Synergistic Effect of Graphite and Carbon Nanotubes on Improved

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Materials and Interfaces

Synergistic Effect of Graphite and Carbon Nanotube on Improved Electromagnetic Interference Shielding Performance in Segregated Composites Li-Chuan Jia, Ding-Xiang Yan, Xin Jiang, Huan Pang, Jie-feng Gao, Peng-gang Ren, and Zhong-Ming Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03238 • Publication Date (Web): 14 Aug 2018 Downloaded from http://pubs.acs.org on August 15, 2018

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Industrial & Engineering Chemistry Research

Synergistic Effect of Graphite and Carbon Nanotube on Improved Electromagnetic Interference Shielding Performance in Segregated Composites

Li-Chuan Jia,† Ding-Xiang Yan,*,‡ Xin Jiang,† Huan Pang,† Jie-Feng Gao,§ Peng-Gang Ren,⊥ Zhong-Ming Li†

†

College of Polymer Science and Engineering, State Key Laboratory of Polymer

Materials Engineering, Sichuan University, Chengdu, 610065, China ‡

§

School of Aeronautics and Astronautics, Sichuan University, Chengdu 610065, China

School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou,

Jiangsu, 225002, China ⊥

Institute of Printing, Packaging Engineering and Digital Media Technology, Xi’an

University of Technology, Xi’an, Shaanxi 710048, China

1

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: Herein, the graphite-carbon nanotube (G-CNT) hybrid loaded ultrahigh molecular weight polyethylene (UHMWPE) composite with a segregated structure was fabricated. In such structure, G-CNT hybrid was selectively distributed at the interfaces of UHMWPE domains to form interconnected networks, as demonstrated by the optical microscopy and scanning electron microscopy. The resultant G-CNT/UHMWPE composite exhibited an excellent electrical conductivity of 195.3 S m-1 and an ultrahigh electromagnetic interference shielding effectiveness (EMI SE) of 81.0 dB. The results were superior to those of single graphite or CNT loaded one, clearly confirming the synergistic effect of graphite and CNT. Amazingly, only 0.5 mm specimen thickness imparted the G-CNT/UHMWPE composite with an EMI SE of 31.8 dB, already matching the requirement for commercial EMI shielding applications. This work highlights the merit of integrating the segregated structure with the synergistic effect of G-CNT hybrid in forming highly conductive networks and developing efficient EMI shielding materials.

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1. INTRODUCTION With the explosive development of electronic equipment and wireless devices, electromagnetic radiation becomes a serious problem. This affects adversely the proper functioning behaviors of electronic devices as well as public health. Thus, substantial efforts have been devoted towards developing high-performance electromagnetic interference (EMI) shielding materials.1-8 As the promising substitutes for the conventional metal-based EMI shielding materials, polymer composites embedded with carbonaceous fillers have attracted significant academic and industrial interests due to the advantages, e.g., lightweight, excellent corrosion resistant, high electrical conductivity and easy processability etc..9-24 Carbon nanotubes (CNTs), with exceptional mechanical and electrical performance, have emerged as highly effective conductive nanofiller for making EMI shielding composites.25-31 A CNT/polypropylene random copolymer composite was reported to exhibit a satisfactory EMI shielding effectiveness (EMI SE) of 23.0 dB with 4.0 wt % CNT.25 Much higher EMI SE of 50.0 dB was obtained in CNT/epoxy composite at improved CNT loading (20.0 wt%).26 Nevertheless, the high cost of CNTs, in comparison to other carbonaceous fillers, like carbon black (CB), graphite, and carbon fibers (CF), limits their widespread applications. It is of great importance for CNT based EMI shielding materials to balance the cost and comprehensive performance (i.e., electrical, mechanical, and EMI shielding performance). In order to achieve this objective, several strategies have been explored such as utilizing the synergistic effect of hybrid fillers with different geometric shapes and sizes 3



32-35

and structuring the specific morphology of conductive networks (i.e., segregated or

double-percolated structures).36 For example, the electrical conductivity for polylactide containing hybrid fillers of CNTs and CB showed 400 times (6 orders of magnitude) that for single CNT (CB) loaded one at the 2.0 wt% filler loading.32 Hybrid CNTs/graphite nanoplatelets (GNP)/epoxy exhibited higher electrical conductivity than that of CNTs/epoxy and GNPs/epoxy by about 2-3 orders of magnitude.33 Aside from the improved electrical conductivity of the composites, the synergistic effect of hybrid fillers contributes to reducing the cost of the final materials, indicating great potential in developing low cost and high-performance EMI shielding materials. Additionally, the formation of segregated structures was also demonstrated to be effective in obtaining high EMI SE at reduced conductive filler loading.37-41 Only 5.0 wt% CNT addition in the segregated CNT/polyethylene realized a superior EMI SE of 46.4 dB, which is 46% higher than that for randomly distributed CNT/polyethylene.39 In our previous work, the segregated graphite/ultrahigh molecular weight polyethylene (UHMWPE) containing economical graphite loading of 15.0 wt% exhibited satisfactory EMI SE of 51.6 dB.41 Unfortunately, limit work was focused on the combination of the synergistic effect of hybrid fillers with the formation of segregated structure. This naturally arouses our curiosity to investigate their combination on EMI shielding performance. In this work, we fully utilized the advantage of synergistic effect of graphite and CNT in the segregated structure to fabricate a highly efficient EMI shielding material. The resultant graphite-CNT/UHMWPE composite (denoted as G-CNT/UH) with 15.0 wt% G-CNT (weight ratio of 1:3) showed a superior EMI SE up to 81.0 dB at 2.1 mm 4


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thickness. Such value was 57.0% and 8.0% higher than that of single graphite and single CNT loaded ones, respectively. The achieved EMI SE was among the highest value even reported for carbonaceous fillers based shielding materials. Moreover, at only 0.5 mm thickness, the G-CNT/UH already showed a satisfactory EMI SE (31.8 dB), exceeding the target EMI SE (20 dB) for commercial shielding applications. Our work highlights the superiority of the combination of synergistic effect of hybrid filler and segregated structure in improving the electrical and EMI shielding performance. 2. EXPERIMENTAL SECTION 2.1 Materials UHMWPE was purchased from Beijing no. 2 Auxiliary Agent Factory, with viscosity average molecular weight of 5.5~6.0×106 g mol-1, density of 0.945 g cm-3, and melting temperature of 137 oC. The average particle size of the UHMWPE is ~250 µm, as shown in Figure 1. Graphite was supplied by Beishu Graphite Co. Shangdong, China, with density of 2.2 g cm-3 and lateral dimension of 20 µm. CNT (Nanocyl NC7000, 90% carbon purity, the average diameter of 9.5 nm, length of 1.5 µm, and surface area of 250~300 m2 g-1) was provided by Nanocyl S.A., Belgium.

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Figure 1. The size distribution of the pure UHMWPE granules. 2.2 Fabrication of G-CNT/UH G-CNT/UH was fabricated by a simple dry mixing (mechanical mixing) method plus hot compaction, as shown schematically in Figure 2a. Initially, the UHMWPE granules (Figure 2b) were mechanically mixed with graphite and CNT to prepared the G-CNT hybrid coated UHMWPE granules using a functional grinder (BJ-100, 25,000 rpm/min) for 2 min. The functional grinder can mix the compounds uniform by using blade rotation at high speed, as shown in Figure S1 (Supporting Information). The detailed formulations of the composites were presented in Table 1. As shown in Figure 2c, numerous graphite and CNT were deposited onto the surfaces of UHMWPE granules. This phenomenon was mainly due to the rough surfaces of UHMWPE granules with a surface area of 0.31 m2/g (Figure 2b and Table S1). It was found that the mixed granules showed a significant increase in surface area compared to the UHMWPE granules due to the large surface area of graphite and CNT (Table S1). Subsequently, the resultant mixture granules were compression molded into a flat plate (Figure 2d) under 10 MPa 6


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and 200 oC for 5 min, after preheating for 5 min. The X-ray photoelectron spectroscopy (XPS) data of UHMWPE granules and UHMWPE sheet prepared by hot compaction were performed to evaluate the chemical state of the samples. As shown in Figure S2, the C/O atomic ratio exhibited a very slight decrease from 12.7 to 11.5 after hot compaction, indicating the retention of chemical state basically. For convenience, the fabricated composites containing graphite and CNT in the weight ratio of 3/1, 1/1, and 1/3 were coded as G3–CNT1/UH, G1–CNT1/UH, and G1–CNT3/UH, respectively. There was no obvious change in the surface area for the UHMWPE sheet and the 4.0 and 15.0 wt% G1–CNT3/UH, which should be attributed to the densification effect of hot compaction technique (Table S1). For comparison, the single graphite and CNT loaded UHMWPE composites were also fabricated under the same conditions and denoted as G/UH and CNT/UH, respectively.

Figure 2. (a) Schematic for the fabrication of segregated G-CNT/UH. SEM images of (b) pure UHMWPE granules, (c) 2.0 wt% G1-CNT3 hybrid coated UHMWPE complex granules. (d) Digital images of the G-CNT/UH with wafer shape. 7



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Table 1. The formulations of the composites. The total weight of the composites is fixed at 100 g and the fillers content is 15.0 wt%. Composites

UHMWE (g)

Graphite (g)

CNT (g)

G/UH

85

15

0

G1–CNT3/UH

85

3.75

11.25

G1–CNT1/UH

85

7.5

7.5

G3–CNT1/UH

85

11.25

3.75

CNT/UH

85

0

15

2.3 Characterization Brunauer–Emmett–Teller (BET) surface areas were obtained by measuring nitrogen sorption isotherms at 77 K on a Micromeritics ASAP 2020 surface area analyzer (Quantachrome, United States). XPS measurement was obtained in an XSAM800 (Kratos Company, UK) using an Al Kα X-ray source. Optical microscopy (OM, Olympus BX51) was used to observe the conductive networks in the G-CNT/UH. The specimens with thickness of 15 µm were prepared using a microtome. Scanning electron microscopy (SEM) observation was performed by a field emission scanning electron microscopy (SEM, Inspect-F, FEI, Finland) at an accelerating voltage of 20 kV. The specimens were cryogenically fractured in liquid nitrogen and then the freshly fractured surfaces were coated with a thin of gold prior to being observed. The electrical conductivity was measured using a Keithley electrometer model 4200-SCS (USA) and a four-point probe instrument (RTS-8, Guangzhou, China). The size distribution of UHMWPE granules was measured using Mastersizer 2000 laser particle size analyzer. 8


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The Raman spectra were recorded using a Labram spectrometer (Super LabRam II system) with 532 nm laser excitation. X-ray diffraction (XRD) data were collected with a DX-1000 diffractometer using CuKa irradiation at 40 kV in a scanning range from 10° to 35°. The EMI SE was measured by using a coaxial test cell (APC-7 connector) in conjunction with a vector network analyzer (Agilent N5230) (Figure S3), (Schematic of measurement setup was shown in our previous work).37 The relevant parameters of the APC-7 connector were presented in Table S2. The APC-7 connector is a precision coaxial connector that has been widely used on laboratory microwave test equipment and can be utilized at frequencies from 2 GHz up to 18 GHz.42 The Agilent N5230 vector network analyzer was calibrated using the standard open, short, and 50 Ω load components. The intermediate frequency bandwidth was set as 1 kHz during the measurement and 201 points were collected for each specimen. Thus, the frequency dependence of EMI SE in the frequency range of 8.2–12.4 GHz (X-Band) obtained here is accurate and reliable. Samples with 10 mm diameter and various thicknesses were placed in the specimen holder, which were connected through Agilent 85132F coaxial line to separate VNA ports. The scattering parameter (S11 and S21) obtained in the frequency of 8.2–12.4 GHz were used to calculate the reflected power (R), transmitted power (T), absorbed power (A), EMI SE (SETotal), microwave reflection (SER) and 2

microwave absorption (SEA), using the following equations: R = S11 , T = S 21 , 2

A = 1 − R − T , SER = −10lg (1 − R ) ,

 T

,  1− R 

SE A = −10 lg 

SETotal = SER + SEA + SEM ,

where SEM is the microwave multiple internal reflections, which can be can be negligible when SETotal≥10 dB.43 9



3. RESULTS AND DISCUSSION To evaluate the distribution of G-CNT hybrid in G-CNT/UH, OM observations were carried out and the results were shown in Figure 3. A typically segregated structure was constructed in the G1-CNT3/UH, in which the G-CNT hybrid was squeezed surrounding UHMWPE domains rather than randomly distributed throughout the whole system. The formation of segregated structure was instrumental in significantly increasing the efficiency of G-CNT hybrid to construct conductive networks at a very low filler loading. As shown in Figure 3a, only 0.1 wt% G-CNT hybrid addition in G1-CNT3/UH realized the continuous pathways. With further increasing G-CNT hybrid to 0.5 and 2.0 wt%, the conductive layer became thicker and denser. It indicated the formation of more perfect interconnected networks (Figure 3b and c).

Figure 3. OM images of the G1-CNT3/UH with (a) 0.1 wt%, (b) 0.5 wt%, and (c) 2.0 wt% filler content, respectively. SEM analysis was further conducted to explore the detailed microstructure of the G-CNT networks in G-CNT/UH, as shown in Figure 4. The G-CNT/UH exhibited a typical faceted structure and very thin interfaces were observed between the adjacent individual polyhedron (Figure 4a, b and c). The formation of such a unique microstructure could be attributed to the fabrication process. During hot compression, UHMWPE granules exhibited the feature of ultrahigh melt-viscosity gel due to low 10


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shear and experienced the plastic deformation to polyhedrons. The G-CNT hybrid was difficult to penetrate into the interior of UHMWPE polyhedrons but preferably distributed at their interfaces. Such state was preserved after undergoing the polymer crystallization upon cooling, as shown in Figure 4a', b' and c'. The corresponding magnified images distinctly revealed a complex microstructure of conductive pathways containing two-dimensional (2D) graphite flakes and one-dimensional (1D) CNTs, in which 1D CNTs bridged the adjacent 2D graphite flakes (Figure 4a'', b'' and c''). This implied the cooperative effect between CNT and graphite on forming conductive networks. It may be anticipated that the unique segregated conductive networks combination with the cooperative effect of G-CNT hybrid have positive influence on electrical conductivity and EMI SE of G-CNT/UH.

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Figure 4. SEM images of the G3-CNT1/UH (a), G1-CNT1/UH (b), G1-CNT3/UH (c) with G-CNT hybrid content of 2.0 wt%. Raman spectroscopy is a fast and powerful tool for the characterization of carbon materials. Figure 5a showed the Raman spectra of the CNT, graphite, CNT/UH, G-CNT/UH and G/UH. A similar feature of the Raman spectra can be observed for the G-CNT/UH: ~1350 cm-1 corresponding to the D band and ~1580 cm-1 corresponding to the G band, respectively. Note that the D band became weaker and wider gradually, whereas the G-band exhibited an opposite trend for the G-CNT/UH with increasing the relative amount of graphite to CNT. The G-band was related to the graphitic structure and the D-band was associated with disordered graphitic structure. Generally, the 12


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intensity ratio of D-band to G-band (i.e., ID/IG) was used to evaluate the relative extent of graphitic defects.44,45 The ID/IG ratio decreased to 0.49 for the G3-CNT1/UH from 1.24 for the G1-CNT3/UH, indicating the increased graphitic domains in the G-CNT/UH with increasing the relative amount of graphite to CNT. This phenomenon should be attributed to the presence of better graphite structure in pure graphite compared to the pure CNT. XRD pattern was further used to characterize the G-CNT/UH and the results were shown in Figure 5b.46,47 The XRD patterns revealed the presence of three peaks at ~21.4°, 23.8°, and 26.5 for the G-CNT/UH, which corresponded to (110) and (200) crystal planes of UHMWPE and typical diffraction peak of graphite. Whereas, no characteristic diffraction peak of CNT can be observed, indicating that the CNT is well dispersion in UHMWPE matrix, as demonstrated by the XRD pattern of the CNT/UH. This should be attributed to the fact the CNT was easily wrapped by UHMWPE matrix, due to its nano-scale feature.48

Figure 5. (a) Raman spectra and (b) XRD patterns of pure CNT and graphite, CNT/UH, G-CNT/UH and G/UH with 15 wt% filler content.

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To investigate the effect of segregated structure and the G-CNT hybrid on the electrical performance, Figure 6 displayed the electrical conductivity of G-CNT/UH, G/UH and CNT/UH with various filler loadings. All composites presented a typical percolation behavior, i.e., nearly 12 orders of magnitude increase in electrical conductivity in a very narrow filler content range (0 to 0.3 wt%). The electrical conductivity increased progressively with the increased filler loading, and the increase rate of G-CNT/UH was larger than that of the G/UH (Figure 6). For example, only 0.3 wt% G-CNT hybrid addition in G1-CNT3/UH realized an electrical conductivity of 0.073 S m-1, which was about 85 times higher than that for G/UH one. These results indicated that G1-CNT3/UH provided a more efficient conductive network. This can be attributed to the higher inherently conductivity of CNT compared to graphite. According to the percolation theory, the dependence of electrical conductivity on filler loading can be predicted using the scaling law as follows:15,16,49

σ = σ0 (ϕ −ϕc )t where σ

is the composite electrical conductivity, σ 0 is a constant related to the

intrinsic conductivity of filler, ϕ is the volume fraction of filler, ϕc is the percolation threshold, and t is a critical exponent. The fitting ϕc was listed in Table 2. The G-CNT/UH exhibited an ultralow ϕc compared to G/UH one. The low percolation threshold for the G-CNT/UH was mainly attributed to the intensively increased utilization efficiency of hybrid fillers in the segregated structure composites. The t value was estimated to be 1.93 for the G3-CNT1/UH, 1.97 for the G1-CNT1/UH and 1.74 for the G1-CNT3/UH, respectively, indicating the formation of three-dimensional 14


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conductive networks.50 It was evident that the decrease in G-CNT weight ratio resulted in the increased electrical conductivity at the same hybrid filler loading. For example, when the G-CNT hybid loading was fixed at 2.0 wt%, the electrical conductivity increased from 4.6 to 11.9 S m-1 with decreasing the G/CNT weight ratio from 3:1 to 1:3, already exceeding the target value (1.0 S m-1) for EMI shielding applications.51 Amazingly, it is worthy noting that the G1-CNT3/UH exhibited higher electrical conductivity than that of CNT/UH when the filler loading was above 0.3 wt%. For instance, the 15.0 wt% G1-CNT3/UH showed an electrical conductivity of 195.3 S m-1, superior to that of the CNT/UH (179.4 S m-1, see Table 2). This value is also much higher than that of the previosuly reported work at similar filler content (7 S m-1 @ 15 wt% for graphene/poly lactide,52 20 S m-1 @ 15 wt% for CNT/epoxy53).The interesting phenomena could be ascribed to the cooperative behaviors of 2D large-sized graphite flakes and 1D small-sized CNTs on structuring segregated conductive pathways in UHMWPE matrixes.

Figure 6. (a) Electrical conductivity as a function of filler content for G-CNT/UH, G/UH, and CNT/UH. The inset presents a log–log plot of the conductivity as a function of ϕ − ϕ c ; (b) is the amplification of (a) in the dotted rectangular frame. 15



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Table 2. The conductivity values (fillers content=15.0 wt%) and the percolation threshold of conductive composites. G/CNT

σ ( S m-1)

ϕc

t

graphite

39.5

0.115 vol%

1.76 ± 0.02

3:1

130.8

0.08 vol%

1.93 ± 0.05

1:1

148.5

0.07 vol%

1.97 ± 0.06

1:3

195.3

0.05 vol%

1.74± 0.06

CNT

179.4

0.02 vol%

2.12 ± 0.10

The synergy mechanism in the G-CNT/UH can be interpreted by the schematic illustration of Figure 7. The presence of 1D-CNT played the role of ‘bridge’ in the continous conducting pathways and effectively linked the gaps between adjacent 2D graphite flakes, as observed by SEM (Figure 4). This led to well-developed conducting channels for electron transport compared to single graphite flake filled one. Furthermore, different from the point-to-point conducting contacts in CNT channels, the transport mechanism of G-CNT hybrid manifested a favorable conducting contacts with higher geometrical dimensioanlity. This significantly increased the amount of contact area within hybrid filler networks, resulting in relatively low contact resistance between conducting fillers.54-56 In addition, the incorporation of 1D small-sized CNTs into 2D large-sized graphite flakes was conducive to forming additional conducting pathways.57

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Figure 7. Schematics of conducting networks in the (a) G/CNT/UH, (b) G/UH, and (c) CNT/UH. To identify the superiority of the segregated G-CNT/UH with G-CNT hybrid on EMI shielding performance, the EMI SE of the G-CNT/UH in the frequency range of 8.2–12.4 GHz (Figure 8a) was investigated. For comparison, the EMI SE of the G/UH and CNT/UH was also presented. In consistent with the electrical conductivity results, the more conductive composites showed the higher EMI SE. This implied that the excellent EMI SE for the composites was mainly originated from the well-developed segregated conductive networks of G-CNT hybrid. Notably, 15.0 wt% G-CNT hybrid loaded G1-CNT3/UH achieved the highest average EMI SE of 81.0 dB at a specimen thickness of 2.1 mm. The value showed 57.0 and 8.0% increase compared to that for the G/UH and CNT/UH ones, respectively. Such high EMI SE was far superior to previously reported value for graphite/polymer, and even higher than the results for CNT/polymer or graphene nanosheets (GNS)/polymer at similar filler loadings and composite thicknesses, as shown in Figure 8b and Table 3.4,52,53,58-64 For example, Huang et. al. reported a 15.0 wt% CNT/epoxy composite with EMI SE of 17.0 dB for a thickness of 2.0 mm.53 Hsiao et al. realized an enhanced EMI SE of 38 dB by covalently modifying GNS with 7.7 wt% GNS loading at 2.0 mm thickness.61 The extraordinary 17



EMI SE for G-CNT/UH can mainly result from the following reasons. First, the G-CNT hybrid in the segregated structure contribued to the formation of well-developed conducting channels and led to an improved electrical conductivity. Second, the interfaces between graphite, CNT and UHMWPE matrix were more multiplex compared to single graphite or CNT filled UHMWPE, which would cause much stronger interfacial polarization loss in an electromagnetic field. Third, the special 1D-2D structure (CNT-graphite) helped to extend the propagation path of electromagnetic wave and thus improving microwave absorption.65

Figure 8. (a) EMI SE of the 15.0 wt% filler loaded composites with various weight ration of graphite and CNT; (b) Comparison of EMI SE of the G1-CNT3/UH with other CNT or GNS-based shielding materials.

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Table 3. Average EMI SE in X-band frequency range for the G1-CNT3/UH and the reported CNT or GNS based composites. Composite

EMI SE

Specific EMI SE

conductivity (S/m) (mm)

(dB)

(dB mm-1)

G1-CNT3/UH

195.3

2.1

81.0

39

Present work

G1-CNT3/UH

195.3

0.5

31.8

64

Present work

CNT/PSa)

1.0

2.0

21

11

58

GNS/PMMAa)

10.0

3.4

25.0

7

59

CNT/BRa)

6 × 10-5

1.0

11.0

11

60

GNS/WPUa)

5.1

2.0

35

18

61

GNS/PUa)

7.3 × 10-4

3.0

21

7

42

CNT/PPa)

~

1.0

34

34

4

CNT/PTTa)

20

2.0

39

15

63

CNT/PUa)

79

2.5

41.6

17

64

GNS/PLAa)

7.0

1.5

14

9

52

CNT/Epoxy

20.0

2.0

17

9

53

a

Electrical

Thickness

Reference

PS, PMMA, BR, WPU, PU, PP, PTT and PLA are polystyrene, polymethyl methacrylate, butyl rubber,

waterborne polyurethane, polyurethane, polyproline, poly (trimethylene terephthalate) and polylactic acid, respectively.

In view of G1-CNT3/UH, the variation of the EMI SE with G-CNT hybrid loadings was shown in Figure 9a. The EMI SE presented a substantial increase with increasing G-CNT hybrid loadings. Interestingly, the incorporation of only 1.0 wt% G-CNT hybrid realized a satisfactory EMI SE of 21.3 dB at the frequency of 12.4 GHz, which was sufficient for the commercial applications as EMI shielding materials. With further increasing G-CNT hybrid to 2.0, 4.0 and 8.0 wt%, the average EMI SE increased to 19



29.4, 41.8 and 53.4 dB, respectively. The improved EMI SE should be attributed to the lower skin depth at higher G-CNT hybrid loading (Figure S4) (more detailed discussion on the skin depth was provide in Supporting Information). To clarify the EMI shielding mechanism of the G-CNT/UH, the EMI SE (SETotal), absorption loss (SEA) and reflection loss (SER) as a function of CNT content at the frequency of 8.2 GHz were presented in Figure 9b (detailed calculations for SEA and SER could be found in the our previously reported work37). It was clear that both SETotal and SEA exhibited a striking enhancement with increasing G-CNT hybrid loadings, but the changes in SER were negligible. For instance, the SETotal, SEA, SER of the composite with 15.0 wt% G-CNT hybrid were 81.7, 75.0, and 6.7 dB, respectively, indicating an absorption-dominant shielding mechanism for G-CNT/UH. The absorption-dominant shielding mechanism could be attributed to the specific heterogeneous segregated structure of conducting G-CNT based channels. The formation of the unique segregated structure provided sufficient interface area for internal multiple reflections of G-CNT hybrid layer between UHMWPE domains, thereby markedly enhancing the propagation pathways of the incident electromagnetic waves. The interfacial conductive G-CNT networks could convert the entered electromagnetic waves into heat effectively by the combination of conductive dissipation, multiple reflections and scattering. In brief, the interconnected G-CNT hybrid networks combined with the formation of segregated structure endowed the G-CNT/UH with high electromagnetic wave-absorbing ability and unprecedented EMI shielding performance.

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Figure 9. (a) EMI SE versus CNT loading as a function of frequency for the G1-CNT3/UH; (b) Comparison of the total EMI shielding effectiveness (SETotal), microwave absorption (SEA), and microwave reflection (SER) at the frequency of 8.2 GHz for the G1-CNT3/UH with various filler loadings. The EMI SE not only depended on the electrical conductivity but also the thickness of the shielding materials. As shown in Figure 10a, the increased specimen thickness gave rise to more striking EMI shielding performance, due to the increased conductive networks that interact with the incident electromagnetic microwaves. Amazingly, the average EMI SE of the G1-CNT3/UH at only 0.5 mm thickness reached to 31.8 dB, already meeting the requirement for commercial EMI shielding applications. In addition, the specific EMI SE (EMI SE divided by material thickness) of the G-CNT/UH was much higher than those of reported CNT and GNS based shielding materials. 4,52,53,58-64 These results further highlighted the advantage of G-CNT/UH as a highly efficient EMI shielding material. In addition, it should be noted that the SEA of the composite increases with the increase of thickness while SER remains almost constant (Figure 10b), demonstrating an adsorption-dominated shielding mechanism again.

21



Figure 10. (a) EMI SE for the 15.0 wt% G1-CNT3/UH with specimen thicknesses. (b) Comparison of SETotal, SEA, and SER at the frequency of 10.3 GHz. 4. CONCLUSION We have developed a highly efficient G-CNT/UH shielding material that combines the advantages of the segregated structure with synergistic effect of G-CNT hybrid in electrical and EMI shielding properties. An excellent electrical conductivity of 195.3 S m-1 and a superior EMI SE of 81.0 dB were obtained for the G1-CNT3/UH with 15.0 wt% of filler content, which is among the highest values, even for the reported shielding materials containing CNT or GNS. Our work shows great potentials for the facile fabrication of highly efficient performance EMI shielding materials in academic and actual applications.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 22


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(1) The digital photographs of the functional grinder. (2) The surface area of the granules and sheets. (3) XPS spectra of UHMWPE granules and UHMWPE sheets. (4) The digital photographs of the EMI shielding test equipment. (5) The skin depth (δ) of the G1-CNT3/UH composites. (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 51673134, 51421061, and 21704070), the Programme of Introducing Talents of Discipline to Universities (B13040), and the Science and Technology Department of Sichuan Province (Grant No. 2017GZ0412, 2018RZ0041) and the Fundamental Research Funds for the central Universities (2017SCU04A03, sklpme2017306, 2012017yjsy102).

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