Laser Synthesis Of Powders From Large Molecules

Abstract

Much of the early work in laser-driven powder synthesis has employed as reactants small molecules (especially hydrides) which have a limited range of potential reaction products. We have been interested in powder synthesis reactions beginning with larger molecular species, in order to access a wider range of reactivity and material compositions, and have found that the broader range of gaseous and solid products obtained provides insight into the details of the laser-driven synthesis process. We have dealt mainly with organosilicon compounds, using a cw CO2 laser for material synthesis and a pulsed CO2 laser to probe the reaction kinetics at short times. The cw reactions are carried out on a flowing gas stream at 145-4000 W/cm2 and an exposure time of 20 ± 10 msec. The pulsed experiments are done in a static cell at low pressure using laser pulse widths of 0.5 psec and intensities of 1 MW/cm2. The pulsed experiments provide data on the initial bond-breaking steps which occurs the reactant molecules heat up. We find that, even at ultimate reaction temperatures of 1500 K in cw experiments, the kinetics of the first reaction steps are important to product formation. We also find that quenching in the cw experiments is sufficiently rapid to allow back-calculation of the pyrolysis temperature from the composition of the light hydrocarbon products. The implications of these results for laser driven powder synthesis are discussed in terms of a general mechanism for reaction initiation, powder growth, and reaction quenching.© (1984) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.