Kinetic model and mechanism of Y3Al5O12 formation in hydrothermal and thermovaporous synthesis.

Yu.D. Ivakin, M.N. Danchevskaya, G.P. Muravieva.

3-rd International Conference on Solvothermal Reactions ICSTR - 3 Bordeaux, July 19-23, 1999, p. 63

ABSTRACT. Kinetics and mechanism of fine crystalline yttrium-aluminum garnet (YAG) formation under hydrothermal and thermovaporous treatment were investigated. It was synthesized from the stochiometric mixture of oxides in the temperature range 250-400 C at pressures of water vapor (4,0-26 ) MPa. The structure of synthesized products was studied by X-ray diffraction, thermal analysis, IR-, UV-, and V-spectroscopy and photoluminescence. Size and habitus of crystals were determined by optical and electron microcopy. It was established that in quasi-equilibrium with water vapor conditions the formation of YAG proceeds to produce intermediate substance with Y(OH)3 structure and amorphous aluminous component. The diffusion of this aluminous component into the Y(OH)3 matrix resulted in the reorganization of oxygen sublattice accompanied with dehydration. The kinetics of YAG formation is described by the equation of solid-phase transformation with the limiting stage of nucleation. Synthesized YAG contains (5-7%) of water, which corresponds to a hydrogarnet structure. Unit cell parameters of synthesized YAG are enlarged and decrease to ordinary values after annealing at 1200 C. The optical properties of YAG were modified by introduction of alloying ions Nd (3+) and Cr (3+) into the structure of YAG during synthesis process. This research was made with financial support of RFBR pr. 98-03-32645a.

Laboratory of Catalysis and Gas Electrochemistry