Interface Reaction in Core–Shell Boron–FeF ₃ : Direct Observation of Gas Release and Oxide Removal during Combustion

Boron is a high-energy fuel, but its persistent B 2 O 3 surface oxide shell suppresses ignition and combustion. Here, we present a promising strategy for improving the performance of boron-based nanoenergetics. FeF 3 · x H 2 O-coated boron nanoparticles (B@FeF 3 · x H 2 O) are synthesized with a one-pot method and incorporated into 3D-printed thermite composites. Thermogravimetric analysis and T-jump ignition testing show that the FeF 3 · x H 2 O coating reduces the onset temperature of boron oxidation by >70 °C, while the combustion regression rates of 3D-printed thermites increased by up to 55%. High-speed digital inline holography and color pyrometry capture the in-flight particle fragments leaving the flame front and observe violent droplet explosions due to rapid gas release. T-jump time-of-flight mass spectrometry confirms HF and BF 2 evolution, indicating BF 3 gas generation and B 2 O 3 removal. These findings reveal that fluorinated coating enhances boron combustion via oxide etching and gas expansion, providing both mechanistic and strategic insights for boron-based nanomaterials.