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(1 - 8 of 8)
- Title
- Nanoarchitectured Energy Storage Devices.
- Creator
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Yu, Zenan, Thomas, Jayan, Seal, Sudipta, Zhai, Lei, Fang, Jiyu, Sundaram, Kalpathy, University of Central Florida
- Abstract / Description
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Supercapacitors, the devices that connect the gap between batteries and conventional capacitors, have recently attracted significant attention due to their high specific capacitance, substantially enhanced power and energy densities, and extraordinary cycle life. In order to realize even better performance with supercapacitors, rejuvenated effort towards developing nanostructured electrodes is necessary. In this dissertation, several strategic directions of nanoarchitecturing the electrodes...
Show moreSupercapacitors, the devices that connect the gap between batteries and conventional capacitors, have recently attracted significant attention due to their high specific capacitance, substantially enhanced power and energy densities, and extraordinary cycle life. In order to realize even better performance with supercapacitors, rejuvenated effort towards developing nanostructured electrodes is necessary. In this dissertation, several strategic directions of nanoarchitecturing the electrodes to enhance the performance of supercapacitors are investigated. An introduction and background of supercapacitors, which includes motivation, classification and working principles, recent nanostructured electrode materials studies, and devices fabrication, are initially presented. A facile method, called Spin-on Nanoprinting (SNAP), to fabricate highly ordered manganese dioxide (MnO2) nanopillars is introduced. The SNAP method that is further modified to develop carbon nanoarray electrodes is also discussed. Subsequently, a template-free method to develop high aspect ratio copper oxide nanowhiskers on copper substrate is presented, which boosts the surface area by 1000 times compared to non-nanostructured copper substrate. Electrochemically deposited MnO2 on the nanostructured substrate provided a specific capacitance of about 1379 F g-1 which is very close to the theoretical value (~ 1400 F g-1) due to this efficient nanostructure design. In addition, a novel method to decorate metal nanoparticles on graphene aerogel, which considerably enhances the electronic conductivity and the corresponding specific capacitance, is demonstrated. Moreover, ferric oxide (Fe2O3) nanorods prepared by a simple hydrothermal method is discussed. Asymmetric devices assembled based on Fe2O3 nanorods and MnO2 nanowhiskers show excellent electrochemical properties. The devices not only display the capability to store energy but also transmit electricity through the inner copper core. These two functions are independent and do not interfere with each other. Finally, a summary of this dissertation as well as some potential future directions are presented.
Show less - Date Issued
- 2015
- Identifier
- CFE0006062, ucf:50995
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0006062
- Title
- SCIENCE INQUIRY KITS AND TEACHER PREPAREDNESS TO TEACH SCIENCE AS INQUIRY IN ELEMENTARY CLASSROOMS.
- Creator
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Clayton, Angela, Jeanpierre, Bobby, University of Central Florida
- Abstract / Description
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The National Science Education Standards (1996) indicate that science education should include inquiry instruction. Many teachers still struggle with how to implement inquiry in their classrooms and a lack of high quality inquiry-based instructional materials has been posited as a hindrance. The purpose of this qualitative study was to observe the instructional practices of three elementary teachers when using an inquiry-based science kit program in their fourth grade classrooms. Teacher...
Show moreThe National Science Education Standards (1996) indicate that science education should include inquiry instruction. Many teachers still struggle with how to implement inquiry in their classrooms and a lack of high quality inquiry-based instructional materials has been posited as a hindrance. The purpose of this qualitative study was to observe the instructional practices of three elementary teachers when using an inquiry-based science kit program in their fourth grade classrooms. Teacher practices and their attitudes towards their preparedness to teach science with the support of the curricular program were examined. Data were collected through pre/post survey comparisons, observations, and a focus group session. Results indicated that these teachers' attitudes were positively impacted. Teachers' access to science kits provided resources which facilitated more inquiry experiences with their students; however, resources alone did not fully address teacher science content needs.
Show less - Date Issued
- 2009
- Identifier
- CFE0002568, ucf:52844
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0002568
- Title
- QUANTITATIVE THICKNESS MAPPING IN HIGH-ANGLE ANNULAR DARK-FIELD (HAADF) SCANNING TRANSMISSION ELECTRON MICROSCOPY (STEM).
- Creator
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Nukala, Haritha, Heinrich, Helge, University of Central Florida
- Abstract / Description
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Only a few methods are currently available for the measurement of sample thicknesses in Transmission Electron Microscopy (TEM). These methods, Convergent-Beam Electron Diffraction (CBED) and thickness mapping in Energy-Filtered TEM (EFTEM), are either elaborate or complex. In this present work, I have investigated and come up with a simple straight-forward method to measure the local thickness of a TEM sample with the atomic number (Z-contrast) imaging using High-Angle Annular Dark Field ...
Show moreOnly a few methods are currently available for the measurement of sample thicknesses in Transmission Electron Microscopy (TEM). These methods, Convergent-Beam Electron Diffraction (CBED) and thickness mapping in Energy-Filtered TEM (EFTEM), are either elaborate or complex. In this present work, I have investigated and come up with a simple straight-forward method to measure the local thickness of a TEM sample with the atomic number (Z-contrast) imaging using High-Angle Annular Dark Field (HAADF) Scanning Transmission Electron Microscopy (STEM). HAADF STEM shows atomic number contrast for high scattering angles of the electrons, owing to predominant electron scattering at the potential of the nucleus similar to Rutherford scattering. The characterization of materials by STEM helps to identify microstructures and nanostructures within a sample and to analyze defects in samples. HAADF STEM imaging is capable of resolving atomic structures with better than 2 Å lateral resolution. However, HAADF STEM has so far not been systematically used to measure sample thicknesses. In Z-contrast imaging, it was known that the intensity of the electrons scattered to high angles increases with increase in the atomic number (Z) of the element/compound with increasing thickness of the sample based on the equation, I ~ t.Zα Where t, is the thickness and α, is a parameter between 1 and 2. This project was started with this simple approach, but the experimental results within the thesis show that the relation between the intensity and the atomic number is not well described by this equation. A more reliable parameter, σZ, the interaction coefficient of the material was calculated. Samples containing Ag2Al platelets in Al matrix were used for calibration purposes. Additional samples containing layers of known elements/compounds were obtained from TriQuint Semiconductors and from the Physics department of UCF to calculate σ for various elements/compounds. These experimental values were used to measure the local thicknesses in nanoparticles and also the total volume of the nanoparticles. This quantitative HAADF STEM analysis represents a new method, which can be added to the list of methods used for the purpose of measurement of the local thickness of a sample in the TEM. This method is especially useful for the thickness measurement of nanoparticles. The other two methods, CBED and thickness maps in EFTEM are strongly affected by the sample orientation and therefore not appropriate for the study of nanoparticle thicknesses, whereas orientation effects are negligible for the conditions used in this HAADF STEM analysis.
Show less - Date Issued
- 2008
- Identifier
- CFE0002309, ucf:47859
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0002309
- Title
- METHOD TO DISCRETIZE CONTINUOUS GRADIENT STRUCTURES AND CALCULATE THERMAL RESIDUAL STRESSES WITHIN LAYERED FUNCTIONALLY GRADED CERAMICS.
- Creator
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Neale, Ryan E, Orlovskaya, Nina, University of Central Florida
- Abstract / Description
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Functionally graded materials (FGMs) are an advanced class of material which seeks to leverage the strengths of one material to mitigate the weaknesses of another. This allows for operation in extreme environments or conditions where materials properties must change at various locations within a structure. Fabrication of this advanced class of material is limited due to geometric, economic, and material constraints inherent in the various methods. For this reason, a model was developed to...
Show moreFunctionally graded materials (FGMs) are an advanced class of material which seeks to leverage the strengths of one material to mitigate the weaknesses of another. This allows for operation in extreme environments or conditions where materials properties must change at various locations within a structure. Fabrication of this advanced class of material is limited due to geometric, economic, and material constraints inherent in the various methods. For this reason, a model was developed to discretize continuous gradient curves to allow for the use of a step-wise approximations to such gradients. These alternative step-wise gradients would allow for the use of numerous manufacturing techniques which have improved composition control, cost of processing, cost of equipment, and equipment availability. One such technique, tape casting, was explored due to its robustness and ability to create layered ceramics. Since ceramics are inherently brittle materials, they serve to be strengthened by the thermal residual stresses that form in the creation of these step-wise graded composites. With models to calculate these residual stresses and determine step-wise approximations of various compositional gradients, the process of designing these layered ceramics can be significantly improved.
Show less - Date Issued
- 2019
- Identifier
- CFH2000530, ucf:45633
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFH2000530
- Title
- First Principles Studies of Pattern Formations and Reactions on Catalyst Surfaces.
- Creator
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Le, Duy, Rahman, Talat, Roldan Cuenya, Beatriz, Schelling, Patrick, Sohn, Yongho, University of Central Florida
- Abstract / Description
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This dissertation undertakes theoretical research into the adsorption, pattern formation, and reactions of atoms, molecules, and layered materials on catalyst surfaces. These investigations are carried out from first-principles calculations of electronic and geometric structures using density functional theory (DFT) for predictions and simulations at the atomic scale. The results should be useful for further study of the catalytic activities of materials and for engineering functional...
Show moreThis dissertation undertakes theoretical research into the adsorption, pattern formation, and reactions of atoms, molecules, and layered materials on catalyst surfaces. These investigations are carried out from first-principles calculations of electronic and geometric structures using density functional theory (DFT) for predictions and simulations at the atomic scale. The results should be useful for further study of the catalytic activities of materials and for engineering functional nanostructures.The first part of the dissertation focuses on systematic first-principles simulations of the energetic pathways of CO oxidation on the Cu2O(100) surface. These simulations show CO to oxidize spontaneously on the O-terminated Cu2O(100) surface by consuming surface oxygen atoms. The O-vacancy on Cu2O(100) then is subsequently healed by dissociative adsorption of atmospheric O2 molecules.The second part discusses the pattern formation of hydrogen on two and three layers of Co film grown on the Cu(111) surface. It is found that increasing the pressure of H2 changes the hydrogen structure from 2H-(2 x 2) to H-p(1 x 1) through an intermediate structure of 6H-(3 x 3).The third part compares the results of different ways of introducing van der Waals (vdW) interactions into DFT simulations of the adsorption and pattern formation of various molecules on certain substrates. Examinations of the physisorption of five nucleobases on graphene and of n-alkane on Pt(111) demonstrate the importance of taking vdW interactions into account, and of doing so in a way that is best suited to the particular system in question. More importantly, as the adsorption of 1,4 diaminebenzene molecules on Au(111) shows inclusion of vdW interactions is crucial for accurate simulation of the pattern formation.The final part carries out first-principles calculations of the geometric and electronic structure of the Moire pattern of a single layer of Molybdenum disulfide (MoS2) on Cu(111). The results reveal three possible stacking types. They also demonstrate that the MoS2 layer to be chemisorbed, albeit weakly, and that, while Cu surface atoms are vertically disordered, the layer itself is not strongly buckled.
Show less - Date Issued
- 2012
- Identifier
- CFE0004224, ucf:48991
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0004224
- Title
- Phase Transformations and Microstructural Evolution in the U-10 wt.% Mo Alloy with Various Zr Additions at 900C and 650C.
- Creator
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Eriksson, Nicholas, Sohn, Yongho, Challapalli, Suryanarayana, Coffey, Kevin, University of Central Florida
- Abstract / Description
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The Reduced Enrichment for Research and Test Reactor (RERTR) now known as the Material Minimization and Management Reactor Control program (MMMRC) seeks to replace the use of highly enriched uranium (HEU) fuels used in research and test nuclear reactors around the world. The low enriched uranium (LEU) fuels must have fissionable uranium densities comparable to the HEU fuels. After extensive investigation by various researchers around the world, the U-Mo alloys were selected as a promising...
Show moreThe Reduced Enrichment for Research and Test Reactor (RERTR) now known as the Material Minimization and Management Reactor Control program (MMMRC) seeks to replace the use of highly enriched uranium (HEU) fuels used in research and test nuclear reactors around the world. The low enriched uranium (LEU) fuels must have fissionable uranium densities comparable to the HEU fuels. After extensive investigation by various researchers around the world, the U-Mo alloys were selected as a promising candidate. The Mo alloyed with U allows for the stabilization of the face-centered cubic ?-U phase, which demonstrated favorable irradiation behavior. However, deleterious diffusional interaction between the fuel and the cladding, typically Al-base alloy, remain a challenge to overcome for application of U-Mo alloys as the LEU fuel. Zr has been identified as a potential diffusion barrier between monolithic U-10 wt.% Mo (U10Mo) metallic fuel and AA6061 cladding alloys for the development of a LEU fuel system. However, interdiffusion and reaction between the Zr barrier and U10Mo fuel can produce phases such as Mo2Zr, and promote the destabilization of ?-U phase into ?'-U (U2Mo) and ?-U. In order to better understand this phenomenon, this study examined the phases that are present in the U10Mo alloys with varying Zr concentration, 0, 0.5, 1.0, 2.0, 5.0, 10.0, 20.0 wt.% at room temperature after heat treatment at 900(&)deg;C for 168 hours and 650(&)deg;C for 3 hours. These two temperatures are relevant to fuel plate fabrication process of homogenization and hot-rolling, respectively. Scanning electron microscopy and X-ray diffraction were employed to identify and quantitatively document the constituent phases and microstructure to elucidate the nature of phase transformations. For U10Mo alloys containing less than 1.0 wt.% Zr, there was no significant formation of Mo2Zr after 900?C homogenization and subsequent heat treatment at 650?C for 3 hours. The ?-U phase also remained stable correspondingly for these alloys containing less than 1.0 wt.% Zr. For U10Mo alloys containing 2 wt.% or more Zr, a significant amount of Mo2Zr formation was observed after 900?C homogenization and subsequent heat treatment at 650?C for 3 hours. For these alloys, destabilization of ?-U into ?'-U (U2Mo), UZr2 and ?-U was observed. The alloy containing 20 wt.% Zr, however, did not demonstrate ?-U decomposition even though Mo2Zr was observed after heat treatments. The formation of Mo2Zr effectively reduced the stability of the metastable ?-U phase by depleting the ?-stabilizing Mo. The destabilization of ?-U phase into the ?-U phase is not favorable due to anisotropic and poor irradiation behavior of ?-U phase. Therefore the formation of Mo2Zr at the interface between U10Mo fuel and Zr diffusion barrier must be carefully controlled during the fabrication of monolithic LEU fuel system for successful implementation.
Show less - Date Issued
- 2015
- Identifier
- CFE0005943, ucf:50812
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0005943
- Title
- Theoretical Studies of Nanostructure Formation and Transport on Surfaces.
- Creator
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Aminpour, Maral, Rahman, Talat, Stolbov, Sergey, Roldan Cuenya, Beatriz, Blair, Richard, University of Central Florida
- Abstract / Description
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This dissertation undertakes theoretical and computational research to characterize and understand in detail atomic configurations and electronic structural properties of surfaces and interfaces at the nano-scale, with particular emphasis on identifying the factors that control atomic-scale diffusion and transport properties. The overarching goal is to outline, with examples, a predictive modeling procedure of stable structures of novel materials that, on the one hand, facilitates a better...
Show moreThis dissertation undertakes theoretical and computational research to characterize and understand in detail atomic configurations and electronic structural properties of surfaces and interfaces at the nano-scale, with particular emphasis on identifying the factors that control atomic-scale diffusion and transport properties. The overarching goal is to outline, with examples, a predictive modeling procedure of stable structures of novel materials that, on the one hand, facilitates a better understanding of experimental results, and on the other hand, provide guidelines for future experimental work. The results of this dissertation are useful in future miniaturization of electronic devices, predicting and engineering functional novel nanostructures. A variety of theoretical and computational tools with different degrees of accuracy is used to study problems in different time and length scales. Interactions between the atoms are derived using both ab-initio methods based on Density Functional Theory (DFT), as well as semi-empirical approaches such as those embodied in the Embedded Atom Method (EAM), depending on the scale of the problem at hand. The energetics for a variety of surface phenomena (adsorption, desorption, diffusion, and reactions) are calculated using either DFT or EAM, as feasible. For simulating dynamic processes such as diffusion of ad-atoms on surfaces with dislocations the Molecular Dynamics (MD) method is applied. To calculate vibrational mode frequencies, the infinitesimal displacement method is employed. The combination of non-equilibrium Green's function (NEGF) and DFT is used to calculate electronic transport properties of molecular devices as well as interfaces and junctions.
Show less - Date Issued
- 2013
- Identifier
- CFE0005298, ucf:50504
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0005298
- Title
- Catalytically Enhanced Heterogeneous Combustion of methane.
- Creator
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Terracciano, Anthony, Orlovskaya, Nina, Vasu Sumathi, Subith, Chow, Louis, Kassab, Alain, University of Central Florida
- Abstract / Description
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Heterogeneous combustion is an advanced internal combustion technique, which enables heat recuperation within the flame by utilizing a highly porous ceramic media as a regenerator. Heat released within the gas phase convectively transfers to the solid media. This heat within the solid media then travels towards the inlet, enabling reactant preheating. Such heat redistribution enables stable burning of both ultra-lean fuel/air mixtures, forming a more diffuse flame through the combustion...
Show moreHeterogeneous combustion is an advanced internal combustion technique, which enables heat recuperation within the flame by utilizing a highly porous ceramic media as a regenerator. Heat released within the gas phase convectively transfers to the solid media. This heat within the solid media then travels towards the inlet, enabling reactant preheating. Such heat redistribution enables stable burning of both ultra-lean fuel/air mixtures, forming a more diffuse flame through the combustion chamber, and results in reduced pollutant formation. To further enhance heterogeneous combustion, the ceramic media can be coated with catalytically active materials, which facilitates surface based chemical reactions that could occur in parallel with gas phase reactions.Within this work, a flow stabilized heterogeneous combustor was designed and developed consisting of a reactant delivery nozzle, combustion chamber, and external instrumentation. The reactant delivery nozzle enables the combustor to operate on mixtures of air, liquid fuel, and gaseous fuel. Although this combustor has high fuel flexibility, only gaseous methane was used within the presented experiments. Within the reactant delivery nozzle, reactants flow through a tube mixer, and a homogeneous gaseous mixture is delivered to the combustion chamber. ?-alumina (?-Al2O3), magnesia stabilized zirconia (MgO-ZrO2), or silicon carbide (SiC) was used as the material for the porous media. Measurement techniques which were incorporated in the combustor include an array of axially mounted thermocouples, an external microphone, an external CCD camera, and a gas chromatograph with thermal conductivity detector which enable temperature measurements, acoustic spectroscopy, characterization of thermal radiative emissions, and composition analysis of exhaust gasses, respectively. Before evaluation of the various solid media in the combustion chamber the substrates and catalysts were characterized using X-ray diffraction, X-ray fluorescence, scanning electron microscopy and energy dispersive spectroscopy. MgO-ZrO2 porous media was found to outperform both ?-Al2O3 and SiC matrices, as it was established that higher temperatures for a given equivalence ratio were achieved when the flame was contained within a MgO-ZrO2 matrix. This was explained by the presence of oxygen vacancies within the MgO doped ZrO2 fluorite lattice which facilitated catalytic reactions. Several catalyst compositions were evaluated to promote combustion within a MgO-ZrO2 matrix even further.Catalysts such as: Pd enhanced WC, ZrB2, Ce0.80Gd0.20O1.90, LaCoO3, La0.80Ca0.20CoO3, La0.75Sr0.25Fe0.95Ru0.05O3, and La0.75Sr0.25Cr0.95Ru0.05O3; were evaluated under lean fuel/air mixtures. LaCoO3 outperformed all other catalysts, by enabling the highest temperatures within the combustion chamber, followed by Ce0.80Gd0.20O1.90. Both LaCoO3 and Ce0.80Gd0.20O1.90 enabled a flame to exist at ?=0.45(&)#177;0.02, however LaCoO3 caused the flame to be much more stable. Furthermore, it was discovered that the coating of MgO-ZrO2 with LaCoO3 significantly enhanced the total emissive power of the combustion chamber. In this work as acoustic spectroscopy was used to characterize heterogeneous combustion for the first time. It was found that there is a dependence of acoustic emission n the equivalence ratio and flame position regardless of media and catalyst combination. It was also found that when different catalysts were used, the acoustic tones produced during combustion at fixed reactant flow rates were distinct
Show less - Date Issued
- 2016
- Identifier
- CFE0006508, ucf:51364
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0006508
