Primary particles neck formation

Estimation of Average Primary Particle Size Method of Characteristic Times 346 Primary Particle Size Effects of Aerosol Material Properties 350 Particle Neck Formation 353 Particle Crystal Structure 355 Basic Concepts 355 Experimental Obsetyafions 355 Problems 356 References 357... 

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. 

In commercial applications of submicron powdered materials as additives, fillens, and pigments, nonagglomerated or weakly agglomerated primary particles are usually desired. Quantitative, predictive methods for describing neck formation and the nature of the bonds between particles are not available. In the ab,sence of proven methods, we di.scu.ss some guidelines that may serve as a starting point for future research. 

Orthophosphoric acid is conveniently prepared by boiling one part of red phosphorus with 16 parts of nitric acid—sp. gr. between 1-20 and 1-25—in a bask fitted with a reflux condenser and a ground glass-joint at the neck. Any nitric acid which is volatilized will thus be returned to the flask. Yellow phosphorus is not so quickly attacked by nitric acid as red phosphorus, possibly because the former melts and forms masses which do not present so nearly as large a surface to the action of the acid as do the particles of red phosphorus. The latter, is more expensive. When the phosphorus is all-oxidized, the soln. is evaporated to dryness, and the residue is finally heated in a platinum dish to a temp, not exceeding 180° to make sure that all the nitric acid is driven off. The boiling liquid usually shows the presence of phosphorous acid, which is subsequently oxidized to phosphoric acid. Whether the primary action results in the formation of both acids or of phosphorous acid alone is not clear. If the acid employed for the oxidation be more cone, than that just indicated an explosion may ensue and if a weaker acid be used the action is very slow. 

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