Carbon Nanotubes and Their Assemblies: Applications in Electromagnetic Interference Shielding

CHAPTER

CARBON NANOTUBES AND THEIR ASSEMBLIES: APPLICATIONS IN ELECTROMAGNETIC INTERFERENCE SHIELDING

14

Songlin Zhang, Nam Nguyen, Jin Gyu Park, Ayou Hao, Richard Liang High-Performance Materials Institute (HPMI), FAMU-FSU College of Engineering, Florida State University, Tallahassee, FL, United States

CHAPTER OUTLINE 1 Introduction ....................................................................................................................................335 2 EMI Shielding Mechanism and Measurement Techniques .................................................................. 336 2.1 Fundamental of Shielding Mechanism ............................................................................. 336 2.2 Shielding Effectiveness Measurement .............................................................................. 338 3 CNT EMI Composite Materials ..........................................................................................................339 4 CNT Sheets (Buckypaper) EMI Shielding Materials ............................................................................ 345 5 Future Trend in EMI Shielding Materials ........................................................................................... 349 6 Summary ........................................................................................................................................352 Acknowledgments ...............................................................................................................................353 References ......................................................................................................................................... 354

1 INTRODUCTION Electromagnetic interference (EMI) protection refers to the reflection and/or absorption of electromagnetic waves (EMWs) by a material, which acts as a shield against the penetration of EMWs. EMWs, especially at high frequencies, such as radio and cellular device waves, can interfere with other devices in proximity. The essential purpose of EMI shielding is to reduce the interference by either the reflection or absorption of EMWs using shielding materials. An effective shielding material should be electrically conductive, which can interact with the electromagnetic fields. Therefore, most shielding Nanotube Superfiber Materials. https://doi.org/10.1016/B978-0-12-812667-7.00014-8 Copyright # 2019 Elsevier Inc. All rights reserved.

335

336

CHAPTER 14 APPLICATION OF CNTs IN EMI SHEILDING

materials are metal-based materials, which have high electric conductivity. However, metal-based materials tend to be expensive and add more weight to the system of interest. Recently, lightweight and flexible EMI shielding materials have attracted wide research attention following the demand in reducing the weight of devices. The research is driven by lightweight and multifunctional materials that enable effective EMI block through either reflection or absorption in a broadband frequency of EMWs. Polymer-based composites are ideal candidate materials since they offer several advantages such as flexibility, ease of manufacturing, and low cost. However, absorption of electromagnetic energy in composites remains challenging due to the need of accomplishing both low reflection and high absorption losses for military purposes, such as for stealth aircrafts [1]. To make electrically conductive polymer composites, different conductive fillers, such as metal nanoparticles [2, 3], carbon black [4–6], graphite [2, 7], and carbon fibers [8, 9], are mixed with polymers by various mixing methods. These mixtures can be used to fabricate EMI shielding protection systems [2, 3, 8–10]. Among different fillers, carbon nanotubes (CNTs) have been researched intensively because of their excellent mechanical and electric properties, and more conductive composites could be obtained with lower loading percentage due to their high aspect ratio. These properties make CNT an excellent choice for high-performance EMI shielding materials at low filler loading [11]. Despite having been extensively explored in recent years, several factors, such as layered structures, special foam structures, and hybridized structures, affect the performance of nanocarbon/polymer composites used as shielding materials; so, further studies are required. In this work, we will present some state-of-the-art efforts in the design and characterization of CNT-based materials for EMI shielding applications. Shielding mechanism, advantages of CNT composites, and the effects of different structure designs on shielding effectiveness (SE) will be presented. The future trend of hybridized nanocarbon architecture will also be discussed.

2 EMI SHIELDING MECHANISM AND MEASUREMENT TECHNIQUES 2.1 FUNDAMENTAL OF SHIELDING MECHANISM Generally, EMI shielding can be achieved by either reducing the original source or source system, or enhancing the shielding performance of the area of interest, or a combination of both. This may be achieved by three primary strategies [12]: (1) diverging the electromagnetic field of source from the area of interest, (2) introducing a cutoff barrier between the source system and the area where EMI shielding is needed, or (3) adding an additional electromagnetic source to reduce the EMI effect of original source system on the area of interest. The choice of shielding methods depends on the various characteristics of source system (electromagnetic or physical) and the area of interest (size and weight issue for miniaturized devices). Compared with the other two strategies, the introduction of cutoff barrier, which is usually called EMI shielding material, is particularly effective in reducing EMI interference due to the universal presence of electromagnetic fields in the modern world. EMI SE is measured in terms of reducing the magnitude of incident EMW power upon transmission across the shielding materials. Generally, shielding materials could show a high shielding performance (higher SE) when shielding materials are highly electric conductive. When the incident EMWs reach the surface of shielding materials, several different interactions between the waves and shielding materials can be expected, as shown in Fig. 1 [13]. It identifies three

2 EMI SHIELDING MECHANISM AND MEASUREMENT TECHNIQUES

337

Incident wave (Ei, Hi) Absorbance (A) Reflected wave (Er, Hr) Reflectance (R) Transmitted wave (Et, Ht) Transmittance (T) 2nd Reflection loss

Incident field strength (Ei, Hi)

Internal reflection

2nd Transmission

Skin depth, õ

Remaining field strength (Et, Ht)

Distance from shield face (t)

FIG. 1 Schematic representation of EMI shielding mechanism [13].

major different mechanisms: reflection (R), absorption (A), and multiple internal reflections (MIRs), which correspond to three SE components SER, SEA, and SEM, due to reflection, absorption, and multiple reflections, respectively. Thus, the total shielding effectiveness (SET) of a particular shielding material can be mathematically described in a logarithmic scale as [14]: SET ðdBÞ ¼ SER + SEA + SEM       PT ET HT ¼ 20log 10 ¼ 20log 10 ¼ 10 log 10 PI EI HI

(1)

where PT (ET or HT) and PI (EI or HI) are the power values (electric or magnetic field intensity) of transmitted and incident EM waves, respectively, as shown in Fig. 1. Based on the classical electromagnetic shielding theory for a plane-wave radiation, reflection loss (SER) and absorption loss (SEA) can be expressed in terms of conductivity (σ), frequency (ω), real part of permeability (μ0 ), skin depth (δ), and thickness (t) of the shielding material as [13, 15] 

 σ 16ωε0 μ0   t t σωμ0 1 2 SEA ðdBÞ ¼ 20 log 10 e ¼ 8:68 ¼ 8:68t 2 δ δ SER ðdBÞ ¼ 10 log 10

(2) (3)

where ε0 is the permittivity of air (or free space). The above expressions show that reflection loss is a function of the ratio of conductivity (σ) and permeability (μ0 ) of the shielding material. Absorption loss is proportional to frequency (ω) of radiation and material thickness (t). Therefore, a good absorbing material should possess high conductivity and permeability and sufficient thickness to achieve the required number of skin depths even at the lowest frequency of concern [13]. Multiple internal reflections (SEM) are closely related to absorption loss (SEA) and can be neglected in the case of high SEA so that

338

CHAPTER 14 APPLICATION OF CNTs IN EMI SHEILDING

by the time the wave reaches the second boundary, it is of negligible amplitude. For practical purpose, when SEA is 10 dB, SEM can be safely neglected [15]. However, SEM is very important for porous shielding materials and for specifically designed type of filled composites with certain design geometries [13].

2.2 SHIELDING EFFECTIVENESS MEASUREMENT Vector network analyzer (VNA) is the most common instrument to measure the magnitude and phases of various signals. Fig. 2 shows a typical test setup, including the VNA of two ports, waveguide adapters, and sample holders. Once the incident waves reach the sample, the reflected waves and transmitted waves are detected at port 1 and port 2, which can be represented by complex scattering parameters (or S-parameters), that is, S11 (or S22) and S12 (or S21), respectively. The scattering parameters of Sij, i and j (which are VNA’s port number), mean receiving and sending port of electromagnetic wave, respectively. S11 (or S22) represents an electromagnetic wave sent from port 1 (or 2) and received by port 1 (or 2), which in turn can be conveniently correlated with reflectance (R), that is, R ¼ jER/EI j2 ¼ j S11 j2 ¼ jS22 j2, while S21 (or S12) represents electromagnetic wave sent from port 1 (or 2) and received by port 2 (or 1), which similarly can be correlated with transmittance (T), that is, T ¼ jET/EI j2 ¼ jS12 j2 ¼ j S21 j2. Since the sum of reflectance (R), transmittance (T), and absorbance (A) is equal to 1, A ¼ (1  R  T), SEM becomes negligible and can be omitted when SEA is larger than 10 dB. Thus, SET can be represented as SET ¼ SER + SEA. Furthermore, the relative intensity of the electromagnetic wave inside the shield by excluding the reflected wave from the incident wave is based on the value (1  R), so that the effective absorption, Aeff, can be written as Aeff ¼ [(1  R  T)/(1  R)]. Therefore, the shielding due to reflection, SER, and absorption, SEA, can be expressed as followed: SER ¼ 10 log 10 ð1  RÞ Vector network analyzer

Port 1

Port 2 Transmission line

Sample

Incident wave

Waveguide

adapter

Reflected wave (S11 or S22)

Transmitted wave (S12 or S21)

(A) (B) FIG. 2 (A) Schematic illustration of the EMI shielding measurement setup and (B) the photograph of one of the waveguide adapters [16, 17].

(4)

3 CNT EMI COMPOSITE MATERIALS

 SEA ¼ 10 log 10 ð1  Aeff Þ ¼ 10 log 10

 T 1R

339

(5)

Although the waveguide configuration described above is easy to set up and has been adopted in other research papers [16, 18], the disadvantage of this testing method is that several waveguide adaptors are required to measure the shielding performance in a wide range of frequencies. Other configurations for measuring solid materials include coaxial line [19, 20] and free-space arrangements [21]. For coaxialline configuration, shielding effectiveness can be measured from 0.5 GHz to a very high frequency of 18 GHz on the same sample without changing adaptors. Free-space arrangement is a noncontact measurement, with a sample introduced between two opposing antennae. The advantage of this method is to cover a wide range of frequencies and possibly change the angle of incident wave on the samples. However, comparing with the previous two techniques, this system requires considerably larger samples varying from around one ten of centimeters to several tens of centimeters to cover a few gigahertz to the frequency of near 20 GHz.

3 CNT EMI COMPOSITE MATERIALS With the rapid development of technology, modern systems are heavily equipped with “smart” electronic and electric devices, which radiate and are affected by electromagnetic waves. Conventional metals and metallic composites [22] could solve the EMI issues; however, the disadvantages are also frustrating because of heavier weight to the system, rapid corrosion, and expensive processing techniques. Modern smart devices require the next-generation EMI shielding materials that are lightweight, corrosion-resistant, flexible, and cost-effective. Compared with metal-based shielding materials, intrinsically conducting polymer (ICP) composites offer superior properties since they are lightweight, anticorrosive, and flexible and offer processing advantages. However, the SE is not satisfied due to the limited electric conductivity because EMI SE of composites significantly depends on their conductivity. To make polymer composite highly conductive, metallic particles or other conductive fillers (i.e., carbon-based fillers such as CNT, graphite, and graphene) are normally embedded in the composites [23–29]. CNTs are well-known conductive fillers for fabricating EMI shielding composite materials [24–26, 30, 31] due to their light weight, high conductivity, exceptionally high mechanical properties, etc. Several methods can be used to prepare CNT-based composite, such as solution mixing [25, 27], ball milling [26], and melt extrusion [28]. The key challenge of making CNT-filled composites is to get these nanofillers well dispersed in the polymer matrix. Fig. 3A shows a schematic of randomly dispersed CNTs in a matrix. To obtain a polymer composite with high conductivity and use less CNT fillers, a homogeneous dispersion of CNTs must be guaranteed to form a conductive network. As represented by a conductive pathway, shown in Fig. 3A (connected red (dark gray in print versions) segments), only these CNTs that are close enough to each other to form a continuous network can contribute to the effective conductivity of composites. Different contact patterns between neighboring CNTs, as shown in Fig. 3B, are of importance to affect the final conductivity of composites, and large contact areas are beneficial to improve the conductivity [32–34]. Due to the nanoscale sizes of CNTs, controlling the contact patterns is not practical; however, increasing the CNT loading percent (%) in the

340

CHAPTER 14 APPLICATION OF CNTs IN EMI SHEILDING

FIG. 3 (A) Schematic view of a representative 3D element with randomly dispersed CNTs in a matrix (the connected red (dark gray in print versions) segments represented one of the electron pathways, color version can be found in Ref. [24]). (B) Several representative contact patterns between two CNTs. (C) The relationship between CNT volume fraction (%) and the electric conductivity. The solid curve and isolated dots represented the predicted and measured electric conductivity [24].

composites can enhance the conductivity, as shown in Fig. 3C. A minimum loading percentage exists, which is called percolation threshold. Before the percolation threshold, the polymer composites are still insulating due to the discontinued CNT network. When the CNT loading percent is in the range of percolation threshold, the conductivity increases exponentially as the percentage of CNT fillers increases (Fig. 3C). The conduction behavior of CNT-filled composite could be well explained by a 3D resistor network model as the solid curve shown in Fig. 3C that fitted the experimental results very well [24]. Due to the hydrophobic nature of CNTs, dispersion of CNT in polymer matrix is a key step to obtain highly conductive composite. Several different methods have been attempted, such as using ultrasonication, high shear mixing, and ball milling, to separate the CNTs well distributed in a matrix. Although the final conductivity of composite is also greatly dependent on other parameters of CNTs including the nanotube length and diameter (aspect ratio) [30], the number of walls [21], and chirality [35], homogeneous distribution of CNTs within the matrix is more important than others because other parameters are difficult to control. To this end, chemical functionalization or pretreatment is generally applied to CNTs to prevent agglomeration during the polymer composite manufacturing process [36]. Another advantage of using functionalized CNTs is achieving a high conductivity with less CNTs due to a small percolation threshold. The most common functional groups introduced to CNTs are oxygen-containing groups, such as dCOOH, dOH, and ]CO. Fig. 4 illustrates the interaction between polymer and functionalized

3 CNT EMI COMPOSITE MATERIALS

H2

H2

H2

341

CH3

+ HOOC H2

O

C

C O

OBu

O O

H2

H2 H2

H2

CH3 Ester linkage

O

C

C O

OBu

O CH2 CH

OH

O CH2 OC

FIG. 4 Schematic diagram of the reaction between functional groups on the CNT with the epoxy groups of the RET (reactive ethylene terpolymer) [30].

CNTs. A rupture of the epoxide ring, which is contained in the reactive ethylene terpolymer (RET), would be facilitated by the dCOOH groups on the functionalized nanotubes and then contribute to the bonding of the COOH on the single-walled CNT (SWCNT) with the epoxy group of the RET [30]. The strong interaction between functionalized CNT fillers and polymer matrix would eliminate the most aggregation, which would happen in the case of pristine CNT filler-based composites. If these functional groups are considered to be randomly located on the surface of CNTs, isotropic bonding of the CNTs with the polymer matrix is implied, yielding uniform dispersion and mixing [30, 37]. Generally, covalent bonding stabilization could be beneficial for enhanced electric conductivity and electromagnetic SE [30, 38]. The EMI SE and conductivities of composite depend on several parameters, such as the processing method, polymer matrix, and CNT loading percent, which are listed in Table 1 [22].

Table 1 Electromagnetic Shielding of Some CNT Polymer Composites [22] Matrix PU PU/ PEDOT PMMA Cellulose PS PAN Epoxy

CNT Loading

Thickness (mm)

σ (S/m)

SET (dB)

Frequency (GHz)

Reference

10 wt% 30 wt%

>0.2 2.5

12.4 275

29 45

X-band 12.4

[37] [27]

10 vol% 9.1 wt% 15 wt% 2 wt% 15 wt%

2.1 0.2 N/A N/A 1.5

150 375 0.1 0.006 15

40 20 19 20 30

X-band 15–40 8–12 0.3–3 X-band

[26] [39] [40] [41] [26]

342

CHAPTER 14 APPLICATION OF CNTs IN EMI SHEILDING

FIG. 5 Uniform dispersion of functionalized SWCNTs in the RET polymer is indicated, through scanning electron microscope (SEM) micrographs, at both (A) 0.2 and (B) 2.2 vol% filling fractions of the SWCNTs. On the other hand, (C) nonfunctionalized SWCNTs dispersed into the RET matrix exhibit clumping. (D) Electric conductivity of the functionalized SWCNT/RET composites follows a power-law characteristic of percolation-like behavior. (E) Enhanced SE has been observed for the chemical functional group-reaction-enhanced SWCNT-RET polymer composites (COOH-SWCNT/RET) compared with nonfunctionalized SWCNT-RET composites (pristine SWCNT/ RET) [30].

Fig. 5A and B shows that functionalized SWCNT/RET composites (either low and high SWCNT loading percentage) present an uniform morphology with homogeneous tube dispersion in RET matrix [30]. The benefits of functionalization pretreatment to the nanotubes are more evident through a more homogeneous dispersion of the SWCNTs in the polymer matrix in comparison with the clusters in the case of nonfunctionalized SWCNT-based composites (Fig. 5C). Better dispersion results in a low percolation threshold as the DC electric conductivity (σ) SWCNT/RET composites shown in Fig. 5D. The electric-percolation-like behavior, which could be fitted to a power law of the form: σ  σ 0(p  pc)β, where pc is the percolation-threshold volume fraction ( p) and β is the critical exponent. σ 0 is a constant conductivity for a particular filler-polymer combination. As shown in the Fig. 5D inset through log-log plots, the obtained value of pc (0.11%) is very low, indirectly implying the homogeneous dispersion of SWCNT fillers. For many military and commercial applications, EMI shielding in the range of 8.2–12.4 GHz (X-band) is more important [42]. Fig. 5E clearly shows that higher loading of functionalized SWCNTs in composites is preferred to achieve a higher EMI shielding performance by utilizing the properties of SWCNT. A higher EMI SE was achieved for the dCOOH-functionalized SWCNT/RET composites compared with that of a nonfunctionalized SWCNT/RET counterpart. However, most of CNT composites with a polymer matrix have a maximum loading around less than 10 wt% [22]. Higher loading creates processing issues, agglomeration, and mechanical property deterioration.

3 CNT EMI COMPOSITE MATERIALS

343

FIG. 6 (A) Electric conductivity of MWCNT/WPU composites with various concentrations. SE in the X-band frequency range of MWCNT/WPU composites with various thicknesses and MWCNT loadings: (B) 76.2 and (C) 4.8 wt% [25].

To fully utilize the potential of CNTs as EMI shielding material, ultrahigh loading percent should be achieved to fabricate even thinner and lightweight shielding composites. Recently, Zeng et al. [25] reported a lightweight and flexible multiwalled carbon nanotube (MWCNT)/waterborne polyurethane (WPU) composites, which show superior EMI SE in the X-band even under the thin thickness of samples. The thickness values of 0.05, 0.32, and 0.8 mm correspond to SE of 24, 49, and 80 dB, respectively. This attributes to the extremely high MWCNT loading up to 76.2 wt%. The MWCNT aqueous solution was obtained by ultrasonic treatment with the assistant of noncovalent surfactant (aromatic modified polyethylene glycol ether in aqueous solution). Then, a simple solution mixing was applied with MWCNT aqueous dispersion and waterborne polyurethane (WPU) emulsion and stirred using a magnetic stirrer. Since the MWCNTs were dispersed in water by surfactant, the damage to the intrinsic structure was minimized while keeping the high conductivity of pure MWCNT. The electric conductivity of 76.2 wt% MWCNT loading sample can reach around 2100 S/m, as shown in Fig. 6A, almost 13 orders of magnitude higher than pure WPU matrix and 20 times higher than most of the MWCNT/polymer composites (less than or equal to 100 S/m) with lower CNT loading [22, 24–26, 30, 31, 43]. Huge differences in EMI SE were observed between high CNT loading (76.2 wt% in Fig. 6B) and low CNT loading (4.8 wt% in Fig. 6C) of WPU composites at the similar thickness of samples, which verified the importance of achieving high filler loading. A final thin-film sample with large areas was prepared on various substrates by traditional spread-plate technique after suitable amount of water-based polyurethane thickening agent, as shown in Fig. 7A and B. The shielding effectiveness showed good stability even after bending 1000 times, as shown in Fig. 7C. The strategy of absorption mechanism dominated CNT-based EMI shielding composites is of great concern of research and industry community when the reflection of electromagnetic waves is not desired due to the potential second pollution [22] and more importantly for the purpose of stealth application in defense areas [44]. Metals or metal-filler-based composites could be used as EMI shielding structural materials, but most electromagnetic waves are reflected to the surroundings, which would cause interference for other devices nearby. While CNT composites are verified to be absorptiondominant, Singh et al. [26] used a ball milling method to fabricate MWCNT/PC (portland cement) composites with various loading percent from 0% to 15% (Fig. 8A–H) and found the incorporation of 15 wt% MWCNTs in the PC matrix produced a SE more than 27 dB in X-band (8.2–12.4 GHz). This SE was found to be dominated by absorption mechanism. By gradually increasing the CNT loading percent, the SEA due to absorption was found to vary from 2 to 23 dB, while the SER due to reflection

344

CHAPTER 14 APPLICATION OF CNTs IN EMI SHEILDING

FIG. 7 Thin and flexible 66.7 wt% MWCNT/WPU composite film with large area and (A) 0.1 mm thick on PI substrate and (B) 0.05 mm thick on the cloth substrate; (C) EMI SE in the X-band of thin and flexible MWCNT/WPU composite films on PI substrates before and after repeated bending 1000 times [25].

increased from 1 to 6 dB. Thus, the total SE achieved for the MWCNT/PC (15%) composite was 28 dB. Fig. 8I and J shows that for a conducting MWCNT/PC composite, SE is mainly dominated by absorption, while the SER due to reflection remains nearly constant and nominal and contributes little [26]. Therefore, CNT composites have good absorption properties and better than metals or metal-fillerbased composites in terms of reduced electromagnetic wave reflectance. Another mechanism to explain the enhanced absorption shielding performance of CNT composites could be attributed to the multiple reflection that happens within the composites themselves [16]. Compared with solid materials, porous or foamed materials have advantages of more faces, which act as a reflective surface once the electromagnetic waves enter into the materials, resulting in an absorption shielding. Researchers have tried to show the advantage of foamed composites compared with those of unfoamed ones because of the multiple reflection shielding [31, 45–49]. Soltani Alkuh et al. [31] reported that foaming of PMMA/MWCNT composites resulted in a 60% reduction in electromagnetic reflection and a 96% increase in electromagnetic absorption due to the multiple reflection within the cells, as shown in Fig. 9A [31, 50]. The microstructure of foamed CNT composites matters including the cell size, density, and cell wall thickness [19, 31, 51]. Fig. 9B schematically shows the unfoamed composites (left) and foamed composites with thick cell walls (middle) and thin cell walls (right). Compared with thick walls, composites with thin cell walls resulted in a nonuniform distribution of the CNT

4 CNT SHEETS (BUCKYPAPER) EMI SHIELDING MATERIALS

345

FIG. 8 MWCNT/PC (portland cement) composite with different incorporation of MWCNTs (A) 0%, (B) 1%, (C) 2%, (D) 3%, (E) 4%, (F) 5%, (G) 10%, and (H) 15%. (I) and (J) Dependence of shielding effectiveness (SEA and SER) in the frequency range of 8.2–12.4 GHz with the increase of MWCNT percentage [26].

fillers and an increase in the distance of the adjacent CNTs. They found electromagnetic reflection is independent of cell density and cell size, while absorption improved by about 34% with increasing cell density and decreasing cell size, indicating an enhancement of the multiple reflection mechanism. The advantage of low density of foam materials enables the foamed EMI shielding composites with a high specific shielding effectiveness. Zeng et al. [48] reported MWCNT/WPU (waterborne polyurethane) composites by a facile freeze-drying method, reaching an SE of 50 dB (CNT 76.2 wt%) or 20 dB (CNT 28.6 wt%) in the X-band, while the density is merely 126 or 20 mg/cm3, respectively. The relevant specific SE over density is up to 1148 dB cm3/g, greater than those of other shielding materials ever reported [48].

4 CNT SHEETS (BUCKYPAPER) EMI SHIELDING MATERIALS CNT-based materials show great advantages over conventional metal materials in terms of EMI shielding application, such as lightweight, corrosion resistance, and potential large-scale production capability. As previously discussed, research work has examined the effectiveness of composites when CNTs were used as fillers. These studies showed that the overall high EMI SE could be obtained as the composites contain more CNT fillers [24–28]. The bulk CNT materials, such as pure CNT films

346

CHAPTER 14 APPLICATION OF CNTs IN EMI SHEILDING

Pin Pin

Pre

f

Absorbance area

Po

ut

Pin

Pre

f

Po

ut

Po

ut

Incident wave Reflective wave

Transmitted wave

Incident wave Reflective wave

Transmitted wave

Incident wave

(A) MWCNTs

(B) FIG. 9 (A) The description of increasing multiple reflections in composite foams caused by enhancing the cell density and reducing the cell size. (B) The distribution of MWCNTs in unfoamed composites (left) and foamed composites with thick cell walls (middle) and thin cell walls (right) [31, 50].

(i.e., buckypapers), came to play as an alternative to further improve the EMI shielding performance of CNT-based materials. Compared with bulk CNT materials, one of the great issues of CNT-filled composites is the limited electric conductivity because usually only small weight loading of CNT filler can be achieved during composite processing. That is because of the intrinsic hydrophobic characteristic and easy aggregation of nanotubes with high aspect ratio. As discussed earlier, EMI shielding performance of CNT-filled composites is strongly related with the electric conductivity, which was governed by the weight loading and distribution status of fillers. When the filler loading is high enough and well dispersed, a conductive network can be generated within composites. Therefore, a higher conductivity can be expected at a higher weight loading level of CNT fillers with a well-controlled dispersion process. However, high weight loading of CNT fillers will cause manufacturing process issues to achieve a good dispersion such as unavoidable aggregation. Therefore, directly using CNT thin films (or buckypapers, BPs) or sandwiched with polymer sheets to achieve high electric conductivity with high weight loading of CNT has attracted great attention.

4 CNT SHEETS (BUCKYPAPER) EMI SHIELDING MATERIALS

347

There are two popular ways to fabricate nanotube sheets or BPs: One is vacuum-assisted solution filtration, through which CNTs can be easily assembled together into a thin film, with dense and entangled network [21, 52], and the other is direct collection of large-area CNT film using the floating catalyst chemical vapor deposition (FCCVD) method [53]. By using BPs, high CNT loading can be easily achieved in composite manufacturing, resulting in a high electric conductivity. Since BPs can be infiltrated with resin, the BP composite itself can be directly used as EMI shielding material. The CNT prepregs made from nanotube sheets can be easily incorporated into the conventional fiberreinforced composite manufacturing processes, hence enabling the structural composite with a good EMI shielding performance. Fig. 10 shows the shielding performance of CNT sheets with different thicknesses (different number of layers). The EMI SE was found to be positively related to the numbers of BP layers and the microstructure of BPs themselves. Park et al. [21] investigated the effects of different layers of nanotube sheets on the EMI shielding performance by attaching multiple SWCNT BP layers to the polymethacrylimide (PMI) foam surface using the vacuum bagging process (Fig. 12A left). More layers of BPs attached on the surface of PMI foam could provide a higher SE. By increasing from one layer to two layers of BP on the PMI foam surface, the EMI SE was improved from 22 dB to more than 30 dB, as shown in Fig. 10. However, the SE increase was not proportional to the number of BP layers, and the increment of SE value decreased sharply, to around only 32 dB with three BP layers. This different behavior of shielding performance of multiple-layer structure of BP composite could be explained by the multiple reflection due to the layer-by-layer structure. Instead of stacking multiple layers of nanotube sheets together, directly increasing the thickness of single sheets can also effectively improve the EMI shielding performance. Wu et al. [53] obtained macro-CNT film with an area larger than 900 cm2 (Fig. 11A) using FCCVD method. By varying the collecting process, different thickness of BPs could be obtained. The EMI SE of these sheets were

Shielding effectiveness (dB)

40

30

20

One BP layers Two BP layers Three BP layers

10

0 0

5

15 10 Frequency (GHz)

20

FIG. 10 EMI SE of multiple-layer SWCNT BP composites on the surface of PMI foam. The lines are the theoretical calculation of multiple SWCNT BP layers having different total thicknesses of 15 μm (one layer of BP), 30 μm (two layers of BP), and 45 μm (three layers of BP) with σ ¼ 50 S/cm [21].

348

CHAPTER 14 APPLICATION OF CNTs IN EMI SHEILDING

FIG. 11 (A) Photograph of the as-prepared BP using FCCVD method. (B) EMI SE of different materials in the frequency range of 40–60 GHz, EMI SE of one piece of BP (1, 2, and 4 μm in thickness), and Cu foil (1 mm in thickness) [53].

measured and compared with copper foil, as shown in Fig. 11B. With increasing BP thickness (from 1 to 2 μm), the EMI SE increased from 36 to 43 dB. The EMI SE of a sheet  2 μm thick changed slightly with increasing frequency. When the thickness of BP reached  4 μm, the EMI SE increased dramatically to a maximum value of 48–57 dB in the frequency range of 40–60 GHz. The typical conductivity of BPs when using vacuum-assisted solution filtration process is in the range of 200–1,000 S/cm at a thickness of 15 μm. The typical SE of absorption (SEA) at 1 GHz is between 1.2 and 2.6 dB. Therefore, the SE of multiple reflections (SEM) in BP composites cannot be neglected, which indicates the importance of stacking structure design of BP composites [21, 54, 55]. Fig. 12A shows a typical stacking structure of BP/polymer composite. Due to the different electric properties of different polymer, such as EPON 862 and polyethylene (PE), various EMI shielding performance could be obtained by carefully selecting the polymer layer, the stacking sequence, and the gap distance between two neighboring BP layers. Park et al. [21] reported the comprehensive study of the effect of this stacking structure on SE of composites by using PE and mixed BP (MWCNTs were mixed with SWCNTs at a weight ratio of 5:1). Fig. 12B shows the EMI SE of the mixed BP/PE composites (on the left) with different stacking structures (on the right). As expected, only one-layer BP/PE composite showed the lowest SE at around 20 dB all over the frequency range. Additionally, two layers of BP/PE composites on the surface of the PE substrate show minimal improvement (around 5–7 dB increase). However, by adding 0.5 and 1.5 mm PE layers between the BP/PE layers, as shown in the right side of Fig. 12B, the SE was further improved. The effect of gap distance (small and large) between two neighboring BP/PE layers of the SE difference was negligible at low frequency, but the SE increases faster in the case of a larger gap and was more pronounced at high frequency. These experimental results could be well fitted with theoretical simulation by considering the SE of multiple reflection induced by the spatial layered structure of BP/PE laminate composites. Therefore, stacking conducting BP layer with proper insulation gap is important for higher EMI SE by utilizing multiple reflection shielding effect. Fig. 12C illustrates the effect of electric conductivity of BPs on the EMI shielding performance. By varying the types of CNT during the BP fabrication process, the conductivity can be tuned gradually from 20 to 1000 S/cm, as reported by Park [21]. While conductivity continuously increased, the EMI SE was also improved from around 20 to 60 dB. Obviously, SE increases proportional to the conductivity of BP. Therefore, higher conductivity of BP is an important factor toward achieving high EMI shielding

5 FUTURE TREND IN EMI SHIELDING MATERIALS

(A)

BP

Polyethylene (PE)

Epon 862

PMI foam

50

349

80 3000 S/cm Doped long-MWCNT BP/PE

70

1.5 mm

1500 S/cm

60 0.5 mm

30

20

10

EMI SE (dB)

EMI SE (dB)

40

1000 S/cm 50 40 30

Mixed BP/PE (2 layer with larger gap, 1.5 mm)

20

Mixed BP/PE (2 layer with small gap, 0.5 mm) Mixed BP/PE (2 layer on the surface) Mixed BP/PE (1 layer)

10

50 S/cm 20 S/cm Mixed BP1/PE Mixed BP2/PE

0

0

(B)

600 S/cm Long-MWCNT BP/PE

2

4

6

8

10

12

14

Frequency (GHz)

16

18

20

0

(C)

5

10

15

20

Frequency (GHz)

FIG. 12 (A) Schematic illustration of structures of BP composite laminates with three BP layers on the surface of the PMI foam and BP layers with alternating PEs as separators. (B) EMI SE of a single BP layer and a double BP layer with different gap distances from PE. While increasing the gap distance, the EMI SE increased because of multiplereflection-induced SEM. A schematic illustration of the composite structure is shown on the right. Dashed lines are estimated SE values based on the simulation with BP/PE layer conductivity of 20 S/cm. (C) EMI SE of single-layer BP/PE composites with low-conducting mixed BP and high-conducting long-MWCNT BP. Conductivity is the main factor in improving the EMI SE. Solid lines are based on the conductivity of original BP with 50, 1000, and 3000 S/cm with 25 μm thickness, and dashed lines are the best fit to the data with 20, 600, and 1500 S/cm from the bottom with thickness of 25 μm.

performance. A great deal of research work has reported on the importance of CNT alignment to achieve high conductivity [32, 33, 56]. Mechanical stretching is one effective way to straighten and align CNT bundles in the stretching direction. The conductivity of BP sample along 0 degree direction (parallel to stretching direction) is five times higher than that of BP sample along 90 degrees (perpendicular to stretching direction). By controlling the mechanical stretching process, various degree of alignment could be obtained through different stretching ratio, which determined the degree of how much the CNT bundles were placed straight and tightly packed. As a result, once the CNT bundles were straightened, the effective conductive path within BP became shorter and shorter, and all bundles got closely packed. Both the short conductive path and tightly packed structure are beneficial to the electric conductivity. Meanwhile, shielding effectiveness of BP sample along 0 degree direction is better than that of sample along 90 degrees direction, because of higher conductivity and negative permeability [57].

5 FUTURE TREND IN EMI SHIELDING MATERIALS Due to the fast growth of smart electronic devices in modern life, nanocarbon-based composite materials could play a critical role with the purpose of EMI shielding by providing various advantages over conventional metals and metal-based composites, such as light weight, corrosion resistance, flexibility,

350

CHAPTER 14 APPLICATION OF CNTs IN EMI SHEILDING

and stability. Among them, CNT would continue to contribute to the development of EMI shielding materials for engineering applications due to their high electric conductivity, mechanical properties, and potential large-scale production capabilities. By tuning the manufacturing process and carefully designing microstructure of CNT-based composites, commercial requirement of SE (i.e., 20 dB) could be easily achieved with many other multifunctional properties including high mechanical strength and long durability [25, 27, 30, 31]. Particularly, CNT foams with porous structure, which show a great promise as absorption-dominant EMI shielding materials, possess extremely low densities (less than 0.02 g/cm3) and the higher specific SE over density (obtained of 1100 dB cm3/g) [22]. Other carbon nanomaterials such as carbon black (CB) [4–6, 58] and graphite nanoplates [2, 7] are also investigated for EMI shielding applications. For instance, foam composites containing polyaniline (PANI), poloxalene (POX), and carbon black (CB) were measured to be effective for EMI shielding in a frequency range of 8.2–12.4 GHz [4]. When CB loading was 10 wt%, SE of PANI/POX/CB composites could be achieved at 19.2–19.9 dB over that range. Due to the small aspect ratio and low intrinsic electric conductivity of CB, high CB filler loading is required to form an interconnected conductive CB network in the polymer matrix, which resulted in an increased percolation threshold of conductive fillers. Some graphite nanocomposites could achieve a highly conductive network with a small percolation threshold by tuning the morphology of graphite including thickness and lateral size. Goyal et al. [7] prepared poly(phenylene sulfide) (PPS) nanocomposites filled by expanded graphite (EG), which has a larger aspect ratio than usual graphite, achieving a small percolation threshold of 1 wt% (0.6 vol %). Also, the electric conductivity was improved to 14 orders of magnitude higher than the pure PPS. However, compared with CB and graphite nanocomposites, due to the ultrahigh aspect ratio of CNT and extreme high electric conductivity, CNT nanocomposites are more prominent to show everlasting advantages including low percolation threshold with high shielding performance and costeffectiveness with easiness of manufacturing. As one of carbon allotropy, graphene, the most promising 2D nanosheet, with the large aspect ratios and excellent electric properties [59, 60] allows them to offer great potentials for superior EMI shielding performance [16, 45, 46, 61]. It has been found that graphene and graphene oxide filler-based polymer bulk composites have exhibited high performance in EMI shielding applications [60–63]. For example, poly(methylmethacrylate)-based bulk foams embedded with chemically reduced graphene oxides have shown good EMI performance of 13–19 dB [64]. As reported in literature by Yan et al. [62], reduced graphene oxide (rGO)-based polystyrene (PS) composites (rGO/PS) were absorptiondominant EMI shielding materials with the increase of rGO loading as shown in Fig. 13A. By investigating the effect of “skin thickness” of rGO/PS film on EMI SE of the composites with 3.47 vol% rGO as shown in Fig. 13B, a significant increase of EMI SE can be found from 15.2 to 41.4 dB at 8.2 GHz and from 12.9 to 48.0 dB at 12.4 GHz when the sample thickness increases from 1 to 2.5 mm, due to higher amount of conductive filler that interact with the incoming electromagnetic wave. To better understand the absorption-dominated shielding mechanism, authors interpreted the rGO/PS composite as a “skin” composed of closely packed cells, with dense rGO layers as highly conductive “membranes” (Fig. 13C). The incident electromagnetic waves entering the “skin” are attenuated by reflecting, scattering, and adsorption many times by the multiple layers of membranes [62]. The 2D nature of graphene provides many facets to the bulk composites, which could act as shielding surface. Similar to the CNT laminate composites, the bulk graphene film with layered structure would feature a high EMI shielding performance [16, 60, 65]. Zou et al. [16] applied a layer-by-layer strategy to fabricate graphene oxide composite film on wool fabric. By increasing the number of

(A) Comparison of total EMI shielding effectiveness (SET), microwave absorption (SEA), and microwave reflection (SER) at the frequency of 8.2 GHz for the rGO/PS composites with various rGO loadings. (B) EMI SE as a function of frequency for the composites with various sample thicknesses and the pristine PS with thickness of 2.5 mm. (C) Schematic representation of microwave transfer across the rGO/PS composite. (D) Average shielding effectiveness of GO/polypyrrole (PPy) and PPy wool fabrics for different numbers of deposition cycle. (E) Schematic representation of the thermal dissipation of layered GO/PPy coating under electromagnetic radiation [16, 62].

5 FUTURE TREND IN EMI SHIELDING MATERIALS

FIG. 13

351

352

CHAPTER 14 APPLICATION OF CNTs IN EMI SHEILDING

deposition cycle, the shielding effectiveness could be enhanced to more than 20 dB as shown in Fig. 13D. The multiple reflection between graphene oxide layers within the composites could be dissipated by the fast-thermal loss of GO/PPy composites (Fig. 13E). Hybrids of graphene and CNT with 3D structure have been predicted one of the most promising lightweight, flexible, and cost-effective EMI shielding materials, owing to their unique nanostructures and extraordinary electronic performance [59, 61, 66–72]. Studies proved that GO/CNT and graphene/ CNT hybrid nanomaterials exhibit higher electric conductivities, large specific area, and high mechanical properties compared with either pristine CNTs or GO/graphene [22]. For example, a hybridized structure made of a 3D porous graphene foam and in situ grown CNTs on graphene, which possesses not only an excellent conductivity but also a high absorption loss, has shown a higher EMI shielding performance than other types, including the pure graphene foam and a composite of CNT and graphene without covalent bonding [73]. The fabrication of seamless structure of 3D architecture of graphene and CNT is still a great challenge due to the different synthesis condition by CVD methods. However, progress has been made through a step-by-step method that grows graphene on a substrate first followed by a CNT deposition or vice versa. For example, 3D CNT foam with a freestanding shape, high porosity, and good mechanical properties can be used as an ultralight and high electrically conductive skeleton, on which graphene or multiple-layer graphene can be grown, resulting in a seamless junction via a covalent bonding. This hybridized structure would offer the full advantages of the unique properties of both nanocarbons, opening a new direction for developing ultralight and high-performance EMI shielding materials. Fig. 14A shows the microporous 3D architecture of CNT-MLGEP (multilayered graphene with open graphitic edge plane) foam by depositing the MLGEP on the CNT skeleton [70]. At high magnification (Fig. 14B and C), due to the radical incorporation of MLGEPs with different size and orientations on the outer surface of CNT, the hybrid structure exhibits a nanoporous surface. By changing the deposition time of MLGEP, the density of hybrid foam can be controlled from 0.0010 to 0.0090 g/cm3 because of more MLGEPs deposited per unit volume as reported by Song et al. [70] Also, electric conductivity was found to increase correspondingly. For hybrid foams at a thickness of 1.6 mm, it can be noted that (Fig. 14D and E) the total EMI SE increased with increasing density, electric conductivity, and sample thickness in the same frequency range, reaching at around 50 dB for a sample with a density of 0.00890 g/cm3. The unique hierarchical foam of CNT and graphene hybrid with a good flexibility and an ultralow density could be a good choice for developing ultralight and high-performance EMI shielding materials.

6 SUMMARY Light weight and high performance of CNT/polymer composites for EMI shielding applications including fabrication methods, characterization, and the latest developments are reviewed. By increasing the filler loading of CNTs in composites with well-controlled dispersion within a matrix, higher electric conductivity and better EMI SE could be obtained. Effects of sample thickness and composite structure have also been discussed. Increasing the thickness of composites generally resulted in a better shielding performance. Additionally, by selectively designing the stacking structure of BP and polymer film, ultrahigh SE could be obtained. Due to the large aspect ratio of CNTs and intrinsic high conductivity, lower volume fractions of fillers are needed to reach the percolation threshold in the composite

6 SUMMARY

353

FIG. 14 (A) SEM image of a CNT-multilayered graphene edge plane (MLGEP) hybrid foam and (B) high-resolution SEM image and (C) a cross-sectional view of an individual CNT-MLGEP hybrid. (D) Average SET, SER, and SEA values in the X-band of the CNT-MLGEP foams with various densities and the CNT foam at a thickness of 1.6 mm. (E) Average SET, SER, and SEA values in the X-band of the CNT-MLGEP foams with a density of 0.0058 g/cm3 at different thicknesses [70].

compared with other carbon nanomaterials including carbon black and graphite nanoplates. Homogeneous dispersion of fillers is preferred to achieve high EMI SE, which could possibly minimize the reflection loss in electromagnetic shielding leading to near purely absorbent materials. This is important for stealth application in defense area. For this purpose, CNT-based foams were also discussed in terms of multiple scattering mechanisms due to the porous structure that allows the electromagnetic wave to enter through the porous material and then results in an absorption-dominant mechanism. A potential trend of new carbon nanomaterial composites including graphene and CNT-graphene hybrids has been discussed, and this provides an insight into ways to fabricate new EMI shielding materials with optimal performance, cost-effectiveness, and ease of manufacturing.

ACKNOWLEDGMENTS This research is supported by High-Performance Materials Institute at Florida State University and is partially supported by National Science Foundation (NSF) Scalable Nanomanufacturing Program project (SNM 1344672). Songlin Zhang also thanks the support from China Scholarship Council (CSC no. 201506630022).

354

CHAPTER 14 APPLICATION OF CNTs IN EMI SHEILDING

REFERENCES [1] X.C. Tong, Advanced Materials and Design for Electromagnetic Interference Shielding, CRC Press, Boca Raton, FL, 2016. [2] S. Maiti, et al., Polystyrene/MWCNT/graphite nanoplate nanocomposites: efficient electromagnetic interference shielding material through graphite nanoplate–MWCNT–graphite nanoplate networking, ACS Appl. Mater. Interfaces 5 (11) (2013) 4712–4724. [3] Y.-H. Yu, et al., Electrical, morphological, and electromagnetic interference shielding properties of silver nanowires and nanoparticles conductive composites, Mater. Chem. Phys. 136 (2) (2012) 334–340. [4] A. Kausar, Electromagnetic interference shielding of polyaniline/poloxalene/carbon black composite, Int. J. Mat. Chem. 6 (1) (2016) 6–11. [5] M. Rahaman, T.K. Chaki, D. Khastgir, Development of high performance EMI shielding material from EVA, NBR, and their blends: effect of carbon black structure, J. Mater. Sci. 46 (11) (2011) 3989–3999. [6] M.H. Al-Saleh, U. Sundararaj, Electromagnetic interference (EMI) shielding effectiveness of PP/PS polymer blends containing high structure carbon black, Macromol. Mater. Eng. 293 (7) (2008) 621–630. [7] R.K. Goyal, et al., Electrical properties of polymer/expanded graphite nanocomposites with low percolation, J. Phys. D. Appl. Phys. 43 (36) (2010) 365404. [8] N. Das, et al., Electromagnetic interference shielding effectiveness of carbon black and carbon fibre filled EVA and NR based composites, Compos. A: Appl. Sci. Manuf. 31 (10) (2000) 1069–1081. [9] M.-S. Hong, et al., Electromagnetic interference shielding behaviors of carbon fibers-reinforced polypropylene matrix composites: II. Effects of filler length control, J. Ind. Eng. Chem. 20 (5) (2014) 3901–3904. [10] S. Kwon, et al., Flexible electromagnetic interference shields made of silver flakes, carbon nanotubes and nitrile butadiene rubber, Carbon 68 (2014) 118–124. [11] L. Zou, et al., Superhydrophobization of cotton fabric with multiwalled carbon nanotubes for durable electromagnetic interference shielding, Fibers Polym. 16 (10) (2015) 2158. [12] S. Celozzi, G. Lovat, R. Araneo, Electromagnetic Shielding, Wiley Online Library, 2008. [13] P. Saini, M. Aror, Microwave Absorption and EMI Shielding Behavior of Nanocomposites Based on Intrinsically Conducting Polymers, Graphene and Carbon Nanotubes, IntechOpen, Rijeka, Croatia, 2012. [14] P. Saini, et al., Polyaniline–MWCNT nanocomposites for microwave absorption and EMI shielding, Mater. Chem. Phys. 113 (2–3) (2009) 919–926. [15] P. Saini, et al., Enhanced microwave absorption behavior of polyaniline-CNT/polystyrene blend in 12.4–18.0GHz range, Synth. Met. 161 (15–16) (2011) 1522–1526. [16] L. Zou, et al., Step-by-step strategy for constructing multilayer structured coatings toward high-efficiency electromagnetic interference shielding, Adv. Mater. Interfaces 3 (5) (2016) 1500476. [17] L. Zou, et al., Comparison of polyelectrolyte and sodium dodecyl benzene sulfonate as dispersants for multiwalled carbon nanotubes on cotton fabrics for electromagnetic interference shielding, J. Appl. Polym. Sci. 131 (15) (2014) 40588. [18] Y. Kato, et al., Stretchable electromagnetic-interference shielding materials made of a long single-walled carbon-nanotube–elastomer composite, RSC Adv. 7 (18) (2017) 10841–10847. [19] Y. Xu, et al., Light-weight silver plating foam and carbon nanotube hybridized epoxy composite foams with exceptional conductivity and electromagnetic shielding property, ACS Appl. Mater. Interfaces 8 (36) (2016) 24131–24142. [20] M. Crespo, et al., Ultra-light carbon nanotube sponge as an efficient electromagnetic shielding material in the GHz range, Phys. Status Solidi RRL 8 (8) (2014) 698–704. [21] J.G. Park, et al., Electromagnetic interference shielding properties of carbon nanotube buckypaper composites, Nanotechnology 20 (41) (2009) 415702. [22] M. Gonza´lez, et al., Carbon nanotube composites as electromagnetic shielding materials in GHz range, in: Carbon Nanotubes-Current Progress of their Polymer Composites, InTech, Rijeka, Croatia, 2016.

REFERENCES

355

[23] N. Nguyen, et al., Direct printing of thermal management device using low-cost composite ink, Macromol. Mater. Eng. 302 (2017) 1700135. https://doi.org/10.1002/mame.201700135. [24] N. Hu, et al., The electrical properties of polymer nanocomposites with carbon nanotube fillers, Nanotechnology 19 (21) (2008) 215701. https://doi.org/10.1088/0957-4484/19/21/215701. [25] Z. Zeng, et al., Thin and flexible multi-walled carbon nanotube/waterborne polyurethane composites with high-performance electromagnetic interference shielding, Carbon 96 (2016) 768–777. [26] A.P. Singh, et al., Multiwalled carbon nanotube/cement composites with exceptional electromagnetic interference shielding properties, Carbon 56 (2013) 86–96. [27] M. Farukh, et al., Sandwich composites of polyurethane reinforced with poly(3,4-ethylene dioxythiophene)coated multiwalled carbon nanotubes with exceptional electromagnetic interference shielding properties, RSC Adv. 5 (92) (2015) 75229–75238. [28] P. Verma, et al., Excellent electromagnetic interference shielding and mechanical properties of high loading carbon-nanotubes/polymer composites designed using melt recirculation equipped twin-screw extruder, Carbon 89 (2015) 308–317. [29] N. Li, et al., Electromagnetic interference (EMI) shielding of single-walled carbon nanotube epoxy composites, Nano Lett. 6 (6) (2006) 1141–1145. [30] S.H. Park, et al., Enhanced electromagnetic interference shielding through the use of functionalized carbonnanotube-reactive polymer composites, IEEE Trans. Nanotechnol. 9 (4) (2010) 464–469. [31] M. Soltani Alkuh, et al., The relationship between electromagnetic absorption properties and cell structure of poly(methyl methacrylate)/multi-walled carbon nanotube composite foams, Mater. Des. 100 (2016) 73–83. [32] R. Downes, et al., Strain-induced alignment mechanisms of carbon nanotube networks, Adv. Eng. Mater. 17 (3) (2015) 349–358. [33] Q. Cheng, et al., High mechanical performance composite conductor: multi-walled carbon nanotube sheet/ bismaleimide nanocomposites, Adv. Funct. Mater. 19 (20) (2009) 3219–3225. [34] S. Zhang, et al., Ultra-high conductivity and metallic conduction mechanism of scale-up continuous carbon nanotube sheets by mechanical stretching and stable chemical doping, Carbon 125 (2017) 649–658. [35] J.S. Bulmer, et al., Microwave conductivity of sorted CNT assemblies, Sci. Rep. 4 (2014) 3762. [36] R. Ramasubramaniam, J. Chen, H.Y. Liu, Homogeneous carbon nanotube/polymer composites for electrical applications, Appl. Phys. Lett. 83 (14) (2003) 2928–2930. [37] T.K. Gupta, et al., Improved nanoindentation and microwave shielding properties of modified MWCNT reinforced polyurethane composites, J. Mater. Chem. A 1 (32) (2013) 9138–9149. [38] I.W. Chen, et al., Highly conductive carbon nanotube buckypapers with improved doping stability via conjugational cross-linking, Nanotechnology 22 (48) (2011) 485708. [39] M. Imai, et al., Highly strong and conductive carbon nanotube/cellulose composite paper, Compos. Sci. Technol. 70 (10) (2010) 1564–1570. [40] I.H. Chen, C.C. Wang, C.Y. Chen, Fabrication and structural characterization of polyacrylonitrile and carbon nanofibers containing plasma-modified carbon nanotubes by electrospinning, J. Phys. Chem. C 114 (32) (2010) 13532–13539. [41] H.M. Kim, et al., Electrical conductivity and electromagnetic interference shielding of multiwalled carbon nanotube composites containing Fe catalyst, Appl. Phys. Lett. 84 (4) (2004) 589–591. [42] P.C.P. Watts, et al., High permittivity from defective multiwalled carbon nanotubes in the X-band, Adv. Mater. 15 (7-8) (2003) 600–603. [43] L.-C. Jia, Y.-K. Li, D.-X. Yan, Flexible and efficient electromagnetic interference shielding materials from ground tire rubber, Carbon 121 (2017) 267–273. [44] A. Kolanowska, et al., From blackness to invisibility–carbon nanotubes role in the attenuation of and shielding from radio waves for stealth technology, Carbon (2017). [45] N. Yousefi, et al., Highly aligned graphene/polymer nanocomposites with excellent dielectric properties for high-performance electromagnetic interference shielding, Adv. Mater. 26 (31) (2014) 5480–5487.

356

CHAPTER 14 APPLICATION OF CNTs IN EMI SHEILDING

[46] Y.-J. Wan, et al., Graphene paper for exceptional EMI shielding performance using large-sized graphene oxide sheets and doping strategy, Carbon 122 (2017) 74–81. [47] Y. Yang, et al., Novel carbon nanotube  polystyrene foam composites for electromagnetic interference shielding, Nano Lett. 5 (11) (2005) 2131–2134. [48] Z. Zeng, et al., Lightweight and anisotropic porous MWCNT/WPU composites for ultrahigh performance electromagnetic interference shielding, Adv. Funct. Mater. 26 (2) (2016) 303–310. [49] Y. Li, et al., Ultrathin carbon foams for effective electromagnetic interference shielding, Carbon 100 (2016) 375–385. [50] T. Kuang, et al., Facile preparation of lightweight high-strength biodegradable polymer/multi-walled carbon nanotubes nanocomposite foams for electromagnetic interference shielding, Carbon 105 (2016) 305–313. [51] J. Li, et al., Morphologies and electromagnetic interference shielding performances of microcellular epoxy/ multi-wall carbon nanotube nanocomposite foams, Compos. Sci. Technol. 129 (2016) 70–78. [52] J. Park, et al., EMI shielding property of carbon nanotube buckypaper composites, Proceedings of the International SAMPE Symposium and Exhibition, Shanghai, China, 2008. [53] Z.P. Wu, et al., Electromagnetic interference shielding effectiveness of composite carbon nanotube macrofilm at a high frequency range of 40 GHz to 60 GHz, AIP Adv. 5 (6) (2015) 067130. [54] J.-H. Lin, et al., Polymer composites made of multi-walled carbon nanotubes and graphene nano-sheets: effects of sandwich structures on their electromagnetic interference shielding effectiveness, Compos. Part B 89 (2016) 424–431. [55] I. Arief, S. Biswas, S. Bose, Graphene analogues as emerging materials for screening electromagnetic radiations, Nano-Struct. Nano-Objects 11 (2017) 94–101. [56] Q. Liu, et al., Highly aligned dense carbon nanotube sheets induced by multiple stretching and pressing, Nanoscale 6 (8) (2014) 4338–4344. [57] W. Zhang, et al., Electromagnetic characteristics of carbon nanotube film materials, Chin. J. Aeronaut. 28 (4) (2015) 1245–1254. [58] J.S. Im, J.G. Kim, Y.-S. Lee, Fluorination effects of carbon black additives for electrical properties and EMI shielding efficiency by improved dispersion and adhesion, Carbon 47 (11) (2009) 2640–2647. [59] B. Wen, et al., Reduced graphene oxides: the thinnest and most lightweight materials with highly efficient microwave attenuation performances of the carbon world, Nanoscale 6 (11) (2014) 5754–5761. [60] W.-L. Song, et al., Flexible graphene/polymer composite films in sandwich structures for effective electromagnetic interference shielding, Carbon 66 (2014) 67–76. [61] B. Wen, et al., Reduced graphene oxides: light-weight and high-efficiency electromagnetic interference shielding at elevated temperatures, Adv. Mater. 26 (21) (2014) 3484–3489. [62] D.-X. Yan, et al., Structured reduced graphene oxide/polymer composites for ultra-efficient electromagnetic interference shielding, Adv. Funct. Mater. 25 (4) (2015) 559–566. [63] A.G. D’Aloia, et al., Electromagnetic absorbing properties of graphene–polymer composite shields, Carbon 73 (2014) 175–184. [64] H.B. Zhang, et al., Tough graphene-polymer microcellular foams for electromagnetic interference shielding, ACS Appl. Mater. Interfaces 3 (3) (2011) 918–924. [65] M.-S. Cao, et al., Ultrathin graphene: electrical properties and highly efficient electromagnetic interference shielding, J. Mater. Chem. C 3 (26) (2015) 6589–6599. [66] A.P. Singh, et al., Probing the engineered sandwich network of vertically aligned carbon nanotube–reduced graphene oxide composites for high performance electromagnetic interference shielding applications, Carbon 85 (2015) 79–88. [67] S.-H. Lee, D. Kang, I.-K. Oh, Multilayered graphene-carbon nanotube-iron oxide three-dimensional heterostructure for flexible electromagnetic interference shielding film, Carbon 111 (2017) 248–257. [68] M. Verma, et al., Graphene nanoplatelets/carbon nanotubes/polyurethane composites as efficient shield against electromagnetic polluting radiations, Compos. Part B 120 (2017) 118–127.

REFERENCES

357

[69] B. Zhao, et al., Flexible, ultrathin, and high-efficiency electromagnetic shielding properties of poly (vinylidene fluoride)/carbon composite films, ACS Appl. Mater. Interfaces 9 (2017) 20873–20884. [70] Q. Song, et al., Carbon nanotube-multilayered graphene edge plane core-shell hybrid foams for ultrahighperformance electromagnetic-interference shielding, Adv. Mater. (2017) 1701583. [71] W.L. Song, et al., Interfacial engineering of carbon nanofiber-graphene-carbon nanofiber heterojunctions in flexible lightweight electromagnetic shielding networks, ACS Appl. Mater. Interfaces 6 (13) (2014) 10516–10523. [72] W.-L. Song, et al., Tuning three-dimensional textures with graphene aerogels for ultra-light flexible graphene/texture composites of effective electromagnetic shielding, Carbon 93 (2015) 151–160. [73] L. Kong, et al., Macroscopic bioinspired graphene sponge modified with in-situ grown carbon nanowires and its electromagnetic properties, Carbon 111 (2017) 94–102.