Doctor of Philosophy / Department of Civil Engineering / Robert J. Peterman / This dissertation describes a testing program to evaluate the bond and splitting propensity characteristics of 5.32-mm-diameter prestressing wires. Prestressing wire reinforcement is used primarily in the production of prestressed concrete railroad ties. Twelve different 5.32-mm-diameter wires were tested in this study in order to measure bonding characteristics of the reinforcement. Establishment of the bond-slip characteristics of these reinforcement at both transfer of prestress (transfer bond) and under flexural loading (flexural bond) is necessary to enable the accurate modeling of these ties using finite elements. Transfer bond and flexure bond of various indent patterns were tested using tensioned pullouts. Specimens of various sizes with single or multiple wires were tested to determine the effects of cover and wire number on bond. Indents were machined on smooth prestressing wires to accurately compare indent geometries. Lateral expansion was tested to determine which wires have higher propensity to cause cracking or splitting. Crossties were instrumented to compare resulting lateral expansion with results found in the laboratory. The results from the testing program showed that the tensioned pullout test was able to be used to predict the transfer length of prisms made with the same reinforcement. The results also showed that the indent geometries were able to be used to predict the splitting of specimens based on the amount of slip the wire had experienced. The testing also showed the importance of concrete cover with the relation to splitting potential.

Thesis advisor: Dunwei Wang / Solar water splitting refers to the reaction that converts solar energy into chemical fuel. It is an attractive means to store solar energy. This process, analogous to nature photosynthesis, uses semiconductor to capture and convert solar irradiation and, as such, is called artificial photosynthesis. Despite its promising prospect, the lack of materials that can satisfy all requirements to achieve efficient solar water splitting becomes an important challenge. In this thesis, we aim to develop a strategy of forming heteronanostructure to tackle the challenge faced by metal oxide-based photoanode for water oxidation. The challenge associated with metal oxide-based photoanodes and current approach to alleviate the challenge is first discussed. We propose a strategy of combining multiple components to form heteronanostructure to meet the challenges, in particular the charge transport issue. By introducing a dedicated charge transporter, we fabricate various heteronanostructure including TiO₂/TiSi₂, Fe₂O₃/TiSi₂ and Fe₂O₃/AZO nanotubes to improve the charge collection and therefore overall efficiency. Additionally, the growth of several important metal oxides by atomic layer deposition is developed and its utilization as photoanode for water splitting is studied for the first time. Because this strategy is based on the rational design and synthesis of materials, it has the potential to produce electrodes with a combination of properties that have not been exhibited simultaneously by single-component materials. In addition, the strategy is highly versatile and can incorporate the latest developments produced by parallel efforts. We are confident that the rational design and synthesis of materials such as the strategy proposed here will play an increasingly more important role in energy research. / Thesis (PhD) — Boston College, 2012. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

Thesis advisor: Dunwei Wang / Water splitting has been recognized as a promising solution to challenges associated with the intermittent nature of solar energy for over four decades. A great deal of research has been done to develop high efficient and cost-effective catalysts for this process. Among which tantalum nitride (Ta₃N₅) has been considered as a promising candidate to serve as a good catalyst for solar water splitting based on its suitable band structure, chemical stability and high theoretical efficiency. However, this semiconductor is suffered from its special self-oxidation problem under photoelectrochemical water splitting conditions. Several key unique properties of graphitic carbon nitride (g-C₃N₄) render it an ideal choice for the protection of Ta₃N₅. In this work, Ta₃N₅/g-C₃N₄ hybrid photoanode was successfully synthesized. After addition of co-catalyst, the solar water splitting performance of this hybrid photoanode was enhanced. And this protection method could also act as a potential general protection strategy for other unstable semiconductors. / Thesis (MS) — Boston College, 2018. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

Thesis advisor: Dunwei Wang / Photosynthesis converts solar energy and stores it in chemical forms. It is one of the most important processes in nature. Artificial photosynthesis, similar to nature, can provide us reaction products that can potentially be used as fuel. This process promises a solution to challenges caused by the intermitted nature of solar energy. Theoretical studies show that photosynthesis can be efficient and inexpensive. To achieve this goal, we need materials with suitable properties of light absorption charge separation, chemical stability, and compatibility with catalysts. For large-scale purpose, the materials should also be made of earth abundant elements. However, no material has been found to meet all requirements. As a result, existing photosynthesis is either too inefficient or too costly, creating a critical challenge in solar energy research. In this dissertation, we use inorganic semiconductors as model systems to present our strategies to combat this challenge through novel material designs of material morphologies, synthesis and chemical reaction pathways. Guided by an insight that a collection of disired properties may be obtained by combining multiple material components (such as nanostructured semiconductor, effective catalysts, designed chemical reactions) through heterojunctions, we have produced some advanced systems aimed at solving fundamental challenges common in inorganic semiconductors. Most of the results will be presented within this dissertation of highly specific reaction routes for carbon dioxide photofixation as well as solar water splitting. / Thesis (PhD) — Boston College, 2013. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

Thesis advisor: Dunwei Wang / Semiconductor devices offer promise for efficient conversion of sunlight into other useful forms of energy, in either photovoltaic or photoelectrochemical cell configurations to produce electrical power or chemical energy, respectively. This dissertation examines ionic and electronic phenomena in some candidate semiconductors and seeks to understand their implications toward solar energy conversion applications. First, copper sulfide (Cu₂S) was examined as a candidate photovoltaic material. It was discovered that its unique property of cation diffusion allows the room-temperature synthesis of vertically-aligned nanowire arrays, a morphology which facilitates study of the diffusion processes. This diffusivity was found to induce hysteresis in the electronic behavior, leading to the phenomena of resistive switching and negative differential resistance. The Cu₂S were then demonstrated as morphological templates for solid-state conversion into different types of heterostructures, including segmented and rod-in-tube morphologies. Near-complete conversion to ZnS, enabled by the out-diffusion of Cu back into the substrate, was also achieved. While the ion diffusion property likely hinders the reliability of Cu₂S in photovoltaic applications, it was shown to enable useful electronic and ionic behaviors. Secondly, iron oxide (Fe₂O₃, hematite) was examined as a photoanode for photoelectrochemical water splitting. Its energetic limitations toward the water electrolysis reactions were addressed using two approaches aimed at achieving greater photovoltages and thereby improved water splitting efficiencies. In the first, a built-in n-p junction produced an internal field to drive charge separation and generate photovoltage. In the second, Fe₂O₃ was deposited onto a smaller band gap material, silicon, to form a device capable of producing enhanced total photovoltage by a dual-absorber Z-scheme mechanism. Both approaches resulted in a cathodic shift of the photocurrent onset potential, signifying enhanced power output and progress toward the unassisted photoelectrolysis of water. / Thesis (PhD) — Boston College, 2013. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

We investigate upper crustal anisotropy of the Endeavour Segment of the Juan de Fuca Ridge using shear wave splitting measurements of ~3000 earthquakes recorded during three years using the Keck seafloor seismic network. We apply a new cluster analysis of shear-wave splitting measurements to our database. The methodology reduces the use of subjective criteria and improves the accuracy of measurements in the presence of noisy data. Fast polarization directions at a given seismic station are constant and stable during the deployment; however, fast-polarization directions between stations vary significantly. We presume that the lack of consistency of shear wave splitting among seismic stations reflects the spatial distribution of anisotropy in the vicinity of the ridge axis. We infer that the variation of fast polarization directions and delay times is caused by spatial variations in shallow hydrogeological structures and the stress field. Local faults and fissures are unlikely to be the primary cause of this anisotropy since most of the fast polarization directions are not consistent with the ridge parallel trend of faults. Stress perturbations induced by magmatic injection into the axial magma chamber or spatial variation in the rates of a hydrothermal heat transfer may contribute to the observed heterogeneity in seismic anisotropy.

Em mamíferos, a temperatura corporal (Tc) varia ao longo do dia, de forma rítmica, bem como a atividade locomotora. Essa ritmicidade diária é gerada endogenamente, por osciladores circadianos. Em geral, os valores mais elevados de Tc concentram-se nos mesmos horários em que há atividade locomotora. No entanto, variações diárias de Tc são observadas mesmo em indivíduos imóveis, evidenciando que o ritmo de Tc não é uma mera consequência dos efeitos agudos da atividade locomotora. Este trabalho teve o objetivo de estudar o ritmo circadiano de Tc no roedor Ctenomys aff. knighti (tuco-tuco) e sua relação temporal com o ritmo de atividade locomotora. A caracterização do ritmo de Tc nesse animal é particularmente interessante, pois ele habita o ambiente subterrâneo, que impõe desafios tanto à expressão rítmica (pois os ciclos ambientais são ausentes ou têm baixa amplitude) quanto à termorregulação (devido à características como alta umidade e atmosfera hipóxica e estagnada). Medimos a Tc e a atividade locomotora dos tuco-tucos em condições controladas de laboratório. Na caracterização inicial, constatamos que o ritmo de Tc dos tuco-tucos persiste mesmo em escuridão e temperatura constantes. Quando houve exposição a um ciclo diário de claro/escuro, o ritmo de Tc foi sincronizado, e os valores mais altos ocorreram na fase de escuro. Os ritmos de Tc e de atividade locomotora mostraram-se fortemente associados no tempo. Investigamos, então, se a corrida na roda de atividade modificaria a amplitude do ritmo de Tc, por envolver atividade muscular intensa. Observamos, no entanto, que a amplitude é pouco alterada com a retirada da roda e que na sua ausência os animais substituem a corrida por outras atividades que também causam efeitos agudos na Tc. Em seguida, estudamos a variação da sensibilidade da Tc aos efeitos agudos da atividade ao longo do dia. Através de um método estatístico, que também foi utilizado para filtrar os dados de Tc, verificamos que há maior correlação entre variações de Tc e de atividade no início da noite. Por fim, investigamos se o controle do ritmo de Tc é exercido pelos núcleos supraquiasmáticos, os quais são os osciladores circadianos do ritmo de atividade locomotora. Nossa abordagem utilizou animais neurologicamente intactos, comparando os padrões do ritmo de Tc e de atividade locomotora durante a ocorrência do fenômeno de \"partição do ritmo\". Constatamos que na maioria dos casos os padrões de partição, tanto da Tc como da atividade locomotora são idênticos, sugerindo um controle temporal comum. No entanto, em um dos animais observamos diferenças entre os dois ritmos, sugerindo que esse controle circadiano pode ser ainda mais complexo. / In mammals, body temperature (Tb), as well as locomotor activity, changes during the day, exhibiting a rhythmic pattern. This daily rhythmicity is generated endogenously by circadian oscillators. Usually, the highest Tb values occur simultaneously to locomotor activity. Nevertheless, daily Tb changes are observed even in immobile individuals. This fact is evidence that the Tb rhythm is not a mere consequence of the acute effects of locomotor activity. The present work,which os performed in the research center CRILAR, in Argentina, had the objective of studying the circadian rhythm of Tb. in the rodent Ctenomys aff. knighti (tuco-tuco) and its temporal relationship with the locomotor activity rhythm. The characterization of the Tb rhythm in this animal is especially interesting because it inhabits the subterranean environment, which poses challenges to both rhythmic expression (because environmental cycles are either absent or have low amplitude) and thermoregulation (due to high relative humidity, hypoxic atmosphere and limited ventilation). We measured Tb and locomotor activity of tuco-tucos under controlled laboratory conditions. An initial characterization showed that the tuco-tuco\'s Tb rhythm persisted even in constant darkness and constant temperature. This rhythm synchronized to a daily light/dark cycle, with highest values occurring during the dark phase. Tb and locomotor activity rhythm were robustly associated in time. Then, we investigated whether the wheel running would modify the Tb rhythm amplitude, because this behavior involves intense muscular activity. However, we observed that the amplitude is only slightly altered upon running-wheel removal and that in the absence of the wheel tuco-tucos substitute running with other behaviors which also have acute effects on Tb. Applying a statistical method, which was also used to filter Tb data, we verified that there is a stronger correlation between Tb changes and activity in the beginning of the night. Finally, we investigated whether the Tb rhythm is also under control of the suprachiasmatic nuclei, which are knowingly the circadian oscillators for the locomotor activity rhythm. We used an approach which involved neurally intact animals, by comparing the rhythmic patterns of Tb and locomotor activity rhythms during the occurrence of a phenomenon called \"splitting\". In most cases, splitting patterns of both Tb and locomotor activity rhythms were identical, indicating a common temporal control of these two variables. Nevertheless, we observed, in one animal, differences between the two rhythms, which suggest that the circadian control might be even more complex.

No description available.

Operator splitting methods have been applied to nonlinear partial differential equations that involve operators of different nature. The main idea of these methods is to decompose a complex equation into simpler sub-equations, which can be solved separately. The main advantage of the operator splitting methods is that they provide a great flexibility in choosing different numerical methods, depending on the feature of each sub-problem. In this dissertation, we have developed highly accurate and efficient numerical methods for several nonlinear partial differential equations, which involve both linear and nonlinear operators. We first propose a fast explicit operator splitting method for the modified Buckley-Leverett equations which include a third-order mixed derivatives term resulting from the dynamic effects in the pressure difference between the two phases. The method splits the original equation into two equations, one with a nonlinear convective term and the other one with high-order linear terms so that appropriate numerical methods can be applied to each of the split equations: The high-order linear equation is numerically solved using a pseudo-spectral method, while the nonlinear convective equation is integrated using the Godunov-type central-upwind scheme. The spatial order of the central-upwind scheme depends on the order of the piecewise polynomial reconstruction: We test both the second-order minmod-based reconstruction and fifth-order WENO5 one to demonstrate that using higher-order spatial reconstruction leads to more accurate approximation of solutions. We then propose fast and stable explicit operator splitting methods for two phase-field models (the molecular beam epitaxy equation with slope selection and the Cahn-Hilliard equation), numerical simulations of which require long time computations. The equations are split into nonlinear and linear parts. The nonlinear part is solved using a method of lines combined with an efficient large stability domain explicit ODE solver. The linear part is solved by a pseudo-spectral method, which is based on the exact solution and thus has no stability restriction on the time step size. We have verified the numerical accuracy of the proposed methods and demonstrated their performance on extensive one- and two-dimensional numerical examples, where different solution profiles can be clearly observed and are consistent with previous analytical studies. / Zhuolin Qu

Solar hydrogen has the potential to replace fossil fuels with a sustainable energy carrier that can be produced from sunlight and water via &quotewater splitting&quote. This study investigates the use of hematite (Fe&sub2O&sub3) as a photoelectrode for photoelectrochemical water splitting. Fe&sub2O&sub3 has a narrow indirect band-gap, which allows the utilization of a substantial fraction of the solar spectrum. However, the water splitting efficiencies for Fe&sub2O&sub3 are still low due to poor absorption characteristics, and large losses due to recombination in the bulk and at the surface. The thesis investigates the use of nanostructured composite electrodes, where thin films of Fe&sub2O&sub3 are deposited onto a nanostructured metal oxide substrate, in order to overcome some of the factors that limit the water splitting efficiency of Fe&sub2O&sub3. Doped (Si, Ti) and undoped Fe&sub2O&sub3 thin films were prepared using vacuum deposition techniques, and their photoelectrochemical, electrical, optical and structural properties were characterised. The doped Fe&sub2O&sub3 exhibited much higher photoelectrochemical activity than the undoped material, due to an improvement of the surface transfer coefficient and some grain boundary passivation. Schottky barrier modeling of Fe&sub2O&sub3 thin films showed that either the width of the depletion region or the diffusion length is the dominant parameter with a value around 30 nm, and confirmed that the surface charge transfer coefficient is small. An extensive review of the conduction mechanisms of Fe&sub2O&sub3 is presented. ZnO and SnO&sub2 nanostructures were investigated as substrates for the Fe&sub2O&sub3 thin films. Arrays of well-aligned high aspect ratio ZnO nanowires were optimised via the use of nucleation seeds and by restricting the lateral growth of the nanostructures. The geometry of the nanostructured composite electrodes was designed to maximise absorption and charge transfer processes. Composite nanostructured electrodes showed lower quantum efficiencies than equivalent thin films of Fe&sub2O&sub3, though a relative enhancement ofcollection of long wavelength charge carriers was observed, indicating that the nanostructured composite electrode concept is worthy of further investigation. The rate-limiting step for water splitting with Fe&sub2O&sub3 is not yet well understood and further investigations of the surface and bulk charge transfer properties are required in order to design electrodes to overcome specific shortcomings.