A: Microwave heating of a solid material takes place primarily by energy conversion. The material as a whole absorbs microwave radiation (generated by a magnetron, a high-power vacuum tube producing electromagnetic radiation in the range of 1-40 gigahertz) and converts it into heat. Only materials that absorb microwaves are heated, and the material itself becomes the heat source in the process. In conventional heating, heat is transferred from an external source to the material, with heating occurring from the surface to the inside of the material via thermal conduction.

In electrically nonconductive (dielectric) materials, microwave energy that is converted to heat is called dielectric loss. Dielectric "lossy" materials resist change in their dipole movements, resulting in the formation of heat. Due to their excellent dielectric losses, water, many ceramics, composites, polymers, food, and wood products absorb microwaves and are heated very rapidly and effectively. Metals convert microwave energy to heat by magnetic loss mechanisms due to eddy currents generated.

Heating metals in a microwave leads to different results because metals are electrically conductive. Heating may or may not occur, depending on the form of the metal. Bulk metals (such as aluminum foil and solid steel) at room temperature reflect microwaves due to low penetration depth, setting up a high voltage between the metal and the magnetron. When this voltage surpasses a threshold, it discharges as a visible spark. For that reason, it is advised not to put metal articles into microwave ovens. However, metal powder at room temperature absorbs microwave radiation and is heated and melted very effectively. The actual mechanism of microwave absorption by, and heating of, powdered metal is not fully understood, but theoretical models indicate that if the metal particle size is less than 100 micrometers the particle will absorb microwaves.

Dinesh Agrawal
Professor of Materials
Director of Microwave Processing and Engineering Center
The Pennsylvania State University