Papers by Keyword: Pechini

Abstract: Given the diversity of areas in which applications of ceramic membranes are included, the literature has shown great interest of researchers to obtain these materials with powders synthesized by different chemical methods. Thus, this work aims at the synthesis of alumina by Pechini method, the 2:1 ratio of citric acid: metal cations, calcined to 1100°C, to obtain membranes. The synthesized powders were characterized by thermal analysis, XRD and particle size distribution, since the membranes were characterized by SEM and mercury porosimetry. The results for the obtained powders showed total mass loss of 61.60%, an alpha-alumina phase with crystallite size of 45.2 nm. As for the membranes, these showed heterogeneously distributed particles with different sizes and irregular geometry, reaching average pore diameter of 0.6 μm me 47% porosity, and thus are characterized for application in microfiltration processes.

Abstract: This paper presents the synthesis, characterization and influence of oxidizing atmosphere in the calcinations and sintering processes on microstructural properties of mixed oxides of Cerium and Zinc, obtained by the polymeric precursor method, aiming at a subsequent catalytic application. The material was submitted to calcinations and sintering at 900 and 1050 °C. The samples were characterized by XRD analysis techniques, surface area by BET method and refined by the Rietveld method. The results of X-ray diffraction showed that interest phase was formed, indicating that the method for obtaining the phase is effective. The results of surface area for the system calcined in air were 85 m²/g and 65 m²/g for the system subjected to oxidizing atmosphere both at 900°C, in the same temperature. It was observed that the temperature increase leads to an increase in crystallite size. This fact is in line with other studies in the literature.

Abstract: In this work, samples of 15 mol% Sr-doped Al₂O₃ were synthesized by Pechini's method and calcined at 900°C. DRX analysis showed that only corundum phase is present, and no shift on the main peaks was observed, suggesting that the additive is preferentially located on the surface of nanoparticles as a surface excess. SrO show high basic character and is applied as heterogeneous catalyst in transesterification of soybean oil to produce biodiesel. Since SrO is difficult to obtain through Pechinis method, due to high stability of SrCO₃, the use of Al₂O₃ as a catalyst support produced via polymeric precursor was studied, and the reaction of transesterification was successfully made, because the crystallization of strontium carbonate on the corundum surface was avoided using the washing procedure. Also, the catalyst was characterized by DSC, isoelectric point and surface area.

Abstract: Alumina ceramics are used in many areas of modern industry because of their excellent mechanical, thermal and optical properties. Sintering of alumina usually requires high temperatures (> 1600°C), due to the large and extensive pore network developed during the reconstructive transformation to its stable alpha phase. Consequently, it is very difficult to suppress grain growth during sintering. In this work it was studied the sintering behavior of alumina powders obtained through a modified Pechini-type method, with addition of seeds of calcined alpha-alumina. Powders without addition of seeds were synthesized for comparison. After calcinations at the temperature of 1000°C, the powders were uniaxially pressed. The compacts were sintered in air in the range of 1400-1600°C and characterized by density and porosity measurements, scanning electron microscopy, X-ray diffraction and infrared spectroscopy. The behavior in the sintering of the compacts with addition of seeds was clearly different compared with the compacts with no seeds.

Abstract: In this work, samples of 10 mol% Mg-doped SnO2 were synthesized by Pechini's method and calcined at 500°C. Previous analysis suggests that the additive is preferentially located on the surface of nanoparticles as a surface excess. Since MgO is highly soluble even in weak acid medium, the samples were "washed" with concentrated nitric acid for a few hours in order to remove Mg from the surface. After the lixiviation, the sample was thermally treated again. This procedure was carried out five times using the same sample, and the dependence between macroscopic properties and surface excess was demonstrated, since it was detected a direct relationship on particle size and isoelectric point on each new washing. Also, a new method to measure surface excess in solids was applied.

Abstract: Zinc oxide is widely used in semiconductors fabrication, pigments and mainly in varistors. The electrical properties of ZnO based varistors are related to the grain boundary characteristics, and it can be modified using additives. In this work, zinc oxide based powders were prepared by chemical route derived from the Pechini’s method, and small amounts of tin ions were added to the system. The XRD measurements showed that the tin oxide not forming solid solution in the bulk, and it was observed that the increasing of tin concentration causes an increasing of the surface area at isothermal conditions of calcination. Once the isoelectric point was dislocated to more acid values, the hypothesis of segregation onto surface was considered.

Abstract: La0.8Sr0.2MnO3±d (LSM) powders were prepared by a Pechini-type polymerizable complex process and by amorphous citrate technique. Both processes involve the complexation of the LSM cations from their nitrates or carbonates. The aim of this work is to determinate the differences between the powers obtained by these techniques. The powders synthesized by the Pechini-type process were calcined between 500 °C and 1100 °C. The powders prepared by amorphous citrate technique were calcined at 900 °C. No contamination of either of the powders was observed by X ray fluorescence analysis. X ray diffraction results showed that a perovskitetype structure phase was obtained. BET results showed that the specific surface areas of the powders prepared by the polymerizable complex process and the amorphous citrate route were 6.6 m2g-1 and 5.7 m2g-1, respectively.