New Electromagnetic Interference Shielding Materials: Biochars, Scaffolds, Rare Earth, and Ferrite-Based Materials

Abstract

In this review, a comprehensive systematic study of the research background, developments, classification, trends, and advances over the past few years in research on new electromagnetic interference (EMI) shielding materials will be described. The following groups of new materials for EMI shielding will be discussed: biochars, scaffolds, rare earth, and ferrite-based materials. We selected two novel, organic, lightweight materials (biochars and scaffolds) and compared their shielding effectiveness to inorganic materials (ferrite and rare earth materials). This article will broadly discuss the EMI shielding performance, the basic principles of EMI shielding, the preparation methods of selected materials, and their application prospects. Biochars are promising, eco-friendly, sustainable, and renewable materials that can be potentially used as a filter in polymer composites for EMI shielding, along with scaffolds. Scaffolds are new-generation, easy-to-manufacture materials with excellent EMI shielding performance. Rare earth (RE) plays an important role in developing high-performance electromagnetic wave absorption materials due to the unique electronic shell configurations and higher ionic radii of RE elements. Ferrite-based materials are often combined with other components to achieve enhanced EMI shielding, mechanical strength, and electrical and thermal conductivity. Finally, the current challenges and future outlook of new EMI shielding materials will be highlighted in the hope of obtaining guidelines for their future development and application.

1. Introduction

The human population in urban and rural regions is exposed to non-ionizing electromagnetic fields (EMFs) due to the rapid development of telecommunication technology and digital systems for information transfer [1,2]. The association of the fifth generation (5G) and upcoming sixth generation (6G) will lead to the emergence of innovative applications (Internet of Things and self-driving vehicles) [3,4]. Wearable and portable electronics will soon become more commonly used [4]. The COVID-19 pandemic triggered the implementation of technology trends such as digital payments, telemedicine, and robotics, which use radiofrequency (RF) electromagnetic waves (EMWs) in the 100 kHz–300 GHz frequency range [5]. These technological advances caused an increase in artificial sources of EMF and resulted in the chronic exposure of people and the environment, creating electromagnetic pollution. Electromagnetic pollution is the continuous and uncontrolled exposure to electromagnetic fields from any emitting source of the EMF spectrum [5]. From 1950 to 2010, the levels of exposure to RF-EMF have increased from extremely low natural levels by about 1018 times [6]. Many studies have shown the harmful effects of EMF on human health [7,8,9,10,11] and the environment [12,13,14,15]. In addition to the potential health risks, the interference of EMW worsens heat accumulation, significantly shortening equipment lifespan [16]. Thus, new electromagnetic interference (EMI) shielding materials are urgently needed.

The conventional EMI shielding materials are metals, carbons, ceramics, cement, conductive polymers, and associated composites. Metals and carbons are the most functional materials due to their high conductivity, while ceramics, cement, and (non-conductive) polymers are less efficient unless combined with functional components/materials [17,18]. Shielding barriers should meet the required dimensions and shapes at the lowest possible cost for various applications.

Among the various new materials, organic materials such as graphene, carbon black, and carbon nanotubes are highly conducting, lightweight, flexible, and highly efficient EMI shielding materials. Still, their high price and the complexity of production and purification lead to high cost and limit their use. Inorganic materials, such as metal powders and metal oxides, are highly efficient EMI shielding materials due to their high electrical conductivity accompanied by magnetic permeability. Challenges in their application are associated with their high prices, chemical instability, corrosion, and heavy weight. Thus, in this review paper, we are focusing on new, suitable, low-cost organic materials—biochars and scaffolds. These organic EMI shielding materials are compared to novel inorganic shielding materials—rare earth and ferrite-based composites. By reviewing their production methods, yields, physico-chemical properties, and EMI shielding effectiveness, the future direction in the development of EMI shielding materials is estimated.

According to the reviewed literature, biochars are promising, eco-friendly, sustainable, and renewable materials that can potentially be used as filters in polymer composites for EMI shielding applications in electronic devices and construction materials. Previous research showed that scaffolds are new-generation, easy-to-manufacture materials with excellent EMI shielding performance.

Recently, great attention has been paid to traditional magnetic materials/alloys with excellent absorption of EMWs, but they are often limited by thickness and range of use. However, according to Snoek’s limit, where the direct current (DC) susceptibility and the cut-off frequency are constant, the permeability value decreases in the high-frequency range, which leads to an apparent decrease in magnetic loss capability [19,20,21]. Improving the permeability and magnetic loss of magnetic materials/alloys at high frequencies is crucial for obtaining good electromagnetic/microwave absorption properties [22]. It was shown that the electronic structure and well-chosen methods for designing magnetic materials/alloys can improve magnetism at high frequencies [23].

Therefore, magnetic metal oxide composites or perovskite oxides doped with rare earth element ions have been in development for the last few years due to their specific structures, interfacial polarization, multiple reflections, and excellent conducting and EMI shielding properties [24].

Rare earth materials are often characterized by various names, such as rare earth elements (REEs), rare earth metals (REMs), rare earth oxides, or yttrium-based rare earth material. This group represents a set of 17 chemical elements, as listed in the periodic table, from lanthanum (La) to lutetium (Lu), including 15 lanthanides and scandium (Sc) and yttrium (Y) with a tendency to occur in the same core deposits as lanthanides and exhibiting identical chemical properties [25]. In the crystal structure, REEs are most often found in the trivalent state (RE³⁺), although the divalent state (RE²⁺) is also possible. REEs with strong inter-electron interactions with their localized 3d and 4f electrons as well as strong coupling with magnetic materials have emerged as outstanding material dopants [26]. REEs based on neodymium (Nd), dysprosium (Dy), and samarium (Sm), due to their relatively large number of unpaired electrons in the atomic structure, possess a high remanent magnetization and coercivity value determining the stability of the remanent state. The orbital electron structure of these elements contains many unpaired electrons, which allows them to store large amounts of magnetic energy. The total magnetic moment originates from these unpaired electrons [27,28]. The reviewed literature revealed the growing interest in rare-earth-based materials as materials for EMI shielding applications [29,30]. These materials can be classified into several groups: RE-doped ferrites (RE-Fs), F₂O₃/Fe₃O₄, RE–transition metal intermetallics (RETMIs), RE oxides (REOs), RE-based alloys, RE spinels, RE-doped MoS₂, and RE polymers. The above-mentioned groups of rare-earth-based materials will be discussed in detail throughout this manuscript. The main advantages of rare-earth-based materials over traditional EMI shielding materials are the magneto-dielectric effects and tunable dielectric properties in an electric or magnetic field. These properties make them attractive for exploring their EMI shielding applications.

New nanocomposite-based foams and heterogeneous layered structures have shown outstanding EMI shielding properties. Namely, thermoplastic polyether-block-amide elastomer beads coated with Ti₃C₂T_x showed an EMI shielding efficiency of 44 dB at 8.5–12.5 GHz [29], microcellular aramid nanofiber/Ti₃C₂T_x MXene foams reached a value of 64.9 dB [30], and foams based on the conductive polymer polypyrrole with Fe₃O₄ achieved a value of 41.1 dB [31]. Combining MXenes with carbon nanotubes and silver nanowires in a layer-by-layer architecture, Zhang et al. achieved an EMI shielding efficiency of 53.1 dB in the X-band [32].

The development, classification, trends, and advancements of research on novel shielding materials in electromagnetic irradiation during the past few years will be thoroughly and methodically reviewed herein. The recent progress in the development of new-generation shielding materials based on biochars, 3D network structures (referred to as scaffolds in the following), materials with rare earth (referred to as rare-earth-based materials in the following), and ferrite materials will be described (Figure 1). This paper will comprehensively discuss the EMI shielding performance, basic principles and mechanisms, preparation methods, structure, morphology, and application prospects of these materials.

2. New EMI Shielding Materials: Biochars, Scaffolds, Rare Earth, and Ferrite-Based Materials

2.1. Biochar as EMI Shielding Material

Among conventional EMI shielding materials, metals such as Cu, Ag, Al, etc., are highly efficient EMI shielding materials due to their electrical conductivity and ability to reflect incident EMWs [16]. Regardless, the use of these metals is limited owing to their high density, inconvenient processing, and poor resistance to corrosion [16]. Recently, shielding materials have been investigated in the form of composites with plastic or metals as substrates [34]. As alternative solutions, conductive polymer-based composites, porous conductive foams, and polymers with micro- and nanoscale filters such as carbon material (graphite, graphene, reduced graphene oxide, carbon nanotubes), MXenes, metal nanoparticles, and nanowires have been explored due to their low mass densities, good flexibility, and stretchability [4,35].

One of the latest approaches in developing new, sustainable, and eco-friendly solutions for EMI shielding is the integration of natural, renewable carbon materials—biochars—into polymers [4]. Biochars originate from various biomass sources, such as bamboo, sugarcane, and cork. To create biochars from biomass, biomass resources are converted to carbon materials by pyrolytic processes in an oxygen-limited environment. A schematic presentation of biochar starting materials and production steps is displayed in Figure 2. After high-temperature carbonization, biochars show extraordinary hardness, excellent thermal stability, and high electrical conductivity, making them suitable fillers for polymers [36]. By combining biochars and polymers, the mechanical, physical, and electrical properties of both materials can be upgraded successfully.

The electrical conductivity of biochar is closely related to the source properties and carbonization conditions [37]. Nevertheless, some researchers have disregarded the electrical properties of biochars, so their potential application in EMI shielding has rarely been examined [38,39,40].

In 2018, Li et al. conducted an initial research study [39]. Namely, they used commercially available bamboo charcoal as the biochar filler and/or an ultra-high-molecular-weight polyethylene (UHMWPE)/linear low-density polyethylene (LLDPE) blend as a potential EMI shielding composite. The composite with 80 wt.% of biochar was fabricated using the mass-producing extrusion and hot compression methods. Through the carbonization of bamboo charcoal at 1100 °C, a graphitic-like structure with good electrical conductivity and a high specific surface area was obtained. Namely, an electrical conductivity of 107.6 S/m and EMI shielding effectivity (SE) of 48.7 dB were measured at 1.5 GHz. The specific EMI SE of the composite was 39.0 dB cm³/g. This value is four times higher than the specific EMI SE of copper (10.0 dB cm³/g) [41].

Tolvanen et al. applied the same approach to prepare a biodegradable two- and three-phase composite of graphite, biochar derived from pine chips, and polylactic acid (PLA) [4]. The EMI shielding performances were tested in the K-band (18–26.5 GHz). The composite showed an outstanding EMI SE of >32 dB when the materials were prepared in the form of a film with a thickness of 25 μm and a high specific shielding effectiveness (SSE/t) of >890 dB cm²/g. It can be seen that these composites are convenient for the application of wearable/portable and stationary devices.

Shortly after, Akgül et al. presented a biochar–iron (BC-Fe) material obtained by the pyrolysis of industrial tea waste biomass and the encapsulation of BC-Fe into polymethyl methacrylate (PMMA) polymer [42]. During synthesis, Fe(III) ions promoted the graphitization of the amorphous carbon in biomass and contributed to biochar surface stabilization. The results of EMI SE measurements showed that the reflection component of the EMI SE of pure PMMA was near −10 dB at frequencies of 7.8 and 10.1 GHz, while it was reduced by ~60% when the content of BC-Fe in the polymer was 40 wt.% in the frequency range of 8.5–12.9 GHz.

Liang et al. successfully developed a mortar/brick structure using wood-derived porous carbon (WPC) as a skeleton and conductive 3D MXene aerogel [1]. The MXene aerogel/WPC composite was composed of highly ordered honeycomb cells inside WPC as a microreactor. Herein, a high graphitization level of natural wood was achieved by applying a high carbonization temperature of 1500 °C. The MXene aerogel/WPC composite showed an excellent EMI SE value of 71.3 dB in a frequency range of between 8.5 GHz and 12.5 GHz, while the sample density was only 0.197 g/cm³. In the study, a wall-like mortar brick structure (WPC as mortar and MXene aerogel as brick) solved the instability of the MXene aerogel network as well as prolonged the transmission path of EM waves, dissipating the incident EM waves in the form of heat and electrical energy. The material’s superior EMI shielding performance was achieved thanks to a specific design.

The shielding efficiency of a gypsum–biochar drywall-like composite was investigated by Natalio et al. [43]. They used wooden chips and eucalyptus biochar and combined them with gypsum. An enhancement in the shielding efficiency was recorded with an increased biochar content in the composites. Namely, the EMI SE values of the drywall-like plates with 10%, 20%, and 40% w/w biochar contents were 11.65 ± 1.6, 19.2 ± 5.7, and 19.25 ± 1.8 dB at a frequency of 6 GHz. This investigation contributes to expanding innovative bio-based sustainable materials with EM shielding properties in the microwave region.

Savi et al. used sewage sludge to obtain biochar and investigated its EMI-shielding features [44,45]. The biochar was mixed with epoxy polymer (20 wt%) and cast into a film 4 mm thick. The composite showed a promising EMI SE of −10 dB [44]. Savi et al. reported an outstanding electrical conductivity of 300 S/m [44]. The same group compared the EMI SE properties of biochar produced from carbonized sludge with graphene nanoplatelets [46]. Herein, the authors produced polymer composites with polyvinylidene fluoride (PVDF), while fillers (biochar and graphene) were added at 90 wt%. The biochar–PVDF composite showed transmission scattering of less than −30 dB.

Savi et al. also studied commercial biochar produced from wood biomass to coat several layers of common building components, such as drywall panels [47]. They recorded EMI SE values of 17 dB at 1 GHz and 25 dB at 18 GHz. Drywall panels coated with several layers of biochar are easy to fabricate and a low-cost solution to realize a protected surrounding for healthcare applications (chemotherapy and tomography) to minimize the intensity of the EM field close to electronic equipment. The same research group produced a cement-based composite with commercial lignin-based biochar and polyvinyl chloride (PVC) [48]. They introduced biochar powder into cement paste to improve its shielding properties. PVC, which was used as a filler, was obtained by decommissioning old electrical cables that would have ended up in a landfill. A combination of 10 wt.% biochar and 6 wt.% PVC revealed the best shielding capacity, around 16 dB in the 5.4–8 GHz frequency range. This investigation shows great importance in the EMI shielding application and from the aspect of the circular economy.

Miao et al. fabricated a conductive EBC@CNF@MWCNT composite aerogel by the freeze-drying process, mixing ferrite chloride with electrically conductive bamboo charcoal (EBC), with cellulose nanofibrils (CNFs) as a skeleton and multi-walled carbon nanotubes (MWCNTs) as a conductive enhancer in the freeze-drying process [36]. Afterward, the aerogel was soaked with polydimethylsiloxane and hot-pressed into the membrane. In the composite, EBC and MWCNTs were arranged uniformly in the CNF skeleton to form a 3D conducting network. The composite displayed exceptional electrical conductivity (47 S/m) and a high EMI shielding effectiveness of 39.5 dB, with an adsorption loss of ~75%.

Recently, Nikolopoulus et al. used biochar prepared from olive tree pruning to fabricate composite samples with carbon black and polytetrafluoroethylene as binders [49]. The EMI shielding capacity was measured in the 1–3 GHz frequency range. The results demonstrated that the raw pure biochar had a low EMI SE, between 1.5 and 4 dB, which was enhanced with an increase in the thickness from 0.1 to 0.5 mm. On the contrary, the composite significantly improved the EMI SE, reaching a value of 39 dB. This work indicates that biochar could be used as a basis for developing composites with high EMI SE values.

Milenkovic et al. recently reported that agricultural biowaste collected after apple and quince processing could be converted into EMI shielding material [50]. The starting materials were chopped, homogenized, dried, and mixed with an equal mass of KOH. The mixtures were carbonized at 850 °C, under nitrogen flow. The resulting biochars were mixed with sodium silicate at 40 wt.%. The film of only 0.2 mm thickness showed an EMI SE of 15.5 dB in the 8–12 GHz frequency range. Although moderate EMI SE values were found, the study demonstrates the outstanding potential of selected biowaste in the production of sustainable EMI shielding materials.

Another study by Perumal et al. proposed a new organic waste for biochar production [37]. Within the study, the composite built from Ricinus communis outer shell-based biochar and epoxy was fabricated by slow pyrolysis at 400–700 °C. The results showed that the biochar pyrolysis at 700 °C led to a maximal electrical conductivity of 95 S/m due to the presence of graphitic carbon. A maximum EMI SE of 26.5 dB was found in the X-band frequency range, at 40 wt.% biochar to epoxy matrix.

According to the reviewed literature, biochar is a promising, eco-friendly, sustainable, and renewable material that could be used as a filter for EMI shielding applications in electronic devices and construction materials. The biochar shielding effectiveness is associated with the electrically conductive graphene region in biochars and porous structure [50]. Namely, EMW attenuation is the result of wave reflection from the graphene region and multiple reflections inside the pores. Biochar features rely on two main factors:

(1)

The experimental conditions (temperature, duration of heating, addition of chemical agent for pore formation or activitvation of graphitization, selected gas);

(2)

The starting material’s chemical composition [39].

With an increased temperature, the porosity, specific free space, and carbon content increase too, while the selection of source materials affects both the properties and yields of the resulting biochar [39].

2.2. Scaffolds as EMI Shielding Material

Many structural patterns, including highly porous materials and conductive structures such as multilayer composites obtained by stacking cellulose nanofibers/reduced graphene oxide (rGO), rGO films, and multistage composite foams, are recognized as excellent EMI shielding materials. These structures are obtained using vacuum-assisted filtration, freeze-drying techniques, or electroless deposition [51]. The mentioned procedures allow for a reduction in the reflection coefficient, but they are complex and often not reliable. The innovative manufacturing technologies imply 3D network structures (scaffolds) that can be decorated with metal nanoparticles [52], coated by polymers, encapsulated with paraffin [53], etc. These approaches enable design freedom, flexibility, precise control over the shape and size, and the connectivity of porous structures.

Cellulose scaffolds (CSs) are recognized as a promising material for EMI shielding applications due to the ability of cellulose nanofibers to improve the thermal and mechanical features of nanocomposites. Tran et al. represented a 3D network structure of cellulose scaffolds decorated with silver nanoparticles (AgNPs) [52]. The scaffolds were filtered at the epoxy matrix and cured at 40 °C to fix the nanoparticles. The obtained composite possesses a thermal conductivity of 2.52 W/m/K, which is over 11-fold the thermal conductivity of pure epoxy. The extremely high electrical conductivity of 53.691 S/m caused a remarkable SE value of 69.1 dB. He et al. designed a scaffold connected with metal nanoparticles [53]. In this work, the shape-stable composite was composed of a paraffin impregnated in a biological porous carbon scaffold, followed by a coating with polyurethane and Fe₃O₄ nanoparticles. The biological porous carbon was obtained from a loofah sponge by immersion in a phenolic resin solution followed by carbonization. Scanning electron microscopy (SEM) results revealed that the system kept its original shape with a 3D and honeycomb-like porous structure of single fibers after carbonization. The polyurethane coating provided adequate mechanical support and effective leak-proof performance. The produced framework shows strong EMI shielding performance (up to 32 dB). Another study based on the encapsulation of Fe₃O₄ nanoparticles in carbon scaffolds was conducted by Wei et al. [54]. Herein, laminar cellulose-paper-based scaffolds with bidirectional gradient distributed Fe₃O₄ nanoparticles were constructed via immersion, drying, and carbonization processes. The resulting carbon scaffolds exhibited high in-plane electrical conductivity (96.3 S/m) and high shielding efficiency (1805.9 dB/cm²g).

A 3D cellulose scaffold was combined with a CNT/MXene composite [55]. Nanosheets of the composite were inserted into a cellulose scaffold by vacuum impregnation. Finally, a hydrophobic and multifunctional 3D system was produced by wrapping it with poly (dimethylsiloxane). The composite had a high compressive strength of 1.53 MPa, a maximum strain at fracture of 74.1%, and an outstanding SE (29.3 dB).

Hu et al. reported a dual-ice templating assembly strategy to prepare a dual-interpenetrated scaffold [56]. The scaffold was linked with a high-quality graphene array and porous MXene-Co aerogel. It showed an absorption-dominated EMI SE of 72.86 dB.

Three-dimensional-printing technology was applied for scaffold fabrication based on a triply periodic minimal surface and 70% porosity [42]. The monolithic layered dipping method was used to regulate the gradient distribution of carbon nanotubes on the 3D-printed scaffold surface and ensure integrity. This system blocked 99.9% of EM waves. The value of the SE in this case was 35.9 dB.

Previous research showed that scaffolds are new-generation, easy-to-manufacture materials with excellent EMI shielding performance. Due to their exceptional three-dimensional construction and inner network, they show anisotropic thermal conductivity and allow heat dissipation, making them efficient EMI shielding materials [52,56]. The results from the literature based on biochar and scaffolds in shielding applications are listed in

Table 1. Summarized results for biochar and scaffolds as EMI shielding materials.

Material	Reinforcement and Matrix Thickness	EMI Shielding Effectiveness	Frequency
Bamboo charcoal/HMWPE/LLDPE composite	3 mm, 140 mm	48.7 dB	1500 MHz	[39]
Pine chip/PLA composite	0.25 mm	>32 dB	18–26.5 GHz	[4]
MXene aerogel/WPC composite	3 mm	71.3 dB	8.5–12.5 GHz	[1]
Gypsum–biochar drywall-like composite	>2 mm	11.65 ± 1.6 dB, 19.2 ± 5.7 dB, and 19.25 ± 1.8 dB for 10, 20, and 40% w/w biochar contents	6 GHz	[43]
Drywall panels coated with commercial wood biochar	10 mm	17 dB 25 dB	1 GHz 18 GHz	[47]
Cement-based/commercial lignin-derived biochar/PVC composite	4 mm	16 dB	5.4–8 GHz	[48]
EBC@CNF@MWCNT composite	~8 mm	>32 dB	8–12 GHz	[36]
Olive tree-derived biochar/polytetrafluoroethylene composite	0.1–0.5 mm	39 dB	1–3 GHz	[49]
Apple and quince biowaste-based biochars	0.2 mm	15.5 dB	8–12 GHz	[50]
Ricinus communis outer shell-based biochar/epoxy composite	0.15 mm	26.5 dB	8–12 GHz	[37]
18 wt.% lignin-based biochar/cementitious composite	4 mm	15 dB	10 GHz	[57]
Cashew shell biochar/carbon fiber-reinforced epoxy resin composite	not reported	−48.6 dB	18 GHz	[58]
Jackfruit rag biochar/waste silk fiber-reinforced vinyl ester composite	not reported	31.5 dB 47.25 dB 63 dB 68.25 dB	8 GHz 12 GHz 16 GHz 18 GHz	[59]
Cellulose scaffold/AgNP composite	1 mm	69.1 dB	8.2–12.4 GHz	[52]
Carbon scaffold/polyurethane/Fe3O4 NP composite	~8 mm	32 dB	8.2–12.4 GHz	[53]
Cellulose-paper-based scaffold/Fe3O4 NP composite	1.3 mm	1805.9 dB/cm2 g	10.3 GHz	[54]
3D cellulose scaffold/CNT/MXene composite	0.25 mm	29.3 dB	18–26.5 GHz	[55]
Hybrid scaffold coupled with high-quality graphene array/MXene-Co aerogel	6 mm	72.86 dB	8.2–12.4 GHz	[56]
3D-printed scaffold/CNTs	10 mm	35.9 dB	8.2–12.4 GHz	[51]

.

2.3. Rare-Earth-Based Materials as EMI Shielding Material

Next-generation microwave-absorbing compounds and hybrid materials could potentially outperform traditional materials in terms of their ability to absorb microwaves, their persistence, their stability at high temperatures, weight reduction, their corrosion resistance, and the need to expand the frequency spectrum over which materials can efficiently absorb microwaves. Research and development in the last few years have promoted magnetic metal oxide composites or perovskite oxides doped with REEs as innovative materials for good electromagnetic wave absorbance due to their specific structures, interfacial polarization, and multiple reflections, showing excellent conductive properties and magnetic losses [60]. By doping rare earth elements, the dielectric loss and dipole polarization can be increased, and the wave absorption performance of ferrite can be further improved. Nikzad et al. [61] prepared a Nd³⁺-substituted Gd–Co ferrite composite and found this to be a promising EW absorption material with a wide effective absorption bandwidth in the X-band. On the other hand, it was observed that appropriate La doping enhances the dielectric and conductive mismatch levels of the material. This enhancement leads to a stronger interface polarization capability [62].

Different microstructures and morphologies manifest different EMI shielding properties. Addressing the challenge of creating absorption-dominant EMI shielding requires significant attention to detailed structural design [63]. This strategy aims to produce structures that minimize the reflection of undesirable electromagnetic noise, capturing electromagnetic waves (EMWs) through various attenuation mechanisms.

The rare earths (REs) for electromagnetic wave absorption applications can be classified into several groups: RE-doped ferrites (RE-Fs), RE–transition metal intermetallics (RETMIs), RE oxides (REOs), and other categories [64]. Graphene-based materials, metal–organic frameworks (MOFs), ferrites, molybdenum disulfide, MoS₂, and other innovative materials can significantly enhance EMW absorption properties by doping or substitution with REs due to the magnetic moment originating from their unpaired electrons [65]. The high-frequency permittivity of rare-earth Er-doped MoS₂ films was studied [66]. The size of nanocrystals depends on the reaction time; the reactant concentration and composition control the morphology [67]. With a lowered concentration, the resulting nanoparticles exhibited a spherical morphology but gradually changed to flower-shaped with increasing concentration.

Pure MoS₂ has a relatively homogeneous loss mechanism, limited impedance matching, and a limited ability to absorb EW. Doping is an effective method to improve the electromagnetic properties of materials [68]. For instance, palladium-doped few-layer MoS₂ nanosheets grown on the surface of multi-walled CNTs and the 10% Pd nanohybrid possess a higher EMI shielding effectiveness (SE ∼26.50 dB) than that of SE ∼21.55 dB in the undoped MoS₂/CNT at the same thickness of 1 mm [69].

Laser-cladded novel FeCo-based alloys with different RROs, such as La₂O₃, Y₂O₃, Nd₂O₃, and Gd, possess enhanced electromagnetic shielding performance with a shielding effectiveness (SE) of up to ~96 dB at 23.9 GHz and an ability to absorb 99.9999% of EMWs [70,71].

Li et al. [72] discovered that REE substitution in Fe₄N could modulate magnetic moments and magnetic exchange coupling interactions and the change in spin polarizability. Zhang et al. [73] studied the microwave absorption properties of high-entropy hexaborides containing rare earth, including Ce, Y, Sm, Er, and Yb, prepared by a boron carbide reduction reaction, which showed good absorption properties with an effective bandwidth of 4.3 GHz, while Chen et al. [74] studied the absorption properties of high-entropy rare earth silicide carbides and high-entropy rare earth oxides based on different ratios of Tm, Y, Dy, Gd, Tb, Pr, and Tb ions, prepared by a solid-state reaction method, which showed good absorption properties with an effective bandwidth of 4.5 GHz. Absorbent, magnetic cores made of alloys based on Fe and Nb are suitable for EMI reduction signals with a 6–7 MHz frequency. However, Zamborszky et al. constructed a broadband coaxial probe head allowing the measurement of Fe-based nanocrystalline cores containing Nb up to 1 GHz using a reflectometry method with a vector network analyzer (VNA) [75].

The effects of the 4f-3d interaction on electronic locality, magnetism, and charge migration during hybridization between Er and Fe/Co were studied [22,76]. Due to enhanced EMI shielding properties, La, Ce, Pr, Gd texaphyrin, Tb, and Er were studied and selected as potential new shielding materials, as well as Co–Mg–La ferrite/graphene composites [77,78,79]. Gd³⁺-doped MnFe₂O₄ synthesized by the sol–gel method expressing very low dielectric loss, saturation, and minimum coercivity can be a potential material with applications in storage and microwave absorption devices [80]. Gd-doped Fe₃O₄ oxide (Gd_xFe_3−xO₄) samples with different percentages of Gd contents were prepared by the hydrothermal method [81].

The substitution of smaller Fe ions with RE ions leads to significant changes in physical properties such as the structural distortion of crystal units, the formation of a new phase, and changes in the morphological, optical, electrical, and dielectric properties due to the different ionic radii between ions.

The partial substitution of RE ions, such as with Sm, Ce, Er, Dy, La, and Nd for Fe³⁺, leads to a structural bend in the spinel structure which induces strain and considerably modifies the electrical and dielectric properties [82].

All ferrites exhibit a cubic spinel structure with the Fd-3m space group. The ionic radius of RE³⁺ ions in this environment (Gd³⁺ = 0.94 Å, Eu³⁺ = 0.95 Å, Sm³⁺ = 0.96 Å, Nd³⁺ = 0.96 Å, Pr³⁺ = 0.99 Å) is larger than that of Fe²⁺ ions (0.65 Å) [83]. The substitution of Fe²⁺ by RE³⁺ ions limits solubility in the lattice and inhibits grain growth. The shifting of the positions of diffraction peaks to smaller two-theta angles with an increase in the RE³⁺ concentration results from the increased lattice constant [84,85,86].

RE substitution has been reported to immensely affect cobalt ferrite’s structural, morphological, and physical properties [87,88]. It induces strain, causes the distortion of the crystal lattice, promotes the creation of vacancies, reduces crystallite sizes, increases grain–grain boundary interfaces, and promotes nanoscale magnetic effects such as spin canting and the core–shell effect. The insertion of RE in place of Fe promotes 3d–4f coupling, leading to variation in the magnetic, dielectric, and conduction mechanisms, and an enhancement in the SE can be obtained.

RE substitution can also decrease the grain size, which is an important factor in low-noise media. Cheng et al. obtained an increase in the polar Kerr rotation for Er- and Tm-doped cobalt ferrite films, while no significant changes in the magneto-optical response were observed for the Ho, Yb, and Lu ion substitutions [89].

For example, different Gd³⁺ doping concentrations in the ferrite structure led to noticeable variations in the magnetic saturation values. This can be explained by the finite size effects of nanoparticles, indirect interactions of Fe-Gd and Co-Gd and relatively weaker Fe-Fe interactions, or surface dipole disorder induced by lattice distortions upon dopant substitution with different ionic radii [90]. These interactions can significantly influence the electromagnetic properties of spinel ferrites [91]. A low-density polyethylene/MWCNT/graphene/LaFe₂O₃ composite and La³⁺ ions substituted into CoLa_xFe_2−xO₄ and NiLa_xFe_2−xO₄ resulted in enhanced microwave absorption at an X-band frequency range of 8–12 GHz [92,93,94].

The nanocomposite Gd-doped MoS₂/reduced graphene oxide Gd-MoS₂/(rGO) for high-performance EMI shielding applications was prepared using a hydrothermal method with a varying percentage of Gd doping. The MoS₂/rGO nanocomposite without dopant showed a low total SE, ~16.18 dB, and the one with 20% Gd-doped MoS₂/rGO nanocomposite showed a higher total SE, ~20.47 dB, in the frequency range of 8.0–12.0 GHz due to enhanced electrical conductivity, defect dipole polarization, and interfacial polarization [95]. A 5% Gd-doped MoS₂ electrode compound synthesized using cost-effective one-step hydrothermal methods can find applications in high-performance energy storage systems [96]. The importance and application of MoS₂-based microwave-absorbing materials are in current social needs, including military radar stealth and civil electronic communication [97].

Flower-shaped Gd-doped FeNi₃ and a novel raspberry-like absorbent based on a biomimetic design with high electromagnetic wave (EMW) absorption performance for an ultra-wide bandwidth of 12.24 GHz were designed by thermal catalysis [98]. These materials were investigated in the 2–18 GHz frequency range, while EMW attenuation was assigned to dipole polarization, multiple wave scattering, and reflections. According to free electron theory, closely related to electrical conductivity, the hierarchical flower-like and NiO/rGO composites were designed using the hydrothermal method. The NiO/rGO composites with a thickness of 2 mm showed an improved reflection loss of −60 dB at 9.8 GHz in comparison with a reflection loss of −40 dB at 10.8 GHz for NiO [99]. On the other hand, Pr³⁺-doped Co_0.5Zn_0.5Bi_0.4−xPr_0.1Fe_1.5+xO₄ spinel ferrite series with different contents of Bi synthesized by the micro-emulsion method and Pr-doped (Bi_0.5Na_0.5)_1−xPr_xTiO₃ ceramics obtained by using the conventional solid-state method have remarkable potential for application in EMI shielding in the X-band frequency range of 8.2–12.4 GHz [100,101].

Neodymium (Nd)-doped spinel ferrites induce an increase in the polarization intensity, improving their saturation magnetization, electrical resistivity, thermal stability, and ability to absorb microwave radiation, making them suitable for advanced electronics, telecommunications, and power systems [102,103,104,105]. Classes of Ce-doped W-type barium ferrites, Ba_1−xCe_xNi₂Fe_15.4O₂₇, with varying concentrations of Ce were successfully prepared by a sol–gel approach, whereby a Ce mole fraction of 0.3 gave the minimum reflection loss (RL) value of −52 dB, while a ferrite with Ce mole fraction of 0.4 had an RL value of −38 dB at 1.36 GHz frequency, suggesting the Ce enhancing EMW absorption of ferrites [106].

RE-metal-based MOFs have large specific surface areas. Their composites exhibit good dielectric loss, making them efficient EMW absorbers. The minimum RL value of the Ce(IV)-MOF is −32.12 dB at a frequency of 4.56 GHz at a thickness of 5.5 mm [107,108]. Excellent EMW absorption performance and mechanisms have been unveiled for the first time for Gd₂O₂S/rGO composites with an absorption capacity of −65 dB and an absorption bandwidth of 5.6 GHz. Gd₂O₂S nanosheets with a 1 nm thickness were produced via a facile hot injection method and mixed with rGO [109]. The EMW absorption performances (minimum reflection loss, RL, effective absorption bandwidth, EAB, thickness, and frequency range used) of various RE–ferrite composites, RE–transition metal intermetallics, REO composites, RE-MOFs, and other RE compounds are presented in [64].

2.3.1. Synthesis, Morphology, and EMI Shielding Efficiency of RE-Based Materials

Synthesis of Gd_xFe_3−xO₄, x = 0, 0.02, 0.04, 0.06, 0.08, and 0.1

A series of Gd_xF_e3−xO₄ materials with different Gd contents, x = 0, 0.02, 0.04, 0.06, 0.08, and 0.1, was obtained by using the hydrothermal method at a temperature of T = 180 °C and high vapor pressure. Gd(NO₃)·6H₂O, FeCl₃·6H₂O, and FeCl₂·4H₂O, as precursors of Gd³⁺, Fe³⁺, and Fe²⁺ ions, were dissolved in water separately and then mixed in the corresponding ratio, and the pH was adjusted up to the value of 12.8 using 10 M NaOH. The mixture was transferred to Teflon-lined stainless steel sealed containers and heated for 2 h at a temperature of 180 °C. After cooling, the obtained samples were washed several times with deionized water. For specific absorption rate (SAR) measurement, the obtained samples were functionalized by stirring overnight in a PEG solution [81].

Synthesis of CoFe_2−xDy_xO₄ Nanoparticles

The first series of CoFe_2−xDy_xO₄ nanoparticles with a stoichiometric ratio of Co(NO₃)₂·6H₂O, Fe(NO₃)₃·9H₂O, and Dy(NO₃)₃·H₂O precursor solutions in distilled water was synthesized by the co-precipitation approach. Oleic acid, followed by a liquid ammonia solution, was added dropwise at pH values of between 10 and 12. The obtained precipitates were washed and sintered at 500 °C for 2 h to form a fine powder. The nanocomposites of polypyrrole and 10 wt% CoFe_2−xDy_xO₄ were obtained via in situ chemical oxidative polymerization, and the synthesis is presented in detail in Figure 3 [110]. The obtained nanocomposites are promising candidates for applications in the 12.4 GHz to 18 GHz range, particularly in satellite communication systems, due to their impressive shielding performance, with an SE of approximately 16.3 dB at a shield thickness of 1.5 mm.

Synthesis of Gd- and Er-Doped α-MnO₂ Nanorods

Gd- and Er-doped α-MnO₂ with a homogenously distributed rod-shaped morphology, as can be seen in Figure 4, was prepared by a modified chemical-route-assisted hydrothermal method. Er(CH₃COO)₂, Gd(CH₃COO)₂, and Mn(CH₃COO)₂, as precursors of Er, Gd, and Mn ions, were dissolved in deionized water separately, according to the stoichiometry ratio of 7 wt.% to PVA; KMnO₄ was used as a reaction catalyst. The PVA was dissolved in hot (T = 80 °C), double-distilled water with continuous stirring. When it was dissolved in water, it became transparent. When the PVA became a transparent solution and cooled down to room temperature, the solution of Er/Gd and Mn ions precursors was added to it very slowly with constant stirring, followed by the addition of KMnO₄. The pH value was adjusted to ~10 by adding KOH solution drop by drop, and the mixture of solutions was left for aging. The brownish-black precipitate was centrifuged and washed several times with distilled water, put into the hydrothermal reactor, and heated at 160 °C for 8 h. The brownish-black precipitate was again centrifuged, washed, dried additionally at 80 °C for 18 h, mortared, and annealed at 450 °C for 2 h to acquire a fine powder of Gd- and Er-doped α-MnO₂ samples with good crystallinity. The EMI SE (presented in Figure 5a–d, showing total absorption and reflection and skin depth variation in the frequency range of 8 to 18 GHz) achieved a maximum of −43 dB at 15.3 GHz and −46 dB at 15.3 GHz for thin-layer Gd- and Er-doped α-MnO₂ at a of thickness ~600 μm, respectively. A schematic representation of the impact of hazardous EM waves radiated from various electronic sources on livelihood and a demonstration of the most possible EMI shielding mechanism to prevent it are presented in Figure 5e [111].

Rare earth elements exhibit high magneto-dielectric effects, and their dielectric properties can be tuned by applying an electric or magnetic field. This property makes them attractive for use in electromagnetic shielding applications where the magnetic field can be used to control the dielectric constant of the material. For example, the doping of MnO₂ with Er and Gd can enhance its dielectric properties, which make it a promising material for high-performance electronic devices. The incorporation of Er ions into the MnO₂ matrix can potentially modify the material’s electrical conductivity, magnetic properties, and structural stability, thereby influencing its effectiveness in attenuating electromagnetic waves [111].

2.4. Iron Oxide-Based Materials as EMI Shielding Material

Iron oxides are inorganic compounds of iron and oxygen that exhibit very different stoichiometries due to the difference in the Fe oxidation state. Briefly, they can be classified into three groups: oxides of Fe²⁺ (FeO, wüstite), oxides of Fe³⁺ (various forms of Fe₂O₃, hematite, maghemite), and combined Fe²⁺/Fe³⁺ oxides (Fe₃O₄, magnetite, Fe₄O₅, Fe₅O₆, etc.). Apart from the difference in the Fe oxidation state, many existing crystal structures also enhance the diversity of iron oxides depending on the synthetic routes. For example, a variety of polymorphs of iron (III) oxide has been reported to date [112]. α-Fe₂O₃ is the most common form of iron (III) oxide and occurs as the mineral hematite. It has a rhombohedral structure and shows antiferromagnetic behavior below ~260 K and weak ferromagnetism at temperatures of between 260 and 950 K [113]. Common methods to prepare α-Fe₂O₃ are thermal decomposition and precipitation in the liquid phase. The β-phase of Fe₂O₃ is metastable, and at temperatures above 773 K, it converts to the α-phase. It can be obtained by the thermal decomposition of iron (III) salts. Similar to the β-phase, γ-Fe₂O₃ is also metastable and converts to the α-phase at high temperatures [114]. It has a cubic structure, and in the bulk form, it is ferromagnetic, but γ-Fe₂O₃ nanoparticles less than 10 nm in size show superparamagnetic behavior. The methods of γ-Fe₂O₃ preparation involve the thermal dehydration of gamma iron (III) oxide-hydroxide, the modest oxidation of _Fe3O4, or the thermal decomposition of suitable iron salts [115]. The ε-phase is rhombic with properties between those of the alpha and gamma phases. It is metastable with a tendency to transform to the alpha phase at temperatures of between 773 and 1000 K. The synthesis of pure ε-Fe₂O₃ is challenging and involves the oxidation of iron in an electric arc or sol–gel precipitation from iron (III) nitrate [116]. In addition, an amorphous form of Fe₂O₃ also exists, which can be prepared under high pressure.

Oxides of iron have become interesting candidates for electromagnetic interference (EMI) shielding due to several factors. Fe₂O₃ is a magnetic material, while Fe₃O₄ is a ferromagnetic. Magnetic materials can interact with and absorb electromagnetic waves, especially at lower frequencies (like radio waves), thus reducing EMI. Iron oxides exhibit a certain level of electrical conductivity, allowing them to contribute to EMI shielding by reflecting or absorbing the electrical components of electromagnetic waves. In addition, they can contribute to the absorption of electromagnetic energy through dielectric loss, a mechanism that involves converting electromagnetic energy into heat. This makes it effective in dissipating energy and preventing the transmission of EMI. Finally, iron oxides are abundant, inexpensive, and easy to combine in composite materials, such as polymers or carbon-based materials, and can maintain their properties over time.

For EMI shielding applications, iron oxides are often combined with other materials, such as various polymers, graphene oxide, carbon nanotubes, or similar, to achieve desirable EMI shielding, mechanical strength, and electrical and thermal conductivity. Using an in situ polymerization technique, Azadmanjiri et al. created iron oxide and polypyrrole nanocomposites and examined their EMI shielding properties in the 0.1–18 GHz frequency range [117]. The composites’ intimate contact between the conducting and magnetic phases increased absorption by 10.10 dB at the instrument’s highest frequency limit (17–18 GHz), while iron oxide nanoparticles only increased the absorption by 2.6 dB. As the possible cause of this improvement, they proposed a better match between dielectric loss, magnetic loss, and improved dispersion of the magnetic/conductive nanocomposites in the matrix. Gupta et al. [118] reported that the microwave shielding properties were affected by the various morphologies of iron oxides. Using a two-step sol–gel process, they produced various ferrite structures, including cubes, rods, and flakes, covered with multilayer rGO. Their EMI shielding capabilities were evaluated in the Ku-band frequency range. In comparison to the flake- and cube-shaped iron oxides, the rod-shaped iron oxide covered with rGO sheets had the highest shielding efficiency value of ~33.30 dB (>99.9% attenuation). This resulted from the combined effect of magnetic and dielectric losses. Anisotropy energy in the composites, eddy current effects, and natural resonances were the sources of magnetic loss. The nanoferrite particle content of the composite was the main cause of eddy currents in the microwave ranges. The surface anisotropic field would result in higher anisotropy energy for the small materials due to the small-size effect. The higher anisotropy energy also contributed to greater microwave absorption. Moreover, the magnetic iron oxide was coated with an rGO layer, which increased the interfaces and surface polarization charges. Interfacial polarization is a significant polarization process, and the corresponding relaxation will lead to a loss mechanism. One of the main causes of dielectric loss could be interfacial polarization. It is well known that some heating-induced absorption losses are caused by the interaction of surface-formed molecular dipoles with the microwave field.

Dhawan et al. created a conducting ferrimagnetic PANI nanocomposite implanted with titanium dioxide (70–90 nm) and γ-Fe₂O₃ (9–12 nm) nanoparticles using micro-emulsion polymerization [119]. They discovered that the high shielding effectiveness value of −45 dB owing to absorption (SE_A) was caused by dielectric and magnetic losses that resulted from the combined action of γ-Fe₂O₃ and TiO₂. In contrast, the SE_A of PANI-TiO₂ was around 22.4 dB, but that of PANI-γ-Fe₂O₃ was approximately 8.8 dB. In another work, PANI tubes made of rGO coated with γ-Fe₂O₃ nanoparticles were synthesized and characterized by Singh et al. [120]. The intercalated iron oxide nanoparticles were produced by thermally breaking down ferric acetyl acetonate in a reducing atmosphere. The β-naphthalene sulphonic acid-induced oxidative polymerization of aniline, which produced the core–shell shape, was also used to enclose those nanoparticles. At a thickness of 2.5 mm, the presence of rGO-γ-Fe₂O₃ in the PANI core structures increased the composite’s interfacial polarization and effective anisotropy energy, which increased scattering and produced a high shielding efficiency of about 51 dB.

Iron oxides are frequently combined with different carbon nanomaterials. The composite material can effectively absorb, reflect, and dissipate electromagnetic radiation over a wider frequency range by fusing the electrical conductivity of carbon nanotubes with the magnetic qualities of iron oxides. Fe₃O₄-nanoparticle-loaded functionalized multi-walled carbon nanotubes were produced by Bhaskara Rao et al. [121]. They discovered a high overall specific shielding efficiency of around 49.56 dB/(g cm⁻³), along with improved absorption (15.85 dB) and reflection (9.43 dB). Liu et al. [122] developed trilayer-type laminated nanocomposites with a matching layer of 15 wt.% Fe₃O₄, an absorbent layer of 5 wt.% MWCNTs, and a reflecting layer of 10 wt.% MWCNTs. Their results showed that such trilayer-type laminated nanocomposites have an excellent ability to absorb microwaves up to 40 dB in the 13 GHz to 40 GHz frequency range. Ferroferric oxide (Fe₃O₄) and MWCNTs were integrated by Li et al. into a core–shell system made of high-density polyethylene (HDPE), polyvinylidene fluoride (PVDF), and polystyrene (PS) [123]. The composite with MWCNTs in the PS shell and Fe₃O₄ in the PVDF matrix had the highest SE, measuring 25 dB at 9.5 GHz with 1 vol.% Fe₃O₄ and 1 vol.% MWCNTs. The SE was over 20 dB throughout the tested frequency range (X-band). Prasad et al. [124] developed a facile two-step hydrothermal process for the synthesis of a MoS₂–reduced graphene oxide/Fe₃O₄ (MoS₂-rGO/Fe₃O₄) nanocomposite and its application as an enhanced shielding material against electromagnetic interference. The Fe₃O₄ nanoparticles were spherical and evenly distributed throughout the MoS₂-rGO composite. The MoS₂-rGO/Fe₃O₄ nanocomposite was found to be an extremely effective electromagnetic shielding material in the 8.0–12.0 GHz X-band, according to an examination of its electromagnetic shielding efficiency. The MoS₂-rGO composite showed low shielding performance (SE_T ~3.81 dB) compared to the MoS₂-rGO/Fe₃O₄ nanocomposite (SE_T ~8.27 dB). This was caused by interfacial polarization in the presence of an electromagnetic field.

By the chemical oxidative polymerization of pyrrole, Sambyal et al. created a conducting polymer-based composite encapsulated with barium strontium titanate (BST), rGO, and Fe₃O₄ nanoparticles [125]. Filler components in the conducting polymer matrix produced an absorption-dominated shielding efficiency value of 48 dB in the 8.2–12.4 GHz (X-band) frequency range. Furthermore, the chemical and thermal stability of the composite material was enhanced by the use of magnetic and dielectric fillers. By using a self-assembly process, rGO was also added to latex coupled with magnetic iron oxide (Fe₃O₄) and flexible natural rubber [126]. Compared to NRG composites, Fe₃O₄ enhanced the electromagnetic interference shielding effectiveness (EMI SE) of natural rubber/reduced graphene oxide (NRG) composites. The EMI SE value of the NRMG composite with 10 parts per 100 parts of rubber rGO is 1.4 times higher than that of the NRG composite with the same rGO content in the 8.2–12.4 GHz frequency band. With a specific EMI SE of 26.4 dB mm⁻¹, the NRMG composite outperforms the previously reported polymer/Fe₃O₄@rGO composites with a low rGO concentration. Remarkably, the NRMG composite’s EMI SE only decreases by 3.5% after 2000 bending–release cycles, indicating that it may find application in flexible shielding materials.

In order to create 3D network porous graphene nanoplatelet (GNP)/Fe₃O₄/epoxy nanocomposites with a low density of 0.34–0.73 g/cm³, Liu et al. suggested a new and simple method called epoxy–water–inorganic filler suspended emulsion polymerization [127]. The porous nanocomposite that resulted from loading 7 wt.% graphene nanoplatelets and 7 wt.% Fe₃O₄ nanoparticles showed a satisfactory specific electromagnetic interference (EMI) shielding effectiveness of about 37.03 dB/(g/cm³), which was much higher than that of the solid equivalents (28.30 dB/(g/cm³)). In another study, Fe₃O₄/thermally annealed graphene aerogel (Fe₃O₄/TAGA) was created by first thermally annealing ethylenediamine-functionalized Fe₃O₄ (NH₂-Fe₃O₄) nanoparticles with graphene oxide (GO), followed by the addition of l-ascorbic acid [128]. Then, the Fe₃O₄/TAGA/epoxy nanocomposites were created using a template-casting technique. The resulting Fe₃O₄/TAGA/epoxy nanocomposites achieved the highest electromagnetic interference shielding effectiveness (EMI SE of 35 dB in the X-band) when the mass ratio of GO to NH₂-Fe₃O₄ was 2:1 and the total mass fraction of Fe₃O₄/TAGA was 2.7 wt.% (comprising 1.5/1.2 wt.% Fe₃O₄/TAGA). This was significantly better than epoxy nanocomposites with the same Fe₃O₄/thermally annealed graphene oxide (Fe₃O₄/TAGO) loading, which only showed an EMI SE of 10 dB.

Liu et al. created magnetic reduced graphene oxide rGO@Fe₃O₄ nanoplatelets (NPs), which were used as fillers, by co-precipitation and electrostatic self-assembly [129]. The nanocomposites were made by applying external magnetic fields to align rGO@Fe₃O₄ NPs during epoxy curing. Because of the anisotropic properties of rGO@Fe₃O₄ NPs and external magnetic fields, the nanocomposite containing aligned rGO@Fe₃O₄ NPs showed anisotropic thermal conductivity. The produced sample exhibited exceptional thermal stability and 13.45 dB EMI shielding at 8.2 GHz. Overall, the rGO@Fe₃O₄ NPs’ in-plane interaction was enhanced by aligning them under a magnetic field, which promoted the growth of horizontal thermal conductive networks. Reflection is the main EMI shielding mechanism. Three-dimensional Fe₃O₄-decorated carbon nanotube/reduced graphene oxide foam/epoxy (3D Fe₃O₄-CNT/rGF/EP) nanocomposites with highly aligned three-dimensional structures were created by Liang et al. using a simple template approach [130]. The obtained 3D Fe₃O₄-CNT/rGF/EP nanocomposites with 0.24 wt.% rGF and 2.76 wt.% Fe₃O₄-CNTs showed a remarkable electrical conductivity of 15.3 S/m and an EMI SE of 36 dB within the X-band range, which was a nearly 482% improvement when compared to the EMI SE value of physically blended Fe₃O₄-CNT/EP nanocomposites without a three-dimensional structure (~6 dB). Using a supercritical carbon dioxide (Sc-CO₂) foaming technique, lightweight and flexible methyl vinyl silicone rubber (VMQ)/multi-walled carbon nanotube (MWCNT)/ferriferrous oxide (Fe₃O₄) nanocomposite foams with superior EMI shielding capabilities were created [131]. The addition of a cellular structure and magnetic Fe₃O₄ nanoparticles greatly improved the VMQ/MWCNT/Fe₃O₄ foams’ microwave-absorbing capacity by successfully lowering secondary electromagnetic wave interference brought on by reflection. In the 8.2–12.4 GHz frequency range, these nanocomposite foams, which have an approximate density of 0.48 g/cm³, showed an EMI shielding efficiency (SE) of 27.5 dB and an average absorption ratio of up to 64%. The foams had a specific EMI SE of about 72 dB g⁻¹ cm³ and a high conductivity of about 14.6 S/m with a filler loading of 1.78 vol.%.

Shu et al. [132] used rice-husk-based activated carbon (AC) and produced compo-sites with acicular or octahedral Fe₃O₄ nanoparticles. A significant effect on the EMI shielding effectiveness of the composites was assigned to the morphology of the metallic nanoparticles and the layered structure of the C component of the composites. They achieved an EMI SE of −52.14 dB.

Record values of EMI shielding effectiveness were achieved for MXene by incorporating magnetic nanoparticles as intercalators between layers [133] and 3D N-doped GO with silver nanowires (−79.99 dB) at a thickness of 2.660 mm [134].

3. Conclusions

This review has provided a comprehensive overview of the developments, classification, trends, and advances in new electromagnetic shielding materials. It gives a thorough literature review of the EMI shielding properties of new EMI shielding materials such as biochars, scaffolds, rare earth, and ferrite-based materials. A detailed discussion is given regarding the preparation methods, structure, EMI shielding performance, EMI shielding mechanisms, and application perspectives of these materials.

Looking ahead, the prospects for and research on electromagnetic shielding materials should focus on scalability, improving performance, cost-effectiveness, sustainability, versatility, and production methods to advance new materials like biochars, scaffolds, rare earth, and ferrite-based materials as novel EMI shielding solutions. Biochars are promising eco-friendly, sustainable, and renewable materials that can be potentially used as EMI shielding materials in electronic devices and construction materials, similar to scaffolds, new-generation, easy-to-manufacture materials with excellent EMI shielding performance. Although MXene, carbon nanotubes, and graphene-based composites are highly efficient EMI shielding materials due to their large free surface area, high aspect ratio, chemical stability, and lightweight nature, the large number of synthetic phases leads to their high price. Carbon-based nanomaterials derived from biomass are being developed in a quest for new, cost-efficient, sustainable, and affordable materials.

The current disadvantages of these materials are the thickness of the shielding barrier and lack of transparency, which limit some applications.

Materials doped with rare earth elements have been developing over the last few years. They have specific structures, interfacial polarization, multiple reflections, and excellent conductive and EMI shielding properties. Combining the magnetic properties of ferrite-based materials with the electrical conductivity of carbon nanomaterials, composite materials can efficiently absorb, reflect, and dissipate electromagnetic energy across a broader frequency range. Although the materials showed outstanding EMI shielding performance, the price of REEs will limit the application of these composites to more sophisticated and sensitive fields such as medicine.

Their EMI shielding effectiveness, good magnetic and dielectric properties, excellent thermal stability, and high electrical conductivity and mechanical strength make biochars, scaffolds, rare earth, and ferrite-based materials ecological and sustainable solutions as new products for blocking EMWs.