Hollow MnFe₂O₄@C Graphene Absorbing Aerogels: Ultra‐Broadband and Flexible Compression Resistance

ABSTRACT

Graphene‐based aerogels have received substantial attention in the realm of effective electromagnetic wave absorption due to their lightweight qualities and unusual three‐dimensional network structure. This work creates a Hollow MnFe₂O₄@ C/reduced graphene oxide (RGO)/polyimide (PI) composite aerogel by anchoring hollow ferrite@carbon particles onto graphene using a two‐step hydrothermal process and freeze‐drying. The electromagnetic properties are adjusted by adjusting the thickness of the carbon shell layer, managing the defect density within the aerogel, improving polarization loss, and enriching internal heterogeneous interfaces. This aerogel has a maximum effective absorption bandwidth of 7.70 GHz and a minimum reflection loss of −68.04 dB. Under volumetric deformation, the composite aerogel continues to perform exceptionally well in terms of absorption. Its maximum effective absorption bandwidths are 7.50 and 7.87 GHz, and its minimum reflection loss at 30% and 50% compression is −79.49 and −60.01 dB, respectively. The impedance mismatch disadvantage brought on by aerogel compression is offset by the synergistic improvement of absorption performance through several loss modes. This aerogel can also tolerate more complicated and demanding settings because it has certain compressive strength and flexibility, as well as fire resistance and thermal insulation. It offers a strong basis for multifunctional absorber material research.

In this study, a hollow MnFe₂O₄@C modified RGO composite was proposed to achieve efficient microwave absorption through a synergistic multi‐loss mechanism. It has excellent thermal properties and can still have excellent microwave absorption performance when the volume is compressed by a certain volume. It has broad application prospects in electromagnetic attenuation.

1. Introduction

Today, innovations in technology and the explosive growth of electronic communication technologies have provided countless benefits to human society. However, the dependability of electronic items and EMI (Electromagnetic Interference, which refers to electromagnetic waves created by electronic devices during operation that radiate outward and cause interference to other sections of the device or external equipment) has become difficult challenges [1, 2, 3]. Furthermore, electromagnetic pollution poses a risk to human health and is more difficult to regulate than other types of pollution. Creating lightweight and highly effective electromagnetic wave absorption materials is critical for efficiently minimizing the effects of electrical interference [4, 5, 6]. To this goal, researchers have conducted significant studies aimed at rationally managing electromagnetic radiation and scientifically resolving electromagnetic pollution, balancing its utility with safety concerns [7, 8, 9, 10].

Graphene aerogels have emerged as key materials in microwave absorption due to their superior wave‐absorbing capacity and ultra‐lightweight characteristics [11, 12]. Unfortunately, pure graphene aerogels have drawbacks such as poor impedance matching, low effective absorption bandwidth, and restricted electromagnetic wave loss mechanisms. In order to improve graphene aerogel impedance matching, boost absorption performance, and increase effective absorption bandwidth, researchers have undertaken a great deal of study [13, 14, 15]. Graphene aerogel and magnetic nanoparticles have been found to introduce more efficient electromagnetic loss and increase numerous loss processes in a synergistic way [16]. This technique is an effective strategy to achieve broadband, high‐efficiency microwave absorption and modify impedance matching [17]. Ding [1] and his colleagues enhanced the interfacial polarization of the composite material and optimized the electromagnetic characteristics by uniformly anchoring cobalt magnetic nanoparticles onto graphene sheets using polyvinylpyrrolidone. The CoO/RGO composite material that integrated CoO/Co₃O₄ achieved a minimum reflection loss (RL) of −58.53 dB. Three‐dimensional restored reduced graphene oxide (RGO)@Fe₃O₄ high‐performance microwave absorber was prepared by Zhang [18] et al. The introduction of Fe₃O₄ nanoparticles not only constructs magnetic loss pathways and alleviates the stacking of RGO sheets but also optimizes the overall electromagnetic performance, enabling the composites to reach a maximum RL of −56.25 dB at 12.62 GHz. Techniques for creating nanoparticle structures that can absorb broadband waves have attracted a lot of interest lately. According to research, it has been verified that microwave‐absorbing materials with hollow and core‐shell structures not only possess inherent lightweight advantages but also successfully boost the material's internal multiple reflection losses of electromagnetic waves, exhibiting exceptional electromagnetic waves (EMW) absorption ability. Xu [19] et al. created a three‐dimensional hierarchical porous NRGO/hollow CoFe₂O₄ composite aerogel using a solvothermal‐hydrothermal two‐step method. They increased electromagnetic wave loss and boosted broadband absorption capacity by introducing and controlling the dispersion of hollow CoFe₂O₄ particles. This aerogel produced significant absorption of −44.70 dB and an absorption bandwidth of 5.20 GHz at an ideal thickness of 1.80 mm. Guo [20] et al. used layered interface engineering to create COF‐derived FeNi₃@C composites and hydrothermally produced FeNi₃ alloy microspheres. Numerous interfaces and flaws were produced by the resulting composites' heterogeneous design and distinctive carbon shell structure. With an effective absorption bandwidth of 7.4 GHz at a thickness of just 1.6 mm, the FeNi₃@C composites demonstrated exceptional electromagnetic wave absorption performance. By inducing interfacial polarization, dipole polarization, and their relaxation, the special carbon shell structure improves the material's ability to attenuate electromagnetic waves.

Meanwhile, polyimide has also attracted great interest in the modification of electromagnetic absorbing materials. Polymer modification is an effective approach to improve the mechanical properties of aerogels by constructing cross‐linked polymer networks within the porous aerogel framework, which significantly enhances structural toughness and compressive resistance. Many polymers possess exceptional toughness and flexibility. Among these, polyimide (PI) demonstrates tolerance to high temperatures in addition to introducing flexible segments [21, 22]. PI@SiO₂ composite nanofiber aerogels with a dual‐network, dual‐crosslinked structure was successfully created by Xue [23] et al. The composite aerogel has exceptional compressive and flexural characteristics due to the stable PI nanofiber network. It retains its elasticity in the face of severe temperatures and can sustain 1,000 compression cycles at 50% strain. Consider an elastic aerogel that is exposed to external impacts while it is in use. Its volume varies when compressed, yet it can buffer the applied force to avoid self‐destruction. It is still up for debate how these volumetric changes affect its microwave absorption characteristics. The requirements for absorptive materials are getting stricter as scientific research advances and practical application demands become more complex. The stability of microwave absorbers' performance is increasingly a focus, in addition to their intrinsic absorptive qualities [24]. Therefore, it is of great significance to look at how resistant microwave absorbers are to deformation and how stable their absorbing performance are both before and after deformation [13, 25].

As is widely known, manganese ferrite (MnFe₂O₄), a typical spinel ferrite with low cost and excellent electromagnetic properties, has been widely applied in the field of microwave absorption [26, 27, 28]. The choice of the hollow structure helps to reduce the density of the material. The choice of the core‐shell structure, on the one hand, can prevent the direct contact and interaction between the core ferrite particles through the physical isolation of the shell, thereby inhibiting the occurrence of agglomeration. On the other hand, it is beneficial to the uneven distribution of the charge at the ‘core’ and ‘shell’ interfaces, and promotes the polarization relaxation phenomenon at the interface [29, 30]. Based on this, this study employed a two‐step hydrothermal method to synthesize magnetic particles featuring hollow MnFe₂O₄ as the magnetic core and carbon layers of varying thicknesses as the dielectric shell [31]. Utilizing the directional freezing method, these particles were successfully combined with graphene to produce hollow MnFe₂O₄@C/RGO/polyimide composite aerogels. The homogeneous distribution of shell‐core shaped nanoparticles on the surface of graphene sheets effectively suppresses graphene layer stacking and increases electromagnetic wave multiple reflection losses within the aerogel. The resulting composite aerogel has exceptional absorptive qualities by regulating the thickness of the carbon shell layer and the doping amount of core‐shell structured particles. The minimal reflection loss was −68.20 dB with an effective absorption bandwidth (EAB) of 7.70 GHz at an ideal composite aerogel thickness of 2.80 mm. In the meantime, polyimide gives this composite aerogel exceptional flexibility to endure external pressures and guarantee that its structural integrity is maintained. The aerogel maintains important absorptive qualities even after being crushed by 30% and 50%. The composite aerogel has an effective absorptive bandwidth (EAB) of up to 7.50 GHz and a minimum reflection loss of −79.49 dB at 30% compression. The composite aerogel shows a minimum reflection loss of −63.01 dB and an effective absorption bandwidth of 7.87 GHz when compressed to 50% of its initial volume, preserving comparatively strong absorption performance. The groundwork for investigating more advanced microwave absorbers with stable absorption characteristics is laid by this work.

2. Experimental

2.1. Materials

Potassium permanganate (KMnO₄) was purchased from Aladdin. MnCl₂·4H₂O, FeCl₃·6H₂O, ethylene glycol, urea, polyvinylpyrrolidone (PVP), glucose, pyromellitic dianhydride (PMDA), 4,4'‐diaminodiphenyl ether (ODA), triethylamine (TEA), and N,N‐dimethylacetamide were all sourced from Sinopharm. All chemical reagents were analytical grade and required no further purification. Deionized water was produced in our laboratory with a resistivity of approximately 18.5 MΩ·cm.

2.2. Synthesis of Hollow MnFe₂O₄@C Core‐Shell Nanoparticles

As illustrated in Figure 1, fully dissolve 30 mmol of MnCl₂·4H₂O and 60 mmol of FeCl₃·6H₂O in 220 mL of ethylene glycol. To the mixture, add 0.8 g of polyvinyl pyrrolidone (PVP) as a capping agent and 7.20 g of urea as a reducing agent. Transfer to a polymerization reactor after sonicating at room temperature until completely dissolved. React for 22 h at 180°C. Let the polymerization reactor cool to room temperature. Use magnetic separation to extract the solid from the mixture. The hollow ferrite manganese oxide was formed upon drying following multiple washes with deionized water and anhydrous ethanol.

FIGURE 1.

Preparation flowchart for CGP aerogel.

Weigh 1.0 g of powdered glucose and 0.2 g of hollow ferrite. After dissolving both in an appropriate amount of deionized water and giving it a good stir, move it to a reactor. After 12 h of reaction at 200°C, let the reactor drop to ambient temperature. After removing the solution from the reactor and separating the precipitate, wash it with anhydrous ethanol and deionized water. The prepared sample should next be dried in a drying oven set at 50°C. Hollow manganese ferrite@carbon, or HMFO@C‐A, is the resultant black powder. 0.2 g of hollow ferrite was weighed separately. Hollow ferrite@carbon with different carbon contents, known as HMFO@C‐B, was obtained by changing the ratio with glucose, weighing 1.8 g of glucose, and repeating the previous procedures.

2.3. Synthesis of Hollow MnFe₂O₄@C/RGO/PI Aerogel

Figure 1 illustrates the particular preparation procedure for hollow MnFe₂O₄@C/RGO/PI. The resulting PAA, graphene, and a predefined amount of HMFO@C were dispersed in deionized water using ultrasonic treatment and triethylamine to produce the precursor solution. After that, the precursor solution is put into a specially designed PTFE mold and set on a copper block that has been pre‐cooled to −70°C for a long time. After that, the mold is frozen for an entire day at ‐80°C. The mold was moved to −70°C under sub‐5 Pa conditions until it dried completely after the precursor solution had completely frozen. After that, the sample was heat‐treated for two hours at 300°C in an inert protective environment. Hollow MnFe₂O₄@C/RGO/PI aerogel (CGP) was the result. By combining different HMFO@C materials and changing their content, several CGP variations can be made. Replacing the magnetic core‐shell particles with HMFO@C‐A produced different hollow MnFe₂O₄@C/RGO/PI aerogels when the HMFO@C‐A content was 25, 125, and 500 mg, respectively, while keeping other parameters unchanged. These were identified as CGP‐A1, CGP‐A2, and CGP‐A3. Similarly, distinct hollow MnFe₂O₄@C/RGO/PI aerogels were produced by adding 25, 125, and 500 mg of HMFO@C‐B; these aerogels were named CGP‐B1, CGP‐B2, and CGP‐B3, respectively. All naming details can be found in Supporting Material Tables S1, and S2.

3. Results and Discussion

3.1. Morphology and Structure

The ferrite particles and graphene‐composite aerogel were characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively, in order to examine the microstructure of the MnFe₂O₄@C/RGO/PI composite aerogel material. SEM pictures of the three different magnetic particles are shown in Figure 2a–c. The pure manganese ferrite particles shown in Figure 2a have a hollow spherical structure with a wall thickness of about 64 nm and a particle size of around 400 nm. Figure 2b,c show increasing cross‐sectional thicknesses for HMFO@C‐A and HMFO@C‐B following coating with carbon shells of different thicknesses. These grew to 95 nm and over 106 nm, respectively, in comparison to the 64 nm wall thickness of HMFO, and the spheres' surfaces were smoother. Figure S2 (Supporting Materials) displays the CGP aerogel SEM image. It shows that HMFO@C particles are well‐loaded onto graphene sheets, suggesting that CGP contains a large number of heterogeneous interfaces. Significant interfacial polarization losses may result from these surfaces, and the CGP aerogel's special hollow structure increases the possibility of numerous electromagnetic wave reflections and scattering. Further microscopic examination of the element's distribution and magnetic particle shape within the CGP aerogel is made possible by transmission electron microscopy. The (202) crystal plane of MnFe₂O₄ is represented by the interplanar spacing of 0.29 nm, as seen in Figure 2g. The hollow structure of the synthesized MnFe₂O₄ in our study is further supported by Figure 2d, where microspheres display a clear outer‐dark‐inner‐light phenomena. The presence of lighter‐colored areas in the outer layer of HMFO@C relative to HMFO is further confirmed by Figure 2e,f,h,i. These lighter areas enlarge as the glucose‐to‐HMFO ratio rises. Mn, Fe, and O elements are uniformly distributed throughout the HMFO hollow spheres, according to EDS analysis of HMFO and HMFO@C (Figures 2j–j3). Furthermore, the outer ring region of HMFO@C exhibits C element enrichment (Figures 2k‐k4). EDS energy‐dispersive spectroscopy analysis and transmission electron microscopy observations further substantiate the presence of a carbon shell layer within HMFO@C.

FIGURE 2.

(a‐c) SEM of HMFO, HMFO@C‐A, HMFO@C‐B, (d‐f) TEM of HMFO, HMFO@C‐A, HMFO@C‐B, (g‐I) HRTEM image of HMFO, HMFO@C‐A, HMFO@C‐B, (j, j1, j2, j3) EDS mapping images of HMFO: (j1) Mn, (j2) Fe (j3) O, (k, k1, k2, k3, k4) EDS mapping images of HMFO@C‐A: (k1) Mn, (k2) Fe, (k3) O, (k4) C.

By adjusting the amount of glucose added during the hydrothermal process, hollow ferrite microspheres with different carbon shell thicknesses were produced during the synthesis of magnetic core‐shell particles. The XRD patterns of hollow manganese ferrite (HMFO), hollow manganese ferrite@carbon‐A (HMFO@C‐A, MnFe₂O₄/glucose mass ratio of 1:5), and hollow manganese ferrite@carbon‐B (HMFO@C‐B, MnFe₂O₄/glucose mass ratio of 1:9) are displayed in Figure 3a. According to their XRD analysis, the HMFO diffraction peaks closely match the distinctive peaks of MnFe₂O₄ (PDF#38‐0430), and the absence of any other notable impurity peaks in the XRD patterns successfully confirms the ferrite's successful synthesis [32]. The resulting HMFO@C‐A and HMFO@C‐B samples still showed diffraction peaks that closely matched the distinctive peaks shown on the typical PDF card for MnFe₂O₄ after the outer layer of manganese ferrite was coated with carbon shells of different thicknesses [33]. This suggests that the structure of HMFO was unaffected by the carbon shell coating procedure or the carbon shell layer. The HMFO structure is preserved in HMFO@C‐A and HMFO@C‐B. However, none of the diffraction patterns showed any distinctive peaks connected to carbon, which is probably because the material's carbon crystallinity is smaller than that of MnFe₂O₄.

FIGURE 3.

(a) the XRD patterns of HMFO and HMFO@C, (b) Raman spectra of CGP aerogels, (c) Nitrogen adsorption‐desorption isotherms of CGP‐A3 aerogel, (d) Nitrogen adsorption‐desorption isotherms of CGP‐B3 aerogel, XPS spectra: (e) C 1s, (f) Mn 2p, (g) Fe 2p, (h) O 1s of CGP aerogel.

Thus, the graphene defects and graphitization degree of pure reduced graphene oxide/polyimide aerogel (GP) and hollow manganese ferrite@carbon/reduced graphene oxide/polyimide aerogel (CGP aerogel) with various magnetic particle types and contents were characterized using Raman spectroscopy. Two significant peaks near 1350 and 1580 cm⁻¹, which correspond to the D and G modes, respectively, are visible in the Raman spectra displayed in Figure 3b. The G mode represents the amount of sp2‐hybridized carbon, whereas the D mode is usually linked to the density of material defects. An important metric for assessing the degree of graphitization and defect density in carbon‐based materials is their ratio (ID/IG). Different ID/IG ratios of 1.1, 1.03, and 1.06 are shown by GP, CGP‐A3, and CGP‐B3. Compared to pure graphene/polyimide aerogels without manganese ferrite@carbon doping, the ID/IG ratios of graphene‐reinforced aerogels doped with manganese ferrite@carbon core‐shell particles are lower. This suggests improved graphitization and fewer flaws in CGP. The CGP‐A3 sample, which has the lowest ID/IG value (1.03), yet shows a very high ratio, suggesting that significant flaws in the CGP aerogel still exist. These flaws could act as polarization centers, creating interfacial polarization and improving the ability to attenuate electromagnetic waves.

Furthermore, aerogels' impedance matching and numerous electromagnetic wave reflections are directly influenced by the distribution of pore sizes inside the material. Thus, nitrogen adsorption‐desorption isotherms were used to examine the specific surface area and pore size distribution of CGP composite aerogels; the findings are displayed in Figure 3c,d. Throughout the whole pressure range, the isothermal adsorption‐desorption curves of CGP‐A3 and CGP‐B3 show a downward convex profile with no notable inflection points. The two samples had specific surface areas of 37.8 and 40.1 m²/g, respectively. Both were categorized as mesoporous materials since their pore size distributions were mostly in the 50 nm range. Electromagnetic waves enter the CGP aerogel and move randomly across these many mesoporous channels, frequently running into walls. This greatly raises the likelihood of electromagnetic wave loss inside the aerogel by expanding their propagation routes. Additionally, the presence of pores makes it possible for the aerogel to contain more air, which is highly beneficial for enhancing its impedance matching [34].

The valence states and chemical makeup of the CGP aerogel were examined using XPS characterization methods. Figure 3e–h displays the XPS spectra. Three unique peaks, representing C─C, C═N, and C═O bonds, are visible in the C1s spectra at 284.8, 285.9, and 288.3 eV, respectively. N atoms from the polyimide layer on the aerogel framework are shown by the presence of C═N. There are two distinctive peaks in the Mn 2p spectra (Figure 3e) at 653.4 and 641.6 eV, which correspond to Mn 2p_1/2 and Mn 2p_3/2, respectively. The presence of Mn²⁺ ions is confirmed by these peaks [35]. The Fe 2p spectrum of the CGP aerogel is seen in Figure 3g, with many satellite peaks and two asymmetric peaks that correspond to Fe 2p1/2 and Fe 2p3/2. Higher spin states could be connected to these paired satellite peaks [36]. The existence of Fe³⁺ in the synthesized ferrite is confirmed by peaks at 712.9 and 726.4 eV. In addition to Fe³⁺ and Mn²⁺, the valence state of Fe²⁺ also exists in the xps spectra of CGP aerogels, which may be related to the specific distribution of metal ions in the spinel crystal structure and the redox balance that often occurs in the actual preparation process. Fe³⁺ needs to be reduced to Fe²⁺ to maintain the balance of charge [37]. Lattice oxygen (OL), C═O, and surface‐adsorbed oxygen (OA) in the spinel metal oxide (MnFe₂O₄) lattice of the CGP aerogel are represented by the distinctive peaks at 529.1, 530.5, and 532.7 eV in the O 1s spectrum (Figure 3h) [38, 39, 40].

3.2. Absorption Properties

The intrinsic electromagnetic properties of microwave‐absorbing materials play a major role in determining their absorptive qualities. Examining CGP aerogel's electromagnetic characteristics offers important information for furthering studies on its capacity to absorb microwaves. The real part (ε′) and imaginary part (ε′′) of the material's complex permittivity (ε  = ε′  − jε′′) represent its capacity to store and release electric field energy from the standpoint of electromagnetic characteristics. Similarly, magnetic energy storage and magnetic loss capacities are represented by the real component (μ′) and imaginary part (μ′′) of its complex permeability (μ  = μ′  − jμ′′).

The electromagnetic parameter maps of CGP aerogels with various HMFO@C types and their doping contents are displayed in Figures 4 and 5, respectively. varied HMFO@C types and varied particle doping concentrations lead to different electromagnetic storage and loss capabilities among the generated CGP aerogel samples, according to the electromagnetic parameter maps of CGP aerogels. For every CGP sample, the real and imaginary components of the dielectric constant drop as frequency increases, as seen in Figures 4a and 5a. Compared to pure graphene/polyimide aerogels (GP), the real and imaginary parts of the dielectric constant for GCP aerogels doped with HMFO@C particles are typically higher. The increase in the real part of the dielectric constant is suppressed at the same particle doping mass when the carbon shell thickness is increased. In particular, CGP‐A3, CGP‐A2, and CGP‐A1 have lower dielectric constants than CGP‐B3, CGP‐B2, and CGP‐B1, respectively, suggesting that ε′ is influenced by the thickness of the carbon shell. The magnetic permeability values of CGP‐A3 and CGP‐B3, CGP‐A2 and CGP‐B2, and CGP‐A1 and CGP‐B1 in paired groups do not differ significantly. Whereas μ′′ is between 0 and 0.07, μ′ is between 0.95 and 1.12. Magnetic permeability trends are identical for CGP aerogels doped with the same mass of magnetic particles (Figures 4d–e and 5d,e). As a result, changing the carbon shell thickness of the HMFO@C particle has little impact on magnetic permeability and mainly affects the dielectric characteristics of the CGP aerogel. Changes in the quantity of HMFO@C particle doping are primarily responsible for the fluctuation in magnetic permeability. Figures 4c,f and 5c,f illustrate how tanδ_ε and tanδ_μ. are used to compute dielectric loss and magnetic loss power, respectively. Overall, the tanδ_ε and tanδ_μ curves of the CGP‐A and CGP‐B composite aerogels show several resonance peaks. This phenomena is caused by the magnetic HMFO@C particles in the CGP aerogel, which increase the dielectric and magnetic losses in the CGP composite aerogel by inducing dipole and interfacial polarization. Additionally, tanδ_μ is less than tanδ_ε, suggesting that dielectric loss predominates in CGP composite aerogels. Dielectric loss and magnetic loss may work in concert to improve the material's overall absorptive qualities.

FIGURE 4.

(a) real part of the dielectric constant of GP aerogel and CGP‐A aerogels, (b) imaginary part of the dielectric constant of GP aerogel and CGP‐A aerogels, (c) tangent of the dielectric loss of GP aerogel and CGP‐A aerogels, (d) real part of the permeability constant of GP aerogel and CGP‐A aerogels, (e) imaginary part of the permeability constant of GP aerogel and CGP‐A aerogels, (f) tangent of magnetic permeability for CGP‐A aerogels and GP aerogel.

FIGURE 5.

(a) real part of the dielectric constant of GP aerogel and CGP‐B aerogels, (b) imaginary part of the dielectric constant of GP aerogel and CGP‐B aerogels, (c) tangent of the dielectric loss of GP aerogel and CGP‐B aerogels, (d) real part of the permeability constant of GP aerogel and CGP‐B aerogels, (e) imaginary part of the permeability constant of GP aerogel and CGP‐B aerogels, (f) tangent of magnetic permeability for CGP‐B aerogels and GP aerogel.

It is crucial to do a fair analysis and evaluation of the dielectric loss characteristics and magnetic loss capabilities of CGP aerogel materials in order to properly analyze the absorptive properties of CGP composite aerogel materials. Hollow ferrite magnetic particles contained in the outer carbon layer and adsorbed onto the three‐dimensional graphene framework are the source of the magnetic loss seen in CGP aerogels. Both CGP‐A and CGP‐B aerogels have resonance peaks in two different frequency ranges: 2–10 and 10–18 GHz, as seen in Figures 4e and 5e. Resonance peaks below 5 GHz are linked to wall resonance, according to Aharoni's theory. Peaks above 10 GHz are caused by exchange resonance, and peaks below 10 GHz are caused by natural resonance. The following formula is used to determine the eddy current coefficient:

$$C_0={\mu }^{\prime \prime }{\left({\mu }^{\prime }\right)}^{-2}{f}^{-1}$$ (1)

According to the skin effect, its current coefficient will not change with frequency if eddy current effects are the only cause of magnetic loss. Figure 6a,d display the present coefficients of CGP‐A and CGP‐B. The changes in C₀ values within the 10–18 GHz band are comparatively less than those in the 2–10 GHz frequency band, suggesting that eddy current losses mostly take place in the 10–18 GHz range. Conductivity is the primary determinant of dielectric losses. The following formula can be used to determine the alternating conductivity (α) of CGP aerogels:

$$\alpha =\frac{\sqrt{2}\pi f}{c}\sqrt{\left({\epsilon }^{\prime \prime }{\mu }^{\prime \prime }-{\epsilon }^{\prime }{\mu }^{\prime }\right)+\sqrt{\left({\epsilon }^{\prime \prime }{\mu }^{\prime \prime }-{\epsilon }^{\prime }{\mu }^{\prime }\right)+{\left({\epsilon }^{\prime \prime }{\mu }^{\prime }+{\epsilon }^{\prime }{\mu }^{\prime \prime }\right)}^2}}$$ (2)

FIGURE 6.

(a) frequency dependence of C₀ for CGP‐A composite aerogel samples, (b) attenuation constant curves of CGP‐A composite aerogel samples, (c) Cole‐Cole semicircle of CGP‐A composite aerogel samples in frequency ranging from 2 to 18 GHz, (d) frequency dependence of C0 for CGP‐B composite aerogel samples, (e) attenuation constant curves of CGP‐B composite aerogel samples, (f) Cole‐Cole semicircle of CGP‐B composite aerogel samples in frequency ranging from 2 to 18 GHz.

The electrical conductivity of CGP‐A and CGP‐B composite aerogel progressively rises with frequency in the 2–18 GHz range, as seen in Figure 6b,e. From the standpoint of CGP aerogels' attenuation coefficient, the addition of HMFO@C will raise graphene aerogel'attenuation coefficient to varied degrees when compared to pure graphene/PI aerogels; this increase is positively connected with the doping content. The magnetic core‐shell structure and the numerous heterogeneous contacts created by the even dispersion of nanoparticles may be connected to this [41]. The composite aerogel possesses several loss modes, including good dielectric loss, magnetic loss, and polarization loss, following the addition of hollow MnFe₂O₄@C. On the one hand, manganese ferrite offers strong magnetic losses like natural resonance and eddy current loss, and the combined effect of these three can greatly increase the overall attenuation constant; on the other hand, RGO and carbon shell form a conductive network to form a conductive loss, and the existence of a heterogeneous interface helps to enhance the interface polarization effect. CGP‐A3 and CGP‐B3 exhibit higher attenuation constants than other samples, suggesting superior attenuation capacity. In comparison to other samples, CGP‐A3 and CGP‐B3 show noticeably greater attenuation constants, suggesting better attenuation capabilities.

The relationship between the real part (ε′) and imaginary part (ε′′) of the dielectric constant can be used to further analyze the loss mechanisms of CGP composite aerogels based on the Debye theory and the Cole‐Cole model. This allows for the analysis of various loss mechanisms, including conductive loss and polarization relaxation.

$${\left({\epsilon }^{\prime }-\frac{{\epsilon }_s+{\epsilon }_{\infty }}{2}\right)}^2+{\left({\epsilon }^{\prime \prime }\right)}^2={\left(\frac{{\epsilon }_s-{\epsilon }_{\infty }}{2}\right)}^2$$ (3)

Figure 6c,f illustrate the relationship between the real and imaginary parts of the dielectric constant for CGP‐A and CGP‐B composite aerogel samples in the 2–18 GHz range. A polarization relaxation process is shown by each semicircle that points downward. The figures' several distinguishable semicircles show that CGP displays several polarization relaxation processes in the 2–18 GHz range. Dipole polarization caused by the abundant surface functional groups of RGO, interfacial polarization resulting from the heterogeneous interface at the RGO/HMFO@C boundary, and new interfacial polarization relaxation produced by the heterogeneous interface between the magnetic manganese ferrite and its outer carbon shell following the introduction of glucose into the carbon shell are the main sources of these polarization relaxations. Furthermore, there is a tailing effect in the second half of the CGP's Kerr circle, which corresponds to noticeably higher real and imaginary parts of the dielectric constant. This suggests that the attenuation of low‐frequency electromagnetic waves is significantly influenced by conduction losses. The components of the dielectric loss of CGP composite aerogels, conductivity loss () and polarization relaxation loss (), were analyzed using the following formula in order to illustrate this and clarify the microwave absorption mechanism of these aerogels.

(4)

The percentage of conductivity loss and polarization loss in CGP composite aerogel materials across several frequency bands is shown in Figure 7. In both CGP‐A and CGP‐B composite aerogels, conductive loss makes up a comparatively minor percentage, as seen in Figure 7a,d. The incomplete reduction of graphene oxide within the CGP aerogel may have occurred because the thermal treatment temperature used to prepare the CGP‐A and CGP‐B composite aerogels was 300°C. As a result, CGP's conductivity is comparatively low, and the aerogel's three‐dimensional conductive network is discontinuous, which makes it challenging to create enough efficient conductive pathways and reduces conductive loss. Both CGP‐A and CGP‐B show polarization loss percentages above 97%, suggesting that polarization loss predominates in CGP aerogels (Figure 7b,e). The carbon shell layer in CGP aerogels creates contact sites between the graphene layers by serving as a bridge between HMFO and graphene. The interfacial polarization loss can be enhanced concurrently by the formation of interfacial polarization sites at the interface between the HMFO core and the carbon shell layer. Furthermore, the internal electron cloud distribution is disturbed by the abundance of defects and functional groups in graphene, creating localized dipoles that increase dipole polarization and raise the total dielectric loss of CGP materials. Graphene aerogels develop a continuous three‐dimensional conductive network as a result of the graphene sheets. This network allows electrons to flow, and when they do so in an alternating electric field, a conductive current is produced. Scattering happens when this current passes across faults or interfaces, dissipating electromagnetic energy and turning it into heat. Magnetic manganese ferrite cores have far lower conductivity than graphene. The continuous conductive network is somewhat disrupted by HMFO@C scattered across graphene layers. Thus, the electrical conductance loss of CGP‐A and CGP‐B composite aerogels is lower than that of GP aerogel without HMFO@C addition. Furthermore, the disruption to the CGP aerogel's conductive network is more severe the higher the HMFO@C loading. As a result, both the CGP‐A and CGP‐B composite aerogels have lower electrical conductance losses than the GP aerogel, as seen in Figure 7c,f. The continuous conductive network in CGP‐A is gradually broken in CGP‐A1, CGP‐A2, and CGP‐A3, as evidenced by a slow decline in . The same concept applies to the change in for CGP‐B. The contact surfaces between materials with varied electrical characteristics have a significant impact on polarization loss, especially interfacial polarization. The addition of HMFO@C to CGP‐A and CGP‐B aerogels greatly enhances the number of introduced heterogeneous interfaces when compared to GP aerogel with 0 mg HMFO@C content. The formation of heterointerfaces increases with the HMFO@C content. As seen in Figure 7c,f, the CGP‐A and CGP‐B composite aerogels have greater polarization losses than the GP aerogel, with losses rising with the concentration of HMFO@C doping. Multiple loss pathways are present in CGP aerogel, according to a preliminary study. Based on transmission line theory, the Hollow‐MnFe₂O₄@C/RGO/PI composite aerogel's electromagnetic wave absorption capacity can be intuitively evaluated by calculating its reflection loss and impedance matching utilizing the complicated permittivity and permeability of CGP aerogel. The following is the precise calculation formula:

$$RL\left(dB\right)=20lg\left|\frac{Z_{in}-1}{Z_{in}+1}\right|$$ (5)

$$Z_{in}=Z_0\sqrt{\frac{{\mu }_r}{{\epsilon }_r}}\tan \left[j\left(\frac{2\pi fd}{c}\right)\sqrt{{\epsilon }_r{\mu }_r}\right]$$ (6)

FIGURE 7.

(a) conductive loss of CGP‐A composite aerogel, (b) Polarization loss of CGP‐A composite aerogel, (c) Average conductive loss and average polarization loss of CGP‐A composite aerogel, (d) Conductance loss of CGP‐B composite aerogel, (e) Polarization loss of CGP‐B composite aerogel, (f) Average conductance loss and average polarization loss of CGP‐B composite aerogel.

Good impedance matching is a prerequisite for the composite aerogel to have better electromagnetic wave absorption performance. Compared with pure graphene/PI aerogel, the hollow manganese ferrite@carbon magnetic particles are embedded into the PI/RGO structure, which can adjust the overall dielectric properties of the material and reduce the overall conductivity of the material [48]. The high air content inside the aerogel makes the impedance matching between the CGP aerogel and the free space more closely, and the three‐dimensional porous network also helps to enhance the scattering effect of various electromagnetic waves [49]. These synergistic effects make the complex permittivity and complex permeability of CGP aerogels effectively regulated, the impedance matching state of the air‐material interface has been greatly optimized (Figure S4), the surface reflection of electromagnetic waves is reduced, and more incident electromagnetic waves can effectively enter the material. The reflection loss maps of CGP composite aerogel and GP aerogel samples are shown in Figure 8a–g. In comparison to GP aerogel, all CGP composite aerogel samples showed lower minimum reflection loss values, suggesting that the addition of HMFO@C magnetic core‐shell particles significantly improves graphene aerogel's microwave absorption capabilities. Furthermore, the improvement in absorptive performance differs depending on the type of HMFO@C particle. All things considered, the CGP‐A composite aerogel with HMFO@C‐A has better absorptive qualities than the CGP‐B aerogel with HMFO@C‐B. With a minimum reflection loss of −68.2 dB, CGP‐A3 has the best absorption performance of any CGP aerogel. Its effective absorption bandwidth is 7.70 GHz at a matching thickness of 2.80 mm. Among CGP‐B composite aerogels, CGP‐B3 exhibits a minimum reflection loss of −44.40 dB and an effective absorption bandwidth of 7.49 GHz at a matched thickness of 3.30 mm. Figure 8h,i summarize the statistical analysis of the minimal reflection loss and effective absorption bandwidth (EAB) for all CGP composite aerogel samples and GP aerogel at all matching thicknesses. With increasing HMFO@C doping levels, it was found that both the minimum reflection loss and EAB somewhat increased. This is because after the electromagnetic wave enters the interior of the CGP aerogel, the adjacent RGO sheets are stacked and the three‐dimensional porous network can effectively increase the reflection and scattering times of the electromagnetic wave inside the material, thereby extending its propagation path and achieving the purpose of attenuating the electromagnetic wave energy [50]. In addition, according to Cao's electron hopping model, electrons can absorb the energy of electromagnetic waves, migrate on the surface of RGO, and jump between adjacent RGO sheets, resulting in conductive loss. The electromagnetic wave collides with the lattice during the propagation inside the aerogel, and its electromagnetic energy will be converted into heat energy [51]. As Raman and XPS spectra show, oxygen‐containing functional groups remain on the surface of RGO. Higher ID/IG also indicates a greater degree of defects, which can be used as polarization centers to induce dipole polarization and further attenuate electromagnetic energy. The hollow MnFe₂O₄@C sphere with core‐shell structure is uniformly distributed on the surface of the wrinkled graphene sheet, which can be used as the ‘seed’ of interfacial polarization. And it can contact with the graphene sheet to produce a large number of heterogeneous interfaces. The space charge accumulates at these heterogeneous interfaces to form interfacial polarization and enhance its interfacial polarization loss ability as an EMA material [52, 53]. In addition, the presence of hollow MnFe₂O₄@C will stimulate the magnetic loss, significantly enhance the resonance effect and eddy current loss, and improve the magnetic loss contribution of the composite aerogel. The synergistic effect of magnetic loss and interfacial polarization loss makes the material have excellent electromagnetic wave attenuation performance.

FIGURE 8.

(a‐c) Absorption performance maps of CGP‐A aerogel, (d‐f) Absorption performance maps of CGP‐B aerogel, (g) Absorption performance map of GP aerogel, (h) Minimum reflection loss values of GP, CGP‐A, and CGP‐B aerogels, (i) Effective absorption bandwidths of GP, CGP‐A, and CGP‐B aerogels, (j) Microwave reflection loss of CGP‐A3 aerogel compared with the quarter‐wavelength matching theory, (k) Comparison of the wave absorption performance with other aerogels [42, 43, 44, 45, 46, 47].

The electromagnetic parameter curves of CGP‐A and CGP‐B composite aerogels at various compression levels are displayed in Figure 9a–f. Examining the evolution of CGP composite aerogels' electromagnetic parameters during compression is crucial to comprehending how the volume change following external impact influences their wave absorption characteristics. Volume compression tests were conducted on CGP‐A3 and CGP‐B3 samples from the CGP composite aerogels. CGP‐A3 and CGP‐B3 are the names given to the CGP aerogels in their native state (0% volume compression). CGP‐A3%–30% and CGP‐A3%–50% are samples of CGP‐A3 aerogel that have been compressed by 30% and 50%, respectively. Similarly, the comparable CGP‐B3 samples are indicated by CGP‐B3%–30% and CGP‐B3%–50%. Supporting Material Table S2 has specific naming guidelines. The dielectric constant of all CGP samples in Figure 9a–f falls as frequency increases, whereas the actual part of the CGP dielectric constant increases with increasing compression. Many resonance peaks are dispersed over the 2–18 GHz frequency range in terms of magnetic permeability. Compressed CGP aerogel demonstrates superior electromagnetic wave attenuation capability when combined with analysis of the attenuation constant, polarization loss, and conductive loss ratio (Figure S3, Supplementary Material), with polarization loss continuing to be the predominant form of electromagnetic wave loss.

FIGURE 9.

(a) real part of the dielectric constant of CGP aerogels before and after compression, (b) imaginary part of the dielectric constant of CGP aerogels before and after compression aerogels, (c) tangent of the dielectric loss of CGP aerogels before and after compression, (d) real part of the permeability constant of CGP aerogels before and after compression, (e) imaginary part of the permeability constant of CGP aerogels before and after compression, (f) tangent of magnetic permeability for CGP aerogels before and after compression.

The outstanding absorptive capabilities of the pre‐ and post‐compression CGP composite aerogels are evident (Figure 10a–f). At a matched thickness of 2.40 mm, CGP‐A3%–30% attained a minimum reflection loss of −56.41 dB with an EAB of 6.72 GHz. CGP‐B3%–30% and CGP‐B3%–50% further improved their initial absorptive properties, achieving minimum reflection losses of −79.49 and −63.01 dB, respectively, with effective absorption bandwidths of 7.50 and 7.87 GHz. The minimum reflection loss remained at −20.35 dB when the CGP‐A3 aerogel was compressed by 50%, and the EAB reached 7.54 GHz at a matched thickness of 2.70 mm. These phenomena most likely result from the CGP aerogel's volumetric contraction due to applied tension. This results in richer electrical pathways, increased physical contact between RGO sheets, and decreased interlayer space in graphene. Furthermore, in the CGP aerogel, volumetric shrinkage greatly increases contact chances between magnetic particles and graphene sheets, creating more heterogeneous surfaces and maximizing interfacial polarization loss. On the other hand, severe compression upsets the balance between impedance matching and electromagnetic wave attenuation capacity. The aerogel's pores are squeezed under high strain circumstances, which lowers porosity and quickly deteriorates impedance matching. The CGP aerogel's ability to absorb microwaves may be greatly reduced if its electromagnetic wave attenuation capacity is inadequate to offset the impedance mismatch. When CGP‐A3 aerogel's compression rises from 30% to 50%, this problem appears. The region where Z = 1 on the impedance matching curve of CGP‐A3%–50% quickly decreases and disappears as compression rises, failing to achieve optimal impedance matching, as can be seen when examining the impedance matching performance of these CGP aerogel samples in Figure 10g–l. The maximum EAB of each sample exceeded 6.7 GHz, with the highest reaching 7.87 GHz, despite the fact that all CGP‐A3 and CGP‐B3 samples showed significant minimum reflection loss following external stress application. This suggests that CGP composite aerogels exhibit some degree of absorptive stability under compressive stress conditions, maintaining excellent microwave absorption performance even after volume contraction.

FIGURE 10.

(a,b) Three‐dimensional absorption performance diagrams for CGP‐A3%–30% and CGP‐A3%–50% aerogels, (c) minimum reflection loss values of CGP‐A3 and CGP‐B3 aerogels before and after compression, (d,e) three‐dimensional absorption performance diagrams for CGP‐B3%–30% and CGP‐B3%–50% aerogels, (f) effective absorption bandwidths of CGP‐A3 and CGP‐B3 aerogels before and after compression, (g‐l) impedance matching of CGP‐A3 and CGP‐B3 aerogels.

The remarkable absorptive qualities of CGP aerogel are explained by the cooperative action of several loss processes. The possible loss processes in CGP aerogel are fully depicted in Figure 11. First, electromagnetic waves have more ways to propagate through CGP composite aerogel due to its many holes. In addition to optimizing impedance matching, this allows the greatest amount of electromagnetic energy to enter the material deeply. Furthermore, electromagnetic waves suffer numerous reflection and scattering inside the CGP composite aerogel due to the hollow ferrite magnetic particles, which effectively attenuate the waves [54]. The remarkable absorptive qualities of CGP aerogel are explained by the cooperative action of several loss processes. The possible loss processes in CGP aerogel are fully depicted in Figure 11. First, the many openings in CGP composite aerogel maximize the material's impedance matching while also expanding the electromagnetic wave propagation channels within it. This makes it easier for electromagnetic waves to enter the inside of the material as much as feasible. Furthermore, electromagnetic waves suffer numerous reflections and dispersion within the CGP composite aerogel due to the hollow ferrite magnetic particles, which results in efficient attenuation. Defects and functional groups on the RGO flake surfaces in the CGP composite aerogel, as well as HMFO@C attached to the RGO surfaces, function as polarization centers to produce dipole polarization. Multiple heterogeneous interfaces, such as hollow ferrite cores‐carbon shells and core‐shell particles‐graphene layers, are created within the CGP composite aerogel by the presence of HMFO@C. These interfaces provide extremely large interfacial polarization losses because they are active areas for charge buildup and relaxation. Together, the three‐dimensional graphene network framework inside the CGP aerogel creates a conductive network that promotes electron hopping and migration between graphene surfaces and carbon shells, increasing conductive losses. Lastly, the addition of ferrite magnetic particles allows for the synergistic interplay of several loss mechanisms inside CGP by introducing a magnetic loss mechanism that is different from dielectric loss. This strikes a compromise between robust absorption and broadband performance.

FIGURE 11.

Schematic diagram of CGP aerogel microwave absorption.

3.3. Mechanical and Thermal Insulation Properties

Improving this lightweight CGP aerogel composite's durability and compressive strength can greatly increase its service life, building on its exceptional and steady absorptive qualities under regulated stress. With a density of only 12.33 mg/cm³, the CGP aerogel is remarkably light, as seen in Figure 12a, allowing it to be easily supported by a flower. The compression and subsequent release of CGP aerogel are shown in Figure 12b, highlighting the material's remarkable elasticity. The CGP composite aerogel's cross‐linked polyimide three‐dimensional network is the source of its durability. The CGP composite aerogel framework has some elasticity thanks to the polyimide. The CGP aerogel's three‐dimensional network framework functions as a quick buffer under compressive stress, allowing for uniform stress distribution to maintain the material's structural integrity [55, 56, 57]. The CGP composite aerogel's compressive stress‐strain curve under 50% strain for 20 cycling cycles is shown in Figure 12c. Additionally, this CGP composite aerogel has outstanding thermal insulation and fire resistance qualities. The CGP composite aerogel maintains its structural integrity after 30 s of exposure to an alcohol lamp's outer flame (Figure 12d). The CGP aerogel sample's surface temperature stayed at just 19.3°C when it was placed on a heating platform with an initial temperature of 75°C. The aerogel sample's surface temperature reached 37.9°C after three minutes, when the heating platform temperature rose beyond 100°C. The sample temperatures stayed at 39.2, 41.1, and 45.4°C after 30, 60, and 120 min, indicating exceptional thermal stability, whereas the heating platform temperatures surpassed 129.8, 132.4, and 132.9°C, respectively. The CGP composite aerogel's remarkable thermal insulation qualities are highlighted by the lack of a noticeable increase in surface temperature. Polyimide adds elements to this graphene composite aerogel that have different thermal conductivity characteristics from graphene [58]. The presence of interfacial thermal resistance prevents heat from spreading effectively throughout the material because of the different physical properties of these constituents. Thermal conduction is further inhibited by the aerogel's thick pore structure, which efficiently suppresses convective heat transport inside the substance [21, 59].

FIGURE 12.

(a) digital image of CGP aerogel, (b) digital image of CGP aerogel's force‐induced compression and rebound, (c) cyclic compression test of CGP composite aerogel, (d) digital photos of CGP aerogel before and after burning in flames, (e) thermal insulation performance graph of PGS composite aerogel.

4. Conclusion

In this study, lightweight hollow ferrite/graphene oxide/polyimide composite aerogels (hollow MnFe₂O₄@C/RGO/PI) with favorable elasticity were created using the straightforward thermohydraulic approach and freeze‐drying methodology. The prepared composite aerogel exhibits an extremely low density of merely 12.33 mg/cm³. Significant minimum reflection loss and a wide effective absorptive bandwidth were obtained at comparatively low matching thicknesses by using techniques like adding hollow ferrite@carbon magnetic particles, modifying the magnetic particle content, and changing the carbon shell thickness. The minimum reflection loss is −68.04 dB at a matched thickness of 2.80 mm, and the effective absorption bandwidth (EAB) is up to 7.92 GHz. This aerogel retains its effective absorptive qualities throughout volumetric deformation under compressive stress conditions. At 30% compression, RL achieves −80.0 dB with a maximum EAB of 7.50 GHz in comparison to its native condition. At 50% compression, RL stands at −63.01 dB with a maximum EAB of 7.87 GHz, still demonstrating remarkable absorptive capability. This robustness is probably due to the aerogel's large conductive network and increased polarization loss, where better attenuation capabilities offset the impedance matching degradation that would otherwise reduce absorptive efficiency. In addition to its remarkable absorptive qualities, this aerogel has extraordinary fire resistance and thermal insulation qualities. Its broad‐spectrum absorptive performance, mechanical flexibility, fireproofing, and thermal shielding qualities greatly increase its potential for use in challenging and complicated situations. Thus, this content advances national defense security, electronic information systems, and related sectors.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: smtd70791‐sup‐0001‐SuppMat.docx.

Data Availability Statement

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