Combustion analysis of three different thermites consisting of aluminum (Al) particles with and without surface functionalization combined with molybdenum trioxide (MoO3) was performed to study the effect of surface functionalization on flame propagation velocity (FPV). Two types of Al particles had self-assembled monolayers (SAMs) of perfluoro tetradecanoic (PFTD) and perfluoro sebacic (PFS) acids around the alumina shell, respectively; the other one did not. Flame speeds for Al with PFTD acid combined with MoO3 are 86% higher than Al/MoO3 whereas those for Al with PFS acid combined with MoO3 are almost half of Al/MoO3. The Al–PFTD structure is more sterically hindered and exhibits lower bond dissociation energy. This chemistry promotes increased flame speeds. Thermal equilibrium studies were performed using a differential scanning calorimeter and a thermogravimetric analyzer to determine activation energy (E a) of the thermites. Results are consistent with flame speed observations and showed an inverse relationship between flame speed and E a. This study shows that surface functionalization can be used as an approach to control the reactivity of Al particles.

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Coupling molecular scale reaction kinetics with macroscopic combustion behavior is critical to understanding the influences of intermediate chemistry on energy propagation, yet bridging this multi-scale gap is challenging. This study integrates ab initio quantum chemical calculations and condensed phase density functional theory to elucidate factors contributing to experimentally measured high flame speeds (i.e., >900 m∕s) associated with halogen based energetic composites, such as aluminum (Al) and iodine pentoxide (I2O5). Experiments show a direct correlation between apparent activation energy and flame speed suggesting that flame speed is directly influenced by chemical kinetics. Toward this end, the first principle simulations resolve key exothermic surface and intermediate chemistries contributing toward the kinetics that promote high flame speeds. Linking molecular level exothermicity to macroscopic experimental investigations provides insight into the unique role of the alumina oxide shell passivating aluminum particles. In the case of Al reacting with I2O5, the alumina shell promotes exothermic surface chemistries that reduce activation energy and increase flame speed. This finding is in contrast to Al reaction with metal oxides that show the alumina shell does not participate exothermically in the reaction.

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