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Electrospinning of bioactive dex-PAA hydrogel fibers

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In this work, a novel method is developed for making nano- and micro- fibrous hydrogels capable of preventing the rejection of implanted materials. This is achieved by either (1) mimicking the native cellular environment, to exert fine control over the

In this work, a novel method is developed for making nano- and micro- fibrous hydrogels capable of preventing the rejection of implanted materials. This is achieved by either (1) mimicking the native cellular environment, to exert fine control over the cellular response or (2) acting as a protective barrier, to camouflage the foreign nature of a material and evade recognition by the immune system. Comprehensive characterization and in vitro studies described here provide a foundation for developing substrates for use in clinical applications. Hydrogel dextran and poly(acrylic acid) (PAA) fibers are formed via electrospinning, in sizes ranging from nanometers to microns in diameter. While "as-electrospun" fibers are continuous in length, sonication is used to fragment fibers into short fiber "bristles" and generate nano- and micro- fibrous surface coatings over a wide range of topographies. Dex-PAA fibrous surfaces are chemically modified, and then optimized and characterized for non-fouling and ECM-mimetic properties. The non-fouling nature of fibers is verified, and cell culture studies show differential responses dependent upon chemical, topographical and mechanical properties. Dex-PAA fibers are advantageously unique in that (1) a fine degree of control is possible over three significant parameters critical for modifying cellular response: topography, chemistry and mechanical properties, over a range emulating that of native cellular environments, (2) the innate nature of the material is non-fouling, providing an inert background for adding back specific bioactive functionality, and (3) the fibers can be applied as a surface coating or comprise the scaffold itself. This is the first reported work of dex-PAA hydrogel fibers formed via electrospinning and thermal cross-linking, and unique to this method, no toxic solvents or cross-linking agents are needed to create hydrogels or for surface attachment. This is also the first reported work of using sonication to fragment electrospun hydrogel fibers, and in which surface coatings were made via simple electrostatic interaction and dehydration. These versatile features enable fibrous surface coatings to be applied to virtually any material. Results of this research broadly impact the design of biomaterials which contact cells in the body by directing the consequent cell-material interaction.

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2011

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A theoretical analysis of microchannel flow boiling enhancement via cross-sectional expansion

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Microchannel heat sinks can possess heat transfer characteristics unavailable in conventional heat exchangers; such sinks offer compact solutions to otherwise intractable thermal management problems, notably in small-scale electronics cooling. Flow boiling in microchannels allows a very high heat transfer rate,

Microchannel heat sinks can possess heat transfer characteristics unavailable in conventional heat exchangers; such sinks offer compact solutions to otherwise intractable thermal management problems, notably in small-scale electronics cooling. Flow boiling in microchannels allows a very high heat transfer rate, but is bounded by the critical heat flux (CHF). This thesis presents a theoretical-numerical study of a method to improve the heat rejection capability of a microchannel heat sink via expansion of the channel cross-section along the flow direction. The thermodynamic quality of the refrigerant increases during flow boiling, decreasing the density of the bulk coolant as it flows. This may effect pressure fluctuations in the channels, leading to nonuniform heat transfer and local dryout in regions exceeding CHF. This undesirable phenomenon is counteracted by permitting the cross-section of the microchannel to increase along the direction of flow, allowing more volume for the vapor. Governing equations are derived from a control-volume analysis of a single heated rectangular microchannel; the cross-section is allowed to expand in width and height. The resulting differential equations are solved numerically for a variety of channel expansion profiles and numbers of channels. The refrigerant is R-134a and channel parameters are based on a physical test bed in a related experiment. Significant improvement in CHF is possible with moderate area expansion. Minimal additional manufacturing costs could yield major gains in the utility of microchannel heat sinks. An optimum expansion rate occurred in certain cases, and alterations in the channel width are, in general, more effective at improving CHF than alterations in the channel height. Modest expansion in height enables small width expansions to be very effective.

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2011

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Synthesis and characterization of ordered mesoporous silica with controlled macroscopic morphology for membrane applications

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Ordered mesoporous materials have tunable pore sizes between 2 and 50 nm and are characterized by ordered pore structures and high surface areas (~1000 m2/g). This makes them particularly favorable for a number of membrane applications such as protein separation,

Ordered mesoporous materials have tunable pore sizes between 2 and 50 nm and are characterized by ordered pore structures and high surface areas (~1000 m2/g). This makes them particularly favorable for a number of membrane applications such as protein separation, polymer extrusion, nanowire fabrication and membrane reactors. These membranes can be fabricated as top-layers on macroporous supports or as embedded membranes in a dense matrix. The first part of the work deals with the hydrothermal synthesis and water-vapor/oxygen separation properties of supported MCM-48 and a new Al-MCM-48 type membrane for potential use in air conditioning systems. Knudsen-type permeation is observed in these membranes. The combined effect of capillary condensation and the aluminosilicate matrix resulted in the highest separation factor (142) in Al-MCM-48 membranes, with a water vapor permeance of 6×10-8mol/m2Pas. The second part focuses on synthesis of embedded mesoporous silica membranes with helically ordered pores by a novel Counter Diffusion Self-Assembly (CDSA) method. This method is an extension of the interfacial synthesis method for fiber synthesis using tetrabutylorthosilicate (TBOS) and cetyltrimethylammonium bromide (CTAB) as the silica source and surfactant respectively. The initial part of this study determined the effect of TBOS height and humidity on fiber formation. From this study, the range of TBOS heights for best microscopic and macroscopic ordering were established. Next, the CDSA method was used to successfully synthesize membranes, which were characterized to have good support plugging and an ordered pore structure. Factors that influence membrane synthesis and plug microstructure were determined. SEM studies revealed the presence of gaps between the plugs and support pores, which occur due to shrinking of the plug on drying. Development of a novel liquid deposition method to seal these defects constituted the last part of this work. Post sealing, excess silica was removed by etching with hydrofluoric acid. Membrane quality was evaluated at each step using SEM and gas permeation measurements. After surfactant removal by liquid extraction, the membranes exhibited an O2 permeance of 1.65x10-6mol/m2.Pa.s and He/O2 selectivity of 3.30. The successful synthesis of this membrane is an exciting new development in the area of ordered mesoporous membrane technology.

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2011

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Growth of gaN nanowires: a study using in situ transmission electron microscopy

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Owing to their special characteristics, group III-Nitride semiconductors have attracted special attention for their application in a wide range of optoelectronic devices. Of particular interest are their direct and wide band gaps that span from ultraviolet to the infrared wavelengths.

Owing to their special characteristics, group III-Nitride semiconductors have attracted special attention for their application in a wide range of optoelectronic devices. Of particular interest are their direct and wide band gaps that span from ultraviolet to the infrared wavelengths. In addition, their stronger bonds relative to the other compound semiconductors makes them thermally more stable, which provides devices with longer life time. However, the lattice mismatch between these semiconductors and their substrates cause the as-grown films to have high dislocation densities, reducing the life time of devices that contain these materials. One possible solution for this problem is to substitute single crystal semiconductor nanowires for epitaxial films. Due to their dimensionality, semiconductor nanowires typically have stress-free surfaces and better physical properties. In order to employ semiconductor nanowires as building blocks for nanoscale devices, a precise control of the nanowires' crystallinity, morphology, and chemistry is necessary. This control can be achieved by first developing a deeper understanding of the processes involved in the synthesis of nanowires, and then by determining the effects of temperature and pressure on their growth. This dissertation focuses on understanding of the growth processes involved in the formation of GaN nanowires. Nucleation and growth events were observed in situ and controlled in real-time using an environmental transmission electron microscope. These observations provide a satisfactory elucidation of the underlying growth mechanism during the formation of GaN nanowires. Nucleation of these nanowires appears to follow the vapor-liquid-solid mechanism. However, nanowire growth is found to follow both the vapor-liquid-solid and vapor-solid-solid mechanisms. Direct evidence of the effects of III/V ratio on nanowire growth is also reported, which provides important information for tailoring the synthesis of GaN nanowires. These findings suggest in situ electron microscopy is a powerful tool to understand the growth of GaN nanowires and also that these experimental approach can be extended to study other binary semiconductor compound such as GaP, GaAs, and InP, or even ternary compounds such as InGaN. However, further experimental work is required to fully elucidate the kinetic effects on the growth process. A better control of the growth parameters is also recommended.

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2010

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Optimization of ionic conductivity in doped ceria using density functional theory and kinetic lattice Monte Carlo

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Fuel cells, particularly solid oxide fuel cells (SOFC), are important for the future of greener and more efficient energy sources. Although SOFCs have been in existence for over fifty years, they have not been deployed extensively because they need to

Fuel cells, particularly solid oxide fuel cells (SOFC), are important for the future of greener and more efficient energy sources. Although SOFCs have been in existence for over fifty years, they have not been deployed extensively because they need to be operated at a high temperature (∼1000 °C), are expensive, and have slow response to changes in energy demands. One important need for commercialization of SOFCs is a lowering of their operating temperature, which requires an electrolyte that can operate at lower temperatures. Doped ceria is one such candidate. For this dissertation work I have studied different types of doped ceria to understand the mechanism of oxygen vacancy diffusion through the bulk. Doped ceria is important because they have high ionic conductivities thus making them attractive candidates for the electrolytes of solid oxide fuel cells. In particular, I have studied how the ionic conductivities are improved in these doped materials by studying the oxygen-vacancy formations and migrations. In this dissertation I describe the application of density functional theory (DFT) and Kinetic Lattice Monte Carlo (KLMC) simulations to calculate the vacancy diffusion and ionic conductivities in doped ceria. The dopants used are praseodymium (Pr), gadolinium (Gd), and neodymium (Nd), all belonging to the lanthanide series. The activation energies for vacancy migration between different nearest neighbor (relative to the dopant) positions were calculated using the commercial DFT code VASP (Vienna Ab-initio Simulation Package). These activation energies were then used as inputs to the KLMC code that I co-developed. The KLMC code was run for different temperatures (673 K to 1073 K) and for different dopant concentrations (0 to 40%). These simulations have resulted in the prediction of dopant concentrations for maximum ionic conductivity at a given temperature.

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2011

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Growth, characterization, and thermodynamics of III-nitride semiconductors

Description

III-nitride alloys are wide band gap semiconductors with a broad range of applications in optoelectronic devices such as light emitting diodes and laser diodes. Indium gallium nitride light emitting diodes have been successfully produced over the past decade. But the

III-nitride alloys are wide band gap semiconductors with a broad range of applications in optoelectronic devices such as light emitting diodes and laser diodes. Indium gallium nitride light emitting diodes have been successfully produced over the past decade. But the progress of green emission light emitting devices has been limited by the incorporation of indium in the alloy, mainly due to phase separation. This difficulty could be addressed by studying the growth and thermodynamics of these alloys. Knowledge of thermodynamic phase stabilities and of pressure - temperature - composition phase diagrams is important for an understanding of the boundary conditions of a variety of growth techniques. In this dissertation a study of the phase separation of indium gallium nitride is conducted using a regular solution model of the ternary alloy system. Graphs of Gibbs free energy of mixing were produced for a range of temperatures. Binodal and spinodal decomposition curves show the stable and unstable regions of the alloy in equilibrium. The growth of gallium nitride and indium gallium nitride was attempted by the reaction of molten gallium - indium alloy with ammonia at atmospheric pressure. Characterization by X-ray diffraction, photoluminescence, and secondary electron microscopy show that the samples produced by this method contain only gallium nitride in the hexagonal phase. The instability of indium nitride at the temperatures required for activation of ammonia accounts for these results. The photoluminescence spectra show a correlation between the intensity of a broad green emission, related to native defects, and indium composition used in the molten alloy. A different growth method was used to grow two columnar-structured gallium nitride films using ammonium chloride and gallium as reactants and nitrogen and ammonia as carrier gasses. Investigation by X-ray diffraction and spatially-resolved cathodoluminescence shows the film grown at higher temperature to be primarily hexagonal with small quantities of cubic crystallites, while the one grown at lower temperature to be pure hexagonal. This was also confirmed by low temperature photoluminescence measurements. The results presented here show that cubic and hexagonal crystallites can coexist, with the cubic phase having a much sharper and stronger luminescence. Controlled growth of the cubic phase GaN crystallites can be of use for high efficiency light detecting and emitting devices. The ammonolysis of a precursor was used to grow InGaN powders with different indium composition. High purity hexagonal GaN and InN were obtained. XRD spectra showed complete phase separation for samples with x < 30%, with ~ 9% indium incorporation in the 30% sample. The presence of InGaN in this sample was confirmed by PL measurements, where luminescence from both GaN and InGaN band edge are observed. The growth of higher indium compositions samples proved to be difficult, with only the presence of InN in the sample. Nonetheless, by controlling parameters like temperature and time may lead to successful growth of this III-nitride alloy by this method.

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2011

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Generalized statistical tolerance analysis and three dimensional model for manufacturing tolerance transfer in manufacturing process planning

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Mostly, manufacturing tolerance charts are used these days for manufacturing tolerance transfer but these have the limitation of being one dimensional only. Some research has been undertaken for the three dimensional geometric tolerances but it is too theoretical and yet

Mostly, manufacturing tolerance charts are used these days for manufacturing tolerance transfer but these have the limitation of being one dimensional only. Some research has been undertaken for the three dimensional geometric tolerances but it is too theoretical and yet to be ready for operator level usage. In this research, a new three dimensional model for tolerance transfer in manufacturing process planning is presented that is user friendly in the sense that it is built upon the Coordinate Measuring Machine (CMM) readings that are readily available in any decent manufacturing facility. This model can take care of datum reference change between non orthogonal datums (squeezed datums), non-linearly oriented datums (twisted datums) etc. Graph theoretic approach based upon ACIS, C++ and MFC is laid out to facilitate its implementation for automation of the model. A totally new approach to determining dimensions and tolerances for the manufacturing process plan is also presented. Secondly, a new statistical model for the statistical tolerance analysis based upon joint probability distribution of the trivariate normal distributed variables is presented. 4-D probability Maps have been developed in which the probability value of a point in space is represented by the size of the marker and the associated color. Points inside the part map represent the pass percentage for parts manufactured. The effect of refinement with form and orientation tolerance is highlighted by calculating the change in pass percentage with the pass percentage for size tolerance only. Delaunay triangulation and ray tracing algorithms have been used to automate the process of identifying the points inside and outside the part map. Proof of concept software has been implemented to demonstrate this model and to determine pass percentages for various cases. The model is further extended to assemblies by employing convolution algorithms on two trivariate statistical distributions to arrive at the statistical distribution of the assembly. Map generated by using Minkowski Sum techniques on the individual part maps is superimposed on the probability point cloud resulting from convolution. Delaunay triangulation and ray tracing algorithms are employed to determine the assembleability percentages for the assembly.

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2011

Surface stress during electro-oxidation of carbon monoxide and bulk stress evolution during electrochemical intercalation of lithium

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This work investigates in-situ stress evolution of interfacial and bulk processes in electrochemical systems, and is divided into two projects. The first project examines the electrocapillarity of clean and CO-covered electrodes. It also investigates surface stress evolution during electro-oxidation of

This work investigates in-situ stress evolution of interfacial and bulk processes in electrochemical systems, and is divided into two projects. The first project examines the electrocapillarity of clean and CO-covered electrodes. It also investigates surface stress evolution during electro-oxidation of CO at Pt{111}, Ru/Pt{111} and Ru{0001} electrodes. The second project explores the evolution of bulk stress that occurs during intercalation (extraction) of lithium (Li) and formation of a solid electrolyte interphase during electrochemical reduction (oxidation) of Li at graphitic electrodes. Electrocapillarity measurements have shown that hydrogen and hydroxide adsorption are compressive on Pt{111}, Ru/Pt{111}, and Ru{0001}. The adsorption-induced surface stresses correlate strongly with adsorption charge. Electrocatalytic oxidation of CO on Pt{111} and Ru/Pt{111} gives a tensile surface stress. A numerical method was developed to separate both current and stress into background and active components. Applying this model to the CO oxidation signal on Ru{0001} gives a tensile surface stress and elucidates the rate limiting steps on all three electrodes. The enhanced catalysis of Ru/Pt{111} is confirmed to be bi-functional in nature: Ru provides adsorbed hydroxide to Pt allowing for rapid CO oxidation. The majority of Li-ion batteries have anodes consisting of graphite particles with polyvinylidene fluoride (PVDF) as binder. Intercalation of Li into graphite occurs in stages and produces anisotropic strains. As batteries have a fixed size and shape these strains are converted into mechanical stresses. Conventionally staging phenomena has been observed with X-ray diffraction and collaborated electrochemically with the potential. Work herein shows that staging is also clearly observed in stress. The Li staging potentials as measured by differential chronopotentiometry and stress are nearly identical. Relative peak heights of Li staging, as measured by these two techniques, are similar during reduction, but differ during oxidation due to non-linear stress relaxation phenomena. This stress relaxation appears to be due to homogenization of Li within graphite particles rather than viscous flow of the binder. The first Li reduction wave occurs simultaneously with formation of a passivating layer known as the solid electrolyte interphase (SEI). Preliminary experiments have shown the stress of SEI formation to be tensile (~+1.5 MPa).

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2011

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Assessment of global model simulations of present and future climate

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Climate change has been one of the major issues of global economic and social concerns in the past decade. To quantitatively predict global climate change, the Intergovernmental Panel on Climate Change (IPCC) of the United Nations have organized a multi-national

Climate change has been one of the major issues of global economic and social concerns in the past decade. To quantitatively predict global climate change, the Intergovernmental Panel on Climate Change (IPCC) of the United Nations have organized a multi-national effort to use global atmosphere-ocean models to project anthropogenically induced climate changes in the 21st century. The computer simulations performed with those models and archived by the Coupled Model Intercomparison Project - Phase 5 (CMIP5) form the most comprehensive quantitative basis for the prediction of global environmental changes on decadal-to-centennial time scales. While the CMIP5 archives have been widely used for policy making, the inherent biases in the models have not been systematically examined. The main objective of this study is to validate the CMIP5 simulations of the 20th century climate with observations to quantify the biases and uncertainties in state-of-the-art climate models. Specifically, this work focuses on three major features in the atmosphere: the jet streams over the North Pacific and Atlantic Oceans and the low level jet (LLJ) stream over central North America which affects the weather in the United States, and the near-surface wind field over North America which is relevant to energy applications. The errors in the model simulations of those features are systematically quantified and the uncertainties in future predictions are assessed for stakeholders to use in climate applications. Additional atmospheric model simulations are performed to determine the sources of the errors in climate models. The results reject a popular idea that the errors in the sea surface temperature due to an inaccurate ocean circulation contributes to the errors in major atmospheric jet streams.

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2014

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Electron Microscopy Study of the Phase Transformation and Metal Functionalization of Titanium Oxide Nanotubes

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Titanium oxide (TiO2), an abundant material with high photocatalytic activity and chemical stability is an important candidate for photocatalytic applications. The photocatalytic activity of the TiO2 varies with its phase. In the current project, phase and morphology changes in TiO2

Titanium oxide (TiO2), an abundant material with high photocatalytic activity and chemical stability is an important candidate for photocatalytic applications. The photocatalytic activity of the TiO2 varies with its phase. In the current project, phase and morphology changes in TiO2 nanotubes were studied using ex-situ and in-situ transmission electron microscopy (TEM). X-ray diffraction and scanning electron microscopy studies were also performed to understand the phase and morphology of the nanotubes. As prepared TiO2 nanotubes supported on Ti metal substrate were amorphous, during the heat treatment in the ex-situ furnace nanotubes transform to anatase at 450 oC and transformed to rutile when heated to 800 oC. TiO2 nanotubes that were heat treated in an in-situ environmental TEM, transformed to anatase at 400 oC and remain anatase even up to 800 oC. In both ex-situ an in-situ case, the morphology of the nanotubes drastically changed from a continuous tubular structure to aggregates of individual nanoparticles. The difference between the ex-situ an in-situ treatments and their effect on the phase transformation is discussed. Metal doping is one of the effective ways to improve the photocatalytic performance. Several approaches were performed to get metal loading on to the TiO2 nanotubes. Mono-dispersed platinum nanoparticles were deposited on the TiO2 nanopowder and nanotubes using photoreduction method. Photo reduction for Ag and Pt bimetallic nanoparticles were also performed on the TiO2 powders.

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2014