La première partie de ce travail concerne la théorie des Oscillateurs Paramétriques Optiques (OPO) en régime nanoseconde. On présente tout d'abord des rappels sur les milieux non-linéaires et anisotropes, permettant de caractériser le couplage non-linéaire. Ensuite, les équations paramétriques obtenues par prise en compte ou non de la dispersion des indices et de la divergence du champ électrique sont démontrées. Un état de l'art sur les modèles et sur les techniques d'affinement spectral clôt cette partie. La deuxième partie présente deux schémas numériques d'intégration des équations paramétriques. Le premier, basé sur le principe du " split-step ", consiste à intégrer successivement les équations liées aux effets non-linéaires et à la propagation. Il est possible de tenir compte de la fluorescence paramétrique afin de simuler le démarrage de l'OPO. Le second est basé sur une transformation des équations, permettant ainsi d'intégrer les équations de Schrödinger non-linéaires dérivées par la méthode du point fixe, pour des schémas aux différences finies, avec une discrétisation de type Crank-Nicolson. La troisième partie présente des comparaisons entre les simulations numériques et les réalisations expérimentales d'OPO vérifiant ou non les hypothèses des modèles. Puis, elle introduit le principe (nouveau) d'affinement spectral de sources cohérentes par insertion d'un cristal photoréfractif. Ce principe est appliqué à deux lasers continus, puis deux lasers et un OPO en régime nanoseconde. En régime continu, on observe un affinement du spectre d'émission qui devient monomode, accompagné d'une réduction de la largeur spectrale de la raie d'émission. Concernant le laser impulsionnel, on montre une stabilisation en fréquence d'émission et un affinement de la raie d'émission. Quant aux OPO en régime nanoseconde, on montre un affinement du spectre et un bon accord entre les simulations et les résultats expérimentaux.

Fundação Carlos Chagas Filho de Amparo a Pesquisa do Estado do Rio de Janeiro / As cerâmicas piesoelétricas estudadas neste trabalho (Pb1,03Zr0,53Ti0,47O3, Pb1,03Zr0,525Nb0,05Ti0,465Fe0,005O3, Pb1,03Zr0,515Nb0,015Ti0,465Fe0,005O3, Pb1,03Zr0,525Nb0,005Ti0,455Fe0,015O3 e Pb1,03Zr0,515Nb0,015Ti0,455Fe0,015O3) foram sinterizadas a 1200oC e 1250oC por 3,5 h a fim de que suas propriedades piesoelétricas fossem investigadas. Nas composições dos PZT obtidas, a matriz e os dopantes empregaram óxidos como matérias primas. As misturas dos óxidos precursores foram calcinadas a 850oC por 3,5 h para obtenção da fase PZT. Os precursores, os pós e os corpos de prova de PZT foram caracterizados quanto às microestruturas, densidades e propriedades físicas. Após a conformação dos pós e a sinterização, os materiais cerâmicos foram polarizados para caracterização de suas propriedades piesoelétricas através de um impedancímetro na faixa de freqüência de 100 KHz a 200 KHz. Os resultados de dispersão de laser dos precursores revelaram aglomeração do óxido de chumbo e óxido de zircônio. As composições calcinadas apresentaram tamanho de partícula na faixa de 0,44 μm a 0,63 μm. As análises de densidade por método de Arquimedes indicaram uma boa densificação dos corpos de prova sinterizados e pouca influência da temperatura de sinterização com uma escala de valores de 95,73 a 97,65% da densidade teórica. As análises de microscopia eletrônica de varredura revelaram que os sinterizados contendo concentrações diferentes de dopantes exibem uma correlação do tipo e teor de dopante com a natureza da fratura, sendo transgranular, quando dopante ferro for predominante e intergranular, quando o dopante Nb for predominante. Também, o aumento da temperatura de sinterização resultou em fratura transgranular independente do tipo e da concentração de dopante, exceto para baixo teor de dopante da composição equimolar, cujos resultados não foram consistentes com a literatura sobre o material. No que diz respeito às propriedades piesoelétricas, revelou-se que a combinação da variação da composição com a temperatura foi favorável para o aumento dos valores da constante dielétrica da formulação equimolar com maior percentual de dopantes. Já o efeito da temperatura com a composição surtiu um efeito muito negativo para os valores de fator de qualidade mecânica da formulação dopada com mais ferro. Para os valores de constante de freqüência da formulação com maior percentagem de nióbio, o efeito da temperatura com a composição gerou um efeito positivo. / The piezoelectric ceramics studied in this work, Pb1.03Zr0.53Ti0.47O3, Pb1.03Zr0.525Nb0.05Ti0.465Fe0.005O3, Pb1.03Zr0.515Nb0.015Ti0.465Fe0.005O3, Pb1.03Zr0.525Nb0.005Ti0.455Fe0.015O3 and Pb1.03Zr0.515Nb0.015Ti0.455Fe0.015O3, were all of them sintered between 1200oC and 1250oC for 3.5h. After that, their piezoelectric properties were investigated. In the present work, oxides were used as raw material in both, matrices and dopants. The mixture of the precursor oxides were calcinaned at 850oC during 3.5h for obtaining the PZT phase. The precursor oxides, the powders and the PZT samples went through characterization tests in order to have their microstructures, densities and physical properties correctly determined. After the powders had been conformed and performed the sinterization process, the PZT ceramics were polarized and their piezoelectric properties determined by using an impedancemeter working in the frequency from 100 KHz to 200 KHz. The obtained results from laser dispersion had revealed agglomeration of lead and zirconium oxide. The calcined samples presented particle sizes from 0.44 μm to 0.63 μm. The density analyses using the Archimedes method indicated a good densification of the sintered samples and a weak influence of the sintering temperature on the obtained density values, whose values ranged from 95.73 to 97.65 % of the theoretical density value. Analysis performed using the scanning electron microscopy technique (MEV) revealed that the sintered samples had showed a correlation between the type and concentration of the dopant with their fracture mode, which were transgranular when Fe prevails over the Nb as dopant, and intergranular, when is the Nb that prevails over the Fe as dopant. By the other side, from increasing the sintering temperature resulted transgranular fractures, independently of which type and content of dopant had been used, except for the equimolar case with relatively low content of dopant, whose results were not consistent with the literature related to this material. About the piezoelectric properties, the results had showed that the combination of the dopant composition with the sintering temperature had brought better values of dielectric constant for the equimolar formulation with more content of dopant. Relating to the mechanical quality factor, from the combination of the dopant composition with the sintering temperature had decreased the factor when Fe prevails over Nb and increased the frequency factor when is the Nb that prevails over the Fe.

Made available in DSpace on 2014-12-17T14:55:13Z (GMT). No. of bitstreams: 1 JoemiaLGMM_TESE.pdf: 3140610 bytes, checksum: 8bdf88d29c888d95e74fd24e403e60fc (MD5) Previous issue date: 2013-06-07 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior / Ceramic substrates have been investigated by researchers around the world and has achieved a high interest in the scientific community, because they had high dielectric constants and excellent performance in the structures employed. Such ceramics result in miniaturized structures with dimensions well reduced and high radiation efficiency. In this work, we have used a new ceramic material called lead zinc titanate in the form of Zn0,8Pb0,2TiO3, capable of being used as a dielectric substrate in the construction of various structures of antennas. The method used in constructing the ceramic combustion synthesis was Self- Sustained High Temperature (SHS - "Self-Propagating High-Temperature Synthesis") which is defined as a process that uses highly exothermic reactions to produce various materials. Once initiated the reaction area in the reaction mixture, the heat generated is sufficient to become self-sustaining combustion in the form of a wave that propagates converting the reaction mixture into the product of interest. Were analyzed aspects of the formation of the composite Zn0,8Pb0,2TiO3 by SHS powders and characterized. The analysis consisted of determining the parameters of the reaction for the formation of the composite, as the ignition temperature and reaction mechanisms. The production of composite Zn0,8Pb0,2TiO3 by SHS performed in the laboratory, was the result of a total control of combustion temperature and after obtaining the powder began the development of ceramics. The product was obtained in the form of regular, alternating layers of porous ceramics and was obtained by uniaxial pressing. 10 The product was characterized by analysis of dilatometry, X-ray diffraction analysis and scanning electron microscopy. One of the contributions typically defined in this work is the development of a new dielectric material, nevertheless presented previously in the literature. Therefore, the structures of the antennas presented in this work consisted of new dielectric ceramics based Zn0,8Pb0,2TiO3 usually used as dielectric substrate. The materials produced were characterized in the microwave range. These are dielectrics with high relative permittivity and low loss tangent. The Ansoft HFSS, commercial program employee, using the finite element method, and was used for analysis of antennas studied in this work / Substratos cer?micos t?m sido investigados por pesquisadores de todo o mundo e tem despertado um grande interesse na comunidade cient?fica, pelo fato de apresentarem altas constantes diel?tricas e um excelente desempenho nas estruturas empregadas. Tais cer?micas resultam em estruturas miniaturizadas, com dimens?es bem reduzidas e de alta efici?ncia de radia??o. Neste trabalho, utilizou-se um novo material cer?mico, chamado de titanato de zinco chumbo, na forma de Zn0,8Pb0,2TiO3, capaz de ser utilizado como substrato diel?trico na constru??o de diversas estruturas de antenas. O m?todo utilizado na constru??o da cer?mica foi a S?ntese por Combust?o Auto-Sustentada a Alta Temperatura (SHS Self-Propagating High-Temperature Synthesis ) que ? definido como um processo que utiliza rea??es altamente exot?rmicas para produzir os mais diversos materiais. Uma vez iniciada a rea??o numa ?rea da mistura reagente, o calor gerado ? suficiente para tornar a combust?o auto-sustent?vel, na forma de uma onda que se propaga convertendo a mistura reagente no produto de interesse. Foram analisados os aspectos da forma??o do comp?sito Zn0,8Pb0,2TiO3 por SHS e os p?s obtidos, foram caracterizados. A an?lise consistiu na determina??o dos par?metros da rea??o para a forma??o do comp?sito, como a temperatura de igni??o e os mecanismos de rea??o. A produ??o do comp?sito Zn0,8Pb0,2TiO3 por SHS, realizada em laborat?rio, foi resultado de um controle total da temperatura de combust?o e ap?s a obten??o do p? 8 iniciou-se o desenvolvimento das cer?micas. O produto foi obtido na forma de camadas regulares e altern?ncia de porosidades e as cer?micas foram obtidas por prensagem uniaxial. O produto foi caracterizado atrav?s de an?lises de dilatometria, difra??o de raios-X e an?lise de microscopia de varredura eletr?nica. Uma das contribui??es principais deste trabalho foi o desenvolvimento de um novo material diel?trico, n?o obstante apresentado anteriormente na literatura. Portanto, as estruturas das antenas apresentadas neste trabalho foram constitu?das por novas cer?micas diel?tricas ? base de Zn0,8Pb0,2TiO3, geralmente utilizado como substrato diel?trico. Os materiais desenvolvidos foram caracterizados na faixa de micro-ondas. S?o substratos diel?tricos de alta permissividade relativa e baixa tangente de perda. O software comercial empregado, Ansoft HFSS, baseado no m?todo dos elementos finitos, foi utilizado na an?lise das antenas estudadas neste trabalho

Fundação Carlos Chagas Filho de Amparo a Pesquisa do Estado do Rio de Janeiro / As cerâmicas piesoelétricas estudadas neste trabalho (Pb1,03Zr0,53Ti0,47O3, Pb1,03Zr0,525Nb0,05Ti0,465Fe0,005O3, Pb1,03Zr0,515Nb0,015Ti0,465Fe0,005O3, Pb1,03Zr0,525Nb0,005Ti0,455Fe0,015O3 e Pb1,03Zr0,515Nb0,015Ti0,455Fe0,015O3) foram sinterizadas a 1200oC e 1250oC por 3,5 h a fim de que suas propriedades piesoelétricas fossem investigadas. Nas composições dos PZT obtidas, a matriz e os dopantes empregaram óxidos como matérias primas. As misturas dos óxidos precursores foram calcinadas a 850oC por 3,5 h para obtenção da fase PZT. Os precursores, os pós e os corpos de prova de PZT foram caracterizados quanto às microestruturas, densidades e propriedades físicas. Após a conformação dos pós e a sinterização, os materiais cerâmicos foram polarizados para caracterização de suas propriedades piesoelétricas através de um impedancímetro na faixa de freqüência de 100 KHz a 200 KHz. Os resultados de dispersão de laser dos precursores revelaram aglomeração do óxido de chumbo e óxido de zircônio. As composições calcinadas apresentaram tamanho de partícula na faixa de 0,44 μm a 0,63 μm. As análises de densidade por método de Arquimedes indicaram uma boa densificação dos corpos de prova sinterizados e pouca influência da temperatura de sinterização com uma escala de valores de 95,73 a 97,65% da densidade teórica. As análises de microscopia eletrônica de varredura revelaram que os sinterizados contendo concentrações diferentes de dopantes exibem uma correlação do tipo e teor de dopante com a natureza da fratura, sendo transgranular, quando dopante ferro for predominante e intergranular, quando o dopante Nb for predominante. Também, o aumento da temperatura de sinterização resultou em fratura transgranular independente do tipo e da concentração de dopante, exceto para baixo teor de dopante da composição equimolar, cujos resultados não foram consistentes com a literatura sobre o material. No que diz respeito às propriedades piesoelétricas, revelou-se que a combinação da variação da composição com a temperatura foi favorável para o aumento dos valores da constante dielétrica da formulação equimolar com maior percentual de dopantes. Já o efeito da temperatura com a composição surtiu um efeito muito negativo para os valores de fator de qualidade mecânica da formulação dopada com mais ferro. Para os valores de constante de freqüência da formulação com maior percentagem de nióbio, o efeito da temperatura com a composição gerou um efeito positivo. / The piezoelectric ceramics studied in this work, Pb1.03Zr0.53Ti0.47O3, Pb1.03Zr0.525Nb0.05Ti0.465Fe0.005O3, Pb1.03Zr0.515Nb0.015Ti0.465Fe0.005O3, Pb1.03Zr0.525Nb0.005Ti0.455Fe0.015O3 and Pb1.03Zr0.515Nb0.015Ti0.455Fe0.015O3, were all of them sintered between 1200oC and 1250oC for 3.5h. After that, their piezoelectric properties were investigated. In the present work, oxides were used as raw material in both, matrices and dopants. The mixture of the precursor oxides were calcinaned at 850oC during 3.5h for obtaining the PZT phase. The precursor oxides, the powders and the PZT samples went through characterization tests in order to have their microstructures, densities and physical properties correctly determined. After the powders had been conformed and performed the sinterization process, the PZT ceramics were polarized and their piezoelectric properties determined by using an impedancemeter working in the frequency from 100 KHz to 200 KHz. The obtained results from laser dispersion had revealed agglomeration of lead and zirconium oxide. The calcined samples presented particle sizes from 0.44 μm to 0.63 μm. The density analyses using the Archimedes method indicated a good densification of the sintered samples and a weak influence of the sintering temperature on the obtained density values, whose values ranged from 95.73 to 97.65 % of the theoretical density value. Analysis performed using the scanning electron microscopy technique (MEV) revealed that the sintered samples had showed a correlation between the type and concentration of the dopant with their fracture mode, which were transgranular when Fe prevails over the Nb as dopant, and intergranular, when is the Nb that prevails over the Fe as dopant. By the other side, from increasing the sintering temperature resulted transgranular fractures, independently of which type and content of dopant had been used, except for the equimolar case with relatively low content of dopant, whose results were not consistent with the literature related to this material. About the piezoelectric properties, the results had showed that the combination of the dopant composition with the sintering temperature had brought better values of dielectric constant for the equimolar formulation with more content of dopant. Relating to the mechanical quality factor, from the combination of the dopant composition with the sintering temperature had decreased the factor when Fe prevails over Nb and increased the frequency factor when is the Nb that prevails over the Fe.

O titanato e zirconato de chumbo \'PB\'(\'ZR\'1-y\'TI\'y)\'O IND.3\' é um material ferroelétrico de estrutura perovskita que tem sido aplicado como transdutores, amplificadores, sensores piezoelétricos, piroelétricos e memórias ferroelétricas. É bem estabelecido que a incorporação de íons de \'LA POT.3+\' ou \'BA POT.2+\' nos sítios ocupados pelo \'PB\' no sistema \'PB\'(\'ZR\'1-y\'TI\'y)\'O\' (PZT), formando os sistemas \'PB\'1-x\'LA\'x\'ZR\'1-y\'TI\'y\'O IND.3\' (PLZT) e \'PB\'1-x\'BA\'x\'ZR\'1-y\'TI\'y\'O IND.3\' (PBZT), provoca mudanças significativas nas suas propriedades. No entanto, poucos trabalhos tem sido dedicados a esses sistemas contendo altas concentrações de \'TI\', principalmente no que se refere à estrutura desses materiais. Assim, este trabalho teve por objetivo analisar as propriedades estruturais e suas correlações com as propriedades dielétricas dos sistemas \'PB\'1-x\'LA\'x\'ZR\'0,40\'TI\'0,60\'O IND.3\' (PLZT100x) e \'PB\'1-x\'BA\'x\'ZR\'0,40\'TI\'0,60\'O IND.3\' (PBZT100x) em função da composição e da temperatura. Foram preparadas amostras cerâmicas por meio de sinterização convencional com x variando entre 0,05 e 0,21 para o sistema PLZT e entre 0,10 e 0,50 para o sistema PBZT. Em relação à estrutura a longa distância, medidas de difração de raios X mostraram uma diminuição no grau de tetragonalidade com o aumento da concentração dos cátions substituintes, que foi atribuída à formação de vacâncias no sítio A (caso do \'LA\') e diferença entre o raio iônico (caso do \'BA\'). Estas alterações estruturais em função da composição foram também responsáveis pelo aumento do grau de difusidade das curvas de permissividade dielétrica e pela observação de um estado ferroelétrico relaxor nas amostras contendo altas concentrações de \'LA\' e \'BA\'. Em relação à estrutural local, os resultados obtidos através da técnica de espectroscopia de absorção de raios X (XAS) nas bordas \'K\' do \'TI\' e LIII do \'PB\' mostraram que a incorporação de átomos de \'LA\' ou \'BA\' à estrutura do PZT leva a uma redução no deslocamento do átomo de \'TI\' em relação ao centro do octaedro \'TI\'O IND.6\' e mudanças na ordem local do átomo de \'PB\'. No que tange as composições contendo 21% at. de \'LA\' e 50% at. de \'BA\', diferentemente dos resultados de DRX que mostraram uma simetria cúbica, a técnica de XAS mostrou uma simetria local tetragonal. Em bom acordo com os resultados obtidos pela técnica de espectroscopia Raman, espectros EXAFS medidos em altas temperaturas mostraram também que a estrutura local não é compatível com uma estrutura de simetria cúbica. Espectros XANES medidos na borda \'K\' do oxigênio revelaram uma redução no grau de hibridização entre os estados 2p do \'O\' com 6sp do \'PB\' à medida que a concentração de \'LA\' ou \'BA\' aumenta, que estaria relacionada com o surgimento de comportamento relaxor. Amostras cerâmicas densas nanoestruturadas de composição PZT, PLZT11 e PBZT10 foram preparadas pelo método de spark plasma sintering (SPS) a fim de analisar a influência do tamanho de grão. Foi verificado que as amostras sinterizadas por SPS apresentam tamanho de grão em torno de 60 nm. A caracterização dielétrica destas amostras mostra que a redução do tamanho de grão causa uma redução no valor de máximo da permissividade dielétrica e características difusas da permissividade em função da temperatura devido ao aumento das regiões de contorno de grão. / Lead titanate zirconate (\'PB\'(\'ZR\'1-x\'TI\'x)\'O IND.3\') are ferroelectric materials with perovskite structure which has been used as transducers, capacitors, piezoelectric and pyroelectric sensors and ferroelectric memories. The substitution of \'PB POT.+2\' ions by \'LA POT.+3\' or \'BA POT.+2\' ions in the \'PB\'(\'ZR\'1-x\'TI\'x)\'O IND.3\' (PZT) system, which leads to the formation of the \'PB\'1-x\'LA\'x\'ZR\'1-y\'TI\'y\'O IND.3\' (PLZT) and the \'PB\'1-x\'BA\'x\'ZR\'1-y\'TI\'y\'O IND.3\' (PBZT) systems, induces several changes in the electric and structural properties of these materials. However, PLZT or PBZT systems based on \'TI\'-rich compositions have not been thoroughly investigated and the literature contains few reports concerning their structure. Thus, the main objectives of this doctoral thesis were the synthesis and structural characterization of \'PB\'1-x\'R\'x\'ZR\'0.40\'TI\'0.60\'O IND.3\' ferroelectric ceramic samples, with R = \'BA\' and \'LA\' and x between 0.00 to 0.50 (PLZT100x and PBZT100x). The characterization with X-ray diffraction technique of these samples showed a decrease of the tetragonality degree with increase of the doping cation concentration, which was related to the appearance of defects caused by the incorporation of \'LA\' or \'BA\' cations. These structural modifications were also responsible by the increase of the diffuseness at the dielectric permittivity and a relaxor behavior as a function of the \'LA\' or \'BA\' concentration. Concerning the local structure, XANES spectra in the absorption edge of various elements in PLZT and PBZT samples were performed. In the cases of \'TI\' \'K\'-edge absorption, the doping of \'LA\' and \'BA\' atoms in the PZT structure leads to a reduction of the displacement of \'TI\' atom in the center of the \'TI\'O IND.6\' octahedron. However, even when the crystal structure is cubic, a local octahedron distortion remains. EXAFS measurements in \'PB\' LIII-edge and \'ZR\' \'K\'-edge were performed and also indicate that local structure around lead or zirconium atoms is also affected by the introduction of \'LA\' and \'BA\' atoms in the PZT structure. In addition, XANES spectra measured at \'O\' \'K\'-edge revealed a reduction in the hybridization degree between \'O\' 2p and \'PB\' 6sp states with the addition of \'LA\' or \'BA\' atoms to the structure of PZT. It has been shown that hybridization between these states is essential to ferroelectricity and this reduction would be related to the relaxor behavior. PLZT and PBZT systems were also studied depending on the size of particle size in a nanometer scale. Thus samples PZT, PLZT11 and PBZT10 compositions were prepared using the synthesis method of precursor polymers and the process of sintering by spark plasma. A pronounced decrease in the values of maximum permittivity was observed and the dielectric curve as a function of the temperature exhibits a diffuse behavior. This size-induced diffuse phase transition and the reduction of the permittivity magnitude could be related to the differences between the core grain and the grain boundaries.

The present work aimed to laser sintering and physical characterization of barium titanate ceramics doped with lanthanum, Ba1-xLaxTiO3 (0.1 ≤ x ≤ 0.4) mol%, and co-doped with manganese, in order to study the PTC effect (Positive Temperature Coefficient of Resistance). In electrical and electronic equipments, materials that exhibit PTC effect are generally used in circuit protection, or as temperature sensors (thermistors), whose main advantages of thermistors are no contacts subject to corrosion, high resistance to mechanical shock, the long operating life, low cost and more stable than thermocouples. Additionally, the laser sintering process uses a CO2 laser as the main heating source. This process has been touted as a promising technology for the ceramic processing, which are subjected to extremely high heating and cooling rates (~ 2000 ° C / min). During the processes of synthesis and characterization Differential Thermal Analysis (DTA), Thermogravimetry (TG), Dilatometry, Differential Scanning Calorimetry, X-ray Diffraction, Scanning Electron Microscopy, Impedance Spectroscopy and Conductive Microscopy Atomic Force (CAFM) technique were used. Lanthanum doped barium titanate powders were produced via solid state reaction method and calcined at 1200 ° C for 4 hours. The laser sintering process was efficient to obtain ceramics with relative density of up to 95%. The sintered ceramics presented homogeny microstructure surface under the condition of Pmax = 5.5 W/mm2 for 60 seconds. The 02BT La sample showed the lowest room temperature resistivity (104 .cm). This value is roughly three orders of magnitude lower than that observed for conventionally sintered ceramic in electric furnace and with the same dopant concentration. Finally, it is clear that the sintering conditions strongly modify the PTC behavior of the like-BaTiO3 ceramics, thus allowing the fabrication of ceramic components for the thermistors manufacture with different characteristics. / O presente trabalho teve como objetivos a sinterização a laser e a caracterização físicas de cerâmicas de titanato de bário dopadas com lantânio, Ba1-xLaxTiO3 (0,1 ≤ X ≤ 0,4) mol%, e co-dopadas com manganês, visando estudar o efeito PTC (Coeficiente positivo de resistência com a elevação da temperatura). Em equipamentos eletro-eletrônicos os materiais que exibem o efeito PTC geralmente são utilizados na proteção de circuitos ou como sensores de temperatura (termistores), cujas principais vantagens dos termistores são a inexistência de contatos sujeitos à corrosão, a alta resistência a choques mecânicos, a longa vida de operação, baixo custo, e são mais estáveis que os termopares. Adicionalmente, o processo de sinterização a laser utiliza como principal fonte de calor um laser de CO2. Este processo tem sido apontado como uma técnica promissora no processamento de corpos cerâmicos, os quais são submetidos a taxas de aquecimento e resfriamento extremamente elevadas (~2000 °C/min), podendo provocar assim alterações em suas propriedades físicas. Durante os processos de síntese e caracterização fez-se uso das técnicas de Diferencial de Análise Térmica (DTA), Termogravimétria (TG), Dilatometria, Calorimétria Exploratória Diferencial, Difração de raios X, Microscopia Eletrônica de Varredura e Espectroscopia de Impedância e de Microscopia de Força Atômica em modo condutivo (CAFM). Pós de titanato de bário dopados com lantânio foram sintetizados pelo método da reação de estado sólido e calcinados a 1200 oC por 4 horas. O processo e sinterização a laser se mostrou eficiente para obtenção de cerâmicas com densidade relativa de até 95%. Foi obtida uma homogeneidade na microestrutura em toda a superfície da cerâmica, sob a condição de Pmax = 5,5 W/mm2 por 60s. A amostra 02BT:La apresentou menor valor de resistividade à temperatura ambiente, da ordem de 104 .cm. Este valor é aproximadamente três ordens de grandeza menor que o observado para a cerâmica sinterizada convencionalmente em forno elétrico e na mesma concentração de dopante. Finalmente, é possível afirmar que as condições de sinterização modificam fortemente o comportamento PTC em cerâmicas a base de BaTiO3, possibilitando dessa forma a confecção de cerâmicas para fabricação de componentes termistores com diferentes características.

Submitted by Luciana Ferreira (lucgeral@gmail.com) on 2015-03-30T13:38:33Z No. of bitstreams: 3 1_pdfsam_Tese - Michelly Patrícia Santana de Almeida Fógia - 2014 - (1).pdf: 19290836 bytes, checksum: 2968ddb5cb0f3df6082b9b4716172cff (MD5) 109_pdfsam_Tese - Michelly Patrícia Santana de Almeida Fógia - 2014 - (2).pdf: 5395221 bytes, checksum: 4db4ad13c96adee2667748bc5321c48f (MD5) license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) / Approved for entry into archive by Luciana Ferreira (lucgeral@gmail.com) on 2015-03-30T15:36:48Z (GMT) No. of bitstreams: 3 1_pdfsam_Tese - Michelly Patrícia Santana de Almeida Fógia - 2014 - (1).pdf: 19290836 bytes, checksum: 2968ddb5cb0f3df6082b9b4716172cff (MD5) 109_pdfsam_Tese - Michelly Patrícia Santana de Almeida Fógia - 2014 - (2).pdf: 5395221 bytes, checksum: 4db4ad13c96adee2667748bc5321c48f (MD5) license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) / Made available in DSpace on 2015-03-30T15:36:48Z (GMT). No. of bitstreams: 3 1_pdfsam_Tese - Michelly Patrícia Santana de Almeida Fógia - 2014 - (1).pdf: 19290836 bytes, checksum: 2968ddb5cb0f3df6082b9b4716172cff (MD5) 109_pdfsam_Tese - Michelly Patrícia Santana de Almeida Fógia - 2014 - (2).pdf: 5395221 bytes, checksum: 4db4ad13c96adee2667748bc5321c48f (MD5) license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) Previous issue date: 2014-09-24 / In this work the structural and optical properties of SiO2-ZnO-TiO2 system doped with of Eu3+ (1% in mol) or Er3+ ions (0,2% to 2,0% in mol) were investigated. The compositions xSiO2-([100-x]/2)ZnO-([100-x]/2)TiO2 with x ranging from 40 to 80 were synthesized and studied. A mixed methodology using the traditional sol-gel process and Pechini method was employed to prepare stable and transparent sols, homogeneous gels, powders and transparent thin films without cracks. The prepared powders were heat treated from 100°C to 1100°C. The optical “band gap” values, the lifetime of Eu3+ 5D0 level, the R/O ratio of Eu3+ emissions and refractive index are dependent on the composition and thermal treatment of the powders. Multilayered thin films were deposited onto silica substrates using the “spin coating” technique. Afterwards, the as-prepared thin films were calcined at 500°C followed by annealings from 700°C to 1000°C. The X-ray diffraction of powders showed that the crystallization starts at 800°C with a mixture of zinc titanate phases (ZnTiO3, Zn2TiO4) and titanium rutile (TiO2). In compositions with low silica content (x = 40 or 50) the formation of zinc silicate (Zn2SiO4) occurs at high temperatures. The optical properties were evaluated for both powders and thin films. Particularly, thin films exhibit transmittance above 80% in the visible region. The refractive index at 632.8 nm and thickness of thin films were estimated using the envelope method and m-Line spectrocopy. Furthermore, morphological analysis of the thin films surface was performed by scanning electron microscopy. The characteristic emission peaks of Er3+ or Eu3+ doped powders and thin films were measured. The photoluminescent emission of Eu3+ ions, obtained the highest intensities for the powders treated at 800°C, for films treated at 700°C, under excitation at 394 nm, where the system remains primarily amorphous. The photoluminescent emission of the Er3+ ions was more intense for the powders treated at 1100°C under excitation at 980 nm, mainly due to reduction of hydroxyl groups which act as photoluminescence quenchers. In the film the highest intensity occurred at a temperature of 700°C under excitation at 378 nm and showing only one guided mode. Thus, the obtained thin films can be suitable for application as waveguides in integrated optical systems. Moreover, the Eu and Er doped powders can be used as phosphors in displays. / Este trabalho avalia as propriedades estruturais e ópticas do sistema ternário SiO2-ZnO-TiO2 dopado com íons Eu3+ (1% em mol) ou Er3+ (0,2% a 2,0% em mol). As composições xSiO2-([100-x]/2)ZnO-([100-x]/2)TiO2, com x = 40, 50, 60, 70 e 80 foram estudadas. Através de metodologia mista do processo sol-gel com o método Pechini foi possível obter sóis estáveis e transparentes, géis homogêneos, pós tratados termicamente de 100ºC a 1100°C e filmes finos transparentes e sem trincas. Filmes finos multicamadas foram depositados sobre substratos de sílica usando “spin coating” e calcinados a 500°C, seguido de um tratamento térmico entre 700°C e 1000°C. Por fotoluminescência foram observadas as transições de emissão características de íons Er3+ e Eu3+. A caracterização por difração de raios X dos pós mostrou que a cristalização tem início a partir de 800°C, com uma mistura de fases de titanatos de zinco (ZnTiO3, Zn2TiO4) e segregação de titânio rutilo (TiO2). Em composições com menores proporções de sílica (x= 40 ou 50) também ocorreu a formação de silicato de zinco (Zn2SiO4) em temperaturas mais elevadas. Os espectros de transmissão dos filmes apresentaram transmitância superior a 80%. Os valores de “band gap” óptico, o tempo de vida, as razões R/O e o índice de refração são dependentes do tratamento térmico e da composição dos pós. Usando o método da envoltória, pôde-se calcular o índice de refração a 632,8 nm e a espessura dos filmes finos, sendo que esta também foi estimada por microscopia eletrônica de varredura (MEV) e espectroscopia m-Line. A emissão fotoluminescente dos íons Eu3+ tem maior intensidade nos pós tratados à 800°C e nos filmes tratados à 700°C, sob excitação a 394 nm. Nestas temperaturas o sistema permanece predominantemente amorfo. A emissão fotoluminescente dos íons Er3+ foi mais intensa para os pós tratados a 1100°C, sob excitação a 980 nm, devido à eliminação de grupos OH supressores das emissões de Er3+. Nos filmes a maior intensidade se deu à temperatura de 700°C, sob excitação a 378 nm. Os filmes apresentaram apenas um modo guiado, sendo o mais apropriado para uso como guia de onda e os pós são indicados como fósforos podendo ser aplicados em “displays".

The temperature and/or entropy of electrically polarisable materials can be altered by changing electric field E. Research into this electrocaloric (EC) effect has focussed on increasing the size of the EC effects, with the long-term aim of building a cooler with an EC material at its heart. Materials and experimental methods are briefly reviewed. A ‘resetting’ indirect route to isothermal entropy change ∆S for hysteretic first-order transitions is described. An indirect route to adiabatic temperature change ∆T, without the need for field-resolved heat capacity data, is also described. Three temperature controllers were built: a cryogenic probe for 77-420 K with ∼5 mK resolution, a high-temperature stage with vacuum enclosure for 295-700 K with ∼15 mK resolution, and a low-temperature stage for 120-400 K with electrical access via micropositioners. Automation enables dense datasets to be compiled. Single crystals of inorganic salts (NH4)2SO4 , KNO3 and NaNO2 were obtained. Applying 380 kV cm−1 across (NH4)2SO4 , it was found that |∆S| ∼ 20 J K−1 kg−1 and |∆T | ∼ 4 K, using the indirect method near the Curie temperature TC = 223 K. Without the ‘resetting’ indirect method, |∆S| ∼ 45 J K−1 kg−1 would have been spuriously found. Preliminary indirect measurements on KNO3 and NaNO2 give |∆S| ∼ 75 J K−1 kg−1 for ∆E ∼ 31 kV cm−1 near TC = 400 K and |∆S| ∼ 14 J K−1 kg−1 for ∆E ∼ 15 kV cm−1 near TC = 435 K, respectively. A cation-ordered PbSc0.5Ta0.5O3 ceramic showing a nominally first-order transition at 295 K was obtained. The Clausius-Clapeyron phase diagram is revealed via indirect measurements where |∆S| ∼ 3.25 J K−1 kg−1 and |∆T | ∼ 2 K, and direct measurements where |∆T | ∼ 2 K. Clamped samples show broadening of the field-induced transition. Epitaxial, ∼64 nm-thick SrTiO3 films were grown by pulsed laser deposition on NdGaO3 (001) substrates with a La0.67Sr0.33MnO3 bottom electrode. The indirect method gives |∆S| ∼ 8 J K−1 kg−1 and |∆T | ∼ 3.5 K near 180 K with |∆E| = 780 kV cm−1. Finite element modelling (FEM) was used to optimise the geometry of multilayered capacitors (MLCs) for EC cooling. Intrinsic cooling powers of 25.9 kW kg−1 are predicted for an optimised MLC based on PVDF-TrFE with Ag electrodes.

Ceramics and polymer-ceramic composites associated with high dielectric constants are of both scientific and industrial interest as these could be used in devices such as capacitors, resonators and filters. High dielectric constant facilitates smaller capacitive components, thus offering the opportunity to miniaturize the electronic devices. Hence there is a continued interest on high dielectric constant materials over a wide range of temperatures. Recently, CaCu3Ti4O12 (CCTO) ceramic which has centro-symmetric body centered cubic structure has attracted considerable attention due to its large dielectric constant (ε ~104-105) which is nearly independent of frequency (upto 10 MHz) and low thermal coefficient of permittivity (TCK) over 100-600K temperature range. Apart from the high dielectric ceramics, high dielectric polymer-ceramic composites have also become promising materials for capacitor applications. By combining the advantages of high dielectric ceramics and low leakage behaviour of polymers, one can fabricate new hybrid materials with high dielectric constants, and high breakdown field to achieve high volume efficiency and energy storage density for capacitor applications. The CCTO polycrystalline powders were generally prepared by the conventional solid-solid reaction route with CaCO3, TiO2 and CuO as the starting materials. This method of preparation often requires high temperatures and longer durations. To overcome these difficulties, in the present investigations, an attempt has been made to synthesize CCTO by adopting microwave assisted heating technique and wet chemical synthesis routes. Also the CCTO crystallites (size varying from nano to micrometers) incorporated in the Polyvinyliden fluoride (PVDF) and Polyaniline (PANI) matrix and several composites with high dielectric constants were fabricated and investigated. Further, the high dielectric constant glasses in the system (100-x)TeO2-xCaCu3Ti4O12, (x=0.5 to 3) were fabricated by the conventional melt-quenching technique and their structural and dielectric properties were studied. The results obtained pertaining to these aforementioned investigations are classified as follows. Chapter 1 is intended to give basic information pertaining to the dielectrics and various mechanisms associated with high dielectric constants. Brief exposure to the high dielectric constant materials is also given. The structural aspects of CCTO, various synthetic routes adopted for the synthesis and the origin of the dielectric anomaly in CCTO are elaborated. In addition, basic information about the high dielectric polymer-ceramic composites and glasses are provided. In chapter 2 the various experimental techniques that were employed to synthesize and characterize the materials under investigation were discussed. Chapter 3 reports the synthesis and characterization of CaCu3Ti4O12, (CCTO) powders by microwave assisted heating at 2.45 GHz, 1.1kW. The processing and sintering were carried out at different temperatures for varied durations. The optimum calcination temperature using microwave heating was found to be 950oC for 20 minutes to obtain cubic CCTO powders. This is found to be fast and energy efficient as compared to that of the conventional methods. The structure, morphology and dielectric properties of the CCTO ceramic processed by microwave assisted heating were studied via X-ray diffraction, Scanning electron microscopy (SEM) and impedance analyser. These studies revealed that, the microwave sintered (MS) samples were less porous than that of the conventional ones. Relative density of about 95% was achieved for the MS pellets (1000oC/60min) while for the conventional sintered (CS) pellets (1100oC/2h) it was only 91%. The dielectric constants for the microwave sintered (1000oC/60min) ceramics were found to vary from 11000 to 6950 in the 100 Hz to 100 kHz frequency range. The presence of larger grains (6-10μm) in the MS samples contributed to the higher dielectric constants. Chapter 4 deals with the synthesis of complex oxalate precursor, CaCu3(TiO)4(C2O4)8 • 9H2O, by the wet chemical route. The various trials and the different reaction schemes involved for the preparation of complex oxalate precursor were highlighted. The oxalate precipitate thus obtained was characterized by the wet chemical analyses, X-ray diffraction, FTIR absorption and TG/DTA analyses. The complex oxalate precursor, CaCu3(TiO)4(C2O4 )8.9H2O was subjected to thermal oxidative decomposition and the products of thermal decomposition were investigated employing XRD,TGA, DTA and FTIR techniques. Nanocrystallites of CaCu3Ti4O12 with the size varying from 30-200 nm were obtained at a temperature as low as 680oC. The nanocrystallites of CaCu3Ti4O12 were characterized using Electron Spin Resonance (ESR) and optical reflectance techniques. The selected area electron diffraction (SAED) pattern with the zone axis [012] and spot pattern in electron diffraction (ED) indicate their single-crystalline nature. The optical reflectance and ESR spectra indicate that the Cu (II) coordination changes from distorted octahedra to nearly flattened tetrahedra (squashed) to square planar geometry with increasing heat treatment temperature. The powders derived from the oxalate precursor have excellent sinterability resulting in high density ceramics which exhibited giant dielectric constants upto 40,000 (1 kHz) at 25oC, accompanied by low dielectric loss < 0.07. The effect of calcium content on the dielectric properties of CaxCu3Ti4O12 (x=0.90, 0.97, 1.0, 1.1 and 1.15) derived from the oxalate route was described in Chapter 5. The structural, morphological and dielectric properties of the ceramics were studied using X-ray diffraction, Scanning Electron Microscope along with Energy Dispersive X-ray Analysis (EDX), and Impedance analyzer. The X-ray diffraction patterns obtained for the x= 0.97, 1.0 and 1.1 ceramics could be indexed to a body– centered cubic perovskite related structure associated with the space group Im3. The microstructural studies revealed that the grains are surrounded by exfoliated sheets of Cu-rich phase. The microstructure that is evolved for the Ca0.97 ceramic more or less resembles that of the Ca1.0 ceramic, but the density of such exfoliated sheets of cu-rich phase is lesser for the Ca0.97 ceramic and none for Ca1.1 ceramic. The sintered pellet (x=0.97) was ground and thinned to the required thickness (~ 20nm) and analyzed using Transmission Electron Microscopy (TEM). The current-voltage (I-V) characteristics of the ceramics exhibited non-linear behaviour. The dielectric properties of these suggest that the sample corresponding to the composition x=0.97, has a reduced dielectric loss while retaining its high dielectric constant. Chapter 6 illustrates the results concerning the fabrication and characterization of nanocrystal composites of Polyaniline (PANI) and CaCu3Ti4O12 (CCTO). These were prepared using a simple procedure involving in-situ polymerization of aniline in dil. HCl. The PANI and the PANI-CCTO composites were subjected to X-ray diffraction, Fourier Transform Infrared (FTIR), Thermo gravimetric, Scanning Electron Microscopic (SEM) and Transmission electron microscopic analyses. The FTIR spectra recorded for the composites was similar to that of pure PANI unlike in the case of X-ray diffraction wherein the characteristics of both PANI and CCTO were reflected. The TGA in essence indicated the composites to have better thermal stability than that of pure PANI. The composite corresponding to 50%CCTO-50%PANI exhibited higher dielectric constant (4.6x106 @100Hz). The presence of the nano crystallites of CCTO embedded in the nanofibers of PANI matrix was established by TEM. The AC conductivity increased slightly upto 2kHz as the CCTO content increased in the PANI which was attributed to the polarization of the charge carriers. The value of dielectric constant obtained was higher than that of the other PANI based composites reported in the literature. Chapter 7 deals with the fabrication and characterization of diphasic Poly(vinylidene fluoride) (PVDF)-CCTO composite. The CCTO crystallites (size varying from nano to micrometers) incorporated in the Polyvinylidene fluoride (PVDF) and composites with varying CCTO content were fabricated. The structural, morphological and dielectric properties of the composites were studied using X-ray diffraction, Thermal analysis, Scanning Electron Microscope (SEM), Transmission Electron Microscopic (TEM) and Impedance analyzer. The room temperature dielectric constant as high as 95 at 100Hz has been realized for the composite with 0.55 Vol.fraction of CCTO (micro sized crystallites), which has increased to about 190 at 150oC. Whereas, the PVDF/CCTO nanocrystal composite with 0.13Vol.fraction of CCTO has exhibited higher room temperature dielectric constant (90 at 100Hz). The PVDF/CCTO nanocrystal composite was further investigated for the breakdown strength and electric modulus. The breakdown strength plotted against the dielectric constant evidenced an inverse relationship of breakdown voltage with the dielectric constant. The relaxation processes associated with these composites were attributed to the interfacial polarization or Maxwell-Wagner-Sillars (MWS) effect. Various theoretical models were employed to rationalize the dielectric behavior of these composites. The fabrication and characterization details of optically clear colored glasses in the system (100-x)TeO2-xCaCu3Ti4O12, (x=0.5 to 3 mol%) are reported in Chapter 8. The color varies from olive green to brown as the CaCu3Ti4O12 (CCTO) content increased in TeO2 matrix. The X-ray powder diffraction and differential scanning calorimetric analyses that were carried out on the as-quenched samples confirmed their amorphous and glassy nature respectively. The optical transmittance of the glasses exhibited typical band-pass filter characteristics. The dielectric constant and loss in the 100 Hz-1MHz frequency range were monitored as a function of temperature (323K673K). The dielectric constant and the loss increased as the CCTO content increased in TeO2 at all the frequencies and temperatures under study. Further, the dielectric constant and the loss were found to be frequency independent in the 323-473 K temperature range. The value obtained for the loss at 1MHz was 0.0019 which was typical of low loss materials, and exhibited near constant loss (NCL) contribution to the ac conductivity in the 100Hz-1MHz frequency range. The electrical relaxation was rationalized using the electrical modulus formalism. These glasses are found to be more stable (a feature which may be of considerable interest) as substrates for high frequency circuit elements in conventional semiconductor industries. Thesis ends with summary and conclusions, though each chapter is provided with conclusions and complete list of references.

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