poly dimethylsiloxane composite for high-performance electromagnetic interference shielding

poly dimethylsiloxane composite for high-performance electromagnetic interference shielding

Composites Science and Technology 189 (2020) 108012 Contents lists available at ScienceDirect Composites Science and Technology journal homepage: ht...

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Composites Science and Technology 189 (2020) 108012

Contents lists available at ScienceDirect

Composites Science and Technology journal homepage: http://www.elsevier.com/locate/compscitech

Flexible Fe3O4/graphene foam/poly dimethylsiloxane composite for high-performance electromagnetic interference shielding Shoupu Zhu a, Qing Cheng a, b, Congcong Yu a, Xiaochun Pan c, Xiaobo Zuo c, Jianfei Liu c, Mingliang Chen a, Weiwei Li a, d, Qi Li a, *, Liwei Liu a, d, ** a

Key Laboratory of Nanodevices and Applications, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou, 215123, China School of Nano Technology and Nano Bionics, University of Science and Technology of China, Hefei, 230026, China Institute of Specific Iogistics, Army Academy of Research, Xian, 710032, China d SZGraphene Nanotechnology Co., Ltd., Suzhou, 215123, China b c

A R T I C L E I N F O

A B S T R A C T

Keywords: Graphene foam Fe3O4 Electromagnetic interference Flexible

The combination of electronic and magnetic shielding materials has been attracted considerable attentions because of the high-performance electromagnetic interference (EMI) shielding effectiveness (SE) produced by the synergetic effects between them. Here, the innovative structural Fe3O4/graphene foam (GF) composite is assembled by magnetic Fe3O4 nanoparticles anchoring onto the highly electronic conductive 3D GF. The elec­ trical conductivity of the flexible Fe3O4/GF/poly dimethylsiloxane (PDMS) composite reaches up to 2.5 S cm 1. The EMI SE of Fe3O4/GF/PDMS composite (~1.0 mm) increases from ~26.6 dB for GF/PDMS composite to ~32.4 dB in the frequency range of 8.2–12.4 GHz, which is attributed to the synergistic effect between Fe3O4 nanoparticles and GF. Furthermore, after repeatedly bending for 10000 cycles, the EMI SE of the Fe3O4/GF/ PDMS composite still reaches up to 29.4 dB.

1. Introduction In order to reduce the harmful effects of electromagnetic wave on electronic equipment and human’s health and satisfy the demands for aerospace and flexible electronics, etc., it is becoming more and more urgent to exploit electromagnetic interference (EMI) shielding materials of light weight, flexibility, corrosion resistance and high performance [1–3]. Recently, conductive polymer composites made up of polymers and conducting fillers have been served as a kind of modern EMI shielding materials [2,4–6]. Generally, the polymers are insulating and nonmagnetic, but can endow the composite with the properties of lightness, elasticity, excellent process-ability and mechanical features etc. [7,8] And the conductive fillers of incorporating into the polymer matrices play the key role in attenuating incident electromagnetic wave by the constructed conductive networks. Among the conductive fillers, the 2D graphene has drawn more attentions due to the characteristics of high electrical conductivity, low density, corrosion resistance, high temperature resistance, easy processing and excellent EMI shielding

effectiveness (SE) etc. [1,9–12] Therefore, many graphene-based conductive polymer composites have been fabricated by filling gra­ phene into polymers and obtained high-performance EMI shielding [13–17]. In more recent years, the synergistic effect between graphene and CNTs has been the focus of attention in graphene-based conductive polymer composites. The weak electronic coupling between graphene sheets can be significantly improved by the formed filling and bridging effects via the embedding of CNTs, thus the lightweight and flexible CNTs/graphene/polymers composites with enhanced EMI shielding performance have been designed and fabricated [18–21]. In a typical instance, Zhu et al. reported that the EMI SE of the flexible oxidized MWCNTs (OCNTs)/graphene/poly dimethylsiloxane (PDMS) compos­ ites reached 57.3 dB which exceeded the summation of that of the separate OCNTs/PDMS (2.5 dB) and graphene/PDMS (50.3 dB) [17]. Additionally, it is well known that the magnetic property plays an important role in attenuating the incident EMI wave. As the ferromag­ netic materials have the advantages of high magnetism, eco-friendliness, low cost, rich mineral reserves and so on, the conductive fillers that

* Corresponding author. ** Corresponding author. Key Laboratory of Nanodevices and Applications, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou, 215123, China. E-mail addresses: [email protected] (Q. Li), [email protected] (L. Liu). https://doi.org/10.1016/j.compscitech.2020.108012 Received 11 October 2019; Received in revised form 22 December 2019; Accepted 15 January 2020 Available online 20 January 2020 0266-3538/© 2020 Elsevier Ltd. All rights reserved.

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nickel foams with a diameter of about 20 � 30 mm2 were heated to 950 � C under Ar (30 sccm (standard cubic centimeter per minute)) and H2 (30 sccm) at a heating rate of 20 � C min 1 using a horizontal tube furnace (TF55030C, Lindberg Blue M, USA) equipped a quartz tube with an inside diameter of about 25 mm. After annealing for 10 min at 950 � C, CH4 was introduced into the system at a flow rate of 10 sccm and then maintained for 15 min. Then the graphene-coated nickel foams were obtained after cooling down to room temperature under Ar (30 sccm) and H2 (30 sccm) atmosphere. At last, the nickel skeletons were completely etched away by HCl solution (3 M) at 80 � C for 24 h and GF was obtained after washing and freeze drying.

electronic conductive graphene loading magnetic ferromagnetic mate­ rials such as γ-Fe2O3 [22], Fe3O4 [23–30], carbonyl iron [31,32], etc. have been many researches in electromagnetic shielding. The Fe3O4 exhibits excellent magnetic property, which can lead to magnetic attenuation [33] and produce much better EMI shielding performance [23,24]. Zhan et al. [23] fabricated flexible Fe3O4/rGO/natural rubber (NR) nanocomposites. While the rGO content was 10 wt% of NR, the EMI SE of Fe3O4/rGO/NR reached 26.4 dB mm 1 in the frequency range of 8.2–12.4 GHz, which was 1.4 times of that of rGO/NR nanocomposite at the same rGO content. Although the EMI shielding performance was improved by introducing Fe3O4 attenuating the incident electromag­ netic wave, what should not be overlooked is that the additional Fe3O4 was more than 3.5 times amount comparing with that of rGO in rGO/NR or Fe3O4/rGO/NR nanocomposites. What is worse, the anchored Fe3O4 nanoparticles have adverse effect on the mutual electronic contact of rGO sheets in Fe3O4/rGO/polymer composite. For instance, Shen et al. [24] fabricated polyetherimide (PEI)/[email protected] composite foam, the electrical conductivity of which was lower than that of the PEI/graphene composite at the same filler content. The relative drop electrical conductivity should be due to the prevented mutual direct electrical contact of the graphene sheets by the anchored Fe3O4 nano­ particles [3]. Although the declined conductivity decreased the SE of reflection (SER), the SE of absorption (SEA) was enhanced by increased dielectric loss and magnetic loss [24]. Similarly, the synergistic effect between electronic conductive graphene and magnetic Fe3O4 has been confirmed by amounts of reports in microwave absorption [34–41]. As is well known, the direct physical contact between the conductive fillers can form conductive network [10,42] and the highly conductive network plays a prominent role in attenuation of electromagnetic radi­ ation [8]. However, simply increasing filler content to form conductive network could lead to difficult processing technology and poor me­ chanical properties [3]. More interestingly, the seamlessly inter­ connected 3D conductive network can be constructed by a catalyst chemical vapour deposition (CVD) method [1,43,44]. And the GF would own the good flexibility by coating a thin layer of PDMS [1,43,44]. Chen et al. [1] prepared a flexible, lightweight and interconnected 3D GF/PDMS composite with a low density of 0.06 g cm 3. And the EMI SE is as high as 20 dB in the X-band, which has no obvious degradation even after bending 10000 times. In addition, Kong et al. fabricated a carbon nanowires (CNWs)/GF/PDMS composite with a low density of 97.1 mg cm 3. And the composite exhibited a high EMI SE of 36 dB in the X-band [43]. In this work, the GF was fabricated by a CVD method and the hex­ adecyl trimethyl ammonium bromide (CTAB) - decorated magnetic Fe3O4 nanoparticles are assembled onto electronic conductive GF on the role of mutual electrostatic attraction in aqueous medium. The anchored Fe3O4 nanoparticles have any weakening or strengthening effect on the contact resistance of the fixed shaped graphene in the 3D structural Fe3O4/GF composite. Therefore, the EMI SE of the composite can directly make clear the synergistic effect between electronic conductive graphene and magnetic Fe3O4. Ultimately, the lightweight Fe3O4/GF/ PDMS composite reaches a high EMI SE of about 32.4 dB in the fre­ quency range of 8.2–12.4 GHz. By contrast, the EMI SE of single GF/ PDMS and Fe3O4/PDMS/polytetrafluoroethylene (PTFE) composite reaches 26.7 and 0.15 dB, respectively. Obviously, the results confirmed the synergistic effect between graphene and Fe3O4. Furthermore, the flexible Fe3O4/GF/PDMS composite still keeps an EMI SE of about 31.5 dB after repeatedly bending for 500 cycles.

2.2. Synthesis of CTAB-modified Fe3O4 nanoparticles Fe3O4 nanoparticles were produced by hydrothermal reaction as our previously report, using Fe(OH)3 sol, vitamin C and hydrazine hydrate as raw materials [45]. Then 5 mg Fe3O4 nanoparticles were dispersed into 30 mL deionized water by ultrasonic dispersion for 30 min. 1 mg CTAB was dissolved into 10 mL deionized water by ultrasonic dispersion for 30 min. Then 0.5 mL CTAB solution was mixed together with above Fe3O4 dispersion. After ultrasonic dispersion for 15 min, the mixture was transferred into a 50 mL Telfon-lined stainless-steel autoclave and heated at 120 � C for 2 h in a drying oven. The modified Fe3O4 nano­ particles were collected and washed by deionized water until the su­ pernatant was clear. 2.3. Assembling of Fe3O4/GF The above CTAB-modified Fe3O4 nanoparticles were dispersed into 20 mL deionized water by ultrasonic dispersion for 15 min. After that, the well-dispersed CTAB-modified Fe3O4 solution was obtained and the GF was soaked into the solution and heated at 35 � C for 12 h in a drying oven. The Fe3O4/GF composite was fabricated after washing and sub­ sequent freeze drying. 2.4. Preparation of Fe3O4/GF/PDMS composite Firstly, a diluted PDMS solution was prepared by mixing a PDMS base agent, curing agent (Sylgard 184, Dow Corning) and ethyl acetate at a mass ratio of 10:1:77. The prepared Fe3O4/GF and GF were dipped into the dilute PDMS solution and followed by curing at 110 � C for 3 h. The obtained composites were separately designed as Fe3O4/GF/PDMS and GF/PDMS. Density of Fe3O4/GF/PDMS composite was estimated by the corresponding quality and volume. 8.5 mg Fe3O4 nanoparticles were added into 20 mL above dilute PDMS solution. After ultrasonic disper­ sion for 15 min, the mixture was vacuum-filtrated using a PTFE mem­ brane (pore size ¼ 0.05 μm, Φ ¼ 50 mm). Finally, the Fe3O4/PDMS/ PTFE film with an areal density of Fe3O4 about 1 mg cm 2 was obtained after curing at 110 � C for 3 h. 2.5. Characterization The morphology of the GF, Fe3O4/GF, and Fe3O4/GF/PDMS com­ posite was observed by a scanning electron microscopy (SEM) (Quanta FEG 250, FEI Co., Hillsboro, OR, USA) with an operating voltage of 20 kV. The powder X-ray diffraction (XRD) measurements were carried out on a D8 Advance with lynxEye and SolX (Bruker AXS, WI, USA) (Cu-Kα radiation, λ ¼ 1.5418 Å) from 10 to 80� at a scanning rate of 20� min 1. To observe the proportion of the Fe3O4 in Fe3O4/GF composites, ther­ mogravimetric analysis (TGA) (Pyris 1 TGA, PerkinElmer, USA) was performed under oxygen atmosphere from room temperature to 1000 � C with a heating rate of 20 � C min 1. X-Ray photoelectron spectroscopy (XPS) was performed with Al-Ka (1486.8 eV) X-ray source (ESCALAB 250Xi, Thermo Fisher Scientific, USA) to confirm the crystal phase of the iron oxide. The hysteresis loops were performed at 300 K on a physical property measurement system (PPMS) equipped with a vibration sample

2. Experimental section 2.1. Fabrication of GF Firstly, nickel foams (110 ppi, 500 g m 2 in area density, about 1.6 mm in thickness, HGP technology CO., LTD, China) were successively washed by acetone, ethanol and deionized water. Four pieces of dried 2

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magnetometer (PPMS DynaCool, Quantum Design Inc., USA). The Raman spectra was recorded by a Raman spectrometer (LabRam HR800UV-NIR, HORIBA JobinYvon, France) using 532 nm laser excitation. Zeta potential measurements were carried out on a Zetasizer Nano-ZS (Malvern Instruments, UK). The electrical conductivities of the sam­ ples were conducted using a four-probe meter (ST2258C, Suzhou Jingge Electronic Co. LTD, CHN). The EMI SE was calculated according to the Sparameters measured on a KeysightPNAN5227A Network Analyzer using the waveguide transmission method in the frequency range of 8.2–12.4 GHz. All samples were cut into 22.86 � 10.16 mm2 to well match the size of the waveguide holders (90CLKA1-SRFRF_PB, A-INFO, china) for the EMI SE measurement. The power coefficients of reflection (R), absorption (A), and transmission (T) can be obtained using the equations: R ¼ |S11|2 ¼ |S22|2, T ¼ |S21|2 ¼ |S12|2 and A ¼ 1 - R - T. The SER and SEA can be described as: SER ¼ - 10 log (1-R) and SEA ¼ - 10 log (T/1-R), respectively. And the SET can be described as: SET ¼ - 10 log T ¼ SER þ SEA. The stress-strain curve of the Fe3O4/GF/PDMS composite was measured using a universal testing machine (model 3365, Instron Corporation, USA) at room temperature. The tensile speed was 1 mm min 1, and the length and width of the composite were 10 mm and 1.5 mm, respectively.

GF exhibit a porous and interconnected 3D networks. The novel struc­ ture can provide fast electron transport channels [1]. As can be seen from Fig. 2h and i, the GF reveals a comparative smooth surface. And in contrast, a layer nanoparticles are homogeneously coated on GF surfaces as shown in Fig. 2b, c, d and e, which indicate that the CTAB-modified Fe3O4 nanoparticles are uniformly adsorbed on the surface of GF. As illustrated in Fig. 2e, the stack of Fe3O4 nanoparticles is not massive, which should be ascribed to the mutual electrostatic repulsion of CTAB-modified Fe3O4 nanoparticles in the assembling process. As shown in Fig. 2f, the much higher magnified SEM image of Fe3O4/GF shows that there is a compact monolayer Fe3O4 nanoparticles adsorbing on the surface of GF, which should be attributed to the mutual elec­ trostatic attraction between CTAB-modified Fe3O4 nanoparticles and GF. As exhibited in Fig. 3a, the identified diffraction peaks at 2 θ ¼ 35.42� and 62.52� are assigned to (311) and (440) lattice planes of cubic Fe3O4 (JCPDS card no. 19–0629), respectively. The obvious peaks of GF and Fe3O4/GF composite at about 26.7� are indexed to the (002) lattice plane of hexagonal graphite (JCPDS card no. 41–1487). The successful loading of Fe3O4 nanoparticles on GF is furtherly confirmed by the highresolution Fe 2p spectrum of Fe3O4/GF composite as shown in Fig. 3b. The observed peaks of binding energies located at 711.5 and 724.9 eV correspond to Fe 2p3/2 and Fe 2p1/2, respectively, which demonstrates the iron oxide is Fe3O4 [48,49]. From Fig. S2, no Ni peak appearance indicates that the Ni skeletons have been etched clearly by 3M HCl at 80 � C for 12 h. Fig. S3 shows the hysteresis curves loops of as-prepared Fe3O4 nanoparticles and Fe3O4/GF composite. From Fig. S3a, the Fe3O4 nanoparticles are likely superparamagnetic behavior at room temperature since both the remanence and the coercivity are almost zero [50,51]. As can be seen from Fig. S3b, the hysteresis loop of Fe3O4/GF composite distributes in four quadrants, which should be attributed to the superposition of intrinsic diamagnetism of graphene [52] and paramagnetism of Fe3O4 [50,51]. From Fig. 3c, the GF and Fe3O4/GF show characteristic peaks including G (graphite) peak at about 1581 cm 1 and 2D peak at about 2720 cm 1. Both the absence of D (disorder) peak of presenting disordered and defected carbons and the sharp 2D peak can indicate that the high-quality graphene of the GF has hardly any defect [53,54]. The Raman peak locations at 217.8, 285.8, 398.1 and 1304 cm 1 are consistent with that of Fe3O4 [55]. From Fig. 3d, the thermogravimetric (TG) curve of Fe3O4-CTAB nanoparticles indicates that the weight loss is mainly between 150 and 400 � C, which involves the decomposition and oxidation of CTAB and the oxidation of Fe3O4 into Fe2O3. And the weight percentages of Fe3O4 and CTAB in Fe3O4-CTAB composite are approximately calculated as 82.18 wt% and 17.82 wt%, respectively, based on the residual weight ratio of Fe3O4-CTAB. Similarly, the weight percentages of Fe3O4-CTAB nano­ particles and GF in Fe3O4/GF composite are approximately calculated as

3. Results and discussion The preparation process of Fe3O4/GF/PDMS composite is schemati­ cally depicted in Fig. 1. The GF is grown on the surface of Ni foam by a CVD method, similar to the previous reports [1,43,44]. And the 3D structural GF is obtained after etching the Ni by 3 M HCl at 80 � C for 24 h. The Fe3O4 nanoparticles are synthesized by a hydrothermal method using Fe(OH)3 sol as precursor [45]. After modified by CTAB, the Fe3O4 nanoparticles show positive charge [46,47]. Fig. 1b shows the respective zeta potential of the CTAB-modified Fe3O4 nanoparticles and GF aqueous dispersions at a concentration of about 0.1 mg mL 1. The GF reveals a peak in the negative zeta potential of about - 24.2 mV. In contrast, the CTAB-modified Fe3O4 nanoparticles show a zeta potential of about 26.4 mV. And the high and positive zeta potential should be attributed to the assembled positively charged CTAB. The zeta potential distributions for modified Fe3O4 and GF are shown in Fig. S1. Therefore, due to their opposite zeta potential the assembly process is easily driven by mutual electrostatic interactions between the CTAB-modified Fe3O4 nanoparticles and GF in deionized water [46]. And the much higher temperature of 35 � C than room temperature is used to increase the path of the CTAB-modified Fe3O4 nanoparticles in Brownian motion and promote the absorption of the CTAB-modified Fe3O4 nanoparticles on the surfaces of GF. At last, the Fe3O4/GF/PDMS is obtained by dipping the Fe3O4/GF into dilute PDMS and subsequent curing at 110 � C for 3 h. From Fig. 2a and g, both the SEM image of Fe3O4/GF composite and

Fig. 1. Schematic illustration of the fabrication procedure of Fe3O4/GF composite (a); zeta potential of CTAB-modified Fe3O4 and GF (dispersed in deionized water at a density of about 0.1 mg mL 1) (b). 3

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Fig. 2. Typical SEM images of Fe3O4/GF composite (a, b); the partial magnification of (b) (c); the partial magnification of (c) (d); high magnified SEM images of Fe3O4/GF composite (e, f); typical SEM images of GF (g, h); high magnified SEM image of GF (i).

Fig. 3. The XRD curves of Fe3O4, GF and Fe3O4/GF composite (a); high resolution Fe 2p spectrum of Fe3O4/GF composite (b); the Raman spectrums of GF, Fe3O4 and Fe3O4/GF (c); TG curves of Fe3O4-CTAB nanoparticles and Fe3O4/GF composite (d).

7.10 wt% and 92.90 wt%, respectively. As revealed in Fig. S4, the res­ idue of Fe3O4/GF composite after TGA is reddish-brown powder, which should be Fe2O3 and converted from Fe3O4 of Fe3O4/GF composite in the oxygen atmosphere at the high temperature. As displayed in Fig. S5, the TG curve of GF shows that the specific value of residual mass ratio is down to 0 wt% at about 820 � C. According to the proportions of Fe3O4 and CTAB in Fe3O4-CTAB composite and the weight ratio of Fe3O4-CTAB in Fe3O4/GF, the mass fractions of Fe3O4, CTAB and GF in the Fe3O4/GF

composite are 5.83 wt%, 1.27 wt% and 92.90 wt%, respectively. In general, the SE of incident electromagnetic wave involves reflec­ tion, absorption and multiple reflection (SEM). And the SEM is negligible while the SEA is more than 15 dB [56]. The large surface area and interface area of the 3D structured GF are benefited for the high-performance SEM [1,24] which is considered as the scattering ef­ fect within the materials [9] and should lead to the absorption-dominant EMI shielding [1]. The incident electromagnetic wave is primarily 4

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reflection while contact to the EMI shielding materials. And SER is caused by the impedance mismatch between air and the shield and closely related to the mobile charge carriers (electrons or holes) [9]. The SEA is connected with free charge carriers and electric dipoles of shielding materials [23,28]. Fig. S6 shows the digital photos of Fe3O4/GF/PDMS and Fe3O4/GF pieces with a size of 22.86 � 10.16 mm2 to measure EMI SE. From the Fig. 4a, the average SER, SEA and SET (the total SE) values of GF/PDMS composite are 8.1, 18.5 and 26.6 dB in the frequency of 8.2–12.4 GHz, respectively. The reflection coefficient (R ¼ |S11|2), transmission coefficient (T ¼ |S21|2) and absorption coefficient (A ¼ 1 - T - R) of GF/PDMS composite are 84.5%, 0.2% and 15.3%, respectively, calculated according to the S-parameters. As can be seen from the Fig. 4b, the SER, SEA and SET values of Fe3O4/GF/PDMS composite are 9.2, 23.2 and 32.4 dB, respectively. Obviously, the SEA of Fe3O4/GF/PDMS is increased greatly comparing with that of GF/PDMS, which should be attributed to the absorption of electromagnetic radia­ tion by the introduced magnetic dipoles of Fe3O4 nanoparticles [8,57]. It is clear that the EMI SE of Fe3O4/GF/PDMS increases as high as about 5.8 dB compared with that of GF/PDMS composite. The results really confirm the synergy effect between the GF and Fe3O4 in EMI shielding. From Fig. S7, the SET and SEA of Fe3O4/PDMS/PTFE in which the area density of Fe3O4 exceeds that of Fe3O4/GF/PDMS and reaches about 1 mg cm 2 is only about 0.15 and 0.14 dB, respectively. The results indicate that the proportions of reflected EM wave energy and absorbed EM wave energy by Fe3O4/PDMS/PTFE in the total incident EM wave energy are 0.2% and 3.2%, respectively. So based on the transmission coefficient of GF/PDMS composite (0.2%) and the proportion of absor­ bed EM wave energy by Fe3O4/PDMS/PTFE composite (3.2%), the increased SEA of Fe3O4/GF/PDMS (4.7 dB) is achievable in theory by the absorption of Fe3O4 to electromagnetic radiation through an energy point of view. In addition, the uniform distribution of Fe3O4 on the surfaces of GF should obtain stronger absorption to incident EM wave than that of Fe3O4 in Fe3O4/PDMS/PTFE composite and generate syn­ ergistic effect between graphene and Fe3O4 in EMI SE. And meanwhile, the SER of Fe3O4/GF/PDMS is a little higher than that of GF/PDMS, which indicates that the loaded Fe3O4 nanoparticles has no adverse in­ fluence on the electrical conductivity of GF. As shown in Fig. 4c, the

electrical conductivities of GF, Fe3O4/GF, GF/PDMS and Fe3O4/GF/PDMS are 3.8, 4.2, 2.4 and 2.5 S cm 1, respectively. After coating PDMS, the electrical conductivities of GF and Fe3O4/GF are declined, which should be caused by the poor conductivity of PDMS layer. As exhibited in Fig. 4d, the EMI SE of the Fe3O4/GF/PDMS is also tested during the repeatedly bending to the contact of the two sides of 10.16 mm. The EMI SE of the Fe3O4/GF/PDMS composite is just only decreased 2.8% after repeatedly bending 500 times, which manifests that the composite has excellent structure stability under bending deformation. Even after 10000 bending cycles, the EMI SE of Fe3O4/GF/PDMS composite remains 29.4 dB, which is about 90.7% of original value. The electrical conductivity of Fe3O4/GF/PDMS after repeatedly bending 10000 times (Fe3O4/GF/PDMS-B) declines to about 2.3 S cm 1 as illustrated in Fig. S8, which is ascribed to the partial structural damage of the GF network [1,44]. Fig. S9 shows the digital photographs of the Fe3O4/GF and Fe3O4/ GF/PDMS composite. From Table 1, the density of Fe3O4/GF is only 4.5 mg cm 3. After coating PDMS, the density of the composite increases to 130 mg cm 3. The SSE is a vital criterion to compare shielding perfor­ mance among different EMI shielding materials [58]. In this work, the SSE of Fe3O4/GF/PDMS composite is as high as 249.2 dB cm3 g 1 based on its low density of about 0.13 g cm 3. For comparison, Table 2 sum­ marized the density, thickness, conductivity, flexibility and EMI SE in X-band of variously typical graphene-based EMI shielding materials [1, 17,23,26,43,59,60]. The mechanical characteristics are also necessary for SEI materials used in some special areas. Fig. S10 shows the Table 1 Density and specific SE (SSE) of Fe3O4/GF/PDMS composite. Sizes (mm)

Mass of foam (mg)

Density of foam (mg cm 3)

Mass of PDMS composite (mg)

Density of PDMS composite (mg cm 3)

SSE (dB cm3 g 1)

Fe3O4/ GF

30 � 20 � 1.0 2.7

4.5

78.0

130.0

249.2

Fig. 4. The EMI SE of GF/PDMS composite (a); Fe3O4/GF/PDMS composite (b); electrical conductivities of GF, Fe3O4/GF, Fe3O4/PDMS and Fe3O4/GF/PDMS (c); EMI SE of Fe3O4/GF/PDMS composite under repeated bend cycle and the inset showing a photo of the bending shape (d). 5

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Table 2 Comparison for typical graphene-based EMI shielding materials (G: graphene, NR: natural rubber, PDA: polydopamine). Material

Density (g/cm3)

Thickness (mm)

Conductivity (S/cm)

Flexibility (dB)

EMI SE (GHz)

Frequency range

Ref.

G/PDMS Foam CNTs/G/PDMS cake [email protected]/NR G/Fe3O4/epoxy CNWs/GF/PDMS PDA-rGO Foam G-Foam Fe3O4/G/PDMS

0.06 0.58 1.18 0.46 0.097 0.05 0.06 0.13

~1 ~1 ~1.8 ~2 ~1.6 ~0.032 0.3 ~1

2 15.15 >0.1 >0.01 3.4 0.72 3.1 2.5

good good good some _ _ _ good

20 67.3 ~43 ~17 36 26.5 ~25.2 32.4

8–12 8.2–12.4 8.2–12.4 8–12 8.2–12.4 8.2–12.4 8.2–12.4 8.2–12.4

[1] [17] [23] [26] [43] [59] [60] this work

mechanical property of Fe3O4/GF/PDMS composite. The composite can tolerate a strain of about 80% at a very low stress of less than 0.137 MPa, indicating the Fe3O4/GF/PDMS composite owns excellent flexibility. As shown in Fig. S7, the low stress of Fe3O4/GF/PDMS is ascribed to the amounts of big pores of interconnected 3D network and the low density of the composite. And the present evident decline of tensile stress in stress-strain curve should be attributed to the partial breakage of 3D network skeleton of the composite.

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.compscitech.2020.108012. References [1] Z.P. Chen, C. Xu, C.Q. Ma, W.C. Ren, H.M. Cheng, Lightweight and flexible graphene foam composites for high-performance electromagnetic interference shielding, Adv. Mater. 25 (9) (2013) 1296–1300. [2] Z. Zeng, H. Jin, M. Chen, W. Li, L. Zhou, Z. Zhang, Lightweight and anisotropic porous MWCNT/WPU composites for ultrahigh performance electromagnetic interference shielding, Adv. Funct. Mater. 26 (2) (2016) 303–310. [3] R. Sun, H.B. Zhang, J. Liu, X. Xie, R. Yang, Y. Li, S. Hong, Z.Z. Yu, Highly conductive transition metal carbide/carbonitride(MXene)@polystyrene nanocomposites fabricated by electrostatic assembly for highly efficient electromagnetic interference shielding, Adv. Funct. Mater. 27 (45) (2017) 1702807. [4] J. Liang, Y. Wang, Y. Huang, Y. Ma, Z. Liu, J. Cai, C. Zhang, H. Gao, Y. Chen, Electromagnetic interference shielding of graphene/epoxy composites, Carbon 47 (3) (2009) 922–925. [5] M.H. Al-Saleh, U. Sundararaj, Electromagnetic interference shielding mechanisms of CNT/polymer composites, Carbon 47 (7) (2009) 1738–1746. [6] C.H. Cui, D.X. Yan, H. Pang, L.C. Jia, X. Xu, S. Yang, J.Z. Xu, Z.M. Li, A high heatresistance bioplastic foam with efficient electromagnetic interference shielding, Chem. Eng. J. 323 (2017) 29–36. [7] J.M. Thomassin, C. Pagnoulle, L. Bednarz, I. Huynen, R. Jerome, C. Detrembleur, Foams of polycaprolactone/MWNT nanocomposites for efficient EMI reduction, J. Mater. Chem. 18 (7) (2008) 792–796. [8] P.K.S. Mural, S.P. Pawar, S. Jayanthi, G. Madras, A.K. Sood, S. Bose, Engineering nanostructures by decorating magnetic nanoparticles onto graphene oxide sheets to shield electromagnetic radiations, ACS Appl. Mater. Interfaces 7 (30) (2015) 16266–16278. [9] H.B. Zhang, Q. Yan, W.G. Zheng, Z. He, Z.Z. Yu, Tough Graphene-polymer microcellular foams for electromagnetic interference shielding, ACS Appl. Mater. Interfaces 3 (3) (2011) 918–924. [10] N. Yousefi, X. Sun, X. Lin, X. Shen, J. Jia, B. Zhang, B. Tang, M. Chan, J.K. Kim, Highly aligned graphene/polymer nanocomposites with excellent dielectric properties for high-performance electromagnetic interference shielding, Adv. Mater. 26 (31) (2014) 5480–5487. [11] M. Cao, C. Han, X. Wang, M. Zhang, Y. Zhang, J. Shu, H. Yang, X. Fang, J. Yuan, Graphene nanohybrids: excellent electromagnetic properties for the absorbing and shielding of electromagnetic waves, J. Mater. Chem. C 6 (17) (2018) 4586–4602. [12] Y. Yuan, W. Yin, M. Yang, F. Xu, X. Zhao, J. Li, Q. Peng, X. He, S. Du, Y. Li, Lightweight, flexible and strong core-shell non-woven fabrics covered by reduced graphene oxide for high-performance electromagnetic interference shielding, Carbon 130 (2018) 59–68. [13] C. Liang, H. Qiu, Y. Han, H. Gu, P. Song, L. Wang, J. Kong, D. Cao, J. Gu, Superior electromagnetic interference shielding 3D graphene nanoplatelets/reduced graphene oxide foam/epoxy nanocomposites with high thermal conductivity, J. Mater. Chem. C 7 (9) (2019) 2725–2733. [14] D.X. Yan, H. Pang, B. Li, R. Vajtai, L. Xu, P.G. Ren, J.H. Wang, Z.M. Li, Structured reduced graphene oxide/polymer composites for ultra-efficient electromagnetic interference shielding, Adv. Funct. Mater. 25 (4) (2015) 559–566. [15] J. Ling, W. Zhai, W. Feng, B. Shen, J. Zhang, W.G. Zheng, Facile preparation of lightweight microcellular polyetherimide/graphene composite foams for electromagnetic interference shielding, ACS Appl. Mater. Interfaces 5 (7) (2013) 2677–2684. [16] W.L. Song, M.S. Cao, M.M. Lu, S. Bi, C.Y. Wang, J. Liu, J. Yuan, L.Z. Fan, Flexible graphene/polymer composite films in sandwich structures for effective electromagnetic interference shielding, Carbon 66 (2014) 67–76. [17] S.P. Zhu, C.T. Xing, F. Wu, X.B. Zuo, Y.F. Zhang, C.C. Yu, M.L. Chen, W.W. Li, Q. Li, L.W. Liu, Cake-like flexible carbon nanotubes/graphene composite prepared via a facile method for high-performance electromagnetic interference shielding, Carbon 145 (2019) 259–265. [18] E. Zhou, J. Xi, Y. Guo, Y. Liu, Z. Xu, L. Peng, W. Gao, J. Ying, Z. Chen, C. Gao, Synergistic effect of graphene and carbon nanotube for high-performance electromagnetic interference shielding films, Carbon 133 (2018) 316–322.

4. Conclusion In this work, the Fe3O4/GF/PDMS composite of high EMI SE and excellent mechanical properties was successfully fabricated. The density of the composite is just only 0.13 g cm 3, and the EMI SE is as high as 32.4 dB in the frequency of 8.2–12.4 GHz. Even after 10000 cycles of repeated bending, the EMI SE of the composite remains 29.4 dB. The reflection shielding is not weakened because the loaded Fe3O4 nano­ particles have no influence on the electronic contact of highly conduc­ tive graphene sheets, which is different from that of the composites of filling Fe3O4/rGO into polymers. And the increased EMI SE of Fe3O4/ GF/PDMS comparing with that of GF/PDMS is mainly from absorption shielding, which is attributed to the magnetic attenuation of Fe3O4. Therefore, the novel structural Fe3O4/GF/PDMS composite directly manifests the synergistic effect between conductive graphene and magnetic Fe3O4 nanoparticles in EMI SE. Declaration of competing interest 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. CRediT authorship contribution statement Shoupu Zhu: Data curation, Formal analysis, Writing - original draft. Qing Cheng: Data curation, Formal analysis. Congcong Yu: Data curation, Formal analysis. Xiaochun Pan: Writing - review & editing. Xiaobo Zuo: Writing - review & editing. Jianfei Liu: Writing - review & editing. Mingliang Chen: Writing - review & editing. Weiwei Li: Writing - review & editing. Qi Li: Supervision, Writing - review & editing. Liwei Liu: Supervision. Acknowledgments This work was supported by the National Natural Science Foundation of China [Grant Nos. 61605237 and 51972330], the Military Commis­ sion Logistics Department [Grant Nos. BY117J013], the State Key Pro­ gram of National Natural Science Foundation of China [Grant No. 61734008], Jiangsu Province Postdoctoral Research Funding Scheme [Grant No. 2018K158C], Youth Support Project of Key Laboratory of Nano Devices and Applications of Chinese Academy of Sciences [Grant No. Y4JA21001]. We also thank the technical support of Nano-X, the Platforms of Characterization & Test, and Nanofabrication Facility from Suzhou Institute of Nano-Tech and Nano-Bionics. 6

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