Engenharia Química
URI permanente desta comunidadehttps://repositorio.fei.edu.br/handle/FEI/25
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Resultados da Pesquisa
Artigo de evento 0 Citação(ões) na Scopus Calorimetric measurement of the interface energy of pure and calcium doped magnesium oxide(2009-10-29) CASTRO, R.; Ricardo Torres; GOUVEA, D.Interface energetics is of key importance in understanding the evolution of nanoparticles during sintering. During this process, two types of interfaces control the driving forces: the surface and the grain boundary. The knowledge of their energies is therefore of major importance to obtain reliable sintered bodies. The objective of this work was to present a quantitative relationship between the grain boundary energy and the surface energy of MgO and Ca-doped MgO nanoparticles to provide data to better understand the sintering behavior of this system, in particular delineating the thermodynamic basis of using Ca as sintering add. Using a novel and convenient calorimetric procedure, the ratio between the grain boundary energy and the surface energy was determined to be 1.1 for pure MgO and 0.7 for Ca-doped MgO. Based on these, the grain boundary energy of pure MgO and Ca-doped MgO were estimated to be 1.2 J.m-2 and 0.4 J.m-2, respectively. The grain boundary energy decrease caused by calcium doping was attributed to its interface segregation and influenced the sintering behavior by changing the equilibrium dihedral angle in addition to diffusion parameters.- Surface energy and thermodynamic stability of γ-alumina: Effect of dopants and water(2006-04-04) CASTRO, R. H, R.; USHAKOV, S. V.; GENGEMBRE, L.; GOUVEA, D.; NAVROTSKY, A.Retaining large surface areas in alumina powders during high-temperature annealing is a major challenge in applications as catalyst supports and ceramic precursors. This is because the alumina surface area drastically decreases with transformation from the γ modification (defect spinel structure) into the a modification (corundum structure). The objective of this work is to show the thermodynamic basis of using additives, such as Zr and Mg, to control the γ-Al 2O 3 surface and bulk energetics and to manipulate the transformation temperature and surface area. These additives are observed to change the pattern of phase transformation and densification. Direct measurements of heats of solution in a lead borate melt of pure and doped alumina as a function of surface area enabled us to experimentally derive trends in the surface energies of hydroxylated surfaces. Accounting for heats of water adsorption measured on pure and doped alumina surfaces allowed us to delineate the thermodynamic effects of hydration on surface energies. Zr-doped γ-alumina showed a higher energy of the hydroxylated surface than did pure γ-alumina but showed a lower energy of the anhydrous surface. Mg addition does not change surface energies significantly but decreases the energetic instability of the bulk γ phase. © 2006 American Chemical Society.