J. Electrochem. Soc.: ELECTROCHEMICAL T E C H N O L O G Y
698
2. Y. Okinaka, P a p e r 50, Extended Abstracts of Electrochem. Soc. Meeting, Chicago, Oct. 15-20, 1967.
3. E. Lifshin and J. Weininger, Electrochem. Technol.,
~, 5 (1967).
4. M. Fleischmann, K. S. Rajagopolan, and H. R. Thirsk,
May 1969
Trans. Faraday Soc., 59, 741 (1963).
5. Y. O k i n a k a and D. T u r n e r , P a p e r 28, E x t e n d e d A b stracts of Electrochem Soc. Meeting, Buffalo, Oct.
10-14, 1965.
6. P. E. Lake and J. M. Gooding, Can. J. Chem., 7, 1089
(1958).
The Heat Capacity of Electrolytes Simulating
Those Found in Silver Chloride-Magnes:"Water-Activated Batteries
Duane W. Faletti
Applied Physics Laboratory, University o~ -washington, Seattle, -washington
and Ivan W. Herrick and Mark F. Adams
College of Engineering, Research Division, -washington State University, Pullman, Washington
ABSTRACT
The heat capacities of m a g n e s i u m chloride-sea water solutions with chlorinities v a r y i n g from 4.5 to 19.0 parts per thousand and with 0-70g of m a g n e s i u m chloride per liter of solution were d e t e r m i n e d over a t e m p e r a t u r e
range from 5 ~ to 85~
These solutions simulate the electrolyte conditions
found in silver c h l o r i d e - m a g n e s i u m w a t e r - a c t i v a t e d batteries using sea water.
A n empirical equation was developed which successfully predicts, w i t h i n
1.5%, the heat capacities of such solutions over a r a n g e of chlorinity from 0
to 19 parts per thousand.
Work is in progress to develop an improved design
technique for silver c h l o r i d e - m a g n e s i u m w a t e r - a c t i vated batteries based on a semiempirical m a t h e m a t i c a l
model (1). To d e t e r m i n e b a t t e r y performance, heat
capacity values accurate to a few per cent are r e q u i r e d
to calculate the t e m p e r a t u r e of the electrolyte circulating in flow passages and cells.
AZ61 m a g n e s i u m alloy is c o m m o n l y used as the
anode in silver c h l o r i d e - m a g n e s i u m batteries and sea
water is the common electrolyte (though it is possible
to operate such batteries on fresh w a t e r ) . Since the
p r i m a r y effect of the b a t t e r y reaction is to add m a g n e s i u m chloride to the e n t e r i n g electrolyte, sufficiently
accurate results for the purposes of b a t t e r y design
can be obtained from a study of the heat capacity of
solutions of m a g n e s i u m chloride in sea w a t e r and in
water. Secondary effects include the formation of
precipitates of the hydroxides of magnesium, alum i n u m , zinc, and manganese, and the r e m o v a l of some
of the salts found in the e n t e r i n g electrolytes by,
among other things, retention in the sponge silver
formed by the reduction of the silver chloride. The
solutions studied cover the range of composition and
t e m p e r a t u r e of interest. They were prepared by the
method given in Ref. (2).
Experimental Method
The heat capacity was d e t e r m i n e d by a method
commonly applied to static systems: electrical energy
was converted to heat at a constant, k n o w n rate w i t h i n
the confines of the solution for a k n o w n length of
time.
T e m p e r a t u r e m e a s u r e m e n t s were taken prior to
and after the heating period for d e t e r m i n i n g the t e m p e r a t u r e rise caused by the heat input. The above
i n f o r m a t i o n plus a knowledge of the heat capacity of
the calorimeter and the mass of the fluid in the
calorimeter was sufficient to allow a calculation of the
heat capacity using the equation
E2t
Cp =
4.184 RAT
CpCal
[1]
"Wsoln
K e y w o r d s : h e a t c a p a c i t y , sea w a t e r - m a g n e s i u m chloride solutions, silver c h l o r i d e - m a g n e s i u m b a t t e r y electrolytes.
w h e r e Cp is the heat capacity of the solution, Cal/g
~ E is the potential across the heater in volts, t is
the length of the heating period in seconds, R is the
heater resistance in ohms, a T is the t e m p e r a t u r e rise
resulting from the heat input, CpCal is the "heat
capacity" (note units) of the calorimeter in Cal/~
and Wsoln is the mass of the solution u n d e r study.
Apparatus.--Two types of calorimeters were used in
this study. One type which worked satisfactorily at
at 5 ~ and 25~ was simply a l - l i t e r Dewar flask
capped by a cork into which the thermometer, heater,
and glass stirrer were mounted, a n a r r a n g e m e n t which
can be operated r a p i d l y and easily. However, there
appeared to be no effective way of heating the space
in the calorimeter above the liquid while still m a i n t a i n i n g calorimetric conditions. The resulting condensation of vapor in this space caused significant
errors at t e m p e r a t u r e s above 45~
In order to c i r c u m v e n t these problems, a cavity
calorimeter was constructed for use at t e m p e r a t u r e s
of 45 ~, 65 ~ and 85~
(Fig. 1). Though a g r e e m e n t
b e t w e e n the two calorimeters was acceptable at 45~
only the results obtained with the cavity calorimeter
a r r a n g e m e n t are presented for t e m p e r a t u r e s above
25~
A B e c k m a n differential t h e r m o m e t e r accurate to
0.002~ was used for the t e m p e r a t u r e measurements.
The heaters were glass coils filled with silicone oil
with a n o m i n a l 60-ohm l e n g t h of Nichrome C resistance wire r u n n i n g through them. The resistance
wire ended below the liquid level and was connected
to copper leads which extended to the outside of the
calorimeter.
Electrical energy to the heater was supplied by a
stable power supply of 60v. The resistance of the
heater was m e a s u r e d i m m e d i a t e l y before and after the
heating period, and the potential drop across the
heater was measured d u r i n g the heating period. The
accuracy of the thermometer, electrical components,
and balance weights was checked periodically.
Procedure.~All heat capacity determinations were
made with a m i n i m u m void space above the liquid so
that evaporation effects could be ignored.
To eliminate condensation on the upper portion of
the calorimeter j a r (which occurred at 45~ and
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Vol. 116, No. 5
THERMOMETER
RETAINER
FOAM LID
~
CAVITY~
-~
~
;
)
t
NYLON
- - HEATERCO|L
* ~ POLYURETHANE
~ INS' ATION
INLET
--
ouT .
CO "ARTM "
\
WOOD BASE
Fig. 1. Schematic of cavity calorimeter; for measuring heat ca-
pacity of electrolytes wlth temperatures ranging from 45 ~ to 85~
above), the following procedure was used with the
cavity calorimeter: Samples were preheated to about
I~ below the test t e m p e r a t u r e in a preheating bath,
w h e r e u p o n the top of the calorimeter j a r was heated
with a hot air blower to evaporate a n y condensate
which m a y have accumulated there. The calorimeter
j a r was t h e n placed in the calorimeter with the water
circulating through the outer c o m p a r t m e n t held at
I~ above the test t e m p e r a t u r e d u r i n g the entire test.
After the calorimeter was allowed to equilibrate
for 20 rain, seven t e m p e r a t u r e readings were taken
at 50-sec intervals, w h e r e u p o n the heater resistance
was measured and the heater was started. The voltage
of the heater (held constant to 0.01v or better) was
measured at least three times d u r i n g the 150-sec h e a t ing i n t e r v a l and the results averaged. Upon completion of the heating p e r i o d , the heater resistance was
measured along with an additional seven t e m p e r a t u r e
readings t a k e n at 50-sec intervals.
Data r e d u c t i o n
699
H E A T CAPACITY OF E L E C T R O L Y T E S
a n d error a n a l y s i s . - - T h e t e m p e r a -
ture rise in the calorimeter was d e t e r m i n e d as follows: Separate t e m p e r a t u r e - t i m e curves were d r a w n
t h r o u g h the two sets of t e m p e r a t u r e s recorded prior to
and following the heating period. This could be done
with a m a x i m u m deviation of 0.002~ or less by
excluding the first point taken after the end of the
heating cycle. The t e m p e r a t u r e rise from the heat i n p u t
was t a k e n to be the difference of these two t e m p e r a t u r e - t i m e curves at a time 80 sec after the start of
the heating period.
The time chosen to take the t e m p e r a t u r e rise, using
Challoner's Method (3), was based on (a) an observed
10-see time lag b e t w e e n the start of the heating a n d
the first indication of a t e m p e r a t u r e rise, a n d (b)
the a p p a r e n t Iinearity of the t e m p e r a t u r e rise with
time d u r i n g the heating period. This process yields
t e m p e r a t u r e rises with m a x i m u m u n c e r t a i n t i e s of 0.4%
(0.008~
with about two thirds of the u n c e r t a i n t y
resulting from the extrapolation process.
The heat capacities of both calorimeters were det e r m i n e d from experiments with w a t e r using the techniques described above but solving for the heat capacity of the calorimeter by r e a r r a n g i n g Eq. [1] and adding the k n o w n value of the heat capacity of w a t e r (4).
The values of calorimeter heat capacity obtained
should at least partially compensate for systematic
errors in t e m p e r a t u r e extrapolations, etc. Five values
of calorimeter heat capacity were d e t e r m i n e d at each
t e m p e r a t u r e level studied from which the average
values of calorimeter heat capacity were calculated.
A n error analysis based on the values of m a x i m u m
uncertainties in the five parameters of the e x p e r i m e n t
(Table I) and the u n c e r t a i n t y values calculated for
the heat capacity of the calorimeter (using the values
of the same five parameters) gives a m a x i m u m u n c e r t a i n t y value of _ 1.25% for the heat capacity
results. If the u n c e r t a i n t y value of the calorimeter
heat capacity is excluded, a m a x i m u m u n c e r t a i n t y
v a l u e of _ 0.7% results. The latter value is of some
interest since the values of the calorimeter heat capacities were subjected to averaging.
As a check on the e x p e r i m e n t a l technique, the heat
capacity was d e t e r m i n e d for four solutions w i t h k n o w n
heat capacities. Two of these solutions were sea water;
the agreement obtained is discussed later. Of the
two r e m a i n i n g solutions, one was of 2.44 m / o (mole
per cent) sodium chloride solutions which was r u n
through at 45~
The heat capacity obtained was
0.33%
greater t h a n that found by interpolating ICT
data (5).
The other check was made with a 0.405 molal sodium
chloride solution studied at 85~
The heat capacity
obtained was 0.55% lower t h a n that obtained b y Eigen
and Wicke (6).
Presentation and Discussion of Results
Three d e t e r m i n a t i o n s of the heat capacity were
made for each solution at each temperature. These
three values were averaged and are presented in
Table II and Fig. 2. The total spread in the three
determinations made at a given point n e v e r exceeded
0.7% and was typically 0.3%. Values of the heat
capacity of solutions with zero chlorinity, i.e. m a g n e s i u m chloride-water solutions, over a t e m p e r a t u r e
range of 10~176
were obtained b y crossplotting
the results of Eigen and Wicke (6) and Rutskov (7)
and are shown in Fig. 2.
A n additional check on the accuracy of the results
is provided by the values of heat capacity for artificial
sea w a t e r solutions. The heat capacities d e t e r m i n e d
at 5~ for a solution with a chlorinity of 4.5 parts per
thousand (% o) and for a solution at 25~ with a
chlorinity of 19.0%o were, respectively, 0.39% and
0.27% lower t h a n values obtained by Cox and Smith
(8).
The empirical equation,
Cp = 0.9960--0.001259 ( C M ) - 0.002174 ( C H L ) q- 0 . 0 0 0 0 0 ' 9 5 2 6
( C M ) (CHL)
[2]
Table I. Estimates of maximum uncertainties
Estimated maximum
uncertainty, %
Parameter
Potential (E)
R e s i s t a n c e (R)
Mass(W)
Time (t)
0.03
0.05
0.01
0.07
0.29
T e m p rise (AT)
Table II. Values of heat capacity of simulated electrolytes for
silver-chloride-magnesium sea water-activated batteries (cal/g~
Chlorinity
(% o)
4.5
4.5
4.5
4.5
4.5
4.5
9.0
9.0
19.0
19.0
19.0
19.0
19.0
19.0
Added
MgC12
g/liter
0.0
10.0
20.0
35.0
50.0
70.0
10.0
35.0
0.0
10.0
29.0
35.0
50.0
70.0
5~
0.987
0.978
25~
45~
0.972
0.957
0.984
0.966
0.947
0.958
0.966
0.937
0.953
0.939
0.932
0.970
0.930
0,957
0.939
0.914
65~
0.961
0.944
0.926
85~
0.945
0.925
0.898
0.935
0.934
0.918
0.899
0,916
0.900
0.881
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700
J. Electrochem. Soc.: E L E C T R O C H E M I C A L
'-,~,i
...Q
~--
0
o
~176
e " ~
I
O
o~
" \~
*--'9-0%0
" \~1
93-
,~
i./.o
iI
190 !
870
I0
20
30
40
50
60
May 1969
chlorinity, the use of Eq. [2] to extrapolate to a
chlorinity of 22.5% appears justified.
The electrolyte t e m p e r a t u r e s and compositions covered in this study are typical of those found in highd r a i n silver c h l o r i d e - m a g n e s i u m sea w a t e r - a c t i v a t e d
batteries. The observed 10% variation in heat capacity
(see Table II) indicates that corrections for this v a r i a tion in heat capacity (i.e., Eq. [2]) should be incorporated into design techniques which make use of
material and energy balances (such as the design
techniques u n d e r development at Applied Physics
Laboratory, U n i v e r s i t y of W a s h i n g t o n ) .
0.0 %0
\,..
TECHNOLOGY
70
CM ((]ms Mg Cl2 /LITER ELECTROLYTE}
Fig. 2. Heat capacity as a function of added MgCI2. Lines show
values of heat capacity as predicted by I:q. [2] for the levels of
chlorinity shown, in parts per thousand.
was developed where CM is added MgC12 in g / l i t e r of
sea water or fresh water, and CHL is chlorinity in
parts per thousand. This equation successfully predicts
the heat capacity as a function of chlorinity
(0.0-0.19%o) and added m a g n e s i u m chloride (0-70
g/liter) to within 1%, with two exceptions: The first
is a 1.1% deviation with a solution of 4.5%0 chlorinity
and 10 g / l i t e r added m a g n e s i u m chloride at 45~ the
second is a solution of 19.0% o chlorinity and 10.0
g / l i t e r added m a g n e s i u m chloride at 45~ with a deviation of --1.4%.
The chlorinity of sea water in the world's oceans
varies from about 4.5 to 22.5%0. Since Eq. [2] is based
on a linear interpolation b e t w e e n 0.0 and 19.0%o
Acknowledgments
Dr. William R. Davis, Assistant Director of the
Applied Physics Laboratory, and E n g i n e e r i n g Assistant
Mr. Rodney Lipp contributed to the success of this
effort. Their contributions are gratefully acknovcledged. This work was supported by the B u r e a u of
Naval Weapons, United States D e p a r t m e n t of the
Navy.
Manuscript s u b m i t t e d J u l y 5, 1968; revised m a n u script submitted Jan. 27, 1969.
A n y discussion of this paper will appear in a Discussion Section to be published in the December 1969
JOUENAL.
REFERENCES
1. D. W. Faletti and M. A. Gackstetter, This Journal,
115, 1210 (1968).
2. D. W. Faletti and M. A. Gackstetter, ibid., 114, 209
(1967).
3. R. A. Challoner, H. A. Gundy, and R. A. Meetham,
Phil. Trans., A247, 553 (1955).
4. N. S. Osborne, H. F. Stimson, and D. C. Ginnings,
J. Res. Nat. Bur. Std., 23, 238 (1939).
5. " I n t e r n a t i o n a l Critical Tables," V, 122 (1953).
6. M. Eigen and E. Wicke, Z. Elektrochem., 55, 354
(1951).
7. A. P. Rutskov, Zh. Prikl. Khim. (J. Appl. Chem.),
21, 820 (1948).
8. R. A. Cox and N. D. Smith, Proc. Roy. Soc. (London), A252, 51 (1959).
Mechanisms of Oxidation of
Ta-lOW Alloy Coated With Tungsten Disilicide
Joan B. B e r k o w i t z - M a t t u c k *
Arthur D. Little, Incorporated, Cambridge, Massachusetts
ABSTRACT
Continuous oxygen consumption m e a s u r e m e n t s have been made on two
commercially available W / S i type coatings, the TRW duplex coating a n d
Solar's TNV-13, at t e m p e r a t u r e s between 1000~ and 1800~ and an oxygen
partial pressure of 150 Torr in helium. The coatings were applied by the
vendors to small cylindrical pellets of Ta-10 a/o W. It was shown that the
coated samples undergo significant changes in properties if preheated in n o n oxidizing atmospheres. Although the as-received TRW and TNV-13 samples
are both n o m i n a l l y similar, they exhibit totally different oxidation behavior
u n d e r the conditions investigated. Oxidation of the TNV-13 coated samples is
parabolic b e t w e e n I000 ~ and 1700~ and the parabolic rate constant increases
with temperature. Oxidation of the TRW coated samples is logarithmic, and
the extent of oxidation in a l - h r time period decreases as the t e m p e r a t u r e is
increased from 1000 ~ to 1250~ to 1500~ A t 1700~ the TNV-13 coating provides protection, while the TRW coating fails catastrophically. The results of
electron microprobe analysis suggest that the basic differences in oxidation
m e c h a n i s m b e t w e e n the two coating systems m a y be ascribed to the presence
of a small a m o u n t of t i t a n i u m in the TNV-13 coating.
The poor oxidation resistance of refractory metals
and alloys severely limits their usefulness in oxidizing
e n v i r o n m e n t s at high temperature. Since the metals
themselves are attractive for m a n y projected applica* Electrochemical Society Active Member.
tions due to their high m e l t i n g points and good hight e m p e r a t u r e strengths, considerable effort has been
devoted in recent years to the development of oxidat i o n - r e s i s t a n t coatings. This paper examines the basic
oxidation m e c h a n i s m of two commercial coating sys-
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