Fractal Characteristic of Silica Xerogels with Different Additives

Two step acid-base catalyzed silica xerogels which had integrated structure were prepared through sol-gel and ambient pressure drying. Silica xerogels powder, TiO₂ powder and short fibers were added as the additives in order to improve the properties of silica xerogels. The porous structure was estimated by the means of nitrogen adsorption method and mercury intrusion porosimetry (MIP). According to the results of these two experiments, the surface fractal dimensions were calculated using Frenkel-Halsey-Hill (FHH) method, Menger sponge model and thermodynamic correlations respectively. The difference among them was compared and the influence of additives for pore structure also was analyzed combining with the fractal dimensions. The fractal dimensions exhibit the degree of surface roughness. The strength of silica xerogels is increased by adding the additives, and the shrinkage is also restrained in the course of drying. The fractal dimensions calculated from FHH method and thermodynamic correlations have almost same tendencies. These two models are more appropriate to analyze the porous fractal structures than Menger sponge model