Powder Preparation and Feeding Control

The maximum allowed size of the particles rma is given by the thickness of the molten layer S, and the density p and velocity v of the particles by 

Therefore, one should firstly apply the lowest economic feeding rate and secondly remove the coarse fractions of the powder that should not reach the crystallization front. Special equipments in laboratory scale and industry scale have been designed for fabrication of Verneuil powder and are shown in Fig. 16.9. 

Air separation in the pseudoliquefied layer as shown in Fig. 16.9(a) allows separation of particles with respect to their geometric form and their weight. The particles move in the turbulent gas flow for several hours so that the powder looseness is increased and the specific surface of the particles is reduced. The separation efficiency by air separation is greater than by sieving, as is indicated in Fig. 16.9(b). 

Sinmltaneously, a certain purification of the powder from specific impurities by factors between 2 and 10 is observed as indicated in Table 16.1. 

The looseness of the powder is an important factor to achieve a steady feeding rate in the growth process. The looseness is determined by the forces between powder particles, namely cohesion, capillary and electromagnetic forces, and by chemical interaction of adsorbed surface films. The looseness is increased by reducing the total forces. For the neutralization of static electricity and of cohesion forces the powder is moistened up to the formation of a monomolecular surface layer. For Y-AI2O3 the maximum looseness corresponds to 3-4% humidity, see Fig. 16.10. 

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