CALCE EPSC Graduate Student Theses and Dissertation Abstracts (2022)

Zhao, Beihan (Ph.D.)
Explorations of Carbon-nanotube-graphene Oxide Inks: Printability, Radio-frequency and Sensor Applications, and Reliability

Carbon-Nanotube (CNT) is a novel functional material with outstanding electrical and mechanical properties, with excellent potential for various kinds of industrial applications. Additive manufacturing or 3D printing of CNT-based materials or inks has been studied extensively, and it is vital to have a thorough understanding of the fluid mechanics and colloidal science of CNT-based inks for ensuring optimum printability and the desired functionality of such CNT-based materials. In this dissertation, a custom-developed syringe-printable CNT-GO ink (GO: Graphene Oxide) is introduced and the fluid mechanics and colloidal science of this ink as well as the different devices (e.g., temperature sensor, humidity sensor, and RF antenna) fabricated with this ink are studied. The following topics are discussed in this dissertation: (1) the application and printability (in terms of the appropriate fluid mechanics and colloidal science) of CNT-based inks; (2) development of temperature sensors with CNT-GO inks; (3) development of humidity sensors with CNT-GO inks; (4) development of RF patch antenna with CNT-GO inks; and (5) evaporation-driven size-dependent nano-microparticulate three-dimensional deposits (CNTs serve as one type of nanoparticle examined in this part of the study). In Chapter 1 of this dissertation, a literature review is conducted on the application of CNT-based inks and the fluid mechanics and colloidal science issues dictating the printability and performance of such CNT-based inks. The problem statement and overall research plan are also introduced in this chapter. In Chapter 2, the development of our custom CNT-GO ink is introduced. Detailed material selection and the mechanism of shape-dependent arrest of coffee-stain effect, which ensured that the printable ink led to uniform deposition, are discussed in this chapter. Temperature sensor prototypes printed with the CNT-GO inks are also presented in Chapter 2. From Chapter 3 to Chapter 5, the performances of our CNT-GO based flexible temperature sensor, humidity sensor, and patch antenna prototypes are discussed. The ink printability on flexible thin PET films is studied, and a straightforward ‘peel-and-stick’ approach to use the CNT-trace (or patch)-bearing PET films on surfaces of widely varying wettabilities and curvatures as different prototypes is introduced. Excellent temperature and humidity sensitivity of our CNT-GO based sensors are presented in Chapter 3 and Chapter 4, and the potential of this CNT-GO material for fabrication of ultra-wideband (UWB) patch antennas is discussed in Chapter 5. Furthermore, the stability and reliability of these printed CNT-GO-based prototypes are also explored. In previous Chapters, the printed CNT-GO patterns were cured by evaporation-mediated deposition on flat substrates (i.e., 2D deposition spanning in x and y directions). This motivated the extension of the physics to the 3rd dimension and probing of particle deposition on a 3D substrate and particle deposition in all x, y, and z directions. Therefore, in Chapter 6, we perform an experiment to demonstrate this kind of possibility using three kinds of micro-nanoparticle-laden water-based droplets (i.e. coffee particles, silver nanoparticles, and CNTs). These droplets were first deposited at the bottom of an un-cured PDMS film; these droplets were lighter than the PDMS and hence, they rose to the top of the PDMS where they could have either attained a Neuman like state or simply remained as an undeformed spherical drop with the top of the drop breaching the air-liquid-PDMS interface. The calculations based on air-water, water-PDMS, and air-PDMS surface tension values confirmed that the Neuman like state was not possible, and the droplets were likely to retain their undeformed shapes as they breached the air-PDMS interface. The timescale differences between the fast PDMS curing and the slower droplet evaporation, led to the formation of spherical shape cavities inside the PDMS after completion of the curing, and allowed evaporation-driven deposition to occur in all x, y, and z directions inside the cavity, with the exact nature of the deposition being dictated by the sizes of the particles (as confirmed by the experiments conducted with coffee particles, silver nanoparticles, and CNTs). Finally, in Chapter 7, the major contributions of this dissertation and proposed future studies related to this dissertation work are listed.

Khemani, Varun (Ph.D.)
Prognostics and Secure Health Management of Analog Circuits

Analog circuits are a critical part of industrial circuits and systems. Estimates in the literature show that, even though analog circuits comprise less than 20% of all circuits, they are responsible for more than 80% of faults. Hence, analog circuit Prognosis and Health Management (PHM) is critical to the health of industrial circuits. There are a multitude of ways that any analog circuit can fail, which leads to proportional scaling in the number of possible fault classes with number of circuit components. Therefore, this research presents an advanced Design Of Experiments-based (DOE) approach to account for components that degrade in an individual and interacting fashion, to narrow down the number of possible fault classes under consideration. A wavelet-based deep-learning approach is developed that can localize the circuit component that is the source of degradation and predict the exact value of the degraded component. This degraded value is used in conjunction with degradation models to predict when the circuit will fail based on the source of degradation. Increasing outsourcing in the fabrication of electronic circuits has made them susceptible to the insertion of hardware trojans by untrusted foundries. In many cases, hardware trojans are more destructive than software trojans as they cannot be remedied by a software patch and are impossible to repair. Process reliability trojans are a new class of hardware trojans that are inserted through modification of fabrication parameters and accelerate the aging of circuit components. They are challenging to detect through traditional trojan detection methods as they have zero area footprint i.e., require no insertion of additional circuitry. The PHM approach is modified to detect these hardware trojans in order to incorporate circuit security, resulting in the Prognosis and Secure Health Management (PSHM) framework. Deep neural networks achieve state-of-the-art performance on classification and regression applications but are a black-box approach, which is a concern for implementation. Wavelets are approximations of cells found in the human visual cortex and cochlea. They were used to develop wavelet scattering networks (WSNs), which were intended to be an interpretable alternative to deep neural networks. WSNs achieve state-of-the-art performance on low to moderately complex datasets but are inferior to deep neural networks for extremely complex datasets. Improvements are made to WSNs to overcome their shortcomings in terms of performance and learnability. Further applications of the research are highlighted for rotating machinery vibration analytics, functional safety online estimation etc.

Saadon, Yonatan (Ph.D.)
Quantile based LSTM predictor of remaining useful life

Accurate prediction of the remaining useful life (RUL) of a degrading component is crucial to prognostics and health management for electronic systems, to monitor conditions and avoid reaching failure while minimizing downtime. However, the shortage of sufficiently large run-to-failure datasets is a serious bottleneck impeding the performance of data-driven approaches, and in particular, those involving neural network architectures. Here, this work shows a new data-driven prognostic method to predict the RUL using an ensemble of quantile-based Long Short-Term Memory (LSTM) neural networks, which represents the RUL prediction task to a set of simpler, binary classification problems that are amenable for prediction with LSTMs, even with limited data. This methodology was tested on two run-to-failure datasets, power MOSFETs and filtration system, and showed promising results on both datasets it demonstrates that this approach obtains improved RUL estimation accuracy for both the power MOSFETs and the filtration system, especially with a small training dataset that is characterized by a wide range of the RUL.

Yao, Zhaoxi (Ph.D.)
Thermal Management of Integrated motors for Electric Propulsion

Electrification of traditional combustion power units has been a major trend. The low emissions, low noise and high efficiency characteristic of electrified power, fit the vision of a low carbon emission future. The development of high power density electric motors is key to facilitating large scale, heavy duty applications. The demand for dense power leads to significant heat flux, causing thermal management to become one of the main obstacles in developing high power density electric motors. Multiple components in the motor generate heat. For example, the motor of interest in this paper is a 1 MW, high power density, surface mounted permanent magnet motor, with a segmented and laminated stator on the outside, and a laminated rotor on the inside. Heat is generated in the stator winding, stator core, magnets, rotor core as well as the motor drive. For high speed motors, windage loss could also be significant in the air gap. Among the heat-generating components, the stator winding is the primary heat source. For this study, a comprehensive thermal management solution was developed. The power density of the motor, based on active mass, exceeded 22 kW/kg and majority of the loss came from the stator windings. Thus, a dedicated direct winding cooling combined with an integrated cooling jacket were deployed. Multiple winding cooling schemes were explored, such as investment-casted cooling channels in potting, hollow conductors, flooded slots and Litz-wire-wrapped cooling tubes. The flooded slots with scaffolding-shaped spacer were chosen in the end, which demonstrate good thermal performance, low pumping power, pressure requirements and low risk of partial discharge as the dielectric coolant also served as liquid insulation. A cooling jacket with integrated power module cooling was designed to cool the stator core and power modules. The cooling jacket included a compression sleeve, which served as the mechanical support to hold the stator segments as well as the cooling surface for the stator cores, and nine cold plates, hosting 18 power modules on top, placed around the curved outer surface of the motor. The cooling concepts were designed, simulated and validated by testing. A functioning prototype was constructed and is in the process of testing.

Ahuja, Kunal (Ph.D.)
Ultra-Thin On-Chip Ald Lipon Capacitors for High Energy and High-Frequency Applications

Liquid electrolytes dominate the supercapacitor market due to their high ionic conductivity leading to high energy and power density metrics. However, with the increase in demand for portable and implantable consumer electronics, all solid-state supercapacitor systems with high safety are an attractive option from both application perspectives and their similar charge storage mechanism. For solid state ionic capacitors, there remains significant room for innovation to increase the ionic conductivity and capacitor architecture to enhance the performance of these devices. Nano-structuring along with advanced manufacturing techniques such as atomic layer deposition (ALD) are powerful tools to augment the performance metrics of these all-solid-state capacitors that can compete with state-of-the-art liquid electrolyte-based supercapacitors. This dissertation has two primary objectives; 1) Study the behavior of polymorphs of ALD LiPON as a capacitor material and 2) Enhance the performance metrics using advanced materials and 3D nanostructuring for improved energy storage and high-frequency applications. In this work, ALD LiPON-based solid state capacitors are fabricated with a gold current collector to study the behavior of the solid electrolyte. LiPON shows a dual energy storage behavior, in low frequency (<10 kHz), LiPON shows an ionic behavior with electric double layer type energy storage, beyond this frequency, LiPON shows an electrostatic behavior with a dielectric constant of 14. The capacitor stack’s thin film structure and dual frequency behavior allow for extended frequency operation of these capacitors (100 Hz to 2000 MHz). Next, LiPON’s energy storage metrics are enhanced using pseudocapacitive energy storage behavior and enhanced surface area in ALD oxy-TiN. Finally, new fabrication techniques and ALD recipes are developed and optimized for integration into 3D templates. For fabrication of these capacitors, the material’s chemistry is analyzed, and ALD techniques are developed to enhance the deposition of electrode/electrolyte materials and current collectors into the 3D nanostructures. The intermixing during the ALD processes are studied to understand the behavior and reliability of these thin films. This work highlights LiPON characteristics as a capacitor material for high-energy and high-frequency applications. Though incomplete, we discuss progress towards the development of all ALD solid-state 3D supercapacitors that can compete against state-of-the-art capacitors available in the market.

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