Journal of Materials Science & Technology, 2020, 59(0): 164-172 DOI: 10.1016/j.jmst.2020.04.048

Research Article

Controllable synthesis of mesoporous carbon hollow microsphere twined by CNT for enhanced microwave absorption performance

Minghang Li, Xiaomeng Fan,*, Hailong Xu, Fang Ye, Jimei Xue, Xiaoqiang Li, Laifei Cheng

Science and Technology on Thermostructural Composite Materials Laboratory, Northwestern Polytechnical University, Xi’an 710072, China

Corresponding authors: *E-mail address:fanxiaomeng@nwpu.edu.cn(X. Fan).

Received: 2020-02-13   Accepted: 2020-04-20   Online: 2020-12-15

Abstract

The low dielectric loss of mesoporous carbon hollow microsphere (PCHM) requires high filler loading (higher than 20 wt%) when it is used as microwave absorbers. In order to decrease the filler loading of PCHM, a new strategy for synergistic increase of polarization and conductive loss was developed by twining PCHM with carbon nanotube (CNT) according to theoretic calculation. By the optimization of CNT content, the minimum reflection coefficient was -34.6 dB with a filler loading of only 10 wt%, which was much lower than -2.1 dB of PCHM. In addition, the effective absorption bandwidth was 3.6 GHz at X band with a thickness of 2.8 mm. The enhanced microwave absorption performance can be ascribed to the unique combination of hollow PCHM and one-dimensional CNT with higher graphitization degree, leading to increase of conductivity and heterogeneous interfaces. As a result, the conductive loss increased from 0.12 to 2.27 and polarization loss increased from 0.15 to 0.67, achieving the balance between attenuation ability and impedance match.

Keywords: Microwave absorption ; Interfacial polarization ; Carbon ; Polarization loss

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Cite this article

Minghang Li, Xiaomeng Fan, Hailong Xu, Fang Ye, Jimei Xue, Xiaoqiang Li, Laifei Cheng. Controllable synthesis of mesoporous carbon hollow microsphere twined by CNT for enhanced microwave absorption performance. Journal of Materials Science & Technology[J], 2020, 59(0): 164-172 DOI:10.1016/j.jmst.2020.04.048

1. Introduction

In recent years, electromagnetic (EM) wave pollution has been a serious problem [1,2]. To solve this, tremendous efforts are employed on EM absorbers. Among them, carbon-based materials stand out to be promising EM absorbers because of their low density and adjustable dielectric properties [[3], [4], [5], [6]]. Mesoporous carbon hollow sphere (PCHM), as novel carbon material, has drawn a lot of attention in EM wave absorption field due to high surface areas [5]. However, the filler loading of PCHM is still high, usually higher than 20 wt% due to the low EM attenuation ability resulting from the low crystalline degree of PCHM [5,[7], [8], [9]]. To decrease the filler loading, the EM attenuation ability should be enhanced and the impedance match condition should be improved synergistically to ensure good EM absorption ability [10]. Attenuation constant and impedance match condition are calculated by the following equations:

$\alpha =\frac{\sqrt{2}\pi f}{c}\times \sqrt{(\mu ''\varepsilon ''-\mu '\varepsilon '')+\sqrt{{{(\mu ''\varepsilon ''-\mu '\varepsilon ')}^{2}}+{{(\mu '\varepsilon ''+\mu ''\varepsilon ')}^{2}}}}$
${{Z}_{\text{in}}}=\sqrt{\frac{\mu }{\varepsilon }}\text{tanh}\left[ j\frac{2\pi }{c}\sqrt{\mu \varepsilon }fd \right]$
$\varepsilon =\varepsilon '-i\times \varepsilon ''$
$\mu '=\mu '-i\times \mu ''$

where α is the attenuation constant, Zin refers to the normalized input impedance and |Zin-1| is usually used to evaluate the impedance match condition, ε and μ are the relative complex permittivity and permeability, f is the frequency, d is the thickness, and c is the speed of light in vacuum [[11], [12], [13]]. The materials that we pay close attention to are dielectric materials, so the μ is taken as 1. Higher value of α means greater attenuation ability [14], and value of |Zin-1| closer to zero means better impedance match condition [15]. The calculation results are shown in Fig. 1. It is clear that with increase of permittivity, especially the imaginary part of permittivity, attenuation constant increases. As for impedance match condition, it becomes better at first and then worse with increase of permittivity. The impedance at the solid circle can satisfy the requirement of reflection coefficient (RC) less than -10 dB, as shown in Fig. 1(b). The satisfied values of permittivity are around 4-8 for real part and 2-6 for imaginary part.

Fig. 1.

Fig. 1.   The relationship between permittivity and (a) α and (b) |Zin-1| with a thickness of 3.6 mm and 9 GHz.


According to Debye theory, ε' and ε'' can be calculated using the following equations:

$\varepsilon '={{\varepsilon }_{\infty }}+\frac{{{\varepsilon }_{\text{s}}}-{{\varepsilon }_{\infty }}}{1+{{\omega }^{2}}{{\tau }^{2}}}$
$\varepsilon ''={{\varepsilon }_{\text{p}}}''+{{\varepsilon }_{\text{c}}}''=\frac{{{\varepsilon }_{\text{s}}}-{{\varepsilon }_{\infty }}}{1+{{\omega }^{2}}{{\tau }^{2}}}\omega \tau +\frac{\sigma }{\omega {{\varepsilon }_{0}}}$

where ω is the angular frequency, εs is the static permittivity, ε is the relative dielectric permittivity at high-frequency limit, εp'' represents the polarization loss, εc'' represents the conductive loss, σ is the electrical conductivity and τ is the relaxation time [16]. The real part of permittivity is related to polarization loss according to Eq. (5). It can be deduced that the polarization and conductive loss should be enhanced simultaneously to improve the ε' and ε''. To enhance conductive loss, high conductive phases are needed. To enhance polarization loss, the most effective way is to increase interfacial polarization loss by enriching heterogeneous interfaces.

Metals, metal oxides, sulfides and MXenes are often used to increase the interfaces of carbon-based materials, such as C/Fe [17,18], C/ZnO [7,8,19], C/TiO2 [20], C/Fe3O4 [21] and C/MoS2 [22,23], and C/Ti3C2Tx [24], revealing improved EM wave absorption ability [6]. However, the filler loading didn’t decrease due to the introduction of second phases with a higher density than carbon [5,7,8].

To decrease the filler loading, the attenuation ability of PCHM needs to be enhanced. According to the above analysis, polarization loss and conductive loss of PCHM should increase simultaneously. To realize this strategy, here we proposed the incorporation of carbon nanotube (CNT) with PCHM. On one side, CNT, as one-dimensional material, has low density and high specific surface area, which can enrich heterogenous interfaces and enhance interface polarization. On the other side, CNT has a higher graphitization degree than PCHM (amorphous carbon), and thus increase conductive loss [[25], [26], [27]]. It’s expected that the incorporation of CNT with PCHM can lead to a decrease of filler loading due to combination of diverse carbon phases.

In this work, the PCHM@CNT was prepared as EM absorbers through catalyst chemical vapor deposition (CCVD) method. The microstructure evolution and EM wave absorption properties with tuning deposition time were studied, and the EM wave absorption mechanism was revealed.

2. Experimental

2.1. Chemical reagents

Tetraethyl orthosilicate (TEOS), resorcinol and hydrofluoric acid (HF, 25 wt%) were purchased from Aladdin Industrial Corporation. Formaldehyde was purchased from Tianjin Tianli Chemical Reagent Co., Ltd. Nickel nitrate hexahydrate (Ni(NO3)2·6H2O) was purchased from Guangdong Chemical Reagent Engineering Technology Research and Development Center. Ethylene (C2H2) and hydrogen (H2) were purchased from Xi'an Weiguang Gas Co., Ltd. Anhydrous ethanol and concentrated ammonia aqueous solution (NH3·H2O, 25 %) were purchased from Tianjin Fuyu Fine Chemical Co., Ltd. Deionized water was used for all experiments. All chemical reagents were analytical grade and used directly without any purification.

2.2. Preparation of PCHM@CNT

PCHM was synthesized by a modified Stöber method. Typically, 2.88 mL TEOS, 7 mL H2O, 2 mL NH3·H2O and 55 mL ethanol were mixed under stirring at room temperature. Then, 0.4 g resorcinol and 0.56 mL formaldehyde were added to above solution and was kept stirring for 18 h. The precipitation was washed with water and ethanol and collected by centrifugation. PCHM was obtained after carbonization at 650 °C under Ar for 6 h and then the silica was removed by etching with HF.

The as-prepared PCHM was immersed into the Ni(NO3)2 solution following by stirring for 30 min. After drying, the PCHM with Ni(NO3)2 was placed into furnace. Then, the furnace was heated from room temperature to 700 °C under flowing Ar atmosphere with a heating rate of 10 °C/min and a flux of 150 mL/min. After this process, H2 was introduced for 15 min to reduce Ni(NO3)2 to Ni with a flux of 150 mL/min, followed by introduction of C2H2, and then CNT can be formed by the catalysis of Ni particles. The flux of C2H4 was 40 mL/min. The reaction time of C2H2 was chosen as 10, 15, 20 and 25 min, corresponding to samples S1, S2, S3 and S4, respectively. The pure PCHM was treated like other samples only without introduction of C2H2, which was named as sample S0.

2.3. Characterization

The microstructures of PCHM@CNT were investigated by transmission electron microscopy (TEM; FEI Talos F200X, 200 kV, USA) and scanning electron microscopy (SEM, FEI Helios G4 CX, USA). Phase compositions of all samples were obtained by X-ray diffraction (XRD-7000, Shimadzu, Japan) and state of carbon was investigated by Raman spectroscopy (inVia, Renishaw, U.K., He-Ne laser, 532 nm excitation wavelength). The surface status was examined with FT-IR (iN10MX, Nicolet, USA). The pore structures and surface areas were characterized by nitrogen adsorption and desorption isotherms, using a Micromeritics ASAP 2020 system. All samples were degassed under vacuum at 200 °C for 6 h before analysis. Barrett-Joyner-Halenda (BJH) method was used to calculate pore size distribution and Brunauer-Emmett-Teller (BET) method was used to calculate specific surface area. The S-parameters and complex permittivity of PCHM@CNT with a filler loading of 10 wt% mixed with paraffin were measured by vector network analyzer (VNA, MS4644A, Anritsu). The tanδ and attenuation constant are calculated by following equations:

$\text{tan}\delta =\frac{\varepsilon ''}{\varepsilon '}$

RC can be calculated to evaluate the microwave absorption ability of samples [4]:

$\text{RC}=20\text{log}\left| \frac{{{Z}_{\text{in}}}-1}{{{Z}_{\text{in}}}+1} \right|$

Lower RC means stronger EM wave absorption ability. Effective absorption bandwidth (EAB) means the corresponding frequency band of RC lower than -10 dB [28].

3. Results and discussion

3.1. Characterization of PCHM@CNT

PCHM was synthesized by a modified Stöber method. The preparation process of PCHM@CNT is shown in Fig. 2(a). Abundant surface areas of PCHM can provide a lot of adsorption sites for Ni(NO3)2·6H2O. As a result, after vacuum impregnation in Ni(NO3)2·6H2O solution, there were a lot of Ni(NO3)2·6H2O on the surface of PCHM. These Ni(NO3)2·6H2O particles were reduced to Ni on the surface of PCHM, which were catalysts for the growth of CNT under H2 [29,30]. The CNT grew from the adsorption sites of Ni particles, which dispersed and twined randomly on the surface of PCHM under the catalysis of Ni particles. The C2H2 was empleoyed as source of carbon in our work.

Fig. 2.

Fig. 2.   (a) Schematic for the preparation of PCHM@CNT. SEM images of (b) sample S0, (c) S1, (d) S2, (e) S3, (f) S4. (g) XRD patterns of samples S0-S4.


Fig. 2(b)-(f) shows morphologies of as-fabricated samples. As can be seen in Fig. 2(b), the diameter of PCHM was about 300-400 nm with a uniform spherical structure, and the surface of PCHM was smooth. As reaction time increased, the contents of CNT increased and the diameters were enlarged at the same time [29]. The PCHM began showing a rough surface during this process, which means that a lot of interfaces have been generated. Among different PCHMs, CNT jointed each other, which could form a conductive network and generate many pores. When the reaction time was 25 min, Ni particles were deactivated and a lot of amorphous carbon deposited and covered the CNT and PCHM [29,31]. Fig. 2(g) shows the XRD patterns of PCHM@CNT with reaction time ranging from 0 to 25 min. It is worth noting that there are two peaks between 20° and 30°. One is around 21° and the other is around 26°. The peak at 21° represents amorphous carbon with low conductivity, while the peak at 26° represents crystal carbon with high conductivity in the XRD pattern [8,32], corresponding to the existence of PCHM and CNT, respectively [33]. The contents of CNT and PCHM were calculated based on the areas of two peaks. As shown in Fig. S1 and Table 1, the contents of CNT gradually increased with longer reaction time. However, when the reaction time was 25 min, the content of CNT decreased slightly, which can be attributed to the deactivation of catalysts [29], causing the deposition of amorphous carbon.

Table 1   The contents of CNT with increase of reaction time.

SamplesS0S1S2S3S4
Reaction Time (min)010152025
Contents of CNT (wt%)014.237.6290.9861.52

New window| CSV


Detailed morphology information of PCHM@CNT is shown in Fig. 3. Fig. 3(a) shows the hollow structure of PCHM. Fig. 3(b) exhibits the junction of CNT among different PCHMs. The CNT grew along the surface of PCHM and twined the PCHM. Thus, there were a lot of interfaces between them. Some dark dots can be seen on the surface of PCHM, which were Ni particles according to the EDS image in Fig. 3(g). The high magnification image of CNT exhibits a hollow structure and rough surface, which can increase the transmission route and energy loss. The inset image in Fig. 3(c) shows wall structure of CNT. Lattice stripes can be seen in the wall of CNT, meaning defective multiwalled microstructure [34]. It’s clear that the PCHM was amorphous phase and the CNT was crystalline phase in Fig. 3(c). In Fig. 3(f), the oxygen element shows a uniform distribution on PCHM. It means that many oxygen-containing functional groups exist on the surface of PCHM.

Fig. 3.

Fig. 3.   TEM images of (a) PCHM and (b) PCHM@CNT particles. (c) HRTEM image and (d) HAADF image of PCHM@CNT and the corresponding (e-g) EDS results.


To further investigate the state of carbon, Raman spectra and FT-IR spectra were employed. The D and G bands in Raman spectra are related to the graphitization degree of carbon. The intensity ratios of them are given in Fig. 4(a), which are called as ID/IG. It can be seen that with the increase of deposition time, the values of ID/IG gradually increased. This trend was consistent with an increasing content of CNT with high graphitization degree, according to the study of Ferrari (in Fig. S2, during stage 2, when the amount of nano-crystal graphite increases, the ID/IG ratio increases) [35]. As a result, the attenuation ability was enhanced correspondingly. However, when the deposition time was 25 min, the deposition of CNT was limited, and some amorphous carbon was deposited. So, the value of ID/IG slightly decreased. The attenuation ability was weakened in consequence.

Fig. 4.

Fig. 4.   The (a) Raman and (b) FT-IR spectra of PCHM@CNT of all five samples.


The changes of chemical bonds in all samples are shown in Fig. 4(b). The peaks at 3422, 1618, 1387, 1202 and 1120 cm-1 represent the vibration of O-H, C=C, N-O, C-O and C-C, respectively [6,36]. Before CVD, the peak of N-O was obvious because of the presence of Ni(NO3)2·6H2O. The width of O-H peak was also wider than other samples, meaning more O-H bonds. This phenomenon can be attributed to the crystal water in Ni(NO3)2·6H2O and the defects in PCHM [37]. After CVD, the Ni(NO3)2·6H2O can be reduced to Ni with H2 at high temperature. The width of O-H peak became smaller than sample S0 and the N-O peak disappeared in sample S1. With longer deposition time, the intensities of both C-O and O-H peaks increased, which means the generation of oxygen-containing functional groups. This is consistent with EDS results. These groups can be dipoles, which can enhance the polarization loss ability of absorbers and optimize the impedance match condition [38].

The BET and BJH analyses were used to demonstrate the pore structure of PCHM@CNT according to the N2 adsorption-desorption isotherm. Sample S2 was used as representative of PCHM@CNT. As can be seen from Fig. 5, sample S2 showed an IV-type isotherm, which was the symbol of mesoporous structure and similar to PCHM (Fig. 5). The specific surface area of sample S2 was 341.1 cm2 g-1, which was smaller than that of PCHM (501.5 cm2 g-1). The decrease of specific surface areas can be attributed to the CNT on the surface of PCHM, indicating the increase of heterogenous interfaces between PCHM and CNT (detailed explanation can be seen in Supplementary Information). The BJH pore size distributions of samples S0 and S2 are shown in Fig. 5(b) and (d). Apart from pores with a size smaller than 10 nm, there were larger pores generated by junctions of CNT. As a result, the pore volume of sample S2 (1.09 cm3 g-1) was higher than sample S0 (0.73 cm3 g-1). These results mean that there were more heterogeneous interfaces and pores for PCHM incorporating CNT, which can greatly enhance the EM wave attenuation ability [[39], [40], [41], [42]].

Fig. 5.

Fig. 5.   N2 adsorption-desorption isotherm (a, c) and pore size distribution (b, d) of PCHM@CNT (sample S2) and PCHM (sample S0).


It is manifest from Fig. S4 that the real and imaginary parts of permeability of samples S0-S4 were close to 1 and 0, which means that they are non-magnetic samples and the magnetic loss is negligible, and thus the effect of Ni particles can be ignored in this work. Only the dielectric loss is considered in this work. As shown in Fig. 6, when the deposition time was shorter than 15 min (samples S0 - S2), both real and imaginary parts of permittivity increased with longer deposition time, resulting from increase of the CNT with high conductivity according to Fig. S1. However, the ε′′ values of samples S3 and S4 decreased with increasing of deposition time. Generally, the state of CNT was different with the increase deposition time. Longer deposition can lead to the deactivation of catalyst, causing more defects in CNT. This is harmful to electrical conductivity. Although the CNT content of sample S3 was higher than sample S2, the graphitization of CNT in sample S3 was lower than sample S2. So, it can be deduced that the graphitization degree plays the decisive role to affect permittivity, leading to the higher imaginary part of sample S2 than sample S3, as shown in Fig. 6. Besides, the deactivation of catalyst Ni can lead to the formation of amorphous carbon and cause the reduction of permittivity [29]. According to the analysis in Fig. 1 and Fig. S5(f), the permittivities of samples S2 and S3 can meet the requirement for both high attenuation ability and great impedance match. Fig. 6(c) and (d) show the attenuation ability of all five samples through tanδ and attenuation coefficient α. It is obvious that samples S2 and S3 possess higher attenuation ability, which is in consistent with results shown in Fig. 1 and Fig. S5(f).

Fig. 6.

Fig. 6.   Electromagnetic parameters of all the samples: (a) ε′ value, (b) ε′′ value, (c) tanδ and (d) attenuation constant.


The calculated 3D plots of RC are shown in Fig. 7. The minimum RC of pristine PCHM was only -2.1 dB at 12.4 GHz with a thickness of 3.6 mm. With the incorporation of CNT, the minimum RC decreased to -6.9 dB, revealing the enhanced absorption ability. It can be found that the minimum RC of sample S2 decreased to -34.6 dB at thickness of 3.2 mm with EAB of 3.6 GHz, which was optimal in all samples. When the deposition time was longer than 15 min, the Ni catalysts were deactivated, causing the deposition of amorphous carbon, so the loss ability became weaker and the RC became higher. It also can be noted that the real parts of samples S2 and S3 were equal but the EM absorption ability of sample S2 was stronger than sample S3. According to Fig. 1(a), higher imaginary part of permittivity has more contribution to attenuation ability. Their impedance match conditions are also different according to Fig. S5(f). The |Zin-1| of all the samples are calculated according to Eq.(2). As shown in Fig. 7(f), sample S2 shows the best impedance match condition.

Fig. 7.

Fig. 7.   The 3D plots of RC values of samples with different deposition time: (a) S0, (b) S1, (c) S2, (d) S3, (e) S4 and (f) |Zin-1| of five samples.


The typical EM wave absorption properties of carbon-based EM wave absorbers at X band are summarized [8,16,18,34,[43], [44], [45], [46], [47], [48]]. To evaluate the absorption properties including filler content, the specific value of RCmin divided by filler contents used as one index. In consideration of the practical application for microwave absorbing agents, it is better to realize efficient absorption with lower filler loading. As shown in Fig. 8, sample S2 shows the best EM wave absorption properties in all five samples. Apart from that, PCHM@CNT shows competitive absorption properties compared with other carbon-based absorbers such as carbon/polymer [45,48] and carbon/metal oxides [8,44].

Fig. 8.

Fig. 8.   The typical properties of carbon-based EM wave absorbers.


The polarization loss and conductive loss of all samples were calculated according to the Eqs. (5) and (6). The least-square method was used to get reliable results [6]. The state of CNT was different with the increase of deposition time, and thus affected the polarization and conductive loss. As can be seen in Fig. 9(b), the polarization loss increased as the deposition time increasing, which was consistent with the increase of CNT content (Fig. S1). According to the analysis of N2 adsorption-desorption isotherm in Fig. 5, it can be concluded that more CNT contents would generate more heterogeneous interfaces, enhancing interfacial polarization loss. The defects in CNT and PCHM also can lead to the increase of dipole polarization loss according to the FT-IR spectra. Due to the higher graphitization degree of CNT, the whole graphitization degree of the absorbers increases, according to the Raman spectra in Fig. 4(a). This is responsible for the increase of conductive loss. Based on the synergistic increase of conductive loss and polarization loss, sample S2 showed the highest attenuation ability with the best impedance match condition. However, the deactivation of Ni particles and more defects in CNT (see in Fig. 4(b)) could cause a decrease of conductivity for CNT [44]. As a result, although the CNT content of sample S3 was higher than sample S2, the conductive loss was lower than sample S2 (the electrical conductivities of five samples are shown in Fig.9(a)). Thus, the microwave absorption ability of sample S3 was worse than sample S2. All in all, sample S2 showed the best microwave absorption properties due to the synergistic increase of polarization loss and conductive loss.

Fig. 9.

Fig. 9.   (a) Electrical conductivity of five samples and (b) polarization loss and conductive loss with deposition time.


Based on the above analysis, the main EM wave absorption mechanisms of PCHM@CNT are shown in Fig. 10. The conductive network can greatly increase conductive loss, which can turn EM energy into heat. Multiple scattering in the hollow and mesoporous structure of PCHM can also cause EM wave dissipation and increase propagation path [49]. The interfaces between CNT and PCHM also play an important role in the increase of polarization loss. The in-situ formed CNT on the surface of PCHM can greatly increase interfacial polarization. The balance between polarization and conductive loss can be realized through controlling the contents of CNT, and thus can get excellent EM absorbers. And the EM absorption properties are consistent with the theoretical predictions.

Fig. 10.

Fig. 10.   Schematic of EM wave absorption.


4. Conclusion

In summary, the pure carbon EM absorbers with PCHM and CNT were synthesized through CCVD method. The in-situ formed CNT led to the construction of large amounts of interfaces, and the high conductivity of CNT can strengthen the conductive loss of PCHM. In this way, the PCHM@CNT EM absorbers showed enhanced EM absorption properties with synergistic increase of polarization and conductive loss with low filler loading. The RCmin was -34.6 dB when the filler loading was only 10 wt%. The EAB was 3.6 GHz with a thickness of 2.8 mm. This work provides a new thought to enhance the EM absorption properties of carbon materials.

Declaration of Competing Interest

The authors declare no competing financial interest.

Acknowledgements

This work was financially supported by the National Science Fund for Distinguished Young Scholars (No. 51725205), the National Natural Science Foundation of China (No.51821091) and the Fundamental Research Funds for the Central Universities (No. 3102019TS0410). We would like to thank the Analytical & Testing Center of Northwestern Polytechnical University for the SEM and TEM images. The authors would like to give their special thanks to Prof. Xiaowei Yin for his kind guidance and help on the research work.

Appendix A. Supplementary data

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jmst.2020.04.048.

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The preparation of graphene aerogel by hydrothermal or chemical reduction has been one of the hot topics of research. But in the process of assembly, the random weak connection of GO flakes leads to irreversible deformation under compression, and the mechanical stability of aerogel based on graphene is one of its drawbacks that is hard to overcome. Here, a novel method to prepare graphene aerogel with high mechanical stability was proposed via combining surface support brought by metallic-CNT networks and interfacial cross-linking of GO sheets achieved by nanoparticle selective absorption. Thoroughly dispersed metallic-CNTs absorbed on the basal plane of GO flakes formed continuous network structures, which not only improve the mechanical performance of flakes but also provide steric effects to impel the adsorption of metallic oxide magnetic nanoparticles concentrated on the edge of GO flakes, thereby guaranteeing the interfacial connection of adjacent rGO flakes by nanoparticle cross-linking. Meanwhile, the surface and interface reinforce approach can greatly improve the electrical conductivity and mechanical stability of composites. Owing to the light weight, abundant interface, high electrical conductivity, combined with the superparamagnetic properties brought by the magnetic nanoparticles, composite aerogel with high mechanical stability and excellent microwave absorption was achieved, of which the effective absorption bandwidth of the aerogel is 4.4-18 GHz and the maximum value can reach -49 dB. This approach could not only be used to prepare microwave absorption materials with light weight and high performance but also be meaningful to enlarge the construction and application of carbon-based materials.

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We report herein the synthesis of a novel nest structured electromagnetic composite through in-situ chemical polymerization of 3-methyl thiophene (3MT) in the presence of the BaFe11.92(LaNd)0.04O19-TiO2 (BFTO) nanoparticles and MCNTs. As an absorbing material, the BFTO/MCNTs/P3MT/wax composites were prepared at various loadings of BFTO/MCNTs/P3MT (0.2:0.10:1.0 ~ 0.2:0.30:1.0), and they exhibited strong microwave absorption properties in the range of 1.0-18 GHz. When the loading of BFTO/MCNTs/P3MT is 0.2:0.30:1.0, the composite has a strongest absorbing peak at 11.04 GHz, and achieves a maximum absorbing value of -21.56 dB. The absorbing peak position moves to higher frequencies with the increase of MCNTs content. The mechanism for microwave absorption of these composites has been explained in detail.

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In order to clearly understand the intrinsic microwave absorption properties of carbon nanomaterials, we proposed an efficient strategy to synthesize high purity metal-free carbon nanotubes (CNTs) over water-soluble K2CO3 particles through chemical vapor decomposition and water-washing process. The comparison results indicated the leftover catalyst caused negative effects in intrinsic microwave absorption properties of CNTs, while an enhanced microwave absorption performance could be observed over the metal-free CNT sample. Moreover, the results indicated that the microwave absorption properties could be tuned by the CNT content. Therefore, we provided a simple route to investigate the intrinsic properties of CNTs and a possible enhanced microwave absorbing mechanism.

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Light-weight nanocomposites filled with carbon nanotubes (CNTs) are developed for their significant potentials in electromagnetic shielding and attenuation for wide applications in electronics, communication devices, and specific parts in aircrafts and vehicles. Specifically, the introduction of a second phase into/onto CNTs for achieving CNT-based heterostructures has been widely pursued due to the enhancement in either dielectric loss or magnetic loss. In this work, ferroferric oxide (Fe(3)O(4)) was selected as the phase in multiwalled carbon nanotube (MWCNT)-based composites for enhancing magnetic properties to obtain improved electromagnetic attenuation. A direct comparison between the two-phase heterostructures (Fe(3)O(4)/MWCNTs) and polyaniline (PANI) coated Fe(3)O(4)/MWCNTs, namely, three-phase heterostructures (PANI/Fe(3)O(4)/MWCNTs), was made to investigate the interface influences of Fe(3)O(4) and PANI on the complex permittivity and permeability separately. Compared to PANI/Fe(3)O(4)/MWCNTs, Fe(3)O(4)/MWCNTs exhibited enhanced magnetic properties coupled with increased dielectric properties. Interfaces between MWCNTs and heterostructures were found to play a role in the corresponding properties. The evaluation of microwave absorption of their wax composites was carried out, and the comparison between Fe(3)O(4)/MWCNTs and PANI/Fe(3)O(4)/MWCNTs with respect to highly efficient microwave absorption and effective absorption bandwidth was discussed.

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