Shock initiation of nano-Al + Teflon: Time-resolved emission studies

Laser-launched flyer plates were used to impact-initiate reactive materials consisting of Al nanoparticles (50 nm) and Teflon microparticles (3 μm). The initiation process was probed with time- and wavelength-resolved emission spectroscopy. Teflon particle and Teflon foil samples without Al fuel were also studied. During ∼15 ns duration shocks produced by 50 μm thick flyers, Teflon was heated to a greater extent than Al. Al/Teflon underwent explosive chemical reactions lasting ∼500 ns at an impact velocity threshold of 1.0 ± 0.1 km/s. The Teflon samples emitted a continuum spectrum attributed to triboluminescence. C₂ emission (Swan bands) from the powder Teflon could be detected at impact velocities of 0.7 km/s, after a few microseconds, presumably from gases trapped between the decomposing sample and the observation window, indicating that Teflon can be at least partially decomposed to carbon and fluorine. During the 15 ns shock, the Al/Teflon had almost the same emission temporal and spectral profile and absolute emission intensity as Teflon alone. At the end of the shock, the Al/Teflon exploded twice in succession, creating intense emission bursts much stronger than those from Teflon alone. Each of the two explosions had an abrupt onset of ∼10 ns, and each lasted 100-200 ns. The first explosion was associated with the arrival of the compressive release wave and the second, whose emission burst was ∼1/3 as intense as the first, with the arrival of shear release waves from the flyer plate edges. For Al/Teflon to react, the Al fuel nanospheres must be released from their passivating oxide shells. The abrupt onset of the two explosions led us to propose a mechanism where tensile stresses resulting from compressive and shear waves cause the Al shells to crack open, allowing hot Teflon to react with Al. A limited number of studies using 25 μm thick flyers, which produced 8 ns shocks, show that the pressure threshold for initiation was about twice as large as with 15 ns shocks.