Página 10 dos resultados de 3096 itens digitais encontrados em 0.057 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...

‣ Properties and sensor performance of zinc oxide thin films

Min, Yongki, 1965-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 152 p.; 3587906 bytes; 5309302 bytes; application/pdf; application/pdf
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Reactively sputtered ZnO thin film gas sensors were fabricated onto Si wafers. The atmosphere dependent electrical response of the ZnO micro arrays was examined. The effects of processing conditions on the properties and sensor performance of ZnO films were investigated. Using AFM, SEM, XRD and WDS, the 02/Ar ratios during sputtering and Al dopant were found to control the property of ZnO films. Subsequent annealing at 700 C improved the sensor response of the films considerably although it had only minor effects on the microstructure. DC resistance, I-V curves and AC impedance were utilized to investigate the gas response of ZnO sensors. ZnO films prepared with high O2/Ar ratios showed better sensitivity to various gases, a feature believed to be related to their lower carrier density. Al doped ZnO showed measurable sensitivity even with lower resistance attributable to their porous microstructure. AC impedance identified two major components of the total resistance including Schottky barriers at the Pt-ZnO interfaces and a DC bias induced constriction resistance within the ZnO films. Time dependent drift in resistance of ZnO films has been observed. Without applied bias, the ZnO films showed a fast and a slow resistance change response when exposed to gases with varying oxygen partial pressure with both response components dependent on operating temperature. Even at the relatively low operating temperatures of these thin film sensors...

‣ 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...

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

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

‣ 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...

‣ 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...

‣ The synthesis and assembly of linear-dendritic rod diblock copolymers

Santini, Catherine Marie Bambenek, 1973-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 327 p.; 20066211 bytes; 20066011 bytes; application/pdf; application/pdf
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Dendrimers are three-dimensional, hyperbranched macromolecules that possess a uniform size and shape. Most dendrimers are spherical in shape; however, the shape of the dendrimer can be adjusting using the number and the position of the branching groups in the core. For example, dendritic rods have been prepared by assembling a dendron around each repeat unit of a linear polymer core, and hybrid-linear dendritic diblock copolymers have been prepared by attaching a dendron to the end functional group of a linear polymer. This linear block in the diblock copolymers also adds physical integrity and an assembly mechanism for arrangement of the polymer. Nonetheless, no one has combined the unique shape of the dendritic rod with that of the linear dendritic diblock copolymer. The objective of this research was to prepare a linear-dendritic rod diblock copolymer, and to examine its assembly behavior in solution, at the air/water interface, and in the bulk. These polymers consisted of a linear poly(ethylene oxide)-poly(ethylene imine) diblock copolymer around which poly(amido amine) branches were divergently synthesized. The dendritic branches were terminated with amine and ester groups, as well as alkyl chains of various lengths in order to "tune" the amphiphilic nature of the polymer.; (cont.) A fundamental change in the assembly behavior of the polymers was observed at generation 4.0 (eight end-groups). In solution...

‣ 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...

‣ Synthesis and characterization of nanocrystalline alloys in the binary Ni-Co system/

Wu, Bruce Y. C., 1980
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 100 leaves; 3862057 bytes; 3860069 bytes; application/pdf; application/pdf
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Nanocrystalline Ni-Co alloys were synthesized using pulsed electrodeposition from a Watts-type bath, and the various plating parameters were systematically varied to examine their effect on the composition, quality, and structure of the resulting deposits. Increasing the pH of the plating bath and/or the average plating current density increased the Ni content of the deposits, but both of these parameters exhibited rather narrow processing windows for the preparation of high-quality deposits. The addition of saccharin during plating was observed both to reduce the crystallographic texture of the Ni-Co alloys and to refine the grain structure. Owing to the low stacking fault energy of Co, many of the alloy deposits exhibited a large density of nanoscale twins. The unique dual-scale grain and nano-twin structure found here caused apparent discrepancies in the grain size measured by XRD and by other microscopy techniques, and also resulted in unique trends in hardness. This thesis lays the groundwork for future tailoring of nanostructured alloys to explore how twins and stacking faults influence their strength and ductility.; by Bruce Y.C. Wu.; Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering...

‣ 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).

‣ Development of bi-layer mineralized bone and cartilage regeneration template

Ott, Cassandra Holzgartner
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 94 leaves; 7061413 bytes; 7065297 bytes; application/pdf; application/pdf
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Porous collagen-glycosaminoglycan (CG) scaffolds have been studied extensively and proven to be capable of tissue regeneration in vivo for applications including skin regeneration templates, hollow nerve guides and conjunctiva regeneration. While the current CG scaffold has been thoroughly examined both mechanically and clinically, it has yet to prove appropriate for load- bearing applications. This study will investigate the mechanical properties of a mineralized CG scaffold and its application potential in a load-bearing environment. Through the introduction of calcium-phosphate mineral into the standard CG formulation the matrix analog will be available for bone regeneration. Utilizing a patented triple co-precipitation technique developed at Massachusetts Institute of Technology and Cambridge University, a homogeneous mineralized scaffold will be manufactured. Comparison to healthy trabecular bone as well as the selection of the most appropriate extracellular matrix analog will be presented. The key to commercial success is the introduction of a bi-layer bone and cartilage regeneration template to address concerns and difficulties in cartilage repair today. This dual combination is termed a layered osteochondral scaffold.; (cont.) The commercial viability of this product as well as the company founded on its inception...

‣ Simulations of polymeric membrane formation in 2D and 3D

Zhou, Bo, Ph. D. Massachusetts Institute of Technology
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 213 p.; 5095015 bytes; 5217969 bytes; application/pdf; application/pdf
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The immersion precipitation process makes most commercial polymeric membranes, which enjoy widespread use in water filtration and purification. In this work, a ternary Cahn-Hilliard formulation incorporating a Flory-Huggins homogeneous free energy function is used to model both initial diffusion and the liquid-liquid demixing stage of the immersion precipitation process, which determines much of the final morphology of membranes. Simulations start with a simple non-solvent/solvent/polymer ternary system with periodic boundary conditions and uniform initial conditions with small random fluctuations in 2D. Results in 2D demonstrate the effects of mobilities (Mij) and gradient penalty coefficients (Kij) on phase separation behavior. A two-layer polymer-solvent/non-solvent initial condition is then used to simulate actual membrane fabrication conditions. 2D simulation results demonstrate an asymmetric structure of membrane morphology, which strongly agrees with the experimental observation. A mass transfer boundary condition is developed to model the interaction between the polymer solution and the coagulation bath more efficiently. Simulation results show an asymmetric membrane with connected top layer.; (cont.) Then a wide range of initial compositions are used in both the polymer solution and the coagulation bath...

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

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

‣ Influence on molecular geometry and chain conformation on properties of polyelectrolyte multilayers

Zacharia, Nicole Suzan
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 136 leaves
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The layer-by-layer (LbL) method of self-assembly is a versatile technique for fabricating thin polymer films. This thesis compares the properties of LbLfilms composed of different weak polycations. Slight perturbations in film assembly conditions can lead to large differences in film properties. Polyion bacisity and architecture are less understood variables. The polycations used are of similar chemical composition but different molecular geometries and basicity. Films studied were composed of poly(acrylic acid) (PAA) in combination with linear poly(ethylene imine) (LPEI), poly(allyl amine hydrochloride) (PAH), branched poly(ethylene imine) (BPEI), or poly(amidoamine) (PAMAM) dendrimer, generation four, amine surface. Various properties of these films are compared; including film thickness, chemical functional group availability, and film composition. Carboxylic acid group ionization is found to increase with assembly pH, and PAA content is found to decrease. PAH, the most basic of the polycations, forms films that are the thinnest and the most ionized while PAMAM films have the most free free acid groups and form the thickest films.; (cont.) Permeability to chloroethyl ethyl sulfide vapor was seen to correlate with film ionization and therefore ionic crosslink density; diffusivity was highest in films that deposit in "loopy" layers and solubility was highest in films with the highest degree of ionization. The diffusion of these polycations (specifically in the direction of film growth) is shown to be able to disrupt LbL heterostructures. A model system of a strong polycation (here poly(hexylviologen)) and PAA was used to show rejection of PXV in favor of a weak polycation...

‣ Adsorption and multilayer assembly of charged macromolecules on neutral hydrophobic surfaces and applications to surface patterning

Park, Juhyun, Ph. D. Massachusetts Institute of Technology
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 148 leaves
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Micrometer- and nanometer-scale chemical patterns are indispensable and ubiquitous in a range of applications, such as optoelectronic devices and (bio) chemical sensors. This thesis studies chemical surface patterning utilizing polyelectrolyte multilayers for electronic and biological applications. It focuses on both fundamental study and application development in the field of layer-by-layer self-assembled composite thin films, with the goal of defining new concepts allowing for technological breakthrough. In the process of completing it, a multicomponent patterning technology that has been a bottleneck in realizing practical devices utilizing the multilayers has been developed. To achieve this goal, a multilayer transfer printing concept was applied to serial printing of individual device components. The main achievements include fundamental studies about uniform multilayer assembly of charged macromolecules on neutral hydrophobic surfaces as the principle of the technique, and the demonstration of multicomponent patterning of polyelectrolyte/nanoparticle composite thin films on a flexible substrate.; (cont.) Extending the technique toward nanometer-scale patterning, a new polymeric mold material that was suitable for sub-100 nm structuring was studied and used for chemical patterning for flow control in microfuidic devices and nanoparticle assembly for potential biological applications...

‣ Computational studies of stress and structure development resulting from the coalescence of metallic islands

Takahashi, Andrew Rikio
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 65 leaves
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Thin film component properties are critical design elements in almost all industries. These films are particularly important in the performance of micro- and nano-electromechanical systems (MEMS and NEMS). Residual stress in thin film components is often treated as an unavoidable side effect of processing steps and the degree of residual stress can drastically affect the performance and properties of the final product. While high levels of residual stress are often detrimental to performance, control of the stress and stress gradients can also be used to enhance performance and even generate new capabilities. The work presented in this thesis examines the role of island coalescence in the development of structure and stress in thin films. The primary methods of investigation are molecular dynamics (MD) and finite element analysis (FEA). The semi-empirical MD calculations show that coalescence is a very rapid process for unconstrained spheres and for hemispheres allowed to slide on a frictionless substrate. Particle rotations are commonly observed during the coalescence calculations. The extent of neck formation between 2 particles is consistent with continuum models even down to length scales which would normally be outside the range in which the models might be expected to be applicable. The MD calculations also show that internal island defects may be induced by the island coalescence process...

‣ Presentation and accessibility of surface bound ligands on amphiphilic graft copolymer films

Kuhlman, William A
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 144 p.
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Amphiphilic comb-type graft copolymers comprising a poly(methyl methacrylate) (PMMA) backbone and short, polyethylene oxide (PEO) side chains, PMMA-g-PEO, are proposed to self-organize at the polymer/water interface, resulting in quasi-2D confinement of the backbone at the immediate surface. The branched architecture and amphiphilic chemistry of these polymers results in a dense PEO brush that resists cell adhesion. To facilitate specific cell-surface interactions, small biological molecules such as adhesion peptides can be selectively tethered to PEO chain ends. Quasi-2D confinement of the polymer backbone results in clustering of tethered epitopes on a length scale dictated by the backbone. The present work investigates two aspects of this polymer architecture on organization of tethered ligands: nanometer length-scale clustering through backbone 2D confinement, and tether length effects on the availability of tethered peptides for cell adhesion.; (cont.) To directly probe 2D confined polymer conformations, combs at the film/water interface were labeled with gold nanoparticles and observed by transmission electron microscopy. A 2D radius of gyration (Rg) was calculated by reconstructing nanoparticle-decorated chain trajectories, and compared with Monte Carlo simulations of a 2D melt of similarly broad length distribution. The 2D Rg calculated from observed conformations scaled with the number of backbone segments (N) as Rg - N.69-0.02 Monte Carlo simulations yielded a scaling exponent v = 0.67 + 0.03...

‣ Novel approaches to low temperature transient liquid phase bonding in the In-Sn/Cu and In-Sn-Bi/Cu systems

Fischer, David S., Ph. D. Massachusetts Institute of Technology
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 114 leaves
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A fluxless low temperature transient liquid phase (LTTLP) bonding process was studied as a method of producing Cu/Cu joints below 125°C and 75°C using interlayer alloys from the In-Sn and In-Sn-Bi systems. Using thermodynamic models, three different compositions (wt. %) of base alloys were chosen to accomplish this task: 50In-43.6Sn-6.4Bi (Tm = 110°C) and eutectic 50.9In-49.1Sn (Tm = 120°C) alloys were used for bonding at 125°C and a eutectic 48.3In-15.6Sn-36.1Bi (Tm = 60°C) alloy was used for bonding at 75°C. In addition, novel approaches to TLP bonding, including the addition of base material to the interlayer alloy and application of an electroless Ni diffusion barrier layer, were employed in an attempt to optimize this joining method. The LTTLP processes were assessed based on their abilities to produce joints with minimal thickness, high reflow temperatures, and good mechanical properties at room/elevated temperatures. It was found that interlayer alloys containing higher Bi contents produced the thinnest joints, with the 48.3In-15.6Sn-36.1Bi alloy producing joints on the order of 10 gm. Increases in nominal Cu composition of the interlayer alloy tended to form larger joints. Application of the Ni layer was observed to decrease the growth rate of the eutectic In-Sn joints made with 5 wt % Cu additions. Shear tests were performed on the joints at room (25°C) and operating (service) temperatures (100°C). Most of the TLP joints had room temperature shear strengths around 13...

‣ PH-gated porosity and reversible swelling transitions in polyelectrolyte multilayers

Hiller, Jeri' Ann, 1976-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 149 p.; 12589486 bytes; 12589294 bytes; application/pdf; application/pdf
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This thesis explores the design of novel pH-responsive polyelectrolyte multilayer (PEM) films. The discoveries of discontinuous pH-induced swelling transitions and concomitant conformational memory effects as well as pH-gated nano- and microporosity in PEMs, are described. The PEMs in this thesis are created by an aqueous processing technique, which enables the straightforward manipulation of processing variables to create molecular-level designed polymeric coatings. The first part of this thesis examines discontinuous pH-induced swelling transitions and reversible memory effects in certain PEMs of poly(allylamine hydrochloride) and (poly(styrene sulfonic acid) (PAH/SPS). It is demonstrated that the systematic design of molecular organization, hydrophobic character, and a variable electrostatic nature, enables these phenomena in PAH/SPS films. Consequently, co-existing conformational states result, which allow the regulation of the affinity of molecular species to the PEM as well as their sustained release from the PEM. The second part of this thesis describes phase-separation phenomena in PEMs comprised of PAH and poly(acrylic acid) (PAA). It is shown that specifically designed PAH/PAA films exhibit pH-gated nano- and microporosity. Further...