This project studies the generation of crystalline mesoporous structured titanium dioxide (TiO2) using cellulose nanocrystal (CNC) aerogel hard templates for photocatalytic and biomaterial applications. Suspensions of CNCs in water varying in solid loading from 20 mg/mL to 100 mg/mL were prepared and frozen at three different temperatures (-20 °C, -40 °C, or -80 °C) using four combinations of hollow cylindrical molds and mold plates with different thermal conductivities (stainless-steel or glass) placed on different heat conductive and insulative substrates (aluminum, polystyrene foam and cardboard). Frozen samples were then freeze dried to sublimate the ice and render a multiscale and mesoporous structure with a variety of microstructural features, including lamellar sheeting, flakes, ribbons, or striations. Ceramic green bodies are then produced by reacting Titanium isopropoxide with water through several different processes to generate amorphous TiO2 either in-situ in the CNC aerogel or as a suspension for infiltration under varying pressure. Green bodies are dried at room temperature, and the extent of ceramic coating of the template is visually determined using SEM imaging. Once dried, crystalline TiO2 are produced through a two-step heat treatment with a CNC burnout at 270 °C and crystallization and sintering at 500 °C, 600 °C, or 1000 °C. The final crystallinity and phase composition is examined using XRD, and the final porosity is determined using BET. Results have shown the ability to satisfactorily coat aerogels under 10 mm in one dimension with TiO2. These samples have been successfully heat-treated to produce both anatase and rutile phase TiO2 while maintaining the macrostructure of the CNC aerogel. Multiscale porosity has been achieved, and samples heat treated at 1000 °C have achieved structural integrity. / Master of Science / Titanium Dioxide (TiO2) is a common material in today's world used in a range of applications including pigments, sunscreens, and thin films. It is a chemically and physically stable material, making it ideal for some biomedical applications including bone and cell growth scaffolds. TiO2 is also photocatalytic and has been used in photovoltaic cells and water decontamination systems to take advantage of this property. While TiO2 has been effectively implemented in these applications, the multiscale, controllable porous structure required for these applications has proven complicated to generate. To help improve this process, cellulose nanocrystal (CNC) aerogels were investigated as tunable hard templates for porous TiO2. Controlled ice templating through alteration of the freezing conditions followed by freeze drying provided a reliable method for the production CNC aerogels with repeatable micro and macrostructures. Testing multiple methods for coating the template in TiO2 led to the successful replication of the template in a ceramic part. The final TiO2 exhibited multiscale porosity with micro and macrostructures matching those of the CNC aerogel template. These parts can be tailored to fit a desired application by controlling the structure of the aerogel.

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京都大学 / 0048 / 新制・課程博士 / 博士(エネルギー科学) / 甲第22088号 / エネ博第396号 / 新制||エネ||76(附属図書館) / 京都大学大学院エネルギー科学研究科エネルギー変換科学専攻 / (主査)教授 星出 敏彦, 教授 今谷 勝次, 教授 川那辺 洋 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DGAM

The main objective of the thesis is to study the thermal properties of Si-based polymer derived ceramics (PDCs) at elevated temperatures and to classify the main factors affecting the thermal transport through these ceramics. The polymer derived ceramics with the chemistry Si- O-C-N were prepared starting from commercial polycarbosilane, polysiloxane, and polysilazane precursors. These precursors are cross-linked at room temperature to obtain the preceramic, followed by controlled pyrolysis (at different temperatures ranging from 1200 oC to 1800 oC in argon, nitrogen or carbon-di-oxide atmospheres), to get the final ceramic. The first part of the thesis discusses on development and studies of dense polymer derived thin disks having a basic chemistry, Si-C, Si-O- C, and, Si-C-N-O, developed via a casting technique followed by specific pyrolysis cycles. Having a thickness in the range of 100 μm- 300 μm, these ceramic disks were studied to be nanocrystalline/amorphous at least up to a temperature of 1400 oC and were found to have a significant amount of Cfree phase existing in them along with the intended chemistry. The high-temperature thermal properties were primarily investigated on ceramics prepared at a pyrolysis temperature of 1200 oC (ceramic still in nanocrystalline/amorphous glassy phase). The disks were found to have very low expansion coefficients (CTE) measured up to ~900 oC and the thermal diffusivity (k) and thermal conductivity (l) of these disks were also measured. An attempt to understand the influence of the different phases in a SiOC ceramic (mainly the Cfree phase, studied by enriching the carbon percentages using DVB) in determining the final thermal properties was also conducted. The influence of carbon enrichment on the mechanical properties of these disks is also studied as a sub-part of this work. The second part of the work deals with testing the possibility to use these ceramics for high-temperature insulation applications. ‘Reticulated’ ceramic foams of relatively same chemistries as that of the disks were prepared by a template replica approach, using polyurethane (PU) foams (more open-celled to more closed-celled types of PU foams were used in the study) as the template. Porous structures having densities ranging from as low as 0.02 g.cm-3 to 0.56 g.cm-3 and with a porosity ~ 80 % to ~99% were prepared and tested. The developed foams showed excellent thermal stability up to a temperature of 1400 oC and possessed very low thermal expansion. The thermal conductivity measured on them at RT gave values in the range 0.03 W.m-1.K-1- 0.25 W.m-1.K-1. A Gibson-Ashby modeling approach to explain the thermal conductivity of the porous ceramics was also attempted. The developed foams were also found to be mechanically rigid. In a nutshell, the thesis work studies the thermal properties of Si-O-C- N ceramics in detail and probes into the possibility to develop these class of Si-O-C-N ceramics into promising high-temperature insulation material.

Porous ceramics have successfully been used in a wide variety of highly advanced applications. Current routes to porous ceramics are limited in the types of porosity they can create and no one process is flexible enough to create any desired structure. This study introduces the use of ice particles as pore forming agents to fabricate porous materials. This novel method possesses several advantages over current industrial techniques including environmental friendliness, low cost, and flexibility in size and shape of resulting pores. Porous ceramic structures were created by adding preformed ice particles to an alumina slurry which was quickly frozen, air dried, and then sintered. Porosity was characterized using Scanning Electron Microscopy (SEM), Archimedes measurements, and gas sorption techniques. Small spherical pores were successfully created in the 20-200?m range and larger spherical pores were also created in the 2-3 mm range. Amount of porosity was controlled through specifying the amount of ice added to the ceramic slurry. Samples were prepared with porosity levels ranging from 30-75%. As a completely new process, these initial results are quite promising and further development will allow for even greater morphology control. / Master of Science

A formação de espinélio de aluminato de magnésio (\'MG\'\'AL IND.2\'\'O IND.4\'; espinélio), através da combinação de óxido de alumínio (\'AL IND.2\'\'O IND.3\'; alumina) com geradores de poros à base de óxido de Magnésio (\'MG\'\'O\'; magnésia) ou hidróxido de magnésio (\'MG\'(\'OH)IND.2\'; HM), resulta em estruturas de elevada porosidade (acima de 50%) e com baixa tendência a densificar mesmo em temperaturas elevadas (1500-1650°C). Devido a isso, esse sistema tem grande potencial de aplicação tecnológica para isolamento térmico e filtração em altas temperaturas. Considerando sua utilização em larga escala como isolante térmico verifica-se a necessidade de desenvolvimento de novas rotas de síntese mais eficazes. A reação de hidroxilação do \'MG\'\'O\' pode ser controlada, utilizando como matéria-prima tanto a magnésia sínter como com a magnésia caustica, desta forma, a expansão de peças de cerâmicas porosas feitas a base de \'MG\'\'O\' também podem ser controladas. Deste modo, neste trabalho pretende-se investigar uma nova rota para a obtenção de espinélio utilizando o \'MG\'\'O\' como ligante hidráulico e incorporar poros, in situ, utilizando-se da decomposição do HM em uma matriz de alumina. Os resultados mostraram que a área superficial específica (ASE) da magnésia influencia fortemente nas propriedades mecânicas do material moldado, comprovando que para as ASEs maiores a magnésia é viável como ligante hidráulico, sendo 0,6 MPa para o modulo de ruptura por compressão diametral, o melhor resultado deste trabalho, enquanto o sistema à base de cimento de aluminato de cálcio (CAC) teve 0,5 MPa de módulo de ruptura, e o sistema à base de \'alfa\'-Bond teve 0,3 MPa de módulo, ambos sistemas de ligantes hidráulicos conhecidos na literatura. A grande diferença no módulo de ruptura por compressão diametral mostrou que o tempo de cura e a ASE da magnésia influenciaram-no fortemente.Enquanto, as amostras com alta ASE de magnésia, em torno de ~60 \'M POT.2\'/g, tiveram 0,6 MPa de módulo de ruptura, as amostras com baixa ASE, ~1 \'M POT.2\'/g, não tiveram alteração no módulo de ruptura. Verificou-se também um aumento gradual no módulo de ruptura segundo a ASE, para um valor intermediário de ASE, ~30 \'M POT.2\'/g, o módulo foi de 0,4 MPa, valor ainda comparável a outros sistemas que se utilizam ligantes hidráulicos. Observou-se que a expansão das amostras durante a cura foi influenciada pela sua ASE, quanto maior a ASE, maior a expansão. Os resultados de PTG e do módulo de ruptura combinados com as imagens de MEV e os difratogramas, das amostras calcinadas, mostraram a influência da temperatura e da ASE na formação da cerâmica porosa por essa rota, sendo a temperatura uma variável de controle já conhecida, observada no diagrama de equilíbrio de fases. Entretanto, a influência da ASE do \'MG\'\'O\' para o controle da formação do espinélio é desconhecido da literatura / Magnesium aluminate spinel (\'MG\'\'AL IND.2\'\'O IND.4\'; spinel) formation by the combination of aluminum oxide (\'AL IND.2\'\'O IND.3\'; alumina) with magnesium oxide (\'MG\'\'O\', magnesium) or magnesium hydroxide (\'MG\'(\'OH) IND.2\'; HM), results in structures of high porosity (above 50%) and with low tendency to densify even at elevated temperatures (1500-1650°C). Due to this, this system has great potential of technological application for thermal insulation and filtration in high temperatures. Considering its large scale use as thermal insulation, it is necessary to develop new and more efficient routes of synthesis. The hydroxylation reaction of \'MG\'\'O\' can be controlled using both sinter magnesia and caustic magnesia as a feedstock, so the expansion of porous ceramic pieces made with \'MG\'\'O\'can also be controlled. Thus, in this work we intend to investigate a new route to obtain spinel using \'MG\'\'O\' as a hydraulic binder and to incorporate pores, in situ, using the decomposition of HM in an alumina matrix. The results showed that the specific surface area (ASE) of the magnesia strongly influences the mechanical properties of the molded material, proving that for the larger ASEs the magnesia is viable as a hydraulic binder, being 0.6 MPa for the diametral compression rupture modulus, the best result of this work, while the calcium aluminate cement (CAC) system had 0.5 MPa of modulus of rupture, and the \'alfa\'-Bond based system had 0.3 MPa of modulus, both systems of hydraulic binders known in the literature. The large difference in the diametral compression rupture modulus showed that the curing time and the ASE of the magnesia strongly influenced it. While high ASE magnesia samples, around ~60 \'M POT.2\'/g, had 0.6 MPa of modulus of rupture, samples with low ASE, ~1 \'M POT.2\'/g, had no change in modulus of rupture. There was also a gradual increase in the ASE burst modulus, for an ASE intermediate value ~30 \'M POT.2\'/g, the modulus was 0.4 MPa,a value still comparable to other systems using hydraulic binders. It was observed that the expansion of the samples during curing was influenced by their ASE, the higher the ASE, the greater the expansion. The results of PTG and the rupture modulus combined with SEM images and the diffractograms of the calcined samples showed the influence of temperature and ASE on the formation of the porous ceramic by this route, the temperature being a known control variable, observed in the phase equilibrium diagram. However, the influence of \'Mg\'\'O\' ASE on the control of spinel formation is unknown in the literature

MAX-phase ceramics are a class of ductile ceramic material group with the general molecular formula Mn+1AXn (n = 1, 2, 3….), where M is an early transition element, A is an element from the ‘A’ group of the periodic table and X is either nitride or carbide. One advantage of these materials is that they maintain their strength at high temperatures. In addition these ceramic materials possess the best properties of both ceramics and metals. Some of their important characteristics are low density, high stiffness, machinability, excellent thermal and electrical conductivity and they even exhibit some plasticity at elevated temperature. These amazing combinations of properties have made researchers foresee the technological importance of these materials as a structural ceramic for high temperature application. Since this ceramic is relatively new to the market, only a handful of work has been undertaken on this material and its applications are limited to heating elements. In addition, analysis of the thermodynamic data on this material is incomplete. This PhD work addresses this issue and conducts a complete thermodynamic analysis involved in the formation mechanism of the ternary titanium carbide MAX-phase Ti2AlC ceramic, using Self-propagating High temperature Synthesis (SHS) form of combustion synthesis process, based on the following exothermic reaction: (2+x) Ti + (y) Al + C → Ti2AlC + (x) Ti + (y) Al (i) Where x and y = 0.1. 0.2, 0.3… A thermodynamic model has been formulated to predict the temperature evolution during the reaction (i), for the formation of Ti2AlC using SHS process. In addition the effect of particle size in the elemental reaction has been studied on the formation mechanism of Ti2AlC and methods to control the porosity by fine tuning the particle size has been recognized. Manufacturing processes such as Self-propagating High temperature Synthesis (SHS), foam replication and freeze casting have been developed in this thesis to produce micro and macro porous Ti2AlC ceramic mainly for electrode applications. A systematic material development technique to produce macro porous Ti2AlC ceramic, using a foam replication technique has been established in this research work. The material fabricated by this technique has a uniform pore size (up to 5mm), with open interconnected pores and is ideal for a flow battery application which requires a multifunctional electrode material which is highly porous to allow the flow of electrolyte through it, is corrosion resistant and at the same time being electrically conductive. The mechanical properties of the ceramic produced by this method has been characterised and steps to mitigate the cracks and defects formed during the fabrication process to obtain structurally stable macro porous Ti2AlC ceramic has been reported in this work. This research demonstrates that one of the applications of macro porous Ti2AlC ceramic formed using foam replication technique is as an electrode material in a photo-Microbial Fuel Cell (p-MFC). Graded porosity micro porous Ti2AlC ceramics have also been fabricated using a freeze casting technique, with camphene as the freezing vehicle. A systematic material development process has been tailored for this particular material. A ceramic material with gradient pore size ranging from 27-305µm has been fabricated using this technique. This type of ceramic is a good candidate as an electrode material in micro-redox battery and for sensing applications. A variety of processing parameters such as solid loading (amount of ceramic content in the material), freezing temperature and mould material which affect the pore formation and pore size have been studied in this PhD and the range of porosities achieved by controlling these parameters have been reported.

Made available in DSpace on 2014-10-09T12:27:39Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:04:18Z (GMT). No. of bitstreams: 0 / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Made available in DSpace on 2014-10-09T12:27:39Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:04:18Z (GMT). No. of bitstreams: 0 / A ítria vem sendo muito utilizada como material luminescente e estrutural resistente a altas temperaturas, devido às suas excelentes características ópticas e refratárias. No processamento coloidal da ítria, o controle da estabilidade da suspensão consiste em uma importante etapa quando se objetiva produzir componentes reprodutivos, com densidade controlada, microestrutura homogênea e estabilidade estrutural. Desta maneira, os estudos envolvendo o comportamento de superfície, as condições de estabilidade das suspensões e o comportamento frente às condições de conformação (fluxo), fornecem importantes subsídios para controle dos processos na fabricação de componentes cerâmicos para queimadores de gás. Neste sentido, realizaram-se estudos sobre o comportamento de superfície, de estabilidade e reológico de suspensões aquosas de ítria e do concentrado de terras raras contendo ítria a fim de adequá-las para o processo de conformação por impregnação, onde foram testados diferentes materiais orgânicos como matrizes de réplica, para a confecção de membranas porosas para queimadores de gás. Neste estudo foram avaliados os parâmetros como: pH do meio, concentrações de dispersante, sólidos e ligante na estabilidade e no comportamento ao fluxo das suspensões cerâmicas, e também os aspectos dos distintos materiais orgânicos selecionados como matrizes de réplica para impregnação com as suspensões cerâmicas otimizadas. Os resultados obtidos indicam que se pode confeccionar membranas cerâmicas porosas pelo método de réplica a partir das suspensões aquosas de ítria e do concentrado de terras raras contendo ítria, utilizando-se pH alcalino, polieletrólito aniônico e uma tela mista de nylon-algodão como matriz de réplica. / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Porous ceramic materials are an interesting group of materials due to a wide range of physical properties, low density, and good permeability. Production of a monolith with a shape stability that would also have a high specific surface area and high porosity is a common problem with porous ceramics. The goal of this work was to maintain the high specific surface area and to produce a monolith with a shape stability. Two forms of porous silica nanofibers (as prepared and milled) were used and partially sintered using the Spark Plasma Sintering method (SPS). Different sintering times and temperatures for SPS were tested. The findings revealed that the best SPS conditions were as follows: temperature: 600 °C, sintering time: 5 minutes, pressure: 3 MPa, and the heating rate: 144 °C/min. These sintering conditions resulted in a stable silica based machinable monolith made from fibers or milled fibers. The monoliths have the specific surface area of up to 470 m^2/g and porosity of 72 %, or the specific surface area of up to 422 m^2/g and porosity of 69 % for as prepared fibers and milled fibers, respectively.