How does it work...

Frequency - Penetration of the waves

  • Consequences

Thermal technology and properties of materials

  • Waves frequencies

  • Temperature effect

  • Heating time

Consequences on the control of microwaves

FREQUENCY - PENETRATION OF WAVES


Microwaves are electromagnetic waves whose material heating frequencies are attributed by an International Committee.

  • The frequency of 2.45 GHz (2450 MHz) can be used freely throughout the whole world, which explains its mass distribution (200 million microwave ovens, for example). Its wavelength, in free space, is about 12 cm (the speed of light divided by the frequency). It is reduced by Öe in a material with permittivity e.
  • The frequency of 915 MHz is only freely usable in North America In Europe, its use is severely regulated which places strict limits in its use. Its wavelength, in free space, is about 32 cm.

Consequences

Since the penetration of a wave into a material is of the same order as its wavelength, we conclude immediatly that: 

  • The penetration of microwaves in expressed in cm or dm,
  • The penetration of waves of lower frequencies (HF, MF, LF) is expressed in m,
  • The penetration of light and infra-red waves in expressed in microns.

Consequently, materials to be treated which have widths expressed in cm or dm are coupled best with microwaves.

Materials of larger volume couple best with lower frequencies (HF or radiofrequencees, MF etc).

Very thin materials or materials requiring surface treatment are to be treated by light waves (IR or light).

THERMAL TECHNOLOGY AND PROPERTIES OF MATERIALS

Electromagnetic waves with frequencies limited to a few tens of GHz display their action by their electric field. We are in the field of ELECTRICITY.
With higher frequencies, we are in the field of OPTICAL and then in IONISING RAYS.

In the field of "electric waves", the alternating electric field operates mainly:

1 - By acting on the conducting electric charges (electrons, ions), which produces a friction affect on the charges (Joule effect).

2 - By acting on the molecules known as "polar", which are then oriented in the electric field like a compass in a magnetic field (dipolar effect). Energy is also consumed by a dipole friction image effect (Dissipation by relaxation) .

These 2 types of electrical energy transformation can be also transformed into absorption by the material, thus causing internal heating which depends on: 

  • the type of material,
  • the frequency of the excitation waves,
  • on the temperature.

Waves frequencies

As concerns the frequency of the waves being used, the conduction effects (Joule effect) decrease regularly as the frequency increases. They become rather weak in microwaves but can be intensified if high charges are introduced into the material (rubber).

This phenomenon of relaxation only exists in the "dipolar relaxation bands", where logically the lower the molecular mass, the higher the frequency. This phenomenon starts to come into existence as from 100 MHz; it is at its maximum in the microwave frequencies.

 

Effective lost factor of a hererogeneous dielectric exhibiting 

dipolar and tail end conductivity losses (d'après METAXAS, 1983)
 

For example, we know that the molecules of water are dipolar in nature (connected with the assymetry of the H-O-H structure) and of very low mass. Their dipolar relaxation band therefore has intense microwave absorption characteristics (in excess of 1 GHz), notably for very hydrated products (IAA, for example).

Temperature effect
Since the thermal agitation of the molecules or the electrical charges is added to the excitation electrical energy (by conduction or by relaxation), its additional effect logically moves all the absorption curves towards higher frequencies. This "slippage" of the curves in particularly beneficial if the excitation frequences of the electric field are in the microwave region R (the standardised frequency of 2.45 GHz in this region). In fact, when the temperature of a mixed product (consisting, for example, of a solid in suspension in water) increases, the dielectric absorption of water decreases strongly, whereas that of the solid has only a slight variation (or increases a little).
Under these conditions, the whole product (liquid then solid) will be successively heated by the microwaves.
This unique property of microwaves at about 2.45 HGz can lead either to an increased homogenity of heating for products with a high water content, or to a thermal runaway or even a denaturation of the solid for products with low water content.

Heating time
Since the transformation of electrical energy into heat energy takes place inside the material using the processes that we have described, the time taken for the increase in temperature is very rapid with microwaves, starting immediately (oblique tangent) contrary to other traditional heating processes using a hot wall (horizontal tangent).
CONSEQUENCES FOR THE CONTROL OF MICROWAVES

The selection of microwave frequences for transformation of materials are based on unique criteria, which cannot be transposed for lower frequencies (HF or radiofrequencies, MF, LF) or for higher frequencies (IR, light, UV) .

1 - Microwaves are associated with centimeter or decimeter wavelengths which are within the scales of the material being treated and therefore very suitable for the material, notably for the depth of penetration of the waves.

2 - The heat generation of microwaves uses the very effective phenomenon of dipolar relaxation, which is one of its properties The control of this phenomenon uses knowledge of the thermal variations of absorption for the materials being treated (See "Dielectric measurement"). This control uses the mechanism of liquid solid heating (increased homogeneity or runaway).

3 - The selection of the 2.45 GHz frequency is particularly favourable for heating mixed products (liquid = solid) in a homogenous manner, in a very short space of time.