Silicon carbide nanowire covered by vertically oriented graphene for enhanced electromagnetic wave absorption performance

Silicon carbide nanowire covered by vertically oriented graphene for enhanced electromagnetic wave absorption performance

Chemical Physics 529 (2020) 110574 Contents lists available at ScienceDirect Chemical Physics journal homepage: www.elsevier.com/locate/chemphys Si...

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Chemical Physics 529 (2020) 110574

Contents lists available at ScienceDirect

Chemical Physics journal homepage: www.elsevier.com/locate/chemphys

Silicon carbide nanowire covered by vertically oriented graphene for enhanced electromagnetic wave absorption performance

T

Dan Zhaoa, , Xiaoyan Yuana, Beibei Lib, Fan Jiangc, Yi Liua, Jinying Zhangc, Chunming Niuc, ⁎ Shouwu Guod, ⁎

a

School of Material Science and Engineering, Shaanxi Key Laboratory of Green Preparation and Functionalization for Inorganic Materials, Shaanxi University of Science and Technology, Xi’an 710021, China b China Electric Power Planning & Engineering Institute, Beijing 100120, China c Center of Nanomaterials for Renewable Energy, State Key Laboratory of Electrical Insulation and Power Equipment, School of Electrical Engineering, Xi’an Jiaotong University, Xi’an 710054, Shaanxi, China d Department of Electronic Engineering, School of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China

ARTICLE INFO

ABSTRACT

Keywords: Silicon carbide Graphene Electromagnetic wave absorption Chemical vapor deposition

Silicon carbide nanowires (SiC NWs) covered by vertically oriented graphene ([email protected]) is prepared by a chemical vapor deposition method. As a microwave absorber, [email protected] exhibits a minimum reflection loss (RL) of −16.2 dB at the thickness of 2.5 mm with an absorption bandwidth of 2.64 GHz (RL < −10 dB, 9.5–12.14 GHz), superior than that of the bare SiC NWs. The improved absorption performance is mainly originated from the complex conductive network and the numerous interfaces formed by the vertically grown graphene on the SiC NWs, which are beneficial for multiple scattering and absorptions of the incident microwave.

1. Introduction With the rapid development of communication equipment and electronic devices, electromagnetic radiation interference and pollution problems become increasingly serious [1–4]. The development of highly efficient electromagnetic wave (EM) absorbing materials has attracted much research attention. EM absorption material can convert the incident EM into heat and other forms of energy, fundamentally eliminating the harm of electromagnetic radiation. Basically, EM absorbing materials can be divided into dielectric loss absorbing materials and magnetic loss absorbing materials. Magnetic loss type materials mainly include ferrite, carbonyl iron and magnetic metal, etc., but they have disadvantages such as high density. Dielectric loss-type materials mainly lose EM through effects such as electron polarization, molecular polarization or interfacial polarization of the medium, such as silicon carbide and silicon nitride. In particular, with excellent thermal, chemical stability, low thermal expansion and high mechanical strength, silicon carbide (SiC) is a promising dielectric material [5]. In addition, SiC nanowires (SiC NW) process optimized EM absorption performance compared to bulk



SiC [6–8] because of their high surface areas, abundant stacking faults, and twinning interfaces [9–12]. Graphene is a resistive loss material that can absorb EM by interacting with electromagnetic fields [13,14]. The higher the electrical conductivity of graphene, the greater the macroscopic current caused by the carrier, and most of its electromagnetic energy falls on the resistance of the material. SiC NW/graphene composites, such as simply blended mixture and three dimensional (3D) foam, were reported to process improved EM absorption properties due to improved electrical conductivity and synergetic effect between SiC NWs and graphene [15–17]. Different from conventional graphene, vertically oriented graphene (VG) nanosheets are few-layer graphene sheets with exposed sharp edges, non-stacking morphology, and high specific surface area [18,19]. Thanks to these advantages, VG could be utilized in various fields, such as gas sensors, supercapacitors, and secondary batteries [20,21]. We suppose that combining graphene and SiC [22–29] by introducing VG nanosheets on the surfaces of SiC NWs may be beneficial for increasing the EM absorption property of SiC NWs. The absorption property enhancement shall be associated with the increased interfaces and interpenetrated 3D porous network formed by adjacent SiC NWs that covered by low specific density VG.

Corresponding authors. E-mail addresses: [email protected] (D. Zhao), [email protected] (S. Guo).

https://doi.org/10.1016/j.chemphys.2019.110574 Received 3 July 2019; Received in revised form 8 October 2019; Accepted 17 October 2019 Available online 18 October 2019 0301-0104/ © 2019 Elsevier B.V. All rights reserved.

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Herein, we describe the synthesis and characterizations of SiC NWs covered by vertically oriented graphene ([email protected]). The SiC NWs with diameters of 20–60 nm were prepared from MWCNTs templates by a chemical vapor deposition (CVD) using silicon monoxide as silicon source, and the MWCNTs with uniform diameters of 10 ± 5 nm were prepared by a CVD method. Few layers of graphene nano-sheets were deposited on the surfaces of SiC NWs by a CVD process at 1200 °C using methane as the carbon source. The [email protected] products have been characterized by scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), Raman scattering, elemental mapping, and thermogravimetric analysis (TGA). In this specially designed structure, VG is adopted not only to increase the resistive loss, but also to increase the interfaces to enhance EM attenuation. As a result, the EM absorption performances of the SiC NWs were significantly enhanced by covering VG nanosheets, which facilitated the formation of 3D interpenetrated porous structure of [email protected] in paraffin.

transmission line theory:

RL(dB) = 20log10

Zin = Z0

µr

Zin Z0 Zin + Z0

tanh j

r

2 fd µr c

(1) r

(2)

where Zin is the input impendence of the EM wave absorbing layer; c is the speed of light in vacuum; f is the frequency; d is the thickness of the absorbing layer; r and µr are the complex permittivity ( r = ' − j '' ) and the permeability ( µr = µ' − jµ'') of the absorbing layer, respectively. The attenuation constant (α) is calculated to evaluate the attenuation properties of the samples, and determined as:

= ( 2 f/c) ×



µ )+



µ )2 + ( µ

µ )2 (3)

where the value of α strictly depends on the values of permittivity and the permeability of the absorbing layer.

2. Experimental 2.1. Preparation of materials

3. Results and discussion

MWCNTs with uniform diameters of 10 ± 5 nm were produced by CVD using ethylene as carbon source and Co/Fe-Al2O3 as catalysts. SiC NWs were synthesized by CVD method using silicon monoxide (powder, ~325 mesh, Sigma-Aldrich) as silicon source and MWCNTs as templates. During the reaction, silicon monoxide and MWCNTs were heated at 1320 °C and 1200 °C for 48 h, respectively, under a constant argon flow at 200 sccm. Similar CVD process was carried out at atmospheric pressure to synthesize [email protected]: initially, the system was heated to 1200 °C under the protection of Ar; then the temperature was maintained for 1 h while a mixture of 100 sccm argon, 10 sccm hydrogen and 10 sccm methane were introduced to form VG on the surfaces of SiC NWs; the system was finally cooled to room temperature under the protection of Ar.

3.1. Morphology and structure The SiC NWs with an average diameter of 20–60 nm were converted from MWCNTs, as shown in the SEM and TEM images (Fig. 1a & c), where numerous twisted and jointed nanowires are obviously observed. This structure is beneficial for the formation of a three-dimensional (3D) network. A thin oxidation layer has formed on the surfaces of SiC NW. In an individual nanowire, numerous stacking faults stripes perpendicular to the growth direction are detected. The lattice fringes are shown in the HRTEM (inset of Fig. 1c), where the interplanar distances of 2.5 Å are corresponding to (1 1 1) crystal plane of 3C-SiC. Few layers of graphene nanosheets were deposited on the surfaces of SiC NWs to fabricate [email protected] The structure of [email protected] hybrids was examined by SEM and TEM (Fig. 1b & d). During the CVD process, the graphene nanosheets were first conformal coated on the surfaces of SiC NWs with the a–b plane of graphene in parallel to the axis of SiC NWs. With the aggregation of excess graphene sheets, VG nanosheets with the a–b plane in perpendicular to the axis of SiC NWs were extruded vertically from the surfaces of SiC NWs (inset of Fig. 1d). The measured XRD pattern of SiC NWs (Fig. 2a, red line) confirms the existence of 3C-SiC, while the diffraction peaks at 35.6°, 41.4°, 60°, and 71.8° (marked by *) can be indexed as the (1 1 1), (2 0 0), (2 2 2), and (3 1 1) lattice planes (PDF No. 29-1129), respectively. The weak peak with 2θ at 34° (marked by ■) might originated be from 6H-SiC (JCPDS card no. 75-1541). Apart from the diffraction peaks brought by SiC, the strong peaks located at 26° and 43° (marked by ·) in the measured XRD pattern of [email protected] are associated with the (0 0 2) and (1 0 0) planes of graphite carbon (PDF No. 01-0640), indicating the successful deposition of conformal graphene and VG sheets on the surfaces of SiC NWs. The Raman features of SiC NWs have not been detected due to the strong photoluminescence of SiC [30,31]. The Raman features of [email protected] are shown in Fig. 2b (black line), where the photoluminescence of SiC is quenched by the covering graphene, and the peaks at 1352 and 1591 cm−1 are associated with the D and G band of graphene. The N2 adsorption/desorption isotherm of SiC NWs (Fig. 2c, red line) exhibits multiple hysteretic loops, indicating the hierarchical mesoporous, while the isotherm of [email protected] (Fig. 2c, black line) exhibits a wide single hysteretic loops. The Barret–Joyner–Halenda (BJH) mesopore size distributions calculated from the adsorption curves are shown in Fig. 2d, where two narrow peaks at 2.4 nm and

2.2. Characterization Raman spectroscopies were obtained by a single monochromator with a microscope (Reinishaw inVia) equipped with CCD array detector (1024 × 256 pixels), and 633 nm Argon ion laser was selected for sample excitation. The morphologies of [email protected] were identified by field emission scanning electron microscopy (FESEM, SU4800, HITACHI, Japan) and high resolution transmission electron microscopy (HRTEM, FEI Tecnai G2 F20 S-TWIN, USA). X-ray diffraction (XRD) (D2 PHASER, Bruker) patterns were obtained using Cu/Kα radiation (λ = 1.5418 Å) at 40 kV and 30 mA. Thermogravimetric analysis (TGA) of the [email protected] was performed by METTLER-TOLEDO TGA/DSC 1Star system. N2 adsorption–desorption isotherms were conducted on a Quantachrome Autosorb-iQ automatic surface area and pore size distribution analyzer at liquid N2 temperature (−196 °C). The specific surface areas were determined from the linear portion of the BrunauerEmmett-Teller (BET) plot. Samples were degassed in vacuum at 100 °C for 10 h prior to taking the surface area and pore volume measurement. 2.3. EM absorption measurements The electromagnetic parameters in the frequency of 2–18 GHz were obtained by a vector network analyzer (Anritsu MS46322B) using the coaxial-line method. The SiC NWs or [email protected] absorbing agent were uniformly in paraffin with a weight ratio of 15 wt%. The samples were machined to a to roidal shape of 7.0 mm in outer diameter, 3.0 mm in inner diameter and thickness of about 2.0 mm for the measurement. The reflection loss (RL) is calculated based on the

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Fig. 1. SEM images of (a) SiC NWs and (b) [email protected]; TEM images of (c) SiC NWs and (d) [email protected], the insets are corresponding HRTEM images.

3.5 nm, as well as one broad at 33.6 nm are observed for [email protected], which further confirm its hierarchical size distribution. The BJH pore size distribution of SiC NWs shows only two overlapped broad peaks at 18.3 nm and 27 nm, respectively. As summarized by Table 1, the specific surface area of [email protected] (110.3 m2 g−1) is higher than that of the SiC NWs (66.7 m2 g−1), probably due to the interpenetrated surfaces and porous structure that induced by the VG nanosheets. The content of graphene in [email protected] is estimated from the TGA measurement to be ~63 wt%, as shown in Fig. 2e.

the reflection loss reaches −10.0 dB, 90% of EM waves could be absorbed by the absorption agent. Significantly lower RL values are delivered by [email protected]/paraffin hybrid than that of the SiC NWs/ paraffin at the same thickness of 2.0, 2.5, 2.8, 3.0, and 3.2 mm (Fig. 3d, purple, black, red, blue, and green lines, respectively), which could be attributed to the hybridization effect of SiC with graphene, as well as the the three-dimensional hierarchical micro-nano network induced by VG sheets on adjacent SiC NWs. According to Eqs. (1) and (2), 3D color images of RL as a function of frequency and thickness for [email protected]/paraffin and SiC NWs/ paraffin hybrids are illustrated in Fig. 4. In the range of 2.0–18.0 GHz, SiC NWs exhibit weak attenuation ability to the incident EM waves. For instances, the lowest RL value of SiC NWs/paraffin is only −3.24 dB, which is obtained at 18.0 GHz with a thickness of 8.0 mm, (Fig. 4b). Compared with SiC NWs, [email protected] hybrids exhibit significantly improved EM wave absorption performance in the range of 2.0–18.0 GHz (Fig. 4a). For instance, when the thickness is 2.5 mm, the RL value can reach −16.2 dB at 10.2 GHz, meaning that more than 90% of EM waves could be attenuated. A 2D map of RL is show in Fig. 5a, where the effective absorption range (RL < −10 dB) is observed in a wide thicknesses range. Furthermore, when decreasing the thickness from 3.5 mm to 2.5 mm, the peak of RL shifts to higher frequency and the corresponding minimum RL value are all approximately −15 dB (Fig. 5b). Exempt RL, absorption bandwidth is another important parameter to evaluate the performance of a EM wave absorber. When RL reaches −10.0 dB, absorption bandwidth becomes the main judgment basis. As shown in Fig. 5b, when the thickness of [email protected]/ paraffin hybrid is 2.5 mm (2.0 mm), the effective absorption bandwidth (RL < −10 dB) is approximately 2.64 GHz (3.2 GHz), within which 90% of incident EM wave can be attenuated.

3.2. EM absorption performances The dielectric and EM absorption properties of paraffin hybrids with 15 wt% SiC NWs or [email protected] are illustrated in Fig. 3. As shown in Fig. 3a, b, higher real ( ) and imaginary ( ) part permittivity values are displayed by the [email protected]/paraffin than that of SiC NWs/ paraffin counterpart, indicating that the introducing of graphene sheets can effectively enhance the dielectric properties of the SiC NWs. The enhanced dielectric loss is mainly attributed to the increased electrical conductivity by the conformal and vertically grown few layer graphene sheets on the surfaces of SiC NWs. The relative permeability (µr ) are not discussed here, due to the attenuation of SiC NWs and [email protected] of are mainly caused by dielectric loss for their nonmagnetic property. Attenuation constants (α) of SiC NWs and [email protected] hybrids with paraffin are calculated based on Eq. (3) to evaluate their attenuation properties, which are displayed as a function of frequency in Fig. 3c. Higher α value is obtained by [email protected] hybrid than that of SiC NWs hybrid, indicating the enhanced microwave attenuation ability of [email protected] In the field of EM wave absorption, RL is an important criterion for evaluating the performance of absorbing agent [28]. When

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Fig. 2. (a) XRD patterns, (b) Raman spectra, (c) N2 adsorption–desorption isotherms, and calculated BJH pore-size distributions of [email protected] (black) and SiC NWs (red); (d) TGA data of [email protected] hybrids. Table 1 Summary of surface areas and average pore diameters of SiC NWs and [email protected] graphene. Sample

Specific surface area (m2/g)

Average pore diameter (nm)

SiC NWs [email protected]

66.7 110.3

4.85 3.859

4 K=

M=

µ

| | = |sinh2 (Kfd)-M|

+µ /µ 2

c·cos( / )·cos(µ / µ )

(5)

4µ ·cos( / )·cos(µ /µ ) cos(µ / µ ))2

(µ cos( / ) + tan

In addition to the RL capability, impedance matching degree (| |) is another parameter to evaluate the attenuation ability of absorbing agent [33,34]. The impedance matching degree of is expressed as follows:

/

·sin

/

µ /µ 2

2

(µ cos( / ) + cos(µ /µ ))2

(6)

typically, small | | value indicate good impedance matching degree. Fig. 5c displays the | | values of the [email protected]/paraffin hybrid as a function thickness and frequency. It is obviously that the preferred 2D thickness and frequency region (blue color region where the | | values are less than 0.2) is agree well with the effective absorption in Fig. 5a (blue color region within which RL < −10 dB). Therefore, the optimal absorbing properties of [email protected]/paraffin hybrid obey the impedance matching rule.

(4)

where K and M are determined by the complex permittivity and permeability as: 4

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Fig. 3. The (a) real and (b) imaginary parts of complex permittivity, the (c) attenuation constant and (d) RL values as a function of frequency for [email protected]/ paraffin and SiC NWs/paraffin hybrids at various thicknesses.

Fig. 4. 3D images of the calculated RL for (a) [email protected]/paraffin and (b) SiC NWs/paraffin hybrids.

In addition, based on the quarter wavelength wave theory:

d=

n 4

0 /(| r ||µr |)

1/2

( n= 1, 3, 5, 7…)

As shown in Table 2, the EM wave absorption performance of [email protected] graphene is comparable to some reported SiC/carbon based composites: SiC NWs on carbon fibers (−21.5 dB with a bandwidth of 2.4 GHz at 2.0 mm) [23], SiC fiber/carbon fiber (−19.9 dB with a bandwidth of 2.5 GHz at 2.0 mm) [26], SiC/CNT heterostructure (−27 dB with a bandwidth of ~2.3 GHz at 2.0 mm) [12], and SiC NW/carbon foam (−31 dB with a bandwidth of 4.2 GHz at 3.3 mm) [28,32]. The EM wave attenuation mechanism of [email protected] hybrids is analyzed as shown in Fig. 6. Several benefits were brought by conformal and vertical coating of graphene: firstly, the interpenetrated porous networks formed by VG nanosheets on the surfaces of adjacent SiC NW increased the conductive loss; secondly, the VG nanosheets on the outer surface introduced numerous interfaces to SiC NWs, which could effectively

(7)

when the thickness of the material (d) is equal to an odd number multiple of the quarter-wavelength [35], the EM waves reflected at the air-absorbing material interface and EM waves that re-reflected at the absorbing material-metal substrate shall cancel each other by destructive interference, resulting in stronger RL. In the case of [email protected]/ paraffin hybrid, the dotted vertical lines in Fig. 5b from the RL peaks are extended and crossed with the (1/4)λ curve in Fig. 5d, indicating that destructive elimination also contributes to the EM wave attenuate in the hybrid.

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Fig. 5. The [email protected]/paraffin: (a) 2D color map of RL; (b) RL values at the various thicknesses; (c) 2D color map of impedance match degree; (d) the (1/4)λ curve. The vertical dashed lines in (b) from RL peaks at various thicknesses crossing with the (1/4)λ curve in (d), whereas the crossover points are indicated by black dots. Table 2 Comparison of the EM wave absorption abilities of SiC/carbon-based hybrids. Absorber

Effective absorption bandwidth (GHz)

Thickness (mm)

RL (dB)

Ref.

SiC NWs on carbon fibers SiC fiber/carbon fiber SiC/CNT heterostructure SiC NW/carbon foam SiC/carbon black graphene/SiC networks [email protected] aerogel Vertically oriented graphene on SiC NWs

2.4 2.5 2.3 4.2 6 3.9 4.7 3.2

2.0 2.0 2.0 3.3 2.0 2.35 3.0 2.0

−21.5 −19.9 −27 −31 −41 −69.3 −47.3 −14.5

2.64

2.5

−16.2

[23] [26] [12] [28] [5] [22] [24] This work This work

increase the attenuate path of incident EM wave through multiple scattering between layers; finally, the twisted and jointed SiC NWs with abundant stacking faults benefited to the dielectric loss.

Fig. 6. A schematic for the EM wave absorption mechanisms of [email protected] hybrid.

4. Conclusion

Declaration of Competing Interest

In summary, [email protected] hybrids are successfully synthesized by a CVD method and characterized by SEM, TEM, XRD, BET, TGA and Raman scattering. [email protected] hybrids with vertically grown graphene sheets on its outer surfaces exhibited better EM absorbing performance than bare SiC NWs due to the specially designed structure and morphology. As a result, with weight percent and thickness of 15 wt% and 2.5 mm, a RL value of −16.2 dB is achieved by [email protected] with an effective absorption bandwidth of 2.64 GHz (RL < −10 dB), demonstrating application potential as an absorb agent with low density and strong absorbency.

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 The authors gratefully acknowledge the financial supporting by the Scientific Research Fund of Sanqin Scholars (BJ11-26), and the Natural Science Fund of Shaanxi University of Science and Technology (2018BJ-59). 6

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