UMD Theses and Dissertations

Permanent URI for this collectionhttp://hdl.handle.net/1903/3

New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a given thesis/dissertation in DRUM.

More information is available at Theses and Dissertations at University of Maryland Libraries.

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    IN-OPERANDO ELECTRON MICROSCOPY AND SPECTROSCOPY OF INTERFACES THROUGH GRAPHENE-BASED MEMBRANES
    (2017) Yulaev, Alexander; Leite, Marina S.; Kolmakov, Andrei; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Electron microscopy and spectroscopy (EMS) techniques enable (near-) surface and interfacial characterization of a variety of materials, providing insights into chemical/electrochemical and morphological information with nanoscale spatial resolution. However, the experimental realization of EMS in liquid/gaseous samples becomes problematic due to their incompatibility with high vacuum (HV) conditions. To perform EMS under elevated pressure conditions, electron transparent membranes made of thin C, SiO2 or/and Si3N4 are implemented to isolate a liquid/gas sample from HV environment. Nevertheless, even a few ten nanometer thick membrane deteriorates signal quality due to significant electron scattering. The other challenge of EMS consists in inaccessibility to probe solid state interfaces, e.g. solid-state Li-ion batteries, which makes their operando characterization problematic, limiting the analysis to ex situ and postmortem examination. The first part of my thesis focuses on developing an experimental platform for operando characterization of liquid interfaces through electron transparent membranes made of graphene (Gr)/graphene oxide (GO). The second part is dedicated to probing Li-ion transport at solid-state-battery surfaces and interfaces using ultrathin carbon anodes. I demonstrated the capability of GO to encapsulate samples with different chemical, physical, and biological properties and characterized them using EMS methods. I proposed and tested a new CVD-Gr transfer method using anthracene as a sacrificial layer. Characterization of transferred Gr revealed the advantages of our route with respect to a standard polymer based approach. A novel platform made of an array of Gr-capped liquid filled microcapsules was developed, allowing for a wide eld of view EMS. I showed the capability of conducting EMS analysis of liquid interfaces through Gr membranes using energy-dispersive X-ray spectroscopy, photoemission electron microscopy, and Auger electron spectroscopy. Using operando SEM and AES, I elucidated the role of oxidizing conditions and charging rate on Li plating morphology in all-solid-state Li-ion batteries with thin carbon anodes. Operando EMS characterization of Li-ion transport at battery interfaces with carbon or Gr anodes will provide valuable insights into safe all-solid-state Li-ion battery with enhanced performance.
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    STUDY OF CATALYTIC GROWTH OF OLEFIN NANO FIBRILS AND CARBON NANO FIBRILS OVER SOLID SURFACE AND ITS APPLICATION
    (2015) Lee, Sangyool; Choi, Kyu Yong; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    In this dissertation, the catalytic growth of nano fibrils over solid surface of different geometric types is studied and their applications are also investigated. The new experimental results on olefin polymerization with metallocene catalyst over silica supports of different geometries are presented. Flat surface silica, nano-sized spherical silica, straight cylindrical pore silica, macroporous silica, and conventional silica are used as support materials. The presence or absence of intraparticle monomer diffusion resistance and particle fragmentation has been shown to have significant effects on the catalytic activity. Also the effects of support geometry on the morphology of polymers and intrinsic catalytic activity are analyzed. The catalytic growth of olefin nano fibrils are applied in micro/milli reactors. Unlike many conventional olefin reactors, the reaction temperature, heat transfer and bimodal distribution of polymer molecular weight can easily be controlled in the micro/milli reactor systems developed during this study. The catalytic growth of carbon nano fibrils on silicon has been investigated for application as anode materials in Li-ion batteries. This research is aimed at developing a binder free silicon anode system that consists of a modified Cu foil (current collector), Si nanoparticles (SiNPs), and carbon nanotubes (CNTs). This anode system includes the nanostructured Cu surface layer as a hub for the Si nanoparticles that undergo deformation and fragmentation during the charge/discharge cycles. SiNPs are deposited with Fe-Co bimetallic catalyst and CNTs are grown in situ at the catalyst sites. The surface layer of the Cu is modified via an oxidation and reduction processes to have knife-like nanostructures with high void fractions. The SiNPs are deposited on/in to the nanostructured Cu foil without any binders. The CNTs growing at the surface of the SiNPs serve as the electron conductor and also holds the SiNP during the lithiation/delithiation cycles. Since Si/CNT particles are surrounded by thin protrusions on the surface of Cu current collector, the maximum connectivity between silicon and current collector can be obtained, and excellent cycle stability of the battery can be maintained without any binders.
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    ALD PROCESSES AND APPLICATIONS TO NANOSTRUCTURED ELECTROCHEMICAL ENERGY STORAGE DEVECES
    (2013) Chen, Xinyi; Rubloff, Gary W; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Next generation Li-ion batteries (LIB) are expected to display high power densities (i.e. high rate performance, or fast energy storage) while maintaining high energy densities and stable cycling performance. The key to fast energy storage is the efficient management of electron conduction, Li diffusion, and Li-ion migration in the electrode systems, which requires tailored material and structural engineering in nanometer scale. Atomic layer deposition (ALD) is a unique technique for nanostructure fabrications due to its precise thickness control, unprecedented conformality, and wide variety of available materials. This research aims at using ALD to fabricate materials, electrodes, and devices for fast electrochemical energy storage. First, we performed a detailed study of ALD V2O5 as a high capacity cathode material, using vanadium tri-isopropoxide (VTOP) precursor with both O3 and H2O as oxidant. The new O3-based process produces polycrystalline films with generally higher storage capacity than the amorphous films resulting from the traditional H2O-based process. We identified the crucial tradeoff between higher gravimetric capacity with thinner films and higher material mass with thicker films. For the thickness regime 10-120 nm, we chose areal energy and power density as a useful metric for this tradeoff and found that it is optimized at 60 nm for the O3-VTOP ALD V2O5 films. In order to increase material loading on fixed footprint area, we explored various 3-dimentional (3D) substrates. In the first example, we used multiwall carbon nanotube (MWCNT) sponge as scaffold and current collector. The core/shell MWCNT/V2O5 sponge delivers a stable high areal capacity of 816 μAh/cm2 for 2 Li/V2O5 voltage range (4.0-2.1 V) at 1C rate (nC means charge/discharge in 1/n hour), 450 times that of a planar V2O5 thin film cathode. Due to low density of MWCNT and thin V2O5 layer, the sponge cathode also delivers high gravimetric power density in device level that shows 5X higher power density than commercial LIBs. In the other example, Li-storage paper cathodes, functionalized of conductivity from CNT and Li-storage capability from V2O5¬, presented remarkably high rate performance due to the hierarchical porosity in paper for Li+ migration. The specific capacity of V2O5 is as high as 410 mAh/g at 1C rate, and retained 116 mAh/g at high rate of 100C. We found V2O5 capacities decreased by about 30% at high rates of 5C-100C after blocking the mesopores in cellulose fiber, which serves to be the first confirmative evidence of the critical role of mesoporosity in paper fibers for high-rate electrochemical devices. Finally, we made high density well-aligned nanoporous electrodes (2 billion/cm2) using anodic alumina template (AAO). ALD materials were deposited into the nanopores sequentially - Ru or TiN for current collection, and V2O5 for Li-storage. Ru metal by ALD shows high conductivity and conformality, and serves best as the current collector for V2O5. The capacity of V2O5 reaches about 88% of its theoretic value at high rate of 50C. Such electrodes can be cycled for 1000 times with 78% capacity retention.