# Synthesis, Sintering, and Electrical Properties ofBaCe0.9−xZrxY0.1O3−δ

Authors: S. Ricote; G. Caboche; C. Estournes; N. Bonanos
Journal: Journal of Nanomaterials
 (2008)

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## Abstract

BaCe0.9−xZrxY0.1O3−δ powders were synthesized by a solid-state reaction. Different contents of cerium and zirconium were studied. Pellets were sintered using either conventional sintering in air at 1700∘C or the Spark Plasma Sintering (SPS) technique. The density of the samples sintered by SPS is much higher than by conventional sintering. Higher values of ionic conductivity were obtained for the SPS sample.


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## Body

## 1. Introduction

Fuel cells can exhibit high-energy efficiency, and the only release is clean water
when used with hydrogen. Types of fuel cells are classified according to the
electrolyte material. Solid electrolytes are used in solid oxide fuel cell (SOFC). In that case, the electrolyte conducts oxygen ions from
the cathode to the anode where they react chemically with hydrogen to form water. Oxygen diffusion occurs at roughly 800–900°C for YSZ [1, 2].
In order to lower the operating temperature, it is interesting to use a
proton-conducting ceramic [2, 3]. This kind of fuel cell is called protonic
ceramic fuel cell (PCFC). In that case, protons pass through the electrolyte to
combine with oxygen at the cathode to liberate water. Hydrogen transport requires
lower temperatures than oxygen transport.Many rare-earth-doped perovskite materials seem to be good candidates for this
application, because of their high proton conductivity at a temperature about 600°C and their stability in
reducing atmosphere [4]. Barium cerates are known for their high protonic conductivity but are not chemically stable; they react with carbon dioxide to
form carbonates. Barium zirconates are chemically and mechanically stable but
their protonic conductivity is quite low [5–8]. An oxide with both cerium and
zirconium could exhibit the two required properties for a PCFC electrolyte: a
satisfying protonic conductivity and a sufficient stability [9, 10]. Thus, this
study deals with BaCe0.9−xZrxY0.1O3−δ powders
(x=0.3 (powder referred to BCZY63), x=0.7 (BCZY27), and x=0.9 (BCZY09)). The
minimal value chosen for x was 0.3, because a compound with more than 60% of
cerium atoms is not stable after a CO2 treatment [10], and the
lattice is orthorhombic. Kreuer [5] pointed out a decrease of the mobility of
the protonic defects when the perovskite structures were deviating from cubic
(orthorhombic). A trivalent
dopant, yttrium, is used to create oxygen vacancies by charge compensation
[2, 4, 11, 12]. These oxygen vacancies are necessary for the proton
conduction, as explained afterwards.Protons are incorporated in the material by the absorption of water molecules
in the oxygen vacancies (reaction 1) [13–15].Reaction 1
(1)H2O(g)+VO••+OOx⟷2OHO•.
Proton transfer occurs by the Grotthuss
diffusion mecha-nism. This is a two-step mechanism which involves both a
proton-transfer and a reorientation step [3, 15–18].
Cerates and Zirconates are pure proton conductors only at low
temperature and under specific atmosphere. At high-oxygen partial pressure,
p-type conduction is predominant (reaction 2).Reaction 2
(2)12O2(g)+VO••⟷2h•+OOx.
Thus, the conductivity is given by (3) where σi is the ionic conductivity and σ°p is the p-type conductivity at an oxygen partial pressure of 1 atm [4, 6, 19, 20] as follows:(3)σtotal=σi+σ°p⋅P(O2)1/4.


## 2. Experimental

BaCe0.9−xZrxY0.1O3−δ powders
(x=0.3, 0.7, and 0.9) were synthesized by a solid state reaction. BaCO3 (Aldrich, 99+%), CeO2 (Aldrich, 99.9%, <5 microns), Y2O3 (Sigma-Aldrich, 99.99%), and YSZ (Aldrich, submicron powder; 99.5%, 5.3 weight %
of yttria) and were mixed in stoichiometric proportions for 20 hours in a ball
mill. The resulting powders were calcined in air at 1400°C for 24 hours.Crystallographic phases, lattice parameters, and theoretical density
were determined with a standard X-ray diffractometer INEL CPS 120 using CuKα1 radiation.Scanning Electron Microscope (SEM,
Jeol JSM 6400F) experiments were achieved to assess the particles size and morphology.


### 2.1. Conventional Sintering

A binder was added to the powders to
facilitate the forming. Cylindrical pellets (10 and 20 mm diameter) were pressed
using a hydraulic press (350 MPa) and sintered in air at 1700°C for 6 hours.


### 2.2. Spark Plasma Sintering

SPS was carried out in the equipment
Sumitomo SPS 2080 supplied by (Plateforme Nationale de Frittage Flash (PNF2), Université Paul Sabatier, France). The
precursors, without binding material, were set in graphite die with an inner
diameter of 8 mm and sintered under vacuum (roughly 1 Pa). The experiments were done in the
temperature range 1600–1700°C for
5 minutes. The pressure was monitored at 100 MPa during the process.
The heating rate for all SPS experiments was maintained at around 150°C min-1,
and the natural cooling rate down to 800°C was 100°C min-1. The
temperature was controlled by an optical pyrometer focused on a small hole
located at the surface of the die. Shrinkage was recorded from the dilatometer
provided with the SPS machine.The DC conductivity was measured in a 4-point probe arrangement. Pt
paste was applied to each end of the bar and contacted with Pt leads. Two Pt
potential probes at a fixed distance were set on one side of the bar, as shown
in Figure1. Different oxygen pressures were obtained by mixing O2 and N2 (oxidizing atmosphere), or H2 and N2 (reducing
atmosphere). The gas mixture was moisturized by bubbling through water held at
13°C, giving a water-vapour
partial pressure of ca. 0.015 atm. The isotope effect was measured by using D2 and D2O in place of H2 and H2O. The
conductivity values were not corrected for the porosity of the samples.Figure 1
DC equipment.


## 2.1. Conventional Sintering

A binder was added to the powders to
facilitate the forming. Cylindrical pellets (10 and 20 mm diameter) were pressed
using a hydraulic press (350 MPa) and sintered in air at 1700°C for 6 hours.


## 2.2. Spark Plasma Sintering

SPS was carried out in the equipment
Sumitomo SPS 2080 supplied by (Plateforme Nationale de Frittage Flash (PNF2), Université Paul Sabatier, France). The
precursors, without binding material, were set in graphite die with an inner
diameter of 8 mm and sintered under vacuum (roughly 1 Pa). The experiments were done in the
temperature range 1600–1700°C for
5 minutes. The pressure was monitored at 100 MPa during the process.
The heating rate for all SPS experiments was maintained at around 150°C min-1,
and the natural cooling rate down to 800°C was 100°C min-1. The
temperature was controlled by an optical pyrometer focused on a small hole
located at the surface of the die. Shrinkage was recorded from the dilatometer
provided with the SPS machine.The DC conductivity was measured in a 4-point probe arrangement. Pt
paste was applied to each end of the bar and contacted with Pt leads. Two Pt
potential probes at a fixed distance were set on one side of the bar, as shown
in Figure1. Different oxygen pressures were obtained by mixing O2 and N2 (oxidizing atmosphere), or H2 and N2 (reducing
atmosphere). The gas mixture was moisturized by bubbling through water held at
13°C, giving a water-vapour
partial pressure of ca. 0.015 atm. The isotope effect was measured by using D2 and D2O in place of H2 and H2O. The
conductivity values were not corrected for the porosity of the samples.Figure 1
DC equipment.


## 3. Results and Discussion

### 3.1. Synthesized Powders

The BCZY powders are single-phased
and the structure is cubic (Figure2). A 24-hour dwell is necessary to complete
the reaction. Previous syntheses were achieved with a 12-hour dwell, and peaks of
the precursors were present in the X-Ray patterns.Figure 2
X-Ray patterns for BCZY09, BCZY27 and BCZY63 powders.The results of the lattice parameter (refined with Celref software) and
density are summarized in Table1. The grains are polygonal and nanoscale (about
300 nm for each cerium and zirconium content) (Figure 3).Table 1
BCZY powders lattice
parameter and density.
PowderLattice parameter (Å)Density  (g·cm-3)BCZY094.1996.152BCZY274.2426.207BCZY634.3316.232Figure 3
SEM picture of the BCZY27 powder.


### 3.2. Sintered Pellets

Measurements of the sample
geometry and Archimede's method were run to determine the density. For the SPS
pellets, the two values are similar; the open porosity is negligible. A heat
treatment at 800°C in air is required to remove graphite from the surface of the pellet, followed
by polishing. The BCZY09 pellets are nearly fully transparent (Figure 4), with
a density percentage of 99%.  An SEM
picture was obtained after a thermal etching at 1300°C (Figure 5). The
impurities are due to the polishing and the gold coating. The grain size is
about 500 nm, and no crack is present on the pellet. The BCZY27 and BCZY63
samples disintegrated totally after a few days, revealing a difference of
colors between the bulk and the superficial layers. XRD patterns were recorded
on these broken pellets, after grinding: only the BCZY perovskite phase is
present in the diffractogram. One hypothesis is the partial reduction of cerium
ions because of the highly reductive atmosphere in the graphite die at high
temperature. Ce3+ ions are larger than Ce4+ ions and
mechanical stresses would be generated during the reduction.Figure 4
Picture of the
BCZY09 SPS pellet, after polishing.Figure 5
SEM picture of the
BCZY09 SPS pellet, after a thermal
etching.The pellets made by conventional sintering at 1700°C present a high-open
porosity (Figure 6). Indeed, the density percentage obtained by geometric
measurements is about 80%, whereas the one obtained with Archimede's method is
about 90%. Cracks and defects exist at the surface of the samples; the pressure
is not homogeneously distributed during uniaxial pressing. An SEM picture is
shown in Figure 7: the grains are irregular and bigger than the ones in the SPS
pellet, between 500 nm and 1.5 μm. Furthermore, a change of the superficial
color is obvious, due to an oxygen nonstoichiometry. Polishing is necessary to
remove it. The X-ray patterns show the expected perovskite phase.Figure 6
Picture of the
BCZY09 CS pellet, after polishing.Figure 7
SEM picture of the
BCZY09 CS pellet.Conventional sintering in different
atmosphere (synthetic air or argon) was tried to avoid this superficial
coloration, but with no positive result.DC experiments were tried at 600°C, but the really high density of the
SPS sample required a very long equilibrium time (more than one day for each
gas composition). So the DC curves were obtained at 800°C and are presented in Figure 8.DC curves at 800°C for H2O exchanged state (a) and D2O exchange state (b).
(a)(b)Equation (3) was  adjusted
to the conductivity data in both gas atmospheres and the parameters (σi)H2O,
(σi)D2O,
and σ°p were determined [19, 20]. The results
are summarized in Table 2. The difference between conductivity of D2O
and H2O exchanged states shows the protonic component of the
conductivity.Table 2
Fitted parameters(σi)H2O,
(σi)D2O,
and σ°p for BCZY09 bars (SPS and CS) at 800°C.
Sample(σi)H2O(σi)D2Oσ°p(mS/cm)(mS/cm)(mS/cm)BCZY09 (SPS)4.332.6658.25BCZY09 (CS)1.851.2125.3One can note a higher ionic contribution of the conductivity for the SPS
sample, which can be partly related to the higher density. Grain boundaries in
the SPS samples do not diffuse the light (transparent sample). These grain
boundaries should present different properties than those of the pellet
obtained by conventional sintering. Grain boundaries contribute to the total
conductivity of the sample. So, this could also explain the difference of ionic
conductivity of the two samples.The p-type conductivity is also higher for the SPS sample. The highly
reductive atmosphere at 1700°C creates holes. The fast cooling does not let enough
time for the reoxidation step. Holes are trapped in the material, generating
higher p-type conductivity.


## 3.1. Synthesized Powders

The BCZY powders are single-phased
and the structure is cubic (Figure2). A 24-hour dwell is necessary to complete
the reaction. Previous syntheses were achieved with a 12-hour dwell, and peaks of
the precursors were present in the X-Ray patterns.Figure 2
X-Ray patterns for BCZY09, BCZY27 and BCZY63 powders.The results of the lattice parameter (refined with Celref software) and
density are summarized in Table1. The grains are polygonal and nanoscale (about
300 nm for each cerium and zirconium content) (Figure 3).Table 1
BCZY powders lattice
parameter and density.
PowderLattice parameter (Å)Density  (g·cm-3)BCZY094.1996.152BCZY274.2426.207BCZY634.3316.232Figure 3
SEM picture of the BCZY27 powder.


## 3.2. Sintered Pellets

Measurements of the sample
geometry and Archimede's method were run to determine the density. For the SPS
pellets, the two values are similar; the open porosity is negligible. A heat
treatment at 800°C in air is required to remove graphite from the surface of the pellet, followed
by polishing. The BCZY09 pellets are nearly fully transparent (Figure 4), with
a density percentage of 99%.  An SEM
picture was obtained after a thermal etching at 1300°C (Figure 5). The
impurities are due to the polishing and the gold coating. The grain size is
about 500 nm, and no crack is present on the pellet. The BCZY27 and BCZY63
samples disintegrated totally after a few days, revealing a difference of
colors between the bulk and the superficial layers. XRD patterns were recorded
on these broken pellets, after grinding: only the BCZY perovskite phase is
present in the diffractogram. One hypothesis is the partial reduction of cerium
ions because of the highly reductive atmosphere in the graphite die at high
temperature. Ce3+ ions are larger than Ce4+ ions and
mechanical stresses would be generated during the reduction.Figure 4
Picture of the
BCZY09 SPS pellet, after polishing.Figure 5
SEM picture of the
BCZY09 SPS pellet, after a thermal
etching.The pellets made by conventional sintering at 1700°C present a high-open
porosity (Figure 6). Indeed, the density percentage obtained by geometric
measurements is about 80%, whereas the one obtained with Archimede's method is
about 90%. Cracks and defects exist at the surface of the samples; the pressure
is not homogeneously distributed during uniaxial pressing. An SEM picture is
shown in Figure 7: the grains are irregular and bigger than the ones in the SPS
pellet, between 500 nm and 1.5 μm. Furthermore, a change of the superficial
color is obvious, due to an oxygen nonstoichiometry. Polishing is necessary to
remove it. The X-ray patterns show the expected perovskite phase.Figure 6
Picture of the
BCZY09 CS pellet, after polishing.Figure 7
SEM picture of the
BCZY09 CS pellet.Conventional sintering in different
atmosphere (synthetic air or argon) was tried to avoid this superficial
coloration, but with no positive result.DC experiments were tried at 600°C, but the really high density of the
SPS sample required a very long equilibrium time (more than one day for each
gas composition). So the DC curves were obtained at 800°C and are presented in Figure 8.DC curves at 800°C for H2O exchanged state (a) and D2O exchange state (b).
(a)(b)Equation (3) was  adjusted
to the conductivity data in both gas atmospheres and the parameters (σi)H2O,
(σi)D2O,
and σ°p were determined [19, 20]. The results
are summarized in Table 2. The difference between conductivity of D2O
and H2O exchanged states shows the protonic component of the
conductivity.Table 2
Fitted parameters(σi)H2O,
(σi)D2O,
and σ°p for BCZY09 bars (SPS and CS) at 800°C.
Sample(σi)H2O(σi)D2Oσ°p(mS/cm)(mS/cm)(mS/cm)BCZY09 (SPS)4.332.6658.25BCZY09 (CS)1.851.2125.3One can note a higher ionic contribution of the conductivity for the SPS
sample, which can be partly related to the higher density. Grain boundaries in
the SPS samples do not diffuse the light (transparent sample). These grain
boundaries should present different properties than those of the pellet
obtained by conventional sintering. Grain boundaries contribute to the total
conductivity of the sample. So, this could also explain the difference of ionic
conductivity of the two samples.The p-type conductivity is also higher for the SPS sample. The highly
reductive atmosphere at 1700°C creates holes. The fast cooling does not let enough
time for the reoxidation step. Holes are trapped in the material, generating
higher p-type conductivity.


## 4. Future Work

Three different sets of experiment will be run
to complete this study. First, different temperature dwells, and cooling rates
will be tested to investigate the influence on the mechanical stress releases. Then,
different atmospheres can be used during the SPS sintering. This will modify
the reductive property in the graphite die at high temperature. Finally, the
influence of the grain boundaries on the total conductivity will be determined
by impedance spectra at low temperature with silver electrodes.


## 5. Conclusions

BCZY compounds can be synthesized by a solid-state reaction, with a 24-hour dwell at
1400°C. The XRD analyses reveal a cubic perovskite structure. The sintering can be achieved
by a conventional way, with a 6-hour dwell at 1700°C, followed by a fast
surface polishing. To reduce the superficial defects, isostatic pressing should
be achieved.To increase the density percentage, SPS experiments can be run. When
cerium is present in the sample, the pellet disintegrates. Other SPS procedures
(different temperature dwells, cooling rates, and atmospheres) are now studied
to obtain stable BCZY27 and BCZY63 pellets. The SPS method is very promising
because of the high-densified crackless samples obtained.Conductivity measurements were run on BCZY09 bars. The  higher ionic component of
the conductivity for the SPS sample is beneficial to use the material as
electrolyte for PCFC. The p-type conductivity is also higher for the SPS bar.
This can be changed with a slower cooling rate allowing a reoxidation of the
sample.



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*Source: 354258-2008-10-20.xml*