Abstract: Thermoelectric ceramics are those capable of converting thermal into electricity energy by the Seebeck effect. The non-conventional synthesis of ceramic powders and the doping of these powders with rare earth elements are factors that can improve the efficiency in energy conversion. The objective of this work was to synthesize and analyze pure and doped SrTiO3 powders by solid state reaction (SSR) and ultrasonic synthesis (USS). The precursors of SSR were: strontium carbonate (SrCO3), titanium oxide (TiO2) and lanthanum and neodymium oxide (La2O3 and Nd2O3). They were mixed in a ball mill for 3.5h in stoichiometric ratio to obtain SrTiO3 (STO), Sr0.96La0.04TiO3 (STO-04La), Sr0.96Nd0.04TiO3 (STO-04Nd) and Sr0.96La0.02Nd0.02TiO3 (STO-02La02Nd). The powder mixtures were calcined at 1000°C in a conventional oven for 2h. The USS powders were produced by solubilization of Ti (C4H9O)4 and SrCO3 in an HNO3 aqueous solution. The solution was precipitated with NH4OH and, then, this powder was dried at 70°C for 24h, in air. The dried powder was added to a solution of KOH and subjected to ultrasound irradiation. The same procedure was followed for the synthesis of USS doped powders, STO-04La, STO-04Nd, and STO-02La02Nd powders using as dopant lanthanum and neodymium oxides. The powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and crystallite sizes were calculated by the Scherrer equation to evaluate the differences on structural and microstructural properties.
Abstract: Strontium-modified lead zirconate titanate ceramics was prepared by the polymeric precursor method. The effect of Pb (II) substitution by Sr (II) in A site of the perovskite structure at molar percentages of 0.0, 0.2, 0.4 and 0.6 mol% was investigated through XRD and Rietveld method. The increasing of strontium amount displaces the morphotropic phase boundary toward to tetragonal phase and that results can be associated to the tetragonal lattice expansion besides the rhombohedral opposite phenomenon. The effect of strontium substitution on the dielectric properties for sintered ceramics is also investigated and showed the strontium substitution leads to reduction of weight loss and increasing of density for sintered ceramic. The Kc and Tc oscillates as function of ceramic morphology and is not possible to verify the existence of conductive intergrains defects by loss tangent curves. An intrinsic event can be occurring because considerable lattice distortion was verified in tetragonal and rhombohedral structures for powder samples.
Abstract: Some piezoelectric properties of lead zirconate titanate PbZr0.53Ti0.47O3 ceramic samples sintered at 1100oC for 3 hours has been investigated in this work and compared to the strontium modification at 0.2, 0.4, and 0.6 mol%. Polarization versus Electric Field curve and d33 and Kp piezoelectric parameters was taken at room temperature in order to investigate the correlation with phase amount and lattice parameters of crystalline phases calculated through Rietveld refinement. The results indicated the lead substitution with 0.4 mol% of strontium enhances the d33 and Kp piezoelectric parameters, but there is no systematic variation of the piezoelectric properties with strontium content. By the other hand, tetragonal phase seems plays an important rule on piezoelectric devices operating near the room temperature and some correlations could be found.
Abstract: This work evaluated on the structural modification during high-energy ball milling of the Si-50C, Si-42.9C-19.1B e Si-33.3C-44.4B (at.-%) powder mixtures from elemental powders. Electron images revealed on occurrence of continuous fracture mechanisms in brittle particles during their processing, which presented rounded particles lower than 10 μm. X-ray diffraction results of Si-50C powders indicated that the intensity of Si peaks was slightly reduced after milling for 17 h, which were moved to the direction of larger diffraction angles after 7 h of milling, suggesting that carbon atoms were dissolved into the Si lattice in order to form an extended solid solution. Following, these values were increased due to the discrete exothermic formation of the SiC compound. In Si-C-B powder mixtures, the SiC and B4C compounds were formed after milling for 7 h.
Abstract: The Starch Consolidation method of molding technique is based on the ability of starch swell and gelatinize in water, so the green bodies are formed from suspensions, generating controlled porosity after firing; however, it is strongly influenced by gelatinization temperature, starch used, content and type of dispersant, pH and viscosity. This work involves the statistical analysis of possible relations between viscosity, content of dispersant and pH. Slurries containing alumina, water and starch were prepared, settling the solids in 40% and varying the starch content of 10, 20, 30, 40 and 50%, compared to the solids at the mixture. PH and viscosity measurements were made of the slurries as a function of added dispersant. Were obtained and analyzed measures of each parameter, using the Pearson correlation, verifying the existence of an almost perfect correlation between most of the parameters studied.
Abstract: The use of superconductors of high critical temperatures in applied superconductivity leads to higher operation temperatures and economy of cryogenic fluids. High temperature superconductor materials exhibits limited transport properties due to grain boundary weak-links and anisotropy on the critical currents. The texturing development in these superconductors decreases in an efficient way the number of high-angle grain boundaries, increasing the values of the critical current densities. Several quantitative analysis methods have been developed to study the morphologic and topologic characteristics improving our understanding of structural parameters. In this work the grain size distribution characterization of Bi2Sr2CaCu2O8+δ Bi2212 superconductor bulk samples heat treated under the influence of an external applied magnetic field of 5T was carried out using a processing technique and analysis of images. The analyzed images were obtained by SEM in the c axis direction. All samples showed similar microstructures on both cases, but with the use of an external applied magnetic field during the heat treatments the average grain size decreased.
Abstract: Nanocrystalline zirconium oxide (ZrO2) was synhesized by hydrothermal method in presence of hydrogen peroxide. Surface morphology analysis depicts the formation of the nanorods. The structural analysis confirms that the as-synthesized ZrO2 product is of pure monoclinic phase (m-ZrO2) with crystallite size of about ~8 nm. The product consists of monodispersed nanoparticles of uniform composition, high purity, and crystallinity. The Raman spectra are quantitatively analyzed and the observed peaks are attributed to various vibration modes of the m-ZrO2. Photoluminescence (PL) spectrum of ZrO2 nanostructure showed a strong and broad emission peak at around 534 nm, which can be attributed the participation of several energy levels.
Abstract: Presently, there is a lack of effectiveness in the manufacturing of sensors and sensing systems, with Brazilian technologies and raw-materials, able to measure soil water content, with efficiency and practicality. On account of this, ceramics is selected as sensing elements for this application, based on relevant results obtained by researchers from the TECAMB Group of INPE, along the last 15 years. In this way, the present work tries to bring together the technologies of manufacturing, characterization and development of porous ceramics and the monitoring of soil water content in typical Brazil soils, for several applications, such as irrigation, drainage, building and environmental monitoring as well. And so, in order to improve the sensing capacity of these ceramic devices, electrical measurements were performed in different climatic conditions of controlled relative humidity and room temperature. The scanning electron microscopy technique was applied for the morphological analysis of the sintered ceramic microstructure. Results evidenced that the ceramic devices presented a very promising response to the water molecules contained in soil samples selected for tests, under established climatic conditions.
Abstract: The fracture toughness is one of the requirements for mechanical properties of materials for use in satellites. The ceramic TZP zirconia (tetragonal zirconia polycrystals) have been investigated for applications in ballistic armor. Due to the chemical inertness and fracture toughness, this material has the potential to act as a screen against impacts of micrometeorites and space debris. The ceramic composites of alumina-zirconia 3Y-TZP (tetragonal zirconia polycrystals doped with 3 mol% ytria ) are the materials with the best benefit / cost for this application. This paper presents and discusses the results obtained from the use of two techniques for determining fracture toughness. The composite alumina - 18.5% of 3Y-TZP zirconia nanoparticles obtained from deflocculated powders have been tested for Vickers and the SEVNB penetration method (Single-Edge-Notch Beam V) to obtain the fracture toughness values (KIC). The KIC values obtained were analyzed due to the lower dispersion of experimental values. The SEVNB method showed better reliability in determining the toughness values in the studied ceramics.