Mechanical-dielectric optimized graphene aerogels with strain-tunable microwave attenuation and shielding functions

Abstract

The widespread use of electronic devices significantly improves human activities but also raises concerns about microwave radiation pollution, creating a demand for materials that can effectively attenuate or shield against this radiation. To address this, we have developed innovative graphene aerogels (SCGAs) that incorporate SiC nanowires and carbon nanotubes, featuring a nature-inspired bridge-lamellar microstructure. These aerogels are optimized for both dielectric and mechanical properties, allowing for strain-tunable microwave attenuation and shielding functions. Specifically, our SCGAs demonstrate excellent microwave attenuation, with a minimum reflection loss of −51.6 dB and an effective attenuation bandwidth of 7.62 GHz, and can shift to a shielding mode with a shielding effectiveness of approximately 50.1 dB when compressed to 80%. This strain-responsive behavior remains stable over time, showing minimal degradation even after 1000 compression cycles, indicating exceptional long-term durability. Additionally, the strain-gradient strategy allows for customized low-reflection shielding applications, and the ceramic/carbon composition ensures superior resistance to harsh environmental conditions. Our research introduces a novel solution that provides effective microwave radiation protection across a broad frequency range and holds promise for various high-tech applications.

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1. Introduction

The rapid advancement of electronic technologies in communication systems, wireless networks, and mobile devices greatly enhances our lives but also raises concerns about microwave radiation pollution.^1,2 This invisible radiation can disrupt electronic devices and is associated with serious health risks, including DNA damage and a higher likelihood of cancer.^3,4 In response, a variety of materials have been developed to address these issues, including highly conductive materials such as metals,⁵ carbon-based materials,^6,7 magnetic materials,⁸ dielectric ceramics,⁹ metal oxides,¹⁰ transition metal carbides,^1,11 and composites.^12–14 These materials can either absorb microwave energy to reduce interference or act as barriers to shield against unwanted penetration. However, their performance remains unchangeable post-manufacture, making them less effective in the face of the evolving microwave communication conditions.¹⁵ There is a growing need for intelligent microwave-responsive materials that can dynamically adjust to changing microwave environments, especially in smart homes, industrial settings, and passive communication systems.¹⁶

Materials that can adjust their electromagnetic properties in response to external stimuli offer the potential to switch between functions such as microwave attenuation, transmission, or shielding.^17–21 Mechanical stimuli are particularly advantageous due to their ease of use and low energy consumption, providing greater operational flexibility. For example, Shui et al. developed carbon aerogels that modulate conductivity through compression and decompression, achieving microwave shielding effectiveness (SE) ranging from about 1.5 dB in the “off” state to 25.5 dB in the “on” state.²¹ Similarly, Koga et al. engineered a carbon aerogel that changes its microwave attenuation from −40 dB without strain to allow transmission when compressed by 80%.²² While there has been progress with single-function switches and modulators, the development of materials that can switch between attenuation and shielding functions is still unexplored. Achieving an efficient and direct transition between these functions through mechanical strain requires materials with optimal compressibility, elasticity, adjustable conductivity, and inherent electromagnetic loss properties to ensure both flexibility and effective performance.

Graphene is renowned for its excellent microwave shielding capabilities, thanks to its superior electrical conductivity, especially in its densely stacked film form.²³ When converted into a porous aerogel and combined with functional additives, graphene also shows significant microwave attenuation due to enhanced dielectric or magnetic losses.^24,25 This adaptability in graphene aerogels presents a promising approach for switching between microwave attenuation and shielding by transitioning from porous to dense structures through strain control. However, challenges persist in maintaining material integrity under varying strains and managing the balance between structural and electromagnetic properties. To address these challenges, we propose the incorporation of SiC nanowires as functional additives in combination with porous graphene aerogels. From a materials perspective, SiC is an ideal dielectric material that offers the potential to optimize dielectric loss, thereby enhancing the microwave attenuation properties of porous graphene aerogels.¹³ From a structural perspective, nanowires can reinforce and connect graphene sheets, facilitating the formation of bridge-lamella patterns when assembled into a three-dimensional structure. This configuration mimics the structure of the Thalia dealbata stem, which is recognized for its high resilience and lightweight properties.²⁶ To enhance the tight and orderly integration of SiC nanowires with graphene sheets, we propose incorporating carboxyl-modified carbon nanotubes, which can interact with SiC nanowires at the nanoscale and form strong hydrogen bonds with graphene sheets, thereby improving the overall structural order within the porous graphene aerogels.^7,27 Carbon nanotubes are also known for their exceptional conductivity, which contributes significantly to conduction losses in microwave attenuation. We anticipate that this approach will significantly enhance the mechanical resilience of graphene aerogels and optimize their tunable electromagnetic properties under varying compressive strains, potentially allowing effective switching between microwave attenuation and shielding.

Therefore, we employ bidirectional freeze-casting to architecturally integrate SiC nanowires, carboxyl-modified nanotubes, and graphene sheets into a bridge-lamellar microstructure, aiming to optimize both the dielectric and mechanical properties of graphene aerogels. The resultant SiC NW/CNT/Graphene aerogels (SCGAs) exhibit superior microwave attenuation capabilities, characterized by a minimal reflection loss (RL_min) of −51.6 dB at a thickness of 2.05 mm, and a broad effective attenuation bandwidth (EAB) of 7.62 GHz at 2.45 mm, attributes to the enriched dielectric loss mechanism. The unique bridge-lamellar configuration of the aerogels significantly improves their mechanical compressibility, elasticity, and strain-sensitive conductivity. Upon compression, varying the strain from 0% to 80%, our aerogels undergo a functional transition from a microwave attenuation (MA) state to a microwave shielding (MS) state, achieving an average shielding effectiveness (SE) of 50.1 dB in the X band. The strain-tunable dynamic MA/MS performance of the SCGAs shows minimal degradation even after 1000 compression cycles, highlighting their exceptional long-term compressive cyclic stability. Strategically arranging the aerogels in a gradient from lower to higher strain achieves low-reflection shielding, advancing green shielding solutions. This study introduces a novel approach for achieving a dynamic switch between microwave attenuation and shielding functionalities, while the ceramic/carbon material system further exhibits harsh environmental tolerance, rendering it particularly advantageous for applications in adverse conditions.

2. Results and discussion

2.1 Fabrication and structural characterizations

The fabrication process of the SiC NW/CNT/Graphene aerogels is illustrated in Fig. 1a and detailed provided in Section S1, ESI.† A mixture of SiC nanowires, CNTs, and graphene oxide (GO) suspensions was used to prepare the aerogels through bidirectional freeze-casting, freeze-drying, and subsequent carbonization. To achieve a homogenous and stable suspension, carboxyl-modified CNTs were initially mixed with the GO suspension. The carboxyl functionalities engage in hydrogen bonding with the GO surface groups, facilitating an effective dispersion of CNTs. As shown in Fig. S1a and b,† after dispersing CNTs into the GO solution, the zeta potential of the CNT/GO suspension decreases from −30.3 mV to −42.4 mV, confirming the effective dispersion of CNTs and indicating increased suspension stability. Simultaneously, the viscosity of the suspension decreases from 135 mPa S for GO to 85 mPa S for CNT/GO, which is attributed to the formation of hydrogen bonds between carboxyl-modified CNTs and GO, reducing the agglomeration of GO nanosheets. The improved stability of the CNT/GO suspension is particularly advantageous for the subsequent dispersion of SiC and facilitates the successful execution of directional freezing. Following this, varying quantities of SiC nanowires were dispersed into the suspension, aided by ultrasonic dispersion to prevent agglomeration. As depicted in Fig. S1a and b,† the zeta potential of the SiC/CNT/GO suspension is −31.9 mV, indicating a strong level of stability. Its viscosity is 118 mPa S, which, while slightly higher than that of CNT/GO, has not increased significantly and remains lower than that of pure GO. This suggests excellent dispersion and fluidity of the solution. The stability is primarily due to the repulsive interactions between carboxyl-modified CNTs and the residual functional groups on GO, which effectively maintain the dispersion of SiC nanowires. Additionally, the physical entanglement of CNTs with SiC nanowires further enhances the dispersion, promoting a stable and uniform mixed suspension. In contrast, the SiC/GO suspension, prepared without the prior addition of CNTs, exhibits the weakest zeta potential (−24.3 mV) and the highest viscosity (164.5 mPa S). This behavior is attributed to the unmodified SiC nanowires, which not only fail to mitigate GO agglomeration but are also hard to disperse, resulting in poor suspension quality. Fig. S1c† also shows these four suspensions after shaking, where it is evident that the CNT/GO and SiC/CNT/GO suspensions exhibit minimal wall adhesion and good fluidity. In contrast, the GO suspension displays noticeable wall adhesion due to the tendency of multilayered GO to aggregate. The SiC/GO suspension is the most turbid and exhibits the poorest fluidity. The excellent dispersion and fluidity of the CNT/GO and SiC/CNT/GO suspensions are crucial for achieving an ordered structure in the subsequent bidirectional freeze-casting process. In this stage, a temperature gradient applied both radially and axially to the suspension guides the formation of ice crystals in a layered configuration, which acts as a scaffold for the aerogel structure. The subsequent freeze-drying removes the ice scaffold and yields a lamellar, bridge-connected biomimetic architecture. The following carbonization reduces GO to graphene, resulting in the formation of the final SCGAs. According to the mass ratio of SiC NW : CNT : GO in the prepared suspension, which is 2 : 3 : 2, 4 : 3 : 2, and 6 : 3 : 2, the obtained aerogels are named SCGA-1, SCGA-2, and SCGA-3, respectively. For comparison, CNT/graphene aerogels (CGAs) without SiC nanowires, alongside pristine graphene aerogels (GAs) are prepared using the same procedure.

Fig. 1 (a) Illustration of the fabrication process of the SCGAs. (b) Optical image showing an ultralight SCGA standing on a flower. (c–e) SEM images of sample SCGA-2. (f) XRD pattern and (g) Raman spectra of CGAs and SCGAs. (h) and (i) EDS mapping of a single piece of the hybrid graphene sheet.

The fabricated SCGAs are characterized by their ultralight nature, with densities not exceeding 25 mg cm⁻³, even in the case of SCGA-3, which contains the highest concentration of SiC nanowires. This attribute is visually demonstrated by the ability to place a sample of SCGA-2, with a density of 18 mg cm⁻³, atop a delicate flower without causing damage, as depicted in Fig. 1b. The ultralightweight advantage of the SCGAs is mainly due to their highly porous microstructure. To delve deeper into the microstructural characteristics of the SCGAs, the morphology of all aerogels was investigated using scanning electron microscopy (SEM). The pristine GAs display a disordered morphology, whereas the inclusion of CNTs results in a more organized microstructure within the CGAs, though they lack regular lamellar walls and bridge connections (Fig. S2†). This occurs because the good dispersion and fluidity of the GO/CNT suspension enable the ice to grow smoothly in a layered fashion, compressing the GO sheets and CNTs to align along the direction of ice crystal growth. However, the insufficient length and inherent flexibility of the CNTs, combined with the low viscosity of the solution, result in inadequate resistance during the ice crystal growth process, leading to slippage. Furthermore, during the subsequent drying process, some microstructures may be compromised, resulting in a bridge-lamellar structure that lacks uniformity and definition. The incorporation of SiC nanowires induces the formation of a well-layered, bridge-connected lamellar structure, which becomes increasingly regular with a reduction in interlayer spacing as the content of SiC nanowires increases (Fig. 1c and S3†). This phenomenon can be attributed to the fact that, under the conditions of good dispersion and fluidity in the GO/CNT/SiC suspension, the SiC nanowires, being longer, thicker, and sufficiently stiff, exhibit distinct behaviors during the bidirectional freeze-casting process. Some of the SiC nanowires, along with the CNTs, become embedded within the GA sheets, forming layered SiC/CNT/GA hybrid structures as they are compressed by the growing ice columns. Meanwhile, other SiC nanowires, due to their greater length (50–500 nm) and weight, which exceed the typical interlayer spacing, are not easily compressed by the ice columns and instead intercalate between the layers, creating “bridge” structures. This can be observed in the SEM images in Fig. S3,† where an increase in SiC nanowire content leads to more nanowires intercalating between the layers. These SiC nanowires also tend to intertwine with CNTs, forming additional bridging pillars. The bridge-lamellar structure of the aerogels is detailed and illustrated in Fig. 1d and e of sample SCGA-2, showcasing the hybrid graphene sheets predominantly aligned in parallel, continuous layers perpendicular to the freezing direction, serving as lamellar walls. Meanwhile, the residual SiC nanowires and CNTs intertwine, forming a bridge-connected framework between these layers. This intricate microstructure provides compression space through lamellar walls and added support through bridge connections, significantly enhancing SCGAs' mechanical elasticity. To investigate the intricacies of the SCGAs at a finer scale, the microstructure of the individual hybrid graphene sheets was presented in Fig. 1h and i, that SiC nanowires and CNTs intricately entwined within the graphene sheets, reminiscent of structural reinforcements akin to biological veins. Elemental mapping corroborates these findings, aligning with the observed material composition distribution. This exquisite hybrid structure indicates that high-strength SiC nanowires and CNTs could enhance the inherent stiffness of individual graphene sheets, laying a foundation for the robustness of the overall structure. Furthermore, the realization of a tight and uniform integration among these three materials is conducive to the control of the aerogels' dielectric properties, thus promising to realize simultaneous optimization of the dielectric and mechanical.

The compositional and phase features of the aerogels were analyzed by XRD and Raman spectroscopy. Crystallographic information presented in Fig. 1f reveals the presence of graphene phases in CGAs, confirming the effective thermal reduction from GO into graphene. Upon integration of SiC nanowires, the SCGAs exhibit five distinct peaks and stacking faults characteristic of β-SiC (JCPDS No. 73-1708), with their intensity escalating with the quantity of SiC nanowires incorporated. Raman spectroscopic analysis reveals two pronounced peaks across all aerogel samples at 1350 cm⁻¹ (D-band) and 1590 cm⁻¹ (G-band), indicating the presence of defective carbon structures and a crystalline graphene phase, respectively (Fig. 1g). Moreover, the observation of a 2D band in each sample points to the presence of multilayer graphene sheets.^28,29 Notably, the Raman spectra of the SCGAs also display additional peaks at 796 cm⁻¹ and 968 cm⁻¹, corresponding to the transverse optical (TO) and longitudinal optical (LO) phonon modes of β-SiC, thereby providing further insight into the phase composition of the SiC nanowires.³⁰ To this end, the results from XRD and Raman spectroscopy further confirm the composition of the three components in the SCGAs. The effect of this composition system on mechanical and dielectric properties will be discussed in detail in the following sections.

2.2 Elasticity and strain-tunable conductivity

Compressibility and mechanical elasticity are essential for using strain to modulate the aerogels' electromagnetic properties. Here, the elasticity of our aerogel is demonstrated in Fig. 2a, where sample SCGA-2 quickly recovers from significant compressive deformation. This resilience is attributed to the porous lamellar structure, which provides sufficient space for compression, while the SiC nanowires, CNTs, and hybrid graphene sheets act as bridges to accommodate and release stress, leading to reliable compressibility and rapid rebound (Fig. 2b).^31,32 To systematically investigate the effects of bridge-connected lamellar microstructure and SiC nanowire contents on mechanical performance, the stress–strain (σ–ε) curves of CGA and SCGAs under different compression strains (40%, 60%, and 80%) were measured. Sample CGA exhibits the weakest compressive stress of 1.4 kPa at 80% strain and is unable to fully recover to its initial height (Fig. S4a†). This is because, although CNTs themselves have a high modulus, their low aspect ratio and low rigidity limit their contribution to enhancing the elasticity of the aerogel. Meanwhile, the relatively disordered microstructure of CGA also fails to provide effective assistance in improving mechanical performance. After incorporating different amounts of high-stiffness SiC nanowires, the maximum stress under 80% strain of SCGAs increases to 15.4 kPa for SCGA-1, 25.8 kPa for SCGA-2, and 45.2 kPa for SCGA-3, respectively (Fig. S4b, c† and 2c). The σ–ε curves of these SCGAs present the typical three characteristic distinct stages of elastic porous materials, including the linear elastic region at low strain, plateau region for medium strain, and densification region under high strain.³³ Moreover, the σ curves could return to the original points after unloading for each ε, especially after the 80% strain, indicating superior elasticity. When subjected to a long-time loading-unloading fatigue process, the sample SCGA-2 shows highly reliable elasticity as the σ–ε curves retain a closed loop shape and over 80% σ retention after 1000 cycles of 60% compression ε (Fig. 2d and e). Such excellent mechanical properties confirm that the reinforcement of SiC nanowires and CNTs on bridging and facilitated ordered bridged-lamellar structure effectively contributes to the super elasticity and reliability of SCGAs.

Fig. 2 (a) Photographs illustrating the excellent elasticity of the SCGAs. (b) Elastic mechanism illustration of SCGAs. (c) Compressive stress–strain (σ–ε) curves of SCGA-2 at 40%, 60%, and 80% strain. (d) Cyclic compressive σ–ε curves and (e) stress retention over 1000 cycles of SCGA-2 at 60% strain. (f) Conductivity variation of SCGA-2 at different compressive strains. (g) Illustration of the strain-tunable conductivity mechanism based on the microstructure evolution of SCGA-2. (h) Adjustment of circuit resistance by compressing and releasing the aerogel to tailor the brightness of the LED (i). The real-time relative resistance change (ΔR/R₀) recording of the aerogel under different strains.

The reliable elasticity of SCGAs promises to enable their strain-tunable conductivity. The high conductivity of graphene sheets and CNTs in SCGAs increases with compression as interlayer spacing decreases and connections between graphene sheets and CNTs improve. Therefore, we conducted a strain-conductivity test for sample SCGA-2 under different compression strains to investigate the relationship between the aerogel and compression strain. As shown in Fig. S5a,† both the top and bottom surfaces of the aerogel are covered with copper foil, which is adhered to the aerogel surface using silver paste to ensure good contact. The assembled aerogel is then placed in the center of the sample stage of a mechanical testing machine, with the copper foil at both sides connected to a resistance meter. This setup allows for the measurement of the aerogel's resistance under different strain levels, from which the conductivity of the aerogel under each strain can be calculated (Fig. S5b†). As shown in Fig. 2f, the uncompressed SCGA-2 exhibits a conductivity of 2.1 S m⁻¹, which significantly increases with compression. When the strain increases to 20%, 40%, 60%, and 80%, the conductivity of the aerogel increases to 3.1, 5.1, 8.4, and 15.4 S m⁻¹, respectively. As depicted in Fig. 2g, the uncompressed aerogel comprises stacked hybrid graphene layers, interconnected by SiC nanowires and CNTs. The relatively large interlayer spacing (20–50 μm) limits the formation of conductive pathways, resulting in current primarily flowing through individual graphene layers, which leads to relatively low conductivity. Upon compression, the interlayer spacing is significantly reduced, thereby increasing the contact area between adjacent graphene layers. Furthermore, previously unconnected CNTs become interconnected, substantially enhancing the conductive pathways, which markedly improves the aerogel's overall conductivity. Different strain levels induce varying degrees of interlayer spacing and corresponding changes in the conductive pathways. Fig. 2g illustrates the microstructure of the aerogel under 60% compressive strain, highlighting the effective contact between graphene layers. In comparison, at 40% strain (Fig. S6a†), the interlayer spacing is slightly larger, while at 80% strain (Fig. S6b†), the layers are nearly fully compacted. These variations directly correspond to the changes in conductivity observed under different strain conditions. Since conductivity is a key parameter affecting the microwave response of materials, changes in the conductivity of aerogels under different strains are promising to enable variations in their electromagnetic properties.³⁴ Upon strain release, the bridge-lamellar microstructure of the aerogel returns to its original form, with only minor changes in interlayers. Notably, after releasing from a high strain (80%), some of the SiC nanowires or hybrid graphene sheets connected between layers may slightly bend, but the majority fully recover their initial morphology. This is attributed to the excellent elasticity and structural stability of the bridge-lamellar architecture, further highlighting the importance of the synergistic optimization of material composition and structure in enhancing performance. To visually demonstrate the strain-tunable conductivity of the aerogel, we constructed a basic circuit where the aerogel acts as a switch controlling an LED lamp (Fig. 2h). Compressing the aerogel, which increases its conductivity, caused the LED to brighten; releasing the pressure that dimmed the light once more. This conductivity variation with strain is attributable to microstructural changes within the aerogel during compression. We further validated the stability of the strain-tunable conductivity by conducting real-time relative resistance change (ΔR/R₀) recordings on the assembled aerogel under continuous application of different strain levels, with each strain level maintained for a period of time. The results, as shown in Fig. 2i, demonstrate that the ΔR/R₀ of aerogel remains stable across various strain levels. This stability in strain-tunable conductivity ensures reliable dynamic adjustments in the aerogel's electromagnetic properties, supporting its potential for practical applications.

2.3 Microwave attenuation performance

The performance of microwave attenuation (MA) materials is determined by their electromagnetic parameters, influenced by microstructure and material composition. In the stress-free state, the porous structure of SCGA results in a lower conductivity, which means it can only provide limited conductive loss for microwaves. Therefore, the introduction of functional additives can provide additional dielectric loss mechanisms for the aerogel, offering opportunities for optimizing MA performance. To investigate the MA performance of SCGAs, their complex permittivity (ε_r = ε′ – jε′′) across the frequency range of 2 GHz to 18 GHz was measured as detailed in Section S2, ESI.† The real component, ε′, quantifies the material's capacity to store microwave energy, while the imaginary component, ε′′, reflects its ability to dissipate this energy. With the increasing content of SiC nanowires, both ε′ and ε′′ show an upward trend (Fig. 3a and b). This enhancement is primarily attributed to the introduction of numerous heterogeneous interfaces and defect dipoles by the additional SiC nanowires, which significantly boost polarization. Concurrently, the bridge-lamellar structure of the SCGAs becomes more compact, as the SiC nanowires intertwine with CNTs to form additional bridge connections. This results in increased interlocking among stacked GA sheets and CNTs, creating more three-dimensional conductive pathways and ultimately enhancing electron migration throughout the structure. Specifically, the increase in ε′ indicates an enhanced ability of the aerogel to store electromagnetic energy, primarily due to increased polarization.¹² The presence of high-dielectric CNTs and graphene sheets, combined with low-dielectric SiC nanowires, creates numerous heterogeneous interfaces. According to Maxwell–Wagner polarization theory, the significant dielectric differences at these interfaces lead to the accumulation of free electrons during migration, thereby markedly enhancing interfacial polarization.³⁵ Notably, the frequency dispersion (FD) of ε′ values becomes more pronounced with the incremental addition of SiC nanowires. As indicated by the dielectric spectrum, FD based on ε′ values is typically caused by materials exhibiting interfacial polarization under dielectric relaxation conditions.³⁶ To achieve effective broadband microwave attenuation, a strong FD in the permittivity, particularly in the real part ε′, is desirable, characterized by a noticeable decrease ε′ with increasing frequency.^12,36Fig. 3c illustrates the intensity of the FD effect using Δε′ values. As SiC nanowires are introduced and increased, the Δε′ of the aerogels rises, demonstrating that the heterogeneous interfaces formed by SiC nanowires, CNTs, and graphene sheets in SCGAs effectively induce interfacial polarization relaxation. Additionally, the stacking faults or twins in SiC can create defect dipoles, leading to dipole polarization relaxation. The structural defects in graphene sheets and the retained oxygen-containing groups also contribute to enhancing dipole polarization, which further contributes to the increase in ε′.^35,37,38 The increase in ε′′ is primarily attributed to the enhanced conductive loss within the aerogel. Although SiC nanowires themselves do not possess high electrical conductivity, SEM images indicate that as the quantity of SiC nanowires increases, the bridges within the SCGAs' bridge-lamellar structure become more numerous, and the interlayer spacing decreases. This results in more interconnected graphene sheets and CNTs, leading to the formation of additional conductive pathways and consequently, increased conductive loss. The frequency dispersion (FD) phenomenon of ε′′ also intensifies with the increasing content of SiC nanowire, which can be attributed to the enhanced conductive loss. The dielectric loss tangent (tanδ = ε′′/ε′), illustrated in Fig. 3d, serves as an indicator of the aerogels' intrinsic dielectric dissipation efficiency. The ascending trend from CGAs to SCGAs with increased SiC nanowires loading signifies amplified dielectric loss capabilities. Generally, dielectric loss is predominantly governed by polarization relaxation and conduction loss.³⁹ The Cole–Cole plots (ε′′ versus ε′) for all samples, as shown in Fig. 3e, delineate the relationship between energy dissipation and storage capacity, with semicircular arcs indicative of the Debye relaxation process and linear trajectories signaling conduction loss.⁴⁰ The presence of more semicircular arcs from SCGA-1 to SCGA-3 corroborates intensified polarization relaxation loss with increased SiC nanowires integration, enhancing the total dielectric loss capability. Moreover, the extended linear region at the tail end of the Cole–Cole plots suggests an increase in conductive losses, which, in conjunction with polarization relaxation losses, contributes to the overall enhancement of dielectric loss.

Fig. 3 The frequency dependent electromagnetic parameters include (a) ε′ (b) ε′′, (c) Δε′ and Δε′′, (d) tan δ, and (e) Cole–Cole curves for CGA and SCGAs. The (f) 3D plots, (g) 2D contours, and (h) 2D plots of RL values versus frequency and thickness for sample SCGA-2. Frequency-dependent (i) attenuation constant α, and (j) impedance matching ratio Z_in/Z₀ of CGA and SCGAs. (k) α, Z_in/Z₀, and RL curves comparison of SCGA-2. (l) Comparison of MA performance considering the EAB and thickness of SCGA-2 with reported graphene/carbon-based aerogels. (m) RCS simulation model. 3D RCS simulation chart of (n) PEC and (o) SCGA-2. (p) RCS simulated values of PEC, CGA, and SCGAs with the scanning angle range from −60° to 60°. (q) Schematic illustration of the MA mechanism of SCGAs.

To evaluate the MA performance of the SCGAs, their Reflection Loss (RL) values were calculated based on the metal backplane model and transmission line theory in Section S2, ESI.† The RL values across varying frequencies and thicknesses for all aerogel samples are depicted in 3D plots and 2D contours in Fig. 3f and g, as well as Fig. S7.† CGA exhibits modest MA performance, with the minimum RL (RL_min) not surpassing −20 dB and a narrow effective attenuation bandwidth (EAB) of 3.86 GHz throughout the assessed thickness and frequency spectrum. With the introduction of a smaller quantity of SiC nanowires, the MA performance of SCGA-1 exhibits an improvement, though not significantly, with an RL_min of −35.7 dB and EAB of 6.38 GHz due to the enhanced polarization loss. SCGA-2, with optimized dielectric and polarization loss, achieves an RL_min of −51.6 dB at a thin 2.05 mm thickness, indicating 99.999% microwave attenuation. Its EAB extends to 7.62 GHz at a 2.45 mm thickness, making it suitable for broadband MA applications. However, despite possessing the highest dielectric loss constant, SCGA-3's MA performance did not improve further but slightly decreased, indicated by the RL_min of −17.6 dB and EAB of 7.52 GHz, a consequence of impedance mismatch. Fig. 3h and S8† illustrate the 2D RL plots for these aerogels across different thicknesses ranging from 2 to 3.5 mm, as well as the corresponding thicknesses at which RL_min and EAB are achieved. It is evident that as the sample thickness increases, the attenuation peaks of all aerogels shift toward lower frequencies. This shift is caused by the quarter-wavelength cancellation effect, where lower frequencies have longer wavelengths, necessitating greater thickness to achieve the corresponding quarter-wavelength destructive interference.

The optimal MA performance of SCGA-2 can be explained by the balance between the inherent attenuation constant (α) and impedance matching ratio (Z_in/Z₀), which were calculated by the eqn (S4) in Section S2, ESI.† The Z_in/Z₀ value represents the efficiency with which microwaves can penetrate the material upon incidence, while the α value indicates the material's intrinsic capacity to attenuate microwaves.⁴¹ As shown in Fig. 3i, the α values progressively increase with the addition of SiC nanowires, consistent with the trends observed in complex permittivity and loss tangent. This increase is attributed to enhanced polarization and conduction losses. Among the samples, SCGA-3 exhibits the highest α values, followed by SCGA-2, indicating that SCGA-3 theoretically possesses greater microwave attenuation capability. However, SCGA-3 does not achieve optimal performance, which can be attributed to impedance matching considerations. Fig. 3j illustrates the Z_in/Z₀ of the aerogels at a thickness of 2.45 mm relative to free space. Theoretically, a Z_in/Z₀ value close to 1 facilitates maximum microwave penetration into the material. It is evident that CGA and SCGA-1 exhibit Z_in/Z₀ values significantly below 1 at low frequencies and above 1 at high frequencies, indicating poor impedance matching, which corresponds to their mediocre MA performance. In contrast, SCGA-3 displays Z_in/Z₀ values consistently below 0.85 across the entire frequency range, suggesting suboptimal impedance matching. Consequently, despite its highest α values, SCGA-3 does not achieve the best MA performance. SCGA-2, however, maintains a Z_in/Z₀ ratio within the range of 0.8 to 1.2 over a broad frequency spectrum, achieving the most favorable impedance matching among all samples. Additionally, its attenuation constant is second only to that of SCGA-3, resulting in the most effective microwave attenuation. Furthermore, SCGA-2's RL_min of −51.6 dB at 17.2 GHz and 2.05 mm thickness can be attributed to the optimal coupling of α and Z_in/Z₀. As depicted in Fig. 3k, the α and Z_in/Z₀ curves of SCGA-2 intersect at a specific point at 2.05 mm thickness, corresponding to the frequency at which RL_min occurs, 17.2 GHz. This observation further highlights the importance of optimizing the dielectric properties to achieve perfect balancing between loss mechanisms and impedance matching for MA materials. Hence, SCGA-2 not only demonstrates exemplary MA performance among the aerogels evaluated in this study but also stands competitive in comparison to similar research including graphene aerogels and their hybrids,^{24,25,42–47} as well as other carbon-based aerogel attenuators^48–51 (Table S1†). In a further comparative illustration, Fig. 3l integrates the EAB and sample thickness to afford a comprehensive assessment of the SCGAs' MA efficiency. Here, SCGA-2 distinguishes itself by achieving an impressive EAB at a reduced thickness relative to similar studies, underscoring its exceptional MA efficiency.

To further evaluate SCGAs for MA applications in actual environments, such as target profiling and environmental conditions, real far-field scattering is the most important characteristic to be considered. In this work, the radar cross section (RCS) of all samples is simulated using HFSS software, providing their real far-field response under realistic conditions for monostatic or bistatic radar systems (as detailed in Section S2, ESI†). Here, a square simulation model comprising an upper aerogel attenuator and a bottom PEC substrate was established and positioned on the XOY plane (Fig. 3m). The thickness of the attenuator was set at 3 mm, as all samples demonstrate optimal performance at slightly less than this thickness. Additionally, at a thickness of 3 mm, the frequency points corresponding to RL_min for all samples are within the 8–12 GHz range, making 10 GHz the chosen frequency for RCS simulation. The plane wave propagates vertically along the Z-axis into the model, with phi (θ) defined as the scanning angle. In general, the smaller RCS value represents the more efficient far-field microwave attenuation performance of MA materials. The simulated 3D RCS results of PEC and all the aerogels are shown in Fig. 3n and o, as well as Fig. S9.† As depicted in Fig. 3n, the RCS value of the single PEC plate is the largest among all models, exhibiting strong scattering signals, which is clearly unfavorable for MA applications. The scattering signal of the CGA is slightly reduced compared to PEC but still far from ideal (Fig. S9a†). In contrast, SCGAs can significantly reduce the scattering signal, with SCGA-2 showing the weakest 3D radar wave scattering signal, corresponding well to its excellent MA performance (Fig. 3o). To further analyze their efficiency, simulated 2D RCS curves within a θ range of −60° to 60° at 10 GHz are depicted in Fig. 3p. The RCS values peak at a scanning angle of 0° for all samples and decreases as θ shifts from 0° to ±60°, demonstrating the influence of scanning angles on the scattering signals. The maximum RCS values at 0° scanning angle are 14.05 dB m² for the PEC plate, 9.42 dB m² for CGA, and 4.87, −19.49, and 0.09 dB m² for SCGA-1 to SCGA-3, respectively. These findings align with the MA performance of each sample. Notably, SCGA-2 reduces the maximum reduced RCS by 33.54 dB m² contrasted with the PEC and maintains an RCS value below −18 dB m² across the −60° to 60° scanning angle range, underscoring its enhanced practical far-field MA capability. This further demonstrates their significant potential for practical applications in complex environments.

Based on the previous discussion, the MA mechanism of SCGAs in their strain-free state can be summarized schematically in Fig. 3q. The MA materials are usually applied on the metal surface and their performance evaluation is based on the metal-backed model which is capable of eliminating the transmission of microwave energy. Therefore, the primary objective of MA materials is to maximize the absorption of incident microwaves while minimizing reflection. Accordingly, achieving impedance matching at the air-material interface and optimizing the loss mechanism within the materials are crucial in the development of highly efficient microwave attenuators. The design of our SCGAs successfully realizes these objectives. Firstly, optimal impedance matching ensures efficient microwave penetration into the aerogel, minimizing reflection and enabling initial dissipation through multiple reflections within its porous lamellar microstructure. Secondly, the hybrid graphene walls and bridges form extensive 3D conductive networks, facilitating substantial electron hopping and migration, which enhances conduction loss. On a more microscopic level, the hybrid sheets that comprise SiC nanowires, CNTs, and graphene generate numerous heterogeneous interfaces that accumulate free electrons during migration, amplifying interfacial polarization. Additionally, the presence of stacking faults in SiC, defects in graphene sheets, and retained oxygen-containing groups induce effective dipole polarization, further improving the ability to store and dissipate microwave energy. As a result, these optimized synergistic dielectric loss mechanisms enable efficient attenuation of incident microwaves in the strain-free state of the SCGAs.

2.4 Strain-tunable microwave shielding compatibility

Transforming microwave attenuation (MA) materials into microwave shielding (MS) materials without altering their composition is challenging. In contrast to the mechanism of MA materials, MS materials do not depend on a metallic backing model. Consequently, microwaves do not undergo multiple reflections or energy dissipation within the material due to metal-backed reflection. Instead, MS materials create an intrinsic barrier that blocks microwave penetration. For non-magnetic materials, this process depends entirely on high electrical conductivity, which enables the direct redirection of incident waves through reflection.^52,53 Here, we exploit the strain-tunable conductivity of SCGAs. Applying compression strain to the porous SCGAs densifies their lamellar microstructure, increasing conductivity and activating their MS function, as shown in Fig. 4a. Considering the electromagnetic properties and elasticity comprehensively, SCGA-2 was chosen as the representative sample for a strain-tunable MS performance study. The complex permittivity and scattering (S) parameters of SCGA-2 under different strains (0%, 20%, 40%, 60%, 80%) were tested to study their response to microwaves in the X band (8.2–12.4 GHz), as detailed in Section S2, ESI.† As anticipated, both the ε′ and ε′′ of SCGAs increase with increasing strain, indicating an enhancement in their ability to store and dissipate microwave energy (Fig. S10†). To evaluate the MS performance of the aerogel under different strains, the electromagnetic microwave interference shielding effectiveness (EMI SE) values, including the total (SE_tol), reflection (SE_r), and absorption (SE_a) shielding effectiveness are calculated based on the eqn (S5)–(S7) in Section S2, ESI.† As expected, the SE_tol value of sample SCGA-2 shows an increasing trend as the compression strain enhances from 0% to 80% (Fig. 4b and d). The 0% strain aerogel has a low average SE_tol of 11.9 dB, indicating weak block capability for microwaves.⁵⁴ For 80% strain aerogel, average SE_tol increases to 50.8 dB, showing superior MS abilities capable of blocking 99.999% of incident microwaves, surpassing the standard shielding requirement (>20 dB) for commercial applications.⁵⁵ Additionally, the SE_r and SE_a values consistently increase as the strain enhances, indicating that reflection and absorption collaboratively contribute to the improvement of total shielding effectiveness (Fig. 4c, d and S11†). The ability to modulate shielding efficiency through strain implies that strain can act as a switch for the aerogel to transition between an attenuator and a shield. When the strain is 0%, it corresponds to attenuation (on) and shielding (off), whereas a strain of 80% corresponds to attenuation (off) and shielding (on), realizing dynamic tailorable MA/MS compatibility. To visually study the microwave response of the aerogels at different strains, their responses to microwaves were simulated using HFSS software. The simulated |S21| values, which theoretically equal the SE_tol values, for all the sample models showed good agreement with the experimental results, validating the accuracy of the simulations (Fig. 4e).¹³ The 2D and 3D electric field distributions of air and the aerogel at 10 GHz under different strain conditions are presented in Fig. 4e and S12,† respectively. The microstructural evolution of the aerogel at various strain levels is also shown in Fig. 4e. Using the electric field distribution in the air as a baseline, the interaction between the aerogel and the electromagnetic field at 0% strain reveals a reduction in electric field intensity. This reduction is due to the larger interlayer spacing in the hybrid graphene structure under zero strain, resulting in fewer conductive pathways and lower electrical conductivity. Consequently, microwave energy penetrates the aerogel's surface, leading to attenuation or transmission.⁵⁶ As the compressive strain on the aerogel increases, the interlayer spacing decreases, enabling greater interconnection between graphene sheets and the formation of extensive 3D conductive networks. This enhances conductivity and leads to a corresponding increase in the reflection of incident microwaves, which gradually decreases the electric field intensity on the aerogel surface. At 80% strain, the aerogel becomes nearly fully compacted, achieving the highest number of conductive pathways and maximum conductivity. At this stage, most electromagnetic energy is reflected, a small portion is absorbed, and minimal energy penetrates through, demonstrating excellent MS performance.

Fig. 4 (a) Schematic illustration of the strain-tunable MA/MS transition mechanism of SCGAs. (b) Total shielding effectiveness (SE_tol) and (c) absorption shielding effectiveness (SE_a) of sample SCGA-2 under different compression strains. (d) The average shielding effectiveness of sample SCGA-2 under different compression strains. (e) Simulated |S21|, electric field intensity and microstructure evolution of SCGA-2 under different compression strains when interacting with microwaves. Power coefficients R/A/T of SCGA-2 under (f) 20%, (g) 40%, (h) 60%, and (i) 80% compression strain. (j) Schematic illustration of strain-tunable gradient low-reflection model. (k) Shielding effectiveness values, (l) power coefficients, and (m) simulated electric field intensity of the gradient low-reflection sample.

Thus far, the strain-tunable MS ability of SCGAs has been verified. However, for MS materials, high shielding effectiveness can lead to significant reflection on the material surface, causing secondary microwave pollution. Therefore, achieving low reflection and high absorption of microwaves has been a pursuit of MS material in recent years and the future.⁵⁴ Our SCGAs with strain-tunable MS performance could provide the solution for low-reflection MS design through strain regulation. The power coefficients and their average values for the aerogels at different strains, including R (reflection), A (absorption), and T (transmission), are calculated and presented in Fig. 4f–i and S13.† These coefficients represent the ratios of reflected, absorbed, and transmitted energy to the total incident microwave energy, respectively. Generally speaking, the sum of these three coefficients is 1, indicating a trade-off relationship. At 20% strain, the aerogel shows the highest A and lowest R due to optimal impedance matching from lower conductivity and permittivity, enhancing microwave penetration and absorption. However, this strain level's low conductivity also allows some microwave transmission. As the strain increases, the R part heightens, the A and T parts weaken, and 40% and 60% aerogels show medium A and R. When strain increases to 80%, high R and low A are achieved due to the impedance mismatch caused by high conductivity, indicating effective microwave blocking dominated by surface reflection. This is contrary to the future pursuit of green, low-reflection microwave shielding. Therefore, using the strain-tunable strategy we propose a gradient low-reflection model to achieve low reflection, high absorption, and minimal transmission simultaneously. The gradient model involves arranging SCGA-2 samples with strains of 20%, 40%, 60%, and 80% in sequence from top to bottom, each layer being 2 mm thick, for a total thickness of 8 mm (Fig. 4j). As shown in Fig. S14,† the Z_in/Z₀ of the aerogel at 2 mm thickness exhibits an increasing trend as the strain increases. Therefore, the gradient model is designed to achieve ideal impedance matching on the surface to reduce reflection, allowing maximal microwave penetration into the material and facilitating gradual dissipation. The simulated |S21| value of this gradient model predicts its effective shielding ability, reaching an average value of 35.2 dB (Fig. S15†). The tested EMI SE values of the low-reflection design, shown in Fig. 4k, confirm its excellent MS performance, with an average SE_tol of 39.1 dB. This value is slightly higher than the simulation due to additional interfaces from the gradient-stacked samples. As expected, the R/A/T coefficients reveal an absorption-dominated shielding mechanism within this gradient aerogel configuration (Fig. 4l). These results highlight the gradient structure's enhanced ability to absorb microwaves and effectively minimize transmission. The gradient structure maintains a higher SE_tol than the single-layer sample at 60% strain, while also reducing reflection, increasing absorption, and minimizing leakage. This design offers a new strategy for efficient, green, and low-reflection microwave shielding materials. The electric field distribution simulation of this model when interacting with microwaves further reveals its low-reflection shielding mechanism. As shown in Fig. 4m, initially, the excellent impedance matching of the first 20% strain layer allows more microwave energy to enter the material, while the vertically graded increase in conductivity provides effective microwave attenuation. Finally, the highly conductive bottom layer with 80% strain can reflect microwaves back, preventing spillover and effectively doubling the lossy path. Therefore, strain-tunable shielding not only enables the transformation of SCGAs from MA to MS materials, achieving dynamic compatibility but also facilitates the optimization of green low-reflection shielding. This materials and design approach holds tremendous potential in advanced multi-purpose microwave response systems.

2.5 Cyclic compression durability and harsh environmental tolerance

In practical applications, whether in daily life or engineering, the ability to withstand prolonged cyclic compressive strain is crucial for microwave-compatible materials. To evaluate the compressive strain cyclic stability of the aerogels, we subjected the SCGA-2 sample to 50% compressive strain for up to 1000 cycles using a reciprocating motor, while simultaneously monitoring the ΔR/R₀ with a resistance meter, as shown in the inset of Fig. 5a and S16.† The results demonstrate that the aerogel exhibits a rapid response in ΔR/R₀ to the applied strain, decreasing by approximately 69.8% under 50% strain and returning to its initial value upon strain release. After 1000 compression cycles, the ΔR/R₀ of the aerogel remains stable, showing a value of 62.6% under 50% strain, representing only a 7.2% decrease compared to the first cycle. This indicates that the aerogel possesses excellent cyclic compressive stability in its electrical performance, which is attributable to its superior resilience and inherent electrical stability. The aerogel's long-term compressive cyclic stability is further evidenced by the slight decline in its ε' and ε′′ values after 100 and 1000 compression cycles, while overall performance remains well. This minor decline is consistent with the small decrease in ΔR/R₀ and is primarily due to slight damage to the bridge-lamellar microstructure, leading to a reduction in conductive pathways within the aerogel (Fig. 5b and c). The MA performance of the aerogel also shows a slight decline but remains stable overall, as indicated by an EAB of 7.65 GHz at a thickness of 2.6 mm after 100 compression cycles and an EAB of 7.1 GHz at a thickness of 2.75 mm after 1000 compression cycles (Fig. 5d and e). Compared to its initial uncompressed state, the primary change is a slight increase in thickness, with only a negligible reduction in EAB. Additionally, we compared the conductivity of the aerogel at 20%, 40%, 60%, and 80% strain before and after 100 and 1000 compression cycles, as shown in Fig. 5f. The results indicate that conductivity decreases more significantly as the compression cycles increase, particularly at lower strain levels. At higher strains, the reduction in conductivity is less pronounced, likely because the cyclic compression was performed at 50% strain. Although the aerogel's microstructure is somewhat compromised, large compression at higher strains still facilitates the establishment of numerous conductive pathways, thereby maintaining the original conductivity. These changes in conductivity influence the strain-tunable MS performance of the aerogel. As depicted in Fig. 5g and h, SE_tol decreases with an increasing number of compression cycles, with a more pronounced reduction at lower strains and a smaller reduction at higher strains, consistent with the changes in conductivity. After 100 and 1000 compression cycles, the SE_tol at 80% strain is 50.5 dB and 49.8 dB, respectively, representing only a 0.6% and 2% decrease compared to the pristine value of 50.8 dB, demonstrating the exceptional strain-tunable MS cyclic stability of SCGAs. Fig. S17† also shows changes in SE_r and SE_a, which follow a trend consistent with SE_tol. The RTA values at different strains reveal that, as the compression cycles increase, the R value decreases while the T and A values increase (Fig. S18†). In conclusion, the strain-tunable MA/MS capability of SCGAs is durable enough to withstand long-term cyclic compression, demonstrating their mechanical and performance stability, which offers significant advantages for practical applications.

Fig. 5 (a) The cyclic ΔR/R₀ of the aerogel under 50% compression strain over 1000 cycles. Frequency-dependent electromagnetic parameters, including (b) ε′ and (c) ε′′, for the aerogel before and after 100/1000 compression cycles. The 2D contour plots of RL values as a function of frequency and thickness for the aerogel after (d) 100 and (e) 1000 compression cycles. (f) The conductivity of the aerogel under different compression strains before and after 100/1000 compression cycles. (g) The SE_tol of the aerogel under 0% and 80% strains before and after 100/1000 compression cycles. (h) The average shielding effectiveness of the aerogel before and after 100/1000 compression cycles. (i) Infrared thermal images of an aerogel on a 100 °C and 300 °C heating plate. (j) Digital photographs of the fresh flowers with or without the aerogel as insulation when heated by a flame gun. (k) Fire resistance properties are illustrated by the ethanol absorption-combustion test cycle of an aerogel. (l) Digital photographs of an aerogel and steel block placed on the freezing plate of −60 °C. (m) Digital photographs of the elasticity of an aerogel in liquid nitrogen (−196 °C). (n) The water contact angle of SCGAs, optical images of water droplets on the surface, and humidity test of the aerogel. (o) σ–ε curves of SCGA-2 after harsh condition treatment at 40%, 60%, and 80% strain. (p) 2D contour plots of RL values as a function of frequency and thickness and (q) SE values under 80% strains for SCGA-2 after harsh condition treatment. (r) Radar chart for the overall appreciation of the SCGAs compared with microwave compatible aerogel/foams in recent years.

In addition to the cyclic compression resistance, it is crucial for microwave compatibility applications to adapt to special environments such as high altitudes, aviation, unmanned areas, polar regions, and high-temperature equipment. Therefore, the harsh environmental tolerance of MA/MS materials is essential for maintaining their performance under such conditions. In high-temperature environments, thermal insulation is crucial, and the porous structure of aerogels inherently provides superior heat insulation capabilities. To visually demonstrate the thermal insulation properties of aerogels, a sample of SCGA-2 was placed on heating plates set at 100 °C and 350 °C, respectively. The dynamic heat distribution was monitored using an infrared camera to observe the temperature changes in real time. As shown in Fig. 5i, the top surface of the hybrid aerogels remains at a relatively low temperature of about 27.6 °C and 44.3 °C on 100 °C and 350 °C heating plates, respectively. After 10 minutes, the temperature slightly increases to 39.7 °C and 86.7 °C, then stays constant until 30 minutes. The temperature difference of aerogel with a 350 °C heating plate reaches 261 °C between the upper and lower surface, exhibiting its excellent heat insulation ability. The exceptional thermal insulation properties of aerogels are primarily attributed to their extremely low thermal conductivity. As illustrated in Fig. S19,† SCGA-2 demonstrates a thermal conductivity of just 0.016 W m⁻¹ K⁻¹, which is even lower than that of air. This impressive performance is mainly due to the highly porous structure of the SCGAs, where air serves as the principal insulating phase, effectively impeding heat transfer. Additionally, the pores in the aerogel are predominantly micropores, which hinders gas conduction, while the layered architecture further suppresses air convection. Consequently, the aerogel achieves a thermal conductivity lower than that of air, resulting in outstanding thermal insulation properties. As a proof of concept for thermal insulation application, a fresh flower was placed on a 20 mm height SCGA-2 sample and kept fresh after being heated for 5 min by flame, as shown in Fig. 5j. In contrast, another flower was scorched quickly without the thermal protection of the hybrid aerogel. In addition to thermal protection applications, the extremely low thermal conductivity of SCGAs also promises to enable infrared cloaking. For specific antidetection application scenarios, this property combined with MA is expected to achieve microwave-infrared compatible cloaking.

SCGAs also exhibit excellent fire-resistant properties, which are highly important in practical applications. As presented in Fig. 5k, when the aerogel is fully absorbed with alcohol and ignited, it retains its appearance and volume after the flames are extinguished. Even after repeating the process of alcohol absorption and burning 10 times, the weight of the aerogel remains stable with almost no loss, demonstrating the stability of its structure and components under flame conditions. The exceptional flame-retardant performance of SCGAs is attributed to the synergistic effects of its ceramic/carbon material system and highly porous structure. The SiC nanowires exhibit remarkable high-temperature stability, forming a robust physical barrier during combustion that slows heat transfer and prevents the direct spread of flames.⁵⁷ Additionally, the carbon-based components, including graphene and CNTs, undergo a controlled oxidation process during combustion, generating significant amounts of CO₂ within the aerogel's microporous structure. This CO₂ inhibits the flow of oxygen to the combustion front, enabling the aerogel to self-extinguish internally and preventing the fire from spreading. Furthermore, the interconnected network of micropores and mesopores within the SCGAs effectively traps combustion gases, such as CO₂, creating a protective layer that shields the material from further heat exposure. The high porosity also effectively reduces heat dissipation within the aerogel, further impeding heat propagation. The highly porous ceramic/carbon architecture further endows the aerogels with anti-frosting ability and shows advantageous adaptability in temperature-changeable environments. As shown in Fig. 5l, a steel block, and sample SCGA-2 were placed on the surface of the −60 °C cold copper plate for 1 hour. In contrast to the steel block that was fully covered by a thick layer of ice frost, the appearance of the aerogels remained unchanged, presenting excellent anti-frosting properties. The anti-frosting mechanism of SCGAs is driven by the synergistic interaction between the ceramic/carbon material system and its microstructural characteristics. First, the extremely low thermal conductivity of SCGAs effectively limits the transfer of cold air, which is crucial for preventing frost formation. Additionally, the excellent hydrophobic properties of graphene and carbon nanotubes prevent surface wetting, significantly lowering the likelihood of frost nucleation within the aerogel.⁴¹ As a result, SCGAs not only suppress the onset of frost but also mitigate its spread and accumulation when exposed to sub-zero environments. This makes the aerogel particularly suitable for critical applications where frost formation is a concern, such as in aerospace, refrigeration, and outdoor electronics. Furthermore, as illustrated in Fig. 5m, the aerogel is also capable of withstanding extremely low temperatures. When immersed in liquid nitrogen (−196 °C), it maintains its morphology without hardening and retains a degree of elasticity in the liquid nitrogen, demonstrating its broad-temperature-range mechanical reliability. Given the challenges posed by rain and high humidity in practical applications, a thorough investigation of the hydrophobic properties of SCGAs is crucial. As shown in Fig. 5n, SCGA-2 exhibits a contact angle of 129°, with water droplets remaining stable on its surface without absorption, underscoring its superhydrophobic nature. In Singapore's consistently humid outdoor environment, the aerogel maintained its weight after being exposed to outdoors with 85.8% humidity for a full day and night. The hydrophobic behavior of SCGAs is attributed to the intrinsic hydrophobicity of the carbon-based components and the porous architecture, which significantly reduces the interaction between water and the solid matrix, thereby ensuring exceptional overall hydrophobic performance.

To further assess the stability of SCGAs under harsh environmental conditions, we evaluated their mechanical elasticity and MA/MS performance following exposure to these conditions (details provided in Section S3, ESI†). As illustrated in Fig. 5o, SCGA-2 exhibits remarkable elasticity post-treatment, recovering to its original height under 80% strain, with only a marginal reduction in maximum stress to 24.6 kPa, representing 95% of its pre-treatment value. The aerogel's electromagnetic parameters, depicted in Fig. S20a and b† show that ε′ and ε′′ remain stable before and after extreme treatments, with only slight variations across different frequency ranges. This observation underscores the stability of the aerogel's electromagnetic properties under harsh conditions. SCGA-2 also maintains outstanding MA performance, achieving an EAB of 6.54 GHz at a thickness of 2.25 mm (Fig. 5p). The slight reduction observed can be attributed to minimal loss of carbon components during extreme treatments, particularly combustion. Fig. S20c† presents the MS performance of SCGA-2 post-treatment at 0% strain, where the average SE_tol in the X-band is 11.7 dB, closely matching the untreated value of 11.9 dB. Under 80% strain, the SE_tol achieves an average value of 50.5 dB, maintaining a high standard of performance as well. These findings demonstrate that the mechanical and strain-tunable MA/MS performance of SCGAs remains robust under extreme conditions, highlighting the potential of SCGAs for reliable performance in demanding environments such as aviation, aerospace, navigation, field communication base stations, and outdoor electronic systems. To comprehensively evaluate the performance of SCGAs, we conducted a comparative analysis with recently developed novel smart/dynamic tailorable electromagnetic materials, focusing on innovation, functional designability, and practical applications. Given the demand for ultralight density in electromagnetic materials for practical applications, the comparison was limited to aerogels and foams.^{21,22,58–60} The evaluation was based on four key criteria: dynamic MA/MS compatibility, long-term mechanical reliability, optimal MA/MS performance, and tolerance to harsh environments. The detailed benchmarks for these comparisons are listed in Table S2.† The results in Fig. 5r demonstrate that SCGAs excel in all four criteria, indicating that SCGAs possess comprehensive and robust advantages for next-generation smart microwave applications, especially adaptive to complex, harsh environments.

3. Conclusions

In conclusion, this study introduces a novel strategy for navigating the complexities of microwave radiation environments by developing SiC NW/CNT/Graphene aerogels (SCGAs) for dynamic strain-tunable MA/MS compatibility. Utilizing a bidirectional freeze-casting method, the SCGAs synergistically assembled SiC nanowires, CNTs, and graphene sheets to achieve concurrent optimization of dielectric and mechanical properties. This integrative approach results in optimized impedance matching and polarization loss within the SCGAs, leading to remarkable performance metrics, including an RL_min of −51.6 dB at 2.05 mm, an EAB of 7.62 GHz at 2.45 mm, and a substantially reduced radar cross-section, signifying their efficiency in the strain-free MA state. The structural reinforcement provided by the SiC nanowires and CNTs, alongside an advanced ordered bridge-lamellar architecture, significantly enhanced the SCGAs' mechanical elasticity and strain-sensitive conductivity. Upon applying compressive strain, the conductive interconnections among the hybrid graphene layers are enhanced, enabling a dynamic transition from the MA to the MS state, with a SE_tol reaching 50.1 dB at 80% strain. Furthermore, our innovative strain-tunable approach facilitates low-reflection microwave shielding through a gradient design, arranging aerogels with increasing strains of 20%, 40%, 60%, and 80% from top to bottom, embodying the concept of green shielding. In addition, the strain-tunable microwave attenuation and shielding performance of the SCGAs show minimal degradation even after 1000 compression cycles, highlighting their exceptional long-term compressive cyclic stability, which can be attributed to the optimized robust three-dimensional conductive networks. In addition, the ceramic/carbon hybrid composition of SCGAs further imparts remarkable environmental tolerance, including thermal insulation and resistance to extreme conditions such as fire, frost, rain, humid, and low temperatures, widens their applicability in adverse settings. Thus, this work represents a step forward in smart microwave-compatible materials, offering an integration of high-efficiency microwave attenuation and shielding with enhanced mechanical and environmental robustness. This advancement provides guidance for smart, multifunctional materials designed to meet the demands of increasingly complex electromagnetic environments.

Experimental

All experimental procedures are given in the ESI.†

Data availability

The authors confirm that the data supporting the findings of this study are available within the article and its ESI.† Raw data that support the findings of this study are available from the corresponding author, upon reasonable request.

Author contributions

Y. Zhao: conceptualization, methodology, investigation, writing – original draft, writing – review and editing. N. Ahmad: methodology, validation, data curation. Y. Yong: supervision, formal analysis, writing – review. W. Zhai: conceptualization, supervision, formal analysis, writing – review, editing, and funding.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by the Advanced Research and Technology Innovation Centre (ARTIC), the National University of Singapore under Grant (project number: A-0005947-31-00).

References

1. F. Shahzad, M. Alhabeb, C. B. Hatter, B. Anasori, S. Man Hong, C. M. Koo and Y. Gogotsi, Science, 2016, 353, 1137–1140.
2. Q. Li, Z. Zhang, L. Qi, Q. Liao, Z. Kang and Y. Zhang, Adv. Sci., 2019, 6, 1801057.
3. J. Liu, H.-B. Zhang, R. Sun, Y. Liu, Z. Liu, A. Zhou and Z.-Z. Yu, Adv. Mater., 2017, 29, 1702367.
4. P. Bandara and D. O. Carpenter, Lancet Planet. Health, 2018, 2, e512–e514.
5. D. Wanasinghe and F. Aslani, Composites, Part B, 2019, 176, 107207.
6. Y. Xia, W. Gao and C. Gao, Adv. Funct. Mater., 2022, 32, 2204591.
7. H. Lv, Y. Li, Z. Jia, L. Wang, X. Guo, B. Zhao and R. Zhang, Composites, Part B, 2020, 196, 108122.
8. X.-X. Wang, T. Ma, J.-C. Shu and M.-S. Cao, Chem. Eng. J., 2018, 332, 321–330.
9. Y. Zhang, Y. Zhao, Q. Chen, Y. Hou, Q. Zhang, L. Cheng and L. Zheng, Ceram. Int., 2021, 47, 8123–8132.
10. T. Hou, B. Wang, Z. Jia, H. Wu, D. Lan, Z. Huang, A. Feng, M. Ma and G. Wu, J. Mater. Sci. Mater. Electron., 2019, 30, 10961–10984.
11. Y. Tang, S. Shao, C. Guo, K. Bi, H. Wang, T. Zhao, J. Liu and F. Wang, Carbon, 2024, 228, 119314.
12. J. Liang, F. Ye, Y. Cao, R. Mo, L. Cheng and Q. Song, Adv. Funct. Mater., 2022, 32, 2200141.
13. Y. Zhao, J. W. Chua, Y. Zhang and W. Zhai, Composites, Part B, 2023, 250, 110454.
14. Y. Jiang, Y. Chen, Y.-J. Liu and G.-X. Sui, Chem. Eng. J., 2018, 337, 522–531.
15. X.-X. Wang, W.-Q. Cao, M.-S. Cao and J. Yuan, Adv. Mater., 2020, 32, 2002112.
16. O. Balci, E. O. Polat, N. Kakenov and C. Kocabas, Nat. Commun., 2015, 6, 6628.
17. H. Lv, Z. Yang, P. L. Wang, G. Ji, J. Song, L. Zheng, H. Zeng and Z. J. Xu, Adv. Mater., 2018, 30, 1706343.
18. Z. Cheng, R. Wang, Y. Cao, Z. Cai, Z. Zhang and Y. Huang, Adv. Funct. Mater., 2022, 32, 2205160.
19. Y. Wang, X.-D. Cheng, W.-L. Song, C.-J. Ma, X.-M. Bian and M. Chen, Chem. Eng. J., 2018, 344, 342–352.
20. C. Kuang, S. Chen, M. Luo, Q. Zhang, X. Sun, S. Han, Q. Wang, V. Stanishev, V. Darakchieva, R. Crispin, M. Fahlman, D. Zhao, Q. Wen and M. P. Jonsson, Adv. Sci., 2024, 11, 2305898.
21. X. Liu, Y. Li, X. Sun, W. Tang, G. Deng, Y. Liu, Z. Song, Y. Yu, R. Yu, L. Dai and J. Shui, Matter, 2021, 4, 1735–1747.
22. X. Li, L. Zhu, T. Kasuga, M. Nogi and H. Koga, Chem. Eng. J., 2023, 469, 144010.
23. B. Shen, W. Zhai and W. Zheng, Adv. Funct. Mater., 2014, 24, 4542–4548.
24. J. Xu, X. Zhang, Z. B. Zhao, H. Hu, B. Li, C. L. Zhu, X. T. Zhang and Y. J. Chen, Small, 2021, 17, 2102032.
25. Y. Jiang, Y. Chen, Y.-J. Liu and G.-X. Sui, Chem. Eng. J., 2018, 337, 522–531.
26. M. Yang, N. Zhao, Y. Cui, W. Gao, Q. Zhao, C. Gao, H. Bai and T. Xie, ACS Nano, 2017, 11, 6817–6824.
27. W. Wang, X. Zhao and L. Ye, Small, 2023, 19, 2300234.
28. N. M. S. Hidayah, W.-W. Liu, C.-W. Lai, N. Z. Noriman, C.-S. Khe, U. Hashim and H. C. Lee, AIP Conf. Proc., 2017, 1892, 1.
29. H. Shivakumar, G.-D. Gurumurthy, B.-K. Bommegowda and S. Parameshwara, J. Polym. Mater., 2023, 40, 93–103.
30. Y. Zhao, Y. Zhang, C. Yang and L. Cheng, Carbon, 2021, 171, 474–483.
31. M. Wu, H. Geng, Y. Hu, H. Ma, C. Yang, H. Chen, Y. Wen, H. Cheng, C. Li, F. Liu, L. Jiang and L. Qu, Nat. Commun., 2022, 13, 4561.
32. G. Agarwal, J. Polym. Mater., 2022, 39, 255–267.
33. M. F. Ashby and L. J. Gibson, Cellular Solids: Structure and Properties, Cambridge University Press, Cambridge, 2 edn, 1997.
34. A. Iqbal, F. Shahzad, K. Hantanasirisakul, M.-K. Kim, J. Kwon, J. Hong, H. Kim, D. Kim, Y. Gogotsi and C. M. Koo, Science, 2020, 369, 446–450.
35. X. Huang, G. Yu, Y. Zhang, M. Zhang and G. Shao, Chem. Eng. J., 2021, 426, 131894.
36. M. Chen, E. J. Siochi, T. C. Ward and J. E. McGrath, Prog. Polym. Sci., 1993, 33, 1092–1109.
37. H. Zhang, Y. Xu, J. Zhou, J. Jiao, Y. Chen, H. Wang, C. Liu, Z. Jiang and Z. Wang, J. Mater. Chem. C, 2015, 3, 4416–4423.
38. M. Samet, A. Kallel and A. Serghei, J. Appl. Polym. Sci., 2019, 136, 47551.
39. M. Qin, L. Zhang and H. Wu, Adv. Sci., 2022, 9, 2105553.
40. J. Wei, G. Shao and X. Huang, ACS Appl. Mater. Interfaces, 2024, 16, 20969–20979.
41. Y. Zhao, T. Niu, X. Dong, Y. Yang and W. Zhai, J. Mater. Chem. A, 2023, 11, 23452–23462.
42. J. Tang, N. Liang, L. Wang, J. Li, G. Tian, D. Zhang, S. Feng and H. Yue, Carbon, 2019, 152, 575–586.
43. Y. Wang, X. Gao, Y. Fu, X. Wu, Q. Wang, W. Zhang and C. Luo, Composites, Part B, 2019, 169, 221–228.
44. W. Yu and G. Shao, J. Colloid Interface Sci., 2023, 640, 680–687.
45. J. Zhou, J. Luo, G. Hao, F. Guo, G. Liu, H. Guo, G. Zhang, L. Xiao, Y. Hu and W. Jiang, J. Mater. Chem. A, 2022, 10, 13848–13857.
46. G. Yu, G. Shao, Y. Chen and X. Huang, ACS Appl. Mater. Interfaces, 2023, 15, 39559–39569.
47. J. Qiu, J. Liao, G. Wang, R. Du, N. Tsidaeva and W. Wang, Chem. Eng. J., 2022, 443, 136475.
48. Y. Li, X. F. Liu, X. Y. Nie, W. W. Yang, Y. D. Wang, R. H. Yu and J. L. Shui, Adv. Funct. Mater., 2019, 29, 1807624.
49. W. H. Gu, J. Q. Sheng, Q. Q. Huang, G. H. Wang, J. B. Chen and G. B. Ji, Nanomicro Lett., 2021, 13, 1–14.
50. J. J. Yang, J. Q. Wang, H. Q. Li, Z. Wu, Y. Q. Xing, Y. F. Chen and L. Liu, Adv. Sci., 2022, 9, 2101988.
51. W. Deng, T. Li, H. Li, R. Niu, A. Dang, Y. Cheng and H. Wu, Carbon, 2023, 202, 103–111.
52. D. Zuo, Y. Jia, J. Xu and J. Fu, Ind. Eng. Chem. Res., 2023, 62, 14791–14817.
53. M. Li, B. Yin, C. Gao, J. Guo, C. Zhao, C. Jia and X. Guo, Exploration, 2023, 3, 20210233.
54. X. Jia, Y. Li, B. Shen and W. Zheng, Composites, Part B, 2022, 233, 109652.
55. M. Han, C. E. Shuck, R. Rakhmanov, D. Parchment, B. Anasori, C. M. Koo, G. Friedman and Y. Gogotsi, ACS Nano, 2020, 14, 5008–5016.
56. M. Ding, D. Zhao, R. Wei, Z. Duan, Y. Zhao, Z. Li, T. Lin and C. Li, Exploration, 2024, 4, 20230057.
57. Y. Si, X. Wang, L. Dou, J. Yu and B. Ding, Sci. Adv., 2018, 4, eaas8925.
58. Y. Zhao, H. Qi, X. Dong, Y. Yang and W. Zhai, ACS Nano, 2023, 17, 15615–15628.
59. W. Gu, S. J. H. Ong, Y. Shen, W. Guo, Y. Fang, G. Ji and Z. J. Xu, Adv. Sci., 2022, 9, 2204165.
60. B. Shen, Y. Li, W. Zhai and W. Zheng, ACS Appl. Mater. Interfaces, 2016, 8, 8050–8057.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta06820c

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