Regulation of Impedance Matching and Dielectric Loss Properties of N-Doped Carbon Hollow Nanospheres Modified With Atomically Dispersed Cobalt Sites for Microwave Energy Attenuation

The rational design of lightweight, broad-band, and high-performance microwave absorbers is urgently required for addressing electromagnetic pollution issue. Metal single atoms (M–SAs) absorbers receive considerable interest in the field of microwave absorption due to the unique electronic structures of M–SAs. However, the simultaneous engineering of the morphology and electronic structure of M–SAs based absorbers remains challenging. Herein, a template-assisted method is utilized to fabricate isolated Co–SAs on N-doped hollow carbon spheres (NHCS@Co–SAs) for high-performance microwave absorption. The combination of atomically dispersed Co sites and hollow supports endows NHCS@Co–SAs with excellent microwave absorption properties. Typically, at an ultralow filler content of 8 wt%, the minimum reflection loss and effective absorption bandwidth of the NHCS@Co–SAs are up to −44.96 dB and 5.25 GHz, respectively, while the absorbing thickness is only 2 mm. Theoretical calculations and experimental results indicate that the impedance matching characteristic and dielectric loss of the NHCSs can be tuned via the introduction of M–SAs, which are responsible for the excellent microwave absorption properties of NHCS@Co–SAs. This work provides an atomic-level insight into the relationship between the electronic states of absorbers and their microwave absorption properties for developing advanced microwave absorbers.     

Facile synthesis and microwave absorbability of C@Ni–NiO core–shell hybrid solid sphere and multi-shelled NiO hollow sphere

We reported the preparation of C@Ni–NiO core–shell hybrid solid spheres or multi-shelled NiO hollow spheres by combining a facile hydrothermal route with a calcination process in H₂ or air atmosphere, respectively. The synthesized C@Ni–NiO core–shell solid spheres with diameters of approximately 2–6 μm were in fact built from dense NiO nanoparticles coated by random two-dimensional metal Ni nanosheets without any visible pores. The multi-shelled NiO hollow spheres were built from particle-like ligaments and there are a lot of pores with size of several nanometers on the surface. Combined Raman spectra with X-ray photoelectron spectra (XPS), it suggested that the defects in the samples play a limited role in the dielectric loss. Compared with the other samples, the permeability of the samples calcined in H₂ and air was increased slightly and the natural resonance frequency shifted to higher frequency (7, 11 and 14 GHz, respectively), leading to an enhancement of microwave absorption property. For the sample calcined in H₂, an optimal reflection loss less than − 10 was obtained at 7 GHz with a matching thickness of 5.0 mm. Our study demonstrated the potential application of C@Ni–NiO core–shell hybrid solid sphere or multi-shelled NiO hollow sphere as a more efficient electromagnetic (EM) wave absorber.     

Multi-Scale Design of Ultra-Broadband Microwave Metamaterial Absorber Based on Hollow Carbon/MXene/Mo2C Microtube

Developing various nanocomposite microwave absorbers is a crucial means to address the issue of electromagnetic pollution, but remains a challenge in satisfying broadband absorption at low thickness with dielectric loss materials. Herein, an ultra-broadband microwave metamaterial absorber (MMA) based on hollow carbon/MXene/Mo₂C (HCMM) is fabricated by a multi-scale design strategy. The microscopic 1D hierarchical microtube structure of HCMM contributes to break through the limit of thickness, exhibiting a strong reflection loss of -66.30 dB (99.99997 wave absorption) at the thinnest matching thickness of 1.0 mm. Meanwhile, the strongest reflection loss of -87.28 dB is reached at 1.4 mm, superior to most MXene-based and Mo₂C-based microwave absorbers. Then, the macroscopic 3D structural metasurface based on the HCMM is simulated, optimized, and finally manufactured. The as-prepared flexible HCMM-based MMA realizes an ultra-broadband effective absorption in the range of 3.7-40.0 GHz at a thickness of 5.0 mm, revealing its potential for practical application in the electromagnetic compatibility field.     

Constructing γ-MnO2 hollow spheres with tunable microwave absorption properties

Hollow γ-MnO₂ sphere was obtained by annealing the precursor at 400 °C with different holding time. The influences of different holding time on the morphology and crystalline structure of final products have been discussed in detail, and the microwave absorption properties of the as-products were also investigated. The results exhibited that finer crystalline feature of the γ-MnO₂ and larger pore size in the hollow γ-MnO₂ sphere could be obtained with the extended holding time. The MnO₂/paraffin composites (50 wt% loading) present extraordinary microwave absorption performance, and the minimum reflection loss (RL) values is −51.3 dB at 4.9 GHz with the thickness of 3.5 mm. The excellent electromagnetic absorption properties can be ascribed to the hollow structure, perfect impedance matching behavior and the multiple interface polarization effect.     

Multiphase Molybdenum Carbide Doped Carbon Hollow Sphere Engineering: The Superiority of Unique Double-Shell Structure in Microwave Absorption

In order to achieve excellent electromagnetic wave (EMW) absorption properties, the microstructure design and component control of the absorber are critical. In this study, three different structures made of Mo₂C/C hollow spheres are prepared and their microwave absorption behavior is investigated. The Mo₂C/C double-shell hollow spheres consisting of an outer thin shell and an inner rough thick shell with multiple EMW loss mechanisms exhibit good microwave absorption properties. In order to further improve the microwave absorption properties, MoC_1-x/C double-shell hollow spheres with different crystalline phases of molybdenum carbide are prepared to further optimize the EMW loss capability of the materials. Finally, MoC_1-x/C double-shell hollow spheres with α-phase molybdenum carbide have the best microwave absorption properties. When the filling is 20 wt.%, the minimum reflection loss at 1.8 mm is −50.55 dB and the effective absorption bandwidth at 2 mm is 5.36 GHz, which is expected to be a microwave absorber with the characteristics of “thin, light, wide, and strong”.     

Fabrication of Graphdiyne/Graphene Composite Microsphere with Wrinkled Surface via Ultrasonic Spray Compounding and its Microwave Absorption Properties

Advanced carbon materials are constantly being used in the field of microwave absorption. Herein, in order to enrich the variety and expand the application fields of graphdiyne (GDY), the wrinkled graphene (RGO) nanosheet coated and embedded with GDY porous microspheres (RGO/GDY) are prepared by GDY synthesis, ultrasonic spray, and pyrolysis. The study finds that RGO and GDY have effective synergistic effects. The suitable pores and composition, conductive network formed by overlapping 0D and 2D materials, special surface and internal morphology design, and high-temperature activation process make RGO/GDY exhibit excellent impedance matching and attenuation capabilities. Under the best amount of GDY (20 mg), the particle sizes of the microspheres (≈6 µm), and filler content (27.5%), the minimum reflection loss (RL_min) is −58 dB@8.3 GHz, and the corresponding matching thickness is 2.7 mm. The effective absorption bandwidth is 4.3 GHz as the thickness is 1.9 mm. By adjusting the thickness, the absorber can completely absorb microwaves of all the C, X, and Ku bands. The microwave absorbing mechanisms are elucidated. GDY materials are first applied to the field of microwave absorption, enhancing the absorption performance of RGO/GDY. It provides a new way to manufacture electromagnetic wave absorbers with satisfactory performance.     

Enhanced microwave absorption properties of flake-shaped FePCB metallic glass/graphene composites

A novel kind of composite absorber, i.e. FePCB/graphene composite, with excellent microwave absorption properties was successfully fabricated by a simple and scalable ball milling method. After being milled, the FePCB particles displayed flaky morphology with large aspect ratio. The complex permittivity and permeability of the flaky FePCB distinctly increased compared with those before milling. Furthermore, with the introduction of graphene, the flaky FePCB/graphene composite exhibited excellent microwave absorption performance with strong absorption and wide absorption band. In particular, for FePCB/graphene composite with an absorber thickness of 2 mm, the reflection loss (RL) reached a minimum of −45.3 dB at 12.6 GHz and the effective absorption bandwidth (RL < −10 dB) covered 5.4 GHz. The enhanced microwave absorption performance of the FePCB/graphene composite was attributed to the high magnetic loss and improved impedance matching which were closely related to the flake-shaped FePCB particles and the introduction of graphene sheets.     

Enhanced microwave absorption properties of polymer-derived SiC/SiCN composite ceramics modified by TiC

Porous SiCN(Ti) composite ceramics with good microwave absorbing performance were fabricated by pyrolysis of solid polysilazane modified by tetrabutyl titanate. The introduction of Ti not only acted as active filler to react with free carbon in the matrix to form TiC, but also played the role as catalyst to promote the formation of SiC nanowires. Finally, SiCN(Ti) composite ceramics formed a microstructure containing multi-nanophases and multi-nano heterogeneous interfaces when annealing temperature reached 1500 °C. The complex microstructure annealed at 1500 °C made composite ceramics have good matching impedance, as well as greatly increase the interfacial polarization loss and dipole polarization loss. As a result, the TiC/SiC/SiCN composite ceramics showed the excellent performance of electromagnetic wave absorption in X band. The minimum reflection loss (RL) of samples was ??17.1 dB at the thickness of 1.9 mm, and the maximum effective absorption bandwidth (EAB) of composite ceramics was 3.2 GHz when the thickness of sample was 2.1 mm, which exhibited a promising prospect as a structural and microwave absorbing integration material.

     

Tunable microwave absorption properties of B-doped SiC nanopowders prepared by arc-discharge method

In this study, we adopted a simple strategy of the arc discharge to prepare the B-doped SiC microwave nano-absorbers. Herein, the amorphous boron was selected as a doping B element source to tune the electromagnetic parameters of the SiC nanopowders. The experimental results indicate that the doping B in the SiC can conveniently tailor the microwave absorption capability of the SiC nanopowders. The appropriate doping B in the SiC can contribute to excellent synergistic effects among its defect dipoles, leakage, and magnetism. Among the doped SiC with B doping content of 0, 5, 10, and 15 at.%, the SiC with doping B content of 10 at.% acquired the optimized impedance matching and enhanced microwave absorption properties. And the optimal reflection loss (RL) value ? 51.2 dB at 6.76 GHz with a matching thickness of 2.9 mm is acquired. The RL of the B-doped SiC nanopowders with B doping content of 10 at.% exceeding ? 10 dB is about 3–5 times those of the other three samples in the ranges of 1–18 GHz at a matching thickness of 1.2–6.0 mm. Therefore, this work provided a feasible way to tune the microwave absorption properties of the B-doped SiC, which is of great significance to improve SiC-based high-temperature dielectric ceramics of microwave absorption performances by the defect-engineering strategy.

     

Enhanced microwave absorption performance of nitrogen-doped porous carbon dodecahedrons composite embedded with ceric dioxide

CeO₂/Co/C dodecahedrons composites with excellent microwave absorption performance were synthesized by using a hydrothermal method. ZIF-67/CeO₂ was first prepared by introducing CeO₂ into the precursor of ZIF-67 and then CeO₂/Co/C composite was obtained after heat treatment. Impedance matching of the samples could be well adjusted by controlling the content of CeO₂. Unique dodecahedral structure for more interfacial reflection and cerium dioxide oxygen vacancies enhance microwave absorption performance. Specifically, the CeO₂/Co/C exhibited a minimum reflection loss of ?68.83 dB is observed at 5.92 GHz, while the thickness was 3.69 mm. The introduction of CeO₂ effectively enhanced the impedance matching of the materials and improved the microwave absorption performance. Therefore, this CeO₂/Co/C composite is a promising microwave absorber material with high performance.     

Carbon nanotubes/Ni and chain-like carbon nanospheres/Ni nanocomposites: selective production and their microwave absorption performances

It was well recognized that constructing the dielectric/magnetic nanocomposites was considered as an effective way to develop excellent microwave absorption materials (MAMs). Herein, we proposed a simple water-assisted chemical vapour deposition process to selectively produce carbon nanotubes (CNTs)/Ni and chain-like carbon nanospheres (CCNSs)/Ni nanocomposites in high yield by controlling the decomposition temperature. The ultrahigh yield of CCNSs could be achieved when C₂H₂ was catalytically decomposed at 515 °C, which was up to ca. 211.0. The results suggested that electromagnetic and microwave absorption properties of as-prepared samples were highly dependent on their microstructures and composition parameters, which could be regulated by the introduction of water vapour and decomposition temperature. It was worth mentioning that the obtained CCNSs/Ni nanocomposites could simultaneously present an optimal reflection loss of ? 28.32 dB with a matching thickness of 1.68 mm, and an effective frequency bandwidth of 4.60 GHz with the matching thickness of 1.71 mm. Our results provided an effective and facile strategy to produce CCNSs/Ni in high yield, which provided a new idea for the designing and synthesis of lightweight and excellent MAMs.

     

Surfactant-free synthesis and electromagnetic properties of Co–Ni–B composite particles

The Co–Ni–B composite particles with different mol ratio of Co to Ni were composited of spheres, spheres in-pair, hierarchical assemblies of dentrites, which were surfactant-free synthesized by chemical reduction method in aqueous solution. The complex permeability of the Co–Ni–B composite particles indicated reverse resonant peak at the frequency range of 8–16 GHz, where the complex permittivity showed the positive resonant peak and the μ_r″ of particles showed negative values, caused by the transformation between electric and magnetic energy. The imaginary parts of relative permeability (μ_r″) of Co–Ni–B composite particles indicated one broad resonant peak over the 2–8 GHz range for the high effective anisotropy. A slight decrease in complex permittivity resulted in an excellent impedance matching property. The Co–Ni–B composite alloy particles with mol ratio of 7:3 exhibited reflection loss less than ?20 dB in frequency range of 4.0–14.5 GHz for the absorber thickness of 1.1–3.2 mm, and an optimal RL of ?32.4 dB was obtained at 12.8 GHz with thickness of 1.2 mm. The broadest bandwidth of reflection loss less than ?10 dB from 13.0 to 17.0 GHz, covering almost the whole Ku-band, was obtained for a thickness of 1.1 mm layer.     

Induced Crystallization-Controllable Nanoarchitectonics of 3D-Ordered Hierarchical Macroporous Co@N-Doped Carbon Frameworks for Enhanced Microwave Absorption

The construction of ordered hierarchical porous structures in metal–organic frameworks (MOFs) and their derivatives is highly promising to meet the low-density and high-performance demands of microwave absorption materials. However, traditional methods based on sacrificial templates or corrosive agents inevitably suffer from the collapse of the microporous framework and the accumulation of nanoparticles during the carbonization transformation, resulting in the deteriorating impedance match, which greatly limits the incident and attenuation of microwaves. Herein, an induced crystallization and controllable nanoarchitectonics strategy is employed to replace traditional growing-etching methods and successfully synthesize carbonized 3D-ordered macroporous Co@N-doped carbon (3DOM Co@NDC) based on the 3D-ordered template. The obtained 3D-ordered macroporous structure ensures the stable growth of hybrid carbon frameworks and Co C nanoparticles without collapse, preserves abundant interfaces for both the incident and attenuation performance, so as to significantly improve the impedance matching and absorption properties compared to conventional MOFs derivatives. The minimum reflection loss of 3DOM Co@NDC is −57.36 dB at the thickness of 1.9 mm, and the effective bandwidth is 7.36 GHz at 1.6 mm. Moreover, the innovative strategy to prepare 3D-ordered hierarchical macroporous structures opens up a new avenue for advanced MOFs-derived absorbers with excellent performance.     

Achieving excellent tunability of magnetic property and microwave absorption performance of FeZn-C core–shell nanoparticles by designing the Fe/Zn ratio

Composition design is vital for the excellent microwave absorption (MA) of core–shell nanoparticles (NPs). In this work, FeZn-C core–shell NPs were synthesized by metal organic chemical vapor deposition with the mixture of zinc (II) acetylacetonate and iron acetylacetonate as precursor. The Fe/Zn ratio of the nanocores could be facilely tuned by adjusting the Zn/Fe ratio in the mixture precursor, and their magnetic behavior could therefore be tuned from super-paramagnetic to ferromagnetic. The Fe/Zn ratio might change the resonance intensity and peak position of the nanoparticle absorbers, and thus be able to tune the attenuation property and improve the thickness matching, leading to double reflection loss peaks and broad effective bandwidth. The optimal reflection loss value of ?58.0 dB and effective bandwidth of 8.3 GHz have been achieved from the NP absorbers. These results demonstrated that the introducing of non-magnetic metal atom in C-coated core–shell ferromagnetic NPs endowed them with excellent tunability in magnetic and MA performance, and could also provide a bench for the design of other core–shell NPs.     

Optimizing the electromagnetic wave absorption performance of designed hollow CoFe2O4/CoFe@C microspheres

Whereas hollow composites present some superiorities like abundant micro interfaces,outstanding impedance matching as the responses of electromagnetic wave (EMW),but versatile designs including crystal transformation,heterogeneous structures and magnetic exchange coupling to further contribu-tion are even not designed or stressed together in previous literatures.In this article,rational design on the hollow CoFe2O4/CoFe@C architecture has been conducted by a sequential process of self-sacrifice by combustion,in-suit polymerization and calcination.Results of morphology observation exhibit that heterogeneous CoFe2O4/CoFe@C composites were generated via crystal transformation from CoFe2O4 to CoFe alloys with encapsulated carbon,together with ultimate growth of crystal particles.As for three carbon-based architectures,relatively low-graphitization carbon layers are favorable for enhancing impedance matching and polarization relaxation,but suppressing the conductive loss essentially.Mod-erate carbon content endows sample S2 with the maximum magnetic saturation (Ms) of 152A emu g-1.The optimized RL of sample S3 is up to-51 dB with 30 wt％ loading,and the effective absorption band(EAB) is of 5.9 GHz at the thickness of 2.17 mm,while 6.0 GHz can be reached at 2.5 mm.Therefore,this hollow multi-interfaces design definitely shed light on novel structure for new excellent absorbers.     

Enhanced microwave absorption performance of graphene/doped Li ferrite nanocomposites

In the present study, Microwave absorbing Li-Sr, Li-Co ferrite nanoparticles and RGO/Li-Sr, RGO/Li-Co ferrite nanocomposites containing Li ferrite and reduced graphene oxide (RGO) were synthesized to further improve the microwave absorption performance of Li ferrite nanoparticles (LiFe₅O₈). The Li-Sr and Li-Co nanoparticles were synthesized by thermal treatment method, the RGO/Li-Sr and RGO/Li-Co nanocomposites were obtained by a polymerization method and were characterized by different techniques. The electromagnetic wave absorption properties of the samples were evaluated by vector network analyzer (VNA) in the frequency range of 2–18 GHz. The magnetic and dielectric loss, impedance matching, and electromagnetic wave absorption of the samples are significantly improved through the addition of RGO. Experimental results revealed that the RGO/Li-Co nanocomposite considerably increased microwave absorption. The minimum reflection loss (RL) of RGO/Li-Co also was found to reach −46.80 dB at the thickness of 3 mm and the effective absorption bandwidth (≤-10 dB) amounted to 6.80 GHz (from 10.52 to 17.32 GHz), which was much higher in comparison with pure Li and Li-Co ferrites nanoparticles. Due to the synergistic effect between magnetic loss and dielectric loss and the good impedance matching, the RGO/Li-Co nanocomposite may be regarded as a new candidate for microwave absorbing materials characterized with a broad effective absorption bandwidth at thin thicknesses.     

Three-dimensional network structure Co/CNT derived from bimetal MOFs toward efficient electromagnetic wave absorber

The development of radar stealth technology and the innovation of modern communication technology have put forward new requirements for microwave absorbing materials. Herein, the novel cobalt/carbon nanotube (Co/CNT) material with a three-dimensional (3D) network structure was prepared through the pyrolysis of nanosized bimetallic CoZn-ZIF precursor. The material exhibits excellent performance under the synergistic effect of multiple loss mechanisms. The minimum reflection loss (RL_min) value reaches −43 dB with 3.3 mm at a filled ratio of 20 wt%. In addition, its effective absorption bandwidth (EAB) can reach 4.2 GHz with a thickness of 3 mm. Furthermore, based on the systematic analysis of the absorption mechanism, an MA fabric with EAB covering the entire X band was successfully constructed with polyimide (PI) fabric as the substrate. The interconnected graphite network provides strong conduction loss and polarization loss, and the existence of Co nanoparticles not only provides magnetic loss but also effectively adjusts impedance matching performance. Overall, this research provides a new perspective for the design and application of high-performance microwave absorbing materials.     

Formation and stability of small well-defined Cu- and Ni oxide particles

Well-defined and -structured Cu/Cu₂O and Ni/NiO composite nanoparticles have been prepared by physical-vapor deposition on vacuum-cleaved NaCl(001) single crystal facets. Epitaxial growth has been observed due to the close crystallographic matching of the respective cubic crystal lattices. Distinct particle morphologies have only been obtained for the Ni/NiO particles, comprising truncated half-octahedral, rhombohedral- and pentagonal-shaped outlines. Oxidation of the particles in the temperature range 473–673 K in both cases led to the formation of well-defined CuO and NiO particles with distinct morphologies. Whereas CuO possibly adopts its thermodynamical equilibrium shape, NiO formation is accompanied by entering a Kirkendall-like state, that is, a hollow core–shell structure is obtained. The difference in the formation of the oxides is also reflected by their stability under reducing conditions. CuO transforms back to a polycrystalline mixture of Cu metal, Cu₂O and CuO after reduction in hydrogen at 673 K. In contrast, as expected from theoretical stability considerations, the formation of the hollow NiO structure is reversed upon annealing in hydrogen at 673 K and moreover results in the formation of a Ni-rich silicide structure Ni₃Si₂. The discussed systems present a convenient way to tackle and investigate various problems in nanotechnology or catalysis, including phase transformations, establishing structure/activity relationships or monitoring intermetallic particles, starting from well-defined and simple models.     

Cornstalk-derived macroporous carbon materials with enhanced microwave absorption

Biomass transformation is being considered as a green and sustainable strategy for carbon-based functional materials in many fields. To produce porous structure favorable for microwave absorption, we demonstrate herein the successful synthesis of macroporous carbon materials with cornstalk as a biomass precursor. It is found that two kinds of typical biological structures in cornstalk, linear vascular bundles and parenchyma cells, can be well preserved during high-temperature pyrolysis. Mercury intrusion porosimetry and N₂ adsorption indicate that these cornstalk-derived carbon materials have very high porosity, which is mainly from desirable macroporous structure rather than conventional micro/mesopores. Electromagnetic (EM) analysis reveals that dielectric loss is the only pathway for the consumption of EM energy, and high pyrolysis temperature favors strong dielectric loss through conductive loss and interfacial polarization loss. Meanwhile, pyrolysis temperature also affects the matching degree of characteristic impedance. When the pyrolysis temperature reaches 750 °C, good dielectric loss and impedance matching endow the sample (CSC-750) with excellent microwave absorption performance, including strong reflection loss, broad response bandwidth, and relatively thin absorber thickness. The advantages of macroporous structure are further highlighted in impedance matching and multiple reflection by comparing with a macropore-free counterpart.

     

Microwave absorption properties of double-layer absorbers based on spindle magnetite nanoparticles and flower-like copper sulfide microspheres

In this work, the spindle magnetite nanoparticles (SMNPs) and flower-like copper sulfide microspheres (FCSMSs) were synthesized via hydrothermal method. The structures, chemical composition and morphologies of samples were analyzed and characterized in detail. The microwave absorption properties of single-layer and double-layer absorbers were investigated based on the electromagnetic transmission line theory in the frequency range from 2 to 18 GHz. The results show that the double-layer absorbers consisting of FCSMSs as matching layer and SMNPs as absorbing layer display superior microwave absorbing performance compared to the single-layer ones due to the proper combination of magnetic loss of SMNPs and dielectric loss of FCSMSs, and the improved impedance matching characteristics. When the thicknesses of the absorbing layer and the matching layer are 1.6 and 0.4 mm, respectively, the minimum reflection loss reaches ??74.3 dB at 10.9 GHz, and the efficient absorption bandwidth is up to 5.34 GHz (8.46–13.8 GHz). The optimal SMNPs/FCSMSs double-layer absorbers can become a novel microwave absorption material with strong-absorption and broad-band.