Evolution of particle temperature and internal composition for zirconium burning in air

An experimental study of single zirconium particle combustion in air is presented, with emphasis on understanding the mechanism cansing particle temperature jumps and the subsequent explosions. The experiment involved generating uniform, ∼200 μm-size particles of controlled initial temperature by a pulsed micro-arc, letting them burn in air in free fall, measuring their instantaneous temperatures with a three-wavelength optical pyrometer, quenching the burning particles during varions stages of burning, and subsequently analyzing the particles' internal compositions using a high-resolution X-ray electron microprobe. Special techniques were developed to separate ont the radiation from the gas-phase luminous zone in the temperature determination, and to rapidly quench the particles in order to freeze their internal composition. Results show that during combustion, the particle temperature first increases and then decreases: oxygen and nitrogen are dissolved in the particle interior whereas no oxide (or nitride) shells or inclusions have been detected. The maximum particle temperature is considerably less than the boiling temperatures of zr and zrO₂: It is demonstrated that the occurrence of the particle temperature jumps and explosions is due to the attainment of the eutectic state within the particle interior, which to the and explosions is due to the attainment of the eutectic state within the particle interior, which leads to the precipitous formation of solid or and rO₂ from supersaturated r-O solution, with simulataneous heat release and nitrogen gas release.