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 Firebrand Pile Thermal Characterization and Ignition Study of Firebrand Exposed Western Red Cedar(2021) Alascio, Joseph Anthony; Stoliarov, Stanislav I; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Over the past several decades, the severity of wildfires across the world has grown, resulting in increased number of structures in the Wildland–Urban Interface being destroyed, and lives lost. An ignition pathway that has been identified to contribute to most structures destroyed during a wildland fire is that of firebrand ignition. Firebrands are small burning pieces of vegetative material that are lofted ahead of the fire front. This study seeks to quantify thermal conditions experienced by building materials exposed to accumulated firebrands and to identify conditions that lead to ignition of these materials. A bench scale wind tunnel was used to house a decking material, western red cedar, on which the firebrands were deposited, which allowed for testing at different air flow velocities, while simultaneously analyzing the temperature of the solid substrate and gaseous exhaust flow constituents to identify trends in flaming and smoldering combustion. Higher peak temperatures and larger heating rates were found with the exposure of a higher air flow velocity. An increased air flow velocity also allowed for quicker, more frequent, and longer sustained flaming of the firebrand pile. A Modified Combustion Efficiency (MCE) value of 0.81 ± 0.02 for the firebrand pile across all testing conditions was quantified, which is indicative of a hybrid–smoldering/flaming combustion mode.Item Transient Infrared Absorption Studies of Molecular Super Rotors Prepared in a Tunable Optical Centrifuge(2021) Michael, Tara; Mullin, Amy S.; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In the work presented here, an ultrafast, angularly accelerating optical trap, called an optical centrifuge, rotationally excites gas-phase molecules such as N2O, CO, and CO2 to extremely high-J rotational levels with oriented angular momentum. Molecules in highly energetic rotational levels are known as super rotors and relax to thermal equilibrium. The state-resolved collision dynamics are investigated using polarization-sensitive high-resolution transient infrared absorption spectroscopy. Three different optical centrifuge traps are used to impart angular momentum to the gas-phase molecules: a full optical bandwidth, a reduced optical bandwidth, and a tunable optical bandwidth. New IR spectral lines of N2O with J=140-205 (E_rot=8,200-17,400 cm^(-1)) are reported. Polarization-dependent transient measurements of N2O in J=195 reveal high orientational anisotropy of r=0.85 produced by the centrifuge. The nearly-nascent rotational distributions of CO are investigated using two pressures and two optical centrifuge bandwidths. The shapes of the distributions beyond the peak at J=62 mimic the intensity profile in the fall-off region of the shaped optical pulses. The capture and acceleration efficiencies of CO and CO2 at comparable angular frequencies using three clipped chirps are also investigated. Nascent rotational distributions show that CO2 and CO exhibit narrow and broad distributions, respectively, due to differences in molecular polarizability anisotropy. Surprisingly, the ratio of [CO2]:[CO] trapped by the centrifuge is nearly 1.5, despite CO2 having twice as many states as CO and about 3-fold larger polarizability anisotropy. The relaxation dynamics of CO and N2O with He and Ar buffer gases indicate that He is more efficient at rotational quenching than is Ar, and leads to products with larger recoil.Item TAILORING LOCALIZED SURFACE PLASMON RESONANCES IN METALLIC NANOANTENNAS(2020) Zhang, Kunyi; Rabin, Oded; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The strong localized electromagnetic field achievable with metallic nanoantennas provides new opportunities for harmonics generation and label-free chemical sensing. In this work, the localized surface plasmon resonances (LSPRs) of metallic nanoarcs on dielectric substrates have been systematically investigated with visible and infrared spectroscopy, with the goal of elucidating the relationship between the structural and material parameters of the nanoarcs and their resonances. The transmission spectra provide rich information regarding the fundamental and higher order LSPR modes. Experimental results and numerical simulations demonstrate that the LSPR wavelengths are governed by the mid-arc length of the nanoarcs, and the extinction cross-sections of the different order modes are controlled by the central angle of the nanoarc and the symmetry of the mode. The fundamental and second order LSPR wavelengths can be tuned independently through the design of a non-uniform arc-width profile. Several relationships between features of the LSPR modes and the geometric parameters of nanoarcs are also confirmed by transformation optics analysis. The newly found relationships are then utilized as guidelines for the realization of plasmonic nanoarc antennas exhibiting efficient second harmonic generation (SHG). In another application, strong coupling between LSPRs and molecular vibrations is evident in the IR spectra of plasmonic nanoarcs placed in contact with a thin film of polymer, a native oxide layer or a thiol monolayer, enhancing the vibrational mode signals. This observation suggests that by appropriately tuning the frequency of the LSPR modes, the localized electromagnetic field around nanoarcs can resonantly couple to another emitter to boost its far-field radiation, which could benefit applications requiring highly localized, sensitive and selective chemical detection.Item BETTII: A pathfinder for high angular resolution observations of star-forming regions in the far-infrared(2016) Rizzo, Maxime Jean; Mundy, Lee G; Rinehart, Stephen A; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this thesis, we study clustered star formation in nearby star clusters and discuss how high angular resolution observations in the far-infrared regime could help us understand these important regions of stellar birth. We use the increased angular resolution from the FORCAST instrument on the SOFIA airborne observatory to study 10 nearby star-forming regions, and discuss the physical properties of sources in these regions that we can infer from radiative transfer modeling using these new observations. We discuss the design of BETTII, a pathfinder balloon-borne interferometer which will provide significantly better angular resolution in the far-infrared regime, and pave the way for future space-borne observatories. We elaborate on the details of BETTII's core technique, called Double-Fourier interferometry, and how to accurately compute the sensitivity of instruments which use this technique. Finally, we show our design and implementation results of the control and attitude estimation system for the BETTII payload, which poses unique challenges as an interferometer on a balloon platform.Item TWO-PHASE HEAT TRANSFER MECHANISMS WITHIN PLATE HEAT EXCHANGERS: EXPERIMENTS AND MODELING(2016) Solotych, Valentin; Kim, Jungho; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Two-phase flow heat exchangers have been shown to have very high efficiencies, but the lack of a dependable model and data precludes them from use in many cases. Herein a new method for the measurement of local convective heat transfer coefficients from the outside of a heat transferring wall has been developed, which results in accurate local measurements of heat flux during two-phase flow. This novel technique uses a chevron-pattern corrugated plate heat exchanger consisting of a specially machined Calcium Fluoride plate and the refrigerant HFE7100, with heat flux values up to 1 W cm-2 and flow rates up to 300 kg m-2s-1. As Calcium Fluoride is largely transparent to infra-red radiation, the measurement of the surface temperature of PHE that is in direct contact with the liquid is accomplished through use of a mid-range (3.0-5.1 µm) infra-red camera. The objective of this study is to develop, validate, and use a unique infrared thermometry method to quantify the heat transfer characteristics of flow boiling within different Plate Heat Exchanger geometries. This new method allows high spatial and temporal resolution measurements. Furthermore quasi-local pressure measurements enable us to characterize the performance of each geometry. Validation of this technique will be demonstrated by comparison to accepted single and two-phase data. The results can be used to come up with new heat transfer correlations and optimization tools for heat exchanger designers. The scientific contribution of this thesis is, to give PHE developers further tools to allow them to identify the heat transfer and pressure drop performance of any corrugated plate pattern directly without the need to account for typical error sources due to inlet and outlet distribution systems. Furthermore, the designers will now gain information on the local heat transfer distribution within one plate heat exchanger cell which will help to choose the correct corrugation geometry for a given task.Item OPTIMIZATION OF THE INFRARED ASPHALT REPAIR PROCESS(2015) Leininger, Christopher William; Schwartz, Charles W; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Infrared asphalt repair is an alternative technology that potentially allows for year round pavement patching that can be more durable, less expensive, and longer lasting than conventional techniques. Although infrared repair has been used for over 10 years by state and local agencies and commercial property owners in several areas of the country, some continuing resistance to this technique still remains. The principal reasons for this resistance are the largely unknown engineering properties of the patch material as compared to the native in situ pavement and the lack of standardized methods, specifications and quality assurance procedures. The following is a preliminary assessment of these engineering properties and current QA/QC procedures. A proposed specification for adoption is included in addition to recommendations for improving current practice.Item Advanced Back-End Processing Techniques For Infrared Focal Plane Arrays(2013) Markunas, Justin K.; Melngailis, John; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)With the continued scaling of infrared focal plane array (FPA) pixel pitch down to the diffraction limit, current backend processing techniques are becoming less viable. For one class of detectors, FPAs are formed when pixels of a detector array are electrically connected to analogous elements on a separate readout integrated circuit (ROIC) chip with malleable In bumps. Currently, these In bumps are formed by a standard thick photoresist liftoff process. Maintaining high connectivity with this process becomes difficult or impossible as pitch is reduced because of liftoff failure due to the increased aspect ratio required of bumps. Another class of infrared FPAs epoxies the detector array to the ROIC. A via is etched through the detector material of each pixel down to ROIC contact pads. Interconnects are currently formed by evaporating metal into the via, linking the detector array to the ROIC. As pixel pitch is reduced, obtaining proper interconnect becomes increasingly difficult due to the line of sight requirement of evaporation. In this work, two novel techniques to realize reduced pitch interconnects were developed and demonstrated that do not have the limitations of current techniques. For In-based FPAs, a template transfer process was developed that does not require a thick liftoff process. In this technique, In bumps are formed by electroplating on a separate, patterned template wafer and then transferred to the detector array or ROIC using a flip-chip bonder. A low-friction, amorphous fluoropolymer was used to shape the bumps and encourage transfer from the template wafer. A proof of principle for this process was obtained, demonstrating the transfer of 5.5 micron thick, 10 micron pitch In bumps to a mechanical ROIC. For the via-based FPAs, interconnection was achieved by electrochemical deposition of Ni films. Both electroplated and electroless Ni processes were developed for this purpose. After confirming the compatibility of these processes with detector and ROIC materials, Ni was plated into the vias of active HgCdTe photodetectors. This resulted in diffusion limited I-V characteristics that were stable through thermal cycling. Electroless Ni via contacts formed on an active 5 micron pitch FPA resulted in 99.94% connectivity.