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Microwave is a form of energy (not heat), a form of electromagnetic wave, which can be converted into heat in a medium. The reactions of materials to microwave can be divided into four situations: (1) penetrating microwave (2) reflecting microwave (3) absorbing microwave (4) partially absorbing microwave
Dielectrics can be divided into non-polar and polar molecular dielectrics. Usually they are arranged irregularly. If they are placed in an alternating electric field, the polar molecular orientation of these media will change with the change of electric field polarity, which is called polarization. The stronger the external electric field is, the stronger the polarization is, the faster the polarization of the external electric field changes, and the faster the polarization is, the more intense the thermal motion of the molecule and the friction between the adjacent molecules are. Thus, the conversion of electromagnetic energy to thermal energy can be realized.Material consisting of polar molecules can absorb microwave well. Water is the best medium to absorb microwave. Therefore, any substance containing water must absorb microwave. The other is composed of non-polar molecules, which basically do not absorb or rarely absorb microwave. These substances include polyvinyl fluoride, polypropylene and other plastic products, glass, ceramics, etc. They can pass through microwave, but not absorb microwave. These materials can be used as containers or supports for microwave heating, or as microwave sealing materials. For conductive metal materials, radio waves can not penetrate into the interior and be reflected, and metal materials can not absorb microwave.
Principle of Full-automatic and Continuously Microwave Defrosting and Microwave Heating Machine:
Generally, the materials processed in the field of processing are mostly dielectric materials, which are composed of polar and non-polar molecules and can absorb microwave in varying degrees. The coupling of dielectric materials and microwave electromagnetic field will lead to various power dissipation to achieve the purpose of energy conversion. There are many ways of energy conversion, such as ion conduction, dipole rotation, interface polarization, hysteresis, piezoelectric phenomena, electrostriction, nuclear magnetic resonance and ferromagnetic resonance, among which ion conduction and dipole rotation are the main principles of microwave heating.
Microwave heating machine, which is caused by the loss of electric field energy by the dielectric material itself, is completely different from other conventional heating methods. Traditional heating method is based on the principle of heat conduction, convection and radiation to transfer heat from the outside to the material. Heat is always transferred from the surface to the inside to heat the material. Thermal gradient inevitably exists in the material, so the material heated is not uniform, resulting in local overheating of the material. Microwave heating can raise the temperature of heated material by high frequency reciprocating motion of dipole molecule inside the heated body. Without any heat conduction process, it can make both inside and outside of the material heated at the same time. The heating speed is fast and uniform. The heating purpose can be achieved only by using a fraction or a dozen parts of the energy consumption of the traditional heating method. Theoretical analysis shows that the amount of heat generated by matter in microwave field is closely related to the type of matter and its dielectric properties, that is, microwave has selective heating characteristics for matter.
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