The so-called absorbing material refers to a type of material that can absorb or greatly attenuate the electromagnetic wave energy projected on its surface, thereby reducing the interference of electromagnetic waves. In engineering applications, in addition to requiring the absorbing material to have a high absorption rate of electromagnetic waves in a wider frequency band, it is also required to have the properties of light weight, temperature resistance, humidity resistance, and corrosion resistance.
The principle of the absorbing material is based on the magnetic microwave absorber, which converts the electromagnetic waves emitted by electronic equipment into heat energy by means of insulation loss, magnetic loss and impedance loss to achieve the effect of reducing electromagnetic radiation. It has high permeability and wide selection of frequency bands. And other characteristics, and can be targeted for specific frequency band development.
The absorbing material has good absorption characteristics in the range of 10MHz~6GHz, which can avoid electromagnetic interference or leakage caused by secondary reflection. The products are mainly absorbing patch types, which can also be processed into various shapes according to customer needs. Absorbers can be used in electronic equipment cavities such as notebook computers, mobile phones, and communication cabinets.
The absorbing material has good electromagnetic wave absorption effect and wide absorption frequency. It can be customized according to the frequency band required by the customer. It is thin and cost-effective, and has a wide range of uses and applications.
01Classified 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 carriers (including the current caused by the change of electric field and the eddy current caused by the change of 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 the electrode, that is, the electromagnetic energy is converted into thermal energy dissipation through the "friction" effect generated by the 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 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 aftereffects, etc. Its main source is the magnetic domain turning, the displacement of the magnetic domain wall and the natural resonance of the magnetic domain, which are 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.
02According 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;
3). Ceramic absorbing materials, such as silicon carbide;
4), other types of materials, such as: conductive polymers, chiral materials (left-handed materials), plasma materials.
The absorber used in the microwave anechoic chamber is often made into a wedge shape (the shape of a tower of gold), which is 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 common 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 also greatly reduced.
Single-layer flat shape
The first absorber developed abroad is a single-layer flat plate, and the absorber made later is directly attached to the metal shielding layer, which is thin in thickness and light in weight, but has a narrow working frequency range.
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.
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.
Incorporating absorbing materials into engineering plastics makes them have both absorbing characteristics and load-bearing capacity, which is a direction for the development of absorbing 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.
Coating absorbing materials on various weapons and military facilities such as airplanes, 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 method 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 fairing of the 0H-6 and AH-1G Cobra helicopter engines The infrared radiation of the engine can be reduced 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.
Improve the electromagnetic compatibility 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 the plane or on the ship will also interfere with the radar or communication equipment that comes with it....... 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. For example, coating the absorbing material on the fuselage, antenna and all surrounding interferences of radar or communication equipment 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 the ground station, thereby improving the communication quality.
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 absorbing material; when used, insert the wave absorbing plate 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, "thereby reducing the induced eddy current in the metal plate, thereby reducing the loss of the induced magnetic field." At the same time, because of 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 excellent degree of the characteristics of the absorbing material.
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 their components.
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 the dark room, and 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 degree of radar or communication antennas, missiles, airplanes, spacecrafts, 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 it can also keep secrets.