Phase-stabilized hexa(imidazole) nickel (II) nitrate Complex/Ammonium Nitrate Composite: eco-friendly chlorine-free oxidizer with high energy density and superior decomposition kinetics

Abstract Ammonium nitrate (AN) has gained attention as a promising non-chlorinated oxidizer for solid propellants. However, its practical application is limited due to its high hygroscopicity, low burning rate, endothermic decomposition, and phase transitions near room temperature. This study presents the development of an energetic hexa(imidazole) nickel (II) nitrate (Ni-Im) complex as an energetic phase stabilizer with novel catalytic characteristics. The Ni-Im complex was synthesized through a solvent-free approach, yielding a highly pure crystalline structure with a decomposition enthalpy of 267 J/g. The Ni-Im complex was integrated with AN via the cost-effective dry grinding method, developing an energy-dense phase-stabilized composite. The NiIm/AN composite developed intermolecular interactions eliminating near-room-temperature phase transition and shifting the first phase transition to 92°C. Additionally, the Ni-Im complex reduces the decomposition endothermic peak of AN by 22.2% and introduces an exothermic peak at 281°C. The Ni-Im complex exhibited a strong catalytic effect towards AN decomposition, reducing its decomposition apparent activation energy of AN decomposition by 39.7%, 30.7%, and 31.9% based on Kissinger, it-FWO, and it-KAS models, respectively. The Ni-Im decomposition generates porous nickel oxide nanoparticles serving as effective catalysts for exothermic gaseous reactions. These findings highlight the Ni-Im/AN composite as a promising halogen-free oxidizer for solid propellants.