EXAFS AND XRD ANALYSES OF THE STRUCTURAL EVOLUTIONS INVOLVED DURING DRYING OF SNO2 HYDROGELS

In order to determine structural changes during drying of inorganic ge ls, the local and long-range order structure of SnO2-x(OH)(2x) xerogel s resulting from drying hydrogels with different concentrations of ele ctrolyte (Cl- and NH4+) have been measured by extended X-ray absorptio n fine structure (EXAFS), X-ray diffraction (XRD) and N-2 adsorption t echniques. EXAFS measurements performed at the Sn K edge on the hydrog els and xerogels show the existence of microcrystallites with the cass iterite structure. Two drying modes have been used: freeze drying and drying by evaporation at 45 degrees C. It is shown that the microcryst allite size determined by XRD and EXAFS techniques depends on the dryi ng mode and on the electrolyte concentrations. The microcrystallite si ze measured on the freeze dried xerogels is similar to that of their p arent hydrogels, whatever the concentration of electrolyte; however, d uring drying by evaporation, a preferential growth of microcrystallite s along the c-axis of the cassiterite structure is observed. The size of these crystallites is enhanced with a decrease of the electrolyte c oncentration. Specific surface areas calculated by the Brunauer-Emmett -Teller method indicate that this preferential growth is related to th e improvement of the network connectivity. The comparison of both dryi ng processes indicates that crystallization and polycondensation are i ndependent phenomena.