Particle synthesis: mechanisms aggregation

The macroscopic properties of liquid suspensions of fumed powders of silica, alumina etc. are not only affected by the size and structure of primary particles and aggregates, which are determined by the particle synthesis, but as well by the size and structure of agglomerates or mesoscopic clusters, which are determined by the particle-particle interactions, hence by a variety of product- and process-specific factors like the suspending medium, solutes, the solid concentration, or the employed mechanical stress. However, it is still unclear how these secondary and tertiary particle structures can be adequately characterized, and we are a long way from calculating product properties from them [1,2]. 

At the synthesis beginning solution contains only monomer, which within the frameworks of irreversible aggregation models can be considered as particles, uniting later in a cluster (macromolecular coil). As it is known [21], within the frameworks of the indicated models such mechanism is called mechanism particle-cluster and aggregates with fractal dimension 2.5 is the result of its action. Besides, the value Cj was calculated according to the Eq. (18) with the following parameters using t = 0.5 min, i3r=4.8 mol/l s and Q=8.3><10-3. 

Solid silica spheres are prepared by the Stober8 method. This involves the condensation of TEOS in alcoholic solution of water and ammonia. The mechanism of this synthesis was elucidated by Van Blaaderen et al.20,21 The particle growth is rate-limited by the production of hydrolysed monomer molecules. It involves a surface condensation of monomers and oligomers, while aggregation of particles is only occurring in the early stages of the condensation. 

The collision-coalescence mechanism of particle growth discussed in this chapter is thought to control primary particle size in Hame reactors. The emphasis is on the synthesis of transition metal oxide particles, which are important in the manufacture of pigments, addili ve.s, and ceramic powders. Also discussed are the factors that determine the formation of necks between particles and particle crystallinity. As demands on product quality become more stringent, more research will be needed on particle size, unifonnity. crystallinity, and aggregate formation. 

Figure 2.6 Scheme of the mechanism for the formation of mesoporous silica. Silica polymers formed initially from silica monomers, and associated with surfactant monomers, which form composite self-organised primary particles which can either continue to grow via monomer addition (path 1) or themselves aggregate in a directional fashion (path 2) to form the final mesophase composite. Nondirectional aggregation would cause formation of disordered pore structures. Reprinted with permission from Nooney, R.I. Thirunavukkarasu, D. Chen, Y. Josephs, R. Ostafin, A.E., Synthesis of Nanoscale Mesoporous Silica Spheres with Controlled Particle Size, Chem. Mater., 14, 4721—4728. Copyright (2002) American Chemical Society... 

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