Browsing by Subject "Magnesium"
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Item Characterization of an Orphan Riboswitch: Identification of a Metal-Sensing Regulatory RNA(2011-09-30T18:58:50Z) Wakeman, Catherine Ann; Winkler, Wade C.Riboswitches are RNA-based genetic control elements found in untranslated regions of the mRNA transcript that they regulate. These RNA motifs are highly structured and bind metabolites to elicit control of gene expression. Typically, the metabolite sensed by these RNAs is a component of the metabolic pathway in which the regulated gene product resides. The focus of this project has been the identification of the ligand for a riboswitch that was discovered using bioinformatics-based search methods. This riboswitch was designated the ykoK RNA element due to its location in the 5' UTR of the B. subtilis ykoK (mgtE) gene, which appears to be a magnesium transporter. Therefore, the possibility that this RNA senses magnesium levels was explored. The data revealed that the RNA element imparts magnesium-responsive regulation to the ykoK gene. These data also indicated which portions of the RNA are essential for genetic regulation. The results of a battery of biochemical tests demonstrated that magnesium triggers a concerted conformational change in the RNA such that it adopts a compacted tertiary structure. Resolution of the three-dimensional structure of the RNA in the magnesium bound state revealed the basis of this metal-induced tertiary conformation and how this relates to genetic control. Intriguingly, this structure revealed the presence of six magnesium ions, making this the first example of multiple ligands binding to a single riboswitch aptamer. When individual metal-binding sites were eliminated using phosphorothioate substitutions, it became evident that all six of these magnesium-binding sites and up to three additional metal-binding sites are required for function of this RNA. Therefore, these data demonstrate that the ykoK RNA element, now designated the M-box RNA, directly senses intracellular magnesium levels for the purposes of genetic control. These findings should have broad implications given that this RNA element is wide spread among Gram-positive bacteria and appears to regulate many additional gene categories such as ABC transporters, cell division proteins, and proteins of unknown function. The exploration of the connection between magnesium concentration and the expression levels of these proteins might provide insights into previously undefined functional roles. [Keywords: regulatory RNA; metal homeostasis; RNA structure; magnesium; riboswitch]Item The construction and use of physics-based plasticity models and forming-limit diagrams to predict elevated temperature forming of three magnesium alloy sheet materials(2013-08) Antoniswamy, Aravindha Raja; Taleff, Eric M.Magnesium (Mg) alloy sheets possess several key properties that make them attractive as lightweight replacements for heavier ferrous and non-ferrous alloy sheets. However, Mg alloys need to be formed at elevated temperatures to overcome their limited room-temperature formabilities. For example, commercial forming is presently conducted at 450°C. Deformation behavior of the most commonly used wrought Mg alloy, AZ31B-H24, and two potentially competitive materials, AZ31B-HR and ZEK100 alloy sheets, with weaker crystallographic textures, are studied in uniaxial tension at 450°C and lower temperatures. The underlying physics of deformation including the operating deformation mechanisms, grain growth, normal and planar anisotropy, and strain hardening are used to construct material constitutive models capable of predicting forming for all three Mg alloy sheets at 450°C and 350°C. The material models constructed are implemented in finite-element-method (FEM) simulations and validated using biaxial bulge forming, an independent testing method. Forming limit diagrams are presented for the AZ31B-H24 and ZEK100 alloy sheets at temperatures from 450°C down to 250°C. The results suggest that forming processes at temperatures lower than 450°C are potentially viable for manufacturing complex Mg components.Item Development of new automated flow titration systems(Texas Tech University, 2004-08) Jo, Kyoo DongTitrimetry is one of the oldest analytical methods still in use. It is probably the analytical technique that has changed the least over the years. It remains one of the most widely used tools for making high precision measurement. In classical batch mode titration procedures, the volume of titrant added to a titration vessel is the variable that is controlled and monitored and subsequently used to calculate the original analytical concentration in the test solution. More advanced and automated batch mode titrators have been developed. However, the downside of titrations in conventional batch mode are significant reagent consumption and poor throughput rate. Thus, automation of titrations using the continuous flow mode has been increasingly drawing attention. The new concept for continuous on-line titrations based on feedback-controlled flow-ratiometry and the principle of compensating errors creates a new paradigm for flow titrations by feedback based flow ratiometry in which the delay between the sample-titrant confluence point and the detector is made constant. The error due to lag time is continuously compensated for by averaging rapid backward and forward titrations. This new concept has been developed and applied to titrations followed by potentiometry and photometry. Electrogeneration of titrants have several significant advantages. The most important of these is the elimination of problems associated with the preparation, standardization, and storage of titrant solutions. This advantage is particularly significant when dealing with unstable reagents. Electrogeneration of the titrant allows the transformation of the newly developed error-compensated feedback based flow-gradient method into a concentration gradient method. Coulometric titrant generation transfers the control from the flow control to current control. This system has been applied to various redox titrations using photometric and potentiometric detection methods. An additional new type of flow injection titration system has been developed based on a syringe pump. In this system, the titrant is injected in a triangular programmed flow pattern by a syringe pump in a continuous stream of the sample. This new type of flow injection titration method was applied to acid-base and redox titrations, and the data interpreted by the principle of compensating errors.Item External sulfate attack of concrete : an accelerated test method, mechanisms, and mitigation techniques(2016-05) Aguayo Jr., Federico Macias; Folliard, Kevin J.; Thomas, Michael D.A.; Fowler, David; Juenger, Maria C.G.; Wheat, HarovelSulfate attack of concrete is perhaps the least understood of the major durability mechanisms plaguing reinforced concrete infrastructures. Many studies have attempted to better understand the underlying mechanisms in which the various modes of deterioration by sulfate attack manifest; however, several controversies still exist. Moreover, ASTM C 1012 (2012), which is the most commonly referred standardized laboratory test method to determine sulfate resistance of blended portland cement mixtures, does not always link well to field performance and may take up to 18 months to complete. The research program presented in this dissertation investigates various issues pertaining to the mechanisms, testing methods, and factors influencing external sulfate attack. The primary focus of this research study was to investigate and propose a reliable, and innovative accelerated test method to evaluate the sulfate performance using concrete specimens. The research program was divided into the following four key components: (1) design a method that can obtain results within a reasonable timeframe (less than six months); (2) design a method that uses concrete specimens and thus links more closely to field performance; (3) develop a better understanding on the role and mechanisms of sulfate attack on concrete through a comprehensive research program including field and laboratory investigations; and (4) investigate the use of calcium sulfate (gypsum) used as an additive to mitigate the potential of sulfate attack in blended portland cement mixtures using high-calcium fly ash. The findings in this dissertation led to the development of a potential accelerated test method for determining sulfate resistance by vacuum impregnating concrete (or mortar) samples with sulfate solution to accelerate the ingress and onset of chemical reactions between the hydrated cement paste and sulfate ion (SO42-). The effects of binder type, water-to-cementitious ratio (w/cm), curing regime, sulfate type and concentration are examined. In comparison to the conventional ASTM C 1012 method, results showed a higher rate of expansion with significant distress observed in samples subjected to the accelerated test method and placed in a 5% Na2SO4 solution. Similar trends, but at a relatively lower expansion rate, were also observed in samples placed in a 0.89% Na2SO4 solution. Physical measurements, chemical analysis and microstructural studies were performed periodically on the specimens.Item Mechanical Flow Response and Anisotropy of Ultra-Fine Grained Magnesium and Zinc Alloys(2011-02-22) Al Maharbi, Majid H.Hexagonal closed packed (hcp) materials, in contrast to cubic materials, possess several processing challenges due to their anisotropic structural response, the wide variety of deformation textures they exhibit, and limited ductility at room temperature. The aim of this work is to investigate, both experimentally and theoretically, the effect os severe plastic deformation, ultrafine grain sizes, crystallographic textures and number of phases on the flow stress anisotropy and tension compression asymmetry, and the mechanisms responsible for these phenomena in two hcp materials: AZ31B Mg alloy consisting of one phase and Zn-8wt.% Al that has an hcp matrix with a secondary facecentered cubic (fcc) phase. Mg and its alloys have high specific strength that can potentially meet the high demand for light weight structural materials and low fuelconsumption in transportation. Zn-Al alloys, on the other hand, can be potential substitutes for several ferrous and non-ferrous materials because of their good mechanical and tribological properties. Both alloys have been successfully processed using equal channel angular extrusion (ECAE) following different processing routes in order to produce samples with a wide variety of microstructures and crystallographic textures for revealing the relationship between microstructural parameters, crystallographic texture and resulting flow stress anisotropy at room temperature. For AZ31B Mg alloy, the texture evolution during ECAE following conventional and hybrid ECAE routes was successfully predicted using visco-plastic self-consistent (VPSC) crystal plasticity model. The flow stress anisotropy and tension-compression (T/C) asymmetry of the as received and processed samples at room temperature were measured and predicted using the same VPSC model coupled with a dislocation-based hardening scheme. The governing mechanisms behind these phenomena are revealed as functions of grains size and crystallographic texture. It was found that the variation in flow stress anisotropy and T/C asymmetry among samples can be explained based on the texture that is generated after each processing path. Therefore, it is possible to control the flow anisotropy and T/C asymmetry in this alloy and similar Mg alloys by controlling the processing route and number of passes, and the selection of processing conditions can be optimized using VPSC simulations. In Zn-8wt.% Al alloy, the hard phase size, morphology, and distribution were found to control the anisotropy in the flow strength and elongation to failure of the ECAE processed samples.Item pH-induced flocculation/deflocculation process for harvesting microalgae from water(2014-08) Choi, Jin-Yong, Ph. D.; Kinney, Kerry A.; Katz, Lynn Ellen; Kinney, Kerry A.; Katz, Lynn E.Historically, the presence of microalgae (algae hereafter) in natural waters has been viewed as a nuisance due to its adverse impact on water quality. More recently, however, algae are being investigated as potential sources of biofuel and a range of natural products. These applications require the development of large-scale cultivation systems for mass production that include growth, harvesting, concentration, and product recovery components. While challenges still remain with respect to many of the processes involved in mass production, one of the most technically and economically challenging steps is harvesting the algae from dilute growth cultures, especially in systems where chemical additives are of concern either within the algae concentrate or the effluent water. For this reason, a pH-induced flocculation/deflocculation method using the hydroxides of alkali or alkaline earth metals (e.g., lime, caustic soda) is of particular interest for algae harvesting as Na, Ca and Mg are typically present in natural waters. The goal of this research was to determine the underlying mechanisms responsible for algae coagulation by magnesium and calcium and to evaluate the potential of these mechanisms for harvesting algae for a range of synthetic and field source water chemistries. In the first two phases of this research, the mechanisms for coagulation with magnesium and calcium were studied independently. A series of bench-scale experiments were designed to isolate the potential mechanisms of algae destabilization associated with each of these cations as a function of water chemistry, and microscopic analyses were performed to characterize the flocculated algae/precipitate mixtures. In the third phase of this research, removal of algae in field source waters was evaluated with respect to the underlying science elucidated in the previous phases. The results indicate that the dominant algae destabilization mechanism associated with magnesium shifts from Mg adsorption/charge neutralization to Mg(OH)₂[subscript (S)] precipitation-enhanced coagulation with increasing pH. Moreover, dissolved Mg²⁺ adsorption to the algae surface led to effective algae coagulation, while minimizing the mass of precipitated Mg(OH)₂[subscript (S)] . For Ca, this research identified the importance of the nucleation process (heterogeneous vs. homogeneous nucleation) on algae removal efficiency. Heterogeneous nucleation is a key factor for optimizing algae removal; thus, the degree of oversaturation with respect to CaCO₃[subscript (S)] is a crucial operating parameter. This research demonstrated that the algae harvesting process using pH-induced flocculation/deflocculation method can be optimized for a wide range of source waters if the water chemistry (e.g. pH, ion concentration, alkalinity, ionic strength) is properly incorporated into the system design.Item Physics-based material constitutive models for the simulation of high-temperature forming of magnesium alloy AZ31(2012-08) Carpenter, Alexander James; Taleff, Eric M.; Bourell, David L.; Kovar, Desiderio; Seepersad, Carolyn C.; Engelhardt, Michael D.Magnesium sheet alloys, such as wrought AZ31, have material properties that make them an attractive option for use in automotive and aircraft components. However, the low ductility of magnesium alloys at room temperature necessitates the use of high-temperature forming to manufacture complex components. Finite-element-method (FEM) simulations can assist in determining the optimum processing parameters for high-temperature forming, but only if an accurate material constitutive model is used. New material constitutive models describing the deformation behavior of AZ31 sheet at 450°C are proposed. These models account for both active deformation mechanisms at this temperature: grain-boundary-sliding creep and five-power dislocation-climb creep. Phenomena affecting these deformation mechanisms, such as material anisotropy and grain growth, are also investigated. This physics-based approach represents an improvement over previous material models, which require nonphysical parameters and can only predict forming for a limited range of conditions. Tensile tests are conducted to obtain data used in fitting constitutive models. New models are used in FEM simulations of both tensile tests and biaxial gas-pressure bulge tests. Simulation results are compared to experimental data for validation and determination of model accuracy.Item Role of Crystallographic Texture and Grain Size on Low Temperature Deformation and Formability of a Mg Alloy(2014-12-12) Dogan, EbubekirInterest in Mg alloys has significantly increased in recent years for weight-critical applications. However, Mg alloys show low strength and poor low temperature formability, due to the limited available slip systems and the strong final texture in wrought products. This limits extensive usage of Mg as a structural material. Mg alloys have a hexagonal close-packed crystal structure that shows lower symmetry compared to its cubic counterparts, which causes activities of various Burgers vectors and slip planes under different stress fields. In this study, it was shown that detailed knowledge of multiple deformation mechanisms can be utilized to engineer the microstructure and flow anisotropy of magnesium alloys, subjected to severe plastic deformation, for ultrahigh strength, ductility, and formability. Using a microstructure based visco-plastic self-consistent crystal plasticity model with detailed electron backscatter diffraction analyses and transmission electron microscopy, new equal channel angular processing (ECAP) methodologies were developed in order to achieve the desired microstructure and grain refinement to a few hundred nanometers. Both experimental and simulation results clearly indicated that the formation of compression/double twins causes deformation localization, followed by dynamic recrystallization within the twins. This non-uniform DRX causes local softening and macro shear banding, and eventual failure. However, it was shown that with proper modification of the texture and grain size, the shear localization and macro shear bands can be suppressed by limiting the twinning activity and, instead, promoting non-basal slip activities at low temperatures (<200?C). These studies led to the ultrahigh strength levels in the Mg-3Al-1Zn (AZ31) alloy. To date, ECAP of Mg alloys is limited to bar/billet form, and a scale-up of the ECAP technology was not utilized for Mg plate and sheet processing. This work is one of the initial studies where commercially available wrought AZ31 plates are processed via the equal channel angular plate extrusion (ECAPE) tool in order to refine grains and weaken the strong wrought texture. The ECAPE tool provided scaled-up samples for further tests, such as stretch formability, a capability which was not possible with bar samples. It has been shown that ECAPE processing tremendously increased the room temperature stretch formability of AZ31 alloy sheets.Item Roles for zebrafish trpm7 in growth, skeletogenesis, kidney function and physiological ion homeostasis(2009-12) Elizondo, Michael Reuben; Parichy, David M.; Wallingford, John B.; Fischer, Janice A.; Paull, Tanya T.; Shankland, MartyDevelopment of the adult form requires coordinated growth and patterning of multiple traits in response to local gene activity as well as global endocrine and physiological effectors. In recent years the zebrafish has been utilized as a favorable animal model as a step towards dissecting and better understanding these postembryonic developmental processes. One of the more powerful methods utilized in zebrafish has been the identification of new gene functions through the use of mutant screens. The nutria mutant was recovered from one such screen to identify postembryonic defects in pigment pattern, growth and metamorphosis. These mutants exhibited a pigment cell defect, touch unresponsiveness and severe growth retardation. Here I will discuss my work towards dissecting the underlying developmental processes governing the phenotypic changes in nutria mutants. I characterize gross alterations in skeletal development in nutria mutants that lead to accelerated endochondral ossification but delayed intramembranous ossification. I show that the nutria phenotype results from mutations in trpm7, which encodes a transient receptor potential (TRP) family member that functions as both a cation channel and a kinase. I find trpm7 expression in the fish-specific, ion homeostasis-regulating gland known as the corpuscles of Stannius (CS), and in the mesonephric kidney. I show that mutants also develop kidney stones. Together these results suggest a role for trpm7 activity in regulation of physiological ion homeostasis. Next I confirm that role by identifying late-embryonic and early larval defects in the CS and the kidney, two organs that regulate physiological ion homeostasis. I demonstrate the early larval detection of kidney stones in trpm7 mutants and show that their appearance is presaged by decreased levels of total calcium and magnesium. Furthermore I establish a link between trpm7 function in the CS and stanniocalcin1 (stc1), a potent molecular regulator of calcium homeostasis. Finally, using transgenic overexpression and morpholino-oligonucleotide knockdown, I demonstrate that stc1 modulates calcium and magnesium levels in trpm7 mutant and wild-type backgrounds. Together these analyses establish postembryonic roles for trpm7 function in growth, skeletogenesis, kidney function, and physiological ion homeostasis.Item Texture Control by Selective Deformation Mechanism Activation in Magnesium Alloy(2014-07-01) Foley, David ChristopherThe need for high strength, light weight structures in automotive and aerospace applications has driven a resurgence of interest in magnesium and its alloys. Unlike aluminum, wrought magnesium typically has a high degree of mechanical anisotropy because of its hexagonal close packed structure. Our objectives were to develop high strength (>350MPa yield) in a bulk magnesium alloy using grain refinement and to control the mechanical anisotropy by controlling crystallographic texture. This dissertation covers the development of thermomechanical processing methods used to tailor the strength and anisotropy of a magnesium alloy with 3%Zn and 1%Zr. The areas of focus in this study were as follows. First, we developed severe deformation processing strategies that increase strength in single-phase Mg alloy via grain refinement to submicron average grain size. We also established the achievable crystallographic textures in Mg alloy using 90o equal channel angular extrusion. In support of these first two goals, we determined the deformation mechanisms activated by differing strain paths and temperatures in single phase Mg alloy. Then, we established the effectiveness of these severe deformation processing strategies on both bar and plate workpiece geometries. We generated a wide range of crystallographic textures using thermomechanical processing. Using this knowledge, we established the effect of grain sizes down to submicron levels on room temperature deformation mechanism activity in single phase Mg alloy. We accomplished these goals through the use of equal channel angular extrusion, rolling, and heat treatment coupled with microscopy, diffraction, and mechanical testing. Notable achievements include demonstration of tensile twin suppression by grain refinement, the development of quasi-single-crystal textures, and the capacity to generate material with nearly identical texture but a range of grain sizes spanning almost two orders of magnitude. The experiments also supported (and were supported by) the development of visco-plastic self-consistent crystal plasticity modeling predictions thanks to the efforts of my colleagues. This work will further the development of advanced manufacturing and design using wrought Mg alloys.Item Time-resolved lattice measurements of shock-induced phase transitions in polycrystalline materials(2010-05) Milathianaki, Despina; Ditmire, Todd R.; Bengtson, Roger; Downer, Michael; Marder, Michael; Taleff, EricThe response of materials under extreme temperature and pressure conditions is a topic of great significance because of its relevance in astrophysics, geophysics, and inertial confinement fusion. In recent years, environments exceeding several hundred gigapascals in pressure have been produced in the laboratory via laser-based dynamic loading techniques. Shock-loading is of particular interest as the shock provides a fiducial for measuring time-dependent processes in the lattice such as phase transitions. Time-resolved x-ray diffraction is the only technique that offers an insight into these shock-induced processes at the relevant spatial (atomic) and temporal scales. In this study, nanosecond resolution x-ray diffraction techniques were developed and implemented towards the study of shock-induced phase transitions in polycrystalline materials. More specifically, the capability of a focusing x-ray diffraction geometry in high-resolution in situ lattice measurements was demonstrated by probing shock-compressed Cu and amorphous metallic glass samples. In addition, simultaneous lattice and free surface velocity measurements of shock-compressed Mg in the ambient hexagonal close packed (hcp) and shock-induced body centered cubic (bcc) phases between 12 and 45 GPa were performed. These measurements revealed x-ray diffraction signals consistent with a compressed bcc lattice above a shock pressure of 26.2±1.3 GPa, thus capturing for the first time direct lattice evidence of a shock-induced hcp to bcc phase transition in Mg. Our measurement of the hcp-bcc phase boundary in Mg was found to be consistent with the calculated boundary from generalized pseudopotential theory in the pressure and temperature region intersected by the principal shock Hugoniot. Furthermore, the subnanosecond timescale of the phase transition implied by the shock-loading conditions was in agreement with the kinetics of a martensitic transformation. In conclusion, we report on the progress and future work towards time-resolved x-ray diffraction measurements probing solid-liquid phase transitions in high Z polycrystalline materials, specifically Bi.