The objective of this work was to characterize metal particle emissions from energetic material reactions, in particular from solid propellant combustion, using a non-intrusive optical diagnostic method. The primary focus was on the release of the metallic species of aluminum, copper, lead, and mercury during energetic reactions. The motivation for developing such diagnostic methods is that particulate matter released to the air during reactions of metal-based energetics and pyrotechnics can cause adverse health effects, such as pulmonary and cardiovascular disease, particulate matter-induced allergy, and cancer. In particular, Department of Defense employees working in test ranges, disposal sites, regular warfighters as well as general public in exposed areas are vulnerable to these harmful effects.
The primary technical approach used for this research study was laser-induced breakdown spectroscopy (LIBS). LIBS is a widely used, robust elemental analysis technique, in which high-intensity laser pulses are used to generate a localized plasma in the medium where the composition is to be detected. Light emitted during ionization-recombination following this plasma is then collected and dispersed using a spectrometer onto a charge-coupled device array. The elemental composition can be determined based on the characteristic spectral lines detected and their relative intensities. Two LIBS schemes were used during the current experiments: one using a traditional 10-nanosecond (ns) pulse-duration, 10-Hz repetition-rate, neodymium-doped yttrium aluminum garnet (Nd:YAG) laser, and the second scheme which utilized an 80-femtosecond (fs) pulse-duration, 1-kHz repetition-rate, amplified Ti:Sapphire laser system. To the best of the research team's knowledge, this was the first-time implementation of the fs-LIBS scheme for elemental species detection in hot, gas-phase reaction media. Several advantages of using fs-LIBS in such environments were hypothesized in the current study.
The results of traditional ns-LIBS-based approach as well as novel fs-LIBS scheme were discussed. Before attempting to detect the metallic species in the gas-phase exhaust region during the combustion of laboratory-scale propellant sticks, initial experiments of laser pulse energy dependence and plasma decay time were performed using solid target plates of Al, Cu and Pb. These initial experiments were conducted to determine the optimum laser parameters as well as to optimize the signal collection apparatus. Subsequent experiments were conducted during combustion events of hydroxyl-terminated polybutadiene/ammonium perchlorate (HTPB/AP) propellant samples doped with known quantities of above metals, in particular micron-size Al, Pb (from base metal as well as lead stearate [(C17H35COO)2Pb], a common additive for altering the reaction rate), Cu, and Mercury Chloride (Hg2Cl2).
The ns-LIBS scheme was capable of detecting Al LIBS signal corresponding to the samples with predetermined quantities of Al in the 5–16% range by mass. An aluminum metal concentration study was also performed, which showed that a propellant strand with a higher mass percentage of aluminum is more likely to have a LIBS signal until up to a point where the gas-phase reaction zone begins to act like a homogeneous medium. Subsequently, a comparison of LIBS detection between a ns Nd:YAG laser and fs Ti:Sapphire laser was also performed. While the LIBS scheme using the 10-ns, 10-Hz Nd:YAG laser pulses could not detect any other metal species besides aluminum, the 80-fs, 1-kHz Ti:Sapphire laser was able to detect characteristic signals from all the metallic additives at percentage concentration in the range of 2–16% by mass in the initial solid propellant mix. Methodologies for further enhancement of detection limits, quantitative concentration determination, as well as better characterization of the reacting particle flow field are discussed.
The primary conclusion of this project was the enhanced detection sensitivity of fs-LIBS scheme for airborne metal particles detection and the proof-of-concept demonstration in selected set of experimental conditions. The research team has also identified several critical research areas needing detailed follow-on research to make this technology a viable option for on-field applications. A primary requirement is a method for simultaneous characterization of particle flow field (i.e. particle size, position, velocity and concentration) during metal speciation. For that, the researchers need to discuss incorporating a newly developed optical technique, digital inline holography (DIH) in partnership with Sandia National Laboratories. In addition, further requirements for quantitative concentration determination, plans for generating extended data sets simulating real world conditions for complex thermodynamic model validation, as well as conceptual ideas for eventual on-filed diagnostic platforms are outlined.