UC Riverside

Solid propellants are a class of energetic (combustible) materials that undergo a rapid exothermic chemistry when an activation barrier is overcome. Applications for such energetic materials can be found in space exploration technologies, automobile airbags, material synthesis, ordinance and rare-earth mining. To broaden their applications and capability, various studies have been conducted to safely and controllably release their stored potential energy. These studies have mainly focused on manufacturing techniques to tailor the properties of energetic materials to release their stored potential energy in a predictable fashion. Other studies have focused on the use of electromagnetic stimulation in ultraviolet (UV), visible (vis) and near-infrared (NIR) regions to control ignition and combustion of propellants in-operando; however, the UV-NIR electromagnetic stimulation was observed to be limited to surface level absorption due to the inherent high photon attenuations of the solid propellants. In contrast to UV-NIR electromagnetic radiation, microwave radiation has shown to display rapid, selective and volumetric heating to control and modulate energy release pathways in solid propellants.

In this work, we first explore mechanisms that control the ignition of energetic materials at microwave frequencies. High-speed color camera imaging, infrared pyrometry, temperature jump (T-jump) ignition and differential scanning colorimetry methods are used to understand the mechanisms driving ignition in 3D printed nanoscale titanium (nTi)/ polyvinylidene fluoride (PVDF) films. This work is further expanded to engineer material systems with controllable ignition under stimulation at 2.45 GHz, where manganese oxide (MnOx) was studied as an oxidizing microwave agent. Beyond research on microwave ignition, the effect of microwave heating during in-operando combustion of reactive materials is investigated. First, heat transfer mechanisms influencing the combustion of energetic materials were experimentally studied via microscope imaging and pyrometry tools. Subsequently, infrared thermometry and color ratio pyrometry were employed to study microwave heating rate, flame front and propagation velocity of gasless Al-Zr-C composites. The research explores mechanisms that drive and impede response of solid propellants to microwave energy prior to ignition as well as throughout rapid energy release.