Micro-aluminum powder with bi- or tri-component alloy coating as a promising catalyst: Boosting pyrolysis and combustion of ammonium perchlorate

A novel design of micro-aluminum (μAl) powder coated with bi-/tri-component alloy layer, such as: Ni–P and Ni–P–Cu (namely, Al@Ni–P, Al@Ni–P–Cu, respectively), as combustion catalysts, were introduced to release its huge energy inside Al-core and promote rapid pyrolysis of ammonium perchlorate (AP) at a lower temperature in aluminized propellants. The microstructure of Al@Ni–P–Cu demonstrates that a three-layer Ni–P–Cu shell, with the thickness of ∼100 nm, is uniformly supported by μAl carrier (fuel unit), which has an amorphous surface with a thickness of ∼2.3 nm (catalytic unit). The peak temperature of AP with the addition of Al@Ni–P–Cu (3.5%) could significantly drop to 316.2 °C at high-temperature thermal decomposition, reduced by 124.3 °C, in comparison to that of pure AP with 440.5 °C. It illustrated that the introduction of Al@Ni–P–Cu could weaken or even eliminate the obstacle of AP pyrolysis due to its reduction of activation energy with 118.28 kJ/mol. The laser ignition results showed that the ignition delay time of Al@Ni–P–Cu/AP mixture with 78 ms in air is shorter than that of Al@Ni–P/AP (118 ms), decreased by 33.90%. Those astonishing breakthroughs were attributed to the synergistic effects of adequate active sites on amorphous surface and oxidation exothermic reactions (7597.7 J/g) of Al@Ni–P–Cu, resulting in accelerated mass and/or heat transfer rate to catalyze AP pyrolysis and combustion. Moreover, it is believed to provide an alternative Al-based combustion catalyst for propellant designer, to promote the development the propellants toward a higher energy.