With the rapid development of modern military technology, the defense systems of all countries in the world are more and more likely to be detected, tracked, and attacked by the enemy, and the survivability of military targets and the penetration capabilities of weapon systems are seriously threatened. Stealth technology, as an effective means to improve the survival and penetration of weapon systems, especially the ability of deep strikes, has become the most important and effective breakthrough in a three-dimensional modern war that integrates land, sea, air, space, electricity, and magnetism. Defense tactics and technical means are highly valued by countries all over the world.
Modern warfare places higher and higher requirements on the absorbing performance of absorbing materials, and it is difficult for traditional absorbing materials to meet the needs. Due to the particularity of the structure and composition, nano-absorbent coatings have become a new bright spot in stealth technology. Nanomaterials refer to materials whose three-dimensional dimensions are at least one nanometer-sized, such as films, fibers, ultrafine particles, multilayer films, particle films, and nanocrystalline materials. They are generally composed of materials with a size of 1-100nm. Micro powder system.
2. The absorbing principle and structural characteristics of nanometer absorbing coating
The essence of the absorbing material is to absorb or attenuate the incident electromagnetic wave, and through the dielectric loss of the material, the electromagnetic wave energy is converted into heat energy or other forms of energy to be dissipated. The absorbing material is generally composed of a matrix material (bonding agent) and an absorbing medium (absorbent).
Absorbing materials can be divided into electrical loss type and magnetic loss type. Electrically lossy materials mainly rely on the electronic polarization, ion polarization, molecular polarization or interface polarization of the medium to absorb and attenuate electromagnetic waves. Magnetic loss materials mainly rely on magnetic excitation mechanisms such as hysteresis loss, domain wall resonance and aftereffect loss to cause the absorption and attenuation of electromagnetic waves. Due to the small size of the nanocrystalline grains, the number of atoms on the grain boundaries is more than that inside the grains, that is, a high concentration of grain boundaries is produced, so that nanomaterials have many properties different from those of general coarse-grained materials. Nanoparticles have small size effects, surface and interface effects, quantum size effects, dielectric effects, and macroscopic quantum tunneling effects.
The main reasons why nanomaterials have very good absorbing properties are as follows:
First of all, nanomaterials have a high concentration of grain boundaries. The specific surface area of the crystal interface atoms is large, the dangling bonds are large, the interface polarization is strong, and multiple scattering is easy to occur. Under the action of electromagnetic field radiation, the movement of atoms and electrons is caused by the surface effect of nanoparticles Intensified and magnetized, the electromagnetic energy is converted into heat energy more effectively, and a strong wave-absorbing effect is produced;
Secondly, the existence of quantum size effect causes the electronic energy level of the nanoparticle to split, and the energy level interval of the split is in the energy level range of the microwave (10-2~10-5eV), thus becoming a new wave-absorbing channel for nanomaterials;
In addition, nano-ions have large saturation magnetic induction, high hysteresis loss and coercive force, so that nano-materials have the properties of high eddy current loss, high Curie point, high use temperature, and wide wave absorption frequency.
The structural characteristics of nanomaterials make the nanometer absorbing materials have the characteristics of wide absorption frequency band, good compatibility, light weight and thin thickness. It is easy to meet the requirements of "thin, light, wide and strong" radar absorbing materials. Very promising high-performance, multifunctional absorbent.
3. Types and main development methods of new nanometer absorbing materials
The rapid development of nanotechnology and the excellent electromagnetic absorbing properties of nano-powders have made nano-absorbents a research direction and hot spot at home and abroad.
3.1 Nano metal and alloy absorbent
Nano-metal and alloy absorbing materials mainly absorb and lose electromagnetic waves through mechanisms such as hysteresis loss and eddy current loss. Nano metal powder absorbing materials mainly include two types: nano carbonyl metal powder absorbing materials and nano magnetic metal powder absorbing materials. Nano-carbonyl metal powders mainly include carbonyl Fe, carbonyl Ni, and carbonyl Co, among which nano-carbonyl Fe is the most commonly used. The carbonyl Fe is uniformly blended with DC805 silicone rubber, the amount of wave absorber is 90%, and the reflectivity is lower than -10dB in the frequency range of 2-10GHz. Nano-magnetic metal powders include Co, Ni, CoNi, FeNi, etc., and their electromagnetic parameters are related to their composition and particle size. Nano-metal magnetic materials have high saturation magnetization, which is generally more than 4 times higher than ferrite, can obtain higher magnetic permeability and magnetic loss, and magnetic properties have high thermal stability. Metal nano-powders have excellent attenuation properties for electromagnetic waves, especially electromagnetic waves ranging from high frequency to light wave frequency, but its absorption mechanism is not yet fully understood. It is generally believed that its absorption of electromagnetic wave energy is determined by three effects: electron scattering caused by the thermal vibration of the lattice electric field, electron scattering caused by impurities and character defects, and the interaction between electrons and electrons.
Nano metal and alloy absorbents are mainly Fe, Co, Ni, Cr, Cu and other nano metal powders. Nano-alloy adopts a multi-phase composite method, and its absorbing performance is better than single-phase nano metal powder. The absorption rate is greater than 10dB and the bandwidth can reach 3.2GHz. Proportion, particle size, and microstructure of alloy powder are the main influencing factors of its wave-absorbing performance. Among the nano-alloys, iron-based nano-alloys are the most studied. When the size of the iron-nickel nano-alloy powder reaches the nanometer level, it has a high magnetic energy product, a small dependence of remanence on temperature and good magnetization performance. At present, the methods for preparing nano-iron-based magnetic powder or oxide and alloy particles mainly include soft chemical method, ultrasonic decomposition method, LB film technology assembly, in-situ polymer modification composite technology, sol-gel electrodeposition method, sol-microemulsion chemistry Tailoring method, chemical thermal reduction method and mechanical alloying method, etc.
3.2 Nano-ferrite and its composite absorbent
Nano-ferrite is a kind of double complex medium, which not only has the ohmic loss, polarization loss, ion and electron resonance loss of general dielectric materials, but also the domain wall resonance loss, magnetic moment natural resonance loss and particle resonance characteristic of ferrite. Loss, therefore, is still one of the main components of microwave absorbing materials. There are two types of nano oxide absorbents: single oxide and composite oxide. Single oxide nano absorbents mainly include Fe2O3, Fe3O4, TIO2, Co3O4, NiO, MoO2, WO3 and other nano powders. Absorbing materials made of a single ferrite are difficult to meet the requirements of bandwidth, light weight, and thin thickness. Therefore, some additives are usually added to the ferrite powder to form a composite absorber, which can better match the electromagnetic parameters. Therefore, the actually used ferrite absorbing coating is often not a single ferrite coating, but a composite ferrite absorbing coating composed of a composite. For example, ferrite is compounded with carbonyl iron powder, iron powder, nickel powder, carbon black, graphite, silicon carbide, resin, etc. to form a composite ferrite nano-powder absorbing material. The peak absorption of the ferrite nanocomposite multilayer film in the frequency range of 7～17GHz is -40dB, and the bandwidth of less than -10dB is 2GHz.
The composite oxide nano absorber not only has excellent wave absorbing performance, but also has multiple functions such as suppressing infrared radiation. The composite material made of ferrite nanoparticles and polymer can effectively absorb and attenuate electromagnetic waves and acoustic waves, and reduce reflection and scattering. Therefore, ferrite absorbing materials are more researched and mature absorbing materials. The mechanism of action can be summarized as the magnetic loss and dielectric loss of ferrite to electromagnetic waves.
The nanometerization of ferrite absorbing materials is a promising new research field of stealth materials. Certain researches have been carried out at home and abroad, and certain research results have been obtained. The United States has developed a series of thin-layered ferrite absorbing materials and successfully applied them to F-117A fighter jets. While studying nano-ferrite absorbing materials, researchers have also explored composite absorbing materials formed by the combination of ultrafine ferrite and other materials from various aspects. Xie Jiaying studied the effect of NdO3 doping on the microwave absorption characteristics of nano-lithium ferrites. They prepared nanocrystalline LiFe5O8 and LiFe4.994Nd0.006O8 materials by mechanical alloying and studied their absorbing properties.
3.3 Nano ceramic absorbent
Nano-ceramic powder is a new type of nano-ceramic absorbing material, mainly including SiC, Si3N4 and composite Si/C/N, Si/C/N/O, etc. Its main components are silicon carbide, silicon nitride and Amorphous carbon has the advantages of high temperature resistance, light weight, high strength, and good wave absorbing performance. In particular, Si/C/N absorbing materials not only have the above advantages, but also have a wide operating temperature range (can be used from room temperature to 1000°C), small dosage, adjustable dielectric properties, and can effectively reduce infrared radiation signals. Excellent characteristics. For example: Si/C/N and Si/C/N/O nano-wave absorbing materials have good absorption performance in the centimeter wave band and millimeter wave band; after the composite of nano SiC and magnetic nano absorber (such as nano metal powder, etc.), The absorbing effect is greatly improved. Nano Si3N4 has a relatively large dielectric loss in the range of 102～106Hz. This strong dielectric loss is caused by interface polarization, which is caused by the electric dipole moment formed by dangling bonds. Nano-ceramic absorbents are characterized by strong oxidation resistance at high temperatures and stable microwave absorbing performance.
3.4 Nano graphite absorbent
The earliest application of nano-ceramic absorbents can be traced back to during World War II. Germany added carbon black to the interlayer of aircraft skin to absorb radar waves. Because of its low density, it was often used to fill the honeycomb interlayer structure. Conductive carbon black is also commonly used to compound with polymer materials to adjust the conductivity of polymer composite materials to achieve a good wave absorbing effect. Graphite has now been used in structural absorbing materials. The United States has made great progress in the research of graphite-thermoplastic resin-based composite materials and graphite-epoxy resin-based composite materials. These composite materials still maintain toughness at low temperatures (-53°C), but are more resistant to high temperature and high humidity environments. The metal is slightly sensitive. The "super black powder" nano-wave absorbing material developed by the United States has an absorption rate of up to 99% for radar waves, and has been successfully applied on the B-2 stealth bomber. Currently, it is researching to cover the wavelengths of centimeter wave, millimeter wave, infrared, and visible light. Of nanocomposites. This "super black powder" nano-absorbent material is essentially graphite-thermoplastic composite material and graphite-epoxy composite material made of nano-graphite as an absorbent. It not only has a high absorption rate, but also maintains a very good performance at low temperatures. Good toughness. In addition, graphite and carbon black are also used in doped high-loss absorbing coatings. This type of absorbing coating is composed of conductive fibers and high-loss materials (such as carbon black, ceramics, clay, etc.) and resin. The length of the conductive fiber It is half the wavelength of the radar wave, and the thickness of the high-loss object is preferably an odd multiple of 1/4 of the wavelength of the radar wave. The disadvantage of graphite and acetylene carbon black as high-temperature absorbents is their poor high-temperature oxidation resistance.
3.5 nanometer silicon carbide absorbent
Pure nano-SiC cannot absorb radar waves, and it needs to be doped to a certain extent to improve the conductivity of SiC. Generally, the elements that can be doped in SiC are B, P, N and so on. Northwestern Polytechnical University obtained nano Si/C/N absorber by doping nano SiC, which has good absorbing properties. The main reason that the Si/C/N nanocomposite absorber can absorb waves is that N atoms are dissolved in the SiC lattice formed in the absorber, and the solid solution N atoms replace the positions of the C atoms in the lattice to form a lattice defect. In a normal SiC lattice, each C atom and each Si atom are covalently connected to the surrounding 4 adjacent silicon atoms. Similarly, each silicon atom is also connected to the surrounding 4 adjacent Si atoms. The C atoms are connected by covalent bonds. When the N atom replaces the C atom and enters SiC, since the N atom has only three valences, it can only form bonds with 3 Si atoms, while the other Si atom will have 1 remaining valence electron that cannot form a bond, forming a negatively charged one. Defects. Due to the thermal motion of the atom, this electron can move on the 4 Si atoms around the N atom, and jump from one Si atom to another Si atom. A certain potential barrier must be overcome during the transition, but it cannot be separated from the 4 Si atoms. A small area composed of silicon atoms, therefore, this electron can also be called a "quasi-free electron." In the electromagnetic field, the position of this "quasi-free electron" changes with the direction of the electromagnetic field, resulting in electron displacement. The "quasi-free electron" transitions from one equilibrium position to another equilibrium position, and must overcome a certain potential barrier to move Lagging behind the electric field, strong polarization relaxation occurs, and this polarization relaxation is the main reason for the loss of electromagnetic wave energy.
Studies have shown that Si/C/N not only has the advantages of high temperature resistance, light weight, good toughness, high strength, and good microwave absorption performance, but also has good thermal stability and a wide operating temperature range (can be used from room temperature to 1000 ℃) , The dosage is small, the dielectric properties are adjustable, and the infrared radiation signal can be effectively reduced. Si/C/N and Si/C/N/O nano-absorbers have good absorption properties not only in the centimeter wave band, but also in the millimeter wave band.
This nano Si/C/N absorber has the following advantages.
⑴The dielectric properties are adjustable, and the controllable ranges are ε＇:1～32; ε〃:0～25; ε〃/ε＇:0～2.
⑵ High temperature stability, heat treatment at 700 ℃ for 10 hours, there is no change in microstructure and performance.
⑶The use temperature range is wide, it can be used at room temperature and high temperature, and the maximum use temperature can reach 1000℃.
⑷High-temperature reflectivity is stable. According to actual tests, the reflectivity curve of the absorbing material at 300℃, 500℃, 700℃ is almost the same as the reflectivity curve at room temperature, and the reflectivity changes little with temperature.
⑸The dosage is small, and 3%-10% (mass fraction) of absorbent can be added to the matrix to achieve a good absorbing effect.
⑹The dielectric constant decreases to a certain extent with the increase of frequency, which is beneficial to increase the width of the absorption band.
3.6 Nano conductive polymer wave absorbing material
Conductive polymer is a type of electrical lossy wave absorbing material, mainly polyacetylene, polypyrrole, polythiophene and polyaniline, etc. Its wave absorbing performance is closely related to the dielectric constant and conductivity of the conductive polymer. The structural characteristics are It has a large conjugated π bond system. The electromagnetic parameters of such compounds mainly depend on the main chain structure of the polymer, room temperature conductivity, dopant properties, doping degree and synthesis method. The conductivity of conductive polymers can vary in the range of insulators, semiconductors, and metal states. Different conductivity exhibits different absorbing properties. After conductive polymers are doped, due to the occurrence between the conjugated chain and the dopant The transfer of electrons produces new carriers, such as solitons, polarons or dipoles. The existence and transition of such dipoles increase their conductivity sharply, so they exhibit better absorbing properties. The electrical conductivity depends on the molecular chain length of the conductive polymer and the binding force of the molecular structure on the dipole. Generally, the longer the polymer chain, the higher the structural regularity, and the better the conductivity. The research results show that when the conductivity of the conductive polymer is in the range of 10-5～10-3S/cm, that is, when it is in the semiconductor state, it has a better absorbing effect. Studies have found that the magnetic loss of nano-conductive polymers is much higher than that of non-nano-conductive polymers.
The conductivity of pure conjugated polymer is not high, the highest is not more than 10-3S/cm, and most of them are less than 10-7S/cm, but after compounding with inorganic absorbent, it can obtain better conductivity and wave absorption Performance: The composite conductive polymer wave absorbing material is made of polymer materials and conductive materials through uniform compounding, layering compounding, or forming a surface film. It is mainly composed of the following parts: organic polymer materials mainly include rubber, resin, emulsion, polyacetylene, polypyrrole, polyaniline and polythiophene, etc.; conductive materials mainly include
Fillers such as metals, non-metals and oxides; dopants include hydrochloric acid, concentrated sulfuric acid, ferric chloride and other organic substances. Since the conductive polymer absorbing material has low density, adjustable electromagnetic parameters, good compatibility, low cost, and a wide variety of options, it is expected to develop into a new type of lightweight, broadband absorbing material. The United States has developed a wave-absorbing material composed of conductive polymers and cyanate whiskers. It has optical transparency and can be sprayed on the optically transparent windows of aircraft canopies, precision guided weapons, and cruise missiles. Conductive polymer density is relatively small, generally 1.0 ~ 2.0g/cm3, good mechanical processing performance, good stability in medium and low temperature, and has broad development prospects in electrical loss type wave absorbing materials.
In summary, nanometer absorbing materials have excellent absorbing properties, combined with the characteristics of bandwidth, multi-function, light weight and thin thickness, etc., have excellent absorbing effects on both microwave and infrared, and can also be combined with the structure. The composite of material or structure absorbing material is a kind of high-performance absorbing material with great development potential. High military sensitivity and technical confidentiality make it difficult to collect data on the research and application of high-performance absorbing media. However, countries all over the world are racing to develop high-performance absorbing materials. In future wars, only suitable for one or two corresponding frequency bands of absorbing media, it will be difficult for future detection systems to have practical significance. Nano absorbing materials are expected to develop into a millimeter wave, centimeter wave, and meter wave in the near future. , Visible light, infrared and other multi-band electromagnetic stealth multi-spectrum absorbing materials.