Grain boundary processes play a major role in controlling different rate processes in yttria stabilized tetragonal zirconia. The present study concentrated on rate processes in tetragonal zirconia, which were significantly influenced by the grain boundary processes. In this present study, nanocrystalline zirconia with grain size as low as 66 nm and density as high as 98.5% was processed using a two steps sintering-sinterforging process in the temperature range of 1473K to 1373K. Significant suppression of grain growth was noted in the second step of the two step process. It was observed that two step sintering-sinterforging process can reduce the processing time by an order of magnitude compared to the two step sintering process. A high grain size dependency of 3.3 indicated grain boundary controlled process dominating this technique. The dense nanocrystalline zirconia was used for microstructural and deformation characterization. An influence of electric field on grain growth behaviour was studied by annealing the specimens at 1573K for 10 hours under an applied field of 4 V/cm to 80 V/cm. It was noticed that grain growth was significantly retarded under a very weak field and the magnitude of retardation dependent on the applied voltage, an extensive grain growth was observed on the other occasion when the applied voltage crossed the threshold value of 3.5V. It was proposed that electrical boundary resistance provides minima in the grain boundary energy during annealing and that retards the grain growth. This technique presented a huge potential application in ceramic processing involving rate process. Again the grain boundary process was reported to control this phenomenon. Low temperature creep properties of nanocrystalline zirconia were investigated in great detail in the present study. Grain boundary sliding was noted as the mode of deformation at 1423 K. Study on the specimens with wide range of grain sizes (65 nm to ~0.4 µm) suggested that the deformation mechanism of the nanograin is similar to that of the submicron grain zirconia. A study on the segregation of yttrium ions to the grain boundaries showed that the segregation behaviour of nanograin and submicron grain 3YTZ was similar, which again indicated towards the possibility of nanocrystalline tetragonal zirconia to be superplastic as the scaling law was applicable from submicron to nanocrystalline 3YTZ. Grain boundary sliding is the mode of deformation of 3YTZ at high temperatures. This study aimed at understanding the influence of grain boundary sliding on rate processes at the boundary namely grain boundary diffusion. Grain boundary diffusivity of the deformed specimens was measured using secondary ion mass spectroscopy. The study revealed that the sliding process is much slower compared to the atomic jumps causing grain boundary diffusion, hence no significant influence of the grain boundary sliding on grain boundary diffusion was observed. This present study demonstrated new techniques which have a huge potential application in processing ceramics at low temperatures. This study also developed an understanding of the grain boundary processes which involved in low temperature rate processes of nanocrystalline zirconia.

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Nanocrystalline £\-Zr condensates deposited by ion beam sputtering on the NaCl (100) surfaces and then annealed at 100 oC to 750 oC in air. The phases present were identified by transmission electron microscopy to be nanometer-size £\-Zr+ZrO¡B£\-Zr+ZrO+c-ZrO2¡Bc-ZrO2¡Bc-+t-ZrO2¡Bt-ZrO2¡Band t-+m-ZrO2 phase assemblages with increasing annealing temperature. The zirconia showed strong {100} preferred orientation due to parallel epitaxy with NaCl (100) when annealed between 150 oC and 500 oC in air. The c- and t-zirconia condensates also showed (111)-specific coalescence among themselves. The c- and/or t-ZrO2 formation can be accounted for by the small grain size, the presence of low-valence Zr cation and the lateral constraint of the neighboring grains.

The anode material in a conventional design of solid oxide fuel cell (SOFC) operating above 1123 K is typically made from NiO/Zirconia. NiO/Zirconia anodes are known to perform well in hydrogen but exhibit difficulties when natural gas is used as a fuel. Natural gas is much cheaper than hydrogen and widely available but causes carbon deposition and deactivation of the NiO/Zirconia SOFC anode. One objective of this work was to prepare and characterize NiO/Zirconia anodes both as powders and as applied to extruded zirconia tubes. The problem of carbon deposition when NiO/Zirconia anodes operate in methane, the main component of natural gas, was investigated. Another aim was to address the problem of coking with an effort to moderate carbon deposition by using additives to the NiO/Zirconia anode. Temperature programmed reduction (TPR) was used to study the reaction characteristicso f NiO/Zirconia anodes.T he carbon depositedo n thesea nodesa fter methane decomposition and reforming was characterized using temperature programmed oxidation (TPO). The anodes were placed in a reactor (stainless-steel, alumina or zirconia) tube in a test assembly developed for an extruded tubular SOFC. The reactor inlet was connected to a flexible gas handling system and the exhaust to a continuously sampling mass spectrometer. This system also allowed simultaneous study of electrical and catalytic measurementsThe various conditions for methane reforming have been shown to influence the character of carbon deposited and the quantity deposited. Conditions such as anode calcination temperature, anode reduction regime, reforming temperature and reforming time have all been shown to influence the reactions occuring on the anode including carbon deposition, subsequently characterized by TPO. NiO/Zirconia powders have also been shown to behave differently from NiO/Zirconia anodes adhered to zirconia tubes. An alkali metal additive has been shown to moderate carbon deposition and improve cell performance. Small additions of Li, typically 1 mol %, to the Ni/Zirconia anode cause a decrease in carbon deposition after reforming at temperatures of 1123 K and 1173 K. The activation energy of surface carbon removed by oxygen is lowered by approximately 50 kJ mot' for the 1 mol % Li doped Ni/Zirconia anode compared to the undoped powder. Anodes doped with Li displayed greater cell performances. The improvements seen with these additives show that their use could offer a viable alternative to conventional anodes in current SOFC systems. Tubular solid oxide fuel cells have been tested in a custom built rig whereby electronic and catalytic measurements can be sampled simultaneously. This was used to monitor the influence of drawing current on the reactions occurring on the anode. The presence of alkali Li on the doped Ni anode surface has been shown to interfere with surface reactions under electrochemical load/steam reforming

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Made available in DSpace on 2014-06-11T19:28:38Z (GMT). No. of bitstreams: 0 Previous issue date: 2013-03-14Bitstream added on 2014-06-13T19:37:29Z : No. of bitstreams: 1 ribeiro_brg_me_arafo.pdf: 4642522 bytes, checksum: 066941fb16edc09232849f9d5518af78 (MD5) / O lascamento e a delaminação da porcelana de cobertura são considerados problemas técnicos frequentes em restaurações cerâmicas com infraestrutura em zircônia Y-TZP. O reparo intraoral com resina composta pode ser uma alternativa de tratamento para solucionar esse problema. Esse estudo avaliou o efeito de diferentes tratamentos de superfície e agentes de união sobre a união entre resina composta e zircônia Y-TZP envelhecida. Corpos-de-prova de zircônia Lava (n=14) foram confeccionados em forma de blocos (7,0 x 7,0 x 1,7 mm). Metade dos espécimes foi submetida ao envelhecimento hidrotérmico em autoclave a 134°C, 3 bar e 8 horas, enquanto a outra metade não sofreu envelhecimento (grupo controle). Difração de raios-X mostrou picos característicos das fases tetragonal e monoclínica, após a zircônia ser submetida ao tratamento em autoclave. As amostras foram subdivididas, de acordo com o tratamento de superfície: jateamento com partículas de alumina 50 μm, deposição triboquímica de sílica 30 μm (Rocatec Soft) ou sem tratamento abrasivo. Microscopia eletrônica de varredura, Espectroscopia por Dispersão de Energia (EDS) e Difração de raios-X foram utilizados para caracterização das amostras após os tratamentos. Clearfil Porcelain Bond Activator + Clearfil SE Bond; fina camada dos cimentos resinosos Panavia F ou RelyX U100 foram utilizados como agentes de união, antes da aplicação da resina composta Filtek Z350 XT, na forma de um cilindro (4,0 x 2,0 mm). Os nove subgrupos experimentais foram submetidos à ciclagem térmica (6.000 ciclos, 5 e 55°C, 30 segundos de imersão) antes do ensaio de resistência de união ao cisalhamento, em máquina de ensaios mecânicos EMIC DL2000 a uma velocidade de 0,5 mm/min. Os valores de resistência de união foram avaliados por meio da Análise de... / Chipping and adhesive failure of the veneer porcelain are considered frequent technical problems in zirconia YTZP-based ceramics restorations. The intraoral repair with composite resin become as an alternative treatment to solve this problem. This study evaluated the effect of different surface treatments, hydrothermal aging and bonding agent about the union Y-TZP zirconia/composite resin. 360 Lava zirconia specimens were made in the form of blocks (7.0 x 7.0 x 1.7 mm). Half of the specimens were subjected to hydrothermal aging by autoclaving at 134°C, 3 bar and 8 hs, while the other half were not subjected to aging (control group). X-ray diffraction analysis showed characteristic peaks of tetragonal and monoclinic phase, after the zirconia be subjected to autoclave treatment. The samples were subdivided according to the surface treatment: Sandblasting with 50 μm alumina particles, tribochemical silica coating 30 μm (Rocatec Soft) or without abrasive treatment. Scanning electron microscopy, Energy-dispersive spectroscopy (EDS) and x-ray diffraction were used for surface characterization. After the surface treatment 3 different bonding agents were applied: Clearfil Porcelain Bond Activator + Clearfil SE Bond; thin layer of luting cements Panavia F or RelyX U100. A cylinder (2.0 x 4.0 mm) of composite Filtek Z350 XT was built on the prepared zirconia to simulate the repairing. The nine experimental subgroups (n = 14) were thermal cycled (6,000 cycles, 5 and 55°C, 30-second dwell time) before the test of shear bond strength in mechanical testing machine EMIC DL2000 (0.5 mm/min). Bond strength values were evaluated using 2-way ANOVA and Tukey’s test (α = 5%). The fracture analyzed as adhesive failure, cohesive or mixed. All specimens without abrasive surface treatment debonded during thermal cycling. The results... (Complete abstract click electronic access below)

Made available in DSpace on 2014-06-11T19:28:56Z (GMT). No. of bitstreams: 0 Previous issue date: 2013-03-08Bitstream added on 2014-06-13T20:18:33Z : No. of bitstreams: 1 sciasci_p_me_arafo.pdf: 542826 bytes, checksum: 39a4037cc6365c2fcfc7d2bfaae5f727 (MD5) / A zircônia tetragonal policristalina estabilizada por ítria (Y-ZTP), em virtude de suas propriedades mecânicas, pode ser indicada para restaurações dento ou implantosuportadas. Porém, em função de sua composição, este material apresenta dificuldade de se unir eficazmente com os cimentos, comprometendo assim a longevidade das restaurações. O objetivo desse estudo foi avaliar a influência de diferentes tratamentos de superfície e cimentos na resistência ao cisalhamento (RC) de uma Y-ZTP, assim como a caracterização química da superfície da zircônia após os jateamentos. Duzentos e quarenta discos de Y-ZTP (5 x 2 mm) foram obtidos e aleatoriamente divididos em 5 grupos (n=40) de acordo com os tratamentos de superfície: 1) jateamento com óxido de alumínio (Al2O3) de 50 μm + silano; 2) jateamento com Al2O3 de 120 μm + silano; 3) jateamento com partículas de Al2O3 modificadas por sílica (Rocatec Soft) (30 μm) + silano; 4) jateamento com Al2O3 de 120 μm seguido do jateamento com partículas de Al2O3 modificadas por sílica (110 μm) (Rocatec Plus) e 5) Rocatec Plus + silano. Discos de resina composta (Filtek Z350 XT) foram confeccionados e imediatamente cimentados sobre a superfície da zircônia com os seguintes cimentos (n=12): 1) RelyX Luting 2; 2) RelyX ARC; 3) RelyX U100; e 4) Panavia F 2.0. Os silanos usados foram: 1) RelyX Ceramic Primer para os cimentos RelyX Luting 2, RelyX ARC e RelyX U100 e 2) Clearfil SE Bond Primer/Clearfil Porcelain Bond Activator para o cimento Panavia F 2.0. Após a cimentação, os espécimes foram armazenados por 6 dias em meio seco à 37ºC antes da ciclagem térmica (10.000 ciclos, 5 – 55ºC, 30 s/banho) para então serem submetidas ao teste de RC em máquina universal de ensaios (EMIC DL2000). Os dados foram analisados por meio de ANOVA 2 fatores e teste de Tukey ( =0,05)... / Due to their excellent mechanical properties, biocompatibility, low thermal conductivity, and chemical stability, Y-TZP (yttria-stabilized tetragonal zirconia polycrystal) ceramics have been used to manufacture metal-free tooth- or implantsupported prostheses. However, the conventional surface treatments (hydrofluoric acid etching and silanization) used for silica ceramics are not effective in promoting suitable bonding between resin-based materials and zirconia. Therefore, the purpose of this in vitro study was to evaluate the effect of different surface treatments on the shear bond strength (SBS) of luting cements to yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) and on its surface chemical characterization. Two hundred and forty Y-TZP discs (5 × 2 mm) were obtained and randomly divided into 5 airborneparticle abraded groups (n=48): 1) 50 μm Al2O3 particles; 2) 120 μm Al2O3 particles; 3) 30 μm silica-modified Al2O3 particles (Rocatec Soft); 4) 120 μm Al2O3 particles, followed by 110 μm silica-modified Al2O3 particles (Rocatec Plus); and 5) Rocatec Plus. After airborne-particle abrasion the surfaces were treated with silane coupling agent. Composite resin discs (Filtek Z350 XT) were produced and bonded immediately to the zirconia-treated surfaces (n=12) with: 1) RelyX Luting 2; 2) RelyX ARC; 3) RelyX U100; or 4) Panavia F 2.0. The silanes used were RelyX Ceramic Primer (RelyX Luting 2, RelyX ARC and RelyX U100) and Clearfil SE Bond Primer/Clearfil Porcelain Bond Activator (Panavia F 2.0). The bonded specimens were stored for 6 days in dry conditions at 37ºC before thermal cycling (10,000 cycles, 5 to 55ºC) and then tested for SBS in a mechanical testing machine (EMIC DL2000). Data were analyzed by two-way ANOVA and Tukey HSD post hoc test ( =.05). Failure mode was determined with a stereomicroscope (×20). The... (Complete abstract click electronic access below)

El objetivo del presente trabajo de título es la aplicación de catalizadores de paladio soportado en óxido de zirconia de alta área, a la reducción de emisiones de NOx en procesos de combustión de metano. Esto se fundamenta en los altos niveles que este contaminante presenta en la atmósfera de Santiago. Además, el interés de este estudio radica en la posibilidad de usar gas natural como una solución al problema y en la imposibilidad de utilizar para ello convertidores catalíticos tradicionales. Se estudió como soporte al óxido de zirconia, sintetizándose con diversos métodos de modo estabilizar una alta área superficial y con ello favorecer altas dispersiones de la fase activa de paladio. Con este fin se utilizó azufre, sodio y níquel incorporado a la zirconia en diferentes etapas de la síntesis del catalizador. Utilizando distintas cargas de paladio, estos materiales se utilizaron como catalizadores para efectuar la combustión bajo 600ºC con el fin de evitar la producción de NOx. Asimismo, suponiendo que estos contaminantes han sido emitidos, se utilizaron estos materiales para efectuar la reducción de NO con metano en exceso de oxígeno. Los resultados muestran que cuando se reprime la combustión de metano, mediante el uso de soportes ácidos, los catalizadores muestran actividad en reducción de NO. Esta actividad aumenta en la medida que crece el área del soporte, lo cual se logra introduciendo en la síntesis sodio o níquel como estabilizadores. Este último elemento permite obtener un catalizador altamente activo en reducción de NO y muy estable en el tiempo. En contraste, la reacción de combustión de metano a baja temperatura es independiente del área del soporte. Se concluye que es factible lograr una disminución importante de los contaminantes NOx, ya que es posible obtener catalizadores activos aplicables a los procesos de combustión de metano, a través de la estabilización del óxido de zirconia de alta área, introduciendo sodio o níquel al soporte. Se propone el estudio en condiciones húmedas del catalizador cuyo soporte se ha estabilizado con Ni, ya que en condiciones secas usadas en reducción de NO, su desempeño es promisorio. Lo mismo ocurre con el soporte dopado con sodio aplicado a combustión de metano.