Browsing by Subject "Materials science"
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Item Carbon composite strengthening: Effects of strain rate sensitivity and feature size(2012-05) Brannigan, Eric; Jankowski, Alan F.; Rivero, Iris V.; Idesman, Alexander V.Strain rate sensitivity of strength is analyzed for a bulk, turbostratic carbon reinforced epoxy resin composite. The strength of the composite was measured using a rate-modified version of the standard, 3-point bending test. Rate sensitivity of stress was calculated by varying the strain rate of stress on the samples, and measuring the increase in yield strength. Metal reinforced carbon matrix composite coatings were also examined, with CuC, NiC, and CuNiC samples analyzed using nano-indentation and tapping mode AFM hardness and modulus measurements. The carbon structures within the coatings are nanoscale, and characterization of the carbon features in the coatings and the bulk fiber composite allow for conclusions to be drawn regarding the structured relationship within metallic and non-metallic carbon composites. For the fibers, we find that bending strength is rate sensitive as attributed to the turbostratic carbon-fiber component. The material has a strength to weight ratio comparable to Ti-6Al-4V alloy. For the coatings, we find that the hardness and elastic modulus are dependent on whether the morphology is layered versus particulate, with the nanodisperse morphology having the highest hardness and elastic modulus.Item Data driven analysis of fast oxide ion diffusion in solid oxide fuel cell cathodes(2015-08) Miller, Alexander Scot; Benedek, Nicole; Yu, GuihuaThe goal of this study was to determine whether atomic-scale features (related to composition and crystal structure) of perovskite and perovskite-related materials could be used to predict fast oxide ion diffusion for Solid Oxide Fuel Cell (SOFC) applications; materials that can be used as SOFC cathodes were a particular focus. One hundred and twenty six pairs of diffusion (D*) and surface exchange (k*) coefficients for a variety of materials were collected from literature sources published between 1991 and 2015. A website was created with these data for public viewing. Statistical tests revealed that diffusion measurements have significant differences at 400K, 700K, and 1000K when grouped according to material family and sample type. Models predicting diffusion rates were created from atomic-scale features at several temperatures between 400K and 1000K. Perovskite and double-perovskite models explained >85% of the variance in ln(D*k*) at 800K-1000K, meaning these models successfully predicted ln(D*k*) more than 85% of the time. These models explained 55%-75% of the variance at lower temperatures (400K-700K). Materials whose B-site cations had the highest electron affinities showed the fastest diffusion at all temperatures. Thus, these models suggest using B-site cations with high electron affinities (i.e. atoms that are easily reduced) may increase fuel cell performance, even at low and intermediate temperatures.Item Novel synthesis of nanostructured electrode materials for lithium-ion batteries(2010-08) Theivanayagam, Murali Ganth; Manthiram, ArumugamLithium-ion batteries have revolutionized the portable electronics market, and they are currently pursued intensively for vehicle applications and storage of renewable energies (solar and wind energy). Cost, safety, cycle life, and energy and power densities are the critical parameters for these applications. With this perspective, there has been immense interest to develop new cathode and anode materials as well as to develop novel synthesis and processing approaches. This dissertation explores the use of novel synthesis approaches to obtain high-performance, nanostructured phosphate and silicate cathodes and iron oxide nanowire anodes and investigates their structure-property relationships. First, a novel microwave-solvothermal (MW-ST) approach has been developed to synthesize phase-pure, highly crystalline LiFePO₄ nanorods within 5-15 minutes at low temperatures of < 300 °C, without requiring reducing gas atmospheres. The LiFePO₄ nanorods, after forming a nanocomposite with conducting polymer or multi-walled carbon nanotubes or coating with conductive carbon, offer excellent cycle life and rate performance when implemented as cathodes in lithium-ion cells. In addition, other LiMPO₄ (M = Mn, Co, and Ni) olivine nanorods have also been synthesized by the MW-ST approach and characterized. The MW-ST process has then been extended to prepare a new class of carbon-coated, nanostructured silicates of the formula Li₂MSiO₄ (M = Fe and Mn). These materials have two times higher theoretical capacities (~ 330 mAh/g) than olivine phosphates (~ 170 mAh/g). Li₂FeSiO₄ exhibits practical discharge capacities of 148 mAh/g at room temperature and 203 mAh/g at 55 °C, with good rate capability and stable cycle life. Li₂MnSiO₄, on the other hand, shows higher discharge capacities of 210 mAh/g at room temperature and 250 mAh/g at 55 °C, but it exhibits poor rate performance and rapid capacity fade during cycling. In addition, carbon-coated olivine solid solution nano-particles of the formula LiM[subscript 1-y]M[subscript y]PO₄ (M = Fe, Mn, Co, and Mg), synthesized by a facile, high-energy mechanical milling process (HMME), have also been investigated. The electrochemical data reveal a systematic shift in the redox potential (open-circuit voltage) of the M²⁺/³⁺ couples in the LiM[subscript 1-y]M[subscript y]PO₄ solid solutions compared to those of the pristine LiMPO₄. The shifts in the redox potentials have been explained by the changes in the M-O covalence (inductive effect), which are caused by changes in the electronegativity of M or the M-O bond length or M-O-M interactions. Finally, a two-step microwave-hydrothermal process has been developed to synthesize carbon-decorated, single-crystalline Fe₃O₄ nanowires. The resulting iron oxide nanowires exhibit capacity values > 800 mAh/g with stable cycle life and high rate performance as an anode in lithium-ion cells.Item Test and characterization of engineering nanocoatings for mems and nanoenergetic materials(2012-05) Vijayasai, Ashwin; Dallas, Timothy E. J.; Gale, Richard O.; Pantoya, Michelle; Weeks, Brandon L.; Hase, William L.; Yeo, ChangdongThis dissertation presents the development, test and characterization of engineered nanocoatings for MEMS and nano-energetic reactive materials. Surface modification on MEMS and nano-energetic reactive materials are developed using a commercially available nanocoating tool. Surface modifications include Chemical Vapor Deposition of Fluorocarbon SAM and nanoparticles and Atomic Layer Deposition of thin oxides. Detailed descriptions of the nanocoating process and their chemical reactions are explained. An F-SAM coated MEMS tribogauge is characterized to estimate the adhesive and frictional forces. In-situ frictional measurements were made. Increasing adhesion force was observed for increasing number of load cycles. The tribogauge is later used as an ex-situ characterization tool to observe the performance of various nanocoating recipes for F-SAM coating. Characterization of the tribogauge is performed using an electronic sense tool. Contact angle goniometer was used to characterize the performance of various recipes. Various types of nanocoatings were deposited on witness samples and nano-energetic materials. A comparison study of underwater combustion tests were made on these thermite pellets. An aging study was performed on both nanocoated witness samples and pellets. The aging experiment is performed by submerging them in de-ionized water for 10 days. Contact angle goniometer and few optical microscopes were used to characterize the performance of various recipes. Apart from the nanocoating based projects, this dissertation briefly explains other projects that were part of the graduate program. A brief description and initial results of a few MEMS device designs are explained in this dissertation. As part of future work new MEMS devices were designed that will allow follow-up nanocoatings projects.Item Thermal characterization of InAs interfacial misfit arrays using nanosecond thermoreflectance method(2015-05) Nguyen, Khai Ta; Wang, Yaguo; Bahadur, VaibhavThermal properties are of the utmost importance because of the ever growing demands given to us by high-power and ultrafast electronics. A nanosecond thermoreflectance method was developed to determine the thermal conductivities of InAs interfacial misfit arrays (IMF). These interfacial misfit arrays were designed to improve the optical properties, such as photoluminescence, of these materials in order to improve electronic devices. A study was performed to see if the thermal properties of these materials were affected in any way. The nanosecond thermoreflectance method was benchmarked with control samples of InAs and GaAs substrates, and the thermal conductivities were close to that of bulk value. After performing the experiments, it was found that the thermal conductivity varies inversely with photoluminescence. It was also found that the thermal interface resistance between the growth and the substrate was inversely proportional to the thickness of the IMF growth.