Stealth technology is one of the keys to success in modern warfare. The use of absorbing stealth materials can reduce the radar scattering area of the aircraft with almost no impact on the aerodynamic and strength performance of the aircraft. It is especially suitable for some parts that cannot or are difficult to take shape measures, such as bombs. Wing, wing leading edge, etc.
Ferrite, metal powder, barium titanate, silicon carbide, graphite, conductive fiber, etc. are all traditional absorbing materials, and they usually have shortcomings such as narrow absorption band and high density. New types of absorbing materials include nanomaterials, metal fiber materials, "chiral" materials, conductive polymers and circuit analog absorbing materials, etc. They have a new absorbing mechanism different from traditional absorbing materials.
Radar stealth technology is an indispensable electronic countermeasure technology in modern warfare. The Gulf War and NATO’s attack on Yugoslavia demonstrated that stealth technology has become one of the keys to victory in modern warfare. There are two main technical ways for aircraft (aircraft, missiles) to stealth: one is to reduce the radar cross-section area RCS (Radar CrossSection) of the aircraft through shape design; the other is to use materials that absorb radar waves in the aircraft, namely the radar absorbing material RAM ( Radar Absorption Material), which uses the interaction between the absorber and electromagnetic energy to achieve the purpose of loss of electromagnetic energy. The shape stealth technology can reduce the RCS of the aircraft without increasing the weight of the aircraft, and it is effective in a wide frequency band, but the subsequent problems are the change of aerodynamic performance (generally deteriorated) and the reduction of strength. The use of RAM can reduce the RCS of the aircraft with almost no impact on the aerodynamic and strength performance of the aircraft. It is especially suitable for some parts that cannot or are difficult to take shape measures, such as missile wings, wing leading edges, etc., so research and develop high performance Radar absorbing materials have become a major issue in the field of military technology in various countries.
The study of electromagnetic wave absorption began in the 1930s, when the Dutch developed the first wave-absorbing material. This absorbing material uses high-loss carbon black and high-dielectric constant titanium dioxide as the medium, so that the absorbing material has a smaller thickness.
During World War II (1935-1945), Germany developed the "Wesch" material for submarine stealth, which was composed of about 0.3 inches of rubber-based carboxyl-iron composite panels. Because the surface has a lattice structure, this material can absorb electromagnetic waves in a wide frequency range. In addition, the Germans also invented the Jaumann layer absorbing material composed of multilayer resistors and dielectric material layers alternately stacked. This material can achieve a reflectivity of -20 dB in the radar band. Subsequently, the American Salisbury W. W. invented the λ/4 resonant absorption screen, which can broaden the absorption bandwidth by 25% at the resonance frequency. This structure was named Salisbury screen.
In the 1950s, Sponge Company commercialized the "Spongex" radar absorbing material. When this material was made 2 inches thick, the reflectivity at 2.4~10GHz could reach -20 dB. In addition, Severin and Meyer began to study The circuit simulates absorbing materials, and the absorbing materials loaded by circuit loops, sheets, dipoles, etc. are studied through experiments. This is also the origin of the research on frequency selective surface (FSS) absorbing materials.
Since the 1980s, with the development of computer technology, absorbing materials have entered the stage of precise optimization design. According to the electromagnetic parameters of the material, the absorbing performance at a given thickness and frequency can be calculated through computer-aided design technology, and the absorbing material can be optimized at the same time. For example, using genetic algorithm, finite element method, FDTD and other technologies to optimize the Jaumann layer structure. Conductive polymer-based composite materials and chiral materials are gradually used in the field of absorbing materials, and the effective medium theory is used to calculate the complex permittivity and complex permeability of these new materials.
Structure type and design
According to the absorption mechanism of radar absorbing materials, it can be divided into impedance matching and resonance absorbing materials.
Impedance matching type absorbing material
Cone shape absorber
The cone-shaped absorbing material is a typical structural absorbing material. The cone structure of the material makes the impedance from the cone absorber
There is a gradual process from air to the bottom end of the absorbing material, but its disadvantage is that it is thick and easily broken. After reasonable design and improvement, this absorbing material is widely used in microwave anechoic chambers and other fields.
The wave-absorbing material of the matching layer is based on the cone-shaped wave-absorbing material, which can reduce the thickness of the material without affecting the wave-absorbing effect. This kind of absorbing material is to set an impedance matching layer between incident and absorption. The impedance value of the impedance matching layer is between the air and the absorption layer. When the thickness of the matching layer is λ/4, the matching effect is most obvious.
Resonant Absorbing Material
Resonant absorbing materials use the principle of interference to reduce the reflection of electromagnetic waves. They are also called λ/4 absorbing materials and include Dallenbach layer, Salisbury screen and Iaumann layer. The impedance of this type of material does not match air, and the material has certain requirements for thickness, so it cannot completely absorb all the electromagnetic energy.
Dallenbach layer absorbing material
The Dallenbach layer absorbing structure is composed of a uniform lossy dielectric layer placed in front of the conductive plate.
Studies have found that a single-layer Dallenbach structure cannot obtain a broadband absorbing material. Therefore, people use a multilayer structure to broaden its absorption frequency. Mayer F. Use two or more absorbing layers to increase the absorption bandwidth. The first layer is a absorbing material. The ferrite layer at the interface with air, the second layer is a short fiber layer containing metal. The Lagrangian method can be used to optimize multilayer Dallenbach absorbers. This method has been used to design cone-shaped and λ/4 absorbers."
Salisbury screen absorbing material
The Salisbury screen is a resistive screen with suitable impedance placed at λ/4 of the reflective surface of the metal back to form a resonant wave-absorbing structure. This structure can make the electromagnetic waves reflected from the metal backing and the impedance layer have opposite phases, thereby achieving "zero reflection".
The absorbing performance of the Salisbury screen is related to the thickness of the absorbing body and the dielectric constant of the dielectric layer. Absorb
The increase in the thickness and dielectric constant of the body will increase the electrical length, which will shift the absorption peak to low frequencies and achieve strong low frequency absorption. According to the structure of the Salisbury screen, the resistive film of purely electrical lossy materials should be placed at the maximum electric field, that is, λ/4 away from the metal surface; while the magnetic lossy materials should be placed at the maximum magnetic field, that is, the metal surface is the best. Since the electrical loss screen must be placed at λ/4 above the metal surface, which results in a thicker absorbing structure, this structure can only be applied to occasions where the size of the material is not limited. In addition, the Salisbury screen cannot achieve "zero reflection" for electromagnetic waves outside the resonance frequency, and the bandwidth of the absorption frequency is relatively narrow. Conductive polymers can also be used as absorbing materials to design and prepare Salisbury screens, and use the optical transmission matrix method to study its absorbing properties.
Jaumamn layer absorbing material
On the basis of the Salisbury screen, the absorption bandwidth was improved by increasing the number of electric screen sheets and isolation layers, and thus developed a new absorbing material called Jaumann absorbing material.
Various layers of materials with different electromagnetic properties can be optimized and combined by various algorithms to obtain absorbing materials with appropriate impedance and better performance. Through analysis and study of impedance matching characteristics, it is found that the tapered combination of dielectric layers can meet the impedance matching conditions. At the same time, the introduction of reactive factors such as capacitive reactance and inductive reactance can better improve its thickness and bandwidth characteristics. The tapered dielectric impedance structure can also be used to meet the requirements. The impedance of the dielectric layer changes continuously while the loss increases, thereby realizing the resonance absorption of the structural wave absorbing material in a wider frequency band.
The internal structure of the multilayer Jaumann layer absorbing material is separated by a low-loss dielectric. A 6-layer Jaumann layer absorber has a reflectivity of -30 dB at 7~15GHz. J. K. Nortier et al. used the equivalent transmission line theory to study a 7-layer resistor chip Jaumann type wave absorber, and the result showed that the structure has an ideal absorption bandwidth.
Research on microwave absorbing materials started in the United States and Germany as early as the Second World War, and there have been more than ten kinds of them. The absorbing performance of the absorbing material depends on the loss absorbing ability of the absorber, so the research of the absorber has always been the focus of the research on the absorbing material. At present, the most important absorbents are:
Ferrite series absorbent
Ferrite series absorbents include nickel-zinc ferrite, manganese-zinc ferrite and barium-based ferrite, etc., which are the earliest developed and most widely used absorbents. Due to the strong ferromagnetic resonance absorption and the dispersion effect of magnetic permeability, ferrite absorbing materials have the characteristics of strong absorption and absorption frequency bandwidth, and are widely used in stealth fields, such as the SR-1 high-altitude reconnaissance aircraft in the United States. With a ferrite absorbing coating. At present, ferrite materials are still the main body of developing thin-layer broadband absorbing materials. There are mainly hexagonal ferrite and spinel ferrite. Ferrite materials have high magnetic permeability at high frequencies, and their resistivity is also high, electromagnetic waves are easy to enter and get effective attenuation. In recent years, there have been many studies on flake hexagonal ferrite. However, the ferrite series absorbent has a large specificity and high frequency performance is not very ideal.
Polycrystalline iron fiber series absorbent
Polycrystalline iron fiber series include iron fiber, nickel fiber, drill fiber and alloy fiber. Polycrystalline iron fiber has the advantages of light weight and bandwidth due to its unique shape characteristics and compound loss mechanism (magnetic loss and dielectric loss). Adjusting the length, diameter and arrangement of the fibers can easily adjust the electromagnetic parameters of the absorbing coating. Polycrystalline iron fiber has particularly outstanding absorbing performance in the low frequency band of microwaves. In addition, if other conductive fibers, such as copper fibers, carbon fibers, etc. are added to the absorbing coating, the interaction with the incident electromagnetic field will cause energy absorption and radiation, which can "amplify" the function of the absorber and reduce the thickness of the coating. And weight, can broaden the absorption band.
Conductive polymer absorbing materials use the linear or planar configuration of some polymer with conjugated electrons to interact with polymer charge transfer complexes to design its conductive structure to achieve impedance matching and electromagnetic loss, thereby Absorb radar waves. Its electrical conductivity can vary within the range of insulators, semiconductors, and metal states. The electromagnetic parameters depend on the main chain structure of the polymer, room temperature electrical conductivity, dopant properties, microscopic morphology and other factors. Due to the diversified structure, low density and unique physical and chemical properties of conductive polymer absorbing materials, they have developed rapidly in the past 10 years. A new type of lightweight broadband absorbing material can be developed by combining conductive polymers with inorganic magnetic loss materials.
Studies have shown that chiral materials can reduce the reflection of incident electromagnetic waves and absorb electromagnetic waves. In practical applications, chiral materials can be divided into intrinsic chiral materials and structural chiral materials. The former's own geometric shape makes it a chiral object, and the latter is through its different parts of anisotropy. It forms a certain angle relationship with other parts and produces chiral behavior, which makes it a chiral material. Compared with general absorbing materials, chiral materials have the advantages of high absorbing frequency and bandwidth, and the absorbing characteristics can be improved by adjusting the chiral parameters. It has great potential in improving absorbing performance and expanding absorbing bandwidth. The current method of manufacturing chiral absorbing materials is to add micro-bodies of appropriate size and chirality to ordinary media. The research of chiral materials is currently a hot field. Once chiral materials are put into practical use, stealth technology will be improved to a new level.
Magnetic metal nanoparticle absorbent
This material has a strong surface effect. Under the electromagnetic field radiation, the movement of atoms and electrons is intensified, which promotes magnetization and converts electromagnetic energy into heat energy, which can absorb electromagnetic waves well, so it can be used for millimeter wave stealth and visible light-infrared stealth. During the Gulf War in 1991, the F-117A fighter jet that made the U.S. show the limelight was coated with a variety of ultrafine particles that can absorb infrared and microwaves, especially nanoparticles. They are sensitive to electromagnetic waves of different bands.
It has strong absorbing ability, so it has excellent broadband microwave absorbing ability, which can evade radar surveillance. On the one hand, the principle of wave absorption is that the size of nanoparticles is much smaller than the wavelength of infrared and radar waves, so 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 infrared detection The reflection signal received by the detector and radar becomes very weak, so as to achieve the effect of stealth; on the other hand, the specific surface area of the nanoparticle material is 3 to 4 orders of magnitude larger than that of the conventional coarse powder, and the absorption rate of infrared light and electromagnetic waves is also higher than Conventional materials are much larger, which greatly reduces the intensity of the reflected signals obtained by infrared detectors and radars, so it is difficult to find the detected target, which plays a role of stealth.