High-speed visualization of metal oxide precursor in thermal plasma flow during nanoparticle formation

The objective of the present study is to understand formation mechanism of metal oxide nanoparticles in thermal plasma. High-speed camera technique with appropriate band-pass filters was applied to visualize the precursors of the metal oxide nanoparticles. A multiphase AC arc was generated under atmospheric pressure of air with aluminum raw powder injection to synthesize aluminum oxide nanoparticles. Optical emission spectroscopy of the multiphase AC arc was performed to select the transmission wavelengths as 514.5 nm and 670.0 nm for emissions from aluminum vapor and aluminum monoxide, respectively. Dynamic behavior of aluminum vapor and aluminum oxide was successfully visualized by the high-speed camera observation with these band-pass filters. Relative intensities of aluminum emission to argon emission as well as aluminum oxide emission to argon emission were calculated to estimate relative number densities of aluminum vapor and aluminum oxide. Results clearly revealed that aluminum vapor mainly existed at the higher temperature region than 4,000 K, while aluminum oxide became dominant at the lower temperature region. These obtained results experimentally elucidated that aluminum oxide nucleates at first, and then aluminum oxide condenses onto the nuclei.