Shock initiation of nano-Al/Teflon: High dynamic range pyrometry measurements

In this paper, laser-launched flyer plates (25 μm thick Cu) were used to impact-initiate reactive materials consisting of 40 nm Al particles embedded in Teflon^AF polymer (Al/Teflon) on sapphire substrates at a stoichiometric concentration (2.3:1 Teflon:Al), as well as one-half and one-fourth that concentration. A high dynamic range emission spectrometer was used to time and spectrally resolve the emitted light and to determine graybody temperature histories with nanosecond time resolution. At 0.5 km s^-1, first light emission was observed from Teflon, but at 0.6 km s^-1, the emission from Al/Teflon became much more intense, so we assigned the impact threshold for Al/Teflon reactions to be 0.6 (±0.1) km s^-1. The flyer plates produced a 7 ns duration steady shock drive. Emission from shocked Al/Teflon above threshold consisted of two bursts. At the higher impact velocities, the first burst started 15 ns after impact, peaked at 25 ns, and persisted for 75 ns. The second burst started at a few hundred nanoseconds and lasted until 2 μs. The 15 ns start time was exactly the time the flyer plate velocity dropped to zero after impact with sapphire. The first burst was associated with shock-triggered reactions and the second, occurring at ambient pressure, was associated with combustion of leftover material that did not react during shock. The emission spectrum was found to be a good fit to a graybody at all times, allowing temperature histories to be extracted. At 25 ns, the temperature at 0.7 km s^-1 and the one-fourth Al load was 3800 K. Those temperatures increased significantly with impact velocity, up to 4600 K, but did not increase as much with Al load. A steady combustion process at 2800 (±100) K was observed in the microsecond range. Finally, the minimal dependence on Al loading indicates that these peak temperatures arise primarily from Al nanoparticles reacting almost independently, since the presence of nearby heat sources had little influence on the peak temperatures.