Jingpeng Chen, Ge Song, Zhuo Liu, Lijing Xie, Shoushun Zhang,*, Chengmeng Chen,3,*

1 CAS Key Laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China

2 University of Chinese Academy of Sciences, Beijing 100049, China

3 Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China

Keywords:Flower-like NiO/SiCw Dielectric loss Interfacial polarization Microwave absorption

ABSTRACT Toward the increasingly serious problem of electromagnetic wave pollution,the development of absorbing material with the properties of the light,thin,wide,strong,and multiple applications scenarios is still a huge challenge.Herein,the core-shell nickel oxide/silicon carbide whiskers(NiO/SiCw)with variational NiO morphologies (tulle-, flower, and rod-like) were designed and fabricated via a facile prehydrothermal method and post-annealing process.For the NiO shell morphology, it can effectively be controlled by the proportion of Ni(NO3)2·6H2O,NH4Cl,and CO(NH2)2.The structure of NiO/SiCw samples was investigated by XRD,SEM,XPS,TEM,and N2 absorption-desorption.Compared to other morphologies of NiO, the flower-like NiO/SiCw possess a porous structure and large surface area that can benefit from the multiple reflections and attenuation of the microwave.As an absorber, the composite with the flower-like NiO/SiCw filler loading of 50% manifests a superior microwave attenuation capability due to its special porous structure,good impedance matching, and large dielectric loss induced from heterojunction interfacial polarization.The minimum reflection loss of flower-like NiO/SiCw is up to -56.8 dB with a thickness of 1.9 mm, and the maximum effective absorption bandwidth (EAB) reaches 5.17 GHz.The as-prepared flower-like NiO/SiCw with strong absorption, thin thickness, and width EAB can meet the potential requirements for microwave absorption materials under oxidation environments.

1.Introduction

The research and development of microwave absorption materials with the advantage of lightweight,thin thickness,wide bandwidth, and strong absorption are increasingly concerned owing to the serious electromagnetic wave pollution arising from the electric devices in the wireless communication, industrial application,and military fields [1-4].Thus, abundant microwave absorbing materials involving novel structure and synthesis methods were explored [5-7].Carbon-based materials (graphene, carbon fiber,SiC, and carbon tube) are extensively applied in the microwave absorption field due to their low density, chemical stability, tunable dielectric property, and diverse forms [8-12].

Among the above materials,SiC is considered an ideal candidate material for absorbing electromagnetic waves [13,14].Especially,the one-dimensional (1D) SiC whiskers/nanowires/nanorods present an antenna-like structure in the hybrid composites, which benefits the attenuation of the microwave.Meanwhile, excellent chemical inertness and ablation resistance properties of SiC are more suitable for absorbing-materials that applied in some harsh environments (high-temperature and marine environments) [15-17].Unfortunately, the single absorber usually cannot meet the requirements of thin thickness, light-weight, broadband absorption,and strong attenuation.Thus,various efficient strategies were made to prompt the microwave absorption performance, such as doping and surface modification [18-20].Dong et al.[21]found that the SiC whisker’s surface modified with MnO nanoparticles achieve a good absorbing property compared with the pristine SiC whiskers.Similarly, Liang et al.[22]reported the reflection loss(RL) of SiC nanowires decorated with Fe3O4nanoparticles can up to -51 dB.

Recent researches indicate a rational design of the core-shell structure of 1D SiC has a significant effect to enhance microwave absorption [23,24].Yang et al.[25]revealed that the maximum absorption peak of the core-shell SiC nanorod/Ni is almost three times that of SiC nanorods.Besides, other materials were also applied to construct the shell on the SiC surfaces, such as NiO,ZnO, and Al2O3[26-28].Noteworthy, NiO with the advantages of low cost, adjustable morphology, moderate dielectric loss, and large surface area is an effective shell material to improve the microwave absorption property[29].Moreover,NiO also possesses high Neel′temperature(523 K of bulk materials)than that of other transition metals materials, which maintain good activity at high temperatures[30].Yang et al.[31]analyzed the microwave absorption property of SiC particles covered by ring-like NiO nanoparticles in the frequency range of 8.2-12.4 GHz, 373 to 773 K, and found the reflection loss still reaches -46.9 dB at 673 K.Although the combination of NiO and SiC can be obtained an excellent microwave absorption performance, the microwave absorption property of NiO/SiC in other bands (2-8.2 GHz and 12.4-18 GHz)is still unknown.More importantly, it is unclear whether the variational morphologies of NiO covering on the SiC affect the attenuation nature of the microwave.

Herein,we have designed and fabricated the core-shell NiO/SiC whiskers(NiO/SiCw)with different NiO morphologies through the hydrothermal method and subsequent annealing process, and investigated the microwave attenuation capability of as-prepared absorbers at 2-18 GHz.By adjusting the proportion of chemical agents (Ni(NO3)2·6H2O, NH4Cl, and CO(NH2)2), the regulation and control of different NiO morphologies (tulle-, flower-, and rodlike) can be effectively achieved.Compared with other NiO morphologies covering the SiC whiskers,the flower-like NiO has a porous and high specific surface area.Meanwhile, the construction of the flower-like structure also causes a good impedance matching and large dielectric loss induced by heterojunction interface polarization.The reflection loss (RL) of flower-like NiO/SiCw reaches-56.8 dB at 15.5 GHz with a thickness of 1.9 mm and its effective absorption bandwidth is 5.17 GHz.These reasonable designs and outstanding properties provide a reference for the development of the ideal absorbing materials.

2.Experimental

2.1.Materials

SiC whiskers (Diameter: 50-300 nm; Length: 10-50 μm) were synthesized according to our previous reported works [32].Ni(NO3)2·6H2O,NH4Cl,and CO(NH2)2(analytical reagent grade)were obtained from Tianjin Shuangchuan Chemical Reagent Company,China.

2.2.Preparation of NiO/SiCw samples

Firstly, SiCw (0.05 g) was dispersed in deionized water (80 ml)and then exfoliated through ultrasonication (260 W) for 1 h to obtain uniformly SiCw solutions.Ni(NO3)2·6H2O, NH4Cl, and CO(NH2)2were injected into the SiCw solutions and magnetically stirred for 30 minutes to configure a mixed solution.Subsequently,this as-prepared solution was placed into the homogeneous reactor and reacted hydrothermally under 150°C for 3 h.After the process completes,the products were washed to obtain Ni(OH)2/SiCw samples.Finally, the as-prepared Ni(OH)2/SiCw samples were annealed at 300°C for 4 h under the air atmosphere.To obtain different NiO morphology covering on SiC whiskers, the amounts of all chemical reagents were adjusted.The dosage of the analytical reagent was depicted in Table 1 in detail.A series of NiO/SiCw samples were synthesized by the above method,which was denoted as tulle-like NiO/SiCw-1, flower-like NiO/SiCw-2, and rod-like NiO/SiCw-3,respectively.Moreover,the pure NiO as comparative samples were also prepared.

Table 1 The dosage of the analytical reagent

2.3.Materials characterization

X-Ray diffraction (XRD, Bruker D8 Advance A25) was used to analyze the crystal phase of the sample using Cu-Kα radiation(λ = 0.154 nm) with a scan range from 20° to 80° at a rate of 10(°)·min-1.The chemical composition of NiO/SiCw samples was tested through X-ray photoelectron spectra (XPS, AXIS ULTRA DLD).Field emission scanning electron microscopy (FE-SEM, JEOL JSM-7001F, Japan) equipped with energy-dispersive X-ray spectroscopy (EDX) was applied to observed the morphology at an accelerating voltage of 10 kV.The microstructure of the samples was further characterized by high-resolution transmission electron microscopy (HR-TEM, JEM-2100F, Japan) equipped with selectedarea electron diffraction (SAED).UV-vis diffuse reflectance spectroscopy measurements were carried out to get the energy band structure of the samples using a Lambda 650 spectrophotometer(Perkin Elmer Enterprise Management (Shanghai) Co., Ltd.).In addition, to understand the dielectric property, the as-prepared samples were detected by vector network analyzer (VNA,N5242A PNA-X, Agilent) in the frequency range from 2 to 18 GHz according to the coaxial method.In the process of testing,the samples need to disperse in the melted paraffin wax with a filler ratio of 50%.And then this mixture was solidified in a cylindrical container (Outer diameter: 7.00 mm, Inner diameter: 3.04 mm,and Thickness: 2.00 mm).The RL (dB) of all absorbents can be calculated based on the transmission line theory [33,34].

where Z0is the impedance of the free space, and Zinrepresents the input impedance of the absorber which can be induced by the following formula:[35]

where f,d,and c are the frequency of the microwave,the matching thickness of the absorber, and the velocity of light in free space,respectively.

3.Results and Discussion

The synthesizing process of the NiO/SiCw is displayed in Fig.1(a).Firstly, Ni2+ions are adsorbed on the surface of SiC whiskers and create small nuclei with dissociated OH-during the hydrothermal reaction process.Subsequently,as the reaction time increases,these small nuclei will grow into Ni(OH)2with different morphologies.These different morphologies of Ni(OH)2are controlled by the dissociation rate of the base, which determines the kinetics of the reaction between the nickel precursor and dissociated OH-[36].During the annealing process, the Ni(OH)2precursors undergo pyrolysis and transform into NiO nanocrystals.In summary,the Ni salt precursor goes through the following process:

During the preparation process of NiO/SiCw samples, the dosage of the analytical reagent has a crucial effect on the morphology, as shown in Fig.1(b)-(e).For example, the tulle-like NiO/SiCw-1 with a diameter of 0.1-1.5 μm was created at the low dosage of the analytical reagent.As the dosage of the analytical reagent increases, the NiO nanoplates vertically grow on the SiC whiskers surface and closely overlap each other to form the flower-like NiO/SiCw-2.From the results of SEM and TEM(Figs.S1 and S2, in Supplementary Material), the thickness ofNiO in the NiO/SiCw-2 sample was counted and its thickness is 194-413 nm.This porous flower-like structure with a certain thickness is beneficial for the multiple reflection and scattering of the microwave between whiskers [37,38].Interestingly, the excessive nickel sources will promote the densification of NiO,resulting in the formation of rod-like NiO/SiCw-3.This finding can also be found through the TEM (Fig.1(f)-(h)).Apparently,NiO is uniformly dispersed on the SiC whiskers surface with a tulle-like, flower-like, and rod-like structure.Fig.1(i) displays the HRTEM images of flower-like NiO/SiCw-2,where the crystal lattice fringes with various d-spacing values present.The d-spacings values of 0.209 and 0.24 nm in different regions of the nanosheet correspond to the (200) and (111) crystalline planes of NiO,respectively.Besides,the SAED patterns shown in Fig.1(j)exhibits the diffraction circles, which demonstrates that NiO is polycrystalline.That is,there are many grain boundaries inside NiO and this boundary is conducive to the formation of polarization[5].The element mapping of flower-like NiO/SiCw-2 was measured as shown in Fig.1(k).The carbon, silicon, nickel, and oxygen elements are uniformly distributed in the composite.Compared with the Ni and O, the elements of C and Si are ambiguous, meaning that SiC whiskers were overlaid by NiO.

As shown in Fig.2(a), the diffraction peak of pure SiC whiskers presents four peaks at 35.6°, 41.8°, 60.0°, and 71.6°, which corresponding to the (111), (200), (220), and (311) planes of β-SiC(JCPDS: 75-0254), respectively [39].For NiO/SiCw samples, the three diffraction peaks in the XRD pattern appear at 37.0°, 43.4°and 62.8°,which are assigned to the(111),(200),and(220)planes of the NiO (JCPDS:04-0835), respectively[40].This further proves the presence of NiO in the NiO/SiCw samples.It is noteworthy that a weak additional peak emerges at 33.5°, which is the typical feature of the stacking faults in SiC [41].Fig.2(b) shows the surface electronic states and chemical compositions of SiCw and NiO/SiCw samples.From the full spectra,except for the C and Si peaks,the Ni and O peaks can also be observed in NiO/SiCw samples.The intensity of Ni and O is larger than that of C and Si, implying the SiC whiskers are covered by NiO.This result is consistent with that of SEM.Furthermore, the fine spectrum analysis of Ni2p in the NiO/SiCw samples was performed (Figs.2(c) and S3).Obviously,it reveals that the peaks around 855.7, 853.8 eV (Ni 2p3/2), and 872.8 eV (Ni 2p1/2) are attributed to the characteristic peaks of Ni2+in NiO/SiCw, further indicating the formation of NiO.Fig.2(d)depicts UV-vis diffuse reflectance spectra.The gradual redshift of absorption edge presents from the SiCw to NiO/SiCw-3,indicating the bandgap of samples has been changed after NiO covers on the SiC whiskers.The corresponding energy bandgap was calculated (the specific deduction in Supplementary Material Note 1)as shown in Figs.2(e) and S4.It is found that the energy bandgap increases with the NiO content elevates, meaning that electrons activity was greatly confined and polarization from electron relaxation was reduced.Additionally, the electric conductivity was determined by a powder resistivity tester (Fig.2(f)).As the semiconductor material(NiO)covers the SiC whisker’s surface,the conductivity decrease, implying the NiO with different morphology hinders electrons hopping.

Moreover, the pore structure of the synthesized sample was investigated by the nitrogen adsorption-desorption isotherms as shown in Fig.S5.Obviously, the isotherms of NiO/SiCw present a typical type-IV hysteresis, indicating the presence of the mesoporous in NiO/SiCw samples.This pore mainly derives from the packing effects of the variational NiO morphologies.According to the calculation results of the Brunauer-Emmett-Teller (BET)method, the flower-like NiO/SiCw-2 gives rise to BET surface area of 141.09 m2·g-1and a pore volume of 31.64 m3·g-1, which is higher than other morphological NiO/SiCw samples.Such a large surface and pore volume provide convenience for dissipation and absorption of the microwave.

The dielectric and magnetic properties of as-prepared samples were investigated at 2-18 GHz (Fig.3(a)-(e)).Obviously, for SiCw and NiO/SiCw samples, the dielectric parameter values (ε′,ε′′) are much higher than that of magnetic parameter values(μ′,μ′′).Also,μ′and μ′′values are close to 1 and 0,respectively.It indicates that the magnetic property of NiO can be negligible and dielectric property is the main factor for determining the level of microwave absorption performance.For pure NiO sample, this phenomenon is also the same.Moreover, the microwave absorption capability of NiO, SiCw, and NiO/SiCw samples at the thickness range from 1.0 to 5.5 mm is presented (Fig.3a1-e1 and a2-e2).For pure SiC whiskers and NiO, the microwave absorption ability is poor.The minimum reflection loss (RL) of SiCw and NiO only are -14.3 dB and -7.80 dB, respectively.After NiO with different morphologies covers on SiC whiskers, the microwave absorption performance was enhanced.For the tulle-like NiO/SiCw-1 with few contents of NiO, the minimum RL reaches -28.9 dB at 17.9 GHz in the thickness of 1.4 mm, and its maximum effective absorption bandwidth(EAB)is 4.65 GHz at the thickness of 1.58 mm.When the NiO covering on SiCw surfaces forms the flower-like structure,it exhibits a lower RL value and a wider EAB value than NiO/SiCw-1.The RL of NiO/SiCw-2 is up to -56.8 dB at 15.5 GHz with a thickness of 1.9 mm,and its maximum EAB has 5.17 GHz in 1.94 mm.However,with the continuous increase of NiO content, the RL value of rodlike NiO/SiCw-3 decreases.The RL decreases to -24.3 dB at 8.8 GHz with a thickness of 3.54 mm, and the maximum EAB reduces to 4.56 GHz at 1.98 mm.From the above results, the coverage of NiO with variational morphologies has a significant influence on the microwave absorption performance of SiC whiskers.Especially, the flower-like NiO/SiCw-2 presents the best microwave absorption capability.

The reflection loss curves under a certain thickness of NiO,SiCw,and NiO/SiCw samples were analyzed.From Fig.3a3-e3, it can be observed that the RL peaks shift to the low frequency with the absorber thickness increasing.This can be explained by the quarter-wavelength attenuation theory[42].For the absorber with a certain thickness,when the incident microwave enters the inside of the absorber, except for part of the microwave is absorbed by the absorber, some incident microwaves would be reflected.Once the reflected microwave and the incident microwave with the same frequency are opposite in phase, interference will occur and the RL value reaches its minimum.As the thickness tmof the absorber increases, according to formula (5) [36], the corresponding frequency fmwill decrease.

Clearly,from Fig.3a3-e3,the experimental results are perfectly consistent with the calculated results.Meanwhile, it also found that the microwave absorbing materials can be modulated by the absorber’s thickness to achieve excellent absorption performance at different frequency ranges.Moreover, some similar absorbents about the SiC were compared as listed in Fig.4.Apparently, for most absorbers, the samples prepared by this work achieve high RL value and wide EAB at the thin thickness.Meanwhile,considering the oxidation resistance property of SiC whiskers and NiO,these prepared microwave absorbing materials satisfy the potential requirement in oxidation environments.

Fig.1.(a)The preparation process of NiO/SiCw samples.The SEM and low magnification TEM images of(b)SiCw and(c,f)NiO/SiCw-1(d,g)NiO/SiCw-2,and(e,h)NiO/SiCw-3.(i) The high magnification TEM images of NiO/SiCw-2.(j) The SAED pattern.(k) The corresponding EDX elemental mapping images of elements C, Si, Ni, and O in NiO/SiCw-2.

To further understand the difference in the microwave absorption properties of NiO/SiCw with different morphologies, the microwave attenuation mechanism was analyzed.Firstly, the impedance matching characterizations must be considered, which reflects whether microwaves can enter the interior of the absorbing materials [46,47].For the ideal absorber, the values of Zin/Z0is close to 1, meaning that most of the incident wave enter the absorbers and is absorbed[48].Figs.5 and S6 shows the impedance matching value of NiO,SiCw and NiO/SiCw samples under different thicknesses.The Z value of pure NiO and SiC whiskers is far less than 1, which indicates that most microwaves are reflected.This well explains why the microwave absorption capacity of NiO and SiC whiskers is poor.After the NiO covers on the SiC whiskers,the Z value is improved, which is closer to 1.0.However, for the NiO/SiCw samples with diverse morphology (tulle-like, flowerlike, and rod-like), the degree of impedance matching is different,where the flower-like NiO/SiCw-2 possesses more outstanding impedance matching property than other absorbers.The reason may be attributed to its porous flower-like structure,which is conducive to adjust the impedance of the absorber to match the impedance of free space.

Fig.2.The characterization of SiCw and NiO/SiCw samples.(a)The XRD patterns.(b)XPS full spectra.(c)Ni2p spectra of NiO/SiCw-2.(d)UV-vis diffuse reflectance spectra.(e) Energy bandwidth.(f) Electric conductivity.

Except for the impedance matching property, the attenuation characteristics of the microwave inside the absorber also need to be considered.This determines the degree to which microwave energy is converted into other forms of energy.The microwave dissipation property is highly related to the relative complex permittivity (εr=ε′+jε′′) and relative complex permeability (μr= μ′+jμ′′),where the real part of permittivity (ε′) and permeability (μ′)reflects the storage capability of electric and magnetic energy,and the imaginary part of permittivity (ε′′) and permeability (μ′′)depicts the loss capability of electric and magnetic energy [49].As shown in Fig.6(a)-(b), both the ε′and ε′′values gradually decrease as the frequency increases, revealing that the displacement current cannot catch up with the potential established in the alternating electric field[5].For the SiCw,it shows the highest ε′and ε′′values,which are 39.6-19.3 and 12.9-1.9,respectively.On the contrary, the NiO exhibits the lowest ε′and ε′′values.It indicates that SiCw possesses a large dielectric loss capacity while NiO is weak for dielectric loss.However, excessive-high or low ε′and ε′′values easily cause the surface impedance mismatch of the absorber, resulting in the reflection of the microwave and the low microwave absorption values.This conclusion can be obtained from the above-mentioned experimental data.After the NiO covers on the SiC whisker,the ε′and ε′′values of NiO/SiCw samples show a downward trend, where the ε′and ε′′values gradually decrease from NiO/SiCw-1 to NiO/SiCw-3(the NiO content increases)under the same frequency.It indicates the microwave energy storage and loss capability of NiO/SiCw samples decrease.However, this phenomenon does not imply that the dielectric loss capability of NiO/SiCw samples is reduced from NiO/SiCw-1 to NiO/SiCw-3.The dielectric loss tangent was calculated as shown in Fig.6(c).The order of the average tanδε value of all samples is in the following sequence:NiO/SiCw-2(0.34)＞NiO/SiCw-1(0.32)＞NiO/SiCw-3(0.29),suggesting that flower-like NiO/SiCw-2 has better dielectric loss property than other NiO/SiCw samples.

The dielectric loss ability is relative to the conduction loss and polarization loss [40].For the former, it has a positive correlation with the electric conductivity according to the Debye theory(≈σ/2πε0f) [50].From the results of Fig.2(f), all samples show a lower conductivity, indicating that electric conductivity loss is weak (Fig.6(d)-(g)).That is, the polarization loss determines the attenuation of the microwave.Based on the structure and morphology of NiO/SiCw samples, amounts of grain boundaries and defects present inside SiC whiskers and NiO,which benefit the generation of polarized charges and thus induce the dipole polarization.For this polarization, the formed polarized charges will directionally rotate in the alternating electromagnetic field, which would overcome the intermolecular force and results in microwave energy consumption [51].Besides, after covering NiO on the SiC whiskers,the new heterojunction interfaces between SiC whiskers and NiO appear, thus enhancing the interfacial polarization [52].For the different structures of NiO/SiCw,the formed heterojunction interfaces are different.From TEM and SEM images,the content of NiO in tulle-like NiO/SiCw-1 is little, which means the number of interfaces from tulle-like NiO/SiCw-1 is lower.As the NiO content on the SiCw surface increases,especially for rod-like NiO/SiCw-3,a partial separation occurs between the shell and the SiC whiskers,as shown in Figs.1(h)and S7.Consequently,the interface polarization loss induced by rod-like NiO/SiCw-3 is reduced.In terms of flower-like NiO/SiCw-2, the flower-like NiO can easily prompt abundant heterojunction interfaces,thus NiO/SiCw-2 shows a large dielectric loss than other absorbers.

The microwave absorption mechanism of NiO/SiCw samples was investigated as depicted in Fig.7.Firstly, when the incident microwave radiates to the surface of absorbing materials, the microwave will generate three phenomena: reflection, transmission,and absorption of waves.Varies morphologies of NiO covering on the SiCw surface can adjust the surface impedance of the absorber, which makes it close to the free-space impedance.Consequently, the incident wave easily enters the interior of the absorbent.Moreover, the multiple reflection and scattering of the microwave among the NiO/SiC whiskers also present (Fig.7(a)).Especially, for flower-like NiO/SiCw-2 with a porous structure and large specific surface area, it is more likely to cause reflection of microwaves.When the microwave was absorbed by the NiO/SiCw samples, it induces the generation of the dielectric loss of the medium.The dielectric loss that attenuates microwaves is mainly caused by the heterojunction interfacial polarization between SiC and NiO (Fig.7(b)) and dipole polarization from defects and grain boundaries(Fig.7(c)).Thus,combining the above factors,the NiO/SiCw samples show an excellent microwave attenuation capability.

Fig.3.(a-e)The dielectric and magnetic property,(a1-e1)3D representation of reflection loss curves,(a2-e2)Effective absorption bandwidth(EAB),(a3-e3)Reflection loss curves under a certain thickness of SiCw and NiO/SiCw samples.(a-a3) SiCw, (b-b3) NiO/SiCw-1, (c-c3) NiO/SiCw-2, and (d-d3) NiO/SiCw-3, (e-e3) NiO.

Fig.4.Comparison of the RL and EAB of typical SiC absorbers in previous reports and our work.(1)graphene/SiC nanowires[19],(2)HfC/SiC nanofiber[43],(3)Fe/SiC hybrid fibers [16], (4) ZrN0.4B0.6/SiC Nanohybrid [34], (5) bone-like graphene/SiC aerogel [44], (6) carbon nanotube/SiC nanowires [45], (7) Al2O3/SiCw [26], (8) ZnO/SiC nanowires[28], (9) graphite/SiC hybrid nanowires [3], (10) NiO/SiC powder [31], (11) Ni/SiC nanorods [25].

Fig.5.The impedance matching ratio Z of (a) SiCw, (b) NiO/SiCw-1, (c) NiO/SiCw-2, and (d) NiO/SiCw-3 under the various thickness.

Fig.6.Frequency dependence of the relative complex permittivityandcomplex permeabilityof SiCw,NiO, and NiO/SiCwsamples:(a) real permittivity, (b) imaginary permittivity,(c)dielectricloss tangent.(d-g)Frequency dependenceofthepolarizationloss and conductivitylossofSiCw andNiO/SiCw samples.

4.Conclusions

In summary,the core-shell structure NiO/SiCw with morphologies of tulle-like, flower-like, and rod-like were fabricated by adjustment of the content of the Ni2+sources.With the increasement of the Ni2+sources, the micromorphology of NiO converts from tulle-like to flower-like, and to rod-like.Especially, the flower-like NiO/SiCw-2 presents the shell sizes of 194-413 nm and a large specific surface area of 135.15 m2·g-1.The morphological design of NiO also changes the energy bandgap (an increase from 2.42 to 2.76 eV), electric conductivity (decrease from 0.08 to 0.03 S·m-1), and impedance matching property of NiO/SiCw samples.The flower-like NiO/SiCw-2 exhibits a better impedance matching property than tulle-and rod-like NiO/SiCw.More importantly,the flower-like NiO structure induces abundant heterojunction interfaces, resulting in a large dielectric loss.Benefiting from these characteristics, the flower-like NiO/SiCw samples achieve a high RL value (-56.8 dB) and a wide EAB (5.17 GHz) at the thin thickness.This present work provides new design materials with respect to the high-performance microwave absorbing materials.

Fig.7.The microwave absorption mechanism of NiO/SiCw samples.

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 research was supported by the National Science Foundation for Excellent Young Scholars of China (21922815), National Natural Science Foundation of China (21975275), Scientific Research Foundation for Young Scientists of Shanxi Province(201801D221156),and Research Project Supported by Department of Resource and Social Security of Shanxi Province.Research was provided technical support by ‘‘Ceshigo Research Service Agency”for vector network analyzer, www.ceshigo.com.

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.03.047.