Materials Science Forum Vols. 798-799

Abstract: This paper presents the results of tests for characterization of soil samples collected in Mossoró-RN, UFERSA-RN Campus (5 ° 12'34 .68 "South latitude, 37 ° 19 '5.74 "west longitude), for the purpose of producing soil-cement for the manufacture of pressed blocks. Objective of improving the quality of soil-cement, and provide conditions for the use of the soil making it ideal for the production of soil-cement block. Tests of compaction, particle size analysis, plastic limit, liquid limit and correct particle size, X-ray fluorescence and morphology by scanning electron microscopy (SEM). It was concluded that the soil needs correction particle size, due to the high clay content. The method combined grading, sieving, sedimentation and blooming X-ray as the fastest and most accurate in correcting soil particle size.

Abstract: Pozzolanic materials react with lime produced during curing of cement forming a hard phase similar to hardened cement. There are multiple methods to evaluate the pozzolanic activity of materials and one of these methods is the evaluation of the conductivity in lime solution. This method does not follow a standard procedure and there are multiple parameters that affect the results. The present paper summarizes some of the variations in this method reported in the literature and shows experimental results from tests in siliceous materials with crystalline (sand) or amorphous (silica fume) structure. The test is also used to evaluate the pozzolanic activity of sugar-cane bagasse ash. The effects of temperature, sample size and solution agitation are reported. The results indicates this method permits a rapid evaluation of pozzolanic activity but the testing parameters must be rigorously controlled.

Abstract: This paper presents the study results with glass-ceramics obtained from base glass (MgO-Al₂O₃-SiO₂-Li₂O system) with addition of ZrO₂ as nucleating agent. The glass was melted at 1650 oC for 3 h and at a heating rate of 10 °C/min. The molten glass was poured into a graphite mold to obtain monolithic samples and also in water in order to obtain particulate material. Such material was grinded and then pressed by both uniaxial and isostatic pressing methods before being sintered. Both the monolithic and pressed samples were performed under two different conditions of heat treatment so that their nucleation and crystallization occurred. In the first one, the samples were heated to 1100 oC with a heating rate of 10 oC/min. In the second one, there was an initial heating rate of 10 oC/min up to 780 oC, which was kept for 5 minutes. After that, the samples were heated to 1100 oC at a heating rate of 1 oC/min. Microhardness analyses showed that base glass presented values around 7.0 GPa. The glass-ceramics obtained from the powder sintering showed microhardness values lower than those obtained from monolithic samples. The highest hardness values were observed in the samples which were treated with two heating rates, whose values were around 9.2 ± 0.5 GPa. Moreover, the glass-ceramics which were produced with an only heating rate, presented values around 7.1 ± 0.2 GPa, very close to those observed in the base glass.

Abstract: Silica glass is a fundamental material for the high technology industry due to its physical properties. This material is widely used in the semiconductor, chemical and optical industry. For applications in the optical industry and special lamps, it is necessary that the silica has a reduced content of bubbles and high optical transmittance in the wavelength regions of visible and medium ultraviolet (200 nm – 300 nm). Silica glass samples were produced by the flame fusion Verneuil Method of Brazilian commercial natural quartz powder and imported high purity commercial powder. An acid leaching procedure of the national quartz powder was introduced before the fusion in order to decrease bubbles concentration. The results were compared with the silica glass produced with imported quartz powder and a commercial silica glass used for the fabrication of special lamps, showing an excellent feasibility of application of Brazilian natural quartz to produce high value added silica glass.

Abstract: YAG (Y₃Al₅O₁₂) and Al₂O₃ ceramics have high resistance to oxidation and corrosion in harsh environments and high temperatures, which turns into a quite attractive material as compared to other ceramics. Thus, lately oxide ceramic YAG has been extensively used as reinforcement phase to Al₂O₃ in order to obtain a composite with improved mechanical properties. This research focused on the development of sintered Al₂O₃-Y₂O₃ powder mixtures for the production of Al₂O₃-YAG composite. Powder mixtures composed of 63.65:36.35wt.% and 80.00:20.00wt.% of Al₂O₃ and Y₂O₃, respectively, were milled by planetary milling for 2h. The compositions were compacted by cold uniaxial pressing, at 70 MPa, for 30s. The two mixtures were sintered at 1500 and 1600°C for 3h. The samples were evaluated for relative density, shrinkage, weight loss, and X-Ray Diffraction (XRD). Scanning Electron Microscopy (SEM) was used for microstructural characterization. The X-Ray Diffraction showed the presence of Al₂O₃ and Y₃Al₅O₁₂ as crystalline phases in both compositions. Samples composed by 80:20wt.% of Al₂O₃/Y₂O₃ powder sintered at 1600⁰C-3h presented the higher relative density ranging around 86% of theoretical density.

Abstract: Zirconia-8 mol% yttria porous solid electrolytes were obtained by mixing with different amounts (0 to 5 wt.%) of KI prior to sintering. Potassium iodide acts as sacrificial pore former, being removed from the ceramic pellet upon sintering at 1400 °C/2 h. The alkali halide content was evaluated by X-ray fluorescence analysis, the density of the pellets by the Archimedes method, and the pore content by observation in scanning probe (SPM) and scanning electron (SEM) microscopes of polished and etched surfaces. The oxide ion resistivity was determined by impedance spectroscopy analysis in the 5 Hz-13 MHz frequency range from 300 to 500 °C. Porous specimens with high skeletal density were obtained. The higher is the alkali halide content, the higher is the pore volume and the total electrical resistivity. A correlation is found between the pore content, evaluated by SEM and SPM, and the electrical behavior analysis from the impedance plots of the porous specimens.

Abstract: This paper proposes to investigate the influence of the type of container on the structure, morphology and measures magnetic nanoparticles of the Zn_0.6Fe_0.4O synthesized by combustion reaction, for use as diluted magnetic semiconductors (DMSs). For synthesis were used as containers stainless crucibles and porcelain. The heating was realized in a muffle furnace at 500 °C. During the reactions, were performed measurements of the maximum temperature and total time of combustion reaction. The synthesized samples were characterized by XRD, SEM and magnetic measurements. The maximum temperatures obtained were 478 and 456 °C and total reaction time of 285 and 195 s for the samples synthesized in the crucible of stainless steel and porcelain, respectively. In both containers were possible to obtain single phase material with average crystallite size of 26 to 37 nm, irregular agglomerates formed by, behavior ferrimagnetic whose, saturation magnetization values of 1.6 e 2.0 emu/g respectively.

Abstract: The present work aims to study the magnetic properties of nanoferrita cobalt obtained by combustion reaction. The structural feature as well as the magnetic behavior when in the presence of a magnet and magnetic measurements was investigated. The resulting samples were characterized by X-ray diffraction (XRD), infrared spectroscopy (FTIR), magnetic behavior when in the presence of a magnet and magnetic measurements. The results indicated the phase single the spinel ferrite CoFe₂O₄, with high intensity of diffraction peaks indicating that the samples are crystalline and nanoparticle formation. The characteristic bands of spinel were observed for nanoferritas CoFe₂O₄. The ferrite nanoparticles were strongly attracted when in presence the magnet presenting a saturation magnetization of 58.0 emu/g, coercivity of 1.14 kOe.

Abstract: Investigation on polycrystalline electroceramics involves the synthesis, the consolidation and the analysis of the electrical behavior, along with careful evaluation of the final microstructure. The synthesis of ceramic powders with controlled characteristics is crucial in the study of materials with optimized properties. Distinct properties may be found in ceramic materials prepared by the several existing methods, due to chemical and phase homogeneities, and to the particle size distribution or medium particle size. In this work, yttrium-doped barium zirconate proton conductor was synthesized by spray pyrolysis, and characterized by several techniques aiming identifying the influence of some parameters of this method of synthesis with particle characteristics. Nanocrystalline powders synthesized at 600-700oC were found to be cubic and single phase. Moreover, depending on the gas flow and furnace temperature, spheroid and porous or cubic and solid particles may be obtained.

Abstract: Yttria-stabilized zirconia is a singular polycrystaline ceramic with a range of technological applications. The combination of its physical properties is responsible for application as solid electrolytes in solid oxide fuel cells. High densification is required for this and other applications. The sintering of this solid electrolyte is still a matter of investigation. To reduce the sintering temperature, the introduction of additives is an effective approach. In this work, the effects of lithium addition on microstructure and electrical conductivity of 8 mol% yttria-stabilized zirconia was studied by scanning electron microscopy and impedance spectroscopy, respectively. Cylindrical pellets were prepared by pressing, followed by sintering at 1200oC without and with 1 and 2 mol% lithium (metal basis). As precursor materials both lithium carbonate and lithium fluoride were used. The main microstructure features were correlated with the results of electrical conductivity.