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
More information is available at Theses and Dissertations at University of Maryland Libraries.
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Item INTEGRATION OF SUPERCONDUCTORS INTO WIDE BANDGAP SEMICONDUCTOR ENVIRONMENTS FOR DEPLOYABLE SINGLE PHOTON DETECTORS(2024) Drechsler, Annaliese Grace; Christou, Aristos; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Superconducting nanowire single photon detectors (SNSPDs) are the photon detecting devices of the future. These devices offer exceptional detecting capabilities over a wide range of wavelengths, which will enable next generation systems for optical communications, light detection and ranging, quantum key decryption, and astronomy among others. There are substantial materials, fabrication, and device development challenges that need to be addressed before these devices are ready for large scale deployment in arrays. This dissertation demonstrates novel approach to SNSPD development by monolithically integrating superconducting materials with wide bandgap semiconductor systems to scale these devices. Specifically, this work explores the integration of niobium nitride (NbN) with multi-channel aluminum gallium nitride (AlGaN)/gallium nitride (GaN) superlattice devices to leverage the benefits of materials similarity and lattice matching to provide high quality detector performance in the proposed system. The multichannel superlattice device selected for this work, the superlattice castellated field effect transistor (SLCFET) utilizes a novel δ-doping approach to generate conducting channels. Epitaxial structures were studied between 300K and 4K. This structure exhibits a substantial reduction in epitaxial resistance, determined to be a result of mobility improvement to 4151.5 cm2/Vs through Hall effect analysis. Phonon scattering modelling indicates that the device is limited by polar optical phonon scattering at high temperatures and interface roughness between the channels at cryogenic conditions. Field effect transistors fabricated from this epitaxial structure were tested and shown to exhibit exceptionally high performance at low temperatures, proving feasibility of device integration. A production-scalable NbN deposition process was developed for SNSPD fabrication. Thorough analyses determined the relationship between deposition parameters and the resultant crystallinity, defectivity, and surface morphology. Analysis of ultra-thin films determined that the NbN films grow through a step-flow growth mechanism. This data was used to develop a temperature-dependent empirical model of the kinetics of the surface morphology and growth mechanism evolution based on the Avrami equation. Fabrication processes were developed using these films to pattern SNSPDs with narrow linewidths down to 50 nanometers composing the meander structure for long wavelength performance. Thorough analysis of the impact of electron beam lithography write conditions were conducted to propose ideal fabrication conditions. Methods were proposed and implemented to address defectivity by reducing the impact of elasto-capillary forces on line collapse including chemical surface modification using hexamethyldisilazane and resist thinning using polymethyl methacrylate (PMMA) and ZEP and implementing charge dissipation layers. Additional processes were proposed and implemented to enable integration into the SLCFET fabrication flow. The SLCFET devices and NbN structures were tested and determined to be functional, thus demonstrating the feasibility of integration. An initial integrated device was designed and modelled by combining a SLCFET with NbN SNSPDs, using the RF output as a readout approach. The devices were successfully fabricated using the processes developed within this dissertation. Testing of the devices showed a 30dB signal difference between the normal and detecting states, thus demonstrating the first device of its kind, representing a substantial contribution to the field. This will open the door for full-scale array development using novel on and off chip signal processing approaches proposed in this work.Item Strategies for Improved Fire Detection Response Times in Aircraft Cargo Compartments(2020) Wood, Jennifer Marie; Milke, James A.; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Prompt fire detection in cargo compartments on board transport aircraft is an important safety feature. Concern has been expressed for the activation time of contemporary detection technologies installed on aircraft. This project will deliver a continuation of research on the issues that have been identified relative to fire detection improvements in cargo compartments on aircraft, with a particular emphasis on freighters. Gas sensors and dual wavelength detectors were demonstrated in a previous phase to be responsive to fires in the previous experiment program. Detectors placed inside a Unit Loading Device (ULD) responded quickly to the array of fire sources. Thus, a further exploration of these observations is conducted including wireless technology along with an analysis of the effects of leakage rates on fire signatures inside ULDs. One primary goal is to assess the differences in fire detection time for detectors located within ULD versus those located on the ceiling of the cargo compartment for fires which originate in a ULD. The results indicated the detector location with the shortest activation time is inside of the ULD. Within the ULD, the wireless detector outperformed both air sampling detectors, however, the results could vary if threshold levels were more restrictive.Item OPTIMIZATION AND IMPLEMENTATION OF A THERMOACOUSTIC FLASHOVER DETECTOR(2013) Hamburger, Kenneth A; Sunderland, Peter B; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The thermoacoustic flashover detector is to be a helmet-mounted device that responds to deteriorating conditions in a compartment fire and produces an audible alarm to alert emergency personnel in time for an escape or change in tactics. An operational prototype device was designed at University of Maryland in 2011, and featured an aluminum, copper, and MACOR tube 178 mm long and 25.4 mm in diameter. The prototype was powered by an external heat band, which provided 44 watts of power at 308 deg C. Optimization of the prototype is conducted across several parameters including power consumption and temperature gradient. To that end, two scaled-down models of diameters 22 mm and 17 mm are constructed, both of which fail to produce sustained sound. Adding water to the device reduces the onset power consumption to 22 watts and the maximum temperature to 285 deg C, which represents the most efficient prototype of the device. A system of radiant heat collector panels and copper-water heat pipes is designed to replace the external heat band as a power source. A heat transfer analysis is conducted to determine the necessary size of the collector panels for proper activation, as well as the response time of the system. Total required surface area will depend on future design parameters, but reasonable estimates suggest that it will be between 0.02-0.03 sq meters. An acoustic analysis of the optimized device is conducted, revealing a fundamental frequency of 500Hz at 101 dB.Item Neutron Detection by Scintillation of Noble-Gas Excimers(2012) McComb, Jacob Collin; al-Sheikhly, Mohamad; Nuclear Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Neutron detection is a technique essential to homeland security, nuclear reactor instrumentation, neutron diffraction science, oil-well logging, particle physics and radiation safety. The current shortage of helium-3, the neutron absorber used in most gas-filled proportional counters, has created a strong incentive to develop alternate methods of neutron detection. Excimer-based neutron detection (END) provides an alternative with many attractive properties. Like proportional counters, END relies on the conversion of a neutron into energetic charged particles, through an exothermic capture reaction with a neutron absorbing nucleus (10B, 6Li,3He). As charged particles from these reactions lose energy in a surrounding gas, they cause electron excitation and ionization. Whereas most gas-filled detectors collect ionized charge to form a signal, END depends on the formation of diatomic noble-gas excimers (Ar2*, Kr2*, Xe2*). Upon decaying, excimers emit far-ultraviolet (FUV) photons, which may be collected by a photomultiplier tube or other photon detector. This phenomenon provides a means of neutron detection with a number of advantages over traditional methods. This thesis investigates excimer scintillation yield from the heavy noble gases following the boron-neutron capture reaction in 10B thin-film targets. Additionally, the thesis examines noble-gas excimer lifetimes with relationship to gas type and gas pressure. Experimental data were collected both at the National Institute of Standards and Technology (NIST) Center for Neutron Research, and on a newly developed neutron beamline at the Maryland University Training Reactor. The components of the experiment were calibrated at NIST and the University of Maryland, using FUV synchrotron radiation, neutron imaging, and foil activation techniques, among others. Computer modeling was employed to simulate charged-particle transport and excimer photon emission within the experimental apparatus. The observed excimer scintillation yields from the 10B(n,α)7Li reaction are comparable to the yields of many liquid and solid neutron scintillators. Additionally, the observed slow triplet-state decay of neutron-capture-induced excimers may be used in a practical detector to discriminate neutron interactions from gamma-ray interactions. The results of these measurements and simulations will contribute to the development and optimization of a deployable neutron detector based on noble-gas excimer scintillation.Item Optimization of Carbon Monoxide Detectors in a Residential Layout and Analysis of Dispersion Characteristics(2012) Engel, Derek; Trouvé, Arnaud; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Current building and life safety standards do not specify the optimal placement of carbon monoxide (CO) detectors in residential structures. Currently, the standards advise to follow manufacturer's instructions, place one centrally on each floor and in each HVAC zone. With the use of computation fluid dynamics (CFD) software FLACS, simulations were run to observe and track CO concentrations, generated from a fire source, throughout a demonstrative box as well as a residential structure under different source and ambient conditions. A MATLAB script was developed to represent CO detector functionality. From this it was possible to evaluate detector placement throughout a structure. The time to detection criterion was analyzed as well as order of alarm in relation to other placements. Final recommendations are presented based upon the dispersion patterns observed.Item Prototype Design for Thermoacoustic Flashover Detector(2012) Buda-Ortins, Krystyna Eva; Sunderland, Peter; diMarzo, Marino; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The thermoacoustic flashover detector integrates the phenomenon of thermoacoustics into a fire fighting application. This report presents the prototype design for the thermoacoustic flashover detector to ultimately be implemented in a firefighter's gear. Upon increases in compartment fire heat flux and temperature corresponding to the onset of flashover, the device will produce a loud warning tone to alert the firefighter that flashover is impending. This is critical because post-flashover, the fire transitions to an untenable environment for a firefighter, as well as compromised structural integrity of the building. The current design produces a tone at 115 dB at about 500 Hz upon heating from an external band heater and cooling via an ice/water bath. At 38 mm from the device, this sound level is louder than the 85 dB from fire alarms and distinct from the 3000 Hz tone of smoke detectors. The minimum power input to the device for sound onset is 44 Watts, corresponding to a temperature difference of 150 degrees Celsius at a mean temperature of 225 degrees Celsius across a 2 cm long porous steel wool stack. The temperatures at the hot and cold ends of the stack are 300 and 150 degrees Celsius respectively, which is achieved with a response time of ~100 seconds. The sound is sustained as long as there is a minimum power input of 31 Watts. Although the measurement uncertainties are estimated at 10 degrees Celsius for the temperatures and 5 Watts for the power input, this design provides a foundation for future improvement and quantification of the device. The mechanisms of the thermoacoustics at work and the materials selected for the prototype are presented. Different power level inputs to the device are analyzed and temperatures for operation are determined. Suggestions for future optimization and integration of the device into firefighters' gear are presented.Item Analysis of the Effects of Temperature and Velocity on the Response Time Index of Heat Detectors(2010) Pomeroy, Andrew Tom; Milke, James A; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Recent revisions to NFPA 72, the National Fire Alarm Code, have specified the response time index (RTI) as the sensitivity listing for heat detectors. Originally derived as a sprinkler sensitivity rating, there has been little work performed to validate the use of the RTI rating for heat detectors. RTI values are determined by plunging the devices into a hot wind tunnel at 200 C (392 F) and 1.5 m/s (4.9 ft/s). These test conditions are unrealistically severe for the majority of expected ceiling jet profiles. While the RTI correlation is purported to be independent of temperature and velocity, data from previous studies indicates otherwise. This study examined the effects of low temperature and low velocity plunge test conditions on the constancy of the RTI for several common heat detectors. The RTI correlation was found to be inconsistent across temperature and velocity test conditions.