Polypyrrole-derived N-doped carbon nanoribbon for broadband microwaves absorption

In this work, we successfully synthesize N-doped carbon nanoribbon (NCNR) from polypyrrole precursor and investigate their dielectric and microwaves absorption (MA) properties. NCNR appears as two-dimensional ribbon-like microstructure with tunable N-doping ratio. The dielectric property of NCNR can be tuned by N-doping content controlling. The results demonstrate that NCNR exhibits excellent MA performance at a filler-loading ratio of only 5 wt%. When the sample thickness is 3.3 mm, the maximal absorption reaches ??73.76 dB at 10.48 GHz. The maximum efficient bandwidth gets to 7.4 GHz (10.6–18 GHz), under a sample thickness of 2.7 mm. A model that refers to conductive loss, polarization relaxation, and impedance match is adopted to explain the MA mechanism of NCNR. This research opens up the exploration of NCNR in the field of MA, and provides a new idea for the design of carbon-related broad band MA materials.

     

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.

     

Facile synthesis of N-doped Co/graphite C composites with melamine as carbon and nitrogen source with enhanced microwave absorption performance

Magnetic metals are vital microwave absorbing materials, while they face the problems of a large density and relatively narrow absorption band in practical microwave absorption applications. The combination of magnetic metals with carbon materials can effectively ameliorate the electromagnetic (EM) parameters to resolve these the problems to a certain extent. In this study, we successfully fabricated the N-doped Co/graphite C (Co/NC) composites via directly calcining a dry mixture of Co powders and melamine in an Ar atmosphere. This method is simple and low cost. The effects of the variation in the phase composition and morphology on EM parameters and the microwave absorbing property were investigated. The graphite C content in Co/NC was relatively larger than Co content. Thus, the contribution of the dielectric loss to EM energy dissipation was more than that of the magnetic loss. Compared to pure Co, the Co/NC composites exhibit superior EM wave absorbency and are easier to realize strong absorption and lightweight. A minimum reflection loss of???41.45 dB was obtained at 3.84 GHz. The effective absorbing bandwidth (EABW, RL?≤????10 dB) reached 14.80 GHz (3.20?18.00 GHz) at a coating thickness d of 1.54?5.00 mm. The maximum EABW was as large as 5.50 GHz at only 1.54 mm thick. The main reasons for the enhanced EM wave absorption performance of Co/NC were the well impedance matching and strong attenuation capability (interfacial and dipolar polarization, conduction loss, magnetic loss, and microwave multiple scattering and reflections on rough surfaces). Our study can provide a simple guideline for preparing other light carbon-based magnetic metal composites.

Graphical abstract

The Co/NC composites with melamine as C and N source more favor for realization of strong absorption and lightweight compared to Co.

     

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.

     

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.

     

Structural and microwave properties of Ag-doped strontium hexaferrite

A series of silver-doped strontium hexaferrite with the chemical formula SrAg_ZFe_12-zO₁₉ (0.0?≤?z?≤?1.0) were synthesized by the Co-precipitation method. The crystal structure, morphology, and properties of microwave absorption with Ag concentration were studied. The structural analysis by XRD revealed that the samples are crystallized with an M-type hexagonal structure. The values of lattice parameters, the volume of the unit cell, and X-ray density are increasing with the increase of Ag doping. The least values of Rietveld refinements have confirmed a good correlation between experimental and calculated data. Hexagonal plate-like morphology was observed in SEM images and the grain size decreases with Ag doping. Microwave properties have been measured by a vector network analyzer. Real and imaginary parts of electrical permittivity dependence with the frequencies in X-band (8–12 GHz) have been studied. The Reflection loss (RL) was investigated for all samples in X-band frequencies. Maximum RL of ? 21.95 dB at 10.0 GHz was observed for the composition of silver, z?=?0.4. Improved RL when compared with the pure sample indicating enhanced impedance matching and attenuation constant hence the material can show maximum energy loss for the incident microwaves. The results so obtained are explained based on composition and microwave phenomena. The present studies have confirmed the nature of microwave absorption for Ag-doped strontium hexaferrite.

     

Microwave absorption performance of hierarchical structured composites of greigite and reduced graphene oxide

The greigite (Fe₃S₄)/reduced graphene oxide (RGO) hierarchical structural composites (F–R) with the Fe₃S₄ nanoparticles attached to the RGO layers were successfully prepared via a simple one-pot solvothermal method. The microwave absorption properties were evaluated by calculating the reflection loss (RL) values. The results show that the RGO content and the filler loading of composites in paraffin mixture are very critical to the microwave absorption properties because they can improve the electromagnetic parameters. Sample F–R-3 presents the best microwave absorption capacity, in which an optimum RL value of ??62.3 dB and an effective absorption bandwidth (EAB, RL value?<???10 dB) of 3.04 GHz (14.96–18 GHz) can be obtained when the matching thickness is only 1.29 mm. Meanwhile, the widest EAB reaches 4.08 GHz (13.92–18 GHz) at the matching thickness of only 1.37 mm. Impressively, when the matching thickness is in the range of 1.2–5.5 mm, all RL peaks are below ??20 dB, and the EAB can be 14.98 GHz (3.02–18 GHz), covering the whole C, X and Ku bands. The distinguished absorption property is mainly ascribed to the combined effect of strong loss ability and good impedance matching. Apparently, the F–R composite with strong absorption ability, thin thickness and wide EAB is suitable for the efficient microwave absorber.

Graphical abstract

     

D-xylose-derived carbon microspheres modified by CuFe2O4 nanoparticles with excellent microwave absorption properties

In this work, we studied the preparation and microwave absorption properties of D-xylose-derived carbon materials modified by CuFe₂O₄ nanoparticles and discussed the influence of the addition amount of carbon materials. It was prepared with a simple hydrothermal method, and various methods are used to characterize its microscopic morphology, chemical structure and microwave absorption properties.Results show that the modification of copper ferrite nanoparticles is effective in mitigating polarization and magnetic losses, enhancing the microwave absorption properties of the samples and absorption curves of the composite materials change from a single peak to double peaks. Comparing all samples it can be concluded that the effective absorption bandwidth of CuFe₂O₄ composite materials containing carbon microspheres (CuFe₂O₄/CMs-0.3) is 6.24 GHz. The minimum reflection loss (RL) is???50.4 dB at 14.64 GHz. The thickness of the above corresponds to 2.5 mm. This work not only provides a reference method for the preparation of microwave absorbing materials but also contributes to the understanding of absorption mechanisms.

     

Honey-comb carbon nanostructure derived from peach gum to yield high microwave absorption

Lightweight and highly efficient electromagnetic absorptions are crucial for microwave absorption materials due to the fast development of information technology. Bio-derived carbon materials are ideal resistance loss-type microwave absorbers with lightweight. A honey-comb carbon structure has been obtained from peach gum by facile hydrothermal and pyrolysis processes. The skeleton of the as-derived honey-comb carbon structure is composed of carbon nanoparticles with diameters around 40 nm. The honey-comb structures were further carbonized at 850 °C (NPG-850) to have a large specific surface area of 1401.7 m² g^?1 with an average pore diameter of 3.2 nm. A minimum reflection loss (RL) of???59.4 dB was obtained by NPG-850 at a thickness of 2.0 mm with an effective absorption bandwidth (EAB, RL?<????10 dB) of 4.1 GHz (14.7–10.6 GHz). The RL value of the peach gum-derived honey-comb carbon nanostructures is much higher than the reported carbon nanostructures even with thinner thickness and less mass loading, which might due to the multi-reflection effect of the honey-comb structures and the skeleton composition.

     

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.     

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.     

Constructing and optimizing core–shell structured Co@TiO2 as highly efficient electromagnetic wave absorber

To achieve highly efficient electromagnetic wave absorber, elaborately designing magnetic–dielectric Co@TiO₂ microspheres with core–shell configuration are successfully constructed through a facile sequential process of liquid phase reduction–sol–gel–annealing. Owing to the core–shell configuration and the synergistic effect among magnetic and dielectric components, the annealed sample shows outstanding electromagnetic wave absorption (EMA) in X and K_u band. Impressively, a maximum reflection loss (RL_max) is reached?–56.6 dB at a coating thickness of 2.3 mm with corresponding effective absorption bandwidth (EAB₁₀) of 7.2 GHz (including 65% of K_u band and 82.5% of X band), much stronger than those of as-prepared Co (EAB₁₀ of 1.6 GHz, RL_max of 14.6 dB) and pristine Co@TiO₂ (EAB₁₀ of 2.9 GHz, RL_max of 16.3 dB). An EAB₁₀ covering completely the whole X and K_u band could be obtained by controlling the thickness only from 2.0 mm to 2.5 mm. The composites have both outstanding RL and wide EAB₁₀ with the thin coating thickness, reinforcing that fabricating core–shell configuration composites is an efficient strategy to boost the EMA efficiency.

     

Microwave absorption and mechanical properties of La(NO3)3-doped multi-walled carbon nanotube/polyvinyl chloride composites

Composites that effectively absorb microwave in the frequency range of 2-18 GHz have been strongly demanded during the past decades. To enhance the absorption of microwave, composites are generally capable of wide absorption bandwidth and low reflection loss. Here, we used La(NO₃)₃-doped multi-walled carbon nanotubes (MWCNTs) as absorber and polyvinyl chloride (PVC) as matrix to fabricate MWCNT-La(NO₃)₃/PVC composites, where La(NO₃)₃-doped MWCNTs broadened the absorption bandwidth and decreased the reflection loss of the composites. Due to the insertion of diamagnetic La³⁺, the absorbing properties of as-prepared composites were enhanced, while the mechanical properties of such composite were slightly changed. The amount of doped La(NO₃)₃ was experimentally optimized to be 6 wt.%.     

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.

     

Lightweight multi-walled carbon nanotube buckypaper/glass fiber–epoxy composites for strong electromagnetic interference shielding and efficient microwave absorption

Multi-walled carbon nanotube buckypaper (BP) reinforced glass fiber–epoxy (GF/EP) composites were selected to fabricate electromagnetic interference (EMI) shielding and microwave absorbing materials. Six different composite configurations with 3.0 mm thick have been conceived and tested over the X-band (8.2–12.4 GHz). Flexible and low-density (0.29 g/cm³) BP provided a high specific EMI SE of 76 dB with controlled electrical conductivity. GF/EP/BP₁₁₁ and GF/EP/BP₁₀₁ composites possess EMI SE as high as of 50–60 dB, which can be attributed to the number of BP inserted and variation in the wave-transmitting layer of the laminates. Furthermore, the shielding mechanism was discussed and suggested that the absorption was the dominant contribution to EMI SE. GF/EP/BP₁₁₀ laminate demonstrated suitable EMI performance (~?20 dB), whereas GF/EP/BP₀₁₁ composite revealed excellent microwave performance, achieving an effective ? 10 dB bandwidth of 3.04 GHz and minimum reflection loss (RL) value of ? 21.16 dB at 10.37 GHz. On the basis of these results, GF/EP/BP composites prepared in this work have potential applications as both EMI shielding and microwave absorber materials given their facile preparation and lightweight use.

     

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.     

The characterization and preparation of core–shell structure particles of carbon-sphere@NiFe<Subscript>2</Subscript>O<Subscript>4</Subscript>@PPy as microwave absorbing materials in X band

An excellent PPy/NiFe₂O₄/CS microwave absorbing materials with a three-layer core–shell structure, was synthesized successfully by two reaction steps of solvothermal reaction and in situ polymerization. The surface morphology, phase structure and chemical components of the composite have been characterized by a scanning electron microscope, X-ray diffraction and X-ray photoelectron spectroscope. The results suggest that the surface of CS is covered by NiFe₂O₄ completely and PPy wraps the obtained NiFe₂O₄/CS successfully. The conductivity and the saturation magnetization (Ms) of the resulting PPy/NiFe₂O₄/CS composites are 0.38 S/cm and 46 emu/g, respectively. The vector network analysis shows the composite performs better microwave absorbing ability than that of CS and NiFe₂O₄/CS. The maximum reflection loss of the composite with 1.97 mm coating thickness is ?53 dB at 10.5 GHz and the bandwidth of reflection loss less than ?10 dB is 3.4 GHz (8.9–12.3 GHz). This ternary composite with light weight, thin thickness and strong absorbing capacity can be an attractive candidate in the field of microwave absorption.     

Self-reducing coal-derived carbon/Ni3Fe magnetic composites with frequency-dependent microwave absorption performance

Coal-derived carbon/Ni₃Fe magnetic composites with frequency dependent microwave absorption performance were prepared at low temperatures (750–850 °C) using coal as the carbon source. The Ni₃Fe alloy was successfully formed due to the carbothermal reaction and reducing gas. SEM images indicate the surface becomes rougher and the number of interlayer of the composites increases with increasing reaction temperature. Consistently, high degree of graphitization of the coal-derived carbon was confirmed by using Raman spectroscopy. Specifically, coal-derived carbon/Ni₃Fe magnetic composites exhibit frequency-dependent microwave absorption characteristics at 2–18 GHz, that is, as the reaction temperature rises from 750 °C to 850 °C, the minimum reflection loss gradually shifts to low frequencies. Among them, CC/Ni₃Fe_(8 0 0)-0.4 exhibits a minimum reflection loss of ?60.76 dB at 16.64 GHz, while the thickness is only 1.28 mm. Such a clean strategy provides experience for the environmental application of coal and microwave absorption. Meanwhile, a lightweight, stable and efficient microwave absorber has been developed.     

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.