Reflection Loss is a Parameter for Film,not Material

In studies of microwave absorption in the current literature,theories such as reflection loss,impedance matching,the delta function,and the quarter-wavelength model have been inappropriately applied.As shown in this case study,these problems need to be corrected as they are representative of similar work in the literature.

AN INVERSION METHOD FOR OBTAINING BOTTOM REFLECTION LOSS

A method for the inversion of the transmission losses for the bottom reflection loss is proposed on the basis of the theory of the smooth- averaged sound field. The procedure of the inversion is based on the criterion of the least square error in the transmission losses between calculation and measurement. By using the Gauss - Newton iterative approach, the non - linear least square aloqrithm is equivalent to solving a sequence of lineared least square problems. The physical causes of the instability of the inversion problem are discussed and the stability is improved by means of the Levenberg- Marquardt method. Both numerical simulations with noise and experimental results show that the inversion for the bottom reflection loss of small grazing angle has high precision and the certain perturbation in the measured transmission losses does not lead to serious deviation in the inversion result of the bottom reflection loss.

ENERGY-LOSS FUNCTIONS DERIVED FROM REELS SPECTRA FOR ALUMINUM

The effective energy loss functions for Al have been derived from differential i nverse inelastic mean free path based on the extended Landau approach. It has be en revealed that the effective energy loss function is very close in value to th e theoretical surface energy loss function in the lower energy loss region but g radually approaches the theoretical bulk energy loss function in the higher ener gy loss region. Moreover, the intensity corresponding to surface excitation in e ffective energy loss functions decreases with the increase of primary electron e nergy. These facts show that the present effective energy loss function describe s not only surface excitation but also bulk excitation. At last, REELS spectra s imulated by Monte Carlo method based on use of the effective energy loss functio ns has reproduced the experimental REELS spectra with considerable success.

Synthesis, magnetic and electromagnetic wave absorption properties of planar anisotrop Y_2Co_(17)@SiO_(2) rare earth soft magnetic composites

Intermetallic complexes of rare-earth and 3d transition metals with core-shell structures are commonly employed as microwave absorbing materials due to their high saturation magnetizations and natural resonance in GHz. Hence, we synthesized Y_2Co_(17)alloy via the co-precipitation reduction-diffusion technique, then coated the Y_2Co_(17)magnetic powders with SiO_(2)to create Y_2Co_(17)@SiO_(2) core-shell structures. The formation of Y_2Co_(17)@SiO_(2)/polyurethane(PU) at various volume fractions and their magnetic, electromagnetic properties were investigated using x-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, and vector network analyzer. The microwave absorption characteristics of Y_2Co_(17)@SiO_(2)/PU were also investigated at various volume fractions. We not only investigate the zero-reflection conditions of the samples with different volume fractions, but also show that every absorber has a strong reflection loss value(RL ≤-65.00 d B) and excellent microwave absorption properties with an average RL of Y_2Co_(17)@SiO_(2)/PU being below-10 d B at 8 GHz–18 GHz under different thicknesses, showing that the enhancement of microwave absorption performance arises from the balance between permeability and permittivity of absorber.

A Review on Metal-Organic Framework-Derived Porous Carbon-Based Novel Microwave Absorption Materials

The development of microwave absorption materials(MAMs) is a considerable important topic because our living space is crowed with electromagnetic wave which threatens human’s health.And MAMs are also used in radar stealth for protecting the weapons from being detected.Many nanomaterials were studied as MAMs,but not all of them have the satisfactory performance.Recently,metal-organic frameworks(MOFs) have attracted tremendous attention owing to their tunable chemical structures,diverse properties,large specific surface area and uniform pore distribution.MOF can transform to porous carbon(PC) which is decorated with metal species at appropriate pyrolysis temperature.However,the loss mechanism of pure MOF-derived PC is often relatively simple.In order to further improve the MA performance,the MOFs coupled with other loss materials are a widely studied method.In this review,we summarize the theories of MA,the progress of different MOF-derived PC-based MAMs,tunable chemical structures incorporated with dielectric loss or magnetic loss materials.The different MA performance and mechanisms are discussed in detail.Finally,the shortcomings,challenges and perspectives of MOF-derived PC-based MAMs are also presented.We hope this review could provide a new insight to design and fabricate MOF-derived PC-based MAMs with better fundamental understanding and practical application.

Determination of dielectric properties of titanium carbide fabricated by microwave synthesis with Ti-bearing blast furnace slag

The preparation of functional material titanium carbide by the carbothermal reduction of Ti-bearing blast furnace slag with microwave heating is an effective method for valuable metals recovery;it can alleviate the environmental pressure caused by slag stocking.The dynamic dielectric parameters of Ti-bearing blast furnace slag/pulverized coal mixture under high-temperature heating are measured by the cylindrical resonant cavity perturbation method.Combining the transient dipole and large π bond delocalization polarization phenomena, the interaction mechanism of the microwave macroscopic non-thermal effect on the titanium carbide synthesis reaction was revealed.The material thickness range during microwave heating was optimized by the joint analysis of penetration depth and reflection loss, which is of great significance to the design of the microwave reactor for the carbothermal reduction of Ti-bearing blast furnace slag.

Theoretical calculation and experiment of microwave electromagnetic property of Ni(C) nanocapsules

With the combination of the dielectric loss of the carbon layer with the magnetic loss of the ferromagnetic metal core, carbon-coated nickel （Ni（C）） nanoparticles are expected to be the promising microwave absorbers. Microwave electromag- netic parameters and reflection loss in a frequency range of 2 GHz-18 GHz for paraffin-Ni（C） composites are investigated. The values of relative complex permittivity and permeability, the dielectric and magnetic loss tangent of paraffin-Ni（C） com- posites are measured, respectively, when the weight ratios of Ni（C） nanoparticles are equal to 10 wt%, 40 wt%, 50 wt%, 70 wt%, and 80 wt% in paraffin-Ni（C） composites. The results reveal that Ni（C） nanoparticles exhibit a peak of magnetic loss at about 13 GHz, suggesting that magnetic loss and a natural resonance could be found at that frequency. Based on the measured complex permittivity and permeability, the reflection losses of paraffin-Ni（C） composites with different weight ratios of Ni（C） nanoparticles and coating thickness values are simulated according to the transmission line theory. An ex- cellent microwave absorption is obtained. To be proved by the experimental results, the reflection loss of composite with a coating thickness of 2 mm is measured by the Arch method. The results indicate that the maximum reflection loss reaches -26.73 dB at 12.7 GHz, and below -10 dB, the bandwidth is about 4 GHz. The fact that the measured absorption position is consistent with the calculated results suggests that a good electromagnetic match and a strong microwave absorption can be established in Ni（C） nanoparticles. The excellent Ni（C） microwave absorber is prepared by choosing an optimum layer number and the weight ratio of Ni（C） nanoparticles in paraffin-Ni（C） composites.

Structural and Magnetic Studies of CTAC-assisted NiZn Ferrite Films within 0.1-3.5 GHz

Spinel NiZn ferrite thin films were prepared on glass substrates by spray plating method. Adding cetyltrimethylammoniumchloride （CTAC）, adsorptive energy of substrate surface increased, and smooth surface and uniform columnar film structures were observed. The optimum reaction temperature up to 85℃ and pH up to 7.5 were obtained. As the solution pH value increases from 6.5 to 7.5, the film saturation magnetization increases to 36.1 and the imaginary part μ″ up to 53.2 for NiZn ferrite film at 500 MHz were achieved, and higher magnetic resonance at 508 MHz was observed. As the ferrite plate thickness is 50 μm, the attenuating characteristics for reflection loss ≤-0.8 dB can be obtained in the wide frequency ranging from 0.5 to 2.7 GHz. Theμ″ of thin film has values higher than 20 at the frequencies between 0.5 and 2 GHz, and the thin film can be applied as shielding material in GHz range.

Experimental and Computational Study of the Microwave Absorption Properties of Recycled α-Fe2O3/OPEFB Fiber/PCL Multi-Layered Composites

The aim of this study was to fabricate multi-layered recycled α-Fe2O3/OPEFB fiber/PCL composites for microwave absorbing applications in the 1 - 4 GHz frequency range. The multi-layered composites were 6 mm thick and each consisted of a 2 mm thick layer of recycled α-Fe2O3/PCL composites at various loadings (5 wt% - 25 wt%) of 16.2 nm recycled α-Fe2O3 nanofiller, placed between two layers of 2 mm thick OPEFB fiber/PCL composites blended at a fixed ratio of 7:3. The real (ε') and imaginary (ε") components of the relative complex permittivity were measured using the open-ended coaxial probe technique and the values obtained were applied as inputs for the Finite Element Method to calculate the reflection coefficient magnitudes from which the reflection loss (RL) properties were determined. Both ε' and ε" increased linearly with recycled α-Fe2O3 nanofiller content and the values of ε' varied between 3.0 and 3.9 while the ε" values ranged between 0.26 and 0.64 within 1 - 4 GHz. The RL (dB) showed the most prominent values within the 1.38 - 1.46 GHz band with a minimum of -38 dB attained by the 25 wt% composite. Another batch of minimum values occurred in the 2.39 - 3.49 GHz range with the lowest of -25 dB at 2.8 GHz. The recycled α-Fe2O3/OPEFB fiber/PCL multi-layered composites are promising materials that can be engineered for solving noise problems in the 1 - 4 GHz range.

Recent progress of electromagnetic wave absorbers: A systematic review and bibliometric approach

The electromagnetic wave absorption capabilities of an absorber depend on its dielectric and magnetic compo- nents, which determine its interfacial polarization, conductivity loss, magnetic loss, and other such mechanisms. In this study, a comprehensive review of the electromagnetic wave and material interaction was conducted. Moreover, for a better understanding of the trends and evolutionary developments in the study of the electro- magnetic wave absorber, 23300 documents dated between 1990 and 2020 were examined, which were obtained from Scopus using the keywords of “electromagnetic wave absorption ”and “microwave absorption ”. The data search in Scopus was conducted using the related keywords in the search bar for titles and abstracts. These results demonstrate that the majority of research regarding electromagnetic wave absorbers was conducted in China, which was followed by the United States. The number of published documents regarding the electromagnetic wave absorption field significantly increased between 1990 and 2020;these documents were mostly published as journal articles. With respect to the journal activity, the most productive journal was the “Journal of Alloys and Compounds ”, with a total of 592 articles. In addition, graphene and titanium dioxide were determined to be the materials that were most studied in the field of electromagnetic wave absorption.

Achieving strong microwave absorption capability and wide absorption bandwidth through a combination of high entropy rare earth silicide carbides/rare earth oxides

Developing electromagnetic(EM) wave absorbing materials with low reflection coefficient and optimal operating frequency band is urgently needed on account of the increasingly serious EM pollution. However, the applications of common EM absorbing materials are encumbered by poor high-temperature stability, poor oxidation resistance, narrow absorption bandwidth or high density. Herein, the strong EM absorption capability and wide efficient absorption bandwidth of high entropy ceramics are reported for the first time, which are designed by a combination of the novel high entropy(HE) rare earth silicide carbides/rare earth oxides(RE3 Si2 C2/RE2 O3). Three HE powders, i.e., HERSC-1(HE(Tm0.2 Y0.2 Dy0.2 Gd0.2 Tb0.2)3 Si2 C2),HERSC-2 HE(Tm0.2 Y0.2 Dy0.2 Gd0.2 Tb0.2)3 Si2 C2/HE(Tm0.2 Y0.2 Dy0.2 Gd0.2 Tb0.2)2 O3) and HERSC-3(HE(Tm0.2 Y0.2 Dy0.2 Gd0.2 Tb0.2)3 Si2 C2/HE(Tm0.2 Y0.2 Dy0.2 Gd0.2 Tb0.2)2 O3), are synthesized. Although HERSC-1 exhibits a limited absorption effect(the minimum reflection loss(RLmin) is-11.6 d B at 3.4 mm) and a relatively narrow effective absorption bandwidth(EAB) of 1.7 GHz, the optimal absorption RLminvalue and EAB of HERSC-2 and HERSC-3 are-40.7 d B(at 2.9 mm), 3.4 GHz and-50.9 d B(at 2.0 mm), 4.5 GHz,respectively, demonstrating strong microwave absorption capability and wide absorption bandwidth.Considering the better stability, low density and strong EM absorption effect, HE ceramics are promising as a new type of EM absorbing materials.

Effects of Multi-walled Carbon Nanotubes on the Electromagnetic Absorbing Characteristics of Composites Filled with Carbonyl Iron Particles

The electromagnetic （EM） wave absorbing property of silicone rubber filled with carbonyl iron particles （CIPs） and multi-walled carbon nanotubes （MWCNTs） was examined. Absorbents including MWCNTs and spherical/ flaky CIPs were added to silicone rubber using a two-roll mixer. The complex permittivity and complex permeability were measured over the frequency range of 1-18 GHz. The two EM parameters were verified and the uniform dispersion of MWCNTs and ClPs was confirmed by comparing the measured reflection loss （RL） with the calculated one. As the MWCNT weight percent increased, the RL of the spherical CIPs/silicone rubber composites changed insignificantly. It was attributed to the random distribution of spherical ClPs and less content of MWCNTs. On the contrary, for composites filled with flaky ClPs the absorption bandwidth increased at thickness 0.5 mm （RL value lower than -5 dB in 8-18 GHz） and the absorption ratio increased at lower frequency （minimum -35 dB at 3.5 GHz）. This effect was attributed to the oriented distribution of flaky CIPs caused by interactions between the two absorbents. Therefore, mixing MWCNTs and flaky CIPs could achieve wider-band and higher-absorption ratio absorbing materials.

Microwave Absorption and Shielding Property of Composites with FeSiAl and Carbonous Materials as Filler

Silicone rubber composites filled with FeSiAI alloys and multi-walled carbon nanotubes （MWCNT）/graphite have been prepared for the first time by a coating process. The complex permittivity and permeability of the composites were measured with a vector network analyzer in a 1-4 GHz frequency range, and the DC electric conductivity was measured by a standard four-point contact method. These parameters were then used to calculate the reflection loss （RL） and shielding effectiveness （SE） of the composites. The results showed that the added MWCNT increased the permittivity and permeability of composites in the L-band, while the added graphite increased only the permittivity. The variation lies in the interactions between two carbonous absorbents. Addition of 1 wt% MWCNT enhanced the RL in the L-band （minimum -5.7 dB at 1 ram, -7.3 dB at 1.5 ram）, while the addition of graphite did not. Addition of MWCNT as well as graphite reinforced the shielding property of the composites （maximum SE 13.3 dB at 1 ram, 18.3 dB at 1.5 ram） owing to the increase of conductivity. The addition of these carbonous materials could hold the promise of enforcing the absorption and shielding property of the absorbers.

Effects of Sm-doping on microstructure,magnetic and microwave absorption properties of BiFeO_(3)

In this paper,polycrystalline samples of Bi_(1-x)Sm_(x)FeO_(3)(x=0,0.05,0.1,0.15) were successfully synthesized by sol-gel method.The effects of Sm concentration on the crystal structure,morphology,chemical states,magnetic properties and microwave absorption performance were studied by X-ray diffraction(XRD),scanning electron microscopy(SEM),transmission electron microscopy(TEM),X-ray photoelectron spectroscopy(XPS),a vibrating sample magnetometer(VSM) and a Vector network analyzer(VNA),respectively.The results show that the rare earth Sm doping causes the crystal structure to change.When x≤0.1,Bi_(1-x)Sm_(x)FeO_(3) is the distorted rhombohedral structure with space group R3c.With the increase of Sm doping amount to x=0.15,the phase structure of Bi_(1-x)Sm_(x)FeO_(3) changes from rhombohedral structure to cubic structure with the space group Pm3 m.The particle size decreases with the increase of the Sm doping amount.The analysis results show that Sm doping can effectively reduce the oxygen vacancies and significantly improve its magnetic properties.The results exhibit that moderately doped rare earth Sm element can effectively improve microwave absorption properties of Bi_(1-x)Sm_(x)FeO_(3) powders.When Sm doping amount of x is 0.1,the Bi_(0.9)Sm_(0.1)FeO_(3) compound has good microwave absorption performance,and the minimum reflection loss value of Bi_(0.9)Sm_(0.1)FeO_(3) powder reaches about-32.9 dB at11.7 GHz,and its effective absorption bandwidth(RL <-10 dB) is 2.6 GHz with the optimal matching thickness of 2.0 mm.

Reduced Graphene Oxide/Carbon Nanofiber Based Composite Fabrics with Spider Web‑like Structure for Microwave Absorbing Applications

Composite fabrics with excellent microwave absorbing performance are highly desired.Herein,reduced graphene oxide/carbon nanofiber(rGO/CNF)based composite fabrics with spider web-like structure were successfully synthesized by electrostatic spinning technique.The spider web-like structure in the composite fabrics provides a connected network for efficient conductive loss of microwave energies.Magnetic loss benefits from the deposited nickel nanoparticles(Ni NPs)anchored in the carbon nanofibers.Meanwhile,the deposited thin polypyrrole(PPy)layers on the conductive network acts as a protective layer for Ni NPs as well as provides abundant interfaces for dissipating electromagnetic energies,which endow the composite fabrics stable microwave absorbing performance.Due to the synergistic effect of microwave absorb-ing mechanism,the maximum reflection loss(RL_(max))of the composite fabric at 6.72 GHz is-46.15 dB,and the effective absorption bandwidth(EAB)is as wide as 8.63 GHz(from 9.37 to 18 GHz).What's more,favorable mechanical and heat insulation properties of the composite fabrics reveal its multifunctional advantages.This rGO/CNF based composite fabric demonstrates a new direction for multifunctional and flexible microwave absorbing materials(MAMs).

三维多孔碳基吸波材料的研究进展

随着电磁波(EMW)污染的日益严重,开发高性能电磁波吸收材料(EWAMs)已成为研究热点.碳基EWAMs具有优异的物理、化学稳定性和高导电性带来的强介电损耗.特别是三维(3D)多孔碳基EWAMs在EMW吸收研究领域长盛不衰.3D多孔结构大大降低了材料的密度,有利于EMW的多次反射和散射,优化了阻抗匹配,因此有望实现“低密度、薄厚度、宽吸收带宽、强吸收”的目标.在此,我们首先阐明了相关的理论基础和评价方法.之后,我们以材料来源为主线索,总结了3D多孔碳基EWAMs的最新研究进展,并突出了其中一些独特而新颖的观点.最后,提出了3D多孔碳基EWAMs的挑战和前景,这将有助于高性能EWAMs的进一步发展.

Electromagnetic and microwave absorption properties of Er-Ho-Fe alloys

The ErxHo（2-x）Fe（17）（x = 0.0, 0.1,0.2, 0.3, 0.4） powders were prepared by arc melting and high energy ball milling method. The influence of the Er substitution on phase structure, morphology, saturation magnetization,electromagnetic parameters was investigated by X-ray diffraction（XRD）,scanning electron microscopy（SEM）, vibrating sample magnetometer（VSM） and vector network analyzer（VNA）,respectively. The results show that the saturation magnetization increases and the average particle size increases with the increase of Er content. The minimum absorption peak frequency shifts towards a lower frequency region with the increase of Er content. The ErxHo2-xFe17 powder can achieve the minimum RL of-24.07 dB at 6.96 GHz with a thickness of 2.0 mm and the minimum RL is less than-20 dB at the thickness range from 2.0 to 3.0 mm. The minimum RL of ErxHo2-xFe17 is-37.26 dB at5.68 GHz and the frequency bandwidth of R〈-10 dB reaches about 1.2 GHz with a thickness of 2.4 mm.And the microwave absorbing properties of the composite with different weight ratios of Er0.3Ho1.7Fe17/graphene were researched. The microwave absorbing peaks of the composite shift to lower frequency with the increase of graphene content. The values of the minimum RL of ErxHo2-xFe17/graphene are close to-10 dB with absorbing coating thicknesses increased.

Controllable preparation and microwave absorption properties of shape anisotropic Fe_(3)O_(4) nanobelts

To substantially prevent electromagnetic threatens,microwave absorbing materials(MAMs)are required to eliminate surplus electromagnetic waves.As a typical MAM,Fe_(3)O_(4) particles with complex permittivity and permeability have been widely applied due to the coexistence of magnetic loss and dielectric loss.However,the necessary high mass fraction significantly limited its applications,thus Fe_(3)O_(4) nanostructures have been extensively investigated to overcome this problem.In this work,uniform Fe_(3)O_(4) nanobelts were prepared by electrospinning and two-step thermal treatment.By controlling the composition and viscosity of the electrospinning precursor solution,Fe_(3)O_(4) nanobelts with tunable lateral sizes(200 nme1 mm)were obtained.The samples with low content(only 16.7 wt%)Fe_(3)O_(4) exhibited wide maximum effective absorbing bandwidths(EAB)over 3 GHz,and Fe_(3)O_(4) nanobelts with smaller lateral sizes showed a maximum EAB of 4.93 GHz.Meanwhile,Fe_(3)O_(4) nanobelts with smaller lateral sizes presented superior reflection loss properties,the lowest reflection loss reached-53.93 dB at 10.10 GHz,while the maximum EAB was up to 2.98 GHz.The excellent microwave reflection loss of Fe_(3)O_(4) nanobelts was contributed to the enhanced synergistic effect of magnetic loss,dielectric loss,and impedance matching,originated from the hierarchically cross-linked networks and shape anisotropies.This study could broaden the practical applications of magnetic absorbers,and provided an approach for the development of shape anisotropic magnetic materials.

Magnetic and microwave absorbing properties of Ce-Co-based alloy powders driven with lanthanum content

The composites of Ce-Co-based alloys doped with La content were fabricated via a vacuum arc melting method.The influences of La addition on microstructure,electromagnetic parameters,magnetic property and microwave absorbing property were measured by the corresponding equipment.The morphology characteristics manifest that all samples display sheet structure,and the average particle size of alloy powders increases with increasing La content The saturation magnetization(MS) decreases with increasing La addition as a whole.The minimum reflection loss(RL) of La0.4Ce1.6Co17 alloy powder about-42.29 dB can be obtained about-42.29 dB at 7.84 GHz with the matching thickness of 1.8 mm,and the corresponding effective bandwidth can achieve about 2.24 GHz.In addition,the minimum RL frequency moves towards a lower frequency region as the La content increases.The minimum RL of La0.3Ce1.7Co17alloy powder is less than-20 dB ranging from 1.2 to 2.4 mm in the whole 4-16 GHz.The maximum bandwidth can reach about 4.88 GHz at the given thickness of 1.2 mm.In general,these all indicate that the La addition is beneficial to improving the microwave absorbing performance in both effective bandwidth and absorption intensity.

Dielectric,Electromagnetic Interference Shielding and Absorption Properties of Si_3N_4-PyC Composite Ceramics

Pyrolytic carbon （PyC） was infiltrated into silicon nitride （Si3N4） ceramics by precursor infiltration and pyrolysis （PIP） of phenolic resin, and Ni nanoparticles were added into the phenolic resin to change the electric conductivity of Si3N4-PyC composite ceramics. Dielectric permittivity, electromagnetic interference （EMI） shielding and absorption properties of Si3N4-PyC composite ceramics were studied as a function of Ni content at 8.2-12.4 GHz （X-band）. When Ni nanoparticles were added into phenolic resin, the electric conductivity of the prepared composite ceramics decreased with increasing Ni content, which was attributed to the decrease of graphitization degree of PyC. The decrease in electric conductivity led to the decrease in both permittivity and EMI shielding effectiveness. Since too high permittivity is harmful to the impendence match and results in the strong reflection, the electromagnetic wave absorption property of Si3N4-PyC composite ceramics increases with increasing Ni content. When the content of Ni nanoparticles added into phenolic resin was 2 wt%, the composite ceramics possessed the lowest electric conductivity and displayed the most excellent absorption property with a minimum reflection loss as low as -28.9 dB.