Preparation of trichlorosilane from hydrogenation of silicon tetrachloride in thermal plasma

Article abstract of DOI:10.1134/S0020168510030076

A new method of producing trichlorosilane by hydrogenation of silicon tetrachloride with assistance of DC charged thermo-plasma was proposed. We have studied the dependence of degree of disassociation and ionization of hydrogen on temperature, as well as

Full text of DOI:10.1134/S0020168510030076

ISSN 0020ꢀ1685, Inorganic Materials, 2010, Vol. 46, No. 3, pp. 251–254. © Pleiades Publishing, Ltd., 2010.
Preparation of Trichlorosilane from Hydrogenation of Silicon
1
Tetrachloride in Thermal Plasma
Wu Qingyou, Chen Hanbin, Li Yuliang, Tao Xumei, Huang Zhijun, Shang Shuyong,
Yin Yongxiang, Dai Xiaoyan
College of Chemical Engineering, Sichuan University, Chengdu, China
eꢀmail: hyyx0675@sina.com
Received January 08, 2009
Abstract—A new method of producing trichlorosilane by hydrogenation of silicon tetrachloride with assisꢀ
tance of DC charged thermoꢀplasma was proposed. We have studied the dependence of degree of disassociaꢀ
tion and ionization of hydrogen on temperature, as well as the function of heat capacity, via which the optimal
volume and H : SiCl molar ratio were confirmed. A DC power of 50 kW was equipped and the highest yield
2
4
of trichlorosilane was above 70%, with an average yield about 60% and minimum unit energy expenditure
about 3.2 kWh/kg for SiHCl3. This process is possible to be industrialized.
DOI: 10.1134/S0020168510030076
1
Δ
INTRODUCTION
H + SiCl4
2
HCl + SiHCl3.
(1)
Solar energy, as a sustainable power source, has
In which, as a diatomic molecule, H2 would be disasꢀ
received extensive attention worldwide, owing to the sociated to two atoms when it reaches to a certain temperꢀ
energy crisis. As a result, the demand of polysilicon, the ature and the disassociation degree would increase with
α
most important raw material for solar industry, is increasꢀ temperature. With the process of disassociation, a few
ing. However, great quantity of silicon tetrachloride, molecules and atoms would be ionized to ions and elecꢀ
–5
–4
which is highly poisonous, would be produced in trons, with an ionization degree
β
about 10 –10 . The
advanced Siemens technology [1], which is the most ionization degree would increase conspicuously with
widely utilized to synthesize polysilicon.
temperature after disassociation finished. Disassociated
hydrogen atoms have very high reactivity; as a result, reacꢀ
tion (1) would be much easier to take place. It’s hard to
make H2 disassociate via conventional heating methods,
nevertheless, the temperature of hydrogen would rise
instantaneously to exceed the disassociation temperature
Jing Zou and Susan Kauzlarich [2] presented a process
to synthesize silicon nanoꢀparticles from reduction of
SiCl4 with sodium naphthalide. Huang Rui [3], Lin Xuaꢀ
nying [4] described a PECVD process to synthesize polyꢀ
silicon films directly from SiCl4 and analyzed the microꢀ
structure of the product. Whereas, restricted by cost,
yield, quality, etc., these methods are hard to be industriꢀ
alized. The most effective way to convert SiCl is producꢀ
α, β
4
ing SiHCl3, the raw material in advanced Siemens proꢀ
cess, via hydrogenation. Yet, several companies monopoꢀ
lize the key technologies of the hydrogenation process [5].
1.0
.9
0.8
0
Plasma, soꢀcalled “the fourth state” of substances, is a
special kind of state, within large quantity of active radiꢀ
cals such as disassociated ions, electrons and atoms, so it
is highly reactive. A.V. Gusev [6, 7] reported a new
method of preparation of SiHCl3 by reduction of SiCl4 in
RF plasma and analyzed the conversion of impurities
within. But it’s hard to use RF power for reactors of indusꢀ
trial grade. In this experiment, a DC power was adopted to
generate H2 plasma and an ideal yield of SiHCl3 was
obtained.
0
.7
.6
0
0.5
0.4
0.3
0.2
0.1
αꢀT
ꢀT
β
EXPERIMENTAL
0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
T × 10 , K
–4
The chemical equation of hydrogenation of SiCl4 is as:
1
The article is published in the original.
Fig. 1. Dependence of
α
and on T in H System.
β
2
251

252
WU QINGYOU et al.
slope, which means in this range, the number of
Cp, J/(mol K)
hydrogen atoms with high reactivity increases most
rapidly. So it is obviously that the optimal reaction
temperature must be in this range. The flow chart is
shown in Fig. 3.
In this process, plasma generator and cooling tank
were equipped with water cooling system. DC power was
50 kW with an efficiency of 70%. Ar was used for protecꢀ
tion and the purity of each reactant was 99.99%.
6
5
5
4
4
3
3
2
2
1
1
00
50
00
50
00
50
00
50
00
50
00
The disassociation degree is only about 0.05 at 3000 K
(
see Fig. 1), so in this experiment, temperature was varied
in the range of 3500–5000 K. We have chosen 3500, 4000,
500 and 5000 K as our experimental points. Based on
4
Fig. 1, hydrogen’s disassociation degree is about 0.2
at 3500 K and it is 0.45, 0.75 and 0.9 at 4000, 4500 and
5
0
5000 K respectively, which indicated that the best ratios of
H : SiCl4 would be 5 : 1, 2.2 : 1, 1.3 : 1 and 1.1 : 1 respecꢀ
tively, considering the stoichiometric numbers in equation
0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
T × 10 , K
2
–4
(1). Based on formula (2), hydrogen’s volume
Qat 3500 K
Fig. 2. Dependence of Cp on T in H2 System.
must suffice equation (3), as
2000
⎛
–
3
5
0000 × 70% = Q⎜ 26.936 + 6.1313 × 10 T
when being charged to form plasma. In the light of the
method in [8], the dependence of hydrogen’s disassociaꢀ
∫
⎝
300
tion degree a and ionization degree
α
on temperature was
3500
obtained in Fig. 1, as well as the dependence of hydroꢀ
gen’s constant pressure molar heat capacity Cp on temperꢀ
ature in Fig. 2. The equations of Cp in each temperature
range were induced by the method of least square, as forꢀ
mula
–7
2
–
5.884 × 10 T dT +
248.76
(3)
∫
2000
–
5
2
⎞
ꢀ– 0.2348T + 6.2853 × 10 T dT .
⎠
3
⎧
⎪
⎪
⎪
⎪
⎪
⎪
⎪
⎪
⎪
⎪
⎪
–3
–7
2
So
similarly,
5000 K respectively.
Q
= 0.1828 mol/s, that is, about 14.7 m /h, and
2
6.936 + 6.1313 × 10 T – 5.884 × 10 T
3
Q = 8.9, 6.2 and 5.2 m /h at 4000, 4500 and
300 K ≤ T < 2000 K
–
5
2
2
2
48.76 – 0.2348T + 6.2853 × 10 T
RESULTS AND DISCUSSION
2000 K ≤ T < 4100 K
3994.1 – 1.6509T + 2.2703 × 10 T
Figure 4 showed the yield of SiHCl3 increased with the
ratio of H : SiCl4 at a constant temperature. Besides,
under normal circumstances, the yield of SiHCl3
–
4
2
–
8
3
–
1.0234 × 10 T
increased with temperature at a constant ratio of H :
2
C = ⎨4100 K ≤ T < 7000 K
(2) SiCl4. However, as shown in Fig. 5, the point of the lowest
p
⎪
unit energy expenditure (3.2 kWh/kg) appeared on the
curve of 4000 K, with the ratio of H : SiCl4 at 2.5 (very
close to 2.2). Under this condition, the yield of SiHCl3
–
5
2
⎪
⎪
⎪
⎪
⎪7
⎪
1879.6 – 0.5272T + 4.6231 × 10 T
2
–
9
3
–
1.1277 × 10 T
was 72.9%.
000 K ≤ T < 15400 K
In this experiment, at 4500 and 5000 K, deposition of
solid particles in cooling tank was observed, especially at
000 K, while no such phenomenon occurred at 3500 or
000 K. These particles were sure to be elemental silicon,
either polysilicon or amorphous silicon. So we could conꢀ
clude the mechanism in this experiment as equation (4)–
–
4
2
–
31277 + 5.5924T – 3.227 × 10 T
⎪
5
4
⎪
–
9
3
⎪
+ 6.0791 × 10 T
⎪
⎩1
5400 ≤ T ≤ 20000 K.
(12):
H2 begins disassociation at about 2000 K and Cp starts
to increase remarkably for the first time as lots of
energy is necessary for the disassociation. The curve
of Cp reaches to a peak at 4100 K and when temperaꢀ
ture reaches to 7000 K, H2 has been disassociated
completely. As shown in Fig. 1, in the temperature
Charged
H2
2H•,
+
(4)
Charged
H2
H• + SiCl4
SiCl • + H•
H2
+
e,
(5)
(6)
(7)
SiCl • + HCl,
3
range of 3000–5000 K, the curve of
α
has a largest
3
SiHCl ,
3
INORGANIC MATERIALS Vol. 46
No. 3
2010

PREPARATION OF TRICHLOROSILANE FROM HYDROGENATION
253
2
1
9
8
9
7
10
5
6
3
4
Fig. 3. Flow Chart of Reduction of SiCl4 by Thermoꢀplasma:
Tubular Reactor;
ratus.
1
—Plasma Generator;
2
—DC Power;
3
9
—Cooling Tank;
—Rotameter; 10—GC Appaꢀ
4
—Graphite
5
—Hydrogen Bottle;
6
—Argon Bottle;
7
—Silicon Tetrachloride bottle;
8—Heator;
–
–
+
–
SiCl +
4
e
SiCl4
SiCl + Cl2
,
(8)
(9)
(10)
2
H2
+
Cl2
2HCl.
(12)
SiCl + H
2
SiCl + H•
2
SiH Cl ,
2
2
2
In which, SiCl2 is a reactive intermediate, and considꢀ
ering the similar chemical properties between C and
(11) Si, the formation of SiCl2 is similar as that of CCl2 [9].
SiCl• + HCl,
Si + HCl,
SiCl• + H•
7
8
7
7
6
6
5
5
4
4
3
3
0
5
0
5
0
5
0
5
0
5
0
3500 K
4000 K
4500 K
000 K
6
5
5
4
3
0
1
2
3
4
H : SiCl ratio
5
6
7
0
1
2
3
H : SiCl ratio
4
5
6
7
2
4
500 K 4000 K 4500 K 5000 K
2
4
3
Fig. 4. Dependence of Yield of SiHCl3 on H : SiCl Ratio at
Fig. 5. Dependence of Unit Energy Expenditure on H :
2
4
2
Different Temperature Points.
SiCl4 Ratio at Different Temperature Points.
INORGANIC MATERIALS Vol. 46
No. 3 2010

254
WU QINGYOU et al.
CONCLUSIONS
U, mV
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
The singleꢀpass yield of SiHCl3 in existing industrial
scale processes was only 17–19%, with a unit energy
expenditure about 5 kWh/kg, besides, high pressure (at
least 0.2 MPa) is necessary and the ratio of H : SiCl must
be larger than 4. The singleꢀpass yield of SiHCl3 would
rise above 70% with unit energy expenditure only
2
4
3.2 kWh/kg and H : SiCl4 ratio only 2.5 because of the
2
adoption of thermoꢀplasma. It was shown that the optiꢀ
mal temperature for the preparation of SiHCl3 from
hydrogenation of SiCl4 was about 4000 K. As it is easy to
enlarge DC power, this method is possible to be industriꢀ
alized.
0
1
2
3
4
5
τ, min
6
7
8
9
10 11
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2
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INORGANIC MATERIALS Vol. 46
No. 3
2010