Synthesis of carbon-coated cobalt ferrite core-shell structure composite:A method for enhancing electromagnetic wave absorption properties by adjusting impedance matching

Jing Gao ,Zhijun Ma,*,Fuli Liu ,Cunxin Chen

1 School of Mining,Liaoning Technical University,Fuxin 123000,China

2 School of Chemical and Environmental Engineering,China University of Mining and Technology,Beijing 100083,China

Keywords:Cobalt ferrite Core-shell structure composite Electromagnetic wave absorption Impedance matching

ABSTRACT Cobalt ferrite has problems such as poor impedance matching and high density,which results in unsatisfactory electromagnetic wave (EMW) absorption performance.In this study,the CoFe2O4@C core-shell structure composite was synthesized by a two-step hydrothermal method.X-ray diffraction,transmission electron microscopy,Fourier transform infrared spectroscopy,thermogravimetric analysis,and vector network analysis et al.were used to test the structure and EMW absorption properties of CoFe2O4@C composite.The results show that the reflection loss (RL) of the CoFe2O4@C composite reaches the maximum value of -25.66 dB at 13.92 GHz,and the effective absorbing band (EAB) is 4.59 GHz(11.20-15.79 GHz) when the carbon mass content is 6.01%.The RL and EAB of CoFe2O4@C composite are increased by 219.55% and 4.59 GHz respectively,and the density is decreased by 20.78% compared with the cobalt ferrite.Such enhanced EMW absorption properties of CoFe2O4@C composite are attributed to the attenuation caused by the strong natural resonance of the cobalt ferrite,moreover,the carbon coating layer adjusts the impedance matching of the composite,and the introduced dipole polarization and interface polarization can cause multiple Debye relaxation processes.

1.Introduction

The electromagnetic radiation and interference produced by various electronic and electrical equipment have become a new pollution source with great harm and difficulty to protect.It not only affects normal communication but also directly threatens the health of human beings.Therefore,the prevention of electromagnetic interference pollution has become a hot issue of social and scientific attention [1,2].Ferrite is widely used in information storage systems,wireless communications,and microwave equipment,which has excellent electromagnetic properties,high resistivity,and permeability,excellent mechanical and chemical stability [3].Ferrite is an excellent electromagnetic wave (EMW)absorbing material due to its simple preparation method and low cost.Yousefiet al.prepared Co0.8Zn0.2Fe2O4by the combustion method using Fe(NO3)3,Zn(NO3)2,and Co(NO3)2as raw materials and found that the saturation magnetization and a remanence magnetization of annealing sample at room temperature were 37 and 14 emu·g-1,respectively[4].Sunet al.prepared MgxCo1-xFe2-O4by sol-gel method,and the obtained ferrite saturation magnetization reached 52.98 emu·g-1[5].Knyazevet al.used solid-phase reaction to calcinate Ni0.5Zn0.3Co0.2Fe2O4at 900 °C,and the obtained ferrite saturation magnetization reached 57.10 emu·g-1[6].Nevertheless,ferrite has the problems of poor impedance matching,a narrow effective absorbing band (EAB),and high density.At present,the application of ferrite EMW absorbing materials is mainly concentrated in the high-frequency range,and the EMW absorption performance is not ideal.

Composites can integrate the best properties of individual components,and the interaction between each component can make it have some special functions [7,8].Carbon materials have a low density and a certain concentration of carriers,and most of the carbon atoms are in the form of sp2hybridization,which can more effectively promote the conductivity of the oxide monomer so that the material has a larger dielectric constant [9-11].Therefore,the effective combination of ferrite and carbon materials can theoretically prepare composite absorbing materials with excellent EMW absorption ability and wide EAB [12-14].Liet al.synthesized porous Fe3O4/C microspheres by hydrothermal method and adjusted the absorbing characteristics of the Fe3O4/C through changing the carbon content,and the EAB was widened to 7.76-12.88 GHz[15].Liet al.prepared CoFe2O4@CNFS composite through electrospinning and calcination technology,which reached the maximum reflection loss(RL)value of-14.0 dB at 3.6 GHz[16].Ahangari prepared Ni0.6Zn0.4Fe2O4/MWNT composite by inverse coprecipitation method,whose saturation magnetization reached 15.0 emu·g-1[17].

In order to further improve the EMW absorption performance of cobalt ferrite and reduce the density,the CoFe2O4@C core-shell structure composite was prepared by a two-step hydrothermal method in this study.The crystal structure,microscopic morphology,electromagnetic parameters,and EMW absorption properties of the composite were studied to determine the optimal carbon coating amount.Cobalt ferrite coated with amorphous carbon can reduce the density and obtain excellent EMW absorption performance at the same time.This paper has a positive meaning for EMW absorbing material.

2.Experimental

2.1.Reagents

The reagents used in this experiment included hexadecyl trimethyl ammonium bromide(C19H42BrH),iron chloride(FeCl3·6H2-O),cobalt acetate (C4H6CoO4·6H2O),glucose (C6H12O6·H2O),sodium hydroxide (NaOH),and anhydrous ethanol (CH3CH2OH).The above reagents are all analytical grades produced by Sinopharm Group Co.,Ltd.(China).

2.2.Synthesis

1 g hexadecyl trimethyl ammonium bromide was dissolved in 70 ml deionized water and treated with ultrasonic for 30 min.A certain amount of Co(NO3)2·6H2O and FeCl3·6H2O were added into the above solution according to the molar ratio ofn(Fe):n(Co)=2:1,and ultrasonic treatment was carried out for 30 min to make the solution uniformly dispersed.Then the mixed solution was transferred to a constant temperature water bath at 40°C,the pH value of the solution was adjusted to 11 with 2 mol·L-1NaOH,and the reaction precursor solution was obtained after stirring for 30 min.After aging at room temperature for 8 h,the lower layer precursor was put into a high-pressure reactor of 100 ml,and the crystallization reaction was carried out in an oven at 180 °C for 8 h.After the reaction kettle cooled naturally,the product was washed with anhydrous ethanol until the pH of the solution was neutral.The filter cake was dried in a 60 °C oven for 12 h to obtain cobalt ferrite after solid-liquid separation.

1 g cobalt ferrite was put into a mixture solution of 50 ml water and ethanol(4:1),then 5 g glucose was added into the above solution,and the solution was fully dissolved by ultrasound for 20 min.After stirring for 10 h in a constant temperature water bath at 30 °C,it was put into a high-pressure reactor with a volume of 100 ml and reacted in an oven at 180 °C for 8 h.The filter cake was dried in a 60°C drying oven for 12 h after solid-liquid separation.The reaction product was calcined in a tubular furnace with nitrogen atmosphere at 500 °C for 3 h and then cooled in the furnace (heating rate was 5 °C·min-1).Three kinds of samples were prepared by increasing the amount of glucose by a multiple,and they were named CoFe2O4@C-1,CoFe2O4@C-2,and CoFe2O4@C-3,respectively,and its preparation flowchart is shown in Fig.1.

Fig.1.Preparation flowchart of CoFe2O4@C composites.

2.3.Characterization

The phase compositions of the samples were determined by an X-ray diffractometer (XRD,Empyrean,Panalytical,Netherlands).The measurement conditions are Cu Kα wavelength of 0.15406 nm,40 kV voltage,tube current of 40 mA,scanning speed of 10(°)·min-1,and scanning range of 5°-80°.The morphology was analyzed by transmission electron microscope (TEM,JEM-2010,JEOL,Japan) and scanning electron microscope (SEM,JSM-7610F,JEOL,Japan).The functional groups of the samples were identified by Fourier transform infrared spectrometer (FTIR,NICOLET 50,Thermo Scientific,America).The carbon structure was analyzed by Raman spectroscopy (Raman,inVia,Renishaw,England).The relationship between sample mass and the temperature was measured by a thermogravimetric analyzer (TGA,TG-DTA6300,SEIKO,Japan).The elemental information of the samples was analyzed by X-ray photoelectron spectroscopy(XPS,K-alpha,Thermo Scientific,America).

The electromagnetic parameters between 1 and 18 GHz of samples were tested using a vector network analyzer(VNA,HP8722ES,Agilent,America).The samples were mixed with paraffin(the mass ratio was 6:4) and then pressed into a toroid with an outer diameter of 7.0 mm and an inner diameter of 3.0 mm.Based on the electromagnetic parameters of absorbing materials (μr,εr),RL (dB)value can be calculated by Eqs.(1) and (2) according to the transmission line theory [18,19].

whereZ0is the impedance of free space(approximately 376.73 Ω),Zinanddare the input impedance and thickness of the absorber,respectively,εrand μrare the relative complex permittivity and complex permeability,fandcare the frequency and velocity of electromagnetic waves in free space,respectively.

3.Results and Discussion

3.1.The microstructure of CoFe2O4@C composites

3.1.1.XRD analysis

The XRD pattern of carbon hydrothermally synthesized with glucose under the same conditions without the addition of cobalt ferrite is shown in Fig.2(a).It can be found that there is only a wide dispersion peak and no sharp diffraction peak at 2θ=20°,indicating that the carbon prepared is an amorphous structure.The Raman spectrum of carbon is shown in the inset of Fig.2(a),and two distinct peaks are observed at approximately 1345 cm-1(D band)and 1595 cm-1(G band),which also reveals the carbon composition prepared.

Fig.2.(a) XRD pattern and Raman spectra of carbon;(b) XRD patterns of cobalt ferrite and CoFe2O4@C composites.

The XRD patterns of cobalt ferrite and CoFe2O4@C composites are shown in Fig.2(b).It can be found that the diffraction peak positions of cobalt ferrite correspond to the crystal planes (111),(220),(311),(222),(400),(422),(511),and(440),which are consistent with the spinel structure (JCPDS No.22-1086),indicating that pure phase cobalt ferrite was prepared.The cobalt ferrite in CoFe2-O4@C is decarburized at 500°C under air conditioning for 4 h,and it is found that the diffraction peak position is completely consistent with that of cobalt ferrite,which indicates that the amorphous carbon coating preparation process has no obvious influence on the crystal plane structure of cobalt ferrite.However,the diffraction peak intensity of cobalt ferrite decreases after amorphous carbon coating,and the structural parameters are shown in Table 1.

Table 1 Structure parameters and RL performance of cobalt ferrite and CoFe2O4@C composites

3.1.2.FTIR analysis

FTIR spectra of cobalt ferrite and CoFe2O4@C composites are shown in Fig.3.It can be seen from Fig.3(a) that the peak near 600 cm-1corresponds to the Fe-O of spinel ferrite,and the peak near 3430 cm-1corresponds to the attached water on the surface of cobalt ferrite [20-22].As we can see from Fig.3(b),(c),(d),the peaks near 1062,1125,1662,and 2929 cm-1represent the stretching vibrations of C-OH,-OH,C=C,and C-H,respectively [23,24].These results indicate that CoFe2O4@C composites retain abundant functional groups formed in the hydrothermal carbonization process of glucose,which provides a possibility for the multiple relaxation processes of CoFe2O4@C composites.

Fig.3.FTIR spectra of (a) cobalt ferrite,(b) CoFe2O4@C-1,(c) CoFe2O4@C-2,and (d)CoFe2O4@C-3.

3.1.3.XPS analysis

From the full spectrum of XPS (Fig.4(e)),it can be found that CoFe2O4@C composites have obvious spectral peaks around the binding energy of 284,529,710,and 780 eV,which correspond to the peaks of C 1s,O 1s,Fe 2p,and Co 2p,respectively.Fig.4(a)shows the fine XPS spectrum of Co 2p,and the binding energy of 780.29 and 795.68 eV correspond to the peak of Co 2p3/2and Co 2p1/2respectively,and there are two strong satellite peaks at 785.78 and 802.48 eV,which indicates that Co exists in the form of Co2+oxidation state [25].The asymmetrical shape of the two main peaks of Co 2p indicates that there are many chemical states of Co2+in cobalt ferrite.According to Lorentzian-Gaussian curve fitting method,the main peak of Co 2p3/2was divided into two peaks,and it was found that the two sub-peaks of Co 2p3/2divided peak fitting were located at 779.91 and 781.83 eV,which correspond to octahedral position (OP) and tetrahedral position (TP) in spinel-type cobalt ferrite,respectively[26].The relative percentage of the molar number of Co2+in OP and TP were calculated by the sensitivity factor method as 48.52% and 51.48%,respectively.

Fig.4(b)shows the fine XPS spectrum of Fe 2p,and the binding energy of 710.45 and 724.30 eV correspond to Fe 2p3/2and Fe 2p1/2respectively,and there are two strong satellite peaks at 718.3 and 733.36 eV,which indicates that Fe exists in the form of Fe3+oxidation state [27].The peak-splitting fitting results of Fe 2p3/2show that the two sub-peaks at 710.48 and 712.36 eV correspond to OP and TP,and the relative percentage of a molar number is 53.69% and 46.31%,respectively.In the fine XPS spectrum of O 1s(Fig.4(c)),the peaks located at 530.03,531.50,and 533.10 eV correspond to metal oxides(lattice oxygen),oxygen vacancy,and chemisorbed oxygen(O-H)respectively[28].In the fine XPS spectrum of C 1s (Fig.4(d)),the peaks located at 284.76,285.84 and 288.57 eV correspond to C-C,C-O-C,and O-C=O,respectively,which are consistent with the results of FTIR analysis.

Fig.4.XPS spectra of CoFe2O4@C composites:(a)-(d) fine spectrum,(e) full spectrum.

3.1.4.TEM and SEM analysis

TEM images of cobalt ferrite and CoFe2O4@C composites are shown in Fig.5 respectively.The particle sizes of cobalt ferrite and CoFe2O4@C samples are nanometers in the range of 30-50 nm.Cobalt ferrite is mainly spherical and irregular quadrilateral,and there is a certain degree of agglomeration phenomenon(Fig.5(a)).We can find a clear core-shell structure from the TEM images of the CoFe2O4@C composites,and the thickness of the coating increases gradually with the increase of glucose content(Fig.5(b),(c),(d)).Fig.5(e) and (f) are SEM images of CoFe2O4@C composite and the element distribution on the sample surface,respectively.It can be found that the surface of CoFe2O4@C composite is mainly carbon element,indicating that amorphous carbon successfully coated cobalt ferrite.

Fig.5.TEM images of (a) cobalt ferrite,(b) CoFe2O4@C-1,(c) CoFe2O4@C-2,(d) CoFe2O4@C-3;(e) SEM image of CoFe2O4@C composite;(f) EDS of CoFe2O4@C composite.

3.1.5.TGA analysis

In order to estimate the content of amorphous carbon in CoFe2-O4@C composites,TGA was carried out under air conditions.It can be seen from Fig.6 that the TGA curves of all samples are basically the same,and there are two weightlessness processes.The first weightlessness occurred before 230 °C due to the loss of hydroxyl or adsorbed water on the surface of the CoFe2O4@C composites,which accounted for about 2.53% of the mass.With the further increase of temperature,the second weightlessness occurred between 230 and 400 °C,and this stage is mainly caused by the thermal decomposition of amorphous carbon in CoFe2O4@C composites.According to the TGA curves,the amorphous carbon contents of the composites CoFe2O4@C-1,CoFe2O4@C-2,and CoFe2O4@C-3 are 3.15%,6.01%,and 9.22%,respectively.It is worth mentioning that the density of cobalt ferrite and CoFe2O4@C was measured using a hydrometer,and it was found that the density of cobalt ferrite was reduced by 24.69% at the highest after amorphous carbon coating (Table 1).

Fig.6.TGA curve of CoFe2O4@C composites.

3.2.EMW absorption performance of CoFe2O4@C composites

The RL of cobalt ferrite and CoFe2O4@C composites were calculated according to the method mentioned in 2.3.The EAB represents the width of the frequency range where the RL is below-10 dB,the reason is that the RL below-10 dB is generally considered to mean over 90% of EMW absorption [29].The RL of cobalt ferrite and CoFe2O4@C composites in the frequency range of 1-18 GHz at different absorption layer thicknesses are shown in Fig.7.The maximum RL of cobalt ferrite at the absorption layer thickness of 2.5 mm and the frequency of 13.50 GHz is -8.03 dB,and the EAB is 0 GHz.However,the EMW absorption performance of CoFe2O4@C composites is greatly improved by the amorphous carbon coating.The maximum RL of CoFe2O4@C-1 at the absorption layer thickness of 2.5 mm and the frequency of 11.88 GHz is-13.63 dB,and the EAB is 2.55 GHz (10.69-13.24 GHz).The maximum RL of CoFe2O4@C-2 at the absorption layer thickness of 2.0 mm and the frequency of 13.92 GHz is -25.66 dB,and the EAB is 4.59 GHz (11.20-15.79 GHz).The maximum RL of CoFe2-O4@C-3 at the absorption layer thickness of 1.5 mm and the frequency of 18.00 GHz is -22.68 dB,and the EAB is 2.04 GHz(15.96-18.00 GHz).In summary,the maximum RL and EAB of CoFe2O4@C-2 composites are increased by 219.55% and 4.59 GHz respectively,and the density is decreased by 20.78% compared with cobalt ferrite.Table 2 summarizes the related articles on RL and EAB of carbon-based ferrite composites,and it can be found that CoFe2O4@C composites have certain advantages.

Table 2 Comparison for EMW absorption performances of typical carbon-based ferrite composites

3.3.Electromagnetic parameters and EMW absorption mechanism of CoFe2O4@C composites

3.3.1.Electromagnetic parameters of CoFe2O4@C composites

The important parameters of the interaction between electromagnetic wave and medium are the complex permittivity (εr=-ε′-jε′′) and permeability (μr=μ′-jμ′′),and their real and imaginary parts represent the storage and loss capacity of electromagnetic energy,respectively [35].The electromagnetic parameters of cobalt ferrite and CoFe2O4@C composites at 1-18 GHz are shown in Fig.8.It can be seen from Fig.8(a) that the real part of permittivity (ε′) of CoFe2O4is between 4.3 and 3.95,and there is no significant change in the whole frequency.However,the ε′of CoFe2O4@C composites increases with the increase of amorphous carbon coating amount.The ε′of the three kinds of CoFe2O4@C composites shows a downward trend with the increase of frequency,with the initial value of 8.22,9.11,12.66,and the termination value of 6.89,8.60,8.86,respectively,which indicates that the coating of amorphous carbon causes the typical frequency dispersion behavior of CoFe2O4@C composites,and this phenomenon is beneficial to enhance the EMW absorption performance [36].

It can be seen from Fig.8(b) that the imaginary part of permittivity (ε′′) of cobalt ferrite changes from 0.12 to 0.49,and the ε′′of CoFe2O4@C composites are higher than that of cobalt ferrite.The ε′′is inversely proportional to ρ according to the free-electron theory Eq.(3)[37].Carbon is a good conductor,and its resistivity is lower than that of cobalt ferrite,so as the amount of amorphous carbon coating increases,the ε′′of CoFe2O4@C composites gradually increases [38].The ε′′curves of CoFe2O4@C composites are similar to that of the ε′with the increase of frequency,which indicates that the complex permittivity of cobalt ferrite is increased by the coating of amorphous carbon.

where ε0is the dielectric constant of the vacuum,ρ is the resistivity,andfis the frequency of the electromagnetic wave.

It can be seen from Fig.8(c)and(d)that the real part of permeability (μ′) of cobalt ferrite decreases from 1.16 to 1.05,and the imaginary part of permeability (μ′′) increases from 0.07 to 0.10 within the frequency range of 1-18 GHz.The μ′of CoFe2O4@C composites are all lower than that of cobalt ferrite,and the μ′′are lower than cobalt ferrite in the range of 13-18 GHz,which indicates that the coating of amorphous carbon has a certain decrease in the permeability of cobalt ferrite.

The dielectric loss tangent(tanδe=ε′′/ε′)and magnetic loss tangent (tanδm=μ′′/μ′) are used to characterize the dielectric and magnetic loss capacities [39].It can be seen from Fig.8(e) and (f)that the tanδeof cobalt ferrite is lower than that of CoFe2O4@C composites,because amorphous carbon and cobalt ferrite belong to two heterogeneous media,and the components on both sides of the interface have different polarity and electrical conductivity,which can improve the interface polarization [40].The tanδeof CoFe2O4@C composites increase gradually and tanδmdecrease in the range of 13-18 GHz with the increase of amorphous carbon coating amount,which is consistent with the analysis results of complex permittivity and permeability.The tanδeof the CoFe2O4@C composites are greater than tanδm,which indicates that the dielectric loss of the composites plays an important role in improving the EMW absorption performance.

3.3.2.Debye polarization relaxation

The dielectric loss of the absorbing materials can be further analyzed by Debye polarization relaxation.The complex permittivity of the material can be expressed by Eq.(4).The expressions of ε′and ε′′can be deduced from Eq.(4)as Eqs.(5)and(6),respectively,thus the relationship between ε′and ε′′can be determined(Eq.(7))[41].

where τ is the relaxation time,εsand ε∞are the electrostatic permittivity and the permittivity of infinite frequency,respectively.

It can be seen from Eq.(7) that the ε′and ε′′curves represent a semicircle,usually defined as a Cole-Cole semicircle,and each semicircle corresponds to a Debye relaxation process.The dielectric Cole-Cole semicircle diagrams of CoFe2O4@C composites are shown in Fig.9.Obviously,cobalt ferrite coated with amorphous carbon brings about dielectric relaxation process.In comparison,the CoFe2O4@C-2 has much more different-sized semicircles than those of others,indicating that there is more than one dielectric relaxation process.Debye polarization relaxation includes ionic polarization,interfacial polarization,electron polarization,and dipole polarization.In general,ionic polarization and electronic polarization occur in the high-frequency band (103-106GHz).Therefore,dipole polarization and interfacial polarization are the main causes of multiple relaxation processes of CoFe2O4@C composites.The heterogeneous interface between cobalt ferrite and amorphous carbon,and structural defects caused by functional groups of amorphous carbon act as dipole centers are the reasons for the multiple relaxation processes.The multiple relaxation processes contribute to improving the EMW absorption performance[42].

Fig.7.RL and EAB curves of (a) cobalt ferrite,(b) CoFe2O4@C-1,(c) CoFe2O4@C-2,and (d) CoFe2O4@C-3 with different frequency and thickness.

3.3.3.Impedance matching

The peak RL value of CoFe2O4@C composites moves to the lowfrequency direction with the increase of the absorption layer thickness,which is consistent with the quarter-wavelength matching theory,as shown by Eq.(8) [43].In addition,when the matching thickness of the sample satisfies Eq.(8),the EMW will be reflectedfrom different interfaces,thus attenuating the EMW energy.The quarter-wavelength curves showed in Fig.10 indicating that the experimental matching thickness and peak frequency are in good agreement with the simulation results.

Fig.8.Electromagnetic parameters of cobalt ferrite and CoFe2O4@C composites:(a) real part of permittivity (ε′),(b) imaginary part of permittivity (ε′′),(c) real part of permeability (μ′),(d) imaginary part of permeability (μ′′),(e) dielectric loss tangents (tanδe),and (f) magnetic loss tangents (tanδm).

Fig.9.The dielectric Cole-Cole semicircle diagrams of (a) CoFe2O4@C-1,(b) CoFe2O4@C-2,and (c) CoFe2O4@C-3.

wheretmis the matching thickness,andfmis the frequency of maximum RL peak.

Impedance matching (Z) is an important parameter affecting the absorption performance of CoFe2O4@C composites,which can be expressed by Eq.(9).The EMW absorbing material can obtain better impedance matching characteristics when the value ofZis close to 1,and the calculation results of impedance matching (Z)are shown in Fig.10.

The amorphous carbon with high conductivity can form a conductive network in CoFe2O4@C composites,which improves the electrical conductivity of the composites.Nevertheless,if the complex permittivity of the material is much higher than the complex permeability,most of the incident EMW will be reflected from the material surface because of the high surface resistance,which will result in strong reflection and weak absorption ability of the EMW,thus weakening the impedance matching of the material [44].As can be seen from the impedance matching in Fig.10,the impedance valueZof CoFe2O4@C-2 is close to 1 at different thicknesses,which means that the incident EMW can be completely absorbed by the composites.This result explains the higher RL value of CoFe2O4@C-2,and this is one of the reasons why CoFe2O4@C-2 has a strong EMW absorption performance.

3.3.4.EMW attenuation

The magnetic loss of EMW absorbing materials mainly comes from natural resonance,eddy current loss,and domain wall resonance,but the domain wall resonance usually occurs at 1-100 MHz.The influence of eddy current resonance and natural resonance on the magnetic loss of CoFe2O4@C composites can be reflected by theC0value of Eq.(10).

It can be seen from Fig.11(a)that theC0values of cobalt ferrite and CoFe2O4@C composites vary greatly at the frequency of 1-6 GHz,indicating that the magnetic loss in this frequency range is caused by natural resonance.In addition,the fluctuation range ofC0is small and the value tends to be constant at the frequency of 6-18 GHz,indicating that the magnetic loss in this frequency range is caused by eddy current loss.

The excellent EMW absorption properties of CoFe2O4@C composites are mainly attributed to the synergy between impedance matching and electromagnetic attenuation [45].The attenuation constant (α) can be expressed by Eq.(11) and the calculation results are shown in Fig.11(b).It can be found that the α of CoFe2-O4@C core-shell structure composites formed by cobalt ferrite coated with amorphous carbon are significantly increased in the range of 1-18 GHz.However,the excessive amount of amorphous carbon coating in CoFe2O4@C-3 leads to the reduction of the imaginary part of permeability of the composites,which makes the impedance matching of the CoFe2O4@C-3 weaker than that of CoFe2O4@C-2,and the attenuation constant is reduced.

Fig.10.Reflection loss,the relationship between simulate thickness and peak frequency,and impedance matching curves of (a) cobalt ferrite,(b) CoFe2O4@C-1,(c)CoFe2O4@C-2,and (d) CoFe2O4@C-3.

The coating of amorphous carbon can increase the attenuation constant (α) of cobalt ferrite.The excellent attenuation capability of EMW strongly depends on the effective complementarity between dielectric loss and magnetic loss[46].Amorphous carbon enhances the dielectric loss of cobalt ferrite,which changes the single loss mechanism of cobalt ferrite into a multiple loss mechanism,thus improving the absorption capacity of EMW.On the one hand,amorphous carbon coating can improve the impedance matching of cobalt ferrite,allowing more EMW to enter the composites.On the other hand,amorphous carbon coating can improve the EMW attenuation ability of cobalt ferrite,so that the EMW entering the material can be more attenuated.CoFe2O4@C-2 achieves the best balance between the impedance matching and electromagnetic attenuation,thereby maximizing the effective absorption of EMW.

Fig.11.The C0 values (a) and attenuation constant α (b) curves of cobalt ferrite and CoFe2O4@C composites.

4.Conclusions

The CoFe2O4@C composite with core-shell structure was prepared by a two-step hydrothermal method.The results show that cobalt ferrite coated with amorphous carbon can enhance the dielectric loss and obtain excellent EMW absorption performance through good impedance matching.In addition,the strong natural resonance of the cobalt ferrite,and the dipole polarization and interface polarization introduced by amorphous carbon can cause multiple Debye relaxation processes,which can effectively attenuate EMW.The RL of the CoFe2O4@C composite reaches the maximum value of -25.66 dB at 13.92 GHz,and the EAB is 4.59 GHz(11.20-15.79 GHz) when the mass amount of amorphous carbon coating is 6.01%.The RL value and EAB of CoFe2O4@C composites are increased by 219.55% and 4.59 GHz respectively,and the density is decreased by 20.78% compared with cobalt ferrite.These results indicate that CoFe2O4@C composite can be used as a lightweight EMW absorbing material.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (51372108).