Theses and Dissertations from UMD

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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 give thesis/dissertation in DRUM

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

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    INTERFACIAL INTERACTIONS OF NANOTUBES: AN IN-SITU STUDY OF STRUCTURE AND REACTIONS WITH THEIR ENVIRONMENTS
    (2021) Chao, Hsin-Yun; Cumings, John; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Nanotubes have the potential to be a revolutionary material for many different applications. Though often touted as versatile and tunable materials, the difficulty of their reliable large-scale production for any specified property is a hurdle in their wide-scale implementation. Interactions at nanotube interfaces dictate overall performance of their growth, radiation resistance, and nanofluidics properties. In this dissertation, I present in-situ experiments using an environmental transmission electron microscope (ETEM). Numerous aspects of interfacial mechanisms of nanotubes are examined at the atomic scale and models considered for the observed behavior. First, I study the interface between nanotubes and catalyst particles during single-walled carbon nanotube (SWCNT) growth. The structure and phase transformation of cobalt catalysts are elucidated for inactive, active, and deactivated nanoparticles by ETEM imaging. Through in-depth studies of multiple distinct cobalt nanoparticles, I establish the dominant nanoparticle phase for SWCNT growth. I also identify the preferred lattice planes and a threshold for work of adhesion for the anchoring and liftoff of SWCNTs. Second, the nanotubes are tested for their radiation resistance properties. I study the resistance of nanotube degradation in an ionizing environment with oxygen pressure, where the damage initiates at the interface with the gas phase. Observations show boron nitride nanotubes (BNNTs) have a higher resistance to damage than carbon nanotubes (CNTs). By computing knock-on threshold energies for the atoms impacted by incoming electrons, a model can be formulated for the oxygen-assisted radiation damage pathway. I provide further validation to the model with heating experiments that demonstrate a surprising increase in damage resistance. Lastly, interfaces between nanotubes and water are studied. The goal is to capture the ordering dynamics of water at the BNNT interface using in-situ characterization at cryogenic temperatures. Water is hyper-quenched to liquid nitrogen temperatures for the formation of low density amorphous (LDA) ice. High resolution images are then acquired, preserving the original water structure. Crystallization of LDA ice is induced by both environmental heating and electron beam irradiation. I present a comparison of the structural evolutions of LDA ice with and without the presence of BNNT, which indicates the presence of nascent ordering at the interface.
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    PROGRESSIVE MICROSCOPIC DAMAGE AND THE DEVELOPMENT OF MACROSCOPIC FRACTURE IN POROUS SANDSTONES
    (2011) Tamarkin, Thomas Francis; Zhu, Wenlu; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The precursory phenomena associated with dilatancy have been extensively studied as a potential means of earthquake prediction. It is known that microstructural damage induced dilatancy precedes macroscopic failure of a rock. However, the quantitative relationship between microstructural damage and fault development is not clearly understood. To better understand the mechanics of brittle faulting of rock and the association of precursory phenomena with faulting, a detailed microstructural study was conducted on porous sandstone deformed to different post-failure stages at different strain rates. A lateral relaxation loading configuration was adopted in which a cylindrical sample is deformed under decreasing radial stresses while the axial load remains constant. This loading path was proven to successfully map out the brittle failure envelope. Compared to conventional triaxial deformation testing, the relaxation loading configuration greatly increases the stability of fault growth. A suite of samples were deformed and subsequently unloaded at different post-failure stages, before macroscopic faulting occurred. Progressive microstructural damage was investigated via quantitative characterization of crack damage indices, crack density, and changes in porosity. Ultimately, this research will lead to an improved comprehension of the relationship between microscopic damage and macroscopic fracture development, providing a better insight into the brittle failure process.