature down to room temperature. We explore a range of surface conditions, density and size of heterogeneous nucleation sites in the bulk, and the degree of contamination in order to create a framework in which to examine these mechanisms acting in concert. Such simulations enable process optimization for gettering. For solar cell processing, segregation to an Al back contact layer is routine. We have estimated the segregation coefficient between a p-type Si wafer and a molten Al layer by the Calphad method and use these results to estimate the thermodynamic driving force for redistribution of Fe into the Al layer. We simulate gettering treatments of supersaturated levels of Fe contamination in Si samples with FeSi2 and Al contacts and compare these results with data at various temperatures. The gettering data for FeSi2 contacts follow a simple exponential decay and can be simulated with appropriate choice of internal gettering time constant. We recognize that radiative heating dominates the temperature ramp for samples in evacuated quartz ampoules and use reasonable parameters to include this effect in our simulations. Fitting parameters for [Fe] data taken from heat treatments at 755ʻC on samples with FeSi2 and Al contacts successfully predict the gettering data of Al coated samples treated at 810ʻC.; (cont.) Discrepencies in the data for Al coated samples treated at 6950C and data for Al coated samples treated at 755ʻC after long times have exposed a new mechanism dominating internal gettering processes. We propose the existence of a silicide precipitate growth retardation mechanism as a result of supersaturation of the Si vacancy (V). Accumulation of V reduces the ability of precipitates to relax strain free-energy ([Delta]g strain) by further V emission. We performed Cu gettering experiments on p/p+ epitaxial wafers. Photoluminescence measurements revealed significant Cu removal from the epitaxial region compared to similarly doped uniformly doped float zone (FZ) Si wafers. Step etching revealed haze...

‣ High mobility strained Si/SiGe heterostructure MOSFETs : channel engineering and virtual substrate optimization

Leitz, Christopher W. (Christopher William), 1976-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 174 leaves; 12400141 bytes; 12399898 bytes; application/pdf; application/pdf
Português
Relevância na Pesquisa
87.79906%
High quality relaxed silicon-germanium graded buffers are an important platform for monolithic integration of high speed heterostructure field-effect transistors and III-V-based optoelectronics onto silicon substrates. In this thesis, dislocation dynamics in compositionally graded SiGe layers are explored and mobility enhancements in strained Si/SiGe metal-oxide-semiconductor field-effect transistors (MOSFETs) are evaluated. These results demonstrate the dramatic increases in microelectronics performance and functionality that can be obtained through use of the relaxed SiGe integration platform. By extending and modifying a model for dislocation glide kinetics in graded buffers to SiGe/Si, a complete picture of strain relaxation in SiGe graded buffers emerges. To investigate dislocation glide kinetics in these structures, a series of identical samples graded to 30% Ge have been grown at temperatures between 650ʻC and 900ʻC on (001)-, (001) offcut 6ʻ towards an in-plane <110>-, and (001) offcut 6ʻ towards an in-plane <100>-oriented Si substrates. The evolution of field threading dislocation density (TDD) with growth temperature in the on-axis samples indicates that dislocation nucleation and glide kinetics together control dislocation density in graded buffers. The TDD of samples grown on offcut substrates exhibits a more complicated temperature dependence...

‣ Effects of the polymeric binder system in slurry-based three dimensional printing of ceramics

Holman Richard K. (Richard Kimbrough), 1973-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 154 p.; 12299604 bytes; 12299357 bytes; application/pdf; application/pdf
Português
Relevância na Pesquisa
87.79915%
The factors dictating the minimum feature size in Slurry-based Three Dimensional Printing (S-3DPTM) of ceramics have been examined, focusing on effects related to the polymeric binder system polyacrylic acid (PAA, MW 60000) and its interaction with the ceramic powder bed. Methods for retrieving structures characteristic of the minimum feature, referred to as the "binder primitive," and for characterizing the size and shape of the binder primitive have been developed. Impact-related spreading of the printed binder droplets has been found to play little or no role on the primitive structure. Two dominant factors controlling primitive width were however identified. The first is wetting-induced spreading of the printed binder solution on the surface of the powder bed. The spreading process is halted prior to completion by infiltration of the printed liquid into the pore space. The role this factor plays thus depends on wetting properties and the relative rates of spreading and infiltration. The spreading and infiltration rates were modeled, and a means of predicting the maximum extent of spreading was developed. The second factor controlling minimum feature size is adsorption of the polymeric binder molecules from solution onto the surface of the ceramic particles during infiltration of the printed binder solution. This effectively filters the polymer from solution...

‣ SiGe-on-insulator and strained-Si-on-insulator for strained-Si CMOS and nanocrystalline-Ge waveguides

Taraschi, Gianni, 1973-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 209 p.; 19344063 bytes; 19343818 bytes; application/pdf; application/pdf
Português
Relevância na Pesquisa
87.80217%
SiGe-on-insulator (SGOI) and strained-Si-on-insulator (SSOI) substrates combine both the benefits of a high-quality, monocrystalline SiGe or strained-Si surface layer with the advantages of an insulating substrate. In particular, many electronic and photonic devices are greatly enhanced by the use of such substrates. In this thesis, techniques were developed for the fabrication of SGOI and SSOI substrates for applications including strained-Si complementary metal-oxide-semiconductor (CMOS) field-effect transistors, and nanocrystalline-Ge waveguides and photonic devices. A versatile fabrication technique was pioneered for the fabrication of low defect density SGOI and SSOI substrates, allowing for the creation of ultra-thin SGOI and SSOI, combining both the benefits of high-mobility strained-Si and SOI. The method pioneered employed wafer bonding of polished, relaxed SiGe virtual substrates to oxidized handle wafers. Layer transfer onto insulating handle wafers was accomplished using grind-etchback or delamination via implantation. Both methods were found to produce a rough transferred layer, but chemical mechanical polishing (CMP) was found to be unacceptable due to non-uniform material removal across the wafer and the lack of precise control over the final layer thickness. To solve these problems...

‣ Supramolecular assemblies of conjugated sensory polymers and the optimization of transport properties

Kim, Jinsang, 1969-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 190 p.; 13360341 bytes; 13384555 bytes; application/pdf; application/pdf
Português
Relevância na Pesquisa
87.79911%
(cont.) The vectorial energy transfer design of sensory films to harvest and direct energy to the surface detection layer toward ultimate signal amplification has been discussed. Third, the role of chemical structure of a sensory polymer in the selectivity of a conjugated polymer-based fluorescent sensor has been examined. In two different sensory systems for the detection of potassium ions and a nitroaromatic explosive TNT, respectively, key chemical design parameters governing their selectivity have been rationalized. Finally, the combination of the conclusions of this thesis provided an idealized structure of a fluorescent conjugated polymer-based sensory film with optimized sensitivity and selectivity.; A sensor is one of the many important applications of conjugated polymers. Poly(p-phenylene ethynylene)s (PPEs) have been studied for fluorescence-based sensor applications. The chemical structure and nano-structure of a polymer in the solid-state are two critical parameters that determine sensitivity and selectivity of a conjugated polymer-based sensor. In this thesis, both parameters have been systematically investigated. First, the Langmuir-Blodgett method was used to control the nano-structure of PPEs in the solid-state. Rational design of surfactant PPEs made it possible to control the conformation of a single polymer strand and interpolymer spatial arrangement at the air-water interface. In situ UV-Vis and fluorescence spectroscopy on the Langmuir film in controlled nano-structures revealed the effects of conformation and spatial arrangement of conjugated polymers on their intrinsic optical properties. Since the controlled structure of a monolayer at the air-water interface can be transferred to a solid substrate...

‣ Stress evolution during growth and atomic-scale surface structure effects in transition-metal thin films

Friesen, Cody A. (Cody Alden), 1978-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 2 v. (272 leaves); 11051348 bytes; 11051140 bytes; application/pdf; application/pdf
Português
Relevância na Pesquisa
87.80308%
Thin films are defined by an extremely high ratio of the in-plane dimensions to the thickness, resulting in very high surface-to-volume ratios. For this reason, the surface characteristics of a film play an important role in the properties of the film as a whole. This research focused on the connections between the properties and the surface state of metallic thin films. High resolution in-situ stress measurements were made during the ultra-high vacuum evaporative deposition of polycrystalline Cu films and homoepitaxial (111)-oriented Cu and Ag films. The measurements were enabled through the development of two high resolution in-situ stress monitoring devices that minimized sample placement and vibrational error through compact and monolithic ultra-high vacuum compatible designs. These devices allowed in-situ stress measurements with unprecedented sensitivities while the device electronics enabled the study of systems in real-time with high data acquisition rates. Measurements were made during growth and interruptions of growth as the films formed and thickened. In the earliest stages of a growth cycle, when only a small fraction of a monolayer is deposited, the elastic component of the adatom-surface interaction dominates the stress evolution. The early stage observations are analyzed through a first- order expansion of the thermodynamic surface stress which allows for direct extraction and measurement of the force-dipole associated with the adatom-surface interaction.; (cont.) Values of the force-dipole magnitude determined from experiments compared well with values obtained from embedded atom method molecular dynamics calculations. On a longer timescale...

‣ Microfabricated magnetophoretic focusing systems for the separation of submicrometer particles

Park, Edward S., 1974-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 202 p.; 7598275 bytes; 7595118 bytes; application/pdf; application/pdf
Português
Relevância na Pesquisa
87.79882%
Magnetic separation is an actively researched field due to its broad applicability to the mineral, chemical, and biological industries. The objective of this work was to design, fabricate, and test systems to study magnetophoresis of particles in suspension. To achieve this goal, two system concepts were developed: an Alternating Field System and a Flow System. Both systems consisted of permanent magnets and miniaturized devices (separation chips), which integrated microfluidic channels with ferromagnetic core elements. The systems produced "sawtooth" magnetic fields that were combined with a long-range magnetic field or pressure- driven flow to bring about migration, focusing and trapping of nonmagnetic particles suspended in ferrofluid. A potential application of such systems is high-resolution, size-based separation of DNA, cellular organelles, viruses, and other like-sized biological entities. The systems were designed using finite element analysis and fabricated using IC/MEMS microfabrication techniques. The fabrication process for the separation chips realized a microfluidic channel and electroplating molds in a single layer of SU-8 photoresist on a glass substrate. Nickel core elements were electroplated into the molds, and a PDMS cover substrate was attached using a novel technique involving contact bond and heat cycling. The systems were tested via experiments using optical fluorescence methods to observe the concentration profiles of polydisperse suspensions of polystyrene beads.; (cont.) Alternating Field System involved simple migration under a long-range magnetic field...

‣ Computational studies of cation and anion ordering in cubic yttria stabilized zirconia

Predith, Ashley P. (Ashley Page)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 137 p.
Português
Relevância na Pesquisa
87.79938%
The investigation of ordering and phase stability in the ZrO2-Y203 system involves two sets of calculations. The first set of calculations uses the cluster expansion method. A guide to the practical implementation of the cluster expansion outlines methods for defining a goal and choosing structures and clusters that best model the system of interest. The cluster expansion of the yttria stabilized zirconia system considers 447 configurations across the ZrO2-Y203 composition range. The effective cluster interaction for pair clusters show electrostatic repulsion between anions and little interaction between cations. Triplet anion terms largely modify the energy contributions of the pair terms. Separate cluster expansions using structures at single compositions show that cation clusters become more important at high yttria composition. The cluster expansion led to the discovery of three previously unidentified ordered ground state structures at 25, 29, and 33 % Y on the cubic fluorite lattice. The ground state with 33 % Y is stable with respect to the calculated energies of monoclinic ZrO2 and the Y4Zr3012 ground state. The ground states have the common ordering feature of yttrium and vacancies in [1 1 2] chains, and Monte Carlo simulations show that vacancy ordering upon cooling is contingent on cation ordering.; (cont.) The second set of calculations consider three driving forces for order: ionic relaxation...

‣ Microfabrication methods to improve the kinetics of the yttria stabilized zirconia -- platinum -- oxygen electrode

Hertz, Joshua L. (Joshua Lee)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 194 p.
Português
Relevância na Pesquisa
87.80071%
Solid oxide fuel cells are a potential electrical power source that is silent, efficient, modular, and capable of operating on a wide variety of fuels. Unfortunately, current technologies are severely limited in that they provide sufficient power output only at very high temperatures (>800°C). One reason for this is because the electrodes have very poor (and poorly understood) kinetics. The work described in this dissertation involves the microfabrication of model systems with triple phase boundary lengths that varied over an order of magnitude to systematically quantify and ultimately improve the kinetics of platinum electrodes on the surface of yttria stabilized zirconia electrolytes. Platinum electrodes with well controlled geometry were sputtered onto the surface of bulk YSZ and onto sputtered YSZ thin films. An unexpected result was found whereby YSZ films of composition Y0.09Zr0.91O2-x had an ionic conductivity remarkably enhanced by a factor of 20-30. This is attributed to the films exhibiting nanometric grain sizes and thereby stabilizing the cubic morphology at considerably lower yttrium levels than is normally needed. This metastable cubic phase is suspected of having reduced defect ordering.; (cont.) Grain boundary resistance...

‣ Anisotropic and tunable characteristics of the colloidal behavior of metal oxide surfaces

Bullard, Joseph Warren, 1978-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 128 p.
Português
Relevância na Pesquisa
87.79911%
The electroosmotic behavior of the rutile polymorph of titanium dioxide was explored as a function of crystallographic orientation. Atomic force microscopy (AFM) was employed to make high-resolution force spectroscopy measurements between a silica sphere attached to a traditional, contact-mode AFM cantilever and TiO2 (110), (100), and (001) surfaces in aqueous solutions. Measurements were taken in a variety of solution conditions across a broad range of pH values, and the resultant force-distance curves were used to deduce relative behaviors of each orientation of rutile, with particular interest in changes of the isoelectric point (iep). Differences in iep as a function of orientation are explained in terms of differences in both the coordination number and density of (Lewis) acidic and basic sites on the surface. The results were supported by angle-resolved X-ray photoelectron spectroscopy (XPS) measurements of a nominal monolayer of palladium metal deposited on each of the three orientations studied. The palladium monolayer served as a means of probing the relative electron affinities of the three surfaces studied, which was exhibited in shifts of the palladium XPS peak that corresponded to differences in binding energy as a function of substrate orientation.; (cont.) The correlation between rutile orientation and shift in palladium binding energy corresponded directly to the relationship between isoelectric point and orientation...

‣ LCD, low-temperature soldering and compound semiconductor : the sources, market, applications and future prospects of indium in Malaysia; Liquid crystal display, low-temperature soldering and compound semiconductor : the sources, market, applications and future prospects of indium in Malaysia

Yong, Foo Nun
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 96 leaves
Português
Relevância na Pesquisa
87.80083%
Indium is a minor but very valuable metal. Decreasing supplies of indium from refining and increasing demands from LCD, low-temperature soldering and compound semiconductors have stimulated the indium price increase dramatically. Traditionally, indium is refined as a by-product of zinc refining. However, this type of indium extraction method is expected to last for the next 10-20 years and this opens a window to extract indium from other ores, especially from tin ore. Interestingly, extraction of indium from tin circuit can be considered as the major business and the pure tin metal is just considered as a by-product due to high indium price and low tin price. Relatively high estimated concentration levels of indium in Malaysian tin ore means that Malaysian tin refiners can withstand high degree of fluctuation of price in the free market due to hedge buying and speculation. Note that the business model for global indium market is duopoly or oligopoly but obviously not monopoly. Between duopoly and oligopoly, the most probable model is oligopoly. Thus, the potential competitors and partnership are also discussed here. Besides, LCD and low-temperature soldering and compound semiconductor are three main applications of indium which affect the indium price.; (cont.) The unique properties of indium for three major applications will be discussed from the material engineering prospective. A cost modeling spreadsheet has been built to estimate the cost of production of indium from tin and zinc ores under different conditions. Hence...

‣ Optical properties of nanostructured silicon-rich silicon dioxide; Optical properties of nanostructured SRO

Stolfi, Michael Anthony
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 195 p.
Português
Relevância na Pesquisa
87.79963%
We have conducted a study of the optical properties of sputtered silicon-rich silicon dioxide (SRO) thin films with specific application for the fabrication of erbium-doped waveguide amplifiers and lasers, polarization sensitive devices and devices to modify the polarization state of light. The SRO thin films were prepared through a reactive RF magnetron sputtering from a Si target in an O2/Ar gas mixture. The film stoichiometry was controlled by varying the power applied to the Si target or changing the percentage of 02 in the gas mixture. A deposition model is presented which incorporates the physical and chemical aspects of the sputtering process to predict the film stoichiometry and deposition rate for variable deposition conditions. The as-deposited films are optically anisotropic with a positive birefringence (nTM > nTE) that increases with increasing silicon content for as-deposited films. The dependence of the birefringence on annealing temperature is also influenced by the silicon content. After annealing, samples with high silicon content (>45 at%) showed birefringence enhancement while samples with low silicon content (<45 at%) showed birefringence reduction. A birefringence of more than 3% can be generated in films with high silicon content (50 at% Si) annealed at 11000C.; (cont.) We attribute the birefringence to the columnar film morphology achieved through our sputtering conditions. Er was incorporated through reactive co-sputtering from Er and Si targets in the same O2/Ar atmosphere in order to investigate the energy-transfer process between SRO and Er for low annealing temperatures. By studying the photoluminescence (PL) intensity of Er:SRO samples annealed in a wide range of temperatures...

‣ Versatility of M13 bacteriophage in medicine : vaccine storage and cancer diagnostics

Shi, Amy (Amy J.)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 94 p.
Português
Relevância na Pesquisa
87.80022%
Two novel ways of engineering the filamentous bacteriophage, M13, for the prevention, diagnosis, and treatment of human disease are proposed. Both ways are founded on the unique structural properties of the M13 bacteriophage and the ability of its major and minor coat proteins, p3 and p8, to be manipulated to serve as virus-based multifunctional platforms. The first project addresses the problem of vaccine storage and the cold chain (requirement to store vaccines at 2-80C or lower). The need for refrigeration leads to high cost, difficult field delivery, and high potential for vaccine instability. By capitalizing on the liquid crystalline nature and unique diffraction patterns of phage films, we aim to encapsulate vaccines in a 3-D liquid crystalline matrix that would not only allow for stability at elevated temperatures but would also allow for easy detection of viability by using a laser light and noting the diffraction pattern. We chose luciferase as a model for a protein-based vaccine, and found several phage-borne peptide sequences with increased affinity to luciferase compared to controls. Two of these sequences, CKLHGTSRC and CTHKNQAC were chosen to form luciferase-encapsulated phage films. The second project addresses the need of more sensitive imaging techniques for early detection of cancer.; (cont.) M13 bacteriophage were used in combination with quantum dots and magnetic nanoparticles as bigger and brighter markers for cancer lesions. A 100% expressed p8 library was created for screening against potential cancer markers and work will soon proceed with screening against several cancer cell lines. In addition...

‣ Field and current induced magnetization reversal in patterned Pseudo Spin Valve devices

Colin, Irénée A. (Irénée Anthelme)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 111 p.
Português
Relevância na Pesquisa
87.79975%
The field and current induced magnetization switchings of Pseudo-Spin-Valve (PSV) devices are described in this dissertation. An aligned sequence of three (one optical and two electron-beam) lithographies was used to define the devices and their electrical contacts. The PSV stack comprised a layer of soft ferromagnetic material Ni80oFe20 (NiFe or Py), a non-magnetic spacer layer of Cu, a hard ferromagnetic layer of Co and a capping layer of Au. The current flowed in plane (CIP) and the devices displayed giant magnetoresistance (GMR). Three different shapes were investigated: notched bars, elliptical rings and rhomboidal rings. In the bars, the notches provided strong pinning potential wells for transverse domain walls in the NiFe layer, which, upon cycling an external field, reversed in a step-like fashion, with domain walls nucleating from both ends of the bars, due to strong magnetostatic coupling between both magnetic layers. Additional important magnetostatic coupling effects were measured and micromagnetic simulations confirmed the ubiquity of such coupling. Current induced magnetic switching (CIMS) experiments were conducted, and threshold densities of the order of 1011 A/m2 were used to switch the magnetization under an external bias field...

‣ Fundamental studies of polyelectrolyte multilayer films : optical, mechanical, and lithographic property control; Fundamental studies of PEM films : optical, mechanical, and lithographic property control

Nolte, Adam John
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 225 p.
Português
Relevância na Pesquisa
87.80299%
Polyelectrolyte multilayers (PEMs) are a versatile type of thin film that is created via layer-by-layer assembly of positively and negatively charged polymers from aqueous solutions. Precise control of the PEM thickness, chemical functionality, and molecular architecture is made possible by changing the polyelectrolytes and assembly conditions during film growth, allowing films to be designed with properties suitable for a given application. This thesis elucidates the intra-film structure and interactions of PEMs through the use of optical, mechanical, and chemical techniques. PEM rugate filters, wherein the refractive index varies through the depth of the film in a continuous, periodic fashion, were constructed by confining silver nanoparticle growth to layers of nanometer-scale thickness. The ability to construct such structures is shown to be dependent on the ability to precisely control the concentration of metal-binding carboxylic acid groups throughout the depth of the film. Software to enable the computation design and optical simulation of these and similar structures was developed.; (cont.) A buckling instability technique was used to probe the Young's modulus of PEM assemblies as a function of polyelectrolyte type, assembly pH...

‣ Deformation induced molecular behavior of Cis 1,4-polyisoprene and its nanocomposites monitored by solid-state NMR

Poliskie, Georgia Michelle, 1978-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 121 leaves; 5459327 bytes; 5474470 bytes; application/pdf; application/pdf
Português
Relevância na Pesquisa
87.80092%
Proton spin lattice (T1) relaxation time constants were used to monitor changes in the molecular motion and architecture of polyisoprene, polyisoprene-clay composites and polyhedral oligomeric silsesquioxane (POSS) nanofillers. The high frequency relaxations monitored by NMR are sensitivity to changes in the environment of polyisoprene chains as a function of compressive strain. These are the first experiments to use magic angle spinning nuclear magnetic resonance techniques in situ with compression measurements to identify changes in the chain environment during compressive strain. For the polyisoprene composites, in situ compression measurements were made during the acquisition of NMR relaxation measurements. Therefore, the architecture of polyisoprene-clay composites was monitored as a function of strain. Clay aggregates, composed of stacked clay platelets, were identified in the nanocomposites. Increases in strain resulted in an irreversible, increase in interfacial area between clay and the polymer as the aggregate broke apart. This increase in area could be easily quantified by NMR (- 230%) and was verified with optical microscopy (- 150%). The correlation between NMR and optical microscopy indicates, with certainty, that NMR relaxation measurements can be used to quantify differences in interfacial area of nanocomposites. With this being established...

‣ Effects of doping single and double walled carbon nanotubes with nitrogen and boron

Villalpando Paéz, Federico
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 143 p.
Português
Relevância na Pesquisa
87.80247%
Controlling the diameter and chirality of carbon nanotubes to fine tune their electronic band gap will no longer be enough to satisfy the growing list of characteristics that future carbon nanotube applications are starting to require. Controlling their band gap, wall reactivity and mechanical properties is imperative to make them functional. The solution to these challenges is likely to lie in smart defect engineering. Defects of every kind can induce significant changes on the intrinsic properties of carbon nanotubes. In this context, this thesis analyzes the effects of doping single and double walled carbon nanotubes with nitrogen and boron. We describe the synthesis of N-doped single-walled carbon nanotubes (N-SWNTs), that agglomerate in bundles and form long strands (<10cm), via the thermal decomposition of ferrocene/ethanol/benzylamine (FEB) solutions in an Ar atmosphere at 950°C. Using Raman spectroscopy, we noted that as the N content is increased in the starting FEB solution, the growth of large diameter tubes is inhibited. We observed that the relative electrical conductivity of the strands increases with increasing nitrogen concentration. Thermogravimetric analysis (TGA) showed novel features for highly doped tubes, that are related to chemical reactions on N sites.; (cont.) We also carried out resonance Raman studies of the coalescence process of double walled carbon nanotubes in conjunction with high resolution transmission electron microscope (HRTEM) experiments on the same samples...

‣ Engineering carbon nanostructures : development of novel aerogel-nanotube composites and optimization techniques for nanotube growth

Steiner, Stephen Alan, III
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 233 leaves
Português
Relevância na Pesquisa
87.79963%
Carbon aerogels offer several unique advantages which make them ideal for evaluating a metal's ability to catalyze nanotube growth, including in situ carbothermic reduction of oxidized nanoparticles to their catalytic metallic phase as they form and production of a bulk quantity of nanoparticles which can be easily characterized. In this work, metal-doped carbon aerogels of seven transition metals were synthesized, characterized, and evaluated for their ability to catalyze growth of carbon nanotubes by thermal chemical vapor deposition (CVD). It was found that carbon aerogels doped with Fe, Rh, Re, Au, and Nb all catalyzed the formation of nanotubes in moderate to high yields, resulting in a direct growth of nanotubes on the exterior surfaces of aerogel monoliths. Ta was found to grow nanotubes only after thorough reduction of its oxides. Growth with W was inconclusive. CVD growth of nanotubes throughout the interior porosity of metal-doped carbon aerogels was also achieved by templating a network of interconnected macropores into the monoliths. Surface-based nanoparticles composed of rhenium, gold, and varying combinations of gold and rhenium were investigated for their ability to catalyze carbon nanotube growth.; (cont.) Nanoparticles of these metals were nucleated onto silicon wafers from solutions of anhydrous ReCI5 and AuC13. After deposition...

‣ Magnetic properties of small multi-layered rings

Jung, Wonjoon
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 112 p.
Português
Relevância na Pesquisa
87.79986%
Thin film rings can be an alternative geometry of magnetic memory cells, in which data bits are stored by the chirality of the flux-closed or 'vortex' state of the ring. The absence of the stray field in the vortex state is advantageous of high density data storage. Elliptical rings with 3 / 2 pm major / minor diameter and widths of 300 nm above were fabricated from multi-layer thin film structures such as the ferromagnetic-antiferromagnetic exchange bias bilayer or giant magnetoresistance (GMR) spin valve structure, and their magnetic and magnetoelectric properties were investigated. Exchange-biased elliptical rings show an interplay between shape anisotropy and exchange anisotropy. When both the exchange bias and applied field are oriented along the major axis, an elliptical ring shows a shifted hysteresis loop and strong in-plane anisotropy. The switching behavior and vortex state stability of the rings are strongly dependent on the pinning direction and applied field direction relative to the major axis of the ellipse. It has proven difficult to control the vortex chirality in a simple manner. A model is described that predicts the vortex chirality of an elliptical magnetic ring as a function of the direction of the applied field and of the exchange bias...

‣ Modeling of and experiments characterizing electromigration-induced failures in interconnects

Andleigh, Vaibhav K. (Vaibhav Kumar), 1973-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 333 leaves; 20585471 bytes; 20585227 bytes; application/pdf; application/pdf
Português
Relevância na Pesquisa
87.8013%
As interconnect linewidths continue to scale downward, a detailed knowledge of stress evolution and void growth processes enables a determination of the electromigration-induced failure times of these interconnects. This thesis provides this knowledge through the development of an electromigration simulation MIT/EmSim and through experiments. The stress effect on diffusivity and alloying effects were incorporated into the MIT/EmSim model for Al-based interconnects, demonstrating Cu transport effects consistent with experiments by IBM. The void nucleation and growth process was modeled in long and short lines, and current density exponents for Black's equation determined for several failure conditions. Interconnects can also be immortal, either without void nucleation or by resistance saturation. This complex reliability behavior can be catalogued in the form of failure mechanism maps using simulation or analytical approximations, plotting failure or immortality mode as a function of current density and line length. To extend the MIT/EmSim model to Cu interconnects, experimental observation of void phenomenology and measurement of Cu-based electromigration parameters were performed using Cu/Ta interconnects specifically designed and fabricated at MIT. The knowledge gained from these experiments was used to develop an electromigration model for Cu...