High-efficient electromagnetic absorption and composites of carbon microspheres

The rapid developments of wireless communication and the advent of 5G era inevitably stimulate the extensive utilization of various electronic devices worldwide, and the accompanying electromagnetic (EM) pollution is evolving into one of the environmental problems demanding prompt solutions for human beings.^1–6 Although shielding and absorption have been widely recognized as two effective strategies against EM pollution, the latter gained attention in recent years because it could provide sustainable EM energy conversion rather than physical reflection in conventional shielding strategy.^7–11 The core for EM absorption is to interrupt the transmission of EM waves in a specific medium, which determines that this absorption medium, usually known as microwave absorbing materials (MAMs), should possess the characteristic ability to interact with the electric or magnetic field branch of incident EM waves.^12–14 Therefore, MAMs can be divided into two categories: magnetic loss type and dielectric loss type.^15,16 Magnetic materials, i.e., magnetic metals/alloys and ferrites, are early applied for EM absorption due to their excellent performance,^17,18 while some intrinsic drawbacks, such as large filler loading, high density, and easy corrosion/oxidation, restrain their further applications under some rigorous conditions, such as acidic and oxidative environments, or in some portable devices.^19,20 Along with the increasing demands for the comprehensive performance of MAMs, more and more research groups turn their concerns to dielectric loss materials. Some of them devote their efforts to the combination of magnetic and dielectric materials,^21,22 and the others even develop many high-performance MAMs in the absence of magnetic components.^23,24

In the past decade, carbon materials as typical dielectric loss media were almost dominant in various novel MAMs in virtue of their low density, chemical stability, and tunable EM property. Conventional carbon members, e.g., graphite, carbon black, carbon nanotubes (CNTs), carbon nanofibers (CNFs), and reduced graphene oxide (rGO), all have exhibited their great potential as effective components to dissipate EM energy.^25–28 However, their high conductivity easily induces mismatched characteristic impedance and weakens EM absorption performance, and meanwhile, the irregular morphology, non-uniform size, and poor dispersion of their microparticles may also bring some difficulties for composites construction and devices processing. Recent progress indicates that carbon microspheres display unique advantages in the fabrication of high-performance MAMs, and as compared with those conventional carbon materials, they have some unique advantages as follows: (1) carbon microspheres are usually amorphous, and, thus, it will be easy to create good impedance matching between free space and absorption medium; (2) carbon microspheres have typical spherical morphology, which endows them with good dispersion in many solvents or resin matrix; (3) carbon microspheres possess large particle size over several hundred nanometers, and, thus, there will be considerable opportunities to modify their EM property by creating a hollow cavity, embedding magnetic particles, and depositing additional EM components. In order to better introduce the application prospect of carbon microspheres, we highlight the recent advances of various promising MAMs based on carbon microspheres in this review, and we also propose some disadvantages and challenges in the related field.

As a typical member of the carbon family, carbon microspheres possess all excellent intrinsic characteristics of carbon materials, including good chemical stability and designable microstructure. In particular, tunable dielectric property endows them with good microwave absorption performances even without any modification. Wang et al. ever prepared solid carbon microspheres and confirmed their potential as high-performance MAMs,²⁹ whose minimum reflection loss (RL) and qualified absorption bandwidth (QAB, bandwidth corresponding to RL ≤ − 10.0 dB) were −37.2 dB and 5.7 GHz, respectively, with the absorber thickness of 2.0 mm. Such a performance was superior to those of some common carbon materials, such as rGO and CNTs. However, pristine carbon microspheres only achieved good performance in a Ku band, which restrained the practical application of carbon microspheres to some extent. In view of this fact, rational construction of various microstructures is becoming a dominant strategy to boost the microwave absorption performances of carbon microspheres. With the activation of potassium hydroxide, more mesopores could be created in carbon microspheres due to the reaction between K species and carbon matrix, which significantly contributed to the impedance matching of carbon microspheres and the synergistic effect of various loss mechanisms.³⁰ As a result, the minimum RL value of porous carbon microspheres could be further decreased to −58.0 dB, and the corresponding QAB was extended to 6.3 GHz (11.7–18.0 GHz).

Although the generation of porous configuration can decrease the effective permittivity and improve the impedance matching according to the Maxwell–Garnett theory, thus suppressing the reflection at the front surface of MAMs,^31–34 some transmission behaviors, such as multiple scattering and reflection, conducive to the consumption of EM energy are usually difficult to be realized if the pore size is much smaller than the wavelength of incident EM waves. Therefore, more and more groups pay their attention to the fabrication of hollow carbon microspheres rather than porous carbon microspheres, because hollow cavity with a large internal space can intensify EM attenuation through multiple reflections of incident EM waves.^35–38 Yang et al. prepared hollow graphitic carbon microspheres (HGCs) through a selective chemical etching [Fig. 1(a)], and they found that the etching time and the size of hollow microspheres could affect microwave absorption performance greatly.³⁹ By regulating the process parameters, the optimal sample, porous HGC [PHGC-3-180, etching time: 3 h, size: 180 nm, Figs. 1(b) and 1(c)], generated QAB as broad as 7.7 GHz with the absorber thickness of 2.2 mm [Fig. 1(d)]. In an effort to pursue better microwave absorption, it is of great interest to introduce abundant mesoporous structures in the shells of hollow carbon microspheres.^40–45 Xu et al. comprehensively investigated EM properties of mesoporous carbon hollow microspheres (PCHMs), carbon hollow microspheres (CHMs), and carbon solid microspheres (CSMs), and they found that there was a collaborative contribution between mesoporous carbon shell and interior cavity to impedance matching and multiple reflections, as well as polarization loss [Fig. 1(e)].⁴⁶ As a result, PCHMs presented an impressive microwave absorption performance, including strong RL intensity of −84.0 dB and broad QAB of 8.0 GHz [Figs. 1(f) and 1(g)]. Of note is that either hollow cavity or mesoporous shell just provides an auxiliary effect to enhance microwave absorption, and thus it is still very necessary to strengthen the intrinsic EM properties of hollow carbon microspheres. Apart from the conventional regulation on the relative graphitization degree, some recent studies further proposed heteroatoms substitution strategy,^47–49 where some heteroatoms with large electronegativity in carbon frameworks could induce charge imbalance and generate more polarization sites, resulting in more powerful dielectric loss capability.

In theory, multiple reflection and scattering behavior can greatly extend the propagation distance of incident EM waves in a loss medium, and, thus, an intricate internal microstructure is highly desirable for carbon microspheres with good EM function. Our group previously designed uniform yolk-shell C@C microspheres through a “coating-coating-etching” route [Fig. 2(a)].⁵⁰ Benefiting from the complex yolk-shell microstructure [Figs. 2(b) and 2(c)], the as-prepared product realized ultra-wide response bandwidth with 13.5 GHz (4.5–18.0 GHz) over −20.0 dB, when the thicknesses of absorbers were manipulated from 1.00 to 5.00 mm [Fig. 2(d)]. Subsequently, we simplified the synthesis of yolk-shell carbon microspheres through differential shrinkage of polypyrrole microspheres during the pyrolysis process and harvested similar microwave absorption performance.⁵¹ Tao et al. and Wang et al. further obtained multi-shelled hollow carbon microspheres by finely adjusting process parameters [Figs. 2(e)–2(g)], and both of them proposed that triple-shell hollow carbon microspheres (HPCNs-3) not only had higher conductive loss than single-shell hollow carbon microspheres but also generated more powerful interfacial polarizations [Fig. 2(h)].^52,53 In this context, multi-chamber carbon microspheres were also rationally constructed, and, more importantly, they exhibited attractive advantages in both impedance matching and dielectric loss.⁵⁴ EM measurements indicated that their integrated QABs were as broad as 14.2 GHz. Very interestingly, recent progress indicates that some modifications on the surface of carbon microspheres can also bring significant enhancement in microwave absorption. For example, Xu et al. regulated the conditions for the polymerization of glycidyl methacrylate and 1,1-diphenylethylene and then converted these polymer microspheres into wrinkled polymer microspheres with different surface roughness.⁵⁵ They manifested that the wrinkled degree had a direct impact on RL characteristics and rough surface favored good microwave absorption performance. Zhang et al. further confirmed the importance of the macroscopic structure by theoretical stimulation.⁵⁶ If carbon microspheres were assembled into honeycomb-like structure (inner diameter of 8 mm, a height of 12 mm, and a thickness of 1 mm), the QAB of carbon microspheres could be broadened from 4.4 to 8.3 GHz. These reports provide a new insight for microwave absorption enhancement of carbon microspheres.

In addition to the positive effects of microstructure and morphology, the rational design of chemical composition is also considered as a vital principle to improve microwave absorption performances of carbon-based MAMs. There are many combinations of carbon microspheres and different secondary dielectric components that have been regarded as possible high-performance MAMs. As a kind of common dielectric media, metal oxides possess considerable potential in the field of microwave absorption benefiting from their intrinsic polarization effects and additional conductive loss.^16,57,58 The direct deposition of metal oxides nanoparticles on the external surface of carbon microspheres is the simplest way to produce binary composites with enhanced microwave absorption performance.⁵⁹ For example, Peymanfar et al. revealed that the combination of carbon microspheres and ZnAl₂O₄ nanoparticles could bring both strong RL (−54.9 dB) and broad QAB (5.5 GHz), while this preliminary combination model easily suffered from the serious agglomeration of metal oxides nanoparticles.⁶⁰ In contrast, the embedment of metal oxides nanoparticles in carbon microspheres may be an advanced alternative mode, because it maintains good dispersion and chemical homogeneity of metal oxides/carbon microspheres composites but puts forward higher requirements for the preparative process.⁶¹ Our group ever designed core-shell BaTiO₃@Carbon microspheres through a space-confined synthesis [Figs. 3(a) and 3(b)].⁶² It was confirmed that the introduction of BaTiO₃ cores made a solid contribution to the impedance matching of BaTiO₃@Carbon microspheres, and apart from some intrinsic loss mechanisms, the sufficient heterogeneous interfaces between BaTiO₃ and carbon also resulted in additional interfacial polarization, thus boosting their RL intensity up to −88.5 dB [Figs. 3(c) and 3(d)]. More importantly, such BaTiO₃@Carbon microspheres further demonstrated better corrosion resistance (0.2M, HCl solution or 473 K under air atmosphere) than conventional magnetic MAMs [Figs. 3(e) and 3(f)]. More recently, Song et al. further assembled mesoporous carbon microspheres decorated with ultrafine ZnO nanoparticles into three-dimensional ordered arrays through a reverse duplication of polymethyl methacrylate opal.⁶³ The synergistic effect between ZnO and mesoporous carbon microspheres, as well as the structure effect, endowed the composites with impressive microwave absorption performance, and especially for ZnO/OMCS-40 (ZnO content: ∼40 wt. %), its QAB could reach up to 9.1 GHz [Figs. 3(g) and 3(h)]. This result is in good agreement with the theoretical prediction that the assembly of carbon-based microspheres may bring more benefits for microwave absorption.⁵⁶ Compared with metal oxides, transition metal sulfides are generally considered to be more promising candidates for microwave absorption due to their narrower bandgaps favorable for conductivity loss.^64,65 When MoS₂ nanosheets were encapsulated into hollow carbon spheres (MoS₂@HCS), there would be significant synergistic effects that could ensure rapid electron transmission, expose sufficient polarization centers, and intensify multiple reflections.⁶⁶ These characteristics rendered MoS₂@HCS as promising MAMs with strong RL intensity and effective absorption in a very broad frequency range (3.3–40.0 GHz). Besides metal oxides and sulfides, SiO₂, in some cases, also addressed its effectiveness to consolidate EM attenuation of carbon microspheres, while the wave-transparent nature determined that its contribution mainly embodied in impedance matching and interfacial polarization.⁶⁷ 

Although metal oxides/sulfides can upgrade microwave absorption performance of carbon microspheres to a great extent, the corresponding composites are still incapable of long-standing applications under some harsh conditions (i.e., strong acidity and high temperature), because the dissolution of metal oxides or the oxidation of metal sulfides may pull down their performance.⁶⁸ In this context, carbides with good thermal and chemical stability should be taken as a new kind of dielectric additive for carbon microspheres. Chen et al. anchored SiC whiskers on the surface of hollow carbon microspheres through the spray drying technology and the carbothermal reduction method.⁶⁹ TG results revealed that the onset temperature of weight loss would increase from 402 to 524 °C after the introduction of SiC whiskers, and meanwhile, both RL intensity and QAB of the composite were significantly enhanced as compared with those of individual hollow carbon microspheres (−48.6 dB vs −4.8 dB and 4.34 GHz vs 0 GHz). Our group also found that the embedment of ultra-small Mo₂C nanoparticles into carbon polyhedrons could produce durable EM performance even if the composite was treated under high-temperature and strong acidic conditions (200 °C and 3.0 M HCl).⁷⁰ In the following study, we designed pomegranate-like Mo₂C@C nanospheres with phosphomolybdic acid/polypyrrole composite as a self-sacrificing precursor [Fig. 4(a)].⁷¹ Such a unique Mo₂C/C composite with core-shell configuration possessed strong RL intensity over −40.0 dB and integrated QABs as broad as 14.5 GHz (absorber thickness: 1.0–5.0 mm), which were superior to those of some conventional carbides/carbon composites [Figs. 4(b) and 4(c)]. EM analysis manifested that there were diversified absorption mechanisms responsible for its good performance, including conductivity loss, interfacial polarization, dipole polarization, and multiple reflections.

In most cases, metal oxides/sulfides and carbides reinforce dielectric loss capability of carbon microspheres through their intrinsic dipole polarization and accompanied interfacial polarization. Besides, the improvement of dielectric loss capability for carbon microspheres also can be realized through increasing their conductivity loss. Therefore, some dielectric components with relatively high conductivity, such as polyaniline (PANI) and CNTs, are also utilized to couple with carbon microspheres.⁷² For example, Yu et al. fabricated waxberry-like carbon@polyaniline microspheres by conducting the polymerization of aniline monomers on the surface of carbon microspheres.⁷³ The growth of PANI nanorod arrays not only enhanced charge transfer (conductivity loss) and brought interfacial polarization but also created good impedance matching. As a result, the resultant composite promised better microwave absorption performance than individual carbon microspheres (RL ≥ − 5.0 dB) and PANI, and its strongest RL value and the corresponding QAB were −59.6 dB and 5.4 GHz (12.6–18.0 GHz), respectively, with the absorber thickness of 2.2 mm. Li et al. also confirmed that when hollow carbon microspheres were wrapped by CNTs, their RL characteristics in X-band would be significantly improved from −2.9 to −34.6 dB. Since the successful peeling of carbon monolayer from graphite, graphene became one of the most popular candidates for the fabrication of various carbon/carbon composites applied in diverse fields.^74–76 The advent of graphene further aroused numerous interests in the preparation of carbon microsphere composites, because it could couple with carbon microspheres to build up novel sandwich-like architecture [Fig. 4(d)].⁷⁷ Simulation results revealed that, as compared with individual reduced graphene oxide (rGO) and carbon microspheres, there were two obviously enhanced electric field distributions in the composites, one at the interfaces between rGO and carbon microspheres, and the other at the spherical caps of carbon microspheres along the vertical direction toward propagation of EM waves [Figs. 4(e)–4(h)]. These results suggested that the enhanced EM performance in the composites not only came from the expected interfacial polarization but also depended on multiple reflections of EM waves induced by the separated rGO sheets. By optimizing the relative graphitization degree of carbon microspheres or introducing SiO₂ nanoparticles in carbon microspheres, RL characteristics of carbon microspheres/graphene composites could be further reinforced, especially in the frequency range of 8.0–12.0 GHz.^78,79

It is undisputed that the absence of magnetic loss contribution may more or less weaken the microwave absorption performance of carbon microspheres, and, thus, the introduction of magnetic additives is widely considered to be an effective method to promote EM attenuation.^80,81 In general, there are two common modes to involve magnetic particles. One is to attach magnetic nanoparticles on the surface of carbon microspheres.^82–85 Under the circumstances, hollow carbon microspheres are more popular supports, because they can provide additional loss pathway through structure effect.^9,84,86–88 Although the loading of magnetic nanoparticles on either solid or hollow carbon microspheres produces magnetic loss capability to some extent, such a combination mode determines that there will be a critical loading content for magnetic components, and excessive magnetic nanoparticles will result in serious agglomeration and poor chemical homogeneity. What is worse, the exposed magnetic nanoparticles are easily corroded, making the resultant composites incapable of long-standing application. In view of these facts, the other mode, i.e., encapsulation of magnetic nanoparticles, is dominant in the construction of magnetic carbon microspheres composites.^89,90 Our group pioneered the embedment of hierarchical Fe₃O₄ microspheres into carbon shells to generate novel dual loss media.⁹¹ When the thickness of carbon shells was in the range of 30–66 nm, there would be a good balance between magnetic loss and dielectric loss, thus creating desirable impedance matching and attenuation ability simultaneously. Wang et al. replaced Fe₃O₄ with α-Fe₂O₃ and obtained yolk-shell Fe₃O₄@C microspheres because there was a spontaneous volume contraction during the reduction of Fe₂O₃.⁹² The formation of such yolk-shell configuration extended QAB from ca. 4.0 to 5.4 GHz with the thickness of 2.2 mm.^91,92 Inspired by these early results, some magnetic ferrite/carbon microsphere composites with unique yolk-shell configuration have been successfully developed one after another.^93,94 Ma et al. mediated the coating of phenolic resin shells on hollow Fe₃O₄ microspheres by SiO₂ layers and then converted the intermediates into yolk-shell Fe₃O₄@C microspheres through the collapse of hollow Fe₃O₄ microspheres and the carbonization of phenolic resin shells under high-temperature inert atmosphere [Figs. 5(a)–5(c)].⁹⁵ The carbonization temperature was confirmed to be an extremely important factor for microwave absorption, and the composite which was generated at 650 °C could display top-notch performance in Ku-band (RL intensity: −58.4 dB, QAB: 6.0 GHz) with the thickness of 1.9 mm [Fig. 5(d)]. Different from the design of yolk-shell configuration, Yuan et al. assembled core-shell Fe₃O₄@mesoporous carbon microspheres into three-dimensional ordered array by means of a confined interface co-assembly coating strategy [Fig. 5(e)].⁹⁶ It was found that the intensified reflection and scattering of EM waves inside the ordered array, as well as intrinsic magnetic loss and dielectric loss of core-shell units, were together responsible for its excellent RL characteristics, especially for its remarkable QAB in the middle-frequency range [Figs. 5(f) and 5(g)].

Compared with magnetic ferrites, magnetic metals or alloys are usually taken as more popular candidates to combine with carbon materials, because they possess large saturation magnetization, high Snoek's limit, compatible dielectric loss, and distinguishable permeability in the gigahertz range.^97,98 Although the pre-prepared carbon microspheres can be taken as the scaffold of magnetic metal/alloy nanoparticles,^99–105 the resultant composites suffer from the same dilemma to the composites of magnetic ferrites and carbon microspheres, e.g., limited loading amount, serious nanoparticles agglomeration, and poor corrosion resistance. Therefore, core-shell configuration is even more desirable for the corresponding composites. It is unfortunate that magnetic metal/alloy particles have a low affinity to organic polymers (carbon source) and are easily agglomerated at high temperature, which makes it difficult to embed magnetic metal/alloy particles into carbon spherical shells. Liu et al. coated spherical Co particles with carbon shells through the carbonization of glucose under solvothermal thermal conditions, while the thickness of carbon shells could not exceed 10 nm even if the weight ratio of glucose to Co particles was as high as 10:1.¹⁰⁶ In that case, the contribution of carbon shells to microwave absorption was not illustrated clearly. Arc discharge has been proved to be an effective method to prepare core-shell magnetic carbon-based composites, where the composition of interior metal/alloy particles can be easily manipulated by ingot anode.¹⁰⁷ Li et al. obtained microporous Co@C microspheres through the dealumination of CoAl@C precursors, and they found that there was a positive correlation between microwave absorption performance and carbon shell thickness.¹⁰⁸ When the thickness of carbon shells was about 5.0 nm, the minimum RL could be less than −100.0 dB and the corresponding QAB also reached 13.2 GHz. However, no matter what method is used, there is no way to make the thickness of carbon shells exceed 10 nm.¹⁰⁹ This situation lays a hidden trouble to the durable performance of those composites from the arc-discharge method. In addition, both special equipment and high energy input in this strategy are not favorable for the practical popularization of the corresponding composites.

In order to improve the fabrication of core-shell composites with magnetic metals and carbon microspheres, metal oxides/hydroxides are usually employed as the intermediate precursors of magnetic metal nanoparticles.¹¹⁰ For example, we previously induced the polymerization of resorcinol and formaldehyde on the surface of Co₃O₄ microspheres and then converted the initial composite into core-shell Co@C microspheres.¹¹¹ It was manifested that carbon shells derived from phenolic resin not only introduced proper dielectric loss but also made Co cores survive from high-temperature agglomeration, which subsequently consolidated magnetic loss by suppressing the potential skin effect. As a result, there were comprehensive improvements in both overall loss ability and impedance matching. In the following studies, polydopamine (PDA) has been established as an advanced alternative to phenolic resin due to its mussel-inspired adhesion.^{20,22,112–114} Yu's group designed hollow Fe@C microspheres through the pyrolysis of dual core-shell sulfonated polystyrene (SPS)@Fe(OH)₃@PDA microspheres, and they even achieved the size control from 310 to 780 nm by manipulating the size of SPS microspheres [Figs. 6(a)–6(d)]. EM measurement results indicated that in addition to the intrinsic loss from Fe nanoparticles and carbon shells, the benefit from hollow microstructure was also remarkable [Fig. 6(e)], and especially for the Fe@C microspheres with large hollow cavities, their microwave absorption performance could be further upgraded thanks to the improved impedance matching [Figs. 6(f) and 6(g)].^20,112

Along with the flourish of metal-organic frameworks (MOFs), they have been developed as one kind of extremely attractive precursors for carbon-based functional materials,^115,116 not only for the uniform component distribution benefited from the periodic atom arrangements in MOFs but also for the good preservations of regular morphology and porous microstructure in final products.^97,117,118 Thanks to the widespread utilization of magnetic nodes (such as Fe²⁺, Co²⁺, and Ni²⁺), MOF transformation is also a popular strategy for the fabrication of magnetic metals/carbon microspheres. Among various MOF precursors, Ni-BTC (BTC = benzene-1,3,5-tricarboxylate) microspheres receive much attention due to their uniform spherical morphology, and the resultant Ni/C microspheres usually display waxberry-like microstructure, which artfully integrates numerous core-shell Ni@C nanoparticles.¹¹⁹ With the assistance of electronic holographic technology, the interaction between Ni nanoparticles and carbon shells, as well as magnetic coupling interactions of different Ni nanoparticles, can be clearly observed [Figs. 7(a)–7(f)].¹²⁰ As a result, Ni/C microspheres derived from Ni-BTC could promise better microwave absorption performance than the counterparts from Ni-ZIF (ZIF: Zeolite Imidazole Framework).¹²¹ Based on these findings, some modifications on Ni/C microspheres are further proposed, including partial oxidation or phosphorization of Ni nanoparticles, which can upgrade both RL intensity and QAB of Ni/C microspheres effectively.^122,123 If hollow microstructure is pre-created in MOFs microspheres, the final composites will show their obvious advantages in microwave absorption. For example, Yang et al. and Qiu et al. realized the transformation of hollow Ni-BTC almost at the same time, and the results revealed that hollow Ni/C microspheres could promise QAB as broad as 6.8 GHz, superior to 4.8 GHz of solid Ni/C microspheres.^124–126 It is well known that ZIF-67 and its derivative Co/C composites are usually displayed in the morphology of polyhedron,^127–129 while hollow Co/C microspheres can be also generated from hollow ZIF-67 microspheres by directing the self-assembly of ZIF-67 crystals in the presence of cetyltrimethylammonium bromide.¹²⁹ As expected, when hollow ZIF-67 microspheres were pyrolyzed at a proper temperature, the as-prepared Co/C microspheres would be robust for microwave absorption in the frequency range of 2.0–18.0 GHz. Ouyang et al. further utilized trimetallic FeCoNi-MOF-74 to produce hollow FeCoNi/C microspheres, and they found that structure effect and ternary magnetic nanoparticles were greatly helpful to promote the EM performance of the final product, where the minimum RL intensity and corresponding QAB were −35.0 dB and 8.1 GHz, respectively, with the absorber thickness of 2.47 mm.¹³⁰ Although MOF transformation has made considerable achievements in the design and synthesis of high-performance carbon-based MAMs, there is still a problem that cannot be solved very well, which is how to regulate the carbon content of MOF-derived carbon-based composites without the introduction of heterogeneous components.^131,132 In the latest research, we identified dual functions of glucose responsible for composition-controllable Co/C microspheres for the first time.¹³³ During a solvothermal reaction in isopropyl alcohol, a part of glucose played as the source of gluconate to complex with Co ions and produce uniform Co-MOFs microspheres, and the rest glucose was converted into carbon nanoparticles and accommodated in Co-MOFs microspheres [Fig. 7(g)]. Both gluconate and carbon nanoparticles were fused into the carbon framework after high-temperature pyrolysis, thus realizing the control on the carbon content of Co/C microspheres. A proper dosage of glucose not only ensured high uniformity of Co/C microspheres and high dispersion of Co nanoparticles [Figs. 7(h) and 7(i)] but also created good impendence matching and decent loss capability. As a result, the best candidate among this series of composites displayed both strong RL value of −71.3 dB (thickness: 3.8 mm) and broad QAB of 6.6 GHz [thickness: 2.0 mm, Fig. 7(j)]. This study opened a new avenue for the fabrication of magnetic carbon-based MAMs with controllable chemical composition through MOF transformation.

A lot of examples have fully demonstrated that the synergetic effect between carbon microspheres and additional dielectric/magnetic components would endow the corresponding composites with prominent ability to consume EM energy adequately. Inspired by this fact, more and more groups shift their research interest to the fabrication of multicomponent composites with at least three different EM units.^134,135 The maturity of the preparation technology for various magnetic carbon microspheres makes them very popular in the construction of multicomponent composites, and rGO, MnO₂ nanosheets, PANI, and polypyrrole (PPy) are usually utilized to modify magnetic carbon microspheres.^136–140 However, it has to point out that such ternary composites have not established their superiority in microwave absorption as compared with those binary counterparts, whose QABs with specific absorber thicknesses are usually less than 5.0 GHz. This situation means that the combination of different components in this way cannot maximize their synergies, and, thus, it is necessary to take microstructure effect and modification strategy into account comprehensively. Yang et al. created yolk-shell microstructure in Fe₃O₄@C microspheres with the assistance of SiO₂ and then conducted the growth of MnO₂ nanosheets on their surface.¹⁴¹ The yolk-shell configuration and the vertically crossed nanosheets array stimulated the attenuation of incident EM waves greatly, and as a result, yolk-shell Fe₃O₄@C@MnO₂ microspheres displayed broader QAB in X-band than solid Fe₃O₄@C@MnO₂ (5.4 GHz vs 2.8 GHz).^138,141 More recently, CNTs were proposed to be an advanced alternative to MnO₂ nanosheets, because they can be in situ planted on the surface of magnetic carbon microspheres with the catalytic effect of magnetic nanoparticles.^117,131 As shown in Fig. 8(a), Ni@C microspheres assembled with numerous core-shell units were first generated through MOF transformation, and then they were pyrolyzed again in the presence of melamine to induce the growth of CNTs (Ni@C@CNTs). Both SEM and TEM images clearly recorded the formation of CNTs [Figs. 8(b)–8(d)], and the movement of Ni nanoparticles from the surface of Ni@C microspheres to the end of CNTs suggested a “vapor–liquid–solid” growth mechanism triggered by Ni nanoparticles [Fig. 8(e)]. The appearance of CNTs strengthened the overall loss capability of such “double-hierarchical” Ni/C composites, thus widening QAB from 4.8 to 5.2 GHz even with a lower filling amount and smaller absorber thickness [Fig. 8(f)].

In an effort to produce better microwave absorption performance in multicomponent carbon microspheres composites, some groups attempted to study microstructures in a more systematic way instead of simple modifications on magnetic carbon microspheres.^142–145 For example, An and Zhang designed triple-shelled hollow microspheres by depositing NiCo/Ni layer and carbon layer on the surface of hollow SiO₂ microspheres successively,^146,147 and the unique layer-by-layer configuration and hollow microstructure brought a significant enhancement in microwave absorption, including powerful RL intensity of −55.4 dB (absorber thickness: 2.7 mm) and broad QAB of 6.8 GHz (absorber thickness: 2.3 mm). Zhou et al. directed the nucleation and growth of ZIF-67 nanocrystals on the surface of hollow VO₂ microspheres and then converted the intermediate precursor to hollow V₂O₃@Co/C microspheres through high-temperature pyrolysis [Fig. 8(g)]. Although the final composite just displayed moderate performance (RL intensity: −40.1 dB and QAB: 4.6 GHz), the corresponding absorber thickness was only 1.5 mm [Fig. 8(h)].¹⁴⁸ It was very interesting that Wang et al. not only adopted the layer-by-layer mode to construct a ternary Co@SiO₂@C composite but also assembled them into one-dimensional chain-like morphology [Figs. 8(i) and 8(j)].¹⁴⁹ The integrated advantages of dual core-shell configuration and one-dimensional microstructure endowed this ternary Co@SiO₂@C composite with excellent performance [Fig. 8(k)], whose minimum RL and QAB with the absorber thickness of 1.7 mm were −39.6 dB and 7.6 GHz, respectively. Inspired by the synergy of different EM components, some quaternary composites are occasionally proposed by elaborate design to be high-performance MAMs.¹⁵⁰ For example, Shi et al. prepared triple-layer Fe₃O₄@SiO₂@PDA microspheres and adsorbed Ni²⁺ on their surface, and after high-temperature reduction under H₂/Ar atmosphere, Fe₃O₄@SiO₂@PDA-Ni²⁺ could be converted into quaternary Fe@SiO₂@C-Ni composites with yolk-shell configuration [Fig. 9(a)], where Fe particles acted as the core and SiO₂@C layer decorated by Ni nanoparticles served as the shell [Figs. 9(b)–9(e)].¹⁵¹ Electronic holographic images could record magnetic coupling between neighboring Fe@SiO₂@C-Ni microspheres as well as that between Fe core and the Ni nanoparticles [Figs. 9(f) and 9(g)]. By modulating the contents of carbon shells and Ni nanoparticles, RL intensity and QAB of the optimized quaternary composite could reach up to −45.5 dB and 8.2 GHz [Fig. 9(h), absorber thickness: 2.0 mm], respectively. Its good performance could be attributed to multi-scale magnetic coupling effects, multiple interfacial polarizations, and multiple reflections of incident EM waves.

In this review, we discuss recent advances in the fabrication of high-performance microwave absorbing materials (MAMs) based on carbon microspheres in detail. Although carbon microspheres show great potential in the field of microwave absorption due to their moderate dielectric property, good chemical stability, low density, and high dispersity, pristine carbon microspheres still cannot produce desirable attenuation toward incident electromagnetic (EM) waves. According to the related theories of microwave absorption, the performance of MAMs mainly depends on their intrinsic EM attenuation capability and interfacial impedance matching. No matter how strong EM attenuation capability MAMs possess, there will be no good microwave absorption if their interfacial impedance is mismatched with that of free space because this situation can cause intense reflection of incident EM waves at the front surface of MAMs. That is to say, both EM attenuation capability and impedance matching play critical roles in determining the microwave absorption performance of MAMs. It is widely accepted that EM attenuation capability and impedance matching condition of MAMs are not only affected by their chemical composition but also associated with their size, morphology/shape, and microstructure. Therefore, various modification strategies, including composition optimization and microstructure design, have been intensively applied to enhance microwave absorption performance of carbon microspheres. Desirable MAMs should be labeled with the characteristics of thin applied thickness (d), lightweight (low filling ratio), wide qualified absorption bandwidth (QAB), and strong absorption intensity. We further list these important parameters of some representative samples from four categories mentioned above in Table I to illustrate the effects of different strategies on microwave absorption performance intuitively. One can find that as compared with solid carbon microspheres, some porous or hollow carbon microspheres can widen QAB in different frequency ranges and decrease the filling ratio to some extent, demonstrating that porous or hollow structures may contribute to the lightweight of carbon microspheres. However, the design of microstructure has no obvious effect on the improvements of reflection loss (RL) intensity. The analysis result of wrinkled carbon microspheres indicates that their widened QAB is at the expense of the increase of filling ratio, which suggests that surface engineering is not enough to bring essential performance improvement. When secondary dielectric components are introduced into the carbon microsphere system, RL intensity can be reinforced effectively, and unfortunately, such a combination fails to broaden QAB as expected unless porous carbon microspheres are assembled into a three-dimensional array. By comparison, magnetic components demonstrate their positive effects on both RL intensity and QAB, especially for microporous Co@C microspheres, whose QAB with the absorber thickness of 2.70 mm almost cover C-, X-, and Ku-bands, indicating that the fabrication of magnetic carbon-based microspheres will still be a major research direction for high-performance MAMs. As for multicomponent carbon microsphere composites, they integrate the advantages of diversified loss mechanisms and microstructure effects and thus also achieve excellent performance. It is worth noting that the filling ratios of dielectric microspheres, magnetic carbon microspheres, and multicomponent carbon microspheres are more or less increased, even if porous, hollow, and yolk-shell structures are created therein. This phenomenon may be explained by a higher density of non-carbon dielectric components and magnetic components than that of pure carbon microspheres.

It has to be pointed out that although considerable achievements related to microwave absorption performance of carbon microspheres have been made in the past decade, they still cannot make up the gap to practical application. In view of this fact, we further propose some perspectives based on our experience and hope that these viewpoints may be helpful for the readers who are struggling in this field. First, most MAMs are usually active in 8.0–18.0 GHz and incapable of generating enough QAB to cover the whole frequency range of 2.0–18.0 GHz, and their insufficient loss capabilities in a low-frequency range have to be strengthened by accumulating absorber thickness (even over 5.0 mm). This situation severely restrains the practical application of those developed MAMs and should be urgently untangled through rational design. Second, although the synergistic effects between different components can contribute to the microwave absorption enhancement effectively, the magnetic response of magnetic components may be weakened by various nonmagnetic ingredients, which is not conducive for consuming the energy of the magnetic branch. There are some methods, including improved grain orientation, enhanced shape anisotropy, and alloying design, that should be further applied to promote their magnetic loss ability. Third, magnetic carbon microspheres display good performance in RL intensity and QAB, while such composites cannot be utilized under some harsh conditions, such as acidic and oxidative environments, which may induce the corrosion of magnetic components and consequent performance degradation. Therefore, the fabrication of high-performance carbon microspheres composites with different dielectric components is still meaningful, especially for some candidates with carbides or additional carbon materials. Fourth, multicomponent carbon microsphere composites have not established a clear advantage as compared with binary magnetic carbon microspheres, suggesting the manipulation on their composition, and the microstructure still needs to be improved greatly. Fifth, most MAMs are roughly constructed in terms of some experimental results and conclusions, while their performance usually depends on the coupling effects of multiple factors (i.e., composition, filler loading, absorber thickness) and thus deviates from the expected goals in most cases. A comprehensive theoretical research on the structure–activity relationship of MAMs will be an opportunity to break the performance bottleneck. Sixth, some functional devices may sustain more mechanical loading, such as tension, compression, fatigue, and shear, than conventional powder, and thus it is of great significance to explore rational and simple methods that can process powder materials into functional devices for their further practical application.

This work was financially supported by the National Natural Science Foundation of China (NNSFC) (Nos. 21676065 and 21776053).

Data sharing is not applicable to this article as no new data were created or analyzed in this study.

The authors declare no conflict of interest.

This article does not involve any experiments on humans or animals.