Silica fine-particle cluster

Fig. 3. Schematic representation of the topological space of hydration water in silica fine-particle cluster (45). The processes responsible for the water spin-lattice relaxation behavior are restricted rotational diffusion about an axis normal to the local surface (y process), reorientations mediated by translational displacements on the length scale of a monomer (P process), reorientations mediated by translational displacements in the length scale of the clusters (a process), and exchange with free water as a cutoff limit.

Relaxation dispersion data for water on Cab-O-Sil, which is a monodis-perse silica fine particulate, are shown in Fig. 2 (45). The data are analyzed in terms of the model summarized schematically in Fig. 3. The y process characterizes the high frequency local motions of the liquid in the surface phase and defines the high field relaxation dispersion. There is little field dependence because the local motions are rapid. The p process defines the power-law region of the relaxation dispersion in this model and characterizes the molecular reorientations mediated by translational displacements on the length scale of the order of the monomer size, or the particle size. The a process represents averaging of molecular orientations by translational displacements on the order of the particle cluster size, which is limited to the long time or low frequency end by exchange with bulk or free water. This model has been discussed in a number of contexts and extended studies have been conducted (34,41,43). 

A particularly neat way to make very pure oxide dispersions was devised by Stober, Fink and Bohn in 1968. Tetraethyl silicate was dissolved in alcohol and reacted with water in the presence of acid catalyst. Very fine nuclei of silica were formed, but these aggregated immediately to form uniform spherical clusters 159 nm in diameter which could then be grown further by controlled addition of tetraethyl silicate. The final particles were porous and of high surface area... 

Calcined diatomite - Also known as straight-calcined, this is diatomite which has been calcined at between 870° and 1100°C in a rotary kiln. This process bums off organic matter, converts some of die opaline silica to ciistobalite, shrinks, hardens, and reduces the fine stmcture of individual particles, and forms agglomerates or clusters of particles duough fusion. The overall effect is to decrease surface area but to increase bulk density and void volume due to the nature of agglomerate packing. Calcination generally turns the white to off-white natural diatomite pink fi om iron oxidation. The ealcined diatomite is carefiiUy milled, sereened, and air classified to various size fi actions, primarily for filtration uses. 

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