# Surface Modification of Activated Carbon Fibers with Fe3O4 for Enhancing Their Electromagnetic Wave Absorption Property

Authors: Xuefeng Yan; Tao Ji; Wei Ye
Journal: Journal of Nanomaterials
 (2020)

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## Abstract

In this study, the porous activated carbon fiber (ACF) is prepared by viscose fiber, and Fe3O4 coating is deposited on the surface of ACF through in situ hybridization to prepare carbon/magnetic electromagnetic (EM) wave absorption materials. Compared with pure Fe3O4 and ACF, the EM wave absorption rate is improved. When the solubility of FeCl3 is 2 mol/L and the thickness of the prepared ACF–Fe3O4(3) EM wave absorption material is 3 mm, the EM wave loss at 10 GHz reaches −44.3 dB and effective EM wave absorption bandwidths (reflectionlossRL<−10 dB and RL<−20 dB) reached 4.8 GHz (8.8–13.6 GHz) and 1.1 GHz (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.


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## Body

## 1. 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 [1–3]. 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 [5–9]. 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 [10–13]. 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 [16–18]. 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 [19–22].The current study uses ACF with a large specific surface area as a substrate. In situ hybridization of the fiber to Fe3O4 can significantly improve the EM wave absorption performance of the material. Effective EM wave protection composite materials have research significance.


## 2. Experimental Section

### 2.1. Sample Preparation

Preparation of ACF through industrial process and thermal decomposition promote in situ growth of Fe3O4 on the surface of ACF, and the process route is shown in Figure 1. In detail, the viscose fiber is impregnated with ammonium hydrogen phosphate, dried, and treated at 850°C for 10 minutes; the process is passed through steam to prepare ACF. 0.25 mol glucose was dissolved into 1000 mL distilled water followed by the addition of 2 mol FeCl3 to form a homogeneous solution. ACF felt was added to the solution, in which the rolling surplus rate was 900%. After drying, ACF that contained FeCl3 was heat-treated at 650°C for 60 min in N2 gas, covered the ACF surface with a layer of Fe3O4, and prepared ACF–Fe3O4(3). The amount of Fe3O4 in the composites was regulated by the addition of different amounts of FeCl3 and glucose, specifically 0, 0.5, and 1 mol for FeCl3 and 0, 0.0625, and 0.125 mol for glucose. By using the preceding experimental method, the final products were denoted as ACF, ACF–Fe3O4(1), and ACF–Fe3O4(2), respectively.Figure 1
Preparation process for the ACF–Fe3O4 composites.


### 2.2. Characterization

Phase structural analysis of the prepared ACF–Fe3O4 was performed using X-ray diffraction (XRD; Rigaku D/max-2500PC) with CuKα radiation. A scanning electron microscope (SEM; ZEISS Gemini SEM 300) equipped with an energy dispersive spectrometer (EDS) was used for morphological observations and elemental analyses. Textural characterization was carried out by nitrogen adsorption at 77 K using an ASAP 2020 automatic physisorption analyzer (Micromeritics Instrument Corp., USA). The chemical states were characterized by X-ray photoelectron spectroscopy (XPS; Thermo Fisher Scientific ESCALAB-250) with Cu Kα radiation. The magnetic properties were achieved using a vibrating sample magnetometer (VSM; Quantum Design MPMS) at 300 K. The formation mechanism was analyzed using thermogravimetry/differential scanning calorimetry (TG/DSC; Netzsch 214 Polyma) at a heating rate of 10°C/min from 30°C to 1000°C in the presence of air. The EM parameters (i.e., relative complex permittivity and relative complex permeability) were evaluated using a vector network analyzer (VNA; CeyearAV3672C) in the frequency range of 2.0–18.0 GHz. Prior to the test, the sample was thoroughly mixed with paraffin at a mass ratio of 3 : 7 and pressed thereafter into a coaxial ring with outer and inner diameters of 7 mm and 3.04 mm, respectively.


## 2.1. Sample Preparation

Preparation of ACF through industrial process and thermal decomposition promote in situ growth of Fe3O4 on the surface of ACF, and the process route is shown in Figure 1. In detail, the viscose fiber is impregnated with ammonium hydrogen phosphate, dried, and treated at 850°C for 10 minutes; the process is passed through steam to prepare ACF. 0.25 mol glucose was dissolved into 1000 mL distilled water followed by the addition of 2 mol FeCl3 to form a homogeneous solution. ACF felt was added to the solution, in which the rolling surplus rate was 900%. After drying, ACF that contained FeCl3 was heat-treated at 650°C for 60 min in N2 gas, covered the ACF surface with a layer of Fe3O4, and prepared ACF–Fe3O4(3). The amount of Fe3O4 in the composites was regulated by the addition of different amounts of FeCl3 and glucose, specifically 0, 0.5, and 1 mol for FeCl3 and 0, 0.0625, and 0.125 mol for glucose. By using the preceding experimental method, the final products were denoted as ACF, ACF–Fe3O4(1), and ACF–Fe3O4(2), respectively.Figure 1
Preparation process for the ACF–Fe3O4 composites.


## 2.2. Characterization

Phase structural analysis of the prepared ACF–Fe3O4 was performed using X-ray diffraction (XRD; Rigaku D/max-2500PC) with CuKα radiation. A scanning electron microscope (SEM; ZEISS Gemini SEM 300) equipped with an energy dispersive spectrometer (EDS) was used for morphological observations and elemental analyses. Textural characterization was carried out by nitrogen adsorption at 77 K using an ASAP 2020 automatic physisorption analyzer (Micromeritics Instrument Corp., USA). The chemical states were characterized by X-ray photoelectron spectroscopy (XPS; Thermo Fisher Scientific ESCALAB-250) with Cu Kα radiation. The magnetic properties were achieved using a vibrating sample magnetometer (VSM; Quantum Design MPMS) at 300 K. The formation mechanism was analyzed using thermogravimetry/differential scanning calorimetry (TG/DSC; Netzsch 214 Polyma) at a heating rate of 10°C/min from 30°C to 1000°C in the presence of air. The EM parameters (i.e., relative complex permittivity and relative complex permeability) were evaluated using a vector network analyzer (VNA; CeyearAV3672C) in the frequency range of 2.0–18.0 GHz. Prior to the test, the sample was thoroughly mixed with paraffin at a mass ratio of 3 : 7 and pressed thereafter into a coaxial ring with outer and inner diameters of 7 mm and 3.04 mm, respectively.


## 3. Results and Discussion

### 3.1. Crystal Structure

XRD profiles of ACF, ACF–Fe3O4(1), ACF–Fe3O4(2), and ACF–Fe3O4(3) are shown in Figure 2. ACF felt, FeCl3, and glucose were treated with high temperature at 650°C under the protection of nitrogen, FeCl3 and glucose undergo thermal decomposition, and the reduction reaction produced Fe3O4 (PDF # 74-0748) magnetic particles. The chemical reaction is shown in Eqs. (1)–(4) [23, 24]. Nonmagnetic Fe2O3 (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].
(1)FeCl3+3H2O→∆,N2FeOH3+3HCl↑(2)C6H12O6→∆,N26C+6H2O↑(3)FeOH3→∆,N2Fe2O3+H2O↑(4)Fe2O3+COorC→∆,N2Fe3O4+CO2↑Figure 2
XRD profiles of ACF, ACF–Fe3O4(1), ACF–Fe3O4(2), and ACF–Fe3O4(3).


### 3.2. Morphological and EDS Analyses

Figures3(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 N2 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.Figure 3
SEM images of (a, b) fiber cross-section of ACF and (c) ACF; nitrogen adsorption isotherm and pore size distribution of (d) ACF.After Fe3O4 is loaded, the particles tend to accumulate in the grooves on the fiber surface. Figures 4(a)–4(c) show the fiber surface morphology after being impregnated with different concentrations of the FeCl3 solution and heat-treated. Fe3O4 particles are distributed along the fiber axis. As the proportion of metal salts increases, the distributed particles increase and there is an aggregation phenomenon. XRD analysis shows that the particulate matter contains Fe3O4; it has EM wave absorption properties. When combined with a carbon-based material, the increase in Fe3O4 can effectively increase the EM wave absorption performance of the material [26, 27]. Figure 4(d) shows the elemental analysis of the ACF–Fe3O4(3) energy dispersive spectrometer (EDS). After the in situ hybridization of Fe3O4 on the ACF surface to form an ACF–Fe3O4 composite, there is the presence of Fe and O elements on the fiber surface. The ACF preparation process includes the dip of ammonium dihydrogen phosphate; thus, element P is also detected. These elements constitute the Fe3O4 formed in the XRD analysis, thereby further confirming the possibility of its formation. In addition, Fe3O4 that absorbs EM waves forms a multi-interface structure on the fiber surface with rich interface polarization; this can promote the EM wave absorption of the material.Figure 4
SEM images of (a) ACF–Fe3O4(1), (b) ACF–Fe3O4(2), and (c) ACF–Fe3O4(3); EDS of (d) ACF–Fe3O4(3).


### 3.3. XPS Analyses

XPS will obtain more information on the composition of the ACF and ACF-Fe3O4 material. Figures 5(a) and 5(b) show the X-ray photoelectron spectra of ACF and ACF–Fe3O4 (3) composite. ACF consisted of the elements C and O. The ACF–Fe3O4 (3) consisted of C, O, and Fe. Figure 5(c) shows the O1s XPS spectrum for ACF–Fe3O4(3). 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 Fe3O4 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 Fe3O4 of 0.5, and it is due to the existence of Fe2O3 [31, 32].Figure 5
XPS spectra: wide scan of (a) ACF and (b) ACF–Fe3O4(3); (c) O1s and (d) Fe2p spectra for ACF–Fe3O4(3) composite.
(a)(b)(c)(d)


### 3.4. TG and magnetic hysteresis analyses

The temperature stability of microwave-absorbing materials is an important material property related to practical engineering applications. Figure6 shows the TG analysis of the ACF and ACF–Fe3O4 composites in the air at a heating rate of 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 H2O 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–Fe3O4 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 Fe3O4 is oxidized to Fe2O3 [33, 34].Figure 6
TG curves of the ACF and ACF–Fe3O4 composites in the air.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–Fe3O4 composite materials were studied at room temperature by measuring their magnetization curves. These magnetic properties include Ms, Mr, and Hc (see Figure 7). Table 1 also shows the magnetic parameters corresponding to Figure 7. ACF has no magnetic properties, and pure Fe3O4 is a typical superparamagnetic material that has high saturation Ms and Mr and low Hc [35]. As the (Fe3O4)–ACF ratio increases, the saturation and residual magnetizations of the composite material increase, while the coercive force decreases. ACF–Fe3O4(3) has high Ms (14.435 emu/g) and Mr (2.023 emu/g), while Hc (178.857 Oe) is low, at room temperature, and ACF–Fe3O4 (3) is superparamagnetic. It shows that the introduction of magnetic Fe3O4 causes the difference in the magnetic properties of the composite materials [26, 36].Figure 7
Magnetic hysteresis loops of ACF, ACF–Fe3O4(1), ACF–Fe3O4(2), and ACF–Fe3O4(3) composites.Table 1
Magnetic properties of ACF, ACF–Fe3O4(1), ACF–Fe3O4(2), and ACF–Fe3O4(3) composites at room temperature.
SamplesHc (Oe)Ms (emu/g)Mr (emu/g)ACF207.7240.0210.002ACF–Fe3O4(1)187.9133.2120.392ACF–Fe3O4(2)177.2585.5460.798ACF–Fe3O4(3)175.85714.4352.023


### 3.5. EM Parameters

Figures8(a)–8(d) show the measured dielectric properties of five samples of 30 wt% ACF, Fe3O4, and ACF–Fe3O4 composites in the 2–18 GHz range. Note that samples with a high (ACF)–(Fe3O4) ratio show high values of ε′ and ε″ in the frequency range of 2–18 GHz (see Figures 8(a) and 8(b)). For ACF–Fe3O4 (2), ACF–Fe3O4 (3), and pure Fe3O4, the ε′ value decreases insignificantly with increasing frequency in the 2–18 GHz range. Although the ε″ value is in the 8–14 GHz range (Figure 8(b)), the peak shows the resonance behavior, which is expected when the sample has high conductivity. Free electron theory indicates that ε′=1/2ε0πρf, 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) and 8(b) show that in the frequency range of 2–18 GHz, changes in the ε′ and ε″ curves of ACF and ACF–Fe3O4(1) are similar and fluctuate in the 8–12 GHz range. Figures 8(c) and 8(d) show the μ′ and μ″ values of the relative complex permeability of the ACF, Fe3O4, and ACF–Fe3O4 composites. With the introduction of the Fe3O4 particles, signs of magnetic increase can be observed from the enhanced permeability. For ACF and ACF–Fe3O4(1), the change trend is not substantially different because the condition is that the concentration of Fe3O4 particles is relatively low. Figures 8(c) and 8(d) show that the μ′ and μ″ curves of the composite material do not fluctuate substantially, and their values are approximately 1 and 0, respectively, which are similar to those reported in the literature [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 (tanδE=ε″/ε′) and magnetic loss tangent (tanδM=μ″/μ′) can characterize the dielectric and magnetic losses of the materials. Materials absorb EM waves through two main mechanisms. We calculated tan δE and tan δM for each sample to determine which one dominates the material. We found that the material tan δE is between 0 and 0.8, while tan δM is between 0 and 0.3. Obviously, the role of dielectric loss and magnetic loss in ACF/Fe3O4 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].Figure 8
Frequency dependence on (a) real and (b) imaginary parts of the complex permittivity of samples, (c) real and (d) imaginary parts of the complex permeability, and the (e) corresponding dielectric and (f) magnetic loss tangents of ACF, ACF–Fe3O4(1), ACF–Fe3O4(2), ACF–Fe3O4(3), and Fe3O4.
(a)(b)(c)(d)(e)(f)The Debye dipolar relaxation indicates can be expressed by the following equation [43]:
(5)εr=ε∞+εs−ε∞1+j2πfτ=ε′f+iε′′f,where f is the frequency, τ is the relaxation time, and εs and ε∞ are the stationary and optical dielectric constants, respectively. The following equation can be deduced from Eqn. (5):
(6)ε′f=ε∞+εs+ε∞1+2πf2τ2,(7)ε′′f=2πfτεs−ε∞1+2πf2τ2.Equations (6) and (7) indicate that the relationship between ε′ and ε″ can be deduced as follows:
(8)ε′−ε∞2+ε′′2=εs−ε∞2.Figure9 shows a plot of the ε″ values vs. ε′ values based on Eq. (8). Evidently, the ACF–Fe3O4 composite material obtained seven clear parts of overlapping semicircles. This result indicates a double dielectric relaxation process in the ACF–Fe3O4 composite owing to the overlapping semicircles. 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 Fe3O4 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″=1/2ε0ρπf) 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 Fe3O4 particles may reduce the impedance matching of the material. Accordingly, pure ACF and Fe3O4 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 Fe3O4 particles. Hence, the material can considerably respond to the impedance matching requirements while maintaining the existing dielectric relaxation characteristics.Figure 9
Typical Cole–Cole curves of ACF, CF–Fe3O4(3), and pure Fe3O4.Figure 10
RL and 3D plots of ACF, ACF–Fe3O4(1), ACF–Fe3O4(2), ACF–Fe3O4(3), and pure Fe3O4 over the 2–18 GHz range (a–i); C0−f curves of (k) Fe3O4 and ACF-Fe3O4 composites.
(a)(b)(c)(d)(e)(f)(g)(h)(i)(j)(k)


### 3.6. EM Wave-Absorbing Properties

Figure10 shows the RL curves of the ACF, Fe3O4, and ACF–Fe3O4 composites with different thicknesses, as well as the corresponding 3D surface plots. As shown in Figures 10(a) and 10(j), the Fe3O4 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 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 Fe3O4 particles is low, ACF–Fe3O4(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. Figure 8(g) shows that when the thickness of the ACF–Fe3O4(3) composite coating is 2.0 mm, the reflection loss (RL) at 13–14.3 GHz and 15.1–16.1 GHz is below −10 dB (90% absorption), the minimum value is −15.2 dB at 15.6 GHz, and the bandwidth of the RL is below −10 dB is 2.3 GHz. As the layer thickness increases, the maximum RL moves significantly to a low frequency range. When the coating thickness reaches 3.0 mm, the corresponding ACF–Fe3O4(3) RL at 8.8–13.6 GHz is below −10 dB, while the minimum value at 10 GHz is −44.3 dB. The bandwidths corresponding to RLs below −10 dB and −20 dB (99% absorption) are 4.8 GHz and 1.1 GHz, respectively. The calculation results show that the synthesized ACF–Fe3O4 composite material has better microwave absorption performance than the pure ACF and pure Fe3O4 nanoparticles. Table 2 also shows the comparison of the electromagnetic wave absorption performance with other carbon/magnetic materials; the composite material prepared based on the porous ACF has obvious electromagnetic wave absorption performance. The loss caused by the Eddy current effect can be calculated by C0C0=μ″μ′−2f−1=2πμ0σd2/3, where σ is the electrical conductivity, d is the diameter of the particle, and μ′ is the vacuum permeability. If the magnetic loss is caused by the Eddy loss effect, then C0 is constant when the frequency changes. Figure 10(k) shows the C0−f curve of the Fe3O4 and ACF–Fe3O4 composites. For pure Fe3O4, the C0 value is nearly constant in the frequency range of 6–18 GHz, while the change in C0 is only observed in the range of 2–6 GHz. When the amount of FeCl3 added is 0.5 mol/L, two C0 levels remain in the range of 4–5 GHz and 14–18 GHz. However, at the 2–18 GHz frequency range, C0 shows a significant downward trend as the concentration of the FeCl3 solution increases, particularly for ACF–Fe3O4(3). This characteristic means that pure Fe3O4 has a significant Eddy current effect, which can be reduced by introducing ACF. In addition, the magnetic loss in ACF–Fe3O4 is mainly caused by natural resonance and not caused by the Eddy current effect.Table 2
Comparison of electromagnetic absorption properties of carbon/magnetic composites in the present and related studies.
SamplesLoading ratio (wt%)Thickness (mm)Electromagnetic wave absorptionReferencesCarbon fiber/nano-Fe3O450%3.0<−5 dB (8.9–9.5 GHz)[44]Carbon fiber/carbonyl iron45%2.0<−10 dB (6.6–8.7 GHz)[45]Carbon fiber/Fe–Co30%2.0<−10 dB (8.2–10.3 GHz)[46]Carbon fiber/Fe3O450%4.41<−10 dB (4.4–8.7 GHz)[47]PANI@Fe3O4@CNFs15%2.0<−10 dB (11.9–15.6 GHz)[48]ACF/Fe3O4(3)30%3.0<−10 dB (8.8–13.6 GHz)Present workNumerous defects cause multiple scattering and interface polarization, thereby providing an important absorption mechanism. In addition, the fibrous structure and high specific surface properties of the ACF–Fe3O4 composite material 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 Fe3O4 that in the EM complementation effect previously proposed. Evidently, ACF-Fe3O4 composite material is a lightweight and efficient EM-absorbing material.Figure 11
Scheme of the EM loss in the ACF–Fe3O4 composite felts.


## 3.1. Crystal Structure

XRD profiles of ACF, ACF–Fe3O4(1), ACF–Fe3O4(2), and ACF–Fe3O4(3) are shown in Figure 2. ACF felt, FeCl3, and glucose were treated with high temperature at 650°C under the protection of nitrogen, FeCl3 and glucose undergo thermal decomposition, and the reduction reaction produced Fe3O4 (PDF # 74-0748) magnetic particles. The chemical reaction is shown in Eqs. (1)–(4) [23, 24]. Nonmagnetic Fe2O3 (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].
(1)FeCl3+3H2O→∆,N2FeOH3+3HCl↑(2)C6H12O6→∆,N26C+6H2O↑(3)FeOH3→∆,N2Fe2O3+H2O↑(4)Fe2O3+COorC→∆,N2Fe3O4+CO2↑Figure 2
XRD profiles of ACF, ACF–Fe3O4(1), ACF–Fe3O4(2), and ACF–Fe3O4(3).


## 3.2. Morphological and EDS Analyses

Figures3(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 N2 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.Figure 3
SEM images of (a, b) fiber cross-section of ACF and (c) ACF; nitrogen adsorption isotherm and pore size distribution of (d) ACF.After Fe3O4 is loaded, the particles tend to accumulate in the grooves on the fiber surface. Figures 4(a)–4(c) show the fiber surface morphology after being impregnated with different concentrations of the FeCl3 solution and heat-treated. Fe3O4 particles are distributed along the fiber axis. As the proportion of metal salts increases, the distributed particles increase and there is an aggregation phenomenon. XRD analysis shows that the particulate matter contains Fe3O4; it has EM wave absorption properties. When combined with a carbon-based material, the increase in Fe3O4 can effectively increase the EM wave absorption performance of the material [26, 27]. Figure 4(d) shows the elemental analysis of the ACF–Fe3O4(3) energy dispersive spectrometer (EDS). After the in situ hybridization of Fe3O4 on the ACF surface to form an ACF–Fe3O4 composite, there is the presence of Fe and O elements on the fiber surface. The ACF preparation process includes the dip of ammonium dihydrogen phosphate; thus, element P is also detected. These elements constitute the Fe3O4 formed in the XRD analysis, thereby further confirming the possibility of its formation. In addition, Fe3O4 that absorbs EM waves forms a multi-interface structure on the fiber surface with rich interface polarization; this can promote the EM wave absorption of the material.Figure 4
SEM images of (a) ACF–Fe3O4(1), (b) ACF–Fe3O4(2), and (c) ACF–Fe3O4(3); EDS of (d) ACF–Fe3O4(3).


## 3.3. XPS Analyses

XPS will obtain more information on the composition of the ACF and ACF-Fe3O4 material. Figures 5(a) and 5(b) show the X-ray photoelectron spectra of ACF and ACF–Fe3O4 (3) composite. ACF consisted of the elements C and O. The ACF–Fe3O4 (3) consisted of C, O, and Fe. Figure 5(c) shows the O1s XPS spectrum for ACF–Fe3O4(3). 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 Fe3O4 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 Fe3O4 of 0.5, and it is due to the existence of Fe2O3 [31, 32].Figure 5
XPS spectra: wide scan of (a) ACF and (b) ACF–Fe3O4(3); (c) O1s and (d) Fe2p spectra for ACF–Fe3O4(3) composite.
(a)(b)(c)(d)


## 3.4. TG and magnetic hysteresis analyses

The temperature stability of microwave-absorbing materials is an important material property related to practical engineering applications. Figure6 shows the TG analysis of the ACF and ACF–Fe3O4 composites in the air at a heating rate of 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 H2O 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–Fe3O4 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 Fe3O4 is oxidized to Fe2O3 [33, 34].Figure 6
TG curves of the ACF and ACF–Fe3O4 composites in the air.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–Fe3O4 composite materials were studied at room temperature by measuring their magnetization curves. These magnetic properties include Ms, Mr, and Hc (see Figure 7). Table 1 also shows the magnetic parameters corresponding to Figure 7. ACF has no magnetic properties, and pure Fe3O4 is a typical superparamagnetic material that has high saturation Ms and Mr and low Hc [35]. As the (Fe3O4)–ACF ratio increases, the saturation and residual magnetizations of the composite material increase, while the coercive force decreases. ACF–Fe3O4(3) has high Ms (14.435 emu/g) and Mr (2.023 emu/g), while Hc (178.857 Oe) is low, at room temperature, and ACF–Fe3O4 (3) is superparamagnetic. It shows that the introduction of magnetic Fe3O4 causes the difference in the magnetic properties of the composite materials [26, 36].Figure 7
Magnetic hysteresis loops of ACF, ACF–Fe3O4(1), ACF–Fe3O4(2), and ACF–Fe3O4(3) composites.Table 1
Magnetic properties of ACF, ACF–Fe3O4(1), ACF–Fe3O4(2), and ACF–Fe3O4(3) composites at room temperature.
SamplesHc (Oe)Ms (emu/g)Mr (emu/g)ACF207.7240.0210.002ACF–Fe3O4(1)187.9133.2120.392ACF–Fe3O4(2)177.2585.5460.798ACF–Fe3O4(3)175.85714.4352.023


## 3.5. EM Parameters

Figures8(a)–8(d) show the measured dielectric properties of five samples of 30 wt% ACF, Fe3O4, and ACF–Fe3O4 composites in the 2–18 GHz range. Note that samples with a high (ACF)–(Fe3O4) ratio show high values of ε′ and ε″ in the frequency range of 2–18 GHz (see Figures 8(a) and 8(b)). For ACF–Fe3O4 (2), ACF–Fe3O4 (3), and pure Fe3O4, the ε′ value decreases insignificantly with increasing frequency in the 2–18 GHz range. Although the ε″ value is in the 8–14 GHz range (Figure 8(b)), the peak shows the resonance behavior, which is expected when the sample has high conductivity. Free electron theory indicates that ε′=1/2ε0πρf, 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) and 8(b) show that in the frequency range of 2–18 GHz, changes in the ε′ and ε″ curves of ACF and ACF–Fe3O4(1) are similar and fluctuate in the 8–12 GHz range. Figures 8(c) and 8(d) show the μ′ and μ″ values of the relative complex permeability of the ACF, Fe3O4, and ACF–Fe3O4 composites. With the introduction of the Fe3O4 particles, signs of magnetic increase can be observed from the enhanced permeability. For ACF and ACF–Fe3O4(1), the change trend is not substantially different because the condition is that the concentration of Fe3O4 particles is relatively low. Figures 8(c) and 8(d) show that the μ′ and μ″ curves of the composite material do not fluctuate substantially, and their values are approximately 1 and 0, respectively, which are similar to those reported in the literature [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 (tanδE=ε″/ε′) and magnetic loss tangent (tanδM=μ″/μ′) can characterize the dielectric and magnetic losses of the materials. Materials absorb EM waves through two main mechanisms. We calculated tan δE and tan δM for each sample to determine which one dominates the material. We found that the material tan δE is between 0 and 0.8, while tan δM is between 0 and 0.3. Obviously, the role of dielectric loss and magnetic loss in ACF/Fe3O4 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].Figure 8
Frequency dependence on (a) real and (b) imaginary parts of the complex permittivity of samples, (c) real and (d) imaginary parts of the complex permeability, and the (e) corresponding dielectric and (f) magnetic loss tangents of ACF, ACF–Fe3O4(1), ACF–Fe3O4(2), ACF–Fe3O4(3), and Fe3O4.
(a)(b)(c)(d)(e)(f)The Debye dipolar relaxation indicates can be expressed by the following equation [43]:
(5)εr=ε∞+εs−ε∞1+j2πfτ=ε′f+iε′′f,where f is the frequency, τ is the relaxation time, and εs and ε∞ are the stationary and optical dielectric constants, respectively. The following equation can be deduced from Eqn. (5):
(6)ε′f=ε∞+εs+ε∞1+2πf2τ2,(7)ε′′f=2πfτεs−ε∞1+2πf2τ2.Equations (6) and (7) indicate that the relationship between ε′ and ε″ can be deduced as follows:
(8)ε′−ε∞2+ε′′2=εs−ε∞2.Figure9 shows a plot of the ε″ values vs. ε′ values based on Eq. (8). Evidently, the ACF–Fe3O4 composite material obtained seven clear parts of overlapping semicircles. This result indicates a double dielectric relaxation process in the ACF–Fe3O4 composite owing to the overlapping semicircles. 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 Fe3O4 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″=1/2ε0ρπf) 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 Fe3O4 particles may reduce the impedance matching of the material. Accordingly, pure ACF and Fe3O4 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 Fe3O4 particles. Hence, the material can considerably respond to the impedance matching requirements while maintaining the existing dielectric relaxation characteristics.Figure 9
Typical Cole–Cole curves of ACF, CF–Fe3O4(3), and pure Fe3O4.Figure 10
RL and 3D plots of ACF, ACF–Fe3O4(1), ACF–Fe3O4(2), ACF–Fe3O4(3), and pure Fe3O4 over the 2–18 GHz range (a–i); C0−f curves of (k) Fe3O4 and ACF-Fe3O4 composites.
(a)(b)(c)(d)(e)(f)(g)(h)(i)(j)(k)


## 3.6. EM Wave-Absorbing Properties

Figure10 shows the RL curves of the ACF, Fe3O4, and ACF–Fe3O4 composites with different thicknesses, as well as the corresponding 3D surface plots. As shown in Figures 10(a) and 10(j), the Fe3O4 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 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 Fe3O4 particles is low, ACF–Fe3O4(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. Figure 8(g) shows that when the thickness of the ACF–Fe3O4(3) composite coating is 2.0 mm, the reflection loss (RL) at 13–14.3 GHz and 15.1–16.1 GHz is below −10 dB (90% absorption), the minimum value is −15.2 dB at 15.6 GHz, and the bandwidth of the RL is below −10 dB is 2.3 GHz. As the layer thickness increases, the maximum RL moves significantly to a low frequency range. When the coating thickness reaches 3.0 mm, the corresponding ACF–Fe3O4(3) RL at 8.8–13.6 GHz is below −10 dB, while the minimum value at 10 GHz is −44.3 dB. The bandwidths corresponding to RLs below −10 dB and −20 dB (99% absorption) are 4.8 GHz and 1.1 GHz, respectively. The calculation results show that the synthesized ACF–Fe3O4 composite material has better microwave absorption performance than the pure ACF and pure Fe3O4 nanoparticles. Table 2 also shows the comparison of the electromagnetic wave absorption performance with other carbon/magnetic materials; the composite material prepared based on the porous ACF has obvious electromagnetic wave absorption performance. The loss caused by the Eddy current effect can be calculated by C0C0=μ″μ′−2f−1=2πμ0σd2/3, where σ is the electrical conductivity, d is the diameter of the particle, and μ′ is the vacuum permeability. If the magnetic loss is caused by the Eddy loss effect, then C0 is constant when the frequency changes. Figure 10(k) shows the C0−f curve of the Fe3O4 and ACF–Fe3O4 composites. For pure Fe3O4, the C0 value is nearly constant in the frequency range of 6–18 GHz, while the change in C0 is only observed in the range of 2–6 GHz. When the amount of FeCl3 added is 0.5 mol/L, two C0 levels remain in the range of 4–5 GHz and 14–18 GHz. However, at the 2–18 GHz frequency range, C0 shows a significant downward trend as the concentration of the FeCl3 solution increases, particularly for ACF–Fe3O4(3). This characteristic means that pure Fe3O4 has a significant Eddy current effect, which can be reduced by introducing ACF. In addition, the magnetic loss in ACF–Fe3O4 is mainly caused by natural resonance and not caused by the Eddy current effect.Table 2
Comparison of electromagnetic absorption properties of carbon/magnetic composites in the present and related studies.
SamplesLoading ratio (wt%)Thickness (mm)Electromagnetic wave absorptionReferencesCarbon fiber/nano-Fe3O450%3.0<−5 dB (8.9–9.5 GHz)[44]Carbon fiber/carbonyl iron45%2.0<−10 dB (6.6–8.7 GHz)[45]Carbon fiber/Fe–Co30%2.0<−10 dB (8.2–10.3 GHz)[46]Carbon fiber/Fe3O450%4.41<−10 dB (4.4–8.7 GHz)[47]PANI@Fe3O4@CNFs15%2.0<−10 dB (11.9–15.6 GHz)[48]ACF/Fe3O4(3)30%3.0<−10 dB (8.8–13.6 GHz)Present workNumerous defects cause multiple scattering and interface polarization, thereby providing an important absorption mechanism. In addition, the fibrous structure and high specific surface properties of the ACF–Fe3O4 composite material 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 Fe3O4 that in the EM complementation effect previously proposed. Evidently, ACF-Fe3O4 composite material is a lightweight and efficient EM-absorbing material.Figure 11
Scheme of the EM loss in the ACF–Fe3O4 composite felts.


## 4. Conclusions

This study selected ACF as a substrate and maximized its high specific surface area. By growing Fe3O4 in situ on the surface of ACF, a high-efficiency, wide-band, and light-weight carbon magnetic EM wave-absorbing material based on ACF was prepared. ACF-Fe3O4 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–Fe3O4 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-Fe3O4 has excellent properties such as broadband, high efficiency, stability, and lightness and is a new type of electromagnetic wave-absorbing composite material.



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*Source: 6980730-2020-12-02.xml*