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## Review Paper

Applied Science and Convergence Technology 2022; 31(6): 120-127

Published online November 30, 2022

https://doi.org/10.5757/ASCT.2022.31.6.120

## Recent Progress of Research into Conductive Nanomaterials for Use in Electromagnetic Interference Shields

Ho Kwang Choia , Dong Su Leeb , Sukang Baeb , Byung Joon Moonb , Seoung-Ki Leec , and Tae-Wook Kima , *

aDepartment of Flexible and Printable Electronics, LANL‐JBNU Engineering Institute‐Korea, Jeonbuk National University, Jeonju 54896, Republic of Korea
bFunctional Composite Materials Research Center, Institute of Advanced Composite Materials, Korea Institute of Science and Technology, Wanju 55324, Republic of Korea
cSchool of Materials Science and Engineering, Pusan National University, Busan 46241, Republic of Korea

Correspondence to:twk@jbnu.ac.kr

Received: September 19, 2022; Revised: October 27, 2022; Accepted: November 3, 2022

Since the breakthrough in the field of wireless communication and electronic equipment, a number of materials have been investigated for potential use as electromagnetic interference (EMI) shields. Such shields are critical, as they prevent the detrimental effects of electromagnetic waves on electronics and human health. Metallic materials, carbon-based materials, and MXene have all been studied for incorporation into EMI shielding to improve handleability, processability, efficiency, weight, and cost effectiveness. Developed films exhibit distinct shielding performances rooted in their distinct characteristics, determined by the type of material used, allowing materials to reflect, absorb, or achieve multiple internal reflection of electromagnetic waves. In this paper, we review the state of research concerning EMI shielding.

Keywords: Electromagnetic interference shielding, Metallic materials, Carbon-based materials, MXene

Since commercialization, telecommunications systems including 5th generation (5G) mobile networks and various electronic goods have been widely used; there have been significant considerations of electromagnetic interference (EMI) and radiation on both human beings and operating electronic systems [15]. It is well known that electromagnetic (EM) waves seriously affect normal operation of electronic and electrical equipment as well as people’s daily lives and physical health [69]. There have been a lot of approaches to prevent emission of or shield humans and systems from the harmful effects of EM waves. Among studied methods, conventional metallic materials (e.g., silver, copper, iron, nickel, and their alloys) have been used as EMI shielding materials, exhibiting excellent EMI shielding performances. Recently, research on transparent and flexible EMI shielding films has drawn great attention due to the wide range of potential applications in flexible or soft electronic systems [1016]. Electrically conductive fillers have been widely used as core materials of composite type EMI shielding, which is composed of metallic materials, carbon materials, and MXenes in the polymer matrix [1720]. Especially, both metallic and carbon-based nanomaterials have been very widely used in EMI shielding materials due to their light weight, low cost, and excellent mechanical and EMI shielding properties [1,2163]. Recently, MXenes have been introduced as promising EMI shielding materials that possess excellent electrical conductivity, hydrophilicity, and chemical activity [2,6469]. It is well known that the most important factor to improve EMI shielding performance is the electrical conductivity of the core materials [17]. Several recent studies reveal that the internal structure of EMI shielding film is a critical variable modulating the shielding properties [2,40,41]. Because conductive nanomaterials have different dimensions and shapes, they form various internal structures and geometrical shapes when processed into films. Considering EMI shielding mechanisms (e.g., reflection, absorption, and multiple internal reflection) [1,2,30,35,37,40,41,57,69], the tailoring of the internal structure enables multiple internal reflections to be induced, meaning that repeated reflection occurs inside the shielding film.

In this review, we discuss EMI shielding mechanisms and capabilities depending on the types of materials, including metal, carbon, and MXene based conductive fillers. We also summarize various strategies to achieve high-performance, lightweight, and cost-effective conductive materials for EMI shielding applications.

### 2.1. Electromagnetic interference

EM waves are composed of an electric field (E) and a magnetic field (H); these two components are perpendicular to each other, as shown in Fig. 1(a) [17,70,71]. EM waves propagate perpendicular to the plane that contains the two field components [17,70,71]. EM waves propagate in either a vacuum or a medium, and can be classified into various types according to their frequency (i.e., radio waves, microwaves, visible rays, ultraviolet rays, X-rays, gamma rays, and etc) [18]. It is well known that the composition of the EM band is due to electronic devices operating in various frequency ranges [72,73]. EM bands are widely used in, for example, mobile electronic systems, wireless communication systems, and computer networks [74]. On the other hand, EM waves induce harmful effects on electric circuits due to EM conduction or EM radiation emitted from external sources; this phenomenon is generally called EMI [75]. EM waves have also been reported to have detrimental effects on human health [9,70,76,77].

Figure 1. (a) Schematic diagram of propagation of electromagnetic waves. Reprinted with permission from [71], Copyright 2009, John Wiley and Sons. (b) Schematic diagram of EMI shielding mechanism. Reprinted with permission from [78], Copyright 2015, Springer Nature.

EMI is generally classified based on its frequency or mode of travel [75]. It can be further classified as either conducted or radiated interference depending on the mode of movement. Conducted EMI refers to EM waves that propagate through electronic devices and external connections, while radiated EMI refers to EM waves emitted by devices with interconnected wiring or cables [75]. EMI can be additionally categorized, based on frequency, into broadband or narrowband interference, with broadband interference encompassing a wide frequency range. This can be further subdivided into impulse or random noise [75]. Various forms of EMI can cause electronic equipment to receive a noise signal rather than the desired signal, creating a situation in which the device cannot distinguish the desired signal. This phenomenon, a result of overcrowding of frequency bands, causes interference between devices operating within shared or adjacent frequency bands. This in turn can lead to errors in device operation. The traditional way in which EMI shielding has been utilized is as bulk metal. Recently, with the development of mobile communication, research on EMI shielding has developed in various ways according to application limitations of bulk metal. Accordingly, research on various shielding materials that may improve shielding performance, despite its increasingly thin nature and lighter weight, has picked up.

### 2.2. EMI shielding mechanism

EM waves that impinge on any material surface exhibit four different mechanisms: reflection, absorption, multiple reflection, and transmission [17]. The measurement of shielding efficiency as these mechanisms operate is expressed using data related to attenuation or loss. This can be explained according to the intensity of EM signal changes with and without shielding, and is defined as the ratio of the electric field strengths with and without shielding [18]. The coefficient of the EMI shielding mechanism is expressed in the power balance equation, where the incident power (1) is the sum of the transmission (T), reflection (R), and absorption (A), as shown in Fig. 1(b) [78].

Experimentally, the scattering, or S, parameters (S11, S12 or S21, S22) are directly measured using a network analyzer and yield the reflection (R) and transmission coefficient (T), as in

$R=S112=S222$
$T=S122=S212$

In addition, the effective absorbance (Aeff) can be represented as

$Aeff=1−R−T1−R$

Using the reflection and transmission coefficients and the Aeff, SER, and SEA can be expressed as

$SER=10log11−R=10log11−S11 2$

Total shielding effectiveness (SET) is the sum of contributions due to reflection (SER), absorption (SEA), and multiple reflection (SEM). Multiple reflections are considered as absorption in many cases because multiple internal reflections of double EM waves are absorbed or dissipated as heat in the shielding material. So, the total SE (SET) can be rewritten as

$SET=SER+SEA$

Reflection is the most important and common mechanism in EMI shielding. It can be expressed as

$SER=20logZW4ZM=20logη+η0 24ηη0=39.5+10logσ2fπµ$

Here, ZW is the wave impedance of air, ZM is the wave impedance of the material, η and η0 are the impedances of the shield and air, σ is the conductivity, f is the frequency, and µ is the relative permeability. As can be seen from the above Eq. (8), the reflection loss is attributed to the relative impedance mismatch between the shielding material and the air. This impedance difference is caused by mobile charge carriers (electrons and holes) on the surface of the material, indicating how important the conductivity of the shielding material is in shielding by reflection.

Absorption is the secondary mechanism of EM attenuation. Absorption loss (SEA) in decibels (dB) for conductive materials is written as

Here, d and a represent the thickness and the attenuation constant, respectively. The attenuation constant is defined as the degree to which the intensity of EM radiation decreases as it passes through a material. The absorption loss is influenced by three factors, viz., conductivity, sample thickness, and permeability of the material. Additionally, multiple reflections, the third shielding mechanism, refer to the reflections of incident EM waves at multiple internal surfaces or interfaces within the shielding material. This means that the internal structure of the shielding materials is the major factor enhancing the SEA value of certain shielding materials. To achieve efficient shielding films by using conductive nanomaterials, one should consider both the conductivity of the nanomaterials and the internal structure of the film formed by those materials.

### 3.1. Metallic materials for EMI shielding

Metallic materials have been widely used for EMI shielding for their cost-effectiveness, handleability, and excellent performance [2141]. Metallic materials in particular are easy to form into film shape through various coating processes, including electroless coating, electroplating, evaporation, and spraying [2127,40].

Wang et al. [21] reported a high-quality copper layer on an epoxy resin deposited by electroless plating with potential application as an EMI shield [Fig. 2(a)]. After 40 minutes of electroless copper plating, the deposited copper layer exhibited a low resistance of 2.24 × 10−6 Ω cm and an acceptable EMI shielding effect of over 60 dB within a frequency range of 4 to 18 GHz. Thiosulfate was used to modify the epoxy resin substrate surface, promoting adhesion between the epoxy resin and the deposited copper layer. Building on this work, a few researchers have reported metallic film patterning for transparent EMI shielding applications. Liang et al. [22] reported the creation of highperformance transparent EMI shielding by an ultra-thin metal nanomesh fabricated using an easy and reasonable process that employed ultraviolet lithography and ion-beam etching technology. The primary advantages of this type of EMI shielding film are its transparency and flexibility. More specifically, performance was adjustable via control of line width and spacing. The optimized copper base metal mesh showed an average transmittance of 83.2 % and a shielding efficiency of over 40 dB. Jiang et al. [23] demonstrated a high-performance transparent EMI shielding film based on a unique freestanding nickel (Ni) metal mesh. This ultra-thin, lightweight, stand-alone transparent mesh showed excellent optoelectronic properties (shielding effect approx. 40 dB, transparency 92 %).

Figure 2. (a) Schematic illustration of synthesis and adsorption-modified epoxy resin substrate and electroless copper plating process. Reprinted with permission from [21], Copyright 2017, The Royal Society of Chemistry. (b) Schematic for the preparation of a P@Ni-Co hybrid membrane. Reprinted with permission from [26], Copyright 2019, Elsevier. (c) Fabrication process of AF@Ni/Cu/Ni by electroless plating. Reprinted with permission from [27], Copyright 2021, Elsevier.

Electroless metal plating was introduced to facilitate the development of high-quality EMI shielding fibers. In this process, shielding performance is controlled by modulating various pretreatment conditions, including refining, etching, and catalyst [24]. One material developed using this process exhibited more than 55 dB of EMI shielding effectiveness (SE) at the 100-1,800 MHz range. Lu et al. [25] reported deposition of electroless copper onto chemically grafted modal fabrics (MFs). These copper coated fabrics were smooth, had high electrical conductivity, and had a value of 50.5 dB of EMI SE at 100 MHz. Similarly, metal coated fibers have been prepared by combining electrospinning technology with the electroless deposition process. As shown in Fig. 2(b), a light and flexible Ni-Co alloy coated PAN-PU (P@Ni-Co) nanofiber membrane has already been fabricated and utilized as an effective EMI shielding material [26]. Due to its remarkable conductivity of 1,139.6 S/cm and a satisfactory saturation magnetization value of 49.6 emu/g, this P@Ni-Co hybrid membrane exhibited more than 68 dB of EMI SE across a wide frequency band. Additionally, an aramid fiber (AF) with a composite metal shell composed of amorphous Ni, crystalline Cu and Ni was previously reported Fig. 2(c) [27]. This Ni/Cu/Ni coated AF demonstrates high shielding of 77.8 dB across a wide frequency range (X-band, Ku-band, and K-band). In this design, the multiple interfaces of the fiber (AF/Ni, Ni/Cu, and Cu/Ni) enhance the reflection and absorption of any EM radiation. Processible metallic nanomaterials have been used as conductive filler materials (e.g., metal particle and metal nanowire) for EMI shielding films. Arranz-Andrés et al. [28,29] studied an EMI shielding fabricated using PVDF polymer and its composites with different concentrations of Cu or Al nanoparticles, as shown in Fig. 3(a). Depending on the kind of nanoparticles used, the composite film exhibited 110 or 19 dB of EMI SE at concentrations of 62.5 wt% for Cu and 26.8 wt% for Al, respectively. Especially, the EMI shielding performance of the composite film increased proportional to the concentration of Al filler. However, it showed a saturated EMI SE value above concentration of 26.8 wt% in the case of the Al nanoparticle. As a conventional conductive metal filler, silver nanoparticles (Ag NPs) are also already widely used for EMI shielding applications. Figure 3(b) shows a highly conductive and flexible EMI material comprised of a Ag NPs/elastic polymer composite [30]. The composite film with silver Ag NPs of 66.5 wt% exhibited about 69 dB EMI SE in the X-band range (8-12 GHz). As the amount of silver Ag NPs increased, EMI shielding performance rose, implying that electrical conductivity is a key determinant of EMI shielding. Liu et al. [31] reported a lightweight, high-performance EMI shielding composite developed by coating metal nanoparticles onto a leather matrix (LM) [Fig. 3(c)]. Although the metal/LM membranes contain only 4.58 wt% of metal nanoparticles, this material showed excellent EMI shielding properties of 70.6 and 76.2 dB for Cu and Ag nanoparticles (Cu NPs and Ag NPs), respectively, at 3.0 GHz. High-efficiency EMI shielding film has also been fabricated by electroless deposition of Ag NPs on polydopamine (PDA) functionalized cellulose nanofibrils (CNFs) [Fig. 3(d)] [32]. These authors also reported that this material had excellent electrical conductivity of 1,000,000 S/m when used with CNFs@PDA@Ag NPs film (weight ratio of CNFs:AgNO3 = 1:24), and exhibited EMI SE of 93.8 dB at 8.2 GHz in the X band range.

Figure 3. (a) SEM images of PVDF/Cu and PVDF/Al hybrids composed of different contents of Cu and Al nanoparticles. Reprinted with permission from [28], Copyright 2012, Elsevier. Reprinted with permission from [29], Copyright 2013, Elsevier. (b) Schematic illustration of the fabrication of Ag NPs/SBS. Reprinted with permission from [30], Copyright 2016, The Royal Society of Chemistry. (c) Schematic illustration of proposed shielding mechanism of metal/LM membrane (left), EMI SE performances of Cu/LM and Ag/LM membranes in frequency range of 0.01−3.0 GHz (right). Reprinted with permission from [31], Copyright 2018, American Chemical Society. (d) Fabrication schematic of CNFs@PDA@Ag NPs EMI shielding film. Reprinted with permission from [32], Copyright 2020, Elsevier.

One-dimensional metallic nanofillers (metal nanowires) are also used as conductive filler for EMI shielding applications. Al-Saleh et al. [33] reported an EMI shielding film composed of copper nanowire (Cu NWs)/polystyrene (PS) composite which exhibited 35 dB of EMI SE at the X-band range using 210 µm thick composite films that contained 2.1 vol.% Cu NWs. This is a notable example of an EMI shield that uses metal nanowires and porous structures. Lightweight polypropylene/ stainless steel fiber (PP-SSF) composites have been fabricated using foam injection molding [34]. An EMI SE value of 47.6 dB was achieved using PP-1.5 vol% SSF composite foam. Ma et al. [35] reported ultralight silver nanowire (AgNW) hybrid polyimide (PI) composite foams with a microporous structure and a low density of 0.014- 0.022 g/cm3. This team used a one-pot liquid foaming process to develop their nanowires, shown in Fig. 4(a). Their composite foam, with a loading weight of 20.5 wt% (0.044 vol %), displayed an EMI SE of 45-16 dB across a frequency range of 30.0-1.5 GHz. Due to the effectiveness of the internal structure of the EMI shielding film, there have been a few efforts to evaluate the validity of internal multiplereflections originating from the porosity of the film. Zeng et al. [36] reported water-based polyurethane (WPU)/Ag NW composites fabricated by a simple freeze-drying process, as shown in Fig. 4(b). The nanocomposite films were developed by controlling the amount of Ag NWs. The EMI SEs of the nanocomposite were 64 and 20 dB at densities of 45 and 8 mg/cm3, respectively. According to the Maxwell- Wagner-Sillars polarization principle [7], the conductivity mismatch between the conductive fillers and the polymer matrices in the composites will cause polarization and charge accumulation at the interface between the two materials. In the case of WPU/AgNW nanocomposites, the interaction of AgNW and WPU leads to high charge storage capacity at the interface between AgNW and WPU matrix, which enables absorption of incident EM waves by interfacial polarization of the electric field; the aligned cell wall promoted multiple reflections of waves throughout the porous structures, promoting absorption and rendering this material suitable for use as an EM shielding material.

Figure 4. (a) SEM images of Ag NWs/PI nanocomposite foam sample, specifically EMI SE of Ag NWs/PI nanocomposite foam, with respect to nanowires contents at 200, 600, and 1000 MHz, as well as EMI SE of Ag NWs/PI nanocomposite foam before/after etching or spraying. Reprinted with permission from [35], Copyright 2014, American Chemical Society. (b) Schematic illustration of preparation of unidirectional porous WPU/Ag NW nanocomposites and porous nanocomposites with unidirectional pores, and SEM images of nanocomposites in the X-Z, Y-Z, and X-Y planes (scale bars are 100

Super-stretch and self-healing hydrogel composites have been prepared by in situ polymerization of acrylamide (AAm) and N-acryloyl- 11-aminoundecanoic acid (A-11) on Ag NWs [37]. Additionally, hydrogel composites have shown self-healing capacity (EMI SE healing efficiency 90 %) originating in their reversible hydrophobic bonding and hydrogen bonding interactions. As a conductive filler, metal nanowires have also been used in transparent electrodes. Similarly, a flexible and transparent EMI shielding film has been fabricated using silver nanofibers, as shown in Fig. 4(d) [38]. This film (1 µm thick) exhibited ~20 dB of EMI SE with 89 % transmittance in the visible range. When the thickness of Ag NF film reached 10 µm, SETand the transmittance improved to ~50 dB and 75 %, respectively. Cu nanowires (Cu NWs) have also been used as conductive fillers in transparent and flexible EMI shielding film, as shown in Fig. 4(c) [38]. A PES/Cu NWs/PET sandwich-structured films showed an EMI SE of 30 dB across the entire X band with optical transmittance of 67 %.

Recently, two-dimensional metallic nanomaterials were introduced as a means of enhancing EMI shielding performance. Choi et al. [40] reported a hierarchically structured porous Cu film composed of a single-crystalline metallic nanosheet [Fig. 5(a)]. Hierarchical porous Cu film was prepared by simple assembly of single-crystalline, nanometer-thick, and micrometer-long copper nanosheets. By spray printing the Cu nanosheets, the Cu film formed a hierarchically structured porous Cu film with multilayer stacking, two-dimensional networking, and a layered, sheet-like void architecture. The porosity and density of the prepared Cu film were 75−66 % and 2.3−3.0 g/cm3, respectively. As a result of the layer-by-layer assembly of the Cu nanosheets (Cu NSs), the hierarchically porous Cu film exhibited EMI SE values of 100 and 60.7 dB at 15 and 1.6 µm thickness, respectively, proving that the hierarchical internal structure of the porous Cu film and multiple interactions of incident EM waves resulted in robust radiation absorption, and suggesting a way forward for the design of EMI shielding. Similarly, Sheng et al. [41] reported on EMI shielding efficiencies of printed Cu nanomaterials with the control of its conduc- Figure 5. (a) Cross-sectional SEM image of porous Cu NSs film polyimide substrate with loading weight of 1.8mg/cm2. Electrical conductivity and corresponding EMI SE of porous Cu NS films with various Cu NS loading weights. Reprinted with permission from [40], Copyright 2021, American Chemical Society. (b) Schematic of EMI shielding process of layered copper nanoplates, relationship between EMI shielding, electrical conductivity of layered copper nanoplates film. Ternary EMI shielding plot of EMI shielding films with different mixture ratios of Cu NP, Cu NW, and Cu NPL. Reprinted with permission from [41], Copyright 2022, American Chemical Society. tivities and nanostructured. By increasing the electronic conductivity of the printed Cu film, optimal EMI SE was found to be 65 dB. These authors summarized the relationship between conductivity and architecture in printed copper nanomaterials with an EMI SE decagram, shown in Fig. 5(b). This showed the geometry effect of Cu nanostructures (nanoplates, nanowires, and nanoparticles) on the EMI SE of the printed Cu film, and suggested that the contribution of the Cu nanoplates was a result of their multiple internal reflections and absorption of EM waves.

### 3.2. Carbon materials for EMI shielding

Like their metallic nanomaterial counterparts, carbon-based nanomaterials and their composite materials have been intensively studied for potential EMI shielding use, primarily for their uniquely high electrical conductivity, light weight, high surface area, good flexibility, low density, chemical stability, and good mechanical properties [4244]. It is well known that the microstructures and compositions of carbonbased nanomaterials play important roles in achieving enhanced EMI shielding performance. Representative carbon base nanomaterials such as graphene [5057], carbon nanotubes (CNTs) [58,59], carbon fibers [60], and carbon black [6163] have been extensively reported for possible use as EMI shields. Graphene and its composites in particular have been examined as likely to achieve success as EMI shielding films due to their relatively high conductivity, processing advantages, corrosion resistance, high specific surface area, and good stability [4549]. Graphene film [50], graphene nanosheet [51], graphene foam [52], and graphene/polymer composite [53] have been widely tested and shown to exhibit low density, good flexibility, and excellent absorption-dominant shielding. Shen et al. [54] reported graphene oxide (GO) prepared by direct evaporation of a GO suspension under mild heating and graphite-like graphene films produced by graphitizing the GO films, as shown in Fig. 6(a). The graphene film achieved 20 dB of EMI SE at a thickness of 8.4 µm. Zhang et al. [55] reported novel graphene structures, such as graphene pellets and papers. The graphene pellets were synthesized by chemical vapor deposition (CVD) using inexpensive nickel powder as a catalyst. Graphene pellets were then pressed and further processed into graphene paper. Graphene paper has a high electrical conductivity of up to 1,136 ± 32 S/cm and exhibited an EMI shielding effect of 60 dB at a thickness of 50 µm. Porous graphene foam was then prepared by hydrazine foaming process [56]. Despite its low electrical conductivity (~3.1 S/cm), the prepared graphene foam had a layered graphene structure and, across the entire frequency range, exhibited an improved EMI SE of ~26.3 dB, which was more than that of graphene film (~23.7 S/cm, ~20.1 dB). The significance of this result suggests that the change of the layered graphene film into a porous graphene foam leads to improved EMI shielding, as does the improved internal multiple reflection formation within the large cell matrix interface, which is the result of the presence of microporous structures.

Graphene has also been used as a filler material for composite type EMI shielding applications. A graphene/PDMS foam composite has been shown to work as an effective EMI shield [1]. A composite with a density of 0.06 g/cm3 was shown to possess SE values as high as 30 and 20 dB in the frequency ranges of 30 MHz–1.5 GHz and the Xband, respectively.

Ultralight, high-performance EMI shielding graphene foam (GF)/poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) composites have been developed by drop coating PEDOT: PSS onto cellular-structured freestanding GFs [Fig. 6(b)] [57]. Their ultralight porous structure and effective charge delocalization allowed the composites to exhibit electrical conductivity of 43.2 S/cm and EMI SE of 91.9 dB. Notably, this is the highest SE value among developed carbon-based polymer composites.

CNTs and their composites have also been studied for application as EMI shields due to their high aspect ratio, strong mechanical properties, good conductivity, chemical inertness, low density, and easy processability [7981]. CNT has already been widely used as a conductive filler for composite type EMI shielding applications. Wu et al. [58] reported CNT macrofilms prepared on Al foil by CVD [Fig. 6(c)]. A 4.0 mm thick CNT macrofilm composed of CNT bundles and iron oxide nanoparticles encapsulated in a thin graphite carbon shell showed an EMI SE of 61-67 dB. Lee et al. [59] reported lattice and parallel patterns of emissible multi-walled carbon nanotubes (MWNTs) with BaTiO3 that exhibited a maximized EMI SE up to 20 dB [Fig. 6(d)]. The directionality and motion of the incident EM waves was controlled by tailoring the rotation of the alignment of the MWNTs.

As a one-dimensional conductive carbon material, conventional carbon fiber is also already widely used for EMI shielding applications, as it is lightweight, flexible, and possesses high electrical conductivity [8285]. Solid polypropylene/carbon fiber composites were manufactured via injection molding, as shown in Fig. 7(a) [60]. At 10 vol.% CF, the SE of the foamed composites reached approximately 24.9 dB in the X-band frequency range. Solid composites with the same CF content presented an SE of about 19.8 dB. Foaming the solid samples reduced the density of the composite by 25 % and the electroosmotic threshold from 8.5 to 7 vol% fiber loading; these improvements originated from the improved interconnectivity of carbon fibers in the composite.

Conductive carbon blacks, conventional nanocarbons, have also been used as nanofillers [8689]. Ghosh and Chakrabarti [61] studied the effects of incorporating conducting carbon black as filler on some selected physical and mechanical properties, as well as the EMI shielding character of vulcanizated ethylene-propylene diene monomerbased compounds. After examining the electrical conductivity of the composite by changing the loading amount of carbon black from 0 to 60 %, they found that the percolation threshold was 15-35 % of the carbon black filler in the matrix. To achieve a meaningful EMI SE value of the composite sample, it required the loading of at least of 30-40 % carbon black. The composite exhibited 8 dB of EMI SE at more than 50 % of loading of carbon black in the composite. Rahaman et al. [62] studied the EMI shielding performances of various conductive composites composed of two different types of conductive carbon blacks (Conductex and Printex XE2) filled in matrices such as ethylene–vinyl acetate copolymer, acrylonitrile–butadiene copolymer and their different blends. They found that both the conductivity and the EMI SE of the conductive composites increased as more conductive carbon black filler was loaded. This suggested that the formation of a closed packed conductive network was the most important factor in achieving an efficient EMI shielding performance [Fig. 7(b)]. Similarly, Ravindren et al. [63] reported a flexible composite system that blended ethylene methyl acrylate (EMA) and ethylene octane copolymer with conductive carbon black. The selective distribution of filler Vulcan-XC72 conductive carbon black in the EMA phase facilitated the way for and EMI shielding of the composites [Fig. 7(c)].

### 3.3. MXene based materials for EMI shielding

MXene has attracted a great deal of attention recently for its promising material properties, such as its high electrical and thermal conductivity, tunable electronic band gap, magnetic ordering, and excellent mechanical strength [64,9093]. MXene is now considered a very promising material for EMI shielding applications [65]. As shown in Fig. 8(a), MXene composite systems have already been studied [64]. A 45 µm thick Ti3C2Tx film is shown to have exhibited an EMI SE of 92 dB (>50 dB for a 2.5 µm film). Such outstanding EMI shielding performance is the result of the superior electrical conductivity of the Ti3C2Tx film (4,600 S/cm) and multiple internal reflections of the Ti3C2Tx flakes within the free-standing film.

Liu et al. [2] introduced a hydrazine-induced foaming process to produce a porous and hydrophobic MXene foam that exhibited almost 70 dB of EMI SE, a higher SE than that of the control sample that was not formed through the foaming process (which showed a value of 53 dB). Additionally, heat treatments have been carried out to improve the electrical conductivity of Ti2CNTx [66]. After thermal annealing, both electrical conductivity and EMI SE increased from 1,125 to 2,475 S/cm and 61 to 116 dB, respectively. Zhang et al. [67] reported a highly aligned Ti3C2Tx MXene film using large MXene flakes and a scalable blade coating process [Fig. 8(b)]. They demonstrated remarkable tensile strength of up to 570 MPa using a 940 nm thick film and achieved electrical conductivity of 15 × 100 S/cm using a 214 nm thick film. Their film also exhibited superior EMI shielding performance (50 dB for the 940 nm thick film), surpassing that of other synthetic materials of similar thickness. Recently, Yun et al. [68] reported a systematic study of the EMI shielding behavior of 2D Ti3C2Tx MXene assembled films of various thicknesses. Ultimately, one monolayer assembled MXene film exhibited nearly 20 % shielding from EM waves. By assembling MXene layers, monolayers up to 30 layer were formed that showed 20 dB of EMI SE for a 24 layer film [Fig. 8(c)]. Beyond using MXene for EMI shielding film, Sun et al. [69] attempted to use MXene as a conductive filler material to produce highly conductive polymer nanocomposites. Conductive MXene samples on polystyrene nanocomposites were prepared by electrostatic assembling of negative MXene nanosheets on positive polystyrene microspheres. Due to the high electrical conductivity of MXene, the nanocomposites possessed an efficient conducting network within their polystyrene matrix after the compression molding process. The nanocomposites therefore showed a low percolation threshold of 0.26 vol%, which allowed high conductivity and EMI shielding performance of 1,081 S/m and 54 dB, respectively, in the X-band range.

EMI shielding is essential to electronic devices, and the development of new shielding materials will impact fields as diverse as communication, aircraft, biomedical, computing, space exploration, and military technology. Electronic devices and strategic systems require lightweight EMI shielding films that can be produced efficiently and cheaply. In this literature review, we discussed the capabilities of various EMI shielding mechanisms developed using metal, carbon, and MXene-based conductive fillers. We also summarized various strategies by which high-performance, lightweight, and cost-effective conductive materials can be built. While a great deal of research concerning EMI shielding materials has already been done, much work remains to be done concerning how best to control material shape, dimensions, and percolation.

This research was supported by the National University Development Project at Jeonbuk National University in 2021.

### Conflicts of Interest

The authors declare no conflicts of interest.

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