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- Title
- Teaching electricity to freshman physical science students through constructivism
- Creator
- Van Horn, Jerry
- Date
- 2005
- Collection
- Electronic Theses & Dissertations
- Title
- Theoretical analysis of electronic, thermal, and mechanical properties in gallium oxide
- Creator
- Domenico Santia, Marco
- Date
- 2019
- Collection
- Electronic Theses & Dissertations
- Description
-
In recent years, Ga2O3 has proven to be a promising semiconductor candidate for a widearray of power electronics and optoelectronics devices due to its wide bandgap, high breakdownvoltage, and growth potential. However, the material suffers from a very low thermalconductivity and subsequent self-heating issues. Additionally, the complexity of the crystalstructure coupled with the lack of empirical data, has restricted the predictive power of modellingmaterial properties using traditional...
Show moreIn recent years, Ga2O3 has proven to be a promising semiconductor candidate for a widearray of power electronics and optoelectronics devices due to its wide bandgap, high breakdownvoltage, and growth potential. However, the material suffers from a very low thermalconductivity and subsequent self-heating issues. Additionally, the complexity of the crystalstructure coupled with the lack of empirical data, has restricted the predictive power of modellingmaterial properties using traditional methods. The objective of this dissertation is toprovide a detailed theoretical characterization of material properties in the wide bandgapsemiconductor Ga2O3 using first-principles methods requiring no empirical inputs. Latticethermal conductivity of bulk β − Ga2O3 is predicted using a combination of first-principlesdetermined harmonic and anharmonic force constants within a Boltzmann transport formalismthat reveal a distinct anisotropy and strong contribution to thermal conduction fromoptical phonon modes. Additionally, the quasiharmonic approximation is utilized to estimatevolumetric effects such as the anisotropic thermal expansion.To evaluate the efficacy of heat removal from β − Ga2O3 material, the thermal boundaryconductance is computed within a variance-reduced Monte-Carlo framework utilizingfirst-principles determined phonon-phonon scattering rates for layered structures containingchromium or titanium as an adhesive layer between a β − Ga2O3 substrate and Au contact.The effect of the adhesive layer improves the overall thermal boundary conductancesignificantly with the maximum value found using a 5 nm layer of chromium, exceeding themore traditional titanium adhesive layers by a factor of 2. This indicates the potential ofheatsink-based thermal management as an effective solution to the self-heating issue.Additionally, this dissertation provides a detailed characterization of the effect of strainon fundamental material properties of β−Ga2O3 . Due to the highly anisotropic nature of thecrystal, the effect strain can have on electronic, mechanical, and optical properties is largelyunknown. Using the quasi-static formalism within a DFT framework and the stress-strainapproach, the effect of strain can be evaluated and combined with the anisotropic thermalexpansion to incorporate an accurate temperature dependence. It is found that the elasticstiffness constants do not vary significantly with temperature. The computed anisotropyis unique and differs significantly from similar monoclinic crystal structures, indicating theimportant role of the polyhedral linkage to the reported anisotropy in material properties.Lastly, the dependence of the dielectric function with respect to strain is evaluated using amodified stress-strain approach. This elasto-optic, or photoelastic, effect is found to be significantfor sheared crystal configurations. This opens up a potential unexplored applicationspace for Ga2O3 as an acousto-optic modulation device
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- Title
- Theoretical and Experimental Investigations of Arterial Pulse Wave Velocity (PWV)
- Creator
- Yavarimanesh, Mohammad
- Date
- 2021
- Collection
- Electronic Theses & Dissertations
- Description
-
Pulse transit time (PTT) is the time delay for the energy wave to travel between two sites in the arteries. (The energy wave can be visualized as acute dilation of the arterial wall and usually moves much faster than blood.) PTT in the form of pulse wave velocity (PWV = D/PTT, where D is the distance between the two sites) has proven to be a marker of arterial stiffness and a major cardiovascular risk factor based on a large body of epidemiological data. The Bramwell-Hill (BH) equation...
Show morePulse transit time (PTT) is the time delay for the energy wave to travel between two sites in the arteries. (The energy wave can be visualized as acute dilation of the arterial wall and usually moves much faster than blood.) PTT in the form of pulse wave velocity (PWV = D/PTT, where D is the distance between the two sites) has proven to be a marker of arterial stiffness and a major cardiovascular risk factor based on a large body of epidemiological data. The Bramwell-Hill (BH) equation relates arterial stiffness and radius to PWV. Arterial stiffness positively relates to blood pressure (BP); thus, many have pursued BP monitoring via PWV. We investigated PWV from theoretical, experimental, and application prospective.In the first theory study, we investigated the 100-year Bramwell-Hill equation, which relates PWV to BP and thus represents a basis for cuff-less BP monitoring. However, it has long been known that this equation underestimates PWV in a BP-dependent manner. We developed a new equation that accounts for spatial changes in arterial cross-sectional area. This new equation largely corrected this well-known underestimation and predicted PWV better based on experimental data in the literature. In the second experimental study, we examined the most popular PTT (finger PTT). We hypothesize that whole body PTT could be better than finger PTT due to less smooth muscle contraction. We collected data from 32 participants in a near supine position. We placed sensors including electrodes on the chest for an ECG waveform, clips on the ear, finger, and toe for photo-plethysmography (PPG) waveforms, and a cuff on the arm for auscultation BP. We recorded the waveforms and referenced BP before and after mental arithmetic, a cold pressor test, slow breathing, and nitroglycerin. Conventional PTTs were assessed as markers of BP in human subjects undergoing a battery of interventions to change BP. This experimental study concludes that PTTs through the whole body rather than the arm show the best BP change tracking ability. Thirdly, we repeated the previous study for seven subjects in three sessions one year apart to see how much PTTs and other PPG waveform feature models change after one year. While it is known that the PTTs calibration models must be updated periodically to account for aging effects, data on the time period required for these “cuff re-calibrations” are scant. Our experimental finding suggests that the PTTs model through the whole body could hold up after one year, and calibration may occur every year, which is reasonably practical. Finally, because of the inverse relation of PWV and arterial radius in the BH equation, we investigated innovative applications for PTT and other physiology-inspired features of carotid and femoral waveform for screening and surveillance of aortic abdominal aneurysm diameter. We hypothesized that arterial waveform features such as PTT constitute a non-imaging solution for the aneurysm size of the aorta. The features detected AAA with 72-80% accuracy. In conclusion, from a theoretical standpoint, we improved the 100-year PWV equation by relaxing its main assumption. From an experimental perspective, we showed the best body location for measuring PWV in the sense of BP prediction and robustness after one year. Lastly, we found that PWV could be used as markers of the abdominal aortic aneurysm from an application standpoint.
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- Title
- Thermal design studies in niobium and helium for superconducting radio frequency cavities
- Creator
- Aizaz, Ahmad
- Date
- 2006
- Collection
- Electronic Theses & Dissertations
- Title
- Two Studies in Nonlinear Biological System Modeling and Identification
- Creator
- Yan, Jinyao
- Date
- 2018
- Collection
- Electronic Theses & Dissertations
- Description
-
Biological systems are often complex, nonlinear and time-varying. The modeling of biological systems, therefore, presents significant challenges that are not overcome by the classical linear methods. In recent decades, intensive research has begun to produce methods for analyzing and modeling isolated classes of nonlinear systems. However, this vast class of models still presents many challenges, especially in complex biological systems. In this research, two novel methods are introduced for...
Show moreBiological systems are often complex, nonlinear and time-varying. The modeling of biological systems, therefore, presents significant challenges that are not overcome by the classical linear methods. In recent decades, intensive research has begun to produce methods for analyzing and modeling isolated classes of nonlinear systems. However, this vast class of models still presents many challenges, especially in complex biological systems. In this research, two novel methods are introduced for analyzing time series resulting from nonlinear systems. In the first approach, we study a class of dynamical systems that are nonlinear, discrete and with a latent state-space. We solve the probabilistic inference problem in these latent models using a variational autoencoder (VAE). Compared to continuous latent random variables, the inference of discrete latent variables is more difficult to solve. However, stochastic variational inference provides us with a general framework that tackles the inference problem for this class of model. We focused on an important neuroscience application – inferring pre- and post-synaptic activities from dendritic calcium imaging data. For it, we developed families of generative models, a deep convolutional neural network recognition model, and methods of inference using stochastic gradient ascent VAE. We benchmarked our model with both synthetic data, which resembles real data, and real experimental data. The framework can flexibly support rapid model prototyping. Both the generative model and recognition model can be changed without perturbing the inference. This is especially beneficial for testing different biological hypotheses. As a second approach, we treat a subclass of nonlinear autoregressive models: linear-time-invariant-in-parameters models. This class of models is useful and easy to work with. We propose an identification algorithm that simultaneously selects the model and does parameter estimation. The algorithm integrates two strategies: set-based parameter identification, and evolutionary algorithms that optimize fitness measures derived from these solutions. The algorithm can identify nonlinear models in novel noise scenarios. We show the performance of the algorithm in various simulated systems and practical datasets. We demonstrate its application to identify causal connectivity in a graph. This problem is often posed in recovering functional connectivity in the brain. The main contribution of this thesis is that we provide two framework for identifying nonlinear, biological systems from time series data. These two classes of nonlinear models and their applications are significant as each class is broad enough for modeling many complicated biological systems. We develop general, fast algorithms for learning these systems from data for these two model classes.
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- Title
- WIRELESS SENSING AND ACQUISITION SYSTEM FOR NON DESTRUCTIVE EVALUATION AND STRUCTURAL HEALTH MONITORING
- Creator
- Kumar, Deepak
- Date
- 2021
- Collection
- Electronic Theses & Dissertations
- Description
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Wireless sensing and communication techniques are superior and preferred over tethered counterparts. Wireless system has already replaced or replacing commonly used data transmission methods for applications such as voice communication, internet, sensing, monitoring, etc. Recently, real-time infrastructure or asset monitoring for safety critical application is gaining attention, due to its ease of better visibility and predictive maintenance. A few of those applications belong to industries...
Show moreWireless sensing and communication techniques are superior and preferred over tethered counterparts. Wireless system has already replaced or replacing commonly used data transmission methods for applications such as voice communication, internet, sensing, monitoring, etc. Recently, real-time infrastructure or asset monitoring for safety critical application is gaining attention, due to its ease of better visibility and predictive maintenance. A few of those applications belong to industries such as aerospace, defense, oil and gas, nuclear, etc., which require top of the line materials and an extra level of attention to safety. Every material or product used for such applications need to go through extensive testing and evaluation to mitigate any risk of failure. The next generation nondestructive evaluation system would provide sensing and characterization capabilities from raw materials to the final product for manufacturing on factory grounds as well as monitoring products’ desired operation in their respective in-service conditions. However, the current evaluation and internet-of-things (IoT)-based monitoring techniques have limited capabilities and cannot meet the sensitivity, range, versatility, and scalability requirements for these applications while being economical and maintenance-free. In this work, the current challenges associated with traditional NDE and SHM methods have been described and an alternative approach is proposed using new wireless sensing mechanisms. First, a wireless near-field high-Q sensing resonator is described, which has a high sensitivity to dielectric changes in composites and polymers. Second, a battery-free wireless communication system with enhanced range is demonstrated that overcomes high power requirements and radio frequency (RF) clutter issues. Third, passive (resistive and capacitive) sensors are integrated with the battery-free RFID platform and the sensor information is transmitted via a hybrid data frame with digital ID and analog sensor data. Fourth, the hybrid data frame is exploited further, an active piezoelectric transducer pair is used to actuate an acoustic pulse and measure time-of-flight (ToF) via the battery-free RFID tag, without using a power-hungry analog to digital conversion approach. The RFID-based wireless sensing system is versatile, scalable, and economically superior to battery-based IoT nodes and it can last for decades in field conditions without any routine maintenance that truly opens the pathway for embedded sensing in next generation NDE systems.
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- Title
- Wireless Phase and Frequency Synchronization for Distributed Phased Arrays
- Creator
- Mghabghab, Serge R.
- Date
- 2022
- Collection
- Electronic Theses & Dissertations
- Description
-
Distributed microwave wireless systems have the potential to dramatically reshape wireless technologies due to their ability to provide improvements in robustness, transmit power, antenna gain, spatial and temporal resolutions, size, scalability, secrecy, flexibility, and cost compared to single-platform wireless systems. Traditional wireless systems use a platform-centric model, where improving capabilities generally necessitates hardware retrofitting, which in many cases can result in a...
Show moreDistributed microwave wireless systems have the potential to dramatically reshape wireless technologies due to their ability to provide improvements in robustness, transmit power, antenna gain, spatial and temporal resolutions, size, scalability, secrecy, flexibility, and cost compared to single-platform wireless systems. Traditional wireless systems use a platform-centric model, where improving capabilities generally necessitates hardware retrofitting, which in many cases can result in a bulky, expensive, and inefficient system. Nevertheless, distributed microwave wireless systems require precise coordination to enable cooperative operation. The most highly synchronized systems coordinate at the wavelength level, supporting coherent distributed operations like beamforming. The electric states that need to be synchronized in coherent distributed arrays are mainly the phase, frequency, and time; the synchronization can be accomplished using multiple architectures. All coordination architectures can be grouped under two categories: open loop and closed loop. While closed-loop systems use feedback from the destination, open-loop coherent distributed arrays must synchronize their electrical states by only relying on synchronization signals stemming from within the array rather than depending on feedback signals from the target. Although harder to implement, open-loop coherent arrays enable sensing and other delicate communications applications, where feedback from the target is not possible.In this thesis, I focus on phase alignment and frequency synchronization for open-loop coherent distributed antenna arrays. Once the phase and frequency of all the nodes in the array are synchronized, it is possible to coherently beamform continuous wave signals. When information is modulated on the transmitted continuous waves, time alignment between the nodes is needed. However, time alignment is generally less stringent to implement since its requirements are dependent on the information rate rather than the beamforming frequency, such as for phase and frequency synchronization. Beamforming at 1.5 GHz is demonstrated in this thesis using a two-node open-loop distributed array. For the presented architecture, the phases of the transmitting nodes are aligned using synchronization signals incoming from within the array, without any feedback from the destination. A centralized phase alignment approach is demonstrated, where the secondary node(s) minimize their relative phase offsets to that of the primary node by locating the primary node and estimating the phase shift imparted by the relative motion of the nodes. A high accuracy two-tone waveform is used to track the primary node using a cooperative approach. This waveform is tested with an adaptive architecture to overcome the performance degradation due to weather conditions and to allow high ranging accuracy with minimal spectral footprint. Wireless frequency synchronization is implemented using a centralized approach that allows phase tracking, such that the frequencies of the secondary nodes are locked to that of the primary node. Once the phase and frequency of all the nodes are synchronized, it is possible to coherently beamform in the far field as long as the synchronization is achieved with the desired accuracy. I evaluate the required localization accuracies and frequency synchronization intervals. More importantly, I demonstrate experimentally the first two-node open-loop distributed beamforming at 1.5 GHz with multiple scenarios where the nodes are in relative motion, showing the ability to coherently beamform in a dynamic array where no feedback from the destination is needed.
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