V.A. Kreisberg, V.P. Rakcheev
Diffusion diagnostics of microporosity in nanoporous amorphous silicas and porous glasses
Proceedings of the 6th ESG Conference "2002 Glass Odyssey", Montpellier, France, 2-6 June 2002, v.A5, p. 1-5 (2002)

ABSTRACT.
Polymodal nanoporosity of silica materials has been studied by two new methods. The first method of diffusion diagnostics is based on the high-sensitive mass spectral registration of kinetics of gas desorption from pores into high vacuum. The second method resides in the analysis of equilibrium desorption and adsorption isotherms at low partial pressures. The computer fitting of equilibrium isotherms and diffusion kinetics of nitrogen, oxygen and argon desorption at 77.5 K has revealed the availability of microporous substructure in mesoporous amorphous silicas and porous glasses. Trimodal nanoporous structure of silicas and porous glasses consists essentially of transporting mesopores with mean diameter from 3 to 15 nm and two kinds of adsorbing micropores with diameter 0.3-0.4 and 0.6-0.7 nm what corresponds 1 and 2 diameters of gas molecules. The morphology of pores in porous glasses depends on method of glass producing by leaching of alkali borosilicate glasses. Gas adsorption in micropores has a character of volume filling. Micropore volume in porous glasses is equal to 3-6% from total pore volume. For amorphous silicas the volume of micropores depends on method of synthesis and on the size of globules. Gas diffusion in mesopores is mainly Knudsen diffusion although the effective diffusion coefficient is the complicated function of Knudsen diffusion coefficient, surface diffusion coefficient and Henry's adsorption constant. The mechanism of diffusion in micropores is surface-diffusion one. The perspective direction of using the diffusion method for pore morphology diagnostics is its application for researching the systems with small pore volume: nanoporous coatings, films and surface defective layers on optical materials.