Abstract: Lanthanum chromite (LaCrO3) has been considered a promissing candidate for use as interconnect materials for solid oxide fuel cells (SOFCs), due to their excellent electrical properties. In this work, LaCr1-xSnxO3 (x = 0 and 0.1) ceramic powders were prepared using the following synthesis routes: Flame spraying (FS) and microwave assisted combustion method (MCM). The powders were characterized by TGA, XRD and TEM. The TG curves showed weigh losses corresponding the dehydration of compounds as well as decomposition of secondary phases and organic matter. The XRD patterns indicated the formation of Sn doped LaCrO3 phase by FS. The crystallite sizes of samples are in the range 20-36 nm. The TEM images revealed the presence of particles with spherical shape and uniform particle size distribution.
Abstract: Lanthanum chromite is a perovskite oxide type, which exhibit high mechanical and chemical stability, high electric conductivity for use in SOFC and catalytic activity for the oxidation reaction. However, their optical properties have not yet been studied. The aim of this study was to synthesize lanthanum chromite by the microwave assisted self-combustion method for use as ceramic pigment. The resulting precursor powders were calcined at 800 to 1000 ° C to obtain the perovskite phase and characterized by X-ray diffraction, followed by Rietveld refinement, UV-Visible and colorimetry. The results revealed porous powders, nanometric, and single-phase with orthorhombic perovskite structure. There was few change in the colorimetric parameters, the chromites presented colours with gray tones which became darker in the calcined powder at higher temperatures.
Abstract: The aim of this work was to synthesize eight samples of pseudoboehmite obtained by the sol-gel process under different conditions in a two level factorial experimental design consisting of three variables. It was observed how the properties of pseudoboehmites (crystallinity and morphology) were affected by the three analyzed variables, namely: aging temperature, aging time and addition of polyvinyl alcohol. The samples were characterized by x-ray diffraction, scanning electron microscopy with secondary electrons detector, differential thermal analysis, and thermogravimetric analysis. After being characterized, the pseudoboehmite powders were calcined in an electric furnace at 500°C. After calcination, the resultant powders were analyzed via x-ray diffraction. One of the most important results achieved in this work was the successful synthesis of gamma-alumina from pseudoboehmite, as well as high crystallinity in the pseudoboehmite samples aged at high temperatures.
Abstract: The titanium dioxide phase formation is dependent on the synthesis method, temperature of calcination and modifiers insertion. By using chemical methods, such as Polymeric Precursor Method, the organic impurities or extrinsic defects caused by doping play an important rule on the formation of precursor structure before the phase crystallization above 500 oC. Some dopants can change the decomposition mechanism of the precursor, which affects the anatase-rutile phase transition. In this work, the Polymeric Precursor Method was used to synthesize titanium dioxide powder samples in order to investigate the effects of iron (III) and vanadium (V) dopants on the phase formation. Through thermal analysis of polymeric precursors and X-ray diffractometry for calcined powder samples it was possible to show the existence of antagonistic effects for both investigated dopants. While the iron doping reduces the anatase phase tetragonality and delays the rutile phase conversion, the vanadium one changes the mechanism of decomposition of polymeric precursor and leads to more amount of rutile phase.
Abstract: Alternative synthesis routes to obtain ceramics with advanced electronics applications have been investigated to improve the properties of these materials. Calcination and sintering by microwave irradiation is a very promising production method, which performs a rapid solid-state reaction and consolidation of particles due to the high heating rates (about 1000°C/min) aided by the microwave irradiation effect. To produce strontium titanate powder (SrTiO3 - STO), TiO2 and SrCO3 raw precursors were mixture in a ball mill for 3.5 h and after, this mixture was dried at 70°C for 24h. The resulting mixture was divided in two portions to produce STO powders following two procedures: (i) in the conventional method (STO-cC), the mixture was calcined at 1000°C with a heating rate of 10oC/min for 2h, and (ii) in the microwave method (uC) other part of mixture was calcined at 1000°C with a heating rate of 1000°C/min for 15 min, 30 min and 60 min. The powders obtained by both methods were characterized by X-ray diffraction (XRD) to evaluate the crystalline phase presents in the powders. The STO crystalline phase was identified in both samples, but it was also noted the presence of SrCO3 spurious phase in the STO-cC and STO-uC (15 and 30min). On the other hand, STO-uC powder calcined for 60 minutes did not showed this spurious phase. Dilatometric analysis was performed on STO-cC and STO-uC (60min) discs shaped powders by uniaxial load pressing (180 MPa) in order to determine the sintering temperature. Green bodies were sintered at 1380oC in conventional oven for 2h (cS) and in microwave oven at three different soaking times: 15, 30 and 60 minutes (uS). The microstructures of the cS and uS ceramics were analyzed by scanning electron microscopy (SEM). Significant difference between densification of the STO-cC-cS (88%) and STO-uC-uS (99%) ceramics were obtained to ceramics sintered by 60 minutes. The values of grain size of ceramics obtained by uC/uS indicates that microwave irradiation is an effective technique to avoid grain growth.
Abstract: Rare earth silicate based MnMgSi2O5+n (M = Ca, Sr or Ba and n=1-2) phosphors, have attracted interest of researchers due to their high efficiency as a host, excellent thermal and chemical stability and high brightness adding to their low cost. These phosphors showed great potential in various applications such as fluorescent lamps, white light emitting diodes, and display components. High temperature solid-state reactions are usually employed to synthesize those compounds. This paper proposes an alternative method of obtaining nanophosphor host based on Eu-doped CaMgSi2O6 (CMS:Eu), persistent luminescence phosphor. Sol gel technique combined to a modified molten salt method was used. The resulted powder was calcined for 3h under an atmosphere of 5% H2 and 95% Ar2. Phase identification by XRD and the measurements of photoluminescence (PL) and photoluminescence excitation (PLE) were performed. Single phased CMS:Eu with persistent luminescence characteristics was prepared.
Abstract: The mesoporous silica SBA-15 molecular sieve has been widely studied due to its unidirectional mesoporous structure, its high average pore diameter, its high thermal and hydrothermal stability and its ability to absorb metal ions, allowing its use as support material for catalysts. This study aimed to synthesize the Co/SBA-15 catalyst, and characterize it through the techniques of X-ray diffraction, temperature programmed reduction (TPR) and scanning electron microscopy (SEM). The SBA-15 support was synthesized from the following molar composition of reaction mixture: 1TEOS: 0.017 P123: 5.7 HCl: 173 H2O: 40 EtOH, and after calcined at 550 °C for 6 hours. The Co/SBA-15 catalyst was prepared by incorporating 10% cobalt by wet impregnation. Through the X-ray diffractograms, it was found that the impregnation has not changed the structure of the material. RTP profiles showed the presence of peaks at different temperatures that may be caused by dispersion of the cobalt.
Abstract: MCM-22 is a microporous molecular sieve with a system of sinusoidal three-dimensional channels that do not interconnect themselves, which presents good adsorption capacity and high acidity. Thus, this study aims to evaluate the chemical and structural properties of the catalyst MgO/MCM-22 using wet impregnation method. The synthesis of the zeolite was performed using the static hydrothermal method. Through the XRD patterns, it was possible to observe that the metal incorporation process did not cause structural changes, as well as the analysis showed small peaks related to the magnesium oxide in the structure. The micrographs presented a spherical morphology with depression in the central region. FTIR spectra showed the typical peaks of metal with little differences in the typical peaks of MCM-22.
Abstract: Milling and hydrothermal treatment of alumina powders in aqueous medium can result in surface transformations generating aluminum hydroxides. The aim of this work was to advance the understanding on these transformations. A α-alumina powder was ball milled in water at different pHs for 10 h, and then autoclaved (150 °C, 3 atm, 3 h). The powders were analyzed by transmission electron microscopy, differential scanning calorimetry simultaneously with thermogravimetry, X-ray diffraction, and infrared spectroscopy. It was observed that milling in basic medium caused the formation of doyleite [Al (OH3)] nanoparticles, which were fully converted to boehmite (AlOOH) by hydrothermal treatment. The boehmite fraction determined by thermal analysis was 1.7 wt%. The powder milled in acid medium had no mechanochemical and hydrothermal transformations.