Confined flame propagation of Al/PTFE mechanically activated composites

The structure of mechanically activated (MA) energetic composites are complex, having at the same time intra-particle reactive length scales as small as sub-micrometer and inter-particle length scales as large as 1 mm. This work investigates the combustion propagation of MA aluminum and polytetrafluoroethylene (PTFE) while varying as-milled particle size, stoichiometry, particle bed density, and confinement to better understand combustion propagation. Propagation rates of MA Al/PTFE are four orders of magnitude slower than similar physically mixed nano-Al/PTFE or micron-Al/PTFE compositions reported elsewhere. Combustion of MA composite beds is found to be macroscopically stable within quartz tubes of >3 mm diameter, though microscopic relay-race combustion is observed in which slow combustion transfers between particles preclude rapid intra-particle combustion. Confined burning rate is increased with reduction of packing density below 55% TMD and burning rate is insensitive to tube end confinement. A maximum burning rate is observed at Al/PTFE ratios of 50/50 wt.%, corresponding to conditions predicted to maximize gas production. Packed beds containing larger (> 75 µm) particles produced 50% faster burning rates than smaller (< 25 µm) particles, and absence of confinement increases burning rate by a factor of ten. Taken together, these results show that the propagation of MA Al/PTFE composites contradicts combustion trends of either purely micrometer-Al/PTFE or nanometer-Al/PTFE packed beds. Findings of this work are supportive of separate results in which laser ignition of single MA Al/PTFE particles results in MA particle microexplosion. It is hypothesized that MA Al/PTFE packed bed combustion is controlled by the rate enhancing effects of microexplosion-induced forward propagation of burning ejecta within packed beds, leading to maximum burning rates in conditions which promote ejecta propagation. The findings of this work provide insight into the combustion physics of MA Al/PTFE and help inform use of MA composites in energetic formulations.