Ceramic powder synthesis aggregation

This chapter discusses four methods of gas phase ceramic powder synthesis by flames, fiunaces, lasers, and plasmas. In each case, the reaction thermodynamics and kinetics are similar, but the reactor design is different. To account for the particle size distribution produced in a gas phase synthesis reactor, the population balance must account for nudeation, atomistic growth (also called vapor condensation) and particle—particle segregation. These gas phase reactors are real life examples of idealized plug flow reactors that are modeled by the dispersion model for plve flow. To obtain narrow size distribution ceramic powders by gas phase synthesis, dispersion must be minimized because it leads to a broadening of the particle size distribution. Finally the gas must be quickly quenched or cooled to freeze the ceramic particles, which are often liquid at the reaction temperature, and thus prevent further aggregation. 

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. 

The present applications of ceria-based ceramics impose strict requirements on the quality and purity of the powders used. Several studies have described the synthesis of ceria nanopowders of high quality and with a well-defined morphology. Typical methods of preparation include hydrothermal synthesis [263, 264], the hydrolysis of an alkoxide solution (sol-gel) ]265], chemical precipitation [266], mechanochemical processing ]267], and gas-phase reaction ]268]. Emulsion techniques can also be used, as these reduce not only the production costs of high-purity spherical powders but also the degree of aggregation. Thus, ceria powders with an average particle size <20 nm and a narrow particle size distribution can be... 

Combustion synthesis The reaction time is short and the process is simple Low cost technique Low heating temperature Used to prepare different t5rpes of materials (ceramic, intermetallic, composite) Varied morphology of the produced powder Lower contamination Agglomeration/aggregation can occm Low crystallinity... 

Sasaki T., Watanabe M., Hashizume H., Yamada H., Nakazawa H. Macromolecule-like aspects for a colloidal suspension of an exfoliated titanate. Pairwise association of nanosheete and dynamic reassembling process initiated from it. J. Am. Chem. Soc. 1996 118 8329-8335 Sasaki T., Nakano S., Yamauchi S., Watanabe M. Fabrication of titanium dioxide thin flakes and their porous aggregate. Chem Mater. 1997 9 602-608 Sathiyakumar M., Gnanam F.D. Synthesis of sol-gel derived alumina powder effect of milling and calcination temperatures on sintering behaviour. Brit. Ceram. Trans. 1999 98 87-92... 

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