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Stiff, Thermally Stable and Highly Anisotropic Wood-Derived Carbon Composite Monoliths for Electromagnetic Interference Shielding Ye Yuan, Xianxian Sun, Minglong Yang, Fan Xu, Zaishan Lin, Xu Zhao, Yujie Ding, Jianjun Li, Weilong Yin, Qingyu Peng, Xiaodong He, and Yibin Li* Center for Composite Materials and Structures, Harbin Institute of Technology, Harbin 150080, People’s Republic of China S Supporting Information *

ABSTRACT: Electromagnetic interference (EMI) shielding materials for electronic devices in aviation and aerospace not only need lightweight and high shielding effectiveness, but also should withstand harsh environments. Traditional EMI shielding materials often show heavy weight, poor thermal stability, short lifetime, poor tolerance to chemicals, and are hard-to-manufacture. Searching for high-efficiency EMI shielding materials overcoming the above weaknesses is still a great challenge. Herein, inspired by the unique structure of natural wood, lightweight and highly anisotropic wood-derived carbon composite EMI shielding materials have been prepared which possess not only high EMI shielding performance and mechanical stable characteristics, but also possess thermally stable properties, outperforming those metals, conductive polymers, and their composites. The newly developed low-cost materials are promising for specific applications in aerospace electronic devices, especially regarding extreme temperatures. KEYWORDS: wood, carbon monolith, EMI shielding, anisotropy, thermal stability

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materials in aviation and space industries.11 Recently, 3D graphene monoliths made up of graphene sheets have been attracted much attention due to their interesting physical and chemical properties. The conductive 3D carbon monolith is a highly desirable candidate for lightweight EMI shielding materials.12−15 However, graphene monoliths are poor in mechanical strength due to the weak cross-links between adjacent graphene sheets, which would hamper their further engineering applications. Wood has been a fundamental engineering material since the beginning of humanity, which has remarkable mechanical properties, light weight, and process ability due to the unique structures from its natural growth. Owing to its biocompatibility and abundance, wood-based materials not only possess important engineering applications but also show great potential in many new technology fields, such as electronics, optics, energy storage, and catalyst.16−24 For example, M. Zhu et al. utilize wood to fabricate two types of transparent wood composites which display extraordinary anisotropic optical and mechanical properties.16 Endowing these functional wood-based materials with electromagnetic properties may have an important significance for exploring lightweight and strong EMI shielding materials for aircraft electronic devices. In this study, natural wood was used as an anisotropic honeycomb-like template to fabricate highly aligned carbon

INTRODUCTION Within the limited space in the aircraft systems, densities of electronic devices have been increased dramatically. In order to make the electronic devices function reliably and not interfere with each other in a confined space, good electromagnetic compatibility (EMC) is strongly needed.1−3 Shielding is one of the most important methods to improve the EMC of electronic devices, which can effectively restrain various electromagnetic interference spreading through the space. In the past decade, considerable efforts have been made for developing EMI shielding materials with high performance. Being lightweight, strong, thermally stable, chemically resistant, and low-cost are the basic technical requirements for practical EMI shielding applications, especially in areas of aviation and aerospace electronic devices.4−6 Metals show a good electromagnetic wave attenuation performance due to their superior electrical conductivities. However, light weight is a necessity for aviation and aerospace applications. Therefore, metals possessing high EMI shielding effectiveness values but with larger densities are less desirable.7 Although the conductive polymer composites containing conductive nanofillers have outstanding advantages compared with conventional metal-based EMI shielding materials, thermal stability, poor flame retardancy, short lifetime, and manufacturing difficulty may restrict their use as EMI shielding materials in harsh environments, especially regarding extreme temperatures.8−10 Porous carbon materials are important as electromagnetic wave attenuating, microelectronic, and thermal management © 2017 American Chemical Society

Received: March 30, 2017 Accepted: June 6, 2017 Published: June 6, 2017 21371

DOI: 10.1021/acsami.7b04523 ACS Appl. Mater. Interfaces 2017, 9, 21371−21381

Research Article

ACS Applied Materials & Interfaces

Figure 1. Schematic of wood derived carbon composite monoliths fabrication steps. The wooden corks were regarded as the raw materials and immersed into the saturated urea solution for 24 h. After drying, the wood and urea mixture was carbonized from 600 to 1000 °C step by step under Argon atmosphere to form wood-derived carbon composites which grafted with nitrogen doped graphene sheets (WCM@N-G). Finally, the WCM@N-G was coated with AgNWs network by simply immersing it into the AgNWs solution. Scale bars, 20 μm. Material Characterizations. The morphology of the carbon monolith 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 spectroscopy (XPS) (VG Scientific ESCALAB Mark II spectrometer). The nitrogen sorptions of samples were measured at Autosorb iQ from Quantachrome Instruments. The apparent surface area was calculated using the BET method, at 77 K. The pore size distribution plots were recorded from the desorption branch of the isotherms based on the Barrett−Joyner−Halenda (BJH) model. Material Measurements. EMI shielding effectiveness was measured by a Vector Network Analyzer (Agilent Technologies N5227A, U.S.A.). The test samples were carefully cut into 22.86 × 10.16 mm2 strips to fit the specific waveguide sample holders (8.2− 12.4 GHz). Thermal conductivity was measured using TPS 2500S from Hot Disk at room temperature via the steady state method. Samples were prepared by cutting them into cubes with dimensions of 30 × 30 × 30 mm3. The electrical conductivities were tested using PARSTAT 4000 (Princeton Applied Research). Specimens were cut into 30 × 3 × 2 mm3 strips. During the tests, two silver wires were stuck to both of the small sides of strip by elargol to connect the instrument. Compressive strength was measured using Instron 5944 (Instron Corporation) with a sensor of 2000 N at a compression speed of 1 mm/min in the room temperature. Samples were prepared by cutting them into cubes with dimensions of 20 × 20 × 20 mm3.

monolith and simultaneously grafted nitrogen-doped graphene (N-graphene) sheets on the porous structure, providing more scattering centers and interfaces. To further enhance the EMI shielding properties, silver nanowires (AgNWs) were also used to form a conductive network on the surface. It is also encouraging that the lightweight and stiff wood-derived carbon monolith (WCM) not only exhibits excellent EMI shielding effectiveness, but also shows high thermal stability, all of which have a great potential as a multifunctional EMI shielding material used in aviation and aerospace applications.

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EXPERIMENTAL SECTION

Materials. The wood was taken from the cork of thermos bottles, which was made from mulberry trees, purchased from a local market. All chemicals were of analytical grade and were used as-received. All water used was deionized (DI) water. Preparation of Wood-Derived Carbon Monoliths. First, the wooden corks were carefully cut into appropriate dimensions and dried in an oven at 60 °C for 24 h. Then the wood was immersed into the saturated solution of urea at 50 °C for 18 h. After that, the wood was dried in an oven at 60 °C for 24 h. The samples were then heated to 600 °C and maintained at this temperature for 1 h, then heated to 1000 °C, and held at these temperatures for 3 h. The whole heating process was in the protection of Ar atmosphere. Finally, the carbon composite monolith was immersed into the Ag nanowire solution, whose concentration is about 1.0 mg/mL. A series of AgNWs were synthesized by a simple one-step process. In the typical synthesis procedure, 0.64 mg of FeCl3 was added in 40 mL of EG solution and stirred until FeCl3 was completely dissolved. Afterward, 0.738 g of PVP was added in that solution and stirred until it was completely dissolved. This solution was marked as A1. At the same time, 0.68 g of AgNO3 was added into another 40 mL of EG solution. The solution was continuously stirred until all AgNO3 was dissolved. This solution was marked as A2. Then, A1 was dropped into A2 at a constant rate of 1 mL/min, along with continuous stirring. Finally, the mixed solution was placed into a Teflon autoclave and heated to 160 °C for 2.5 h. The obtained suspension was diluted with DI water.25 In the last step, the wood derived carbon composite products were dried at 60 °C for 24 h for further use.

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RESULTS AND DISCUSSION Figure 1 describes the entire procedure for preparing the wood derived carbon composite monoliths, which starts from a wooden cork. After immersion into a saturated solution of urea and dried in an oven, the samples were carbonized in an Ar atmosphere. For pure wood, the color turned from light brown to a black metallic luster with around 25% volume shrinkage after carbonization. However, the geometry shape of the carbon monolith is well maintained. Two reactions took place during the heating procedure, that is, dehydration and carbonization. Compared with pure WCM, WCM grafted N-graphene 21372


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Figure 2. Typical cross-section SEM images of WCM: (a) to (c) transversal direction; (d) to (f) longitudinal direction. Scale bars, 100 μm (a) and (d), 20 μm (b) and (e), and 5 μm (c) and (f).

Figure 3. (a) An analogy picture for grafting process; typical cross-section SEM images of WCM@N-G (b) and (c), WCM@N-G@AgNWs (d) and (e). Insert picture in (b) and (d) is the high magnification SEM images of N-graphene sheets and AgNWs. Scale bars, 50 μm.

wood. Moreover, it is clear that most of the cells are open structures, indicating high air flow inside WCM. Compared with the transversal section, as indicated by Figure 2(d−f), cross-sectional SEM images of longitudinal section reveal long cylindrical channels with a diameter of a few tens of micrometers, indicating the anisotropic nature of the wood structure. It is known that wood cells exhibit cylindrical structure with a high aspect ratio and primarily run parallel to the trunk of the tree. Interestingly, some smaller pores were found to be trapped in the middle of two adjacent longitudinal cell walls, which built a hierarchical pore structure in the monolith. SEM images in Figure 3 depict the surface morphology of WCM@N-G and WCM@N-G covered with AgNWs (WCM@ N-G@AgNWs). Unlike WCM, here the carbonized wood forms the main framework for the porous network, while Ngraphene sheets were grafted on the surface, which is similar to the moss covered on the gray tile of a traditional Chinese architecture’s roof. Graphene sheets in most of the previously reported aerogels are usually stacked densely into pore walls so as to provide sufficient structural stability. However, in WCM@ N-G, N-graphene sheets are randomly grown from the wood surface and separated from each other. This is favorable for

(WCM@N-G) appear darker, which may be ascribed to the microstructure composed of N-graphene sheets. During carbonization, urea not only serves as a molecular template to form graphene, but also acts as a nitrogen source for Ndoping. Meanwhile, the microfibers in wood act as the carbon source to grow N-graphene in a two-step process on increasing temperature. First, layered g-C3N4 and small carbon clusters were generated from urea and microfibers, respectively, at about 600 °C. Then, when the reaction temperature grows up to 1000 °C, the N-graphene sheets were formed between the gC3N4 layers.26 The above mechanism enables one-step synthesis of hierarchical carbon composite monoliths containing carbonized wood skeletons and N-graphene sheets. The WCM@N-G was immersed into the Ag nanowires solution whose concentration is about 1.0 mg/mL and stirred for a few minutes. The sample was finally taken out and dried at 60 °C for 24 h. Typical anisotropic structures of WCM in transversal and longitudinal sections are shown in Figure 2. In the transversal sections, shown in Figure 2(a−c), the WCM cell exhibited a uniformly rectangular honeycomb structure with average cell size about 50 μm, and the thickness of the cell wall is around 1 μm, which originates from the lingocellulosic cell structure of 21373


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Figure 4. (a) Raman spectrum of WCM and WCM@N-G; (b) XPS survey spectra of WCM and WCM@N-G; (c) High-resolution N 1s spectrum of WCM@N-G with the peak deconvoluted into graphitic N and pyridinic N peaks; (d−f) The nitrogen adsorption/desorption isotherms of WCM, WCM@N-G, and WCM@N-G@AgNWs.

2.94 atom % according to the XPS analysis, as is shown in Figure. 4(b). A certain amount of oxygen was also detected which could be attributed to adsorbed oxygen-containing molecules. The high-resolution N 1s XPS spectrum of WCM@ N-G demonstrates the presence of graphitic N and pyridinic N and it is worth noting that the percentage of graphitic N in WCM@N-G is much higher than pyridinic N, in Figure. 4(c). In order to confirm the existence of the N-graphene sheets, XRD patterns of WCM and WCM@N-G were also compared in Figure S4. It can be observed that there is an obvious difference between WCM and WCM@N-G in around 25°. WCM has a sharp peak, however, the peak of WCM@N-G is gentler, which means more defects existed in the carbon structure of WCM@N-G due to the growth of N-graphene sheets. The nitrogen sorption measurement results and pore distributions of WCM, WCM@N-G, and WCM@N-G@ AgNWs are shown in 4(d−f). The specific surface area of WCM, WCM@N-G, and WCM@N-G@AgNWs is measured to be 71m2/g, 115 m2/g, and 79 m2/g, respectively. It is easy to speculate that the specific surface area of WCM@N-G is larger than WCM because of the contributions of the grafted Ngraphene sheets. However, the specific surface area was decreased after covering the AgNWs. Although the covered AgNWs networks may increase the specific surface area of the sample, the grafted N-graphene would stack together and aggregate on the surface of the walls when immered into the AgNW solution, resulting in a decrease of the specific surface area. The insert images of pore distribution of the three samples indicate that the diameters of a majority of the nanopores ranged from 0 to 5 nm. The nitrogen adsorption/desorption isotherms in Figure 4(d,e) seem to differ a lot from the typical isotherms of mesoporous and microporous materials, it supposes that the pores in WCM formed by the accumulation of tabular materials and the hysteresis loop was formed by capillary condensation effect.27−29 EMI Shielding Effectiveness. Lightweight and strong EMI shielding material is currently in high demand for electronic devices.30 Electromagnetic waves can be attenuated quickly in a good conductor because of the induced current created in the conductor. Large electrical conductivity is typically needed to

maximum exposure and access of the graphene surface area. As the N-graphene sheets are directly grown from the wood surface, the interface is chemically bonded resulting in strong adhesion between the graphene and the wood surface.26 TEM images of the N-graphene sheets and the interface between the graphene sheets and the carbon matrix of WCM@N-G are shown in Figure S1 of the Supporting Information (SI). Meanwhile, the N-graphene sheets not only grow on the surface of the cross-section, but also grow on the surface along pore channel, which can be observed from Figure S2. However, as the diameter of the cell is as small as around 50 μm, it is difficult for g-C3N4 to deposit deeply through the pore channel, especially for thick samples. The different morphologies of WCM@N-G, compared with WCM, indicate that the combination of raw wood and urea is necessary in the synthesis of the “moss on tile” structure. Figure 3(d,e) shows the surface morphologies of WCM@N-G@AgNWs. It clearly shows that the AgNWs randomly cover the surface of WCM@N-G, constructing a network structure. AgNWs at high magnification are shown in Figure S3. These AgNWs with diameter of 200 nm and length of 10 μm were overlapped together on the cell walls. To assess the chemical compositional evolution of the WCM and WCM@N-G, Raman spectra and X-ray photoelectron spectroscopy (XPS) were measured. Figure 4(a) shows the Raman spectra of the WCM sample. The peaks located at around 1320 and 1590 cm−1 are assigned to the characteristic D (defects and disorder) and G 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. In Figure 4(a), the D/G intensity ratio of the WCM was determined to be 0.85. The relatively low D/G intensity ratio might indicate a high degree of carbonization with few defects in the carbon structure. However, the D/G intensity ratio of WCM@N-G was 0.99, which indicates the presence of defects in WCM@N-G because of nitrogen-doping which breaks the sp2 carbon plane. In Figure 4(b,c), X-ray photoelectron spectroscopy (XPS) analysis reveals the introduction of nitrogen atoms into the structure of WCM@ N-G. The content of the nitrogen dopant was estimated to be 21374


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Figure 5. EMI shielding effectiveness of WCM, WCM@N-G, and WCM@N-G@AgNWs in the transversal direction (a) to (c), in the longitudinal direction (d) to (f).

Table S1. A comparison of three common commercial shielding materials and our resultant product were also compared in Table S2. Meanwhile, in Figure 5(d−f), the EMI shielding effectiveness in the longitudinal direction shows similar values and trends. Also, it can be observed from Figure 5 that the difference of the average shielding values between WCM and WCM@N-G@AgNWs is more than 5 dB. Thus, taking the balance of the shielding performance and cost into consideration, for high-end electronic devices in spacecraft which need shielding materials with high performance, WCM@N-G@ AgNWs is a good candidate. It is known that the high electrical conductivity is the key factor determining the shielding ability.31 The electrical conductivities of WCM, WCM@N-G, and WCM@N-G@AgNWs in the longitudinal direction were measured to be 65.84 S/cm, 65.92 S/cm, and 69.79 S/cm, respectively. Meanwhile, the electrical conductivities of WCM, WCM@N-G, and WCM@N-G@AgNWs in the transverse direction were measured to be 65.13 S/cm, 66.69 S/cm, and 68.73 S/cm. The electrical conductivities of samples measured in the transverse and longitudinal directions have little difference, which is beyond our expectations. It is known that the electrical conductivity of silver is about 6.3 × 107 S/m, and the electron mobility of graphene is about 15 000 cm2/(V·s), which means both the silver and the graphene are good conductors. The propagation of electronic waves in any medium can be described by Maxwell’s equation:32

obtain high performance EMI shielding materials. However, electrical conductivity is just the first step 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 shielding material are strongly needed, which can result in multiple scattering to absorb more electromagnetic waves. Inspired by the EMI shielding mechanism, a highly aligned and stiff EMI shielding material with hierarchical pore structure is fabricated. In this wood-derived EMI shielding material, the aligned carbon channels act as highly conductive frameworks and loading structures. The grafted N-graphene sheets introduced thousands of interfaces and defect sites on the surface of the WCM without adding much weight. Also, the covered AgNWs act as a conductive network in improving the EMI shielding performance. As discussed above, the EMI shielding properties of WCM, WCM@N-G and WCM@N-G@AgNWs were investigated in the frequency range of 8−12 GHz (X band), as is shown in Figure 5. During the preparation of EMI shielding materials, wood with similar density was cut into thin pieces with certain thicknesses. After carbonization, the length and width was modified to fit the size of the waveguide. The thickness of every piece of material was controlled as ∼1.5 mm by trial and error. In the transverse direction, the average EMI shielding values band for one piece of WCM, WCM@N-G, and WCM@N-G@ AgNWs are 39.5 dB, 41.0 and 44.2 dB, respectively. To further improve the EMI shielding effectiveness of the carbon composite monoliths, multiple monoliths were stacked together. As expected, the stacked monoliths show a large improvement in EMI shielding effectiveness. For example, for three pieces of WCM@N-G@AgNWs, the EMI shielding values were more than 60 dB. EMI shielding effectiveness for one piece of pure wood and WCM carbonized at 300 °C in the transversal direction were also compared in Figure S5. It can be observed that there is almost no shielding ability for them, this mainly attributes to their poor electrical conductivities. It is worth noting that for three pieces of WCM@N-G@ AgNWs, the specific EMI shielding effectiveness is as high as 465.1 dB·cm3·g−1 (density is around 0.13 g/cm3), which is higher than most of reported shielding materials, as shown in

∇ × E = −jωμH

(1)

∇ × H = −jωε E

(2)

∇·E = 0

(3)

∇·H = 0

(4)

The electromagnetic wave that propagates in a good conductor will produce heat in the good conductor by inducing the vibration of atoms. In this way, the electromagnetic wave attenuates quickly in a good conductor. The attenuation constant α of the electromagnetic wave in good conductor can be expressed as follows: α= 21375

ωμσ 2

(5) DOI: 10.1021/acsami.7b04523 ACS Appl. Mater. Interfaces 2017, 9, 21371−21381

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Figure 6. (a) Comparison of total EMI shielding effectiveness (SET), microwave absorption (SEA), and microwave reflection (SER) at the frequency of 9 GHz for WCM, WCM@N-G, and WCM@N-G@AgNWs in the transversal direction (a) and in the longitudinal direction (b); schematic of electromagnetic wave transfer across the carbon monolith in the transversal direction (c) and in the longitudinal direction (d).

where ω is the angular frequency, which equals 2πf, μ is the permeability of the good conductor, and σ is the electrical conductivity of the good conductor. Larger α values always mean more energy loss for electromagnetic waves. For silver, graphene, and WCM, μ are around 1.0, and σ are large values. Thus, in the X band, according to eq 5, the electromagnetic wave attenuates quickly in the good conductor. According to the equation of skin depth δ of good conductors, δ=

1 fπμσ

second layer and cell wall structure act as a reflecting surface and give 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. In the longitudinal direction, however, some incident waves strike the top surface of the cell wall and some are immediately reflected. Other incident waves propagate through the channel and reflected back and forth between the internal multilayers. The special microstructure provides WCM@N-G@AgNWs with the unique advantage to behave as a multilevel shield. When an 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. The total EMI shielding 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 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 = −10log(1 − R), and SEA = −10log(1 − Ae) = −10log[T/(1 − R)].33 Therefore, absorptivity (A), reflectivity (R), and transmissivity (T) can be obtained. Figure 6(a,b) shows the variation of SET, SEA, and SER of three pieces of WCM, WCM@N-G, and WCM@N-G@AgNWs at a frequency of 9 GHz. It is clear that the contribution of absorption to the EMI shielding is much higher than that of reflection. For instance, in the longitudinal direction, the

(6)

where f is the frequency, μ is the permeability, and σ is the electrical conductivity. For the same group of samples with different directions, μ is the same and σ is similar. Thus, it can be assumed that the skin depth is almost the same in the longitudinal direction and in the transversal direction. For instance, the skin depth is estimated to be 0.1 μm at X band for a pure WCM (suppose f = 10 GHz, σ = 65.92 S/cm, μ is about 1.0), which is much thinner than the cell wall. The schematic diagram of the EMI shielding mechanism of WCM@N-G@AgNWs is indicated in Figure 6(c,d). In the transversal direction, as electromagnetic incident waves strike the top surface, some waves are immediately reflected. The remaining waves pass through the AgNWs network and the interaction with the high electron density of N-graphene sheets induces currents in the porous structure, resulting in a drop in energy of the electromagnetic waves. The surviving waves, after passing through the two layers of conductive network, encounter the relatively thicker cell wall. As the cell wall is also highly conductive, the phenomenon of electromagnetic wave reflection and attenuation repeats. Simultaneously, the 21376


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Figure 7. Stress−strain measurements of WCM in the longitudinal direction (a) and in the transversal direction (b) with different bulk densities; More than 2000 g weight standing on a WCM sample (c) and the deformation analysis by FEM method (d).

Figure 8. (a) Thermal conductivities of WCM, WCM@N-G, and WCM@N-G@AgNWs in the transverse direction (a) and in the longitudinal direction (b); heat transfer analysis by FEM method (c) and (d).

absorption loss (49.31 dB) and reflection loss (4.86 dB) of WCM@N-G@AgNWs contribute to 91.8% and 8.2% of the total EMI shielding effectiveness, respectively. This suggests that the highly conductive hierarchical structure benefits from the multiple reflections of the incident microwaves inside the structure, and consequently are responsible for the absorptiondominant EMI shielding. Mechanical Strength. In addition to the excellent EMI shielding properties, the carbon composite also provides remarkable mechanical stability. Figure 7(a) shows the

compressive properties of the WCM. In the longitudinal direction, the compressive strength of the WCM was 1.6 MPa (WCM-1), 2.4 MPa (WCM-2), and 3.3 MPa (WCM-3), with the densities of 0.115 g/cm3, 0.133 g/cm3, and 0.148 g/cm3, respectively. On the contrary, the compressive strength in the transversal direction was dropped to 0.12 MPa (WCM-1), 0.57 MPa (WCM-2) and 0.84 MPa (WCM-3), with the densities of 0.115 g/cm3, 0.133 g/cm3 and 0.148 g/cm3, respectively. The difference of the compressive strength indicates the highly anisotropic structure of WCM. The finite element method 21377


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Figure 9. Developed heat transfer unit model according to the microstructure of WCM@N-G.

Furthermore, the grafted N-graphene sheets may stack together with each other, which may degrade its thermal insulating ability. The FEM was also employed to investigate the mechanism of anisotropic thermal conductivity of the material, as is shown in Figure 8. Although the boundary conditions are consistent, it can be observed that the temperature at the top surface is 33.16 °C (transversal direction) and 27.31 °C (longitudinal direction), respectively. According to the equation of Fourier law:34

(FEM) was employed to investigate the micro deformation induced by the anisotropic structure (using commercial FEM software ANSYS Workbench 16.0 as the analysis tool), as is shown in Figure 7(d). After the pressure was applied in the longitudinal direction, there was almost no visible deformation. However, after equal pressure was applied in the transversal direction, obvious deformations of the cell wall could be observed, which was about 3 orders of magnitude higher than that in the longitudinal direction. After grafting the N-graphene sheets and covering the AgNWs onto the WCM structure, the compressive strength did not have obvious changes, as is shown in Table S3. This is mainly ascribed to the fact that the amount of the grafted N-graphene sheets is tiny and the trace of Ngraphene sheets that bonded onto the surface of the cell wall had little or no effect on the mechanical properties of the monoliths. The load bearing ability of the carbon monolith is shown in Figure 7(c). It is clear seen that the carbon monolith is stiff enough to carry more than 2000 g weight without any deformation. Thermal Performance. EMI shielding materials need to be thermally stable and fire retardant when they are used in some extreme environments for aviation and aerospace applications. The anisotropic thermal properties of WCM, WCM@N-G, and WCM@N-G@AgNWs are investigated, as hown in Figure 8. The thermal conductivity of WCM is 0.102 W/m·K and 0.135 W/m·K with a density of around 0.129 g/cm3 in the transverse and longitudinal directions, respectively. After introducing the N-graphene sheets, the thermal conductivity of the WCM@NG tended to decrease to 0.089 W/m·K (transversal direction) and 0.127 W/m·K (longitudinal direction). This is mainly attributed to the porous thin film formed by the grafted Ngraphene sheets, which could act as an obstacle during the heat transfer. However, the thermal conductivity of the WCM@NG@AgNWs increased to 0.113 W/m·K (transversal direction) and 0.141 W/m·K (longitudinal direction). The AgNWs have a higher thermal conductivity and the covered AgNWs network plays an important role in improving the thermal conductivity.

ke =

QL A(Thot − Tcold)

(7)

Where ke is the effective thermal conductivity, Q is heat flow, and L is the thickness that heat transfer, A is the surface area that the heat applied, and Thot and Tcold is the temperature at the top and bottom surface of the model. It can easily be deduced that the thermal conductivity in the transversal direction is lower than that in the longitudinal direction. The results of measured and calculated thermal conductivities of WCM are shown in Table S4. It can be seen there are some differences between them. This is mainly due to the simplified model and the estimated thermal conductivity of the cell wall introducing errors for the final results. According to the cross-sectional SEM images of WCM and WCM@N-G, a unit cell model of WCM@ N-G in Figure 9 was developed to estimate the effective thermal conductivity of the anisotropic structures at room temperature. It is assumed that the void space of the unit cell model is saturated with air. The effect of convection and radiation are negligible due to the narrow pore space and the low environment temperature. The side length and thickness of the WCM and N-graphene sheets film of the unit cell model are marked in Figure 9. With the assumption of one-dimensional heat conduction in the direction where heat is imposed, the anisotropic thermal resistance of the developed model is obtained by considering the parallel or series resistances. The effective thermal 21378


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Figure 10. Thermal properties of the WCM@N-G. (a−d) The leaf placed on the steel plate on the flame of an alcohol lamp is burnt in a long period of time; (e−h) the leaf placed on the WCM@N-G can be well protected from the flame of an alcohol lamp even after being heating for 180 s; and (i−l) Photographs of WCM@N-G on a hot flame with time ranging from 0 to 300 s.

conductivity can be derived from the analogy between thermal and electrical resistances.34 If the heat is imposed at the right plane as shown in Figure 9(a), then the equivalent thermal circuit will consist of five components as shown in Figure 9(c). The overall effective thermal resistance of the unit cell in the transversal direction, Ret, is given by the following: R et =

Let ketAet

ket =

kNG

where ket is the overall thermal conductivity of the unit cell in the transversal direction, Let and Aet are the heat transfer distance and areas, respectively. The overall effective thermal resistance, Ret, can be expressed by the total parallel thermal resistances from S1 to S5,

kel =

(10)

Finally, the overall effective thermal conductivity of the unit cell is given by (detailed calculation process is shown in the SI): 2kNG + +4

a b

1 1 kNG

+

a 1 2b k WCM

+

a

(

2b

1 k WCM

+

1 kNG

+

1 kair

a

(

2b

1 k WCM

+

1 kNG

+

1 kair

)

(12)

(13)

A piece of green leaf was directly placed on a steel plate which was heated by an alcohol burner, and the leaf was totally scorched after 180 s. However, when a piece of WCM@N-G (3 mm in thickness) was placed in between the leaf and the steel plate, as shown in parts (e−h) of Figure 10, the leaf was well protected when the heating time was up to 90 s, and it was a little damaged even after heating to 180 s, indicating that the WCM@N-G has a low thermal conductivity and thermal stable property. Parts (i−l) of Figure 10 show the photographs for a WCM@N-G directly burning on a hot flame at different times. The shape of the carbon monolith maintained well after burning on the outer flame of the alcohol lamp for 180 s, and it was only a little damaged even after heating to 300 s, which demonstrates a high thermal stability of WCM@N-G. The thermal tolerance of WCM and WCM@N-G were characterized by TGA in air and is shown in Figure S6. It can be seen that both WCM and WCM@N-G have a mass loss less than 10% below 450 °C. These results indicate that the WCM@N-G is a good candidate that can be used as EMI shielding materials in high temperature environment.

As S1 is similar to S4 and S2 is similar to S5, eq 9 can be expressed as follows:

ket =

+

4b(a + b) 4b(a + 3b) a2 + k k + 2 WCM 2 NG (a + 4b) (a + 4b) (a + 4b)2 kair

(9)

1 2 2 1 = + + R et R s1 R s2 R s3

+

a 1 2b k WCM

where kWCM, kNG, and kair are the thermal conductivity of wood derived carbon, N-graphene film, and air, respectively. Similarly, the overall effective thermal conductivity of the unit cell in longitudinal direction, kel, is given by the following (a detailed calculation process is shown in SI):

(8)

1 1 1 1 1 1 = + + + + R et R s1 R s2 R s3 R s4 R s5

1 1

)

(11)

As observed from Figure 2, the diameter of the cell is much larger than its thickness, that is, a ≫ b. Also, the thermal conductivity of graphene foam is low according to previous reports.35 Finally, the first item in eq 11 can be negligible and can be expressed as follows: 21379


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(5) Wen, B.; Cao, M.; Lu, M.; Cao, W.; Shi, H.; Liu, J.; Wang, X.; Jin, H.; Fang, X.; Wang, W.; Yuan, J. Reduced Graphene Oxides: LightWeight and High-Efficiency Electromagnetic Interference Shielding at Elevated Temperatures. Adv. Mater. 2014, 26, 3484−3489. (6) Yousefi, N.; Sun, X.; Lin, X.; Shen, X.; Jia, J.; Zhang, B.; Tang, B.; Chan, M.; Kim, J. K. Highly Aligned Graphene/Polymer Nanocomposites with Excellent Dielectric Properties for High-Performance Electromagnetic Interference Shielding. Adv. Mater. 2014, 26, 5480− 5487. (7) Yang, Y.; Gupta, M. C.; Dudley, K. L.; Lawrence, R. W. Novel Carbon Nanotube-Polystyrene Foam Composites for Electromagnetic Interference Shielding. Nano Lett. 2005, 5, 2131−2134. (8) Yang, Y.; Gupta, M. C.; Dudley, K. L.; Lawrence, R. W. Conductive Carbon Nanofiber-Polymer Foam Structures. Adv. Mater. 2005, 17, 1999−2003. (9) Li, N.; Huang, Y.; Du, F.; He, X.; Lin, X.; Gao, H.; Ma, Y.; Li, F.; Chen, Y.; Eklund, P. C. Electromagnetic Interference (EMI) Shielding of Single-Walled Carbon Nanotube Epoxy Composites. Nano Lett. 2006, 6, 1141−1145. (10) Zhang, H. B.; Yan, Q.; Zheng, W. G.; He, Z.; Yu, Z. Z. Tough Graphene-Polymer Microcellular Foams for Electromagnetic Interference Shielding. ACS Appl. Mater. Interfaces 2011, 3, 918−924. (11) Chung, D. D. L. Electromagnetic Interference Shielding Effectiveness of Carbon Materials. Carbon 2001, 39, 279−285. (12) Cao, M. S.; Wang, X. X.; Cao, W. Q.; Yuan, J. Ultrathin Graphene: Electrical Properties and Highly Efficient Electromagnetic Interference Shielding. J. Mater. Chem. C 2015, 3, 6589−6599. (13) Kumar, P.; Shahzad, F.; Yu, S.; Hong, S. M.; Kim, Y.-H.; Koo, C. M. Large-area Reduced Graphene Oxide Thin Film with Excellent Thermal Conductivity and Electromagnetic Interference Shielding Effectiveness. Carbon 2015, 94, 494−500. (14) Balandin, A. A. Thermal Properties of Graphene and Nanostructured Carbon Materials. Nat. Mater. 2011, 10, 569−581. (15) Du, X.; Skachko, I.; Barker, A.; Andrei, E. Y. Approaching Ballistic Transport in Suspended Graphene. Nat. Nanotechnol. 2008, 3, 491−495. (16) Zhu, M. W.; Song, J. W.; Li, T.; Gong, A.; Wang, Y. B.; Dai, J. Q.; Yao, Y. G.; Luo, W.; Henderson, D.; Hu, L. B. Highly Anisotropic, Highly Transparent Wood Composites. Adv. Mater. 2016, 28, 5181− 5187. (17) Si, Y.; Wang, X.; Yan, C.; Yang, L.; Yu, J.; Ding, B. Ultralight Biomass-Derived Carbonaceous Nanofibrous Aerogels with Superelasticity and High Pressure-Sensitivity. Adv. Mater. 2016, 28, 9512− 9518. (18) Li, T.; Zhu, M. W.; Yang, Z.; Song, J. W.; Dai, J. Q.; Yao, Y. G.; Luo, W.; Pastel, G.; Yang, B.; Hu, L. B. Wood Composite as An Energy Efficient Building Material: Guided Sunlight Transmittance and Effective Thermal Insulation. Adv. Energy Mater. 2016, 6, 1601122. (19) Wang, Y. G.; Sun, G. W.; Dai, J. Q.; Chen, G.; Morgenstern, J.; Wang, Y. B.; Kang, S. F.; Zhu, M. W.; Das, S.; Cui, L. F.; Hu, L. B. A High-Performance, Low-Tortuosity Wood-Carbon Monolith Reactor. Adv. Mater. 2017, 29, 1604257. (20) Pan, Z. Z.; Nishihara, H.; Iwamura, S.; Sekiguchi, T.; Sato, A.; Isogai, A.; Kang, F. Y.; Kyotani, T.; Yang, Q. H. Cellulose Nanofiber as a Distinct Structure-Directing Agent for Xylem-Like Microhoneycomb Monoliths by Unidirectional Freeze-Drying. ACS Nano 2016, 10, 10689−10697. (21) Zhu, H. L.; Fang, Z. Q.; Wang, Z.; Dai, J. Q.; Yao, Y. G.; Shen, F.; Preston, C.; Wu, W. X.; Peng, P.; Jang, N.; Yu, Q. K.; Yu, Z. F.; Hu, L. B. Extreme Light Management in Mesoporous Wood Cellulose Paper for Optoelectronics. ACS Nano 2016, 10, 1369−1377. (22) Zhu, M. W.; Li, T.; Davis, S. C.; Yao, Y. G.; Dai, J. Q.; Wang, Y. B.; AlQatari, F.; Gilman, W. J.; Hu, L. B. Transparent and Haze Wood Composites for Highly Efficient Broadband Light Management in Solar Cells. Nano Energy 2016, 26, 332−339. (23) Liu, K. K.; Jiang, Q. S.; Tadepalli, S.; Raliya, R.; Biswas, P.; Naik, R. R.; Singamaneni, S. Wood-Graphene Oxide Composite for Highly Efficient Solar Steam Generation and Desalination. ACS Appl. Mater. Interfaces 2017, 9, 7675−7681.

CONCLUSIONS In summary, lightweight, thermally stable, and stiff highperformance EMI shielding wood derived carbon composite monoliths have been successfully prepared via a facile strategy. A high compressive strength of 3.3 MPa and a high EMI shielding effectiveness of more than 60 dB in the X band were achieved. The carbon composite monoliths also show the specific EMI shielding effectiveness as high as 465.1 dB·cm3· g−1. Moreover, the low thermal conductivity and high flame retardancy of the foam allows it to be used in high temperature environments. The high EMI shielding effectiveness, plus the thermal stable feature, make the wood derived carbon composite monoliths useful for EMI shielding applications particularly in aviation and aerospace areas.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b04523. TEM images of WCM@N-G, SEM images of WCM@NG and AgNWs, XRD patterns of WCM and WCM@NG, EMI shielding effectiveness in the transversal direction of pure wood (a) and WCM carbonized at 300 °C, the thermal tolerance of WCM and WCM@N-G, EMI shielding performance of typical carbon-based composites, compressive stress of WCM@N-G and WCM@NG@AgNWs, measured results, and FEM results of thermal conductivities of WCM@N-G, details of thermal conductivity calculation process of WCM@N-G (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel/Fax: +86-451-86402326. E-mail: [email protected] (Y.L.). ORCID

Ye Yuan: 0000-0002-1656-9053 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 (20122302110065).

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REFERENCES

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