At present, there are many kinds of absorbing materials, which can be roughly divided into the following categories:
1、 According to material forming process and bearing capacity
Absorbing materials can be divided into coating absorbing materials and structural absorbing materials.
2、 According to the principle of absorbing wave
1. The basic types of absorbing materials include absorbers with the same complex permeability and complex permittivity, broad-band absorbers with gradual impedance and thin-layer absorbers that attenuate surface current.
2. The interference absorbing material is to eliminate the interference by using the equal amplitude and opposite phase of the reflected waves on the surface and bottom of the absorbing layer.
3、 According to material loss mechanism
1. Resistance type loss, such absorption mechanism is related to the conductivity of the material, the higher the conductivity, the greater the macro current caused by carriers, which is conducive to the conversion of electromagnetic energy into heat energy. Such as silicon carbide, graphite, etc. belong to resistance type absorbing materials.
2. Dielectric loss, which is a kind of dielectric loss absorption mechanism related to electrode, is to convert electromagnetic energy into thermal energy consumption and dissipate it through the "friction" produced by repeated polarization of medium. Such as barium titanate is a dielectric type absorbing material.
3. This kind of absorption mechanism is a kind of magnetic loss related to the dynamic magnetization process of ferromagnetic medium, which can be refined into: hysteresis loss, damping loss, rotating magnetic eddy current, and post magnetic effect. The main sources are domain turn, domain wall displacement and natural resonance of magnetic domain which are similar to the hysteresis mechanism. For example, ferrite and hydroxy iron belong to magnetic loss absorbing materials.
4、 By study period
1. Traditional microwave absorbing materials, such as ferrite, barium titanate, metal powder, graphite, silicon carbide, conductive fiber and so on, belong to traditional microwave absorbing materials. Among them, ferrite absorbing materials and metal micro powder absorbing materials have more research and application and better performance.
2. New absorbing materials, including nano materials, chiral materials, conductive polymers, polycrystalline iron fibers and circuit simulation absorbing materials, have different absorbing mechanism from traditional absorbing materials. Among them, nano materials and polycrystalline iron fibers are two kinds of new wave absorbing materials.
5、 By material
1. Ferrite absorbing material, which is a kind of complex dielectric material, can absorb electromagnetic wave with polarization effect and magnetic loss effect. With the advantages of high absorptivity, thin coating and wide frequency band, it is widely used in various fields.
2. The particle size of metal powder is 0.5-20 μ M. Metal powder microwave absorbing materials have the advantages of high Curie temperature, good temperature stability, high magnetization, high microwave permeability and high dielectric constant in magnetic materials, so they are widely used in the field of microwave absorbing materials. It mainly absorbs electromagnetic wave through hysteresis loss and eddy current loss. At present, the size of metal powder is usually 1-10 μ M.
3. The absorbing mechanism of polycrystalline iron fiber absorbing material is eddy current loss and hysteresis loss. In addition, it is a good conductor with strong dielectric loss absorption performance. Under the action of external alternating electric field, the electrons in the fiber vibrate and the electromagnetic energy part is transformed into heat energy. Polycrystalline iron fiber has unique shape anisotropy, which can achieve high absorption in a wide frequency band. The mass of polycrystalline iron fiber is 40% - 60% less than that of traditional metal powder materials, and overcomes the serious defects of most magnetic materials. Polycrystalline iron fiber absorbing material has the advantages of light weight, small area density (can be reduced to 1.5 ~ 2kg / m2), wide frequency band (4 ~ 18GHz), and the electromagnetic parameters of the material can be adjusted by adjusting the length, diameter, arrangement of the fiber and the content of dispersant.
4. Nano wave absorbing material refers to that the size of the material is nanometer (usually 1-100nm). The unique structure of nanomaterials makes them have the characteristics of tunneling effect, quantum effect, small size effect and interface effect. When nano materials are used as absorbent, the coatings can not only absorb electromagnetic wave well, but also have thin coating and wide absorption frequency band.
5. Absorbing structure composite material is a mixture of absorbing materials and resin foam fibers. The most commonly used materials are carbon fiber and silicon carbide fiber composite.
6. Plasma absorbing material, the absorption band of plasma stealth material is (3mhz-300ghz), it doesn't need to change the appearance of aircraft. It is cheap and easy to maintain. It has high potential application value and has become the development trend of stealth technology in the future.
6、 By shape
1, wedge shaped, microwave darkroom absorber often used to make wedge (gold tower shape), mainly from polyurethane foam, non-woven fabric refractory, silicate plate metal film assembly type. The length of the absorber increases with the decrease of the frequency (the increase of the wavelength). The general wedge type absorber has the approximate formula L / λ ≈ 1. Therefore, at 100MHz, the length of the wedge reaches 3000mm, which is not only difficult to realize in technology, but also greatly reduces the available space of microwave anechoic chamber.
2. The first absorber developed in foreign countries is the single-layer flat type. Later, the absorbers are directly attached to the metal shielding layer, which has thin thickness, light weight, but narrow working frequency range.
3. The absorber can work in a wide frequency range and can be made into any shape. Ferrite and short metal fibers are evenly dispersed in suitable organic polymer resin to make composite materials. The working frequency band can be widened by 40% - 50%. Its disadvantages are large thickness, complex process and high cost.
4. Coating shape, in the aircraft table
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