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Abstract
Artificially designed metamaterial structures can manipulate electromagnetic waves, endowing them with exotic physical properties that are not found in natural materials, such as negative refractive index, superlens, and inverse Doppler effect. These characteristics are widely applied in various engineering and military applications. Due to increasingly complex application environments and innovation in radar detection technology, the combination of broadband absorption performance under thin thickness and efficient preparation methods at low cost is often the focus of research on new generation stealth materials. Here, we propose Al@SiO2 composite conductive film metamaterial (Al@SiO2 CCFM) to achieve wideband absorption of electromagnetic waves. This metamaterial structure combines two resonant units, resulting in three absorption bands in the absorption curve. The results show that the absorption rate of the metamaterial is above 90% in the frequency range of 10.6 GHz to 26.0 GHz. The resonance mechanism between multiple structures is a prerequisite for achieving wideband absorption. The materials Al and SiO2 used in Al@SiO2 CCFM are inexpensive and abundant, and the fabrication method is simple. Therefore, they hold great potential for large-scale applications in the multispectral stealth and electromagnetic shielding field.
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1. Introduction
Electromagnetic wave absorbing material is a type of material that is capable of absorbing electromagnetic waves. It can convert electromagnetic waves into thermal energy or other forms of energy, thereby reducing the reflection and scattering of electromagnetic waves in the material. This achieves the effective reduction of electromagnetic interference, improves electromagnetic compatibility and stealth capabilities, and is widely used in the fields of electronics, communications, aerospace, and other related areas [1–4]. For instance, electronic devices require protection against electromagnetic interference, communication equipment demands signal stability, and aerospace technology necessitates stealth effect, all of which rely on the support of absorbing materials [5–7]. Therefore, the study and application of absorbing materials hold significant practical significance and broad development prospects.
In recent years, metamaterials have attracted widespread attention as a new type of absorbing material. By manipulating the artificial structure of metamaterials to regulate the equivalent electromagnetic parameters, people can control electromagnetic waves [8]. A typical three-layer Metamaterial is proposed for the first time to achieve the wave absorption effect [9]. The absorption of electromagnetic waves in different frequency bands is achieved through designing the shape and size parameters of the top resonant unit. The resonant unit is generally made of metal [10,11], resistive film [12–15], or some lumped elements such as capacitors [16], inductors [17], diodes [18–21], etc. Absorbing metamaterials based on the resonance mechanism usually have narrow absorption peaks [22,23]. In order to meet the needs of practical applications, broadening the absorption band has become a research hotspot in the field of absorbing Metamaterial. Yang et al. [24] design a radar absorbing metamaterial that is transparent in the visible and near-infrared regions. The top resonant unit is composed of a resistive silver nanowires film, which achieves over 90% absorption in the frequency range of 4.1 GHz to 18.2 GHz, with 75% visible light transmittance and 66% near-infrared transmittance. In addition to the three-layer structure, the proposal of multi-layered and three-dimensional structures [25,26] successfully expands the absorption bandwidth in the low-frequency range but inevitably increases the thickness of the metamaterial. Jiang et al. [27] use 3D printing technology to prepare a three-dimensional honeycomb structure metamaterial composed of resistive film, which can achieve over 90% electromagnetic absorption in the frequency range of 3.5 GHz-24.0 GHz with the thickness of 15.51 mm. Furthermore, due to the importance of the metamaterial structure, researchers have also obtained ideal structures through artificial intelligence [28] and biomimetics [29,30]. Huang et al. [31] take inspiration from the wings of the Pachliopta aristolochiae and combine microwave absorbers with biomimetic superstructures to prepare a biomimetic metamaterial with effective radar stealth absorption in the range of 2.0 GHz to 18.0 GHz at the maximum thickness of 0.04 λ. By using the enhanced resonance in absorption through the biomimetic structure, the metamaterial breaks through the Planck-Rosenthal limit.
In this paper, the Al@SiO2 CCFM was achieved by optimizing the size and sheet resistance of the composite resonant unit, wideband absorption with absorption rate greater than 90% in the frequency range of 10.6 GHz to 20.0 GHz was achieved in the three-layer metamaterial structure. The two resonator structures are rationally distributed in the top plane of the metamaterial using planar design of broadband metamaterial absorbers. The electromagnetic coupling effect between the two structures is the key to achieving broadband absorption. Using low-cost Al and SiO2 as raw materials, the design requirements of A are achieved by rationally configuring the volume content of the two materials to achieve impedance matching between air and materials. A broadband radar absorbing metamaterial was prepared using simple physical vapor deposition technology. The wideband absorption function was verified by experiments, and this research provides a new approach to design and prepare broadband microwave absorbing metamaterials.
2. Structure and model design
The structure of Al@SiO2 CCFM is shown in Fig. 1(a). It is composed of the top layer of windmill structure and hexagon structure, the middle layer of poly methyl methacrylate (PMMA) dielectric, and the bottom metal reflective backplane. The electromagnetic simulation software CST Microwave Studio 2021 was used and the frequency-domain solver was selected in the simulation. Periodic boundary conditions were set along the x and y directions, and the Flouquet port excitation was applied along the z direction with open boundary conditions. The square resistance of the composite conductive film and the size of the metamaterial were used as variables in the optimization process (see the appendix for details). After simulation optimization, the specific parameters of the Al@SiO2 CCFM are as follows: the structure parameter are p = 12 mm, a = 1 mm, b = 4 mm, c = 1.9 mm, d = 3.8 mm, g = 2 mm, h1 = 200 nm, h2 = 2.6 mm, and h3 = 0.05 mm. The Al@SiO2 conductive composite film have sheet resistance of 60 Ω/sq.
Fig. 1. (a) the schematic of the composite structure metamaterial, (b) the absorption performance of the individual hexagonal metamaterial structure, (c) the absorption performance of the individual windmill-shaped metamaterial structure, and (d) the absorption performance of the composite structure metamaterial.
We simulated the absorption properties of the metamaterials with top layer of single hexagon structure, single windmill structure and combined structures when the electromagnetic wave is vertically incident on the surface of the metamaterial. As shown in Fig. 1(b), the individual hexagon structure generated absorption rate greater than 80% in the frequency range of 11.0 GHz to 19.1 GHz and the absorption peak close to 100% at 23.0 GHz. As shown in Fig. 1(c), the individual windmill structure generated two weak absorption peaks at 17.5 GHz and 22.1 GHz. When the two structures were combined, the absorption band of the metamaterial was greatly expanded, achieving broadband absorption with absorption rate greater than 90% in the frequency range of 10.6 GHz to 26.0 GHz (Fig. 1(d)), which covers the entire Ku band and part of the X and K bands. Due to the strong absorption of the hexagon structure at 12.5 GHz, the combined structure also generated strong absorption at 12.5 GHz. It is interesting that strong resonant absorption was achieved with combined structures at 20.7 GHz, but the absorption intensity of the two individual structures was both weak.
3. Results and discussion
In order to gain a deeper understanding of the absorption mechanism of the metamaterial, we simulated the electric field distribution, magnetic field distribution, and power loss density distribution, as well as the surface current distribution on the upper and lower surfaces of the Al@SiO2 CCFM at different heights (h = 2.6 mm for the upper surface of the metamaterial and h = 0.1 mm for the lower surface of the metamaterial) at frequencies of 12.5 GHz, 20.7 GHz, and 24.4 GHz under normal incidence of TE polarized waves. At 12.5 GHz (Fig. 2(a)), the electric field exhibits strong coupling effect at the junction of the long side of the hexagon structure and the windmill structure. As the cross-sectional height gradually decreases into the interior of the Al@SiO2 CCFM, the electric field strength gradually weakens but still resonance along the edge of the hexagon structure. The magnetic fields localized at the vertices of the hexagon structure on the top surface of the Al@SiO2 CCFM. As the cross-sectional height moves down to the lower layer, The magnetic field is positioned at the center of the corresponding position in the hexagonal structure, and the magnetic field strength continuously increases as the cross-sectional height decreases. The power loss density at the junction between the hexagonal structure and the windmill structure is strong, indicating that at 12.5 GHz, the high loss ofAl@SiO2 CCFM to electromagnetic waves is caused by electromagnetic coupling between the two structures. In Fig. 2(d), the direction of the current at the edge of the hexagon on the top surface of the metamaterial at 12.5 GHz is opposite to the direction of the current in the ground metal layer at the bottom. The reverse parallel current leads to the magnetic dipole resonance inside the material. The power loss density distribution is basically consistent with the electric field distribution, and the power loss decreases gradually from the upper surface to the lower surface. This indicates that most of the electromagnetic waves are absorbed in the upper part of the Al@SiO2 CCFM, and the power losses mainly caused by the ohmic losses at the edges of the hexagonal resonant unit at 12.5 GHz. This corresponds to the strong absorption peak phenomenon of the individual hexagon structure at 12.5 GHz. At 20.7 GHz (Fig. 2(b)), the electric field is strong at the edge of the windmill structure on the top surface of the Al@SiO2 CCFM, and the intensity of the electric field gradually decreases as the height of the cross section decreases towards the interior of the Al@SiO2 CCFM. The magnetic field is strong at the center of the windmill structure and the center of the hexagon structure. As the height of the cross section gradually decreases towards the interior of the Al@SiO2 CCFM, the magnetic field gradually moves from the center of the resonant structure to the position between two adjacent hexagonal structures. The power loss density at the connection between different patterns is strong, indicating that the power loss of the Al@SiO2 CCFM is mainly caused by the electromagnetic coupling between the hexagon structure and the windmill structure. Single structure cannot cause high electromagnetic wave absorption at this frequency, indicating the near-field coupling effect between resonant units, successfully expands the absorption band of the Al@SiO2 CCFM. At 24.4 GHz (Fig. 2(c)), the electric field is strong at the center of the square hole in the windmill structure and the intensity of the electric field gradually decreases as the height of the cross section decreases towards the interior of the metamaterial. The magnetic field is strong at the windmill structure and at the edge of the hexagon structure. As the height of the cross section gradually decreases towards the interior of the Al@SiO2 CCFM, The magnetic field is located in the gap between adjacent patterns. The power loss density is also consistent with the distribution of the electric field. The center of the hexagon structure and the overall windmill structure together cause high electromagnetic wave absorption. In Fig. 2(d), the current direction at the windmill structure on the top surface of the metamaterial is the same as that on the ground metal at 20.7 GHz and 24.4 GHz, indicating the occurrence of electric resonances within the metamaterial. Under the joint excitation of the electromagnetic resonances, the metamaterial has three absorption peaks, and broad-band absorption is achieved based on the superposition of multiple different resonance modes or peaks.
Fig. 2. Under normal incidence of TE waves, the field distribution (a-c) and surface current distribution(d) of the metamaterial at different cross-sectional heights are shown at frequencies of 12.5 GHz, 20.7 GHz, and 24.4 GHz.
In practical applications, it is usually required that the absorbing material has polarization insensitivity and wide-angle incidence performance. Therefore, we simulated the absorptive properties of the Al@SiO2 CCFM under different polarization angles and different incident angles. As shown in Fig. 3(a), the Al@SiO2 CCFM exhibits strong absorption (>90%) within the frequency range of 10.4 GHz to 26.0 GHz, regardless of the polarization angle. This indicates that the Al@SiO2 CCFM has good polarization insensitivity performance. Figure 3(b) and (c) respectively demonstrate the relationship between the incidence angle and absorption performance of electromagnetic waves under TE and TM modes. For the TE mode, at an incidence angle less than 40°, the Al@SiO2 CCFM can achieve strong absorption (>90%) within the frequency range of 10.9 GHz to 26.2 GHz. When the incidence angle is less than 70°, the Al@SiO2 CCFM can achieve absorption >60% within the frequency range of 10.1 GHz to 27.1 GHz. For the TM mode, at an incidence angle less than 40°, the Al@SiO2 CCFM can achieve strong absorption (>90%) within the frequency range of 11.6 GHz to 26.1 GHz. When the incidence angle is less than 70°, the Al@SiO2 CCFM can achieve absorption >60% within the frequency range of 11.3 GHz to 28.0 GHz. This indicates that the Al@SiO2 CCFM exhibits good wide-angle incidence performance in both TE and TM modes.
Fig. 3. Polarization insensitivity and wide angle incidence performance of Al@SiO2 CCFM (a) The polarization insensitivity performance of the metamaterial; (b) The wide-angle incidence performance of the metamaterial under TE wave incidence; (c) The wide-angle incidence performance of the metamaterial under TM wave incidence.
We chose aluminum and silica, two low-cost materials, as the constituent materials of the composite conductive film. By controlling their content, we regulated the square resistance of the conductive film. Al@SiO2 composite conductive film with the thickness of 200 nm was deposited on the polymethyl methacrylate (PMMA) substrate using magnetron sputtering method, and copper foil with the thickness of 0.05 mm was attached to the bottom as the reflecting backplate. The square resistance of the Al@SiO2 composite conductive film is 60 Ω/sq, and the complex dielectric constant of the PMMA is 2.25 (1-j0.001). As shown in Fig. 4(a), the laser-etched stainless steel mask plate (0.1 mm thickness. The thickness of the stainless steel mask is much smaller than the size of the top layer pattern of Al@SiO2 CCFM, so there is no shadow effect on the film) was adhered to the cleaned dielectric layer material (PMMA), and the two layers were tightly combined and placed in the vacuum chamber of the magnetron sputtering system. Al@SiO2 composite film was deposited onto the dielectric material through co-sputtering of the Al target and the SiO2 target. Figure 4(b) indicates that the square resistance of the composite conductive film can be regulated by different Al contents. After the deposition process, the mask plate was removed, and the top patterned structure of the metamaterial was obtained. The copper backplate with thickness greater than the skin depth at the operating frequency (0.05 mm in this experiment) was attached to the back of the sample to prevent the transmission of electromagnetic waves (see the appendix for details). The sample with the size of 200 mm × 200 mm, is shown in Fig. 4(c). The transmission coefficient of the sample was measured using the free-space method in a microwave darkroom. The measurement was conducted using Agilent (P9377B) vector network analyzer and two pairs of horn antennas with frequencies of 8.0 GHz to 12.0 GHz and 12.0 GHz to 18.0 GHz, respectively. The angle between the two horn antennas was 10°, and they were connected to the vector network analyzer via phase-stable cables. The absorption performance of the Al@SiO2 CCFM is shown in Fig. 4(d). The absorption curve within 8.0 GHz to 18.0 GHz is in good agreement with the simulation results. The Al@SiO2 CCFM achieved wideband absorption (absorption rate greater than 90%) within 10.6 GHz to 18.0 GHz and produced the absorption peak close to 100% at 12.6 GHz. With the increase in frequency, the absorption strength decreased but still maintained absorption strength over 90%.
Fig. 4. (a) The preparation process of the metamaterial; (b) The resistance control of the square Al@SiO2 nanocomposite film; (c) Comparison of the simulated and experimental absorption rates of the sample under normal incidence. (d) Comparison of experimental and simulation results of Al@SiO2 CCFM
4. Conclusion
In this paper, a broadband radar absorbing metamaterial based on Al@SiO2 composite conductive film was designed. Both simulation and experimental results indicate that the absorber achieved wideband absorption greater than 90% within 10.6 GHz to 26.0 GHz. The thickness of Al@SiO2 CCFM is only 2.6 mm. The superposition of different resonant units generates near-field coupling, greatly broadening the absorption band of the Al@SiO2 CCFM. Regardless of the polarization angle, Al@SiO2 CCFM can achieve more than 90% absorption within the frequency range of 10.6 GHz to 26.0 GHz. For TE mode electromagnetic waves, when the incident angle is below 40°, Al@SiO2 CCFM can achieve more than 90% absorption within the frequency range of 10.9 GHz to 26.2 GHz. For TM mode electromagnetic waves, when the incident angle is below 40°, Al@SiO2 CCFM can achieve more than 90% absorption within the frequency range of 11.6 GHz to 26.1 GHz. The low-cost Al and SiO2 materials can be directly used to prepare structured surfaces using the mask. The simple fabrication method and excellent absorption performance support the large-scale application of Al@SiO2 CCFM in aerospace, communication facilities, electromagnetic shielding, and other fields.
Funding
National Natural Science Foundation of China (52003039, 52272078); Fundamental Research Funds for the Central Universities (N2109001, N2125018).
Disclosures
The authors declare that they have no conflict 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 authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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