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L'émergence des nanotubes de carbone a ouvert de nouveaux champs d'application dans le domaine des matériaux polymères. L'ajout de ces charges carbonées au sein de polymères permet la mise en œuvre de composites aux propriétés électriques optimisées. La conductivité de ces matériaux dépend en grande partie de l'organisation des charges dans la matrice, notamment de la présence de réseaux percolants. L'objectif du présent travail de thèse est de comprendre les mécanismes de structuration des nanotubes de carbone au sein de différents milieux. L'architecture de ces réseaux de charges a principalement été révélée par le biais de mesures électriques et diélectriques. L'originalité de nos travaux réside dans l'utilisation de matrices liquides, notamment des huiles de silicone, afin de s'affranchir des contraintes présentes dans les plastiques d'une part, et de simplifier les processus de mise en œuvre d'autre part. Le manuscrit de thèse est articulé autour de six chapitres. Une première partie bibliographique aborde les propriétés des nanotubes de carbone ainsi que les phénomènes que sont la percolation et la percolation dynamique. Le second chapitre, matériel & méthode, présente les matériaux employés ainsi que les différentes techniques de caractérisation utilisées au cours de la thèse. Le troisième chapitre de la thèse aborde, à travers des mesures de conductivité, la percolation dynamique des nanotubes de carbone sein d'huiles de silicone. Le chapitre 4 propose une modification la loi de puissance de Kirkpatrick, afin de décrire la conductivité en fonction du temps et du taux de charge. L'exposant critique de percolation, caractérisant la transition isolant conducteur, se révèle être un indicateur de l'état de dispersion des nanotubes à travers la matrice. Le chapitre 5 démontre la possibilité de contrôler l'organisation des charges par l'application d'un champ électrique. L'application d'un champ élevé permet une augmentation de plusieurs ordres de grandeur de la conductivité ainsi qu'une diminution des charges nécessaire à la formation d'un réseau percolant. Nous avons notamment déterminé des seuils de percolation de l'ordre de 0.005% massique en nanotube de carbone. Enfin, l'influence des propriétés intrinsèques de la matrice, telles la viscosité et la tension de surface, est étudié dans le chapitre 6. La dispersion des nanotubes de carbone s'avère être favorisée au sein de liquides ayant des tensions de surface proches de celle des tubes. Au contraire, une agrégation de charge est rapidement observée dans le cas ou la différence de tension de surface charge-matrice est importante. Nous avons également observé que la percolation des nanotubes est défavorisée au sein de milieux visqueux. / The rise of carbon nanotube has open possibility for composites polymers. Mixing this carbonaceous filler with polymer medias leads to an optimization of the electrical properties. Then, conductivity mainly depends of the filler architecture, especially the presence of percolating networks. The objective of this work is to understand the percolation mechanisms of the carbon nanotubes in different media. During this study, filler network has been revealed by the mean of electrical and dielectrical measurements. The originality of our work lies in the use of liquid matrices, such as silicone oils, in order to overcome the stresses in the plastic on the one hand, and to simplify the processing in other hand. This thesis is organized around six chapters. The first bibliographic part discusses the carbon nanotubes properties as well as percolation and dynamic percolation phenomena. The second chapter, matériel & méthode, presents the materials used and the different characterization techniques employed. The third chapter of the thesis talks about dynamic percolation of carbon nanotubes in silicone oil, probed by conductivity measurements. Chapter 4 provides a change of the power law Kirkpatrick to describe the conductivity as a function of time and filler content. The critical exponent of percolation is proving to be an indicator of the dispersion state of nanotubes throughout the matrix. In the Chapter 5, electric field is depicted as a tool to control the organization of fillers. The application of a high field increases the conductivity of several orders of magnitude and decreases the percolation threshold. Percolation thresholds close to 0.005 wt % have been determined. At last, the influence of the intrinsic properties of the matrix, such as viscosity and surface tension, is discussed in Chapter 6. Carbon nanotubes dispersion appears to be favored if the difference of surface tension between filler and liquid is low. In contrast, a filler aggregation is rapidly observed in the case where the difference in surface tension is important. We also observed that the percolation of the nanotubes is favored in viscous media.

Submitted by Luciana Ferreira (lucgeral@gmail.com) on 2017-11-16T09:48:38Z No. of bitstreams: 2 Dissertação - Arthur Melo Pereira - 2017.pdf: 8324435 bytes, checksum: a36f9e13810ace212ce9d44a66fc2fb4 (MD5) license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) / Approved for entry into archive by Luciana Ferreira (lucgeral@gmail.com) on 2017-11-16T09:49:04Z (GMT) No. of bitstreams: 2 Dissertação - Arthur Melo Pereira - 2017.pdf: 8324435 bytes, checksum: a36f9e13810ace212ce9d44a66fc2fb4 (MD5) license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) / Made available in DSpace on 2017-11-16T09:49:04Z (GMT). No. of bitstreams: 2 Dissertação - Arthur Melo Pereira - 2017.pdf: 8324435 bytes, checksum: a36f9e13810ace212ce9d44a66fc2fb4 (MD5) license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) Previous issue date: 2017-09-26 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / In a society dependent on electric energy for the execution of various daily activities, it is normal that its use is increasingly increasing over time. In order to carry out the transportation of all electric energy, it is essential to use transmission lines, which with increasing energy demand inevitably have tended to multiply throughout the world, and especially in Brazil, given its continental dimensions. Considering the function of the transmission lines for the electrical system, its importance in the context of the electric power supply is remarkable. However, the lines constitute one of the great emitting sources of electric and magnetic fields of low frequency, which has caused concern and has been motivating fact of several studies, like the realized in this work. Therefore, in view of the scenario presented in the previous paragraph, it is necessary to establish ways of calculating the fields more and more precisely. For the calculation of the electric field is used the Image Method, the Maxwell Potential Coefficients Method and the Coulomb Law, and for the magnetic field the Biot-Savart's Law is used. The results obtained for the electric and magnetic fields were for infinite rectilinear geometries, finite rectilinear and for the conductors taking the form of a catenary, the latter geometry being the most real model for the arrangement of the conductors in a line. In all cases treated, an analytical and numerical approach was performed, in order to allow the calculation of the three geometries with accuracy. Taking advantage of the methodology of calculation of the fields, in addition this work proposes a method of support to the monitoring of transmission lines. The method consists of using the Genetic Algorithm associated to the values of the electric and magnetic fields measured to determine the parameters of the line, such as: phase spacing, cable-soil height, equivalent conductor diameter, current and operating voltage. Given the simplicity of implementation when compared to other methods, the achievement of satisfactory results and the need for a single measuring device to monitor the transmission line, the proposed method proves to be viable and promising to carry out the line monitoring process. / Em uma sociedade dependente da energia elétrica para a execução de diversas atividades do cotidiano, é normal que a sua utilização seja cada vez mais crescente no decorrer do tempo. Para realizar o transporte de toda energia elétrica é imprescindível o uso de linhas de transmissão, que com o aumento da demanda de energia inevitavelmente tenderam a se multiplicar pelo mundo e em especial pelo território brasileiro, dadas as suas dimensões continentais. Tendo em vista a função das linhas de transmissão para o sistema elétrico, é notável a sua importância no contexto do fornecimento de energia elétrica. No entanto, as linhas constituem uma das grandes fontes emissoras de campos elétricos e magnéticos de baixa frequência, o que tem causado preocupação e tem sido fato motivador de diversos estudos, como o realizado neste trabalho. Portanto, diante do cenário apresentado no parágrafo anterior, se faz necessário estabelecer formas de calcular os campos de maneira cada vez mais precisa. Para o cálculo do campo elétrico utiliza-se o Método das Imagens, o Método dos Coeficientes de Potencial de Maxwell e a Lei de Coulomb, já para o campo magnético a Lei de Biot-Savart é empregada. Os resultados obtidos para os campos elétricos e magnéticos foram para as geometrias retilíneas infinitas, retilíneas finitas e para os condutores assumindo a forma de uma catenária, sendo que essa última geometria constitui o modelo mais real quanto à disposição dos condutores em uma linha. Em todos os casos tratados foram realizadas uma abordagem analítica e numérica, de maneira a possibilitar o cálculo das três geometrias com exatidão. Aproveitando-se da metodologia de cálculo dos campos, adicionalmente este trabalho propõe um método de apoio ao monitoramento de linhas de transmissão. O método consiste em utilizar o Algoritmo Genético associado aos valores dos campos elétrico e magnético medidos para determinar os parâmetros da linha, como: espaçamento entre fases, altura cabo-solo, diâmetro equivalente dos condutores, corrente e tensão de operação. Dada a simplicidade de implementação quando comparado a outros métodos, a obtenção de resultados satisfatórios e a necessidade de um único aparelho de medição para monitorar a linha de transmissão, o método proposto mostra-se viável e promissor para realizar o processo de monitoramento de linhas.

Made available in DSpace on 2016-03-15T19:35:52Z (GMT). No. of bitstreams: 1 JOSE CARLOS TACZA ANAYA.pdf: 7682166 bytes, checksum: f3eebed2cf5cb0f5ecda9415f8754978 (MD5) Previous issue date: 2015-01-19 / Conselho Nacional de Desenvolvimento Científico e Tecnológico / In this dissertation, we present the capability of a new network of sensors to monitor the atmospheric electric field at various locations in South America. The main goal is to obtain the characteristic Universal Time daily curve of the atmospheric electric field in fair-weather. That curve is known as the Carnegie curve, which is related to the currents flowing in the Global Atmospheric Electric Circuit. This has been accomplished using monthly, seasonal and annual averages. After obtaining our standard curve of variation of the electric field in fair-weather, the deviations related to phenomena such as solar flares, solar protons events, geomagnetic storms, total solar eclipse and seismic activity are analyzed and commented. / Neste trabalho de dissertação apresenta-se a capabilidade de uma nova rede de sensores para monitorar o campo elétrico atmosférico em vários locais na América do Sul. O objetivo principal é obter a curva diária do campo elétrico atmosférico de tempo bom. Para isto foram realizadas médias mensais, sazonais e anuais. Essa curva é comparada com a curva característica em Tempo Universal conhecida como a Curva de Carnegie, a qual é relacionada com as correntes fluindo no Circuito Elétrico Atmosférico Global. Depois de obter a curva padrão de variação do campo elétrico atmosférico de tempo bom, foram analisados e comentados os desvios relacionados a explosões solares, eventos de prótons solares, tempestades geomagnéticas, eclipse solar e atividade sísmica.

Materials with appropriate combinations of multifunctional properties (strength, toughness, electrical conductivity and piezoelectricity) together with desired biocompatibility are promising candidates for biomedical applications. Apart from these material properties, recent studies have shown the efficacy of electric field in altering cell functionality in order to elicit various cell responses, like proliferation, differentiation, apoptosis (programmed cell death) on conducting substrates in vitro. In the above perspective, the current work demonstrates how CaTiO3 (CT) addition to Hydroxyapatite (HA) can be utilised to obtain an attractive combination of long crack fracture toughness (up to 1.7 MPa.m1/2 measured using single edge V-notch beam technique) and a flexural strength of 155 MPa in addition to moderate electrical conductivity. The enhancement of fracture toughness in HA-CT composites has been explained based on the extensive characterization of twinned microstructure in CT along with the use of theoretical models for predicting the enhancement of toughening through crack tip tilt and twist mechanisms. Subsequent in vitro studies on HA-CT composites with human Mesenchymal Stem cells (hMSCs) in the presence of electric field has shown enhanced differentiation towards bone like cells (osteogenic lineage) as evaluated by ALP activity, Collagen content and gene expression analyses through Polymerase Chain Reaction (PCR) at the end of two weeks. he extracellular matrix mineralization analysis at the end of 4 weeks of hMSC culture further substantiated the efficacy of electric field as a biochemical cue that can influence the stem cell fate processes on conducting substrates. The electric field stimulation strategy was also implemented in in vitro studies with C2C12 mouse myoblast (muscle) cells on elastically compliant poly(vinylidene difluoride) (PVDF)-multiwall carbon nanotube (MWNT) composite substrates. PVDF is a piezoelectric polymer and the addition of MWNTs makes the composite electrically conducting. Upon, electric field stimulation of C2C12 mouse myoblast cells on these composites, has been observed that in a narrow window of electric field parameters, the cell viability was enhanced along with excellent cell alignment and cell-cell contact indicating a potential application of PVDF-based materials in the muscle cell regeneration. In an effort to rationalise such experimental observations, a theoretical model is proposed to explain the development of bioelectric stress field induced cell shape stability and deformation. A single cell is modelled as a double layered membrane separating the culture medium and the cytoplasm with different dielectric properties. This system is linearized by invoking Debye-Huckel approximation of the Poisson-Boltzmann equation. With appropriate boundary conditions, the system is solved to obtain intracellular and extracellular Maxwell stress as a function of multiple parameters like cell size, intracellular and extracellular permittivity and electric field strength. Based on the stresses, we predict shape changes of cell membrane by approximating the deformation amplitude under the influence of electric field. Apart from this, the shear stress on the membrane has been used to determine the critical electric field required to induce membrane breakdown. The analysis is conducted for a cell in suspension/on a conducting substrate and on an insulating substrate to illustrate the effect of substrate properties on cell response under the influence of external electric field.

La composition interne de notre planète est un vaste sujet d’étude auquel participent de nombreuses disciplines scientifiques. Les conditions extrêmes de pression et de température qui règnent à l’intérieur du noyau (constitué principalement de fer et de nickel) et du manteau terrestre (à base de pérovskites) rendent très difficile la détermination de leur compositions exactes. Ce projet de thèse contribue aux études récentes dont l’enjeu est de déterminer plus précisément le chimisme des minéraux présents. Il a pour objet le développement d’un outil de calcul des vitesses de propagation des ondes sismiques a l’aide d’une méthode fondée sur les simulations ab initio. Ces vitesses sont déduites du tenseur élastique complet, incluant la relaxation atomique et les modifications induites du champ cristallin. Nous utilisons l’approche de la théorie de perturbation de la fonctionnelle de la densité (DFPT) qui permet de s'affranchir des incertitudes numériques qu’impliquent les méthodes classiques basées sur des différences finies. Nous combinons cette approche avec le formalisme « Projector Augmented-Wave » (PAW) qui permet, avec un coût de calcul faible, de prendre en compte tous les électrons du système. Nous avons appliqué la méthode sur des matériaux du noyau et du manteau terrestre. Nous avons déterminé les effets de différents éléments légers (Si, S, C, O et H) sur les vitesses de propagation des ondes sismiques dans le fer pur ainsi que celui de l’aluminium dans la pérovskite MgSiO3. / The internal composition of our planet is a large topic of study and involves many scientific disciplines. The extreme conditions of pressure and temperature prevailing inside the core (consisting primarily of iron and nickel) and the mantle (consisting mainly of perovskites) make the determination of the exact compositions very difficult. This thesis contributes to recent studies whose aim is to determine more accurately the chemistry of these minerals. Its purpose is the development of a tool for the calculation of seismic wave velocities within methods based on ab-initio simulations. These velocities are calculated from the full elastic tensor, including the atomic relaxation and induced changes in the crystal field. We use the approach of the density functional perturbation theory (DFPT) to eliminate numerical uncertainties induced by conventional methods based on finite differences. We combine this approach with the « Projector Augmented-Wave » (PAW) formalism that takes into account all the electrons of the system with a low computational cost. We apply the method on core and mantle materials and we determine the effects of various lights elements (Si, S, C, O and H) on the seismic wave velocities of pure iron, as well as the effect of aluminum in the perovskite MgSiO3.

In this thesis the major objective is the implementation of the inverse neural network concept in the design of printed lens (transmitarray) antenna. As it is computationally extensive to perform full-wave simulations for entire transmitarray structure and thereafter perform optimization, the idea is to generate a design database assuming that a unit cell of the transmitarray is situated inside a 2D infinite periodic structure. This way we generate a design database of transmission coefficient by varying the unit cell parameters. Since, for the actual design, we need dimensions for each cell on the transmitarray aperture and to do this we need to invert the design database. The major contribution of this thesis is the proposal and the implementation of database inversion methodology namely inverse neural network modelling. We provide the algorithms for carrying out the inversion process as well as provide check results to demonstrate the reliability of the proposed methodology. Finally, we apply this approach to design a transmitarray antenna, and measure its performance.

The developments of new types of conductors and increase of voltage level have driven the need to carry out research on evaluating overhead line acoustic noise. The surface potential gradient of a conductor is a critical design parameter for planning overhead lines, as it determines the level of corona loss (CL), radio interference (RI), and audible noise (AN). The majority of existing models for surface gradient calculation are based on analytical methods which restrict their application in simulating complex surface geometries. This thesis proposes a novel method which utilizes both analytical and numerical procedures to predict the surface gradient. Stranding shape, proximity of tower, protrusions and bundle arrangements are considered within this model. One of UK National Grid's transmission line configurations has been selected as an example to compare the results for different methods. The different stranding shapes are a key variable in determining dry surface fields. The dynamic behaviour of water droplets subject to AC electric fields is investigated by experiment and finite element modelling. The motion of a water droplet is considered on the surface of a metallic sphere. To understand the consequences of vibration, the FEA model is introduced to study the dynamics of a single droplet in terms of phase shift between vibration and exciting voltage. Moreover, the evolution of electric field within the whole cycle of vibration is investigated. The profile of the electric field and the characteristics of mechanical vibration are evaluated. Surprisingly the phase shift between these characteristics results in the maximum field occurring when the droplet is in a flattened profile rather than when it is ‘pointed’.Research work on audible noise emitted from overhead line conductors is reviewed, and a unique experimental set up employing a semi-anechoic chamber and corona cage is described. Acoustically, this facility isolates undesirable background noise and provides a free-field test space inside the anechoic chamber. Electrically, the corona cage simulates a 3 m section of 400 kV overhead line conductors by achieving the equivalent surface gradient. UV imaging, acoustic measurements and a partial discharge detection system are employed as instrumentation. The acoustic and electrical performance is demonstrated through a series of experiments. Results are discussed, and the mechanisms for acoustic noise are considered. A strategy for evaluating the noise emission level for overhead line conductors is developed. Comments are made on predicting acoustic noise from overhead lines. The technical achievements of this thesis are summarized in three aspects. First of all, an FEA model is developed to calculate the surface electric field for overhead line conductors and this has been demonstrated as an efficient tool for power utilities in computing surface electric field especially for dry condition. The second achievement is the droplet vibration study which describes the droplets' behaviour under rain conditions, such as the phase shift between the voltage and the vibration magnitude, the ejection phenomena and the electric field enhancement due to the shape change of droplets. The third contribution is the development of a standardized procedure in assessing noise emission level and the characteristics of noise emissions for various types of existing conductors in National Grid.

The aim of my diploma thesis is to analyze the antenna array radiation and to produce the antenna array radiation computation program. The program should display the electric and magnetic intensity and radiation power density distribution over the planar or cylindrical surface in the near-field area. The antenna array analyzed in this paper consists of a number of elementary dipoles placed in a row with a reflector which represents the conductive board situated in parallel to the row of dipoles. The demonstration program was built in MATLAB and allows computing and displaying the computed values components of antenna array radiation on the planar or the cylindrical surface. The program also allows saving computed values, saving displayed graphs, or saving the adjusted parameters of antenna array. The created program can be used for the presentation of the antenna array radiation in a school laboratory or for the graphical representation when a new antenna is designed.

This master‘s thesis deals with measurement of liquid dielectric materials (insulators). Above all, it is the different kinds of clean and drinking water, but also transformer oils. There was done theoretical information retrieval about the given topic in this project and on the basis of theory there were realized the relevant measurements of selected properties of liquid dielectric, such as permittivity, capacitance, loss number, conductivity, but also temperature dependence of these parameters. The measurements were realized with the help of product manufactured at the faculty FEKT VUT and with the help of measuring system AGILENT 16452A. The measured results were evaluated, graphically processed and compared.