polymer composite with extremely low graphene loading

Journal Pre-proof High-efficiency electromagnetic interference shielding realized in nacre-mimetic graphene/polymer composite with extremely low graphene loading Weiwei Gao, Nifang Zhao, Tian Yu, Jiabin Xi, Anran Mao, Mengqi Yuan, Hao Bai, Chao Gao PII:

S0008-6223(19)31066-8

DOI:

https://doi.org/10.1016/j.carbon.2019.10.051

Reference:

CARBON 14713

To appear in:

Carbon

Received Date: 4 September 2019 Revised Date:

18 October 2019

Accepted Date: 19 October 2019

Please cite this article as: W. Gao, N. Zhao, T. Yu, J. Xi, A. Mao, M. Yuan, H. Bai, C. Gao, Highefficiency electromagnetic interference shielding realized in nacre-mimetic graphene/polymer composite with extremely low graphene loading, Carbon (2019), doi: https://doi.org/10.1016/j.carbon.2019.10.051. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier Ltd.

Graphical Abstract

High-performance EMI shielding was realized in nacre-mimetic graphene/polymer composites with extremely low graphene loading. With such a nacre-mimetic, highly aligned network, the flexible composites exhibit anisotropic conductivities, mechanical properties and therefore remarkable EMI shielding effectiveness. Specifically, the biomimetic composites with 0.42 wt% conductive filler has an enhanced EMI shielding effectiveness of ~65 dB, which is comparable to the copper foil.

High-efficiency Electromagnetic Interference Shielding Realized in NacreMimetic Graphene/Polymer Composite with Extremely Low Graphene Loading Weiwei Gao1*, Nifang Zhao2, Tian Yu1, Jiabin Xi1, Anran Mao2, Mengqi Yuan2, Hao Bai2 and Chao Gao1* 1

MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of

Polymer Science and Engineering, Key Laboratory of Adsorption and Separation Materials & Technologies of Zhejiang Province, Zhejiang University, Hangzhou 310027, China 2

State Key Laboratory of Chemical Engineering, College of Chemical and Biological

Engineering, Zhejiang University, Hangzhou 310027, China *Corresponding authors. E-mail: [email protected] (Weiwei Gao); [email protected] (Chao Gao)

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Abstract: Electromagnetic interference (EMI) shielding performance of composites are usually limited by their electrical conductivity and permeability, which largely depend on the conductive filler content, aspect ratio, magenetic permeability, etc. Higher filler content usually leads to high cost, poor dispersion and easy agglomeration, making the polymer composites mechanically brittle and difficult to process. Therefore, it is highly desirable to develop composite with low conductive filler content while maintaining its high EMI shielding performance. Here, in our work a high-performance EMI shielding was realized in nacre-mimetic graphene/polymer composites with extremely low graphene loading. A nacre-mimetic 3D conductive graphene network with biaxial aligned lamellar structure was prepared by a unique bidirectional freezing technique. With such a nacre-mimetic, highly aligned network, our graphene/polymer composites exhibit anisotropic conductivities, mechanical properties and therefore remarkable EMI shielding effectiveness at an extremely low graphene content. Specifically, the biomimetic composites with 0.42 wt% graphene content shows an enhanced EMI shielding effectiveness of ~65 dB after annealing the graphene aerogels at 2500 ℃, which is comparable to the copper foil. More remarkably, as the composite is low in density, its specific shielding effectiveness is even higher than that of metal foils and solid materials with high conductive filler content.

Keywords:

graphene, nacre-mimetic, anisotropic, biaxial aligned lamellar structure,

electromagnetic interference shielding

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1. Introduction High performance electromagnetic interference (EMI) shielding materials are of great importance for many strategic fields including expensive electronic devices and systems, personal protection, aircraft and so on [1-5]. Superior than traditional metal-based EMI shielding materials [6] which are usually limited by high mass density, poor flexibility and undesirable corrosion susceptibility, electrically conductive polymer composites have recently aroused much attention [2,4,7-17] for their low density, excellent flexibility, easy processability and low cost, which are all crucial factors for the practical applications of EMI shielding materials required in the areas such as aerospace, mobile phones, and most recently, flexible and soft electronics [2,18]. As one of the most attractive carbon-based materials, graphene materials attract many attentions because of their particular electromagnetic functions [19,20-22], especially graphene aerogel shows great potential as the conductive filler in the EMI shielding materials due to its ultralow density, super-elasticity, chemical stability and so on [23-28]. The EMI shielding performance of conductive polymer composites largely rely on the conductivity, aspect ratio, magnetic permeability, conductive filler content and its connectivity [3,8,12-14,29,30]. In a regular polymer composite, conductive fillers are usually covered with polymer layers and randomly distributed/oriented in the polymer matrix. Poor connectivity between the filler particles, which forming interfacial impedance mismatch and high electrical percolation, largely reduces the conductivity of the composites. Therefore, high filler content is always required to achieve high conductivity, based on the electron percolation between adjacent filler particles. For example, it has been reported that reduced graphene oxide (rGO)/epoxy composites with high filler content (15 wt%) show an EMI shielding effectiveness (SE) of 21 decibels (dB) with a

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conductivity of ~10 S m-1 [11]. 15 wt% carbon nanofiber/polystyrene (PS) foam has an EMI SE of ~19 dB in the X-band frequency region (8.2~12.4 GHz) [8]. However, higher filler content usually leads to high cost, poor dispersion and easy agglomeration, making the polymer composites mechanically brittle and difficult to process [11]. To tackle these problems, nanocomposites with aligned conductive fillers have been developed, where the key is to design a nacre-mimetic 3D conductive network with highly aligned structure. The well-built 3D conductive network is beneficial for easy processability and low cost production of materials. Kim et al [7]. reported a highly aligned 2 wt% graphene/polymer nanocomposites prepared by the aqueous casting method, exhibiting high performance EMI shielding efficiency of 38 dB. Among other methods, ice-templating is an effective technique to build highly ordered 3D structure. As an example, unidirectional freezing has been applied to fabricate anisotropic porous MWCNT/WPU composites with giant EMI SE of 20~50 dB depending on the filler density of 20~126 mg cm-3 [9]. However, unidirectional freezing only allows for conductive fillers to align in the freezing direction. Instead, a bidirectional freezing technique has been developed to make functional materials with long-range aligned lamellar structure [31-38]. Biaxially aligned lamellar structure with controllable interlayer spacing and layer thickness has giant aspect ratio and is therefore beneficial for EMI shielding applications. Here, a high-performance EMI shielding polymer composite with a nacre-mimetic 3D conductive network was realized through fabricating a graphene oxide (GO) aerogel with biaxially aligned lamellar structure by bidirectional freezing [31,32], followed by thermal reduction and Polydimethylsiloxane (PDMS) infiltration. Graphene has been chosen as the conductive filler in our polymer composites for its high intrinsic conductivity, excellent mechanical properties, high aspect ratio, lightweight, flexibility and so on [39]. By assembling

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graphene into such nacre-mimetic highly aligned 3D conductive network, the as-prepared flexible graphene/PDMS composites exhibit anisotropic conductivities, mechanical properties and remarkable EMI shielding effectiveness at an extremely low graphene content. The aligned composites with 0.4 wt% conductive filler has an excellent EMI SE of ~42 dB (with a conductivity of ~0.32 S m-1) along the direction that perpendicular to freezing direction and rapidly decreased SE of ~ 15 dB along the freezing direction. After annealing aerogels at 2500 ℃ high temperature, the composites show an enhanced EMI SE of ~65 dB, and a corresponding conductivity of ~0.5 S m-1. With ultralow 0.15 wt% conductive filler, the composite, treated at conventional 800 ℃, displays an EMI SE of ~25 dB, which approaches the target value of the EMI shielding effectiveness needed for commercial around 20 dB [40,41]. High efficiency EMI shielding was realized in our anisotropic graphene/PDMS composites by structural control at extremely low graphene concentration. By building a more efficient nacre-mimetic 3D conductive network, our research provides an effective approach to enhance the EMI shielding performance of polymer based composites, making them both economically and functionally promising for civil-military applications.

2. Experimental section 2.1. Preparation of graphene oxide precursor suspensions Graphene oxide (GO) suspension was purchased from Hangzhou Gaoxi Technology Company. The concentrated GO suspensions of 16 mg mL-1 were obtained by centrifugation. Aqueous Poly (vinyl alcohol) (PVA, Mw = 20 500, 99%, Aladdin Chemistry Co., Ltd, China) solution (10 mg mL-1) was added to the GO suspension. The ratio of GO: PVA was kept at 5 : 3

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in weight. Some typical GO concentrations were selected, including 10 mg mL-1, 5 mg mL-1, 3 mg mL-1, 2.5 mg mL-1 and 2 mg mL-1. 2.2. Preparation of anisotropic graphene aerogels and graphene/PDMS composites GO hydrogels were fabricated via bidirectional freeze casting technique. GO dispersions (10 mg mL-1, 5 mg mL-1, 3 mg mL-1, 2.5 mg mL-1 and 2 mg mL-1) were poured into a square tube and then frozen at four typical temperature of -196 ℃ (Liquid Nitrogen, LN), -110 ℃, -90 ℃, and -50 ℃. After the precursor suspension was frozen entirely, the sample was tapped out of the mould and freeze-dried for more than 48 h at -80 ℃ with Freeze dryer under 0.05 mbar pressure (Labconco 8811, Kansas City, USA). Finally, the graphene aerogels were reduced by thermal treatment at 800 ℃ for 2 h in a H2/Ar mixed gas. Polydimethylsiloxane (PDMS, Sylgard 184) were combined with graphene aerogel by selfassembly at atmosphere environment. The composites were uniform, and initial porous and lamellar structure are visible, indicating the intact graphene framework structure in composites. 2.3. Characterization Scanning electron microscopy (SEM) images were obtained by Hitachi S-3700 at an acceleration voltage of 10 kV in secondary electron mode. Mechanical properties were investigated in the tensile mode by an electronic universal testing machine (UTM2102, ShenZhen Suns Technology Stock Co., Ltd, China) with gauge length of 10 mm, and loading rate of 0.5 mm min-1. All the samples were cut into strips with a length of 30 mm and a width of 3-4 mm. At least five samples were tested for each experimental condition to obtain statistically reliable values. Electrical conductivities of composites were measured by Keithley 2632 (Keithley, USA) under a current range of 2 mA. At least five samples were tested for each condition to obtain statistically reliable values.

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Electromagnetic interference (EMI) shielding measurements were carried out using ZVA 40 vector network analyzer (Rohde & Schwarz, Germany) in X-band frequency range (8.2-12.4 GHz). The composites were cut into a rectangular shape with the size of 22.86 mm × 10.16 mm × 5 mm for EMI measurement. 3. Results and discussion

Fig. 1. (a-d) Schematic illustration of fabrication process of anisotropic graphene/PDMS composite. (a) Graphene oxide (GO) aerogels were fabricated by bidirectional freeze casting technique. The inset is schematic of dispersed GO sheets. (b) GO aerogels with lameller structure were formed after low temerature freezing dry at 10 Pa. (c) Graphene aerogels were achieved by further thermal reduction of GO aerogel. (d) Graphene/PDMS composites were finally obtained by infiltrating PDMS into graphene aerogel. Digital photos of GO aerogel (e), graphene aerogel (f), and flexible graphene/PDMS composite (g), corresponding to the fabrication process in (b-d). SEM images of cross section parallel to copper

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plates of graphene oxide aerogel (h) and corresponding graphene/PDMS composites (i), both of them showing long-range ordered lameller structure. (j) High electeomagnetic interference (EMI) shielding performance of 0.42 wt% graphene/PDMS composites. (k) Proposed EMI shielding mechanism.

Fig. 1 illustrates the fabrication process of our biaxially aligned graphene/PDMS composites. GO aerogel was firstly fabricated by a bidirectional freezing technique (Fig. 1a). Briefly, a suspension of well-dispersed GO sheets was firstly filled into a plastic mold on a cold copper plate with a PDMS wedge as a spacer. When cooling down the copper plate, the PDMS helped in generating both vertical and horizontal temperature gradients. As such, ice crystals grew into a biaxially lamellar pattern with GO sheets expelled and assembled to replicate the ice morphology. In contrary to the conventional approach for the fabrication of porous aerogels which could only achieve short-range ordered structure [20], the bidirectional freezing technique has been proven to be very effective for fabricating long-range aligned lamellar structure, capable of controlling over multiscale architectures including lamellar thickness, interlayer spacing and so on [31], which is highly related to the EMI shielding performance of the composites. After freeze-drying, a GO aerogel with biaxially aligned lamellar structure was obtained (Fig. 1b and 1e). The aerogel changed from brown to black color after thermal reduction (Fig. 1c and 1f). The flexible and bendable graphene/PDMS composites were finally made by infiltrating PDMS into the reduced graphene aerogel (Fig. 1d and 1g). Graphene aerogel exhibits nacre-mimetic long-range ordered lamellar structure (Fig. 1h) and it is noteworthy that the aerogel with aligned structure was not damaged during infiltration (Fig. 1i and S1). As illustrated in Fig. 1k, such biaxially aligned lamellar structure indicates many effective interfaces were formed inside the composites, which provides the possibility of multiple interlayer reflection. It could effectively weaken secondary reflections of electromagnetic waves, and enhance the effective absorption of electromagnetic waves [42]. Therefore, the composite shows an excellent high efficiency EMI shielding

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performance in the direction perpendicular to the layers owing to the ‘multiple interlayer reflection’. The EMI SE is as high as ~65 dB for an extremely low graphene loading of ~0.42 wt%, which is higher than those of other carbon-based foam materials, and is comparable with that of solid materials with high conductive filler content or metal materials [41,43,44]. According to Simon formula [45,46] originated from Maxwell’s equations and previous reports, EMI shielding performance mainly relies on its electrical conductivity of non-magnetic materials. So far there has few evidence that structural control of materials can play a key role in the EMI shielding. As reported in literature [25], the microstructure of the graphene aerogels can be effectively adjusted by changing the fabrication parameters including freezing temperature, dispersion concentration/viscosity and so on. In order to reduce the conductivity effect on the EMI, here a series of graphene aerogels with different interlayer spacing were fabricated by bidirectional freezing casting of dispersion with same concentration at different temperature. The microstructure of the graphene aerogels fabricated at -196 ℃ (Liquid nitrogen, LN), -110 ℃, and -50 ℃ were shown by the Scanning Electron Microscopic (SEM) images in Fig. 2a-c, respectively. All the aerogels were prepared with 5 mg mL-1 GO : 3 mg mL-1 PVA dispersion, and the corresponding apparent density of 3D graphene aerogel is ~4.9 mg cm-3 and ~4.2 mg cm-3 after 800 ℃ and 2500 ℃ thermal treatment, respectively. SEM images were taken on the cross-section parallel to the copper plates. All the aerogels have shown obvious long-range aligned lamellar structure. With increasing freezing temperature, the interlayer spacing of the lamellar structure increases from ~50 to ~150 µm together with more dendrites and interlayer bridges, and the estimated layer density (Layer number per mm) has a range of 20~7 layers mm-1 (Fig. 2d).

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Composites were finally achieved after thermal reduction of aerogels and PDMS infiltration. The lamellar structures were almost reserved in the composites, and the aerogels didn’t collapse during infiltration, also demonstrated in Fig. S1 in the Supporting Information.

Fig. 2. (a-c) SEM images of cross section parallel to copper plates of graphene oxide aerogels at the freezing temperature of LN (a), -110 ℃ (b), and -50 ℃ (c), respectively. Graphene oxide aerogels showing long-range aligned lamellar structure. The freezing temperature has a great effect on the lamellar structure of aerogels, obvious increasement in interlayer spacing, and more dendrites and interlayer bridges with increasing freezing temperature. (d) Summarized interlayer spacing/layer density and (e) conductivity as a function of freezing temperature. (f) EMI shielding performance of graphene/PDMS composite frozen at different temperature. (g) Summarized EMI shielding effectiveness as a function of freezing temperature. An enhanced EMI shielding effectiveness corresponding to the composite with more lamellar interfaces.

Owing to the nacre-mimetic highly-aligned architecture of graphene network, the composites are still conductive at an extremely low conductive filler content (~0.42 wt% in our case), as

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shown in Fig. 2e. With increasing freezing temperature, the conductivity of the graphene/PDMS composites, treated at 800 ℃, show a slightly change in the range of 0.32~0.39 S m-1. Note that conductivity was measured in the direction parallel to the lamellar layers. For the aerogel prepared at LN temperature (Fig. 2a), dendrites are not long enough to connect adjacent lamellar layers, resulting in lower conductivity than those with longer dendrites (Fig. 2b-c). As expected, the composites show an excellent EMI shielding performance in Fig. 2f-g. In the frequency ranging from 8.2 to 12.4 GHz, the composites treated at 800 ℃ display an enhanced EMI SE from ~31.5 dB to ~41 dB as increasing the layer density from ~7 to ~20 layers mm-1. Although all the composites have same amount of conductive fillers (0.49 wt%) and similar electrical conductivities, they exhibit different EMI shielding performance, which demonstrates the importance of the architecture of the 3D conductive network. This can be attributed to the ‘multiple interlayer reflection’ resulted from the nacre-mimetic long-range aligned lamellar structure of the 3D conductive network embedded within the composites. High layer density indicates the more lamellar interfaces formed inside the composite per unit volume, which could effectively decrease secondary reflections and enhance the effective absorption of electromagnetic waves [42]. Besides, nacre-mimetic aligned lamellar structure is beneficial to build conductive networks, which is good for electrons to absorb more electromagnetic energy to migrate in interlayer channel, and then convert electromagnetic energy to heat energy [47]. More lamellar interfaces lead to more energy loss when electromagnetic waves pass through the composites. Therefore, with more layers, the composite prepared from LN frozen aerogel shows the highest EMI shielding efficiency. At the same time, in the sense that structure of 3D conductive network has a significant impact on the EMI shielding performance of the final composites.

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Besides, aerogels show porous structure as increasing GO suspension to 10 mg mL-1 (Fig. S2, Supporting Information) because high viscosity of dispersion hinders ice crystals to grow preferentially along the wedge under dual temperature gradients leading to randomly growth of ice crystals, thus porous structure. The pore size is controllable and has a range of 12 µm ~55 µm with increasing freezing temperature from LN to -50 ℃.

Fig. 3. Morphologies of graphene/PDMS composites (LN-0.49 wt%) in the XY plane (a), ZX plane (b), and YZ plane (c). Inset in (a) is schematic of anisotropic composite structure. (d-f) Magnified images of composites corresponding to that of composites in (a-c). Uniform composite seen in YZ plane. Visible lamellar structured composite in both XY and ZX planes. (g) Anisotropic mechanical property, (h) anisotropic conductivity, and (i) anisotropic EMI shielding effectiveness of graphene/PDMS composite.

The biaxially aligned lamellar structure makes our graphene/PDMS composite a highly anisotropic material. As shown by the SEM images in Fig. 3a-c, the graphene/PDMS composite made from LN frozen aerogels (LN, 5 mg mL-1 GO : 3 mg mL-1 PVA dispersion) exhibit

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anisotropic cross-sectional structure in both the through-plane (XY and ZX) and in-plane (YZ) directions, which perpendicular and parallel to ice crystal growth directions, respectively. Fig. 3d-f are the magnified images corresponding to those of Fig. 3a-c. After PDMS infiltration, long range aligned lamellar structures were well retained in both XY and ZX planes of composites, and uniform structure in YZ plane. Owing to such anisotropic structure, the composite has anisotropic mechanical property, conductivity and EMI shielding performance in Fig. 3g-i. The tensile properties of composites in XY, ZX, and YZ planes are compared together with that of pure PDMS. As shown by the typical stress-strain curves in Fig. 3g, graphene largely enhanced the modulus of the composite in all three planes, comparing with that of pure PDMS. Moreover, the composite shows obvious anisotropy in both the through-plane (XY and ZX) and in-plane direction (YZ), illustrated by the inset of Fig. 3a, the strengths and elongations are ~0.3 MPa and ~142% in XY plane, ~0.25 MPa and ~126% for ZX plane, and ~0.084 MPa and ~63% for YZ plane, respectively. Besides mechanical properties, enhancement and anisotropy were also found for composites’ conductivity. With an extremely low filler content (0.49 wt%), the graphene/PDMS composite has an obviously enhanced conductivity ranging from ~0.32 to ~0.007 S m-1, i.e., 3.2 х 1013 - 7 х 1011 times of pure PDMS (10-14 S m-1). Moreover, the conductivities measured in ZX and XY planes are ~0.12 and ~0.32 S m-1, respectively, which are 1-2 orders of magnitude higher than the conductivity in YZ plane. Most importantly, the composite also has anisotropic EMI shielding performance with ~42 dB in the YZ plane as at least 3 times of that for ZX (~16 dB) and XY (~13 dB) planes in Fig. 3i. It is noteworthy that, by building such a well-ordered structure, the conductivity and EMI shielding performance has been dramatically enhanced at an extremely low filler content, which is beneficial for developing lowcost, easy processing and flexible polymer based EMI shielding materials.

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Fig. 4. Conductive filler concentration effect on the EMI shielding effectiveness of graphene/PDMS -1

composites. (a-c) SEM images of cross-section of graphene oxide (GO) aerogels with 10 mg mL (a), 5 -1

-1

mg mL (b), and 3 mg mL (c). Aerogels showing isotropic porous structure (a) and long range ordered lamellar structure (b-c) while reducing GO content. (d-f) Morphologies of graphene/PDMS composites corresponding to that of aerogels in (a-c). (g) EMI shielding effectiveness as a function of frequency for the composites with different conductive filler concentration at LN freezing temperature. (h) Sumarized EMI shielding effectiveness and (i) conductivity as a function of GO concentration.

As filler content is important for EMI shielding performance of the composites, we further fabricated a series of graphene/PDMS composites starting from 10, 5, 3, 2.5, and 2 mg mL-1 GO in aqueous suspension with the ratio of GO : PVA kept at 5 : 3, respectively. All the aerogels were frozen at LN temperature. Fig. 4a-c show the structural variation of the aerogels changing from an isotropic random porous (Fig. 4a) to dendritic lamellar structures (Fig. 4b-c) with decreasing GO concentration. At high GO concentration, the high viscosity of dispersion hinders

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ice crystals to grow into a lamellar pattern, instead, resulting in an isotropic random porous structure in Fig. 4a. With decreasing GO concentration, ice crystals grow preferentially into biaxially aligned lamellar layers with more layers and dendrites in Fig. 4b-c. The lamellar structure can be obtained as low as 2 mg mL-1 GO. Below this concentration, aerogel will collapse after thermal reduction. After PDMS infiltration, porous structure is visible in composite with high graphene content (Fig. 4d), and lamellar structure in that with low graphene content (Fig. 4e-f). As expected, the composite conductivity decrease from ~3.41 to ~0.03 S m-1 with GO concentration decreasing from 10 to 2 mg mL-1. As reported in literature [8], high conductive filler content is required to achieve high conductivity and EMI shielding performance. However, in Fig. 4g-h, it’s found that high performance EMI SE of ~42 dB was observed in 0.49 wt% graphene/PDMS composite, which is higher than the random porous graphene/PDMS composite, about ~32 dB with 1 wt% of graphene filler. Therefore, EMI shielding performance depends on both conductivity and aligned structure of conductive network. The nacre-mimetic lamellar structure exhibits more effective EMI shielding effectiveness than that of random porous structure. Such nacre-mimetic biaxially aligned lamellar structure provides multilayer interfaces, leading to more reflection loss during the electromagnetic wave pass. In our composites, even the filler content is as low as 0.2 wt%, it still reaches an EMI shielding efficiency of ∼20 dB, which is comparable with commercial EMI materials [40,41]. All the EMI shielding measurements were accomplished at the frequency ranging from 8.2 to 12.4 GHz.

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Fig. 5. Comparison of EMI shielding effectiveness among graphene-based polymer composites, films and our long range aligned lamellar structured graphene/PDMS composites. (a) EMI shielding effectiveness versus conductive filler concentration of different materials. (b) EMI shielding effectiveness versus conductivities of different materials. Ultralow percolation threshold is achieved in our long-range aligned lamellar structured composite by bidirectional freezing casting technique. Specific long-range aligned lamellar structure contributes effective multiple interlayer reflection, and more lamellar interfaces result in more energy loss when electromagnetic wave passes through the composites. Finally High performance EMI shielding realized in our composites at extremely low graphene concentration by structural control.

In Fig. 5, EMI shielding effectiveness of our graphene/PDMS composite was compared with those of other reported EMI shielding materials with high conductive fillers or copper foils (SE=70 dB; SSE=7.8 dBcm3g-1) [2,6,7,9-11,43,44,48-54]. Moreover, Specific shielding effectiveness (SSE) is higher than that of solid materials with high mlconductive filler concentration. Generally, highly efficient EMI shielding requires higher conductive filler concentration, which leads to high cost, poor dispersion and easy agglomeration. However, high performance EMI shielding effectiveness is realized in the long-range lamellar structured graphene/PDMS composites at an extremely low graphene concentration. Especially, for the ~0.42 wt% graphene/PDMS composite treated at 2500 ℃, EMI SE and SSE reach ~65 dB and ~100 dB cm2 g-1 respectively. Ultralow percolation threshold is achieved in our long-range aligned composite by structural control. Moreover, specific long-range aligned lamellar structure

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contributes multiple interlayer reflection, and more lamellar interfaces result in more energy loss when electromagnetic wave passes through the composites.

3. Conclusion

In conclusion, we have realized high performance EMI shielding graphene/PDMS composites at an extremely low graphene content, such as ~65 dB in the ~0.42 wt% graphene/PDMS composites. The long range lamellar structured graphene network with ultralow percolation threshold was fabricated by a bidirectional freezing technique. By infiltrating PDMS into nacremimetic highly aligned 3D graphene network, the flexible graphene/PDMS composites exhibit anisotropic conductivities, mechanical properties and remarkable EMI shielding effectiveness at an extremely low graphene content. Ultralow percolation threshold is achieved in the long-range aligned composite by structural control. Moreover, specific long-range aligned lamellar structure contributes multiple interlayer reflection, and more lamellar interfaces result in more energy loss when electromagnetic wave passes through the composites. By building a more efficient nacremimetic 3D conductive network, our research provides an effective approach to enhance the EMI shielding effectiveness of polymer based composites, making them both economically and functionally promising for civil-military applications.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Nos. 51603183, 51603182, 21674098, 51873191, 51722306, 21625402, 51533008), the Fundamental Research Funds for the Central Universities (No. 2018XZZX002-15), the National Key R&D Program of

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China (No. 2017YFC1103900, 2016YFA0200200) and the State Key Laboratory of Chemical Engineering (No. SKL-ChE-16T02).

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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: