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Synthesis of absorbing coating based on magnetorheological gel withcontrollable electromagnetic wave absorption propertiesTo cite this article: Li-Rui Wang et al 2019 Smart Mater. Struct. 28 044001

 

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Synthesis of absorbing coating based onmagnetorheological gel with controllableelectromagnetic wave absorption properties

Li-Rui Wang1, Miao Yu1 , Ping-An Yang1,2, Song Qi1 and Jie Fu1

1Key Lab for Optoelectronic Technology and Systems, Ministry of Education, College of OptoelectronicEngineering, Chongqing University, Chongqing 400044, People’s Republic of China2 School of Automation, Chongqing University of Posts and Telecommunications, Chongqing 400065,People’s Republic of China

E-mail: yumiao@cqu.edu.cn

Received 29 October 2018, revised 6 January 2019Accepted for publication 13 February 2019Published 14 March 2019

AbstractOwing to the increasingly complicated electromagnetic environment and changeablebattleground, it is especially urgent to study electromagnetic wave absorbing coating withenhanced absorbing capacity and adaptive ability. Magnetorheological gel (MRG) is prepared bymixing magnetic particles with high viscosity polymer matrix. Under magnetic field, thesemagnetic particles move to form columnar or chains, which allows electromagnetic (EM) wavesto penetrate and scatter along the chains. Accordingly, the MRG-based coating that absorbselectromagnetic waves was successfully synthesized. Moreover, flower-like carbonyl ironparticles (FCIPs), as the filled magnetic particles, were prepared by in situ method. We studiedthe effect of magnetic field, particle concentration and matrix curing on electromagnetic (EM)absorbing properties of the MRG-based coating. With the purpose to test absorption performanceof the MRG-based coating under different magnetic field intensity, a device that regulates themagnetic field is essential. Thus, we built the device that contains permanent magnet, frame,lifting table. From the results, magnetic field can make absorption peak frequency (APF) movetoward high frequency. The amount of shifting to high frequency increases as the magnetic fieldincreases. The magnetic-induced frequency offsets of four samples with different mass fractions(40 wt%, 50 wt%, 60 wt%, 70 wt%) are 2.88 GHz, 1.26 GHz, 1.52 GHz and 0.64 GHz from 0 to300 mT, respectively. Meanwhile, a suitable magnetic field is beneficial for the coating to absorbmore electromagnetic waves. This characteristic of MRG will make it have a wide applicationprospect in real-time electromagnetic protection and electromagnetic stealth.

Keywords: magnetorheological gels (MRGs), absorbing coating, microwave absorption,magnetic-induced frequency shifting

(Some figures may appear in colour only in the online journal)

1. Introduction

Wireless technology is widely used in military field or civildomain [1, 2], such as radar detection, locking and tracking,communication base station, electric rail transit, medical andscientific research equipment and various household appliances[3–6]. These devices generally operate at gigahertz (GHz) range[7, 8]. There is no doubt that excessive electromagnetic (EM)

wave generated from these electronic devices bring electro-magnetic radiation that has greatly threatened human health anddisturbed various commercial or industrial equipment [9–14].Worse yet, electronic equipment work in various frequencybands and radar detection technology has also been greatlyupgraded, so electromagnetic environment (military and civi-lian) is becoming more and more complicated [8]. Absorbingmaterials can absorb unwanted electromagnetic wave energy

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impinged onto the surface and transform it into heat and/orother types of energy [1]. Most of the existing absorbingmaterials that absorb electromagnetic (EW) waves in a part-icular frequency band are unable to meet the requirements formulti-band or full coverage. While in some cases, especially inthe military application of anti-radar camouflage and stealth, thefrequency that needs to be absorbed is varied. This forces us tolook for absorbing material with variable microwave-absorptionperformance.

Generally, magnetorheological (MR) materials containmagnetorheological elastomer (MRE), MR foam, magne-torheological gel (MRG), magnetorheological fluid (MRF)[15–20]. They are a type of smart hybrid material. They are allmade of magnetic particles distributed in non-magnetic matrix[21–23]. To our best knowledge, the MR materials possess soexcellent magnetic-control properties that MR materials exhibitgreat applications in the fields of noise reduction, vibrationattenuation, smart sensing, etc. The same is true for electro-magnetic protection and stealth, etc [24–26]. That’s becausecarbonyl iron particles (CIPs) in MR materials present highfrequency of Snoek’s limit, high saturation magnetization, highrelative permeability at radar wave frequency, and high Curietemperature [27–29]. Besides, CIPs possess both magnetic anddielectric properties, which is good for electromagnetic impe-dance matching [30]. All of these render CIP a promisingmicrowave absorber. Meanwhile, the CIPs in MR materials canresponse to magnetic field (MF) to form chains that parallel tothe direction of magnetic field [15, 31, 32]. The movement ofthe CIPs will result in the transformation of microstructure ofMR materials [33]. Hence, the interaction between particles andparticles or matrix changes accordingly, as well as electro-magnetic parameters (conductivity, permeability, permittivityand so on). While, the microwave-absorption capability of theMRG-base coating is decided by the complex permeability andpermittivity at a given frequency and layer thickness [1, 30, 34].Therefore, we can use the magnetic field to control the particlesto get the desired absorbing performance. Min Dandan et alprepared the oriented FCI/EP composites. The measurementresults showed that they obtained the higher permeability andmodest permittivity of the composites after orientation in thefrequency range of 2–18 GHz. The calculated absorptionproperties indicated that the orientation plays an important rolein decreasing the absorber thickness and broadening theabsorption bandwidths [7]. Chen Qianwang et al fabricated acomposite with highly aligned array of Fe3O4/CNT coaxialnanofibres in poly (methyl methacrylate) matrix under lowmagnetic field. It was found that microwave absorption of themagnetically aligned composite at 8.5–12.5GHz was evidentlyenhanced [35]. Yu miao et al studied MREs’ absorbing prop-erties and discussed the effect of magnetic field intensity anddirection angle on the absorbing. Compared to 0mT, theeffective absorption bandwidth (below −20 dB) of 400 mTextended by 40%. The microwave-absorbing peaks movedtowards a higher frequency of 10.46%. While, in the process ofvarying the magnetic field direction angle (0°–90°), the peakfrequencies reduced by 13.75%, and the value of the minimumRL decreased by 45.78% (−25.866 dB for the 0° sample;−37.708 dB for the 60° sample) [36].

From these works, in order to achieve desired absorbingperformance, magnetic field drives the particles to move tochange micro-internal structure of composites from isotropyto anisotropy [33]. While, the particles could no longer movewhen these materials solidified. The absorbing performancecould not be further adjusted. Fortunately, like other MRmaterials, MRG is also made from CIPs and non-magneticmatrix, while its matrix is high viscosity polymer [19, 37, 38].The CIPs are not easy to settle and matrix do not solidify. Sothe CIPs can move over and over. Microstructures of MRGcan be regulated continuously, rapidly, and reversibly undermagnetic field [39]. These inspired us to prepare desiredMRG-based absorbing coating.

We did the following two jobs: one is to modify CIP(type EW) by iron nano-flakes to improve electromagneticloss capacity. The flower-like CIPs prepared have both goodelectromagnetic wave loss ability and high saturation mag-netization. Only in this way can we get a large regulationrange of absorbing properties in a small magnetic field. Theother is to prepare and test the MRG-based coatings. Theabsorption of the MRG-based coatings in different magneticfields (from 0 to 300 mT) is tested by arcuate method. Themagnetic field is provided by permanent magnet. After that,the effects of magnetic field, particle concentration and matrixcuring on electromagnetic (EM) absorbing property of theMRG-based coatings were analyzed.

2. Experimental

2.1. Materials

The raw materials required were listed as follows: diphe-nylmethane diisocyanate (MDI: 4,4-≈50%, 2,4-≈50%)was offered by Yantai Wanhua Polyurethanes Co. Ltd China,while castor oil (CO) was supplied by and Sinopharm Che-mical Reagent Co. Ltd China. All chemical reagents wereanalytically pure and used as received.

CIP: soft magnetic carbonyl iron particles (CIP: typeEW) were provided by BASF Corporation, Germany. Thesize is d50=6.5 μm. Sodium borohydride (NaBH4), Ferroussulfate heptahydrate (FeSO4·7H2O), Polyvinylpyrrolidone(PVP), hydrochloric acid (HCl) and absolute ethyl alcoholwere obtained from Kemiou (Tianjin, China).

Table 1. Composition of the MRG-based absorbing coatings.

Samples PU 40 wt%

Grouping Number mCO/mMDI

Content of flower-likeCIP, wt%

The first group 1 10:1 402 10:1 503 10:1 604 10:1 70

The secondgroup

5 20:1 60

6 15:1 607 20:3 60

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2.2. Preparation of EW-type flower-like CIP

For the synthesis of flower-like CIPs, Fe2+ is reduced into Fenuclei with excessive sodium borohydride solution. These Fenuclei gathers into sheet structure on the surface of the rawCIPs. The details can be seen in our previous research [21].

2.3. Preparation of MRG-based absorbing coating

A series of MRG-based coatings were synthesized to discuss themagnetron absorption performance. The preparation processcould be described as follows: the first step is to obtain PU matrix.Before the cross-linking reaction of CO and MDI, the waterinside CO was evaporated completely in drying oven. Then, theCO and MDI were dumped into a beaker to mix uniformity. Thenext step was to add the flower-like CIPs into the above mixture,following by stirring until the mixture appears well-distributed.After that, the mixture last was transferred to the drying oven forabout 2 h and the vulcanization temperature was set at 80 °C.During the whole curing process, the mixture was taken out every15min and was stirred for 2min with the aim of particles uniformdispersion. Finally, the steady MRG-based coatings came intobeing after placed several days at room temperature. In thispaper, seven samples were prepared through this method, and thespecific composition of them were listed in table 1. The fabri-cation process is shown schematically in figure 1.

2.4. Establishment of magnetron absorbing coating testingdevice

Because MRG is viscoplastic fluid, arcuate method is selectedinstead of T/R to measure the absorbing performance. With the

Figure 1. MRG absorbing coating. (a) Preparation process of MRG absorbing coating. (b) Photograph of MRG with and without magneticfield; (c) Internal microstructure with and without magnetic field.

Figure 2. Photograph of magnetron absorbing coating testing device.

Figure 3. Hysteresis loop of the EW-type flower-like CIP.

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purpose to test absorption performance of the MRG-basedcoatings under different magnetic field intensity, a device thatregulates the magnetic field is essential. Thus, based on arcuatetest system, the device (figure 2) that contains permanent magnet,frame, lifting table was built. The device is supposed to providevariable magnetic field for these MRG-based coatings. The sizeof the permanent magnet is 210*210*30mm. The liftingtable can adjust the distance between the magnet and sample tovary the magnetic field intensity. To the best of our knowledge,calibrating the magnetic field should be done before testing. Anacrylic cavity frame, whose cavity size is 210*210*2mm, isplaced on the top of the frame, and the cavity is unloaded withMRG absorbing coating during calibration. Then the PF-035 typedigital tesla meter serves to measure the magnetic field at thebottom of the cavity. Record the position of the upper surface ofthe lifting table when the magnetic field at the bottom of thecavity frame is 0, 50, 100, 150, 200 and 300mT.

2.5. Characterization

The morphology and size distribution of flower-like CIPs werecaptured under a JEOL JSM-7800F field-emission scanningelectron microscope (FE-SEM) at an acceleration voltage of30.0 kV. The magnetic performance (M−H curve) of the

flower-like CIPs and MRG-based coatings were characterizedat room temperature using a vibrating sample magnetometer(VSM Lake Shore 7407). The viscosity of the MRG-basedcoatings was characterized by an advanced commercialrheometer (Modal: MCR301, Anton Paar). The rheometerworks in the mode of rotational shear, and the test temperatureis at 25 °C. The test gap is 1 mm. The shear rate is 10 s−1.Scanning range of magnetic field is 0–1 T. The scanningelectron microscope (SEM) images of MRG were recordedafter gold sputtering treated. However, it could not exertmagnetic field on SEM in the detection process, that is, it isimpossible to observe the particle distribution of MRG-basedabsorbing coating under magnetic field. For this reason, put anappropriate amount of MRG-based absorbing coating betweentwo glass slides. The distribution of particles in the MRGabsorbing coating before and after the application of themagnetic field were further observed by the optical digitalmicroscope (KEYENCE, VHX-600E). The absorption char-acteristics of these MRG-based coatings in the range of2–18GHz were measured using bow method with a vectornetwork analyzer. Using the reflection loss RL (RL=101gΓp,dB) to express the performance of absorbing electromagneticwaves. Here, Γp is power reflectivity of absorbing materials.

Figure 4. (a) The microstructure image in SEM without magnetic field; (b) The photo of optical digital microscope without magnetic field;(c), (d) Typical microstructure of absorbing coatings under a magnetic field: (c) The original image; (d) The image after image segmentation.

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3. Results and discussion

3.1. Characterization of flower-like CIP

From figure 3, there are plenty of nanoflakes evenly sur-rounded on the surface of CIP. Iron nanoflakes greatly changeits surface into globular flowers. The flower-like CIP (typeEW) is a kind of soft magnetic particle with narrow hysteresisloop, small enclosing area, high permeability, easy magneti-zation and demagnetization. The value of Ms reaches193.41 emu g−1 which is nearly 25 emu g−1 higher than thatof flower-like CIP (type CN). Relatively high magnetizationproduces greater force between particles and particles, adja-cent chains will merge to form a longer and thicker columnarstructure, resulting in highly shape anisotropy of micro-structure of the MRG-based coating [40]. This is favor to geta large regulation range of absorbing property.

3.2. Characterization of MRG coatings

Figure 4(a) presents SEM image of MRG-based coatingwithout magnetic field. It is plain to see the particles in thepolyurethane matrix are relatively evenly distributed withlittle aggregation. With the aid of optical digital microscope,the distribution of particles in the absence of magnetic field isalso shown in figure 4(b). The highlight is absorber particles,

while yellow background is polyurethane matrix. The dis-tribution of particles in optical digital microscope is similar toSEM image. Once a uniform magnetic field is applied acrossthe slide on the optical digital microscope, the micro-morphology of the MRG-based absorbing coating is shown infigure 4(c). At the same time, image analysis technology is todistinguish the absorber particles from the polyurethanematrix (figure 4(d)). White is the absorber particle, and blackrepresents polyurethane substrate. From these two pictures,the absorbing particles redistribute from uniformity intochains or columnar structure under a magnetic field. Com-pared with the distribution in the zero field, the microstructurechanges from isotropic distribution to anisotropic chains. The‘phase transition’ phenomenon that is called magnetroncharacteristic appears. The dramatic change of MRG-basedcoating under magnetic field is the basis for achieving real-time and reversible magnetically controlled electromagneticwave absorption.

3.3. Microwave absorption properties

3.3.1. Magnetron absorbing properties of MRG-based coatingwith flower-like CIPs of 40 wt%, 50 wt%, 60 wt% and 70 wt%.The magnetized flower-like CIPs can be simplified asmagnetic dipoles whose magnetic dipole moments areconsistent with the applied magnetic field [41, 42]. There

Figure 5. Reflection losses (RL) curves of MRG absorbing coating with different flower-like CIPs (the quality of CO and MDI is 10:1):(a) 40 wt%; (b) 50 wt%; (c) 60 wt%; (d) 70 wt%.

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are interaction force between the magnetic dipoles [24, 43].The flower-like CIPs can be driven by interaction force toform chains or columnar structure, resulting in shapeanisotropy of microstructure of the MRG-based coating[33, 40]. The appearance of these anisotropic chainsindicates that the particles have been rearranged. Theinteraction between the particles and the particles or matrixwill change, which undoubtedly changes the physicalproperties (conductivity, complex permittivity, complexpermeability) of the MRG-based coating itself. Generally,microwave absorption properties were mainly associated withimpedance matching and loss factor [27, 44, 45]. Theimpedance matching ensures that electromagnetic waves canenter absorbing materials, while loss factor α represents losscapability. The equations are as follows [27, 46]:

=-+

( )Z Z

Z ZR 1in 0

in 0

m me e

= ( )Z 2r

rin

0

0

me

= ( )Z 300

0

where Z0 is the intrinsic impedance of free space, Zin isthe input impedance of the absorber. εr (εr=ε′−jε″) is thecomplex permittivity of MRG-based coating; ε0 is thecomplex permittivity of free space. μr (μr=μ′−jμ″) isthe complex permeability of MRG-based coating; μ0 is thecomplex permeability of free space.

Essentially, the impedance matching and loss factordepend on complex permittivity and permeability. This willultimately affect the reflection loss RL. These test resultsconfirm that the magnetic field has a great influence on themicrowave absorption performance (figures 5, 6). From theminimum RL value, the samples 1, 2 and 3 have the similartrend, with the increase of the magnetic field, the minimumRL values decrease first and then rise. According to Debyetheory, the imaginary permittivity can be defined as follows[7, 47]:

e e e w t s we = - + +¥( ) ( ) ( )/ 1 , 5s2 2

0

where εs is the static permittivity; ε∞ is the relative dielectricpermittivity at the high frequency limit; ω is the angularfrequency; τ is the polarization relaxation time; and σ is theelectrical conductivity. In the MRG-based coating, electricalconductivity σ is a highly significant factor that influences the

Figure 6.MRG absorbing coating with different flower-like CIPs: (a) Optimal reflection loss RL and (b) absorption peak frequency vary withthe magnetic field; (c) magnetic hysteresis loop; (d) viscosity varies with the magnetic field (the quality of CO and MDI is 10:1).

*ap

m e m e m e m e m e m e= - ¢ ¢ + - ¢ ¢ + ¢ + ¢( ) ( ) ( ) ( )f

c

2, 42 2

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imaginary part of permittivity ε″. When the magnetic field isapplied, isolated particles gather into chains. Furthermore,with the increase of magnetic field, there will build up muchmore chains and the growing interaction between particlesand small chains make the main chain grow thicker. It isreasonable for chains formed by CIPs to have moreopportunities to establish conductive paths at the same massfraction, thus achieving enhanced electrical conductivitycompared to the evenly distributed composites [40]. There-fore, the improved electrical conductivity mainly results in theenhancement of the imaginary permittivity of the MRG-basedcoating. A high ε″ value brings high loss factor, which is infavor of dissipation of electromagnetic waves. As a result, thevalue of reflection loss RL decreases.

It was proved that the magnetic moments lie preferen-tially in the easy planes; the anisotropy chains formed by theflower-like CIPs is ascribed to easy magnetization and resultin the enhancement of permeability [7, 48, 49]. In summary,the appropriate magnetic field makes complex permittivityand permeability increase. It is helpful to improve theimpedance matching and loss ability of absorbing materials,enhance microwave absorbing intensity. CIPs are magneticloss materials, whose complex permeability is higher than thecomplex permittivity. Nevertheless, excessive magnetic fieldsmakes impedance matching worse, which is not conducive to

absorbing electromagnetic waves. As for sample 4, duo tohigh particles’ content, the viscosity is much higher than theother samples. The particles are difficult to move. The numberand size of chains are inferior to those of other samples whenthe magnetic field is same. In other words, sample4 need alarger magnetic field to form conductive channels. At0–300 mT, the complex permittivity and permeabilityincreases slowly. It is likely to avoids the impedancematching deterioration. Thus, the RL of sample4 keepsdecrease at 0–300 mT.

The absorption peak frequency (APF) shifts according to theactual situation to absorb the target frequency bands or frequencypoints. It is an important aspect of intelligent controllability.Magnetic field can make APF move toward high frequency. Theamount of shifting to high frequency increases as the magneticfield increases. The microwave absorption induced by naturalferromagnetic resonance of magnetically anisotropic composite isstrongly affected by the applied external magnetic field and theinternal anisotropy Ha. During the chaining process, the magneticeasy axes of the chains aligned mainly parallel to the surface ofthe MRG-based coating, which increases the internal anisotropyfield evidently [35]. So the natural resonance frequencyfr ( fr=γHa/2π) moves to high frequency. Moreover, themagnetic-induced frequency offsets of the four samples

Figure 7. Reflection losses (RL) curves of MRG absorbing coating with different matrix solidification degree (the content of flower CIP is60 wt%): (a) 20:1; (b) 15:1; (c) 10:1; (d) 20:3.

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were 2.88GHz (40wt%:10.32–13.2GHz), 1.26 GHz (50wt%:10.1–11.36GHz), 1.52 GHz (60wt%:8.24–9.76GHz), and0.64GHz (70wt%:7.76–8.4 GHz), respectively. The generaltrend is that the offset of APF decreases with the particlecontent. This is mainly due to the different viscosity andmagnetization caused by flower CIP content. The movingvelocity of CIPs in a magnetic field is as follows [50]:

mh

df= ( )M

av18

, 6s0 2

where a denotes the radius of the microparticles. The gradient ofthe magnetic field and the volume fraction of the microparticlesare denoted by δ and ∅, respectively. η denotes the viscosity ofthe sample. The movement speed of the particles affects theformation of the chains. Under the low content, the viscosity andsaturation magnetization are low (figures 6(c), (d)). The particlesgradually form a chain structure under the magnetic field thatalways cause the movement of APF. Nevertheless, at the highcontent, viscosity is relatively high, which will impede themovement of particles. Thus, the APF does not change much.The viscosity of sample 4 is much higher than that of the otherthree samples. Therefore, the magnetic-induced offset frequencyof sample 4 is the smallest.

3.3.2. Magnetron absorbing properties of MRG-based coatingwith curing degree. It is known from the above analysis thatthe viscosity has a great influence on the absorptionproperties, and the curing of the polyurethane matrix is

closely connected the viscosity. Therefore, the MRGabsorbing coatings with different matrix curing are preparedby adjusting the mass ratio of CO and MDI. From figures 7, 8,the minimum RL value varies with the magnetic field. Likethe first group of samples, the peak moves to high frequencyalong with magnetic field. It can be seen from figures 7 and8(a), the minimum RL value is not very regular.Theoretically, if the flower-like CIP content is the same, thedifference of minimum RL value is not large. However, theminimum RL value of sample 6 is obviously lower than thatof other samples. The reason is that there is a thickness errorof 0.2 mm in the acrylic cavity used to hold the MRG coating,so that the minimum RL value is deviated. Fortunately, thechange trend of these samples with magnetic field has littlerelationship with the thickness of the coating.

The APF of all the samples shifts faster before 100 mTand changes slowly afterwards. Magnetic particles canrespond quickly to magnetic fields. Once the magnetic fieldapplied, the particles are arranged along the magnetic field.The microstructure of magnetorheological coating has a greatchange from homogeneous distribution to anisotropic struc-ture. This brings great frequency shift. Continue to increasemagnetic field, the movement of chain particles has beenrestricted, and the microstructure changes are not significant.The frequency changes are relatively gentle. The frequencyoffsets are 1.36 GHz, 1.52 GHz, 1.52 GHz and 1.4 GHzrespectively for the four samples whose mass ratios of COand MDI are 20:1, 15:1, 10:1 and 20:3. The magnitude of

Figure 8.MRG absorbing coating with different matrix curing: (a) Optimal reflection loss RL (b) absorption peak frequency and (c) viscosityvaries with the magnetic field; (d) frequency offset (the content of flower CIP is 60 wt%).

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magnetic-induced frequency offset is similar, which indicatesthat when the content of CIP is the same, the saturatedmagnetic field is the same, simply changing the curing degreeof the matrix has little effect on the frequency offsets. But inpractical application, the change of curing will affect thedensity, adhesion and packaging of the coating, so it isnecessary to select the best one according to the need.

4. Conclusion

In order to achieve real-time and reversible control ofabsorbing performance, the intelligent EW absorption ofMRG coating has been explored preliminarily in this paper.Microwave absorption of magnetically anisotropic compositeare strongly affected by the applied external magnetic field.The appropriate magnetic field is helpful to improve theimpedance matching and loss ability of MRG-based coating,enhance microwave absorbing intensity. What’s more, duringthe reorientation process, the particles aligned mainly parallelto the surface of the sample, which increases the internalanisotropy field evidently. So the natural resonance frequencyfr ( fr=γHa/2π) move toward high frequency. This suggeststhat MRGs are magnetic-field-sensitive electromagneticwave-absorbing material and they can adjust the microwave-absorption peak as needed. At the same time, suitable particleconcentration and matrix curing can bring large magnetic-induced frequency range. Consequently, this paper offers apromising strategy to design novel intelligent absorbingcoating based on MRG for electromagnetic protection andantiradar camouflage. What’s more, MRG absorbing coatingreported here expected to be useful in future investigations onthe controllability of MRG electromagnetic properties.

Acknowledgments

The financial support from the National Natural Science Foun-dation of China (Grant No. 51775064) is highly acknowledged.

ORCID iDs

Miao Yu https://orcid.org/0000-0002-8547-6432Jie Fu https://orcid.org/0000-0002-7297-6729

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