A. James Clark School of Engineering

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The collections in this community comprise faculty research works, as well as graduate theses and dissertations.

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    Explorations of Carbon-Nanotube-Graphene-Oxide Inks: Printability, Radio-Frequency and Sensor Applications, and Reliability
    (2022) Zhao, Beihan; Das, Siddhartha SD; Dasgupta, Abhijit AD; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    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.
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    ALD-ENABLED CATHODE-CATALYST ARCHITECTURES FOR LI-O2 BATTERIES
    (2015) Schroeder, Marshall Adam; Rubloff, Gary W; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The Li-O2 electrochemical redox couple is one of the prime candidates for next generation energy storage. Known for its impressive theoretical metric for specific energy, even current practically obtainable values are competitive with state of the art Li-ion intercalation chemistries and the achievable performance of batteries featuring this nascent technology will continue to improve as fundamental scientific challenges in each component of the device are addressed. The positive electrode is particularly complicated by its role as a scaffold for oxygen reduction and evolution, exhibiting sluggish kinetics, poor chemical stability, and limited cyclability due to parasitic side reactions. Fortunately, recent Li-O2 research has shown some success in improving the performance and cyclability of these O2 cathodes by shifting toward nanostructured architectures with catalytic functionalizations. Atomic layer deposition (ALD) is one of the most promising enabling technologies for fabricating these complex heterostructures. Offering precise control of film thickness, morphology, and mass loading with excellent conformality, this vapor-phase deposition technique is applied in this work to deposit thin film and particle morphologies of different catalyst chemistries on mesostructured carbon scaffolds. This thesis dissertation discusses: (1) development of a lab-scale infrastructure for assembly, electrochemical testing, and characterization of Li-O2 battery cathodes including a custom test cell and a state of the art integrated system for fabrication and characterization, (2) design, fabrication, testing, and post-mortem characterization of a unique 3D cathode architecture consisting of vertically aligned carbon nanotubes on an integrated nickel foam current collector, (3) atomic layer deposition of heterogeneous ruthenium-based catalysts on a multi-walled carbon nanotube sponge to produce a freestanding, binder-free, mesoporous Li-O2 cathode with high capacity and long-term cyclability, (4) evaluation of dimethyl sulfoxide as an electrolyte solvent for non-aqueous Li-O2 batteries, and (5) investigation of the relative importance of passivating intrinsic defects in carbon redox scaffolds vs. introduction of heterogeneous OER/ORR catalysts for improving the long-term stability and cyclability of these Li-O2 electrodes.
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    High Frequency Electrical Transport Properties of Carbon Nanotubes
    (2010) Cobas, Enrique Darío; Fuhrer, Michael S; Takeuchi, Ichiro; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Carbon nanotubes (CNTs) have extraordinary electronic properties owing to the unique band structure of graphene and their one-dimensional nature. Their small size and correspondingly small capacitances make them candidates for novel high-frequency devices with cut-off frequencies approaching one terahertz, but their high individual impedance hampers measurements of their high-frequency transport properties. In this dissertation, I describe the fabrication of carbon nanotube Schottky diodes on high-frequency compatible substrates and the measurement of their rectification at frequencies up to 40GHz as a method of examining the high-frequency transport of individual CNTs despite their high impedance. The frequency dependence of the rectified signal is then used to extract the Schottky junction capacitance as a function of applied bias and ambient doping and to look for resonances which might be a signature of a room-temperature Luttinger Liquid.
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    Structure and Properties of Nanocomposites Containing Anisotropic Nanoparticles
    (2007-10-08) Cipriano, Bani Hans; Raghavan, Srinivasa R; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    This dissertation deals with polymeric materials containing dispersed anisotropic nanoparticles such as nanotubes, nanofibers, and nanoplatelets. Such polymer nanocomposites have attracted much attention since they exhibit a host of superior properties over the parent polymer. For example, nanoparticles can impart flame-retardancy, high electrical conductivities, and high mechanical stiffness to the polymer. Despite the growing interest in these materials, many aspects remain poorly characterized, and the connection between properties and microstructure is still not fully understood. This provides the motivation for the present study. In the first part of this study, we focus on the flammability behavior of polymer nanocomposites containing multi-walled carbon nanotubes (MWNTs). It has been shown that MWNTs impart flame-retardancy to the polymer at low loadings, and moreover, the flame-retardancy correlates with the rheological properties of the nanocomposite. Here, we show that the aspect ratio of MWNTs is a key parameter in controlling both the rheology and flammability. Particles with a larger aspect ratio impart much higher storage moduli and complex viscosities to the nanocomposites compared to equivalent mass loadings of particles with a smaller aspect ratio. Additionally, in flammability experiments, the larger-aspect-ratio particles lead to a greater reduction in mass loss rate, i.e., they are more effective at reducing flammability. In the second part of this study, we focus on the conductivity of nanocomposites containing particles such as MWNTs or carbon nanofibers (CNFs). When these materials are processed by compression molding or melt extrusion, the conductivities of the resulting composites are often found to be disappointingly low. Here, we show that the conductivities can be increased, sometimes by orders of magnitude, simply by subjecting the sample to quiescent annealing at temperatures above the polymer's glass transition temperature (Tg). We demonstrate these results for both MWNT and CNF-based composites in polystyrene (PS). The mechanism behind the conductivity increase is shown to involve an increase in the connectivity of the particle network, which is reflected in dynamic rheological measurements as an increase in the plateau modulus at low frequencies. In the final part of this study, we present a simple method to improve the rheology and flammability properties of nanocomposites formed from polymers and clay platelets. These materials are usually made by combining a polymer with a commercial organoclay powder. We show that by fractionating the clay to exclude low-aspect ratio particles and aggregates, we can improve their dispersion (exfoliation) in the polymer. The resulting composites have higher optical transparency and better rheological properties for a given mass loading of clay. When these composites are subjected to a flame, we find a more uniform residue when compared to samples with the commercial organoclay. The fractionated clay also shows an interesting behavior when dispersed in water, where it forms birefringent gels at high particle loadings.
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    Synthesis and integration of one-dimensional nanostructures for chemical gas sensing applications
    (2007-04-30) Parthangal, Prahalad Madhavan; Zachariah, Michael R; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The need for improved measurement technology for the detection and monitoring of gases has increased tremendously for maintenance of domestic and industrial health and safety, environmental surveys, national security, food-processing, medical diagnostics and various other industrial applications. Among the several varieties of gas sensors available in the market, solid-state sensors are the most popular owing to their excellent sensitivity, ruggedness, versatility and low cost. Semiconducting metal oxides such as tin oxide (SnO2), zinc oxide (ZnO), and tungsten oxide (WO3) are routinely employed as active materials in these sensors. Since their performance is directly linked to the exposed surface area of the sensing material, one-dimensional nanostructures possessing very high surface to volume ratios are attractive candidates for designing the next generation of sensors. Such nano-sensors also enable miniaturization thereby reducing power consumption. The key to achieve success in one-dimensional nanotechnologies lies in assembly. While synthesis techniques and capabilities continue to expand rapidly, progress in controlled assembly has been sluggish due to numerous technical challenges. In this doctoral thesis work, synthesis and characterization of various one-dimensional nanostructures including nanotubes of SnO2, and nanowires of WO3 and ZnO, as well as their direct integration into miniature sensor platforms called microhotplates have been demonstrated. The key highlights of this research include devising elegant strategies for growing metal oxide nanotubes using carbon nanotubes as templates, substantially reducing process temperatures to enable growth of WO3 nanowires on microhotplates, and successfully fabricating a ZnO nanowire array based sensor using a hybrid nanowire-nanoparticle assembly approach. In every process, the gas-sensing properties of one-dimensional nanostructures were observed to be far superior in comparison with thin films of the same material. Essentially, we have formulated simple processes for improving current thin film sensors as well as a means of incorporating nanostructures directly into miniature sensing devices. Apart from gas sensing applications, the approaches described in this work are suitable for designing future nanoelectronic devices such as gas-ionization, capacitive and calorimetric sensors, miniature sensor arrays for electronic nose applications, field emitters, as well as photonic devices such as nanoscale LEDs and lasers.