NiO/Ni Heterojunction on N-Doped Hollow Carbon Sphere with Balanced Dielectric Loss for Efficient Microwave Absorption

Hollow carbon spheres are potential candidates for lightweight microwave absorbers. However, the skin effect of pure carbon-based materials frequently induces a terrible impedance mismatching issue. Herein, small-sized NiO/Ni particles with heterojunctions on the N-doped hollow carbon spheres (NHCS@NiO/Ni) are constructed using SiO₂ as a sacrificing template. The fabricated NHCS@NiO/Ni displayed excellent microwave absorbability with a minimum reflection loss of −44.04 dB with the matching thickness of 2 mm and a wider efficient absorption bandwidth of 4.38 GHz with the thickness of 1.7 mm, superior to most previously reported hollow absorbers. Experimental results demonstrated that the excellent microwave absorption property of the NHCS@NiO/Ni are attributed to balanced dielectric loss and optimized impedance matching characteristic due to the presence of NiO/Ni heterojunctions. Theoretical calculations suggested that the redistribution of charge at the interfaces and formation of dipoles induced by N dopants and defects are responsible for the enhanced conduction and polarization losses of NHCS@NiO/Ni. The simulations for the surface current and power loss densities reveal that the NHCS@NiO/Ni has‑ applicable attenuation ability toward microwave under the practical application scenario. This work paves an efficient way for the reasonable design of small-sized particles with well-defined heterojunctions on hollow nanostructures for high-efficiency microwave absorption.     

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.     

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”.     

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.     

Dielectric properties and microwaves response behavior of polypyrrole-derived N-doped carbon nanotubes

A series of N-doped carbon nanotubes (NCNTs) have been synthesized via a temperature-controlled annealing of polypyrrole (PPy) hierarchical nanostructures. The microstructures, dielectric properties and microwaves response behavior of these NCNTs were systematically investigated. The results indicate that initial pyrrolic N in PPy was gradually converted into pyridinic and graphitic N during the annealing process. NCNT prepared at 700 °C exhibits excellent broadband microwave absorption performance, where its effective absorption bandwidth (reflection loss value lower than ? 10 dB) reaches 8.20 GHz in the frequency range of 9.80–18.00 GHz. A model refers to conductive loss and polarization relaxation was adopted to explain the high-performance microwave absorption. This research greatly expands the development of N-doped CNTs for the application in microwave areas.

     

Ferromagnetic and microwave absorption properties of copper oxide/cobalt/carbon fiber multilayer film composites

The copper oxide/cobalt/carbon fiber multilayer film composites were synthesized by thermal oxidation route. In order to investigate the intrinsic reasons for microwave absorption properties of absorbers, the complex permittivity, complex permeability and the microwave absorption properties of composites were studied in the 1-18 GHz range. The strongest reflectivity loss (RL) of microwave absorber was further enhanced to − 42.7 dB (microwave absorption rate > 99.9%) at 10.8 GHz for a layer of 2.0 mm thickness, and the strong absorption (RL < − 10 dB) was obtained between 8.72 and 18 GHz for the thickness of 1.3-2.2 mm. The results indicated that the dielectric loss and magnetic loss led to the excellent microwave absorption property of CuO/Co/CF composites. It is believed to be ideal for making a lightweight, strong absorption and wide-frequency microwave absorbing material.     

Enhancement of the electromagnetic wave absorption via the bamboo-like SiC whiskers with high-density stacking faults

It is a focus of electromagnetic wave-absorbing materials to control the microscopic appearance and structure design of materials to achieve good absorbing performance. Herein, we synthesized the bamboo-like β-SiC whiskers with numerous stacking faults using bamboo pulp paper. The results show that the bamboo-like β-SiC whiskers stacking faults are mostly concentrated at the bamboo nodes of the whiskers which had a significant impact on conductive and polarization losses. The composite with β-SiC whiskers/paraffin mass ratio of 0.5 shows good EM wave absorption capacity with a minimum reflection loss (RL_min) of ? 46.62 dB at 2.35 mm, and the effective absorption bandwidth (EAB) is 3.4 GHz (8.3–11.7 GHz) at 2.50 mm. The conductance loss, dipole polarization, polarization relaxation loss, and interfacial polarization induced by the bamboo-like structure are the major factors to improve its microwave absorption performance. This work provides a new idea for designing biomass-derived materials for excellent microwave absorbers.

     

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.     

A review on carbon/magnetic metal composites for microwave absorption

At present,developing high-efficiency microwave absorption materials with properties including light-weight,thin thickness,strong absorbing intensity and broad bandwidth is an urgent demand to solve the electromagnetic pollution issues.An ideal microwave absorber should have excellent dielectric and magnetic loss capabilities,thereby inducing attenuation and absorption of incident electromagnetic radiation.Recently,various carbon/magnetic metal composites have been developed and expected to become promising candidates for high-performance microwave absorbers.In this review,we introduce the mechanisms of microwave absorption and summarize the recent advances in carbon/magnetic metal composites.Preparation methods and microwave absorption properties of carbon/magnetic metal com-posites with different components,morphologies and microstructures are discussed in detail.Finally,the challenges and future prospects of carbon/magnetic metal absorbing materials are also proposed,which will be useful to develop high-performance microwave absorption materials.     

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.     

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.     

Preparation and microwave absorption of porous hollow ZnO by CO2 soft-template

The porous hollow ZnO samples were prepared by calcination of ZnCO₃ precursor at 450 °C. The structural properties were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), thermogravimetric analysis and differential thermal analysis (TG-DTA). A possible mechanism for the formation of porous hollow microstructure was proposed. The microwave absorption properties of the porous hollow structural ZnO have been investigated. The reflection loss (RL) of the ZnO was calculated based on the relative complex permeability and permittivity. A minimum reflection loss of the wax-composite with 25 wt% porous hollow ZnO is −36.3 dB at 12.8 GHz with a thickness of 4.0 mm. The results indicate that porous hollow structural ZnO can be used as a desirable material for the microwave absorption.     

Boron-doped helical carbon nanotubes: lightweight and efficient microwave absorbers

Boron-doped helical carbon nanotubes (B-HCNTs) were obtained by annealing HCNTs under the boric oxide presence. The morphology and structure of HCNTs remained unchanged even after annealing and B-doping. HCNTs displayed excellent electromagnetic wave (EMW) absorption, judging by the corresponding optimal reflection loss and the absorption bandwidth values equal to ? 47.86 dB and 3.20 GHz, respectively. Quick and straightforward synthesis process, excellent chemical stability and low density make our B-HCNTs promising as lightweight and efficient microwave absorbers.

     

Co含量对轻质微波吸收剂C/Co纳米纤维吸波性能的影响

采用静电纺丝法结合热处理制备了一种可应用于2~18 GHz频段的高性能轻质微波吸收剂C/Co纳米纤维, 详细研究了金属Co含量对纳米纤维的电磁特性及微波吸收性能的影响。相对于纯碳纳米纤维, C/Co纳米纤维的微波吸收性能得到显著加强, 其主要吸波机制仍是介电损耗。随着Co含量的增加, C/Co纳米纤维的电磁衰减能力逐渐下降, 而微波吸收却先增强后减弱, 含37.8wt% Co的C/Co-5纳米纤维因金属Co粒子和纳米碳纤维的良好结合与协同效应, 以及纤维中特殊的Co粒子@石墨核壳结构所带来的良好阻抗匹配与足够高的电磁衰减能力而表现出最好的吸波性能。模拟计算结果表明, 涂层厚度在1.1~5.0 mm间变化时, 填充5wt% C/Co-5纳米纤维的硅胶吸波涂层的反射损耗(RL)值超过-20 dB的频率范围在3.2~18 GHz, 最小RL值达到-78.8 dB, 其中当涂层厚度仅为1.5 mm时, RL值低于-20 dB的吸收带宽可达6.0 GHz (12~18 GHz)。C/Co纳米纤维优异的微波吸收性能表明, 这些磁性碳杂化纳米纤维有望成为一种极具应用前景的新型吸波材料。     

Robust magnetic and electromagnetic wave absorption performance of reduced graphene oxide loaded magnetic metal nanoparticle composites

With the increasing demand for microwave absorbing materials, to develop a microwave absorber with a simple synthesis process is of great significance to the field of protection. Herein we have successfully loaded iron-cobalt-nickel oxide (FeCoNiO_x) onto the as-prepared polydopamine-reduced graphene oxide (PDA-rGO) through a two-step process. The preparation method has mild reaction conditions without high temperature and pressure compared with the traditional method, which is conducive to large-scale production. Based on effectively combining dielectric loss and magnetic loss mechanism, the obtained material possesses excellent electromagnetic waves absorbing performance with the minimum RL value of ?36.28 dB. The results proved that the composites can be endowed with various microwave absorption effects by the adjustion of different component ratios. In addition, the microwave absorption mechanism was dicussed in detail, and we believe that the results of our research have certain guiding significance for preparation of efficient microwave absorbers.     

磁性多孔rGO@Co/CoO复合材料的制备和吸波性能

使用氧化石墨烯和乙酰丙酮钴为原料,用溶剂热和高温煅烧法合成了一系列三维多孔rGO@Co/CoO纳米复合材料。采用X射线衍射(XRD)和X射线光电子能谱(XPS)表征了材料的晶体结构及元素组成,用拉曼光谱分析了材料内部的石墨化程度及结构缺陷,用扫描电镜(SEM)和透射电镜(TEM)观察和分析了材料的形貌及微观结构。结果表明,煅烧温度为350、500及650℃制备的产物分别为rGO@CoO复合材料、面心立方(fcc型)rGO@Co纳米复合材料以及双晶型(fcc和hcp型)rGO@Co纳米复合材料。填充量(质量分数)为10%时,S500的吸波性能最优异,RL_min值为-74.5 dB,对应的频率为13.9 GHz,匹配厚度为2.5 mm,有效吸收带宽为6.1 GHz(10.7~16.8 GHz)。这种材料优异的吸波性能,可归因于多种损耗机制共同作用和独特的三维孔隙结构。     

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.     

Microwave absorption properties of multiferroic BiFeO3 nanoparticles

Multiferroic BiFeO₃ (BFO) nanoparticles ranging from 60 nm to 120 nm were synthesized successfully by a sol-gel method, and the microwave absorption properties of BFO nanoparticles were investigated in the range of 12.4 GHz to 18 GHz. The reflection loss of BFO nanoparticles is more than 10 dB (or more than 90%) in the 13.1 GHz-18 GHz range and reaches to 26 dB at 16.3 GHz, which indicated that the BFO is a good candidate for microwave absorption application. The excellent microwave absorption properties of BFO nanoparticles could be attributed to the good electromagnetic match as a consequence of the coexistence of ferroelectric and weak ferromagnetic order in BFO nanoparticles, which has been confirmed by electric and magnetic measurement. Moreover, the nanosize-confinement effect may also have contribution to the high reflection loss of BFO nanoparticles.     

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.