Carbon-based microwave-absorbing materials (MAMs) have received attention in recent decades, from fundamental theoretical study to experimental design and analysis, due to the pursuit of this type of MAM for their excellent microwave-absorbing ability, and broad efficient bandwidth features, and lightweight and thin format (Zhao et al., 2016; Guan et al., 2021). Although conjugated carbonaceous materials with different morphologies have high dielectric losses, single dielectric loss shows inefficient bandwidth in electromagnetic absorption (EMA) owing to impedance mismatching. Metals, metal oxides, metal sulfides, elements, and other magnetic and dielectric materials have been anchored to the carbonaceous materials including carbon nanotubes (CNTs) (Munir, 2017; Peymanfar et al., 2019b; Peymanfar et al., 2019c; Mo et al., 2019; Chen et al., 2021a; Liu et al., 2021; Wang et al., 2022), carbon fibers (CFs) (Peymanfar and Moradi, 2020; Wang et al., 2020; Chen et al., 2021b; Chen et al., 2021c; Gunwant and Vedrtnam, 2021; Song et al., 2021; Wu et al., 2021), carbon spheres (Ning et al., 2020a; Zhang et al., 2020a; Peymanfar et al., 2021a; Song et al., 2021), carbon micro-tubes (Peymanfar et al., 2021d), activated carbon (Mahmoodi et al., 2022), graphene (Li et al., 2017; Meng et al., 2018; Wang et al., 2019a; Peymanfar et al., 2019d; Zhi et al., 2021; Wu et al., 2022), carbon net-like morphology (Peymanfar and Ghorbanian-Gezaforodi, 2021), and carbon black (Ibrahim et al., 2020; Lalan and Ganesanpotti, 2020). This phenomenon can effectively enhance the microwave-absorbing ability of the conjugated structures due to the tunable electromagnetic properties and emergent synergetic loss mechanisms in the tailored composites (Duan et al., 2018; Chen et al., 2021b; Guan et al., 2021; Kumar et al., 2021).

In recent years, conjugated carbonaceous materials composited with different structures that provide outstanding microwave absorption have been reported. For instance, Che et al. architected a special conductive network with two types of carbonaceous structures with increased conductive and dielectric features. The ternary-phased C@Fe₂O₃@MWCNTs (CFC) showed diverse mechanisms consisting of the combination of relaxation and conductive losses, leading to thin, lightweight, and high-performance microwave absorbers. Figure 2A shows the possible microwave-absorbing mechanisms in the absorbing medium.

As shown in Figure 2Ba, higher MWCNT content causes more permittivity, suggesting a more conducive polarization loss mechanism.

Figure 2Bc shows a diagram of RL versus the frequency of microwave absorbers with diverse MWCNT ratios. The optimized absorber showed high-performance MA, with an RL of −49.9 dB at 2.0 mm in thickness. The multiphase component augmented the heterogeneous interfaces desirable for interfacial polarization (Wang et al., 2019a).

Wang et al. used two fascinating ternary structures, Ni@carbon nano-onions and Ni/carbon nanotubes anchored to graphene surfaces (Ni@C/G and Ni/CNT/G), tailored through an atomic layer deposition (ALD) NiO. Two different carbonaceous morphologies comprising carbon nano-onions and CNTs were grown on the surface of graphene using a chemical vapor deposition scenario. The unique morphologies provided the absorbers with multiple reflections and scattering, relaxation loss, natural and exchange resonance, eddy current loss, impedance matching, and conductive networks, which promoted microwave absorption. The results showed that the hybrid architectures demonstrated proper microwave-absorbing features with low filling ratios at low-frequency ranges compared to the related structures. The associated microwave-absorbing mechanisms are summarized in Figure 3A.

The ε′ and ε" of NCG with different Ni ratios are shown in Figures 3Ba, b. The imaginary and real parts of the permittivity are tunable by varying the Ni content. The permittivity curves essentially originated from the conductive loss and polarization. The complex permeability curves of NCG samples are shown in Figures 3Ca, b. The bumps are associated with natural and exchange resonances due to the presence of Ni. NCG 100 showed a maximum RL of −45.5 dB at 6.2 GHz with efficient bandwidth as wide as 5.6 GHz at a thickness of 2.5 mm. Figure 3D compares the RLs of the samples with diverse Ni contents, demonstrating the effect of Ni amount on microwave absorption (Xu et al., 2020).

Peymanfar et al. designed an interesting microwave-absorbing nanocomposite by wrinkling a nickel nanosheet and using it to cover grape-like carbon microspheres (CMSs). They also investigated the effects of the absorbing medium on the characteristics of microwave absorption. In the reported work, 2D Ni nanostructures were first prepared. Next, CMSs fabricated by the conventional hydrothermal method were decorated with the wrinkled nickel nanosheets using an ultrasonic and hydrothermal procedure. The special morphology obtained by wrinkled structure enhanced the polarization loss, boosting the microwave absorption. CMSs have a high surface area, low density, and proper dielectric properties owing to their conjugated structures. The effects of the combination of CMSs and Ni in PS and PVDF matrices showed improved electromagnetic absorption. Loading the CMSs diminished the matching thickness by modulating the complex permittivity and permeability, governed by the quarter wavelength mechanism. Compared to the sample fabricated by PS, PVDF showed increased impedance matching and magnetic loss while reducing the relaxation loss. All the tailored samples exhibited narrow band gaps and substantial magnetic features. Notably, the results demonstrated that the architected microwave-absorbing and shielding structures showed considerable reflection loss, efficient bandwidth, and shielding properties with thin thickness owing to the wrinkled morphology of Ni and the applied media. Figure 4A showed the possible microwave-absorbing mechanisms in the absorbing matrix. As shown in Figure 4C, the Ni-containing composites exhibited diverse notches due to their natural and exchange resonances. Substitution of the PS by PVDF increased the imaginary part of the permeability but diminished both real and imaginary parts of the permittivity. The results also demonstrated that loading the CMSs generally boosted the real part of the permittivity compared to the Ni/PS nanocomposite. Moreover, the significant real part of the Ni(OH)₂/PS was established by dipole polarization and enhanced n → π* transitions owing to the existing hydroxyl functional groups. Figure 4B shows the diagram of RL versus the frequency of microwave absorbers for the combination of different components. A comparison of the curves demonstrates the importance of carbon-based microspheres in ameliorating the microwave absorption. The maximum RL of the wrinkled Ni/PS was −50.53 dB at 11.52 GHz while that for the CMS/Ni/PVDF was −90.29 at 10.68 GHz (Peymanfar et al., 2021a; Lv et al., 2021).

Diao et al. (2021) were the first to design functional Bi₂Te_2.7Se_0.3/graphene foam (BTSGF) composites with thermoelectric materials Bi₂Te_2.7Se_0.3 (BTS) and GF showing satisfactory conductive and dielectric properties. In this structure, GF is the main component, with a conjugated backbone and high porosity that ameliorates the wave path, impedance matching, and RL. This material has intrinsic electromagnetic-absorbing properties due to its conjugated structure, resulting in conductive and relaxation losses as well as multiple reflections and scattering. To further improve the absorbing strength of the GF, the authors combined the BTS and graphene to form Bi₂Te_2.7Se_0.3/GF (BTSGF) composites through a solvothermal self-assembly method. The BTSGF composites showed enhanced microwave attenuation (MA) properties due to the various synergistic effects of BTS and GF, including conductive and polarization losses. More importantly, the BTS susceptibility to energy conversion is desirable for electromagnetic wave attenuation. Therefore, the authors proposed a new electromagnetic wave thermo-electric loss MA mechanism, which is of great significance to the study of thermoelectric materials as high-efficiency MA materials.

Figure 5A shows the frequency dependence of the ε′ of BTSGF for different BTS loadings and annealing temperatures. As the calcination temperature increases, the ε′ of BTSGF with a lower BTS loading is basically unchanged, while the ε′ of BTSGF with a higher BTS loading (especially 90 wt%) increases significantly. The representative RL curves of BTS9GF-200 with different thicknesses are shown in Figure 5B. BTSGF with high BTS loading and annealing temperatures showed increasing thermoelectric properties and improved microwave absorption. Therefore, both the BTS loading and annealing temperature help to amplify the electromagnetic elimination of BTSGF. The optimized absorber showed high-performance MA properties with an RL of −73.0 dB at 5.0 mm in thickness (Diao et al., 2021).

Cao et al. (2015) (https://pubs.rsc.org/en/content/articlelanding/2015/tc/c5tc02185e/unauth) produced ultrathin graphene as a lightweight microwave absorber with high efficiency in thermally harsh environments by fabricating graphene oxide (GO) according to the modified Hummers’ method and subsequent solution-based GO reduction. Graphene is a fascinating carbon-based material for microwave absorption owing to its lightweight structure; high specific surface area; notable charge carrier mobility; abundant defects; and numerous functional groups including hydroxyl, epoxy, and carboxyl groups, which increase relaxation loss. The composite exhibited different dependencies on changing temperature and concentrations toward imaginary permittivity and microwave absorption.

Figure 6A shows the imaginary parts of permittivity versus temperature for different frequencies and ratios of graphene composites. The graph demonstrates that the conductivity and polarization tune the imaginary part of permittivity. The higher the graphene content, as the conjugated and conductive parts, the higher the permittivity, indicating more conductive and polarization losses. The imaginary permittivity of the graphene (3 wt%) composite was lower than others because the conductive network is not formed in the composite; thus, polarization loss is the main factor. Conductive networks form in the graphene (7 wt%) composite, augmenting the conductive loss. Therefore, a balance is formed between the increasing conductivity and decreasing polarization with increasing temperature; hence, the imaginary permittivity remains stable. In addition, the conductivity of the composite is higher in the 11 wt% composite, indicating that the imaginary permittivity increased with increasing temperature. Figure 6B demonstrates the reflection loss versus frequency for different graphene ratios. However, the expected result was better RL in the 11 wt% composite due to better dipole polarization and hopping conductivity; at some point, more GO will lead to more impedance mismatching and more reflection. Therefore, the 7 wt% composite showed better microwave-absorption properties compared to the 3 wt% and 11 wt% composites, showing the best RL of −42 dB with 2.1 mm thickness at 413 K. Accordingly, the fabricated ultrathin graphene composites showed highly efficient microwave absorption and thermal-stable permittivity at elevated temperatures.

Ji et al. designed a microwave absorber consisting of conjugated organic and inorganic sulfide materials to enhance MA performance and reduce IR emissivity. The authors modified the phase components (reduced GO (rGO) and CuS phases) and micro/nanostructure (microporous and nanosheet) by adjusting the amounts of CuS. Additionally, they changed the method of reduction (hydrothermal and ascorbic acid reduction), resulting in variations of pore structure and defects and modulation of the complex permittivity and microwave absorption. According to the structure of the composite aerogels, including rC-3 (hydrothermal reduction, 30 mg CuS), rC-4 (hydrothermal reduction, 60 mg CuS), and RC-4 (ascorbic acid reduction, 30 mg CuS), the EM parameters were further explored. As shown in Figures 7a–d, various CuS@rGO composite aerogel structures can be obtained by adjusting the amount of CuS and reduction methods. In addition, as illustrated in Figures 7d, e, f, rC−4 had the largest average dielectric complex permittivity, implying the stronger dielectric loss due to the higher CuS content in rC-4 compared to rC-3 due to more interfacial polarization. Furthermore, rC−4 formed more defects than RC-4 because the rGO in rC−4 was reduced at 120°C, compared to 95°C for RC-4. Moreover, rC−4 had a larger pore diameter than RC−4, which allowed higher attenuation of electromagnetic waves (EMWs). Although rC−4 showed excellent dielectric loss, RC-4 displayed excellent attenuation capacity (α) and impedance matching (Z) among the composite aerogels, with good absorbing performance. The superior efficiency may have occurred due to the higher number of functional groups and defects. As shown in Figure 7B, CuS@rGO composite aerogel obtained using ascorbic acid as a reductive agent showed an RL of −60.3 dB with a lower filler content (2 wt%) at a thickness of 3.5 mm. Thus, this work offered a facile method to design and develop porous rGO-based composite aerogel absorbers with radar-IR compatible stealth (Wu et al., 2022).

Cao et al. (2010) prepared a short carbon fiber/silica composite via conventional ceramic processing for practical applications in high-temperature microwave absorption. Carbon fibers (CFs) and their composites are attractive candidates as microwave absorbers due to their excellent electrical and mechanical properties for countless domestic and commercial applications. Moreover, their material properties can be predicted by calculating the reflection loss from the permittivity and permeability, which decreases the number of experiments required. The real part of the permittivity increases with increased temperature, which is ascribed to decreasing the relaxation time of electron polarization, and the imaginary part augments as well, which in this case originates from the enhanced electrical conductivity of the carbon fibers. Figures 8Aa, b, Ba, b show the RL dependence on temperature at various frequencies. The RL-temperature curves at five different frequencies imply a maximum CF/SiO₂ absorption peak around −10.22 dB at 9.90 GHz at 30°C, and −8.69 dB at 9.25 GHz at 600°C, indicating that the peak slightly shifts toward lower frequency with increasing temperature. These results showed that the microwave absorption of CF/SiO₂ at lower temperatures is greater than others at higher temperatures.

Zhao et al. (2019) fabricated a hierarchically porous magnetic carbon (HPMC) structure by introducing a magnetic substance on microstructure biomass material, which demonstrated excellent EMW performance. Biomass-derived materials provide novel microstructures, including heterostructures with unique morphology such as microtubular channels, which are challenging to produce by routine chemical synthesis (Peymanfar and Mirkhan, 2022). The authors used rice as an example of biomass materials and conducted pre-treatment by a simple dipping and sintering method to attain porous carbon with the benefits of Ni²⁺ incorporation as a magnetic resource. The regulation of the microstructure and composition of the samples was synchronously and simply achieved by modulating the ratio of KOH to rice treated with Ni²⁺ (NTR) powder. This work paved the path for the development of biomass materials as green, renewable, low-cost, and high-performance carbon-based microwave absorbers. Figure 9A, B shows the complex permittivity curves versus frequency for different KOH to NTR powder ratios. The HPMC-1.5 sample is composed of high-density metal Ni NPs; thus, it is reasonable that the low filler ratio of 15 wt% for HPMC-1.5 in the paraffin matrix results in low permittivity properties. Nevertheless, both the ε′ and ε″ values of the other three specimens increase with increasing metal ratios. At low filler loading (15wt%), HPMC-1.0 showed better microwave absorption, which was mainly ascribed to the significant dielectric loss with polarization relaxation, conductive loss, multiple reflections, and synergistic magnetic loss. Figure 9B shows the remarkable RLs of HPMC-1.0 due to its fascinating hierarchical porous channels in nano/micron-scale, three-dimensional (3D) interconnected network, and favorable nickel content, which improved impedance matching. The sample showed a maximum RL of −52 dB at only 1.7 mm and wide efficient bandwidth of 5 GHz at a low filler content of 15 wt%.

Synthetic polymers such as polyesters and polyamides offer several advantages compared to natural polymers, including the easier synthesis of large quantities, non-significant batch-to-batch variations, and countless possibilities for modifications to achieve the desired properties (Maitz, 2015; Englert et al., 2018). As only the formulation parameters control the final properties of the materials, the chemistry of conductive polymers offers wide diversities of synthetic methods, allowing both the integration of conductive elements in different media and the construction of complex structures. This ability not only provides microwave-absorbing characteristics but also conforms to environmental constraints (Olmedo et al., 1993; Olmedo et al., 1995). These factors make polymers promising candidates to solve numerous problems in the microwave-absorbing field.

Gu et al. fabricated a metamaterial composite (metacomposite) in which carbon nanospheres (CNS) were discovered and supported on nanosilver-polydopamine (CNS-PDA/Ag). The impedance and optical band gap indicated that the CNS-PDA/Ag metacomposite broadened the conductive pathway to promote electrical conductivity. The remarkable point of this study was the high negative permittivity obtained from the merged secondary fields due to the unique structure of the hydrothermal and high-temperature annealing method.

As shown in Figures 10A, B, CNSP showed positive permittivity, while CNS showed negative permittivity. The explanation for this finding is the higher graphitization of CNS, which increases the electrical conductivity. By modifying the surface of CNS using PDA, the oxygen-containing functional groups were enriched, leading to a reduction in the electrical conductivity of CNS, regulating the conductivity in CNS-PDA. In CNS-PDA/Ag, the Ag nanoparticles are deposited on the surface of the CNS-PDA. Ornamenting the structure with PDA decreases the conductivity and expands the CNS-PDA structure. The negative permittivity in the CNS and CNS-PDA/Ag is caused by the increased electrical conductivity. The carbonization of CNSP and deposition of Ag nanoparticles facilitate charge circuits. The increased negative permittivity in CNS-PDA/Ag can be ascribed to the Maxwell–Wagner–Sillars effect. All three components—CNS, PDA, and Ag—expose anisotropy in dielectric properties and electrical conductivity, further impacting the electrical features of the final metacomposite (Gu et al., 2019; Lv et al., 2020).

In the last decade, polypyrrole (PPy) as a conjugated organic polymer with N as a heteroatom has gained attention as a microwave absorber (Peymanfar et al., 2018). Qi et al. fabricated a microwave absorber using conjugated conductive polymers. Conjugated microporous polymers (CMPs) are suggested due to their conjugate molecular fragments and permanent micropores. The process and precursors used to prepare the CPTPB, CPTPA, and CPTB are depicted in Figure 11A. The authors evaluated the permittivities of CPTPB (TB/pyrrole1:1), CPTPA (Tris (4-(PTPA)/pyrrole 1:3), and CPTB (PTB/pyrrole 1:3) with different ratios of pyrrole as a monomer. As shown in Figures 11Ca–f, CPTPB showed higher permittivity compared to that of pure PPy. Interestingly, the permittivity decreased with increasing amounts of pyrrole amount from CPTPB-3 to CPTPB-5, as shown by the reduced conductive and relaxation losses. Figures 11Ce, f confirm the increased permittivity by substituting CPTPB and CPTB with CPTPA. The synergic effect produced by the π-π stacking in the carbon-based conjugated structures was precisely dissected. This phenomenon tunes the heterogeneous interfaces, benefiting the interfacial polarization (Jiao et al., 2020).

To evaluate the effect of the doping agent ratio on the microwave-absorbing characteristics and energy band gap, Javanshir and coworkers used dodecylbenzenesulfonic acid (DBSA) as a doping agent to regulate polyaniline (PANi) features. The results showed that the amount of doping agent is a compromise with the energy band gap, DC electrical conductivity, and microwave absorption performance. Noticeably, polyacrylonitrile (PAN) was used as an absorbing medium instead of conventional paraffin wax. Paraffin has poor mechanical properties, which limit its practical applications. PAN can be used as a polymeric matrix in two ways: first, the useful mechanical features of PAN promote its practical applications. Second, the presence of nitrile functional groups allows the n→σ*, n→π*, and π→π* transitions to augment the polarization, relaxation loss, electron hopping, micro currents, and conductive networks desirable for microwave attenuation. Figure 12Aa–d shows the dependence of frequency on the complex permittivity of the sample. The regulation of the molar ratio between aniline and DBSA (Aniline: DBSA = 10.0, 7.5,5.0.2.5, denoted as PANi10.0, 7.5, 5.0, and 2.5) modifies the morphology and microwave-absorbing ability of the tailored conjugated polymers. The microwave-absorbing features affect the specific surface area, modulating polarization loss and conductive capacity, which are adjusted by oxidative doping. The results showed that the amount of oxidative dopant regulates the energy bandgap, influencing the relaxation loss. Figure 12B presents the microwave absorption performance of the samples with different molar fractions of the doping agent. The specific mechanisms originating from the architected morphologies and chemical structures led to the metamaterial features and permeability of the samples, promoting impedance matching and microwave-absorbing properties (Peymanfar et al., 2021b; Yang et al., 2022b) (https://www.sciencedirect.com/science/article/abs/pii/S0032386120308065).

Researchers have also assessed 2D nanomaterials. Carbon nanotubes and carbon-based structures (exclusively graphene and g-C₃N₄) have attracted attention due to their high degrees of microwave absorption (Peymanfar and Ghorbanian-Gezaforodi, 2020; Peymanfar et al., 2021e). Mohammadi et al. fabricated g-C₃N₄/polythiophene (PTh) composites utilizing various mass fractions of conjugated structures, resulting in efficient bandwidths for all samples. This occurred for polarization and conductive losses, which was promoted by π→π* and n→π* interactions between the PTh and nanosheets, as well as the metamaterial features in the composites, which provided permeability in the microwave absorbers and improved impedance matching and microwave attenuation. As shown in Figure 13Aa–d the real and imaginary parts of permittivity in the g-C₃N₄/PTh (30%)/PS increased due to interactions between the 2D structures and PTh, boosting the dielectric characteristics. The charge transition between the conjugated structures establishes conductive networks leading to the negative part of permeability of the g-C₃N₄/PTh (70%)/PS. Figure 13B shows the graph of RL versus the frequency of g-C₃N₄/PTh (30%)/polystyrene (PS). PS was used as an absorbing matrix, which has practical applications for the final product due to its mechanical features and the regulation of microwave absorption owing to its chemical structure (Peymanfar et al., 2020a).

Chang and co-workers designed a Fe₃O₄/polypyrrole/carbon nanotube ternary composite in which Fe₃O₄ nanoparticles were synthesized by a hydrothermal method at different sizes (H- Fe₃O₄, 100 nm (and (C- Fe₃O₄, 20 nm). First, the Fe₃O₄/PPy composite was prepared; CNTs were then added to the final composition as a reinforcer. In this composite, PPy and CNT increased the permittivity with the dielectric properties. In addition, the amount of PPy directly affected the RL. The results showed that when PPy is composited with Fe₃O₄, the magnetic properties of Fe₃O₄ and the permeability decreased. Furthermore, increasing the amount of CNT showed no significant change in μ′ and μ″, suggesting that Fe₃O₄ nanoparticles played a crucial role in complex permeability. The value of ε′ was less than the value of ε″ and the HPC series composite showed the highest ε′ and ε″ values. The results indicated that with increasing PPy, ε″ increased; however, the presence of polypyrrole without magnetic material was not efficient and the combination was required to improve ε′ and ε″ and, ultimately, microwave absorption. Figure 14Aa–c shows that, among the samples, the HP3C composite with 20% CNT showed the best absorption, with an RL of −25.9 dB at 2.0 mm in thickness (Yang et al., 2016).

Zhang et al. reported a conductive network constructed by a binary core-shell PPy/Fe3O4 composite prepared by chemical oxidative polymerization with polyvinyl alcohol and p-toluenesulfonic acid. The dielectric properties of PPy and the magnetic loss of iron affected the absorption parameters; thus, the combination of these materials created efficient microwave absorption. The diameter of the PPy shell varied from 20 to 80 nm to regulate the microwave absorption properties. Important points in the preparation of PPy/Fe₃O₄ composites are physical and chemical approaches. The preparation of PPy/Fe₃O₄ composites by chemical method demonstrated better RL performance from the broadened interfaces. Figure 15 shows that by increasing the mass fraction of PPy, ε′ and ε″ are enhanced because PPy increases the conductive loss due to the presence of conjugated bonds and dielectric properties; however, μ′ and μ″are decreased due to the reduction of magnetic filler. Figure 15 shows the best RL of -31.5 dB (15.42 GHz) at 2.5 mm thickness for a PPy shell thickness in pyrrole@Fe3O4 composites of 80 nm (Qiao et al., 2016).

The magnetic shortcomings leading to the impedance mismatching of the conjugated polymers as dielectric structures can be addressed by inserting magnetic structures. Moreover, the polarization mechanisms and impedance matching can be tuned by loading other dielectric materials. Table 1 shows some binary and ternary composites designed to mitigate the EMWs.

Meta-materials (MTMs) are the new generation of artificially manufactured materials that are not found in the natural environment (Jia et al., 2018). In recent years, MTMs have been the subject of extensive studies to enhance and improve their characteristics and applications (Zhang et al., 2020c). MTMs are important due to their unusual electromagnetic properties including backward propagation, negative refraction index, reverse Doppler effect, negative permeability, negative permittivity, and perfect absorber action (Chen et al., 2006; Zhou et al., 2009; Qin and Brosseau, 2012; Song et al., 2013; Cheng et al., 2017a). Instead of relying on their combination, the exotic electromagnetic characteristics depend on the sub-wavelength geometry of periodic resonance structures (Xin et al., 2017). Due to the unique properties of MTMs, they have been employed in vast and various fields such as filters (Sabah and Uckun, 2009), electromagnetic cloakings (Cummer et al., 2006; Schurig et al., 2006; Rajput and Srivastava, 2014; Colombi et al., 2015), super-lenses (Aydin et al., 2007; Amireddy et al., 2016), radars (Kurihara et al., 2005), antennas (Enoch et al., 2002; Alici and Özbay, 2007), solar cells (Hao et al., 2010; Bian et al., 2013), concealments (Tak et al., 2016), reduction of automotive false imaging, and microwave absorbers (Xin et al., 2017).

Landy et al. (2008) were the first to introduce the perfect metamaterial absorber (MMA) consisting of split-ring resonance structures with a middle dielectric layer. Generally, MMAs include three layers: a dielectric spacer, metallic structures, and a metallic mirror, which can be used to form MMAs with various designs (Duan et al., 2014; Yan et al., 2014; Zhou et al., 2015). The nearly perfect absorption, ease of fabrication, lower cost, and small thickness are some advantages of MMAs over conventional absorbers (Sood and Tripathi, 2015; Lee et al., 2016). Despite the many advantages of MMAs, this type of microwave absorber has a narrow absorption bandwidth, which is mainly related to the strong magnetic and electric resonances (Sood and Tripathi, 2015; Cheng et al., 2017a; Xin et al., 2017). To solve this issue, researchers have designed dual, multi, and broadband MMAs with high absorbing performance (Hu et al., 2016; Lian et al., 2016; Rufangura and Sabah, 2016; Wang, 2016). Some researchers have proposed enhancing the bandwidth by employing multi-resonance units with diverse geometric dimensions (Li et al., 2011; Ghosh et al., 2013; Park et al., 2013; Viet et al., 2014). Other researchers have extended the efficient bandwidth by constructing multi-layer absorbers by vertically stacking multiple various metallic resonance structures (Xiong et al., 2013; Yang et al., 2013; Bhattacharyya et al., 2015). Some bandwidth enhancement approaches, including the frequency tunable method and lumped elements loading technique, have also been utilized for bandwidth enhancement (Kim et al., 2016; Zhang et al., 2016). These methods have disadvantages such as increasing unit dimension, greater thickness, and formation difficulty that limit their application. Consequently, there is an urgent need to design a single-layer MMA with high efficiency and broadband absorption (Xin et al., 2017).

Sun et al. (2021) fabricated a low-frequency ultra-broad bandwidth metamaterial for microwave absorption based on activated waxberry. Applying natural hierarchical biomass is one solution for the contradiction between electromagnetic loss and impedance matching in microwave absorbers. The metamaterial demonstrated ultrabroad bandwidth properties, with an efficient absorbing band covering 1–40 GHz. An incident-angle independent feature (from 5° to 40°) was also detected. The high performance of this metamaterial was the result of good interface impedance matching of the fabricated metastructure and chemical activation, leading to a high dielectric loss. Additionally, the carbonized waxberry gradient radial microstructure is one essential reason for the broadband microwave absorption (Figure 16).

Xu et al. (2021) also fabricated a metamaterial microwave absorber based on planar indium tin oxide (ITO) and low-loss glass substrates that showed polarization insensitivity, wide operation angle, broad bandwidth, and high absorbing properties. After preparation and conducting relevant assessments such as transmission line theory and equivalent circuit model, the metamaterial showed >90% absorption at 8–20 GHz with a vast angular range from 0 to 60°. In addition, by applying transparent substrates such as quartz glass and soda-lime glass, high optical transmittance (approximately 80%) was achieved. The lower dielectric loss of quartz glass compared to other dielectric substrates indicated that the high microwave absorption mainly contributed to the planar indium tin oxide metamaterial structure.

Ning et al. (2020b) manufactured a step-structured metamaterial electromagnetic absorber composed of magnetic materials including carbonyl iron powder blended in resin. The metamaterial showed >90% electromagnetic absorption in the frequency range of 1.23–19 GHz with a measured thickness of 6 mm. The broadband and strong electromagnetic absorption were mainly related to the structure-induced multi-resonances, the edge diffraction effects of the stepped structures, and the high magnetic loss of the magnetic materials. Additionally, the absorber showed a good angular performance, with incident angles of up to 70° for transverse magnetic (TM)-polarized incidence and 45° for transverse electric (TE) case (Figure 17).

Cheng et al. (2017a) described an ultra-thin broadband microwave absorber based on a magnetic rubber plate and cross-shaped metamaterial structure. Low-frequency absorption was easily achieved by adjusting the geometric specifications of the cross-shaped metamaterial structure and the magnetic rubber plate thickness. The authors reported a broader bandwidth (2.5 GHz) with a thickness of 2 mm. The bandwidth expansion resulted from the overlap of two resonance absorption peaks from the magnetic rubber plate and cross-shaped structure metamaterial, respectively.

Huang et al. (2014) continued their work by proposing a second-order cross-fractal of magnetic materials (Figure 18) to decrease the magnetic material thickness and broaden the bandwidth. The results showed that the prepared MTM thickness decreased to 1 mm, with a bandwidth that broadened by 1.09 GHz compared to the unloaded magnetic layer. After analyzing the metamaterial functions, the authors concluded that the MTM possessed two functions, namely, the addition of an extra absorption band and a slightly moved absorption band, due to the magnetic material.

Zhang et al. (2020c) reported the successful fabrication of a metamaterial microwave absorber with high efficiency in the low-frequency regions, especially in the S-band (2–4 GHz). The absorber was formed by stacking layers of metamaterials and magnetic coatings. The results demonstrated that the manufactured metamaterial showed good impedance matching and broadband absorption (2.2–9.5 GHz) by using magnetic materials with high dielectric substrates and well-designed structures.

A polarized structure under an input electromagnetic wave can induce a secondary field by producing a quasi-antenna structure, producing permeability and improving impedance matching, eventually boosting microwave attenuation.

Electrostatic interactions produced by lone pairs of electrons (non-bonding) on doped heteroatoms, heteroatoms in the polymer backbone, or anti-bonding orbitals containing σ* and π*, develop electron hopping, conductive loss, micro currents, polarization paths, secondary fields, and conductive networks. Figure 19 shows the electrostatic susceptibility in g-C₃N₄ (Peymanfar et al., 2022).

The charge flow under an alternating field generates quasi-antennas, creating secondary magnetic fields that interact with and degrade the magnetic part of incident electromagnetic waves, producing permeability, strengthening impedance matching, and promoting microwave absorption (Peymanfar et al., 2022).

The heteroatoms in conjugated structures act as polarization centers and tune the energy gap, regulating conductivity and polarizability. Interestingly, both features can augment the quasi-antenna property in an alternating field. However, the tailored morphology of the dielectric structure plays a crucial role in producing quasi-antennas in an absorbing medium (Peymanfar and Mirkhan, 2022). Under an alternating field, the electron cloud migrates toward the atom with greater electronegativity. Thus, the heteroatom as a polarization center develops quasi-antenna paths, generating secondary magnetic fields, which can be deduced by Oersted’s law.

Zhou et al. (2006) studied the microwave-heat transformation of the ZnO whisker and its composite to explain how tetra needle-shaped semiconductor networks absorb microwaves and how quasi-antenna composites create diffuse reflections.

When the material is subjected to an electromagnetic field, the charge concentration at the tips of the T-ZnO whisker needles increases and they function as multipoles, tuned by the incident microwaves, contributing to microwave absorption. Comparison of T-ZnO to 1D ZnO (Figure 20) shows that the former has extremely sharp tips, which contribute to the charge concentration and quasi-antenna effects. Furthermore, the quasi-antenna semiconducting crystalline structures of T-ZnO whiskers emit microwaves with random distributions, resulting in diffuse reflections of incident microwaves.

Cheng et al investigated the microwave-absorbing properties of three-dimensional ZnO micro/nanorod networks. The random distribution of semi-conducting zinc oxide crystals in an isotropic quasi-antenna leads not only to the scattering of incident microwaves and attenuation of EM energy but also acts as microwave receivers by producing vibrating microcurrents in local networks (Li et al., 2010). The vibrating microcurrent induces the vortex magnetic field around the nanotrees, while the alternating magnetic field induces the vortex electric field around itself. The alternating vortex electric and magnetic fields induce each other and then generate EM radiation. This is justified by antenna theory. With the random orientation of these isotropic quasi-antennas, a large part of the EM radiation counteracts each other (Zhuo et al., 2008). Figure 21 shows the SEM and TEM images of the as-synthesized products, in which the nanotrees show isotropic crystal symmetry like isotropic antennas, which may play an important role in microwave attenuation (Zhuo et al., 2008).

By receiving EM energy as an antenna and transforming it into dissipated current, netlike ZnO nanostructures act as a transmitting antenna and transform vibrating current into EM radiation (Li et al., 2010). The microwave-absorbing properties of nanostructured ZnO whiskers were explained by Xiao-Ling et al. (2007) using a quasi-antenna model. Due to their high length-to-diameter (LD) ratio and the n-type semiconductor property, polarized charges are readily formed at the sharp needlepoints of nanostructured ZnO whiskers during EMW transmission. Therefore, at high frequencies, a single leg of ZnO whiskers can be treated as a vibrating electric dipole, and a large number of nanostructured ZnO whiskers can be polarized as a large number of individual radiation sources. According to common knowledge, the established electric dipole is the fundamental cell of antenna radiation.

Cheng et al. designed novel 1D mesoporous MoO₂/C hetero-nanowires. Figure 22 shows the mechanisms to elucidate the superior dielectric loss of the absorber. The 1D nanowires can be considered quasi-antenna acceptors based on the theory of short dipole antennas. Additionally, charge transitions and hopping will form conduction currents along with conjugated nanowires and established networks. This causes Joule-level heating in an alternating EM field. Lastly, interfacial polarization derived from multiple interfaces induced by MoO₂, C, dipole polarization from MoO₂, defects, and remaining heteroatoms convert microwave energy into thermal energy, leading to increased microwave absorption (Cheng et al., 2017b; Hou et al., 2019).

The evaluated mechanisms demonstrate that the metamaterial and quasi-antenna characteristics are efficient mechanisms to be considered in developing conjugated organic polymers to promote their microwave-absorbing features. These features can be inserted into their structures, particularly by modifying their morphology and elemental doping.