Among all kinds of absorbing coatings, the earliest developed and most widely used coatings are those made of various metals or alloy powders, ferrites, etc. According to the analysis of existing foreign data, ferromagnetic materials are still the main body of developing thin-layer broadband coatings.
Ferrite materials have high magnetic permeability at high frequencies, and their resistivity is also high (108~1010 electromagnetic waves are easy to enter and are effectively attenuated. The main problems are high density and poor temperature stability. , The United States, Japan, Britain, Russia and other countries are developing new ferrite absorbing coatings. The main tasks include: manufacturing ferrite containing a large number of free electrons or adding a small amount of radioactive substances to ferrite to improve its absorbing performance , Manufacture hollow microsphere ferrite or ferrite hollow sphere to reduce its density and strengthen its absorbing effect. Combine ferrite and resistance wire to deal with different frequency radar or frequency agile radar: by changing the ferrite The chemical composition, particle size and distribution, particle morphology, and surface treatment technology of the alloy have improved its loss characteristics. It is reported that in 2~40GHz, its bulk material||>5. It should be pointed out that at low frequencies (f<1GH2), ferrite has a higher p value and c, which is smaller. So as a matching material, it has Obvious advantages over metal powder. In addition, from the perspective of broadening the absorption band of the wave-absorbing coating towards low frequencies, ferrite materials have good application prospects.
Magnetic metal and alloy powder have multiple functions such as absorption, transmission and polarization of electromagnetic waves. The basic requirements for using it to absorb electromagnetic wave energy are: the particle size of the metal powder should be smaller than the skin depth at the high-end frequency of the working frequency band, and the thickness of the material It should be greater than the skin depth at the low end of the working frequency band, which not only ensures the absorption of energy, but also prevents electromagnetic waves from penetrating the material. Magnetic metal (alloy) powder has good temperature stability and large dielectric constant, which make it widely used in wave-absorbing coatings. At present, there are mainly micron (1~10m) pure Fe, Ni, Co powders and their alloy powders, and nano-scale powders used for absorbing coatings. In recent years, G.Vian of the University of Paris, France, has studied the microwave absorbing properties of micron-sized Ni and Co powders and found that they have a maximum value in the range of 1~8GHz. For pure metal powder, when the particle size is 1.9μm and frequency f=6.5GHz, the theoretical permeability μ'=4, μ”=6.1; for Ni powder with a particle size of 1.4pm, at f=1.4GHz When, μ'=8, μ”=5.
Nanomaterials research is a frontier topic in modern international science. In recent years, reports on the ability of nanomaterials to absorb electromagnetic waves have aroused great interest of researchers. They proposed and explored the idea of using nanomaterials as a new generation of stealth coatings. The research fields are concentrated on magnetic nanoparticles, granular films and multilayer films. French scientists have recently developed a broadband microwave absorbing coating, which is composed of nano-scale fillers and adhesives. The filler is composed of an ultra-thin amorphous magnetic layer and an insulating layer, and the thickness of the magnetic layer is only 3m. The thickness of the insulation layer is 5 nm. The production method of this coating: the CoNi alloy and SiN are deposited on the substrate by a vacuum deposition method to form an ultra-thin electromagnetic absorption sandwich structure, and then the ultra-thin sandwich structure is pulverized into debris, and mixed with an adhesive to prepare a coating . According to reports, the structure of this sandwich film has a high magnetic permeability, and the n and "are both greater than 6 within the frequency of 0.1-18GHz. The resistivity of the coating material after mixing with the adhesive is higher than 50 cm The material has good absorbing properties in 50MHz~50GHz. Another method for preparing multi-layer nanoparticle film reported abroad is: using 3m glass hollow microspheres as a carrier, using electroless plating, sputtering and other processes to remove Ni, Metals such as Al generate nano-level electromagnetic loss layer, matching layer and protective layer. The thickness of the electromagnetic loss layer is about 10mm. When the filling volume fraction is 50%, the coating density is 0.40~0.46g/cm3. Under 2mm, the absorption rate within 8-18GHz can reach 10dB.
High permeability absorber is the key material for the development of high-performance absorbing coatings. The "super black" coating developed by the National Bureau of Standards and Standards of America is easy to spray on the metal surface. Can absorb 99% of radar energy. This material is considered to be very likely to belong to nanomaterials. Surface wave absorbing materials (SWAC) are made of high permeability materials, which can effectively suppress or eliminate backscattering caused by surface waves or creeping waves.