Engenharia de Materiais
URI permanente desta comunidadehttps://repositorio.fei.edu.br/handle/FEI/17
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7 resultados
Resultados da Pesquisa
- Surface segregation in SnO2-Fe2O3 nanopowders and effects in mössbauer spectroscopy(2005-06-01) CASTRO, R. H. R.; HIDALGO, P.; COAQUIRA, J. A. H.; BETTINI, J.; ZANCHET, D.; GOUVEA, D.SnO2-Fe2O3 nanopowders prepared by the polymeric precursor method were studied by combined conventional and high-resolution techniques. The powders treated at 500°C were analyzed by EDS local probe associated with HRTEM to directly detect surface segregation of Fe ions onto SnO2 nanoparticles over a broad range of concentrations. The segregation of these ions controls the system microstructure by changing the surface energies and acting as nucleation sites for the formation of a Fe oxide phase (magnetite) at high Fe concentrations. A technologically interesting core-shell-type particle structure, with a magnetic shell and semiconductor core, was observed for the first time. The influence of the segregated Fe ions in Mössbauer spectra is also addressed as a new proposal for the interpretation of the effects of composition changes in both the bulk and at the interface of particles. In this proposal, the two observed sites in Mössbauer spectra would be independently related to bulk-substituted and surface-segregated Fe ions. © Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005.
- Surface segregation and consequent so2 sensor response in SnO2-NiO(2005-08-09) HIDALGO, P.; CASTRO, R. H. R.; COELHO, A. V. C.; GOUVEA, D.Many studies have been carried out concerning the development of SO 2 sensors to detect and avoid its health prejudicial effects. However, there is still a lack of reliable, high-speed-response sensors that work at room temperature. In this work, the segregation of Ni in the SnO 2-NiO system is used to obtain a rapid SO2 sensor response. Segregation and its structure consequences were studied by electron dispersive spectroscopy, infrared spectroscopy, and high-resolution transmission electron microscopy in nanopowders of SnO2-NiO with different compositions prepared by a polymeric precursor method. The sensor activity of SnO2-1 mol % Ni was studied and a linear calibration curve was formed with a maximum limit response of 32 ppm SO2. © 2005 American Chemical Society.
- A new method for obtaining adsorption isotherms on colloidal suspensions via electrokinetic sonic amplitude measurement(2005-12-06) GUIMARAES, K. L.; CASTRO, R. H. R.; GOUVEA, D.The standard methods for obtaining adsorption isotherms on colloidal suspensions are usually very time consuming and involve a large number of steps and assumptions that increase the experimental errors. In this work, an alternative method is proposed to evaluate the adsorption behavior of electrostericstabilized systems based on electrokinetic sonic amplitude signal measurements. The new method, entitled "zeta-sorption", is noticeably less time-consuming when compared to conventional procedures but showed great precision and reliability confirmed by comparison with data obtained from conventional routes on alumina-polyacrylate and alumina-citric acid aqueous suspensions. The experimental conditions that restrict the applicability of the new method were identified and justified by discussing the possible ion exchanges. © 2005 American Chemical Society.
- Surface reactivity and electrophoretic deposition of ZrO2-MgO mechanical mixture(2007-08-05) CASTRO, R. H. R.; MARCOS, P. J. B.; SAKAMOTO, E. K.; GOUVEA, D.Electrophoretic deposition (EPD) is a precision technique useful for obtaining high quality ceramic bodies with controlled dimensions and smooth coatings. The electrophoretic deposition rate is highly dependent on the surface chemistry of the powders, especially when dealing with multi-component systems. The objective of this work is to study the surface reactivity of both ZrO 2 and MgO in ethanol suspension to provide experimental benchmarks to control EPD of a ZrO2-3 wt% MgO mechanical mixture (Z3M) in ethanol. Infrared spectroscopy (FTIR) showed that ZrO2 surface spontaneously reacts with ethanol, generating negative electrophoretic mobility of the particles (-0.07 × 10-8 V-1 s-1) measured by Electroacoustic Sonic Amplitude (ESA). MgO surface also spontaneously reacted with ethanol, but a positive electrophoretic mobility was observed in this case (0.26 × 10-8 V-1 s-1). Scanning Electron Microscopy of Z3M dried from ethanol suspension showed that MgO particles were located around the ZrO2 particles, forming composite agglomerates, probably due to the electrostatic attraction between MgO and ZrO2 particles. Homogeneous deposits could be obtained from EPD of Z3M ethanol suspensions. Mercury intrusion porosimetry showed that the ZrO 2-MgO green deposited bodies using different voltages had similar pores diameters distributions, indicating that the ZrO2-MgO agglomerates are not affected by the increasing deposition rates. © Springer Science+Business Media, LLC 2007.
- Electrophoretic deposition of ZrO2-Y2O3: A bi-component study concerning self-assemblies(2009-01-05) CASTRO, R. H. R.; KODAMA, P. K.; GOUVEA, D.; MUCCILLO, R.There are many industrial advantages of using mechanical multi-oxides mixtures to obtain ceramic parts by electrophoretic deposition (EPD). This is mainly because one could avoid complex chemical synthesis routes to achieve a desirable composition. However, EPD of these suspensions is not an easy task as well since many different surfaces are present, leading to unexpected suspension behavior. The particles surface potentials and interactions can, however, be predicted by an extension of the DLVO theory. Using this theory, one can control the suspension properties and particles distribution. The objective of this work was to apply the colloidal chemistry theories to promote the formation of a heterocoagulation between ZrO2 and Y2O3 particles in ethanol suspension to achieve a suitable condition for EPD. After identifying a condition where those particles had opposite surface charges and adequate relative sizes, heterocoagulation was observed at operational pH 7.5, generating an organized agglomerate with ZrO2 particles surrounding Y2O3, with a net zeta potential of -16.6 mV. Since the agglomerates were stable, EPD could be carried out and homogeneous deposits were obtained. The deposited bodies were sintered at 1600 °C for 4 h and partially stabilized ZrO2 could be obtained without traces of Y 2O3 second phases. © 2009 Springer Science+Business Media, LLC.
- Quantification of MgO surface excess on the SnO 2 nanoparticles and relationship with nanostability and growth(2011-02-15) GOUVEA, D.; PEREIRA, G. J.; GENGEMBRE, L.; STEIL, M. C.; ROUSSEL, P.; RUBBENS, A.; HIDALGO, P.; CASTRO, R. H. R.In this work, we experimentally showed that the spontaneous segregation of MgO as surface excess in MgO doped SnO 2 nanoparticles plays an important role in the system's energetics and stability. Using X-ray fluorescence in specially treated samples, we quantitatively determined the fraction of MgO forming surface excess when doping SnO 2 with several different concentrations and established a relationship between this amount and the surface energy of the nanoparticles using the Gibbs approach. We concluded that the amount of Mg ions on the surface was directly related to the nanoparticles total free energy, in a sense that the dopant will always spontaneously distribute itself to minimize it if enough diffusion is provided. Because we were dealing with nanosized particles, the effect of MgO on the surface was particularly important and has a direct effect on the equilibrium particle size (nanoparticle stability), such that the lower the surface energy is, the smaller the particle sizes are, evidencing and quantifying the thermodynamic basis of using additives to control SnO 2 nanoparticles stability. © 2010 Elsevier B.V.
- Quantificação da segregação de MgO em pós nanométricos de SnO2 preparados por método químico(2009) GOUVEA, D.; PEREIRA, G. J.;Pereira, Gilberto J.;PEREIRA, GILBERTO JOSÉ