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Synthesis and characterization of doped apatite-type lanthanum silicatesfor SOFC applications
H. Gasparyan a,b, S. Neophytides b, D. Niakolas b, V. Stathopoulos c,d, T. Kharlamova e, V. Sadykov e,O. Van der Biest f, E. Jothinathan f, E. Louradour g, J.-P. Joulin g, S. Bebelis a,a Department of Chemical Engineering, University of Patras, GR 26504 Patras, Greeceb Institute of Chemical Engineering and High Temperature Chemical Processes (FORTH/ICE-HT), GR 26504 Patras, Greecec CERECO S.A.-Ceramics and Refractories Technological Development Company, GR 34100 Chalkida, Greeced Department of Applied Sciences, Technological Educational Institute of Chalkida, GR 34400 Psahna, Greecee Boreskov Institute of Catalysis SB RAS, 630090 Novosibirsk, Russiaf Department of Metallurgy and Materials Engineering, Katholieke Universiteit Leuven, B-3001 Heverlee, Belgiumg
CTI
Cramiques Techniques et Industrielles SA, F-30340 Salindres, France
a b s t r a c ta r t i c l e i n f o
Article history:
Received 1 September 2009
Received in revised form 25 August 2010
Accepted 22 November 2010
Available online xxxx
Keywords:
Apatites
Lanthanum silicates
Oxide ion conductors
Solid oxide fuel cells
SOFC
A series of iron- and/or aluminium-doped apatite-type lanthanum silicates (ATLS) La9.83Si6x yAlxFeyO26 (x=0, 0.25, 0.75, and 1.5, y=0, 0.25, 0.75, and 1.5) were synthesized using the mechanochemical activation
(MA), solid state reaction (SSR), Pechini (Pe) and solgel (SG) methods. The total conductivity of the prepared
materials was measured under air in the temperature range 600850 C using 4-probe AC impedance
spectroscopy. Its dependence on composition, synthesis method, sintering conditions and powder particle size
was investigated. It was found that for electrolytes of the same composition, those prepared via
mechanochemical activation exhibited the highest total specific conductivity, which was improved with
increasing Al- and decreasing Fe-content. The highest conductivity value at 700 C, equal to 2.04102 S cm1,
was observed for the La9.83Si5Al0.75Fe0.25O26 electrolyte. La9.83Si4.5Fe1.5O26 electrolyte samples synthesized
using the Pechini method exhibited higher conductivity when sintered conventionally than when spark-plasma
sintering (SPS) was used. 2010 Elsevier B.V. All rights reserved.
1. Introduction
Solid oxide fuel cells (SOFCs) have experienced phenomenal
progress in the last two decades because of their high efficiency, fuel
flexibility and low pollutant emissions. Yttria stabilized zirconia ZrO2(Y2O3) or YSZ, an O
2 conductor, is used as solid electrolyte in the
state-of-the-art SOFCs. However, YSZ is relatively expensive and
performs efficiently at high temperatures (8001000 C), at which
degradation of SOFC components is fast, resulting in reduction of the
SOFC useful life. Development of low cost solid electrolytes exhibiting
high ionic conductivity at reduced temperatures is one of the key
issues for development and commercialization of intermediate
temperature (600850 C) solid oxide fuel cells (IT-SOFCs).
Apatite-type (general formula 10xM6O26, where A=rare earth
or alkaline earth, M=Si, Ge, P, V, Zn) lanthanum silicates (ATLS)
[19] exhibit high oxide ion conductivity at intermediate tempera-
tures (e.g. ~0.03 S cm1 at 700 C, for La10Si5.5Al0.5O26.75 [7,8]) and
thus have attracted significant attention as promising electrolytes,
alternative to YSZ, for IT-SOFCs [9,10]. The ionic conduction in apatites
is dominated by the interstitial migration mechanism [8,11,12]. The
apatite structure is tolerant to extensive aliovalent doping, which has
been applied in the case of lanthanum silicates for improving oxide
ionic conductivity [614]. In this work are presented results concerning
the effect of composition (doping), method of synthesis and sintering
procedure on the conductivity of iron- and aluminium-doped ATLS
La9.83Si6 x yAlxFeyO26 (x=0, 0.25, 0.75, and 1.5, y=0, 0.25, 0.75,
and 1.5).
2. Experimental
2.1. Preparation of samples
detailed list of the tested apatitic electrolytes, including their
nominal compositions and preparation data, is presentedin Table 1. The
powders of the apatite samples were prepared via the mechanochem-
ical activation (MA) [2,15], solgel (SG) [3], solid state reaction (SSR)
[6,16] and Pechini (PE) or modified solgel [1719] methods. To
synthesize doped ATLS via mechanochemical activation amorphous
SiO2 (REACHIM, 99.9%), La2O3 (VEKTON, 99.99%), Al(OH)3 (REACHIM,
99.5%)and Fe2O3 (REACHIM, 99.5%)were used. SiO2 andLa2O3 were not
Solid State Ionics xxx (2010) xxxxxx
Corresponding author.
E-mail address: [email protected] (S. Bebelis).
SOSI-12136; No of Pages 5
0167-2738/$ see front matter 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ssi.2010.11.025
Contents lists available at ScienceDirect
Solid State Ionics
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / s s i
Please cite this article as: H. Gasparyan, et al., Solid State Ion. (2010), doi:10.1016/j.ssi.2010.11.025
http://dx.doi.org/10.1016/j.ssi.2010.11.025http://dx.doi.org/10.1016/j.ssi.2010.11.025http://dx.doi.org/10.1016/j.ssi.2010.11.025mailto:[email protected]://dx.doi.org/10.1016/j.ssi.2010.11.025http://www.sciencedirect.com/science/journal/01672738http://dx.doi.org/10.1016/j.ssi.2010.11.025http://dx.doi.org/10.1016/j.ssi.2010.11.025http://www.sciencedirect.com/science/journal/01672738http://dx.doi.org/10.1016/j.ssi.2010.11.025mailto:[email protected]://dx.doi.org/10.1016/j.ssi.2010.11.025 -
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calcined prior to mechanochemical activation, while all parent
compounds were characterized by X-ray diffractometry (XRD), infrared
(IR) spectroscopy and thermal analysis in order to obtain the
stoichiometry of the target sample. Then mixtures of the parent
compounds were activated for 35 min in a high-power (1200 rpm)
planetary ball mill AGO-2 (steel drums and balls, sample-to-ball mass
ratio 1/20) and then calcinedat 1200 C for6 h. Inthe caseof the solgel
method, stoichiometric amounts of La(NO3)36H2O (Alfa Aesar, 99.9%),
Al(NO3)39H2O (Merck, for analysis,98.5%) and TEOS (Aldrich, puriss
grade,99.0%) were dissolved in absolute ethanol (Merck, for analysis,
min. 99.8% ), glacial acetic acid (Merck, for analysis, min. 99.8%) and
double distilled water. The resulted sol was aged overnight (T N60 C).
After drying, the precursor was calcined at 1000 C in air. In the case of
the Pechini (modified solgel) method the procedure was similar to the
aforementioned solgel route, the difference being that ethylene glycol
and citric acid were used as gelling agents, as described in detailelsewhere [18,19]. The gel was calcined at 900 C in air and then ball
milled for 48 h [18,19]. In the solid state reaction method appropriate
amounts of the relevant oxides, namely SiO2 (Rhodia, 99%), La2O3(Rhodia, 99.9%), Fe2O3 (Minraux Industrielles de Gaillon, 98.5%) and
Al2O3 (Rio Tinto Alcan,99.8%) were homogenizedby ball milling,firedat
1400 C and then crushed, milled, homogenized andfired once again at
1400 C. After this firing, they were crushed again andfinally milled into
powder. La2O3 was calcined in air at 700 C for 2 h prior to mixing with
theother oxides. Thepowderreceived in this wayis denoted as coarsein Table 1. Part of the coarse powder was further milled to form the
fine powder (Table 1). The powder of each material prepared via the
MA,SG andSSR methodswas pressed into pellets (disks) andsinteredin
air at temperatures 1200 to 1600 C (heating and cooling rates of
10 C min
1) to form dense structures (approximately 9598% relativedensity), as shown using scanning electron microscopy (SEM) and
porosity measurements. In the case of the powders prepared via the
Pechini method, spark-plasmasintering(SPS) [19] in vacuum at 1200 C
(60 MPa pressure, 2 min dwell time, 100 C min1 heating rate) in
addition to conventional (pressureless) sintering (1550 C for 5 h,
heating and cooling rates of 20 C min1) was applied, resulting in
structures of approximately 97 and 94% relative density, respectively.
The aim of additionally using SPS was to obtain samples having relative
density higherthan 95%aftersinteringat lower temperatures compared
to conventional sintering as well as to obtainfine grain size(nanometer
size) in order to study the influence of the grain size on conductivity via
comparison with the conventionally sintered samples which had
coarser grain size, as shown below. The grain size of the sintered
samples was checked using SEM while the particle size of the
corresponding powders was determined using a Mastersizer particle
analyzer (Malvern Instruments,UK). In all cases theformationof apatite
structure was evidenced by XRD. The presented XRD data were
collected (Siemens D 500 X-ray diffractometer) at room temperature
over the 2 range of 1580 at a rate equal to 0.02 s1, using
monochromated Cu-K radiation.
2.2. Experimental set-up
Total specific conductivity measurements of apatite pellets (disks)
were carried out in a single chamber cell of approximately 30 cm3
volume, described in detail elsewhere [20]. The impedance spectra
were obtained in air at temperatures 600 to 850 C, using a symmetric
set-up of two platinum electrodes deposited on the two sides of the
electrolyte. The Pt porous electrodes were deposited by applying thin
coatings of a Pt organometallic paste (Engelhardt M603B) on theapatite disks, followed by calciningfirst in 400 C for 60 min and then
in 830 C for 30 min. The samples were clamped inside the single
chamber cell using two Au plates pressed on both sides of the disk
between two non-conductive ceramic slabs. These Au plates covered
the Pt electrodes almost entirely and were connected via 4-Au wires
(two for each plate) with the external electric circuit. The AC
impedance measurements were carried out using a Princeton Applied
Research 263A potentiostatgalvanostat combined with a Princeton
Applied Research 5210 dual phase lock-in amplifier. The applied
stimulus amplitude in the AC impedance measurements was 10 mV
and the widest frequency range was 10 mHz100 kHz. The ohmic
resistance of each sample, on which the specific conductivity
calculation was based, was determined from the intersection of the
corresponding Nyquist plot with the Zreal (real part of the impedance)axis at high frequencies,referring to that part of theplot which didnot
change upon polarization. The magnitude of the capacitances
corresponding to the individual arcs in the Nyquist plots was taken
into account in order to assign them to contributions of the different
processes [3,16].
3. Results and discussion
In Fig. 1a and b are shown scanning electron microscopy (SEM)
pictures of the top view (Fig. 1a.1 and b.1) and cross section (Fig.1a.2
and b.2) of the La9.83Si4.5Fe1.5O26 electrolyte samples prepared via
mechanochemical activation (MA) and solid state reaction (SSR),
respectively, whilein Fig.1c are shown SEM picturesof the top viewof
the La9.83Si4.5Fe1.5O26 samples prepared via the Pechini method
Table 1
Composition, powder preparation method and particle size, total conductivity at 700 C and activation energy for conduction corresponding to the tested ATLS samples.
Composition Particle size
D0.5/m
Sintering temperature
Tsint/C
Conductivity at 700 C
/S cm1Activation energy
for conduction
Ea/eV
La9.83Si5Al0.75Fe0.25O26 43.5 1600 (4 h) 204 104 0.57
La9.83Si5Al0.25Fe0.75O26 47.6 1600 (4 h) 151 104 0.61
La9.83Si4.5Fe1.5O26 20.7 1600 (4 h) 116 104 0.66
La9.83Si5Al0.25Fe0.75O26 5.6 1500 (1 h) 1.07 104 0.99
La9.83Si4.5Fe1.5O26
14.0 1550 (1 h) 1.29 104
0.96La9.83Si4.5Fe1.5O26 (fine)
9.8 1500 (1 h) 0.41 104 1.03
La9.83Si4.5Fe1.5O26 (coarse) 47.0 1500 (1 h) 0.45 104 0.94
La9.83Si4.5Al1.5O26 (fine) 4.0 1500 (1 h) 1.18 104 0.71
La9.83Si4.5Al1.5O26 (coarse) 20.9 1500 (1 h) 0.79 104 0.73
La9.83Si4.5Fe1.5O26 (fine) 9.8 1450 (1 h) 0.87 104 1.00
La9.83Si4.5Al1.5O26 (fine) 4.0 1450 (1 h) 0.55 104 0.98
La9.83Si4.5Al1.5O26 (fine) 4.0 1450 (4 h) 1.45 104 0.70
La9.83Si4.5Al1.5O26 (fine) 4.0 1450 (10 h) 1.01 104 0.75
La9.83Si4.5Fe1.5O26 ** (SPS) 0.8 1200 (2 min) 12.2 104 1.12
La9.83Si4.5Fe1.5O26 ** 2.5 1550 (5 h) 23.5 104 0.93
La9.83Si4.5Al1.5O26 * 20.1 1500 (4 h) 1.96 104 0.75
Mechanochemical activation (MA), Solid state reaction (SSR), *Solgel method (SG), and **Pechini method (PE).
D0.5: 50 % of particles under this size, SPS: Spark-plasma sintering.
2 H. Gasparyan et al. / Solid State Ionics xxx (2010) xxx xxx
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followed by pressureless (Fig. 1c.1) or spark-plasma sintering
(Fig. 1c.2), respectively (Table 1). The SEM pictures (Fig. 1) show
the existence of significant differences in the microstructure of the
dense pellets depending on composition, synthesis method and
sintering conditions. In Fig. 1b, for the sample prepared by SSR is
observed great variation regarding grain size as well as sinterability.
This may be due to local variations in composition causing the
formation of phases with differences in sintering behavior. As solgel
or MA methods achieve material precursors of high chemical
homogeneity, no such side reactions are expected and a much more
uniform grain microstructure is observed (Fig. 1a and c). The
conventionally and spark-plasma sintered (SPS) samples (Fig. 1c)
exhibit uniform microstructure but with significantly different grain
size. However such small grain microstructure is expected for the SPS
sample (Fig. 1c.2) in comparison with the conventionally sintered
sample (Fig. 1c.1) due to the rapid sintering step.
In Fig. 2 are presented typical normalized XRD patterns of the
La9.83Si4.5Fe1.5O26 electrolyte sample prepared via solid state
reaction and sintered at 1550 C for 1 h, of the La9.83Si4.5Fe1.5O26,
La9.83Si5Al0.75Fe0.25O26 and La9.83Si5Al0.25Fe0.75O26 samples pre-
pared via mechanochemical activation and sintered at 1600 C for 4 h
as well of the La9.83Si4.5Al1.5O26 sample prepared via the solgel
Fig. 1. SEM images of selected ATLS samples (Table 1): (a) La9.83Si4.5Fe1.5O26 (MA) (a.1: top view, a.2: cross section), (b) La9.83Si4.5Fe1.5O26 (SSR, Tsint: 1550 C ) (b.1: top view,b.2: cross section) and (c) La9.83Si4.5Fe1.5O26 (PE) (c.1: conventional sintering, c.2: SPS).
3H. Gasparyan et al. / Solid State Ionics xxx (2010) xxxxxx
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method and sintered at 1500 C for 4 h (Table 1). These XRD patterns
match the JCPDF file card 49-0443 corresponding to the standard
pattern for undoped lanthanum silicate (La9.33Si6O26) and thus reveal
that the samples presented in Fig. 2 were mainly comprised of the
apatite-phase. This was also the case for all tested materials prepared
via the aforementioned methods as well as those prepared via the
Pechini method [18,19]. It is noted, that the higher background
observedat low 2 inthecaseof the La9.83Si4.5Fe1.5O26 prepared via
is due to the fact that part of the sample holder was exposed to the
X-ray beam as its diameter was smaller than that of the specific
sample.
Fig. 3a shows in the form of an Arrhenius plot, in the temperature
range 600850 C, a comparison of the calculated total specific
conductivities in air of the best performing ATLS samples prepared
using different methods (Table 1) as described above. In Fig. 3b and c
are presented complex impedance plots (Nyquist plots) obtained at600 C in air with a selection of the aforementioned samples (Fig. 3b)
and Nyquist plots obtained in air at different temperatures with
sample La9.83Si5Al0.75Fe0.25O26 prepared via MA (Fig. 3c), which
exhibited the highest total specific conductivity among all samples. As
shown in Fig. 3a, for electrolytes of the same composition, powders
prepared via mechanochemical activation resulted in pellets exhibit-
ing specific conductivity higherthan that of pellets prepared using the
other methods. The conductivity of the apatite electrolytes prepared
via MA seems to be improved with increasing Al- and decreasing Fe-
content (Fig. 3a), the highest conductivity obtained with sample
La9.83Si5Al0.75Fe0.25O26 . The same conclusions result from compa-
rison of the total conductivity at 700 C, the value of which for all
tested samples is listed in Table 1 along with the corresponding
activation energy for conduction Ea. Increase in total conductivitywith increasing Al- and decreasing Fe-content was also observed for
apatite electrolytes prepared from powders of similar particle size (4
to 9.8 m) synthesized via SSR and sintered at 1500 C for 1 h. This is
shown in Table 1 for 700 C and was also the case for lower
temperatures. However, the opposite trend was observed by
comparing samples sintered at 1450 C for 1 h ( Table 1), which does
10 20 30 40 50 60 70 80
La9.83
Si4.5
Al1.5
O26
1500oC (4h - SG)
(e)
(d)
(c)
(b)
La9.83
Si5Al
0.75Fe
0.25O
261600
oC (4h - MA)
La9.83
Si5Al
0.25Fe
0.75O
261600
oC (4h - MA)
La9.83
Si4.5
Fe1.5
O26
1600oC (4h - MA)
In
tensity
(a.u)
2, deg
La9.83
Si4.5
Fe1.5
O26
1550oC (1h - SSR)
(a)
200 a.u.
Fig. 2. Typical XRD patterns of electrolytes prepared via solid state reaction (SSR) and
sintered at 1550 C for 1 h, via mechanochemical activation (MA) and sintered at
1600 C for 4 h, as well as via the solgel method (SG) and sintered at 1500 C for 4 h
(Table 1).
Fig. 3. (a) Temperature dependence of the total specific conductivity in air of the best
performing ATLS samples prepared via different methods (Table 1). The dotted line
corresponds to YSZ [21] and is shown here for comparison. (b) Complex impedance
plots (Nyquist plots) obtained at 600 C in air with a selection of the aforementioned
samples (the same symbols as in (a) are used). (c) Nyquist plots obtained in air at
different temperatures with sample La9.83Si5Al0.75Fe0.25O26 prepared via MA (the
best performing among all samples).
4 H. Gasparyan et al. / Solid State Ionics xxx (2010) xxx xxx
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not allow for drawing general conclusions and implies the need for
further systematic study. Comparison of the conductivities of the
La9.83Si4.5Fe1.5O26 electrolytes prepared via the Pechini (modified
solgel) method shows that pressureless sintering resulted in a
sample with conductivity higher than that of the sample sintered
using the SPS technique (Fig. 3a). The conductivity of the former
sample at 700 C was close to that of sample of the same composition
prepared by Shaula et al. [6] via a SSR route. Compared with samples
of the same composition prepared via SSR, the conductivity of sample
La9.83Si4.5Al1.5O26 prepared via the solgel route was higher than
that of samples prepared in the present work (Table 1) but
significantly lower than that of samples prepared by Shaula et al. [6].
Comparison of the conductivities of samples prepared from solelyFe-doped or Al-doped electrolyte powders of the same or similar
particle size synthesized via SSR and sintered for the same time (1 h)
at different temperatures or at the same temperature (1450 C) for
different times, respectively, reveals the existence of a set of optimal
sintering conditions in each case. For the SSRpreparedsamples doped
solely with Al the optimal sintering conditions corresponded to
1450 C for 4 h, while for those doped solely with Fe they
corresponded to 1550 C for 1 h. The effect of electrolyte powder
particle size on conductivity is shown in Fig. 4, where the total
conductivities of La9.83Si4.5Al1.5O26 samples prepared using the SSR
method and the same sintering conditions (1500 C for 1 h) but
corresponding to different powder particle size (4 m or 20.9 m,
Table 1) are compared at different temperatures. The inset in Fig. 4
shows the corresponding Nyquist plots obtained at 600 C. Thecomparison shows that the La9.83Si4.5Al1.5O26 sample prepared
starting from fine powder performed better than that prepared
starting from coarse powder. Obviously, additional experiments with
electrolyte powders of varying composition and well defined particle
sizes, in combination with determination of the grain sizes in the
sintered pellets, are needed in order to reach a solid conclusion
concerning the effect of particle size on the conductivity of the dense
electrolyte samples.
4. Conclusions
Comparison of the total conductivities of ATLS electrolyte samples
prepared using powders synthesized via mechanochemical activation
(MA), solid state reaction (SSR), solgel (SG) or Pechini (modifiedsolgel) route showed that mechanochemical activation results in
electrolyte samples exhibiting the best performance concerning
conductivity, their conductivity characteristics being improved by
increasing Al- and decreasing Fe-content. Concerning the electrolyte
samples prepared using SSR, for those doped solely with Al the
optimal sintering conditions corresponded to 1450 C for 4 h, while
for those doped solely with Fe they corresponded to 1550 C for 1 h.
Furthermore, for the electrolytes doped solely with Al it was found
that a smaller particle size of the powder resulted in an increase of the
total conductivity. In the case of Fe-doped electrolytes prepared using
the Pechini method, conventional (pressureless) sintering resulted in
samples with higher conductivity compared to that of samples of the
same composition prepared using spark-plasma sintering (SPS).
Acknowledgement
Financial support by the European program STREP: MATSILC
033410 Novel Materials for Silicate-Based Fuel Cells is gratefully
acknowledged.
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Fig. 4. Temperature dependenceof the total specific conductivityof La9.83Si4.5Al1.5O26 samples prepared via solid state reaction from powders of different particle size ( fine:
4 m, coarse: 20.9 m, Table 1). Inset: Corresponding Nyquist plots obtained in air at
600 C.
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