Charging and coagulation during flame synthesis of silica

The effect of charging during flame synthesis of silica particles is investigated. 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) across the flame. As a result, these electric fields not only charge the newly formed particles, but they also reduce the flame temperature and the particle residence time at high temperatures. However, the field created 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 separated from that of convection (ionic wind). A negative electric field creates the most drastic effect on the characteristics of the product particles. The primary particle size decreases with increasing field strength regardless of polarity or electrode configuration. Repulsion of the charged particles resulted in electrostatic dispersion, slower coagulation and smaller primary particle sizes. Transmission electron micrographs show that the extent of agglomeration is greatly influenced by electrode polarity and configuration during electrically assisted flame synthesis of silica powders. Copyright (C) 1996 Elsevier Science Ltd