Journal of New Materials for Electrochemical Systems 9, 201-208 (2006)
© J. New. Mat. Electrochem. Systems
Elaboration, Characterisation and Electrical Properties of ZrO2 -In2 O3 Compounds
with Different Compositions for Intermediate Temperature SOFC
∗
1
A. Ringuedé1 , P. Mourot1 , C. Alvarez Lugano1 , J-C. Badot2 and M. Cassir1
Ecole Nationale Supérieure de Chimie de Paris- ENSCP
Laboratoire d Electrochimie et de Chimie Analytique - UMR7575 CNRS-ENSCP-Paris6
2
Laboratoire de Chimie Appliquée à l’Etat Solide — UMR 7574 CNRS-ENSCP-Paris6
ENSCP, 11 rue Pierre et Marie Curie, 75231 Paris Cedex 05, France
Received: March 07, 2006, Accepted: May 26, 2006
Abstract: Different compositions of the ZrO2 -In2 O3 system were synthesized by a wet co-precipitation technique, using chlorides
as precursors. The electrical behaviour of the corresponding sintered pellets was studied by impedance spectroscopy under air, as a
function of the temperature and the In2 O3 content. A transition from ionic to electronic conductivity was clearly revealed, for an
In% close to 50 % at 600◦ C. Impedance diagrams shapes vary from typical ionic behaviour, i.e. two separated semi-circles at low
In%, to pure electronic resistance. Concerning the intrinsic value of the total conductivity, it varies rapidly from 3.10−4 S.cm−1
(electrolyte ionic conductivity) to around 0.3 S.cm−1 (electrode conductivity) for 50 to 60 In%, respectively. Despite of indium
volatilisation from the surface during the high temperature sintering, these materials are promising for intermediate-temperature
SOFC, in particular with low-temperature thin layers synthesis techniques
Keywords: mixed conductivity, SOFC, electrolyte, electrode, indium oxide, zirconia, impedance spectroscopy
conductivity. In an intermediate domain, both cubic ZrO2
and In2 O3 coexist, showing a mixed ionic and electronic behaviour. These mixed-conducting oxides are of particular
interest for SOFC applications, i.e. for both cathode and
electrolyte materials, reducing nonohmic polarisation at
the interface and improving the performance of the whole
cell [6-9]. Another benefit of such oxides is the chemical
and mechanical compatibility between the electrode and
electrolyte materials. In effect, with an ionic radius of
0.77 Å, In3+ is closer to that of Zr4+ , 0,79 Å, than all the
other usual dopants, except Sc3+ (RSc 3+ = 0,81 Å). According to the phase diagram established by Sasaki et al [4],
the ZrO2 -In2 O3 system enables the thermodynamic coexistence between the ionic and electronic phases. This system includes monoclinic, tetragonal and cubic ZrO2 solid
solutions and cubic In2 O3 solid solution. Gauckler and
Sasaki have synthesised different pellets of mixed In2 O3 ZrO2 powders, in different proportions, and analysed their
structural and electrical properties at 1000◦ C [7]. Their results are in accordance with the theoretical predictions and
seem promising. Nevertheless, even though In2 O3 -doped
ZrO2 solid solutions are stable, the problem of the stability
of In2 O3 in In2 O3 -rich mixtures at high temperatures (i.e.
1. INTRODUCTION
Doped indium oxide, In2 O3 , has been proposed very
early as SOFC cathode, i.e. In2 O3 -ZrO2 , In2 O3 -ZrO2 PrO2 and In2 O3 -PrO2 -HfO2 [1-3]. In the typical SOFC
conditions (>800◦ C), In2 O3 is not stable and is expensive, which explains that this oxide has not been used as a
cathode material in recent SOFC devices. However, some
authors have analysed more accurately ZrO2 -In2 O3 system, which presents a high electronic and/or ionic conductivity according to the proportions of both oxides [4-7].
In2 O3 -doped ZrO2 , with relatively low dopant ratios (< 40
%), stabilises solid solutions of cubic or tetragonal-ZrO2
phases, which may exhibit conductivities comparable to
that of yttria-doped zirconia [4]. According to the sintering
temperature (around 1500◦ C) and the phase diagram[4],
the lowest india content to stabilise cubic zirconia is around
18%, which is very close to the formation of the tetragonal ZrO2 phase, the highest india content being of 40%.
The cubic-ZrO2 eutectoid composition contains 23.5% of
india. In the case of ZrO2 -doped In2 O3 , a single In2 O3 cubic phase is observed, with an increased n-type electronic
∗ To
whom corresponding to:
201
202
A. Ringuedé et al. / J. New Mat. Electrochem. Systems
1000◦ C), has to be solved.
The aim of the present study, 10 years after the previous pioneer works, is to analyse thoroughly the behaviour of such oxide mixtures at lower temperatures, i.e.
600◦ C, corresponding to a new generation of intermediatetemperature SOFC. A special attention is given here to the
electrical properties which are analysed here in details at a
temperature where few data are available in the literature.
Furthermore, two synthesis processes are compared, coprecipitation and co-milling. This study is the first of a serial where other synthesis routes will be investigated, such
as thin layer techniques allowing low-temperature syntheses (< 400◦ C), avoiding high-temperature treatments,
around 1500◦ C, favouring In2 O3 partial evaporation.
2. EXPERIMENTAL
2.1. Powders synthesis
Classical ceramic route, by mixing commercial oxide
powders (indium (III) oxide, 99.99%, Aldrich; zirconium
(IV) oxide, 99.99%, Aldrich) in the right proportion, was
followed to get some reference data. Chemical coprecipitation was used to synthesize all the compositions, from 25 to
90 In %, according to the procedure, detailed by Sasaki et
al [4-7]. Zirconyl chloride (ZrOCl2 -8H2 O, purity > 99.0%,
Fluka) and indium chloride (InCl3 -2.5 H2 O) were used as
metals salts precursors. They were mixed in stoichiometric
amounts and dissolved in deionised water. Precipitation of
the corresponding hydroxides was obtained by adding a
28% NH4 OH solution in the initial mixture, then washed
in water and ethanol, heat-treated at 110◦ C during a night,
milled and calcined at 800◦ C, during 2 hours.
2.2. Pellets fabrication
Powders were uniaxially pressed in a 13 mm diameter
stainless steel mould, applying a 0.5 tons pressure. They
were sintered at 1490◦ C, during 10 hours, with a temperature increase of 2 ◦ C.min−1 . The final dimensions of the
pellets were dependent on the composition: an increase of
the shrinkage was observed when the indium content was
higher.
2.3. Microstructural characterisations
X-Ray diffraction was performed using a Philips PW
1390 diffractometer with CoKα radiation source (λ =
1.7890 Å). The specific experimental conditions were as
follows: ∆2Θ=0.02◦ , 30-75◦ 2Θ range, step 0.02 sec.
Scanning electron Microscopy (SEM — Hitachi S2500),
coupled to EDS, was used to observe the microstructure
(grain size and porosity), and to evaluate the composition.
Pellets were polished up to 1 μm using diamond paste and
annealed for one hour at 1440◦ C to reveal the grains and
their boundaries.
2.4. Electrical characterisations
The electrical conductivity was determined from impedance spectroscopy measurements, under ambient air atmosphere and varying the temperature from 293 to 1000
K. It was carried out using an Autolab PGSTAT 30, Eco-
Table 1. Intensity ratio between (200) peak and (400)
peak, corresponding to zirconia and india phases, respectively.
In%
I(400) / I(200)
25
-
40
1/10
50
1/3
60
1
75
2
90
4
chemie BV. Depending on the In2 O3 content, a 5 or 200
mV ac signal amplitude was applied, respecting the system
linearity. No dc polarisation was imposed. All impedance
diagrams were recorded in the 1 MHz- 0.1 Hz frequency
range, using 11 points per decade of frequencies. A symmetrical two-electrodes configuration was used. Platinum
paste was painted and fired on both faces of the pellets.
Thus, the electrical conductivity was calculated from the
measured resistance R, using the classical formula:
σ=
1 e
R S
(1)
where e is the sample thickness and S the electrode surface
area.
Impedance diagrams have been decomposed using
EQUIVCRT.Pas, commercialized by B.A. Boukamp [10].
3. RESULTS AND DISCUSSION
3.1. Crystallographic structure and microstructure
X-Ray diffraction patterns, shown in Figs 1a and 1b,
clearly reveals that for values higher than 40 In%, the
ternary system is composed of two separate phases, indexed using the cubic zirconia JCPDS file and the indium
oxide file [11,12]. No real variations regarding the zirconia
peaks position are observed, whatever the indium content.
A global shift can however be noted for the 60 and 75%
samples when analysing pellets instead of powders. This
displacement is not observed anymore after milling the sintered samples, as reported in Figure 1b, where the analysis
is focused on the most characteristic peaks of both phases,
between 34 to 42◦ as 2Θ range. As expected, only the
relative intensity ratio between (400) c-In2 O3 and (200) cZrO2 peaks is modified and increases with respect to In %
substitution, as reported in Table 1.
Referring to the JCPDS file for both phases, the theoretical cell parameters are 0.5128 nm and 1.0119 nm for
the cubic zirconia phase [11] and the cubic indium oxide
phase [12], respectively. The medium experimental values
obtained by us are (0.5117 ± 0.0003) nm and (1.011 ±
0.002) nm, as shown in Fig. 2, even if slightly lower than
the theoretical ones, confirm the existence of both cubic
phases.
.Concerning the compositions, analysis of the bulk (heart
of the pellet) and the surface, in contact with air or in
contact with a platinum foil, was performed by EDS. The
results are summarised in table 2. The bulk of the sintered
pellets present an In % close to the expected one, whereas
a large difference can be observed for the surface area in
Elaboration, Characterisation and Electrical Properties of ZrO 2 -In 2 O 3 Compounds with
Different Compositions for Intermediate ... / J. New Mat. Electrochem. Systems
a
203
b
Figure 1. a- X-Ray diffraction patterns —(a) 90% (crushed pellet) — (b) 75% (pellet) - (c) 60% (pellet) - (d) 50% (crushed
pellet) — (e) 40% (pellet)- (f) 25% (pellet) ; c-ZrO2 peaks and *c-In2 O3 peaks.
b- Details X-Ray diffraction patterns on crushed pellets— (a) 90% — (b) 75% - (c) 60% - (d) 50% — (e) 40% - (f) 25% ;
(222) and (400) *c-In2 O3 peaks and (111) and (200) c-ZrO2 peaks.
Table 2. EDS analysis (at %) of the pellet inside and the surface in contact with air.
Expected In %
Pellet inside
25
40
50
60
75
90
28
43
56
61
79
85
Surface in contact
with air, after sintering
10h at 1490◦ C
24
35
31
31
36
35
Figure 2. Cell parameters according to c-ZrO2 and c-In2 O3
phases, as a function of the In2 O3 content.
Surface of sintered pellets
after polishing and annealing
1 hour at 1440◦ C
20
34
37
64
74
70
contact with air at 1490◦ C during the sintering process.
For indium contents higher than 40%, it can be noticed
that the indium content on the surface remained close to
30-35 % whatever the expected initial In percentage. It appears that indium oxide proceeding from the pellet surface
in contact with air was volatilised until reaching a stable state, where the remaining amount of this oxide corresponds to its solubility in the cubic zirconia phase, leading
to a cubic solid solution. [4]. When the same pellets were
polished (up to 1 μm using diamond paste) and annealed
for only one hour at 1440◦ C, the indium content at the
surface of the pellets was the expected one, except for the
90% pellet, with a value of 70% after the annealing treatment. These observations tend to prove the influence of the
high-temperature thermal treatment on the evaporation of
In2 O3 . The indium oxide phase, as confirmed by the XRay diffraction experiments, was separately observed for
the pellets containing more than 40 In %, leading to a
bi-phased system. It should be noticed that the indium
evaporation concerns specifically the surface of the pellets
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A. Ringuedé et al. / J. New Mat. Electrochem. Systems
a
b
Figure 3. Micrograph of the 90 In% pellet:
a- inside of the pellet,
b-surface in contact with air during sintering.
Figure 4. Mapping of Zr, In and O on the 40 In% pellet.
a
b
Figure 5. Micrographs of the 40 In% pellet, after polishing and annealing 1 h at 1440◦ C:
a- magnification 7000
b- magnification 10000.
Elaboration, Characterisation and Electrical Properties of ZrO 2 -In 2 O 3 Compounds with
Different Compositions for Intermediate ... / J. New Mat. Electrochem. Systems
205
Figure 6. Impedance diagram recorded for:
a) 25 In%, 359◦ C, under air; b) 40 In%, 350◦ C, under air (as a function of the signal amplitude, from 50 to 300 mV) ; c)
50 In%, 350◦ C, under air ; d) 60 In%, 45 ◦ C under air (as a function of the signal amplitude, from 5 to 30 mV).
The frequency logarithms are reported in the figure.
during the sintering process (at 1490◦ C) and not the bulk.
In the usual SOFC conditions, the temperature is below
900◦ C and even much lower for intermediate-temperature
SOFC (< 700◦ C) and the evaporation process should become negligible.
It is also well-known than zirconia can be ionic conductive while In2 O3 is mainly electronic. It was checked at
ambient temperature that up to 40 In%, the pellets are
insulating. This is not the case anymore for ratios higher
than 60 In%, when testing the surface in contact with the
platinum foil during sintering. In these cases, a resistance
lower than 1 Ω was measured. Concerning the other face,
in contact with air, the insulating behaviour remained. As
confirmed by SEM observations, shown in Fig. 3, the microstructure of this pellet side is completely modified: the
material looked as reduced to its skeleton, made of the cubic solid solution india-doped zirconia with only 35 % of
indium incorporated. The porosity most probably results
from the departure of the indium rich-phase.
In the case of low indium contents, i.e. 40 In%, as depicted in Fig. 4, the mapping of the different basic elements, O, In and Zr shows an homogeneous distribution.
Fig. 5a allows the observe a very weak and small porosity in the case of 40 In%, even after a heat treatment
at 1440◦ C: this is typical of a residual inter and intragranular porosity after a sintering process. In the micrograph, shown in Fig. 5b, the microstructure is largely observable: in the middle, a quasi hexagonal grain can be
observed and just above appears a powder particle which
was probably not involved in the sintering process. Thus,
we have roughly evaluated the powder grain size (0.7 μm)
and the grain size after annealing (3.4 μm).
3.2. Electrical behaviour
Fig. 6 shows impedance diagrams relative to In2 O3 ZrO2 pellets, with different proportions of In2 O3 . The material contribution, when the sample presents some ionic
conductivity, was identified as the high frequency part,
non-influenced by the signal amplitude variation. As illustrated in Fig. 6b, the high frequency part of the diagrams,
up to 1 kHz, is related to the material behaviour, while below 1 kHz the influence and the response of the platinum
are observed. This is typical of an ionic conductor. In the
case of an electronic conductor, i.e. a 60 In% pellet shown
in Fig. 6d, even using a very small amplitude (5 mV), the
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A. Ringuedé et al. / J. New Mat. Electrochem. Systems
1.6
co-milling
co-precipitation
Literature[13]
1.5
Ea (eV)
1.4
1.3
1.2
1.1
1
0.9
0.8
0
10
20
30
40
50
In mol %
Figure 7. Total conductivity as a function of the temperature, for all the compositions.
Figure 9. Activation energy of the total conductivity as
a function of the indium content: comparison between experimental data and the literature [13].
0
-1
-2
-3
-4
-5
-6
-7
-8
-9
8
10
12
14
16
18
20
co-precipitation 25%
co-milling 20%
co-milling 30%
Figure 8. Total conductivity as a function of the synthesis process in Arrhenius coordinates in the case of 90% of
In2 O3 .
impedance diagrams in all the frequency range are never
superimposed. These diagrams exhibit an inductive electrical behaviour at the highest frequency range, ending on
the real axis as a pure resistance, correlated to the sample
resistance itself. In effect, this resistance value slightly decreases from 0.85 Ω to 0.83 Ω when the signal amplitude
varies from 5 to 30 mV.
As depicted by the four diagrams in Fig. 6, the electrical behaviour is clearly dependent on the In%, and the
diagram shape varies from two semi-circles to one pure
resistance. Two semi-circles are observed for ionic compositions (due to grain and grain boundary responses), wellseparated for 25 In% while the grain boundary relaxation
frequency is shifted to the highest frequencies for higher
In%, i.e. 40 In%. For 50 In%, only one major semi-circle is
clearly observed, and this dielectric behaviour completely
disappears for a 60 In% sample, an electronic conductor.
0.2
log σ (S/cm)
log σ (S/cm)
10000/T (1/K)
Co-milling
0
-0.2
11
16
21
26
10000/T (1/K)
31
36
-0.4
-0.6
Co-precipitation
-0.8
Figure 10. Conductivity variation as a function of the In%,
at 600◦ C.
The dielectric constant was calculated from the high frequency semi-circle, associated to the grains response, and
increases from 39 to 980 when the In% varies from 25 to
50%, which is coherent with the evolution of the conduction nature from ionic to electronic.
The total conductivity, calculated from impedance spectroscopy measurements, is reported in Arrhenius coordinates in Fig. 7. The activation energy is deduced from
the slope of the linear variations. Two groups are distinguished, depending on the composition: the typical ionic
conduction behaviour up to 50 In%, with an activation
energy close to 1.3 eV; beyond 50 In%, no obvious conductivity variation is observed when the temperature is modified. However, pseudo activation energy, corresponding to
the electronic process, close to 10 meV, can be deduced.
In the case of the high india-content polycrystalline compounds, more accurate conductivity measurements can be
obtained using high frequency impedance spectroscopy, up
Elaboration, Characterisation and Electrical Properties of ZrO 2 -In 2 O 3 Compounds with
Different Compositions for Intermediate ... / J. New Mat. Electrochem. Systems
207
4. CONCLUSION
Figure 11. Conductivity variation as a function of the In%,
at 600◦ C.
to 10 GHz [13]. Work is in progress on this aspect and will
be presented in a forthcoming paper.
3.3. Influence of the powders synthesis process: coprecipitation and co-milling
Low impact of the powder fabrication process is observed
in terms of ionic conductivity, while the electronic conductivity is 6 times higher when the pellets are produced by
co-mixed oxide powders, as shown in Figs 8 and 9, respectively. For compositions where the ionic conduction type
is predominant, it can be noticed that the In% has more
effect on the activation energy value than the powder fabrication process, even if the activation energy is slightly
lower for co-precipitation synthesis (Fig. 10).
As reported in Fig. 11 for a temperature of 600 ◦ C, the
conductivity presents an abrupt change at a value close to
50 In %: it varies rapidly from 3.10−4 S.cm−1 to around
0.3 S.cm−1 for 50 to 60 In%, respectively. The conduction
nature is thus completely modified, justifying these differences. At lower content, the electrical behaviour is ionic,
close to the behaviour of doped zirconia, while at the highest ratios, the electrical behaviour is electronic, as for a
pure indium oxide phase. These properties are quite interesting, allowing some ionic/electronic conduction transition, only by gradually modifying the composition of the
material. This transition can be considered in terms of
percolation when both zirconia and india phases coexist in
the high india-content region (> 45%), which was identified by XRD. Very small variations in the india content
could dramatically change the conductivity, which could
be very sensitive to the sample preparation and justify the
differences between co-milling and co-precipitation.
Comparing our diagram at 600 ◦ C with the same diagram obtained at 1000 ◦ C by Sasaki et al [6], the general
shape is similar; nevertheless, the ionic domain is extended
up to 50 In% in our case, instead of 45 In% and the transition range is slightly narrower.
The co-precipitation synthesis of ZrO2 -In2 O3 materials
allowed us to obtain powders corresponding to the expected composition, with about 90% accuracy. After an
annealing treatment of 10 hours at 1490◦ C, dense materials are obtained. These materials are homogeneous, with a
very low porosity for compositions < 50 In% and a higher
porosity for all the compositions above this value. A surface evaporation was observed will all the compositions
above 40 In%, which was due to the sintering process (10 h
at 1490◦ C). This evaporation became negligible after polishing the pellets and submitting them to an annealing
treatment during 1 h at a lower temperature, 1440◦ C.
The study of ZrO2 -In2 O3 pellets, with different compositions, allowed us to confirm at 600◦ C the existence of
two separate conductivity domains, exhibiting an ionic or
an electronic behaviour. At 600◦ C, the ionic domain extends from 20 to 50 In%, with conductivities comparable
to that of yttria-doped zirconia. Beyond 60 %, the electronic conductivity domain reaches a value of 100 S/m,
which is slightly weak for an electronic conductor. Nevertheless, the co-milling synthesis, used here as a comparison
reference, allows to obtain in the same composition range
values 6 times higher. It should be stated here that our
co-precipitation synthesis route should be optimised.
Contrarily to Sasaki el al [4,7], we have found two different phases, ZrO2 and In2 O3 , even in the In2 O3 richest
studied compositions, which means that no solid solution
was found with the highest amounts of indium oxide. The
electronic-ionic transition domain was slightly narrower at
600◦ C in our study than at 1000◦ C [4].
This first study shows the interest of such mixed compounds and, in particular, for intermediate-temperature
SOFC, enabling the improvement of cell performance. The
use of low-temperature synthesis routes, i.e. thin layer
technologies, could solve the indium oxide evaporation
problem and will be soon presented.
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