Theses and Dissertations from UMD

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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM

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    A SUNLIGHT TO MICROWAVE POWER TRANSMISSION MODULE PROTOTYPE FOR SPACE SOLAR POWER
    (2013) Jaffe, Paul; Granatstein, Victor L; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The prospect of effectively limitless, continuous electricity from orbiting satellites for use on earth has captured many people's interest for decades. The proposed approach typically entails collection of solar energy, its conversion to microwave energy, and the wireless transmission of the microwaves to the earth. This offers the benefit of providing baseload power while avoiding diurnal cycle and atmospheric losses associated with terrestrial solar power. Proponents have contended that the implementation of such systems would offer energy security, environmental, and broad technological advantages to those who would undertake their development, while critics have pointed out economic, political, and logistical barriers. Niche applications, such as provision of power to remote military bases, might better tolerate the higher energy costs associated with early operational systems. Among recent implementations commonly proposed for solar power satellites, highly modular concepts have received considerable attention. Each employs an array of modules for performing conversion of sunlight into microwaves for transmission to earth. This work details results achieved in the design, development, integration, and testing of photovoltaic arrays, power electronics, microwave conversion electronics, and antennas for 2.45 GHz microwave-based "sandwich" module prototypes. Prototypes were fabricated and subjected to the challenging conditions inherent in the space environment, including solar concentration levels in which an array of modules might be required to operate. This testing of sandwich modules for solar power satellites in vacuum represents the first such effort. The effort culminated with two new sandwich module designs, "tile" and "step", each having respectively area-specific masses of 21.9 kg/m2 and 36.5 kg/m2, and mass-specific power figures of 4.5 W/kg at minimum one sun and 5.8 W/kg at minimum 2.2 suns (AM0) simulated solar illumination. The total combined sunlight to microwave efficiency of the modules was shown to be on the order of 8% and 7% for vacuum operation in the 10-6 torr regime. These represent the highest reported combined sandwich module efficiencies under either ambient or vacuum conditions, nearly quadrupling the previous efficiency record. The novel "step" concept was created to address thermal concerns and resulted in a patent publication. Results from module characterization are presented in context and compared with figures of merit, and practical thresholds are formulated and applied. The results and discussion presented provide an empirical basis for assessment of solar power satellite economic models, and point to several opportunities for improvements in area-specific mass, mass-specific power, and combined conversion efficiency of future prototypes.
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    Superconducting Logic Circuits Operating With Reciprocal Magnetic Flux Quanta
    (2011) Oberg, Oliver Timothy; Wellstood, Frederick; Herr, Anna; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Complimentary Medal-Oxide Semiconductor (CMOS) technology is expected to soon reach its fundamental limits of operation. The fundamental speed limit of about 4 GHz has already effectively been sidestepped by parallelization. This increases raw processing power but does nothing to improve power dissipation or latency. One approach for increasing computing performance involves using superconducting digital logic circuits. In this thesis I describe a new kind of superconducting logic, invented by Quentin Herr at Northrop Grumman, which uses reciprocal pairs of quantized single magnetic flux pulses to encode classical bits. In Reciprocal Quantum Logic (RQL) the data is encoded in integer units of the magnetic flux quantum. RQL gates operate without the bias resistors of previous superconducting logic families and dissipate several orders of magnitude less power. I demonstrate the basic operation of key RQL gates (AndOr, AnotB, Set/Reset) and show their self-resetting properties. Together, these gates form a universal logic set and provide memory capabilities. Experiments measuring the bit error rate of the AndOr gate extrapolated a minimum BER of 10-480 and a BER of 10-44 with 30% margins on flux biasing. I describe an analytic timing model for RQL gates which demonstrates the self-correcting timing features. From this model I derive equations for the timing behavior and operating limits. Using this timing model I ran simulations to determine correction factions for more accurate predictions at higher frequencies. Using these results, I also develop Very High Speed Integrated Circuit (VHSIC) Hardware Description Language (VHDL) models to describe the combinational logic of RQL gates. To test the timing predictions of the timing model, I performed three experiments on Nb/AlOx/Nb circuits at 4.2 K. The first measured the time of output. The second measured the operating margins of the circuit. The third measured the maximum frequency of operation for RQL circuits. Together, these three experiments showed quantitative agreement with the model for the timing output, qualitative agreement with the limits of operation, and a projected speed limit of 50 GHz for the Hypres 4.5 kA/cm2 process. To power RQL circuits I describe a new design for power splitters and combiners which minimize standing waves. I describe a new kind of Wilkinson power splitter which required numerical optimization but proved to be adequate. I experimentally tested two new designs of the power splitter. Both showed less than 10% variation in standing waves between power splitter and combiner, making it adequate for RQL circuits. I also compared these results with the S-parameters of the power network, which also indicated that the design was adequate for RQL circuits. Finally, I tested an 8-bit Kogge-Stone architecture carry-look ahead adder designed using VHDL models. The adder contained 815 Josephson junctions and was fully functional at 6.21 GHz with a latency of 1.25 clock cycles. The adder produced the correct logical output, had a measured optimal operating point within 8% of the optimal simulated operating point, and measured power margins of 1 dB. It operated best at the designed clock amplitude of 0.88Ic and dissipated 0.570 mW of power.
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    Characterization and Modeling of High Power Microwave Effects in CMOS Microelectronics
    (2010) Holloway, Michael Andrew; O'Shea, Patrick G; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The intentional use of high power microwave (HPM) signals to disrupt microelectronic systems is a substantial threat to vital infrastructure. Conventional methods to assess HPM threats involve empirical testing of electronic equipment, which provides no insight into fundamental mechanisms of HPM induced upset. The work presented in this dissertation is part of a broad effort to develop more effective means for HPM threat assessment. Comprehensive experimental evaluation of CMOS digital electronics was performed to provide critical information of the elementary mechanisms that govern the dynamics of HPM effects. Results show that electrostatic discharge (ESD) protection devices play a significant role in the behavior of circuits irradiated by HPM pulses. The PN junctions of the ESD protection devices distort HPM waveforms producing DC voltages at the input of the core logic elements, which produces output bit errors and abnormal circuit power dissipation. The dynamic capacitance of these devices combines with linear parasitic elements to create resonant structures that produce nonlinear circuit dynamics such as spurious oscillations. The insight into the fundamental mechanisms this research has revealed will contribute substantially to the broader effort aimed at identifying and mitigating susceptibilities in critical systems. Also presented in this work is a modeling technique based on scalable analytical circuit models that accounts for the non-quasi-static behavior of the ESD protection PN junctions. The results of circuit simulations employing these device models are in excellent agreement with experimental measurements, and are capable of predicting the threshold of effect for HPM driven non-linear circuit dynamics. For the first time, a deterministic method of evaluating HPM effects based on physical, scalable device parameters has been demonstrated. The modeling presented in this dissertation can be easily integrated into design cycles and will greatly aid the development of electronic systems with improved HPM immunity.
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    Critical behavior of superconductors and electrical transport properties of carbon nanotube thin films
    (2007-08-28) Xu, Hua; Anlage, Steven M; Lobb, Christopher J; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    With AC microwave measurements from 10 MHz up to 50 GHz and DC nano-volt level measurements we have investigated the superconducting phase transition of YBa 2 Cu 3 O 7-δ films in zero magnetic field and electrical transport properties of single walled carbon nanotube networks. We studied the microwave conductivity of YBa 2 Cu 3 O 7-δ thin films around Tc for different incident microwave power and observed that the microwave fluctuation conductivity deviates from scaling theory at low frequency around Tc. We systematically investigated the length scales involved in AC measurements and found the probed length scale depends on both frequency and current. At low current density J but high frequency ω, we observed critical behavior without hindrance from finite-size effects. However, at low current density J and low frequency ω, the experimentally probed length scale LAC may approach the thickness d of the sample, and then the critical behavior will be destroyed by finite-size effects. In this regime, we can not observe the phase transition. With very small applied microwave power, specifically -46dBm, and high frequency data, we have investigated the critical fluctuations of YBa2 Cu 3 O 7-δ thin films around Tc. It is shown that the determination of Tc is crucial for obtaining critical exponents. Improved temperature stability and conductivity calibration allow us to take high quality data at small temperature intervals (50mK). This improves the conventional data analysis method and allows a new method of extracting exponents to be developed. With these two methods, consistent values of Tc and the critical exponent were precisely determined. Experiments on 6 samples have been done and the results give a dynamical scaling exponent z=1.55±0.15. The scaling behavior of the fluctuation conductivity is also established. We have also investigated fluctuation effects of YBa2 Cu 3 O 7-δ by doing frequency-dependent microwave conductivity measurements and dc current-voltage characteristics on the same film. The dc measurement verified that the applied microwave power -46dBm in our ac measurement is small enough for the correct determination of Tc and critical exponents. For both dc and ac experiments the scaling behavior of the data was investigated. We found that the dc measurement could be affected by disorder. For high quality YBCO films and crystal, the critical exponent z is also around 1.5, which is consistent with ac measurement. Finally, using our broadband experimental technique and DC current-voltage characteristic measurement system, we measured the transport properties of single-walled carbon nanotube films. Based on the real and imaginary parts of the microwave conductivity, we calculated the shielding effectiveness for various film thickness. Shielding effectiveness of 43 dB at 10 MHz and 28 dB at 10 GHz is found for films with 90% optical transmittance, which suggests that single walled carbon nanotube(SWCNT) films are promising as a type of transparent microwave shielding material. We also investigated the frequency and electric field dependent conductivity of single walled carbon nanotube networks of various densities. We found the ac conductivity as a function of frequency follows the extended pair approximation model and increases with frequency above an onset frequency ω0 which varies over seven decades with a range of film thickness from sub-monolayer to 200 nm. The nonlinear electric field-dependent conductivity shows strong dependence on film thickness as well. Measurement of the electric field dependence of the resistance allows for the determination of the localization length scale L of localized states, which is found to systematically decrease with increasing film thickness. The onset frequency ω0 of enhanced ac conductivity and the localization length scale L of SWCNT networks are found to be correlated, and an empirical formula relating them has been proposed. Such studies will help the understanding of transport properties and broaden the applications of this novel material system.
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    Measurements of doping-dependent microwave nonlinear response in cuprate superconductors
    (2007-04-25) Mircea, Dragos Iulian; Anlage, Steven M; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Near-field microwave techniques have been successfully implemented in the past for the local investigation of magnetic materials and high-temperature superconductors. This dissertation reports on novel phase-sensitive linear- and nonlinear response microwave measurements of magnetic thin films and cuprate superconductors and their interpretation.