V.A. Kreisberg, V.P. Rakcheev, T.V. Antropova
XIX International Congress on Glass, July 1-6, 2001, Extended Abstracts, Oral and Poster Sessions, Society of Glass Technology, Edinburgh, Great Britain (Scotland), p. 749 - 750
ABSTRACT. In studies of glass surface and films on glass the main attention is often paid to chemistry of surface and near the surface region. The problems of morphology and geometrical structure of surface are mainly staying in the shadow. The questions of nanoporous surface structure are not investigated because the methods for researching micro porosity are not developed and the common adsorption methods do not work in this case. Some samples of porous glasses with various porous structure produced by the leaching of alkali borosilicate glasses were investigated by the new method of diffusion diagnostics of micro porosity of solids and films. The method is based on very sensitive registration of desorbing gases using desorption kinetic mass spectrometry. After preliminary adsorption of a gas at temperature of liquid nitrogen, the pumping out and mass spectral recording of gas desorbed into the high vacuum are carried out. Distinguishing in the diffusion drag of the gas transport in the pores of various morphology results in the essential difference in the type of isothermal kinetic curves of desorption of which the corresponding diffusion coefficients can be calculated. Oxygen, nitrogen and argon diffusion coefficients at temperature 77.5 K from nanoporous samples of porous glasses have been measured in Knudsen regime of gas flow in vacuum. On the basis of the model of consistency between diffusion-transport characteristics and parameters of ultraporous structure, the characteristics of porous structure of the investigated glasses were determined. These parameters (volume, size, volume distribution) are consistent with those obtained from equilibrium isotherms of gas adsorption. The use of different gas diffusing molecules allowed to estimate the influence of shape, size and value of quadrupole moment of molecules on diffusion-transport characteristics of porous glass and thus to take into account energetic factors of interaction adsorbent - adsorbate in process of gas diffusion. The diffusion characteristics of porous glasses with skeleton texture were compared with mean diameter of pore entrance for silica gels of globular structure and kinetic diameter of zeolite channels. The proposed kinetic desorption method does not diagnose macropores thus allowing to study micropores in the presence of macropores. Owing to the high sensitivity of mass spectral analysis one can determine the surface micropores of extremely small capacity (1e-10 cm3/g).