This research project is a part of our endeavor to developing a new hydrogen sulfide (H2S) splitting cycle for hydrogen production. In view of that the Bunsen reaction is the key step for the overall efficiency, the objective of this research is to develop an effective and efficient process to carry out the Bunsen reaction in the presence of organic solvents. Organic solvents can help dissolve iodine crystal, lower the reaction temperature and reduce the corrosiveness accompanying the reaction. Through screening of the ordinary organic solvents, aromatic hydrocarbons stood out and toluene was used in this project.<p> In order to study the Bunsen reaction rate in the presence of toluene, the iodine solubility in HI solution was extensively explored at room temperature. Although the iodine solubility in water is small (0.3404g/L at 25â), it was found that the iodine solubility in HI solution increases greatly as the [HI] increases. At lower [HI] (0~0.238 M), the iodine solubility is linear to [HI] with a relationship of [iodine solubility] = 0.57[HI] + 0.0030; at higher [HI] (0.238 ~7.6 M), the relationship of the iodine solubility and [HI] conforms to [iodine solubility]/[HI] = 0.190[HI] + 0.58.<p> In the second part, the iodine distribution behavior between HI solution and toluene phase was studied at room temperature. It was determined that the iodine distribution coefficient (D = [I2]HI solution/[I2]toluene) increases as the increase of [HI]. At lower [HI] (0~1.89 M), the distribution coefficient has a quadratic relationship with [HI] as D = 1.4027[HI]2 + 0.8638[HI] + 0.0088; at higher [HI] (1.89~7.54 M) the distribution coefficient is linear to [HI] with a relationship of D=5.5937[HI]-3.5632.<p> On the basis of the above work, in a semi-batch reactor, the Bunsen reaction rate in the presence of toluene was measured. In a mixture of toluene and water, iodine prefers to stay in toluene phase. The Bunsen reaction was readily initiated by feeding SO2 into water phase. Experimental results indicated that the rate of the Bunsen reaction in the presence of toluene is equal to the molar flow rate of feeding SO2 when the iodine concentration is higher than a certain value. This specific value depends on the reaction conditions, such as the interface area between water and toluene phase, the dispersion efficiency and the flow rate of SO2.

Distributed hydrological models have been used for decades to calculate and predict the movement of water and energy within watersheds. These models have evolved from relatively simple empirical applications into complex spatially distributed and physically-based programs. However, the evolution of distributed hydrological models has not involved the improvement of the numerical methods used to calculate the redistribution of water and energy in the watershed. Because of this, many models still use numerical methods that are potentially inaccurate. In order to simulate the transport of water and energy in a hydrological model, typical numerical methods employ an operator splitting approach. Operator splitting (OS) essentially breaks down the set of coupled ordinary differential equations (ODEs) that define a hydrological model into separate ODEs that can be solved individually. The dominant operator splitting method in surface water models is the ordered series approach. Because the ordered series approach treats parallel hydrological processes as if they happen in series, it is prone to errors that can significantly reduce the accuracy of model results. The impact that operator splitting errors have upon hydrologic model results is, to date, unknown. Using a new distributed hydrological model, Raven, the impact of operator splitting errors is investigated. Understanding these errors will lead to better numerical methods for reducing errors in models and to shed light on the shortcomings of hydrological models with respect to numerical method choice. Alternative numerical methods - the explicit Euler and the implicit iterative Heun methods - are implemented and assessed in their ability to minimize errors and produce more accurate distributed hydrological models.

Distributed hydrological models have been used for decades to calculate and predict the movement of water and energy within watersheds. These models have evolved from relatively simple empirical applications into complex spatially distributed and physically-based programs. However, the evolution of distributed hydrological models has not involved the improvement of the numerical methods used to calculate the redistribution of water and energy in the watershed. Because of this, many models still use numerical methods that are potentially inaccurate. In order to simulate the transport of water and energy in a hydrological model, typical numerical methods employ an operator splitting approach. Operator splitting (OS) essentially breaks down the set of coupled ordinary differential equations (ODEs) that define a hydrological model into separate ODEs that can be solved individually. The dominant operator splitting method in surface water models is the ordered series approach. Because the ordered series approach treats parallel hydrological processes as if they happen in series, it is prone to errors that can significantly reduce the accuracy of model results. The impact that operator splitting errors have upon hydrologic model results is, to date, unknown. Using a new distributed hydrological model, Raven, the impact of operator splitting errors is investigated. Understanding these errors will lead to better numerical methods for reducing errors in models and to shed light on the shortcomings of hydrological models with respect to numerical method choice. Alternative numerical methods - the explicit Euler and the implicit iterative Heun methods - are implemented and assessed in their ability to minimize errors and produce more accurate distributed hydrological models.

Two problems in computational wave dynamics are considered: (i) the use of Klemp-Wilhelmson time splitting at large scales and (ii) analysis of wave-wave instabilities in nonhydrostatic and rotating mountain waves. The use of Klemp-Wilhelmson (KW) time splitting for large-scale and global modeling is assessed through a series of von Neumann accuracy and stability analyses. Two variations of the KW splitting are evaluated in particular: the original acousticmode splitting of Klemp and Wilhelmson (KW78) and a modified splitting due to Skamarock and Klemp (SK92) in which the buoyancy and vertical stratification terms are treated as fast-mode terms. The large-scale cases of interest are the problem of Rossby wave propagation on a resting background state and the classic baroclinic Eady problem. The results show that the original KW78 splitting is surprisingly inaccurate when applied to large-scale wave modes. The source of this inaccuracy is traced to the splitting of the hydrostatic balance terms between the small and large time steps. The errors in the KW78 splitting are shown to be largely absent from the SK92 scheme. Resonant wave-wave instability in rotating mountain waves is examined using a linear stability analysis based on steady-state solutions for flow over an isolated ridge. The analysis is performed over a parameter space spanned by the mountain height (Nh/U) and the Rossby number (U/fL). Steady solutions are found using a newly developed solver based on a nonlinear Newton iteration. Results from the steady solver show that the critical heights for wave overturning are smallest for the hydrostatic case and generally increase in the rotating wave regime. Results of the stability analyses show that the wave-wave instability exists at mountain heights even below the critical overturning values. The most unstable cases are found in the nonrotating regime while the range of unstable mountain heights between initial onset and critical overturning is largest for intermediate Rossby number.

The field of solar energy conversion has experienced resurgence in recent years due to mounting concerns related to fossil fuel consumption. The sheer quantity of available solar energy and corresponding opportunity for technological improvement has motivated extensive study of novel light-absorbing semiconductors for solar energy conversion. Often, these studies have focused on new ways of synthesizing and altering thin film semiconductor materials with unique compositions and morphologies in order to optimize them for higher conversion efficiencies. In this dissertation, we discuss the synthesis and electrochemical characterization of a variety of candidate semiconductor materials exhibiting promising characteristics for photoelectrochemical solar energy conversion. Three specific methods of thin film deposition are detailed. The first is a physical vapor deposition technique used to independently tune the morphology and composition of hematite (α-Fe2O3) based materials. Because of hematite’s poor electronic properties, these modifications were able to significantly improve its performance as a photoanode for water oxidation. The second technique is electrodeposition, which was employed to deposit the novel ternary metal oxide, CuBi2O4. The study of these films, along with those prepared by physical vapor deposition, provided insight into the factors limiting the ability of this photo-active material to function as a photocathode for hydrogen evolution from water. The third technique is chemical spray pyrolysis, which was employed to deposit and optimize films of the bismuth chalco-halides BiOI and BiSI. These studies were used to obtain previously unknown properties of these materials relevant to their utilization in photoelectrochemical cells. The manipulation of deposition temperature had significant effects on these properties and dictated the films’ overall photoconversion performance. / text

Photoelectrochemical studies were performed on bismuth vanadate (BiVO₄) to understand chemical and physical properties of the photocatalysts, and to improve the photoactivity for water oxidation. Scanning electrochemical microscopy (SECM) was used to screen various dopants for BiVO₄, to calculate the photoconversion efficiencies to chemical energy at BiVO₄ electrodes, and to study the water oxidation intermediate radicals at the surface of BiVO₄. Tungsten and molybdenum doped BiVO₄ (W/Mo-BiVO₄) shows a photocurrent for water oxidation that is more than 10 times higher than undoped BiVO₄. Photoelectrochemical measurements and material analysis were done to discuss the factors that affect performance of BiVO₄. Finite elements analysis was also performed to explain the electron-hole transport and electrochemical reactions at W/Mo-BiVO₄ electrodes in solutions. Addition of conductive or electron accepting materials, e.g. reduced graphene oxide, into BiVO₄ was tried to study the electron-hole transport phenomena in the metal oxide electrodes. Surface adsorbed radicals produced during the water oxidation at W/Mo-BiVO₄ were interrogated by using SECM that the surface coverage and decay kinetics of adsorbed hydroxyl radicals at W/Mo-BiVO₄ were measured. The quantum efficiencies of the injected photon conversion to chemical energy were obtained from the photoelectrochemical measurements by using SECM. SECM techniques and finite elements analysis were also used to measure the faradaic efficiency of water oxidation at W/Mo-BiVO₄ under irradiation. Finally, unbiased water splitting to generate hydrogen and oxygen from water splitting only using photon energy at W/Mo-BiVO₄ electrodes was demonstrated in a dual n-type semiconductor or Z-scheme device. / text

Στα δίκτυα κινητής τηλεφωνίας τρίτης γενιάς, όπου η απαίτηση για την επέκταση της χωρητικότητας του συστήματος αυξάνεται με τον αριθμό των χρηστών καθώς και με την ποσότητα της πληροφορίας που απαιτείται για ένα δεδομένο εξυπηρετητή, η απαίτηση για επέκταση του εύρους ζώνης είναι υψίστης σημασίας. Παρόλα αυτά, τα προβλήματα της συμφόρησης είναι συχνά αναπόφευκτα, ιδιαίτερα στα ασύρματα κανάλια, όπου η χωρητικότητα είναι πιο σπάνια, αλλά και στα αστικά κέντρα όπου το συνδρομητικό φορτίο είναι ιδιαίτερα ψηλό. Στις μέρες μας, η αυξανόμενη πυκνότητα χρηστών, αλλά και οι τεχνολογίες επείγουσας κατανάλωσης εύρους ζώνης δημιουργούν μια ακόμη μεγαλύτερη ανισορροπία. Καθώς όμως το εύρος ζώνης παραμένει σπάνιο και σταθερό, νέες μέθοδοι θα πρέπει να ευρεθούν για την αντιμετώπιση των προβλημάτων συμφόρησης. Μια από αυτές είναι η κυψελοειδής διάσπαση, μια μέθοδος η οποία επιτρέπει τη δημιουργία μικρότερων κυψελών. Έτσι ο ίδιος αριθμός καναλιών χρησιμοποιείται για μικρότερη περιοχή, ενώ για την ίδια πιθανότητα συμφόρησης περισσότεροι χρήστες μπορούν να ανατεθούν στην κυψέλη. / -

In this thesis, we study a linear fractional Fokker-Planck equation that models non-local (`fractional') diffusion in the presence of a potential field. The non-locality is due to the appearance of the `fractional Laplacian' in the corresponding PDE, in place of the classical Laplacian which distinguishes the case of regular (Gaussian) diffusion. Motivated by the observation that, in contrast to the classical Fokker-Planck equation (describing regular diffusion in the presence of a potential field), there is no natural gradient flow formulation for its fractional counterpart, we prove existence of weak solutions to this fractional Fokker-Planck equation by combining a splitting technique together with a Wasserstein gradient flow formulation. An explicit iterative construction is given, which we prove weakly converges to a weak solution of this PDE. / Graduate

Shear-wave splitting is a phenomenon observed in almost all in situ rocks. Due to propagation through stress-aligned and fluid-saturated microcracks and fractures the initial shear wave splits into two almost orthogonal waves which propagate with different velocities along similar ray paths. The process is characterized by the polarization direction of the faster split shear wave, which is parallel to the orientation of the cracks, and the time delay between the onsets of the two waves. The analysis of shear-wave splitting has been conducted over records of 233 microearthquakes in the vicinity of five seismic stations in SW Iceland. Visual methods have been applied to the data to retrieve the final results for polarization directions and time delays. The main polarization azimuth for the leading split wave is N30°- 60°E which is in full agreement with the mapped alignments of normal faults and volcanic fissures in the surface. The time delays measured at different sites vary in the range of 10-100 ms for the events of best quality. In general, splitting times do not show a clear pattern at all recording sites with increasing depth. The only firm conclusion that can be drawn from the time delays is that at station BLF in the Brennisteinsfjöll fissure swarm, the time delays are smaller than in the Hengill area and therefore the strength of anisotropy beneath that station appears to be lower.

While prices of solar energy are becoming cost competitive with traditional fossil fuel resources, large scale deployment of solar energy has been limited by the inability to store excess electrical energy efficiently. One promising route towards both the capture and storage of solar energy is through photoelectrochemical water splitting, a process by which a semiconducting material can collect energy from the sun and use it to directly split water (H2O) into hydrogen fuel and oxygen. Unfortunately, photoelectrochemical water splitting devices are limited by the low efficiencies and high overpotentials of the oxygen evolution reaction (OER). To improve kinetics of OER, different electrocatalyst are often coated on the semiconductor. However, the role of the catalyst and the mechanism of charge transfer at the semiconductor|catalyst interface is not clear. It is important to understand this interface if we are to rationally design high performance water splitting cells. The research presented in this dissertation takes on two aims: 1) obtaining a fundamental knowledge of the charge transfer processes that take place at the semiconductor catalyst interface of photoanodes and 2) developing new experimental approaches that can be applied towards achieving the first aim. Specifically, this dissertation begins with a prospectus that outlines the state of the field, and the what was known about the semiconductor|electrocatalyst interface at the outset of the presented work (Chapter II). Next, the testing and application of new nanoelectrode AFM probes to study an array of electrochemical phenomena will be discussed (Chapter III). These probes will then be applied towards the study of hematite (Fe2O3) semiconductors coated with cobalt phosphate (oxy)hydroxide (CoPi) electrocatalyst (Chapter IV) and bismuth vanadate (BiVO4) semiconductors coated with CoPi electrocatalyst (Chapter 5). This dissertation includes previously published and unpublished co-authored material. / 2020-01-11