Up to now, the range of X-band of electromagnetic waves is still the most critical frequency of electromagnetic waves in civil and military aerospace and space technology (Micheli et al., 2010; Vinayasree et al., 2013; Silva and Rezende, 2021). Though with increasing convenience to human life, the ensuing electromagnetic pollution can greatly impact human health and sophisticated electronic devices. Thus, microwave absorbing materials have drawn the attention of researchers worldwide. A microwave absorbing material with lightweight, strong absorption, small thickness, and wide absorption bandwidth is considered ideal for attenuating electromagnetic energies (He et al., 2020; Liang et al., 2020; Wang et al., 2020).

As one of the essential absorbing properties, impedance matching determines whether electromagnetic waves can enter from free space into the absorber, which is an essential factor that must be considered when designing an absorber. The current single absorbing materials, including magnetic and electric loss absorbing material, can not achieve good impedance matching conditions due to the large gap between complex permittivity and permeability (Li et al., 2018). Apart from that, the attenuation constant is another factor that must be considered for designing a microwave absorbing material. When electromagnetic waves pass through the surface of the absorber and enter the interior, it is vital that incident waves can be attenuated rapidly to achieve effective absorption (Li et al., 2022). The perfect combination of attenuation constants and impedance matching will be a top priority for scholars designing ideal absorbers (Shen et al., 2021).

Typical nanostructured carbon, including graphite, CB and carbon nanotubes (CNTs) materials, are usually applied as microwave absorber given their excellent dielectric properties, low density and other unique physical and chemical properties (Li et al., 2018). In recent years, graphene, as a new type of nanostructured carbon material, has become a focus of attention for its excellent electrical, thermal, mechanical properties and high specific surface area (Song et al., 2021). Pure graphene absorbers mainly rely on conduction losses to attenuate electromagnetic wave energies (Zhang et al., 2019). However, good electrical conductivity hampers impedance matching, leading to the inability of electromagnetic waves to cross the surface of the absorber. This means that the best way to take advantage of graphene’s superb dielectric loss is to improve impedance matching. As another member of the carbon family, carbon black possesses both excellent dielectric properties and low density. One of the benefits of using carbon black as a filler between graphene sheets is that it preserves graphene’s dielectric properties while also reducing its conductivity, thus improving the wave absorption properties of graphene.

As of now, the most widely studied magnetic microwave-absorbing materials include Fe₃O₄, cobalt-zinc ferrite, magnetite nanoparticles, Ni, and carbonyl iron powders (CIP) (He et al., 2017; Li et al., 2017; Liao et al., 2018; Xu et al., 2018; Wang et al., 2019; Wang et al., 2020; Reyes-Ortega et al., 2021; Wang et al., 2021). Carbonyl iron powders (CIP) are commonly used in composites because of their high saturation magnetization strength, low coercivity, and good magnetic properties (Sista et al., 2021). However, the high density and filling ratio of CIP prevent it from being widely used. In general, it is hard to achieve the desired electromagnetic wave absorption properties solely by filling absorbers with dielectric or magnetic media. Thus, researchers attempt to combine magnetic and carbon materials in composite or use hybridized approaches to improve the impedance matching as well as its microwave loss performance (Yang et al., 2020; Han et al., 2021; Jang et al., 2021). Recently, Liu et al. (Liu et al., 2010) investigated the microwave absorption performance of monolayer coatings consisting of CIP and CB in the range of 2–18 GHz. They found that at a sample thickness of 1.5 mm, the absorption bandwidth broadened with increasing CB content, especially when the ratio of CIP and CB was 25 wt.% respectively, and the absorption bandwidth (RL < −4 dB) reached 10.1 GHz (7.9–18 GHz). Lopes et al. (Lopes et al., 2020) prepared hybrid composites with 30 wt.% MnZn ferrites, 50 wt.% CIP and 1 wt.% CNTs and investigated their electromagnetic absorption properties on silicone rubber in the X-band (8.2–12.4 GHz). The RL_max was found at 2.3 mm thickness, with a value of −21 dB. Xu et al. (Xu et al., 2015)evaluated the microwave absorption performance of the graphene doped with CIP and found that the CIP/GO composites filled with 5 vol% had excellent wave absorption performance at 2–18 GHz, with the RL_max of - 25.14 dB at 6 mm and -26.52 dB at 8 mm. The above studies showed that many scholars have tried to combine the properties of CIP and carbon-based materials to prepare an ideal microwave absorbing material and have indeed achieved some progress. However, there still exist deficiencies such as the lack of absorbing bandwidth, the low absorbing intensity, and the excessive thickness.

In order to overcome these deficiencies, the present RGO-CB/CIP composite was synthesized via a three-step method. GO was prepared based on the Hummers method in the first step and followed by the one-pot method to attach the CB with RGO during the reduction of GO to GRO. Finally, the group on RGO-CB was activated by acidification treatment while roughening the CIP surface to facilitate the binding of CIP to RGO-CB. This work concentrates on the electromagnetic and microwave absorption properties of RGO-CB/CIP composites in 8–12.4 GHz. The effects of CIP loading and absorber thickness on the microwave absorption properties of the composites were discussed, and the microwave absorption mechanism was analyzed in depth.

RGO-CB/CIP composite’s XRD pattern was depicted in Figure 2. It showed the presence of four phases of carbon in the RGO-CB/CIP composite by comparing the test data with the PDF#89-8487 standard card, with 2θ values equal to 26.55°, 42.37°, 50.71°, and 59.91°, which can be attributed to (002), (100), (102) and (103) crystal planes of RGO or CB, respectively. Compared with the standard card PDF#72-1081, there were majorly four CIP crystal planes in the RGO-CB/CIP composite, (−111), (002), (−112) and (130), corresponding to the 2θ values of 16.88°, 20.06°, 23.26°, and 40.57°, respectively. The characteristic diffraction peaks show that the CIP successfully grew in the composite.

Figure 3 shows the RGO-CB/CIP composite’s SEM image. Obviously, there are abundant crumples on the surface of RGO, and multiple heterogeneous interfaces are formed through the contact between individual folds or the connection of CB and CIP, which would lead to multiple interface polarization. Meanwhile, CB, CIP and RGO were superimposed in sequence, forming a multi-stage structure. The interface polarization and multiple scattering characteristics of the multi-stage structure might effectively improve the absorbing properties (Weng et al., 2022). It was evident that the nanospheres were tightly adhered to the RGO surface, especially between the individual folds. Therefore, the two-dimensional graphene nanosheets are developed into three-dimensional structural composite materials.

Figure 4 shows the morphology of the RGO-CB/CIP composite taken by the transmission electron microscope. From Figures 4A,B, it can be intuitively observed that the soft RGO film is like a giant net covering CB and CIP, forming a cladding structure. This structure would easily lead to the confinement effect (Hao et al., 2016) and could significantly improve the electromagnetic wave attenuation capability of the RGO-CB/CIP composite. By performing a high-resolution TEM (Figure 4D) of the interface shown in Figure 4C, it can be confirmed that CIP and CB were tangled with each other by the lattice fringe spacing of ∼0.52 and ∼0.33 nm corresponding to the d-spacing of CIP(−111) and carbon (002), respectively. It provides visual structural evidence for the existence of interface polarization to attenuate microwaves in the composite.

The as-prepared samples were applied to the test of magnetic properties at room temperature and the magnetic field was 6,000 Oe. The hysteresis loops of the samples are shown in Figure 5. It is evident that the growth of CB has limited influence on the magnetic properties of RGO, while the saturation magnetization of the RGO-CB/CIP is much higher than that of RGO and RGO-CB samples since the CIP is a typical magnetic material, which shows that the CIP is successfully growing with the RGO-CB.

RL (reflection loss) was calculated from the relative complex permittivity ( ε r ) and relative complex permeability ( μ r ) measured by the vector network analyzer. According to transmission line theory, RL can be calculated by. RL ( dB ) = 20lg | Z in -1 Z in +1 | (1) Z i n = μ r ε r tanh [ j ( 2 f π c d ) μ r + ε r ] (2) Where Z _in is the normalized input impedance, and d is the thickness of the material. Effective microwave absorption is defined as less than 10% reflectance, i.e., RL < −10 dB, and the corresponding frequency range is the effective absorption bandwidth (EAB). Absorption and reflection losses are shown in Figure 6 for the various thicknesses of samples, and all samples have good microwave absorption in X-band. Based on transmission line theory and calculated S1’s electromagnetic parameters, it is found that the EAB of S1 is equal to 4.2 GHz, which covers the X-band (8.2–12.4 GHz) with a maximum of RL_max of −16.88 dB at 2.6 mm (Figure 6A). The RL_max of −18.59 dB and −17.28 dB are achieved by S2 at 2.6 and 2.7 mm thicknesses, respectively, with full EAB coverage of the X-band (Figure 6B). RL_max of −19.78 dB and -20.92 dB for S3 and S4 were achieved (Figures 6C,D). Meanwhile, the EAB of both S3 and S4 covered the X-band at 2.7 mm. Aside from this, it is clear from the graph that the increase of CIP content is positively associated with the maximum intensity of RLmax for the four samples in the 2.0–3.0 mm range, with RLmax values of −19.00 dB, −21.14 dB, −22.59 dB, and −29.17 dB, corresponding the thickness of 2.4 mm (S1), 2.4 mm (S2), 2.4 mm (S3) and 2.5 mm (S4,) respectively.

The absorption properties of electromagnetic waves are closely related to the complex permittivity and complex permeability. In general, the real part of the complex permittivity ( ε r = ε ′ − i ε ′ ′ ) and the real part of the complex permeability ( μ r = μ ′ − j μ ′ ′ ) represents the material’s ability to store electrical and magnetic energy, respectively. In contrast, the imaginary part of the complex permittivity ( ε ′ ′ ) and the imaginary part ( μ ′ ′ ) of the complex permeability represents the ability of the material to attenuate electrical energy and magnetic energy, respectively. According to Figure 7A, the real ( ε ′ ) and imaginary ( ε ′ ′ ) parts of the dielectric constant decrease with increasing frequency, which is typical of dispersive behaviour and exacerbates polarization hysteresis at high frequencies. Figure 7A shows that the highest values of S2 are followed by S1, S3, and S4 in ascending order. Moreover, as CIP content increases, permittivity falls steadily apart from an exception for S2, which may result from moderate conduction loss and polarization loss.

Based on the conductive network manner proposed by Cao and his coworker, the electron hopping between CB-RGO layers significantly affects the conductivity of the composite (Wang et al., 2018). When introduced, CIP exhibits resistive properties that hinder inter-layer electron hopping and intra-layer electron migration, reducing conductive network connections and lowering the conductivity of the composites.

According to the Debye theory, the ε ′ and ε ′ ′ of the dielectric constants can be described as follows. ε ′ = ε ∞ + ε s − ε ∞ 1 + ω 2 τ 2 (3) ε ′ ′ = ε s − ε ∞ 1 + ω 2 τ 2 ω τ + σ ω ε 0 (4)

Based on the two equations above, the relationship between ε ′ and ε ′ ′ can be calculated as ( ε ′ − ε s + ε ∞ 2 ) 2 + ( ε ′ ′ ) 2 = ( ε s − ε ∞ 2 ) 2 (5)

In the above equations, ω = 2 π f and f is the frequency. Where τ , ε ∞ and ε s are the relaxation time, the permittivity at an infinite frequency and the static dielectric constant, respectively. The relationship graph between ε ′ and ε ′ ′ was shown in Figure 7B, and there are a considerable number of semicircles (denoted as Cole-Cole semicircle), which indicates the corresponding Debye relaxation process (Deng et al., 2020). In addition to the dipole polarization caused by graphene defects and functional groups, from the four samples in this paper, it can be observed that S1 does not have a distinct Debay ring. With the increase in the amount of CIP, S2 shows a few semicircles, S3 has the highest number of semicircles, and the number of semicircles in S4 decreases when the proportion of CIP and CB/RGO reaches 3:1. It can be speculated that the addition of CIP forms a interface between RGO-CB, leading to the aggregation and inhomogeneous distribution of charges at the interface between RGO-CB/CIP, which generates macroscopic electric moments. Under the precondition of ensuring the dielectric properties of RGO-CB, increasing the amount of CIP can form more interface, resulting in more charge accumulation at the interface and enhanced interfacial polarization within the material. Furthermore, a visible peak appears in the curve of S3 at 10 GHz, which indicates the most robust interfacial polarization in S3. Conversely, as can be seen from S4, a further increase in the CIP content diminishes the electrical conductivity of the composite, resulting in weaker interfacial polarization.

As can be seen in Figure 8A, the mean values of μ ′ and μ ′ ′ for all samples generally appear to increase with the amount of CIP and show clear resonance peaks, indicating that the CIP causes magnetic resonance. Sample S2 has the highest resonance peak, which means that the magnetic loss is the strongest. The manners of magnetic loss generally include hysteresis loss, domain wall resonance, natural resonance, and eddy current loss. In the frequency range of 8–12.4 GHz, the ways of magnetic loss are mainly natural resonance and eddy current loss (Zhao et al., 2019). According to the principle of eddy current loss, the value C 0 = μ ′ ′ ( μ ′ ) − 2 f − 1 would remain constant with frequency (Zhou et al., 2019). The value C 0 of the four samples, as shown in the Figure 8B, fluctuates within the 8–12.4 GHz frequency, indicating that the electromagnetic magnetic loss manner of the RGO-CB/CIP composite in this X-band does not contain eddy current loss or is not strong enough. It can be inferred from above that the primary manner of magnetic loss for RGO-CB/CIP composite is natural resonance.

Figure 9 shows the dielectric loss tangent and the magnetic loss tangent of the RGO-CB/CIP composite from 8 to 12.4 GHz. The dielectric loss tangent is much larger than the magnetic loss tangent, indicating that the dielectric loss is dominant among the two loss manners. As CIP is a kind of magnetically lossy absorbing material, the average value of the magnetic loss tangent increases gradually as the CIP content. However, something is unexpectable that the content of CIP increasing makes the dielectric loss tangent of S2 larger than that of S1. These results suggested that adding an appropriate amount of CIP could increase the number of heterogeneous interfaces in the composite, enhancing the interfacial polarization of the composite and improving its dielectric loss strength. However, with the continuing increase of CIP, the dielectric loss tangent value of S3 decreases abruptly and reaches its lowest value at S4. This result also corroborates the previous finding of the decrease in S3 and S4 Debay rings. In short, the excessive addition of CIP weakens the connection points between graphene layers, and widens their distance, leading to the difficulty of electron hopping between the layers, thus hindering the interfacial polarization. As the CIP is in a resistive state on graphene, excessive concentration also inevitably leads to a less efficient conductive network and makes it difficult for conductive losses to complete for electromagnetic waves attenuation. M η = 2 × Re [ μ r ε r ] | μ r ε r | 2 + 1 (6) Where Re [X] is taken as the real part of the complex number X, ε r denotes the relative complex permittivity and μ r is the relative complex permeability. The ideal impedance matching condition is that the relative wave impedance of the medium is equal to the relative wave impedance of the air, so the closer the wave impedance matching coefficient ( M η ) is to 1, the better the electromagnetic impedance matching of the material. It can be seen from the graph (Figure 10) that there was a tendency for the wave impedance matching coefficient to increase and then decrease for all thicknesses, especially for the samples of thickness from 2.5 to 2.8 mm, which all had values equal to 1 at a specific frequency range before decreasing with increasing frequency. As the thickness increases, the specific frequency range, at which the M η value of S2 reaches 1, shifts continuously to the left. Comparing M η at the thicknesses from 2.0 to 3.0 mm, it can be found that only when thicknesses are between 2.6 and 2.7 mm, can every point’s values reach 0.8 or more. That can explain why the absorption bandwidth of each sample tended to increase and then decrease as the thickness increased, with only the thicknesses of 2.6–2.7 mm meeting the entire frequency range of 8.0–12.4 GHz.

RGO-CB/CIP composite absorption of electromagnetic waves was summarized in Figure 11. Impedance matching was critical to ensure the vast majority of electromagnetic waves penetrate the composite’s interior through the air. The energy of the incident electromagnetic waves was then converted into heat dissipation inside the absorber by various absorption mechanisms, as elaborated below.

1 Conductive losses. In the composite material, graphene, carbon black and CIP formed a three-dimension conductive network of limited resistance. When electromagnetic waves passed through the composite material, induced currents were formed within the material and converted to thermal energy as the induced currents passed through the resistive conductive network.

In summary, RGO-CB/CIP composite was prepared by hummers method, one-pot method, and acidified mixing method. GO was grafted with CB during the reduction process of GO to RGO, forming an RGO-CB composite, which would be tightly CIP-coated. By combining the magnetic loss material with the dielectric loss material, the RGO-CB/CIP composite achieved better impedance matching performance. Most of the electromagnetic waves of the X-band can enter the material. The redox process of graphene led to the formation of many defects on the surface of graphene, which became active centres for dipole polarization. In addition, the heterogeneous interface formed between RGO, CB, and CIP provided conditions for the accumulation of charges, and the multi-heterogeneous interface dramatically enhanced the interface polarization. The results demonstrated that the sample, whose proportion of CIP and RGO-CB is 1:1, had an absorption bandwidth covering the X-band at 2.6 and 2.7 mm, and had microwave absorption peaks at −18.59 dB and -17.28 dB, respectively. This research provided a method for making X-band-targeted absorbing materials, as well as an in-depth analysis of the absorption mechanism, in the hopes of serving as a guide for future researchers.