Bidwell and Speiser 1964

THE RELATIVE THERMODYNAMIC PROPERTIES OF
SOLID NICKEL-PALLADIUM ALLOYS*
L. R. BIDWELL? and R. SPEISER$
The relative thermodynamic properties of solid nickel-palladium alloys were determined in the
temperature range 700-1200°C using a galvanic cell with a solid electrolyte. Within experimental error,
the relative partial molar free energies of nickel were linear functions of the temperature, signifying
that AC, above 700°C is essentially zero and that the specific heats at elevated temperatures can be
approximated by the Kopp-Neumann rule. Nickel-rich solutions were found to exhijit endothermic
enthalpies of mixing and positive deviations from Raoult’s law for the activity of the solvent and
negative deviations for the activity of the solute. palladium-rich solutions, on the other hand, exhibit
exothermic enthalpies of mixing and negative deviations from Raoult’s law for the activity of the solvent
and solute alike. The excess entropies of mixing are positive for all compositions. This is considered to be
primarily a result of the ferromagnetic properties of the alloys. From a consideration of magnetic,
electronic, vibrational, and lattice distortional factors it has been concluded that nickel-palladium
alloys, particularly those rich in palladium, exhibit significant short-range order.
PROPRIETES THER~~ODYNAMIQUES DES ALLIAGES NICKELPALLADIUM
Les auteurs ont Btudie les propriirtcs thermodynamiques relatives des atliages nickel-paladin B,
l’etat so&de, dans le domaine de temperature de 700 B. 12OO”C, en utilisant une cellule Zt electrolyte
solide. Dans les limites des erreurs experimentales, ils ont observe que les energies libres molaires par-
tie&s du nickel etaient des fonctions lineaires de la temperature, ce qui implique que AC, est essentiel-
lament nul au-dessus de 7OO”C, et qu’on peut obtenir, de man&e approchee, les chaleurs specifiques aux
temperatures Blevees en utilisant la regle de Kopp-Neumann. Les autcurs ont observe que les solutions
riches en nickel montraient des enthalpies de melanges endothermiques, des deviations positives de la loi
de Raoult pour l’activitb du solvant et des deviations negatives pour l’activite du solute. D’autre part,
les solutions riches en palladium montrent des enthalpies de melange exothermiques et des deviations
negatives de la loi de Raoult aussi bien pour la solvant que pour lc solute. L’exces de l’entropie de
melange est positif pour toutes les compositions. Ces proprietes semblent &re au premier chef une
consequence des proprietes ferromagnetiques des alliages. En prenant en consideration les facteurs
magnetiques, electroniques, vibratoires, et la distorsion du reseau, les auteurs concluent que les alliages
nickel-palladium, et particulierement ceux riches en palladium, possedent un ordre rl petite distance
significatif.
DIE RELATIVEN THERMODYNAMISCH~~ EIGENSCHAFTEN FESTER NICKEL-
PALLADIUM-LEGIERUNGEN
Die relativen thermodynamischen Eigenschaften fester Nickel-Palladium-Legierungen wurden im
Temperaturbereich 700-1200°C mit Hilfe einer galvanischen Zelle mit festem Elektrolyt bestimmt.
Innerhalb der Fehlergrenzen warm die relativen partiellen molaren freien Energien von Nickel linearo
Funktionen der Temperatur. Dies bedeutet, d& AC, oberhalb 700°C im wesentlichen Null ist und daB
die spezifischen Warmen bei hohen Temperaturen durch die Kopp-Neumann-Regel approximiert
werden k&men. Nickelreiche Legierungen zeigten endotherme ~~hungsenthalpien, positive Abwei-
chungen vom Raoult’schen Gesetz fdr die Aktivitat des Lijsungsmittels und negative Abweichungen fur
die Aktivitiit dss gel&ten Stoffes. Palladiumreiche Legierungen zeigten andererseits exotherme
Mischungsenthalpien und negative Abweichungen vom Raoultschen Gesetz fur die Aktivitat sowohl
des Losungsmittels als such des geliisten Stoffes. Die Uberschuflmisohungsentropien sind fur alla
Zusammensetzungen positiv. Dies wird in erster Linie auf die ferromagnetischen Eigenschaften der
Legierungen zurtickgefiihrt. Es wird auf Grund &nor Betrachtung dsr magnet&hen, elektronischen,
Schwingungs- und Gitter-Ve~rrun~fak~~n geschlossen, de0 in Nickel-Pall~i~-Legierungen,
insbesondere in den pall~iu~eichen, starke Nahordnung auftritt.
INTRODUCTION well homogenized alloys that had been annealed for
THE nickel-palladium system, unlike the closely periods up to 2 weeks at 600°C and 3 weeks at 4OO”C,
related Fe-I’d, Fe-P& Co-Pt, and Ni-Pt systems, they found that the cell Dimensions followed a
has generally been regarded as a complete series of smooth curve as a function of composition with a
solid solutions in which no superlattices are formed.(l) considerable positive deviation from Vegard’s law.8
The most prominent evid,enoe for the absence of They could not 6nd any evidence for the existence of
ordering in the system is the careful X-ray diffraction ordering or for a miscibility gap that had been
work of Hulgtren and Zapffe.f2) Using a series of suggested earlier by Fraenkel and Stern.t4) In recent
* Received May 4, 1964; revised June 22, 1964. years, however, several Russian investigate have
This paper is based in part on a dissertation submitted noted anomalous electrical and magnetic behavior at
by L. R. Bidwell to the Ohio State University in partial
fulfillment of the requirements for the Ph.D. degree. the stoichiometric composition NiPd, which they
t Aerosuace Research Laboratories. Office of Aerosnace conclude is due to the existence of a superstructure
Rekearch,-Wright-Patterson Air Force ‘Base, Ohio. *
$ Department of Met.a~~gieal Engineering, The Ohio at that composition. Among the effects observed are
State University, Columbus, Ohio. 5 X-ray studies by the authors confirm this observation.(3’
ACTA METALLURGICA, VOL. 13, FEBRUARY 1965 61

62 ACTA METALLURGICA, VOL. 13, 1965
a satellite peak in the variation of the thermoelectrio
force as a function of longitudinal and transverse
magnetic field strength ;(S) a small but sharp variation
in the Curie temperature and a maximum in the
temperature coefficient of the longitudinal galvano-
magnetic effect;@*‘) a sharp change in the magnitude
of the odd Nernst-Ettingshausen effect;(s)* and a
sharp change in magnetostriction in both longitudinal
and transverse magnetic fields.@) Although it is
questionable whether or not these observations are
related to the presence of a superlattice, particularly
in view of the negative but more direct X-ray results,
they do suggest that a tendency toward ordering,
perhaps in the form of increased short-range order,
may exist in palladium-rich alloys. This possibility
becomes especially attractive when the nickel-
palladium system is compared with the iron-pal-
ladium system. While iron has a larger number of
positive holes in the d-band than nickel, there are
many obvious similarities between the two systems,
e.g., their small difference in atomic size, the confor-
mation of their phase diagrams,“) and their deviations
from Vegard’s law. t2) The iron-palladium system
shows strong ordering effects, with superlattices
stable to SOO-900°C being formed at compositions
roughly corresponding to FePd and FePd,. It was
expected that a knowledge of the solution properties
of solid nickel-palladium alloys would contribute
substantially to a better understanding of their
alloying behavior and might also provide further
insight into the question of ordering in the system.
EXPERIMENTAL PROCEDURE
The relative thermodynamic properties of solid
nickel-palladium alloys were derived from measure-
ments of the reversible potential of the solid galvanic
cell
Pt 1 Ni, NiO 1 0.85Zr0, * 0.15CaO 1 Ni-Pd, NiO 1 Pt
as a function of composition and temperature. Solid
galvanic cells employing calcia stabilized ziroonia as
the electrolyte have been used with considerable
success by Kiukkola and Wagner(lO~ll) in determining
the standard free energies of formation of several
oxides at elevated temperatures and by Rapp and
Maak(12) in an investigation of the thermodynamic
properties of solid copper-nickel alloys. It can be
shownds) that under conditions of pure anionic
conductivity of the electrolyte, the activity (aNi) and
relative partial molar free energy (Apx,) of nickel in
the nickel-palladium alloy can be obtained directly
* Reference 8 refers to Ni,Pd, however, the experimental
data clearly indicate that Ni Pd, is intended.
from the open circuit potential of the cell using the
relation
E= -Elna, = _A’Ni
2F 21r” (1)
where E is in volts, T is the absolute temperature, R
is the gas constant, and F is the Faraday oonstant.
The standard state for this case is solid nickel saturated
with oxygen. However, since the solubility of oxygen
in nickel is less than 0.08 at.% in the 700-1200°C
temperature range covered,(i) this is essentially
equivalent to a standard state corresponding to pure
solid nickel. The nearly pure anionic conductivity
of the electrolyte in this temperature range and in
the range of oxygen partial pressures imposed by the
electrodes (Ni-NiO equilibria for 0.04 < aNi < 1.0)
has been well established by Kingery et uZ.(13) and by
Schmalzried.04) According to the relative values of
the standard free energies of formation of ZrO,,
NiO, and Pd0,d5) mixed potentials arising from
displacement reactions involving the cell components
would not be expected.
The cell apparatus consists of a cell holder and an
impervious mullite reaction tube that serves as a
gas-tight furnace enclosure for maintaining the cell
in an inert environment, The reaction tube is graded
to Pyrex joints at each end and.is encased in electri-
cally grounded platinum foil in order to shield the
measuring circuits from induced a.c. potentials. The
essential features of the cell holder, which is a
modification of the configuration employed by
Schmalzried,06) are shown in Fig. 1. The tapered
brass fixture serves as the seal for one end of the mullite
reaction tube and as an exit for the electrical leads.
In addition, it aligns the two small concentric mullite
tubes which clamp the galvanic cell. A portion of
the outer tube, which is held to the brass fixture
under spring tension, is cut away to facilitate place-
ment of the cell and to provide a seat for the alundum
thrust block. The inner tube, which acts as a pushrod,
is positioned via a compression spring by a hollow
threaded screw located in the brass fixture. The
hross Alignment Fixture
Towed to fit Pyrex
Ground Gloss Joint
Mullitr Tuber
FIG. 1. Schematic diagram of galvanic cell holder.

BIDWELL AND SPEISER: THERMODYNAMICS OF Ni-Pd ALLOYS 63
tablets and platinum electrodes which make up the
cell are held against the alundum thrust block by
applying a moderate pressure with the pushrod.
The cell, which consists of a stabilized zirconia
electrolyte tablet sandwiched between metal plus
nickel oxide electrodes backed with platinum contacts,
is roughly 4 mm long by 8 mm dia., thus minimizing
the effects of thermal gradients. The cell assembly
was heated by a tubular resistance furnace which was
held to within f 1.5”C by a proportional band
controller. Cell temperatures were measured by means
of a Pt-13% Rh thermocouple placed in the alunclum
thrust block, l-2 mm from the alloy electrode. The
thermocouple used for this purpose was calibrated
against the melting points of pure zinc and gold.
Cell potentials were measured with a Leeds and
Northrup K-3 potentiometer and a high sensitivity
galvanometer using a three wire circuit in which the
platinum leg of the thermocouple acted as the positive
lead to the galvanic cell. The negative lead to the
cell was grounded through a 0.01 ,uF capacitor as a
further precaution against the introduction of induced
potentials.
The alloys were prepared at nominal 10 at.%
intervals from high purity carbonyl nickel and
commercial high purity palladium sponge by induction
melting in vacua. The alloy ingots were cold forged
and then homogenized in vacua beginning with 5 hr
at 1075”C, followed by successively higher tempera-
tures, and ending with a 1 hr treatment within 100°C
of the solidus. Chemical analyses of the alloys
indicated that they contained less than 200 ppm
impurities, exclusive of oxygen. The electrode
tablets were prepared from reagent grade NiO and
from metal powder ground from the homogenized
ingots with a tungsten carbide dental drill. Prio;
to use, the NiO was heated in purified helium several
hours at 1000°C in order to free it of excess oxygen.
Tablets 6 mm dia. by 2 mm thick were cold-pressed
at 5000 psi in a hardened steel die. It was f0und
from experience that metal to oxide ratios of 5 : 1 to
10: 1 were preferable to lower ratios because of the
improved sensitivity in the measurements made
possible by the lower circuit resistance.
The electrolyte tablets were prepared from high
purity Wah Chang (99.98%) zirconia and reagent
grade calcium carbonate. The appropriate mixtures
of zirconia and calcium carbonate were compacted
by cola-pressing ad calcinecl at 1300°C for 12 hr.
The calcined material was ground in a boron carbide
mortar, cold-pressed into tablets 1-2 mm thick by
9 mm dia., and then sinterecl in air at 1750°C for
36 hr.
In starting an experiment, the mull&e reaction
tube was alternately evacuated with a mechanical
pump and purged with purifiecl helium several times
in order to obtain a clean system. The helium was
purified by passing it over anhydrous magnesium
perchlorate, ascarite, hydrogen-reduced copper cata-
lyst at 15O”C, oxidized copper catalyst at 15O”C,
again over anhydrous magnesium perchlorate and
ascarite, and finally through a liquid-nitrogen cold
trap. In a few of the earlier experiments the cell was
heated to 700°C and data recorded with increasing
temperature. In all subsequent ones however, the
initial equilibration of the cell was hastened by
making the first measurements at 1100°C. The cells
were usually allowed to equilibrate 8-12 hr (overnight)
at this temperature. After the initial equilibration
period, the cdl potential was measured to the nearest
0.01 mV every 10-20 min over a period of several
hours. When the voltage had attained a constant
value the furnace temperature was lowered and the
cell allowed to equilibrate at the new temperature.
Data on the nominal 20, 40, 60 and 80 at.% compo-
sitions were recorded both on cooling and heating.
The data on the nominal 10, 30, 50, 70 and 90 at.%
compositions were checked with measurements on a
duplicate cell. The time required for the cells to
reach equilibrium, for all but the initial equilibration,
varied from 1 to 2 hr at the higher temperatures to
4 to 8 hr at the lower temperatures.
EXPERIMENTAL RESULTS AND DISCUSSION
Within experimental error, the cell potentials were
linear functions of the temperature. Values taken
from the smoothed experimental data at even 100°C
intervals are shown in Table 1. The overall repro-
ducibility of the experimental measurements was
exceptionally good. In the earlier experiment,s, where
only normal precautions were observed in the prepa-
ration and manipulation of the cell components and
in the assembly of the cell, the scatter in the data was
generally less than &2.5%. In later experiments,
where improved techniques were employed, the scatter
was in all cases considerably less than f 1 Oh.
In general, the observed cell voltages were very
stable over’ long periods of time. For example, the
pot.ential of a cell containing a 47.4 at. y. Pa electrode
equilibrated at 12OO”C, changed by only 0.04 mV
in over 28 hr. However, cells involving 77.7 and
86.5 at.% Pd electrodes were exceptions to this rule
at temperatures of 1000°C and above. After one of
these cells appeared to reach equilibrium, the voltage
drifted downward, continuously, at the rate of several
hundredths of a millivolt per hour. In all cases the

64 ACTA METALLURGICA, VOL. 13, 1965
TABLE 1. Cell potentials for nickel-palladium alloys
NPd
0.107
0.191
0.287
0.382
0.474
0.564
0.681
0.777
0.866
700°C
3.51
6.25
10.45
17.43
26.75
41.40
66.30
93.40
132.85
Cell potential (mV)
800°C 900°C 1ooo”c
4.09 4.64 6.27
7.32 8.39 9.47
12.30 14.15 15.98
20.13 22.76 25.41
30.40 34.00 37.66
46.40 51.40 56.35
73.05 79.80 86.50
102.15 110.86 119.55
144.05 155.10 166.25
llOO°C 1200°C
6.86 6.45
10.55 11.63
17.83 19.68
28.03 30.69
41.30 44.90
61.35 66.35
93.30 100.05
128.20 136.90
177.26 188.35
NiO content at the electrolyte interface appeared to
decrease on the alloy side and to increase on the
nickel side. This behavior was attributed to concen-
tration polarization of the electrodes due to the small
but finite electron conduction in the electrolyte. By
allowing these cells to remain at 1000°C and above
only long enough to achieve practical equilibrium,
the drop in potential attributable to polarization
could be held well within the limits of reproducibility
dictated by the other experimental errors.
Although there is no direct experimental criterion
for cell reversibility, reproducibility is generally
accepted as a strong indication of it. Cells were
checked for thermal reversibility by recording
potential measurements with increasing as well as
decreasing temperature. They were also checked for
electrical reversibility by momentarily unbalancing
the potentiometer, causing a small current of several
microamps to flow according to the direction of
unbalance. When the cells were displaced from
equilibrium in this manner, they rapidly returned to
their initial voltage, usually within a matter of
minutes.
The activities (aNi) and relative partial molar free
energies (ApNi) of nickel, taken relative to solid
nickel saturated with oxygen, were calculated from
equation (1). The corresponding quantities for
palladium were evaluated from a graphical integration
of the Gibbs-Duhem equation in the form :
s
Nm
1% yPd = @NilNNiNPd - aNi dNPd, (2)
1
where the Ni are the appropriate mole fractions,
ypd is the activity coefficient of palladium (apd/Npd),
and aNi E log yNi/NPd2 (see Tables 2 and 3). The
activities of nickel and palladium calculated from
the smoothed data at 900°C are shown in Fig. 2.
Also included in this figure are the values for a,,
calculated directly from the experimental data. In
most cases these values fall nearly on top of each
other but have been spread out somewhat in order to
show the number of measurements. Figures 3 and 4
illustrate the manner in which the activities and
relative molar free energies change with temperature
from 700 to 12OOW.
From a consideration of the phase diagram, i.e.,
the absence of miscibility gaps, compounds, or super-
lattices, and the similarities in the atomic size and
electronic structure of nickel and palladium, the small
deviations of the activities from ideal solution
behavior seen in Fig. 3 are not surprising. However,
the details of the observed deviations are noteworthy,
particularly in the case of nickel, which exhibits both
positive and negative departures from Raoult’s law.
Although this type of behavior is unusual, it is not
without precedent in systems involving transition
metals with nearly or completely filled d-shells.
Similar behavior has been observed by Weibke and
Matthea in Cu-Pt alloys, Schmahl(ls) in Au-Pd
alloys, Orianios) in Co-Pt alloys, and by Aukrust and
Muant20) in Fe-Pd alloys.
The relative partial molar entropies of nickel
(A&) were calculated from the slopes of potential-
temperature curves using the relation
=2p aE
p. Npd [ I
6'T P,Npd' (3'
The values for palladium were determined in a similar
manner from the slopes of APpd-temperature curves
(Fig. 5). The relative partial molar enthalpies were
evaluated from equations of the form :
A& = AFi + TA& (4)
Like the activities, the enthalpies of mixing exhibit
rather unusual behavior (Fig. 6) in that they are
endothermic for nickel-rich solutions and exothermic
for palladium-rich solutions. It is worthy of note,
although no explanation can be offered, that the
partial enthalpies of mixing of nickel and palladium
pass through maximum positive and negative values,

BIDWELL AND SPEISER: THERMODYNAMICS OF Ni-Pd ALLOYS 65
TABLE 2. Activities in nickel-palladium alloys
NPd
0.107
0.191
0.287
0.382
0.474
0.564
0.681
0.777
0.865
0.107
0.191
0.287
0.382
0.474
0.564
0.681
0.777
0.865
0.107
0.191
0.287
0.382
0.474
0.564
0.681
0.777
0.865
-_
T”C
700
800
900
aNi oPd
0.920 0.072
0.862 0.105
0.779 0.145
0.660 0.201
0.528 0.272
0.373 0.372
0.206 0.530
0.108 0.680
0.042 0.824
0.915 0.070
0.854 0.104
0.766 0.148
0.647 0.206
0.518 0.278
0.367 0.381
0.206 0.540
0.110 0.687
0.044 0.827
0.912 0.068
0.847 0.105
0.756 0.151
0.638 0.211
0.510 0.284
0.362 0.390
0.206 0.548
0.112 0.694
0.047 0.830
!PC
1000
1100
1200
aNi aPd
0.908 0.068
0.841 0.106
0.749 0.154
0.629 0.217
0.503 0.292
0.358 0.400
0.207 0.556
0.113 0.701
0.048 0.833
0.906 0.067
0.837 0.106
0.740 0.157
0.623 0.221
0.498 0.298
0.355 0.408
0.207 0.565
0.115 0.705
0.050 0.835
0.903 0.066
0.833 0.106
0.733 0.160
0.617 0.225
0.493 0.303
0.352 0.413
0.207 0.570
0.116 0.709
0.051 0.836
TABLE 3. Relative molar quantities for nickel-palladium alloys at 900°C
h’d
0.107
0.191
0.287
0.382
0.474
0.564
0.681
0.777
0.865
Cal/mole cal/deg-mole Cal/mole
_ -
AFm A Fm AFJf Ai& Ai& AS” - -
AHNI AHpd AH=
-214 -6255 -860 0.271 5.652 0.847 104 375 133
-387 -5255 -1317 0.497 4.406 1.243 195 -87 141
-653 -4407 -1731 0.852 3.220 1.531 346 -630 66
-1050 -3626 -2034 1.222 2.522 1.719 384 -668 -18
- 1569 -2930 -2214 1.678 1.902 1.784 399 -699 -121
-2372 -2194 -2271 2.302 1.315 1.746 329 -652 -225
-3682 -1405 -2132 3.110 0.825 1.554 -34 -437 -309
-5112 -851 -1801 4.614 0.516 1.296 -403 -246 -281
-7156 -435 -1342 5.131 0.336 0.983 -1138 -39 -189
0.4 0.6 0.0
Fm. 2. Activities in nickel-palladium alloys at 900°C. FIG 3. Temperature variation of the activities in nickel-
palladium alloys.