Tailored Multi‐Scale and Flexible Metamaterial for Broadband Electromagnetic Wave Absorption and Infrared Stealth

ABSTRACT

Radar‐infrared compatible stealth materials are widely used in applications such as electromagnetic protection and infrared thermal surveillance. However, harmonizing radar absorption, infrared stealth, and mechanical flexibility is a difficult challenge. In this work, a flexible metamaterial absorber (SMMA) with a skin‐like multi‐scale structure capable of achieving radar‐IR compatible stealth was fabricated by utilizing freeze‐drying and screen‐printing techniques. The geometric parameters of the resonant metasurface and the dielectric constant of the SiC@Polyurethane (SiC@Pu) aerogel were optimized through electromagnetic simulations. By integrating metamaterial and SiC@Pu aerogel, this architecture synergizes structural and material losses to significantly enhance electromagnetic wave absorption (MA) performance. The skin‐like multi‐scale design ensures that incident electromagnetic wave undergo effective attenuation and dissipation. The final SMMA achieves excellent broadband MA performance with an effective bandwidth of 9.27 GHz and a peak reflection loss of −44.3 dB, while significantly reducing the radar cross‐section. Meanwhile, the hierarchically porous aerogel exhibits excellent thermal insulation, maintaining a surface temperature nearly 48°C lower than a 100°C heat source, thereby effectively suppressing the infrared signature. This research demonstrates a design strategy for skin‐like, multi‐scale metamaterial absorbers, providing a pathway toward advanced, flexible, radar‐infrared compatible stealth materials.

A bio‐inspired SiC‐based metamaterial absorber (SMMA) achieves simultaneous electromagnetic wave absorption and infrared stealth. By mimicking the hierarchical structure of human skin, this device integrates a thermally insulating, porous aerogel with a resonant metasurface. This multi‐scale synergistic design offers a high‐efficiency, broadband solution for advanced radar and thermal signature suppression.

1. Introduction

With the rapid advancement of information technology and electromagnetic equipment, electromagnetic radiation has emerged as a significant threat to both equipment reliability and human health. Consequently, the effective management and attenuation of electromagnetic wave has become a critical area of research. The development of next‐generation high‐performance materials that integrate electromagnetic wave absorption (MA) with infrared stealth has thus emerged as an urgent and pivotal focus of current investigation [1, 2]. As an emerging class of engineered composites, metamaterials exhibit precise control over the propagation and absorption characteristics of electromagnetic wave (EMW) across multiple frequency bands by virtue of their carefully designed artificial atom structures. This distinctive electromagnetic manipulation capability endows these materials with broad potential for application in diverse areas, including electromagnetic wave absorption [3, 4], communication systems [5, 6], and radiative cooling [7, 8]. In 2008, Landy et al. [9] experimentally demonstrated a metamaterial absorber that achieved near‐perfect absorption through the independent tuning of permittivity and permeability of the structure. Subsequently, Park [10] reported that the incorporation of multiple resonators of varying sizes enables the achievement of dual‐, triple‐, and even quad‐band near‐perfect absorption within the microwave regime.

Although conventional periodic electromagnetic absorbers exhibit excellent performance at specific resonant frequencies, their practical applications are limited by narrow absorption bandwidth and high sensitivity to both the incidence angle and polarization. To achieve effective control over EMW, researchers have proposed various design strategies for metamaterial absorbers. One approach involves modulation of the geometry of resonant units to excite multiple adjacent resonant modes. Su et al. [11] addressed the issues of low efficiency and restricted functionality in near‐field modulation by developing a multi‐dimensional multiplexed metasurface via physics‐driven deep learning, achieving simultaneous high‐performance microwave absorption, polarization conversion, and anomalous reflection. Similarly, Guan et al. [12] addressed the restricted integration of surface and space waves by developing a multiplexed metasurface that utilizes a triple‐layer metallic‐dielectric structure. This architecture enables independent control over guided wave propagation and space wave reflection or absorption. The results confirm that the metasurface achieves broadband microwave absorption and a significant reduction in the radar cross section. However, such approaches often lead to complex designs of the unit cell and hinder the formation of smooth and continuous absorption spectra. An alternative strategy involves the optimization of the dielectric layer thickness based on the quarter‐wavelength interference principle to achieve impedance matching [13]. Although this method can improve absorption efficiency within specific frequency bands, it exhibits inherent frequency selectivity and may increase the overall thickness of the device. These characteristics consequently limit the applicability of such devices in lightweight and flexible systems.

To overcome these challenges, recent efforts have focused on the enhancement of absorber performance through the integration of multiple energy dissipation mechanisms [14]. Among these, a critical strategy involves the precise modulation of surface electrical loss. This modulation is typically accomplished by constructing the top resonant structure from materials with specific sheet resistance, such as indium tin oxide (ITO) films [15] or graphene synthesized by chemical vapor deposition (CVD) [16]. In contrast to highly conductive metals, these resistive materials convert incident electromagnetic energy into thermal energy more effectively through Ohmic loss. This mechanism is widely recognized as an effective pathway for achieving broadband absorption [17, 18]. In parallel, the optimization of energy loss within the dielectric matrix is of equal importance. The incorporation of fillers, such as carbonyl iron powder (CIP) [19], ferrites [20], and SiC [21], represents another key strategy for achieving broadband and efficient absorption. SiC, a wide‐bandgap semiconductor, exhibits excellent thermal stability, chemical inertness, and mechanical robustness [22, 23]. Furthermore, the significant dielectric loss capability and tunable electrical properties of SiC render it an ideal candidate for the design of high‐performance electromagnetic wave absorbers. Han [24] fabricated 3D braided SiC composites via precursor infiltration and pyrolysis (PIP). The results demonstrate that a five‐directional architecture incorporating axial yarns performs more effectively than its four‐directional counterpart, achieving a minimum reflection loss of −16.1 dB at a thickness of 5 mm. Similarly, Yu [25] fabricated Ni‐doped SiC fibers using the electrospinning method. The resulting fibers achieved a wide effective absorption bandwidth of 6.3 GHz at a thickness of 2.0 mm, which is attributed to the enhancement of the inherent dielectric loss capabilities. However, the introduction of magnetic fillers to broaden the bandwidth inevitably increases material density and compromises the lightweight advantage. Additionally, Mo [26] engineered a SiC fiber/Si₃N₄ composite featuring a “low‐high‐low” dielectric sandwich profile. This configuration exhibited excellent electromagnetic properties at room temperature and maintained robust absorption at temperature up to 600°C. Despite these promising results, the complex preparation process and high cost associated with methods hinder the large‐scale application of SiC‐based absorbing materials. Therefore, achieving a balance between absorption performance and material flexibility, fabrication costs, and environmental adaptability remains a critical challenge for the advancement of this field.

In this work, a flexible SiC‐based metamaterial absorber (SMMA) was fabricated through directional freeze‐drying and bonding processes, achieving both broadband electromagnetic wave absorption and infrared stealth. Inspired by the hierarchical architecture of human skin, this skin‐like metamaterial consists of resonant layers and SiC@Polyurethane (SiC@Pu) dielectric layers. Similar to the epidermis, the SiC@Pu layer provides structural support while contributing to electromagnetic wave absorption and the enhancement of infrared stealth. Resembling the dermis, the resonant layer further modulates and attenuates incident EMW. The design parameters of the model were optimized by numerical simulations, and broadband absorbing metamaterials were subsequently fabricated according to the optimized results. Subsequent experimental characterizations demonstrate that the measured performance of the sample is highly consistent with the simulation results. The resulting metamaterial efficiently attenuates EMW and provides excellent thermal insulation, which significantly reduces the infrared signature of the SMMA. The final SMMA achieves excellent broadband radar absorption, with an effective bandwidth of 9.27 GHz and a peak reflection loss of −44.3 dB. Furthermore, the hierarchically porous aerogel exhibits superior thermal insulation, maintaining a surface temperature nearly 48°C lower than that of a 100°C heat source. Moreover, the metamaterial exhibits excellent flexibility, allowing it to conform to complex surfaces. The proposed metamaterial shows significant potential for applications in electromagnetic stealth, personal protection, and electromagnetic compatibility, offering a high‐performance stealth solution for both military and civilian technologies.

2. Results and Discussion

2.1. Design and Fabrication of SiC‐Based Metamaterial Absorber

The design of this flexible SiC‐based metamaterial absorber (SMMA) is bio‐inspired by the hierarchical structure of human skin, integrating a resonant metasurface with a dielectric lossy layer through a synergistic strategy. The core design involves the fabrication of a SiC@Pu aerogel with a custom‐tuned dielectric constant and the engineering of a metasurface composed of a periodic array of resonant units, which was precisely optimized via full‐wave electromagnetic simulation software. Figure 1 illustrates the design process of SMMA. The SiC powders were pre‐hydrophilized to prevent agglomeration and enhance dispersion stability before being dispersed into an aqueous WPU solution to form a stable colloid (Figures S1 and S2). As shown in Figure S3, the successful stabilization was further confirmed by the pronounced Tyndall effect observed when a laser beam passed through the dispersion. The SiC@Pu aerogels with a vertical microporous structure were obtained through directional freeze‐drying technology. The absorbing metasurface was fabricated by screen‐printing a simulation‐optimized square resonant ring array onto a PET film using a conductive copper paste. The exact dimensions of the screen are shown in Figure S4. The final SMMA device was assembled by precisely aligning and bonding the constituent layers, namely the SiC@Pu aerogel layer, the metasurface, and a copper backplate.

FIGURE 1.

Schematic diagram of the SiC‐based Metamaterial Absorber (SMMA) preparation process.

Structurally, the SiC@Pu aerogel supports the metasurface while simultaneously functioning as both a thermal insulator and a wave absorber. The broadband electromagnetic wave absorption and infrared stealth performance of SMMA originate from the synergy between the macroscopic periodic structure and the microscopic porous material across different physical scales. For electromagnetic wave absorption, the material employs a dual‐attenuation strategy, combining microscopic conductive and polarization losses with macroscopic resonant absorption. First, incident EMW penetrate the top low‐dielectric SiC@Pu aerogel, which serves as an initial impedance matching layer and induces pre‐attenuation. The transmitted waves then reach the core metasurface structure for further resonant absorption. Therefore, the synergy between microscopic losses (multiple reflection and interfacial polarization) and macroscopic resonant absorption endows the SMMA with its excellent broadband, high‐efficiency electromagnetic wave absorption performance.

Furthermore, the exceptional thermal insulation performance is attributed to the highly ordered and vertically aligned lamellar porous network of the SiC@Pu aerogel. First, solid‐state thermal conduction is confined within the tortuous and low‐volume‐fraction SiC skeleton, which significantly reduces both solid and gaseous thermal conduction. Second, the micro‐ and nano‐scale closed pores suppress macroscopic thermal convection. Consequently, the SiC@Pu aerogel exhibits superior thermal insulation at the macroscopic level, leading to a substantial reduction in the infrared signature of the material.

2.2. Characterization of SMMA

The SiC@Pu aerogels with varying SiC contents were systematically characterized by Raman and XRD spectroscopy (Figures S5 and S6). All samples exhibit distinct diffraction and vibrational features attributable to silicon carbide, confirming the successful incorporation of the SiC phase into the aerogel framework. Notably, as the SiC content increases, the intensities of the characteristic peaks in both the XRD and Raman spectra progressively increase, indicating a higher loading of the SiC phase within the composite structure.

The overall electromagnetic response of the metasurface is dictated by the collective resonance of the periodic unit cells. Therefore, the structural integrity and dimensional uniformity of the resonant array are paramount for achieving the designed absorption performance. As observed in the optical photograph (Figure 2a), the periodic array of resonant units is well‐ordered with uniform dimensions. This macroscopic periodicity and high pattern fidelity are further corroborated by infrared (IR) thermal imaging (Figures 2b,c), confirming that the fabrication precision satisfies the stringent requirements for operation in the millimeter‐wave band.

FIGURE 2.

Morphological and compositional characterization of the SMMA. (a) Optical photograph of the resonant ring array. (b,c) Infrared thermal images of the resonant ring array. (d) SEM image of the metal backplate. (e,f) SEM images of the resonant ring. (g) SEM image of the silicon carbide aerogel and the corresponding EDS mapping images for (h) Si, (i) C, and (j) O elements. (k‐m) Photographs of the SMMA.

The copper backplate, which serves as the perfect electric conductor (PEC) layer, was fabricated via spray‐coating of a conductive copper paste. The corresponding SEM image reveals the uniform deposition of copper particles (Figure 2d), forming a continuous and dense conductive network that effectively reflects incident EMW (Figure S7). Figure 2e,f present the detailed morphology of the screen‐printed resonant rings. The conductive copper particles are uniformly and densely dispersed on the PET substrate, forming sharp and well‐defined pattern boundaries. This structural integrity is essential for achieving an efficient and stable electromagnetic resonant response. Furthermore, elemental mapping of the SiC@Pu aerogel demonstrates that the key constituent elements of the framework‐silicon (Si), carbon (C), and oxygen (O)‐exhibit a highly homogeneous distribution (Figure 2g–j). Such compositional uniformity is critical for maintaining a stable and uniform dielectric constant across the layer, which is paramount for achieving precise impedance matching for the absorber.

The uniformity and structural integrity of the large‐area SiC@Pu aerogel samples provide a reliable foundation for the dielectric layer in high‐performance electromagnetic wave absorbers (Figure S8). Finally, the individual layers were integrated via a composite assembly process to produce the final large‐scale metamaterial absorber prototype with both structural and functional integration. As demonstrated in Figure 2k–m, the resulting device is not only structurally intact but also exhibits excellent flexibility, withstanding significant bending without apparent damage. As shown in Figure S9, the designed SMMA adapts to variations in the curvature of a cylinder, achieving seamless coverage on target substrates. This outstanding mechanical compliance highlights the considerable potential of conformal applications.

2.3. Dielectric and Electromagnetic Wave Absorption (MA) Performances of SiC@Pu Aerogel

As illustrated in Figure 3a–c, the reflectance, transmittance, and absorption of EMW are primarily determined by the real and imaginary components of complex permittivity of a material. Specifically, Figure 3a shows that the reflectance (R_ik ) strongly depends on the real permittivity (ε′): as ε′ decreases from 10 to 1, R_ik significantly drops from ∼27.5% to ∼0.5%, indicating that higher ε′ values lead to stronger EMW reflection. To suppress surface reflection to below 10%, ε′ must be maintained at approximately 3.5 or lower. Figure 3b further confirms that when ε′ is below this threshold, more than 90% of incident EMW can penetrate the material for subsequent effective absorption. The relationship between the permittivity and R_ik and absorbance (A_ik ) of the material is calculated according to Equations (1) and (2) [27]:

$$R_{ik}={\left(\frac{n_{ki}-1}{n_{ki}+1}\right)}^2={\left(\frac{\sqrt{\frac{{\epsilon }_k}{{\epsilon }_i}}-1}{\sqrt{\frac{{\epsilon }_k}{{\epsilon }_i}}+1}\right)}^2$$ (1)

$$A_{ik}=1-R_{ik}$$ (2)

where n_ki represents the refractive index of medium, k relative to medium i. Concurrently, ɛ_i and ɛ_k denote the complex permittivities for each of the two media, respectively.

FIGURE 3.

Analysis of electromagnetic wave absorption performance and mechanism. (a–c) Theoretical calculation results showing the relationship between (a) reflectance (R), (b) absorbance (A), and the complex permittivity (real permittivity ε′ and loss tangent tanδ), and (c) the relationship between absorber thickness, frequency, and real permittivity ε′. (d–f) Electromagnetic parameters of different SiC@Pu aerogel: d) real part ε′, (e) imaginary part ε′′, and (f) dielectric loss tanδ. (g–i) 2D contour maps illustrating the (g) reflectance (R), (h) absorbance (A), and (i) iransmittance (T) of the SiC‐2@Pu composite as a function of frequency and material thickness.

Figure 3c demonstrates that the required absorber thickness is inversely related to permittivity. Consequently, materials with a higher ε′ can achieve efficient EMW attenuation at a reduced thickness. The governing equation for this principle is expressed as Equation S1. By leveraging the inherently low permittivity of the selected SiC powder (Figure S10), its composite with Pu enables effective tuning of electromagnetic parameters of the final aerogel within a controllable range. Figures 3d–f depict the dielectric properties of SiC‐loaded aerogels with varying mass fractions. Figure 3d shows that the ε′ increases with SiC content and exhibits strong frequency dispersion, rising steeply in the 2–6 GHz range and leveling off at higher frequencies. This trend indicates that precise tuning of ε′ in the range of 1 to 5 is enabled by increased SiC loading. The imaginary permittivity (ε″) and dielectric loss tangent tanδ also exhibit similar frequency‐dependent behaviors (Figure 3e,f). These parameters show moderate growth between 2–4 GHz and a stable plateau above 8 GHz, suggesting consistent dielectric relaxation mechanisms across all compositions [28, 29, 30].

Figure 3g–i illustrate the thickness‐dependent EMW absorption characteristics of the aerogel composites. The reflectance (R), absorbance (A), and transmittance (T) are calculated using Equations S2–S5. As shown in Figure 3g, the SiC‐2@Pu maintains reflectance below 10% across the entire 2–18 GHz range for thicknesses between 0 and 5 mm, enabling more than 90% of the incident EMW to penetrate the material. Correspondingly, the absorbance gradually increases with thickness, exceeding 40% at thicknesses above 5 mm, while the transmittance concurrently decreases to approximately 60% (Figure 3h). This enhancement is attributed to the low ε′ of SiC, which suppresses surface reflection and facilitates wave penetration. Additionally, the vertically aligned porous structure promotes multiple internal reflections and scattering, effectively extending the propagation path of EMW and boosts absorption efficiency [31, 32]. However, excessive SiC content can induce a significant rise in permittivity, leading to impedance mismatch and elevated reflectance. This effect is evident in the SiC‐5@Pu sample, which exhibits a reflectance that exceeds 10% in certain frequency bands (Figure S11). Moreover, a high SiC loading leads to a denser microstructure, which, although beneficial for mechanical robustness, can constrain the propagation of EMW and reduce absorption efficiency. These findings underscore the importance of synergistically optimizing SiC content and structural morphology to achieve a balance between impedance matching, effective wave attenuation, and broadband absorption in electromagnetic functional materials [29, 33, 34].

2.4. Ma Performances of SMMA

To optimize the broadband absorption performance of the designed SMMA, a comprehensive parametric study of the key structural unit parameters was conducted using CST electromagnetic simulation software. A structure composed solely of a lossless dielectric layer and a copper backplate exhibits negligible electromagnetic wave absorption (Figure S12). The introduction of a single metallic resonant ring induces strong, frequency‐selective absorption via intense resonance; however, this effect is typically confined to a narrow bandwidth (Figures S13 and S14) [35, 36, 37]. Crucially, it is found that stacking multiple resonant structures, such as in a dual‐ring design, effectively generates additional absorption peaks. This principle of exciting multiple resonant modes to merge their respective absorption bands provides an effective strategy for the rational design of high‐performance broadband electromagnetic absorbers (Figure S15).

Figure 4a clearly shows the unit cell structure of the SMMA, which consists of a double‐layer metallic resonant ring integrated with a low‐loss SiC@Pu dielectric layer. The geometric dimensions of the metallic resonant rings are critical parameters that determine the resonant frequencies and the corresponding intensities of these resonances. As shown in Figure 4b,c, the side lengths of the first‐ and second‐ layer metallic resonant rings (parameters c and d, respectively) were systematically varied in the simulation. The results reveal that these two parameters exhibit a different tuning effect on the distinct resonant modes. Specifically, as the side length d of the second metallic ring increases (Figure 4c), the low‐frequency absorption peak at approximately 4 GHz first strengthens and then weakens, while progressively shifting toward lower frequencies. Concurrently, the high‐frequency absorption peak demonstrates a trend of shifting toward higher frequencies. This indicates that parameter d primarily governs the low‐frequency resonant mode, with its variation directly modulating the effective path length for the equivalent inductor‐capacitor resonance [35, 38]. In contrast, variations in the side length c of the first metallic ring exert a more pronounced influence on the position and intensity of the high‐frequency absorption peak, while the low‐frequency peak remains relatively stable (Figure 4b).

FIGURE 4.

Structural design, parametric optimization, and performance verification of the SMMA. (a) Schematic of the SMMA unit cell, illustrating the dual‐layer metallic resonant rings and dielectric layers. (b–e) Reflection loss with different structural parameters: (b) side length c of the first layer metallic ring, (c) side length d of the second layer metallic ring, (d) thickness t₁ of the first dielectric layer, and (e) thickness t₂ of the second dielectric layer. (f) Photograph of the experimental setup for reflectivity measurement in an anechoic chamber. (g,h) Comparison of simulated (dashed lines) and measured (solid lines) reflection loss curves for the conventional MMA and the optimized SMMA.

Figure 4d,e demonstrate the influence of varying the thicknesses of the first and second SiC@Pu dielectric layers (t₁ and t₂ , respectively) on the reflection loss. As either t₁ or t₂ increases, the absorption peaks collectively shift toward lower frequency regions. This phenomenon is attributed to the extended effective optical path as EMW propagate through a thicker dielectric medium. According to the quarter‐wavelength interference theory, this extended path leads to a decrease in the resonant frequency required to satisfy the phase cancellation condition. Furthermore, a moderate increase in thickness enhances the internal Fabry‐Pérot cavity resonance, which prolongs the propagation and oscillation time of EMW within the structure. This significantly augments the energy dissipation arising from localized surface plasmon resonance (LSPR) and dielectric loss [39].

Finally, a sample was fabricated based on the optimal parameters, and the reflectivity of the sample was measured in an anechoic chamber (Figure 4f). Figure 4g,h present a comparison between the simulated and measured results for both a conventional metamaterial absorber (MMA) without the SiC@Pu layer and the optimized SMMA. The excellent agreement between the experimental data and the simulation curves strongly validates the accuracy of the design and analysis. Owing to the synergistic interplay between structural resonance and material loss, the performance of the SMMA represents a significant advancement over that of the conventional MMA. Specifically, the effective absorption bandwidth (reflection loss below −10 dB) is broadened from 3.48 GHz for the conventional MMA to 9.27 GHz for the SMMA, and a maximum reflection loss of 44.3 dB is achieved at 3.79 GHz. This enhancement is primarily attributed to the incorporation of the SiC@Pu dielectric layer: the low permittivity (ε < 3.5) of the layer optimizes impedance matching with free space, thereby significantly improving the coupling efficiency of incident waves. Concurrently, the intrinsic polarization and conduction loss characteristics exert a pre‐attenuation effect on transmitted waves at higher frequencies. Cole‐Cole plots further confirm that interfacial relaxation processes are the primary contributors to the dielectric loss tangent (Figure S16). Moreover, the highly ordered and vertically aligned lamellar structure of the aerogel ensures a high density of hetero‐interfaces, which promotes multiple internal reflections. This configuration effectively traps and dissipates electromagnetic wave within the lossy framework. In summary, the effective balance between the structural resonance loss and dielectric loss mechanisms is successfully achieved through the synergistic optimization of the structural parameters (c, d, t₁ , t₂ ) and the intrinsic material properties.

Using CST Microwave Studio simulations, the electromagnetic field distributions of the conventional MMA and SMMA were systematically investigated by placing field monitors at the characteristic frequencies corresponding to the reflection loss (RL) peaks. The results demonstrate a strong correlation among the electric field intensity distribution, energy dissipation patterns, and the magnitudes of the RL peaks. The samples exhibiting stronger RL peaks show more pronounced electric field localization and energy dissipation (Figure 5a,b). Notably, the introduced low‐permittivity SiC@Pu surface layer and the periodic resonant structure collectively establish a multi‐stage loss mechanism. The SiC@Pu layer exhibits dual functionality owing to the unique dielectric properties of the material. On one hand, acting as an impedance gradient layer, it achieves excellent impedance matching with free space (377 Ω) due to its relative permittivity (ε < 3.5), significantly minimizing electromagnetic wave reflection at the air‐material interface and thereby facilitating efficient wave penetration into the absorber. On the other hand, the dipole polarization relaxation of SiC contributes to moderate dielectric losses, which pre‐attenuates electromagnetic energy before it reaches the underlying resonant structure. These pre‐attenuated waves subsequently undergo strong resonant absorption within the underlying MMA structure, where LSPR and Fabry‐Pérot cavity effects synergistically generate intense electric field enhancement and eddy current losses at the target frequency (3.61 GHz) [38, 39]. This innovative graded‐loss design endows the SMMA with superior absorption performance compared to the conventional MMA, including an approximately 30 dB reduction in the RL peak and a roughly 32% broader effective absorption bandwidth. These improvements primarily arise from two contributions of the SiC@Pu layer: the optimized impedance matching and a complementary broadband loss mechanism that works in synergy with the resonant structure. This work provides a novel strategy for designing high‐performance metamaterial absorbers, demonstrating a full‐process optimization from impedance matching to energy dissipation via the synergistic integration of dielectric gradient layers and resonant structures.

FIGURE 5.

Analysis of the electromagnetic absorption mechanism and far‐field scattering performance of the SMMA. (a,b) Simulated field and power loss density distributions for the MMA and SMMA at their resonant frequencies: (a) Electric field (E‐field) distribution, (b) Power loss density distribution. (c‐g) RCS simulations to evaluate far‐field performance. c) Schematic of the RCS measurement setup. (d,e) 3D far‐field scattering patterns for (d) a reference perfect electric conductor plate and (e) the SMMA, respectively. (f,g) Quantitative comparison of the 2D RCS for the PEC and the SMMA in (f) Cartesian and (g) polar coordinates.

To assess the electromagnetic absorption performance of the material in practical applications, the radar cross‐section (RCS) values of both a PEC and the SMMA structure were simulated using CST Microwave Studio. An RCS monitor was employed at 3.83 GHz to characterize the angular scattering behavior of the materials across a full azimuthal range (0°–360°). As depicted in the 3D and 2D radar wave scattering patterns, the RCS variation exhibits strong angular dependence (Figure 5c–g). The PEC exhibits consistently high RCS signals at all angles, whereas the SMMA‐1 sample shows significantly suppressed RCS levels across the full angular range. At the target frequency of approximately 3.83 GHz, SMMA‐1 achieves a significant RCS reduction of ‐6.93 dBm², confirming the efficient absorption and dissipation of the incident EMW. Crucially, the consistently low RCS values of the SMMA across all observation angles suggest that the reduction is dominated by an absorption mechanism rather than by multi‐directional scattering. This behavior aligns with the low RL performance discussed earlier, providing a theoretical foundation for the applicability of the material in stealth in far‐field scenarios.

2.5. Infrared Stealth Properties and Applications in Complicated Environments of SMMA

The IR stealth capability of a material is governed by the Stefan‐Boltzmann law, which relates the total radiant power (H) to the infrared emissivity (ε_IR ) and the fourth power of the surface absolute temperature (T) via the equation H = ε _IRσT⁴ [40, 41, 42]. Consequently, reducing either T or the ε_IR of the material represents the fundamental strategy for achieving IR stealth. To investigate the IR stealth performance of the SiC@Pu aerogel, the thermal insulation capability of the material in a high‐temperature environment was evaluated, with the experimental device shown in the Figure S17. Infrared thermal imaging tests demonstrate the enhanced thermal insulation performance of the SiC@Pu aerogel compared to that of a pure Pu aerogel. As depicted in Figure 6a,b, when placed on a heating stage maintained at 100°C, the surface temperature of the pure Pu aerogel rose rapidly, stabilizing at approximately 73.6°C within 20 min. In contrast, the surface of the SiC@Pu aerogel stabilized at a significantly lower temperature of approximately 52.1°C, thereby maintaining a temperature difference of nearly 50°C relative to the heat source throughout the 20‐min test. The SiC@Pu aerogel exhibits a controlled apparent emissivity of approximately 0.55–0.65, which is lower than that of the pure Pu aerogel (Figure S18). Therefore, the lower emissivity characteristics, combined with the highly ordered and vertically aligned network of the SiC@Pu aerogels (Figure 6c), effectively modulate the radiative heat transfer. This leads to a more attenuated thermal signal on the surface of the SiC@Pu aerogel relative to the pure Pu counterpart. Furthermore, when the lightweight SiC@Pu aerogel is placed on a palm or finger, it effectively insulates against body heat (approximately 33°C) and conceals the human thermal signature (Figure 6d). Consequently, the surface exhibits a low‐temperature “blue” profile in the infrared camera, blending with the ambient background and effectively achieving infrared stealth.

FIGURE 6.

Thermal insulation performance and microstructural characterization of the SiC@Pu aerogels. (a) Infrared thermal images of the pure PU aerogel and SiC@Pu aerogel, respectively, on a 100°C hot plate. (b) Corresponding radiation temperature‐time heating curves for the aerogel samples with different SiC contents. (c) SEM images at different magnifications for the SiC‐3@Pu and SiC‐5@Pu aerogels, respectively. (d) Optical and thermal photographs of the aerogel. (e) SMMA samples subjected to water immersion and rinsing. (f) Water contact angles of copper backplate and SiC@Pu aerogel after water rinsing for 15 min. (g) Mechanism of radar‐infrared compatible stealth for SMMA.

Considering practical application environments, moisture adhesion and immersion influence the ability of devices to reflect or absorb electromagnetic wave. Therefore, water immersion tests exceeding 30 min demonstrate that the stealth device maintains the structural integrity of the resonant units during prolonged use (Figure 6e). Furthermore, the SiC@Pu layer and the resonant surface exhibit excellent hydrophobicity, with the water contact angle decreasing only slightly after 15 min of water rinsing (Figure 6f). Through the deposition of soil particles onto the surface of the SMMA to simulate environmental contamination scenarios, it is observed that the incident water flow rapidly carries away the soil pollutants. Notably, this effective removal of surface contaminants demonstrates the robust self‐cleaning capability of the composite, ensuring the operational stability of the material in complex outdoor environments.

Furthermore, accelerated UV aging tests are conducted to simulate 20 and 40 h of intense solar irradiation. As shown in Figure S19, SEM images reveal no discernible structural damage or microcracks within the vertically aligned lamellar pores or the metallic backplate. Meanwhile, the emissivity of the top‐layer SiC@Pu aerogel does not exhibit a significant decrease after exposure to UV light (Figure S20).

In curved application scenarios for stealth technology, devices require a certain degree of mechanical flexibility to conform to complex surfaces. The cyclic bending performance of the materials is evaluated using a flexible device testing system (Figure S21). Experimental results indicate that the SiC@Pu aerogel and the copper backplate exhibit excellent mechanical flexibility. After undergoing 500 bending cycles at an angle of 60°, the electrical conductivity remains nearly constant with negligible degradation (Figure S22). SEM images demonstrate that the copper particles maintain a uniform distribution on the PET surface without evidence of significant delamination. Furthermore, the aerogel preserves an intact microstructure, and no structural collapse of the pore channels is observed (Figure S23). In addition, the stress‐strain curve of the SMMA exhibits characteristic elastomeric behavior, with an elongation at break of approximately 1400% (Figure S24). A sample with a width of 5 mm can easily lift a 1 kg water bottle (Figure S25). This significant extensibility indicates that the material possesses exceptional toughness.

In this work, radar‐infrared compatible stealth is achieved through the synergy of multi‐scale mechanisms involving material composition, architecture, and meta‐structures (Figure 6g). Regarding material composition, the absorption performance of the SiC@Pu composite is primarily governed by dielectric loss, which arises from intense interfacial polarization at the high‐density heterogeneous interfaces between the SiC nanoparticles and the Pu matrix [32]. In terms of architectural design, the highly oriented vertical channel structure significantly reduces the effective permittivity at the material surface, thereby optimizing impedance matching and allowing electromagnetic wave to enter the aerogel with minimal reflection [28, 33]. Furthermore, the parallel longitudinal microchannels form semi‐enclosed spaces similar to microwave anechoic chambers, forcing electromagnetic wave to undergo scattering and multiple reflections between the pore walls. This mechanism substantially extends the effective propagation path of electromagnetic wave within the lossy skeleton, enabling efficient energy dissipation through continuous networks formed along the oriented framework. Regarding the meta‐structure, intense equivalent inductance‐capacitance resonance occurs within the structure when the frequency of the incident wave matches the natural frequency of the resonant rings, inducing highly localized electromagnetic fields at the gaps and edges of the units [20, 29]. This localized enhancement triggers significant ohmic loss through circular current paths.

In the field of infrared stealth, the vertical channels of the material effectively confine stagnant air, which significantly reduces gaseous thermal conductivity by suppressing convective heat transfer and limiting the movement of gas molecules within micro‐nano pores smaller than the mean free path of the gas. Simultaneously, the highly oriented yet low‐volume‐fraction solid skeleton greatly extends the heat transfer distance of the solid‐phase heat flow by increasing phonon scattering and constructing tortuous conduction paths, thereby achieving excellent thermal insulation performance [43, 44]. These features ensure that the outer surface of the metamaterial absorber maintains a low characteristic temperature under high‐temperature backgrounds, leading to effective stealth performance in complex infrared detection environments.

3. Conclusion

In this study, a flexible and multifunctional metamaterial absorber with radar‐infrared compatible stealth capabilities was developed by utilizing a skin‐like, multi‐scale architecture. By synergistically integrating a macroscopic dual‐layer resonant metasurface with a hierarchically porous SiC@Pu aerogel, the SMMA achieves an ultra‐broad effective absorption bandwidth of 9.27 GHz and a peak reflection loss of −44.3 dB. This composite structure also provides a significant RCS reduction over a wide angular range and demonstrates excellent thermal insulation, maintaining a surface temperature nearly 50°C lower than that of a 100°C heat source, which effectively suppresses its infrared signature. The excellent performance stems from a multi‐scale design paradigm that bridges macroscopic resonant engineering with microscopic material architecture to create a unique graded‐loss mechanism. Within this framework, the SiC@Pu aerogel serves a trifunctional role as an impedance‐matching layer, a dielectric loss medium, and a thermal insulator. This multi‐scale design overcomes the inherent bandwidth limitations of conventional metamaterials, and the resulting lightweight, flexible composite holds immense potential for applications in electromagnetic wave absorption, personal thermal management, and conformal stealth skins.

4. Experimental Section

4.1. Materials

Water‐based polyurethane (WPU, 32 wt% solid content) was purchased from Shenzhen Jitian Chemical Co., Ltd. Nano‐sized silicon carbide SiC powder was obtained from Zhongyan Nano New Material Co., Ltd. Polystyrene sulfonic acid solution (PSS, MW = 10,000) was supplied by Shanghai Titan Technology Co., Ltd. The conductive copper paste (model: NX‐202) was purchased from Shenzhen Nanxiang Conductive Material Technology Co., Ltd.

4.2. Preparation of SiC@Pu Aerogels

The raw SiC powder was first treated by mixing it with a certain amount of PSS solution, followed by ball milling (The detailed synthesis procedure is provided as described in the Supporting Information). The treated powder was then washed with ethanol, centrifuged, and dried. Subsequently, 0.2−1.0 g of the treated SiC powder was dispersed into the WPU solution and stirred for 6 h to obtain homogeneous dispersions with varying weight percentages (e.g., 1 wt.%, 2 wt.%, 4 wt.%, etc.), which were designated as SiC‐n@Pu (where n = 1, 2, 3…), respectively. The resulting dispersions were then processed using a directional freezing technique to create ice templates, which were subsequently freeze‐dried for 72 h to obtain the final SiC@Pu aerogels.

4.3. Fabrication of the Metamaterial Absorber

The conductive resonant rings with specific dimensions were fabricated on a substrate using screen printing with the NX‐202 copper paste. The conductive backplate was prepared by spray‐coating the copper paste. The final metamaterial absorber was assembled by integrating the multi‐layered aerogels and the conductive resonant ring structures using an adhesive. (Specific structural parameters and models are detailed in the S4).

4.4. Characterizations

The sample morphology was examined using a Scanning Electron Microscope (SEM, Hitachi TM3030) and a Field Emission SEM (FESEM, Hitachi SU8010) equipped with an Energy Dispersive X‐ray Spectrometer (EDS). The crystal structure was analyzed by X‐ray Diffraction (XRD, Bruker D8) with Cu‐Kα radiation. The chemical structure of the materials was characterized using a laser Raman spectrometer (Thermo Scientific DXRxi). The electrical conductivity of the samples was detected by a four‐probe resistivity meter (MCP‐T370). Surface temperature changes of the materials were recorded using a thermal IR imager (FLUKE T540). The infrared reflectance (8‐14 µm) was measured using FT‐IR (Nicolet IS50). The cyclic folding performance of the composite material is measured using a flexible material and device testing system (FlexTest). The mechanical performance of the SMMA was characterized by universal dual‐column material testing machine (Tiniius Olsen H10K‐S). The electromagnetic parameters of the SiC@Pu aerogels were measured over the 2–18 GHz range using the coaxial method. The samples were compressed into a toroidal shape (outer diameter: 7 mm, inner diameter: 3.04 mm) for testing with a vector network analyzer (VNA, Rohde & Schwarz ZNB40). The microwave absorption performance was evaluated by measuring the reflectivity of 180×180 mm samples in an anechoic chamber using the arch method, in accordance with the GJB2038A‐2011 standard. The measurements were conducted over the 2–18 GHz frequency range using a VNA (Keysight E5063A). Numerical simulations for reflection loss, radar cross‐section (RCS), E‐field distribution, and power loss density analysis were performed using CST Studio Suite 2022.

The practical infrared stealth performance for the human body was evaluated through experiments involving volunteers, who applied SMMA to clothing or skin surfaces exposed to the outdoor environment. Infrared images were captured using an infrared thermal imager to compare the distribution of surface temperatures.

All tests involving human volunteers complied fully with local laws and institutional ethical guidelines. Each subject gave full informed consent before participation.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: smll73926‐sup‐0001‐SuppMat.docx.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.