Submitted by Fabio Sobreira Campos da Costa (fabio.sobreira@ufpe.br) on 2017-07-11T12:20:54Z No. of bitstreams: 2 license_rdf: 811 bytes, checksum: e39d27027a6cc9cb039ad269a5db8e34 (MD5) Diego_Lopes_Dissertacao.pdf: 4018850 bytes, checksum: a5aa0e7bd17e5fbdc23ed085f38eca0c (MD5) / Made available in DSpace on 2017-07-11T12:20:54Z (GMT). No. of bitstreams: 2 license_rdf: 811 bytes, checksum: e39d27027a6cc9cb039ad269a5db8e34 (MD5) Diego_Lopes_Dissertacao.pdf: 4018850 bytes, checksum: a5aa0e7bd17e5fbdc23ed085f38eca0c (MD5) Previous issue date: 2016-07-19 / CAPES / A presente dissertação aplica um modelo computacional para mapeamento da distribuição de potencial e campo elétrico ao longo de dispositivos isolantes na presença de uma camada condutiva em sua superfície. Na natureza, a formação dessa camada se dá através da deposição de poluição juntamente com a umidade ambiental. A pesquisa teve como base outros estudos presentes na literatura que abordam a mesma temática. A avaliação do comportamento da distribuição de potencial e campo elétrico em peças isolantes é de fundamental importância na determinação de possíveis regiões, as quais esses dispositivos apresentem maior estresse elétrico, tais regiões podem levar a descargas parciais e possíveis desgastes dos acessórios levando assim à sua falha. O modelo aplicado foi implementado utilizando o MEF (Método dos Elementos Finitos) através da construção do ambiente nosoftwareCOMSOLMultiphysicse da representação gráfica do acessório noAutoCAD. O estudo foi realizado em isoladores de vidro e polimérico, bem como em espaçadores, sob diferentes níveis de poluição. Os resultados obtidos são coerentes e apresentam consonância com a teoria envolvida neste estudo. / This dissertation applies a computational model to map the voltage distribution and electric field along dielectric devices in presence of a thin conductive layer on its surface. In the nature this thin layer may appear due to pollution deposition with environmental humidity. The base of this research are another studies found in literature with the same theme. The behavior of voltage distribution and electric field in dielectric devices has significant importance helping to determine possible regions where this devices present higher stress levels, causing partial discharges and possible damage to the accessories leading them to a failure condition. The applied model was implemented using FEM (finite element method) building an environment in COMSOL Multiphysics software and graphically representing each accessory through using AutoCAD. The present study was done in insulators made of glass and polymer, as well as spacers, under several pollution levels. The obtained results are consistent and show consonance with the whole theory involved in this research.

A bipolar charge transport (BCT) model is used to simulate charge injection and transportdynamics inside insulation material which are used in a high voltage direct current (HVDC)cable. Gaining knowledge about space charge density and electric field distribution in theinsulation material enables minimising charge injection at the metal-insulator interface andavoiding unnecessary energy loss. Simulation methods using the numerical Finite ElementMethod (FEM) are implemented in COMSOL multiphysics in order to investigate the effect ofchemical structure such as dipoles, physical defects such as interface roughness and impurityconcentration leading to ions, on the potential barrier and charge injection at the interface.Interface dipoles such as surface dipoles or chemical dipoles can increase or decrease thepotential barrier at the interface depending on direction/orientation of the dipole. Moreover,using a field enhancement factor to include the effect of interface roughness at the interfaceyields increased charge injection when higher values of the field enhancement factor wereused. The barrier height becomes therefore locally lower where the degree of roughness ishigher. Including the effect of ions, the electric field was observed to be enhanced near theelectodes, where it was weakened in the middle of the insulation, depending on the amount ofthe impurity concentration inside the insulation. Improvement on the charge injection lows isalso done using a combination of both Richardson-Schottky and Fowler-Nordheim chargeinjection laws to include both a classical and a quantum mechanical description in the BCTmodel. Solving for the transmission coefficient from Schrödinger equation could improve theaccuracy of Fowler-Nordheim as well. Including potentials due to image effect or chemicalcompositions such as water dipoles will affect the charge injection barrier and the transmission coefficient.

The terms &lsquo / ground-probing radar&rsquo / , &lsquo / ground penetrating radar (GPR)&rsquo / , &lsquo / sub-surface radar&rsquo / or &lsquo / surface-penetrating radar (SPR)&rsquo / refer to various techniques for detecting and imaging of subsurface objects. Among those terms GPR is preferred and used more often. In this thesis, the depth and the position of the buried circular cylinder are determined by a GPR system which comprises of an optical fiber sensor (OFS). The system is a combination of OFS, GPR and optical communication link. In order to determine the depth and the position, first of all the electric field distribution at the OFS is obtained by integrating the Green&rsquo / s function over the induced current distribution. Those distributions are observed for different frequency and depth values. The voltages inside the distribution are measured by OFS. By changing the depth of the cylinder and the frequency of the system, various plots showing x axis displacement versus measured voltages are obtained. Those plots are related to interference fringe patterns. The position and the depth of the cylinder are obtained using interference fringe patterns. All of the studies mentioned are performed in MATLAB R2007b program. The noises of the system due to OFS are extracted using OPTIWAVE OPTISYSTEM 7.0 program. By adding those noises to the measured voltage values, the operating frequency of the system is observed.

The photoconductive method is one of the oldest methods for the generation of THz room temperature operated THz electromagnetic waves. The THz photoconductive source has operated at a lower power level in the order of hundreds of nW. In addition, the energy conversion of optical to THz efficiency has remained extremely low. One of the most efficient THz photoconductive sources is a trap-enhanced field (TEF) effect source. The field is measured to contain more than 90% of the total DC bias within the first 5 µm of an 80 µm gap between the electrodes reaching kV/cm with only a modest bias. The overall THz power, however, has remained low, due to its rapid saturation. To date, there has been a limited understanding of the TEF effect. In this thesis, a more detailed experimental investigation of TEF effect current transport and field distribution based on annealing is presented to explain some of the underlining physics of TEF effect. A spatially extended line excitation is introduced to effectively reduce the screening effect while still exploiting the TEF region to maintain high efficiency and reach the µW regime. The record efficiency reached by this method is demonstrated. An experimental demonstration with a numerical analysis of the line excitation is presented. The spectral analysis of both a point and a line excitation demonstrate that the line excitation spectrum is not only comparable to that of the point excitation, but also extends the range of useful lower frequency content. To further improve the THz efficiency, the line excitation THz array is investigated.

Poptávka ze strany vesmírného výzkumu, zdravotnictví a bezpečnostního průmyslu způsobila v posledních letech zvýšený zájem o vývoj materiálů pro detekci a zobrazování vysokoenergetického záření. CdTe a jeho slitina CdZnTe. jsou polovodiče umožnují detekci záření o energiích v rozsahu 10 keV až 500 keV. Šířka zakázaného pásma u CdTe / CdZnTe je 1.46 -1.6 eV, což umožňuje produkci krystalů o vysoké rezistivitě (10^10-10^11 cm), která je dostačující pro použití CdTe / CdZnTe při pokojové teplotě. V mé práci byly zkoumány detektory CdTe/CdZnTe v různých stádiích jejich poruchovosti. Byly použity velmi kvalitní spektroskopické detektory, materiál s nižší rezistivitou a výraznou polarizací, detektory s asymetrií elektrických parametrů kontaktů a teplotně degenerované vzorky. Z výsledků analýzy nízkofrekvenčního šumu je patrný obecný závěr, že zvýšená koncentrace defektů způsobí změnu povahy původně monotónního spektra typu 1/f na spektrum s výrazným vlivem generačně-rekombinačních procesů. Další výrazná vlastnost degenerovaných detektorů a detektorů nižší kvality je nárůst spektrální hustoty šumu typu 1/f se vzrůstajícím napájecím napětí se směrnicí výrazně vyšší než 2. Strukturální a chemické analýzy poukázaly, že teplotní generace detektorů způsobuje difuzi kovu použitého při kontaktování a stopových prvků hlouběji do objemu krystalu. Část mé práce je věnována modifikaci povrchu svazkem argonových iontů a jejímu vlivu na chemické složení a morfologii povrchu.