Combustion acceleration in Al/CuO nanothermite films via gap-induced particle-advection mechanism

Al/CuO nanothermite films are prone to cracks/gaps that alter combustion behavior. This study investigates nanothermite films with engineered gaps, focusing on particle advection. Using high-speed microscopy (70,000 fps, submicron resolution), we find gaps induce a “V-shaped” reaction front, boosting the combustion rate by 6-fold (929 vs. 153.8 mm/s for gap-free films). Maximum enhancement occurs at gaps of 0–200 μm, where molten Al₂O₃/Cu particles (∼4.8 μm in diameter, ∼7 m/s in velocity) traverse the gaps to ignite unreacted regions, acting as the primary driver for accelerated combustion. Beyond 200 μm, enhancement diminishes as particles solidify upon cooling to the phase transition temperature, rebound upon reaching the opposite side, and ultimately cause ignition failure. A condensation model with experimental parameters as input explains this phenomenon: Al₂O₃ solidifies within the 200 μm range, while Cu solidifies within the 600 μm range. Simulation results confirm that combustion acceleration is significant before both particles are fully solidified, with the enhancement diminishing after Al₂O₃ solidifies and nearly no combustion acceleration occurring after Cu solidifies. These findings clarify the structure-property relationship of energetic films, laying a solid foundation for the regulation of ignition timing based on the gap-burning rate correlation. The prepared lightweight, thin, and coatable films are suitable for microdevices, further opening up a new path for the development of multi-timing complex logic ignition networks.