Fluxing system for reactors for production of silicon

A process is disclosed for the production of elemental silicon that utilizes a non-reactive condensible gas as a means for purging the reactor of ambient air prior to the introduction of an alkali metal and a silicon tetrahalide.

BACKGROUND OF THE INVENTION 
The reduction of a gaseous transition metal halide such as silicon with an 
alkali metal such as sodium has been described in the prior art. In that 
procedure, a sweep of the reactor with dry argon has been utilized to 
exclude atmospheric moisture and oxygen before initiating the reaction. 
This is done to prevent the alkali metal from reacting with water and 
forming undesirable by-products. In an alternative procedure, a vacuum 
pump has been used to evacuate the reactor before introduction of the 
reactants in order to remove atmospheric moisture and oxygen. The use of 
dry argon results in the discharge into the ambient atmosphere of argon 
which is not recycled. The use of a vacuum chamber requires the use of a 
high strength reactor and sealing means that prevent the entry of 
atmospheric moisture and oxygen. The expense of constructing specialized 
vacuum reactors and the introduction of contaminants from seal lubricants 
or gaskets have rendered these approaches unsatisfactory. The applicants 
have discovered a process for removal of the ambient air and moisture from 
a reactor which avoids the need to use argon or a high vacuum. The process 
comprises the use of a non-reactive, condensible gas that is passed into 
the reactor to expel ambient air. Thereafter the reactor is filled with 
the gaseous transition metal halide and the non-reactive condensible gas 
is transferred to a condensing zone where it is separated from the gaseous 
transition metal halide. Thereafter the separated gaseous transition metal 
halide is recycled to the reactor and the non-reactive, condensed gas is 
placed in a reservoir from which it is recycled via an expansion valve 
when it is required to expel atmospheric moisture and oxygen from the 
reactor.

DETAILED DESCRIPTION OF THE INVENTION 
In the U.S. Pat. No. 4,442,082, which is incorporated by reference, there 
is described a general reaction scheme for the reduction of a transition 
metal halide using an alkali metal. This reaction is also described in 
Final Report, "Novel Duplex Vapor - Electrochemical Method for Silicon 
Solar Cell", by: L. Nanis, A. Sanjurjo, K. Sancier and R. Bartlett, March 
1980, which is incorporated by reference. 
The invention may be practiced by the production of substantially pure 
elemental silicon, by a process comprising the steps of: 
(a) passing a non-reactive gas that is condensible at a temperature from 
about -90.degree. C. to +50.degree. C. and preferably -60.degree. C. to 
+20.degree. C. at atmospheric pressure into a reaction zone to displace 
ambient air; 
(b) passing a silicon, tetrahalide into said reaction zone to displace said 
non-reactive condensible gas; 
(c) passing an alkali metal into said reaction zone to obtain a mixture of 
silicon, and an alkali metal halide. 
For this system to work, the non-reactive, condensible gas (NRCG) must be 
removed by condensation at a sufficiently low temperature so that its 
vapor pressure is very low, typically less than 1 torr. In this way, the 
NRCG (1) will not be lost during displacement of air from the reactor and 
(2) it will not contaminate the halogenated silane (SiF.sub.4). The lowest 
possible temperature for condensation must be at least slightly above 
-94.8.degree. C. at which temperature the vapor pressure of the SiF.sub.4 
is 1 atmosphere, otherwise SiF.sub.4 will condense. Therefore, we must 
select the candidates for the NRCG so that their vapor pressures are 
sufficiently low at the temperature of condensation. 
Also, the flushing of the reactor with the NCRG must be done after the 
reactor is cool enough to prevent (1) thermal decomposition of the NRCG or 
(2) its reaction with SiF.sub.4 or oxygen from air. 
The non reactive condensible gas that is condensible at a temperature of 
-90.degree. C. to +50.degree. C. may be one of but not limited to: 
______________________________________ 
b.p.(.degree.C.) 
(760 mm Hg) 
______________________________________ 
trichlorofluoromethane 
25.degree. 
dichlorodifluoromethane 
-29.degree. 
chlorodifluoromethane 
41.degree. 
1,1-diflourethane -25.degree. 
1-chloro-1,1-difluoroethane 
-10.degree. 
1,1,2-trichloro-1,2,2- 
48.degree. 
trifluoroethane 
1,2-dichloroo-1,1,2,2- 
3.8 
tetrafluoroethane 
______________________________________ 
and mixtures thereof. 
The especially preferred non-reactive condensible gases are those which 
will condense between -30.degree. and 5.degree. C., and that have very low 
vapor pressures in the condensed phase. Other gases such as sulfur dioxide 
may be utilized. The process is carried out by passing the non-reactive, 
condensible gas into a reaction zone so that it displaces the ambient air. 
Thereafter the inlet for the non-reactive condensible gas is closed and 
the tetrahalide compound is passed into the reaction zone where it mixes 
with and expels the non-reactive condensible gas. The flow of tetrahalide 
compound is continued for a period of time so that substantially all of 
the reaction zone will be filled with the tetrahalide gas. 
The non-reactive condensible gas and the mixture of the non-reactive 
condensible gas with air or the tetrahalide compound are transferred from 
the reaction zone to a suitable condensing zone. The condensing zone will 
have means for venting ambient air that is separated from the non-reactive 
condensible gas and the tetrahalide gas. In addition means will be 
provided for the separation of the tetrahalide gas from the non-reactive, 
condensible gas. The non-reactive condensible gas is liquified in a 
condenser and the liquid is passed to a reservoir from which it may be 
recycled and expanded to a gas for reuse in the reaction zone. 
After the tetrahalide gas displaces substantially all of the non-reactive 
condensible halide, the reaction may be initiated by introducing the 
alkali metal. The reaction is terminated by turning off the inlet of 
alkali metal and before removal of the reactants from the reactor any 
tetrahalide gas is removed by using an inert gas which moves the 
tetrahalide gas from the reactor to means for condensing the tetrahalide 
gas from the streams of inert gas and tetrahalide gas. The invention may 
be utilized in batch type reactors or in continuous production reactors. 
The inert gas is separated in the means for condensing the inert gas and 
is passed to a reservoir in which it may be stored and recycled to the 
reactor when needed. 
The temperature in the condensing means will be low enough to cause the 
inert gas that is being utilized to condense. The gas should have a very 
low vapor pressure typically below 1 torr and preferably under 1 m torr. 
The preferred gaseous halide is silicon tetrafluoride and the preferred 
alkali metal is sodium. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 illustrates a batch type reactor for use in the practice of the 
invention. 
To start a reactor that has been opened to the air, the non-reactive 
condensible gas such as dichlorodifluoromethane is passed into the reactor 
through line 17 after being expanded by valve 6. The 
dichlorodifluoromethane pushes out the ambient air from the reactor 
through valve 2. 
The ambient air passes through line 13 into the condenser 22 which is 
operated at a temperature that does not condense the air but does condense 
the dichlorodifluoromethane and the air passes through the condenser and 
is pumped out of the system through valve 3 in line 14. The 
dichlorodifluoromethane is condensed in the condenser and passed through 
line 15 to a reservoir 23. The non-reactive condensible gas is recycled to 
the reactor through line 16 using a pump and valve 5 to control the flow 
to the reactor. When the partial pressure of 0.sub.2 in the reaction zone 
is below 1 m torr, as determined by a standard oxygen sensor, valve 5 is 
closed and valve 1 is opened to allow the silicon tetrahalide gas to flow 
into the reactor through line 11 and expel the dichlorodifluoromethane 
through valve 2. The mixture of SiF.sub.4 and the non-reactive condensible 
gas are separated in the condenser and the SiF.sub.4 is passed through 
valve 4 to line 12 where it is recycled to the reactor by means of a pump. 
Valve 1 is a constant pressure valve set to deliver SiF.sub.4 at 1 
atmosphere. As the condensible gas is condensed the pressure drops and 
valve 1 automatically admits more SiF.sub.4. When substantially all of the 
dichlorodifluoromethane is removed from the reactor, as determined by no 
flow of SiF.sub.4 into the system through valve 1, valve 2 is closed and 
sodium metal is introduced through valve 18 to initiate the reaction. 
Before introducing sodium, the amount of air in the reactor can be 
monitored using a conventional on-line gas sampling means such as gas 
chromatography or mass-spectroscopy. The procedure for removing the 
ambient air is repeated before restarting the reactor anytime that the 
reactor is shut down and air is introduced by opening removal door 19. 
The line 14 is provided with off take valve 4 which is connected to line 12 
to recycle separated silicon tetrafluoride gas to intake line 11. Valve 1 
controls the flow of silicon tetrafluoride from intake line 11 and recycle 
line 20. 
The means for purging silicon tetrafluoride gas from the reactor when it is 
shut down comprise a valve 24 and inert gas reservior 25. The reaction is 
terminated by closing valves 1 and 18. Any unreacted silicon tetrafluoride 
gas is removed from the reactor by opening valve 24 and 21 and causing an 
inert gas such as argon to push the silicon tetrafluoride through line 26 
to a condenser 27 that condenses the silicon tetrafluoride but not the 
inert gas. Valve 24 is a constant pressure valve set to deliver argon at 1 
atm of pressure. The separated inert gas is pumped to a reservoir 25 to be 
recycled and the condensed silicon tetrafluoride can later be passed via 
line 20 to expansion valve 28 where it is expanded and recycled to the 
reactor. Once the reactor is filled with argon, the reactor can then be 
opened to the atmosphere.