Formation of mixed oxide powders in flames: Part I. TiO₂−SiO₂

Abstract

Mixed oxide powders, e.g., Al₂O₃−TiO₂, SiO₂−GeO₂, and TiO₂−SiO₂, are used in industry to produce ceramics, optical fibers, catalysts, and paint opacifiers. The properties of these products depend upon the morphology of the powders. Ceramics and optical fibers are produced using either a uniform mixture of multicomponent particles or a uniform solution. The desired morphology for catalysts is a high surface area and many active sites. TiO₂ coated with a layer of SiO₂ is the desired structure for use as a paint opacifier. In this paper, TiO₂−SiO₂ mixed oxide powders were synthesized using a counterflow diffusion flame burner. TiCl₄ and SiCl₄ were used as source materials for the formation of oxide particles in hydrogen-oxygen flames. In situ particle sizes were determined using dynamic light scattering. A thermophoretic sampling method also was used to collect particles directly onto carbon coated grids, and their size, morphology, and crystalline form examined using a transmission electron microscope. A photomultiplier at 90° to the argon ion laser beam was used to measure the light-scattering intensity. The effect of temperature and of Si to Ti concentration ratio on particle morphology was investigated. Strong temperature dependence was observed. At high temperatures, TiO₂ particles were covered with discrete SiO₂ particles. At low temperatures, the structure changes to TiO₂ particles encapsulated by SiO₂. TEM diffraction pattern measurements showed that the TiO₂ is rutile and the SiO₂ is amorphous silica. At high Si to Ti ratios, SiO₂-encapsulated TiO₂ particles form. At low Si to Ti ratios, one obtains TiO₂ particles covered with discrete SiO₂ particles.