The feasibility of using alkali metal-silicon halide diffusion flames to produce solar-grade silicon in large quantities and at low cost is demonstrated. Prior work shows that these flames are stable and that relatively high purity silicon can be produced using Na + SiCl4 flames. Silicon of similar purity is obtained from Na + SiF4 flames although yields are lower and product separation and collection are less thermochemically favored. Continuous separation of silicon from the byproduct alkali salt was demonstrated in a heated graphite reactor. The process was scaled up to reduce heat losses and to produce larger samples of silicon. Reagent delivery systems, scaled by a factor of 25, were built and operated at a production rate of 0.5 kg Si/h. Very rapid reactor heating rates are observed with wall temperatures reaching greater than 2000 K. Heat release parameters were measured using a cooled stainless steel reactor tube. A new reactor was designed

Miller, W. J.

Olsen, D. B.

English

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Produced by the NASA Center for Aerospace Information (CASI) 
https://ntrs.nasa.gov/search.jsp?R=19790008187 2020-03-22T01:45:09+00:00Z
,,.^-oChem TN -201	 DOE /JPL 954777-79/6
Distribution Cote9ory' UC-63
SILICON HALIDE -ALKALI METAL
FLAMES AS A SOURCE OF
SOLAR GRADE SILICON
(NASA-CG-158091)
	 STLICGN HALIDE-ALvALI
	
N79- 163E8
METAL FLAMES AS A SOUNCE CF SCLAS GRADE
SILICON Quarterly Rel:ort (AeroChem Research
Labs_, Inc.)
	 15 F HC 402/MF A01	 CSCL 10A	 Onclas
G3/44 43f16
SIXTH QUARTERLY REPORT	
^'arLc^ to 0 11 J
D. 8 OLSON AND W. J MILLER
iANUARY 1979
Al,
The JPL Low-Cost Silicon Solar Array Project is
sponsored by the 1). S. Departuent of Energy and
fors part of the Solar Photovoltaic Conversion
Prograa to initiate a savor effort towerd the
developneot of low-cost solar arrays. This work
was perforsed for the Jet Piopulsion Laboratory,
California Institute of Technology by agreement
between NASA and DoE.
04vel6mv Rosoarch Laboratories, Inc.
Princeton, Now Jersey
FOREWORD AND ACKNOWLEDGMENTS
This is the sixth quarterly progress report on a program which began
17 May 1977; it covers the period 1 October 1978 through It December 1978.
Major contributions to the experimental effort on the program lur frip
this report period were made by G. Rolland anal L. hoonig.
TN-201
ABSTRACT
This program is dot,ig,ned to demonstrate the feasibility of using alkali
metal-RiIicon halide diffusion flames to produce solar-grade silicon in large
quantities and at low cost. Prior work has shown that these flames are stable
and that relatively high purity silicon can be produced using Na + SiCl,, flames.
Silicon of :similar purity is obtained from Na + Si F,, flames although yields
are lower and product separation and collection are less thermochemically
favored. Continuous separation of silicon front 	 byproduct alkali salt has
been demonstrated in a heated graphite reactor.
During the current reporting period the process has been scaled up to
reduce heat losses and to produce larger samples of silicon. Reagent delivery
systems, scaled by a factor of 25, have been built and operated at a production
rate of 0.5 kg Si h - '. Very rapid reactor heating rates are observed with wall
temperatures reaching } 2000 K. Host ri-loose parameters wcrc rioasured using a
cooled stalnic-is steel r :-tor tube. Fxtvrnal heat Input is not required. A
new reactor has been designed based on these measurements and construction is
underway.
iii
TN-201
TABLES CONTENTS
Page
ABSTRACT
	
I.	 INTRODUCTION	 1
	II.	 TECHNICAL. DISCUSSION	 1
A. Reagent Delivery Systems 	 1
B. Scaled-Up Na + SIC1,, Experiments	 2
C. Process Variable Measurements	 3
	
III.	 PLANS	 4
	
IV.	 NEW TECHNOLOGY
	
V.	 REFERENCES	 5
LIST OF I LUSTRAI10,5-
Figure
1	 REACTOR WALL, TEMPERATURE PROFILE DURING A Na + SIC1,. RUN 	 6
	
2	 SOLIC PRODUCT DISTRIBUTION	 7
	
3	 DISTRIBUTION OF SILICON DEPOSITION	 7
	
4	 REACTOR CONFIGURATION FOR MEASURING HEAT RELEASE RATES 	 8
	
5	 TEMPERATURE PROFILES OF THE WALL COOLING AIR FOR THE REACTOR
AS SHOWN IN FIG. 4	 9
	
6	 HEAT PRODUCTION PROFILES	 10
	
7	 WALL HEAT FLUX PROFILES	 11
	
8	 VOLUMETRIC HEAT RELEASE PROFILES 	 11
I
v
1TN-201
I. INTRODUCTION
The object of this program is to determine the feasibility of using
continuous high-temperature diffusion flamers of alkali metals and silicon
halides to produce silicon in large quantities and of suitable purity for use
in the production of photovoltaic solar cells. Reactions of gaseoue, Na or K
with silicon halides (e.g., SiC14, SiHC!s, or S1F4) are highly exothermic,
luminescent' - ' and, in fact, have many characteristics in common with conven-
tional oxidation flames. High adiabatic flame temperatures have been calcu-
lated' for these systems from reliable thermochemical data and, at these
temperatures, the product alkali salt exists as a gas while the silicon is
present as a condensed phase. A process is envisioned in which the difference
in physical state of desired and undesired products is utilized to sfparate
the silicon from the byproduct salt in a continuous and efficient manner.
The experimental program is designed to demonstrate the viability of the
production/separation processes and to obtain physical and chemical variables
useful for this and other high-temperature silicon production processes. Prior
work' -4
 showed chat relatively pure silicon could be produced in low tempera-
ture reactors where the silicon and salt particles codeposited onto the walls.
More recent experin;vnts' using a heated graphite reaction tube have demonstrated
continuous high temperature separation of the silicon from the byproduct alkali
metal salt. Results obtained during the present reporting period, discussed in
detail in Section II, center on the scale -tip of the experiment to alleviate
problems; due to heat losses and to produce larger amounts of silicon. Larger
reagent delivery systems have been built and operated at a production rate of
0.5 kg Si h- '. Neat-release parameter measurements and their use in designing
an optimized reactor are also discussed in Section Il.
II. TECHNICAL DISCUSSION
A. REAGENT DELIVERY SY ST EMS
During this reporting period we have scaled up the experiment to alleviate
problems due to heat losses and to produce larger amounts of silicon. As an
initial guideline we cLose a scale factor such that the reaction exothermici.ty
(without salt condensation) would be about equal. to the electrical power pre-
viously used to heat the graphite flow reactors, about 1.2 kW, i.e., a scale-up
1I
TN-201
by about a factor of 25 to a production rate of silicon of — 0.5 kg h-'.
To produce silicon at this rate, 3.1 kg h- ' of SIC14 is consumed requiring ..
150 W for its vaporization. This energy flux could not be maintained across
the glass vaporizer walls so a platinum wire immersion heater was built. A
SiCl,, recharging system was also added since the 150 cm' capacity of our
vaporizer was inadequate at the much higher flow rate. With this system the
reaction is not limited by SiCl,, flow rate or capacity. Flow meters were in-
stalled in the halide delivery line to allow close adjustment of the Na/SiCl,,
equivalence ratio.
The required 1.7 kg h - ' of Na corresponds to a flow rate of 220 k min-'
of vapor at 1200 K and 0.5 atm. It was decided that an experimental confirma-
tion of the Na flow rate would be required. The heat input needed to vaporize
this quantity of Na is 2.0 kW and was found to be easily within our present
vaporizer capability. Experiments were performed in which the gaseous Na de-
livered during a ten minute period was condensed and weighed to determine the
flow rate. An orifice with 3.0 mm diam was found to produce the desired flow
rate with one atmosphere Na vapor pressure in the vaporizer. (Higher pres-
sures are not used for reasons of safety.) The 0.8 k capacity of the liquid
reservoir in this vaporizer allows runs of about 30 min duration without re-
charging. This is adequate for our present experiments and no significant
problems are foreseen in constructing a liquid sodium recharging system in
the future when even larger scale runs are undertaken.
B, SCALED-UP Na + SiCl/. EXPERIMENTS
In the first test of these scaled-up systems the reactor was a 1.5 cm i.d.,
0.3 cm wall thickness graphite tube loosely held inside a stainless steel tube.
This combination was heated to 1200 K by a "clam shell" resistance heater prior
to beginning the experiment. The Na + SiCl,, flame was run for six minutes before
solid product deposits clogged the reaction tube. Approximately 4.7 moles Na
and 1.4 mole SiCl,, were delivered, corresponding very closely to the desired
rate of silicon production. The graphite tube was found to be fractured into
pieces and saturated with silicon. Both reagent systems performed without
problems.
Subsequent experiments were carried out in larger (4.3 cm 1.d.) alumina
reaction tubes. With the tube preheated to 1400 K, introduction of the reagents
quickly raised the wall temperature to 1900 K where it stabilized for the
2
TN-201
remainder of the experiment. From this initial wall heating rate, the
reaction heat release was esti mati'd to he 1.2 kW, corresponding closely to
that computed ast;uming no Nall condensation. Samples of deposits from the
middle of the reaction zone were found to be aw 90% silicon and w 10% NaCl.
Near the cool rva^,-io inlet flange and at the downstream end of the flow tube
the product was +152 silicon. Thus good separation of the silicon from the
alkali :salt is occurring in the high temperature region of the reactor.
An expt^riment was also run without preheating the reactor tube. Figure
1 shows the measured wall temperature profile. Initially a rapid wall heating
rate (800 K min -1 ) was observed during the period in which both salt and sili-
con condensation would occur ("1" in Fig. 1). The reaction exothermicity
under these conditions and flow rates 1s about 5.7 kW. The temperature then
stabilized briefly at the boiling point of NaCl (I1), and from there rose
above 2000 K where the Pt/Pt-107 Rh thermocouple failed. Thus the ultimate
reactor temperature could not be measured. The experiment was terminated due
to excess henttng of the outer glass vacuum system wall. Although the reaction
tube deposits contain a high portion (- 757)of NaCl, the amount of silicon
recovered as elemental Si in this experiment was about 90% of the total which
could have been produced. Figure 2 shows that the solid praducts (NaCl and
Si) were predominantly collected in the section of the reactor near the inlet,
while Fig. 3 indicates whe.- t;-.: silicon deposited.
Thus input of additional energy to preheat the reactor does not appear
to be necessary. However, longer run times are needed to reach steady-state
in the reactor to achieve good separation of the silicon from the byproduct
salt. Nucleation and condensation rates appear to be large enough for effi-
cient silicon collection under experimental conditions of about 160 ms
residence time and 0.5 atm in the flow tube.
C. PROCESS VARIABLE MEASUREMENTS
Heat release parameters necessary to optimize the reactor size or further
scale up to produce larger amounts of silicon have also been measured. To do
this a 6 cm i.d. stainless steel tube with four separate cooling coils was
installed into the reactor, cf. Fig. 4, and wall cooling air temperature pro-
files for each section were measured during reactor operation, from a cold
start. These profiles are shown !n Fig. 5. The total heat production was
computed from the temperature profiles and the heat capacity of the COO11T1g
cr
3
"W-P-
	 I
TN-201
air and reactor, and is shown in Fig. 6 for each reactor suction and the total
(3:). Most of tht- heat is seen to be produced in zones A and B which are 10
and 16 cm ?ong, respectively. 1'rom the reactant Mow rates we calculate an
exothermicity of 4.2 kW, in good agreement with the experimental results.
Measured heat flux through the reactor walls is presented in Fig. 7. In
the first 25 cm of the reactor an average value of `i W cm - ' was obtained. Volu-
metric heat release data is Riven in 1'1g. 8. An average value of 3 W cm-' was
measured for the portion of the reactor near the inlet. These data clearly
show that the reaction takes place in sections A and B of this reactor, confirm-
ing that the kinetics of the reaction are rapid, probably mixing limited. Thus
smaller reactor volumes can he used, simplifying reactor design and construction.
III-	 PLANS
An optimized reactor is now under construction to 1.^ used to demonstrate
production of larger quantities of silicon. Our aim is to collect high purity
liquid silicon at the reactor outler with the NaCl (antt hopt-ful.ly mw>st t t1Ler
impurities) being collected on coaled surfaces far front 	 Si ccillector. A
small (8 cm diam, 25 cm long) cylindrical graphite reactor has been designed.
The reactor will operate near 0.5 atm and reaction products (NaCl(g) and Si(t))
will leave the reactor through a small converging nozzle exhausting into a
large volume held at a low (,< 10 Torr) pressure. The jtt from the reactor
impinges on a graphite crucible where Si(Q) droplets will be collected. A
large (65 cm diam, 80 cm long) all-metal vacuum housing is being acquired to
contain the reactor and Si co-lector. Its water cooled walls will provide Sur-
faces for Na ca collection for the time being. More detailed design calculations
are being carried out. Our efforts in the next quarter will be directed toward
construction of this system and making it operational. Important questions to
be answered experimentally are whether the thick walled, nigh density graphite
will withstand the environment and how efficiently the silicon particles are
collected.
IV,  NSW T^CNNULUG"
No reportable items of new technology have been identified.
4
PINW- 7
TN-201
1. Miller,	 W.J.,	 "Silicon Halide-Alkali	 Metal	 Flames as a Source of Solar
Grade	 Silicon," First Quarterly Report, AeroChem TN-178, September 1977.
2. Miller,	 W.J.,	 "Silicon Halide-alkali Metal
	 Flames as a Source of Solar
Grade Silicon," Second Quarterly Report, AeroChem TN-182,
	
December 1977.
3. Olson,	 D.B.	 and Miller,	 W..1.,	 "Silicon	 Halide-Alkali	 Metal	 Flames as a
Source of Solar Grade Silicon," Third quarterly Report,	 AeroChem TN-187,
FRDA/JPL 954117- 78/3,	 March	 1978.
4. Olson,
	 D.B.	 and Miller,	 W.J.,	 "Silicon Halide-Alkali Metal 	 Flames as a
Source of
	 Solar Grade. Silic,)n," Fourth Quarterl y Report,	 AeroChem TN-192,
DOE/JPL 954777-78/4, June	 1078.
5. Olson,	 D.B.	 and Miller,	 W.J.,	 "Silicon halide-Alkali	 Metal	 Flames as a
Source of
	 Solar Grade Silicon," Fifth Quarterly Report,
	
AeroChem TN-199,
DOF/JPL 954777-76/5, October	 1978.
4
41
5
TN-?AI
79-18
2000
II
Y
~ 1000
I
300
0	 1	 2	 3	 4
TIME, min
FIGURE 1 REACTOR WALL TEMPERATURE PROFILE DURING A Na + SiCl. RUN
h
r1
TN-201
113-I•
rz
7
0
0	 10	 20	 30	 40
	
REACTOR	 STANCE , cm
FIGURE 2 SOLID PROI11 Cl' DISTRIBUTION
W ^U
W
a
ca
W 
2JR
0 75
	 79
-1
z 0.50
UQ
LC
la.
z
0.25
_J
0
0	 10	 20	 30	 40
REACTOR DISTANCE, cm
FIGURE 3 DISTRIBUTION OF SILICON DEPOSITION
7
a_zJ00U
0
U
T
m
1
- r
T
a
1%,-
	 `^^
U C: -	 -^
l/Y
a
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ORTrr'.`-?^ PAGE IS
or
FIGURE 4 REACTOR CONFIGURATION FOR MEASURING NEAT RELEASE RATES
8
2	 3	 4	 5
1100
900
x 700
500
300
0
TN-201
TIME, min
FIGURE 5 TEMPERATURE PROFILES OF THE WALI, COOLING Alit 1'UR THE REACTOR
AS SHOWN IN FIG. 4
9
—13
4
Itis
i ,
0
U
O
Q
W 1
Q
0
0
U 1	 z	 3	 4 5
T14-20i
TIME , min
FIGURE 6 HEAT PRODUCTION PROFILES
1.0
W-
I'1 N-"OI
H
N
E
3
X
J
k
ra
? 4
J
A
0
C
I	 2	 3	 4
TIME,  min
F 1ta1la 7 WALL. HF:A -f FLUX PROFILES
2
5
79-11M
U
cn
d 4
^N
J
U)
r-a
ws
2
U_
a
F-
w
2DI
0
> O
0
A
B
2	 3	 4
TIME , min
FIGURF. R VOLUMETRIC HEAT RFI.FASF. PROH11-1
11
5


Silicon halide-alkali metal flames as a source of solar grade silicon

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