Absorbing paint

Introduction
Absorbing coating, also known as microwave absorbing coating or radar wave absorbing coating. It is a functional material used to cover the target with a coating layer to achieve radar stealth. It can convert the incident radar wave energy into heat energy and dissipate it or eliminate or weaken it through resonance effect to achieve the purpose of effective absorption and attenuation. Composed of film-forming substances, absorbers, etc. The former is organic polymer or inorganic adhesive,

The latter are powders, short fibers or chiral materials that have loss function to electromagnetic waves. The main performance characterization parameters include reflectance (-dB), absorption bandwidth (Δf), coating thickness (d) and areal density (kg/m2), etc. The basic meaning is shown in the figure. Generally, under the condition that the coating density and thickness are strictly controlled, it is required to have a lower reflectivity in a wider frequency band. The purpose of the coating is to reduce the radar cross-sectional area (RCS) of the target, so that military targets such as aircraft and ships have stealth performance; it can be used to eliminate electromagnetic interference of radar systems and electromagnetic wave shielding of microwave equipment. The comprehensive application of stealth technologies such as wave-absorbing coatings, structural design, and electronic countermeasures has enabled the self-protection and attack capabilities of aircraft and other weapons to achieve a qualitative leap. The development of new absorbents is the key to further improving the performance of the coating. Nano absorbing coatings, chiral absorbing coatings, polycrystalline iron fiber absorbing coatings, functional polymer absorbing coatings and intelligent absorbing coatings are being studied. .
Composition of absorbing coating

Absorbing paint is formed by dispersing powder (absorbent) with specific dielectric parameters in a matrix (binder). Generally speaking, the substrate plays the role of bonding, strength and resistance to the environment, and the absorbent plays the role of electromagnetic loss.

Commonly used absorbent coatings:
1. Ferrite series are divided into spinel type and ferrite containing large-size divalent metal ions: ①Stone type ferrite: such as Mn-Zn, Ni-Zn, Li-Ti and other series, application widely. But its ε and μ are small, it is difficult to meet the principle of ∣μr-εr∣ as small as possible, so other magnetic powder additives are often needed; ②Large-size divalent metal ion ferrites, such as barium, strontium, calcium, lead, etc. Hexagonal crystal structure.
2. Ceramic series, mainly SiC ceramics, with high temperature resistance, high strength, corrosion resistance, low creep, low expansion coefficient, and good chemical stability. It is a good absorbent for making multi-band (cm, mm, infrared).
3. Conductive polymer series. Conductive polymer is made of an insulating polymer material with a conjugated main chain through chemical or electrochemical methods and a charge transfer compound with dopants. The change in conductivity can be controlled, and the main obtained are: retinoid Schiff base and its complexes.
4. Metal powder series, including carbonyl iron powder, carbonyl nickel powder, permalloy, etc. Its characteristics are as follows: ①High activity; ②Good electromagnetic parameters and easy to adjust.
5. Polycrystalline iron fiber series include iron fiber, nickel fiber, drill fiber and alloy fiber. Its characteristics are as follows: ① light weight (area density less than 2kg/m2); ② frequency bandwidth (4～18GHz); ③ easy to adjust electromagnetic parameters (change fiber length, diameter and arrangement).
6. Nano-material series, using the special surface effect and volume effect of nano-materials, with good wave-absorbing characteristics.
classification
According to the different coating absorbers, the following mainly introduces several types of commonly used absorbing coatings: ferrite absorbing coatings, nano absorbing coatings, polycrystalline iron fiber absorbing coatings and conductive polymer absorbing coatings.

Ferrite absorbing coating
Ferrite absorbing material is a more mature and well-researched absorbing material. Because of its high permeability and resistivity at high frequencies, electromagnetic waves are easy to enter and decay quickly, and are widely used in In the field of radar absorbing materials.
Compared with magnetic metal powder, ferrite material has better frequency characteristics, its relative permeability is larger, and its relative permittivity is smaller. It is suitable for making matching layer and has good application prospects in the aspect of low frequency and broad band. The main disadvantages are high density and poor temperature stability. For this reason, researchers from various countries hope to improve the loss characteristics and reduce the density by adjusting the chemical composition, particle size and distribution, particle morphology and dispersion technology of the material itself.
Ferrite absorbing materials are usually divided into two types: spinel ferrite and hexagonal ferrite. Among them, spinel ferrite has a long history of application, but spinel ferrite has a long history of application. The electromagnetic parameters (permittivity and permeability) are relatively small, and it is difficult to meet the principle that the relative permittivity and relative permeability are as close as possible. Therefore, a single ferrite material cannot meet the absorption frequency bandwidth, thin thickness and areal density. Small request. However, the composite ferrite material made by dispersing ferrite powder into magnetic particles can control its electromagnetic parameters through the particle size and composition of the ferrite powder.

Nano absorbing coating
Nanomaterials have excellent absorbing properties, as well as broadband, good compatibility, light weight and thin thickness. The United States, Russia, France, Germany, Japan and other countries have all studied nanomaterials as a new generation of absorbing materials. And explore.
Due to the quantum size effect and tunneling effect caused by the special structure of nanomaterials, it produces many specific energies that are different from conventional materials. On the one hand, because the size of nanoparticles is 1-100nm, which is much smaller than the wavelength of electromagnetic waves emitted by radar, 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 reflection signal becomes very weak, so as to achieve the effect of stealth; on the other hand, the specific surface area of nano-particle 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. The infrared light emitted by the detected 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 the particles, the surface effects and quantum size effects of the particles 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 the nanoparticles to split. The interval is in the microwave energy range (10-2～10-5eV), which leads to new absorbing channels. The transmittance and absorption rate of nano-materials to electromagnetic waves are much greater than that of micron-sized powders. The "super black powder" nano-wave absorbing material developed by the United States has an absorption rate of more than 99% for radar waves. Magnetic nanoparticles, nanoparticle films and multilayer films are the main forms of nanomaterials used as stealth materials.

Polycrystalline iron fiber absorbing coating
The new polycrystalline iron fiber absorbent is a light-weight magnetic radar wave absorbent. This polycrystalline iron fiber is a carbonyl iron monofilament with a diameter of 1 to 5 μm and a length of 50 to 500 μm. The fiber density is low and the structure is Isotropic or anisotropic. Research shows that: polycrystalline iron fiber absorbing material has the advantages of wide absorption frequency band, low density and high absorption performance. China is vigorously carrying out research on polycrystalline iron fiber absorbing materials, that is, using magnetic fibers in radar absorbing materials. This material loses electromagnetic wave energy through various mechanisms such as eddy current loss, so it can achieve broadband and high absorption, and can reduce 40% to 60% weight compared with general absorbing coatings, and overcomes the serious shortcomings of most magnetic absorbents.

Conductive polymer wave-absorbing coating
This type of absorbing material uses the linear or planar configuration of certain polymers with conjugated electrons and the transfer of polymer charges to the complex to design the conductive structure of the polymer to achieve impedance matching and electromagnetic loss. At present, the research on organic polymers with microwave electrical and magnetic loss properties has attracted more and more attention from all over the world. The absorbing coating made of retino-based Schiff base from Garnegie-Mellon University in the United States can reduce the target's RCS (radar effective cross section) by 80%, while the specific gravity is only 10% of ferrite. A transparent absorbing material has been developed in China, which is a mixture of conductive polymer aniline and cyanate whiskers, suspended in a polyurethane or other polymer matrix. This material can be sprayed or combined with The material constitutes a laminated material. The characteristic of this kind of coating is that the absorbing agent is evenly distributed in the coating, which has changed the shortcomings of the uneven distribution of the components of the traditional absorbing material. Therefore, there is no need to increase the thickness to increase the bandwidth, and the process is simple. The spray gun can be sprayed on any part of the aircraft (including the nose, tail, rivets, seams, etc.), which is especially suitable for stealth modification of old aircraft. In addition, this absorbing coating is optically transparent, which is suitable for hiding the electromagnetic window of the cockpit cover and night vision infrared device.

Factors affecting performance
In the preparation process of the absorbing coating, the factors that affect the performance of the coating are:
(1) The dispersion and stability of the wave absorber. The dispersion of the absorbent particles in the coating has an important impact on the absorbing performance of the sample, and the highly dispersed state of the particles can give full play to the superiority of the nano absorbent. The density of nano-particle metal composite powder is relatively high, it is easy to settle in the coating matrix, and it has poor compatibility with the coating matrix, and it is difficult to uniformly disperse in the coating matrix. In order to improve the dispersion and stability of the nano-wave absorbing agent in the resin, it can be modified by surface coupling agent modification and polymer coating.
(2) The choice of base material. In the actual application process, the absorbent will be compounded with other matrix materials to prepare a wave absorbing material. The choice of matrix material should focus on minimizing the impact on the absorbent in the absorbing material, giving full play to the performance of the absorbent, and comprehensively considering the structural requirements, adhesion, abrasion resistance, aging resistance, and resistance of the absorbing composite material. Factors such as scouring performance. Polymers with rigid groups should be preferred to meet the needs of preparing structural stealth materials, and matrix materials that have certain microwave absorbing properties and whose absorption frequency bands are best complementary to the absorbent should be preferred to achieve the purpose of broadband absorption. The matrix material can be thermosetting resin, such as commonly used epoxy resin, unsaturated polyester resin, phenolic resin, etc., or thermoplastic resin, such as rubber, nylon, polystyrene, etc. In the coating type absorbing material. The absorbent is dispersed in the binder to make a wave-absorbing coating, which is coated on the application target. The most commonly used binder is the resin matrix. Commonly used matrixes include epoxy resin, methacrylic resin, phenolic resin, and vinyl ester resin.
(3) The effect of the amount of curing agent and curing temperature on the absorbing properties of the sample. The contact state of the absorbing agent particles in the coating has an important influence on the absorbing performance of the sample, and the curing temperature and curing time of the coating have an important influence on the contact state of the particles, which in turn affects the absorbing performance of the coating. The curing temperature of the coating affects the curing speed of the coating. The higher the curing temperature, the faster the curing speed; the curing time is short, but the rapid volatilization of the solvent will cause pinholes in the coating, causing poor contact between the absorber particles and coating conductivity. Poor: If the curing temperature is too low, the curing will take a long time, which will cause the particles of the wave absorbing agent to settle and agglomerate, and the epoxy resin will float on the surface of the coating, which will reduce the wave absorbing performance of the coating. The amount of curing agent also affects the curing speed of the coating. If the amount of curing agent is large, the curing speed of the coating will be faster and the curing time will be short, but it will have an adverse effect on the mechanical properties of the coating. It is not conducive to the coating operation; if the amount of curing agent is small, the curing speed is slow, and the curing time is long. When choosing epoxy resin E-44 as the base material of the wave-absorbing coating, HF-1 is chosen as its curing agent. HF-1 curing agent is a room temperature curing agent, and its use concentration is generally 13% to 20%. Since more solvents are used in the dispersion of the absorber particles, the curing agent is taken as 16%, and the coated aluminum plate is placed in an oven at 30°C for a little drying to evaporate the solvent and then solidify at room temperature. To shorten the curing time.

Application of microwave absorbing coating
One of the main functions of ultra-fine powder is the combination of ultra-fine powder and surface technology to form a surface composite coating. This coating can improve the mechanical, physical and chemical properties of the substrate surface, and endow the substrate surface with new mechanical, thermal, optical, electromagnetic and catalytic sensitivity functions. Ultra-micro or nano-coating materials can be divided into structural coatings and functional coatings according to their uses. Structural coatings include high-strength, high-hardness and wear-resistant coatings, heat-resistant, high-temperature and oxidation-resistant coatings, corrosion-resistant, protective and decorative coatings; functional coatings include thermal coatings, optical coatings, electrical coatings, Magnetic coating and catalytic sensitive coating, etc.

Ultrafine powders and nano coatings are widely used as protective coatings for aerospace vehicles, microwave filtering and wave absorbing coatings, ultraviolet protective coatings and "stealth" coatings. For example, the surface effect of nanomaterials is used to prepare nanocomposite coatings that absorb electromagnetic waves in different frequency bands. Nano powders that can be used as radar wave absorbers include: composite powders of nano metals (Fe, Co, Ni, etc.) and alloys, and nano oxides (Fe3O4, Fe2O3, ZnO, NiO2, TiO2, MoO2, etc.) powder, nano graphite, nano silicon carbide and mixed powders. The composite powder of nano-alumina, iron oxide, silicon oxide and titanium oxide combined with polymer fibers has strong absorption performance in the mid-infrared band. This composite powder has a good shielding effect on infrared detectors in this band . Nano-magnetic materials, especially nano-magnetic materials similar to ferrite, are filled into the coating, which not only has good functions of absorbing and dissipating infrared rays, but also has good characteristics of absorbing radar waves. In addition, the density is small, and it is used in stealth. Certain advantages. Nano-scale borides and carbides, including nanofibers and carbon nanotubes, will also be useful in the application of stealth materials.

Microwave absorbing material is a kind of functional material that can absorb electromagnetic waves with little reflection, scattering and transmission. Electromagnetic pollution has become one of the most important environmental pollution in the world in this century. The most effective way to control electromagnetic wave pollution is to use absorbing materials to absorb it. For example, composite materials made of iron oxide nanoparticles and polymers can absorb and attenuate electromagnetic waves and sound waves, reduce reflection and scattering, and are considered to be an excellent wave absorbing material. According to data reports, the absorption peak of this nanocomposite multilayer film at the frequency of 7GHz～17GHz is as high as 14dB, the absorption level of 10dB is 2GHz, and the multilayer film with a thickness of tens of nanometers is equivalent to the usual absorption of tens of microns. The effect of wave material. Microwave absorbers are the main body that absorbs electromagnetic waves and should have the characteristics of high absorption, wide frequency bandwidth, and light weight. However, traditional wave absorbers have their own shortcomings. For example, the thickness of dielectric absorbers is large and the absorption band is narrow; although ferrite absorbers have the advantages of strong absorption, bandwidth, and low cost, they have high density and are not resistant to high temperatures. , When the temperature rises, its wave-absorbing performance is greatly reduced; the microwave permeability of ultrafine metal powder is high, and the temperature stability is good, but its anti-oxidation, acid and alkali resistance is poor, the dielectric constant is large, and the frequency spectrum characteristics Poor, low frequency absorption performance is poor, and the density is high, the volume duty ratio of the absorbent is generally greater than 50%. In recent years, some new absorbing materials have been reported in the literature, such as chiral media absorbents, conductive polymer absorbents, retinyl Schiff base salts, nano absorbents, whisker absorbents, polycrystalline iron fibers and radioisotopes, and Rare earth element absorbents, etc. Among them, the nano-microwave absorber has the characteristics of wide frequency band, good compatibility, light weight, thin thickness, etc., while having good absorbing performance. It is a kind of stealth material with great development prospects. It is used in military, environmental protection, and human protection. It has good application prospects and has become a research hotspot in the United States, Japan, Russia, France, Germany and other countries. The main aspects of current research include: nano-metal and alloy absorbing materials, nano-ceramic absorbing materials, nano-oxide absorbing materials, transition metal sulfide nano-absorbing materials and nano-composite absorbing materials. Different types of nano-absorbent materials have their own advantages and disadvantages in physics and chemistry. Metal nano-absorbing

The wave material has the characteristics of large specific surface area and relatively more surface atoms of the particles, but its magnetic loss is not large enough, and the magnetic permeability decreases slowly with the increase of frequency, which is not good for frequency broadening, and its chemical stability and corrosion resistance Not ideal. Nano-ceramic absorbing materials have the characteristics of high temperature resistance, light weight, high strength and good absorbing performance, but the preparation process needs further improvement, and the infrared absorption of this material is strongly dependent on the annealing temperature. Nano-oxide absorbing materials have good absorbing performance and the function of inhibiting infrared radiation. Transition metal sulfide and its doped absorbing materials have absorption properties in ultraviolet, near ultraviolet, visible light and near infrared light regions.

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