Solid State Ionics xxx (2013) xxx–xxx

SOSI-13119; No of Pages 5

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Solid State Ionics

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Microstructure and electrical properties of the composites based onSrTi0.5Fe0.5O3 − δ and Ce0.8(Sm0.8Sr0.2)0.2O2 − δ

E.Yu. Pikalova a,b,⁎, А.А. Murashkina a, D.А. Medvedev a, P.S. Pikalov b, S.V. Plaksin a

a Institute of High Temperature Electrochemistry, Ural's Branch of Russian Academy of Science, 20 Academicheskaya st., Yekaterinburg, Russiab Ural federal University, 19 Mira st., Yekaterinburg, Russia

⁎ Corresponding author. Tel.: +7 343 3623394.E-mail address: e.pikalova@list.ru (E.Y. Pikalova).

0167-2738/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.ssi.2013.10.036

Please cite this article as: E.Y. Pikalova, et al.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 14 May 2013Received in revised form 26 August 2013Accepted 14 October 2013Available online xxxx

Keywords:MIEC membranesSOFCComposite anode materialsFluoritePerovskite

New composite materials with mixed oxygen ionic and electronic conductivity based on perovskiteSrTi0.5Fe0.5O3 − δ and fluorite Ce0.8(Sm0.8Sr0.2)0.2O2 − δ were prepared by mixing base components synthesizedby the solid state reaction methodwith following joint calcination at 1573 K and sintering in a gas-tight ceramicat 1623–1823 K. The phase content, microstructure and surface morphology, thermal expansion of sinteredsamples and their electrical properties were investigated by XRD, scanning electron microscopy, dilatometricmethod and four-probe dc technique, respectively. It was found that the composite materials are two-phaseperovskite–fluorite systems with chemically stable content, characterized by the smallness of grain size relativeto Ce0.8(Sm0.8Sr0.2)0.2O2 − δ and considerably higher microhardness in comparison with SrTi0.5Fe0.5O3 − δ. Theypossess intermediate values of thermal and chemical expansion, and high level of ionic and n-type electronicconductivity, making these composite materials promising candidates as mixed ionic and electronic conductingceramicmembranes for a wide variety of gas separation applications, including oxygen separation, partial oxida-tion of methane and hydrogen separation and as anodes for solid oxide fuel cells (SOFCs).

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Materials with mixed oxygen ionic and electronic conductivity(MIEC) are increasingly attracting research interest because of theirpossible use as membranes in the partial oxidation of primary organicfuel to syngas and hydrogen production by electrochemical conversion[1]. Iron-doped strontium titanates are prospective MIEC materials foroxygen and hydrogen separation by electrochemical reformers and aselectrode materials in solid oxide fuel cells (SOFC) due to the highvalues of total conductivity under oxidizing and reducing conditionsand considerable oxygen permeation [2–4]. The main disadvantageof these materials is their low stability in a reducing atmosphere at ele-vated temperatures due to the reduction of iron ions to metallic statewith the next localization at the grain boundaries [5]. It was foundthat the total conductivity of SrTi0.5Fe0.5O3 − δ decreases by 20% during600 h in wet hydrogen at 1173 K [6]. One possible way to improvethe stability of strontium titanates is by the partial substitution of Srwith rare earth elements [7,8]. In [9] it was shown that an increase inthe total resistance of Sr1 − хGdхTi0.5Fe0.5O3 − δ during long-term treat-ment in hydrogenwas lower than that of SrTi0.5Fe0.5O3 − δ by a factor of2. However, the introduction of gadolinium and other lanthanides leadsto a decrease in the conductivity of the base material [7]. The secondmethod to improve both the chemical stability andmechanical strengthof the membrane material and also to increase the ionic conductivity of

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MIEC is by a combination of a mixed-conductor with perovskite struc-ture (titanate, cobaltite, ferrite, etc.) with an electrolyte material thathas high ionic conductivity and stability in a hostile environment[10]. It is known that a combination of dopants makes it possibleto increase the ionic conductivity of CeO2-based materials and theirstability in reducing atmospheres [11–13]. Such electrolytes can besuccessfully used as components of composite electrodes. In [14] a sig-nificant increase in the electrochemical reaction rate with compositeanode Sr0.92Y0.08TiO3 − δ/Sm0.2Ce0.8O2 − δwas reported in comparisonwith the base titanate.

In present work the synthesis and study of the structural, thermaland electrical properties of a new composite system (1 − x)SrTi0,5Fe0.5O3 − δ–x Ce0.8(Sm0.8Sr0.2)0.2O2 − δ (х = 0, 0.25, 0.5, 0.75 and 1) wereconducted from the perspective of using them as oxygen-permeableMIEC membranes applicable in electrochemical reformers for extrapure hydrogen or oxygen production and partial oxidation of methane.

2. Experimental

SrTi0.5Fe0.5O3 − δ and Ce0.8Sm0.18Sr0.02O2 − δ were synthesized bythe solid state reaction method from oxides and strontium carbonatewith a high degree of purity at 1573 K and 1423 K for 10 h, respectively.The powders were weighted according to (1 − x)SrTi0.5Fe0.5O3 − δ–

xCe0.8(Sm0.8Sr0.2)0.2O2 − δ (0 ≤ х ≤ 1, Δх = 0.25) composition andmixed in the planetary mill (Pulverisette-6) with zirconia balls in alco-hol media during 1 h at 300 min−1. The mixtures were calcined at1573 K for 10 h with subsequent milling for 2 h. Ceramic samples

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Table 1Structural properties of (1 − х)SrTi0.5Fe0.5O3 − δ–хCe0.8(Sm0.8Sr0.2)0.2O2 − δ composites.

x Sinteringtemperature, K

Unit cell parameter, Ǻ Relativedensity, %

Microhardness,kg/mm2

Fluorite Perovskite

1 1823 5.437 – 94.2 1150 ± 300.75 1773 5.439 3.905 92.9 1097 ± 160.5 1773 5.429 3.899 93.4 1129 ± 270.25 1723 5.432 3.909 93.9 1107 ± 300 1623 – 3.903 94.1 942 ± 40

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were prepared by the tape rollingmethodwith sintering at 1623–1823 Kfor 3 h [15]. For different kinds of investigations the samples were madein bar, plate and tube shape. The density of the sintered samples wasmeasured by employing both the Archimedes principle and the calcula-tion from the weight and the dimensions of the specimens. It wasfound that both methods of obtaining the density provided almost thesame value. The average value derived from the two methods for eachspecimen was compared to the theoretical one, calculated from thelattice parameters. The theoretical density of composite materials wascalculated as a partial sum of the theoretical density of each phase. Therelative density of (1 − x)SrTi0.5Fe0.5O3 − δ–xCe0.8(Sm0.8Sr0.2)0.2O2 − δ

ceramiс samples decreased with a growth of fluorite content. Thesintering temperatures of the samples for electrical investigationswere experimentally chosen to reach at least 93–94% of relative densityfor every composition (Table 1).

XRD was performed on the milled powders of specimens aftersintering and after long-term treatment in reducing atmosphere(pO2 = 1 × 10−18 atm, T = 1173 K, 600 h) by using a DMAX-2500,Rigaku Co. Ltd. diffractometer with Ni-filtered CuKα radiation in therange of 15° ≤ 2θ ≤ 85°. The surfacemorphology and chemical contentof ceramic samples were determined by scanning electron microscopeJSM-5900LV (JEOL) and INCA ENERGY-200 spectrometer. Microhard-ness was evaluated using the Vickers hardness test. The thermal expan-sion and isothermal expansion of ceramicsweremeasured by TesatronicTT-80 dilatometer in air during heating from room temperature to1173 K at a rate of 150 K/h and at 1173 K in range of oxygen partialpressure change from 0.21 to 1 × 10−18 atm, respectively. The conduc-tivity of the ceramic samples 0.1 × 0.5 × 2.5 cm was investigated bythe standard four probe dc-method utilizing the microprocessorsystem ZIRCONIA-318 in a wide range of temperatures (673–1173 K)in oxidizing (air, pO2 = 0.21 atm) atmosphere, as well as in a widerange of oxygen partial pressures (1 × 10−19–0.21 atm) at 1173 K[15]. To organize the electrodes in the conductivity measurements Ptwire 0.01 cm in diameter was applied in four points across the sampleand covered by Pt paste with following sintering at 1173 K for 1 h forensuring of the contact.

Fig. 1. XRD patterns of (1 − x)SrTi0.5Fe0.5O3 − δ–xCe0.8(Sm0.8S

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3. Results and discussion

3.1. Structural properties

The XRD analysis of (1 − x)SrTi0.5Fe0.5O3 − δ–xCe0.8(Sm0.8Sr0.2)0.2O2 − δ sintered at 1673–1823 K showed that SrTi0.5Fe0.5O3 − δ andCe0.8(Sm0.8Sr0.2)0.2O2 − δ crystallize in structures of perovskite (spacegroup Pm3m) and fluorite (space group Fm3m), respectively (Fig. 1)In X-ray patterns of composites there are characteristic peaks of thesamephaseswith slight shift. The deviation of cell parameters offluoriteand perovskite phases in composites from those of individual phases isinsignificant (Table 1). Therefore the two-phase systems are character-ized by a good chemical compatibility and any sign of an interphaseinteraction with a subsequent formation of impurity phases after thesintering was not detected. Two kinds of grains with differing contentwere observed in BSE images of sintered samples (Fig. 2). The elementaldispersive spectrum indicated the presence of Sr, Ti, Fe and O in darkgrains (perovskite phase) and Ce, Sm and О (fluorite phase) in brightgrains. Visual image of Sr in fluorite phase is not clear because of itslow content (4 at.%). There are no visible open pores on the surfacesof sintered samples, which is in good agreement with the calculatedrelative density (Table 1).

Detailed analysis of grain size distribution has shown that themean size of fluorite grains decreases significantly in composites incomparison with Ce0.8(Sm0.8Sr0.2)0.2O2 − δ. The sintering temperatureof CeO2-based materials prepared by the solid state reaction method iscomparatively high (1823 K) which leads to prodigious grain growthandpoormechanical properties of ceria ceramic.With an increase of pe-rovskite content the sintering temperature decreases and in compositesthe mean grain sizes of both phases become almost equal (Fig. 3).It should be noted that the minimal mean size of grains was foundin 0.5SrTi0.5Fe0.5O3 − δ–0.5Ce0.8(Sm0.8Sr0.2)0.2O2 − δ. During sinteringthe grain growth is determined by diffusion processes and it dependson temperature, sintering time and the nature of thematerial (chemicalcomposition and particle size). In the case of single-phase material,sintering and grain growth are more intense due to the large contactsurface of the powder particles of one kind. In the case of composites(two-phase materials), grain growth is limited by diffusion difficultiesassociatedwith the presence of particles from another phase. Therefore,in the case of equimolar ratio of strontium titanate and cerium oxidegrain size from each of the phases is minimized. Note that fine-grained materials have higher strength properties due to the increasedcontact area between their grains. In Table 1 the microhardness of(1 − x)SrTi0.5Fe0.5O3 − δ–xCe0.8(Sm0.8Sr0.2)0.2O2 − δ is presented. Theintroduction of 25 mol% of the fluorite phase (х = 0.25) leads to asharp 20% increase in microhardness of strontium titanate and thenwith the further increase of x it changes insignificantly.

r0.2)0.2O2 − δ sintered at 1623–1823 K in air atmosphere.

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X=0

X=1

X=0.25

X=0.50 X=0.75

Fig. 2. Scanning electron microscope images of (1 − x)SrTi0.5Fe0.5O3 − δ–xCe0.8(Sm0.8Sr0.2)0.2O2 − δ, back-scattered electron regime.

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3.2. Thermal and chemical expansion

Fig. 4 presents the temperature dependence of thermal expansionof (1 − x)SrTi0.5Fe0.5O3 − δ–xCe0.8(Sm0.8Sr0.2)0.2O2 − δ in the range of300–1173 K. All the dependences except for Ce0.8(Sm0.8Sr0.2)0.2O2 − δ

displayed nonlinear behavior and therefore coefficient of thermal ex-pansion (CTE) values were calculated in low temperature (300–873 K)and high temperature (873–1173 K) regions. From the concentration

Fig. 3. Concentration dependences of the mean grain size on fluorite phase content in(1 − x)SrTi0.5Fe0.5O3 − δ–xCe0.8(Sm0.8Sr0.2)0.2O2 − δ.

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dependences it can be seen that with an increase in x the CTE changesat low temperatures to a small degree, but at high temperatures the con-centration dependence becomes remarkable (from 17.6 × 10−6 K−1 to12.0 × 10−6 K−1) (Fig. 4, inset). It is likely that the deviation of thelinear behavior is connected with the oxygen desorption from anionsublattice under heating [16,17]. To maintain crystal electroneutralitythe oxygen desorption can be accompanied by reduction of ions with

Fig. 4. Thermal expansion of (1 − х)SrTi0.5Fe0.5O3 − δ–хCe0.8(Sm0.8Sr0.2)0.2O2 − δ and theconcentration dependence of calculated TEC values (α) in a low- and high temperatureregion (inset).

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Fig. 6. Dependence of total conductivity of (1 − х)SrTi0.5Fe0.5O3 − δ–хCe0.8(Sm0.8Sr0.2)0.2O2 − δ on oxygen partial pressure at 1173 K.

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variable valence (in this case iron and titanium ions) to a lower oxida-tion state. As far as reduced ions are of greater size this leads to increasedCTE values in the high temperature region.

Chemical stability and mechanical stability under high partial pres-sure gradient or redox cycles at high temperatures are required proper-ties of ceramic materials in electrochemical devices. In order to ensurethe constructional integrity of electrochemical cells the chemical expan-sion of constituentmaterialswhile transitioning froman oxidizing atmo-sphere (pO2 = 0.21 atm) to a reducing one (pO2 b 1 × 10−15 atm)must not exceed 0.2% [18]. In Fig. 5 the dependences of the relative linearexpansion of (1 − х)SrTi0.5Fe0.5O3 − δ–хCe0.8(Sm0.8Sr0.2)0.2O2 − δ on ox-ygen partial pressure measured at 1173 K are presented. The expansionof SrTi0.5Fe0.5O3 − δ takes place both in oxidizing (1 × 10−5 ≤ рО2

≤0.21 atm) and reducing (1 × 10−18 ≤ рО2≤1 × 10−13 atm) regions,0.09% and 0.13%, respectively. This behavior can be associated with thereduction of Fe4+ to Fe3+ at high pO2 and Fe3+ to Fe2+ and Ti4+ toTi3+ at low pO2. With an increase in the fluorite component the inten-sity of the sample expansion in an oxidizing atmosphere decreases.In reducing conditions contrarily, because of the reduction of Сe4+ toСe3+ in Ce0.8(Sm0.8Sr0.2)0.2O2 − δ,, the expansion of fluorite-reachmate-rials increases and becomes unacceptably high at x = 1 (0.3%). Thus,the application of (1 − х)SrTi0.5Fe0.5O3 − δ–хCe0.8(Sm0.8Sr0.2)0.2O2 − δ

with x ≥ 0.5 in multilayered electrochemical devices is possible onlyat intermediate temperatures (773–1023 K), where reduction of ceriain the fluorite phase is moderate [19]. In electrochemical reformers,where MIEC materials are used individually and their need to expandis not critical, all the raw of investigated composites are acceptable.

3.3. Electrical properties

The investigation of dependence of total conductivity of compositeson oxygen partial pressure was carried out for the establishment of thenature of transport properties. It was found that with an increase in thefluorite content the total conductivity of composites decreases in air(pO2 = 0.21 atm) because of the reduction in p-type electronic (hole)conductivity (Fig. 6). At the same time an ionic conductivity areawidensand ionic contribution to the total conductivity increases with x. Thedependence of n-type electronic conductivity on pO2 changes fromthe power−1/4 to−1/5with x growth. The values of n-type electroniccontribution calculated by fitting change from 0.23 S/cm at x = 0 to1.43 S/cm at x = 1 (pO2 = 1 × 10−18 atm, T = 1173 K).

Based on an analysis of the dependencies according to empiricalEq. (1) [1,20] the transport numbers of oxygen ions and electroniccharge carriers were calculated at the different pO2 values (Eq. (2)).

σtotal ¼ σ ion þ σh;o � pO2ð Þ1=4 þ σe;o � pO2ð Þ−1=4 ð1Þ

Fig. 5. Relative linear expansion as a function of pO2 for (1 − х)SrTi0.5Fe0.5O3 − δ–

хCe0.8(Sm0.8Sr0.2)0.2O2 − δ ceramic samples at 1173 K.

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where σtotal — total conductivity, σion — ionic conductivity (const),and σh,o and σe,o — hole conductivity and electron conductivity atpO2 = 1 atm, respectively.

ti ¼σ i

σtotal: ð2Þ

The following trends with an increase of fluorite phase in thecomposites can be distinguished: ionic transport number increasesat pO2 = 0.21 atm from 0.019 to 0.996 and at intermediatepO2 = 1 × 10−8 atm from 0.638 to 0.905 and at the low pO2 = 1 ×10−18 atm changes insufficiently in the range 0.069–0.091. An elec-tronic p-type conductivity prevails in perovskite-rich materials inair and hole transport number tp reaches 0.981 at x = 0. Electronicn-type conductivity dominates in all the investigated samples in re-ducing conditions.

The results of long-term testing of composite ceramic materialsat high temperatures (1173 K) and under reducing atmosphere(pO2 = 1 × 10−18 atm) for 600 h has shown that the total conductivi-ty does not vary significantly for all samples (no more than 5%), but theimproved stability of composites with increased fluorite content shouldbe noted (Fig. 7). Under the same conditions the total conductivity ofSrTi0.5Fe0.5O3 − δ decreases by 15%. In reducing atmosphere strontiumtitanate samples lost their mechanical strength and crumbled to apowder,while the other ceramic samplesmaintained their integrity. Ac-cording to SEManalysis, the composites after long-term testing retainedtheir gas-tightness and had no volumedefects (Fig. 8). Net of crackswasclearly observed on the surface of Ce0.8(Sm0.8Sr0.2)0.2O2 − δ. Decrease inthe total conductivity of fluorite during the test was 7%. Thus, in thermal

Fig. 7. Percentage change of total resistance of (1 − х)SrTi0.5Fe0.5O3 − δ–хCe0.8(Sm0.8Sr0.2)0.2O2 − δ during long-term treatment at 1173 K and pO2 = 1 × 10−18 atm.

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Fig. 8.Microstructure of 0.5SrTi0.5Fe0.5O3 − δ–0.5Ce0.8(Sm0.8Sr0.2)0.2O2 − δ and Ce0.8(Sm0.8Sr0.2)0.2O2 − δ before (а, c) and after (b, d) long-term testing, respectively.

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or redox cycles composite materials are more stable in comparisonwith the basic oxides. By XRD it was revealed that the phase contentof composites after treatment during 600 h in reducing atmospherewas closed to both the calculated and those after the sintering inair indicating a good compatibility composite components andtheir weak interaction with each other regardless of the ambientatmosphere.

4. Conclusions

The new composite materials (1 − x)SrTi0.5Fe0.5O3 − δ–xCe0.8(Sm0.8Sr0.2)0.2O2 − δ (х = 0, 0.25, 0.5, 0.75 and 1) were prepared bythe solid state reaction method. XRD and SEM revealed the formationof a two-phase perovskite–fluorite system with the specified stoichi-ometry. The system displayed high structural stability after sintering(1623–1823 K, 5 h) and in long-term testing in a hostile atmosphere(1173 K, 600 h, рО2 = 1 × 10−18 atm) and no secondary phases wereobserved after the treatment. It was found that the composite materialsare characterized by good mechanical properties that are attributable totheir microhardness increasing in comparison with SrTi0.5Fe0.5O3 − δ

from 940 to 1100–1130 kg/mm2 and an improvement in the intergraincontact relative to Ce0.8(Sm0.8Sr0.2)0.2O2 − δ. With an increase in the fluo-rite content (х = 0, 0.25, 0.5, 0.75 and 1) a decrease in p-type electronic(hole) conductivity was detected in air (1.061, 1.025, 0.574, 0.157and 0.147 S/cm at 1173 K, respectively), also widening of the ionicconductivity area and an increase in the ionic conductivity (0.062,0.068, 0.077, 0.111 and 0.157 S/cm at 1173 K) in the range of inter-mediate pO2 values and an increase in n-type electronic conductivityat low рО2 (0.227, 0.418, 0.703, 1.009 and 0.157 S/cm at 1173 K,рО2 = 1 × 10−18 atm). Increased ionic–electronic transport underreducing conditions as well as amoderate level of chemical and ther-mal expansion of the composites means they can be regarded as pro-spective MIEC membrane materials in the production of hydrogen orsyngas using the electrochemical conversion method and as anodematerials for SOFCs.

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Acknowledgments

The present work was financially supported by Council of thePresident of the Russian Federation (grant no. СП-44.2012.1), theRussian Foundation for Basic Research and the Government of Sverd-lovsk Oblast (grants nos. 12-03-33002, 13-03-00065, 11-08-00099,13-03-96098).Weare grateful toMr. Peter RoyOrman (BAVictoria Uni-versity, Wellington, NZ) for his constructive advice on the textpresentation.

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