Abstract:
To study the crystallization mechanism of zirconia under the glycothermal condition, zirconia powders were produced by the reaction of zirconium tetra n-propoxide (ZNP) in glycol. It was found that the crystallization behavior depended on the carbon number of glycol used. Amorphous product was obtained for the reaction in ethylene glycol. The use of 1,3-propanediol resulted in crystalline glycol complex and zirconia was not formed even by the reaction at 300ํC. When 1,4-butanediol was employed, crystalline glycol complex was initially formed, which then transformed to amorphous phase and the tetragonal zirconia subsequently crystallized from the amorphous phase through the solid-state transformation mechanism. However, the formation of crystalline zirconia in 1,5-pentanediol and 1,6-hexanediol proceeded via a soluble intermediate (i.e., glycoxides). Synthesis conditions, i.e., ZNP concentration and drying method, affected the properties of zirconia powders. For the use of 1,4-butanediol, crystallite size, microsphere particle size of zirconia and BET surface area increased with increasing the ZNP concentration. Glycol removal at reaction temperature did not change the pore system of the powders because the aggregation of primary particles probably occurred during the reaction process. In contrast, physical properties of zirconia obtained in 1,5-pentanediol were not affected by ZNP concentration, whereas the pore system of the powders was improved when the glycol was removed from the autoclave by flash evaporation due to the reduction of aggregation among the ultrafine particles during drying. The effect of silica on the BET surface area of tetragonal zirconia and its thermal stability was studied. Silica-modified zirconia with the Si/Zr ratios of 0.01-0.15 were prepared by the reaction of ZNP and tetraethyl orthosilicate (TEOS) in 1,4-butanediol. With increasing TEOS amount, the BET surface area drastically increased and tetragonal-to-monoclinic transformation temperature shifted toward higher temperatures. Infrared spectroscopy indicated the presence of Si-O-Zr bonds in the samples, resulting in the retardation of crystallite growth during calcination
