Design of magnetic triple-shell hollow structural Fe3O4/FeCo/C composite microspheres with broad bandwidth and excellent electromagnetic wave absorption performance

A three-step strategy combining solvothermal and liquid phase reduction method had been developed for preparation of magnetic triple-shell hollow structural Fe₃O₄/FeCo/C (TSH–Fe₃O₄/FeCo/C) composite microspheres. FeCo was used to enhance electromagnetic (EM) wave absorption in different frequency band and broaden effective absorbing bandwidth, while carbon was used to improve impedance matching. Triple-shell hollow structure was designed to enrich the multiple interfaces to favor the interfacial polarization, increase the multiple reflections and scattering, and provide physicochemical protection to Fe₃O₄ core from oxidation. The microstructure and morphology of TSH-Fe₃O₄/FeCo/C composite microspheres were characterized by TEM, XRD and Raman in detail. The results indicated that magnetic Fe₃O₄ was completely covered by FeCo and carbon via layer by layer. As an EM wave absorber, the maximum reflection loss of TSH-Fe₃O₄/FeCo/C composite microspheres was up to -37.4 dB due to better normalized characteristic impedance at a thickness of 2.2 mm and the bandwidth less than -10 dB even reached up to 5.9 GHz. The excellent EM wave absorption performance was attributed to the combination of shell materials (Fe₃O₄, FeCo and carbon) and unique triple-shell hollow structure, which lead to multiple relaxation processes and good impedance matching. Consequently, this work would contribute to the design and preparation of high performance EM wave absorbent with outstanding absorbing property and wider absorption range.     

Controllable dielectric properties and strong electromagnetic wave absorption properties of SiC/spherical graphite-AlN microwave-attenuating composite ceramics

Fully dense SiC/spherical graphite-AlN microwave-attenuating composite ceramics were manufactured via hot-pressing sintering, in which, apart from the primary SG (spherical graphite) attenuating agent, 5–30 wt% semiconductive α-SiC was employed as the second attenuating agent. The incorporation of SiC contributed to a slightly decreasing electrical conductivity and enhanced polarization relaxation. Controllable complex permittivities were obtained, namely, both the real and imaginary permittivities exhibit first a decrease and then an increase with the SiC addition, and which delivers an optimized impedance matching of the composites. RL_min values below ?10 dB (more than 90% absorption) were achieved by all the composites containing 5–20 wt% SiC with the sample thickness of 1–1.4 mm, and the absorption performance characteristics were significantly tunable by controlling the of SiC content at 8.2–12.4 GHz. Impressively, a superior reflection loss of ?46 dB (1.1 mm) and wide effective absorption bandwidth of 2.1 GHz were achieved at a 5 wt% SiC content, respectively, rendering SiC/SG–AlN composites a potential ultra-thin and highly efficient microwave-attenuating ceramic candidate.     

Carbonized foams from graphene/phenolic resin composite aerogels for superior electromagnetic wave absorbers

Ultralight graphene/phenolic resin composite aerogels (GPFs) were prepared through the chemical reduction and self-assembly of graphene oxide (GO) in water-soluble phenolic resin, followed by a freeze-drying process; carbonized foams (GPFs(T)) were obtained by the subsequent heat treatment of GPFs at a relatively low temperature (500–700 °C). Although GPFs do not show the qualified reflection loss value of below −10dB, GPFs(T) achieve the greatly enhanced electromagnetic-wave absorbing performance. Specifically, the minimum reflection loss value of GPF₁ (500) reaches −22.7 dB at 14.4 GHz with the absorber thickness of 2.0 mm and the effective absorption bandwidth is up to 5.4 GHz (12.4–17.8 GHz). The evolution of electromagnetic-wave absorbing properties from GPFs to GPFs(T) at different temperatures related with different graphene content is explored. GPFs(T) are expected to exhibit high thermal stability and excellent corrosion resistance property, and especially still maintain ultralight nature (e.g the density of GPF₁ (500) is only 24.3 mg/cm³). Most importantly, little graphene (as low as 7.5 wt% of GO addition for GPF₁(T)) in GPFs(T) guarantees the facile formation of three-dimension (3D) skeleton network and greatly cut downs the carbonization temperature of phenolic resin to achieve the required electromagnetic-wave energy losing ability. The present work provides an effective method to fabricate an ultralight material with exceptional performances including the good electromagnetic-wave absorbing property and the high stability.     

Enhanced electromagnetic wave absorption performance of SiCN(Fe) fibers by in-situ generated Fe3Si and CNTs

The SiCN(Fe) fibers with excellent one-dimensional microstructure and electromagnetic wave (EMW) absorption performance were synthesized by combining polymer-derived ceramics (PDCs) method and electrospinning. The in-situ generation of Fe₃Si and CNTs by adding ferric acetylacetonate (FA) into the raw materials effectively improved the dielectric properties, magnetic properties and the impedance matching performance of the SiCN(Fe) fibers. The EMW absorption performance of SiCN(Fe) fibers were mainly based on dipole polarization loss, interface polarization loss and eddy current loss. The RL_min value of SiCN(Fe) fibers reached ?47.64 dB at 1.38 mm and the effective absorption band (EAB, RL ≤ ?10 dB) reached 4.28 GHz (13.72–18 GHz, 1.35 mm).     

High electromagnetic wave absorbing performance of activated hollow carbon fibers decorated with CNTs and Ni nanoparticles

Activated hollow carbon fibers (ACHFs) decorated with carbon nanotubes (CNTs) and nickel nanoparticles (CNTs–Ni–ACHFs) were prepared by thermal reduction and chemical vapor deposition technique. Microwave reflection loss, permittivity and permeability of CNTs–Ni–ACHFs composites as novel electromagnetic wave absorbents were studied in the frequency range of 2–18 GHz. It was demonstrated that CNTs–Ni–ACHFs absorbents possessed the best microwave absorbing performances whose minimum reflection loss was −43.457 dB at 13.10 GHz with a thickness of 2.0 mm, which is much better than those of Ni–ACHFs and ACHFs samples. The dielectric polarizations and magnetic loss derived from the effect of the porous structures, Ni nanoparticles, and defects in the CNTs–Ni–ACHFs composites are playing an important role for the excellent microwave absorbing performances.     

In-situ synthesis of ternary layered Y3Si2C2 ceramic on silicon carbide fiber for enhanced electromagnetic wave absorption

A novel ternary layered ceramic of Y₃Si₂C₂ was successfully in-situ synthesized on the surface of home-made third-generation KD-SA SiC fiber for the first time by molten salt method aimed at improving the electromagnetic wave (EMW) absorption. After in-situ synthesis of Y₃Si₂C₂ ceramic layer on SiC fiber (SiC_f/Y₃Si₂C₂), significantly improved EMW absorption performance was obtained. The minimum reflection loss (RL_min) of ?16.97 dB was reached in SiC_f/Y₃Si₂C₂ composites at the thickness of only 2.19 mm, and the effective absorption bandwidth (EAB) was up to 5.44 GHz (12.56–18 GHz) at a thin thickness of 2.64 mm. The improvement in EMW absorption of SiC_f/Y₃Si₂C₂ is mainly attributed to enhanced dielectric loss and conduction loss resulting from increased heterogeneous interfaces and multiple reflections and scattering originating from net structure. The SiC_f/Y₃Si₂C₂ could be a promising EMW absorber for application in high-performance EMW absorbing materials.     

Single crystal to polycrystal: Enhanced dielectric loss and electromagnetic wave absorption of MoO2 ceramic at Gigahertz

In this work, the dielectric loss behavior and electromagnetic wave absorption (EMA) property of MoO₂ ceramic from single crystal to polycrystal have been in-depth investigated. It is found that polycrystal MoO₂ shows better dielectric loss ability than single crystal MoO₂, leading to greatly enhanced EMA performance. It is determined that polarization loss occupies the dominant position in the dielectric loss of MoO₂ ceramic. This research provides new insights in understanding of dielectric loss behavior and EMA mechanism for dielectric MoO₂ crystals.     

In-situ growth of bamboo-shaped carbon nanotubes and helical carbon nanofibers on Ti3C2Tx MXene at ultra-low temperature for enhanced electromagnetic wave absorption properties

The potential of two-dimensional layered MXenes in electromagnetic wave (EMW) absorption needs further development. Herein, we carried out the in situ growth of carbon nanotubes (CNTs) on the surface of Ti₃C₂T_x MXene at ultra-low temperature via chemical vapor deposition. The obtained CNTs exhibited a bamboo-like structure and were accompanied by helical carbon nanofibers. The ultra-low temperature solved the problem that the high temperature required in the traditional CNT growth process would destroy the structural integrity of MXene. The lush CNT forest cross-linked the MXene layers, transforming the two-dimensional layered structure into a three-dimensional conductive network, providing abundant conductive channels for carriers, optimizing the impedance matching of the CNT/MXene hybrid, and resulting in a significant dielectric loss. The as-prepared CNT/MXene hybrid exhibited a minimal reflection loss of ?52.56 dB (99.9994% EMW absorption) in the X-band. This work proposes a new idea to enhance the EMW absorption properties of Ti₃C₂T_x MXene and fabricate high-performance MXene-based EMW absorbers.     

Hierarchical brain-coral-like structure (3D) vs rod-like structure (1D): Effect on electromagnetic wave loss features of SrFe12O19 and CoFe2O4

Morphological configuration plays a vital role in regulating the absorption performance of magnetic materials. Herein, a novel challenge is discussed on electromagnetic loss features of two hard and soft magnetic materials with hierarchical brain-coral like structure and rod-like structure. In this study, pure SrFe₁₂O₁₉ (Sr) as hard magnetic component and CoFe₂O₄ (Co) as soft magnetic component with two distinct morphologies were successfully synthesized by facile hydrothermal and solvothermal methods. In the first approach, electromagnetic loss features of rod and brain-coral-like particles were investigated, and in second approach -according to the obtained results-microwave absorption performance of a mixture of hard/soft magnetic components with hierarchical structure were evaluated. The minimal reflection loss (RL) for brain-coral-like particles of individual Sr and Co samples were −17.6 dB (at 18.8 GHz with 9 mm thickness) and −31.2 dB (at 8.1 GHz with 10 mm thickness), respectively, which show far better performance than rod-like structure. Remarkably, the composite of Sr and Co micro-particles with hierarchical structure exhibited strong RL value of −38 dB with 2.6 GHz effective absorption bandwidth at the thickness of 2.5 mm, with a filling ratio of 40 wt%. According to the results, it is founded that the electromagnetic loss features are crucially boosted via hierarchical configuration of magnetic materials. Increment in complex permittivity and permeability, accounting for the formation of cross-linked networks in the hierarchical structure, promoted the interfacial polarization phenomena with different relaxation times and appearance of multi resonance peaks.     

Ethylene–(vinyl acetate) copolymer/carbon fiber conductive composite: effect of polymer–filler interaction on its electrical properties

Ethylene–(vinyl acetate) (EVA)/carbon fibre (CF) composites were prepared by changing the content of CF in the composite. To investigate the effect of the interaction between EVA and CF on the electrical properties of the composite, the CF was treated in nitric acid. The interaction between EVA and CF was examined by a solvent-extraction method. It was found that the interaction of EVA with CF was enhanced due to the chemical absorption of EVA on CF. The correlation of CF content, electrical properties and the formation of polymer–filler gel for the composite with oxidized CF was studied. Although the composites filled with treated CF exhibit a slightly higher resistivity than those filled with untreated CF at room temperature, they show the improved electrical properties, including elimination of the negative-temperature-coefficient (NTC) effect, high electrical reproducibility after thermal cycles, and independence of the conductivity on time, which improves the practical applications of positive-temperature-coefficient (PTC) materials. Copyright © 2004 Society of Chemical Industry