Browsing by Subject "microstructure"
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Item Antimicrobial Activity of Cationic Antiseptics in Layer-by-Layer Thin Film Assemblies(2010-07-14) Dvoracek, Charlene M.Layer-by-layer (LbL) assembly has proven to be a powerful technique for assembling thin films with a variety of properties including electrochromic, molecular sensing, oxygen barrier, and antimicrobial. LbL involves the deposition of alternating cationic and anionic ingredients from solution, utilizing the electrostatic charges to develop multilayer films. The present work incorporates cationic antimicrobial agents into the positively-charged layers of LbL assemblies. When these thin films are exposed to a humid environment, the antimicrobial molecules readily diffuse out and prevent bacterial growth. The influence of exposure time, testing temperature, secondary ingredients and number of bilayers on antimicrobial efficacy is evaluated here. Additionally, film growth and microstructure are analyzed to better understand the behavior of these films. The antimicrobial used here is a positively-charged quaternary ammonium molecule (e.g. cetyltrimethylammonium bromide [CTAB]) that allow assemblies to be made with or without an additional polycation like polydiallyldimethylamine. While films without this additional polymer are effective, they do not have the longevity or uniformity of films prepared with its addition. All of the recipes studied show linear growth as a function of the number of bilayers deposited and this growth is relatively thick (i.e. > 100 nm per bilayer). In general, 10-bilayer films prepared with CTAB and poly(acrylic acid) are able to achieve a 2.3 mm zone of inhibition against S. aureus bacteria and 1.3 mm against E. coli when test are conducted at body temperature (i.e. 37oC). Fewer bilayers reduces efficacy, but lower test temperatures improve zones of inhibition. As long as they are stored in a dry atmosphere, antimicrobial efficacy was found to persist even when films were used four weeks after being prepared. The best films remain effective (i.e. antimicrobially active) for 4-6 days of constant exposure to bacteria-swabbed plates. This technology holds promise for use in transparent wound bandages and temporary surface sterilization.Item Coastal Microstructure: From Active Overturn to Fossil Turbulence(2012-02-14) Leung, Pak TaoThe Remote Anthropogenic Sensing Program was a five year effort (2001- 2005) to examine subsurface phenomena related to a sewage outfall off the coast of Oahu, Hawaii. This research has implications for basic ocean hydrodynamics, particularly for a greatly improved understanding of the evolution of turbulent patches. It was the first time a microstructure measurement was used to study such a buoyancy-driven turbulence generated by a sea-floor diffuser. In 2004, two stations were selected to represent the near field and ambient conditions. They have nearly identical bathymetrical and hydrographical features and provide an ideal environment for a control experiment. Repeated vertical microstructure measurements were performed at both stations for 20 days. A time series of physical parameters was collected and used for statistical analysis. After comparing the data from both stations, it can be concluded that the turbulent mixing generated by the diffuser contributes to the elevated dissipation rate observed in the pycnocline and bottom boundary layer. To further understand the mixing processes in both regions, data were plotted on a Hydrodynamic Phase Diagram. The overturning stages of the turbulent patches are identified by Hydrodynamic Phase Diagram. This technique provides detailed information on the evolution of the turbulent patches from active overturns to fossilized scalar microstructures in the water column. Results from this study offer new evidence to support the fossil turbulence theory. This study concluded that: 1. Field Data collected near a sea-floor outfall diffuser show that turbulent patches evolve from active (overturning) to fossil (buoyancy-inhibited) stages, consistent with the process of turbulent patch evolution proposed by fossil turbulence theory. 2. The data show that active (overturning) and fossil (buoyancy-inhibited) patches have smaller length scales than the active+fossil (intermediate) stage of patch evolution, consistent with fossil turbulence theory and with laboratory studies. 3. Compared to a far-field reference, elevated dissipation rates near the diffuser were found in the seasonal pycnocline as well as in the bottom boundary layer. 4. More than 90% of the turbulent patches observed in the water column were non-overturning (active+fossil and fossil). Such patches can provide significant mixing in the interior of the ocean, far from surface and bottom boundary layers.Item Developing & tailoring multi-functional carbon foams for multi-field response(2009-05-15) Sarzynski, Melanie DianeAs technological advances occur, many conventional materials are incapable of providing the unique multi-functional characteristics demanded thus driving an accelerated focus to create new material systems such as carbon and graphite foams. The improvement of their mechanical stiffness and strength, and tailoring of thermal and electrical conductivities are two areas of multi-functionality with active interest and investment by researchers. The present research focuses on developing models to facilitate and assess multi-functional carbon foams in an effort to expand knowledge. The foundation of the models relies on a unique approach to finite element meshing which captures the morphology of carbon foams. The developed models also include ligament anisotropy and coatings to provide comprehensive information to guide processing researchers in their pursuit of tailorable performance. Several illustrations are undertaken at multiple scales to explore the response of multi-functional carbon foams under coupled field environments providing valuable insight for design engineers in emerging technologies. The illustrations highlight the importance of individual moduli in the anisotropic stiffness matrix as well as the impact of common processing defects when tailoring the bulk stiffness. Furthermore, complete coating coverage and quality interface conditions are critical when utilizing copper to improve thermal and electrical conductivity of carbon foams.Item Development of imaging methods to quantify the laminar microstructure in rat hearts(Texas A&M University, 2004-11-15) Hudson, Kristen KayThe way in which the myocardium responds to its mechanical environment must be understood in order to develop reasonable treatments for congestive heart failure. The first step toward this understanding is to characterize and quantify the cardiac microstructure in healthy and diseased hearts. Myocardium has a laminar architecture made up of myolaminae, which are sheets of myocytes surrounded by a collagen weave. By enhancing the contrast between the myocytes and the surrounding collagen, the myocardium can be investigated and its laminar structure can be quantified. Many of the techniques that have been used to view the microstructure of the heart require the use of toxic or caustic chemicals for fixation or staining. An efficient imaging method that uses polarization microscopy and enhances the contrast between the collagen and myocytes while minimizing the use of harmful chemicals was developed in this research. Collagen is birefringent; therefore its visibility should be enhanced through polarization microscopy and image processing. The sheet angles were viewed directly by cutting slices of a rat septum perpendicular to the fiber angle. Images of different polarization combinations were taken and a region of interest was selected on the sample. Image processing techniques were used to reduce the intensity variation on the images and account for the variable gain of the camera. The contrast between the collagen and myocytes was enhanced by comparing adjusted images to the background and looking at a single image this comparison produced. Although the contrast was enhanced, the embedding media reduced the collagen signal and the enhancement was not as striking as expected.Item Effects of Water Content and Alumino-Silicate Sources on the Structure and Properties of Geopolymers(2012-10-19) Lizcano, MaricelaGeopolymers (GPs) are a special class of inorganic polymers with unique properties. Their 3-D amorphous structure and properties are often attributed to SiO2/Al2O3 molar ratios. However; contradictory results reported in literature on the structure and properties, do not conclusively support these reported findings. Furthermore, alternative processing methods are necessary for synthesizing pure geopolymers without impurities often found in precursor material. A rigorous study on chemical composition and processing parameters as well as alternative processing methods are necessary for advancing GPS in various engineering applications. The effects of H2O/(SiO2 + Al2O3) and SiO2/Al2O3 molar ratios , as well as precursor material on the density, open porosity, microstructure and the thermal and mechanical properties in K and Na activated geopolymers is investigated. X-ray diffraction, Nuclear Magnetic Resonance as well as alcohol immersion to determine density and open porosity is utilized for structural characterization. Thermogravimetric analysis and Thermomechanical analysis are used to investigate thermal behavior. Thermal conductivities and mechanical properties were measured using Thermal Constant analysis and compression testing respectively. Conclusive results demonstrate that the amount of water used to process GPs is the governing factor affecting their structure while SiO2/Al2O3 molar ratio plays no significant role. The K- and Na-activated samples have similar amounts of residual water after aging for 21 days at ambient conditions. In addition, the effects of the initial water content, SiO2/Al2O3 ratio, and alkaline activator (Na or K) on the thermal and mechanical properties of GPs, indicate that the dominant factor controlling thermal conductivity is H2O/(SiO2 + Al2O3) ratio used in processing, and to a lesser degree, the type of activation ion (Na or K). The SiO2/Al2O3 ratio did not have an effect on thermal conductivity. However, GPs compressive strengths are strongly affected by H2O/(SiO2 + Al2O3) ratio, especially at higher water ratio. At high and intermediate H2O/(SiO2 + Al2O3) ratios, liquid/solid ratio is the most important factor controlling the strength of GPs. At low H2O/(SiO2 + Al2O3) ratios, SiO2/Al2O3 ratio also plays an important role. Finally, partial geopolymer synthesis was possible using pure SiO2 and Al(OH)3 precursors, providing a possible low temperature alternative to other aluminosilicate precursors.Item Mechanical Properties, Thermal Stability and Radiation Damage of Ferritic Steels Processed by Thermal Mechanical Treatments(2014-08-04) Song, MiaoThe dramatically increasing demand for energy stimulates scientists all over the world to consider the possible ways to meet future energy needs. The application of atomic energy has been demonstrated to be a reliable, economic, and environmental friendly choice by the reactors operating today for half century. Lessons have been learned during the operation of generation I-III reactors. To avoid an unanticipated failure, high performance structural materials still represent a crucial component for extending the operation life of current reactors and the design of generation IV nuclear reactors and future fusion reactors. Due to their superior radiation tolerance, ferrite steels are currently the primary candidate under examination for generation IV or future fusion reactors. Whereas considerable data have been published on modification of the chemical composition of ferrite steels and evaluation of their overall performance, few attempts have been tried to process the existing ferrite steels by plastic deformation and evaluate the performance and properties improvements. The deformation technique introduced here is equal channel angular extrusion (ECAE). During ECAE, severe shear strain (on the order of several hundred percent) is introduced into a material by extrusion through a channel of constant cross section that contains an abrupt angle. The work presented in this dissertation is an attempt to apply ECAE to reactor steels of interest-namely T91 and 12Cr ODS. T91 (modified 9Cr-1Mo steel) is widely used and a commercialized material. 12Cr ODS is an oxide dispersion strengthened ferrite steel developed based on the reduce activation concept and targeted for cladding at elevated temperature. The pros and cons for such an approach were evaluated based on the influence of such a plastic deformation on mechanical performance, thermal stability and radiation damage or tolerance. In general, ECAE processed materials show improved strength and radiation tolerance. However, thermostability and ductility are sacrificed. The latter is not a potential problem because during service at elevated temperatures, the ECAE processed materials can regain ductility within a short period. Attention should be paid to the degeneration of thermostability after ECAE deformation.Item Multifunctional Composites and Devices for Sensing and Energy Harvesting(2011-08-08) Cleveland, Michael AllenThis research investigates a novel class of active materials for energy and sensing applications. Magnetocaloric alloys, Gd5Si2Ge2, were developed into a composite with poly(vinylidine flouride) (PVDF), piezoelectric polymer. The giant megnetocaloric property combined with the piezoelectricity creates extraordinary properties for composite materials. The research approach was primarily experimental. Activities include synthesis, characterization, and device design and evaluation. Using the arc melting method, the magenetocaloric samples were created. Multi-length scales characterized using atomic force microscopy (AFM), optical microscopy, scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS), X-Ray diffraction (XRD), and X-Ray Photoelectron spectroscopy (XPS). The prototype devices were evaluated for their power generation and efficiency. Through those techniques, the fundamental understanding in the new materials was obtained. The relationships between process-microstructures, microstructure-properties, and structure-power generation were established. Results showed that the phase transformation of the magnetocaloric material at its Curie temperature induced a significant increase in power generation in the peizeoelectric polymer. Such transition was also beneficial for a laminated device for energy harvesting. In addition, it was found that the oxidation that occurred during high temperature melting stabilized the orthorhombic phase at room temperature. The multifunctional composites as well as the laminated structure use the thermal expansion of the magnetocaloric material for energy harvesting, cyclic monitoring, and/or thermal switching. This thesis consists of six chapters. Chapter I provides a history and explanation of the materials used. Chapter II provides an explanation of the motivation for this work. Chapter III addresses the experimental procedures. The results of which are presented in Chapter IV and discussed in Chapter V. The research is summarized and future recommendations are given in Chapter VI.Item Nanoengineered Thin Films for Solid Oxide Fuel Cells(2013-11-21) Su, QingSolid oxide fuel cells (SOFCs) are very attractive as energy generation devices because of their high energy efficiency, flexible fuel selections and clean energy conversion. To avoid cell cracking and formation of non-conducting compounds at electrolyte/electrode interfaces issues caused by high operating temperatures (~1000 ?C for conventional SOFCs), intermediate temperature SOFCs (ITSOFCs) in the range of 500-700 ?C have attracted extensive research interests. However, the polarization loss of cathode and ohmic loss of electrolyte significantly increases under reduced temperatures which lead to decreased cell performance and power output. To address the above issues, the efforts in this work are focused on engineering microstructure of cathode, electrolyte and their interface to achieve high performance. First, a bi-layer method has been developed to prepare La0.5Sr0.5CoO3 (LSCO) cathode by combining a pulsed laser deposition (PLD) technique and a screen printing method. It provides a cost-effective approach to fabricate thick and high quality cathode films and the method could also be applied to many other cathode systems. Second, detailed PLD interlayer thickness effect is investigated. The mechanical and electrochemical properties of those hybrid cathodes are examined and correlated with the microstructure of the cells with different interlayer thicknesses. Third, partial oxygen pressure AC impedance study has been carried on those bi-layer cathodes with different interlayer thicknesses. The guidelines for designing high-performance bi-layer cathodes with optimum performance and low cost are proposed. Fourth, the design of a La0.8Sr0.2MnO3-?/Zr0.92Y0.08O2 thin interlayer with a vertically-aligned nanocomposite (VAN) structure between the electrolyte and oxygen electrode is demonstrated for solid oxide reversible fuel cells. The VAN structure significantly improves the overall cell performance and also acts as a transition layer that improves adhesion and relieves both thermal stress and lattice strain. Fifth, Two-phase (Ce0.9Gd0.1O1.95)0.5/(Zr0.92Y0.08O1.96)0.5 nanocomposite thin films with vertically aligned structure are grown as the electrolyte for thin film solid oxide fuel cells (TFSOFCs). More than 50% increase in overall power density is achieved compared with that of the cells without VAN electrolyte.Item Remodeling of fiber and laminar architecture of rat heart septum in a transitional normal state between pressure overload hypertrophy and failure(2009-06-02) Hegde, Bharati KrishnaCongestive Heart Failure (CHF) is a major fatal disease today in the United States. The heart's function is a mechanical one. To diagnose and treat CHF effectively there is a need to understand at the microstructural level, the differences in the response of the myocardium to a change in its mechanical environment. Hence to assess growth and remodeling processes in the myocardium, the fiber and myolaminar structure of two groups of Dahl salt-sensitive rats were compared: low salt (LS) normal controls and a high salt (HS) group with hearts in "transitional eutrophy" defined by normal size and shape but in transition from pressure overload hypertrophy to dilated hypertrophy. To create the HS group with transitional eutrophy, we fed Dahl salt-sensitive rats, a sustained high salt diet from age 6 wks till sacrifice at age 11-13 wks. Such rats have a heart that transitions from too thick (pressure overload hypertrophy at about age 9 wks) to too thin (dilated hypertrophy at about age 15 wks to death) with a transitional period (age 11-13 wks) having normal size and shape. Fiber angles, sheet angles, number and thickness of sheets were measured in the septum at four transmural quarters (TQ1 to TQ4 with TQ1 being closest to LV and TQ4 closest to RV). A uniformity index was defined to characterize sheet angle dispersion. Upon comparison to LS controls, the HS group had normal size hearts with normal shape. However, there was a significant increase in the number of sheets, which corresponded with a significant decrease in the thickness of sheets in all quarters in HS group. Differences in fiber angles were significant in TQ1, TQ2, and TQ4 with fiber angles more positive in HS group. Differences in sheet angles and uniformity index were not significant. Despite having a normal size and shape, we found that hearts in a state of transitional eutrophy have a significantly different fiber and sheet morphology. The experimental data was used to develop a model that represents the path to failure that may be taken by the myolaminae when the heart is subjected to excessive pressure overload.Item The development of processing methods for a quantitative histological investigation of rat hearts(Texas A&M University, 2004-11-15) Jetton, Emily HopeIn order to understand the mechanical functions of the cardiac muscle it is important to first understand the microstructure of the tissue. Young et al. (1998) realized that quantitative three-dimensional information about the ventricular myocardium is necessary to analyze myocardial mechanics. They developed a technique using confocal fluorescence laser scanning microscopy to obtain three-dimensional images. While this method worked well in rebuilding the myocardial tissue image by image, it was quite extensive and costly. Costa et al. (1999) developed a method that was used to perform three-dimensional reconstruction as well. Their method, while less expensive and much less time consuming, required sheet assumptions and did not look directly at the cross-fiber plane. From Dr. Criscione's previous work on canines (Ashikaga et al., 2004), we found that the sheet structure can be accurately determined from cross-fiber sections without making any sheet assumptions. We have now expanded on those ideas and created a method to perform the quantitative histological investigation of the rat hearts in a way that is both timely and cost effective. We developed a processing method that preserves the orientation of the fiber and sheet angles. This method was carried out using plastic embedding since the dehydration process used in paraffin embedding has a tendency to grossly distort tissue. Once the heart was fixed in formalin, we then removed the septum and sliced it several times vertically. This allowed us to image the tissue at several depths and find an average fiber angle for each slice. Next, the specimen was hardened, and the sheet orientation was evaluated using polarized light. Once both fiber and sheet angles were obtained from several depths within the septum, we then constructed a three-dimension model of the wall. This method was both cost effective and less time consuming than previous ones and will be a method that can be used in the future to compare the myocardial tissue of diseased and healthy rat hearts so that we may better understand the mechanical functions of the heart as it remodels due to disease.Item The effect of strain and path change on the mechanical properties and microstructural evolution of ultrafine grained interstitial free steel during equal channel angular extrusion (ECAE)(Texas A&M University, 2007-04-25) Sutter, Steven GeorgeThe objectives of this study were to examine the effect of strain and path change on the microstructural evolution of ultrafine grained interstitial free (IF) steel during equal channel angular extrusion (ECAE); to determine the mechanical properties; to observe the resulting texture; and to perform optical and electron microscopy of the resulting material. The effects of different routes of extrusion (A, B, C, C' and E), heat treatment and plastic strains from 1.15 to 18.4 were examined. Monotonous tensile testing was used to determine mechanical behavior of processed materials. X-ray diffraction and TEM analyses were performed to evaluate the effect of processing on texture and grain morphology. Hardness measurements were performed to determine recrystallization behavior of the processed material. Optical microscopy was conducted on heat treated samples to determine their grain size and refinement. Monotonous tensile testing of processed materials showed that there was significant strengthening after the first extrusion. Further processing resulted in increasing values of yield strength and ultimate tensile strength, with ductility at failure varying depending upon which processing route was used. The best tensile strength results were obtained after processing Routes 8C' and 16E, due to the significant grain refinement these routes produced. X-ray diffraction revealed increases in strength of preferred texture along the directions [111] and [001], perpendicular to the transverse plane, for all specimens that were processed using ECAE. TEM observations showed a consistent refinement of grain size as the amount of processing increased, especially within Routes C' and E. Hardness measurements of heat treated specimens showed that the onset of recrystallization occurred at approximately the same temperature of recrystallization as that of pure iron, 450????C. The recrystallization curves for all samples showed that grain growth begins at a temperature of around 700????C. The low carbon content of IF steel made optical microscopy challenging. The grain size of annealed materials becomes finer and more uniform, ranging between 60 and 90 ????m2, at high strain levels under Routes C' and E, due to the many potential nucleation sites developed in highly worked material.