FORMATION AND THERMAL-STABILITY OF GELS IN THE SIO2-TIO2-ZRO2 SYSTEM

Gels were prepared in the SiO2-TiO2-ZrO2 system (SiO2, TiO2 and ZrO2=0 -100 mol%) from Si(OC2H5)(4), Ti (O-isoC(3)H(7))(4) and Zr (O-nC(3)H(7 ))(4) a solutions with or without H2O and CH3COOH. Without H2O and CH3 COOH transparent bulk gels were formed in the range 30-100 mol% SiO2, 0-70 mol% TiO2 and 0-50 mol% ZrO2 and opaque bulk gels in the rest com positions. X-ray diffraction measurement indicated that all the gels w ere non-crystalline even after drying at 50 degrees C for 3 days. Crys tallization peak temperatures T-c were determined by DTA on the dried gels preliminarily heat-treated at 200 degrees C for 1h. T-c was low i n the TiO2-ZrO2 rich compositions (T-c=366 degrees C for TiO2 and 1219 degrees C for ZrO2). T-c increased up to 1258 degrees C with increasi ng SiO2 content. From T-c and liquidus temperature T-L, T-c/T-L ratio was calculated. T-c/T-L ratio means the thermal stability of the amorp hous solids. It was discovered that TiO2 and ZrO9 alone are thermally unstable (T-c/T-L= 0.3 for TiO2 and 0.23 for ZrO2) but mixing TiO2 wit h ZrO2 enhances the thermal stability (for example, T-c/T-L=0.45 for 5 0 TiO2.50ZrO(2)) with increased ability of forming amorphous phase (am orphous-forming ability). This is true in the SiO2-TiO2-ZrO2 system fo r fixed SiO2 concentrations. T-c/T-L increased with SiO2 content and r eached the highest value of 0.86 at 90SiO(2).10TiO(2) composition, whi ch is higher than that of SiO2 (0.77). Yet it seems likely that there are more stable compositions centering around the 80SiO(2).10TiO(2).10 ZrO(2) in the SiO2-TiO2-ZrO2 system. It is expected that compositions of high T-c/T-L value have a high amorphous-forming ability.