Numerical Study of the Nanoparticle Formation Mechanism in a Titania Flame Combustion Synthesis Process

In this study, a monodisperse particle formation (MPF) model was built and a five-zone diagram (FZ diagram) was used to examine the Titania (TiO2) combustion synthesis process. The effects of chemical reactions, Brownian motion, sintering reactions and diffusion were considered, while the polydispersity of aggregates and primary particles were neglected in the MPF model. The effective collision frequency was used to modify the collision frequency influenced by van der Waal interactions. There were precursor-heated, chemical reaction/nucleation, high-temperature, coagulation/coalescence and aggregation zones in FZ diagram. Results of the FZ diagram as well as particle size (d(p)) were investigated via three parameters in the MPF model: particle number density (N), total aggregate volume per unit mass of gas (V) and total aggregate surface area per unit mass of gas (A). The inlet oxygen/nitrogen ratios (O-2/N-2) change from 20/80 to 50/50 will enhance the high-temperature zone, which increases the collective particle sizes from 81.4 to 120.9 nm; the increasing Titanium isopropoxide (TTIP) concentrations (X-TTIP) will also increase the particle sizes from 85.7 to 99.3 nm, due to the reinforcement in the chemical reaction/nucleation zone. The particle sizes increase rapidly as the height of particle collection becomes higher, which showed an important factor about choosing a flame type to synthesis particles that are small enough.