Electromagnetic wave absorption properties of polymer-derived magnetic carbon-rich SiCN-based composite ceramics

In this paper, the mixture of Fe and Ni nanoparticles (abbreviated as FeNi) was added to liquid polysilazane (PSZ) as a magnetic source, to prepare a series of magnetic carbon-rich SiCN-based composite ceramics by adjusting the mass ratio of FeNi through the polymer derivation method. The phase composition, microstructure, conductivity, electromagnetic wave (EMW) absorption performance and mechanism of composite ceramics prepared were discussed. The analysis shows that the introduction of magnetism has adjusted the impedance matching and improved the magnetic loss performance of composite ceramics on the whole, and the dielectric loss ability of composite ceramics has been strengthened benefiting from the formation of conductive path of CNTs precipitated by FeNi catalysis in the matrix. Therefore, the addition of magnetic particles improves the EMW absorption peak intensity and effective absorption bandwidth (EAB) of composite ceramics. When the addition amount of FeNi was 5 wt%, the sample 5# exhibited the best comprehensive EMW absorption performance: Its minimum reflection loss (RL_min) was ?18 dB and the EAB was 2.5 GHz when the thickness was 1 mm, the EAB covering the C, X and Ku bands can be obtained by adjusting the thickness from 1.0 mm to 4.0 mm. Through calculation, the EAB (EAB_tf) of 5# with a thickness of 1 mm and a filling rate of 1 wt% can reach 50, which is significantly higher than that of a series of SiCN-based composite ceramics previously reported. In addition, the density of 5# was 2.3 g/cm³, and its compressive strength (CS) can reach 337 MPa. The data shows that the composite ceramic 5# prepared in this experiment has the merits of light weight, excellent comprehensive EMW absorption performance and good compression resistance, and is expected to be one of the promising materials in the field of new-generation EMW absorbers.     

Synthesis of nitrogen-doped graphene-based binary composite aerogels for ultralightweight and broadband electromagnetic absorption

The development of ultralightweight and broadband electromagnetic wave (EMW) absorbing materials remains a big challenge. In this work, porous magnesium ferrite microspheres decorated nitrogen-doped reduced graphene oxide (NRGO/MgFe₂O₄) composite aerogels were prepared by a two-step route of solvothermal synthesis and hydrothermal self-assembly. Results of microscopic morphology characterization showed that NRGO/MgFe₂O₄ composite aerogels had a unique hierarchical porous structure. Moreover, the influence of additive amounts of graphene oxide on the electromagnetic parameters and EMW absorption properties of NRGO/MgFe₂O₄ composite aerogels was explored. Remarkably, the attained binary composite aerogel with the content of NRGO of 70.21 wt% exhibited the best EMW absorption performance. The minimum reflection loss reached up to −55.7 dB, and the corresponding effective absorption bandwidth was as large as 5.36 GHz at a thin matching thickness of 1.98 mm. Furthermore, when the matching thickness was slightly increased to 2.29 mm, the widest effective absorption bandwidth was enlarged to 7.1 GHz, covering the entire Ku-band. The magnetodielectric synergy and unique hierarchical porous structure in NRGO/MgFe₂O₄ composite aerogels not only improved the impedance matching, but also greatly enhanced the EMW absorption capacity. It was believed that the results of this work could be helpful for the preparation of graphene-based magnetic composites as broadband and efficient EMW absorbers.     

Morphology-controlled preparation and tunable electromagnetic wave absorption performance of manganese dioxide nanostructures

The development of electromagnetic wave (EMW) absorption materials with lightweight, wide absorption bandwidth, thin thickness, and strong EMW absorption performance has become a hotspot. Herein, the morphology-controlled preparation of α-manganese dioxide (α-MnO₂) was successfully obtained via a facile hydrothermal method, and the EMW absorption performance of α-MnO₂ was investigated in detail. The results indicated the as-obtained Mn-1.0-120 possessed the best EMW absorption performance with minimum reflection loss of −53.43 dB at about 5.2 GHz with a thickness of 4.1 mm originated from the synergistic effects of multiply scattering, dielectric loss, and magnetic loss. This contribution demonstrates that the MnO₂ has promising candidates with a tunable EMW absorption performance for applications in the electromagnetic field in the future.     

Fabrication of Ni/ZnO/C hollow microspheres decorated graphene composites towards high-efficiency electromagnetic wave absorption in the Ku-band

Developing light-weight, thin thickness and high-efficiency electromagnetic wave (EMW) absorbers is an effective strategy for dealing with the increasingly serious problem of electromagnetic radiation pollution. Herein, nickel/zinc oxide/carbon (Ni/ZnO/C) hollow microspheres decorated graphene composites were facilely prepared through the high-temperature pyrolysis of bimetallic NiZn metal-organic frameworks (MOFs) precursors. Morphological characterization results manifested that the Ni/ZnO/C microspheres with unique hollow structure were almost evenly anchored on the wrinkled surfaces of flake-like graphene. Moreover, the influences of additive amounts of graphene oxide (GO) in the MOFs precursors on the crystal structure, graphitization degree, micromorphology, magnetic properties, electromagnetic parameters and EMW absorption performance were investigated in detail. It was found that the superior EMW absorption performance could be achieved through facilely adjusting the additive amounts of GO in the precursors. As the additive amount of GO was equal to 60 mg, the obtained composite showed the comprehensive excellent EMW absorption performance. Notably, the optimal minimum reflection loss reached ?57.5 dB at 16.5 GHz in the Ku-band under an ultrathin matching thickness of merely 1.34 mm, and the broadest effective absorption bandwidth achieved 5.6 GHz (from 12.4 to 18 GHz) when the thickness was 1.5 mm. Furthermore, the underlying EMW absorption mechanisms of as-prepared composites were revealed. It was believed that our results could be valuable for the structural design and EMW absorption performance modulation for MOFs derived magnetic carbon composites.     

Design of hierarchical core-shell ZnFe2O4@MnO2@RGO composite with heterogeneous interfaces for enhanced microwave absorption

In order to overcome the problems caused by electromagnetic pollution, the design and development of high-performance microwave absorbers is urgently required. In this work, a hierarchical ZnFe₂O₄@MnO₂@RGO composite was successfully fabricated via a facile and rapid hydrothermal method. Its unique core-shell structure and synergistic effect between multiple components are beneficial for electromagnetic wave absorption. The morphology, elemental composition, microstructure and microwave absorption characteristics were systematically studied. With a filler loading of 20 wt%, the composite presents a minimum reflection loss (R_Lmin) of ?46.7 dB and an effective absorption bandwidth (EAB) as wide as 5.2 GHz at a thickness of 2.5 mm. The superior absorption ability profits from a special microstructure, good impedance matching, multiple attenuation features, interfacial polarization, and the synergistic effect of dielectric and magnetic loss. Consequently, this work lays a foundation for the design of high-performance electromagnetic wave absorbers with multicomponent heterogeneous structures.     

Core-shell MXene/nitrogen-doped C heterostructure for wide-band electromagnetic wave absorption at thin thickness

MXenes have been utilized to fabricate electromagnetic wave (EMW) absorbers owning to large aspect ratio, high electronic conductivity, and favorable hydrophilicity. In this work, the core-shell MXene/nitrogen-doped (N-doped) C heterostructure was firstly prepared via HCl and LiF etching, in-situ polymerization, and carbonization. When mixed with paraffin at a low filler loading of 30 wt%, the MXene/N-doped C hybrid reached a wide effective absorption bandwidth of 5.0 GHz (13.0 GHz–18.0 GHz) at a thin thickness of 1.72 mm. The stronger ability of attenuating EMWs promoted the absorption performance of MXene/N-doped C, overcoming the deficiency in the characteristics of impedance compared with its counterparts. This work provides a new insight in manufacturing MXene-based absorbers to alleviate EMW pollution by delicate structural design and effective multi-component strategy.     

Enhanced electromagnetic microwave absorption of Fe/C/SiCN composite ceramics targeting in integrated structure and function

The Fe/C/SiCN composite ceramics were synthesized by polymer-derived method to obtain the integration of structure and functions. The electromagnetic waves (EMW) absorption properties at X and Ku bands were investigated. The addition of nano-sized Fe particles improved the magnetic loss and impedance matching, and the carbon nanotubes generated by the iron in-situ catalysis increased the internal relaxation polarization and interfacial polarization, which together improved the EMW absorption properties significantly. In particular, the Fe/C/SiCN-9 showed the optimum reflection loss (RL) of ?31.06 dB at 10.03 GHz with an effective absorption bandwidth (EAB, RL < ?10 dB) of 3.03 GHz at 2.51 mm, indicating the excellent EMW absorption properties of Fe/C/SiCN composite ceramics.     

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).     

Synthesis of carbon-coated cobalt ferrite core–shell structure composite: A method for enhancing electromagnetic wave absorption properties by adjusting impedance matching

Cobalt ferrite has problems such as poor impedance matching and high density, which results in unsatisfactory electromagnetic wave (EMW) absorption performance. In this study, the CoFe₂O₄@C core-shell structure composite was synthesized by a two-step hydrothermal method. X-ray diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy, thermogravimetric analysis, and vector network analysis et al. were used to test the structure and EMW absorption properties of CoFe₂O₄@C composite. The results show that the reflection loss (RL) of the CoFe₂O₄@C composite reaches the maximum value of -25.66 dB at 13.92 GHz, and the effective absorbing band (EAB) is 4.59 GHz (11.20-15.79 GHz) when the carbon mass content is 6.01%. The RL and EAB of CoFe₂O₄@C composite are increased by 219.55% and 4.59 GHz respectively, and the density is decreased by 20.78% compared with the cobalt ferrite. Such enhanced EMW absorption properties of CoFe₂O₄@C composite are attributed to the attenuation caused by the strong natural resonance of the cobalt ferrite, moreover, the carbon coating layer adjusts the impedance matching of the composite, and the introduced dipole polarization and interface polarization can cause multiple Debye relaxation processes.     

Metal ions induced dielectric and magnetic loss in Co3O4 for electromagnetic wave absorption

Despite Co₃O₄ has been widely applied in electromagnetic wave (EMW) absorbers, single Co₃O₄ doesn't have excellent EMW absorbing performance. Modification of Co₃O₄ with other metal ions addition is an effective way to improve its impedance matching and EMW attenuation. Herein, CuO/Cu_0.3Co_2.7O₄/Co₃O₄ and NiCo₂O₄/Co₃O₄ composites have been obtained via a facile two-stage strategy, and the influence of Cu²⁺ and Ni²⁺ on the high-frequency and low-frequency EMW absorbing performance of the composites has been investigated as well. The electromagnetic parameters of samples are regulated by adding different metal ions to achieve optimum impedance matching. Dipole polarization and magnetic resonance are the main loss mechanisms. The composite with Cu²⁺ and Ni²⁺ additions exhibits the best EMW absorption with an effective absorption bandwidth (EAB) of 10.8–18.0 GHz for 2.1 mm thickness at high-frequency and 4.5–8.5 GHz for 4.9 mm thickness at low frequencies, respectively. This work offers an effective method for preparing composite materials with multicomponent broadband absorption of oxides.     

The effect of TiN deposition time on the field-emission performance coated on ZnO nanorod arrays

ZnO nanorod arrays (NRs) with a large number of sharp tips and uniform shapes were grown on the carbon cloth (CC) by a simple hydrothermal method. Titanium nitride (TiN) nanoparticles with various thicknesses were deposited on the ZnO NRs by magnetron sputtering to obtain ZnO/TiN core-shell arrays. Field emission (FE) performance of ZnO NRs show close dependence on TiN coating thickness. The turn-on field first decreases and then increases with increasing TiN coating thickness from 60 nm to 300 nm. The arrays with a design architecture can strike a balance between increased emission sites and limited field shielding effects. ZnO/TiN240 core-shell NRs have the lower turn-on electric field at 0.79 V/μm and the higher current densities at 9.39 mA/cm². The field enhancement factor (β) of ZnO/TiN240 is about 3.2 times that of the bare ZnO NRs. On the other hand, the electrochemical properties were improved due to the formation of core-shell heterojunction on the ZnO/TiN interface and porous structure, which makes the ion and charge transport more convenient. Hence, this work not only revealed that the ZnO/TiN core-shell structure exhibited excellent improvement in both FE and supercapacitors applications, but also that growing arrays on CC was expected to achieve flexible display.     

Sandwich-like preparation of Ti3C2Tx@CoFe@TiO2 nanocomposites for high-performance electromagnetic wave absorption

Electromagnetic wave (EMW) absorbing materials have been widely applied in the fields of military and engineering areas. It is of great significance to develop high-performance EMW absorbing materials. This work assembled the sandwich-like Ti₃C₂T_x based nanocomposites by the microwave-assisted annealing of CoFe-MOF@Ti₃C₂T_x (CFMF@Ti₃C₂T_x) precursors at different temperatures. Results show that, as the heat treatment temperature is 450 °C, the sandwich-like Ti₃C₂T_x@CoFe@TiO₂ nanocomposites present better EMW absorption properties. The minimum reflection loss (RL) value was ?62.9 dB at 17.95 GHz with a thin thickness of 1.2 mm. Moreover, the effective absorption bandwidth (EAB) value was 5.02 GHz (12.74–17.76 GHz) with a thickness of 1.4 mm. The application of microwave-assisted annealing contributed to the formation of CoFe nanoparticles and TiO₂ nanoparticles because of the ultra-fast heating rate. The introduction of the nanoparticles enhanced the multiple polarization, optimized the impedance matching and introduced magnetic loss, leading to the improvement of EMW absorption. When the annealing temperature further increased to 550 °C, the EMW absorbing performance was weakened, which was mainly correlated with the decrement of the interface area due to the increase of the TiO₂ nanoparticle size and CoFe nanoparticle size. Thus, the loss effect of the multiple interface polarization weakens in the EMW absorption. In addition, the high permittivity of Ti₃C₂T_x disappears, which deteriorated the impedance matching and attenuation ability of EMW. Ultimately, sandwich-like Ti₃C₂T_x@CoFe@TiO₂ nanocomposite with satisfactory EMW absorbing properties is established, promising for various EMW absorbing applications.     

Preparation of SiCf/Si–O–C composites by impregnation pyrolysis of polysiloxane and its electromagnetic wave absorption properties

High-temperature structural electromagnetic wave (EMW) absorption materials are increasing in demand because they can simultaneously possess the functions of mechanical load-bearing, heatproof, and EMW absorption. Herein, SiC_f/Si–O–C composites were prepared by precursor impregnation pyrolysis using continuous SiC fibers needled felt as reinforcement and polysiloxane as a precursor, respectively. The phase composition, microstructure, complex permittivity, and EMW absorption properties of SiC_f/Si–O–C composites after annealing at various temperatures were investigated. The annealing at 1400–1500°C affects positively the EMW absorption performance of the composites, because the β-SiC microcrystals and SiC nanowires were generated by the activation of carbothermal reduction reaction in the composites, and the aspect ratio of SiC nanowires increased with the rise of temperature. The composites exhibit optimal EMW absorption performance, with the effective absorption bandwidth covering the entire X-band and the minimum reflection loss (RL_min) of −32.8 dB at 4.0 mm when the annealing temperature is raised to 1500°C. This is because that the impedance matching is improved as the rising of ε′ and decreasing of ε″ due to the conversion of free carbon in the composite into SiC nanowires.     

A design of core-shell structure for γ-MnO2 microspheres with tunable electromagnetic wave absorption performance

Manganese dioxide (MnO₂) has been widely utilized in the electromagnetic wave (EMW) absorption field because it exhibits numerous crystal types including α-MnO₂, β-MnO₂, γ-MnO₂, and δ-MnO₂, and is environmentally friendly. To enhance the EMW absorption performance of this material, we combined the precipitation method and calcination process to obtain γ-MnO₂ microspheres, and developed a core-shell structure of γ-MnO₂@SiO₂ and γ-MnO₂@SiO₂@TiO₂ microspheres via the sol-gel process. Based on the synergistic effects between the core-shell structure and dielectric loss, γ-MnO₂@SiO₂ with a thickness of 2.85 mm achieved the minimum reflection loss of ?60.2 dB, demonstrating that these microspheres are excellent candidates for EMW absorbers.     

Magnetic sputtering of FeNi/C bilayer film on SiC fibers for effective microwave absorption in the low-frequency region

It is a great challenge in promoting a microwave absorber with excellent absorbing properties in the low-frequency region. Herein, SiC fibers (SiC_f) coated by a bilayer of FeNi/C (SiC_f/FeNi/C) are fabricated via a two-step magnetron sputtering method. Owing to the improved dielectric loss, magnetic loss, and impedance matching, the reflection loss of SiC_f/FeNi/C is remarkably enhanced. Accordingly, the minimum reflection loss of SiC_f/FeNi/C reaches ?26.18 dB at a low-frequency region of 3.44 GHz. Besides, the mechanic strength of SiC_f/FeNi/C maintains at 2.32 GPa as compared to as-received SiC_f. Thus, SiC_f/FeNi/C is expected to be an ideal structure material to meet low-frequency microwave absorption requests.     

Polymer-derived Co2Si@SiC/C/SiOC/SiO2/Co3O4 nanoparticles: Microstructural evolution and enhanced EM absorbing properties

In this work, porous core-shell structured Co₂Si@SiC/C/SiOC/SiO₂/Co₃O₄ nanoparticles were fabricated by a polymer-derived ceramic approach. The in situ formation of mesopores on the shell, microstructural, and phase evolution of resulting nanoparticles were investigated in detail. The obtained nanoparticles-paraffin composites possess a very low minimum reflection coefficient (RC_min) −60.9 dB, broad effective absorption bandwidth 3.50 GHz in the X-band and 15.5 GHz in the whole frequency range (from 2.5 to 18 GHz). The results indicate outstanding electromagnetic wave (EMW) absorbing performance among all the reported cobalt-based nanomaterials, due to the reasons as follows: (a) The unique core-shell structure as well as complex phase composition of SiC/C/SiOC/SiO₂/Co₃O₄ in the shell, result in a large number of heterogeneous interfaces in the nanoparticles; (b) Nanoparticles have both dielectric and magnetic loss; (c) Mesopores in the shell prolong the propagation path of EMW, thereby increasing the absorption/reflection ratio of EMWs. Thanks to the material structure design, the resulting core-shell structured cobalt-containing ceramic nanoparticles have great potential for thin and high-performance EMW absorbing materials applied in harsh environment.     

Interface evolution of a C/ZnO absorption agent annealed at elevated temperature for tunable electromagnetic properties

Excellent electromagnetic (EM) wave absorption agents that attenuate EM waves by mechanisms based on impedance matching, conductive loss and polarization loss instead of destructive interference are urgently needed but remain challenging. Here, an EM wave absorption agent with a tailorable heterogeneous interface is designed and prepared by the in situ growth of ZnO nanoparticles on the surface of mesoporous carbon hollow microspheres (PCHMs@ZnO) via hydrothermal synthesis followed by annealing. A controlled interface evolution associated with abundant heterogeneous interfaces plays a crucial role in optimized impedance matching and enhanced interfacial polarization loss. With this method, targeted EM wave absorption agents with an excellent absorption ability that is derived mainly from polarization loss and conductive loss rather than destructive interference are successfully obtained. When the PCHMs@ZnO annealed at 700°C were combined with paraffin, the effective absorption bandwidth of the resultant composites covers the whole X band, and the mean value of reflection loss (RL) reaches −12 dB, exceeding those of other reported ZnO-based materials. When the thickness of the composites varies from 3.3 to 4.3 mm, the value of the RL is lower than −8 dB in the whole X band. This work provides a promising model for preparing high-performance EM wave absorption agents.     

Si3N4-BN-SiCN ceramics with unique hetero-interfaces for enhancing microwave absorption properties

SiCN-based ceramics with broadband and strong microwave absorption properties are desired for the structural and functional integration of ceramic matrix composites. The elemental composition and thermal expansion coefficients of the ceramics matrix crucially affect its microstructure and electromagnetic wave (EMW) absorption properties. BN layer with lower electrical conductivity and higher specific area, exhibits both the impedance matching characteristic and EMW attenuation in the process of multiple reflections, electrical conductivity loss, dipole polarization and interfacial polarization. Therefore, Si₃N₄-BN-SiCN ceramics, which were synthesized using chemical vapor infiltration (CVI) method, construct unique hetero-interface of Si₃N₄-BN, Si₃N₄–SiCN and BN-SiCN. Therefore, the Si₃N₄-BN-SiCN ceramics have outstanding EMW absorption performance and realize an effective absorption bandwidth (EAB) that covers the whole X band and the minimum reflection coefficient (RC) reaches -18.43 dB at a thickness of 3.37 mm.     

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.     

High performance X-band electromagnetic wave absorption of SiBCN metamaterials fabricated by direct pyrolysis

Electromagnetic wave (EMW)-absorbing ceramic metamaterials without doping absorbers were prepared by direct pyrolysis of polymer-derived SiBCN ceramics (PDC-SiBCN). The pyrolytic behavior of the ceramics, the effect of the pyrolysis temperature on the mechanical strength, and the EMW absorption properties of the metamaterials are discussed in detail. The mechanism of the enhanced EMW absorption by the metamaterials was further analyzed. The results indicated that PDC-SiBCN demonstrated the optimum impedance matching and absorption performance at a pyrolysis temperature of 1500 °C. The metamaterial can effectively absorb the full X-band when its thickness is in the range of 4.3–5.7 mm. When the thickness was adjusted to 4.7 mm, the minimum reflection coefficient (RC) reached − 26.6 dB. In addition, the flexural strength of the metamaterial was 31.2 MPa, and the compressive strength was 96.9 MPa, which is 60 times higher than that of the solid structure. This study provides a feasible guide for the simple fabrication of lightweight, high-strength, and high-performance EMW-absorbing materials.