Development of next generation pyrotechnic emitters with dynamic, on-command selectable light emission characteristics requires a shift away from the “emission control exclusively via formulation” practice that is effective for passive emitter design. High-brilliance pyrotechnic emitters have also traditionally required addition of toxicological and environmentally harmful additives in order to achieve necessary color purity and brilliance. A number of studies have been previously conducted on microwave emission stimulation within a multimodal microwave cavity that demonstrates that with pyrotechnic flames containing weakly ionized species, microwave field irradiation can produce a ‘shift’ in light emission color, whereas in pyrotechnic flames containing strongly ionized species (i.e. flames with a high electron population), microwave field irradiation produces light amplification without color shift.

The objective of the project was to develop on-command microwave-selectable color and intensity emissions within a single pyrotechnic formulation by exploiting electromagnetic-flame interactions. The development of this technology will enable tunable, multi-color, and multi-purpose pyrotechnics, where environmental impact could be achieved via (1) reductions in size/weight of ordnance to produce desired photoemission impulse, (2) reduced inventories due to multi-purpose use, and (3) the ability to produce spectral photoemission from environmentally friendly formulations that, without microwave irradiation, are incapable of satisfying photoemission color purity/intensity requirements or through minimization of the use of environmentally harmful and toxicologically harmful additives.

This effort studies the microwave amplification of pyrotechnic light emission through combined experimental and computational approaches. Experiments are conducted on small pyrotechnic articles in free-space and multimodal microwave fields. Computational efforts have led to the development of a suite of capabilities capable of predicting the degree of microwave-induced light emission enhancement occurring from pyrotechnics. Additionally, a premixed, gas-phase, particle seeded burner has been developed that will be utilized in later studies to establish greater control over experimental conditions and to validate modeling efforts.

Efforts have demonstrated that microwave light emission enhancement can be extracted from a number of pyrotechnic formulations illuminated by a free-space microwave field and a number of different effects have been demonstrated. Notably, it was shown that using a small quantity of alkali dopant (up to 5 wt.%) in a magnesium/polytetrafluoroethylene pyrotechnic, one can drastically alter the time-stability of microwave enhancement, the chromaticity of the pyrotechnic flame, and the type of effect microwave illumination has on the pyrotechnic (luminosity enhancement or color switching), suggesting that many different colors and light emission characteristics may be possible from a single pyrotechnic formulation having small changes in type and quantity of dopant addition. High degrees of light amplification of 300% to 400% were found to be possible in boron containing formulations. Modeling capabilities have been developed toward the eventual goal of two-dimensional transient simulation of microwave modulated pyrotechnic emission. A one-dimensional model of electric field-modified pyrotechnic light emission was developed, from which several discoveries were made. The model indicates electromagnetic shielding in high electron population flames can be significant. The model also shows, counterintuitively, that atomic transitions of lower wavelength (higher energy) produce a greater degree of light intensity modulation. Degree of light amplification is found to optimize at higher field strength and for flames of lower flame temperature.

Taken together, these results mitigate much risk associated with pursuit of dynamic control of pyrotechnic light emission with electromagnetic fields, though additional study is necessary. The concurrent development of modeling capabilities is critical to understanding microwave enhanced emission, optimizing processes, would be helpful in application, and is also useful to the design of traditional, non-microwave enhanced pyrotechnics. A number of applications have been identified from this effort, which are described in detail within the Final Report.

References:

¹ Barkley, S. J., Miklaszewski, E. J., Dilger, J., Michael, J. B., and Sippel, T. R., “Microwave Enhancement of Flare Combustion,” AIChE Annual Meeting, 2016, pp. 1–29.