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Theses and Dissertations

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Browsing Theses and Dissertations by Author "Bailey, Travis, committee member"

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    Cationic-doping of mayenite electride: synthesis, processing, and effect on thermal stability
    (Colorado State University. Libraries, 2021) DeBoer, Brodderic, author; Ma, Kaka, advisor; Weinberger, Chris, committee member; Bailey, Travis, committee member; Bandhauer, Todd, committee member
    Mayenite electride is an electrically conductive ceramic developed from its parent phase, oxy-mayenite (12CaO•7Al2O3, commonly referred to as C12A7). C12A7 has a unique unit cell that consists of a positively charged [Ca24Al28O64]4+ framework containing twelve cages and two extra-framework O2- ions located inside two cages. The extra-framework O2- ions can be replaced with electrons when C12A7 is heated in a reducing environment, and those extra-framework electrons act like anions, forming the mayenite electride phase, denoted as C12A7:e- hereafter. The anionic electrons enable peculiar properties of C12A7:e- such as high electrical conductivity and low work function, making it a promising material for field emission devices, thermionic-cooling, and as a hallow cathode for electrical propulsion. Compared to other electride materials such as Ca2N, which barely sustain their electride properties even at ambient conditions, C12A7:e- has been reported to be stable up to 400 °C. This temperature is yet not high enough to enable its applications in the technologies mentioned above. Doped derivatives of C12A7:e- emerged in recent years to improve its electronic properties, mainly electron density and electrical conductivity. However, the effects of doping on the oxidation resistance and thermal stability of C12A7:e- remained unclear. Experimental effort on cationic doping of C12A7:e- was particularly lacking in the literature. Therefore, the goal of this study is two-fold: (1) to develop processing routes for successful cationic doping of C12A7:e-, and (2) to test if cationic doping can improve the thermal stability of C12A7:e-. Copper (Cu) and niobium (Nb) were selected as cationic dopants in this study to elucidate how cationic doping affects the thermal stability of the mayenite electride. First, effort was focused on developing synthesis and processing methods to effectively dope Cu and Nb into C12A7:e-. Three different methods were investigated, including diffusion doping; in conventional furnace or via spark plasma sintering (SPS), single-step in-situ formation via SPS, and a solid-state reaction (SSR) synthesis followed by reduction. The phase constitutions, lattice parameters, and microstructure of the various C12A7:e- samples fabricated via the aforementioned methods were characterized to verify if cationic doping was successfully achieved. Electrical conductivity was measured to verify the electride phase is sustained after the doping. Thermal analysis was performed to determine the thermal stability of the cation-doped C12A7:e- compared to undoped counterparts, including onset temperature and peak temperature of oxidation, oxidation rate, mass gain percentage resulted from oxidation, and any decomposition reaction. The key findings of this study include: (1) both Cu-doping and Nb-doping improved the thermal stability of the C12A7:e- by increasing the onset temperatures of oxidation; (2) Cu-doping was effectively and efficiently achieved via the novel SPS diffusion doping method. SPS diffusion doping of Cu at 800 °C gave rise to a minimum lattice parameter (a = 11.942 Å) of C12A7:e-, the lowest oxidation rate, and the smallest mass gain percent at 1050 °C; (3) Using oxy-mayenite and Nb2O5 as precursor for reaction sintering and in-situ reduction in SPS led to successful Nb-doping into the C12A7:e-. Despite the increased onset oxidation temperature resulted from Nb addition, pest oxidation occurred in Nb-doped C12A7:e- samples, leading to high oxidation rate, high total mass gain percentage, and fracture of the solid samples at temperature above 700 °C. In conclusion, Cu-doping was experimentally proved to be an effective approach to improve the thermal stability of C12A7:e- and meanwhile increase the electrical conductivity.
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    Preparation and characterization of poly lactic-co-glycolic nanoparticles encapsulated with gentamicin for drug delivery applications
    (Colorado State University. Libraries, 2019) Sun, Yu, author; Li, Yan Vivian, advisor; Bailey, Travis, committee member; Wang, Zhijie, committee member
    Wound treatment has always been a popular topic around the world. Since the emergence of nanotechnology, the development and design of novel wound dressing materials have been dramatically improved. The ues of nanoparticles encapsulated with antibiotics to deliver drugs has been shown to be a potentially effective approach to control bacterial infections at a wound position. Recently, biodegradable and biocompatible polymers have drawn lots of attention for the manufacture of drug-loaded nanoparticles in the pharmaceutical industry. In this work, poly-lactic-co-glycolic acid (PLGA) was used in nanoparticle synthesis due to its biodegradability, biocompatibility, and nontoxicity. For this work, gentamicin was loaded into the PLGA nanoparticles as an antibiotic because it is a broad-spectrum antibiotic effective in wound treatments. PLGA nanoparticles were prepared while gentamicin was loaded in the nanoparticles via a double emulsion evaporation method. Poly vinyl alcohol (PVA) was a surfactant that was an important factor in determining the most probable nanoparticle size and morphology. When the PVA concentrations were 9% and 12%, the nanoparticles demonstrated a spherical structure with a porous surface. The porous surface of a nanoparticle was promising for the purpose of releasing encapsulated antibiotics. Another important factor in determining the formation of nanoparticles was the PLGA concentration. Poly lactic-co-glycolic acid (PLGA) was the main material affecting PLGA nanoparticles' properties. PLGA nanoparticles would have various release profiles, morphology, and size distribution with different PLGA concentrations. The results suggested that different PLGA concentrations can endow PLGA nanoparticles with various properties which can lead to different applications of PLGA nanoparticles.
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    Stable and unstable tiling patterns of ABC miktoarm triblock terpolymers studied via GPU-accelerated self-consistent field calculations
    (Colorado State University. Libraries, 2022) Hawthorne, Cody, author; Wang, David, advisor; Bailey, Travis, committee member; Miyake, Garrett M., committee member; Szamel, Grzegorz, committee member
    Block copolymers are macromolecules formed from linking together two or more chemically distinct types of polymers. Provided the different monomers that make up each polymer are immiscible enough, melts of these molecules will self-assemble into highly ordered, periodic structures at length scales typically on the order of nanometers. The exemplary and simplest material in this respect is the AB diblock copolymer, a linear macromolecule formed by bonding together two immiscible polymers (or 'blocks') A and B. This material is capable of assembling into lamellar, cylindrical, spherical, and networked morphologies depending on the length of the A block and degree of immiscibility between A and B. The ability to control bulk properties of block copolymers via tuning these molecular properties, as well as the length scales that these ordered structures form at, makes them intriguing candidates for next generation technological applications in lithography, photonics, and transport. In order to realize these applications it is imperative to have an intimate understanding of the phase behavior of the materials such that the morphology that will form at a given combination of parameters can be predicted reliably. Self-consistent field theory, or SCFT, has emerged as a useful theory for investigating block copolymer phase behavior. This statistical-mechanical theory has been successfully used to construct phase diagrams of the self-assembled morphologies of various block copolymer systems. These phase diagrams provide the connection between molecular properties (such as block lengths, block incompatibility, and chain architecture) and bulk properties necessary in order to control the behavior of the material. The theory must, in general, be solved numerically – an open-source software termed 'PSCFPP' has recently been made available for this purpose, capable of implementing high-performance SCFT calculations for arbitrarily complex acyclic block copolymers by taking advantage of the massive parallelization of GPUs. In this work, PSCFPP is used to apply SCFT to a neat melt of complex ABC miktoarm triblock terpolymers, which are an interesting class of block copolymer formed by linking three distinct polymers A, B, and C at a single junction point. The resulting star-shaped macromolecule is referred to as a 'miktoarm' and exhibits unique morphologies such as the Archimedean tiling patterns that cannot be found in other block copolymer materials. To focus on the effect of composition, which has not yet been fully elucidated, we restrict the interaction parameters between monomers ABC to the symmetric case where all are equivalent. The central region of the phase diagram, where the effect of the miktoarm architecture is most significant, is mapped out in detail and a 3D morphology previously thought to be metastable is shown to be a stable phase. Further, discrepancies in the literature concerning the stability of multiple 2D tiling patterns are resolved such that the phase diagram presented is the most accurate for the system to date. Finally, a 2D morphology of some interest owing to the possibility of exhibiting photonic band gaps is definitively shown to be stable in this system and its thermodynamic properties analyzed to ascertain what drives its formation. These results provide a solid foundation for further refinement of our understanding of ABC miktoarm phase behavior and demonstrate the utility of a software such as PSCFPP for obtaining high-accuracy SCF results.