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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
11

Desenvolvimento de processo alternativo para a fabricação de materiais cerâmicos envolvendo as técnicas de conformação por consolidação com amido e colagem

Fernandes, Rodrigo Sampaio [UNESP] 01 December 2008 (has links) (PDF)
Made available in DSpace on 2014-06-11T19:32:50Z (GMT). No. of bitstreams: 0 Previous issue date: 2008-12-01Bitstream added on 2014-06-13T19:03:29Z : No. of bitstreams: 1 fernandes_rs_dr_guara.pdf: 3696333 bytes, checksum: 967ad7b5dd1ef455a42e079e5e339fe8 (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / A conformação por consolidação com amido é uma técnica que utiliza o amido como ligante e elemento formador de poros e é usada geralmente para confecção de cerâmicas porosas. Essa técnica pertence à família dos métodos de conformação direta que utilizam moldes impermeáveis. Por outro lado, a obtenção de cerâmicas pelo método da colagem de barbotinas é extremamente conhecido e amplamente utilizado na indústria. Baseia-se no ato de verter a barbotina em moldes porosos de gesso. O objetivo deste trabalho foi desenvolver um novo método para a obtenção de cerâmicas de alumina, que consiste na junção dos processos de colagem e de conformação com amido. Ou seja, suspensões contendo amido foram vertidas em moldes porosos de gesso e submetidas ao processo de gelatinização, visando a consolidação das peças e futura sinterização. Para atingir esta meta, o trabalho foi dividido em três etapas: a primeira, um estudo detalhado da conformação por consolidação com amido; posteriormente, uma análise de peças cerâmicas produzidas com a utilização de um molde com base porosa e laterais impermeáveis, a fim de gerar um gradiente de porosidade; por fim, um estudo das cerâmicas obtidas a partir do vertimento de suspensões contendo amido no teor de 10% a 40%, em moldes de gesso de consistência 70 e 90. As cerâmicas produzidas na primeira e terceira etapas foram caracterizadas por meio do ensaio de flexão por três pontos, e em determinados casos os resultados foram tratados estatisticamente pelo método de Weibull, pela determinação da porosidade aparente, baseado no Princípio de Arquimedes, por microscopia óptica e microscopia eletrônica de varredura. Uma análise comparativa mostrou que as cerâmicas produzidas por este novo método proposto apresentaram maior resistência mecânica que as obtidas somente por conformação por consolidação amido ou somente por colagem. / The formation by starch consolidation is a technique which uses the starch as a binder and porous form element and is generally used to make porous ceramics. This technique belongs to the direct consolidation methods that use mould impermeable. On the other hand, the ceramic obtained by the slip casting method is extremely known and largely used in the industry. It is based on the act of pouring the slip in cast porous moulds. This work was intended to develop a new method for obtaining alumina ceramics, that consists on joining of slip casting processes and starch consolidation, that is, suspensions containing starch were poured in cast porous moulds and submitted to the gelling process, in order to obtain the sample consolidation and future sintering. For reaching this purpose, the work was divided in three stages: the first one, a detailed study of the starch consolidation; later, an analysis of ceramic samples produced by using a mould porous base and impermeable laterals, in order to generate a porosity gradient; finally, a study of the ceramics obtained from suspension flowing containing starch of 10% to 40% content in cast mould of 70 and 90 consistence. The ceramics produced in the first and third stage were characterized by means of threepoint flexural test and in certain cases the results were statistically treated by the Weibull method, by the determination of the apparent porosity, based on Archimedes principle, by optical microscopy and scanning electronic microscopy. The comparative analysis showed that the ceramics produced by this new proposed method presented better mechanical resistance than the ones obtained only by starch consolidation or only by slip casting.
12

Membranas suportadas de alumina, obtidas pelo método da sedimentação gravitacional. / Alumina supported membranes, obtained by the method of sedimentation.

Robson de Miranda Soares 10 September 1998 (has links)
Este trabalho trata da preparação de membranas suportadas em substratos com alta permeabilidade, alta porosidade e razoável resistência mecânica., pelo método da sedimentação gravitacional. Os substratos foram obtidos através da prensagem de composições contendo poli(metacrilato de metila) e ácido esteárico dissolvidos no dicloro metano e alumina APC-SG (d50 = 3 &#956m ) calcinada a 1600&#176C durante 1h. As partículas de alumina após calcinação são classificadas em peneiras da série Tyler e utilizadas aquelas cuja distribuição se encontravam na faixa granulométrica de 250 a 500 &#956m. Os substratos após a prensagem a 33 MPa são calcinados e sinterizados a 1600&#176C durante 3h e apresentam \"alta resistência mecânica\", taxa de fluxo de 404 m3/m2h , porosidade de 59 % e distribuição de poros nas faixas de 10 a 100 &#956m e de 0.4 a 2.0 &#956m. Devido a grande diferença entre os diâmetros dos poros do substrato e o tamanho das partículas de alumina AKP-50 (d50 = 0.3 &#956m), utilizada para a obtenção da membrana, foi necessário acrescentar uma camada intermediária de alumina APC-SG entre o substrato e a membrana para viabilizar o processo de sedimentação. A camada intermediária foi depositada por sedimentação gravitacional e sinterizada a 1500&#176C durante 1h. A porosidade do substrato/camada intermediária é igual a 56 %, taxa de fluxo de 10 m3/m2h e distribuição de poros estreita na faixa de 0.4 a 2 &#956m. As membranas foram sinterizadas a 100&#176C durante 0.5h e se caracterizam por possuir uma distribuição de poros estreita na faixa de 0.04 a 0.09 &#956m, espessura média de 57 &#956m e taxa de fluxo de 0.4 m3/m2h. / This work is concerned with ceramic membrane preparation supported on a ceramic substrate having high permeability, high porosity and reasonable mechanical strength. Membranes were prepared from AKP-50 alumina by the gravitational sedimentation method followed by sintering at 1000&#176C for 0.5 h. Membranes showed a unimodal narrow pore size distribution in the 0.04 to 0.09 range, thickness of 57 &#956m and 0.4 m3/m2h. flow rate. Substrates were prepared by uniaxial compression, at 33 MPa, of alumina PMMA stearic acid powder composites followed by organic burning and final sintering at 1600&#176C for 3h. In order to increase alumina APC-SG grain size distribution to 250- 500 &#956m range, alumina was previously calcined at 1600&#176C for 1h. Substrates show high mechanical strength, 4.0 to 2 m3/m2h flow rate, 60% porosity and pore size distribution in the range of 10-100 &#956m and from 0.4 to 2.0 &#956m. Due to the large difference between the substrate pore diameter and the AKP-50 alumina grain size distribution (d50 = 0.3 &#956m) used in the membrane preparation, it was necessary to add an intermediary APC-SG layer. This layer was prepared also by the gravitational sedimentation method and sintered at 1500&#176C for 1h. The intermediary layer/substrate showed 56% porosity, 10 m3/m2h flow rate and a narrow pore sue distribution in the 0.4-2.0 &#956m range.
13

Carboximetilquitosanas: preparação, caracterização e aplicação como agentes de estabilização de suspensões aquosas de alumina / Carboxymethylchitosan: synthesis, characterization and application as stabilizers of aqueous alumina suspensions

Juliana Colombo Lamas 16 May 2008 (has links)
Suspensões coloidais encontram um grande interesse prático e apresentam aplicações na área de material cerâmico. No processamento deste material, a qualidade do produto final depende das propriedades iniciais da suspensão. Portanto, a desaglomeração e dispersão do pó cerâmico são cruciais para que os defeitos sejam minimizados. Para isto, estabilizantes eficientes devem ser utilizados. A carboximetilquitosana (CMQ) é a uma cadeia macromolecular, que apresenta grupos funcionais (carboximetila e amino) que podem adquirir cargas quando dissolvidos em meio aquoso e, por isso, pode ser usada no controle de propriedades de suspensões em que a presença de grupos com cargas seja importante para os mecanismos de estabilização. Neste trabalho, a CMQ foi preparada pela reação entre quitosana e diferentes proporções de ácido monocloroacético. A reação foi realizada em dois meios, um em que ocorreu previamente a dissolução da quitosana em um sistema de solventes (dimetilacetamida/cloreto de lítio) e outro em meio heterogêneo, no qual o material permaneceu em suspensão. A carboximetilação com prévia dissolução da quitosana pode levar a uma distribuição mais homogênea dos grupos carboximetila, quando comparado à preparação em meio heterogêneo, o que poderá influenciar na eficiência da CMQ como estabilizante. As amostras de CMQ foram caracterizadas pelas técnicas de espectroscopia na região de infravermelho (IV), ressonância magnética nuclear (RMN) e análise termogravimétrica (TGA). A técnica de IV, aplicada também para as amostras na forma ácida, permitiu a determinação do grau de substituição médio (GS ), sendo encontrado valores que variaram de 0,6 a 1,1. A eficiência da CMQ como dispersante foi avaliada por meio de medidas viscosimétricas, de potencial zeta e diâmetro médio de partícula. Em geral, houve uma diminuição da viscosidade da suspensão de alumina com a adição da CMQ, com certo destaque para as amostras com GS em torno de 1,0. As medidas de potencial zeta mostraram um aumento da magnitude das cargas superficiais, quando na presença de CMQ, o que proporciona repulsão eletrostática entre as partículas. As medidas de diâmetro de partícula mostraram que, além da diminuição do valor médio na região do ponto de carga zero (pcz), em que a agregação é máxima, ocorreu uma diminuição na distribuição dos diâmetros das partículas, em todos os pHs considerados. Estes resultados confirmam a eficiência das amostras de CMQ como estabilizantes para suspensões aquosas de alumina. / Colloidal suspensions are of great practical interest, including applications in ceramic materials. In colloidal ceramic processing, the quality of the final product will depend on the properties of the initial powder suspension. Thus, an efficient deagglomeration and dispersion of the ceramic powder in a solvent is crucial for minimizing defects. This requires efficient dispersants. Carboxymethylchitosan (CMCh) corresponds to macromolecular chains containing groups carboxymethyl and amino, that can acquire charges when dissolved in water. Then, it can be used for the control of the properties of surface, when the presence of charged groups is important to the mechanisms of particles dispersion. In this work, carboxymethylchitosan was prepared reacting chitosan and different amount of monochloroacetic acid, in two different media. In the first one, chitosan was previously dissolved in a solvent system (dimethylacetamide/ lithium Chloride) and in the second one, corresponding to heterogeneous media, the reactants and products remained suspended during the whole process. The reaction with previous dissolution of chitosan may generate a more homogeneous distribution of carboxymethyl groups, which in turn can influence the CMCh efficiency as dispersant. The CMCh sample was characterized by infrared spectroscopy (IR), nuclear magnetic resonance (NMR) and thermogravimetric analyses (TGA). The obtainment of IR spectra considering the samples of CMCh in their acid form, also allowed determining the average degree of substitution (DS), that ranged from 0.6 to 1.1. These samples were used as dispersants for aqueous alumina suspensions. In this work the efficiency of the carboxymethylchitosans as stabilizing agents was analyzed considering rheological, zeta potential and average particle size measurements. The addition of CMCh, mainly that with DS near 1.0, led to a suspension with lower viscosity. Zeta potential measurements show that the addition of CMCh results in an increase in the absolute value of zeta potential at the point of zero charge (pzc), what promote the electrostatic repulsion between the particles. The presence of CMCh led also to a suspension with lower average diameter of particles and narrow particle size distribution in all pH values considered in the present work. The results showed that the charged macromolecules (CMCh) obtained from a renewable source (chitosan), present a good performance as deflocculating agents of alumina suspensions.
14

Nanofeatures of Biomaterials and their Impact on the Inflammatory Response

Pujari-Palmer, Shiuli January 2016 (has links)
Nanomaterials offer an advantage over traditional biomaterials since cells naturally communicate via nanoscale interactions. The extracellular matrix, for example, modulates adhesion and cellular functions via nanoscale features. Thus incorporating nanofeatures into biomaterials may promote tissue regeneration, however in certain forms and doses nanomaterials can also cause harm. A thorough understanding of cell-nanomaterial interactions is therefore necessary to better design functional biomaterials. This thesis focuses on evaluating the effect of nanofeatures on inflammation using two different models: nanoporous alumina and hydroxyapatite nanoparticles (HANPs). The inflammatory response caused by in vitro exposure of macrophages to nanoporous alumina, with pore diameters of 20nm and 200nm, was investigated. In addition in vivo studies were performed by implantation of nanoporous membranes in mice. In both cases the 200nm pore diameter elicited a stronger inflammatory response. Nanoporous alumina with 20, 100 and 200nm pores were loaded with Trolox, a vitamin E analogue, in order to scavenge ROS produced by primary human polymorphonuclear (PMNC) and mononuclear (MNCs) leukocytes. Unloaded alumina membranes stimulated greater ROS production from PMNCs cultured on 20nm versus 100nm pores. This trend reversed when PMNCs were cultured on Trolox loaded membranes since Trolox eluted slower from 20nm than 100nm and 200nm pores. ROS produced from PMNCs was reduced between 8-30% when cultured on Trolox loaded membranes. For MNCs, ROS production was not affected by pore size. However when the alumina was loaded with Trolox ROS production was quenched by 95%. HANPs with distinct morphologies (long rods, sheets, dots, and fibers) were synthesized via hydrothermal and precipitation methods. The HANPs were then exposed to PMNCs, MNCs, and the human dermal fibroblast (hDF) cell line. Changes in cell viability, ROS, morphology, and apoptotic behavior were evaluated. PMNC and hDF viability decreased following exposure to fibers, while the dot particles reduced MNC viability. Fibers stimulated greater ROS production from PMNCs and MNCs, and caused apoptotic behavior in all cell types. Furthermore, they also stimulated greater capsule thickness in vivo, suggesting that nanoparticle morphology can significantly influence acute inflammation. The outcome of this thesis, confirms the importance of understanding how nanofeatures influence inflammation.
15

The effect of superplasticising admixtures on the performance of ciment fondu

Gill, Susan Margaret January 1987 (has links)
No description available.
16

Electron spin resonance studies of doped alumina and aluminium nitride

Mushait, A-N. January 1988 (has links)
No description available.
17

The electrochemical breakdown of beta alumina

Gilmour, Alison January 1987 (has links)
No description available.
18

Élaboration d'une céramique dentaire par stéréolithographie / Development of a stereolithography-manufactured dental ceramic

Dehurtevent, Marion 04 December 2017 (has links)
La stéréolithographie, technique additive de mise en forme couche par couche, permet de limiter les contraintes de l’usinage mais doit répondre aux exigences des normes dentaires en vigueur pour son utilisation clinique. L’effet de la formulation de différentes céramiques alumineuses mises en barbotines et de l’orientation des modèles lors de la mise en forme ont été évalués. Ces travaux ont permis de développer et d’optimiser les différentes étapes de stéréolithographie à masque dynamique sans racleur et les propriétés physiques et mécaniques des céramiques densifiées adaptées à une application dentaire. Pour cette application, la mise en forme des barbotines était assurée lorsque la viscosité est inférieure ou égale à 138 mPa.s. Lorsque la viscosité était supérieure ou égale à 151 mPa.s, la mise en forme est impossible. L’augmentation du taux de matière sèche des barbotines d’alumine a augmenté la viscosité et la durée minimale nécessaire à la mise en repos avant photo-polymérisation. La diminution de la proportion de la phase organique a permis de limiter l’apparition de défauts et d’augmenter les propriétés mécaniques des céramiques. Le compromis entre un taux de matière sèche élevé et une viscosité maximale à ne pas dépasser a permis de sélectionner la barbotine avec un taux matière sèche de 80% et une grande granulométrie, comme meilleure formulation pour une utilisation dentaire. Lors de la mise en forme, l’augmentation de la surface des couches polymérisées a augmenté la durée de mise en repos des barbotines mais également le risque d’apparition de défauts dans les pièces. Ceci est à l’origine d’une diminution des propriétés mécaniques des céramiques. La surface maximale des couches pour ce type de procédé doit donc être limitée. Parallèlement, l’analyse microstructurale de l’alumine pure densifiée a montré une texturation. En effet, l’axe principal des grands grains suit l’orientation des couches, ce qui peut induire des propriétés mécaniques anisotropes. Plus précisément, lorsque ces grains étaient orientés perpendiculairement à la charge, la ténacité (par flexion) des céramiques a augmenté par rapport à celles des céramiques dont l’axe principal des grains était orienté parallèlement à la charge. Enfin, la répartition du stress dans une infrastructure de bridge, analysée par la méthode des éléments finis, était comparable entre les céramiques mises en forme par stéréolithographie et les céramiques usinées commercialisées. En vue d’une utilisation clinique, les céramiques mises en forme par stéréolithographie sans racleur doivent donc être orientées afin de présenter la plus faible surface de couche. Si l’axe principal des grands grains dans la céramique suit l’axe de la mise en forme, ces particules doivent être orientées perpendiculairement à la charge occlusale. Enfin, des études complémentaires sont nécessaires pour étudier la précision d’adaptation d’une infrastructure sur la préparation dentaire afin d’optimiser son ajustage. / Stereolithography is a layer-by layer additive manufacturing method. It allows to reduce the strains that occur with milling methods. Its use for clinical purposes needs to follow dental material standards. The impact of slurry composition and the influence of layer orientation on mechanical and physical properties have been studied. This work permitted to develop and optimize a stereolithographic manufacturing process with digital light processing with no blade in order to obtain suitable physical and mechanical properties of alumina ceramics for dental applications. The slurries could be processed by stereolithography when their viscosity was below 138 mPa.s. Viscosity values beyond 151 mPa.s caused deformations in the printed layers. High dry matter content increased the viscosity and the minimal time needed to relax the slurries before photopolymerization. Reduction of the organic phase reduced the formation of defects and thus increased mechanical strength of the ceramics. The compromise between a high dry matter content and low viscosity that is compatible with stereolithography manufacturing was found for 80% dry matter content slurries having large particle size. The great layer surfaces increased the period needed to relax the slurries between two polymerizations, and the risk of defects to appear. This can reduce mechanical strength of the ceramics. To avoid this drawback, the maximal surface of the printed layers should be limited. In addition, microstructural analysis showed a texturation of pure densified alumina. In fact, the main axis of large particles followed the orientation of the printed layers. This led to anisotropic mechanical properties. Moreover, an orientation of these particles perpendicular to the load increased fracture toughness (measured by flexural strength) of the ceramics compared to those of ceramics presenting particles oriented parallel to the load. Finally, compared finite element analysis of a bridge framework showed similar strain repartition between stereolithographic and subtractive methods. In conclusion, stereolithography with no blade of dental ceramics need to orientate the models in order to present the smallest layer surfaces. If the axis of the large particles is the same as the printing axis, these particles must be oriented in order to be perpendicular to masticatory forces. More studies are required to evaluate the marginal gap between prosthetic frameworks and dental crown preparations to allow clinical use.
19

Investigation and Optimization of a Porous Sintered Ceramic Material in a Grinding Application

Clark, Laura A 03 May 2009 (has links)
Porous sintered ceramic foam made of 99.8% Alumina (AL2O3) was investigated for grinding applications in metallic bearings. The properties of this alumina foam were investigated and correlated to the grinding performance in an I.D. grinding application. The mechanical properties, such as density and Young's Modulus, were correlated to grinding performance. The physical properties, investigated with Scanning Electron Microscope (SEM) analysis and a CT Scan, were used to study sintering behavior and porosity size and distribution. This work found an optimal friability and sintering temperature for the alumina foam grinding wheels. This will enable the alumina foam to become a viable grinding product and compete with the conventional vitrified products. This would be significant since the alumina foam has mainly one ingredient and is manufactured with a simpler process. It can also support a higher porosity than conventional grinding wheels, which has proven to provide better grinding performance and coolant access, keeping the work piece cooler and avoiding thermal damage.
20

Development and Characterization of Novel Alumina Based Ceramic Matrix Composites for Energy Efficient Sliding Applications

Paluri, Rajeshwari S. Lakshmi 2011 August 1900 (has links)
Friction, wear, and lubrication have direct influence on performance, reliability, and service life of mechanical systems with moving components. The useful life of these systems and their efficiency can be improved by improving the surface properties/ performance at sliding interfaces. Further, the usage of materials for sliding systems is limited in extreme environments, such as high temperature, and space, etc., due to their limited surface properties. This thesis focuses on the development of a new class of composites with superior surface properties, i.e., low friction and high wear resistance for extreme environmental conditions. Alumina, a well understood material for its tribological performance, is a merit choice for applications where high wear resistance is required, such as pump bearings, seal rings, valve seats, piston components, gears, cutting tool inserts and artificial joints. We propose to develop a novel alumina based ceramic composite to enhance its surface and tribological properties using a powder compaction technique. The newly developed composites will be characterized by X-ray Diffraction (XRD), Fourier Transform Infrared spectroscope (FTIR), Optical microscope, Environmental Scanning Electron Microscope (E-SEM), Goniometer and Surface profilometer. In-situ formation of high temperature stable phases, effect of sintering temperature, and percentage of reinforcement on phase formation will be studied. Investigation of effect of sintering temperature and percentage of reinforcement on density, porosity, and grain size will be conducted. The composites will be characterized for their tribological properties (friction and wear). The mechanisms for modified friction and wear will be proposed. The process parameters and compositions will be optimized. XRD results confirmed the formation of Al18B4O33, and AlB2 and FTIR confirmed the presence of B2O3. Increase in sintering temperature and wt % of boron affected the porosity, grain size, and hardness of the composites. The coefficient of friction was lower for the composites compared to pure alumina ceramic. The coefficient of friction decreased with increase in sintering temperature. The wear mechanism was found to be micro-fracture using ESEM and SEM studies.

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