SAXS MEASUREMENTS OF THE POROSITY IN CU(II)-DOPED SNO2 XEROGELS DURING CRYSTALLIZATION

The small-angle X-ray scattering (SAXS) technique was used to study th e porosity which develops in Cu(II)-doped SnO2 monolithic xerogels dur ing crystallization. The influence of the upper temperature of heat-tr eatment and of Cu(II) content on the structure was determined. Previou s studies of the porosity in undoped SnO2 samples treated at temperatu res ranging from 300 up to 600 degrees C demonstrated the existence of a bimodal size distribution (one distribution was due to intra-aggreg ate and the other to inter-aggregate pores). However, the SAXS data fr om Cu(II)-doped samples heated to 500 degrees C had a single mode dist ribution due only to inter-aggregate pores. Doped samples isothermical ly treated at 500 degrees C were studied by the in situ SAXS technique . The time evolution of the scattering intensity function, or structur e function of the porous material, exhibits a dynamical scaling proper ty. The asymptotic behavior at high q (wave numbers) of the scaled fun ction and consequently the nature and morphology of the porosity inter face are a function of Cu(II) content. The kinetic exponents predicted by the statistical theory for the structure function suggest that the mechanism of porosity coarsening is controlled by surface diffusion. (C) 1997 Elsevier Science B.V.