Monolithic samples of SnO2 xerogel were produced by careful control of
the gelation and drying steps of material preparation. In these sampl
es, small and nanoporous aggregates stick together, yielding a monolit
hic (nonpowdered) material. The material was analyzed by in situ small
-angle X-ray scattering (SAXS) during isothermal treatment at temperat
ures ranging from 473 to 773 K. At 473 K, the SAXS intensity does not
change significantly with time. All experimental scattering intensity
functions for T > 473 K are composed of two wide peaks, which evolve w
ith increasing time. Each of them was associated with one of the modes
of a bimodal distribution of pore sizes corresponding to a fine (intr
a-aggregate) and a coarse (inter-aggregate) porosity. The SAXS intensi
ties of the maxima of both peaks increase with increasing treatment ti
me, while the position of their maxima, associated with an average cor
relation distance, decreases. The time dependences of the SAXS intensi
ty corresponding to both families of pores qualitatively agree with th
ose expected for a two-phase separating system exhibiting dynamic scal
ing properties. The time evolutions of the several moments of the stru
cture function of samples heat treated at 773 K exhibit a good quantit
ative agreement with the theory of dynamic scaling for systems evolvin
g by a coagulation mechanism. The kinetic parameters are the same for
both peaks, indicating that the same mechanism is responsible for the
structural evolution of both families of pores.