Engenharia de Materiais
URI permanente desta comunidadehttps://repositorio.fei.edu.br/handle/FEI/17
Navegar
2 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.
- 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.