(632f) Tuning of Energetic Material Microwave Enhancement through Micro/Nanostructure

Dynamic techniques to control the combustion of energetic materials are desired to enable in situ tuning of energetic output (e.g. solid rocket motor thrust and pyrotechnic emission). Previous work has demonstrated that microwave irradiation of aluminized ammonium perchlorate composite propellant flames of propellants doped with readily ionizing alkali earth metals (e.g. sodium in form of sodium nitrate, NaNO₃), can result in the enhancement of atmospheric pressure flame structure, bulk flame temperature, and burning surface regression rate without significant effect on equilibrium performance.^1–3 It has found that microwave energy is primarily absorbed by the flame structure rather than the condensed phase propellant and microwave irradiation is proposed to primarily result in enhancement through (1) plasma formation within the flame and (2) dielectric absorption in hot aluminum oxide features of aluminum particle diffusion flames, resulting in enhanced aluminum particle combustion. In these processes, plasma formation is precluded by sodium ionization and microwave energy absorption of free electron populations within the flame, and high energy loss to aluminum oxide features within aluminum flames are expected to be a result of the exponential temperature dependence of the aluminum oxide loss tangent at propellant-relevant temperatures.⁴ Together, these effects enhance heat feedback to the burning surface of the propellant, resulting in enhanced regression rate. In consideration of the overall flame structure of a composite propellant, the aluminum droplet diffusion flame, with a local motor-pressure flame envelope temperature of ~3800 K⁵ and hot aluminum oxide product stream having a temperature bounded by the oxide boiling temperature, is the highest temperature feature of the composite solid propellant flame. As such, it is expected that local, high temperatures of aluminum diffusion flames and direct aluminum diffusion flame heating, which may occur prior to buildup of critical plasma-level electron populations, may enhance free electron populations and result in reduction in both plasma kernel formation times and the minimum required dopant concentration.

To investigate plasma formation near aluminum particle combustion, we target sodium ionization to aluminum diffusion flames within burning propellants through development of Al/NaNO₃ mechanically activated composites. Ball milled Al/NaNO₃ composites are characterized via scanning electron microscopy and x-ray energy dispersive spectroscopy, X-ray diffraction, and calorimetry. Microwave-enhancement of atmospheric pressure burning rate, kernel formation time, and minimum required dopant concentration are investigated using 900 W continuous wave and microsecond-pulsed (100 kW peak power) S-band irradiation of propellant flame structures within resonant cavities. Propellants containing balled milled Al/NaNO₃ are compared to propellants of the same formulation having physically mixed ingredients. The degree of combustion enhancement and flame structure is observed using high speed video, two-color pyrometry, and UV-VIS spectroscopy, and time-resolved diode measurements of propellant flame microwave absorption to explore the ability to optimize microwave plasma enhancement through micro/nanostructure control of energetic materials within the model system of a composite solid propellant.

Bibliography

¹ S. Barkley, K. Zhu, J. Lynch, M. Ballestero, J. Michael, and T. Sippel, in AIAA Aerosp. Sci. Meet. (2016), pp. 1–9.

² S.J. Barkley, K. Zhu, J.B. Michael, and T.R. Sippel, in 55th AIAA Aerosp. Sci. Meet. (2017), pp. 1–8.

³ S.J. Barkley, K. Zhu, J.B. Michael, and T.R. Sippel, in 52nd AIAA/SAE/ASEE Jt. Propuls. Conf. AIAA (2016), pp. 1–11.

⁴ W.H. Sutton, Am. Ceram. Soc. Bull. 68, 376 (1989).

⁵ W.E. Price, in Fundam. Solid-Propellant Combust., edited by K. Kuo (AIAA, New York, 1984), pp. 479–513.