3.1.4 Polymer-based composites containing magnetic particles composites

A binary or even ternary component consists of two or more types of the fillers provide an effective way to bypass the inherent shortcomings of a single-filler composite. The incorporation of magnetic components will supplement the attenuation properties of a carbon-based EMI shielding material.

Physical blending or deposition of metallic particles within a polymer blend or structure is the most direct way of incorporation a third element, however, such method faces the problem of uniform dispersion and deposition at the bottom layers due to the higher density of metallic particles. Electroless plating of metals on carbon substrates provides a neat way of incorporating metal components uniformly into a system without excessive weight addition. Works by Kim et al. [26] and Yim et al. [27] dispersed nickel coated MWCNT through electroless plating in epoxy and high-density polyethylene, respectively. Figure 5a gives an illustration of

#### Figure 5.

(a) Schematic diagram of the electroless Ni-plating process; (b) SEM images of (1) pristine MWCNTs and (2) Ni-coated MWCNTs, respectively; (c) comparison of the EMI SE of MWCNTs/HDPE and Ni-MWCNTs/ HDPE and (d) the proposed shielding mechanism of Ni-MWCNTs/HDPE [27].

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

the nickel coated MWCNTs. It is apparent that the nickel coated MWCNTs appeared rougher comparing to the pristine ones due to the presence of nickel particles as shown Figure 5b. Yim achieved 140% (at 1 GHz, Figure 5c) in enhancement of the EMI SE compared to the pristine MWCNT/polymer composites. The enhancement was attributed to the increased surface conductivity. Figure 5d shows the proposed shielding mechanism of Ni-MWCNTs/HDPE. EM wave was firstly reflected at the composite surfaces upon reaching the surface of the composite. When the penetrated EM wave meets the nickel layer on the MWCNTs, the metallic layer functioned as EM absorbable or reflective fillers. It is evident that the EMI absorbing nature of the metallic layer can be used as an effective additional shielding material despite the small amount present in the systems.

#### 3.2 Foams and aerogels used in EMI shielding

hollow cylinder, that is, single-walled carbon nanotubes (SWCNTs) or of a collection of graphene concentric cylinders, that is, multi-walled carbon nanotubes (MWCNTs) [21, 22]. Graphene sheet (GS), an atomically thick two-dimensional structure, exhibited excellent mechanical, thermal and electrical properties [23]. Both CNTs and graphene offer substantial advantages over conventional carbon fillers and the percolation threshold can be achieved by both at very low content if

In general, carbon fillers with high aspect ratio are generally more effective in imparting electrical conductivities to a polymer matrix, hence it is no surprise to observe the highest SE from fillers with the highest aspect ratio, that is, SWCNTs > MWCNTs > CNFs > CB when the volume fraction of the fillers is the same. The different methods of fillers dispersion and various carbon filler surface modification methods were comprehensively reviewed in the published paper and will not be

A binary or even ternary component consists of two or more types of the fillers

Physical blending or deposition of metallic particles within a polymer blend or structure is the most direct way of incorporation a third element, however, such method faces the problem of uniform dispersion and deposition at the bottom layers due to the higher density of metallic particles. Electroless plating of metals on carbon substrates provides a neat way of incorporating metal components uniformly into a system without excessive weight addition. Works by Kim et al. [26] and Yim et al. [27] dispersed nickel coated MWCNT through electroless plating in epoxy and high-density polyethylene, respectively. Figure 5a gives an illustration of

(a) Schematic diagram of the electroless Ni-plating process; (b) SEM images of (1) pristine MWCNTs and (2) Ni-coated MWCNTs, respectively; (c) comparison of the EMI SE of MWCNTs/HDPE and Ni-MWCNTs/

HDPE and (d) the proposed shielding mechanism of Ni-MWCNTs/HDPE [27].

provide an effective way to bypass the inherent shortcomings of a single-filler composite. The incorporation of magnetic components will supplement the attenu-

discussed in detail here [3, 24]. The EMI shielding performance of the polymer/carbon-fillers composites can also be found in Ref. [3, 7, 24, 25].

3.1.4 Polymer-based composites containing magnetic particles composites

ation properties of a carbon-based EMI shielding material.

properly dispersed.

Electromagnetic Materials and Devices

Figure 5.

220

In view of the rigid index of fuel-economy in the applications of automobile and aerospace, lightweight EMI shielding materials with the combination of reduced density and high EMI SE are much preferred. In this section, we aim to provide a general overview on the preparation of foam and aerogel materials used in EMI shielding and the advantages and challenges faced by each category and possible strategies towards enhancing their EMI shielding performances. The specific EMI SE, defined as the ratio of the EMI SE to the density (SSE) or both density and thickness (SSE/t), is a more appropriate criterion to compare the EMI shielding performance with those of other typical materials for the applications where lightweight is required.

#### 3.2.1 Polymer-based composite foams

Conductive polymer-based composites foams offer significant reduction in weight, while the pores decrease the real part of the permittivity, accordingly reducing the reflection at the material surface. The porous structure enhances the energy absorption through wave scattering in the walls of the pores. Electrically conductive fillers, including CNFs, CNTs and graphene sheets, are commonly used to form a desirable conducting network within the inherently insulating polymer foam matrix. Yang et al. [28] first reported CNFs reinforced polystyrene (PS) composite foam as a conductive foam for EMI shielding application. The EMI SE of PS/CNFs foam containing 1 wt% CNFs was less than 1 dB, upon increasing CNFs content to 15 wt%, EMI SE increased to 19 dB. Following this work, the authors reported PS/CNTs composite foam with varying CNTs contents from 0 to 7 wt% [4]. The PS/CNTs composite foam achieved a higher EMI SE of above 10 dB compared to 3 dB for the PS/CNFs composite foam at the same filler content of 3 wt%. The difference in the results originated from the remarkable electrical and structural properties of CNTs, such as larger aspect ratio, smaller diameter, higher electrical conductivity and strength, compared to CNFs.

#### 3.2.2 Syntactic foams

Syntactic foam, filling hollow spheres in a matrix, is a kind of lightweight composite materials. The approaches to enhance the EMI SE of syntactic foams include (i) hollow particles made of a conductive material; (ii) coating a conductive layer onto the surface of hollow particles and (iii) adding a second conductive filler in syntactic foam matrix.

Zhang et al. [29] added a second conductive filler, (CNFs, chopped carbon fiber (CCF), and long carbon fiber (LCF)), into syntactic foams containing conductive


transmittance (T) in Figure 6c. The specimens were both reflective and absorptive towards EM radiation at silver content less than 17.8 wt%. The contribution of reflection (0.83) towards EMI SE surpassed that from absorption (0.16) when silver content increased to 28.5%. The dense and thick electrically conductive silver formed due to further increasing the silver content to 30.5 wt% increased the R to

Xu et al. [31] fabricated syntactic foams ("hybridized epoxy composite foams" according to authors) through impregnating expandable epoxy/MWCNT/microsphere blends into a preformed, highly porous, and 3D silver-coated melamine foam (SF) sponge. The highly conductive SF resolved the problem of the foam reduction of high filled epoxy blends and provided channels for rapid electron transport. MWCNTs were used to offset the loss of conductive pathways due to the crystal defects in the silver layer and the insulating epoxy resin. As a result, the EMI SE of 68.1 dB was achieved with only 2 wt% of MWCNTs and 3.7 wt% of silver due

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

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

developed by Zhang provides an excellent low-density and high-performance EMI

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,

(a) Schematic representation of the preparation of PN-based carbon foams and (b) EMI SE of carbon foams [32].

shielding material for use in areas where mechanical integrity is desired.

/g so far when mechanical property was considered. The carbon foam

) carbon foam by direct

and electrical conductivity, and high temperature resistance. Zhang et al. [32] prepared a novel ultralight (0.15 g/cm<sup>3</sup>

foams and aerogels) emerged as an efficient approach.

0.97 and resultant in reflection as the dominant shielding mechanism.

Lightweight Electromagnetic Interference Shielding Materials and Their Mechanisms

to the synergy of the MWCNT and SF.

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

3.2.3 Carbon foams

341.1 dB cm3

Figure 7.

223

3.2.4 CNTs/graphene foams

Table 2.

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

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 due to the larger aspect ratio of CNFs.

Zhang et al. [30] also demonstrated the effect of functionalization of HCMs on the EMI SE of the epoxy-HCMs syntactic foam. HCMs were coated with 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. The SSE reached up to 46.3 dB cm3 /g, demonstrating the prospect of epoxy/Ag-PDA-HCMs syntactic foam as a lightweight high-performance EMI shielding material. The corresponding EMI shielding mechanism of this syntactic foam was analyzed by comparing the values of reflectance (R), absorptance (A), and

#### Figure 6.

(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].

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

transmittance (T) in Figure 6c. The specimens were both reflective and absorptive towards EM radiation at silver content less than 17.8 wt%. The contribution of reflection (0.83) towards EMI SE surpassed that from absorption (0.16) when silver content increased to 28.5%. The dense and thick electrically conductive silver formed due to further increasing the silver content to 30.5 wt% increased the R to 0.97 and resultant in reflection as the dominant shielding mechanism.

Xu et al. [31] fabricated syntactic foams ("hybridized epoxy composite foams" according to authors) through impregnating expandable epoxy/MWCNT/microsphere blends into a preformed, highly porous, and 3D silver-coated melamine foam (SF) sponge. The highly conductive SF resolved the problem of the foam reduction of high filled epoxy blends and provided channels for rapid electron transport. MWCNTs were used to offset the loss of conductive pathways due to the crystal defects in the silver layer and the insulating epoxy resin. As a result, the EMI SE of 68.1 dB was achieved with only 2 wt% of MWCNTs and 3.7 wt% of silver due to the synergy of the MWCNT and SF.
