Artigos
URI permanente para esta coleçãohttps://repositorio.fei.edu.br/handle/FEI/795
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Resultados da Pesquisa
- Direct measurement of interface energies of magnesium aluminate spinel and a brief sintering analysis(2017) Pereira G.J.; Bolis K.; Muche D.N.F.; Gouvea D.; Castro R.H.R.© 2017Surface and grain boundary energies are key parameters for understanding and controlling microstructural evolution. However, reliable thermodynamic data on interfaces of ceramics are relatively scarce, limiting the realization of their relevance in processes such as sintering and grain growth. In this work, the heat of sintering itself was used to quantify both surface and grain boundary energies in MgAl2O4 spinel. Nanoparticles were compacted and heated inside a Differential Scanning Calorimeter (DSC) when densification and grain growth were observed. The evolved heat signal was quantitatively attributed to the respective microstructural evolution in terms of interfacial area change, allowing determination of average surface and grain boundary energies for MgAl2O4 as 1.49 J m−2 and 0.57 J m−2, respectively. The data was then used to interpret the thermodynamics involved in density and grain growth during isothermal sintering of MgAl2O4.
- Interface energy measurement of MgO and ZnO: Understanding the thermodynamic stability of nanoparticles(2010) Castro R.H.R.; Torres R.B.; Pereira G.J.; Gouvea D.Nanomaterials have triggered excitement in both fundamental science and technological applications in several fields. However, the same characteristic high interface area that is responsible for their unique properties causes unconventional instability, often leading to local collapsing during application. Thermodynamically, this can be attributed to an increased contribution of the interface to the free energy, activating phenomena such as sintering and grain growth. The lack of reliable interface energy data has restricted the development of conceptual models to allow the control of nanoparticle stability on a thermodynamic basis. Here we introduce a novel and accessible methodology to measure interface energy of nanoparticles exploiting the heat released during sintering to establish a quantitative relation between the solid-solid and solid-vapor interface energies. We exploited this method in MgO and ZnO nanoparticles and determined that the ratio between the solid-solid and solid-vapor interface energy is 1.1 for MgO and 0.7 for ZnO. We then discuss that this ratio is responsible for a thermodynamic metastable state that may prevent collapsing of nanoparticles and, therefore, may be used as a tool to design long-term stable nanoparticles. © 2010 American Chemical Society.
- Effects of dependence between solid solution and surface excess in nanoparticles(2010) Pereira G.J.; Gouvea D.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. © (2010) Trans Tech Publications.
- Surface modification of SnO 2 nanoparticles containing Mg or Fe: Effects on sintering(2007) Castro R.H.R.; Pereira G.J.; Gouvea D.Controlling the surface chemistry of oxide systems has emerged an effective tool to obtain desirable nanostructures and macro properties. A relatively simple way to achieve this is by using dopants that are prone to segregate to the surfaces of the powders. In this work, we delineate the effect of Mg and Fe on SnO 2 nanopowders focusing on the surface modifications caused by surface segregation. The effects of increasing the temperature of calcinations are particularly addressed to evaluate the surface modifications at high temperatures. The powders were studied by infrared spectroscopy, zeta potential measurements, X-ray diffraction, and specific surface area measurements. Since sintering is a high-temperature process strongly dependent on surface characteristics, we drawn a relationship between the final densities after sintering and the surface chemistry of the doped powders. Doped SnO 2 pellets were sintered to over 95% of the theoretical density within a few seconds (fast firing) when significant surface modifications were observed. © 2006 Elsevier B.V. All rights reserved.
- Alumina foam coated with strontium oxide as a heterogeneous catalyst(2012) Bassetti F.B.; Innocentini M.D.M.; Pereira G.J.; Ortega F.S.The most common method to produce biodiesel is the transesterification of oils in the presence of methanol and a catalyst. Catalysts may be either homogeneous or heterogeneous, whereas the heterogeneous consume fewer resources, energy and are reusable, being considered an environmentally attractive approach. This research presents the production of a heterogeneous catalysis system for biodiesel synthesis, consisting of alumina foam with a thin coating of SrO. The support, obtained by direct foaming of suspensions, presented approximately 90% vol. of highly connected pores. The SrO film was obtained by soaking the ceramic foam into either a Pechini resin with SrO precursors or an aqueous solution of strontium nitrate, followed by heating to 1400°C for 2 hours. The density and permeability of samples were evaluated and microstructure was characterized by EDS and SEM. The results show that the use of ceramic foams as catalyst support is feasible using the proposed route. © (2012) Trans Tech Publications, Switzerland.