Titania and silica powders produced in a counterflow diffusion flame
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Titania and silica powders produced in a counterflow diffusion flame Aaron J. Rulison,a) Philippe F. Miquel,b) and Joseph L. Katzc) Department of Chemical Engineering, The Johns Hopkins University, Baltimore, Maryland 21218 (Received 14 February 1996; accepted 1 July 1996)
Earlier publications describe the counterflow diffusion flame burner and its unique capability to produce oxide particles having certain structures, such as spheres of one material coated with another, spheres of one composition with attached bulbs of another composition, and uniform multicomponent mixtures. Here we describe the production and properties of bulk quantities of powders produced using this burner. Measurements were made of specific surface area and, for titania, of phase composition. It was found that the controls over powder characteristics used in other forms of flame-synthesis are equally effective in the counterflow diffusion flame burner. We found that the specific surface area of both silica and titania powders decrease with increasing precursor concentrations. Transmission electron microscopy analysis of the titania powders indicates that the mean size of the particles that comprise these powders increases with increasing concentration. These trends are consistent with the collision-coalescence theory of particle growth. In addition, the crystalline phase of titania can be controlled by selecting the appropriate feed stream. For example, over the ranges TiCl4 precursor concentrations tested, feeding it only into the oxidizer stream yields mainly anatase TiO2 powders, while feeding only into the fuel stream yields mainly rutile TiO2 powders. These trends can be explained by the known atmosphere-dependent anatase-rutile transformation. The present data demonstrate that, in addition to its unique capability to produce certain particle shapes and morphologies, the counterflow diffusion flame burner can be manipulated to produce either of the major commercial titania phases, and also silica, with a wide range of specific surface areas.
I. INTRODUCTION
Titania and silica powders are used to produce paint opacifiers, catalysts, catalyst supports, ceramic membranes, fiber optics, liquid thickeners, varistors, capacitors, etc. New processes are needed to produce these powders at lower cost and without the environmentally detrimental side streams generated by traditional methods. For most applications, a specific particle size, morphology, or crystalline structure is required. For example, for use as light-scattering pigments, rutile (one of the crystalline forms of TiO2 ) particles with a narrow size distribution around 200 nm are desired.1 In catalysis, on the other hand, anatase (another TiO2 crystalline form) particles coated with a monolayer of vanadium oxide show higher activity and selective oxidation of o-xylene than do their rutile counterparts.2 a)
Present address: Microgravity Ventures, Space Systems/Loral, Mail Stop G19, 3825 Fabian Way, Palo Alto, California 94303-4604. b) P
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