139 SYNTHESIS OF TITANIA POWDER BY TITANIUM TETRACHLORIDE OXIDATION IN AN AEROSOL FLOW REACTOR M. Kamal Akhtar, Yun Xiong, and Sotiris E. Pratsinis Center for Aerosol Processes, Department of Chemical Engineering, University of Cincinnati, Cincinnati, OH 45221 ABSTRACT Vapor phase synthesis of titania particles by oxidation of titanium tetrachloride (TiCI 4 ) was studied in an aerosol reactor between 1200 K and 1723 K. The effect of process variables (reactor residence time, temperature, reactant concentration) on powder size and phase characteristics was investigated using the differential mobility particle sizer, scanning electron microscopy and X-ray diffraction. The morphology of the particles remained unchanged under the process conditions investigated; titania particles were primarily anatase though the rutile weight fraction increased with increase in reactor temperature. The geometric number average diameter of the particles was between 0.13 itm and 0.35 lim and the geometric standard deviation of the particle size distribution was about 1.4. The average particle size increased with increasing temperature, TiCI 4 concentration and residence time. The observed changes in the particle size distribution were compared with those predicted by solving the aerosol dynamic equation by a sectional method and accounting for coagulation and first order chemical reaction. INTRODUCTION The preparation of ceramic powders has been moving increasingly towards a broader diversity of preparation processes with emphasis on more sophisticated chemical processing (Rice, 1990). While chemical and phase composition are basic requirements, control of particle size and morphology is of increasing importance. Ultrafine titania particles are routinely produced on a large scale by the oxidation of titanium tetrachloride vapor, the so-called "chloride" process. Reactions in the gas phase in the temperature range 973-1673 K result in the formation and growth of fine titania particles. Titanium dioxide pigments are used in coatings to provide maximum light scattering with virtually no absorption. The pigmentary properties of titania, for instance, hiding power (opacity) and gloss, are directly related to particle size and polydispersity (Kingery et al., 1976). In this paper a systematic experimental investigation of the formation of titania particles was carried out by the reaction between titanium tetrachloride and oxygen in a tubular aerosol flow reactor. Tubular reactors allow operation under well defined stoichiometries, temperatures and residence times and provide excellent control of the process variables affecting powder characteristics. Titania was chosen as the model compound for investigation not only because of its bulk use in the pigment industry but also because the reaction kinetics and mechanisms for its formation are reasonably well understood (Pratsinis et al., 1990). EXPERIMENTAL Clean, dry argon carrier gas (Wright Brothers, 99.8%) was bubbled through a fritted glass outlet into a gas washing bottle containing TiCI 4 (Aldrich, 99.9%) at room temperature (= 296 K). The mixture of Ar-TiCI 4 was diluted with a controlled amount of additional argon, Mat. Res. Soc. Symp. Proc. Vol. 249. 01992 Materials Research Society