Distribution coefficients in the systems SiF4-Si 2OF6 and SiF4-SiOF2

Article abstract of DOI:10.1023/A:1017596315429

The liquid-vapor distribution coefficients in the SiF₄-Si ₂OF₆and SiF₄-SiOF₂systems were determined between 183 and 250 K. The results were used to evaluate the enthalpies of Si₂OF₆

Full text of DOI:10.1023/A:1017596315429

Inorganic Materials, Vol. 37, No. 4, 2001, pp. 413–415. Translated from Neorganicheskie Materialy, Vol. 37, No. 4, 2001, pp. 498–500.
Original Russian Text Copyright © 2001 by Devyatykh, Pryakhin, Bulanov.
Distribution Coefficients in the Systems SiF4–Si2OF6
and SiF4–SiOF2
G. G. Devyatykh, D. A. Pryakhin, and A. D. Bulanov
Institute of High-Purity Substances, Russian Academy of Sciences,
ul. Tropinina 49, Nizhni Novgorod, 603600 Russia
Received May 25, 2000; in final form, October 5, 2000
Abstract—The liquid–vapor distribution coefficients in the SiF4–Si2OF6 and SiF4–SiOF2 systems were deter-
mined between 183 and 250 K. The results were used to evaluate the enthalpies of Si2OF6 and SiOF2 vapori-
zation.
The purity of silicon tetrafluoride for electronic and tion coefficients α in the systems SiF4–Si2OF6 and
fiber optics applications must be very high. Silicon tet-
rafluoride prepared from alkali- and alkaline-earth flu-
orosilicates typically contains volatile compounds of
sulfur, hydrogen, and carbon as impurity phases [1, 2],
because the starting fluorosilicates are fluorine isola-
tion products in processing natural phosphates by the
nitrate route. The content of these impurity species is
not very high, and they can be removed easily by both
chemical and physicochemical means [3]. Water plays
a special role in determining the impurity composition
of SiF4 [4]. Even small amounts of water lead to the for-
mation of siloxane species [5]:
SiF4–SiOF2.
In our experiments, we used silicon tetrafluoride
prepared by thermal decomposition of sodium hexaflu-
orosilicate and purified by low-temperature rectifica-
tion. According to IR spectroscopic analysis (IFS-113V
spectrophotometer), the Si2OF6 content of the sample
was 0.08 mol %.
The liquid–vapor distribution coefficients were
determined by the static equilibration and Rayleigh dis-
tillation methods [8]. The experimental setup used is
shown in Fig. 1.
To determine the distribution coefficient as a func-
tion of temperature by the static equilibration method,
the thermostat was connected directly to the inlet of the
mass spectrometer. After the sample was thermostated
SiF4 + H2O = SiF3OH + HF,
SiF3OH + SiF4 = SiF3OSiF3 + HF.
In SiF4 + Si2OF6 + H2O gas mixtures, siloxane may
hydrolyze further,
6
SiF3OSiF3 + H2O = SiF3OSiF2OH + HF,
SiF3OSiF2OH + SiF4 = SiF3OSiF2OSiF3 + HF,
8
7
to form higher molecular (including cyclic) compounds
[6] which differ markedly in properties from silicon tet-
rafluoride and tend to further polymerize and conden-
sate. In view of this, these compounds can be removed
readily by cryofiltration or common sublimation.
Hexafluorodisiloxane, Si2OF6, and silicon oxyfluoride,
SiOF2, are the impurities closest in properties to SiF4
and, hence, the most difficult to separate. According to
IR spectroscopy data [7], the Si2OF6 content of silicon
tetrafluoride varies from a few molar percent to hun-
dredths of a molar percent. The problem of reducing the
content of these impurities continues to be a challenge.
One potentially attractive approach is low-temperature
rectification. The implementation of this method
depends crucially on precise knowledge of the relevant
liquid–vapor distribution coefficients. The purpose of
this work was to experimentally determine the distribu-
To vacuum pump
To mass spectrometer
1
2
3
4
5
9
10
Fig. 1. Schematic of the experimental setup: (1) tempera-
ture-controlled chamber, (2) cryostat, (3) electric heater,
(4) metallic Dewar, (5) heat-insulating case, (6) manometer,
(7, 8) valves, (9, 10) cylinders.
0020-1685/01/3704-0413$25.00 © 2001 MAIK “Nauka/Interperiodica”

414
DEVYATYKH et al.
and then equilibrated for 3 h, the vapor phase was fed
α
2.6
directly into the ionization chamber of the mass spec-
trometer. Experiments were performed at temperatures
in the range 185–257 K.
2.4
2.2
2.0
1.8
1.6
1.4
1.2
1.0
The distribution coefficient was calculated by the
formula
α = x/x0,
4
2
where x and x0 are the impurity concentrations in the
vapor and liquid in equilibrium. The impurity concen-
tration in the liquid phase was assumed to be equal to
that in the starting silicon tetrafluoride. We also
assumed that the composition of the liquid phase
remained unchanged when the vapor sample was taken,
since a very small amount of the substance was taken
for analysis. The results are shown in Fig. 2.
In the Rayleigh distillation method, silicon tetraflu-
oride was fed from cylinder 10 to cylinder 1 precooled
to 153 K. The amount of SiF4 was determined from the
change in the weight of cylinder 10. Next, cylinder 1
was heated to the preset temperature, which was then
stabilized by heater 3, connected to a PROTERM 100
microprocessor controller. During experiments, the
temperature was maintained constant to within 1 K and
was measured by a copper–Constantan thermocouple
with an accuracy of 0.2 K.
3
1
180 190 200 210 220 230 240 250 T, K
Fig. 2. Temperature dependences of the liquid–vapor distri-
bution coefficient: (2, 4) static equilibration method,
(1, 3) Rayleigh distillation (w = 0.19 g/min) methods;
(3, 4) SiOF , (1, 2) Si OF .
2
2
6
α
2.4
2
1
2.2
2.0
1.8
1.6
1.4
1.2
1.0
After equilibration for 2–3 h, we set a constant flow
rate from cylinder 1 to cylinder 9 and performed distil-
lation for 4–5 h. Under the assumption that the flow rate
was constant throughout the process, the distillation
rate was determined as the ratio between the distillate
weight and the distillation time. The impurity concen-
trations in the starting sample and distillate were deter-
mined by mass spectrometry (MI-1309 instrument,
electron-impact ion source). The concentrations were
0
10
20
30
40
50
1/w, min/g
Fig. 3. Distribution coefficient vs. inverse distillation rate
for (1) SiOF and (2) Si OF at 203 K.
expressed as the percentage ratio of the 28SiF4+ (m/e =
2
2
6
+
+
104) to 28Si2 OF6 (m/e = 186) or 28Si32OF2 (m/e = 82)
32
logp [Pa]
5.9
ion intensity. The ionizing voltage in the ion source was
50 V, the collector current was 1.5 mA, and the acceler-
ating voltage was 1.5 kV.
The preliminary results obtained at a distillation rate
w = 0.19 g/min and different temperatures are pre-
sented in Fig. 2. It can be seen that the distribution coef-
ficients of the two impurities are notably lower than
those determined by the static equilibration method. In
view of this, subsequent experiments were carried out
at lower distillation rates. The 203-K distribution coef-
ficients as functions of the inverse distillation rate are
shown in Fig. 3. At low distillation rates (<0.03 g/min),
α varies very little. The α values obtained by the Ray-
leigh distillation method at w < 0.03 g/min coincide
with those determined by the static equilibration
method.
5.7
5.5
5.3
5.1
1
2
4.9
4.4
4.6
4.8
5.0
5.2
103/T, K–1
Using the present results and the temperature depen-
dence of SiF4 vapor pressure reported earlier [9], we
calculated the temperature-dependent SiOF2 and
Fig. 4. Temperature dependences of (1) SiOF and
(2) Si OF vapor pressures.
2
2
6
INORGANIC MATERIALS Vol. 37 No. 4
2001

DISTRIBUTION COEFFICIENTS IN THE SYSTEMS SiF4–Si2OF6 AND SiF4–SiOF2
415
5. Sennikov, P.G., Ikrin, M.A., Ignatov, S.K., et al., IR
Spectroscopic and Quantum-Chemical Studies of Sili-
con Tetrafluoride Hydrolysis Products at Low Water
Concentrations, Izv. Akad. Nauk, Ser. Khim., 1999, no. 1,
pp. 92–96.
Si2OF6 vapor pressures (Fig. 4) and evaluated the
198-K vaporization heats of these substances:
∆vH(Si2OF6) = 18.4 kJ/mol and ∆vH(SiOF2) =
15.9 kJ/mol.
6. Catrett, F.D. and Margrave, J.L., A New Fluorosiloxane
Synthesis, Synth. Inorg. Metalorg. Chem., 1972, vol. 2,
no. 4, pp. 329–333.
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INORGANIC MATERIALS Vol. 37
No. 4 2001