Lightweight, thermally insulating and stiff carbon honeycomb-induced graphene composite foams with a horizontal laminated structure for electromagnetic interference shielding

Lightweight, thermally insulating and stiff carbon honeycomb-induced graphene composite foams with a horizontal laminated structure for electromagnetic interference shielding

Accepted Manuscript Lightweight, thermally insulating and stiff carbon honeycomb-induced graphene composite foams with a horizontal laminated structur...

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Accepted Manuscript Lightweight, thermally insulating and stiff carbon honeycomb-induced graphene composite foams with a horizontal laminated structure for electromagnetic interference shielding Ye Yuan, Liyang Liu, Minglong Yang, Tieliang Zhang, Fan Xu, Zaishan Lin, Yujie Ding, Chunhui Wang, Jianjun Li, Weilong Yin, Qingyu Peng, Xiaodong He, Yibin Li PII:

S0008-6223(17)30742-X

DOI:

10.1016/j.carbon.2017.07.060

Reference:

CARBON 12226

To appear in:

Carbon

Received Date: 9 June 2017 Revised Date:

11 July 2017

Accepted Date: 17 July 2017

Please cite this article as: Y. Yuan, L. Liu, M. Yang, T. Zhang, F. Xu, Z. Lin, Y. Ding, C. Wang, J. Li, W. Yin, Q. Peng, X. He, Y. Li, Lightweight, thermally insulating and stiff carbon honeycomb-induced graphene composite foams with a horizontal laminated structure for electromagnetic interference shielding, Carbon (2017), doi: 10.1016/j.carbon.2017.07.060. 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.

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Graphical Abstract

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Lightweight, Thermally Insulating and Stiff Carbon Honeycomb-Induced

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Graphene Composite Foams with a Horizontal Laminated Structure for

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Electromagnetic Interference Shielding

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Lightweight, Thermally

Insulating and

Stiff

Carbon

Honeycomb-Induced Graphene Composite Foams with a Horizontal

Laminated

Structure

for

Electromagnetic

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Interference Shielding Ye Yuan a, Liyang Liu b, Minglong Yang a, Tieliang Zhang b, Fan Xu a, Zaishan Lin a, Yujie Ding a, Chunhui Wang a, Jianjun Li a, Weilong Yin a, Qingyu Peng a, Xiaodong

Center for Composite Materials and Structures, Harbin Institute of Technology,

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a

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He a, and Yibin Li a, *

Harbin 150080, People’s Republic of China b

Shenyang Aircraft Design Institute, Aviation Industry Corporation of China,

Shenyang 110035, People’s Republic of China

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*Corresponding author. E-mail: [email protected] (Yibin Li)

Abstract: In this study, lightweight but stiff carbon monolith with unique laminated

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inner structure for high electromagnetic interference (EMI) shielding is designed and

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prepared. The carbon monolith is consisting of carbon honeycomb structure which was filled with a horizontal laminated reduced graphene foam structure that is perpendicular to the cell wall. The stiff carbon honeycomb (CH) structure not only acts as a conductive and load-bearing framework, but also as the induced source for the growth of ice crystals to make a highly aligned laminated structure in the reduced graphene oxide (rGO) foam, by utilizing the big difference in thermal conductivity between air and the carbon cell wall. Consequently, a EMI shielding effectiveness of

ACCEPTED MANUSCRIPT around 36~43 dB and remarkable specific EMI shielding effectiveness of 688.5 dB·cm3·g-1 in the X-band is achieved. More importantly, the addition of CH structure significantly increases the compressive stress of rGO foam to 2.02 MPa, almost which

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is 6,760 times higher than common rGO foam. Moreover, the carbon monolith shows a thermal conductivity of 0.057 W/(m·K) and exhibits good flame retardancy. These results indicate that the carbon monolith is an ideal component for high-performance

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EMI shielding material, which has great potential applications in aviation and the

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aerospace industry. 1. Introduction

In recent years, considerable efforts have been made to develop high performance EMI shielding materials due to the rapid progress of modern electronics. Being

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lightweight, strong, multifunctional and easy-to-fabricate are essential technical requirements for practical EMI shielding applications, especially in areas of aircraft

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and aerospace electronic devices. For example, navigation devices used in spacecraft require EMI shielding and thermostabilized working environment, as well as

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lightweight, strong but free-standing structures to avoid the devices being damaged. Nevertheless, searching for EMI shielding materials with such features is still a great challenge.1-7 Metals show a good electromagnetic wave attenuation performance for their superior electrical conductivities. However, light weight is a necessity for aviation and aerospace applications. Therefore, metals (such as copper) possessing high EMI shielding effectiveness values but with low specific EMI shielding effectiveness are less useful (considering a material’s density).8 Although metallic

ACCEPTED MANUSCRIPT materials can be coated on the surfaces of lightweight porous materials by using electroplating or vacuum deposition for EMI shielding, these materials are commonly associated with problems such as poor chemical resistance, oxidation, corrosion and

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difficulty in processing.9-11 As a promising EMI shielding substitute for traditional metals, polymer composites with features such as low density, excellent corrosion resistance, tunable electrical

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conductivity and ease of processing have attracted much attention in recent years.12-17

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The nanofillers in polymer composites, such as carbon black, carbon nanotubes (CNTs) and graphene, are expected to exhibit high efficiency electromagnetic wave attenuations because of their unique structure and rich electronic properties.18-23 However, it is worth noting that several critical disadvantages have limited the

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development of polymeric EMI shielding composites. First, the intrinsic electrically insulating features of most polymers can dramatically suppress the electrical conductivity of the EMI shielding composites. Thus, high nanofiller loading is needed

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to improve EMI shielding effectiveness to form a continuous percolating network.

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This can decrease the specific EMI shielding effectiveness and the mechanical properties because of the aggregation problem. Second, for most conventional polymers, their thermal stability, poor flame retardancy and short lifetime may restrict their use as EMI shielding materials for uses in harsh environments, especially regarding high temperature. Porous materials are important as electromagnetic wave attenuating, thermally insulating and microelectronic materials in aviation and space industries. However,

ACCEPTED MANUSCRIPT they nonetheless have some general disadvantages. For example, porous organic and metal-organic solids have enormous structural diversity but do not exhibit the high temperature stability of some purely metals or inorganic materials. Porous inorganic

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materials, such as fibrous insulation, are usually electron insulating and soft. Thus, it is difficult to find a porous EMI shielding material that integrated electron conductivity, thermal stability and stiffness to meet the actual engineering

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requirements.24,25 There are hence many motivations to explore new EMI shielding

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materials. Macroscopic porous graphene foams made up of graphene sheets have been developed and found to possess interesting and promising applications. In large-scales, in situ formation of graphene porous networks in an aqueous graphene oxide (GO) precursor solution has been considered as a promising approach achieving lightweight

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graphene products.26-31 The porous and conductive 3D graphene monolith is a highly desirable candidate for lightweight EMI shielding and thermally insulating materials.5 However, graphene foams always suffer from poor mechanical strength due to the

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weak crosslinks between adjacent graphene sheets, which can significantly limit their

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practical applications.32-33 Moreover, graphene (or rGO) foams derived from a freeze drying method are also associated with poor electrical conductivity and uncontrollable pore shapes, which may degrade their EMI shielding abilities. In this work, a convenient strategy is presented for preparation of lightweight and

stiff hybrid graphene foam with a unique aligned porous structure for EMI shielding applications, using a freeze-drying method with a carbonized honeycomb as the conductive framework and load-bearing structure, as well as the induced source for

ACCEPTED MANUSCRIPT the laminated porous rGO structure. Interestingly, the lightweight and porous carbon architecture exhibits excellent EMI shielding effectiveness, mechanical and thermal stability, all of which have a great potential in aviation and aerospace applications.

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2. Experimental Section 2.1 Materials.

The graphene oxide suspension was purchased from C6G6 Technology Co., Ltd,

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China. The Nomex honeycomb was purchased from Aramicore Composite Co., Ltd.

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All chemicals were of analytical grade and were used as received. All water used was deionized (DI) water (18.2 MΩ, Milli-Q, Millipore Co.). 2.2 Preparation of the Carbon Monolith.

In a typical process, a CH structure is derived from a Nomex honeycomb. The

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honeycomb is mainly consisted of poly (m-phenylene isophthalamide) (PMIA) short fibers and phenolic resin. After drying at 60

for 24 hours in an oven, the

honeycomb was taken out into a tube furnace in an Argon gas environment for direct

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carbonization at 1000 oC for 1 hour. The CH structure was then dipped into a

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graphene oxide suspension until all of the cells were fully filled in a plastic culture dish. The graphene oxide suspension was purchased from C6G6 Technology Co., Ltd, China, whose concentration was 24.3 mg/cm3. The suspension was diluted to 5 mg/cm3, 10 mg/cm3 and 20 mg/cm3 by long time stirring for further use. The size and size distributions of graphene oxide sheets can refer to the report from Prof. Gao’ s group.26 Then, the culture dish was put into a bubble chamber ensure a slow temperature decrease as well as to avoid anisotropy when freezing. Next, the bubble

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) for 10 hours after they have been completely

frozen. Samples were then dried by a vacuum freeze dryer for 4 days until completely dried. Finally, the monolith was reduced by superfluous hydrazine monohydrate vapor

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(hydrazine monohydrate was purchased from Aladdin, ≥98%) at 90 oC for 24 hours in

using the same method as the CH-rGO foams. 2.3 Characterization and Measurements.

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a glass desiccator, obtaining the CH-rGO foam.26 Pure rGO foams were prepared

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The surface morphology of the CH-rGO foam was characterized by field-emission scanning electron microscopy (FESEM) (Carel Zeiss, supra55). Raman spectra were obtained with a Lab RAM HR800 from JY Horiba. The carbon (C), nitrogen (N) and oxygen (O) contents of the samples were analyzed using X-ray photoelectron

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spectroscopy (XPS) (VG Scientific ESCALAB Mark II spectrometer). Element analysis of the samples was also measured by EDS (Oxford Instruments). EMI shielding effectiveness was measured by a Vector Network Analyzer (Agilent

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Technologies N5227A, USA). The test samples were carefully cut into 22.86 ×10.16

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mm2 strips to fit the specific waveguide sample holders (8~12 GHz). Thermal conductivity was measured using TPS 2500S from Hot Disk at room temperature (24oC) via the steady state method. Samples were prepared by cutting them into cubes with dimensions of 30 mm×30 mm×10 mm. The electrical conductivity of the cell wall of CH was tested using PARSTAT 4000 (Princeton Applied Research). Specimens were cut into 30 mm×3 mm×3 mm strips. During the tests, two silver wires were stuck to both of the small sides of strip by elargol to connect the

ACCEPTED MANUSCRIPT instrument. 3. Results and discussion Fig. 1 describes the entire procedure for preparing the CH-rGO foam structure,

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which starts from a Nomex honeycomb. After heat treatment, the carbonized honeycomb showed no apparent difference from the original one, except that the color turned from brown to a black metallic luster with a 10% volume shrinkage. The side

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length of the cell wall is measured to be around 3 mm. Two reactions took place

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during the carbonization procedure, that is, dehydration and carbonization. The CH structure was then subjected to GO suspension after completely filling up the cells in a plastic culture dish. In order to obtain an accurate result in various tests, the freezing process needs to be slow and homogenous to avoid large pores throughout some of

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the thin samples. Thus, the culture dish was put into a bubble chamber to ensure that the temperature dropped slowly as well as to avoid anisotropy while freezing. Then, the bubble chamber was put in a freezer (-15

) for 10 hours until completely frozen.

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After freeze drying, a CH-GO monolith with hierarchical pore structure was obtained.

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Finally, superfluous hydrazine monohydrate vapor was utilized to reduce the CH-GO foams into CH-rGO foams.26 Obviously, the geometry of the CH-rGO foam is easy to control via facile processing of the honeycomb structure. Additionally, the preparation process of CH-rGO foam is rather simple to be completed. The top view of the CH-rGO foam was observed using scanning electron microscopy (SEM), shown in Fig. 2. Fig. 2 (a) shows the cross section of the CH-rGO foams. The CH-rGO foam exhibits a structure formed by physical connections with

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Fig. 1. Schematic of CH-rGO foam fabrication steps: the Nomex honeycomb was carbonized at 1000 oC in Argon atmosphere; the carbon monolith was then immersed

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into the GO suspension with different densities and freeze in an isotropic cold environment; the ice crystal was grown perpendicularly to the cell wall due to the different thermal conductivities between air and carbon cell wall; the CH-rGO foam with a horizontal laminated structure perpendicular to the cell wall was finally

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500 µm.

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obtained by the freeze-drying and hydrazine monohydrate vapor reduction. Scale bar,

small gaps between the CH network and the rGO foam. The CH structure acts as

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highways for electron transport (electrical conductivity of the cell wall is measured to be 1.14×103 S/m) and framework to resist external loading, implying its potential in improving the electrical and mechanical properties of the composite foams. Pure CH structures are shown in Fig. 2 (b) and (c). It is observed that the original structure was well maintained after carbonization, and the thickness of the cell wall is around 50~150 µm. Fig. 2(d) is a high magnification image of the cross section of the CH cell wall. The network consisted of short carbon fibers is exposed and the fibers are

ACCEPTED MANUSCRIPT covered with carbon coatings. The Nomex honeycomb is made up of PMIA short fibers and phenolic resin, thus, the carbon fibers were from the carbonization of PMIA fibers and the covered carbon coatings were residual from the carbonized phenolic

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resin.34-36 These interconnected carbon fibers and coatings in the cell wall construct a strong and conductive carbon structure (Fig. S1). These images also show that the CH structure not only consists of macro regular geometric pores, but also is made of

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plenty of micro pores.

Fig. 2. Typical top-view SEM images of CH-rGO foams (a), CH structure (b) and (c); (d) Cross-section morphology of magnified SEM image of CH structure. Scale bars, 200 µm (a), 1 mm (b), 500 µm (c) and 20 µm (d).

Fig. 3 shows the side view SEM images of the CH-rGO foams and common rGO foams. It is notable to observe that the rGO foams located in the CH cell show a

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Fig. 3. SEM images of typical side views of CH-rGO foams from (a) to (c), and common pure rGO foams from (d) to (f); (g) left: schematic showing the formation mechanism of the horizontal laminated graphene foam structure in CH-rGO foams by

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freeze-drying method; right: schematic showing the formation mechanism of the

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porous structure in common pure rGO foams by freeze-drying method. Scale bars, 1 mm (a), (b), (d), (e) and 200 µm (c), (f).

highly aligned laminated microstructure perpendicular to the cell wall. Each layer in the laminated structure is a porous rGO film. High magnification of SEM images of the rGO is shown in Fig. S2. Generally, the inner pore structure of rGO foams should be homogeneous if an isotropic cooling environment is designed. However, the rGO

ACCEPTED MANUSCRIPT microstructure shown in Fig. 3(a) to (c) exhibits highly anisotropic characteristics towards the cell wall. Here, the principle of the freeze-casting process that has previously been proposed for nanoparticle suspensions and was employed to explain

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the formation of such a unique laminated structure.31, 37 The microstructure of the rGO foam is mainly determined by the ice crystal template through the freeze-drying method. During the freezing process, the only difference between the CH-rGO foam

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and the common rGO foam is the introduction of the CH structure. Actually, it is

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known that the thermal conductivity of the cell wall is much larger than that of air, which means heat transfers faster in the cell wall than in air.38 Thus, as schematically illustrated in Fig. 3(c), the growth rate of the ice crystals at the interface of the cell wall shows high anisotropic growth kinetics during solidification. Then, the GO

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sheets aligned along the growth direction of the ice due to the squeezing effect and a continuous aligned laminated microstructure is finally formed. Conversely, for common rGO foams, heat transfer is almost isotropic in all freezing directions,

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yielding a homogenous and porous foam structure, as is shown in Fig. 3(d) to (f).

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To assess the chemical compositional evolution of the CH-rGO foams, Raman spectra and X-ray photoelectron spectroscopy (XPS) of CH and rGO were measured respectively. Fig. 4(a) shows the Raman spectra of the CH sample. The peaks located at around 1320 cm-1 and 1590 cm-1 are assigned to the characteristic D (defects and disorder) and G (graphitic) bands of the carbon materials, respectively. The D/G ratio of band intensities indicates the degree of structural order with respect to a perfect graphitic structure. Here, the D/G intensity ratio of the sample was determined to be

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(d).

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Fig. 4. Raman spectrum of CH (a) and rGO (c); C1s XPS spectra of CH (b) and rGO

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0.88. The relatively low D/G intensity ratio might indicate a high degree of carbonization with few defects in the carbon structure. In Fig. 4(c), the characteristic

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G and D bands of the Raman spectra for rGO foams after hydrazine vapor treatment clearly indicate the formation of rGO in the foams. The D/G intensity ratio was 1.12, which indicates the degree of disorder and average size of the sp2 domains. X-ray photoelectron spectroscopy (XPS) analysis of the CH and rGO sample was revealed in Fig. 4(b) and (d). For the CH samples, after carefully being fitted toward C 1s, it can be divided into three obvious peaks centering at ca. 284.58, 285.48 and 286.20 eV. The binding energy at ca. 284.58 eV can be attributed to sp2 C, ca. 285.48 eV

ACCEPTED MANUSCRIPT corresponding to sp3 C bonds, whereas ca. 286.20 eV assignable to C-O bonds. In Fig. (d), C 1s of the rGO samples also can be divided into three peaks centering at ca. 284.6, 285.5 and 287.6 eV. The binding energy at ca. 284.6 eV can be attributed to

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C-C bonds, ca. 285.5 eV corresponding to C=O bonds, whereas ca. 287.6 eV assignable to the C-N bonds.39 X-ray diffraction (XRD) of rGO and pure carbon honeycomb was also characterized (Fig. S3).

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EMI Shielding Effectiveness. Strong and lightweight EMI shielding is currently in

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high demand for both commercial and defense purposes. An effective EMI shielding material must protect the component from stray external signals. Electromagnetic waves can be attenuated quickly in a good conductor because of the induced current created in the conductor. Thus, shielding materials need to be electrically conductive.

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However, electrical conductivity is not enough for a good EMI shielding material. In order to absorb electromagnetic wave radiation, the radiation should interact with the material’s electric and magnetic dipoles. Besides, interfaces or defect sites within the

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shielding material are strongly needed, which can result in multiple scattering to

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absorb more electromagnetic waves.1,15,40 In general, excellent shielding can be achieved by using thick materials. However, conventional EMI shielding materials are relatively heavy or unable to sustain a considerable load, which put them at a disadvantage for use in aviation and aerospace applications. Inspired by the EMI shielding mechanism, a highly aligned porous carbon EMI shielding material is designed. In this carbon monolith, the CH structure acts as a highly conductive framework and loading structure. RGO foams with a laminated

ACCEPTED MANUSCRIPT structure and thousands of interfaces and defect sites were introduced into the CH structure to enhance the absorption effect without adding too much weight. In order to obtain a monolith with high electrical conductivity, the graphene oxide foam was

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completely reduced by hydrazine hydrate.

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Fig. 5. EMI shielding effectiveness measurement results in 8 ~ 12 GHz (X band): (a)

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EMI shielding effectiveness of pure CH structure (density is 38 mg/cm3); EMI shielding effectiveness of CH-rGO foams: (b) density is ~42 mg/cm3, inner filled rGO foam with the primary GO density of 5 mg/cm3; (c) density is ~48 mg/cm3, inner filled rGO foam with the primary GO density of 10 mg/cm3; (d) density is ~61 mg/cm3, inner filled rGO foam with the primary GO density of 20 mg/cm3.

X band (in the frequency range of 8-12 GHz) is the most important band for

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S4. It is known that the measured EMI shielding result is sensitive to the sample state and environmental conditions. Thus, at least three samples for one control group were prepared and measured to ensure the accuracy of results. As is shown in Fig. 5 (a), the

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mean value of EMI shielding effectiveness of the pure CH (cell wall conductivity is

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1.14×104 S/m) with a thickness of 2 mm is measured to be ~14dB. When the thickness increases to 3 mm and 5 mm, this value increases from 17 dB to 21 dB, respectively. Generally, during EMI shielding measurements, samples need to be of equal size with waveguide to ensure the accuracy of the measured results. However,

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pure rGO foams with very low density such as 5 mg/cm3 or 10 mg/cm3 (unlike CH-rGO foams, which have the protection of a CH structure) are always fragile and easy to break during measurement, which may lead to inaccurate measuring results.

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Thus, pure rGO foam with the primary GO solution density of 20 mg/cm3 was

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selected to make a comparison with the CH-rGO foam. In Fig. S4, pure rGO foam with electrical conductivity of ~ 9.7 S/m shows a similar EMI shielding ability and variation trend with the CH structure. Although the CH structure is highly conductive compared with the pure rGO foam (almost 200 times higher), they shared the same level of EMI shielding ability. It is speculated that the visible cells and fewer interfaces in the CH structure make it hard to prevent some electromagnetic waves from escaping throughout the structure. Fig. 5(b) to (d) show the EMI shielding

ACCEPTED MANUSCRIPT effectiveness values of the CH-rGO foam with different rGO foam densities. It can be seen that the EMI shielding effectiveness values are enhanced when the CH structure is filled up with rGO foams, even though the primary GO solution density of the rGO

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foam is only 5mg/cm3. When the density rises to 20 mg/cm3, as is shown in Fig. 5(d), the mean value of the EMI shielding effectiveness of the CH-rGO foam (thickness of 5 mm) significantly increased to as good as ~41 dB, which is 20 dB higher than that

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of the CH structure or rGO foam. It is believed that the increase was the result of a

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synergistic shielding effect from conductive CH framework and porous rGO foam. Meanwhile, it can be observed that the EMI shielding effectiveness also increases with the sample thickness. For example, the mean value of the EMI shielding effectiveness of the CH-rGO foam (10mg/cm3, Fig. 5(c)) with a thickness of 2 mm is

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measured to be ~18dB. When the thickness increases to 3 mm, the sample shows an EMI shielding effectiveness value of ~24 dB. This value tends to be ~28 dB when increasing the thickness to 5 mm. The EMI shielding effectiveness of CH-rGO foams

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reduced by the hydrazine hydrate with different reduction time were also compared in

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Fig. S5. It can be observed that the CH-rGO foam has an optimal effect by reducing 24 hours.

High EMI shielding effectiveness values are useful to estimate a material’s EMI

shielding ability. However, after taking density into consideration, the specific EMI shielding effectiveness is more appropriate to compare with the shielding performance between different materials. The specific EMI shielding effectiveness of the CH-rGO foam (20 mg/cm3) with a thickness of 3 mm can reach ∼ 442.6 dB·cm3·g-1, and ∼

ACCEPTED MANUSCRIPT 688.5 dB·cm3·g-1 with a thickness of 5 mm. These two values are much higher than those of typical metals (10 dB·cm3/g for solid copper) and some of carbon/polymer composites (33.1 dB·cm3/g for foam composites containing 7 wt% CNTs).2 The

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excellent specific EMI shielding effectiveness is one of the most outstanding advantages of CH-rGO foam as a high-performance EMI shielding material, which allows it to be used in aviation and aerospace applications. The comparison of the

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specific EMI shielding effectiveness of our data with other reported results was shown

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in Fig. S6.

Fig. 6. (a) Comparison of total EMI shielding effectiveness (SET), microwave absorption (SEA), and microwave reflection (SER) at the frequency of 9 GHz for CH-rGO foams with various rGO density; (b) Comparison of total EMI shielding effectiveness (SET), microwave absorption (SEA), and microwave reflection (SER) as a function of thickness at 9 GHz; (c) schematic representation of electromagnetic

ACCEPTED MANUSCRIPT wave transfer across the CH-rGO foam with the horizontal laminated structure. To further explore the EMI shielding mechanism of the CH-rGO foam, the measured EMI shielding results were analyzed systematically.

When an

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electromagnetic radiation incident occurs on a shielding material, the sum of absorptivity (A), reflectivity (R), and transmissivity (T) must add up to 1, that is, T+R+A=1. Usually, absorptivity (A), reflectivity (R), and transmissivity (T)

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coefficient were obtained by using four S parameters. The total EMI shielding

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effectiveness (SET) is the sum of the reflection from the material surface (SER), the absorption of electromagnetic energy (SEA), and the multiple internal reflections (SEM), which can be expressed as: SET = SEA + SER + SEM. SEM is usually negligible when SET ≥15 dB. Thus, SET can be expressed as: SET ≈ SEA + SER. The effective

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absorbance (Ae) can be described as Ae = (1−R−T)/(1−R). With regards to the power of the effective incident electromagnetic wave inside the shielding material, the reflectance and effective absorbance can be conveniently expressed as SER =

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−10log(1–R), and SEA = −10log(1–Ae) = −10log[T/(1–R)].5,21 Therefore, absorptivity

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(A), reflectivity (R), and transmissivity (T) can be obtained. Fig. 6(a) shows SET, SEA, and SER of the CH-rGO foam as a function of rGO foam density at 9 GHz. It can be observed that, as the rGO foam density increases, both the SET and SEA increase while SER slightly decreases. It is noted that the contribution of absorption to the EMI shielding effectiveness is larger than that of reflection, even for pure CH structure. For CH-rGO foams with rGO foam density of 5 mg/cm3, SET, SEA, and SER are calculated to be ∼19, 13.5 and 5.5 dB, respectively. For a larger density of 10 mg/cm3,

ACCEPTED MANUSCRIPT SET, SEA, and SER are ∼21.04, 18.3 and 2.74 dB, respectively. This suggests that the porous structure in the honeycomb benefit from the multiple reflections of the incident microwaves inside the pore, and consequently are responsible for the EMI

shielding.

The

electromagnetic

wave

shielding

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absorption-dominant

effectiveness of carbon nanomaterials may change obviously or keep as a constant with the variation of frequency.41 In this study, the average EMI shielding

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effectiveness of CH-rGO foam is almost constant in the testing frequency range.

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The schematic diagram of EMI shielding mechanism is indicated in Fig. 6(c). As electromagnetic incident waves strike the surface of CH-rGO foam, some waves are immediately reflected at the surface of the highly conductive CH cell wall or the rGO sheets. The remaining waves pass through the surface and the interaction with the

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high electron density of rGO sheets induces currents in the porous rGO film, resulting in a drop in energy of the electromagnetic waves. The surviving waves, after passing through the first layer of the porous rGO film, encounter the next porous layer, and

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the phenomenon of electromagnetic wave attenuation repeats. Simultaneously, the

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second layer and cell wall of CH structure act as a reflecting surface and gives rise to multiple internal reflections. The electromagnetic waves can be reflected back and forth between the internal multilayers until they are completely absorbed within the structure. This is in stark contrast to pure rGO foams that have a regular porous structure and no laminated structures available to provide the internal multiple reflections. Thus, the special microstructure provides CH-rGO foams with the unique advantage to behave as a multilevel shield.42,43 Fig. 6(b) shows the variation of SET,

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Mechanical Strength. In addition to the excellent EMI shielding properties, the

Fig. 7. Stress-strain measurement results of samples with different bulk densities: rGO foams (a) and CH-rGO foams (b); (c) Compressive mechanical analysis of CH-rGO foams; A 100 g weight standing on a rGO foam (d) and a CH-rGO foam (e); A 1000 g weight standing on a rGO foam (f) and a CH-rGO foam (g).

ACCEPTED MANUSCRIPT CH-rGO foam also provides remarkable mechanical stability compared with rGO foam, which is necessary for shielding materials that are used in areas where mechanical properties are important. Fig. 7(a) shows the compressive properties of

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the pure rGO foams. The compressive strength of the pure rGO foam (5 mg/cm3) was 2.96 kPa under 50% deformations. After increasing the foam density, compressive strength of the rGO foam was increased to 4.31 kPa (10 mg/cm3) and 18.91 kPa (20

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mg/cm3) under the same 50% deformation. Fig. 7(b) shows the compressive

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properties of the CH structure and CH-rGO foam. The pure CH structure showed a compressive strength of 1.13 MPa. After introducing the CH structure into the rGO foam, a sharp rise was found in both compressive strength and Young’s modulus compared with the pure rGO foam. For example, the compressive strength and

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Young’s modulus of the CH-rGO foam with a rGO density of 10 mg/cm3 is 1.42 MPa and 37.87 MPa, respectively. On the contrary, the compressive strength and Young’s modulus of pure rGO foam with a density of 10 mg/cm3 is 0.21 kPa and 5.57 kPa. The

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CH-rGO foam significantly increases the compressive strength by 6,760 times

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compared with pure rGO foam. Meanwhile, the density of the carbon monolith is only 5 times higher than pure rGO foams. It is known that rGO or graphene foam always exhibits poor mechanical stability because of the weak Van der Waals force between neighboring rGO sheets. However, honeycomb is mechanically stable and light structure. Thus, the introduction of the CH structure into the rGO foam can largely enhance the stiffness of the entire structure. Fig. 7 (c) shows the interface load transfer models of the CH-rGO foam. When outside loads are applied on the CH-rGO foam, it

ACCEPTED MANUSCRIPT is the CH structure rather than the rGO foam that sustains almost all the load. Nevertheless, loads can be effectively transferred from the CH cell wall to the rGO foam through the adjacent interfaces. Although the rGO foam is weaker than the CH

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structure, the load transfer makes the CH-rGO foam stronger, compared with the pure CH structure. The comparison of the load bearing ability between the rGO foam and CH-rGO foam is shown in Fig. 7(d) ~ (g). It can be observed from Fig. 7(d) and (e)

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that either the rGO foam or CH-rGO foam can sustain a 100 g weight and maintain its

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original structure. However, the rGO foam was squashed badly after adding a 1000 g weight on it. Contrarily, the CH-rGO foam is still stiff enough to carry the same 1000 g weight without any deformation.

Thermal Performance. EMI shielding materials need to be thermal insulating and

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fire retardant when they are used in some special environments for aviation and aerospace applications, such as navigation devices. Thus, the thermal properties of CH-rGO foams are investigated. Fig. 8(a) shows the thermal conductivity of the rGO

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foam and CH-rGO foam (in the axial direction). The rGO foam possess a thermal

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conductivity of 0.021, 0.023 and 0.028 W/m·K with densities of 4.9, 9.6 and 19.3 mg/cm3, respectively. After introducing the CH structure, the thermal conductivities of the CH-rGO foams tended to increase gradually from 0.045, 0.052 to 0.057 W/m·K, with densities of 43, 54 and 61 mg/cm3. It is quite clear that the introduction of the CH structure enhanced the thermal conductivity of the rGO foam, which is mainly attributed to a relatively higher thermal conductivity of the CH structure. Additionally, the thermal conductivity of CH-rGO foam is still low enough to be a thermal

ACCEPTED MANUSCRIPT insulating material. Thermal conductivities of CH-rGO foams reduced by the hydrazine hydrate with different reduction time was shown in Fig. S7. According to Fig. 8(b), thermal conductivity of CH-rGO foam in the axial direction can be

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predicted by a parallel conduction model:

Fig. 8. (a)Thermal conductivities of pure rGO foams and CH-rGO foams with different original GO solution densities: 5mg/cm-3, 10mg/cm-3 and 20mg/cm-3; (b) Schematic of contributions to thermal conductivity in the foam with laminated pore structure; (c-f) The flame of an alcohol lamp burning a Chaenomeles speciose flower standing on a thin CH-rGO foam plate with time ranging from 0 to 15 s; (g-j) Photographs of CH-rGO foam on a hot flame with time ranging from 0 to 30 s.

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kCH-rGO = kCH (1-VrGO) + krGO VrGO

(1)

where kCH-rGO, kCH, krGO are the thermal conductivities of the CH-rGO foam, the CH

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structure and the rGO foam respectively, and ArGO is the cross section fraction of the rGO foam, VrGO≤1.

When kCH≥krGO, the thermal conductivity of the CH-rGO foam, kCH-rGO, should be

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higher than that of pure rGO foam. However, what’s more interesting is that the

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laminated structure in the CH-rGO foam is different from common rGO foam microstructure. Thus, the thermal conductivity of the rGO foam in the CH structure can be expressed by a series model as is shown in Fig. 8(b): krGO = l / (l1/kl+l2/kair)

(2)

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where krGO, kl, kair are the thermal conductivities of the rGO foam, the layer structure in the rGO foam and the air respectively, l is the total length of the rGO foam in the CH structure, l1 is the thickness of the layer structure in rGO foam and l2 is the gap

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distance between the two layered structure. Detailed description of equation (1) and (2)

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was exhibited in supporting information. According to equation (2), the inner laminated microstructure of CH-rGO foam

may generate a larger thermal resistance compared with pure rGO foam. The relatively large thermal resistance, caused by the laminated porous structure, contributes to the low thermal conductivity of CH-rGO.44 Parts (b)-(e) of Fig. 8 display the photographs for an CH-rGO foam with a thickness of 8 mm on a hot flame at different times. On the top of the foam, a

ACCEPTED MANUSCRIPT Chaenomeles speciose flower is on it. It can be seen that the flower is still fresh after the foam burning on the flame for 15 s, which shows a favorable thermal insulating ability of the foam. Parts (f)-(i) of Fig. 8 show the photographs for a CH-rGO foam

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directly burning on a hot flame at different times. The shape of the foam maintained well after burning on the outer flame of the alcohol lamp for 30 s. The burning experiment shows the foam has high flame-retardance performance. These results

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indicate that the CH-rGO foam is a good candidate for EMI shielding material for use

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in high temperature environment. 4. Conclusions

In summary, lightweight, thermally insulating and stiff high-performance EMI shielding CH-rGO foams have been successfully prepared via a facile freeze-drying

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strategy, which has advantages in terms of manufacturing simplicity and scalability. A high compressive stress of 2.02 MPa and a high EMI shielding effectiveness of around 36~42 dB in the X band were achieved. The CH-rGO foam also shows the

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specific EMI shielding effectiveness as high as 688.5 dB·cm3·g-1. Moreover, the low

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thermal conductivity and high flame retardancy of the foam allows it to be used in high temperature environments. The high EMI shielding effectiveness, plus an ultralight, compressible stable structure and thermal stable feature, make CH-rGO foams useful for EMI shielding applications particularly in aviation and aerospace areas.

ASSOCIATED CONTENT Supporting Information

ACCEPTED MANUSCRIPT SEM images of CH structure and CH-rGO foam, EMI shielding effectiveness of pure rGO foams with primary GO solution density of 20 mg/cm3, XRD pattern of carbon honeycomb and rGO, thermal conductivities and EMI shielding effectiveness of

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CH-rGO foams reduced by the hydrazine hydrate with different reduction time, details of parallel and series conduction models of CH-rGO foams. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

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Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

This work was supported by the Natural Science Foundation in China (NSFC 11272109) and the Ph. D. Programs Foundation of Ministry of Education of China

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(20122302110065). The authors also acknowledge Li Huang in Harbin Institute of

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Technology for her characterization help of scanning electron microscope.

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