Synthesis and electromagnetic wave absorption properties of peanut shell-like SiC fibers

Synthesis and electromagnetic wave absorption properties of peanut shell-like SiC fibers

Journal Pre-proofs Synthesis and electromagnetic wave absorption properties of peanut shell-like SiC fibers Bin Zhu, Yi Cui, Dongfeng Lv, Pan Liu, Hen...

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Journal Pre-proofs Synthesis and electromagnetic wave absorption properties of peanut shell-like SiC fibers Bin Zhu, Yi Cui, Dongfeng Lv, Pan Liu, Hengyong Wei, Jinglong Bu PII: DOI: Reference:

S0167-577X(19)31920-2 MLBLUE 127288

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Materials Letters

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5 November 2019 8 December 2019 30 December 2019

Please cite this article as: B. Zhu, Y. Cui, D. Lv, P. Liu, H. Wei, J. Bu, Synthesis and electromagnetic wave absorption properties of peanut shell-like SiC fibers, Materials Letters (2019), doi: 2019.127288

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Synthesis and electromagnetic wave absorption properties of peanut shell-like SiC fibers Bin Zhu, Yi Cui*, Dongfeng Lv, Pan Liu, Hengyong Wei, Jinglong Bu College of Materials Science and Engineering, Hebei Provincial Key Laboratory of Inorganic Nonmetallic Materials, North China University of Science and Technology, Tangshan 063210, Hebei, People's Republic of China *Corresponding author: [email protected] (Y. Cui) Abstract: The peanut shell-like SiC fibers were prepared in order to improve the electromagnetic wave absorption properties. The phase of fibers was mainly β-SiC and less amorphous SiO2. The peanut shell-like holes with the average size of 89 nm appeared in the surface of SiC fibers. Their optimal reflection loss was high as -48 dB at 11.1 GHz as absorber thickness was 2.53 mm. The SiC fibers exhibit excellent electromagnetic wave absorption ability in X-band. It was attributed to the fibers’ biomimetic structure, which caused the microwave multiple reflection, scattering and Debye relaxation. Key words: Biomimetic, Surfaces, SiC fiber, Absorbing properties 1. Introduction As development of the advanced radar detection technology, the radar stealth materials with excellent electromagnetic wave (EMW) absorption properties become more important for the progress of military [1-2]. Common carbon and magnetic materials are limited by low oxidation temperature and low curie temperature respectively [2-4]. SiC material has many excellent properties such as low density, high oxidation resistance, chemical and thermal stability as well as dielectric lossy 1

characteristics [1-6]. For example, Kuang et al. and Chen et al. prepared worm-like SiC nanowires and B-doped SiC materials respectively, which showed good EMW absorption properties [2-3]. Nevertheless, due to the single polarization and low conductance of SiC material, its absorbing properties still need to be improved. The electromagnetic wave absorption properties of materials could be enhanced via the bionic structure design. For examples, the tremella-like NiCo/C composites, starfish-like C/CoNiO2 heterostructure, honeycomb-like carbon/Fe3O4 composites, wormhole-like mesoporous carbons and waxberry-like Ni/C microspheres all displayed excellent EMW absorption properties [7-11]. It was because that the bionic structures would introduce amounts of rough interfaces and holes which could achieve multiple reflections, scatter of EMW and also conducive to dielectric loss and impedance matching of materials [7-12]. If a bionic structure SiC fiber with rough interfaces and holes was designed, its absorbing properties may be enhanced. In this work, a novel bionic structure of SiC fiber with peanut shell-like rough and holes surface had been prepared and its electromagnetic wave absorption properties were studied in depth. 2. Experimental section Firstly,











polyvinylpyrrolidone (Mw=1,300,000) and 1ml N, N-dimethylformamide were uniformly mixed in a beaker to obtain the spinning precursor solution. Using six-needle electrospinning device (Needle size is 0.6 mm), the SiO2 precursor fibers were prepared at spinning voltage of 25 kV and with feed rate of 4 mL/h. Then the 2

as-spun fibers were cured in flowing air for 2 h at 200 °C to remain the fiber shape. Finally, the cured fibers were carbonization reducted at 1550 °C for 2 h in Ar atmosphere, to obtain SiC fibers. The SiC fibers were heated at 400 °C for 2 h in air to remove carbon. They were immersed in 10 vol% HF solution for 12h and then washed to pH~7 by deionized water. The resultant SiC fibers were obtained after dried at 80 °C. The crystal phase and chemical state of sample was tested using XRD (D/MAX2500PC, Japan) and XPS (PHI5300C, USA). The morphology of sample was observed through DB-FIB (JEM-2800F, USA) and TEM (JEM-2010, Japan).. Subsequently SiC fibers were mixed with paraffin in mass ratio of 1:4 to measure the complex permittivity by vector network analyzer (AV 3656D, CETC 41st institute, China) 3. Results and discussion Fig. 1(a) and S1 present XRD patterns of SiC fibers before and after HF etched respectively. The diffraction peaks at 35.7°, 41.4°, 60.0°, and 71.8° are corresponding to (111), (200), (320) and (311) planes of β-SiC crystal (JCPDS#65-0360). The peak of SF could be caused by stacking faults [3,6]. The broad peak at 20° to 30° belongs to amorphous SiO2 phase becomes weaker after HF etching. The binding energies of 101.2 eV and 103.6 eV correspond to Si−C and Si−O bonds in XPS narrow spectrum of Si2p. The peaks at 284eV, 285.2 eV and 286.6 eV are assigned to Si-C, Si-C-O and C=O bonds respectively in C1s spectrum. The peaks locate at 532.8 and 534.4 eV, are attributed to Si-O-C and Si-O as shown in O1s spectrum [13,14]. These results indicated that the fiber as-prepared comprises a lot of β-SiC crystal and some 3

amorphous SiO2 phase. The SEM images (Fig. 1, S1) of the SiC fibers before and after HF etched show that the fibers have high aspect ratio. The SiC fibers before HF etched have smooth surfaces and well-developed SiC grains as shown in Fig.S1. The measured lattice spacing of 0.25 nm indicates that the SiC fibers grew along the (111) crystal plane. Besides that, the stacking faults were observed in the fibers. When the fibers were etched by HF, the SiO2 phase near the fibers’ surfaces reacted with HF and was removed. As a result, some SiC grains exposure in the fibers’ surfaces and thus there appeared a lot of peanut shell-like holes with the average size of 89 nm. Meanwhile, the mass ratios of Si and O elements in the SiC fibers before HF etched was 0.81: 2.28 and it increased to 4.94: 2.18 in the fibers after HF etched. The values of ε′ and ε″ represent storage and loss capability of EMW energy respectively. With the frequency increased in 2-8 GHz, the ε′ values have no obvious fluctuation, but the ε′′ values basically increase in Fig. 3a. However, as the frequency furtherly increases to 8-12.4 GHz, the ε′ and ε′′ both have formants response near 9 GHz and 11 GHz, which indicates relaxation polarization process occurring in SiC fibers [13]. Fig. 3(b) provides the loss tangent (tan δ = ε′′ / ε′) which generally increases with frequency, showing a strong dielectric loss effect in range of 8-12.4 GHz. This implied that SiC fibers have strong EMW attenuation in X-band. In order to identify the EMW absorption capacity, the reflection loss (RL) values of SiC fibers are calculated according to the transmission-line theory [6]. Z in 

r  2 tanh  j r  c

 r  r fd  



RLdB   20 log10

Z in  Z 0 Z in  Z 0


Where Zin and Z0 refer to the impedance of normalized input and free space, f is the applied frequency of EMW, d is the thickness of the material and c presents the velocity of EMW in free space. As shown in Fig. 3(c) the RL values of 2.5-6.5 mm (Spacing 0.5 mm) unequal thickness in 0-18 GHz are calculated. It is found that the sample perform strong absorbing property when the absorbers thickness varies from 2.5 to 3.5 mm at X-band (8.2-12.4 GHz). The value of RL is −36 dB as the absorbers thickness is 3.1 mm at the frequency of 9.1 GHz, and the RLmin reaches -48 dB at 11.1GHz (2.53 mm) as displayed in Fig. 3(d-e). As the absorber thickness is 3.13 mm, the effective absorption (RL<−10 dB) bandwidth is 2.5 GHz (7.8-10.3 GHz), and the RL reaches to −39 dB at 9.0 GHz (Fig. 3f). By contrast with others' researches (Table S1), the SiC fibers shows outstanding EMW loss capability, and the loss mechanisms are illustrated in Fig. 3(j). The network of SiC fibers with high aspect ratio is beneficial to multiple microwave reflection. The electrons transmitted in a single fiber, and also between adjacent SiC fibers to cause the conductivity loss [2]. The stacking faults in the fibers could form polarization centers to induce polarization relaxation under an alternating electromagnetic field to benefit dielectric loss [1,6]. The presence of small pores in fibers’ surfaces could make charges accumulate which may form many dipoles to promote dipolar polarization [1]. The small pores could also enhance the multiple microwave scattering and reflection [7-15]. Furthermore, the large number of interfaces between the pores and air bring high specific surface 5

area, which also facilitate interface relaxation polarization and increase the skin depth to benefit impedance matching characteristics [1,3,10-12]. The relaxation process can be described by the Cole-Cole semicircle. The relationship between ε' and ε" are expressed as the equation on the basis of Debye relaxation theory [5].  ' s         '' 2      s  2    2 



Where εs and ε∞ are the dielectric constants of static and extreme. Each semicircle referring to one Debye relaxation process. Obviously, three semicircles are shown in Fig. 4(a), suggesting that there are three kinds of Debye relaxation polarization mechanisms in the SiC fibers. It is indicated that the dielectric loss is mainly due to the relaxation loss. However, the Cole-Cole semicircle is close to a straight line at low frequencies, which indicates the conductivity loss plays a dominant role in the low frequency range [2,5]. The microwave absorption properties of the SiC fibers are also determined by the combination of microwave loss and impedance matching characteristics. The microwave loss characteristics are reflected through attenuation factor α [10].


2 f c

(  ''  ''  '  ')  (  ''  ''  '  ') 2  (  ''  '  '  '') 2


As shown in Fig. 4(b), the response peak appeared in the C-band (4-8 GHz) is caused by the conductivity loss. It can be found that the trend of α curve in 8-12.4 GHz is highly consistent with that of dielectric loss tangents (Fig. 3b), which also verifies that dielectric loss plays a major role for the attenuation of EMW in X-band. 6

A delta-function method was proposed to examine the matching degree of characteristic impedance [12]. △  sinh 2 Kfd   M



4 ' cos E  ' cos M  2 2  E  '  '  ' cos E    ' cos M    tan M   cos E    cos M  2  


  E  sin  M  4 2   ' ' K  (7) c cos E  cos M  The delta value is usually defined less than 0.4 meaning a desirable impedance matching [10]. Peanut shell-like SiC fibers have rich pores and interfaces, which are beneficial for impedance matching [12]. It can be observed that the coverage of the matching region is as high as 84% (Fig. 4c). The small amount of amorphous SiO2 phase not completely etched by HF, which also has a positive influence on impedance matching [14]. 4. Conclusion In summary, the SiC fibers as-prepared had peanut shell-like surfaces with large amounts of holes and interfaces. The bionic structure of SiC fibers bring microwave scattering and enhance multiple reflection mechanisms but also benefit Debye relaxation and matched impedance. As a result, the peanut shell-like SiC fibers shows outstanding microwave absorbing property in X-band. Acknowledgements The research was supported by the National Natural Science Foundation of China (51472072 and 51302064). References [1] Z. Shen, J. Chen, B. Li, et al. J. Alloy. Compd. 815 (2020) 152388. 7

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Figure Captions Fig. 1. XRD pattern and XPS narrow spectra of Si2p, C1s, O1s and SEM,TEM images of SiC fibers. Fig. 2. (a) The complex permittivity, dielectric loss tangent (b), RL 3D diagrams (c-d), RL curves (e-f) of SiC fibers and the schematic illustration of the microwave absorption mechanism (j). Fig. 3. Cole-Cole semicircle(a), attenuation constant(b) and the calculated delta map of SiC fibers(c).