IntroductionCoagulation is an omnipresent mechanism in all gas phase particle generation processes which determines size and shape ( external structure) of the product particles. If the coagulation of ultrafine particles can systematically be controlled i.e. increased or suppressed, it should be possible to form structured particles of any size.Gutsch (1995) showed that electrostatic conditioning of ultrafine aerosols has a considerable potential with respect to the systematic manipulation of the agglomerate size and structure.The subject of the present research is a systematic investigation of electrical effects during particle synthesis in a TiO 2 flame reactor. The reaction zone is supplied with additional charge carriers via a corona discharge in immediate proximity to the flame. The resulting agglomerate size distributions are measured in terms of mobility equivalent diameters and geometric structure datas.Generator and Experimental MethodsThe experimental apparatus is shown in Fig. 1. It consists of the following three basic components: Particle synthesis of TiO 2 through gas phase reactions in the flame Electrical aerosol conditioning Particle analysisThe influence of the electrical flame conditioning on agglomerate size or structure is analysed by means of mobility analyses, SEM and TEM micrographs, by measurements of the specific surface area as well as x-ray defraction analyses. Samples were taken via a thin probe directly from the flames. Dilution and stabilization of the size distribution were subject to a careful investigation.Results - Experiments without electrical conditioningMobility analyses have shown that the average particle size as well as the standard deviation of the distribution increase with an increasing reactor residence time. The observed particle growth is due to size enlargement of the primary particles on one hand (comp. fig. 2) and to their agglomeration on the other. The agglomeration is also responsible for the broadening of the size distribution.Furthermore, with increasing reaction temperature smaller mobility equivalent diameters were observed for almost all investigated residence times. This observation may be explained by a reduced agglomeration at higher temperatures which appears to outweigh the effects of an increased primary particle diameter observed both in electron microscopic analysis and in an attendant decrease in specific surface area.The reduced agglomeration is apparently also responsible for the narrower distribution width. For identical residence times, mobility analyses showed that the particle size increased with increasing TiCl 4 -concentration.A higher TiCl 4 -concentration resulted in an accelerated growth of the primary particles, as shown by specific surface measurements and TEM micrographs, as well as in an increased agglomeration. This assumption is compatible with the observed broadening of the size distribution with increasing TiCl 4 concentrations.Results - Experiments with electrical conditioningBy applying a voltage between two opposing needle electrodes (Fig. 1) a corona discharge occurs at their tips resulting in a bipolar charging zone. From the emerging charge carriers and the electric field formed between the electrode tips, an ionic wind develops which was changing the flame geometry significantly.As a first approach to study the electric influence on particle growth, a flame setting was chosen for which, at least optically, a difference with or without electric influence was not discernible. The mobility analyses have shown that the electric influence results in a reduction of the particle diameter of up to 40%. This observation is explainable by the electrostatic particle interactions. Close to the electrode tips an unipolar particle charging occurs. The repulsive forces acting between the particles lead on one hand to a reduced collision rate, and on the other hand to particle losses (electrostatic dispersion). At higher residence times the oppositely charged particles collide resulting in more open agglomerate structures consisting of smaller primary particles.Furthermore, the decrease in mobility equivalent diameter was a function of the applied voltage, the distance and the positioning of the electrodes in the reactor.