At present, the use of electronic equipment is more widely used, and the use of increased intensity, but also accompanied by improved performance of electronic equipment, miniaturization and low power consumption, for the requirements of electronic circuits will be more and more high, the circuit complexity, precision design and operation of the speed of the increase will lead to an increase in the number of sources of noise, electromagnetic interference is more severe, the electronic products on the noise tolerance will also be reduced.
As electronic products increasingly utilize low-power, high-speed, and highly integrated circuits, these devices are more vulnerable than ever to the threat of electromagnetic interference. At the same time, the increase in high-power home appliances and office automation equipment, as well as mobile communications, wireless networks are widely used, and greatly increase the source of electromagnetic interference. The use of certain technical means, so that the same electromagnetic environment of a variety of electronic and electrical equipment can work normally, and does not interfere with the normal work of other equipment, which is electromagnetic compatibility (EMC).
Elimination of electromagnetic interference and protection against electromagnetic radiation can be through the reflection and absorption of electromagnetic waves by two means. Reflection loss process mainly through the reflection of electromagnetic waves to achieve the purpose of shielding, effective reflection shielding needs materials can reflect most of the incident electromagnetic waves. For high conductivity media such as silver, copper and other materials, the formation of a continuous conductive path on the surface of the media will form an effective electromagnetic wave reflection loss, reflection shielding plays a major role; for high permeability media such as iron and magnetic steel and other materials, absorption shielding plays a major role. Electromagnetic shielding through the reflection loss, will produce a series of practical applications, such as reflecting out of the electromagnetic wave to the outside world of electronic devices and devices within the normal work of the impact, resulting in secondary electromagnetic radiation interference.
According to the theory of electromagnetic waves and the principle of interaction between materials and electromagnetic waves, a more effective way is to enhance the shielding material on the electromagnetic wave absorption efficiency, so that the electromagnetic radiation energy as much as possible to lose in the material inside, reducing the interference of the surrounding devices. It is also because of the existence of these problems, NFC absorber has a place.
NFC absorber is generally made of matrix materials and absorbing medium composite, can be projected onto the surface of the electromagnetic wave energy absorption, and through the material of the dielectric loss of electromagnetic wave energy into heat or other forms of energy. Good wave-absorbing materials with lightweight, temperature, moisture and corrosion resistance and other properties, today’s electronic parts thin and light, short trend, the development of wave-absorbing materials towards “thin materials, light weight, wide frequency band, strong strength” and so on. Wave-absorbing materials are generally made of base material (or adhesive) and absorbing medium (absorber) composite.
NFC absorber characteristics
Maximize the incident electromagnetic wave into the wave-absorbing material, thereby reducing the direct reflection of electromagnetic waves.
NFC absorber on the incident electromagnetic wave can produce effective absorption or attenuation, that is, electromagnetic loss, so that the electromagnetic wave energy is converted into heat or other forms of energy, so that the electromagnetic wave in the medium is maximized absorption.
NFC absorber types, the mainstream classification of the following types: ① according to the material loss mechanism can be divided into resistive, dielectric and magnetic media type. Silicon carbide, graphite, etc. belongs to the resistive type, electromagnetic energy is mainly attenuated in the resistance; barium Chinate and so on belongs to the dielectric type, its mechanism relies on the electronic polarization of the medium, ion polarization, molecular polarization or interface polarization, such as chiropractic, attenuation, absorption of electromagnetic waves; ferrite, ultrafine metal powder, carbonyl iron, etc. belongs to the magnetic medium type, with a high angle tangent of the magnetic loss, relying on hysteresis loss, domain wall resonance and natural resonance, Magnetic polarization mechanisms such as after-effect loss attenuation, absorption of electromagnetic waves. ② According to the principle of wave absorption can be divided into wave-absorbing and interference type. The former is the material itself on the absorption of electromagnetic waves, the latter is the use of surface and bottom two columns of reflected waves interfere with each other to offset. ③ According to the material molding process and load-bearing capacity, can be divided into coated type and structural type. Coating type is the binder and metal, alloy powder, ferrite, conductive fibers and other wave-absorbing agents mixed to form a wave-absorbing coating.
Common NFC absorbers
1.Magnetic metal micropowder NFC absorber
Magnetic metal micropowder is a very important class of electromagnetic wave absorbers, mainly referring to Fe, Co, Ni metal monomers and their alloy microparticles. Metal powders have high magnetic permeability imaginary part and magnetic loss angle tangent value, mainly through the hysteresis loss, eddy current loss and natural resonance loss and other mechanisms to absorb and attenuate electromagnetic waves.
There are two types of metal micropowder absorbers in common use: one type is carbonyl metal powder, such as carbonyl Fe powder, carbonyl Ni powder and carbonyl Co powder, etc., with most of the particle sizes distributed in the range of 0.5-20 μm. The other type is magnetic metal ultramicron powder, with particle sizes generally in the range of 20 nm to 1.5 μm, which can be obtained by evaporation, reduction, organic alcohol salts and other methods.
Magnetic metal micropowder NFC absorber in use there are some problems, such as easy oxidation, poor corrosion resistance, easy to produce yield effect, higher density, low-frequency band absorption performance is poor, and can not be used alone to get a wide bandwidth of wave-absorbing materials. Therefore, reduce its particle size, its surface modification, doping, coating or fiberization is the main direction of development of such materials.
2.Ferrite NFC absorber
Ferrite belongs to the double complex dielectric materials, both ferrimagnetic and dielectric properties, the main mechanism of electromagnetic wave absorption is the self-polarization effect of dielectric properties and magnetic properties of hysteresis loss, domain wall resonance and natural resonance effect. According to the different crystal structure, ferrite can be divided into spinel type, magnetite type and garnet type, used as an absorber is mainly spinel type and magnetite type.
Although ferrite NFC absorbers have the advantages of excellent absorption performance and low cost, their disadvantages such as high density, poor high-temperature performance, and narrow absorption band also limit their scope of use. At present, most of the research on ferrite NFC absorbers focuses on nanosizing, doping treatment, surface modification and composite with other materials.
3.Nano NFC absorber
Nano NFC absorber is a new type of functional material at the nanoscale, which not only has a large magnetic loss, but also has a variety of functions, such as wave absorption, wave transmission, polarization, etc. It can be fused with structural or coating materials, and combines the advantages of strong absorption, wide bandwidth, and good compatibility, so it is also a very promising absorber material. At present, domestic and foreign research on nano NFC absorbers mainly focus on nano-metal and alloy absorbers, nano oxide absorbers, nano-ceramic absorbers, nano conductive polymers and nano-metal and insulating medium composite absorbers.
4.Lightweight NFC absorber
Carbon-based electromagnetic wave NFC absorber has the advantages of light weight, corrosion resistance and easy processing, etc. The portable and flexible development of electronic equipment has put forward more application requirements for lightweight carbon-based wave absorbing materials. Carbon-based wave-absorbing materials include carbon fiber/polymer composites, carbon nanotubes/polymer composites, graphene/polymer composites, carbon nanotubes/metal composites, graphene/metal composites, and multifunctional carbon-based materials.
Among them, graphene NFC absorber possesses more conductive channels and effectively shields electromagnetic waves through multiple reflection losses and absorption losses. Graphene wave-absorbing material synthesis methods include reduced graphene oxide method, liquid phase exfoliation method, and chemical vapor deposition method. In order to improve the wave-absorbing properties of the material, material designers have improved its wave-absorbing properties by preparing few-layer graphene (FLG) and utilizing an effective conductive network.
5.Conductive Polymer NFC Absorbers
Conductive polymers are characterized by low density, easy processing, low cost, easy to coat large areas, and diverse structures, which as wave-absorbing materials can lead to a significant reduction in product quality. Conductive polymers have electron conjugate system, its conductivity can change in the insulator, semiconductor and metal range, its electromagnetic parameters depend on the main chain structure of polymer, intrinsic conductivity, dopant properties and other factors, is an ideal alternative to the traditional metal wave-absorbing materials, a new type of electromagnetic shielding materials.
Conductive polymer NFC absorbers are divided into intrinsically conductive polymers and composite conductive polymer materials. Polyaniline, polypyrrole, polythiophene and other modified intrinsic polymers have a high dielectric loss, can be electromagnetic waves generated by the conversion of electrical energy into thermal energy, and easy to control the conductivity and dielectric constant through chemical reactions. Composite conductive polymers also include metal-based conductive composites, carbon based conductive composites and other conductive composites. By adding metals, metal oxides or carbon fibers in the conductive or non-conductive polymer matrix can effectively improve the material’s wave-absorbing properties.
6.Ceramic NFC absorber
Most of the ceramic NFC absorbers belong to dielectric loss type wave absorbing materials, compared with magnetic metal micropowder, ferrite, etc. In addition to the wave absorption effect, they can also effectively attenuate the infrared radiation, which is one of the main components of the fabrication of multi-band absorbers. In addition, they also have a small density, dielectric constant with different sintering temperatures have a wide range of changes and other characteristics, so the microstructure and electromagnetic parameters can be regulated by controlling the preparation conditions, and then obtain excellent performance of the wave-absorbing materials. At present, domestic and foreign research and development of ceramic wave-absorbing materials mainly include silicon carbide, silicon nitride, alumina, aluminum borosilicate and barium titanate, etc., of which the higher degree of concern is SiC, but the conventional preparation of SiC powder wave-absorbing properties of the lower, must be doped in order to obtain a satisfactory wave-absorbing effect!