Broadband electromagnetic wave absorption of Ni0.5Zn0.5Nd0.04Fe1.96O4 ferrites modified by nano-silver particles

Ferrite materials have the potential to become excellent absorbing materials due to their high magnetic loss and good impedance matching. However, the disadvantages of high density and lack of dielectric loss capability limit its application. Herein, we used the citric acid sol-gel method and the self-propagating combustion method to prepare neodymium-doped nickel-zinc ferrite (NZNF), then the target effect of Sn²⁺ and an improved electroless silver plating process was used to plate a layer composed of silver nanoparticles (Ag NPs) with strong dielectric loss on the NZNF, and a magnetic/dielectric composite material (NZNF@Ag) with a heterogeneous structure was prepared. The number and particle size of Ag NPs on the surface of NZNF can be precisely controlled, thereby greatly enhancing the dielectric loss capability with little impact on the magnetic loss. The huge difference in conductivity between conductors and semiconductors promotes the occurrence of polarization at the heterogeneous interface and significantly enhances the electromagnetic wave absorption ability of the composite material. In the 2–18 GHz frequency band, the best sample can obtain an effective bandwidth of 6.82 GHz when the matching thickness is 2.1 mm. Combining conductors with semiconductor materials to obtain significantly enhanced interfacial polarization provides a new idea for improving the performance of wave absorbing materials.     

3D printed crack-free SiOC(Fe) structures with pyrolysis-induced carbon nanowires for enhanced wave absorption performance

High-performance SiOC(Fe) wave-absorbing ceramics, containing a large number of carbon nanowires, were successfully prepared using a combination of photopolymerization 3D printing technology and the polymer-derived ceramic pyrolysis method. By employing an optimized segmented slow heating scheme with extended holding time, the pyrolysis of SiOC(Fe) ceramics at 1000 °C facilitated the growth of carbon nanowires, Fe₃C and SiO₂ grains. These carbon nanowires were interlaced and interconnected within the samples, forming abundant conductive networks. This highly conducive network efficiently converted electromagnetic energy into thermal energy, effectively dissipating electromagnetic waves, and consequently enhancing the microwave absorption performance of ceramics. Moreover, this approach not only reduced ceramic cracks but also improved the dielectric loss performance of the materials, achieving a minimum reflectivity value of −35.72 dB. The SiOC(Fe) ceramics added with 5 wt% VcFe effectively enhanced the magnetic loss of the material, reduced the difference between the relative complex permeability (μ_r) and the relative complex dielectric constant (ε_r), and improved the impedance matching between the material surface and air, thereby further improving its microwave absorption performance. This resulted in an increase in the maximum effective absorption bandwidth of the material to 12.7 GHz at 5 mm. This study offers a promising solution for the preparation of ceramic matrix composite materials incorporating carbon nanowires, magnetic particles and ceramic precursors, which would be potentially valuable for radar detection and sensor applications.     

Tailoring microwave absorption bandwidth of SiCN based composite ceramic fibers by tuning the embedding ratio of Ni3Si

In order to expand the application prospects of SiCN ceramics in the field of microwave (MW) absorption materials, a series of Ni₃Si embedded SiCN ceramic fibers composites (NSF) were prepared by controlling Ni conversion rate through the electrospinning technique and polymer derivation, with the intention of improving the impedance matching degree, enhancing the conductivity and polarization, and further promoting the dielectric loss ability and MW absorption performance of ceramic materials. The microstructure, phase composition, conductivity, MW absorption properties and mechanism of the material were analyzed by a variety of characterization methods. The results show that NSF exhibited high dielectric loss efficiency and desirable effective absorption bandwidth (EAB) when the conversion rate of Ni was 0.5 wt%: The MW of the entire Ku band (12–18 GHz, 6 GHz) could be effectively absorbed by the sample with a thickness of 2.64 mm, and its EAB could cover 6–18 GHz by adjusting its thickness from 1 mm to 5 mm, so its performance is significantly superior to a number of similar SiCN based composite ceramic materials previously reported. To sum up, the NSF prepared in this work exhibits suitable impedance matching degree, good conductivity, obvious polarization effect, excellent dielectric loss ability, and gratifying EAB in MW, and it is expected to become a powerful candidate in the field of broadband MW absorption materials in the future.     

Carbon doped with binary heteroatoms (N,X–C,where X = P,B, or S) derived from polypyrrole for enhanced electromagnetic wave absorption at microwave frequencies

Dual heteroatom-doped carbon materials show great promise as electromagnetic wave absorbers. However, synthesizing carbons containing multiple heteroatoms at controlled heteroatom doping levels has provided challenges to date. Herein, we report a simple method for manufacturing dual heteroatom doped carbons (N,X–C, where X = P, B, or S) by direct carbonization of polypyrrole synthesized in the presence of H₃PO₄, H₃BO₃, or H₂SO₄, respectively. The heteroatom content of the N,X–C products could be precisely tuned by varying amounts of acid dopant used in the polypyrrole synthesis. The N,X–C materials showed excellent electromagnetic wave absorption properties, especially N,S₁–C (prepared using equimolar amounts of pyrrole and H₂SO₄) which offered a wide absorption bandwidth up to 6.6 GHz (11.38–18 GHz), and a RL_min of ?32.3 dB (14.2 GHz) at 2.5 mm at a ?ller loading of 9.0 wt%. The outstanding electromagnetic wave absorption performance of N,S₁–C was attributed to the presence of N dopant species, defects, C–S, and C–SO_x groups, which optimized dipole polarization and conduction loss in the dielectric loss leading to excellent impedance matching.     

Synthesis and tunable electromagnetic shielding and absorption performance of the three-dimensional SiC nanowires/carbon fiber composites

Silicon-carbide nanowires (SiCnws) have been considered as dielectric loss materials for application in the field of electromagnetic wave (EMW) attenuation. In this study, SiCnws/carbon fiber (CF) composites were fabricated using precursor infiltration and pyrolysis process for the in-situ growth of SiCnws. The SiCnw fraction of the SiCnws/CF composites could be adjusted using various catalysts. At a small SiCnw fraction (38 wt%), the composites exhibited excellent EMW absorption performance with the minimum reflection loss of ? 18.3 dB when their thickness was only 1.2 mm and can cover the entire X and Ku bands by adjusting the material thickness. They transformed from EMW absorption performance to electromagnetic interference (EMI) shielding property with the increase in SiCnw fraction from 38 wt% to 73 wt%, reaching an EMI shielding effectiveness of 31.25 dB. In addition, the density of the SiCnws/CF composites was only 0.31–0.41 g/cm³, and their compressive strength can reach 0.61–0.99 MPa with excellent high-temperature stability. Therefore, the SiCnws/CF composite presents a promising EMW absorption and EMI shielding material that can be applied in harsh environments.     

Tunable design of ZnFe2O4@C@BPC hybrid with rich heterogeneous interfaces as a hydrophobic electromagnetic wave absorber

Environmentally friendly microwave absorbers with superior electromagnetic wave absorption, lightweight and hydrophobic ability have received considerable attention in practical applications. However, addressing the above-mentioned characteristics is simultaneously a tremendous challenge. Along these lines, in this work, a lightweight and efficient hybrid material was fabricated by employing simple self-assembly of core-shell ZnFe₂O₄@C nanospheres embedded within longan shell-derived honeycomb-like porous carbon. The results display that the carbon skeleton not only improves the conduction loss, but also promotes the reflection and scattering of EM wave. In addition, the core-shell ZnFe₂O₄@C microspheres are conducive to enhancing the ability of interface polarization and magnetic loss, and further improving the synergistic effect between the dielectric loss and magnetic loss. Furthermore, the unique structure of the ZnFe₂O₄@C@BPC endows it excellent hydrophobicity and avoids water vapor contamination in practical applications. Precisely, at a thickness of 3.4 mm, the minimum reflection loss (RL) is up to ?58.6 dB at 12.9 GHz. Notably, the effective absorption bandwidth (EAB) is as wide as 9.1 GHz (8.9–18.0 GHz), covering almost the entire X and Ku bands. Consequently, this outstanding performance renders the ZnFe₂O₄@C@BPC composite quite attractive for a broad range of applications in lightweight, hydrophobic microwave absorption materials.     

Structure regulation and microwave absorption property of SiCN ceramic aerogels produced by catalytic pyrolysis

In the present work, polymer-derived SiCN ceramic aerogels (PDCA-SiCN) were fabricated via a combined sol-gel/freeze drying/polymer precursor conversion method. The microstructure of PDCA-SiCN was studied by regulating the synthesis temperature and time during the sol-gel process. PDCA-SiCN showed a unique three-dimensional network structure, and the specific surface area and pore size of PDCA-SiCN prepared at 150 °C for 20 h were 134 g/m² and 18 nm, respectively. To assess the electromagnetic wave absorption (EMA) properties of PDCA-SiCN, the materials were uniformly blended with common paraffin, and the influence of PDCA-SiCN/paraffin ratio on the EMA properties was also investigated. The sample with a PDCA-SiCN/paraffin ratio of 20:80 exhibited the best EMA performance, with a minimum reflection loss (RL) of −43.37 dB at 7.6 GHz and electromagnetic absorption bandwidth of 3.8 GHz, which correspond to an absorption of 99.99 % of the electromagnetic waves. The excellent EMA properties of PDCA-SiCN could be attributed to a synergistic effects of good impedance matching, multiple reflections and high dielectric loss.     

竹节状碳化硅晶须吸波性能研究

采用电磁波吸收材料来降低电磁波对设备的干扰及对人体的伤害，是目前常用的电磁波防护手段之一。特定结构碳化硅晶须作为一种一维介电材料，其优于普通的晶须、块状和颗粒状的吸波性能引起了研究人员的关注。本研究以生竹粉、硅粉和二氧化硅为原料，通过碳热还原法在不同温度下制备了竹节状碳化硅晶须，并对其结构和吸波性能进行了检测分析。结果表明：以1400℃烧结的竹节状晶须制备的试样在厚度为3 mm，频率为9.1 GHz时，最小反射损耗达到-14.4 dB，有效吸收带宽为1.8 GHz，吸波性能最好，具有进一步研究价值。     

Formation of Sn filled CNTs nanocomposite: Study of their magnetic,dielectric properties and enhanced microwave absorption performance at gigahertz frequencies

The Simplistic formation, advantageous configuration, non-colossal magnetoresistance and broadband absorption are important parameters for microwave absorbent materials. In this study, a core-shell nanocomposite comprising of Sn-filled carbon nanotubes (Sn/CNTs) was prepared by arc discharge method. The microstructure, morphology and surface composition of Sn/CNTs-based core-shell nanocomposites were characterized in detail. Sn/CNTs nanocomposite showed a magnetic signal due to the broken bonds and defects at interfaces in Sn/CNTs. The weak ferromagnetism was found to be helpful in improving magnetic permeability in the Sn/CNTs which confirms its role as a magnetic loss material under incident electromagnetic wave. Sn-filled CNTs revealed an appropriate value of dielectric constant, which plays an important role in impedance matching upon incident electromagnetic wave. The composite of Sn-CNTs and paraffin with a 50 wt % loading showed the lowest reflection loss (RL) of ?43.87 dB at 10 GHz, with a wide effective absorption band (RL ≤ ?10 dB) of 3 GHz in thickness of 2.3 mm. This enhanced performance is attributed to the combined effect of the conduction loss in one-dimensional core-shell architecture, the interfacial loss Sn-CNT interface, the magnetic loss due to defects-induced ferromagnetism in Sn shell, and in the carbon-containing atomic layers of CNTs.     

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.     

One-step synthesis of Fe nanoparticles wrapped in N-doped carbon nanohorn microspheres as high-performance electromagnetic wave absorber

Designing cost-effective and eco-friendly electromagnetic absorbing materials is important for their widespread practical applications. Herein, Fe nanoparticles wrapped in carbon nanohorn microspheres enriched with N (Fe@NCNHs) were produced by a simple one-step method, which is a nonequilibrium strategy that involves evaporating a graphite anode and Fe wire in an arc plasma. The total pore volume for CNHs was calculated to be 0.38 cm³ g^?1, corresponding to its naturally inherited micropore and mesopore-dominate porosity. In addition, the N-doping content reached up to 9.3 at%. The electromagnetic wave-absorption performance of Fe@NCNHs can be controlled using the size and loading amount of Fe nanoparticles wrapped in CNHs, depending on the number of Fe wires inserted into the anode. When two Fe wires are inserted into the anode, Fe nanoparticles with uniform size are well wrapped in CNHs, exhibiting excellent electromagnetic wave absorption property with a minimum reflection loss (RL) of ?44.52 dB at 10.86 GHz matching an extremely low thickness of 1.6 mm at X band. The effective absorption bandwidth (RL < ?10 dB) was up to 13.86 GHz, and the matching thickness ranged between 1.2 and 5.0 mm. The results obtained in this study indicate that Fe@NCNHs are promising microwave-absorbing materials with enhanced dielectric loss and good impedance matching, which is attributed to the multiple reflections induced by the hollow structure of CNHs, interfacial polarization between the CNHs and Fe nanoparticles, dipole polarization induced by N-doping, and pentagonal and hexagonal defects on CNHs.     

Dielectric and microwave absorption properties of polymer-derived TiC/SiC/SiOC multiphase ceramics in X band

Polymer-derived TiC/SiC/SiOC ceramics were prepared using tetrabutyl titanate (TBT)-modified polysiloxane (PSO) as precursor. The effects of heat treatment temperature and TBT content in precursor on the microstructure, phase composition, and microwave absorbing properties of TiC/SiC/SiOC ceramics were investigated. The crystallinity of the ceramics increases with the increase of heat treatment temperature. With the increase of TBT content, the TiC content of the ceramics increases and the SiC content decreases. When the TBT content ranges from 1 to 5 wt.%, the increase of TBT content has little effect on the real part of the dielectric constant of TiC/SiC/SiOC ceramics. When the TBT content is 7 wt.%, the imaginary part of the dielectric constant of the ceramics changes. For TiC/SiC/SiOC ceramic obtained from the pyrolysis of PSO-TBT precursor with 7 wt.% TBT, the dielectric constant is within the target electromagnetic parameters. Therefore, it has an effective absorption bandwidth of 4.2 GHz, covering the entire X band, showing an excellent microwave absorbing performance.     

Enhanced electromagnetic microwave absorption properties of SiCN(Fe) ceramics produced by additive manufacturing via in-situ reaction of ferrocene

SiCN(Fe) ceramics with excellent electromagnetic wave (EMW) absorption performance were successfully prepared from a preceramic polymer doped with ferrocene. Additive manufacturing (Digital Light Processing), providing enhanced structural design ability, was employed to fabricate samples with complex architectures. During pyrolysis, ferrocene catalyzed the in-situ formation of a large amount of turbostratic carbon, graphite and SiC nanosized phases, which formed carrier channels in the electromagnetic field and increased the conductivity loss. Meanwhile, it also increased the dipole polarization, interface polarization and the dielectric properties of the material, which finally enhanced the EMW absorption capacity of SiCN(Fe) ceramics. When containing 0.5 wt% ferrocene, the material showed good performance with EAB 4.57 GHz at 1.30 mm, and RL_min −61.34 dB at 2.22 mm. The RL_min of 3D-SiCN-0.5 ceramics was −6 dB, and the RL of the X-band was lower than −4 dB at 2 mm.     

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).     

A nanocomposite of dual-phase Fe/TiCN wrapped in nitrogen-doped carbon with magnetic and dielectric characters for superior microwave absorption

A careful approach to the optimization of magnetic and dielectric losses in nanomaterials can improve the electromagnetic wave absorption loss performance for certain microwave absorption applications. In this study we prepared dual core (Fe/TiCN) coated with nitrogen (N) doped carbon shell nanocomposite by arc-discharge method under mix atmospheres of working gases and with varying elemental compositions. Among all nanocomposites, (Fe/TiC_0.7N_0.3)@N–C dual-core@ N- doped shell nanocomposite exhibits enhanced microwave absorption. Owing to the novel dual-core@ N-doped shell structure and numerous defects induced by doping N in carbon shells, an improved dielectric relaxation in composite is observed and the minimum reflection loss was reached −44.36 dB at 5.3 GHz for 4.8 mm thickness.     

Designing of an rGO-based heterostructure for highly efficient microwave absorption performance and flame retardancy

To meet the increasingly serious challenges of electromagnetic pollution and fire hazards, highly efficient electromagnetic absorbing materials with good flame retardancy have been accurately developed. A complex high-performance electromagnetic absorber was designed by doping graphene oxide heterostructures (including zero-dimensional nanoparticles and two-dimensional graphene) containing cobalt, nickel and iron oxyhydroxide through the coordination effect. When 20 wt% CNFO@rGO was incorporated into paraffin, the minimum reflection loss (RL) reached -51.6 dB at a thickness of 3.5 mm, showing a high electromagnetic absorption efficiency. It was concluded that the excellent impedance matching, the best dielectric loss and the higher magnetic loss caused by the heterostructure combined with the electromagnetic parameters, Cole-Cole semicircle, impedance matching (Z) and attenuation coefficient resulted in excellent electromagnetic wave absorption performance. Moreover, the peak heat release (PHRR) and heat release rate (HRC) decreased by 55.5% and 55.1%, respectively, when 10 wt% CNFO@rGO was incorporated into the epoxy resin (EP), indicating enormous potential for the enhancement of flame-retarding electromagnetic absorbing materials.     

Dielectric properties and electromagnetic wave transmission performance of polycrystalline mullite fiberboard at 2.45 GHz

Refractory lining is an indispensable part of high temperature microwave heating equipment, and its wave transmission performance exerts an important impact on the mode and efficiency of microwave heating, while the complex dielectric constant (dielectric constant and dielectric loss) of the material is the decisive factor in determining the wave transmission performance of the material. In this work, we measured the complex dielectric constant of polycrystalline mullite fiber board (PMF) in the temperature range of 25–1000 °C, and the effect of temperature on dielectric constant and dielectric loss was analyzed; The wave-transmission properties of the material were calculated according to the theory of electromagnetic wave transmission line, and the effects of temperature, material thickness, polarization modes of electromagnetic wave and incident angle on the wave transmission performance were analyzed. The results reveal that the dielectric constant of PMF does not change much with the increase of temperature, which is about 1.6; The dielectric loss does not change much within 200 °C, but when the temperature is higher than 200 °C, the change presents approximately exponential increase with the rise of temperature. The wave transmission performance fluctuates with the increase of the thickness, and there are maximum value and minimum value, and the overall wave transmission performance decreases with the increase of the material thickness. In a transverse electric (TE) field, the overall wave transmission performance decreases with the increase of the incident angle, and better wave transmission performance can be obtained by priority selection of vertical incidence of electromagnetic wave. In a transverse magnetic (TM) field, with the increase in the incident angle, the wave transmission performance firstly climbs up then declines, and there is an optimal incident angle where total transmission can occur. Finally, this work selected the thickness corresponding to different temperature as the preferred thickness. This work is of important theoretical significance for understanding the mechanism of the dynamic change of the wave transmission performance of the thermal insulation materials in microwave heating, and provides important practical guidance for the design and optimization of microwave heating equipment.     

A low-cost lightweight microwave absorber: Silicon carbide synthesized from tissue

Recently, microwave absorbing materials have been widely used with the development of electromagnetic wave technology such as 5G communication. It is urgent to develop low-cost electromagnetic wave absorbing materials to meet the increasing civil market demand. Herein, we report a novel and simple strategy to synthesize lightweight silicon carbide microwave absorber by calcining tissue and glass microspheres at high temperature. The results show that higher synthesis temperature can improve the crystallinity of SiC, generate more whiskers and reduce the impurity content. This in turn increases the dielectric loss of the material. The products synthesized at 1600 °C have excellent microwave absorption properties. At the thickness of 1.3 mm, it achieves a reflection loss of ?32.5 dB and an effective absorption bandwidth of 4.1 GHz. This study broadens the way for the potential reuse of waste paper and glass, and provides a useful reference for the preparation of low-cost microwave absorbing materials.     

Embedded SiO2 interface in SiCnws/Ba0.75Sr0.25Al2Si2O8 ceramic with enhanced electromagnetic absorption at elevated temperature

For high-temperature electromagnetic absorption materials, higher polarization loss is needed to balance the impedance mismatch due to greater conduction loss at elevated temperatures. Here, a SiO₂ interface was introduced into a SiCnws/BSAS ceramic based on wide bandgap and low dielectric constant characteristics of SiO₂. The interface structure was tailored by changing the SiO₂ content. When the SiO₂ content reached 15 vol%, three-phase interlaced interfaces were formed, which produced many nano-heterointerfaces that increased the polarization loss by 77.5 %. The optimized SiCnws/SiO₂-BSAS ceramic achieved enhanced electromagnetic absorption from 298 K to 873 K, and its effective absorption bandwidth reached 4.1 GHz at 873 K. The electromagnetic absorption mechanism was analyzed from the perspectives of electron transport and space charges. This heterointerface design strategy provides a new method for the development of high-temperature electromagnetic absorption materials.     

MXene-derived TiC/SiBCN ceramics with excellent electromagnetic absorption and high-temperature resistance

Demand for high-performance electromagnetic (EM) wave absorbing materials with high-temperature resistance is always urgent for application in a harsh environment. In this contribution, two-dimensional material, Ti₃C₂T_x MXene, was introduced into a hyperbranched polyborosilazane. After pyrolyzation, the as-prepared TiC/SiBCN ceramics present excellent EM wave absorption in X-band. The TiC nanograins appearing after annealing provide multilevel reflection and interface polarization. Dipole polarization formed at interface defects, in company with interfacial polarization, also makes a great contribution to enhanced EM wave absorption. The TiC/SiBCN nanocomplex prepared with 5 wt% Ti₃C₂T_x MXene possesses a minimum reflection coefficient of −45.44 dB at 10.93 GHz and abroad bandwidth 8.4 and 12.4 GHz, almost covering the entire X-band. Tuning the thickness in the range of 2.35-2.54 mm, the effective absorption band can achieve the entire X-band. And the EM wave absorbing performance has been maintained to a large extent at 600°C with the minimum reflection coefficient of −26.12 dB at 12.13 GHz and the effective absorption bandwidth of 2 GHz. Last but not the least, TiC/SiBCN ceramics offer a good thermal stability in argon as well as in air atmosphere, making it possible to serve in high-temperature detrimental environments. This study is expected to provide a new perspective for the design of high-performance absorbing materials that are able to be used in harsh environments, especially in high temperatures.