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1 Introduction
With the continuous development of military technology in various countries in the world, modern detection and guidance technologies such as radar, infrared, and laser are widely used in weapon systems, which greatly threatens the survival of army weapon systems. As an effective means to improve the survival, penetration and deep strike capabilities of weapon systems, the absorbing stealth technology has been attached great importance by various countries.Among them, radar stealth accounts for more than 60%, so the focus of stealth lies in radar stealth[1].In recent years, the world’s military powers have been competing to study radar absorbing stealth materials and stealth technology. Traditional absorbing materials, such as ferrite and metal ultrafine powder, have been difficult to meet due to reasons such as density and absorption frequency. Stealth materials are required to be “thin, light, wide and strong”. Therefore, the development of new radar absorbing materials has become the main research direction.[2,3].
The absorbing material is generally composed of a matrix material (or a binder) and an absorbing medium (absorbing agent). The existing radar absorbing coatings have problems such as poor weather resistance, low adhesion, and poor environmental adaptability, which requires the development of new coatings to meet the needs of army equipment surfaces.
At present, while further improving and improving the performance of traditional absorbing materials at home and abroad, on the one hand, they are working on the exploration of a variety of new materials, such as carbon nanotube materials.[4], hollow bead material[5],Conductive polymers[6],nanomaterials[7]It is gradually applied to radar stealth materials. On the other hand, it is committed to solving the problems of optimizing the formulation system and construction technology of absorbing coatings in engineering applications, and striving to make the absorbing coatings firmly adhere to the surface of the object to be coated. Continuous film formation, good weather resistance, strong environmental adaptability, etc.[8].
Hollow microbeads are a kind of potential absorbing material[9], because its intrinsic characteristics including shape, composition, structure and wall thickness can directly affect the fluctuation of its own permittivity and permeability, and then control the quality of absorbing performance, therefore, in special fields such as electromagnetic waves, hollow core Microbeads have also been widely studied and applied.
2. Overview of research on absorbing coatings
2.1 Research situation abroad
The research on absorbing coatings began during World War II, originated in Germany, developed in the United States and expanded to developed countries such as Britain, France, Russia and Japan. The developed wave-absorbing coatings have been successfully applied to military equipment such as aircraft, ships and ground armored vehicles. Army equipment such as Russia’s T-80 main battle tank, American M113 armored transport vehicle, etc., and other stealth weapons, such as stealth vehicles and stealth missile launchers developed by the United Kingdom and the United States. At present, the research on radar absorbing coatings abroad mainly focuses on ferrite absorbing coatings, carbonyl iron absorbing coatings, metal ultrafine powder or metal oxide magnetic ultrafine powder absorbing coatings, ceramic absorbing coatings, nano absorbing coatings Coatings, radioisotope absorbing coatings, conductive polymer absorbing coatings, retinyl Schiff base salt absorbing coatings, chiral absorbing coatings, doped high loss absorbing coatings, rare earth element absorbing coatings, etc. Research.
Japan is a world leader in the development of ferrite, and has developed a double-layer structure high-efficiency broadband absorbing coating composed of an impedance transformation layer and a low-impedance resonant layer, which can absorb radar waves from 1 to 2 GHz with a reflectivity of -20dB , is currently the best absorber. Singh et al.[10,11]CoTi-doped M-type calcium ferrite La(CoTi)xFe12-2xO19 composite lanthanum oxide and NiTi-doped M-type calcium ferrite La(NiTi)xFe12-2xO19 composite lanthanum oxide were studied in the frequency range of 8~12.4GHz The electromagnetic and absorbing properties of the material, with the increase of the amount, the dielectric loss keeps increasing, the magnetic loss is basically unchanged, the resonance frequency moves to the low frequency, and the matching thickness keeps increasing.
Shen et al.[12]W-type BaCo2Fe16O27 ferrite and BaO·7LaO·3Co2Fe16O27 ferrite were synthesized by sol-gel method, and the reflectivity was calculated by transmission line theory at 12.4~18GHz. The doped ferrite is large. By designing the double-layer structure of doped ferrite and chopped carbon fiber, the broadband absorption effect of the effective bandwidth of 5.2 GHz with reflection loss less than -10 dB in 12.4~18 GHz is obtained. The disadvantage of ferrite absorbing materials is that the relative density is high, which increases the weight of the components, and even affects the overall performance of the components, and the high-frequency effect is not ideal.
Carbonyl iron absorbent is a typical magnetic loss type wave absorbing material, and the magnetic loss angle can reach about 400. However, due to the large specific gravity of carbonyl iron absorbent, the volume duty ratio in the coating is generally greater than 40%, resulting in This wave absorbing coating still has the disadvantage of high areal density. Recently, the European GAMMA company has developed a new type of wave absorbing coating, which uses polycrystalline iron fiber mainly carbonyl iron monofilament as the absorbing agent, which can achieve high absorption rate in a wide frequency band. The volume duty cycle of the absorbent is 25%, so the weight can be reduced by 40% to 60%.
At present, the absorbing paint has been applied to the missiles and aircraft of the French National Strategic Defense Forces.
Radar absorbing materials used in high-speed aircraft components have to withstand long-term high temperature work, while ceramic materials have excellent mechanical properties and thermophysical properties, especially high temperature resistance, high strength, low creep, small expansion coefficient, resistance to It has strong corrosiveness and good chemical stability, and at the same time has the function of absorbing waves, which can meet the requirements of stealth, so it has been widely used as an absorbent. Ceramic absorbing materials mainly represent silicon carbide absorbing materials and silicon carbide composite absorbing materials. The French Alcole company uses ceramic composite fibers to manufacture unmanned stealth aircraft. This ceramic composite fiber is composed of glass fiber, carbon fiber and aramid fiber. After adding TiO2, it can withstand a high temperature of 1200 ° C. Sung-SooKima, etc.[13]Co and Co-Fe films are deposited on hollow ceramic substrates by electroless plating process. The prepared absorbers have low density and strong wave absorbing ability. The absorption peak and spectrum of the absorbers can be changed by adjusting the content of Co in the films. When the thickness of the coating is 1.5mm, the absorption capacity can reach -20dB, and when the thickness is 2.5mm, the absorption capacity can reach -25dB.
Because nanomaterials have the characteristics of good wave absorption, wide frequency band, good compatibility, low areal density and thin coating, the United States, Russia, France, Germany, Japan and other countries have regarded nanomaterials as a new generation of stealth materials. research and explore. At present, the so-called “super black powder” nano-absorbing material developed by the United States can absorb up to 99% of radar waves. France has developed a broadband microwave absorption coating, which is composed of adhesives and nano-particle filling materials. The structure formed by stacking multiple layers of thin films has good magnetic permeability and good wave-absorbing performance in the range of 50MHz to 50GHz.
Conductive polymer absorbing coatings use the linear or planar configuration of conjugated π electrons of some polymers and the transfer of polymer charges to complexes to design the conductive structure of the polymer to achieve impedance matching and Electromagnetic loss[14]. American Signal Products Company (SignatureProductsCompany) has developed a wave-absorbing coating that can be used to adapt to 5~200GHz radar. It is based on a polymer with spraying function as the matrix, and uses cyanate ester with excellent radar wave absorption properties. The composite of whiskers and conductive high polymer polyaniline is used as absorbent. However, because the research on the synthesis of conductive polymers used in such wave-absorbing coatings has just started, it is a new research field of polymer materials, and further theoretical and experimental research is needed.
Retinyl Schiff base salt is an organic polymer with carbon-nitrogen double bond structure, which has strong polarity. When radar waves are absorbed by this material, the energy can be quickly converted into heat energy and dissipated.[15]. The absorbing coating made of retinyl Schiff base salt at Carnegie-Mel-lon University in the United States can reduce the target RCS by 80%, and the specific gravity is only 10% of ferrite.
Chiral absorbing coatings are a new type of absorbing coatings. Numerous research results show that chiral materials can reduce the reflection of incident electromagnetic waves and absorb electromagnetic waves. In 1990, the absorbing effect of chiral materials was publicly reported for the first time abroad. The results showed that chiral absorbing materials have the characteristics of high absorbing frequency and absorption bandwidth. However, the research on chiral absorbing coatings is still in its infancy, and there are still many problems to be solved.
2.2 Domestic research situation
The domestic research work on absorbing coatings started from the “Seventh Five-Year Plan”. After more than ten years of development, a scientific and technological pattern combining basic research in universities and local research institutes with applied research in national defense research institutes has been formed. A new stage of transition from pre-research of basic materials to equipment application. With the implementation of my country’s “863” plan, the research on absorbing materials has also been vigorously developed.For example, universities and scientific research units such as National University of Defense Technology, Northwestern Polytechnical University, Harbin Institute of Technology, Aerospace Materials Research Institute are currently conducting research on absorbing materials, especially structural absorbing materials. Carbon fiber or silicon carbide fiber reinforced plastics are used as aircraft structural parts, which have both wave absorbing properties[16]. The State Key Laboratory of Solidification Technology of Northwestern Polytechnical University has successfully developed a new type of high temperature resistant absorbent mainly composed of SiC crystallites of solid solution N.
At present, the domestic research on radar absorbing coatings mainly focuses on ferrite series absorbing coatings, carbonyl metal powder absorbing coatings, nano absorbing coatings, conductive polymer absorbing coatings, polycrystalline iron fiber absorbing coatings, etc.[17-19]However, due to the shortcomings of these radar absorbing coatings, such as anti-oxidation, poor acid and alkali resistance, poor low-frequency absorption performance, high density, and large absorbent volume, there is no large-scale application in our military equipment.
Among the ceramic absorbing materials, silicon carbide is the main component for making multi-band absorbing materials. It has the possibility to achieve light-weight, thin-layer, broadband and multi-band absorption, and has broad application prospects.Wang Jun et al[20]Ultrafine metal cobalt powder with an average particle size of 30nm was uniformly dispersed into polycarbosilane by ultrasonic waves, and the mixed magnetic silicon carbide ceramics with good mechanical properties and continuously adjustable resistivity were prepared by melt spinning and sintering. fiber.The 3-layer structure absorbing material prepared by arranging this fiber orthogonally and compounding with epoxy resin has good microwave absorption characteristics, and its reflection attenuation in the range of 8.0~12.4GHz is more than -12dB, and the maximum can reach -16.3dB. Among them, the bandwidth less than -15dB is about 1.2GHz; Sun Jingjing et al.[21]The effect of Al doping amount on the dielectric properties of nmSiC powder was studied. In the range of 8.2~12.4 GHz, the relative permittivity and dielectric loss tangent of β-SiC (undoped Al) were higher than those of Al-doped samples. And these two parameters decrease with the increase of aluminum content.
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