CONTROL OF SIZE AND MORPHOLOGY OF NANO PARTICLES USING CO2 LASER DURING FLAME SYNTHESIS

An attempt has been made to control size and morphology of nanoparticles generating and growing in a co-flow oxy-hydrogen flame combining CO2 laser irradiation. The coalescence characteristic time could be controlled nearly independent of the collision characteristic time of particles by raising particle temperature rapidly with laser beam irradiation during flame synthesis. Silica aggregates generated by hydrolysis of SiCl4 in a flame are irradiated by a high-power CO2 laser beam, and rapidly heated up to temperatures that are high enough to enhance sintering, but lower than evaporation temperature. Sintering of aggregates is rapidly enhanced to change aggregates into more spherical particles. Since spherical particles have much smaller collision cross sections than volume-equivalent aggregates, much slower growth of particles is observed when the flame is irradiated by CO2 laser beam and as a result, the diameter of resulting spherical particles decreases to about 60% of the size observed without CO2 laser beam as the laser power increases. A higher precursor flow rate even resulted in the change of non-spherical particles into smaller spherical particles as CO2 laser power increased. Light-scattering measurement with an Ar-ion laser and TEM observation through a localized thermophoretic sampling were used to confirm the above effects of CO2 laser irradiation in a flame. Depending on the irradiation height of CO2 laser beam in a flame, significantly different mechanisms were found. The radial distributions of scattering intensity and morphological change were also studied. The proposed method controlling the sintering characteristic time of particles using CO2 laser irradiation in a flame seems to be promising to produce smaller and, at the same time, spherical nanoparticles even for high carrier gas flow rate.