Mechanical Engineering
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Item ULTRA-THIN ON-CHIP ALD LIPON AS SOLID-STATE ELECTROLYTE FOR HIGH ENERGY AND HIGH FREQUENCY CAPACITOR APPLICATIONS(2022) Ahuja, Kunal; McCluskey, F. Patrick; Rubloff, Gary W.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)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 by pseudocapacitive energy storage behavior and increased surface area using 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 for 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.Item Soot Oxidation in Flames: Nanostructure, Morphology, and Chemical Kinetics(2019) Anderson, Paul Marcus; Sunderland, Peter B.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Soot produced from the combustion of hydrocarbons is of immense scientific interest owing to its deleterious effects on human health and the environment. Despite decades of research, existing soot models are accurate across only a narrow range of combustion conditions. A substantial portion of this inaccuracy is rooted in the multitude of factors affecting soot oxidation that remain ill-understood. In the current work, a novel flame system allowed soot oxidation to be observed in isolation from competing soot formation processes. Measurements tracked evolving soot structures, oxidation rates, temperatures and gas species concentrations. Transmission electron microscopy (TEM) was used to characterize soot structure at aggregate, primary particle, and nanostructural scales. For this, a program called Aerosol Image Analyzer was developed, incorporating new algorithms for processing and measuring TEM images of mass-fractal aerosols, like soot. For the first time, TEM image measurement uncertainties incorporating sample, operator, and random effects, were quantified through gage repeatability and reproducibility analysis. Successful methods for reducing operator bias were presented, and automated measurement methods from literature were tested and found to be unreliable. Measurements of surface area by N2 adsorption validated TEM as a technique for determining soot specific surface area, provided that the polydispersity and partial sintering of primary particles is taken into account. TEM measurements of soot undergoing oxidation showed continuously decreasing primary particle size distributions and increasing specific surface area. Measurements of soot aggregate morphology found a fractal dimension of 1.74 that was unchanged by oxidation. The breakup of aggregates by oxidative fragmentation was observed for the first time using methods that combined TEM analysis with laser extinction. Soot nanostructure was characterized through high resolution TEM measurements of lattice fringe length, tortuosity, orientation, and separation distance. It was observed that primary particles could be divided into an inner 80%, where lattice fringes showed greater graphitic order with increasing radial location, and an outer 20%, where this trend was reversed. While oxidation proceeded in a shrinking-sphere manner at the particle surface, the interior underwent thermal and oxidation-induced graphitization, challenging the assumption that the nanostructure of mature soot is “locked-in.” This results in a surface nanostructure that is effectively unchanging from the perspective of the oxidizing gases and corresponds to a constant collision efficiency kinetics model.Item Electroosmotic Soft Actuators(2017) Sritharan, Deepa; Smela, Elisabeth; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation details the research involved in creating the first paper-based soft actuator driven by electroosmosis. To accomplish this, research breakthroughs were made in the fields of electrokinetic pumping and device manufacturing using soft materials. Electroosmosis is an electrically induced microfluidic flow phenomenon. When an electric field is applied to the fluid, across the microchannels, electroosmotic flow occurs in the direction of the applied electric field. In this work, liquid was electroosmotically displaced within a flexible microfluidic device to actuate an elastomeric membrane. The goal of this work was to create a fully sealed fluidic actuator. It was therefore necessary to encapsulate the pumping fluid within the device, and to maximize pressure it was necessary to eliminate compliance caused by trapped gases. Electrolytic gas formation is well known to disrupt pumping in DC electroosmotic systems that use water as the pumping liquid. In this work, electrolysis was eliminated by replacing water with propylene carbonate (PC): PC was determined to be electrochemically stable up to at least 10 kV, in the absence of moisture or salt contaminants. Bubble-free electroosmotic pumping with PC was achieved within sealed miniature actuators, which could be continuously operated for at least one hour. Benchtop fabrication techniques were developed to build encapsulated fluidic actuators composed entirely of soft, flexible materials. Stretchable electrochemically stable electrodes were made using a conductive paint made by mixing carbon nanoparticles into a silicone base. High-density microchannel networks were incorporated by using paper and other flexible porous materials, instead of conventional planar replica-molded microchannels. The device was filled with pumping fluid without the use of external tubing, and then encapsulated by casting a film of elastomer over the filled reservoir to form the actuating membrane. The resulting actuators were flexible and stretchable, demonstrating significant membrane deformations (hundreds of micrometers) within seconds of applying the electric field and ability to lift large loads (tens of grams). These polymeric electroosmotic actuators are unique among electroactive polymer actuators because they are able to simultaneously generate high force as well as large stroke. It is envisioned that this research will pave the way for the creation of artificial muscles and smart shape-changing materials that can be actuated by electroosmosis.Item IMPACT OF DUST ON THE RELIABILITY OF PRINTED CIRCUIT ASSEMBLIES(2013) Song, Bo; Pecht, Michael G; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Dust is a ubiquitous component of the environments in which we live and work. It can deposit on printed circuit assembly to act as a source of ionic contamination. Two common consequences of dust contaminations in the printed circuit boards are loss of impedance (i.e., loss of surface insulation resistance) and electrochemical migration between traces and component leads. Both failure mechanisms involve the contamination forming a current leakage path on a printed circuit board. Based on studies on ionic contaminations, researchers have argued that the impact of dust in these two failure mechanisms is dependent on its pH, its hygroscopic compositions, and the critical relative humidity of the salts in it. However, due to the lack of experimental results and the complexity of dust compositions, the argument is not substantiated. Very few papers concerning the impact of different natural dusts on these two failure mechanisms can be found in the literature. In practice, mixtures of Arizona dust and salts are used as a substitute for dust in experiments. In this research, natural dusts were collected from four locations: natural outdoor and indoor dust samples from Massachusetts, U.S., natural outdoor dust from Tianjin, China, and the ISO standard test dust (Arizona test dust). Loss of impedance in dust contaminated printed circuit boards was investigated under controlled temperature (20ºC to 60ºC) and relative humidity (50% to 95%) ranges. The impact of dust on electrochemical migration and corrosion was evaluated under temperature-humidity-bias tests (50ºC, 90% RH, and 10 VDC). In addition to the conventional DC measurement where only resistive data can be obtained, electrochemical impedance spectroscopy were adopted to obtain nonlinear equivalent circuit models of the electrochemical process, which helps to understand the underlying physics-of-failure. The variation of impedance with relative humidity exhibited a transition range. Below the range, the impedance was constant, and above it, the impedance degraded by orders of magnitude. The value of the transition range decreased with an increase of dust deposition density. The equivalent circuit modeling showed that the dominant resistive path gradually shifted from the bulk to the interfacial with the increase of temperature from 20 ºC to 60 ºC. There were big variations among different dusts, which were quantified using the degradation factor introduced in the research, the critical transition range, and time-to-failure. This result demonstrated that a single salt or a mixture of compounds can not be representative of all dusts. It also indicated that using the ISO standard test dust in place of natural dust samples for reliability evaluation could lead to inaccurate results. Dust should be collected from the field in order to evaluate its impact. It is showed in this thesis that some critical characteristics of dust can be used to classify different dusts for the failure mechanisms of interest. Moisture sorption capability of dust can be used to classify different dusts regarding the loss of impedance failure. The dust with the highest moisture sorption capability had the highest degradation factor. Ion species/concentration or conductivity of dust aqueous solution can be used to classify dust regarding the electrochemical migration related failures. Dust with the highest ion concentration and conductivity had the lowest time-to-failure. The underlying principals behind those critical characteristics were described and discussed based on the physics-of-failure.Item Probing the ignition mechanism of aluminum nanothermites(2012) Chowdhury, Snehaunshu; Zachariah, Michael R; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Nanothermites are defined as intimate mixtures of metal and metal oxidizer particles usually below 100 nm in diameter. They belong to a class of energetic materials which have been of recent interest due to their high amounts of stored energy, and their potential for future use in a variety of applications. Once ignited, nanothermites undergo self-sustaining reactions. Such reactions are very poorly understood due to the lack of proper diagnostic techniques replicating the heating rates in self-sustaining reactions. We use a temperature jump (T-jump) technique by heating a thin platinum wire to study the nanothermite reactions at heating rates of 10 5 K/s. First we study the ignition initiation mechanism in Al-CuO nanothermites and show that there is an inherent ignition delay, i.e., ignition occurs after the electric pulse is shut off. This ignition delay increases progressively as the oxide shell thickness is increased, suggesting that the reacting species have to move across the shell. T-jump time of flight mass spectrometry (T-jump TOFMS) is used qualitatively to support such a claim. Several nanothermites are also tested for their ignition temperature. The oxidizers were chosen based on their behavior towards heating. For several oxidizers (CuO, Fe2O3, KClO4 etc.) ignition in the nanothermites is noticed to occur when the oxidizers release oxygen using T-jump TOFMS. Complementary electron microscopy techniques show that Al-CuO reactions can occur even in the absence of oxygen, via reactive sintering mechanism. Furthermore, electron microscopy techniques are used to show evidence of condensed phase initiation in other nanothermites. The role of positive ions in correlation to ignition in nanothermites is also studied for selected nanothermites using the T-jump TOFMS. Positive ions are seen to be generated during the ignition interval and are found to consist primarily of Na+ ions. A hypothesis for such observation is proposed and is seen to be consistent with molecular dynamics simulations from literature.