Controlling Microwave Energy Absorption in Aluminum Particle Compacts by Tuning the Crystallinity of the Alumina Shell

Abstract Microwave-to-thermal energy conversion in aluminum particle compacts can be controlled by modifying the phase and thickness of the native alumina passivation shell. In this study, the alumina shell surrounding aluminum nanoparticles was altered through thermal annealing in air at 400 °C for thermal treatment times of 30, 60, 90, and 120 min. Transmission electron microscopy (TEM) revealed shell growth from an average thickness of 2.76 nm to 9.63 nm with increasing annealing time. Thermal treatment also induced a transition from an initially amorphous shell to increasingly crystalline alumina, as confirmed by X-ray Diffraction (XRD). Compacts fabricated from the annealed powders were exposed to microwave radiation at 2.45 GHz for 5 min in an electromagnetic exposure chamber. Transient surface temperatures were recorded in situ using infrared thermograph. Compacts prepared from the as-received powders exhibited significantly higher steady-state temperatures than those made from annealed powders. The transition from an amorphous to a crystalline alumina shell resulted in an approximately 25 % reduction in steady-state temperature during microwave exposure. Microwave energy absorption is inferred from comparative thermal response under identical exposure conditions, rather than from direct measurements of the dielectric loss or absorbed microwave power. These results demonstrate that microwave heating of aluminum particle compacts is strongly influenced by the phase of the alumina shell and that shell crystallinity can be used to tailor material response to microwave energy.