Effect of covalent organic framework COF-43 on the thermal decomposition and phase transition of ammonium nitrate

• COF-43 completely suppresses ammonium nitrate's detrimental low-temperature phase transitions (AN IV ↔AN III & AN III ↔AN II ), addressing the key instability hurdle for its use in propellants. • At a 10 wt% loading, COF-43 catalytically shifts AN decomposition from endothermic to exothermic, while a 5 wt% loading only lowers the onset temperature. • COF-43 serves as a dual-functional additive, providing simultaneous phase stabilization and catalytic enhancement of AN decomposition. • COF-43 acts as a dual-functional, metal-free additive, providing simultaneous phase stabilization (via nanoconfinement) and catalytic enhancement for AN. • The decomposition onset temperature is reduced by 17 °C and the T₅₀% by 21 °C for the 10 wt% composite, indicating dramatically accelerated kinetics. Ammonium nitrate (AN) is a promising chlorine-free oxidizer for green solid propellants (GSPs), but its practical application is limited due to undesirable polymorphic phase transitions at low temperature and endothermic decomposition. This study introduces a covalent organic framework (COF-43) as a multifunctional additive to simultaneously address these issues. AN/COF-43 composites with 5 and 10 wt% loadings were prepared via a co-solvent method and characterized using XRD, FT-IR, FE-SEM, and TG-DSC. The results show that COF-43 completely suppresses the AN IV ↔AN III and AN III ↔AN II transitions up to 125 °C and eliminates the associated volume change that causes microcracking in propellants. Furthermore, COF-43 catalyzes the thermal decomposition of AN, reducing the onset temperature by 17 °C and changing the decomposition pathway for the 10 wt% composite from endothermic to exothermic. These dual function-phase stabilization and catalytic decomposition-are attributed to the nanoscaling effects and interfacial hydrogen bonding between AN ions and polar functional groups of COF-43. This work highlights the potential of COFs as advanced additives for the development of high-performance and phase-stable AN-based green solid propellants (GSPs). The metal-free composition and clean combustion profile of COF-43 distinguish it from conventional metal-based additives, offering a promising pathway toward environmentally sustainable AN-based propellants.