Enhanced microwave absorbing properties of PVP@multi-walled carbon nanotubes/graphene three-dimensional hybrids

Abstract

An absorber hybrid was fabricated by the incorporation of PVP treated multi-walled carbon nanotubes (PVP@MWNTs) and graphene nanoplatelets (GNPs) using an ultrasonication filtration method. The microwave absorbing properties of PVP@MWNTs/GNPs hybrids were investigated in the frequency range of 8.2–12.4 GHz. The structure and morphology of the PVP@MWNTs/GNPs hybrids was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectroscopy. The SEM and TEM results showed that GNPs were covered by PVP@MWNTs; the embedding of PVP@MWNTs into GNPs layers endows PVP@MWNTs/GNPs hybrids with optimum dispersion, which is helpful to the significant improvement in electron transfer effectiveness. The PVP@MWNTs/GNPs hybrids reveal excellent microwave absorbing properties. The maximum reflection loss value (RL) of PVP@MWNTs/GNPs is −26.5 dB at 11.29 GHz with a thickness of 2 mm, and the effective absorption (<−10 dB) bandwidth reaches 1.6 GHz. However, the RL of pristine MWNTs is about −5 dB at 12 GHz, and GNPs is −4.43 dB at 12.23 GHz. The results indicate that the combination of PVP@MWNTs and GNPs have a synergetic effect on the improved microwave absorbing properties.

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1. Introduction

With the rapid development of modern science and technology, serious electromagnetic (EM) interference problems have stimulated intensive research in EM wave absorption materials in recent years,^1,2 especially attracting much more attention in military and civil fields.^1,3,4 The electromagnetic absorbing materials can be used to minimize the EM reflection from metal plates such as aircrafts, ships, tanks, and the walls of echoic chambers and electronic equipment. The strong absorbing capability of EM is mainly based on the attenuation mechanism of either dielectric loss or magnetic loss.^5,6

Carbon nanotubes and graphene, which are dielectric loss absorbers, are some of the most intensively explored carbon allotropes in materials science because they exhibit unique properties, such as higher specific surface area, lower density and higher conductivity.^7–12 Nowadays, the enhancement of polymer composites properties with graphene–CNTs mixed fillers has attracted considerable attention because of its unique 3-D nanostructure and extraordinary properties.¹³ The combination of one-dimensional (1-D) CNTs and two-dimensional (2-D) graphene would endow graphene–CNTs mixed fillers with additional performance. The remarkable synergistic effect between graphene and CNTs in improving the mechanical properties and thermal conductivity of filled polymer composites has been investigated. For example, Yang et al.¹⁴ carefully demonstrated the synergistic effect between multi-graphene platelets (MGPs) and chemically functionalized multi-walled carbon nanotubes (GD400-MWCNTs) in improving the mechanical properties and thermal conductivity of epoxy composites. Zhang and coworkers¹⁵ investigated the synergistic effects of functionalized graphene and functionalized MWNTs on the electrical and mechanical properties of poly(ether sulfone) composites. The results showed that a considerably higher tensile strength, a better tensile modulus and higher conductivity were obtained by the incorporation of CNTs and GNPs. The enhanced thermal conductivity of epoxy composites filled with graphite nanoplatelet–SWCNT fillers was reported by Yu et al.¹⁶

Based on the theory of hybrid structure,¹⁷ this novel kind of PVP@MWNTs/GNPs hybrids materials can combine the advantages of carbon nanotubes and graphenes, which would make this unique hybrids structure possess the potential application in a wide field. As we know, the synergistic effect on the mechanical properties, thermal conductivity and the electrical conductivity has been studied.^1,18–20 The aim of this paper is to investigate the synergistic effect of MWNTs and GNPs on the enhancement of the microwave absorption properties. How would the synergy between the two mixed fillers affect the microwave absorbing properties while combine MWNTs and GNPs? On the one hand, the combination of MWNTs and GNPs can increase the multiple interfacial polarizations, which plays an important role in the regulation of dielectric parameter and the enhancement of microwave absorbing properties. On the other hand, the combination of MWNTs and GNPs can effectively impede the stacking of graphene sheets through enlarging the space between graphene sheets and decrease the agglomeration of graphene, which is favorable to the formation of more conductive paths. Moreover, MWNTs can also link the narrow gaps between graphene sheets and bridge the remote GNPs to form multiple reflection interface, which is beneficial to the enhancement microwave absorbing properties of the MWNTs/GNPs hybrids. In this paper, a convenient route of ultrasound filtration method was used to fabricate thin hybrid films of MWNTs and graphene, in which MWNTs density can be manipulated properly using GNPs via vacuum filtration method. A schematic illustration of the combination of PVP@MWNTs and GNPs is shown in Fig. 1.

Fig. 1 Combination mechanism of PVP@MWNTs with GNPs.

2. Experimental

2.1. Materials

Multi-walled carbon nanotubes (MWNTs, 30–50 nm diameter, 10–20 μm long), with a purity of 95%, were provided by Chengdu Organic Chemicals Co., Ltd, Chinese Academy of Sciences. Graphene nanoplatelets (GNPs, KNG-180) were purchased from Xiamen Knano Graphene Technology Corporation Limited. Polyvinylpyrrolidone (PVP) K13-18 was obtained from Aladdin Chemistry Co. Ltd, Fengxian District. Other chemical reagents were all of analytical grade and used without further purification.

2.2. Fabrication of PVP@MWNTs/GNPs hybrids

0.3 g graphene nanoplatelets without purification was dispersed in 50 mL deionized water for 30 minutes by sonication to form a dispersion of GNPs/H₂O solution. The 0.3 g pristine MWNTs were non-covalently modified with 0.1 g PVP polymer using the method in ref. 2, which the content of PVP was 8.0 wt%. The PVP polymer treated MWNTs (0.1 g) were added into the above GNPs solution and sonicated for 4 hours. The resulting mixtures were treated by vacuum filtration through a 0.2 μm membrane filter and dried under vacuum at 40 °C for 16 h.

2.3. Characterization

The morphologies of the samples were observed by scanning electron microscopy (SEM, S-4700, Hitachi, 15 kV) and transmission electron microscope (TEM, H-600, resolution is 0.204 nm). X-ray diffraction patterns were obtained using an X-ray diffractometer (X'Pert Pro, PANalytical) with Cu Kα as X-ray radiation source to identify the phase structure. Raman spectroscopy (514 nm, Ar⁺ ion laser) was used to study the doping defect structure. The Raman spectra were obtained using a laser confocal Raman spectrometer (Horiba Jobin Yvon, HR 800) in the range from 1000 to 3500 cm⁻¹. The electrical conductivities of the hybrids were measured using a SZ-82 digital four probes resistance tester (Suzhou Electronic Equipment Factory, China). The complex permittivity of the samples was measured by a network analyzer (Agilent technologies E8362B: 10 MHz to 20 GHz). The complex permittivity was extracted from the scattering parameter measurements according to the ASTM 5568 standard using waveguide method.⁴ The samples were prepared by mixing the PVP@MWNTs/GNPs hybrids in paraffin with a ratio of 1 : 9. Then press the mixture into rectangular model with the dimensions of 22.86 mm in length, 10.16 mm in width and 2 mm in thickness, and set of filled the rectangular model for tests in X (8.2–12.4 GHz) bands. Three different measurements were performed and data were averaged, in order to reduce uncertainty.

As a single-layer absorber, reflection loss (RL) of the sample is determined from the measured relative complex permeability and permittivity according to the following formulas:²¹

RL (dB) = 20 log₁₀|(Z_in − 1)/(Z_in + 1)| (1)

where Z_in is the normalized input impedance at the free space and material interface, ε_r (ε_r = ε′ − jε′′, where ε′ is the real part, ε′′ the imaginary part) is the complex permittivity, μ_r (μ_r = μ′ − jμ′′, where μ′ is the real part, μ′′ the imaginary part) is the complex permeability of absorber,²² f is the frequency of EM wave in free space, d is the thickness of the absorber, and c is the velocity of light in free space. Considering that there were no magnetic materials added into the absorber, moreover, the magnetic loss is so small that μ′ would be taken as 1 and μ′′ is taken as 0.

3. Results and discussions

3.1. Morphology and microstructure

Fig. 2(a)–(c) show the morphology of the GNPs, MWNTs and PVP@MWNTs/GNPs hybrids, respectively. The image in Fig. 2(a) shows that the GNPs are flaky structure, they overlap with each other and like folds of tissue slice. However, the pristine graphene sheets always have the pronounced tendency to form agglomeration (stacks of graphene sheets). This is because graphene nanoplatelets can be easily attracted to each other due to their very high specific surface area and high surface energy. The image of Fig. 2(b) reveals that MWNTs are long and stringy. Fig. 2(c) is the morphology of PVP@MWNTs/GNPs hybrids. Both the laminated structure of GNPs and line structure MWNTs could be clearly found. It also can be found that MWNTs were attached on GNPs, which will facilitate the dispersion of GNPs because the long and tortuous MWNTs can construct hierarchical MWNTs/multiple graphene layer architecture and inhibit the stacking of multiple graphene layers. Fig. 2(c) illustrates the formation of an interconnected hybrids network between GNPs and MWNTs. The nanotubes form junctions among the nanoplatelets by connecting two or more together.

Fig. 2 SEM images of (a) GNPs; (b) MWNTs; (c) PVP@MWNTs/GNPs hybrids.

Fig. 3 is the TEM images of GNPs, MWNTs and PVP@MWNTs/GNPs hybrids. As shown in the Fig. 3(a), the pristine GNPs tend to aggregate due to van der Waals forces and strong π–π interactions between the GNPs sheets.²³ When the tubular shape MWNTs are incorporated into laminar GNPs, long MWNTs (Fig. 3(b)) penetrate into the interlayers of GNPs and construct a hierarchical MWNTs/GNPs layer architecture to inhibit the stacking of multiple graphene layer, which is favorable to the formation of more conductive paths. Fig. 3(c) clearly reflects that MWNTs formed multiple junctions among themselves and graphene nanoplatelets. On the other hand, the MWNTs attached on the surface and the edge of GNPs can bridge the broad gaps between graphite sheets, resulting in a remarkable improvement of the conductivity. Moreover, the one-dimensional MWNTs might have bridged adjacent two-dimensional sheet of GNPs and provided additional channels for electron transport. The three-dimensional structure of PVP@MWNTs/GNPs hybrids is more important for the formation of electron transport path and the enhancement of microwave absorbing.

Fig. 3 TEM images of (a) GNPs; (b) MWNTs; (c) PVP@MWNTs/GNPs hybrids.

Fig. 4 shows the XRD patterns of GNPs, MWNTs and PVP@MWNTs/GNPs hybrids. The peaks corresponding to GNPs (Fig. 4(a)) and MWNTs (Fig. 4(b)) are present at 2θ = 26.4° and 2θ = 54.5°. The diffraction peak at 2θ = 26.4 are assigned to (002) plane, and the diffraction peak at 2θ = 54.5° are assigned to (004) plane. It suggests that the crystal structures of MWNTs and GNPs are complementary to each other, which is helpful to the absorbing of electromagnetic wave. However, the intensity of the peak at 26.4° for PVP@MWNTs/GNPs hybrids (Fig. 4(c)) is lower than that of the pristine GNPs, which could be possibly attributed to low weight fraction of GNPs in the PVP@MWNTs/GNPs hybrids. The results also confirm the existence of graphite layers of GNPs in PVP@MWNTs/GNPs hybrids. From Sherrer's analysis of X-ray diffraction patterns, the 17.8 nm thickness of GNPs can be estimated, which corresponds to about 53 layers. For the PVP@MWNTs/GNPs hybrids, 12.9 nm in thickness, about 38 layers, have been estimated. This means that the stacked layers of the graphene sheets decreased slightly due to the presence of the MWNTs. This is because MWNTs were attached on the surface of the GNPs and filled in the gaps between the GNPs sheets, and then prevented the aggregation of GNPs, resulting in the decrease the thickness of GNPs.²⁴ Consequently, the agglomeration of the MWNTs/GNPs hybrids is much looser than that of the GNPs alone, meaning that MWNTs could also effectively improve the dispersion of the GNPs. Hence the XRD results also verify the combined effect of dual filler for the better dispersion of PVP@MWNTs/GNPs hybrids.

Fig. 4 XRD patterns of (a) GNPs; (b) MWNTs; (c) PVP@MWNTs/GNPs.

Fig. 5 is the Raman spectrum of GNPs, MWNTs and PVP@MWNTs/GNPs samples. Raman spectroscopy is a quick and non-destructive technique to analyze carbon sample. It can be seen in Fig. 5 that two characteristic peaks, corresponding to the D-band and G-band, were observed at approximately 1350 cm⁻¹ and 1580 cm⁻¹, respectively. The D peak is assigned to the vibrations of sp³ carbon atoms of disordered graphite and is a breathing mode or k-point photons of A_1g symmetry, whereas the G peak corresponds to the first-order scattering of the E_2g mode.^25,26 The intensity ratio of the D and G band (I_D/I_G) is a measure of disorder degree and average size of the sp² domains in the graphene sp² network.²⁷ The ratio value I_D/I_G of pristine MWNTs (Fig. 5(a)), GNPs (Fig. 5(b)) and PVP@MWNTs/GNPs hybrids (Fig. 5(c)) are 0.74, 0.33 and 0.63, respectively. The 2D band at around 2700 cm⁻¹ is a major fingerprint of graphene. The ratio of G band and 2D band intensity I_G/I_2D is related to the graphene layers; the smaller the ratio, the fewer the graphene layers. The ratio I_G/I_2D value of pristine MWNTs, GNPs and PVP@MWNTs/GNPs hybrids are 1.29, 2.94 and 1.24, respectively. This result demonstrates that the combination of GNPs with MWNTs still keep a good crystallinity and purity.

Fig. 5 Raman spectra of (a) GNPs; (b) MWNTs; (c) PVP@MWNTs/GNPs hybrids.

3.2. Dielectric properties

The electrical conductivities of MWNTs, GNPs and PVP@MWNTs/GNPs hybrids were measured using a SZ-82 digital four probes resistance tester. The samples were prepared by uniformly mixing the MWNTs, GNPs and PVP@MWNTs/GNPs hybrids in a paraffin matrix with a ratio of 1 : 9, respectively. Results are shown in Fig. 6. The conductivity of MWNTs hybrids is 5.12 × 10⁻⁶ S cm⁻¹, while the conductivity of GNPs hybrids is 7.84 × 10⁻⁶ S cm⁻¹. However, the incorporating of MWNTs and GNPs makes the conductivity values of PVP@MWNTs/GNPs hybrids slight higher than of pristine MWNTs and GNPs. This is attribute to the construction of three-dimensional structure of PVP@MWNTs/GNPs hybrids, which is favorable to the formation of more conductive paths and is more important for the enhancement of microwave absorbing.

Fig. 6 The DC conductivity of the PVP@MWNTs/GNPs hybrids as a function of the weight ratio of PVP@MWNTs/GNPs in paraffin at a mixed filler loading of 10 wt%.

According to the transmission line theory,²⁸ when an electro-magnetic wave transmits through a medium, its reflection is affected by many factors, such as permittivity, permeability, sample thickness, specific surface area, and the frequency of the electromagnetic wave.³ The permittivity was measured using a waveguide method in this paper. Fig. 6 is the frequency dependence of real and imaginary part of relative complex permittivity of GNPs, MWNTs and PVP@MWNTs/GNPs hybrids. For the sample of GNPs (Fig. 7(a and a′)), the real and imaginary permittivity values show slight variation, the ε′ is from 8.05 to 7.33, and the ε′′ is from 1.98 to 1.27. For MWNTs (Fig. 7(b)), the real part of complex permittivity (ε′) declines from 9.64 to 8.03 over the frequency of 8.2–10.5 GHz, and then increases to 8.6 in the frequency of 10.5–12.4 GHz. The imaginary part of complex permittivity (ε′′) (Fig. 7(b′)) increases from 0.83 to 2.53 in the frequency range of 8.2–10.24 GHz, and then declines to 1.25 over 10.24–12.4 GHz. However, for the sample of PVP@MWNTs/GNPs hybrids, the real and imaginary permittivity values show a complex variation. The real permittivity (Fig. 7(c)) value decreases from 13.7 to 10.86, and two broad peaks occur in the frequency of 8.2–9.8 GHz and 9.8–11.5 GHz. The imaginary permittivity (Fig. 7(c′)) value increases from 1.75 to 4.8, and the curve exhibits two peaks in 9.0–10.0 GHz and 10.8–11.6 GHz. The significant fluctuations revealed in the range of 8.2–12.4 GHz is ascribed to the strong interfacial polarization occurred in the interfaces of the PVP@MWNTs/GNPs hybrids.

Fig. 7 Frequency dependence of real and imaginary part of relative complex permittivity of (a and a′) GNPs; (b and b′) MWNTs; (c and c′) PVP@MWNTs/GNPs hybrids.

The dielectric loss tangent (tan δ_ε = ε′′/ε′) of GNPs, MWNTs and PVP@MWNTs/GNPs hybrids is shown in Fig. 8, which is calculated based on the real and imaginary permittivity. The tangent loss can be used to evaluate the performance of EM wave absorption when the permittivity meets the impedance matching requirements.¹² The dielectric loss tangent of GNPs (Fig. 8(a)) and MWNTs (Fig. 8(b)) show less variation, exhibit a peak in 9.6–11.1 GHz and 9.0–10.9 GHz, respectively. For the sample of PVP@MWNTs/GNPs hybrids (Fig. 8(c)), the values of tangent loss exhibit nonlinear behavior which is larger than that of GNPs and MWNTs. It reveals two broad peaks in 9.0–12.4 GHz, which is attributed to interfacial polarization. The peaks values are 0.41 and 0.33 at the frequency of 9.69 GHz and 11.33 GHz, respectively. Of interesting, the fluctuation peaks position of dielectric loss tangent corresponds to the fluctuations position of imaginary permittivity, which indicated that the dielectric loss mainly come from the interfacial polarization. There are many factors making contributions to the dielectric properties: dielectric relaxation, resonance, the motion of conduction electrons, defects in the nanotubes, the length and diameters of MWNTs, etc. In this paper, the combination of graphene sheets and the PVP@MWNTs makes the heterogeneous system possess more complicated interfaces.²⁹ The multipole interfacial polarization loss is favorable for improving the microwave absorption properties. The general permittivity behaviors of dielectric materials can be described by the Cole–Cole model.¹⁹

Fig. 8 The dielectric loss of (a) GNPs; (b) MWNTs; (c) PVP@MWNTs/GNPs hybrids.

Firstly, the interfacial polarization (called as the Maxwell–Wagner effect) and the associated relaxation will generate because of the interface formation between graphene sheets and MWNTs. The interfacial polarization and proper dielectric loss are favorable for improving the microwave absorption properties. Secondly, the synergistic effects of graphene sheets and MWNTs improve the impedance matching. Thus, it is reasonable to expect that the PVP@MWNTs/GNPs hybrids should have excellent microwave absorption properties. Thirdly, Debye dipolar relaxation is an important mechanism for dielectric loss material to absorb EM waves. On the basis of Debye theory for dielectric loss behavior, is known as the following equation:³⁰

ε_r = ε_∞ + (ε_s − ε_∞)/(1 + j2πfτ) (3)

where f, ε_s, ε_∞ and τ are the frequency, static permittivity, relative dielectric permittivity at the high-frequency limit and polarization relaxation time. ε′ and ε′′ can be described as:

ε′ = ε_∞ + (ε_s − ε_∞)/(1 + (2πf)²τ²) (4)

ε′′ = 2πfτ(ε_s − ε_∞)/(1 + (2πf)²τ²) (5)

According to the eqn (4) and (5), the relationship between ε′ and ε′′ can be expressed as follow:

(ε′ − (ε_s + ε_∞)/2)² + (ε′′)² = ((ε_s − ε_∞)/2)² (6)

The plot of ε′ versus ε′′ would be a single semicircle, which can be denoted as the Cole–Cole semicircle. Each semicircle corresponds to one Debye relaxation.¹ It is common knowledge that relaxation is usually caused by a delay in polarization with respect to changing electrical field in a dielectric medium. Fig. 9 shows the ε′–ε′′ curves of three samples of GNPs, MWNTs and PVP@MWNTs/GNPs hybrids. The ε′–ε′′ plot of the hybrids exhibits a succession of semicircles corresponding to the Debye relaxation process, which can be ascribed to relaxation phenomena due to the contribution of heterogeneous interface polarization. For the samples of GNPs (Fig. 9(a)) and MWNTs (Fig. 9(b)), only one conspicuous Cole–Cole semicircle was found in the ε′–ε′′ curves, which suggests that there is sole relaxation process for GNPs and MWNTs. For the sample of PVP@MWNTs/GNPs hybrids (Fig. 9(c)), two obvious semicircles were found. The presence of two semicircles suggests that there are two dielectric relaxation processes, and each semicircle corresponds to a Debye dipolar relaxation, representing the contribution of Debye relaxation to the enhanced dielectric properties of the PVP@MWNTs/GNPs hybrids. In the hybrids, the existence of interfaces between MWNTs and GNPs gives rise to interfacial polarization. This also confirms that the enhanced absorption is related to the number of semicircles.³¹ In this present, the synergistic effect between MWNTs and GNPs also aids to the enhancement of microwave absorption properties; the dielectric relaxation is the main reason for the microwave absorbing of PVP@MWNTs/GNPs hybrids.

Fig. 9 ε′–ε′′ curves of (a) GNPs; (b) MWNTs; (c) PVP@MWNTs/GNPs hybrids.

3.3. Electromagnetic wave absorbing properties

Fig. 10 shows the reflection loss of GNPs, MWNTs and PVP@MWNTs/GNPs hybrids in the frequency range of 8.2–12.4 GHz with a thickness of 2 mm. It is clearly seen that the microwave absorption properties of the pristine MWNTs and GNPs are very weak. The microwave absorbing properties of GNPs (Fig. 10(a)) is about −4.43 dB at 12.23 GHz, and the reflection loss of MWNTs (Fig. 10(b)) is nearly about −5 dB at 12 GHz. However, the sample of PVP@MWNTs/GNPs hybrids (Fig. 10(c)) has a considerable improvement in the microwave absorbing. The maximum reflection loss is −26.5 dB at 11.29 GHz. It is noteworthy that two absorption peaks appear in the frequency of 8.8–12.4 GHz. The first absorption peak at 9.5 GHz, which reflection loss reaches −11.54 dB, and the effective absorption bandwidth with the reflection loss below −10 dB is 0.2 GHz. The second absorption peak at 11.29 GHz, which reflection loss is −26.5 dB, and the effective absorption bandwidth (<−10 dB) is 1.6 GHz. Here, the bandwidth is defined as the frequency width in which the reflection loss is less than −10 dB, it means that the frequency bandwidth can achieve 90% of reflection loss.^32,33 Furthermore, the absorption value will be as high as 99% if the reflection value is below −20 dB.³² The results indicate that the incorporation of MWNTs and GNPs is helpful for the enhancement of microwave absorption properties. This is due to that the incorporation of MWNTs and GNPs, it can enhance the dispersion of MWNTs and GNPs to build effective conductive network (shown in Fig. 11), which probably endows the hybrids with much higher microwave absorbing performance. On the other hand, the incorporation of MWNTs and GNPs might form multipole interfacial between GNPs and MWNTs, which is beneficial to enhancing dielectric loss. The interfacial polarization plays a crucial role for the enhancement of microwave absorption. Meanwhile, three-dimensional conductive network structure of PVP@MWNTs/GNPs hybrids was constructed when one-dimensional MWNTs penetrated the adjacent two-dimensional sheet of GNPs, which is more important for the formation of electron conductive path and the electromagnetic wave absorbing.

Fig. 10 Reflection loss of (a) GNPs; (b) MWNTs; (c) PVP@MWNTs/GNPs hybrids.

Fig. 11 Attenuation schematic of microwave in PVP@MWNTs/GNPs hybrids.

4. Conclusions

The novel absorber of PVP@MWNTs/GNPs hybrids were prepared by ultrasonication filtration method and their electromagnetic wave absorbing properties were investigated in the frequency range of 8.2–12.4 GHz. The SEM and TEM results indicated that the MWNTs were attached on the surface of GNPs. The long MWNTs penetrated into the interlayers of GNPs, and the hierarchical PVP@MWNTs/GNPs layer architecture was constructed. The three-dimensional structure is more important for the formation of electron conductive path and the enhancement of microwave absorbing properties. In the comparison of MWNTs, GNPs and PVP@MWNTs/GNPs hybrids, the PVP@MWNTs/GNPs hybrids have much better microwave absorbing properties. The maximum reflection loss is −26.5 dB at 11.29 GHz. Two absorption peaks appeared in the frequency of 8.8–12.4 GHz. The first absorption peak at 9.5 GHz, which reflection loss reaches −11.54 dB, and the effective absorption bandwidth with reflection loss below −10 dB is 0.2 GHz. The second absorption peak at 11.29 GHz, which reflection loss is −26.5 dB, and the effective absorption frequency width (<−10 dB) is 1.6 GHz. The results indicated that the combination of MWNTs and GNPs has synergetic effect on the improved electromagnetic wave absorbing properties.

Acknowledgements

The authors are grateful for the financial support from National Natural Science Foundation of China (No. 51373136, No. 51373137), Shaanxi Natural Science Foundation (2014JM6241) and NWPU Graduate student Entrepreneurship Seed Fund (Z2014169). Thanks Mr Bobhueftle for the help to revise the manuscript.

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