Powder Temperature, Size, and Number Density in Laser-Driven Reactions

Abstract

A technique to accurately measure the temperature of powders in a laser-driven reaction has been developed. Particles are formed by heating reactant gases with a 150-W CO2 laser. The brightness temperature of the particulate cloud was measured with a micro-optical pyrometer. The emissivity was determined from scattering and transmission measurements. A correction for high turbidity was derived. The scattering and transmission measurements also allow the determination of the size and number density of the particles. The temperature and particle size as a function of height are reported for five silicon powder reaction conditions, and for one silicon carbide reaction. The measurements indicate that particles are often nucleated before the reactant gas has reached the CO2 laser beam. The reaction zone temperature decreases once most of the reactant gas is consumed, unless the produced powder absorbs 10.6 μm radiation as does SiC. The silicon particles nucleate as amorphous silicon, and then crystallize as they move into hotter regions of the reaction zone.