Página 18 dos resultados de 2573 itens digitais encontrados em 0.199 segundos

‣ High-strain actuation of lead-free perovskites : compositional effects, phenomenology and mechanism

Soukhojak, Andrey N. (Andrey Nestorovich), 1972-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 80 p.; 30728469 bytes; 30728227 bytes; application/pdf; application/pdf
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An experimental study was carried out to map the compositional dependence of electromechanical behavior and ferroelectric phase stability in the barium, zirconium-codoped sodium bismuth titanate (BNBZT) system for barium concentrations up to 18 mol.% and zirconium concentrations up to 4 mol.%. A number of polycrystalline BNBZT samples has been electromechanically tested under applied electric fields of different frequencies (0.2-47 Hz). A novel model of electromechanical response capable of describing both dynamic and static hysteresis for pure and mixed cases of ferroelectric, antiferroelectric, ferroelastic and paraelectric behavior has been developed. Major electromechanical properties of polycrystalline BNBZT have been identified and compositionally mapped. The peak of electromechanical response (d33 = 400 pC/N) has been found at the composition (Bil/2Nal/2)0.93Bao.07Zro.02Tio.9803. The compositional dependence of ferroelectric phase stability has been mapped by means of a Landau type free energy expansion. A nanodomain relaxation mechanism of frequency dependent electromechanical response of BNBZT has been suggested and is supported by optical and transmission electron microscopy.; by Andrey N. Soukhojak.; Thesis (Ph.D.)--Massachusetts Institute of Technology...

‣ Fabrication, characterization, and micromagnetic analysis of lithographically defined particle arrays for applications in data storage

Hwang, Minha, 1973-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 103 leaves; 10354567 bytes; 10354329 bytes; application/pdf; application/pdf
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In this thesis, the magnetic behavior of nanostructured ferromagnetic particle arrays are studied by experiments and numerical micromagnetics for ultra-high-density data storage applications. 1 00nm or 200nm period arrays of nanostructured nickel, cobalt, and cobalt phosphorus are fabricated by the techniques of interference lithography combined with evaporation and electrodeposition. The nanomagnet arrays are characterized by bulk magnetometry and magnetic force microscopy. The remanent states of evaporated conical particles and electrodeposited cylindrical particles are studied by micromagnetic simulations and compared with experimental measurements. For electrodeposited particles, the influence of size, aspect ratio and microstructure on switching field is also investigated. Finally, the effect of demagnetizing magnetostatic interactions and switching field spread on the squareness and the shape of hysteresis loops is studied with the help of an Ising-like interaction model. Based on these observations, a stability condition for patterned media is found and used for determining the optimum spacing between nanomagnets.; by Minha Hwang.; Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering...

‣ Silicon carbide process development for microengine applications : residual stress control and microfabrication

Choi, Dongwon, 1973-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 181 p.; 10893062 bytes; 10915808 bytes; application/pdf; application/pdf
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The high power densities expected for the MIT microengine (silicon MEMS-based micro-gas turbine generator) require the turbine and compressor spool to rotate at a very high speed at elevated temperatures (1300 to 1700 K). However, the thermal softening of silicon (Si) at temperatures above 900 K limits the highest achievable operating temperatures, which in turn significantly compromises the engine efficiency. Silicon carbide (SiC) offers great potential for improved microengine efficiency due to its high stiffness, strength, and resistance to oxidation at elevated temperatures. However, techniques for microfabricating SiC to the high level of precision needed for the microengine are not currently available. Given the limitations imposed by the SiC microfabrication difficulties, this thesis proposed Si-SiC hybrid turbine structures, explores key process steps, identified, and resolved critical problems in each of the processes along with a thorough characterization of the microstructures, mechanical properties, and composition of CVD SiC. Three key process steps for the Si-SiC hybrid structures are CVD SiC deposition on silicon wafers, wafer-level SiC planarization, and Si-to-SiC wafer bonding. Residual stress control in SiC coatings is of the most critical importance to the CVD process itself as well as to the subsequent wafer planarization...

‣ Surface kinetics modeling of silicon oxide etching in fluorocarbon plasmas

Kwon, Ohseung, 1969-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 127 p.; 5149343 bytes; 5148978 bytes; application/pdf; application/pdf
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Fluorocarbon plasma for silicon oxide etching is a complicated system involving many ion and neutral species. Depending on the plasma condition, many difficulties arise such as RIE lag, etch stop, and low selectivity to photoresist. For a better understanding of the process it is necessary to have an appropriate physical model to describe the surface kinetics including simultaneous etching and deposition. A novel surface kinetic model, the translating mixed-layer (TML) model, has been developed. ABACUSS II, a modeling environment and simulator was used for solving differential algebraic equations that describes the surface kinetics. In the modeling, the effect of many variables were investigated including neutral and ion fluxes to the surface, sticking probabilities, surface composition, sputter etching reactions, ion enhanced chemical etching reactions and neutral-to-ion flux ratio. The model has been applied to various systems including silicon etching with chlorine chemistry, silicon oxide etching with fluorine chemistry and silicon oxide etching with fluorocarbon plasma. The verification of the model was done using measured etching yield data determined by quartz crystal microbalance (QCM) in conjunction with plasma neutral and ion concentrations/fluxes determined by mass spectrometry. The etching and deposition rates have been measured as functions of ion impinging angle...

‣ An investigation into cellular attachment and contraction in collagen-GAG scaffolds with characterized pore sizes

Albers, Andrew Michael, 1979-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 121 p.; 8696165 bytes; 8711447 bytes; application/pdf; application/pdf
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Scaffolds fabricated from natural and man-made polymers have historically been used in partial- and full-thickness dermal wound beds to inhibit contraction and promote natural healing of tissue. By encouraging fibroblast migration and proliferation in the scaffolds, it is possible to reduce scar tissue formation and regenerate functioning dermis. A series of experiments were performed to determine the effects of average pore size and available surface area in a lyophilized Collagen-GAG scaffold on the infiltration and attachment of dermal fibroblasts. An updated design of the Cell Force Monitor (CFM) was used to quantify bulk cellular contractile behavior in seeded scaffolds. The effect of scaffold geometry on fibroblast contractile behavior was also investigated with the CFM. Results show that cellular seeding methods employed led to cellular agglomeration on the surfaces of the scaffolds, negating any possible correlation between internal available surface area and cellular attachment. It was also discovered that cell culture passaging techniques have more of an influence on cellular contractile behavior than scaffold pore size, given the seeding techniques employed in this study. No correlation was found between contractile behavior and scaffold geometry in the CFM.; by Andrew Michael Albers.; Thesis (S.M.)--Massachusetts Institute of Technology...

‣ 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
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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...

‣ Applications of lattice dynamics theory : calculating vibrational entropy in alloys and dielectric losses in ceramics

Wu, Eric John, 1974-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 114 leaves; 4160027 bytes; 4159832 bytes; application/pdf; application/pdf
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This thesis applies the theory of lattice dynamics towards two applications: vibrational entropy in alloys and dielectric absorption in oxides. Vibrational entropies between ordered-L12 and disordered phases in the Au3Cu, Cu3Au, Cu3Pd, Pd3Cu, Cu3Pt, Au3Pd, and Pd3Au systems are calculated. This study was done using first-principles energy calculations and the supercell method. Calculated values of ASʻrder-disorder ranged between -0.05 - 0.07 kB. Length-dependent transferable force constants are used to predict vibrational entropies in the Au-Cu, Au-Pd, and Cu-Pd systems. The stiffness of these force constants is obtained from a function that depends on bond length; this function is determined by fitting polynomials to a small set of calculated force constants. Once a function that describes force constant stiffness vs. bond length is determined for a particular pair type, the function can be transferred between different configurations and chemical systems to construct force constants for an arbitrary structure. These transferable force constants are shown to accurately predict vibrational entropies of L12-ordered and disordered phases in Cu3Au, Au3Pd, Pd3Au, Cu3Pd, and Pd3Au. The dielectric absorption at microwave frequencies in binary oxides is calculated within an empirical Buckingham energy model. For MgO...

‣ Mechanics aspects of water thermocompression bonding

Stamoulis, Konstantinos, 1970-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 81 leaves; 3712800 bytes; 3721575 bytes; application/pdf; application/pdf
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Wafer-level, thermocompression bonding is a promising technique for microelectromechanical systems (MEMS) packaging. The process is a form of solid-state joining and requires the simultaneous application of temperature and pressure to wafers patterned with metallic thin films in order to bring the mating surfaces into atomic proximity. The quality of the resulting bond is critically dependent on the interaction between flatness deviations that range from wafer bow to surface roughness, the thin film properties and the process parameters and tooling used to achieve the bonds. Hitherto there has been limited modeling applied to understand the relative contributions of these effects. This thesis addresses the above issue through the development of a mechanics-based framework that allows the effect of flatness deviations to be assessed for typical geometries and process conditions. The strain energy release rate associated with the elastic deformation required to overcome wafer bow is calculated. A contact yield criterion is used to examine the pressure and temperature conditions required to flatten surface roughness asperities in order to achieve bonding over the full apparent area. The results are compared to experimental data of bond yield and toughness obtained from four-point bend delamination testing...

‣ Synthesis and electrochemical characterization of lithium vanadium phosphate

Hsiung, Chwan Hai H. (Chwan Hai Harold), 1982-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 41 leaves; 2494839 bytes; 2493201 bytes; application/pdf; application/pdf
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In a world where the miniaturization and the portability of electronic devices is king, batteries play an ever-increasingly important role. They are vital components in many consumer electronics such as cell phones and PDAs, in medical devices, and in novel applications, such as unmanned vehicles and hybrids. As the power demands of these devices increases, battery performance must improve accordingly. This thesis is an introductory investigation into the electrochemical properties of a promising new battery cathode material: lithium vanadium phosphate (Li3V2(PO4)3) (LVP). Studies of other members of the phospho-olivine family, which LVP is a part of, indicate that the olivines have high lithium diffusivity but low electronic conductivity. LVP is part of the phosphor- olivine family, which traditionally has been shown to have high lithium diffusivity but low electronic conductivity. LVP was synthesized via a solid-state reaction and cast into composite cathodes. (90/5/5 ratio of LVP, Super P Carbon, and PVDF.) These composite cathodes were used in lithium anode, LiPF6 liquid electrolyte, Swage-type cells that were galvanostatically cycled from 3.OV to 4.2V and from 3.4V to 4.8V at C/20 rates. Electrochemical impedance spectroscopy was carried out on an LVP / liquid electrolyte / LVP cells from 0.01Hz to 1MHz. Finally...

‣ Characterization of human skin emanations by Solid Phase Microextraction (SPME) extraction of volatiles and subsequent analysis by Gas Chromatography-Mass Spectrometry (GC-MS)

Akin James (James J.)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 99 p.; 4559379 bytes; 4563473 bytes; application/pdf; application/pdf
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An experimental study was performed to develop and validate a collection and analysis protocol for human skin emanations. The protocol developed included the rubbing of glass beads on the palms and backs of hands for 20 minutes. The volatile headspace above samples were extracted by a solid-phase microextraction fiber which incorporated a composite coating of liquid polymer matrix and solid porous particles. This protocol provided robust and convenient signatures of human skin emanations and was applied to two experiments for validation. In one experiment, a set of twins donated samples and results suggested qualitative differences between samples of twins. The second experiment involved collections from four unrelated individuals over a period of one month. Multivariate analysis was applied to this data set and indicated a stable signature that can be ascribed to the individual, confirming that the protocol developed here can be extended to larger sample sets of MHC typed individuals.; by James Akin.; Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2005.; Includes bibliographical references (p. 80-82).

‣ Initiated chemical vapor deposition of functional polyacrylic thin films; iCVD of functional polyacrylic thin films

Mao, Yu, 1975-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 113 leaves; 5225859 bytes; 5230548 bytes; application/pdf; application/pdf
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Initiated chemical vapor deposition (iCVD) was explored as a novel method for synthesis of functional polyacrylic thin films. The process introduces a peroxide initiator, which can be decomposed at low temperatures (<200⁰C) and initialize addition reaction of monomer species. The use of low temperatures limits the decomposition chemistry to the bond scission of initiator, while retaining functional groups of monomers, which has been confirmed in the infrared spectroscopy, nuclear magnetic resonance, and x-ray photoelectron spectroscopy of iCVD poly(glycidyl methacrylate) (PGMA) thin films. Studies of PGMA iCVD deposition kinetics and molecular weights indicate a free radical polymerization mechanism and provide guide for vapor-phase synthesis of other vinyl monomers. The retained epoxy groups can crosslink under e-beam irradiation, resulting in e-beam patterning of iCVD PGMA thin films with 80 nm negative-tone features achieved. iCVD copolymerization was also investigated to further tune film composition and properties. A surface propagation mechanism was proposed based on the study of the monomer reactivity ratios and the copolymer molecular weights during iCVD copolymerization.; (cont.) The synthesized acrylic copolymers have been investigated in applications as positive-tone e- beam resists...

‣ Sensor applications of carbon nanotubes

Rushfeldt, Scott I
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 69 leaves (some folded); 3696226 bytes; 3699036 bytes; application/pdf; application/pdf
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A search of published research on sensing mechanisms of carbon nanotubes was performed to identify applications in which carbon nanotubes might improve on current sensor technologies, in either offering improved performance, reduced cost of manufacture, or both. Using this overview of carbon nanotube-based sensors, specific sensor technologies that could benefit from the use of newly developed techniques for producing aligned and ordered bundles of carbon nanotubes were selected. Reports of chemical/gas, biological, optical, mechanical, and a few other sensor applications of carbon nanotubes are reviewed. Only a few of these applications might benefit from aligned and ordered bundles of carbon nanotubes. Of these potential applications, only applications in semiconducting gas sensors, DNA sensors, and infrared sensors appear to have clearly defined market niches and are sufficiently technologically mature to allow a detailed assessment of commercial potential. It is argued that DNA and infrared sensors have good commercial potential with a medium amount of risks, while gas sensors have a smaller potential. Finally, DNA sensors are believed to derive the most value from aligned and ordered bundles of carbon nanotubes.; by Scott I. Rushfeldt.; Thesis (M. Eng.)--Massachusetts Institute of Technology...

‣ Chemistry of airborne particles from metallurgical processing

Jenkins, Neil Travis, 1973-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 189 p.
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Airborne particles fall into one of three size ranges. The nucleation range consists of nanoparticles created from vapor atom collisions. The decisive parameter for particle size and composition is the supercooling of the vapor. The accumulation range, which comprises particles less than 2 micrometers, consists of particles formed from the collision of smaller primary particles from the nucleation range. The composition of agglomerates and coalesced particles is the same as the bulk vapor composition. Coarse particles, the composition of which is determined by a liquid precursor, are greater than 1 micrometer and solidify from droplets whose sizes are controlled by surface, viscous, and inertial forces. The relationship between size and composition of airborne particles could be seen in welding fume, a typical metallurgical aerosol. This analysis was performed with a cascade impactor and energy dispersive spectrometry with both scanning electron microscopy (SEM-EDS) and scanning transmission electron microscopy (STEM-EDS). Other methods for properly characterizing particles were discussed. In the analysis, less than 10% of the mass of fume particles for various types of gas metal arc welding (GMAW) were coarse, while one-third of flux cored arc welding (FCAW) fume particles were coarse. Coarse particles had a composition closer to that of the welding electrode than did fine particles. Primary particles were not homogeneous. Particles larger than the mean free path of the carrier gas had the same composition as that of the vapor...

‣ Mapping of elastic modulus and hardness in Trochus niloticus seashell nacre by nanoindentation

Villarreal, Julián Enrique
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: :
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Positionally-sensitive nanoindentation was carried out in the freshly-cleaved nacre found in the shell of the gastropod mollusk Trochus niloticus. Nacre is a hierarchical biocomposite composed of mineral tablets of 95 weight % calcium carbonate (CaCO3) in the aragonite mineral form and a biomacromolecular organic matrix. Nanoindentation was carried out in a pattern of square grids of 256 indents at maximum loads of 1 mN and 500 gN. The average elastic modulus and hardness for the 1 mN indents were found to be 97.8 GPa + 6.41 GPa and 5.41 GPa ± 0.49 GPa, respectively, and for the 500 gN indents average elastic modulus of 94.8 GPa ± 7.28 GPa and hardness of 4.89 GPa ± 0.53 GPa. Maps of the 2-D spatial distribution of elastic modulus and hardness for the indent areas were generated. Tapping mode Atomic Force Microscopy was performed on the indented nacre after a treatment of surface etching, which revealed the tablet boundaries in order to correlate qualitatively the topographical features with the properties distribution. The properties distribution maps revealed a non-uniform distribution of nanomechanical properties as well as highly-localized regions in which the values of the properties differed from the average values. Future studies may point to a direct correlation between structural heterogeneity and the properties distribution.; by Julián Enrique Villarreal.; Thesis (S.B.)--Massachusetts Institute of Technology...

‣ Computational structure analysis of multicomponent oxides

Hinuma, Yoyo
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 138 p.
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First principles density functional theory (DFT) energy calculations combined with the cluster expansion and Monte Carlo techniques are used to understand the cation ordering patterns of multicomponent oxides. Specifically, the lithium ion battery cation material LiNi0.5Mn0.5O2 and the thermoelectric material P2-NaxCoO2 (0.5 =/< x =/< 1) are investigated in the course of this research. It is found that at low temperature the thermodynamically stable state of LiNi0.5Mn0.5O2 has almost no Li/Ni disorder between the Li-rich and transition metal-rich (TM) layer, making it most suitable for battery applications. Heating the material above ~600°C causes an irreversible transformation, which yields a phase with 10~12% Li/Ni disorder and partial disorder of cations in the TM layer. Phase diagrams for the NaxCoO2 system were derived from the results of calculations making use of both the Generalized Gradient Approximation (GGA) to DFT and GGA with Hubbard U correction (GGA+U). This enabled us to study how hole localization, or delocalization, on Co affects the ground states and order-disorder transition temperatures of the system. Comparison of ground states, c lattice parameter and Na1/Na2 ratio with experimental observations suggest that results from the GGA...

‣ First principles study of effect of surface structure on chemical activity of Pt electrocatalysts in fuel cells

Han, Byungchan
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 165 p.
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To facilitate commercialization of fuel cell systems as alternative energy device, the enhancement of Pt electrocatalysts activity is one of the most challenging issues. The first step to the solution is elucidating relationship between surface structure and chemical reactivity as electrocatalysis occurs on its surface. However, in spite of concerted experimental and theoretical research over the last decades, the detailed mechanism is still in debate. This thesis explores a structural sensitivity of the chemical reactivity in the Pt-based alloy electrocatalysts by combining ab-initio density functional theory (DFT) with relevant thermodynamic and kinetic approach. We developed a rigorous statistical mechanical formalism, which can parameterize the energetics obtained by first principles calculations as a function of surface topologies. This methodology enables kinetic Monte Carlo simulations to provide thermally equilibrated structures as a function of partial pressures of adsorbates and alloy compositions. With our consistent methods, we characterize surface structures on the atomic scale, and quantify chemical reactivity of various Pt-alloy model systems. Our methodology reproduced accurate and consistent results of available experimental measurements. We find that our methodology is considerably useful for studying the structural effect on the heterogeneous catalysis. Through the thesis...

‣ Instabilities of jets of non-Newtonian fluids impacting a plate

Varagnat, Matthieu
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 111 p.
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The problem of buckling and coiling of jets of viscous, Newtonian liquids impacting a plate has received a substantial level of attention over the past two decades, both from experimental and theoretical points of view. Nevertheless, most industrial and everyday life fluids are non-newtonian, and their rheological properties affects their behavior in this problem. The present work aims at studying the instabilities of jets of such fluids falling on a plate, via both phenomenological descriptions and theoretical analysis of jet motion and shape. Several fluids with different rheological properties, including viscous Newtonian oil, model non-Newtonian fluids, and commercial shampoos, are used and different dynamical regimes are documented. A special focus is placed on viscoelastic, shear-thinning cetylpyridinium (CPyC1) solutions. In concentrated solutions, CPyCl surfactant molecules have been shown to assemble in long wormlike micellar structures, which gives the fluid its non-Newtonian properties. Jets of CPyCl solution show several novel shapes and dynamical regimes not observed in the case of Newtonian fluids. The present study provides quantitative experimental measurements and mechanisms for these novel features.; by Matthieu Varagnat.; Thesis (S.M.)--Massachusetts Institute of Technology...

‣ Merging quadratic programming with kernel smoothing for automated cluster expansions of complex lattice Hamiltonians

Okan, Osman Burak
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 48 p.
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We present a general outline for automating cluster expansions of configurational energetics in systems with crystallographic order and well defined space group symmetry. The method presented herein combines constrained optimization techniques of positive-definitive quadratic forms with the mathematical tool of Tikhonov regularization (kernel smoothing) for automated expansions of an arbitrary general physical property without compromising the underlying physics. Throughout the thesis we treat formation energy as the fundamental physical observable to expand on since the predominant application of cluster expansions is the extraction of robust approximations for configurational energetics in alloys and oxides. We therefore present the implementational aspects of the novel algorithmic route on a challenging material system NaxCoO2 and reconstruct the corresponding GGA ground state line with arbitrary precision in the formation energy-configuration space. The mathematical arguments and proofs, although discussed for cases with arbitrary spin assignments and multiple candidate species for single site occupancy, are eventually formulated and illustrated for binary systems. Various numerical challanges and the way they are resolved in the framework of kernel smoothing are addressed in detail as well. However...

‣ 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
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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...

‣ Deformation behavior of cylindrical block copolymer bicrystals : pathway to understanding block copolymer grain boundaries

Wanakamol, Panitarn
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 169 leaves
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Model bicrystals made by adhering pieces of near-single-crystal styrene-isoprene-styrene (SIS) cylindrical block copolymer (BCP), produced by a roll-casting process; yield various types of pure tilt grain boundaries. The study of the deformation of the bicrystals, each containing one grain boundary, enables a deeper understanding of the influences grain boundaries and the incompatibilities between them have on mechanical behavior. Mechanical properties and deformation of near-single-crystal systems provide a reference base for the expected bicrystal behavior. We consider various aspects of incompatibility that can arise from joining two highly anisotropic grains together (i.e. Young's modulus, Poisson's ratio, deformation mode(s)). Experimentally, the structure of grain boundaries was characterized using atomic force microscopy (AFM). In deformation experiments, optical microscopy was employed to examine the deformation gradient in the specimen and in situ small angle x-ray scattering (SAXS) was used to monitor the microdomain structural evolution. Finally, finite element simulations illustrated the state of strains of the bicrystal. The symmetric (45-45) bicrystal turns out to be the most complex system, despite of the simplest geometry.; (cont.) Due to the opposite orientation of the grains...