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Preparation and tailoring electromagnetic shielding and microwave absorbing performance of Fe3O4 modified activated carbon foam based on mesophase coal pitch pyrolysis foaming
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Yuanyuan GE1, Yuzhe WANG1, Guozhong XU1, *, Yaming ZHU1, Xia YUAN2, *, Guimei SHI3, Xiangyun ZHONG1
Journal of Fuel Chemistry and Technology | 2026, 54(1) : 20250120-120 - 20250120-134
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Journal of Fuel Chemistry and Technology | 2026, 54(1): 20250120-120-20250120-134
• RESEARCH PAPER •
Preparation and tailoring electromagnetic shielding and microwave absorbing performance of Fe3O4 modified activated carbon foam based on mesophase coal pitch pyrolysis foaming
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Yuanyuan GE1, Yuzhe WANG1, Guozhong XU1, *, Yaming ZHU1, Xia YUAN2, *, Guimei SHI3, Xiangyun ZHONG1
Affiliations
  • 1Engineering Research Center of Coal Chemistry for Liaoning Province, Professional Technology Innovation Center of Coal Chemical Industry for Liaoning, School of Chemical Engineering, University of Science and Technology Liaoning, Anshan 114051, China
  • 2School of Materials Science and Technology, Shenyang Ligong University, Shenyang 110159, China
  • 3School of Science, Shenyang University of Technology, Shenyang 111003, China
Published: 2026-01-01 doi: 10.1016/S1872-5813(25)60593-7
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The development of materials with excellent microwave absorption (MWA) and electromagnetic interference (EMI) shielding performances has currently received attention. Herein, mesophase pitch-based carbon foam (MPCF) with 3D interconnected pore structure was prepared through the high pressure pyrolysis of mesophase coal tar pitch. It is found that the 3D interconnected cellular pores of MPCF facilitate multiple reflections of electromagnetic waves, which results in the minimum reflection loss (RLmin) value of MPCF reaches −37.84 dB with the effective absorption bandwidth (EAB) of 5.44 GHz at a thickness of 2.70 mm, and the total average electromagnetic shielding effectiveness (SET) under 3.00 mm thickness achieves 26.52 dB in X-band. Subsequently, MPCF is activated by KOH to obtain activated carbon foam (A-MPCF). The average SET of A-MPCF achieves 103.00 dB for abundant nanopores on the pore cell walls, which leads to a transition from the multiple reflections of electromagnetic waves on the walls to diffuse reflection. Unfortunately, the reflection coefficient (R) of A-MPCF increases from 0.78 to 0.90. To reduce the R value, Fe3O4/A-MPCF was fabricated via the in situ growth of nano Fe3O4 on A-MPCF. Consequently, the R value of Fe3O4/A-MPCF was reduced from 0.90 to 0.74, whereas the MWA performance was only slightly decreased. This work proposes a simple strategy for simultaneously adjusting MWA and EMI shielding performances of materials.

carbon foam  /  microwave absorption  /  electromagnetic interference shielding  /  mesophase pitch
葛媛媛, 王禹哲, 徐国忠, 朱亚明, 袁霞, 史桂梅, 钟祥云. Fe3O4/中间相煤沥青基活性复合炭泡沫制备及其电磁屏蔽和吸波性能调控. 燃料化学学报（中英文）, 2026 , 54 (1) : 20250120-120 -20250120-134 . DOI: 10.1016/S1872-5813(25)60593-7
Yuanyuan GE, Yuzhe WANG, Guozhong XU, Yaming ZHU, Xia YUAN, Guimei SHI, Xiangyun ZHONG. Preparation and tailoring electromagnetic shielding and microwave absorbing performance of Fe3O4 modified activated carbon foam based on mesophase coal pitch pyrolysis foaming[J]. Journal of Fuel Chemistry and Technology, 2026 , 54 (1) : 20250120-120 -20250120-134 . DOI: 10.1016/S1872-5813(25)60593-7
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The speedy advancement of communication technology has not only enhanced the convenience of human lives, but also filled the surrounding environment with electromagnetic waves. These waves pose a certain hazard to our bodies and interfere with the functioning of precision instruments[1−2]. Consequently, researchers have paid widespread attention to the effective dissipation of electromagnetic waves. Currently, there are two primary methods to evaluate the ability of materials to dissipate electromagnetic waves, namely electromagnetic interference (EMI) shielding and microwave absorption (MWA)[3−4]. The MWA performance for materials is evaluated by attenuation constant (α) and impedance matching (Zin/Z0) together[5]. The α value typically correlates positively with the conductivity. Nevertheless, excessively high conductivity can generate impedance mismatch, leading to electromagnetic waves being reflected away from the material’s surface. The EMI shielding effectiveness of materials (SET) is related to their reflection loss (SER), absorption loss (SEA), and multiple reflection loss (SEM)[6−8]. The SER represents how much of incident electromagnetic waves are reflected. The SEA reflects the absorption of electromagnetic waves within the material, driven by polarization loss, magnetic resonance, or eddy current loss. The SEM represents the multiple reflections and scattering losses of electromagnetic waves at heterogeneous interfaces or defects within the material[9]. Unlike MWA materials, materials with high conductivity usually exhibit impressive EMI shielding performance. However, this is predominantly driven by SER, which can generate significant secondary pollution to the space. Thus, preparing materials with excellent EMI shielding and MWA properties simultaneously poses a significant challenge and is also a long-standing objective pursued by researchers. To obtain materials with excellent MWA performance, both a good impedance matching level and a sufficiently high attenuation constant are essential[10]. A good impedance matching level can ensure that incident electromagnetic waves efficiently enter the material’s interior and are subsequently dissipated. Conversely, if the impedance matching level is not ideal, electromagnetic waves will not enter into the material but will reflect back into the free space. Such materials often have a large minimum reflection loss (RLmin) value and are not suitable for MWA applications. Therefore, for the purpose of constructing materials with impressive MWA and EMI shielding performance, it is rational to design EMI shielding materials dominated by SEA.
In recent years, numerous studies have demonstrated that loading magnetic components such as Fe[11], Co[12] and Ni[13] on the carbon materials can optimize the impedance matching, thereby facilitating the dissipation of electromagnetic waves in the form of SEA. Liu et al.[14] mixed Ni/Zn ferrite with coal tar pitch to prepare composite carbon foam through foaming and carbonization process. It was illustrated that the complex permittivity and permeability of carbon foam were improved after introducing Ni/Zn ferrite. The results showed the average SET of the carbon foam reached 42.0 dB in the X-band and the EMI shielding mechanism was dominated by absorption. Liu et al.[1] fabricated Fe3O4/carbon composite foam with the EMI shielding mechanism dominated by absorption through introducing Fe3O4 nanoparticles on the surface of melamine-based carbon foam. The study pointed out that the excellent EMI shielding performance of Fe3O4/carbon foam is ascribed to the synergistic effect between the magnetic loss of Fe3O4 and the dielectric loss of carbon foam matrix. It was shown that the average SET reached 21.0 dB in the X-band when the thickness was 4.0 mm.
In addition, the nanoporous structure of the material also exerts a significant influence on its MWA and EMI shielding performance. Kim et al.[15] pointed out that the nanoporous structure can increase the transmission path of the electromagnetic waves within the material, which promotes multiple reflections and scattering, thereby enhancing the material’s absorption ability for electromagnetic waves. This finding has been confirmed by many reports as well. Luo et al.[16] systematically investigated the effect of the pore structure for activated carbon (AC) on the MWA performance by adjusting the activation temperature. It was revealed that the MWA performance of AC increased with the increase of the total pore volume (vpore) for AC, which was attributed to the interfacial polarization effect inspired by the hierarchical porous structure of AC. When the vpore of AC reached 0.71 cm3/g, the RLmin and EAB achieved −57.75 dB and 9.86 GHz, respectively.
Mesophase pitch-based carbon foam (MP-CF) is a bulk carbon material with a closed-cell structure. It possesses many advantages such as good electrical conductivity, high chemical stability, strong acid and alkali resistance, and an easily tailored pore structure. Our team used mesophase coal pitch as the raw material and tubular carbon fibers as the additive agent to fabricate MP-CF via high pressure pyrolysis. By adjusting the additive amount of tubular carbon fibers, the effective regulation of the pore cell structure, MWA and EMI shielding performance for the carbon foam was successfully achieved. The results indicated that the MWA and EMI shielding performance of the carbon foam with tubular carbon fibers were significantly enhanced. Compared with the pure MP-CF, the average SET of the MP-CF with the tubular carbon fiber doping amount of 9% increased from 13.54 dB to 26.52 dB. Meanwhile, Deng et al.[17] systematically investigated the effect of graphitization degree on the MWA and EMI shielding performances by varying the carbonization temperature during the preparation process of MP-CF. It was demonstrated that with an increase graphitization degree, dipole polarization decreased and the interfacial polarization enhanced. When the optimal graphitization degree is achieved, the average SET, RLmin and EAB of MP-CF attained 35.13, −16.69 dB and 3.63 GHz, respectively. In brief, the unique pore cell structure of the MP-CF endows it with significant potentials in the fields of MWA and EMI shielding. However, there are few reports on the simultaneous application of MP-CF for MWA and EMI shielding performance.
Based on the aforementioned considerations, in this study, a mesophase coal pitch-based carbon foam (MPCF) was fabricated via high pressure pyrolysis to serve as a framework for electromagnetic wave loss. Furthermore, in order to further enhance its electromagnetic waves loss ability by increasing the probability of contact between electromagnetic waves and pore walls, the MPCF was activated by KOH to obtain an activated MPCF (A-MPCF). Subsequently, the A-MPCF was modified with Fe3O4 by in situ growing magnetic nano-Fe3O4 on the pore walls to improve the overall loss capability. The mechanism of electromagnetic waves loss for the three carbon foams prepared was revealed by analyzing the EMA and EMI shielding performance, as well as the structure activity relationship. The purpose of this study is to provide a simple and feasible strategy for preparing materials with excellent EMA and EMI shielding performance simultaneously.
1　Experimental
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1.1　Materials
The mesophase coal pitch (MP) was purchased from China Jining Keneng New Carbon Material Technology Co., Ltd., with its property parameters detailed in Table S1. Tubular carbon fibers were prepared by carbonated medical cottons at 600 °C, and medical cottons were purchased from local pharmacies. Potassium hydroxide (KOH), hydrochloric acid (HCl), trisodium citrate (C6H5Na3O7·2H2O), ferric chloride (FeCl3·6H2O), sodium acetate (C2H3NaO2·3H2O), ethyl alcohol (CH3CH2OH) and glycol (C2H6O2) were purchased from Sinopharm Chemical Reagent Co., Ltd.
1.2　Preparation of mesophase coal pitch-based carbon foam
First, the MP was pulverized to pass 150–200 mesh using a ball mill. Then, 40 g of MP powder and 3.6 g of tubular carbon fibers were thoroughly mixed. Subsequently, the mixture was placed into an autoclave, and subjected to high pressure pyrolysis at a foaming temperature of 490 °C and a foaming pressure of 7 MPa to obtain the green carbon foam. Finally, the resultant green carbon foam was carbonized at 900 °C under a nitrogen atmosphere to obtain the mesophase coal pitch-based carbon foam (named as MPCF). The detailed preparation process of MPCF can be found in previous work[18].
1.3　Preparation of mesophase coal pitch-based activated carbon foam
The MPCF was cut into a cuboid with a dimension of 22.86 mm×10.16 mm×3.00 mm. Next, the rectangular MPCF was subjected to ultrasonic treatment in a 1.8 mol/L KOH ethanol solution for 4 h. After that, the mixture was transferred into a drying oven to evaporate the solution at 70 °C for 10 h. Subsequently, the impregnated rectangular MPCF was activated at 850 °C for 2 h in a tubular furnace under a nitrogen atmosphere. Finally, the activated sample was washed to neutrality using 0.1 mol/L HCl solution and deionized water to obtain activated MPCF (named as A-MPCF).
1.4　Preparation of Fe3O4/A-MPCF
A mixed solution was prepared by adding FeCl3·6H2O (1.14 g), C6H5Na3O7·2H2O (0.72 g) and C2H3NaO2·3H2O (4.00 g) into ethylene glycol (40 mL), and then stirred vigorously for 0.5 h using a magnetic stirrer. A-MPCF was placed in a hydrothermal reactor containing the mixed solution, and then the hydrothermal reactor was placed in an oven set at 220 °C to react for 6 h. After the hydrothermal reaction, the prepared carbon foam was successively washed three times with ethanol and deionized water. Finally, the washed sample was dried to obtain the Fe3O4 decorated A-MPCF (named as Fe3O4/A-MPCF). The strategy for preparation of MPCF, A-MPCF and Fe3O4/A-MPCF is shown in Figure 1. The characterization methods of the samples can be seen in the Support Information.
1.5　MWA and EMI shielding effectiveness measurement
1.5.1　MWA performance
The electromagnetic parameters of the carbon foams were measured by a Vector Network Analyzer (Agilent E5071C, USA) using the coaxial line method. First, the samples for testing were ground to pass 60–80 mesh. Then, the ground carbon foam powders were mixed with paraffin in a mass ratio of 7∶3 to prepare toroidal specimens (7.00 mm outer diameter, 3.04 mm inner diameter and 2.00 mm thickness). The specimens were measured in a coaxial test fixture to obtain the relative complex permittivity (εr) and complex permeability (μr) in the frequency range of 2–18 GHz. The dielectric loss tangent (tan δε) and magnetic loss tangent (tan δμ) were calculated through the following equations:
$ \mathrm{tan}\;{{\delta }}_{\text{ε} }\text=\frac{{\varepsilon }{{'}{'}}}{{\varepsilon }{{'}}} $
$ \mathrm{tan}\;{{\delta }}_{\mu }\text=\frac{{\mu }{{'}{'}}}{{\mu }{{'}}}$
Based on the transmission-line theory, the reflection loss (RL) was simulated calculated using the tested ε and μ values[19−20]:
$ {Z}_{{\mathrm{in}}}\text={Z}_{0}\text{×}\sqrt{\frac{{\mu }_{{\mathrm{r}}}}{{\varepsilon }_{{\mathrm{r}}}}}{\times }{\tan{\mathrm{h}}}\left(j\frac{2\pi fd\sqrt{{\mu }_{{\mathrm{r}}}{\varepsilon }_{{\mathrm{r}}}}}{c}\right) $
$ RL\text=20 \lg \left|\frac{{Z}_{{\mathrm{in}}}-{Z}_{0}}{{Z}_{{\mathrm{in}}}+{Z}_{0}}\right| $
where Z0, Zin, f, c and d are the impendence of free space, the input impendence of absorber, the frequency of microwave, the speed of light in vacuum and the coating thickness. The attenuation constant (α) was determined using following equation[21]:
$\begin{split} &\qquad \alpha \text=\frac{\sqrt{2}\pi f}{c}\text{×}\\& \sqrt{\left({\mu }{{'}{'}}{\varepsilon }{{'}{'}}-{\mu }{{'}}{\varepsilon }{{'}}\right)+\sqrt{{\left({\mu }{{'}{'}}{\varepsilon }{{'}{'}}-{\mu }{{'}}{\varepsilon }{{'}}\right)}^{2}+{\left({\mu }{{'}{'}}{\varepsilon }{{'}{'}}+{\mu }{{'}}{\varepsilon }{{'}}\right)}^{2}}} \end{split}$
1.5.2　EMI shielding performance
The S parameter of samples with a thickness of 3 mm was tested by a vector network analyzer (Agilent N5234A, USA) using the waveguide method in the X-band (8.2–12.4 GHz). The carbon foams were cut into 22.86 mm×10.16 mm×3.00 mm to fit the size of waveguide cavity. Absorption coefficient (A), reflection coefficient (R) and transmission coefficient (T) were calculated according to Equations (6)−(8), respectively. Absorption loss efficiency (SEA), and reflection loss efficiency (SER) were calculated according to Equations (9) and (10), respectively[22−23].
$ R\text={\left|{S}_{11}\right|}^{2} $
$ T\text={\left|{S}_{21}\right|}^{2} $
$ A=1-R-T $
$ {S E}_{{\mathrm{R}}}\text=-10\;\text{lg}\;\left(1-R\right) $
$ {S E}_{{\mathrm{A}}}\text=-10\;\text{lg}\;\left(\frac{T}{1-R}\right) $
When SEA > 15 dB, the SEM is negligible. Therefore, the SET of the composite material can be expressed as[14].
$ {S E}_{{\mathrm{T}}}={S E}_{{\mathrm{R}}}+{S E}_{{\mathrm{A}}} $
2　Results and discussion
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2.1　Microstructure characterization of carbon foams
The pore morphology of MPCF is very uniform, as shown in Figure 2(a), with a significant decrease in pore diameter and a sharp rise in pore number in contrast with carbon foam without added tubular carbon fibers (Figure S1). As shown in Figure 2(b), the pore size of MPCF is mainly distributed between 50–250 μm, and the pore cell density (Ncell, cell/cm3), bulk density (g/cm3), porosity (%), and average pore cell diameter (μm) are also listed in Figure 2(b). A-MPCF exhibits similar pore cell morphology to that of MPCF, as observed by comparing Figure 2(a) and (d). Additionally, numerous wrinkles are observed on the surface of the pore cells in MPCF (Figure 2(c)) and A-MPCF (Figure 2(e)).
As shown in Figure 2(e) and (f), the pore wall surface of Fe3O4/A-MPCF is covered by dense nanoparticles with an average diameter of 109.40 nm. It can be found from Figure 2(g) and (h) that Fe and O elements are homogeneously dispersed on the surface of Fe3O4/A-MPCF, suggesting the successful loading of nano Fe3O4 on the A-MPCF pore surface.
The texture of three kinds of composite carbon foam (MPCF, A-MPCF and Fe3O4/A-MPCF) was measured using low-temperature nitrogen adsorption/desorption test. The adsorption/desorption isotherms and the pore size distributions of the samples are displayed in Figure S2 and Figure 3(a), respectively, and other textural parameters are listed in Table 1. From Figure 3(a), it can be found that two peaks of pore size distribution in 1–2 and 2–6.5 nm are displayed for A-MPCF, which is absent for MPCF and Fe3O4/A-MPCF. In addition, it can also be observed from Table 1 that compared to MPCF, the specific surface area (SBET) of A-MPCF has increased from 2.24 to 45.96 m2/g, micropore volume from 0 to 0.011 m3/g, mesopore volume from 0.0080 to 0.036 m3/g, which suggests MPCF has been successfully activated by KOH. Moreover, the SBET, micropore volume and mesopore volume for Fe3O4/A-MPCF all have reduced in contrast with A-MPCF, which illustrates that the nano-Fe3O4 preferentially grows on the surfaces of highly active micropores and mesopores of A-MPCF, thereby covering both micropores and mesopores.
Subsequently, XRD tests were performed for MPCF, A-MPCF and Fe3O4/A-MPCF, and the results are presented in Figure 3(b). Prepared carbon foams display characteristic peaks of C (002) and (100) at approximately 26° and 43°, which are typical features of amorphous carbon[17,24]. Compared to MPCF, the (002) diffraction peak of A-MPCF is significantly wider, indicating an increase in disorder[10], which is related to the increase in the number of micropores and defects generated after activation. The XRD pattern of Fe3O4/A-MPCF shows characteristic diffraction peaks belonging to the (311), (400) and (440) crystal planes of Fe3O4 at 2θ=35.48°, 43.12° and 62.62° (PDF#97-0.15-9963), respectively.
Figure 3(c) displays the Raman spectra for carbon foams. All samples exhibit two peaks at 1350 and 1580 cm−1, which correspond to the D and G bands, respectively.
Usually, the D band refers to the defect degree in carbon materials, while the G band represents sp2 carbon networks[25−26]. The intensity ratio of the D and G peak (ID/IG) serves as an indicator of the graphitization degree or defect degree[27]. Compared with that of MPCF (1.11), the ID/IG value of 1.15 for A-MPCF has significantly increased, indicating an increase in the disorder and defect degree of A-MPCF, which is aligned with the XRD results. After the in situ growth of nano-Fe3O4, the ID/IG value achieved 1.19, which suggests that the introduction of nano-Fe3O4 improved the defect degree of Fe3O4/A-MPCF. Liu et al.[28] pointed out that increasing the defect level of materials is beneficial for improving their EMW absorption and EMI shielding performance.
XPS tests were performed to analyze the surface chemical states of elements. Figure 3(d) exhibits XPS full-scan spectra for three samples. The XPS full-scan spectra of MPCF and A-MPCF present the characteristic peaks at 284.8 and 532.0 eV assigning to the C 1s and O 1s, indicating the presence of abundant C and O elements[26]. As depicted in Figure 3(e), the O element contents for A-MPCF is slightly increased, meaning that KOH activation introduces new oxygen functional groups for A-MPCF. In addition to the characteristic peak of C 1s and O 1s, the XPS full-scan spectra of Fe3O4/A-MPCF display the characteristic peak of Fe 2p (711 eV), suggesting that C, O and Fe elements coexist in Fe3O4/A-MPCF. As displayed in Figure 3(f), the high-resolution C 1s spectra of MPCF, A-MPCF and Fe3O4/A-MPCF each show peaks of C−C (284.80 eV) and C−O (286.54 eV)[29]. The C 1s spectrum of Fe3O4/A-MPCF exhibits the peak at 288.89 eV, corresponding to the signal of O−C=O[30]. The O 1s spectra of MPCF, A-MPCF and Fe3O4/A-MPCF (Figure 3(g)) show characteristic peaks of C=O and C−O at 531.96 and 533.42 eV, respectively[31]. In addition, Fe3O4/A-MPCF shows the characteristic peak of O−Fe at 529.88 eV[32]. The high-resolution Fe 2p spectrum for Fe3O4/A-MPCF is shown in Figure 3(h), in which six characteristic peaks are observed. The peaks at 710.03 and 723.34 eV are attributed to Fe 2p3/2 and Fe 2p1/2 of Fe3+, and those at 711.24 and 724.99 eV are assigned to Fe 2p3/2 and Fe 2p1/2 of Fe2+, respectively[33]. The peaks at 719.25 and 732.88 eV are satellite peaks caused by shake-up excitation of high-spin oxidized Fe species. These XPS results further reveal that the ferrite in Fe3O4/A-MPCF exists in the form of Fe3O4.
Figure 3(i) shows magnetic hysteresis loops of the samples. The saturation magnetization (Ms) of MPCF and A-MPCF is near zero, indicating these carbon foams are non-magnetic. Fe3O4/A-MPCF displays a superparamagnetic behavior with the Ms of 45.53 emµ/g. Observing the illustration in Figure 3(i), it can be found that Fe3O4/A-MPCF can be instantly picked up by a magnet. These characterization results confirm that Fe3O4 nanoparticles have been successfully grown on the surface of A-MPCF. This is of great significance for optimizing the impedance matching level.
Figure 4 displays the conductivity of the samples. It can be found that KOH activation has little effect on the conductivity of carbon foam. Compared to MPCF and A-MPCF, the conductivity of Fe3O4/A-MPCF decreases significantly. This is ascribed to the fact that Fe3O4 is a semiconductor material with conductivity less than carbon materials, which results in the lower conductivity of Fe3O4/A-MPCF.
2.2　MWA performance of carbon foams
Figure 5(a)−(c) present the 3D RL curves of three carbon foams in the frequency of 2–18 GHz. Compared to MPCF, the RLmin of A-MPCF decreases from −37.34 to −48.26 dB, indicating that the nano-pores generated by the KOH activation improve the probability of incident electromagnetic waves entering the material’s interior. Moreover, the MWA performance of Fe3O4/A-MPCF is slightly inferior to that of MPCF and A-MPCF.
The reason may be that the introduction of nano-Fe3O4 endows magnetism for Fe3O4/A-MPCF, while reduces its conductivity (Figure 4), leading to the weakened dielectric loss ability. To evaluate the MWA performance of the samples, a 3D column graph containing |RLmin|, EAB, and coating thickness was presented in Figure 5(d). Compared to other similar MWA materials, A-MPCF achieves the RLmin of −48.26 dB and an EAB of 3.91 GHz at a coating thickness of 3.1 mm.
Generally, the MWA performance of materials correlates closely with the relative complex permittivity and permeability[34−35]. The complex permittivity includes the real part (ε') and the imaginary part (ε"), and the complex permeability contains the real part (μ') and the imaginary part (μ"). The real part and imaginary part respectively represent the storage capacity and loss capacity to the electromagnetic wave for the absorbents[36−37]. Materials with higher conductivity and saturation magnetization exhibit enhanced dielectric and magnetic loss capabilities. Figure 6(a) and (b) show the complex permittivity of samples. As frequency increases, both the ε′ and ε′′ of samples exhibit a downward trend. This is commonly associated with the delayed response of dipole polarization to potential changes in the GHz range[38]. All samples have multiple resonance peaks in the frequency range of 2–18 GHz, indicating that the prepared carbon foams display multiple polarization loss behaviors. As shown in Figure 6(c), the variation trend of dielectric loss angle tangent (tan δε) is essentially in line with that of ε′′ for samples. Among them, the tan δε value of Fe3O4/A-MPCF is always lower than that of others, confirming that Fe3O4/A-MPCF has the poor dielectric loss capacity. Figure 6(d) and (e) display the complex permeability for carbon foams. The μ' of MPCF and A-MPCF are close to 1, while μ'' are about 0, indicating that MPCF and A-MPCF are not magnetic. While there are multiple formants for Fe3O4/A-MPCF, this indicates the existence of magnetic resonance loss[39].
In addition, the value of μ′ and μ′′ for Fe3O4/A-MPCF show an increasing trend in the high frequency range. Recent studies have established that the applied electromagnetic field can make the composite electrically polarized and magnetically polarized simultaneously, thus forming electromagnetic coupling and improving the complex permeability of the composite at the high frequency region[40]. The μ′′ value of three samples is negative in several frequency region. This phenomenon may be attributed to the phase lag between the capacitor and the inductor on the porous structure[28]. Furthermore, the value of tan δμ can reflect the magnetic energy dissipation of the material. Figure 6(f) reveals that tan δμ exhibits little difference among materials in the high frequency region. However, the tan δμ of Fe3O4/A-MPCF in the low frequency region is significantly higher than that of the other two materials, meaning that it exhibits the strongest magnetic loss capacity. Finally, the tan δε values for all samples are significantly higher than tan δμ, indicating that dielectric loss is the dominant attenuation mechanism.
For the other hand, it has been widely accepted that the magnetic loss mechanisms in MWA materials mainly consist of hysteresis loss, eddy current loss, domain wall loss, and natural resonance[41]. The hysteresis loss primarily occurs near 100 MHz, rather than in the microwave frequency range. Meanwhile, the domain wall loss occurs only in multi-domain structured materials[42]. Thus, in the GHz frequency range, natural resonance and eddy current loss dominate the loss mechanisms for Fe3O4/A-MPCF. The eddy current loss value (C0) can be described by Equation (12).
${{C}}_{\text{0}}\text={\mu }^{{''}}{\text{(}{\mu }{{'}}\text{)}}^{-\text{2}}{{f}}^{-1} $
If the eddy current loss is the sole attenuation mechanism, the C0 value does not change with the frequency variation[43]. As presented in Figure 7(a), the C0 values of Fe3O4/A-MPCF vary with frequency, indicating that the eddy current loss plays a very limited role. Consequently, the dominant mechanism of magnetic loss in Fe3O4/A-MPCF is natural resonance.
The dielectric loss primarily includes the polarization relaxation and conduction loss. The polarization relaxation primarily consists of the electron, ion, dipole, and interface polarization. However, the electron and ion polarization typically occur in the THz and PHz frequency ranges. Therefore, in the GHz frequency range, the polarization relaxation primarily results from the dipole and interface polarization[44]. Generally, Debye theory is commonly employed to analyze the dielectric relaxation process[45]. Based on Debye theory, the relationship between the ε′ and ε′′ is expressed by Equation (13):
$ {\left({\varepsilon }{{'}}-\frac{{\varepsilon }_{{\mathrm{s}}}+{\varepsilon }_{\text{∞} }}{2}\right)}^{2}+{\left({\varepsilon }{{'}{'}}\right)}^{2}={\left(\frac{{\varepsilon }_{{\mathrm{s}}}-{\varepsilon }_{\text{∞} }}{2}\right)}^{2} $
where εs represents the static permittivity, and ε∞ represents the relative permittivity at high-frequency limit. The curve of ε′ vs. ε" is known as the Cole-Cole curve, where each semicircle indicates the presence of a Debye relaxation process. From Figure 7(b)–(d), multiple semicircles exist in all carbon foams, showing that they exhibit multiple Debye relaxation processes, which means that all samples display multiple polarization loss behaviors. In addition, the Cole-Cole curves of all samples show a straight line at the tail, which suggests that all carbon foams dissipate electromagnetic wave in the form of conduction loss[32].
Figure 8(a)−(c) display the characteristic impedance matching (Zin/Z0) curves of samples, and Figure 8(d) shows the corresponding attenuation constant (α) curves. When the Zin/Z0 value approaches 1, it indicates that most electromagnetic waves are incident inside the material[46]. A higher α value facilitates the dissipation of electromagnetic waves within the material. Fe3O4/A-MPCF has the best impedance matching level, but it has the lowest α value. MPCF shows the highest α value, but its impedance matching level is poor. By contrast, the α value and impedance matching level of A-MPCF are in the middle level.
As shown in Figure 4(d), the RLmin of Fe3O4/A-MPCF (−37.34 dB) and MPCF (−37.84 dB) are very close, while A-MPCF exhibits the optimal RLmin (−48.26 dB). This highlights the importance of balancing impedance matching and attenuation constant to achieve optimal MWA performance.
Based on the above, the differences in the MWA performance of the prepared carbon foams are primarily attributed to the following reasons: After KOH activation, the SBET of A-MPCF significantly increases compared to that of MPCF, rising from 2.24 to 45.96 m2/g. This increase means the generation of abundant nanopores on the pore cell walls of A-MPCF.
When electromagnetic waves are incident on the pore cell walls of A-MPCF, these nanopores increase the number of reflections and diffuse reflections within A-MPCF. This enhances the interaction between electromagnetic waves and A-MPCF, resulting in greater energy absorption and dissipation, thereby improving the MWA performance of A-MPCF. However, the specific surface area results show that the BET value of Fe3O4/A-MPCF obviously decreases from 45.96 to 5.24 m2/g after the introduction of Fe3O4. This indicates that the Fe3O4 nanoparticles have blocked the partial nanopores of the A-MPCF matrix. In addition, the conductivity test results demonstrate that the conductivity of Fe3O4/A-MPCF reduces from 180.35 to 158.75 S/m. These results suggest that the introduction of Fe3O4 nanoparticles endows Fe3O4/A-MPCF with magnetic loss ability, it also decreases the probability of electromagnetic wave reflection and scattering within the Fe3O4/A-MPCF, and weakens its conduction loss ability. Therefore, the impedance matching of Fe3O4/A-MPCF is improved, while its MWA performance is diminished. Consequently, based on the aforementioned discussion and the results of MWA performance test, we conclude that A-MPCF exhibits the optimal MWA performance.
2.3　EMI shielding performance
Figure 9(a) and (b) respectively present the SET curves and the average values of SEA, SER and SET for the samples. All samples exhibit certain EMI shielding performance. Notably, the average SET of A-MPCF and Fe3O4/A-MPCF reaches 103.0 and 96.9 dB, respectively, significantly outperforming most EMI shielding materials reported in previous studies (see Table 2 for comparison). The A-MPCF exhibits the optimal average SET, which can be attributed to its unique three-dimensional hierarchical porous structure. This structure effectively extends the transmission path of electromagnetic waves, thereby enhancing the structural loss ability of A-MPCF. Consequently, the EMI shielding performance of A-MPCF is significantly improved. For Fe3O4/A-MPCF, although its magnetic loss capability for electromagnetic waves is enhanced, its structural and conductive loss capabilities are somewhat reduced, which in turn leads to a slight decrease in its average SET.
T, R and A of carbon foams are depicted in Figure 9(c). The T values of all materials are near zero, suggesting that almost no electromagnetic waves pass through the materials. Although R values of all materials are greater than their A values, which does not prove that the primary loss mechanism for electromagnetic waves is surface reflection. This is because the R value includes power reflected from the foam material’s outer surface, as well as multiple reflections and scattering from its interior[47]. Among them, Fe3O4/A-MPCF has the lowest R value, which mainly attributed to two factors: (1) The introduction of Fe3O4 nanoparticles can optimize the impedance matching level, reducing electromagnetic waves reflected from the surface of A-MPCF; (2) nano Fe3O4 causes the blockage in micropores for A-MPCF, resulting in a reduction of the multiple reflection and scattering of electromagnetic waves within Fe3O4/A-MPCF. Based on the above discussion, it is concluded that the Fe3O4/A-MPCFpossesses the optimal EMI shielding performance.
2.4　Analysis of electromagnetic wave dissipation mechanism
Electromagnetic waves dissipation mechanism for Fe3O4/A-MPCF is shown in Figure 10. It is mainly attributed to the following aspects:
(1) When electromagnetic waves enter the 3D interconnected pore structure, they will be reflected and scattered on the pore cell wall for many times.
(2) Electromagnetic waves react with the electron carrier on the conductive network, converting the electromagnetic waves into heat energy by induced micro-current (i.e., conduction loss).
(3) The abundant defects can serve as the polarization centers, triggers the dipole moment imbalance and dipole polarization, effectively dissipating incident electromagnetic waves under alternating electromagnetic fields.
(4) There are rich heterogeneous interfaces (Fe3O4, MP carbon and hollow carbon fiber) in Fe3O4/A-MPCF. Accumulation of the space charge at the heterogeneous interface boundary creates a capacitor-likely structure, which attenuates the power of the incident electromagnetic waves through the arrangement of charges or polar bonds, resulting in the interface polarization loss.
(5) A large number of nano Fe3O4 can be used as magnetic loss centers to attenuate electromagnetic waves, and the loss mechanism can be attributed to natural resonance (Figure 7(a)).
In brief, the loss mechanism of Fe3O4/A-MPCF to electromagnetic waves is the synergistic effect of the conduction loss, multiple reflection and scattering, dipole polarization, interface polarization, and natural resonance, in which the multiple reflections and scattering as well as the dipole polarization are the main electromagnetic wave loss mechanisms for Fe3O4/A-MPCF.
To objectively evaluate the MWA and EMI shielding performances of prepared carbon foams, the MWA and EMI shielding performances of the carbon foams prepared in this work were compared with those of the reported materials, as shown in Table 2.
3　Conclusions
Less
In conclusion, three kinds of mesophase pitch-based carbon foams were successfully prepared by high-pressure pyrolysis, KOH activation and hydrothermal methods. The KOH activation provides a unique pore structure for the carbon foam, that is, there are 3D connected micron sized pore cells and abundant micropores on the pore walls. This unique pore structure optimizes the impedance matching level, and also enhances the dielectric loss capacity of the activated carbon foam, thereby improving the electromagnetic wave loss capacity. The MWA test results show that A-MPCF exhibits the best MWA performance, with the RLmin of −48.26 dB at a coating thickness of 3.1 mm and an EAB of 3.91 GHz. Subsequently, the introduction of nano Fe3O4 endows the composite carbon foam with magnetic loss capability. Compared to A-MPCF, the MWA performance of Fe3O4/A-MPCF slightly decreases. However, the R value of Fe3O4/A-MPCF notably reduces from 0.90 to 0.74. When the thickness of Fe3O4/A-MPCF is 4.6 mm, its RLmin achieves −37.34 dB with an EAB of 2.21 GHz. The average SET of Fe3O4/A-MPCF with the thickness of 3.0 mm achieves 96.93 dB in X-band, which is superior to the EMI shielding performance of materials reported in the literature. This work proposes a simple strategy for simultaneously adjusting MWA and EMI shielding performances of MWA materials.
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Year 2026 volume 54 Issue 1
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doi: 10.1016/S1872-5813(25)60593-7
  • Receive Date：2025-05-16
  • Online Date：2026-09-23
  • Published：2026-01-01
Article Data
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  • Received：2025-05-16
  • Revised：2025-07-04
  • Accepted：2025-07-16
Affiliations
    1Engineering Research Center of Coal Chemistry for Liaoning Province, Professional Technology Innovation Center of Coal Chemical Industry for Liaoning, School of Chemical Engineering, University of Science and Technology Liaoning, Anshan 114051, China
    2School of Materials Science and Technology, Shenyang Ligong University, Shenyang 110159, China
    3School of Science, Shenyang University of Technology, Shenyang 111003, China

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表12种不同金属材料的力学参数
科
Family		 属数
Number of
genus		 种数
Number of
species		 占总种数比例
Percentage of
total species (%)		 属
Genus		 种数
Number of
species		 占总种数比例
Percentage of total
species (%)
鹅膏菌科Amanitaceae 2 11 5.26 鹅膏菌属 Amanita 10 4.78
小菇科 Mycenaceae 2 12 5.74 丝盖伞属 Inocybe 5 2.39
多孔菌科 Polyporaceae 8 14 6.70 蜡蘑属 Laccaria 5 2.39
红菇科 Russulaceae 3 23 11.00 小皮伞属 Marasmius 6 2.87
小菇属 Mycena 11 5.26
光柄菇属 Pluteus 5 2.39
红菇属 Russula 17 8.13
栓菌属 Trametes 5 2.39
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