The so-called absorbing material refers to a type of material that can absorb or greatly reduce the electromagnetic wave energy received on its surface, thereby reducing the interference of electromagnetic waves. In engineering applications, in addition to requiring a high absorption rate of electromagnetic waves in a wide frequency band, the absorbing material is also required to have the properties of light weight, temperature resistance, humidity resistance, and corrosion resistance.
With the development of modern science and technology, the impact of electromagnetic radiation on the environment is increasing. At the airport, the flight cannot take off due to electromagnetic wave interference, and it is delayed; at the hospital, mobile phones often interfere with the normal operation of various electronic diagnosis and treatment equipment. Therefore, the treatment of electromagnetic pollution and the search for a material that can withstand and weaken electromagnetic wave radiation-absorbing materials have become a major issue in materials science.
Electromagnetic radiation causes direct and indirect damage to the human body through thermal, non-thermal, and cumulative effects. Studies have confirmed that ferrite absorbing materials have the best performance, which has the characteristics of high absorption frequency band, high absorption rate, and thin matching thickness. Applying this material to electronic equipment can absorb leaked electromagnetic radiation and achieve the purpose of eliminating electromagnetic interference. According to the law of electromagnetic waves propagating in the medium from low magnetic to high magnetic permeability, high magnetic permeability ferrite is used to guide electromagnetic waves, through resonance, a large amount of radiant energy of electromagnetic waves is absorbed, and then the energy of electromagnetic waves is converted into heat energy through coupling.
Two issues should be considered when designing the absorbing material
1) When electromagnetic waves encounter the surface of the absorbing material, pass through the surface as much as possible to reduce reflection;
2) When the electromagnetic wave enters the inside of the absorbing material, the energy of the electromagnetic wave should be lost as much as possible;
Classified by the loss mechanism of the absorbing material:
1) Resistive loss, this kind of absorption mechanism is related to the resistive loss of the material's conductivity, that is, the greater the conductivity, the greater the macro current caused by the carrier (including the current caused by the change of the electric field and the eddy current caused by the change of the magnetic field). Large, which is conducive to the conversion of electromagnetic energy into thermal energy.
2). Dielectric loss, which is a kind of dielectric loss absorption mechanism related to electrodes, that is, through the "friction" effect of repeated polarization of the medium, the electromagnetic energy is converted into heat and energy consumption is dissipated. The dielectric polarization process includes: electron cloud displacement polarization, polar dielectric moment turning polarization, electric ferrite domain turning polarization and wall displacement.
3) Magnetic loss. This type of absorption mechanism is a type of magnetic loss related to the dynamic magnetization process of ferromagnetic media. This type of loss can be refined into: hysteresis loss, gyromagnetic eddy current, damping loss, and magnetic after-effects, etc. , Its main source is the magnetic domain turning, magnetic domain wall displacement and natural resonance of the magnetic domain similar to the hysteresis mechanism. In addition, the latest microwave loss mechanism of nanomaterials is a hot spot in the analysis of microwave absorbing materials.
According to the element classification of the absorbing material:
1), carbon-based absorbing materials, such as: graphene, graphite, carbon black, carbon fiber, carbon nanotubes;
2), iron-based absorbing materials, such as: ferrite, magnetic iron nanomaterials;
3), ceramic absorbing materials, such as: silicon carbide;
4), other types of materials, such as: conductive polymers, chiral materials (left-handed materials), plasma materials;