Development of alternative cathodes that have high capacity and long cycle life at an affordable cost is critical for next generation rechargeable batteries to meet the ever-increasing requirements of global energy storage market. Lithium-sulfur batteries, employing sulfur cathodes, are increasingly being investigated due to their high theoretical capacity, low cost, and environmental friendliness. However, the practicality of lithium-sulfur technology is hindered by technical obstacles, such as short shelf and cycle life, arising from the shuttling of polysulfide intermediates between the cathode and the anode as well as the poor electronic conductivity of sulfur and the discharge product Li2S. This dissertation focuses on overcoming some of these problems. The sulfur cathode involves an electrochemical conversion reaction compared to the conventional insertion-reaction cathodes. Therefore, modifications in cell-component configurations/structures are needed to realize the full potential of lithium-sulfur cells. This dissertation explores various custom and functionalized cell components that can be adapted with pure sulfur cathodes, e.g., porous current collectors in Chapter 3, interlayers in Chapter 4, sandwiched electrodes in Chapter 5, and surface-coated separators in Chapter 6. Each chapter introduces the new concept and design, followed by necessary modifications and development. The porous current collectors embedded with pure sulfur cathodes are able to contain the active material in their porous space and ensure close contact between the insulating active material and the conductive matrix. Hence, a stable and reversible electrochemical-conversion reaction is facilitated. In addition, the use of highly porous substrates allows the resulting cell to accommodate high sulfur loading. The interlayers inserted between the pure sulfur cathode and the separator effectively intercept the diffusing polysulfides, suppress polysulfide migration, localize the active material within the cathode region, and boost cell cycle stability. The combination of porous current collectors and interlayers offers sandwiched electrode structure for the lithium/dissolved polysulfide cells. By way of integrating the advantages from the porous current collector and the interlayer, the sandwiched electrodes stabilize the dissolved polysulfide catholyte within the cathode region, resulting in a high discharge capacity, long-term cycle stability, and high sulfur loading. The novel surface-coated separators have a polysulfide trap or filter coated onto one side of a commercial polymeric separator. The functional coatings possess physical and/or chemical polysulfide-trapping capabilities to intercept, absorb, and trap the dissolved polysulfides during cell discharge. The functional coatings also have high electrical conductivity and porous channels to facilitate electron, lithium-ion, and electrolyte mobility for reactivating the trapped active material. As a result, effective reutilization of the trapped active material leads to improved long-term cycle stability. The investigation of the key electrochemical and engineering parameters of these novel cell components has allowed us to make progress on (i) understanding the materials chemistry of the applied functionalized cell components and (ii) the electrochemical performance of the resulting lithium-sulfur batteries. / text

Transparent electrodes (TEs) require materials that have both transparency and electrical conductivity, a combination not usually found in nature. They are in increasing demand for use in solar cells, touch screens, displays, transparent heating films and several other devices. Most TEs used today are made of indium tin oxide (ITO). However, it has several disadvantages, such as high fabrication cost, rigidity and brittleness. Many ITO alternatives are being pursued, among which metallic nanowire (NW) networks on transparent substrates such as glass or polymer, have received much attention. This thesis demonstrates ordered silver NW networks on polyimide, fabricated by the field-directed chaining technique. We achieved a sheet resistance of 27 Ω/sq and 95.4% transparency at 550nm, with a Figure of Merit (FOM) 0.023Ω-1, which is higher than the FOM of commercial ITO, 0.005Ω-1. We have demonstrated that ordered NW networks, directed by alternative current (AC) electric fields, are easy to fabricate over a large area and at low cost, on rigid and flexible substrates. The AC electric field changes with different experiment setup. In this work, the effect of polymer thickness, electric field frequency, and gap size between electrodes are explored by COMSOL simulation and validated experimentally. By choosing the appropriate frequency and gap size, ordered NW networks are successfully created on a 23μm polyethylene terephthalate (PET) sheet. Fluid motion is one of the disruptors during NW chaining. We demonstrate control of this disruptor by the use of sandwiched channels for the NW suspension. Post-fabrication treatments are important and necessary for improving the connectivity and conductivity of Ag NW networks. In this work, we explore Joule heating and show its potential to improve the conductivity over other post-treatment approaches. However, Joule heating can also cause failures of NW networks. Ordered NW networks present better optical-electrical properties than random NW networks. Post-fabrication treatment can improve the properties, but there is a limit. In this work, a mathematical model is built for optical-electrical properties of perfectly ordered NW networks, which sets the upper bound of performance for transparent electrodes made of NW networks. A linear relationship is found between the transmittance and inverse sheet resistance. The model is then modified with factors to account for departure from the ideal. / Graduate / 2019-12-12

This thesis reports the theoretical investigations aimed at understanding the structure, stability and properties of a few inorganic compounds. The first chapter presents an introductory overview of the theories used to solve the questions addressed in the thesis. A brief discussion of the work is also presented here. The second chapter deals with electron reservoirs which have been one of the basic motifs of single-electron device. Mononuclear vinylidene complexes of type Mn(C5H4R’)(R” 2 PCH2CH2PR "2)= C = C(R1)(H) were synthesized and reported [Venkatesan et al, Organometallics 25, 5190 (2006)] as potential electron reservoirs capable of storing and releasing electrons in a reversible fashion. These compounds have been of great interest because their red-ox chemistry (reversible oxidative coupling and reductive decoupling) is governed by the C - C bond. However slow oxidation of the mononuclear vinylidene complexes leads to undesired product. In our model compound Mn(C5H5)(PH3)2 = C = C(R1)(H), we substituted the cyclopentadienyl moiety by isolobal dianionic dicarbollyl ligand Dcab2- (C2B9H2-11 ). This simple substitution could reduce the production of undesired product. Calculations of vertical detachment energy, thermodynamic feasibility and molecular orbital analysis showed that this substitution was thermodynamically feasible and led to easy oxidation and dimerization of the parent compound accompanied with better reversibility of the reaction. The effect of substituents (R = H,Me,Ph) on Cβ atom of our model system was also analyzed. The substituent on β carbon had a great effect on the stability and reactivity of these complexes. Our comparative study between Mn(C5H5)(PH3)2 = C = C(R)(H) and Mn(Dcab)(PH3)2 = C = C(R)(H)−1 (where R = H,Me,Ph) predicted the latter to be a more potential electronic reservoir. Gas-phase observations on MAl 4- (M = Li, Na, Cu) and Li3Al-4 coupled with computations led to the conclusion that Al42− [Boldyrev and Wang et al, Science 291, 859 (2001)] is “aromatic” while Al44- is “antiaromatic” [Boldyrev and Wang et al, Science 300, 522 (2003)]. It has been reported by Pati et al [J. Am. Chem. Soc. 125, 3496 (2005)] that co-ordination with a transition metal can stabilize the “antiaromatic” Al4Li4. In the first section of chapter three, it has been reported that Al4Li4 can also be stabilized by capping it with main group element like C and its isoelectronic species BH. Calculations of binding energy, nuclear independent chemical shift (NICS), energy decomposition analysis and molecular orbital analysis supported the capping induced stability, reduction of bond length alternation and increase of aromaticity of these BH/C capped Al4Li4 systems. The interaction between px and py orbitals of BH/C and the HOMO and LUMO of Al4Li4 was responsible for such stabilization. Calculations suggested that capping might introduce fluxionality in the molecule at room temperature. Al has valence electronic configuration of s2p1 and Al42− has been shown to have multiple aromaticity [Boldyrev and Wang et al, Science 291, 859 (2001)]. Analogy between electronic configuration s2pof Al and d1sof Sc/Y prompted us to explore the aromaticity of M42− clusters (M = Sc, Y ) which have been described in the second section of chapter three. Different geometries of M42− clusters (M = Sc, Y ) were explored, and the planar butterfly-like D2h geometry (two fused triangles) was found to be the most stable isomer. This is unlike the case of Al42− where D4h isomer was the most stable one as reported in the literature. In D2h geometry of M42− clusters (M = Sc, Y ), significant electron delocalization in each wing of the butterfly indicated fused d aromaticity. Atomization energy and chemical hardness supported the preference of D2h geometry over the D4h geometry. Molecular orbital analysis showed that the d-electrons were delocalized in each triangle of D2h geometry. Our interest in the search of new kinds of binuclear sandwich compounds led us to consider sandwiched metal dimers CB5H6M - MCB5H6 (M = Si, Ge, Sn) which are at the minima in the potential energy hypersurface with a characteristic M - M single bond. This work has been described in the first section of chapter four. The NBO analysis and the M - M distances ( ˚A) (2.3, 2.44 and 2.81 for M= Si, Ge, Sn respectively) indicated substantial M - M bonding. Consecutive substitution of two boron atoms in B7H7−2 by M (Si, Ge, Sn) and carbon respectively led to neutral MCB5H7, where M - H bond bent towards the carbon side of the five membered ring. Dehydrogenation of two MCB5H7 might lead to our desired CB5H6M - MCB5H6 where similar bending of M -M bond has been observed. The bending of M - M bond in CB5H6M -MCB5H6 was more than the M - H bending in MCB5H7. Molecular orbital analysis has been done to understand the bending. Larger M - M bending observed in CB5H6M - MCB5H6 in comparison to M - H bending observed in MCB5H7 was suspected to be favored by stabilization of one of the M - M π bonding MO’s. Preference of M to occupy the apex of pentagonal skeleton of MCB5H7 over its icosahedral analogue MCB10H11 has been observed. Structures of sandwiched binuclear L- M – M - L where M = Ti, Zr and L = Cp, C3B3H6 were also investigated as described in second section of chapter four. We found that these compounds having bent geometry with short M - M distance (1.87˚A for M=Ti and 2.29˚A for M=Zr) lie at the minima in the potential energy hypersurface. Bending from the linear geometry led to the stabilization of M - L antibonding interaction in L - M – M - L. Molecular orbital analysis, NBO calculations, Wiberg bond index and charge analysis suggested M2+ unit to be embedded in between two L’s in L - M – M - L. Molecules that have the ability to perform interesting mechanical motions have always been of great interest. Umbrella inversion of ammonia is one of the most interesting and well studied phenomena. This study has led to the development of the MASER. The possibility of inversion of the molecule C9H9−Li+ by the movement of Li+ through the C9H9−ring was studied earlier [Das et al, Chem. Phys. Lett. 365, 320 (2002)]. In the fifth chapter theoretical investigation on a B12 cluster has been reported, which could exhibit a through ring umbrella inversion. Calculations showed that a part of the molecule, consisting of a three membered boron ring could invert through the rest, viz., a nine membered boron ring. Using a simple model, the double well potential for the motion was calculated. The barrier for inversion was found to be 4.31 kcal/mol. The vibrational levels and tunneling splitting were calculated using this potential. It was found that the vibrational excitation to the v = 17 level caused large amplitude “inversion oscillation” of the molecule. After considering the tunneling effect, inversion rate at 298K was calculated by using transition state theory and was found to be 1.17 x 1010/s. Finally, in the last chapter the main results of the thesis have been summarized.

Understanding the fundamental physics in light absorption and perfect light absorption is vital for device applications in detector, sensor, solar energy harvesting and imaging. In this research study, a large area fabrication of Al-doped ZnO/Al2O3/graphene/Al2O3/gold/silicon device was enabled by a spin-processable hydrophilic mono-layer graphene oxide. In contrast to the optical properties of noble metals, which cannot be tuned or changed, the permittivity of transparent metal oxides, such as Al-doped ZnO and indium tin oxide, are tunable. Their optical properties can be adjusted via doping or tuned electrically through carrier accumulation and depletion, providing great advantages for designing tunable photonic devices or realizing perfect absorption. A significant shift of Raman frequency up to 360 cm-1 was observed from graphene in the fabricated device reported in this work. The absorption from the device was tunable with a negative voltage applied on the Al-doped ZnO side. The generated absorption change was sustainable when the voltage was off and erasable when a positive voltage was applied. The reflection change was explained by the Fermi level change in graphene. The sustainability of tuned optical property in graphene can lead to a design of device with less power consumption.

In dieser Dissertation analysieren wir drei Aspekte von Konforme Feldtheorien (CFTs). Erstens betrachten wir Korrelationsfunktionen von sekundären Zuständen (SZ) in zweidimensionalen CFTs. Wir diskutieren eine rekursive Formel zu ihrer Berechnung und erstellen eine Computerimplementierung dieser Formel. Damit können wir jede Korrelationsfunktion von SZ des Vakuums erhalten und für Nicht-Vakuum-SZ den Korrelator als Differentialoperator, der auf den jeweiligen primären Korrelator wirkt, ausdrücken. Mit diesem Code untersuchen wir dann einige Verschränkungs- und Unterscheidbarkeitsmaße zwischen SZ, i.e. die Rényi-Entropie, den Spurquadratabstand und die Sandwich-Rényi-Divergenz. Mit unseren Ergebnissen können wir die Rényi Quanten-Null-Energie-Bedingung testen und stellen neue Werkzeuge zur Analyse der holographischen Beschreibung von SZ bereit. Zweitens untersuchen wir vierdimensionale Weyl-Fermionen auf verschiedenen Hintergründen. Unser Interesse gilt ihrer Spuranomalie, und der Frage, ob die Pontryagin-Dichte auftritt. Deshalb berechnen wir die Anomalien von Dirac-Fermionen, die an vektorielle und axiale Eichfelder gekoppelt sind, und dann auf einem metrisch-axialen Tensor Hintergrund. Geeignete Grenzwerte der Hintergründe erlauben es dann, die Anomalien von Weyl-Fermionen, die an Eichfelder gekoppelt sind, und in einer gekrümmten Raumzeit zu berechnen. Wir bestätigen das Fehlen der Pontryagin-Dichte in den Spuranomalien. Drittens liefern wir die holographische Beschreibung einer vierdimensionalen CFT mit einem irrelevanten Operator. Wenn der Operator eine ganzzahlige konforme Dimension hat, modifiziert sein Vorhandensein in der CFT die Weyl-Transformation der Metrik, was wiederum die Spuranomalie ändert. Unter Ausnutzung der Äquivalenz zwischen Diffeomorphismen im Inneren und Weyl-Transformationen auf dem Rand, berechnen wir diese Modifikationen mithilfe der dualen Gravitationstheorie. Unsere Ergebnisse repräsentieren einen weiteren Test der AdS/CFT-Korrespondenz. / Conformal field theories (CFTs) are amongst the most studied field theories and they offer a remarkable playground in modern theoretical physics. In this thesis we analyse three aspects of CFTs in different dimensions. First, we consider correlation functions of descendant states in two-dimensional CFTs. We discuss a recursive formula to calculate them and provide a computer implementation of it. This allows us to obtain any correlation function of vacuum descendants, and for non-vacuum descendants to express the correlator as a differential operator acting on the respective primary correlator. With this code, we study some entanglement and distinguishability measures between descendant states, i.e. the Rényi entropy, trace square distance and sandwiched Rényi divergence. With our results we can test the Rényi Quantum Null Energy Condition and provide new tools to analyse the holographic description of descendant states. Second, we study four-dimensional Weyl fermions on different backgrounds. Our interest is in their trace anomaly, where the Pontryagin density has been claimed to appear. To ascertain this possibility, we compute the anomalies of Dirac fermions coupled to vector and axial non-abelian gauge fields and then in a metric-axial-tensor background. Appropriate limits of the backgrounds allow to recover the anomalies of Weyl fermions coupled to non-abelian gauge fields and in a curved spacetime. In both cases we confirm the absence of the Pontryagin density in the trace anomalies. Third, we provide the holographic description of a four-dimensional CFT with an irrelevant operator. When the operator has integer conformal dimension, its presence in the CFT modifies the Weyl transformation of the metric, which in turns modifies the trace anomaly. Exploiting the equivalence between bulk diffeomorphisms and boundary Weyl transformations, we compute these modifications from the dual gravity theory. Our results represent an additional test of the AdS/CFT conjecture.

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The present work is a new attempt to interpret on the episode in Genesis 18:1-15 by the method of narrative criticism. The general tendency on the narrative had focused on the exemplary act of Abraham’s hospitality interpreting it as his righteousness by the perspective of NT (Heb 13:2) or by the test motive of Greek Myth (the birth of Orion). The retributive theology was considered too much in interpreting the Fellowship narrative (Gn 18:1-15).These interpretations conflict with the narrator’s own theological views: (1) righteousness by faith (Gn 15:6), (2) God’s mercifulness to save Lot (Gn 19:29), and (3) God’s being gracious to make Sarah conceive (Gn 21:1). This study attempts to find out the author’s own interpretative view indicated in the whole Abraham narrative (Gn 11:27-25:11) as well as in the Fellowship Narrative itself (Gn 18:1-15).The present work is an attempt to interpret on the narrative by the method of narrative criticism. This study pays attention to the narrator’s various literary skills: “linking structure with preceding episode” (Gn 18:1a); “Sandwiched structure” of the larger context (Gn 18:1-21:7); Unique Plot Sequence; and Repeated Clue word and phrase (“laugh,” “Sarah,” ”this time next year”). These literary skills are understood to indicate the faithfulness of the Lord who tries to fulfill what he promised. The conclusion of this study overturns the traditional interpretations on the Fellowship Narrative. This work attests that Abraham showed his righteousness not by doing hospitality but by obeying God’s new command of circumcision out of willing heart as he used to obey the Lord’s commands having faith in the promise of the Lord (ch.5.3.2.2). The motive of God’s visit is to have the covenantal fellowship with obedient Abraham (ch. 2.1.6; 2.1.6.1). Abraham’s first moment recognition of deity is attested by interpreting of the technical pair verb, “And he lifted up his eyes and he saw and Lo!” (Gn 18:2a), which depicts prophetic experience of Abraham (ch.3.3). Abraham’s manner for the visitors is relevant to the higher ones (ch.3.3.2.1). The futile human endeavor without having faith is considered as the reason of being delayed of fulfillment of God’s promise (Gn 16). The fulfillment of the promised son was not attained by any human effort, but only by God’s merciful intervention in the Abraham narrative (ch. 4.4.4 and 4.4.5). / Thesis (PhD)--University of Pretoria, 2010. / Old Testament Studies / unrestricted