Laser-induced-breakdown-spectroscopy-based detection of metal particles released into the air during combustion of solid propellants.

Numerous metals and metal compounds are often added to propellants and explosives to tailor their properties such as heat release rate and specific impulse. When these materials combust, these metals can be released into the air, causing adverse health effects such as pulmonary and cardiovascular disease, particulate-matter-induced allergies, and cancer. Hence, robust, field-deployable methods are needed to detect and quantify these suspended metallic particles in air, identify their sources, and develop mitigation strategies. Laser-induced breakdown spectroscopy (LIBS) is a technique for elemental detection, commonly used on solids and liquids. In this study, we explored nanosecond-duration LIBS for detecting airborne metals during reactions of solid propellant strands, resulting from additives of aluminum (Al), copper, lead, lead stearate, and mercury chloride. Using the second harmonic of a 10-ns-duration 10-Hz, Nd:YAG laser, plasma was generated in the gas-phase exhaust plume of burning propellant strands containing the target metals. Under the current experimental conditions, the ns-LIBS scheme was capable of detecting Al at concentrations of 5%, 10%, and 16% by weight in the propellant strand. As the weight percentage increased, the LIBS signal was detected by more laser shots, up to a point where the system transition from being nonhomogeneous to a more-uniform distribution of particles. Further measurements and increased understanding of the reacting flow field are necessary to quantify the effects of other metal additives besides Al.