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.

A Theoretical Study on Gas-Phase Coating of Aerosol Particles

“…Equations [6] - [9], In the absence of surface reaction the results were compared to the simulation results by Friedlander et al (12) for [12a], [13], and [14] along with the auxiliary equations were solved simultaneously using the IMSL subroutine DIV-a purely scavenging process. This comparison is shown in Fig.…”

Synthesis of TiO2 nanoparticles containing Fe, Si, and V using multiple diffusion flames and catalytic oxidation capability of carbon-coated nanoparticles

“…Silica coating on titania nanoparticles is well known from previous literautre. Such coating could occur due to differences in chemical reaction rates of the precursors (Hung and Katz 1992). At small HMDSO concentrations, few and small SiO2 particles or aggregates can also deposit on the surface of titania particles (Powell et al 1997).…”

“…At small HMDSO concentrations, few and small SiO2 particles or aggregates can also deposit on the surface of titania particles (Powell et al 1997). Also, the oxidation of TTIP along with the formation of TiO2 is faster as compared with the oxidation of HMDSO for the formation of SiO2 (Hung and Katz 1992;Xiong and Pratsinis 1993). Hence, SiO2 ends up coating the TiO2 nanoparticles.…”

Flame-coating of titania particles with silica

“…16,17,20 For example, the oxidation rate of TiCl 4 is much faster than that of SiCl 4 and silica formed from SiCl 4 deposits on titania particles formed at lower temperatures. 16 This was observed in co-flow 17 and counter-flow 16 diffusion flames (ജ60 wt% TiO 2 ).…”