Quarter-wavelength Cancellation Research Articles

Magnetic exchange coupling via constructing hard/soft magnetic interface for stable microwave absorption

Magnetic exchange coupling interaction at hard/soft magnetic interface can expand the frequency range of magnetic resonance, providing the stable microwave absorption performance that is insensitive to matching thickness. A novel nanocomposite, in which shell/core structured Sm2Co17/Co(Sm) hard/soft magnetic nanoparticles are embedded into Sm2O3-doped porous carbon, is prepared. The magnetic exchange coupling between hard magnetic Sm2Co17 shell and soft magnetic Co(Sm) core results in extending resonance frequency. Due to the synergistic effect of quarter-wavelength cancellation and electromagnetic loss, the nanocomposites deliver the optimal reflection loss (RLm) of −38.87 dB and effective absorption bandwidth (EAB) of 3.36 GHz at thickness of 2.5 mm and 2.7 mm, respectively, and stable microwave absorption performances (RLm ≤ -20 dB and EAB ≥ 3.0 GHz) in thickness of 2.4–3.0 mm.

Magnetic and absorption properties in hundreds of megahertz band for a FeSi@SiO2 composite with brick stacking structure and crystallographic orientation

Electromagnetic (EM) pollution in the 0.1–1 GHz band is increasing with the rapid advancement in 5G communication technology. However, there are barely any absorption materials in the said band. Herein, we report a high-performance EM wave absorption composite with brick stacking structure and crystallographic orientation prepared using flake-like FeSi@SiO2 particles. The permeability of this composite with such novel structure is increased by more than ten times compared with conventional composites, reaching 42 at 1 MHz. The reflection loss (RL) can reach −40 dB around 0.3 GHz and the effective absorption bandwidth (RL 〈−10) can cover 0.2–1 GHz, which is significantly better than previous reports. Such a high permeability is ascribed to easy magnetization plane and low demagnetization factor parallel to the flake-like particles plane. The excellent absorption is attributed to the intrinsic loss introduced by the enhanced domain wall resonance, and the interference loss generated by quarter-wavelength cancellation. Particularly, the composite proposed in this study has great business and application value in the field of EM wave absorption, and the preparation method can be extended to other EM wave absorption composites, which provides a considerable guidance for the preparation of magnetic metallic absorption composites.

3D core-shell Fe3O4@SiO2@MoS2 composites with enhanced microwave absorption performance

In this work, a 3D ternary core–shell Fe3O4@SiO2@MoS2 composite is synthesized by a hydrothermal technique and a modified Stöber method, where magnetic Fe3O4@SiO2 microsphere with the core of raspberry-like Fe3O4 nanoparticles is completely coated by the flower-like MoS2. Herein, the electromagnetic parameters of the composites are effectively tuned by the combination of magnetic Fe3O4 with dielectric SiO2 and MoS2. The obtained ternary composites exhibit remarkable enhancement of microwave absorption. The measurement results indicate that the minimum reflection loss (RL) of Fe3O4@SiO2@MoS2 composites reaches −62.98 dB at 1.83 mm with the effective absorption bandwidth (RL < −10 dB) of 5.76 GHz (from 11.28 to 17.04 GHz) at 1.92 mm, much higher than those of pure Fe3O4 particles and Fe3O4@SiO2 microsphere. It is believed that the improved performances come from the specific structural design and the plentiful interfacial construction. Further, the synergistic effect of the dielectric and magnetic loss as well as the promoted impedance matching also help to enhance the microwave absorption of the composites. The microwave absorption behavior of the composites conforms to the quarter-wavelength cancellation theory. Our study offers an effective and promising strategy in the structural design and interfacial construction of the novel magnetic/dielectric composites with high-efficiency microwave absorption.

Fe nanoparticles embedded in polyaniline-derived carbon fibers as broad bandwidth microwave absorbers for GHz electromagnetic wave

Magnetic-dielectric composites attract numerous attentions for good microwave absorption performances based on magnetic-dielectric loss mechanism and tunable electromagnetic parameters. Fe/carbon fibers are prepared through carbonization of Fe doped polyaniline nanotubes, in which Fe nanoparticles are embedded in carbon fibers. The introduction of Fe nanoparticles results in good electromagnetic impedance and proper quarter-wavelength cancellation induced, which ensure that microwave penetrate composites to the great extend and is consumed as much as possible, leading to enhanced microwave absorption performance. The minimal reflection loss of Fe/carbon fibers composites starts being less than −10 dB from 1.7 mm and is up to −44.09 dB at 6.62 GHz and 4.1 mm and the effective bandwidth reaches 6.0 GHz with 2.4 mm.

Chiffon cake-derived hierarchically porous carbon with efficient microwave absorption properties

In this work, hierarchically porous carbons were synthesized by carbonizing chiffon cake at different carbonization temperature from 600 to 1200 °C. The synthesis method exhibits that bio-derived food waste is more nature-friendly as no external hazardous activators are used during the pyrolysis process, and it takes less cost. The corresponding microwave absorption properties were proved to be dependent on the carbonization temperature, as well as resulting crystal structures and graphitization degrees. With the carbonization temperature of 1000 °C, the obtained samples (C1000) showed excellent microwave absorption performance with a maximum reflection loss − 49.52 dB at 17.7 GHz. The broad effective absorption bandwidth reached 5.1 GHz (12.4–17.5 GHz) at the thickness of 1.52 mm. The optimal absorbing property can be attributed to the strong dielectric loss, good impedance matching and quarter-wavelength cancellation. This work provides a way to prepare low cost, green and sustainable microwave absorbent with efficient microwave absorption property.

High-Temperature Oxidation-Resistant ZrN0.4B0.6/SiC Nanohybrid for Enhanced Microwave Absorption.

Most microwave absorbers lose their function under harsh working conditions, such as a high temperature and an oxidative environment. Here, we developed a heterogeneous ZrN0.4B0.6/SiC nanohybrid via combined catalytic chemical vapor deposition (CCVD) and chemical vapor infiltration (CVI) processes using ZrB2 as the starting material. The composition and structure of the ZrN0.4B0.6/SiC nanohybrid were controlled by tuning the CCVD and CVI parameters, such as reaction temperature, time, and reactant concentration. The optimal heterogeneous ZrN0.4B0.6/SiC nanohybrids were obtained initially by preparing ZrB2@C via the CCVD process at 650 °C for 30 min and the subsequent CVI at 1500 °C, where the ZrB2@C reacted with Si under N2. The ZrN0.4B0.6/SiC nanohybrid exhibited enhanced microwave absorption ability with a minimum reflection loss value of approximately -50.8 dB at 7.7 GHz, a thickness of ∼3.05 mm, and antioxidation features at a high temperature of 600 °C. The heterogeneous ZrN0.4B0.6/SiC nanohybrid possessed reasonable conductivity, leading to dielectric loss, whereas SiC nanofibers formed a three-dimensional network that brought higher dipole moments, whereas a small part of the ZrN0.4B0.6/SiC nanohybrid structure generated an effective interface for higher attenuation of microwaves. Therefore, these material features synergistically resulted in a well-defined Debye relaxation, Maxwell-Wagner relaxation, dipole polarization, and the quarter-wavelength cancellation, which accounted for the enhanced microwave absorption.

Effects of calcination temperature on the electromagnetic properties of carbon nanotubes/indium tin oxide composites

Carbon nanotubes (CNTs)/Indium tin oxide (ITO) composites have been successfully synthesized via coprecipitation and calcination processes. The effects of calcination temperature on the electromagnetic properties of the CNTs/ITO composites were investigated. The real part of the permittivity and the dielectric loss increase with increasing calcination temperature, which can be attributed to the enhanced dielectric relaxation and space charge polarization. The CNTs/ITO composites synthesized at 600 °C can achieve lower reflection loss (RL) value at the thickness of 2–4 mm, while the composites synthesized at 850 °C exhibit broader bandwidth, corresponding to an RL value below −10 dB at a thin coating thickness of 1–1.5 mm. The microwave absorption properties of the CNTs/ITO composites at different frequencies can be tuned by controlling the calcination temperature and the thickness of the absorbing coating. The dielectric loss, quarter-wavelength cancellation, and well-matched characteristic impedance in the air-absorber interface are believed to contribute to the superior microwave absorption performance of the composites. The effects of the secondary phase ITO on the electromagnetic properties of the CNTs/ITO composites were also discussed in this study.

Enhanced electromagnetic properties of carbon nanotubes and SiO2-coated carbonyl iron microwave absorber

The carbon nanotubes/SiO2-coated carbonyl iron (CNTs/CI@SiO2) composites were fabricated as microwave absorption materials (MAMs) by a simple mechanical mixing method for the purpose of the MAMs with lightweight, thin thickness, broad bandwidth and high oxidation resistance. The effects of different concentration of CNTs on the electromagnetic properties of the CNTs/CI@SiO2 composites were investigated. The results indicate that the complex permeability does not change obviously with the increasing concentration of CNTs because of the constant concentration of CI@SiO2 in the composites. While the complex permittivity and attenuation constant increase with the increasing concentration of CNTs, and the impedance matching ratio decreases with the increasing concentration of CNTs. The reflection loss (RL) calculation results show that 2 wt% CNTs mixed with 65 wt% CI@SiO2 has the optimum microwave absorption properties. The minimum RL is −51.54 dB at 15.44 GHz with a matching thickness of 1.41 mm. The bandwidth corresponding to the RL below −10 dB is more than 6.08 GHz with a thickness of 1.5 mm. The enhanced microwave attenuation ability, improved impedance matching in air-absorber interface and the quarter-wavelength cancellation are believed to contribute the excellent microwave absorption properties of the CNTs/CI@SiO2 composites.

Effects of composition on the microwave absorbing properties of FexNi100−x (x = 0–25) submicro fibers

The morphology and composition of a material have important influences on its electromagnetic parameters and microwave absorbing property. In this paper, the FexNi100−x (x = 0–40) powders were prepared by an oxalate precipitation-thermal decomposition process. When x = 0–25, the powders show submicro fibrous morphologies, however, when x > 25, some granular particles appear due to the weak coordination ability of ethylenediamine with Fe2+ in the oxalate precipitation process. The electromagnetic parameters and microwave absorbing properties of the composites of the FexNi100−x (x = 0–25) submicro fibers with 80 wt% paraffin were studied in 2–18 GHz. Compared with the Ni/paraffin composite, enhanced electromagnetic losses and microwave absorbing properties are observed for the Fe10Ni90/paraffin and Fe25Ni75/paraffin composites. The Fe10Ni90/paraffin shows the best microwave absorbing property that at the composite thickness of 2.0 mm, the effective bandwidth and minimum RL reach 3.78 GHz (12.21–15.99 GHz) and −45.37 dB at 13.90 GHz, respectively. The excellent microwave absorbing property of the Fe10Ni90/paraffin composite is from its high loss factor and the quarter-wavelength cancellation mechanism.

Application of yolk–shell Fe3O4@N-doped carbon nanochains as highly effective microwave-absorption material

Yolk–shell Fe3O4@N-doped carbon nanochains, intended for application as a novel microwave-absorption material, have been constructed by a three-step method. Magnetic-field-induced distillation-precipitation polymerization was used to synthesize nanochains with a one-dimensional (1D) structure. Then, a polypyrrole shell was uniformly applied to the surface of the nanochains through oxidant-directed vapor-phase polymerization, and finally the pyrolysis process was completed. The obtained products were characterized by X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), and thermogravimetric analyses (TGA) to confirm the compositions. The morphology and microstructure were observed using an optical microscope, scanning electron microscope (SEM), and transmission electron microscope (TEM). The N2 absorption–desorption isotherms indicate a Brunauer–Emmett–Teller (BET) specific surface area of 74 m2/g and a pore width of 5–30 nm. Investigations of the microwave absorption performance indicate that paraffin-based composites loaded with 20 wt.% yolk–shell Fe3O4@N-doped carbon nanochains possess a minimum reflection loss of −63.09 dB (11.91 GHz) and an effective absorption bandwidth of 5.34 GHz at a matching layer thickness of 3.1 mm. In addition, by tailoring the layer thicknesses, the effective absorption frequency bands can be made to cover most of the C, X, and Ku bands. By offering the advantages of stronger absorption, broad absorption bandwidth, low loading, thin layers, and intrinsic light weight, yolk–shell Fe3O4@N-doped carbon nanochains will be excellent candidates for practical application to microwave absorption. An analysis of the microwave absorption mechanism reveals that the excellent microwave absorption performance can be explained by the quarter-wavelength cancellation theory, good impedance matching, intense conductive loss, multiple reflections and scatterings, dielectric loss, magnetic loss, and microwave plasma loss.

Monodisperse NixFe3-xO4 nanospheres: Metal-ion-steered size/composition control mechanism, static magnetic and enhanced microwave absorbing properties

An easy metal-ion-steered solvothermal method was developed for the one-step synthesis of monodisperse, uniform NixFe3-xO4 polycrystalline nanospheres with tunable sphere diameter (40–400nm) and composition (0≤x≤0.245) via changing just Ni2+/Fe3+ molar ratio (γ). With g increased from 0:1 to 2:1, sphere diameter gradually decreased and crystal size exhibited an inversed U-shaped change tendency, followed by increased Ni/Fe atom ratio from 0% to 0.0888%. An in situ-reduction, coordination-precipitation transformation mechanism was proposed to interpret the metal-ion-steered growth. Size- and composition-dependent static magnetic and microwave absorbing properties were systematically investigated. Saturation magnetization declines with g in a Boltzmann model due to the changes of crystal size, sphere diameter, and Ni content. The coercivity reaches a maximum at γ=0.75:1 because of the critical size of Fe3O4 single domain (25nm). Studies on microwave absorption reveal that 150–400nm Fe3O4 nanospheres mainly obey the quarter-wavelength cancellation model with the single-band absorption; 40–135nm NixFe3-xO4 nanospheres (0≤x≤0.245) obey the one and three quarter-wavelength cancellation model with the multi-band absorption. 150nm Fe3O4 nanospheres exhibit the optimal EM wave-absorbing property with an absorbing band of 8.94GHz and the maximum RL of −50.11dB.

Microwave Absorbing Properties of NiFe2O4 Nanosheets Synthesized Via a Simple Surfactant-Assisted Solution Route

NiFe2O4 nanosheets have been synthesized via a simple surfactant-assisted solution route, which are confirmed by X-ray powder diffraction, scanning electron microscopy and transmission electron microscopy. The NiFe2O4 sample exhibits the sheet-like structure, with width varying from 200 to 800 nm and thickness ranging from 20 to 60 nm. The electromagnetic properties of NiFe2O4 nanosheets-paraffin composites have been deeply investigated. The multiple dielectric relaxation loss in NiFe2O4 nanosheets is attributed to the size distribution and morphology of the NiFe2O4 nanosheets. The magnetic loss in the present system is caused mainly by the natural resonance. An absorber with a thickness of 4.3 mm exhibits an optimal reflection loss (RL) value of -47.1 dB at 7.67 GHz. RL values exceeding -20 dB in the 2.68-17.96 GHz range are obtained by choosing an appropriate absorption-layer thickness between 1.9 and 10 mm. Not only the RL peak frequency but also the number of the peaks can be well explained by the quarter-wavelength cancellation model.

Enhanced Microwave Absorption Properties of Flexible Polymer Composite Based on Hexagonal NiCo2O4 Microplates and PVDF

Hexagonal NiCo2O4 microplates were synthesized via a facile one-pot hydrothermal method and followed by a subsequent annealing process. The complex permittivity and permeability of a NiCo2O4 and polyvinylidene fluoride (PVDF) composite were investigated over 2–18 GHz. The experiment indicated that the good microwave absorption performance of NiCo2O4@PVDF depends on dielectric loss and quarter-wavelength cancellation. Our results show that the absorption frequency bandwidth of reflection loss (RL) less than −20 dB for the NiCo2O4@PVDF composite can be measured over the frequency range of 3–15.5 GHz with an absorbing thickness that varies in the range of 1.25–5 mm. Furthermore, an optimal RL of −44.8 dB was observed at 10.7 GHz with a thickness of 1.75 mm. The loss mechanism is also discussed.

Magnetic, electrical and microwave properties of Mn–Co substituted NixZn0,8-xFe2O4 nanoparticles

Mn–Co substituted spinel Ni–Zn ferrite nanoparticles were synthesized in the form of NixZn0,8-xMn0.1Co0.1Fe2O4 (0 ≤ x ≤ 0.8) using the solid state reaction route. 1 wt.% B2O3 (boron) was added to initial mixture to inhibit crystal growth. Structural characterization showed that single phase NiZn-ferrite nanoparticles were obtained upto the nickel concentration of x = 0.5 and above that a small fraction of hematite phase (α-Fe2O3) was detected. It was observed that ac conductivity of nanoparticles decreases with increasing nickel amount giving rise to better stability of electrical bonds between Ni and Fe ions. Besides, ac conductivity also decreases with decreasing frequency which is considered as a signature of an ionic conductivity. Magnetic measurements revealed that as nickel concentration increases up to x = 0.5, the saturation magnetization of the samples increases. Further increase results in a decrease in the magnetization. The microwave characterization showed that the samples of Ni0.6Zn0.2Mn0.1Co0.1Fe2O4 and Ni0.8Mn0.1Co0.1Fe2O4, have the best absorption properties with minimum reflection losses of −25 dB at 10 GHz and 12 GHz, respectively. The mechanism of absorption appears to be quarter wavelength cancellation at the matching thickness, which increases exponentially with decreasing microwave frequency.

Hydrothermal Synthesis of Three-dimensional Butterfly-like Ni Architectures as Microwave Absorbers

Three-dimensional butterfly-like Ni architectures were fabricated by a surfactant-assisted hydrothermal method. The Ni architectures, with lengths of about 20 μm and widths of 4-6 μm, were assembled from tens of Ni nanorods with the averaged diameter of about 200 nm. The Ni nanorods were coated by oxide shells. The saturation magnetization of the Ni architectures was found to be 66.2 emu/g. The magnetic loss in the Ni architectures was mainly caused by the natural resonance, while the dielectric relaxation loss was originated from the interfacial relaxation between the oxide shells and the Ni nanorods in addition to the size distribution and morphology of the Ni architectures. Absorbers with a thickness of 2.1 mm exhibited an optimal reflection loss (RL) value of -38.9 dB at 12.8 GHz. RL values exceeding -20 dB in the 8.0-17.8 GHz range were obtained by choosing absorber thicknesses between 1.5 and 3.2 mm. A quarter-wavelength cancellation model was used to explain the observed thickness dependence of RL peak frequency.

Large Scale Synthesis of Superparamagnetic Face-centered Cubic Co/C Nanocapsules by a Facile Hydrothermal Method and their Microwave Absorbing Properties

A large scale direct hydrothermal method has been used to prepare face-centered cubic (FCC) Co/C nanocapsules with a core of FCC-Co nanoparticles and a shell of amorphous C. The size distribution is 5-20 nm. The TB of FCC-Co/C nanocapsules is determined as 85 K, indicating that FCC-Co/C nanocapsules are superparamagnetic at 300 K. For FCC-Co/C nanocapsules-paraffin composite, an optimal reflection losbs (RL) of -33.4 dB is observed at 9.6 GHz for the 2.6 mm thick layer. RL values exceeding –20 dB in the 7.4 –15 GHz range are obtained by choosing an appropriate absorption-layer thickness between 1.7 and 3.3 mm. The good microwave absorbing properties of FCC-Co/C nanocapsules is considered to result from the excellent synergetic effect of the multi-dielectric relaxation loss and the multi-magnetic resonance loss. Quantitative calculation demonstrated that not only RL peak position but also the number of the peaks are determined by the quarter-wavelength cancellation model.

Enhanced microwave absorption properties in GHz range of Fe3O4/C composite materials

2-D Fe3O4/C composite materials were synthesized simultaneously through a single heating procedure using sodium chloride salt powder as a sacrificial template. Electromagnetic parameters of composite were measured by a vector network in the range of 2–18 GHz. The obtained composite materials have excellent electromagnetic wave absorption properties. The maximum value of reflection loss of Fe3O4/C-1.0 is −46 dB at 12.8 GHz when the thickness of absorber adjusts to 2.9 mm and the absorbing bandwidth exceeded −10 dB in a broad range from 10.3 to 17.1 GHz. Moreover, the effect of amorphous carbon and magnetite on microwave absorption properties had been studied, respectively. The relationship between the RL peak frequency and the absorber thickness was successfully explained by the quarter-wavelength cancellation.

Improvement of microwave-absorbing properties of Co2Z barium ferrite composite by coating Ag nanoparticles

Co2Z barium ferrite particles with coated Ag nanoparticles were prepared to increase permittivity. The microwave-absorbing properties of the Ag-coated Co2Z particle composite showed that the stronger reflection loss (RL) peak and smaller absorber thickness could be achieved simultaneously compared with the pure Co2Z barium ferrite particles with the same permeability but smaller permittivity. This enhanced absorption phenomenon was discussed using quarter-wave length cancellation. The separated electromagnetic wave energy reflected at the air-absorber (AA) interface and absorber-metal plate (AM) interface was achieved by measuring reflection coefficients of the composites without (S11-OPEN) and with (S11-SHORT) a backed metal plate to investigate the RL enhancement.

Synthesis and electromagnetic properties of Fe3S4 nanoparticles

Fe3S4 nanoparticles with a narrow diameter distribution of 50nm have been synthesized by an environmentally benign chemical method and the electromagnetic properties were investigated in the frequency range of 0.03–18GHz. Theoretical approach, based on the Cole–Cole semicircle, evidently indicates that the dielectric relaxation process corresponds to the Debye dipolar relaxation, meanwhile accompanying with a suppressed eddy current phenomenon. Additionally, Fe3S4 nanoparticles possess significant magnetic losses mainly arising from the natural resonance, resulting in an ideal electromagnetic match. The reflection loss is optimized to be −36.03dB at 12.61GHz for a Fe3S4-paraffine composite layer of 3.0mm in thickness. The electromagnetic absorption properties are ascribed to a synergetic effect of dielectric loss, magnetic loss and the quarter-wavelength cancellation.

Enhanced microwave absorption of flower-like FeNi@C nanocomposites by dual dielectric relaxation and multiple magnetic resonance

Flower-like FeNi@C nanocomposites can be a potential material for fabricating microwave devices.