What Are Microwaves?

Microwaves are electromagnetic waves in the frequency band from 300 MHz to 300 GHz. Industrial microwave processing is usually accomplished at frequencies of 915 MHz or 2,450 MHz.

Microwave energy can cause an electric field to be generated. When this field interacts with other materials, certain responses are seen based on the intrinsic properties of the material:

• Conductors: ​

Electrons move freely in the material in response to the electric field. This results in an electric current. The average conductor has a certain electrical resistance within it. When electrons move through the material, they move against this resistance, which leads to heat generation. This phenomenon is called Resistive Heating and is the process that is used in a typical electric heating element such as a stovetop.

• Insulators

Electrons do not flow freely, but re-orientation or distortions of induced, or permanent dipoles, can give rise to heating.​

Electromagnetism and Microwaves

Microwaves are similar to sound; they travel in waves.

The similarities end here. Microwaves can travel through vacuums, such as space, in the absence of air. While sound waves create slight pressure differences as they travel, microwaves create electromagnetic waves, which can result in fields being generated.

Commonly occurring electromagnetic waves include:

• Light
• Radio station signals
• Cell phone signals
• Wi-Fi

Microwaves are largely reflected from metallic conductors, but interact well with dipoles (asymmetrically charged molecules), such as water. Microwaves are an efficient method of heating non-conducting materials know as dielectrics, which are not heated as efficiently by conventional convective methods.

Microwaves generate rapidly changing electric fields and dipoles rapidly change their orientations in response to the changing fields. If the field change is occurring near the natural frequency at which reorientation occurs, then a maximum utilization of energy is realized and optimum heating occurs. The material is then said to be ‘well coupled’ with the microwaves.

The material properties of greatest importance for insulators, or dielectrics, are the permittivity and loss factor (measures of the materials ability to absorb and store energy). Especially important is a factor termed tan delta-d (the materials ability to convert stored energy into heat). Hence, for optimum coupling a balanced combination of moderate permittivity and high loss/tan delta-d is required.

Microwave Power

At the microwave frequencies of 915 MHz or 2,450 MHz, the magnetron is the economic product of choice for the generation of the required power and frequency. The magnetron is a diode-type electron tube, which is used to convert high voltage DC power under the influence of strong permanent magnets into the microwave frequencies and power of choice. At 2,450 MHz, which is the frequency used at EWI, magnetron powers from 0.2-10 kW can be obtained. The EWI RP process uses arrays of magnetrons with variable power output to a maximum of 3 kW each, depending on the application.

If you’re just being introduced to the world of microwaves and their many uses, a lot of this may have been very complicated. Microwaves and the energy they emit are the basis upon which all of EWI’s units operate. They are incredibly powerful, precise, and can be used safely in the absence of high temperatures. EWI’s Reverse Polymerization process uses microwaves to reduce waste in many forms.