English

• With the rapid advancement of the Internet era and the increasing importance of military detection technologies, electromagnetic wave absorbing (EMWA) materials play a crucial role in both civilian and military applications. However, conventional EMWA materials often suffer from limitations such as narrow absorption bandwidth, high density, and low environmental stability, which restrict their practical applicability [1-3].

  A well-designed structure and optimized composition are essential for achieving superior EMWA performance. Metal-organic frameworks (MOFs) have emerged as promising precursors for constructing high-performance EMWA materials due to their structural diversity, tunable chemistry, and high porosity [4]. On one hand, metal ions in MOFs can be readily converted into metal compounds embedded within a porous carbon matrix, increasing heterogeneous interfaces and promoting interfacial polarization. On the other hand, the porous structure of MOFs offers abundant active sites for interactions with adjacent ions and molecules, enhancing Maxwell-Wagner polarization and improving the capability of the material to dissipate electromagnetic waves (EMWs) [5]. For instance, Xie et al. developed a bimetallic NiZn-MOF-derived composite that achieved a minimum reflection loss (RL_min) of −56.8 dB at a thickness of 2.4 mm with 20 wt% filler loading [6]. Similarly, Wang et al. synthesized TiO₂/Co/C composites using ZIF-67/MIL-125 as precursors, resulting in an RL_min of −48.97 dB and a maximum effective absorption bandwidth (EAB, RL ≤ −10 dB) of 5.41 GHz at 30 wt% filler loading [7].

  V₂O₃, a prototypical material for the Mott transition (a fundamental electronic interaction and relaxation phenomenon) shows considerable promise as an EMWA material owing to its exceptional dielectric loss properties [8-11]. Vanadium-based compounds can be conveniently prepared via the pyrolysis of V-MOFs, a simple and efficient approach [12]. For example, Zhou et al. constructed hierarchical MOF-derived Co/C@V₂O₃ hollow spheres, achieving an RL_min of −40.1 dB with a wide EAB of 4.64 GHz at 50 wt% filler loading [13].

  Biomass-derived materials are also gaining attention as EMWA candidates, given their low cost, eco-friendliness, large surface area, high porosity, and straightforward synthesis processes [14,15]. For instance, Mou et al. prepared boron-carbon nanosheets derived from coconut shells, achieving an RL_min of −54.24 dB at a thickness of 2.5 mm [16]. Our research group designed a rail-like heterostructure derived from V-MOF/butterfly wings (BWs), demonstrating an ultralow RL_min of −59.3 dB and an EAB_max of 6.56 GHz at 30 wt% filler loading and a thickness of 2.34 mm [17]. These studies suggest that combining MOFs with three-dimensional (3D) porous carbon materials can enhance electron transfer efficiency, prevent nanoparticle agglomeration, and ultimately achieve excellent EMWA performance.

  Additionally, the biomass surface contains numerous functional groups that facilitate the formation of stable crosslinking gel networks through hydrogen bonding. After high-temperature carbonization, biomass-derived carbon aerogels maintain high conductivity while enhancing their porous structure [18]. For example, Wang et al. synthesized CNT/cellulose aerogels via low-temperature carbonization, achieving an RL_min of −43.6 dB and an EAB_max of 7.42 GHz at a thickness of 3.0 mm and a filler loading of 10 wt% [19]. Similarly, Zhou et al. developed an Fe₃O₄/carbonized cellulose composite with a self-assembled porous structure, achieving a wide EAB of 6.72 GHz and RL_min of −42.25 dB at a 20 wt% filler loading [20]. Therefore, combining MOFs with cellulose aerogels provides a pathway toward achieving high EMWA performance at lower filler loadings.

  In today’s rapidly evolving scenario, the surface temperature of inverter electronic equipment under various operating conditions often deviates significantly from ambient levels, resulting in marked radiation contrasts in thermographic imaging. This effect can compromise the thermal infrared stealth capabilities of certain military and industrial targets, highlighting the need for advanced thermal insulation materials to evade infrared detection [21-23]. Aerogels are recognized for their exceptional thermal insulation properties. For example, Gao et al. developed chitosan-derived carbon/rGO aerogels with a thermal conductivity as low as 0.057 W m⁻¹ K⁻¹, while Du et al. synthesized SiC-carbon fiber aerogels with a thermal conductivity of 0.046 W m⁻¹ K⁻¹ [24,25]. These aerogels not only absorb EMWs efficiently but also offer substantial thermal insulation, making them promising candidates for infrared stealth applications. Additionally, the hollow tubular structure of Juncus effusus extends EMW and heat radiation transmission paths while minimizing weight, thereby enhancing the capacity of the material for EMW and heat dissipation.

  In this study, we produced 3D hollow tubular V₂O₃/carbon aerogels (VCA) through freeze-drying and high-temperature pyrolysis. The Juncus effusus cellulose (JEC)-derived carbon aerogels act as conductive loss scaffolds, while anchored V₂O₃@C structures provide dielectric loss. As a result, VCA exhibits excellent EMWA performance, effectively integrating radar stealth and thermal insulation capabilities, paving the way for its practical application in EMWA technology.

  Metal ion agglomeration during MOF carbonization can impact material stability. In this study, we used the cost-effective and widely available Juncus effusus to prepare JEC through a straightforward chemical treatment process. The JEC was then composited with V-MOF, flash-frozen in liquid nitrogen, freeze-dried, and subjected to high-temperature carbonization to produce a 3D porous network. This structure ensured effective dispersion of V₂O₃ within the carbon matrix, enhancing conductive loss and interfacial polarization. The hollow structure and high porosity of VCA improved impedance matching between the absorber and air, facilitating multiple reflections, scattering, and interfacial polarization, which in turn strengthened EMWA performance. Fig. 1a illustrates the straightforward preparation process of VCA composites.

  Figure 1

  

  Figure 1.  (a) Schematic diagram of VCA fabrication process. (b) XRD, (c) XPS spectra of VCA-2.

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  X-ray powder diffraction (PXRD) analysis was used to confirm the chemical composition of the samples. V-MOF was prepared following our previous methodology, with XRD patterns consistent with prior reports, confirming successful V-MOF synthesis (Fig. S1a in Supporting information). The high-temperature pyrolysis product, V₂O₃@C, showed eight characteristic peaks at 24.3°, 33.0°, 36.2°, 41.2°, 49.8°, 53.9°, 63.1°, and 65.2°, corresponding to the (101), (012), (104), (110), (113), (024), (116), (214), and (300) planes of V₂O₃ (JCPDS No. 34−0187), respectively (Fig. 1b). Additionally, diffraction peaks around 22°−26° are attributed to graphitized carbon in CA and V-MOF. The VCA-1, VCA-2, and VCA-3 composites primarily consist of V₂O₃ and carbon, with a decrease in V₂O₃ peak intensity as JEC content increases.

  X-ray photoelectron spectroscopy (XPS) analysis was conducted to determine the surface elements and valence states. As shown in Fig. 1c, the XPS survey spectrum of VCA-2 reveals characteristic peaks at 517.5, 284.8, and 530.4 eV, corresponding to V 2p, C 1s, and O 1s, respectively. The high-resolution V 2p spectrum (Fig. S1b in Supporting information) shows four peaks at 515.6, 517.3, 524.3, and 530.7 eV, attributed to V³⁺ 2p_3/2, V⁴⁺ 2p_3/2, V³⁺ 2p_1/2, and V⁴⁺ 2p_1/2, respectively [26]. In the high-resolution C 1s spectrum (Fig. S1c in Supporting information), peaks at 284.8, 286.4, and 288.3 eV correspond to C—C/C=C, C—O, and O=C—O bonds, respectively [27]. In Fig. S1d (Supporting information), peaks at 530.4, 531.9, and 533.3 eV correspond to the V-O band, oxygen vacancies, and adsorbed water [28]. These results confirm the successful synthesis of V₂O₃@C.

  Scanning electron microscope (SEM) and transmission electron microscopy (TEM) analyses revealed the microstructure and morphology of the samples. As shown in Figs. 2a-c, pure CA exhibits a 3D porous network composed of interconnected JEC tubes, with individual tubular apertures around 2 µm. These tubular structures form a regular array of macropores. In Figs. 2d-f, V₂O₃ microspheres are uniformly distributed on the 3D CA backbone, preserving the porous structure and enhancing heterogeneous interfaces. The porous VCA forms an extensive 3D conductive network that promotes extended electron migration paths and supports multiple EMW scattering and reflection events. Further structural analysis of VCA-2 was carried out using TEM, high-resolution TEM (HRTEM), and selected-area electron diffraction (SAED), as shown in Figs. 2g-i. The HRTEM images reveal a lattice spacing of 0.169 nm, corresponding to the (116) crystal plane of V₂O₃. The SAED patterns display bright rings associated with the (012), (104), (006), and (116) planes of V₂O₃. EDS mapping confirms the uniform distribution of the V, C, and O elements across the carbon surface, further validating the homogenous uniform of V₂O₃ dispersion within the 3D tubular CA structure.

  Figure 2

  

  Figure 2.  SEM of (a-c) CA, (d-f) VCA. (g) TEM and EDS, (h) HRTEM, and (i) SAED of VCA-2.

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  EMWA properties are influenced by complex permittivity (ε_r = ε′ − jε″) and complex permeability (μ_r = μ′ − jμ″), where ε′ and μ′ represent electrical and magnetic energy storage, respectively, and ε″ and μ″ indicate electrical and magnetic energy dissipation, respectively. The electromagnetic parameters and tangent values of the composite materials over the 2−18 GHz range are presented in Fig. S2 (Supporting information). Specifically, Figs. S2a and b show that ε′ and ε″ decrease with increasing frequency for all samples. Notably, pure CA displays the highest ε′ and ε″ values due to its intrinsic conductivity. As the JEC content increases, dielectric constants significantly increase from VCA-1 to VCA-3. Figs. S2d and e reveal μ′ and μ″ values close to 1 and 0, indicating minimal magnetic properties. In Figs. S2c and f, tanδ_μ is nearly 0 across all samples, while tanδ_ε is substantially higher, indicating that dielectric loss primarily contributes to EMW attenuation.

  To assess EMWA performance, two crucial parameters are commonly evaluated: RL_min (Eqs. S1 and S2 in Supporting information) and EAB_max [29]. Fig. 3 displays the calculated RL curves across the 2−18 GHz frequency range with a 12 wt% fill ratio. As shown in Figs. S3a and d (Supporting information), the poor EMWA performance of pure CA arises from the high mobility of carbon carriers, which drives strong dielectric losses. However, excessive conductivity disrupts impedance matching, reducing the capacity of the material to absorb incident EMWs. With V-MOF compositing, VCA samples exhibit significantly improved EMWA performance. Specifically, VCA-1 demonstrates moderate EMWA performance, achieving an RL_min of −11.6 dB and an EAB_max of 3.36 GHz at a 2.5 mm thickness. With increasing JEC content, VCA-2 shows even greater EMWA performance, achieving an RL_min of −63.92 dB at a thickness of 2.0 mm and an EAB_max of 3.68 GHz. Furthermore, the EAB_max expands to 8.24 GHz (9.76−18 GHz) at a 2.44 mm thickness, covering nearly half of the tested frequency band. Owing to increased CA content, VCA-3 experiences impedance mismatch, resulting in an RL_min of −33.4 dB at 3.2 mm and an EAB_max of 4.72 GHz at 1.8 mm. Thus, rational design of EMWA materials in terms of composition and structure enables tailored impedance matching, enhanced interfacial polarization, and multiple reflections and scatterings, collectively achieving outstanding EMWA performance. Fig. S3g and Table S1 (Supporting information) show that the EMWA performance of VCA-2 surpasses that of most previously reported MOF- and carbon-based composite aerogels, demonstrating its superior EMWA properties [30-39].

  Figure 3

  

  Figure 3.  (a) 2D RL plots, (b) 3D RL plots of VCA-2, (c) attenuation constant, (d) polarization percentage, (e) 3D |Z_in/Z₀| plots, and (f) smith charts of VCA-2.

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  To elucidate the dynamic behavior of dielectric loss, we analyzed the percentage contributions of ɛ_c″ and ɛ_p″ using least-squares fitting [40]. The contribution of ɛ_p″ was quantified, and the ratio ɛ_p″/ɛ″ was defined as the polarization percentage. With increasing JEC content, the sphere orbits expand from small to large, and their color shifts from light to dark red. As shown in Fig. 3d and Figs. S5a-c (Supporting information), the polarization loss of VCA-2 and VCA-3 is significantly higher than that of VCA-1, attributed to the increasing number of heterogeneous interfaces. Additionally, the conductive loss of VCA exhibits a gradual increase owing to the rising carrier concentration and enhanced electron hopping.

  Two key factors are crucial for optimizing EMWA performance: impedance matching (Z) and attenuation constant (α) (Eqs. S7 and S8 in Supporting information). Strong attenuation and well-matched Z are essential for enhanced EMWA [41-43]. Additionally, |Z_in/Z₀| significantly influences EMWA performance. Generally, when |Z_in/Z₀| is close to 1, more incident EMWs can penetrate the material. A range of 0.8–1.2 indicates good impedance matching. Fig. 3e and Figs. S5d-f (Supporting information) display the relationship between thickness, frequency, and |Z_in/Z₀|, with the grey-shaded regions representing well-matched impedance. Fig. 3c shows that the four samples exhibit increasing attenuation with frequency, with the order of α values being CA > VCA-2 > VCA-3 > VCA-1. CA and VCA-2 maintain high α values across the frequency range, indicating strong electromagnetic dissipation capacity. VCA-2 demonstrates the best EMWA performance, attributed to a synergistic effect of optimal impedance matching and strong attenuation.

  The Smith chart provides a visual representation of the impedance matching of EMWA materials at various thicknesses. As shown in Fig. 3f and Figs. S5g-i (Supporting information), the normalized input impedance (Z_in) is depicted within the Smith chart, where the point at the center of the circle represents the free-space impedance (Z₀) [44]. When the Z_in of the material approaches the air impedance, it signifies that more EMWs penetrate the material. The green circle indicates good impedance matching, allowing EMWs to enter the material, while areas outside the green circle represent impedance mismatches, preventing EMW entry. For the VCA-2 sample at a thickness of 2.0 mm, most Z_in points fall within the circle, demonstrating optimal impedance matching at this thickness.

  To further elucidate EMW loss mechanisms, finite element simulations at 2, 16.72, and 18 GHz were conducted (Fig. S7 in Supporting information) [45]. The electric field distribution, related to polarization loss, reflects dielectric attenuation capacity. VCA-2 exhibits strong interfacial polarization, especially at the carbon aerogel skeleton and V₂O₃ interfaces, while CA shows weaker interfacial polarization across all frequencies. Surface current density correlates with impedance matching, where lower current density indicates better impedance matching. VCA-2 achieves the best impedance matching at high frequencies. The power loss density distribution, which represents attenuation capacity, indicates that VCA-2 has a larger power attenuation area than CA, confirming the superior EMW loss capability of VCA-2.

  Based on the above results, the EMWA mechanism of VCA is systematically illustrated in Fig. 4a. First, the hollow, reciprocally crosslinked tubular structure of the JEC-derived carbon aerogels serves as a stable backbone for in situ V-MOF growth on the CA surface, preventing spontaneous aggregation of V₂O₃ microspheres. This stable structure promotes the formation of a conductive network while minimizing local impedance mismatches caused by uneven material distribution [46]. Second, the unique porous, hollow structure with high surface area and numerous cavities not only provides a 3D conductive network for electron migration and hopping within the electromagnetic field but also extends the transmission path of incident EMWs [47]. This structure facilitates multiple reflections and scattering, creating abundant opportunities to dissipate EMW energy. Finally, the rich interfacial regions in VCA introduce interfacial, defect, and dipole polarizations that further enhance impedance matching, contributing to improved EMWA performance.

  Figure 4

  

  Figure 4.  (a) Proposed EMWA mechanism of VCA composites, (b) gradient structure design, (c) VSWR, (d, e) 2D and 3D RCS plots, (f) temperature-time plot, and (g) thermal transfer mechanism of VCA-2.

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  In addition to intrinsic electromagnetic properties, structural design can further enhance EMWA performance, making these materials more suitable for practical applications. A gradient structure design, which broadens EAB, was simulated using CST Studio Suite 2022 [48]. For single-layer materials, thickness is typically optimized in accordance with the λ/4 cancellation theory. Here, a gradient structure with layer thicknesses of 2, 2.5, and 2 mm (from top to bottom) was designed. Upon tuning the thickness of each layer, multiple λ/4 interactions occur between adjacent coatings, generating multiple EMWA peaks and broadening the EAB. This gradient structure achieved an EAB_max of 12.56 GHz, nearly covering the X and Ku bands (Fig. 4b). The voltage standing wave ratio (VSWR) further confirmed impedance matching, with the VSWR value of the gradient structure being closer to 1 compared to that of the single-thickness structure (Fig. 4c), indicating superior impedance matching and improved broadband EMWA performance [49].

  Radar cross-section (RCS, Eq. S9 in Supporting information) is an essential parameter for evaluating EMWA and reflection capabilities in real-world applications. To assess the potential of EMWA materials in practical scenari os, RCS simulations were conducted on a perfect electrical conductor (PEC) substrate with and without CA and VCA-2 coatings using CST [50]. The substrate dimensions were 20 × 20 cm², with incident EMWs along the Z-axis. Figs. 4d and e, and Fig. S8 (Supporting information) show the 2D and 3D RCS simulation results and RCS data in polar coordinates for the unmodified PEC substrate, CA, and VCA-2. In the 2D plot, RCS values are displayed over a range of −90°−90°. The 3D RCS plot visualizes reduced EMW scattering signals for CA and VCA-2 coatings on the PEC substrate, while the PEC alone shows high EMW scattering. In the 2D RCS plot, the maximum RCS reduction values for CA and VCA-2 are 15.25 and 29.40 dB m², respectively, indicating that VCA-2 exhibits excellent radar attenuation performance, suitable for practical applications.

  Superior thermal infrared stealth properties are also critical for EMWA materials, as they can protect electronic devices and military assets from infrared detection [51]. To demonstrate the infrared stealth capabilities of VCA-2, a sample was placed on a 160 ℃ thermal platform for 30 min, and its real-time infrared images and surface temperatures were recorded using thermal infrared imaging equipment (Fig. S9 in Supporting information). As shown in Fig. 4f, the surface temperature of VCA-2 increased slowly, reaching a maximum of 79.2 ℃, significantly lower than that of the heating platform. Thermal transfer typically occurs via conduction, convection, and radiation. As illustrated in Fig. 4g, the porous structure of VCA-2 delays heat transfer from the thermal platform. Its large specific surface area promotes heat dissipation, while the abundant air pockets within the porous structure slow thermal conduction, keeping the surface temperature low. Overall, the 3D porous structure provides VCA-2 with excellent thermal insulation properties, making it well-suited for high-temperature or harsh environments.

  In summary, VCA composites were synthesized by in situ growth and high-temperature carbonization. By optimizing the JEC to V-MOF ratio, an RL_min of −63.92 dB and an EAB_max of 8.24 GHz were achieved with a minimal 12.0 wt% filler loading. High-temperature carbonization enabled the formation of an extensive conductive network and numerous defects, promoting strong polarization responses. The unique hollow and porous structure of carbon aerogels enhanced EMW scattering, reflection, and impedance matching. Additionally, the high surface area, multiple heterogeneous interfaces, and 3D porous structure of VCA confer lightweight, superior EMWA, and excellent thermal insulation performance. The evaluation of far-field behavior using RCS simulations shows that VCA-2 achieves an impressive radar attenuation, with a maximum RCS reduction of 29.4 dB m². The gradient metamaterial design further broadens the EAB to 12.56 GHz, nearly covering X and Ku bands. This work introduces an innovative design for high-performance, lightweight, and multifunctional EMWA materials, paving the way for practical applications.

  CRediT authorship contribution statement

  Jingyuan Luo: Writing – review & editing. Liping Wu: Writing – review & editing, Writing – original draft. Jinxi Yan: Methodology. Xintong Lv: Conceptualization. Yuqi Luo: Formal analysis. Wei Jiang: Software. Zhiqiang Xiong: Data curation. Anqi Ni: Software. Chongbo Liu: Writing – review & editing, Investigation, Funding acquisition. Renchao Che: Writing – review & editing.

  Acknowledgments

  This work was supported by the National Natural Science Foundation of China (No. 22265021), the Aeronautical Science Foundation of China (No. 2020Z056056003). The authors would like to thank Shiyan Jia Lab (https://www.shiyanjia.com) for assistance with XPS analysis and MJEditor (http://www.mjeditor.com) for their linguistic assistance during the preparation of this manuscript.

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2025.111065.

  
  
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  Figure 1  (a) Schematic diagram of VCA fabrication process. (b) XRD, (c) XPS spectra of VCA-2.

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  Figure 2  SEM of (a-c) CA, (d-f) VCA. (g) TEM and EDS, (h) HRTEM, and (i) SAED of VCA-2.

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  Figure 3  (a) 2D RL plots, (b) 3D RL plots of VCA-2, (c) attenuation constant, (d) polarization percentage, (e) 3D |Z_in/Z₀| plots, and (f) smith charts of VCA-2.

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  Figure 4  (a) Proposed EMWA mechanism of VCA composites, (b) gradient structure design, (c) VSWR, (d, e) 2D and 3D RCS plots, (f) temperature-time plot, and (g) thermal transfer mechanism of VCA-2.

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