Commercial polysilsesquioxanes filled with silicon and aluminum particles were prepared in the form of a thin sheet by tape casting and pyrolyzed in nitrogen atmosphere at various temperatures up to 1500°C. Two silicone resins with different carbon content were used to prepare slurries containing 30vol% of Si and Al as filler with volume ratios of 1:1 and 1:3. The crystalline phases that formed in the polymer derived ceramic (PDC) substrates pyrolyzed at different temperatures were identified using x-ray diffraction patterns. Substrates pyrolyzed at 1500°C were characterized with respect to their density, total open porosity and thermal expansion coefficient. The resulting microstructures were examined using scanning and transmission electron microscopy. The main crystalline phases in substrates pyrolyzed at 1000°C were identified as Si, β-SiC and AlN. These gradually converted to β-SiC/SiAlON composites at 1500°C. The higher carbon content favored formation of the phases β-SiC and AlN. The β-SiAlON phase predominated in substrates with higher Si content while SiALON polytypoids were more significant in substrates with higher Al content. The 21R (SiAl6O2N6), 12H (SiAl5O2N5) and 15R (SiAl4O2N4) SiAlON polytypoids were observed in all the samples and were identified by high resolution electron microscopy (HREM) in samples pyrolyzed at 1500°C.

A new method to prepare silicon oxycarbide (SiOC) foams has been developed and it consists of electron beam irradiation of a methylsilicone preceramic polymer followed by pyrolysis in an inert atmosphere. Methylsilicone resin foams were prepared by simultaneous curing and foaming, without the addition of calalysts or blowing agents. The polymer precursor was irradiated with 1.5 MeV EB up to a dose of 7.0 MGy and at a dose rate of 2.8 kG/s, in air. During irradiation the polymer melted, due to rapid increase in temperature, and simultaneously crosslinked by interaction with the ionizing radiation. Crosslinking occurred mainly by poly-condensation reactions and gaseous condensation products were released. The latter acted as an intrinsic foaming agent in the molten polymer. Foams obtained with radiation doses higher than 3.5 MGy showed a high degree of crosslinking with a ceramic yield of over 89% at 1,000 °C. Pyrolysis at 1,200–1,500 °C resulted in SiOC ceramic foams with dense struts and walls, with bulk density around 0.3 g/cm3 and total porosity of 84%. Foams pyrolyzed at 1,200 °C revealed compression strength of 6.8 MPa.

A ceramic matrix for carbon fiber-reinforced ceramic matrix composites (CMCs) has been developed from poly(methylsilsesquioxane)/silicon mixtures, using a low-cost process. In this process the space in two-dimensional carbon fiber preform was filled with a slurry composed by Si powder dispersed into poly(methylsilsesquioxane)/trietoxysilane solutions. Three different volume ratio of Si:polymer were used to stack eight-harness plain weave of carbon fiber, forming laminates composites, which were pressed and cured up to 200°C. The compact bodies were first pre-pyrolyzed at 1000°C and then pyrolyzed at 1450°C/2h and 1500°C/1h. On pyrolysis, the polymer-filler mixture was converted to a multiphase ceramic matrix through reactions between Si, gaseous and solids products from the polymer degradation and the N2 atmosphere. Pyrolysis led to conversion of the initial matrix into silicon oxide (SiO2), silicon carbide (SiC) and silicon oxinitride (Si2ON2), though after pyrolysis at 1450°C metallic silicon was still detected. With one cycle of infiltration the composite characteristics were followed by bulk density and open porosity measurements, X-ray diffraction, microscopy and mechanical testing.

A técnica de processamento de colagem por fita (tape casting) tem sido amplamente utilizada na obtenção de cerâmicas para diferentes aplicações: substratos cerâmicos e estruturas planares em multicamadas para circuitos integrados e capacitores; eletrólitos sólidos para células a combustível e sensores; cerâmicas piezoelétricas para atuadores e transdutores; membranas de separação para micro-filtragem; compósitos estruturais e trocadores de calor. Neste trabalho, a técnica convencional de colagem por fita foi adaptada com a utilização do processo de pirólise controlada de misturas de polímeros e carga, empregando-se polissiloxanos e cargas de silício e alumina nas suspensões. Foram preparadas suspensões com 60% vol. de fase polimérica (polissiloxanos e metil-trietoxi-silano) e 40% vol. de carga (Si e Al2O3), com diferentes concentrações dos polímeros e das cargas. As amostras na forma de substratos foram pirolisadas em atmosfera de nitrogênio a 1400 °C/2 h e 1500 °C/2 h, sendo convertidas em materiais cerâmicos no sistema Si-Al-O-N-C. O processo de pirólise foi caracterizado até a temperatura de 1000 ºC por análise termogravimétrica. As cerâmicas foram caracterizadas quanto às fases formadas, microestrutura, massa específica aparente e condutividade térmica. Os substratos cerâmicos apresentaram porosidade relativamente alta (entre 12 e 22%) e baixa condutividade térmica (entre 3 e 8 W/m.K), sendo constituídos por fases cristalinas de beta-SiC, Si2ON2, O'-SiAlON, Al2O3, mulita e fase amorfa de SiOC; o Si foi observado nas amostras pirolisadas a 1400 ºC.

A técnica de processamento de colagem por fita (tape casting) tem sido amplamente utilizada na obtenção de cerâmicas para diferentes aplicações: substratos cerâmicos e estruturas planares em multicamadas para circuitos integrados e capacitores; eletrólitos sólidos para células a combustível e sensores; cerâmicas piezoelétricas para atuadores e transdutores; membranas de separação para micro-filtragem; compósitos estruturais e trocadores de calor. Neste trabalho, a técnica convencional de colagem por fita foi adaptada com a utilização do processo de pirólise controlada de misturas de polímeros e carga, empregando-se polissiloxanos e cargas de silício e alumina nas suspensões. Foram preparadas suspensões com 60% vol. de fase polimérica (polissiloxanos e metil-trietoxi-silano) e 40% vol. de carga (Si e Al2O3), com diferentes concentrações dos polímeros e das cargas. As amostras na forma de substratos foram pirolisadas em atmosfera de nitrogênio a 1400 °C/2 h e 1500 °C/2 h, sendo convertidas em materiais cerâmicos no sistema Si-Al-O-N-C. O processo de pirólise foi caracterizado até a temperatura de 1000 ºC por análise termogravimétrica. As cerâmicas foram caracterizadas quanto às fases formadas, microestrutura, massa específica aparente e condutividade térmica. Os substratos cerâmicos apresentaram porosidade relativamente alta (entre 12 e 22%) e baixa condutividade térmica (entre 3 e 8 W/m.K), sendo constituídos por fases cristalinas de beta-SiC, Si2ON2, O'-SiAlON, Al2O3, mulita e fase amorfa de SiOC; o Si foi observado nas amostras pirolisadas a 1400 ºC.The tape casting technique has been widely used to prepare ceramic tapes for different applications: ceramic substrates and multilayer planar structures for integrated circuits and capacitors, solid electrolytes for fuel cells and sensors, piezoelectric ceramics for actuators and transducers, membrane systems for micro-filtration, structural composites and heat exchanger. In this work the conventional tape casting technique was adapted to utilize the active filler controlled polymer pyrolysis process making use of polyssiloxanes and fillers in the initial suspension. Suspensions were prepared with 60 vol. % of polymeric phase (polyssiloxanes and methyl-trietoxy-silane) and 40 vol. % of fillers (Si and Al2O3) with different contents of polymer and fillers. The cured tapes were pyrolysed In nitrogen atmosphere at 1400 ºC/2 h and 1500 ºC/2 h, converting the material to ceramic tapes in the Si-Al-O-N-C system. The pyrolysis process was characterized by thermogravimetry up to 1000 ºC and pyrolyzed tapes were characterized according to phase formation, microstructure, density and thermal conductivity. The converted tapes showed relatively high porosity (between 12 and 22%) and low thermal conductivity (between 3 and 8 W/m.K.). The tapes are mainly composed by crystalline phases of beta-SiC, Si2ON2, O'SiAlON, Al2O3 and mullite and a SiOC amorphous phase; Si was observed in tapes pyrolysed at 1400 ºC.

Manufacturing and microstructure of novel materials in the Si-Al-O-N-C system derived from polymer-filler mixtures were investigated. Silicon (Si) and aluminium (AI) were added to a poly(methyl and phenyl)siloxane matrix to react with the carbon-bearing products that result from the decomposition of the latter during pyrolysis. This was carried out in nitrogen atmosphere up to 1500 degrees for 2h. Silicon Carbide (SiC) particles were used as inert filler embedded in the polymer/active filler mixture. Green bodies of the metal/polysiloxane mixtures were pressed and cured at 170 degreesC for 30min. Microcrystalline composites, such as SiC, AlN and SiAlON, from the filler reaction products embedded in a silicon oxycarbide glass matrix could be formed with complex geometry through near-net-shape process of polymer/ceramic conversion.

Sintering behavior of Al(2)O(3)-NbC powders, mixtures with Y(2)O(3) addition was investigated under condition of constant heating rate in dilatometric experiments. Three different compositions containing 10, 20 and 40 wt% of niobium carbide were prepared. The amount of yttria, which was used as a sintering aid, was kept constant at 3 wt%. Identical sintering schedule was applied to all compositions. The constant heating rate of 20 degreesC/min up to 1750 degreesC was applied followed by an isothermal dwell time of 15 minutes. Experimental results imply that NbC modifies the Al(2)O(3) sintering behavior by shifting the sintering stages to higher temperatures, thus hindering densification of the matrix. Mixtures with yttria additions exhibited systematic delay of the initial shrinkage stage. An anomalous peak on the densification rate curves on a certain densification stage is also observed. These characteristics indicate that yttria modifies the diffusion process of the alumina matrix.