At an aviation expo in Beijing, the 17th, graphene products became the focus of attention, the exhibition shows the five generations of Chinese machine on graphene article wave absorption, and this material may be fighters - 20 fuselage with stealth material. The display of the product in such an open manner shows that our national defense industry is steadily improving.
The graphene material presented here does not have a performance profile, but it may be inferred from the general properties of the stealth material. Radar absorbing materials reduce the energy reflected back to radar waves by absorbing the energy of incoming radar waves. As the electric current passes through the resistance, the electrical energy is converted to heat dissipation in the air, and the radar wave energy is converted to the ohm loss through the absorbing material. Because the radar waves that illuminate the plane are only a tiny fraction of the radiation energy, there is no case of the body being heated.
According to the mechanism of electromagnetic loss, absorbing material is divided into electric absorbing absorbing material and magnetic absorbing absorbing material. Conductive carbon black or graphite is used in most absorbents of electric absorbing and absorbing materials. The magnetic absorbing material is usually iron oxide. Obviously, the Beijing air show is showing electrical absorption, not magnetic absorption.
The absorption bandwidth of absorbing materials can be divided into broad band and narrowband. Narrowband absorbing material is also called the resonant absorbing material, is often only in one or more discrete frequencies can be achieved on the absorbing, if the other side of the radar has the wide frequency agility ability or the combination of several different frequency radars detection, this material will not work. Broadband absorbing material is usually made up by absorbing material unit, to make it within a wider band absorbing ability, although at very high and very low band cannot stealth, but due to the spectrum radar detecting target itself, so it can play a role in stealth. At present, it is not yet known that the frequency range of graphene absorber bands in the five generation machines displayed at the Beijing air show is too sensitive to be published.
The absorbing materials can be divided into two types according to the method of use: the first is the surface coating type, which is generally applied to the metal part of the body surface to absorb radar waves; The second is the structural type, that is, the material is an important part of the body, which is used directly to make the front edge of the aircraft, the pelvic fin and other parts that have the bearing capacity and need to absorb the wave. The five - generation graphene absorbent bars displayed at the Beijing air show are the second category. It is estimated that the j-20 and the leading edge of the wings and the pelvic fins have been used, which can effectively reduce the edge diffraction.
In order to introduce the radar wave into the absorbing material and dissipate its energy, the absorbing materials generally have two characteristics, impedance matching and attenuation characteristics. The impedance matching property is to create certain conditions for the radar wave to have the minimum reflectivity of the absorbing material surface, as little as possible to reflect the radar wave, as much as possible into the absorbing material. Attenuation is the ability of absorbing materials to dissipate radar energy in the interior.
To make radar wave completely drawn into, namely perfect impedance matching characteristics is very difficult, the need of absorbing material is equal to the relative dielectric constant and relative magnetic permeability, looking for this kind of material is very difficult, scientists can only find the two parameters is matching material, as much as possible to inhale radar wave.
To increase the attenuation characteristics of absorbing material must at the same time improve medium conductivity and magnetic permeability, often take way to increase the conductivity, but when the material conductivity increases, the impedance matching characteristics of the material is very poor, the material can absorb a lot of radar wave, but also to reflect more radar wave. To do this, you have to find a material that compares the conductivity of the intermediate values, and finally the scientists chose the carbon (graphene is also carbon).
In order to increase the absorptive capacity of the carbon materials, the scientists will also carbon materials must be geometry shape, increase the absorption of the radar wave area, shown below this kind of structure is the most commonly used geometry, radar, once the incident to the shape, will be wedge multiple reflection, cause many times to absorb, greatly enhance the absorption effect. The height of the wedge to let opponents radar wavelength, generally in order to reflect more electromagnetic wave, the height of the wedge is no more than 1 times the wavelength, but to achieve very high absorption rate, we must increase the wedge height reach 7 to 10 times the wavelength, apparently, greater than the little is not reasonable. If only absorption ability is considered, and the opponent is using UHF band radar, then the stealth material should be a meter thick?
In general, in view of the airborne fire control radar in x-band, absorbing ability of carbon material between - db ~ 20-30 db, on display in Beijing show in China will not exceed this value, this kind of material can be reduced to between 1/1000 ~ 1/100 of the original RCS, it has been pretty good.