Carbon nanofibers (CNFs) have emerged as promising candidates for realizing lightweight and high-performance electromagnetic (EM) wave absorbing materials owing to their obvious merits, such as long-range conductive networks, tunable dielectric properties, and atomic-scale composition regulation. The existing challenges are how to optimize surface impedance matching through structural design and realize multifrequency response characteristics by EM synergistic effects. In this study, we propose a confined-coordination growth strategy to anchor small-sized Co nanoparticles and simultaneously introduce structural defects on the surface of CNFs to realize lightweight and superior EM wave absorption. Interestingly, these post-coordinated metal–organic framework (MOF)-derived small Co nanoparticles can balance surface impedance, strengthen interfacial polarization, and promote interfacial electric field polarization, and the sublimation of Zn species introduces structural defects to regulate the dielectric constant and trigger defect polarization. Benefiting from the combined advantages of matched impedance, long-range conductive networks, abundant dielectric‒magnetic heterointerfaces, and structural defects, the minimum reflection loss (R_L) of the CNFs reached as high as −51.0 dB, and the effective absorption bandwidth (EAB) covered the entire Ku band with a broad bandwidth of 7.33 GHz. This strategy provides integrated insight into optimizing the impedance characteristics of CNFs and manipulating the EM wave absorbing performance.

With the increasing seriousness of electromagnetic pollution in civil applications and national defense, current radar absorbing structures (RASs) with narrow absorption performance and high density are inadequate to meet the demands for excellent electromagnetic absorption performance. Therefore, achieving broadband absorption capabilities in RASs across the frequency range of 2 to 40 GHz is a pressing issue and a topic of significant interest. This review article summarizes the multi-dimensional design of broadband RASs by integrating materials, structures, and manufacturing processes, promoting the application of novel materials in three-dimensional structures through advanced manufacturing processes in the future. Meanwhile, the multi-scale absorption mechanism, including the micro-scale absorption attenuation mechanism and macro-scale absorption resonance, has been discussed. Finally, the major challenge of current RASs and their relatively new frontier has been discussed, highlighting their potential for diverse applications across multiple fields.

Hollow engineering is considered to be an essential subfield in promoting electromagnetic (EM) wave absorption intensity and realizing lightweight characteristics. However, the enhancement of the effective absorption bandwidth (EAB) still faces considerable challenges. Herein, hollow carbon nanocages with CoFe₂Se₄ quantum dots (HCNs@CoFe₂Se₄-QDs) with superior EM wave absorption intensity and ultra broadband EAB are produced by using tightly arranged SiO₂ spheres as hard-template materials. Specifically, the removal of SiO₂ templates inevitably results in the formation of a hollow cavity, which is favorable for optimizing impedance matching and increasing the absorption intensity. In addition, the incorporation of selenium powder effectively increases the number of heterogeneous interfaces by forming CoFe₂Se₄ quantum dots (QDs) during the pyrolysis process, leading to strengthened interfacial polarization and ultra broadband EAB. As a result, superior EM wave attenuation with a minimum reflection loss (R_L) of −67.6 dB and an EAB of 11.4 GHz is achieved with only a 20 wt% filler ratio. This design concept of hollow engineering with magnetic QDs provides inspiration for optimizing the EM wave absorption intensity and simultaneously promoting the absorption bandwidth.

Owing to the tunable compositions and versatile functionality, the development of eco-friendly metal–phenolic coordination crystals derivatives is highly anticipated for electromagnetic wave absorption. In this study, three kinds of magnetic hollow carbon spheres (HCSs) with macro-meso-microporous characteristics, including Fe/HCS, Co/HCS, and CoNi/HCS, are successfully fabricated via the co-operative hard template and self-assembling process, in which magnetic particles are encapsulated in carbon shell matrix after the pyrolysis of metal–polyphenol coordination crystals and further subsequent template removal. On the one hand, hierarchical macro-meso-micropores effectively balance the impedance gap between absorbers and air and introduce structural defects or distortion, leading to matched impedance and enhanced dipolar/defect polarization. On the other hand, wrapped magnetic particles provide uncountable hetero-interfaces and induce ferromagnetic resonance, resulting in strengthened interfacial polarization and additional magnetic loss. In particular, enhanced minimum reflection loss (R_L,min) and broadband effective absorption bandwidth (EAB) are achieved with only 10 wt.% filler loading. Specifically, the R_L,min and EAB values are −57.5 dB and 7.2 GHz for Fe/HCS, −50.0 dB and 5.8 GHz for Co/HCS, and −52.1 dB and 6.7 GHz for CoNi/HCS, respectively. Moreover, this work provides us a modular-assembly strategy to regulate the hollow cavity of absorbers and simultaneously manipulates the chemical components of absorbers to regulate electromagnetic wave absorption performance.

Light-weight and exceptional microwave absorption are two vital characteristics for microwave absorbers in practical applications, but still face challenges. Herein, we employ a sacrificial template strategy to fabricate heteroatoms-doped carbon nanocages (CNs) via chemical vapor deposition, in which heteroatoms are simultaneously doped into the carbon frameworks by bubbling flowing source liquid. Compared with CNs, doped heteroatoms, accompanied with the inevitably defective arrangements in the lattice, not only decrease the electrical conductivity and balance the impedance characteristics, but also introduce structural-chemical defects and trigger dominant dipolar/defect polarization. As a result, both the minimum reflection loss (R_L,min) and effective absorption bandwidth (EAB) greatly increase at an ultralow filler loading of 5 wt.% owing to internal hollow void and high specific surface area. The R_L,min values reach −53.6, −43.2, and −50.1 dB for N-CNs, S-CNs, and N,S-CNs with the corresponding EAB of 4.9, 2.5, and 3.1 GHz, respectively. Furthermore, this work provides an effective strategy for the construction of heteroatoms-doped hollow carbon frameworks in large-scale production and the obtained doped carbon nanocages can be used as light-weight and high-performance microwave absorbers.