Graphene-Based Sandwich Structures for Frequency Selectable

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Graphene-based sandwich structures for frequency selectable electromagnetic shielding Weili Song, Congcheng Gong, Huimin Li, Xiao-Dong Cheng, Mingji Chen, Xujin Yuan, Hao-Sen Chen, Yazheng Yang, and Daining Fang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08229 • Publication Date (Web): 25 Sep 2017 Downloaded from http://pubs.acs.org on September 29, 2017

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ACS Applied Materials & Interfaces

Graphene-based sandwich structures for frequency selectable electromagnetic shielding

Wei-Li Song,a,b Congcheng Gong,c Huimin Li,a,b,d* Xiao-Dong Cheng,a,b Mingji Chen,a,b,e* Xujin Yuan,a,b Haosen Chen,a,b,e* Yazheng Yang,a,d,e Daining Fang a,b,d,e

a

Institute of Advanced Structure Technology, Beijing Institute of Technology, Beijing 100081, P.R.

China. b

Beijing Key Laboratory of Lightweight Multi-functional Composite Materials and Structures,

Beijing Institute of Technology, Beijing 100081, P.R. China. c

Beijing University of Civil Engineering and Architecture, School of Electrical and Information

Engineering, Beijing, 100044, P.R. China. d

State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing

100081, China. e

Collaborative Innovation Center of Electric Vehicles in Beijing, Beijing 100081, China.

*Corresponding Authors: Tel/Fax:

+86 10 6891-3022. Email: [email protected] (H. Li);

[email protected] (M. Chen); [email protected] (H. Chen).

-1ACS Paragon Plus Environment


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Abstract Because of the substantial development of electronics and telecommunication techniques, materials of electromagnetic interference (EMI) shielding performance are significant in alleviating the interference impacts induced from a remarkable variety of devices. In the work, we propose novel sandwich structures for manipulating the EM wave transport, which holds unique EMI shielding features of frequency selectivity. By employing electrical and magnetic loss spacers, the resultant sandwich structures are endowed with tunable EMI shielding performance, showing substantial improvements in overall shielding effectiveness along with pronounced shielding peak shift. The mechanisms suggest that the multiple interfaces, electromagnetic loss media and changes of representative EM wavelength could be critical roles in tailoring the EMI shielding performance. The results provide a versatile strategy that could be extended in other frequency ranges and various types of sandwich structures, promising great opportunities for designing and fabricating advanced electromagnetic attenuation materials and devices.

Key words: Sandwich structure; Multiple interfaces; Graphene; Resonance; Frequency selectivity


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1. Introduction Because the progressive advancement have been achieved in the advanced electronic and telecommunication industries, the electromagnetic (EM) signals lead to electromagnetic emission and interferences in the communicating apparatus, which are considered as a significant concern. For addressing such issues, a variety of lightweight materials with electromagnetic interference (EMI) shielding properties have been largely developed to meet the requirements in the current industries.1-11 In addition to traditional copper, carbon materials (carbon nanotubes, graphene and carbon filers) that have delocalized π electronic networks have drawn significant attention in the scientific communities since the unique advantages in electrically conductive carbon are ideal allow for achieving the EMI shielding materials with exclusive characteristics.12-17 According to the fundamental principles of EMI shielding materials, the primary mechanisms are known to involve reflection, absorption attenuation and multiple reflections. For absorption dominant shielding materials, electric dipole and electrical conductance is critical for electric materials and meanwhile magnetic dipole is responsible for magnetic materials. Generally, such materials hold large skin depth that allows for consuming EM energy inside the absorbing materials (Figure 1a).18-20 Recently, extensive explorations have been carried out for developing carbon materials with various morphology and prototypes.18,

21

Among these materials,

graphene-based composites receive significant interests due to the exclusive physical and chemical properties, offering tunable electrical conductivity, favorable features for integrating hetero-materials of different properties, and excellent process ability. In typical instance, 3D porous nickel framework was introduced for growing a 3D graphene framework by chemical vapor deposition (CVD), and the resulting graphene/poly(dimethyl siloxane) (PDMS) foam composites with density of 0.07 g cm-3 exhibited EMI shielding performance of 28 dB and 36 dB at thickness of 2 mm and 3 mm, respectively.4 Li and Yan have utilized specific technologies to fabricate polymeric carbon bulk materials, where the homogenous dispersed active fillers are able to generate considerable EMI



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shielding capability in the robust composites.22-23 Alternatively, Song and coworkers have demonstrated a simple method to fabricate reduced graphene oxide (RGO) aerogel-carbon hybrid textiles (0.07 g cm-3 in density), which present highly effective electromagnetic interference (EMI) shielding performance up 37 dB.24 For reflection loss dominant shielding materials, on the other hand, continuous electrically conductive paths and large interfacial impedance mismatching conditions are required to be established by conductive fillers in the composites.18, 21 In general, such materials possess highly small skin depth because major electromagnetic waves are required to be reflected on the air-solid interfaces, as illustrated in Figure 1b. For examples, Song and coworkers utilized chemically exfoliated graphene nanosheets (GN) to fabricate flexible polymeric graphene composite films, which deliver the effective EMI shielding performance (shielding effectiveness (SE) values greater than 20 dB) in the X band.20 Lately, further improvements have been made in the conductive thin films by using magnetic nanoparticles25 and introducing unique heterostructures.26 In the studies by Shen et al.27-28 lightweight carbon-based thin films were fabricated via thermal treatment in the inert environment. Whereby, the as-fabricated thin films enable to create strong EM wave reflection because of the highly conductive feature, which offers a universal strategy for substantially promoting the EM reflection. In addition to the uniform bulk materials and thin films, gradient materials or sandwich structures have been widely used electromagnetic technology.29-31 In particular, Jaumann screen that is able to generate quarter-wavelength resonance between multiple resistance sheets is generally used in microwave absorption. By borrowing this concept, here we demonstrate novel sandwich structures for EMI shielding based on graphene films and dielectric spacers (Figure 1c). The frequency selective EMI shielding performance has been observed, which presents a novel mechanism in EMI shielding materials. In particular, the various types of dielectric spacers have been fabricated using electrical loss and magnetic loss materials, showing considerable improvements in the shielding


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effectiveness. Implication of the results and mechanism offers an interesting plateau for fabricating new generation EMI shielding structures with tunable and designable advantages.

Figure 1 Schemes of typical types of EMI shielding materials and structures with three mechanisms: (a) Thick materials with absorption dominant shielding mechanism, (b) thin films with reflection dominant shielding mechanism and (c) sandwich structures with coupling multi-mechanism.

2. Results and discussion For preparing the sandwich structures, both electrically insulting GO films and electrically conductive GN films have been prepared. Specifically, various spacers, including polymeric dielectric spacers, electrical loss dielectric spacers and magnetic loss dielectric spacers were also fabricated via different materials and methods. In the assembly of the sandwich structures, graphene-based films (GO or GN) and various spacers were glued with polymeric solution, and the thickness of all the sandwich structures almost remains at around ~2 mm. Initially, two different types of graphene and corresponding films were prepared via different method. Electrically conductive GN was prepared by the direct chemical exfoliation in the acid solution,26, 32 while the electrically insulating GO was prepared by the modified Hummers’ method.33 Subsequently, both aqueous GN suspensions and GO solutions (with 50 mg solid mass) were then filtrated to fabricate free-standing GN and GO films, respectively. As shown in Figure 2a



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and 2b, the typical photographs of the flexible GO and GN films are given. The well aligned GO and GN nanosheets in the films are exhibited in the corresponding SEM cross-section images, as demonstrated in Figure 2c and 2d, respectively. Representative TEM image of the GN films also suggests the anisotropic features of the stacked GN films, which is consistent with the previous studies.26, 32 . Additionally, the anisotropic feature would allow EM wave travel perpendicularly into the graphene layers, where EM scattering from other angles would be eliminated. The cross-section SEM images of the sandwich structure shows a multi-layer feature of the GN-glue-D interfaces (Figure 2f). X-ray photoelectron spectroscopy (XPS) spectra of both GO and GN films were plotted in Figure 2g and 2h, which indicates that the presence of small amount of oxygen-containing functional groups has slightly impact on the electrical conductivity of the GN films (24000 ± 2000 S/m).32 X-ray diffraction (XRD) spectra (Figure 2i) also imply that the shift and broadened graphtic peaks in GO and GN, respectively, which is also consistent with previous studies.26, 32


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Figure 2 Materials and characterizations: (a) Electrically insulating GO film and (b) electrically conductive GN films; SEM cross-section views of (c) GO films and (d) GN films; (e) TEM cross-section view of GN films; (f) SEM of the sandwich structures; XPS C1S spectra of (g) GO and (h) GN films; (i) XRD spectra of (002) in GO and GN films.

GN-based sandwich structures with various GN layers were fabricated using the polymeric dielectric spacers (stable complex permittivity in X band: ε*= 2.6 + j0.08). The schematics of the sandwich structures are illustrated in Figure 3a-3c, and the inset in Figure 3a shows the typical electromagnetic transport feature at the conductive graphene-air interface. Specifically, the electrically conductivity of bulk GN films (24000 ± 2000 S/m) allows them for reflecting partial incident electromagnetic waves, with rest traveling through the nanosheets/films. Compared to multi-layer sandwich structures (Figure 3d), the single-layer sample exhibits the inferior EMI shielding performance (below 20 dB). Such ineffective performance is linked with the insufficient electrical conductivity,34 and thus the skin effect is unexpected. In the contrary, the two-layer and three-layer GN-based sandwich structures apparently enable to highly effective EMI shielding performance, with the peak values beyond 40 dB around 11 GHz. Implication of the results suggests that the two-layer and three-layer sandwich structures are able to generate frequency selective electromagnetic absorption. Figure 3d-3f collectively show that the SE absorption in GN-D-GN and GN-D-GN-D-GN is massively different from that in 1-layer GN samples, and SE absorption was observed to govern the overall shielding mechanism as expected. Additionally, comparison between GN-D-GN and GN-D-GN-D-GN manifests that the insertion of one central GN layer has no pronounced impact on the entire EMI shielding performance. According to the scheme in Figure 3b and 3c, the major reason of reduced EMI SE peak in GN-D-GN-D-GN should be related to the fact that insertion of the GN in the central layer may slightly interrupt the formation of the resonance,



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where the impedance condition would be altered. Establishment of resonance-like sandwich structure based on conductive GN films as the EM reflective surface layer possesses non-linear improvement in shielding performance. For bettering understand the resonance-like feature, Cu-foil-D-Cu foil sandwich structures were proposed for interpreting the GN-D-GN sandwich structures with two conductive surfaces, as exhibited in Figure S1. Apparently, a much resonance feature could be observed using the metal-based sandwich structure with exactly the same dielectric space. Because of resonance-like feature, EMI shielding peaks (Figure 3d) in the sandwich structure is associated with the resonance. Upon adding another conductive thin layer into the sandwich structures, the interruption mainly impacts the generation of resonance-like EMI shielding performance owing to the medium condition change of creating resonance, and thus the peak shielding performance would be influenced, rather than the entire EMI shielding performance. Thus, such improvements have greatly promoted the peak absorption EMI SE, suggesting the advantages of such unique GN-based sandwich structures. For understanding the difference between 16 dB and maximum 48 dB in GN-D-GN (Figure 3d), the metal-based sandwich structure Cu-foil-D-Cu is also given. As is shown in Figure S1, the Cu-foil-D-Cu foil also show very similar tendency to the GN-D-GN samples, clearly exhibiting a resonance-like characteristic around 9.5 GHz. Therefore, it is suggested that the graphene-based sandwich structures with semi-metallic electrical conductivity are more likely to create resonance-like chambers. In addition, the utilization of spacers with dielectric or magnetic loss characteristics would substantially interrupt the resonance-like feature, which in turn broadens the entire effective shielding performance via scattering the EM wave in the spacers.


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Figure 3 GN-based sandwich structures with various GN layers and EMI shielding performance: Schemes from the cross-section view of (a) D-GN-D (1-layer GN), (b) GN-D-GN (2-layer GN) and (c) GN-D-GN-D-GN (3-layer GN) sandwich structures. (d) Total SE, (e) SE absorption and (f) SE reflection of the corresponding sandwich structures. Note that the illustrated electromagnetic waves in the schemes are not to the scale, and green and pink waves in the schemes indicate the transmittance and reflected waves, respectively.

For proving the frequency selectivity along with the improved performance based on the GN-based sandwich structures, four different types of graphene-based structures were used for direct comparison (Figure 4a-4d). Firstly, the symmetric GO-D-GO sandwich structures with poor electrical conductivity in GO was anticipated to deliver extremely weak EMI shielding performance (below 2 dB), indicating both low electromagnetic loss features in both GO and polymeric dielectric spacers. In addition, EMI shielding performance of two asymmetric sandwich structures (GO-D-GN and GN-D-GO) were obtained (Figure 4e), showing that both exhibited much suppressed shielding performance similar to 1-layer GN sandwich structure (Figure 3d). The performance difference between GO-D-GN and GN-D-GO should be attributed to the presence of various transmittance and -9ACS Paragon Plus Environment


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reflection features at different interfaces in the multi-layer structures. In the GO-D-GN, the electromagnetic waves travel from insulator (GO) - insulator (D) – conductor (GN); however, in the GN-D-GO, they travel from conductor (GN) - insulator (D) - insulator (GO). Thus, the surface refraction index of GO and GN is completely different, leading to different performance in the EMI shielding. In the contrary, comparison among these four types of sandwich structures indicates the effectiveness of creating a unique structure for substantially enhancing EMI shielding performance, in particular the contribution in the SE absorption (Figure 4f). As exhibited in Figure 4g, it is suggested that GN-D-GO holds the highest SE reflection because of the largest impedance mismatching condition between GN-air interfaces. With the similar top interfaces between GN-D-GO and GN-D-GN, comparison from SE reflection indicates that a portion of electromagnetic energy has been consumed at the GN-air interfaces mainly owing the establishment of resonance-like feature in the unique GN-D-GN structure. Hence, the performance from these four samples implies the frequency selectivity in the GN-D-GN sandwich structure for largely consuming or absorbing electromagnetic energy in various approaches.


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Figure 4 Two-layer sandwich structures with various graphene films and EMI shielding performance: Schemes from the cross-section view of (a) GO-D-GO, (b) GN-D-GN, (c) GO-D-GN and (d) GN-D-GO sandwich structures; (e) Total SE, (f) SE absorption and (g) SE reflection of the corresponding sandwich structures. Note that the illustrated electromagnetic waves in the schemes are not to the scale, and green and pink waves in the schemes indicate the transmittance and reflected waves, respectively.

Since the frequency selective EMI shielding feature has been found in the GN-D-GN sandwich structures as aforementioned, spacer effects on the EMI shielding have been further discussed via varying the spacers. Specifically, two representative categories of spacers, i.e. electrical loss and magnetic loss, were fabricated into the GN-based sandwich structures. In the preparation of electrical loss spacers, as shown in Figure 5a, polymeric textile was initially immersed into GO aqueous solution, followed by in situ reducing into RGO within the textile networks. Upon the dried RGO-textile integrated networks was obtained, phenol formaldehyde (PF) resin/ethanol (EtOH) precursor solutions were added into the mixture networks. Until the curing process was completed, the polymeric RGO dielectric composites were obtained. As exhibited in Figure 5b and 5c, cross-section views of RGO-textile networks and polymeric RGO dielectric composites are given, showing typical textile-based composite features. XRD spectra in Figure 5d demonstrate that the polymeric RGO dielectric composites have both features from neat RGO and polymeric dielectric matrices (PF+textile). EMI shielding performance in Figure 5e indicates that both the polymeric RGO dielectric composites with 2 wt% and 3 wt% RGO loadings are poor in EMI shielding because of their dielectric characteristics. Additional measurements on the complex permittivity also confirm that both of these two polymeric RGO dielectric composites are typical dielectric materials with both real and imaginary parts lower than 4 in the X band (Figure 5f and 5g).



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Figure 5 Electrical loss dielectric spacers for sandwich structures: (a) Scheme of fabricating the polymeric RGO dielectric composites from polymer textile, GO solution and PF resin; SEM image of (b) textile with RGO network and (c) the as-fabricated RGO dielectric composites; (d) XRD spectra of the samples as marked. (e) Total SE of the two types of RGO dielectric composites; (f) real permittivity and (g) imaginary permittivity of the two types of RGO dielectric composites.

The as-prepared polymeric RGO dielectric composites were utilized as the spacers for fabricating into the two pieces of GN films to form sandwich structures (Figure 6a). Interestingly, total SE shielding performance of both GN-RGO(1)D-GN and GN-RGO(2)D-GN was observed to massively change in the investigated region, with the presence of electrical loss spacers. Firstly, the resonance-like absorption peak in GN-D-GN was found to shift toward the lower frequency via substituting the polymeric dielectric spacers with RGO dielectric composite spacers. This shift could be understood in terms of the factor that the employment of the electrical loss spacers would -12ACS Paragon Plus Environment

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essentially vary the dielectric constant of the transport medium (spacer), which results in the changes in wavelength and velocity. As a consequence, the initial solid-air interface matching conditions would be significantly changed, leading to the resonance peak shift towards lower frequency owing to the decreased wavelength of traveling waves in the medium (Figure 6d). Furthermore, EMI shielding performance of the sandwich structures with electrical loss spacers was observed to be entirely promoted, with overall total SE beyond 20 dB and 30 dB achieved in GN-RGO(1)D-GN and GN-RGO(2)D-GN sandwich structures, respectively (Figure 6e). Such substantial shielding promotion is largely owing to the presence of electrical loss spacers, which enable to convert electromagnetic energy into other forms (Figure 6d). Moreover, curves in Figure 6f and 6g also suggest that the dominant SE absorption presents a resonance-like feature and all the sandwich structures hold similar SE reflection owing to similar air-solid interface condition. Note that the position of the resonance peak would be out of the investigated region mainly because of the greater change in complex permittivity.

Figure 6 Sandwich structures and EMI shielding performance using electrical loss dielectric spacers: Schemes of (a) assembly of the sandwich structure with RGO dielectric composite spacer between -13ACS Paragon Plus Environment


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two pieces of GN films, (b) GN-RGO(1)D-GN and (c) GN-RGO(2)D-GN sandwich structures; (d) Electrical loss mechanisms using the RGO dielectric composite spacer; (e) Total SE, (f) SE absorption and (g) SE reflection of the corresponding sandwich structures. Note that the illustrated electromagnetic waves in the schemes are not to the scale, and green and pink waves in the schemes indicate the transmittance and reflected waves, respectively.

Beside the electrical loss spacers, magnetic loss spacer was also prepared for fabricating into the GN-based sandwich structures. In the synthesis of the Fe3O4 nanoparticles, precursor solution was firstly prepared, followed by transferring into the autoclave for hydrothermal process (Figure 7a). The dried powders were then dispersed into the PF resin precursor solution, which was subsequently cured to obtain polymeric Fe3O4 dielectric composites. As exhibited in Figure 7a, SEM image of the Fe3O4 nanoparticles is demonstrated, showing particles with size of 200-300 nm in diameter. XRD spectra in Figure 7c confirm that the typical peaks from the samples are assigned to be Fe3O4 phase (JCPDS card no. 88-0315). As exhibited in Figure 7c, pronounced peaks at 30.28o (d = 2.949) and 35.69 o (d = 2.513) correspond to (220) and (311) crystalline planes, along with (400), (511) and (440) planes at 43.02o, 57.24o and 63.10 o, respectively. In addition, Figure 7d displays the magnetic properties of the as-prepared Fe3O4 nanoparticles. As plotted in Figure 7e, polymeric Fe3O4 dielectric composites apparently shows very poor EMI shielding performance lower than 2 dB. This observation is also consistent with the dielectric features from both complex permittivity (Figure 7f) and complex permeability (Figure 7g).


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Figure 7 Magnetic loss dielectric spacers for sandwich structures: (a) Scheme of fabricating the polymeric Fe3O4 dielectric composites from Fe3O4 nanoparticles and PF resin; (b) SEM image, (c) XRD pattern and (d) magnetic hysteresis loop of the hydrothermally prepared Fe 3O4 nanoparticles; (e) Total SE of the two types of Fe3O4 dielectric composites; (f) real permittivity and (g) imaginary permittivity of the two types of Fe3O4 dielectric composites.

Likewise, the as-prepared polymeric Fe3O4 dielectric composite spacers were fabricated into GN-based sandwich structures (Figure 8a) and the corresponding EMI shielding performance is plotted in Figure 8e-8g. The improvement in the EMI shielding performance of the GNFe3O4(1)D-GN and (c) GN- Fe3O4(2)D-GN sandwich structures is very similar to that in the GN-based sandwich structure using magnetic loss spacers. Apparently, the overall improvement in EMI shielding is also linked with the presence of magnetic loss from magnetic Fe3O4 (Figure 8d). -15ACS Paragon Plus Environment


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Compared to the case by using electrical loss spacers (Figure 6e), the analogous peak shift in Figure 8e should be attributed to changes in wavelength and velocity as well (Figure 8d). Therefore, the air-solid interface conditions would be remarkably varied, leading to the peak shift towards lower frequency as expected (Figure 8e). Additionally, the SE absorption (Figure 8f) and SE reflection (Figure 8g) curves suggest that the SE absorption based on the resonance-like mechanism is considered as the dominant role in attenuating the electromagnetic energy.

Figure 8 Sandwich structures and EMI shielding performance using magnetic loss dielectric spacers: Schemes of (a) assembly of the sandwich structure with polymeric Fe3O4 dielectric composite spacer between two pieces of GN films, (b) GN- Fe3O4(1)D-GN and (c) GN- Fe3O4(2)D-GN sandwich structures; (d) Magnetic loss mechanisms using the Fe3O4 dielectric composite spacer; (e) Total SE, (f) SE absorption and (g) SE reflection of the corresponding sandwich structures. Note that the illustrated electromagnetic waves in the schemes are not to the scale, and green and pink waves in the schemes indicate the transmittance and reflected waves, respectively.


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As listed in Table 1, the typical EMI shielding materials (mainly on carbon-based materials) with various materials and features have been summarized.35-54 Performance comparison in Table 1 provides an overview of several types of EMI shielding materials, with absorption dominant and reflection dominant mechanisms in the traditional materials. Note that the measurements mainly contain absorption and reflection branches, and thus the internal multiple reflections and resonance attenuations would be involved into the absorption and reflection parameters. The mechanism categories are established primarily on the materials and structures. Uniform bulk and thin films are more likely to be homogeneous with single phase and one air-solid-air interface. Alternatively, the sandwich structures with hetero-phase and multiple interfaces (air-solid 1-solid 2-air) would be more complicated than the case in the single-phase EMI shielding materials. Besides the uniform bulk and thin film materials, the exceptional sandwich structure in this work have endowed the EMI shielding materials with frequency selectivity, which is owing to the employment of the resonance mechanisms and structures from microwave absorption materials.35,36 Since frequency selectivity is sensitive to the material thickness and electromagnetic parameters, such design strategy is easily extended to other frequency ranges. This unique universal approach opens up a completely novel design for fabricating EMI shielding materials with specific performance.



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Table 1. EMI Shielding performance of typical materials. (PP: polypropylene, PU: polyurethane, PS: poly styrene, PANI: poly-aniline, PC: polycarbonate; CT: carbon textile, PMMA: polymethyl methacrylate; PS:poly styrene; PEI: poly etherimide; PDMS: poly dimethylsiloxane; TMAH: tetramethylammonium hydroxide.) Materials or structures

Mechanism categories

Frequency selectivity

Graphene foam/PEI

Absorption dominant

Not obvious

MWCNT/PU

Absorption dominant

Not obvious

RGO/SiO2

Absorption dominant

Not obvious

CVD graphene/PDMS

Absorption dominant

Not obvious

MWCNT/PS

Absorption dominant

Not obvious

MWCNT/PU

Absorption dominant

Not obvious

MWCNT/PC

Absorption dominant

Not obvious

Graphene/PMMA

Absorption dominant

Not obvious

Graphene/PS

Absorption dominant

Not obvious

MWCNT/polymer

Absorption dominant

Not obvious

MWCNT/polymer

Absorption dominant

Not obvious

Graphene aerogel/CT

Absorption dominant

Not obvious

CNT/epoxy

Absorption dominant

Not obvious

Fe3O4/RGO/PEI

Absorption dominant

Not obvious

Fe3O4/RGO/TMAH

Absorption dominant

Not obvious

Fe3O4/RGO/PANI

Absorption dominant

Not obvious

Types & Thickness (mm) Uniform bulk 2.3 Uniform bulk 1.5 Uniform bulk 1.5 Uniform bulk 2 Uniform bulk 2 Uniform bulk 2 Uniform bulk 1.85 Uniform bulk 2.5 Uniform bulk 2.5 Uniform bulk 3.8 Uniform bulk 3.8 Uniform bulk 2 Uniform bulk 2 Uniform bulk 2.5 Uniform bulk 3 Uniform bulk


Shielding Performance (dB)

Refs

8-13

37

28-29

38

34

5

24-28

4

17

39

15-17

40

~25

41

13-19

42

25-29

43

46-52

44

42-45

44

26-27

24

30-35

45

14-18

46

41

47

29-31

48

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3.34 PVdF-HFP/LiBF4/EMIMBF4/CNT

RGO/PU foam Fe3O4/GN CNF-GN-CNF GN/PEVA RGO films RGO/PU GN-D-GN GN- RGO(1)D-GN GN- Fe3O4(1)D-GN

Uniform Absorption dominant

Not obvious

Absorption dominant

Not obvious

Reflection dominant Reflection dominant Reflection dominant Reflection dominant Reflection dominant Multiple mechanisms Multiple mechanisms Multiple mechanisms

Not obvious Not obvious Not obvious Not obvious Not obvious 11 GHz 10 GHz 9.5 GHz

bulk 2.6 Uniform bulk 20-60 0.20-0.25 Thin film 0.22-0.27 Thin film 0.36 Thin film 0.0084 Thin film 0.02 Sandwich 2 Sandwich 2 Sandwich 2

41-47

49

20-57.7

50

21-24

25

25-28

26

23-27

20

18-22

27

35-50

13

17-49 23-41 29-46

This work This work This work

Conclusions In summary, a series of graphene-based sandwich structures have been fabricated by assembling graphene films and various spacers. The results suggest that the sandwich structures enable to offer effective EMI shielding performance greater than 20 dB in the X-band. Compared to the conventional uniform bulk and thin film shielding materials, the sandwich structures were able to present unique frequency selectivity owing to employment of the resonance features. Further improvements could be achieved via tuning spacers with either electrical loss or magnetic loss properties, which highlight an exclusive platform for designing advanced EMI shielding materials.

Acknowledgments Financial support from 973 Project (2015CB932500), NSF of China (51302011), the Project of Beijing Municipal Science and Technology Commission (Z161100001416007), the Project of State Key Laboratory of Explosion Science and Technology (ZDKT17-02) and Beijing Natural Science Foundation (16L00001) is gratefully acknowledged. -19ACS Paragon Plus Environment


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Experimental Section GN films: Neat GN films were fabricated based on our previous work.26, 32 Briefly, the commercial graphite of 500 mg (grade 3805 from Asbury Carbons) was initially treated in the mixture solution of alcohol and water for stirring and sonication. Subsequently, the dried powders were added in a mixture solution of sulfuric acid (60 ml) and nitric acid (20 ml), followed by sonicating for 2 days. Upon direct exfoliation in the acid environment, the as-treated powders were then transferred into abundant water, and washed for several times. In the fabrication of GN films, the as-fabricated aqueous suspension with 50 mg powders were used for filtration, followed by drying in the oven and then peeling off to achieve free-standing GN films. Graphene oxide (GO) and GO films: GO was prepared according to the modified Hummers method.33 Briefly, graphite powder of 2 g and NaNO3 powder of 1 g were added to 120 mL of H2SO4 in an ice bath, followed by gradually adding 6 g KMnO4 powder under temperature of 0 oC. Until the mixture was stirred for 2h, the mixture was heated to stay at 30 oC for 30 min. Subsequently, the above mixture was dropwise added with 150 mL water, followed by adding 50 mL H2O2 (5%). Ultimately, the mixture solution was washed with water and 5% HCl to obtain the GO aqueous solution. In the fabrication of GO films, the as-fabricated aqueous solution with 50 mg GO was directly filtrated, followed by drying in the oven and then peeling off to obtain free-standing GO films. Dielectric spacers: The polymeric dielectric spacers were prepared by directly curing the polymer in the mould. Typically, a portion of the phenol formaldehyde (PF) resin/ethanol (EtOH) (PF%=30 wt%) precursor solutions were directly cast into the mould with dimensions of 22.86 × 10.16 × 2 mm3. Then, the mould was then transferred into an oven, and was heated to 100 oC for resin curing (~6 h). The dried samples were taken off and used as the dielectric spacers. RGO dielectric composite spacers: RGO dielectric composites with polymer matrices were prepared according to our previous study.55 In the typical preparation, a portion of polypropylene


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textile was immersed into the mixture solution of GO and hydroquinone (GO: hydroquinone=1:5 wt/wt) under sonication. Until the solution was stabilized under the sonication, the mixture was sealed and transferred into the oven, followed by heated up to 100 oC. After treating for 12 h, the samples were then washed and dried. The samples were then transferred into the phenol formaldehyde (PF) resin/ethanol (EtOH) (PF%=30 wt%) precursor solutions, followed by curing at 100 oC for 6 h. Two types of RGO dielectric composite spacers were fabricated via using different amount of starting GO solution with different concentrations. The RGO dielectric composite spacers with GO concentrations of 5 mg/ml and 7.5 mg/ml possessed RGO loadings around 2 wt% and 3 wt%, which were assigned as RGO(1)D and RGO(2)D, respectively. Fe3O4 dielectric composite spacers: Firstly, Fe3O4 nanoparticles were synthesized by the hydrothermal method. Briefly, FeSO4·7H2O (70 mg) was added into 14 ml water, followed by stirring until FeSO4·7H2O was completely dissolved. Subsequently, 20 mg NaOH powders were dissolved in 4 ml H2O, followed by mixing with the above solution. Then, the mixture solution was then transferred to a Teflon-lined stainless steel autoclave, which was heated at 180 oC for 10 h. The as-prepared samples were transferred in a filter and washed with water for several times. Afterward, the dried Fe3O4 nanoparticles were then dispersed into the phenol formaldehyde (PF) resin/ethanol (EtOH) (PF%=30 wt%) precursor solutions under strong mechanical stirring. Until stable suspension was obtained, the mixture suspension was casted into the mould (dimensions of 22.86 × 10.16 × 2 mm3), followed by curing for achieving the Fe3O4 dielectric composite spacers. Two types of Fe3O4 dielectric composite spacers (with dimension of 22.86 × 10.16 × 2 mm3) were fabricated by adding various amounts of Fe3O4 nanoparticles around 20 mg and 40 mg, which were assigned as Fe3O4(1)D and Fe3O4(2)D, respectively. Sandwich structures: For accurate measurement of S parameters, sandwich structures have been prepared for achieving bulk samples that are able to be positioned in the testing chamber.25 In the



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fabrication of glue, poly (ethylene-vinyl acetate) (PEVA) (DuPont Company) was employed for sticking to form sandwich structures. Initially, PEVA was dissolved toluene under vigorously stirring, followed by evaporating most of solvent to obtain viscous PEVA/toluene glue. In the specific fabrication, both GN and GO films (thickness ~50 microns) were processed into the dimension of 22.86 × 10.16 mm2, followed by sticking with various spacers to form the resultant sandwich structures. All the sandwich structures were dried in an ambient condition for obtaining electromagnetic testing samples. In particular, the excessive glue would be removed upon compacting, ensuring that the glue has limited impact on the overall thickness. Note that the thickness of all the sandwich structures stays around 2 mm. (1) For the sandwich structures with various GN layers, the PF dielectric spacers were used. Three types of sandwich structures were fabricated and the resulting dielectric spacer (1 mm in thickness)-GN-dielectric spacer (1 mm in thickness) (1-layer GN), GN-dielectric spacer (2 mm in thickness)-GN (2-layer GN) and GN-dielectric spacer (1 mm in thickness)-GN-dielectric spacer (1 mm in thickness)-GN (3-layer GN) were abbreviated as D-GN-D, GN-D-GN, GN-D-GN-D-GN, respectively. (2) For the sandwich structures with various graphene films, the PF dielectric spacers were used. Four types of sandwich structures were fabricated and the resulting GO-dielectric spacer (2 mm in thickness)-GO (symmetric type), GN-dielectric spacer (2 mm in thickness)-GN (symmetric type), GO-dielectric spacer (2 mm in thickness)-GN (asymmetric type) and GN-dielectric spacer (2 mm in thickness)-GO (asymmetric type) were abbreviated as GO-D-GO, GN-D-GN, GO-D-GN, GN-D-GO, respectively. (3) For the sandwich structures with RGO dielectric composite spacers, two types of sandwich structures were fabricated. The resulting GN-RGO(1)D (2 mm in thickness)-GN and GN-RGO(2)D (2 mm in thickness)-GN were abbreviated as GN-RGO(1)D-GN and (c) GN-RGO(2)D-GN, respectively.


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(4) For the sandwich structures with Fe3O4 dielectric composite spacers, two types of sandwich structures were fabricated. The resulting GN-Fe3O4(1)D (2 mm in thickness)-GN and GN-Fe3O4(2)D (2 mm in thickness)-GN were abbreviated as GN-Fe3O4(1)D-GN and (c) GN-Fe3O4(1)D-GN, respectively. Characterizations: Field emission scanning electron microscopy (FE-SEM) characterizations were performed on a ZEISS supra 55 system. Transmission electron microscopy (TEM) images were carried out on a JEOL JEM-2010 scanning TEM system. The field dependent magnetization for the samples was measured by a Lake Shore 7410 VSM at room temperature. X-ray photoelectron spectroscopy (XPS) was performed on PHI-5300 system. X-ray diffraction (XRD) characterization was applied on a PANalytical X’ Pert PRO MPD diffraction system. Electrical conductivity (σ) was determined using the classical four-probe method, obtained by a multimeter Keithley 2400 and a multiheight probe. EMI shielding: The sandwich structures were applied on an Anritsu 37269D vector network analyzer (VNA) for acquiring S parameters (S11 and S21) using the wave guide method in X-band.25 Particularly, the power coefficients, reflection coefficient (R) and transmission coefficient (T), could be calculated by the equations of R=|S11|2 and T=|S21|2, respectively. Absorption coefficient (A) was achieved from the relation of A=1-R-T.25 Electromagnetic shielding effectiveness (SE Total) refers to the logarithm of the ratio of the incident wave PI to the transmitted wave PT, and it could be determined by the equation of SE Total=10log(PI/ PT)dB. The total experimental SE is the sum of reflection (SE Ref) and absorption (SE Abs), which can be given as SE Ref=−10log(1-R)dB and SE Abs=−10log(T/(1−R))dB, respectively.25

Supporting Information Available: SE shielding performance of Cu-foil-D-Cu foil sandwich structure.



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Graphene-Based Sandwich Structures for Frequency Selectable

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