Dissertations / Theses on the topic 'Ceramic'

The use of sintered cordierite/enstatite glass-ceramics as interlayers for joining silicon nitride to itself and to metals has been investigated. The role of the additives B203 and P205, which control the dynamics of sintering and crystallisation, has been studied using SEM, XRD and non-isothermal DTA-based measurements of activation energy. The measured activation energies for the crystallisation of μ-cordierite, for compositions with no additives, with B203 only, and with P205 only, did not differ significantly and were in the range 415-460 kJ mol-1. When both B203 and P205 were present this was increased to 503-524 kJ mol-1. The activation energy for α-cordierite formation when no additives were present was 952 ± 57 kJ mol-1. This was substantially reduced by the presence of B203 (540 ± 27 kJ mol-1), P205 (668 ± 41 kJ mol-1) and when both were present (352 ± 26 kJ mol-1). Cordierite/enstatite glass-ceramics have been successfully used to join silicon nitride to itself. Joining at 1050-1100'C in N2 with an applied load of ~ 2.5 MPa, resulted in joint strengths, measured in 4-pt bending, of 110-170 MPa. This is comparable to the intrinsic strength of the glass-ceramic and sufficient for practical applications. These strengths were obtained using an interlayer with a TCE (5.7 MK-1) greater than that of the silicon nitride (3.0 MK-1). Suggestions for further improvements to the joint strength are discussed. The use of a glass-ceramic joint with graded thermal expansion to bridge a TCE mismatch is discussed, and the geometrical restrictions on the joint, which limit possible practical applications, are outlined. The concept of a ceramic/metal compression joint with a glass-ceramic interlayer has been demonstrated for joining silicon nitride to both Nimonic alloy 80A and Ti. The requirements for continuity of electronic structure at the Nimonic 80A/glass-ceramic and the Ti/glass-ceramic interfaces are satisfied by reaction between the glass/glass-ceramic and, the pre-oxidised surface of the Nimonic alloy to form a MgTi205-Al2TiO5 solid solution phase, and the Ti to form Ti5Si3. For the lower WE mismatch (Ti-silicon nitride) the residual joining stresses generated on cooling were marginally too high and need to be further reduced, either by a slight alteration to the joint geometry and/or a smaller WE mismatch.

Herein, we study the contact damage performance of two armour ceramics, alumina and silicon carbide, with varying microstructures and one particle-reinforced ceramic nanocomposite, alumina/silicon carbide, in an attempt to understand the microstructural mechanisms that affect plasticity and cracking under quasi-static and dynamic conditions. Quasi-static contact damage was imitated using Vickers indentation over a varying load regime. Numerical analysis of the indentation size effect, performed using the proportional specimen resistance model, allowed the contributions of plastic deformation and cracking to be separated into two individual values. In all three samples, higher levels of surface energy were found to correlate with increased amounts of cracking per unit area of indentation impression. Analytical modelling of crack initiation during Vickers indentation together with quantitative measurements of surface flaw populations revealed that such an increase in cracking damage was the result of higher densities of larger flaws. The hardness of the monolithic ceramics was found vary based on grain size and porosity levels, a smaller average grain size and lower porosity levels resulting in higher hardness values. In the nanocomposite materials, hardening was found to occur with further additions of silicon carbide nanoparticles. Such an effect has been attributed to the increased dislocation densities, as measured using Cr3+/Al2O3 fluorescence spectroscopy, and the impedance of dislocation movement within the lattice due to the presence of silicon carbide nanoparticles. In order to simulate dynamic contact damage, a low velocity, scaled-down drop-weight test was designed and developed. The dynamic contact damage resistance was determined based on the depth of penetration of a blunt indenter. In the monolithic ceramics, the indenter penetration was found to be shallower in materials of higher hardness. However, the nanocomposite materials displayed an opposing trend, the indenter penetration becoming deeper in the samples of higher hardness. The macro-scale fracture patterns produced during drop-weight impacts were seen to vary based on flaw populations and indenter penetration. In certain microstructures, extensive micro-cracking was also observed.

Ceramics and Ceramic Matrix Composites (CMC) are well known as useful materials for harsh environment applications. Ceramic components have been widely used as abrasives, cutting tools, nuclear fuel elements, catalyst supports and astronomical telescope mirrors. Generally speaking, ceramics and CMC are excellent candidates for high-temperature applications because of their good mechanical properties and thermal stability at elevated temperatures. However, to produce large-size and complex components, development and testing of joints for ceramics and CMC are fundamental. Joining materials and techniques currently available to join ceramic and CMC include diffusion bonding using various active fillers, transient eutectic phase methods such as nano-infiltration and transient eutectic-phase (NITE), laser joining, selected area chemical vapor deposition , glass-ceramic joining , solid state displacement reactions, preceramic polymer routes, reaction forming, brazing. Adhesive joining materials (e.g. epoxy resins) are used to join ceramics only for applications at temperature lower than 150 °C, i.e. adhesively bonded joints are widely used for automotive, aerospace, electronic and packaging applications. Together with the need of a reliable joining method, a widely accepted standard to test the mechanical strength of joined ceramics and composites is still unavailable and measuring the shear strength of the same joining material with different test methods could lead to different results. The aim of this Thesis is to compare and discuss several different shear strength tests used to join ceramics and CMC, with the final goal to provide designers and scientific community a widely accepted, reliable test method. A homogeneous pure shear stress state is not obtainable with most of the currently used single or double lap tests, which give apparent and not pure shear strength of the joined samples; in addition, the presence of several different apparent shear strength tests in the literature makes comparison quite impossible. The asymmetrical four point bending test (ASTM C1469-10) is recommended as a standard test by the ASTM to test joined ceramics and composites, but notches have to be cut in the joined area when the joining material strength is high. When the shear strength of the joining material approaches that of the substrate to be joined, ASTM C1469-10 is not suitable. With torsion tests a pure shear loading strength can be measured without using notches. One of the very first proposals for torsion test for epoxy bonded aluminum alloy samples was reported by M. Ouddane et al.: as thoroughly discussed there, torsion results were considered more reliable than those obtained by standard lap tests, the improvement mainly due to the fact that lap tests induce non uniform stress concentrations that affect the reliability of results. Recent papers deal with torsion tests on joined hourglass shaped samples : preliminary results suggest that the torsion test method with a miniature specimen has a potential to evaluate the shear properties of the joint interface, provided that the fracture occurs in the joined area. Torsion tests have been thoroughly analyzed in this Thesis: a pure non-uniform shear stress distribution is obtained with torsion tests; together with asymmetrical four point bending test (ASTM C1469-10), torsion test is the only one able to measure pure shear strength of joined ceramics and composites. The torsion test method is proposed in some ASTM standards but none of them is directly applicable to joined ceramics: the main result of this Thesis is to demonstrate the reliability of torsion as a method to measure the shear strength of joined ceramics and CMC. Two torsion standards have been adapted to joined C/C and ceramics, first by preparing square section samples (TS), rods (TC), tubes (TT), then by mechanically shaping the joined samples as a hourglass in different shapes. The choice of an epoxy resin (Araldite AV119) to join SiC in this PhD thesis was done in order to have a “model” brittle joining material to obtain a statistically relevant number of joined samples in a reasonable time, to compare shear strength results . More than one hundred joined samples have been prepared and tested at room temperature by asymmetrical four point bending (A4PB) (ASTM C1469-10), torsion on square section samples (TS), torsion on circular section rods (TC), torsion on hourglass shaped samples (THG, TDHG, TRHG), torsion of tubes (TT), single lap in compression (SL), single lap off set in compression (SLO), double lap off set in compression (DLO), Brazilian test (BT), double notch (DN) methods. A modified ASTM B898 standard has been used as a further example of single lap test in compression to complete the comparison work (B898).

De nos jours, la consommation des condensateurs céramiques multicouches (MLCCs) augmente en raison de leur efficacité et leur fiabilité. La miniaturisation résultant dans une plus grande dissipation volumique de chaleur et les nouvelles applications demandent des MLCCs qui peuvent travailler de 300 à 350°C, au-delà des limites actuelles de 200 – 250°C. De plus, les exigences environnementales augmentent également avec les réglementations REACH et RoHS qui interdisent l'utilisation du plomb en Europe. Il est donc impératif de créer des nouveaux matériaux sans plomb capables de répondre aux nouvelles attentes des MLCC.Cependant, la compatibilité avec les méthodes de production industrielle, ainsi que les prix du marché sont des limites importantes. Trois familles de matériaux sans plomb ont été examinées : celle du BaTiO3 (BT), du K0.5Na0.5NbO3 et du Na0.5Bi0.5TiO3 (NBT). Le NBT-BT à la MPB (6 % BT) a été choisi comme matériau diélectrique de base. Plusieurs méthodes et paramètres de synthèse ont ensuite été étudiés pour déterminer les meilleures conditions de synthèse. La synthèse à l'état solide et le frittage traditionnel ont été choisis pour les échantillons en massif et le coulage en bande a été choisi pour les couches. Pour éviter l'évaporation des espèces volatiles, le frittage a été effectué en couvrant le NBT-BT par une poudre de ZrO2. Tous les échantillons présentaient des phases secondaires contenant du Ba (Ba2TiO4 et Ba2Ti9O20) en raison de l'évaporation du Na pendant le frittage. Un effet de peau créé par la coexistence des phases tétragonale, rhomboédrique et cubique a également été observé, lié à la concentration locale de Ba dans le réseau cristallin du NBT. Les effets des paramètres de synthèse et la stœchiométrie des réactifs sur les propriétés diélectriques, la résistance d'isolement et la séparation des phases ont été analysés. La stœchiométrie nominale Na0.44Bi0.48Ba0.06TiO3 était la plus appropriée pour les MLCC en raison de sa résistance d'isolement élevée, de ses faibles pertes diélectriques et de sa stabilité de la permittivité en température. La présence de phases secondaires est initialement bénéfique en raison de l'élimination des lacunes d'oxygène. Au-delà d'une fraction volumique critique (2.5 à 3.0 %) et d'une taille de grain moyenne critique (0.9 à 3.0 m2), la tendance s'inverse en raison de la nature conductrice des phases secondaires.Pour atteindre la fraction volumique et la taille de grain critiques, un agent dispersant a été utilisé lors du broyage dans la jarre de YSZ, avec du MEK et de l'éthanol comme solvants, et sans sécher les réactifs avant la pesée. Enfin, la relaxation des contraintes a été réalisée à 400°C pendant 3 heures. Trois modèles ont été utilisés pour expliquer la dispersion des propriétés diélectriques en fréquence : le modèle de Maxwell-Wagner, le modèle de Nyquist et la loi de Curie-Weiss modifiée. Les incompatibilités entre les propriétés diélectriques du NBT-BT rapportées dans la littérature ont ensuite été analysées soulignant l'importance d'avoir des méthodes de synthèse et de mesure strictes. Les trois principaux facteurs affectant les propriétés diélectriques induisant ces incompatibilités étaient la stœchiométrie, la méthode de métallisation et la fixation des fils électriques à l'aide de la laque d'argent. Des pertes diélectriques croissantes à haute température a aussi été observée après chaque cycle thermique dépassant les 300°C, indiquant une dégradation thermique des échantillons.Enfin, les monocouches céramiques ont montré une faible densité (62%) après frittage, limitant la plage de température correspondant aux spécifications d'Exxelia. Cependant, en utilisant un pressage des couches avant frittage, l'échantillon multicouche fritté présentait une densité élevée (89%). Une mesure des propriétés diélectriques doit être effectuée sur cet échantillon multicouche synthétisé, afin de déterminer sa compatibilité avec les spécifications d’Exxelia
MLCC consumption is today increasing due to their high efficiency, reliability and frequency characteristics. MLCCs that can work from 300 to 350°C are required both for miniaturization, resulting in greater volume heat dissipation and for new applications. Moreover, environmental requirements are also increasing, the REACH and RoHS regulations prohibiting the use of lead in Europe. It is imperative to create new lead-free materials that are able to meet those requirements.However, the compatibility with the production methods, price, and market are important industrial limitations that need to be considered.Three families of lead-free materials were examined: BaTiO3-based, K0.5Na0.5NbO3-based and Na0.5Bi0.5TiO3-based materials. NBT-BT at the morphotropic phase boundary (6% BT) was chosen as the base dielectric material.Several synthesis methods and parameters were studied to determine the best synthesis conditions. Solid-state synthesis and traditional sintering were chosen for the bulk samples and tape casting was chosen for the layer samples preparation. Sintering was done under ZrO2 powder to prevent the evaporation of volatile species.All samples had secondary Ba-containing phases (Ba2TiO4 and Ba2Ti9O20) formed because of the evaporation of Na during sintering. A skin-effect was observed due to a phase coexistence (tetragonal, rhombohedral, and cubic) due to the local concentration of Ba in the NBT lattice.The effects of the synthesis parameters and the stoichiometry of the reactants on dielectric properties, insulation resistance, and phase separation were analysed.The Na0.44Bi0.48Ba0.06TiO3 nominal stoichiometry was the most suitable for the MLCCs due to its high insulation resistance, low dielectric losses, and stability of permittivity in temperature.The phase separation was initially beneficial, due to the resulting elimination of oxygen vacancies. Above a critical volume fraction (2.5 to 3.0%) and a critical mean surface area (0.9 to 3.0 m2), the trend was reversed due to the conductive nature of the secondary phases.To achieve the critical volume fraction and surface area of the secondary phases, a dispersing agent was used during ball-milling in YSZ jar, with MEK and ethanol as solvents, and without drying the reactants prior to weighing. Finally, a strain relaxation was done at 400°C for 3 hours.Three models explained the frequency dispersion of the dielectric properties: the Maxwell-Wagner model, the Nyquist plot and the modified Curie-Weiss law.Incompatibilities between the dielectric properties of NBT-BT reported in the literature were then analysed, showing the importance of maintaining strict synthesis and measurement methods. The three main factors affected the dielectric properties, creating these incompatibilities in the bulk samples. There were the stoichiometry, the metallization method, and the fixing of the electrical leads using silver paste.An increase of the high-temperature dielectric losses after each thermal cycle reaching more than 300°C was observed, indicating a thermal degradation of the material.Finally, the sintered ceramic monolayers showed a low density (62%), limiting the temperature range corresponding to Exxelia’s specifications. However, after pressing the layers together before sintering, the sintered multilayer sample showed a high density (89%). Dielectric property measurement should be carried out for these synthesized multilayers

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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)
Neste trabalho, apresenta-se uma rota alternativa para produção de cerâmicas de mulita (3Al2O3.2SiO2), a partir da mistura de pós de alumina (Al2O3) e sílica (SiO2), para uma possível aplicação em um dispositivo de perfuração de rochas por jato supersônico quente (thermal spallation). Os pós precursores foram utilizados de dois modos diferentes: no primeiro com tamanho micrométrico, tal como fornecido pelos fabricantes; no segundo, a alumina foi moída e misturada separadamente com ácido silícico e aerosil nanométricos, ambos usados como fontes de sílica. O processo consistiu basicamente na mistura a úmido dos pós, secagem, prensagem e sinterização. Além do tamanho das partículas dos pós, foi avaliada a influência da pressão de prensagem (40 a 300 MPa), dos aditivos de sinterização (MgO, CaO e Y2O3), do meio de dispersão (água e álcool), da calcinação dos pós, da temperatura (1600 e 1650 ºC) e do tempo (1 e 3h) de sinterização. As cerâmicas obtidas foram caracterizadas de acordo com a contração, perda de massa, porosidade e densidade aparente e resistência à flexão. A microestrutura foi caracterizada por meio da microscopia óptica e microscopia eletrônica de varredura (MEV), e complementada com difração de raios X. Os resultados obtidos mostram que cerâmicas de mulita para aplicações comerciais, que requerem resistência mecânica até aproximadamente 207 MPa, podem ser obtidas utilizando pós de alumina moída e aerosil 380, com 1 % de CaO, homogeneizadas com álcool, calcinadas a 600 ºC, prensadas com 160 MPa (ou mais), pré-sinterizadas a 1000 ºC por 1h e sinterizadas a 1650 ºC por 1h. Estas cerâmicas demonstram também, grande potencial para uso em queimadores para fornos e tubeiras para thermal spallation.
The present study was made in order to obtain an alternative process to produce mullite ceramic (3Al2O3.2SiO2), from powder mixture of alumina (Al2O3) and silica (SiO2), for a possible use in a device for rock drilling hot supersonic jet (thermal spallation). The precursors powders were employed in two different ways: the first powder, in micrometric size, was used as supplied by the manufacturer; the second, milled alumina was alternated with silicic acid and nanometric aerosil®, both used as silica sources. The ceramic processing consisted basically of four steps: mixture of humid powders, drying, pressing and sintering. Besides the powder particle size, it was also evaluated the influence of the pressing (40 to 300 MPa), the sintering additives (MgO, CaO and Y2O3), the middle of dispersion (water and alcohol), the powder calcination and the time (1 and 3h) and sintering temperature (1600 and 1650 ºC). The obtained ceramics were characterized according to the contraction, mass loss, porosity, densification and resistance to flexing. The microstructure was analyzed by light microscopy, scanning electron microscopy (SEM) besides X-ray diffraction. The obtained results show that mullite ceramic for commercial applications requiring mechanical resistance up to about approximately 207 MPa, it can be obtained using milled alumina powder and aerosil 380® with 1 % CaO, homogenized with alcohol, calcined in 600 ºC, pressed with 160 MPa (or more), pre-sintered to 1000 ºC for 1h and sintered to 1650 ºC for 1h. These ceramic also show, great potential to be used in burners for ovens and nozzles for thermal spallation.

A method has been developed to produce bulk ceramic components from a class of ceramics known as polymer derived ceramics. In the past polymer derived ceramics have been limited to thin film applications or in the fabrication of MEMS devices. The reason being that when the polymer is into a ceramic, large quantities of gas are generated which produce internal pressure that fractures the ceramic components. The method developed here solves that issue by casting into the polymer a 3 dimensional network of polymer fibers in the form of a foam which, during pyrolysis, burns out and leaves a network of open channels that allows decomposition gases to escape thus preventing pressure from building up. The inclusion of the polymer foam allows for the formation of strong plastic like green bodies which can be machined into any shape. The green bodies are then pyrolized into ceramic components. This process allows for the simple and inexpensive fabrication of complex ceramic components that have the potential to replace current components that are made with traditional methods.
M.S.M.S.E.
Department of Mechanical, Materials and Aerospace Engineering;
Engineering and Computer Science
Materials Science & Engr MSMSE

In the last two decades additive manufacturing (AM) has emerged as a highly important and influential technology. A large range of approaches to AM have been developed which give rise to hundreds of distinct techniques. Many of these are specific to one material system, and only a handful have been successful at producing ceramic parts. Robocasting is one such technique, having been used to produce complex ceramic parts with reasonable mechanical properties. In this thesis robocasting is investigated further, firstly by characterising the rheology of the robocasting paste, and then by measuring the strength and reliability of ceramic parts produced by robocasting. The critical defects associated with the process are identified, and efforts have been made to eliminate them. Furthermore, it was possible to produce a new class of ceramic composites consisting of alumina platelets aligned by the shear forces that arise during printing. These platelets themselves and the composites were extensively characterised. A new in-situ double cantilever test was developed in order to study the fracture behaviour of the composites. Lastly, the principle of using the printing process to align platelets was applied to fibres in order to create printed fibre reinforced ceramic matrix composites, and printed carbon fibre reinforced epoxy.

This paper is in support of my thesis exhibition “Indirect Measure” May 5th – June 3rd 2017, at the Reece Museum, located on the campus of East Tennessee State University. This document is an account of my examination into what constitutes art and the change in my perception of the utilitarian ceramic vessel during my research into the perceived difference between craft and art. Using broad definitions, I define what I believe art to be and how it is different from, and the same as craft.

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A cerâmica Titanato Zirconato de Chumbo, PZT, se destaca por suas notáveis características piezoelétricas, sendo candidata a várias aplicações tecnológicas como transdutores, dispositivos ressonantes, entre outras, devido também a seu baixo custo e facilidade na fabricação (DESHPANDE, 1994). A adição de íons modificadores pode melhorar as propriedades da cerâmica, tornando-a mais eficiente. Inúmeros íons como nióbio (Nb+5) e estrôncio (Sr+2) podem ser usados para alterar parâmetros estruturais ad estrutura perovskita A(B’B”)O3 . O método Pechini é um dos mais empregados para síntese de pós cerâmicos com homogeneidade adequada ao estudo da dopagem. Neste trabalho, este método foi utilizado para síntese de pós de PZTS e PZTSN seguindo a composição Pb(1-x)Srx(Zr0,5Ti0,5)O3 e Pb(1-x)Srx[(Zr0,5Ti0,5)Nb0,04]O3, respectivamente, onde 0,01Lead zirconate titanate, PZT, is well known because of its excellent piezoelectric properties and has been rised for several technologic applications including: transducers, ressonant devices, and advantages such as low cost and ease to fabricate. The addition of modifiers ions could improve the ceramic properties favoring their applications. Several ions as niobium (Nb+5) and strontium (Sr+2) could be used to change structural parameter of perovskite cell A(B’B”)O3. The Pechini’s method is one of the most important method to obtain ceramic powders with adequated homogeneity. Related to this method was used to synthetise PSZT and PSZTN. The dopant powders following the compositions Pb(1-x)Srx(Zr0,5Ti0,5)O3 and Pb(1-x)Srx[(Zr0,5Ti0,5)Nb0,04]O3 , respectively, where 0,01