Materials Science Forum Vols. 727-728

Abstract: In the present work, by selecting Si₃N₄, TiB₂ and Al₂O₃ as binding agents as well as La₃O₂ as an additive, sintered wBN composites were studied. By modifying the number of sintering cycles, the composites processed at 4.5GPa and 1800^°C showed improved mechanical properties. The degree of transformation of the wBN, as well as the chemical reactions during the sintering process were discussed. This new composite material was found to present polycrystalline structure that provides superior cutting properties. Moreover, owing to superior properties, the wBN composite sharpens itself during cutting.

Abstract: The powder compaction process is the most widely used forming technique for shaping ceramic products, since traditional ceramic as floors until advanced ceramics for technical applications. This process is characterized by high productivity but it has a serious problem, namely the density gradient originated during the die cavity filling and the subsequent compaction operation^[. Therefore, the process requires a high control of all parameters involved in order to guarantee to obtain samples with high homogeneity of the green density, which ensures a sintered body of high quality and low population of microstructural defects. Thus, it is extremely important to understand the variable that controls the compaction mechanisms and interfere in the homogeneity of the green density along the compacted body.

Abstract: Yttria stabilized zirconia (YSZ) is the most common electrolyte in solid oxide fuel cells (SOFC). The planar configuration is widely used for designing single cells, in which a thick cathode layer can be used as the supporting electrolyte film. The manufacture of the semi-cell formed by anode and electrolyte has been widely studied and there are many works dealing with its colloidal processing. However, the semi-cell formed by cathode and electrolyte has received much lower attention. This work deals with the manufacture of a semi-cell consisting of YSZ as electrolyte and strontium-doped lanthanum manganite (LSM) as a cathode through a colloidal processing route. The colloidal behavior of diluted suspensions of YSZ and the rheology of their concentrated suspensions were studied as a function of deflocculant content, mixing time by using ultrasounds probe and ageing time. The colloidal stability of aqueous suspensions of LSM was studied by measuring the zeta potential as a function of pH and deflocculant content. These concentrated suspensions were used to obtain thick self-sustained substrates by casting methods. The YSZ electrolyte and LSM cathode were prepared by tape casting in water medium.

Abstract: In the last years many efforts have been spent to develop colloidal processes that use water instead of organic solvents. The aim of this study was to develop aqueous tape casting systems to produce zirconia tapes. Different types of binder were tested. Rheological properties of the slurries were measured at room temperature. The slurries were cast on a PET carrier with/without a silicone-based coating. Interactions between the slurry and the polymeric substrate were evaluated. It was found that an acrylic binder is more appropriate for colloidal processing. The acrylic-based suspensions showed a pseudoplastic behavior and remained stable after hours. Tapes with thicknesses between 90 and 200 µm were produced. Optimized tapes showed smooth and homogeneous surfaces and adequate plasticity.

Abstract: A filter box was used to obtain the experimental data of filtration operation with different air velocities as 5, 10 and 15 cm/s. The experimental data were compared to the simulated ones using CFD technique. For all range of air velocity and dust cake porosity and permeability employed, data of pressure drop in filter box could be satisfactorily predicted by simulations in CFD. Furthermore, the CFD simulations showed that undesirable patchy cleaning might be overcome using air velocities lower than 15cm/s.

Abstract: The Ba_(x)Sr_(1-x)Co_(y)Fe_(1-y)O_3-d (BSCF) powder, used with cathode in intermediated temperature solid oxide fuel cells (ITSOFCs), has been prepared by Citrate-EDTA method and calcined at 900°C for 5h. Among the parameters to be checked in a liquid phase reaction, the pH of precursor solution is very important to control the disposal of organic, crystal structure and powders morphology. In this work, precursor solution pH has been variegated 2, 4, 6 and 8. The thermal decomposition has been observed in infrared, two absorption peaks are observed at 1436 and 860 cm^-1, after calcination, corresponding to the presence of CO₃^2-, in the samples with pH 2, 6 and 8. The single phase of Perovskite crystalline structure is seen only in the synthesis at pH 4 and 6, verified by X-ray diffraction (XRD). Morphological analysis, by scanning electron microscope (SEM), shows the powder is agglomerated and has similar trend independent of precursor solution pH.

Abstract: Solid oxide fuel cells (SOFCs) have emerged as an efficient way to transform chemical energy into electrical energy. However, a major disadvantage of this technology is related to the high temperatures required for SOFC operation. In this way, new materials are necessary to maintain the electrical properties of the cell at intermediate temperatures. Based on these ideas, it is necessary to study both the structural variation of the cells components at different temperatures and their electrochemical behavior. In this work, a crystallographic characterization is presented, which was performed in a commercial SOFC cell using X-ray diffraction (XRD). An equivalent linear electrical model to predict SOFC losses is developed as well. Keywords: Solid oxide fuel cells (SOFCs); AC impedance; Electrochemical impedance spectroscopy (EIS); Equivalent circuit models.

Abstract: Paraiba State is a major instance of non-metallic mineral mainly of bentonite clay, balls clays, kaolin, feldspar, quartz, limestone, mica, etc. Recently it was discovered new deposits of clay in the region of Cubatí and Pedra Lavrada, PB, thereby providing an expansion of mineral inputs in the region. The aim of thi study is to characterize the clays from the city of Pedra Lavrada - PB, to be used as ceramic raw materials. The mineralogy of the clays was performed using the following techniques: laser diffraction (AG), thermogravimetric and differential thermal analysis (TG and DTA), chemical analysis (EDX), X-ray diffraction (XRD), cations exchange capacity (CEC) and surface area (AE). The specimens were molded by compression of 2 MPa, with dimensions of 60mm x 20 mm x 4 mm. The followed physical and mechanical properties were determined: firing shrinkage, water absorption, bending stress rupture and modulus after calcined at 800, 900, 1000, 1100 and 1200 °C. The results showed that the clays present smectite and kaolinite in their mineralogical composition and through the physical and mechanical properties this clays could probably be used as ceramic raw materials.

Abstract: A wide variety of synthesis methods have been proposed for obtaining mullite, however, some techniques produce powders with heterogeneous morphology and a large amount of glassy phase, forming large clusters making it difficult to obtain nanopowders. The microwave synthesis offers advantages over conventional methods, the heating is rapid and uniform, avoiding an undesirable grain growth. The aim of this work was the synthesis of nanoscale mullites powders from delaminated bentonites. For the process of delamination samples were initially treated, then rehydrated, frozen and deagglomeration in ball mill and subsequently subjected to centrifugation, in order to produce a nanosized precursor for the synthesis of mullite. The powders obtained after peeling were characterized chemical and mineralogical. The synthesis of mullite was performed in a microwave oven and the powders were characterized by XRD and SEM. The results showed that the delamination of the bentonite and microwave synthesis were essential in obtaining nanopowders mullite.

Abstract: The most common method to produce biodiesel is the transesterification of oils in the presence of methanol and a catalyst. Catalysts may be either homogeneous or heterogeneous, whereas the heterogeneous consume fewer resources, energy and are reusable, being considered an environmentally attractive approach. This research presents the production of a heterogeneous catalysis system for biodiesel synthesis, consisting of alumina foam with a thin coating of SrO. The support, obtained by direct foaming of suspensions, presented approximately 90% vol. of highly connected pores. The SrO film was obtained by soaking the ceramic foam into either a Pechini resin with SrO precursors or an aqueous solution of strontium nitrate, followed by heating to 1400°C for 2 hours. The density and permeability of samples were evaluated and microstructure was characterized by EDS and SEM. The results show that the use of ceramic foams as catalyst support is feasible using the proposed route.