The rapid development of wireless communications and radar detection has created an urgent demand for lightweight microwave absorption materials that simultaneously possess broad bandwidth, strong attenuation, reliable impedance matching, and radar scattering control capability. Carbon-based and carbon-metal composite systems have attracted considerable attention because carbon skeletons provide lightweight conductive and dielectric pathways, while metal or magnetic domains introduce heterointerfaces and synergistic magnetic responses. Enhancing conductive connectivity can strengthen internal attenuation, but it simultaneously aggravates surface reflection and limits microwave entry. Insufficient interfacial polarization, in turn, restricts internal dissipation. How to spatially decouple dense conductive domains while preserving the connections among distributed metal/N-C polarization centers to maintain internal dissipation has become a core challenge in the design of such materials.

MOF-derived architectures are suitable platforms because their precursors combine metal species, heteroatom-doped carbon, and pore-forming processes, and the contributions of their multiple mechanisms to microwave loss depend on whether local charge displacement and polarization relaxation can occur under GHz alternating electromagnetic fields while impedance matching is maintained. An effective architecture should organize conductive pathways, interfacial polarization potential gradients, and polarization centers to synergistically support charge displacement, dipole relaxation, impedance matching, and microwave excitation. However, although existing strategies based on compositional regulation, interface engineering, and hierarchical assembly can tune conductivity, magnetic response, and interfacial polarization, the spatial relationship between conductive pathways and polarization-active interfaces remains difficult to fully control. Introducing hollow gaps between functional domains can alleviate this limitation, but spatial separation may also isolate discrete polarization units and weaken their electrical coupling. Therefore, it is crucial to construct structural units that can maintain conductive connectivity while exhibiting unconventional electromagnetic coupling.

To address the above issues, a joint team including Sichuan Agricultural University conducted in-depth research using ZEPTOOLS' ZEM series desktop scanning electron microscope. The team proposed a scaffold-guided growth-reconstruction-conversion strategy to construct CoNi/N-doped carbon heterostructures with gradient hollow cavities on helical carbon nanocoil scaffolds. Through cavity expansion, they achieved spatial decoupling of conductive domains and helical reconnection of polarization centers, constructing a coherent electromagnetic loss network that combines strong absorption, broad bandwidth, and good impedance matching.

Title: Helical Conductive Pathways Bridge Polarization Centers at Hollow‐Confined Metal/N‐C Interfaces to Form a Coherent Electromagnetic Loss Network 
Journal: Advanced Functional Materials 
URL:https://doi.org/10.1002/adfm.79316

The study first focused on the construction and morphological/structural characterization of the heterostructures. The research team used helical carbon nanocoils (CNCs) to guide the growth of ZIF-67 precursors, followed by Ni-assisted cation substitution and etching and subsequent annealing at 700°C in Ar to prepare the hollow CNC/CoNi@C-1~3 series, while using unetched, Ni-free CNC/CoNi@C-4~6 as dense solid controls. The microscopic morphology and microstructure of the samples were observed with a ZEPTOOLS ZEM18 desktop scanning electron microscope. Combined SEM and TEM images clearly show cavities expanding from the compact collapsed shells with local cavities in CNC/CoNi@C-1, to larger cavities in CNC/CoNi@C-2, and further to oversized cavities surrounding the helical scaffold in CNC/CoNi@C-3. X-ray diffraction peaks were conservatively assigned to fcc Co-related metal domains. HRTEM revealed local structures such as fcc Co/CoNi(111) 0.204 nm, fcc Co/CoNi(200) 0.179 nm, hcp Co(101) 0.188 nm, and (Co,Ni)O-related (200) 0.216 nm, in which Ni incorporation is supported jointly by the synthesis route, elemental mapping, and XPS. Elemental distribution shows that Co, Ni, and N are continuously distributed along the helical scaffold, providing a structural basis for subsequent interfacial polarization and loss analysis.

The study then focused on the structural origin of interfacial polarization and dielectric response. Nitrogen adsorption-desorption tests showed that the hollow-structured CNC/CoNi@C-1~3 had significantly higher specific surface areas than the dense controls CNC/CoNi@C-4~6, indicating that Ni-assisted reconstruction generated more accessible pores. Raman spectra showed that ID/IG decreased from 1.187 (C-1) and 1.071 (C-2) to 0.981 (C-3), indicating that the hollow-optimized sample had more ordered sp² carbon and better continuous electronic coupling. KPFM line scanning further showed that CNC/CoNi@C-3 exhibited a more continuous, step-like surface potential variation, whereas CNC/CoNi@C-1 exhibited discrete and strong potential fluctuations, indicating that cavity expansion altered the mesoscopic surface potential landscape. KPFM only reflects quasi-static surface potential and work function differences and does not directly quantify interfacial polarization or built-in fields at GHz; the GHz response is independently supported by complex permittivity and electric modulus analyses. XPS analysis of Co2p, Ni2p, and O1s surface chemical states revealed the coexistence of metal/N-C interfaces and (Co,Ni)O surface species. Electrochemical impedance spectroscopy showed that CNC/CoNi@C-3 had a larger low-frequency impedance arc, indicating higher electrochemical interfacial resistance. The electric double-layer capacitance Cdl was 1.20 F·g⁻¹ (6.13 F·g⁻¹ for CNC/CoNi@C-1), indicating reduced electrolyte-accessible charge storage and leakage, and a changed interfacial space-charge environment consistent with the KPFM surface potential reconstruction. EIS/CV are used only as low-frequency auxiliary interfacial probes and are not included in the GHz dielectric and reflection loss analyses.

The study further revealed the formation mechanism of the interfacial electronic structure and polarization centers. Work function analysis showed that the work functions of N-doped carbon, CoNi, and solid Co were 3.69 eV, 4.63 eV, and 4.99 eV, respectively. Because the work function of N-doped carbon is lower than those of CoNi and Co, electrons migrate from N-C to the metal phase, forming a built-in electric field directed from N-C to the metal. PDOS results show that Co3d and C/N2p undergo orbital hybridization near the Fermi level. Charge density difference maps further show that the hollow CoNi/N-C model has a wider charge separation region than the solid Co/N-C model, with |Δz| increasing from 0.31 Å to 1.11 Å and |μ| approximately 2.16 times that of the solid model. This indicates that the hollow structure does not maximize static charge transfer, but rather provides a spatially extended charge separation region, thereby providing distributed polarization centers for interfacial charge displacement and dipole relaxation under microwave excitation.

The study ultimately focused on macroscopic microwave absorption performance and loss mechanisms. CNC/CoNi@C-3 achieved a minimum reflection loss RLmin = -54.80 dB at a thickness of 3.33 mm, an effective absorption bandwidth EAB of 6.29 GHz at a thickness of 2.27 mm, and an absorption window of 8.31–12.31 GHz at a thickness of 3.00 mm, approaching full X-band coverage. According to Figure 5, the RLmin values of CNC/CoNi@C-1 and CNC/CoNi@C-2 are approximately -12.56 dB and -29.04 dB, respectively. The |Zin/Z0| and quarter-wavelength matching curves of the powder samples indicate that C-3 has a more suitable impedance state. In the CNC/CoNi@C-3-based metamaterial model, the real part of Zeff is close to 1 and the imaginary part is close to 0, and the Smith chart trajectory is close to the matching region, indicating that its electromagnetic advantages can be retained at the structural level. Electromagnetic simulations show that the surface current of C-3 is more continuous and delocalized, and power loss is dissipated volumetrically inside the absorber rather than concentrated on the surface. Among the compared samples, RCS simulations show that it has the strongest radar scattering suppression. The mechanism can be summarized as a three-step architecture-interface-dissipation sequence: cavity expansion suppresses dense shell stacking and surface current accumulation and improves microwave entry and impedance matching. Work function mismatch and interfacial electronic coupling establish metal/N-C polarization centers and built-in potential gradients. Helical CNC conductive pathways reconnect spatially separated polarization centers into an extended coherent electromagnetic loss network. Defect/dipole polarization and magnetic loss serve as secondary attenuation channels. Corrosion resistance tests show that the corrosion current density of CNC/CoNi@C-3 is 1.18×10⁻⁶ A·cm⁻², on the order of 10⁻⁶ A·cm⁻². Although higher than that of C-1, it represents an acceptable performance-durability trade-off while maintaining microwave absorption performance.

The ZEPTOOLS ZEM series scanning electron microscope is a highly integrated, portable, and cost-effective scientific research instrument. It features rapid vacuum pumping, high imaging speed, and a variety of signal detector options, making it suitable for morphology observation and composition analysis, and it can also accommodate various in-situ experimental requirements. The equipment has low installation environment requirements, is not floor-sensitive, and is simple to operate; even non-specialists can quickly get started. It can be more widely applied in new material R&D, life sciences, failure analysis, industrial quality inspection, and many other fields, providing research institutes and enterprise users with a powerful microcharacterization solution that combines high performance and high cost-effectiveness.