Abstract

Motivated by the increase of stress over electromagnetic pollution issues arising from the fast-growing development and need for electronic and electrical devices, the demand for materials with high electromagnetic interference (EMI) shielding performance has become more urgently. Considering the energy consumption in real applications, lightweight EMI shielding materials has been attentive in this field of research. In this chapter, first of all, the EM theory will be briefly discussed. Secondly, the EMI shielding performance and corresponding mechanisms of three categories of lightweight materials, such as polymer-based composites, foams and aerogels, are reviewed. Finally, the summary and conclusions of this field will be addressed.

Keywords: lightweight, EMI shielding, polymer-based composite, foam, aerogel

#### 1. Introduction

Electromagnetic (EM) waves are generated when an electric field comes in contact with a magnetic field. The oscillations of the electric field and the magnetic field are perpendicular to each other and they are also perpendicular to the direction of EM waves propagation. EM waves travel with a constant velocity of 3.0 <sup>10</sup><sup>8</sup> m/s in vacuum. Unlike mechanical waves (sound waves) which need a medium to travel, EM waves can travel through anything, such as air, water, a solid material or vacuum. EM radiation refers to the EM waves, propagating through space–time, carrying EM radiant energy [1]. It is a form of energy that is all around us. Human activities like using global positioning system (GPS) device to navigate precise location, heating up a food in a microwave or using X-rays detection by a doctor would be impossible without EM radiation. Figure 1 shows the EM spectrum used to describe different types of EM energy according to their frequencies (or wavelengths). The EM spectrum ranges from lower energy waves (longer wavelength), like radio waves and microwaves, to higher energy waves (shorter wavelength), like X-rays and gamma rays. As for the radiated emission which is focused on in this chapter, the frequency locates in the radio frequency spectrum (3 KHz–300 GHz).

Electromagnetic interference (EMI) is a disturbance generated by conduction or external radiation that affects an electrical circuit. The interference emission

2. EMI shielding theory

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

(Po) of an EM wave as [4]:

2.1 Far field and near field

Shielding effectiveness and attenuation %.

Wave impedance in far field and near field [5].

tions as follows,

Table 1.

Figure 2.

213

The EMI capability of a material is called shielding effectiveness (SE). It is defined in terms of the ratio between the incoming power (Pi) and outgoing power

Lightweight Electromagnetic Interference Shielding Materials and Their Mechanisms

SE <sup>¼</sup> 10 log Pi

The unit of EMI SE is given in decibels (dB). According to Eq. (1), how much

According to the distance r between the radiating source and the shield material, an EM wave can be divided into near field wave and far field wave relative to the total wavelength λ of the EM wave. As shown in Figure 2, the region within the distance r > λ/2π is the far field while the distance r < λ/2π is the near field. In far field, the EM waves can be regarded as plane waves and EMI should consider both electric field (E) and magnetic field (H) effects. It fulfills the condi-

<sup>Z</sup> <sup>¼</sup> j j <sup>E</sup>

SE (dB) 20 30 40 50 60 70 Attenuation % 99 99.9 99.99 99.999 99.9999 99.9999

attenuation is blocked at given SE is given in Table 1.

P0

(1)

j j <sup>H</sup> (2)

E ⊥ H (3)

#### Figure 1.

A diagram of the EM spectrum showing various properties across the range of frequencies and wavelengths.

sources are from the conducted emission (several KHz–30 MHz) to the radiated emission (30 MHz–12 GHz) [2]. The conducted emission is the noise which is internally generated from the poor designed electrical circuit such as electrical cables and power wires. The radiated emission that is externally generated is in the form of transmitting EM waves such as the intended EM radiation from the radio broadcasting antenna and the unintended EM radiation from the high-speed transceivers. While detecting the EMI shielding of the device, it is usually relevant to the radiated emission lonely. The conducted emission is another subject especially for the noise prevention in system level.

EMI is encountered by all of us in our daily life and are expected to face exponential rise in future due to the growing numbers of wireless devices and standards, including cell phones, GPS, Bluetooth, Wi-Fi and near-field communication (NFC). Great effort has been dedicated for the development of EMI shielding materials. EMI shielding can be achieved by prevention of EM waves passing through an electric system either by reflection or by absorption of the incident radiation power. In the past, metals were conveniently used in many occasions. Among them, galvanized steel and aluminum are the most widely used. Copper, nickel, pre-tin plated steel, zinc and silver are also used for some purposes. When the trend in today's electronic devices become faster, smaller and lighter, metals are disadvantageous in weight consideration. Moreover, the EM pollution is not truly eliminated or mitigated since the EM signals are almost completely reflected at the surface of the metal protecting the environment only beyond the shield [3]. Hence, intensive research efforts have been focused on the development of EMI shielding materials that work by tunable reflection and absorption based on novel materials that possess lightness, corrosion resistance, flexibility, easy processing, etc.

This chapter is divided into two sections. In the next section, we will describe the EMI shielding theory in details and the parameters that influence the shielding by reflection and absorption. After that, we introduce three categories of lightweight EMI shielding materials, namely, polymer-based composites, foams and aerogels.

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

### 2. EMI shielding theory

The EMI capability of a material is called shielding effectiveness (SE). It is defined in terms of the ratio between the incoming power (Pi) and outgoing power (Po) of an EM wave as [4]:

$$SE = 10\log\left(\frac{P\_i}{P\_0}\right) \tag{1}$$

The unit of EMI SE is given in decibels (dB). According to Eq. (1), how much attenuation is blocked at given SE is given in Table 1.

#### 2.1 Far field and near field

According to the distance r between the radiating source and the shield material, an EM wave can be divided into near field wave and far field wave relative to the total wavelength λ of the EM wave. As shown in Figure 2, the region within the distance r > λ/2π is the far field while the distance r < λ/2π is the near field.

In far field, the EM waves can be regarded as plane waves and EMI should consider both electric field (E) and magnetic field (H) effects. It fulfills the conditions as follows,

$$Z = \frac{|E|}{|H|} \tag{2}$$

$$E \perp H \tag{3}$$


Table 1.

sources are from the conducted emission (several KHz–30 MHz) to the radiated emission (30 MHz–12 GHz) [2]. The conducted emission is the noise which is internally generated from the poor designed electrical circuit such as electrical cables and power wires. The radiated emission that is externally generated is in the form of transmitting EM waves such as the intended EM radiation from the radio broadcasting antenna and the unintended EM radiation from the high-speed transceivers. While detecting the EMI shielding of the device, it is usually relevant to the radiated emission lonely. The conducted emission is another subject especially for

A diagram of the EM spectrum showing various properties across the range of frequencies and wavelengths.

EMI is encountered by all of us in our daily life and are expected to face exponential rise in future due to the growing numbers of wireless devices and standards, including cell phones, GPS, Bluetooth, Wi-Fi and near-field communication (NFC). Great effort has been dedicated for the development of EMI shielding materials. EMI shielding can be achieved by prevention of EM waves passing through an electric system either by reflection or by absorption of the incident radiation power. In the past, metals were conveniently used in many occasions. Among them, galvanized steel and aluminum are the most widely used. Copper, nickel, pre-tin plated steel, zinc and silver are also used for some purposes. When the trend in today's electronic devices become faster, smaller and lighter, metals are disadvantageous in weight consideration. Moreover, the EM pollution is not truly eliminated or mitigated since the EM signals are almost completely reflected at the surface of the metal protecting the environment only beyond the shield [3]. Hence, intensive research efforts have been focused on the development of EMI shielding materials that work by tunable reflection and absorption based on novel materials that possess lightness, corrosion resistance,

This chapter is divided into two sections. In the next section, we will describe the EMI shielding theory in details and the parameters that influence the shielding by reflection and absorption. After that, we introduce three categories of lightweight EMI shielding materials, namely, polymer-based composites, foams and aerogels.

the noise prevention in system level.

Electromagnetic Materials and Devices

Figure 1.

flexibility, easy processing, etc.

212

Shielding effectiveness and attenuation %.

Figure 2. Wave impedance in far field and near field [5].

where Z is the intrinsic impedance or what is sometimes called wave impedance. |E| and |H| are the electric and magnetic fields' amplitudes, respectively. For air (σ = 0, μ = μ0, ε = ε0), the wave impedance (Z0) is always equal to 377 Ω and can be expressed as

$$z\_0 = \sqrt{\frac{j o \mu \mu}{\sigma + j o \epsilon \varepsilon}} = \sqrt{\frac{j o \mu \mu\_0}{j o \epsilon \varepsilon\_0}} = \sqrt{\frac{\mu\_0}{\varepsilon\_0}} \approx 377 \Omega \tag{4}$$

direction with the electric field Er and magnetic field Hr. Other portions of the EM wave are transmitted though the material with the electric field Et and magnetic

Lightweight Electromagnetic Interference Shielding Materials and Their Mechanisms

SE <sup>¼</sup> 20 log Ei

SE <sup>¼</sup> 20 log Hi

The EMI SE of the material depends on the distance between radiation source and the shielding material. When the radiation source is far from the shielding material, the SE is called as far field SE. In the case of the short distance between radiation source and the shielding material, the SE is called as near field SE. Figure 3b illustrates three EMI shielding mechanisms in a conductive shield material. When an EM wave strikes the left boundary of the homogenous conductive material, a reflected wave and a transmitted wave will be created at the left external and right external surface, respectively. As the transmitted wave propagates within the shield material, the amplitude of the wave exponentially decreases as a result from absorption, and the energy loss due to the absorption will be dissipated as heat [6]. Once the transmitted wave reaches the internal right surface of the shield (t), a portion of wave continues to transmit from the shield material and a portion will be reflected into the shield material. The portion of internal reflected wave will be re-reflected within the shield material, which represents the multiple-reflections mechanism. The skin effect would affect the effect of multiplereflections to the overall shielding to a great extent. The depth at which the electric field drops to (1/e) of the incident strength is call the skin depth (δ), which is given

<sup>δ</sup> <sup>¼</sup> <sup>1</sup>

where f is frequency (Hz). μ and σ are the magnetic permeability and the electrical conductivity of the shield material, respectively. If the shield is thicker than the skin depth, the multiple-reflections can be ignored. However, the effect of multiple-reflections will be significant as the shield is thinner than the skin depth. As shown in Figure 3b, in case the shield material is a good conductor, Zm ≪ Z0,

ffiffiffiffiffiffiffiffiffiffi

<sup>π</sup><sup>f</sup> σμ <sup>p</sup> (10)

Theoretically, the SE of a material is contributed from three mechanisms including reflection, absorption and multiple-reflections., the materials with mobile charge carriers (electrons or holes) can interact with the incoming EM wave to facilitate reflection. Absorption depends on the thickness of the shield materials. It increases with the increase of the thickness of the shield materials. For significant absorption, the shield materials possess electric and/or magnetic dipoles which could then interact with the EM fields. Multiple-reflections is the third shielding mechanism, which operates via the internal reflections within the shield material.

Et � � � �

Ht � � � �

� � � �

SEoverall ¼ SER þ SEA þ SEMR ð Þ dB (9)

� � � �

(7)

(8)

field Ht. The electric field SE can be expressed as:

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

The magnetic field SE can be expressed as:

Therefore, the overall SE is the sum of all the three terms:

as follows [7]:

then [8].

215

where σ is the electrical conductivity, μ<sup>r</sup> is the relative permeability (μ = μ0μr), μ<sup>0</sup> is the permeability of air (4<sup>π</sup> � <sup>10</sup>�<sup>7</sup> H/m), <sup>ε</sup><sup>0</sup> is the permittivity of air (8.85 � <sup>10</sup>�<sup>12</sup> F/m).

In near field, the wave front is curved instead of planar, so the wave front is not parallel to the surface of the shielding material. In this case, the wave impedance (|E|/|H|) is not constant and depends on the distance and the dominant field. For an electrical radiation source, the electrical field dominates. The wave impedance is higher than 377 Ω and decreases as the distance r increases. It can be expressed as [5].

$$z\_0 = \frac{1}{2\pi fer} \tag{5}$$

For a magnetic radiation source, the near field is mainly magnetic. The wave impedance is lower than 377 Ω and increases as the distance r increases, it can be expressed as [5].

$$Z\_0 = 2\pi f \mu r \tag{6}$$

In this chapter, all the formulations and results are taken based on far field condition because a distance of 48 cm associated with operating at a frequency of 100 MHz is already considered as far field.

#### 2.2 EMI shielding mechanisms for homogeneous shield materials

Figure 3a illustrates the reflection and transmission of an EM wave when it strikes on a shield material. The uniform EM wave with the electric field Ei and magnetic field Hi is normal incident to the material. When the EM wave strikes the left boundary of the material, portions of the EM wave are reflected in the opposite

#### Figure 3.

(a) Schematic illustration of EM plane wave is normal incident to a material with thickness t and (b) schematic illustration of attenuation of an incident EM wave by a shield material (thickness of shield material = t).

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

direction with the electric field Er and magnetic field Hr. Other portions of the EM wave are transmitted though the material with the electric field Et and magnetic field Ht. The electric field SE can be expressed as:

$$SE = 20\log\left|\frac{E\_i}{E\_t}\right|\tag{7}$$

The magnetic field SE can be expressed as:

where Z is the intrinsic impedance or what is sometimes called wave impedance. |E| and |H| are the electric and magnetic fields' amplitudes, respectively. For air (σ = 0, μ = μ0, ε = ε0), the wave impedance (Z0) is always equal to 377 Ω and can be

> ffiffiffiffiffiffiffiffiffi jωμ<sup>0</sup> jωε<sup>0</sup>

where σ is the electrical conductivity, μ<sup>r</sup> is the relative permeability (μ = μ0μr), μ<sup>0</sup>

In near field, the wave front is curved instead of planar, so the wave front is not parallel to the surface of the shielding material. In this case, the wave impedance (|E|/|H|) is not constant and depends on the distance and the dominant field. For an electrical radiation source, the electrical field dominates. The wave impedance is higher than 377 Ω and decreases as the distance r increases. It can be

> <sup>z</sup><sup>0</sup> <sup>¼</sup> <sup>1</sup> 2π f εr

For a magnetic radiation source, the near field is mainly magnetic. The wave impedance is lower than 377 Ω and increases as the distance r increases, it can be

In this chapter, all the formulations and results are taken based on far field condition because a distance of 48 cm associated with operating at a frequency of

Figure 3a illustrates the reflection and transmission of an EM wave when it strikes on a shield material. The uniform EM wave with the electric field Ei and magnetic field Hi is normal incident to the material. When the EM wave strikes the left boundary of the material, portions of the EM wave are reflected in the opposite

(a) Schematic illustration of EM plane wave is normal incident to a material with thickness t and (b) schematic illustration of attenuation of an incident EM wave by a shield material (thickness of shield

2.2 EMI shielding mechanisms for homogeneous shield materials

¼

ffiffiffiffiffi μ0 ε0 r

Z<sup>0</sup> ¼ 2π fμr (6)

≈377Ω (4)

(5)

s

expressed as

(8.85 � <sup>10</sup>�<sup>12</sup> F/m).

expressed as [5].

expressed as [5].

Figure 3.

214

material = t).

z<sup>0</sup> ¼

Electromagnetic Materials and Devices

100 MHz is already considered as far field.

s

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi jωμ σ þ jωε

is the permeability of air (4<sup>π</sup> � <sup>10</sup>�<sup>7</sup> H/m), <sup>ε</sup><sup>0</sup> is the permittivity of air

¼

$$SE = 20\log\left|\frac{H\_i}{H\_t}\right|\tag{8}$$

Theoretically, the SE of a material is contributed from three mechanisms including reflection, absorption and multiple-reflections., the materials with mobile charge carriers (electrons or holes) can interact with the incoming EM wave to facilitate reflection. Absorption depends on the thickness of the shield materials. It increases with the increase of the thickness of the shield materials. For significant absorption, the shield materials possess electric and/or magnetic dipoles which could then interact with the EM fields. Multiple-reflections is the third shielding mechanism, which operates via the internal reflections within the shield material. Therefore, the overall SE is the sum of all the three terms:

$$\text{SE}\_{overall} = \text{SE}\_{R} + \text{SE}\_{A} + \text{SE}\_{MR} \text{ (dB)} \tag{9}$$

The EMI SE of the material depends on the distance between radiation source and the shielding material. When the radiation source is far from the shielding material, the SE is called as far field SE. In the case of the short distance between radiation source and the shielding material, the SE is called as near field SE.

Figure 3b illustrates three EMI shielding mechanisms in a conductive shield material. When an EM wave strikes the left boundary of the homogenous conductive material, a reflected wave and a transmitted wave will be created at the left external and right external surface, respectively. As the transmitted wave propagates within the shield material, the amplitude of the wave exponentially decreases as a result from absorption, and the energy loss due to the absorption will be dissipated as heat [6]. Once the transmitted wave reaches the internal right surface of the shield (t), a portion of wave continues to transmit from the shield material and a portion will be reflected into the shield material. The portion of internal reflected wave will be re-reflected within the shield material, which represents the multiple-reflections mechanism. The skin effect would affect the effect of multiplereflections to the overall shielding to a great extent. The depth at which the electric field drops to (1/e) of the incident strength is call the skin depth (δ), which is given as follows [7]:

$$\delta = \frac{1}{\sqrt{\pi \xi \sigma \mu}} \tag{10}$$

where f is frequency (Hz). μ and σ are the magnetic permeability and the electrical conductivity of the shield material, respectively. If the shield is thicker than the skin depth, the multiple-reflections can be ignored. However, the effect of multiple-reflections will be significant as the shield is thinner than the skin depth.

As shown in Figure 3b, in case the shield material is a good conductor, Zm ≪ Z0, then [8].

$$\mathrm{SE}\_R = 20 \log \left| \frac{E\_i}{E\_t} \right| = 20 \log \left| \frac{(Z\_0 + Z\_m)^2}{4Z\_m Z\_0} \right| \cong 20 \log \left| \frac{Z\_0}{4Z\_m} \right| \tag{11}$$

<sup>ε</sup><sup>i</sup> <sup>¼</sup> <sup>ε</sup><sup>0</sup> � <sup>j</sup>ε<sup>00</sup> <sup>¼</sup> <sup>ε</sup><sup>0</sup> � <sup>j</sup> <sup>σ</sup>

<sup>T</sup> <sup>¼</sup> <sup>T</sup>1T2e�γmD

where T<sup>1</sup> and T<sup>2</sup> are the transmission coefficients at the boundary 0 and t, respectively. R<sup>1</sup> and R<sup>2</sup> are the reflection coefficients at the boundary 0 and t, respectively. γ<sup>m</sup> is the complex propagation constant. The T1,T2, R1, and R<sup>2</sup> can

> <sup>T</sup><sup>1</sup> <sup>¼</sup> <sup>2</sup> Zm Zm þ Z<sup>0</sup>

> <sup>T</sup><sup>2</sup> <sup>¼</sup> <sup>2</sup> <sup>Z</sup><sup>0</sup> Zm þ Z<sup>0</sup>

> <sup>R</sup><sup>1</sup> <sup>¼</sup> Zm � <sup>Z</sup><sup>0</sup> Zm þ Z<sup>0</sup>

<sup>R</sup><sup>2</sup> <sup>¼</sup> <sup>Z</sup><sup>0</sup> � Zm Zm þ Z<sup>0</sup>

where Z<sup>0</sup> and Zm are the impedance of the air and the composite material, respectively. Z<sup>0</sup> can be expressed in Eq. (4) and Zm can further be expressed as:

> ffiffiffiffiffiffi μr εeff r

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

eff � jε<sup>00</sup>

eff <sup>r</sup> � � (23)

SE ¼ �20 log Tð Þ j j (24)

ε0μ<sup>0</sup> ε<sup>0</sup>

When modern electronic devices are designed, high performance EMI shielding materials are highly demanded. In addition, lightweight is one additional important technical requirement for potential applications especially in the areas of automobile and aerospace. In the following section, we will briefly review state-of-the-art research work regarding polymer-based composite, foams and aerogels used for

Polymer/conductive fillers composites was seen as a promising advanced EMI shielding materials since the discovery that an insulating polymer would allow the

Zm ¼ Zo

The propagation constant γ<sup>m</sup> can be expressed as [10]:

So, the SE can be calculated in terms of T,

3. Lightweight EMI shielding materials

EMI shielding.

217

3.1 Polymer-based composites

γ<sup>m</sup> ¼ jω

in Figure 3b, the transmission coefficient T can be expressed as [10]:

Lightweight Electromagnetic Interference Shielding Materials and Their Mechanisms

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

further be expressed in terms of the impedance Z<sup>0</sup> and Zm [10]:

where ε<sup>0</sup> and ε<sup>00</sup> are the real and imaginary part of the complex relative permittivity of the filler, respectively. σ is the electrical conductivity of the filler. As shown

ωϵ<sup>0</sup>

<sup>1</sup> <sup>þ</sup> <sup>R</sup>1R2e�2γmD (17)

(16)

(18)

(19)

(20)

(21)

(22)

where Zm ¼ ffiffiffiffiffiffiffiffiffiffi jωμ <sup>σ</sup> <sup>þ</sup> <sup>j</sup>ωε <sup>q</sup> ffi ffiffiffiffiffi jωμ σ q , Z<sup>0</sup> ¼ ffiffiffiffi μ0 ε0 q . So SEMR can be expressed as [8].

$$\text{SE}\_{\text{MR}} = 20 \log \left( \frac{1}{4} \sqrt{\frac{\sigma}{o \mu\_r \varepsilon\_0}} \right) = 168 + 10 \log \left( \frac{\sigma\_r}{\mu\_\text{g} f} \right) \tag{12}$$

where ω = 2πf, σ<sup>r</sup> = σ/σCu is the relative conductivity of the material, it is related to the electrical conductivity of the copper, the electrical conductivity of copper is <sup>σ</sup>Cu = 5.8 � <sup>10</sup><sup>7</sup> S/m. If the shield material possesses electric and/or magnetic dipoles, the attenuation of incident EM wave happens inside the shield material due to the absorption and multiple-reflections, the amplitude of the EM wave declines during wave traveling, and it can be expressed as [8].

$$SE\_A = \mathbf{131.4t} \sqrt{f \mu\_r \sigma\_r} \tag{13}$$

$$\text{MSE}\_{\text{MR}} = 20 \log \left| 1 - \left( \frac{Z\_0 - Z\_m}{Z\_0 + Z\_m} \right)^2 e^{-2t/\delta} e^{-2j\beta t} \right| \cong 20 \log \left| 1 - e^{-2t/\delta} e^{-2jt/\delta} \right| \tag{14}$$

where t is the thickness of the shield material, δ is the skip depth under the operation frequency, β is the propagation constant.

The mechanism of multi-reflections is complicated. For a good conductor material, the multiple-reflection is usually insignificant because most of the incident EM waves are reflected from the external conductive surface of the shield material, and only few penetrated EM waves can be retained for multiplereflections. The influence is more important for a material that has high permeability and low electrical conductivity. In this case, EM waves can easily penetrate through the external surface of the shield material and most penetrated EM waves are reflected from the second surface of the shield material. The influence is more important in low frequency and is reduced when the frequency gets higher because the ratio between material thickness and skin depth (t/δ) become larger as the frequency increases.

#### 2.3 EMI shielding mechanisms for composites

Composites are made from fillers and matrices with significantly different physical or chemical properties. Hence, EMI shielding mechanisms are more complicated than those for homogeneous shield materials because of the huge surface area available for reflection and multiple-reflections. The EMI SE of composites can be measured experimentally, and it also can be calculated theoretically. The effective relative permittivity εeff of composites, which is one of the most important parameters in the calculation, can be approximately calculated from the Maxwell Garnett formula as [9]:

$$
\varepsilon\_{\rm eff} = \varepsilon\_{\rm t} + \mathfrak{H} \,\varepsilon\_{\rm t} \frac{\varepsilon\_{\rm i} - \varepsilon\_{\rm t}}{\varepsilon\_{\rm i} + 2\varepsilon\_{\rm t} - f(\varepsilon\_{\rm i} - \varepsilon\_{\rm t})} \tag{15}
$$

where ε<sup>e</sup> is the relative permittivity of the matrix, ε<sup>i</sup> is the relative permittivity of the filler and f is the volume fraction of the filler. If the filler are electrical conductive particles, the relative permittivity ε<sup>i</sup> can be expressed as [10]:

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

$$
\varepsilon\_{\dot{\imath}} = \varepsilon' - j\varepsilon'' = \varepsilon' - j\frac{\sigma}{a\varkappa\_0} \tag{16}
$$

where ε<sup>0</sup> and ε<sup>00</sup> are the real and imaginary part of the complex relative permittivity of the filler, respectively. σ is the electrical conductivity of the filler. As shown in Figure 3b, the transmission coefficient T can be expressed as [10]:

$$T = \frac{T\_1 T\_2 e^{-\gamma\_m D}}{1 + R\_1 R\_2 e^{-2\gamma\_m D}}\tag{17}$$

where T<sup>1</sup> and T<sup>2</sup> are the transmission coefficients at the boundary 0 and t, respectively. R<sup>1</sup> and R<sup>2</sup> are the reflection coefficients at the boundary 0 and t, respectively. γ<sup>m</sup> is the complex propagation constant. The T1,T2, R1, and R<sup>2</sup> can further be expressed in terms of the impedance Z<sup>0</sup> and Zm [10]:

$$T\_1 = \frac{2Z\_m}{Z\_m + Z\_0} \tag{18}$$

$$T\_2 = \frac{2Z\_0}{Z\_m + Z\_0} \tag{19}$$

$$R\_1 = \frac{Z\_m - Z\_0}{Z\_m + Z\_0} \tag{20}$$

$$R\_2 = \frac{Z\_0 - Z\_m}{Z\_m + Z\_0} \tag{21}$$

where Z<sup>0</sup> and Zm are the impedance of the air and the composite material, respectively. Z<sup>0</sup> can be expressed in Eq. (4) and Zm can further be expressed as:

$$Z\_m = Z\_o \sqrt{\frac{\mu\_r}{\varepsilon\_{\mathcal{G}f}}} \tag{22}$$

The propagation constant γ<sup>m</sup> can be expressed as [10]:

$$\gamma\_m = j\alpha \sqrt{\varepsilon\_0 \mu\_0 \left(\varepsilon\_{\rm eff}^{\prime} - j\varepsilon\_{\rm eff}^{\prime\prime}\right)}\tag{23}$$

So, the SE can be calculated in terms of T,

$$\text{SE} = -2\mathbf{0}\log(|T|) \tag{24}$$

#### 3. Lightweight EMI shielding materials

When modern electronic devices are designed, high performance EMI shielding materials are highly demanded. In addition, lightweight is one additional important technical requirement for potential applications especially in the areas of automobile and aerospace. In the following section, we will briefly review state-of-the-art research work regarding polymer-based composite, foams and aerogels used for EMI shielding.

#### 3.1 Polymer-based composites

Polymer/conductive fillers composites was seen as a promising advanced EMI shielding materials since the discovery that an insulating polymer would allow the

SER <sup>¼</sup> 20 log Ei

ffi

SEMR <sup>¼</sup> 20 log <sup>1</sup>

wave traveling, and it can be expressed as [8].

Z<sup>0</sup> þ Zm � �<sup>2</sup>

operation frequency, β is the propagation constant.

2.3 EMI shielding mechanisms for composites

SEMR <sup>¼</sup> <sup>20</sup> log <sup>1</sup> � <sup>Z</sup><sup>0</sup> � Zm

the frequency increases.

Garnett formula as [9]:

216

� � � � �

ffiffiffiffiffiffiffiffiffiffi jωμ σ þ jωε q

Electromagnetic Materials and Devices

where Zm ¼

Et � � � �

ffiffiffiffiffi jωμ σ q

, Z<sup>0</sup> ¼

4

� � �

� <sup>¼</sup> <sup>20</sup> log ð Þ <sup>Z</sup><sup>0</sup> <sup>þ</sup> Zm <sup>2</sup>

ffiffiffiffi μ0 ε0 q

ffiffiffiffiffiffiffiffiffiffiffiffi σ ωμrε<sup>0</sup> � � r

SEA ¼ 131:4t

where t is the thickness of the shield material, δ is the skip depth under the

The mechanism of multi-reflections is complicated. For a good conductor material, the multiple-reflection is usually insignificant because most of the incident EM waves are reflected from the external conductive surface of the shield material, and only few penetrated EM waves can be retained for multiplereflections. The influence is more important for a material that has high permeability and low electrical conductivity. In this case, EM waves can easily penetrate through the external surface of the shield material and most penetrated EM waves are reflected from the second surface of the shield material. The influence is more important in low frequency and is reduced when the frequency gets higher because the ratio between material thickness and skin depth (t/δ) become larger as

Composites are made from fillers and matrices with significantly different physical or chemical properties. Hence, EMI shielding mechanisms are more complicated than those for homogeneous shield materials because of the huge surface area available for reflection and multiple-reflections. The EMI SE of composites can be measured experimentally, and it also can be calculated theoretically. The effective relative permittivity εeff of composites, which is one of the most important parameters in the calculation, can be approximately calculated from the Maxwell

> ϵ<sup>i</sup> � ϵ<sup>e</sup> ε<sup>i</sup> þ 2ε<sup>e</sup> � fð Þ ε<sup>i</sup> � ε<sup>e</sup>

where ε<sup>e</sup> is the relative permittivity of the matrix, ε<sup>i</sup> is the relative permittivity of the filler and f is the volume fraction of the filler. If the filler are electrical conduc-

εeff ¼ ε<sup>e</sup> þ 3f ε<sup>e</sup>

tive particles, the relative permittivity ε<sup>i</sup> can be expressed as [10]:

e �2t=δ e �2jβt

where ω = 2πf, σ<sup>r</sup> = σ/σCu is the relative conductivity of the material, it is related to the electrical conductivity of the copper, the electrical conductivity of copper is <sup>σ</sup>Cu = 5.8 � <sup>10</sup><sup>7</sup> S/m. If the shield material possesses electric and/or magnetic dipoles, the attenuation of incident EM wave happens inside the shield material due to the absorption and multiple-reflections, the amplitude of the EM wave declines during

> ffiffiffiffiffiffiffiffiffiffiffi f μrσ<sup>r</sup> q

> > ffi 20 log 1 � e

� � � � �

� � � � �

4ZmZ<sup>0</sup>

� � � � �

<sup>¼</sup> <sup>168</sup> <sup>þ</sup> 10 log <sup>σ</sup><sup>r</sup>

ffi <sup>20</sup> log <sup>Z</sup><sup>0</sup>

. So SEMR can be expressed as [8].

4Zm � � � �

μrf � �

�2t=δ e �2jt=<sup>δ</sup> � � �

� � � �

(11)

(12)

(13)

(15)

� (14)

flow of current through the conductive network stablished by conductive fillers above the percolation threshold. The conductive composite materials preserve the advantages of lightness of polymers, low cost, design flexibility and ease of processing, and the incorporation of conductive fillers circumvent intrinsic nature of polymers being transparent to EM waves through interaction between EM wave and the conductive fillers. Metallic fillers, intrinsically conductive polymers and carbon based electrically conductive fillers are discussed in this section with specific examples. Polymer/magnetic particles composites will also be briefly introduced as magnetic portion is an important component in EM waves that should not be ignored. This section aims to provide a general overview on the preparation of polymer-based EMI shielding materials and the advantages and challenges faced by each category and possible strategies towards enhancing the EMI shielding performances.

3.1.2 Intrinsically conductive polymers-based composites

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

structure, hence the EMI shielding properties of the material.

about 36 dB over a wide frequency range up to 1.5 GHz [13].

quency specific material is ideal for shielding at 2.2 and 8.8 GHz.

mance and complex processing procedures involved.

level is usually needed for acceptable performances.

219

3.1.3 Polymer-based composites containing carbon-based fillers

Blends of a polymer with an intrinsically conductive polymer results in a composite combining the desired properties of the two components, that is, adequate mechanical properties of the polymer matrix for mechanical support and the electrically conducting component for interaction with the EM radiation. Conducting polymers are conjugated polymers, which on doping exhibit electronic conductivity. Distinctive to metallic fillers, the electrical conductivity of conducting polymers arises from the polymer molecular structure. Alteration of parameters such as chain size, doping level, dopant type and the synthesis route directly affect the molecular

Lightweight Electromagnetic Interference Shielding Materials and Their Mechanisms

Among the available conducting polymers, polypyrrole (PPY) and polyaniline (PANI) are the most widely used conductive fillers for EMI shielding purposes. PPY is known to possess high conductivity, easy synthesis, good environmental stability and less toxicological problem. Chemical and electrochemical polymerization of PPY on a polyethylene terephthalate (PET) fabric is given as an example for electrically conducting composite. Pyrrole was first dissolved in an aqueous solution containing 10 wt% polyvinyl alcohol (PVA) and sprayed on the PET fabric before subject to electrochemical polymerization at room temperature under a constant current density. The resultant PPY coated PET fabric was shown to exhibit EMI SE

PANI was studied extensively for its various structures, unique doping mechanism, excellent physical and chemical properties, stability, and the readily obtainable raw materials. Lakshmi et al. [14] prepared PANI-PU composite film by adding aniline to polyurethane (PU) solution in tetrahydrofuran (THF). Doping of composites was done by adding camphor sulfonic acid to the composite solution. The EMI SE of the PU-PANI film was found to increase with thickness and the fre-

Other intrinsically conducting polymers, such as poly(p-phenylene-vinylene) [15, 16] and poly(3-octylthiophene) [17], were also investigated for EMI shielding applications, but too much lesser extent, mainly due to the unsatisfactory perfor-

In general, the EMI shielding performance arises by the addition of conductive polymer consequently dominated by reflection mechanism due to the increase of the level of impedance mismatch with air. One obvious advantage of such polymerpolymer system is the lightweight being preserved, also there is no issue on substrate flexibility as those associated with metallic or carbon-based fillers. However, the main drawbacks of such composites include (1) poor mechanical properties of the most of the intrinsically conducting polymers require a matrix material for structural support; (2) the insoluble and infusible characteristics caused conducting polymers to exhibit poor processability and (3) high filler (conducting polymer)

Similar to metallic fillers, carbon-based fillers come in various shapes and aspect ratios. Carbon black (CB), including graphite and CB, is the generic name given to small particle size carbon pigments which are formed in the gas phase by thermal decomposition of hydrocarbons [18]. Carbon fibers (CFs) are 1D carbon structure of diameter generally lies between 50 and 200 nm and aspect ratios around 250 and 2000, largely produced by chemical vaporization of hydrocarbon [19, 20]. Carbon nanotubes (CNTs) can be considered as rolled-up hollow cylinders of graphene sheets of very high aspect ratio due to the small diameter, constituted of a single

#### 3.1.1 Polymer-based composites containing metallic fillers

Metals are typical wave-reflection materials used for EMI shielding purpose owing to their abundance in mobile charge carriers that can interact with the incident EM radiation. Metallic fillers of various physical forms, such as fibers or nanoparticles, were dispersed in the polymer matrix to increase the interaction with the incident EM radiation. Injection-molding provides a direct method to disperse metallic fillers into a polymer matrix. Stainless steel fibers (SSF) introduced into polycarbonate matrix through injection molding shown that EMI SE is heavily dependent on the molding parameters which would give an optimum electrical conductivity [11]. Blended textiles of polyester fibers with SSF showed that the EMI SE is more than 50 dB in the frequencies ranging from 30 MHz to 1.5 GHz [12] (see Figure 4a). As shown in Figure 4b and c, comparison of reflectance, absorbance and transmittance, (identified as reflectivity, absorptivity and transmissibility in Figure 4) for SSF and SSF/ polyester fiber fabrics as a function of frequency revealed absorption as the dominant EMI shielding mechanism. In the case of SSF/polyester with 10 wt% SSF, EMI shielding by absorption increased from 30 MHz to maximum at 500 MHz and then decreased with the increase in frequency.

The challenges in achieving a good dispersion of metallic fillers and the weight increase make polymer/metallic fillers composites a less popular choice. Much attention was switched to intrinsically conductive polymers (including polyaniline, polyacetylene, and polypyrrole), carbon-based materials (including carbon fibers, carbon black, graphite, graphene, carbon nanotubes and mesoporous carbon), and magnetic materials like carbonyl iron and ferrites (including Fe3O4 and α-Fe2O3).

#### Figure 4.

(a) The EMI SE of the SSF/PET fabric as a function of frequency; (b) reflectivity/absorptivity/transmissibility of SSF fabric and (c) SSF/PET fabric with 10 wt% SSF as a function of frequency [12].

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

#### 3.1.2 Intrinsically conductive polymers-based composites

flow of current through the conductive network stablished by conductive fillers above the percolation threshold. The conductive composite materials preserve the advantages of lightness of polymers, low cost, design flexibility and ease of processing, and the incorporation of conductive fillers circumvent intrinsic nature of polymers being transparent to EM waves through interaction between EM wave and the conductive fillers. Metallic fillers, intrinsically conductive polymers and carbon based electrically conductive fillers are discussed in this section with specific examples. Polymer/magnetic particles composites will also be briefly introduced as magnetic portion is an important component in EM waves that should not be ignored. This section aims to provide a general overview on the preparation of polymer-based EMI shielding materials and the advantages and challenges faced by each category and possible strategies towards enhancing the EMI shielding

Metals are typical wave-reflection materials used for EMI shielding purpose owing to their abundance in mobile charge carriers that can interact with the incident EM radiation. Metallic fillers of various physical forms, such as fibers or nanoparticles, were dispersed in the polymer matrix to increase the interaction with the incident EM radiation. Injection-molding provides a direct method to disperse metallic fillers into a polymer matrix. Stainless steel fibers (SSF) introduced into polycarbonate matrix through injection molding shown that EMI SE is heavily dependent on the molding parameters which would give an optimum electrical conductivity [11]. Blended textiles of polyester fibers with SSF showed that the EMI SE is more than 50 dB in the frequencies ranging from 30 MHz to 1.5 GHz [12] (see Figure 4a). As shown in Figure 4b and c, comparison of reflectance, absorbance and transmittance, (identified as reflectivity, absorptivity and transmissibility in Figure 4) for SSF and SSF/ polyester fiber fabrics as a function of frequency revealed absorption as the dominant EMI shielding mechanism. In the case of SSF/polyester with 10 wt% SSF, EMI shielding by absorption increased from 30 MHz to maximum at 500 MHz and then

The challenges in achieving a good dispersion of metallic fillers and the weight increase make polymer/metallic fillers composites a less popular choice. Much attention was switched to intrinsically conductive polymers (including polyaniline, polyacetylene, and polypyrrole), carbon-based materials (including carbon fibers, carbon black, graphite, graphene, carbon nanotubes and mesoporous carbon), and magnetic materials like carbonyl iron and ferrites (including Fe3O4 and α-Fe2O3).

(a) The EMI SE of the SSF/PET fabric as a function of frequency; (b) reflectivity/absorptivity/transmissibility

of SSF fabric and (c) SSF/PET fabric with 10 wt% SSF as a function of frequency [12].

3.1.1 Polymer-based composites containing metallic fillers

decreased with the increase in frequency.

performances.

Electromagnetic Materials and Devices

Figure 4.

218

Blends of a polymer with an intrinsically conductive polymer results in a composite combining the desired properties of the two components, that is, adequate mechanical properties of the polymer matrix for mechanical support and the electrically conducting component for interaction with the EM radiation. Conducting polymers are conjugated polymers, which on doping exhibit electronic conductivity. Distinctive to metallic fillers, the electrical conductivity of conducting polymers arises from the polymer molecular structure. Alteration of parameters such as chain size, doping level, dopant type and the synthesis route directly affect the molecular structure, hence the EMI shielding properties of the material.

Among the available conducting polymers, polypyrrole (PPY) and polyaniline (PANI) are the most widely used conductive fillers for EMI shielding purposes. PPY is known to possess high conductivity, easy synthesis, good environmental stability and less toxicological problem. Chemical and electrochemical polymerization of PPY on a polyethylene terephthalate (PET) fabric is given as an example for electrically conducting composite. Pyrrole was first dissolved in an aqueous solution containing 10 wt% polyvinyl alcohol (PVA) and sprayed on the PET fabric before subject to electrochemical polymerization at room temperature under a constant current density. The resultant PPY coated PET fabric was shown to exhibit EMI SE about 36 dB over a wide frequency range up to 1.5 GHz [13].

PANI was studied extensively for its various structures, unique doping mechanism, excellent physical and chemical properties, stability, and the readily obtainable raw materials. Lakshmi et al. [14] prepared PANI-PU composite film by adding aniline to polyurethane (PU) solution in tetrahydrofuran (THF). Doping of composites was done by adding camphor sulfonic acid to the composite solution. The EMI SE of the PU-PANI film was found to increase with thickness and the frequency specific material is ideal for shielding at 2.2 and 8.8 GHz.

Other intrinsically conducting polymers, such as poly(p-phenylene-vinylene) [15, 16] and poly(3-octylthiophene) [17], were also investigated for EMI shielding applications, but too much lesser extent, mainly due to the unsatisfactory performance and complex processing procedures involved.

In general, the EMI shielding performance arises by the addition of conductive polymer consequently dominated by reflection mechanism due to the increase of the level of impedance mismatch with air. One obvious advantage of such polymerpolymer system is the lightweight being preserved, also there is no issue on substrate flexibility as those associated with metallic or carbon-based fillers. However, the main drawbacks of such composites include (1) poor mechanical properties of the most of the intrinsically conducting polymers require a matrix material for structural support; (2) the insoluble and infusible characteristics caused conducting polymers to exhibit poor processability and (3) high filler (conducting polymer) level is usually needed for acceptable performances.

#### 3.1.3 Polymer-based composites containing carbon-based fillers

Similar to metallic fillers, carbon-based fillers come in various shapes and aspect ratios. Carbon black (CB), including graphite and CB, is the generic name given to small particle size carbon pigments which are formed in the gas phase by thermal decomposition of hydrocarbons [18]. Carbon fibers (CFs) are 1D carbon structure of diameter generally lies between 50 and 200 nm and aspect ratios around 250 and 2000, largely produced by chemical vaporization of hydrocarbon [19, 20]. Carbon nanotubes (CNTs) can be considered as rolled-up hollow cylinders of graphene sheets of very high aspect ratio due to the small diameter, constituted of a single

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 properly dispersed.

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

Lightweight Electromagnetic Interference Shielding Materials and Their Mechanisms

shielding material despite the small amount present in the systems.

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 light-

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

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

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

3.2 Foams and aerogels used in EMI shielding

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

weight is required.

3.2.2 Syntactic foams

in syntactic foam matrix.

221

3.2.1 Polymer-based composite foams

electrical conductivity and strength, compared to CNFs.

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 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].
