V. Briois et al., EXAFS AND XRD ANALYSES OF THE STRUCTURAL EVOLUTIONS INVOLVED DURING DRYING OF SNO2 HYDROGELS, Journal of non-crystalline solids, 191(1-2), 1995, pp. 17-28
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.