Preparation and properties of functional particle Fe₃O₄-rGO and its modified fiber/epoxy composite for high-performance microwave absorption structure

The commercial graphene oxide suspension (GO) (1 mg ml⁻¹) was supplied by Tangshan Jianhua Technology Co., Ltd, Chine. Ferric chloride hexahydrate (FeCl₃ · 6H₂O) was purchased from Tianjin Damao Chemical Reagent Factory, China. Ferrous chloride tetrahydrate (FeCl₂ · 4H₂O) was obtained from Tianjin Beichen Founder Reagent Factory, China. Sodium hydroxide (NaOH) was purchased from Tianjin Jinfeng Chemical Co., Ltd, China. All the above reagents were of analytical grade and used directly without further purification.

Epoxy resin E51 (EP, Average epoxy resin value of 0.51) was purchased from Nantong Xingchen Synthetic Material Co., Ltd, China. Polyetheramine (PEA) (Jeffamine D400, Mn = 400 g mol⁻¹, PDI = 1.1) was bought from Shanghai Jingchun Chemical Reagent Co., Ltd, China. Carbon fiber cloth was purchased from Jiangsu Tianniao High technology Co., Ltd, China. E-glass fiber cloth was supplied by Jiaxing Baotai Composites Co., Ltd, China.

Fe₃O₄-rGO was prepared by a co-precipitation method. Weigh 2.04 g of ferric chloride hexahydrate and 1.50 g of ferrous chloride tetrahydrate, add them into 100 ml of GO solution with a concentration of 1 mg ml⁻¹, and agitate ultrasonically for 20 min. In a nitrogen atmosphere, drop 0.2 g ml⁻¹ of sodium hydroxide solution until pH value of the solution mixed of 10, stir at 90 °C for 2 h. Add 50 mg of reducing agent ascorbic acid (L-AA) and stir at 80 °C for 4 h to fully reduce GO. The target product Fe₃O₄-rGO was obtained after magnetic separation and freeze-drying.

Preparation of single-layer composite Fe₃O₄-rGO@EP/GF: a certain mass of absorbing functional particles Fe₃O₄-rGO was added into the mixture of epoxy resin and curing agent PEA with mass ratio of 1:0.55 and dispersed uniformly by ultrasonic agitation, and then the Fe₃O₄-rGO@EP resin obtained was brushed manually on glass fiber cloth, the structure-function composite integrated Fe₃O₄-rGO@EP/GF was prepared by a hot pressing process, the specific process included: the prepreg obtained by brush coating was pre-cured at 90 °C for 35 min to reach the gel state, then heated to 135 °C and cured at 5 MPa for 1 h, finally cured at 135 °C and 15 MPa for 2 h, then cooled to obtain the final composite.

Preparation of multi-layer composite Fe₃O₄-rGO@EP/fiber: absorbing functional particles Fe₃O₄-rGO with mass fraction of 10 wt% and 12 wt% were added into the mixture of epoxy resin and curing agent PEA with mass ratio of 1:0.55 respectively and dispersed uniformly by ultrasonic stirring, and then these Fe₃O₄-rGO@EP resins were manually brushed, respectively, on glass fiber cloth, then 10 wt% Fe₃O₄-rGO@EP/GF prepreg and 12 wt% Fe₃O₄-rGO@EP/GF prepreg were obtained. CF/EP prepreg was prepared by hand brushing mixture of epoxy resin and curing agent PEA with the mass ratio of 1:0.55 onto carbon fiber cloth. In order to control the filling amount of functional particles in each layer of composite, Fe₃O₄-rGO@EP/GF prepreg prepared was heated to gel state, then CF/EP prepreg, 12 wt% Fe₃O₄-rGO@EP/GF gel prepreg and 10 wt% Fe₃O₄-rGO@EP/GF prepreg were laid, respectively, then the structure-function integrated multi-layer fiber/EP composite was prepared by a hot pressing process, the specific process of which is the same as that of the above-mentioned composite Fe₃O₄-rGO@EP/GF.

In order to ensure the contents of fiber, functional particle and resin in prepared composites Fe₃O₄-rGO@EP/GF and Fe₃O₄-rGO@EP/fiber, and the repeatability of the above-mentioned preparation processes, the following procedures should be strictly implemented: (1) Before being manually brushed with resin, the fiber cloth was weighed to obtain its initial mass A. (2) Prepare the mixed solution of resin and particle in strict accordance with the content of functional particle. Of course, in order to fix the wet resin content in the brushed fiber cloth, more mixture was needed, because during the brushing process, a certain amount of mixed solution was left in the container and brushing tool used. After many experiments, the residue was determined to be about 2.3 wt%. (3) During the subsequent hot pressing process, strictly control its temperature and pressure to prevent the infiltration resin extruding from the fiber cloth, and finally obtain the required composite, weigh it and express as B. By comparing A and B, the contents of resin and functional particle could be determined, and only which with the appropriate contents of resin and functional particle could be used for the subsequent experiments.

Crystallization characteristic of Fe₃O₄-rGO was determined by an x-ray diffraction (XRD, HAOYUAN DX-2700B) using CuKα radiation (λ = 1.5406 Å). The magnetic property of material was determined with a vibrating sample magnetometer (VSM, Lake Shore7410, America Micro Sense) at room temperature. The surface compositions and elemental distributions of materials were examined by an x-ray photoelectron spectroscopy (XPS, EscaLab 250Xi). A Fourier transform infrared spectrometer (Nicolet IS50) was used to record the chemical structure and bonding property of sample in a wavenumber range from 400 to 4000 cm⁻¹. Morphologies and surface characteristics of Fe₃O₄-rGO and rGO were investigated by a Field emission scanning electron microscopy (SEM, Hitachi SU8010) at accelerating voltages of 15 kV. A transmission electron microscope (TEM, JEOL-2100F) was used to observe the micro-morphology of sample at an accelerating voltage of 20 kV.

The permittivity and permeability of all materials were obtained with a vector network analyzer (Agilent N5244A, Keysight, USA). For EMW absorbing functional particles, they were mixed with paraffin at a mass ratio of 3:7, melted and pressed into a cylindrical ring with an inner diameter of 3.04 mm, an outer diameter of 7.00 mm and a thickness of 2.50 mm, and then their electromagnetic parameters were measured in the frequency range of 2–18 GHz. For the EP/fiber composites, they were cut into a 22.70 × 10.00 × 2.50 mm rectangular parallelepiped and their electromagnetic parameters were obtained by the waveguide method in the frequency range of 8.2–12.4 GHz. The permittivity and permeability of more than 10 samples were tested for each composite, and their average values were the electromagnetic parameters of the composite in this frequency range. Figure 1(a) is the schematic diagram of the reflectance test principle of the arch method and figure 1(b) is the microwave darkroom environment diagram. In actual performance measurement, according to the Chinese military standard GJB2038A-2011 on the RAM (radar absorber material) reflectance arch method test method, the fiber/EP composites with a size of 180 × 180 × 35 mm were prepared, and then they were placed on a good-conductor support with the same size to study their absorption properties at 8.2–12.4 GHz.

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Simulations were performed using FEKO 2017. In the specific simulation process, the vertical scanning simulations of the multi-layer composites were performed using the adaptive scanning technology AFS (Function Method of MoM Green-SGF), which could obtain the final microwave absorbing performances of multi-layer composites through their electromagnetic parameters. The measured permittivity and permeability of Fe₃O₄-rGO@EP/GF were used as the EM parameter inputs of materials. The plane wave excitation was selected, the observation points were set in the tree structure, and the final absorption performances were calculated through the physical optics method (PO). By simulating the absorbing properties of multi-layer composites composed of electromagnetic parameter layers with different thicknesses, the structure of composites with the best absorbing properties was obtained and then used to guide the experiment.

Figure 2(a) is the XRD spectrum of absorbing functional particle Fe₃O₄-rGO prepared, and all of its peaks correspond to the Fe₃O₄ standard card (JCPDS 88-0315). Peaks at 2θ = 18.1, 30.1, 35.4, 43.1, 53.4, 57.0, 62.6 and 74.0° can be attributed to the (111), (220), (311), (400), (422), (511), (440) and (533) planes of the cubic lattice structure of Fe₃O₄. There is no weak broad peak in the range of 20°–30°, indicating that Fe₃O₄ loaded on rGO sheets can prevent their re-stacking effectively [16–22]. No other peak appears, indicating that the pure and impurity-free Fe₃O₄-rGO is successfully synthesized. XRD patterns of magnetite Fe₃O₄ and hematite γ-Fe₂O₃ are very similar [16], in order to further verify that the crystals loaded on the surface of rGO are pure magnetite Fe₃O₄, XPS tests were performed, and the results are shown in figures 2(b) and (c). The main constituent elements of Fe₃O₄-rGO are C, O and Fe, as shown in figure 2(c), its XPS spectrum has two characteristic peaks 711.18 and 724.88 eV from Fe 2p_3/2 and Fe 2p_1/2 respectively [23], and no γ-Fe₂O₃ peak is observed, indicating that there is no γ-Fe₂O₃ in the material system [21–25], thereby further confirming that the oxide loaded on the surface of rGO is magnetite Fe₃O₄ [24–29]. Figure 2(d) is the FITR spectra of Fe₃O₄, rGO and Fe₃O₄-rGO. Pure Fe₃O₄ shows its unique lattice absorption peak at 592 cm⁻¹. Peaks of –C–O stretching vibration, –C–OH deformation vibration, –COO– stretching vibration and C=C bond of carbon skeleton in rGO appear at 1066, 1286, 1560 and 1618 cm⁻¹ [30, 31], indicating that rGO is reduced by ascorbic acid, but not completely. Compared with the other two materials, the lattice peaks of Fe₃O₄ also appear in the infrared spectrum of Fe₃O₄-rGO, indicating that Fe₃O₄ is synthesized on the surfaces of rGO sheets. In addition, the peaks of –CO stretching vibration and –C–OH deformation vibration almost disappear, the peak of –COO– stretching vibration decreases slightly, and the peak of C=C bond of carbon skeleton almost remains unchanged, indicating that most of the Fe-O bonds are formed by Fe₃O₄ and –C–OH on the surface of GO and a few by -COOH, which can further increase the reduction degree of GO and facilitate the dispersion of rGO. Figure 2(e) is SEM images of Fe₃O₄-rGO and GO, the one with relatively great magnification, after being reduced by ascorbic acid. It can be seen that GO has certain folds, which are conducive to the composite modification by particles [32]. In addition, Fe₃O₄ is formed on the surface of rGO sheets, almost completely covering them. Combined with XRD results, it can be known that Fe₃O₄ is generated between rGO sheets too, which can improve the dispersion uniformity of rGO and Fe₃O₄ [33]. Figure 2(f) is a TEM image of Fe₃O₄-rGO, showing that Fe₃O₄ nanoparticles with the particle size of 10–25 nm are relatively evenly distributed on the surface of rGO. Figure 2(g) shows the magnetic characteristics of the material. Fe₃O₄-rGO possesses superparamagnetism, Hc = 45.3 Oe, Ms = 52.7 emu g⁻¹, indicating that this functional particle is soft magnetic material, which will contribute to the energy conversion, transmission and attenuation of EMW [17, 34]. Figure 2(h) shows the EMW absorption performances of Fe₃O₄-rGO/paraffin composites with different thicknesses calculated by the transmission line theory based on the electromagnetic parameters measured by the coaxial method of VNA. It can be seen that when the test thickness is 2.5 mm, the material has effective absorption (RL less than 10 dB) from 10.92 to 17.18 GHz, most covering X-band and Ku-band; when the test thickness is 3 mm, RL_min of this material is −35.95 dB at 10.64 GHz. All these show that the absorbing particles prepared possess good absorption performance, and can be used as effective functional particles for preparing structural absorbing composites.

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According to the transmission line theory, microwave absorption characteristics are highly dependent on the complex permittivity and complex permeability of materials. The electromagnetic parameters of functional particle Fe₃O₄-rGO are quite different from those of resin and fiber, while the electromagnetic parameters of resin/fiber composites can be improved to a certain extent by increasing its filling amounts [35, 36], thereby obtaining better EMW absorbing abilities. The dielectric properties of GF/EP composites filled with different functional particle Fe₃O₄-rGO loadings were tested by VNA to guide the design and preparation of final composite, and figure 3 is the electromagnetic parameters of GF/EP composites filled with Fe₃O₄-rGO from 2 to 12 wt%. It can be seen from figures 3(a) and (b) that when the filling amounts of Fe₃O₄-rGO increase from 2 wt% to 12 wt%, values of the real part of dielectric constant (ε') of Fe₃O₄-rGO@EP/GF composites in X-band fluctuate between 4.17–4.43, 4.39–4.76, 4.40–4.79, 4.67–4.97, 5.17–5.68 and 5.26–5.69 respectively, while values of their imaginary part ε'' change between 0.16–0.43, 0.18–0.4, 0.16–0.42, 0.17–0.54, 0.61–1.23 and 0.75–1.38 respectively. As expected, with the increase of the amounts of Fe₃O₄-rGO added, ε' values of composites increase from 4.2 to 5.5, while ε'' values from about 0.2 to 1.2 on average. It can also be seen from figures 3(c) and (d) that when the filling amounts of Fe₃O₄-rGO increase from 2 wt% to 12 wt%, values of the real part of permeability (μ') of Fe₃O₄-rGO@EP/GF composites in X-band fluctuate between 0.97–0.99, 0.97–1.02, 0.95–1.05, 0.98–1.07, 0.96–1.12 and 0.96–1.19 respectively, while values of their imaginary part (μ'') change between 0.01–0.05, 0–0.04, 0–0.04, 0–0.07, 0.02–0.24 and 0.01–0.22 respectively. At low addition, that is, less than 8 wt%, there are little changes in values of μ' and μ'' of composites, but when additional amounts of functional particles Fe₃O₄-rGO are higher than 8 wt%, values of μ' and μ of composites will be greatly improved. As is known to all, the real parts (ε' and μ') of electromagnetic parameters represent their storage capacities to electromagnetic energy, while the imaginary parts (ε'' and μ'') show their loss capacities to electromagnetic energy [37]. Therefore, the addition of absorbing functional particles improves the electromagnetic balance in composite EP/GF and increases their loss ability to EMW, which can also be confirmed from figures 3(e) and (f). The functional particles in this study are composed of Fe₃O₄ and rGO. For component rGO, on the one hand, they help to construct the conductive networks in composites, on the other hand, the defects and oxygen-containing functional groups on their surfaces tend to accumulate electrons and cause electron polarization, thus giving materials a high degree of polarization, which will lead to stronger polarization relaxation. At the same time, its polar oxygen-containing functional groups can also generate stretching vibration and bending vibration in the electromagnetic field, which should also be one of the reasons for the sudden changes of dielectric loss tangent tanδ_E in some frequency band, and these can improve the effective absorption to EMW and the ability to dissipate electromagnetics. In addition, polar oxygen-containing functional groups can lead to additional charge rearrangement and orbital hybridization, making the electric dipole polarized, which is very important for improving the dielectric constant and microwave absorption performances. The contribution of dipole polarization to dielectric loss is usually explained by Cole-Cole semicircles, each of which represents a Debye relaxation process. The relationship between ε' and ε'' can be expressed as follows.

$$\begin{eqnarray}&&{\left(\varepsilon ^{\prime} -\displaystyle \frac{{\varepsilon }_{s}+{\varepsilon }_{\infty }}{2}\right)}^{2}+{\left(\varepsilon ^{\prime\prime} \right)}^{2}={\left(\displaystyle \frac{{\varepsilon }_{s}-{\varepsilon }_{\infty }}{2}\right)}^{2}\end{eqnarray} \tag{ 1 }$$

Where: ε_s and ε_∞ are the static permittivity and relative permittivity at the high frequency limit.

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Figure 4 shows the ε' − ε'' curves for 2 wt% Fe₃O₄-rGO@EP/GF and 12 wt% Fe₃O₄-rGO@EP/GF. It can be seen that both composites have two distinct Cole-Cole semicircles, corresponding to two Debye relaxation processes, which should be attributed to the polarization relaxation of rGO or Fe₃O₄, respectively. As is known to all, the relaxation process is usually caused by the delay of molecular polarization relative to the changing electric field in a medium. With the increase of additional amounts of functional particle Fe₃O₄-rGO, the polarized groups and defects in rGO and the interface between Fe₃O₄ and rGO all increase, leading to a large number of interface polarization and strong dipole polarization, therefore, the Cole-Cole semicircles of 12 wt% Fe₃O₄-rGO@EP/GF composite is stronger [38].

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Functional particle Fe₃O₄-rGO exhibits superparamagnetism, so with the increase of its contents in composites and the boundary conditions of material compositions (hysteresis effect and magnetic induction intensity of the magnetic domain wall), values of μ' and μ'' will increase. In particular, as can be seen from figures 3(c) and (d), when the contents of the functional particles greater than 8 wt%, values of μ' and μ'' of composites will increase significantly. Magnetic losses are caused by eddy current effects, natural and exchange resonances. Figure 3(f) shows multiple resonances in 9–12 GHz. Eddy current loss comes from changes in magnetic flux density in an alternating electromagnetic field. If the magnetic loss of material is only caused by eddy current loss (${{\rm{C}}}_{0}=\mu ^{\prime\prime} {\left(\mu ^{\prime} \right)}^{-2}{{\rm{f}}}^{-1}=2\pi {\mu }_{0}\sigma {{\rm{d}}}^{2}$) [39], its value should be kept constant, independent of frequency change [38, 40]. From figure 5, it can be seen that the C₀ of 12 wt% Fe₃O₄-rGO@EP/GF in 8.2–9 GHz is a constant. Therefore, its magnetic loss in the relevant frequency band should mainly be eddy current loss. In other frequency bands, the loss values keep changing, inferring that natural resonance and exchange resonance exist in the material system [6, 16]. In addition, it can be seen from figure 5 that when the addition amounts of functional particles more than 8 wt%, the contribution of natural resonance and exchange resonance in magnetic loss increases.

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According to the transmission line theory, the dielectric constant and permeability measured by the coaxial method are used to calculate the EMW reflection loss value of composite EP/GF according to formulas (2) and (3).

$$\begin{eqnarray}&&{R}_{L}(dB)=20\,\mathrm{log}\left|\displaystyle \frac{{Z}_{in}-1}{{Z}_{in}+1}\right|\end{eqnarray} \tag{ 2 }$$

$$\begin{eqnarray}&&{Z}_{in}=\sqrt{\displaystyle \frac{{\mu }_{r}}{{\varepsilon }_{r}}}\,\tanh \left[j\left(\displaystyle \frac{2\pi fd}{c}\right)\sqrt{{\mu }_{r}{\varepsilon }_{r}}\right]\end{eqnarray} \tag{ 3 }$$

Where: Z_in is the normalized input impedance of the metal-supported microwave absorption layer, ε_r and μ_r are the complex permittivity and permeability of composite, c is the velocity of EMW in the free space, f is the frequency of EMW and d is the thickness of the absorbing material.

Figure 6 shows the relationship between frequency and RL of 10 wt% Fe₃O₄-rGO@EP/GF and 12 wt% Fe₃O₄-rGO@EP/GF with different thicknesses calculated. It can be seen that the two composites all have certain absorption ability in X-band. When the thickness of composite 10 wt% Fe₃O₄-rGO@EP/GF is 3 mm, its RL_min is only −15.46 dB at 12.07 GHz, and effective absorption bandwidth is 2.56 (9.84–12.40) GHz. When the thickness of composite 12 wt% Fe₃O₄-rGO@EP/GF is 3 mm, its RL_min is only −17.79 dB at 11.85 GHz, and effective absorption bandwidth is 1.83 (9.98–10.61, 11.16–12.36) GHz. All of these show that the single-layer composite Fe₃O₄-rGO@EP/GF possesses poor absorbing performance and needs further improvement.

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Compared with the single-layer composites, the double-layer composites are more conducive to the flexible design of the overall dielectric properties of the system, and the requirements to the dielectric parameters of each layer in it are not as high as those in single-layer composites, so it is easier to achieve high performances under the low process requirements. Therefore, a two-layer structure absorbing composite composed of an absorbing layer Fe₃O₄-rGO@EP/GF and a reflecting layer CF/EP was designed, its electromagnetic parameters measured by the above-mentioned VNA are used to simulate and design the thickness matching of each layer in the double-layer composite and the influence of the contents of absorbing functional particles on absorbing performance of composite through FEKO, so as to obtain the composite have the best absorbing performance in X-band by adjusting its thickness of each layer and the contents of functional particles.

Through genetic algorithm, it is finally determined that when the absorbing layer is 12 wt% Fe₃O₄-rGO@EP/GF with a thickness of 3.3 mm and the reflecting layer CF/EP with a thickness of 0.2 mm, the double-layer composite has the best EMW absorption performance in X-band, as shown in figures 7(a) and (b), which has effective absorption from 8.33 to 12.34 GHz and RL_min of −35.99 dB at 10.20 GHz. However, although the broadband absorption performance in X-band is obtained by matching the thickness of the absorbing layer and the reflective layer, its absorbing performance is still not ideal because the maximum absorbing intensity is still not high enough. According to formula 4, the input impedance of the multi-layer absorbing layer with pure reflective metal as the base can be obtained.

$$\begin{eqnarray}&&{\rm{Zn}}={\eta }_{n}\displaystyle \frac{{Z}_{n-1}+{\eta }_{n}th\left({\gamma }_{n}{d}_{n}\right)}{{\eta }_{n}+{Z}_{n-1}th\left({\gamma }_{n}{d}_{n}\right)}\end{eqnarray} \tag{ 4 }$$

Among them:

$$\begin{eqnarray}&&{\eta }_{n}=\sqrt{\displaystyle \frac{{\mu }_{n}}{{\varepsilon }_{n}}}=\sqrt{\displaystyle \frac{{\mu }_{n}^{{\prime} }-j{\mu }_{n}^{{\prime\prime} }}{{\varepsilon }_{n}^{{\prime} }-{\varepsilon }_{n}^{{\prime\prime} }}}\end{eqnarray} \tag{ 5 }$$

$$\begin{eqnarray}&&{\gamma }_{n}={\rm{j}}\displaystyle \frac{2\pi f}{c}\sqrt{\left({\mu }_{n}^{{\prime} }-j{\mu }_{n}^{{\prime\prime} }\right)\left({\varepsilon }_{n}^{{\prime} }-j{\varepsilon }_{n}^{{\prime\prime} }\right)}\end{eqnarray} \tag{ 6 }$$

Here: n is the number of layer of material, η_n is the characteristic impedance of the nth layer of material, γ_n is the propagation constant of the nth layer material, d_n is the thickness of the nth layer, and Z_n−1 is the input impedance of the n-1th layer material, f is the frequency of the incident EMW, and c is the speed of light.

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Matching the impedance with the intrinsic impedance of the air is very important for the design of microwave absorbing composites, which can be achieved by adjusting the dielectric constant of the absorbing material or designing multi-layer absorbing material [1]. By adjusting the dielectric constant of the surface layer, the impedance of the whole multi-layer absorber can be adjusted, then the reflection caused by the sudden change of the dielectric of the surface layer can be reduced, the incidence of EMW can be increased, and the absorbing performance to EMW of composites can be improved [41]. Therefore, the above-designed wave-absorbing layer of the double-layered structure is further subdivided into multiple functional layers, and while keeping the bottom absorbing layer unchanged, then the dielectric constant of the upper absorbing layer can be reduced, which can reduce the surface reflection to EMW and then have absorbing performance improved, so as to obtain a composite with higher absorbing ability.

Therefore, the structure of the composite is further optimized by genetic algorithms. For the thickness of the single-layer glass fiber cloth is 0.25 mm, the simulation value of the thickness of each layer of the absorbing layer is set to an integral multiple of 0.25. The optimal structure of composite obtained through FEKO simulation is: the surface layer is 10 wt% Fe₃O₄-rGO@EP/GF with a thickness of 0.5 mm, the middle layer is 12 wt% Fe₃O₄-rGO@EP/GF with a thickness of 2.8 mm, and the bottom layer is still CF/EP with a thickness of 0.2 mm. Figure 7(d) is the FEKO modeling diagram of the designed wave-absorbing composite, and figure 7(c) is its modeling wave-absorbing performance. It can be seen that by adjusting the dielectric properties of the surface, although the effective absorption bandwidth of the composite does not change much (8.38–12.33 GHz), its RL_min increases to −60.35 dB at 10.25 GHz.

Figure 8 is the comparisons of experimental and simulative RL-f of two-layer and three-layer absorbing structure composites. It can be seen that the frequencies corresponding to RL_min from experiments of the two composites are all close to their simulation values, especially for the three-layer composite system; their RL_min measurement values are higher than the simulation values, and the three-layer composite has the relatively larger increase. The ideal thickness of each layer is designed to achieve accurate impedance matching. However, the thicknesses of the manufactured absorbing composites slightly deviate from those of their simulations due to the tiny change in the thickness of each layer caused by the hot pressing process. However, all the errors are small. Specifically, for the double-layer absorption structure composite, its frequency corresponding to RL_min from experiment is 10.50 GHz, possessing a small change compared with the simulation value 10.20 GHz, RL_min increases from −35.99 dB of simulation to −28.31 dB of experiment, and the effective absorption bandwidth, 8.65–12.40 GHz from experiment, changes little too. For the three-layer absorption structure composite with the frequency corresponding to RL_min of 10.37 GHz from experiment, the change is also small compared to the simulated value, its RL_min is increased to −34.60 dB and the effective absorption bandwidth is 8.43–12.40 GHz from experiment. Based on these, it can be considered that the simulation in this study is scientific and accurate.

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