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 aggl omerated 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 transf orm infrared (FTIR) spectroscopy in the presence of particles. At each samp ling location, the particle size distribution was measured by closely contr olled thermophoretic sampling on transmission electron microscopy (TEM) gri ds, followed by image analysis and statistical evaluation of 200 to 350 pri mary particles. That way, particle inception, growth by coagulation and rap id coalescence, followed by agglomeration at the later process stages was v isualized. Initially, the primary particle size increases rapidly up to abo ut the end of the luminous flame, but then levels off. Limited particle agg regation was observed on the collected particles on the filter. The geometr ic standard deviation (ok) of the primary particle size inside the flame re mained below the value for a self-preserving distribution, indicating the s ignificance of surface growth and/or sintering. The crystallinity and speci fic surface area of the particles collected on filters were measured by X-r ay diffraction and nitrogen adsorption, respectively. (C) 2001 by The Combu stion Institute.