The basic principle of microwave heating

Microwaves have characteristics of short wavelength (1 m to 1 mm), high frequency (300 MHz to 300 GHz), and obvious quantum characteristics. Microwave technology is widely used in radar, navigation, multi-channel communication, remote sensing and television. Beginning in the 1960s, microwave heating technology was gradually applied to physical processing such as paper, wood, and resin extrusion. In recent years, the introduction of microwave heating technology in the production process of molybdenum industry not only can effectively improve the reaction conversion rate and selectivity, but also reflect many advantages such as energy saving and environmental protection. It has been widely recognized as one of the means to realize green technology. .

Microwave is a form of energy (rather than heat) but can be converted to heat in a medium. The reaction of materials to microwaves can be divided into four cases: (1) penetrating microwaves; (2) reflecting microwaves; (3) absorbing microwaves. (4) Partial absorption of microwaves.

Generally, in the field of microwave energy processing, most of the materials processed are dielectric materials, and the dielectric materials generally absorb microwave energy to different extents. The dielectric materials and microwave electromagnetic fields are coupled to each other, which will form various power dissipations to achieve energy conversion. . There are many ways to convert energy, such as ion conduction, dipole rotation, interfacial polarization, hysteresis, piezoelectric phenomenon, electrostriction, nuclear magnetic resonance, ferromagnetic resonance, etc., where ion conduction and dipole rotation are microwaves. The main principle of heating. Microwave heating is a heating method that relies on the absorption of microwave energy by an object to convert it into heat energy and heat the whole body at the same time, which is completely different from other conventional heating methods. The traditional heating method is to transfer heat from the outside to the inside of the material according to the principle of heat conduction, convection and radiation. The heat is always transferred from the surface to the inside to heat the material. There is inevitably a temperature gradient in the material, so the heated material is uneven, resulting in unevenness. The material is locally overheated, affecting the quality of the material and the reaction process, and the heating rate is slow and the energy consumption is high. The microwave heating technology is different from the traditional heating method in that it generates a "internal friction heat" by the high frequency reciprocating motion of the dipole molecules inside the heating body, so that the temperature of the heated material is raised, and the heat transfer process can be performed without any heat conduction process. The external heating at the same time and the simultaneous heating, the heating speed is fast and uniform, and only a fraction or a fraction of the energy consumption of the conventional heating method can achieve the purpose of heating.

The relationship between the heat (P) generated in the microwave per unit volume of the dielectric per unit time and the electric field strength (E), frequency (f), and dielectric loss coefficient (tgδ) of the dielectric

P = f·E2 · εr · tgδ (1)

Εr in the formula represents the dielectric constant of the substance.

It can be seen from the above formula that the amount of heat generated by a substance in a microwave field is highly dependent on the type of substance and its dielectric properties, that is, the characteristic that the microwave selectively heats the substance. One-third or one-tenth of a degree can be used for heating purposes.

The relationship between the heat (P) generated in the microwave per unit volume of the dielectric per unit time and the electric field strength (E), frequency (f), and dielectric loss coefficient (tgδ) of the dielectric

P = f·E2 · εr · tgδ (1)

Εr in the formula represents the dielectric constant of the substance.

It can be seen from the above formula that the amount of heat generated by a substance in a microwave field is highly dependent on the type of substance and its dielectric properties, that is, the characteristic that the microwave selectively heats the substance.