Enhanced microwave heating of high-conductive liquids using gradient-permittivity impedance matching

Microwave heating is an efficient and environmentally benign thermal energy technology; however, its effectiveness is severely limited in the processing of high-conductive liquids. The significant difference in dielectric properties between air and sample leads to impedance mismatch, causing substantial microwave reflection at the interface. To address this issue, a gradient relative permittivity impedance matching technique based on pyramid array structure is proposed in this work. A high-efficiency microwave heating system integrating a metallic cavity and the proposed matching layer is developed to validate the heating performance. The underlying mechanism is analyzed through a two-dimensional infinite-interface model and further confirmed by multi-physics simulations. Experimental tests on liquids with diverse relative permittivity consistently yield heating efficiencies above 88%, thereby validating the accuracy of the simulation results. Moreover, the system's performance is evaluated under variations in liquid surface height and matching layer properties, demonstrating remarkable operational robustness and adaptability. The proposed design offers a scalable and efficient solution for microwave heating of large-volume, high-conductive liquids, facilitating the synergistic use of ionic liquids and microwave energy in chemical processes and advancing the development of emerging microwave chemical engineering technologies.