Página 19 dos resultados de 2573 itens digitais encontrados em 0.176 segundos

‣ Fluid flow behavior of semi-solid aluminum at high shear rates

Yurko, James Andrew, 1975-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 127 leaves; 9903812 bytes; 9903572 bytes; application/pdf; application/pdf
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The rheological behavior and microstructure of semi-solid aluminum alloys were studied using a novel apparatus, the Drop Forge Viscometer (DFV). The viscometer determines force from the curvature of displacement data allowing calculations of viscosities at shear rates in excess of 1000 s-1. Alternatively, the DFV can be operated like a conventional parallel-plate compression viscometer, attaining shear rates as low as 10-5 s-1. Durations of an experiment range between approximately 5 ms and 24 hours. Most rapid compression tests resulted in periods of first rapidly increasing shear rate followed by rapidly decreasing shear rate. Viscosity during the increasing shear rate period decreased by 1-2 orders of magnitude. The viscosity during the decreasing shear rate was an order of magnitude smaller (relative to another experiment) when it achieved a 75% greater maximum shear rate. The DFV was used to calculate viscosity as a function of shear rate for Al-Si and Al-Cu alloys that were rheocast with the commercial SIMA and MHD processes, as well as the recently developed MIT method. Experiments were conducted between fractions solid of 0.44 and 0.67. Viscosity of A357 produced by the three processing routes all had similar viscosities, ranging from 300 Pas at 120 s-1 to 2.2 Pas at 1500 s-1. The final height of compressed Al-Cu was always greater than Al-Si for a given set of experimental conditions. Segregation was not observed in rapid compression experiments shorter than 10 ms...

‣ Scanned pulsed laser annealing of Cu thin films; Scanned pulsed laser annealing of copper thin films

Verma, Harsh Anand, 1980-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 52 leaves; 1915947 bytes; 1920318 bytes; application/pdf; application/pdf
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As the microelectronics industry has moved to Cu as the conductor material, there has been much research into microstructure control in Cu thin films, primarily because grain sizes affect resistivity. Also with Cu-based interconnects, interfacial electromigration is the dominant mechanism, and therefore the crystallographic orientation of the grains could influence reliability. Scanned laser annealing can, in principle, be used to develop test structures with extremely large grains which could be used to quantify the effects of grain boundary scattering and the role of crystallographic orientation in interfacial electromigration. Earlier research on scanned continuous laser annealing suggested that control of the thermal gradient would lead to an improved ability to manipulate grain structures. A scheme to alter the thermal profile during scanned pulsed laser annealing is implemented in the present work. Annealing of samples with Cu thin films over Si substrates with and without a silicon dioxide layer were investigated. Samples were scanned at three different velocities over a range of powers. Ablation was observed at high powers. Quasi-periodic agglomerated structures were observed below threshold powers for ablation, and grain growth was observed at lower powers. The powers at which these regimes occur...

‣ Fundamental studies of pH-sensitivity in polyelectrolyte multilayers

Choi, Jeeyoung, 1974-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 136 leaves; 5694002 bytes; 5691600 bytes; application/pdf; application/pdf
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This thesis investigated the fundamental characteristics of the pH-sensitive behavior of weak polyelectrolytes, such as poly(acrylic acid)(PAA) and poly(allylamine hydrochloride) (PAH) from two perspectives of the assembled multilayers - functional group composition and layer growth. In the first part of this thesis, with respect to the composition control of the functional groups, the assembly conditions to create surfaces that were dominated by specific functional groups were investigated, either to promote or inhibit adsorption of other entities onto the multilayer surfaces. After selective irreversible anchoring, the multilayer surface then acquires the specific physical or chemical properties of the newly incorporated molecules. For example, polystyrene-block-poly(acrylic acid)(PS-PAA) selectively adsorbs to the PAH-rich surfaces of PAA/PAH multilayer films thus rendering the originally hydrophilic PAH-rich regions hydrophobic. Furthermore, by using micro-patterned multilayers via ink-jet printing, two co-existing surface regions that selectively dictate the adsorption behavior of amphiphilic block copolymers were successfully generated. In the second part, the effects of the charge density of weak polyelectrolytes on multilayer growth were quantified by assembling PAA or PAH with several strong polyelectrolytes into multilayers and comparing the results with that of PAA/PAH multilayers.; (cont.) Specifically...

‣ Nanoscale properties of poly(ethylene terephthalate) vascular grafts

Macias, Celia Edith, 1982-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 48 leaves; 2843923 bytes; 2844240 bytes; application/pdf; application/pdf
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Vascular grafts are prosthetic tubes that serve as artificial replacements for damaged blood vessels. Poly(ethylene-terephthalate), PET, has been successfully used in large diameter grafts; however, small caliber grafts are still a major challenge in biomaterials. Due to surface forces, blood plasma proteins adsorb to the graft, resulting in inflammation, infection, thrombus formation, and ultimately, vessel reclosure. The object of this project was to characterize and analyze the nanoscale surface properties of three different commercial vascular grafts, woven collagen-coated, knitted collagen- coated, and knitted heparin-bonded, all PET-based. The study was performed in order to ascertain differences in biocompatibility due to surface coating and morphology. Scanning Electron Microscopy, Atomic Force Microscopy and High Resolution Force Spectroscopy techniques were used to characterize the surface of the samples as well as to measure the forces between these surfaces and blood plasma proteins. The results will serve as a basis for the understanding of the nanoscale interactions between the biomaterial and blood plasma proteins. Such interactions are brought about by the different surface topologies and components, therefore a thorough understanding of surface properties will act as a building block for further changes in small caliber vascular grafts in order to enhance their biocompatibility.; by Celia Edith Macias.; Thesis (S.B.)--Massachusetts Institute of Technology...

‣ Characterization of polystyrene-block-poly (acrylic acid) micelles

Kohen, Naomi (Naomi T.)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 38 leaves; 1835300 bytes; 1834853 bytes; application/pdf; application/pdf
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Several parameters that affect the formation, size and spatial distribution of micelles of poly(styrene-block-acrylic acid) (PS-b-PAA) in organic solvents or assembled on solid substrates have been investigated. The micelles were characterized in the solvated state using Dynamic Light Scattering, and were imaged and characterized in the dry thin film state using Atomic Force Microscopy. Micelle size in solution followed scaling laws based on the ratio of the two block copolymer segments. Micelle size was not affected by the addition of PS homopolymer or salt, whereas micelle diameter did increase with the addition of PAA homopolymer both in solution and in the dry state on sold supports. Furthermore, micelles formed in toluene, but they did not form in tetrahydrofurane, chloroform or hexane. In terms of spatial distribution in the dry state, the only parameters which affected spacing, and therefore density, were annealing conditions and addition of PAA homopolymer. Annealing near or below the glass transition temperature for 16 hours increased the order of the films, as was demonstrated by Fast Fourier Transforms of their AFM images. Annealing for longer periods of time or at temperatures significantly above the glass transition temperature destroyed the micelles.; by Naomi Kohen.; Thesis (S.B.)--Massachusetts Institute of Technology...

‣ Electrodeposition of amorphous matrix Ni-W/Wp̳ composites

Jenket, Donald R. (Donald Robert)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 30 p.; 3023736 bytes; 3022575 bytes; application/pdf; application/pdf
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An amorphous Ni-W alloy matrix was incorporated with W particulate through two types of electrodeposition. The plating bath for the electrodeposition contained nickel sulfate, sodium tungstate, sodium citrate, ammonium chloride, and a variable amount of 1 gm tungsten particulate ranging in concentration from about 5g/L to 15g/L.The first method was electrodeposition with only moderate stirring of the plating bath. The second method had a forced flow of solution on the substrate via a pump. The results showed incorporation in both methods, but the flowed method resulted in more incorporation. The amount of incorporation increased with the amount of particulate in solution until a limit that lies somewhere between 10g/L and 15g/L of particle concentration. At this point, the incorporation became hindered by the excess amount of particulate in solution. It was also shown that an increase of particulate concentration caused more voids in the material, and the flowed method caused less voids than the normal method. A tapering in the amount of incorporation between the substrate side and the surface side of the deposit was observed; the area close to the substrate had a higher incorporation than the area near the surface. Hardness testing showed mechanical property differences through the thickness of the deposit with the area near the substrate being softer than the area near the surface. Compression testing showed an increase in the strain and a decrease in the stress before failure...

‣ Electrochemical kinetics of thin film vanadium pentoxide cathodes for lithium batteries

Mui, Simon C., 1976-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 154 p.; 8232120 bytes; 8239199 bytes; application/pdf; application/pdf
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Electrochemical experiments were performed to investigate the processing-property-performance relations of thin film vanadium pentoxide cathodes used in lithium batteries. Variations in microstructures were achieved via sputtering and anneal treatments, resulting in films with different morphologies, grain size distributions, and orientations. Key findings included (1) grain size distributions largely did not affect the current rate performance of the cathodes. Rather, the film orientation and the ability to undergo rapid phase transformation were more vital to improving performance; (2) interfacial resistance and ohmic polarization were also dominant at the high current rates used (> 600 [mu]A/cm²) in addition to solid diffusion; and (3) optimization of thin film batteries requires that film thickness be < 500 nm to avoid diminishing returns in power and energy densities. Kinetic parameters including the transfer coefficient ([alpha] = 0.90± 0.05) and standard rate constant (k⁰ [approx.] 2 x 10⁻⁶ cm/s) for vanadium pentoxide films were quantified using slow scan DC cyclic voltammetry and AC cyclic voltammetry. The reaction rate was found to be potentially limiting at moderate to high current rates (> 200 [mu]A/cm²).; (cont.) An analysis of the wide variation in current-rate performance for different V₂0₅ architectures (including composite...

‣ Hydrogen degradation and microstructural effects of the near-threshold fatigue resistance of pressure vessel steels

Fuquen-Molano, Rosendo
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 200 leaves; 8895488 bytes; 8903973 bytes; application/pdf; application/pdf
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Safety of pressure vessels for applications such as coal conversion reactors requires understanding of the mechanism of environmentally-induced crack propagation and the mechanism by which process-induced microstructures as in thick section weldments affect the fatigue resistance of the structure. At low stress intensities near-threshold [delta]K₀ ([delta]K = Kmax - Kmin), water vapor in the environment was found to produce a pronounced effect on the fatigue resistance for partial pressures as low as 10 torr. In 2 1/4Cr-1%Mo SA378-2-22 steel the crack propagation rates at high load ratio (R = Kmin-Kmax) are increased in the presence of water vapor and the opposite effect is observed at low load ratio. It is proposed that water vapor-containing environments give rise to two mechanisms affecting crack growth rates: embrittlement caused by hydrogen produced in the water-metal reaction; and crack closure, enhanced by the increased surface roughness and the wedging action of the oxidation product. The microstructure is proposed to affect crack propagation rates mainly through crack closure induced by the synergistic effect of fracture surface roughness and oxide produced by fretting.; by Rosendo Fuquen-Molano.; Thesis (Ph. D.)--Massachusetts Institute of Technology...

‣ Electronic properties of phenylated ligand-capped nanoparticle films

Schilling, Thomas C
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 37 leaves; 1769036 bytes; 1768489 bytes; application/pdf; application/pdf
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An investigation was carried out of the electronic characteristics of drop-cast films comprised of phenylated ligand-capped gold nanoparticles. In homoligand-type films, the dominant mechanism of charge transfer was expected to involve orbital overlap and end group-effected wave function displacement, whereas heteroligand-type films were expected to conduct through less efficient hopping mechanisms. Films utilizing the former mechanism are expected to have great applicability within microelectronics and rapid-prototyping technologies due to the small scale (2-6nm) of functionalized nanoparticles and the structural flexibility of interdigitation as a form of inter-particle bonding. The comparative conductances of the cast films reveal a strong correlation with the ligand Hammaker constant (effectively a measure of the work function of the conjugated bond with the gold core of the nanoparticle and the charge displacement effected by the electronegativity or polarity of the ligand end group). The conductance was also greatly affected by the size of ligand end groups - a rough measure of the close-packing ability of a given ligand both within the ligand shell and amongst the shells of adjacent nanoparticles. The following experiments illustrate these correlations...

‣ Rate dependent rupture of solid-supported phospholipid bilayers.

Ng, Sarah S
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 31 leaves; 1306277 bytes; 1305094 bytes; application/pdf; application/pdf
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An experimental study on solid-supported phospholipid bilayers was performed in order to investigate rate-dependent behavior of force and probability of bilayer rupture. 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) solid-supported lipid bilayers were created on mica using vesicle fusion technique and then ruptured normal to the surface using a silicon nitride cantilever tip (radius#80nm). High resolution force spectroscopy was performed using the Molecular Force Probe (1D) to obtain force versus distance curves between the tip and substrate, varying the rate of penetration between a range of 250 nm/sec to 8.0 pm/sec. Statistical analysis was used to find distributions for average yield distance and yield force at different rates to find correlations in our data. Lastly, experimental data was compared to proposed theoretical models that describe rupture probability as a function of activation energy. A two yield force profile on approach was achieved with consistency at all rates. The yield forces occurred at statistical significant distances of around 4 nm and 9 nm, which are consistent with bond calculations of the phospholipid. However, no relationship was found between force and tip velocity within the range of experimentation.; (cont.) Because rupture occurred even at the lowest penetration rates...

‣ Biomechanics of the human chorioamnion

Prévost, Thibault Philippe
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 115 leaves
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The human fetal membrane, namely the chorioamnion, is the structural soft tissue retaining the amniotic fluid and the fetus during pregnancy. Its biomechanical integrity is crucial for maintaining a healthy gestation and a successful delivery. The premature rupture of the fetal membrane (PROM) can result in serious perinatal complications. Despite extensive research in this field, the mechanical and biochemical processes governing the membrane deformation and failure remain poorly understood. The aim of this study is to characterize the mechanical behavior of the chorioamnionic tissue along with its biochemical properties, through mechanical testing and biochemical analyses. In order to accomplish this goal, specific mechanical and biochemical testing protocols were developed. In vitro mechanical testing was performed on samples from seven patients under different uniaxial and biaxial loading conditions. Significant relaxation was noted under uniaxial loading while very limited creep was observed under biaxial loading. Biochemical measurements such as collagen and sulfated glycosaminoglycan contents were also obtained. In addition, a microstructurally based constitutive model for the fetal membrane is proposed.; (cont.) The model allows for nonlinear hyperelastic response at large deformation. We also propose a framework to capture the time-dependent response of the tissue. The model was implemented in a finite element formulation to allow three-dimensional simulations of membrane deformation.; by Thibault Philippe Prévost.; Thesis (S.M.)--Massachusetts Institute of Technology...

‣ Assessment of colloidal self-assembly for photonic crystal

Yip, Chan Hoe
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 93 p.
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A suspension of monodisperse colloids has an interesting property of self-assembling into a three-dimensional ordered structure. This crystalline material has attracted significant interest on the implementation of photonic crystals, which have practical applications in reflectors, filters, resonators, and waveguides. In this thesis, self-assembly of colloidal crystals and photonic crystal technologies are reviewed. Potential colloidal photonic and non-photonic devices were presented and their values/limitations were discussed. Colloidal photonic crystals were assessed on their technical capabilities, growth techniques and fabrication cost. In this assessment, the bulk colloidal photonic crystals are found to be inherently robust against stacking disorder, cracks and voids. The high reflectance performance and lattice parameter tailoring are useful for implementing reflectors, optical switch and sensors. Besides, the anomalous dispersion characteristic near to the band edges or near to flat bands of the photonic band diagram is suited for superprism and light harvesting applications. Potentially, the unique characteristics of colloidal photonic crystal could be capitalized in a low cost micro-fabrication model. Finally, the study has shown that it is more technically and commercially viable to implement bulk colloidal photonic crystal applications rather than lithographically-defined types.; by Chan Hoe Yip.; Thesis (M. Eng.)--Massachusetts Institute of Technology...

‣ Chemical vapor deposition of functionalized isobenzofuran polymers

Olsson, Ylva Kristina
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 48 leaves
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This thesis develops a platform for deposition of polymer thin films that can be further tailored by chemical surface modification. First, we explore chemical vapor deposition of functionalized isobenzofuran films using two different functional groups: pentafluorophenolate ester and alkyne. Both functional groups can be further modified using either ester substitution or click chemistry, respectively. The resulting thin films are characterized extensively using nuclear magnetic resonance (NMR), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). We show that the functional groups remain on the surface post deposition for both films at cracking temperature not exceeding 6000C. However, gel permeation chromatography(GPC) measurements of the pentafluorophenolate ester films show only marginal polymerization. On the other hand, the alkyne films appear crosslinked and showed defect formation. Films deposited at ambient temperature show formation of a large number of micro defects. Increasing the deposition temperature, in addition to increasing the growth rate, also leads to formation of films with two distinct domains: one smooth domain with no micro defects and another filled with defects.; (cont.) Analogous to the deposition of unmodified isobenzofuran films...

‣ Bronze metallurgy in Iron Age central Europe : a metallurgical study of Early Iron Age bronzes from Stična, Slovenia.; Metallurgical study of Early Iron Age bronzes from Stična, Slovenia

Cooney, Elizabeth Myers
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 2 v. (381 leaves)
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The Early Iron Age (750-450 BCE) marks a time in the European Alpine Region in which cultural ideologies surrounding bronze objects and bronze production were changing. Iron was becoming the preferred material from which to make many utilitarian objects such as weapons and agricultural tools; this change can be clearly seen in the different treatments of bronze object deposits from the Late Bronze Age to the Early Iron Age. The Early Iron Age hillfort settlement of Sticna in what is now southeastern Slovenia was one of the first incipient commercial centers to take advantage of the new importance placed on iron, conducting trade with Italy, Greece, the Balkans, and northern Europe. This metallurgical study of bronze funerary objects from Sticna identifies construction techniques, use patterns, and bronze metallurgical technologies from the ancient region of Lower Carniola. This information is then used to explore the cultural importance of bronze at Early Iron Age Sticna and to compare the bronze work of Lower Carniola with that of other regions in central Europe and Italy from this time of great change in Iron Age Europe.; Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2007.; This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.; Includes bibliographical references (v. 2...

‣ Quantitative studies in effects of additives on protein aggregation

Shinde, Chetan (Chetan Ulhas)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 62 p.
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Rational design of protein additives has been limited by the understanding of mechanism of protein and additive interaction. In this work we have applied molecular dynamics with all atom potentials in order to study the thermodynamic effect of additives on proteins. The method is based on statistical mechanical model that characterizes the preferential binding of proteins to either water or additives. Extensive study was done on model systems comprising of additives urea, glycerol & arginine hydrochloride and proteins RNaseT1 and hen egg lysozyme. Trajectories in range 10-19 nanoseconds were analyzed in order to validate this method and compared with the experimental results. The method was found to agree with experimental results for the first 2 nanoseconds and the extended runs were studied further to narrow down the cause of deviations. Protein RNaseT1 was found to be very unstable and consequently showed very high deviations in preferential binding for longer runs. Constraining the protein using harmonic potential has resulted in better averages for RNase T1.; (cont.) Lysozyme has been found to be very stable and the calculations are in good agreement with experimental values. Local preferential binding calculations showed the importance of structure as well as sequence in prediction of preferential binding of protein.; by Chetan Shinde.; Thesis (S.M.)--Massachusetts Institute of Technology...

‣ Exploration of catalysis activation emergency as a function of gold nanoparticle surface morphology

Stefanescu, Cristina F
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 45 p.
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The application of rippled gold nanoparticles with bi-ligand surface morphology as a catalyst was tested. The hydrolysis of 2,4-dinitrophenyl acetate (DNPA) served as the catalytic reaction being analyzed and the bi-ligand composition used was 16-mercaptohexadecanoic acid to imidazole thiol (MHA to IT). The influence of temperature on catalytic reaction of DNPA with the MHA: IT system was tested for ligand rations of 2:1, 1:2, and 1:1 by monitoring the catalytic system on a UV-VIS spectrometer. Catalytic rate constants were obtained and found to increase with increased temperature. The measured catalytic rate constants were greatest overall for the 1:1 system, followed by the 1:2 system, and lastly the 2:1 system. The activation energy for each ligand-ratio system was measured and found to be 22.17 kJ/mol for the 2:1 system, 14.7 kJ/mol for the 1:2 system, and 26.52 for the 1:1 system. The 2:1 and 1:2 systems followed the trend of lower activation energy values for systems with faster rates; however the 1:1 system did not fit this trend as it resulted in the highest activation energy value as well as the fastest reaction rates.; by Cristina F. Stefanescu.; Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering...

‣ The impact of mass decompounding on assessing the value of vehicle lightweighting

Bjelkengren, Catarina
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 125 p
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Among consumers and manufacturers alike, there is an increasing realization about the need for fuel efficient vehicles. One effective way to accomplish this is through vehicle lightweighting, which can be achieved by material substitution, novel vehicle component design, and changes in processing. Although primary vehicle mass reduction is often associated with additional costs to the automaker, a decision to lightweight may, depending on when in the vehicle development process the decision is taken, result in additional secondary mass savings such that the value derived from lightweighting is greater than the costs. In this study, the concept of secondary mass savings, or mass decompounding, is developed using regression analysis. Moreover, the full, both primary and secondary, mass savings potential is assessed at different times in the vehicle development process. Lastly, powertrain and market trend modeling are employed to estimate the value of the compounded mass savings in terms of improved fuel economy and acceleration. This methodology is applied to a collected vehicle dataset in order to generate a model by which the value of and the subsystem-specific amount of secondary mass savings may be easily estimated during the early stages of vehicle development. In summary...

‣ Temperature effects on the electronic conductivity of single-walled carbon nanotubes

Mascaro, Mark Daniel
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 59 p.
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The room-temperature electronic conductivity and temperature dependence of conductivity were measured for samples of carbon nanotubes of three types: pristine; functionalized with a nitrobenzene covalent functionalization, which are expected to display poor electronic conductivity; and functionalized with a carbene covalent functionalization, which are expected to display pristine-like conductivity. Measurements were taken via four-point probe measurement across palladium contacts on a silicon surface coated with a distribution of nanotubes. Room-temperature measurements indicate that carbene-functionalized tubes do exhibit significantly greater conductivity than nitrobenzene-functionalized tubes, but also significantly less than pristine tubes. Statistically different distributions of resistances observed in similarly prepared samples indicate that this measurement technique is strongly affected by uncontrollable and so far uncharacterizable parameters of the employed sample preparation technique. Measurements at varying temperature indicate the expected linear relationship of resistance and temperature is dominated in pristine and carbene samples by a effect, possibly contamination-related, which significantly and permanently increases the resistance of samples after cycling to high temperatures...

‣ Electrospun nanofibers : opportunities in environment and energy

Mukherjee, Kunal
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 60 leaves
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One-dimensional nanostructures have several attractive material properties compared to their bulk counterparts and have found applications in many novel devices. They have especially shown promise in the fields of renewable energy generation and environmental monitoring by improving solar cells and gas sensors. However difficulties in large scale manufacturing of these nanostructures have prevented most of these devices from entering the marketplace. Electrospinning is a simple, versatile and scalable technique to fabricate one-dimensional nanostructures in the form of nanofibers. It has recently been successfully used to produce semiconducting metal oxide nanofibers which have been used in chemiresistive gas sensors and dye-sensitized solar cells. Electrospun gas sensors and solar cells have already been shown to perform better than many of its competitors in a very short period of time. This coupled with low manufacturing costs and well protected intellectual property makes electrospinning a good candidate to bring the advantages of one-dimensional nanostructures into the marketplace.; by Kunal Mukherjee.; Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2008.; Includes bibliographical references (leaves 53-56).

‣ Metallographic study of gamma - gamma prime structure in the Ni-based superalloy GTD111

Kountras, Apostolos, 1970-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 73 leaves; 3974231 bytes; 3972540 bytes; application/pdf; application/pdf
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The potential for land-based turbine buckets material rejuvenation presents a significant commercial and scientific interest. Ni-based superalloy GTD111 is used at a number of GE-manufactured power generation turbines. The outstanding creep resistance features of Ni-based superalloys can be attributed to a large extent, to the gamma prime ([gamma]') precipitates found within the FCC [gamma]-matrix. Service-induced material degradation mainly involves coarsening and shape transformation of [gamma]'-phase precipitates; therefore, any bucket repair attempt should primarily address the restoration of [gamma]' precipitates to the original configuration. In the present study a quantitative metallographic analysis of GTD111 alloy under different conditions was performed. Several micrographs were taken and analysed using image analysis software. Gamma prime precipitate size was measured and compared between the different alloy conditions, leading to useful conclusions concerning material degradation as a result of high-temperature service exposure. In addition, microstructural transformations observed as a result of different heat treatments, formed the basis for investigation of procedures that can potentially restore the alloy microstructure in the original condition. High temperature solution and aging heat treatments sequence is considered as potentially sufficient for restoring the GTD111 microstructure. Finally...