(632g) Burning Rate Control of Energetic Materials with Thermally Switchable Microwave Properties

The dynamic control of energetic material burning rate remains an elusive goal. Previous work has demonstrated that continuous/pulsed microwave irradiation of doped ammonium perchlorate composite propellants can enhance flame structure, bulk flame temperature, and increase propellant regression rates at atmospheric conditions.^1–3 In these efforts, increases in propellant burning rate were found to be in part a result of dielectric absorption in hot aluminum oxide combustion features of aluminum droplet diffusion flames and subsequent increase in heat feedback to regression surfaces. Energy deposition to hot metal oxide features are hypothesized to be a result of the exponential temperature dependence of aluminum oxide dielectric loss at propellant flame temperatures,⁴ where loss is due to aluminum oxide electron promotion to a valencene shell, resulting in 3000% more efficient energy deposition at 1500 K. While in these studies energy depositions was observed in burning aluminum agglomerates within the bulk of the propellant flame, more efficient combustion enhancement may be realized by localization of temperature-dependent deposition directly to an energetic material burning surface rather than utilization of indirect enhancement through thermal radiation feedback. Additionally, rather than exploiting thermal runaway effects of a combustion product (e.g. aluminum oxide), it may be possible to effect regression surface energy deposition through incorporation of materials in to energetics that have thermally switchable microwave properties.

To investigate the ability to control burning rate through microwave energy deposition directly to an energetic material regression surface, we formulate AP composite propellants containing nanostructured/nanoscale additives of (1) ball milled Al/MoO₃ nanostructured energetics or (2) graphene oxide nanoscale platelets. Ball milled Al/MoO3 nanostructured composite particles are hypothesized to effect microwave energy deposition to/near the burning surface through rapid ignition and production of aluminum oxide and flame temperatures in excess of the bulk propellant flame temperature. Separately, graphene oxide (GO) upon heating to propellant burning surface temperatures is converted from its microwave-reflective form of GO to microwave-absorptive reduced-graphene oxide (r-GO) structures^5–7 In order to explore the ability to enhance propellant burning rates, experiments will be conducted with continuous wave, 900 W and microsecond-pulsed (100 kW peak power) S-band microwave radiation propagated within a resonant cavity. Burning rate enhancement and near-burning surface flame temperatures are measured using high speed video and two-color pyrometry, respectively. Condensed phase dielectric properties of propellants containing Al/MoO3, GO, and r-GO as well as forward/reflected microwave power measurements are used to explore microwave absorption of the condensed phase. Additionally, the microwave ignition delay of GO and r-GO structures and the use of microwave energy as a deflagration ignition source using r-GO burning surface chars are explored. The prognosis for use of microwave energy deposition coupled with thermally tuned energetic material nanostructures as a means for use in energetic material ignition/ extinguishment/ reignition as well as regression rate throttling applications is discussed.

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 52nd AIAA/SAE/ASEE Jt. Propuls. Conf. AIAA (2016), pp. 1–11.

³ S.J. Barkley, K. Zhu, J.B. Michael, and T.R. Sippel, in 55th AIAA Aerosp. Sci. Meet. (2017), pp. 1–8.

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

⁵ O. Akhavan, Carbon N. Y. 48, 509 (2009).

⁶ M. Han, X. Yin, L. Kong, M. Li, W. Duan, L. Zhang, and L. Cheng, J. Mater. Chem. A 2, 16403 (2014).

⁷ L.N. Wang, X.L. Jia, Y.F. Li, F. Yang, L.Q. Zhang, L.P. Liu, X. Ren, and H.T. Yang, J. Mater. Chem. A 2, 14940 (2014).