Unlocking controllable ignition and combustion in onion-structured Al@Na3AlF6@AP composites through Na3AlF6 “Temperature Switch”

The combustion of aluminum in solid propellants is inherently hindered by its surface oxide shell, leading to ignition delay, particle agglomeration, and low combustion efficiency. Herein, an onion-structured Al@Na 3 AlF 6 @AP composite is synthesized via a two-step evaporation-induced self-assembly process, sequentially depositing cryolite (Na 3 AlF 6 ) and ammonium perchlorate (AP) onto the aluminum surface. The Na 3 AlF 6 interlayer functions as a temperature switch inducing the transformation of the Al 2 O 3 into Al 6 NaO 9.5 at 700 °C and simultaneously exposing the active aluminum core. Therefore, the oxidation efficiency of Al@Na 3 AlF 6 is as high as 96.75 %, significantly surpassing the values achievable by pure aluminum and conventional aluminum alloys. Furthermore, the AP-encapsulated structure of Al@Na 3 AlF 6 @AP constructs a self-sustaining oxygen supply network during aluminum combustion and reduces the diffusion time of oxidative gases to the aluminum surface by 29.6 %. As a result, the size of aggregates during Al@Na 3 AlF 6 @AP combustion is approximately 10 μm, and the combustion heat (19.532 ± 0.133 MJ kg −1 ) is 8.4 % higher than that of the physically mixed sample Al@Na 3 AlF 6 /AP. The unique onion structure and Na 3 AlF 6 “Temperature Switch” both fundamentally reshape combustion kinetics, shifting the rate-limiting step from oxygen diffusion to aluminum oxidation activation.