1. Introduction
An electromagnetic (EM) fields are everywhere in this universe; it occurs naturally as well as artificially. Earths’ magnetic fields and lightning are the natural sources of EM field and all electronic devices and electric power transmission lines are the artificial or man-made sources of EM field [
1]. EM ionized radiation is divided into four categories, static field, extremely low-frequency EM fields, intermediate frequency EM fields and radio frequency EM fields [
2]. The intermediated and radio frequency EM radiations maybe harmful to living beings as well as electronic devices and also it travels at the speed of light. Living beings exposed to an extreme level of EM radiations may cause cancer, tissue damage, lymphoma, leukemia etc., [
3] and the electronic devices are damaged with EM radiations because it creating electromagnetic interference (EMI) [
4]. The shielding of the living beings and the electronic devices are important to protect from EM radiation and its interferences.
Metals are the best material for EM shielding applications which are silver, copper, stainless steel, iron, gold, nickel, brass, graphite, etc., are good conductors of electricity as well as reflect and absorb most of the EM radiations [
5]. It is having some drawbacks like it is heavier, able to get corrode, not flexible etc. The use of metal-coated metal textile materials is overcoming the drawbacks of the metal properties i.e., it becomes flexible, less weight, and porous. Particularly in textile materials different techniques are used to convert the conventional textile material to conductive textile material; the techniques used are coating conductive polymers [
6,
7], metal coating of fiber, yarn & fabric [
8,
9], metal fibers blending [
10,
11], metal core wires [
12,
13], and carbonization of fabric, yarn & fibers [
14].
Textile material itself has many different structures and designs, the major structures are woven, knitted, and nonwoven. Many studies are there on textile structures and designs on EM shielding. Kan Lai et al. [
15] used the four metals (silver (Ag), copper (Cu), titanium (Ti), and aluminum(Al) to coat on the surface of the polyester (PET) filament using the vacuum evaporation deposition technique and produced woven fabric and tested it for electromagnetic shielding effectiveness (EM SE) as per ASTM D 4935 method. At 2.45 GHz frequency the Ag/PET, Cu/PET, Al/PET, and Ti/PET is shown as 80 dB, 72 dB, 64 dB, and 26 dB SE respectively for approximately 30 µm thickness of the metal coating. The Ag and Cu has very good EM SE among the set of metals. Jung-Sim Roh et al. [
16] used the PET/Cu/PET filament covered yarn and PET/stainless steel (SS)/PET filament covered yarn to produce the woven plain fabric at different opening. The Cu yarn fabric has SE of 46 dB at 1.5 GHz frequency and the SS yarn fabric has 41 dB at 1.5 GHz frequency. For SS yarn fabric, without the grid, grid opening area of 0.16, 0.63 and 2.53 mm
2 has SE of 41, 36, 32 and 26 dB at 1.5 GHz respectively. This study concluded that, the EM SE value can change by using the grid structure for composite applications. Kai Yang et al. [
17] have developed the Cu/Ni coated Milife nonwoven fabric using the electroless plating method for EM shielding application. At maximum 20 wt% Ni/80 wt% Cu coated fabric has a total SE of 25 dB at 1.5 GHz frequency among it 14 dB was reflected radiation (SE
R).
Some of the works in textile materials studied the increase in the number of layers and its angle against the EM SE and it shows interesting results. Zhi-Cai Yu et al. [
18] studied the warp knitted fabric along with conductive weft yarn was inserted to produce the fabric using the crochet machine. The warp yarn is polyester and the weft yarns are the rubber yarn, crisscross section PET (CSP), antibacterial nylon (AN) wrap yarns, stainless steel wire (SSW) as core (CSP/AN/SSW) hybrid yarn A and polyester yarn, bamboo charcoal polyester yarn (BC-PET), cotton yarn and jade fiber yarn as hybrid yarn B. EM SE of the warp knitted fabric prepared with CSP/AN/SSW and BP-PET as weft yarn and PET as warp yarn has 10 dB at 1.5 GHz frequency. The fabric was laid at two layers and three layers and the laid angles are 0°/0°, 0°/0°/0°, 0°/90°, 0°/90°/0°, 0°/45°, and 0°/45°/0°. For two-layer and three-layer fabric the laid angle 0°/90° and 0°/90°/0° has highest SE value of 28 and 40 dB at 3 GHz frequency. Laying angle of 90° has more efficient for EM shielding. Olena Kyzymchuk et al. [
19] prepared the SS wire and cotton yarn 1 × 1 rib knitted fabric using a flat knitting machine. Single-layer and double-layer fabric samples are tested for EM SE and the result for 0°/0° and 0°/90° has higher SE than the single layer that is 3–5 dB and 10–15 dB at 1.5 GHz frequency respectively.
In this work, the copper-coated Milife
® fabric named as the ultrathin nonwoven fabric was taken and cut into strips to study the effect of the gap, thickness and angle of the strips on the EM SE. The EM SE is measured as per the ASTM standard [
20]. To compare the effect of the parameters on EM SE, the design of experiment (DoE) more precisely screening design is implemented. MiniTab
® software is used for the analysis of DoE.
Electromagnetic Shielding Mechanism
An EM field is the combination of electric
E and magnetic
H fields that are perpendicular to each other. Due to voltage differences, the electric field is creating and its moving charges are creating the magnetic field. The current contains the electric as well as magnetic fields. There are two regions in the electromagnetic interference (EMI) shielding; the near field shielding region and the far-field shielding region. If the distance from the source is less than the wavelength (
λ) divided by
2π is called a near or induction field and greater distance is called a far or radiation field. The reduction in the electric and magnetic field is caused by shielding because the EM wave is reflected from the shield surface, multiple reflections in-between the shield and absorption of the shield. The EMI shielding is mainly based on far-field radiation, so the plane wave is used for measurement. In this case, the multiple reflection loss is neglected because the reflecting surface is larger than the skin depth,
δ (m), is defined as (Equation (1))
where,
f (Hz) is frequency and
µ is the magnetic permeability equal to
µ0.
µr,
µ0 is the absolute permeability of free space (vacuum,
µ0 = 4π 10
−7 H/m) and
K (S m
−1) is the electrical conductivity. An electric field at a high frequency penetrates only the near-surface region of a conductor. The amplitude of the wave decreases exponentially as the wave penetrates the conductor. The depth at which the amplitude is decreased to
1/e of the value at the surface is called the “skin depth,” and the phenomenon is known as the “skin effect” [
4].
EMI shielding value is represented in decibel (dB) and its effectiveness is mentioned by total shielding effectiveness
SET, in (Equation (2)):
where,
P1 is power without shield specimen (W/m
2),
P2 is power with the presence of shield specimen (W/m
2) and log x is decimal logarithm [
21].
EM SE of the specimen is influenced by its electrical conductivity, permeability, and permittivity, properties of ambient surroundings, and parameters of the EM source. There are many studies in textile materials that related the EM SE with electrical conductivity [
22,
23,
24,
25], and opening area [
15,
26,
27] of the specimen. The EM SE is measured by various methods and its varying according to the specimen application. These EM SE testing techniques are open field test [
28], coaxial transmission line test [
20,
29,
30], shielded box test [
31] and shielded room test [
32].
The main motivation of this work is to simulate the conductive material structural parameters against the electromagnetic shielding property. EM shielding is needed in many industrial as well as commercial applications and each one needs a various level of shielding effectiveness (ref. Tables 4 and 5). According to the requirement, the use of conductive materials can be optimized using the proposed model and thereby make the design of the shielding counter more effective. “Effectively” in this means of cost wise as well as resources wise. In this experiment, the copper/ nickel-coated ultra-thin nonwoven material strips were used for modeling. The simulated structures were formed by the conductive strips’ material at the different thickness, gap, and angle to study their effects on electromagnetic shielding effectiveness. Another goal is to find out the optimal model based on the requirement.
2. Materials
The copper/nickel (Cu/Ni) coated ultrathin nonwoven fabric (Cu/Ni NW) (Milife
®) named as ‘MEFTEX 20’ was procured from the BOCHEMIE a.s., Bohumin, Czechia. Milife
® is the registered trademark of JX NIPPON ANCI Corporation, Chiba, Japan, contains 100% polyester nonwoven made with the combined orientation of spinning technology. The Milife
® fabric is thin, smoothy and it has a silk-like appearance achieved from laying perpendicular polyester filaments. Metallization of Cu/Ni on ultrathin nonwoven (Milife
®) was done with chemical and electrochemical metal deposition method using ‘roll on roll’ coating technique. The fabric is also finished with anti-corrosion resistance on the metal surface. The ‘Cu/Ni NW’ fabric parameters are given in
Table 1.
The scanning electron microscopic images of the uncoated and coated ultrathin nonwoven fabric are shown in
Figure 1. Coating of the material is evenly applied on the nonwoven surface see
Figure 1c,d.
Many studies are about the effect of EM SE on the gap between the conductive material in textile structures but there are very few studies for the gap between and angle of laying conductive material in textile structures [
18,
19]. In this study, the ‘Cu/Ni NW’ sample is cut into strips at 3, 6 & 9 mm thick and laid with 3, 6, & 9 mm gap between each other. Each thickness of strips was laid at the above gaps and form a single layer of the sheet as shown in
Figure 2 and with help of CREO
® CAD, the graphical model of strips was created. For example, the single-layer 3 mm thick strip was laid at 3, 6, & 9 mm gap was shown in
Figure 2a–c. Likewise, 3 mm thick strips, 6 mm and 9 mm thick strips were laid at 3, 6, & 9 mm gap. Two-layer strips were formed by laying two single layer strips at 90°, 60° & 45° angle with same gap, 9 mm thick strips of 3 mm gap laid at 90°, 60° & 45° angle are shown in
Figure 2d–f. Moreover, 9 mm thick strips of 6 mm and 9 mm gap are laid at 90°, 60° & 45° angle to form two-layer strip samples. Likewise, 9 mm thick strips, the 3 mm and 6 mm thick strips were formed as two-layer strip samples with various gap and laid angle. The two-layer samples were taken for the EM SE test as per ASTM method to analyze the effect of gap between and angle between the conductive strips’ material. Each thickness of the strips laid in 9 different ways and tested for EM SE. So, a total of 27 samples were prepared from the combination of strip thickness, gap between the strips, and angle between the two layers of strips for testing EMI shielding and the sample codes or names are given in
Table 2. The overlaying of the strips at various angles is prepared with the same strip thickness as well as the same gap between the strips as shown in
Figure 2d–f. For reference, the one-layer and two-layer strips laid at 0° angle was prepared for EM SE test and the sample code is given in
Table 2.
5. Conclusions
The Copper/Nickel coated 100% polyester ultrathin nonwoven fabric was taken in this study. For reference purposes, the single-layer and two-layer fabric samples were tested for EM SE and it exhibits 53 and 73 dB at 1.5 GHz frequency and graded as ‘very good’ according to the general requirement [
35]. All the single layer strips samples (SL) and two-layer strips (TL) laid at 0° angle samples had the EM SE of less than 12.5 and 9 dB respectively at 1.5 GHz frequency. Among TL samples, TL390 exhibits the highest SE of 42 dB at 1.5 GHz frequency. The EM SE (at 1.5 GHz frequency) is increasing with an increase in percent strips cover area (A
c) for the TL samples; A linear correlation between A
c and EM SE has been found (0.98 ≤ R
2 ≤ 1). A maximum A
c of 93.75% for TL3 series samples and recorded the highest SE value. An increase in area per aperture (A
a) of the TL samples has decreased in EM SE value; An exponential relationship of A
a and EM SE was found (0.87 ≤ R
2 ≤ 0.91). The TL945, TL960, and TL990 samples have A
c of 75% but A
a of 114, 93, and 81 mm
2 which has EM SE of 22, 23, and 28 dB respectively at 1.5 GHz frequency. So, the decrease in SE for the same A
c of samples is because of the decrease in A
a. The influence of percent cover area and area per aperture parameters has a significant effect on EM SE results.
The screening factorial design (SFD) model from the design of experiment (DoE) technique is used for the analysis of factors having a significant effect on EM SE at 1.5 GHz frequency. The three main factors are thickness (A), gap (B), and laid angle (C) of the strips at three levels were used for DoE analysis. The SFD model has 13 base runs and 39 total runs in the experiment. In ANOVA, the significant effect of SE on factors was calculated with reference to P-value. Good predictability of EM SE with the regression equation model of factors was found (R2 = 0.92). The factors which are A, B, and C has a significant effect on the SE value has found in the Pareto chart; In a normal probability plot, it found that the factors A, and C has positive significant effect and factor B has negative significant effect on SE value. The main effect plot fits confirms that the SE value was increased with an increase in factor A, decreases with increases in factor B, and decreases initially then increases with increases in factor C. The best SE value with respect to factors interaction was found in the interaction plot, the highest SE of 38 dB was found in the interaction of higher C (i.e., 90°) and a lower B (i.e., 3 mm).
Hence, the higher percent cover area and lower area per aperture structure have been recommended for achieving higher EM SE. DoE is concluded that the combination of the larger strip thickness, the lower gap between the strips, and the higher laid angle of strips has an excellent EM SE. This model could be helpful to construct the optimal fabric or composite structures based on the required level of shielding for electromagnetic shielding application.