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Microwaves, also known as the radio frequency (RF) waves, are electromagnetic waves with frequencies in the range of 500 MHz to 100 GHz. Microwaves have been used primarily in the field of telecommunications. After the invention of a magnetron, an efficient high frequency microwave generator, microwaves have been increasingly used for heating applications with first microwave ovens introduced to the market in late 1960s.
Microwave heating is based on absorption of RF energy by dielectric materials. Molecules of polar components, called susceptors, vibrate when placed in electromagnetic fields. This vibration causes an increase in the molecule's kinetic energy which is dissipated as heat.
The dielectric properties of a material are characterized by its complex permittivity ε :
(1)ε = ε' - i ε''
where the real part (ε') is the dielectric constant and the imaginary part (ε'') is the dielectric loss factor. The dielectric constant is an indication of the amount of energy that can be stored in a material in the form of electric field, while the dielectric loss factor is a direct measure of how much energy a material can dissipate in the form of heat.
When a material is heated by microwave irradiation, the amount of RF power converted to heat per unit volume of a susceptor is a function of the electric field frequency, its intensity, and the dielectric loss factor, as given by the following equation:
(2)P = 2 π f ε0 ε'' E2
P - power, W/m3
f - field frequency, Hz
ε0 - absolute permittivity, 8.854·10-12 F/m
ε'' - dielectric loss factor, dimensionless
E - electric field intensity, V/m
Materials that exhibit magnetic properties, e.g. ferrite, can be heated by both the electric (E) and magnetic (H) components of an electromagnetic field. RF power associated with the H-field component can be expressed by a relationship similar to Eq. (2), in which the dielectric loss factor (ε'') and the electric field intensity (E) are replaced by magnetic loss factor (µ'') and magnetic field intensity (H), respectively.
Microwave heating differs significantly from conventional heating. During conventional heating of a solid material, the heat is first transferred to its surface, typically by a combination of convection and radiation mechanisms. The inside of the material is then heated through conduction from the surface. In microwave heating, however, the energy deposition is concentrated in the material itself, resulting in heating from inside out.
Another characteristic feature of microwave heating is its selectivity. Some materials are strong absorbers of RF energy, while others are not. Materials of high dielectric constants and high dielectric loss factors are most effectively heated by microwaves. Examples of such materials include water, carbon black, as well as diesel soot. Most ceramic materials, due to their low dielectric constant and loss factor, are practically transparent to RF energy. This class of materials includes cordierite, a ceramic used in some diesel particulate filter substrates, which is virtually transparent to microwaves. Metals, on the other hand, are nearly perfect reflectors of microwave energy. Therefore, metal ducts can be used as waveguides for RF energy.
Specific data on complex permittivities of materials is somewhat difficult to find in the published literature. Available data is often incomplete. It is also inconsistent, presumably due to the use of different measurement techniques and parameters by different authors. Dielectric properties of some selected materials, including diesel soot, are listed below. This data, despite some discrepancies, confirms that diesel particulates are an RF absorber, while cordierite is not (this characteristic can be also used for the measurement of soot loading in ceramic diesel particulate filters by microwaves ).
|Material||Dielectric constant, ε'||Dielectric loss factor, ε''||Reference|
|Diesel soot 1||10.695||3.561|||
|Diesel soot 2||9.578||2.854|
|Diesel soot 3||8.6||7.4|||
The deposition of microwave energy within a heated medium is dependent on the medium geometry, sometimes called an RF cavity (in the case of a diesel filter, the cavity would be defined by the metallic filter container). If RF energy launched down a metal waveguide strikes a metal wall, the energy will be reflected off the wall, setting up a pattern of E- and H-field standing waves. The E-field distribution in a microwave oven can be pictured as a two-dimensional array of E-field peaks of high electrical field intensity and valleys where the field intensity is low. Materials with dielectric loss properties will be heated only when placed in an E-field standing wave peak and not if located in a valley. Consumers microwave oven manufacturers attempt to compensate for this uneven heating by moving the food through the electrical field on a rotating platform and/or by utilizing rotating metal vanes to periodically redistribute the field.