METASTABILITY OF TETRAGONAL ZRO2 DERIVED FROM ZR-N-PROPOXIDE ACETYLACETONE WATER ISOPROPYL-ALCOHOL

ZrO2 nanopowders derived from zirconium n-propoxide [Zr(OC3H7)(4)]-ace tylacetone-water-isopropanol have been investigated with respect to th eir tetragonal metastability on heating-cooling processes. The transfo rmation temperature of metastable tetragonal to monoclinic (t' --> m) phase is found to be governed by ultimate firing temperature, time, an d atmospheres employed. Crystallite growth is fastened with increase i n calcination temperatures over 1000-1400 degrees C, and the t' --> m transformation temperature is correlated linearly with crystallite siz e in the studied range of 12-20 nm. Heating in an oxygen environment i ncreases the size of the final crystallites and hence the rate of the t' --> m transformation. It is revealed that the t' --> m transformati on temperature depends largely on the heating atmosphere, but only wea kly on the cooling one. Based on the findings of this work, surface ox ygen deficiencies are attributed to be responsible for low-temperature tetragonal metastability. A crystallite growth model to explain the d ecline of t'-ZrO2 phase is proposed, Kinetic and thermodynamic factors are also discussed in connection with the existing theories of tetrag onal metastability.