Unagglomerated nanoparticles have attracted much attention from many researchers in various fields because of their inherent potentials and good sinterability of suppressing rapid grain growth during their sintering process. Production of the nanoparticles at high concentrations is of course needed for practical applications. Therefore myself and Prof. Choi proposed a new method for synthesis of unagglomerated smaller spherical nanoparticles at high concentrations utilizing laser irradiation on small aggregates in a flame.

The morphology and size of particles are determined by competition between collision and coalescence during gas-phase synthesis. When early-stage small aggregates are irradiated by high power laser beam to rapidly coalesce and become spheres, the spheres have much smaller collision cross-sections than the original aggregates. Therefore, much slower growth of nanoparticles can be achieved and we called this principle as ��Coalescence effect��. We applied this method to the synthesis of silica and titania nanoparticles.

For silica, we could reduced particle sizes maintaining their spherical shape by about 2 times. Also, we found that different mechanisms could occur depending on laser irradiation position from the burner surface and justified the results by measuring volume fraction and number density of particles in flame.

For titania, we could transform original aggregates to spheres with much smaller volume (25 times) and proved that the morphology change and size reduction was solely due to the coalescence effect. As for the controllability of crystalline phase, the laser irradiation yielded very interesting results: rutile contents in particles decreased with increasing the laser power.

This was understood by 1) melting and subsequent quenching at high laser power 2) cure of the oxygen-deficient defect or strains in the anatase particles at low power not enough to melt the particles. Raman scattering analysis and Fourier analysis for XRD peak broadening verified this. We are now applying this method to produce unagglomerate alumina nanoparticles and also control their crystalline phase.

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