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Adhesion as a tool for in-built nanotechnology design in cementitious materials

ROSSETTO, Hebert L.; Souza , Milton Ferreira de; PANDOLFELLI, Victor C.
Fonte: KOREAN ASSOC CRYSTAL GROWTH, INC Publicador: KOREAN ASSOC CRYSTAL GROWTH, INC
Tipo: Artigo de Revista Científica
ENG
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Confined water, such as those molecules in nanolayers of 2-3 nm in length, plays an important role in the adhesion of hydrophilic materials, mainly in cementitious ones. In this study, the effects of water containing kosmotropic substances on adhesion, known for their ability of enhancing the hydrogen bond (H-bond) network of confined water, were evaluated using mechanical strength tests. Indeed, to link adhesion provided by water confined in nanolayers to a macro-response of the cementitious samples, such as the bending strength, requires the evaluation of local water H-bond network configuration in the presence of kosmotropes, considering their influences on the extent and the strength of H-bonds. Among the kosmotropes, trimethylamine and sucrose provided a 50% increase in bending strength compared to the reference samples, the latter just using water as an adhesive, whereas trehalose was responsible for reducing the bending strength to a value close to the samples without any adhesive. The results attained opened up perspectives regarding exploring the confined water behavior which naturally occurs throughout the hydration process in cement-based materials.; Fundacao de Amparo Pesquisa do Estado de Sao Paulo (FAPESP); Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP); Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq); Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPq)[02/10492-3]; Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq); Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPq)[151329/2007-9]

Regulação do balanço vegetativo-reprodutivo pelo crescimento semi-determinado em tomateiro (Solanum lycopersicum L. cv Micro-Tom) e seu impacto na produtividade e eficiência no uso da água; Regulation of the vegetative-to-reproductive balance through the semideterminate growth habit in tomato (Solanum lycopersicum L. cv Micro-Tom) and its impact in the productivity and water use efficiency

Vicente, Mateus Henrique
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Dissertação de Mestrado Formato: application/pdf
Publicado em 29/08/2013 PT
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O hábito de crescimento influencia o balanço entre o desenvolvimento vegetativo e o reprodutivo das plantas. Este, por sua vez, está diretamente vinculado a diversas variáveis de interesse agronômico, como produtividade e consumo de água. Em tomateiro, existem três hábitos de crescimento conhecidos: determinado, indeterminado e semi-determinado, sendo esse último na verdade determinado, porém com uma extensão do ciclo vegetativo. Cultivares de crescimento determinado são largamente utilizadas para produção de tomates destinados à indústria (molhos e ketchups), e as indeterminadas destinadas ao consumo in natura. Por outro lado, genótipos de crescimento semi-determinado, embora ainda sejam pouco explorados, apresentam-se como ótimas opções tanto para indústria quanto para o consumo in natura. Diante disto, o presente trabalho teve por objetivo avaliar a implicação do hábito de crescimento semi-determinado no desempenho produtivo e na eficiência no uso da água de plantas de tomateiro. Para tal, genótipos com diferentes hábitos de crescimento foram produzidos, por meio de introgressões de variações alélicas que afetam o hábito de crescimento das plantas, no background genético da cultivar miniatura de tomateiro Micro-Tom (MT). A caracterização desses genótipos demonstrou que os materiais de crescimento semideterminado apresentam um atraso no florescimento...

Modeling the growth of LT and TL-oriented fatigue cracks in longitudinally and transversely pre-strained Al 2524-T3 alloy

Maduro, L. P.; Baptista, C. A R P; Torres, M. A S; Souza, R. C.
Fonte: Universidade Estadual Paulista Publicador: Universidade Estadual Paulista
Tipo: Conferência ou Objeto de Conferência Formato: 1214-1219
ENG
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The aluminum alloy 2524 (Al-Cu-Mg) was developed during the 90s mainly to be employed in aircraft fuselage panels, replacing the standard Al 2024. In the present analysis the fatigue crack growth (FCG) behavior of 2524-T3 was investigated, regarding the influence of three parameters: load ratio, pre strain and crack plane orientation of the material. The pre strain of aluminum alloys is usually performed in order to obtain a more homogeneous precipitates distribution, accompanied by an increase in the yield strength. In this work, it was evaluated the resistance of Al 2524-T3 sheet samples to the fatigue crack growth, having L-T and T-L crack orientations. FCG tests were performed under constant amplitude loading at three distinct positive load ratios. The three material conditions were tested: as received(AR), pre strained longitudinally (SL) and transversally (ST) in relation to rolling direction. In order to describe FCG behavior, two-parameter kinetic equations were compared: a Paris-type potential model and a new exponential equation introduced in a previous work conducted by our research group. It was observed that the exponential model, which takes into account the deviations from linearity presented by da/dN versus AK data...

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

BaTiO₃ based materials for piezoelectric and electro-optic applications

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

Melt growth of BSO (Bi₁₂SiO₂₀) : critical issues for growth in a micro-g environment

Zheng, Yu, 1970-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 122 leaves; 11843653 bytes; 11843413 bytes; application/pdf; application/pdf
ENG
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by Yu Zheng.; Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1999.; Includes bibliographical references (leaves 117-122).

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

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
EN_US
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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...

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
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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...

Next generation optical receivers : integration and new materials platform

Zhang, Yiwen, 1978-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 81 leaves; 4643551 bytes; 4652424 bytes; application/pdf; application/pdf
EN_US
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Future growth of optical communication into new application and market space is highly dependent on the ability of optical receivers to increase functionality while reducing price and physical size. Current hybrid receiver technology is inadequate in meeting the cost and performance demands of future market. Monolithic integration and new material systems are potential solutions and have been the focus of research investigation. This thesis summarizes the research progress of monolithic integration on InP, and the achievements in realizing 1.55[micro]m photodetector on GaAs and Si and their potentials for monolithic opto-electronic integrated circuits. The overall trend for next generation receivers is to move towards higher levels of integration, with investigation in new material systems that have the potentials for lower cost and larger scale integration. The impact of monolithic integration optical receiver components is analyzed in a cost analysis model.; by Yiwen Zhang.; Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2004.; Includes bibliographical references (leaves 77-81).

Stress and structure evolution during Volmer-Weber growth of thin films

Seel, Steven Craig, 1972-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 2 v. (294 leaves); 13148711 bytes; 13148511 bytes; application/pdf; application/pdf
ENG
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To investigate stress evolution during film deposition, a novel electrical technique for in situ thin film stress measurements was developed utilizing piezoresistive silicon microcantilevers, or piezocantilevers. In addition to the thin film stress measurements of Cu made with the piezocantilevers, our collaborators at Sandia National Laboratories have studied thin film growth stresses for Al, Ag, and amorphous Ge (a-Ge) films measured using conventional laser deflectometery. A computer simulation of thin film formation by the Volmer-Weber mechanism was developed to model the nucleation and growth of individual islands that impinge and coalesce to form a continuous film. By including a size-dependent lattice contraction in pre-coalescence islands, the simulation can be used to qualitatively reproduce the measured compressive stress behavior of Al at low film thicknesses. In contrast to Al films that support stress from the very onset of deposition, Ag films exhibit no measurable stress until larger film thickness are achieved. This difference in behavior was attributed to shear occurring at the Ag-SiO2 interface, which is suppressed at the much stronger Al-SiO2 interface. Tensile stress generation resulting from island coalescence was modeled analytically and by finite element methods (FEM) as the energetic balance between interfacial energy reduction and strain energy generation resulting from grain boundary formation. The magnitude of the island-coalescence stress calculated using FEM was found to decrease dramatically with decreasing island-substrate contact angle.; (cont.) Using the contact-angle-dependent FEM calculations of island-coalescence stress...

MOCVD growth of In GaP-based heterostructures for light emitting devices; Metalorganic chemical vapor deposition growth of In GaP-based heterostructures for light emitting devices

McGill, Lisa Megan, 1975-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 205 p.; 9286017 bytes; 9285825 bytes; application/pdf; application/pdf
ENG
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In this work, we examine fundamental materials processes in the growth of indium gallium phosphide (InGaP) via metalorganic chemical vapor deposition (MOCVD). In particular, we realize improvements in the epitaxial integration of high-quality InGaP device materials on non-standard platforms, such as GeSi graded buffers orSi substrates, and InGaP or indium aluminum gallium phosphide (InAlGaP) graded buffers on GaP substrates. We apply these improvements to the design and implementation of strained-InGaP quantum-well light emitting diodes (LEDs) operating in the yellow-green region of the visible spectrum. The innovative use of these traditional materials is intended to provide a solution for bright green solid-state light emitters. Initial modes of InGaP lattice-matched epitaxy on GeSi were studied. Three- dimensional growth was observed over a wide range of deposition temperatures and V/III ratios. Pre-growth thermal cycling in a H2 plus PH3 ambient led to a large increase in surface roughness and the formation of surface mesas. Thermodynamic simulations suggest that these mesas may be P clusters or GeP solid complexes. They may also be surface oxides formed in conjunction with water vapor in the deposition chamber. Such surface degradation prior to the initiation of epitaxy is unfavorable for monolayer growth. The development and evolution of defect microstructures in relaxed...

The role of hydrogen in the growth of carbon nanotubes : a study of the catalyst state and morphology

Kim, Jin Suk Calvin
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 52 leaves; 2308459 bytes; 2309295 bytes; application/pdf; application/pdf
ENG
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The role of hydrogen in chemical vapor decomposition (CVD) of C2H4 for growth of carbon nanotubes (CNTs) was investigated. Fe/A1203 (1/10 nm) catalyst layers were used for growth on Si substrates and the times at which H2 was introduced during the 40 minute temperature ramp, 15 minute annealing (without C2H4), and 15 minute growth (during which C2H4 was flowing) stages was varied. When H2 was introduced before heating, CNTs grew to a length of [approx.] 0.3 mm. However, CNT growth was severely suppressed when H2 was introduced at different points during temperature ramp. Recovery of CNT growth was observed when H2 was introduced during the annealing and growth stages. Under optimum conditions, an [approx.] 1 mm thick carpet of CNTs could be obtained. The chemical state and morphology of the catalysts as a function of the time of H2 introduction were examined using XPS and AFM, respectively. We found that the as-deposited state of Fe was an iron oxide, due to reaction with 02 in the atmosphere, and that the H2 reduced the iron oxide to different oxidation states, depending on the time of H2 introduction. AFM inspection showed that surface roughness could also be correlated with areas of vertical CNT growth.; (cont.) A preliminary model for CNT growth in which the oxidation state of iron determines its catalytic activity is proposed...

Advanced engineered substrates for the integration of lattice-mismatched materials with silicon

Isaacson, David Michael, 1976-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 171 p.
ENG
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The dramatic advances in Si/SiO2-based microelectronic processing witnessed over the past several decades can largely be attributed to relatively material-independent device scaling. However, with physical and economic limitations to the continued scaling of such devices appearing on the horizon, it is likely that near-term advances will come from the integration of novel and previously underrepresented materials. One of the most promising ways to enhance performance comes from the integration of judiciously chosen lattice-mismatched materials with Si. However, the integration of such structures typically poses significant technical challenges. The work contained in this thesis seeks to address several of these important issues, primarily through the use of relaxed, graded SiGe buffers on Si (i.e. Vx[Si1-xGex]/Si). Several new phenomena in relaxed graded SiGe buffers are developed in this thesis. A rise in threading dislocation density was observed in high-Ge content relaxed graded SiGe layers grown at relatively high temperatures, which was attributed to dislocation nucleation. This observation is contrary to conventional graded buffer theory in which high growth temperatures are expected to result in reduced threading dislocation densities (TDDs).; (cont.) Additionally...

Block copolymer-templated iron oxide nanoparticles for bimodal growth of multi-walled carbon nanotubes

Yazzie, Kyle E
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 74 leaves
ENG
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Since their discovery carbon nanotubes (CNTs) have sparked great interest due to their exceptional mechanical, electrical, and thermal properties. These properties make carbon nanotubes desirable for numerous applications including: nanoelectronics, high-strength composites, energy storage, superhydrophobic surfaces, sensors, and biomaterial interfaces. Bulk synthesis of carbon nanotubes with controlled physical features, i.e. length, diameter, multiwalled vs. single walled, carbon nanotube chirality, etc. is necessary to make full use of carbon nanotubes exceptional properties in commercial aspects. Typical carbon nanotube synthesis processes use chemical vapor deposition (CVD), arc-discharge, and laser ablation. Synthesizing carbon nanotubes via CVD typically involves depositing a thin metal film on a silicon substrate, and heating the substrate so that the thin metal film dewets and forms metallic nanoparticles. A hydrocarbon gas is then flowed over the nanoparticles to initiate carbon nanotube growth. Though these thin metal film catalysts are easy to prepare, they offer poor control over nanoparticle diameters and areal density. It has been shown that physical properties of carbon nanotubes, such as diameter and uniformity of growth...

Effects of catalyst pretreatment for carbon nanotube growth; Effects of catalyst pretreatment for CNT growth

Morgan, Caitlin D
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 37 leaves
ENG
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The effects of pretreatment of iron catalyst for carbon nanotube (CNT) growth was studied. CNTs were grown on Fe/A1203 (1/10 nm) thin-film catalyst deposited on silicon substrates via exposure to C2H4 in a thermal chemical vapor deposition (CVD) furnace. During CVD, the sample was exposed to a carrier gas (Argon) for the 35-minute temperature ramp, and 15-minute anneal, then to a mix of carrier gas and ethylene for a 15-minute growth stage. Experiments were performed varying the amount of oxygen contaminant in the carrier gas, and the time of hydrogen introduction. Samples were characterized via atomic force microscopy (AFM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was found that the later hydrogen was introduced, the higher the catalyst density and the taller the CNT carpet. The catalyst efficiency was also shown to increase with later hydrogen introduction. No clear trend was observed between the amount of oxygen in the carrier gas and the height of CNT growth. Data points to the model of catalyst coarsening being crucial to the nucleation and growth of CNTs and the parameters of CNTs grown. Variations in trends are discussed.; by Caitlin D. Morgan.; Thesis (S.B.)--Massachusetts Institute of Technology...

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
ENG
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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...

Catalytic properties, densification and mechanical properties of nanocrystalline yttria-zirconia-based materials

Cui, Jianyi
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 117 p.
ENG
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Alumina, titania, ceria and manganese oxide were either coated onto or doped in cubic 7 mol% Y203-ZrO2 (7YZ) nanocrystals to form nanocomposites for methane combustion. These novel catalysts were very active and thermally stable. In particular, 25 wt% Mn203-coated 7YZ and 25 wt% Mn203-doped 7YZ showed remarkably low light-off temperatures of 3750C and 3580C, respectively. These catalysts were highly attractive as they were competitive with the much more expensive supported noble metal catalysts. Their catalytic activity could be attributed to the availability of active surface oxygen species, which facilitated the methane activation at low temperatures. Nanocrystalline 3 mol% and 8 mol% Y203-ZrO2 (3YZ and 8YZ) were successfully densified with an ultrafine grain size of < 90 nm by pressureless sintering at 11000C and 11500C, respectively. The low-temperature sinterability could be attributed to the well-defined nanocrystalline particles obtained via hydrothermal synthesis, and the effective elimination of secondary porosity through the dry compact processing. Submicron-sized 3 mol% Y203-ZrO2 ceramics with a grain size of - 150 nm was also obtained with commercial TOSOHC powders. Grain growth during densification of TOSOH© powders was successfully suppressed by presintering to 93% density under an argon atmosphere...

Growth and characterization of bismuth perovskite thin films for integrated magneto-optical isolator applications

Taussig, Alexander R
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 156 leaves
ENG
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In this thesis, we discuss the motivation for integrated magneto-optical isolators and explain why the orthoferrite is such an attractive materials class for this purpose. We then derive from first physical principles the dependence of Faraday rotation, absorption, and certain figures of merit on the material's dielectric tensor elements. Next, we use pulsed laser deposition to grow thin films of BiFeO3 on MgO (001) and SrTiO3 (001) substrates. After optimizing growth conditions to obtain high quality films, we characterize the films' crystal structure with two-dimensional x-ray diffraction. We then examine the magnetic, optical, and magneto-optical properties of these films. We find that the highly textured films grown on SrTiO3 are monoclinic with an out-of-plane c-axis aligned with the (001) direction of the substrate and approximate pseudocubic lattice parameters of a = b = 4.04 A, c = 3.95 A, and 90° - [beta] = -0.88°. These films are weakly magnetic, with a magnetization of 1.2 emu/cm3 at an applied field of 10 kOe; highly absorptive, with an average absorption coefficient of 910 cm-1; and possess a low specific Faraday rotation of 320/cm at 1.8 kOe of applied field. As expected, we find that the magneto-optical figure of merit is negligible for this material due to its high absorption...

A Short History of an Elusive Yet Ubiquitous Structure in Chemistry, Materials and Mathematics

Hyde, Stephen; O'Keeffe, M; Proserpio, Davide M.
Fonte: Wiley-VCH Verlag GMBH Publicador: Wiley-VCH Verlag GMBH
Tipo: Artigo de Revista Científica
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Beauty in the sciences: The extraordinary history of a three-periodic net and its associated surface, the gyroid, is recounted (see picture). These structures appear in diverse contexts in mathematics, as the topology for crystal structures in materials,