CHARGING AND COAGULATION DURING FLAME SYNTHESIS OF SILICA

The effect of charging during flame synthesis of silica particles is i nvestigated. These particles are made by SiCl4 oxidation/hydrolysis in a premixed, CH4-O-2-N-2 flat flame. Unipolar and bipolar DC electric fields are created across the flame axis using various combinations of needle and plate electrodes. Needle electrodes create highly focused electric fields resulting in the onset of convection (ionic wind) acro ss the flame. As a result, these electric fields not only charge the n ewly formed particles, but they also reduce the flame temperature and the particle residence time at high temperatures. However, the field c reated with this configuration is not stable. Using a plate as one of the electrodes increases the stability of the electric field. Using a plate/plate electrode configuration suppresses the onset of the ionic wind, so the effect of field charging on particle growth may be separa ted from that of convection (ionic wind). A negative electric field cr eates the most drastic effect on the characteristics of the product pa rticles. The primary particle size decreases with increasing field str ength regardless of polarity or electrode configuration. Repulsion of the charged particles resulted in electrostatic dispersion, slower coa gulation and smaller primary particle sizes. Transmission electron mic rographs show that the extent of agglomeration is greatly influenced b y electrode polarity and configuration during electrically assisted fl ame synthesis of silica powders. Copyright (C) 1996 Elsevier Science L td