What Are Microwaves?

Microwave refers to the portion of the electromagnetic spectrum between 300 MHz and 300 GHz.
Microwave radiation travels at speed of light, i.e., c = 300000 km/s in free space and the relationship between wavelength (l) and frequency (f) is : l = c / f

Principle of Microwave Heating

To avoid conflict with communications equipment, several frequency bands have been set aside for industrial microwave processing. Traditionally Microwaves are used to heat materials that are electrically non-conducting (dielectrics) and composed of polar molecules. Polar molecules /domains have an asymmetric structure and align themselves to an imposed electric field. When the direction of the field is rapidly alternated, the molecules /domains move in synchronization producing heat in the material.

However in recent years, other aspects of microwave interaction with materials has been exploited to extend the microwave processing arena for applications that were once thought to be impossible like Powder Metallurgy. In this case the magnetic dipole movement and hysterisys losses leading to generation of heat is the dominant method of heating. The effect of eddy current losses also contributes significantly in such cases where the processed materials are electrically conductive. The ongoing research in this area is ever expanding the horizon of microwave processing applications.

When materials are exposed to microwave radiation, microwaves partially get Reflected (R), Absorbed (A) or Transmitted (T) depending on the dielectric properties of the material – Permittivity (ɛ) and Permeability (μ); which is a function of chemical composition, phase and temperature of the material. The absorbed portion of the incident microwaves, heats the material by polarization of the atomic / molecular structure or through dipole movement. As the microwaves travel through the material, it gets attenuated, resulting in volumetric heating.

Microwaves are generated by magnetron tubes, which are composed of a rod-shaped cathode surrounded by a cylindrical anode. Electrons flow from the cathode to the anode, creating an electric and magnetic field. The field frequency is a function of the dimension of the slots and cavities in the magnetron. Oscillations in the slots and cavities form microwaves, which are then radiated out through an antenna projecting out of the cathode space.

A Microwave Processing System Usually Comprises Four Components:

1. Generator. This consists of a magnetron, power supply and a waveguide assembly to carry the waves. Magnetron is typically water or air-cooled and is a consummable component.

2. Applicator. This is the processing chamber in which the material is processed. Waveguides direct microwaves to the product being heated in this space.

3. Control System System that monitors, measures, controls and manages the complete heating process. This is usually in the form of a computer, a PLC or an Human Machine Interface (HMI) with touchscreen.

4. Materials Handling System System that positions the product inside the applicator or exposure area. This is usually a conveying arrangement, a rotary table or a trolly system with associated controls to manage material handling into and out of the system.