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.

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Subject: search.filters.subject.Nanocomposites

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    Understanding the Relationships Between Architecture, Chemistry, and Energy Release of Energetic Nanocomposites
    (2017) DeLisio, Jeffery Brandon; Zachariah, Michael R; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Energetic nanocomposites are a class of reactive material that incorporate nanosized materials or features in order to enhance reaction kinetics and energy densities. Typically, these systems employ metal nanoparticles as the fuel source and have demonstrated reactivities orders of magnitude larger than more traditionally used micron-sized metal fuels. One drawback of using nanosized metals is that the nascent oxide shell comprises a significant weight percent as the particle size decreases. This shell also complicates the understanding of oxidation mechanisms of nanosized metal fuels. In this dissertation, I apply a two-fold approach to understanding the relationships between architecture, chemistry, and energy release of energetic nanocomposites by 1) investigating alternative metal fuels to develop a deeper understanding of the reaction mechanisms of energetic nanocomposites and 2) creating unique microstructures to tailor macroscopic properties allowing for customizability of energetic performance. In order to accurately study these systems, new analytical techniques capable of high heating rate analysis were developed. The oxidation mechanisms of tantalum nanoparticles was first probed using high heating rate TEM and Temperature-Jump Time-of-Flight Mass Spectrometry (T-Jump TOFMS) and shell crystallization was found to plan an important role in the mechanism. An air-sensitive sample holder was developed and employed to analyze the decomposition and oxidation of molecular aluminum compounds, which theoretically can achieve similar energy release rates to monomolecular explosives in addition to much higher energy densities. In order to obtain simultaneous thermal and speciation data at high heating rates, a nanocalorimeter was integrated into the TOFMS system and measurements were performed on Al/CuO nanolaminates to probe the effect of bilayer thickness on energy release. An electrospray based approach to creating energetic nanocomposites with tunable architectures is also described. An in depth study on the electrospray synthesized nAl/PVDF thin film reaction mechanism was performed using T-Jump TOFMS. The nAl/PVDF system was also studied using a Molecular Beam Sampling Time-of-Flight Mass Spectrometer designed and built primarily to investigate the reaction mechanisms of energetic nanocomposites at 1 atm in both aerobic and anaerobic environments.
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    NANOTUBE-MATRIX INTERPLAY AND TUNABILITY IN ULTRAHIGH VOLUME-FRACTION ALIGNED CARBON NANOTUBE POLY(URETHANE-UREA) NANOCOMPOSITES
    (2017) Gair, Jeffrey Lynn; Bruck, Hugh A.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The present dissertation seeks to better understand the nature of biphasic poly(urethane-urea) (PUU) interactions in materials with densely packed, aligned carbon nanotubes (CNTs). Of particular interest are the CNT-matrix interactions with in-situ polymerized PUU of various stoichiometric ratios. A novel synthesis method for PUU which permits fabrication of PUU-based polymer nanocomposites (PNCs) has been developed. Study of the thermal and multiscale mechanical behavior of stoichiometrically varied PUU materials has been conducted to demonstrate significant interaction between the matrix and CNTs, both in terms of morphology and mechanical reinforcement. PNCs with CNT Vf up to 30% have been achieved with excellent wetting confirmed via Micro-CT. TGA and DSC have revealed that CNTs stabilize thermal degradation of PUU by inducing crystallinity and reducing phase-mixing. AFM confirmed this by visualizing the crystals present in the matrix materials. CNT-induced crystallinity and phase-separation are attributed to the binding of CNTs to hard segments, which limit chain mobility during polymerization. Higher CNT Vf PNCs were found to increase soft-segment crystallinity, though with diminishing returns. Extreme crystallinity was found at 10% Vf CNTs which is though to arise due to an optimized spacing to permit ordered crystal formation of the PUU. Enhancements to indentation modulus of up to 1600% in the transverse orientation and 3500% in the axial orientation have been recorded via quasi-static nanoindentation. Greater CNT Vf and greater hard-segment composition lead to reduced chain mobility, and in some instances, can reduce CNT effectiveness in mechanical enhancement. The 10% CNT Vf exhibits greater indentation and storage moduli arising which is thought to arise from an optimized balance of inter-CNT spacing and chain mobility. Furthermore, PUU with higher hard-segment content is highly anisotropic and highly rate-sensitive, indicating significant morphological interactions with inter-CNT spacing of ~18nm. Degradation and increased loss modulus are seen in similar PUU with 20% loading, pointing to weak chain interactions and reduced hydrogen, likely do to confinement and reduced mobility. A model has also been developed which sheds light on the evolution of CNT-matrix interactions across a wide range of CNT volume-fractions.
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    SIZE MODIFICATION AND COATING OF TITANIUM DIOXIDE USING A PREMIXED HYDROGEN/AIR FLAME
    (2006-08-22) Lee, Seungchan; Ehrman, Sheryl H; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    A study was conducted of the effect of flame processing on the size distribution of titania nanoparticles, and a flame process was developed for producing TiO2/SiO2 coreshell particles from aqueous suspensions of TiO2 and SiO2 nanoparticles. Both were performed using a premixed hydrogen/air flame. At the adiabatic flame temperature of 2400 K, the number mean diameter of titania primary particle increased considerably from an initial value of 44 nm to 96 nm, presumably by atomic diffusion, and viscous flow coalescence. Moreover, the majority of product particles from this high flame temperature were smooth and spherical. Based on the results of size modification experiments, coating experiments were performed. The dominant morphology observed in the product particles from coating experiments was silica coated titania. The silica coating was very smooth and dense. The total particle size and the shell volume of the product particles were in reasonable agreement with values predicted from the atomized droplet size distribution and the droplet concentration.