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Abstract
A multilayer transparent microwave absorber that possesses a broadband, is bendable, and covers the frequency band near 5 GHz is proposed in this work. The absorber uses three layers of 110 Ω/sq etched patterned indium–tin–oxide-polyethylene terephthalate (ITO-PET) films as the resonant layers. The infinite cycle structure of the absorber is simulated by using the CST STUDIO SUITE. The absorber shows >90% absorption effect in the frequency band 4.16-7.48 GHz and a 99% absorption effect in the band near 5 GHz (4.98-5.83 GHz). Furthermore, by replacing the intermediate medium with polydimethylsiloxane (PDMS), the absorber obtains excellent bendability characteristics.
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1. Introduction
Due to the advancements in 5G communication technology, the applications of the internet of everything have increased manifold. However, an extensive utilization of wireless communication devices leads to serious electromagnetic contamination [1,2]. The normal operation of electronic equipment and communication systems is directly affected by the electromagnetic interference and may cause serious accidents [3]. In the sophisticated electromagnetic environments, the immunity of electronic devices against electromagnetic interference is considered to be one of the important indicators regarding the reliability of future communication systems. The electromagnetic shield is commonly employed in medical facilities and devices, military installations, communication equipment, and other practical fields. These shields have the capability to safeguard wireless communication systems and delicate instruments against the electromagnetic pollution by enhancing the anti-electromagnetic interference capability of the electronic equipment [4].
It is worth mentioning that the 5G communication technology and WIFI communication are closely related, as they both use the same electromagnetic frequency band, i.e., near 5 GHz. Nowadays, a growing number of smart wireless devices are accessing this band [5,6]. The source of electromagnetic interference may arise from smart appliances, such as mobile phones, TVs, and even refrigerators [7]. An access to a large number of signals may lead to electromagnetic pollution, such as abnormal connection of communication signals, blockage of network channels, and an increase in connection delay. As a result, there is an urgent need for a low-band microwave absorber, which can operate in the frequency band near 5 GHz in order to ensure the normal operation of wireless communication systems.
Please note that the electromagnetic shielding device (absorber) should be thin and light weighted. In addition, it should have a broad band, high-performance, mechanical flexibility, and optically transparent characteristics. As compared to the traditional electromagnetic waves absorbing materials, such as Dallenbach layers [8], Salisbury screens [9], and Jaumann absorbers [10], the metamaterials are capable of overcoming the existing limitations, and gain distinctive characteristics exceeding the characteristics of natural materials. These characteristics include adjustable electromagnetic parameters and negative refractive index, etc. The design of the metamaterial absorber is considered to be a better broadband solution [11,12]. A typical metamaterial absorber design comprises a sandwich structure with frequency selective surface (FSS)-substrate dielectric layer-perfectly conductive backplane. Please note that most of the proposed metamaterial absorbers are built on flat arrays of individual metal patterns or on glass substrates [13–16]. These absorbers are rigid materials, which are non-bendable and opaque, thus making the application of metamaterial absorbers extremely limited. Recently, there has been a wide interest in Indium-Tin-Oxide (ITO) films, which are highly accurate, easy to fabricate, flexible, and transparent [17,18]. In addition, the use of flexible substrates including Polyethylene Terephthalate (PET) and Polydimethylsiloxane (PDMS) is also considered a better design solution for flexible transparent absorbers [19]. The previous works show that the absorber structures designed based on ITO-PET films have excellent absorption capability [20–23].
In this work, we propose a multilayer transparent bendable broadband microwave absorber, which operates in the frequency band near 5 GHz. It uses an ITO-PET film with excellent optical transparency and high flexibility. The absorber possesses greater than 90% absorption capability covering the frequency range of 4.16-7.48 GHz, essentially covering the C-band (4-8 GHz). It is worth mentioning that the absorber has greater than 99% absorption capability in the band near 5 GHz (4.98-5.83 GHz). The thickness of the absorber is only 0.101 times the top cut-off frequency wavelength (72.12 mm). It keeps relatively steady absorption in the incident angle range of 0° to 60°, while it is non-sensitive towards the polarization over the incident wave. Moreover, after replacing the intermediate medium with PDMS, the absorber gains excellent absorption capability even at large bending angles. Both simulation and experiments reveal that the absorber proposed in this work has excellent absorption performance. Moreover, it showcases an excellent optical transparency and mechanical flexibility. The absorber has extensive application promise in various areas, such as selectable absorption of electromagnetic signals from mobile phones and the normal use of equipment dashboards under strong electromagnetic interference.
2. Numerical simulations and discussions
The structure of the proposed multilayer transparent bendable broadband microwave absorber covering the frequency band near 5 GHz is presented in Fig. 1(a). The cell structure is presented in Fig. 1(b) and Fig. 1(c). Please note that the cell structure of the proposed absorber is composed of three layers of patterned ITO-PET (ITO square resistance value of 110 Ω/sq). A PMMA substrate is used as an intermediate medium between each layer. The ground plane is a complete layer of ITO-PET (ITO square resistance value of 8 Ω/sq). The ITO-PET materials used in the experiments performed in this work all conform to the commercial standard (HG/T 5299-2018). Therefore, the absorber has a good light transmission ability and is flexible as well. In the proposed design, the 110 Ω/sq ITO pattern is effectively matched to the free-space impedance in order to reduce reflectivity and allow more electromagnetic waves to enter the dielectric layer. The 8 Ω/sq ITO film serves as the ground plane. This plane has good conductivity and reflects the incident beams completely, thus ensuring a zero transmittance of ground plane.
Fig. 1. The structure of the proposed system; (a) total structure of the absorber; (b) cell structure; (c) three-dimensional separation of unit structure; (d) top view of the first layer of the unit structure; (e) top view of the second layer of the unit structure; (f) top view of the third layer of the unit structure.
The thickness parameters of the unitary structure are presented in Fig. 1(c), where hp denotes the thickness of the PET layer along with the permittivity and permeability of PET, i.e., 3 and 0.06, respectively [24]. h1 denotes the thickness of the first PMMA layer, h2 denotes the thickness of the second PMMA layer, h3 denotes the thickness of the third PMMA layer, and the permittivity and permeability of PMMA are 2.25 and 0.01, respectively [24]. The thickness of ITO films is generally several tens of nanometers, which is negligible as compared to the thickness of the PET layer considered in the simulations performed in this work. The parameters of the first layer of the pattern are presented in Fig. 1(d), where p denotes the period of the absorber cell structure, a denotes the long axis of the pattern, and b denotes the short axis of the pattern. On the center line of the pattern, an identical loop is combined at the top and the bottom, with c being the length of the outer loop edge and d being the length of the inner loop edge. The second layer pattern is presented in Fig. 1(e). This pattern is obtained by rotating the first layer pattern clockwise by 90° with the same pattern parameters. The third layer of the pattern is presented in Fig. 1(f), where e denotes the side length of the outer ring and w denotes the width of the ring. The specific parameters of the unit structure include hp = 0.3 mm, h1 = 1.7 mm, h2 = 1.7 mm, h3 = 2.7 mm, p = 11.2 mm, a = 10 mm, b = 2.4 mm, c = 4 mm, d = 1.6 mm, w = 1.2 mm, and e = 9 mm.
In this work, the analysis of the absorber is performed based on the 3D electromagnetic simulation software, namely CST Studio Suite. The master-slave boundary, Floquet port, and adaptive grid are configured in the program, and the polarization directions of the electric and magnetic fields are excited along the x and y directions, respectively, for simulating the infinite period cell of the absorber. The S-parameters of the absorber are obtained based on the numerical simulations, as shown in Fig. 2(b). The absorbance of the absorber is defined as $A(w) = 1 - R(w) - T(w)$, where $R(w) = {|{{S_{11}}} |^2}$ denotes the reflectance and $T(w) = {|{{S_{21}}} |^2}$ denotes the transmittance. To take into account the effect of the polarization-conversion of unit cell, R(w) in the above equation is defined as $R(w) = |{{S_{11}}} |_{xx}^2 + |{{S_{11}}} |_{xy}^2$. The absorption simulation results of the absorber are presented in Fig. 2(a). It is evident that the absorption of the microwave absorber proposed in this work exceeds 90% in the frequency range of 4.16-7.48 GHz. It basically covers the C-band (4-8 GHz) and has a very good absorption performance (absorption rate >99%) for the frequency band near 5 GHz (4.98-5.83 GHz). The S11 parameter reaches a value of -33.8 dB at the resonant frequency of 5.38 GHz, where the reflectance and transmittance are close to 0. This shows that the input impedance and the free-space impedance are matched almost perfectly. Please also note that the transmittance of the absorber is close to 0 throughout the absorber's broadband, which indicates that the low-resistance ITO film (8 Ω/sq) has good anti-transmittance characteristics for avoiding the secondary scattering of electromagnetic waves at the absorber's ground plane.
Fig. 2. The simulation results obtained using the proposed model; (a) absorbance, reflectance, and transmittance curves of the absorber; (b) S-parameters of the absorber.
Generally, it is difficult to achieve a wide bandwidth, while maintaining a small gap between the bandwidth and the thickness of the absorber. In order to investigate the relationship between thickness of absorbers and absorption, we use the following mathematical expressions [25]:
Fig. 3. Valid parameters (a) Normalized equivalent impedance; (b) Equivalent dielectric constant, equivalent permeability.
The equivalent parameters of the absorber proposed in this work are presented in Fig. 3(b). The equivalent parameters are extracted directly by the CST software. At the resonance point frequency of 5.38 GHz, the real parts of the effective permittivity and effective permeability are identical. Moreover, their imaginary parts are also similar, which means that the impedance and the air are perfectly matched at this point and the absorption process is optimal. It is noteworthy that the effective permittivity and effective permeability of other frequencies in the operating band have a close value, which maintain good wave absorption performance.
In order to enhance the absorption in the most commonly used 5 GHz band, this work explores the current flow and loss distribution of the absorber at 5 GHz, as presented in Fig. 4. The current flow in Fig. 4(a) shows that the bottom and third layers possess the highest density, and they mainly reflect the electromagnetic waves to prevent their penetration out of the absorber. The currents in the first and second layers are relatively sparse, and their main function is to allow the electromagnetic waves to enter and vanish inside the absorber. The loss distribution in Fig. 4(b) shows that the first and the third layers are mainly responsible for the electromagnetic wave loss in the x-direction, whereas the second layer is responsible for the electromagnetic wave loss in the y-direction. The low-resistance ITO film on the bottom surface does not participate in the process of electromagnetic wave loss.
In order to quantify the loss capacity of the patterned ITO film on the absorber surface, the quantification factor Q is computed as follows [26]:
where, f denotes the resonant frequency, $\varDelta f$ denotes the absorption bandwidth, ${P_r}$ denotes the stored energy, and ${P_a}$ denotes the dissipated energy. It is evident from (3) that the dissipation energy and the absorption bandwidth are proportional to each other. In order to investigate the relationship between the square resistance of ITO and the absorption bandwidth, this work performs a sweep simulation of the resistance value of the surface ITO film in the range of 80-150 Ω/sq with a step size of 5, as presented in Fig. 5. As the resistance value of the ITO film increases, the absorption band shifts in the direction of high frequencies. As this work only focuses on the absorption in the frequency band near 5 GHz, an ITO film with a square resistance of 110 Ω/sq is considered. This film has the best absorption performance near 5 GHz.In z-direction, the absorber structure has a pattern with 90° relative rotations. As presented in Fig. 6(a), the absorption capacity is relatively close under the irradiation of TE and TM modes of the normal incident plane wave. As the patterns of the first and second layers are not in the same plane, they may lead to some deviations in the performance of TE and TM patterns. On the whole, as presented in Fig. 6(b), the absorber proposed in this work maintains relatively similar excellent absorption for the incident electromagnetic waves with polarization angles $\varphi $ ranging from 0° to 90°. This shows that the proposed pattern with 90° relative rotation of absorber structure is polarization-insensitive.
Fig. 6. (a) The absorption rate of TE and TM modes for normal incident plane waves; (b) absorption rate of absorber with frequency and polarization angle $\varphi $.
Let us assume that the angle of incidence of electromagnetic waves is random. Then, the design of the absorber should consider the absorption of electromagnetic waves incident at different angles. In order to investigate the relationship between the absorption of the absorber and the incident angle, the absorption of polarized waves incident in the range of 0° to 60° is simulated in this work for TE mode and TM mode, as shown in Fig. 7(a) and Fig. 7(b). It is evident that the proposed absorber has a good absorption effect under the polarized waves in both TE and TM modes for the electromagnetic wave incident in the range of 0° to 40°. When the incident angle $\theta $ is greater than 40°, the absorption bandwidth of the absorber under TE mode polarization becomes narrower, while the absorption band of the absorber under TM mode polarization is slightly shifted toward higher frequency. However, the absorber still maintains good absorption effect in the band near 5 GHz. Generally, the absorber is stable in terms of absorption effect at different angles of incidence, especially in the frequency band near 5 GHz. This shows that the proposed absorber possesses the characteristics of incident angle stability.
Fig. 7. The simulated absorption of the incident waves in the range of 0° to 60° for (a) TE mode and (b) TM mode.
3. Bending absorber design
During the experiments, we observe that the PDMS material (dielectric constant of 2.35 and magnetic permeability of 0.02 [27]) performed significantly better as compared to PMMA in terms of bendability. Therefore, in the experiments involving the bending absorber, the PMMA on the absorber structure is replaced with PDMS of same thickness.
where, l denotes the absorber length, n denotes the bending angle (the raised bending angle is denoted as α and the depressed bending angle is denoted as β), and r represents the radius of the laminated cylinder. In this work, by using the bending angle n as the independent variable and considering a constant length l of the absorber, we obtain cylinders with different radii r based on calculations. Afterwards, we fit the absorber toward the edge of the cylinder to obtain the bent absorber. In this paper, a bent 10${\times} $ 6 cell absorber is treated as a single cell in the simulation. the CST software still uses the infinite cycle array approach for the simulation. Therefore, the master-slave boundary, Floquet port and adaptive grid are configured in the CST program.As presented in Fig. 8(a), a higher bending angle α results in a more pronounced protrusion of the absorber. Figure 8(b) shows the effect of the bending angle of the projection on absorption. It is noteworthy that the wider the bending angle of the absorber projection, the wider is the absorption bandwidth. Also note that the absorption peak is almost constant. Due to the physical advantages of raised bending absorber, the reflected electromagnetic waves of each part do not interfere with each other. The surface of raised bending absorber can be simply divided into several approximate equivalent planes. As the orientation of the surface resonant pattern changes with bending, it may change the direction of reflection of the incident electromagnetic waves. This moves the reflected wave away from the direction of the receiving antenna, achieving better absorption as compared to a flat surface.
Fig. 8. (a) The bending of the absorber body with upward projection; (b) the effect of the angle of projection on the absorber body.
As presented in Fig. 9(a), a higher bending angle β results in a more prominent depression of the absorber structure. Figure 9(b) shows the effect of the angle of the recessed bend on the absorption. It is evident that the absorption curve becomes more complex as compared to the situation when the bend is raised. This is because of the physical nature of the depressed and curved inner trap, due to which the reflected electromagnetic waves may interfere with each other, and there may even be multiple reflections. The electromagnetic waves are reflected many times from the surface of the recessed absorber, which may lead to multiple losses, ultimately achieving a better absorption performance as compared to a flat surface.
Fig. 9. (a) The downward concave bending of the absorber; (b) effect of the angle of concavity on the absorber.
4. Fabrication and characterization
In order to experimentally verify the designed absorption performance of the proposed absorber, a transparent absorber sample with a size of 268.8 mm × 134.4 mm is manufactured. The ITO was sputtered onto PET substrate, followed with wet-etching to obtain the layer of PET-ITO. The PET-ITO layer and the PMMA layer were manually bonded together using an adhesive. The whole fabrication process was very straightforward and used mature industrial techniques. As presented in Fig. 11(d), the emblem of Guangdong Polytechnic Normal University is evident on the paper printed below the sample, which proves that the absorber is extremely transparent. Due to the limitation of the experimental conditions, only the absorber samples in the planar condition are experimentally examined in this work. Four 134.4 mm × 67.2 mm absorber samples are prepared and then stitched together in a complete absorber sample by stitching treatment. A pair of C-band (4-8 GHz) horn antennas are used for performing the tests. During the tests, the distance between the antenna output surface and the sample is 60 cm. The experimental process is shown in Fig. 10 (c). The transmitting antenna radiates electromagnetic waves towards the absorber. The reflected electromagnetic energy is collected by using the receiving antenna. The resulting data is imported in the vector network analyzer (VNA) for performing analysis. In order to reduce the experimental error, a 268.8 mm × 134.4 mm copper foil is used as the reference for performing calibration. The test is normalized for the 268.8 mm × 134.4 mm copper foil and the 268.8 mm × 134.4 mm sample with ITO on the back side.
Fig. 10. The experimental verification; (a) copper plate test as a normalization standard; (b) absorber sample test; (c) experimental simple view.
Fig. 11. (a) A comparison of experimental and simulated absorbance results; (b) A comparison of experimental and simulated reflection coefficient results.
The experimental parameters of the transparent absorber are compared with the simulated parameters. The corresponding results are presented in Fig. 11. The results show that the trends of experimental and simulated parameters are similar. The absorption curves presented in Fig. 11(a) show that the experimental detection results of both peak absorption and absorption bandwidth are consistent with the software simulation results. From the reflection coefficient curve presented in Fig. 11(b), it is evident that the experimental results are consistent with the resonance point frequency of the simulation results. Both are 5.38 GHz, and the reflection coefficient reaches -33.8 dB. Even the experimental detection results reach a lower reflection coefficient.
As compared with the absorbers proposed in the earlier literature, the absorber proposed in this work has a higher absorption peak at the resonance point. As presented in Table 1, the reflection coefficient in this work reaches -33.8 dB at the resonance point frequency (which can be converted to more than 99% absorption). The relative thickness of the absorber proposed in this work is only 0.101 times the wavelength of the upper cutoff frequency. The FoM (Figure of Merit) metrics in the table allow for a comprehensive consideration of the relationship between thickness and bandwidth. This means that absorbers can be compared under a fairer metric system. The proposed absorber achieves a FoM value of 5.84, which is a relatively excellent performance assessment. It also has mechanical flexibility, thus making it suitable for various practical applications.
Table 1. Comparison of Absorbers with Similar Products in This Work.
5. Conclusions
In this work, a multilayer transparent bendable broadband microwave absorber covering the frequency band near 5 GHz is designed and validated. The absorber is a multilayer PET-ITO structure. It has a high absorption effect in the 4.16-7.48 GHz band and > 99% absorption effect in the 4.98-5.83 GHz band. The thickness (7.3 mm) of the absorber is only 0.101 times the upper cut-off frequency wavelength (72.115 mm). It is notable that the absorber is insensitive to polarization, and the change in absorption is small at incident angles < 40°. After replacing the intermediate medium with PDMS, the absorber achieves excellent absorption performance even at large bending angles. The simulation results are in good agreement with the experimental results.
Funding
Guangdong Province for Science and Technology Innovative Young Talents (No. 2019KQNCX069); Special Fund Project for Science and Technology Innovation Strategy of Guangdong Province (Climbing plan, pdjh2020b0344); Guangzhou Science and Technology Innovation Development Special Fund Project (202102020307); Open funding of Guangdong Provincial Key Laboratory of Millimeter-Wave and Terahertz (2019B030301002KF2105).
Acknowledgments
We would like to thank Prof. Wenhua Gu and Dr. Chen Fu from Nanjing University of Science and Technology for their help with the testing of the experiment.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the corresponding author upon reasonable request.
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