Neste trabalho foram preparados e caracterizados catalisadores de platina suportados em g-alumina (g-Al2O3), óxido de zircônio (ZrO2) e em suportes mistos de óxido de zircônio e g-alumina. Os suportes bi-componente contendo (15%) ZrO2/g-Al2O3 foram preparados por dois métodos distintos: no primeiro, utilizou-se uma suspensão ácida de óxido de zircônio comercial e no segundo, óxido de zircônio precipitado a partir do acetato. Os catalisadores foram submetidos a ensaios catalíticos de reforma a vapor de etanol a fim de verificar o efeito do suporte sobre a distribuição dos produtos. As técnicas de caracterização utilizadas foram área superficial específica (B.E.T), redução a temperatura programada (RTP), difração de raios-X (DRX) e espectroscopia de absorção molecular na região UV-VI, que identificou a espécie [PtOxCly]s2-. A conversão do etanol e a distribuição dos produtos variaram para cada suporte testado e para os diferentes tempos de contatos (W/F) utilizados. O catalisador (1%) Pt/g-Al2O3 apresentou a melhor seletividade para a produção de hidrogênio e a menor conversão para o etanol. Os catalisadores que utilizaram o suporte bi-componente (15%) ZrO2/g-Al2O3 e ZrO2 pura apresentaram alta conversão de etanol e baixa seletividade para hidrogênio, além de alta tendência à formação de bio-produtos. / In this work catalysts of platinum supported on g-alumina (g-Al2O3), oxide of zirconium (ZrO2) and mixed oxide of zirconium and g-alumina had been prepared and characterized. The bi-component supports containing ( 15%) ZrO2/g-Al2O3 had been prepared by two different methods : in the first, an acid suspension of comercial oxide of zirconium was used and in the second, oxide of zirconium was precipitated from acetate. The catalysts were tested in ethanol steam reforming reaction to verify the effect of the support on the distribution of the products. For the characterization, the following techniques were used. Superficial specific area (B.E.T,), temperature programmed reduction (TPR.), X-ray powder diffraction (XRD) and spectroscopy of molecular absorption in the UV-vis, for identification of [PtOxCly]s 2- species. The ethanol conversion and product distribution were different for each support tested and for different time of contact (W/F) used. The catalyst (1%) Pt/g-Al2O3 presented the best selectivity for the hydrogen production and the lower conversion for ethanol. The catalysts with bi-component support (15%) ZrO2/ g-Al2O3 and pure ZrO2 presented high ethanol conversion and lower selectivity for hydrogen production with high selectivity and formation of bio-products.

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In part I, MgO and Co1-xO powders in 9:1 and 1:9 molar ratio (denoted as M9C1 and M1C9 respectively) were sintered and homogenized at 1600oC followed by annealing at 850 and 800oC, respectively to form defect clusters and precipitates. Analytical electron microscopic (AEM) observations indicated the protoxide remained as rock salt structure with complicated planar diffraction contrast for M9C1 sample, however with spinel paracrystal precipitated from the M1C9 sample due to the assembly of charge- and volume-compensating defects of the 4:1 type, i.e. four octahedral vacant sites surrounding one Co3+-filled tetrahedral interstitial site. The spacing of such defect clusters is 4.5 times the lattice spacing of the average spinel structure of Mg-doped Co3-dO4, indicating a higher defect cluster concentration than undoped Co3-dO4. The {111} faulting of Mg-doped Co3-dO4/Co1-xO in the annealed M1C9 sample implies the possible presence of zinc blend-type defect clusters with cation vacancies assembled along oxygen close packed (111) plane. In part II, the Mg2TiO4/MgO composites prepared by reactive sintering MgO and TiO2 powders (9:1 molar ratio) at 1600oC and then air-cooled or further aged at 900oC were studied by X-ray diffraction and (AEM) in order to characterize the microstructures and formation mechanism of nanosized Mg2TiO4 spinel precipitated from Ti-doped MgO. Expulsion of Ti4+ during cooling caused the formation of (001)-specific G.P. zone under the influence of thermal/sintering stress and then the spinel precipitates, which were about 30 nm in size and nearly spherical with {111} and {100} facets to minimize coherency strain energy and surface energy. Secondary nano-size spinel was precipitated and became site saturated during aging at 900oC, leaving a precipitate free zone at the grain boundaries of Ti-doped MgO. The intergranular spinel became progressively Ti-richer upon aging 900oC and showed <110>-specific diffuse scatter intensity likely due to short range ordering and/or onset decomposition. In part III, the Co1-xO/Co2TiO4 composite prepared by reactive sintering CoO and TiO2 powders (9:1 molar ratio) at 1450oC and then air-cooled were studied by X-ray diffraction and AEM in order to characterize the microstructures and formation mechanism of nanosized Co2TiO4 spinel precipitated from Ti-doped Co1-xO. Slight expulsion of Ti4+ during cooling caused the precipitation of nanosize Co2TiO4 spinel. Bulk site saturation also caused impingement of the Co2TiO4 precipitates upon growth. The Co3-dO4 spinel, as an oxidatin product of Co1-xO, was found to form at free surface and the Co1-xO/Co2TiO4 interface. The Co2TiO4 spinel particles formed by reactive sintering rather than precipitation were able to detach from the Co1-xO grain boundaries to reach parallel epitaxial orientation with respect to the host Co1-xO grains via Brownian-type rotation of the embedded particles. In part IV, AEM was used to study the defect microstructures of Zr-dissolved TiO2 prepared via reactive sintering the ZrO2 and TiO2 powders (8:92 in molar ratio, designated as Z8T92) at 1600oC for 24 h and then aged at 900oC for 2-200 h in air. The Zr-dissolved TiO2 with rutile structure showed dislocation arrays, defect clusters, G.P. zone, superlattice, nanometer-size domains incommensurate and commensurate superstructure, may be the precursor of ZrTi2O6 precipitates at 900oC. The rutile showed diffuse diffractions along [001] direction as a result of Zr4+ substitution for Ti4+ with volume compensating defect clusters. Incommensurate and commensurate structures, as indicated by diffraction splitting and extra diffraction along <100> and <010> directions may be attributed to the ordering and clustering process of Zr and Ti atoms in these directions. Part V, deals with the reactive sintering of ZrO2 and TiO2 powders (1:4 molar ratio) at 1400 to 1600oC in air to form orthorhombic ZrTiO4 (a-PbO2-type structure, denoted as a) and to study its epitaxial reorientation in the matrix of tetragonal TiO2 (rutile) grains with Zr4+ (15 mol %) dissolution. The epitaxial relationship of intragranular ZrTiO4 and Zr-dissolved rutile (denoted as r) was determined by electron diffraction as [010]a//[011]r; (001)a // (011)r (i.e. [100]a // [100]r; (001)a // (011)r). The reorientation of the intragranular particles in the composites can be reasonably explained by rotation of the nonepitaxial particles above a critical temperature (T/Tm > 0.8) and below a critical particle size for anchorage release at interface with respect to the host grain. Reactive sintering facilitated the reoreientation process for the particles about to detach from the grain boundaries. The Brownian rotation of the confined ZrTiO4 particles in rutile grains was activated by a beneficial lower interfacial energy for the epitaxial relationship, typically forming lath-like ZrTiO4 with (101)a/(211)r habit plane having fair match of oxygen atoms at the interface. Further aging at 900oC for 50 h in air caused modulated and periodic antiphase domains in ZrTiO4 matrix, as likely precursor of equilibrium ZrTi2O6.

Made available in DSpace on 2016-06-02T20:34:36Z (GMT). No. of bitstreams: 1 4430.pdf: 7319951 bytes, checksum: b21bea92cf852895996f96fd4057b891 (MD5) Previous issue date: 2012-05-09 / Universidade Federal de Minas Gerais / In this work the fundamental aspects of electrolytic breakdown in ZrO2 films were investigated. The electrolytic breakdown is a type of dielectric breakdown that occurs during galvanostatic growth of anodic oxides. To perform this investigation, the study was divided in two parts: (i) the modification of ZrO2 films by anodic doping and (ii) characterization of the electrical discharges (sparks) that appears on the electrode surface during zirconium anodization. The anodic doping study aimed to monitor the system and analyze the influence of current density, temperature and dopant concentration on the electrochemical, morphological and microstructural properties of ZrO2 films doped with calcium and niobium ions. The factorial design technique was used to optimize the number of experiments and quantify the effects of experimental variables on the responses. The results showed that all variables influenced the electrochemical response, leading to changes in morphology and microstructural properties of the ZrO2 doped films. This result was observed because these variables can affect the mobility of ionic species, ionic transport inside the oxide, the electrolyte resistivity and the dissolution process of the oxide. The spark characterization study aimed to investigate the evolution of electrical discharges during the experiment and correlate them with the processes observed in the anodization curve of zirconium. Hence, the electrode surface was recorded during the experiments using a high definition and high speed video camera. The image analysis allowed the calculation of the size, quantity and lifetime of the sparks discharges. Due to the enormous amount of data, it was necessary to develop a new methodology to analyzing the results. The methods used were based on techniques of imaging processing and multivariate data analysis. The use of these techniques coupled to electrochemical ones provided new insights that elucidated some aspects of the breakdown phenomenon. Furthermore, it provided guidelines for the implementation of future work. / Neste trabalho foram investigados aspectos fundamentais do fenômeno de ruptura eletrolítica do ZrO2, que é um tipo de ruptura dielétrica observada durante o crescimento galvanostático de óxidos anódicos. Para isso foram feitos dois estudos dirigidos, um sobre a modificação do ZrO2 por dopagem anódica e outro de caracterização das descargas elétricas (sparks) visualizadas na superfície do eletrodo durante a anodização do Zr. O objetivo desse primeiro estudo foi mapear o sistema, analisando a influência da densidade de corrente, temperatura e concentração do dopante sobre o comportamento eletroquímico e sobre as propriedades morfológicas e microestruturais dos filmes de ZrO2 dopados com íons cálcio e nióbio. A técnica de planejamento fatorial foi utilizada com o intuito de otimizar o número de experimentos e quantificar os efeitos das variáveis de preparação sobre as respostas investigadas. Os resultados mostraram que todas as variáveis de preparação investigadas afetaram as respostas eletroquímicas, visto que influenciaram a mobilidade das espécies iônicas, o transporte de íons dentro do óxido, a resistividade do meio e a dissolução do óxido, levado a modificações nas propriedades morfológicas e microestruturais dos filmes formados. Já o estudo de caracterização dos sparks teve como objetivo estudar a evolução das descargas elétricas ao longo do experimento e correlacioná-las com os processos observados na curva de anodização do zircônio. Desta forma, a superfície do eletrodo foi filmada durante os experimentos, em definição e alta velocidade de captura de imagens, o que permitiu o cálculo do tamanho, quantidade e tempo de duração dos sparks. Devido à enorme quantidade de dados gerados, foi necessário o desenvolvendo de novas metodologias para a análise dos resultados, que foram baseadas em técnicas de tratamento de imagens e análise multivariada de dados. A utilização dessas técnicas acopladas às técnicas eletroquímicas, forneceram novos insights que auxiliaram na elucidação de alguns aspectos do fenômeno de ruptura eletrolítica no ZrO2. Além disso, forneceram diretrizes para a execução de trabalhos futuros.

Catalisadores bifuncionais a base de óxido de zircônio modificado por óxidos de tungstênio (W = 10, 15 e 20 %) ou molibdênio (Mo= 10, 15 e 20 %) contendo platina (Pt = 1 %) foram preparados pelo método dos precursores poliméricos. Por comparação, catalisadores a base de tungstênio também foram preparados pelo método de impregnação. Após calcinações a 600, 700 e 800 ºC, os catalisadores foram caracterizados por difração de raios-X, espectroscopia de absorção na região do infravermelho, análise termogravimétrica, análise térmica diferencial, adsorção de nitrogênio e microscopia eletrônica de varredura. Os perfis de redução dos metais foram determinados por redução a temperatura programada. Os catalisadores sintetizados foram testados na isomerização do n-heptano. Os difratogramas de raios-X dos catalisadores Pt/WOx-ZrO2 e Pt/MoOx-ZrO2 revelaram a presença do ZrO2 tetragonal e da platina metálica em todas as amostras calcinadas. Picos de difração referentes ao WO3 e ao ZrO2 monoclínico também foram observados em algumas das amostras dos catalisadores Pt/WOx-ZrO2. Nos catalisadores do tipo Pt/MoOx-ZrO2 também foram observados picos de difração referente ao ZrO2 monoclínico e ao óxido Zr(MoO4)2. O aparecimento destas outras fases contidas nos catalisadores Pt/WOx-ZrO2 e Pt/MoOx- ZrO2 variaram de acordo com o teor de W ou Mo e de acordo com a temperatura de calcinação. Os espectros de absorção na região do infravermelho exibiram bandas de absorção referentes as ligações O-W-O e W=O nos catalisadores Pt/WOx-ZrO2 e referentes as ligações O-Mo-O, Mo=O e Mo-O nos catalisadores Pt/MoOx-ZrO2. A área superficial específica dos catalisadores Pt/WOx-ZrO2 variou de 30-160 m2 g-1 e para os catalisadores do tipo Pt/MoOx-ZrO2 variou de 10-120 m2 g-1. O teor de metais (W ou Mo) e a temperatura de calcinação exercem uma influência direta no valor da área superficial específica das amostras. Os perfis de redução dos catalisadores Pt/WOx-ZrO2 exibiram dois picos a baixas temperaturas, os quais são atribuídos a redução da platina. A redução das espécies WOx foi evidenciada por dois picos de redução a altas temperaturas. No caso dos catalisadores Pt/MoOx-ZrO2, os perfis de redução mostram três eventos de redução, os quais são atribuídos a redução das espécies MoOx depositadas no suporte e em algumas amostras um dos picos é relacionado com a redução do óxido Zr(MoO4)2. Os catalisadores Pt/WOx-ZrO2 foram ativos para a isomerização do n-heptano com alta seletividade a 3-metil-hexano, 2,3-dimetil-pentano e 2-metil-hexano entre outros hidrocarbonetos ramificados. Os catalisadores Pt/MoOx- ZrO2 praticamente não apresentaram atividade para a isomerização do n-heptano, gerando principalmente produtos oriundos do craqueamento catalitico. _________________________________________________________________________________________ ABSTRACT: Bifunctional catalysts based on zircon oxide modified by tungsten (W = 10, 15 and 20 %) and by molybdenum oxide (Mo= 10, 15 e 20 %) containg platinum (Pt = 1%) were prepared by the polymeric precursor method. For comparison, catalysts the tungsten base was also prepared by the impregnation method. After calcinations at 600, 700 and 800 ºC, the catalysts were characterized by X-ray diffraction, fourier-transform infrared spectroscopy, thermogravimetric and differential thermal analysis, nitrogen adsorption and scanning electron microscopy. The profile of metals reduction was determined by temperature programmed reduction. The synthesized catalysts were tested in n-heptane isomerization. X-ray diffractogram of the Pt/WOx-ZrO2 and Pt/MoOx-ZrO2 catalysts revealed the presence of tetragonal ZrO2 and platinum metallic phases in all calcined samples. Diffraction peaks due WO3 and ZrO2 monoclinic also were observed in some samples of the Pt/WOx-ZrO2 catalysts. In the Pt/MoOx-ZrO2 catalysts also were observed diffraction peaks due ZrO2 monoclinic and Zr(MoO4)2 oxide. These phases contained on Pt/WOx-ZrO2 and Pt/MoOx-ZrO2 catalysts varied in accordance with the W or Mo loading and in accordance with the calcination temperature. The infrared spectra showed absorption bands due O-W-O and W=O bonds in the Pt/WOx-ZrO2 catalysts and due O-Mo-O, Mo=O and Mo-O bonds in the Pt/MoOx-ZrO2 catalysts. Specific surface area for Pt/WOx-ZrO2 catalysts varied from 30-160 m2 g-1 and for the Pt/MoOx-ZrO2 catalysts varied from 10-120 m2 g-1. The metals loading (W or Mo) and the calcination temperature influence directly in the specific surface area of the samples. The reduction profile of Pt/WOx-ZrO2 catalysts showed two peaks at lower temperatures, which are attributed to platinum reduction. The reduction of WOx species was evidenced by two reduction peak at high temperatures. In the case of Pt/MoOx-ZrO2 catalysts, the reduction profile showed three reduction events, which are attributed to reduction of MoOx species deposited on the support and in some samples one of the peak is related to the reduction of Zr(MoO4)2 oxide. Pt/WOx-ZrO2 catalysts were active in the n-heptane isomerization with high selectivity to 3-methyl-hexane, 2,3- dimethyl-pentane, 2-methyl-hexane among other branched hydrocarbons. The Pt/MoOx-ZrO2 catalysts practically didn't present activity for the n-heptane isomerization, generating mainly products originating from the catalytic cracking.

Neste trabalho foram preparados e caracterizados catalisadores de platina suportados em g-alumina (g-Al2O3), óxido de zircônio (ZrO2) e em suportes mistos de óxido de zircônio e g-alumina. Os suportes bi-componente contendo (15%) ZrO2/g-Al2O3 foram preparados por dois métodos distintos: no primeiro, utilizou-se uma suspensão ácida de óxido de zircônio comercial e no segundo, óxido de zircônio precipitado a partir do acetato. Os catalisadores foram submetidos a ensaios catalíticos de reforma a vapor de etanol a fim de verificar o efeito do suporte sobre a distribuição dos produtos. As técnicas de caracterização utilizadas foram área superficial específica (B.E.T), redução a temperatura programada (RTP), difração de raios-X (DRX) e espectroscopia de absorção molecular na região UV-VI, que identificou a espécie [PtOxCly]s2-. A conversão do etanol e a distribuição dos produtos variaram para cada suporte testado e para os diferentes tempos de contatos (W/F) utilizados. O catalisador (1%) Pt/g-Al2O3 apresentou a melhor seletividade para a produção de hidrogênio e a menor conversão para o etanol. Os catalisadores que utilizaram o suporte bi-componente (15%) ZrO2/g-Al2O3 e ZrO2 pura apresentaram alta conversão de etanol e baixa seletividade para hidrogênio, além de alta tendência à formação de bio-produtos. / In this work catalysts of platinum supported on g-alumina (g-Al2O3), oxide of zirconium (ZrO2) and mixed oxide of zirconium and g-alumina had been prepared and characterized. The bi-component supports containing ( 15%) ZrO2/g-Al2O3 had been prepared by two different methods : in the first, an acid suspension of comercial oxide of zirconium was used and in the second, oxide of zirconium was precipitated from acetate. The catalysts were tested in ethanol steam reforming reaction to verify the effect of the support on the distribution of the products. For the characterization, the following techniques were used. Superficial specific area (B.E.T,), temperature programmed reduction (TPR.), X-ray powder diffraction (XRD) and spectroscopy of molecular absorption in the UV-vis, for identification of [PtOxCly]s 2- species. The ethanol conversion and product distribution were different for each support tested and for different time of contact (W/F) used. The catalyst (1%) Pt/g-Al2O3 presented the best selectivity for the hydrogen production and the lower conversion for ethanol. The catalysts with bi-component support (15%) ZrO2/ g-Al2O3 and pure ZrO2 presented high ethanol conversion and lower selectivity for hydrogen production with high selectivity and formation of bio-products.

The master thesis deals mainly with the optimization of machining of ceramic semi-finished products from ZrO2 and Al2O3. The first part with literary research, which describes the steps of the technology of forming ceramic bodies, especially the gelcasting method. The thesis describes possibilities of milling of ceramic materials using CNC technology. The thesis describes the dependence of acquired roughness on bodies and used machining strategies. The best roughness parameters on ZrO2 samples were obtained after they were calcined at 900 ° C, while the lowest roughness was obtained on the samples in the unprotected state for the Al2O3 samples. Samples machined with a spherical milling cutter showed surface roughness Ra = 1 m for ZrO2 and Ra = 1.3 m for Al2O3. It also describes the possibility of machining sharp-edged shapes and fine details where the best results are achieved on ZrO2 samples engraved at 900 and 1100 °C. Tool wear was particularly important when machining samples of Al2O3 ignited at temperatures above 800 °C.

The work was focused on the preparation of layered ceramic materials and their characterizations. The direct measurment of weight deposite for enhanced description of one component system was studied within this work. The kinetics of electrophoretic deposition obtained from theoretical calculation and from experimental values were confronted. It was prepared a lot of depositions for described kinetic of electrophoretic deposition with applied constant currents. The relative density and porosity were determined on the annealled and sintered bodies. The hardness measurments were performed on sintered bodies and then resulted values were confronted with the used currents. A ceramic composite based on Al2O3 and ZrO2 was prepared by using of precision describtion of electrophoretic deposition kinetic.

Composite materials with laminated structure provide advantages which are utilised during component design. Low density, temperature and chemical stability are the profitable properties predetermining application of ceramic laminates. The main obstacle for wide spread of ceramic materials is their inherent brittleness. Therefore, in this thesis, the crack propagation in ceramics laminates has been extensively studied. Laminated structures with various volume fractions of components (alumina and zirconia) were prepared by electrophoretic deposition. Evaluation of crack propagation through the interface and determination of basic mechanical properties was conducted on the basis of extensive literature search. Crack deflection originated in both presence of internal stresses and differences in elastic modulus during the crack interface passing was monitored. A special type of specimen geometry was employed with the aim to set arbitrary angle between crack and interface. It was experimentally found that the degree of crack deflection is dependent on entering angle and volume fraction of components. Higher crack deflection was already found in the bulk of the test piece comparing to the test piece surface. The 3D fracture surface reconstruction generated using laser confocal microscopy was used in this detailed crack propagation study. Further basic elastic and strength characteristics of laminates were determined and compared to those obtained from monolithic materials. The validity of the mix rule for elastic characteristics was confirmed by comparing of elastics modulus. The most reliable method for elastic modulus determination was marked the dynamic resonance method due to low scatter and consistency in measurement. The flexural strength of all laminates tends to be close to the flexural strength of the weakest component. Therefore the mix rule is not applicable for flexural strength estimation on the contrary of elastic characteristics. The change of component volume fraction leads only to change of flexural strength scatter. Thanks to gained knowledge about crack propagation and basic characteristic determination will be possible to design ceramic laminates more efficiently for given needs of application.