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Abstract
The prevalent use of multispectral detection technology makes single-band camouflage devices ineffective, and the investigation of technology for camouflage that combines multispectral bands becomes urgent. The multifunctional-hierarchical flexibility metasurfaces (MHFM) for multispectral compatible camouflage of microwave, infrared, and visible, is proposed, fabricated, and measured. MHFM is primarily composed of an infrared shielding layer (IRSL), a radar absorbing layer (RAL), and a visible color layer (VCL). Among them, IRSL can block thermal infrared detection, and RAL can efficiently absorb microwave band electromagnetic (EM) waves. The VLC can display black (below 28°C), purple (28°C∼31°C), green (31°C∼33°C), and yellow (above 33°C) at different temperatures to achieve visible camouflage. Simulation results show that MHFM can achieve absorption higher than 90% in the 2.9∼13.9 GHz microwave band. Theoretically, the emissivity of MHFM in the infrared spectral range 3∼14 µm is less than 0.34. In addition, the MHFM consists of high-temperature-resistant materials that can be used normally at temperatures up to 175°C, providing excellent high-temperature stability. The measurement results show that the camouflage performance of the MHFM is in excellent agreement with the proposed theory. This study proposes a new method for multispectral camouflage that has broad engineering applications.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Multispectral high-tech detection combined with one another has become the prevailing detection technology, and the study of camouflage technology has become particularly important [1–3]. Microwave detection technology is highly developed and widely employed as the predominant detection method [4,5]. With its ability to provide high target resolution, passive detection, and position concealment, infrared detection technology has become increasingly valuable in a variety of applications [6,7]. The extensive use of multispectral detection technology that combines microwave, infrared, and visible bands can expose the positions of combat weapons on the battlefield and increase the possibility of tracking and identification. Therefore, research into microwave, infrared, and visible multispectral camouflage technology cannot be delayed [8–10].
Microwave camouflage [11–13] is accomplished by utilizing materials with microwave camouflage properties to decrease the radar scattering cross section (RCS) of the target [14–16]. Accordingly, Microwave camouflage materials must be designed and fabricated to fulfill the demands of absorbing electromagnetic (EM) waves efficiently while reflecting them minimally. Infrared camouflage [17,18] is accomplished by incorporating materials that have a low infrared emissivity, which helps to minimize the object's surface infrared radiation. To be effective, materials used for infrared camouflage must exhibit both low absorption and high reflection of infrared radiation [19,20]. Dynamic color-changing materials are used in visible camouflage techniques to decrease the contrast of the material with its surroundings. This is achieved by adjusting the color of the material to match the color of the background. [21–23]. Organic reversible temperature-sensitive color-changing microencapsulated powders (OTSMP) is dynamic color-changing material in which the brightness and chromaticity change with temperature [24]. However, the existing OTSMP has the defects that it can only achieve monochromatic transformation and the temperature of color change is mostly in the high temperature area, which cannot be widely used in complex and changing background environment [25,26]. Therefore, designing materials that can simultaneously exhibit high EM wave absorption in the microwave band, strong reflection in the infrared band, and adaptability to changing environmental backgrounds is an exceedingly difficult task.
Current research on multispectral camouflage, including microwave, infrared, and visible, mainly focuses on materials with surface coatings and structured metasurfaces. Materials with surface coatings offer various benefits, such as an easy and cost-effective preparation method, the capability of being applied regardless of geometry, and more. A new type of absorbing material is proposed by Huang et al., which utilizes reduced graphene oxide as a fundamental element [27]. The effective absorption bandwidth of the microwave camouflage band is 5.75 GHz, and the infrared emissivity in the infrared camouflage band is 0.59 within the range of 8∼14 µm, realizing the function of infrared and microwave-compatible camouflage. Zhang et al. effectively reduced the infrared emissivity in ZrB2 by preparing Ag-ZrB2 nanocomposite films. As the silver content is increased, the infrared emissivity is reduced to less than 0.11, and the nanocomposite films reflected 65% of visible, realizing the function of infrared and visible compatible camouflage [28]. The above latest research shows that the coated material can achieve dual-band compatible camouflage. Nonetheless, this material cannot achieve the function of multispectral compatibility for microwave, infrared, and visible light camouflage simultaneously. It has the drawbacks of limited absorption in the microwave band and high infrared emissivity, thereby restricting its utilization on various platforms and occasions.
The microwave EM waves absorbers based on the metasurfaces can achieve high-efficiency absorption of EM waves across a wide range of frequencies in the microwave band [29–31]. Frequency-selective surface (FSS) is a type of metasurface that exhibits selectivity for EM waves at various frequencies [32–34]. Therefore, the integration of metasurfaces with different functions with functional materials can achieve the function of multispectral compatible camouflage. Based on an all-metal wide-angle metasurfaces polarization camouflage technique, Ma et al. proposed an electrodynamic resistance-reduction mechanism to avoid significant polarization-dependent infrared absorption/radiation, which can almost completely eliminate the pseudo-Brewster effect, leading to a significant reduction in thermal emission and depolarization [35]. An et al. achieved microwave and infrared compatible camouflage by combining a new flexible high-temperature resistant SiO2 fiber paper with a metasurfaces microstructure [36]. Jagyeong Kim et al. designed metal-semiconductor-metal metasurfaces with multiple plasmon resonance modes and printed camouflage patterns on the metasurfaces to achieve infrared and visible compatible camouflage [37]. The above studies achieved dual-band compatible camouflage using a combination of a metasurface and functional material but could not simultaneously achieve microwave, infrared, and visible compatible camouflage. Li et al. designed a multilayer integration of resistive metasurfaces with a composite material made of polyethylene filled with conductive carbon black film that could achieve multispectral compatible camouflage in the microwave, visible, and near-infrared bands with a thickness of 17 mm [38]. Zhu et al. achieved microwave (8-12 GHz), infrared, and visible compatible camouflage by overlaying multiple layers of ZnS/Ge on the metasurfaces [39]. The above studies used a combination of metasurfaces and functional materials to achieve the function that microwave, infrared, and visible camouflaged with multispectral compatibility, which, however, has the disadvantages of narrower microwave camouflage bands and thicker metasurfaces.
In this study, a multifunctional-hierarchical flexibility metasurfaces (MHFM) for multispectral compatible camouflage of microwave, infrared, and visible, is proposed, fabricated, and measured for the first time. The principle of how this is achieved can be seen in Fig. 1. MHFM can achieve more than 90% efficient absorption in microwave band 2.9∼13.9 GHz and infrared camouflage band 3∼14µm to achieve infrared emissivity lower than 0.34. MHFM can display black (below 28°C), purple (28°C∼31°C), green (31°C∼33°C), and yellow (above 33°C) in different temperatures, which has good visible camouflage performance and can be widely used in the night sky, woodland, desert, and other background environments. In this study, the microwave, infrared, and visible camouflage performance of MHFM is measured separately, and the measurements all match the theoretical design. Meanwhile, the study measured the absorption characteristics of MHFM at 25°C∼175°C, and the results showed that MHFM has excellent high-temperature stability. The MHFM proposed in this study can achieve multispectral camouflage in various environmental backgrounds and proposes a new idea for researching multispectral compatible camouflage technology.
Fig. 1. The schematic diagram of the MHFM is presented in this paper for achieving multispectral camouflage.
This paper is structured as follows. The experimental preparation of the visible color layer (VCL) and the design simulation of MHFM is carried out in section 2. In Section 3, the impact of structural parameters on the absorptivity of MHFM is analyzed, along with the absorption mechanism. Additionally, equivalent circuit diagrams are used to simulate the composition of MHFM. In Section 4, the microwave, infrared, and visible camouflage performance and the high-temperature stability of MHFM are measured separately.
2. Design and methods
To achieve excellent performance, MHFM must satisfy the demands of high absorption in the microwave camouflage band, low infrared emissivity in the infrared band, and visible camouflage simultaneously. This study combines a method with different functional layers to achieve a multispectral combined camouflage function.
2.1. Design and fabrication of VCL
In the visible range, the contrast characteristics of luminance and chromaticity between the target and the background are essential detection features of the detection system. The principle behind the implementation of visible light camouflage is to use dynamic color-changing materials to alter the brightness and chromaticity of the target surface. This causes the brightness and color of the object surface to change correspondingly with the change of the background color, thereby achieving dynamic camouflage. OTSMP is a category of materials whose color and brightness change with temperature within a certain temperature range. It has superior characteristics such as obvious brightness and chromaticity change, high-temperature resistance (220°C), reversible color change, and sensitive color change. The OTSMP used in this study consists of microcapsules containing thermochromic pigments inside. The microcapsules are spherical particles with an average diameter of 2 to 7 µm, and the outer layer is a transparent shell about 0.2 to 0.5 µm thick that neither dissolves nor melts. Its function is to protect the internal thermochromic pigment from the erosion of other chemical substances. Thermochromic pigments consist of electron donors, electron acceptors, modifiers, sensitizers, and other solvents. The principle of OTSMP reversible color change is the electron transfer phenomenon between the electron donor and electron acceptor due to temperature change. During the electron transfer process, light of a certain wavelength is absorbed or radiated, resulting in a noticeable color change.
Polydimethylsiloxane (PDMS) is an organic non-metallic polymer with high flexibility, high transparency, thermal stability (-55°C∼200°C), and easy operation, which can be used as the main material for preparing substrates [40]. VCL is fabricated by blending PDMS with various OTSMPs through a process. The addition of multiple OTSMPs can achieve multiple color changes of OTSMPs, effectively solving the problem of the single-color change of OTSMPs currently. The fabrication process is as follows: using PDMS as the main substrate material, mixing multiple organic reversible temperature-sensitive color-changing microcapsule powders (OTSMP) in equal proportions, and then mixing them with different proportions of PDMS. The optimal ratio is determined through visual observation of the appearance color and measurement of the relative dielectric constant.
The experiments are conducted on the principle of proportional grouping and controlled variables. Firstly, equal proportions of various OTSMPs are mixed. Then the PDMS: mixed powders are divided into sample 1 (9.5:0.5), sample 2 (9.0:1.0), sample 3 (8.5:1.5), and sample 4 (8.0:2.0) according to different ratios with 10 g per group as the standard value. To ensure the flexibility of the dielectric substrate, the minimum percentage of PDMS is set to 80%. Also, 10 g of PDMS samples are prepared to compare the relative dielectric constant measurements. The experimental fabrication process is divided into different ratios of PDMS doped mixed powder samples and PDMS sample fabrication. The fabrication process and the fabricated samples are shown in Fig. 2. Observing the fabricated samples, the color of the samples with different ratios is found to be the same and the samples remain very flexible after doping with the mixed powder. The VCL fabricated in this study can realize multiple color changes at different temperatures and has excellent characteristics such as high-temperature resistance (200°C), flexibility, and good conformability.
The five fabricated samples are cut into small pieces, and the values of relative dielectric constant and loss angle tangent are measured using the coaxial dielectric probe measurement method (1GHz∼18 GHz) and waveguide transmission/reflection method (18∼25 GHz) for each of the five samples. The measurement results of relative dielectric constant and loss angle tangent values and color detection at different temperatures are shown in Fig. 3. Figures 3(a) and 3(b) show that the relative dielectric constant and loss angle tangent increase after PDMS is doped with mixed powder, and the more the mixed powder is doped, the larger the relative dielectric constant and loss angle tangent. Overall, the variation of relative dielectric constant and loss angle tangent values for samples 1 to sample 4 with different ratios is small. Therefore, the ratio of sample 1 (9.5:0.5) with less doped mixed powder is chosen to fabricate VCL to better retain the flexibility of PDMS. Figure 3(c) shows that the values of relative dielectric constant and loss angle tangent of sample 1 (9.5:0.5) at the temperatures of 27°C, 29°C, 32°C and 35°C change very little, indicating that sample 1 (9.5:0.5) has good high-temperature stability. Figure 3(e) shows that the prepared sample 1 (9.5:0.5) can clearly show black (27°C), purple (29°C), green (32°C), and yellow (35°C) at different temperatures. Therefore, the VCL fabricated using the proportion of sample 1 (9.5:0.5) has excellent color change and better temperature stability, which can well meet the demand for visible camouflage.
Fig. 3. Measured (a) relative dielectric constant (b) loss angle tangent of samples 1∼4 and PDMS; (c) measured relative dielectric constant and loss angle tangent of sample 1 at different temperatures; (d) material measurement site diagram of coaxial dielectric probe measurement method (1GHz∼18 GHz) and waveguide transmission/reflection method (18∼25 GHz); (e) color change of the fabricated sample 1 at different temperatures.
2.2. Structure design and simulation analysis
To achieve infrared camouflage while ensuring smooth microwave transmission to the surface of the radar absorbing layer (RAL), the infrared shielding layer (IRSL) of MHFM must satisfy the demands of both having a low ability to emit infrared radiation and a high ability to allow microwave transmission. Additionally, to display the color change of the VCL, the IRSL must be fabricated with transparent materials. Based on the Hagen-Rubens approximation formula [41].
Among them, E, ε, and c represent the emissivity, the permittivity of free space, and the velocity of light, respectively. ρ and λ stand for the resistivity and wavelength, respectively. From the formula, it can be observed that the emissivity of metals is low. However, when an oxide layer will be formed on the surface of the metal, the resistivity of the oxide layer is higher, and therefore its emissivity will be elevated. Indium tin oxide (ITO) is a thin film material that has both electrical conductivity and transparency, making it useful for various applications such as in touchscreens and solar cells. The dielectric constant ε of ITO in the infrared region can be explained by the Drude model [42].
Fig. 4. (a) The unit structure of the IRSL as viewed from the front. (b) The unit structure of the RAL is seen from the front. (c) The structural unit of the MHFM is shown in its deconstructed form. (d) A side view of the structural unit of the MHFM.
The RAL and reflective layers of MHFM are made of highly transparent surface resistive ITO. The dielectric layer uses both Polyethylene naphthalate two formic acid glycol ester (PEN), and PDMS materials. PEN and PDMS have high transparency and flexibility. In addition, the heat resistance temperature of PEN is 175°C and the heat resistance temperature of PDMS is 200°C. The permittivity values of PEN and PDMS are 4.0(1-j0.06) and 2.7(1-j0.03), respectively. Furthermore, the dielectric material produced and evaluated in section 2.1 is referred to as dielectric COLOR, with an approximate permittivity of 2.8(1-j0.04) as shown in Fig. 3. The COLOR is placed between the underlying PET and PDMS for better proximity to the heat source and to enhance the color switching speed. The design of the RAL structure is based on the factors of excellent absorption performance and easy fabrication of the structure. Therefore, we choose the more common and simple structures of square ring and circle for the RAL structure. After simulating the absorptivity of the three combinations of the square ring, circle, and circle nested inside a square ring, we finally chose a circle nested inside a square ring as the structural pattern of the RAL. Figure 4(b) displays the RAL structure that has been designed for this purpose, where parameter c and parameter t denote the side and width of the square outer frame, respectively, and parameter d denotes the diameter of the circular structure. To ensure that the camouflage performance is not affected, a dielectric material (PDMS) has been inserted between the IRSL and RAL to prevent direct contact between them. This is necessary because the ITO film resistors used in the design have a high conductivity. ITO thin film resistors need to be sputtered onto the PEN dielectric. The decomposition diagram of the MHFM structural unit designed is shown in Fig. 4(c). Using CST Microwave Studio for EM simulation, analysis, and parameter optimization, the optimal parameters for achieving the best absorption performance have been determined: The ITO resistance values for the resonant structure surface (R1 = 100 Ω/sq), the reflecting layer (R3 = 6 Ω/sq), and the FSS (R2 = 6 Ω/sq) are specified, and the thickness of all PENs is h2. Figure 4(d) illustrates a side view of the MHFM structural cell, while Table 1 presents the remaining parameters for the structure.
Table 1. The proposed MHFM's geometric parameters (mm)
The theoretical emissivity of the IRSL can be expressed by the following equation [43].
The IRSL's effective emissivity is determined by εFSS, whereas εITO and εPEN correspond to the emissivities of ITO and PEN, respectively. The percentage of area covered by ITO and PEN in the IRSL is represented by SITO and SPEN, respectively. ITO with a resistance of 6 Ω/sq has an emissivity of approximately 0.1, whereas the emissivity of PEN is less than 0.9 [44]. Taking into account the precision limitations of the sample fabrication process and the impact of FSS on the transmittance of EM waves in the microwave camouflage band, the resulting sizes of the ITO patches and gap widths are presented in Table 1. The final theoretical calculation resulted in an emissivity is less than 0.34 for the IRSL.
The absorption characteristics of the RAL are simulated and the structural parameters of the MHFM RAL are optimized using CST Microwave Studio [45].
Among them, R(ω) and T(ω) denote the reflectance and transmittance of the MHFM, respectively. S11 and S21 denote the reflectance and transmittance coefficients of the MHFM, respectively. The absorptivity, S11, and S21 characteristics of the transverse electric (TE) mode EM waves for normal incidence are illustrated in Fig. 5(a). The graph indicates that the absorptivity of MHFM exceeds 90% within the frequency band range of 2.9 to 13.9 GHz. The absorption bandwidth is 11 GHz and the relative absorption bandwidth is 131%. There are strong absorption peaks at frequency points: 3.9 GHz and 11.04 GHz. To further explore the camouflage absorption properties of MHFM, the RCS of MHFM and the equivalent size perfect electric conductor (PEC) in TE mode are simulated at 2∼20 GHz using CST Microwave Studio software. Figure 5(b) illustrates that the MHFM achieves a significant reduction in RCS compared to an equivalent-size PEC metal plate. The MHFM exhibits an RCS reduction over -10 dB within the frequency range of 3.1 to 16 GHz. Figures 5(c)∼(e) and 5(f)∼(g) show the comparison of 2D bistatic scattering patterns and 3D bistatic scattering patterns of MHFM with equivalent size PEC at 8 GHz, 11 GHz, and 14 GHz under the normal incidence of EM waves, respectively. According to the simulation results, the designed MHFM has demonstrated outstanding microwave camouflage capabilities.
Fig. 5. (a) S11, S21, and absorptivity for normal incidence of EM waves in TE mode. (b) RCS of MHFM compared with equivalent size PEC metal plate. The comparison of (c)∼(e) 2D bistatic scattering patterns and (f)∼(g) 3D bistatic scattering patterns of MHFM with equivalent size PEC metal plate at 8 GHz, 11 GHz, and 14 GHz for normal incidence of EM waves, respectively.
3. Results and discussions
3.1. Parameterized simulation analysis
The broad incident angle characteristics and the polarization insensitivity characteristics are vital indicators of the reliable functionality of MHFM. Figures 6(a) ∼ (c) depict the simulation outcomes of MHFM in TE and TM modes across a range of incident angles spanning from 0° to 60°, as well as in TE mode across a range of polarization angles spanning from 0° to 90°. As the incident angle increases, the absorption performance of MHFM in TE mode gradually decreases, and that of MHFM in TM mode gradually increases. The relationship between the equivalent impedance and the angle of incidence for TE and TM modes can be formulated by the following equation [46].
where R represents the equivalent impedance, while θ represents the incidence angle. An increase in the incidence angle in TE mode leads to a significant increase in the equivalent impedance. This, in turn, results in a decrease in the degree of impedance matching of the equivalent impedance of the MHFM and the air wave impedance, thereby causing a reduction in absorption performance. Contrary to the TE mode, a slight decrease in the equivalent impedance is observed in the TM mode as the angle of incidence increases. The degree of impedance matching between the MHFM's equivalent impedance and the air wave impedance undergoes only minimal changes, resulting in a consistently stable absorptivity. However, over the range of incidence angles from 0 to 60°, the absorptivity of MHFM in both modes is higher than 80%, demonstrating good stability of the incidence angle. Furthermore, the MHFM's resonant structure, as designed in this study, exhibits centrosymmetric behavior, resulting in identical absorptivity for TE mode polarization angles spanning from 0° to 90°. This outstanding feature highlights the excellent polarization insensitivity of the MHFM.Fig. 6. Comprehensive analysis of the absorption characteristics of MHFM (a) absorptivity in TE mode and (b) in TM mode over the range of incidence angles from 0 to 60°, (c) absorptivity for TE mode polarization angles ranging from 0° to 90°. (d) The impact of varying widths a of the ITO patch on the FSS transmittance, (e) the influence of different infrared emissivity on the absorbance characteristics of MHFM, (f) the impact of varying resistance values R1 of the resonant structure ITO film on the absorbance, (g) the impact of the resonant structure parameter c on the absorbance. The effect of different (h) relative dielectric constants and (i) loss angle tangents of VCL on the absorbance of MHFM.
The impacts of various parameters of MHFM on the EM waves absorption performance are simulated in Figs. 6(d)∼(i). The gap width b between ITO patches on the FSS is set to 0.1 mm due to the inherent constraints of the sample fabrication process accuracy. Figure 6(d) shows the transmittance of the FSS with different widths a of the ITO patch. Based on the figure, it is evident that decreasing the width a of the ITO patch results in a higher transmittance of the FSS to the microwave EM waves and an increase in the infrared emissivity. Considering the needs of transmittance and infrared emissivity, the value of a is chosen to be 0.56. The influence of different infrared emissivity on the absorption characteristics of MHFM is demonstrated in Fig. 6(e), indicating that the absorption performance improves with an increase in infrared emissivity. Setting the infrared emissivity to 0.34 enables the attainment of lower levels of infrared emissivity and broadband absorption. The impact of varying ITO resistance values R1 on absorptivity is depicted in Fig. 6(f), revealing that higher resistance values lead to better absorptivity and a wider absorption bandwidth. A narrower absorption bandwidth is observed when the ITO resistance value R1 exceeds 100 Ω/sq. In addition, due to the limitation of the process, the upper limit of the resistance value of the PEN-based ITO resistive film can be fabricated to 100 Ω/sq. So, the resistance value of R1 is chosen to be 100 Ω/sq. Figure 6(g) shows that the larger the resonant structure parameter c of the MHFM is, the better the absorption performance is. The value of c is taken as 9.5 mm due to the limitation of the cell structure boundary. Figures 6(h) and 6(i) show the effect of different relative dielectric constants and loss angle tangents of the dielectric of VCL on the MHFM absorptivity. From the figures, it can be seen that the absorptivity of MHFM remains constant when the values of relative dielectric constant and loss angle tangent are varied in a certain range.
3.2. EM field analysis of the EM waves absorption mechanism
In this study, the mechanism of MHFM to achieve ultra-broadband efficient absorption is analyzed by the EM field. Figure 7(a) simulates the capability of the MHFM's film resistance and dielectric to absorb EM waves. The “power stimulated” indicates that the incident power at the default port is 0.5 W; the “metasurface overall” indicates the power of the EM waves absorbed by the MHFM (the part above 0.45 W is the part with absorptivity of more than 90%). The figure shows that the film resistance of MHFM plays a major role in the absorption of EM waves, and the dielectric of MHFM absorbs fewer EM waves. Figure 7(b) simulates the EM waves absorption ability of MHFM with various layers of film resistance (FSS, RAL, and reflective layer) and different dielectrics (PDMS, PEN, and COLOR). The figure shows that RAL plays the main role in the absorption of EM waves by the film resistance, while FSS and the reflective layer play a relatively minor role in the absorption process. This result verifies the important role of RAL in realizing the microwave camouflage function. Due to the minor role of FSS on EM waves loss and the resonant structure's symmetry, only the EM field results of RAL and Reflective layer in TE mode are simulated and analyzed. Figures 7(c) and (d) present the EM field simulation results of the front surface current of the RAL, surface energy loss of the RAL, and backside surface current of the reflective layer at the resonance frequency peaks of 3.9 GHz and 11.04 GHz in TE mode, respectively. The information in the figure indicates that the strongly induced current and energy loss is generated at the focal point of the resonant architecture of the RAL, suggesting that the center exhibits a strong ability to attenuate EM waves. The absorption of EM waves is attributed to both magnetic and electric resonances, as evidenced by the presence of surface currents on the front and back surfaces of the reflective layer that flow in opposite directions. Concurrently, the dissipation of energy from the resonant pattern of RAL at 11.04 GHz is significantly stronger than that at 3.9 GHz, which can clearly explain why the absorbance at 11.04 GHz is higher than that at 3.9 GHz.
Fig. 7. EM field simulation results of MHFM for (a) the thin-film resistance and dielectric (b) the ability of each layer of material to absorb EM waves; Peak frequency points at (c) 3.9 GHz and (d) 11.04 GHz for the front surface current of the RAL, surface energy loss of the RAL and backside surface current of the reflective layer in TE mode, respectively. The simulation results of the CST temperature field solver at (f) 3.9 GHz and (g) 11.04 GHz for the peak frequency points, respectively.
The equation for power loss, Ploss = I2R (where Ploss represents the power lost by the EM waves, and R and I denote the surface resistance and surface current, respectively), indicates that the induced current generated will result in the dissipation of EM waves as heat due to ohmic losses. The simulation results verify the role of ohmic heat loss in EM waves absorption by MHFM through the CST temperature field. The initial conditions of the CST temperature field simulation are set as follows: the simulation boundaries are all Open, the initial temperature is 294 K, the mesh type is Tetrahedral (the number of meshes is about two hundred and sixty thousand), and the solver is selected as the Thermal steady state solver. The thermal conductivity (W/m-K) of each material layer is shown in Table 2 [47–49]. The simulation results of the temperature field solver for the RAL layer of MHFM are presented in Fig. 7(f) and (g). The temperature field simulation results show that the maximum temperature of the RAL at 11.04 GHz is stronger than that at 3.9 GHz, and the ohmic loss effect is stronger at 11.04 GHz, which further explains the stronger absorption performance at 11.04 GHz than that at 3.9 GHz.
Table 2. Thermal conductivity of each layer material
3.3. S-parameter inversion and simulation analysis of equivalent circuit model
This study employs the S-parameter inversion to analyze the EM response properties of the MHFM and creates a circuit model to emulate the EM wave absorption effect of the MHFM. The interrelation between the reflection coefficient S11, transmission coefficient S21, as well as equivalent impedance Z(ω) and equivalent refractive index n(ω) of MHFM can be expressed as follows [50].
The reflectance and equivalent impedance of MHFM is expressed as [52].
Among them, Z0 (377 Ω) and θ denote the wave impedance in a vacuum and the incidence angle of EM wave, respectively. Figures 8(a) and (b) show the outcomes of the ε(ω), µ(ω), and Z(ω) obtained through a combined simulation and inversion process utilizing both the CST and Matlab software. The figures demonstrate a close resemblance between the ε(ω) and μ(ω) values. Additionally, the real part of Z(ω) is close proximal to 1, and the imaginary part is almost 0, thereby signifying that the MHFM design has achieved well-matched impedance. The two-port equivalent circuit model is capable of simulating the scattering phenomenon that occurs upon the incidence of EM waves on the surface of the MHFM. The equivalent circuit uses Z0 to represent the airwave impedance in a vacuum and Zin to represent the input impedance. The reflective layer of the MHFM is modeled in the equivalent circuit using a short-circuited two-port network; a combination of resistors, inductors, and capacitors connected in series to mimic a single-layer thin-film resistive structure, and multiple circuits consisting of resistors, inductors, and capacitors connected in parallel to mimic a multilayer thin-film resistive structure; and a resistor to simulate a dielectric material layer [53]. The designed equivalent circuit model is shown in Fig. 8(c), and the parameters of each equivalent circuit component are shown in Table 3. Figure 8(d) illustrates the S11 and absorptivity outcomes obtained from both EM simulation using CST Microwave Studio and circuit simulation using Advanced Design System (ADS), from which it is shown that the simulation results obtained from the equivalent circuit model are in good concurrence with those obtained from the EM simulation.
Fig. 8. (a) ε(ω) and μ(ω), (b) Z(ω) calculated by the combined simulation. (c) Simulated equivalent circuit of MHFM. (d) Absorptivity and S parameters are acquired through simulations using both CST and ADS.
Table 3. The element parameters of the equivalent circuit model
4. Measurements
To verify the actual stealth performance, a sample of size 180 mm × 180 mm is fabricated, with reference to Fig. 9(a). From the figure, it is apparent that the MHFM sample fabricated possesses excellent flexibility. The fabrication flow of the PDMS dielectric layer and VCL is shown in Fig. 2. The PEN substrate is coated with FSS, RAL, and a reflective layer through the process of plasma sputtering by utilizing ITO targets of thicknesses 185 nm, 20 nm, and 185 nm, respectively. Following this, the FSS and RAL microstructures are created using high-precision laser etching techniques. Finally, MHFM samples with multispectral compatible camouflage of microwave, infrared, and visible are fabricated by cutting, stacking, and gluing.
Fig. 9. (a) Sample diagram, microstructure diagram of FSS, RAL, resistance measurement diagram, bending diagram, and horizontal placement diagram of MHFM sample. (b) The measurement result under natural ambient light indoors of visible camouflage performance of the MHFM sample.
4.1. Measurement of visible camouflage performance
Visible camouflage performance is measured by the color and the time required for a complete color change of the MHFM sample under natural ambient light indoors. The measurement processes are: heating the heating plate temperature to 27°C and keeping it constant, placing the MHFM sample on the heating plate, recording the time required for the complete color change, and taking pictures of the sample. After the color change, the temperature of the heating plate is heated to 29°C and kept constant. The above processes are repeated to measure the time required for the color change of the sample when the constant temperature of the heating plate is 29°C, 32°C, and 35°C, respectively, and to take pictures of the sample. The measurement results under natural ambient light indoors of the visible camouflage performance of MHFM samples are shown in Fig. 9(b). The measurement results demonstrate that the overall color of the MHFM samples is black at a temperature of 28°C. When the temperature of the MHFM sample is 29°C, 32°C, and 35°C, the overall color of the MHFM sample will change to purple, green, and yellow. The average time required for a complete color change of the samples is 15 s. Under natural ambient light indoors, the clear color change at different temperatures shows that the MHFM samples have excellent visible camouflage properties.
4.2. Measurement of microwave absorption camouflage performance
Microwave camouflage performance is measured by the arch frame measurement method. The primary elements of the measurement system are depicted in Fig. 10(a). Figure 10(b) illustrates a comparison between the absorption measurement outcomes of MHFM samples for EM waves within the microwave frequency range and the simulation results. The information in the figure indicates that the MHFM samples have a slightly lower absorption performance for low-frequency EM waves, as well as a slightly narrower absorption bandwidth, compared to the simulation results. The absorption performance for high-frequency EM waves is also slightly lower, but the absorption bandwidth is wider. This is due to the errors in sample preparation and measurement. The simulation and measurement results of the MHFM samples on the absorption of microwave frequency band EM waves are in excellent agreement, and the MHFM samples have excellent microwave camouflage performance. To verify the high-temperature stability of the MHFM samples, the absorptivity of the MHFM samples at ambient temperatures ranging from 25°C to 175°C is measured in this study. The measurement procedure is as follows: a constant temperature baking table is heated to 25°C, 75°C, 125°C and 175°C and then keep constant; the MHFM samples are placed onto the constant temperature baking table, and the temperature of the surface of the MHFM samples is measured by a thermometer. When the temperature on the surface of the MHFM sample no longer increased, the MHFM sample is transferred to a calibrated arch frame measurement platform for the EM wave absorptivity measurement (the measurement process is less than 10 s). Figure 10(c) shows the absorptivity measurements of the MHFM samples at 25°C∼175°C. The measurement results show that as the temperature increases, the absorption performance of MHFM for low frequency EM waves slightly increases, and the bandwidth of high frequency band slightly narrows. The absorption of EM waves is significantly influenced by the RAL, which plays a crucial role in this process. A minor alteration in the bandwidth of absorption and absorption capability occurs due to a slight rise in the ITO resistance within the temperature range of 25°C to 175°C [54]. However, overall, the MHFM samples showed a slight difference in absorbance from 25°C to 175°C, and all of them have good absorbance performance and excellent high-temperature stability.
Fig. 10. (a) Measurement system of arch frame measurement method. (b) Measurement results of MHFM sample absorptivity and S11 at room temperature of 25°C compared with simulation results. (c) Measurement results of MHFM sample absorptivity at 25°C∼175°C.
4.3. Measurement of infrared camouflage performance
A calibrated TSS-5X-3 infrared emissivity tester is used to measure the infrared emissivity of IRSL. As depicted in Fig. 11(a), the measured frontal infrared emissivity of FSS is 0.37, which exhibits a slight deviation from the theoretically calculated value of 0.34. The inaccurate fabrication of the FSS sample, and the error introduced by the existence of particulate matter on its surface during measurement, explain this outcome. According to Fig. 11(b), infrared emissivity at the back side of the FSS is measured to be 0.84. This is because the front-facing surface of the FSS features an ITO film resistor structure which reduces its infrared emissivity, while the back side lacks any such structure. Additionally, the FLIR ONE PRO thermal imaging camera for infrared detection is used to measure the effectiveness of IRSL to reduce infrared emissivity. The experimental process includes setting the heating plate to a temperature of 100°C and applying non-woven fabric on it to ensure that the temperature is evenly distributed across the surface, thereby reducing experimental errors. Next, the FSS and the comparison PEN materials are placed on the heating plate and the temperature of both materials is measured using an infrared thermometer. The measurement results, after reaching a steady-state temperature, are shown in Figs. 11(c)∼(h), and the measured temperatures are 97.3°C and 61.3°C when the comparison of PEN material and FSS are placed, respectively. As depicted in the figure, the temperature recorded when the FSS is placed on the heating plate is significantly lower than the temperature recorded when the comparison PEN material is placed on the plate. The temperature reading presented in Fig. 11(g) is used to calculate the FSS design's ability to emit infrared radiation. This emissivity value is then calculated using the following formula [55].
Fig. 11. The results of the infrared emissivity measurements (a) front side and (b) underside of the sample. Images of (c) the PEN material and (d) the FSS are captured by the camera. Images of (f) the PEN material and (g) the FSS are captured using the infrared thermal imager. (e) Magnified view of the FSS cell structure. (h) Infrared thermal imaging camera magnification of FSS microstructure under heat.
Based on the given formula, which involves converting the Celsius temperature (°C) readings into Kelvin (K), the infrared emissivity of the FSS is estimated to be 0.35, by using the temperature measurements obtained from the infrared thermal imaging camera (TIr = 61.3°C), the ambient temperature (TAm = 25°C), and the true temperature (TRe = 96.3°C). It is noteworthy that this estimated value is quite close to the measurement result obtained from the infrared emissivity tester, which is 0.37. The experimental results indicate that the FSS can significantly decrease infrared emissivity and exhibit favorable practical effects. Table 4 presents a comparison between the MHFM developed in this study and the multispectral camouflage metasurfaces that have been designed in other research studies [37–39]. The comparison shows that the MHFM designed in this study can simultaneously realize the combined microwave, infrared, and visible multispectral camouflage [56–58].
Table 4. Comparison between the MHFM developed in this study and the multispectral camouflage metasurfaces that have been designed in other research studies
5. Conclusion
This study describes new methods for simultaneously realizing microwave, infrared, and visible light camouflage. The MHFM designed by this method can achieve over 90% absorption efficiency within the microwave frequency range of 2.9 to 13.9 GHz and exhibit an infrared emissivity of less than 0.34 in the infrared camouflage band of 3 to 14 µm. MHFM is available in black, purple, green, and yellow colors at different temperatures for excellent visible camouflage. The measurements of MHFM microwave, infrared, and visible camouflage performance are in accordance with the theoretical design. In addition, after measuring, MHFM still exhibited excellent absorption performance at 175°C, demonstrating outstanding high-temperature stability. The MHFM proposed in this study can be used in various background environments to meet the needs of microwave, infrared, and visible multispectral combined camouflage. It has a wide range of potential applications in the field of engineering platforms for multispectral combined camouflage technology. The multispectral camouflage implementation method proposed in this research can provide a new idea for the research of multispectral combined camouflage technology.
Funding
Fund for Shanxi Key Subjects Construction (20210302123074); Shanxi Provincial Key Research and Development Project (201803D421043).
Acknowledgments
The authors thank the National Natural Science Foundation of China for help identifying collaborators for this work.
Disclosures
There are no conflicts to declare.
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.
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