Thesis (MScEng)--University of Stellenbosch, 2005. / ENGLISH ABSTRACT: The calibration and field evaluation of an Insulator Pollution Monitoring Relay (IPMR) were the main aims of this research programme. A repeatable artificial wetting test method was developed after several modifications were made to the steam system, test chamber and the test routine. The IPMR was successfully calibrated with insulators that were artificially polluted according to the solid layer method. Linear and polynomial relationships were determined after curve-fitting techniques were performed on the results. The calibration showed that the IPMR is capable as a device relating the maximum conductivity during artificial wetting to the ESDD, a severity classification parameter. The IPMR was successfully used in a salt fog chamber to determine if the device is capable to evaluate the severity of an instantaneous pollution event. The IPMR was successfully installed at a natural pollution test site along the Cape west coast. The conductivity measurements with natural wetting showed good correlation to flashovers experienced. A rule of thumb, developed to indicate a possible risk of flashover, was based on observations made on the relationship between humidity and surface conductivity. The measured IPMR data was successfully applied to quantify the site severity according to the conductivity measurement with natural wetting. This calculated severity value could be used in the assessment of flashover probability of high voltage insulators. / AFRIKAANSE OPSOMMING: Die hoofdoel van die tesis was die kalibrasie en veldtoetse van 'n isolatorbesoedelingsmonitor (IPMR). 'n Herhaalbare nagebootste benatting-toetsmetode is ontwikkel na veranderings aan die stoomstelsel, toetsruimte en die toetsproses. Die IPMR is suksesvol gekalibreer met isolators wat besoedel was met 'n nagebootste besoedeling volgens die "solid layer method". Liniêre sowel as kwadratiese verwantskappe is ontwikkel na krommepassings op die resultate uitgevoer was. Die kalibrasie het gewys dat die IPMR in staat is om die maksimum geleidingsvermoë wat d.m.v. nagebootste benatting verkry is, met die ESDD, 'n besoedelingsklassifikasie, kan vergelyk. Die apparaat is ook suksesvol gebruik tydens soutmistoetse om te bepaal of dit in staat is om 'n skielike besoedelingsgebeurtenis te kan meet. Na die afhandeling van laboratorium werk is die apparaat by 'n natuurlike isolator besoedeling-toetsstasie langs die Kaapse weskus geïnstalleer. Die geleidingsvermoë metings met natuurlike benatting het goeie korrelasie getoon met isolator oorvonkings. 'n Skattingsmetode wat ontwikkel is om moontlike oorvonkings te voorspel, is gebaseer op waarnemings wat gemaak is van die humiditeit sowel as die oppervlakte geleidingsvermoë. Die IPMR se geleidingsvermoë metings met natuurlike benatting is aangewend om die besoedelingsgraad van die gebied te bepaal. Die bepaalde besoedelingsgraad kan verder gebruik word om die waarskynlikheid van die oorvonking van isolators vas te stel.

This thesis outlines the development of two new techniques that exploit very small structures, on the micro- and nano-scale, to enable innovative electrical transport measurements on a variety of materials of current interest in condensed matter physics. The first technique aims to apply the versatility of electron-beam lithography for micro-fabrication of patterned electronic circuitry to the problem of performing transport experiments on individual crystallites taken from a typical powder sample. We show that these small samples, tens of microns in size, are actually often very high quality single crystals and can be exploited for measurements of electrical transport in materials of which no larger crystals are available. By way of demonstration, we present the results of preliminary transport measurements on a crystallite of the layered oxide chalcogenide Sr₂MnO₂Cu_1.5Se₂. We report a phase transition in the resistivity at 213K which may correspond to the onset of previously reported short-range order in copper and vacancy sites in the Cu_1.5Se₂ planes. The second technique is designed to investigate the topological protection of surface transport in 3-D topological insulators. We decorate the surfaces of single-crystal samples with two different species from a well-characterised family of single-molecule magnets. The two coatings have an electrostatically identical influence on the sample surface, but differ in that one species carries a spin and the other is spinless. The spinless molecule acts as a control, to allow us to cleanly determine the influence of the magnetic component of a scattering potential on transport in the surface. With this technique we investigate proposed topological Kondo insulator SmB₆. We find that the surface state dominates low-temperature transport and demonstrate that the momentum relaxation is very sensitive to a spin degree of freedom in the scatterer, in keeping with expectations of a topological insulator.

Les progrès en biotechno]ogie ont permis le développement d’outils pour le transfert de gène intégratif en transgénèse, bioproduction et thérapie génique. Cependant, trois challenges majeurs doivent être relevés pour garantir un système sécurisé : l’innocuité et l’efficacité du transfert, l’intégration ciblée et contrôlée dans le génome, le niveau et la durée d’expression du transgène au cours du temps. Dans ce but, mes travaux de thèse ont consisté à tester des solutions pour améliorer la biosécurité du transposon piggyBac qui nécessite un plasmide porteur du gène d’intérêt à insérer dans le génome et une source de transposase catalysant la réaction d’intégration du transgène. Une des stratégies de ma thèse repose sur l’apport de la source de transposase sous forme d’ARN messager au lieu d’ADN afin d’améliorer la stabilité de l’intégration et de réduire les effets génotoxiques en limitant la transposase dans les cellules. Pour la première fois, la biodisponibilité de l’ARNm de la transposase et les conditions optimales d’utilisation en cellules humaines ont été déterminées pour augmenter la biosêcurité du système. Le second objectif de mes travaux consiste à améliorer l’expression du transgène en ajoutant des insulateurs connus pour s’opposer à l’extinction de l’expression des gênes. En termes de biosécurité, cette stratégie permet de réduire le nombre de copies du transgène nécessaires pour obtenir une expression suffisante. Deux candidats ont été identifiés pour améliorer l’expression du transgène. La combinaison des approches ARNrn et insulateurs est prometteuse pour sécuriser le transfert de gène médié par piggyBac et pour maintenir l’expression du gène d’intérêt. / Advances in biotechnology have enabled the development of tools for gene transfer applicable to transgenesis, bioproduction and gene therapy. But, 3 major challenges must be met to ensure a secure system: the safety and effectiveness of the transfer. the targeted and controlled integration into the genome. and the level of transgene expression over time. In this aim, my thesis project was to validate solutions to improve the biosafety of the piggyBac transposon, which requires a plasmid carrying the gene of interest to be inserted in the genome, and a source of transposase which catalyzes the transgene integration. One approach of my thesis work is to deliver the source of piggyBac transposase as an mRNA molecule instead of DNA. This strategy aims to improve the stability of the integration and reduce the genotoxic effects by limiting the transposase in the cells. For the 1st time, the bioavailability of the transposase rnRNA and the optimal conditions for its use in human cells were determined to increase the biosafety of the transposon system. The 2nd objective ofmy project is to improve the expression of the transgene by adding insulators known to counteract the transgene silencing. This strategy reduces the number of integrations required ta get a sufficient expression of the transgene and thus, improve biosecurity. Two candidates have been identified to improve transgene expression. The combination of the mRNA and insulator strategies is promising to secure the piggyBac-mediated gene transfer and to maintain the expression of the gene of interest

Cette thèse porte sur l'exploration de différentes phases topologiques présentes dans des hétérostructures à base de mercure, cadmium et tellure (HgCdTe). Ces systèmes sont de parfaits cas d'études des états topologiques dans la matière condensée. En effet, leur structure de bande peut aisément être modifiée d'inversée à non-inversée par le biais de paramètres internes ou externes.Lorsqu'un système présente une structure de bande inversée, il a une topologie non triviale. Il est impossible de modifier cet ordre topologique sans fermer son gap, ce qui inévitablement entraîne l'apparition de particules sans masse dans son volume. Un système présentant une structure de bande inversée et un gap d'énergie finie dans lequel se trouve le niveau de Fermi, est appelé isolant topologique. Ce nouveau type de matériau est isolant dans son volume, mais abrite des états métalliques sans gap sur ses bords. Ces derniers ont une relation de dispersion linéaire et sont protégés des effets liés au désordre et de la rétrodiffusion par des impuretés non magnétiques. Ces états particuliers apparaissent à l'interface de matériaux présentant des ordres topologiques différents. Ainsi, un isolant topologique 2D se caractérise par des canaux 1D de conductance polarisés en spin à ses bords, alors qu'un isolant topologique 3D accueille des fermions de Dirac 2D, polarisés en spin, aux surfaces.L'existence de fermions sans masse 2D et 3D a déjà été démontrée expérimentalement. Cependant, la transition de phase topologique durant laquelle apparaissent les particules sans masse n'a que très peu été explorée. Il est possible de modifier la structure de bande de HgCdTe d'inversée à non inversée par le biais de la composition chimique, la pression, la température ou le confinement quantique. Ces paramètres permettent ainsi de sonder le système au voisinage de différentes transitions de phase topologiques. Dans ce travail, l'utilisation de la température comme paramètre d'ajustement continu du gap permet d'étudier au point de transition de phase l'apparition de fermions semi-relativistes de Dirac (2D) et de Kane (3D) ainsi que leurs propriétés.Les systèmes étudiés au cours de ces travaux de recherche sont des cristaux massifs de Hg1-xCdxTe et des puits quantiques HgTe/CdTe présentant des structures de bandes inversées et non inversées, ainsi que des couches minces de HgTe contraintes pouvant être considérées comme des isolants topologiques 3D ayant un confinement quantique résiduel. Tous ces systèmes possèdent des propriétés topologiques. L'interprétation des résultats s'appuie sur les prédictions théoriques basées sur le modèle de Kane. En annexe, une vue d'ensemble des puits quantiques composites InAs/GaSb, structures également identifiées comme isolants topologiques, est présentée, comportant les résultats préliminaires obtenus sur ces dernières.Toutes les structures ont été étudiées par magnétospectroscopie en transmission dans les domaines de fréquence terahertz et infra-rouge moyen à l'aide d'un dispositif expérimental spécifiquement conçu pour permettre des mesures sur une large plage de températures. / This thesis presents an investigation of different topological phases in mercury-cadmium-telluride (HgCdTe or MCT) based heterostructures. These solid state systems are indeed a perfect playground to study topological states, as their band structure can be easily varied from inverted to non-inverted, by changing internal or external parameters.If a system has an inverted band ordering, its electronic structure has a non-trivial topology. One cannot change its topological order without closing the band gap, which is inevitably accompanied with the appearance of massless particles in the bulk. A system, that has an inverted band structure and a finite gap in which the Fermi level is positioned, is called a topological insulator. These novel materials are insulators in the bulk, but host gapless metallic states with linear dispersion relation at boundaries, protected against disorder and backscattering on non-magnetic impurities. These states arise at the interfaces between materials characterized by a different topological order. A 2D topological insulator is thus characterized by a set of 1D spin-polarized channels of conductance at the edges, while a 3D topological insulator supports spin-polarized 2D Dirac fermions on its surfaces.The 2D and 3D massless fermions have already been demonstrated experimentally in HgCdTe-based heterostructures. However, the topological phase transitions during which the massless particles appear remain barely explored. The HgCdTe band structure can be tuned from inverted to non-inverted using chemical composition, pressure, temperature, or quantum confinement. These parameters therefore allow to probe the system in the vicinity of different topological phase transitions. In this thesis, the use of temperature as continuous band gap tuning parameter allows to study the appearance and the parameters of semi-relativistic 2D Dirac and 3D Kane fermions emerging at the points of phase transitions.The systems investigated were Hg$_{1-x}$Cd$_x$Te bulk systems and HgTe/CdTe quantum wells characterized by an inverted and regular band order, and strained HgTe films which can be considered as 3D topological insulators with a residual quantum confinement. All these systems exhibit topological properties, and the experimental results are interpreted according to theoretical predictions based on the Kane model. This thesis is complemented by an overview and the preliminary results obtained on a different compound -- a InAs/GaSb broken-gap quantum well, which was also identified as a topological insulator. The structures were studied by means of terahertz and mid-infrared magneto-transmission spectroscopy in a specifically designed experimental system, in which temperature could be tuned in a broad range.

Recently high temperature vulcanized (HTV) silicone rubber (SIR) / polymeric/composite insulators are gaining wider acceptance as overhead transmission line insulators for extra high voltage (EHV) and ultra-high voltage (UHV) systems due to some promising features like hydrophobicity recovery, light weight, ease of handling and installation, better pollution ashover performance, admirable resistance against vandalism etc. Since polymeric insula-tors are of recent origin, their long-term eld performance is yet to be understood. Owing to their organic nature, and exposure to environmental stresses like pollution, temperature, UV radiation, humidity, fog, rain etc., the insulator performance degrades over a period. The sheds/petticoats of the insulators become wettable leading to frequent ashover in humid and contaminated environment. Hence, long term reliability of the composite insulators is of foremost concern to the power utilities. The available literature on the long term eld performance of these insulators for di erent climatic conditions and under multiple environ-mental stresses for both the HTV SIR and Liquid Silicone Rubber (LSR) is scant. Also there is no reference standard for evaluation of these insulators for pollution/contamination test methods in the laboratory. However currently, CIGRE Work Group is working towards the standardization of the test methods for arti cial pollution tests for polymeric insulators. The thesis addresses some of the issues in detail. In the first part of the thesis, a new and simple pre-treatment methodology to achieve uniform contamination layer on inherently hydrophobic HTV SIR Insulator samples is presented for laboratory pollution performance evaluation. The surface water level di usion in the dipping period is found to make the insulator surface temporarily hydrophilic. Then the uniform contamination layer is applied by dipping the sample immediately in the pollution slurry. Exhaustive experiments were conducted on full scale SIR insulators as well as SIR slabs to investigate the hydrophilicity appearance on the SIR surface. A specially fabricated arrangement for assessment of Wettability Class (WC) is made as per IEC stds. The results of WC measurement and wet ashover studies support the temporary reduction in hydrophobicity of SIR due to dipping phase in the proposed pre-treatment methodology. The next part of the thesis presents the results for the effeect of long term thermal aging experimentation conducted on HTV SIR with difffeerent degrees of pollution (medium, heavy), the effeect of arid desert climate on polymeric insulators is studied. The experimental set-up consists of controlled HVAC source, temperature controlled furnace with a provision for high voltage (HV) and Leakage Current (LC) monitoring, a Digital Storage Oscilloscope (DSO), compact DAQ-9201 of National Instruments operated in LabVIEW platform etc. Two types of HTV SIR Insulators are considered for the study. Flat slabs as well as full-scale insulator samples of creepage length 725 mm are stressed simultaneously to simulate the in-service condition. The experimentation is conducted for about 575 hours with application of 21.0 kVrms at 60oC. The results of the hydrophobicity recovery for thermally aged contaminated polymeric insulators are reported. Besides, monitoring electrical and mechanical proper-ties, changes in material properties of SIR are also analyzed using Physiochemical analysis techniques like Fourier transform infrared (FTIR) spectroscopy, X-Ray Photoelectron Spectroscopy (XPS), Scanning Electron Microscopy (SEM), Thermo-Gravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC). Some of the key findings of the study are increased surface oxidation, surface roughness and mechanical stress due to thermal aging of polymeric insulators. Experimental investigations show that the characteristics of power frequency component of leakage current can be linked with thermal aging of SIR. Further, a unique climatic aging experimental facility is established to evaluate the long-term reliability of SIR under environmental stresses like UV, Humidity, temperature and applied electric stress. The investigations are conducted on two different types of HTV SIR and LSR at samples as well as full-scale insulator samples. The experimentation is conducted for 500 hours with 10.0 kVrms at 50oC, with 85% humidity and 1 W/m2 UV ir-radiation which is in accordance with the aging cycle specified in IEC standard. The results of the comparative studies conducted for the electrical, mechanical and material properties indicate leakage current pulses, brittleness, Salt deposition for multistress aged samples. In summary, an attempt has been made to contribute a pollution methodology with sim-ple pre-treatment technique for inherently hydrophobic HTV SIR surface to achieve better uniformity of contamination layer. Also, electro-thermal and multiple stresses investigations were conducted for long term performance on polymeric insulators.

Made available in DSpace on 2014-10-09T12:51:13Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:10:37Z (GMT). No. of bitstreams: 1 11257.pdf: 9782449 bytes, checksum: 9692d0a5b3355b0291e58320e6dc078d (MD5) / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Made available in DSpace on 2014-10-09T12:51:13Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:10:37Z (GMT). No. of bitstreams: 1 11257.pdf: 9782449 bytes, checksum: 9692d0a5b3355b0291e58320e6dc078d (MD5) / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

No description available.

This thesis presents the study for the temperature-driven and current-driven metal-insulator phase transition (MIT) in Ca₂RuO₄ via home-built variable temperature Scanning Tunneling Microscope. Atomically resolved topography images along with temperature dependence of resistivity are taken verifying the quality of the single crystals used in this experiment. Tunneling spectra are measured under various temperatures across the Tmi = 357K, which clearly shows spectra evolution with temperature and the difference between the room- temperature insulator phase and the high-temprature metal phase. Compared with DMFT calculation, the STS indicates lattice structure plays a vital role in the phase transition. Same measurement is conducted on the crystals under a DC current, thanks to a custom designed sample holder. The evolution of the tunneling spectra with source current demon- strates similarity with the one of temperature-driven MIT. The comparison between the spectra taken at high-temperature metalic state and the high-current metalic state high- lights the similarity of these 2 phases, with both showing a DOS transfer from 1eV to lower energy, when compared with the ground state. Combined with a variety of other studies via transport, scattering technique and infrared thermal imaging, it is found that the local temperature dominates both temperature-driven and current-driven MIT. It is very likely that the current-driven is caused by the inevitable Joule heating generated by the current, indicating the high-current metallic phase might be the same with high-current metallic phase. Finally, surface roughness and autocorrelation length analysis suggests an inhomo- geneous surface topography stemmed from the coexistence of the insulating S* phase and conducting L* phase under current.

No description available.