Improved ignition and combustion performances of aluminum nanoparticles by in-situ partial conversion of native alumina to surface Al-MOF shell

Although aluminum nanoparticles (n-Al) as the promising metal fuel have attracted much attention, it still remains great challenge to efficiently extract chemical energy from n-Al. The energetic performance of n-Al is limited by native alumina layer, which slows down mass/heat transport, hinders sufficient ignition/combustion. In this study, we report the in-situ partial conversion of native alumina to surface Al-MOF shell (MIL-53(Al)), the resulted n-Al@Al-MOF has been confirmed by TEM, XRD and XPS. The effects of treatment conditions on the thermochemical reactivity, ignition/combustion of n-Al@Al-MOF and comparison samples have been investigated by TGA-DSC-MS, CO2 laser ignition, and constant volume combustion experiments. Tuning organic linker (terephthalic acid, H2BDC) amount and treatment time, the onset temperatures of n-Al@Al-MOF oxidation in air are lowered by about 18.0–38.0 °C than that of pristine n-Al (529.2 °C). Moreover, n-Al@Al-MOF with suitable content of surface Al-MOF shell exhibits remarkably improved ignition and combustion performances in comparison with n-Al, representing a shorter ignition delay time (tig), higher peak pressure (Pmax) and higher pressurization rate (e.g. 4 ms, 638.2 kPa, 3088.6 kPa/ms for n-Al@Al-MOF vs. 20 ms, 106.3 kPa, 349.8 kPa/ms for n-Al). Importantly, violent deflagration even occurs for n-Al@Al-MOF modified with appropriate surface Al-MOF shell. However, shortened tig, enhanced pressurization rate and deflagration behavior have not been observed by physical mixtures of n-Al with MIL-53 (or H2BDC). The surface Al-MOF shell should decompose as CO2 gases to form voids, playing unique roles in facilitating mass/heat transfer and improving ignition/combustion. In addition, n-Al@Al-MOF exhibits a decreased combustion heat but a good stability in air. This work provides a facile, mild and effective route to enhance the ignition and combustion performances of metal fuels.