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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: simeon@chemeng.upatras.gr (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

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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.

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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).

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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).

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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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