carbonyl iron nanocomposites for efficient electromagnetic interference shielding

carbonyl iron nanocomposites for efficient electromagnetic interference shielding

Accepted Manuscript Magnetic and electrically conductive epoxy/graphene/carbonyl iron nanocomposites for efficient electromagnetic interference shield...

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Accepted Manuscript Magnetic and electrically conductive epoxy/graphene/carbonyl iron nanocomposites for efficient electromagnetic interference shielding Yu Chen, Hao-Bin Zhang, Yaqin Huang, Yue Jiang, Wen-Ge Zheng, Zhong-Zhen Yu PII:

S0266-3538(15)30071-3

DOI:

10.1016/j.compscitech.2015.08.023

Reference:

CSTE 6185

To appear in:

Composites Science and Technology

Received Date: 28 May 2015 Revised Date:

24 August 2015

Accepted Date: 30 August 2015

Please cite this article as: Chen Y, Zhang H-B, Huang Y, Jiang Y, Zheng W-G, Yu Z-Z, Magnetic and electrically conductive epoxy/graphene/carbonyl iron nanocomposites for efficient electromagnetic interference shielding, Composites Science and Technology (2015), doi: 10.1016/ j.compscitech.2015.08.023. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Magnetic and electrically conductive epoxy/graphene/carbonyl iron nanocomposites for efficient electromagnetic interference shielding Yu Chena, Hao-Bin Zhanga,*, Yaqin Huanga, Yue Jianga, Wen-Ge Zhengb, Zhong-Zhen Yua,* a

State Key Laboratory of Organic-Inorganic Composites, College of Materials Science and

b

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Engineering, Beijing University of Chemical Technology, Beijing 100029, China

Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences,

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Ningbo 315201, China

Abstract

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Magnetic and electrically conductive epoxy nanocomposites are fabricated by compounding thermally reduced graphene oxide (TGO) and magnetic carbonyl iron (CI) using a solvent-free and efficient centrifugal mixing method. The addition of TGO sheets not only forms an interconnected conducting network within the epoxy matrix, but also prevents the

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aggregation of heavy CI components. The incorporation of CI components leads to obvious increases in permeability, magnetic loss, and electromagnetic interference (EMI) shielding properties. Among different shapes of CI components, spherical CI particles result in the best

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EMI shielding performance. The ternary nanocomposites exhibit excellent shielding

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effectiveness (>36 dB within 9.5-12 GHz) and a maximum value of ~40 dB at 11.7 GHz, much higher than that (~20 dB) of epoxy/TGO nanocomposite with the same content of TGO. Wave absorption loss is confirmed to be the main EMI shielding mechanism for the epoxy nanocomposites. The high EMI shielding performance makes the epoxy nanocomposites promising for EMI shielding applications. Keywords: A: Polymer-matrix composites (PMCs); A: Functional composites; B: Electrical properties

_____________________________________________________ Correspond author: Fax: +86-10-64428582 E-mail: [email protected] (H.-B. Zhang); [email protected] (Z.-Z. Yu)

ACCEPTED MANUSCRIPT 1. Introduction Electromagnetic radiation has become a serious pollution with the rapid popularity of electronic equipments and personal electronic devices, which would affect the function of sensitive apparatus and the health of human beings. Hence, great efforts have been devoted to

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designing and developing electromagnetic interference (EMI) shielding materials/composites to eliminate the harmful effects of electromagnetic radiation. Polymer based composites are considered as promising candidates for EMI shielding because of their advantages, such as

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low density, resistance to corrosion, and adjustable mechanical and functional properties. To replace heavy metal fillers, lightweight carbon fillers including carbon black nanoparticles

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[1-4], carbon fibers [5] and carbon nanotubes (CNTs) [6-8] are commonly used in EMI shielding polymer composites. Recently, graphene becomes a promising filler of polymers due to its large specific surface area, high aspect ratio, and extraordinary electrical and thermal conductivities [9-14]. It has been confirmed that graphene is efficient in improving

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electrical and EMI shielding properties of polymers [15-18], because of the high dipole polarization and hopping conductivity [16,19]. However, further enhancement of EMI shielding performance is difficult, especially at high filler contents, because of the polymer

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processing difficulty resultant from the large specific surface area of graphene sheets.

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EMI shielding properties of polymer composites are mainly determined by dielectric and magnetic properties of fillers [20,21]. Thus, various magnetic components, such as Fe3O4 [18,21-23], α-Fe2O3 [24], γ-Fe2O3, Co3O4 [25], Fe [26,27] and [email protected] architectures [28] are integrated with dielectric graphene to optimize the impedance matching and absorption capability. Note that electrical conductivity of absorption materials needs to be tuned for impedance matching, while both high electrical conductivity and strong magnetic loss contribute to total EMI shielding performance. Previously, we fabricated polystyrene (PS) nanocomposites with thermally reduced graphene oxide (TGO) and modified Fe3O4

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ACCEPTED MANUSCRIPT nanoparticles [21]. The addition of Fe3O4 enhances the EMI shielding effectiveness (SE) of PS/TGO nanocomposites, which is ascribed to the unique microstructure formed by the percolated graphene network and the surrounded magnetic nanoparticles. Compared to Fe3O4 nanoparticles, carbonyl iron (CI) exhibits much higher magnetic properties and it may endow

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polymer nanocomposites with superior EMI shielding properties [20,29-31]. Till now, there is few literature about polymer/graphene/CIs for EMI shielding despite to the report of graphene/magnetic nanoparticles for wave absorption.

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Herein, we demonstrate a facile and efficient method to fabricate epoxy nanocomposites with high EMI shielding performance by incorporation of both conductive graphene and

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magnetic CIs. The influences of three types of CIs on electrical conductivity, permittivity, permeability and EMI shielding performance of the epoxy nanocomposites are explored. The combination of dielectric loss TGO sheets and magnetic loss spherical CIs would benefit the enhancement of EMI shielding performances. The EMI shielding mechanisms of the

2. Experimental 2.1 Materials

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nanocomposites are also discussed.

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Flaky, ellipsoidal and spherical CIs were supplied by Nanjing University (China) and

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denoted as FCI, ECI and SCI, respectively. Graphite flakes (300 mesh) were bought from Huadong Graphite Factory (China). Epoxy resin (DER 332) was obtained from Dow Chemical Company. Methyl hexahydrophthalic anhydride as curing agent, tris-(dimethyl laminomethyl) phenol as accelerator, and acetone were purchased from Beijing Chemical Factory (China). 2.2 Preparation of TGO and its epoxy nanocomposites TGO was obtained by oxidizing graphite flakes according to a modified Staudenmaier method followed by thermal exfoliation and reduction in a muffle furnace at 1050 oC [32, 33].

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ACCEPTED MANUSCRIPT Epoxy monomer, TGO/CI fillers, curing agent and accelerator were compounded with a planetary centrifugal vacuum mixer at 25 oC for 10 min and the resultant mixture was put into mold and cured in a vacuum oven at 80 oC for 4 h followed by 120 oC for 2 h. In addition to the centrifugal mixing method, two other methods were also adopted to

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prepare epoxy/TGO nanocomposites. By mechanical mixing method, epoxy, TGO and relevant components were mechanically stirred in acetone at 300 rpm/min for 10 min with an IKA Eurostar 20 digital agitator; while in the ultrasonic mixing method, epoxy, TGO and

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relevant components were ultrasonicated with JY92-IIDN (Scientz, China) at 270 W for 30 min in acetone. Subsequently, acetone was evaporated during stirring. Note that the

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centrifugal method is superior to the other two methods as it is facile, efficient and solvent-free. To highlight the advantages of centrifugal mixing, the electrical and EMI shielding properties of the nanocomposites prepared by the three methods were compared. 2.3 Characterizations

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X-ray diffraction (XRD) patterns of CIs were recorded with a Rigaku D/Max 2500 VB2+/PC X-ray diffractometer. Magnetic properties of CI and its epoxy nanocomposites were measured with a Lake Shore 7410 vibrating sample magnetometer (VSM). Morphology

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of CIs and microstructure of epoxy nanocomposites were observed with a JEOL JSM-7800F

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scanning electron microscope (SEM). The morphology of TGO was further observed with an H-800 transmission electron microscope (TEM). Electrical conductivities of the nanocomposites were measured with a 4-probes-Tech RST-8 resistivity meter (> 10-4 S/m) and ZC-90G resistivity meter (< 10-4 S/m). EMI SE of the nanocomposites was measured by an Agilent N5242A vector network analyzer in the frequency range of 8-12 GHz. The electromagnetic parameters of CI and its epoxy nanocomposites were obtained using an Agilent HP-8722ES vector network analyzer in the frequency of 1-18 GHz at room temperature.

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ACCEPTED MANUSCRIPT 3. Results and discussion 3.1 Characterization of graphene and CI As the integration of dielectric and magnetic components is favorable for the efficient electromagnetic wave attenuation, both graphene sheets and magnetic CIs are used to

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improve EMI shielding performance of epoxy. Fig. 1 shows SEM images of graphene sheets and three different shaped CIs. Graphene sheets exhibit a typical layered structure while CIs are similar in average size (1~2 um) but different in shapes, i.e. flaky, ellipsoidal and

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spherical. Fig. 2a and b show XRD patterns and magnetic properties of FCI, ECI and SCI, respectively. The similar XRD characteristics with peaks localizing at 44.6o and 65.0o,

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corresponding to (110) and (200) reflections of cubic α-Fe, respectively, suggesting similar crystalline structures of FCI, ECI and SCI despite their different morphologies. VSM results reveal that FCI, ECI and SCI exhibit different saturation magnetization and coercivity. SCI shows a higher saturation magnetization (222.1 emu/g) than FCI (206.9 emu/g) and ECI

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(217.9 emu/g), while its coercivity (22.2 Oe) is lower than those of FCI (28.4 Oe) and ECI (33.1 Oe). The excellent magnetic properties may make SCI a promising candidate for microwave absorption [20,34,35]. The addition of CIs effectively improve the magnetic

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properties of epoxy/TGO nanocomposites (Fig. 2c, d), which almost do not have magnetic

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properties in the absence of CIs. When the CI content increases from 10% to 30%, the saturation magnetization of the corresponding epoxy/TGO nanocomposite increases from ~20 to ~60 emu/g. It is also seen that, at the same content, the three CIs exhibit different enhancement in saturation magnetization of their epoxy nanocomposites due to their different intrinsic magnetic properties. 3.2 Microstructures of epoxy nanocomposites Fig. 3 shows SEM images of epoxy/TGO and epoxy/TGO/CI nanocomposites. Graphene is well dispersed in epoxy matrix and no large aggregates are observed (Fig. 3a). In contrast,

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ACCEPTED MANUSCRIPT SCI particles are not well dispersed as they tend to precipitate during curing process and are selectively located in the back side of the epoxy/SCI composite due to their high density (Fig. 3b). Interestingly, in the presence of 3 wt% graphene sheets, the dispersion of SCI particles is greatly improved, evidenced by a uniform dispersion of the particles in the matrix (Fig. 3c).

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The three-dimensional network formed by graphene sheets plays a crucial role in preventing the precipitation and aggregation of SCI particles. Under high magnification, it is clearly seen that the isolated SCI particles are surrounded by interconnected graphene sheets, and thus an

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unique microstructure consists of interconnecting graphene network and magnetic particles is formed (Fig. 3d), which is expected to be efficient in improving electromagnetic wave

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attenuation by dielectric and magnetic losses. The electrically conductive graphene network provides high conductivity and thus strong polarization for electromagnetic wave attenuation, while the magnetic SCI particles act as dipoles which are polarized in the presence of electromagnetic wave, also resulting in high wave attenuation. Once electromagnetic wave

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gets into the numerous spaces consisting of interconnected graphene sheets and surrounded SCI particles, it is multiply reflected by the conducting graphene sheets and then repeatedly interact with the magnetic particles and attenuated again and again. With a similar

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microstructure, in our previous work, magnetic Fe3O4 nanoparticles showed high efficiency in

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improving the EMI shielding properties of polymer/graphene composites [21]. Zhu et al [20] also proposed a cross-linked frame work structure with magnetic particles for the enhancement of wave absorption capacity. 3.3 Effects of compounding approaches on electrical and EMI shielding properties of epoxy/TGO nanocomposites A solvent-free, environmentally benign and efficient centrifugal method is employed to prepare epoxy/TGO nanocomposites using a planetary centrifugal vacuum mixer. The electrical and EMI shielding properties of as-prepared epoxy nanocomposites are compared

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ACCEPTED MANUSCRIPT to those of the nanocomposites prepared by ultrasonic and mechanical mixing methods (Fig. 4). The epoxy/TGO nanocomposites exhibit higher electrical conductivities than their counterparts prepared by ultrasonic and mechanical mixing methods (Fig. 4a). A high conductivity of 1.43 S/m is obtained with only 2 wt% graphene, higher than 0.15 and 0.098

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S/m for the nanocomposites prepared by ultrasonic and mechanical mixing, respectively. The conductivity difference of the three types of nanocomposites becomes smaller with the increase of TGO contents.

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In accordance with the electrical conductivities, better EMI shielding performance is also observed for epoxy nanocomposites prepared by centrifugal mixing than those prepared by

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the other two methods. As shown in Fig. 4b, with only 3 wt% TGO, EMI SE is higher than 20 dB in the whole frequency range of 8-12 GHz, which is higher than the reported values of nanocomposites filled with reduced graphene oxide [13], TGO [9,11], graphite nanosheets [36]. With 5 wt% of TGO, the EMI SE is ~27 dB in the frequency range of 8-12 GHz and a

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maximum value of 30.7 dB is achieved at 9.4 GHz. In contrast, the epoxy nanocomposites prepared by ultrasonic and mechanical methods show lower EMI SE values (Fig. 4c and d). For example, when graphene content is 3 wt%, the average EMI SE values of the epoxy/TGO

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nanocomposites prepared by ultrasonic and mechanical mixing are only ~18 and ~16 dB,

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respectively; while the average EMI SE of the epoxy/TGO nanocomposite prepared by centrifugal mixing is more than 20 dB. The advantages of centrifugal mixing technique become more valuable when considering the solvent-free, high efficiency and feasibility for mass production. Therefore, this work provides a promising method for the preparation of polymer nanocomposites with high electrical and EMI shielding properties. 3.4 Dielectric and magnetic properties of epoxy/TGO/SCI composites To reveal the contribution from dielectric and magnetic losses, Fig. 5 shows complex permittivity and permeability of SCI particles and their epoxy/TGO nanocomposites in the

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ACCEPTED MANUSCRIPT frequency range of 1-18 GHz. As shown in Fig. 5a, c and e, the real part of permittivity (ε’) of SCI is lower than 2.5, and its imaginary part of permittivity (ε”) and dielectric loss (tan δE = ε”/ε’) are close to zero, suggesting the weak dielectric loss of SCI. However, the addition of TGO leads to great increases in ε’, ε” and tan δE. The values of ε’ are in the ranges of

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9.47-26.81 and 13.04-34.36 for epoxy nanocomposites with 1.69 and 6.27 vol% of SCI, respectively. This is associated with the strong polarization of graphene sheets due to their high specific surface area, structural defects and retained oxygen-containing groups.

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Moreover, the values of ε” for corresponding epoxy nanocomposites with 1.69 and 6.27 vol% SCI are in the ranges of 6.59-11.99 and 9.30-15.61, respectively, much higher than that of

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SCI itself. The obviously increased ε” is attributed to the conducting graphene network and high electrical conductivity, which induce strong electron polarization and space charge polarization [20]. Furthermore, the value of tan δE for epoxy/TGO nanocomposites is above 0.30 and increases with frequency, implying the strong dielectric loss properties derived from

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the dielectric graphene sheets. Additionally, with the same graphene content, the composites with higher SCI contents show increases in ε’ and ε”, but nearly unchanged tan δE. This

sheets [16].

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further confirms that the dielectric loss of the nanocomposites originated mainly from TGO

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The real part (µ’) and imaginary part (µ”) of the complex permeability and the magnetic loss (tan δM = µ”/µ’) of SCI and its epoxy/TGO nanocomposites are compared (Fig. 5b, d, f). Compared to SCI and epoxy/TGO, epoxy/TGO/SCI nanocomposite exhibits higher values of µ’, µ” and tan δM. The increase of SCI content leads to great increase in µ’, µ” and tanδM, suggesting the notable influence of magnetic SCI on permeability and magnetic loss. Therefore, the improved magnetic loss is achieved by the incorporation of magnetic SCI and thereby contributes to the EMI shielding performance. The magnetic loss mechanism can be mainly ascribed to natural resonance in microwave frequency band [20]. The combination of

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ACCEPTED MANUSCRIPT dielectric loss graphene sheets and magnetic loss SCI facilitates the enhancement in EMI shielding properties of the epoxy nanocomposites. 3.5 EMI shielding performances of epoxy/TGO/SCI nanocomposites As reported [9,11,21], EMI shielding performance of polymer nanocomposites closely

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correlates with the loading of conductive fillers. However, it is not always possible to increase the loading of fillers because of the increased polymer processing difficulty and rather slow increment of conductivity, especially at high loadings. In the present work,

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magnetic CIs are incorporated into epoxy/TGO nanocomposites and their effects on electrical conductivity and EMI shielding properties are explored. The addition of 10 wt% CIs does not

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affect the conductivity and EMI SE of the epoxy/3%TGO nanocomposite (Fig.6a). By increasing the content of ECI and SCI to 30 wt%, the conductivity of epoxy/TGO nanocomposite is increased from 4.35 S/m to 6.06 and 8.38 S/m, respectively; while slight decrease in conductivity is observed for the epoxy/TGO/FCI nanocomposite. Interestingly, an

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obvious increase in EMI SE is achieved, and CIs with different shapes show different influences on their EMI SE. As shown in Fig. 6b, SCI filled nanocomposite exhibits higher EMI shielding efficiency while FCI nanocomposite presents the lowest shielding efficiency

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among the nanocomposites filled with SCI, EMI and FCI. Even so, a significant increase of

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~10 dB in SE is observed for FCI-filled nanocomposite in relative to the epoxy/TGO nanocomposite, and the bandwidth for SE above 30 dB is about 3 GHz (9-12 GHz). Note that the values of SE increase more rapidly with frequency after 9.5 GHz. ECI composite exhibits higher SE values than 32 dB within the frequency range of 9.5-12 GHz, while SCI-filled nanocomposite shows SE values above 36 dB within the similar frequency range and a maximum value of ~40 dB is achieved at 11.7 GHz. These results are much higher than those of epoxy/TGO nanocomposites and may be one of the best EMI shielding performance reported for graphene based polymer nanocomposites [9,11,13,20,21,34-37].

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ACCEPTED MANUSCRIPT Because thickness plays an important role on EM wave attenuation properties, the SE values of epoxy/TGO/SCI nanocomposites with various thicknesses (1, 2, 3 and 4 mm) are compared (Fig. 6c). The SE value increases from ~12 dB for 1 mm thick sample and over 22 dB for 2 mm and 3 mm thick samples. The 4 mm-thick sample shows even higher SE values

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over the entire frequency range explored. As reported [37], the increased EMI SE with sample thickness derived mainly from the increment of absorption loss and only small part from reflection loss, which is due to the shallow skin depth and high electrical conductivity in

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the microwave range.

To reveal the EMI shielding mechanism, the values of absorption (SEA) and reflection

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(SER) for epoxy/TGO and epoxy/TGO/CI nanocomposites are compared (Fig. 6d). Electromagnetic wave attenuation from absorption is higher than that from reflection for each nanocomposite, indicating an absorption EMI shielding mechanism for graphene filled epoxy composites [9,11]. Moreover, the presence of magnetic CIs further increases the contribution

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of absorption to EMI shielding performance due to natural resonance in microwave waveband, as evidenced by the noticeably higher SEA values. These results are consistent with the increased values of µ’, µ” and tan δM. Consequently, the excellent EMI shielding

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mechanism of epoxy/TGO/CI nanocomposites can be attributed to natural resonance, domain

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wall resonance, and electric current loss. As far as different CIs are concerned, epoxy/3%TGO/30%SCI nanocomposite exhibits a higher SEA as compared to its counterparts filled with ECI and FCI, which suggests a larger contribution of absorption loss to EMI shielding performance for SCI-filled nanocomposite. These results may be correlated with the different electrical conductivities of the epoxy nanocomposites and various magnetic properties of different shaped CIs [35,36]. Although it is still unclear that how magnetic CIs with different morphologies exactly influence EMI shielding properties of epoxy/TGO nanocomposites, it can be confirmed that epoxy/TGO/CI

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4. Conclusion

Epoxy nanocomposites are prepared with conductive TGO sheets and magnetic CIs by a facile, efficient and solvent-free centrifugal method. It is interesting that graphene sheets

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effectively prevent the aggregation of heavy CI particles. Compared to epoxy/TGO nanocomposites, CI-filled composites exhibit obviously increased permeability and magnetic

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loss, leading to improved impedance matching and EM attenuation. SCI shows the greatest influence on EMI shielding performance and leads to SE values above 36 dB within 9.5-12 GHz and a maximum value of SE (~40 dB) is achieved at 11.7 GHz. The different influences of CIs on SE may originate from the different magnetic properties of CI and composite

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conductivity. The addition of SCI further increases the contribution from absorption loss to EMI shielding performance. This work provides a promising approach for the preparation of excellent EMI shielding materials and may open up new opportunities for the practical

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applications of graphene materials.

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Acknowledgements

Financial support from the National Natural Science Foundation of China (51103007, 51125010 and 51373011), the Fundamental Research Funds for the Central Universities (YS201402), and the State Key Laboratory of Organic-Inorganic Composites (201501007) is gratefully acknowledged References [1] Al-Saleh MH, Sundararaj U. Electromagnetic interference (EMI) shielding effectiveness of PP/PS polymer blends containing high structure carbon black. Macromol Mater Eng

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Figure captions: Fig. 1 (a) TEM and (b,c,d) SEM images of (a) TGO, (b) FCI, (c) ECI and (d) SCI.

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Fig. 2 (a) XRD patterns and (b) hysteresis loops of FCI, ECI and SCI; hysteresis loops of epoxy/3%TGO nanocomposites filled with (c) 10 % and (d) 30 % CIs.

Fig. 3 SEM images of (a) epoxy/TGO (3%), (b) epoxy/SCI (10%) and (c,d) epoxy/TGO/SCI

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nanocomposites under different magnifications.

Fig. 4 (a) Electrical conductivity of epoxy/TGO nanocomposites; EMI SE of epoxy/TGO

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nanocomposites prepared by (b) centrifugal, (c) ultrasonic and (d) mechanical mixing. The inset of Fig. 4b is the photograph of the specimen used for EMI SE measurement. Fig. 5 Frequency dependence of (a,b) real and (c,d) imaginary parts of (a,c) complex

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permittivity and (b,d) permeability of SCI and its epoxy nanocomposites; (e) dielectric and (f) magnetic losses of SCI and its epoxy nanocomposites. The inset of Fig. 5a is the photograph of specimen used for the measurement of dielectric and magnetic parameters.

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Fig. 6 EMI SE of epoxy/TGO (3%) nanocomposites with (a) 10 and (b) 30 % of FCI, ESI and

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SCI; (c) effect of thickness on EMI SE of epoxy/TGO (3%)/SCI (30%) nanocomposite; and (d) comparison of SEA and SER of epoxy/TGO (3%) and epoxy/TGO (3%)/CI (30%) nanocomposites.

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