The 'artifact' for improving the survival ability of weapons and equipment on the battlefield - radar absorbing materials

Radar absorbing materialIt is a material designed to absorb or reduce the electromagnetic wave energy received on its surface, thereby reducing electromagnetic radiation or interference. Conventional metal materials generate reflections when receiving electromagnetic waves, while absorbing materials can allow the target surface to absorb and convert electromagnetic waves into thermal energy, reducing or obscuring reflected signals and achieving reduced or obscuring radar detection.

1、 Testing method and principle

Radar detection is the process of emitting high-frequency electromagnetic waves in a certain spatial direction and detecting the direction of a target object by receiving reflected electromagnetic wave signals. If the energy of the reflected wave received by the radar receiver can be reduced or the reflected wave can be reduced to the point where the received signal is weak enough to be unrecognizable, then the goal of radar stealth has been achieved.

There are many indicators that characterize the stealth effect of radar, and the most commonly used one is electromagnetic wave reflectivity. Assuming that the power of the radar electromagnetic wave emitted from the radar transmitter is Pi, and the power of the electromagnetic wave reflected back after passing through the target is Pr, then the power reflectivity is Rp=Pr/Pi. Obviously, radar stealth requires a small reflectivity. For ease of comparison, the reflectance R is usually expressed in decibels (dB), where R=10lgRp. In this way, since the power reflectance is less than 1, R is negative. Therefore, for a certain target object, it is desirable for its R value to be as small as possible. If adoptedradar stealth materialSo this material should be able to absorb or transmit radar waves, minimizing the reflection waves used for detection. For general targets, it is usually difficult to transmit a large amount of radar waves, so the materials used for radar stealth are mainly absorbing materials.

The bow method is commonly used to test the reflectivity of flat plates. Its principle is that under the same wavelength and polarization conditions, electromagnetic waves of the same power are incident on the surface of the radar absorbing material and the surface of the test aluminum plate from a certain angle. The ratio of the power reflected by the radar absorbing material and the aluminum plate mirror is defined as the reflectivity. The principle of bow test.

2、 Traditional coated absorbing materials

According to the material forming process and load-bearing capacity, radar absorbing materials can be divided into coating type absorbing materials and structural type absorbing materials. This issue mainly introduces traditional coated radar absorbing materials.

1.Nano absorbing materialsMaterials with dimensions on the nanometer scale in a certain direction (between 0.1-100nm) are called nanomaterials, which are located in the transition region between atoms and macroscopic objects. Due to their unique structure, nanomaterials possess some special properties, such as quantum size effect, surface and interface effect, small size effect, macroscopic quantum tunneling effect, etc. The radar absorption performance of nano absorbing materials is excellent, so many research institutions abroad have focused on their research and application.
It is reported that a research institution in the United States has developed a nano absorbing material called "super black powder", which has an absorption rate of up to 99% for radar waves and has been successfully applied on the B-2 stealth bomber. Currently, research is underway on nano composite materials covering centimeter wave, millimeter wave, infrared, visible light and other wavelengths. At present, the main research on nano absorbing materials includes nano metal and alloy absorbing materials, nano ferrite and its composite absorbing materials, nano ceramic absorbing materials, nano graphite absorbing materials, nano silicon carbide absorbing materials, nano conductive polymer absorbing materials, etc.

2.Ferrite absorbing materialThe research on ferrite absorbing materials started earlier and their application time is relatively long. According to their different structures, they can be roughly divided into two types: spinel ferrite absorbing materials and hexagonal ferrite absorbing materials. Among them, spinel ferrite has lower cost, simpler preparation process, and the longest application history. However, spinel ferrite has lower magnetic permeability, and the relative dielectric constant and relative magnetic permeability are difficult to match, making it difficult to achieve good absorption performance.
When developing the F-117A fighter jet, Lockheed Martin of the United States used a large amount of ferrite absorbing material on the surface of the fuselage to achieve good radar stealth effect. NEC Corporation has made improvements to ferrite absorbing materials, achieving a -10dB absorption effect in the 6-13GHz range, with a -20dB absorption effect in the 8.5-12.2GHz range. The total thickness is about 4.7mm, with a unit area mass of 8kg/m2. At present, the research and application level of ferrite absorbing materials by domestic research institutions is lower than that of foreign countries. In the frequency range of 8-18GHz (X, Ku band), the electromagnetic wave reflectivity can only reach the level of -10dB.

3.Chiral absorbing materialChiral materials refer to materials that do not have geometric symmetry and cannot be completely aligned with an object's mirror image through translation, rotation, and other means. As early as the 1980s, research on chiral absorbing materials gradually became a focus of attention. The ability to generate cross polarization under the action of electromagnetic fields is one of the important reasons why it can absorb electromagnetic waves and has good radar absorption performance.Compared with other types of absorbing materials, it has two obvious advantages:

(1) It is easier to adjust the chiral parameter ratio of chiral materials and thus adjust the dielectric and magnetic permeability of the materials, which can achieve non reflective absorption effects over a wide frequency band;

(2) Chiral absorbing materials have lower frequency sensitivity than dielectric constant and magnetic permeability, making it easier to achieve wideband electromagnetic wave absorption. Therefore, chiral materials have great potential in expanding the absorption frequency band and low-frequency absorption.

Compared to other types of absorbing materials, chiral absorbing materials can enhance the absorption performance to a certain extent, broaden the existing absorption frequency band, and utilize the adjustability of chiral parameters of chiral materials to prepare high-performance radar absorbing materials. But at the same time, its preparation process is relatively complex and the preparation cost is high, which limits its application scope.

 4.Polycrystalline iron fiber absorbing materialPolycrystalline iron fiber refers to fibers including Fe, Ni, Co, and their alloys, and its absorption mechanism mainly includes eddy current loss, hysteresis loss, and dielectric loss. As an absorbent for coating type absorbing materials, it can reduce the density of the coating, broaden the absorption frequency band, and achieve many advantages such as better oblique incidence characteristics. Foreign research institutions have conducted earlier studies on the absorbing properties of polycrystalline iron fibers, but due to technological limitations, there are relatively few relevant literature. According to reliable sources, a French research institution has successfully developed a new type of polycrystalline iron fiber radar absorbing material, which has been successfully applied to missiles and re-entry vehicles of its national strategic defense forces.

5.Conductive polymer absorbing materialConductive polymers refer to a class of polymers with π - electron conjugated chains that have been chemically or electrochemically doped. The excited transitions on the conjugated chains after doping can transfer charges, allowing the conductivity of conductive polymers to be adjusted between insulators, semiconductors, and metal bodies. Therefore, they are also known as "organic metals". Conductive polymers have physical and chemical properties such as diverse structures, good processing performance, and controllable electromagnetic parameters. After years of research, conductive polymers have made significant progress in technological exploration and practical applications.
In recent years, infrared detectors have been continuously installed on various types of weapons and equipment, and infrared stealth has become a research hotspot. Therefore, materials that can combine infrared stealth with good radar absorption performance have gradually become the direction of exploration for many research institutions. However, in principle, infrared stealth materials have high reflectivity and low emissivity, while radar absorbing materials have low reflectivity and high absorption performance. The two have a direct conflict in their working principles. Conductive polymer materials exhibit extremely unique infrared absorption and reflection characteristics due to their high conductivity and certain metal like properties. Their infrared reflectivity is much lower than that of ordinary polymers. Therefore, conductive polymers can make significant progress in the integration of infrared absorption. Through multi-layer design, it is possible to achieve lightweight, wideband, and multispectral stealth functions.