Particle coalescence, growth size, determination

The process by which vapor-deposited material forms two-dimensional arrays of spherical particles just beneath the substrate surface has been studied in detail. A model has been proposed in which the numbers and sizes of particles are determined by the coupled processes of particle growth (by capture of diffusing molecules) together with particle coalescence. Expressions have been derived for particle size and number density as functions of deposition parameters. Experimental evidence is presented in support of the model for the case of selenium physically vapor-deposited onto a heated thermoplastic substrate. Finally, the technological application of the deposit morphology as dry microfilm is reviewed. 

After an initial period, increasing retention time has a small impact on the rate of particle growth.J Thus, for practically sized treaters with retention times of 10 to 60 minutes, retention time is not a determinant variable. Intuitively, one expects viscosity to have much greater effect upon coalescence than would temperature. With this in mind, the following equation appears to give reasonable results ... 

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. 

Stability of a macroemulsion is an important factor as this determines its extent of usability for particle preparation or various other applications. Instability is basically coalescence of the dispersed phase droplets or Ostwald ripening (growth of large droplets at the expense of much smaller ones). When this process goes on, the emulsion eventually breaks into two layers. Other processes related to stability but considered less important [3] are (a) creaming or sedimentation, the rate of which is dependent on the difference in density between the continuous and dispersed phases, droplet size, viscosity of the continuous phase and interdroplet interaction and (b) flocculation, dependent on colloidal interactions between the droplets [8, 12]. Several factors determine the stability of macroemulsions these are discussed here in brief. This discussion is largely derived from Rosen [3] and some subsequent investigations [e.g. 6, 7, 13-15]. 

Ulrich (447) has further considered the factors governing particle size as condensed from vapor and concluded that the initially formed, very small particles assume the translational velocities of large gas molecules and that final particle size is determined by collision and coalescence. The logarithm of final size is propor tional to logarithm of growth time. This also explains why there is an increase in particle size with increase in silica concentration. This suggests that the final particles are likely to have a microporous structure. However, since the primary particles are only 10-20 A in diameter, and are closely packed, the internal pores of the final particles are usually impervious to nitrogen adsorption and detectable only by water adsorption.. ... 

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