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- Title
- Local transient characterization of thermofluid heat transfer coefficient at solid-liquid nano-interfaces.
- Creator
-
Mehrvand, Mehrdad, Putnam, Shawn, Peles, Yoav, Orlovskaya, Nina, Abdolvand, Reza, University of Central Florida
- Abstract / Description
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The demands for increasingly smaller, more capable, and higher power density technologies in microelectronics, energy, or aerospace systems have heightened the need for new methods to manage and characterize extreme heat fluxes (EHF). Microscale liquid cooling techniques are viewed as a promising solution for removing heat from high heat flux (HHF) systems. However, there have been challenges in physical understanding and predicting local thermal transport at the interface of micro and...
Show moreThe demands for increasingly smaller, more capable, and higher power density technologies in microelectronics, energy, or aerospace systems have heightened the need for new methods to manage and characterize extreme heat fluxes (EHF). Microscale liquid cooling techniques are viewed as a promising solution for removing heat from high heat flux (HHF) systems. However, there have been challenges in physical understanding and predicting local thermal transport at the interface of micro and nanoscale structures/devices due to ballistic effects and complex coupling of mass, momentum, and energy transport at the solid-liquid-vapor interfaces over multiple time and length scales. Moreover, it's challenging to experimentally validate new HHF models due to lack of high resolution techniques and measurements.This dissertation presents the use of a high spatiotemporal and temperature resolution measurement technique, called Time-domain Thermoreflectance (TDTR). TDTR is used to characterize the local heat transfer coefficient (HTC) of a water-cooled rectangular microchannel in a combined hot-spot heating and sub-cooled channel-flow configuration. Studies focused on room temperature, syringe-pumped single-and two-phase water flow in a ?480 ?m hydraulic diameter microchannel, where the TDTR pump heating laser induces local heat fluxes of ?0.5-2.5 KW/cm2 in the center of the microchannel on the surface of a 60-80 nm metal or alloy thin film transducer with hot-spot diameters of ?7-10 ?m. In the single-phase part, a differential measurement approach is developed by applying anisotropic version of the TDTR to predict local HTC using the measured voltage ratio parameter, and then fitting data to a thermal model for layered materials and interfaces. It's shown that thermal effusivity distribution of the water coolant over the hot-spot is correlated to the local HTC, where both the stagnant fluid (i.e., conduction and natural convection) and flowing fluid (i.e., forced convection) contributions are decoupled from each other. Measurements of the local enhancement in the HTC over the hot-spot are in good agreement with established Nusselt number correlations. For example, flow cooling results using a Ti metal wall support a maximum HTC enhancement via forced convection of ?1060(&)#177;190 kW/m2?K, where the well-established Nusselt number correlations predict ?900(&)#177;150 kW/m2?K.In the two-phase part, pump-probe beams are first used to construct the local pool and flow boiling curves at different heat fluxes and hot spot temperatures as a function of HTC enhancement. At a same heat flux level, it's observed that fluid flow enhances HTC by shifting heat transfer mechanism (or flow regime) from film boiling to nucleate boiling. Based on observations, it's hypothesized that beyond an EHF flow may reduce the bubble size and increase evaporation at the liquid-vapor interface on three-phase contact line, but it's unable to rewet and cool down the dry spot at the center due to the EHF. In the last part of two-phase experiments, transient measurements are performed at a specific heat flux to obtain thermal temporal fluctuations and HTC of a single bubble boiling and nucleation during its ebullition cycle. The total laser power is chosen to be between the minimum required to start subcooled nucleation and CHF of the pool boiling. This range is critical since within 10% change in heating flux, flow can have dramatic effect on HTC. Whenever the flow gets closer to the dry spot and passes through it (receding or advancing) HTC increases suddenly. This means that for very hot surfaces (or regions of wall dry-out), continuous and small bubbles on the order of thermal diffusion time and dry spot length scales respectively could be a reliable high heat flux cooling solution. This could be achieved by controlling the bubble size and frequency through geometry, surface structure and properties, and fluid's thermos-fluid properties.
Show less - Date Issued
- 2017
- Identifier
- CFE0006765, ucf:51832
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0006765
- Title
- Manufacturing of Single Solid Oxide Fuel Cells.
- Creator
-
Torres-Caceres, Jonathan, Orlovskaya, Nina, Xu, Yunjun, Das, Tuhin, University of Central Florida
- Abstract / Description
-
Solid oxide fuel cells (SOFCs) are devices that convert chemical energy into electrical energy and have the potential to become a reliable renewable energy source that can be used on a large scale. SOFCs have 3 main components; the electrolyte, the anode, and the cathode. Typically, SOFCs work by reducing oxygen at the cathode into O2- ions which are then transported via the electrolyte to the anode to combine with a fuel such as hydrogen to produce electricity. Research into better materials...
Show moreSolid oxide fuel cells (SOFCs) are devices that convert chemical energy into electrical energy and have the potential to become a reliable renewable energy source that can be used on a large scale. SOFCs have 3 main components; the electrolyte, the anode, and the cathode. Typically, SOFCs work by reducing oxygen at the cathode into O2- ions which are then transported via the electrolyte to the anode to combine with a fuel such as hydrogen to produce electricity. Research into better materials and manufacturing methods is necessary to reduce costs and improve efficiency to make the technology commercially viable.The goal of the research is to optimize and simplify the production of single SOFCs using high performance ceramics. This includes the use of 8mol% Y2O3-ZrO2 (YSZ) and 10mol% Sc2O3-1mol%CeO2-ZrO2 (SCSZ) layered electrolytes which purport higher conductivity than traditional pure YSZ electrolytes. Prior to printing the electrodes onto the electrolyte, the cathode side of the electrolyte was coated with 20mol% Gd2O3-CeO2 (GDC). The GDC coating prevents the formation of a nonconductive La2Zr2O7 pyrochlore layer, which forms due to the interdiffusion of the YSZ electrolyte ceramic and the (La0.6Sr0.4)0.995Fe0.8Co0.2O3 (LSCF) cathode ceramic during sintering. The GDC layer was deposited by spin coating a suspension of 10wt% GDC in ethanol onto the electrolyte. Variation of parameters such as time, speed, and ramp rate were tested. Deposition of the electrodes onto the electrolyte surface was done by screen printing. Ink was produced using a three roll mill from a mixture of ceramic electrode powder, terpineol, and a pore former. The pore former was selected based on its ability to form a uniform well-connected pore matrix within the anode samples that were pressed and sintered. Ink development involved the production of different ratios of powder-to-terpineol inks to vary the viscosity. The different inks were used to print electrodes onto the electrolytes to gauge print quality and consistency. Cells were produced with varying numbers of layers of prints to achieve a desirable thickness. Finally, the densification behaviors of the major materials used to produce the single cells were studied to determine the temperatures at which each component needs to be sintered to achieve the desired density and to determine the order of electrode application, so as to avoid over-densification of the electrodes. Complete cells were tested at the National Energy Technology Laboratory in Morgantown, WV. Cells were tested in a custom-built test stand under constant voltage at 800(&)deg;C with 3% humidified hydrogen as the fuel. Both voltage-current response and impedance spectroscopy tests were conducted after initial startup and after 20 hours of operation. Impedance tests were performed at open circuit voltage and under varying loads in order to analyze the sources of resistance within the cell. A general increase in impedance was found after the 20h operation. Scanning electron micrographs of the cell microstructures found delamination and other defects which reduce performance. Suggestions for eradicating these issues and improving performance have been made.
Show less - Date Issued
- 2013
- Identifier
- CFE0004946, ucf:49641
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0004946
- Title
- Mechanical Properties and Thermal Residual Stresses of ZrB2-SiC Ceramic Composites for Hypersonic Vehicle Applications.
- Creator
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Stadelmann, Richard, Orlovskaya, Nina, Kumar, Ranganathan, Raghavan, Seetha, University of Central Florida
- Abstract / Description
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Ultra High Temperature Ceramic (UHTC) ZrB2- 10, 20, 30wt%SiC composites are of high interest for use in hypersonic air-breathing vehicles. In this work, ZrB2- 10, 20, 30wt%SiC UHTC composites were produced by the Spark Plasma Sintering (SPS) technique. After sintering, almost dense ceramics with ~ 5-8% porosity were produced. Their mechanical properties, such as Young's, shear, and bulk moduli, along with Poisson's ratio, 4-point bending strength, and single edge V-notched beam (SEVNB)...
Show moreUltra High Temperature Ceramic (UHTC) ZrB2- 10, 20, 30wt%SiC composites are of high interest for use in hypersonic air-breathing vehicles. In this work, ZrB2- 10, 20, 30wt%SiC UHTC composites were produced by the Spark Plasma Sintering (SPS) technique. After sintering, almost dense ceramics with ~ 5-8% porosity were produced. Their mechanical properties, such as Young's, shear, and bulk moduli, along with Poisson's ratio, 4-point bending strength, and single edge V-notched beam (SEVNB) fracture toughness were measured. In addition, in-situ bending experiments under a Raman microscope were performed to determine the piezo-spectroscopic coefficients of SiC Raman active peaks for calculation of thermal residual stresses. The results show that these materials are possible candidates for hypersonic air-breathing vehicles due to their high Young's modulus, ability to withstand high temperatures, and relatively low densities.
Show less - Date Issued
- 2013
- Identifier
- CFE0005060, ucf:49945
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0005060
- Title
- Piezospectroscopic Calibration of Alumina-Nanocomposites for the Development of Stress-Sensing Structures.
- Creator
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Fugon-Dessources, Daniela, Raghavan, Seetha, Gou, Jihua, Orlovskaya, Nina, University of Central Florida
- Abstract / Description
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Alpha-alumina is known to exhibit photo-luminescent (PL) properties, mainly characteristic R-lines that shift according to applied stress. In addition to showing excellent PL properties, polymers with embedded alumina nanoparticles have been shown to improve the overall composite mechanical properties. While the use of the PL properties to develop stress-sensing materials using an alumina-epoxy material has been success- fully shown in compression, the properties have not been developed for...
Show moreAlpha-alumina is known to exhibit photo-luminescent (PL) properties, mainly characteristic R-lines that shift according to applied stress. In addition to showing excellent PL properties, polymers with embedded alumina nanoparticles have been shown to improve the overall composite mechanical properties. While the use of the PL properties to develop stress-sensing materials using an alumina-epoxy material has been success- fully shown in compression, the properties have not been developed for tension. In this study, the PL response of variable volume fraction alumina-epoxy composites will be determined under tensile conditions. It is expected that increasing the volume fraction of alumina nanoparticles will increase the sensitivity of the particles PL emission shift to applied stress. Three tensile alumina-epoxy specimens of 21.0%, 31.2%, and 34.5% volume fractions were manufactured and tested under tensile static loads. The results of this experiment will determine the piezospectroscopic (PS) coefficient and calibration of bulk alumina nanocomposites in tension. A linear region was identified in the PS response of the nanocomposite to the applied tensile load. The PS coefficient of this linear region increased as the volume fraction of the nanocomposite increased. To demonstrate the application of structural composites with stress sensing capabilities, alumina nanoparticles were integrated in the manufacturing of a carbon fiber composite specimen. The results of the stress-sensing composite mechanical experiment showed that alumina nanoparticles were able to detect changes in stress. The results for both the bulk nanocomposite calibrations and the application of stress-sensing alumina nanoparticles in a carbon-fiber composite will advance the development of this novel stress-sensing method.
Show less - Date Issued
- 2014
- Identifier
- CFE0005168, ucf:50661
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0005168
- Title
- Mechanical Properties of Brittle Ceramics: Case Study of Boron Rich Ceramics and Acropora cervicornis Coral Skeleton.
- Creator
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Carrasco-Pena, Alejandro, Kwok, Kawai, Orlovskaya, Nina, Gou, Jihua, Uribe Romo, Fernando, University of Central Florida
- Abstract / Description
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Ceramics are ubiquitous in man-made and natural structures. Their mechanical properties highly depend on their composition, microstructure and level of defects in the bulk of the material, the latter affecting the integrity of the components; such is the case of boron-rich ceramics where large agglomerates create high stressed regions, or coral skeleton where porosity determines their strength against hydrodynamic forces present in the ocean tides. Therefore, studying the properties of...
Show moreCeramics are ubiquitous in man-made and natural structures. Their mechanical properties highly depend on their composition, microstructure and level of defects in the bulk of the material, the latter affecting the integrity of the components; such is the case of boron-rich ceramics where large agglomerates create high stressed regions, or coral skeleton where porosity determines their strength against hydrodynamic forces present in the ocean tides. Therefore, studying the properties of ceramic materials using invasive and non-invasive methods helps in the understanding of the link between the properties and the performance of the structures. The aim of this research was to test the novel ceramic component ZrB2-30wt%SiB6 and Acropora cervicornis coral skeleton using non-conventional techniques that allow for the study of their mechanical properties and their behavior when exposed to external loads present in their environments of application. The first part of this study focuses on understanding the effects of adding SiB6 to enhance the mechanical properties of ZrB2 ceramics for their ultra-high temperature use. The second part will emphasize in the behavior of Acropora cervicornis coral skeleton when exposed to compressive forces and the effects porosity has on this structure when subjected to such loads. It was found that the SiB6 phase was not stable after sintering of the composite and large agglomerates were present in the surface of the material acting as stress concentrators, thus compromising the biaxial strength of the component that resulted to be 224.9 MPa. It was also found that coral skeletons are highly susceptible to porosity which creates variability on the elastic modulus ranging from 60-1 GPa for simulated porosity of 0-90% respectively and a strength of 3.56 (&)#177; 0.31 GPa obtained through Vickers indentation. Finite element models were developed and validated against experimental results for the ZrB2-30wt%SiB6 and Acropora cervicornis coral skeleton.
Show less - Date Issued
- 2019
- Identifier
- CFE0007440, ucf:52696
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0007440
- Title
- Simultaneous Measurement of Isotope-free Tracer and Interdiffusion Coefficients in Sandwich Type Diffusion Couples.
- Creator
-
Schulz, Esin, Sohn, Yongho, Bai, Yuanli, Florczyk, Stephen, Coffey, Kevin, Orlovskaya, Nina, University of Central Florida
- Abstract / Description
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The experimental determination of the tracer or self-diffusion coef?cient as a function of composition can be quite burdensome in alloys since separate measurements must be carried out for each alloy composition. A new formalism recently developed by I.V. Belova, N.S. Kulkarni, Y.H. Sohn and G.E. Murch, based on linear response theory combined with the Boltzmann(-)Matano method allows determination of tracer and interdiffusion coef?cients simultaneously from a single, isotope-free solid to...
Show moreThe experimental determination of the tracer or self-diffusion coef?cient as a function of composition can be quite burdensome in alloys since separate measurements must be carried out for each alloy composition. A new formalism recently developed by I.V. Belova, N.S. Kulkarni, Y.H. Sohn and G.E. Murch, based on linear response theory combined with the Boltzmann(-)Matano method allows determination of tracer and interdiffusion coef?cients simultaneously from a single, isotope-free solid to solid diffusion couple experiment. In this study, for the first time, an experimental investigation with an analytical approach based on the new formalism has been carried out in the binary Cu-Ni system. Pure Cu and Ni thin films were deposited in between several binary diffusion couples with varying terminal alloy compositions (such as Cu, Cu-25Ni, Cu-50Ni, Cu-75Ni, Ni). Diffusion couples were then annealed at 800(&)deg;C, 900(&)deg;C and 1000(&)deg;C. After annealing, the couples were water quenched, cross-sectioned, and prepared for compositional characterization. Scanning Electron Microscopy was employed to examine the interdiffusion zone. Energy Dispersive X-ray Spectroscopy was conducted to obtain concentration profiles for quantitative analysis. The superposition of the concentration profiles of thin film and interdiffusion were analyzed for the simultaneous determination of tracer and interdiffusion coefficients. The tracer diffusion coefficient of Cu, tracer diffusion coefficient of Ni and inter-diffusion coefficients simultaneously determined using the experimental methodology based on the novel formalism derived, and produced results consistent with previously reported values determined independently by radiotracer and interdiffusion experiments.
Show less - Date Issued
- 2018
- Identifier
- CFE0007368, ucf:52101
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0007368
- Title
- Processing, Characterization and Performance of Carbon Nanopaper Based Multifunctional Nanocomposites.
- Creator
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Liang, Fei, Gou, Jihua, Su, Ming, Fang, Jiyu, Orlovskaya, Nina, Xu, Yunjun, University of Central Florida
- Abstract / Description
-
Carbon nanofibers (CNFs) used as nano-scale reinforcement have been extensively studied since they are capable of improving the physical and mechanical properties of conventional fiber reinforced polymer composites. However, the properties of CNFs are far away from being fully utilized in the composites due to processing challenges including the dispersion of CNFs and the viscosity increase of polymer matrix. To overcome these issues, a unique approach was developed by making carbon nanopaper...
Show moreCarbon nanofibers (CNFs) used as nano-scale reinforcement have been extensively studied since they are capable of improving the physical and mechanical properties of conventional fiber reinforced polymer composites. However, the properties of CNFs are far away from being fully utilized in the composites due to processing challenges including the dispersion of CNFs and the viscosity increase of polymer matrix. To overcome these issues, a unique approach was developed by making carbon nanopaper sheet through the filtration of well-dispersed carbon nanofibers under controlled processing conditions, and integrating carbon nanopaper sheets into composite laminates using autoclave process and resin transfer molding (RTM). This research aims to fundamentally study the processing-structure-property-performance relationship of carbon nanopaper-based nanocomposites multifunctional applications: a) Vibrational damping. Carbon nanofibers with extremely high aspect ratios and low density present an ideal candidate as vibrational damping material; specifically, the large specific area and aspect ratio of carbon nanofibers promote significant interfacial friction between carbon nanofiber and polymer matrix, causing higher energy dissipation in the matrix. Polymer composites with the reinforcement of carbon nanofibers in the form of a paper sheet have shown significant vibration damping improvement with a damping ratio increase of 300% in the nanocomposites. b) Wear resistance. In response to the observed increase in toughness of the nanocomposites, tribological properties of the nanocomposite coated with carbon nanofiber/ceramic particles hybrid paper have been studied. Due to high strength and toughness, carbon nanofibers can act as microcrack reducer; additionally, the composites coated with such hybrid nanopaper of carbon nanofiber and ceramic particles shown an improvement of reducing coefficient of friction (COF) and wear rate. c) High electrical conductivity. A highly conductive coating material was developed and applied on the surface of the composites for the electromagnetic interference shielding and lightning strike protection. To increase the conductivity of the carbon nanofiber paper, carbon nanofibers were modified with nickel nanostrands. d) Electrical actuation of SMP composites. Compared with other methods of SMP actuation, the use of electricity to induce the shape-memory effect of SMP is desirable due to the controllability and effectiveness. The electrical conductivity of carbon fiber reinforced SMP composites can be significantly improved by incorporating CNFs and CNF paper into them. A vision-based system was designed to control the deflection angle of SMP composites to desired values. The funding support from National Science Foundation and FAA Center of Excellence for Commercial Space Transportation (FAA COE CST) is acknowledged.
Show less - Date Issued
- 2012
- Identifier
- CFE0004569, ucf:49194
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0004569
- Title
- Super-adiabatic combustion in porous media with catalytic enhancement for thermoelectric power conversion.
- Creator
-
Mueller, Kyle, Orlovskaya, Nina, Chen, Ruey-Hung, Kapat, Jayanta, University of Central Florida
- Abstract / Description
-
The combustion of ultra-lean fuel to air mixtures provides an efficient way to convert the chemical energy of hydrocarbons into useful power. Conventional burning techniques of a mixture have defined flammability limits beyond which a flame cannot self-propagate due to heat losses. Matrix stabilized porous medium combustion is an advanced technique in which a solid porous matrix within the combustion chamber accumulates heat from the hot gaseous products and preheats incoming reactants. This...
Show moreThe combustion of ultra-lean fuel to air mixtures provides an efficient way to convert the chemical energy of hydrocarbons into useful power. Conventional burning techniques of a mixture have defined flammability limits beyond which a flame cannot self-propagate due to heat losses. Matrix stabilized porous medium combustion is an advanced technique in which a solid porous matrix within the combustion chamber accumulates heat from the hot gaseous products and preheats incoming reactants. This heat recirculation extends the standard flammability limits and allows the burning of ultra-lean fuel mixtures, conserving energy resources, or the burning of gases of low calorific value, utilizing otherwise wasted resources. The heat generated by the porous burner can be harvested with thermoelectric devices for a reliable method of generating electricity for portable electronic devices by the burning of otherwise noncombustible mixtures.The design of the porous media burner, its assembly and testing are presented. Highly porous (~80% porosity) alumina foam was used as the central media and alumina honeycomb structure was used as an inlet for fuel and an outlet for products of the methane-air combustion. The upstream and downstream honeycomb structures were designed with pore sizes smaller than the flame quenching distance, preventing the flame from propagating outside of the central section. Experimental results include measurements from thermocouples distributed throughout the burner and on each side of the thermoelectric module along with associated current, voltage and power outputs. Measurements of the burner with catalytic coating were obtained for stoichiometric and lean mixtures and compared to the results obtained from the catalytically inert matrix, showing the effect on overall efficiency for the combustion of fuel-lean mixtures.
Show less - Date Issued
- 2011
- Identifier
- CFE0004142, ucf:49043
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0004142
- Title
- Re, Os, Al and Mg boron rich ceramic compounds for structural application.
- Creator
-
Xie, Zhilin, Orlovskaya, Nina, Blair, Richard, Gou, Jihua, Raghavan, Seetha, University of Central Florida
- Abstract / Description
-
Hard and ultra-incompressible materials are of great interest due to their important applications in industry. A common route to design hard materials is combining transition metals with light and small covalent elements. Light elements such as carbon, oxygen, nitrogen and boron have been considered as good candidates. This study includes the synthesis of ReB2, OsB2 and another higher boride AlMgB14.Most of the techniques used for ReB2 synthesis reported 1:2.5 Re to B ratio because of the...
Show moreHard and ultra-incompressible materials are of great interest due to their important applications in industry. A common route to design hard materials is combining transition metals with light and small covalent elements. Light elements such as carbon, oxygen, nitrogen and boron have been considered as good candidates. This study includes the synthesis of ReB2, OsB2 and another higher boride AlMgB14.Most of the techniques used for ReB2 synthesis reported 1:2.5 Re to B ratio because of the loss of the B during high temperature synthesis. However, as a result of B excess, the amorphous boron, located along the grain boundaries of polycrystalline ReB2, would degrade the ReB2 properties. Therefore, techniques which could allow synthesizing the stoichiometric ReB2 preferably at room temperature are in high demand. This thesis reported the synthesis of ReB2 powders using mechanochemical route by milling elemental crystalline Re and amorphous B powders in the SPEX 8000 high energy ball mill for 80 hours. The formation of boron and perrhenic acids are also reported after ReB2 powder was exposed to the moist air environment for a twelve months period of time. The synthesized ReB2 powder was characterized by X-ray diffraction, scanning electron microscope, transmission electron microscope, secondary ion mass spectrometry and Raman spectroscopy. OsB2 also shows its attractive properties. The hardness of orthorhombic OsB2 was reported to be 37 GPa, when the applied load is lowered to 0.245N. However, only one of the three predicted phases has been synthesized. In this study, the hexagonal OsB2 has been synthesized by the mechanochemical method. The lattice parameters of the Hex-OsB2 are ?=?=90(&)deg;, ?=120(&)deg;; a=b=2.9047 (&)#197;, c=7.4500 (&)#197;. The synthesized OsB2 powder was annealed at 1050(&)deg;C for 6 days, but no phase change was found. This shows that the Hex-OsB2 is very stable. Another promising hard material, AlMgB14, was also studied in this thesis. The AlMgB14 was synthesized at 1050 (&)deg;C under normal pressure. Several different routes were tried and compared. It shows AlMgB14 cannot be synthesized merely by ball milling, which can bring the risk of oxidization. Magnesium metal is preferred to use as one of the raw materials.
Show less - Date Issued
- 2012
- Identifier
- CFE0004261, ucf:49500
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0004261
- Title
- Rhenium, osmium and iridium diborides by mechanochemistry: Synthesis, structure, thermal stability and mechanical properties.
- Creator
-
Xie, Zhilin, Orlovskaya, Nina, Blair, Richard, Gou, Jihua, Raghavan, Seetha, An, Linan, University of Central Florida
- Abstract / Description
-
Borides are implemented in a range of industrial applications due to their unique mechanical, electrical, thermal and catalytic properties. In particular, transition metal diborides are of special interest. In the recent years, borides of rhenium (Re), osmium (Os) and iridium (Ir) have been studied as for their ultra-hardness and superior stiffness. In this dissertation, a mechanochemical method is introduced to produce rhenium diboride (ReB2) powder, a novel hexagonal osmium diboride (h-OsB2...
Show moreBorides are implemented in a range of industrial applications due to their unique mechanical, electrical, thermal and catalytic properties. In particular, transition metal diborides are of special interest. In the recent years, borides of rhenium (Re), osmium (Os) and iridium (Ir) have been studied as for their ultra-hardness and superior stiffness. In this dissertation, a mechanochemical method is introduced to produce rhenium diboride (ReB2) powder, a novel hexagonal osmium diboride (h-OsB2), and iridium boride powders. Densification by Spark Plasma Sintering (SPS), thermal stability and mechanical properties of h-OsB2 were also studied. ReB2 was recently reported to exhibit high hardness and low compressibility, which both are strong functions of its stoichiometry, namely Re to B ratio. Most of the techniques used for ReB2 synthesis reported 1:2.5 Re to B ratio because of the loss of the B during high temperature synthesis. However, as a result of B excess, the amorphous boron, located along the grain boundaries of polycrystalline ReB2, would degrade the ReB2 properties. Therefore, techniques which could allow synthesizing the stoichiometric ReB2 preferably at room temperature are in high demand. ReB2 powder was synthesized at low temperature using mechanochemical route by milling elemental crystalline Re and amorphous B powders in the SPEX 8000 high energy ball mill for 80 hours. The formation of boron and perrhenic acids are also reported after ReB2 powder was exposed to the moist air environment for a twelve month period of time.Hexagonal osmium diboride (h-OsB2), a theoretically predicted high-pressure phase, has been synthesized for the first time by a mechanochemical method, i.e., high energy ball milling. X-ray diffraction (XRD) indicated the formation of h-OsB2 after 2.5 hours of milling, and the reaction reaches equilibrium after 18 hours of milling. The lattice parameters of the h-OsB2 are a=2.916(&)#197; and c=7.376 (&)#197;, with a P63/mmc space group. Transmission electron microscopy confirmed the appearance of the h-OsB2 phase. The thermal stability of h-OsB2 powder was studied by heating under argon up to 876 (&)deg;C and cooling in vacuo down to -225 (&)deg;C. The oxidation mechanism of h-OsB2 has also been proposed. The hexagonal phase partially converted to the orthorhombic phase (20 wt.%) after spark plasma sintering of h-OsB2 at 1500(&)deg;C and 50MPa for 5 minutes. Hardness and Young's modulus of the h-OsB2 were measured to be 31 (&)#177; 9 GPa and 574 (&)#177; 112 GPa, respectively by nanoindentation method.Prior to this research a number of compounds have been prepared in Ir-B system with lower than 2 boron stoichiometry, and no IrB2 phases have been synthesized experimentally. In this dissertation, three new iridium boride phases, ReB2-type IrB2, AlB2-type IrB2 and IrB have been synthesized with a similar mechanochemical method. The formation of these three phases has been confirmed by both X-ray diffraction (XRD) and transmission electron microscope (TEM) after 30 hours of ball milling and 48 hours of annealing. The IrB2 phases have hexagonal crystal structures and the new IrB phase has an orthorhombic crystal structure. The segregation of iridium from iridium borides' lattices has also been studied by high resolution TEM.
Show less - Date Issued
- 2014
- Identifier
- CFE0005911, ucf:50832
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0005911
- Title
- Catalytically Enhanced Heterogeneous Combustion of methane.
- Creator
-
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
- Title
- Electrical properties of polymer-derived silicoaluminum carboxide ceramics and their applications in micro-sensors.
- Creator
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Cao, Yejie, An, Linan, Fang, Jiyu, Zhai, Lei, Chen, Quanfang, Orlovskaya, Nina, University of Central Florida
- Abstract / Description
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Polymer-derived ceramic (PDC) is a new kind of material which is directly synthesized by the thermal decomposition of polymer precursors. Due to their unique structure, which consists of the amorphous matrix phase and free-carbon phase, PDCs exhibit many distinguished properties even at high-temperature environment such as oxidation and creep resistance, amorphous semiconducting behavior as well as piezoresistive behavior. These outstanding properties make PDCs become promising candidates for...
Show morePolymer-derived ceramic (PDC) is a new kind of material which is directly synthesized by the thermal decomposition of polymer precursors. Due to their unique structure, which consists of the amorphous matrix phase and free-carbon phase, PDCs exhibit many distinguished properties even at high-temperature environment such as oxidation and creep resistance, amorphous semiconducting behavior as well as piezoresistive behavior. These outstanding properties make PDCs become promising candidates for various applications especially for high-temperature microsensors. However, most common used PDCs in the market now are SiC, SiCN and Si(M)CN ceramics, the high price and toxicity of their raw materials as well as strict operating requirements limit their applications. SiCO ceramics are appealing increasing attentions because they can cover these shortcomings of non-oxide ceramics, but their oxidation and corrosion resistance is so weak. In this dissertation, SiAlCO ceramics are chosen as main material. The addition of Al can improve the oxidation and corrosion resistance of SiCO ceramics. In this dissertation, the SiAlCO ceramics are synthesized by using silicone resin and aluminum tri-sec-butoxide (ATSB), then ceramic samples are obtained by pyrolyzing disk green bodies at 1000, 1100, 1200, 1300, 1400?C. Firstly, the composition, microstructure and structure evolution of SiAlCO ceramics are characterized via X-Ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and Impedance spectroscopy (IS). The results indicate all ceramic samples pyrolyzed below 1400?C are amorphous and a sudden structure change point around 1100?C is observed due to the increase of degree of ordering. Si-C, Si-O, C-C/H, and C=C bonds are observed within the materials.Secondly, the room-temperature and temperature-dependent conductivity of the SiAlCO ceramics are studied. The optical absorption spectra are also measured. The conductivity increases by ~6 orders of magnitude when pyrolysis temperature increases from 1000 to 1400?C. A very high activation energy of 7.15eV is observed, and the redistribution of oxygen within the material is found to be responsible for it. Amorphous semiconductor behavior which follows the band-tail hopping (BTH) process is observed within this material. And the BTH process is resulted from unique electronic structures of the materials.Thirdly, SiAlCO ceramic exhibits extraordinary piezoresistive behavior with an extremely high gauge factor in range of 7000 ~16000, which is higher than that of any previously reported high-temperature materials. The coupling effect of pressure and temperature on the piezoresistive behavior is also studied. The piezoresistive stress coefficient increases with increasing temperature, which is contradictive to other reported materials. Such change of the piezoresistive stress coefficient is due to the change in the characteristic temperature, which is reversely related to the density of state within the band-tail level.In addition, SiAlCO also shows anomalous piezo-dielectricity with the positive pressure coefficient of the dielectric constant as high as 0.10-0.25 MPa-1, which is much higher than that of other high-temperature materials. The polarizability of the material also increases with increasing pressure. There behaviors are attributed to the unique cell-like structure of the materials.In the end, a pressure sensor is successfully developed. A supportive circuit is designed and the relationships among pressure, resistance and output voltage of the system are tested. The sensitivity of the sensor is calculated to be ?1 V=?15.125 Pa, indicating the SiAlCO ceramics are promising candidates for pressure sensor materials.
Show less - Date Issued
- 2016
- Identifier
- CFE0006275, ucf:51052
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0006275
- Title
- Interdiffusion, Crystallography and Mechanical Properties of Nickel Manganese Gallium Alloys.
- Creator
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Zhou, Le, Sohn, Yongho, Heinrich, Helge, Coffey, Kevin, An, Linan, Orlovskaya, Nina, University of Central Florida
- Abstract / Description
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NiMnGa Heusler alloys, functioning as either ferromagnetic shape memory alloys or mangetocaloric materials, have both practical applications and fundamental research value. The functional properties of NiMnGa alloys are closely related to the martensitic transformation from high temperature austenitic phase to low temperature martensitic phase. Alloys can be used for room temperature or high temperature applications, depending on the martensitic transformation temperature, which is...
Show moreNiMnGa Heusler alloys, functioning as either ferromagnetic shape memory alloys or mangetocaloric materials, have both practical applications and fundamental research value. The functional properties of NiMnGa alloys are closely related to the martensitic transformation from high temperature austenitic phase to low temperature martensitic phase. Alloys can be used for room temperature or high temperature applications, depending on the martensitic transformation temperature, which is compositional sensitive. The microstructure and crystallography of the martensites can be very complex but are crucial to the optimization of the material performance. In this study, for the first time, a combinatorial study by combining solid-to-solid diffusion couples and various characterization techniques was carried out to fundamentally investigate the NiMnGa ternary alloys. Phase equilibria, interdiffusion behavior, microstructural and crystallographic development, and mechanical properties in NiMnGa alloys were systematically examined. Selected diffusion couples between pure Ni, Ni25Mn75 and four ternary off-stoichiometric NiMnGa alloys (i.e., Ni52Mn18Ga30, Ni46Mn30Ga24, Ni52Mn30Ga18, Ni58Mn18Ga24 in atomic percent) were assembled and annealed at 800, 850 and 900 (&)deg;C for 480, 240 and 120 hours, respectively. The microstructure and concentration profiles of the interdiffusion zone were examined by scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS). Concentration profiles across the interdiffusion zone were further quantified by electron probe micro analysis (EPMA). Detailed microstructure and crystallography of the austenite and martensite were investigated using transmission electron microscopy (TEM). TEM thin foils were prepared by using focused ion beam (FIB) in situ lift out (INLO) technique, which is able to select desired composition from diffusion couples. The mechanical properties, namely reduced elastic modulus and hardness, as a function of composition were assessed via nanoindentation. Solubility values obtained for various phases were mostly consistent with the existing isothermal phase diagrams, but the phase boundary of the ?(Mn) + ? two-phase region was slightly modified. In addition, equilibrium compositions for the ?(Ni) and ?' phases at 900 (&)deg;C were also determined for the respective two-phase regions. Both austenitic and martensitic phases were found at room temperature in each diffusion couple with a clear interphase boundary. The compositions at the interfaces corresponded close to valence electron concentration (e/a) of 7.6, but decreased to lower values when Mn concentration increased to more than 35 at. %. Average effective interdiffusion coefficients for the ? phase over various compositional ranges were determined and reported in the light of temperature-dependence. Ternary interdiffusion coefficients were also determined and examined to assess the ternary diffusional interactions among Ni, Mn and Ga. Ni was observed to interdiffuse the fastest, followed by Mn then Ga. Interdiffusion flux of Ni also has strong influences on the interdiffusion of Mn and Ga with large and negative cross interdiffusion coefficients. The main ternary interdiffusion coefficients exhibited minimum values near 52 at. % Ni concentration. Extensive TEM analyses have been performed for the study of microstructure and crystallography of austenite and martensite from all diffusion couples. Crystallographic variations in martensitic phase, including non-modulated (NM) martensite, modulated (5M or 7M) martensite, were found in the diffusion couples. The 5M and 7M martensites were only found near the interface between austenite and martensite, corresponding to compositions with lower e/a ratio. The NM martensites were found mostly away from the interface region, with high e/a ratios. The tetragonality ratio (c/a) for NM martensite generally increases with e/a ratio, but also depended on the composition. All martensitic microstructure consists of twinned variants with different orientations that were documented using electron diffraction. The twinning relationship along with the c/a ratio was correlated to martensitic transformation temperature. In addition, pre-martensitic state has been clearly observed in the cubic austenitic phase region, with distinctive tweed microstructure originating from the local lattice distortions. Mechanical properties including reduced elastic modulus (Er) and hardness (H) as a function of composition were measured and analyzed by nanoindentation. A decrease of Er and H was observed with Mn or Ni substituting Ga, and Ni substituting Mn for the austenitic phase. However, an opposite trend was found for the martensitic phase. The softening of the elastic constants near the vicinity of martensitic transformation contributed to the sharp decrease in Er and H near the interface region. The measured Er and H had larger scatter for the martensitic phase than those for the austenitic phase. The scatters observed were attributed to the martensitic variants with different orientations. Contribution from the variation in grain orientation or shape memory effect was determined to be small in this investigation.
Show less - Date Issued
- 2016
- Identifier
- CFE0006204, ucf:51108
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0006204
- Title
- Scandia and ceria stabilized zirconia based electrolytes and anodes for intermediate temperature solid oxide fuel cells: Manufacturing and properties.
- Creator
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Chen, Yan, Orlovskaya, Nina, An, Linan, Chen, Quanfang, Sohn, Yongho, Raghavan, Seetha, Huang, Xinyu, University of Central Florida
- Abstract / Description
-
Scandia and ceria stabilized zirconia (10 mol% Sc2O3 (-) 1 mol% CeO2 (-) ZrO2, SCSZ) has superior ionic conductivity in the intermediate temperature range for the operation of solid oxide fuel cells, but it does not exhibit good phase stability in comparison with yttria stabilized zirconia (8 mol% Y2O3 (-) ZrO2, YSZ). To maintain high ionic conductivity and improve the stability of the electrolyte, layered structures with YSZ outer layers and SCSZ inner layers were designed, along with the...
Show moreScandia and ceria stabilized zirconia (10 mol% Sc2O3 (-) 1 mol% CeO2 (-) ZrO2, SCSZ) has superior ionic conductivity in the intermediate temperature range for the operation of solid oxide fuel cells, but it does not exhibit good phase stability in comparison with yttria stabilized zirconia (8 mol% Y2O3 (-) ZrO2, YSZ). To maintain high ionic conductivity and improve the stability of the electrolyte, layered structures with YSZ outer layers and SCSZ inner layers were designed, along with the referential electrolytes containing pure SCSZ or YSZ. The electrolytes were manufactured by tape casting, laminating, and pressureless sintering techniques. After sintering, while the thickness of YSZ outer layers remained constant at ~30 ?m, the thickness of inner layers of SCSZ for the 3-, 4- and 6-layer designs varied at ~30, ~60 and ~120 ?m, respectively. Selected characterizations were employed to study the structure, morphology, impurity content and the density of the electrolytes. Furthermore, in situ X-ray diffraction, neutron diffraction and Raman scattering were carried out to study the phase transition and lattice distortion during long-term annealing at 350 (&)deg;C and 275 (&)deg;C for SCSZ and YSZ, respectively, where the dynamic damping occurred when Young's modulus was measured.In YSZ/SCSZ electrolytes, thermal residual stresses and strains were generated due to the mismatch of coefficients of thermal expansion from each layer of different compositions. They could be adjusted by varying the thickness ratios of each layer in different designs of laminates. The theoretical residual stresses have been calculated for different thickness ratios. The effect of thermal residual stress on the biaxial flexural strength was studied in layered electrolytes. The biaxial flexure tests of electrolytes with various layered designs were performed using a ring-on-ring method at both room temperature and 800 (&)deg;C. The maximum principal stress during fracture indicated an increase of flexural strength in the electrolytes with layered structure at both temperatures in comparison with the electrolytes without compositional gradient. Such an increase of strength is the result of the existence of residual compressive stresses in the outer YSZ layer. In addition, Weibull statistics of the strength values were built for the layered electrolytes tested at room temperature, and the effect of thermal residual stresses on Weibull distribution was established. The calculation of residual stress present at the outer layers was verified. The high ionic conductivity was maintained with layered electrolyte designs in the intermediate temperature range. It was also established that the ionic conductivity of layered electrolytes exhibited 7% (-) 11% improvement at 800 (&)deg;C due to the stress/strain effects, and the largest improvements in a certain electrolyte was found to nearly coincide with the largest residual compressive strain in the outer YSZ layer.In addition to the study of layered electrolytes, mechanical properties of porous Ni/SCSZ cermet were studied. The anode materials were reduced by 65 wt% NiO (-) 35 wt% SCSZ (N65) and 50 wt% NiO (-) 50 wt% SCSZ (N50) porous ceramics in the forming gas. Young's modulus as well as strength and fracture toughness of non-reduced and reduced anodes has been measured, both at room and high temperatures. High temperature experiments were performed in the reducing environment of forming gas. It was shown that while at 700 (&)deg;C and 800 (&)deg;C the anode specimens exhibited purely brittle deformation, a brittle-to-ductile transition occurred at 800 (-) 900 (&)deg;C, and the anode deformed plastically at 900 (&)deg;C. Fractography of the anode specimens were studied to identify the fracture modes of the anodes tested at different temperatures.
Show less - Date Issued
- 2013
- Identifier
- CFE0005090, ucf:50750
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0005090
- Title
- Mechanochemistry for Solid-State Syntheses and Catalysis.
- Creator
-
Restrepo, David, Blair, Richard, Kuebler, Stephen, Miles, Delbert, Clausen, Christian, Orlovskaya, Nina, University of Central Florida
- Abstract / Description
-
Traditional methods of synthesizing inorganic materials, such as hydrothermal, sol-gel, calcination and grinding steps, can typically require use of high temperatures, expensive precursors or use of solvents. Because of the energy-intensive nature or environmental impact these techniques, there is a push, especially from an industrial perspective, to move towards greener approaches. Mechanochemistry is a solvent-free alternative technique that can be used to synthesize a variety of materials...
Show moreTraditional methods of synthesizing inorganic materials, such as hydrothermal, sol-gel, calcination and grinding steps, can typically require use of high temperatures, expensive precursors or use of solvents. Because of the energy-intensive nature or environmental impact these techniques, there is a push, especially from an industrial perspective, to move towards greener approaches. Mechanochemistry is a solvent-free alternative technique that can be used to synthesize a variety of materials under ambient conditions. Due to this, there is an increase in attention towards the use of this approach in both solid-state inorganic and organic chemistry.This dissertation reports the mechanochemical synthesis of a few inorganic materials without the need of using high temperatures or solvents. Additionally, examples are presented in which mechanochemistry is used in conjunction with a secondary technique. This mechanical activation of the precursors lead to a decrease in calcination temperature and reactions times, as well as alteration of properties or unique reaction products. The synthesis of kaolinite, vanadia nanostructures, and spinels were carried out in this fashion. Mechanical activation of the precursors allowed for reduced hydrothermal treatment times in case of both kaolinite and vanadia nanostructures and the spinels are calcined at lower temperature for shorter periods of time. In addition, we report alternative template agents than previously reported for the formation of vanadia nanotubes, and report the formation of nanorods. Choosing the appropriate amine template can alter the structure and size of the material.Isomorphously substituted mixed oxides, kaolinite and spinels (MgAl2O4 and ZnAl2O4) were synthesized through a mechanically assisted process. Kaolinites are treated hydrothermally for 1 week at 250 (&)#186;C to produce an X-ray pure crystalline material. The spinels undergo calcination as low as 500 (&)#186;C to produce a nanocrystalline material. Rare-earth metals and transition metals were used as the substitutional atom. The substituted kaolinites exhibit strong order along the c axis, but less ordering along the a and b axes. Trivalent chromium and trivalent rare-earth metals, such as La, Ce, Pr, Nd, Eu, Gd, Ho, and Er, are used to replace aluminum in the structure. Likewise, divalent and trivalent transition, such as Mn, Ni, Cu and Cr, are used as the substitutional atoms in MgAl2O4 and ZnAl2O4. Cathodoluminescence studies on the substituted Spinel structure show that Mn2+ ions can occupy both the tetrahedral or octahedral holes to give a green and red emission, respectively. On the other hand, Cr3+ ions only occupy the octahedral holes to yield a red emission, similar to that in ruby. These isomorphously substituted materials may have potential applications in catalysis or glaze materials in ceramics.Oxidized graphite, an alternative to graphite oxide and graphene, can be synthesized rapidly by mechanochemical means. Grinding urea hydrogen peroxide adduct with graphite without the need of a solvent produces a product with an oxygen content of 5-15 wt%. The byproducts of this reaction are urea and water. This material is oxidized along the edges of the sheets, allowing it to be hydrophilic while retaining the conductivity. The material can suspend in water and processing allows for films of resistivities between 50 ? cm-2 and 10 k? cm-2. It was determined that the edges are fully oxidized to yield (-)COOH groups. This process offers a scalable, environmentally benign route to large quantities of oxidized graphite.An alternative method for the synthesis of nanostructured vanadia is reported. This process involves mechanical grinding of vanadium pentoxide, V2O5, with an amine template, such as diphenylamine, theophylline, rhodamine 6G and rhodamine, prior to hydrothermal treatment. This allows for the synthesis of VOx nanotubes and nanorods dependent on which template is used. Diphenylamine, theophylline, and rhodamine B produce nanorods. Use of rhodamine 6G produces asymmetric VOx nanorods.In addition to the mixed metals oxides mentioned above, sodium and calcium tantalates are synthesized mechanically. This route does not require the need of elevated temperatures or expensive and hazardous materials. X-ray diffraction analysis of NaTaO3, Ca2Ta2O7, Ca4Ta2O9 and CaTa2O6 shows that these are the only phases detected after 4 h, 10 h, 27 h and 10 h of milling, respectively. During the synthesis of Ca2Ta2O7, an intermediate phase, Ca4Ta2O9, forms within 1 h, which reacts after 5 h to form the desired product. Reference Intensity Ratio analysis shows that the material synthesized mechanically is nanocrystalline Ca2Ta2O7.Nanocrystalline ZrSi2 can also be obtained through mechanochemical synthesis. This method allows for size control and results in crystallites ranging from 9 to 30 nm. Dilution with CaCl2 enables the size control process. A linear relationship exists between the concentration of CaCl2 and the crystallite size. Contrary to a typical self-propagating metathesis reaction, this process does not allow for self-propagation and requires continuous input of mechanical energy to continue. However, this method allows for non-passivated nanoparticles of ZrSi2, which can be incorporated into composites as a reinforcement material for several applications.Hard and ultra-compressible borides, such as ReB2 and OsB2, can be synthesized mechanically. The traditional synthesis of ReB2 requires excess boron due to treatment at high temperatures. This can lead to amorphous boron aggregating at the grain boundaries, which in turn, this would degrade the properties of the material. The mechanochemical approach requires mechanical treatment of Re and B powders in stoichiometric quantities for 80 h. Mechanical synthesis of OsB2 powders requires a 1:3 ratio of Os and B powders. After 12 h of milling time, h-OsB2 begins to form, and is the major phase present after 18 h. The lattice parameters corresponding to the hexagonal OsB2 were determined to be a = b = 2.9047 (&)#197;, c = 7.4500 (&)#197;, ? = ? = 90(&)#186;, ? = 120(&)#186;. Treatment of the OsB2 powder at 1050 (&)#186;C under vacuum for 6 days did not induce a phase change, suggesting the hexagonal phase is very stable.Mechanocatalysis of the depolymerization of cellulose and hydrogenation of olefins over BN are reported as well. Heterogeneous catalysis is difficult to apply to solids, such as cellulose. However, mechanical grinding of kaolin and cellulose allows for the catalysis to occur in the solid state. This process allows for a variety of different biomasses to be used as feedstock without inhibition. Kaolinite was found to be the best acid catalyst due to high surface acidity and its layered structure, allowing for up to 84% conversion of the cellulose to water-soluble compounds. This process allows for reduction of waste, insensitivity of feedstock, multiple product pathways and scalability.Hydrogenation reactions are carried out using transition-metals catalysts. These metals have desirable catalytic properties not seen in main group elements, but there is growing concern over their use. A metal-free heterogeneous hydrogenation catalyst based on frustrated Lewis pairs would significantly reduce the health, environmental, and economic concerns associated with these metal-based catalysts. We report the first metal-free heterogeneous hydrogenation catalyst. Hydrogenation of trans-cinnamic acid is carried out over defect-laden h-BN. The reactor we use is designed to maximize the defects produced in BN sheets. The introduction of defects in BN creates frustrated Lewis pairs. DFT calculations show that the carbon double bond is weakened over boron substitution for nitrogen sites, vacancies of both boron and nitrogen, and Stone-Wales defects.A new method for crystalline germanium deposition occurring at lower temperatures (210-260 (&)#186;C) is reported. This method involves mechanical treatment of the precursors to reduce the particle size. A ground mixture of Ge and CuI are heated under vacuum to synthesize GeI2. In situ disproportionation of this compound at 210 (&)#186;C allows for the deposition of polycrystalline Ge films onto a both glass and polymer substrates. The rate of deposition is found to be 25 ng min-1. The byproducts of this process are GeI2, GeI4 and Cu3Ge, which are valuable precursors for the synthesis of germanium nanostructures and organogermanium compounds.Mechanochemistry is also utilized for the synthesis of trisubstituted pnictides. Mechanochemical treatment of bromobenzene with either Na3Sb or Na3Bi allows for the formation of triphenylstibine or triphenylbismuthine, respectively. The synthesis of the alkali metals pnictide precursors is reported as well. The synthesis of triphenylstibine produces SbPh3 as the major product from the reaction. The synthesis of triphenylbismuthine produces more Wurtz-type coupling products, which are due to the BiPh3 acting as a catalyst. Tributyl and triphenyl analogues are reported as well. The trialkylated analogues for both Sb and Bi produce more Wurtz type coupling products. This would allow for a more cost effective and scalable, alternative methods than what is currently in use today.
Show less - Date Issued
- 2013
- Identifier
- CFE0004741, ucf:49785
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0004741
- Title
- Perovskite catalysts enhanced combustion on porous media and thermoelectric power conversion.
- Creator
-
Robayo, Manuel, Orlovskaya, Nina, Chen, Ruey-Hung, Kapat, Jayanta, Vasu Sumathi, Subith, University of Central Florida
- Abstract / Description
-
A combustion chamber incorporating a high temperature porous matrix was design and tested. The effects and merits of combining combustion on porous media and catalytic enhancement were explored, in addition to the proof of concept of integrating these technologies with simple heat engines, such as thermoelectric generators, to generate efficient and reliable power. The direct observation of the flame during the combustion becomes possible due to a specially designed stainless steel chamber...
Show moreA combustion chamber incorporating a high temperature porous matrix was design and tested. The effects and merits of combining combustion on porous media and catalytic enhancement were explored, in addition to the proof of concept of integrating these technologies with simple heat engines, such as thermoelectric generators, to generate efficient and reliable power. The direct observation of the flame during the combustion becomes possible due to a specially designed stainless steel chamber incorporating a quartz window where the initiation and propagation of the combustion reaction/flame was directly visible. The simple design of the combustion chamber allowed for a series of thermocouples to be arranged on the central axis of the porous media. With the thermocouples as output and two flow controllers controlling the volumetric flow of fuel and air as input, it was possible to explore the behavior of the flame at different volumetric flow ranges and fuel to air ratios. Additionally the design allowed for thermoelectric modules to be placed in the walls of the combustion chamber. Using combustion as a heat source and passive fins for cooling, the device was able to generate enough power to power a small portable electronic device. The effects of La-Sr-Fe-Cr-Ru based perovskite catalysts, on matrix stabilized combustion in a porous ceramic media were also explored. Highly porous silicon carbide ceramics are used as a porous media for a catalytically enhanced superadiabatic combustion of a lean mixture of methane and air. Perovskite catalytic enhancement of SiC porous matrix with La0.75Sr0.25Fe0.6Cr0.35Ru0.05O3, La0.75Sr0.25Fe0.6Cr0.4O3, La0.75Sr0.25Fe0.95Ru0.05O3, La0.75Sr0.05Cr0.95Ru0.05O3, and LaFe0.95Ru0.05O3 were used to enhance combustion. The flammability limits of the combustion of methane and air were explored using both inert and catalytically enhanced surfaces of the porous ceramic media. By coating the SiC porous media with perovskite catalysts it was possible to lower the minimum stable equivalence ratio and achieve more efficient combustion.
Show less - Date Issued
- 2014
- Identifier
- CFE0005543, ucf:50315
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0005543
- Title
- DESIGN AND DEVELOPMENT OF HETEROGENOUS COMBUSTION SYSTEMS FOR LEAN BURN APPLICATIONS.
- Creator
-
Terracciano, Anthony, Orlovskaya, Nina, Vasu Sumathi, Subith, Chow, Louis, Kassab, Alain, University of Central Florida
- Abstract / Description
-
Combustion with a high surface area continuous solid immersed within the flame, referred to as combustion in porous media, is an innovative approach to combustion as the solid within the flame acts as an internal regenerator distributing heat from the combustion byproducts to the upstream reactants. By including the solid structure, radiative energy extraction becomes viable, while the solid enables a vast extension of flammability limits compared to conventional flames, while offering...
Show moreCombustion with a high surface area continuous solid immersed within the flame, referred to as combustion in porous media, is an innovative approach to combustion as the solid within the flame acts as an internal regenerator distributing heat from the combustion byproducts to the upstream reactants. By including the solid structure, radiative energy extraction becomes viable, while the solid enables a vast extension of flammability limits compared to conventional flames, while offering dramatically reduced emissions of NOx and CO, and dramatically increased burning velocities. Efforts documented within are used for the development of a streamlined set of design principles, and characterization of the flame's behavior when operating under such conditions, to aid in the development of future combustors for lean burn applications in open flow systems. Principles described herein were developed from a combination of experimental work and reactor network modeling using CHEMKIN-PRO. Experimental work consisted of a parametric analysis of operating conditions pertaining to reactant flow, combustion chamber geometric considerations and the viability of liquid fuel applications. Experimental behavior observed, when utilizing gaseous fuels, was then used to validate model outputs through comparing thermal outputs of both systems. Specific details pertaining to a streamlined chemical mechanism to be used in simulations, included within the appendix, and characterization of surface area of the porous solid are also discussed. Beyond modeling the experimental system, considerations are also undertaken to examine the applicability of exhaust gas recirculation and staged combustion as a means of controlling the thermal and environmental output of porous combustion systems. This work was supported by ACS PRF #51768-ND10 and NSF IIP 1343454.
Show less - Date Issued
- 2014
- Identifier
- CFE0005269, ucf:50549
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0005269