Comprehensive review on polymer composites as electromagnetic interference shielding materials

Abstract

Modern electronic devices need effective shielding from external Electromagnetic Interference (EMI) to function correctly. The Electromagnetic (EM) waves emerging from communication systems and medical devices must be isolated to safeguard humans from exposure. The mechanics of EMI shielding as well as several characterization methods are discussed in this paper. Existing trends and practices of designing, fabrication and use of polymer matrix composites for EMI shielding applications are covered. Several approaches and new solutions for fabricating composites either by modification of filler or matrix are discussed and prominent features of EMI shielding material are also discussed. Conducting polymers such as polyaniline, polypyrrole and polythiophene make them adept for EMI shielding applications. An assessment of the different factors affecting the performance of EMI shielding materials is also presented.

Keywords

Electromagnetic interference shielding polymer composites shielding effectiveness absorption reflection multiple reflections

Introduction

Innovation in technology has insisted a drastic transformation in the present electronic devices and enabling them to be used by people across all generations. Electronic devices increase pollution in the environment by emitting electromagnetic radiations. Exposure to electromagnetic emissions is detrimental to the health of living beings.^1-3 Presently, electronic systems are used in automation industries health care, airports, automobiles, aviation sector, railways, satellites, home and offices. Extensive use of electronic control and communication devices in numerous fields emit EM radiations in a variety of frequencies and generate electromagnetic interference (EMI). EMI interferences affect the usual operation of electronic devices in the system. EM waves further produce noise owing to the intervention with critical electronic devices and affects their sensitivity. Radars, antenna systems, military control systems, security devices, and banking are some of the important organizations that are prone to electromagnetic interference. The performance of systems associated with networking devices and data center servers also degrade with the interference of EM waves. Therefore, for smooth operation of electronic devices in the system, it is necessary to develop effective EMI shielding materials.

Electromagnetic interference shielding materials must have good electrical conductivity and low magnetic permeability so that the permeation of EM waves is low.^4-6 Although metals like steel, copper and aluminum are suitable for EMI shielding; higher density, corrosion, oxidation, high forming and material cost restrict the use of these materials in outdoor shielding applications. Innovative electronic systems apart from effective EM shielding demand properties such as ease of manufacturing, flexibility in material design, low thermal expansion coefficient, non-corrosiveness. Additionally, high strength to weight ratio, lightweight and reliable materials is required to meet the needs of advanced electronic systems. Presently, intrinsically conducting polymers like polyaniline and polypyrrole are majorly used for EMI shielding owing to their excellent electrical conductivity.^7-11 Electrically conducting and magnetic fillers reinforced polymer composites are also widely considered for shielding purposes.^12-14 Polymer composites reinforced with carbon nano tubes, graphene, carbon black and graphite are also extensively used for EMI shielding and other potential applications like lithium ion batteries, sensors, solar cells. Off late, polymer syntactic foams and fibers sandwiched with composites cores have gained significant interest in EMI shielding applications attributed to their high inbuilt porosity enabling them to undergo multiple reflections and effectively shield against EMI.¹⁵ Most recent ingress as an effective shielding material is MXenes; a conducting transition metal 2D carbide similar to graphene.^16,17

Ever increasing demands for high performance facilities with modern technology necessitates need for new materials. Achieving such enhanced performance standards with one material is a difficult and a challenging task. Thereby novel constituents are processed by mixing two or more materials called as composite materials to provide exclusive blend of properties that cannot be achieved from conventional materials. Composite materials offer advantages such as higher strength, enhanced modulus, better bending stiffness and corrosion and chemical resistance.¹⁸ Additionally, light weightiness and good mechanical properties enable them to be used in applications of aeronautics and space sector.^19-22 Polymer based materials are used in applications such as stealth technology wherein electromagnetic waves need to shield mainly through absorption. Furthermore, reinforcing suitable fillers in polymer composites apart from providing tailor made physical and mechanical properties; can also assist in achieving desired EMI shielding by tuning the permittivity, electrical conductivity and permeability based on the specific requirements. Polymer and polymer composites reveal immense capabilities to be a efficient EMI shielding, light weight, thermally stable material that can be used in a number of applications such as radars, electronic devices, aerospace and military applications and stealth technology (Figure 1). In the present review, focus is envisioned on polymer matrix composites that can serve as giant market for EMI shielding. Based on the various reinforcements, our study has been categorized into different types based on the type of fillers used in polymer matrix composites namely metallic fillers, intrinsically conducting polymers, magnetic particles and porous materials. Further, the different factors affecting the performance of EMI shielding materials are also discussed and finally, the prominent features of EMI shielding material are discussed.

Figure 1.

Schematic representation of various electromagnetic bands with applications.²³

Electromagnetic compatibility (EMC), measurement, methods and standards

Electromagnetic Compatibility has gained a significant importance owing to the tremendous increase in wireless devices. Electromagnetic compatibility allows several electronic devices to be used without common intervention when operated in confined vicinity. All the circuits of electronic devices are prone to gather radiation of undesirable electrical intervention that compromises the working of one or more circuits.

Electromagnetic compatibility definition: Electromagnetic compatibility is defined as capability of electronic systems and devices to perform without any errors and malfunctions when operating within a confined electromagnetic surroundings. Basically, electromagnetic compatibility is comprised of two components, namely emissions and susceptibility or immunity.

Emissions: Electromagnetic energy generated due to the electromagnetic interference are referred as emissions. These emissions are undesired and must be kept below the standard limits so that the functioning of other equipment are not disrupted.²⁴

Susceptibility & immunity: The response of electronic devices to the undesired electromagnetic energy is termed susceptibility. Electronic devices are designed in such a way as to maintain a higher level of immunity against these undesired signals.²⁴

Electromagnetic compatibility standards

Rising awareness and requisite of maintaining higher electromagnetic compatibility standards, many standards have been put forth to enable manufacturers meet the required levels to maintain full electromagnetic compatibility.²⁵ Electromagnetic compatibility has become an essential part of every electronics project.²⁶ For new products it is mandatory to meet the standards of electromagnetic compatibility so that the product can perform under all possible circumstances (Tables 1, 2, 3, and 4).²⁷

Table 1.

Electromagnetic compatibility standards.²⁸

Area	Standard	Details
Aerospace	DO-160	Aircraft EMC requirements
	SAE ARP5412B	Aircraft lightning environment and related test waveforms
	SAE ARP5416A	Aircraft lightning test methods
Automotive	SAEJ1113	General automotive EMC
Commercial	ANSI C63.4	Methods of measurement
	CISPR 11	ISM equipment EN 55011
	CISPR 16	Methods of measurement
	CISPR 22	ITE equipment EN 55022
	FCC Part 15B	ITE equipment
	IEC 61000-3-2	Harmonics
	IEC 61000-3-3	Flicker
	IEC 61000-4-2	Electrostatic Discharge, ESD
	IEC 61000-4-3	Radiated immunity
	IEC 61000-4-4	Electrically Fast Transient
	IEC 61000-4-5	Surge (lightning)
	IEC 61000-4-6	Conducted immunity
	IEC 61000-4-8	Magnetic immunity
	IEC 61000-4-11	Voltage dips, interrupts & variations
Medical	IEC 60601-1-2	Medical products
Military	MIL STD 461F	EMC test requirements

Table 2.

EMI SE of different polymer composites with metallic fillers.

Ref.	Materials	Processing method	Frequency	Thickness (mm)	Shielding effectiveness (dB)
35	Poly carbonate/Stainless Steel fibers (6 wt.%)	Injection molding	100 kHz – 1.8 GHz	1.5	30-40
36	Polyester fibers/316L stainless steel fibers (10, 20, 30 and 40 wt.%)	NR	300 MHz – 1500 MHz	—	50
37	Polyether sulphone/Aluminum flakes (5, 10, 15, 20, 30 and 40 vol.%)	Hot pressing the mixture in matched- metal die	1–2 GHz	2.82–2.95	26
	Polyether sulphone /Steel fibers (10 and 20 vol.%)				42
	Polyether sulphone /10 μm diameter Carbon fibers (100μm long – 10, 20, 30 and 40 vol.% 200μm long – 10, 20, 30 and 40 vol.% 400μm long – 10, 20, 30 and 40 vol.% 800μm long – 10 and 20 vol.% 3000μm long – 5, 10, 15 and 20 vol.%)				19
	Polyether sulphone /Nickel particles (10 and 20 vol.%)				23
38	Polyether sulphone/Carbon filaments (3, 7, 13 and 19 vol.%)	Wet mixing in blender	1–2 GHz	2.85	32
	Polyether sulphone/Nickel fibers (3, 7, 13, 19, 25, 37 and 43 vol.%)	Dry mixing of polymer powder and filler and hot pressing in steel mold			58
	Polyether sulphone/Nickel filaments (3, 7, 13 and 19 vol.%)	Ball milling			87

Table 3.

EMI SE of different intrinsically conducting polymer composites.

Ref.	Materials	Processing method	Frequency	Thickness (mm)	Shielding effectiveness (dB)
39	Polyethylene terephthalate/Polypyrrole	Chemical and electrochemical polymerization of Polypyrrole on a Polyethylene terephthalate fabric in sequence	50 MHz – 1.5 GHz	—	36
40	Polyurethane/Polyaniline	NR	S (1.9 – 3.9 GHz) and X (7 – 13 GHz) band	0.62, 1.26 and 1.9	26.7 and 15.5
41	Polystyrene/Poly(3-octylthiophene) (20 wt.%)	Blends were prepared using Brabender mixer and further pressed into sheets	100 kHz – 1 GHz	3	43
	Polyvinyl chloride/Poly(3-octylthiophene) (5, 10 and 20 wt.%)				47
	Ethyl-vinyl acetate/Poly(3-octylthiophene) (20 wt.%)				51

Table 4.

Polymer composites having good EMI SE.

Ref.	Materials	Processing method	Frequency	Thickness (mm)	Shielding effectiveness (dB)
39	Polyester fabric/polypyrrole	Chemical and electrochemical polymerization of Polypyrrole on a Polyethylene terephthalate fabric in sequence	50 MHz-1.5 GHz	—	36
46	Polypyrrole impregnated conducting polymer composites	Composite was prepared by imbibing a porous host polymer with a pyrrole solution and then with an oxidant solution.	1–2 GHz	—	26
47	Polypyrrole impregnated microporous polyethylene tapes	NR	10 kHz to 1 GHz	—	40–50
48	Galvanostatically synthesized conducting polypyrrole thin films	NR	10 kHz to 1 GHz	50–100	40–50
49	Laminate consisting on a PANI camphor sulfonic acid	NR	Far-field	1–30	30
50	Polypyrrole-γ-Fe₂O₃-Flyash nano-composites	In-situ emulsion polymerization	12.4−18 GHz	2	∼25.5
51	Polypyrrole nylon 6 Composite Fabrics	Electrochemical polymerization and Chemical oxidative polymerization	—	0.066	16
52	Polyaniline with polyvinyl chloride	NR	2.2-3 GHz	—	4
52	Polypyrrole with polyvinyl chloride	NR	2.2-3 GHz	—	4
53	γ -Fe2O3 decorated reduced graphene oxide in polyaniline core-shell tubes	Chemical oxidative polymerization	8–12.5 GHz	2.5	∼51
54	Flexible multilayer graphene nanosheets (10, 30, 50 and 60 vol.%)/polymer composite films	Wetting casting method and sandwich method	8.2–12.4 GHz	1	27
55	Monolayer graphene	Inductively coupled plasma chemical vapor deposition method	2.2–7 GHz	—	2.27
56	Graphene nanosheet/water-borne polyurethane composite	Solution mixing	8.2–12.4 GHz	2	∼32
57	Polyaniline composite containing. 5.0 wt.% Ag@graphene	In-situ polymerization	0.45–1.5 GHz	1	29.33
42	Polypyrrole hollow microsphere/silver nano-composite (2, 5 and 10 wt.%)	Tollen’s reagent and centrifugation	0.5-8 GHz	1	∼59-23
58	Solid copper	Mixing of polymer powder and filler and then hot pressing in steel mold at 310°C	1–2 GHz	3.1	90.2
	7 vol.% 2 μm Ni fiber/Polyethersulfone composites		1–2 GHz	2.85	58.1
	19 vol.% carbon fiber/Polyethersulfone composites		1–2 GHz	2.85	73.9
59	2 wt.% aligned reduced Graphene Oxide/epoxy composites	Aqueous casting method	0-1 KHz	>0.1	38
60	7 wt.% Carbon Nano Tube/Polystyrene foam	Hot pressing	8.2–12.4 GHz	1.2	18.2−19.3
61	Multi Wall Carbon Nano Tubes reinforced carbon foam	Chemical vapor deposition	8.2–12.4 GHz	2.75	85
62	1.8 vol.% graphene/polymethyl methacrylate foam	Solution blending and melt compounding	8–12 GHz	4	13–19
63	Functionalized Graphene sheet (3.2 and 5.6 vol.%)/Polystyrene foam	Solution mixing, mechanical mixing and salt leaching methods	8.2–12.4 GHz	2.5	29
43	Silver nanoparticles coated hollow carbon sphere/epoxy foam	NR	8–12 GHz	1.5	60.2
64	Magnetite-zinc oxide/carbon foam	Sacrificial template route	8.2–12.4 GHz	2.75	48.5
65	2 vol.% Carbon Nanofiber/epoxy foam	Mechanical dispersion	30 MHz to 1.2 GHz	3	25
66	Graphene/poly(dimethyl siloxane) foam	Chemical vapor deposition	30 MHz–1.5 GHz and 8–12 GHz	1	∼30 and ∼20

Additionally, several international establishments, national organizations, trade associations and insurance bodies have setup the regulations for electromagnetic compatibility, some globally known organizations are

• International Electro technical Commission

• Federal Communications Commission standard

• European standard and

• Military standard.

Electromagnetic interference, shielding mechanism and characterization methods

Electromagnetic interference, EMI

Electromagnetic radiation is a type of energy with the wave nature either emitted or absorbed by charging constituents. Presently artificial electromagnetic radiation created by electric appliances, radios, mobiles, TVs and mobile towers has started to emerge and accumulate in the environment. These radiations are termed as electromagnetic interference (EMI). Electromagnetic Interference is stated as the emission of conduct or radiated electromagnetic signals during operation that lead to inappropriate working of electrical devices and harm living creatures.²⁹ Such interferences need to be lowered so that different electronic equipment is compatible and function properly in the presence of each other. Numerous means are available through which electromagnetic interference is passed from one electronic device to other. Therefore, understanding these methods is very important to mitigate the effects of electromagnetic interference.

Electromagnetic interference can be broadly divided into two types, namely continuous and impulse interference.

Continuous interference: The continuous EMI refers to EMIs that are continuously emitted by a source. The source may be either man made or natural but the interference is continuously experienced, for as long as a coupling mechanism exists between the EMI source and the receiver.³⁰ Sustained interferences like oscillation or radio signals are termed as continuous interference. This interference may be owing to an unscreened oscillator or wideband noise.

Impulse interference: Impulse EMI is EMIs that occur intermittently or within a very short duration. Impulse EMIs could also be occurring naturally or man-made.³⁰ Short impulse such as electrostatic discharge, lightning or switching a circuit is termed as impulse interference.

Apart from knowing the form of interference, travelling path of interference from the transmitting device to receiving device also needs to be known. Inopportunely it is not easy to determine because defining the paths is a challenging task. Nevertheless good preliminary design can lessen several problems.

Shielding mechanism

The capacity of a shielding material is usually expressed in terms of shielding effectiveness (SE). Output is measured in decibel (dB) that depicts the extent of its opaqueness to an EM wave within a specified frequency bandwidth. Shielding effectiveness is the ratio of impinging energy to residual energy.³¹ SE is defined as the ratio of incident EM waves entering (E_e) and departing (E_d) the material and is given by

S E = 10 lo g_{10} \frac{E_{e}}{E_{d}}

(1)

Where,

$E_{e}$ - Electromagnetic waves entering the material

$E_{d}$ - Electromagnetic waves coming out of the material

Three primary interactions are observed within the material when an incident EM wave passes through the material, namely, absorption (A), reflection (R) and transmission (T)^32,33 (Figure 2). Reflected wave comprises of reflection from the material surface and multiple reflection occurring within the material. For monolithic isotropic materials, absorption can be calculated by determining the transmittance and reflection and is given by

A = 1 - R - T

(2)

For composite or hollow materials, reflections from the inner surface demonstrate significant drop in the transmitted signal. Such a mechanism cannot be termed as absorption but instead the effective attenuation coefficient (α) can be calculated by combining the effect of reflection mechanism with the absorption of the porous material as

α = 1 - R - T

(3)

Schelkunoff’s theory reveals that total shielding effectiveness (SE_T) of a material is due to absorption, reflection and multiple reflections. Further it states that, if absorption loss is less than 10 dB, the effect of multiple reflections can be neglected and the cumulative shielding effectiveness can be estimated using reflection and absorption losses only.

S E_{T} = S E_{A} + S E_{R}

(4)

Mechanism of EMI shielding can be also understood by measuring their dielectric and magnetic properties

Relative complex permittivity ε_{r} = \frac{ε}{ε_{0}} = (ε^{'} - j ε^{″})

(5)

where,

Figure 2.

Representation of various mechanism due to propagation of incident wave through the composite material.³⁴

ε – Permittivity of the medium

ε₀ – Permittivity of free space

ε_rʹ - Real part of permittivity – represents electric charge storage capacity

ε_r^″ - Imaginary part of permittivity – represents dielectric dissipation

Relative complex permeability μ_{r} = \frac{μ}{μ_{0}} = (μ^{'} - j μ^{″})

(6)

where,

μ - Magnetic Permeability

μ₀ - Permeability of free space

μ’ - Real part of permeability– represents magnetic storage

μ” - Imaginary part of permeability– represents magnetic losses.

Further, the extent of losses can be measured from the tangent of dielectric loss and magnetic loss given by equation (7) and (8), respectively.

\tan δ_{D} = \frac{ε^{″}}{ε^{'}}

(7)

ta n_{M} δ = \frac{μ^{″}}{μ^{'}}

(8)

High dielectric loss means more amount of incident EM wave energy will dissipate in the form of heat and high SE can be achieved.

Characterization methods

The commonly used EMI shielding test methods are

• Open field or free space method

• Shielded box method

• Shielded room method

• Co-axial transmission line method.

The open field or free space method is used to measure the EMI shielding effectiveness of finished product as per the envisaged application (Figure 3(a)). This method mainly focusses on the shielding of the system and does not emphasis on the performance of a given material. The product to be tested is placed at a distance of 30 m from a receiving antenna and the radiated emissions are recorded as depicted in the figure. This method is not preferred for specimens made of different shielding materials.

Figure 3.

(a). Schematic of Open Field method.³¹ (b). Schematic of Shielded box method.³¹ (c). Schematic of shielded room method.³¹ (d). Schematic of Coaxial transmission method.³¹

This method is utilized for comparing measurements of test specimens made up of different shielding materials (Figure 3(b)). Test specimen is fixed at the periphery of the metallic box and a receiver antenna is placed inside to collect the generated EM waves. A transmitter antenna is located outside the metallic box to measure the transmitted EM waves from the shielding material. This method however has the following shortcomings

• Difficult to attain good electrical connection between the test specimen and shielding box.

• Operating frequency of the EM wave is limited to 500 MHz.

• Results acquired from different test centers cannot be compared.

• It is not possible to estimate EMI SE by reflection, absorption and transmission separately.

It is the most advanced method to measure EM shielding and is developed to overcome the shortcomings of the shielded box method (Figure 3(c)). The overall principle is similar to shielded box method, major difference is the isolation of the measuring system, signal generator, transmitter antenna, receiver antenna and recorder in separate shielded rooms to remove the probability of interference. Additionally, the antennas are situated in room sized sound absorbent chambers and the size of test specimen is significantly increased, typically around 2.5 m² in area. Frequency range of the EM wave is greatly enhanced and the reproducibility of the data is significantly improved.

Coaxial transmission line method is the widely used method to measure EMI shielding owing to the numerous advantages. In this method, shielding effectiveness due to absorption, transmission and reflection can be resolved independently (Figure 3(d)). In addition, the results obtained from different test center can also be compared and this method is convenient for nano reinforced materials attributed to the comparatively small test specimen requirement. Co-axial transmission cables can generate EM waves over a larger frequency range with smaller losses than antennas.

Many standards are available to estimate the electromagnetic shielding effectiveness of materials namely

• ASTM D4935-99

• ASTM ES7-83

• MIL-STD-188-125A

• IEEE-STD-299-1991

• MIL-STD-461C and

• MIL-STD-462.

Factors affecting the performance of EMI shielding materials

Electromagnetic interference shielding effectiveness of polymer composites are dependent on the electrical and magnetic properties of constituents. Manufacturing methods, filler content in the composites and physical properties of constituents also strongly influence the EMI shielding effectiveness. For designing a proper shielding material optimizing the volume/weight fraction of constituents, fabrication process parameters and methods are very important. In the following section an attempt is made to study the important factors that influence the EMI shielding effectiveness. To design an effective shielding material it is necessary to optimize the ingredients, manufacturing parameters and methods considering their influence (individual or synergetic) on shielding effectiveness. The factors such as influencing properties, methods of manufacturing, type, shape and quantity of ingredients, and type of composite by structure and architect mentioned as shown in Figure 4, are very important for designing effective EMI shielding materials.

Figure 4.

Factors affecting the performance of EMI shielding materials.

Polymer composites for EMI shielding

When modern electronic devices are designed, high performance EMI shielding materials are highly demanded. Additionally, lightweight is one additional important technical requirement for potential applications especially in the areas of automobile and aerospace. Polymer composites are widely employed for EMI shielding owing to the attractive properties offered in terms of low density and effective shielding. In this section, polymer based composites reinforced with inherent metallic fillers, intrinsically conductive polymers and porous materials are dealt in detail to know the effectiveness of these composites for EMI shielding.

Polymer-based composites containing metallic fillers

Polymer based composites reinforced with conductive fillers are seen as favorable and advanced EMI shielding constituents attributed to the innovation of insulating polymer that permits current flow through the conductive setup established by conductive fillers beyond the threshold percolation. Conductive composites offer advantages of low density, low cost, flexibility in design and processing ease. Further, reinforcing conductive polymers reduces the transparency of polymers to EM waves considerably by interacting with EM waves. Metal possess unique feature of reflecting incident EM waves, thereby enabling the metallic fillers to be ideally suited for EMI shielding. Metallic fillers possess large quantity of mobile charge carriers that interact with the incident EM wave. Metallic fillers in the form of fibers, micro and nano particles are used as reinforcement in polymer matrix to enhance the interactions with EM wave.

Attaining good dispersion with high density metallic fillers is a challenging and difficult task. Therefore, metallic fillers reinforced polymer composites are less popular.

Intrinsically conductive polymers-based composites

Polymers blends are reinforced with intrinsically conductive polymer to enhance their mechanical properties and electrically conducting constituent for interacting with the incident EM wave. Conductive polymers are conjugated polymers that exhibit electronic conductivity on doping. Unlike metallic fillers wherein electrical conductivity is distinctive feature, the molecular structure of polymer enables conductive polymers to be electrically conductive. The molecular structure of polymers is greatly affected by varying the parameters like chain size, level of doping, type of dopant and synthesis method and thus the shielding properties.

Polypyrrole and polyaniline are extensively used as intrinsically conducting polymers for EMI shielding purposes. Polypyrrole is the preferred one owing to its excellent properties of high conductivity, ease of processing, ecological stability and less toxicity whereas polyaniline is used for its various structures, distinctive doping mechanism, exceptional physical and chemical properties, stability and ease of availability.

In general, intrinsically conductive polymers possess good EMI shielding effectiveness as a result of reflection mechanism dominance during shielding attributed to the increasing level of impedance incompatibility with air. Additionally, lightweightness and availability of substrates increase the viability of these composites in envisaged applications. However, the main shortcomings associated with these composites are listed here

• Most of the conducting polymers depict inferior mechanical properties. Therefore, a proper matrix material is essential for structural support

• Conducting polymers exhibit inferior process capability due to insolubility and non-fusible features

• High filler loadings are required for achieving suitable execution.

Polymer matrix based porous composites

In view of the stiff guidelines to meet economic concerns of using low fuel consumption in automobile and aerospace applications, lightweight EMI shielding materials with high EMI shielding effectiveness are favored. The specific EMI shielding effectiveness is defined as the ratio of the EMI shielding effectiveness to the density or both density and thickness. It is an apt criteria to relate the EMI shielding performance with other materials for applications demanding lightweightness.

Porous EMI shielding materials

Numerous methods are available to lightweight electrically conducting materials possessing higher EMI shielding abilities. Most polymer resins have densities in the range of 900 – 1200 kg/m³ that is substantial lower than conducting metals density like copper (8960 kg/m³). Reinforcing conducting fillers with polymer matrix will increase the density of the composites. Therefore, inventive methods have been established to incorporate porosity within the composite to reduce the composite density below the density of the neat resins and obtain effective EMI shielding.

Syntactic foam composites are lightweight, flexible and porous materials possessing high thermal and electrical conductivity. Syntactic foams structures are widely preferred for EMI shielding applications. Syntactic foams are a particular class of foam structure realized by mixing hollow spheres in a continuous polymer matrix. These foams are considered as closed cell foams for the porosity in these materials is enclosed within hollow particles (Figure 5). The closed-pore structure gives advantages of low density, low moisture uptake and excellent mechanical properties. The pore structure in foam composites helps energy absorption by electromagnetic wave scattering in the wall of pores in addition to contributing low weight. Syntactic foams are widely used in aerospace, automobile and flexible and portable electronics and wearable devices in defense. Syntactic foams are also being explored as multifunctional composite materials due to their attractive mechanical, electrical, and thermal properties.

Figure 5.

Microstructure of syntactic foam composites.

Many approaches are available to improve the EMI shielding effectiveness of syntactic foams. Hollow particles made of a conductive material Or Surface of hollow particles coated with conductive material

Hollow microspheres like glass microballoons and cenospheres can be metal coated via several techniques. These methods are broadly classified as

• Chemical in-situ synthesis as metal coating followed by inner organic template removal

• Chemical synthesis including precipitation on the microspheres and

• Physical vapor deposition on the microspheres.

Several studies have suggested and demonstrated promising EMI SE by using metal coated hollow particles as fillers for making syntactic foam EMI shields. Panigrahi et al.⁴² studied the electromagnetic shielding effectiveness of silver coated polypyrrole microballoons. Ultrasound aided emulsion polymerization technique was used to fabricate the nano-composites. In-situ chemical oxidative copolymerization of pyrrole was performed on the surface of sulfonated polystyrene microspheres. Further, hollow polypyrrole spheres were formed by liquefying sulfonated polystyrene inner core in tetrahydrofuran. Hollow polypyrrole spheres reinforced with 2,5 and 10 wt.% of silver were fabricated. The developed nano-composites were investigated for EMI shielding in the frequency range of 0.5–8 GHz. It is inferred in the study that occurrence of reflection mechanism is owing to the impedance difference between the corresponding layers of air and interacting medium while absorption mechanism is attributed to the transfer of electromagnetic energy to be dissolute as heat energy. Internal multiple reflections involve multiple reflections between the opposite layers of a material. Increasing the incident frequency decreases reflection in all the samples. Substantial decrease in reflection is observed for hollow polypyrrole spheres (−0.1 to −6 dB) as compared with polypyrrole (−0.02 to −2 dB). Electromagnetic waves penetrate through hollow polypyrrole spheres and cause arbitrary internal reflections. These trapped radiations further enhance the reflection as the probability of escaping the outer surface of hollow polypyrrole spheres decreases considerably. Reflection values of hollow polypyrrole spheres reinforced silver particles with 2, 5 and 10 wt.% lie in the range of −0.25 to −3.5, −0.15 to −2 and −0.35 to −3 respectively. Results show reflectance of hollow polypyrrole spheres reinforced with 5 and 10 wt.% silver particles are substantially high in comparison with other samples owing to the highly conducting silver particles coated on the outer surface of hollow polpyrrole spheres. Absorbance tests reveal that polypyrrole exhibit lowest absorption followed by hollow polpyrrole spheres and nanocomposites with 2, 5 and 10 wt.% of silver. Coating of silver particles is effective in reducing the absorbance of electromagnetic waves.

Electromagnetic shielding effectiveness of polypyrrole, hollow polpyrrole spheres and hollow polpyrrole spheres reinforced with 2, 5 and 10 wt.% of silver particles lie in the range of 20 to 5, 34.5 to 6, 36.5 to 11.5, 55.78 to 20 and 59 to 23 dB respectively. All nano-composites reinforced with silver particles reveal higher shielding effectiveness in comparison with polypyrrole and hollow polpyrrole spheres. Higher filler loading of silver particles ensures simultaneous contribution due to internal reflection and reflection from outer surface of spheres, thereby significant increase in shielding effectiveness. Such high EMI shielding capacity using conducting polymers are rarely reported.

Shielding effectiveness of epoxy/hollow carbon microballoons syntactic foam composites was investigated by Zhang et al.⁴³ Hollow carbon microballoons were coated with polydopamine by self-polymerization of dopamine to enhance the bonding between constituents and assist in uniform dispersion of fillers to have good stress transfer between the constituents. Silver nanoparticles were developed on hollow carbon microballoons by scattering polydopamine coated hollow carbon microballoons in varying concentration of silver nitrate solution (Figure 6). Silver nanoparticles get reduced on the polydopamine surface in different densities, producing silver nanoparticle coated HCMs (Ag-PDA-HCM). Shielding effectiveness of the syntactic foam increases by the deposition of silver nanoparticles on hollow carbon microballoons. Shielding effectiveness of polydopamine coated hollow carbon microballoons syntactic foams with 28.5 and 30.5 wt.% of silver nanoparticles investigated for a frequency range of 8–12 GHz is around 49.5 and 60.2 dB respectively.

Figure 6.

EMI SE of syntactic foams containing pristine hollow carbon microballoons and silver polydopamine hollow carbon microballoons with varying silver content. “Reprinted with (adapted) permission from,⁴³ Copyright 2014, American Chemical Society.”

Shielding effectiveness of nickel coated cenospheres through ultrasonic aided direct current magnetron sputtering technique was investigated by Xiaozheng et al.⁴⁴ Cenospheres having diameter in the range 20–100 μm are coated with nickel films in varying concentration by controlling the sputtering power (99, 142 and 187 W) to achieve layer thickness of 26, 42 and 85 nm respectively. Nickel coated cenospheres are mixed in molten paraffin matrix placed in a mold with dimensions of 150×60 mm. Composites developed at 187 W sputtering power reveal shielding effectiveness of around 25 dB tested over a frequency range of 80–110 GHz.Enhancement of shielding effectiveness is owing to the conductive nickel films deposited on cenosphere particles that effectually increase the electrical response of as received cenosphere particles. Usefulness of nickel coated particles even at low volume compositions for electromagnetic shielding interference is established in this study.

Zhang et al. investigated the EMI shielding effectiveness of carbon foam developed by direct carbonization of phthalnitrile.⁴⁵ Developed foams had very low density of 150 g/m³. EMI Shielding effectiveness of 51.2 dB was exhibited by the carbon foams tested over a frequency range of 8.2–12.4 attributed to conducting carbon matrix formed after full carbonization and the effects of polarization induced by the intrinsic nitrogen containing structure. The carbon foams offer an ultra-lightweight and high performance EMI shielding material that can be utilized for applications demanding mechanical integrity.

Composites with carbon fillers

Carbon reinforced polymer composite materials for EMI shielding applications. Invention of unique allotropic carbon forms such as carbon nanotubes, graphene, fullerene, graphite, etc. have etc. have racked up the area of advanced materials and have significantly contributed in the development of numerous fields of science and technology [19]. Carbon black, carbon nanofibers, carbon nanotubes, graphene, graphite are the widely used carbon fillers. Carbon black among all the aforementioned materials is considered as a primitive material utilized in EMI shielding applications.

Carbon black is a familiar filler material reinforced in polymer matrix to attain desirable EMI shielding effectiveness. Formation of carbon black is mainly attributed to the thermal disintegration of hydrocarbons in the gaseous phase leading to fine size carbon pigments.³⁴ Due to their antistatic protection ability, carbon black reinforced composites are widely used for electrostatic discharge and EMI shielding applications.^31,67 Carbon nanofiber reinforced composites are apparently considered as potential materials for diverse range of applications including EMI shielding.⁶⁸ Carbon fibers are formed by the interlocking sheet of carbon atoms having regular hexagonal pattern.³¹ CNTs are considered to be very strong and stiff materials.^34,69 Both single walled and multi walled carbon nanotube are considered as best alternatives for EMI shielding as compared with conductive fillers due to large surface area and high aspect ratios.³⁴ Graphene reinforced composites are also one of the widely used EMI shielding materials owing to their lightweight, high aspect ratio and economic feasibility.⁷⁰ Some important studies carried out with carbon black particles are reported in Table 5

Table 5.

EMI SE of carbon black, carbon fiber, single and multi walled carbon nanotubes and graphite reinforced polymer composites.

Ref.	Materials	Processing method	Frequency	Thickness (mm)	Shielding effectiveness (dB)
71	Acrylonitrile Butadiene Styrene/Carbon black (10, 15, 20, 25, 30, 35 and 40 wt.%)	Plastic extruder and hot pressing	0.2 – 1.6 GHz	1.5	06
72	Polyaniline/Poloxalene/Carbon black (0, 1, 3, 5 and 10 wt.%)	Solution blending method	8.2 – 12.4 GHz	--	20
73	High Density Polyethylene/Carbon black/MWCNT	Melt compounding approach	6.8 GHz	0.35	27.26
	High Density Polyethylene/Carbon black/PANI coated MWCNT		6.8 GHz	0.35	27.63
	High Density Polyethylene/Carbon black/MWCNT		6.8 GHz	0.35	27.27
74	Epoxy/Carbon black (30 wt.%)	Mechanical mixing	1-10	1	44
75	Acrylonitrile butadiene styrene/Carbon black (15 wt%.)	Solution mixing	8.2–12.4	1.1	20
76	Styrene-b-ethylene-ran-butylene-b-styrene (SEBS)/Carbon black (15 wt%.)	Melt blending	8-12	5	∼20
77	Styrene butadiene Rubber/Carbon black (34 wt%.)	NR	7.8–12.4	70	67
78	Poly vinylidene fluoride/Carbon fiber (9 wt.%)	Mechanical stirrer	0–1.6	0.05	∼12
79	Polystyrene/Carbon fiber (15 wt.%)	Hot compression molding	8.2–12.4	--	19
80	Polypropylene/Carbon fiber (10 vol.%)	Injection molding	8.2–12.4	3.2	25
81	Epoxy/Carbon fiber (47 vol.%)	Hot pressing	0–1.5	2.08	∼114
82	Liquid crystal polymer/Carbon fiber (40 vol.%)	Miniaturized internal mixer	0–1.5	1.45	41
82	Epoxy/Single wall carbon nanotube (15 wt.%)	Mechanical mixing	8.2–12.4	2	28
83	Poly methyl methacrylate/Single walled carbon nanotube (20 wt.%)	Coagulation method	8-12	4.5	40
84	Reactive ethylene terpolymer/Single walled nano tubes (4.5 vol.%)	Hot pressing	8.2–12.4	2	∼32
85	Polyaniline/Single wall carbon nanotube (25 wt.%)	Alcohol assisted dispersion and pressing process	2-18	2.4	31.5
86	Epoxy/Single walled carbon nanotube (15 wt.%)	Mechanical mixing	0.01–1.5	1.5	49
87	Polyurethane/Single walled carbon nanotube (20 wt.%)	Conventional process	8.2–12.4	2	17
88	Epoxy/Multiwalled carbon nanotube (0.66 wt.%)	Infiltration method	8-12	2	33
89	Polyethylene dioxythiophene/Multiwalled carbon nanotube (15 wt.%)	In situ emulsion polymerization	12.4-18	2.8	58
90	Poly trimethylene terephthalate/Multiwalled carbon nanotube (10 wt.%)	Melt mixing	12.4-18	2	42
91	Polycarbonate/Multiwalled carbon nanotube (20 wt.%)	Solvent casting and compression molding techniques	8.2–12.4	2	43
92	Polystyrene/Multiwalled carbon nanotube (20 wt.%)	Injection and compression molding	8.2–12.4	2	30
93	Polyaniline/Multiwalled carbon nanotube (25 wt.%)	In-situ polymerization	12.4-18	2	39.2
94	High density polyethylene/Graphite (30 wt.%)	Hot compression molding	8-12	3	33
95	Ultrahigh molecular weight Polyethylene/Graphite (7.05 vol.%)	Mechanical mixing and hot compaction	8.2–12.4	2.5	51.6
96	Epoxy/Graphite (2 wt.%)	Solution processing technique	8.2-18	5	11
97	Epoxy/Solution Processable Functionalized Graphene (15 wt.%)	In-situ process	8.2–12.4	-	21
98	Polystyrene/reduced graphene oxide (7 wt.%)	High pressure solid phase compression moulding	8.2–12.4	2.5	45.1
99	Polyethylene oxide/reduced graphene oxide (2 vol.%)	Aqueous mixing method	2-18	1.8	48.8
100	Water-Borne Polyurethane/reduced graphene oxide (7.5 wt.%)	--	8.2–12.4	1	34

Challenges and future work

In the recent times, significant progress has been achieved in the field of electromagnetic shielding mechanisms. However, they are many challenges that need to be answered. Some of the critical challenges are listed below

• Shielding by absorption has become the most enviable shielding mechanism as it can capably absorb electromagnetic wave and attenuate. Even though many materials and composites demonstrate high absorption, shielding is generally restricted to thin frequency range. Therefore, developing new composite materials with appropriate permeability, permittivity, conductivity, thickness and frequency characteristics is need of the hour and future trend.

• The commercial range for composites shielding effectiveness is considered to be around 20–25 dB. Therefore, composite materials should be designed or prepared to meet the desired shielding effectiveness.

• Materials like hydrogels, aerogels, multilayer 3D composite, and composite foams demonstrate high EMI shielding capabilities and therefore more research should be carried out to meet the specific needs of envisaged applications.

Conclusions

• Electromagnetic interference (EMI) shielding materials are required to relieve the increasing stress over electromagnetic pollution problems arising from the growing demand for electronic and electrical devices.

• Commercial applications require EMI shielding of around ∼20 dB that is equal to or less than 1% transmittance of electromagnetic wave. Most of the polymer based composites with metallic fillers, intrinsically conducting polymer and porous fillers are efficient enough to meet the envisaged shielding.

• High conductance and dielectric properties of conducting polymers such as polyaniline, polypyrrole and polythiophene reinforced with coated nanoparticles make them adept for EMI shielding applications.

• Carbon nanotubes and graphene based composites provide excellent EMI shielding effectiveness.

• Different factors that affect the performance of EMI shielding materials are also presented. Prominent features of EMI shielding material are also discussed.

Footnotes

Acknowledgements

The authors gratefully acknowledge the Deanship of Scientific Research, King Khalid University (KKU), Abha-Asir, Kingdom of Saudi Arabia for funding this research work under the grant number RGP.2/58/42.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This study was supported by Deanship of Scientific Research, King Khalid University (KKU), Abha-Asir, Kingdom of Saudi Arabia for funding this research work under the grant number RGP.2/58/42.

ORCID iD

Kiran Shahapurkar

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