Materials Science Forum Vols. 530-531

Abstract: Hydrocyclones are centrifugal devices employed on the solid-liquid and liquid-liquid separation. The operation and building of these devices are relatively simple, however the flow inside them is totally complex and its prediction is very difficult. The fluid moves on all possible directions (axial, radial and swirl), the effects of turbulence can not negligible and an air core along the center line of the hydrocyclone can appear when the operational conditions are favorable. For that reason, the most models that are used to predict the hydrocyclone performance are empirical and require the collection of the main operational and geometric variables in order to validate them. This work objectified to apply Computational Fluid Dynamics (CFD) on Bradley Hydrocyclone and compare the results from this technique to empirical models. The numerical simulation was made in a computational code called Fluent® that solves the transport equation by finite volume technique. The turbulence was described by Reynolds Stress Model (RSM) and the liquid-gas interface was treated by Volume of Fluid Model (VOF). In agreement with the results from the simulation, it was possible to predict the internal profiles of velocity, pressure, air core, particle trajectories, efficiencies, pressure drop and underflow-to-throughput ratio.

Abstract: The spouted bed has been used in drying, granulation, catalytic polymerization, residues treatment and coating of several materials. The success of its applications is attributable to the solids circulation characteristics and excellent gas-particle contact. The pattern of solid and gas flows in a spouted bed was numerically simulated using a Eulerian multiphase model. The typical flow pattern of the spouted bed was obtained in the present calculation for axisymetric gas-solid flows. The simulated velocity profiles and the voidage profiles presented a good agreement qualitative and quantitative with the experimental results obtained by He et al. [4].

Abstract: ZrO2: 3mol% Y2O3 powders were mixed in different proportions to ZrO2: 8 mol% MgO, pressed and sintered at 1500°C for producing composites with oxygen ion conductivity better than that of ZrO2: 8 mol% MgO and thermal shock resistance better than that of ZrO2: 3 mol% Y2O3. The electrical conductivity was evaluated by the impedance spectroscopy technique at 600°C as a function of the partial pressure of oxygen using zirconia-based oxygen pump and sensor. The thermal shock resistance was studied by room temperature- 1550°C dilatometry on sintered pellets. Moreover the composites were studied by X-ray diffraction and scanning electron microscopy. All composites are partially stabilized in the monoclinic-tetragonal phase and the apparent density is higher than 90% of the theoretical density. The thermal shock behavior of the composites is similar to that of ZrO2:8 mol% MgO materials used in disposable high temperature oxygen sensors. The electrical response of the composites at high temperatures is better than the electrical response of ZrO2:8 mol% MgO.

Abstract: Ceramic ZrTiO4 powders were prepared by a modified sol-gel method using zirconium oxychloride and titanium tetraisopropoxide. In situ high temperature X-ray diffraction results show that crystallization of the amorphous gel starts at 400 °C. Singlephase ZrTiO4 nanoparticles were obtained after heat treatment at 450 oC for 1 h. An average particle size of 46 nm has been determined by nitrogen adsorption analysis. After pressing these sinteractive powders, pellets with controlled pore size distribution were obtained by sintering at temperatures as low as 400 oC. The analysis of pores by mercury porosimetry shows an average porosity of 45 %. Pressing and sintering the nanosized powders prepared by that modified sol-gel technique produced pellets that are good candidates to be used in humidity sensing devices.

Abstract: ZnO varistors are nonlinear resistors used as surge arresters in power transmission and distribution for the protection of electronic devices. Electrical characteristics of these materials have been extensively studied, but their mechanical behavior is not completely understood. It has been suggested that the breakdown of ZnO varistors is related to microstructure heterogeneities and processing defects. These defects are the same that usually control the mechanical strength of ceramic materials. In this work, mechanical properties (flexural strength, fracture toughness, elastic constants, and hardness) of five commercial blocks of ZnO varistors (class I) from different producers were measured and correlated to their microstructure. Pore fraction and size significantly affected the flexural strength.

Abstract: Due to the necessity of the automation and control of processes in agriculture, as well as to the crescent interest for the environmental monitoring, efforts have been demanded in the development of more versatile, reliable sensors and sensor systems with smaller cost [1-2]. In this sense, the search of new materials, the modeling study of sensor and the development of new measurement techniques and processing of signs have been orientating the progress in this area [3-4]. In this work, the results of the characterization analyses of sensor elements of ZrO2-TiO2 porous ceramic for application as soil humidity sensor, are shown and discussed. These ceramics were obtained from the mechanical mixture of ZrO2- TiO2 powders and sintered at 1000, 1100 and 1200 oC, for obtaining different porosities. The characterization of the ceramic was carried out using measurements of B.E.T.; nitrogen and mercury porosimetry; scanning electronic microscopy and X-ray diffraction. The porous ceramic characterization as soil humidity sensor element was accomplished through capacitance and impedance measurements using a RLC bridge. The ceramic sensor elements were immersed in the selected and previously characterized soils, the humidities of which were defined in accordance with Atterberg limits, more exactly liquid limit. The results obtained for specific surface area, distribution curves of pore size, microstructure, crystalline phases and sensibility to the soil humidity showed that the ZrO2-TiO2 porous ceramic sintered at 1100 oC presents a great potential to be applied as sensor element for soil humidity monitoring.

Abstract: Zirconia-toughened alumina (ZTA) ceramics with Ce-TZP (tetragonal zirconia polycrystalline doped with ceria) volume fraction on 33% were prepared with the addition.. The influence of glass infiltrated method was investigated. Coprecipitated Zr and Ce hydroxide mixture was obtained from ZrOCl2.8H2O and CeCl3. 7H2O aqueous solution. CeO2-ZrO2 calcinated powder was compacted and the compacted samples were sintered at 1180°C. Powder samples were characterized by scanning electronic microscopy (SEM), The volume fraction of each phase, the grains size and shapes, and the porosity were investigated with SEM. The relative density and shrinkage was investigate too. The results showed that the crystalline matrix was composed by SiO2 -B2O3-La2O3-Al2O3-Ce-TZP and revealed the important role played the glassy phase in the densification of this ceramic composite.

Abstract: In this work, commercial alumina fine powders were used as raw materials, namely two tabular alumina fractions (–500 mesh and –230 mesh) and a reactive alumina. Statistical modelling and the Response Surface Methodology (Statistica, Mixtures Designs and Triangular Surfaces module) were applied to three-component mixtures and used to calculate the various property-composition surfaces. To that aim, the various mixtures were prepared, cast, dried, fired and characterised. The particle size distribution modulus, q, was determined for all mixtures using the software LISA. The various response surfaces were then combined, so that the water content in the mixture could be minimised and the matrix flowability maximised. The properties of the resulting test-bricks (linear shrinkage, mechanical strength, apparent density and porosity) were also modelled and response surfaces were obtained. Combined results enabled the definition of an optimised particle size composition range, which guarantees the presence of a low water flow-bed that enables the aggregate self-flow.

Abstract: Recycling of residues from different industrial process as new raw materials has been studied by several institutions to be applied in the production of ceramic bricks, roof tile. The aim of this work is to study the potentiality of the residues from granite and kaolin industries as raw materials to obtain ceramic tile. The raw materials were fired at temperatures of 1100oC, 1120oC and 1150oC. After fired the specimens were submitted to a physical and mechanical tests, such as, linear shrinkage, mechanical resistance, water absorption and apparent density. The results show that the residues can be used in the composition of the paste to produce ceramic tiles.