Gas-phase coating of TiO 2 with SiO 2 in a continuous flow hot-wall aerosol reactor
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Gas-phase coating of TiO2 with SiO2 in a continuous flow hot-wall aerosol reactor Quint H. Powell, George P. Fotou, and Toivo T. Kodasa) Department of Chemical and Nuclear Engineering, University of New Mexico, Albuquerque, New Mexico 87131
Bruce M. Anderson Kemira Pigments, Inc., P.O. Box 368, Savannah, Georgia 31402
Yongxiang Guo Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131 (Received 27 December 1995; accepted 18 October 1996)
The feasibility of in situ coating of titania particles with silica using a high-temperature, gas-phase process was demonstrated. Titania was produced from the reaction of TiCl4 and O2 in a hot-wall, tubular, aerosol reactor and directly coated in the gas phase via the reaction of O2 with SiCl4 vapor. Rough SiO2 coatings were obtained at 1300 ±C while uniform, dense coatings were obtained at all conditions examined for 1500 ±C. The presence of water in the reactor significantly influenced the morphology of the coatings and resulted in smooth, dense, and uniform coatings at 1300 ±C. Coating thicknesses could be controlled from 5 nm to roughly 100 nm, corresponding to growth rates on the order of 10–100 nmys. The characteristics of the coatings depended upon the concentration of SiCl4 and the coating temperature. These process variables influenced the coating mechanism, growth rate, and densification which directly influenced the coating uniformity and thickness.
I. INTRODUCTION
Titania is industrially produced by a gas-phase route which involves the reaction of titanium tetrachloride with oxygen, often called the chloride route, and a wet chemistry process, commonly called the sulfate route.1 In the chloride route, pigmentary titania is produced by reacting TiCl4 with O2 at temperatures in the range 1500–2000 ±C.1,2 This is normally accomplished by mixing streams of preheated TiCl4 vapor and oxygen. Upon reaction, TiO2 monomers are formed which collide to form particles that grow in size by sintering. After particle formation and cooling of the aerosol stream, the TiO2 powder is separated from the gas and treated further in liquid-phase processes which usually include the application of an insoluble hydrous metal oxide coating followed by drying and milling.3 These coatings are applied on titania particles to improve dispersion, pigment durability (reduce photoactivity), and resistance to increases in viscosity encountered in aging paint.4 Normally, coatings are obtained by precipitating SiO2 , Al2 O3 , or Al2 O3ySiO2 mixtures from silicon and/or aluminum hydroxide solutions.5,6
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J. Mater. Res., Vol. 12, No. 2, Feb 1997
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Coating by liquid-phase routes has several disadvantages. For most applications, compact and coherent metal oxide coatings are desirable because denser coatings result in improved durability7 and lower oil adsorption. To achieve high quality coatings with t
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