Influence of drying methods on fractal geometric characteristics of mesoporous silica aerogels

Abstract

Chemical modification/ambient drying method and freeze drying method were introduced to research the synthesis of mesoporous silica aerogels. By analyzing N₂ gas adsorption/desorption isotherms, the fractal geometric characteristics of gels were focused. The overall surface fractal dimensions were determined by analyzing N₂ gas adsorption branch and a Frenkel-Halsey-Hill (FHH) equation was empolyed to determine surface fractal dimension D _f. It is found that, during ambient drying process, V _TMCS/V _Wetgel ratio plays a crucial role in the changes of geometric feature, the key point is 50%, when the ratio is lower, and surface roughness increases with the ratio, when it exceeds 50%, the surface is almost unaffected by the modification. While freeze drying always tends to get larger D _f, freeze drying process could cause a rough surface of the gels. Compared with traditional porosity and specific surface area analyses, fractal geometry may be expected to be favorable for mesoporous structural analyses of materials.

Keywords

silica aerogel / N₂ gas adsorption / surface fractal dimension / mesoporous structure

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