Investigation of Thermal Behavior in Combustion of Al/Fe3O4 Nanothermite Film

Abstract

Aluminum-based nanothermites are promising energetic materials for rapid heat release and microscale combustion applications, but the mechanisms governing their combustion morphology and flame propagation remain incompletely understood. This thesis investigates Al/Fe3O4 nanothermite thin films, focusing on two controlling factors: equivalence ratio (ER) and particle morphology. Al/Fe3O4 thin films provide a suitable nanothermite platform for studying the transition between destructive multiphase burning and near-gasless condensed-phase propagation due to their high energy release and relatively limited gas production. Specifically, the work examines how ER drives the transition between destructive particle-ejection combustion and structurally preserved near-gasless propagation, and how core–shell (CS) and physically mixed (PM) architectures modify the combustion event. Polymer-assisted Al/Fe3O4 thin films were fabricated with controlled ER values and particle morphologies. Their combustion behavior was characterized using synchronized high-speed optical imaging and infrared thermography, combined with post-combustion SEM/EDS analysis and inverse thermal modeling. For PM films, ER < 3 produced destructive combustion with particle ejection, localized hot spots, and partial removal of the energetic layer. In contrast, ER ≥ 3 led to preserved combustion, where the reacted layer remained attached to the substrate and formed a measurable cooling zone. This transition indicates that increasing ER improves condensed-phase continuity and shifts the combustion mechanism from reactive sintering to diffusion-driven reaction. Particle morphology also strongly affected flame propagation. At ER = 3, CS films propagated faster than PM films, with velocities of 11.93 cm/s and 6.11 cm/s, respectively. The inverse-modeled reaction rate of CS films was also more than twice that of PM films, indicating stronger reaction–transport coupling due to improved fuel–oxidizer contact and shorter transport distances. Cooling-zone analysis showed that preserved reacted layers act as thermal reservoirs, redistributing heat toward the reaction and preheating zones and contributing to flame-front stability. Overall, this thesis demonstrates that both ER and particle morphology can be used to tune combustion mechanism, propagation behavior, and post-combustion structure in Al/Fe3O4 nanothermite thin films.