Vol. 133, No. 2
THERMODYNAMICS
perimental trends in nucleation rate with temperature
and composition. A n important property of the Si-C1-H
system is that the equilibrium deposition efficiency is independent of C1/H ratio for concentrated mixtures of
chlorosilane in hydrogen, implying that free-space reactor
throughput increases linearly with chlorosilane loading of
the feed. This observation, combined with the inhibiting
effect of chlorine on nucleation in the Si-C1-H system,
suggests that addition of chlorine to free-space reactors
may yield more efficient polysilicon production. However, in light of the sensitivity to the thermodynamic data,
the results of these calculations should be interpreted
carefully. The general trends with composition and temperature are most likely valid, but the absolute values
may be in error due to incorrect thermodynamic data.
Manuscript submitted April 4, 1985; revised manuscript
received Oct. 20, 1985.
Massachusetts Institute of Technology assisted in meeting the publication costs of this article.
REFERENCES
1. C. L. Yaws, K. Y. Li, J. R. Hopper, Y. S. Fang, and
K.C. Hansen, J P L Final Report no. NASA-Cr-.
164009 (1981).
2. J. R. Lay and S. K. Iya, 15th IEEE Photovoltaic Specialists Conference, p. 565 (1981).
3. R. Lutwack, in Third European Communities Photovoltaic Solar Energy Conference, W. Palz, Editor, p.
220, Reidel Publishing, (1980).
4. G. S. Springer, Adv. Heat Transfer, 14, 281 (1978).
5. J. L. Katz and M. D. Donohue, J. Colloid Interface
Sci., 85, 267 (1982).
6. J. Bloem and L. J, Giling, in "Current Topics in Mate-
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
O F Si-C1-H S Y S T E M
425
rials Science," Vol. I, E. Kaldis, Editor, p. 147,
North-Holland, Amsterdam (1978).
L. P. H u n t and E. Sirtl, This Journal, 119, 1741 (1972).
C. S. Herrick and R. A. Sanchez-Martinez, ibid., 131,
455 (1984).
D. W. Woodruff and R. A. Sanchez-Martinez, ibid.,
132, 706 (1985).
H. P. Meissner, C. L. Kusik, and W. H. Dalzell, Ind.
Eng. Chem., Fundam., 8, 659 (1969).
M. Farber and R. D. Srivastava, J. Chem. Thermodyn.,
11, 939 (1979).
K. R. Sarma and M. J. Rice, This Journal, 128, 2647
(1981).
T. U. M. S. Murthy, N. Miyamoto, M. Shimbo, and J.
Nishizawa, J. Cryst. Growth, 33, 1 (1976).
P. Van der Putte, L. J. Giling, and J. Bloem, ibid., 31,
299 (1975).
F. C. Eversteijn, Philips Res. Rep., 26, 134 (197i).
C. S. Herrick and D. W. Woodruff, This Journal, 131,
2417 (1984).
C. E. Morosanu, D. Iosif, and E. Segal, J. Cryst.
Growth, 61, 102 (1983).
J. Delong, Solid State Technol., 15, 29 (October 1972).
A. Lekholm, This Journal, 119, 1122 (1972).
P. H. Robinson and N. Goldsmith, J. Electron. Mater.,
4, 313 (1975).
Y. S. Chiang, RCA Rev., 38, 500 (1977).
J. Bloem, J. Cryst. Growth, 18, 70 (1973).
A. J. Pesthy, R. C. Flagan, and J. H. Seinfeld, J. Colloid Int. Sci., 82, 465 (1981).
M. K. Alam and R. C. Flagan, Aerosol Sci. Technol., 2,
271 (1983).
M. K. Alam and R. C. Flagan, J. Colloid Interface Sci.,
97, 232 (1984).
M. K. Alam, PhD Thesis, California Institute of Technology, Pasadena, CA (1984).
A. A. Chernov and M. P. Rusaikin, J. Cryst. Growth,
45, 73 (1978).
Thermochemical Stability of/3- and '-Alumina Electrolytes in
Na/S Cells
I. The Activities of Na20 and AI203
N. S. Choudhury*,'
General Electric Corporate Research and Development, Schenectady, New York 12301
ABSTRACT
The Na20 activity in certain compositions of fl-, j3"-alumina ceramic electrolytes was determined by open-circuit emf
measurements. The A1,.,Q activity was estimated with the assumptions that the fl-, and fi"-alumina phases may be treated
as line compounds belonging to the pseudobinary Na~O-AI~O~system. These thermochemical parameters are utilized in
Part II to assess the stability of the electrolytes in Na]S cells.
The degradation behavior of fl- and ~'-alumina electrolytes in Na]S cells has been studied by several investigators, and various mechanisms have been proposed. Notable among these mechanisms are impurity diffusion (1-4),
dendritic penetration of metallic sodium during charge
(5-15), and electron injection (16). However, little attention
has been paid to the thermodynamic stability of these
electrolytes in Na/S cells. The lack of attention may be
partly due to the absence of all the pertinent thermochemical data for the various phases in the Na~O-A1203
system. This series of papers addresses the issue of the
thermochemical stability of the solid electrolytes. Part I
of the series is concerned with the determination and estimation of the activities of Na~O and A1203 in fl-, ~'a l u m i n a electrolytes. These results are used in Part II to
assess the thermochemical stability of the electrolytes in
the environment of the Na]S cells.
* Electrochemical Society Active Member.
' Present address: General Electric/Knolls Atomic Power Laboratory, Schenectady, New York 12301
Review of Pertinent Thermochemical Data
The ceramic electrolytes of various compositions used
in the Na/S cells are mainly composed of ~'-alumina and
fl-alumina phases. I n addition to Na~O and A1203, these
compositions also contain a few weight percent of
dopants, e.g., Li20, MgO, etc. However, at 600 K (the cell
operating temperature) the dopants are most likely to be
"frozen-in," and thus the ~'- and fl-alumina electrolytes
may be considered to belong to the pseudo binary Na~OA1203 system, in which a-Al~O3,B-alumina,/3"-alumina,and
NaA102 are the consecutive phases with increasing Na20
content (17). The occurrence of other soda-rich phases,
e.g., NasA10,, NaTAl~O8 (18), etc., have been reported.
However, these phases are not germane to the present
analysis.
The thermochemical data on A1203, Na~O, and NaA102
may be obtained from various compilations, e.g., by
Barin, Knacke, and Kubaschewski (19). The only other
thermochemical information for the Na~O-A1203 system
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J. E l e c t r o c h e m . Soc.: S O L I D - S T A T E S C I E N C E A N D T E C H N O L O G Y
426
F e b r u a r y 1986
Table I. Thermochemical date for NaAlO~, ~"-alumina and/~-alumina at 600 K
Phase, j
x~
NaA102
/~'-alumina
B-alumina
1
~5
~9
c~, (kcal/mol) =
RT In (aNa2o 9 a~Al~Oa)
-43.281
--80.5
-65.15
-52.65
Na~O-rich
coexistent
phase
AlzOs-rich
coexistent
phase
NaTA13Os
NaA102
~'-alumina
fi"-alumina
fl-alumina
~-A1203
reported in the literature is the Na20 activity for a-A1203,
/3-alumina coexistence, as given in Table I.
The chemical potential of any composition, Na20
xA1,.,O3 in the pseudobinary system (where x varies from 0
to infinity) may be represented by
[s176
xA1203
=
~s
+
Xls176
+ R T In aNa20 + x R T in aA1203
[1]
where /X~
and ~~
are the chemical potentials of
Na20 and A12Oa in the standard states and aNa20 and
aA~203are the prevailing Na20 and A120~ activities at that
composition. If for a given phase, j, with composition
Na20 9xj A1203, xj can be treated as a constant, or in other
words, if the phase has a limited stoichiometry, Eq. [1]
may be modified as follows
RT In aNa20 + xjRT in aAl203= Cj
_
o
--
o
--
o
~
[2]
'
where cj - (~ Na20"xjAl203 ]s Na20 X ] ~ A~O~)and IS a constant for that phase. The ranges of RT In aN~2o and RT In
aAl2O3 in Eq. [2] for the phase, j, are given by the equilibrium values fixed by the two-phase coexistence at both
sides of the single-phase field. The fl-, B"-alumina and
NaA102 phase fields are quite narrow (17) and, therefore,
Eq. [2] should be valid for each of these intermediate
phases. Table I lists the values of xj and the available
thermochemical information in terms of Eq. [2]. It is apparent from the table that additional information on either the anglo o r aAl2O a for any one pair of coexistent
phases (e.g., aN~2o in/3-, /3"-alumina) is sufficient to estimate the activities of Na20 and A1203 for all the pairs of
coexistent phases (e.g., aA~2O3in /9-, ~'-alumina; aNa20 and
aA,203 in fl"-alumina, NaA102) between NaA102 and
a-A1203, with the aid of Eq. [2]. The Na20 activity in elec-
I. ELECTROLYTE TUBE
2. I" DIA ~rALUMINA TUBE
3. I/4"DIA a-ALUMINA TUBE
4. e-ALUMINA FURNACE TUBE
5. GAS INLETS
6. GAS OUTLETS
7. LEAD WIRES
8. FEED-THRUS
9. THERMOCOUPLE
I0. GLASS SEAL
I1. GAS-TIGHT O-RING SEALS
12. PLATINUM MESH WRAPPED AROUND
PLATiNIZED SURFACE
13. W,WS2, No2 S ELECTRODE
RT in aN~2Olimits
(kcaYmol)
Upper
Lower
----~
RT in aA~o3limits
(kcaYmol)
Lower
Upper
---80.5
-65.15
-52.65
--
Reference
(19)
h
0
0
0
(20)
(21, 22)
(23)
trolytes, composed mainly of fl- and fl"-alumina phases,
was determined to provide the necessary information.
Experimental M e t h o d s for N a 2 0 Activity D e t e r m i n a t i o n
The Na20 activity was determined from open-circuit
emf's of two types of cells operating at 1020-1220 K and at
-623 K.
Cell arrangement for emf measurements at 10201220 K . - - T h e cell: W, WS2, Na2S/electrolyte/Po2 was employed for the higher temperature measurements. The
cell arrangement is shown in Fig. 1. The electrolyte was-in
t h e form of a closed end, 33 m m diam, N18 in. long tube.
The open e n d of the electrolyte tube was sealed to a 1 in.
diam a-alumina tube with a sealing glass. The cell was positioned inside an a-alumina furnace tube with appropriate gas-tight seal made on the side of the 1 in. diam
a-alumina tube. The arrangement enabled the outside gas
(O2 or 90 ppm 02 + argon mixture fixing the Po2) passing
through the furnace tube to be isolated from the inside of
the electrolyte. A mixture of W, WS2, Na2S (1:1:4 molar)
was placed inside the electrolyte tube and served to establish the Na chemical potential at the inner electrode. A
cover gas of argon (purified by passing through titanium
chips held at 675 K) was admitted through the 1/4 in. diam
a-alumina tube. The bottom end of the 1/4 in. a-alumina
tube was positioned about 1/2 in. above the electrode mixture by the gas-tight sealing arrangement at the open end
of the 1 in. diam a-alumina tube.
The outer surface of the electrolyte tube at the closed
end was platinized and electrical contact with the platin u m lead wire was established by wrapping the outer
platinized surface with a platinum mesh spot welded to
the lead wire. A tungsten lead wire embedded in the W,
WS2, Na2S mixture provided electrical contact to the inner electrode. The thermocouple e m f s between the W
and Pt lead wires were separately determined in the
operating temperature range of the cell and appropriate
corrections to the observed cell e m f s were made.
The cell emf's were monitored with a digital electrometer (input impedance 1014~) and were recorded after waiting for at least 1/2h at each temperature. The attainment
of equilibration was indicated by the reproducibility of
the recorded emi's on cycling the cell temperature within
the range of measurements.
Cell arrangements for
emf measurements
at
~623 K . - - T h e open-circuit emf's between the reference
electrode (Na) and test electrodes (Hg or In) in a molten
NaNO~ bath operating at -623 K were used to compute the
Na20 activity at 623 K. A schematic diagram showing the
REFERENCE
ELECTRODE ,I 1,
II
ELECTROLYTETUBE
!I
CONTAININGNa
U
'~
GLASSENCLOSURE
] _ ~ - ~ COLINTERELECTRODE
.~ _ _
PLATINUMFOIL
I
I
~
I~
II I
--ELECTROLYTETUBE
CONTAINING
TEST
ELECTRODE
NoNO3 BATH
B
Fig. 1. Schematic of the cells used to determine Na20 activity between
1020 and 1220 K.
Fig. 2. Schematic of the cells used to determine NazO activity at
- 6 2 3 K.
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Vol. 133, No. 2
B- AND #"-ALUMINA ELECTROLYTES
427
Table II. List of ceramic electrolyte compositions used for Na~O-activity determination
Composition
code
Na20
CSPL 2/8
G8
G8B
Ceramatec
Starting a-AI.203 powder
Phases, % (~)
Nominal composition, w/o
MgO
Li20
A1203
fi"
~
8
9
9
2
1
1
-0.3
0.3
90
89.7
89.7
50
70
70
50
30
30
8.85
--
0.75
90.4
99
Tr.
Estimated total
oxide impurity,
w/o
Source
Alcoa Al6SG
Alcoa Al7
Baikowski
Type AS2
Baikowski*
Type AS2
0.206
0.146
0.021
0.021
* The starting powder used by Ceramatec, Incorporated (supplier) is believed to be Baikowski Type AS2 or of similar purity.
local a r r a n g e m e n t of t h e t e s t half-cell, r e f e r e n c e e l e c t r o d e
a n d a c o u n t e r e l e c t r o d e i n t h e NaNO3 b a t h is s h o w n i n
Fig. 2. T h e t e s t half-cells w e r e p r o d u c e d f r o m c l o s e d end,
10 m m d i a m fl, fl"-alumina e l e c t r o l y t e t u b e s o f a p p r o p r i ate c o m p o s i t i o n s . T h e o p e n e n d of t h e e l e c t r o l y t e t u b e
w a s s e a l e d to a n o p e n - e n d e d glass t u b e (FN glass), w h i c h
h a d a R o d a r alloy f e e d - t h r u (to m a k e e l e c t r o n i c c o n t a c t
w i t h t h e t e s t e l e c t r o d e ) s e a l e d t h r o u g h t h e wall. A n a p p r o p r i a t e a m o u n t of e i t h e r H g or I n (in s m a l l c h u n k s ) w a s
p l a c e d i n s i d e t h e half-cell a n d t h e cell w a s e v a c u a t e d b y a
m e c h a n i c a l p u m p for a b o u t a n h o u r a n d t h e n t h e t o p of
t h e glass t u b e w a s s e a l e d off. T h e p r i m a r y p u r p o s e of t h i s
cell a r r a n g e m e n t w a s to i n v e s t i g a t e t h e a n o d i c d e g r a d a tion of the ceramic electrolytes at Hg and In electrodes;
d e t a i l s will b e p u b l i s h e d i n s e p a r a t e r e p o r t s (24).
Various electrolyte compositions were investigated.
T a b l e II lists t h e n o m i n a l c o m p o s i t i o n s , a m o u n t s o f diff e r e n t p h a s e s (#"-alumina, B - a l u m i n a ) e s t i m a t e d b y x-ray
diffraction, a n d s o u r c e a n d p u r i t y of a - a l u m i n a s t a r t i n g
p o w d e r u s e d i n t h e m a n u f a c t u r e of t h e electrolytes. T h e
d e n s i t i e s of t h e e l e c t r o l y t e t u b e s w e r e b e t w e e n 3.25 a n d
3.26 g / c m 3 (>/99% theoretical).
Results and Discussions
Na,20 activity in fl"-fl-alumina.--Results f r o m e m f measurements at 1020-1220K.--Stable a n d r e p r o d u c i b l e e m f ' s
w e r e o b t a i n e d for t h e W, WS2, Na2S/electrolyte/po.~ cells
w i t h t h e C S P L 2/8 e l e c t r o l y t e t u b e s . T a b l e s I I I a n d I V list
t h e e m f d a t a ( c o r r e c t e d for t h e W/Pt t h e r m o c o u p l e emf)
t o g e t h e r w i t h t h e c a l c u l a t e d R T In aNa2O for t w o cells. R T
i n ~N,~o w a s c a l c u l a t e d f r o m t h e f o l l o w i n g e q u a t i o n , app r o p r i a t e for t h e cell.
R T In aN,2o
=
AG~
--
1/2 hG~
AG~
+ 1/2 R T i n Po2 - 2FE
[3]
Table Ill. Open-circuit emf data for W, WS2,
Na2S/CSPL 2/8 electrolyte/po2 cell no. 1
Temp, K
p~, atm
EMF, V
(corrected)
RT in aN~o,
kcal/mol
1019
1053
1094
1120
1137
1172
1200
1
1
1
1
1
1
1
1.290
1.264
1.230
1.198
1.192
1.164
1.139
-46.752
-46.307
-45.649
-44.750
-44.851
-44.336
-43.804
Table IV. Open-circuit emf data for W, WS~,
Na2S/CSPL 2/8 electrolyte/po2 cell no. 2
Temp, K
1108
1136
1169
1176
1198
1218
p~, atm
90
90
90
90
x 10 -6
x 10 -6
x 10 -6
• 10 -6
1
1
EM:F,V
(corrected)
RT In aN~o,
kcaYmol
0.996
0.954
0.929
0.924
1.135
1.128
-45.422
-44.366
-44.251
-44.240
-43.575
-43.696
E l r e f a i e a n d S m e l t z e r (21) u s e d t h e W, WS2, Na2S elect r o d e for t h e aN~2o d e t e r m i n a t i o n i n e l e c t r o l y t e s c o m p o s e d
of fl- a n d ~ - a l u m i n a p h a s e s . S u b s t i t u t i n g t h e d a t a for
hG~
hG~
a n d AG~
t a b u l a t e d i n t h e i r p a p e r , Eq.
[3] r e d u c e s to
R T In anglo = (35,351 - 22.189T + 1/2 R T i n Po2 - 2FE)
caYmole
[4]
T h e l e a s t s q u a r e s b e s t fit o f t h e R T In a~a2o vs. T d a t a i n
T a b l e s III a n d I V gives t h e f o l l o w i n g e q u a t i o n
R T In aN~2o = (-63.551 + 0.0165T) k c a l ] m o l e
[5]
w i t h a c o r r e l a t i o n c o e f f i c i e n t o f 0.9791. T h e R T i n aNa2o
v a l u e e x t r a p o l a t e d to 600 K is -53.65 kcal]mole.
Results f r o m e m f measurements at -623 K . - - O p e n - c i r c u i t
e m f ' s at - 6 2 3 K w i t h t h e H g e l e c t r o d e , a f t e r a f e w a n o d i c
s w e e p s (to -3.0V), w e r e i n t h e r a n g e o f 2.79-2.85V, a n d
w e r e m o s t likely c o n t r o l l e d b y t h e f o l l o w i n g r e a c t i o n
[Na20] + H g = H g O + 2Na § + 2e
[6]
w h e r e [Na20] r e p r e s e n t s Na20 i n t h e e l e c t r o l y t e at t h e
p r e v a i l i n g activity. H o w e v e r , t h e initial o p e n - c i r c u i t
e m f s , b e f o r e t h e a n o d i c s w e e p s , w e r e b e t w e e n 2.2 a n d
2.4V a n d are b e l i e v e d to b e m i x e d p o t e n t i a l s ; t h e c a t h o d i c
reaction may be the amalgamation reaction
N a + + e = [Na],g
[7]
F l u c t u a t i o n s of - 5 0 m V w e r e o b s e r v e d o n t h e o p e n c i r c u i t e m f s a n d are b e l i e v e d to b e c a u s e d b y r a p i d obs e r v e d c o n v e c t i o n of H g w i t h i n t h e e l e c t r o l y t e / g l a s s enc l o s u r e a s s e m b l y (Fig. 2). T h u s , t h e o p e n - c i r c u i t e m f d a t a
for t h e H g e l e c t r o d e (after a n o d i c s w e e p s ) p r e s e n t e d i n
T a b l e V r e p r e s e n t t h e m e a n o b s e r v e d e m f s . T a b l e V also
s h o w s t h e c o r r e s p o n d i n g R T In aNa2o values. T h e a p p r o priate thermodynamic data used in the computation (and
also t h a t for t h e I n e l e c t r o d e ) w e r e e s t i m a t e d b y l i n e a r int e r p o l a t i o n at 623 K f r o m t h e c o m p i l a t i o n s o f Barin,
K n a c k e , a n d K u b a s c h e w s k i (19).
Table V. Open-circuit emf data for Na/electrolyte/Hg cells
Electrolyte
composition
G8
G8
G8B
Ceramatec
Ceramatec
Temp, K
EMF, V
RT in aN~2o,
kcaYmol
623
625
623
623
624
2.85
2.85
2.79
2.80
2.79
- 57.473
- 57.473
-54.706
-55.167
-54.706
Table Vl. Open-circuit emf data for No/electrolyte/In cells
Electrolyte
composition
G8
G8
Temp, K
EMF, V
RT In a ~ o ,
kcal/mol
620
622
1.647
1.605
-54.215
-52.278
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J. Electrochem. Sac.: SOLID-STATE SCIENCE AND TECHNOLOGY
428
February 1986
Table Vii. Estimates of Na20 and AI~O3 activities for coexistent phases at 600 K
Coexistent
phases
RT in aN~o
kcal/mol
aNazO
(log mean)
R T ] n aAl20a
{~ht203
kcaYmol
(log mean)
NaA102,
fl"-alumina
/]"-alumina,
B-alumina
B-alumina,
a-alumina
-38.198 • 1.540
1.21 • 10-14
-5.083 -+ 1.540
0.014
-55.559 -!--0.798
5.75 x 10-z~
-1.870 -+ 0.990
0.208
-80.5'
-65.152
4.72 x 10-30
1.84 x 10-24
0
0
1
1
' Ref. (20).
2Ref. (21, 22).
~The open-circuit e m f data at ~623 K with the In electrode are listed in Table VI; the RT In aN~2o values were
calculated on the basis of the following reaction
3[Na20] + 2In = In203 + 6Na § + 6e
[8]
It is apparent from Tables V and VI that the Na20 activities of various electrolytes at -623 K are in reasonable
agreement with each other and with that of the C S P L 2/8
electrolyte extrapolated to 623 K using Eq. [5]. The least
squares best fit of the RT In aNa~OVS. T data pooled from
Tables III through VI gives the following equation
RT in aN~2o = (-67.556 + 0.0200T) kcaYmole
[9]
with a correlation coefficient of 0.9790. Equation [9] is
graphically illustrated in Fig. 3, which also shows the
m a x i m u m likelihood estimates for the 95% confidence
band. The 95% confidence range at 600 K is -54.761 to
-56.357 kcaYmole. The regression analyses were performed with the GE S T A T P A C computer program (25).
The above results indicate that the purity, composition,
and changes in the phase content of the ceramic electrolytes do not have significant effect on the Nasa activity at
~600 K, i.e., the various electrolytes appear to belong to a
pseudobinary, two-phase (fl"- and B-alumina) system with
the impurities and dopants "frozen-in."
Estimates of the Na20 and A1203 activities.--In order to
estimate the AloO3 activity for fl"-alumina, B-alumina coexistence from the present result of Na20 activity for fl"alumina, B-alumina, and the Nasa activity for/3-alumina,
a-alumina coexistence (see Table I); thermochemical consideration requires that RT In aNa2o for B-alumina, fl"alumina coexistence be greater than the RT in aN~o for
B-alumina, a-alumina coexistence. This precludes the possibility of using the data of Ref. (23) in conjunction with
the present results for the estimate of the A1203 activity
"40
-50
z
o
I
I
I
S
I
600
I
TOO
I
B
A
N
D
I
800
I
900
I
I000
I
I100
Conclusions
The Na20 activity at 600 K for fl"-, B-alumina ceramic
electrolytes is -5.75 x 10 -~1, the estimated AlzO3 activity
being -0.208. The estimated Na20 and A1203 activities at
600 K for NaA102, fl"-alumina equilibrium coexistence are
-1.21 x 10 -'4 and 0.014, respectively.
Manuscript submitted J u n e 20, 1985; revised manuscript received Oct. 28, 1985.
General Electric Company assisted in meeting the publication costs of this article.
b-t~
-60
for fl"-alumina, B-alumina coexistence. Although the reason for the discrepancy between Choudhury's (20) results
'(RT In aNa2o = -80.5 kcaYmole and those of Elrefale and
Smeltzer (21) and Rag et al. (22) (RT In aNa2o = -65.15
kcal/mole) for B-alumina, a-alumina coexistence is not
known, both of these values are used to estimate the
A1203 activity for fl'-alumina, B-alumina coexistence and
the Na20 and the AI~O~ activities for NaA102, fl"-alumina
coexistence, as discussed below.
Although B-alumina and fl"-alumina exhibit limited stoichiometric ranges, two values each for x~ (9 and 10) and
for x~" (5 and 6) were assigned for the estimates. Starting
with Eq. [2] for the B-alumina phase, the values of x~, c~
(-80.5 and -65.15 kcal/mole), and the high and low values
for R T In aNa2O(-54.761 and -56.357 kcal/mole) were combined to yield the R T In aA~2Oaestimates for fl"-, B-alumina
coexistence; the estimated value is -1.870 -- 0.990
kcal/mole, c," values were then c o m p u t e d for x~" = 5 and
6. The estimate o f R T In aN~2o and R T In aA,~03 for NaA102,
fl"-alumina coexistence were then m a d e by simultaneous
solutions of Eq. [2] for NaA102 and fl"-alumina. The estimate of RT In aNa2Ois -38.198 -+ 1.540 kcal/mole, the corresponding estimate of R T In aA12o3 being -5.083 -+ 1.540
kcal/mole. Table VII summarizes the estimates of the
Na20 and A1203 activities at 600 K for pairs of coexistent
phases between NaA102 and a-A120 in the pseudobinary
system.
1200
TEMP, K
Fig. 3. Estimate of RT In ONa20VS. temperature showing the 95% confidence band.
REFERENCES
1. F. G. Will, This Journal, 123, 834 (1976).
2. T. Kaneda, J. B. Bates, J. C. Wang, and H. Engstrom,
Mater. Res. Bull., 14, 1053 (1979).
3. I. Yasui and R. H. Doremus, J. Am. Ceram. Sac., 60,
296 (1977).
4. I. Yasui and R. H. Doremus, This Journal, 125, 1007
(1978).
5. R. D. Armstrong, T. Dickinson, and J. Turner,
Electrochim. Acta, 19, 187 (1974).
6. G. J. Tennenhouse, R. L. Ku, R. H. Richman, and T. J.
Whalen, Bull. Am. Ceram. Sac., 53, 523 (1975).
7. R . H . Richman and G. J. Tennenhouse, J. Am. Ceram.
Sac., 58, 63 (1975).
8. D. K. Shetty, A. V. Virkar, and R. S. Gordon, in "Fracture Mechanics of Ceramics," No. 4, R. L. Bradt and
D. P. H. Hasselman, Editors, p. 652, P l e n u m Press,
New York (1978).
9. A. V. Virkar and G. R. Miller, in "Fast Ion Transport
in Solids," P. Vashista, J. N. Mundy, and G. K.
Shenoy, Editors, p. 87, Elsevier North Holland, Inc.,
New York (1979).
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Vol. 133, No. 2
/3- AND fl"-ALUMINA ELECTROLYTES
10. A. V. Virkar, L. Viswanathan, and D. R. Biswas, J.
Mater. Sci., 15, 302 (1980).
11. A. V. Virkar, ibid., 16, 1142 (1981).
12. R. W. Davidge, G. Tappin, J. R. McLaren, and G. J.
May, J. Am. Ceram. Soc., 58, 771 (1979).
13. M. P. J. Brennan, Electrochim. Acta, 15, 621 (1980).
14. L. C. DeJonghe, L. Feldman, and P. Millett, Mater.
Res. Bull., 14, 589 (1979).
15. N. K. Gupta and G. J. Tennenhouse, This Journal, 126
1451 (1979).
16. L. C. DeJonghe, L. Feldman, and A. Buechele, J. Mater. Sci., 16, 780 (1981).
17. R. C. DeVries and W. L. Roth, J. Am. Ceram. Soc., 52,
364 (1969).
18. M. G. Barker, P. G. Gadd, and M. J. Begley, J. Chem.
Soc., Chem. Comm., 379 (1981).
19. I. Barin and O. Knacke, "Thermochemical Properties
of Inorganic Substances," Springer-Verlag, New
York-Heidelberg (1973), and I. Barin, O. Knacke,
and O. Kubaschewski, "Thermochemical Proper-
20.
21.
22.
23.
24.
25.
429
ties of Inorganic Substances, Supplement,"
Springer-Verlag, New York-Heidlberg (1977).
N. S. Choudhury, This Journal, 120, 1663 (1973).
F. A. Elrefaie and W. W. Smeltzer, ibid., 128, 1443
(1981).
G. Rog, S. Kozinski, and A. Kozlowska-Rog,
Electrochim. Acta, 28, 43 (1983). The Na20-activity
for a-alumina, B-alumina coexistence determined
by these authors are essentially in agreement with
Ref. (21).
A. Dubreuil, M. Malenfant, and A. D. Pelton, This
Journal, 128, 2006 (1981).
M. W. Breiter, N. S. Choudhury, and E. L. Hall, To be
published.
W. B. Nelson, C. B. Morgan, and P. Caporal, "1979
STATPAC Simplified--A short introduction to
how to run STATPAC, a general statistical package
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Electric Corporate Research and Development,
Schenectady, NY (Dec. 1978).
Thermochemical Stability of B- and fi"-Alumina Electrolytes in
Na/S Cells
II. Assessment
N. S. Choudhury*'1
General Electric Corporate Research and Development, Schenectady, New York 12301
ABSTRACT
The degradation of B- and fl"-alumina electrolytes in Na/S cells is assessed from a thermochemical viewpoint. Two
probable schemes for the S-side degradation are considered: (i) Na20-leaching and (ii) free A1203 liberation. The probable
degradation mechanism at the Na electrode, considered in this assessment, involves reactions leading to the liberation
of metallic A1. The assessment indicates that a- and B-alumina with low Na~O activity are thermodynamically stable,
while the other soda-rich phases in the Na=,O-A120~ system are unstable to the Na20 leaching reactions. On the other
hand, thermodynamic stability to the reaction with Na is favored by high Na20 activity and qualifies soda-rich fi"alumina and other soda-rich compounds as stable phases. Thus, the existence of any phase in the Na20-Al~O3 system
which would be stable at both the Na and the S electrode, is negated by the assessment.
The estimates of the Na20 and A1203 activities given in
Table VII, Part I (1) are utilized to examine the thermodynamic feasibility of probable chemical reactions involving
the components of the electrodes of the Na]S cells and the
B- and ~"-alumina electrolytes. The pertinent free energy
data used in this assessment are taken from the compilations of Barin, Knacke, and Kubaschewski (2). The cell
operating temperature is assumed to be 600 K and the potential for S, Na2S~ coexistence is taken as 2.08V with respect to Na, i.e., the cell open-circuit emf in the "twophase" region.
reactions. These two reaction schemes are considered in
further detail in the following paragraphs.
Na20 "leaching" mechanism.--In this situation, the
various possible reactions can be written as follows
3[Na~O] + C = Na2CO3 + 4Na T + 4e
[1]
3[Na20] + S = Na2SO3 + 4Na § + 4e
[2]
4[Na20] + S = Na~SO4 + 6Na+ + 6e
[3]
[Na~O] + C = CO + 2Na + + 2e
[4]
Probable Reactions at the S Electrode
2[Na20] + S = SO2 + 4Na+ + 4e
[5]
I n addition to the presence of sulfur, carbon is also
present at this electrode as current collector. Because
A120~ is relatively stable, it is assumed that no compounds
of Al with S or C are formed. Thus, the probable reactions
of interest may involve the formation of the condensed
phases, e.g., Na2CO3, Na2SO4, Na2SO3, or gaseous products
SO2, CO, and finally O2~ Two possible reaction schemes
may be considered. One involves progressive quasisteady-state "leaching" of Na~O from the electrolyte to
form the various reaction products, together with the formation of a soda-poor electrolyte, without significant
change in aA~203-This mechanism may be particularly applicable to the compositions in B"- + B-alumina two-phase
field or for infinitesimal change in the electrolyte composition at any particular instant of time. The other mechanism involves liberation of free A1203 as a result of the
2[Na20] = O2 + 4Na + + 4e
[6]
*Electrochemical Society Active Member.
1Present address: General Electric/Knolls Atomic Power Laboratory, Schenectady, New York 12301.
where [Na20] represents Na20 in the electrolyte at the
prevailing activity. Treating aNa2o as a variable, the reactions may be graphically represented in a plot of RT In
aN~2o vs. E (where E is the potential at the electrode with
respect to Na) as in Fig. 1. It is apparent from the figure,
that for the stability against the Na20 leaching reactions
(at the cell open-circuit potential), a RT In aNa2o value of
less than - -60 kcaYmole (aNa2o -< 1.4 • 10 -22) is desirable.
A lower value may be needed if the cell charging practice
demands the S-electrode potential to deviate significantly from the cell open-circuit potential.
With reference to Table VII, Part I (1), soda-poor
B-alumina (i.e., in coexistence with ~-A1203) should be stable, while soda-rich /~'-alumina unstable to many of the
above reactions. The estimated RT In aNa2() for B-, ~"alumina coexistence (the material of practical interest) is
shown in Fig. 1; which indicates that the B-, /3"-alumina
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