Fabrics with electromagnetic shielding, wave absorbency and wave transparent properties

The theoretical research on absorbing fabrics mainly aimed to calculate the EM parameters of fabrics base on various equivalent methods so as to obtain high loss Angle fibre materials.

As a carrier of information transmission, EM waves transmission have been exploded with the development of electronic equipment and communication technology. As shown in Figure 1, according to the different frequencies and wavelength of EM waves, in the range of 10³–10¹⁴ Hz EM waves can be divided into radio waves, microwaves and infrared, and the generated radiation is non-ionizing radiation. Frequencies between 10¹⁵–10²² Hz can be divided into ultraviolet rays, X-rays and gamma rays, and the generated radiation is ionizing radiation. EM waves with different frequencies are applied in different scenarios and affect the development of human society to different degrees. On the one hand, the thermal and non-thermal effects of EM waves on biological tissues will lead to damage to the human body, such as fatigue, insomnia, tension, headaches and other symptoms will occur in the human body, and even increase the risk of leukemia and tumors under long-term exposure to EM radiation. On the other hand, high frequencies of EM waves will interfere with the normal operation of electronic equipment and communication, even damage equipment and cause communication information leakage. EM shielding materials, wave-absorbing materials and wave-transparent materials are the key to control EM waves transmission and pollution. The development of EM materials that can work at wide frequencies, have low density, thin thickness and good protective performance are a hot spot of current research. Among them, fabric is favoured by researchers for its low density, excellent mechanical properties and structure diversified.

Figure 1: Schematic diagram of EM waves spectrum

It is called fabrics that the fibres or fibres bundles with specific properties are arranged and combined in 2D or 3D space according to specific rules to form a collection of fibre bundles with a certain thickness, shape and geometric size by textile technology. As base materials with specific properties, fabrics can be made into fabric reinforced composites, fabric preforms and other fabric-based composites. According to the different fabric structures, it can be divided into woven fabrics, braid fabrics, knitted fabrics, etc., as shown in Figure 2. Figure 2a is a 3D angular interlocking woven fabric, the red and yellow in which represent yarns of different warp and weft directions. Figure 2b is a 3D braided fabric based on the symmetry of space group P4, different colours in which represent different paths of braided yarns. Figure 2c is a kind of knitted fabric, green and blue in which represent different types of coils that can be connected with each other to form a knitted fabric. Due to its low-density gravity and excellent mechanical properties, fabrics are widely used in aerospace, transportation, building construction, EM waves protection and other fields. Fabrics also have excellent designability, by changing fibres characteristics, fabrics structures, fabrics layers, yarn density and direction Angle (or braiding Angle) to meet the ideal application conditions [25]. With the rapid development of fibres metallization, fibres modification and conductive polymer polymerization, fabrics have been widely used in the field of EM waves. A series of fabrics composites with EM function, flexibility and light weight have been formed. According to the specific application scenarios, fabrics that meet the EM properties can be divided into EM shielding fabrics, wave-absorbing fabrics and wave-transparent fabrics.

Figure 2: Schematic diagram of three kinds of fabrics. (a) is a 3D woven fabric; (b) is a 3D braided fabric]; (c) is a knitted fabric.

The mechanisms of fabrics on EM waves can be divided into the macro level and the micro level. In macro level, Maxwell’s equations revealed the relationship between electric fields and magnetic fields, as well as the interaction law between fields and mediums by mathematical methods. A changing electric field gives rise to a changing magnetic field, which in turn gives rise to a changing electric field, so that the magnetic field alternates with the electric field, excites each other, and travels away at a certain speed, producing electromagnetic waves. Based on Maxwell’s equations, scholars have deduced many mathematical theories and methods for the interaction between EM waves and mediums, among which, transmission line theory has become a mainstream analysis method with easy understanding, convenient calculation and high precision. Figure 3 shows the mechanisms of EM waves on material based on transmission line theory, including reflection loss on material surface, absorption loss of material and multiple reflection loss inside the material.

Figure 3. Schematic diagram of EM waves transmission on materials.

In micro level, skin effect, polarization and magnetic loss will occur when EM waves interact with materials. Skin effect will cause thermal effect caused by an increase of conductor resistance due to uneven current in the conductor. Polarisation will cause Debye relaxation and dielectric loss and the loss is related to imaginary part of the complex dielectric constant of materials. The magnetic loss will produce resonance loss and eddy current loss and other losses, which is related to loss Angle tangent and imaginary part of complex permeability. According to specific application scenarios, fabrics with EM properties can be divided into EM shielding fabrics, wave-absorbing fabrics and wave-transparent fabrics. The following describes the research status of these three fabrics, respectively.

1. EMI Shielding fabrics

As one of the effective means to restrain EM interference and realize EM protection, EM shielding means to limit the transmission of EM energy from one side of the material to the other side. The mechanisms of EM shielding can be analyzed by transmission line method. Materials with high conductivity are usually used to restrain EM radiation, with the reflection effect of conductor on EM waves. Shielding effectiveness (SE) is usually used to represent the shielding ability and effect of materials on EM. Although traditional metals and alloy materials have a good EM shielding effect, their development is limited by the disadvantages of heavy weight, high cost and poor corrosion resistance. Novel EM shielding materials with lightweight characteristics are becoming more and more popular. EM shielding fabrics have the advantages of low density, good flexibility and light weight, which are widely used in the manufacture of EM protection products such as protective clothing, shielding tents and shielding gun-suit. EM shielding fabrics also have strong one-time molding ability, excellent designability, breathable fabrics properties, both soft and EM shielding properties, which can be made into different geometry to shield radiation source, but also can be processed into shielding suit and shielding cap to make the staff from EM radiation. In addition, metal fibers fabrics also have other functions, such as antistatic, antibacterial and deodorant. EM shielding fabrics are the ideal shielding materials with outstanding properties. The research of EM shielding fabrics can be divided into theoretical calculation and experimental measurement.

1.1. Theoretical calculation of EM shielding fabrics

The current theoretical calculation methods of EM shielding fabrics are to directly equivalent the conductive yarns with shielding performance to metal plates, and then the equivalent calculation is carried out according to the fabric structures corresponding to metal plate structures, such as no pore, pore structure, metal grid, layered parallel array and other structures, so as to calculate SE of fabrics. Based on transmission line theory, there are three different mechanisms for EM waves attenuation by the shielding body: reflection attenuation, absorption attenuation and multiple reflection attenuation. Firstly, metal plates are classified into no pore, pore structure, metal grid, layered parallel array (as shown in Figure 4), then the theoretical formulas or semi-empirical formulas of EM shielding are derived based on transmission line theory and equivalent circuit methods.

Figure 4: Schematic diagram of parallel array structures of metal.

The method of equivalent metal yarns to pores structure metal plates provides an idea for solving the SE of EM shielding fabrics. However, these models have certain limitations, requiring that the whole fabric has good electrical connectivity, resistance must equal to that of metal plates, fabrics must have a certain thickness, and the pores in fabrics need to be regular. In addition, simply approximating the shape of a single pore to a rectangle or a circle will cause a large error. When metal fibres content is too low, these models are not applicable, which is not conducive to the development of EM shielding fabrics.

2. Wave absorbing fabrics

Wave-absorbing materials can effectively absorb EM radiation, reduce EM pollution, protect the ecological environment, protect all kinds of electronics and electrical equipment from EM interference, avoid equipment failure or aging, maintain the normal operation of equipment, and can provide effective protective measures for the human body. It is one of the important ways to control EM waves transmission and prevent EM waves pollution to prevent the human body from being harmed by EM radiation in a strong radiation environment. With the rapid development of electronic information technology, the application of EM wave-absorbing materials is not limited to stealth military, but deep into communication anti-interference, electronic information confidentiality, environmental protection, human protection and many other fields. The application of fabrics in the manufacture of wave-absorbing materials has the characteristics of good electromagnetic absorption capacity, strong designability, low manufacturing difficulty and low cost, and has high application value, such as the manufacture of aircraft fuselage skin, aircraft engine and radar stealth military tent.

3. Wave transparent fabrics

Wave-transparent materials play an important role in national defence and military, aerospace, national economy and other fields, such as the manufacturing of radar radome, wave-transparent wall, protective wall and so on. The wave- transparent materials with high transmittance, low reflectivity and loss, good mechanical properties, good structural stability and fatigue resistance are the focus of recent research. Fabrics have the advantages of simple structure, easy processing, strong designability, one-time moulding, excellent mechanical properties and structural stability, and low manufacturing cost, which has great application potential in transmitting materials such as missiles, carrier rockets, aircraft, microwave towers, microwave relay station, communication antenna radome and antenna window radome, and transmitting wall manufacturing, etc.

4.  Conclusions

Fabrics, which have been widely employed in EM waves field, can be divided into EM shielding fabric, wave-absorbing fabric and wave-transparent fabric based on different application scenarios. The development statuses of these three fabrics were analysed and summarized. With regard to the EM shielding fabric, current theoretical methods aimed to equivalent the yarns with metal properties to metal plates, perforated metal plates or metal grid structure on the basis of the permutation and combination. In addition, those methods are suitable for single-layer or double-layer fabric composite materials, and it is required that the intersection points of fabric grid should be conductive, which has certain limitations. In detail, for 2D fabrics with a large degree of buckling or multi-layer fabrics, those methods will cause a large error. The fabrication of EM shielding fabrics can be processed by means of surface metallization, metal coating or woven fabrics with conductive fibres. However, the experiment without structure optimization will lead to both low efficiency and high production cost. In addition, the EM shielding mechanism and analysis methods of EM properties based on fabric structures are not only hardly reported, but also lack of optimum design method for shielding fabrics.

Furthermore, the theoretical research on absorbing fabrics mainly aimed to calculate the EM parameters of fabrics base on various equivalent methods so as to obtain high loss Angle fibre materials. However, the EM parameters obtained by means of equivalent methods couldn’t meet the requirements of wide frequencies and ideal EM parameters. In terms of experiments, absorbing fabrics were mainly studied on fibre materials, in which the ideal complex dielectric constant and complex permeability could be achieved through surface treatment or modification of the fibres or fibres base. However, there are problems of high experimental costs and complicated procedures. In addition, there are few reports on theories based on the structures of fabrics, and different structures have different effects on absorbing performance. Therefore, current research should lay emphasis on the development of the design method of a fabric structure that meets the requirements of absorbing performance. Moreover, the theoretical methods of wave-transparent fabrics were mature. The key factors affecting wave-transparent properties were dielectric constant and tangent of loss Angle.

At present, the research on the development of wave-transparent fabrics mainly involved how to reduce the dielectric constant and loss Angle tangent, and meet the mechanical properties and ablative properties simultaneously. There are few reports on the influence of fabric structure on wave-transparent properties. Therefore, the development and optimization of the wave-transparent fabric structure design can make the wave-transparent fabric structure more diversified. In order to meet the needs of different EM properties, fabrics will be diversified, intelligent, as well as meet the direction of multi-working conditions, such as from 2D to 3D. 3D fabrics can not only enhance the mechanical properties and thermal stabilities of composite materials, but also reduce the vertical stratification, which can meet the diversified requirements of EM properties in structural design. The fabric structures with EM properties will be designed from large to small and from heavy to light. In addition, more and more attentions should be paid to the research on fabric structures that satisfy EM properties. In brief, by optimizing fabric structure design, the ideal EM parameters of EM fabrics that meet the requirements of total reflection, zero reflection or total transmission are expected to be obtained.

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About the author:

• R Sasirekha, Sona M Anton and Z.Shahanaz are from the Department of Fashion Design, Hindustan Institute of Technology and Science, Chennai, Tamil Nadu.
• Dr D Madwesaran and S Ragupathy are from the Department of Electrical Engineering and Electronics, SSM College of Engineering, Komarapalayam, Tamil Nadu.
• J Lavanya is from the Department of Fashion Design, SRM Institute of Science and Technology, Kattankalathur, Chennai, Tamil Nadu.
• V Sathya is from the Department of Fashion Design, SRM Institute of Science and Technology, Ramapuram, Chennai, Tamil Nadu.
• Dr D Anita is from the Rachel School of Fashion Design, Footwear Design and Development Institute, Chennai, Tamil Nadu.
• M Manoj Prabagar is from the Department of Costume Design and Fashion, Vivekananda College of Arts and Science, Tiruchengode, Tamil Nadu.
• S R Viswanath is from the Department of Textile Technology, Park College of Engineering and Tekhnology, Coimbatore, Tamil Nadu.