Preparation of non-spherical nano-Fe₃O₄ and its effect on thermal decomposition of AP and combustion performance of composite fuels

Based on the aqueous sol-gel method, three types of nano-Fe₃O₄ particles with different microstructures were synthesized with FeSO₄ solution as the precursor. The structural characteristics of these Fe₃O₄ particles and their effects on the high-temperature thermal decomposition of ammonium perchlorate (AP) and the combustion performance of AP/aluminum (Al) composite fuels were analyzed with scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), surface area analysis, differential scanning calorimetry (DSC), and high-speed imaging. The results indicate that at a pH of 8, the synthesized nanoparticles comprise bulk and fibrous particles with an average particle size of approximately 70 nm, exhibiting some degrees of agglomeration. As the pH increases, the particle size shows a growing trend, with a gradual reduction in the quantity of fibrous particles. At a pH of 12, the morphology of the synthesized nanoparticles is predominantly bulk, characterized by an average particle size of 80 nm and no significant agglomeration; the bulk particles are identified as Fe₃O₄, while the fibrous particles are FeOOH. With increasing pH, the specific surface area of the samples exhibits a decreasing trend, with no significant differences in specific surface area observed between the samples prepared at pH 10 and pH 12. The addition of three different sets of Fe₃O₄ nanoparticles led to a noticeable reduction in the high-temperature decomposition peak temperatures of AP, which decreased to 363.82 °C, 389.46 °C, and 404.54 °C, corresponding to reductions of 89.2 °C, 63.56 °C, and 48.48 °C, respectively; no significant differences were observed in the low-temperature decomposition peak temperatures. The heat release of the samples was measured at 635.56 J g^-1, 1040.66 J g^-1, and 985.34 J g^-1, showing improvements of 60.08 J g^-1, 465.18 J g^-1, and 409.86 J g^-1 over pure AP. The maximum weight loss rates for the three samples were recorded at 363.91 °C, 388.76 °C, and 401.13 °C, corresponding to 69.1% min^-1, 18% min^-1, and 14% min^-1. The combustion process of the composite fuels was divided into five stages: ignition, flaming, steady burning, decay, and extinguishment. The ignition delay time of composite fuels containing the nano-Fe₃O₄ catalyst decreased as the content of nano-Fe₃O₄ increased from 0.5 wt% to 1 wt% and 2 wt%, decreasing to 130 ms, 114 ms, and 95 ms, respectively, compared to reductions of 2 ms, 18 ms, and 37 ms for the composite fuel without nano-Fe₃O₄. With the increased addition of nano-Fe₃O₄ particles, both the flame height and width during the steady burning stage increased significantly, resulting in a substantial enhancement of combustion duration. When the addition of nano-Fe₃O₄ reached 1 wt%, further increases in the particle content resulted in a stabilization of the flame area with no significant increases. Employing nano-Fe₃O₄ as a catalyst effectively enhances the combustion efficiency of composite fuels using AP as an oxidizer, suggesting good application prospects for nano-Fe₃O₄ particles as catalysts in solid rocket propellants.