A study has been made of the properties of thin yttrium oxide dielectric films prepared by the electron beam evaporation of high purity Y₂O₃ powder. Films deposited on freshly cleaved NaCl crystals and on polished n-type silicon were examined in the electron microscope. The specimens were found to be polycrystalline, with a crystal size of the order of 100 Å. The structure was found to be essentially the same as found for bulk Y₂O₃. D.C. conduction measurements were made on films of various thicknesses. The characteristics were found to be bulk-limited, with the conductivity decreasing at lower pressures. An activation energy of 0.6 eV was found. The conduction mechanism was believed to be Poole-Frenkel emission of electrons from donor centers into the Y₂O₃ conduction band. The donor centers were believed to be interstitial yttrium atoms rather than oxygen vacancies because of the pressure dependence observed in conductivity. Step response measurements were made, and the results explained on the basis of a loss peak with a most probable relaxation time of 200 seconds. The relaxation of oxygen atoms dissolved in the anion defective Y₂O₃ lattice was assumed to be the mechanism. The results of step response and A.C. bridge loss measurements indicated that different relaxation mechanisms are dominant in different frequency ranges. Internal photoemission measurements were made on Al-Y₂O₃-Al sandwiches. The energy barrier between the electrodes was found to be trapezoidal, with barrier heights of 3.14 and 3.72 eV. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

Devido a crescente busca por fontes de energia que utilizem combustíveis renováveis, a utilização do biogás (mistura de gases produzida durante a digestão de matéria orgânica) proveniente do tratamento anaeróbio de águas residuárias e esgotos, surge como um processo promissor para a produção de gás de síntese (mistura de H2 e CO), contribuindo para a geração de produtos de maior valor agregado e para o aproveitamento de rejeitos industriais e domésticos. O biogás normalmente é composto de 60-65% de CH4 e 30-35% de CO2 e, de acordo com a composição do biogás, pode-se combinar processos de reforma do metano com CO2 e reforma a vapor do metano de modo a maximizar o consumo do CH4 excedente presente no biogás, para a geração de gás síntese. Catalisadores baratos e eficientes devem ser desenvolvidos para estas aplicações. Este trabalho tem como objetivo o estudo das melhores combinações destas reações em função da composição de alimentação do reator, aliado ao desenvolvimento de catalisadores de níquel, ítrio e alumínio, buscando minimizar a deposição de carbono, que é o principal problema encontrado nestes processos. Os catalisadores foram preparados pela técnica de coprecipitação de óxido de ítrio e alumínio, no qual se impregnou o óxido de níquel. Os catalisadores foram caracterizados por diversas técnicas: EDX, método B.E.T., DRX, RTP, XAS, além dos ensaios catalíticos para as reações de reforma do metano: com CO2, a vapor e oxidativa. Os resultados mostraram que é possível a preparação de catalisadores de níquel suportado em mistura de Y2O3-Al2O3, e que eles são ativos para as reações de reforma do metano. / Due to the considerable growth in the demand for energy sources that use renewable fuels, the use of biogas (a mixture of gases produced during digestion of organic matter) from the anaerobic treatment of wastewater and sewage, appears as a promising process for the production of synthesis gas (mixture of H2 and CO), contributing to the generation of products with higher value and the use of industrial wastes and domestic. Biogas is typically composed of 60-65% of CH4 and 30-35% of CO2 and, according to the composition of biogas, it can combine the reform processes of methane with CO2 and steam reforming of methane to maximize the consumption of CH4 excess present in the biogas to the production of the synthesis gas. Cheap and efficient catalysts must be developed for these applications. This paper aims to study the best combinations of these reactions depending on the feed composition of the reactor, coupled with the development of catalysts of nickel, yttrium and aluminum in order to minimize the carbon deposition, which is the main problem in these processes. The catalysts were prepared by the technique of co-precipitation of yttrium oxide and aluminum, which is impregnated nickel oxide. The catalysts were characterized by different techniques: EDX, BET method, XRD, TPR, XAS, and catalytic tests for methane reforming reactions: CO2, steam and oxidative. The results showed that it is possible to prepare nickel catalysts supported on mixed Y2O3-Al2O3, and they are active for the methane reactions.

Devido a crescente busca por fontes de energia que utilizem combustíveis renováveis, a utilização do biogás (mistura de gases produzida durante a digestão de matéria orgânica) proveniente do tratamento anaeróbio de águas residuárias e esgotos, surge como um processo promissor para a produção de gás de síntese (mistura de H2 e CO), contribuindo para a geração de produtos de maior valor agregado e para o aproveitamento de rejeitos industriais e domésticos. O biogás normalmente é composto de 60-65% de CH4 e 30-35% de CO2 e, de acordo com a composição do biogás, pode-se combinar processos de reforma do metano com CO2 e reforma a vapor do metano de modo a maximizar o consumo do CH4 excedente presente no biogás, para a geração de gás síntese. Catalisadores baratos e eficientes devem ser desenvolvidos para estas aplicações. Este trabalho tem como objetivo o estudo das melhores combinações destas reações em função da composição de alimentação do reator, aliado ao desenvolvimento de catalisadores de níquel, ítrio e alumínio, buscando minimizar a deposição de carbono, que é o principal problema encontrado nestes processos. Os catalisadores foram preparados pela técnica de coprecipitação de óxido de ítrio e alumínio, no qual se impregnou o óxido de níquel. Os catalisadores foram caracterizados por diversas técnicas: EDX, método B.E.T., DRX, RTP, XAS, além dos ensaios catalíticos para as reações de reforma do metano: com CO2, a vapor e oxidativa. Os resultados mostraram que é possível a preparação de catalisadores de níquel suportado em mistura de Y2O3-Al2O3, e que eles são ativos para as reações de reforma do metano. / Due to the considerable growth in the demand for energy sources that use renewable fuels, the use of biogas (a mixture of gases produced during digestion of organic matter) from the anaerobic treatment of wastewater and sewage, appears as a promising process for the production of synthesis gas (mixture of H2 and CO), contributing to the generation of products with higher value and the use of industrial wastes and domestic. Biogas is typically composed of 60-65% of CH4 and 30-35% of CO2 and, according to the composition of biogas, it can combine the reform processes of methane with CO2 and steam reforming of methane to maximize the consumption of CH4 excess present in the biogas to the production of the synthesis gas. Cheap and efficient catalysts must be developed for these applications. This paper aims to study the best combinations of these reactions depending on the feed composition of the reactor, coupled with the development of catalysts of nickel, yttrium and aluminum in order to minimize the carbon deposition, which is the main problem in these processes. The catalysts were prepared by the technique of co-precipitation of yttrium oxide and aluminum, which is impregnated nickel oxide. The catalysts were characterized by different techniques: EDX, BET method, XRD, TPR, XAS, and catalytic tests for methane reforming reactions: CO2, steam and oxidative. The results showed that it is possible to prepare nickel catalysts supported on mixed Y2O3-Al2O3, and they are active for the methane reactions.

No description available.

Materials with length scales in the nanometer regime demonstrate properties that are remarkably different from analogous bulk matter. As a result, researchers are striving to catalog the changes in properties that occur with decreasing size, and more importantly, understand the reason behind novel nanomaterial properties. By learning the true nature of nanomaterials, scientists and engineers can design better materials for a variety of applications. Inert gas-phase condensation synthesis of metal oxide nanoparticles was used to develop materials to explore the optical and chemical properties of metal oxide nanoparticles. One potential application for nanomaterials is use in optical applications. The possibility of interparticle energy transfer was investigated for lanthanide-doped yttrium oxide nanoparticles using laser spectroscopy. Experimental evidence collected with this study indicates that interparticle, lanthanide-mediated energy transfer may have been observed. In addition, lanthanide-doped gadolinium oxide nanoparticles were synthesized and investigated with optical spectroscopy to identify the best potential candidates for bioanalytical applications of this material. The influence of particle annealing and dopant concentration were also studied. Nanoparticle film structure was investigated with scanning electron microscopy. Two different film structures composed of oxide nanoparticles were found to grow under different synthesis conditions. The film structure was found to be determined by the degree of particle aggregation in the gas phase during synthesis. Aggregation of the particles was found to be controlled by a combination of gas pressure and properties. Chemical properties of metal oxide nanoparticles also are very important. Reflection-absorption Infrared Spectroscopy and vacuum surface analytical techniques were used to explore the chemistry of the chemical warfare agent dimethyl methylphosphonate (DMMP) on yttrium oxide as well as other metal oxide nanoparticles. DMMP was found to dissociate at room temperature on several types of metal oxide nanoparticles. Hydroxyl groups were found to be critical for the adsorption of DMMP onto the particles. Finally, the reactivity of the nanoparticles was found to increase with decreasing particle size. This was attributed to a relative increase in the number of high-energy surface defects for the smaller particles. / Ph. D.

Conselho Nacional de Pesquisa e Desenvolvimento Científico e Tecnológico - CNPq / In recent years, the use of synthesis routes with low environmental impact has emerged as a differential and is widely proposed by the scientific community for the production of various materials. In this scenario, modified sol-gel processes have gained evidence for using alternative agents in place of the metal alkoxides employed in the conventional method. The route of synthesis proposed in this work is one of the slopes of the modified sol-gel, as it explores the complexing capacity of the natural organic matter (NOM) contained in river water for the production of nanomaterials. The present work deals with the development of this new route of ecologically friendly synthesis for the production of neodymium doped pure yttrium oxide nanoparticles. Y2O3 (or yttria) has some particularly interesting physical and chemical properties for numerous technological applications such as wide band gap, low phonon energy, high thermal stability, and is still considered an excellent host matrix for rare earth ions. Calcination temperature, concentration and pH of the initial solution were the parameters investigated initially. Through the XRD measurements it was found that the single crystalline phase was obtained at the temperature of 450 º C for all pure samples, without significant influences of pH and concentration. Samples doped with Nd3+ were calcined at 450, 700 and 1000º C. Samples with estimated crystallite sizes between 10 and 25 nm were obtained. The diffraction patterns of these samples did not show secondary phases related to the dopant, confirming that it was suitably incorporated in the matrix. The fluorescence emission spectra showed higher intensity at concentrations of 0.5 and 1 mol% of Nd3+. On the other hand, the samples with concentrations of 2 and 3 mol% of Nd3+ presented low luminescent intensity due to the processes of cross relaxation and excitation migration. Influence of crystallite size on luminescent intensity was also observed. The samples with smaller sizes, calcined at 450º C, presented low emission intensity due to the high concentration of adsorbed OH radicals, representing a channel of fluorescent suppression in nanoparticles. Through the heat treatment carried out at low temperatures, the suppression effects caused by the hydroxyls were reduced, allowing the obtainment of nanoparticles of Y2O3: Nd3+ with higher luminescent efficiency and reduced particle sizes. / Nos últimos anos, o uso de rotas de síntese com baixo impacto ambiental vem despontando como um diferencial e sendo largamente proposta pela comunidade cientifica para a produção de vários materiais. Neste cenário, processos sol-gel modificados tem ganhado evidência por utilizar agentes alternativos em substituição aos alcóxidos metálicos empregados no método convencional. A rota de síntese proposta neste trabalho é uma das vertentes do sol-gel modificado, pois explora a capacidade de complexação da matéria orgânica natural (MON) contida em água de rio para a produção de nanomateriais. O presente trabalho aborda o desenvolvimento dessa nova rota de síntese ecologicamente amigável para a produção de nanopartículas de óxido de ítrio puro e dopado com neodímio. O Y2O3 (ou ítria) apresenta algumas propriedades físicas e químicas particularmente interessantes para inúmeras aplicações tecnológicas como band gap largo, baixa energia de fônons, alta estabilidade térmica, e ainda é considerada uma excelente matriz hospedeira para íons terras-raras. Temperatura de calcinação, a concentração e pH da solução inicial foram os parâmetros investigados inicialmente. Através das medidas de DRX constatou-se que a fase cristalina única foi obtida na temperatura de 450º C para todas as amostras puras, sem influências significativas do pH e da concentração. As amostras dopadas com Nd3+ foram calcinadas em 450, 700 e 1000º C. Foram obtidas amostras com tamanhos de cristalito estimados entre 10 e 25 nm. Os padrões de difração destas amostras não apresentaram fases secundárias relacionadas ao dopante, confirmando que este foi adequadamente incorporado na matriz. Os espectros de emissão fluorescente revelaram maior intensidade nas concentrações de 0,5 e 1 mol% de Nd3+. Por outro lado, as amostras com concentrações de 2 e 3 mol% de Nd3+ apresentaram baixa intensidade luminescente devido aos processos de relaxação cruzada e migração de excitação. Influência do tamanho de cristalito na intensidade luminescente também foi observada. As amostras com menores tamanhos, calcinadas a 450º C, apresentaram baixa intensidade nas emissões devido à alta concentração de radicais OH adsorvidos, que representam um canal de supressão fluorescente em nanopartículas. Através do tratamento térmico realizado posteriormente a baixas temperaturas, os efeitos de supressão causados pelas hidroxilas foi reduzido, possibilitando a obtenção de nanopartículas de Y2O3:Nd3+ com maior eficiência luminescente e tamanhos reduzidos de partículas.

Production of top quality castings for applications in energy, aerospace and medical industries by the addition of ceramic cores to investment casting presents many metallurgical and technological problems. Ceramic cores applied in investment casting are subjects to many requirements in terms of their inertness to the casted alloy and at the same time to their final removal from the casting cavity in such a way that does not damage the casting. Both of these assumptions can be achieved by applying a thin layer of inert oxide ceramic coating to the surface of a ceramic core, made out of a chemically removable – leachable oxide ceramic.

Le titanate de baryum dopé (BaTiO3, BT) avec des éléments de terres rares est utilisé comme diélectrique dans la fabrication de condensateurs céramiques multicouches (MLCC). L'oxyde de terres rares le plus couramment utilisé comme dopant dans la formulation commerciale de la poudre BT pour la fabrication de MLCC est Y2O3, car il a des propriétés similaires à celles de l'ajout de Ho2O3, Er2O3 ou Dy2O3 et il est moins coûteux. D'autres additifs, tels que Mn, Mg et Ca contribuent à la répartition globale des défauts électroniques pouvant être générés lorsque les ions dopants sont insérés dans le réseau, tandis que SiO2 est utilisé comme additif de frittage. La structure pérovskite du BT peut héberger une large gamme de dopants pouvant remplacer Ba ou Ti dans le réseau. Aussi, des phases secondaires, notamment celles connues sous le nom de pyrochlores Y2Ti2O7, associées à l'Y2O3 en tant que dopant BT, supposées être á l'origine des mécanismes de défaillance à long terme des MLCC, apparaissent. Le but de ce travail est de caractériser le BT dopé avec différentes concentrations d'Y2O3, en validant son éventuelle contribution à la formation de phases secondaires et en évaluant la fiabilité des MLCC préparées avec ce type de matériaux. Le rôle de Y2O3 a été évalué sur deux types de matières premières, le premier est du BaTiO3 pur (<100 ppm en Y) et le second est une formulation commerciale conçue pour les MLCC connus sous le nom de X7R (-55 °C et 125 °C, ±15 %) qui, entre autres éléments, contient déjà 1% en poids de Y2O3. Des poudres et des céramiques avec différentes concentrations d'Y3+, telles qu'Y2O3, de dopage (1% en poids à 20% en poids) ont été préparées puis traitées thermiquement ou frittées. Le traitement thermique de la poudre a été effectué à l'air, tandis que le frittage de la céramique (poudre compactée à 2 MPa) a été effectué à la fois dans l'air et dans une atmosphère réductrice (1310 °C à l'air pendant 3 h, deux étapes: 1310 °C puis 1150 °C 15 h et une atmosphère réductrice N2, H2, H2O à 1310 °C pendant 3 h). En ce qui concerne l'addition d'Y2O3, la transition de phase de tétragonal à un mélange de tétragonal et de cubique a été observée lorsque la concentration en Y2O3 augmentait dans la poudre traitée thermiquement et dans la céramique correspondante. [...] / Doped barium titanate (BaTiO3, BT) with rare-earth elements (REE) is used as dielectric in the manufacture of multilayer ceramic capacitors (MLCCs). The most common REE oxide employed as dopant in the commercial formulation of BT powder for fabrication of MLCCs is Y2O3, because it results in similar properties than adding Ho2O3, Er2O3 or Dy2O3, and it is less expensive. Other additives, such as Mn, Mg, and Ca contribute to the global distribution of the electronic defects that can be generated when the doping ions are inserted into the lattice, while SiO2 is used as a sintering additive. The perovskite structure of the BT can host a wide range of dopants that can substitute either Ba or Ti in the lattice. There are reports of secondary phases, particularly those known as pyrochlores Y2Ti2O7, related to Y2O3 as BT dopant, that are supposed to be the cause of long term failure mechanisms of MLCC's under nominal operation. The purpose of this work is to characterize BT doped with different concentrations of Y2O3, validating its possible contribution to the formation of secondary phases, and evaluating reliability of MLCCs prepared with this kind of materials. The role of Y2O3 was evaluated on two kinds of raw materials, the first one is pure BaTiO3 (< 100 ppm Y) and the second kind is a commercial formulation designed for MLCCs known as X7R (-55 °C and 125 °C, 15% tolerance) which, among other elements, already contains 1 wt% of Y2O3. Powders and ceramics with different Y3+, as Y2O3, doping concentration (1 wt% up to 20 wt%) were prepared and subsequently thermally treated or sintered, respectively. Heat treatment of powder was conducted on air, while sintering of ceramics (powder compacted at 2 MPa) was carried out both, in air and reducing atmosphere (1310 °C in air for 3 h, two steps: 1310 °C then 1150 °C 15 h, and a reducing atmosphere N2, H2, H2O at 1310 °C for 3 h). Regarding Y2O3 addition, the phase transition from tetragonal to a mixture of tetragonal and cubic was observed as Y2O3 concentration increases in the thermally treated powder and in the corresponding ceramics. [...]

La modification des diagrammes de phase sous pression dans les nanomatériaux en comparaison de matériaux massifs est généralement reliée de façon univoque à l'énergie de surface. L'objectif de ce travail a consisté à étudier l'influence de l'énergie d'interface, définie par l'état de surface (défauts cristallins et chimie de surface) et l'énergie de surface, sur le comportement à haute pression de différents nanomatériaux pour lesquels un effet de taille est fréquemment rapporté. Le contrôle et la caractérisation de l'état de surface de nanoparticules d'Y2O3 nous ont permis de montrer que l'amorphisation sous pression rapportée dans la littérature n'est pas uniquement due à la réduction de la taille de grain mais nécessite une densité de défauts initiale, généralement non contrôlée. Une forte dépendance en taille de la pression de transition dans ZnO est avancée dans la littérature. L'étude sous pression de différents échantillons nanométriques de ZnO (issus de diverses voies de synthèse) a été effectuée par spectroscopie Raman et diffraction de rayons X. La qualité cristalline de ces derniers a été estimée par Photoluminescence, XPS, Raman et IR. Tandis que les échantillons présentant une forte densité de défauts conduisent à une augmentation de la pression de transition, le comportement d'un échantillon "sans défauts" ne diffère que peu de celui de ZnO massif. Différentes approches et extensions de modèles thermodynamiques sont proposées: modèles de Gibbs, Landau et Ginzburg-Landau. Ces derniers ouvrent la voie à la définition d'un protocole expérimental permettant d'obtenir des données fiables pour étudier les transitions de phase de nanomatériaux sous pression / The modification of phase diagrams under pressure into nanosized materials in comparison with bulk ones is usually attributed to surface energy. The goal of this work has consisted into studying the influence of interface energy, which includes both the surface state (crystalline defects and surface chemistry) and surface energy, on the high-pressure behavior of several nanomaterials for which size effects has been reported. The control and characterization of the surface state for Y2O3 nanoparticles has enabled us to show that the pressure induced amorphization reported into literature is not only linked to size reduction but require an initial density of defects A strong size dependence of ZnO transition pressure is claimed into literature. The high-pressure study of different ZnO nanometric samples (obtained through several ways of synthesis) has been performed by Raman spectroscopy and X-ray diffraction. The crystalline quality of our samples has been investigated by photoluminescence, XPS, Raman and IR. Whereas samples exhibiting a high density of defects lead to an increase of pressure transition, the behaviour of “defect free” nanoparticles is rather equivalent to bulk one. Several approaches and extensions of thermodynamic models are submitted: model of Gibbs, Landau and Ginzburg-Landau. Those models open the way to the definition of an experimental protocol which allow to obtain reliable data in order to study phase transitions of nanomaterials under pressure

No / A new dual-log spiral geometry is proposed for microstrip resonators, offering substantial advantages in performance and size reduction at subgigahertz frequencies when realized in superconducting materials. The spiral is logarithmic in line spacing and width such that the width of the spiral line increases smoothly with the increase of the current density, reaching its maximum where the current density is maximum (in its center for ¿/2 resonators). Preliminary results of such a logarithmic ten-turn (2 × 5 turns) spiral, realized with double-sided YBCO thin film, showed a Q.-factor seven times higher than that of a single ten-turn uniform spiral made of YBCO thin film and 64 times higher than a copper counterpart. The insertion loss of the YBCO dual log-spiral has a high degree of independence of the input power in comparison with a uniform Archimedian spiral, increasing by only 2.5% for a 30-dBm increase of the input power, compared with nearly 31% for the uniform spiral. A simple approximate method, developed for prediction of the resonant frequency of the new resonators, shows a good agreement with the test results.