Tesi sul tema "Zirconia"

Made available in DSpace on 2014-10-09T12:37:38Z (GMT). No. of bitstreams: 0
Made available in DSpace on 2014-10-09T14:00:18Z (GMT). No. of bitstreams: 1 05330.pdf: 1385235 bytes, checksum: 83cf2a9d722cc0781168d05ce0005ce0 (MD5)
Dissertacao (Mestrado)
IPEN/D
Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

Thermal mismatch between different components of a system could cause of problems like residual stress induced cracking, thermal fatigue or even optical misalignment in certain high technology applications. Use of materials with customized thermal expansion coefficient is a counter-measure to resolve such problems. Zirconium tungstate (ZrW2O8) with negative thermal expansion coefficient is capable of being used in synthesis of composites with tailored coefficient of thermal expansion (CTE). In this work, the sol-gel method which had been already set up in our group was characterized and the sources of the factors imposing impurities in the product were distinguished in all the steps of precursor preparation and heat treatment. In the second part of study, zirconium tungstate particles synthesized by the sol-gel method were utilized as core in synthesis of ZrW2O8&ndash
ZrO2 core&ndash
shell composite particles. Shell layer was composed of ZrO2 nanocrystallites and precipitated from an aqueous solution by urea hydrolysis. Volume of the shell was effectively controlled by concentration of the initial zirconium ion in the solutions. The rate of precipitation was a function of the ratio of initial urea concentration to zirconium ion. It is hypothesized that isolation of the ZrW2O8 within a layer of ZrO2, will be a key element in solving problems associated with reactivity of ZrW2O8 towards other components in sintering of ceramic&ndash
ceramic composites with tuned or zero thermal expansion coefficient.

The objectives for the thesis were to generate tough ceramics utiising the toughening mechanisms inherent to zirconia materials. The aims have been realised with the successful fabrication of hot pressed silicon nitride / zirconia composite ceramics. The zirconia was prestabilised with two different types of dopant additives, yttria and ceria, with the intention of understanding the chemical compatibility with the silicon nitride matrix and the overall effect on the subsequent mechanical properties. The volume fraction of added zirconia was also varied. The increased toughness over silicon nitride materials alone was attributed to the toughening agents inherent to zirconia which existed either in the form of the tetragonal polymorph or the monoclinic variant. The toughening modes were dependent on initial chemistry of the composite system. When the zirconia was prestabilised with yttria the tetragonal polymorph was retained within the composite. The enhanced toughness was attributed to a transformation toughening mechanism. However, when the zirconia was prestabiised with ceria the depletion of Ce from solid solution with the zirconia during processing resulted in the formation of the unstabiised monoclinic variant. The enhanced toughness was attributed, in this case, to a microcrack type energy absorption mechanism, similar to several ZTA composite ceramics. Additionally, an experiment using ultrasound non-destructive testing, indicated that Tetragonal Zirconia Polycrystals (TZP) is ferroelastic and, as such, can provide a significant contribution to enhanced levels of fracture toughness in these materials or composites containing the same. Further work has been conducted to actually observe, as a function of applied unia.xial stress, the crystallographic changes occurring within the bulk of a 3Y-TZP ceramic via neutron elastic scattering at the ILL, Grenoble, France. This experiment has provided clear direct proof of the ferroelastic nature of zirconia. A similar experiment will be carried out at the Rutherford Laboratory, though with significantly improved statistics. An approach to improve the high temperature properties of TZP via the chemical alteration of the grain boundary phase was also considered. As a preliminary step the grain boundary volume was increased through controlled additions of the grain boundary composition in the form of both a premilled and a premelted glass. Poor fired densities were attained, however, due to the solute additive partitioning from the generation of an enhanced grain boundary phase to overstabilisation of the zirconia resulting in the formation of cubic stabilised zirconia. Furthermore, the incorporation of nitrogen within the grain boundary phase, via sintering TZP with sole additions of A1N, resulted in the attainment of poor fired densities and hence was not considered a suitable method for grain boundary modification.

The above factors emphasize the scope of this thesis for further investigations on zirconia, the improvement of all-ceramic zirconia restorations, and especially the interaction of zirconia and veneering and its influence on the performance of the whole restoration. The introduction, chapter 1, gave a literature overview on zirconia ceramics. In chapter 2, the objective of the study was to evaluate the effect of abrading before and after sintering using alumina-based abrasives on the surface of yttria-tetragonal zirconia polycrystals. Particular attention was paid to the amount of surface stress–assisted phase transformation (tetragonal→monoclinic) and the presence of microcracks. Chapter 3 is based on the idea that the conventional sintering techniques for zirconia based materials, which are commonly used in dental reconstruction, may not provide a uniform heating, with consequent generation of microstructural flaws in the final component. As a consequence of the sintering system, using microwave heating, may represent a viable alternative. The purpose of the study was to compare the dimensional variations and physical and microstructural characteristics of commercial zirconia (Y-TZP), used as a dental restoration material, sintered in conventional and microwave furnaces. Chapter 4 described the effect of sandblasting before and after sintering on the surface roughness of zirconia and the microtensile bond strength of a pressable veneering ceramic to zirconia.

The above factors emphasize the scope of this thesis for further investigations on zirconia, the improvement of all-ceramic zirconia restorations, and especially the interaction of zirconia and veneering and its influence on the performance of the whole restoration. The introduction, chapter 1, gave a literature overview on zirconia ceramics. In chapter 2, the objective of the study was to evaluate the effect of abrading before and after sintering using alumina-based abrasives on the surface of yttria-tetragonal zirconia polycrystals. Particular attention was paid to the amount of surface stress–assisted phase transformation (tetragonal→monoclinic) and the presence of microcracks. Chapter 3 is based on the idea that the conventional sintering techniques for zirconia based materials, which are commonly used in dental reconstruction, may not provide a uniform heating, with consequent generation of microstructural flaws in the final component. As a consequence of the sintering system, using microwave heating, may represent a viable alternative. The purpose of the study was to compare the dimensional variations and physical and microstructural characteristics of commercial zirconia (Y-TZP), used as a dental restoration material, sintered in conventional and microwave furnaces. Chapter 4 described the effect of sandblasting before and after sintering on the surface roughness of zirconia and the microtensile bond strength of a pressable veneering ceramic to zirconia.

Made available in DSpace on 2014-10-09T12:38:11Z (GMT). No. of bitstreams: 0
Made available in DSpace on 2014-10-09T14:04:43Z (GMT). No. of bitstreams: 1 01779.pdf: 1867284 bytes, checksum: 561bc30f019f8009c1ab308bcee6e3ca (MD5)
Dissertacao (Mestrado)
IPEN/D
Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

Made available in DSpace on 2014-10-09T12:45:07Z (GMT). No. of bitstreams: 0
Made available in DSpace on 2014-10-09T14:06:57Z (GMT). No. of bitstreams: 1 07016.pdf: 7788234 bytes, checksum: f8eff0ec5a7b678c12027aceb93c324d (MD5)
Dissertacao (Mestrado)
IPEN/D
Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

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)

The aim of this study was to investigate a zirconia-matrix reinforced with metal powder (chromium, iron and stainless steel (AISI 316)) including processing, characterisation, and measurements of their properties (mechanical, thermal and electrical). Zirconia stabilised with 5.4 wt% Y₂0₃ (3 mol%) as the matrix was first studied and followed by an investigation of the effects of metal reinforcement on zirconia-matrix composites. Monolithic zirconia was pressureless sintered in air and argon to observe the effect of sintering atmosphere, while the composites were pressureless sintered in argon to avoid oxidation. Sintering was carried out at various temperatures for 1 hour and 1450°C was chosen to get almost fully dense samples. The density of the fired samples was measured using a mercury balance method and the densification behaviour was analysed using TMA (Thermo-mechanical Analysis). The TMA was also used to measure the coefficient of thermal expansion. In addition, thermal analysis using DTA and TGA was performed to observe reactions and phase transformations. Moreover, optical microscopy and SEM were used to observe the microstructures, XRD was used for phase identification, and mechanical properties including Vickers hardness, fracture toughness and bending strength were measured. The effect of thermal expansion mismatch on thermal stresses was also analysed and discussed. Finally, thermal diffusivity at room temperature and as a function of temperature was measured using a laser flash method, and to complete the study, electrical conductivity at room temperature was also measured. The investigation of monolithic zirconia showed that there was no significant effect of air and argon atmosphere during sintering on density, densification behaviour, microstructures, and properties (mechanical and thermal). Furthermore, the results were in good agreement with that reported by previous researchers. However, the presence of metal in the composites influenced the sintering behaviour and the densification process depends on the metal stability, reactivity, impurity, particle size, and volume fraction. Iron reacted with yttria (zirconia stabiliser), melted and reduced the densification temperature of monolithic zirconia, while chromium and AISI 316 did not significantly affect the densification temperature and did not react with either zirconia or yttria. AISI 316 melted during fabrication. Moreover, all of the metal reinforcements reduced the final shrinkage of monolithic zirconia. In terms of properties, the composites showed an increase in fracture toughness, and a reduction in Vickers hardness and strength with increasing reinforcement content. In addition, the thermal diffusivity of the composites showed an increase with reinforcement content for the zirconia/chromium and zirconia/iron composites, but not for the zirconia/AISI 316 composites due to intrinsic mircocracking. Furthermore, all the composites became electrically conductive with 20 vol% or more of reinforcement. It has been concluded that of those composites the zirconia/chromium system may be considered as having the best combination of properties and although further development is needed for such composites to be used in real applications in structural engineering, the materials may be developed based on these findings. In addition, these findings may be used in development of ceramic/metal joining as composite interlayers are frequently used.

Mestrado em Materiais e Dispositivos Biomédicos
Zircónia e alumina são biocerâmicos bastante conhecidos, e são usados principalmente em aplicações ortopédicas. Estes materiais têm sido aplicados em implantes de anca e joelho graças à reduzida taxa de desgaste e à excelente biocompatibilidade que apresentam. No entanto, estes cerâmicos apresentam também algumas limitações: a fragilidade da alumina ao impacto e a sensibilidade da zircónia ao envelhecimento. Devido a estas limitações, o objectivo passou por desenvolver implantes mais resistentes e com uma resposta inflamatória menos intensa. Surgiram, então, estudos de compósitos de zircónia e alumina. Neste estudo, foram desenvolvidas três diferentes composições de Alumina reforçada com Zircónia (ZTA) com 80 wt% a 90 wt% de alumina, e três composições de Zircónia reforçada com Alumina (ATZ) com 80 wt% a 90 wt% de zircónia. Foram usados dois tipos diferentes de zircónia estabilizada: zircónia estabilizada com 3 mol% de yttria (3YSZ) que foi usada nos compósitos ATZ, e zircónia estabilizada com 2 mol% de yttria (2YSZ) aplicada nos compósitos ZTA. A composição ATZ com o melhor conjunto de propriedades foi também testada com a zircónia 2YSZ, de forma a produzir um compósito com melhores propriedades mecânicas e uma resistência à degradação semelhante à apresentada pelo compósito ATZ, com a zircónia 3YSZ. Foram seleccionados dois aditivos, óxido de lântanio e pentóxido de tântalo, que foram depois adicionados aos compósitos ATZ e ZTA com o melhor conjunto de propriedades (composição 80:20), com o objectivo de melhorar a resistência ao envelhecimento e as propriedades mecânicas dos materiais produzidos. Após uma etapa de moagem, os pós compósitos foram obtidos por atomização, a partir de suspensões estabilizadas, com uma distribuição de tamanho de partícula nanométrica controlada. Estes pós foram caracterizados através de várias técnicas tais como microscopia electrónica de varrimento, difracção de raios-X, fluorescência de raios-X, densidade real, e área superficial específica. De forma a aumentar a densidade dos corpos em verde, foram efectuadas duas diferentes prensagens, prensagem uniaxial e prensagem isostática a frio (CIP). Foram obtidos cerâmicos com uma elevada densidade (com densidade relativa entre 97% e 99%) a uma baixa temperatura de sinterização (1400ºC). O tamanho de grão das amostras sinterizadas foi observado por SEM e, de forma a verificar as fases cristalográficas presentes, foi realizada difracção de raios-X. Em todos os compósitos foi obtida uma microestrutura dispersa, com um tamanho de grão nanométrico (abaixo dos 500 nm). Este conjunto de etapas de produção aplicado levou à obtenção de compósitos com propriedades mecânicas melhoradas. Foram estudadas a dureza de Vickers, a tenacidade à fratura e resistência à flexão das amostras sinterizadas. Os compósitos ATZ atingiram os melhores valores de tenacidade à fractura e resistência à flexão (acima de 5 MPa.m1/2 e 1394 MPa respetivamente), enquanto os compósitos ZTA apresentaram os melhores valores de dureza (acima de 1846 HV). Como era esperado, o compósito ATZ com zircónia 2YSZ apresentou melhores propriedades mecânicas, tendo sido obtidos 7.94 MPa.m1/2 de tenacidade à fratura e 1498 MPa para a resistência à flexão. A aplicação dos dopantes nos compósitos ZTA e ATZ induziram alterações nas suas propriedades. A adição de Ta2O5 melhorou, com sucesso, as propriedades mecânicas dos dois tipos de compósitos. Foi verificado um aumento dos valores de dureza, tenacidade à fratura e resistência à flexão em relação às amostras sem dopantes. A adição de La2O3 não levou a melhorias nas propriedades mecânicas mas, no entanto, também não teve um efeito prejudicial, o que levou à sua preservação. Foram realizados testes de envelhecimento de acordo com a norma ISO13356 (2008) em todos os compósitos produzidos. A quantidade de zircónia monoclínica, indicador de degradação, foi determinada por difracção de raios-X após 5,12,24,48 e 96 horas de testes de envelhecimento. Foi determinado que os compósitos ZTA não dopados, não apresentaram zircónia monoclínica após 96 horas em ambiente agressivo. Para os compósitos ATZ, apesar de a quantidade de zircónia monoclínica aumentar proporcionalmente ao conteúdo de zircónia presente no compósito, foi verificado que a extensão da degradação foi mínima, e relegada apenas para a superfície do material. Esta evidência permitiu que as propriedades mecânicas se mantivessem durante todo o período dos testes de degradação. Como era esperado, o compósito com a zircónia menos estável, o ATZ com a zircónia 2YSZ, apresentou o conteúdo mais elevado de zircónia monoclínica. No entanto, o facto de as propriedades mecânicas se manterem ao longo dos testes de degradação, confirmou que a degradação, mais uma vez, não se expandiu para o interior do material. A adição dos dois dopantes levou a melhorias na resistência à degradação dos compósitos ATZ, que apresentaram um conteúdo de zircónica monoclínica menor em comparação com as amostras não dopadas, após 96 horas de testes de degradação. No entanto, a adição de Ta2O5 teve um efeito desestabilizador na zircónia 2YSZ presente no compósito ZTA, sendo que foi detectada 10% de zircónia monoclínica após as 96 horas de testes de degradação. De novo, foi confirmado que esta degradação esteve presente apenas à superfície do material, visto que as propriedades mecânicas se mantiveram após estes testes. Foi também testada a biocompatibilidade destes compósitos. Células MG63 foram cultivadas nas amostras sinterizadas e foram realizados ensaios MTT e ensaios de atividade da fosfatase alcalina. Para todos os compósitos produzidos foi verificado que a viabilidade/proliferação celular aumentou significativamente desde o dia 1 para o dia 4. Os compósitos ZTA, que possuíam um maior número de locais de adesão, apresentaram uma maior adesão e proliferação celular, em comparação com os compósitos ATZ. A adição de La2O3 e Ta2O5 não induziu diferenças significativas na viabilidade celular dos compósitos ATZ. No entanto, no compósito ZTA, a adição de Ta2O5 levou a um pior desempenho devido à sua verificada hidrofobicidade. O presente estudo mostra que podem ser obtidas composições óptimas destes compósitos, com excelentes propriedades mecânicas, resistência à degradação e biocompatibilidade satisfatória.
Zirconia and alumina are well known bioceramics, used in the field of orthopedics. These materials have been used as hip and knee bearings thanks to their reduced wear rate and excellent biocompatibility. However, these ceramics presented some limitations: the brittleness of alumina and the aging sensitivity of zirconia. The aim became to develop long-lasting hip implants, with less inflammatory response and better designs. Zirconia alumina composites were then studied. In this study, three different grades of alumina toughened zirconia composites (ATZ) from 80 wt% to 90 wt% of zirconia, and three grades of zirconia toughened alumina (ZTA) from 80 wt% to 90 wt% of alumina were developed. Two different types of stabilized zirconia were used: 3 mol% yttria stabilized zirconia (3YSZ) was applied on the ATZ samples, and a 2 mol% yttria stabilized zirconia (2YSZ) on the ZTA samples. The ATZ with the best set of properties (80Z20A) was also tested with the 2YSZ, in order to produce a composite with improved mechanical properties and similar aging resistance to the ATZ with 3YSZ. Two selected additives, lanthanum oxide and tantalum pentoxide were added to the best ATZ and ZTA composite (80:20 composition) with the aim of enhance the aging resistance and mechanical properties of the produced materials. After a wet milling stage, the composite powders were achieved by spray-drying, from stabilized suspensions with a controlled nanometric particle distribution. The obtained composite powders were characterized through several techniques, such as scanning electron microscopy, X-ray diffraction, X-ray fluorescence, true density and specific surface area. Two stages of pressing, uniaxial pressing and cold isostatic pressing, were performed in order to improve the density of the green pieces. High density ceramics (with a relative density between 97% and 99%) were achieved with a low sintering temperature (1400ºC). The grain size of the sintered pieces was determined by SEM, and X-ray diffraction was performed in order to verify the present crystallographic phases. A disperse microstructure was obtained for all composites, with a nanometric grain size (under 500 nm). This set of producing stages, lead to the obtention of composites with enhanced mechanical properties. The Vickers Hardness, fracture toughness and flexural strength of the sintered samples were evaluated. Higher values of fracture toughness and flexural strength were achieved for the ATZ samples (up to 5 MPa.m1/2 and 1394 MPa respectively), while ZTA samples presented higher values of hardness (up to 1846 HV). As expected, the ATZ with 2YSZ presented enhanced mechanical properties, with an outstanding fracture toughness of 7.94 MPa.m1/2, and 1498 MPa of flexural strength. The addition of the two dopants to both ZTA and ATZ composites induced changes in their properties. The addition of Ta2O5 successfully improved the mechanical properties of both composites. In comparison with the undoped ATZ and ZTA composites, improvements of the hardness, fracture toughness and flexural strength were verified. The addition of La2O3 did not lead to an enhancement of the mechanical properties; however, it did not led to a deleterious effect either, and these properties were maintained. Accelerated aging tests were made on all produced composites, accordingly to ISO13356 (2008). The amount of monoclinic zirconia, which is an indicator of degradation on these composites, was quantified by X-ray diffraction analysis for 5,12,24,48 and 96 hours of aging tests. It was determined that, the undoped ZTA samples did not present monoclinic zirconia after 96 hours on an aggressive environment. Regarding the ATZ composites, even though the monoclinic zirconia content increased proportionally to the zirconia content present in the composite, it was found that the extent of degradation was minimal, since it was relegated to the material surface. This fact allowed the maintaining of the mechanical properties of the material throughout all the duration of the aging tests. As expected, the less stable composite, the ATZ with 2YSZ, presented the highest content of monoclinic zirconia. Nonetheless, the mechanical properties tested on the aged composite confirmed that the degradation did not expand to the material bulk. The addition of both dopants, successfully improved the aging resistance of the ATZ composite, presenting a lower amount of monoclinic zirconia after 96 hours of aging tests in comparison with the undoped ones. However, the addition of Ta2O5 destabilized the 2YSZ present on the ZTA composites, and 10% of monoclinic zirconia was detected after 96 hours of aging tests. Still, the mechanical properties were maintained on all the doped composites, which again confirmed the presence of degradation only at the material surface. The biocompatibility of these composites was also tested. MG63 cells were seeded on the sintered samples and MTT and alkaline phosphatase activity (ALP) assays were performed. The cell viability/proliferation increased significantly from day 1 to day 4 for all the produced composites. The ZTA composites, with more anchorage sites, presented higher cell adhesion and proliferation in comparison with the ATZ composites. The addition of La2O3 and Ta2O5 did not induced significant changes on the cell viability of the ATZ composites. However, the addition of Ta2O5 on the ZTA composite led to a poor performance, due to its verified hydrophobicity. The present study shows that optimal compositions of these composites can be achieved, with improved mechanical properties, hydrothermal degradation resistance and satisfactory biocompatibility.

Mestrado em Engenharia de Materiais
A Zircónia é um cerâmico avançado que se tem destacado como um material versátil e promissor, apresentando uma combinação interessante de propriedades térmicas, elétricas, óticas e mecânicas, pouco comuns nos materiais cerâmicos, sendo por isso utilizada em diversas aplicações. A empresa INNOVNANO produz pós de Zircónia com diferentes características e atualmente tem clientes interessados em cerâmicos à base de Zircónia estabilizada com ítria de coloração bege com propriedades mecânicas específicas. Assim, o trabalho realizado neste estágio pretendeu desenvolver um material que cumpra os requisitos solicitados, tendo como referência uma amostra cedida por um cliente da INNOVNANO cuja cor se pretendia reproduzir, mas cuja composição e processamento eram desconhecidos. Deste modo, o trabalho teve início com a caracterização da amostra de referência através de Microscopia Eletrónica de Varrimento, Difração de raios X, espectroscopia de Raman, Fotoluminescência e Refletância Difusa de modo a permitir compreender o mecanismo de coloração, o qual mostrou ser baseado em defeitos estruturais, desenvolvidos em condições redutoras. A introdução de defeitos extrínsecos pela dopagem com óxido de ferro (Fe2O3) na matriz da Zircónia estabilizada com 2 mol % de ítria (2YSZ) foi a alternativa selecionada para a reprodução da cor solicitada. As amostras foram estudadas do ponto de vista microestrutural, estrutural e ótico. A dopagem com óxido de ferro revelou ser um mecanismo de coloração adequado, reprodutível e irreversível, permitindo o desenvolvimento de um material cromaticamente estável no que concerne à sua utilização em diferentes condições de processamento, tais como diferentes atmosferas e intervalos de temperatura. A estabilidade da cor foi confirmada com tratamentos térmicos em atmosferas oxidantes e redutoras. O efeito da adição do dopante nas propriedades mecânicas da Zircónia foi estudado, avaliando-se a tenacidade à fratura (KIC), a dureza de Vickers (HV10) e a resistência à flexão (σflexural) em amostras com elevado teor de Zircónia tetragonal ( >92 %) e com elevada densidade relativa ( >96%). Os materiais desenvolvidos preenchem os requisitos previamente definidos pela INNOVNANO, tendo-se observado, contudo, um ligeiro decréscimo da tenacidade à fratura com a adição de dopante. Já a dureza Vickers e a resistência à flexão não foram significativamente afetadas pela adição de Fe2O3.
Zirconia is a very versatile advanced ceramic that offers an interesting combination of thermal, chemical, electrical, mechanical and optical properties which are uncommon to find in ceramic materials and therefore is used in several applications. INNOVNANO is one of the main Zirconia powders suppliers in the market with potential clients for beige YSZ ceramics with specific mechanical properties. Thus, the work performed during this internship aimed to develop a material that meets the requested requirements, having as reference a sample provided by an INNOVNANO’s client which colour was intended to be reproduced but which composition and processing were unknown. In this way, the work began with reference material characterization through Scanning Electron Microscopy, X-ray Diffraction, Raman Spectroscopy, Photoluminescence and Diffuse Reflectance in order to understand the colouring mechanism, which was shown to be based on structural defects developed under reducing conditions. The introduction of extrinsic defects by doping with iron oxide (Fe2O3) in the matrix of 2 mol % yttria-stabilized Zirconia (2YSZ) was the alternative selected for the reproduction of the requested colour. The samples were studied from the microstructural, structural and optical point of view. Doping with iron oxide has proved to be a suitable, reproducible and irreversible colouring mechanism allowing the development of a chromatically stable material with respect to its use in different processing conditions such as different atmospheres and temperature ranges. Colour stability was proved by thermal treatments in oxidizing and reducing atmospheres. The effect of dopant addition on the mechanical properties of Zirconia was studied by evaluating the fracture toughness (KIC), Vickers hardness (HV10) and flexural strength (σflexural) in samples with high tetragonal Zirconia content (> 92%) and high relative density (> 96%). The developed material fulfils the requirements previously defined by INNOVNANO, but a slight decrease of the fracture toughness with the addition of dopant was observed while Vickers hardness and flexural strength were not significantly affected by the addition of Fe2O3.

Industrial exploitation of ceramic nanopowders is inhibited by their poor flowability and strong tendency to agglomerate. To achieve good flowability and die-filling characteristics, controlled agglomeration is required whilst the strength of the agglomerates is minimised so that they crush into primary particles when die pressed. Yttria stabilised zirconia nanopowders with a primary particle size of ~16 nm were obtained through different drying routes from an aqueous suspension and characterised in terms of flowability and agglomerate strength.

Advanced zirconia-based materials have many important applications in electronics and medical applications, and of most interest to this research, solid oxide fuel cells (SOFC) which is a key technology for alternative and hydrogen-based energy generation. The SOFC in its most basic form is a device for converting hydrogen and oxygen into water with a resulting generation of power. Most SOFC manufacturers/developers are using zirconia doped with yttria as the electrolyte with variations on the amount of yttria. The SOFC places high demands on the ceramic components, placing significant demands on powder processing technology to enable fabrication of reliable components. It has been shown that the process of co-precipitation of three initially mixed chlorides, aluminium chloride, yttrium chloride and zirconium oxychloride in aqueous solutions, can produce an oxide powder that can be used in SOFC manufacture. Zirconia powders synthesised from aqueous solution in this way have been found, however, to include hard agglomerates which are detrimental to further processing and applications.Industrial manufacture of zirconia and zirconia-yttria products can best be summarised as four step operation; (1) hydrolyse of zirconyl chloride and mixing of other solutions, (2) precipitation, (3) calcination (4) forward processing for particle size, surface area and handle-ability characteristics . The use of aqueous solutions allows for lower costs of production and reduced waste. However such production is hampered by limited understanding of the fundamental chemistry particularly during aqueous processing which limits the development of better powders for the widespread use of SOFC’s. The aim of this project was to develop an understanding of these problems based on an industrial process that is in use within Western Australia. The work has been broken up into five sections, with the first four dealing with predominately non-stabilized zirconia and tracks the process from aqueous chemistry through to final ceramic. The final section does the same for a 3 mole% yttrium partially stabilised zirconia.The influence of concentration and added chloride salts on the solution speciation of zirconyl chloride solutions, and the precipitate formed upon addition of aqueous ammonia, has been investigated using a combination of techniques, such as SAXS, DLS, ICP-OES, TEM and SEM.To further investigate the precipitation process the effect of pH of precipitation, starting solution concentration, and agitation levels on the particle size of hydrous zirconia precipitates have been investigated. The pH of precipitation was also found to have a significant impact on the type of hydrous zirconia produced. TGA/DTA, micro combustion and TEM / EDS were used to investigate the difference in the powders produced at pH 3 and 12.The two hydrous zirconium manufactured at pH 3 and 12 have been studied as further processing consistent with industrial procedures was undertaken, including how the differences in structure due to the pH of precipitation, may effect the calcination, in situ and ex situ x-ray diffraction was used for this.With the knowledge developed thus far, two 3 mol% partially stabilised zirconia (P-SZ) samples suitable for the SOFC market were manufactured from solutions through to ceramics.The combination of SAXS, DLS, in situ XRD, TEM, ICP, TGA/DTA, micro combustion, and standard ceramic testing was found to be excellent for providing comprehensive information on changes through an industrial process and will allow optimisation to produce powders suitable for SOFC applications.

Ferrosilicon containing 50-75% Si and 1.0-5.0% Zr is used as inoculant in the cast iron industry. Zr can be added to liquid ferrosilicon by use of Zr metal or zirconium ferroalloy (FeSiZr). Then the recovery of Zr, i.e. the fraction of Zr transferred from the additive to the ferrosilicon, as well as the hit rate on specification is high. The aim of this study was to investigate the recovery of Zr from zircon sand, ZrSiO4, and zirconia, ZrO2, in comparison to zirconium ferroalloy when added to liquid ferrosilicon with 75% Si at 1600⁰C.  Also the refining effect of the different additives on Al was investigated. The experiments were carried out by stirring samples of controlled amounts of ferrosilicon and Zr additive in a graphite crucible at 1600⁰C and under inert Ar atmosphere for certain amounts of time. The reaction between ferrosilicon and Zr additive was stopped by rapid cooling of the samples. ICP-OES provided the concentration of Zr and Al and LECO O/N the concentration of O. SEM-ESD was used to examine the microstructures of ferrosilicon and Zr additive after experiments. It was found that ZrO2 was reduced by Si at the particle surface to yield dissolved Zr and ZrSiO4. The ZrSiO4 additive decomposed via two simultaneous reactions, one yielding ZrO2, Si and O2 and the other Zr, Si and O2. The recovery of Zr from ZrO2 and ZrSiO4 was significantly lower than from FeSiZr. Of ZrO2 and ZrSiO4, ZrO2 yielded the highest Zr recovery; the difference was much bigger than predicted by thermodynamics. It was discussed if that could be due to a higher reaction rate of the ZrO2, caused by the smaller size (APS 1 µm compared to d50 91 µm) and larger surface area of this addition. It was also found that utilization of density differences to separate the ferrosilicon and Zr additive did not work for zirconia under the same conditions as it worked for zircon sand, although zirconia has a higher density than zircon sand. The reason was the smaller particle size of the ZrO2 powder. No refining of Al was observed.

In the last decades there has been growing interest in developing ceramic materials with high fracture toughness (Klc) and strength for structural applications. In the specific case of 3 mol% yttria-doped tetragonal zirconia (3Y-TZP), KIC can be increased by promoting phase transformation from tetragonal (t) to monoclinic (m) phase in front of a propagating crack tip referred to as transformation toughening. However, the stronger the tendency for stress induced transformation, the higher the risk for prematura spontaneous t-m transformation in humid atmosphere. This phenomenon, which is referred to as ageing, hydrothermal degradation or low temperatura degradation (LTD) induces microcracking and loss of strength and limits the use of 3Y-TZP. The resistance to L TO can be increased by reducing the grain size into the nanoscale by using Spark Plasma Sintering (SPS). However, the reduction of grain size may reduce t-m phase transformation in front ofthe crack tip and therefore fracture toughness may decrease. One way to enhance KIC is the incorporation of a second phase as a toughening mechanism into zirconia matrix. In the present study, M.JltiWalled Carbon Nanotubes (l'v1VVCNTs) were used to reinforce zirconia matrix. A novel method was developed in this project in order to measure the "true" fracture toughness of small cracks of 3Y-TZP/CNT composites. The method is based on producing a very sharp notch using Ultra-short Laser Ablation (UPLA). The same method was also applied to a high toughness zirconia ceramic 12Ce-Zr02 with 300 nm grain size, which has much higher plateau fracture toughness than SPSed 3Y-TZP with 177 nm grain size. Moreover, the wear behaviour of zirconia/CNT composite was investigated by studying the effect of CNTs on the friction coefficient and the wear rate of the composites. The wear behaviour was investigated with scratch tests and reciprocating sliding. The machinability of zirconia/CNTs using Electrical Discharge Machining (EDM) was evaluated by studying the electrical conductivity, the thermal conductivity and the damage produced after machining. Besides that, the influence of grinding, thermal etching, and annealing of SPS zirconia with different grain sizes were studied. lt has been found that by inducing a very sharp notch using UPLA, the "true" KIC of SPSed 3Y-TZP and 3Y-TZP/CNT composites for small cracks is low and independent of the added CNT amount. On the contrary, Vickers indentation KIC is higher and increases with CNT content, which is attributed to the larger crack sizes studied in indentation and toan increase in the resistance to cracking under Sharp contact loading induced by the presence of CNT. Moreover, only 10% of difference in strength was found in 12Ce-Zr02 and 3Y-TZP using UPLA method indicating that the "true" KIC of both materials is almost similar. Thus, the beneficia! effect of higher indentation KIC in 12Ce-Zr02 reported in literature has a very small effect on the "true" KIC that determines the strength of unshielded small cracks. The incorporation of CNTs into zirconia matrix increases the friction coefficient and drastically decreases the wear rate when the amount of CNT reaches the percolation value (2 wt% CNT) under relatively low loads. However, during scratch test and under high loads, the composites develop chipping and brittle fracture. The addition of CNTs strongly enhances the electrical conductivity of the composite and induces slight changes in the termal conductivity which results in successful EDM machining of the composites with 1 wt% and 2 wt% CNT. The thermal etching of ground SPS zirconia at 1100 ºC for 1 hour in air induces a surface nanograin layer with crystallized grains of about 60 nm sizes and a thickness of less than few hundred nanometers, which is independent of the original grain size of the bulk material. If thermal etching is carried out at much higher temperatura, 1575 ºC for 1 hour, ground and polished zirconia reaches similar grain size.
En las últimas décadas ha habido un creciente interés en el desarrollo de materiales cerámicos con alta tenacidad a la fractura (KIC) y alta resistencia para aplicaciones estructurales. En el caso del óxido de circonio dopado con 3% molar de itria (3Y-T2P), KIC aumenta con el volumen de fase tetragonal (t) que se transforma a monoclínica (m) frente a la punta de las grieta, debido a las altas tensiones presentes. Sin embargo, cuanto mayor es la tendencia a transformase por influencia de la tensión, también mayor es el riesgo de degradación superficial por transformación t--m en presencia de humedad. Este fenómeno, que se conoce como degradación a baja temperatura {LTD), induce microfisuras superficiales que limita el uso de 3Y-T2P. La resistencia a LTD puede aumentarse mediante la reducción del tamaño de grano, pero el mecanismo de aumento de tenacidad por transformación de fase deviene menos efectivo. Una manera de mejorar KIC es la incorporación de una segunda fase que pueda producir un aumento de KIC sin afectar la resistencia a L TO. Con este objetivo, en esta tesis se han utilizado nanotubos de carbono (CNT) de pared múltiple como refuerzo y se ha utilizado un nuevo método con el fin de medir la "verdadera" KIC de grietas pequeñas en 3Y-T2P/CNT. El método se basa en producir una entalla microscópica superficial muy afilada utilizando ablación mediante un femtoláser (UPLA). Se ha aplicado también a 12Ce-Zr02 con mayor KIC y los resultados se han comparado con los obtenidos en 3Y-T2P producida por sinterización mediante corrientes pulsadas (SPS). Por otra parte, se ha estudiado el coeficiente de fricción y la resistencia al desgaste de los compuestos 3Y-T2P/CNT mediante ensayos de rayado y ensayos de desgaste por deslizamiento alternativo. Se ha determinado la conductividad térmica y eléctrica de los compuestos 3YTZP/CNT y se ha evaluado experimentalmente la posibilidad de ser mecanizados por electroerosión. Finalmente se han estudiado los cambios superficiales que induce el ataque térmico usual para revelar los bordes de grano de muestras pulidas, cuando se aplica a probetas desbastadas. Entre los resultados encontrados cabe mencionar que se ha determinado que partiendo de una entalla microscópica muy afilada introducida mediante UPLA. el "verdadero" valor de KIC de 3Y-T2P y 3Y-T2P/CNT es inferior a los obtenidos generalmente mediante el método de indentación y es casi independiente de la concentración de CNTs. Por otra parte, se ha encontrado que a pesar del alto valor de KIC reportado en la literatura en grietas grandes en 12Ce-Zr02 con respecto a 3Y-T2P utilizando los métodos convencionales de mecánica de la fractura, en este trabajo se pone de manifiesto que la diferencia en la resistencia a flexión es relativamente pequeña si ambas cerámicas poseen un tamaño de grano submicrométrico. Este resultado se discute en términos de una KIC muy similar para grietas muy pequeñas tal como se determina mediante el método UPLA. Por lo tanto, el efecto beneficioso de una superior KIC in 12Ce-Zr02 reportada en la literatura tiene un efecto muy pequeño sobre la "verdadera" KIC que determina la resistencia de grietas pequeñas. Se concluye además que el método de indentación no es adecuado para la determinación de KIC. La incorporación de nanotubos de carbono en circona aumenta el coeficiente de fricción y disminuye drásticamente la tasa de desgaste cuando se aplican cargas de contacto relativamente bajas. La adición de pequeñas fracciones de CNT aumenta notablemente la conductividad eléctrica de 3Y-T2P lo cual explica que el material compuesto pueda ser en principio mecanizado mediante EDM tal como se ha puesto de manifiesto con concentraciones de sólo 1 o 2% en peso de CNT. En cuanto a la influencia del desbaste en el ataque térmico a 1100 ºC durante 1 hora, en 3Y-T2P este tratamiento térmico origina nucleación de granos nanométricos en la superficie desbastada independientemente del tamaño de grano original.

Tetragonal polycrystalline zirconia, commonly stabilised with 3 mol% yttria (3Y-TZP), became one of the most interesting ceramics for biomedical applications due to its biocompatibility and high mechanical properties. Among them, its high fracture toughness should be highlighted, which is due to the stress-induced tetragonal-to-monoclinic (t-m) phase transformation near a crack tip. However, the tetragonal grains on the Surface can also spontaneously transform to monoclinic phase in a humid environment, phenomenon known as low-temperature degradation (LTD), which is an important issue for applications in which water is present. Several methods have been proposed to increase the LTD resistance in zirconia, which range from improving the fabrication process in terms of grain size, density or residual stresses, to doping zirconia with other oxides, like magnesia or ceria. Particularly, ceria-stabilised zirconia (Ce-TZP) possesses higher LTD resistance and fracture toughness than 3Y-TZP, but lower fracture strength and hardness, partly because of its larger grain size. The approach proposed in this work consists in the improvement of mechanical properties of Ce-TZP by reducing grain size, without reducing LTD resistance. With this objective, starting from two compositions of Ce-TZP (10 and 12 mol% CeO2), different amounts of CaO and Y2O3 have been added to reduce the grain growth during sintering, and thus increasing the critical t-m transformation stress, and consequently flexural resistance and hardness. On the other hand, as flexural resistance is determined by fracture toughness from small superficial cracks, a novel reproducible methodology to produce superficial micro-notches by means of ultra-short pulsed laser ablation has been developed to measure this property. It has been highlighted that with the developed methodology very sharp cracks are produced, with the size in the order of natural cracks. Results show that small-crack fracture toughness is very different from values measured from methods using large cracks or from the indentation method. This thesis is submitted for the degree of Doctor of Philosophy in the program "Materials Science and Engineering" at the Universitat Politècnica de Catalunya as a compendium of published articles. The research described in this work was carried out by the author during the period from December 2012 to October 2017 under the supervision of Prof. M. Anglada in the Department of Materials Science and Metallurgical Engineering at the Universitat Politècnica de Catalunya, and during 3 research stays during 2015, 2016 and 2017 (406 days in total) in the Department of Materials Science (MTM) at KU Leuven (Belgium) under the supervision of Prof. J. Vleugels. The work described in this dissertation is original, unless otherwise detailed references are provided.
L'òxid de zirconi estabilitzat amb un 3 mol% d'itria és una ceràmica policristal·lina amb estructura tetragonal (denominada freqüentment zircònia, o bé 3Y-TZP) presenta propietats molt interessants per a aplicacions biomèdiques degut a la seva biocompatibilidad i altes propietats mecàniques. Entre elles, destaca la seva alta tenacitat de fractura, la qual és deguda a la transformació de fase tetragonal a monoclínica (t-m) induïda sota tensió al voltant de la punta d'una esquerda. No obstant, els grans tetragonals superficials també poden transformar-se de forma espontània a fase monoclínica en ambients humits, fenomen conegut com a degradació a baixa temperatura (LTD, per les seves sigles en anglès), la qual és un problema important en aplicacions on l'aigua hi és present. Diversos mètodes s'han proposat per tal d'augmentar la resistència a la LTD de la zircònia, des de la millora dels processos de fabricació en termes de tamany de gra, densitat o tensions residuals, fins al dopatge de la zircònia amb altres òxids, com magnèsia o cèria. Particularment, la zircònia estabilitzada amb cèria (Ce-TZP) posseeix una alta resistència a la LTD i major tenacitat de fractura que la 3Y-TZP, però menor resistència a fractura i duresa, en part degut a la seva major grandària de gra. L'estratègia seguida en aquest treball consisteix en la millora de les propietats mecàniques de la Ce-TZP mitjançant la reducció de la grandària de gra, sense disminuir la resistència a la LTD. Amb aquest objectiu, partint de dues composicions de Ce-TZP (10 i 12 mol% de CeO2), s'han afegit diferents quantitats de CeO i de Y2O3 per a reduir el creixement de gra durant la sinterització i així augmentar la tensió crítica per a la transformació t-m, i per tant la resistència a flexió i duresa. Per altra banda, donat que la resistència a flexió ve determinada per la tenacitat de fractura d'esquerdes petites superficials, s'ha desenvolupat una metodologia reproduïble per a produir micro-entalles superficials mitjançant l'ablació làser de puls ultra-curt per així mesurar aquesta propietat. S'ha posat en relleu que amb la metodologia desenvolupada es produeixen esquerdes molt afilades i de dimensions de l'ordre de les esquerdes naturals presents. Els resultats posen de manifest que la tenacitat de fractura per esquerdes petites són molt diferents de les obtingudes per mètodes amb esquerdes grans o a partir del mètode d'indentació. Aquesta tesis és dipositada pel grau de doctor en el programa "Ciència i Enginyeria dels Materials" de la Universitat Politècnica de Catalunya com a compendi de publicacions. La investigació descrita en aquest treball es va desenvolupar per l'autor durant el període de desembre de 2012 a octubre de 2017 sota la supervisió del professor M. Anglada al Departament de Ciència dels Materials i Enginyeria Metal·lúrgica de la Universitat Politècnica de Catalunya, i durant tres estades d'investigació durant 2015, 2016 i 2017 (406 dies en total) al Departament de Ciència de Materials (MTM) de KU Leuven (Bèlgica), sota la supervisió del professor J. Vleugels. El treball descrit en aquesta dissertació és original, llevat quan es proporcionen referències detallades.

This research studies the chemical nature of zirconia and the complex surface chemistry of zirconia in order to better comprehend its behavior under chromatographic conditions. This research shows how the physical and chemical properties of zirconia depend strongly on the thermal treatment during synthesis. The morphology of the samples was also studied. The absorption capability of Adenosine Triphosphate (ATP) on zirconia was also monitored and spectrally characterized. The results of this research showed how the properties of zirconia vary with thermal treatment. It was observed that the zirconia prepared at a higher temperature had lower surface area, lower pore size and pore volume as compared to the zirconia prepared at a lower temperature. The morphology studies showed the porosity of the zirconia. The results from the absorption experiments showed that zirconia prepared at a higher temperature absorbed more ATP than the zirconia prepared at a lower temperature. Significant changes were also observed on the pellets of zirconia pre and post absorption experiments. I hope that this research sheds more light on the complex properties of zirconia’s surface chemistry and the results of this study could better help in the application and use of zirconia in chromatography to separate proteins.

Zirconia is one of the most promising column packing materials for High Performance Liquid Chromatography (HPLC). The perfect HPLC support material should be energetically homogenous, have a high surface area on which different chemical species can reversibly attach and be physically and chemically stable over a wide range of pH, temperature and solvent conditions. Most existing supports do not have all of these properties. This project is also focused on a proteomics study. Zirconia, hafnium oxide and titanium oxide which are some of the more promising materials currently available, can be used for the separation and analysis of phosphorylated proteins. Adenosine triphosphate, Adenosine diphosphate and Adenosine monophosphate were used as prototypes for phosphorylated proteins. Separation, absorption, fluorescence and SEM studies were performed to determine the adsorption of Adenosine phosphates species at a particular pH on Zirconia. Zirconia was also used for the purification of Fibrinogen Growth Factor (FGF) protein, which are a family of growth factors involved in angiogenesis, wound healing, and embryonic development. The sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) technique was used to analyze the off-column purification and separation of this protein. This research suggests that, at acidic conditions, adenosine monophosphate has more favorable absorption on the Zirconia surface. On the other hand, the separation study suggests that basic conditions are more favorable for the absorption of ATP, ADP and AMP when mixed together on Zirconia 500. Furthermore, it was found that Zirconia is a very promising material for the purification of FGF protein.

Sulfated zirconia has attracted extensive attention due to its superactivity to isomerize alkane and alkenes. Platinum or iron/manganese promoted sulfated zirconia has been shown to increase the reaction rate. These catalysts are normally activated to 650-725°C in air before use. Through on-line analysis of evolved gas species during dynamic heating of the catalysts, some significant information can be obtained concerning the activation mechanism. In the present investigation, combined TG-FTIR, TG-MS and TGDTA techniques were utilized to measure the weight loss of the samples, analyze the evolved gas species and to monitor the phase transformation temperature of the solid. Four samples [(1%) 5% Pt/S04 2-VZr02, 2% Fe/0.5% Mn (iron(III)nitrate, maganese(II) nitrate treated/iron(III) sulfate, maganese(II) sulfate treated)/S04 2-/Zr02] were studied by thermal analysis. Also, the catalytic activity of 0.4% Pt/S04 2-/Zr02 was evaluated the isomerization and oligomerization of 1-hexene. It was found that this type of catalyst has high catalytic activity even at ambient and subambient temperatures.

Reaction and conversion of CO2 to chemicals is a challenging area of research. The objective of this work is to study and investigate the use of mixed metal oxide Zr/Ti oxide and related catalysts for the conversion and utilisation of CO2. The first reaction studied was propane dehydrogenation using CO2 to produce propene. Then, the study extended to investigate the direct reaction of CO2 as whole molecule with methane, ethane, acetylene, ethylene and propane to synthesis carboxylic acids. The catalysts were prepared in several ways. Four methods were based on coprecipitation of the mixed catalyst from solutions of zirconium (IV) oxynitrate hydrate and titanium (IV) chloride. Other methods involved impregnation, based on titanium (IV) oxide. Catalysts were characterised by nitrogen adsorption, by powder X-ray diffraction, by ammonia temperature programmed desorption and, ultimately, in terms of catalytic activities. The powder X-ray diffraction patterns of the impregnated titania-rich Zr/Ti oxide catalysts showed that ZrO2 dissolved in the solid anatase phase of titania. At higher concentrations, the ZrO2 appeared as a separate tetragonal phase. Low zirconia content Zr/Ti oxide catalysts showed significantly increased surface areas and higher acidities than the individual oxides. A range of other metal oxides were added as third metal oxides in these mixtures, but none had significant impacts on surface areas or on surface acidities. Propane dehydrogenation is thermodynamically limited. The only possible route is a radical mechanism for H2 removal via a surface process. The catalytic activities at low CO2:propane ratio showed that Zr/Ti oxide exhibited the higher activity than single oxides, but activities were all too low to be of economic significance. In contrast, using higher CO2:propane ratio improved the propene yield and selectivity to values comparable to those achieved with the industrial chromium based catalyst. The catalyst showed selectivity to C-H bond breaking to form propene over C-C bond breaking to make ethene. The study demonstrated that CO2 was utilised mainly for the reverse water gas shift reaction (RWGS) to remove hydrogen from the catalyst surface. The study showed that the Zr/Ti oxide catalysts exhibited higher stability compared to the industrial catalysts at slightly higher gas space velocity. Thermogravimetric analysis showed that Zr/Ti oxide catalyst assists coke gasification in the presence of CO2 at 600 oC. The other mixed oxide catalysts generally showed lower surface acidities and higher selectivities to C-C bond breaking products over the desired propene product. The second study was the direct reaction of CO2 with CH4 to produce acetic acid. Again, this reaction is thermodynamically unfavourable and the only possible route must involve a radical species by which reactants are concentrated on the catalyst surface. Evidence of methyl surface species formation in the presence of methane was indeed found over the Zr/Ti oxide catalyst. With CO2 methane reacted with CO2 to form acetic acid over Zr/Ti oxide catalysts. The C-C insertion mechanism is proposed by which methyl surface species formed on the catalyst and reacted with CO2. This was followed by hydrogenation to form acetic acid. Reactions of CO2 with ethane, ethylene, acetylene and propene were also studied, in the hope of observing direct insertion to produce the corresponding carboxylic acid. In fact, lower acids were formed in all cases, suggesting a radical mechanism involving C-C bond breaking over Zr/Ti oxide catalyst. Interestingly, acetic acid was formed with all these precursor hydrocarbons, and it appears that it occurs via C≡C, C=C, C-C and C-H bond breaking.

The term nanoceramics is well known in the ceramic field for at least two decades. Even though there are many reports that nanoceramics are superior in terms of mechanical and other properties, no comprehensive and conclusive study on the grain size dependent variation in mechanical properties. So this study was an attempt to study the property variation with grain size and yttria content for a well known ceramic, yttria stabilised zirconia. High solids content but low viscosity YSZ nanosuspensions have been slip cast into -52% dense, very homogeneous green bodies in sizes up to 60 mm in diameter. Sintering cycles have been optimised using both hybrid and conventional two-step heating to yield densities >99.5% of theoretical whilst retaining a mean grain size of <100 nm. The sintered samples have been characterised for hardness, toughness, strength, wear resistance and hydrothermal ageing resistance. The results have been compared with that of a submicron zirconia ceramic prepared using a commercial powder. The strength of the nanoceramics has been found to be very similar to that of conventional submicron ceramics, viz. -10Pa, although the fracture mechanism was different. Two toughness measurement approaches have been used, indentation and surface crack in flexure. The results indicate that the nano 1.5YSZ ceramics may be best viewed as crack, or damage, initiation resistant rather than crack propagation resistant; indentation toughness measurements as high as 14.5 MPa m 112 were observed. Micro-Raman mapping was demonstrated to be a very effective technique to map the phase transformations in zirconia. The wear mechanism of nanozirconia has been observed to be different compared to that in conventional, submicron YSZ and the wear rates to be lower, particularly under wet conditions. In addition, and potentially most usefully, the nan03YSZ ceramics appear to be completely immune to hydrothermal ageing for up to 2 weeks at 245°C & 7 bar; conditions that see a conventional, commercial submicron ceramic disintegrate completely within 1 hour.

Interatomic potential and quantum mechanical simulation techniques have been applied to both the bulk and surfaces of yttrium-stabilized cubic zirconia (YSZ) with special reference to its role in partial oxidation reaction at high temperature. The stabilities of pure ZrO2 phases and surfaces have been examined. The bulk structures of low pressure phases are reproduced with the observed order of stability: monoclinic > tetragonal > cubic zirconia. The relative stability of plane cubic ZrO2 surfaces is predicted to be (111)c > (110)c > (100)c > (310)c. In addition, the stability of topological surfaces is found to be reduced with the coordination number of Zr ions at the topological sites in the order: plane > step > kink > corner. The dispersion of defects in YSZ systems has been studied, considering both the dopant content and surface segregation. In the bulk, the yttrium dopants tend to form a pair with two yttrium atoms close to each other and preferentially occupying the 1st or 2nd nearest neighbour (NN) sites to the compensating oxygen vacancy. At the surface, yttrium segregates to the top layers (up to 4-5 Å) of the dominant (111) surface of YSZ. The composition of the outermost surface of YSZ is predicted to be independent of Y bulk concentration and reach a maximum Y/Zr ratio of 1:1. In relation to catalytic oxidation, the interaction between oxygen molecules and the (111) surfaces of pure c-ZrO2 and YSZ have been investigated using quantum-mechanical DFT-GGA methods. The adsorption states of oxygen and the pathways for dissociation have been indentified on the plane and stepped (111) surfaces. In general, the creation of oxygen vacancies by yttrium doping provides an active site for oxygen adsorption. In addition, the low-coordinated Zr cations on the YSZ surfaces can attract strongly reduced oxygen species and the most stable adsorption state of oxygen is adopted to achieve a higher bond saturation of the neighbouring Zr site.

The phase relations, electrical properties and structural characteristics of doped cubic stabilised Zirconia based electroceramic materials have been investigated using a number of characterisation techniques. The phase relations of the ternary systems ZrO2 -Y2O3 -TiO2 and ZrO2 -Gd2 O3 - TiO2 at 1500°C have been investigated. Electrical characterisation in air and in low oxygen partial pressures has been carried out using 2-probe A.C. Impedance Spectroscopy and 4-probe D.C. resistivity measurements to ascertain whether compositions within these systems could be utilised as the anode materials in Solid Oxide Fuel Cells. The effect of porosity on the ionic and electronic conducting properties of the ZrO2 -Y2 O3 -TiO2 system has been investigated to provide a clearer understanding of the effect of the porosity within candidate anode materials. The effect of Al2O3 additions on the electrical properties and stability of the Solid Oxide Fuel Cell material of choice, 8 mol% Yttria stabilised Zirconia, has been investigated. Al2 O3 has been found to remain primarily as a second phase within the 8YSZ, however a small quantity of Al3+ does dissolve into the fluorite matrix. Al2 O3 has been found to have a negligible effect on the high temperature ionic conductivity of 8YSZ and improves the resistance of 8YSZ to hydrothermal degradation by stabilising the cubic structure. High temperature Time of Flight Neutron Diffraction has been used to link the change in activation energy observed in 8YSZ to a break down in local ordering of oxygen ions. Extended X-ray absorption Fine Structure Spectroscopy has been used to characterise the local structure of the cations in 8 mol% Yttria-stabilised Zirconia. Analysis of the high temperature data reveals that the local structure is quite different from the average crystallographic structure. The oxygen vacancies were determined to be associated with Zirconium ions and found to disorder at high temperatures.

mestrado em Ciência e Engenharia de Materiais
A realização deste trabalho tem como objetivo a produção e caracterização de compósitos de metal em matriz cerâmica (CERMETOS). Assim sendo, e tendo como base pós cerâmicos e compósitos produzidos pela INNOVNANO, foram também produzidos pós por mecanossíntese. Os pós à base de zircónia estabilizada com ítria (YSZ), produzidos por estes dois métodos, foram caracterizados cristalográfica, química, morfológica, reológica, térmica e magneticamente. Os pós foram compactados e sinterizados em vácuo, seguindo-se a sua caracterização, principalmente estrutural, microestrutural, mecânica e magnética. Todos os resultados foram analisados com olhar crítico e tentando relacionar as propriedades físicas e químicas dos pós e compactos verdes com as propriedades finais dos sinterizados. A comparação entre cerâmicos e compósitos (quer pós, quer sinterizados) foi sempre o principal objetivo deste trabalho. Os resultados incluem a avaliação da importância das várias etapas de preparação dos pós, essencialmente no processo produtivo da INNOVNANO, a avaliação da estabilização da fase tetragonal da zircónia, a resposta magnética e a interpretação do decréscimo nas propriedades mecânicas verificado nos compósitos.
This work main goal is to develop metal composites on a ceramic matrix, designated as CERMETs. Hence, and having as starting materials the ceramic and composite powders produced by INNOVNANO, mechanosynthesis powders were also prepared. The yttria stabilized zirconia (YSZ) based powders (produced by those two methods) were crystallographic, chemically, morphologically, rheological, thermal and magnetically characterized. Pressed compacts from the previous powders were prepared and sintered in vacuum conditions, followed by their characterization namely in terms of structure, microstructure, mechanical and magnetic behavior. The results were analyzed with a critical mind, trying to co-relate the physical and chemical properties of the powders and green compacts with the final sintered properties. The comparison between ceramic and composites (either powders, either sintered compacts) was always the main goal during the development of this work. The results include the evaluation of the meaning and importance of the several powders preparation steps that are conducted in INNOVNANO’s producing method, the importance of the tetragonal zirconia phase stabilization, the magnetic response and the interpretation of the decrease in mechanical resistance verified in CERMETs.

The recently discovered flash sintering technique has shown that the application of a sufficiently large dc electric field (E-field) to a ceramic during sintering can cause sintering at temperatures several hundred degrees below conventional temperatures with sintering rates that allow for sintering in seconds rather than hours. This technique has already been demonstrated in wide range of ceramic materials including ionic conductors, electronic conductors, semi-conductors and insulators. The application of this techniques to a large range of materials and the dramatic lowering of sintering temperatures and times means this technique has the possibility to revolutionize the ceramics field. Because the field of flash sintering is still young, the dominant mechanisms are still yet to be identified. So far, the mechanisms active during flash sintering remain only hypothetical. The key to the mechanism is that it must connect two phenomena controlled by different species: conductivity, which is determined by the fast moving species, and sintering, which is controlled by the slowest moving species. At this point the primary hypothesis is that the high E-fields used cause the nucleation and separation of Frenkel defects allowing for both high sintering rates and high conductivities. This work aims to identify the mechanisms involved in the flash sintering of cubic 8 mol% yttria stabilized zirconia (8YSZ). This materials has been selected because it was one of the first materials show flash sintering properties, and that it has been very well characterized because of its use as a fast oxygen ion conductor. This work began with the construction and initial testing of flash sintering equipment. Mechanistic investigations were performed on fully dense presintered samples of various grain sizes and single crystalline samples subject to the voltages similar to those used during flash sintering at constant temperatures between 600°C and 1000°C. Because flash sintering has a sintering component and electrochemical component, dense samples were used to remove the sintering so observations can be made only on the electrochemical behaviors during these electrical treatments. Additionally, the range of E-fields used for flash sintering of green 8YSZ samples was increased by an order of magnitude from the 150 V/cm to 2250 V/cm. Treatment of fully dense and single crystal samples show that the electrolytic reduction of ZrO2 to ZrO2-δ give I-V relations and power dissipations that are similar to those observed in flash sintering of powder samples. Flash sintering using E-field up to 2250 V/cm reveals several relationships between temperature, E-field, current density and power dissipation and their relation to the onset of sintering and densification. Most importantly the onset of sintering can be determined by the power relationship TOnset = 2440 E-1/5.85, where TOnset is the furnace temperature when flash sintering starts in K and E the applied E-field. Based on these observations, a new mechanism based on electrolytic reduction in the material is proposed to explain the onset and early stages of flash sintering and the observed electrical behaviors. Based on this, the movement of reduced zirconia in the E-field is proposed as a possible sintering mechanism.

The recently discovered flash sintering technique has shown that the application of a sufficiently large dc electric field (E-field) to a ceramic during sintering can cause sintering at temperatures several hundred degrees below conventional temperatures with sintering rates that allow for sintering in seconds rather than hours. This technique has already been demonstrated in wide range of ceramic materials including ionic conductors, electronic conductors, semi-conductors and insulators. The application of this techniques to a large range of materials and the dramatic lowering of sintering temperatures and times means this technique has the possibility to revolutionize the ceramics field. Because the field of flash sintering is still young, the dominant mechanisms are still yet to be identified. So far, the mechanisms active during flash sintering remain only hypothetical. The key to the mechanism is that it must connect two phenomena controlled by different species: conductivity, which is determined by the fast moving species, and sintering, which is controlled by the slowest moving species. At this point the primary hypothesis is that the high E-fields used cause the nucleation and separation of Frenkel defects allowing for both high sintering rates and high conductivities. This work aims to identify the mechanisms involved in the flash sintering of cubic 8 mol% yttria stabilized zirconia (8YSZ). This materials has been selected because it was one of the first materials show flash sintering properties, and that it has been very well characterized because of its use as a fast oxygen ion conductor. This work began with the construction and initial testing of flash sintering equipment. Mechanistic investigations were performed on fully dense presintered samples of various grain sizes and single crystalline samples subject to the voltages similar to those used during flash sintering at constant temperatures between 600°C and 1000°C. Because flash sintering has a sintering component and electrochemical component, dense samples were used to remove the sintering so observations can be made only on the electrochemical behaviors during these electrical treatments. Additionally, the range of E-fields used for flash sintering of green 8YSZ samples was increased by an order of magnitude from the 150 V/cm to 2250 V/cm. Treatment of fully dense and single crystal samples show that the electrolytic reduction of ZrO2 to ZrO2-δ give I-V relations and power dissipations that are similar to those observed in flash sintering of powder samples. Flash sintering using E-field up to 2250 V/cm reveals several relationships between temperature, E-field, current density and power dissipation and their relation to the onset of sintering and densification. Most importantly the onset of sintering can be determined by the power relationship TOnset = 2440 E-1/5.85, where TOnset is the furnace temperature when flash sintering starts in K and E the applied E-field. Based on these observations, a new mechanism based on electrolytic reduction in the material is proposed to explain the onset and early stages of flash sintering and the observed electrical behaviors. Based on this, the movement of reduced zirconia in the E-field is proposed as a possible sintering mechanism.

This research investigates the impact of zirconia nanoparticle in conductivity, water uptake, fuel crossover and fuel efficiency of modified Nafion® membranes. Synthesized water-retaining mesoporous zirconia nanoparticles (ZrO2) were used to modify Nafion® membrane in order to enhance the thermal properties, water uptake, proton conductivity and mechanical strength of composited membrane for fuel cell applications. Recast and impregnation methods were used to prepare a nanocomposite membrane with required weight% of zirconia nanoparticles. The mechanical stability of modified membranes has become a priority for fuel cell applications as the membranes must endure all the fuel cell operations (to prevent crossover of the fuel while still conducting). Their mechanical stress and yielding stress in the recast and impregnation methods compared with the commercial Nafion® membrane were observed under tensile tests. The incorporated membrane with zirconia nanofiller shows an improvement in mechanical strength, due to the hydrophilic phase domains in the nanocomposite membrane. The water contact angle and water uptake of the composited membrane were measured. The modified membranes with zirconia nanoparticles showed a significant improvement in water uptake and contact angle leading to enhanced hydrophilicity when compared to unmodified hydrophobic Nafion® membrane. This shows the potential for use as electrolytes in fuel cell applications. Zirconia nanoparticles were further impregnated with sulfuric acid and phosphoric acid to introduce the additional acid sites for absorption of water. In addition, zirconium phosphates (ZrP) and sulfated zirconia (S-ZrO2) were incorporated into Nafion® 117 membrane by impregnation method to obtain a reduced methanol permeation and improved proton conductivity for fuel cell application. The mechanical properties and water uptake of Nafion® membrane incorporated with zirconium phosphates and sulfonated zirconia nanoparticles were much more improved when compared to the commercial Nafion® 117, due to the presence of acid site within the nanoparticles. Furthermore, the results showed that incorporating ZrP and S-ZrO2 nanoparticles enhanced proton conductivity and IEC of modified Nafion ® membrane as they sustain water affinity and strong acidity. The results show that nanocomposite membranes have low water content angle, improved thermal degradation, higher conductivity and lower methanol permeability than commercial Nafion® 117 membrane, which holds great promise for fuel cell application. The Nafion®/ sulfated zirconia nanocomposite membrane obtained a higher IEC and water uptake due to the presence of SO 2-4 providing extra acid sites for water diffusion. To reduce the agglomeration of ZrO2 nanoparticles and improve the water diffusion, ZrO2 was electrospun with polyacrylonitrile (PAN) solution to obtain a 1D morphology. The recast method was used to synthesize the high thermal and mechanical stability of Nafion® membrane incorporated with polyacrylonitrile (PAN) nanofibers. The modified Nafion® membranes exhibited improved fuel cell efficiency when tested in direct methanol fuel cells with a high proton conductivity due to incorporating PAN/Zr nanofibers that retain water within the membrane. Moreover, nanocomposite membranes achieved a reduced methanol crossover of 4.37 x 10-7 cm2 /s (Nafion®-PAN/ZrP nanofibers), 9.58 x 10-8 cm2 /s (Nafion®- PAN/ZrGO nanofibers) and 5.47 x 10-8 cm2 /s (Nafion®-PAN/Zr nanofibers), which is higher than 9.12 x 10-7 cm2 /s of recast Nafion® membrane at the higher concentration of 5M. All the blended membranes showed increase in power density at a temperature of 25 °C in comparison with pristine recast Nafion® membrane (76 mW·cm−2 , 69 mW·cm−2 , 44 mW·cm−2 , 18 mW·cm−2 ). Finally, incorporating electrospun PAN/ Zr nanofibers into Nafion® membrane has successfully reduced the use of Nafion® solution that will eliminate the cost problems, while improves the protons conductivity and the methanol permeability which influence the fuel cell efficiency and long-term stability.
Physics

There are two industrial sources of zirconia: zircon and baddeleyite [1-5]. The baddeleyite reserves in Phalaborwa (the world’s major baddeleyite source) are expected to be depleted by the year 2005 [1-3]. This leaves the Russian Baddleyite (Kola Peninsula) and zircon as the only industrial sources of zirconia. The major drawback to zircon use is the large amounts of impurities it is found concentrated with, especially radioactive impurities (Uranium and Thorium) [2-3]. Acid leaching of zircon does not remove these impurities [4-5]. The impurities are usually included in the zircon lattice. The tetragonal structure of zircon with the high coordinated bisdisphenoids ZrO8 and low coordinated tetrahedra SiO4 create a safe (inaccessible and stable) habitat for these impurities [7]. Processes for the recovery of zirconia and zirconium chemicals rely heavily on precipitation or cyrstallisation techniques for purification [8-16]. Precipitation techniques need to be repeated to obtain the required purity. The purity of products from such methods is still suspect, as there still remains a high radioactivity content after purification [2]. The long process time is another disadvantage of these precipitation processes. These factors together are the reason for the high cost of zirconia and zirconium chemicals. Zirconium and its compounds are regarded to be of low toxicity [1-6]. This implies that they have a great potential of replacing numerous high toxic chemicals. Prominent examples are seen in leather tanning and paints. In leather tanning chromium chemicals can be replaced. In paints lead driers and chromium chemicals for corrosion resistance can be replaced. The objective of this study was to characterise and optimise the De Wet’s zirconium extraction processes for the beneficiation of zircon sand into high purity zirconia and zirconium chemicals. However, at each process step some factors were varied e.g. fusion temperature, reactant mole ratios and composition of leach solutions. Attention was also paid to reducing the total number of process steps. The products produced at each step were analysed. Particular attention was given to the fate of the radioactive impurities. Characterisation of the decomposition step, showed that within the zircon tetragonal structure, the SiO4 bisdisphenoids linkages. This was shown by the preference of sodium for the SiO4 tetrahedra. Fusion for 336 hours with periodic intermediate milling proved the preference of sodium for attacking the SiO4 tetrahedra linkages. This selectivity was clearly demonstrated when decomposing zircon in sodium poor(<4 moles NaOH per mol of zircon) and low temperature (e.g. 650°C) reaction conditions. The advantage of fusing at 650°C with a mole (or even two moles) of sodium hydroxide is that it leads to minimal (<5% m/m Na2O) sodium in the insoluble solids after the removal of soluble silicates. This is a solution to alkali fusion processes, as high amounts of water are usually required to wash out the neutralised sodium salt e.g. 50g of NAC1 usually requires a litre of distilled water to reach levels below 600 ppm NA2O. This reaction condition can be employed when synthesising products where low amounts of sodium are required in the final products e.g. when synthesising zirconia for the ceramic industry. When fusing for two hours without the intermediate milling step the following results were observed. The reaction at 850°C when fusing a mole of zircon with two moles of sodium hydroxide, was the most efficient in consuming sodium hydroxide. Near complete zircon decomposition was at 850°C when fusing a mole of zircon with six moles of sodium hydroxide. Characterisation with XRD, Raman and IR spectroscopy was misleading as complex spectra were measured, indicating many different phases present. The inconsistency was partly attributed to non-homogeneity in the samples due to NaOH migration. When fusing for 336 hours with the intermediate milling step the following results were observed. The reaction at 850°C when fusing a mole of zircon with a mole of sodium hydroxide was the most efficient in consuming sodium hydroxide. This reaction condition was able to liberate 0.58 moles of zirconia per mole of sodium hydroxide. The highly improved efficiency was attributed to the formation of phases Na2ZrSiO5, Na4Zr2Si3O12 and SrO2. The process is pseudo-catalytic as it liberates zirconium while showing minimal sodium consumption. Decomposition at 650°C also showed improved efficiency but not as efficient as the 850°C sub-stoichiometric fusion. The improved decomposition was attributed to the polymerisation of the orthosilicate monomers Na4SiO4 to the metasilicate chains Na2SiO3.
Dissertation (MSc (Chemical Technology))--University of Pretoria, 2007.
Chemical Engineering
unrestricted

Electrochemical anodization is a facile technique to synthesize ordered oxide nanostructures. Though the number of materials exhibiting anodized nanostructures has increased considerably in the recent years, only nanoporous alumina and nanotubular titania have been investigated extensively for various applications. Anodized nanostructures, nanotubes and nanopores, of zirconia are also of considerable interest for applications such as templates, sensors and solid-oxide fuel cells. In spite of the potential applications of zirconia, these nanostructures have been barely studied. As most of these applications require elevated temperatures in excess of 400C, thermal stability becomes an important attribute. Even though zirconia (Tm=2715C) has as higher melting point than alumina(Tm = 2072C), literature reports and initial research showed that the thermal stability of anodized zirconia was limited to 500C-1 h compared to 1000C-4 h for alumina. The work carried out as a part of this research showed that halide ions used in the synthesis are the possible cause for the lower thermal stability. Chemical treatment of the zirconia membranes to neutralize the halide ions helped enhance the stability to 1000C-1 h, thus, improving their usability for most of the applications mentioned above. Most of the current reported work on aluminum, zirconium, and titanium is predominantly limited to anodization of foils which can only yield free-standing nanostructures. As synthesis of these nanostructures on a substrate would further facilitate their usage, supported anodized zirconia nanostructures were synthesized by anodizing sputtered zirconium films. This study showed that the anodized morphology depends strongly on the sputtered film microstructure, which changes in accordance with the Thornton’s zone diagrams. A general approach thus developed is expected to be applicable to anodization of all metallic films. Most applications involving zirconia also require stabilization against a tetragonal-monoclinic phase transformation by suitable alloying such as with yttria. Towards this end, routes to develop anodized yttria-stabilized zirconia nanostructures, which are nonexistent, were explored. The synthesis of yttria stabilized zirconia nanostructures with no detectable monoclinic phase was achieved. Yttrium alloying using a solution treatment was found to enhance stability of the supported nanostructures to 900C-16 h, which makes it possible to now evaluate these nanostructures, especially for micro-SOFC applications.

Electrochemical anodization is a facile technique to synthesize ordered oxide nanostructures. Though the number of materials exhibiting anodized nanostructures has increased considerably in the recent years, only nanoporous alumina and nanotubular titania have been investigated extensively for various applications. Anodized nanostructures, nanotubes and nanopores, of zirconia are also of considerable interest for applications such as templates, sensors and solid-oxide fuel cells. In spite of the potential applications of zirconia, these nanostructures have been barely studied. As most of these applications require elevated temperatures in excess of 400C, thermal stability becomes an important attribute. Even though zirconia (Tm=2715C) has as higher melting point than alumina(Tm = 2072C), literature reports and initial research showed that the thermal stability of anodized zirconia was limited to 500C-1 h compared to 1000C-4 h for alumina. The work carried out as a part of this research showed that halide ions used in the synthesis are the possible cause for the lower thermal stability. Chemical treatment of the zirconia membranes to neutralize the halide ions helped enhance the stability to 1000C-1 h, thus, improving their usability for most of the applications mentioned above. Most of the current reported work on aluminum, zirconium, and titanium is predominantly limited to anodization of foils which can only yield free-standing nanostructures. As synthesis of these nanostructures on a substrate would further facilitate their usage, supported anodized zirconia nanostructures were synthesized by anodizing sputtered zirconium films. This study showed that the anodized morphology depends strongly on the sputtered film microstructure, which changes in accordance with the Thornton’s zone diagrams. A general approach thus developed is expected to be applicable to anodization of all metallic films. Most applications involving zirconia also require stabilization against a tetragonal-monoclinic phase transformation by suitable alloying such as with yttria. Towards this end, routes to develop anodized yttria-stabilized zirconia nanostructures, which are nonexistent, were explored. The synthesis of yttria stabilized zirconia nanostructures with no detectable monoclinic phase was achieved. Yttrium alloying using a solution treatment was found to enhance stability of the supported nanostructures to 900C-16 h, which makes it possible to now evaluate these nanostructures, especially for micro-SOFC applications.

Wang Jun.
Thesis (M.Phil.)--Chinese University of Hong Kong, 2003.
Includes bibliographical references (leaves 74-79).
Abstracts in English and Chinese.
Acknowledgements --- p.i
List of publications --- p.ii
Abstract (In English) --- p.iii
Abstract (In Chinese) --- p.v
Table of contents --- p.vii
List of figures --- p.x
List of tables --- p.xiv
Chapter Chapter I --- Introduction --- p.1
Chapter Chapter II --- Dye-doped sol-gel zirconia and zirconia-organically modified silicate waveguides --- p.6
Chapter 2.1 --- General sol-gel process --- p.6
Chapter 2.2 --- Dye-doped sol-gel zirconia and zirconia-ORMOSIL waveguides --- p.7
Chapter Chapter III --- Basic theory and experiment setup of distributed feedback waveguide lasers --- p.13
Chapter 3.1 --- Coupled-wave theory of distributed feedback lasers --- p.13
Chapter 3.2 --- Introduction on the theory of planar optical waveguide --- p.16
Chapter 3.3 --- Experiment setup design of DFB waveguide lasers --- p.19
Chapter Chapter IV --- Zirconia and zirconia-organically modified silicate distributed feedback waveguide lasers tunable in the visible --- p.23
Chapter Chapter V --- Tunable multi-wavelength distributed feedback zirconia waveguide lasers --- p.35
Chapter 5.1 --- Brief introduction on multi-wavelength lasers --- p.35
Chapter 5.2 --- R6G-doped zirconia multi-mode waveguides --- p.36
Chapter 5.3 --- Experimental results and discussion --- p.39
Chapter 5.3.1. --- Dual- and quadruple-wavelength DFB waveguide lasers --- p.39
Chapter 5.3.2. --- The dispersion characteristics of multi-wavelength DFB waveguide lasers --- p.44
Chapter 5.3.3. --- Determination of waveguide parameters by the DFB technique --- p.48
Chapter 5.4 --- Summary --- p.51
Chapter Chapter VI --- Distributed feedback laser action in sol-gel glass symmetric waveguides --- p.53
Chapter 6.1 --- The fabrication of dye-doped glass symmetric waveguides --- p.54
Chapter 6.2 --- Experimental results and discussion --- p.56
Chapter 6.2.1. --- DFB laser action in sol-gel glass symmetric-asymmetric waveguide --- p.56
Chapter 6.2.2. --- Dispersion characteristics of DFB symmetric and asymmetric waveguide lasers --- p.61
Chapter 6.3 --- Summary --- p.65
Chapter Chapter VII --- Summary --- p.69
References --- p.74

碩士
國立東華大學
材料科學與工程研究所
89
The developments of future gate oxide of MOS device for ULSI technology and next-generation capacitor of DRAM devices for memory storage require new dielectric materials. There is significant interest in using ZrO2 for the fabrication of charge storage insulators in the next generation memories. In this study, thin films of zirconia, silica, and zirconium silicate were deposited on Si(100) and glass substrates by R.F. magnetron sputtering from zirconia, silica and zirconium silicate composite ceramic targets instead of metal targets. This research was focused on the evaluations of film growth kinetics, microstructure, mechanical properties, optical properties, and electrical properties. The results showed the films were structurally amorphous, besides 90 ZrO2-10 SiO2 system. The surface morphology of all films were smooth and dense. Their growth rates increased as the silica content and R.F. power increased. The maximum growth rate was nearly 10 nm/min. Regarding to mechanical properties, residual stress was compressive for all films, the hardness of thin films was in the range of 2~12 GPa, and there was a good adhesion to glass substrates with a the maximum critical load of 21.2 N. The refraction index showed a maximum value of 2.17 for pure ZrO2 system. The transmittance with a maximum value of 90 % for pure SiO2 system was influenced by ZrO2 content in targets. In respect of electric properties, the dielectric constant which varied with target composition had a highest of 32. The resistivity was higher than 109 ohm-cm, with a higher value after annealing. The breakdown fields were meassured with values higher than 3 MV/cm.

碩士
國立中央大學
化學工程與材料工程學系
103
The synthesis of sub-10 nm zirconia nanocrystals, as well as the dispersion of them in polar and non-polar solvents after proper surface modification, had already been accomplished in our previous studies. By adding comparable resin to the dispersion, we could prepare a transparent composite having 47 vol% ZrO2 nanoparticles and a refractive index of 1.69. Such a high reflection index material is useful as brightness enhancement film for FPD, a simpler lens-stack for cellular phone, or a more efficient second order optical design for the LED lighting systems. The index of the ZrO2/resin composite could be increased further, by loading with more nanoparticles. This was impossible previously since the viscosity increased exponentially as the inorganic content approaching a threshold. Electrophoretic deposition (EPD) may be a way to overcome this problem. By depositing the organic modified nanoparticles at the electrode under the compression of the electric field, the particle may self-assemble into a densely-packed film on the electrode. In this study, electrophoretic deposition was used to preparing a zirconia nanoparticles/polymer composite with a high inorganic fraction. We start by substituting the previous silanes modification scheme with a non-hydrolytic one. We found that this could approach save about 20% silane and take less time to react. We also measured the zeta potential of the modified particles with two different instruments, and confirmed that they were positively charged. Finally, we successfully made a continuous transparent nanocomposite which had nearly 82 wt% / 64 vol% filler loading, and establish a guideline for controlling the properties of the EPD nanocomposite film.