A. James Clark School of Engineering
Permanent URI for this communityhttp://hdl.handle.net/1903/1654
The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
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Item Phonon Transport and Nonequilibrium Kinetics with Stimulation Modeling in Molecular Crystals(2024) Liu, Zhiyu; Chung, Peter W.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)An important family of materials known as molecular crystals has been used extensively in fields such as organic semiconductors, energy, optoelectronics, and batteries. Due to their periodic crystal structure, phonons are the predominant heat and energy carriers. Phonons and their transport behaviors are crucial to the performance of semiconductors, the figure of merit of thermoelectrics, shock-induced properties of molecular crystals, and light-matter interactions of materials. Recent decades have seen significant advancements in the understanding of the phonon transport behaviors in inorganic crystals. However, a comprehensive understanding of phonon properties in molecular crystals is still lacking. While various theoretical models and computational simulations have been developed to study vibrational energy transfer in molecular crystals and to correlate vibrational structure with the stability of materials, these approaches often suffer from limitations. Many of these studies either neglect anharmonic scattering entirely or rely on simplified representations of phonon scattering. In this dissertation, we focus on investigating the phonon transport and nonequilibrium kinetics in molecular crystals. In the first work, we study the harmonic phonon properties of cellulose Iβ using tapered reactive force fields (ReaxFF). While geometry optimization with the original ReaxFF potential often results in structures with negative eigenvalues, indicating structural instability, the modified potential with a tapering function yields structures with no associated negative eigenvalues. Three ReaxFF parameterizations are evaluated by comparing lattice properties, elastic constants, phonon dispersion, temperature-dependent entropy, and heat capacity with experimental results from the literature. In the second study, we study the phonon transport behavior of Si, Cs2PbI2Cl2, cellulose Iβ, and α-RDX by calculating the thermal conductivity using different thermal transport models including the Phonon gas model, Cahill-Watson-Pohl, and the Allen-Feldman model and the Wigner formulation. By comparing the calculated thermal conductivity with experimental values, we highlight the significant contributions of wave-like heat carriers in cellulose Iβ and α-RDX. We show how different phonon properties influence particle-like and wave-like behavior in various materials and reveal unusual mechanisms present in molecular crystals. Lastly, we investigate nonequilibrium phonon kinetics resulting from direct vibrational excitations by employing the phonon Boltzmann transport equations. The results of our mid-IR pump-probe spectroscopy simulations align closely with experimental data from the literature. Additionally, by exciting different phonon modes at varying frequencies, we uncover distinct stages and pathways of vibrational energy transfer. To gain insights into the decomposition mechanism of RDX under excitation, we further calculate the bond activities of the N-N and N-O bonds, identifying possible stimuli that could trigger bond cleavage.Item MEASURING AND MODELING ELECTROMAGNETIC FORCES THAT INFLUENCE GRANULAR BEHAVIOR(2024) Pett, Charles Thomas; Hartzell, Christine M; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)On the surfaces of small, airless planetary bodies, forces other than gravity, such as cohesive, magnetic and electrostatic forces, may dominate the behavior of regolith. Yet, the magnitude of these forces remains uncertain, as well as the link between grain-scale and bulk-scale physics. In this work, techniques for measuring and modeling electromagnetic forces that influence granular behavior are developed. We discuss an experimental method for measuring interparticle cohesion by breaking cohesive bonds between grains with electrostatic forces. The centroid positions of the lofted grains at the moment of detachment are imaged in order to numerically calculate initial accelerations to solve for cohesion. We propose the design of a payload that would be deployed on the Moon or an asteroid and use an electrically biased plate to induce electrostatic dust lofting and measure interparticle cohesion in situ. We would call the system \textbf{Small--FORCES} because it would be able to image \textbf{Small} \textbf{F}orces \textbf{O}ptically \textbf{R}esolved for \textbf{C}ohesion \textbf{E}stimation via \textbf{E}lectrostatic \textbf{S}eparation. We numerically integrate Poisson's equation and develop a model for the potential distribution of a photoelectron sheath as a function of distance from surfaces. We use this model to gauge the extent to which the solar wind will perturb the Small-FORCES electric field that is used to loft charged regolith inside the sheath and obtain suitable trajectories for imaging lofted regolith that will be used to measure cohesion. We then derive a formula to quantify the maximum region of our system's electric field that we predict can be shielded from the ambient solar wind, which depends on system dimensions and applied voltage. In another experiment, we investigated the affect of magnetic cohesion on the avalanching behavior of magnetic grains. We will introduce an instrument and novel method for characterizing the bulk magnetic susceptibility of granular mixtures by submerging an inductor coil in a bed of metallic beads. In prior works, the magnetic force on grains was calculated based on the magnetic susceptibility of a single grain, but our coil uniquely quantifies effects from void spaces and demagnetization in the bulk. Compared to both a commercial Terraplus Inc. KT-10 meter and theoretical approximations, we report similar trends in susceptibility values measured as a function of mass of ferromagnetic material per volume. We conclude the talk with a discussion on a conductive model we developed to simulate surfaces other than dielectrics in the solar wind. We use a 2D grid-free treecode to enable complex surface geometries that would be computationally intensive for traditional PIC codes. Instead of using the capacitance matrix method to calculate the induced surface charge magnitudes, we discretized the conductor surface into point charges and allow them to have Coulomb interactions with the external plasma particles. The linear system used to explicitly solve for the induced surface charge magnitudes couples the interaction between surface charges and plasma particles self-consistently via the conductive boundary condition. The model has been validated thus far with image charge theory.Item Understanding the effect of fabrication conditions on the structural, electrical, and mechanical properties of composite materials containing carbon fillers(2022) Morales, Madeline Antonia; Salamanca-Riba, Lourdes G; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Carbon structures are commonly used as the reinforcement phase in composite materials toimprove the electrical, mechanical, and/or thermal properties of the matrix material. The structural diversity of carbon in its various forms (graphene, carbon nanotubes, graphite fibers, for example) makes it a useful reinforcement phase, as the properties of the composite material can be tailored for a specific application depending on the structure and properties of the carbon structure used. In this work, the incorporation of graphene/graphitic carbon into an aluminum metal matrix by an electrocharging assisted process (EAP) was investigated to create a composite material with enhanced electrical conductivity and yield strength. The increased electrical conductivity makes the composite suitable for application in more efficient power transmission lines. The increased strength makes it useful as a lightweight structural material in aerospace applications. The EAP involves applying a direct current to a mixture of molten aluminum and activated carbon to induce the crystallization of graphitic sheets/ribbons that extend throughout the matrix. The effect of processing conditions (current density, in particular) on the graphitic carbon structure, electrical properties, and mechanical properties of the composite material was investigated. The effect of porosity/voids and oxide formation was discussed with respect to the measured properties, and updates to the EAP system were made to mitigate their detrimental effects. It was found that the application of current results in some increase in graphitic carbon crystallite size calculated from Raman spectra, but many areas show the same crystallite size as the activated carbon starting material. It is likely that the current density used during processing was too low to see significant crystallization of graphitic carbon. There was no increase in electrical conductivity compared to a baseline sample with no added carbon, most likely due to porosity/voids in the samples. The mechanical characterization results indicated that the graphitic carbon clusters formed by the process did not act as an effective reinforcement phase, with no improvement in hardness and a decrease in elastic modulus measured by nanoindentation. The decreased elastic modulus was a result of compliant carbon clusters and porosity in the covetic samples. The porosity/voids were not entirely eliminated by the updates to the system, thus the electrical conductivity still did not improve. Additionally, a multifunctional composite structure consisting of a carbon-fiber reinforced polymer (CFRP) laminate with added copper mesh layers was investigated for use in aerospace applications as a structural and electromagnetic interference (EMI) shielding component. The CFRP provides primarily a structural function, while the copper mesh layers were added to increase EMI shielding effectiveness (SE). Nanoindentation was used to study the interfacial mechanical properties of the fiber/polymer and Cu/polymer interfaces, as the interfacial strength dictates the overall mechanical performance of the composite. Further, a finite element model of EMI SE was made to predict SE in the radiofrequency to microwave range for different geometry and configurations of the multifunctional composite structure. The model was used to help determine the optimum design of the multifunctional composite structure for effective shielding of EM radiation. It was found from nanoindentation near the fiber/polymer and Cu/polymer interfaces that the carbon fibers act as an effective reinforcement phase with hardness in the matrix increasing in the interphase region near the carbon fibers due to strong interfacial adhesion. In contrast, the Cu/polymer interface did not exhibit an increase in hardness, indicating poor interfacial adhesion. The EMI SE model indicated that the combination of CFRP layers, which primarily shields EMI by absorption, and Cu mesh, which predominantly shields by reflection, provided adequate SE over a wider frequency range than the individual components alone. Further, it was found that the SE of the CFRP layers were improved by including multiple plies with different relative fiber orientations.Item Electromagnetic Characterization of Misaligned Serpentine Waveguide Structures in Traveling-Wave Tubes at Microwave Frequencies(2022) Kuhn, Kyle; Antonsen, Jr., Thomas M; Beaudoin, Brian L; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Modern-day millimeter and microwave source technology has advanced considerably over the past century, but to meet the defense industry’s demand for high power and large bandwidth, vacuum electronic devices (VEDs) are still the ideal candidate to fulfill such requirements as opposed to their solid-state semiconductor counterparts. Of the numerous VEDs available, the traveling-wave tube (TWT) amplifier provides novel solutions in areas where size, weight, and power (SWaP), and bandwidth are of great importance such as on satellites and in electronic warfare applications. The advancement in computer-aided design (CAD) and simulation has allowed for increasingly complicated device configurations to be designed with ease. Instead, challenges arise in fabrication as extremely tight manufacturing tolerances on the order of micron to submicron levels are necessary due to the very short wavelengths in the mm-wave and sub-mm-wave regimes. Without this level of manufacturing precision, VEDs will not operate at optimal levels in power, bandwidth, and efficiency. We present a serpentine waveguide (SWG) design to be used as the slow-wave structure (SWS) in a TWT amplifier. Manufacturing techniques for the design are discussed, and a detailed study into how one-dimensional and two dimensional misalignments in the circuit’s half-plane affect the radio frequency (RF) signal that propagates through the device. Figures of merit include the device’s reflected power, or S11, the transmitted power through the SWG, or S21, the device’s cutoff frequency, and the SWG’s dispersion curves. Computer simulations using Ansys’s High Frequency Structure Simulator, or HFSS, and cold test laboratory measurements for aligned and misaligned Ka-band (26.5 GHz – 40 GHz) SWG circuits are presented. Upon completing a thorough RF characterization of the Ka-band device, efforts will shift focus to designing a SWG circuit for a W-band (75 GHz – 110 GHz) TWT amplifier prototype.Item An Integrated Photonic Platform For Quantum Information Processing(2021) Dutta, Subhojit; Waks, Edo EW; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Quantum photonics provides a powerful toolbox with vast applications ranging from quantum simulation, photonic information processing, all optical universal quantum computation, secure quantum internet as well as quantum enhanced sensing. Many of these applications require the integration of several complex optical elements and material systems which pose a challenge to scalability. It is essential to integrate linear and non-linear photonics on a chip to tackle this issue leading to more compact, high bandwidth devices. In this thesis we demonstrate a pathway to achieving several components in the quantum photonic toolbox on the same integrated photonic platform. We focus particularly on two of the more nontrivial components, a single photon source and an integrated quantum light-matter interface. We address the problem of a scalable, chip integrated, fast single photon source, by using atomically thin layers of 2D materials interfaced with plasmonic waveguides. We further embark on the challenge of creating a new material system by integrating rare earth ions with the emerging commercial platform of thin film lithium niobate on insulator. Rare earth ions have found widespread use in classical and quantum information processing. However, these are traditionally doped in bulk crystals which hinder their scalability. We demonstrate an integrated photonic interface for rare earth ions in thin film lithium niobate that preserves the optical and coherence properties of the ions. This combination of rare earth ions with the chip-scale active interface of thin film lithium niobate opens a plethora of opportunities for compact optoelectronic devices. As an immediate application we demonstrate an integrated optical quantum memory with a rare earth atomic ensemble in the thin film. The new light matter interface in thin film lithium niobate acts as a key enabler in an already rich optical platform representing a significant advancement in the field of integrated quantum photonics.Item Testing and Optimization of Optical Illumination to Extract Cancer Cells from Tissue Samples(2020) Han, Chang-Mu; Waks, Edo; Shapiro, Benjamin; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This study aims for mathematical modeling, experimental validation, and systematic optimization of Expression Microdissection (xMD). xMD selectively heats targeted cancer cells via a photothermal effect to enable their procurement from tumor tissue samples to provide highly pure cell populations for molecular analysis so that misdiagnoses caused by non-neoplastic cell contamination can be reduced. Several theoretical models have been validated for the photothermal effect in tissues. However, these models are not generally applicable to the physics behind the process of xMD. In this study, we proposed a mathematical model that analyzes the spatial and temporal temperature distribution and heat melt track in an xMD sample that is composed of a thermoplastic film and a tumor tissue section sandwiched by two glass slides. To experimentally validate the model, we designed and built a continuous-wave laser raster system from scratch to implement xMD on the sample in which the tissue was replaced by a tissue-mimicking phantom fabricated by spin-coating. The phantom is used to imitate the physical properties of an immunohistchemically-stained tissue, such as thickness, light absorption, and scattering, etc. Moreover, we proposed an indirect method that uses absorbance spectral slope of the xMD-treated film as temperature indicator for the sample in order to overcome the challenge of temperature measurement on a multilayered micro-scaled medium and experimentally validate the model. The result shows that the experimentally measured temperature of the phantom and melt track width on the film were in good consistency with those predicted by the model. Furthermore, based on the validated model, we systematically optimized the xMD process under realistic tissue variations for commercially-available laser raster systems, featuring two laser types (pulse v.s. continuous-wave) and two system configurations (top-down v.s. bottom-up). Specifically, we analyzed the temperature distribution of the xMD sample under three cases of the variations: (1) size of the stained cancer cell, (2) tissue section thickness, and (3) tissue stain intensity to find the optimal xMD operation space (i.e., laser intensity, scan speed, and pulse-on time) for the systems. In the optimization results, the optimal laser intensity and pulse time of the pulse xMD system for the sample variations range from 6 x 10^7 W/m^2 to 13 x 10^7 W/m^2 and from 2 ms to 10 ms, respectively. However, over-melting problem may occur when dealing with thicker tissue samples. The result suggests the pulse time of less than 0.8 ms. Similarly, for the continuous-wave xMD system, the optimal range of the intensity and speed are from 7 x 10^7 W/m^2 to 1 x 10^8 W/m^2 and from 60 mm/s to 100 mm/s, respectively. These ranges are overlapped by the specifications of the continuous-wave systems, indicating they are capable of processing the samples in real clinical practice. Furthermore, no obvious difference of the optimal range can be seen between the laser systems of the two configurations when extracting cancer cells from the thin tissues (5 um). When processing the think tissues (15 um), our simulations however show that the bottom-up pulse xMD system has better heating efficiency in tissue than the other xMD systems do, indicating it has smaller optimal operation window. Additionally, the top-down xMD system induces higher temperature on the film/tissue interface. Such result points out that the top-down xMD system can provide better xMD performance than the bottom-up system does. Our model demonstrated its validity to describe the xMD mechanism. The optimization results revealed the optimal xMD range for the varying realistic tissue samples. We anticipate the xMD model and parametric simulations enable researchers to facilitate the cell retrieval process and maximum the xMD performance without contaminating subsequent molecular profiling of cancer and other diseases so that cancer patients can receive molecular medical treatments in a timely manner.Item SYNTHESIS AND CHARACTERIZATION OF ENERGETIC NANOMATERIALS WITH TUNABLE REACTIVITY FOR PROPULSION APPLICATIONS(2020) Kline, Dylan Jacob; Zachariah, Michael R.; Liu, Dongxia; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Combustion is the world’s leading energy conversion method in which a fuel and oxidizer react and release energy, typically in the form of heat. Energetic materials (propellants, pyrotechnics, and explosives) have combustion reactions that are so fast that they are generally limited by how quickly the fuel and oxidizer can reach each other. Recent research has employed nanomaterials to reduce the distance between reactants to increase energy release rates. This dissertation attempts to uncover and quantify structure-function relationships in energetic nanomaterials by modifying chemical and physical properties of the materials and characterizing the observed changes using new diagnostic tools. This dissertation begins with the development of diagnostic tools that can capture the dynamics of energetic material combustion using a high-speed color camera to measure temperature. This tool has also been modified into a high-speed microscope that allows for spatial and temperature measurements at microscale length (µm) and time (µs) scales. Changes to chemical formula have been explored for energetic nanomaterial systems, though visualization of the reaction dynamics limited detailed results on reaction mechanisms. The first study performed here probed the role of gas generation vs. thermal effects in energy release rate where it was found that combustion inefficiencies from reactive sintering could be mitigated by introducing a gas-generating oxidizer. To explore combustion improvements in the fuel, a metal fuel nanoparticle manufacturing method was explored, though the combustion performance was again limited by reactive sintering. Another effort to reduce reactive sintering with a gas generator proved successful, but also unveiled the importance of different heat transfer mechanisms for propagation. The role of physical architecture on propellant combustion was also investigated to improve efficiency and versatility in solid propellants. It was found that addition of a poor thermal conductor to a propellant mixture increased the propagation rate of the material and this was attributed to the result increase in burning surface area resulting from inhomogeneous heat transfer. Lastly, this dissertation explores a method to remotely ignite materials using microwaves and titanium nanoparticles. This work sets the stage for a remotely staged solid propellant architecture that would provide control over solid propellant combustion in-operando.Item Highly Efficient, Megawatt Class RF Power Sources for Mobile Ionospheric Heaters(2020) Appanam Karakkad, Jayakrishnan Appanam; Antonsen Jr., Thomas M; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this thesis, we consider the development of a highly efficient, grid-less tetrode as a megawatt-level RF source in the 3 to 10 MHz range for application in a mobile ionospheric heater. Such a heater has potential advantages over the stationary facilities, such as HAARP (High-Frequency Active Auroral Research Program), found at high latitude. The considered device operates in class D mode with an annular electron beam allowing realization of high efficiency. The beam current is controlled using an annular modulating electrode (mod-anode) placed around the annular emitter on the cathode. This feature removes the traditional semi-transparent grid and the problems associated with interception of current beam at the grid. Three different device configurations based on differing magnetic field confinement were considered. Model A, which has a constant focusing magnetic field and no beam compression, offers the highest interaction efficiency. However, to generate a uniform and constant magnetic field over the whole device length would require the use of a large and bulky solenoid. This makes the setup in the case of Model A much larger (and much heavier). Model B has a magnetic field that is up-tapered from the cathode towards the anode and collector where the bulk part of the solenoid is located. This configuration retains the compression of the electron beam to maintain a high efficiency while keeping the size of the device manageable. It has a lower efficiency than Model A, but it provides a larger cathode area in than in Model A which mitigates cathode loading. In the case of Model C, there no guiding magnetic field and is the most compact, but its interaction efficiency is the lowest among the three device types. Model C also uses two modulating anodes maintained at varying voltages to provide electrostatic focusing of the electron beam. It is still operated in the class D regime by switching the two mod-anodes of Model C on and off together. However, the voltage swing will be much larger compared to Model A. A theoretical analysis to find the optimal operating point for model A is presented. In particular, the trade-off between the peak current and the duration of the current pulse is analyzed. The beam distributions in axial and transverse momenta and in total electron energies, before and after the decelerating gap, calculated using the Michelle code are presented for Model A. Using static-case Michelle simulation results, the instantaneous and average device efficiencies of the three models were maximized while reducing the device size by studying the influence of electrode geometry (Anode-Cathode Gap, and Anode-Collector gap and shapes) on the device efficiency. After optimizing the device geometry for these three different models, time-domain simulations with secondary electrons were performed. For model A, it is found that during the portion of the RF cycle when the beam current is on, secondaries emitted from the collector are driven back into the collector by the incoming primary beam. When the beam is switched off, secondaries can stream back into the tetrode and have a small negative impact on efficiency. We present a design in which the secondary electrons are eventually absorbed at the collector, rather than at the cathode or anode. For model B, most of the secondary electrons are trapped in the collecting region due to an effect called magnetic mirroring from the up tapering of the magnetic field towards the collector region. In Model C, the secondary electrons are largely scattered throughout the tetrode due to the lack of magnetic field confinement making it much harder to prevent the loss of efficiency. In short, three different versions of the grid-less tetrode have been proposed and studied. The optimized version of these devices have efficiencies ranging from 81% to 91.5%. The choice of the optimal design for real systems may depend on a number of tradeoffs. In the situations where the weight and size of a system play a crucial role, Model C could be more preferable with the penalty of lower efficiency. In turn, Model A can offer the highest efficiency, but the solenoids required for maintaining a constant magnetic field along the entire device could be very heavy and bulky. In comparison, Model B offers a middle ground among the three models on compactness and efficiency.Item On Engineering Risks Modeling in the Context of Quantum Probability(2020) Lee, Yat-Ning Paul; Baecher, Gregory B; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Conventional risk analysis and assessment tools rely on the use of probability to represent and quantify uncertainties. Modeling complex engineering problems with pure probabilistic approach can encounter challenges, particularly in cases where contextual knowledge and information are needed to define probability distributions or models. For the study and assessment of risks associated with complex engineering systems, researchers have been exploring augmentation of pure probabilistic techniques with alternative, non-fully, or imprecise probabilistic techniques to represent uncertainties. This exploratory research applies an alternative probability theory, quantum probability and the associated tools of quantum mechanics, to investigate their usefulness as a risk analysis and assessment tool for engineering problems. In particular, we investigate the application of the quantum framework to study complex engineering systems where the tracking of states and contextual knowledge can be a challenge. This study attempts to gain insights into the treatment of uncertainty, to explore the theoretical implication of an integrated framework for the treatment of aleatory and epistemic uncertainties, and to evaluate the use of quantum probability to improve the fidelity and robustness of risk system models and risk analysis techniques.Item Effects of Ferroelectric Properties on Mechanical Behavior of Class II Multilayer Ceramic Capacitors(2019) Li, Nga Man Jennifa; McCluskey, Patrick; Das, Diganta; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Class II Multilayer Ceramic Capacitors (MLCCs) are one of the most widely adopted types of passive components in modern electronic systems due to their high volumetric efficiency. Mechanical failures are dominant in MLCCs due to the brittle nature of the ceramic dielectric. Over the years, there have been many studies on the effect of design parameters, assembly parameters on crack susceptibility of these parts. Barium Titanate (BaTiO3) based ceramic is used as the dielectric in Class II MLCCs. This material is responsible for capacitance aging and temperature dependent properties in these units. Changes in mechanical properties due to electrical and mechanical loading or loading history is widely reported in the literature for bulk BaTiO3 and other ferroelectric materials. However, these effects have yet to be reported for Class II MLCCs components in the literature. With the indentation technique becoming more popular in research and development, more studies have adopted the technique for assessing mechanical properties of MLCCs and other electronic components. However, indentation measured properties are dependent on test parameters. In the case of ferroelectric materials, the properties also depend on texture of the specimen and other electromechanical coupling effects. In this study, baseline measurements of mechanical properties under various test parameters as well as treatment history of MLCCs are established using the Oliver-Pharr method. Two mechanisms are evaluated for the potential change in flexural strength for MLCCs with DC voltage history. The first one is the fracture toughness anisotropy caused by domain switching. The second one is the change in stress distribution in the MLCC body with the increase in effective dielectric texture (crystallinity) due to poling under a bending load. Finally, flexural strength is measured for commercial MLCCs with different volumetric ratio of effective dielectric, and an empirical relationship is developed to relate the change in flexural strength to the applied poling voltage and the volumetric ratio of the tested units.