Electromagnetic Textile Research And Development Is Deepening

Electromagnetic radiation refers to the phenomenon that electromagnetic waves are emitted or leaked into the air. There are different academic viewpoints at home and abroad about the damage of electromagnetic radiation to human body. As for the harm of electromagnetic radiation, we need neither to turn pale nor to ignore its existence.

With the rapid development of information technology, Spin The material needs to further consider its electromagnetic wave transmission performance. With the popularity of the "people-oriented" concept and the enhancement of the individual electromagnetic protection requirements, the textiles, which are commonly used as protective materials for human body, need to study the electromagnetic shielding performance and give consideration to the heat and moisture permeability and comfort of the textiles. Therefore, starting from the requirements of electromagnetic compatibility, electromagnetic shielding, electromagnetic stealth, radar reconnaissance camouflage, electronic warfare attack and defense, and considering that the material structure of a certain condition can affect the electromagnetic wave permeability better than the physical properties of the material itself, it is urgent to carry out electromagnetism research of other textile materials involved in the complex structure design and special electromagnetic properties, providing more scientific contents for the research of electrical properties of textiles, and laying the foundation for the development of electromagnetic textiles with special electromagnetic functions.

Where do we start developing electromagnetic radiation protection materials?

The research status of electromagnetic radiation protection materials is to develop "light, soft, thin and wide" new materials, that is, to pursue lightweight, flexible, thin-walled, wide-band structural characteristics, and adopt gradient structure to absorb and reflect electromagnetic waves, so as to achieve high shielding effectiveness. When talking about the future direction of electromagnetic radiation protection textiles, Zhou Guotai, academician of the Chinese Academy of Engineering said.

Although not all radiation can cause harm to the human body, but in the face of the ubiquitous radiation in nature, electromagnetic radiation protective clothing, one of the important equipment for personal protection, has become the focus of attention in the textile industry and even the mass consumption.

What progress has been made in the study of shielding materials for electromagnetic radiation protection at present stage? In this regard, Dr. Shi Meiwu, senior engineer of the Quartermaster Equipment Research Institute of the General Logistics Department, gave us a more detailed description: "at present, besides the electromagnetic radiation protective clothing materials such as stainless steel wire blended fabric, electroless silver plated fabric, electroless copper plated (nickel) fabric, conductive acrylic fabric and polyion fabric, some researchers have used tin instead of nickel to avoid heavy metal research. clothing Finally, the final product takes into account the electromagnetic shielding effectiveness and comfort of electromagnetic radiation protective clothing. Especially when protective clothing requiring high shielding effectiveness is required, because such protective clothing must be made into an electric sealing structure, it is easy to cause the problem of poor permeability and comfort. Therefore, the use of porous fabrics to metallized is beneficial to the protection effect and comfort.

In addition, researchers have designed skin core structures for synthetic fiber materials used for metallization. The core layer is a conventional polyester, and the cortex is made into pore master batch and blended with ordinary polyester to form a composite core sheath composite filament. After alkali treatment, there are many pits on the surface of the fabric. The metallization is carried out on the uneven surface of the fiber. The metal layer is riveted to the pits of the fiber, and the problem of metal layer falling is avoided because of the riveting effect. This metallized fabric has better durability.

Standards need to be unified and perfected.

Zhou Guotai said that formulating a scientific and reasonable exposure limit is the basis for distinguishing whether the environment of the personnel is basically safe (except for electromagnetic sensitive people), whether the electromagnetic radiation of the working environment needs protection and the electromagnetic radiation protective clothing that should be equipped with much shielding effectiveness.

Since 1988, China has drafted 6 national standards on electromagnetic radiation by the Ministry of health, the State Environmental Protection Administration and the Ministry of electronics. Aiming at the problem of long and confused contents, the General Armament Department took the lead in the unification of the 7 national military standards related to electromagnetic protection since 1984. It considered the concept of "exposure" and "limit value" and redefined the "exposure limits" of the GJB 5313 "electromagnetic radiation exposure limits and measuring methods" standard, and issued a formal version in 2004.

The two "electromagnetic radiation exposure limits and measuring methods" standard after merging and merging separately are the basic basis of environmental electromagnetic field allowable intensity and human exposure limit in China. Although one of them is the national standard, it still stays in the stage of soliciting comments, but its predecessor is 6 national standards, so its technical content can still be trusted. If there are any uncertain factors, it is just because different management departments may have some differences on the control limits of exposure limits due to different management angles. Shi Meiwu believes that the exposure limit value of GJB5313 is moderately strict compared with the previous draft, which can serve as the "upper standard" for shielding effectiveness design of electromagnetic radiation protective clothing.

The newly developed GB/T 23463-2009 protective clothing microwave protective clothing standard stipulates that the shielding effectiveness of electromagnetic radiation protective clothing should be determined according to the exposure limits of GJB5313 and the electromagnetic radiation field strength in the workplace.

Similarly, whether the public needs to wear electromagnetic shielding protective clothing, as long as the exposure limits of the corresponding frequency under the standard and the actual electromagnetic intensity of the environment are concluded.

Clear sense of trend concept

In view of the development trend of electromagnetic radiation protection, the two experts have the following suggestions: first, improve the knowledge level of the public and professionals through science popularization education, so that the public and professionals can have a clear understanding of the concept of electromagnetic radiation, the mechanism of damage and the limits of exposure, and strictly control the media's exaggerated coverage of the hazards of electromagnetic radiation.

The two is to strictly distinguish the basic characteristics of ionizing radiation and non ionizing radiation, and gradually change the term "electromagnetic radiation" into electromagnetic "exposure", so as to express that the harm to non ionizing radiation is not as serious as that of ionizing radiation, and the effect of human exposure to microwave is similar to that exposed to light waves. Actually, electromagnetism The wavelength of a wave is longer than that of visible light, and its damage is not great.

Three, we should set up a unified national standard for electromagnetic radiation limits as soon as possible, and formulate a national law on the prevention and control of electromagnetic radiation pollution as soon as possible.

The four is to make scientific and specific product standards and methods for polluting sources such as induction cookers and microwave ovens, which are prone to electromagnetic leakage and frequently used by the public. To eliminate the serious unqualified products that electromagnetic waves leak out, use accessories with good cooking pots and other accessories, and establish reasonable use methods to prevent these products from hurting the human body.

The five is to formulate relevant laws and regulations, and set strict regulations for electromagnetic radiation protection clothing for workers. When the electromagnetic field intensity exceeds the exposure limits in the working environment, electromagnetic radiation protective clothing shall be provided in accordance with the regulations of GB/T 23463-2009 "protective clothing microwave protective clothing" standard.

　 Three directions for developing electromagnetic textiles

General Logistics Department Quartermaster Equipment Research Institute Shi Mei Wu Xiao Hong Beijing University of Technology Wang Qun

Study on relationship between cavity size and cut-off frequency of metallized textiles

According to the theory of pinhole coupling in electromagnetic compatibility theory, the size is far smaller than that of the wavelength, so the slot is equivalent to the electric dipole and the magnetic dipole. According to the theory and the electromagnetic leakage practice of metal plate holes, the factors affecting the shielding effectiveness of metal plate include electrical conductivity, thickness, pore shape, size and arrangement. The shielding effectiveness of metal mesh is related to the wire diameter, spacing and the resistance of unit length.

It can be seen that due to the large number of influencing factors, even a well structured metal plate pore or metal mesh can not establish a complete theoretical equation to express the relationship between the gap state and shielding effectiveness. But one thing is clear: according to cut-off waveguide management theory, cut-off waveguide as a high pass filter can not pass through a certain shape structure below the cut-off frequency. As the observation window of household microwave oven, observers can observe the holes on metal mesh, but the electromagnetic wave of 2.45GHz in microwave oven does not leak out.

Therefore, we first study the relationship between the different shape, size and arrangement of the holes on the foil and its shielding effectiveness. At the high frequency end, the electromagnetic shielding effectiveness is tested by the arch method test system, and the flange coaxial method is used to detect the low-frequency end. The basic conclusion is that the material and thickness of various metal foil have little effect on the electromagnetic shielding effectiveness. The larger the diameter of the hole and the smaller the distance between the holes, the lower the shielding effectiveness of the conductor, and the lower shielding efficiency for the high frequency end.

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On the basis of the above, the porous fabric made of three kinds of porous fabric woven by porous structure in Figure 1 is metallized by electroless copper plating and nickel plating. The relationship between the hole size and electromagnetic shielding effectiveness is shown in Figure 3. Obviously, the processing method, shown in Figure 2, can take into account the shielding effectiveness and thermal comfort of electromagnetic radiation protective clothing. When necessary, laser drilling can also be used for thermoplastic synthetic fabrics to form more complex holes.

Study on electromagnetic properties of textiles with planar periodic structure

According to the theory of frequency selective surfaces, periodic conduction of a non conducting element (or pore cell) on a conductor plane produces a band-pass effect or a high pass effect on the conductor plane. Periodically conducting a conductive unit (or a resistance unit with poor electrical conductivity) on a dielectric plane will cause the dielectric plane to form a band stop or low pass effect. The resulting frequency selective penetrating effect of electromagnetic waves is very meaningful in selecting useful signals and cutting off interference signals. This precise control of electromagnetic waves is more applicable than shielding interference. How to make textiles have the frequency selective permeability function of electromagnetic waves has not been noticed.

In fact, the frequency selective surface (FSS) made of textiles has its convenience and unlimited design. It can be flexibly combined with different materials, unit pattern and size, arrangement density and arrangement, and make the final product soft and soft with other materials.

There are more ways to establish conductive periodic structure units on the dielectric plane, for example, local chemical plating or electroplating, partial sputtering of conductive materials, local coating printing, adhesion of conductive materials or conductive polymer materials. These processes are very easy to implement both for traditional textile processes and for new technologies.

Through a series of contrastive experiments, it can be seen that textile can establish periodic structure conductive units on its insulation surface by various means, or non-conductive units with periodic structure on its conductive surface, and make flexible band pass filters that can be selected through electromagnetic wave frequency. It can be applied to the accurate control of electromagnetic signal transmission and energy transmission, so as to achieve the purpose of blocking or clearing specific frequency electromagnetic waves.

　 Study on electromagnetic properties of textiles with three-dimensional periodic structure

The establishment of a three-dimensional periodic structure unit on the fabric will form more effective electromagnetic fabric with frequency selective penetration. There are many mature processing techniques for the establishment of three-dimensional periodic structure of textiles. For example, the conductive fabric made of silver plated nylon filament on the plush loom is 8mm in length. The silvered filaments are consolidated in the ground when processing, so the silver plated filament is not linked to the bottom of the fabric and becomes a basic vertical U shaped conductor.

For the plush fabric, the E8257D signal transmitter and E7405A spectrum analyzer are used as the test equipment, and the transmission power measured by the arch method is used to test the system. It shows that in the wide frequency range, most of the incoming electromagnetic waves enter into the floss, and the energy is reflected repeatedly and consumed. Only a very small proportion of the electromagnetic energy can be reflected to the receiver. That is to say, this state is related to fiber size, and the fiber size affects the incident impedance of electromagnetic wave on the surface of plush fabric, thus affecting the frequency selectivity.

The above example is a vertical villi with one end in a free state. We also use warp knitted spacer fabric, with silver plated nylon filament as spacer yarn, and polyester filament as raw material for two ends, and separate the spacer yarns to two ends. In order to obtain high scattering effect, that is, to reduce the emissivity and to achieve radar camouflage, we have set up the consolidation method of conductive spacer yarns. The conductive yarns are arranged in a group way. Each group of conductive yarns are arranged in a circular form, but the yarns are fixed on a small circumference on the bottom of the spacer fabric, while the spacer yarns are on the larger circumference on the top of the spacer fabric. Therefore, the separated yarns form an open and flat cone on the top of the spacer yarn to facilitate the scattering of electromagnetic waves to a wider range, so as to reduce the specific reflectivity of the specific direction and achieve the effect of radar camouflage.

In addition, the manufacture of left-handed materials by chemical metallization technology and textile processing technology is an important topic worthy of study. Left-handed materials are called metamaterials because their properties exceed the materials in nature. However, the left-handed material has been etched by the micro printed circuit board, such as the open resonator ring, and so on. Therefore, little progress has been made in the construction technology of left-handed materials.

The existing chemical fiber textile processing technology can realize the three-dimensional periodic structure more easily. For example, on the basis of metallizing the fiber surface to form metal columns, or local metallization to form an open ring resonator, and by twisting and twisting to achieve the chirality, using the weaving method of plush, velvet, towel and spacer fabric to make the fibers fixed in a relatively regular form, it is possible to realize the structure needed for left-handed materials, and this process is much easier, especially the line degree of these fibers can be conveniently coincide with the electromagnetic wave length which needs to produce double negative effects.

In the above cycle structure textile The research and development of left-handed materials based on textiles will have important academic significance and application value.