Microwave
In space, including the Earth we live on, electromagnetic waves of various frequencies exist in a mixed state.
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Electromagnetic wave spectrum
High-frequency/high-energy bands
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γ-rays, x-rays, ultraviolet rays, visible light, infrared rays, etc. (high energy levels can have significant or potentially significant effects on the human body)
Communications/broadcasting/medical bands
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Electromagnetic waves (including microwaves) used in communications, radio, TV broadcasting, satellite radar, mobile phones, and medical devices.
Low-frequency/noise bands
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Unnecessary radiation from office equipment, high-voltage power lines, cosmic noise, etc.
Definition of
Microwave
Frequency Bands
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Microwaves are electromagnetic waves with a frequency ranging from 300 MHz to 300 GHz.
Microwave Applications
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They are used in a variety of fields, including dielectric heating, wireless communications such as Wi-Fi, radar, satellite communications, and obstacle detection for autonomous vehicles (AGVs).
Frequencies Commonly Used in Dielectric Heating
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The 915 MHz and 2.45 GHz bands are primarily used.
Microwave
absorption
mechanism
When microwave energy strikes a material, the following phenomena occur:
Reflection
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Energy bounces off the surface of the material. This phenomenon is typically seen in metals (conductors).
Transmission
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Energy passes through the material and exits through the other side. This phenomenon is typically seen in weak dielectrics (such as Teflon).
Absorption
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Energy is consumed within the material, converted into heat energy, and heats the material. This phenomenon is called dielectric heating.
The microwave loss factor(εr)of a dielectric is an indicator of how well a material absorbs microwave energy and converts it into heat.
Low-Loss Dielectrics
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Almost all microwave energy is transmitted, with the material's temperature remaining largely unchanged. (Typical materials: Teflon, PP, Quartz, etc.)
High-Loss Ferroelectrics
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Efficiently absorb microwave energy, heating the material itself. Only the remaining unabsorbed energy is transmitted. (Typical materials: water, SiC, etc.)
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Three Microwave Phenomena
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Microwave absorption mechanism
Microwave
Dielectric
heating
Microwave heating utilizes the principle of dielectric heating.
Rotation of Polar Molecules
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When polar molecules, such as water (H₂O), contained in a target object are exposed to a high-frequency electromagnetic field (microwaves, typically 2.45 GHz), the alternating positive and negative magnetic fields cause them to rotate in response to the changing frequency.
Heat generation
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As these polar molecules rotate or vibrate billions of times per second (about 2.45 billion times) along the direction of the rapidly changing electromagnetic field, friction between the molecules occurs, and this friction is converted into heat energy, heating the object.
Internal heating
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Energy directly penetrates into the object to heat it, so it has the characteristic of a fast heating rate.
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Comparison of external heating and microwave heating
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Dielectric heating principle by dipole change
Microwave
Heating
Calculation
When microwave energy is applied to a material, the average microwave power (P) absorbed per unit volume is expressed by the following equation:
P = 2 π f ε0 εr' tanδ E2 [w/m3]
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P = Volumetric energy density [W/m3]
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f = Frequency [Hz]
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ε0 = Permittivity of free space ( 8.85 x 10 -12 [F/m] )
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εr' = Relative dielectric constant (real part of complex relative permittivity)
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tanδ = Dielectric loss tangent (loss factor)
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E = Electric field intensity [V/m]
Microwave
Penetration
Depth
The penetration depth (D) of microwaves is defined as the depth at which the microwave power penetrating the material is reduced to half of its initial value, expressed by the following equation:
D = 3.31 x 107/f√(εrtanδ) [m]
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D = Penetration depth [m]
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f = Frequency [Hz]
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εr' = Relative dielectric constant (real part of complex relative permittivity)
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tanδ = Dielectric loss tangent (loss factor)
Microwave
Heating
Features
Features of Microwave Heating
Rapid Heating (High-speed Heating)
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Since heat is generated internally and simultaneously, it can reach high temperatures in a significantly shorter time compared to external heating methods, thereby reducing overall process time. (No preheating required)
Uniform Heating
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Theoretically, the entire interior generates heat uniformly. However, in practice, heating variations may occur depending on the microwave wavelength, penetration depth (typically 2.5–3.8 cm from the surface), and the composition of the material, such as moisture content and density.
High Thermal Efficiency
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Energy loss to the surrounding environment is minimal as the energy is concentrated directly on the material being heated. This results in superior energy efficiency compared to conventional methods. (Approximately 80% efficiency of applied power)
Selective Heating
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Microwaves effectively transfer energy only to substances with polar molecules (dielectrics). This allows for the intensive and rapid heating of materials with high moisture content or ferroelectric properties.
Easy Control and Rapid Response
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Heating speed and temperature can be precisely and promptly controlled through electrical management, such as power ON/OFF and output modulation.
Clean Working Environment
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The heating furnace itself barely gets hot and does not generate harmful gases, significantly improving the overall working environment.
Microwave
Applications
The applicable types of microwaves are as follows:
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Batch type
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Conveyor type
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Slot waveguide type
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Shutter type
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Microwave Plasma
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Continuous liquid reactor
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Batch type reactor
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Vacuum type dryer/reactor
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Continous vacuum type