The main methods to achieve the target stealth technology are shape stealth technology and material stealth technology. Material stealth technology occupies an important position in stealth technology because of its simplicity and ease of implementation. Stealth materials can be divided into structural stealth materials and coated stealth materials according to their application forms.
Absorbing composite materials generally require the following characteristics:
(1) Thin thickness requires that the incident electromagnetic wave is quickly lost and absorbed in the thinnest possible absorbing composite material in the stealth electromagnetic wave frequency band. The level of material loss absorption is generally determined by the material's magnetic loss and electrical loss. Therefore, the researches mostly focus on improving the absorptivity of the material and reducing the thickness of the coating from the two aspects of impedance matching and improving the electromagnetic parameters of the material.
(2) The frequency bandwidth requires that the frequency band with attenuation ≤ 10dB be as wide as possible. In a sufficiently wide working frequency band, a good match between the material and the air is required, so that the total reflection between the air and the material interface is small. This requires the material to have better frequency characteristics, and make full use of the material's performance through reasonable design.
(3) Light weight requires low surface density and light weight per unit area of the absorbing composite material.
(4) The strong function requires that the absorbing composite material can be used as a absorbing material as well as a structural material, with high mechanical properties and good environmental adaptability and physical and chemical properties.
Absorbing composite materials can be divided into coating type and structural type according to the material molding process and carrying capacity. Coated absorbing composite materials are mixed absorbent and binder and then coated on the target surface to form a absorbing composite coating; while structured absorbing composite materials usually disperse the absorbent in special fibers (such as quartz). Fiber, glass fiber, etc.)-reinforced composite material with a wave-absorbing structure formed by the composite material, which has the dual functions of carrying and absorbing radar waves.
The absorbing composite material used for the control of the target characteristic signal is mainly composed of absorbent, matrix (binder) and additives. Among them, the quantity, performance and matching of the absorbent are important links in the absorbing material, which provides the absorbing performance required by the absorbing composite material; the matrix acts as a skeleton in the absorbing composite material, and it provides the absorbent on which it functions The carrier; additives can improve the production process, improve construction conditions, improve product quality, and give special functions to the absorbing composite material.
1. Carbon fiber absorbing composite material
The carbon fiber structure absorbing composite material is a kind of multifunctional composite material, which has the dual functions of bearing and reducing the radar reflection cross section. Carbon fiber absorbing composite materials have the advantages of high strength, high modulus and light weight. They are not only widely used in general aircraft and missiles, but also increasingly appear in stealth weapons. The United States has adopted a large number of carbon fiber structure absorbing composite materials on stealth aircraft such as B-2 and F-117A.
The emergence of high-performance carbon fiber truly provides a structural material that can replace metal as the main bearing member, and it also ushered in a new era of rapid development and application of structural absorbing materials. The high-performance carbon fiber has a low resistivity, about 10-2Ω. Cm, and is a strong reflector of radar waves. The electrical conductivity of the high-temperature treated carbon fiber is suitable as a conductive reflective material and reinforcement of a wave absorbing material. Only specially treated carbon fiber can absorb radar waves. Adjusting the resistivity of carbon fiber can make it have a wave-absorbing function. In 1982, a Japanese patent reported a laminated flat wave-absorbing composite material, composed of 50% (volume fraction) carbon fiber and epoxy resin, with a thickness of 3mm, a density of 0.7g/crr13, and a metal plate with a back surface of 1mm. In the range of ~12GHz, the reflection attenuation is greater than 15dB. The key to this technology is the use of a special wave-absorbing carbon fiber. There are many ways to make carbon fiber have a suitable resistivity to absorb radar waves. Reduce the carbonization temperature of the carbon fiber, make the cross section of the carbon fiber into a triangle or an angular square, modify the surface, coat the surface of the resin with electromagnetic loss substances, deposit a layer of carbon powder with tiny holes, spray nickel or Nitride treatment can greatly improve the absorbing performance of carbon fiber.
Common carbon fiber absorbing composite materials are as follows: ① Carbon/carbon absorbing composite material (abbreviated as C/C) is an excellent high temperature resistant absorbing material, which is composed of a carbon fiber skeleton and a carbon matrix. The characteristics of high temperature, low linear expansion coefficient at high temperature, good thermal conductivity and increase in strength with increasing temperature. At present, C/C absorbing composite materials have been used in long-range missiles, rocket nozzles, nose cones, and space shuttles. The machine is on the front line, which can well suppress infrared radiation and absorb radar waves; ②The carbon/thermoplastic resin absorbing composite material has excellent absorbing performance, which can greatly attenuate pulses with a frequency of 0.1MHz~50GHz, and the carbon fiber composite material can Absorb radiant heat without reflecting radiant heat, which can reduce the characteristics of radar waves and infrared rays. It can be used to make engine compartment skins, wing front lines and even the front section of the fuselage; ③The carbon fiber hybrid absorbing composite material is adopted Absorbing composite materials that meet special performance requirements or have better comprehensive performance are manufactured with a certain mixing ratio between fibers and structural design forms. The resistivity of carbon fiber is very low, and the resistivity of SiC fiber is high, and the absorbing effect is not good. By artificially designing carbon fiber and SiC fiber in different ratios to control their resistivity, it can be made into a composite material with high temperature resistance, oxidation resistance, excellent mechanical properties and good wave absorbing properties.
2. Silicon carbide fiber absorbing composite material
Silicon carbide (SiC) series fibers have high strength. High modulus, low thermal expansion coefficient, adjustable resistivity and other characteristics, high temperature resistance, small diameter, easy to braid waiting point, is an ideal reinforcing agent for high-performance composite materials. SiC fiber can be used as a wave absorber as well as a reinforcing material. The electromagnetic wave absorber made from it and the resin matrix is not only excellent in strength, heat resistance, and chemical resistance, but more importantly, it has excellent electromagnetic wave absorption ability in the frequency range of 8~16GHz. The shortcomings that the previous materials are difficult to overcome. Since the frequency used by current military radars is usually 6~16GHz, this electromagnetic wave absorber has a unique stealth effect when applied to military aircraft or other aerial targets.
3. Nano absorbing composite material
The quantum size effect and tunneling effect caused by the special structure of nanomaterials cause it to produce many special properties that are different from conventional materials. On the one hand, the size of nanoparticles is 1~100nm, which is much smaller than the wavelength of electromagnetic waves emitted by radar. Therefore, the transmittance of nanoparticle materials to this wave is much stronger than that of conventional materials, which greatly reduces the reflectivity of the wave and makes the radar The received reflected signal becomes very weak, so as to achieve the effect of stealth; on the other hand, the specific surface area of nanoparticle materials is 3 to 4 orders of magnitude larger than that of conventional particles, and the absorption rate of electromagnetic waves and infrared light waves is also greater than that of conventional materials. Many, the infrared light emitted by the detection object and the electromagnetic wave emitted by the radar are absorbed by the nanoparticles, making it difficult for infrared detectors and radars to detect the detected target. In addition, with the refinement of particles, the surface effects and quantum size effects of the particles have become prominent. The interface polarization and multiple scattering of the particles can become important absorption mechanisms. The quantum size effect causes the electronic energy levels of nanoparticles to split. The interval is in the microwave energy range (10-2~10-5eV), thus forming a new absorbing channel. The rapid development of nanotechnology and the excellent electromagnetic absorbing properties of nano-powders have made nano absorbing composite materials a hot spot in the research of absorbing composite materials at home and abroad. Nano absorbing composite materials have become an important new in the research and development of absorbing composite materials. Field.