Applied Science and Convergence Technology 2022; 31(6): 120-127
Published online November 30, 2022
Copyright © The Korean Vacuum Society.
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
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 [1–5]. 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 [6–9]. 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 [10–16]. 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 [17–20]. 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 [1,21–63]. Recently, MXenes have been introduced as promising EMI shielding materials that possess excellent electrical conductivity, hydrophilicity, and chemical activity [2,64–69]. It is well known that the most important factor to improve EMI shielding performance is the electrical conductivity of the core materials . 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.
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) . 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 . 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 . EM waves have also been reported to have detrimental effects on human health [9,70,76,77].
EMI is generally classified based on its frequency or mode of travel . 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 . 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 . 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.
EM waves that impinge on any material surface exhibit four different mechanisms: reflection, absorption, multiple reflection, and transmission . 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 . 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 (
Experimentally, the scattering, or S, parameters (S11, S12 or S21, S22) are directly measured using a network analyzer and yield the reflection (
In addition, the effective absorbance (
Using the reflection and transmission coefficients and the
Total shielding effectiveness (
Reflection is the most important and common mechanism in EMI shielding. It can be expressed as
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 (
Here, d and
Metallic materials have been widely used for EMI shielding for their cost-effectiveness, handleability, and excellent performance [21–41]. Metallic materials in particular are easy to form into film shape through various coating processes, including electroless coating, electroplating, evaporation, and spraying [21–27,40].
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 . One material developed using this process exhibited more than 55 dB of EMI shielding effectiveness (SE) at the 100-1,800 MHz range. Lu
One-dimensional metallic nanofillers (metal nanowires) are also used as conductive filler for EMI shielding applications. Al-Saleh
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 . 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) . 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,
Recently, two-dimensional metallic nanomaterials were introduced as a means of enhancing EMI shielding performance. Choi
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 [42–44]. 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 [50–57], carbon nanotubes (CNTs) [58,59], carbon fibers , and carbon black [61–63] 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 [45–49]. Graphene film , graphene nanosheet , graphene foam , and graphene/polymer composite  have been widely tested and shown to exhibit low density, good flexibility, and excellent absorption-dominant shielding. Shen
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 . 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)] . 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 [79–81]. CNT has already been widely used as a conductive filler for composite type EMI shielding applications. Wu
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 [82–85]. Solid polypropylene/carbon fiber composites were manufactured via injection molding, as shown in Fig. 7(a) . 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 [86–89]. Ghosh and Chakrabarti  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
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 [64,90–93]. MXene is now considered a very promising material for EMI shielding applications . As shown in Fig. 8(a), MXene composite systems have already been studied . 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.
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.
The authors declare no conflicts of interest.