Bidwell et al. 1970

THE THERMODYNAMIC PROPERTIES OF COBALT-PALLADIUM SOLID SOLUTIONS*
L. R. BIDWELL,? F. E. RIZZO$ and J. V. SMITHS
The thermodynamic activities of cobalt in palladium, relative to pure f.c.c. ferromagnetic cobalt,
have been determined from e.m.f. measurements on solid-electrolyte oxygen concentration cells over
the temperature range SOO-1100°C. The activity of cobalt in palladium exhibits positive departures
from ideal solution behavior in cobalt-rich alloys and negative departures in palladium-rich alloys.
The activities of palladium, deduced from the experimental data, indicate negative departures from
ideality at all compositions. Cobalt-palladium alloys were of particular interest because their unusually
high Curie temperatures permitted compositions up to 40 at. % palladium to be studied in the ferro-
magnetic state. The experimental data were of sufficient precision that the ferromagnetic-paramagnetic
transition could be detected in alloys containing nominally 20 and 30 at. ‘4 palladium. The relative
integral molar excess entropies, determined from the changes in the partial molar free energies
of mixing as a function of temperature, are positive for all the compositions studied. The heats of
mixing are endothermic for cobalt-rich compositions and exothermic for palladium-rich compositions.
An extrapolation of the experimental values of the partial molar free energies of cobalt to lower tempera-
tures, suggests the presence of a at high
du cobalt le palladium, au cobalt
z% partir de f.e.m. B oxygene zt
et 1100°C. du cobalt le palladium
au comoortement de en
cobalt, et nbgatifs en palladium. du palladium,
& la
un in&& de Curie
il est B l’&at ferromag&tique, des alliages dont les
compositions vont jusqu’8 40% at. de
la transformation ferromagn&ique-paramagn&ique puisse atre detectbe dans les
alliages contenant normalement 20 & 30 de palladium.
B partir des variations des Energies libres molaires partielles du en fonction de
de melange
en cobalt et exothermiques en
palladium. du cobalt
la presence de miscibilite de 600°C
en cobalt.
in Palladium
an festen Sauerstoffkonsentrationszellen
im im Palladium
in kobaltreichen
in palladiumreichen
zu ‘A Palladium
im ferromagnetischen in
Legierungen 30 At. ‘A Palladium zu erlauben.
in kobaltreichen in palladiumreichen zu tiefen auf
das Vorhandensein einer Mischungsliicke unterhalb 600°C bei hohen Kobaltkonzentrationen hin.
INTRODUCTION first-period transition elements. Alloys with chrom-
The normally paramagnetic properties of pure pal- ium and manganese, for example, tend toward anti-
ladium are radically altered by the addition of the ferromagnetic behavior, while those with iron, cobalt
* Received October 24, 1969; revised March 6, 1970.
t Metals and Ceramics Division, Air Force Materials
Laboratory, Wright-Patterson Air Force Base, Ohio 45433.
Formerly al,: Aerospace Research Laboratories, W-PAFB,
Ohio.
$ Department of Metallurgical Engineering, University of
Texas at El Paso, El Paso, Texas 79999. Formerly at: Aero-
space Research Laboratories, Wright-Patterson Air Force
Base, Ohio.
For!e%aseFOhio 45433.
eras ace Research Laboratories, Wright-Patterson Air
and nickel become ferromagnetic at very low con-
centrations of the 3d element. In the latter alloys,
the bulk saturation magnetization is much larger
than can be accounted for by the 3d element alone.(l)
In order to rationalize the distribution of the atomic
moments in these alloys, it is necessary to assume
that magnetic moments are induced in the palladium
atoms.(z) The neutron scattering measurements,
ACTA METALLURGICA, VOL. 18, SEPTEMBER 1970 1013

ACTA METALLURGICA, VOL. 18, 1970
3 .
5 1000 “\_ a
oz 0 l k.0 0 0 00 0
F 000. 1. CURIE TEMP
5soo_ 63-01
I- 400\+ ‘-\\\\.
200 %/> \a -6‘ ‘\
<.I> >, ‘Y
Ob.‘, 1 I ’ h
CO 20 40 60 00 Pd
ATOMIC % PALLADIUM
FIG. 1. Phase equilibrium diagram for the cobalt-
palladium system (redrawn from Hansen and Anderko’5’).
Alloys investigated are indicated on 900°C isotherm.
both in the presence and in the absence of an applied
magnetic field, conducted by Cable et aZ.t3) indicate
that this assumption is correct.
In an earlier study of the thermodynamic properties
of palladium solid solutions containing nickel by one of
US,(~) rather unusual behavior was observed in that
the heats of mixing deduced from the rate of change
of the free energy with temperature were negative for
palladium-rich solutions and positive for nickel-rich
solutions. The excess entropies of mixing, however,
were positive at all compositions. A theoretical
analysis of the expected ferromagnetic contributions
to these properties indicated that the observed positive
excess entropies could be almost totally ascribed to
the results of electron-spin disordering.
The present study ofcobalt-palladiumsolidsolutions
was undertaken as a further step in an investigation
of the role of ferromagnetism in the thermodyna-
mics of alloying. The cobalt-palladium system was
especially attractive because, as shown in the phase
diagram(s) in Fig. 1, the ferromagnetic state persists
to relatively high temperatures and palladium con-
tents. In addition, the system is similar to the nickel-
palladium system at elevated temperatures, with
minima in the liquidus and solidus curves near
50 at. % palladium and a complete series of solid
solutions. At temperatures below 6OO”C, however,
the phase equilibria for cobalt-rich alloys are rather
poorly defined. Past studies of this region have led to
ambiguous results since the cobalt f.c.c. to h.c.p.
transformation involves a great deal of hysteresis and
does not occur isothermally. It was hoped that the
e.m.f. technique employed in this investigation might
be sufficiently sensitive to follow the course of both
the magnetic and structural transformations in
cobalt-rich alloys.
EXPERIMENTAL PROCEDURE
The chemical activities (a,,) and relative partial
molar free energies (A(?,,) of cobalt in cobalt-
palladium solid solutions were determined from the
reversible, open-circuit, electrical potentials of the
concentration cell
Co, Co0 (Zr,.,,Ca,,,BO,.sjl Co-Pd, Co0
as functions of both composition and temperature in
an atmosphere of purified helium. The reference
state for this cell is solid, f.c.c., ferromagnetic cobalt
saturated with oxygen. Since the solubility of oxygen
in cobalt is less than 0.06 at. ‘A at all temperatures
investigated,c5) the reference state is equivalent to
unit activity for cobalt. The essentially pure oxygen
ionic conductivity of the cell electrolyte(6.7) allows
a,, and AC,,, to be determined from the open-circuit
potential by means of the relations
RT A%,
E= - ---lnaco= ___
2F 2F (1)
where E is in volts, T is the absolute temperature,
R is the gas constant and F is the Faraday constant.
The details of the apparatus and its calibration,
the methods for purifying the helium and preparing
the electrolyte and cell electrode tablets, and the
operating procedures employed in this investigation
have been adequately described in previous publica-
tions.(4ns) The alloys studied were prepared at nom-
inal intervals of 10 at. % from low-nickel reagent
grade cobalt and from palladium sponge obtained
from the International Nickel Co. They were vacuum
melted in high-purity recrystallized alumina crucibles,
surface ground, severely cold-worked, homogenized
for 24 hr at 50°C below the solidus temperature,
and powdered with a tungsten carbide dental drill.
Chemical analyses of the alloys vr-ere performed by
Battelle Memorial Institute. A spectrographic im-
purity analysis of the alloys and starting materials
indicated that 0.2 wt. ‘A nickel was the major impurity.
Reagent grade Co0 was used in the preparation of the
metal-metal oxide cell electrodes. Prior to its use,
the Co0 was heated at 1100°C for 24 hr in a stream
of purified helium in order to remove any excess
oxygen that may have been present.
The temperature range over which reliable data
were obtained (800-l 100°C) was established by several
factors. Attempts to measure cell potentials for
cobalt-rich alloys below 6OO”C, which presumably
would have penetrated the two-phase field of the

BIDWELL et al.: THERMODYNAMIC PROPERTIES OF Co-Pd SOLID SOLUTIONS 1015
f.c.c. to h.c.p. transformation (see Fig. l), were
unsuccessful. Even with an especially high imped-
ance circuit employed successfully at low temperatures
in a previous investigation of copper-platinum
alloys,(g) the cells polarized easily and did not yield
data that could be regarded as thermodynamically
reversible. At 1200°C and palladium contents greater
than 70 at. %, the e.m.f. values tended to be time
dependent, i.e. they decreased with time at a rate
that increased with palladium content. The 90.3 at. %
palladium alloy behaved in a similar fashion at 1000°C
and above. The decrease in e.m.f. was generally
irreversible and appeared to be attributable to the
formation of a thin electrical blocking layer of oxide
at the reference-electrode/electrolyte interface.
This situation can arise when the diffusion of oxygen
within the reference electrode is not sufficiently
rapid to compensate the open-circuit transport of
oxygen that is permitted by the very small, but non-
zero, electronic conductivity of the electrolyte.
The magnitude of this leakage current increases
with temperature and with cell potential. It can
damage a cell irreversibly if the oxygen cannot be
distributed within the electrode rapidly enough to
maintain two-phase metal-metal oxide equilibrium
at the electrolyte interface. By limiting the experi-
mental temperature range to 1100°C and avoiding
long excursions above 1000°C with the 90.3 at. %
palladium alloy, reliable data were obtained at
all compositions and temperatures reported.
EXPERIMENTAL RESULTS AND DISCUSSION
Cell potentials taken from the smoothed experi-
mental data and their respective rates of change with
temperature are tabulated in Table 1. A typical
e.m.f.-temperature curve, showing the extent of the
scatter in the data, is presented in Fig. 2. The data
shown are for four separate cells, out of a total of
seven, with cobalt-30.7 at. ‘A palladium electrodes.
TABLE 1. Cell potentials for cobalt-palladium alloys
Cell potential
(mV) dE
Npd -- z?
800°C 900°C 1000°C 1100°C hVl°C)
0.082 3.13 3.72 4.32 4.92 5.97
0.217 5.41 6.43 7.50 9.24 17.7*
0.307 6.73 8.21 10.7 13.3 25.6?
0.401 10.3 13.9 17.5 21.2 36.4
0.510 23.6 29.0 34.3 39.6 53.4
0.600 36.0 42.8 49.4 56.2 67.2
0.714 73.7 82.4 91.1 99.8 87.0
0.800 140 151 162 173 109
0.903 226 241 256 271 147
* For T > 994°C; for T < 994”C, dE/dT = 10.1 ,JV/“C.
t For 21 > 906°C; for T < 906”C, dE/dT = 14.8 pV/“C.
5
‘411
13
I
O-A3
0 -A4
8-83
12 m-84 A
ml
-2 I / I
7:
pp’
8600 900 1000 1100
TEMPERATURE ,“C
FIG. 2. Partial e.m.f. data for a cobalt-30.7 at.%
palladium alloy. Data are from four out of a total of
seven cells.
The data for the remaining cells fall on top of those
shown and were omitted for clarity. In general, the
reproducibility of the data for a given cell, for measure-
ments taken randomly with respect to heating or
cooling, was fl per cent. Occasional cells, e.g.
B3 in Fig. 2, while yielding excellent results by
conventional e.m.f. standards, were unaccountably
more erratic than the others, particularly at the higher
temperatures. The reproducibility of data from
separate cells of the same nominal composition was
f3 per cent or better, which in most cases can be
accounted for by small differences in composition of
*0.2 at. % or less.
The Curie temperatures of the 21.7 and 30.7 at. %
palladium alloys are well within the 800-1100°C
experimental temperature range.‘@ Figure 2 clearly
shows the transition from ferromagnetic to paramag-
netic behavior in the 30.7 o/o alloy. While this transition
is of second order and the change in cell potential
(partial molar free energy) should be a smooth non-
linear curve, the nonlinearity is small and the data
are too scattered to justify anything but a linear
approximation. The two line segments in Fig. 2 were
drawn through the data from all seven cells. The
points of intersection occurred at 906°C for this alloy
and at 994°C for the 21.7 at. ‘A palladium alloy.
These temperatures are remarkably close to the
990°C and 900°C Curie temperatures reported by
Grube and Kastnero’) for alloys containing nominally
20 and 30 at. % palladium respectively. While the
closeness of the agreement is doubtlessly somewhat

1016 ACTA METALLURGICA, VOL. 18, 19’70
a
ATOMIC % PALLADIUM
FIG. 3. Activities of cobalt-palladium alloys. I-this in-
vestigation, 1100°C; a--Sohwerdtfeger and Muan,
1200°C.“e’
fortuitous in view of the linear treatment of the data
required in the present work, it nevertheless gives
powerful support to the reliability and sensitivity of
the data.
The activities (a,,) and partial molar free energies
(AB,,) of cobalt, relative to solid cobalt saturated
with oxygen, were calculated from equation (1).
The related quantities for palladium were evaluated
from a graphical integration of the Gibbs-Duhem
equation in the form :
log YPd = -%o’NCoNPd + s
Nco %o’~~co (2)
0
where the N, are the constituent mole fractions,
ypd is the activity coefficient (apd/Npd) of palladium
and c(’ E log yc0/NPd2. The activities of cobalt
determined at 1100°C are compared in Fig. 3 with
values obtained by Schwerdtfeger and Muan at
1200°C. The data shown for these investigators are
02 20 40 60 00 Pd
ATOMIC % PALLADIUM
FIG. 4. Comparison of cobalt-palladium and nickel-
palladium(4) activities at 900°C.
largely the results of gas equilibration
except at three highest contents. The
are from galvanic cell
very similar those used this investigation.
agreement between two studies excellent.
If is assumed the temperature from
the work are at 12OO”C, correction
for difference in results in
closer agreement.
principal feature the chemical of
cobalt a positive from ideality cobalt-
rich and a departure in
rich solutions. shown in 4, this similar to
behavior exhibited nickel in except
7, ( , , , , , , , I ,
I I I I I I I
CO 20 40 60 80 Pd
ATOMIC % PALLADIUM
FIG. 5. Entropies of mixing of cobalt-palladium alloys
at 1000°C.
that the positive deviations from ideality of cobalt
are much larger. The activities of palladium indicate
negative departures from ideality in both systems
but significantly more so in the case of the cobalt
alloys.
The partial molar entropies (A&‘,,), excess entropies
(ASooxs ) and enthalpies (Ai7,,) of mixing of cobalt
were calculated from the rate of change of cell poten-
tial with temperature (see Table 1) using the relations
Afl CO (3)
and A80,xs = Asco + R In Nc,, (4)
A&, = AQco f T AS,, (5)

BIDWELL et GE.: THERMODYNAMIC PROPERTIES’OF Co-Pd SOLID SOLUTIONS 1017
co 20 40 60 80 Pd
ATOMIC % PALLADIUM
The corresponding partial molar quantities for pallad-
ium were evaluated from a graphical integration of the
Gibbs-Duhem equation in the farm
+ s
Nco &$yoXs
() (1 - No,)2 dNco C6)
and the free energies deduced from equation (2).
The partial and integral molar properties calculated
at 1000% are illustrated in Figs. 5 and 6.
GENERAL DISCUSSION
The s~g~~~a~t ~hara~~ristics of the the~od~amie
properties of cobalt-palladium solid-solutions are
- -600
1
i!
\
jj-800
r-
I
0
Q-1000
20 40 60 80 Pd
ATOMIC Y. PALLADlUM
Fm. 7. Comparison of the excess free energies of mixing
of cobalt-palladium and nick&-palladium~~) alloys at
1000°C.
20 40 60 80
ATOMIC % PALLADIUM
FIG. 8. Compwison of the excess entropies of mixing of
eob~t-palladia and nic~al-palladiuml*) alloys at
1000°C.
best ilIustrated by referring to the pertinent excess
quantities. These are compared with the values pre-
viously obtained for nickel-palladium alloys(Q in
Figs. 7-9. The deviations from ideal alloying behavior
are similar for both systems, although subs~nt~a~~
larger for the cobalt dloys. The integral excess free
energies of mixing or iron-palladium alloys, reported
by Aukrust and Muan, exhibit a similar composi-
tional dependency to that shown in Fig. 7, in&dmg
the presence of an inflection point at low pa~dium
contents and a maximum negative value at high
palladium oontents. The absolute value of the latter
exceeds 3000 cal/mol and suggests an increasingly
negative departure from ideality for the iron-group
elements in palladium in the order nickel, cobalt,
iron.
As indicated in Fig. 8, the positive excess entropies
of mixing of cobalt alloys are nearly twice those for
400
-800
’ ‘ 1 s 8 ’ s r /
20 40 60 80 ’ Pd
ATOMIC % PALLADIUM
Fm. 9. Comparison of the enthalpies of mixing of cobalt-
palladium and nickal-pall~ium~6~ alloys at 1000°C.