Vol. 112, No. 8
VACUUM
DEPOSITED
d e n u m films are compatible with passivated silicon
substrates a n d m a y be used for integrated circuit i n terconnections. I n contact with bare silicon, the applicability is t e m p e r a t u r e limited on account of the
formation of b r i t t l e m o l y b d e n u m silicides through
solid-state reaction. The films exhibit an interesting
stress behavior insofar as the intrinsic component is
n e a r l y constant up to a deposition t e m p e r a t u r e of
500~
t h e n decreases and changes from tensile to
compressive stress at 625~ This is accompanied b y a
transition from almost p u r e (110) to more r a n d o m
crystallite orientation i n the same t e m p e r a t u r e i n t e r val. A casual relationship b e t w e e n the two p h e n o m e n a
is suggested in accordance with concepts about the
origin of stress proposed by Hoffman et al. The i n creased atomic mobility at the higher temperatures
allows consolidation of lattice defects by reorienting
small regions o~ m a t e r i a l and s i m u l t a n e o u s l y r e d u c i n g
tensile stresses. It is also shown that crystallite orientation and generation of stress are not d e t e r m i n e d by
the first l a y e r of deposit attached to the substrate, but
are subject to change d u r i n g the entire film growth
if t e m p e r a t u r e conditions are altered.
Acknowledgment
The authors would like to t h a n k Mrs. C. Walter for
stress measurements, Messrs. B. L. Graves for sample
preparation, B. N a r k e n for expansion measurements,
E. J. Taschler for v a l u a b l e help in the assembly of the
v a c u u m system and evaporation source, Ch. H. Ma,
J. S. Makris, a n d N. R. Stempel for x - r a y diffraction
work.
Manuscript received Feb. 17, 1965; revised m a n u script received April 2, 1965. This paper was presented
at the San Francisco Meeting, May 9-14, 1965.
A n y discussion of this paper will appear in a Discussion Section to be published in the J u n e 1966 JOVRNAL.
REFERENCES
1. "The Metal Molybdenum," p. 200, J. Harwood,
Editor, A m e r i c a n Society for Metals, Cleveland
(1958).
2. J. C. Kelly, J. Sci. Instr., 36, 89 (1959).
3. N. J. Maskalick and C. W. Lewis, Trans. 8th Nat.
Vac. Syrup., 2, 874 (1961), P e r g a m o n Press, 1962.
4. R. A. Holmwood and R. Glang, to be published.
J. Chem. and Engng. Data.
5. N. Schwartz and R. Brown, Trans. 8th Nat. Vac.
Syrup., 2, 836 (1961).
6. J. E. Kunzler, i n " U l t r a - H i g h - P u r i t y Metals," p.
171, Am. Soc. for Metals, Metals Park, Ohio
(1962).
7. J. Bardeen, J. Appl. Phys., 11, 88 (1940).
MOLYBDENUM
FILMS
831
8. H.
Mayer, " S t r u c t u r e and Properties of T h i n
Films," p. 225, J o h n Wiley & Sons, Inc., New
York (1959).
9. I. G. Young and C. W. Lewis, Trans. lOth Nat. Vac.
Symp., p. 428, MacMillian C o m p a n y (1964).
10. W. Geiss and J. A. M. vanLiempt, Z. Physik, 41,
867 (1927).
11. R. W. Roberts and T. A. Vanderslice, "Ultrahigh
V a c u u m and its Applications," p. 27, Prentice
Hall, Inc., Englewood Cliffs, N. J. ~1963).
12. P. E. Blackburn, M. Hoch, and H. L. Johnston, J.
Phys. Chem., 62, 769 (1958).
13. R. E. M a r i n g e r and A. D. Schwope, J. Me~als,
March 1954, p. 365.
14. L. Holland, "Vacuum Deposition of T h i n Films," p.
99, J o h n Wiley & Sons, Inc., New York (1958).
15. C. S. Barrett, "Structure of Metals," p. 444, McG r a w - H i l l Book Co., Inc., New York (1952).
16. I. R. Landau, "Metals and Alloys," Vol. 9, 73, 100
(1938).
17. R. E. Thun, i n "Physics of T h i n Films," Vol. 1,
p. 226 Academic Press, New York (1963).
18. R. W. Hoffman, R. D. Daniels, and E. C. Crittenden,
Jr., Proc. Phys. Soc., London, B67, 497 (1954).
19. J. D. F i n e g a n and R. W. Hoffman, Trans. 8th Nat.
Vac. Syrup., 2, p. 935, P e r g a m o n Press, New York
(1961).
20. J. W. Mentor and D W. Pashley in "Structure and
Properties of T h i n Films," p. 111, Proc. Int. Conf.,
Bolton Landing, 1959, J o h n Wiley & Sons, Inc.,
New York.
21. H. B l a c k b u r n and D. S. Campbell, Trans. 8th Nat.
Vac. Syrup., 2, p. 943, P e r g a m o n Press, New York
(1961).
22. R. Glang, R. Holmwood, and R. L. Rosenfeld, Rev.
Sci. Instr., 36, 7 (1965).
23. J. Harwood, "The Metal Molybdenum," p. 10,
A m e r i c a n Society for Metals, Cleveland (1958).
24. J. W. Menter and D. W. Pashley, 1.c., p. 137.
25. H. S. Story a n d R. W. Hoffman, Proc. Phys. Soc.
London, BT0, 950 (1957).
26. H R. Clauser, "The Encyclopedia of E n g i n e e r i n g
Materials a n d Processes," p. 430, Reinhold P u b lishing Corp., New York (1963).
27. L. Maissel, J. Appl. Phys., 31, 21I (I960).
28. Physikalisch-Technisches Reichsamt, Z. Instr., 40,
156 (1920).
29. J. Disch, Z. Physik, 5, 173 (1921).
30. P. H i d n e r t a n d W. B. Gero, Sci. Pap. Bur. Stand.,
19, 429 (1924).
31. R. S. Archer in "Rare Metals H a n d Book," p. 291,
2nd ed., Reinhold P u b l i s h i n g Corp., New York
(1961).
32. Climax M o l y b d e n u m Co., " M o l y b d e n u m Metal,"
Climax, Ohio, 1960.
33. N. Cabrera, p. 42/43 i n " S t r u c t u r e and Properties
of T h i n Films," Proc. of a n Intern. Conf. at
Bolton Landing, New York, 1959.
34. H. A. Robinson, J. Phys. Chem. Solids, 26, 209
(1965).
Contact Potential Difference between Silver Nitrate and Silver
James L. Copeland 1 and Ralph I.. Seifert
Depv~rtment of Chemistry, Indiana University, Bloomington, Indiana
ABSTRACT
The silver n i t r a t e - s i l v e r contact potential was d e t e r m i n e d from 190 ~ to
280~ by the Zisman d y n a m i c condenser method. At the m e l t i n g point of the
salt the contact potential of liquid silver n i t r a t e relative to silver is --530 mv,
with a t e m p e r a t u r e coefficient of 0.14 m v / d e g , and that of the solid salt is
--608 my. The discontinuity of 78 m v at the m e l t i n g point is a t t r i b u t e d p r i m a r i l y to a greater excess surface concentration of silver ions i n the liquid
t h a n in the solid salt.
Of the various methods that have been used to
measure contact potential b e t w e e n two dissimilar substances, only the electron b e a m deflection method (1),
the K e l v i n v a r i a b l e condenser method (2, 3), and the
1 P r e s e n t a d d r e s s : D e p a r t m e n t of C h e m i s t r y , K a n s a s
v e r s i t y , M a n h a t t a n , K a n s a s 66504.
State Uni-
Zisman d y n a m i c condenser method (4) do not i n volve ions a n d / o r electrons i m p i n g i n g on the test s u r faces d u r i n g part of the procedure. Therefore these
methods are most acceptable for study of the contact
potential of a metal and its salt, either molten or in
solution. 'The Zisman d y n a m i c condenser method was
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832
JOURNAL
OF
THE
ELECTROCHEMICAL
used in this study because it permits greatest precision
of m e a s u r e m e n t .
One plate of the d y n a m i c condenser is v i b r a t e d
r e l a t i v e to t h e other. If t h e r e is a difference in the
Volta (or outer) potentials of the facing surfaces, the
periodic change in capacitance of the condenser p r o duces an a l t e r n a t i n g c u r r e n t in a high resistance in
t he circuit j o i n i n g t h e plates. T h e a - c p o t e n ti a l across
the resistor is amplified. A v a r i a b l e e m f source bet w e e n one plate and the resistor p e r m i ts equalization of the Volta potentials as indicated by zero a-c
signal. This balancing e m f at zero signal equals the
contact potential b e t w e e n the substances of which
the plates are m a d e if t h e r e is no n e t t h e r m a l emf
in the e x t e r n a l circuit and the e r r o r due to fringing
field (5) is avoided. To t h e e x t e n t that any net e r r o r
f r o m these two sources can be held constant in two
successive series of m e a s u r e m e n ts , the e r r o r can be
cancelled out by using one condenser plate as a r e f erence surface, d e t e r m i n i n g the a p p a r e n t contact potential of each of the tw o desired substances r e l a t i v e
to the r e f e r e n c e plate (6), and calculating the contact potential of the two substances.
To d e t e r m i n e the contact potential b e t w e e n silver
and s i l v e r n i t rat e in contact w i t h the silver, a silver
v i b r a t i n g plate w a s used as a reference. Its contact
potential was d e t e r m i n e d r e l a t i v e to the surface of a
stationary silver plate and then r e l a t i v e to silver
n i t r a t e c o v e r i n g the stationary silver plate.
Experimental
Materials.--Reagent grade silver n i tr a te (Baker
A n al y zed ) was used w i t h o u t purification other than
drying. H e l i u m used as an inert atmosphere had a
stated i m p u r i t y content of 0.01%.
The t wo plates comprising the d y n a m ic p a r a l l e l plate condenser w e r e fabricated f r o m silver with a
p u r i t y of 999-F fine as quoted by the B a k e r P l a t i n u m
Division of E n g e l h a r d Industries, Inc. Before all runs
the plates w e r e a b r a d e d with No. 1 steel wool and
boiled successively in the f o l lo w i n g concentrated
aqueous solutions for about 30 m i n each, with i n t e r v e n i n g rinsings in distilled water: (a) a m m o n i u m h y droxide, (b) hydrochloric acid, and (c) potassium
cyanide. A f t e r thorough w a s h i n g in distilled water, the
plates w e r e treated as above but in r e v e r s e order for
the solutions, rinsed in distilled water, heated in air
at 500~176
for a p p r o x i m a t e l y 4 hr, and t h e n t r a n s f e r r e d to a desiccator w h i l e still hot. D u r i n g and foll o w i n g this t r e a t m e n t the plates w e r e handled only
w i t h clean tongs or surgical l a t e x gloves, care being
taken not to touch the active surfaces.
Apparatus.--The v i b r a t i n g condenser was m o u n t e d
inside a cubic brass v a c u u m chamber, designed to
p e r m i t all operations during a r u n to be p e r f o r m e d
w i t h o u t opening the chamber. V i e w i n g ports w e r e
p r o v i d e d in the f r o n t of t h e chamber, and t h r ee ad justable supports p e r m i t t e d l e v e l i n g the c h a m b e r to
insure that the l o w e r surface of the u p p e r condenser
plate wo u l d be p a r a l l e l to the surface of the m o l t e n
silver nitrate. Details of construction are given in
ref. (7).
The silver plates of the dynamic condenser are
shown in Fig. 1, w i t h the m o u n t i n g assembly and
h e a t e r f o r the v i b r a t i n g plate. U p p e r p l a t e (G) was v i brated at a f r e q u e n c y of 40 cps, with an a m p li tu d e of 2
ram, by means of a d r i v e shaft w h i c h linked connector
( A ) , t h r o u g h a S y lp h o n bellows, to an e x t e r n a l 1/~ hp
synchronous motor, g e a r e d to d e l iv e r 2400 rpm. H e a t e r
(C) was supported by its h e a v y copper leads and did
not touch any part of the v i b r a t i n g plate assembly.
S i l v e r cylinder (H), s e r v i n g as stationary plate,
was m o u n t e d above a l e v e l i n g p l a t f o r m by t h r e e silica
tubes and a transite ring. The l e v e l i n g p l a t f o r m could
be raised and l o w e r e d t h r o u g h a distance of 3.2 cm
f r o m outside the v a c u u m chamber. W h e n (H) was in
its l o w e r position a r e c t a n g u l a r stainless steel salt
pan could be m o v e d b e t w e e n the condenser plates. A
August 1965
SOCIETY
B
K
D
E
F
G
M
N
0
H
P
I
Q
~
0
2cm
Fig. 1. Plates of dynamic condenser and exploded view of the
mounting assembly and heater for the vibrating upper plate: (A)
boron nitride connector; (B) leads from thermocouple (D); (C)
heater, No. 26 nichrome wire on boron nitride frame; (D) chromelalumel thermocouple in alumina insulator; (E) alumina washer;
(F) silver washer; (G) vibrating silver plate of dynamic condenser;
(H) stationary plate of dynamic condenser, upper surface of solid
silver cylinder; (I) insulated chromel-alumel thermocouple; (J)
threaded hole for reciprocating drive shaft; (K) grounded shield
wire; (L) silver foil reflector (unwrapped); (M) quartz thermocouple well; (N) silver wire lead to vibrating plate; (O) 1/4-28
thread on extension from silver condenser plate, for screwing into
holder (A); (P) rim of cavity for holding silver nitrate; (Q) silver
wire lead to stationary plate.
stud on the sliding bottom of the pan caught on r i m
(P), d u m p i n g silver nitrate into the plate cavity as
the pan was pushed over it. The l o w er portion of
cylinder (H) was s u r r o u n d e d by a small cylindrical
f u r n ace positioned inside the t h r e e silica supports but
not touching the cylinder. T h e r m o c o u p l e (D) e x t e n d e d
to w i t h i n 4 m m f r o m the l o w e r surface of plate (G),
and t h e r m o c o u p l e (I) was about 0.8 m m b el ow the
center of the top surface of c y l i n d e r (H).
Storage batteries w e r e used for h e a t e r c u r r e n t d u r ing measurements. A l t e r n a t i n g c u r r e n t was used d u r ing prolonged periods w h e n m e a s u r e m e n t s w e r e not
being made. One side of each electrode h e a t e r circuit
and of each t h e r m o c o u p l e circuit was grounded. A
grounded w i r e shield b e t w e e n each plate and its h e a t e r
eliminated an observed c h a n g e in m e a s u r e d contact
potential w h i c h was p r o p o r t i o n a l to the d-c h e a t e r
current. 'The shields also reduced the slight dependence
of m e a s u r e d contact potential on m e a n plate separation
to less than the r a n d o m er r o r in the measurements.
This dependence of apparent contact potential on plate
separation (5) had al r ead y b e e n greatly diminished
by m a k i n g the v i b r a t i n g plate smaller than t h e stationary one.
F o r balancing the contact potential b e t w e e n the
plates a K - 2 p o t e n t i o m e t e r was connected by a r e v e r s ing switch to the v i b r a t i n g p l at e and to ground. The
off-balance 40-cycle signal was passed t h r o u g h a 50
megohms i n p u t resistor of a h i g h - g a i n amplifier with
a band-pass filter tuned to 40 cps. T h e amplified signal
was displayed on an oscilloscope. A l t h o u g h shielded
wires w e r e used t h r o u g h o u t t h e circuit for signalc a r r y i n g leads and batteries w e r e used for the heaters
and amplifier, a small a m o u n t of 60-cycle pickup was
present in the amplified signal.
Procedure.--To detect a v e r y small a m p l i t u d e 40
cps signal in the presence of the 60 cps pickup, use was
m a d e of the fact that the combined signal was r e p e t i -
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VoI. 112, No. 8
SILVER
NITRATE-SILVER
rive e v e r y 1/20 sec. W i t h oscilloscope t r ig g e r e d by a
60 cps signal and set at a sweep rate of 0.1 msec/cm,
a 40 cps component in the input signal produced t h r ee
almost linear and p a r a l l e l traces, w i t h vertical disp l a c e m e n t proportional to th e a m p l i t u d e of the 40 cps
component. The null point, at w h i c h a single trace
was observed, could be detected w i t h i n • 0.2 my.
H o w e v e r , over periods of 2-5 min, the applied potential necessary to obtain a null signal was observed
to ex h i b i t r a n d o m fluctuations, sometimes as lar g e as
3.5 m y but for the m a j o r i t y of e x p e r i m e n t a l points
less than 0.8 my.
W i t h the system e v a c u a te d to less t h a n 0.1 m m Hg,
the plates w e r e heated f o r 1 hr at about 300~
The
decomposition pressure of silver oxide at 300~ is
19.8 at m (8). H e l i u m was then a d m i t t e d to about 50
m m H g above atmospheric pressure. Contact potential
m e a s u r e m e n t s w e r e m a d e at r e g u l a r i n t e r v a l s until the
values became constant. To establish a b l a n k c u r v e
for the contact potential of the silver surface of the
stationary plate r e l a t i v e to the v i b r a t i n g r e f e r e n c e
plate, m e a s u r e m e n t s w e r e then m a d e at plate t e m p e r atures c o m p a r a b l e to those at w h ic h m e a s u r e m e n t s
w e r e l a t e r to be m a d e w i t h silver n i t r a t e on the stat i o n a r y plate.
H e l i u m was chosen as a m b i e n t gas because studies
by Mignolet (9) showed t h a t helium, w i t h the lowest
polarizability of a n y gas, should h a v e undetectable
effect on the m e a s u r e d contact potentials of metals.
Since its dielectric constant of 1.0000684 at S.T.P. is
the smallest of any gas, it w o u l d best a p p r o x i m a t e the
dielectric properties of a v a c u u m w h il e decreasing the
r a t e of loss of salt by vaporization. A t no t i m e d u r i n g
the course of a c o m p l e t e r u n was the condenser c h a m ber opened or the a m b i e n t gas altered (except for h y d r o g e n t r e a t m e n t in the last r u n ) .
A f t e r completion of m e a s u r e m e n t s for the b l an k
curve, the stationary p la te was lowered, and silver
n i t r a t e was placed in the plate cavity and m e l t e d
before the plate assembly was raised to m e asu r i n g
position. The d y n a m ic c o n d e n s e r was now composed of
the r e f e r e n c e plate and th e surface of th e liquid silver
n i t r a t e in contact w i t h the silver surface for which
the b l a n k cu r v e had been established. Contact p o t e n tial m e a s u r e m e n t s w e r e m a d e at a t e m p e r a t u r e near
250~ u n t i l a constant v a l u e was attained, and then
at different t em p e r a t u r e s . F o r all th e silver n i t r at e
measurements, except those in r u n 3 n e a r the mel t i n g
point [209.7~ (10)], the t e m p e r a t u r e of the v i b r at i n g
r e f e r e n c e plate was h e l d 10 ~ -A-_I ~ above that of the
s t a t i o n a r y plate to p r e c l u d e possible condensation of
va p o r f r o m the m o l t e n salt. F r o m 199 ~ to 214~ in
the t h i r d silver n i t r a t e r u n some m e a s u r e m e n t s w e r e
m a d e w i t h a t e m p e r a t u r e difference of only 2 ~ but
the a v e r a g e difference was about 3.5~
Many runs
w e r e t e r m i n a t e d before constant values w e r e attained
at 250~
owing to such factors as loss of salt by
creepage and possibly evaporation, f a i l u r e of electrical
connections within the housing, and electrical shorting by the creeping salt. Generalizations concerning
b e h a v i o r during the b l a n k m e a s u r e m e n t s are t h e r e fore based on m a n y m o r e runs than those associated
w i t h the t h r e e co m p l e t e d salt runs.
In the final r u n the b a r e plates w e r e heated in h y d r o g e n as well as h e l i u m before constant values w e r e
obtained. P r i o r to this r u n the stationary plate had
been c o n t a m i n a t e d by silver chloride, w h ic h p e r m e a t e d the surface. Since the thermocoup]e channel
was exposed w h e n the c o n t a m i n a t e d surface was cut
away, the plate cavity was r e f u r b i s h e d w i t h m o l t e n
silver and subsequently f a c e d - o f f and polished prior
to r o u t i n e plate preparation. Since m o l t e n silver dissolves m o r e t h a n 20 times its own v o l u m e of oxygen
(S.T.P.) and solid silver just below its m e l t i n g point
dissolves its o wn v o l u m e (11), the o x y g e n content
of the surface s t r a t u m of the r e f u r b i s h e d plate must
h a v e been considerably g r e a t e r than that in earlier
runs. This was b o r n e out by the initial s i l v e r - s i l v e r
CONTACT
833
POTENTIAL
contact potential of about --75 m v at 300~ 2 w hi c h
did not change for several days. The h e l i u m was then
replaced by d r y electrolytic hydrogen, w h e r e u p o n the
contact potential a b r u p t l y ch an g ed polarity and then
r e m a i n e d f a i r l y constant at about 100-130 mv. This
positive potential of the stationary plate surface, r e l a tive to the s m a l l e r v i b r a t i n g plate, could result f r o m
reaction of h y d r o g e n w i t h t h e g r e a t e r concentration
of o x y g e n diffusing to the surface of the m o r e massive
silver cylinder, f o r m i n g t r a n s i e n t l y adsorbed w a t e r
molecules w i t h the positive end of t h ei r dipoles outward. On r e p l a c e m e n t of the h y d r o g e n by helium, the
contact potential became e v e n m o r e n e g a t i v e than
before, possibly owing to accelerated depletion o.f oxy g e n f r o m the small r e f e r e n c e plate by the h y d r o g e n
treatment. A second h y d r o g e n t r e a t m e n t g a v e the
same behavior, but to less extent. W h e n stability in
the vicinity of zero contact potential was finally attained, the system was e v a c u a t e d at less than 0.1 m m
Hg for 24 hr, flushed with electrolytic hydrogen, again
ev acu at ed f o r 24 hr, and then flushed and filled w i t h
helium. The plates w e r e aged t h e r e i n for 48 hr at
about 500~
S u b s e q u e n t a g r e e m e n t of m e a s u r e m e n t s
in this r u n with the two e a r l i e r completed runs i ndicates that an y effect that o x y g e n m a y h a v e had on
the earlier completed runs was v e r y small.
Th e s i l v e r - s i l v e r contact p o t en t i al m e a s u r e m e n t s in
each b l an k r u n indicated t h e t i m e w h e n both plates
h ad aged a d e q u a t e l y to n e a r l y identical states, and
providec~ a correction c u r v e for application to the
silver n i t r at e data to compensate for possible t e m p e r a t u r e - d e p e n d e n t voltages ap p ear i n g e l s e w h e r e in
the circuit.
Results
Condenser stabilization. F i g u r e 2 is a plot of observed contact potential vs. t i m e for a typical bl a nk
d u r i n g its stabilization period and illustrates the long
time r e q u i r e d to attain constant values as well as
the t e n d e n c y to proceed f r o m n e g a t i v e values to zero
or slightly positive ones. Since the concentration of
o x y g e n absorbed d u r i n g the p r e l i m i n a r y p l at e t r e a t m e n t w o u l d decrease r e l a t i v e l y r a p i d l y in the small
r e f e r e n c e plate, the slowly d i m i n i sh i n g n e g a t i v e contact p o t en t i al m u s t h a v e r e s u l t e d f r o m continuing diffusion of o x y g e n to the surface of the massive silver
cylinder f o r m i n g the stationary plate. T h e final constant contact potentials obtained in practically all the
b l an k runs w e r e close to zero, indicating that the two
silver surfaces w e r e finally almost identical.
Each d y n am i c condenser composed of silver ni t r a t e
surface and v i b r a t i n g r e f e r e n c e plate r e q u i r e d t w o to
t h r ee weeks to attain constant contact potential at
constant t e m p e r a t u r e . Th e v a r i a t i o n of observed values
w i t h time did not display a q u a l i t a t i v e l y r e p r o d u c i b l e
trend. Th e g e n e r a l t e n d e n c y was to b eg i n at n e g a t i v e
values, to rise w i t h time to less n e g a t i v e values, and to
fall back e v e n t u a l l y to a constant v a l u e n e a r the inis T h e s i g n of t h e c o n t a c t p o t e n t i a l is t a k e n as t h e s i g n of t h e p o t e n t i a l a p p l i e d t o t h e v i b r a t i n g r e f e r e n c e p l a t e to a c h i e v e a n u l l
p o i n t , i.e., t h e c o n t a c t p o t e n t i a l is t h e V o l t a p o t e n t i a l of t h e s t a t i o n a r y plate (silver or silver nitrate) r e l a t i v e to that of the v i b r a t ing reference plate.
E
:
20
l
I
I
I
I
I
I
I
I
I
I
I
2
I
4
I
6
I
8
I
I0
I
12
I
14
I
16
I
i8
I
20
I
22
o
~_ - 2 0
~
-40
~
-80
I--
-tO0
o
0
TIME
24
(days)
Fig. 2. Typical variation of contact potential between silver
plates during stabilization at 280~ for blank run.
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JOURNAL OF THE ELECTROCHEMICALSOCIETY
834
tial values. H o w e v e r , plots of the data w e r e erratic,
w i t h n u m e r o u s m a x i m a and minima.
Contact potentials.--Results obtained in the t h r ee
c om p l et ed runs are shown in Fig. 3, 4, and 5. Each
plotted point represents the a v e r a g e of at least t h r ee
readings t a k e n o v e r a period of s e v e r a l minutes at
the g i v en t e m p e r a t u r e , w h i c h is that for the stationary
plate. T i m e i n t e r v a l s b e t w e e n points at different t e m p e r a t u r e s v a r i e d f r o m 30 m i n to s e v e r a l hours. A f t e r
the initial stabilization period, each of the b l a n k runs
r e q u i r e d two days for completion, and the t h r e e silver
n i t r a t e runs r e q u i r e d three, six, and two days, respectively. R ep r o d u cib il it y of both b l a n k and salt runs
was verified by taking data both w i t h descending and
ascending t e m p e r a t u r e s - - c i r c l e s and dots, respectively,
in Fig. 3, 4, and 5.
The l o wer curves in Fig. 3, 4, and 5 give the c o n tact potential, or difference in Volta potential, b e t w e e n
t he silver n i t r a t e surface and th e surface of the r e f e r e n c e plate. Subtraction of t h e potential of t h e stationary silver plate r e l a t i v e to the r e f e r e n c e plate at
each t e m p e r a t u r e (upper curves) gives the desired
contact potential, w h i c h is the Volta potential of the
surface of the silver nitrate, WAgNO~, minus the Volta
potential of the silver p la te in contact w i t h the salt,
WAg. This difference in Volta potentials is g iv e n as a
function of t e m p e r a t u r e in Fig. 6, as d e t e r m i n e d f r o m
the smoothed curves in Fig. 3, 4, and 5.
The c u r v a t u r e observed above 214~ in r u n 1 (Fig.
3) for both the b l a n k and silver n it r a t e data was u n usual, b ei n g observed in n e i t h e r the other two completed runs nor in the n u m e r o u s sets of b l a n k m easu r e m e n t s m a d e in i n c o m p le ta b l e runs. This c u r v a t u r e
must h a v e resulted f r o m some t e m p e r a t u r e - d e p e n d e n t
vol t ag e bias not connected with the silver and silver
n i t r a t e surfaces of the stationary plate, since it cancelled out in the calculation of the s i l v e r - s i l v e r nitrate
contact potential, giving a r e s u lt a n t c u r v e similar
to those of runs 2 and 3, as shown in Fig. 6. This illustrates the a d v a n t a g e of using a r e f e r e n c e plate.
A b o v e about 217~
the b l a n k and s i l v e r - n i t r a t e
=I
E
August 1965
'
'
Reference
--
'
'
I
260
I
.J
"3_-52o"
~
-540
]
9
o -560
~o - 5 8 0
~-600
0r
I
200
0
o -620
.
I
I
I
I
220
240
TEMPERATURE
I
Fig. 5. Contact potentials obtained in run 3. Temperature difference between plates only 2~176 for salt data between 199 ~
and 214~
I
I
I
I
I
I
-520
,uL _ 5 4 0
~
-560
o
-600
j
Run :5
I ~ I
Run __
Run I
-580
2
1"3,- 6 2 0
o>
P
200
I
I
I
I
220
240
TEMPERATURE
I
I
260
o
gNO 3 - - Reference
- 6 0 0 |` oL~ J
,
200
I
,
,
220
240
TEMPERATURE
Fig. 3. Large temperature
served only in run 1.
--]
dependence
'
'
I "~/
260
(~
280
of contact
potential
ob-
WAgNOa
(I)
--
WAg =
>
40I
.
20
I
I
Run 2:cAgNO3
I
I
I
I
I
(s)'--
cAg
Run 3:~AgNO3
,o
~ -560
I
200
I
I
I
220
240
TEMPERATURE
(s) --
Y~Ag =
--(622.0___ 11.9) d- (0.043 _ 0.058)t
i- -540
o
o0 - 5 8 0
I
I
260
[3]
I
Ag - Reference
o ~'
-500
w
c~ - 5 2 0
n
[2]
w h e r e t is t e m p e r a t u r e in ~ at the position of the
t h e r m o c o u p l e in the stationary plate. Equation [1] is
based on salt data for t = 218~176
and Eq. [2],
218~176
The e r r o r terms are standard deviations
calculated for the least squares evaluations of the
constants. S t a n d a r d deviations of e x p e r i m e n t a l points
f r o m the calculated equations for the blank and salt
m e a s u r e m e n t s were, respectively, for r u n 2, 2.3 and 1.9
my, and for r u n 3, 0.12 and 0.74 my.
Corresponding calculations for linear portions of
the curves below the m el t i n g point of silver nitrate
gave:
--(570.5 _+ 19.8) -- (0.127 ~ 0.096)t
I
280
curves for runs 2 and 3 are linear w i t h i n e x p e r i m e n t a l
error. The best linear equation was d e t e r m i n e d by the
m e t h o d of least squares for each of the four sets of
data and the equations for the blank and salt m e a s u r e ments in each r u n w e r e combined to give the f o l l o w ing:
Run 2:WAgNO3 (1) - - WAg
--(541.9 • 11.5) + (0.085 • 0.046)t
[1]
--(563.2 _+ 5.8) + (0.155 _ 0.025)t
-56
I
(~
Fig. 6. Volta potential of silver nitrate surface minus that of
silver in contact with silver nitrate.
Run 3:
i:! .....
280
(~
I
280
(~
Fig. 4. Contact potentials obtained in run 2. Ten degrees temperature difference between plates for all points.
[4]
S t a n d a r d deviations of e x p e r i m e n t a l points for the
solid salt m e a s u r e m e n t s w e r e 0.73 m v for run 2 and
0.56 m y for r u n 3.
If values g i v e n by Eq. [1] to [4] are weighted by
the reciprocal of t h ei r variance, the following a v e r a g e
values are obtained. A t the m el t i n g point of pure silv e r nitrate, 209.7~ (10), the Volta potential of the
liquid salt r e l a t i v e to silver is about --530 my, with a
t e m p e r a t u r e coefficient of about 0.14 m v / d e g , and t h a t
of the solid salt r e l a t i v e to silver is about --608 my,
w i t h a t e m p e r a t u r e coefficient of --0.003 m v / d e g . In
each phase the silver n i t r at e is sat u r at ed with silver,
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Vol. 112, No. 8
SILVER
NITRATE-SILVER
helium at about 800 m m Hg, and nitrogen dioxide
and o x y g e n at their decomposition pressures for the
s i l v e r - s a t u r a t e d solution.
Discussion
Temperature gradients in silver nitrate.--Vertical
and radial t e m p e r a t u r e gradients in the m e l t resulted
f r o m the higher t e m p e r a t u r e of the r e f e r e n c e plate
and t h e cooling of the edge of the stationary plate by
convection currents in the helium. Resulting co n v ection currents in the m e l t m a y h a v e produced transient
local variations in the liquid salt surface during the
stabilization period. Mixing by convection m a y h a v e
accelerated the approach to near steady states for
processes occurring in the melt, although t e m p e r a t u r e
d e p e n d e n t states w o u l d continue to be affected slightly
by the t e m p e r a t u r e gradients.
Silver-silver nitrate interface. The surface of the
stationary plate after a r u n had the same appearance
as fo l l o wi n g its initial preparation, except that small
a d h e r e n t crystals of silver f o r m e d on the inner s u r face of the cavity r i m (P in Fig. 1) at a p p r o x i m a t e l y
the position of the initial molten salt surface. They
w e r e f o r m e d in this region of lowest t e m p e r a t u r e by
crystallization of dissolved silver a n d / o r by electrochemical deposition resulting f r o m the radial t e m p e r a t u r e gradient in the silver. Some localized solution and recrystallization of silver m a y h a v e occurred
at all parts of the interface due to the different w o r k
functions of different crystallographic faces. It p r o v e d
to be impossible to r e m o v e the last trace of silver
n i t r a t e f r o m the stationary plate by creepage and
e v a p o r a t i o n before the creeping salt caused electrical
l e a k a g e inside the condenser chamber. T h e r e f o r e no
m e a s u r e m e n t s w e r e m a d e of the contact potential of
the final silver surface r e l a t i v e to the r e f e r e n c e plate.
If it w e r e different than the value obtained initially,
all the calculated contact potentials w o u l d be in e r r o r
by a p p r o x i m a t e l y the same small value. The m a g n i tude of the observed discontinuity n e a r 210~ would
not be influenced significantly.
Solutes in silver nitrate.--After the long stabilization period the m e l t must have been saturated, at the
stabilization t e m p e r a t u r e , w i t h helium, silver dissolved
from the stationary plate and formed f r o m decomposition of the melt, and the nitrogen dioxide and o x y g e n
from decomposition of the melt. No d e t e r m i n a t i o n s of
solubilities of these substances in silver n i tr a te h a v e
been made.
Since the final stabilized contact potential for the
salt runs was always close to the values obtained at
the b eg i n n i n g of the stabilization period, the less n eg ative values for the i n t e r v e n i n g period w e r e caused
either by transient effects such as u n k n o w n trace i m purities in the silver n i tr a te diffusing to the surface
and e v e n t u a l l y vaporizing, or by differing rates of
increase of the effects due to decomposition of melt,
solubility of silver and helium, and electrochemical
solution and deposition of silver at the s a l t - m e t a l
interface. If the latter, the influences of the different
effects on the final e q u i l i b r i u m state a p p r o x i m a t e l y
cancelled each other. A f t e r a steady state was finally
achieved at the stabilizing t e m p e r a t u r e , any f u r t h e r
changes w i t h t e m p e r a t u r e of effects influencing contact potential must h a v e been r e v e r s i b l e and h a v e occ u r r e d r ap i d l y enough to be completed by the time
the plate t e m p e r a t u r e s w e r e stabilized, since only
small r a n d o m changes w e r e observed during the 2-5
rain periods that m e a s u r e m e n t s w e r e m a d e at each
e x p e r i m e n t a l t e m p e r a t u r e . A n y changes occurring
m o r e slowly and any i r r e v e r s i b l e t e m p e r a t u r e dep e n d e n t changes must h a v e had no effect on contact
potential in v i e w of the observed r e p r o d u c i b i l i t y of
m e a s u r e m e n t s with ascending and descending t e m peratures. Thus the presence of any nitrogen dioxide
and o x y g e n from decomposition of the m e l t m u s t not
have influenced the m e a s u r e m e n t s significantly. R e ported decomposition pressures (12) increase f r o m 7
m m Hg at 248~ w h i c h is near the stabilization tern-
CONTACT
POTENTIAL
835
p er at u r es for the t h r ee runs, to 80 m m Hg at 280~
the highest t e m p e r a t u r e at w h i c h contact potential
m e a s u r e m e n t s w e r e made. If the pressure of d e c o m position products in the condenser c h a m b e r increased
d u r i n g h i g h - t e m p e r a t u r e m e a s u r e m e n t s and these
products did affect the m e a s u r e d contact potential,
values d e t e r m i n e d initially at the stabilization t e m p e r atures w o u l d not h a v e been r e p r o d u c e d later.
Discontinuity o~ contact potential at the melting
point of silve~- nitrate.--Because of the radial t e m p e r ature gradient in the silver nitrate, its t e m p e r a t u r e
b el o w the center of the v i b r a t i n g plate was higher
than that n e a r t h e edge. Therefore, contact potentials
obtained near the m e l t i n g point w e r e influenced by
the fraction of the salt surface that had solidified with
descending t e m p e r a t u r e s and the fraction that had
m e l t e d w i t h ascending temperatures. D u r i n g r u n 2
(Fig. 4) t h i r t y m i n u t es elapsed b e t w e e n the m e a s u r e ments at 203.5 ~ 207 ~ and 209.5~ m a d e w i t h ascending t em p er at u r es. This was twice the time allowed for
stabilization b e t w e e n descending t em p er at u r es. T h e r e fore the l o w e r cu r v e must r ep r esen t a closer approach to equilibrium. W h e n a still closer approach to
u n i f o r m t e m p e r a t u r e was achieved near the me l t i ng
point in r u n 3 by r e d u c i n g the t e m p e r a t u r e difference
b e t w e e n plates and w ai t i n g about an hour for stabilization at each t e m p e r a t u r e , the resulting data (Fig.
5) indicated that there must be a discontinuity in the
contact potential at the m e l t i n g point. As m e a s u r e d by
the stationary plate thermocouple, the t e m p e r a t u r e at
the sharp rise in Fig. 5 is about 2~ above the m e l t i n g
point of p u r e silver n i t r a t e (10). This is the combined er r o r r esu l t i n g f r o m t e m p e r a t u r e gradients and
use of an u n c a l i b r a t e d s m a l l - w i r e thermocouple.
T h e observed discontinuity in v a l u e of the contact
potential indicates that the surface potentials of solid
and liquid salt at the h e l i u m interface differ by about
78 mv. The o b s e r v e d difference m a y result f r o m the
influence of surface concentrations of the solutes discussed above, w h i c h w o u l d be different for the two
phases, a n d / o r from p r e f e r e n t i a l concentration of silv e r ions in the surface of the m e l t to a g r e a t e r degree than in the solid a n d / o r a similar concentration
of nitrate ions in the surface of the solid. Th e concentrations of gases in the m e l t must h a v e been small
and w e r e p r o b a b l y e x c e e d e d by the concentration of
silver. Since solidification of the silver n i t r at e progressed t o w a r d the w a r m e r r e f e r e n c e plate, any silver
forced out of solution w o u l d tend to collect on the surface of the salt u n d e r the r e f e r e n c e plate. This w oul d
cause a discontinuity in the opposite direction f r o m
that observed. T h e r e f o r e it is concluded that the m a jor cause of t h e observed discontinuity in the contact
potential is a larger co n cen t r at i o n of Ag + ions than
N O 3 - ions in the free surface of the melt.
Acknowledgment
The authors wish to express their thanks to the
Directorate of Chemical Sciences, A i r Force Office of
Scientific Research and to the Atomic E n e r g y Commi s sion for their support of the research r e p o r t e d in this
paper.
Manuscript r e c e i v e d Oct. 27, 1964; r ev i sed m a n u script r e c e i v e d A p r i l 19, 1965.
A n y discussion of this paper will appear in a Discussion Section to be published in the J u n e 1966 JOURNAL.
REFERENCES
1. B. J. Rothlein and P. H. Miller, Jr., Phys. Rev., 76,
1882 (1949).
2. L o r d K e l v i n , Phil. Mag., 46, 82 (1898).
3. W. E. M e y e r h o f and P. H. Miller, Jr., Rev. Sci.
Instr., 17, 15 (1946).
4. W. A. Zisman, Rev. Sci. Instr., 3, 367 (1932).
5. K. W. Bewig, " I m p r o v e m e n t s in the V i br a t i ng
Condenser Method of Measuring Contact P o tential Difference," N a v a l Research Lab., R e p o r t
5096, Feb. 4, 1958; D o c u m e n t PB131530, O.T.S.,
U. S. Dept. of Commerce, Washington, D. C.
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JOURNAL
836
OF
THE
ELECTROCHEMICAL
6. K. W. B ewi g and W. A. Zisman, J. Phys. Chem.,
67, 130 (1963).
7. J. L. Copeland, "The Contact Potential Difference
b e t w e e n Metallic S i l v e r and Molten S i l v e r Salts,"
Ph.D. thesis, I n d i a n a University, Bloomington,
Ind. (1962). A v a i l a b l e f r o m U n i v e r s i t y Microfilms, A n n Arbor, Mich., order no. 625022.
8. F. G. Key es and It. Hara, J. A m . Chem. Soc., 44,
479 (1922).
9. J. C. P. Mignolet, Discussions Faraday Soc., 1950
(8), 105, 326; J. Chem. Phys., 21, 1298 (1953).
SOCIETY
August
1965
10. K. J. Macleod and F. E. W. Wetmore, Ann. N. Y.
Acad. Sci., 79, 873 (1960). [C. Sinistri and P.
Franzosini, Ric. Sci. Rend. Sez, A3 (4), 419
(1963) give the v a l u e 209.4 ~ -~ 0.2~
11. J. W. Laist, "Copper, Silver, and Gold," Vol. II of
" C o m p r e h e n s i v e Inorganic Chemistry," M. C.
Sneed, J. L. Maynard, and R. C. Brasted, Editors,
p. 141, D. v a n Nostrand Co., Inc., N ew York
(1954).
12. M. C e n t n e r s z w e r and M. B l u m e n t h a l , Bull. intern.
acad. polon, sci., Classe sci. math. nat., 1936A,
No. 8-9, 482.
The Anodic Reaction Mechanism in Fused Silver Nitrate
Nirmal Gupta 1 and Benson R. Sundheim
D e p a r t m e n t of Chemistry, N e w Y o r k University, N e w Yo~k, N e w Y o r k
ABSTRACT
T h e oxidation of the n i t r a t e ion at a Pt anode in m o l t e n AgNO3 has been
studied by s t e a d y - s t a t e and f a s t - s w e e p v o l t a m m e t r y . Two mechanisms ar e
found, both leading to NO2 ~- u 02. T h e first is susceptible to t r an sp o r t control and has an na v a l u e of 1.17 at 238~ It is postulated t h a t this m e c h a n i s m
involves the p r e l i m i n a r y dissociation of the n i t r a t e ion to f o r m NO2 + -~- O2 =,
the latter (possibly bound as orthonitrate) being the el ect r o act i v e species. The
second m e c h a n i s m is not subject to t r an sp o r t control and has an n~ v a l u e of
0.90. This is t h o u g h t to i n v o l v e the f o r m a t i o n of NO3 radical followed by its
decomposition to form the products. If the Eo values of 0.357v and 1.25v (vs.
A g / A g +) are adopted, the two mechanisms are characterized by e x c h a n g e
c u r r e n t densities of 2.4 x 10-~ and 0.14 a m p / c m 2, respectively. Th e corresponding values calculated for ko, t h e h et er o g en eo u s rate constant, are 1.3 x 10 -7
and 1.1 x 10 -7 cm/sec.
Th e objective of this study is to devise and test a
me ch an i s m f o r the electrochemical oxidation of a nit ra t e ion on a p l a t i n u m electrode in m o l t e n silver nitrate. Th e i n f o r m a t i o n obtained f r o m the p r e l i m i n a r y
investigation of p o te n t ia l vs. c u r r e n t curves in the
s t e a d y - s t a t e condition led to t h e exploration of the
possibility of t h e r e being m o r e than one reaction
m e c h a n i s m in different potential regions.
Experimental Procedures
Reference e l e c t r o d e . - - A 1 cm 2 piece of silver m et al
dipping in the molten silver nitrate was used as a
r e f e r e n c e electrode. In a p r e l i m i n a r y test a c u r r e n t
voltage cu r v e w h e r e i n the potential of a silver cathode
was m e a s u r e d w i t h respect to a resting silver r e f e r e n c e
electrode was carried out over a considerable r a n g e of
c ur r en t densities. A graph of c u r r e n t vs. observed potential showed a straight line corresponding to an app a r e n t resistance of 0.53 ohm. On this basis, it was
concluded that the r e f e r e n c e electrode showed no
noticeable polarization and t h e r e f o r e could be used
safely. T h e stability of th e p o te n t ia l of two such silver
electrodes w i t h respect to one a n o th e r over a period of
time was excellent.
I n s t r u m e n t a t i o n . - - T h e t e m p e r a t u r e of the cell was
m a i n t a i n e d at an a r b i t r a r i l y selected t e m p e r a t u r e of
238.6 ~ in a l arg e pot m e l t i n g f u r n a c e fitted w i t h t e m p e r a t u r e r e g u l a t i n g and stirring equipment. An eq u i m o l a r m i x t u r e of sodium n it r it e and sodium n i t r a t e
was used as t h e melt. The assembly was capable of
ma i n t ai n i n g the t e m p e r a t u r e to w i t h i n 0.02 of a degree
of the desired t e m p e r a t u r e . Since the t e m p e r a t u r e
m e a s u r e d in these e x p e r i m e n t s was actually that of
the bath and not of the silver nitrate, the cells w e r e
allowed to r e m a i n in the m e l t for at least 2 hr in
or d er to attain the c o r r e c t t e m p e r a t u r e before any
m e a s u r e m e n t s w e r e made.
Preparation of silver n i t r a t e . - - S i l v e r n i tr a te of analytical q u al i t y ( B a k e r A n a l y z e d ) was m e l t e d in a
cell and subjected to continuous evacuation by a m e chanical v a c u u m p u m p for s e v e r a l hours. D u r i n g this
1Present
60616.
address:
Institute
of G a s T e c h n o l o g y ,
Chicago, Illinois
time, gases, mostly w a t e r vapor, w e r e evolved. Two
electrodes of p l a t i n u m and silver, respectively, and of
3.5 cm 2 geometrical surface area w e r e then inserted
in the m e l t and an electrolysis car r i ed out for 48 hr
u n d e r v a c u u m at a c u r r e n t density of 4.5 m a / c m 2.
This p r e - e l e c t r o l y s i s p r o c e d u r e was f o u n d to be necessary in order that the c u r r e n t vs. potential m e a s u r e m en t s be reproducible. P r e s u m a b l y , h e a v y m e t a l ions
w e r e r e m o v e d by this procedure. T h e silver n i t r a t e
thus obtained was t r a n s p a r e n t and colorless and could
be subjected to a n u m b e r of m e l t i n g and solidification cycles in t h e presence of light u n d er v a c u u m
w i t h o u t any p er cep t i b l e change in color. F i g u r e 1
shows a P y r e x cell f o r potential vs. cu r r en t m e a s u r e m en t s fitted w i t h a m u l t i p l e pulley w h i ch was used to
rotate at various speeds, the l ar g e p l a t i n u m electrode
h a v i n g a geometrical surface area of 3.45 cm 2. The t w o
silver electrodes w e r e placed s y m m e t r i c a l l y w i t h r e spect to t h e rotating p l a t i n u m electrode. All t h r e e of
the electrodes w e r e c o m p l e t e l y i m m e r s e d in the molten
silver n i t r at e during t h e experiment. It was found that
reproducible results could be obtained only if t he cell
was continuously evacuated. A satisfactory r o t a t i n g
seal was made by choosing the glass tube and bottom
glass j o i n t for a snug fit. Both of the joints had the
same i n n er diameter. Such a contact could be used
w i t h m e r c u r y at speeds up to 2000 rpm. At higher r o tation speeds, the glass tube was coated with a thin
l ay er of p l a t i n u m in place of t h e tungsten spiral
around the glass tube. N u m b e r 8 cup grease was used
as a l u b r i can t b e t w e e n the cup and the glass tubing.
For rotating the p l a t i n u m m i c r o e l e c t r o d e at speeds
g r eat er than 1200 rpm, a Bodine synchronous m o t o r
was used at 1800 rpm. The a r m a t u r e of this motor was
connected to a gear a r r a n g e m e n t w i t h a 1:1 ratio. One
of these gears had a h o l l o w center shaft which was
used to hold the p l a t i n u m microelectrode. The rotating
contact was established by inserting copper w i r e in
the m e r c u r y pool.
Analysis of reaction p r o d u c t s . - - T h e cell shown in
Fig. 2 was used to collect the reaction products. The
cell had one p l a t i n u m electrode and one silver electrode. B r e a k seals in t h e side arm w e r e used to col-
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