ULTRAFINE REFRACTORY PARTICLE FORMATION IN COUNTERFLOW DIFFUSION FLAMES

The effect of process variables (percursor concentration, residence time, temperature, and electric field) on ceramic powder formation was studied using a counterflow diffusion flame burner. GeCl4and SiCl4were used as source materials for the formation of GeO2 and SiO2 in hydrogen—oxygen flames. In-situ particle size and number density were determined using dynamic light scattering and 90° light scattering. A thermophoretic sampling method also was used to collect particles directly onto carbon coated grids, and their size and morphology examined using transmission electron microscopy. Increasing the precursor concentration caused larger particles to form and enhanced surface growth effects; decreasing residence time favored homogeneous nucleation as the particle formation route; using higher temperatures resulted in a larger size for the fundamental particles but less aggregation. Using GeCl4in the presence of a I250V/cm electric field, a three fold increase in particle size was obtained. Process variables also determined the degree of crystallinity; crystalline particles were usually found when mostly small particles were produced; amorphous particles were found when high precursor concentration, long residence time, and low temperatures were used. These results show how desired particle characteristics (particle size, particle morphology, crystalline or amorphous) can be generated through control of flame conditions. © 1992, Taylor & Francis Group, LLC. All rights reserved.