Engenharia de Materiais
URI permanente desta comunidadehttps://repositorio.fei.edu.br/handle/FEI/17
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8 resultados
Resultados da Pesquisa
- Interface excess and polymorphic stability of nanosized zirconia-magnesia(2008-05-27) Castro R.H.R.; Marcos P.J.B.; Lorriaux A.; Steil M.C.; Gengembre L.; Roussel P.; Gouvea D.Controlling the phase stability of ZrO2 nanoparticles is of major importance in the development of new ZrO2-based nanotechnologies. Because of the fact that in nanoparticles the surface accounts for a larger fraction of the total atoms, the relative phase stability can be controlled throughout the surface composition, which can be tuned by surface excess of one of the components of the system. The objective of this work is to delineate a relationship between surface excess (or solid solution of MgO relative to ZrO2 and the polymorphic stability of (ZrO2) 1-x-(MgO)x nanopowders, where 0.0 ≤ x ≤ 0.6. The nanopowders were prepared by a liquid precursor method at 500 °C and characterized by N2 adsorption (BET), X-ray diffraction (XRD), X-Ray photoelectron spectroscopy (XPS), and Raman spectroscopy. For pure ZrO 2 samples, both tetragonal and monoclinic polymorphs were detected, as expected considering the literature. For MgO molar fractions varying from 0.05 to 0.10, extensive solid solution could not be detected, and a ZrO 2 surface energy reduction, caused by Mg surface excess detected by XPS, promoted tetragonal polymorph thermodynamic stabilization with relation to monoclinic. For MgO molar fractions higher than 0.10 and up to 0.40, Mg solid solution could be detected and induced cubic phase stabilization. MgO periclase was observed only at x = 0.6. A discussion based on the relationship between the surface excess, surface energy, and polymorph stability is presented. © 2008 American Chemical Society.
- Relationship between surface segregation and rapid propane electrical response in Cd-doped SnO2 nanomaterials(2008-07-05) Castro R. H. R.; Hidalgo P.; Perez H.E.M.; Ramirez-Fernandez F.J.; Gouvea D.Controlling the surface properties of nanoparticles using ionic dopants prone to be surface segregated has emerged as an interesting tool for obtaining highly selective and sensitive sensors. In this work, the surface segregation of Cd cations on SnO2 nanopowders prepared by the Pechini's method was studied by infrared spectroscopy, X-ray diffraction, and specific surface area analysis. We observed that the surface chemistry modifications caused by the surface segregation of Cd and the large specific surface area were closely responsible for a rapid and regular electrical response of 5 mol% Cd-doped SnO2 films to 100 ppm propane and NOx diluted in dry air at relatively low temperature (100 °C). © 2008 Elsevier B.V. All rights reserved.
- 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.
- Erratum: Quantification of MgO surface excess on the SnO 2 nanoparticles and relationship with nanostability and growth (Applied Surface Science (2011) 257 (4219-4226))(2014) Gouvea D.; Pereira G.J.; Gengembre L.; Steil M.C.; Roussel P.; Rubbens A.; Hidalgo P.; Castro R.H.R.
- 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.
- The rheological behavior and surface charging of gelcasting alumina suspensions(2008) Ortega F.S.; Castro R.H.R.; Gouvea D.; Pandolfelli V.C.Aim of this work is to investigate the effect of monomers containing either carboxylate (ammonium acrylate) or acrylamide (hydroxymethylacrylamide) functional groups on the surface charging and rheological behavior of alumina suspensions. The rheological behavior was investigated by changing the concentrations of dispersant (ammonium polyacrylate) and monomers in the suspensions. The zeta potential of alumina suspensions containing each of the different monomers was measured as a function of dispersant additions. The suspension rheological behavior varied significantly depending on the monomer type, which could be explained in terms of repulsive forces, pH changes and additive interactions. © 2006 Elsevier Ltd and Techna Group S.r.l.