Monitoring the flame synthesis of TiO2 particles by in-situ FTIR spectroscopy and thermophoretic sampling

The evolution of flame-made titania particles from spherical to weakly agglomerated ones has been studied in a premixed flame of methane and oxygen by oxidizing titanium tetraisopropoxide vapor at various concentrations. The axial and radial flame temperatures were measured by in situ Fourier transform infrared (FTIR) spectroscopy in the presence of particles. At each sampling location, the particle size distribution was measured by closely controlled thermophoretic sampling on transmission electron microscopy (TEM) grids, followed by image analysis and statistical evaluation of 200 to 350 primary particles. That way, particle inception, growth by coagulation and rapid coalescence, followed by agglomeration at the later process stages was visualized. Initially, the primary particle size increases rapidly up to about the end of the luminous flame, but then levels off. Limited particle aggregation was observed on the collected particles on the filter. The geometric standard deviation (ok) of the primary particle size inside the flame remained below the value for a self-preserving distribution, indicating the significance of surface growth and/or sintering. The crystallinity and specific surface area of the particles collected on filters were measured by X-ray diffraction and nitrogen adsorption, respectively. (C) 2001 by The Combustion Institute.