Absorbing material introduction and application

1. Absorbing material introduction

1.1 With the development of modern science and technology, the impact of electromagnetic radiation on the environment is increasing. At the airport, aircraft flights were delayed due to electromagnetic wave interference. In hospitals, mobile phones often interfere with the normal operation of various electronic diagnostic and therapeutic equipment. Therefore, to control electromagnetic pollution and find a material that can withstand and weaken the radiation of electromagnetic waves, wave-absorbing materials, has become a major issue in materials science.

1.2 The so-called wave absorbing material refers to a type of material that can absorb the electromagnetic wave energy projected onto its surface. In engineering applications, in addition to requiring the absorbing material to have a high absorption rate for electromagnetic waves in a wider frequency band, it is also required to have properties such as light weight, temperature resistance, moisture resistance, and corrosion resistance.

1.3 Electromagnetic radiation causes direct and indirect damage to the human body through thermal, non-thermal and cumulative effects. Studies have confirmed that ferrite absorbers have the best performance, and they have characteristics such as high absorption band, high absorption rate, and thin matching thickness. The application of this material to electronic devices can absorb leaked electromagnetic radiation and can achieve the purpose of eliminating electromagnetic interference. According to the law of electromagnetic wave propagation in the medium from the low magnetic guide to the high magnetic guide, high-permeability ferrite is used to guide the electromagnetic wave. Through resonance, a large amount of radiant energy of the electromagnetic wave is absorbed, and then the energy of the electromagnetic wave is converted into heat energy through coupling.

2, the shape of the absorbent material and its application

2.1 Spiked

Absorbers used in microwave darkrooms are often made of sharp-shaped pods. They are made by adding carbon powder to the foam and then applying a layer of high-strength foam to the outer layer to protect the absorber. The absorber will not be damaged even if it is impacted by the outside world. . However, as the frequency decreases (wavelength increase), the length of the absorber also greatly increases, and the ordinary sharp-shaped absorber has an approximate relationship of L/λ≈1, so that at 100 MHz, the length of the cusp reaches 3 cm; at 60 MHz, the cusp The length of 5cm is not only difficult to achieve in the process, but also the available space in the microwave darkroom is greatly reduced. 2.2 Single-layer flat plate The earliest developed absorber in foreign countries is a single-layer flat plate. The absorbers that are made later are directly attached to the metal shield layer. The thickness is thin and light, but the operating frequency range is narrow.

2.2 Double or multilayer flat

The absorber can work over a wide range of operating frequencies and can be made into any shape. For example, NEC Corporation of Japan can make ferrite and metal short fibers evenly dispersed in suitable organic polymer resin to make composite materials, and the working frequency band can be widened by 40% to 50%. The disadvantages are the large thickness, complicated process and high cost.

2.4 Coating

Only coating-type absorbers can be used on the surface of the aircraft. In order to broaden the frequency band, composite coatings are generally used. If the thickness of the lithium cadmium ferrite coating is 2.5mm to 5mm, it can be attenuated by 8.5dB in centimeters; when the thickness of spinel ferrite is 2.5mm, it can be attenuated by 24dB at 9GHz; ferrite can be added to chlorine. When the butyl rubber coating thickness is 1.7 mm to 2.5 mm, the attenuation is about 30 dB at 5 GHz to 10 GHz.

2.5 Structure

The incorporation of absorbent materials into engineering plastics has both absorption properties and load capacity, which is a direction for the development of absorbent materials.

In recent years, in order to further improve the performance of absorbing materials, several types of complicated absorbers have been developed abroad. For example, in Japan, microwave anisotropic chambers made of such absorbers have performances of 136 MHz, 25 dB, 300 MHz, 30 dB, 500 MHz, 40 dB, 1 GHz to 40 GHz, and 45 dB.

3 Engineering Applications of Absorbing Materials

In the increasingly important stealth and electromagnetic compatibility (EMC) technology, the role and position of electromagnetic wave absorbing materials is very prominent. It has become a magic weapon and a “secret weapon” for electronic warfare in the modern military. Its engineering applications are mainly in the following aspects.

3.1 Stealth Technology

By coating absorbing materials on aircraft, missiles, tanks, warships, warehouses and other weapons and equipment and military facilities, it can absorb reconnaissance waves and attenuate reflected signals to break through the defense zones of enemy radar. This is a kind of anti-radar reconnaissance. Powerful means to reduce a weapon system's exposure to infrared-guided missiles and laser weapons. For example, the US B-1 strategic bomber has an effective reflection cross-section of only 1/50 of that of the B-52 bomber due to the coating of the absorbing material; after applying the absorbing material on the cowling of the 0H-6 and AH-1G Cobra helicopter engines The engine's infrared radiation can be reduced by about 90%. In the Gulf War of 1990, the first U.S. F-117A aircraft that entered the territory of Iraq was an invisible aircraft coated with absorbing materials. They effectively avoided radar monitoring in Iraq. It is reported that the world’s first stealth battleship has been put into use by the Swedish Navy in recent years. The United States, Britain, Japan, and Russia have all developed their own stealth tanks and other stealth combat vehicles. In addition, electromagnetic wave absorbing materials can also be used to conceal equipment such as landing lights and other airport navigation equipment and other ground equipment, ship masts, decks, submarine periscope brackets and ventilation ducts.

3.2 Improve overall performance

The false signal generated by reflection of electromagnetic waves from the aircraft's airframe may lead to false or false tracking of high-sensitivity airborne radars. When an aircraft or several radars on a ship are working at the same time, the crosstalk between radar antennas is sometimes very serious. The jammer that comes on board or on board can also interfere with its own radar or communication equipment.... In order to reduce such interferences, magnetic shields are often used in foreign countries to improve the performance of radar or communication equipment. If radar or communication equipment fuselage, antenna and all surrounding interference materials are coated with absorbing materials, they can make them more sensitive and accurate to find enemy targets; coating absorbing materials on the walls around radar parabolic antenna openings, It can reduce the interference of the side lobe to the main lobe and increase the working distance of the transmitting antenna. It can reduce the interference effect of the false target reflection on the receiving antenna. The application of absorbing materials in the satellite communication system will avoid interference between the communication lines and improve The sensitivity of the satellite-borne communicators and ground stations enhances the quality of communication.

3.3 Security Protection

Due to the application of high-power radars, communications equipment, microwave heating and other equipment, preventing electromagnetic radiation or leakage and protecting the health of operators is a new and complex subject. Absorbing materials can achieve this goal. In addition, electromagnetic radiation is a common problem in current home appliances, and the use of absorbent materials and their components can also be effectively suppressed.

3.4 Microwave Darkroom

The space formed by the wall surface decorated by the absorber is called a microwave darkroom. An equivalent non-reflective free space (no-noise zone) can be formed in a dark room. The electromagnetic waves reflected from the surrounding area are much smaller than the direct electromagnetic energy and can be ignored. The microwave anechoic chamber is mainly used for the measurement of characteristic impedances and couplings such as radar or communication antennas, missiles, aircrafts, spacecrafts, and satellites, astronaut measurements of antenna patterns for back shoulders, and installation, testing and adjustment of spacecrafts. It can eliminate external clutter and improve measurement accuracy and efficiency (indoors can work around the clock). It can also keep secrets.