Y2O3:Eu particles are phosphors that have found wide applications. Flamesynthesized Y2O3:Eu particles may have either the cubic or the monoclinic structure. The effects of particle size and Eu doping concentration on crystal structure and the surface elemental composition of the flame-synthesized Y2O3:Eu particles had not been previously reported. In this study, a flame aerosol process was used to generate polydisperse Y2O3:Eu particle. H2 was used as the fuel gas, with either air or O2 gas as the oxidizer. The precursor was aqueous solutions of the metal nitrates, atomized using a 1.7-MHz ultrasonic atomizer. The product particles were analyzed by transmission electron microscopy (TEM), X-ray diffractometer (XRD), Selected area electron diffraction (SAED), X-ray photoelectron spectroscopy (XPS), fluorescence spectrophotometer, and inductively coupled plasma mass spectrometer (ICP-MS). The Y2O3:Eu particles generated in H2/O2 flames were spherical and fully dense, with diameters in the range of 10~3000 nm. In particle samples with lower Eu doping concentrations, a critical particle diameter was found, whose value increased with increasing Eu doping concentration. Particles well below the critical diameter had the monoclinic structure; those well above the critical diameter had the cubic structure. At sufficiently high Eu doping concentrations, all Y2O3:Eu generated in H2/O2 flames had the monoclinic structure. On the other hand, all particles generated in the H2/air flames had the cubic structure. For the Y2O3:Eu particles generated in H2/O2 flames, XPS results showed that the surface Eu concentration was several times higher than the doping concentration. For Y2O3:Eu particles generated in H2/air flames, the surface Eu concentration was equal to the doping concentration. For both types of particles, the photoluminescence intensity reached a maximum corresponding to a surface Eu concentration 40~50%. The photoluminescence intensity then decreased rapidly with higher Eu doping concentration. The effect of particle size and Eu doping concentration on crystal structure may be explained by the interplay between surface energy and polymorphism. A mechanism for this surface enrichment phenomenon was proposed based on the binary Eu2O3-Y2O3 phase diagram.

Abstract The purpose of this thesis is to study the electrical characteristics of ultra-thin high-k gate oxide-semiconductor interfaces. The measured samples are Y2O3/Si¡BGd2O3/GaAs¡BGa2O3(Gd2O3)/GaAs MOS capacitors. An accurate C-V relation has been obtained consistently by using a model that includes both series and shunt parasitic resistances. Using the semiconductor parameters and the oxide parameters, an ideal C-V curve with Dit = 0 is fitted to the accurate capacitance data, and the interface state density is deduced by Terman method. After post - metallization annealing (PMA) at 425¢J, the oxide charge density, interface state density and leakage current were reduced. The results are following : (1) For Y2O3/Si MOS capacitors, we obtained a oxide charge density ~ 7.7 x 1010 cm-2, an interface state density ~ 3.6 x 1010 cm-2ev-1, and an equivalent oxide thickness ~ 52Å; (2) For Gd2O3/GaAs MOS capacitors, we obtained a oxide charge density ~ 9.8 x 1011 cm-2, an interface state density ~ 2 x 1011 cm-2ev-1, and an equivalent oxide thickness ~ 57Å; (3) For Ga2O3(Gd2O3)/GaAs MOS capacitors, we obtained a oxide charge density ~ 4.2 x 1012 cm-2, an interface state density ~ 6 x 1011 cm-2ev-1, and an equivalent oxide thickness ~ 91Å. The dielectric constants obtained from our data are smaller than the reported values. A possible explanation is that an interfacial layer formed at the oxide/semiconductor interface to reduce equivalent dielectric constant.

Neste trabalho, foi estudada a sinterização de Nanoestruturas de zircônia estabilizada com ítria, ZrO2-Y2O3. Pós de ZrO2-3%mol de Y2O3 com tamanhos nanométricos e área superficial específica de 16,2 m2/g, foram compactados uniaxialmente em pressões variando entre 12,3MPa e 73,5MPa. Nestas pressões, as amostras compactadas atingiram densidade à verde entre 33% e 44,3%. Os compactos foram sinterizados por dilatometria de 1.250ºC até 1.4000C e os resultados indicaram que a temperatura de início de densificação está próxima a 1.0000C, independente da pressão de compactação aplicada. Temperaturas da ordem de 1.4000C são necessárias para a densificação total do compacto. Comparativamente pós microestruturados de ZrO2-3%mol Y2O3 (área superficial de 7,0 m2/g), foram compactados a 73,5MPa e alcançou densidade a verde de 44,2%. Este material apresentou início da retração próximo a 1.2000C, sugerindo que o uso de pós nanoestruturados reduziu a temperatura de sinterização em 2000C. A sinterização convencional dos pós nanoestruturado foi realizada em temperaturas entre 1.2500C e 1.4000C, com patamares entre 2 e 16h. Os pós microestruturados foram submetidos à sinterização em temperaturas de até 1.6000C, com os mesmos patamares. Os resultados indicaram que em todas as condições de sinterização e indiferentemente do pó utilizado, apenas a fase ZrO2 tetragonal foi encontrada. Além disso, a densidade relativa (DR) dos nanopós variou entre 92%(1.2500C-0h) e superior a 99%(acima de 1.3500C- 4h). Análises microestruturais indicaram a presença de grãos refinados com tamanho médio de 0,18?m (DR=92%,) para nanopós sinterizados a 1.2500C-0h, e tamanho médio de 0,95?m (DR=100%, 1.4000C-16h). Os pós micrométricos apresentaram tamanho de grão médio de 0,39?m (DR=98,8%) para cerâmicas sinterizadas a 1.5300C-0h e tamanho de grão médio de 1,84?m (DR=100%) para cerâmicas sinterizadas a 1.600-0h. Os expoentes de crescimento de grão calculados foram da ordem de 2,8 e 2,3 para. nanoparticulas e microparticulas, respectivametne, indicando que mecanismo de difusão pelos contornos de grão foi preponderante nos materiais estudados. Os valores de energia de ativação para o crescimento de grão calculados foram de 141,3kJ/mol e 244,7kJ/mol, respectivamente, indicando que os pós micrométricos necessitam de maior consumo energético para promover o crescimento de grão. Os resultados são discutidos em função de associar os fenômenos de densificação e crescimento de grão com o tamanho das partículas utilizadas. / We studied the sintering of nanoparticles of yttria stabilized zirconia, ZrO2-Y2O3. Powders of ZrO2-Y2O3 3 mol% with nanometric sizes and specific surface area of 16.2 m2/g, were uniaxially compacted at pressures ranging between 12.3 MPa and 73.5 MPa. Such pressures, the compressed samples reached the green density between 33% and 44.3%. The compacts were sintered by dilatometry up 1.400ºC and the results indicated that the onset temperature of densification is next to 1.000ºC, regardless of the compaction pressure applied. Temperatures of 1.400ºC are required to complete the densification of the compact. Compared microparticulate powders of ZrO2-Y2O3 3 mol% (specific surface area of 7.0 m2/g) were compressed to 73.5 MPa and green density reached 44.2%. This material exhibited the beginning of the next retraction 1.200ºC, suggesting that the use of powder nanopatticulados reduced sintering temperature of 200ºC. The conventional sintering of nanoparticle powders was undertaken at temperatures between 1.400ºC and 1.250ºC, with levels between 2 and 16h. The microparticulate powders were subjected to sintering at temperatures up to 1.600ºC with the same levels. The results indicated that under all conditions, and sintering the powder used interchangeably, only tetragonal ZrO2 phase was found. Furthermore, the relative density (RD) of nanoparticles ranged from 92% (1.250ºC- 0h) and greater than 99% (above 1.350ºC-4h). Microstructural analysis indicated the presence of refined grains with average size of 0.18 ? m (DR = 92%) for nanoparticles sintered at 1.250ºC-0h, and average size of 0.95 ?m (DR = 100%, 1.400ºC -16h). The powders were micron average grain size of 0.39 ?m (DR = 98.8%) to the sintered ceramic 1.530ºC-0h and the average grain size of 1.84 ?m (DR = 100%) of sintered ceramics the 1.600 ºC-0h. The grain growth exponents calculated were the order of 2.8 and 2.3 for nanoparticles and microparticles, respectively, and indicating that the mechanism of grain boundary diffusion was predominant in the studied materials. The values of activation energy for the grain growth were calculated 141.3 kJ/mol and 244.7 kJ/mol, respectively indicating that the powder micrometric require more energy to promote grain growth. The results are discussed in terms of the associated phenomena of densification and grain growth in the particle size used.

Neste trabalho, foi estudada a sinterização de Nanoestruturas de zircônia estabilizada com ítria, ZrO2-Y2O3. Pós de ZrO2-3%mol de Y2O3 com tamanhos nanométricos e área superficial específica de 16,2 m2/g, foram compactados uniaxialmente em pressões variando entre 12,3MPa e 73,5MPa. Nestas pressões, as amostras compactadas atingiram densidade à verde entre 33% e 44,3%. Os compactos foram sinterizados por dilatometria de 1.250ºC até 1.4000C e os resultados indicaram que a temperatura de início de densificação está próxima a 1.0000C, independente da pressão de compactação aplicada. Temperaturas da ordem de 1.4000C são necessárias para a densificação total do compacto. Comparativamente pós microestruturados de ZrO2-3%mol Y2O3 (área superficial de 7,0 m2/g), foram compactados a 73,5MPa e alcançou densidade a verde de 44,2%. Este material apresentou início da retração próximo a 1.2000C, sugerindo que o uso de pós nanoestruturados reduziu a temperatura de sinterização em 2000C. A sinterização convencional dos pós nanoestruturado foi realizada em temperaturas entre 1.2500C e 1.4000C, com patamares entre 2 e 16h. Os pós microestruturados foram submetidos à sinterização em temperaturas de até 1.6000C, com os mesmos patamares. Os resultados indicaram que em todas as condições de sinterização e indiferentemente do pó utilizado, apenas a fase ZrO2 tetragonal foi encontrada. Além disso, a densidade relativa (DR) dos nanopós variou entre 92%(1.2500C-0h) e superior a 99%(acima de 1.3500C- 4h). Análises microestruturais indicaram a presença de grãos refinados com tamanho médio de 0,18?m (DR=92%,) para nanopós sinterizados a 1.2500C-0h, e tamanho médio de 0,95?m (DR=100%, 1.4000C-16h). Os pós micrométricos apresentaram tamanho de grão médio de 0,39?m (DR=98,8%) para cerâmicas sinterizadas a 1.5300C-0h e tamanho de grão médio de 1,84?m (DR=100%) para cerâmicas sinterizadas a 1.600-0h. Os expoentes de crescimento de grão calculados foram da ordem de 2,8 e 2,3 para. nanoparticulas e microparticulas, respectivametne, indicando que mecanismo de difusão pelos contornos de grão foi preponderante nos materiais estudados. Os valores de energia de ativação para o crescimento de grão calculados foram de 141,3kJ/mol e 244,7kJ/mol, respectivamente, indicando que os pós micrométricos necessitam de maior consumo energético para promover o crescimento de grão. Os resultados são discutidos em função de associar os fenômenos de densificação e crescimento de grão com o tamanho das partículas utilizadas. / We studied the sintering of nanoparticles of yttria stabilized zirconia, ZrO2-Y2O3. Powders of ZrO2-Y2O3 3 mol% with nanometric sizes and specific surface area of 16.2 m2/g, were uniaxially compacted at pressures ranging between 12.3 MPa and 73.5 MPa. Such pressures, the compressed samples reached the green density between 33% and 44.3%. The compacts were sintered by dilatometry up 1.400ºC and the results indicated that the onset temperature of densification is next to 1.000ºC, regardless of the compaction pressure applied. Temperatures of 1.400ºC are required to complete the densification of the compact. Compared microparticulate powders of ZrO2-Y2O3 3 mol% (specific surface area of 7.0 m2/g) were compressed to 73.5 MPa and green density reached 44.2%. This material exhibited the beginning of the next retraction 1.200ºC, suggesting that the use of powder nanopatticulados reduced sintering temperature of 200ºC. The conventional sintering of nanoparticle powders was undertaken at temperatures between 1.400ºC and 1.250ºC, with levels between 2 and 16h. The microparticulate powders were subjected to sintering at temperatures up to 1.600ºC with the same levels. The results indicated that under all conditions, and sintering the powder used interchangeably, only tetragonal ZrO2 phase was found. Furthermore, the relative density (RD) of nanoparticles ranged from 92% (1.250ºC- 0h) and greater than 99% (above 1.350ºC-4h). Microstructural analysis indicated the presence of refined grains with average size of 0.18 ? m (DR = 92%) for nanoparticles sintered at 1.250ºC-0h, and average size of 0.95 ?m (DR = 100%, 1.400ºC -16h). The powders were micron average grain size of 0.39 ?m (DR = 98.8%) to the sintered ceramic 1.530ºC-0h and the average grain size of 1.84 ?m (DR = 100%) of sintered ceramics the 1.600 ºC-0h. The grain growth exponents calculated were the order of 2.8 and 2.3 for nanoparticles and microparticles, respectively, and indicating that the mechanism of grain boundary diffusion was predominant in the studied materials. The values of activation energy for the grain growth were calculated 141.3 kJ/mol and 244.7 kJ/mol, respectively indicating that the powder micrometric require more energy to promote grain growth. The results are discussed in terms of the associated phenomena of densification and grain growth in the particle size used.

Les propriétés structurales et de luminescence de couches minces de TiO2 et Y2O3 dopées terres rares (thulium, terbium et ytterbium) ont été étudiées en vue de les intégrer dans une cellule photovoltaïque comme couche de conversion spectrale du proche UV vers l’infrarouge afin d’en améliorer l’efficacité. Ces couches minces ont été synthétisées par dépôt chimique en phase vapeur à pression atmosphérique à l’aide de précurseurs organo-métalliques et assisté par aérosol (aerosol assisted MOCVD). Les couches minces sont partiellement cristallisées dès la synthèse (400°C pour le TiO2 en phase anatase, 540°C pour Y2O3 en phase cubique). Après traitement thermique la cristallisation est largement améliorée et la luminescence des ions dopant terres rares est obtenue dans les deux matrices oxydes. Le thulium émet dans une large bande située vers 800 nm et l’ytterbium vers 980 nm. Le terbium quand à lui émet dans une gamme située principalement dans le visible. Les spectres d’excitation ont montré que l’absorption des photons se fait via la matrice. En matrice TiO2 une efficacité de transfert d’énergie du Tm3+ vers l’Yb3+ de l’ordre de 20 % a été déterminée pour des teneurs de 0,8 % des deux dopants, ce qui correspond à la limite d’auto-extinction. Le rendement global mesuré est faible, nous avons montré que les causes probables de cette faible valeur sont le manque d’absorption des couches minces pour obtenir l’excitation de l’ion sensibilisateur ainsi que des processus de luminescence et de down conversion pas assez efficaces. / Structural and luminescence properties of rare-earth-doped (thulium, terbium and ytterbium) thin films of yttrium oxide and titanium oxide were studied as a down-converting layer from near-UV to infrared for integration in solar cells to improve their yield. These thin films were synthesized by chemical vapor deposition at atmospheric pressure with organo-metallic precursors and assisted by aerosol (aerosol assisted MOCVD). The thin films were partially crystallized as deposited (400°C in the anatase phase for TiO2 , 540°C in the cubic phase for Y2O3). After annealing the crystallization is greatly improved and the rare-earth ion luminescence is obtained in both oxide matrices. The thulium emits in a large band centered around 800 nm and the ytterbium at 980 nm. The terbium emits mainly in the visible range. Excitation spectra showed that the photon absorption occurs in the matrix. In the TiO2 matrix a transfer rate from Tm to Yb of 20 % was measured for doping of 0,8 % for both rare-earth, which corresponds to the quenching limit. The overall measured yield is low, we showed that the probable reasons were the thin films’ lack of absorption to obtain the excitation of the sensitizer ion and a low efficiency of luminescence and down-conversion processes.

[pt] Materiais à base de zircônia apresentam excelentes propriedades mecânicas, estabilidade química e dimensional, tenacidade, juntamente com um módulo de Young na mesma ordem de grandeza de ligas de aço inoxidável. Devido à essas características, a zircônia tem sido utilizada em uma ampla gama de aplicações, incluindo a fabricação de peças protéticas. As modificações na composição da zircônia e o desenvolvimento do CAD-CAM (computer assisted design) fizeram com que as próteses à base de zircônia se tornassem um procedimento clinico frequentemente realizado na atual Odontologia restauradora. Blocos pré-sinterizados de zircônia, fabricados industrialmente, homogêneos e com falhas mínimas são usinados em um ambiente industrial controlado para receberem a forma desejada. Após a usinagem, as peças em zircônia são sinterizadas para promover sua densificação final. Este processo reduz os possíveis defeitos criados por etapas laboratoriais manuais e intermediárias na confecção de restaurações dentárias. O objetivo deste estudo foi avaliar o efeito da degradação hidrotérmica e do jateamento com óxido de alumínio nas propriedades mecânicas de três tipos de zircônia tetragonal policristalina estabilizada por ítria (ZTPI). Três tipos de zircônias contendo diferentes concentrações de ítria e grau de translucidez foram utilizadas: ZTPI com 5,2 por cento mol (Prettau Anterior Super-Translúcida da Zirkonzahn), ZTPI com 3 por cento mol de Y2O3 (VIPI Block Zirconn) e ZTPI com 3 por cento mol de Y2O3 com translucidez melhorada (VIPI Block Zirconn Translucent). Todas indicadas para uso em próteses odontológicas monolíticas. 15 corpos de prova de cada zircônia testada foram sinterizados e divididos em três grupos de acordo com otratamento de superfície e processo de degradação. No Grupo I as zircônias sofreram jateamento com óxido de alumino. No grupo II as zircônias foram lixadas e polidas. No grupo III as zircônias foram lixadas, polidas, recuperadas e degradadas em reator hidrotérmico por 5 horas a 134 graus Celsius e 2 bar. Para a caracterização microestrutural foram realizados ensaios de difração de raios-X, espectrometria Raman, densidade relativa, microdureza Vickers, rugosidade superficial e análise de tamanho médio de grão por MEV. Os resultados mostraram que a densidade de todos os grupos ficou acima de 99 por cento e que a degradação promoveu a transformação de fase monoclínica e afetou os valores de resistência à flexão. No entanto, a indicação clínica das cerâmicas não foi alterada. O tamanho médio de grão ficou entre 0,367 e 0,621 micrômetros. A dureza e tenacidade à fratura do material não sofreram alterações em função do teor de ítria ou das fases cristalinas presentes. A degradação não afetou de forma significativa os valores de rugosidade, porém, o jateamento com óxido de alumínio não só afetou os valores de flexão como alterou a indicação clínica de cerâmicas utilizadas neste trabalho. / [en] Zirconia-based materials present excellent mechanical properties, tenacity, chemical, and dimensional stability as well as their Young modulus is similar to stainless steel alloys. Due to these characteristics, zirconia has been largely applied in the fabrication of dental prostheses. Modifications in the zirconia composition and the development of CAD-CAM systems has allowed zirconia-based restorations to become a common clinical procedure in current dentistry. Fully sintered and homogeneous blocks from industry are machined at controlled environments seeking to obtain the desired geometry of the restorations. After the machining process, the restorations are sintered to improve their density. This process reduces the failures caused by manual and laboratorial manipulations during fabrication procedure. The aim of this study is to evaluate the effect of hydrothermal aging and sandblasting with aluminum oxide on the mechanical properties of three Yttria-containing tetragonal zirconia polycrystalline (Y-TZP). Three types of Y-TZP, containing different yttria dioxide, were used in the study: Y-TZP with 5,2 per cent mol Y2O3 (Prettau Anterior Super-Translúcida da Zirkonzahn), Y-TZP with 3 per cent mol de Y2O3 (VIPI Block Zirconn) and Y-TZP with 3 per cent mol Y2O3 and optimized translucency (VIPI Block Zirconn Translucent). All the zirconias are indicated for being used as monolithic restorations. 15 specimens from each zirconia tested were sintered and distributed to three groups according to the surface treatment and the aging process. In the group I, the specimens were sandblasted with aluminum oxide particles. For the group II, the zirconia specimens were abraded and polished. In the group III, the specimens were abraded, polished, recovered and aged in a reactor controller for 5 hours at 134 degrees Celsius and 2 bar. The characterization of the zirconia microstructure has been carried out by means of: X-ray diffraction, relative density, Raman spectroscopy, Vickers microhardness, superficial roughness. In addition, the average grain size analysis by scanning electron microscopy were made. The results show that the relative density values were above 99 per cent, and that the hydrothermal aging causes monoclinic phase transformation and affects flexural strength values. However, the clinical indications of materials did not change. The average grain size was between 0.367 and 0.621. The microhardness and tenacity values of materials did not change by yttria concentration or crystallographic phases. The roughness values were not significantly affected by the hydrothermal aging. On the other hand, the flexural strength values were affected by sandblasting process and clinical indication of ceramics tested was changed.

In the present dissertation the formation of microstructure, the kinetics of densification and the formation of surface layers developing during liquid phase sintering of silicon carbide are studied. The focus is on the additive systems Al2O3 plus Y2O3 and AlN plus Y2O3. Phase and especially liquid phase formation in both of the systems SiC, Al2O3 , Y2O3 and AlN, Al2O3 , Y2O3 are investigated in detail examining 12 espectively 17 different compositions per system. Melting temperatures have been determined by TG/DTA, in both systems for the first time. Phase composition of samples was analysed by the combination of XRD, SEM and EDX. In the system SiC, Al2O3 , Y2O3 the formation of the phases expected from the quasibinary Al2O3 , Y2O3 could be observed thus silicon carbide has to be in equilibrium with the oxide additives. The low solubility of SiC in the oxide melt, which was suggested by Hoffmann and Nader, could be confirmed. In the system AlN, Al2O3 , Y2O3 the formation of phases as stated by Medraj was confirmed, except for the dimension of the stability region of the γ- spinel and YAG which is wider in the present work. For the first time diffusion coefficients of the species Y3+ and Al3+ in the oxide melt formed by Al2O3 and Y2O3 at temperatures above 1825 ◦ C were determined. The values are in the order of 2 · 10−6 cm2 /s which results in a diffusion length of 14.1 μm for a diffusion time of one second. This allows the fast equilibration of Y and Al deficiencies. Kinetics of densification was modeled by kinetic field, master curve and thermokinetic method, based on detailed experimental investigation of the shrinkage during liquid phase sintering of SiC. It could be proved that the first 30 − 40 % of densification are controlled by solid phase reactions which accelerate particle rearrangement without presence of a liquid phase. During the remaining 60 − 70 % of densification a liquid is present, resulting in the predominance of mechanisms of liquid phase sintering. The models deliver activation energies in the range from 608 KJ/mol to 1668 kJ/mol and allow, within the scope of validity of each method the prediction of densification during liquid phase sintering of silicon carbide. When sintering silicon carbide with Al2O3 plus Y2O3 the formation of several surface layers, depending on atmosphere, maximum temperature, dwelling time and amount and composition of additives was observed. In nitrogen atmosphere with low partial pressures a surface layer consisting of AlN is forming whilst at high partial pressures SiAlON- polytypes occur. After sintering in Argon or Ar-CO- atmosphere three main types of surface layers are present. One consists of alumina, one contains only YAG and one shows highly porous, additive depleted regions. An explanation for the formation of the several surface layers could be given by the combination of the determined diffusion coefficients with the results achieved in the thermodynamics part. The results achieved in this work can be a contribution to the knowledge based design of the production process of liquid phase sintering of silicon carbide.

Os objetivos gerais deste trabalho foram estudar o comportamento de densificação e as propriedades mecânicas do nitreto de alumínio (AlN) com aditivo de sinterização. Os objetivos específicos foram estudar: 1) o efeito do teor de Y2O3 na densificação do AlN; 2) o efeito da adição de carbono na densificação do AlN com CaO; 3) o efeito da adição simultânea de Y2O3 e CaO na densificação do AlN; 4) o efeito da porosidade nas propriedades elásticas do AlN com Y2O3 ou CaO; e 5) o efeito do teor de Y2O3 ou CaO nas propriedades mecânicas do AlN. As amostras foram sinterizadas sob atmosfera inerte em várias temperaturas (1100-2000ºC) e analisadas por determinação de densidade, difração de raios X e microscopia eletrônica de varredura. As propriedades determinadas à temperatura ambiente foram: constantes elásticas pelo método de pulso-eco ultra-sônico, resistência à flexão biaxial, dureza Vickers e tenacidade à fratura pelos métodos Indentation Fracture (KIc-IF) e Indentation Strength (KIc-IS). O AlN com Y2O3 apresentou significativa densificação por sinterização no estado sólido, mas completa densificação ocorreu somente por sinterização via fase líquida. A densificação do AlN não foi influenciada pelo aumento de 0,5% a 4% de Y2O3, pois a temperatura de formação da fase líquida (~ 1725ºC) não variou nesta faixa de teor de aditivo. Porém, a adição de carbono levou à formação de segundas-fases refratárias no AlN com 4% de Y2O3, o que atrasou sua densificação devido ao aumento da temperatura de formação da fase líquida. A adição de carbono não mudou a temperatura de formação da fase líquida do AlN com 4% de CaO sinterizado acima de 1650ºC, mas sua densificação também atrasou devido à diminuição da fração de fase líquida e ao aprisionamento de gás nos poros fechados. A adição simultânea de CaO e Y2O3 diminuiu significativamente a temperatura de sinterização do AlN em relação às adições individuais destes aditivos. O AlN com mistura de CaO e Y2O3 atingiu quase completa densificação a partir de 1650ºC devido à formação de fase líquida com composição próxima da fase CaYAl3O7, que apresentou boas características de molhamento e espalhamento nesta temperatura de sinterização. A adição de carbono mudou a composição das segundas-fases de CaYAl3O7 para CaYAlO4 e, então, para YAM (2Y2O3.Al2O3) no AlN com mistura de CaO e Y2O3, o que atrasou sua densificação em baixas temperaturas de sinterização (< 1700ºC) devido à alta viscosidade e/ou baixa molhabilidade da fase CaYAlO4 e à alta refratariedade da fase YAM. As propriedades elásticas, bem como a dureza Vickers do AlN foram influenciadas pela porosidade, mas não foram influenciadas pelo tipo (CaO e Y2O3) e teor (0,5% a 8% em massa) de aditivo de sinterização. A resistência 7 à flexão foi influenciada pelo tipo e teor de aditivo de sinterização, sendo observado um teor crítico de aditivo, acima do qual esta propriedade diminuiu significativamente e, então, tendeu a permanecer constante em função do teor de aditivo. Não se observou correlação entre tenacidade à fratura e resistência à flexão. Os valores de KIc-IS, que não variaram com o tipo e teor de aditivo de sinterização, foram considerados mais consistentes do que os valores de KIc-IF. / The general objectives of this work were to study the densification behavior and the mechanical properties of aluminum nitride (AlN) with sintering aid. The specific objectives studied were: 1) the effect of Y2O3 content on the densification of AlN, 2) the effect of carbon addition on the densification of AlN with CaO, 3) the effect of simultaneous addition of Y2O3 and CaO on the densification of AlN; 4) the effect of porosity on elastic properties of AlN with CaO or Y2O3, and 5) the effect of Y2O3 or CaO content on the mechanical properties of AlN. The samples were sintered under inert atmosphere at various temperatures (1100ºC-2000ºC) and analyzed by density measurement, X-ray diffraction and scanning electron microscopy. The properties measured at room temperature were: elastic constants by the ultrasound pulse-echo method, biaxial flexural strength, Vickers hardness and fracture toughness by the Indentation Fracture (IF) and Indentation Strength (IS) methods. AlN with Y2O3 presented significant densification by solid-state sintering, but full densification occurred only by liquid-phase sintering. The densification of AlN was not influenced by the increase of 0.5% to 4% Y2O3, since the liquid-phase formation temperature (~ 1725ºC) did not change in this range of additive content. However, the addition of carbon to AlN with 4% Y2O3 led to formation of refractory second-phases, which delayed the densification due to an increase of the liquid-phase formation temperature. The addition of carbon did not change the liquid-phase formation temperature of AlN with 4% CaO sintered above 1650ºC, but its densification delayed significantly due to the diminution of liquid-phase fraction and trapped gas in closed pores. The simultaneous addition of CaO and Y2O3 significantly diminished the sintering temperature of AlN in relation to the individual additions of these additives. AlN with simultaneous addition of CaO and Y2O3 achieved almost full densification at 1650ºC due to formation of liquid-phase with composition close to CaYAl3O7 phase, which had good wetting and spreading characteristics at this temperature. The addition of carbon changed the composition of second-phases from CaYAl3O7 to CaYAlO4 and then to YAM (2Y2O3.Al2O3) in AlN with simultaneous addition of CaO and Y2O3, delaying its densification at low sintering temperatures (< 1700ºC) due to the high viscosity and/or low wettability of CaYAlO4 phase and high refractoriness of YAM phase. The elastic properties, as well as Vickers hardness were influenced by the increase of porosity, but were not influenced by type (CaO and Y2O3) and content (0.5% to 8% in mass) of sintering aid. Flexure strength was influenced by the type and content of sintering aid. It was observed a critical additive content, 9 above which the strength decreased significantly and then tended to remain almost constant as a function of additive content. No relationship was observed between fracture toughness and flexure strength. The KIc-IS values, which did not vary with type and content of sintering aid, were found to be more consistent than the KIc-IF values.

Ce travail s’inscrit dans le programme de recherche sur les réacteurs nucléaires de « génération IV » à caloporteur sodium. Un des verrous technologiques à lever concerne le contrôle de la corrosion des matériaux (matériaux de structure, gaines des combustibles, etc.) par le sodium liquide. Un des paramètres clé est la teneur en oxygène dissous dans le caloporteur. Cette thèse porte sur l’élaboration et la caractérisation de matériaux céramiques de type ThO2 dopé avec Y2O3 pour la réalisation de sondes potentiométriques à oxygène utilisables dans le sodium liquide. Des travaux ont été effectués et des sondes ont été testées par le passé, cependant les sondes présentaient généralement, à l’époque, une absence de fiabilité dans le temps. L’objectif de cette thèse est d’élaborer et de caractériser des électrolytes à base d’oxyde de thorium dopé à l’oxyde d’ytrium à l’aide de techniques de synthèse permettant de contrôler la pureté, la taille des grains, la compacité, etc. Pour mettre en place les protocoles expérimentaux une céramique modèle a été choisie, la cérine dopée à l’oxyde d’yttrium. Les processus de transport ont été étudiés à l’aide de la spectrocopie d’impédance. Une interprétation des phénomènes de blocage de la conduction ionique dans les deux céramiques étudiées en fonction de la pression partielle d’oxygène a pu être proposée. / This PhD thesis was prepared within the framework of the research program on "Generation IV" nuclear reactors with sodium as coolant. One of the main technological problem concerns the control of the corrosion processes of the materials (structural materials, fuel claddings, etc.) by liquid sodium. A key parameter is the dissolved oxygen content in the coolant. This thesis focuses on the development and characterization of ceramic materials based on ThO2 doped with Y2O3 for making potentiometric oxygen sensor used in liquid sodium. Work has been carried out and probes were tested in the past, however, the probes had at the time, a lack of reliability. The objective of this thesis is to develop and characterize electrolytes based on thorium oxide doped with yttrium oxide using specific synthesis techniques to control purity, grain size, compactness, etc. To develop experimental protocols a ceramic model has been chosen, i.e., yttria-doped ceria. Transport processes were studied using the impedance spectroscopy technique. An interpretation of the blocking phenomena of the ionic conduction in both ceramics as a function of the oxygen partial pressure has been given.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / In this work, the frequency upconversion phenomenon of yttrium oxide (Y2O3) nanocrystals doped with Er3+ 2%, at three different sizes (17, 28 e 36 nm) was studied. Spectroscopic studies were performed by exciting the samples with a CW diode laser, emitting at 800 nm wavelength and measuring their emission spectra. It was also analyzed the behaviour of the luminescent emission as a function of the excitation laser power and the influence of the size of the nanocrystals on the observed luminescent emission. The performance of these nanophosphors as nanometric temperature sensors was also investigated. For this, the behaviour of the light emission spectrum of this system was analyzed, subjecting the nanoparticles to different temperatures. In these systems, the basic principle of operation is the change of the fluorescence intensity ratio (FIR) from the following levels' pairs 2H11/2 and 4S3/2, 2H11/2 and 4F9/2, and 4S3/2 and 4F9/2 of erbium ions with the sample temperature. From the obtained curves, it was possible to obtain the characteristic parameters of the sensors and their respective sensitivities. We found that the sensitivity of the temperature sensor using such crystals is rather high and this magnitude depends strongly on the crystallite size. For nanotermometers based on FIR thermally coupled between the levels (2H11/2 and 4S3/2), the sensitivity is higher for larger size nanocrystals. On the other hand, when it is used the FIR between the emitting levels thermally uncoupled (2H11/2 and 4F9/2), the highest sensitivity is obtained for the smaller nanocrystals. It was also proposed a new methodology for temperature measurement exploring, simultaneously, multiple spectral lines associated with Stark sublevels of erbium ions. This new method made it possible to obtain temperature sensors with high sensitivity over a wide temperature range. The results indicate that nanocrystals of yttria-doped erbium are very promising for thermal sensing nanoscale. / Neste trabalho foi caracterizado o fenômeno de conversão ascendente de frequências de nanocristais de óxido de ítrio (Y2O3) de três tamanhos diferentes (17, 28 e 36 nm) dopados com 2% de Er3+. Os estudos espectroscópicos foram realizados excitando as amostras com um laser de diodo CW, emitindo no comprimento de onda de 800 nm e medindo os seus espectros de emissão. Analisou-se, também, o comportamento da emissão luminescente em função da potência do laser de excitação e a influência do tamanho dos nanocristais sobre a emissão luminescente observada. Além disso, investigou-se também o desempenho desses nanofósforos como sensores de temperatura nanométricos. Para isso, o comportamento do espectro de emissão de luz desse sistema foi analisado, submetendo as nanopartículas a diferentes temperaturas. Nesses sistemas, o princípio básico de funcionamento do sensor é a variação da razão da intensidade de fluorescência (RIF) proveniente dos seguintes pares de níveis 2H11/2 e 4S3/2, 2H11/2 e 4F9/2, e 4S3/2 e 4F9/2 dos íons de érbio com a temperatura da amostra. A partir das curvas obtidas, foi possível obter os parâmetros característicos dos sensores e suas respectivas sensibilidades. Verificamos que a sensibilidade do sensor de temperatura usando esses cristais é bastante elevada e que essa grandeza depende fortemente do tamanho do cristalito. Para nanotermômetros baseados na RIF entre os níveis termicamente acoplados (2H11/2, 4S3/2), a sensibilidade é maior para nanocristais de maior tamanho. Por outro lado, quando se usa a RIF entre os níveis emissores não acoplados termicamente (2H11/2, 4F9/2), a sensibilidade mais alta é obtida para os nanocristais de menor dimensão. Foi proposta, também, uma nova metodologia para medida de temperatura explorando, simultaneamente, múltiplas linhas espectrais associadas aos subníveis Stark dos íons de érbio. Esse novo método possibilitou a obtenção de sensores de temperatura com alta sensibilidade por um grande intervalo de temperatura. Os resultados obtidos indicam que nanocristais de ítria dopados com érbio são muito promissores para sensoriamento térmico em nanoescala. / São Cristóvão, SE