Interface Reaction in Core-Shell Boron-FeF₃: Direct Observation of Gas Release and Oxide Removal during Combustion

Boron is a high-energy fuel, but its persistent B₂O₃ surface oxide shell suppresses ignition and combustion. Here, we present a promising strategy for improving the performance of boron-based nanoenergetics. FeF₃·xH₂O-coated boron nanoparticles (B@FeF₃·xH₂O) are synthesized with a one-pot method and incorporated into 3D-printed thermite composites. Thermogravimetric analysis and T-jump ignition testing show that the FeF₃·xH₂O 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 BF₂ evolution, indicating BF₃ gas generation and B₂O₃ 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.