Absorbing material

Introduction
1.1 With the development of modern science and technology, the impact of electromagnetic wave 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.
1.2 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 permeability ferrite is used to guide electromagnetic waves, through resonance, a large amount of electromagnetic wave radiation energy is absorbed, and then the electromagnetic wave energy is converted into heat energy through coupling.
1.3 When designing the absorbing material, two issues should be considered: 1) When the electromagnetic wave encounters 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 electromagnetic waves is lost as much as possible;

classification
2.1 Classified by the loss mechanism of absorbing materials:
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 electric field change and the eddy current caused by the magnetic field change) Large, so as to facilitate the conversion of electromagnetic energy into thermal energy.
2) Dielectric loss, which is a kind of dielectric loss absorption mechanism related to the electrode, that is, the electromagnetic energy is converted into thermal energy dissipation through the "friction" effect generated by repeated polarization of the medium. 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 kind of absorption mechanism is a kind of magnetic loss related to the dynamic magnetization process of ferromagnetic medium. This kind of loss can be refined into: hysteresis loss, gyromagnetic eddy current, damping loss and magnetic aftereffect, 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 absorbing materials today.
2.2 According to the element classification of the absorbing material:
1) Carbon-based absorbing materials, such as graphene, graphite, carbon black, carbon fiber, carbon nanotube;
2) Iron-based absorbing materials, such as ferrite, magnetic iron nanomaterials [1];
3), ceramic absorbing materials, such as: silicon carbide [2];
4), other types of materials, such as: conductive polymer [3], chiral materials [4] (left-handed materials), plasma materials [5];

shape
3.1 Wedge shape
The absorber used in the microwave anechoic chamber is often made into a wedge shape (golden tower shape), mainly composed of polyurethane foam type, non-woven fabric flame-retardant type, and silicate plate metal film assembly type. As the frequency decreases (wavelength increases), the length of the absorber also increases greatly. The ordinary wedge-shaped absorber has an approximate relationship of L/λ≈1, so at 100MHz, the wedge length reaches 3000mm, which is not only difficult to achieve in the process , And the effective available space of the microwave anechoic chamber is greatly reduced.
Wedge absorbing material
Wedge absorbing material
3.2 Single-layer flat shape
The first absorber developed abroad is a single-layer flat plate, and the absorbers made later are directly attached to the metal shielding layer, which is thin and light, but the working frequency range is narrow.
3.3　Double or multilayer flat shape
This kind of absorber can work in a wide operating frequency range and can be made into any shape. For example, NEC Corporation of Japan disperses ferrite and short metal fibers evenly in a suitable organic polymer resin to make composite materials, and the working frequency band can be expanded by 40% to 50%. Its disadvantages are large thickness, complicated process and high cost.
3.4　Coating shape
Only coating-type absorbing materials can be used on the surface of the aircraft, which broaden the frequency band and generally use composite coatings. For example, when the thickness of the lithium cadmium ferrite coating is 2.5mm～5mm, it can attenuate 8.5dB in the centimeter band; when the thickness of the spinel ferrite coating is 2.5mm, it can attenuate 24dB at 9GHz; the ferrite is chlorinated When the thickness of the butadiene rubber coating is 1.7mm～2.5mm, the attenuation is about 30dB at 5GHz～10GHz.
3.5　 structure
Incorporating absorbent materials into engineering plastics makes them both absorbent and load-bearing, which is a direction for the development of absorbent materials.
Nowadays, in order to further improve the performance of absorbent materials, several complex types of absorbent bodies with combinations of shapes have been developed abroad. For example, a microwave anechoic chamber made of this type of absorber in Japan has performances: 136MHz, 25dB; 300MHz, 30dB; 500MHz, 40dB; 1GHz～40GHz, 45dB.

Engineering Applications
In the increasingly important stealth and electromagnetic compatibility (EMC) technology, the role and status of electromagnetic wave absorbing materials are very prominent, and they have become the magic weapon and "secret weapon" of electronic countermeasures in modern military. Its engineering applications are mainly in the following aspects.
4.1 　 Stealth Technology
Coating absorbing materials on various weapons and military facilities such as aircraft, missiles, tanks, ships, warehouses, etc., can absorb reconnaissance radio waves and attenuate reflected signals, thereby breaking through enemy radar defenses. This is a kind of anti-radar reconnaissance A powerful means to reduce the weapon system from being attacked by infrared guided missiles and laser weapons. For example, because the American B-1 strategic bomber is coated with absorbing material, its effective reflection cross section is only 1/50 of that of the B-52 bomber; after coating the absorbing material on the cowl of the 0H-6 and AH-1G Cobra helicopter engines It can reduce the infrared radiation of the engine by about 90%. In the 1990 Gulf War, the first US F-117A aircraft that entered the territory of Iraq were stealth aircraft coated with absorbent materials. They effectively avoided Iraq’s radar monitoring.
It is reported that the world's first stealth warship successfully developed by the Swedish Navy has been put into use. The United States, Britain, Japan, Russia and other countries have all developed their own stealth tanks and other stealth combat vehicles. In addition, electromagnetic wave absorbing materials can also be used to conceal landing lights and other airport navigation equipment and other ground equipment, ship masts, decks, submarine periscope brackets and air ducts and other equipment.
4.2 　 Improve the electromagnetic compatibility performance of the whole machine
The false signals generated by the reflection of electromagnetic waves by the aircraft fuselage may lead to false interception or false tracking of highly sensitive airborne radars; when several radars on an aircraft or a ship are working at the same time, the crosstalk between the radar transmitting and receiving antennas is sometimes very serious , The jammer on board or on the ship will also interfere with the radar or communication equipment on board... In order to reduce such interference, the excellent magnetic shielding of absorbing materials is commonly used abroad to improve the performance of radar or communication equipment. If the radar or communication equipment fuselage, antenna and all surrounding interferences are coated with absorbing material, it can make them more sensitive and accurate to find the enemy target; coating the absorbing material on the surrounding wall of the opening of the radar parabolic antenna, It can reduce the side lobe's interference to the main lobe and increase the distance of the transmitting antenna, and it can reduce the interference of false target reflection on the receiving antenna; the application of absorbing materials in the satellite communication system will avoid the interference between communication lines and improve The sensitivity of the spaceborne communicator and ground station, thereby improving the communication quality.
4.3　 RFID antenna anti-metal isolation application
This application mainly uses the high track rate characteristics of a type of high magnetic track rate, low loss type wave absorbing material; when used, insert the wave absorber between the 13.56MHz loop antenna and the metal substrate to increase the induced magnetic field through the wave absorbing material In itself, it reduces the chance of passing through the metal plate, 　 thus reduces the induced eddy current in the metal plate, thereby reducing the loss of the induced magnetic field. At the same time, due to the insertion of the absorber, the measured parasitic capacitance will also be reduced and the frequency offset will be reduced. It is consistent with the resonant frequency of the card reader, thereby improving the reading distance. Of course, the degree of improvement depends on the quality of the absorbing material.
4.4　Safety protection
Due to the application of high-power radars, communication devices, microwave heating and other equipment, preventing electromagnetic radiation or leakage and protecting the health of operators is a new and complex topic. Absorbing materials can achieve this goal. In addition, there is a widespread problem of electromagnetic radiation in today's household appliances, which can be effectively suppressed through the rational use of absorbing materials and components.
4.5　Microwave anechoic chamber
The space formed by the wall surface decorated by the absorber is called a microwave anechoic chamber. An equivalent non-reflective free space (no noise zone) can be formed in a dark room. The electromagnetic waves reflected from the surroundings are much smaller than the direct electromagnetic energy and can be ignored. Microwave anechoic chambers are mainly used for the measurement of characteristic impedance and coupling of radar or communication antennas, missiles, aircraft, spacecraft, satellites, etc., the measurement of astronauts' shoulder antenna pattern, and the installation, testing and adjustment of spacecraft, etc. It can eliminate external clutter interference and improve measurement accuracy and efficiency (indoors can work around the clock), and keep secrets.
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