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 Simplified Reflection Fabry-Perot Method for Determination of Electro-Optic Coefficients of Poled Polymer Thin Films(MDPI, 2011-08-18) Park, Dong Hun; Luo, Jingdong; Jen, Alex K.-Y.; Herman, Warren N.We report a simplified reflection mode Fabry-Perot interferometry method for determination of electro-optic (EO) coefficients of poled polymer thin films. Rather than fitting the detailed shape of the Fabry-Perot resonance curve, our simplification involves a technique to experimentally determine the voltage-induced shift in the angular position of the resonance minimum. Rigorous analysis based on optical properties of individual layers of the multilayer structure is not necessary in the data analysis. Although angle scans are involved, the experimental setup does not require a θ-2θ rotation stage and the simplified analysis is an advantage for polymer synthetic efforts requiring quick and reliable screening of new materials. Numerical and experimental results show that our proposed method can determine EO coefficients to within an error of ∼8% if poled values for the refractive indices are used.Item THERMAL HYDRAULIC CHARACTERIZATION AND VALIDATION OF HEAT EXCHANGERS BASED ON TRIPLY PERIODIC MINIMAL SURFACE(2021) Dharmalingam, Lalith Kannah; Aute, Vikrant C; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Rapid growth in the field of additive manufacturing has set off a stream ofresearch into complex shapes and geometries for engineering applications. Triply Periodic Minimal Surfaces (TPMS) are a class of differential surfaces that are gaining such increased interest in the past few years. Of the most commonly studied TPMS, Schwarz-D TPMS has been shown to out-perform traditional Heat eXchanger (HX) designs in recent research. This research examines some of the under-studied TPMS structures for HX applications. Fischer-Koch S, C(Y), and C(±Y) TPMS structures were numerically analyzed to predict their thermal-hydraulic performance and the results were compared with a Schwarz-D TPMS HX. The under-studied TPMS HXs showed a 1.5 to 5 times increase in overall thermal performance while maintaining similar pressure drops when compared to the Schwarz-D TPMS HX. Furthermore, thermal-hydraulic characterization of a full-scale TPMS based HX design was carried out for high temperature (> 900 °C) applications and a parametric HX design solver was developed to predict its performance within ±5% deviation.Item Device and Circuit Level EMI Induced Vulnerability: Modeling and Experiments(2021) Cui, Yumeng; Goldsman, Neil; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Electro-magnetic interference (EMI) commonly exists in electronic equipment containing semiconductor-based integrated circuits (ICs). Metal-oxide-semiconductor field-effect-transistors (MOSFETs) in the ICs may be disrupted under EMI conditions due to transient voltage-current surges, and their internal states may change undesirably. In this work, the vulnerabilities of silicon MOSFETs under EMI are studied at the device and the circuit levels, categorized as non-permanent upsets (``Soft Errors'') and permanent damages (``Hard Failures''). The Soft Errors, such as temporary bit errors and waveform distortions, may happen or be intensified under EMI, as the transient disruptions activate unwanted and highly non-linear changes inside MOSFETs, such as Impact Ionization and Snapback. The system may be corrected from the erroneous state when the EMI condition is removed. We simulate planar silicon n-type MOSFETs at the device level to study the physical mechanisms leading to or complicate the short-term, signal-level Soft Errors. We experimentally tested commercially available MOSFET devices. Not included in regular MOSFET models, exponential-like current increases as the terminal voltage increases are observed and explained using the device-level knowledge. We develop a compact Soft Error model, compatible with circuit simulators using lumped (or compact-model) components and closed-form expressions, such as SPICE, and calibrate it with our in-house experimental data using an in-house extraction technique based on the Genetic Algorithm. Example circuits are simulated using the extracted device model and under EMI-induced transient disruptions. The EMI voltage-current disruptions may also lead to permanent Hard Failures that cannot be repaired without replacement. One type of Hard Failures, the MOSFET gate dielectric (or ``oxide'') breakdown, can result in input-output relation changes and additional thermal runaway. We have fabricated individual MOSFET devices at the FabLab at the University of Maryland NanoCenter. We experimentally stress-test the fabricated devices and observe the rapid, permanent oxide breakdown. Then, we simulate a nano-scale FinFET device with ultra-thin gate oxide at the device level. Then, we apply the knowledge from our experiments to the simulated FinFET, producing a gate oxide breakdown Hard Failure circuit model. The proposed workflow enables the evaluation of EMI-induced vulnerabilities in circuit simulations before actual fabrication and experiments, which can help the early-stage prototyping process and reduce the development time.Item Broadband Permittivity Characterization of Tunable Dielectric Thin Films for Millimeter-wave Devices(2020) Marksz, Eric; Takeuchi, Ichiro; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The use of millimeter-wave carrier frequencies has the potential to revolutionize wireless telecommunications by providing a massive increase in available bandwidth. However, millimeter-wave communications are hindered by poor atmospheric and building penetration, and require complicated RF front-end architectures. Tunable dielectric thin films offer a fast, compact, and cost-effective way to overcome many of the challenges facing the use of millimeter-wave spectrum. Few materials have been characterized in the millimeter-wave regime where measurements become increasingly challenging as test signal wavelengths approach the physical size of devices. The few tunable dielectric materials that have been studied at these frequencies suffered from high dielectric loss or other limitations. In this dissertation, we address both the measurement and materials challenges that have limited the commercial implementation of tunable millimeter-wave devices. In this work, we describe our implementation of a unified on-wafer approach to measure the relative permittivity of thin films and substrates across a continuous frequency band from 100 Hz to 110 GHz. We achieve this ultra-wide bandwidth by combining electrical measurements of on-wafer planar capacitors and transmission lines, and use finite-element simulations to connect our electrical measurements to material properties. Motivated by the need for better tunable dielectrics, we also developed a high throughput technique to accelerate the discovery of tunable dielectric thin films. We discuss this technique, which is inspired by the principles of combinatorial materials science and the “Materials Genome Initiative”. Our technique enables the characterization of many unique material compositions using a single 10 mm composition-spread thin film chip. In addition to speeding up the synthesis, fabrication, and measurement steps, the single-sample nature of this approach provides extreme consistency in the processing variables that impact dielectric properties. Finally, we present another approach to tunable dielectric materials discovery with our development of (SrTiO3)n−1(BaTiO3)1SrO thin films incorporating “targeted chemical pressure”. These atomically-precise, strain-engineered superlattices achieve unparalleled performance, with measured relative tunability of almost 50 % and low dielectric loss even beyond 100 GHz. We discuss our use of the materials-by-design approach, which incorporates collaboration between theory, synthesis, and characterization, to overcome barriers to commercial integration without sacrificing advantageous material properties.Item Characterization and Modeling of Brushless DC Motors and Electronic Speed Controllers with a Dynamometer(2019) Brown, Robert; Chopra, Inderjit; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The global drone market is expected to grow from $4.9 billion to $14.3 billion within the next decade, indicating a heavy demand for high performance electric aircraft. Modern drones are propelled with brushless DC (BLDC) motors and electronic speed controllers (ESCs). However, a current lack of information concerning the performance and efficiency of BLDC motors and ESCs prevents their use in rigorous aircraft design. Low cost hobby ESCs and BLDCs are typically used in research aircraft, but few technical details are released by their manufacturers. To better understand these devices, a custom dynamometer was constructed to study the performance of ESCs and BLDC motors. By properly recording the DC, AC, and mechanical power, information on peak efficiency and performance for the ESCs and BLDC motors are determined experimentally. Motors between 920 KV to 2500 KV were tested with 18 A, 30 A, and 40 A ESCs. A combination of these tests were carried out at 7.2 V, 11.1 V, and 14.8 V DC to explore trade offs in the design process. While typically neglected in formal analysis, this work seeks to better understand the power loss mechanisms in ESCs, as it was found that ESCs could have efficiencies as low as 65%, reducing the overall efficiency of the system considerably. This custom dynamometer features a load varying device, power analyzers, and a unique two DAQ setup to properly capture the high frequency electrical signals of BLDC motors. From the sets of experimentally recorded motor and ESC tests, a novel analytical model is developed to predict the performance of ESCs and BLDC motors. At the heart of this modeling effort is describing the 3 phase AC circuit as a single equivalent circuit, which encapsulating the motor’s performance. This work is critical in the design process, as properly sizing ESCs, motors, and rotors for an electric aircraft can improve aircraft endurance and range. Performance metrics are extracted from experimental results and are fit into the analytical model. Predictions for the system’s mechanical power, AC power, and DC power agree well with experimental results, demonstrating applicability of the robust model.Item Statistical Characterization and Prediction for a Stochastic Sea Environment(2012) Chang, Che-yu; Ayyub, Bilal M.; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Designing marine and maritime systems requires the probabilistic characterization of sea waves in the time-history and spectral domains. These probabilistic models include parameters that can be empirically estimated based on limited data in durations, locations and applicability to particular designs. Characterizing the statistical uncertainties associated with the parameters and the models is an essential step for risk-based design methods. A framework is provided for characterizing and predicting the stochastic sea-state conditions using sampling and statistical methods in order to associate confidence levels with resulting estimates. Sea-state parameters are analyzed using statistical confidence intervals which give a clear insight for the uncertainties involved in the system. Hypothesis testing and goodness-of-fit are performed to demonstrate the statistical features. Moreover, sample size is required for performing statistical analysis. Sample size indicates the number of representative and independent observations. Current practices do not make a distinction between the number of discretization points for numerical computations and the number of sampling points, i.e. sample size needed for statistical analysis. Sample size and interval between samples to obtain independent observations are studied and compared with existing methods. Further, spatial relationship of the sea-state conditions describes the wave energy transferred through the wave movement. Locations of interest with unknown sea-state conditions are estimated using spatial interpolations. Spatial interpolation methods are proposed, discussed, and compared with the reported methods in the literature. This study will enhance the knowledge of sea-state conditions in a quantitative manner. The statistical feature of the proposed framework is essential for designing future marine and maritime systems using probabilistic modeling and risk analysis.Item Characterization and Modeling of the Magnetomechanical Behavior of Iron-Gallium Alloys(2006-08-31) Atulasimha, Jayasimha; Flatau, Alison; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Magnetostrictive Iron-Gallium alloys (Galfenol) demonstrate moderate magnetostriction (~350 ppm) under very low magnetic fields (~100 Oe), have very low hysteresis, high tensile strength (~500 MPa), high Curie temperature (~675°C), are in general machinable, ductile and corrosion resistant. Therefore, they hold great promise in active vibration control, actuation, stress and torque sensing in helicopters, aircrafts and automobiles. To facilitate design of magnetostrictive actuators and sensors using this material, as well as to aid in making it commercially viable, it is necessary to perform a comprehensive characterization and modeling of its magnetomechanical behavior. This dissertation addresses some of these issues, focusing primarily on quasi-static characterization and modeling of the magnetomechanical behavior of single-crystal FeGa alloys with varying gallium content and along different crystallographic directions, and studying the effect of texture on the magnetomechanical behavior of polycrystals. Additionally, improved testing and modeling paradigms for magnetostrictive materials are developed to contribute to a better understanding and prediction of actuation and sensing behavior of FeGa alloys. In particular, the actuation behavior (λ-H and B-H curves) for 19, 24.7 and 29 at. % Ga <100> oriented single crystal FeGa samples are characterized and the strikingly different characteristics are simulated and explained using an energy based model. Actuation and sensing (B-σ and є-σ curves) behavior of <100> oriented 19 at. % Ga and <110> oriented 18 at. % Ga single crystal samples are characterized. It is demonstrated that the sensing behavior can be predicted by the model, using parameters obtained from the actuation behavior. The actuation and sensing behavior of 18.4 at. % Ga polycrystalline FeGa sample is predicted from the volume fraction of grains close to the [100], [110], [210], [310], [111], [211] and [311] orientations (obtained from cross-section texture analysis). The predictions are benchmarked against experimental actuator and sensor characteristics of the polycrystalline sample.