Cyclic corrode and exfoliate combustion obstacle triggered by tetrafluoroborate to improve release energy

The passivation layer of metal oxide that forms from the exterior inward in the combustion process of metal fuel reseals the reactive fuel, resulting in inadequate energy release. Here, by introducing BF₄^- into the system and intervening in the metal combustion process, a cyclic reaction is activated that overcoming the regenerative passivation layer through in situ corrosion and desquamation. Microspheres (Al@BF₄^-/AP) composed of NH₄BF₄ and LiBF₄ containing BF₄^-, aluminum (Al), and ammonium perchlorate (AP) were synthesized utilizing the solvent evaporation self-assembly method. The direct effect of BF₄^- intervention is to impart flammability to Al@BF₄^-/AP microspheres, leading to a substantial increase in heat release in combustion process and a marked reduction in the size of combustion residues. The results indicate that the cyclic corrosion and desquamation originate from the thermal decomposition of BF₄^-, with the subsequently generated HBF₄ reacting with Al₂O₃ to form Al(BF₄)₃. Subsequently, Al(BF₄)₃ decomposes to form BF₃, which then reacts with water to regenerate HBF₄, initiating a new cycle that leads to corrode and desquamate. In addition, the combination of various cations with BF₄^- can result in distinct promoting effects, and eutectic salts can be tailored to meet the specific requirements of the propellant. This work provides practical solutions and innovative approaches to address the unavoidable combustion challenges that occur in situ.