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

Microwave absorbing materials have been becoming a countermeasure and security media for radio environment, and also been becoming the main military force attached by the world. In this study, experimental data, theoretical researches and FEKO—a 3D structure and electromagnetic field analysis simulation tool, namely, FEldberechnung bei Korpern mit beliebiger Oberflache, are effectively combined. Through systematic studies on electromagnetic parameters and microwave absorbing properties of high-performance absorbing functional particle Fe3O4-rGO (i.e. reduced graphene oxide), single-layer composite Fe3O4-rGO@EP/GF (epoxy/glass fiber) and multi-layer structure composite Fe3O4-rGO@EP/fiber, combined with the simulation calculations of material structures, design on the microwave absorption performances of fiber/resin structural composites is achieved. In particular, under the guidance of FEKO simulations, a multi-layer structure composite with two layers of Fe3O4-rGO@EP/GF with different Fe3O4-rGO added amounts as the absorbing layer and carbon fiber (CF) reinforced EP as the reflecting layer is prepared, which has an effective absorption bandwidth of 8.43–12.40 GHz, almost covering the whole X-band, and RLmin (minimum reflection loss) up to −34.60 dB at 10.37 GHz. The relevant experimental results are basically consistent with the FEKO simulation results. This study is helpful to improve the developing efficiencies and properties of microwave absorbing structure composites with both excellent mechanical properties and outstanding microwave absorbing performances, especially conducive to industrial productions.


Introduction
In today's world, radar, infrared detection technology and electronic technology are developing rapidly. For the electromagnetic wave (EMW) can be absorbed effectively by absorbing materials to improve the performance of electronic equipment or hide targets in the radar system, a lot of researches have been conducting on effective EMW absorbing materials in the fields of electromagnetic interference and stealth technology. At present, EMW absorbing materials have been becoming a countermeasure and security media for the radio environment, and also been becoming the main military force attached by the world.
At present, researches on microwave absorbing fiber/resin composites mainly include two directions, one is a multi-layer structure absorbing material, and the other is a periodic structural absorbing material. Jae-Hun Choi et al [1] on the basis of obtaining ideal SiC/EP single-layer dielectric material, by using CST simulation and ideal dielectric constant guidance obtained a multi-layer alternating structure, thereby further improving absorbing performance of the composite, its effective absorption bandwidth reached 3.4 GHz, and strongest stir at 80°C for 4 h to fully reduce GO. The target product Fe 3 O 4 -rGO was obtained after magnetic separation and freeze-drying.

Preparation of fiber/EP absorbing composite
Preparation of single-layer composite Fe 3 O 4 -rGO@EP/GF: a certain mass of absorbing functional particles Fe 3 O 4 -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 Fe 3 O 4 -rGO@EP resin obtained was brushed manually on glass fiber cloth, the structure-function composite integrated Fe 3 O 4 -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 Fe 3 O 4 -rGO@EP/fiber: absorbing functional particles Fe 3 O 4 -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 Fe 3 O 4 -rGO@EP resins were manually brushed, respectively, on glass fiber cloth, then 10 wt% Fe 3 O 4 -rGO@EP/GF prepreg and 12 wt% Fe 3 O 4 -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, Fe 3 O 4 -rGO@EP/GF prepreg prepared was heated to gel state, then CF/EP prepreg, 12 wt% Fe 3 O 4 -rGO@EP/GF gel prepreg and 10 wt% Fe 3 O 4 -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 abovementioned composite Fe 3 O 4 -rGO@EP/GF.
In order to ensure the contents of fiber, functional particle and resin in prepared composites Fe 3 O 4 -rGO@EP/GF and Fe 3 O 4 -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.

Characterizations
Crystallization characteristic of Fe 3 O 4 -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 Fe 3 O 4 -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.

EMW absorbing property test
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.

Simulation for absorption performance and field distribution by FEKO
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 Fe 3 O 4 -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.

Results and discussion
3.1. Composition and absorbing performance of functional particle Fe 3 O 4 -rGO There is no weak broad peak in the range of 20°-30°, indicating that Fe 3 O 4 loaded on rGO sheets can prevent their re-stacking effectively [16][17][18][19][20][21][22]. No other peak appears, indicating that the pure and impurity-free Fe 3 O 4 -rGO is successfully synthesized. XRD patterns of magnetite Fe 3 O 4 and hematite γ-Fe 2 O 3 are very similar [16], in order to further verify that the crystals loaded on the surface of rGO are pure magnetite Fe 3 O 4 , XPS tests were performed, and the results are shown in figures 2(b) and (c). The main constituent elements of Fe 3 O 4 -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-Fe 2 O 3 peak is observed, indicating that there is no γ-Fe 2 O 3 in the material system [21][22][23][24][25], thereby further confirming that the oxide loaded on the surface of rGO is magnetite Fe 3 O 4 [24][25][26][27][28][29].  [30,31], indicating that rGO is reduced by ascorbic acid, but not completely. Compared with the other two materials, the lattice peaks of Fe 3 O 4 also appear in the infrared spectrum of Fe 3 O 4 -rGO, indicating that Fe 3 O 4 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 -COOstretching 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 Fe 3 O 4 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 Fe 3 O 4 -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, Fe 3 O 4 is formed on the surface of rGO sheets, almost completely covering them. Combined with XRD results, it can be known that Fe 3 O 4 is generated between rGO sheets too, which can improve the dispersion uniformity of rGO and Fe 3 O 4 [33]. 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].  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.

Absorbing property and mechanism of single layer Fe 3 O 4 -rGO@EP/GF
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 Fe 3 O 4 -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   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 Fe 3 O 4 -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 Fe 3 O 4 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. Where: ε s and ε ∞ are the static permittivity and relative permittivity at the high frequency limit. Figure 4 shows the ε′−ε″ curves for 2 wt% Fe 3 O 4 -rGO@EP/GF and 12 wt% Fe 3 O 4 -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 Fe 3 O 4 , 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 Fe 3 O 4 -rGO, the polarized groups and defects in rGO and the interface between Fe 3 O 4 and rGO all increase, leading to a large number of interface polarization and strong dipole polarization, therefore, the Cole-Cole semicircles of 12 wt% Fe 3 O 4 -rGO@EP/GF composite is stronger [38].
Functional particle Fe 3 O 4 -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 , its value should be kept constant, independent of frequency change [38,40]. From figure 5, it can be seen that the C 0 of 12 wt% Fe 3 O 4 -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.
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).
in r r r r 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% Fe 3 O 4 -rGO@EP/GF and 12 wt% Fe 3 O 4 -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% Fe 3 O 4 -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

Microwave absorbing properties of multi-layer fiber/EP composite
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 Fe 3 O 4 -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% Fe 3 O 4 -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. 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.
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 waveabsorbing 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% Fe 3 O 4 -rGO@EP/GF with a thickness of 0.5 mm, the middle layer is 12 wt% Fe 3 O 4 -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.

Conclusions
In brief, experimental data, theoretical researches and FEKO simulation are effectively combined to improve the developing efficiencies and properties of microwave absorbing structure composites. Through the preparation of high-performance absorbing functional particles and the combination of their filling amounts, composite structure and thickness of each layer, the design to the EMW absorbing capability of structural composite is realized. The pure magnetite Fe 3 O 4 in the wave-absorbing functional particle Fe 3 O 4 -rGO prepared by the coprecipitation method almost completely covers on the surfaces of rGO sheets and is also formed between its sheets, which help to improve the dispersion uniformity of rGO and Fe 3 O 4 ; its RL min is −35.95 dB and effective absorption can cover most the whole X-band and Ku-band. The addition of absorbing functional particles improves the electromagnetic balance of composite EP/GF and increases its EMW absorbing ability. With the increase of Fe 3 O 4 -rGO filling amount, the polarized groups and defects in rGO and the interface between Fe 3 O 4 and rGO all increase, resulting in a large number of interfacial polarization and strong dipole polarization, and therefore higher dielectric relaxation and polarization relaxation. However, single-layer composite Fe 3 O 4 -rGO@EP/GF has a poor absorbing performances, so the matching thickness and absorbing functional particle content of each layer in double-layer and three-layer composites are designed by FEKO simulation, then a multi-layer structure composite is prepared according to the structure of composite with the best EMW absorption performances obtained by FEKO simulation, which has RL min of −34.60 dB, and effective absorption bandwidth of 8.43-12.40 (3.97) GHz, almost covering the whole X-band. The experimental results are basically consistent with those from FEKO simulation. Therefore, a structure composite possessing the outstanding wave absorbing performance is obtained with high efficiency, especially with a simple preparation process.