3.2.3 Carbon foams

hollow carbon microspheres (HCMs). The EMI SE values of used syntactic foams at the same filler content were compared, as shown as Table 2. The results showed that CNFs is more effective in providing EMI shielding compared to CCF and LCF

CCF Aspect ratio: 6–50

LCF Aspect ratio: 150–750

Zhang et al. [30] also demonstrated the effect of functionalization of HCMs on

/g, demonstrating the prospect of epoxy/Ag-

polydopamine (PDA) via the self-polymerization of dopamine. The PDA coating promotes dispersion and served as a reducing agent to deposit silver (Ag) particles on the surface of HCMs as illustrated in Figure 6a. The average EMI SE of the epoxy-HCMs syntactic foam containing Ag-PDA-HCMs with 28.5 and 30.5 wt% of silver in the X-band achieved 49.5 and 60.2 dB, respectively as shown in Figure 6b.

PDA-HCMs syntactic foam as a lightweight high-performance EMI shielding material. The corresponding EMI shielding mechanism of this syntactic foam was ana-

(a) Schematic illustration of the procedure for preparation of PDA-HCMs and Ag-PDA-HCMs; (b) EMI SE in the frequency range from 8 to 12 GHz for syntactic foam containing pristine HCMs and Ag-PDA-HCMs with different silver contents; and (c) reflectance (R), absorbance (A), and transmittance (T) of EM radiation

over syntactic foams containing Ag-PDA-HCMs with different silver content at 10 GHz [30].

the EMI SE of the epoxy-HCMs syntactic foam. HCMs were coated with

Aspect ratio: 500–1700

Comparison of the EMI SE (dB) of CNF, CCF, and LCF reinforced syntactic foam.

0.5 5.2 2.2 2.8 1.0 11.3 3.4 4.4 1.5 16.4 3.7 6.5 2.0 24.9 4.3 7.5

lyzed by comparing the values of reflectance (R), absorptance (A), and

due to the larger aspect ratio of CNFs.

Table 2.

Figure 6.

222

Filler content (vol%) CNF

Electromagnetic Materials and Devices

The SSE reached up to 46.3 dB cm3

Carbon foam is a class of three-dimensional (3D) architecture consisting of a sponge-like interconnected network of porous carbon. Carbon foams have been wildly used as candidates for realistic EMI shielding applications due to their excellent properties, such as low density, resistance to chemical corrosion, high thermal and electrical conductivity, and high temperature resistance.

Zhang et al. [32] prepared a novel ultralight (0.15 g/cm<sup>3</sup> ) carbon foam by direct carbonization of phthalonitrile (PN)-based polymer foam, as shown in Figure 7a. High EMI SE of 51.2 dB (see Figure 7b, C1000 was labeled as the carbonization of 1000°C) was contributed by the high graphitic carbonaceous species and the intrinsic nitrogen-containing structure. The carbon foams showed the best SSE of 341.1 dB cm3 /g so far when mechanical property was considered. The carbon foam developed by Zhang provides an excellent low-density and high-performance EMI shielding material for use in areas where mechanical integrity is desired.

### 3.2.4 CNTs/graphene foams

The EMI SE of carbon foams was closely related to the char yield of polymer precursors and the demanding carbonization conditions. Therefore, a new kind of filler-free lightweight EMI shielding material, is in demand, which can be prepared without the stringent processing conditions. In view of the lightweight requirement, assembling one dimensional (1D) CNTs and two-dimensional (2D) graphene sheets into three dimensional (3D) macroscopic porous structures (e.g., sponges, foams and aerogels) emerged as an efficient approach.

Lu et al. [33] synthesized a flexible CNTs sponge with a density of 10.0 mg/cm<sup>3</sup> via chemical vapor deposition (CVD) process, composed of self-assembled and interconnected CNT skeletons. The freestanding CNTs sponge showed the high EMI SE and SSE of 54.8 dB and 5480 dB cm<sup>3</sup> /g in X-band, respectively. After composited with polydimethylsiloxane (PDMS) by directly infiltrating method, the CNT/PDMS composites still exhibited excellent EMI SE (46.3 dB) at the thickness of 2.0 mm, while the CNT loading content was less than 1.0 wt%.

the conductive network while maintaining the advantage of light carbon textile. Singh et al. [37] studied the EMI SE of pure GA, which was 20 dB, with a density 75 mg /cm3 and a thickness of 2 mm. They discussed the EMI shielding mechanism by correlating the EM wave interaction with the 3D porous structure. Zeng et al. [38] fabricated an ultralight and highly elastic rGO/lignin-derived carbon (LDC) composite aerogel with aligned microspores and cell walls by directional freezedrying and carbonization method. The EMI SE of rGO/LDC composite aerogels with a thickness of 2 mm could reach up to 49.2 and 21.3 dB under ultralow

Lightweight Electromagnetic Interference Shielding Materials and Their Mechanisms

, respectively. The graphitization of GAs facilitates to improve its electrical conductivity, thus improving the EMI SE. Liu et al. [39] reported an effective method of manufacturing an integrated graphene aerogel (IGA) using a complete bridge between rGO sheets and polyimide macromolecules via graphitization at 2800°C, as shown in Figure 9a. The rGO sheets were efficiently reduced to graphene during graphitization, while the polyimide component was graphitized to turbostratic carbon to connect the graphene sheets, resulting in a high EMI SE of 83 dB in X-band at a low density of 18 mg/cm3

as shown in Figure 9b. The EMI shielding mechanism analysis for the porous IGA revealed that most of the incident EM wave was dissipated through absorption, thus

EMI SE. As shown in Figure 10a, two types of ultralight (4.5–5.5 mg/cm3

Different reduction process of graphene oxide (GO), including chemical reduction and thermal reduction would affect the EMI shielding performance of GAs. Bi et al. [40, 41] carried out a comprehensive study of EMI shielding mechanisms of GAs solely consisted of graphene sheets to determine the main parameters of high

were prepared by chemical reduction and thermal reduction of GO aerogels. The EMI SE reached 27.6 and 40.2 dB for chemically reduced graphene aerogel (GAC) and thermally reduced graphene aerogel (GAT), respectively. The distinct graphene surface resulted from different processing pathway led to different EM wave

(a) Schematic illustration for fabricating IGA and (b) effect of annealing temperature on EMI shielding

(a) Schematic representation of the preparation process of GAC and GAT [42] and (b) R & A of GA9 and

forming an absorption-dominant EMI shielding mechanism.

,

) 3D GAs

densities of 8.0 and 2.0 mg/cm<sup>3</sup>

DOI: http://dx.doi.org/10.5772/intechopen.82270

Figure 9.

Figure 10.

GA9F [41].

225

performance of IGAs [39].

Surface modification is employed to increase the EMI shielding ability of graphene foams. Zhang et al. [34] prepared surfaced modified 3D graphene foams via self-polymerization of dopamine with a subsequent foaming process, as shown in Figure 8a. The polydopamine (PDA) served as a nitrogen doping source and an enhancement tool to achieve higher extent of reduction of the graphene through providing wider pathways and larger accessible surface areas. The enhanced reduction of graphene sheets and the polarization effects introduced by PDA decoration compensated the negative effect of the barrier posed by PDA. As a result, the resultant EMI SE showed 15% improvement compared to PDA-free graphene foam as shown in Figure 8b. Wu et al. [35] also fabricated an ultralight, high performance EMI shielding graphene foam (GF)/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) composites by drop coating of PEDOT:PSS on the freestanding cellular-structured GFs, as illustrated in Figure 8c. The GF/PEDOT: PSS composites possess an enhanced electrical conductivity from 11.8 to 43.2 S/cm after the incorporation of PEDOT:PSS. The modified grapheme foam with a density of 18.2 <sup>10</sup><sup>3</sup> g/cm<sup>3</sup> provide a remarkable EMI SE of 91.9 dB (identified as SET in Figure 8d).

## 3.2.5 Graphene aerogels

Aerogel is a synthetic porous ultralight material derived from a gel, in which the liquid component used in gel are replaced by air. In recent years, the great potential of graphene aerogel (GAs) in EMI shielding applications has been confirmed by several researchers. Song et al. [36] reported that the EMI SE of GA-carbon textile hybrid with a thickness of 2 mm was 27 dB. The 3D scaffold GA greatly enhances

#### Figure 8.

(a) Schematic representation of the preparation of PDA-GO and PDA-rGO; (b) EMI SE of rGO foam and PDA-rGO foam [34]; (c) schematic procedure of the preparation of GF/PEDOT:PSS composites; (d) EMI SE of GF/PEDOT:PSS composites as a function frequency [35].

#### Lightweight Electromagnetic Interference Shielding Materials and Their Mechanisms DOI: http://dx.doi.org/10.5772/intechopen.82270

the conductive network while maintaining the advantage of light carbon textile. Singh et al. [37] studied the EMI SE of pure GA, which was 20 dB, with a density 75 mg /cm3 and a thickness of 2 mm. They discussed the EMI shielding mechanism by correlating the EM wave interaction with the 3D porous structure. Zeng et al. [38] fabricated an ultralight and highly elastic rGO/lignin-derived carbon (LDC) composite aerogel with aligned microspores and cell walls by directional freezedrying and carbonization method. The EMI SE of rGO/LDC composite aerogels with a thickness of 2 mm could reach up to 49.2 and 21.3 dB under ultralow densities of 8.0 and 2.0 mg/cm<sup>3</sup> , respectively.

The graphitization of GAs facilitates to improve its electrical conductivity, thus improving the EMI SE. Liu et al. [39] reported an effective method of manufacturing an integrated graphene aerogel (IGA) using a complete bridge between rGO sheets and polyimide macromolecules via graphitization at 2800°C, as shown in Figure 9a. The rGO sheets were efficiently reduced to graphene during graphitization, while the polyimide component was graphitized to turbostratic carbon to connect the graphene sheets, resulting in a high EMI SE of 83 dB in X-band at a low density of 18 mg/cm3 , as shown in Figure 9b. The EMI shielding mechanism analysis for the porous IGA revealed that most of the incident EM wave was dissipated through absorption, thus forming an absorption-dominant EMI shielding mechanism.

Different reduction process of graphene oxide (GO), including chemical reduction and thermal reduction would affect the EMI shielding performance of GAs. Bi et al. [40, 41] carried out a comprehensive study of EMI shielding mechanisms of GAs solely consisted of graphene sheets to determine the main parameters of high EMI SE. As shown in Figure 10a, two types of ultralight (4.5–5.5 mg/cm3 ) 3D GAs were prepared by chemical reduction and thermal reduction of GO aerogels. The EMI SE reached 27.6 and 40.2 dB for chemically reduced graphene aerogel (GAC) and thermally reduced graphene aerogel (GAT), respectively. The distinct graphene surface resulted from different processing pathway led to different EM wave

Figure 9.

Lu et al. [33] synthesized a flexible CNTs sponge with a density of 10.0 mg/cm<sup>3</sup>

composited with polydimethylsiloxane (PDMS) by directly infiltrating method, the CNT/PDMS composites still exhibited excellent EMI SE (46.3 dB) at the thickness

Aerogel is a synthetic porous ultralight material derived from a gel, in which the liquid component used in gel are replaced by air. In recent years, the great potential of graphene aerogel (GAs) in EMI shielding applications has been confirmed by several researchers. Song et al. [36] reported that the EMI SE of GA-carbon textile hybrid with a thickness of 2 mm was 27 dB. The 3D scaffold GA greatly enhances

(a) Schematic representation of the preparation of PDA-GO and PDA-rGO; (b) EMI SE of rGO foam and PDA-rGO foam [34]; (c) schematic procedure of the preparation of GF/PEDOT:PSS composites; (d) EMI SE

of GF/PEDOT:PSS composites as a function frequency [35].

/g in X-band, respectively. After

via chemical vapor deposition (CVD) process, composed of self-assembled and interconnected CNT skeletons. The freestanding CNTs sponge showed the high

Surface modification is employed to increase the EMI shielding ability of graphene foams. Zhang et al. [34] prepared surfaced modified 3D graphene foams via self-polymerization of dopamine with a subsequent foaming process, as shown in Figure 8a. The polydopamine (PDA) served as a nitrogen doping source and an enhancement tool to achieve higher extent of reduction of the graphene through providing wider pathways and larger accessible surface areas. The enhanced reduction of graphene sheets and the polarization effects introduced by PDA decoration compensated the negative effect of the barrier posed by PDA. As a result, the resultant EMI SE showed 15% improvement compared to PDA-free graphene foam as shown in Figure 8b. Wu et al. [35] also fabricated an ultralight, high performance EMI shielding graphene foam (GF)/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) composites by drop coating of PEDOT:PSS on the freestanding cellular-structured GFs, as illustrated in Figure 8c. The GF/PEDOT: PSS composites possess an enhanced electrical conductivity from 11.8 to 43.2 S/cm after the incorporation of PEDOT:PSS. The modified grapheme foam with a density of 18.2 <sup>10</sup><sup>3</sup> g/cm<sup>3</sup> provide a remarkable EMI SE of 91.9 dB (identified as SET in

of 2.0 mm, while the CNT loading content was less than 1.0 wt%.

EMI SE and SSE of 54.8 dB and 5480 dB cm<sup>3</sup>

Electromagnetic Materials and Devices

Figure 8d).

Figure 8.

224

3.2.5 Graphene aerogels

(a) Schematic illustration for fabricating IGA and (b) effect of annealing temperature on EMI shielding performance of IGAs [39].

#### Figure 10.

(a) Schematic representation of the preparation process of GAC and GAT [42] and (b) R & A of GA9 and GA9F [41].

response upon striking the graphene/air interface. Nitrogen-doping and side polar groups induced strong polarization effects in GAC. Higher extent of reduction of the grapheme sheets in GAT left a smaller amount of side polar groups and formed more sp2 graphitic lattice, both favored π-π stacking between the adjacent graphene sheets. The enhanced polarization effects and the increased electrical conductivity of GAT contributed to better EMI shielding performance. Bi further investigated the effect of porosity on EMI shielding mechanisms compressing the aerogel (GA9) into thin film (GA9F), as shown in Figure 10b. The highly connected conducting network resulted in a significant increase in the electrical conductivity of GA9F, while the EMI SE remained unchanged at constant rGO content. The observation was contradictory to the previous outcomes that higher electrical conductivity or better-connected network contributed to higher EMI SE. Hence, the fact can be believed that the EMI SE is highly dependent on the effective amounts of materials response to the EM waves. Despite the similar intrinsic properties of rGO, the amount of absorption of EM waves in GA9 was much higher than that in GA9F when the EM waves penetrated through the porous structure. The cavities within the highly porous GA absorbed the EM waves through multiple internal reflections and eventually depleted the energy. Hence, the tightly connected conducting network within GA9F changed the EMI shielding mechanism from absorption to reflection.

reference papers listed in this chapter. Although the data are not involved all the published results, they are representative to the library of lightweight EMI shielding materials. The reported EMI SEs of polymer-based composites containing conductive fillers varied in the range of 20–60 dB corresponding to the densities higher than 0.8 g/

Lightweight Electromagnetic Interference Shielding Materials and Their Mechanisms

comparable EMI SE of 20–80 dB with the lower density (<0.8 g/cm3

\*†, Shuguang Bi2† and Ming Liu<sup>3</sup>

\*Address all correspondence to: lyzhang@dhu.edu.cn

These authors contributed equally to this work.

provided the original work is properly cited.

1 Center for Civil Aviation Composites, Donghua University, Shanghai, China

3 Temasek Laboratories, Nanyang Technological University, Singapore

2 Chemistry and Chemical Engineering College, Wuhan Textile University, Wuhan,

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

. Polymer-based foams reinforced with additional conductive fillers and carbon foams outperform polymer-based composites in terms of EMI SE. They possessed

mer- and carbon-based foams, indicating they can be used as an ideal potential lightweight EMI shielding materials though the mechanical properties of aerogels still

Liying Zhang would like to acknowledge the support by the initial research funds for young teachers of Donghua University. Shuguang Bi would like to acknowledge the financial support of Wuhan Engineering Center for Ecological Dyeing & Finishing and Functional Textiles, Key Laboratory of Textile Fiber & Product (Wuhan Textile University), Ministry of Education, Hubei Biomass Fibers and Eco-dyeing & Finishing Key Laboratory. Zhang and Bi would also thank the funding support by State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University (KF1827). Ming Liu would like to acknowledge the support from School of Materials Science and Engineering at Nanyang Technological University for this work.

) exhibited high EMI SEs in the same range of poly-

). Aerogels with

cm3

ultralow densities (<100 mg/cm3

DOI: http://dx.doi.org/10.5772/intechopen.82270

remain a big issue.

Acknowledgements

Conflict of interest

Author details

Liying Zhang<sup>1</sup>

China

†

227

No conflict of interest.
