Development needs of electromagnetic wave absorbing materials

The electromagnetic environment safety of electronic equipment is also under research in the field of civil electronic information products. The difference is that civil electronic information products mainly consider not only their own information leakage, but whether external electromagnetic fields will affect the normal operation of their own equipment And whether the electromagnetic field emitted by itself will affect the normal operation of other electronic information products is called EMC technology. The exact meaning of the EMC darkroom is: when a device and other devices are in a common electromagnetic environment, the device will not suffer unallowable degradation due to the electromagnetic emission of other devices in the same environment, and at the same time it will be electromagnetically under normal working conditions. Radiation will not cause unallowable degradation of other equipment in the same environment. In order to achieve "electromagnetic compatibility" for different electrical and electronic equipment in the same electromagnetic environment, corresponding EMC standards must be specified for different types of equipment, resulting in a large number of international, regional (such as Europe), national and industry standards. Some are recommended standards, and some are mandatory standards. As of August 1999, my country has issued a total of 76 EMC national standards.

The research of electromagnetic wave absorbing materials involves material science, electromagnetic field theory, electromagnetic wave absorbing materials and absorber theory, computational mathematics, etc. With the gradual importance of material design theory and methods, the research of electromagnetic wave absorbing materials has gradually become one of EMC and material science Important branch. Theoretically speaking, EMC technology has two basic requirements for electromagnetic wave absorbing materials:
(1) No reflection (that is, complete absorption);
(2) The absorption frequency band is as wide as possible.
Finding a new design method for non-reflective absorbing materials has always been the goal that people seek, but absorbing materials also have the same problem of reflection of electromagnetic waves as shielding materials. Although many electromagnetic wave absorbing materials have been studied so far, they have not been able to achieve non-reflective absorption. However, in practical applications, electromagnetic wave absorbing materials required by electronic and electrical equipment are mostly electromagnetic wave absorbing materials with low reflectivity.

Currently, there are mainly the following types of absorbing materials and absorbing bodies that are being developed and put into practical use abroad:
(1) Ferrite absorbing materials (nickel-zinc ferrite, manganese-zinc ferrite, barium ferrite, etc.): due to the resonance absorption of ferromagnetic materials and the dispersion effect of magnetic permeability, ferrite materials have The advantages of strong absorption and bandwidth are widely used in various stealth technical fields. The ferrite absorbing material studied by NEC Corporation of Japan is composed of two layers with a thickness of 3.8mm and 0.9mm. The mass per unit area is 8kg/m2. The bandwidth of attenuation -20dB is 8.5～12.2GHz, and the bandwidth of -10dB is 6～13GHz. .
(2) Micro-powder absorbing materials: Micro-powder materials (especially nano-absorbing materials) have received attention from all aspects due to their peculiar physical and chemical properties. They have low reflection and high absorption of electromagnetic waves. It is a new material worthy of attention. The surface atoms in the particles of the ultrafine powder material account for a large proportion of the whole particle atoms. Due to the large number of dangling bonds and empty bonds, the activity of surface atoms is greatly increased. When electromagnetic waves are incident on such particles, the movement of molecules and electrons intensifies, the conversion of electromagnetic energy into heat energy is highly efficient, and the electromagnetic loss is large. Its transmission and absorption performance depends on the size of the particle size. This feature can be used to achieve interlayer turn matching. The purpose of broadening the frequency band. For example, the electromagnetic wave absorber made of nano-silicon carbide fiber as the base material has a bandwidth of -15dB absorption greater than 1GHz in the frequency range of 8-12GHz. The specially treated carbon fiber has good application value in the radar band. For metal powders such as hydroxy Fe powder, Ni powder, and Co powder, the particle size is generally between 10-50nm, which has also been extensively studied, but its applicability is limited due to poor oxidation resistance.
(3) Polycrystalline ferromagnetic metal fiber: Polycrystalline ferromagnetic metal fiber has unique shape characteristics and compound loss mechanism (magnetic loss and dielectric loss), light weight (density <2kg/m2), frequency bandwidth (4～ 18GHz) and good performance of oblique incidence, and the electromagnetic parameters of the absorber can be adjusted by adjusting the length, diameter and arrangement of the fiber. It is a kind of absorber worth studying.
(4) Schickoff base retinoids: black like graphite, absorbing performance is better than other materials, and the weight is only 1/10 of iron ball absorbing materials. The absorbing frequency bandwidth of this material is effective from long wave to 8mm band. It converts all electromagnetic wave energy into heat energy through ion displacement method, but the temperature rise of the material itself is not obvious.
(5) Dielectric ceramic absorbing materials: PZT (lead zirconate titanate), BaTiO3 and other dielectric materials also have good absorbing effects, but the absorption bandwidth is small.
(6) Conductive polymer material: Compared with other absorbing materials, it has the characteristics of low density (only 1/5 of ferrite), and its absorbing performance can be controlled by adjusting the conductivity by doping. Foreign reports have -10dB and 12GHz bandwidth.
(7) Chiral absorbing material: Compared with ordinary materials, it has chiral parameters. The electromagnetic wave propagating in it can only be left-handed or right-handed circularly polarized waves. Its advantage is that the chiral parameters can be adjusted. Impedance matching is much easier than adjusting μ and ε; in addition, it has low frequency sensitivity and is easy to achieve broadband absorption. Once the material's practical technology has a breakthrough, it will have a significant impact on EMC technology.
At present, the United States, Japan, and Western European countries are leading the world in the research of electromagnetic wave absorbers. They have separately developed civil electromagnetic wave absorbers with a thickness of millimeters. The most advanced absorber structure is the electromagnetic wave absorber structure used in military stealth aircraft in the United States. This structure can reduce the reflection of radar waves by 7-10dB in a wider frequency band.
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