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Title: Design and Characterization of High Temperature Packaging for Wide-Bandgap Semiconductor Devices.
Creator: Grummel, Brian, Shen, Zheng, Sundaram, Kalpathy, Yuan, Jiann-Shiun, University of Central Florida
Abstract / Description: Advances in wide-bandgap semiconductor devices have increased the allowable operating temperature of power electronic systems. High-temperature devices can benefit applications such as renewable energy, electric vehicles, and space-based power electronics that currently require bulky cooling systems for silicon power devices. Cooling systems can typically be reduced in size or removed by adopting wide-bandgap semiconductor devices, such as silicon carbide. However, to do this, semiconductor...
Show moreAdvances in wide-bandgap semiconductor devices have increased the allowable operating temperature of power electronic systems. High-temperature devices can benefit applications such as renewable energy, electric vehicles, and space-based power electronics that currently require bulky cooling systems for silicon power devices. Cooling systems can typically be reduced in size or removed by adopting wide-bandgap semiconductor devices, such as silicon carbide. However, to do this, semiconductor device packaging with high reliability at high temperatures is necessary. Transient liquid phase (TLP) die-attach has shown in literature to be a promising bonding technique for this packaging need. In this work TLP has been comprehensively investigated and characterized to assess its viability for high-temperature power electronics applications. The reliability and durability of TLP die-attach was extensively investigated utilizing electrical resistivity measurement as an indicator of material diffusion in gold-indium TLP samples. Criteria of ensuring diffusive stability were also developed. Samples were fabricated by material deposition on glass substrates with variant Au(-)In compositions but identical barrier layers. They were stressed with thermal cycling to simulate their operating conditions then characterized and compared. Excess indium content in the die-attach was shown to have poor reliability due to material diffusion through barrier layers while samples containing suitable indium content proved reliable throughout the thermal cycling process. This was confirmed by electrical resistivity measurement, EDS, FIB, and SEM characterization. Thermal and mechanical characterization of TLP die-attached samples was also performed to gain a newfound understanding of the relationship between TLP design parameters and die-attach properties. Samples with a SiC diode chip TLP bonded to a copper metalized silicon nitride substrate were made using several different values of fabrication parameters such as gold and indium thickness, Au(-)In ratio, and bonding pressure. The TLP bonds were then characterized for die-attach voiding, shear strength, and thermal impedance. It was found that TLP die-attach offers high average shear force strength of 22.0 kgf and a low average thermal impedance of 0.35 K/W from the device junction to the substrate. The influence of various fabrication parameters on the bond characteristics were also compared, providing information necessary for implementing TLP die-attach into power electronic modules for high-temperature applications. The outcome of the investigation on TLP bonding techniques was incorporated into a new power module design utilizing TLP bonding. A full half-bridge inverter power module for low-power space applications has been designed and analyzed with extensive finite element thermo-mechanical modeling. In summary, TLP die-attach has investigated to confirm its reliability and to understand how to design effective TLP bonds, this information has been used to design a new high-temperature power electronic module.
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Date Issued: 2012
Identifier: CFE0004499, ucf:49276
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0004499

Title: MAC LAYER AND ROUTING PROTOCOLS FOR WIRELESS AD HOC NETWORKS WITH ASYMMETRIC LINKS AND PERFORMANCE EVALUATION STUDIES.
Creator: Wang, Guoqiang, Marinescu, Dan, University of Central Florida
Abstract / Description: In a heterogeneous mobile ad hoc network (MANET), assorted devices with different computation and communication capabilities co-exist. In this thesis, we consider the case when the nodes of a MANET have various degrees of mobility and range, and the communication links are asymmetric. Many routing protocols for ad hoc networks routinely assume that all communication links are symmetric, if node A can hear node B and node B can also hear node A. Most current MAC layer protocols are unable to...
Show moreIn a heterogeneous mobile ad hoc network (MANET), assorted devices with different computation and communication capabilities co-exist. In this thesis, we consider the case when the nodes of a MANET have various degrees of mobility and range, and the communication links are asymmetric. Many routing protocols for ad hoc networks routinely assume that all communication links are symmetric, if node A can hear node B and node B can also hear node A. Most current MAC layer protocols are unable to exploit the asymmetric links present in a network, thus leading to an inefficient overall bandwidth utilization, or, in the worst case, to lack of connectivity. To exploit the asymmetric links, the protocols must deal with the asymmetry of the path from a source node to a destination node which affects either the delivery of the original packets, or the paths taken by acknowledgments, or both. Furthermore, the problem of hidden nodes requires a more careful analysis in the case of asymmetric links. MAC layer and routing protocols for ad hoc networks with asymmetric links require a rigorous performance analysis. Analytical models are usually unable to provide even approximate solutions to questions such as end-to-end delay, packet loss ratio, throughput, etc. Traditional simulation techniques for large-scale wireless networks require vast amounts of storage and computing cycles rarely available on single computing systems. In our search for an effective solution to study the performance of wireless networks we investigate the time-parallel simulation. Time-parallel simulation has received significant attention in the past. The advantages, as well as, the theoretical and practical limitations of time-parallel simulation have been extensively researched for many applications when the complexity of the models involved severely limits the applicability of analytical studies and is unfeasible with traditional simulation techniques. Our goal is to study the behavior of large systems consisting of possibly thousands of nodes over extended periods of time and obtain results efficiently, and time-parallel simulation enables us to achieve this objective. We conclude that MAC layer and routing protocols capable of using asymmetric links are more complex than traditional ones, but can improve the connectivity, and provide better performance. We are confident that approximate results for various performance metrics of wireless networks obtained using time-parallel simulation are sufficiently accurate and able to provide the necessary insight into the inner workings of the protocols.
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Date Issued: 2007
Identifier: CFE0001736, ucf:47302
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0001736

Title: ADAPTIVE SELF-REGULATION AND ORGANIZATIONAL POLITICS: INVESTIGATING THE EFFECTS IN THE ACCOUNTING PROFESSION.
Creator: Howell, Sharon, Peggy D. Dwyer, Robin W. Roberts, University of Central Florida
Abstract / Description: The purpose of this dissertation is to investigate whether or not perceptions of organizational politics mediate the relationships between accountants' personality and interpersonal traits and their perceptions of a superior's leadership ability and performance. An accountant who has a higher degree of confidence in his or her superior's abilities is more likely to be committed to a given project, resulting in a better project outcome. This benefits the client and ultimately society as a...
Show moreThe purpose of this dissertation is to investigate whether or not perceptions of organizational politics mediate the relationships between accountants' personality and interpersonal traits and their perceptions of a superior's leadership ability and performance. An accountant who has a higher degree of confidence in his or her superior's abilities is more likely to be committed to a given project, resulting in a better project outcome. This benefits the client and ultimately society as a whole. This study contributes to the accounting and psychology literatures because extant research views perceptions of leadership ability and performance from the perspective of the individual agent, with little or no recognition that social action and interaction shape and mold both the individual agent's actions and perceptions of those actions. Perceived leadership and perceived performance are important in accounting for several reasons. First, individuals act in part in relation and response to the expectations of others. Thus, the perception of effective leadership and performance is gained by meeting the expectations of others. Secondly, accountants with reputations for effectiveness have been found to be more successful in their careers. Finally, the reputation for effectiveness in performance and leadership ability has been shown to increase those abilities. This study draws on the adaptive self-regulation framework as well as other theoretical models of perceived performance. The study results indicate that certain manageable personality, interpersonal, and contextual variables affect how accountants view the level of organizational politics within the workplace. In turn, the accountant's view of the organizations' politics is shown to very strongly affect how the accountant perceives his or her superiors' performance and leadership ability.
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Date Issued: 2005
Identifier: CFE0000507, ucf:46458
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0000507

Title: PARTITIONING A GRAPH IN ALLIANCES AND ITS APPLICATION TO DATA CLUSTERING.
Creator: Hassan-Shafique, Khurram, Dutton, Ronald, University of Central Florida
Abstract / Description: Any reasonably large group of individuals, families, states, and parties exhibits the phenomenon of subgroup formations within the group such that the members of each group have a strong connection or bonding between each other. The reasons of the formation of these subgroups that we call alliances differ in different situations, such as, kinship and friendship (in the case of individuals), common economic interests (for both individuals and states), common political interests, and...
Show moreAny reasonably large group of individuals, families, states, and parties exhibits the phenomenon of subgroup formations within the group such that the members of each group have a strong connection or bonding between each other. The reasons of the formation of these subgroups that we call alliances differ in different situations, such as, kinship and friendship (in the case of individuals), common economic interests (for both individuals and states), common political interests, and geographical proximity. This structure of alliances is not only prevalent in social networks, but it is also an important characteristic of similarity networks of natural and unnatural objects. (A similarity network defines the links between two objects based on their similarities). Discovery of such structure in a data set is called clustering or unsupervised learning and the ability to do it automatically is desirable for many applications in the areas of pattern recognition, computer vision, artificial intelligence, behavioral and social sciences, life sciences, earth sciences, medicine, and information theory. In this dissertation, we study a graph theoretical model of alliances where an alliance of the vertices of a graph is a set of vertices in the graph, such that every vertex in the set is adjacent to equal or more vertices inside the set than the vertices outside it. We study the problem of partitioning a graph into alliances and identify classes of graphs that have such a partition. We present results on the relationship between the existence of such a partition and other well known graph parameters, such as connectivity, subgraph structure, and degrees of vertices. We also present results on the computational complexity of finding such a partition. An alliance cover set is a set of vertices in a graph that contains at least one vertex from every alliance of the graph. The complement of an alliance cover set is an alliance free set, that is, a set that does not contain any alliance as a subset. We study the properties of these sets and present tight bounds on their cardinalities. In addition, we also characterize the graphs that can be partitioned into alliance free and alliance cover sets. Finally, we present an approximate algorithm to discover alliances in a given graph. At each step, the algorithm finds a partition of the vertices into two alliances such that the alliances are strongest among all such partitions. The strength of an alliance is defined as a real number p, such that every vertex in the alliance has at least p times more neighbors in the set than its total number of neighbors in the graph). We evaluate the performance of the proposed algorithm on standard data sets.
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Date Issued: 2004
Identifier: CFE0000263, ucf:46225
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0000263

Title: DISPERSION-MANAGED BREATHING-MODE SEMICONDUCTOR MODE-LOCKED RING LASER: EXPERIMENTAL STUDY, NUMERICAL SIMULATIONS AND APPLICATIONS.
Creator: Resan, Bojan, Delfyett, Peter J., University of Central Florida
Abstract / Description: A novel dispersion-managed breathing-mode semiconductor mode-locked ring laser is developed. The "breathing-mode" designation derives from the fact that intracavity pulses are alternately stretched and compressed as they circulate around the ring resonator. The pulses are stretched before entering the semiconductor gain medium to minimize the detrimental strong integrating self-phase modulation and to enable efficient pulse amplification. Subsequently compressed pulses facilitate bleaching...
Show moreA novel dispersion-managed breathing-mode semiconductor mode-locked ring laser is developed. The "breathing-mode" designation derives from the fact that intracavity pulses are alternately stretched and compressed as they circulate around the ring resonator. The pulses are stretched before entering the semiconductor gain medium to minimize the detrimental strong integrating self-phase modulation and to enable efficient pulse amplification. Subsequently compressed pulses facilitate bleaching the semiconductor saturable absorber. The intracavity pulse compression ratio is higher than 50. Down chirping when compared to up chirping allows broader mode-locked spectra and shorter pulse generation owing to temporal and spectral semiconductor gain dynamics. Pulses as short as 185 fs, with a peak power of ~230 w, and a focused intensity of ~4.6 gw/cm2 are generated by linear down chirp compensation and characterized by shg-frog method. To our knowledge, this is the highest peak power and the shortest pulse generation from an electrically pumped all-semiconductor system. The very good agreement between the simulated and the measured results verifies our understanding and ability to control the physical mechanisms involved in the pulse shaping within the ring cavity. Application trends such as continuum generation via a photonic crystal fiber, two-photon fluorescence imaging, and ultrafast pulse source for pump-probe experiments are demonstrated.
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Date Issued: 2004
Identifier: CFE0000176, ucf:46155
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0000176

Title: Appropriate Adjective: Executive Authority and the Classification of Enemy Combatants.
Creator: Davis, Taraleigh, Merriam, Eric, Bledsoe, Robert, Edwards, Barry, University of Central Florida
Abstract / Description: Is the term enemy combatant an established legal category of persons under international law? Has the President exceeded his constitutional authority in classifying United States citizens who are suspected terrorists as enemy combatants? In 2018 a U.S. citizen was released after being held for 13 months as an enemy combatant. He was detained without being charged with a crime and without the ability to challenge the legality of his detention. This thesis serves two purposes. First, it will...
Show moreIs the term enemy combatant an established legal category of persons under international law? Has the President exceeded his constitutional authority in classifying United States citizens who are suspected terrorists as enemy combatants? In 2018 a U.S. citizen was released after being held for 13 months as an enemy combatant. He was detained without being charged with a crime and without the ability to challenge the legality of his detention. This thesis serves two purposes. First, it will seek to trace the history of the term enemy combatant and highlight the evolution of its use by the executive branch. This thesis then examines whether the executive has exceeded his constitutional authority to classify a United States citizen as an enemy combatant. While most of the literature focuses on the treatment and detention of enemy combatants, existing scholarship largely overlooks the issue of authority to classify enemy combatants. This thesis will argue that the executive is overstepping the boundaries of its presidential power when the executive branch creates the criteria (a legislative function) for enemy combatants and applies the criteria in the classification of enemy combatants (a judicial function). This qualitative study will use normative legal research focusing on the principles of the law in classifying a suspected terrorist as an enemy combatant as well as the legal history of the term. The analysis of the legal history of the term enemy combatant will be completed by content analysis using Nvivo 12 software of various government documents as well as case studies of enemy combatant cases.
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Date Issued: 2019
Identifier: CFE0007448, ucf:52703
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0007448

Title: Effects of Rest Position on Ultrasound-Derived Morphological Characteristics of the Vastus Lateralis and Lower-Body Force Production.
Creator: Varanoske, Alyssa, Wells, Adam, Fukuda, David, Stout, Jeffrey, Mangine, Gerald, University of Central Florida
Abstract / Description: INTRODUCTION: Ultrasound assessment of the lower body typically encompasses 10-15 minutes of supine rest prior to examination because of the potential influence of gravitational fluid shifts on tissue size and composition. However, examination of the vastus lateralis (VL) muscle requires individuals to lay in the lateral recumbent position, and this change in position may influence muscle morphological characteristics and their ability to predict muscle function.PURPOSE: The purpose of this...
Show moreINTRODUCTION: Ultrasound assessment of the lower body typically encompasses 10-15 minutes of supine rest prior to examination because of the potential influence of gravitational fluid shifts on tissue size and composition. However, examination of the vastus lateralis (VL) muscle requires individuals to lay in the lateral recumbent position, and this change in position may influence muscle morphological characteristics and their ability to predict muscle function.PURPOSE: The purpose of this investigation was to examine the effect of rest position on ultrasound-derived morphological characteristics of the VL and to determine whether or not rest position affects the relationships between muscle morphological characteristics of the VL and lower-body force and power production. METHODS: Thirty-one resistance-trained males (age: 23.0 (&)#177; 2.1 years; height: 1.79 (&)#177; 0.08 m; body mass: 87.4 (&)#177; 11.7 kg) participated in this investigation. Muscle morphological characteristics, including cross-sectional area (CSA), muscle thickness (MT), pennation angle (PA), echo intensity (UnCorEI), subcutaneous adipose tissue thickness (SFT), and EI corrected for SFT (CorEI) of the VL were assessed in the dominant limb after 15 minutes of rest in 3 recumbent positions: supine (SUP), dominant lateral recumbent (DLR), non-dominant lateral recumbent (NDLR), as well as after 15 minutes of standing (ST) and immediately after laying down (IP). Following ultrasound assessments, participants completed unilateral performance assessments, including vertical jumps (UVJ), isometric/isokinetic testing, and a 1-repetition maximum (1-RM) leg press. RESULTS: A repeated-measures analysis of variance revealed significantly different (p (<) 0.05) CSA, MT, PA, UnCorEI, and SFT in ST compared to recumbent positions after 15 minutes of rest (NDLR, DLR, and SUP). Additionally, significant differences were observed between recumbent positions for CSA, CorEI, and UnCorEI; however, no differences were observed for MT, PA, and SFT. Different magnitudes of relationships were observed between muscle morphological characteristics measured after rest in different positions and performance variables. However, muscle morphology after IP generally appears to be the best predictor of performance for most variables, although utilizing the NDLR and DLR positions may provide comparable, or potentially stronger results for variables such as IsokPF. The relationship between muscle morphology and various performance variables in ST were weaker compared to the recumbent positions examined, specifically for IsokPF, 1-RM leg press, and for all UVJ variables, except total work.CONCLUSIONS: Muscle morphology differs depending on the rest position utilized prior to ultrasound analysis. These rest positions also affect the ability for muscle morphology to predict lower-body force production. Future research should consider evaluation of muscle morphology of the VL after IP in the prediction of muscle function; however, NDLR and DLR may also be used. SUP and ST provide significantly weaker relationships for some performance variables compared to the other recumbent positions.
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Date Issued: 2019
Identifier: CFE0007550, ucf:52610
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0007550

Title: Thermodynamic Modeling and Transient Simulation of a Low-Pressure Heat Recovery Steam Generator Using Siemens T3000.
Creator: Caesar, Andres, Das, Tuhin, Bhattacharya, Samik, Putnam, Shawn, University of Central Florida
Abstract / Description: With world energy consumption rising, and nonrenewable energy resources quickly depleting, it is essential to design more efficient power plants and thereby economically utilize fossil fuels. To that end, this work focuses on the thermodynamic modeling of steam power systems to enhance our understanding of their dynamic and transient behavior. This thesis discusses the physical phenomena behind a heat recovery steam generator (HRSG) and develops a mathematical description of its system...
Show moreWith world energy consumption rising, and nonrenewable energy resources quickly depleting, it is essential to design more efficient power plants and thereby economically utilize fossil fuels. To that end, this work focuses on the thermodynamic modeling of steam power systems to enhance our understanding of their dynamic and transient behavior. This thesis discusses the physical phenomena behind a heat recovery steam generator (HRSG) and develops a mathematical description of its system dynamics. The model is developed from fundamentals of fluid dynamics, phase change, heat transfer, conservation laws and unsteady flow energy equations. The resulting model captures coupled physical phenomena with acceptable accuracy while achieving fast, and potentially real-time, simulations. The computational HRSG model is constructed in the Siemens T3000 platform. This work establishes the dynamic modeling capability of T3000, which has traditionally been used for programming control algorithms. The validation objective of this project is to accurately simulate the transient response of an operational steam power system. Validation of the T3000 model is carried out by comparing simulation results to start-up data from the low-pressure system of a Siemens power plant while maintaining the same inlet conditions. Simulation results well correlate with plant data regarding transient behavior and equilibrium conditions. With a comprehensive HRSG model available, it will allow for further research to take place, and aid in the advancement of steam power system technology. Some future research areas include the extension to intermediate and high-pressure system simulations, combined simulation of all three pressure stages, and continued improvement of the boiler model. In addition to enabling model-based prediction and providing further insight, this effort will also lead to controller design for improved performance.
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Date Issued: 2018
Identifier: CFE0007562, ucf:52599
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0007562

Title: CLASS-E CASCODE POWER AMPLIFIER ANALYSIS AND DESIGN FOR LONG TERM RELIABILITY.
Creator: Kutty, Karan, Yuan, Jiann-Shiun, University of Central Florida
Abstract / Description: This study investigated the Class-E power amplifier operating at 5.2 GHz. Since the operation of this amplifier applies a lot of stress on the switching transistor, a cascode topology was applied in order to reduce the drain-source voltage stress. Such an amplifier was designed and optimized in order to improve stability, power added efficiency, and matching. A layout for the said design was then created to be fabrication-ready using the TSMC 0.18 um technology. Post-layout simulations were...
Show moreThis study investigated the Class-E power amplifier operating at 5.2 GHz. Since the operation of this amplifier applies a lot of stress on the switching transistor, a cascode topology was applied in order to reduce the drain-source voltage stress. Such an amplifier was designed and optimized in order to improve stability, power added efficiency, and matching. A layout for the said design was then created to be fabrication-ready using the TSMC 0.18 um technology. Post-layout simulations were performed in order to realize a more realistic circuit performance with the layout design in mind. Long-term stress effects, such as oxide breakdown, on the key transistors were modeled and simulated in order to achieve an understanding of how leakage currents affect the overall circuit performance. Simulated results were compared and contrasted against theoretical understanding using derived equations. Recommendations for future advancements were made for modification and optimization of the circuit by the application of other stress reduction strategies, variation in the class-E topology, and improvement of the driver stage.
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Date Issued: 2010
Identifier: CFE0003360, ucf:48477
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0003360

Title: DENSE SPECTRAL BEAM COMBINING WITH VOLUME BRAGG GRATINGS IN PHOTO-THERMO-REFRACTIVE GLASS.
Creator: Andrusyak, Oleksiy, Zeldovich, Boris, University of Central Florida
Abstract / Description: Beam combining techniques have become an important tool in the design of high-power high-brightness laser systems. Spectral beam combining (SBC) is an incoherent combining technique that does not require phase control of sources, allowing for a stable and robust system. Using SBC, beams from an array of lasers with each element operated at a different wavelength are combined into a single near-diffraction-limited beam with the same aperture using dispersive optical elements. SBC by means of...
Show moreBeam combining techniques have become an important tool in the design of high-power high-brightness laser systems. Spectral beam combining (SBC) is an incoherent combining technique that does not require phase control of sources, allowing for a stable and robust system. Using SBC, beams from an array of lasers with each element operated at a different wavelength are combined into a single near-diffraction-limited beam with the same aperture using dispersive optical elements. SBC by means of volume Bragg gratings (VBGs) utilizes unique spectral response of VBGs: diffraction efficiency is close to unity when the Bragg condition is satisfied and is close to zero at multiple points corresponding to particular wavelength offsets from Bragg condition. High-efficiency VBGs can be recorded in UV-sensitive photo-thermo-refractive (PTR) glass. Narrow-band reflecting VBGs allow multi-channel SBC with high spectral density of channels. In this dissertation, experimental results of SBC with high spectral density of combined channels in two spectral regions of interest (1064 and 1550 nm) are reported. The behavior of narrow-band VBGs under high-power laser radiation is investigated. A laser system with kW-level output power and near-diffraction-limited divergence of spectrally-combined output beam is demonstrated. The system combines five randomly-polarized Yb-doped fiber lasers with 0.5 nm spectral separation in central wavelengths using narrow-band reflecting VBGs with absolute efficiency of combining > 90%. A novel design of a multi-channel high-power SBC system is suggested. In this approach, a common-cavity is created for all channels such that wavelengths of the sources are passively controlled by the combination of a common output coupler and intra-cavity VBGs which also act as combining elements. Laser wavelengths are automatically selected to match resonant wavelengths of respective VBGs. We report successful demonstration of a passively-controlled SBC system consisting of two amplifiers in a common cavity configuration. A compact and rugged monolithic SBC module based on multiplexed VBGs is introduced. Experimental results of a four-channel implementation of such module are discussed. Modular design of high-power laser systems is suggested with multiple modules arranged in a series. We show that with basic combining parameters achieved up to date, laser systems with 10 kW output power can be constructed using this arrangement. Further scaling to 100 kW power level is discussed.
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Date Issued: 2009
Identifier: CFE0002662, ucf:48189
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0002662

Title: FINITE ELEMENT METHOD MODELING OF ADVANCED ELECTRONIC DEVICES.
Creator: Chen, Yupeng, Wu, Thomas, University of Central Florida
Abstract / Description: In this dissertation, we use finite element method together with other numerical techniques to study advanced electron devices. We study the radiation properties in electron waveguide structure with multi-step discontinuities and soft wall lateral confinement. Radiation mechanism and conditions are examined by numerical simulation of dispersion relations and transport properties. The study of geometry variations shows its significant impact on the radiation intensity and direction. In...
Show moreIn this dissertation, we use finite element method together with other numerical techniques to study advanced electron devices. We study the radiation properties in electron waveguide structure with multi-step discontinuities and soft wall lateral confinement. Radiation mechanism and conditions are examined by numerical simulation of dispersion relations and transport properties. The study of geometry variations shows its significant impact on the radiation intensity and direction. In particular, the periodic corrugation structure exhibits strong directional radiation. This interesting feature may be useful to design a nano-scale transmitter, a communication device for future nano-scale system. Non-quasi-static effects in AC characteristics of carbon nanotube field-effect transistors are examined by solving a full time-dependent, open-boundary Schrödinger equation. The non-quasi-static characteristics, such as the finite channel charging time, and the dependence of small signal transconductance and gate capacitance on the frequency, are explored. The validity of the widely used quasi-static approximation is examined. The results show that the quasi-static approximation overestimates the transconductance and gate capacitance at high frequencies, but gives a more accurate value for the intrinsic cut-off frequency over a wide range of bias conditions. The influence of metal interconnect resistance on the performance of vertical and lateral power MOSFETs is studied. Vertical MOSFETs in a D2PAK and DirectFET package, and lateral MOSFETs in power IC and flip chip are investigated as the case studies. The impact of various layout patterns and material properties on RDS(on) will provide useful guidelines for practical vertical and lateral power MOSFETs design.
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Date Issued: 2006
Identifier: CFE0001389, ucf:46987
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0001389

Title: HIGH CURRENT DENSITY LOW VOLTAGE ISOLATED DC-DC CONVERTERSWITH FAST TRANSIENT RESPONSE.
Creator: Yao, Liangbin, Batarseh, Issa, University of Central Florida
Abstract / Description: With the rapid development of microprocessor and semiconductor technology, industry continues to update the requirements for power supplies. For telecommunication and computing system applications, power supplies require increasing current level while the supply voltage keeps decreasing. For example, the Intel's CPU core voltage decreased from 2 volt in 1999 to 1 volt in 2005 while the supply current increased from 20A in 1999 to up to 100A in 2005. As a result, low-voltage high-current...
Show moreWith the rapid development of microprocessor and semiconductor technology, industry continues to update the requirements for power supplies. For telecommunication and computing system applications, power supplies require increasing current level while the supply voltage keeps decreasing. For example, the Intel's CPU core voltage decreased from 2 volt in 1999 to 1 volt in 2005 while the supply current increased from 20A in 1999 to up to 100A in 2005. As a result, low-voltage high-current high efficiency dc-dc converters with high power-density are demanded for state-of-the-art applications and also the future applications. Half-bridge dc-dc converter with current-doubler rectification is regarded as a good topology that is suitable for high-current low-voltage applications. There are three control schemes for half-bridge dc-dc converters and in order to provide a valid unified analog model for optimal compensator design, the analog state-space modeling and small signal modeling are studied in the dissertation and unified state-space and analog small signal model are derived. In addition, the digital control gains a lot of attentions due to its flexibility and re-programmability. In this dissertation, a unified digital small signal model for half-bridge dc-dc converter with current doubler rectifier is also developed and the digital compensator based on the derived model is implemented and verified by the experiments with the TI DSP chip. In addition, although current doubler rectifier is widely used in industry, the key issue is the current sharing between two inductors. The current imbalance is well studied and solved in non-isolated multi-phase buck converters, yet few discusse this issue in the current doubler rectification topology within academia and industry. This dissertation analyze the current sharing issue in comparison with multi-phase buck and one modified current doubler rectifier topology is proposed to achieve passive current sharing. The performance is evaluated with half bridge dc-dc converter; good current sharing is achieved without additional circuitry. Due to increasing demands for high-efficiency high-power-density low-voltage high current topologies for future applications, the thermal management is challenging. Since the secondary-side conduction loss dominates the overall power loss in low-voltage high-current isolated dc-dc converters, a novel current tripler rectification topology is proposed. Theoretical analysis, comparison and experimental results verify that the proposed rectification technique has good thermal management and well-distributed power dissipation, simplified magnetic design and low copper loss for inductors and transformer. That is due to the fact that the load current is better distributed in three inductors and the rms current in transformer windings is reduced. Another challenge in telecommunication and computing applications is fast transient response of the converter to the increasing slew-rate of load current change. For instance, from Intel's roadmap, it can be observed that the current slew rate of the age regulator has dramatically increased from 25A/uS in 1999 to 400A/us in 2005. One of the solutions to achieve fast transient response is secondary-side control technique to eliminate the delay of optocoupler to increase the system bandwidth. Active-clamp half bridge dc-dc converter with secondary-side control is presented and one industry standard 16th prototype is built and tested; good efficiency and transient response are shown in the experimental section. However, one key issue for implementation of secondary-side control is start-up. A new zero-voltage-switching buck-flyback isolated dc-dc converter with synchronous rectification is proposed, and it is only suitable for start-up circuit for secondary-side controlled converter, but also for house-keeping power supplies and standalone power supplies requiring multi-outputs.
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Date Issued: 2007
Identifier: CFE0001814, ucf:47336
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0001814

Title: Ignition Studies of Oxy-Syngas/CO2 Mixtures Using Shock Tube for Cleaner Combustion Engines.
Creator: Barak, Samuel, Vasu Sumathi, Subith, Kapat, Jayanta, Ahmed, Kareem, University of Central Florida
Abstract / Description: In this study, syngas combustion was investigated behind reflected shock waves in order to gain insight into the behavior of ignition delay times and effects of the CO2 dilution. Pressure and light emissions time-histories measurements were taken at a 2 cm axial location away from the end wall. High-speed visualization of the experiments from the end wall was also conducted. Oxy-syngas mixtures that were tested in the shock tube were diluted with CO2 fractions ranging from 60% - 85% by volume...
Show moreIn this study, syngas combustion was investigated behind reflected shock waves in order to gain insight into the behavior of ignition delay times and effects of the CO2 dilution. Pressure and light emissions time-histories measurements were taken at a 2 cm axial location away from the end wall. High-speed visualization of the experiments from the end wall was also conducted. Oxy-syngas mixtures that were tested in the shock tube were diluted with CO2 fractions ranging from 60% - 85% by volume. A 10% fuel concentration was consistently used throughout the experiments. This study looked at the effects of changing the equivalence ratios (?), between 0.33, 0.5, and 1.0 as well as changing the fuel ratio (?), hydrogen to carbon monoxide, from 0.25, 1.0 and 4.0. The study was performed at 1.61-1.77 atm and a temperature range of 1006-1162K. The high-speed imaging was performed through a quartz end wall with a Phantom V710 camera operated at 67,065 frames per second. From the experiments, when increasing the equivalence ratio, it resulted in a longer ignition delay time. In addition, when increasing the fuel ratio, a lower ignition delay time was observed. These trends are generally expected with this combustion reaction system. The high-speed imaging showed non-homogeneous combustion in the system, however, most of the light emissions were outside the visible light range where the camera is designed for. The results were compared to predictions of two combustion chemical kinetic mechanisms: GRI v3.0 and AramcoMech v2.0 mechanisms. In general, both mechanisms did not accurately predict the experimental data. The results showed that current models are inaccurate in predicting CO2 diluted environments for syngas combustion.
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Date Issued: 2018
Identifier: CFE0006974, ucf:52909
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0006974

Title: Effect of Nonclassical Optical Turbulence on a Propagating Laser Beam.
Creator: Beason, Melissa, Phillips, Ronald, Atia, George, Richardson, Martin, Andrews, Larry, Shivamoggi, Bhimsen, University of Central Florida
Abstract / Description: Theory developed for the propagation of a laser beam through optical turbulence generally assumes that the turbulence is both homogeneous and isotropic and that the associated spectrum follows the classical Kolmogorov spectral power law of . If the atmosphere deviates from these assumptions, beam statistics such as mean intensity, correlation, and scintillation index could vary significantly from mathematical predictions. This work considers the effect of nonclassical turbulence on a...
Show moreTheory developed for the propagation of a laser beam through optical turbulence generally assumes that the turbulence is both homogeneous and isotropic and that the associated spectrum follows the classical Kolmogorov spectral power law of . If the atmosphere deviates from these assumptions, beam statistics such as mean intensity, correlation, and scintillation index could vary significantly from mathematical predictions. This work considers the effect of nonclassical turbulence on a propagated beam. Namely, anisotropy of the turbulence and a power law that deviates from . A mathematical model is developed for the scintillation index of a Gaussian beam propagated through nonclassical turbulence and theory is extended for the covariance function of intensity of a plane wave propagated through nonclassical turbulence. Multiple experiments over a concrete runway and a grass range verify the presence of turbulence which varies between isotropy and anisotropy. Data is taken throughout the day and the evolution of optical turbulence is considered. Also, irradiance fluctuation data taken in May 2018 over a concrete runway and July 2018 over a grass range indicate an additional beam shaping effect. A simplistic mathematical model was formulated which reproduced the measured behavior of contours of equal mean intensity and scintillation index.?
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Date Issued: 2018
Identifier: CFE0007310, ucf:52646
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0007310

Title: CLASS F AND INVERSE CLASS F POWER AMPLIFIER SUBJECT TO ELECTRICAL STRESS EFFECT.
Creator: Skaria, Giji, Yuan, Jiann, University of Central Florida
Abstract / Description: This study investigated the Class F and inverse Class F RF power amplifier operating at 5.8 GHz. The major challenging issue in design and implementation of CMOS power transistor is the breakdown voltage especially in sub-micron CMOS technologies. In order to eliminate this problem a Cascode topologies were implemented to reduce the Drain-to-Source voltage (stress). A Cascode Class F & Inverse Class F RF power amplifier were designed, and optimized in order to improve efficiency and...
Show moreThis study investigated the Class F and inverse Class F RF power amplifier operating at 5.8 GHz. The major challenging issue in design and implementation of CMOS power transistor is the breakdown voltage especially in sub-micron CMOS technologies. In order to eliminate this problem a Cascode topologies were implemented to reduce the Drain-to-Source voltage (stress). A Cascode Class F & Inverse Class F RF power amplifier were designed, and optimized in order to improve efficiency and reliability using 0.18[micro]m CMOS technology process. A 50% decrease in the stress has been achieved in the Cascode class-F and Inverse class F amplifiers. The sensitivity and temperature effect were investigated using BSIM-4 model. Such an amplifier was designed and optimized for a good sensitivity. A substrate bias circuit was implemented to achieve a good sensitivity. Recommendations were made for future advancements for modification and optimization of the class F and inverse class F circuit by the application of other stress reduction strategies, and improvement of the substrate bias circuit for a better sensitivity.
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Date Issued: 2011
Identifier: CFE0004030, ucf:49161
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0004030

Title: Normally-Off Computing Design Methodology Using Spintronics: from Devices to Architectures.
Creator: Roohi, Arman, DeMara, Ronald, Abdolvand, Reza, Wang, Jun, Fan, Deliang, Del Barco, Enrique, University of Central Florida
Abstract / Description: Energy-harvesting-powered computing offers intriguing and vast opportunities to dramatically transform the landscape of Internet of Things (IoT) devices and wireless sensor networks by utilizing ambient sources of light, thermal, kinetic, and electromagnetic energy to achieve battery-free computing. In order to operate within the restricted energy capacity and intermittency profile of battery-free operation, it is proposed to innovate Elastic Intermittent Computation (EIC) as a new duty-cycle...
Show moreEnergy-harvesting-powered computing offers intriguing and vast opportunities to dramatically transform the landscape of Internet of Things (IoT) devices and wireless sensor networks by utilizing ambient sources of light, thermal, kinetic, and electromagnetic energy to achieve battery-free computing. In order to operate within the restricted energy capacity and intermittency profile of battery-free operation, it is proposed to innovate Elastic Intermittent Computation (EIC) as a new duty-cycle-variable computing approach leveraging the non-volatility inherent in post-CMOS switching devices. The foundations of EIC will be advanced from the ground up by extending Spin Hall Effect Magnetic Tunnel Junction (SHE-MTJ) device models to realize SHE-MTJ-based Majority Gate (MG) and Polymorphic Gate (PG) logic approaches and libraries, that leverage intrinsic-non-volatility to realize middleware-coherent, intermittent computation without checkpointing, micro-tasking, or software bloat and energy overheads vital to IoT. Device-level EIC research concentrates on encapsulating SHE-MTJ behavior with a compact model to leverage the non-volatility of the device for intrinsic provision of intermittent computation and lifetime energy reduction. Based on this model, the circuit-level EIC contributions will entail the design, simulation, and analysis of PG-based spintronic logic which is adaptable at the gate-level to support variable duty cycle execution that is robust to brief and extended supply outages or unscheduled dropouts, and development of spin-based research synthesis and optimization routines compatible with existing commercial toolchains. These tools will be employed to design a hybrid post-CMOS processing unit utilizing pipelining and power-gating through state-holding properties within the datapath itself, thus eliminating checkpointing and data transfer operations.
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Date Issued: 2019
Identifier: CFE0007526, ucf:52619
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0007526

Title: The Effects of Facial Cues on Consumer Judgment and Decision-Making.
Creator: Liu, Fan, He, Xin, Wang, Ze, Mao, Huifang, Sivo, Stephen, University of Central Florida
Abstract / Description: This dissertation investigates the roles of facial cues in consumer behavior. Specifically, the research examines the effect of facial structural resemblance, facial expressions, and other perceptual cues(-)in both individual and group settings(-)on consumer judgment and decision-making. Essay 1 examines the influence of facial resemblance on consumers' product purchase likelihood. This effect is moderated by consumers' mental construal, such that the effect of increased facial resemblance on...
Show moreThis dissertation investigates the roles of facial cues in consumer behavior. Specifically, the research examines the effect of facial structural resemblance, facial expressions, and other perceptual cues(-)in both individual and group settings(-)on consumer judgment and decision-making. Essay 1 examines the influence of facial resemblance on consumers' product purchase likelihood. This effect is moderated by consumers' mental construal, such that the effect of increased facial resemblance on product purchase likelihood occurs among consumers with high-level construals but not among those with low-level construals. Results of three experimental studies show that increased facial resemblance among team members enhances the perceived entitativity of the group, which in turn leads to more favorable intention of purchasing the product offered by the group. Essay 2 investigates the differential effects of recipients' group entitativity on two types of donation (time vs. money). Through three studies, the research demonstrates that high (versus low) group entitativity among the recipients increases donation of time but decreases donation of money. Such differential effects on donation of time versus money are driven by consumers' emotional or cognitive well-being associated with time or money donations. In essay 3, the effect of smile intensity on customer behavior is shown to be moderated by power and salience of ulterior motive. When employees' ulterior motive is not salient to customers, low-power customers evaluate the employee with intensified smiles more favorably compared to high-power customers. In contrast, when ulterior motive is made salient, high-power rather than low-power customers react more positively to smile intensity. Results show that the interactive effects between smile, power, and ulterior motive are driven by customers' warmth and competence perceptions. Collectively, this dissertation focuses on consumers' face-based judgments of individuals and teams, and investigates how such facial cues might influence consumers' attitude, purchase intention, and prosocial behavior.
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Date Issued: 2015
Identifier: CFE0005828, ucf:50930
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0005828

Title: Disciplinary Mythologies: A Rhetorical-Cultural Analysis of Performance Enhancement Technologies in Sports.
Creator: Lamothe, John, Scott, Blake, Janz, Bruce, Campbell, James, Oliveira, Leonardo, University of Central Florida
Abstract / Description: In sports discourse, the relationship between athletics and technology is often paradoxical. On the one hand, modern sports rely on technology at every level, from training and tracking of players to the equipment and apparel used by athletes to the game strategies and playing fields themselves. Nearly all of these technologies are intended to increase athletic performance on some level. And yet, certain performance enhancement technologies can be criticized for being antithetical to the...
Show moreIn sports discourse, the relationship between athletics and technology is often paradoxical. On the one hand, modern sports rely on technology at every level, from training and tracking of players to the equipment and apparel used by athletes to the game strategies and playing fields themselves. Nearly all of these technologies are intended to increase athletic performance on some level. And yet, certain performance enhancement technologies can be criticized for being antithetical to the spirit of sports, which is framed as being a strictly natural and pure human endeavor. Using a rhetorical-cultural methodological approach, popular sports discourse is analyzed to investigate how arguments in contested spaces between sports and technologies get (re)negotiated and (re)articulated to fit within a sports social language that emphasizes (")pure(") and (")natural(") ideals of sport. This often results in a dichotomy where the sport/technology relationship is either black boxed, thus being subsumed in the sport social language and becoming transparent and the relationships unarticulated, or the technology is regulated out of the sport through rules and bans. The reason for this articulation is attributed in large part to the deep humanism embedded in the sport social language. How a shift to a posthuman perspective would effect sports discourse is explored. These conclusions about underlying values in sports discourse lead to the formation of a new theoretical framework called disciplinary mythologies. Building off of Foucault's disciplinary power, Scott's disciplinary rhetorics, and Barthe's mythologies, disciplinary mythologies are discrete units of persuasion that both construct and constitute claims by drawing upon layered narratives and shifting associations that lose their context when entering the realm of myth. Two specific disciplinary mythologies are discussed(-)the level-playing-field topos and the nostalgia enthymeme(-)and it is shown how sports discourse often draws upon them to shape arguments and actions.
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Date Issued: 2015
Identifier: CFE0005970, ucf:50773
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0005970

Title: Towards Energy-Efficient and Reliable Computing: From Highly-Scaled CMOS Devices to Resistive Memories.
Creator: Salehi Mobarakeh, Soheil, DeMara, Ronald, Fan, Deliang, Turgut, Damla, University of Central Florida
Abstract / Description: The continuous increase in transistor density based on Moore's Law has led us to highly scaled Complementary Metal-Oxide Semiconductor (CMOS) technologies. These transistor-based process technologies offer improved density as well as a reduction in nominal supply voltage. An analysis regarding different aspects of 45nm and 15nm technologies, such as power consumption and cell area to compare these two technologies is proposed on an IEEE 754 Single Precision Floating-Point Unit implementation....
Show moreThe continuous increase in transistor density based on Moore's Law has led us to highly scaled Complementary Metal-Oxide Semiconductor (CMOS) technologies. These transistor-based process technologies offer improved density as well as a reduction in nominal supply voltage. An analysis regarding different aspects of 45nm and 15nm technologies, such as power consumption and cell area to compare these two technologies is proposed on an IEEE 754 Single Precision Floating-Point Unit implementation. Based on the results, using the 15nm technology offers 4-times less energy and 3-fold smaller footprint. New challenges also arise, such as relative proportion of leakage power in standby mode that can be addressed by post-CMOS technologies. Spin-Transfer Torque Random Access Memory (STT-MRAM) has been explored as a post-CMOS technology for embedded and data storage applications seeking non-volatility, near-zero standby energy, and high density. Towards attaining these objectives for practical implementations, various techniques to mitigate the specific reliability challenges associated with STT-MRAM elements are surveyed, classified, and assessed herein. Cost and suitability metrics assessed include the area of nanomagmetic and CMOS components per bit, access time and complexity, Sense Margin (SM), and energy or power consumption costs versus resiliency benefits. In an attempt to further improve the Process Variation (PV) immunity of the Sense Amplifiers (SAs), a new SA has been introduced called Adaptive Sense Amplifier (ASA). ASA can benefit from low Bit Error Rate (BER) and low Energy Delay Product (EDP) by combining the properties of two of the commonly used SAs, Pre-Charge Sense Amplifier (PCSA) and Separated Pre-Charge Sense Amplifier (SPCSA). ASA can operate in either PCSA or SPCSA mode based on the requirements of the circuit such as energy efficiency or reliability. Then, ASA is utilized to propose a novel approach to actually leverage the PV in Non-Volatile Memory (NVM) arrays using Self-Organized Sub-bank (SOS) design. SOS engages the preferred SA alternative based on the intrinsic as-built behavior of the resistive sensing timing margin to reduce the latency and power consumption while maintaining acceptable access time.
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Date Issued: 2016
Identifier: CFE0006493, ucf:51400
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0006493

Title: Design and modeling of a heat exchanger for porous combustor powered steam generators in automotive industry.
Creator: Dasgupta, Apratim, Orlovskaya, Nina, Gou, Jihua, Vasu Sumathi, Subith, University of Central Florida
Abstract / Description: A major challenge faced by automobile manufacturers is to achieve reduction of particulate emission to acceptable standards, as the emission standards become more and more stringent. One of the ecologically friendly options to reduce emissions is to develop external combustion in a steam engine as a replacement of the internal combustion engine. There are multiple factors, other than pollution that need to be considered for developing a substitute for Internal Combustion Engine, like specific...
Show moreA major challenge faced by automobile manufacturers is to achieve reduction of particulate emission to acceptable standards, as the emission standards become more and more stringent. One of the ecologically friendly options to reduce emissions is to develop external combustion in a steam engine as a replacement of the internal combustion engine. There are multiple factors, other than pollution that need to be considered for developing a substitute for Internal Combustion Engine, like specific power, throttle response, torque speed curve, fuel consumption and refueling infrastructure. External combustion in a steam engine seems to be a bright idea, for a cleaner and more environment friendly alternative to the IC engine that can satisfy the multiple technology requirements mentioned. One way of performing external heterogeneous combustion is to use porous ceramic media, which is a modern and innovative technique, used in many practical applications. The heterogeneous combustion inside ceramic porous media provides numerous advantages, as the ceramic, acts as a regenerator that distributes heat from the flue gases to the upstream reactants, resulting in the extended flammability limits of the reactants. The heat exchanger design is the major challenge in developing an external combustion engine because of the space, such systems consume in an automobile. The goal of the research is to develop a compact and efficient heat exchanger for the application. The proposed research uses natural gas as a fuel that is mixed with air for combustion and the generated flue gases are fed to a heat exchanger to generate superheated system for performing engine work to the vehicle. The performed research is focused on designing and modeling of the boiler heat exchanger section. The justification for selection of working fluid and power plant technology is presented as part of the research, where the proposed system consists of an Air and Flue Gas Path and a Water and Steam Path. Models are developed for coupled thermal and fluid analysis of a heat exchanger, consisting of three sections. The first section converts water to a saturated liquid. The second portion consists of a section where water is converted to saturated steam. The third section is the superheater, where saturated steam is converted to superheated steam. The Finite Element Model is appropriately meshed and boundary conditions set up to solve the mass, momentum and energy conservation equations. The k-epsilon model is implemented to take care of turbulence. Analytical calculations following the established codes and standards are also executed to develop the design.
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Date Issued: 2017
Identifier: CFE0006579, ucf:51308
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0006579

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