A zircônia tetragonal estabilizada por ítria (Y-TZP) têm sido utilizada na área odontológica para próteses livres de metais devido à estética associada ao alto desempenho mecânico. Porém, a presença de ambiente úmido pode causar a transformação acelerada da fase tetragonal para monoclínica e consequente falha catastrófica deste material, processo este conhecido como degradação a baixa temperatura ou envelhecimento. A cinética desta transformação é função da composição química da cerâmica e sua microestrutura. Tendo em vista que métodos químicos permitem a síntese de pós cerâmicos à base de zircônia de dimensões nanométricas, cuja microestrutura da cerâmica sinterizada é constituída por grãos submicrométricos quimicamente homogêneos, e que a presença de alumina é indicada para evitar a degradação de fases da zircônia, o objetivo deste estudo foi verificar a degradação a baixa temperatura e ambiente úmido de cerâmicas de zircônia estabilizada com 3 mol% de ítria (Y-TZP) e do compósito Y-TZP/Al2O3, proveniente de pós sintetizados pela rota de coprecipitação. A concentração de alumina na Y-TZP foi estudada na faixa de 0,05 a 20% em massa. A eficiência do processo desenvolvido foi verificada pela avaliação das características físicas dos pós obtidos (granulometria, área de superfície específica, estado de aglomeração e estrutura cristalina). As amostras cerâmicas foram prensadas, sinterizadas e avaliadas quanto à densidade aparente e microestrutura. Após a caracterização inicial das cerâmicas a degradação das amostras foi estudada in vitro em reator hidrotérmico pressurizado a 150°C. As amostras (n=4) foram submetidas à análise de difração de raios X de acordo com o tempo de envelhecimento, acompanhando a curva cinética de transformação de fase. A porcentagem de cada fase cristalina foi determinada pelo Método de Rietveld. A relação entre o tempo de envelhecimento e a concentração de fase monoclínica foi determinada pela equação de Avrami modificada por Kolmogorow (Johnson-Mehl- Avrami-Kolmogorow JMAK). Após envelhecimento a 150°C por 70 horas, todas as amostras contendo alumina apresentaram menor concentração de fase monoclínica, comparativamente à cerâmica Y-TZP, que apresentou 66,5% dessa fase. Menores porcentagens de fase monoclínica após o envelhecimento hidrotérmico foram obtidas com a adição de 10 e 20% em massa de alumina na matriz de zircônia, sendo esses valores 59,1 e 52,9%, respectivamente. Deve-se considerar, no entanto, que a diminuição da degradação total é consequência da menor porcentagem de zircônia na matriz em função da adição de alumina. Neste contexto, o efeito benéfico da adição de alumina ocorre apenas no início do envelhecimento. / The yttria tetragonal zirconia polycrystal (Y-TZP) is used in dentistry for metal free prosthesis due to esthetics associated with a high mechanical performance. However, the presence of humid environment can cause an accelerated tetragonal to monoclinic (t-m) phase transformation and consequent catastrophic failure of this material. This process is known as low temperature degradation (LTD) or aging. The kinetics of phase transformation is a function of the chemical composition of the ceramic and its microstructure. Considering that chemical methods allow the synthesis of nanometric zirconium-based ceramic powders, which microstructure of the sintered ceramic consists of submicrometric chemically homogeneous grains, and that the presence of alumina is indicated to delay the tetragonal phase degradation, the aim of this study was to verify the degradation at low temperature in humid environment of 3mol% yttria stabilized zirconia ceramics (Y-TZP) and the Y-TZP/Al2O3 composite prepared from coprecipitated powders. The addition of alumina at Y-TZP was studied in the range of 0.05 to 20wt%. The efficiency of the developed process was verified by the evaluation of the physical characteristics of the obtained powders (granulometry, specific surface area, agglomeration state and crystalline structure). The ceramic samples were pressed, sintered and submitted to apparent density and microstructure evaluation. After the initial characterization of the ceramics, the in vitro degradation of the samples was studied in a hydrothermal pressurized reactor at 150°C. The samples (n = 4) were submitted to X-ray diffraction analysis according to the aging time, followed by the determination of the kinetic curve of phase transformation. The Rietveld Method was employed to determine the percentage of each crystalline phase. The relationship between the aging time and the percentage of monoclinic phase was determined by the Johnson- Mehl-Avrami-Kolmogorow equation (JMAK). After 70 hours aging at 150°C, all the alumina-containing samples presented a lower concentration of monoclinic phase, compared to the Y-TZP ceramics, which monoclinic phase concentration was 66.5%. The lower percentages of monoclinic phase after hydrothermal aging were obtained with the addition of 10%wt and 20 wt% alumina in the zirconia matrix (59.1% and 52.9%, respectively). This behavior is due to the lower concentration of zirconia in the composite containing alumina. In this point of view beneficial effect due to alumina addition occurs in the early stage of aging.

A zircônia tetragonal estabilizada por ítria (Y-TZP) têm sido utilizada na área odontológica para próteses livres de metais devido à estética associada ao alto desempenho mecânico. Porém, a presença de ambiente úmido pode causar a transformação acelerada da fase tetragonal para monoclínica e consequente falha catastrófica deste material, processo este conhecido como degradação a baixa temperatura ou envelhecimento. A cinética desta transformação é função da composição química da cerâmica e sua microestrutura. Tendo em vista que métodos químicos permitem a síntese de pós cerâmicos à base de zircônia de dimensões nanométricas, cuja microestrutura da cerâmica sinterizada é constituída por grãos submicrométricos quimicamente homogêneos, e que a presença de alumina é indicada para evitar a degradação de fases da zircônia, o objetivo deste estudo foi verificar a degradação a baixa temperatura e ambiente úmido de cerâmicas de zircônia estabilizada com 3 mol% de ítria (Y-TZP) e do compósito Y-TZP/Al2O3, proveniente de pós sintetizados pela rota de coprecipitação. A concentração de alumina na Y-TZP foi estudada na faixa de 0,05 a 20% em massa. A eficiência do processo desenvolvido foi verificada pela avaliação das características físicas dos pós obtidos (granulometria, área de superfície específica, estado de aglomeração e estrutura cristalina). As amostras cerâmicas foram prensadas, sinterizadas e avaliadas quanto à densidade aparente e microestrutura. Após a caracterização inicial das cerâmicas a degradação das amostras foi estudada in vitro em reator hidrotérmico pressurizado a 150°C. As amostras (n=4) foram submetidas à análise de difração de raios X de acordo com o tempo de envelhecimento, acompanhando a curva cinética de transformação de fase. A porcentagem de cada fase cristalina foi determinada pelo Método de Rietveld. A relação entre o tempo de envelhecimento e a concentração de fase monoclínica foi determinada pela equação de Avrami modificada por Kolmogorow (Johnson-Mehl- Avrami-Kolmogorow JMAK). Após envelhecimento a 150°C por 70 horas, todas as amostras contendo alumina apresentaram menor concentração de fase monoclínica, comparativamente à cerâmica Y-TZP, que apresentou 66,5% dessa fase. Menores porcentagens de fase monoclínica após o envelhecimento hidrotérmico foram obtidas com a adição de 10 e 20% em massa de alumina na matriz de zircônia, sendo esses valores 59,1 e 52,9%, respectivamente. Deve-se considerar, no entanto, que a diminuição da degradação total é consequência da menor porcentagem de zircônia na matriz em função da adição de alumina. Neste contexto, o efeito benéfico da adição de alumina ocorre apenas no início do envelhecimento. / The yttria tetragonal zirconia polycrystal (Y-TZP) is used in dentistry for metal free prosthesis due to esthetics associated with a high mechanical performance. However, the presence of humid environment can cause an accelerated tetragonal to monoclinic (t-m) phase transformation and consequent catastrophic failure of this material. This process is known as low temperature degradation (LTD) or aging. The kinetics of phase transformation is a function of the chemical composition of the ceramic and its microstructure. Considering that chemical methods allow the synthesis of nanometric zirconium-based ceramic powders, which microstructure of the sintered ceramic consists of submicrometric chemically homogeneous grains, and that the presence of alumina is indicated to delay the tetragonal phase degradation, the aim of this study was to verify the degradation at low temperature in humid environment of 3mol% yttria stabilized zirconia ceramics (Y-TZP) and the Y-TZP/Al2O3 composite prepared from coprecipitated powders. The addition of alumina at Y-TZP was studied in the range of 0.05 to 20wt%. The efficiency of the developed process was verified by the evaluation of the physical characteristics of the obtained powders (granulometry, specific surface area, agglomeration state and crystalline structure). The ceramic samples were pressed, sintered and submitted to apparent density and microstructure evaluation. After the initial characterization of the ceramics, the in vitro degradation of the samples was studied in a hydrothermal pressurized reactor at 150°C. The samples (n = 4) were submitted to X-ray diffraction analysis according to the aging time, followed by the determination of the kinetic curve of phase transformation. The Rietveld Method was employed to determine the percentage of each crystalline phase. The relationship between the aging time and the percentage of monoclinic phase was determined by the Johnson- Mehl-Avrami-Kolmogorow equation (JMAK). After 70 hours aging at 150°C, all the alumina-containing samples presented a lower concentration of monoclinic phase, compared to the Y-TZP ceramics, which monoclinic phase concentration was 66.5%. The lower percentages of monoclinic phase after hydrothermal aging were obtained with the addition of 10%wt and 20 wt% alumina in the zirconia matrix (59.1% and 52.9%, respectively). This behavior is due to the lower concentration of zirconia in the composite containing alumina. In this point of view beneficial effect due to alumina addition occurs in the early stage of aging.

The present thesis focuses on the preparation of tetragonal zirconia doped with yttria oxide, ceric dioxide, and magnesium oxide. In its theoretical part the thesis covers the characteristics of doped ceramics, the possible methods of powders preparation and of their further processing. The experimental part describes the precipitation synthesis of zirconia in base environment and further processing for ceramics. Also studied were the influences of dopant and of the processing technique used on the phase composition and microstructure of the resulting ceramics. To describe the powder structure, the thermal, dilatometric, surface area, and x-ray analyses were performed; as well as both the scanning and transmission electron microscopy employed. Dry processed ceramics showed the average grain size of 87–94 nm and the relative density in the range of 84,0–99,3 %. In comparison, the ceramics produced of hydrothermally synthesised powders proved the average grain size of 75–85 nm and the relative density between 92,7-99,9 %. As the next step, samples were subjected to low-temperature degradation for the time of 5 and 15 hours. Samples consisting of pure tetragonal ZrO2 were resistant to 5 and 15 hours of low-temperature degradation at 180 °C.

OBJECTIVES: This study examined mechanical, chemical and microstructural properties of multilayer zirconia materials that are composed of layers of different forms of zirconia with varying translucency. Their resistance to low temperature degradation and their properties were compared to each other, and to monolithic zirconia. METHODOLOGY: “ZirCAD Prime” from Ivoclar Vivadent, “AxZir XT Multilayer Dental Zirconia” from Axsys Dental Solutions, and “inCoris ZI” from Dentsply Sirona, were examined. Twenty specimens were created from each material, half of which were aged. Specimens were examined for the following: Three point bending flexural strength, grain size, microhardness, indentation fracture toughness, warp and elemental composition. RESULTS: inCoris ZI had significantly higher flexural strength than ZirCAD Prime, which in turn had significantly higher strength than AxZir XT. The flexural strength values were 1113.55MPa, 857.21MPa and 625.77MPa, respectively. Grain size patterns were noted in multilayer specimens; more translucent layers had significantly larger grain sizes. AxZir XT’s incisal most layer average grain size was 0.988μm, whereas ZirCAD Prime’s was 1.172μm. The dentin most layer of AxZir XT average grain size was 0.529μm whereas ZirCAD Prime’s was 0.470μm. Microhardness results showed few significant differences between layers. The highest microhardness was found in AxZir XT’s incisal most layer, after aging, with a value of 13.502 GPa. The lowest was found in the aged inCoris ZI specimen, with a value of 10.775 GPa. In the ZirCAD Prime, fracture toughness was highest in the dentin most layer with a value of 8.88 MPa m¹/², compared to its incisal most layer that had a value of 4.92 MPa m¹/². This pattern was not seen in AxZir XT, where the dentin most layer had a value of 8.36 MPa m¹/², and the incisal most layer had a value of 6.40 MPa m¹/². Hydrothermal aging had detrimental and significant impacts on fracture toughness of all materials. Elemental composition analysis revealed predictable levels of elements or molecules in ZirCAD Prime. and inCoris ZI, but not within the AxZir XT. 5Y levels were seen in ZirCAD Prime’s incisal layer, and 2.5-3Y in the dentin most layer. inCoris ZI had constant levels of 3Y, but AxZir XT had no distinct level of Yttria in its layers. CONCLUSIONS: 1. Flexural Strength of multilayer materials was significantly lower than monolithic zirconia. 2. Grain sizes appeared largest in translucent incisal-most layers, with significant differences between them and the opaque dentin-most layers. 3. The elemental composition analysis showed an expected level of 3 mol% Yttria in the inCoris ZI with varying amounts by layer in the ZirCAD Prime from 3Y (cervical) to 5Y (incisal), but there was no clear gradation in the AxZir XT. 4. Some significant differences were seen between the materials and their layers in the microhardness tests. inCoris ZI had significantly lower values than both ZirCAD Prime and AxZir XT. The highest values were found within AxZir Xt. 5. Fracture toughness was significantly higher in the dentin-most layer compared to the incisal most-layer of ZirCAD Prime but not in AxZir XT. 6. LTD significantly decreased some fracture toughness test values. inCoris ZI, AxZir XT’s 1st incisal layer and ZirCAD Prime’s 2nd transition layer had significant decreases in fracture toughness after aging. 7. LTD had no impact on flexural strength or microhardness values. 8. LTD significantly decreased grain size of inCoris ZI.

[EN] Zirconia has become a widely utilized structural ceramic material with important applications in dentistry due to its superb mechanical properties, biocompatibility, aesthetic characteristics and durability. Zirconia needs to be stabilized in the t-phase to obtain improved mechanical properties such as hardness and fracture toughness. Fully dense yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) materials are normally consolidated through the energy-intensive processing of powders at very high temperatures (>1000 °C). Innovative non-conventional approaches are being developed to reduce time and energy consumption and, consequently, environmental impact in ceramic powder processing. Microwave sintering is one such approach aimed at fully-densifying ceramics by using a different heating mechanism based on the material's inherent dielectric properties. The main purpose of this work is to obtain highly dense Y-TZP dental materials from commercial and lab-prepared sources via microwave sintering with mechanical and microstructural qualities that are similar or even improved with respect to their conventionally sintered counterparts. Therefore, its effect on important aspects related to dental applications has been studied. First, Y-TZP ceramics have been characterized upon sintering to determine whether the resulting properties meet the minimum mechanical requirements for structural dental applications. Second, the influence of microwave sintering on hydrothermal degradation, a spontaneous ageing phenomenon that affects zirconia materials in wet conditions, has been investigated. And third, the behavior under fretting wear conditions of microwave and conventionally sintered materials has been assessed to evaluate their durability and performance. The main conclusions indicate that microwave sintering allows proper consolidation of dental Y-TZP materials resulting in a finer microstructure due to shorter processing time and mechanical properties comparable, and in some cases enhanced, to those obtained in conventional sintered materials at lower dwell temperatures. Additionally, a higher resistance to hydrothermal degradation has been determined for microwave sintered materials due to a finer grain size and lower sintering temperatures that reduce the presence of cubic phase, which is responsible for destabilizing neighboring tetragonal phase grains. Finally, a similar wear rate has been obtained between microwave and conventional sintering of zirconia materials under fretting wear conditions. In addition, humidity can reduce the wear volume loss due to the lubricative effect of water and wear of degraded materials might increase the resistance due to the formation of a protective debris layer. In general, microwave sintering can be an interesting alternative for obtaining fully-densified Y-TZP dental materials providing certain advantages over conventional methods. Nonetheless, more studies are still necessary to have a better understanding of the advantages and disadvantages of microwave sintering of zirconia ceramics. / [ES] La circona es un material ampliamente utilizado como cerámica estructural con aplicaciones en el ámbito dental debido a sus propiedades mecánicas, biocompatibilidad, características estéticas y durabilidad. Para poder aprovechar las altas propiedades mecánicas de la circona, es necesario estabilizarla en su fase tetragonal. Los materiales de circona policristalina estabilizada con itria (Y-TZP) se consolidan normalmente a través de polvos mediante procesos energéticamente intensivos a altas temperaturas (>1000 °C). Actualmente, se están desarrollando técnicas basadas en métodos no convencionales para reducir el tiempo y el consumo energético en el procesado de polvos cerámicos. La sinterización por microondas tiene por objetivo la densificación completa mediante la utilización de mecanismos de calentamiento basados en las propiedades dieléctricas del material. El objetivo principal es la obtención de materiales dentales de Y-TZP altamente densos mediante la sinterización por microondas con propiedades mecánicas y microestructurales similares, o incluso por encima de las obtenidas por el método convencional. Para ello, se estudian aspectos relevantes al ámbito dental. En primer lugar, los materiales son caracterizados con el fin de determinar si las propiedades finales cumplen con los requisitos mecánicos para aplicaciones dentales. Además, se ha investigado la influencia de la sinterización por microondas en la degradación hidrotérmica, un fenómeno espontáneo de envejecimiento que afecta a los materiales de circona en condiciones de humedad. Finalmente, se ha evaluado el comportamiento en condiciones de desgaste fretting de los materiales sinterizados para determinar su durabilidad. Las conclusiones principales indican que la sinterización por microondas permite la consolidación adecuada de estos materiales, resultando en una microestructura más fina debido a los tiempos más cortos de procesado y en propiedades mecánicas comparables a las de materiales obtenidos mediante el método convencional, incluso a temperaturas más bajas. Una mayor resistencia a la degradación hidrotérmica se ha determinado en materiales sinterizados por microondas. Al emplear temperaturas más bajas se reduce la presencia de fase cúbica, la cual es responsable por la desestabilización de granos adyacentes de fase tetragonal. Tasas de desgaste similares han sido observadas entre materiales sinterizados por microondas y convencionalmente bajo condiciones de desgaste fretting. Adicionalmente, la humedad puede reducir sustancialmente la pérdida de volumen de desgaste debido al efecto lubricante del agua y los materiales degradados pueden aumentar la resistencia a este tipo de desgaste como consecuencia de la formación de una capa protectora de material que se desprende más fácil. En general, la sinterización por microondas es una alternativa interesante para obtener materiales dentales de Y-TZP altamente densos con ciertas ventajas sobre los métodos convencionales pero deben considerarse también las desventajas de esta técnica. / [CAT] La circona és un material àmpliament utilitzat com a ceràmica estructural amb aplicacions en l'àmbit dental a causa de les seues propietats mecàniques, biocompatibilidad, característiques estètiques i durabilitat. Per a poder aprofitar les altes propietats mecàniques de la circona, és necessari estabilitzar-la en la seua fase tetragonal. Els materials de circona policristalina estabilitzada amb itria (Y-TZP) es consoliden normalment mitjançant processos energèticament intensius a altes temperatures (>1000 °C). Actualment, s'estan desenvolupant tècniques basades en mètodes no convencionals per a reduir el temps i el consum energètic en el processament de la pols ceràmicas. La sinterització per microones té per objectiu la densificació completa mitjançant la utilització de mecanismes d'escalfament basats en les propietats dielèctriques del material. L'objectiu principal d'aquesta tesi és l'obtenció de materials dentals de Y-TZP altament densos mitjançant la sinterització per microones amb propietats mecàniques i microestructurals superiors a les obtingudes per mètodes convencionals. En primer lloc, els materials seràn caracteritzats per a determinar si les propietats finals compleixen amb els requisits mecànics per a aplicacions dentals. En segon lloc, s'investigarà la influència de la sinterització per microones en la degradació hidrotèrmica, un fenomen espontani d'envelliment que afecta als materials de circona en condicions d'humitat. I en tercer lloc, s'avaluarà el comportament en condicions de desgast fretting dels materials sinteritzats per a determinar la seua durabilitat. Les conclusions principals indiquen que la sinterització per microones permet la consolidació adequada i millorada de materials de Y-TZP, amb una microestructura més fina a causa dels temps més curts de processament i propietats mecàniques comparables a les de materials obtinguts mitjançant el mètode convencional, fins i tot a temperatures més baixes. Un factor positiu ha sigut la major resistència a la degradació hidrotèrmica en materials sinteritzats per microones. A més, al emprar temperatures més baixes es redueix la presència de fase cúbica, la qual és la responsable de la desestabilització de grans adjacents de fase tetragonal. Finalment, sota condicions de desgast fretting, s'han observat taxes de desgast similars entre materials sinteritzats per microones i via convencional. Addicionalment, en condicions de 100% d'humitat es pot reduir substancialment la pèrdua de volum de desgast a causa de l'efecte lubrificant de l'aigua i materials degradats, els quals poden augmentar la resistència a aquest tipus de desgast com a conseqüència de la formació d'una capa protectora de material que es desprèn amb més facilitat. En general, la sinterització per microones és una alternativa molt interessant per a obtindre materials dentals Y-TZP òptims i amb certes avantatges sobre els mètodes convencionals, però han de considerar-se també algunes desavantatges d'aquesta tècnica. / Presenda Barrera, Á. (2016). ADVANCED CERAMIC MATERIALS FOR DENTAL APPLICATIONS SINTERED BY MICROWAVE HEATING [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/68510 / TESIS

[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.

Indiana University-Purdue University Indianapolis (IUPUI)

In the iron production processes, sinters and pellets are mostly used as raw materials due to their consistency with respect to physical and chemical properties. However, natural iron ores, as mined, are rarely used directly as a feed material for iron processing. This is mainly due to the fact that they have small contents of iron and high concentration of impurities. Moreover, they swell and disintegrate during the descent in the furnace as well as due to low melting and softening temperatures. This work involves an investigation of the parameters that influence the use of natural iron ores as a direct feed material for iron production. Furthermore, it points out ways in which these can be mitigated so as to increase their direct use in iron production. Natural iron ore from Muko deposits in south-western Uganda was used in this study. Initially, characterisation of the physical and chemical properties was performed, to understand the natural composition of the ore. In addition, investigations were done to study the low temperature strength of the ore and its behaviour in the direct reduction zone. Also, simulations were performed with three models using the experimental data from the direct reduction experiments in order to determine the best model for predicting the direct reduction kinetics of natural iron ores. Chemical analyses showed that the Muko ore represents a high grade of hematite with an Fe content of 68% on average. The gangue content (SiO2+Al2O3) in 5 of the 6 investigated iron ore samples was &lt; 4%, which is within the tolerable limits for the dominant iron production processes. The S and P contents were 0001-0.006% and 0.02-0.05% respectively. These can be reduced in the furnace without presenting major processing difficulties. With respect to the mechanical properties, the Muko ore was found to have a Tumble Index value of 88-93 wt%, an Abrasion Index value of 0.5-3.8 wt% and a Shatter Index value of 0.6-2.0 wt%. Therefore, the ore holds its form during the handling and charging processes. Under low temperature investigations, new parameters were discovered that influence the low temperature strength of iron oxides. It was discovered that the positioning of the samples in the reduction furnace together with the original weight (W0) of the samples, have a big influence on the low temperature strength of iron oxide. Higher mechanical degradation (MD) values were obtained in the top furnace reaction zone samples (3-25% at 500oC and 10-21% at 600oC). These were the samples that had the first contact with the reducing gas, as it was flowing through the furnace from top to bottom. Then, the MD values decreased till 5-16% at a 500oC temperature and 6-20% at a 600oC temperature in the middle and bottom reaction zones samples. It was found that the obtained difference between the MD values in the top and other zones can be more than 2 times, particularly at 500oC temperature. Furthermore, the MD values for samples with W0 &lt; 5 g varied from 7-21% well as they decreased to 5-10% on average for samples with W0 ≥ 5 g. Moreover, the MD values for samples taken from the top reaction zone were larger than those from the middle and bottom zones. During direct reduction of the ores in a H2 and CO gas mixture with a ratio of 1.5 and a constant temperature, the reduction degree (RD) increased with a decreased flow rate until an optimum value was established. The RD also increased when the flow rate was kept constant and the temperature increased. An optimum range of 3-4g was found for natural iron ores, within which the highest RD values that are realised for all reduction conditions. In addition, the mechanical stability is greatly enhanced at RD values &gt; 0.7. In the case of microstructure, it was observed that the original microstructure of the samples had no significant impact on the final RD value (only 2-4%). However, it significantly influenced the reduction rate and time of the DR process. The thermo-gravimetric data obtained from the reduction experiments was used to calculate the solid conversion rate. Three models: the Grain Model (GM), the Volumetric Model (VM) and the Random Pore Model (RPM), were used to estimate the reduction kinetics of natural iron ores. The random pore model (RPM) provided the best agreement with the obtained experimental results (r2 = 0.993-0.998). Furthermore, it gave a better prediction of the natural iron oxide conversion and thereby the reduction kinetics. The RPM model was used for the estimation of the effect of original microstructure and porosity of iron ore lumps on the parameters of the reduction process. / <p>QC 20130531</p> / Sustainable Technology Development in the Lake Victoria Region

Ce travail de thèse est une étude par des méthodes de simulation de structure électronique du phénomène de dégradation en milieu aqueux de la zircone yttriée. La zircone yttriée est notamment utilisée pour la fabrication de prothèses dont la durée de vie dépend du matériau et de son environnement. Pour ces applications, la zircone yttriée est de structure tétragone, mais en fonction du dopage en yttrium et de l'environnement, cette phase est en compétition avec des structures monoclinique et cubique. Cette compétition est cruciale dans ce travail car elle peut à la fois, augmenter la résistance, ou favoriser la détérioration de ce matériau. L'étude réalisée se concentre sur des taux de dopage inférieurs à 20% mol. en oxyde d'yttrium Y2O3.Dans un premier temps, les structures atomiques d'équilibre à basse température ont été déterminées dans les trois phases en utilisant une méthode de calcul basée sur la Théorie de la Fonctionnelle de la Densité, dans l'approximation de la densité locale. Les résultats incluent de nouvelles structures qui n'ont jusqu'alors pas été discutées dans la littérature et qui sont cohérentes avec le diagramme de phase de la zircone yttriée à basse température. Une analyse plus détaillée suggère que le mécanisme de stabilisation de la zircone yttriée tetragonale est un effet du dépistage ionocovalent particulièrement efficace dans cette phase et dont la signature dans la structure s'exprime par la présence d'ions zirconium en coordination 7. Ceci représente un point important permettant de lier stabilité et structure dans ces systèmes. Une deuxième partie de ce travail est consacrée à l’effet de l'eau dans la zircone yttriée en volume. Pour conclure, ce manuscrit représente une contribution originale à la compréhension de mécanismes à l'échelle atomique qui sont à l'origine des propriétés de la zircone yttriée et de ses applications.yttriée et de ses applications. / This PhD thesis is an electronic structure simulation work to study the stabilization mechanism of yttria stabilized zirconia (YSZ) and the phenomena of the degradation of YSZ due to the presence of an aqueous media. YSZ is used especially in dental and orthopedics application sbut its service depends on the environment. For these biomedical applications a crystallographic tetragonal YSZ (t-YSZ) structure is used, but depending on the concentration of yttria (Y2O3) and the environment, this tetragonal structure is thermodynamically in competition with a monoclinic and cubic phases. This competition is crucial in this work, because it has both effects : increase the resistance or promote the degradation of this material. The study is conducted for Y2O3 concentrations less than 20% mol. First, atomic structures of the three phases at low temperature were determined using density functional theory (DFT)with the local density approximations (LDA). The results include new structures that were not discussed in the literature and which are consistent with the phase diagram of YSZ at low temperature. A more detailed analysis suggests that the stabilization mechanism in YSZ is due to an effective ionocovalent screening : particularly in t-YSZ phase, its signature is expressed in Zr-ions with a coordination number of 7. This represents an important point for linking stability and structure in these systems. A second part of this work is devoted to the effect of water on YSZ bulk systems which leads to low temperature degradation (LTD) of YSZ based bioceramics materials. In conclusion, this PhD thesis represents an original contribution to the understanding of the mechanism and properties of YSZ and its applications at the atomic scale.