Materials Science Forum Vols. 591-593

Abstract: The addition of zirconia to alumina can improve the chemical properties, mechanical stability, acidity and surface area of alumina. The aim of this work is to evaluate the influence of 0.005 moles of zirconia addition to the structure and morphology of alumina powders by combustion reaction. The compositions were called AP (alpha-Al2O3) and AZ (Zr0.005Al1.995O3) and were prepared by using a silica container with direct heating on a hot plate at 480oC until selfignition. The powders were characterized by X-ray diffraction and nitrogen adsorption by BET analysis. The composition AZ showed high surface area (47 m2/g) and small crystallite size (60 nm). The alpha-alumina AP showed a wide agglomerates distribution with small agglomerates size when compared with AZ composition.

Abstract: Potassium hexa and octatitanate fibers have been proposed as reinforcement for friction materials. The aim of this work was to establish a calcination route to produce these fibers, using commercial anatase and potassium carbonate powders. These powders were dry mixed with TiO2/K2O molar ratio, n, of 3.0, 3.5, and 4.0, and then calcined at 950, 1050, and 1150°C for 3 h. Calcined powders were milled, washed in warm water with different pHs, and heat treated to crystallize the fibers. The best conditions to growth long fibers were n=3.0 and 1050°C, in the twofase field (liquid + K2Ti4O9). Controlled ion-exchange with water removed K+ ions from K2Ti4O9 fibers resulting in potassium hexa or octatitanate fibers after the second heat-treatment. Fibers with sub-micrometer thickness (~0.6 μm) and average length of ~20 μm could be prepared.

Abstract: Silicon Nitride is considered as an important material for use in structural applications. Its performance is severely influenced by modern synthesis processes. In the present work, silicon nitride powder synthesis was performed using liquid silicon tetrachloride and gaseous ammonia, at low temperature and inert atmosphere. Diimide pyrolisis was made on temperature between 1300 and 1500 0C. A Taguchi design of experiments methodology was applied, aiming to obtain powders with appropriated characteristics for structural applications. On pyrolisis, the use of alumina based substrates resulted on SIALON phase formation, probably originated from oxygen reaction, provided from alumina. Silicon carbide substrates and alumina recovered with silicon nitride enhance synthesis of pure silicon nitride powder.

Abstract: The addition of nanometric particles of a second phase into ceramics matrix is one of the most recent alternatives in the development of materials with high mechanical properties and wear resistance. These nanostructured materials can be defined as systems that have at least one microstructural characteristic of nanometric dimensions (less 100nm). In this work aluminadiamond nanocomposites were produced using diamond nanometric powders obtained by high energy milling. Diamond powder was produced in the SPEX shaker/mill during 6h, with a ball-tomass ratio of 4:1. The crystallite size was 30nm. After the elimination of the Fe deriving of the contamination during the milling, and desaglomeration, this nanometric powder was added in the alumina matrix in the ratio of 5wt%. The powder densification was performed by hot pressing sintering. The obtained nanocomposites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and microhardness, and they have promising characteristics regarding abrasion and wear resistance.

Abstract: Zircon sand was reacted with liquid caustic soda (50% NaOH) in open vessel at 600 oC for 2h. The effect of NaOH/ZrSiO4 reactant ratio on the yield of zirconia recovery was verified. Samples of fusion products water washed were characterized by X-ray diffraction (XRD) to identify the main compounds formed. Silica powders were obtained via acid catalyzed reaction and zirconia powders were resulted from crystallization of zirconium oxychoride. Both zirconia and silica powders were analyzed by XRF (X-ray fluorescence) and BET method. Laser Quasi Elastic Light Scattering (QLS) technique was used for agglomerate size distribution determination. High purity and fine zirconia and silica powders were obtained. The specific surface area of zirconia calcined at 550 oC reached ~ 70m2g-1. Silica powder calcined at 800 oC presented a high specific surface area ~ 500 m2g-1.

Abstract: Nickel oxide-yttria stabilized zirconia (NiO-YSZ) anode materials were synthesized by combustion process involving metal nitrate-urea decomposition. The precursor was heated to evaporate water undergoing dehydration, and then the mixtures froth and swell. Further heating produces large amounts of gases that ignite at ambient atmosphere. Different nitrate/urea initial molar ratios were employed in order to investigate the influence of the reaction temperature in the physical characteristics of the powder composite. The flame temperature was measured by infrared optical pyrometer. The combustion was rapid and self-sustaining, with flame temperatures ranging from 870 to 1330°C. The as-synthesized powders were uniaxially pressed and sintered in air. Powders were characterized by X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), helium picnometry for density measurements and gas adsorption technique (BET). The ceramic samples were evaluated by SEM and Archimedes density measurements.

Abstract: Some authors showed that the results obtained for the movement of particle fall in liquids could not be extended to describe the rising of light particles with the same diameter in the same difference of density conditions, especially in the region of higher Reynolds number. The main objective of this study was the investigation of the dynamic behavior of light spheres and of gas bubbles isolated in stagnant liquids. The experimental data of the rising of light spheres and bubbles were obtained with photographic techniques and a stroboscope. The obtained results showed that there were significant differences between the behavior of the ascending gas bubbles and the solid particles in free fall.

Abstract: Bentonites have been used for several purposes, due to their excellent physical, chemical, and structural properties. The main applications of bentonites in Brazil are, among others, as agglomerants for foundry molds, in iron ore pelletizing, as drilling mud in oil and water wells, as wine clarifiers, in catalysis, at the chemical and pharmaceutical industries, for pollutants removal in water treatment plants, and in cosmetics. Since the 1980’s, bentonites have been used as barriers in landfills, thanks to the attractive characteristics of the montmorillonite, the principal clay mineral in the smectitic clays (popularly called bentonites). The inadequate accumulation of residues in soils and waters is identified as one of the most serious current problems. Nowadays, geosynthetic clay liners, or GCL’s, have been used as a geo-synthetic hydraulic barrier. They are constituted by a sodium or calcium bentonite layer encapsulated in geo-textiles or attached to geo-membranes, which are secured together by physical or chemical methods. This paper discusses GCL’s, their uses, the influences of the type of clay, the working principles, the related aspects to landfills projects, and also the main Brazil’s bentonite exploitation areas.

Abstract: Mullite is a versatile material used in traditional and advanced ceramic, due to low thermal expansion, high thermal shock and creep resistance. The production of waste from ore industry is a problem that is increasing nowadays. The kaolin processing industry produces residue rich in Al2O3 and SiO2. These oxides are good precursors to produce mullite. The aim of this work is to use the residue from kaolin industry to produce mullite ceramic bodies. It was studied alumina, clay and residue from kaolin processing as raw materials. The material was dried and pressing in uniaxial press (30MPa). The samples were sintered at temperatures of 1450oC, 1500oC, 1550oC and 1600oC. The ceramic bodies were characterized by X-ray diffraction. The density was measured by Archimedes method and the flexural strength by the three point bending technique. The results showed that is possible to produce mullite with high amount of waste from kaolin processing.

Abstract: Today, the phosphate rock processed in the world represents around some 20 billion dollars per year. Some exploitable deposits are characterizing by low grade phosphate rocks, composed of the apatite group in association with a wide assortment of accessory minerals. Usually a combination of beneficiation techniques is used to process a phosphate rock since run of mine, and then a flotation is applied to recovery apatite. This work proposes an evaluation of particle size for apatite recovery by column flotation in bench-scale. The variables collector dosage and depressant dosage were investigated experimentally applying a factorial design. According to the experimental results, the particle size range between 37&m and 105&m is an optimum size used to obtain simultaneously a product with the industry demand grade (at least 33%) and phosphorus recovery, upper than 60%.