Effects of interfacial structure and catalyst addition on the auto-ignition behaviors of aluminum and ammonium perchlorate mixture in an optical rapid compression machine

Catalysts and interfacial structure modification are typically applied to resolve the problems of low combustion efficiency caused by agglomeration in aluminum (Al) and ammonium perchlorate (AP) propellants combustion scenarios. In this work, the auto-ignition behaviors of Al and AP mixtures with different interfacial structures and catalysts were investigated under rapid thermal stimulus by using an optical rapid compression machine (RCM). The ignition and non-ignition cases are distinguished by analyzing high-speed images and pressure curves. The critical ignition conditions and the ignition delay times (IDTs) are obtained and used to study the effects of interfacial structures and catalysts on the auto-ignition. Besides, thermal analysis and mechanistic discussions are further conducted to support the findings. Results show that the critical ignition temperature for all mixtures decreases with the increase of pressure. Mixtures with different interfacial structures show different auto-ignition sensitivities. The core-shell structured Al@AP are much easier to ignite than mechanically mixed Al/AP. Because Al particles in core-shell Al@AP can catalyze the decomposition of HClO₄ which is the intermediate product of AP, thereby reducing the activation energy of AP decomposition and increasing its low-temperature decomposition (LTD) stage. In addition, both CuO and Fe₂O₃ catalysts promote the ignition, which decrease IDT of samples 18 % and 15 % at 2.5 MPa and 960 K, respectively. And the thermal analysis results also show that both catalysts can significantly reduce the chemical activation energy and accelerate the LTD and high-temperature decomposition (HTD) stages of AP. Finally, the pressure dependence of core-shell structures on IDT is explored. The fitting results between pressure and IDT for core-shell structured mixture with different catalysts show that the addition of both CuO and Fe₂O₃ reduces the pressure sensitivity of Al@AP, and the addition of CuO has a more significant effect.