The objective of this project was to assess the use of three different optical diagnostic techniques for the standoff detection of hazardous combustion products produced from the reaction of energetic and pyrotechnic formulations. By performing laboratory scale testing for proof-of-concept, useful data will be obtained for comparison with thermochemical calculations and insight toward the miniaturization and simplification of the diagnostic techniques for eventual application in a field-ready system.
A four-Liter combustion chamber was instrumented for the simultaneous application of three optical diagnostic techniques: ultraviolet-visible (UV/VIS) spectroscopy, light extinction, and laser induced incandescence. The techniques were selected because they are optical, non-intrusive, and can be used to characterize the atomic and molecular species, soot, and condensed phase particles present in the combustion products of metal-based energetic and pyrotechnic formulations. High-speed UV-VIS emission spectroscopy performed using a spectrometer-coupled streak camera was used to assess the production and lifetimes of metal species of interest during the reactions. Light extinction measurements provide a qualitative assessment of soot production as a function of time. Laser induced incandescence allows the characterization of particle size distribution of metals/soot over time during a combustion event.
Samples of BKNO3, IM-23, IM-68, and Hercules Red Dot were initiated at atmospheric pressure using an electric match. An ultrafast laser system was used to provide the input source for both light extinction and laser induced incandescence measurements. For light extinction measurements an integrating sphere and photodiode were used to detect the transmitted laser intensity through the reaction path. Sheet forming optics were used for two-dimensional laser induced incandescence (LII) measurements and the signal was detected in the direction perpendicular to the laser sheet using an imaging detector and laser line filter. A fiber optic cable and collimator enabled signal collection for the spectroscopic measurements. In addition to experimental work, the thermochemical code CHEETAH was used to determine the combustion products and temperature of the energetic material reactions.
Spectroscopic measurements were consistent with CHEETAH calculations of combustion products. The emission spectrum obtained from BKNO3 contains atomic lines of potassium at 766.4 and 769.8 nm as expected. Measurements from IM-23 show atomic lines of magnesium and potassium. The emission spectrum from IM-68 shows features from diatomic oxygen, nitrogen, magnesium, and barium. Measurements of the Red Dot reaction show nitrogen and cyanide. In addition spectra obtained from IM-23, IM-68, and Red Dot all have contamination from sodium which is evident by the emission line at 588 nm. Sodium is a common contaminant in energetic materials.
Light extinction measurements and high speed video recording of the reactions showed materials with more condensed phase products caused greater attenuation of the laser signal. Results showed that the fastest reaction is complete in ~0.02 seconds, indicating a data acquisition rate of 500 Hz should be sufficient for future measurements.
For the laboratory proof-of-concept LII measurements the detector repetition rate was too slow to resolve the signal. The 5 kHz pulsed laser system and speed of the energetic material reactions caused difficulty for adequate signal detection to record the LII signal decay and determine particle size. However, it was clear from the recorded images that additional information is obtained when compared to the standard high speed video images. In the LII images fine smoke particles are visible which are otherwise not detectable by the standard high speed camera. Based on the laser source parameters and detection scheme for this experimental setup we can visualize fine smoke particles as small as 0.3 µm in diameter.
This optical methodology for emissions characterization will enable standoff detection of hazardous combustion products, providing data regarding metal species and soot generated as a function of time. The techniques used can be readily integrated into a ruggedized field-use system that is mobile and compact. Measurements by such a system would allow the Department of Defense (DoD) to model and predict potential range contamination, occupational exposure levels on ranges, and the migration of airborne contaminants. This also may serve as a methodology to assess new technologies employed for reducing the long-term health effects for DOD firing ranges.