PHASE-SEPARATION IN GEL-DERIVED MATERIALS, SEPARATION AND CRYSTALLIZATION OF SNO2 WITHIN AN AMORPHOUS SIO2 MATRIX

On heating a gel of composition 33.5SnO2-66.5SiO2 (wt. %) up to 1050-degrees-C, SnO2 crystallization occurred with different mechanisms according to thermal treatments. Thermal analysis, SAXS (Small Angle X-Ray Scattering), WAXS (Wide Angle X-Ray Scattering), and TEM (Transmission Electron Microscopy) results obtained for samples treated at different temperatures demonstrated that the SnO2 load is divided into two moieties, one composed of isolated SnO2 particles suitable for primary crystallization yielding crystallites with 4.0 nm of average size; the other, being dissolved in SiO2, remains in the amorphous SiO2/SnO2 solid solution up to the highest temperature. The presence of these two phases accounts for SnO2 surface segregation at the expense of the SnO2 concentration of neighboring outer layers and the independence of apparent density on temperature.