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Magnetic Absorbing Materials and Applications

Author:admin   AddTime:2021-02-22

1 Introduction
Stealth technology is an emerging edge science, involving multiple disciplines and technical fields, and is widely used. From various weapons and equipment, aircraft stealth to modern electronic information equipment anti-jamming systems are indispensable practical technologies and components.
As far as weapons are concerned, stealth technology is a technology that achieves stealth by reducing the light, electricity, and heat detectability of electrical appliances, weapons, or aircraft; in other words, it uses a variety of technical measures to reduce external signals (light, electricity, etc.). , Magnetic waves, infrared rays, etc.), which makes the reflected signal difficult to distinguish from the background signal where it is located, and minimizes its own characteristic signal to achieve its own concealment effect. Stealth technology can be divided into active stealth technology and passive stealth technology. The so-called active is the use of computers to analyze external detection signals and actively transmit corresponding interference signals in time to achieve their own concealment. The passive stealth technology is a passive stealth technology, which includes stealth structure technology and stealth material technology. The stealth structure technology is to reduce its own characteristic signal under the condition of not affecting the function as much as possible, and try to reduce the radar reflection cross-sectional area, which is particularly important in the military. It can be seen that stealth structure technology and stealth material technology are two inseparable parts of stealth technology, and stealth materials play an important role in achieving stealth, and they are also one of the main contents of stealth technology research.
With the rapid development of electronic technology and the popularization of electronic products, especially mobile communications, computers, and household appliances, people’s living environment is severely polluted by electromagnetic waves. The increase in urban high-rise buildings has caused the deterioration of the electronic environment. How to reduce electromagnetic interference has become a whole A general concern in the world's electronics industry. Stealth materials are also one of the effective methods to solve the anti-electromagnetic interference of electronic products.
Stealth materials are also called absorbing materials, whose role is to convert external electromagnetic wave energy into heat energy, reduce the intensity of reflected waves, and achieve the effect of stealth or anti-interference. According to the loss mechanism of the absorbing material, it can be divided into: resistance type, dielectric type and magnetic medium type. In order to achieve the best stealth effect, a variety of absorbing materials are often combined to form a composite absorbing material, which is widely used in radar, aerospace, microwave communications, electronic countermeasures, and electronically compatible absorption shielding.
This article specifically introduces the general appearance, application and development of magnetic media, mainly ferrite absorbing materials.
2 Magnetic Absorbing Material
2.1 The basic principle of the work of absorbing materials
The so-called absorbing means absorbing electromagnetic waves. The basic working principle of absorbing materials is:
For general materials, the dielectric constant ε and magnetic permeability μ of the material can be written in the following plural forms:
(1) In the formula: ε'and μ'are the variables of polarization and magnetization generated by the absorbing material under the action of electric or magnetic field, and ε" is the rearrangement caused by the electric couple moment of the material under the action of an external magnetic field. A measure of loss, μ" is a measure of loss caused by the rearrangement of the magnetic even moment of a material under the action of an external magnetic field. As far as the medium is concerned, ε" and μ" are responsible for the electromagnetic wave absorbing function. They cause energy loss. The loss factor tanδ can be expressed by the following formula:

(2) It can be seen that tanδ increases with the increase of ε" and μ".
In addition to improving the loss as much as possible when designing the absorbing material, another key factor must be considered, namely, the problem of wave impedance matching, so that the reflection coefficient (γ) of the medium surface to the wave is 0 or minimum, and electromagnetic waves are incident on the medium and absorbed. The definition of reflection coefficient γ is shown in formula (3):

(3) When a plane electromagnetic wave with a wave impedance of Zo is perpendicularly incident from a free space (μ0, ε0) to an absorbing medium with a permeability of μr and a dielectric constant of εr, reflection and transmission will occur on the interface. The reflection loss of the absorbing medium to the electromagnetic wave is represented by R, the unit is dB:

(4) where Zo is the impedance of free space,
Zin is the input wave impedance,
In the formula, μr and εr are the relative permeability and relative permittivity of the material respectively.
To obtain zero reflection loss (in fact it is impossible but can only reach the minimum), the wave impedance of the absorbing medium must match the wave impedance of the free space, that is, Zin=Zo. At this time, the reflection loss is the smallest, indicating that the electromagnetic wave energy is absorbed by the medium. absorb.
For an absorbing medium with finite thickness, the input wave impedance can be expressed as:
(5) In the formula, f-electromagnetic wave frequency:
   d — the thickness of the absorption medium;
   c — the speed of light.
Substituting Eq. (5) into Eq. (4), the reflection loss equation becomes a complex transcendental function, which indicates that there is a complex functional relationship between the reflection loss and the electromagnetic parameters of the medium, and only specific boundary conditions can be effectively solved.
When the thickness d of the absorbing medium is infinite, the input wave impedance must meet the minimum reflection loss requirements:
(6) In fact, the εr and μr values of general media are quite different. Only by using specific media and special process design to adjust εr and μr can the wave impedance of the absorption medium match the wave impedance of free space. For example, if ferrite powder wants to improve its absorption performance, it is compounded with conductive polymer to reduce the dielectric constant of ferrite, while metal magnetic powder is added with insulating resin to increase the dielectric constant, or the shape and size of the powder are changed. Adjust εr and μr to meet the requirements of (6), thereby reducing reflection loss and improving absorption performance. The actual operation is quite complicated, but for general communication equipment, as long as the reflection loss is less than 20dB, the electromagnetic wave is 99% absorbed by the medium.
The matching conditions of ferrite absorbing materials are:
(7) Where: λ is the electromagnetic wavelength in free space; d is the thickness of the absorber. It can be seen that the following requirements must be met in order to obtain high-performance absorbing materials:
(1) The incident wave enters the material to the maximum without being reflected on its front surface, that is, the matching characteristics of the material;
(2) The electromagnetic wave entering the material can be absorbed and attenuated quickly by the material, which is the attenuation characteristic of the material. But like many engineering problems, these two requirements are often contradictory.
2.2 Classification and characteristics of magnetic absorbing materials
Generally speaking, magnetic materials (mediums) with absorbing properties can be divided into two categories: metal and non-metal (ferrite), which are introduced below.
2.2.1 Ferrite absorbing materials
Ferrite absorbing materials are the earliest, most widely used and more mature type of magnetic materials. They have good absorbing properties even at low frequencies and thin thickness. Its basic principle is the ferromagnetic natural resonance absorption of ferrite materials.
Without an external constant magnetic field, when the angular frequency of the incident alternating magnetic field is equal to the intrinsic angular frequency determined by the equivalent magnetic anisotropy field Hk of the crystal, the ferrite absorbing material will absorb a large amount of electromagnetic waves energy. The intrinsic angular frequency ωk is determined by ωk = γHk, and γ is the gyromagnetic ratio. In the range of meter wave to centimeter wave, the reflected energy can be attenuated by 17-20DdB, and it is still widely used since the 1950s. According to the different microstructures, it can be divided into hexagonal crystal system, spinel type, garnet type and magnetoplumbite type ferrite. The most widely used as absorbing materials are the hexagonal crystal system and the spinel crystal system, because the hexagonal crystal system has a plate-like structure, and the plate-like structure is the best shape of the absorbent. Spinel-type ferrite has a relatively low dielectric constant ε'and magnetic permeability μ'. A composite ferrite made of pure ferrite powder dispersed in a non-magnetic body can be passed through ferrite powder The particle size, the mixing ratio of ferrite powder and non-magnetic material and the composition of ferrite to control its electromagnetic parameters. Ferrites such as Ni-Zn, Li-Zn, Ni-Mg-Zn, Li-Cd, Ni-Cd, Co-Ni-Zn, Mg-Cu-Zn, etc. have been developed and widely used.
The ferrite absorbing material absorbs electromagnetic wave energy by using the magnetic loss and the dispersion of magnetic permeability at high frequency. From the dispersion curve of magnetic permeability (Figure 1), it can be seen that the curve is divided into 5 areas. The absorption mechanism of these five areas is different. In the figure, μ'is the real part of the permeability, and μ" is the imaginary part of the permeability. In the low frequency region (I) f<104Hz, μ'hardly changes with frequency, and the change of μ" is also very small; When the region (II) f is l04~106Hz, the change of μ'and μ" is also very small, but there is a peak in μ", which is the absorption caused by the sample's size resonance, which is caused by the sample's geometric size and the propagating electromagnetic wave The resonant absorption caused by the standing wave generated by the half-wavelength phase of φ is not related to the material properties; when the region (Ⅲ) f is l06~l08Hz, μ'drops sharply, and μ'increases rapidly, and the magnetic loss increases rapidly. Resonance absorption, which is mainly caused by the displacement of the domain wall; when the region (IV) f is l08~l011Hz, μ'continues to decrease, and the peak value of μ'appears in this region, which is due to the rotation of the magnetic moment in the magnetic domain The natural resonance absorption caused by it; in the extremely high frequency region (V)f>1010Hz, it belongs to natural exchange resonance, and experimental observation is rare. For ferrite powder materials, the absorption mechanism is mainly natural resonance. Ferrites of different structures have different natural resonance frequencies.
The resonance frequency of the traditional cubic crystal system spinel ferrite (Zn-Ni, Li-Zn) can be expressed as:

(8) It can be seen from the formula that its natural resonance frequency is directly proportional to the Ms of the material and inversely proportional to the magnetic permeability μr. That is, the material with high magnetization has high fc, and the material with high magnetic permeability has low fc. For spinel iron The oxygen body can generally only be used in the frequency band less than 3GHz. The resonance frequency is also related to magnetic anisotropy. For example, for planar hexagonal ferrite, there is strong uniaxial anisotropy. For example, Co2Z-type Ba3Co2Fe24O41 ferrite, its resonance frequency can be expressed as:
(9) Where Hkθ is the magnetic anisotropy field with the axial angle θ; Hkφ is the magnetic anisotropy field with the axial angle φ, Hkθ is two orders of magnitude larger than Hkφ. Table 1 shows the comparison of the resonance frequencies of the two crystal systems. It can be seen that the resonance frequency of the high permeability is low. The resonance frequency of the hexagonal system is 2 to 3 orders of magnitude higher than that of the cubic system, which is suitable for ultra-high frequency absorption materials. .
2.2.2 New ferrite absorbing materials
1) Metal magnetic ultrafine powder absorbing material
Stealth technology began in World War II. As an effective means to improve the survivability and penetration capabilities of weapon systems, it has been regarded as a key military high-tech developed by all countries in the world. Especially with the development of radar detection technology, the original stealth technology is facing great challenges. There is an urgent need for new stealth materials with thin thickness, light weight, bandwidth and multiple functions.
New-type absorbing materials require thin, light, wide and strong, that is, they require low density, light weight, wide absorption frequency, and strong absorption capacity. If the relevant data is substituted for (4), then the absorption loss of the material is expressed as

(10) In the formula, R is the absorption loss of the material, σr is the conductivity constant of the material, and μr is the magnetic permeability of the material.
Since the σr and μr of ferrite materials are lower than that of metal materials, the only way to improve the absorption loss of ferrite materials is to increase the thickness d of the material, which will obviously affect its application. Theoretically speaking, metal materials can withstand high temperatures due to the high Curie point (770K), and Ms can reach 3 to 4 times that of ferrite. It can be seen from formula (9) that the natural resonance frequency of metal is much higher than that of ferrite. It has better absorption performance, but the bulk metal absorbing material will be limited by the metal skin effect. As the particle size of the metal or alloy decreases, the electromagnetic wave absorption performance of the material gradually increases, and the reflection performance gradually decreases. A study by the University of Paris in France pointed out that micron-sized Ni and Co magnetic powders have strong absorption performance at 1-8 GHz. Metal ultrafine powder absorbing material, also known as magnetic medium absorbing material. Research on metal powders at home and abroad believes that the absorbing ability of magnetic metal powders is better than that of ordinary metals. This is because in addition to electron absorption, there are also magnetic loss and frequency dispersion, and the shapes of rods, flakes, and fibers are anisotropic. Magnetic powder is better than granular magnetic powder. It can be seen that metal ultrafine powder is an extremely important absorption material, which can make up for the shortcomings of ferrite's heavy weight and unsatisfactory high-frequency characteristics.
2) Magnetic nano absorbing materials
In recent years, with the cross-discipline research, absorbing materials have more space in the choice of materials, especially the combination with materials with different characteristics, which makes the performance of absorbing materials have greater progress. In recent years, the focus of material technology is the successful application of nanotechnology in the preparation of microwave absorbing materials, which makes the nature of the absorbing materials show an amazing leap. The vigorous development of computer-aided design and the application of optimization theory help to determine the influence of the medium on the microwave absorbing performance when the medium parameters ε* (complex permittivity) and μ* (relative complex permeability) change with frequency. The relationship between various formulations and media parameters, in-depth discussion of various mechanisms that affect media, so as to adjust the material parameters as needed, are of great help, and can also guide the direction of experiments and speed up the development process.
The following example is the application of nanotechnology in absorbing materials, which shows the optimization of absorbing performance when the absorbing material ferrite enters the nanometer level. Figure 2 shows the ferrite and conductive composite with a 20% mass fraction of nanocomposites (where the nanoparticles are 10nm ferrite prepared by co-precipitation) and non-nanocomposites on electromagnetic waves in the range of 8-12GHz The reflection coefficient R varies with frequency f. It can be seen from the figure that the curve trends of the two are basically the same. The difference is that the absorption rate of the nanocomposite in the entire frequency band is higher than that of the non-nanocomposite.
When the particle size is reduced to 10~100nm, the physical and chemical properties of the particles change greatly. As the size of the particles decreases, the surface area of the particles increases, the proportion of surface atoms becomes larger, and the constraints of internal atoms become smaller, crystal defects increase, and the chemistry is extremely active. The energy band of conducting electrons of metal atoms changes from continuous to split energy level. Different energy level transitions can absorb energy in different bands. If the particle size of the nanoparticles is distributed according to the size and mixed with polyurethane to form a composite absorber, it can have strong absorption of millimeter waves, far infrared and near infrared, and its absorbing frequency band is much wider than the above-mentioned absorbing materials. It can be described as a broadband absorbing material. And it has good compatibility, low quality and thin thickness. For example, the size of nano γ-FeNi metal powder is less than 10nm, and the highest absorption rate for centimeter and millimeter waves is as high as 99.95%. Recently, people have carried out research on the nanometerization of ferrites, combining nano-ferrites and conductive polymers into novel nano-composite microwave absorbing materials. Theoretically, they have the absorption function of magnetic loss and dielectric loss and have the properties of nanoparticles themselves. Absorbing performance. Synthetic material is a kind of absorbing material with light weight, wide bandwidth and excellent performance. High attention has been paid to nano-wave absorbing materials at home and abroad. The United States has developed the fourth generation of nano-wave absorbing materials, which can absorb up to 99% of radar waves, and its thickness is only micron level.
3) Composite magnetic absorbing material

Various materials have different absorbing characteristics and adapt to different wavebands. At present, one of the main research directions of absorbing materials is multi-frequency. Therefore, if these materials can be combined, the application range of absorbing materials will be greatly expanded. These materials are not a simple addition of the inorganic phase and the organic phase. There are only strong or weak chemical bonds between the two phases. Their combination will realize a new material that integrates many specific properties of inorganic, organic, and nano-particles. Especially the inorganic and organic interface characteristics make it have a broader application prospect. The excellent optical properties, high elasticity and toughness of organic materials, as well as ease of processing, can improve the brittleness of inorganic materials; more importantly, the presence of organic materials can provide an excellent carrier environment

, To improve the stability of the nano-scale inorganic phase, so as to achieve its unique micro-control, the characteristics of photoelectromagnetic catalysis and other aspects are better played, and new materials with strange characteristics may even be produced.
However, simple inorganic nanoparticles are not easy to disperse in organic matter, and there is often serious phase separation between organic matter and inorganic particles. There should be a strong interaction between the organic and inorganic phases in order to make better use of the organic matrix to prevent the agglomeration of inorganic nanoparticles, so that the nanoparticles can exist stably for a long time. Therefore, the preparation of composite absorbing materials is not only the nanotechnology of the inorganic phase and the organic phase, but also the composite nanotechnology. The molecular design of the material is very important.
The preparation methods of composite materials developed in recent years are also diverse, which can be roughly classified into four categories: direct blending of nano-units and polymers; in-situ generation of nano-units in the polymer matrix; monomers in the presence of nano-units The molecules are polymerized in situ to generate polymers; nano-units and polymers are simultaneously generated. The core idea of various preparation methods of nanocomposite materials is to effectively control the geometric parameters, spatial distribution parameters and volume fraction of the nanounits in the composite system, especially through the preparation conditions (space constraints, reaction dynamics). The control of scientific factors, thermodynamic factors, etc.) to ensure that a certain composition of the system has a one-dimensional size at least within the nanoscale range (that is, to control the primary structure of the nano unit), and then consider controlling the secondary structure of the nano unit aggregate.
Nano ferrite wave absorbing composite material
Ferrite absorbing composite material is a double complex medium with a certain dielectric constant and dielectric loss, but also a certain magnetic permeability and magnetic loss. Its mechanism of action can be summarized as the magnetic loss and dielectric loss of ferrite to electromagnetic waves. Loss, the nano-composite absorbing material made of ferrite nanoparticles and polymer compound can effectively absorb and attenuate electromagnetic and acoustic waves, and is considered an excellent absorbing material. The peak absorption of the ferrite nanocomposite multilayer film in the frequency range of 7~17GHz is (-40dB), and the frequency bandwidth less than (-10dB) is 2GHz.
Magnetic nano metal powder wave absorbing composite material
From the analysis of metal electronic energy level transition, atomic relative vibration optical wave, atomic rotation energy level and atomic magnetic energy level, it can be seen that the magnetic metal ultrafine particles have a strong interaction with electromagnetic waves and have a large amount of electromagnetic wave energy absorption. Under the conditions, nano-metal powder absorbing composite materials have outstanding advantages such as high microwave permeability and good temperature stability (the Curie temperature is as high as 770K), and they have been widely used. Nano-metal powder absorbing composite materials mainly include carbonyl nano-metal powder composite materials and nano-magnetic metal powder composite materials. The carbonyl nano-metal powder mainly includes carbonyl Fe, carbonyl Ni, and carbonyl Co. Nano-magnetic metal powder mainly includes Co, Ni, CoNi, FeNi, etc.
3 Application of magnetic absorbing materials
The application of magnetic absorbing materials is divided into military and civilian aspects.
In terms of its military use, for example, ferrite absorbing materials have been widely used in stealth technology. They have the characteristics of strong absorption, wide frequency band and low cost, but they also have the disadvantages of large density and poor high temperature characteristics. At present, the coating-type absorbing material with practical value is a magnetic loss-type coating or film made of magnetic compounds such as ferrite or carbonyl iron as the absorbent and natural rubber or artificial rubber as the base material. Such materials can be used not only to suppress mirror echoes, but also to suppress traveling waves, creeping waves and edge diffraction echoes. The absorption effect of this type of material is closely related to the frequency of the incident wave and the thickness of the coating. Taking the products currently available at home and abroad as an example, the coating thickness of 1.5-2mm is between 8-12GHz, and the peak absorption rate can reach 98%-99% at the selected two frequencies. Outside the two peaks, it can absorb 90%-97%. There is also a thin product, a thin film with a thickness of 0.5-1.5mm, which can obtain 97% absorption at 10-12GHz. When it drops to 6GHz or rises to 16GHz, the absorption drops to 75%. The advantage of the coated absorbing material is that RCS can be reduced without changing the shape of the aircraft.
The microwave anechoic chamber made of magnetic medium absorbing material can be widely used in the interference of electronic equipment, the measurement of the coupling degree of characteristic impedance of the antenna missile of radar or communication equipment, aircraft and satellite, and the measurement of the pattern of the astronaut's back shoulder antenna , Spacecraft installation test and adjustment, etc. In addition, magnetic medium absorbing materials also have broad space in improving the compatibility of airborne and shipborne radar equipment and improving the performance of the entire aircraft.
Coating magnetic dielectric absorbing materials on the surface of various military equipment can eliminate radar tracking of the equipment, so that these military equipment can easily break through the enemy’s radar defense zone and defeat the enemy. It is a powerful means of anti-radar reconnaissance. It is also a way to protect military equipment from being destroyed by infrared and laser guided weapons. In addition, the absorbing materials can also be used for concealed landing and other airport navigation equipment and other ground terminal equipment, ship masts, decks, submarine periscope brackets or air ducts and other equipment. This is the stealth equipment. It has been used in actual combat during the Gulf War, and it will have a wider and wider application space in the military field in the future.
In terms of civilian use, with the rapid development of the telecommunications industry, the application of microwave absorbing materials has broken through the scope of military invisibility, and has penetrated into many fields such as communication anti-interference, electronic information confidentiality, environmental protection and human protection.
As we all know, electronic and electrical equipment not only produce electromagnetic interference, but also cause electromagnetic pollution to the ecological environment. It mainly comes from various electrical equipment around us, such as color TVs, mobile phones, phone sets, computers, refrigerators, and launching stations (stations) of various telecommunication systems, etc. It interferes with scientific surveys, hospitals, navigation systems, and electronic systems in various important projects, causing huge economic losses. If the magnetic medium absorbing material is applied to electronic equipment, it can absorb the interference of external electromagnetic radiation, and also absorb the electromagnetic radiation leaked by itself, so as to eliminate electromagnetic interference (electromagnetic pollution), purify human living space, and avoid bringing to society Huge economic loss.

We know that electromagnetic radiation causes direct and indirect damage to the human body through thermal, non-thermal, and cumulative effects. Applying absorbing materials to home appliances such as TVs, stereos, computers, game consoles, microwave ovens, VCD players, mobile phones, etc. Reduce electromagnetic wave leakage below the national health safety limit (less than 38 microwatts/cm2) to ensure people's health. Applying it to high-power radars, microwave medical devices, and microwave crushers can prevent their electromagnetic radiation from leaking and protect operators from electromagnetic radiation.
The development of the modern telecommunications industry has made all our communication methods related to electromagnetic waves; the advent of the network information era has made electronic money and paperless trade a reality; how to prevent electromagnetic waves from leaking and ensure information security has become the focus of attention in this field. The application of absorbing materials can solve these problems very well, so that the implementation of my country's "Golden Bridge, Golden Card, Golden Gate, and Golden Tax" project can be more reliably guaranteed.
As the development direction of today's automobile industry, the electric vehicle industry has been put on the global economic strategic agenda by major automobile manufacturers. However, although the replacement of the internal combustion engine by the electric motor solves the chemical pollution of automobile exhaust, the physical pollution caused by the use of the electric motor-electromagnetic pollution is also a thorny problem that humans must solve. The application of magnetic medium absorbing materials makes electronic vehicles and electric vehicles more advanced, safer and more environmentally friendly.
At present, the output of devices and absorbing materials used to reduce electromagnetic interference is increasing day by day, and new products are constantly emerging. As early as 1981, for the first time in the rice field area of Osaka Prefecture, Japan, 40,000 tiles of ferrite absorbing materials were installed on the outer layers of high-rise buildings, which greatly improved the TV viewing effect in the city. Later, it was used on the outer layer of Aomori Prefectural Hospital and Tokyo's new office building, and the effect was quite obvious. Since Japan implemented electromagnetic compatibility regulations for electronic products in 1996, the demand for microwave absorbing materials has increased sharply. In 1996, market sales increased by 121.6% to approximately 4.5 billion yen: the market size increased to 6.5 billion yen in 1999. Metal and alloy series absorbing materials have also attracted much attention because of their wide absorption frequency and simple technology. In recent years, the Technology Development Institute of Datong Special Steel Company of Japan has developed a variety of electromagnetic wave absorbing materials to meet the needs of the market. The first is the use of FeCr series electromagnetic stainless steel, flat powder and polyester rubber are mixed and rolled into thin sheets to form a soft rubber containing two phases It can absorb electromagnetic waves of several hundred MHz, and the maximum attenuation reaches -20dB. In order to meet the needs of waveguide impedance matching and modulation and amplification couplers in microwave communication, Datong Company has also developed Fe7Cr9AI soft magnetic metal powder (average particle size 15μm) into PVC and mixed and rolled into thin plates. The frequency needs to adjust the filling rate of the alloy powder and the plate thickness to meet the maximum attenuation rate in the 10GHz~28GHz frequency band. Then successfully developed a variety of heat-resistant sheets (called DPR-HT, -HTY, -HTZ), which are mixed with metal powder (Fe-Si-Al) and rubber, and then rolled into a sheet with a thickness of 100~200μm, or It is directly liquefied into a slurry and coated on the device to absorb electromagnetic waves, and the use temperature can reach above 100 ℃. The resistivity is as high as 10~108Ω.cm. Because of its light weight, softness and good absorption, it is widely used in mobile phones, computer CPUs, CD-R, CD-RW and digital camera CCD charge couplers and LCD video cables. , There are considerable market prospects. Other companies are not far behind and are developing and producing absorbing materials. TDK uses FeSi series alloy powders to make absorbing materials called "IR-L", which can meet the absorption of 10MHz~10GHz high video zone. Then, the FeSiAl metal magnetic powder and resin are mixed and rolled into a plate of about 1 mm, which can obtain an attenuation effect of 7 to 8 dB. Another example is Tokin's development of an electromagnetic wave absorption sheet, which uses a few micrometers of FeSiAl alloy flakes mixed with organic solvents and coated on a polymer film to form an absorption sheet, which is said to absorb radiation from 300MHz to 3GHz. These thin absorbing materials are mainly used for those devices that have adopted anti-EMI countermeasures, but the radiation is still not fully up to the standard, the use effect is particularly obvious. This kind of thin absorbing sheet can reduce radiation by 3~5dB. The absorbing structure composite materials not only have good absorbing properties, but also have good rigidity, moisture resistance and high temperature resistance. Now the absorbing structure composite materials are beginning to be used in mechanical foundation devices that generate large vibrations and ship and vehicle transmission equipment to reduce vibration noise , Is a good sound-absorbing material.
It can be seen that absorbing materials have broad application prospects and a broad market in anti-interference of electronic products.
4 Development prospects of magnetic absorbing materials
According to research, there are three most promising high-tech industries in this century: biotechnology, electronic information industry, and materials science. Today is the era of the electronic information industry. However, the short product life cycle and high investment in technology development have made the competition in this field intensified; while biotechnology is still in its infancy; electromagnetic wave absorbing material technology as a material science The branch, its wide application and close correlation with its electronic information industry will make it a rising emerging industry. Materials are the foundation of technological progress. New material technology is the foundation of modern technological revolution and an important milestone in human progress. For example, the development and utilization of steel materials drove the first industrial revolution, and the invention of silicon semiconductor materials promoted electronics, computers, and information technology. The development of the industry and the invention of magnetic medium absorbing materials will surely drive the next industrial revolution.
With the rapid development of modern science and technology, absorbing composite materials have greatly improved in terms of preparation technology, performance and application, and are developing rapidly in the direction of composite, intelligent, and broadband.
According to the current development status of absorbing materials, it is difficult for one type of material to meet the comprehensive requirements of thinness, width, lightness, and strength proposed by the increasing stealth technology. Therefore, different types of powders need to be processed according to specific requirements. Forms of compounding to obtain the best absorption performance. For example, the combination of ferromagnetic Mn-Zn, Ni-Zn ferrite and ferroelectric BaTiO3 can greatly improve the absorbing performance, and organic-inorganic nanocomposite technology can also be used. This method can easily adjust the electromagnetic of the composite Parameters to meet the requirements of impedance matching, and can greatly reduce the quality.
Intelligent absorbing composite materials and structures are emerging high-tech fields that have gradually formed and received much attention in the 1980s. They have both sensing functions,

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