Ce travail concerne l'étude du phénomène de décharges partielles dans les matériaux isolants utilisés en électronique de puissance. En utilisant des méthodes de détection électrique et optique, le mécanisme de décharge partielle sur des substrats d'AlN dans l'huile silicone a été étudié sur un grand nombre d'échantillons. La variation de la nature du substrat (AlN, Al2O3 et composite verre/époxy) et du matériau d'encapsulation (huile silicone, huile de colza, huile minérale de transformateur, liquide d'imprégnant du condensateur Jarylec et Ugilec) met en évidence l'origine des décharges partielles de l'ensemble substrat – encapsulant. Les décharges partielles sur les substrats céramiques frittés ne dépendent pas du passivant, et se produisent dans le volume du substrat. L'évolution temporelle de la lumière émise dans les liquides en configuration pointe – plan et sur le substrat dans différents liquides montre que l'émission de lumière est un phénomène très complexe influencé par de nombreux paramètres : électroluminescence du solide, de l'encapsulant, décharges partielles, absorption des matériaux. Le phénomène d'électroluminescence du liquide est activé par une illumination extérieure. Les mesures de spectroscopie diélectrique haute tension n'apportent pas d'information supplémentaire sur le phénomène de décharges partielles, car les pertes correspondantes sont très faibles. / This work concerns the study of partial discharge phenomena on insulating materials used in power electronics. Using electrical and optical detection methods, the mechanism of partial discharges on AlN substrates in silicon oil is studied with a large number of samples. Changing the nature of substrates (AlN, and Al2O3 composite glass/epoxy) and of encapsulating materials (silicone oil, rapeseed oil, mineral transformer oil, capacitor impregnating liquids Jarylec and Ugilec) provides a number of conclusions about the origin of partial discharges on embedded substrates. Partial discharges on sintered ceramic substrates do not depend on the encapsulating material nature, but on the nature of the substrates themselves. The temporal evolution of light emitted by the liquid in a point - plane configuration, and on embedded substrates shows that the light during emission is a very complex phenomenon influenced by many parameters: electroluminescence of the solid, of the encapsulating material, partial discharges, and light absorption of materials. The electroluminescence of the liquid is activated by external light. Measurements of high voltage dielectric spectroscopy are also performed, but no additional information on partial discharges is obtained since corresponding losses are very low.

Made available in DSpace on 2014-12-17T14:54:55Z (GMT). No. of bitstreams: 1 RanilsonCF_TESE.pdf: 2036029 bytes, checksum: f7ba8fa789f5420decc75fb98f14f807 (MD5) Previous issue date: 2010-07-27 / The main purpose of this work was the development of ceramic dielectric substrates of bismuth niobate (BiNbO4) doped with vanadium pentoxide (V2O5), with high permittivity, used in the construction of microstrip patch antennas with applications in wireless communications systems. The high electrical permittivity of the ceramic substrate provided a reduction of the antenna dimensions. The numerical results obtained in the simulations and the measurements performed with the microstrip patch antennas showed good agreement. These antennas can be used in wireless communication systems in various frequency bands. Results were satisfactory for antennas operating at frequencies in the S band, in the range between 2.5 GHz and 3.0 GHz. / O objetivo principal deste trabalho foi o desenvolvimento de substratos diel?tricos cer?micos de niobato de bismuto (BiNbO4) dopados com pent?xido de van?dio (V2O5), com alta permissividade el?trica, usados na constru??o de antenas patch de microfita com aplica??es em sistemas de comunica??es sem fio. A alta permissividade el?trica do substrato cer?mico proporcionou uma redu??o no tamanho das antenas. Os resultados num?ricos obtidos nas simula??es e medi??es realizadas com as antenas patch de microfita mostraram boa concord?ncia. Essas antenas podem ser usadas em sistemas de comunica??es sem fio em v?rias faixas de freq??ncias. Foram obtidos resultados satisfat?rios em antenas com freq??ncias de opera??o na banda S, na faixa compreendida entre 2,5 GHz e 3,0 GHz.

Directing differential cellular response by manipulating the physical characteristics of the material is regarded as a key challenge in biomaterial implant design and tissue engineering. In developing various biomaterials, the influence of substrate properties, like surface topography, stiffness and wettability on the cell functionality has been investigated widely. However, such study to probe into the influence of substrate conductivity on cell fate processes is rather limited. The need for such an understanding is based on the fact that specific tissues in the body are electrically active in nature, such as in brain, heart and skeletal muscle. These tissues make use of electrical conductivity as an effective cue for tissue homeostasis, development, regeneration and so on. Moreover, understanding the importance of underlying conductivity in basic biological processes is essential in developing electrically conductive biomaterials with the ability to simulate normal electrophysiology of the body by interfacing with bioelectric fields in cells and tissues. Electrical stimulation and charge conduction can regulate numerous intracellular signalling pathways, can interact with cytoskeleton proteins to modulate the morphology, increase protein synthesis and on the more can favor the ECM protein conformational changes. On these grounds, the present dissertation illustrates that persistent electrical activation influences the multipotency of hMSCs and acts like a promoter towards selective differentiation of hMSCs into neural/cardiomyogenic or osteogenic lineage. Besides, continual exposure to electric field stimulated conducting culture environments lead to growth arrest while enhancing differentiation. In total, this dissertation suggests the dominant role of conductivity in inducing my oblast differentiation and hMSc lineage commitment that involves EF stimulated in vitro culture conditions. Also, a knowledge base with qualitative and quantitative understanding of stem cells and their response to substrate physical properties and external field effect was developed through this comprehensive study. Such an improved understanding of the ability of hMSCs in sensing electrical conductivity may lead to the development of culture additives/conditions that better induce directed stem cell differentiation.