Materials Science Forum Vols. 591-593

Abstract: In the municipal area of Campos dos Goytacazes–RJ several ceramic industries are involved with a significant production of bricks and tiles. These ceramic products can serve as matrices for the incorporation of industrial residues such as the ash of sugarcane bagasse used as fuel in the boilers of the sugar and alcohol plants. The incorporation of ashes in ceramics is a solution that eventually can cause an improvement on the properties of the material. The objective of this work is to characterize granulometric fractions of sugarcane bagasse ash with particle sizes less than 149, 75 and 44 μm aiming at the ash incorporation into red ceramics. Chemical, mineralogical and microstructural characterization were carried out in the granulometric fractions. The results showed that the granulometric fractions present as main crystalline phase the cristobalite and that the ashes with smaller particle sizes can improve the densification of the ceramic body by the formation of more liquid phase during firing, due to the presence of less silica and more alkaline- and alkaline-earth oxides than the original ash.

Abstract: The mineral constituents of a typical clay from Campos dos Goytacazes, Brazil, were investigated in terms of their nature and morphology by analytical methods including transmission electron microscopy and energy dispersive spectroscopy. The clay was found to have a large amount of finer, < 2 μm, aluminumsilicate particles of kaolinite and, in smaller proportion, halloysite, which is also an aluminumsilicate. At 1050oC the clay shows a consolidated structure with continuous glassy matrix in which Ti, Fe and Si oxides as well as mullite crystals are thoroughly embedded.

Abstract: Gelcasting is a novel method of forming ceramics and has been increasingly employed in preparing complex-shaped, near-net-shape advanced materials. In this work, alumina, zirconia and alumina-zirconia bioceramics were shaped by gelcasting, using MAM-MBAM monomer system, with subsequent sintering and characterization. High purity tetragonal ZrO2 (3mol%Y2O3) and .Al2O3 powders were used as starting powders. Powder mixture containing 80 wt.% of alumina and 20 wt.% of tetragonal zirconia were obtained by milling/homogenization. In all cases, the suspension obtained had at least 55 vol.% solid loading. Ceramic bodies were demolded, dried and characterized by green relative density. The compacts were sintered in air at 1550 and 15750C, for 120 minutes, with controlled heating-rate to facilitate organic compounds removal. Sintered samples were characterized by relative density, and analyzed by X-Ray diffraction and scanning electronic microscopy. Hardness and fracture toughness were determined using Vicker’s indentation method.

Abstract: This work has for objective to characterize the clay from Vale do Mulembá-ES. The Vale do Mulembá is located in Joana D´Arc, Vitória in Espírito Santo State, Brazil. That clay is used in the production of the traditional clay pans of the State, what contributes to the economy of the area. The clay from Vale do Mulembá presents characteristics and behavior different from the other clays used for the production of clay pans. The characterization studies were carried out through chemical analysis, X-ray diffraction, particle size distribution, plasticity and thermal analyses (DTA, TGA). The results showed that the clay is typical kaolinitic, present relatively to the amount high of Al2O3 and high plasticity.

Abstract: Boron carbide (B4C) and silicon carbide (SiC) are materials with high hardness and low density what make them very useful in various applications, such armor plates. Hot press sintering of B4C-SiC composites was studied. The powder mixture was obtained in situ by carbothermal reduction using B2O3, SiO2 and carbon black as precursors. Seeds of commercial B4C and SiC were used to accelerate the synthesis reaction. Compositions were prepared to obtain powders after carbothermal reduction with 10, 30 and 50 wt% of SiC. Carbothermal reaction was conducted in argon atmosphere at temperatures up to 1700 °C. The synthesized powders were analyzed by DRX and SEM. Hot pressing at 1850 °C/30 min (20 MPa) in argon atmosphere was applied. Densities higher than 93% of theoretical density and microhardness of 34 GPa were achieved for hot pressed samples.

Abstract: Cellular ceramics have been produced by several methods, giving a wide range of macrostructures and properties. This work describes a novel route to produce cellular ceramics based on the gel casting of emulsions consisting in an aqueous ceramic suspension containing water-soluble organic monomers and an emulsified insoluble liquid phase. The effects of solids loading and kerosene fraction on the rheological properties of emulsions were characterized. Samples with different kerosene additions (20, 30, 40 and 50 %vol.) were produced and their green and sintered densities were measured. The results have shown a good correlation between the volumetric fraction of kerosene into the emulsion and the porosity of samples. Diametrical compression was used to evaluate the strength of sintered samples, which varied with the porosity between 18 and 37 MPa. The cellular structure was analyzed by SEM and revealed isolated pores for samples with low porosity, which changed to an interconnected network of pores as the porosity increased.

Abstract: The densification behavior of mixtures of nanocrystalline and conventional microcrystalline zirconia powders undergoing uniaxial pressing was investigated. The mixtures were obtained with ratios between 10 and 80 wt.% of nanoparticulate powder added to the microparticulate powder. Nitrogen and Mercury porosimetry were used to measure the pore size distributions of the compacts. The powders and the fracture surfaces of the compacts were observed via SEM. The results showed that compacts of the powder mixtures attain higher densities during pressing than the unmixed powders. The powder mixture with 80 wt. % of nanoparticles showed the better particle packing efficiency. A comparison between density values of uniaxial and isostatic pressing also was done and indicated that the best results were obtained by the isostatic method.

Abstract: Porous materials are of significant interest due to their wide application in catalysis, separation, lightweight structural materials, biomaterials and other areas. Porous ceramics are produced within a wide range of porosities and pore sizes depending on the application intended. Porosity and pore size distribution can be carefully controlled by the choice of organic composite and the amount added. The material may have two types of pores: open and closed pores. The open pores, also called interconnected pores, are those which are in contact with the external surface of the material, being very useful for the manufacture of ceramic filters. A high number of closed pores are important for the manufacturing of materials used in thermal applications. There are many methods for obtaining porous ceramics, in general consisting in adding to the ceramic matrix organic particles, which volatilize during the first heat-up. The objective of this study was to produce ceramic composite nanostructure of alumina and yttria stabilized zirconia (Y-TZP) with micrometric pore sizes. The effects of ZrO2 additions in the mechanical properties of Al2O3 have been intensively investigated, due to the possible increase of the mechanical strength of this material. The organic particles used to create the pores were CMC and PVC. The microstructure of the porous ceramic samples obtained was evaluated considering the degree of sinterization of the nanoparticles, pores formation, porosity, specific surface of the pores and the distribution of the interconnecting pores.

Abstract: A new method for SOFC fuel cell anode preparation is proposed where the main difference lies over cermet powder processing by high energy milling. Yttria stabilized zirconia powder and metallic nickel undergo co-milling in a vibratory device employing zirconia bead media. Dispersed and homogeneous powders are therefore obtained. The material is pressed uniaxialy and sintered at 1350°C for 0,5 h in air and under argon and hydrogen. In the former case, partial nickel oxidation occurs before sintering leading to small shrinkage down to 2% and porosity about 38%. Linear shrinkages from 5 to 7% after sintering in both inert and reduced atmospheres were observed not demanding pore-former additives. Conventional YSZ, Ni and NiO powder mixtures were prepared for comparison purpose. The high energy milling process is able to reduce the starting sintering temperature by 130° C besides a higher densification compared to the simple mixtures YSZ+Ni. The excessive sintering and particle coalescence is absent in high energy milled material, where the metal is well dispersed and the microstructure is highly homogenous. The high energy milling process is a promising route to prepare with excellent performance anode materials for SOFC cells.

Abstract: In this work, it was investigated the behavior of a ceramic composition in pressing and firing process, related to its pyroplasticity, to permit the production of roof tiles in roller kiln. Experiments were realized varying the parameters of pressing, sintering and composition, adding a refractory material. With this study, it became possible to select the optimal parameters of pressing and firing, and to start the pilot industrial tests. The developed process has advantages, in the process and the final product, related with the standard process (extrusion and firing in tunnel kiln). The process is energetically more efficient, because there are no movable in the kiln. It was measured a flexural strength above 40 MPa with 12mm thickness, saving material. Also, the product has a higher quality in size, shape and dimensional precision (linear thermal shrinkage lower than 1,5%); tonality, porosity (water absorption lower than 10%), and frost resistance.