The influence of thermal treatment on the phase development in HfO2-Al2O3 and ZrO2-Al2O3 systems

Amorphous precursors of HfO2-AlO1.5 and ZrO2-AlO1.5 systems covering the whole concentration range were co-precipitated from aqueous solutions of the corresponding salts. The thermal behaviour of the amorphous precursors was examined by differential thermal analysis, X-ray powder diffraction (XRD), laser Raman spectroscopy and scanning electron microscopy. The crystallization temperature of both systems increased with increase in the AlO1.5 content, from 530 to 940degreesC in the HfO2-AlO1.5 system, and from 405 to 915degreesC in the ZrO2-AlO1.5 system. The results of phase analysis indicate an extended capability for the incorporation of Al3+ ions in the metastable HfO2- and ZrO2-type solid solutions obtained after crystallization of amorphous co-gels. Precise determination of lattice parameters, performed using whole-powder-pattern decomposition method, showed that the axial ratio c(1)/a(f) in the ZrO2- and HfO2-type solid solutions with 10mol% or more of Al3+ approach 1. The tetragonal symmetry of these samples, as determined by laser Raman spectroscopy, was attributed to the displacement of the oxygen sublattice from the ideal fluorite positions. It was found that the lattice parameters of the ZrO2-type solid solutions decreased with increasing Al3+ content up to similar to10 mol%, whereas above 10 mol%, further increase of the Al3+ content has very small influence on the unit-cell volume of both HfO2- and ZrO2-type solid solutions. The reason for such behaviour was discussed. The solubility of Hf-4+ and Zr4+ ions in the aluminium oxides lattice appeared to be negligible. (C) 2004 Elsevier B.V. All rights reserved.