progress in low-frequency microwave absorbing materials · 17124 journal of materials science:...

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Journal of Materials Science: Materials in Electronics (2018) 29:17122–17136https://doi.org/10.1007/s10854-018-9909-z

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REVIEW

Progress in low-frequency microwave absorbing materials

Zirui Jia1 · Di Lan1 · Kejun Lin1 · Ming Qin1 · Kaichang Kou1 · Guanglei Wu1,2 · Hongjing Wu1

Received: 30 June 2018 / Accepted: 20 August 2018 / Published online: 23 August 2018 © Springer Science+Business Media, LLC, part of Springer Nature 2018

AbstractElectromagnetic wave equipment and devices working at low frequency of 0.5–8 GHz have been extensively used in wire-less data communication systems, local area network, household appliances and so on. It is found that the extensive use of such devices have a terrible pollution to their surroundings and moreover threaten the health of human being by weakening biological immune systems, breaking DNA strands, promoting cancers. A key solution to this problem is to develop materi-als that are able to attenuate the harmful electromagnetic waves pollution. This review aims at summarizing the progresses obtained in conventional materials and new emerging structures for microwave absorption at low frequency. The ultimate aim of these materials is to realize a wider effective absorption frequency bandwidth (fE) at a thinner coating thickness (d). Typical and well-received component and construction of composite, synthesis methods, and fE are summarized in several tables in detail. The different characteristics of different types of absorbing materials are given much attention in this review.

1 Introduction

As is known to all, with the rapid development of electronic equipment and wireless information technology, the electro-magnetic interference issue has become a serious problem which cannot be ignored [1–3]. Hence, electromagnetic wave absorbers (EMWAs) have aroused due attention at home and abroad. It is well established that ideal EMWAs should exhibit thin matching thickness, lightweight, broad absorb-ing bandwidth, and strong EM absorption [4–6]. To meet the ever-stricter requirements, a number of global research efforts have been made to develop new EM wave absorbing materials and plenty of novel EMWAs with advanced micro-wave properties have emerged. However, there is much less attention focusing on the application for microwave absorb-ing at low frequency.

In this review, we present the recent efforts and pro-gresses obtained at low frequency and try to give some reli-able suggestions for future studies of new promising systems in this field.

Electromagnetic absorbing materials are able to convert electromagnetic waves into thermal energy or other forms of energy by means of their dielectric or magnetic loss ability. Electric permittivity

(r =

− j) and magnetic perme-

ability (r = − j) are important parameters of a certain

absorbing material, and are directly associated with their absorbing properties. According to the theory of electro-magnetic absorption, ε’ and µ’ refer to the ability to store electrical energy, while ε″ and µ″ represent the efficiency of converting electromagnetic energy into heat. In view of achieving good microwave absorption, high value of ε″ and µ″ are expected. However, the usual case is that larger ε″ and µ″ values may lead to a high reflection of electromag-netic wave at the interface of an absorber, which means a poor impedance matching and thus greatly limits its absorp-tive capacity. To improve the absorbing performance, many types of composite materials with complex morphology and nanostructure were designed and prepared to change imped-ance matching characteristics and tune the electromagnetic absorption ability.

In general, electromagnetic absorption performance is primarily characterized by the coaxial-line and the free-space methods, the electromagnetic parameters are obtained

Zirui Jia and Di Lan have contributed equally to this work.

* Kaichang Kou [email protected]

* Hongjing Wu [email protected]

1 School of Sciences, Northwestern Polytechnical University, Xi’an 710072, People’s Republic of China

2 Institute of Materials for Energy and Environment, Growing Base for State Key Laboratory, College of Materials Science and Engineering, Qingdao University, Qingdao 266071, People’s Republic of China

http://orcid.org/0000-0002-5575-3224

http://crossmark.crossref.org/dialog/?doi=10.1007/s10854-018-9909-z&domain=pdf

17123Journal of Materials Science: Materials in Electronics (2018) 29:17122–17136

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first and then input into the following equations to calculate the reflection loss (RL) [7–12]:

While Zin represents the input impedance of the absorber, Z0 means the impedance of free space (377 Ω), r

(r =

− j) and r(r =

− j) are the complex permittivity and permeability of the absorber, f is the fre-quency of electromagnetic wave, d means the coating thick-ness of the absorber, c is the velocity of light in vacuum. The reflection loss (RL) value usually represents the micro-wave absorption capacity of the absorber, for example, the RL value of − 10 dB is comparable to 90% of microwave absorption.

It is known that microwave absorbing materials are mainly composed of dielectric loss materials including con-ductive polymers, carbon-based absorbents, ceramics, and magnetic loss materials including ferrites, ultra-fine metal powder, metal oxides based on their primary EM absorp-tion mechanism [13–18]. Table 1 lists the classification and characteristics of EMWAs [19].

With these in mind, this review is organized as fol-lows. In the section on ‘EM wave absorption performance of dielectric loss absorbers’, several representative micro-wave absorbers involving dielectric loss will be introduced. The influences of morphological and structural change of absorber will also be discussed. In the section on ‘EM wave absorption performance of magnetic loss absorbers’, some detailed strategies will be proposed for magnetic loss absorbers to allow impedance matching, enhancing EM absorption, and broadening the EM absorption frequencies. In the section on ‘meta-material microwave absorbers’, meta-material microwave absorbers are introduced, which have the ability to create independent tailored electric and magnetic responses to incident radiation.

(1)Zin = Z0(r/r

)1∕2tanh

[j

(2fd

(rr

)1∕2/c)]

(2)RL (dB) = 20 log ||(Zin − Z0)/(Zin + Z0)||

2 EM wave absorption performance of dielectric loss‑dominant absorbers

2.1 Conductive polymers

Conductive polymers have been widely studied thanks to their light weight, corrosion resistant, good and flexible processability and variability of conductivity [20]. Among them, polyaniline (PANI) has been frequently employed as a well-known absorbent in terms of its easy of synthesis, broad response bandwidth, environmentally stable, low monomer cost, and controllable dielectric loss ability [21–23]. Con-sequently, PANI-based composites are extremely advanta-geous over other conductive polymers for the above-men-tioned merits. However, conductive polymers have their own drawbacks, such as the fact that their high complex permittivity (ε = εʹ + iεʺ) and quite low complex permeabil-ity (μ = μʹ + iμʺ) prevents them from well-matching imped-ance, thus resulting in weak absorbing performance [24].

Wang et al. [25] fabricated a ternary microwave absorb-ing material composing of Mn–Zn ferrite, PANI and epoxy resin, and they found that Mn–Zn ferrite was self-assembled on the surface of PANI nanocomposites. The results indi-cated that when the matching thickness was 2 mm, the com-posite with the PANI of above 28.12 wt% could effectively broaden the absorbing band with RL of − 23 dB at frequen-cies around 700 MHz. The introduction of Mn–Zn ferrite are beneficial to improving impedance matching, then has a positive effect in enhancing microwave absorbing property of the nanocomposites.

Zhou et al. [26] reported Fe3O4–poly(3,4-ethylenedioxy-thiophene) (PEDOT) hybrids possessed attractive microwave absorbing properties at low frequency. The EM absorbing performance of hybrids composites, which had been fab-ricated by a simply mechanical mixing method, exhibited great enhancement compared to that of Fe3O4 or PEDOT individually. When the thickness was 4 mm, the RL of the sample reached − 31.4 dB at 4.5 GHz with PEDOT/Fe3O4

Table 1 Classification and characteristics of EMWAs

Categories Low density Broad band-width

Strong absorp-tion

High mechani-cal perfor-mance

Low cost Thermal and chemical sta-bilities

Dielectric Conductive polymers √ X X √ X X Carbon-based √ X X √ √ √ Ceramics √ X X √ X √

Magnetic Ferrites X √ √ X √ X Metal powder X √ √ X X X Metal oxides X √ √ X √ X

17124 Journal of Materials Science: Materials in Electronics (2018) 29:17122–17136

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ratio of 2.0. The excellent reflection loss of the hybrids resulted from the conjugation of conducting polymers and inorganic magnetic nanoparticles, including additional inter-face polarization and matching impedance as well as the thickness of the coating.

Table 2 lists an overview of EM absorption properties of conductive polymer hybrids. If there are no special instruc-tions, the effective absorption band refers to RL < − 10 dB (90% energy absorption).

2.2 Carbon‑based absorbents

Compared with conductive polymer-based composites, carbonaceous microwave absorbers have aroused inten-sive attention due to their tunable surface properties, high strength-to-weight ratio, anti-corrosion properties, ultrathin structural features, ultrahigh carrier mobility (~ 200,000 cm2 V−1 s−1), highest thermal conductivity (~ 5300 W m−1 K−1), etc. [29–31].

Among them, porous carbon nanowires, porous carbon fibers, carbon nanotubes, and graphene foams play a promi-nent role in recent years because of their excellent physi-cal and chemical properties, including high specific surface area, light weight, good electrical conductivity, mechani-cal strength, thermal stability, and corrosion resistance [13, 32–34].

Synergistically assembled multiwalled carbon nanotube (MWCNT)/graphene foams (GFs) with highly efficient microwave absorption was prepared by Chen et al. [35] through a facile solvothermal method. The composites with (CGFs) and without (C@GFs) solvothermal process, with different mass ratios of modified MWCNT/GFs and vari-ous annealing temperatures were investigated. C@GFs was obtained through freeze-drying of MWCNT/GO aqueous solution directly followed by annealing process. The samples were denoted as C@GxF-y or CGxF-y, where x is the mass ratio of GFs/MWCNT, y annealing temperature.

As can be seen in Fig. 1a, the qualified absorption band-width of C@GFs was much narrower than that of CGFs, while the absorption of C@GFs at low frequency was much weaker. Besides, CGFs with lower annealing temperature and higher MCNT loading showed stronger absorbing peaks at low frequency (see in Fig. 1b). Compared with

other absorbents, CGFs exhibits incredibly high AAI, which means the advantage of broad qualified bandwidth and ultra high average absorption intensity (Fig. 1c). A more realis-tic parameter of specific microwave absorbing performance (SMAP) was defined as the ratio of AAI to the density and thickness of the product (AAI/(d·t)). The SMAP for CGFs were much higher than other reported absorbents (Fig. 1d), and the SMAP of CG2F-200 was approaching 104, which indicated that CGFs showed incredible absorbing perfor-mance with ultrathin thickness and low density. Among the group of CGFs, CG2F-200 is superior to other kinds of CGFs with minimum RL value of ~ − 32.5 dB (d = 2 mm) at the frequency of ~ 4.5 GHz. The effective absorbing band (RL < − 10 dB) almost covered the whole frequency range of 2–18 GHz.

The authors believed the excellent absorbing performance of the CGFs could be attributed to three aspects, namely well balanced matching of impedance, special multilevel structure, giant 3D cross-linked and intricate loss network. As shown in Fig. 2a, most incident microwaves were able to penetrate into the inside of porous CGFs and scattered repeatedly. Besides, massive positive and negative charged domains were formed on the pore wall under alternating electromagnetic field, leading to responsive polarization losses, which in turn improved the overall absorption. What’s more, the integrated MWCNT/graphene network formed a giant 3D cross-linked and intricate conductive network (see in Fig. 2b). This created extremely long and complex transmission channel for the incoming electromag-netic waves.

Table 3 lists an overview of EM absorption properties of carbon-based composites. If there are no special instruc-tions, the effective absorption band refers to RL < − 10 dB (90% energy absorption).

2.3 Ceramics

Ceramics have also attracted more and more attention as a potential candidate for EM wave absorbing. A great deal of ceramics have been investigated such as SiC [44], SiCf [45], Si3N4 [46], Al2O3 [47], SiO2 [48], SiOC [49], SiBC [50], SiBCN [51], and BaTiO3 [37, 52]. As a typical dielectric absorber, SiC can be decorated by other magnetic absorbers

Table 2 EM wave absorption properties of conductive polymers absorbers with different chemical structures, doping or with different fillers

NP nanoparticle, HDs hematite dendrites

Materials d (mm) f (GHz) RL (dB) Band (GHz) Percentage Refs.

PANI-NP 5 5.8 − 40.5 – 30 wt% [21]Ferrite/PANI 2 0.7 − 23 0.3 PANI–28.12 wt% [25]Fe3O4/PEDOT 4 4.5 − 31.4 – 50 wt% [26]HDs/PANI 5.3 4.6 − 54.0 6.5 50 wt% [27]CNT@BaTiO3@PANI 6 3.7 − 22.27 1.5 20 wt% [28]


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such as metal particles (e.g. Fe, Co and Ni) or their metal oxides to adjust the impedance matching characteristics.

Zhang et al. [44] studied the ß-SiC nanowires (NWs) with different states of stacking faults fabricated by varying the heating temperature. The ß-SiC nanowires prepared under 1400 °C showed most stacking fault content as well as the best EM absorbing performance. In details, the minimum RL value for the SiC NW-1400 sample was measured to be − 30 dB at 6.7 GHz. Moreover, an effective absorption band (RL < − 10 dB) covering a frequency range of 6.0–9.7 GHz was obtained for SiC NW-1400 with a thickness of 4.6 mm. The stacking fault content has a great impact on the reflec-tion loss of the SiC NW, because it can induce numerous dipole and electrons moment, which contribute to the dielec-tric loss, resulting in the enhanced energy dissipation ability.

Hou et al. [45] have fabricated Fe/SiC hybrid fibers by electrospinning and subsequent high-temperature (1300 °C) pyrolysis in Ar atmosphere using polycarbosilane (PCS) and Fe3O4 precursors. It was found that the introduction of Fe particles had greatly facilitated the growth of SiCO nanow-ires on the surface of the hybrid fibers. Besides, both the permittivity and permeability were strongly enhanced after the introducing Fe into SiC in almost the entire frequency

band (2–14 GHz). As shown in Fig. 3, the Fe/SiC hybrid fib-ers (35 wt% fiber, PCS:Fe = 3:0.5) with various thicknesses exhibited remarkably strong absorption at low frequency, and the minimum RL value of − 46.3 dB was reached at 6.4 GHz with the thickness of 2.25 mm. The effective absorption (< 20 dB) in a low-frequency covered the fre-quency of 4–9.6 GHz with varied thickness. Such wonder-ful performance are related to the enhancement of the per-mittivity and permeability, which is greatly influenced by considerable amount of Fe3Si phase (converted from Fe3O4 powders during the pyrolysis process), free carbon and Fe. Furthermore, the well matching thickness is also good for the absorption ability.

Dong et al. [53] studied the role of in situ grown Si3N4 nanowires (Si3N4nws) and SiC nanowires (SiCnws) in enhancing the EM absorbing performance when decorated with three-dimensional (3D) hierarchical graphene foams (GFs). The Si3N4nws–GFs and SiCnws–GFs hybrid were prepared by a one-step carbothermal reduction process under flowing N2 and Ar, respectively. Their morphologies and structures were shown in Fig. 4a, b and c, d. The two kinds of hybrid composites showed similar morphologies, with lots of NWs of 200 nm in diameter grown into the porous

Fig. 1 a, b RL of CGFs and C@GFs with different compositions and heat treatment, c, d superiority of CGFs and C@GFs compared with other reported literatures (Reproduced with permission from Ref. [35], copyright 2017 Elsevier)


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GFs in a random distribution. From Fig. 4e, f, detailed dif-ferences between Si3N4nws and SiCnws can be observed: the surface of Si3N4nws was covered by an ~ 2 nm amorphous SiOx layer, while the SiCnws was composed of a crystal-line SiC core coated with a thin SiOx amorphous shell layer (~ 25 nm).

As shown in Fig. 5, both of the hybrids showed strong EM absorption at low frequency. The minimum RL of the Si3N4nws–GF composite was about − 48.8 dB at 6.4 GHz

with a thickness of 2.36 mm, while the SiCnws–GF exhib-ited stronger RL of − 67.8 dB at 5.9 GHz with 2.6 mm thickness. Compared with other excellent absorbers reported before (Fig. 6), Si3N4nws–GF and SiCnws–GF have more competitive absorbing performances. The reason of the outstanding absorbing performance can be attributed to several aspects, including interfacial polari-zation, dipole polarization, unique space structure, lots of stacking faults and so on.

Fig. 2 a Schematic representation of electromagnetic wave attenua-tion mechanism of CGFs. b Schematic illustration of the formation of numerous resistance–inductance–capacitance coupled circuits in 3D

CGF for responding to incident electromagnetic wave (Reproduced with permission from Ref. [35], copyright 2017 Elsevier)

Table 3 EM wave absorption properties of carbon based absorbers with different chemical structures, doping or with different fillers

BCNTs/MVQ bamboo-like carbon nanotube/methyl vinyl silicone composite, EG/Fe3O4 NR expanded graphite/Fe3O4 nanoring composites


MWCNT/GFs 2 ~ 4.5 ~ − 32.5 16 MWCNT/GFs = 1:2 [35]Ag/CNT 6.3 2.8 − 51.1 0.8 30 wt% [36]BaTiO3/MWCNTs 3 6.8 − 33.5 0.5–1 5 wt% [37]BCNTs/MVQ 2.056 5.92 − 38.2 ~ 4.8 7 wt% [38]EG/Fe3O4 NR 2.6 6.8 − 24.8 8 (< 20 dB) 10 wt% [39]GO/ferrites 3 3.3 − 17.15 1 60 wt% [40]Fe3O4/CNT 5 4 − 25 ~ 1 88 wt% [33]NiO@C 2 2 − 40 > 6 50 wt% [41]MWCNTs/Fe 4.27 2.68 − 39 1.43 (< 20 dB) 60 wt% [42]Co/C 4.0 5.8 − 35 5.80 (2.5 mm) 40 wt% [43]


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Table 4 lists an overview of EM absorption properties of ceramic-based composites. If there are no special instruc-tions, the effective absorption band refers to RL < − 10 dB (90% energy absorption).

3 EM wave absorption performances of magnetic loss‑dominant absorbers

3.1 Ferrites

Ferrites have been studied intensively as one of the most concerns for the following merits including cheaper raw materials, simple preparation craft and good magnetic loss.

Cai et al. [54] found that amorphous Li–Zn ferrite pre-pared by self-reactive quenching technology were of hol-low microstructure, and with the increase of heat-treatment temperatures (from 300 to 1200 °C), amorphous Li–Zn ferrites would transform into complete crystallization state and the surface were composed of polygon plate crystals. Besides, when the heat-treatment temperature was 1200 °C, the space-charge polarization enhanced, thus leading to the excellent microwave absorbing properties. When the thickness was 5 mm, the RL reached to its peak value of

Fig. 3 The RL of Fe/SiC fibers with different thicknesses (Repro-duced with permission from Ref. [45], copyright 2017 American Chemical Society)

Fig. 4 a, b SEM images of Si3N4nws/GFs at different magnifications, c, d SEM images of SiCnws/GFs at different magnifications, e TEM image of the in situ grown Si3N4nws, and f TEM image of the in situ

grown SiCnws (Reproduced with permission from Ref. [53], copy-right 2017 Royal Society of Chemistry)


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− 49 dB at 3.4 GHz with the effective absorption bandwidth of 1.3 GHz. The absorbing property of the ferrites can be adjusted by simply changing the heat-treatment temperature.

Apart from changing the heat-treatment temperature, doping a small amount of the divalent metallic ions or rare-earth ions in ferrites [55–58] is proven to be a effec-tive strategy for enhancing attenuation capacity. Wang

et al. [56] studied Mn–Zn ferrite prepared by sol–gel combustion technique. The result showed that satisfac-tory value of − 17 dB at 800 MHz and the effective band-width from 700 MHz to 1 GHz could be obtained by the Mn0.8Zn0.2Fe2O4/EP composite. Stergiou [57] investigated rare earth doped Ni–Co–Zn spinel ferrites, and it showed that loss peaks of RL < − 46 dB occurred at 4.1 GHz of 2 mm and 5 GHz of 1.8 mm for Y and La-doped Ni–Co–Zn spinels, respectively.

Wu et al. [59] synthesized CuxFe3−xO4@Cu hollow spherical chains (HSCs) with tunable composition by simply changing the Cu2+ concentration. It was prepared through facile liquid-phase reduction with Fe3O4 HSCs as cores and templates fabricated through a magnetic field-induced Oswald ripening process. Cu1.300Fe1.700O4@Cu HSCs with 44 wt% showed strongest EM properties, with the minimum RL of − 45.1 dB at 3.44 GHz (d = 4.4 mm) (see in Fig. 7). The authors believed that its excellent microwave absorbing performance may be ascribed to the hollow/porous core–shell structure, Cu2+ substitution, and Cu0 shell with high conductivity.

Table 5 lists an overview of EM absorption properties of ferrite-based composites. If there are no special instruc-tions, the effective absorption band refers to RL < − 10 dB (90% energy absorption).

Fig. 5 The RL of a Si3N4nws–GF and b SiCnws–GF, respectively (Reproduced with permission from Ref. [53], copyright 2017 Royal Society of Chemistry)

Fig. 6 A comparison of several excellent EM absorber (Reproduced with permission from Ref. [53], copyright 2017 Royal Society of Chemistry)

Table 4 EM wave absorption properties of ceramics absorbers with different chemical structures, doping or with different fillers


ß-SiC nanowires 4.6 6.7 − 30 3.7 50 wt% [44]Fe/SiC 1.5–3.5 6.4 − 46.3 5.6 (< 20 dB) 35 wt% [45]Si3N4nws–GFs 2.36 6.4 − 48.7 – 50 wt% [53]SiCnws–GFs 2.6 5.9 − 67.8 – 50 wt% [53]


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Fig. 7 RL of 44 wt% Cu1.300Fe1.700O4@Cu/paraffin samples versus frequency (S4) and various thicknesses. a 3D curves, b 2D contour curves, c 2D curves, d calculated match-ing thickness, and e the mecha-nism of microwave absorption of CuxFe3−xO4@Cu HSCs (Reproduced with permission from Ref.[59], copyright 2017 John Wiley & Sons, Inc.)

Table 5 EM wave absorption properties of ferrites with different chemical structures, doping or with different fillers


LiZn ferrites/paraffin 5 3.4 − 49 1.3 60 wt% [54]Mn1−xZnxFe2O4/EP 2 0.4 − 17 0.3 9.1 wt% [56]Y-doped Ni–Co–Zn spinels 2 4.1 < − 46 – – [57]La-doped Ni–Co–Zn spinels 1.8 5 < − 46 – – [57]CuxFe3−xO4@Cu 4.4 3.44 − 45.1 – 44 wt% [59]


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3.2 Magnetic metallic materials

Magnetic metals and metallic alloys have composed the sec-ond main group of magnetic loss absorbing materials owing to their high curie temperature and the good temperature stability, high saturation magnetization (Ms), and composi-tion controllability. Currently, iron (Fe), cobalt (Co), nickel (Nd) and corresponding metallic alloy are considered to be the most employed magnetic materials. In terms of design-ing EM absorbing materials, metallic materials have some serious drawbacks, such as narrow bandwidth, which have provoked scientists to think in new directions and design new materials for EM absorbing.

The morphology and structure of the material, concentra-tion effect are considered as the determining factors of the EM absorbing performance. So many researchers have tried to improve their EM absorption performances by modifying the morphology and structure.

Core–shell structure is among one of the many attempts. Planar anisotropy carbonyl-iron (PACI) particles coated with ZnO nanoshells were prepared by Han et al. [60]. When the thickness was 2.5 mm, the optimal RL of PACI@ZnO com-posites reached to − 31.93 dB at 1.96 GHz. The reflection loss exceeding − 20.0 dB (99% microwave absorption) was obtained in the 1.5–2.3 GHz range for an absorber thick-ness of 2.1–3.3 mm. What’s more, after the PACI at ZnO composite was rotationally oriented in an external mag-netic field, its microwave absorbing property was further improved. The minimum RL reached to − 40.06 dB and the matching thickness reduced to 2.2 mm with a slight varia-tion of matching frequency. The corresponding bandwidth (RL < − 20 dB) shifted from 1.8 to 2.7 GHz for an absorber thickness of 1.7–2.5 mm. By coating ZnO nanoshells, the complex permittivity of PACI was effectively suppressed, which was helpful to fulfill impedance matching characteris-tic and the attenuation characteristic. In the case of oriented PACI@ZnO composites, the impedance matching was fur-ther achieved and optimal RL was further improved.

Sodium stearate (SS) surfactant with various amount was used by Zare et al. [61] as a modifier to FeCo alloy particles. The result showed that the 80 wt% FeCo/paraffin compos-ites with 0.004 mol SS had the minimum RL of − 44 dB, which was obtained at 5 GHz with the thickness of 2.7 mm and the corresponding bandwidth was 2.2 GHz. The reason of enhanced absorption lies in the fact that the concentra-tion of SS acts as a tuning parameter of the permittivity and conductivity, which consequently influences the impedance matching characteristic and microwave absorbing property.

Chen et al. [62] synthesized Co7Fe3 nanospheres with diameters ranging from 350 (CF350) to 650 nm (CF650) via liquid-phase reduction method. Figure 8 showed the struc-ture, morphology, and compositions of Co7Fe3 nanospheres. The Co7Fe3 nanospheres had uniform size and smooth

surface. The diameter of Co7Fe3 spheres can be adjusted by changing the amount of N2H4·H2O as the reducing agent. It was found that the CF350/paraffin with 50 vol% exhibited broad bandwidth of 6.7 GHz as well as incredible minimum RL of − 78.4 dB the thickness of 1.93 mm. The good per-formance of Co7Fe3 nanospheres can be attributed to the reduced effect of eddy current, which is beneficial to elec-tromagnetic matching.

Table 6 lists an overview of EM absorption properties of magnetic metallic materials. If there are no special instruc-tions, the effective absorption band refers to RL < − 10 dB (90% energy absorption).

3.3 Transition metal oxides and binary metal oxides

Transition metal oxides and binary metal oxides stand out as high-performance EM wave absorbers in view of their excellent dielectric and/or magnetic properties, such as high stability, low cost and easy fabrication. Considerable efforts have been devoted to the investigation of metal oxides, such as ZnO [65], Fe3O4 [66], Co3O4 [67], MnO2 [68], NiO [69], Ni2O3 [70], MnFe2O4 [71], NiFe2O4 [72, 73], CoO [74],and CoFe2O4 [75].

Liu et al. [76] proposed that flower-like NiCo2O4/Co3O4/NiO composite was fabricated through a facile hydrother-mal method, followed by a post-annealing treatment. Fig-ure 9a–d demonstrates the synthetic processes of NiCo2O4/Co3O4/NiO composite and the structural transformations taken place in the process. After a series of variations, the final composites displayed a hierarchical flower shape with radically assembled porous nanosheets.

As shown in Fig. 9e, the hybrid/paraffin mixture was then pressed into a toroidal shape for measurement of EM param-eters. When the filler loading was as low as 20 wt%, the composite/paraffin mixture with the matching thickness of 4.93 mm showed an extremely strong EM wave absorption of − 57.0 dB, which occurred at 5.92 GHz.

The improved EM absorbing capability of the NiCo2O4/Co3O4/NiO composite would be explained by the synergistic effect among multiple components. The dielectric loss of the composites was greatly enhanced by additional interfacial polarization and defect polarization relaxations (shown in Fig. 9f). Moreover, the hierarchical structure with assembled nanosheets could provide more active sites for wave reflec-tion and scattering, thus increasing the wave propagation paths and further promoting the EMW attenuation.

NiFe2O4 nanoparticles and its nanohybrids using reduced graphene oxide (r-GO) sheets were prepared by Ameer and Gul [73] via solvothermal synthesis. The almost spherical NiFe2O4 nanoparticles with an average particle size ranging from 30 to 58 nm have uniformly grown on the surface of graphene sheets. The NiFe2O4/r-GO with 15 wt% of r-GO composites showed effective absorbance from 1 MHz to


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3 GHz and the minimum reflection loss was as low as − 68 dB at 1.11 GHz with the matching thickness of 2 mm. The excellent and unusual absorbing property may be ascribed to the nanohybrid system as well as special type of hybrid multilayer stacking, which cause enhanced electric and dipo-lar polarization coupled with continuous path to scatter the incident energy.

Table 7 lists an overview of EM absorption properties of oxide-based composites. If there are no special instructions,

the effective absorption band refers to RL < − 10 dB (90% energy absorption).

4 Meta‑material microwave absorbers

Meta-material is a kind of new-emerging materials, which is unavailable in natural surroundings. It is an arrange-ment of artificial structural elements designed to achieve

Fig. 8 a SEM image of CF350 nanospheres, b SEM image of single CF350 nanosphere, and c the RL of CF350 (50 wt%)/paraffin with various thicknesses (Reproduced with permission from Ref. [62], copyright 2017 American Chemical Society)

Table 6 EM wave absorption properties of metal powders with different chemical structures, doping or with different fillers


PACI@ZnO 2.5 1.96 − 31.93 – 35 vol% [60]Oriented PACI@ZnO 2.2 ~ 1.96 − 40.06 – 35 vol% [60]FeCo/paraffin 2.7 5 − 44 2.2 80 wt% [61]Co7Fe3/paraffin 1.93 5.8 − 64 6.7 50 vol% [62]Co/paraffin 4 3.4 ~ − 30 1.2 15 vol% [63]Ce0.2Nd1.8Fe17 2.4 5.1 − 22.5 1.3 80 wt% [64]


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advantageous and unusual properties, such as a negative refractive index, negative permittivity and negative perme-ability [77]. However, meta-material suffers from narrow absorbing band [78].

Cheng et al. [79] employed an ultra-thin low-frequency broadband microwave absorber based on a magnetic rubber plate (MRP) and cross-shaped structure (CSS) metamaterial (MM) numerically and experimentally. The low-frequency absorption could be easily adjusted by tuning the geometric parameter of the CSS MM and the thickness of MPR. A relatively broader bandwidth of 2.5 GHz could be obtained with the total thickness of about 2 mm in experiments. The expansion of the bandwidth was attributed to the overlap of two resonant absorption peaks originated from MRP and CSS MM, respectively.

Huang et al. [80] proposed a second-order cross frac-tal meta-material structure (see in Fig. 10) to decrease the thickness of magnetic material and meanwhile broaden the bandwidth. Compared to the unloaded magnetic layer, the

thickness of loaded meta-material structure was decreased to 1 mm while the bandwidth was verified by experiment to be 1.09 GHz wider than the unloaded one. The authors analyzed the functions of meta-material and concluded that it has two function, the first one is to add an addition absorp-tion band and the second is to slightly move the absorption band created by the magnetic material.

Table 8 lists an overview of EM absorption properties of meta-materials. If there are no special instructions, the effec-tive absorption band refers to RL < − 10 dB (90% energy absorption).

5 Conclusions

This review has introduced three types of absorbers and their development situation, including dielectric loss absorb-ers, magnetic loss absorbers and meta-material microwave absorbers. In each type, several representative microwave

Fig. 9 a–d Synthetic processes of NiCo2O4/Co3O4/NiO composite, e sample for measurement of EM parameters, and f the mechanisms of microwave absorption (Reproduced with permission from Ref. [76], copyright 2017 Royal Society of Chemistry)

Table 7 EM wave absorption properties of transition-metal oxides and binary metal oxides with different chemical structures, doping or with different fillers


ZnO nanotrees 4.0 4.2 − 58 – 60 vol% [65]NiCo2O4/Co3O4/NiO 4.93 5.92 − 57.0 2.32 20 wt% [76]NiFe2O4/r-GO 2 1.46 − 68 ~ 3 r-GO 15 wt% [73]


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absorbers have been introduced. Various synthesis methods, electromagnetic attenuation theories, and electromagnetic absorption properties of absorbers have been discussed too. Moreover, the key electromagnetic parameter information is compared in several tables to express the characteristics of various absorbers intuitively. To achieve a broader fE value and a thin coating layer, these electromagnetic absorb-ers were prepared to strike a balance between impedance matching and EM attenuation ability (including magnetic and dielectric loss ability). Based on the above factors, the

development of low-frequency electromagnetic absorbers can be well understood. In our opinion, all types of low-fre-quency EMWAs mentioned above have good development potential in different application conditions. Some methods like: changing external conditions such as heat-treatment temperature, magnetic materials doped in dielectric loss absorbers or dielectric material doped in magnetic loss absorbers to improve impedance matching and take advan-tages of both materials, designing complex morphology and nanostructure to create more interfaces and scatter the EM waves, creating new absorbing mechanism to broaden the frequency band and shift it to low frequency, exploit-ing meta-material microwave absorbers and so on, provide valuable practical experience and exploring ideas for the design and development of new low-frequency absorbing materials. Among all the divisions discussed above, car-bonaceous materials are the most researched divisions for their considerable advantages. However, even for EMWAs

Fig. 10 Structure and its S11 curve: a meta-material structure, b overall configuration, and c S11 curve of different structures (Reproduced with permission from Ref. [80], copyright 2014 Springer International Publishing AG)

Table 8 EM wave absorption properties of meta-materials

Materials d (mm) f (GHz) RL (dB) Band (GHz) Refs.

CSS MM 2 3.64 − 25 2.5 [79]Second-order

cross fractal2.6 3.4 − 37.5 2.3 [80]


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as talented as carbon-based materials, they still suffer sev-eral problems like narrower bandwidth coupled with higher concentrations, which greatly restricts their applications. It is challenging and demanding for the researchers to further extend the frequency band with relatively lightweight and meanwhile retain their excellent attenuation characteristic. In the future study, more and more attention may be paid to the application in extreme conditions, such as absorb-ing materials working at elevated temperature and the effect of temperature on the absorption ability [81–84], or more focusing on the materials related to daily life, like design-ing wearable microwave absorption cloth [85], or trying to develop microwave absorbing materials with tunable elec-tromagnetic parameters for frequency-selective microwave absorption [86–90] and so on.

Acknowledgements Financial support was provided by National Natu-ral Science Foundation of China (Nos. 21806129, 51872238, 50771082 and 60776822), the Fundamental Research Funds for the Central Uni-versities (3102018zy045), and Natural Science Basic Research Plan in Shaanxi Province of China (Program No. 2017JQ5116).

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