ISOTHERMAL STRUCTURAL EVOLUTION OF SNO2 MONOLITHIC POROUS XEROGELS

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.