Microwave Power Absorption Mechanism of Metallic Powders

Microwave processing of metallic powders, both in multimode and single-mode cavity, has attracted wide interest. However, the mechanism of interaction between microwaves and metal powders is not yet very well understood. In this paper, the microwave power absorption mechanism in metallic powders has been systematically studied. For this, mixtures of copper and alumina powders have been processed in a 2.45-GHz microwave single mode cavity in either air or forming gas (N2 + 5 wt% H2). In the nominal pure magnetic field (H-field) region in the single-mode microwave cavity, pure copper powder cannot be heated in the reductive (forming gas) atmosphere while it will heat in air atmosphere. Although neither alumina nor copper separately heat up in forming gas in the H-field, mixtures of copper and alumina powders heat up readily. These behaviors can be explained by the presence of oxide layers found on many metallic particles, which play a vital role in the microwave heating of metal powders. The oxide layers isolate the particles and confine the electric current to the surface of every particle, thereby allowing microwaves to penetrate into the sample. Since the particle size is quite small and the surface area is high, the heat produced by the sum of the surface currents on all of these particles is significant. Concurrently, the oxide layer on each metal particle may act as thermal insulation material to help maintain the temperature as the metal particles heat up since many metal oxides have low thermal conductivity. We propose a new mathematical model and derive the power dissipation equation which provides the fundamental basis for the analysis, calculation, and simulation of heating of metallic powders in microwaves.