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Journal of Membrane Science 383 (2011) 235– 240

Contents lists available at SciVerse ScienceDirect

Journal of Membrane Science

jo u rn al hom epa ge: www.elsev ier .com/ locate /memsci

attice characteristics, structure stability and oxygen permeability ofaFe1−xYxO3−ı ceramic membranes

iaotong Liua, Hailei Zhaoa,c,∗, Jianying Yanga, Yuan Lia, Ting Chena, Xionggang Lub,eizhong Dingb, Fushen Lia,c

School of Material Science and Engineering, University of Science and Technology Beijing, Beijing 100083, ChinaSchool of Materials Science and Engineering, Shanghai University, Shanghai 200072, ChinaBeijing Key Lab of New Energy Material and Technology, Beijing 100083, China

r t i c l e i n f o

rticle history:eceived 27 June 2011eceived in revised form 21 August 2011ccepted 27 August 2011vailable online 2 September 2011

a b s t r a c t

BaFe1−xYxO3−ı (x = 0–0.2) materials were synthesized by conventional solid-state reaction process foroxygen separation application. The effects of Y-doping on the crystal structure development, electricalconductivity and oxygen permeability were evaluated. Yttrium introduction effectively stabilize the cubicstructure of BaFe1−xYxO3−ı. With Y-doping, the oxidation state of Fe ions reduces, resulting in the increasein oxygen vacancy concentration as charge compensation and the decreases in electrical conductivity.

eywords:xygen permeation membraneerovskiteixed conductor

Y-doping enhances the structural stability of BaFe1−xYxO3−ı in reducing atmosphere but decreases theoxygen permeability. Both of them are attributed to the strong binding energy of Y–O bond. The cobalt freemembrane BaFe0.95Y0.05O3−ı shows good structural stability under reducing atmosphere and acceptableoxygen permeation flux of 0.798 ml (STP) min−1 cm−2 at 900 ◦C for 1.1 mm thick membrane, making it a

uture

tructural stabilitylectrical conductivity

promising candidate for f

. Introduction

Nowadays, Fischer Tropsch synthesis (F-T), which uses a mix-ure of CO and H2 (syngas) to produce hydrocarbons, chemicalsnd liquid fuels, has become once more an appealing technol-gy accompanying with the continuously rising of the crude oilrices [1]. Syngas manufacture is responsible for more than a halff the F-T investments and thus becomes the key part of thehole process [2–4]. Compared with other industrial scale pro-uction methods, partial oxidation of methane (POM) based onembrane is a promising alternative process to produce syngas

onsidering its low cost [4], energy saving (the reaction is exother-ic) [5], relatively simple process (both separation and catalytic

rocesses are achieved in a single step) [6] and proper ratio ofroduct (CO:H2 = 1:2) [7]. In a membrane reactor for POM, oxy-en separation membrane made by mixed oxide-ion and electrononductor (MIEC) is the key component of the whole reactor, andts materials have been thoroughly and extensively studied in past

ecades.

Among the many candidate materials studied, perovskitexide material remains to be the most promising one for future

∗ Corresponding author at: School of Material Science and Engineering, Universityf Science and Technology Beijing, Beijing 100083, China. Tel.: +86 10 82376837;ax: +86 10 82376837.

E-mail address: hlzhao@ustb.edu.cn (H. Zhao).

376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.memsci.2011.08.059

practical applications.© 2011 Elsevier B.V. All rights reserved.

applications due to its mixed ionic and electronic conductingbehavior. By doping in the A/B sites of ABO3 perovskite oxides, notonly the ionic and electronic conductivity but also the catalyticactivity can be adjusted. Numerous investigations have focusedon the improvement of various properties of perovskite oxygenpermeation materials by doping strategy, and many encouragingresults in oxygen permeability were achieved [8,9]. In the vari-ety of peroskite-type oxygen permeable materials, cobalt-basedperovskites attracted much attention due to the high oxygen per-meability and excellent surface oxygen exchange kinetics.

Industrial applications, however, set many strict requirementson the membrane, including the good chemical and mechanicalstability under harsh operating conditions of high temperatures(750–900 ◦C) and low oxygen partial pressures, where maintain-ing the stability of the toxic and expensive Co-rich materials isvery challenging [10,11]. Therefore, cobalt free materials attractedconsiderable interests recent years and many systems have beeninvestigated, such as Ba–Ce–Gd–O [12], Y–Ba–Cu–O [13] andLa–Sr–Fe–Ga–O [14]. In addition to the structural stability limita-tions under application conditions mentioned above, large scalepreparation of these types of materials is another challenge. Toomany elements may make it difficult to get homogeneous materi-als. The desired oxygen separation membrane should be composed

of less element species and have excellent structural stability andhigh oxygen permeability.

BaFeO3−ı is a good candidate for oxygen permeable mem-brane considering its relatively strong tolerance against reducing

2 rane Science 383 (2011) 235– 240

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1

2

3

t[Ybatwrdiwocpo

2

2

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Table 1Sintering temperature and relative density of dense BaFe1−xYxO3−ı samples.

Samples Sintering temperature (◦C) Relative density (%)

BaFeO3−ı 1300 91.9BaFe0.95Y0.05O3−ı 1320 92.6

36 X. Liu et al. / Journal of Memb

tmosphere and acceptable oxygen permeability at POM operationemperatures. Besides, Fe is environmental friendly and abundantn resources. The known problem of this base material is that ithows hexagonal structure at low temperature with limited oxy-en permeability [15,16]. The hexagonal structure will transformo cubic structure when the temperature increases up to ca. 850 ◦C,hich may cause the membrane material to become unstable for its

ntended use. Compared with hexagonal structure, cubic structuresually exhibits high oxygen permeability owing to its isotropictructure, i.e. the randomly distributed oxygen vacancy. Doping is

good strategy to suppress the phase transformation of materials.any attempts have been made to preserve the high temperature

ubic phase of BaFeO3−ı down to room temperature, such as La,a, Y, Rb, Ca-doping in A-site [15], and Ce, Zr, Ni-doping [16–18]

n B-site. We have noticed, however, that doping in A-site ofteneads to the formation of impurity phase, which could decrease thexygen permeability [15]. Therefore, B-site doping appears to be aore advisable approach to stabilize the cubic structure. To achieve

good structural stability without too much oxygen permeabilityacrificed, the candidate elements for B-site doping are expected toave the following properties:

) Large ionic radius. From the viewpoint of geometric structure,BaFeO3−ı has a tolerance factor t > 1, as expressed in Eq. (1). Dop-ing B-site with large ion may decrease the tolerance factor andlead it to be close to unity, thus resulting a cubic structure.

) Low electronegativity. This may decrease the average oxida-tion state of iron due to its weaker attractivity towards electronunder the hypothesis that doping will not affect the oxidationstate of oxygen ions seriously.

) Low valence of doping cation. It should be not higher than three.Together with requirement (2), this may increase the oxygenvacancy concentration (ı) as charge compensation.

t = rA + rO√2(rB + rO)

(t = 1 is ideal cubic perovskite phase) (1)

where rA, rB and rO represent the radius of cation in A/B site andoxygen ion. Tolerance factor t is usually used to characterize thelattice structure symmetry [19].

According to these pre-conditions set forth, yttrium appearso be one of the most suitable candidates with ionic radii = 0.9 A21], electronegativity = 1.22, and common oxidation valance = 3.ttrium substitution in B-site of BaCo0.7Fe0.3O3−ı has been reportedeing successful [22]. But to our best knowledge, this is the firstttempt to dope yttrium in the B-site of cobalt-free BaFeO3−ı. Inhe present work, cubic single phase BaFe1−xYxO3−ı (BFY, x = 0–0.2)ere synthesized successfully via conventional solid state reaction

oute. The effects of Y-doping on the phase structure, electrical con-uctivity and chemical stability in severe reducing atmosphere;

nitial oxygen nonstoichiometry and oxygen permeability of BFYere investigated. An excellent structural stability with moderate

xygen permeability was achieved by BFY membrane. A possibleharge compensation mechanism associated with Y-doping wasroposed to elucidate the evolution of electrical conductivity andxygen permeability of BFY membranes with Y-doping level.

. Experimental

.1. Powder synthesis and membrane preparation

The BFY (x = 0–0.25) series powders were prepared by con-

entional solid state reaction method. Stoichiometric reagents ofaCO3, Fe2O3 and Y2O3 (all in A.R. purity) were mixed by ball milling

n ethyl alcohol for 3 h at a speed of 200 rpm. After drying, the pow-ers were screened and calcined at 1000 ◦C in air atmosphere for

BaFe0.9Y0.1O3−ı 1350 91.0BaFe0.85Y0.15O3−ı 1400 89.4

10 h. The calcined powders were thoroughly ground with a mortarand pestle to break the soft agglomerations. The obtained ceramicpowders were press-formed into disks (∼�19, ∼1.5 g) and bars(4 mm × 7.5 mm × 42 mm) under uniaxial pressure in a stainlesssteel mold with an appropriate amount of PVA (1 wt%). The greensamples were then sintered at 1300–1400 ◦C for 10 h in air with theheating and cooling rates of 3 ◦C/min to achieve a similar relativedensity. Sintering temperatures for each composition are shown inTable 1.

2.2. Characterization

The phase structure evolution of the sintered samples and thesamples after reducing test were examined by X-ray diffraction(XRD) using a RIGAKU D/MAX-A diffractometer with the Cu K�radiation. The lattice structure and parameters were analyzed withJade software based on the XRD data. X-ray photoelectron spec-tra (XPS) of the samples were obtained using PHI Quantera SXMspectrometer with the Al K� radiation, and charge referencingwas done against the binding energy (BE) of adventitious carbon(C 1s = 284.6 eV). To investigate the structural stability under thereducing atmosphere, the samples were annealed at 900 ◦C for 10 hin flowing 5%H2/Ar. Electrical conductivity of the samples was mea-sured by a four-probe dc method with the bar at the temperatureranging from 200 to 900 ◦C under different atmospheres. The bulkdensity of the samples was measured by Archimedes’ method usingpure water as medium. The relative density was derived from themeasured bulk density and the theoretical density determined byusing XRD data.

2.3. Oxygen permeability measurement

The oxygen permeability of the BFY membrane was measuredby the gas chromatography (GC) method. All the samples were pol-ished with an emery paper to the thickness of 1.1 mm first, andthen install into a vertical high-temperature oxygen permeationsystem. The samples were loaded between a quartz tube (low oxy-gen partial pressures side) and an alumina tube (high oxygen partialpressures side). A silver ring with a 13 mm diameter serves as thesealing agent between the quartz tube and the sample. Air wassupplied at a flow rate of 90 ml/min [STP], while, the sweep gas,helium, was applied at a flow rate of 60 ml/min [STP]. The oxy-gen permeation measurement was carried out from high to lowtemperatures. Good sealing without nitrogen leakage was guaran-teed before oxygen permeation flux data were recorded. The mixedgases of sweep helium and permeated oxygen were introduced intothe gas chromatography equipment to determine the oxygen con-centration. The oxygen permeation flux was calculated accordingto Eq. (2).

JO2 (ml cm−2 min−1) = CO2 × F

S × (1 − CO2 )(2)

where CO is the measured oxygen concentration in the outlet gason the sweep side; F is the flow rate of the helium; S represents theeffective membrane surface area for permeation. The measurementwas performed in 780–900 ◦C.

X. Liu et al. / Journal of Membrane Science 383 (2011) 235– 240 237

Fi

3

3

3

BdsctcirtBiiooL(

pats

Fh

Table 2Oxygen nonstoichiometry (ı) in BaFe1−xYxO3−ı measured by iodometry titrationtechnique.

Samples � in BaFe1−xYxO3−ı

BaFeO3−ı 0.380BaFe Y O 0.387

ig. 1. XRD patterns of BaFe1−xYxO3−ı samples sintered at the temperature indicatedn Table 1. (a) 2� = 10–100×; (b) 2� = 28–32◦ .

. Result and discussion

.1. Phase development and structure characterization

.1.1. Phase structure and relative factorsFig. 1 shows the room-temperature XRD patterns of sintered

aFe1−xYxO3−ı (x = 0–0.2) samples. Without doping, pure BaFeO3−ı

isplays a hexagonal perovskite structure. As expected, quite amall amount of yttrium (x = 0.05) doping into B-site of BaFeO3−ı

an stabilize the cubic structure down to room temperature. Allhe XRD peaks of samples with x = 0.05–0.2 can be indexed asubic perovskite structure (Pm3m, 221) without any detectablempurity, suggesting that yttrium has dissolved into the host mate-ial BaFeO3−ı and occupies the B-site rather than A-site owingo the absence of BaO impurity. With increasing yttrium level inaFe1−xYxO3−ı, the peaks shift to small angles gradually, as shown

n Fig. 1(b), indicating the expansion of crystal cells. This is anotherndication that the dopant Y does take the B-site instead of the A-sitef BaFe1−xYxO3−ı, taking into account of the ion size, the ion radiusf Y (0.9 A VI, 1.075 A IX) is larger than that of Fe3+ (HS: 0.645 A VI,S: 0.55 A VI) and Fe4+ (0.585 A VI) but smaller than that of Ba2+

1.61 A XII).The cell parameters of BaFe1−xYxO3−ı were derived from XRD

atterns. According to Eqs. (3) and (4) [20], the critical radius (rc)nd the free volume of cell (Vf) were calculated for the situationshat Fe ions take valence 3+ or 4+, respectively. The results arehown in Fig. 2. The ion radius is chosen according to Shanon’s

ig. 2. Calculated rc and Vf of BaFe1−xYxO3−ı with different Y contents under theypothesis that Fe ions take valence 3+ or 4+, respectively.

0.95 0.05 3−ı

BaFe0.9Y0.1O3−ı 0.399BaFe0.85Y0.15O3−ı 0.438

previous work [21]. Since the ion radius is affected by the spinstate and Fe ions easily take high spin state at high temperatures,to approach the actual situation, the high spin state of both Fe3+

and Fe4+ ions is selected here for structure calculation [23,24]. Theresults indicate a clear increasing trend of Vf and rc with yttriumdoping no matter the change of oxidation of Fe ion. Therefore,it is reasonable to state that the actual BaFe1−xYxO3−ı materialswith mixed Fe3+/Fe4+ ions will have a similar increasing trend.The increase in rc and Vf should be beneficial to the oxygen iontransportation in the lattice of BFY materials.

rc = −rA2 + 3

4 (a2) −√

2a · rB + r2B

2rA +√

2a − 2rB

(3)

VF = a3 − 43

�(r3A + r3

B + (3 − ı)r3O) (4)

where a represents the lattice parameter of cubic structure, ı is theoxygen vacancy concentration.

3.1.2. Initial oxygen nonstoichiometryOne of the yittrium doping aims in this work is to increase

the initial oxygen nonstoichiometry (ı) of BaFe1−xYxO3−ı, takingadvantage of the low electronegativity of yttrium together withits lower valence (III) than B-site ideal valence in the A(II)B(IV)O3structure. X-ray photoelectron spectroscopy (XPS) measurementson samples with different yttrium contents were carried out toinvestigate the changing tendency of electron distribution and theoxidation state of each element affected by yttrium doping. Fig. 3exhibits the XPS results of O 1s and Fe 2p for samples BaFe1−xYxO3−ı

with x = 0.05 and 0.15. There is no obvious change in the bindingenergy of O 1s between the two samples (Fig. 3(a)), suggesting thatyttrium doping has negligible impact on the electron distributionaround oxygen atoms. However, an obvious variation is detectedfor the binding energy of Fe 2p electrons, as shown in Fig. 3(b). It iswidely known that the binding energy of inner electron increaseswith the oxidation state raising. According to Yang’s [25] previousresults, the binding energy of Fe 2P1/2 and Fe 2P3/2 for BaFeO3 is724.7 and 711.1 eV, respectively. With partial substitution of Y forFe, the corresponding binding energy of Fe 2P1/2/Fe 2P3/2 changes tobe 724.4/723.2 eV, and 710.6/710.4 eV for samples BaFe0.95Y0.05O3and BaFe0.85Y0.15O3, respectively. These results indicate that theoxidation state of Fe decreases with Y doping. As a result, the ı inBaFe1−xYxO3−ı increases with Y substitution in order to keep theelectroneutrality of material, which is confirmed by the iodometrytitration measurement on the oxygen nonstiochiometry of sam-ples. As listed in Table 2, the measured ı increases gradually withY doping level.

The increase in Vf, rc and ı with Y-doping is undoubtedly a goodsignal for oxygen permeability.

3.1.3. Structural stabilityGood structural stability is an essential requirement for the

long-term operation of oxygen permeation membranes. Y-doping

is expected to be capable of enhancing the structural stabil-ity BaFe1−xYxO3−ı due to the large ionic radius of yttrium andthe strong Y–O bond. The results in Fig. 1 indicate that Y sub-stitution can stabilize the cubic phase of BaFe1−xYxO3−ı, while

238 X. Liu et al. / Journal of Membrane Science 383 (2011) 235– 240

F0

trotB9piYostiea

m1Bloodt

ig. 3. XPS spectra of O 1s (a) and Fe 2p (b) for samples BaFe1−xYxO3−ı (x = 0.05,.15).

he structural stability under low oxygen partial pressure stillemains to be investigated. To evaluate the structural stabilityf BaFe1−xYxO3−ı in a short time, a harsh experimental condi-ion was employed in this work. The sintered dense samplesaFe1−xYxO3−ı (x = 0–0.2) were heat-treated in flowing 5%H2/Ar at00 ◦C for 10 h. After cooling down to room temperature, the sam-les were subjected to XRD examination. The results are shown

n Fig. 4. An obvious split of diffraction peaks for samples with-doping is observed, indicating the phase transition of cubic per-vskite structure after heat treatment. Nevertheless, the perovskitetructure characteristic is still kept. Some impurities, assignableo Fe and Ba3Fe2O6, are detected, which disappear gradually withncreasing Y-doping amount, demonstrating that Y-doping cannhance the tolerance of BaFe1−xYxO3−ı materials to reducingtmosphere.

Compared with other reported oxygen permeation membraneaterials, such as BaCo0.9−xFexNb0.1O3−ı (5%H2/Ar at 850 ◦C for

0 h) and BaCo0.7Fe0.3−xNbxO3−ı (5%H2/Ar at 900 ◦C for 5 h) [26,27],aFe1−xYxO3−ı shows a really good performance and is an excel-

ent candidate for practical POM application from the viewpoint

f structural stability. Considering that the oxygen ions continu-usly transport from the high oxygen side to the low oxygen sideuring the operation of membrane, the oxygen partial pressure onhe membrane surface of low oxygen side should be much higher

Fig. 4. XRD patterns of densified BaFe1−xYxO3−ı samples after heat-treated in5%H2/Ar at 900 ◦C for 10 h.

than that in 5%H2/Ar atmosphere. Therefore, the structural stabil-ity (reduction resistance) of BaFe1−xYxO3−ı should be much betterin practical application environment than the result shown here[27,28].

3.2. Electrical conductivity

The total conductivity of the samples BaFe1−xYxO3−ı (x = 0–0.15)were measured at temperature ranging from 200 to 900 ◦C underdifferent oxygen partial pressures. The results are shown inFig. 5. Generally, the electronic conductivity in perovskite mixedconductor is far higher than the ionic conductivity which isattributed by the presence of oxygen vacancies. Thus, the mea-sured values can be assumed to be the electronic conductivityalone.

Fig. 5(a) shows the temperature dependence of the conduc-tivity of BaFe0.9Y0.1O3−ı measured in different atmospheres, andthe corresponding Arrhenius plots of ln(�T) vs. 1/T are shown inFig. 5(b). It is a typical p-type semiconductor that the conductivityincreases with increasing oxygen partial pressures. At the tem-peratures lower than 400 ◦C, the conductivity shows an increasingdependence on temperature. This semiconducting behavior can beexplained by a thermally activated p-type small polaron-hoppingmechanism. Average activation energy of ∼0.355 eV was deter-mined from the linear part of the Arrhenius plot in the range of200–400 ◦C. After the conductivity value reaches the highest pointat about 400–450 ◦C, the curve change direction apparently evenin the pure oxygen atmosphere. This decrease in electrical conduc-tivity is believed to be attributed to two reasons. One is the releaseof lattice oxygen, which will lead to the partial annihilation of elec-tronic holes, lots of material existing this phenomenon at around400–650 ◦C, as described by Eq. (5) [29]. The other is the iron ionsspin transition, which happens around 500–600 ◦C as reported inprevious works of relative materials. The transition from low-spinto high-spin of Fe 3d electrons will lead to more overlap betweenFe 3d and O 2p electrons, and thus result in the narrowed energygap, making the material show the electronic conduction behav-ior of metallic materials [23,24]. These two factors influence thetrend of conductivity together. It needs to emphasize that, underAr atmosphere, the lattice oxygen releases much faster and eas-

ily reaches the equilibrium state (ca. 600 ◦C), after that the smallpolaron-hopping mechanism works again, resulting in the slightincrease of conductivity at high temperature. At relatively high

X. Liu et al. / Journal of Membrane Science 383 (2011) 235– 240 239

Ff

ot

O

Ttxleitddicyte8tclibp

the ability of oxygen ion migration in the lattice. The ABE of Fe–Oand Y–O is calculated by using Eq. (6) [20], which is −200.6 kJ/moland −291.5 kJ/mol, respectively. High ABE is apparently unfavor-able for the oxygen ion migration. This huge difference in ABE can

ig. 5. Temperature dependence of electrical conductivity of BaFe0.9Y0.1O3−ı in dif-erent oxygen partial pressures (a) and the Arrhenius plot (b).

xygen partial pressure, the lattice oxygen release becomes moreardily due to the dense state of samples.

×O + 2h• → V

••O + 1

2O2 (g) (5)

he electrical conductivity of BaFe1−xYxO3−ı in air as functions ofemperature and Y content is shown in Fig. 6. For the samples with

= 0.05–0.15, the stable cubic structure made them have a simi-ar conductivity change tendency. With increasing Y content, thelectrical conductivity of BaFe1−xYxO3−ı decreases gradually. Thiss attributed to the decreased charge carrier concentration andhe reduced charge moving path resulting from the Y-doping. Asemonstrated in Figs. 3 and 6, BaFe1−xYxO3−ı is a p-type semicon-uctor and Y-doping leads to the decrease of oxidation state of Fe

ons, which indicates the decease of charge carrier (electron holes)oncentration. On the other hand, the steady valence of elementttrium localizing at B-site will lead to the interruption of the elec-ron hopping along –B–O–B– bond, making the effective ways forlectron transport decrease. The sudden conductivity change at ca.00 ◦C for sample without Y-doping is associated with the phaseransform from low temperature hexagonal to high temperatureubic, since high symmetry leads to more electron clouds over-

◦

ap [30]. The electrical conductivity of pure BaFeO3 above 800 Cs higher than that of others owing to the absence of lattice distur-ance from Y-doping. Similar phenomenon was observed in oxygenermeation test, as discussed in Section 3.3.

Fig. 6. Temperature dependence of electrical conductivity of samples BaFe1−xYxO3−ı

in air.

3.3. Oxygen permeability

Fig. 7 shows the temperature dependence of oxygen perme-ation flux through 1.1 mm thick BFY ceramic membranes. It issimilar to the conductivity curve that the BaFeO3−ı is special toothers due to its phase transformation to a higher symmetric crys-tal system at high temperature. Careful inspection reveals thatBaFe0.95Y0.05O3−ı exhibits higher oxygen permeability than cubicBaFeO3−ı at 900 ◦C. One possible reason is the uncompleted phasetransition of BaFeO3−ı. The low oxygen partial pressure in thesweep side of the membrane may impede the phase transition ofBaFeO3−ı from hexagonal to cubic. Another reason is the increasedoxygen vacancy concentration (ı), as shown in Section 3.1. Theyare believed to be favorable for the oxygen ion migration and hencethe improvement of oxygen permeability. Nevertheless, the oxygenpermeability decreases obviously with Y-doping level, suggestingthat some other factor may exert negative influence on the oxygenpermeation process. The average bonding energy (ABE) of metal-oxygen in perovskite structure is another vital factor that affects

Fig. 7. Temperature dependence of oxygen permeation flux through 1.1 mm thickBaFe1−xYxO3−ı membranes.

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40 X. Liu et al. / Journal of Memb

e accounted for the decreasing oxygen permeability of samplesith high Y content. This is reflected in the calculated activation

nergy of oxygen permeation, as shown in the inset of Fig. 7. Amonghese samples, BaFe0.95Y0.05O3−ı membrane with 1.1 mm thick-ess shows the highest oxygen permeation flux of 0.798 ml (STP)in−1 cm−2 at 900 ◦C, which is not high but comparable with other

eported BaFeO3-based membranes [17,18].

BE = 112m

(�HAmOn − m�HA − n

2DO2

)

+ 16m′

(�HBm′ On′ − m′�HB − n′

2DO2

)(6)

here �HAmOn and �HBm′ On′ are the formation heats of AmOn andm′ On′ oxides, respectively; �HA and �HB stand for the sublima-ion heats of A and B metals, respectively; and DO2 represents theissociation energy of oxygen.

. Conclusions

The cubic perovskites BaFe1−xYxO3−ı (x = 0.05–0.15) were syn-hesized successfully by solid state reaction method. Little amountf yttrium can prevent the phase transformation of BaFe1−xYxO3−ı

rom cubic to hexagonal perovskite structure during cooling pro-ess. Y-doping significantly enhances the structural stability ofaFe1−xYxO3−ı in reducing atmosphere. The pure BaFeO3−ı expe-iences a phase transition from hexagonal to cubic phases at ca.00 ◦C, which leads to the increase in both electrical conductiv-

ty and oxygen permeability of material. Due to the relativelyow electronegativity of Y element, Y-doping causes the reduc-ion of the oxidation state of Fe ions, and thus the augment ofxygen vacancy concentration. The structural analyses reveal that-doping leads to the increase in the critical radius rc and freeolume Vf of the perovskite structure. All these factors shoulde favorable for oxygen ion migration. However, the oxygen per-eability of BaFe1−xYxO3−ı decreases with Y-doping level. This isainly attributed to the strong binding energy of Y–O bond, which

estricts the migration of oxygen ion around Y atoms and thusncreases the activation energy of oxygen ion migration in mem-rane material. Nonetheless, BaFe0.95Y0.05O3−ı membrane showsoderate oxygen permeation flux of 0.798 ml (STP) min−1 cm−2

t 900 ◦C and good structural stability under low oxygen partialressure. Considering the practical operation requirements andhe optimal balance between oxygen permeability and stability,he BaFe0.95Y0.05O3−ı material is a promising candidate for futurepplications.

cknowledgements

The authors of this work appreciate the financial support from63 Program of National High Technology Research Developmentroject of China (No. 2006AA11A189), Beijing Municipal Natu-al Science Foundation (No. 2102031), National Nature Scienceoundation of China (No. 20973021) and Outstanding Doctoral Dis-ertation Guidance Foundation of Beijing (YB20091000802).

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