Mixed oxides SiO2-ZrO2 and SiO2-TiO2 by a non-hydrolytic sol-gel route

SiO2-ZrO2 and SiO2-TiO2 mixed oxides with various metal contents have been prepared by a non-hydrolytic sol-gel route involving the condensation between chloride and isopropoxide functions at 110 degrees C. Well condensed, monolithic gels were obtained in one step, without the use of additives. The Si/M ratio of the oxide may be controlled easily by the composition of the starting mixture. The Si/Zr oxides remained amorphous after calcination for 5 h at 600 degrees C; IR and Si-29 NMR spectroscopy showed a large amount of Si-O-Zr bonds, indicating a homogeneous distribution of the components on the atomic scale. The crystallization of tetragonal zirconia took place at higher temperature; the transformation of tetragonal to monoclinic zirconia was strongly retarded and did not take place after 2 h at 1300 degrees C. The crystallization of zircon (for a sample containing 50 mol% Zr) started at 1500 degrees C and was completed after 20 h at 1500 degrees C. IR spectroscopy indicated the presence of a limited number of Si-O-Ti bonds in all the Si/Ti oxides after calcination for 5 h at 500 degrees C. The sample within the stable glass region(5 mol% Ti) appeared perfectly homogeneous: it crystallized at 900 degrees C as single-phase cristobalite oxide, with Ti4+ ions substituting Si4+ ions at random. On the other hand, the precipitation of anatase was observed for the Si/Ti oxides with a high Ti content (20-50 mol% Ti), which are outside the stable glass region. The transformation of anatase to rutile was not observed even after 2 h at 1300 degrees C.