Preparation and fine purification of SiF4 and 28SiH4

Article abstract of DOI:10.1023/A:1014735203351

Silicon tetrafluoride was obtained by the thermal decomposition of pure-grade sodium hexafluorosilicate and purified by low-temperature distillation. Next, SiF₄ was ²⁸Si-enriched by centrifugation and converted to silane by calcium h

Full text of DOI:10.1023/A:1014735203351

Inorganic Materials, Vol. 38, No. 3, 2002, pp. 283–287. Translated from Neorganicheskie Materialy, Vol. 38, No. 3, 2002, pp. 356–361.
Original Russian Text Copyright © 2002 by Bulanov, Balabanov, Pryakhin, Troshin.
Preparation and Fine Purification of SiF4 and 28SiH4
A. D. Bulanov, V. V. Balabanov, D. A. Pryakhin, and O. Yu. Troshin
Institute of Chemistry of High-Purity Substances, Russian Academy of Sciences,
ul. Tropinina 49, Nizhni Novgorod, 603950 Russia
e-mail: bulanov@ihps.nnov.ru
Received July 11, 2001
Abstract—Silicon tetrafluoride was obtained by the thermal decomposition of pure-grade sodium hexafluoro-
silicate and purified by low-temperature distillation. Next, SiF4 was 28Si-enriched by centrifugation and con-
verted to silane by calcium hydride reduction. To remove hydrocarbons, the silane was distilled.
Silicon tetrafluoride, owing to its high saturation not yield a sufficiently pure product either. For this rea-
vapor pressure and chemical stability and the fact that son, we turned to fractional distillation (hereafter, dis-
19F is the only fluorine isotope, is a convenient com-
pound for separating silicon isotopes with the purpose
tillation). Earlier, only one work [23] was devoted to
SiF4 distillation, but its purpose was silicon isotope sep-
of obtaining chemically pure monoisotopic silicon, a aration rather than SiF4 purification.
promising electronic material.
The setup we used to purify SiF4 is schematized in
On a laboratory scale, SiF4 can be obtained by a
variety of methods, in particular by reacting silicon and
fluorine [1, 2], fluorinating silicon oxide or tetrachlo-
ride with various agents [3–13], and decomposing fluo-
rosilicic acid with concentrated acids [14, 15]. The
main industrial sources of SiF4 are Li, Na, K, Ca, and
Ba fluorosilicates. The most common of them is the
sodium hexafluorosilicate waste in the conversion of
apatites to phosphoric acid or mineral fertilizers [16].
Thermal decomposition of Na2SiF6 affords 99.999%-
pure SiF4 because most of the impurities concentrate in
the sodium fluoride [17].
In this work, SiF4 was obtained by decomposing
pure-grade Na2SiF6 predried at 250–300°C in vacuum
or flowing nitrogen for several hours. The reaction was
run in vacuum at 500 to 620°C in a resistance-heated
stainless steel reactor. The resulting SiF4 was continu-
ously removed and directed to a metallic cylinder
cooled with liquid nitrogen.
Fig. 1. SiF4 was distilled in a middle-fed stainless steel
column with a foamed-plastic insulation. The rectify-
ing sections (1) were packed with 3 × 3 × 0.25-mm
nichrome prismatic coils. First, the condenser (2) was
cooled to the set temperature by liquid nitrogen fed at a
controlled rate. Next, up to 1.5 kg of SiF4 was intro-
duced into the column from cylinder 6 through the con-
denser. Liquid SiF4 was fed from the tank (4) into the
bottom section through a flow-control valve (3), ini-
tially at a high rate to wet the packing and then at
1.5 l/h. The SiF4 pressure was measured with gage 5
and controlled by varying the liquid nitrogen supply to
the condenser. The reboiler (14) was maintained at a
constant temperature with the use of a Proterm-100
microprocessor-based temperature-controller.
The condenser temperature was –70°C, and the
gage pressure was 0.3 MPa. The column was operated
in the closed-distillation mode for 4 h, and then the
overhead and bottoms were simultaneously drawn off
at a constant rate into cylinders 7 and 8, respectively.
The withdrawal rate was controlled with metering
valves 9 and measured with flowmeters 10. The with-
drawal ratio was Ӎ0.002. The overhead, bottoms, and
main product were sampled at intervals for IR spectro-
scopic analysis into cell 11 through a sampling line.
Hydrogen fluoride, silane, trifluorosilane, and methane
accumulated in the overhead; hexafluorodisiloxane,
carbon dioxide, and silicon hydroxyfluoride, in the bot-
Note that the starting salt contained about 0.2 wt %
moisture and, even after predrying, yielded a product
contaminated with oxygen-containing compounds,
e.g., hexafluorodisiloxane, because of the high hydro-
lyzability of SiF4.
Since heavy impurities make the centrifugal isoto-
pic enrichment of SiF4 difficult, it was necessary to fur-
ther purify the SiF4.
Purification by chemical means (passing SiF4 over toms. The desired product stayed in the feed tank. A
typical plot of the Si2OF6 concentration in the bottoms
iron powder at 800°C [18] or through concentrated sul-
furic acid [19]) and by impurity adsorption on silica gel as a function of distillation time is presented in Fig. 2.
[20] or activated charcoal [21] proved inefficient. Phys- The process was terminated after the Si2OF6 concentra-
icochemical methods such as outgassing of solid SiF4,
fractional sublimation, and simple distillation [22] do ferred through Petryanov filter 12 to cylinder 13.
tion ceased changing. The purified product was trans-
0020-1685/02/3803-0283$27.00 © 2002 MAIK “Nauka/Interperiodica”

284
BULANOV et al.
5
2
12
13
7
3
to forepump
10
9
1
4
11
IR
8
14
6
28
Fig. 1. Schematic of the SiF and SiH distillation system: (1) rectifying sections, (2) condenser, (3) flow-control valve, (4) feed
4
4
tank, (5) pressure gage, (6–8) cylinders, (9) metering valves, (10) flowmeters, (11) IR spectroscopic cell, (12) Petryanov filter,
(13) receiving cylinder, (14) reboiler.
The IR spectroscopic analysis of the purified prod-
uct is presented in Table 1. Distillation reduced the
Si2OF6 concentration in SiF4 by one order of magni-
tude. The purified product was isotopically enriched by
centrifugation at OAO GAZ. Four enriched SiF4 speci-
The processes of the third group are rather diverse.
One of them is plasma conversion of SiF4 to silane. It is
characterized by a low silane yield of Ӎ25% per cycle
and gives fluorosilanes (SiH2F2, SiH3F) as by-products,
which have to be converted to silane by reaction with
NaF [32]. Another approach is to reduce SiF4 with an
alkali or alkaline-earth hydride or an alkali or alkaline-
earth aluminum hydride [33–35]. It requires a very dry
solvent (ether), which has to be recovered after each
use. Moreover, ethers may contaminate silane with car-
bon. The third process is hydrogen reduction of SiF4 in
the presence of calcium [36]. It combines calcium
28
mens were obtained, in which the Si abundance was
99.6, 99.82, 99.93, and 99.96%. Note that the concen-
tration of Si2OF6 (which was 28Si-enriched to the same
extent) in the enriched SiF4 specimens was 10–1 to
10−2 mol %.
The numerous processes for converting SiF4 into Si
can be divided into three groups (Table 2). The first
includes direct conversion processes: laser decomposi-
tion [24], developed at the General Physics Institute,
Russian Academy of Sciences, which requires sophisti-
cated equipment and is difficult to implement; ammo-
nia reduction of SiF4 [25]; and sodium reduction of SiF4
[26, 17]. The processes of the second group involve
SiO2 as an intermediate compound. SiO2 is produced by
either SiF4 hydrolysis in an alkali solution or oxidation
with oxygen, air, or water vapor. Next, SiO2 is reduced
with either hydrogen plasma [27] or a solid reductant
such as carbon [28], boron [29], magnesium [30], and
aluminum [31], which are controlled impurities in sili-
con. The third group of processes involve SiF4-to-SiH4
conversion followed by silane decomposition to poly-
crystalline silicon and hydrogen. An advantage of these
processes is that the silane can be purified before
decomposition to afford a purer final product.
2
6
40
35
30
25
20
15
10
5
0
5
10
15
20
25
Time, h
Fig. 2. Time dependence of the Si OF concentration in the
2
6
SiF distillation bottoms.
4
INORGANIC MATERIALS Vol. 38 No. 3 2002

PREPARATION AND FINE PURIFICATION OF SiF4 AND 28SiH4
285
eutectic is hygroscopic and, therefore, needs to be thor-
oughly dried. Furthermore, it has to be filtered often to
remove the calcium fluoride precipitate.
Table 1. IR spectroscopic analysis of SiF4
Molar percent
before distillation after distillation
Impurity
We reduced SiF4 with CaH2 in the absence of a sol-
vent [38]. With commercial CaH2, silane formed in a
low yield, apparently because CaH2 contained the prod-
ucts of its reaction with atmospheric moisture—cal-
cium oxide and hydroxide. The reaction of these com-
pounds with SiF4 is thermodynamically plausible. To
obtain a purer CaH2, we reacted distilled calcium with
Pd-diffused hydrogen.
Si2OF6
SiF3OH
SiF3H
H2O
1.8
–
0.1
2 × 10–3
<3 × 10–5
2.3 × 10–3
<1 × 10–3
2 × 10–3
<8 × 10–4
4.7 × 10–4
–
–
CH4
<1 × 10–3
6 × 10–3
<2 × 10–3
1 × 10–3
CO2
The silane resulting from the reaction
CO
28SiF4(g) + 2CaH2(s)200°C 28SiH4(g) + 2CaF2(s)
HF
was found to be isotopically diluted by less than 0.1%.
Its analysis is presented in Table 3. The SiF4 content,
determined by IR spectroscopy, was below 0.01 mol %,
but the total fluorine content, determined by ion-
exchange chromatography, was 0.1 wt %.
hydriding and silane synthesis. This process is very
exothermic and may cause partial silane decomposi-
tion. The fourth process is SiF4 reduction with calcium
hydride. This hydride is cheaper and safer to handle
than LiH, NaH, LiAlH4, and NaAlH4. Fadeev et al.
[37] proposed to react SiF4 gas with CaH2 in a molten
LiCl (60%) + KCl (40%) eutectic mixture. However,
The reaction yield was 90–98% in terms of SiF4 and
6–9% in terms of CaH2. The capacity of our apparatus
was about 10 g/h. The basic disadvantages of this pro-
cess are a low yield in terms of CaH2 and the necessity
this approach requires laborious preparations. The of grinding the hydride in a very dry atmosphere. Small
Table 2. Methods for converting SiF4 to Si
Possibili-
Product
Method
t, °C
Other reagents
Product
Impurities ty of puri-
fication
yield, %
SiF4
Si
1. Laser decomposition
2. SiF4 + NH3
–
H2, Ar
–
Si powder
–
–
–
850–900 NH3
99.9 99.9% 28Si film
3. SiF4 + Na
300–600 Na, Ar
–
Si + NaF
SiF4
SiO2
SiO
Si
1. SiO2
Si in plasma
1) 4400 H2
2) 500
99.5 Si powder
10–6%
–
2. SiO2 + C in an arc furnace
3. SiO2 + B
–
C
–
Si + SiO
C
B
–
–
–
–
1527 ± 20 B
1433 Mg
1100 Al
90–95 28–30Si film
4. SiO2 + Mg
–
–
Si + MgO
28Si
Mg
B
5. SiO2 + Al
SiF4
SiH4
Si
1. In plasma
2. MH, MAlH4
3. H2 + Ca
4. CaH2
300–3000 H2
25
SiH2F2, SiHF3
10–5%
+
+
+
+
60–250 Ethers
97–100 SiH4
C
–
H2, Ca
–
SiH4
360–390 CaH2, LiCl, KCl
180–200 CaH2
97
SiH4 99.999%
5. CaH2
90–98 SiH4
ca. 10–5%
+
INORGANIC MATERIALS Vol. 38 No. 3 2002

286
BULANOV et al.
Table 3. Analysis of isotopically enriched silane
sorbed by sodium fluoride or further hydrogenated with
CaH2. As a result, the total fluorine content was as low
Concentration
as 10–4 wt %. To remove the hydrocarbons, 28SiH4 was
distilled in a quartz or metallic column similar to the
column in Fig. 1, but with a smaller diameter of the rec-
tifying sections and a smaller feed tank (300 cm3). The
charge weight was 70–180 g. The condenser tempera-
ture was about –110°C, and the pressure was slightly
above atmospheric. The reflux rate was 0.8 of the max-
imum and was Ӎ900 ml/h in the metallic column and
360 ml/h in the quartz column. The column was oper-
ated in the closed-distillation mode for 3 h, and then the
overhead and bottoms were simultaneously drawn off
at a constant rate. The withdrawal ratio was 0.001.
Methane and hydrogen accumulated in the overhead;
the other hydrocarbons and disilane, in the bottoms.
The purified product was transferred through a Petry-
anov filter to a receiver.
Analytical
method
Impurity
after
after
synthesis purification
28SiF4
<10–2
IR spectroscopy
CH4
2.4 × 10–2
1 × 10–5 Gas chromatog-
raphy
C2H6
3.0 × 10–4 1.6 × 10–5
7.1 × 10–4 2 × 10–5
C2H4
C3H8
3.3 × 10–4 <5 × 10–6
3.1 × 10–4 <5 × 10–6
2.6 × 10–5 <5 × 10–6
1.9 × 10–4 <5 × 10–6
C3H6
n-C4H10
i-C4H10
The resulting isotopically enriched silane was used
Other hydrocar- Ͼ6.4 × 10–4 <5 × 10–6
bons in terms
of C5H12
to obtain 28Si by thermal decomposition.
ACKNOWLEDGMENTS
Total F–
10–1
3 × 10–4 Ion-exchange
chromatography
This work was supported by the Russian Foundation
for Basic Research, grant no. 01-03-33208.
Mg
7 × 10–8 Atomicemission
spectroscopy
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