Compacts of pyrogenic powders

Adsorptive properties of porous silicas compacts of pyrogenic powders, 506, 507f precipitated silicas, 507-509 silica gels, 509, 510-512 zeolitic silicas, 512-514 Adsorptive properties of silicas, challenges for improvement, 505 Aerogel(s) definition, 7, 620 porosities, 379, 380 ... 

Four main types of porous silica adsorbents have been identified compacts of pyrogenic powders, precipitated silicas, silica gels, and zeolitic silicas. The importance of porosity relative to the adsorptive properties of each group is reviewed, with particular reference to the adsorption of nitrogen, argon, and water vapor. The differences in size and specificity of these adsorptive molecules may be exploited to explore the surface properties of each grade of silica. A notable feature cf Silicalite I, which is the best known of the zeolitic silicas, is its remarkable hydrophobic character. Furthermore, the uniform tubular pore structure of this microporous silica is responsible for other highly distinctive properties. 

Note that the fractal DLCA-like aggregates which are generated in the flame frequently agglomerate to large, micrometre-sized clusters. This may either occur during the combustion process or later when the pyrogenic powders are stored in silos or big packs (Kammler et al. 2004 Tsantilis and Pratsinis 2004 Ibaseta and Biscans 2010). These agglomerates are relatively compact with fractal dimensions of 2.5. 

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