1. About Microwave

　Microwaves are a type of electrical wave, and electrical waves are a type of electromagnetic wave.

　　Electromagnetic waves propagate through the interaction of electric and magnetic fields, and therefore can also propagate in a vacuum.

　　Electromagnetic waves are “waves” and therefore have the characteristics of both wavelength and frequency.

　　Wavelength is the length of a wave from one peak to another, and frequency is the number of waves produced in one second.

　　The speed of an electromagnetic wave is independent of frequency and is about 300,000 km/s. The speed divided by the frequency is the wavelength.

　　As shown in Figure 1, electromagnetic waves can be categorized according to frequency and are used in different fields according to their respective properties.

　　Light is also a type of electromagnetic wave.

　　Electromagnetic waves with a frequency below 3000 GHz are generally referred to as electric waves. Electric waves with frequencies between 300MHz and 300GHz (wavelengths between 1m and 1mm) are called microwaves.

Figure 1 Classification and application of electromagnetic waves

　　Microwaves are not only used in the field of communication, but are also used in radio telescopes and radars for celestial observation and monitoring of moving objects. At the same time, there is also the familiar GPS navigation, positioning system is also the application of microwaves. In addition, another important application of microwaves is heating.

　　2. Microwave frequency bands that can be used for microwave heating devices

　　The ITU (International Telecommunication Union) has divided the frequency range in which radio waves can be used according to their purpose. In the end, each country has made its own laws based on this classification.

　　For microwaves with frequencies between 300 MHz and 300 GHz (wavelengths from 1 m to 1 mm), the ITU has classified the available frequency ranges for industrial, scientific, and medical applications as shown in Table 1.

　　433.92MHz is designated as the ISM band for some countries in Zone I (Europe), and 915MHz is designated as the ISM band for Zone II (North and South America).The band above 2450MHz is the ISM band common worldwide.

Table 1 ISM band delineation of microwave

　　In addition, in order to avoid interference with communications, the Airwaves Act was enacted, which defines the leakage of airwaves in a very strict manner.

　　However, the law does not specify the amount of leakage in the ISM band of 2450 MHz.

　　Therefore, microwave devices that use this band (called ISM devices here) are designed to meet the safety limits.

　　For microwave devices that use frequencies other than the ISM band, it is necessary to comply with the provisions of the Radio Wave Law on countermeasures against large-scale leakage of radio waves, such as the installation of a radio wave protection layer in all rooms and buildings in which the devices are placed.

　　This is why the ISM band is used for domestic microwave ovens and industrial heating equipment.

　　The 2450MHz band is widely used not only because it is the world's common ISM band, but also because of the birth of the inexpensive, lightweight magnetron (power: 300W-10kW) with built-in permanent magnets shown in Fig. 2.

Figure 2 2450MHz band magnetron (power 2KW water-cooled)

　　3. Microwave heating principle

　　In the event of heavy rainfall, satellite television programs often can not be viewed, most of us must have encountered this situation.

　　It is generally believed that the frequency of microwaves in the 18GHz or so is the most easily absorbed by the water.

　　Compared to a microwave oven with a microwave frequency of 2.45GHz (2450MHz), a satellite TV station transmits at a frequency of about 12GHz, and when it rains, the waves are absorbed by the rain, so it is not difficult to understand why we can't watch satellite TV programs.

　　Since the radio waves emitted by the satellite are very weak, the rain will not be heated, but theoretically, the rain will emit heat after absorbing the radio waves emitted by the satellite.

　　In the following, we will explain the [microwave heating principle] [dielectric power to absorb microwaves] [depth of microwave penetration into the dielectric] [dielectric properties of the dielectric].

　　(1) Microwave heating principle

　　IEC (International Conference on Electrical Standards) definition of microwave heating: dielectric in 300MHz ~ 300GHz electromagnetic waves under the action of molecular motion and ionic conduction and generate heat to achieve heating.

　　This dielectric microwave heating principle is very complex, it is difficult to explain in a few words, the following is a rough introduction.

　　[Permanent dipole following the microwave electric field changes and do the pole movement, due to the permanent dipole speed can not keep up with the vibration of the electric field changes, thus generating a phase delay. This delay becomes a resistance to the change in the microwave electric field, and the permanent dipole in motion is heated.

　　Simply put, the permanent dipole is forced to vibrate while being resisted, thus generating heat. The following is an illustration of the principle of microwave heating.

　　Illustration of the principle of microwave heating

　　Figure 3 shows the structure of a water molecule as an example of a permanent dipole.

　　A water molecule consists of one oxygen atom and two hydrogen atoms.

　　Although the water molecule is not electrically charged from the viewpoint of the entire molecular structure, since the two hydrogen atoms and the oxygen atom form an angle of 104.5°, each will carry a small amount of positive and negative charge, forming a dipole.

　　As shown in Figure 4, the dipole of water is distributed in a disordered state without the action of an external electric field, but when placed in an electric field, it will change direction with the direction of the electric field.

　　If the water is irradiated with microwaves, the electric field formed is an alternating current field, and in the case of a microwave oven, 2.45 billion times of alternating positive and negative electric fields are generated in one second.

　　Figure 5 shows the state of a permanent dipole under low-frequency electric wave irradiation.

　　In this case, the change in direction of the permanent dipole can quickly follow the change in the upper electric field, and at this time the water does not produce heat.

　　Figure 6 shows the state of the permanent dipole under the irradiation of electric wave with too high frequency.

　　Because the electric field changes too quickly, the dipole can not follow the electric field changes in response, this condition, the water also does not produce heat.

　　Figure 7, on the other hand, shows the state of a permanent dipole irradiated by an electric wave with a moderate rate of change in the electric field.

　　At this point, the permanent dipole changes with the electric field at a slightly slower rate. The water molecules undergo a delayed effect in their movement, during which time they absorb the energy of the electric wave and emit heat. This band of electric waves is what we call microwaves.

　　(2) Microwave heating method and the dielectric properties of substances

　　Through the above description, many people may think that only dielectric can be heated by microwave. In fact, like iron, stainless steel and other metals and iron ore and other metal oxides can also be heated by microwave.

　　For metal ions microwaves can indeed penetrate into the interior of the metal ions and heating, but the microwave can not penetrate the metal plate, basically all will be reflected back.

　　Here only on the [dielectric microwave absorption] [microwave penetration to the depth of the dielectric] [dielectric properties of the dielectric] to explain, metal is not introduced.

　　(a) dielectric material microwave absorption (formula)

　　Equation 1 is the theoretical formula for the microwave absorption of the dielectric P1.

　　P1=K-f-εr-tanδ-f-E2 [W/m3]

　　K:0.556×10-10

　　εr:dielectric constant of the dielectric

　　tanδ:dielectric loss angle of the dielectric

　　f:frequency

　　E:electric field strength [V/m]

Equation 1

　　In Equation (1), the relative dielectric constant ε and the dielectric loss angle tanδ are values specific to the substance (dielectric). In addition, the product of the two, εr・tanδ, is called the dielectric loss coefficient (referred to as the loss coefficient), which indicates the degree of absorption of microwaves by the dielectric.

　　E represents the strength of the electric field acting on the dielectric and depends on the design of the device.

　　Equation (1) is only a theoretical formula, in practice, the electric field strength E acting on the dielectric can not be calculated. Therefore, when calculating the microwave absorption of the dielectric, it is mainly obtained by the heat calculation of formula (b).

　　(b) The microwave absorption of the dielectric (calculated by the heat method)

　　A container containing w [ g ] liquid (initial temperature T1 [ °C ]) is placed inside the application cavity. The liquid is subjected to

　　[W] microwave irradiation t [s], the temperature rises to T2 [°C].

　　The specific heat of the liquid is C [ J / (kg・K) ], and according to equation (2) the microwave energy absorbed by the liquid is

　　[W], and the heating efficiency can be calculated according to Equation (3).

　　For example, the liquid is the case of water, according to the specific heat of water 4180 [ J / (kg・K) ], you can calculate the microwave absorption.

　　(c) the depth of microwave penetration of dielectrics

　　Microwave penetration depth from the surface of the dielectric to the interior is the microwave energy is reduced to 50% of the depth of penetration, so it is also known as the energy halving depth.

　　According to formula (4) can calculate the energy halving depth D

　　(d) Dielectric properties of matter

　　It has been shown in (a) that the relative dielectric constant ε and and dielectric loss are two values specific to dielectrics. FIG. 8 is a graph representing the properties of relative permittivity and dielectric loss for a variety of substances. The upper horizontal axis of the graph represents the energy halving depth D, and the vertical diagonal line is a scale with the same energy halving depth.

　　From the figure we can roughly see, the upper right of the energy halving depth of the lower material on the microwave absorption of more, the lower left of the energy halving depth of the higher material on the microwave absorption of less.

　　4. The characteristics of microwave heating

　　Microwave heating has the advantages that other heating methods do not have, such as:

　　・Internal heating

　　High-speed heating and selective heating

　　High heating efficiency, rapid response and temperature controllability.

　　Uniform heating, energy saving and environmental protection

　　Easy to operate and good working environment

　　The following is a description of the characteristics

　　(1) Internal heating

　　As shown in Fig. 9, the microwave reaches the object to be heated rapidly with the speed of light. As explained in Chapter 3 (2), the microwaves are absorbed by the object while entering the interior of the object, causing the object to emit heat.

　　For example, water, from Figure 8 can be seen in water microwave energy halved depth of about 1cm. That is to say, microwaves from the surface of water into the water inside 1cm, 50% of the microwave energy will be absorbed by the water, the water is heated, the remaining 50% of the microwave continues to enter to the water more than 1cm deeper. By the same token, when entering from the surface to 3 cm inside, 12.5% of the microwaves can still continue to enter the inside of the water and cause it to be heated. Thus, as represented in FIG. 10, microwaves can internally heat an object.

　　(2) High-speed heating

　　As shown in Fig. 10, the conventional heating method realizes the increase of temperature by diffusion of heat from the surface of the object to be heated to the inside of the object through conduction.

　　The microwave can be directly penetrated into the heated object inside, if the volume of the heated object size can be completely penetrated by the microwave, then the heated object as a whole will produce heat, heat conduction and diffusion time is negligible, and therefore can achieve high-speed heating.

　　Even if the volume of the object is very large, although the microwave can not penetrate to the center of the object to be heated, but as long as the microwave penetration of the place will emit heat, this part of the heat will be diffused to the entire object to make the internal temperature rise. As shown in Figure 11, and only through the surface of the object heated by the traditional heating method, compared to this microwave heating from the inside of the object heated more quickly.

　　(3) Selective heating

　　As shown in Figure 8, different substances have different absorption properties for microwaves. For example, borosilicate glass, shown in Fig. 8, is sold as special glassware for microwave heating. This is because such containers hold water for microwave heating, borosilicate glass for microwave absorption is only 1 in 3000 of water, completely negligible, so only water will be heated.

　　In other words, as shown in Figure 12, as long as you choose the material of the container, you can do only want to heat the object to be heated, in essence, to achieve high efficiency heating.

　　(4) Highly efficient heating

　　Microwaves enter the interior of the object to be heated at the speed of light, and since the object is absorbing microwaves and then emits heat itself, the heat consumption of the heating furnace and the air heating inside the furnace is very small and negligible, achieving highly efficient heating.

　　(5) Rapid response and temperature controllability

　　Microwave is the speed of light into the interior of the heated object so that the object itself heats up, the reaction speed is very fast. For example, it can be started and stopped instantly. In addition, the power of the microwave can be adjusted to control the amount of heat generated inside the object being heated. As shown in Fig. 13, the temperature change of the object to be heated reacts instantaneously according to the set temperature and can be maintained at the set temperature.

　　(6) Uniform heating

　　Since heat is generated simultaneously inside the object being heated, even objects with complex shapes are heated uniformly. Uneven heating due to wavelength can be adjusted by means of stirrers, turntables, conveyors, etc.

　　(7) Energy saving and environmental protection

　　Microwave is transmitted through the electric and magnetic fields and interactions, so it does not require a medium, and can be transmitted in a vacuum. Microwave heating is directly on the heated object, penetrate into the object and attenuation, without heating the air in the environment. The object to be heated absorbs the microwaves and converts them into thermal energy to generate heat. In this process, because there is no need to heat the surrounding air, so it can be said to be energy-saving and environmentally friendly energy.

　　(8) Easy operation and good working environment

　　Traditional heating must have a heat source, the heat source is not only on the object to be heated, but also will heat the heating furnace. If the heating furnace is placed in the room, due to heat radiation around the ambient temperature will be higher, so the operability and operating environment becomes a problem. In contrast, with microwave heating, microwaves are emitted as soon as the electricity is turned on and only the object to be heated is heated. The heater does not generate high temperatures and there is no heat radiation, so good maneuverability and operating environment can be ensured.

　　5. The basic structure of microwave applications and microwave components

　　There are many electronic components that generate microwaves, such as magnetrons, speed control tubes, cyclotron and so on.

　　One of the cheaper and can emit high-power microwave is the magnetron. Magnetrons are a type of vacuum tube and are used in microwave ovens. Microlife manufactures and sells microwave application equipment and devices with outputs ranging from 300W to 300kW, mainly using magnetrons in the 2450MHz band.

　　(1) Basic structure of microwave application equipment

　　The basic structure of microwave application equipment is shown in Figure 14. Microwaves are emitted from the magnetron inside the microwave source, and the microwaves pass through the [waveguide], [isolator], [power detector], [waveguide], and [EH tuner] to the [application chamber] to heat the object to be heated.

　　The microwaves emitted from the microwave source toward the isolator are herein referred to as traveling (or incident) waves. Microwaves reflected back from the application cavity in the opposite direction are called reflected waves. The value obtained by subtracting the power of the reflected wave from the power of the incident wave shown on the power detector is the value of the applied wave.