A J-20 stealth fighter jet flew over the radar monitoring area, but there was almost no echo signal on the radar screen - its radar cross section was less than 0.01 square meters. Behind this is not any mysterious' optical invisibility ', but a carefully designed radar absorbing material at work.

Conventional metal materials generate strong reflections when receiving electromagnetic waves. Radar absorbing materials are completely different - they do not reflect, but rather 'absorb'. When the electromagnetic waves emitted by the radar are incident on the surface of the absorbing material, the material converts the electromagnetic energy into thermal energy through an internal loss mechanism, causing a significant attenuation of the signal reflected back to the radar receiver. If the energy of the reflected wave is weak enough to be unrecognizable by radar, the target achieves "radar stealth".

2、 The two "hard indicators" of absorbing materials: allowing waves to enter and making waves disappear

Starting from electromagnetic wave theory, in order for a material to become an excellent radar absorbing material, it must simultaneously meet two conditions:

• Firstly, impedance matching - making electromagnetic waves' willing to come in '.When electromagnetic waves are incident on the surface of a material from air (with an impedance of about 377 Ω), if the electromagnetic impedance of the material differs too much from that of air, most of the energy will be reflected back at the surface. The ideal absorbing material requires material design to make its characteristic impedance as close as possible to free space, allowing electromagnetic waves to enter the interior of the material to the maximum extent possible, rather than being reflected off the surface.
• Secondly, electromagnetic losses prevent incoming waves from escaping.Once electromagnetic waves enter the interior of materials, they need to be effectively absorbed or attenuated through mechanisms of dielectric loss (ε″) and magnetic loss (μ ″). The larger the imaginary parts of the dielectric constant and magnetic permeability of the material, the more significant the attenuation of electromagnetic waves propagating inside the material.

3、 Coating type vs structural type: differences and complementarity between two technical routes

According to the material forming process and load-bearing capacity, radar absorbing materials can be divided into two categories: coating type and structural type.

• Coating type absorbing materialIt is an absorbing coating formed by mixing absorbents and binders and applying them to the target surface. Its advantages lie in its simple preparation process, low maintenance cost, and suitability for complex surfaces and locally strong reflection areas of stealth aircraft. Ferrite absorbing coatings have been widely used on stealth aircraft such as F-117A. However, coating materials also have inherent shortcomings: narrow absorption band, heavy weight, and easy detachment under high-speed airflow erosion.
• Structural absorbing materialsThen the absorption function and load-bearing function will be combined into one - the material itself is both a structural component and an absorber. It solves the problems of significant weight gain and easy detachment of traditional coating materials under high-speed airflow while retaining the advantages of lightweight and high strength through the collaborative design of mechanics and electromagnetics. Structural absorbing materials not only increase the absorption loss of electromagnetic waves through impedance matching, but also have high specific strength and stiffness, as well as excellent fatigue resistance. The honeycomb absorbing structure is a typical representative among them. The new structural material developed by Russia consists of expanded graphite, barium aluminum hexagonal ferrite filler, and polyvinyl fluoride resin, which can absorb 99.9% of incident electromagnetic waves.

Engineering Practice:The two technological routes are often not a "choice between two", but a "collaborative use" - using coating type in complex surfaces and locally strong reflection areas, and using structural type in structural parts that need to be supported.

4、 From Ferrite to Nanomaterials: The Evolution of Absorbents

The performance of radar absorbing materials is rooted in their absorbent system. From early to contemporary times, absorbents have undergone multiple generations of evolution:

• ferriteIt is the earliest absorbent to achieve engineering applications. According to their different structures, they can be divided into spinel ferrite and hexagonal ferrite. Spinel ferrite has a low cost and simple preparation process, but its magnetic permeability is low, making it difficult to achieve good absorption performance. NEC Corporation has made improvements to ferrite absorbing materials, achieving a -10dB absorption effect in the 6-13GHz range, with -20dB in the 8.5-12.2GHz range. The F-117A fighter jet extensively uses ferrite absorbing coatings on its fuselage surface.
• Nano absorbing materialsIt has been a research focus in recent years. Nanomaterials have special properties such as quantum size effect, surface and interface effect, and small size effect, and have excellent radar absorption performance. The "ultra black powder" nano absorbing material developed by a research institution in the United States has a radar absorption rate of up to 99% and has been successfully applied on the B-2 stealth bomber. The current research on nano absorbing materials includes nano metals and alloys, nano ferrites and their composites, nano ceramics, nano graphite, nano silicon carbide, nano conductive polymers, etc.
• Chiral absorbing materialUtilizing the cross polarization property of chiral materials under electromagnetic field to achieve absorption. Chiral materials have two significant advantages: firstly, they can adjust the dielectric parameters and magnetic permeability by adjusting the chiral parameters, achieving non reflective absorption effect over a wide frequency band; Secondly, it has lower frequency sensitivity and is easier to achieve wideband absorption.

5、 Key Performance: What Engineers Should Pay Attention to

6、 Research on Platinum Brand Radar Absorbing Materials: From Laboratory to Engineering

Advanced Institute (Shenzhen) Technology Co., Ltd. (referred to as "Advanced Institute Technology") was established in 2016. It is a national high-tech enterprise specializing in shielding materials, absorbing materials, flexible substrate coatings, and precious metal pastes. It has the independently registered trademark "Research Platinum".

In the field of radar absorbing materials, Advanced Institute Technology relies on its technological accumulation in the field of electromagnetic materials to launch the Research Platinum brand series of radar absorbing material products:

• Research Platinum brand high magnetic permeability absorbing materialBy adopting advanced nanotechnology, the grain size of ferrite is controlled within the range of tens of nanometers. Compared with micrometer sized grains, the magnetic permeability is increased by about 30%, and the stability is better at different temperatures. This material exhibits excellent absorption characteristics over a wide frequency range and can provide significant absorption effects in the high frequency range of GHz. More than 20dB of reflection loss can be achieved within the target frequency band, and at least 99% of the incident electromagnetic wave energy is absorbed by the material.
• Yanbo brand electromagnetic protection absorbing coatingIt exhibits excellent adaptability in complex electromagnetic environments, with wideband absorption performance and good environmental adaptability. The high-performance absorber and unique coating structure design used in the coating enable efficient electromagnetic wave absorption and attenuation over a wide frequency range.
• Research Platinum Brand Radar Absorbing MaterialsIt has been applied in various electromagnetic stealth technologies such as military aircraft, ships, communication base stations, and satellites. With the continuous development of materials science and electromagnetic technology, the research of platinum brand radar absorbing materials is moving from the laboratory to a wider range of engineering applications.

7、 Conclusion

The essence of radar absorbing materials is to transform the physical concept of "reflection countermeasure" into an engineering solution of "absorption and resolution". It does not pursue to "block back" electromagnetic waves, but rather allows them to "come in, consume, and no longer go out" - by impedance matching, the waves enter the interior of the material, and through dielectric and magnetic losses, the energy of the waves is converted into thermal energy.

From the ferrite coating of F-117A to the nano absorbing material of B-2, from the multi-layer composite absorbing structure of J-20 to the next generation of intelligent adjustable metamaterials - radar absorbing materials have always been the core material basis of stealth technology. Understanding the dual implementation conditions of impedance matching and electromagnetic loss, the technical differences between coating and structural types, and the performance boundaries of different absorbent systems - these are the key to upgrading radar absorbing materials from "functional materials" to "stealth engineering decisions".