Phase development and activation energy of the Y2O3-Al2O3 system by a modified sol-gel process

Abstract

The phase development of Y3Al5O12 (YAG), YAlO3 (YAP), and Y4Al2O9 (YAM) synthesized by a modified sol-gel method has been studied. A chelate agent (ethyl acetoacetate) is added to control the hydrolysis and to modify the sol-gel process. The sol-gel-derived powder is then heat-treated to induce crystallization. The X-ray diffraction method is employed to determine the crystalline phases of the resulting powders. The formation of YAG, YAP, and YAM is described. For both YAP and YAM compositions, there is a phase transformation from YAP polymorphism to perovskite YAP. The activation energies of YAG are estimated at about 69 kcal/mol by the isothermal process as fitted with the Johnson-Mehl-Avrami equation and about 222 +/- 2 kcal/mol by the continuous heating method as fitted with Kissinger and Sotgiu plots. The difference of activation energies calculated from the two methods is mainly due to the complicated crystallization of YAG. (C) 1998 Elsevier Science S.A. All rights reserved.