Si-N-C Nanowires Derived by Polyhydridomethylsilazane Pyrolysis

doi:10.4236/msa.2011.27124

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Materials Sciences and Applicatio n, 2011, 2, 936-939

doi:10.4236/msa.2011.27124 Published Online July 2011 (http://www.SciRP.org/journal/msa)

Si-N-C Nanowires Derived by

Polyhydridomethylsilazane Pyrolysis

Gongjin Qi

Beijing Aeronautical Technology Research Center, Beijing, China.

Email: [email protected]

Received November 10th, 2010; revised January 24th, 2010; accepted May 19th, 2011.

ABSTRACT

The preceramic polymer, polyhydridomethylsilazane, was synthesized and pyrolyzed at 1873 K in nitrogen atmosphere

to prepare Si-N-C nanowires without using any catalysts. The diameters of the nanowires were from tens of nanometers

to several microns, and the maximum length of the nanowires reached several hundred microns. The nanowires had no

bulbs or droplets on the tips. However, two different bulbs with diameters of several microns were observed, and one

bulb was covered by nanowires all around, while the other bulb had nanowires just covered on its han dle.

Keywords: Si-N-C Nanowires, Polyhydridomethylsilazane, Pyrolysis

1. Introduction

Silicon nitride possesses excellent thermal and mechani-

cal properties and is one of the most important engineer-

ing ceramics. Synthesis of one-dimensional nanostruc-

tured silicon nitrides has attracted tremendous interest in

recent years [1-8], because of their potential in applica-

tions where mechanical strength and high-temperature/

corrosive durability are required. What’s more, it has been

demonstrated that wide-band-gap single-crystalline sili-

con nitride can be excellent hos material with high doping

levels similar to GaN and AlN. The synthesis methods for

one-dimensional silicon nitride include chemical vapor

deposition process, nitridation of silicon/silica powders or

silicon substrates, combustion synthesis, and polymeric

precursor pyrolysis method, etc. However, most of the

synthesis methods utilize catalysts such as iron, gallium,

gallium nitride, and FeCl2.

In our previous paper [9], a preceramic polymer, per-

hydropolysilazane (PHPS), was used to prepare silicon

nitride powders by thermal decomposition. A novel phe-

nomenon was found that silicon nitride-based nanowires

appeared on the surface of the polymer-derived ceramic

powders although there were no catalysts at all. No bulbs

or droplets were observed on the tips of the nanowires,

and two gas-solid mechanisms were proposed to explain

their growth.

In this paper, we present Si-N-C nanowires derived by

another preceramic polymer, polyhydridomethylsilazane

(PHMS). The diameters of the nanowires were from tens

of nanometers to several microns, and the maximum

length reached several hundred microns. The nanowires

had no bulbs or droplets on the tips. However, a novel

phenomenon was observed that different bulbs with di-

ameters of several microns appeared in addition to the

nanowires.

2. Experimental

The precursor methylhydridocyclosilazane (MHS) [10],

with the structure unit of [CH3SiHNH]n, was synthesized

and crosslinked into polyhydridomethylsilazane (PHMS).

The polymer PHMS was pyrolyzed at 1873K in nitrogen

atmosphere (99.99%) for 2 hours to obtain ceramic nano-

wires.

The morphology of polymer-derived ceramic powders

and nanowires were characterized using field emission

scanning electron microscope (FESEM, Sirion 200, FEI

Company, Netherlands) equipped with energy dispersive

X-ray (EDX) spectrometer (Genesis 60S, EDAX Com-

pany, USA).

3. Results and Discussion

FESEM micrographs of the PHMS-derived ceramic

products are shown in Figure 1. After pyrolysis at 1873

K in nitrogen atmosphere, the PHMS-derived products

were irregular powders with the maximum size of about

500 microns (Figure 1(a)). Among the ceramic products,

the surfaces of some powders showed hair-like mor-

phology (Figures 1(b,c)). Magnification of the Fig-

Si-N-C Nanowires Derived by Polyhydridomethylsilazane Pyrolysis 937

Figure 1. FESEM micrographs of the PHMS-derived ceramic products. (a) micrographs of ceramic powders; (b)(c)

nanowires on some powders; (d)(e)(f) magnification of the one-dimensional nanostructures in (c).

ure 1(c) exhibited one-dimensional nanostructures at

different sites with different sizes and shapes (Figures

1(d)-( f) ). The diameters of the nanowires were from tens

of nanometers to several microns, and the maximum

length reached several hundred microns. There were no

bulbs or droplets on the tips of the nanowires.

In order to determine the composition of the nanowires,

EDX spectrum was conducted on the nanowires in Fig-

ure 1(c). The composition of the nanowires was silicon,

nitrogen and carbon, suggesting that the nanowires were

Si-N-C (see Figure 2).

As was seen in Figure 1, the nanowires appeared

without bulbs on the tips. In addition to these nanostruc-

tures, however, a novel phenomenon arised that few

ball-like structures were observed (Figure 3). As shown

in Figure 3(a), there was a ball with the diameter of

about five microns. Magnification of the ball indicated

that the ball was covered by nanowires all around (see

Figure 3(b)). Different from Figure 3(a), another bulb

structure like a football was found in Figure 3(c), which

was a little smaller with the diameter of about three mi-

crons. Although there was no nanowires covered on its

surface, yet magnification of the bulb indicated that the

handle was covered by nanowires in parallel circles (Fig-

ure 3(d)).

In Ref. [8], silicon nitride nanowires have been syn-

thesized using a polysilazane pyrolysis method, and

FeCl2 catalyst must be used during the process otherwise

no nanowires could be obtained. However, in our study,

the nanowires were obtained without any catalysts.

Several mechanisms have been proposed for the

growth of silicon nitride nanowires, including vapor-

liquid-solid (VLS), solid-liquid-gas-solid (SLGS), and

vapor-solid (VS) mechanisms. As shown in Figure 1, the

nanowires had no bulbs or droplets at the tips, suggesting

a growth mechanism different from the VLS mechanism.

As for the bulbs in Figure 3, it was a just special case.

What’s more, the sizes of the bulbs were several microns,

which had exceeded the size of NANO-wires. The two

bulbs in Figure 3 showed different morphology, and they

probably had different compositions and different growth

mechanisms. In a word, this paper is still a preliminary

investigation, and further study on TEM observation and

growth mechanism are in progress.

Si-N-C Nanowires Derived by Polyhydridomethylsilazane Pyrolysis

938

Figure 2. EDX spectrum of the nanowires derived by PHMS pyrolysis.

Figure 3. FESEM micrographs of the ball-like structures in PHMS-derived products. (a)(b)ball-like structure covered by

nanowires ; (c)(d) football-like structures with the handle covered by nanowires.

4. Conclusions

In summary, we have synthesized Si-N-C nanowires by

polymeric precursor pyrolysis method using polyhydric-

domethylsilazane without any catalysts. The diameters of

the nanowires were from tens of nanometers to several

microns, and the maximum length reached several hun-

dred microns. There were no bulbs or droplets on the tips

Si-N-C Nanowires Derived by Polyhydridomethylsilazane Pyrolysis939

of the nanowires. In addition to the nanowires, few ball-

like structures appeared with the diameters of several

microns. One bulb was covered by nanowires all around,

while the other bulb just had nanowires covered on its

handle.

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