Development and Application of Microwave Technology - Part II

- Sep 17, 2020-

 (Continued)

2. Characteristics of microwave (I)
Microwave usually presents three characteristics: penetration, reflection and absorption. For glass, plastic and porcelain, microwave almost penetrates and is not absorbed; for water and food, it absorbs microwave and makes itself heat; for metal things, they reflect microwave. From the point of view of electronics and physics, microwave has the following important characteristics different from other bands:

1) Penetration
When microwave irradiates a dielectric object, it can penetrate into the interior of the object. This property is called penetrability. Microwave is the only electromagnetic wave (except light wave) that can penetrate the ionosphere in radio frequency spectrum.
Compared with other electromagnetic waves used for radiation heating, such as infrared, far-infrared, etc., microwave has longer wavelength and better penetrability. When microwave penetrates into the medium, the rise of the medium temperature caused by the dielectric loss makes the interior and exterior of the dielectric material heat up almost at the same time, forming a volume heat source, which greatly shortens the heat conduction time in the conventional heating, and the internal and external heating of the material is uniform.

2) Heating-selectivity
The ability of a material to absorb microwave is mainly determined by its dielectric loss factor. The material with large dielectric loss factor has strong microwave absorption capacity. On the contrary, the material with small dielectric loss factor has weak ability to absorb microwave. Due to the difference of dielectric loss factors of various materials, microwave heating shows the characteristics of heating-selectivity. Different materials produce different thermal effects. Water molecule is a polar molecule with high dielectric constant and dielectric loss factor. It has strong microwave absorption ability. However, the dielectric constant of protein and carbohydrate is relatively small, and its microwave absorption capacity is much smaller than that of water. Therefore, for food, the amount of water content has a great impact on the microwave heating effect.

3) Low thermal inertia
Microwave heating of dielectric materials is instantaneous, and the heating speed is fast. On the other hand, the output power of microwave can be adjusted at any time, the temperature rise of medium can be changed without inertia, and there is no "waste heat" phenomenon, which is very conducive to the needs of automatic control and continuous production.

(Translated from Elecfans Forum)

(To be continued)