Microwave Stimulation of Energetic Al-Based Nanoparticle Composites for Ignition Modulation

Although electromagnetic stimulation promises safe and controlled ignition of energetic materials, ultraviolet (UV) to infrared (IR) wavelength sources experience significant photon attenuations in energetic materials. Conversely, radiation in microwave frequencies is recognized for instantaneous volumetric heating capabilities. However, many energetics are poor microwave heaters, and to accelerate the heating, recent efforts focused on adding microwave susceptors that do not participate in the energetic reaction. This is the first effort to demonstrate that nanoscale aluminum (nAl)/manganese oxide (MnOx) can be rapidly heated at rates ∼104 °C/s under microwave radiation without addition of inert microwave susceptors. Detailed analysis of nanoscale MnOx was performed via X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The samples of MnOx at different loadings of nAl were three-dimensionally (3D) printed into composite films and tested with a microwave applicator at a 2.45 GHz frequency. Infrared thermometry experiments showed that with an increase in MnOx content the heating rate in the samples increases by orders of magnitude. Computational modeling based on the dielectric and thermophysical properties of the materials showed that an electric field is the dominant mechanism accounting for ∼96% of the heating of the nAl/MnOx composites at microwave frequencies. The microwave ignition mechanism was deconvoluted via high-speed IR imaging, in situ time-of-flight mass spectroscopy (TOFMS), temperature-jump (T-jump), and thermogravimetric analysis/differential scanning calorimetry (TGA/DSC) analysis. The results show that microwave stimulation can effectively heat and control ignition in nAl-based thermites with MnOx, where the oxidizer acts dually as a microwave susceptor and an ignition driver.