Initial response and combustion behavior of microscale Al/PTFE energetic material by nanosecond laser ignition

Abstract

Al/PTFE reactive material has potential applications in defense and military due to its good thermodynamic properties and strong stability. To clarify its microscopic reaction process, the initial response and combustion behavior from microsecond up to several milliseconds are fully exhibited via laser-induced plasma spectroscopy and high-speed schlieren photographs while varying the ignition energy. Totally different plasma chemistry and combustion behavior of microscale Al/PTFE (mole ratio of 4:3) from those of pure Al are recorded. The fading of AlO and CN emissions and enhancement of C2 emission from the composite plasma with increase of laser energy prove that Al-F exothermic combination dominate the initial response since the decomposed gases of PTFE keep most Al away from air to be oxidized. The acceleration of laser-induced shockwave by a local high temperature reaction zone at the expanding plume front along the direction of laser beam is illustrated. As the plasma quenched and transformed to combustion at later stage, an ellipsoidal flame generally occurs above the surface for PTFE composite due to the upward move of Al vapor with PTFE decomposition gas while the combustion from pure Al exhibits a horizontal expansion along the surface. Self-sustained combustion of micro-Al/PTFE lasts around 1 ms is observed at extreme high energy of 1100 mJ. Inside flame plume, the transmission of decomposition matter and energy between different flame parts are clarified. Finally, a qualitative reaction model is built to successfully describe the response and combustion behavior after nanosecond laser ignition in detail. This study provides the support for the application of these insensitive reactive materials in laser ignition.