GDC/LSCF Composite Nanopowder Preparation for IT-SOFC by Induction
Plasma Spraying
Yan Shen and François Gitzhofer*
Energy, Plasma and Electrochemistry Research Centre (CREPE), Chemical and Biotechnical Engineering
Department, Université de Sherbrooke, Québec, Canada, J1K 2R1, 001-819-821-7841;
[email protected]
Ce0.8Gd0.2O1.9 (GDC) and La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) composite nanopowders were successfully synthesized
using induction plasma by axial injection of a solution. The resulting nanocomposite powders consisted of two
kinds of nanopowders with different mass ratio of GDC/LSCF, such as 3/7 and 6/4. The morphological
features, crystallinity and the phases of the synthesized powders were characterized by scanning electron
microscopy (SEM), transmission electron microscopy (TEM), local energy-dispersive X-ray spectroscopy
(EDS) analysis and X-ray diffraction (XRD). The nanopowders are almost spherical with a diameter in the
range of 10-60 nm and their BET specific areas are around 20 m2/g. The GDC and LSCF phases are
homogeneously distributed. Preliminary results of the suspension plasma spray coatings are discussed. The
coatings are homogeneous and porous (51% open porosity) with cauliflower structures.
Keywords: Induction Plasma, Nanocomposite Powder, Gadolinium Doped Ceria (GDC), Lanthanum
Strontium Cobalt Iron Oxide (LSCF)
1. Introduction
Lanthanum
associated with any mechanical mixing process.
strontium
cobalt
iron
oxide,
Suspension plasma spray (SPS), invented in the
LaxSr1-xCoyFe1-yO3-δ is a mixed ionic-electronic
middle of the 1990s by Université de Sherbrooke [2],
conductor with significant electronic conductivity, as
eases the fabrication of nanostructured coatings was.
La0.6Sr0.4Co0.2Fe0.8 O3-δ (LSCF) has a high electrical
This technique provides a cost effective way to
-1
conductivity of 275 Scm at 600°C [1]. It is a very
produce the coatings without long-time sintering. In
popular material for intermediate temperature solid
this article preliminary results of SPS composite
oxide fuel cell (SOFC). However, the ionic
cathode coatings using the previous synthesized
conductivity of LaxSr1-xCoyFe1-yO3 drops rapidly
nanocomposite are presented.
with temperature. Hence in order to enhance the
ionic conductivity, gadolinium doped ceria is added
2. Experiments
into LaxSr1-xCoyFe1-yO3-δ. In this study, LSCF mixed
2.1. Powder Synthesis Using SolPS
with 30wt% GDC and LSCF mixed with 60wt%
LSCF metal nitrates were stoichiometrically mixed
GDC were synthesized by induction solution plasma
and dissolved into distilled water to obtain a
spray (SolPS). This method promises nanosized
concentration of 1.1 M solution. GDC metal nitrate
particle powders with the exact compositions and
were then added to the LSCF nitrate solution
also provides an efficient way to synthesize the
according to the theoretical mass ratios of GDC to
uniformly
LSCF in GDC6/LSCF4 and GDC3/LSCF7. Finally,
distributed
composite
nanopowders
avoiding long sintering time and contaminations
Glycine (98.5%, Alfa Aesar, Wardhill, MA) was
1
added to the solution at a concentration of 1.45 M.
Fig. 2 Induction plasma deposition system
3. Results and Discussions
3.1.
Morphologies,
Composition
and
Phase
Structure Study of the SolPS Powders
No significant morphology differences were found
between the nanopowders collected from those three
Fig. 1 Induction plasma nanopowder synthesis system
places (Fig. 3). All powders are almost globular with
diameters from 10 to 60 nm exhibiting no
Fig. 1 presents the induction plasma powder
pronounced agglomeration. Referring to Bouchard’s
synthesis system. The plasma was generated by a
Tekna
Plasma
Systems
(Sherbrooke,
work
Québec,
[3],
La0.8Sr0.2MO3
(M=Sr,
Fe,
Co)
nanopowders had exactly the same particle size
Canada) PL50 torch. The torch was connected to a 3
distribution. In this study, GDC nitrates were added
MHz LEPEL HF power generator. The plasma
into the LSCF nitrate solution, the concentrations of
plume was formed inside the torch by partially
GDC nitrates were modified and the injection flow
ionizing the central gas (Argon) and the sheath gas
rate was reduced from 20 ml/min to 5 ml/min after a
(Oxygen). The system contained a reactor and filter
preliminary test. The results show that different
unit, which included porous metal filters. Both the
concentrations of GDC metal nitrates in the
reactor and filter unit were continually water-cooled
precursor solution did not affect the size or the shape
with a double wall system during the synthesis.
of the synthesized nanopowders. The synthesized
After some chemical and physical reactions, the
GDC3/LSCF7 nanopowders have almost the same
nanopowders were adsorbed on the inner-faces of
particle size ranges and shapes as GDC6/LSCF4.
both of the reactor and the filter unit as well as on
The BET specific surface areas of as-synthesized
the surface of the filters.
GDC6/LSCF4 and GDC3/LSCF7 nanopowders are
2.2.
20.8 m2/g and 19.6 m2/g respectively. Due to the low
Cathode Deposition Using SPS
12wt%
of
nanopowders
as-synthesized
were loaded
into
feeding rate, high plasma temperature and long
GDC6/LSCF4
ethanol.
residence time of the material in the plasma, it is
The
reasonable that the variation between the metal
deposition was carried out in a vacuum chamber. A
nitrate precursor solutions did not have a strong
programmable sting moves the substrates under the
influence on the thermal history in the plasma. Thus
plasma plume to build coatings.
the
morphologies
of
the
final
nanopowders are similar, as expected.
2
synthesized
(a)
with 7* in Fig. 4(a), 5* in Fig. 4(b). Eliminating the
overlapping effect, the mixed phases of the particles
are quite limited and almost disappeared in the
nanopowders after calcinations. In addition, the
LSCF and GDC phases are homogeneously
distributed, potentially increasing the TPB of the
cathodes as the ion and electron conductors will be
in close contact at the nanoscale.
(b)
(c)
Table 3 EDS local composition analysis of synthesized
nanopowders (* denotes overlapping nanoparticles)
EDS Label #
Composition
Fig. 3 SEM Micrograph of as-synthesized GDC6/LSCF4
nanopowder collected from (a) filters, (b) the reactor and (c) the
GDC
filter unit
LSCF
Fig. 4 (a)
Fig. 4 (b)
3, 4, 5, 6, 9,
1, 2, 3, 4, 6, 8,
11, 15, 16
9, 10, 11
2, 14, 17
7
(a)
GDC/LSCF
1, 7*, 8, 10,
12, 13
5*
The XRD diffraction patterns of these two mixed
powders are presented in Figs. 7 and 8. The patterns
exhibit mainly a perovskite structure of LSCF and a
fluorite structure of GDC. Before the calcinations,
limited quantities of undesired phases are revealed.
These are probably some metastable phases related
(b)
to the mixed GDC/LSCF composition particles
indicated in EDS analysis figures.
They appear
during plasma spray and are back to equilibrium
after heat treatment. Because of the lower GDC
concentration in the GDC3/LSCF7 powder, some
peaks of GDC are not quite distinguished from
LSCF. The powders after 2h calcination at 1000°C
in air display a better crystallinity and only two pure
Fig. 4 TEM EDS local analysis figure of (a) as-sythesized and (b)
calcined (1000°C, 2h) GDC6/LSCF4 nanopowder (see Table
phases are presented, indicating that the proper
3)
stoichiometric ratio of metal nitrates was added into
the primary solution precursor as well as the absence
Fig. 4 and Table 3 show the results of local EDS
phase analysis of the GDC6/LSCF4 particles.
Separated GDC and LSCF phases are identified
from those pictures. Some of the nanoparticles
indicate the mixed phase of GDC/LSCF. Obviously,
most of mixed compositions are the consequence of
overlapping particles, such as the particles denoted
of preferential evaporation of one of the elements.
The grain sizes of LSCF and GDC are around 17 and
22 nm respectively in GDC6/LSCF4, as well as 21
and 14 nm in GDC3/LSCF7.
3
Fig. 5 XRD patterns of GDC6/LSCF4 as-synthesized (a) and
calcined at 1000°C for 2h (b)
Fig. 7 SEM micrograph of the SPS cathode coating (a) surface
X250, (b) surface X1.00k and (c) polished cross-section of SPS
cathode coating
4.
Conclusion and Future Work
Using induction plasma SolPS, it is possible to
achieve
a
homogeneously
mixed
nanosized
composite GDC/LSCF powder without using a
Fig. 6 XRD patterns of GDC3/LSCF7 as-synthesized (a) and
prolonged period of mechanical mixing. The
calcined at 1000°C for 2h (b)
nanopowders exhibit a perovskite structure and a
3.2.
Morphology and Microstructure of the
fluorite structure as well as separated GDC and
Coating
LSCF phases. The SPS deposited coating is expected
As shown in Fig.
9,
a homogeneous
to have enlarged TPBs due to the homogeneous
and
cauliflower-structure cathode coating was obtained
nanoparticle
distribution
and
fine
coating
by the SPS induction plasma spray process with
microstructure. In the future, the cathode layer will
as-synthesized GDC6/LSCF4 nanopowder. The
be electrochemically characterized.
average open porosity is 51%. Micron-size channels
exist between the “micro-cauliflower plants” (Fig.
5.
Acknowledgement
15), facilitating the air and the oxygen-depleted air
This research was partly supported through funding
flow in the cathode. Because of the nano-cauliflower
by the NSERC Solid Oxide Fuel Cell Canada.
structures and the well mixed ion and electron
conductor phases, the TPBs are expected to be
6.
References
extended. No delaminations are found at the
[1] A. Petric, P. Huang, F. Tietz (2000), Solid State
interface between cathode coating and LSGFM
Ionics , 135 (1-4), p 719-725
electrolyte. Furthermore, the electrolyte, after the
[2] F. Gitzhofer, E. Bouyer, and M. I. Boulos (1997),
cathode deposition, is free from any cracks, showing
U.S. Patents, #5, 609, 921
the experiment parameters for the cathode deposition
[3] D. Bouchard, L. Sun, F. Gitzhofer and G. M.
had no adverse influence on this electrolyte.
Brisard (2006), J. Therm. Spray Techn., 15 (1), p
37-45
4