The research on the safety of electronic equipment in the electromagnetic environment originated from the research on electromagnetic information leakage prevention of military equipment, called TEMPEST technology (electromagnetic information leakage prevention technology), which has a history of more than 40 years. Information in electronic equipment leaks to the outside through conduction and radiation. For information security, electromagnetic radiation is easier to detect than conduction. It has always been the focus of TEMPEST technology research. The United States has always been in a leading position in principle and technology research.
The electromagnetic environment safety of electronic equipment is also being studied in the field of civil electronic information products. Different from TEMPEST technology, the main consideration of civil electronic information products is not only its own information leakage problem, but whether the external electromagnetic field will affect the normal operation of its own equipment and whether the electromagnetic field emitted by itself will affect the normal operation of other electronic information products. , Called EMC (ElectromagneTIc CompaTIbility) technology. The exact meaning of EMC 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 it will also emit electromagnetic radiation under normal working conditions. It will not cause other equipment in the same environment to suffer unallowable degradation. 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.
What is the research status of electromagnetic wave absorbing materials in EMC? Is there any problem?
1 Research status of electromagnetic wave absorbing materials
1.1 Electromagnetic wave absorbing material
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, it has not been possible to achieve non-reflective absorption. However, in practical applications, most of the electromagnetic wave absorbing materials required by electronic and electrical equipment are electromagnetic wave absorbing materials with low reflectivity.
At present, there are mainly the following types of absorbing materials and absorbing bodies that are being developed and put into practical use abroad:
(1) Ferrite series 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 It has the advantages of strong absorption and wide frequency bandwidth, and is 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. The reflection of electromagnetic waves is small and the absorption is high. 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 entire particle atoms. Due to the large number of dangling bonds and empty bonds, the activity of the 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 inter-layer turn matching. The purpose of widening 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's base retinoid: black like graphite, absorbing performance is better than other materials, and the weight is only 1/10 of the iron ball absorbing material. The absorbing frequency bandwidth of this material is effective from long wave to 8mm wave 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 through 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 it can be adjusted by adjusting the chiral parameters. 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.
1.2 Electromagnetic wave absorber
The above-mentioned types of materials are the main absorber materials currently researched and developed. However, as far as the current research and production level is concerned, it is unrealistic to use a single material to make the absorber to achieve broadband absorption, and it cannot solve the problem of non-reflection. problem. In practical applications, a single material and direct use are generally less used, but in the form of electromagnetic wave absorbers.
Electromagnetic wave absorber is a structured electromagnetic wave absorbing material in order to obtain the best electromagnetic wave absorption effect, and it can appear in the form of a commercial product. In the world, composite and structural design methods are often used to solve the absorption problem of a certain frequency band, and it is the first to obtain applications in military use. Stealth aircraft such as B-2, YF-22, YF-23 and other stealth aircraft use structured absorbers. .
The research of electromagnetic wave absorbers is based on the research of absorbing materials. The structures of absorbers that have been put into practical use are:
(1) Single-layer structure: a single-layer coating and a single-layer absorber of composite materials.
(2) Multi-layer structure: It is composed of a wave-transmitting layer, an impedance matching layer, an absorbing layer and a reflective backing. In the design, the incident wave and the reflected wave are often used to cancel each other. Although the corresponding absorption peak will appear at this time, the absorption bandwidth will be affected.
At present, the United States, Japan, and Western European countries are leading the world in the research of electromagnetic wave absorbers. They have respectively 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.
The laboratory research on microwave absorbing materials and electromagnetic wave absorbers in my country began in the 1980s, and entered a more comprehensive research stage in the middle and late 1990s. Compared with foreign countries, both the material research and the research of electromagnetic wave absorbers are in the tracking and exploratory stage as a whole, but good progress has been made in some aspects, and some of their own characteristics have been formed. The research focus is mostly on a certain kind of absorbing material, and relatively little research on the design method of the absorber. The research on the design method of absorber mainly focuses on the design of single-layer and multi-layer structure. However, due to the absorption bandwidth of the material itself, impedance matching, and the addition of adhesives, the absorption characteristics are reduced.
Generally speaking, the theoretical research and application research of absorbing materials and electromagnetic wave absorbers are still under development, and mature theories have not yet been formed. When electromagnetic wave absorbers are used in different electrical products, the frequency band and bandwidth required for EMC can be achieved through material improvements. The biggest problem is impedance matching. However, it is gratifying that many electrical appliance manufacturers in my country have gradually paid attention to product EMC issues and invested in research. However, the research on electromagnetic wave absorbing materials used as commercial electronic products still needs to be strengthened.
2 Trends and problems in the research of electromagnetic wave absorbing materials
Due to historical reasons, the development of military TEMPEST technology is the first, and the development of civilian EMC technology is later. The research of military electromagnetic wave absorbing materials has always been ahead of the research of civil electromagnetic wave absorbing materials, and the same is true for the research of electromagnetic wave absorbers. On the whole, due to the requirements of technological development, higher requirements are put forward for the performance of electromagnetic wave absorbing materials and the design of electromagnetic wave absorbers, and a single material and structure can no longer meet the requirements.
For military electromagnetic wave absorbing materials, the development from single frequency band, narrow frequency band materials to multiple frequency bands and broad frequency bands. Ultra-fine structures and composite structures have become important development directions, and new electromagnetic wave absorbing materials and electromagnetic wave absorbing structures have become research hotspots.
Electromagnetic wave absorbers can not only be used as absorbing materials for electromagnetic wave shielding darkrooms, but also can be used for electromagnetic wave absorption and electromagnetic field design in various electronic devices. Electromagnetic wave absorber is an indispensable absorbing material for electromagnetic wave shielding anechoic chambers. There are pyramidal composite sponge absorbers impregnated with absorbing powder. Its size is related to the absorbing frequency. The shape is designed to match the impedance of electromagnetic waves. It can meet the electromagnetic wave absorption rate of 30MHz-40GHz and 10-20dB; the plate-shaped, grid-shaped and double-layer ferrite absorbing materials can meet the electromagnetic wave absorption rate of 30MHz-2GHz and 10dB. The electromagnetic wave absorber used in the electromagnetic wave shielding darkroom is adapted to the size of the darkroom, and the thickness is continuously reduced in order to effectively use the space.
Electromagnetic wave absorbers for electromagnetic wave shielding darkrooms cannot be used in civil electronic information products due to their volume and weight limitations. Electromagnetic wave absorbers with a thickness of millimeters have been used in military applications, but their development in civilian use is relatively slow due to various reasons. However, Japan and Western European countries have already exhibited electromagnetic wave absorbers with a thickness of millimeters. Products are rare.
In addition, in the actual electromagnetic leakage protection technology, different methods must be taken into account for the specific circuit conditions. Since the electromagnetic interference of electronic and electrical equipment is conducted in two ways: conduction and radiation, there are also two types of responses to EMC technology. (1) Conduction: Harmonics and clutter different from the frequency of the power supply are transmitted to the power grid through wires, and thus have an impact on other equipment and cause information leakage. The conducted electromagnetic waves are generally low in frequency and can be suppressed by the design of the filter. (2) Radiation, the high-frequency electromagnetic oscillation inside the equipment radiates outward in the form of waves, directly affecting other equipment. In order to reduce the electromagnetic radiation of the equipment, the following methods are generally used to suppress:
(1) Low-radiation circuit design, (2) Equipment shielding, including case shielding, wiring holes and seam shielding, (3) Isolation technology, including equipment working environment shielding (for example, the Pentagon in the United States installed electromagnetic wave shielding materials) , (4) Shielding of power lines and signal lines, (5) Filtering of power lines and signal lines, (6) Methods of electromagnetic wave absorbing materials.
Due to the limitation of circuit design technology and cost, many products need to use shielding and wave absorption methods when they fail to meet EMC standards. The purpose of shielding and wave absorption is to confine electromagnetic waves in a certain area, the difference is that shielding mainly uses the reflection of conductive materials on electromagnetic waves to limit the propagation of electromagnetic waves. Generally, the entire radiation source needs to be shielded, otherwise, the phenomenon of weakening in one direction and increasing in other directions will occur.
Wave absorption is the use of materials to absorb electromagnetic waves to convert the electromagnetic field energy of electromagnetic waves into other forms of energy (usually into
Heat). Electromagnetic wave absorbing materials are not required to have high conductivity, they are relatively easy to apply in equipment, and the technical requirements for use are lower. Research on EMC materials is currently mainly focused on shielding materials, and relatively little research on electromagnetic wave absorbing materials and absorbers. In fact, electromagnetic wave absorbing materials and absorbers play an irreplaceable role in EMC technology, such as shielding materials, and are currently becoming a hot spot in the development of EMC disciplines.
However, practical electromagnetic wave absorbing materials not only have specific requirements for the performance of the material, but also have specific requirements for the material in the use environment, such as use temperature and use size. These are the problems that must be solved for the practical use of electromagnetic wave absorbing materials. , Is also the research goal of a future stage.
3 concluding remarks
The research of electromagnetic wave absorbing materials began with military TEMPEST technology, and has been widely used in military stealth aircraft, stealth warships, stealth military vehicles, and stealth fortifications. It has become an important technology for information countermeasures in electronic warfare and has been highly regarded by all countries. Pay attention.
Due to the advancement of civil product EMC technology and the continuous formulation and implementation of standards, civil EMC technology and products have also received more and more attention. Especially for commercial confidentiality, there is a strong demand for electromagnetic wave absorbing materials. Electromagnetic wave absorbing materials can not only eliminate radar virtual images and ghost images, but also can be used for the elimination of TV ghost images, electromagnetic information leakage prevention in large commercial buildings, and the EMC of civil electronic products. It can also be used for the health of operators in electromagnetic environments. protection.
In the research of electromagnetic wave absorbing materials, on the one hand, improving the performance of existing materials and designing various types of absorber structures, on the other hand, is also stepping up the development of new electromagnetic wave absorbing materials. At present, the various electromagnetic wave absorbing materials mentioned above are not enough in the civil EMC product market due to various reasons, such as thickness, specific gravity, anti-aging performance, absorption bandwidth, etc., and they also have important development prospects.