Thesis (D. ENG.)--Johns Hopkins University, 1934. / Cover title. Vita. Caption title: Dielectric properties of cellulose paper ... by J.B. Whitehead ... [and] E.W. Greenfield ... Bibliography: p. 8, 14.

Thesis (Ph. D.)--Anōtatē Scholē Ēlektrologōn Mēchanikōn E.M. Polytechneiou, 1981. / Summary in English. Includes bibliographical references (p. 139-142).

The three key areas of this thesis are crystal synthesis strategies, growth mechanisms, and new types of quantum materials nanowires. The highlights are introduction of a new catalyst (TiO2) for nanowire growth and application to Bi2Se3, Bi2Te3, SnO2, and Ge nanowires; demonstration of step-flow growth, a new growth mechanism, for Bi2Te3 sub-micron belts; and the characterisation of the first quasi-one dimensional topological insulator (orthorhombic Sb-doped Bi2Se3) and topological Dirac semimetal nanowires (Cd3As2). Research into new materials has been one of the driving forces that have contributed to the progress of civilisation from the Bronze Age four thousand years ago to the age of the semiconductor in the 20^th century. At the turn to the 21^st century novel materials, so-called quantum materials, started to emerge. The fundamental theories for the description of their properties were established at the beginning of the 20^th century but expanded significantly during the last three decades based, for example, on a new interpretation of electronic states by topological invariants. Hence, topological insulator (TI) materials such as mercury-telluride are one manifestation of a quantum material. In theory, TIs are characterised by an insulating interior and a surface with spin-momentum locked conduction. In real crystals, however, the bulk can be conducting due to crystal imperfections. Nanowires suppress this bulk contribution inherently by their high surface-to-volume ratio. Additionally, trace impurity elements can be inserted into the crystal to decrease the conductance further. These optimised TI nanowires could provide building blocks for future electronic nanodevices such as transistors and sensors. Initial synthesis efforts using vapour transport techniques and electronic transport studies showed that TI nanowires hold the promise of reduced bulk contribution. This thesis expands the current knowledge on synthesis strategies, crystal growth mechanisms, and new types of quantum materials nanowires. Traditionally, gold catalyst nanoparticles were used to grow TI nanowires. We demonstrate that they are suitable to produce large amounts of nanowires but have undesired side-effects. If a metaloxide catalyst nanoparticle is used instead, quality and even quantity are significantly improved. This synthesis strategy was used to produce a new TI which is built from chains of atoms and not from atomic layers as in case of previously known TIs. The growth of large nanowires with a layered crystal structure leads to step-flowgrowth, an intriguing phenomenon in the growth mechanism: New layers grow on top of previous layers with a single growth frontmoving fromthe root to the tip. These wires are ideal for further electronic characterisation that requires large samples. The nanowire growth of tin-oxide will also be discussed, a side project that arose from my growth studies, which is useful for sensor applications. Under certain conditions it forms tree-like structures in a single synthesis step. All of the aforementioned growth studies are carried out at atmospheric pressure. A separate growth study is carried out in ultra-high vacuum to assess the transferability of the growth process towards the cleanliness requirements of the semiconductor industry. If two quantum materials are joined together, exotic physics may emerge at the interface. One of the goals of TI research is the experimental observation of Majorana fermions, exotic particles which are their ownantiparticles with potential applications in quantum computing that may appear in superconductor/TI hybrid structures. We have synthesised such structures and initial characterisation suggests that the resistivity increases when they are cooled below the critical temperature of the superconductor. Beyond TIs, a new type of quantum material, called a topological Dirac semimetal, opens new realms of exotic physics to be discovered. Nanowires are grownfroma material which has recently been discovered to be a topological Dirac semimetal. Their growth mechanism is characterised and an extremely high electron mobility at room temperature is measured. The contribution of this thesis to the field is summarised in Fig. 1. Its core is the study of the growth mechanism of quantum materials which will be vital for future development of applications and fundamental research.

The deterioration of electrical insulating oils results in the formation of complex oxidation products, many of which are polar in structure. The significance of the microwave-frequency dielectric-loss measurement, when applied to the evaluation of aged transformer oils, is investigated. A cylindrical cavity, operating in the TE₀₁ mode, is used to measure the loss tangent of aged transformer oils. Q-factor measurements are made by a dynamic method which is described. The problem of mode interference in the cavity is investigated in detail. It is found that the loss tangent of transformer oils, measured at X-band, increases as the oil deteriorates through oxidation. The increase is influenced by several factors but closely parallels the increase in acidity. Sludge particles do not in themselves cause a significant increase in the dielectric losses. An indirect correlation between the loss tangent and the sludge content of an oil may exist but has not been established. The change in the dielectric constant of an oil caused by the presence of dissolved water, or by the ageing process, is too small to be measured by the method used. A small but measureable increase in the loss tangent is produced by the presence of water in concentrations of approximately 75 parts per million. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

The work contained in this thesis is concerned mainly with conduction mechanisms and polarization processes in thin amorphous insulating films. A model has been proposed and its d.c. conduction characteristics computed. The numerical results show the possibility of obtaining either space charge, Schottky, or Poole-Frenkel characteristics depending on the model parameters. The transient electronic discharge current has been analysed and the results show that this electronic current is approximately independent of the preapplied voltage in contrast to the ionic discharge current which is linearly dependent on preapplied voltage. This result, together with the experimental results obtained on Ta/Ta₂O₅/Au diodes, suggests that the calculations of low frequency dielectric losses using step response measurements are complicated by space charge effects only when the preapplied field is relatively low (≤lMV/cm for Ta₂0₅ films). Ta/Ta₂O₅/Au diodes were prepared by solution anodization or plasma anodization. All prepared diodes exhibited a rectification behaviour. Over the frequency range 100 Hz -100 kHz capacitance and loss tangent were found to decrease slightly with increasing frequency while the equivalent series resistance was found to be approximately proportional to ω⁻¹‧º⁵. All prepared diodes, with gold counter electrodes less than 1000 Å thick, were found to withstand, under a slowly applied field, field strengths approaching the formation field value. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

A dissertation submitted in fulfilment of the requirements for the degree of Master of Science in Engineering to the Faculty of Engineering and the Built Environment, School of Electrical and Information Engineering, University of the Witwatersrand, Johannesburg, 2018 / Power cable terminations and joints experience high electrical stress due to the abrupt change in geometry of the cable; hence the insulation at these points is more prone to partial discharges and has been reported as the main root cause of power cable system failures. Electrical failure of the insulation is known to occur due to a phenomenon of Partial Discharges (PD). Modern practice (especially in higher voltage installations) entails installation of PD sensors at strategic locations during installation of electrical equipment such as cable joints and terminations. This enables continuous monitoring of PD events in the plant, and this is termed on-line PD diagnosis. However, with limited knowledge to interpret the meaning of certain PD changes during the service period, this practice remains limited. It is therefore the interest of the study to understand the time evolution behaviour of PD characteristics in order to discern the insulation condition or deteriorating stages. The present study is on XLPE power cables, focusing on PD in artificial defects in the cable termination insulation that in most cases arise from poor workmanship. The power cables were subjected to accelerated ageing to emulate their ageing under service conditions. PD measurements were then conducted at periodic time intervals and characterized PD in terms of PD Inception Voltage (PDIV), maximum apparent PD magnitude (Qmax), Pulse Repetition Rate (PRR) and Phase-Resolved-PartialDischarge-Pattern (PRPDP). The findings are that, Qmax, PRR and PDIV did not show any time-evolution trends unique to a defect, the general trends observed were that of a fairly constant PDIV with several fluctuations of a 5 kV band. Qmax showed a decreasing trend over ageing time. The PRR decreased overall, with a pick up increase near the end of the tests. Qmax and PRR were noted significantly fluctuative between 23% and 57% of the total ageing period, distinct characteristics were that, the tram line had the largest PRR which is expected since it is a flat cavity, and iii the PRPDP appeared more skewed than other defects. The semicon feather had a PRPDP that seemed like a combination of a void discharge and corona discharge. The ring cut PRPDP was similar to that of the tram line except that it was not skewed. Furthermore, a capacitance PD model was constructed in Matlab R Simulink R to emulate experimental observed PD behaviour and therefore confirm the theory explaining the observed time-dependency of PD phenomena. Simulated void discharge PRPDP which corresponded with experimentally measured PRPDP were obtained for the unaged, moderately aged and severely aged cavity defect. The corona-surface discharge effect observed in the semicon PRPDP was also successfully emulated. The study outcomes suggest that PD characteristics evolve over time, and that the behaviour of the observed trend is unique at different stages during ageing. The time evolution characteristics of PD are The PRPDP signatures did not change with time of ageing despite the variations in Qmax and PRR. This means that, defect signatures obtained prior ageing or in-service operation of the cables can still serve as a good reference of identifying the nature of the defect at different ageing stages except in the event of PD evanescence. From the simulations, it was derived that the PD region surface conductivity as well as the geometry of the defect are the main contributing factors to the unique signatures observed at different stages and per defect. / XL2019

No description available.

In transgenic research the precise control of transgene expression is crucial in order to obtain transformed organisms with expected desirable traits. A broad range of transgenic plants use the constitutive cauliflower mosaic virus (CaMV) 35S promoter to drive expression of selectable marker genes. Due to its strong enhancer function, this promoter can disturb the specificity of nearby eukaryotic promoters. When inserted immediately downstream of the 35S promoter in transformation vectors, special DNA sequences called insulators can prevent the influence of the CaMV35S promoter/enhancer on adjacent tissue-specific promoters for the transgene. Insulators occur naturally in organisms such as yeasts and animals but few insulators have been found in plants. Therefore, the goal of this study is to identify DNA sequences with insulator activity in Arabidopsis thaliana. A random oligonucleotide library was designed as an initial step to obtain potential insulators capable of blocking enhancer-promoter interactions in transgenic plants. Fragments from this library with insulator activity were identified and re-cloned into pB31, in order to confirm their activity. To date, one insulator sequence (CLO I-3) has been identified as likely possessing enhancer-blocking activity. Also, two other oligonucleotide sequences (CLO II-10 and CLO III-78) may possess insulator activity but more sampling is needed to confirm their activity. Further studies are needed to validate the function of plant insulator(s) and characterize their associated proteins.

Currently research has focused on insulators from non-plant species such as the fruit fly, Drosophila melanogaster. The accumulated data suggests that many different insulator sequences exist in D. melanogaster, each one containing its own different primary binding protein, while sharing similar secondary binding proteins. Together, they produce chromatin loops separating enhancers and promoters into distinct domains preventing cross-talk between them. Is this the case in plants? To approach this question, we have investigated enhancer-blocking insulators in the model plant Arabidopsis thaliana using two unrelated approaches. Firstly, we have developed an assay for the direct selection of insulators in Arabidopsis thaliana using a random oligonucleotide library. This assay helped us to define four novel insulator sequences named InI-3, InII-12, InIII-50, and InIII-78. Secondly, we have used genetic analyses to characterize potential insulator sequences originally from three non-plant species: UASrpg from the fungus Ashbya gossypii, BEAD1c from human T-cell receptors, and gypsy from D. melanogaster, that have been reported to function in A. thaliana. Our findings suggest that non-plant insulators and their protein binding sites function in plants and support the model of multiple, functional, different insulator sequences as was found in D. melanogaster. They also argue for the conservation of insulator mechanisms across species.

Thesis advisor: Vidya Madhavan / Spin-orbit coupling (SOC) is the interaction of an electron's intrinsic angular momentum (spin) with its orbital momentum. The strength of this interaction is proportional to Z<super>4</super> where Z is the atomic number, so generally it is stronger in atoms with higher atomic number, such as bismuth (Z=83) and iridium (Z=77). In materials composed of such heavy elements, the prominent SOC can be sufficient to modify the band structure of the system and lead to distinct phase of matter. In recent years, SOC has been demonstrated to play a critical role in determining the unusual properties of a variety of compounds. SOC associated materials with exotic electronic states have also provided a fertile platform for studying emergent phenomena as well as new physics. As a consequence, the research on these interesting materials with any insight into understanding the microscopic origin of their unique properties and complex phases is of great importance. In this context, we implement scanning tunneling microscopy (STM) and spectroscopy (STS) to explore the surface states (SS) of the two major categories of SOC involved materials, Bi-based topological insulators (TI) and Ir-based transition metal oxides (TMO). As a powerful tool in surface science which has achieved great success in wide variety of material fields, STM/STS is ideal to study the local density of states of the subject material with nanometer length scales and is able to offer detailed information about the surface electronic structure. In the first part of this thesis, we report on the electronic band structures of three-dimensional TIs Bi₂Te₃ and Bi₂Se₃. Topological insulators are distinct quantum states of matter that have been intensely studied nowadays. Although they behave like ordinary insulators in showing fully gapped bulk bands, they host a topologically protected surface state consisting of two-dimensional massless Dirac fermions which exhibits metallic behavior. Indeed, this unique gapless surface state is a manifestation of the non-trivial topology of the bulk bands, which is recognized to own its existence to the strong SOC. In chapter 3, we utilize quasiparticle interference (QPI) approach to track the Dirac surface states on Bi₂Te₃ up to ~800 meV above the Dirac point. We discover a novel interference pattern at high energies, which probably originates from the impurity-induced spin-orbit scattering in this system that has not been experimentally detected to date. In chapter 4, we discuss the topological SS evolution in (Bi_1-xIn_x)₂Se₃ series, by applying Landau quantization approach to extract the band dispersions on the surface for samples with different indium content. We propose that a topological phase transition may occur in this system when x reaches around 5%, with the experimental signature indicating a possible formation of gapped Dirac cone for the surface state at this doping. In the second part of this thesis, we focus on investigating the electronic structure of the bilayer strontium iridate Sr₃Ir₂O₇. The correlated iridate compounds belong to another domain of SOC materials, where the electronic interaction is involved as well. Specifically, the unexpected Mott insulating state in 5<italic>d</italic>-TMO Sr₂IrO₄ and Sr₃Ir₂O₇ has been suggested originate from the cooperative interplay between the electronic correlations with the comparable SOC, and the latter is even considered as the driving force for the extraordinary ground state in these materials. In chapter 6, we carried out a comprehensive examination of the electronic phase transition from insulating to metallic in Sr₃Ir₂O₇ induced by chemical doping. We observe the subatomic feature close to the insulator-to-metal transition in response with doping different carriers, and provide detailed studies about the local effect of dopants at particular sites on the electronic properties of the system. Additionally, the basic experimental techniques are briefly described in chapter 1, and some background information of the subject materials are reviewed in chapter 2 and chapter 5, respectively. / Thesis (PhD) — Boston College, 2014. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.