22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Synthesis of beta silicon carbide nanoshells
L. Mangolini1,2, D. Coleman1, O. Yasar-Inceoglu2 and T. Lopez2
1
Materials Science and Engineering Program, UC Riverside, Riverside, CA, U.S.A.
2
Mechanical Engineering Department, UC Riverside, Riverside, CA, U.S.A.
Abstract: Silicon carbide nanoshells have been produced via a two-steps process
involving the non-thermal plasma synthesis of silicon nanoparticles and their rapid in-flight
carbonization in a second non-thermal plasma reactor. Our analysis indicates that the
hollow shell structure is a result of rapid diffusion of carbon into the silicon lattice and of
the rapid nucleation of beta silicon carbide crystals in the outer shell of the silicon particle.
The highly energetic carbonization reaction leads to the production of high-quality
crystalline silicon carbide structures.
Keywords: silicon carbide, nanoshells, energetic reactions
1. Introduction
Silicon carbide is relevant for a variety of applications
including; high power electronics, optoelectronics,
catalysis, etc. Production in the form of nanopowder,
allowing for control on size and size distribution would
offer new opportunities in all of these applications.
Previous attempts at the synthesis of silicon carbide
nanopowder include the laser pyrolysis of silane-methane
mixtures [1] and both thermal [2] and non-thermal plasma
[3] based approaches. In particular, previous reports
indicate that non-thermal plasmas can lead to the
formation of ultrafine (< 50 nm) amorphous silicon
carbide powders [3].
The formation of crystalline silicon nanoparticles in
non-thermal plasma reactors has been reported by several
groups [4-6].
More recently, the formation of
nanoparticles of even higher melting point materials such
as carbon diamond [7] has been observed in non-thermal
plasma reactors. The presence of a crystalline silicon
carbide phase in a sample composed of carbon-coated
silicon nanocrystals obtained via non-thermal plasma
synthesis and processing has also been recently reported
[8]. These results motivate our focus on the non-thermal
plasma synthesis of silicon carbide nanoparticles. In this
contribution we report the successful production of beta
silicon carbide nanopowders using a two-step process
composed of (a) the synthesis of silicon nanocrystals
using silane precursor and (b) the carbonization of these
particles to produce hollow silicon carbide nanoparticles.
These steps proceed in series in two continuous flow nonthermal plasma reactors.
2. Experimental
A two-step reactor has been designed and optimized for
the production and in-flight carbonization of silicon
particles to give silicon carbide nanocrystals. A reactor
schematic is shown in Fig. 1. The first stage of the
reactor is designed according to already published
specifications [6]. To summarize, 100 sccm of silane is
flown through a 2.54 cm diameter Pyrex tube with a
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copper ring electrode wrapped around it. A 13.56 MHz
signal with a power of 100 W is applied to the electrode.
The pressure in the first stage is 800 Pa. The distance
between the ring electrode and the metal flange to which
the plasma is coupling is 7 cm. After being produced in
the first reactor, the silicon nanoparticles are accelerated
through an orifice and injected into the second reactor,
which is also made of a 2.54 cm diameter Pyrex tube.
The pressure in the second stage is automatically
maintained at 460 Pa using a butterfly valve. A copper
coil electrode (18 cm length) is wound around the second
Pyrex tube to sustain a second non-thermal plasma.
Methane with a flow rate of 5 sccm is added immediately
downstream of the orifice that separates the two reaction
volumes. The nanoparticles are collected downstream of
the reaction volume using a filter. Transmission electron
microscopy (TEM) and x-ray diffraction (XRD) analysis
are performed to access particle size, morphology and
structure.
3. Results and discussion
In Fig. 2 we show the XRD scan for two samples:
sample (a) is produced with only the first plasma on,
while for sample (b) a power of 40 W is provided to the
coil electrode, generating a non-thermal plasma in the
second stage.
The diffraction pattern for sample (a) corresponds to
that of silicon with a diamond structure, while the
spectrum for sample (b) is found to match that of beta
silicon carbide, as verified by comparing it with standard
diffraction patterns from the Inorganic Crystal Structure
Database (ICSD FIZ Karlsruhe). In sample (b) it is
possible to distinguish a residual signal from the silicon
nanoparticles, although with low intensity compared to
that from silicon carbide. This suggests that the second
discharge activates the carbonization of the silicon
particles leading to their efficient conversion to crystalline
silicon carbide.
The diffraction pattern for sample (a) corresponds to
that of silicon with a diamond structure, while the
1
spectrum for sample (b) is found to match that of beta
silicon carbide, as verified by comparing it with standard
diffraction
patterns
A TEM image for silicon carbide nanoparticles
produced under the conditions corresponding to spectrum
(b) of Fig. 2 is shown in Fig. 3. The particles are hollow
and have a shell morphology with an outer diameter of
roughly 30 nm and an inner diameter of roughly 20 nm.
Higher resolution images not shown here for brevity
confirm that the shell is polycrystalline with beta silicon
carbide structure.
20 nm
Fig. 1. Schematic of a two-steps plasma system for the
synthesis of silicon nanocrystals and for their in-flight
carbonization.
Fig. 2. XRD spectra for powder produced without (a),
and with (b) RF power supplied to the second discharge.
from the Inorganic Crystal Structure Database (ICSD FIZ
Karlsruhe). In sample (b) it is possible to distinguish a
residual signal from the silicon nanoparticles, although
with low intensity compared to that from silicon carbide.
This suggests that the second discharge activates the
carbonization of the silicon particles leading to their
efficient conversion to crystalline silicon carbide.
2
Fig. 3. TEM micrograph of silicon carbide nanoshells.
Previous reports of nanostructured materials with
hollow or shell-like morphology usually explain the
formation of such structures in terms of the nanoscale
Kirkendall effect [9]: mass transfer can lead to a hollow
structure if the diffusion of the core element in the shell
material is faster than the diffusion of the shell element in
the core material. This leads to out-diffusion of the core
element and to the formation of a void in the core of the
particle. An extensive review of the available literature
on diffusion in the silicon-carbon system rules out such an
explanation. The diffusion coefficient of carbon in silicon
is several orders of magnitude larger than that of silicon in
carbon [10]. The diffusion coefficient of carbon in silicon
carbide is also almost two orders of magnitude larger than
that of silicon in silicon carbide [11]. Therefore an
alternative explanation of the mechanism leading to the
formation of hollow silicon carbide nanoshells is needed.
Our investigation has found that the molar specific
volume of silicon carbide is slightly higher than that of
silicon (12.46 cm3/mole versus 12.05 cm3/mole
respectively, corresponding to a 3.3% increase). This has
important consequences on the particle morphology.
Under the reasonable assumption that the carbonization
proceeds radially inward via the diffusion of carbon into
the silicon particles, it is reasonable to expect that the
saturation of the silicon matrix with carbon and the
consequent formation of silicon carbide domains occur at
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the outer edge of the original silicon particle. Upon
nucleation of the silicon carbide, the small volume
expansion is sufficient to lead to the formation of a void
within the silicon core of the particle. Our prediction is
based on simple geometrical considerations, i.e., on the
conservation of silicon volume for the case in which an
outer shell with a given thickness undergoes a uniform
volume expansion. For instance, if we assume that a
25 nm silicon particles is carbonized to the point that a
3.5 nm outer shell is saturated with carbon, we find that
the uniform conversion of the outer shell into silicon
carbide leads to the opening of a ∼6 nm void inside the
silicon core. The thickness value of 3.5 nm is justified by
the XRD analysis of samples treated at intermediate
power between those shown in Fig. 2. At the onset of
carbonization, the size of the silicon carbide crystalline
domain is around 3.5 nm, as determined via the Scherrer
equation. Continuous diffusion of carbon toward the
silicon core leads to conversion to silicon carbide and
further expansion of the void till the carbonization is
complete.
This process occurs at low pressure leading to poor
thermal contact with the background gas and low cooling
rates. Since the enthalpy of reaction for Si + C → SiC is
high (-71.55 KJ/Kmol, from the NIST Chemistry
WebBook), the nanoparticle reaches very high
temperature during the carbonization process. We have
performed a time-dependent solution of the nanoparticle
energy balance and included the kinetics of carbon
diffusion into the model to predict the time-scale
necessary for saturation and nucleation of silicon carbide
in silicon. We have found that the rapid release of the
enthalpy of formation of silicon carbide heats the
nanoparticle to temperatures as high as 2200 K. The
intense heating mechanism accelerates the kinetics of
carbon diffusion leading to the efficient and complete
conversion of the silicon particle into a silicon carbide
hollow particle. The intense heating is likely the reason
crystalline particles of silicon carbide, a strong covalently
bound material with high melting point (3000 K), can be
produced in a non-thermal process.
4. Conclusions
Several studies have shown that non-thermal plasmas
are a promising tool for the synthesis of nanopowders, in
particular because of their capacity to offer a high degree
of control over the particle size while maintaining a
narrow size distribution. This work extends the range of
materials that are achievable via non-thermal plasmas by
demonstrating the synthesis of beta silicon carbide
nanocrystals. The proposed process is based on the
synthesis of silicon nanocrystals and their subsequent
carbonization using methane as the carbon precursor.
After carbonization, the silicon carbide particles have a
hollow nanoshell morphology. This is likely the result of
the volume expansion associated with the conversion of
silicon to silicon carbide. The nucleation and growth of
the silicon carbide phase is also accompanied by a
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significant release of energy which contributes to the
formation of a crystalline structure.
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