microwave absorbing material

Microwave absorbing material is a kind of material that can absorb microwave and electromagnetic energy with less reflection and scattering. Also known as radar absorbing material or radar stealth material. The basic principle of microwave absorption is to convert microwave energy into other forms of motion energy through a certain physical mechanism, and convert it into heat energy through the dissipation of the motion. All forms of lossy motion excited by microwaves can become absorption mechanisms. Common mechanisms include electric induction, magnetic induction, electromagnetic induction, and electromagnetic scattering. The microwave absorbing materials used in practical applications often have multiple mechanisms at work.

Claim 
The microwave absorbing material should have good absorbing performance, that is, have a microwave absorption rate higher than the required threshold and a wide absorption band. In addition, this material should also have a small thickness and areal density, good mechanical properties and environmental resistance, and a price acceptable to users.
In order to counter the reconnaissance and tracking of radar and other electronic equipment, the selection of suitable microwave absorbing materials combined with the target shape design can better reduce the radar cross section (RCS) and thus play a stealth effect. This radar stealth technology is not only used for aerospace and aviation aircraft, but also for military ships on the sea, various military vehicles on land, and ground launching facilities that use microwave absorbing materials, and even used on iron towers and bridges in civil buildings.

Brief history
In the mid-1930s, various countries began to study electromagnetic wave absorbing materials. The Netherlands produced the first microwave absorbing material, and Tokyo Institute of Technology in Japan developed ferrite, which was patented in 1932. During World War II, significant progress was made in radar detection of aircraft and surface ships, forcing countries to do everything possible to find ways to reduce the detection of aircraft and submarines. Among them is the German "Chimney Scanning" project. The main purpose is to develop materials that absorb radar waves to equip submarines with snorkels and periscopes. Two materials "Wesch" and "Zhuman" have been developed. (Jnuman), the thickness is 7.6mm and 76mm respectively. The Massachusetts Institute of Technology in the United States has developed a 0.60～1.88mm microwave absorbing material. In the 1960s, Tokyo Institute of Technology in Japan made great progress in researching ferrites. Since the mid-1970s, countries such as Japan, the United States, and the former Soviet Union have developed a 3.2-20GHz series of microwave absorbing materials and used them as stealth materials for various military equipment. This period is the stage of comprehensive development and application of stealth technology and microwave absorbing materials. The United States has developed a variety of structural microwave absorbing materials, such as the SF-RB material developed by the American Emerson Cumming Company with a thickness of 3 to 5 mm; Rockwell Company has developed a complex honeycomb. The United States has also developed RAc0 material with a thickness of 10-20mm and a reflection frequency of 2-15GHz.
Since the mid-1980s, China has made great progress in the development of microwave absorbing materials.

Constitute
The main component of microwave absorbing materials is absorbent, followed by binders and related additives. The absorbing performance of microwave absorbing materials mainly depends on the absorber and its preparation process. Absorbent is the main base material that absorbs electromagnetic waves and is prepared through specific process technology. Absorbents are usually powdered or fibrous, such as ferrite powder, carbonyl iron powder, various ultrafine metal powders, silicon carbide powder, silicon carbide fibers, carbon fibers, metal fibers, and organic high molecular polymers. New absorbents include composite ferrite, ultrafine metal particles, silicon carbide, organic polymer (functional polymer) nanomaterials, etc.

Kind
According to the application, it can be divided into coating type, patch type and structure type microwave absorbing materials; according to the working principle can be divided into interference type and absorption type microwave absorbing materials.
Coated microwave absorbing material
It is made by using polymer solutions or emulsions such as resin type and rubber type as the base material, and adding the absorbent in a certain proportion through a specific process. The coatings that have been widely used are usually coated absorbing materials with various ferrites, iron and alloy powders as the main absorbers. Japan and the United States have developed a series of ferrite microwave absorbing materials, such as lithium cadmium ferrite, lithium zinc ferrite, nickel cadmium ferrite, and ceramic ferrite. Coated microwave absorbing materials are easy to construct, low in cost, and suitable for objects with complex shapes. The disadvantage is that weight, thickness, and adhesion often affect the aerodynamic characteristics of the equipment.
SMD as wave absorbing material
The patch is not suitable for very complex shapes. But it can realize complex structural design, and the patches used are rubber, ceramic and plastic.
Structural microwave absorbing material
The non-metal composite material is used as the carrier and the absorbent is added. Usually epoxy resin and thermoplastic materials are used as the matrix, filled with absorbents such as ferrite, graphite, and carbon black, such as carbon fiber/epoxy resin, graphite/thermoplastic material, boron fiber/epoxy resin, graphite fiber/epoxy resin, etc. . This type of material can not only reduce the scattering of electromagnetic waves, but also bear a certain load. It is lighter than general metal materials, has a certain strength and rigidity, and has been used.
Interference type as wave absorbing material
Interference-type microwave absorbing materials are mainly based on the interference of electromagnetic waves. At the center frequency point, the incident electromagnetic wave and the reflected electromagnetic wave have opposite phases (phase difference 180°) and cancel each other. This type of material is characterized by a narrow frequency range, can be used under high frequency, and the material can be made thinner.
Absorption type as wave absorbing material
The large dielectric loss and magnetic loss of the incident electromagnetic wave in the object are used to convert electromagnetic energy into heat or other forms of energy. Absorptive microwave absorbing materials are divided into electrical loss type and magnetic loss type. Generally, electrical loss-type materials are better at absorbing high frequencies; while magnetic loss-type materials are more effective at absorbing low frequencies. If the two are combined, a wave-absorbing material with a wider frequency range can be made. The electrical loss type microwave absorbing material is based on carbon powder or metal particles, and a graded dielectric performance absorbing material is prepared by changing the thickness and depth and changing the type of filler. Magnetic loss type microwave absorbing material is a thin layer material filled with magnetic materials such as ferrite or carbonyl iron powder, such as epoxy polysulfide, silicone rubber, urethane and fluoroelastomer.

application
Mainly used for aircraft, missiles, ships and other equipment.
aircraft
The United States used wave absorbing materials on the SR-71 Blackbird reconnaissance aircraft in the 1950s, and used absorbing coatings and structural absorbing materials on the A-12 naval attack aircraft in the 1960s. The American F-117A stealth fighter, B-2 stealth strategic bomber and YF-22 stealth fighter represent the advanced level of stealth technology in the world today. F-117A is an aircraft that has been put into actual combat. Its main structure surface is coated with "iron ball" absorbing materials. Its vertical tail and wing front edges are equipped with triangular absorbing plastic liners, and its high temperature parts are mainly reinforced with carbon fiber. Thermoplastic composites. The radar absorption rate of the aircraft is as high as 98%. The B-2 second-generation stealth bomber has been successfully developed, and the first aircraft has entered service. Most of the structure of B-2 uses plastic, graphite/epoxy resin, carbon fiber and ceramic materials. The exterior of the engine is coated with ultra-high density carbonaceous absorbing material. The radar cross-sectional area of this aircraft is less than 0.1m2. The YF-22 stealth fighter uses a large number of advanced composite materials, accounting for 35% to 40% of the entire aircraft, including rock climbing epoxy resin and thermoplastic carbon fiber composite materials. This machine has better stealth performance. In addition, stealth aircraft developed by other countries include the former Soviet Union’s Tu160 pirate flag, Tu-95B/C stealth bomber, An-24 stealth transport aircraft, MiG-29, MiG-3l and Su-27 stealth fighters. The Phantom-200 stealth fighter in France and the F-SX stealth fighter in Japan also use microwave absorbing materials to varying degrees.
missile
The United States, the former Soviet Union, and Japan all regard stealth performance as an important aspect of missile advancement in the development of a new generation of missiles. The new generation of missiles have stealth capabilities, and various advanced composite materials and wave-absorbing materials have been widely used in missiles. The United States and the former Soviet Union are paying particular attention to the stealth capability of the cruise missile. The radar scattering area of the American Tomahawk cruise missile is only 0.05m2, and the radar scattering area of the new generation cruise missile AGM-129 will be even smaller.
Ship
General countries have strengthened the research and development of stealth ships, and some stealth ships have come out and have shown good stealth performance. The US "Seashadow" stealth ship uses special composite materials to limit heat dissipation and avoid infrared detection. Microwave absorbing materials are pasted into the pillars on both sides of the ship to prevent the formation of reflected electromagnetic waves. The main body of the Swedish "Smyge" stealth test ship is made of glass fiber reinforced plastic, reinforced plastic and Kevlar fiber using composite sandwich technology production. All air passages are covered with radar reflection net or microwave absorbing material. In order to avoid infrared detection, the entire ship is painted with infrared camouflage materials. The stealth performance requirements put forward by the United States in the 21st Century Stealth Submarine Program, the noise radiation value is less than 20dB, and it has the means to effectively control the propagation of magnetic and optical signals.
Other microwave absorbing materials can also be used in electronic equipment, components, microwave reflection chambers, and to improve the performance of radar and microwave equipment.

development trend
The main aspects of future development and research are: exploring new microwave absorption theories and new absorbents. Foreign countries are studying coatings containing radioisotopes and semiconductor coatings. Its advantages are bandwidth absorption, high reflection attenuation rate and long service life, which can better meet the aerodynamic requirements of supersonic flight.
Future stealth materials must have wide frequency band characteristics, and new stealth materials compatible with microwaves and infrared are required.
With the increasing application of composite materials in aircraft, the use of structural microwave absorbing materials has become an important development direction for aircraft materials. The promotion and application of a new generation of thermoplastic resins such as polyether ether ketone (PEEK), polyether sulfone (PES), polyaryl sulfone (PEI) and polyethyleneimine (PAS) are used to improve the physical properties of structural microwave absorbing materials. Performance, processing methods and cost reduction provide favorable conditions.
It has been recognized that the manufacturing process, particle size and shape of the absorbent have a great influence on the absorbing performance. Research on microparticles and ultrafine particles is being carried out abroad, including research on new types of magnetic particles, ultrafine metal powders and composite powders. In addition, researches on nanomaterials and nanocomposite materials have been carried out. Nanocomposite materials have the characteristics of wide frequency band (covering P, S, X, Ku millimeter wave bands), multi-functionality (both absorbing radar waves and infrared radiation), light weight, thin thickness, etc. They are a kind of promising development. High-performance absorbing material.
Ceramic microwave absorbing materials have excellent environmental resistance that other microwave absorbing materials cannot compare. They are becoming one of the development focuses of many manufacturers. Among them, electrical loss ceramic products (such as SiC) have light weight and good heat resistance, which are attractive Attention.