(526b) Metal-Rich Aluminum-PTFE Reactive Composite Material
Reactive materials combining aluminum with polytetrafluoroethylene (PTFE) with different compositions have been manufactured earlier. Expected advantages of such materials include high reactivity and greater volatility of high-temperature combustion products leading to reduced condensation and two-phase losses in propulsion systems. However, previously synthesized materials included substantial amounts of PTFE varied from 35 to 70 wt %, and thus have low density and low energy density. Materials with greater aluminum concentrations would have more attractive characteristics, but are difficult to prepare. In particular, mixing aluminum and PTFE is challenging because of substantial difference in their mechanical properties. In this study, composite Al·PTFE powders with 90 wt% Al are prepared using mechanical milling at 77 K, the liquid nitrogen temperature (cryomilling). Reference samples of the same composition are also prepared by room-temperature milling. Prepared powders are characterized using thermo-analytical measurements in both oxidizing and inert gas environments. The volatile species released upon heating are identified using mass spectrometry. Prepared and reacted materials are analyzed using electron microscopy and x-ray diffraction. Finally, the powders are ignited using different ignition stimuli. Stability and reactivity of the prepared powders are compared to those reported earlier for the mixtures of nano-aluminum and PTFE and for ball-milled Al-PTFE composites with 70 wt% Al. The mixing of PTFE is more homogeneous in the cryomilled than in room-temperature milled materials. PTFE is retained in both cryomilled and reference room-temperature milled materials upon their heating up to 820 °C in inert environment. Conversely, neat PTFE or PTFE included in the Al·PTFE composites prepared elsewhere decomposed at approximately 490 °C. In an oxidizing environment, cryomilled materials exhibit greater reaction rates as compared to room-temperature milled powders. The oxidation for all materials started well below the aluminum melting point. Finally, cryomilled samples can be ignited by electrostatic discharge (spark), whereas reference room-temperature milled materials cannot. However, when placed on a burning paper above a metal mesh, room-temperature milled materials ignited but their cryomilled counterparts did not.