Non-equilibrium phase synthesis in Al2O3–Y2O3 by spray pyrolysis of nitrate precursors Original Research Article

The phase evolution in the Al2O3–Y2O3 system has been studied for 4, 10, 15 and 37.5% Y2O3 amorphous powders prepared by spray pyrolysis of nitrate precursor solutions. Two distinct metastable transformation sequences were identified for the amorphous powders upon heat treatment. Crystallisation accompanied by partitioning leads to a mixture of hexagonal YAlO3 and γ-Al2O3 (spinel structure). Partitionless crystallisation, on the other hand, leads directly to a γ-Al2O3 solid solution in dilute alloys, and garnet at temperatures as low as 800°C in the stoichiometric composition provided segregation is avoided during decomposition. Further heat treatment of a yttria-supersaturated γ-Al2O3 leads to the precipitation of the orthorhombic YAlO3 that is stable up to temperatures as high as 1600°C, while hexagonal YAlO3 converts to garnet, Y3Al5O12. A rationalisation of the phase evolution sequence has been attempted on the basis of kinetic considerations, cation coordination and semi-quantitative free energy–composition curves for the various competing phases.