Browsing by Subject "Carbon"
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Item 1st Annual Jackson School Research Symposium(2012-02-04) Jackson School of Geosciences; The University of Texas at AustinItem Aqueous formate solution for geological carbon storage : numerical simulation and geochemical interaction studies(2023-05-04) Oyenowo, Precious Olufemi; Okuno, Ryosuke, 1974-; Mirzaei-Paiaman, AbouzarCarbon storage in geologic formations has been considered an important technology that reduces the carbon intensity of fossil fuels-based industrial processes. Carbon capture and storage (CCS) conventionally uses carbon dioxide (CO₂) as a carbon carrier. However, various shortcomings of the conventional CCS are related to the physical properties of CO₂, such as low carbon density at low to moderate pressure, low mass density, low viscosity, immiscibility with water, and corrosivity. In particular, CO₂ injection often results in inefficient use of pore space in the formation under subsurface heterogeneities. This report is centered on the novel idea of using a formate solution as an aqueous carbon carrier for geologic carbon storage. Formate is the conjugate base of formic acid. Formate can be produced from CO₂ via electrochemical reduction (CO₂ ECR). The CO₂ ECR technology is not yet industrialized, although it has been substantially improved over the past few years in the energy transition with the current technology readiness level of 5 to 6. The cost of formate produced industrially using the technology is unknown. We measured the viscosities and densities of formate solutions in brine, over a range of formate concentrations and temperatures. The measured data were used in numerical reservoir simulations of formate injection: (i) into an aquifer, and (ii) into an oil reservoir. Compared to simulations of CO₂ injection using the same reservoirs, results consistently showed that the formate injection case resulted in more stable fronts of oil and water displacement. The more stable fronts yielded the oil recovery and carbon storage that were insensitive to the injectant breakthrough. Cost-revenue analysis using the simulation results showed the formate breakeven cost for the oil reservoir case was within the literature estimates of the cost of formate production via CO₂ ECR. The results support the necessity of research and development for efficient CO₂ ECR systems. Geochemical interaction studies were carried out to understand the effect of formate injection (at concentrations up to 30-wt%) on carbonate rock, and the effect on the rock wettability. Experimental data from Amott wettability tests and core floods with limestone cores were analyzed to mechanistically understand the wettability alteration observed in the experiments. Static calcite dissolution tests showed that the degree of calcite dissolution increased with increasing formate concentration in a NaCl brine even with an initially neutral pH. Geochemical modeling indicated that the increased calcite dissolution could be caused by the formation of calcium formate complexes that reduced the activity coefficient of the calcium ion and drove the calcite dissolution. The Amott test results and history matching of the core flooding data showed that high-concentration formate solutions rendered the initially oil-wet core to a more water-wet state.Item Biotic and abiotic controls on carbon dynamics in a Central Texas encroaching savanna(2014-12) Thijs, Ann; Hawkes, Christine V.; Litvak, Marcy E.Anthropogenic activities are responsible for increases in atmospheric CO₂ and climate change. These increases are partly counterbalanced by natural processes, such as carbon uptake in land surfaces. These processes are themselves subject to climate change, creating a coupled carbon-climate system. I investigated the carbon sink that woody encroachment represents, using a Central Texas savanna as study site, and studied how climatic factors influence this carbon sink. Woody plant encroachment, a worldwide structural change in grassland and savanna ecosystems, alters many ecosystem properties, but the net effect on the carbon balance is uncertain. Woody encroachment represents one of the key uncertainties in the US carbon balance, and demands a more detailed understanding. To come to a process-based understanding of the encroachment effect on carbon dynamics, I analyzed patterns of carbon exchange using eddy-covariance technology. I expected the imbalance between carbon uptake and release processes associated with the encroaching trees specifically, to be responsible for the carbon sink. I also expected that the sink would vary in time, due to strong links between carbon fluxes and soil water in this semi-arid ecosystem. I further studied the ecophysiology of the dominant species, as well as soil respiration processes under different vegetation types, and scaled these findings in space and time. I found that the ecosystem was a significant carbon sink of 405 g C m⁻² yr⁻¹. The encroaching trees increased photosynthesis by 180% and decreased soil respiration by 14%, compared to the grassland, resulting in a strong carbon sink due to the encroachment process. The encroaching process also altered carbon dynamics in relation to climatic drivers. The evergreen species Ashe juniper effectively lengthened the growing season and widened the temperature range over which the ecosystem acts as a carbon sink. The drought resistance of the encroaching trees reduced the sensitivity of this savanna to drought. I conclude that encroachment in Central Texas savannas increased the carbon sink strength by increasing the carbon inputs into the ecosystem. Woody encroachment also reduced the sensitivity to climatic drivers. These two effects constitute a direct effect, as well as a negative feedback to the coupled carbon-climate system.Item Carbon based materials for electrodes in electrochemical double layer capacitors(2012-12) Murali, Shanthi; Ruoff, Rodney S.; Goodenough, John B.; Bielawski, Christopher W.; Korgel, Brian A.; Johnston, Keith P.Electrochemical double layer capacitors (EDLCs, also called supercapacitors or ultracapacitors) are high power density energy storage devices that operate through the separation of charge at the electrochemical interface between an electrode and a supporting electrolyte. Numerous types of carbon materials with high surface area and internal porosity, such as activated carbon, carbon fabrics, nanotubes, and reduced graphene oxide have been studied as electrode materials. Electrolytes such as aqueous alkaline and acid solutions usually give high capacitance, while organic and ionic liquids provide a wider operation voltage. Graphene, due to its high theoretical surface area of 2630 m2/g, good electrical conductivity, and relatively low density, is being studied as an electrode material in EDLCs. The objective of this dissertation is thus to study effective methods for synthesis of graphene-based materials, and to investigate their behavior in EDLCs. This work explored microwave assisted synthesis of graphite oxide (‘MEGO’, prepared in less than one minute by irradiation of graphite oxide by microwave). This material was further chemically activated to obtain a unique carbon material, activated microwave exfoliated graphite oxide (‘a-MEGO’) with specific surface areas up to 3100 m2/g. Gas adsorption measurements were used to study the specific surface area and porosity of a set of a-MEGO samples, which were also studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) for their structure, and by combustion analysis (i.e., elemental analysis) and X-ray photoelectron spectroscopy (XPS) to understand their elemental composition. Cyclic voltammetry (CV), galvanostatic charge/discharge, and frequency response, tests were done in order to study the performance of these new carbon materials as electrodes in both aqueous and organic electrolytes in a two electrode cell set up.Item Carbon Emissions Trading in Indonesia: An Analysis of Its Feasibility and Rationale(2020-04) Brink, AnnekaOver the past decade the United Nations has developed a framework consistent with conserving carbon and biodiversity-rich forests while simultaneously reducing global greenhouse gas emissions. The project, known as REDD+ (Reducing Emissions from Deforestation and Forest Degradation in Developing Countries), was created as an international framework to halt deforestation, specifically in tropical nations. By valuing forests in terms of their ecosystem services, as opposed to solely their export value, REDD+ aims to encourage financially developing countries to preserve natural carbon sinks and decrease the number of forests converted to alternate uses. Putting a price tag on the role forest ecosystems play in carbon capture and storage allows forest conservation to compete economically with historically more lucrative forest exploitation practices. The first country to facilitate REDD+ implementation was Indonesia in 2009. As the world’s top greenhouse gas emitter from deforestation, degradation, and land-use change, Indonesia has a unique opportunity to achieve substantial emissions reductions at relatively low abatement costs. In 2011, Indonesia and Norway signed a landmark deal stipulating that Norway would pay Indonesia USD $1 billion if the country verifiably reduced carbon emissions stemming from deforestation and peatland conversion. After nearly a decade of ineffective policies and reforms ridden with loopholes, Indonesia finally recorded three consecutive years of declining emissions, beginning in 2017. While this pollution prevention represents a success for the environment, climate change mitigation, and REDD+ stakeholders around the world, there is room for improvement. If Indonesia is to have any hope of achieving its 2030 emissions reductions targets, more extensive reforms and stronger regulatory forces ought to be enacted. This paper discusses the feasibility and rationale behind the current REDD+ offset project operating in Indonesia. The challenge for Indonesia is for such an offset system to achieve economic growth, social equity, and reduced forest cover loss simultaneously. If not, the policy could reveal that the triple-win scenario is rooted in inherently contradictory goals, thus undermining effective policymaking for conserving Indonesia’s imperiled tropical forests.Item Characterization of the High Island 24L Field for modeling and estimating CO₂ storage capacity in the offshore Texas state waters, Gulf of Mexico(2019-07-25) Ruiz, Izaak; Meckel, Timothy AshworthCarbon, Capture, and Storage (CCS) is considered an essential technology that can contribute to reaching the IPCC’s target to limit global average temperature rise to no more than 2.0°C. The fundamental purpose of CCS is to reduce anthropogenic CO₂ emissions by capturing gas from large point sources and injecting it into deep geologic formations. In the offshore Texas State Waters (10.3 miles; 16.6 kilometers), the potential to develop CO₂ storage projects is viable, but the size of storage opportunity at the project level is poorly constrained. This research characterizes the High Island 24L Field, a relatively large historic hydrocarbon field, that has produced mainly natural gas (0.5 Tcf). The primary motivation for this study is to demonstrate that depleted gas fields can serve as volumetrically significant CO₂ storage sites. The stratigraphy of the inner continental shelf in the Gulf of Mexico has been extensively explored for hydrocarbon for over 50 years, and this area is well suited for CCS. Lower Miocene sandstones beneath the regional transgressive Amphistegina B shale have appropriate geologic properties (porosity, thickness, extent) and can be characterized utilizing 3D seismic and well logs in this study. Identifying key stratigraphic surfaces, faults, and mapping structural closure footprints illustrates the field’s geologic structure. The interpreted stratigraphic framework can then be used to model three different lithologic facies and effective porosity to calculate CO₂ storage capacity for both the ~200-ft (60-m) thick HC Sand (most productive gas reservoir) and the overlying thicker 1700 ft (520 m), but non-productive, Storage Interval of Interest. Four different methodologies are utilized to achieve confidence in the CO₂ storage capacity estimates. A storage capacity of 15 – 23 MT is calculated for the HC Sand and 108 – 179 MT for the Storage Interval of Interest by applying interpreted efficiency factors. This study evaluates the accuracy of these storage capacity methodologies to better understand the key geologic factors that influence CO₂ storage in a depleted hydrocarbon field for CCSItem Climate action strategies for the University of Texas at Austin(2010-05) Hernandez, Marinoelle; Eaton, David J.; Walker, Jim H.This report analyzes the current greenhouse gas emissions inventory for The University of Texas at Austin (UT-Austin), reviews the carbon reduction strategies being implemented at UT-Austin and other peer institutions, and offers recommendations for strategies that could reduce greenhouse gas emissions at UT-Austin in the future.Item Electrical transport measurements of individual bismuth nanowires and carbon nanotubes(2005) Jang, Wan Young; Yao, Zhen, Ph. D.Nanostructures are defined by reducing dimensions. When the reduced size of materials is comparable to the Fermi wavelength, quantum size effect occurs. Dimensionality plays a critical role in determining the electronic properties of materials, because the density of states of materials is quite different. Nanowires have attracted much attention recently due to their fundamental interest and potential application s. A number of materials have been tried. Among them, bismuth has unique properties. Bismuth has the smallest effective mass as small as 0.001me. This small effective mass of Bi nanowires allows one to observe the quantum confinement effect easily. Also Bi nanowires are good candidates for a low-dimensional transport study due to long mean free path. Because of these remarkable properties of Bi nanowires, many efforts have been made to study Bi nanowires. However, because bismuth is extremely sensitive to the oxide, it is very difficult to make a reliable device. So far, array measurements of Bi nanowires have been reported. The study is focused on the synthesis and electric transport measurements of individual Bi nanowires. Bi nanowires are synthesized by electrodeposition using either anodic aluminum oxide (AAO) templates or commercially available track etched polycarbonate membranes (PCTE). The desired nanowire has a heterostructure of Au – Bi – Au. Au wires on both sides serve as contact electrodes with Bi. To extract nanowires from PCTE or AAO, several attempts have been made. Devices consisting of single Bi nanowires grown by hydrothermal method are fabricated and electrical measurements have been carried out after in-situ deposition of Pt electrodes. The temperature dependence of resistance of majority of nanowires increases with decreasing temperature, showing polycrystalline nature of nanowires. However, some nanowires show resistance peaks at low temperature, suggesting quantum size effect (QSE). Magnetoresistance (MR) has also been measured. We have also studied electric transport measurements of carbon nanotubes grown in AAO templates. These vertically grown carbon nanotubes (CNTs) are useful for field emission device. In addition, ultra-density vertical CNT transistor arrays have also been proposed based on these nanotube structures. To realize these interesting electronic applications, a detailed understanding of the electronic transport properties of the nanotubes is needed. In particular, nanotubes grown in the AAO templates are known to possess significant amount of structural disorder. It is thus important to elucidate the effect of disorder on the electronic properties of these nanotubes. Electrical transport measurements of individual carbon nanotubes are studied, The four-terminal resistance at room temperature scales linearly with the nanotube length indicating diffusive nature of transport. The conductance shows an exp[(-1/T)1/3] dependence on temperature T, suggesting that two-dimensional variable-range hopping is the dominant conduction mechanism. The maximum current density carried by these nanotubes is on the order of 106 A/cm2.Item First-principles investigation of carbon-based nanomaterials for supercapacitors(2016-06-29) Pak, Alexander Jin; Hwang, Gyeong S.; Ekerdt, John G; Manthiram, Arumugam; Akinwande, Deji; Ren, PengyuSupercapacitors are electrochemical energy storage devices known for their large power densities and long lifetimes yet limited energy densities. A conventional understanding of supercapacitors relates the high power to fast ion accumulation at the polarized electrode interface, forming the so-called electric double layer (EDL), and the low energy to limited electrode surface area (SA). To improve the energy density, the capacitance may be enhanced by using high SA electrode materials such as carbon-based nanomaterials. While promising results have been experimentally reported, capacitances have also been noted to exhibit a highly non-linear relationship with SA. These interesting observations suggest that a gap exists in our fundamental understanding of charge storage mechanisms in the EDL of carbon nanomaterials. Given that EDLs are typically on the order of 1-3 nm thick, theoretical simulations can elucidate these unknown physical insights in order to identify new design principles for future electrode materials. In this dissertation, we explore two broad types of carbon-based nanomaterials, which are separated into two Parts, using a combined density functional theory and classical molecular dynamics computational approach. In Part I, we study the capacitance using various chemically and/or structurally modified graphene (or graphene-derived) materials which is motivated by previous accounts of the limited capacitance using pristine graphene. Our analysis demonstrates the viability of dramatically improving the capacitance using graphene-derived materials owing to enhancements in the quantum capacitance with marginal effects on the double layer capacitance. In Part II, we investigate the capacitance using nanoporous carbon materials which is motivated by experimental observations that relate capacitance to pore width rather than SA. Our findings confirm that promoting ion confinement through pore width control can enhance capacitance, but also identify pore shape dispersity as another important structural feature that facilitates fast ion dynamics during charging/discharging. The work in this dissertation presents an overview of new insights into charge storage mechanisms using low-dimensional carbon-based nanomaterials and future directions for materials development. Moreover, we anticipate that the established methodologies and analyses can be broadly applicable to the study of other applications utilizing electrified interfaces, including capacitive deionization and liquid-gated field effect transistors.Item Hierarchical three-dimensional Fe-Ni hydroxide nanosheet arrays on carbon fiber electrodes for oxygen evolution reaction(2014-05) O'Donovan-Zavada, Robert Anthony; Manthiram, ArumugamAs demands for alternative sources of energy increase over the coming decades, water electrolysis will play a larger role in meeting our needs. The oxygen evolution reaction (OER) component of water electrolysis suffers from slow kinetics. An efficient, inexpensive, alternative electrocatalyst is needed. We present here high-activity, low onset potential, stable catalyst materials for OER based on a hierarchical network architecture consisting of Fe and Ni coated on carbon fiber paper (CFP). Several compositions of Fe-Ni electrodes were grown on CFP using a hydrothermal method, which produced an interconnected nanosheet network morphology. The materials were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Electrochemical performance of the catalyst was examined by cyclic voltammetry (CV) and linear sweep voltammetry (LSV). The best electrodes showed favorable activity (23 mA/cm², 60 mA/mg), onset potential (1.42 V vs. RHE), and cyclability.Item High order stratigraphic framework of intraslope growth faulted subbasins offshore Matagorda Bay, Texas(2021-12-09) Franey, John D.; Meckel, Timothy AshworthCarbon capture and storage (CCS) is currently one of the leading atmospheric emission mitigation technologies. To have meaningful impact on the atmosphere CO₂ concentrations, megatons (10⁶) of CO₂ must be removed from the carbon cycle permanently. This requires a subsurface geologic storage sites that are both volumetrically significant and secure over geologic time-scales. The northern Gulf of Mexico (GOM) has the ability to serve as a major location for CCS. Miocene strandplain systems in the GOM are an ideal stratigraphy for such storage due to their proximity to emissions sources, quality sand reservoirs, and depth relative to overpressure. This study focuses on a suite of strike parallel subbasins within the Lower Miocene offshore Matagorda Bay, TX. Each subbasin has potential to serve as a future carbon sequestration site. Accurate mapping of subbasins’ stratigraphy is necessary to understand the variable thickness and associated risk of reservoir-sealing shale intervals, recognizing that injection beneath thicker, more uniformly distributed shales is more favorable. These intervals must be mapped at high resolution (4th order cyclicity) to understand the individual components in assessment and risk analysis. This research generates a novel dip-steered seismic volume which is leveraged to improve seismic attribute calculations and mapping at the 4th order. The dip-steered seismic volume records the seismic dip in the inline and crossline direction of seismic features at the intersection of every inline, crossline, and seismic sample. This volume is used to generate a model of dense, 3D, auto-tracked horizons across each subbasin. The models better connect high resolution, but sparse, well log data and low resolution, but continuous, seismic data. Thickness distributions and shale interval maps generated from the models aid in risk assessment. Based on the resulting shale thicknesses, the suite of subbasins should be further considered as future storage sitesItem Ice-free lagoon sediment in areas of continuous Arctic permafrost revealed through electrical resistivity imaging(2020-09-14) Pedrazas Hinojosa, Micaela Nicole; Cardenas, Meinhard Bayani, 1977-The Arctic is undergoing profound changes due to amplification of global warming in northern latitudes. One of the key features in the Arctic that remains understudied is ice-bonded subsea permafrost. This coastal feature is assumed to be ice-rich and underlies the many coastal lagoons in the Arctic. Subsea permafrost, is estimated to store as much organic carbon as Earth’s atmosphere and protects Arctic coastlines from erosion. However, subsea frozen sediment near the shoreline has not been thoroughly mapped and how much thawed sediment exists beneath coastal lagoons remains unclear. The presence or absence of ice beneath the surface, and its thawing are vital information that potentially represent a positive carbon feedback to the global climate system. Through modeling and direct observations of electrical resistivity across a lagoon on the Alaska Beaufort Sea coast during the summer, we found that the subsurface is ice-free down to at least 17 m under the lagoon and down to 22 m at the beach. This finding contrasts with the broadly held idea of continuity of ice-rich permafrost across the land-sea interface extending from land to offshore in the near-surface. Since the subsurface beneath the beach and the lagoon are unfrozen to depths of at least ~ 20 m, there exists a significantly thawed portion of sediment beneath the lagoons, which may serve as a source and conduit for heat, water, and carbon exchange.Item The Limits of Liability in Promoting Safe Geologic Sequestration of CO2(2011-01-01) Adelman, David E.; Duncan, Ian J.Item Magnetic, thermoelectric, and electronic properties of layered oxides and carbon materials(2007) Caudillo, Roman, 1977-; Goodenough, John B.; Jose-Yacaman, MiguelThe structure and physical properties of layered oxides and carbon materials were studied. Two layered carbon materials were studied: carbon nanotubes (CNTs) synthesized by electron irradiation from amorphous carbon in situ in a transmission electron microscope (TEM) and a carbon and silver nanocomposite consisting of graphitic carbon nanospheres encapsulating Ag nanoparticles. In the CNT experiments, the effect of electron irradiation in the TEM is shown to alter drastically their structure and properties, even being able to transform amorphous carbon into a CNT. This suggests a possible alternative synthesis technique for the production of CNTs, in addition to providing a method for tailoring their properties. The structure and magnetic properties of the carbon and silver nanocomposite was characterized with x-ray diffraction, scanning and transmission electron microscopy techniques, and magnetic susceptibility measurements with a superconducting quantum interference device (SQUID) magnetometer. While the sp² bonding gives a grapheme sheet its mechanical properties, the p[subscript pi] electrons are responsible for its electronic and magnetic properties. In a flat graphene sheet the p[subscript pi] electrons are itinerant, but in a narrow p[subscript pi] band. The introduction of curvature to the graphene sheets that encapsulate the Ag nanoparticles is demonstrated to narrow the p[subscript pi] band sufficiently to result in "ferromagnetic" behavior. A model that is able to explain spin localization and ferrimagnetic spin-spin interactions in graphitic materials with positive curvature is presented. Layered oxides from the family of the P2 Na[subscript x]CoO₂ structure were synthesized and their properties studied. Na[subscript x]CoO₂ has a rich phase diagram ranging form a promising Na-rich thermoelectric composition to the hydrated Na-poor composition Na[subscript 0.33]CoO₂· 1.3H₂O that is superconductive. Intermediate to these two Na compositions exists an insulating phase with x [approximately equal to] 0.5 that presents a variety of interesting structural, magnetic, thermoelectric, and electronic behavior. Investigations of Na[subscript x]CoO₂ that probe the role of H₂O in the superconductive Na[subscript 0.33]CoO₂· 1.3H₂O are presented and conclude that H₂O plays a more active role than a passive lattice spacer. The relationship between Na ordering and an interesting magnetic behavior observed with [chi](T) measurements of annealed NaxCoO₂ and Sr[subscript x/2]CoO₂ samples is determined and found to correspond to a (2a x 2a) superstructure. The properties of NaxCoO₂ (x [approximately equal to] 0.5) are reviewed and thermoelectric S(T) measurements are made in order to develop a model that is able to explain the salient features of the NaxCoO₂ (x [approximately equal to] 0.5) phase.Item Municipal economic growth through green projects and policies(2012-05) Lindner, Harry Dreyfus; Gamkhar, ShamaCities generally need economic growth. Green policies and projects are environmentally beneficial, desirable, expected by the public, and pragmatic in the long term. However, there is insufficient research on what, if any, municipal green projects and policies generate economic growth. To address this question, the author created a database of green and economic indicators, and modeled the green indicators to predict the economic indicators. The database included carbon usage, transportation metrics, water usage, the number of green jobs, and the gross domestic product (GDP) for the 100 largest cities, defined by metropolitan statistical area (MSA), in the U.S. To gather data on green indicators, existing green rankings, indices, and reports were evaluated for methodology and usability for this paper. The results of the data-gathering step show the need for more and better data collection. That means an increased number of green indicators should be collected, and data should be collected at the MSA (or county) level for more of the largest cities. Specifically to name some green indicators, data collection on energy usage, buildings, waste, land use, air quality, and food could be improved. Those green indicators would likely be included in a model that uses green indicators to predict green jobs or GDP. However, those were not included for the regressions in this paper. The results of the regressions in this paper show two indicators with promise for predicting economic growth as defined by GDP and number of green jobs: (1) percent of people using public transportation, biking, or walking to work, and (2) public water consumption per person. The first explanatory variable indirectly measures the adoption of policies that promote public transportation, biking, and walking. The results suggest that these policies have a positive effect on GDP and number of green jobs. This means the results suggest that as the percent increases, so does GDP and number of green jobs. The second explanatory variable measures the water conservation policies. The results suggest this variable has a negative, albeit weaker relationship with GDP per person. This means the results suggest as water conservation increases (less water usage per person), the GDP per person increases. This paper offers a methodology and some of the groundwork for building a model to show which, if any, municipal green projects and policies predict economic growth.Item Nanoscale graphene for RF circuits and systems(2013-08) Parrish, Kristen Nguyen; Akinwande, DejiIncreased challenges in CMOS scaling have motivated the development of alternatives to silicon circuit technologies, including graphene transistor development. In this work, we present a circuit simulator model for graphene FETs, developed to both fit measured data and predict new behaviors, motivating future research. The model is implemented in Agilent ADS, a circuit level simulator that is commonly used for non-standard transistor technologies, for use with parameter variation analyses, as well as easy integration with CMOS design kits. We present conclusions drawn from the model, including analyses on the effects of contact resistance and oxide scaling. We have also derived a quantum-capacitance limited model, used to intuit intrinsic behaviors of graphene transistors, as well as outline upper bounds on performance. Additionally, the ideal frequency doubler has been examined and compared with graphene, and performance limits for graphene frequency multipliers are elucidated. Performance as a demodulator is also discussed. We leverage this advancement in modeling research to advance circuit- and system-level research using graphene transistor technology. We first explore the development of a GHz planar carbon antenna for use on an RF frontend. This research is further developed in work towards the first standalone carbon radio on flexible plastics. A front end receiver, comprised of an integrated carbon antenna, transmission lines, and a graphene transistor for demodulation, are all fabricated onto one plastic substrate, to be interfaced with speakers for a full radio demo. This complete system will motivate further research on graphene-on-plastic systems.Item New insights into the carbon isotope composition of speleothem calcite : an assessment from surface to subsurface(2012-05) Meyer, Kyle William; Banner, Jay L.; Breecker, Dan O.; Musgrove, MaryLynnThe purpose of this study was to provide new insights into the interpretation of speleothem (cave calcite deposit) δ13C values. We studied two caves in central Texas, which have been actively monitored for over 12 years. We compared δ13C values of soil CO2 (δ13Cs), cave drip water (δ13CDIC), and modern cave calcite (δ13Ccc). Measured average δ13C values of soil CO2 were -13.9 ± 1.4‰ under mixed, shallowly-rooted C3-C4 grasses and were -18.3 ± 0.7‰ under deeply-rooted ashe juniper trees (C3). The δ13CDIC value of minimally-degassed drip water in Natural Bridge Caverns was -10.7 ± 0.3‰. The carbon isotope composition of CO2 in equilibrium with this measured drip water is -18.1 ± 0.3‰. The agreement between juniper soil CO2 and drip water (within ~0.2‰) suggests that the δ13C value of drip water (δ13CDIC) that initially enters the cave is controlled by deeply-rooted plants and may be minimally influenced by host-rock dissolution and/or prior calcite precipitation (PCP). At Inner Space Caverns, δ13CDIC values varied with vegetation above the drip site, distance from the cave entrance, and distance along in-cave flow paths. Whereas CO2 derived from deeply-rooted plants defines the baseline for drip water δ13CDIC entering the caves, kinetic effects associated with the degassing of CO2 and simultaneous precipitation of calcite account for seasonal variability in δ13CDIC and δ13Ccc. We documented increases in δ13CDIC at a rate of up to 0.47‰/hour during the season of peak degassing (winter), suggesting that δ13CDIC variations may be controlled by total elapsed time of CO2 degassing from drip water (Ttotal). We also observed seasonal shifts in the δ13C values of modern calcite grown on glass substrates that are correlated with shifts in drip water δ13CDIC values and drip-rate. Therefore, we suggest that increased aridity at the surface above a given cave results in, slower drip-rates, higher Ttotal, and therefore higher δ13CDIC values. We propose that large variability (>2‰) in speleothem δ13Ccc values dominantly reflect major vegetation changes, and/or increasing Ttotal by slowing drip-rates. Based on these findings, variability in speleothem carbon isotope records may serve as a proxy for paleoaridity and/or paleovegetation change.Item No evidence for millimeter continuum source overdensities in the environments of z [greater than approximately] 6 quasars(2018-09-05) Champagne, Jaclyn Bradli; Casey, Caitlin M.Bright high-redshift quasars (z > 6), hosting supermassive black holes (M [subscript BH] > 10⁸M [subscript circled dot]), are expected to reside in massive host galaxies embedded within some of the earliest, most massive galaxy overdensities. We analyze 1.2mm ALMA dust continuum maps of 35 bright quasars at 6 < z < 7 and search the primary beam for excess continuum emission as evidence for early protoclusters. We compare the detection rates of continuum sources at [greater than or equal to] 5σ significance in the fields surrounding the quasars (A [subscript eff] = 4.3 arcmin²) with millimeter number counts in blank field surveys. We discover 15 sources in the fields (excluding the quasars themselves), corresponding to an overdensity δ [subscript gal] [triple bar] (N [subscript gal] – N [subscript exp]) / N [subscript exp] = -0.07 ± 0.56, consistent with no detected overdensity of dusty galaxies within 140 physical kpc of the quasars. However, the apparent lack of continuum overdensity does not negate the hypothesis that quasars live in overdense environments, as evidenced by strong [CII] overdensities found on the same scales to similarly-selected quasars. The small field of view of ALMA could miss a true overdensity if it exists on scales larger than 1 cMpc, if the quasar is not centered in the overdensity, or if quasar feedback plays a role close to the quasar, but it is most likely that the large line of sight volume probed by a continuum survey will wash out a true overdensity signal.Item Patterns in iscoscapes and N:P stoichioscapes of the dominant seagrasses (Halodule wrightii and Thalassia testudinum) in the western Gulf of Mexico(2018-06-26) Cuddy, Meaghan Ruth; Dunton, Kenneth H.Seagrasses assimilate carbon (C), nitrogen (N), and phosphorus (P) from the water column and sediment, making their tissue nutrient content an excellent bioindicator of long-term, system-wide environmental conditions. I examined the role of seagrasses as ecological indicators of water quality and nutrient loading in three Texas estuarine systems through examination of their tissue isotopic (δ¹³C and δ¹⁵N) and stoichiometric (N:P) ratios over a period of five years using maps of spatial patterns in isotopic and stoichiometric regimes (“isoscapes” and stoichioscapes”). Leaf tissue samples were collected from the dominant Texas seagrasses, Halodule wrightii and Thalassia testudinum, at 567 stations during annual sampling between 2011 and 2015. Tissues were analyzed for C, N, and P content and C and N isotopic composition. Data were used to develop interpolated maps of variations in seagrass δ¹³C and δ¹⁵N signatures and N:P ratios in the Mission-Aransas NERR, Corpus Christi Bay, and upper and lower Laguna Madre. Regions where seagrasses had significantly enriched δ¹⁵N signatures, depleted δ¹³C signatures, or elevated N:P ratios were often associated with areas of urbanization or development. This was supported by significant relationships between δ¹⁵N and δ¹³C clusters and distance from outlets draining high population watersheds. I also documented a distinct temporal shift in δ¹³C signatures and N:P ratios across the study areas. The change in δ¹³C signatures was particularly notable in H. wrightii in 2015, when δ¹³C signatures became more depleted and N:P ratios were elevated, following an influx of freshwater and nutrients ending a three-year drought in south Texas. The spatial and temporal variation in seagrass tissue C content reported here reflects inputs of freshwater and riverine DIC as well as changes in the benthic light environment, while the N and P dynamics reported suggest that N:P ratios and δ¹⁵N signatures of seagrasses on the Texas coast are accurate bioindicators of nutrient loading in these estuaries. These metrics may thus serve as early indicators of changes in water quality.Item Preparation and characterization of nitrogen doped carbon nanotube electrode materials(2006) Maldonado, Stephen; Stevenson, Keith J.This dissertation describes the preparation and characterization of nitrogen doped carbon nanotube films by a chemical vapor deposition (CVD) process. Thorough description of the preparation method is given. A variety of techniques are used to demonstrate the relationship between the physicochemical properties and electrochemical properties of nitrogen doped carbon nanotubes, shedding insight to the influence of nitrogen doping on observed electrocatalysis phenomena. Chapter 1 comprises a general overview of the presented material and scope of the work. Chapter 2 details the CVD preparation of nitrogen doped carbon nanotubes. Scanning electron microscopy, thermal gravimetric analysis, and x-ray photoelectron spectroscopy are used to demonstrate the degree of control afforded by the CVD process over the resultant properties of the prepared carbon nanotubes. The direct preparation of carbon nanotube films on a current collector is shown. Chapter 3 consists of a thorough characterization and comparison of nitrogen doped carbon nanotubes and non-doped carbon nanotubes. Transmission electron microscopy, thermal gravimetric analysis, and Raman spectroscopy demonstrate the increased disorder caused by nitrogen doping into the graphitic lattice structure of carbon nanotubes. X-ray photoelectron spectroscopy highlights the existence of multiple carbon-nitrogen surface functionalities that change in relative abundance as the nitrogen content is varied. Titration analyses indicate that nitrogen doped carbon nanotubes are basic and acquire a cationic surface charge in solutions of neutral pH. Chapter 4 presents a collection of voltammetric responses of several outer-sphere and innersphere redox probes. The similarities and differences between the responses at non-doped carbon nanotube electrodes and nitrogen doped carbon nanotube electrodes are noted and discussed in the context of the conclusions of the physical characterizations. A detailed mechanistic analysis of the O2 reduction process at nitrogen doped carbon nanotube electrodes is presented. Chapter 5 details continuing work with carbon nanotube materials for fundamental and applied studies.