Ultrafast Production of Silicon via Aluminothermic Reduction of
Tetrachlorosilane in Thermal Plasma Jet
Kentaro Shinoda1*, Hideyuki Murakami1, Yoshinari Sawabe2, and Kunio Saegusa2
1
National Institute for Materials Science (NIMS), Tsukuba, Ibaraki, Japan
2
Sumitomo Chemical Co., Ltd., Tsukuba, Ibaraki, Japan
Abstract: A new route to produce silicon at a high rate via reduction of
tetrachlorosilane with aluminum molten particles by utilizing a thermal plasma
jet is proposed. High purity aluminum particles were melted and sprayed by
atmospheric dc plasma jet in an argon atmosphere in an airtight chamber.
Tetrachlorosilane vapor was injected at the tail of the plasma jet and reacted with
the aluminum molten particles. The reacted particles were collected with a
carbon filter located downstream. Silicon generation was confirmed in the
particles by X-ray diffraction and fluorescence methods. Silicon morphology in
the collected particles was not whiskers, which are typical morphology of vapor
phase growth, suggesting liquid or solid phase growth of silicon. The time for the
reduction reaction was estimated from the residence time of aluminum particles
in the dc plasma jet by a numerical simulation, which turned out to be on the
order of millisecond. This reaction rate was a few orders of magnitude larger than
the conventional thermal decomposition processes. This study has shown the
feasibility of the aluminothermic reduction process by thermal plasma jet for a
high-rate silicon production.
Keywords: aluminothermic reduction, silicon production, thermal plasma jet
1. Introduction
A high productivity process for silicon reduction,
which can substitute thermal decomposition
processes such as the Siemens process [1], is
demanded for the supply of solar-grade silicon.
Silicon production processes based on the
aluminothermic reduction of tetrachlorosilane have
higher productivities because of the large driving
force for reactions [2]. The aluminothermic
reduction is expressed by
these processes requires special attentions such as
the selection of container materials and
contamination from the containers. In this regard,
thermal plasma processes seem to have some
advantages because they can offer a containerless
reaction between molten particles and gas at high
rates in thermal plasma jets. Therefore, in this paper,
we propose a new route to produce silicon at a high
rate via reduction of tetrachlorosilane with
aluminum molten particles by utilizing a thermal
plasma jet.
3SiCl4 (g) + 4Al (l) = 3Si (s) + 4AlCl3 (g).
The standard Gibbs energy of this reaction is -579 kJ
at 1300 K [3], which is much negatively larger
compared to those in the Siemens process. However,
the use of high-temperature reaction containers in
*
Current affiliation: National Institute of
Advanced Industrial Science and Technology
(AIST), Tsukuba, Ibaraki, Japan
2. Materials and Methods
Figure 1 shows a schematic diagram of the
experimental apparatus developed for this study. A
10-kW class dc plasma torch (SPG-30N2,
Technoserve Co. Ltd., Toyohashi, Aichi) was used
to generate thermal plasma jets. This plasma torch
equipped two powder feed tubes inside the anode,
which enabled internal injections of the powder. The
nozzle diameter was 6 mm. A sheath gas
surrounding the plasma torch can be flowed to
protect the torch from chloric corrosions. An electric
furnace having a quartz tube 144 mm in internal
diameter was connected to the plasma torch as a
reaction chamber. The chamber length was 950 mm.
This system was designed to be air-tight and
vacuumed with a rotary pump so experiments could
be conducted under controlled atmospheres. The
quartz tube can be heated to avoid a precipitation on
the internal surface of the quartz tube. Two stainless
steel tubes were installed between the plasma torch
and the quartz tube so that reaction gases can be
flowed. A carbon filter ~10 mm thick was used to
collect reacted particles. Reacted gases were sent to
an exhaust system in which chloride gases were
filtered through diluted sodium hydroxide. A
photograph of the actual apparatus is shown in Fig. 2.
fluorescence (XRF) method. The phase was
analyzed by a powder X-ray diffraction (XRD)
method using a Cu Kα radiation at 40 kV and 300
mA. Energy disperse spectroscopy (EDS) was also
conducted to examine the distribution of Si and Al
elements in the particles.
Fig. 1. Schematic diagram of experimental setup.
The chamber air was replaced to argon (Ar).
Because of the exhaust system, the chamber pressure
was slightly higher than the atmospheric pressure
(~1.05 atm). An Ar dc plasma jet was generated with
the plasma torch at a current of 300 A and 15
standard liters per minute (SLM) of Ar. Sheath Ar
gas flow rate was 5 SLM.
High purity aluminum (Al) powder (purity >
99.99 %, 25-45 µm) was fed through the powder
feed tube at a rate of 3 g/min with a carrier Ar gas
flow of 2 SLM. Melted and accelerated Al powder
particles were reacted at the tail of the plasma jet
(120 mm from the exit of the torch nozzle) with
tetrachlorosilane (SiCl4) vapors, which were injected
with a carrier Ar gas at a rate of 600 standard cubic
centimeters per minute (SCCM) through the
stainless steel tube. The reacted particles were
collected with a carbon filter located 380 mm
downstream from the torch so that reactions could
complete before the particles reached the filter. This
filter location was decided by a preliminary Al spray
test, in which almost all the Al particles solidified at
the position of 320 mm.
The collected samples were observed with an optical
microscope and a scanning electron microscope. The
composition was determined by an X-ray
Fig. 2. Photograph of panoramic view of experimental setup.
3. Results
The samples collected with a carbon filter exhibited
black to dark brown colors as shown in Fig. 3 (a).
Meanwhile, white vapors were observed on the
chamber wall as shown in Fig. 3 (b). Note that the
white vapors deposited only at the bottom side of the
tube (> 640 mm from the nozzle exit in this case)
and no deposits were observed in the region close to
the torch.
Compositions of these samples were summarized in
Table 1. Main components of the sample consist of
Si and Al for the one from the filter, while Cl and Al
from the wall. Since AlCl3 has a sublimation point of
~160-190 °C, it is natural to think that the white
vapors depositing on the wall were AlCl3.
(a)
(I) Al melting zone (< 120 mm)
Al (s) → Al (l)
(II) Aluminothermic reaction zone (120-380 mm)
(b)
Fig. 3. Photographs of (a) carbon filter and (b) quartz tube after
trial. The diameter of the carbon filter was 140 mm. The
photograph of the quartz tube was taken from the bottom side.
The nozzle of a plasma torch can be seen at the other side of the
tube.
Table 1. Compositions of collected samples determined by XRF.
Samples
Al
Si
Cl
(a) on filter
35.8
49.3
9.0
(b) on wall
21.2
1.5
75.0
*
Other elements contain Fe, Cr, Ni, Zn, and Cu.
Others*
5.9
2.3
Fig. 4. Bright-field image of collected powder observed with an
optical microscope.
The sample collected from the filter was examined
in more detail. Figure 4 shows an optical micrograph
of the sample. Spherical shape particles with dusts
on their surface were observed. The spherical shape
suggests that reacted particles solidified before they
reached to the filter, i.e., the reaction has completed
before they were collected by the filter as planned.
Figure 5 shows an XRD pattern of the sample. It
consisted of peaks from Si and Al, which did not
conflict with the XRF result. It also suggests that no
chlorides or oxides formed.
Fig. 5. XRD pattern of collected powder.
Finally, cross sections of the sample particles were
examined by SEM and EDS. Pure Si regions were
observed as well as Al regions. The Si shapes were
not whiskers, which are typical morphology in a
vapor phase growth, suggesting liquid or solid phase
growth of Si.
4. Discussion
4.1. Reaction mechanisms in this process
Silicon was successfully produced through this
process. Summarizing the results, this process can be
classified into the following three regions:
Fig. 6. BEI and EDS of cross section of collected powder.
4Al (l) + 3SiCl4 (g) → 3Si (s) + 4AlCl3 (g)
Al (l) → Al (s)
(III) Precipitation zone (> 640 mm)
AlCl3 (g) → AlCl3 (s)
4.2. Ultrafast production of silicon
The time for the reduction reaction was estimated
from the residence time of aluminum particles in the
dc plasma jet by a numerical simulation (Jets &
Poudres, SPCTS, University of Limoges), which
turned out to be on the order of a few milliseconds
(Fig. 7). This reaction rate was a few orders of
magnitude larger than the conventional thermal
decomposition processes such as the Siemens
process.
Fig. 7. Numerical simulation results.
5. Conclusions
Silicon was successfully produced via reduction of
tetrachlorosilane with aluminum molten particles by
utilizing a thermal plasma jet. The reaction time was
estimated to be a few milliseconds, suggesting an
ultrafast production of silicon in this method. This
study showed the feasibility of the aluminothermic
reduction process by thermal plasma jet for a highrate silicon production.
Acknowledgements
We appreciate Ms. Kaori Nakane, Mr. Akihiro
Yamaguchi, Mr. Yoshito Yasui, and Mr. Ryohei
Tansho for their help in experiments.
References
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