Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2007. / Clear procedures for designing dielectric resonator oscillators (DROs) are presented in this thesis, including built examples to validate these design procedures. Both series and parallel feedback DROs are discussed and the procedures for building them are presented. Two examples at different frequencies for each type of DRO are constructed and tested with the results shown. The first is at a frequency of approximately 6.22 GHz and the second for the higher frequency of 11.2 GHz. The DROs for the desired frequencies are designed using the Microwave Office (MWO) software by AWR with the design based on the small-signal model (scattering parameters). Oscillators are produced using the negative resistance method. The circuit achieves low noise by using a dielectric resonator with a high Q factor. Both the series and parallel feedback DRO circuits can be mechanically tuned around the resonant frequency to maximize performance.

Este trabalho visou ao desenvolvimento de antenas de ressoador dielétrico de banda dupla em frequências de micro-ondas por meio do estabelecimento de uma metodologia de projeto para as mesmas e da proposição de configurações inéditas de antenas desse tipo. Estudos realizados sobre antenas de ressoador dielétrico e sobre suas técnicas de projeto forneceram a base para a elaboração de um procedimento de projeto para antenas de banda dupla que explora a flexibilidade das antenas de ressoador dielétrico. Empregando-se esse procedimento de projeto, duas antenas com configurações originais foram desenvolvidas para operar em frequências de micro-ondas. Uma delas é uma antena de ressoador dielétrico cilíndrica sobre plano de terra, destinada a operar nas frequências centrais de 3,94 GHz e de 5,42 GHz, com diagrama de radiação direcional e polarização circular na primeira banda de operação e com diagrama de radiação omnidirecional e polarização linear na segunda. A outra antena consiste em uma antena de ressoador dielétrico cilíndrica sobre substrato dielétrico aterrado, cujas frequências centrais de operação são 3,99 GHz e 6,20 GHz, com diagrama de radiação direcional e polarização circular em ambas as bandas de operação. O projeto das duas antenas também levou a contribuições novas adicionais em alguns temas específicos relacionados: estruturas de excitação para antenas de ressoador dielétrico, circuitos acopladores híbridos de 90° de banda dupla, circuitos de alimentação para antenas de banda dupla e modelos matemáticos para o ressoador dielétrico cilíndrico isolado no espaço livre. Os comportamentos previstos teoricamente para as antenas projetadas são confirmados por meio de resultados obtidos por simulação eletromagnética e por medição experimental, o que indica a validade das hipóteses adotadas, das configurações de antena propostas e da metodologia de projeto concebida, demonstrando também a flexibilidade desta para variadas especificações de antena. Os avanços obtidos com este trabalho e as melhorias que podem ser efetuadas sobre o mesmo são identificados examinando-se resultados recentemente publicados na literatura técnica. Este trabalho foi realizado na forma de uma pesquisa de doutorado sanduíche, desenvolvida parcialmente na University of Houston sob a supervisão do Prof. Stuart A. Long. / This work aimed at the development of microwave dual-band dielectric resonator antennas by means of establishing a design methodology for them and of proposing novel configurations of this antenna type. Studies on dielectric resonator antennas and on their design techniques provided the ground for elaborating a design procedure for dual-band antennas that exploits the flexibility of dielectric resonator antennas. By employing this design procedure, two antennas with original configurations have been developed for operation at microwave frequencies. One of them is a cylindrical dielectric resonator antenna on a ground plane, intended for operation at the center frequencies of 3.94 GHz and 5.42 GHz, with a directional radiation pattern and circular polarization in the first operating band and with an omnidirectional radiation pattern and linear polarization in the second one. The other antenna consists of a cylindrical dielectric resonator antenna on a grounded dielectric substrate, whose operating center frequencies are 3.99 GHz and 6.20 GHz, with a directional radiation pattern and circular polarization in both operating bands. The design of the two antennas also led to additional new contributions in some related specific subjects: excitation structures for dielectric resonator antennas, dual-band 90°-hybrid couplers, feeding circuits for dual-band antennas, and mathematical models for the cylindrical dielectric resonator isolated in free space. The behaviors theoretically expected for the designed antennas are confirmed by means of results obtained from electromagnetic simulation and from experimental measurements, which indicates the validity of the assumed hypotheses, of the proposed antenna configurations and of the conceived design methodology, also demonstrating the flexibility of the latter for various antenna specifications. The advances obtained with this work and the improvements that can be made on it are identified by examining results recently published in the technical literature. This work was carried out as a sandwich doctoral research, developed at the University of Houston under the supervision of Prof. Stuart A. Long.

Neste trabalho, cerâmicas de BaTiO3 foram preparadas pelo método de Pechini com o propósito de estudar as suas propriedades dielétricas, incluindo as anomalias dielétricas de origem interfacial nestes ferroelétricos. Após calcinação das soluções precursoras a 600 oC por 2h, os pós compactados foram sinterizados a várias temperaturas entre 1150 e 1450 oC. Subseqüentemente, foram realizadas caracterizações (micro)estruturais aplicando as técnicas de microscopia eletrônica de varredura e difração de raios X, além de estudos dielétricos utilizando a técnica de espectroscopia de impedância. Destas caracterizações, verificou-se a obtenção de materiais cerâmicos com tamanhos de grãos variando entre 0,8 e 37 micrometros ao aumentar a temperatura de sinterização. Os resultados de dinâmica térmica de densificação indicam a contribuição de dois mecanismos independentes: (i) sinterização com participação de fase líquida, induzindo altas densidades dos materiais a partir de ~1300 oC (perto do ponto eutético), e (ii) sinterização via reação de estado sólido, implicando alta sinterabilidade mesmo abaixo dos 1300 oC, em se tratando de pós nanométricos, ou seja, com alta densidade de defeitos e, portanto, processos de difusão iônica intensificados. Por outro lado, a resposta dielétrica dos grãos evidenciou o cumprimento da lei de Curie-Weiss, típica de ferroelétricos, enquanto um fenômeno de anomalia dielétrica (não cumprimento desta lei) foi observado para as respostas totais destes materiais em altas temperaturas e baixas freqüências. A análise dos resultados de espectroscopia de impedância via circuito equivalente, sendo considerado o modelo de camadas (grãos e contornos de grãos) em série, revelou a predominância das propriedades dielétricas das interfaces intergranulares como a causa desta anomalia. Considerando a relação entre propriedades macroscópicas e microscópicas no modelo, a variação da intensidade da anomalia é discutida em função do tamanho dos grãos, sendo este parâmetro uma medida (função inversa) da densidade das interfaces intergranulares. Em geral, este trabalho revela uma excelente correlação entre microestruturas, circuito equivalente e propriedades dielétricas exibidas por estes materiais eletrocerâmicos. / BaTiO3 ceramics were prepared in this work, following the Pechinis method, with the purpose of studying their dielectric properties, including especially the occurrence and characteristics of dielectric anomalies associated to internal interfaces, as often found in ferroelectrics. After calcination of the precursor solutions at 600 oC for 2h, the synthesized green compacts were sintered at several temperatures from 1150 to 1450 oC. Subsequently, (micro)structural characterizations were carried out by applying scanning electron microscopy and X-ray diffraction techniques, followed by dielectric measurements using the impedance spectroscopy technique. From these characterizations, the production of ceramic bodies with average grain sizes varying from 0.8 to 37 micrometer was verified when increasing the sintering temperature. The results of thermal dynamics of densification indicate the contribution of two independent mechanisms, namely (i) liquid phase-assisted sintering, inducing high materials density from ~1300 oC (close to the eutectic point) above, and (ii) solid-state sintering, implying high sinterability even below 1300 oC, as dealing with nanoparticles-sized powders, meaning high density of surface defects and, thus, enhanced ionic diffusion processes. On the other hand, while the dielectric response of the grains satisfied the Curie-Weiss law, typical of ferroelectrics, a dielectric anomaly (departure from this law) phenomenon was observed for the total dielectric response of these materials towards higher temperatures and lower frequencies. The analysis of the results from impedance spectroscopy via equivalent circuit, involving consideration of the series layer (grains and grain boundaries) model, revealed that the predominance of the dielectric properties from the intergranular interfaces is responsible for occurrence of this anomaly. Considering the relation between macroscopic and microscopic properties holding in this model, the variation of the anomaly strength is here discussed in terms of the value of average grain size in these ceramics, a microstructural parameter that represents a measure (inverse function) of the density of the intergranular interfaces. In general, the present study reveals a really good correlation between microstructures, equivalent circuit and dielectric properties exhibited by these electroceramic materials.

Este trabalho visou ao desenvolvimento de antenas de ressoador dielétrico de banda dupla em frequências de micro-ondas por meio do estabelecimento de uma metodologia de projeto para as mesmas e da proposição de configurações inéditas de antenas desse tipo. Estudos realizados sobre antenas de ressoador dielétrico e sobre suas técnicas de projeto forneceram a base para a elaboração de um procedimento de projeto para antenas de banda dupla que explora a flexibilidade das antenas de ressoador dielétrico. Empregando-se esse procedimento de projeto, duas antenas com configurações originais foram desenvolvidas para operar em frequências de micro-ondas. Uma delas é uma antena de ressoador dielétrico cilíndrica sobre plano de terra, destinada a operar nas frequências centrais de 3,94 GHz e de 5,42 GHz, com diagrama de radiação direcional e polarização circular na primeira banda de operação e com diagrama de radiação omnidirecional e polarização linear na segunda. A outra antena consiste em uma antena de ressoador dielétrico cilíndrica sobre substrato dielétrico aterrado, cujas frequências centrais de operação são 3,99 GHz e 6,20 GHz, com diagrama de radiação direcional e polarização circular em ambas as bandas de operação. O projeto das duas antenas também levou a contribuições novas adicionais em alguns temas específicos relacionados: estruturas de excitação para antenas de ressoador dielétrico, circuitos acopladores híbridos de 90° de banda dupla, circuitos de alimentação para antenas de banda dupla e modelos matemáticos para o ressoador dielétrico cilíndrico isolado no espaço livre. Os comportamentos previstos teoricamente para as antenas projetadas são confirmados por meio de resultados obtidos por simulação eletromagnética e por medição experimental, o que indica a validade das hipóteses adotadas, das configurações de antena propostas e da metodologia de projeto concebida, demonstrando também a flexibilidade desta para variadas especificações de antena. Os avanços obtidos com este trabalho e as melhorias que podem ser efetuadas sobre o mesmo são identificados examinando-se resultados recentemente publicados na literatura técnica. Este trabalho foi realizado na forma de uma pesquisa de doutorado sanduíche, desenvolvida parcialmente na University of Houston sob a supervisão do Prof. Stuart A. Long. / This work aimed at the development of microwave dual-band dielectric resonator antennas by means of establishing a design methodology for them and of proposing novel configurations of this antenna type. Studies on dielectric resonator antennas and on their design techniques provided the ground for elaborating a design procedure for dual-band antennas that exploits the flexibility of dielectric resonator antennas. By employing this design procedure, two antennas with original configurations have been developed for operation at microwave frequencies. One of them is a cylindrical dielectric resonator antenna on a ground plane, intended for operation at the center frequencies of 3.94 GHz and 5.42 GHz, with a directional radiation pattern and circular polarization in the first operating band and with an omnidirectional radiation pattern and linear polarization in the second one. The other antenna consists of a cylindrical dielectric resonator antenna on a grounded dielectric substrate, whose operating center frequencies are 3.99 GHz and 6.20 GHz, with a directional radiation pattern and circular polarization in both operating bands. The design of the two antennas also led to additional new contributions in some related specific subjects: excitation structures for dielectric resonator antennas, dual-band 90°-hybrid couplers, feeding circuits for dual-band antennas, and mathematical models for the cylindrical dielectric resonator isolated in free space. The behaviors theoretically expected for the designed antennas are confirmed by means of results obtained from electromagnetic simulation and from experimental measurements, which indicates the validity of the assumed hypotheses, of the proposed antenna configurations and of the conceived design methodology, also demonstrating the flexibility of the latter for various antenna specifications. The advances obtained with this work and the improvements that can be made on it are identified by examining results recently published in the technical literature. This work was carried out as a sandwich doctoral research, developed at the University of Houston under the supervision of Prof. Stuart A. Long.

O aquecimento por micro-ondas (MO) é uma tecnologia emergente para o processamento térmico contínuo de alimentos líquidos. A taxa d aquecimento e a profundidade de penetração das MO dependem das propriedades dielétricas (PD) do alimento (permissividade elétrica relativa e fator de perda dielétrica ). As PD de água de coco verde e de sucos de frutas cítricas (laranja, limão e tangerina) foram estudadas empregando a técnica de cabo coaxial. As medições foram realizadas em 101 frequências na faixa de 500 MHz até 3000 MHz e em temperaturas entre 0 °C e 90 °C, o que permitiu estudar o efeito da frequência e da temperatura. A profundidade de penetração foi calculada com uma expressão derivada das equações de Maxwell para eletromagnetismo. O trabalho experimental começou com as PD de: água de coco verde simulada, água destilada, solução de açúcares e sais da água de coco verde e água de coco verde natural. Resultados foram comparados para avaliar a contribuição de cada componente (água, açúcares e sais). PD de sucos de diferentes variedades de laranjas, tangerinas e limões, assim como misturas de sucos de duas variedades de laranja, foram também avaliadas. A condutividade elétrica das amostras foi medida nas diferentes temperaturas e o seu efeito sobre o fator de perda foi analisado. Os resultados foram comparados nas frequências comerciais de 915 MHz e 2450 MHz. O comportamento dielétrico das amostras estudadas foi caracterizado por dois mecanismos principais: a condução iônica de sais dissolvidos e a rotação dipolar da molécula de água, os quais têm predominância variável dependendo da frequência e da temperatura. A permissividade elétrica relativa de todas as amostras decresceu com o incremento da temperatura e da frequência, enquanto que o comportamento do fator de perda com a temperatura e frequência foi determinado pela composição. Os sais apresentaram maior efeito que os açúcares sobre as PD da água de coco verde. A profundidade de penetração foi maior na frequência de 915 MHz, associada com o aumento no comprimento de onda. Correlações polinomiais foram ajustadas nas frequências comerciais para representar a dependência com a temperatura. / Microwave (MW) heating is an emergent technology for thermal processing of liquid food products. Heating rate and MW penetration depth depending on dielectric properties (DP) of food material (relative electrical permittivity () and dielectric loss factor ()). DP for green coconut water (GCW) and citrus fruits juices (orange, tangerine and lemon) were studied by open-ended coaxial line probe technique. The measurements were carried out for frequencies between 500 MHz and 3000 MHz at 101 points in this range and temperatures from 0 °C to 90 °C, which allowed studying temperature and frequency effect. The power penetration depth was calculated using an expression derived from the Maxwell equations for electromagnetism. Experimental work started with DP measurement for: simulated (GCW), distilled water, sugars and salts solutions of (GCW) and natural (GCW). Results were compared in order to evaluate the each component contribution (water, sugars and salts). DP for juices from different orange, tangerine and lemon varieties as well as mixtures of orange juice from two varieties, were also evaluated. Electrical conductivity was measured and its effect on dielectric loss factor was analyzed. Obtained results were compared at commercial frequencies of 915 MHz and 2450 MHz. Dielectric behavior of studied samples was characterized by two main mechanisms: ionic conduction of dissolved salts and dipole rotation of water molecules, which presented variable predominance, depending on frequency and temperature. Relative electrical permittivity decreased with increase on both temperature and frequency for all tested samples, whereas dielectric loss factor behavior with temperature and frequency was determined by composition. Salts showed higher effect than sugars on dielectric properties of GCW. Penetration depth was higher at 915 MHz, associated to increasing wave length. Polynomial correlations for DP were adjusted at commercial frequencies in order to illustrate the temperature dependence.

This thesis deals with the investigations on the dielectric properties of polycrystalline ceramics having uncommon substitutions in barium titanate and other related phases of BaTiO3-CaTiO3, MgTiO3-CaTiO3 and MgTiO3-BaTiO3 systems. After presenting a brief introduction on the ceramic materials studied in terms of their crystal structures, electrical properties, nonstoichiometry and microstructural characteristics. The thesis describes the synthesis of the ceramics and the methodology of different techniques utilized in characterizing the samples. Barium calcium titanate was synthesized through novel wet chemical techniques and the dielectric properties of calcium substituted barium titanate do not reveal multi-site occupancy whereas they are predominantly influenced by the A/B cationic ratio. The role of transition metals of the 3d series from vanadium (Z=23) to zinc (Z=30) in modifying the crystallographic phase content, microstructure and the dielectric properties of BaTiO3 ceramics containing 10 at% impurities were studied. All the transition metals brought about the phase conversion to hexagonal BaTiO3, although no systematics could be arrived at relating the hexagonal content to the 3d electronic configuration of the impurities. The relaxor dielectrics arising from the titanate solid solution with uncommon substitution and its interconversion to normal ferroelectrics is studied. The effects of cationic substitutions of iron and niobium for titanium in BaTiO3 pervoskite lattice in crystal symmetry and dielectric properties were investigated. The above dielectric characteristics are comparable in a converse way to those of the well known Pb(Mg1/3Nb2/3)O3-PbTiO3 system wherein the relaxor behaviour occurs within the lower lead titanate compositional limits. The modification in -T characteristics of positive temperature coefficient in resistance (PTCR) by the addition of segregative additives such as B2O3, Al2O3 etc in BaTiO3 and its conversion to grain boundary layer capacitance is studied. The presence of Al-related hole centers at the grain boundary regions resulted in charge redistribution across the modified phase transition temperatures due to symmetry-related vibronic interactions, which result in broad PTCR characteristics extending to higher temperatures. The processing of high permittivity ceramics by the manipulation of microstructural features in semiconducting BaTiO3 is studied wherein the grain boundary layer effect superimposed with the contributions from the barrier layers formed during electroding related to microstructure is proposed to be responsible for the unusual high permittivity in semiconducting BaTiO3. The influence of Mg2+ as a substituent in modifying the crystallographic phase contents, microstructure and the dielectric properties of (Ba1-xMgx)TiO3 ceramics, (x ranging from zero to 1.0 ) is studied. The results point to the dual occupancy of Mg2+ both in A and B sublattice and the role of oxygen vacancy as well as (Ti3+ –VO) defects in stabilization of hexagonal phase to lower temperatures. The microwave dielectrics of the BaMg6Ti6O19 phase formed in the compositional range of x=0.4 to 0.7 was investigated for suitable application in microwave dielectrics. Extensive miscibility between the ilmenite-type MgTiO3 and perovskite-type CaTiO3 over a wide compositional range is brought about by the simultaneous equivalent substitution of Al3+ and La3+. The resulting Mg1-(x+y)CaxLay)(Ti1-yAly)O3 ceramics exhibit improved microwave dielectric properties by way of high permittivity, low TCK and high quality factor. The elemental distribution reveals the complexity in the Mg/Ca distribution and its correlation with the solid state miscibility as well as dielectric properties. Microwave dielectric property of Mg4Al2Ti9O25 which is detected as secondary phase is studied in detail.

Ces travaux de thèse ont consisté à mener une étude approfondie des propriétés physico-chimiques duparylène C, qui est un polymère chloré, en lien avec ses propriétés électriques et diélectriques. Ces dernièresont cerné principalement le comportement de la constante diélectrique et des pertes diélectriques enfonction de la température (de l’azote jusqu’à 300 °C) sur une plage de fréquence étendue (10-4 Hz – 1 MHz).Les analyses par diffraction des rayons X ont montré que ce polymère présentait une structure cristalline a-monoclinique avec un taux de cristallinité de 45 % après élaboration. Ce taux est faiblement dépendant del’épaisseur pour des couches d’épaisseurs supérieures à 50 nm. Des recuits spécifiques appliqués sur leparylène C au dessus de la température de transition vitreuse ont permis de modifier le taux de cristallinité etdes valeurs de 30% à 75 % ont pu être obtenues. Une relation linéaire entre le taux de cristallinité et latempérature de recuit a été proposée. Les analyses diélectriques ont permis de mettre en évidence troismécanismes principaux de relaxation : La relaxation b, la relaxation g et la relaxation a. Par ailleurs, unmécanisme de polarisation interfaciale de type Maxwell-Wagner-Sillars (MWS) a été identifié à hautetempérature (au-delà de la transition vitreuse). La mobilité moléculaire des chaînes autour de la températurede transition vitreuse a été analysée en profondeur et les résultats (indice de fragilité, paramètresthermodynamiques…) ont été positionnés par rapport aux données de la littérature concernant d’autrespolymères. / This work of thesis consisted in undertaking a thorough study of the physicochemical properties of theparylene C, which is a chlorinated polymer, in relationship with its electric and dielectric properties. These lastdetermined mainly the behavior of the permittivity and the dielectric losses according to the temperature (ofnitrogen up to 300 °C) on a wide frequency range (10-4Hz – 1 MHz). The analyses by x-rays diffraction showedthat this polymer had a a - monoclinical crystalline structure with a rate of crystallinity of 45 %. This rate isslightly depending on the thickness for layers thicknesses higher than 50 nm. Specific annealing applied to theparylene C made it possible to modify the rate of crystallinity and values from 30% to 75 % of crystallinitycould be obtained. A linear relation between the rate of crystallinity and the temperature of annealing wasproposed. The dielectric analyses made it possible to highlight three principal mechanisms of relaxation: b, g,a. In addition, a mechanism of interfacial polarization (Maxwell-Wagner-Sillars) was identified at hightemperature (beyond the glass transition). The molecular mobility of the chains around the glass transitionwas analyzed in-depth and the results (index of fragility, thermodynamic parameters...) were positionedcompared to the data of the literature concerning of other polymers.

O aquecimento por micro-ondas (MO) é uma tecnologia emergente para o processamento térmico contínuo de alimentos líquidos. A taxa d aquecimento e a profundidade de penetração das MO dependem das propriedades dielétricas (PD) do alimento (permissividade elétrica relativa e fator de perda dielétrica ). As PD de água de coco verde e de sucos de frutas cítricas (laranja, limão e tangerina) foram estudadas empregando a técnica de cabo coaxial. As medições foram realizadas em 101 frequências na faixa de 500 MHz até 3000 MHz e em temperaturas entre 0 °C e 90 °C, o que permitiu estudar o efeito da frequência e da temperatura. A profundidade de penetração foi calculada com uma expressão derivada das equações de Maxwell para eletromagnetismo. O trabalho experimental começou com as PD de: água de coco verde simulada, água destilada, solução de açúcares e sais da água de coco verde e água de coco verde natural. Resultados foram comparados para avaliar a contribuição de cada componente (água, açúcares e sais). PD de sucos de diferentes variedades de laranjas, tangerinas e limões, assim como misturas de sucos de duas variedades de laranja, foram também avaliadas. A condutividade elétrica das amostras foi medida nas diferentes temperaturas e o seu efeito sobre o fator de perda foi analisado. Os resultados foram comparados nas frequências comerciais de 915 MHz e 2450 MHz. O comportamento dielétrico das amostras estudadas foi caracterizado por dois mecanismos principais: a condução iônica de sais dissolvidos e a rotação dipolar da molécula de água, os quais têm predominância variável dependendo da frequência e da temperatura. A permissividade elétrica relativa de todas as amostras decresceu com o incremento da temperatura e da frequência, enquanto que o comportamento do fator de perda com a temperatura e frequência foi determinado pela composição. Os sais apresentaram maior efeito que os açúcares sobre as PD da água de coco verde. A profundidade de penetração foi maior na frequência de 915 MHz, associada com o aumento no comprimento de onda. Correlações polinomiais foram ajustadas nas frequências comerciais para representar a dependência com a temperatura. / Microwave (MW) heating is an emergent technology for thermal processing of liquid food products. Heating rate and MW penetration depth depending on dielectric properties (DP) of food material (relative electrical permittivity () and dielectric loss factor ()). DP for green coconut water (GCW) and citrus fruits juices (orange, tangerine and lemon) were studied by open-ended coaxial line probe technique. The measurements were carried out for frequencies between 500 MHz and 3000 MHz at 101 points in this range and temperatures from 0 °C to 90 °C, which allowed studying temperature and frequency effect. The power penetration depth was calculated using an expression derived from the Maxwell equations for electromagnetism. Experimental work started with DP measurement for: simulated (GCW), distilled water, sugars and salts solutions of (GCW) and natural (GCW). Results were compared in order to evaluate the each component contribution (water, sugars and salts). DP for juices from different orange, tangerine and lemon varieties as well as mixtures of orange juice from two varieties, were also evaluated. Electrical conductivity was measured and its effect on dielectric loss factor was analyzed. Obtained results were compared at commercial frequencies of 915 MHz and 2450 MHz. Dielectric behavior of studied samples was characterized by two main mechanisms: ionic conduction of dissolved salts and dipole rotation of water molecules, which presented variable predominance, depending on frequency and temperature. Relative electrical permittivity decreased with increase on both temperature and frequency for all tested samples, whereas dielectric loss factor behavior with temperature and frequency was determined by composition. Salts showed higher effect than sugars on dielectric properties of GCW. Penetration depth was higher at 915 MHz, associated to increasing wave length. Polynomial correlations for DP were adjusted at commercial frequencies in order to illustrate the temperature dependence.

Neste trabalho, cerâmicas de BaTiO3 foram preparadas pelo método de Pechini com o propósito de estudar as suas propriedades dielétricas, incluindo as anomalias dielétricas de origem interfacial nestes ferroelétricos. Após calcinação das soluções precursoras a 600 oC por 2h, os pós compactados foram sinterizados a várias temperaturas entre 1150 e 1450 oC. Subseqüentemente, foram realizadas caracterizações (micro)estruturais aplicando as técnicas de microscopia eletrônica de varredura e difração de raios X, além de estudos dielétricos utilizando a técnica de espectroscopia de impedância. Destas caracterizações, verificou-se a obtenção de materiais cerâmicos com tamanhos de grãos variando entre 0,8 e 37 micrometros ao aumentar a temperatura de sinterização. Os resultados de dinâmica térmica de densificação indicam a contribuição de dois mecanismos independentes: (i) sinterização com participação de fase líquida, induzindo altas densidades dos materiais a partir de ~1300 oC (perto do ponto eutético), e (ii) sinterização via reação de estado sólido, implicando alta sinterabilidade mesmo abaixo dos 1300 oC, em se tratando de pós nanométricos, ou seja, com alta densidade de defeitos e, portanto, processos de difusão iônica intensificados. Por outro lado, a resposta dielétrica dos grãos evidenciou o cumprimento da lei de Curie-Weiss, típica de ferroelétricos, enquanto um fenômeno de anomalia dielétrica (não cumprimento desta lei) foi observado para as respostas totais destes materiais em altas temperaturas e baixas freqüências. A análise dos resultados de espectroscopia de impedância via circuito equivalente, sendo considerado o modelo de camadas (grãos e contornos de grãos) em série, revelou a predominância das propriedades dielétricas das interfaces intergranulares como a causa desta anomalia. Considerando a relação entre propriedades macroscópicas e microscópicas no modelo, a variação da intensidade da anomalia é discutida em função do tamanho dos grãos, sendo este parâmetro uma medida (função inversa) da densidade das interfaces intergranulares. Em geral, este trabalho revela uma excelente correlação entre microestruturas, circuito equivalente e propriedades dielétricas exibidas por estes materiais eletrocerâmicos. / BaTiO3 ceramics were prepared in this work, following the Pechinis method, with the purpose of studying their dielectric properties, including especially the occurrence and characteristics of dielectric anomalies associated to internal interfaces, as often found in ferroelectrics. After calcination of the precursor solutions at 600 oC for 2h, the synthesized green compacts were sintered at several temperatures from 1150 to 1450 oC. Subsequently, (micro)structural characterizations were carried out by applying scanning electron microscopy and X-ray diffraction techniques, followed by dielectric measurements using the impedance spectroscopy technique. From these characterizations, the production of ceramic bodies with average grain sizes varying from 0.8 to 37 micrometer was verified when increasing the sintering temperature. The results of thermal dynamics of densification indicate the contribution of two independent mechanisms, namely (i) liquid phase-assisted sintering, inducing high materials density from ~1300 oC (close to the eutectic point) above, and (ii) solid-state sintering, implying high sinterability even below 1300 oC, as dealing with nanoparticles-sized powders, meaning high density of surface defects and, thus, enhanced ionic diffusion processes. On the other hand, while the dielectric response of the grains satisfied the Curie-Weiss law, typical of ferroelectrics, a dielectric anomaly (departure from this law) phenomenon was observed for the total dielectric response of these materials towards higher temperatures and lower frequencies. The analysis of the results from impedance spectroscopy via equivalent circuit, involving consideration of the series layer (grains and grain boundaries) model, revealed that the predominance of the dielectric properties from the intergranular interfaces is responsible for occurrence of this anomaly. Considering the relation between macroscopic and microscopic properties holding in this model, the variation of the anomaly strength is here discussed in terms of the value of average grain size in these ceramics, a microstructural parameter that represents a measure (inverse function) of the density of the intergranular interfaces. In general, the present study reveals a really good correlation between microstructures, equivalent circuit and dielectric properties exhibited by these electroceramic materials.

"Il ya beaucoup de place vers le bas!", ait déclaré Richard Feynman dans son discours sur les nanotechnologies en 1959, ouvrant un nouveau monde de la science et de la technologie! L'idée d'utiliser des nanoparticules afin d'améliorer les propriétés diélectriques des polymères qui étaient déjà en cours d'utilisation a suscité l'intérêt des chercheurs dans les deux dernières décennies. Nanocharges tels que la silice, l'alumine, l dioxyde de titan, etc, mais aussi des particules plus grosses comme les argiles ou les nanotubes de carbone ont été mélangés avec les polymères «classiques» afin d'améliorer les propriétés du polyéthylène, des résines époxy, de polypropylène, etc.De nos jours, le rendement de conversion d'énergie de générateurs électriques est limitée par des problèmes thermiques et électriques, limitations étant surtout liées à la qualité des rubans isolants appliqués sur les barres en cuivre. En conséquence, des rubans isolants innovants basés sur des de matériaux nanostructurés ont été envisagés pour augmenter le rendement énergétique des alternateurs et le but de ce travail est d'étudier ces matériaux innovants et de comparer leurs propriétés avec celles des matériaux déjà utilisés, afin d'aider à choisir le meilleur matériau composite pour les futurs rubans. Après une brève introduction sur le contexte de ce travail, on a présenté bref état de l'art les propriétés des polymères époxy, avec un débat sur les propriétés électriques de la matrice polymère choisie (résine époxyde), ses propriétés chimiques et thermiques. Ensuite, les nanocharges choisies et leurs propriétés spécifiques sont présentés, en discutant les différentes étapes du procédé de fabrication, aussi qu'un débat sur les phénomènes qui apparaissent à l'échelle nanométrique et leur éventuelle influence sur les propriétés du matériau composite fini.Différents groupes de matériaux composites à base d'époxy remplis de silice nanométrique, argile organique ou de nitrure de bore sont analysés dans ce travail. Afin de caractériser et interpréter leurs propriétés, plusieurs outils ont été utilisés: la microscopie imagerie, la caractérisation thermique ainsi que les méthodes d'investigation à fort ou faible champ électrique. Leur caractéristiques sont ressemblés et différents observations sur des propriétés «générales» ou «spécifiques» des matériaux composites ont été observés et discutés par rapport à l'influence du type de charge utilisée, de son traitement ou de son poids total sont débattues.Enfin, un modèle numérique basé sure une généralisation de la loi des mélanges sera utilisée afin de prédire la réponse diélectrique des matériaux composites ainsi que les paramètres (taille, permittivité) de l'interphase, «l'ingrédient« magique du mélange matrice de remplissage. Le modèle présenté nous a permis de donner un lien entre les différents matériaux et de valider les résultats obtenus expérimentalement. Une approche par éléments finis est utilisée.Ce manuscrit s'achève par des conclusions sur le travail présenté et il laisse entrevoir les perspectives dans l'analyse complexe des polymères nanocomposites . / “There's plenty of room of the bottom!” said Richard Feynman in his talk on top-down nanotechnology in 1959, bringing into the spot light a new world of science and technology ! The idea of using nanoparticles in order to improve the dielectric properties of the polymers that were already in use attracted the interest of researchers for the last two decades. Nanofillers such as silica, alumina, titania etc, but also larger particles such as clays or carbon nanotubes were mixed with the “classic” polymers in order to improve the properties of polyethylene, epoxy resins, polypropylene etc. Since nowadays the energy conversion efficiency of electrical generators is restricted by thermal and electrical issues, these limitations can be related to the electrical insulator tapes themselves. Thus, innovative insulating tapes based on nanostructured material scenarios to address the energy saving concern are intended and the purpose of this work is to investigate these innovative materials and to compare their properties with those of the materials already in use, in order to help choosing the best composite material for the future tapes.This works begins with a state of the art regarding the properties of epoxy polymers. Their chemical, thermal and dielectric properties are presented. Afterwards, the chosen fillers and their specific properties are presented. The influence of the chosen fillers as well as different steps of the nanocomposite materials manufacturing process are presented and the discussion ends with a debate on the phenomena appearing at the nanometric scale and their possible influence on the properties of the finite composite material .Different materials groups of epoxy based composites filled with nanometric silica, organoclay or boron nitride are analyzed afterwards. In order to characterize and interpret their properties, several tools were used: imaging microscopy, thermal characterization as well as high and low electric field investigation methods. A debate trying to distinguish between so called “general” or “specific” behavior of the composite materials with respect to the normal, unfilled polymer is also presented. The influence of the type of filler, its treatment or its weight total percentage will be are chosen as comparison criteria. Finally, a numerical model based on Finite Element Method approximation was used in order to predict the dielectric response of the composite materials as well as the specific parameters (size, permittivity) of the interphase, the magic “ingredient” of the matrix-filler mix. The presented model allowed us to give a connection between the different materials and validate the experimentally obtained results. This manuscript ends with conclusions on the presented work and suggests possible future works in the complex analysis of polymer nanocomposites.