Surface Modification of Activated Carbon Fibers with Fe 3 O 4 for Enhancing Their Electromagnetic Wave Absorption Property

In this study, the porous activated carbon ﬁ ber (ACF) is prepared by viscose ﬁ ber, and Fe 3 O 4 coating is deposited on the surface of ACF through in situ hybridization to prepare carbon/magnetic electromagnetic (EM) wave absorption materials. Compared with pure Fe 3 O 4 and ACF, the EM wave absorption rate is improved. When the solubility of FeCl 3 is 2mol/L and the thickness of the prepared ACF – Fe 3 O 4 (3) EM wave absorption material is 3mm, the EM wave loss at 10GHz reaches − 44.3dB and e ﬀ ective EM wave absorption bandwidths ( reflection loss ð RL Þ < − 10 dB and RL < − 20 dB) reached 4.8 GHz (8.8 – 13.6GHz) and 1.1GHz (9.3 – 10.4 GHz), respectively. The prepared ACF-based composite material has a light structure and strong absorption bandwidth. Findings can provide references for the research on other EM wave-absorbing materials.


Introduction
Given the rapid development of electronic and electrical technologies, the scope of electromagnetic (EM) energy utilization has been continuously expanded, although EM radiation pollution has followed [1][2][3]. The problem of EM pollution has become the fifth largest public hazard after wastewater, exhaust gas, solid waste, and noise. Relevant studies have indicated that EM pollution will replace noise pollution in the current century and become the leading physical pollution. At present, an effective method is to use EM wave absorption materials to reduce or eliminate EM wave pollution [4].
Compared to traditional ferrite, carbonyl iron magnetic EM wave absorption materials, carbon-based EM wave absorption materials have the advantages of being light weight, having adjustable frequency range, and with good compatibility with the organic/inorganic phase interface of the matrix [5][6][7][8][9]. Graphite powder, carbon black, carbon nanotubes, chopped carbon fiber, and activated carbon fiber (ACF) have been reported as carbon-based EM wave protection functional fillers [10][11][12][13]. We know that ACF has a large surface area, which is a factor that cannot be disregarded [14]. Given the numerous polar groups on the surface, multiple polarization effects occur on the surface of EM wave-absorbing materials, thereby causing absorption attenuation owing to relaxation effects [15]. The main factor that determines the absorption characteristics of ACF is resistance, but its conductivity is high, easily reflects EM waves, and affects absorption efficiency [16][17][18]. Moreover, ACF and other carbon materials have extremely low magnetic permeability; hence, they have nearly no effect on magnetic signals, and achieving broadband absorption is difficult [16,17]. Accordingly, many studies load magnetic particles on carbon materials, including graphite powder nickel plating, carbon nanotube nickel plating, and loading nanoferrite particles, to improve the EM wave absorption performance of materials [19][20][21][22].
The current study uses ACF with a large specific surface area as a substrate. In situ hybridization of the fiber to Fe 3 O 4 can significantly improve the EM wave absorption performance of the material. Effective EM wave protection composite materials have research significance.    3 Journal of Nanomaterials thereafter into a coaxial ring with outer and inner diameters of 7 mm and 3.04 mm, respectively.  Figure 2. ACF felt, FeCl3, and glucose were treated with high temperature at 650°C under the protection of nitrogen, FeCl 3 and glucose undergo thermal decomposition, and the reduction reaction produced Fe 3 O 4 (PDF # 74-0748) magnetic particles. The chemical reaction is shown in Eqs. (1)-(4) [23,24]. Nonmagnetic Fe 2 O 3 (PDF # 72-0469) is also present. The diffraction peaks of ACF at 26.5°and 43.3°are typical diffraction peaks of carbonaceous fibers [21].

Morphological and EDS Analyses. Figures 3(a) and 3(b)
show a SEM of the fiber cross-section of ACF. We found that there are many tiny pores inside the fiber, and the presence of a large number of pores means that the fiber has more interfaces. Figure 3(c) shows that the ACF surface has evident grooves, which is a typical viscose-based ACF after drawing treatment [25]. At the same time, it can be seen from the ACF N 2 adsorption isotherm and pore size distribution diagram ( Figure 3(d)) that ACF has a large specific surface area and micropores smaller than 2.5 nm. This is because after the activation process of viscose fibers, a large number of micropores are generated, which increases the specific surface area of the fibers. After Fe 3 O 4 is loaded, the particles tend to accumulate in the grooves on the fiber surface.   . The spectrum can fit to three peaks with binding energies of 529.50, 530.86, and 532.75 eV. The lattice oxygen at 530.86 eV is comparable to macroscaled crystallite binding energy values for magnetite [28]. The spin-orbit peaks of Fe2p1/2 and Fe2p3/2 are shown in Figure 5(d); Fe2p peaks at 710.89 eV and 724.46 eV are characteristic peaks of X-ray photoelectron spectroscopy [29,30]. We found that Fe 3 O 4 is composed of three forms of iron, namely, Fe2 + octahedron, Fe3 + octahedron, and Fe3 + tetrahedron. The Fe2p3/2 derived from the binding energy of 710.89 eV was reasonably divided into peaks to obtain three main peaks and two satellite peaks. The Fe2 + /Fe3 + ratio is 0.63, which is slightly larger than the theoretical value of Fe 3 O 4 of 0.5, and it is due to the existence of Fe 2 O 3 [31,32].

TG and magnetic hysteresis analyses.
The temperature stability of microwave-absorbing materials is an important material property related to practical engineering applications. Figure 6 shows the TG analysis of the ACF and ACF-Fe 3   5 Journal of Nanomaterials 10°C/min. The mass loss process is following three stages. The slight mass loss below 110°C is due to the evaporation of the sample water. Thereafter, a weight loss gradually occurred from 110°C to 410°C, which can be attributed to the removal of the unstable oxygen-containing functional groups from the sample and H 2 O vapor caused by the destruction of the oxidized functional groups. Lastly, ACF showed a significant weight loss between 410°C and 720°C, thereby indicating that ACF was oxidized and decomposed in the air. The apparent weight loss of the ACF-Fe 3 O 4 composite material is from 410°C to 540°C. The reason is that the presence of metal ions promotes the accelerated oxidation of ACF, while Fe 3 O 4 is oxidized to Fe 2 O 3 [33,34].
In general, ACF-based materials have striking magnetic properties because of their fibrous structure and high specific surface area. Evidently, these characteristics can affect the magnetic properties of the material. The magnetic properties of the ACF-Fe 3 O 4 composite materials were studied at room temperature by measuring their magnetization curves. These magnetic properties include M s , M r , and H c (see Figure 7). Table 1 also shows the magnetic parameters corresponding to Figure 7. ACF has no magnetic properties, and pure Fe 3 O 4 is a typical superparamagnetic material that has high saturation M s and M r and low H c [35]. As the (Fe 3 O 4 )-ACF ratio increases, the saturation and residual magnetizations of the composite material increase, while the coercive force decreases. ACF-  (Figure 8(b)), the peak shows the resonance behavior, which is expected when the sample has high conductivity. Free electron theory indicates that ε ′ = 1/2ε 0 πρf, where ε ′ is the dielectric constant of vacuum, ρ is the resistivity, and f is the frequency of the microwave. The conductivity of ACF is extremely high and will form a large conductive network. Thus, the resistivity of the composite material ρ decreases, and as the ACF content increases, ε′ becomes considerably high. Figures 8(a) [37,38]. In terms of the key factor of absorption, magnetic resonance peaks are evident near 9 GHz and 15 GHz, which are the surface effects of the magnetic particles and spin wave excitation [39]. Dielectric loss tangent (tan δ E = ε″/ε′) and magnetic loss tangent (tan δ M = μ″/μ′) can characterize the dielectric and magnetic losses of the materials. Materials absorb EM waves through two main mechanisms. We calculated tan δ E and tan δ M for each sample to determine which one dominates the material. We found that the material tan δ E is between 0 and 0.8, while tan δ M is between 0 and 0.3. Obviously, the role of dielectric loss and magnetic loss in ACF/Fe 3 O 4 composites is limited. High dielectric constant materials affect impedance matching and have strong reflection and weak absorption [40,41]. The material itself and its structure can enhance the EM wave absorption performance of the material [42].
The Debye dipolar relaxation indicates can be expressed by the following equation [43]: where f is the frequency, τ is the relaxation time, and ε s and ε ∞ are the stationary and optical dielectric constants, respectively. The following equation can be deduced from Eqn. (5):  Journal of Nanomaterials Equations (6) and (7) indicate that the relationship between ε ′ and ε ″ can be deduced as follows: Accordingly, the presence of the large specific surface area ACF improves the intensity of the Debye dipole relaxation process, and the interface between ACF and Fe 3 O 4 particles is the cause of double dielectric loss. To describe the dielectric relaxation process in detail, the typical Cole-Cole curve of ACF is also shown in Figure 9. The relaxation process (multiple semicircles) of ACF is evidently caused by defects and groups. We know that ACF is prepared by viscose fiber impregnated with ammonium dihydrogen phosphate and activated by high-temperature water vapor. Numerous micropores and oxygen-containing groups will cause many defects on the ACF surface. Thereafter, defects can act as polarization centers will produce polarization relaxation under changing EM fields and attenuate EM waves; this will have a profound impact on the loss of electromagnetic waves. Free electron theory (ε ″ = 1/2 ε 0 ρπf ) shows that the ε ″ is proportional to the specific conductance. Pure ACF has a high conductivity, thereby resulting in a high ε ″ value. Thus, ACF has strong dielectric loss. Therefore, the ACF arc is larger than other samples. However, the high ACF conductivity may also cause a significant skin effect because the surface is exposed to EM waves. The high ε ″ of ACF or low ε ″ of the Fe 3 O 4 particles may reduce the impedance matching of the material. Accordingly, pure ACF and Fe 3 O 4 particles show extremely poor microwave absorption performance, as shown in Figure 10. In our case, a new composite material was obtained using the appropriate amount of ACF and Fe 3 O 4 particles. Hence, the material can considerably respond to the impedance matching requirements while maintaining the existing dielectric relaxation characteristics.
3.6. EM Wave-Absorbing Properties. Figure 10 shows the RL curves of the ACF, Fe 3 O 4 , and ACF-Fe 3 O 4 composites with different thicknesses, as well as the corresponding 3D surface plots. As shown in Figures 10(a) and 10(j), the Fe 3 O 4 RL is relatively poor at frequencies between 2 GHz and 18 GHz and is above −5.0 dB in the RL range. Moreover, RL of ACF is only 0.8 GHz with a bandwidth below −10.0 dB. Dielectric loss is the main microwave absorption mechanism of ACF owing to weak magnetic properties. Previous studies have concluded that the local state close to the Fermi level can be achieved by introducing the lattice defects in the carbonaceous material and when radiation is incidental on the surface of the absorber, thereby causing a large radiation absorption. Consequently, the existence of high specific surface area and defects is an important reason for the     Journal of Nanomaterials improvement of the electromagnetic wave absorption capacity of ACF. The previous analysis of the EM parameters shown in Figure 8 indicates that when the concentration of the Fe 3 O 4 particles is low, ACF-Fe 3 O 4 (1) has evident resonance peak at 6 GHz to 10 GHz, as shown in Figure 10(c), and−45.9 dB appears at 6.9 GHz absorption peak.      11 Journal of Nanomaterials may cause multiple reflections in the absorber. The result is an extended propagation path of the EM waves in the material, thereby further enhancing the absorption capacity of the composite material. Figure 11 shows a diagram that intuitively presents the EM wave absorption mechanism. In general, the enhanced of EM wave absorption performance of the composite materials is attributed to the compensation characteristics of ACF and Fe 3 O 4 that in the EM complementation effect previously proposed. Evidently, ACF-Fe 3 O 4 composite material is a lightweight and efficient EMabsorbing material.

Conclusions
This study selected ACF as a substrate and maximized its high specific surface area. By growing Fe 3 O 4 in situ on the surface of ACF, a high-efficiency, wide-band, and lightweight carbon magnetic EM wave-absorbing material based on ACF was prepared. ACF-Fe 3 O 4 has more interfaces and defects, as well as multiple reflection losses, which in turn enhances the EM wave absorption performance. When the thickness of the prepared ACF-Fe 3 O 4 is 3 mm, the minimum RL reached −44.3 dB at 10 GHz, and the effective bandwidth of the RLs < −10 dB and <−20 dB is 4.8 GHz and 1.1 GHz, respectively. The prepared ACF-Fe 3 O 4 has excellent properties such as broadband, high efficiency, stability, and lightness and is a new type of electromagnetic wave-absorbing composite material.

Data Availability
All data used to support the findings of this study are included within the article.

Conflicts of Interest
The authors declare that they have no competing interests.