Review on microwave–metal discharges and their applications in energy and industrial processes

Although microwave heating or processing is rapidly emerging as an effective and efficient tool in various technological and scientific fields, the understanding of the interaction mechanisms between microwaves and various matters is still insufficient. For instance, in case of microwave–metal (MW–m) interaction, the phenomena and effects are far more complicated than the conventional microwave heating. Particularly, when metals with sharp edges, tips or submicroscopic irregularities are subjected to microwave irradiation, electrical discharges, including corona, spark or arc discharges, could occur and consequently trigger complicated effects like the appearance of hot spots, the plasma effect and the photo-catalytic effect. This paper focuses on this process and presents a critical review on the state-of-the-art theoretical background as well as the influencing factors of MW–m discharges, followed by a careful discussion of the special associated effects from the heating effect, plasma effect and potential photo-catalytic effect. An overview on the applications of MW–m discharges is provided and followed by the identification of the associated negative aspects and challenges. This work presents a format to better understand the MW–m discharge phenomena. It will be beneficial to extend the discharge phenomena and their featured effects into applications such as microwave-assisted pyrolysis, pollutants removal, material synthesis, and many other fields.