In-situ growth of SiC nanowire arrays on carbon fibers and their microwave absorption properties

Accepted Manuscript In-situ growth of SiC nanowire arrays on carbon fibers and their microwave absorption properties Renbing Wu, Zhihong Yang, Maosen Fu, Kun Zhou PII:

S0925-8388(16)31830-8

DOI:

10.1016/j.jallcom.2016.06.106

Reference:

JALCOM 37975

To appear in:

Journal of Alloys and Compounds

Received Date: 8 April 2016 Revised Date:

10 June 2016

Accepted Date: 11 June 2016

Please cite this article as: R. Wu, Z. Yang, M. Fu, K. Zhou, In-situ growth of SiC nanowire arrays on carbon fibers and their microwave absorption properties, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.06.106. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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In-situ growth of SiC nanowire arrays on carbon fibers and their microwave absorption properties Renbing Wu,a,c,d∗ Zhihong Yang,b Maosen Fu,c Kun Zhoud,*

b

Department of Materials Science, Fudan University, Shanghai 200433, P. R. China

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College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 211100, P. R. China

State Key Laboratory of Solidification Processing, Northwestern Polytechnical University,

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Xi’an 710072, P. R. China

School of Mechanical & Aerospace Engineering, Nanyang Technological University,

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Singapore 639798, Singapore

Abstract

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Large-scale SiC nanowire arrays have been directly grown on the surface of carbon fibers via a molten-salt-assisted chemical vapor deposition process at relatively low temperature (1200 °C). The morphology, microstructure, and phase composition of the as-grown products

electron

microscopy

(HRTEM)

and

X-ray

diffraction

(XRD).

The

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transmission

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were characterized by field-emission scanning electron microscopy (FESEM), high-resolution

characterization results indicate that the as-grown nanowires were of single-crystalline β-SiC phase with the growth direction along [111], lengths up to several tens of micrometers and diameters of 50–100 nm. The growth of the nanowires was governed by the molten-mediated vapor-liquid-solid (VLS) mechanism. The microwave absorption properties of the as-fabricated SiC nanowires/carbon fibers (SiCnw/Cf) composites were investigated over 2–12

*Corresponding authors: E-mail address: [email protected] (R. Wu), [email protected] (K. Zhou) 1

ACCEPTED MANUSCRIPT GHz and a minimum reflection loss of –21.5 dB at 7.7 GHz could be achieved with 30 wt% SiCnw/Cf as fillers in the silicone matrix.

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Keywords: In-situ growth, SiC, Nanowires, Microwave absorption

1. Introduction

Microwave absorbing materials that can dissipate incidental electromagnetic (EM) waves by

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the mechanism of magnetic or dielectric loss have received considerable attention due to the increasing demand for innovative EM interference shielding [1−6]. Conventional absorbing

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materials such as ferrites and magnetic metals have been extensively studied in view of their excellent microwave absorption properties [7−11]; however, they usually suffer from large specific gravity and poor corrosion resistance, which greatly restrict their applications [12−

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14].

Silicon carbide (SiC) possesses low density, good thermal and chemical stability, superior mechanical strength and tunable band gap [15]. These unique properties make SiC

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recognized as promising candidates for lightweight and stable microwave absorbers [16−21].

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In particular, with low dimensionality and shape effects, SiC nanowires are expected to exhibit better EM wave absorption abilities than bulk SiC particles [22−25]. Various methods including chemical vapor deposition (CVD) [26−28], carbothermal reduction [29−31], arc-discharge [32] and melting solution [33] have been utilized to synthesize SiC nanowires. Nevertheless, most of these reported techniques involve sophisticated facilities and complicated processes. On the other hand, recent studies indicated that SiC nanowires could directly grow on the surface of a carbon fabric by a simple CVD approach [34−36]. Using this

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required. Furthermore, although SiC nanowires/carbon fabric (SiCnw/Cf) composites have been used in the field emission and electrochemical capacitor [37−41], almost no attempt has been made to use such composites as microwave-absorbing fillers.

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This paper reported the in-situ growth of SiC nanowire arrays on the surface of carbon

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fabric by a simple molten-salt-assisted CVD method at a relatively low temperature. The morphology, microstructure, and phase composition of the as-grown products were comprehensively characterized by means of complementary analytical ways. The microwave absorption properties of SiCnw/Cf composites were investigated in terms of complex

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permittivity and permeability. The study suggests that such composites are particularly promising for lightweight microwave absorption materials.

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2.1 Synthesis

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2. Experimental section

SiC nanowire arrays were grown by a simple molten-salt-assisted CVD on the carbon fabric substrate. In a typical experimental process, a carbon fabric substrate was immersed in saturated nickel nitrate for 5 min and then dried in air naturally at room temperature. The treated substrate was placed on the alumina boat, in which commercial silicon (Si), sodium chloride (NaCl), and sodium fluoride (NaF) powders were ground together and loaded in advance. Subsequently, the boat with the substrate was pushed into the center of a tubular

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ACCEPTED MANUSCRIPT furnace. Before heating, the furnace tube was pumped to 0.1 Pa and flushed twice by high-purity argon to remove moisture and oxygen. The furnace was heated up to 1200 °C and maintained at this temperature for 6 h in an argon flow. Finally, the power was switched off

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and the furnace naturally cooled down to room temperature. 2.2 Characterizations

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The obtained products were characterized by X-ray diffraction (XRD, Rigaku) with Cu Kα radiation (λ=1.5406 Å), field-emission scanning electron microscopy (FESEM, JEOL

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JSM-7600), equipped with energy dispersive X-ray spectroscopy (EDS), and a high-resolution transmission electron microscopy (HRTEN, JEOL, JEM-2100F). The EM wave absorption studies of SiC nanowires on carbon fibers were conducted using a vector network analyzer (Agilent HP8722D) in the transmission reflection mode in the frequency of

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2–12 GHz. The samples were prepared by mixing SiCnw/Cf composites in a silicone matrix and then pressed them into a ring with an outer diameter of 7 mm and an inner diameter of 3

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mm. According to the transmission line theory, the reflection loss (RL) curves were calculated

RL(dB) = 20 log ( Z in − Z 0 ) ( Z in + Z 0 )

(1)

Z in = Z 0 ( µ r ε r ) tanh[ j (2π fd c)( µr ε r )1 2 ]

(2)

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by [4]:

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where Zin is the input impedance of the absorber, Z0 is the impedance of free space, f is the frequency of the EM wave, d is the thickness of the absorber, c is the velocity of light in free space,

µr = µ '− j µ '' is the complex permeability, and ε r = ε '− jε '' is the complex

permittivity.

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3. Results and discussion 3.1 Structural and morphology characterization A commercial carbon fabric woven by carbon fibers with smooth surface was used as the

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substrate for the CVD synthesis of SiC nanowires (Figs. 1a-c). After the CVD process, the carbon fabric changed from original black to grey in color due to the growth of SiC nanowires (Fig. 1d). Figs. 1e-g show representative FESEM images of the SiC nanowires grown on a

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carbon fabric under different magnifications, suggesting that a high density of wire-like

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structures entirely and uniformly covered the whole carbon fibers. Notably, most nanowires stood vertically on the carbon fibers and thus were formed into nanoarrays. These nanowires were approximately several tens of micrometers in length and had a very large aspect ratio. Two magnified FESEM images of SiC nanowires as shown in Figs. 1h and i further show that

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their diameters ranged from 50 to 100 nm and the surface of each nanowire was very smooth and clean. The droplet particles were also found to be attached to the tips of the nanowires,

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suggesting that the growth of SiC nanowires was governed by the vapor-liquid-solid (VLS) process. The EDS analyses of the droplet and nanowire reveal that the droplet contained C, Si,

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O and Ni, while the nanowire was mainly composed of C, Si and a little O element (Fig. S1, Supporting Information).

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Fig. 1 (a) Photograph of a carbon fabric, (b) and (c) FESEM images of the carbon fabric, (d) a photograph of a carbon fabric after reaction; (e)-(i) FESEM images of SiC nanowires grown

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on the fabric under different magnifications.

Fig. 2 shows the XRD patterns of the as-grown nanowires. Besides the peak from the residual carbon fibers, other diffraction peaks can be indexed to the lattice planes (111), (220), (311) and (222) of cubic SiC (3C-SiC) (JCPDS Card no.29-1129). A small peak marked with ‘‘SF’’ is likely ascribed to the stacking faults within the crystals. The strong and sharp diffraction peaks indicate that the as-grown SiC nanowires had good crystallinity. 6

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(220)

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Fig. 2 XRD pattern of the SiC nanowires grown on the carbon fabric.

HRTEM analyses were further employed to investigate the interior structures of the as-grown SiC nanowires. Fig. 3a is a typical TEM image of a single nanowire under a low magnification, confirming a smooth surface along the whole length and a small droplet

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particle attached to the tip of the nanowire, which is in good agreement with the FESEM observations. Fig. 3b is a high-magnification TEM image of the SiC nanowire, in which dense stacking faults and twins along the growth direction of nanowire are visible. Fig. 3c presents

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an HRTEM image from the area marked with a white square in Fig. 3b. Both

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defect-containing (e.g. a high density of stacking faults in region 1) and defect-free (e.g. perfect 3C domains in region 2) segments with a distinctive atomic structure can be observed. The region with faceted surfaces that is correlated with the location of twin boundaries (TB) is also displayed. The zigzag angle is 141o (70.5 o + 70.5o), consistent with the rational angle of the (111) twin crystals in face-centered cubic structure. The measured lattice fringe spacing is ~ 0.25 nm, corresponding to the interspacing of the (111) lattice planes of 3C-SiC, suggesting that the wires grew along the [111] direction. The fast Fourier transformed (FFT) 7

ACCEPTED MANUSCRIPT diffraction pattern of region 1 (Fig. 3d) contains both diffraction spots and streaks, indicating the presence of a high density of stacking faults, i.e. the absence of long-range order in the close-packed layers; while that of region 2 (Fig. 3e) reflects the absence of defects in the

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3C-structure of the segment.

Fig. 3 SiC nanowire microstructure: (a) low-magnified TEM image, (b) high-magnified TEM image showing high-density stacking faults and micro-twins within nanowires, (c) HRTEM image recorded from the white square area in (b), (d) and (e) corresponding the FFT diffraction patterns obtained from defect-containing (1) and defect-free (2) regions in (c), respectively.

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ACCEPTED MANUSCRIPT 3.2 Growth mechanism Based on the above morphology and structural analyses, a possible VLS growth mechanism can be proposed for the SiC nanowires. At elevated temperature, the molten Ni

4Ni (NO3)2 → 4NiO + 8NO2 + 2O2 2NiO + 3C → 2Ni + 3CO2

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catalytic droplets were formed on the surface of carbon fibers through the following reactions: (1) (2)

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Meantime, the solid salt (NaCl and NaF) began to melt and provide the molten media for the

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mixed Si powder. Such a medium facilitated the migration and evaporation of Si, and thus the Si vapor reacted with the oxygen to yield SiO vapor at relatively low temperature: 2Si + O2 → 2SiO

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At this stage, the generated SiO and C from the carbon fibers were dissolved in the formed Ni

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liquid droplets, forming a Ni-Si-O-C system. Along with the continuous dissolution of SiO and C into the liquid droplet, it would become supersaturated, and SiC nuclei were then

SiO (v) + 2C (v or l) → SiC (s) + CO (v)

(4)

SiO (v) + 3CO (v) → SiC (s) + 2CO2 (v)

(5)

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formed:

where s, l and v refer to the solid, the liquid and the vapor state, respectively. The precipitated SiC nucleus would preferentially grow in the [111] direction, which has lower surface energy than others in 3C-SiC [42, 43]. The preferential growth as well as continuous generation from the reaction among SiO, CO and C in the liquid droplets was the main reason that SiC nanowires could grow so long on the surface of carbon fibers. It is noted that SiC nanowires still grew on the surface of the carbon fabric when the same reaction conditions were taken 9

ACCEPTED MANUSCRIPT but using iron nitrate as catalyst (Fig. S2, Supporting Information) 3.3 Microwave absorption properties The EM wave absorption properties of the SiCnw/Cf samples were investigated by mixing

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them in a silicone resin matrix to form the composite. Figs. 4(a)-(c) show the frequency dependences of the measured complex permittivity real part ε', permittivity imaginary part ε'', permeability real part µ', and permeability imaginary part µ'' of the SiCnw/Cf–silicone

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composites. The values of ε' and ε'' for all composite samples exhibit an obvious resonance

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and both of them increased with increasing SiCnw/Cf loading concentration in the range of 2–12 GHz. In contrast to the features of ε' and ε'', both µ' and µ'' are independent of the investigated frequency and the concentration of SiCnw/Cf. Thus, the EM wave absorption properties of SiCnw/Cf–silicone composites result mainly from dielectric loss rather than

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magnetic loss.

Fig. 4d shows the dielectric loss tangents ( tan δε = ε ''/ ε ' ) of the composites versus

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frequency. Generally, a higher tangent value means a higher loss and a better microwave attenuation capability. It can be found that the values of the dielectric loss tangent increase

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with increasing concentrations of SiCnw/Cf in the composites. The peak frequency positions of tan δε are at 10.11, 9.48 and 6.83 GHz for 10, 20, and 30 wt% concentrations of SiCnw/Cf in the composites, respectively. The dielectric loss is mainly due to polarization. Firstly, a high density of SFs and TBs within SiC nanowires can give birth to the space charge polarization and relaxation, leading to the enhanced dipole polarization loss [44, 45]. Secondly, the high aspect ratio of nanowires is believed to play an important role in the microwave absorption due to the formation of a large amount of interfacial polarization at the interfaces between the 10

ACCEPTED MANUSCRIPT SiC nanowires and the silicone. Furthermore, carbon fibers have good electric conductivity and their bonding to the SiC nanowires can also increase the microwave attenuation. 25

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dielectric loss tangents of the SiCnw/Cf –silicone composites.

To further reveal the microwave absorption properties of the SiCnw/Cf composites, the reflection loss (RL) was calculated according to the transmission line theory. Fig. 5a shows the dependence of calculated RL on frequency for SiCnw/Cf-silicon composites with different concentrations and Cf–silicone composites with 30 wt% at a thickness of 2 mm. It was found that the EM wave absorption depended on the amount of SiCnw/Cf in the composites. The RL peaks shifted to lower frequency when the concentration of SiCnw/Cf in the composites 11

ACCEPTED MANUSCRIPT increased from 10 to 30 wt%. Furthermore, the RL values of the 30 wt % SiCnw/Cf-silicon composites were lower than those of the 30 wt % Cf-silicon composites, demonstrating that the growth of SiC nanowires could enhance the microwave absorption ability of Cf. Fig. 5b

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shows the calculated RL of the 30 wt % SiCnw/Cf-silicon composites for various thicknesses (1–4 mm). The peak value of RL moved toward a lower frequency region with the increasing thickness, suggesting that one can tune the range of absorption frequency by adjusting the

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thickness to reach an optimal match. The RL value reached the minimum of –21.5 dB at 7.7

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GHz with the thickness of 2 mm, which is superior to that of previously reported SiC nanowires (–13.9 GHz, 2.0 mm) [23]. Moreover, the absorption bandwidth with the RL smaller than –10 dB (over 90% microwave absorption) was 2.45 GHz (between 5.85 and 8.30 GHz). The microwave absorption properties of S SiCnw/Cf–silicone and Cf–silicone

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composites were summarized in Table S1 (Supporting Information). The good absorbing performance of the SiCnw/Cf composites renders them useful as lightweight microwave absorbers with a lower weight percentage in contrast to other absorbing materials

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4. Conclusions In summary, single-crystalline β-SiC nanowires with diameters of 50–100 nm and lengths of up to several tens of micrometers were successfully in-situ grown on the surface of

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the carbon fabric via a facile molten-assisted CVD method. The VLS growth mechanism was responsible for the initial nucleation and formation of nanowires with large aspect ratio. The as-grown SiCnw/Cf–silicone composites exhibited excellent microwave absorption properties,

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which mainly resulted from dielectric loss. A minimum RL –21.5 dB at 7.7 GHz and the

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absorbing bandwidth of 2.4 GHz with RL blow –10 dB were achieved for the composites containing 30 wt% SiCnw/Cf with the thickness of 2 mm. Considering the convenient fabrication process of SiCnw/Cf composites as well as their low density and good thermal stability, the current study presents them as attractive candidates for thin and light-weight

Acknowledgements

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materials in EM wave absorption applications.

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This work was financially supported by the National Natural Science and Foundation of China (Grant No. 51102278), Research Grant for Talent Introduction of Fudan University,

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China (Grant No. JJH2021103), the Fund of the State Key Laboratory of Solidification Processing in NWPU (Grant No. SKLSP201616) and Ministry of Education, Singapore (Academic Research Fund TIER 1 – RG128/14).

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ACCEPTED MANUSCRIPT Highlights 1. A convenient molten-assisted chemical vapor deposition method has been developed to synthesize SiC nanowire arrays. 2. SiC nanowires/carbon fibers composites exhibited excellent microwave absorption properties.

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3. A minimum reflection loss –21.5 dB at 7.7 GHz could be achieved with SiC nanowires/carbon fibers.