Materials Science Forum Vols. 591-593

Abstract: The influence of Mn addition (from 1 up to 10 mol%) on densification and electrical conductivity of CeO2 was studied in detail. Bulk specimens were prepared by the conventional technique of mixing suitable amounts of cerium dioxide and manganese carbonate, followed by pressing and sintering. The apparent density of sintered specimens was determined by the immersion method and electrical characterization was carried out by impedance spectroscopy measurements in the 300 to 550 °C range. High densification (> 95% of the theoretical density) was obtained for small addition of Mn (1 mol%) to CeO2 sintered at 1300 °C for only 0.1 h. Electrical conductivity changes in sintered pellets depend on the sintering procedure, but not on the manganese content.

Abstract: A new class of hybrid ruteno-cuprates – such as Ru-1212 and Ru-1222 – was discovered in 1995 by Bauerfeind and collaborators. These materials present superconducting and magnetic states at low temperatures, an atypical duality in other superconductors. The superconductivity is more easily observed in Ru-1222, while Ru-1212 is a more problematic case, due to the strong effects of the preparation details in its superconducting properties, becoming the material superconductor or not. Ru-1212 presents a critical temperature that can vary between 0 and 46 K, depending on the preparation conditions, and a temperature of magnetic transition of around 132 K. The samples were prepared through solid state reactions, by using a mixture of high purity powders, followed by calcination and sinterization in the nitrogen and oxygen atmospheres. This paper shows the preparation process of Ru-1212 samples, followed by their structural and magnetic characterization.

Abstract: The chemical methods allow obtaining powders with high reactivity and chemical homogeneity. This work studied the sintering of Y2O3-stabilized ZrO2 powders produced by polymerization routes. In the three methods used were obtained powders via formation of gels, which were characterized by spectroscopy in the infrared. After the gel calcinations, the powders obtained were characterized by X-ray diffraction and scanning electronic microscopy (SEM). Among the different chemical methods, there were differences in the coordination of the metallic ions, which caused differences in the thermal behavior, reactivity, size and form of the particles of the powders. The powders were pressed and sintered at 1400 and 1600°C for 2 hours. The bodies sintered were characterized by SEM, apparent density and dilatometry. The average size of the particles varied with the method, and the smallest particles were obtained by Pechini method. PCS method showed a strong reactivity of the powder and it was already partially sintered even in the step of elimination of organic substances. Similarly, the sintered compacts presented different properties with each other, and the compacts obtained by PEG/FA method had the best microstructure.

Abstract: Polycrystalline SiC-diamond composites have been fabricated by high pressure and high temperature, HPHT, sintering using a Si infiltration method. However, infiltration of liquid silicon around the diamond particles results not only in SiC but also in free silicon, which causes deterioration of the composite properties. In this work, a novel sintering procedure was developed to avoid the formation of free silicon in the composite structure. A disc composed of a mixture of graphite and silicon was first press-molded at room temperature. The disc was then placed above the diamond powder inside a high pressure chamber used for the HPHT sintering process. This arrangement permitted to preferentially form liquid Si in a C solution, which infiltrates in between the diamond particles. Using this procedure, free silicon formation is inhibited and the SiC-diamond composite forms a rigid structure with improved properties.

Abstract: Ceramic components are frequently used as substrates for the production of temperature sensing devices in petroleum industries, in view of their high thermal and electrical resistance and inertness in hostile ambient conditions. Complex cubic perovskite oxide ceramics have a great advantage in terms of their varied physico-chemical characteristics with the substitution of structural elements in the respective formula units. In this context, we have produced Ba2AlSnO5.5 ceramics by solid state reaction process. Structural characteristics, studied by powder x-diffraction, reveal that Ba2AlSnO5.5 ceramic has an ordered complex cubic perovskite structure. Ba2AlSnO5;5 ceramics were sintered in the form of circular discs of 10 and 15 mm diameters and 2 mm thickness, in the temperature 1200 to 14000C by normal sintering process. The microstrutural characteristics were studied by scanning electron microscopy on both polished and fracture surfaces. The SEM micrographs show homogenous surface morphology and particle size distribution. Mechanical hardness of the sintered Ba2AlSnO5.5 ceramics were studied by Vicker´s hardness tests. The results of these studies have been presented and discussed in this work

Abstract: Commercial α−Si3N4, Al2O3 and a mixed yttrium and rare earth oxides, RE2O3, were used as starting-powders. Powder batches were milled using different Al2O3/RE2O3 contents, as additive. Hot-pressing was done at 1750oC-30 min-20MPa in N2 atmosphere. Specimens neat to 6x3x3mm3 were polished and characterized by XRD and SEM. Specimens were submitted to creep tests, under compressive stresses between 100 and 350 MPa at temperatures ranging from 1250 to 1300oC in air. Higher additive amounts resulted in larger grains of higher aspect ratios and in a decreased anisotropy in the hot-pressed ceramics. The compressive creep behavior depends on the intergranular phase content. While higher amounts of additives resulted in higher creep rates, • ε , and higher stress exponents, n, the activation energy Qss, has been inferior for samples with lower additive contents. Grain sliding has been identified to be the predominant mechanism responsible for creep deformation of these ceramics.

Abstract: The direct incorporation of Gd2O3 powder into UO2 powder by dry mechanical blending is the most attractive process for producing UO2-Gd2O3 nuclear fuel. However, previous experimental results by our group indicated that pore formation due to the Kirkendall effect delays densification and, consequently, diminishes the final density of this type of nuclear fuel. Considering this mechanism as responsible for the poor sintering behavior of UO2-Gd2O3 fuel prepared by the mechanical blending method, it was possible to propose, discuss and, in certain cases, preliminarily test feasible adjustments in fabrication procedures that would minimize, or even totally compensate, the negative effects of pore formation due to the Kirkendall effect. This work presents these considerations.

Abstract: In the present study, the waste scrap agate, obtained as a by-product of rock agate beneficiation as decorating ends and media for wet grinding was used as inert filler in a traditional triaxial red stoneware tiles composition. The effects on the technological properties have been investigated. The study has been carried out using the mixtures experiments method for the experimental design. Characterization results were used to calculate statistically significant and valid regression equations, relating dried and fired body properties with clay, feldspar and scrap agate contents in the unfired mixture. The regression models were then used simultaneously to delimit the combinations of those three raw materials most adequate to produce a ceramic body with specified properties. The use of mathematical optimisation shown that, for the particular raw materials under consideration, there is a rather forgiving composition range of clay (20-68 wt.%), feldspar (17-50 wt.%) and scrap agate (15-45 wt.%) contents within which it is possible to simultaneously specify the technological requirements for triaxial red stoneware tiles bodies, not only of the fired products but also of the intermediate materials at important stages of the processing.

Abstract: The presence of pores in ceramics is directly related to the chosen forming process. So, in the starch consolidation method, the ceramics show, after burning, pores with morphology similar to that presented by this organic material. On the other hand, the increase in solid load leads up to alterations in dispersion viscosity, increasing the thermal stresses during drying and sintering processes. In order to verify the solid percentage influence in ceramic final properties, samples were prepared with silicon carbide in different compositions using or not starch as binder agent and pore forming element. The characterization of the ceramic pieces was performed by superficial roughness measurements, porosity besides by optical and scanning electron microscopy. The results showed ceramics with SiC and starch presented physical and microscopic properties slightly higher in relation to those with only ceramic powder in their composition. The presence of organic material, agglomerated and foam during the forming were essential for the final properties of the studied samples.

Abstract: The aim of this work was to study the effect of temperature and heating rate on the densification of two leucite-based dental porcelains: one low-fusion and one high-fusion commercial leucite porcelains (Dentsply-Ceramco). Porcelain powders were characterized by differential thermal analysis (DTA), X-ray diffraction (XRD), particle size distribution, and helium picnometry. Bar samples were sintered from 650 to 1050oC, using heating rate of 55oC and 10oC/min. Sintered samples were characterized in terms of bulk density, measured by the Archimedes method in water, and fractured surface microstructure by scanning electron microscopy (SEM). The results show that densification increases with increasing temperature and the increase in heating rate has no effect on the densification of the porcelains studied.