Review of carbon-based electromagnetic shielding materials: film,composite,foam,textile

Abstract

Carbon-based electromagnetic shielding materials are reviewed in terms of their performance, type, and preparation. They include film, composite, foam, and fabric with particular attention on their frequency selectivity ascribed to the periodic structure. The SE/t, referring to shielding effectiveness per unit thickness (dB/mm), and SSE, referring to shielding effectiveness per unit density (dB·cm³/g), are summarized. The main conclusions of this work are as follows: (1) large area film shows higher SE/t, in which carbon nanotube (CNT) film is endowed with the most attractive value (19,500 dB/mm); materials containing CNTs achieve higher shielding efficiency, ascribe to a high specific surface area, have a greater length–diameter ratio, and a one-dimensional continuous-oriented structure; (2) notably, frequency selectivity based on varied period structures has been widely studied; the method includes multilayer structure/printing/cutting/backfilling and, especially, woven fabric; (3) favorable shielding effectiveness is attributed to the shielding material's intrinsic electrical conductivity and structural integrity. Based on these developments, this paper aims to provide some valuable data, highlight the important research direction, and advance the development of carbon-based electromagnetic shielding materials.

Keywords

electromagnetic interference shielding carbon-based shielding effectiveness periodic structure frequency selective

Electromagnetic interference (EMI) shielding material has caught the attention of researchers, because electromagnetic radiation often interferes with electronic devices, such as electronic products, radar, and wireless communication devices, resulting in abnormal operation and can even be harmful to the human body.^1–7 Electromagnetic pollution and EMI occur when the strength of electromagnetic radiation exceeds the range that the human body or device can withstand.⁸ Recently, the widespread development of gigahertz electronic systems and telecommunications devices has pushed electromagnetic pollution to an unprecedented level; hence, EMI shielding has undoubtedly come to be a basic requirement for innumerable electronic elements devices and equipment and as an immediate area of research focus.^9,10

The principle of EMI shielding is that shielding materials reflect or absorb electromagnetic radiation to achieve a shielding effect.¹⁰ Electromagnetic waves, also known as electromagnetic radiation, are transverse waves, existing in an alternating electromagnetic field, wherein the electric field and magnetic field are perpendicular to the wave propagation direction and change with time and space periodically.¹¹ In 1864, a complete theory of electromagnetic waves was established by the British scientist Maxwell and it was found that electromagnetic waves and light share the same speed of propagation, while the existence of electromagnetic waves was confirmed experimentally by the German physicist Hertz in 1887. Electromagnetic waves from low frequency to high frequency are radio waves, microwaves, infrared rays, visible light, ultraviolet rays, X-rays, and gamma rays. Among these, X-rays and gamma rays belong to high-frequency electromagnetic radiation, and high-frequency electromagnetic radiation is also called ionizing radiation. Ionizing radiation is able to cause ionization phenomena, which are widely used in biomedical fields involving radiodiagnosis and radiotherapy.¹² Ionizing radiation shielding materials are largely high-density materials, such as lead bricks and high-density concrete.¹³ However, humans are susceptible to non-ionizing radiation within our everyday life.¹⁴ Therefore, the electromagnetic shielding materials reviewed in this paper are targeted at non-ionizing radiation.

Electromagnetic shielding materials

The most common electromagnetic shielding materials include metal,^15,16 conductive polymers based on electrical additives or intrinsically,^17,18 and carbon-based materials.^19–67 Metal is the most conventional electromagnetic shielding material. However, ascribable to its heavy, expensive, rigid, and corrosive nature, it has been gradually eliminated.^68–70 Conductive polymers based on electrical additives are thought to be an alternative to metal materials in terms of light weight, flexibility, and decay resistance.⁷¹ Nevertheless, conductive polymer composites retain a poor high temperature resistance.⁷²

Carbon-based materials, such as graphene (GN), carbon nanotubes (CNTs), carbon fiber (CF), and carbon black (CB), have developed into one of the most popular components in electromagnetic shielding, by reason of their high electrical conductivity, good mechanical performance, lightweight, flexibility, and large aspect ratio.⁷³ The application of CB and CF came earlier. The chain-like aggregation structure of CB particles forms a stronger conductive network than that of other carbon-based materials and, at the same time, the higher filler content leads to little combining with the polymer matrix.⁷⁴ CF has become an attractive shielding material due to its high specific strength and high conductivity. However, the absorption capability of CF is poor, which results in poor further attenuation of electromagnetic waves when used as a single medium.^75,76 Therefore, CF is commonly combined with dielectric fibers or a matrix to achieve better EMI shielding effectiveness (SE). In recent years, CNTs and GN have received extensive attention due to their great aspect ratio and excellent transmission performance.⁷⁷ CNTs can be viewed as curled GN sheets, including single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs).⁹ GN is a single layer of carbon atom flakes firstly obtained by British scientists Andre Geim and Konstantin Novoselov through mechanical peeling method in 2004, which retain fantastic electrical conductivity and mechanical properties.⁷⁸ The application of carbon materials to electromagnetic shielding can be traced back to 1989. Chiou et al.⁷⁹ demonstrated that adding CF to cement can enhance its SE. Figure 1 shows the search results of carbon-based electromagnetic shielding materials archived on the Web of Science in the past five years.^80–85 The research of carbon-based electromagnetic shielding materials is booming, with a total of 1286 retrieval results in 2019, which is far more than that of metal and conductive polymer shielding materials. In addition, the diverse types of carbon-based electromagnetic shielding materials, such as film, composite, foam, and textile, lead to their different advantages and characteristics. Film is well known to be a good choice for EMI shielding because of its ultrathin plane structure, lightweight, ﬂexibility, and simple preparation process.¹⁹ The composite attributes that have drawn most attention are high strength and modulus and corrosion resistance.⁸⁶ Foam is widely used in electromagnetic shielding to ascribe lightness and a large surface area.⁸⁷ On account of the softness, flexibility, breathability, low cost, and simple preparation of textile, it has developed into a research hotspot for electromagnetic shielding materials.^88–90 Herein, we summarize the electromagnetic shielding properties of some carbon-based materials in terms of their performance, structure, type, and preparation.
Figure 1.
Development trend of carbon-based shielding materials in the last five years. GN: graphene; CNT: carbon nanotube; CF: carbon fiber; CB: carbon black.

Mechanisms of shielding

The EMI SE depends on many factors, such as conductivity, permeability, and structure, attributed mainly due to EMI shielding based on absorption, reflection, and transmission. Among them, the reflection and absorption of electromagnetic waves are the main electromagnetic attenuation mechanisms of the EMI shielding effect.^91,92 Figure 2 displays a schematic illustration of the EMI shielding mechanism.⁹³
Figure 2.
Schematic illustration of the electromagnetic interference shielding mechanism.⁹³

The reflection coefficient (R) and transmission coefficient (T) are obtained by the S parameter, and the calculation formula is as follows
$R = | S_{11} |^{2} = | S_{22} |^{2}$
(1)

$T = | S_{12} |^{2} = | S_{21} |^{2}$
(2)

Figure 3 demonstrates the two-port network and the S parameter (scattering parameter) matrix, where |S₁₁|² represents the reflected power from port 1 to itself and |S₁₂|² represents the transmitted power from port 2 to port 1.⁹⁴
Figure 3.
Two-port network and the S parameter (scattering parameter) matrix.

The absorption coefficient (A) is given by
$A = 1 - R - T$
(3)

The propagation loss formula of the electromagnetic wave is as follows
$Loss (dB) = - 10 log_{10} (\frac{P}{P_{i}})$
(4)
where P is the output power of an electromagnetic wave and P_i is the receiving power of an electromagnetic wave.

The electromagnetic shielding efficiency is calculated as follows
$SE = - 10 \lg T$
(5)

Carbon-based electromagnetic shielding materials

Herein, carbon-based shielding materials are classified structurally. Figure 4 shows the search results of different types of electromagnetic shielding materials based on the Web of Science in the past five years.^95–98 The studies related to textiles show ∼1600 retrieval papers in 2019, which is a great deal more than the studies on film, foam, and composites. This result can be ascribed to the light weight, flexibility, and other advantages of textiles. In addition, the booming research on flexible and wearable electronics devices is an important factor.^99–102
Figure 4.
Development trend of different types of carbon-based electromagnetic shielding materials in the past five years.

Recently, frequency selectivity, based on varied period structures, has been extensively investigated. Frequency selective surfaces (FSSs) are related to two-dimensional periodic arrangement structures with electromagnetic wave filtering characteristics.¹⁰³ In previous studies, most FSS structures were made of metal, which makes it difficult to meet the lightweight requirements of new wave-absorbing materials.^104,105 Therefore, new FSSs have been recently thrived based on conductive polymers, CF-reinforced composite mesh, or frequency selective fabrics (FSFs).¹⁰⁶ Here, whether film and composites have periodic structures or not is reviewed separately.

Film

Film is proverbially a good option for electromagnetic shielding due to its ultrathin plane structure, light weight, and simple preparation process.^19,107 To date, several methods of preparing thin film have been developed, including vacuum ﬁltration,^20,27 microwave irradiation,²¹ centrifugal evaporating,^22,24 electrophoretic deposition,¹⁹ chemical vapor deposition (CVD),²⁵ and layer-by-layer (LbL) assembly.^23,33,39 Film was discussed with respect to electromagnetic shielding properties and with special attention to the periodic structure.

Aperiodic structure

Films are mainly combined with other materials, such as ferrites and metals, to improve the electromagnetic shielding properties. Some works have been performed by means of changing the structure to increase the surface area.

Ferrite possesses absorption and loss characteristics to electromagnetic waves without electrical conductivity, while carbon materials are light and conductive.¹⁰⁸ Therefore, a great deal of research has combined the two materials. For example, Lee et al.²¹ prepared GN/CNT/Fe₂O₃ films by the microwave method, which achieved 130–134 dB EMI SE at 0.6 mm thickness. Yuan et al.²⁶ prepared reduced graphene oxide (RGO)/Fe₃O₄/SiO₂/polypyrrole non-woven composite film (FSPG film), which showed an EMI SE of 32 dB, thickness of 0.27 mm, and absolute SE (SSEt) of 12,608 dB·cm²/g. However, in most situations, high conductivity is believed to be the key factor affecting electromagnetic shielding characteristics. Therefore, plating with metals such as Cu, Ag, and Ni is reported as an effective way to improve electromagnetic shielding efficiency. Wang et al.²⁴ use copper deposition to improve the EMI SE of GN film, which reports brilliant EMI shielding performance (63 dB) and conductivity ((5.88 ± 0.29) × 10⁶S/m) by centrifugal evaporation. Xing et al.¹⁹ prepared carbon-fabric/waterborne polyurethane (CEF-NF/WPU) film using Ag deposition, which achieved 103 dB at 0.183 mm. Zeng et al.²⁷ made a polyvinylidene fluoride (PVDF)/CNT/Ni film through vacuum ﬁltration, which showed a maximum SE/t (shielding effectiveness per unit thickness) of 103 dB/mm. The deposition of the metal gives the film superior electrical conductivity and ductility without adding much weight. Among those metals, silver is widely favored due to its excellent electrical conductivity and ductility.¹⁰⁹

From a structural point of view, a large area film is proved to be a feasible method to improve electromagnetic shielding performance. This is an effective solution to the problem of uneven conductive ﬁller distribution. Wu et al.²⁵ produced CNT macro-film via CVD that exhibits an EMI SE of 78 dB and SSEt value of 1,950,000 dB·cm²/g. Kumar et al.²⁰ prepared a large area of RGO by vacuum ﬁltration, which has excellent conductivity (243 S/cm) and EMI SE (20 dB). Shen et al.²² fabricated large area graphene oxide (GO) ﬁlm via evaporation, exhibiting excellent EMI SE of about 20 dB and thickness of 8.4 µm. Although possessing many favorable features, most EMI shielding properties for large area film are still inadequate for application in ﬁelds such as mobile phones, remote control toys, radio transmitters, and other portable electronic devices.^20,22,25

Periodic structure

The periodic structure of the film is mainly realized by the multilayer structure; a few studies have examined printing.^110–112 The interactions of the electromagnetic wave and material can be categorized into reflections, absorption, and multilayer reflections. The multilayer reflection depends mainly on the geometry of the shielding material.¹¹³ Therefore, a multilayer structure generates multiple reflections between the interface layers, which improves the electromagnetic shielding efficiency.¹¹⁴ In recent years, LbL assembly technology has been widely used in film manufacturing by a step-by-step process thanks to the low cost, simple process, and being “green.”¹¹⁵ Here we discuss electromagnetic shielding properties of film with multilayer structures. For example, Song et al.²³ prepared a sequence of sandwich structures via assembling GN film and a dielectric spacer, which has a unique frequency selectivity. Lu et al.³³ reported 19 dB EMI SE by stacking four layers of 1 mm thick GN/polyethylene terephthalate (PET) film. Zhang et al.³⁹ prepared CNT/polymer film that achieves 35 dB EMI SE at 0.15 mm. In the multilayer structure, the shielding efficiency strengthens with the number of layers, but the thickness also increases. In addition, enlarging the distance between conductive layers is beneficial to improve microwave absorbance. This has great application potential for transparent materials, for example, broadband radar absorbing materials (RAMs).¹¹⁶

Electromagnetic shielding film is widely used in military, medical, electronic information, and other fields on account of its high flexibility, light weight, good tensile strength, and other advantages, and can be used for aerospace flexible electronic products, commercial portable electronic products, flexible wearable electronic devices, and many more.

Composites

Composites usually involve two main components: matrix material and reinforcement material.¹¹⁷ There is a third material between the matrix and the filler: the interfacial zone, which determines the macroscopic properties of the composite.⁹³ The matrix mainly involves synthetic resin, ceramics, graphite, carbon, etc. The reinforcement material includes but is not limited to glass fiber, CF, silicon carbide, etc. Generally speaking, the large size and the FSS result in high strength, high modulus, corrosion resistance, and other advantages.¹¹⁸ Herein, we divide the composite into the periodic structure and the aperiodic structure for discussion.

Aperiodic structure

An emerging area of carbon-based composites was developed with the advent of nanomaterials. Carbon nanostructures have high tensile strength and electrical conductivity, are lighter, more flexible, have higher heat conductivity, and can be used as electromagnetic shielding materials.^119,120 CNTs with large specific surface area and length–diameter ratio contribute to the conductive network and induced current, which is considered excellent for carbon nanostructure conductive fillers. The high conductivity and connectivity of the conductive fillers are profitable to the enhancement of the shielding performance of EMI.¹²¹ We discuss the composite materials with CNTs as conductive fillers. Sun et al.³⁴ prepared a graphene foam (GF)/CNT/polydimethylsiloxane (PDMS) composite by computer-aided design (CAD), which showed the electrical conductivity of 31.5 S/cm and an outstanding SE of 75 dB. The corresponding shielding effectiveness per unit density (SSE) was 833 dB·cm³/g. Jia et al.³¹ studied separation structure carbon nanotube/polyethylene (s-CNT/PE) composites exhibits a high EMI SE (46.4 dB) at 2 mm thickness. Kong et al.³² prepared a CNT/RGO foam composite by freeze-drying and in-situ growth, showing that the EMI SE of the 2 mm thick composite is 31 dB and the SSE is 547 dB·cm³/g. Huangfu et al.³⁰ reported polyaniline (PANI)/MWCNT/thermally annealed graphene aerogel (TAGA)/epoxy composites that show EMI SE of 42 dB at the X-band for 3 mm thick samples and conductivity of 52.1 S/cm. Yang et al.³⁷ made CNT/GF/SiC composite material by in-situ growth, which achieves 32 dB SE and 34 dB·cm³/g SSE. Wang et al.⁵³ prepared porous composites based on PVDF and MWCNTs, which realized 57 dB EMI SE with a density of 0.79 g/cm³ and thickness of 2 mm. Zhang et al.⁵⁵ studied cellulose porous composites based on CNTs, achieving 35 dB SE with 2.5 mm thickness. Zeng et al.⁵⁴ prepared MWCNT/waterborne polyurethane (WPU) porous composites by the freeze-drying method, which reported 50 dB SE and 1148 dB·cm³/g SSE. As a one-dimensional, high aspect ratio sp²-allotrope shielding material, CNTs are widely used in EMI and radar absorption.¹²² However, their dispersion in the polymer matrix has been a major concern of researchers. In composite materials, CNTs as the electromagnetic absorption phase in strengthening matrix can expand the conductive network, form the electronic propagation, and introduce a large number of interfaces to the matrix in order to enhance the surface current and dissipation of electromagnetic waves.

For composites, porosity has been proved to be an effective factor to improve electromagnetic shielding efficiency. In this work, the SE/t and SSE of porous composites^{32,34,53–55} are compared with those of other composites (Figure 5). Apart from the hollow porous carbon fiber (HPCF) composite,³⁶ there is no significant difference in the SE/t of composites. For the FSS, it is found that the density of porous composites is far below that of others. Therefore, as an ultralight EMI shielding material, porous composite material has a broad application prospect in the aerospace field due to its effective microwave attenuation and extremely low density.³⁴
Figure 5.
(a) SE/t (shielding effectiveness per unit thickness) and (b) SSE (shielding effectiveness per unit density) of porous composites and other composites.

Periodic structure

The periodic structure of a composite is mainly realized by cutting,^36,38 the multilayer structure,^28,35 or backfilling.⁴⁰ Cutting is to knife the fabric, including non-fabric and braid, into frequency selected surface elements, and then put it into the mold injection matrix. This method is simple, easy to operate, and low cost. The multilayer structure is stacked layers, which enhance the tensile strength of the material.¹²³ The backfilling strategy uses reinforcing material to form the conductive skeleton and then fills the matrix. Zhang et al.³⁸ designed and fabricated CF/epoxy composite microwave-absorbing materials with a grid structure, which show the reflectivity is less than –10 dB in the range of 10–18 GHz with a three layer grid structure. Wang et al.³⁵ produced a GN/thermoplastic polyurethane (TPU) composite by solution mixing, which achieved 27 dB reflection loss with three layers. Wei et al.³⁶ enhanced the microwave-absorbing properties of composite materials by embedding the FSS in HPCF, which exhibits 10 dB bandwidth. Fan et al.²⁸ studied a three-dimensional (3D) interlock CF woven fabric/epoxy composite, a 3D interlock CF woven fabric with filled warp/epoxy composite, and a 3D orthogonal CF woven fabric/epoxy composite with a free-space measurement system, which showed that the electromagnetic wave absorption characteristics are greater than 90% (less than 10 dB) in the bandwidth of 11.2–18 GHz, and greater than 60% (less than 4 dB) in the bandwidth of 8–12 GHz. Zhao et al.,⁴⁰ through a facile backfilling strategy, prepared flexible 3D PDMS/RGO/SWCNT composites with excellent EMI SE (31 dB) and mechanical properties. The size and position of FSSs has an important influence on the absorption performance. Generally speaking, the large size and the FSS being located on the surface provide a better frequency selection.³⁸ The 3D network can not only act as a fast channel for electron transfers, but also transfer the external load into the FSS effectively to improve the absorbing performance of composite materials.⁵³ This provides a feasible method to study the electromagnetic shielding performance in the future.

Ascribe to their light weight and low density, porous composites have matured into the research hotspot of electromagnetic shielding materials, and have high potential for application in advanced shielding fields such as aerospace and automotive mobile electronic equipment.

Foam

Foam is a concentrated dispersal system of insoluble gas dispersed in a liquid. Due to its lightness and large area of special surface, foam is widely used in electromagnetic shielding.¹²⁴ In past research on electromagnetic shielding, polymer was often used as the matrix and carbon-based material was used as the reinforcement. The polymer matrix contains a porous structure that increases the reflection of multiple layers. Here we discuss the electromagnetic shielding properties of polymer-based foam.

Aperiodic structure

Carbon-based polymer composite foam has received extensive attention of researchers due to its outstanding characteristics and the introduction of nanoparticles.¹²⁵ For example, Fan et al.²⁹ prepared MXene/GN mixed foam (MX/RGO), which exhibited 51 dB SE at 3 mm thick and 1000 S/cm conductivity. Zhang et al.⁴⁹ prepared a super light (0.15 g/cm³) carbon foam-based phthalonitrile (PN), which reached 51 dB SE and 341 dB·cm³/g SSE. Duan et al.⁴² fabricated epoxy/nickel carbon ﬁber (EP/NCCF) foam, which exhibited 40 dB SE and 194 dB·cm³/g SSE. Shen et al.⁴⁸ prepared polyetherimide (PEI)/GN foam with low density (about 0.28–0.4 g/cm³), which showed the 14–18 dB SE and 42 dB·cm³/g SSE. Polymer foam has the advantages of low density, excellent heat and sound insulation, high ratio strength, and corrosion resistance.¹²⁶ Using a supercritical fluid, such as CO₂, gives polymer foams a lighter weight and higher dimensional stability than solid foam.¹²⁷ Polymer foam enables expression by density, average cell size, and cell density.¹²⁸

Periodic structure

The periodic structure of foam comes from its conductive/dielectric periodic structure, which is usually employed by filling conductive fillers such as GN, CNT, or CF in the polymer foam. The electromagnetic waves are absorbed or reflected by the conductive filler; in addition, multiple reflections occur in the foam pores. GN is considered to be a new material for preparing polymer composite foams due to its outstanding mechanical and electrical properties, high specific surface area, and layered structure.¹²⁹ The addition of GN enables one to stabilize the porous structure of the foam. Li et al.⁶¹ prepared polyimide (PI)/GN foam by thermal decomposition, and achieved high SE (24 dB) at an ultrathin thickness of 24 µm. Wang et al.⁶⁴ used vacuum-assisted impregnations to prepare annealed sugarcane (ASC)/RGO hybrid foam to achieve ultralight density (0.047 g/cm³) and an excellent SE value (53 dB). Wang et al.⁶⁵ prepared GN/PANI composite foam by in-situ polymerization, which showed a high SE of 52.5 dB. Eswaraiah et al.⁵⁸ prepared GN/PVDF composite foam, which has an EMI SE of 20 dB in the X-band. It is worth noting that the aggregation of GN in the polymer matrix affects the performance of the foam, so the “grafting” method is extensively used in the preparation of GN polymer composite foam.¹²⁹ GN/polymer composite foam is beneficial to applications of composite foam in the aerospace and automotive fields on account of its light weight and excellent electrical conductivity.

Electromagnetic shielding foam retains potential application prospects in aerospace and next-generation smart devices, including portable electronic devices, auto parts, and precision instruments, resulting from its low density and high performance.

Textiles

Textiles are products made from textile fiber after processing and weaving, including woven fabrics, knitted fabrics, and non-woven fabrics, as well as textile raw materials and so on.¹³⁰ Recently, textiles have developed into a research hotspot for electromagnetic shielding materials, due to their advantages of being soft, flexible, breathable, low cost, and requiring simple preparation. A large part of the carbon-based materials of the textile structure are made by electrospinning technology^56,59,60 and wet spinning technology to prepare carbon-based yarn, and endow electromagnetic shielding textiles with different structures. Some carbon-based electromagnetic shielding materials obtain conductivity and electromagnetic shielding performance by coating conductive materials on the surface of the textile. This paper classifies and discusses the preparation methods of carbon-based electromagnetic shielding textiles, consisting of electrospinning, wet spinning, and coating.

Electrospinning

Electrospinning is a widely used fiber manufacturing process that utilizes electricity to produce fibers with diameters ranging from nanometers to micrometers.¹³¹ Electrospinning fibers have been used in the field of electromagnetic shielding on account of their smaller pore diameters, higher specific surface area, and interconnected porous networks.¹³² Im et al.⁵⁹ carried out electrospinning and heat treatment on carbon nanofibers as a matrix for EMI shielding, and found that the conductivity of carbon nanofiber reaches 38 S/cm and 50 dB SE. Chen et al.⁵⁶ prepared CNT non-woven fabric by electrospinning technology, which reached 20.42 dB SE at a thickness of 2 mm. Kang and Jin⁶⁰ prepared MWCNT fiber non-woven fabric with the electrospinning method, and found its electrical conductivity was increased to 2.4×10^–4 S/cm, which verified the possibility of EMI applications. In the electrospinning process, the collected fibers were arranged randomly in the form of a non-woven mat, which is low cost and has a fast production rate.¹³³ Electrospinning is more flexible in the aspect of the controllability of fiber diameter, and provides more possibilities for nanostructure materials.¹³⁴

Wet spinning

Wet spinning technology is a method in which the spinning dope extruded from the spinneret forms a thin stream and is then solidified into fibers, which is low cost but requires complex equipment.^135,136 Wet spinning is suitable for the preparation of hollow fibers, for example, Wei et al.⁶⁶ used wet spinning technology to prepare polyacrylonitrile (PAN) hollow fibers as the precursor of a CF composite, and achieved a favorable electromagnetic SE. In addition, the production of CF precursors mainly uses wet spinning, for example, Oroumei et al.⁶³ employed lignin to produce CF precursors by wet spinning, with low cost and sustainable raw materials. Continuous carbon-based fibers enable preparation using wet spinning technology.¹³⁷ Zhou et al.⁶⁷ used wet spinning technology to prepare SWCNT/permalloy nanoparticle/poly(vinylalcohol) (SWCNT/PNP/PVC) nanocomposite fibers, which exhibited ultra-high conductivity (96,700 S/m) and the EMI attenuation rate of the fabric was close to 100% under the action of electromagnetic waves with a frequency exceeding 6 GHz. Lv et al.⁶² adopted wet spinning and heat treatment to prepare carbon nanofibers and found that composite material with a carbon nanofiber (CNF) content of 8 wt% and 2.1 mm thickness showed a maximum reflection loss of –34 dB and a –10 dB bandwidth at 10.5 GHz. Cong et al.⁵⁷ used wet spinning technology to successfully fabricated flexible and macroscopic GN fibers, laying a foundation for the application of advanced composite materials. Carbon-based fibers prepared by wet spinning technology are conductive, lightweight, corrosion-resistant, and flexible, and are extensively used in multifunctional textiles and wearable electronics.¹³⁷

Coating

Currently, the majority of electromagnetic shielding fabric are mostly prepared by coating the fabric with conductive materials, including CNT, GO, carbon nanoparticles, metal particles, and so on. For instance, Lan et al.⁴⁴ prepared high-loaded CNT/polymer composite coating via capillary-assisted assembly technology on cellulose fabrics, showing that the EMI SE is 12 dB. Liu and Kang⁴⁶ used two-component spray deposition technology to prepare conductive silver film on CF fabric, and reported 80 dB SE at 0.25 mm thick. Lin et al.⁴⁵ coated polypropylene (PP)/MWCNT on PET yarn to make guide motor fabric/knitted fabric with 20 dB SE. Islam et al.⁴³ dyed cotton fabric with GO for continuous dyeing, which achieved 26–35 dB SE in the X-band and 2.3 × 10^–1 S/cm conductivity. Pothupitiya Gamage et al.⁴⁷ prepared MWCNT-coated fabrics with high transparency, low density (0.066–0.1 g/cm³), and low thickness (0.12–0.20 mm) by the dip coating process with a high SE (68 dB), SSE (486.54 dB·cm³/g), and SSEt (35,000 dB·cm²/g). Ding et al.⁵⁰ prepared Al₂O₃ fiber fabric with pyrolysis carbon (PyC) coating by CVD, achieving a reflection loss of 40 dB at 4.5 mm thick in the X-band. Sun et al.⁵² prepared cotton fabric flexible wave-absorbing material-coated copper–cobalt–nickel ferrite/GO/polyaniline by using the two-step method, resulting in a 2 mm sample with a reflection loss of 47 dB at 300 kHz–3.0 GHz. Haji et al.⁵¹ coated amine functionalized multi-wall carbon nanotube (NH2-MWCNT) on PET fabric after plasma treatment, showing the reflection loss was 18 dB in the X-band. FSFs have not only the filtering characteristics of FSSs but also the softness of fabrics, and are easy to transport and apply. They can be formed by tailoring, coating, weaving, or other means in the fabric to form a periodic conductive area where electromagnetic waves are absorbed, resulting in frequency selection.^116,138 Compared with the FSS of other structures, they have certain advantages and would be irreplaceable in practical and commercial applications, which opens up the possibility for lightweight, flexible EMI shielding performance materials, such as wearable smart textiles.¹³⁹ Furthermore, as the weave structure and matrix maintain the original shape, the FSFs can be self-supporting.¹⁴⁰ FSFs can replace traditional weaving technology and are expected to be batch produced, which has a high application prospect in advanced application fields, such as aerospace, automobile mobility, and so on.

Electromagnetic shielding effectiveness of competing carbon-containing materials

Table 1 summarizes the key properties of carbon-based materials. The listed attributes include type, preparation, band, SE, including SE_max and SE_ave, SE/t, SSE, and SSEt, and electrical conductivity. For electromagnetic shielding materials all properties need higher values.
Table 1.
Electromagnetic interference shielding effectiveness (SE) of carbon-based materials

Type Material Method Band (GHz) SE_max (dB) SE_ave (dB) SSE (dB·cm³/g) SE/t (dB/mm) SSEt (dB·cm²/g) Electrical conductivity (S/m) Periodic structure Frequency selective Ref.

Film CEF-NF/WPU/Ag Ag deposition 0–1.5 103 – 60 563 3310 37,660 – – ¹⁹

Textile MWCNT/CF Coating 0–3 68 – 487 570 35,000 1642 Yes Yes ⁴⁷

Film GO Vacuum filtration 0–4 20 – – 13,300 – 24,300 ± 1200 – – ²⁰

Textile GO/cotton Coating 0.3–3 47 – – 24 – – Yes Yes ⁵²

Textile CF Electrospinning 0.8–4 50 – – – – 3800 – – ⁵⁹

Textile CF/Silver Spray deposition 0.9–1.5 – 80 102 323 4082 – Yes – 46

Textile PP/MWCNT Coating 1–3 – 20 – 36 – – Yes Yes ⁴⁵

Film GN/Cu Centrifugal evaporating 1–12 63 – 30 8070 38,500 (5.88 ± 0.29) × 10⁶ – – ²⁴

Composite GN/TPU Solution mixing 2–18 52 – – 8 – – Yes Yes ³⁵

Foam PANI/GN In-situ polymerization 2–18 53 – – 26 – – Yes Yes ⁶⁵

Composite HPCF Dry wet spinning 4–18 29 – 147 145 7315 – Yes Yes ³⁶

Textile CNT/polymer Capillarity assisted assembly 8–12 – 5 47 130 11,860 2 Yes – ⁴⁴

Foam PEI/GN WVIPS 8–12 18 – 41 7 164 – – – ⁴⁸

Textile NH2-MWCNT/PET Coating 8–12 18 – – – – – Yes Yes ⁵¹

Film GO Evaporation 8–12 20 – – 2380 – 100,000 – – ²²

Foam GN/PVDF Foaming 8–12 20 – – – – 10⁵ Yes – ⁵⁸

Textile CNT Electrospinning 8–12 20 – – 10 – – – – ⁵⁶

Foam PI/GN Thermal decomposition 8–12 24 – 33 1000 13,750 23 × 10⁴ Yes – ⁶¹

Composite CNT/RGO In-situ growth 8–12 – 31 547 15 2735 97 – – ³²

Composite RGO/SWCNT/PDMS Backfilling 8–12 31 – 111 15 554 120 Yes – ⁴⁰

Film RGO/Fe₃O₄/SiO₂ Hydrazine hydrate 8–12 – 32 340 120 12,608 71 – – ²⁶

Composite CNT/GF/SiC In-situ growth 8–12 32 – 34 13 135 224 – – ³⁷

Textile CF Wet spinning 8–12 34 – – 17 – – – – ⁶²

Film CNT/polymer LbL 8–12 35 – 20 230 1370 20 Yes – ³⁹

Composite CNT/cellulose Dip coating 8–12 35 – 950 14 3800 39 – – ⁵⁵

Textile GO/cotton Coating 8–12 35 – – 32 – 50 Yes Yes ⁴³

Foam EP/NCCF/CNT Mold pressing 8–12 – 40 194 20 971 10 – – ⁴²

Textile Al₂O₃/PyC Coating 8–12 40 – – 9 – 33,400 Yes Yes ⁵⁰

Composite PANI/MWCNT/ TAGA/epoxy Template casting 8–12 42 – – 14 – 52 – – ³⁰

Textile Wood-pulp Carbonize 8–12 – 45 159 150 5298 7768 Yes – ⁴¹

Composite CNT/PE Mechanical blending 8–12 – 46 – 22 – 57 – – ³¹

Film GN/Dielectric layer LbL 8–12 49 – – 25 – 24,000 ± 2000 Yes Yes ²³

Composite MWCNT/WPU Freeze-drying 8–12 – 50 1148 11 2550 45 – – ⁵⁴

Foam MX/RGO Reduction heat 8–12 51 – 11,021 17 36,737 1000 – – ²⁹

Foam Carbon/PN Heat treatment 8–12 – 51 341 25 1705 240 – – ⁴⁹

Foam ASC/RGO Vacuum-assisted impregnations 8–12 – 53 1128 18 3830 600 Yes – ⁶⁴

Composite MWCNT/PVDF Selective etching 8–12 – 57 72 28 360 100 – – ⁵³

Composite GF/CNT/PDMS CVD 8–12 – 75 833 47 5206 31,500 – – ³⁴

Film CNT CVD 8–12 78 – 780 19,500 1,950,000 – – – ²⁵

Film GN/CNT/Fe₂O₃ Microwave irradiation 8–12 135 – – 225 – 22,781 – – ²¹

Composite Interlock woven fabric/epoxy Injection 8–18 63 – – 16 – – Yes Yes ²⁸

Composite CF/epoxy Mixing 8–18 64 – – 16 – – Yes Yes ³⁸

Film GN/PET LbL 18–26.5 19 – – 19 – – Yes – ³³

Film PVDF/CNT/Ni Vacuum filtration 18–26.5 51 – – 100 – 29 – – ²⁷

SSE: shielding effectiveness per unit density; SE/t: shielding effectiveness per unit thickness; CEF-NF/WPU: carbon-fabric/waterborne polyurethane; MWCNT: multi-walled carbon nanotube; CF: carbon fiber; GO: graphene oxide; PP: polypropylene; GN: graphene; TPU: thermoplastic polyurethane; PANI: polyaniline; HPCF: hollow porous carbon fiber; CNT: carbon nanotube; PEI: polyetherimide; PET: polyethylene terephthalate; PVDF: polyvinylidene fluoride; PI: polyimide; PN: phthalonitrile; RGO: reduced graphene oxide; SWCNT: single-walled carbon nanotube; PDMS: polydimethylsiloxane; GF: graphene foam; EP/NCCF: epoxy/nickel-coated carbon fiber; PyC: pyrolysis carbon; TAGA: thermally annealed graphene aerogel; PE: polyethylene; MX: MXene; PN: ; ASC: annealed sugarcane; WVIPS: water-vapor induced phase separation; LbL: layer-by-layer; CVD: chemical vapor deposition.

Of the carbon-based materials listed in Table 1, GN/CNT/Fe₂O₃ film gives the best value of SE (135 dB),²¹ and CEF-NF/WPU/Ag film gives the next highest value of 103 dB.¹⁹ On the other hand, in relation to the SSE, except MX/RGO foam, which gives the best value of 11,021 dB·cm³/g,²⁹ the other high ones are all porous composites, including the MWCNT/WPU composite (1148 dB·cm³/g),⁵⁴ CNT/cellulose composite (950 dB·cm³/g),⁵⁵ GF/CNT/PDMS composite (833 dB·cm³/g),³⁴ and CNT/RGO composite (547 dB·cm³/g).³² With respect to the SE/t, CNT and GO film give the highest value (19,500 dB/mm)²⁵ and the second highest value (13,300 dB/mm),²⁰ respectively. In addition, CNT film also gives the highest SSEt (1,950,000 dB·cm²/g).²⁵ Concerning the conductivity of materials, GN/Cu film gives the highest value of (5.88 ± 0.29) × 10⁶S/m.²⁴

Materials containing CNTs have higher shielding efficiency. For instance, Wu et al.¹⁴¹ prepared a CNT mat and Fe/CNT mat, showing ultra-high SE/t of 30,000 and 40,000 dB/mm, respectively. The electromagnetic shielding properties of carbon-based materials were studied on the basis of whether they contained CNTs, including the SE/t and SSE, respectively, as shown in Figures 6(a) and (b). It is noteworthy that due to ultrathin thickness (4 and 1 µm, respectively), the CNT materials reported in these two articles have ultra-high SE/t.^25,141 However, other materials contain CNTs without superior SE/t, which results from different preparation methods, material types, and even testing methods. For SSE, with the exception of MX/RGO foam, porous composites containing CNTs exhibited great attraction. This is attributed to the structural morphology of the fiber, which gives not only a high specific surface area, but also even more important are the great length–diameter ratio and one-dimensional continuous-oriented structure, which contribute to the conductive network and induced current. Among the four electromagnetic shielding types, film is the most attractive choice owing to its thin plane structure, light weight, flexibility, and simple preparation process.^19,107
Figure 6.
(a) SE/t (shielding effectiveness per unit thickness) and (b) SSE (shielding effectiveness per unit density) of material containing carbon nanotubes (CNTs) and others.

The electromagnetic SE of carbon-based materials, including the SE/t and SSE, respectively, are shown in Figures 7(a) and (b). Thanks to the reduced thickness, the film and fabric are more attractive for SE/t, especially for a large area film.^20,22,25 By contrast, the results of composite and foam are unsatisfactory. For SSE, MX/RGO foam that has been freeze-dried and heating reduced is far superior to other electromagnetic shielding materials.²⁹ The introduction of MX increases the conductivity, making it a promising application in aerospace and intelligent devices. Of particular concern is that porous ultralight composites exhibit gigantic advantages in terms of SSE, which enables them to be applied in aerospace structures.^32,34,54,55
Figure 7.
(a) SE/t (shielding effectiveness per unit thickness) and (b) SSE (shielding effectiveness per unit density) of carbon-based material.

Conclusion

The electromagnetic shielding properties of carbon-based electromagnetic shielding materials, including film, composites, foam, and textiles, are reviewed. Carbon-based material is a promising electromagnetic shielding material with advantages of light weight, excellent electrical conductivity, heat resistance, corrosion resistance, various molecular structures, high specific strength, vibration attenuation, etc.

For electromagnetic shielding behavior, the essence of the shielding effect is the intrinsic conductivity and structural integrity of the material. Carbon-based materials have become a hotspot due to their excellent electrical conductivity and relatively complete structure. Many research scholars have concentrated on the high specific surface area. For example, GN with scattered layers increases the specific surface area but causes structural damage, which leads to a decrease in the current conducting structure. This shielding mechanism is superficial and general, with controversies and shortcomings. In fact, a high specific surface area refers to a complete structured large specific surface area film, rather than realizing a larger surface area of the surface through a mold. Large area films refer to perfect structures with properties such as conductivity. Apart from large area films, surface deposition and coated fabrics are considered as means to improve SE, which is attributed to the skin effect. The skin effect means that a large amount of charge is accumulated on the surface of the shielding material, which enhances the electrical conductivity, thereby improving the SE.

The comprehensive shielding performance of the film is the greatest, especially with a large area of film and the deposition of metal. Due to the reduced thickness, large area film has excellent electromagnetic shielding performance, but is not suitable for application in ﬁelds such as mobile phones, remote control toys, radio transmitters, and other portable electronic devices. In future electromagnetic shielding research, we need to consider how to reduce the film area while also reducing the thickness. The method of surface deposition also involves controlling the thickness of the coating as closely as possible. In addition, heat treatment can effectively improve the electromagnetic shielding performance of the carbon-based material.

The electromagnetic shielding performance of composites is relatively poor, which corresponds to the relatively low electrical conductivity but high weight. In this case, the porosity can be introduced into the composites to reduce their density and obtain high EMI shielding. The application of porosity can not only reduce the weight of the composites, but also form multiple reflections between the interfaces. In addition, large aspect ratio reinforced materials, such as CNTs, can effectively form a conductive network in the polymer, increasing its conductivity.

FSFs, materials endowed with the flexibility of textiles and the electromagnetic effect of FSSs, possess advantages over other FSSs and will be irreplaceable in specific fields, such as absorbing materials, electronic fabric wires, wings, and communication windows.

The rapid development of modern industry and the electronics industry has brought serious electromagnetic radiation, which has a severe impact on precision electronic equipment an even the human body. Therefore, great strides have been made in the progress of highly efficient EMI shielding materials nowadays. Apart from the high SE of electromagnetic shielding materials, light weight and flexibility are two other vital demands for EMI shielding applications, such as wearable devices and portable electronic products, especially in aerospace and next-generation flexible electronic products.

Type	Material	Method	Band (GHz)	SE_max (dB)	SE_ave (dB)	SSE (dB·cm³/g)	SE/t (dB/mm)	SSEt (dB·cm²/g)	Electrical conductivity (S/m)	Periodic structure	Frequency selective	Ref.
Film	CEF-NF/WPU/Ag	Ag deposition	0–1.5	103	–	60	563	3310	37,660	–	–	¹⁹
Textile	MWCNT/CF	Coating	0–3	68	–	487	570	35,000	1642	Yes	Yes	⁴⁷
Film	GO	Vacuum filtration	0–4	20	–	–	13,300	–	24,300 ± 1200	–	–	²⁰
Textile	GO/cotton	Coating	0.3–3	47	–	–	24	–	–	Yes	Yes	⁵²
Textile	CF	Electrospinning	0.8–4	50	–	–	–	–	3800	–	–	⁵⁹
Textile	CF/Silver	Spray deposition	0.9–1.5	–	80	102	323	4082	–	Yes	–	46
Textile	PP/MWCNT	Coating	1–3	–	20	–	36	–	–	Yes	Yes	⁴⁵
Film	GN/Cu	Centrifugal evaporating	1–12	63	–	30	8070	38,500	(5.88 ± 0.29) × 10⁶	–	–	²⁴
Composite	GN/TPU	Solution mixing	2–18	52	–	–	8	–	–	Yes	Yes	³⁵
Foam	PANI/GN	In-situ polymerization	2–18	53	–	–	26	–	–	Yes	Yes	⁶⁵
Composite	HPCF	Dry wet spinning	4–18	29	–	147	145	7315	–	Yes	Yes	³⁶
Textile	CNT/polymer	Capillarity assisted assembly	8–12	–	5	47	130	11,860	2	Yes	–	⁴⁴
Foam	PEI/GN	WVIPS	8–12	18	–	41	7	164	–	–	–	⁴⁸
Textile	NH2-MWCNT/PET	Coating	8–12	18	–	–	–	–	–	Yes	Yes	⁵¹
Film	GO	Evaporation	8–12	20	–	–	2380	–	100,000	–	–	²²
Foam	GN/PVDF	Foaming	8–12	20	–	–	–	–	10⁵	Yes	–	⁵⁸
Textile	CNT	Electrospinning	8–12	20	–	–	10	–	–	–	–	⁵⁶
Foam	PI/GN	Thermal decomposition	8–12	24	–	33	1000	13,750	23 × 10⁴	Yes	–	⁶¹
Composite	CNT/RGO	In-situ growth	8–12	–	31	547	15	2735	97	–	–	³²
Composite	RGO/SWCNT/PDMS	Backfilling	8–12	31	–	111	15	554	120	Yes	–	⁴⁰
Film	RGO/Fe₃O₄/SiO₂	Hydrazine hydrate	8–12	–	32	340	120	12,608	71	–	–	²⁶
Composite	CNT/GF/SiC	In-situ growth	8–12	32	–	34	13	135	224	–	–	³⁷
Textile	CF	Wet spinning	8–12	34	–	–	17	–	–	–	–	⁶²
Film	CNT/polymer	LbL	8–12	35	–	20	230	1370	20	Yes	–	³⁹
Composite	CNT/cellulose	Dip coating	8–12	35	–	950	14	3800	39	–	–	⁵⁵
Textile	GO/cotton	Coating	8–12	35	–	–	32	–	50	Yes	Yes	⁴³
Foam	EP/NCCF/CNT	Mold pressing	8–12	–	40	194	20	971	10	–	–	⁴²
Textile	Al₂O₃/PyC	Coating	8–12	40	–	–	9	–	33,400	Yes	Yes	⁵⁰
Composite	PANI/MWCNT/ TAGA/epoxy	Template casting	8–12	42	–	–	14	–	52	–	–	³⁰
Textile	Wood-pulp	Carbonize	8–12	–	45	159	150	5298	7768	Yes	–	⁴¹
Composite	CNT/PE	Mechanical blending	8–12	–	46	–	22	–	57	–	–	³¹
Film	GN/Dielectric layer	LbL	8–12	49	–	–	25	–	24,000 ± 2000	Yes	Yes	²³
Composite	MWCNT/WPU	Freeze-drying	8–12	–	50	1148	11	2550	45	–	–	⁵⁴
Foam	MX/RGO	Reduction heat	8–12	51	–	11,021	17	36,737	1000	–	–	²⁹
Foam	Carbon/PN	Heat treatment	8–12	–	51	341	25	1705	240	–	–	⁴⁹
Foam	ASC/RGO	Vacuum-assisted impregnations	8–12	–	53	1128	18	3830	600	Yes	–	⁶⁴
Composite	MWCNT/PVDF	Selective etching	8–12	–	57	72	28	360	100	–	–	⁵³
Composite	GF/CNT/PDMS	CVD	8–12	–	75	833	47	5206	31,500	–	–	³⁴
Film	CNT	CVD	8–12	78	–	780	19,500	1,950,000	–	–	–	²⁵
Film	GN/CNT/Fe₂O₃	Microwave irradiation	8–12	135	–	–	225	–	22,781	–	–	²¹
Composite	Interlock woven fabric/epoxy	Injection	8–18	63	–	–	16	–	–	Yes	Yes	²⁸
Composite	CF/epoxy	Mixing	8–18	64	–	–	16	–	–	Yes	Yes	³⁸
Film	GN/PET	LbL	18–26.5	19	–	–	19	–	–	Yes	–	³³
Film	PVDF/CNT/Ni	Vacuum filtration	18–26.5	51	–	–	100	–	29	–	–	²⁷

Footnotes

Declaration of conflicting interests

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

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Natural Science Foundation of China (NSFC 51803185), the Key Laboratory of Advanced Textile Materials and Manufacturing Technology, Ministry of Education, China (No. 2017QN05), the Postdoctoral Foundation of Zhejiang Sci-Tech University Tongxiang Research Institute (TYY202013), and the Science Foundation of Zhejiang Sci-Tech University (No. 18012107-Y).

ORCID iD

Xinghua Hong

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