Recent Advances in Polymer Nanocomposites for Electromagnetic Interference Shielding: A Review.

The mushrooming utilization of electronic devices in the current era produces electromagnetic interference (EMI) capable of disabling commercial and military electronic appliances on a level like never before. Due to this, the development of advanced materials for effectively shielding electromagnetic radiation has now become a pressing priority for the scientific world. This paper reviews the current research status of polymer nanocomposite-based EMI shielding materials, with a special focus on those with hybrid fillers and MXenes. A discussion on the theory of EMI shielding followed by a brief account of the most popular synthesis methods of EMI shielding polymer nanocomposites is included in this review. Emphasis is given to unravelling the connection between microstructures of the composites, their physical properties, filler type, and EMI shielding efficiency (EMI SE). Along with EMI shielding efficiency and conductivity, mechanical properties reported for EMI shielding polymer nanocomposites are also reviewed. An elaborate discussion on the gap areas in various fields where EMI shielding materials have potential applications is reported, and future directions of research are proposed to overcome the existing technological obstacles.

Introduction

The rapid growth of technology and proliferation of electronic devices in recent years have yielded a novel class of pollution coined as electromagnetic interference (EMI). These interferences may be mainly caused by radio frequency interference, electrostatic discharge, electromagnetic coupling, and electromagnetic conduction or induction from various sources. Besides, the introduction and development of 5G technology have also led to an increase in the presence of high-energy electromagnetic (EM) signals in the atmosphere. Mutual interference among EM radiations emitted from devices can sabotage device performance.[1] EMI has dreadful effects on electronic devices and electrical systems used in high-end applications like communication, military, medical, and remote sensing.[2−7] Since the interference of EM radiation occurs at the high-frequency radio frequency (RF) and microwave bands, they have adverse effects on the human body.[8−11] A prolonged exposure to EM radiation augments the risk of cancer, asthma, heart diseases, migraines, and even miscarriages.[12] Considering the extent of the threat EMI can cause, it is obligatory to reduce the electromagnetic radiations effectively. The practice of jamming EM radiations by means of barriers fabricated from conducting or magnetic materials is called EMI shielding.[13,14] EMI shielding of a material is understood with reference to shielding efficiency (SE) and is the ability of the material to attenuate the incident EM radiation. It can be defined mathematically aswhere PI (EI or HI) and PT (ET or HT) are the power (electric or magnetic field intensity) of incident and transmitted electromagnetic waves, respectively.[15−17] The unit of SE is prescribed as decibel (dB). EMI can be classified in two ways: one is based on its mode of propagation, and the other is based on its characteristic frequency. The mode of propagation is further classified into two: radiated and conducted interference. Radiated interference is attributed to the EM radiations emitted from any device, whereas conducted interference is the energy emitted through an external connection. Based on the characteristic frequency, EMI can be categorized as narrow and broadband. As the name suggests, broadband interference is present over a broad frequency range. Different frequency ranges have diverse applications. The L band is used by low earth orbit satellites and wireless communication; the S band is used in multimedia applications such as mobile phones and television; the C band is used for long-distance radio telecommunication and wi-fi devices; the X band is for weather monitoring and air traffic control; and the Ku band is used for extremely small aperture systems, satellite communication, and so on. The design recommendations for EMI shielding material are distinct for different bands of frequencies. The applications of EM waves in different bands of frequencies are illustrated in Table .[13]

Table 1

Applications of EM Waves in Very Low Frequency (VLF), Low Frequency (LF), Medium Frequency (MF), High Frequency (HF), Very High Frequency (VHF), and Ultra High Frequency (UHF)

band name	band frequency	applications
VLF	3–30 kHz	navigation, submarine communication
LF	30–300 kHz	AM long-wave broadcast, navigation
MF	300–3 MHz	AM medium-wave broadcast
HF	3–30 MHz	AM short-wave broadcast, radio frequency identifications, marine and mobile radio telephones
VHF	30–300 MHz	FM radio broadcast, television broadcast
UHF	300 MHz–1 GHz	television, microwave oven, mobile phones
L band	1–2 GHz	low earth orbit satellites, mobile phones, wireless LAN, radars, GPS, communication, etc.
S band	2–4 GHz	multimedia applications like mobile, TV, cordless phones
C band	4–8.2 GHz	long-distance radio telecommunications, satellite communication, wi-fi devices, etc.
X band	8.2–12.4 GHz	weather monitoring, air traffic control, defense tracking, satellite communication, etc.
Ku band	12.4–18 GHz	very small aperture terminal systems
K band	18–27 GHz	radar and satellite applications
Ka band	27–40 GHz	satellite communication
V band	40–75 GHz	military and research
W band	75–110 GHz	military and research

In the past, metals and their composites were used as EMI shielding materials. Such materials have shown high EMI shielding effectiveness on account of their enhanced conductivity (σ), better mechanical properties, and good permeability.[18] The effective shielding properties in metals are due to their enhanced reflection mechanism. However, they faced severe shortcomings such as poor mechanical flexibility, high density, affinity to corrosion, etc. During the last two decades, researchers have been focusing on polymeric materials which are capable of overcoming all the shortcomings of metal-based shielding materials due to their innate flexibility, light weight, easy processability, chemical resistance, and ultimate scalability.[14−19] Polymers can be commonly categorized into two: insulating polymers and intrinsically conducting polymers (ICPs). Polystyrene (PS), poly(vinylidene fluoride) (PVDF), polypropylene (PP), poly(methyl methacrylate) (PMMA), poly(vinyl alcohol) (PVA), polyethylene (PE), poly(vinylpyrrolidone) (PVP), and epoxy are insulating polymers, and their conductivity can be enhanced by adding conducting fillers.[20] Most polymers comprised of polyaniline (PANI), polypyrrole (PPy), poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS), and polythiophene (PTP) are generally intrinsically conducting polymers, and they play a major role in EMI shielding studies. In polymeric materials, EMI shielding is attained mainly through absorption and reflection mechanisms. Intrinsically conducting polymer (ICP)[21−23] based EMI shielding materials possess much better performance in corrosion resistance and electrical conductivity. The electrical conductivity can be further enhanced by adding suitable conducting fillers.[24−31] There are a lot of reports on polymer nanocomposites which use carbon-based nano-/microstructures such as CNT, carbon fiber, graphene, graphene oxide, and carbon black as fillers.[32−34] Carbon-based fillers can be easily dispersed in a polymer matrix, producing an electrically conductive network, and thus they can be used as lightweight EMI shielding materials.[35,36] As the filler content in the matrix increases, the conductivity also improves and reaches a certain level called percolation limit. Filler addition beyond the percolation limit causes a drastic change in electrical conductivity.[20] The basic requirement for enhanced EMI SE is acquiring a percolation threshold with low filler content which could be obtained by uniform dispersion of fillers. Proper dispersion of fillers can be accomplished by adding a suitable amount of surfactants, surface modification, and functionalization of fillers. Structural design of polymer composites is an important parameter, and the properties arising from filler–filler or filler–polymer interactions give rise to openings to create hybrid filler composites. Hybrid fillers constitute two or more fillers, and it is the synergistic effect of the polymer matrix and this hybrid filler that endows outstanding shielding efficiency to the composite. MXenes have been recently launched as appropriate fillers for EMI shielding applications owing to their high electrical and thermal conductivity (k), mechanical stability, etc. The good flexibility and layered structure of MXenes, together with their 2D morphology, make them a unique filler.[37]

Realizing the potential of polymer nanocomposites for EMI shielding applications, scientists are making efforts to fabricate novel polymer nanocomposites with diverse fillers for high-end scientific and industrial applications. Hence, an elaborate review on the recent developments in this field would be of great interest to the scientific world. In this review, the theoretical aspects of EMI shielding are discussed followed by a detailed review of the recent works conducted on filler-based polymer nanocomposites for shielding application.

Electromagnetic Shielding Theory and Mechanism

When EM radiation is incident on a shielding material with different intrinsic impedance compared with the propagating medium, two waves are produced at the external surface. One is reflected, and the other is transmitted, as shown in Figure . The amplitudes of these waves depend on the intrinsic impedance of shielding material and its surroundings. When the transmitted waves travel from the external surface to shielding material, the amplitude of the waves decreases exponentially due to absorption. Total SE, as expressed in eq , can be written as a function of impedance mismatch between the medium and the material (η0 and η), sample thickness (d), and skin thickness (skin depth δ).[38]The three terms are: shielding by reflectionshielding by absorptionand shielding by multiple reflectionsSkin thickness or skin depth is defined as the depth at which the field decreases to 1/e from its preliminary value and is a function of frequency (f), permeability (μ), and conductivity (σ):The amount of energy absorbed increases with an increase in shield thickness and a decrease in skin depth. Skin depth decreases with an increase in shield conductivity, magnetic permeability, and EM wave frequency.

Figure 1

Schematic illustration of the electromagnetic shielding mechanism.

When a plane wave is incident on a shielding material, the separation between the absorption point and the radiating source is called the EM radiating region (r). It can be divided into three proportionately to the total wavelength of the EM wave. When the distance between the source and shield is lower than , it is labeled as a near-field shielding (λ is the total wavelength of EM waves in the region within the distance). In this region, the theory of electric and magnetic dipoles and the ratio E/H ≠ Z0 are used. Here, E represents the strength of the electric field; H is the magnetic field strength; and Z0 is intrinsic impedance of free space (Z0 = 377 Ω). If E/H > Z0, the EM wave is electric field prominent, and if E/H < Z0, it is magnetic field prominent. So, in the near-field region, SEE ≠ SEH (SEE – SE due to the E field and SEH – SE due to the H field). Conversely, if space is greater than , it is called far-field shielding where the electromagnetic plane wave theory is functional, i.e., the ratio E/H = Z0. So, in the far-field region, the plane wave exists, and SEE = SEH. If , then it is called the transition region.

The EMI shielding effectiveness of a system is monitored by three major mechanisms, namely, reflection, absorption, and multiple internal reflections. Material properties like conductivity, permittivity, permeability, and thickness play a major role in governing EMI shielding effectiveness. When a plane wave is incident on the shielding material, most of the radiation is reflected or absorbed by the shield. The reflection power is managed by surface reflection (SER) and multiple reflections (SEM); however, absorption is managed by some loss mechanisms connected with electric polarization and magnetization. According to the Schelkunoffs theory, the total SE is represented as eq .where SET is the total shielding effectiveness; SER represents the shielding effectiveness due to reflections; SEA is the shielding effectiveness due to absorption; and SEM is shielding effectiveness due to multiple reflections. When SEA is greater than 10 dB, SEM becomes insignificant and can be ignored in eq . The magnitude of reflection is dependent on surface conductivity and is given by eq where σT is total conductivity; μr is relative permeability of space; and ω is frequency in Hz. Hence, for a stable value of σT and μr, the ratio of conductivity and permeability will be constant, and SER is inversely proportional to frequency.[39] On the other hand, absorption can decay throughout the thickness and is given by eq where t is thickness; σT is total electrical conductivity; ω is frequency; and μr is relative permeability of space. Since the total conductivity is also expressed as σΤ = 2πfε0ε″ for εr = ε′ – jε″, the shielding mechanism is connected with relative permittivity and permeability values.

Experimental Calculation of EMI Shielding Efficiency

Depending on the type of material and the frequency range, there are four most commonly used EMI shielding measurement techniques: (1) open-field or free space test, (2) shielded box test, (3) coaxial transmission line test, and (4) shielded room test. The coaxial transmission line method is the most commonly used one, and scattering parameters are used for calculating EMI SE.[40,41] In this method, a reference test sample is mounted on a special holder, and voltages at different frequencies are recorded. Then, the reference sample is replaced by a load sample, and the same measurements are taken. The ratio of power received by the reference and the load sample gives the SE of load material. The main advantage of this technique is that the obtained data can be resolved into the reflected, absorbed, and transmitted components. For the generation and the measurement of the EM waves that pass through and are reflected from the specimen, a vector network analyzer (VNA) is generally used. Figure represents the block diagram of a VNA arrangement. The incident and transmitted waves in a two-port VNA can be mathematically expressed by complex scattering parameters (or S-parameters), i.e., S11 (or S22) and S12 (or S21), correspondingly.

Figure 2

Schematic representation of the vector network analyzer.

Complex scattering parameters are used to link reflectance (R) and transmittance (T), which successively provide absorbance (A).[42] The transmittance and reflectance are represented aswhere S11, S22, S12, and S21 are known as scattering parameters (i.e., S11 and S22 are the reflection coefficients and S12 and S21 are the absorption coefficients).SE is due to reflection and absorption with respect to the power of the incident electromagnetic waveR is reflectance; A is absorbance; and T is the transmittance coefficientEffective absorbance is given bywhere Aeff gives the power which is absorbed by the EMI shielding material.[43]

EMI Shielding Properties of Polymer Nanocomposites

Polymers are basically insulators, and their properties can be enhanced through the formation of a filler–matrix interface by adding nanofillers.[44,45] The introduction of nanofillers alters the degree of crystallinity and glass transition temperature (Tg) of the polymer matrix.[46] Due to their enhanced conducting and flexible properties, polymeric materials have opened a new perspective in the field of EMI shielding. A composite is a combination of two or more nonmiscible materials forming a new material with properties that are comparatively diverse from those of individual ones. The matrix may be a polymer, glass, or ceramic, while the filler or reinforcement material can be particles, ribbons, flakes, fibers, platelets, or tubes.[47] EMI shielding of a material depends mainly on the aspect ratio, size, dielectric constant, magnetic properties, filler’s intrinsic conductivity, and physical geometry.[36,48]

Polymer nanocomposites can be synthesized by different methods according to the polymer and filler perspectives. Based on the filler viewpoint, the synthesis of polymer composites can happen in two ways: (1) scaffold impregnation and (2) filler mixing. Scaffold impregnation involves the fabrication of a filler scaffold followed by the infiltration of the polymer into the voids of the filler structure. The major advantage of this method is the liberty to tailor tune the geometry and morphology of the filler. From this perspective, one-dimensional, two-dimensional, and three-dimensional scaffolds can be prepared. The voids within the scaffold adversely affect the mechanical properties of the material. Hence, impregnation with polymer and ensuring a proper interfacial bonding of the scaffold with the polymer matrix strengthen the mechanical properties as well as ensure adequate stress transfer from the filler to the matrix. It is the quality of the polymer infiltration that decides the performance of the polymer composite. In a filler mixing process, the dispersed filler and the polymer solution are mixed together. This technique is much faster than scaffold impregnation, but the final end product is comparatively less dense. This is due to the dispersion limit of the filler.[49]

Based on the polymer viewpoint, there are three main methods for the preparation of polymer nanocomposites.

Solution Blending

Solution blending is the most effective fabrication method for polymer nanocomposites since it is accessible to even small sample sizes. It consists of mainly three steps: (1) Dispersion of filler in an adequate solvent. (2) Mixing of polymer via an ultrasonication process, magnetic stirring, or high-speed shear mixing (at room temperature). (3) Recovering the composite by precipitation.[50,51] This method is mainly used to synthesize polymer nanocomposites of a range of polymers such as poly(vinyl alcohol) (PVA),[52−54] polyvinyl fluoride (PVF),[55] polyethylene (PE),[56,57] poly(methyl methacrylate) (PMMA),[58] poly(ethyl methacrylates) (PEMA),[59] and polyurethane (PU),[60] etc. Graphene-based polymer nanocomposites can also be prepared by this method. The major drawbacks of this approach include small-scale production, environmental unfriendliness due to toxic solvents, and the difficulties involved in the removal of solvents.

In Situ Polymerization

In situ polymerization is a useful method for the preparation of biopolymers in which fillers are uniformly dispersed in a polymer matrix, ensuring a powerful interaction between them. The initiator added to the solution activates the reaction between the filler and monomer, after which polymerization begins. This technique is used for the preparation of thermally unstable polymers since they cannot be dissolved by melt or solution blending. In situ polymerization methods are popularly used for the synthesis of epoxy nanocomposites and graphene-based polymer nanocomposites.[61] This method can be used to fabricate nanocomposites of graphene with polymers like epoxy,[62−65] PMMA,[66] nylon-6,[67] polyurathene (PU),[68] poly(butylene terephthalate) (PBT),[69] polyaniline (PANI),[70] PE,[71] etc. The main disadvantage of this method is that as polymerization advances the viscosity of the sample solution simultaneously increases, ultimately preventing the homogeneous dispersion of fillers in polymers.[50,72]

Melt Blending

Melt blending is one of the most extensively used methods for the preparation of polymer nanocomposites since it is eco-friendly and supports mass production. In this technique, the polymer is melted at high temperature, and filler is mixed to this polymer melt under shear. The polymer chains penetrate in between the nanofillers, and it does not require any solvent for polymer or filler. The main disadvantage of this approach is the poor dispersion of various fillers in polymers.[73−76]

EMI Shielding Properties of Polymer Nanocomposites with Various Fillers

According to the EM wave percolation theory, the electrical conductivity of composites is determined by the formation of conductive networks.[77] The concentration of filler at which the conductivity lane is formed inside the insulating matrix is called the percolation threshold. By incorporating suitable fillers, the percolation threshold can be attained for conduction through polymer nanocomposites, thereby ensuring an increase in EMI shielding. The main requirement is to achieve the percolation threshold with a minimum amount of fillers which can be made possible by proper dispersion of fillers. However, an increase in filler concentration can lead to poor dopant dispersion in the matrix and an increase in the viscosity of the medium.[78] A proper dispersion can be attained through surface adaptation and functionalization of fillers. Apart from the percolation threshold, the intrinsic conductivity of the filler also plays a key role in modifying the conductivity and EMI SE of the composite. The polymer (matrix) phase and the reinforcement (filler) phase are the two core material constituents of a composite. Adequate interface bonding between the two phases is needed for the load transfer from the matrix to reinforcement. This interfacing is responsible for the macroscopic properties and proper functioning of the nanocomposite.[79] The electrical conductivity of polymer nanocomposites can be modified by tuning parameters like doping level, morphology, etc.[80] Researchers are showing a profound interest in developing tunable polymer nanocomposites with enhanced shielding properties for high-end potential applications.

Metallic Fillers

Metals are the traditional materials used for EMI shielding applications where EMI shielding is attained by the mechanism of reflection. The mobile charge carriers in the shielding material interact with incoming waves, thereby enhancing the reflection shielding effectiveness. The outstanding electrical conductivity of metals is the foremost criteria for using them as shielding material. Metallic-filler-based polymer nanocomposites have shown amazing EMI shielding performance due to enhanced conducting properties, and thus they can be used as a successful shielding material. Frequently used metallic fillers are silver, aluminum, copper, and nickel in diverse forms.

Among the metal-based fillers, silver having ultrahigh electrical conductivity is considered as one of the potential materials for shielding applications. Silver-based nanostructures are successfully used for making composite materials for EMI shielding applications.[81−85] Particularly, silver nanowire polymer nanocomposites have been extensively investigated as shielding materials since a high aspect ratio of a nanowire is favorable to build a highly percolated network structure and attain high conductivity. Ma et al. reported an EMI SE of 1210 dB g–1 cm3 at 200 MHz for an ultralightweight silver nanowire (AgNW)/polyimide composite foam with microcellular structure.[86] In the following year, the same authors portrayed an ultralightweight polyimide (PI) composite foam filled with three different shapes of silver nanofillers, nanospheres (PIF-P), nanowires (PIF-W), and nanowires–nanoplatelets (PIF-WS) via a simple and effective one-pot liquid foaming procedure.[87] The distribution of nanofillers in composite foam arrangements is revealed in Figure (a). For the nanosphere composite (PIF-P), silver nanospheres (AgNSs) were uniformly placed on the cell walls and cell membranes with intense distribution. However, the AgNSs were not linked to each other, which resulted in an unsuccessful EM wave attenuation. On the contrary, a well-connected network was created in the silver nanowire (AgNW) composite (PIF-W). This interconnected network of AgNWs characterizes the higher aspect ratio of nanowires which provides fast electron transport channels in the composite foam. As a modification to the above sample, the incorporation of silver nanoplatelets on PIF-W (PIF-WS) paved the way for an enhanced electrical conductivity of about 3.2 × 10–7 S/m. This is due to the high surface area/aspect ratio and the outstanding mobility of silver nanoplatelets compared to AgNWs. Meanwhile, the nanowires in PIF-WS maintained the bridging between nanoplatelets, thereby aiding in higher electrical conductivity and enhancing its EMI shielding feature. The results demonstrated that PIF-WS composites were suitable for EMI shielding in advanced applications like aircrafts and spacecrafts.

Figure 3

(a) Schematic image of the allocation of silver nanocompositions in composite foams for PIF-P, PIF-W, and PIF-WS. Inset: indicates the 3D model of the allocation of silver nanofillers in composite foams. Specific EMI SE of composite foams for PIF-P, PIF-W, and PIF-WS calculated in frequency ranges (b) 30 MHz–1.5 GHz and (c) X-band. Inset (b) displays their specific EMI SE at 200, 600, and 1000 MHz. Inset (c) displays the specific EMI SE of composite foams at 9.6 GHz. Reprinted with permission from (87). Copyright 2015, Royal Society of Chemistry.

Copper (Cu) is also a suitable metallic filler capable of enhancing the electrical conductivity of the composite. Compared to pristine copper, Cu used in polymer nanocomposites possessed properties such as flexibility, low weight, resistance to corrosion, and good processability. Kim et al. reported EMI SE of translucent and flexible silver nanowire/polyimide (AgNW/PI) composites using plasma-treated and electroless Cu-plated nanowires (Cu/AgNWs/PI).[88] An EMI SE of 55 dB was reported in the L-band of frequency. The value they reported was two times greater than that of AgNW/PI with the same thickness. Liu et al.[89] successfully demonstrated an EMI SE of approximately 100 dB in the X-band for a lightweight and wearable Cu@Ag nanoflake-coated leather matrix composite (LM-Cu@Ag), which was approximately 5 times that of the EMI SE value essential for commercial shielding applications. In this composite, conducting nanofillers were uniformly distributed only at the interfaces of the polymer matrix and not inside the volume. The surface conductivity of the sample was 78 500 S/m, and the continuous surface-conducting networks were suggested to be the reason for its high EMI SE. The sample when subjected to bending 25 000 times exhibited no cracking, which is an indication of its excellent mechanical strength.

Nickel (Ni) is another popular metal used to improve the efficiency of composite materials since it can improve magnetic permeability which is responsible for EM absorption. Researchers had fabricated a variety of Ni/polymer composites for high EMI shielding applications.[90−92] Copper- and nickel-metalized polymethacrylimide (PMIF) foams (PMIF@Cu and PMIF@Ni, respectively) were fabricated through an electroless plating method by Jianwei et al.[93] These plated foams had good electrical conductivity of about 1.06 × 104 S/m for PMIF@Cu and 9.15 × 103 S/m for PMIF@Ni. Additionally, a shielding effectiveness of 52 and 43 dB was exhibited by Cu- and Ni-plated foams, respectively. A major outcome of this study was the excellent radiation performance of the plated foams, thus enabling them to be used as monopole antennas. Moreover, these metal-based foams weighed 25–30 times less than the traditional copper antennas. This provided a great opportunity in designing lightweight telecommunication devices. In another work, lightweight and flexible nanoporous silver (Ag) membranes prepared by sequential vacuum filtration of a solution consisting of bacterial cellulose and Ag nanoparticles followed by a hot pressing were fabricated by Guh-Hwan et al.[94] The nanoporous structure of Ag led to an increase in multiple reflections of electromagnetic waves. By adjusting the thickness of the nanoporous Ag layers, the conductivity as well as EMI SE were tailor tuned. For a thickness of 1.2 μm, a shielding effectiveness of ∼53 dB was obtained in 0.5–18 GHz. Moreover, the nanoporous Ag membranes displayed good electromechanical durability and fast heat dissipation, making them suitable for the development of next-generation electronic devices. Yadong et al. prepared the first expandable microsphere (EM)/liquid metal (LM) monolith with a finite package without leakage.[95] The composite exhibited a good shielding effectiveness (98.7 dB) over a broad frequency range of 8.2–40 GHz and a high strength (3.43 MPa). The structural adjustment of liquid metal architecture provided great advantages in electromagnetic shielding and sealing. Table summarizes values obtained for important parameters such as EMI SE, SSE, and conductivity for various metallic-filler-incorporated polymer composites.

Table 2

Summary of Electrically Conductive EMI Shielding Polymer Compositesa

sample details	filler	Bυ	EMI SE (dB)	SSE (dB cm3 g–1)	σ (S/m)
AgNW/PI[86]	AgNW	X		1210.00
AgNW/PI[87]	AgNW				3.2 × 10–7
AgNWs/PI[88]	AgNW	L	55.00
Cu@Ag nanoflakes coated LM-Cu@Ag[89]	Cu@Ag	X	100.00	120.00	7.9 × 104
PMIF@Cu[93]	Cu		52.00		1.1 × 104
PMIF@Ni[93]	Ni		43.00		9.2 × 103
nanoporous Ag membranes/BC[94]	nanoporous Ag	C, X, and Ku	∼53.00		2.9 × 104
EM/LMm[95]	liquid metal	X, Ku, K, and Ka	98.70

Bυ, band of frequency. σ, conductivity. SSE, specific EMI shielding efficiency.

The data presented here reveal that metal fillers can provide very good EMI SE in most cases with a maximum of 100 dB in the case of Cu@Ag nanoflakes coated with LM. The large EMI SE of metallic-filler-dispersed polymers might be due to their high electrical conductivity which is well over 103 S/m. Furthermore, in most of the samples presented in Table showing high EMI SE, Ag nanoparticles are present. Silver is highly conductive and has a lower market price than that of many carbon fillers like CNT and graphene. Despite all of these, the preparation of polymer composites with metallic fillers is not getting much attention recently as evidenced by the very few number of publications on them. There are many reasons for this downward swing in the popularity of metallic fillers. Low propensity to corrosion, high density, lack of flexibility, moderate mechanical properties, and high rigidity of metal-based shielding materials negatively influence the performance of shielding. Also, the shielding mechanism in metal-based shielding material is mainly due to reflection which results in secondary rays, which is a big drawback as reflected EM waves can cause a secondary EM radiation effect. However, this property can be exploited if they are used in the backside of the shield which can facilitate rereflections and helps the inner absorptive layer to absorb and attenuate more EM waves.

Magnetic Fillers

For significant absorption of the radiation, the shield must require electric and/or magnetic dipoles which interact with the EM fields in the incident radiation. Materials with high values of dielectric constant (SiO2, ZnO, TiO2, and BaTiO3) and magnetic permeability (Ni, Co, and Fe3O4) are commonly used for the development of EMI absorption materials. Among them, the magnetic fillers can realize the absorption-dominated shielding for a broad frequency range due to their high permeability. Bayet et al. investigated the effect of particle size of the magnetic nanofiller on the EMI SE of Fe3O4/PANI/DMF composites for the X band of frequencies.[96] For this purpose, different compositions of magnetite (Fe3O4) nanoparticles having two size regimes, 10–20 nm (super paramagnetic) and 20–30 nm (ferromagnetic), were used. A higher concentration of Fe3O4 assisted in increasing the conductivity and magnetic permeability of the composite, thereby enhancing the shielding by an absorption mechanism. The total shielding efficiency of the composite was modified to 68 dB from 47 dB with variation in particle size. The resulting composite exhibited a reasonably good electrical conductivity (9.2 ± 0.5 S/cm) along with good magnetic strength. Polymers with magnetic fillers exhibit good EMI and dielectric properties, thereby facilitating their use in multifunctional applications. In another work, GF/h-Fe3O4/PDMS (graphene foam/hollow Fe3O4/polydimethylsiloxane) composites were fabricated through a novel method: in situ growth of a hollow Fe3O4 sphere onto a three-dimensional graphene foam (GF) surface followed by its filling with polydimethylsiloxane (PDMS).[97] By varying the orientation of GF and by altering the morphology of in situ grown Fe3O4 nanospheres, the EMI SE of 70.37 dB was obtained in the X band. Here, h-Fe3O4 alone can be considered as a filler since GF serves only as a template to arrange the former. The electrical conductivities of these samples were close to 84.02 ± 8.385 S/cm. When used as thermal interface materials in electronic devices, they showed excellent cooling efficiency. Critical data of certain magnetically conductive EMI shielding polymer composites are provided in Table .

Table 3

Summary of Magnetically Conductive EMI Shielding Polymer Compositesa

sample details	filler	Bυ	EMI SE (dB)	SSE (dB cm3 g–1)	σ (S/m)
Fe3O4/PANI/DMF(96)	Fe3O4 NPs	X	68.00		(9.2 ± 0.5) × 102
GF/h - Fe3O4/ PDMS(97)	Hollow Fe3O4	X	70.37		8.4 × 103
PANI/BaTiO3/ Fe3O4(98)	BaTiO3	Ku	–16.80	17.00	1.7 × 10–1
	Fe3O4		–19.40	20.00	9.4 × 10–1
Ni0.4Co0.6Fe2O4/PAPY(99)	Ni0.4Co0.6 Fe2O4	X	29.40		(1.3–2.9) × 102 (DC)

Bυ, band of frequency. σ, conductivity. SSE, specific EMI shielding efficiency. PANI, poly(aniline). Ni0.4Co0.6Fe2O4/PAPY,[99] nickel-doped cobalt ferrite/poly(ani-co-Py).

The papers reviewed in this part unravel the fact that iron oxides in nanoscale are still attractive to researchers, and their dispersion in polymers can give high values of EMI SE (maximum is 70.37). These samples show good electrical conductivity as well. However, it is mainly the magnetic permeability of the fillers that provides them the absorption-dominated shielding capability. It is known that, like magnetic dipoles, electric dipoles can also enhance EMI shielding by absorption. However, it is seen from ref (98) that despite the addition of magnetic filler Fe3O4 and colossal dielectric material BaTiO3 the EMI SE of the sample is very low. This is probably due to the decrease in conductivity, as shown in Table . This means that along with good magnetic/dielectric properties a moderate electrical conductivity should also be maintained for high performance. The magnetic fillers, despite their advantages, have a major drawback in that their dispersion in polymers is mostly improper, which results in poor mechanical properties. This can be resolved to an extent by using carbon-based fillers along with magnetic fillers.

Carbon-Based Fillers

Carbon-based fillers like carbon nanotube (CNTs), graphene/graphene oxide (GO), carbon fiber (CF), carbon black (CB), and graphite are good conductors of electricity and also excellent absorbers of electromagnetic radiation with a broad range of frequencies. There has been enormous progress in the field of allotropic modifications of carbon and its correlated structures owing to their powerful mechanical, thermal, and electrical properties, better conductivity, low density, and high permittivity. One of the main advantages of using carbon-based fillers is that most of their unique properties are structure dependent. Hence, only a very small quantity of carbon fillers is required to improve the transport properties of polymer nanocomposites.[100−102]

Carbon Nanotubes

Carbon nanotube, an allotrope of carbon, is the strongest and stiffest material discovered to date. They have gained wide attention due to their excellent properties such as high Young’s modulus (Y) (1.25–1.80 TPa),[16] tensile strength (T) (50–200 GPa), room-temperature conductivity (around 6000 W/m K), and electrical property (102–106 S/cm). The properties of the CNT/polymer nanocomposites are strongly dependent on the dispersion and orientation of CNTs in the host matrix. CNTs can be classified into single-walled carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes (MWCNTs). Due to their smaller diameter and higher aspect ratio, the electrical properties of SWCNTs and MWCNTs are different. The intrinsic conductivity and conductive path inside the host matrix play a vital role in enhancing the electrical features of the composite and thereby provide a superior EMI shielding capability. Owing to the CNT’s intrinsic conductivity and aspect ratio, polymer nanocomposites with good EMI shielding performance can be prepared even by low loading of the filler.[103] Different approaches are employed to categorize CNT-based polymer nanocomposites. Grouping can be made on the basis of classification, dispersion, and concentration of the CNT and on account of the host matrix.

An EMI shielding efficiency of 34 dB was attained for the poly(vinylidene fluoride)/ethylene-α-octene block copolymer/MWCNT (PVDF/OBC/MWNCT) composite in the X-band.[104] The sample showed reasonably good tensile yield strength (∼65 MPa) and elastic modulus (λ) (∼2400 MPa). In another work, it has been shown that the EMI shielding effectiveness can be increased by combining stainless steel (SSF) and CNTs along with the polymer.[105] Nanocomposites of polypropylene (PP) reinforced with SSF and CNT together exhibited a better EMI shielding of 57.4 dB in the X-band than those PP composites reinforced with SSF alone and CNT alone.[105] This was due to the formation of a conductive filler network formed by the bridging between SSFs and CNTs. However, the conductivity of the sample never exceeded 1 S/m. Yet, the EMI SE due to absorption remained dominant. Furthermore, for all filler loading, the hybrid sample exhibited more yield strain than composites with a single filler. Hui et al. in 2020 synthesized CNT film (CNTf) reinforced epoxy (EP) multilayer composites with good impact resistance behavior and EMI shielding efficiency.[106] A SE of 52.31 dB in the X-band was attained by introducing a three-layer CNTf into the interlayer of EP plates. EM waves entering the multilayered CNTf encountered sufficient attenuation through repeated reflection, adsorption, and scattering. Yeiping et al. synthesized a polysulfone (PSU)/CNT composite foam via solid-phase milling and supercritical CO2 (scCO2) foaming.[107] Prominent absorption was attributed by the synergistic effect of CNTs and microcellular structures in PSU domains. Figure shows a comparison of different EMI SEs and coefficients of A, R, and T values. The prepared composites with good EMI SE have potential applications in aerospace and military applications. To sustain the long-term usage of EMI shielding composites, Wang et al. developed a conducting composite with both self-healing and EMI shielding properties. The self-healing polyurethane bearing Diels–Alder (DA) bond (PUDA)/CNT composites exhibited an EMI SE of 35.9 dB in the X-band with 7.0 wt % CNT loading.[108] The highest conductivity (close to 10 S/m) was obtained for a sample with 7 wt % of CNT. Even after harsh mechanical damage, the prepared composite possessed outstanding self-healing capacity to recover the electrical, mechanical, and EMI shielding properties. 3D printing is an alternative emerging technology adopted to enhance the conductivity of polymer composites. A 3D-printed, segregated CNT/polylactic acid (PLA) composite translated to an interconnected conductive network after compression was reported by Yan et al.[109] With 5 wt % of CNT loading, an EMI SE of 67.0 dB in the X-band was attained. The 3D-printed PLA scaffold with sustained structure provided mechanical robustness and tunable EMI SE value. The 3D-printed sample showed higher electrical conductivity (20 S/m), Young’s modulus (4.43 GPa), and bending strength (BS) (∼87.8 MPa), which were 101% and 43% higher than 43.7 MPa and 3.08 GPa for the conventional CNT/PLA composite. Due to these properties, the prepared composite can be used for different radiation source fields and electronic devices. Information on a number of carbon-based polymer nanocomposites is tabulated in Table .

Figure 4

Resemblance of SET, SEA, and SER at the frequency of 10 GHz for the (a) solid (s-PC and e-PC) and (b) s-PCF (segregated PSU/CNT composite foam) and e-PCF (conventional extruding PSU/CNT composite foam) with various CNT loadings. Resemblance of T, R, and A at the frequency of 10 GHz for the (c) solid and (d) s-PCF and e-PCF composites with various CNTs loadings. Reproduced with permission from ref (107). Copyright 2020, Royal Society of Chemistry.

Table 4

Summary of Carbon Nanotube Mixed EMI Shielding Polymer Compositesa

sample details	filler	Bυ	EMI SE (dB)	SSE (dB cm3 g–1)	σ (S/m)	λ (MPa)	Y (MPa)	T (MPa)	BS (MPa)
PVDF/OBC/MWCNT[104]	MWCNT	X	34.00			∼2400.00		∼65.00
PP/SSF/CNT[105]	CNT	X	57.40
CNTf-reinforced EP multilayer[106]	CNT	X	52.31	261.60 dB/cm			428.00
PSU/CNT[107]	CNT		23.7		5.2 × 101
PUDA/CNT[108]	CNT	X	35.90		1.0 × 101
3D-printed segregated CNT/PLA[109]	CNT	X	67.00		20 × 101		4430.00		∼87.80
MWCNT/PMMA (in situ)[110]		Ku	58.73
MWCNT/PMMA (ex situ)[110]			32.06
PTT/PP/MWCNT[111]	MWCNT	S	40.00
PVDF/PS/MWCNT[112]	MWCNT	X	43.03	∼61.47

Bυ, band of frequency; σ, conductivity; SSE, specific EMI shielding efficiency; λ, elastic modulus; Y, Young’s modulus; T, tensile strength; BS, bending strength; SCS, specific compressive strength; PMMA, poly(methyl methacrylate); PTT, poly(trimethylene terephthalate); PP, polypropylene; PVDF, poly(vinylidene fluoride); PS, polystyrene.

Graphene/Graphene Oxide

Graphene is well thought out as the fundamental structural unit of every graphitic material like fullerenes, charcoal, graphite, and CNTs consisting of a single layer of sp2-hybridized bonded carbon atoms. The carbon atoms are well placed in a honeycomb hexagonal structure. Graphene is treated as the world’s stiffest, thinnest, and strongest nanomaterial, and it also acts as an outstanding conductor of electricity and heat. The percolation threshold can be attained with very low content due to its ultrahigh specific surface area. Owing to their exceptional charge carrier mobility and other properties, graphene can be considered as a suitable candidate for nanoelectronics, super capacitors, batteries, solar cells, hydrogen storage, sensors, and flexible displays. It has admirable mechanical properties. Graphene is harder than diamond and about 200 times stronger than steel. Nevertheless, graphene is said to have zero band gap, and consequently its utilization in semiconductor technologies has been highly inadequate.[113−115]

Grafting amino ethyl methacrylate (AEMA) onto graphene nanosheets (GNSs) through covalent modification strengthened the interactions and provided compatibility of GNSs in waterborne polyurethane (WPU).[116]Figure (a) depicts the preparation procedure of P-GN and AEMA-GN, and Figure (b) shows the electrostatic attraction between AEMA-GN and WPU. The electrostatic attraction between AEMA-GN and WPU enabled a continuous dispersion of GNS in WPU due to which a conductivity as high as 43.64 S/m and an EMI SE of 38 dB in the X-band were obtained. Despite the complicated structure developed through tedious methods, it can be seen that the EMI SE is still not sufficient for many applications. However, later, Ying et al. fabricated an EMI shielding graphene foam (GF)/poly(3,4-ethylene dioxy thiophene):poly(styrenesulfonate) (PEDOT:PSS) composites via drop coating of PEDOT:PSS on cellular-structured, self-supporting GFs which showed a better EMI SE.[117] Here, graphene foams were functionalized with 4-dodecyl benzenesulfonic acid in order to enhance the interfacial bonds with PEDOT:PSS. The high porosity (98.8%) and improved electrical conductivity (35.2.S/cm) of GF/PEDOT:PSS composites lead to excellent EMI SE of 91.9 dB in the X-band. SEA contributed more than 80% of the total shielding effectiveness which suggested that the shielding is predominantly due to attenuation of microwave energy into thermal energy. Another study shows that the conductivity of samples with graphene and rGO as fillers could be further increased by the use of thermal treatment.[118] In this work, a 6.6 μm thick nitrogen-doped graphene film (rGO–EDA-2) was synthesized by pressure-assisted self-assembly filtration, followed by thermal annealing.[118] Chemical reaction between oxygenated groups of GO and amine groups of ethylene diamine (EDA) resulted in the formation of GO-EDA structures. After thermal treatment, a compression required composite was obtained with an EMI SE of 58.5 dB. The sample had a conductivity of 8796 S/cm, and it was able to withstand a tensile strain up to 32.6%. Data of some polymeric composites consisting of graphene/graphene oxide are summarized in Table .

Figure 5

(a) Scheme of the procedure for preparing P-GN and AEMA-GN. (b) The AEMA-GN attached to the sulfonated functional groups of WPU through electrostatic attraction for better compatibility. Reprinted with permission from ref (116). Copyright 2015 American Chemical Society.

Table 5

Summary of Graphene/Graphene Oxide Mixed EMI Shielding Polymer Compositesa

sample details	filler	Bυ	EMI SE (dB)	SSE (dB cm3 g–1)	σ (S/m)	T (MPa)
GNS/WPU[116]	GNs	X	38.00		4.4 × 101
GF/PEDOT:PSS[117]	GF	X	91.90	3124.00	3.5 × 103
nitrogen-doping rGO-EDA-2[118]	rGO		58.50	29. 00	8.8 × 105
PVA/SRGO[119]	SRGO	S and C	∼25.00		2 × 10–2 (DC)	90.00
GNS/TPU[120]	GNS	X	∼20.00		>1 × 101
PEI/rGO[121]	rGO	X	22.00–26.00		1 × 10–4
epoxy/glass fiber nanocomposites reinforced with graphene[122]	graphene	X	–27.00–30.00
PEK/graphene[123]	graphene	X	∼33

Bυ, band of frequency; σ, conductivity; SSE, specific EMI shielding efficiency; T, tensile strength; PVA, polyvinyl alcohol; SRGO, selectively reduced graphene oxide; GNSs, graphene nanosheets; TPU, thermoplastic polyurethane; PEI, polyetherimide).

Carbon Nanofibers and Carbon Black

The interlocked sheets of carbon atoms or graphene having customary hexagonal patterns form carbon fiber.[124] Carbon nanofiber (CNF) based composites have been used in a diverse range of applications such as sensors, energy storage and conversions, electrode materials for batteries, fillers for composites, super capacitors, and various electronic devices. Usual carbon fibers (CFs) and CNFs are relatively divergent from each other; for instance, the dimension of CF is in a μm series, while that of CNFs is in the nm range.[125,126] In CNFs, a foremost distinction from CNTs is the poorer uniformity in graphene layer orientation and graphitic edge terminations seen on their exterior.[127] Carbon black (CB) is a member of the carbon family, which is used as a filler in polymers, plastics, and elastomers to modify the electrical, mechanical, and optical properties of materials. Generally, CB is formed in the gas phase by thermal decomposition of hydrocarbons.[16] CB is crystalline in structure and contains short-range graphitic ordering, which results in low resistivity. CB consists of elemental carbon particles fused to form aggregates.[128]

Carbon nanofiber reinforced PEK/CNF composites having an EMI SE of −40 dB (>99.99% attenuation) in the Ka-band were fabricated by Chauhan et al. using a corotating twin screw extruder.[129] Absorption loss was 12 times greater than reflection loss. The resulting nanocomposites exhibited reasonably good conductivity (10–3 S/cm) along with excellent mechanical strength (tensile strength ∼112 MPa, tensile modulus ∼7.2 GPa, and thermal stability (up to almost 580 °C)), which made them applicable as good EMI absorbers in aerospace and defense applications. Ghosh et al. developed a cost-effective and industrially useful EMI shielding composite material by incorporating a new-generation conductive filler Ketjen 600 JD-CB (K-CB) with carboxylated nitrile butadiene rubber (XNBR).[130] XNBR/K-CB composites prepared with very low percolation threshold provided an excellent EMI SE value of 43.39 dB in the X-band. Compared with other composite materials, even after deformation of XNBR/K-CB composites, the EMI SE value was retained, and the flexible structure of the material was preserved. The sample with the highest EMI SE was endowed with a DC conductivity close to 10–1 S/cm and, surprisingly, a much higher high-frequency AC conductivity of the order of 104 S/cm. Data of some important polymeric composites consisting of CNF and CB are summarized in Table .

Table 6

Summary of Polymeric Composites Consisting of Carbon Nanofibers and Carbon Blacka

sample details	filler	Bυ	EMI SE (dB)	σ (S/m)	T (MPa)
PEK/CNF[129]	CNFs	Ka	–40.00	∼1.0 × 10–1	∼112.00
XNBR/K-CB[130]	K-CB	X	43.39	1.0 × 101 (DC)
				1.0 × 106 (AC)
PSU/CNFs[131]	CNFs	X	45.00	8.7 × 10–1	78.00
HS-CB/PP[132]	HS-CB	X	43.00	4.4 × 101
CPE/K-CB[133]	K-CB	X	38.40	1.0 × 100 (DC)	11.00
PLA/TPU/CB[134]	CB	X	–27.00

Bυ, band of frequency; σ, conductivity; SSE, specific EMI shielding efficiency; T, tensile strength; PSU, polysulfone; HS-CB, high structure carbon black; CPE, chlorinated polyethylene; PLA, polylactic acid.

The discussions presented in Sections 4.3.1, 4.3.2, and 4.3.3 help us to compare the performance of various carbon-based fillers. From the tables it is clear that the carbon-based fillers will not only enhance the conductivity by delocalization of charge carriers but also improve the mechanical properties of the composite. Apart from this, the EMI SE values for samples reviewed here indicate that while MWCNT-incorporated composites put up a consistent yet moderate performance graphene-dispersed composites show inconsistent EMI SE but give the single highest value of EMI SE (91.90 dB) among carbon fillers. The EMI SE of rGO-filled composites is much inferior to graphene unless it is doped to improve the conductivity, as done in ref (118). CB and CNT, on the other hand, consistently give arguably moderate EMI SE values. CB has the added advantage that since it is an excellent UV absorber it can protect the composite shield from UV exposure. However, its electrical conductivity is low compared to CNT and graphene, which is a disadvantage. CNF has a conductivity similar to the CB. However, since its microscopic crystals are perfectly aligned in the direction of their long axis, CNF has high mechanical strength. This is evident from the data presented in Table as the tensile strengths for CNF reinforced composites are much higher than composites with CB as filler. Yet, CNFs have the added disadvantage that they have weak magnetism and moderately high conductivity, which would increase the skin depth, resulting in an impedance mismatch. Due to this, their EMI shielding performance would be mediocre. Clearly, the EMI SE performance is not solely decided by the nature of fillers in it. Rather, it also depends on the structural features of the composite shields which can enhance the filler–polymer interaction and result in a continuous dispersion. Efforts to enhance the compatibility of filler and polymer to improve the dispersion of the former have met with considerable success as evidenced by the work presented in ref (116). Furthermore, advanced methods like 3D printing can be seen to bring about surprising performance improvement in both EMI SE and mechanical properties.

Hybrid Fillers

Hybrid fillers are a combination of carbon-based fillers with other fillers such as metal oxides, ferrites, metallic particles, etc. Hybrid fillers provide excellent mechanical and physical properties, and they are also proficient in tuning the permittivity, permeability, and electrical and thermal conductivities of composites. A suitable combination of magnetic characteristics of metal-based fillers and dielectric properties of carbon-based fillers provide better attenuation of EM waves. A flexible and wideband high performance electromagnetic interference (EMI) shielding film composed of 3D graphene/CNT/iron oxide (3D G-CNT-Fe2O3) structures and poly(3,4-ethylene dioxythiophene) and poly(4-styrenesulfonate) was fabricated by Lee et al.[135] Multilevel absorption, reflection, and scattering processes were accomplished on the surfaces and the interlayers of the 3D G-CNT-Fe2O3 heterostructure, due to the coupling of hysteresis loss, conduction loss, and multiple scattering. This resulted in an outstanding EMI shielding effectiveness of 130 dB in the X-band. The resulting composite exhibited reasonably good electrical conductivity due to a well-dispersed network structure. These properties made them applicable as an EMI film for the next generation of soft and wearable electric devices. The absorption capacity of composites can be increased by increasing the number of filler layers. Xu et al.[136] fabricated a flexible waterborne polyurethane composite film (rGO@Fe3O4/T-ZnO/Ag/WPU) with different filler layers of varying density, as shown in Figure . The top layered rGO@Fe3O4 enhanced the EM wave absorption capacity, and the bottom conducting layer T-ZnO/Ag improved the reflection ability of the film. A continuous “absorb–reflect–reabsorb” process led to an EMI SE of 87.2 dB against the X-band with the lowest reflection efficiency of 2.4 dB.

Figure 6

Illustration of the shielding mechanism and EMI SE of the rGO@Fe3O4/T-ZnO/Ag/WPU composite film and SEM image of the fracture surface of the double-layer structure. Reprinted with permission from ref (136). Copyright 2018 American Chemical Society.

3D porous graphene nanoplatelets/reduced graphene oxide foam/epoxy (GNPs/rGO/EP) nanocomposites with improved EMI shielding performance and outstanding electrical (maximum value of 179.2 S/m) and thermal conductivities were synthesized through a template method by Chaobo et al.[137]Figure depicts the movement of electromagnetic waves through the 3D GNPs/rGO/EP nanocomposites. Due to an impedance mismatch between the composite and air, a fraction of the incident wave was reflected, and the remaining traveled into the composite. The conducting network was formed by proper dispersion of filler and was useful to block the waves emerging through the composite. This provided a high electrical conductivity to 3D GNPs/rGO/EP nanocomposites which in turn executed proper reflection and absorption of EM radiation. The porous nature of the GNPs/rGO foam further attenuated all the interference waves and eventually yielded better EMI SE values. The prepared composite showed an EMI SE of around 51 dB in the X-band as illustrated in Figure .

Figure 7

Schematic sketch of the EM waves transfers throughout the 3D GNPs/rGO/EP nanocomposites. Reproduced with permission from ref (137). Copyright 2019, Royal Society of Chemistry.

Figure 8

EMI SET of the 3D GNP/rGO/EP and rGO/EP nanocomposites in the X-band. (B) Evaluation of microwave reflection (SER) and absorption (SEA) at the X-band. (C) Evaluation of EMI SET of 3D GNP/rGO/EP and GNP/EP nanocomposites fabricated with two various methods in the experimental part. (D) The absorption, reflection, and transmission against the rGO/EP and 3D GNPs/rGO/EP nanocomposites at 12.4 GHz. Reprinted with permission from ref (137). Copyright 2019, Royal Society of Chemistry.

Xu et al. enhanced the EMI SE of microwire/graphene/silicone rubber composites by artistically aligning magnetic microwires (M) and graphene fibres (G).[138] Randomly arranged M and G provided an EMI SE of only 6 dB in which polarization effects at the interface between the two regions contributed to the SE value. However, an equal amount of MMMGGG periodic arrays with merely 0.059 wt % filler loading gave an EMI SE of 18 dB (98.4% attenuation). Figure depicts a schematic illustration of the mechanism of EM wave distribution through these filler-reinforced polymer composites. In Figure (a), microwires played the role of an initial absorbing layer. The subsequent graphene fiber layer activated an absorb–reflect–reabsorb mechanism, leading to good SE. The randomly distributed microwires and graphene fibers in Figure (b) result in reduced SE due to low polarization and aspect ratio effects. The normalized shielding effectiveness of these composites was of the order of two to four times that of the other shielding materials. These materials are very attractive for various applications due to the high SE, simple structure, low loading, and multifunctionality. Meanwhile, an ultrathin flexible carbon fabric/Ag/waterborne polyurethane film (CEF-NF/Ag/WPU film) formed by porous structured electroless Ag plating combined with the enhanced-pressing process was reported by Di Xing et al.[139]Figure provides a detailed explanation of the shielding mechanism through CEF-NF/Ag/WPU films by migration and hopping of electrons and EM wave multiple reflections. The Ag particles provided better electrical conductivity (11986.8 S/cm) and an outstanding shielding effectiveness of 102.9 dB at 30–1500 MHz. This composite outperforms most of the reported works and can be considered as a suitable shielding material in extreme environments. The authors also pointed out that the pressing process is the key step toward obtaining such a highly efficient EMI shielding material, as samples with an enhanced pressing process showed an increase in EMI SE of 37.5% compared to samples without enhanced pressing.

Figure 9

Schematic illustration of the mechanism of electromagnetic wave propagation through filler-reinforced polymer composites containing (a) orderly distributed microwires/graphene fibers MMMGGG and (b) randomly distributed microwires/graphene fibers M +G. Reproduced with permission from ref (138). Copyright 2020, Elsevier.

Figure 10

Schematic illustration of the shielding mechanism for the CEF-NF/Ag/WPU films transferring across the material. Reproduced with permission from ref (139). Copyright 2020, Elsevier.

In another work, Jiajun et al. fabricated lightweight, multifunctional polypropylene/carbon nanotube/carbon black (PP/CNTs/CB) nanocomposite foams with low loading (5 wt %) hybrid fillers.[140] The small cell-conductive pathways provided the samples with an outstanding specific EMI shielding efficiency of ∼72.23 dB cm3/g in the X-band. Figure depicts the propagation of electromagnetic microwave across the foam. The incident wave was reflected and absorbed by a hybrid filler layer of composite foams with low filler content. On the other hand, a huge number of small cells in the conductive channel exhibited a multireflection and absorption of the electromagnetic microwaves in the case of foams with high hybrid filler content. Furthermore, the microcellular foams showed a higher electrical conductivity (2.85 × 10–3 S/m) compared to its solid nanocomposite counterpart. It is the peculiar structure of PP/CNTs/CB carbon foams that bestowed on it these remarkable properties. Due to its superior thermal insulation and compressive properties, the authors claim that the sample is suitable for aircraft and spacecraft applications. A lightweight, flexible, and absorption-dominated composite was fabricated by Acharya et al. by incorporating CuAl2Fe10O19 (CFA) nanoparticle-decorated rGO filler in polyvinylidene fluoride (PVDF)[141] through chemical reduction in the presence of hydrazine. An EMI SE of ∼60 dB in the X-band and ∼50 dB in the Ku-band was reported. Strong absorption was the result of multiple relaxation mechanisms at interfaces of rGO, CFA, and PVDF and the synergetic effect of fillers. The electrical conductivity of the sample with better EMI SE was between 2 and 7 S/m. A further increase in filler loading improved the conductivity and EMI SE due to absorption. However, the total EMI SE remained inferior.

Figure 11

Schematic illustration of electromagnetic microwave distribution in the PP/CNT/CB nanocomposite foams. Reproduced with permission from ref (140). Copyright 2020, Elsevier.

Recently, Wei Hu et al.[142] prepared a flexible lignin-based electromagnetic shielding polyurethane (FeCLPU) film. The addition of lignin was reinforced with CNTs and aminated ferroferric oxide nanoparticles (Fe3O4). Figure shows the EMI shielding improvement mechanism of FeCLPU with CNTs and Fe3O4. Owing to the impedance matching, Fe3O4 can reduce the reflection of EM waves. Also, the phenyl group of lignin conjugates with CNT to promote uniform dispersion of CNTs, which resulted in an EMI SE of 37.5 dB. Concurrently, in another work, Jie et al.[143] fabricated polyoxymethylene (POM)/multiwalled carbon nanotube (MWCNT) and POM/graphene nanoplate (GNP) composites (PMCNT and PMGNP) with an EMI SE of 45.7 and 44.7 dB, respectively. Their respective electrical conductivities were 3484 and 2695 S/m. Figure illustrates the interfacial and dipole polarization loss, polarization loss, multiple reflections/scatterings, conduction loss, etc., that happened in the composite structures. They found that composites with high filler loading attenuated EM waves the most. 3D-assembled graphene structures play an important role in EMI shielding due to their exceptional properties.

Figure 12

Electromagnetic shielding mechanism of the FeCLPU biocomposite. Reproduced with permission from ref (142). Copyright 2021, Elsevier.

Figure 13

Schematic illustration of the shielding mechanism of the composites with high MWCNT or GNP loadings. Reproduced with permission from ref (143). Copyright 2021, Elsevier.

In a recent work, lightweight silver/reduced graphene oxide-coated carbonized melamine (CMF/rGO/Ag) hybrid foams were fabricated by a simple one-step heat treatment method, and an EMI shielding of 63.2 dB was attained in the X band.[144] Due to the effective reinforcement of rGO between Ag nanoparticles and carbonized melamine frameworks, the composite had outstanding structural stability and mechanical property. The unique porous structure provided an ultrahigh specific EMI SE of 7616 dB cm2 g–1 and high absorption coefficient of about 0.51. The outstanding properties of the composites made them promising materials in the field of electronic packaging. Information about various polymer composites with hybrid fillers reported in the past few years is summarized in Table .

Table 7

Summary of Polymeric Composites Consisting of Hybrid Fillersa

sample details	filler	Bυ	EMI SE (dB)	SSE (dB cm3 g–1)	σ (S/m)	SM (MPa)	Y (MPa)	T (MPa)	FS (MPa)	CS (MPa)
3D G – CNT- Fe2O3 poly (3, 4–ethyle Nedioxythiophene poly (4–styrene sulfonate)[135]	CNT, Fe2O3	X	130.00
rGO@ Fe3O4/T-ZnO/Ag/WPU[136]	rGO@Fe3O4, ZnO/Ag	X	87.20
3D porous G nanoplatelets/RGO foam/EP[137]	3D porous G, RGO	X	51.00		1.8 × 102
microwire/graphene/silicone rubber[138]	microwire/graphene		18
CEF – NF/Ag/WPU film[139]	CEF, Ag	UHF	102.90	106.30	1.2 × 106			8.80–10.10
PP/CNTs/CB[140]	CNTs/CB	X		∼72.23	2.85 × 10–3
CuAl2Fe10O19/rGO/PVDF[141]	CuAl2Fe10O19/rGO	X	60		2 × 100
		Ku	∼50		7 × 100
FeCLPU/CNTs/Fe3O4[142]	CNTs/Fe3O4		37.50
POM/PMCNT[143]	MCNT		45.70		3.5 × 103
POM/PMGNP[143]	MGNP		44.70		2.7 × 103
CMF/rGO/Ag[144]	rGO/Ag	X	63.20
rGO/γ-Fe2O3/CF[145]	rGO, γ-Fe2O3, and CF	X	45.26		4.8 × 101–1.7 × 104				67.00
EP/rGO – CF[146]	rGO, CF	X	37.60	39.00	7.2 × 100
MWCNT-Fe3O4 @ Ag/EP[147]	MWCNT,Fe3O4@Ag	X	35.00		2.8 × 101		4.6 × 103
PVDF/FLG-3/NSF-30[148]	FLG-3/NSF-30	S, C, and X	∼45.00		1.8 × 10–1 (DC)
			∼53.00
PVC/MLG/MWCNT	MLG, MWCNT	X	43.00		>1.0 × 100		1.7 × 103	39.75
PMMA/MLG/MWCNT[149]							1.6 × 103	39.68
Flexible G/MWCNT/PDMS[150]	G, MWCNT	X	54.43	87.86	1.0 × 102					1.40–1.94
carbonyl iron powder - carbon fiber felt/epoxy resin[151]	carbonyl iron powder/carbon fiber felt	X	53.9
PVA/MLG/MWCNT[152]	MLG, MWCNT	X	39.00		4.9 × 10–3		3.5 × 102	20.00
PNTMn-Fe/PCF@NiP/PNTMn-Fe[153]	NiP, Fe	Ku	52.00			3.8 × 103
TPU/CB/PPy/CNTs[154]	CB	X	–20.00		7.6 × 102
	CNTs
CPE/FCNF/K-CB[155]	FCNF/K-CB	X	33.00		2.2 × 100 (AC)			∼27.00
ACET in EP/NCCF/CNT blend foam[156]	NCCF,CNT		40.80
PU/3D fire-retardant carbon – CNTs[157]	3D fire-retardant carbon,CNTs		–57.20		4.8 × 103					6.50
PDMS+ PTSA doped PPy[158]	MnFe2O4	X	–21.00		2.4 × 103		9.3 × 10–1	2.62
PU/rGO/Fe2O3 hollow microspheres[159]	rGO, Fe2O hollow microspheres	X	37.45
PVDF/rGO/BaZrFe11O19[160]	rGO, BaZrFe11O19	X	48.59
CuNWs – TAGA/epoxy[161]	CuNWs, TAGA		47.00		1.2 × 102
CNT/graphene/polyimide (PI)[162]	CNT/graphene		28.2
Acrylonitrile-butadiene-styrene/MWCNT/CNF/HS-CB[163]	MWCNT/CNF/HS-CB	X	50		1 × 10–1 –1 × 102

Bυ, band of frequency; σ, conductivity; SSE, specific EMI shielding efficiency; SM, storage modulus; T, tensile strength; Y, Young’s modulus; FS, flexural strength; CS, compressive strength; FLG, few layered graphene nanosheets; NSF, nickel spinal ferrites; PVC, polyvinyl chloride; MLG, multilayered graphene; PNTMn-Fe-Mn (manganese)-ferrite nanoparticles incorporated in a thermoplastic matrix (PVDF) along with conductive MWNTs; PCF@NiP-PVDF films sandwiched with a Ni (nickel)-deposited woven carbon fiber (CF) mat; PPy, polypyrrole; CPE, chlorinated polyethylene; FCNF, functionalized carbon nanofiber; ACET, 0D acetylene black; NCCF, nickel-coated carbon fiber; PDMS, polydimethylsiloxane; PTSA, paratoluene sulfonic acid; TAGA, thermally annealed graphene aerogel; CuNWs, copper nanowires.

The long list of hybrid-filler-incorporated polymer nanocomposites in Table explains that these composites are widely developed and studied recently. The reason is self-explanatory from the table as the EMI SE of a few samples is well over 100 dB. The highest value reported is 130 dB, which is remarkably high.[135] This huge value is probably a result of coupling the magnetic and conducting fillers and the formation of their effective conductive network through the development of a 3D structure. The data presented imply that EMI SE is not always proportional to the electrical conductivity of the samples. Furthermore, the combinations of conducting and magnetic fillers do not always lead to a huge EMI SE. These might be due to the fact that hybrid filler combination, polymer–filler compatibility, and the structure of the shield can also be vital factors in defining the EMI SE. More efforts are required to obtain a trade-off between these factors and maximize the shielding performance. Hybrid-filler-incorporated polymer nanocomposites have undoubtedly better EMI shielding performance than composites with a single filler. However, the data presented here show that they have very ordinary mechanical properties, which might be overcome by the right combination of fillers and their better interaction with the matrix.

MXenes

MXenes are a novel two-dimensional ceramic material, composed of transition metal carbides or carbonitrides with the formula MAX, where M represents a transition metal; A is C and/or N; and X indicates the surface termination group (−OH, =O, and/or −F).[164,165] MXenes are made from MAX phases by selectively removing A phases (exfoliation process) which are the layered carbides, nitrides, or carbonitrides. The intergrowth of close-packed A planar atomic layers and alternative hexagonal MX layers leads to the formation of a MAX phase. The M–X bonds are covalent/ionic in character, while the M–A bonds are ionic in nature. Compared with M–X bonds, M–A bonds are weaker, thereby enabling easy removal of the A layer with appropriate chemicals without destroying M–X bonds. By changing the class of transition metal, altered forms of activities of layers could be achieved in MXene. Enhanced electrical and thermal conductivity, hydrophilicity, higher specific surface area, and excellent film-forming ability of MXenes make them extremely versatile materials. MXenes are also capable of accommodating various ions and molecules between their layers by a process called intercalation, which helps to enhance the inherent properties of materials. Due to their excellent features, MXenes are a suitable substitute to the conventionally used metals and carbon materials in EMI shielding applications. By a close comparison with previously reported literature, it could be observed that composites with MXenes surpass other conductive materials (i.e., graphite, graphene, CNFs, and CNTs) with identical thickness and are almost similar to metals (e.g., Ag and Cu) in their electrical conductivity. MXenes possess good electrical and thermal conductivity, high strength, light weight, good thermal stability, and easy processability. Porous and segregated structures of MXenes are achieved by their tunable surface chemistry. This clearly depicts that MXenes are the best candidate for lightweight EMI shielding applications.

A lot of work has been done in the last lustrum with MXene-based composites for EMI shielding applications. Yan-Jun reported an EMI shielding efficiency of 40.5 dB and a tensile strength of 38.5 ± 2.9 MPa for the MXene/PEDOT:PSS (poly(3,4-ethylenedioxythiophene)/poly(styrene sulfonate) composite film with a thickness of ∼6.6 μm.[166] In order to improve the shielding effectiveness of the composite, it is treated with concentrated H2SO4, which takes away the nonconductive PSS, thereby enhancing the maximum conductivity to a mammoth 438 000 S/m. This study provides a good balance between shielding performance and mechanical property, which has not been acquired in the reported literature under the same conditions. In another work, lightweight, compressible, and electrically conductive (conductivity as high as 2211 S/m) polydimethylsiloxane (PDMS)-coated MXene foams were synthesized by performing MXene aerogels with sodium alginate (SA) by Xinyu et al.[167] However, the PDMS coating improved the structural ability and durability of the composite. Additionally, this coating introduced 3D conductive networks facilitating an average EMI shielding efficiency of 70.5 dB. Only a slight change in EMI SE occurred after 500 compression–release cycles. In yet another report, TCTAs (thermally annealed cellulose nanofibers (CNFs)/Ti3C2T MXene aerogels/epoxy nanocomposites) were fabricated with an EMI shielding effectiveness of 74 dB in the X-band, which is nearly the highest value compared to formerly reported works with similar filler content.[168] The corresponding electrical conductivity and storage modulus were 1672 S/m and 9.79 GPa. The samples also showed good thermal stability due to which it can be used for a multitude of applications. A flexible, lightweight MXene/AgNW composite film with nanocellulose (NC) as a binder was fabricated by Miao et al. via an aqueous filtration process.[169] The fabrication of these composite films is depicted in Figure . The intercalated AgNWs between Ti3C2Tx MXene sheets enabled us to assemble a brick-and-mortar-like composite structure which eventually strengthened the fabricated composite. The porous nature of the NC produced more interfaces at which EMWs were reflected and scattered efficiently. An MXene/AgNW hybrid film (TN 0.167A) with 16.9 μm thickness provided an EMI SE of ∼42 dB in the X-band. Figure illustrates the electrical conductivity, EMI SE, and its comparison with various composite films. The brick-and-mortar-layered structure of these films provided a tensile strength as high as 63.8 MPa. Due to these remarkable properties, the reported composite films can be used in aerospace, smart electronics, and wearable devices.

Figure 14

Schematic explanation for fabrication of MXene/AgNW composite films. Reproduced with permission from ref (169). Copyright 2020, Royal Society of Chemistry.

Figure 15

(a) Comparison of electrical conductivities and thickness of pure MXene film (TC) and MXene/AgNW hybrid films (TN 0.5A, TN 0.25A···) with diverse nanocellulose (NC) to MXene weight ratios. (b) EMI SE of MXen/AgNW hybrid films in the X-band. (c) Total EMI SE (SET) and its reflection (SER) and absorption (SEA) of MXene/AgNW and pure MXene film composite films at a frequency of 8.2 GHz. (d) Comparison of SET, SEA, and SER of TC film and TN0.167A composite film in the X-band. Reproduced with permission from ref (169). Copyright 2020, Royal Society of Chemistry.

Pritom et al. reported an EMI SE of ∼41 dB for the Ti3C2T MXene interlayered cross-linked PEDOT:PSS nanocomposite film.[170] The addition of the cross-linker, divinyl sulfone, into PEDOT:PSS made it water insoluble and also helped in the efficient interconnection of Ti3C2TMXene flakes creating more absorption sites, thereby enhancing electrical conductivity and EMI SE (99.999%) in the X-band. Figure shows the shielding mechanism of PEDOT:PSS-Ti3C2TMXene films in which multiple internal reflections take place from the consequent layers. EM waves which escaped from one layer get reflected back and forth between several layers until they get completely absorbed. The conductivity of the sample which exhibited the maximum EMI SE was 388 S/cm. The increasing thickness of the sample initially increased the EMI SE, but after reaching a maximum value it started decreasing slowly. In the recent past, Van-Tam et al. fabricated electrically conductive composite inks, a mixture of carbon nanotubes (CNTs), and heat-treated Ti3C2T MXene in a waterborne polyurethane (WPU) matrix.[171] An EMI shielding film with SE of 70 dB in overall X-band and Ka-band can be prepared through the doctor blade printing method. CNTs acted as conductive bridges in composites to enhance the electrical and thermal conductivity. The resulting composite exhibited a reasonably good electrical conductivity (3278.3 S/cm) along with good tensile strength (64.5 MPa). Data of certain polymeric composites consisting of MXenes are summarized in Table . Furthermore, Figure gives a schematic of the turning up of different MXene structures for efficient EMI shielding and the hike in the number of publications focused on MXene-based EMI shielding.[172]

Figure 16

Schematic illustration of EMI shielding mechanisms of cross-linked PEDOT:PSS-Ti3C2T MXene. Reproduced from reference (170) with permission from the Royal Society of Chemistry.

Table 8

Summary of MXene-Based EMI Shielding Polymer Compositesa

sample details	filler	Bυ	EMI SE (dB)	SSE (dB cm2 g–1)	σ (S/m)	Y (MPa)	T (MPa)	flexural strength (MPa)
MXene/PEDOT/PSS[166]	MXene		40.50		>4.4 × 105		38.50 ± 2.90
MXene/Sodium alginate polydimethylsiloxane (PDMS) - coated MXene[167]	MXene	X	70.50	26.90 dB/mm	2.2 × 103
TCTA CNF/Ti3C2Tx aerogel/EP[168]	CNF/Ti3C2Tx	X	74.00	37.00 dB/mm	1.7 × 103
MXene/AgNW composite films with NC as a binder[169]	MXene, AgNW	X	42.00	16724.00	3.0 × 104		63.80
PEDOT:PSS- Ti3C2Tx MXene films[170]	Ti3C2Tx, MXene	X	41.00	89924.00	3.9 × 104
Ti3C2Tx MXene/CNT/WPU[171]	Ti3C2Tx/CNTs	X and Ka	70.00		3.3 × 105		64.50
MXene – PAT – PANI – PpAP[173]	MXene	X and Ku	45.18		7.8 × 102		1.50
cellulose/MXene[174]	MXene	X and Ku	43.00		2.8 × 103
polyamide-imide (PAI)/Ti3C2Tx MXene[175]	Ti3C2Tx MXene	X	>43.00		2.3 × 101– 3.2 × 103			6.36 × 102
PVA/multilayered films[176]	MXene		44.40	9343.00	7.2 × 102
polydimethylSiloxane/MXene[177]	MXene		∼23.5
			–39.8
Ti3C2Tx MXene/epoxy[178]	Ti3C2Tx MXene	X	41.00		1.1 × 102	4.3 × 103

Bυ, band of frequency; σ, conductivity; SSE, specific EMI shielding efficiency; T, tensile strength; Y, Young’s modulus; FS, flexural strength; PpAP, poly(p-aminophenol); PAT polymer, a new type of polymer composition.

Figure 17

2D MXenes carried out as shields against EMI. (a) The class of various MXene structures for efficient EMI shielding and (b) the number of periodicals concentrated on MXenes for EMI shielding. Reproduced with permission from ref (172). Copyright 2020, Wiley.

Like CNT- and graphene-based composites, MXene-based polymer composites have also been getting a lot of attention recently which is clear from Table . The table also reveals that most of the samples have an EMI SE over 40 dB with the highest reported value of 74 dB. Nevertheless, the shielding performance is still inferior to the best performing MWCNT and graphene-dispersed polymer composites, which were discussed earlier. Furthermore, it is obvious that as observed in the case of composites with hybrid fillers the sample with the highest conductivity does not have the highest EMI SE in this case too, which is an indication of other factors like structural peculiarity, magnetic properties, and dielectric constants that also play a pivotal role in deciding the shielding efficiency of the shield composites. Also, from Table , it is apparent that the mechanical properties of all the MXene-dispersed composites are ordinary, which is contrary to the nature of MXenes themselves. This might be because a high amount of MXenes are usually required to build their effective conductive network in the polymer, and this can be a detriment to the mechanical properties. Due to this issue, the cost of MXene-filled polymer composites would be very high. This can be overcome to an extent by preparing the MXene–polymer composite using methods like freeze-drying through which the percolation threshold can be reduced by the creation of a 3D conductive network while maintaining a porous structure. However, porosity generates the risk of quick oxidation of MXenes. Another useful technique that can be used to prepare MXene-based polymer nanocomposites with good mechanical strength is vacuum-assisted filtration. However, in this case, the possibility of the formation of multilayer structures through the conductive networks will be difficult. Thus, in order to utilize the exceptional conductivity and mechanical properties of MXene-dispersed EMI shielding composites, new preparation methods and innovative structural designs need to be developed.

Other Conducting Fillers

The electromagnetic interference (EMI) shielding behavior of composites is mainly controlled by their electrical properties and the conductive filler properties. Greater filler content leads to mechanically brittle composites which are very difficult to process due to poor dispersion and easy agglomeration. Therefore, it is suitable to develop composites with low and diverse conductive filler contents. Weiwei et al. synthesized a nacre-mimetic 3D conductive graphene network with a biaxial-aligned lamellar structure via a unique bidirectional freezing technique.[179] With a low filler content, the composite exhibited anisotropic mechanical properties, conductivity (0.5 S/m), and also an enriched EMI SE of ∼65 dB in the X-band. Concomitantly, Junchen and his co-workers fabricated a flexible EMI shielding material with a silver nanowire (AgNW)/polyvinyl butyral (PVB) ethanol solution and textile substructure through a facile immersing technique.[180] This material with a thickness of 1.4 mm exhibited an EMI SE of 59 dB in the frequency range 5–18 GHz. Interestingly, it was observed that the conductivity of the AgNW/PVB textile in no way changes even after washing with water. Additional features like flexibility and resistance to oxidation make this material suitable for mass production of shielding materials for potential applications.

A 3D-expanded graphite (EG) network by premelt blending of EG with stearic acid and polyethylene wax, followed by powder mixing and thermal molding with linear low-density polyethylene (LLDPE) particles, was synthesized by Baojei et al.[181] The construction of 3D networks in the composite provided better conductivity (4000 S/m), and thus an EMI SE of 52.4 dB at 12.4 GHz was obtained. Meanwhile, polymer composite membranes having enhanced mechanical and EMI shielding properties were fabricated by compounding thermoplastic polyurethane (TPU) with flake-shaped nanographite by Xinyang and co-workers.[182] Flake-shaped nanographites act as heterogeneous cell-nucleating agents which provided better cellular structure and EMI SE to the TPU matrix. Ultralight polyurethane foams were prepared from a layer of biomass-derived glucaric acid–chitosan/single-walled carbon nanotubes (SWNTs/GA-chitosan) and a crystalline layer of paraffin. An EMI SE of 56 dB is exhibited by the foam coated with synthesized conductive composite layers. The incorporation of the paraffin layer introduced a shape memory feature to the foam. Details of some important composites with fillers other than those reported in the previous sections are summarized in Table .

Table 9

Summary of Polymer-Based Shielding Materials with Various Fillersa

sample details	filler	Bυ	EMI SE (dB)	SSE (dB cm2 g–1)	σ (S/m)	T (MPa)	k (W/mK)
nacre-mimetic 3D conductive graphene network with biaxial aligned lamellar structure[179]	nacre-mimetic 3D conductive graphene	X	∼65.00	∼100.00	∼5.0 × 10–1
AgNWs/PVB ethanol[180]	AgNWs	C, X, and Ku	59.00
polymer composites with an enhanced 3D EG network[181]	3D EG network	X	52.40		4.0 × 103	10.00
biomass-derived GA-chitosan/SWCNTs and a crystalline layer of paraffin[182]	SWCNTs		56.00	467.00	7.4 × 102
carbon scaffold based on natural wood[183]	carbon scaffolds	X	27.80		1.3 × 101	24.90	0.58
two- and three-phase composites of poly(lactic acid), graphite, and biochar[184]	graphite and biochar	K	30.00	890.00	4.2 × 101
PANI/V2O5[185]	V2O5	Ku	∼17.00–19.00

Bυ, band of frequency; σ, conductivity; SSE, specific EMI shielding efficiency; T, tensile strength; k, thermal conductivity; V2O5, vanadium pentoxide.

From the discussions above, it can be inferred that with the use of special yet complicated architectures like nacre’s multiscale architecture and 3D expansion methods the EMI SE of single-filler polymer nanocomposites can be improved. Shape improvements like cellular and scaffold structures and the use of foams are also found to be beneficial. However, the use of semiconductors alone as filler seems not to work well. The mechanical properties of most of the samples presented in Table are not investigated, which is puzzling as unique architectures like nacre–mimetic composites are expected to give exceptionally good rigidity and toughness.

Conclusions and Future Prospects

In this article, the recent research advances in polymer-nanocomposite-based EMI shielding materials have been reviewed. An overview of EMI shielding performance of polymer nanocomposites with various fillers such as metallic, magnetic, inorganic, and organic carbon nanostructures and hybrid materials has been incorporated along with various ways in which morphological, structural, and processing parameters are modified to optimize the EMI shielding performance. This review gives a clear indication toward the next steps that are to be undertaken for the development of polymer-nanocomposite-based EMI shielding materials for futuristic applications.

One of the major challenges that future technologies have to deal with is the EMI. By now, it is clear that artificial intelligence (AI) based technologies and autonomous systems will play vital roles in human life in the decades to come.[186] The large-scale implementation of such technologies could be seen in the automobile, electronic, space, defense, and biomedical industries, which would necessitate the use of high voltage electronics. This would lead to enhanced EMI that can disrupt applications which are very sensitive to EMI. For instance, in the automobile industry, the performance of electronic technologies like advanced driver-assistance systems is adversely affected by EMI.[187] The main technique used these days to bring down the harmful EMI is to provide electronic equipment with EMI shielding enclosures, usually made of metals. However, due to their heaviness, lack of flexibility, less resistance toward corrosion, and difficulty in tuning EMI shielding effectiveness, scientists are now trying to replace metals with polymer-based EMI shielding materials. Figure shows the number of publications in the past six years in areas related to EMI shielding of polymers/polymer nanocomposites collected using three different keywords. The graph suggests that works on polymer/polymer nanocomposite-based EMI shielding materials are steadily increasing every year and have shown the largest growth in the last lustrum in 2021.

Figure 18

Bar diagram representing the number of publications with three different keyword combinations, i.e., “EMI shielding” together with “Polymer”, “EMI shielding” with “polymer” and “filler”, and “Polymer nanocomposites” with “EMI shielding” in the last 6 years (until February 2022) from the Scopus database.

Pure polymers (insulating or conducting) or their blends filled with one or more conductive fillers such as metals, metal oxides, and various carbon forms are the finest candidates to replace metal-based EMI shielding materials due to their light weight, mechanical flexibility, noncorrosiveness, low environmental degradation, high EMI shielding absorption, and marketable viability. From this review, it becomes obvious that polymer composites with conventional metallic fillers have an EMI SE consistently over 40 dB. However, works on such composites have been continuously decreasing for the past few years as very few options are left to explore in the pursuit of improving their shielding performance. Studies on polymer composites with carbon-based fillers have also hit a high already, which has now shifted the attention of the scientists to MXenes and hybrid fillers. The highest EMI SE obtained for polymer composites with carbon-based fillers in the past few years is 91.9 dB, in which case graphene was used as the filler. However, except for CNTs, no other carbon-based fillers including graphene can be seen to provide EMI SE consistently over 40 dB. Interestingly, the data presented suggest that even with a higher electrical conductivity than CNTs graphene-based fillers fail to provide the polymer nanocomposite with an EMI SE which is desirable for many advanced applications. The lack of sufficient efforts to enhance the EMI SE by adopting steps like proper exfoliation of fillers, optimization of filler concentration to attain a percolation threshold, 3D architecture of conducting pathways, and enhancing polymer–filler compatibility with suitable chemical additives might be the reason behind this. In contrast, hybrid and MXene fillers are found to provide exceptional EMI shielding capability to the polymers. From the literature reviewed here, the highest value of EMI SE reported for composites with hybrid fillers is 130 dB, while that for composites with MXenes is 74 dB. While the consistent good EMI SE performance of polymer nanocomposites with MXene fillers can be attributed to the exceptionally high conductivity of MXenes, the huge EMI SE values shown by samples with hybrid fillers could be due to the synergistic effects of magnetic and conducting fillers and the structural peculiarity endowed to the composite by these fillers. In other words, the hybrid fillers offer conducting pathways of various nature and provide the sample with the structural distinctiveness to elevate the possibilities of multiple internal reflections and absorptions, leading to the dissipation of the incoming signal. Besides, this also gives the opportunity to use multiple mechanisms to shield the incoming radiation; for instance, using the CNT and a magnetic filler together enhances the SE via attenuation of the signal by both magnetic loss and conducting networks. In spite of all these, only a small number of research studies have been reported for the MXene-based ultrathin EM interfaces. Also, many different possible combinations of hybrid fillers and polymers are yet to be studied. Considering the importance of this area of research, all the above said unexplored possibilities could be probed in about a lustrum itself.

The real challenge in the years ahead is restricted not only to enhancing the EMI SE further but also improving the mechanical properties, chemical stability, thermal stability, and thermal conductivities of the polymer nanocomposites, which are important features that futuristic applications require. One of the major problems associated with many research publications reviewed here is that although most of them focus on improving EMI SE very few focus on the aforementioned aspects. The flexibility of polymer composites and their ability to withstand force are basic qualities required for many advanced applications. Therefore, any attempt to improve the EMI SE of polymer nanocomposites ignoring the mechanical performance would be pointless. Examples include the use of EMI shielding polymer nanocomposite materials in aerospace applications. In this case, composite coatings or sheets can be used on the exterior surface and interior passenger cabin to curtail the EMI generated externally by lightning strikes, high intensity radiated fields, etc. and internally by passenger carry-on devices, respectively.[188] For these roles, the EMI shielding materials should have mechanical properties such as high strength, high stiffness, good fatigue resistance, and exceptional fracture toughness to mention a few, since they have to withstand forces like compressive stress, internal tensile stress, and tension.[189] Interestingly, the tables presented in this review point to the fact that out of the 79 articles only 19 report mechanical properties. It can also be noted that their tensile strength (the highest being 112 MPa) and Young’s modulus (the highest being 4.6 GPa) are much smaller than those of aluminum composites and carbon fiber composites, which limit their scope as fuselage alternatives, yet many of them have the potential to be used in other parts of aircrafts like the air-cabin and wings. Therefore, this is still an under-researched area, and more efforts are needed to address the concerns regarding the strength, stiffness, long-term durability, and nonvisible impact damage of polymer nanocomposite-based EMI shielding materials in order to facilitate their large-scale use in the aviation industry. However, the efforts to improve mechanical properties of polymer nanocomposite-based EMI shielding materials will be beneficial not only to the aviation industry alone but also to other industries like automobile manufacturing, space, and defense along with others.

In addition to good mechanical properties, for many EMI shielding applications, it is imperative that the materials which replace metals have high thermal stability and chemical resistance. High thermal conductivity is yet another feature that is required to control the heat generated in many devices. Take, for example, the case of polymer nanocomposite-based EMI shielding material being used in satellite applications. In this case the material might have to withstand temperatures ranging from −200 °C to +350 °C and have to undergo numerous heating and cooling cycles. Therefore, it is imperative that the EMI shielding material is capable of withstanding low and high temperatures and resisting performance degradation during thermal cycles.[190,191] Thus, improving the thermal stability and thermal conductivity of polymer nanocomposite-based EMI shielding materials will definitely be an important area of research in the coming days. Another issue that must be taken care of is that when used in satellites polymers will be exposed to high energy radiations like γ rays and UV rays which might result in their degradation.[192] Also, the atomic oxygen which is abundant in space can corrode the surface of the polymer, and the resulting scattered impingement can trigger the degradation of a sensitive interior surface.[193] Hence, substantial improvements in the structural, mechanical, and thermal properties of polymer nanocomposites are required before they can be used as replacements of metal EMI shields in various applications.

Since a technological proliferation is taking place in the electronic industry, consumer electronic market size is expected to grow rapidly in the coming decades. The resulting use of electronic/electrical systems and technologies on a scale never seen before will eventually end up in a high level of EMI. This concern has resulted in a global effort to promote commercialization of EMI shielding materials, leading to an escalation in EMI shielding market growth. The global EMI shielding market had reached a value of US$ 6.69 billion in 2021. It is expected to reach US$ 8.84 billion by 2027, with a compound annual growth of 4.9% for the period 2022–2027.[194] By the end of 2030, the global EMI shielding market is expected to have an overwhelming hike of US$ 10.6 billion.[195] Governments all across the world are currently contemplating the implementation of stringent regulations to protect the users from EMI which might result in an improved growth rate compared to what is predicted. Therefore, this area of research is set to thrive further in the years to come, leading to the development of improved and innovative technologies which could make the EMI shielding market a potential hub of profitable opportunities. The future research on EMI shielding polymer nanocomposites will strive to fill the previously mentioned gap areas in various ways. Advanced synthesis methods like 3D printing along with other novel synthesis techniques and various improved postsynthesis treatment methods could be used to develop polymer nanocomposites with ameliorated properties. Adjusting polymer–filler compatibility, varying composite architecture, and obtaining synergy between various fillers are also different methods to improve the performance of the material. The next few years will also witness an increase in efforts to develop cutting-edge EMI shielding polymer nanocomposites with self-healing and shape memory capabilities, as these emerging technologies, once fully developed, have the potential to transmogrify the EMI shielding industry.