Particle production in the gas phase is carried out either by gas-to-particle conversion or by droplet-to-particle conversion. In this study, the prediction of particle size and morphology was performed by modeling and experiments for these two conversion processes in the gas phase, and proposed the optimal operating conditions and control methods of particle size and morphology.
In order to investigate the effect of the particle transport and deposition to the reactor wall on the particle size distribution, a simple model for simultaneous aerosol growth by pure birth process and transport by the combination of convection, diffusion and thermophoresis in a tubular aerosol reactor are presented. At the initial stage of the growth, the size distribution becomes broader with residence time. However, when the residence time is sufficiently long, the size distribution becomes narrower. Thermophoretic and diffusional loss to the reactor wall make the size distribution broader when the residence time is short. Sufficiently long residence time is required to produce particles with narrow size distribution when thermophoresis and diffusion is significant. Controlling thermophoresis is more important than the diffusion since the thermophoresis gives a more profound effect on the growth than the diffusion.
Nanosized ZnO particles prepared by the oxidation of zinc vapor. This process was simulated by the coagulation model. Prepared nanosized ZnO particles have higher photocatalytic activities than nanosized $TiO_2$ particles.
In order to explain the decreasing of $TiO_2$ particle size with increasing the reactor temperature in the gas-to-particle conversion process, direct surface reaction(DSR) model was introduced. DSR model include the particle growth mechanism by surface reaction as well as the growth by coagulation. The particle size could decrease with increasing the reactor temperature in DSR model, while the particle size always increases with increasing the reactor temperature in coagulation model.
Droplet drying model was introduced in order to predict the particle morphology in the droplet-to-particle conversion process(spray pyrolysis). The simulation indicated that the hollow particles in spray pyrolysis is formed if the system pressure is low or the system temperature is high or the droplet number concentration is low or the carrier gas flow rate is high.
The particle morphology was controlled by changing the precursor type. For preparation of $Ni_x Zn_{1-x} Fe_2 O_4$ particles, the proper selection of precursor combination was nickel nitrate, zinc nitrate and ferric nitrate, and the particles had the maximum magnetic property at x = 0.5. For preparation of $Sr_5 (PO_4 )_3 Cl:Eu$ phosphor particles, pure halophosphate crystal phase was formed by excess chloride source, but excessive chloride source made the particles hollow.
The particle size and morphology were controlled by changing the solvent evaporation rate of droplet in the spray pyrolysis process. Extra water droplets were added into the system and less volatile liquid was mixed with solvent in order to change the solvent evaporation rate of droplet. The prepared particles had smaller particle size and denser morphology. The droplet drying modeling was performed for the prediction and the explanation for these phenomena.
Finally, the droplet collision frequency in the spray pyrolysis reactor was characterized with $Y_2O_3 :Eu$ particles. The droplet collision frequency data are useful to scale-up of spray pyrolysis process.
