Silicon carbide nanowires synthesized with phenolic resin and silicon powders

Đây là một bài báo khoa học về dây nano silic trong lĩnh vực nghiên cứu công nghệ nano dành cho những người nghiên cứu sâu về vật lý và khoa học vật liệu.Tài liệu có thể dùng tham khảo cho sinh viên các nghành vật lý và công nghệ có đam mê về khoa học

Silicon carbide nanowires synthesized with phenolic resin

and silicon powders

Division of New Materials, Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 102201, China

a r t i c l e i n f o

Article history:

Received 13 October 2008

Received in revised form

10 December 2008

Accepted 11 December 2008

Available online 24 December 2008

PACS:

61.46 w

81.05.Je

Keywords:

Silicon carbide

Nanostructure

Scanning electron microscopy

High-resolution electron microscopy

a b s t r a c t Large-scale silicon carbide nanowires with the lengths up to several millimeters were synthesized by a coat-mix, moulding, carbonization, and high-temperature sintering process, using silicon powder and phenolic resin as the starting materials Ordinary SiC nanowires, bamboo-like SiC nanowires, and spindle SiC nanochains are found in the fabricated samples The ordinary SiC nanowire is a single-crystal SiC phase with a fringe spacing of 0.252 nm along the [111] growth direction Both of the bamboo-like SiC nanowires and spindle SiC nanochains exhibit uniform periodic structures The bamboo-like SiC nanowires consist of amorphous stem and single-crystal knots, while the spindle SiC nanochains consist of uniform spindles which grow uniformly on the entire nanowires

&2008 Elsevier B.V All rights reserved

1 Introduction

Since the discovery of carbon nanotubes (CNTs) in 1991,

increasing attention has been given to nanometer-scaled

one-dimensional materials because of their unique properties and

wide variety of potential applications[1–5] Silicon carbide (SiC)

possesses a range of excellent physical, chemical, mechanical, and

electronic properties These properties make SiC nanowires an

attractive candidate material for many applications, such as

reinforcement material, catalysis supports, and next-generation

high-temperature, high-power, and high-frequency electronic

devices[6–8]

Various methods have been reported to fabricate SiC

nano-wires One-dimension SiC nanotubes and nanowires with various

shapes and structures have been synthesized at different

temperatures using carbon nanotubes as templates [9] Pan

et al.[10]have successfully synthesized oriented SiC nanowires

by reacting aligned carbon nanotubes template with SiO These

oriented nanowires may have promising applications for vacuum

microelectronic devices due to their excellent field emission

performances Core–shell SiC/SiO2nanowires have been

success-fully fabricated by directly heating the NiO-catalysed silicon

substrate under reductive environments using the carbothermal

reduction [11] Helical crystalline core–shell SiC/SiO2 nanowires and amorphous SiC nanosprings have also been synthesized by a chemical vapor deposition technique[12,13] A screw-dislocation-induced growth mechanism was proposed for the formation of the novel structure Hao et al [14] have prepared beaded SiC nanochains, which consist of a stem and uniform beads A simple chemical vapor deposition route has been employed to fabricate BN-nanotube-encapsulated SiC nanowires which possess an unusual gap between the BN-nanotube sheath and the SiC core

[15,16] They have also developed an efficient route to synthesize coaxial nanocables composed of a single-crystalline SiC core, an amorphous SiO2intermediate layer, and a graphitic carbon outer sheath[17] Carbon-rich SiC nanowires have been synthesized by

a method, which is based on the ability to form and manipulate the properties of ethyl alcohol nanometer-sized bridges [18] Electrospun PAN fibers have been employed as templates to fabricate SiC nanowires with a uniform cross-section, well-ordered structure, and a very low concentration of stacking faults

[19] Niu and Wang [20,21] synthesized scales of high-quality crystalline silicon carbide nanowires with small diameters by direct thermal evaporation of ferrocene or ZnS onto a silicon wafer

at high temperature This kind of SiC nanowires possessed a uniform size, crystalline structure, and a thin oxide layer A tentative growth model according to the vapor–liquid–solid (VLS) mechanism was also proposed b-SiC nanowires can also be directly synthesized by heating single-crystal silicon wafer and graphite without metal catalysts [22] The diameter of SiC

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Corresponding author Tel./fax: +86 10 89796090.

E-mail address: hshzhao@tsinghua.edu.cn (H Zhao).

nanowires is in the range of 10–30 nm, and the length is up to a

few millimeters The protective gas, the high temperature, the

closed space, and the slow-cooling rate are key factors for the

formation ofb-SiC nanowires

This paper proposed a method to synthesize SiC nanowires

with phenolic resin and silicon powders without using any metal

catalysts The advantages of this method are as follows: (1) The

process is simple and the starting materials are commercially

available Therefore, SiC nanowires can be fabricated at a low cost

(2) SiC nanowires can be produced in a relatively high quantity

(3) Three types of SiC nanowires can be obtained (4) The lengths

of the SiC nanowires can be up to several millimeters (5) No metal

catalysts are required during the fabrication of SiC nanowires

2 Experimental

The preparation of SiC nanowires includes the following steps:

(1) As is described in our previous work[23], precursor powders,

core/shell silicon/phenolic resin powders, were fabricated by

coating each silicon powder with a homogeneous phenolic resin

shell (2) The obtained precursor powders were pressed to form

cylindrical green compacts (3) The as-prepared green compacts

were subsequently heated at 800 1C in Ar atmosphere to get

carbonized compacts (4) The carbonized compacts, which were

held in a graphite crucible with a graphite lid, were sintered in a

graphite furnace in Ar atmosphere Here, the graphite lid is used

as the substrate to grow SiC nanowires The sintering temperature

was 1500 1C A soaking time of 2 h was sufficient to synthesize a

large quantity of SiC nanowires

X-ray diffraction (XRD) patterns of the prepared nanowires

were recorded on a Japan D/max-IIIA X-ray diffractometer using

standard Cu Karadiation A Hitachi S-3000N SEM operated at an

accelerating voltage of 10 KV was employed to investigate the

morphology of the products Further structure characterization

and selected area electron diffraction (SAED) patterns were

performed on a Tecnai TF20 high-resolution transmission electron

microscopy (HRTEM) operated at 200 KV During the HRTEM

sample preparation, the nanowires were first dispersed in ethanol

under ultrasonic vibration over 15 min and then placed onto

standard carbon-coated copper grids

3 Results and discussion

It can be observed obviously that the as-synthesized sample on

the graphite lid is a light green fluffy material and the wires have

the length of up to several millimeters SEM image shown inFig 1

reveals that most of the materials are wire-like structures with

diameters in the nanometer range At the same time, it can be

found that the nanowires have several types of structure This can

be further evidenced by the TEM analysis

The XRD pattern shown inFig 2 verifies that the fabricated

sample is cubicb-SiC Moreover, the stronger (111) diffraction

peak indicates that the [111] is the dominant growth direction of

the SiC nanowires

Using HRTEM, we found that the fabricated SiC nanowires can

be grouped into three types of one-dimension structures They are

described as follows:

(1) Ordinary SiC nanowires, as is indicated by (1) inFig 3a: this

figure presents a typical morphology of the ordinary SiC

nanowires Most of the synthesized nanowires belong to this

structure They are uniform in diameter along their entire

length The HRTEM lattice image and SAED pattern shown in

Fig 4 indicate that the single-crystal SiC nanowire with a

fringe spacing of 0.252 nm grows along the [111] direction It

is known that, among the SiC surfaces, the {111} surfaces have the lowest surface energy [24] Thus, when the SiC

Fig 1 A typical SEM image of SiC nanowires.

Fig 2 XRD pattern of SiC nanowires.

Fig 3 A typical morphology of the ordinary SiC nanowires: (a) bamboo-like structure and (b) spindle nanochain structure.

nanowires grow parallel to the {111} surfaces, the system’s

energy can be reduced significantly It also shows that the SiC

nanowire is covered with a continuous SiO2amorphous layer

with a thickness of 1 nm, which is probably formed during the

TEM sample preparation

(2) Bamboo-like SiC nanowires as is indicated by (2) inFig 3a:

this figure presents a TEM image of the bamboo-like SiC

nanowire with a stem diameter of about 40 nm Larger

diameter knots, like bandages wrapped around the nanowires,

grow periodically along the entire length of the bamboo-like

SiC nanowire The knots possess a high density of stacking

faults which are perpendicular to the axis of the bamboo-like

SiC nanowire The formation of these stacking faults is

generally thought to be attributed to the thermal stress

during the nanowire growth[25] However, it is interesting to

find that the stacking faults only exist in the knots instead of

the whole bamboo-like SiC nanowire

(3) Fig 5a and b shows the HRTEM lattice images taken from the

area marked with circles (S and K) inFig 3a, corresponding

to the stem and knot of the bamboo-like SiC nanowire,

respectively.Fig 5a suggests that the stem of the bamboo-like SiC nanowire is amorphous SiC without stacking faults and planar defects This can be confirmed by the corresponding SAED pattern shown inFig 5c It can be found fromFig 5b that the knots are single-crystalline b-SiC with a fringe spacing of 0.252 nm, which corresponds well to the {111} plane of cubicb-SiC Stacking faults exist in the whole knot This can be further evidenced by the SAED pattern of the knot (recorded from the area marked with circled K in Fig 3a) shown in Fig 5d The SAED pattern of the knot shows bright spots and streaks, indicating that defects exist in the knot area

(4) Spindle SiC nanochains: this type of SiC nanowires also has a uniform periodic structure As is indicated by (3) inFig 3b, the spindle nanochain possesses a thinner stem with a diameter

of 20 nm and uniform spindles with a diameter of 50 nm The distance between two neighboring spindles is about 250 nm Therefore, the period of the spindle nanochain can be regarded as 250 nm Similar to the beaded SiC nanochains reported in Ref [14], the mechanical interlocking between spindles can enhance the interfacial adhesion between the nanochains and the matrix Thus, the unusual morphology of spindle SiC nanochains may endow them with an excellent reinforcing effect By simply laying the spindle SiC nanochains

on a flat metal surface, a high density of semiconductor–metal junctions can be easily fabricated

4 Conclusions

In summary, we have demonstrated a process for the fabrication of SiC nanowires with three distinct structures,

by using commercially available phenolic resin and silicon powders as starting materials The first type of SiC nanowire, regarded as ordinary SiC nanowire, is single-crystal SiC phase with a fringe spacing of 0.252 nm along the [111] growth direction The second type of SiC nanowire, namely bamboo-like SiC nanowire, possesses amorphous SiC stem decorated periodi-cally by larger diameter single-crystal SiC knots along its whole length The third type of SiC nanowire, spindle SiC nanochain, consists of uniform spindles which grow evenly on its entire length These nanowires may find applications in composite materials or electronic devices Given the simplicity of the procedures and the unique morphology of the synthesized materials, the method described here would attract a great deal

of attention

Acknowledgments The authors would like to acknowledge the National Natural Science Foundation of China (contract No 50802052) and the Key Faculty Support Program of Tsinghua University for providing financial support

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