A Review of Electromagnetic Shielding Fabric, Wave ...

As one of the effective means to restrain electromagnetic (EM) interference and achieve EM protection, EM shielding refers to limiting the transmission of EM energy from one side of a material to the other side. The mechanisms of EM shielding can be analyzed using the transmission line method. Typically, materials with high conductivity are employed to reduce EM radiation, utilizing the reflection properties of conductors on EM waves. Shielding effectiveness (SE) is commonly used to represent the ability and effectiveness of materials in shielding against EM energy.

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Although traditional metals and alloy materials provide excellent EM shielding, their development is constrained by disadvantages such as heavy weight, high cost, and poor corrosion resistance. Novel EM shielding materials that are lightweight are increasingly popular. EM shielding fabrics exhibit low density, good flexibility, and light weight, making them widely used in manufacturing EM protection products such as protective clothing, shielding tents, and shielding gun suits. Additionally, EM shielding fabrics possess strong one-time molding capability, excellent design adaptability, and breathable properties. They can be crafted into various geometries to shield radiation sources and can also be processed into shielding suits and caps to protect personnel from EM radiation. Furthermore, fabrics made from metal fibers offer additional functions like antistatic properties, antibacterial capabilities, and deodorizing effects. Consequently, EM shielding fabrics are ideal materials due to their outstanding properties. Research on EM shielding fabrics can be categorized into theoretical calculations and experimental measurements.

2.1. Theoretical Calculation of EM Shielding Fabrics

The shielding effect of materials can be measured using the transmission coefficient (T) and Shielding Effectiveness (SE). The transmission coefficient (T) is defined as the ratio of the electric field intensity (Et) or magnetic field intensity (Ht) at a location with a shield to the electric field intensity (E0) or magnetic field intensity (H0) at the same location without a shield. The formula is as follows:

T = Et/E0 = Ht/H0

(1)

Shielding Effectiveness (SE) represents the shielding capacity of a shielding body against EM interference. It is frequently expressed logarithmically, as defined below:

SE = 20lg(E0/Et) = 20lg(H0/Ht) = 10lg(P0/Pt) = 20lg(1/|T|)

(2)

Here, lg = log10; P0 is the power density without shielding; Pt is the power density with a shielding body at the same location. For ease of calculation, the most commonly used formula is SE = 20lg(E0/Et).

According to the literature, SE can be divided into reflection loss (SER), absorption loss (SEA), and multiple reflection loss (SEM):

SE = SER + SEA + SEM

(3)

The current theoretical calculations of EM shielding fabrics mimic conductive yarns' shielding performance to metal plates, and equivalent calculations are conducted based on fabric structures corresponding to metal plate structures, such as no pore, pore structure, metal grid, and layered parallel array. This approach calculates the SE of fabrics. Based on transmission line theory, there are three different mechanisms for EM wave attenuation by the shielding body: reflection attenuation, absorption attenuation, and multiple reflection attenuation. Initially, metal plates are classified into structures with no pores, porous structures, metal grids, and layered parallel arrays. Theoretical formulas or semi-empirical formulas of EM shielding are derived from transmission line theory and equivalent circuit methods.

Under far-field plane wave conditions, the transmission coefficient for non-porous metal plates is defined as follows:

(T = 4ηeηmγd(ηe + ηm)²/[1 + (ηm/ηe - ηm)²e²γd]; ηm = 3.69 × 10⁷fμrσ = 377Ω; γ = (1+j)πμfσ)

(4)

Here, ηe is the impedance of the EM wave; ηm is the impedance of the metal plate; γ is the propagation constant of the EM wave in metal; d is the thickness of the metal plate; μr and σr are the relative permeability and relative conductivity of the metal plate. Using Equation (2), the SE of non-porous metal plates can be derived.

For porous structures in metal plates, the transmission coefficient of pores (Th) is obtainable from literature:

Th = 4n(qF)^(3/2). (Circular pore)

(5)

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Th = 4n(kqF)^(3/2). (Rectangular pore)

(6)

In these equations, q denotes the area of a single circular pore, and q' indicates the area of a single rectangular pore. n is the number of holes, F stands for the area of the metal plate, while k = baφ23, with a and b being the dimensions of the rectangular pore. For a square, φ = 1; for b/a > 5, φ = b²/a ln(0.63b/a). The transmission coefficient for porous metal plates is Th = T + Th. With Equation (2), the SE for metal plates with porous structures can be calculated.

Henn et al. were the first to propose that metallized fabrics are equivalent to porous structure metal plates, deriving formulas for metallized fabrics' SE by calculating the SE for porous structure metal plates. Additionally, Safarova et al. utilized this methodology to compute the SE for metal fiber-blended fabrics, analyzing fabric pore shape using image processing technology and approximating irregular shapes as rectangles to establish SE models based on fabric porosity, thickness, and fiber volume.

The equivalent method of metal yarns to porous structure metal plates offers a direction for evaluating EM shielding fabrics' SE. However, these models have limitations, necessitating good electrical connectivity throughout the fabric, resistance equal to that of metal plates, a certain fabric thickness, and regular pore structures. Simplifying the shape of single pores to rectangles or circles may result in significant errors, particularly when metal fiber content is low, presenting challenges to EM shielding fabric development.

SE formulas for metal grids have been documented in literature.

SE = Aa + Ra + Ba + K1 + K2 + K3

(7)

In this equation, Aa represents absorption loss from pores, Ra denotes reflection loss from pores, Ba is the loss from multiple reflections, K1 is a modification related to unit area and pore number, K2 deals with skin depth adjustments, and K3 concerns adjacent pore coupling. Each item's calculation formula can be referenced from the respective literature.

Table 1

Symbols of the Calculation Formula and Instructions Aa 27.3dw,(rectangular);32dD,(circular) Here, d is the depth of pores in cm, and D represents the diameter of a circular hole. Ra 20lg|1 + 4K2/4K| For rectangular pores: K = j6.69 × 10^−5fw
For circular pores: K = j5.7 × 10^−5fw Ba 20lg|1 - (K₁K + 1)²10^−0.1Aa| f in MHz K₁ 10lg(a/n), r r refers to the distance between the shield and field source; a signifies the area of a single pore in cm²; n is the number of pores per square centimeter. K₂ 20lg(1 + 35p².3) P = Width of conductor between holes; Skin depth K₃ 20lg[coth(Aa/8.686)]

SE = 20lg(1/s[0.265 × 10^−2Rf]² + [0.265 × 10^−2Xf + 0.333 × 10^−8f(ln(sa/1.5))]²

(8)

In this equation, s is the pitch of the metal grid, Rf denotes AC resistance per unit length of the metal grid, a is the radius of the metal fibers, and Xf represents the reactance per unit length of the metal grid.

Chen et al. made conductive fabrics woven from polypropylene fibers with copper wire and stainless-steel wire, respectively, proposing a metal grid structure and calculating the conductive fabrics' SE using metal grid structure formulas from existing literature. Within the frequency range of 30 MHz to 1.5 GHz, measured values differed significantly from theoretical values, likely due to poor contact or low conductivity at yarn intersections. Cai et al. used a metal grid structure model to calculate the SE of stainless-steel fibers blended fabrics. When the stainless-steel fiber content was 5%, 10%, and 15%, the computed results aligned closely with experimental values under low-frequency conditions. Rybicki et al. developed an equivalent circuit model of conductive grid yarns based on a periodic metal grid structure, asserting that SE is influenced by grid size, thickness, and grid material resistivity. This approach showed a degree of feasibility when compared to simulation experiments.

While the metal mesh structure closely resembles real 2D fabrics, this method requires that the fabric grid's intersection points be conductive, the pores must be regular, and the conductive fibers' percentage should not be minimal. Moreover, yarns containing metal fibers usually constitute a blend of metal and other fibers, which can alter EM parameters and cause significant inaccuracies. This model does not suffice for fabrics with high buckling degrees or 3D structures, thus restricting EM shielding fabric development.

Other optimization methods to calculate SE include Sabrio's metal parallel array method. This technique divides the metal grid into two periodic arrays of parallel metal plates at varying angles, allowing for SE calculations for each array. Liang et al. derived an SE model for 2D metal fibers blended woven fabrics using this methodology. A comparison of theoretical and measured values revealed that yarn diameter, electrical conductivity, and weaving angle significantly impact SE. Notably, the yarn crossing point’s conductivity does not affect this model, suggesting high applicability.

Yin et al. established an SE model for plain weave fabrics through a weighted average based on the buckling surface equations and fabric structures. This model elucidated the mathematical relationship between SE and parameters such as pitch, thickness, and fiber volume content in plain weave fabrics. The model trend closely matched experimental results, offering theoretical guidance for effective EM shielding fabric design with significant buckling.

The metal yarns were equivalent to structures with no pores, pores, and metal grids, requiring conductive crossing points in yarns and a specific fabric thickness. This requirement can restrict fabric design and development, as well as present several limitations. The method akin to a parallel metal array structure appears more accurate and unaffected by whether the yarn crossing points are conductive; however, it remains unsuitable for 2D fabrics with high buckling and 3D fabrics. Currently, research into SE is predominantly focused on 2D fabrics, with a scarcity of reports on 3D fabrics. 3D fabrics possess greater developmental potential and enhanced functionality relative to 2D fabrics. Examining the effects of fabric structure on SE will have theoretical significance for advancing 3D EM shielding fabrics.

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