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Phases transition and oxygen permeating propertiesof SrFeGa0.25O3-d

Min Jae Shin a,b, Ji Haeng Yu b,*, Shiwoo Lee b

aAdvanced Energy Technology, University of Science and Technology, 113, Gwahangno, Daejeon 305-333, Republic of KoreabReaction and Separation Materials Research Center, Korea Institute of Energy Research, 71-2, Jang dong, Daejeon 305-343, Republic of Korea

a r t i c l e i n f o

Article history:

Received 19 February 2010

Received in revised form

20 April 2010

Accepted 20 April 2010

Available online 20 May 2010

Keywords:

Mixed conductor

Oxygen transport membrane

SrFeGa0.25O3-d

Phase transition

* Corresponding author.E-mail address: jhyu@kier.re.kr (J.H. Yu).

0360-3199/$ e see front matter ª 2010 Profedoi:10.1016/j.ijhydene.2010.04.123

a b s t r a c t

Mixed conducting materials have potential for application of the membrane reactor under

large chemical potential gradients. A perovskite-related SrFeGa0.25O3-d was prepared by the

conventional solid-state reaction and its phase under air, He and 8% H2 was analyzed by in-

situ high-temperature XRD. The perovskite structure in air transformed to orthorhombic

brownmillerite at 600 �C in He atmosphere, and to cubic brownmillerite at around 1000 �C.

In 8% H2, the SrFeGa0.25O3-d decomposed into other phases that were completely different

from the perovskite and the brownmillerite above 900 �C. The phase decomposition in

hydrogen caused a drastic change both in electrical conductivity and surface morphology

of the membrane. The electrical and oxygen-transporting characteristics correlated with

crystal structure of SrFeGa0.25O3-d were discussed.

ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

1. Introduction alternative. Reactors, which are equipped with a dense

With the recent shift from a carbon economy to a hydrogen

economy, many studies have been recently published on

hydrogen production, management, and storage. In order to

produce hydrogen, water or steam electrolysis, thermo-

chemical reaction, and photocatalysis techniques as well as

hydrocarbon reforming are being developed to enhance the

efficiency, and thus, lower the production cost, which is the

barrier towards the use of hydrogen for clean and high effi-

cient devices like fuel cells. However, hydrocarbon reforming,

such as steam reforming (CH4 þ 2H2O 4 4H2 þ CO2), partial

oxidation (POx, CH4 þ O2 4 2H2 þ CO2), and combined auto-

thermal reforming (ATR, 2CH4 þ 2H2O þ O2 4 6H2 þ 2CO2) is

still the most widely used technique to produce the

commercial scale of hydrogen in a cost-efficient manner. In

order to developmore efficient POx or ATR reactors, the use of

pure oxygen, instead of air, has been suggested as a promising

ssor T. Nejat Veziroglu. P

membrane to separate the oxygen from air, can directly

produce syngas without any oxygen supply utility, like cryo-

genic plants [1].

Mixed Oxygen-Ionic and Electronic Conducting (MIEC)

oxides have been intensively researched for oxygen-trans-

porting membrane reactors since the ionic diffusion through

the membrane provides production of nearly pure oxygen

[2e5]. Among the MIEC materials, SrFeO3-d perovskite is

a promising parent oxide for developing the membrane

reactor [6e9]. SrFeO3-d generally undergoes phase transition

from a perovskite to a brownmillerite structure by decreasing

the oxygen partial pressure and thus the oxygen content [10].

The phase transformation produces an oxygen vacancy

ordering, which causes degradation in ionic conductivity due

to the partial trapping of oxygen vacancies in the local cluster

[6]. The disordereorder transition can be partially suppressed

by substituting the foreign cations. Several studies have

ublished by Elsevier Ltd. All rights reserved.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 5 ( 2 0 1 0 ) 7 5 1 2e7 5 1 8 7513

shown that the substitution of La into A sites and Cr, Al, Ga, Ti,

or Mo into B sites enhances ionic transport as well as struc-

tural stability [9,11e18]. For instance, La1-xSrxFe1-yGayO3-

d (x ¼ 0.6e0.9; y ¼ 0.2e0.5), in which the crystal lattice was

identified as brownmillerite, showed a significant oxygen

permeability combined with a sufficient stability under the

membrane operation condition.

Some perovskite-related oxides have been of interest as

potential membrane reactors under large oxygen potential

gradient. SrFeCo0.5Ox exhibits high oxygen ion conductivity

with structural stability even in reducing atmospheres

[19e22]. In addition, the methane conversion efficiency of the

tubular SrFeCo0.5Ox membrane was 98% over 1000 h at 850 �C.The SrFeCo0.5Ox material consists of a brownmillerite phase

(Sr4Fe6-xCoxO13�d) and a perovskite phase (SrFe1-xCoxO3-d)

depending on synthesis methods. Because these perovskite-

related materials have polymorphs depending on the ther-

modynamic parameters, the oxygen permeability of the

membrane is strongly influenced by the operating condition.

In this study, we synthesized SrFeGa0.25O3-d by adding

excess Ga into SrFeO3-d using solid-state reaction and inves-

tigated its high-temperature phases and oxygen permeating

properties in oxidizing and reducing atmosphere. Some

researchers have reported on the electrical and structural

properties of SrFe1-xGaxO3-d (x¼ 0.1, 0.2) [23,24], but there have

been no report on the structural and permeating properties of

overstoichiometric SrFeGaxO3-d. We examined the effects of

the phase change on the electrical conductivity and the

oxygen permeation flux of SrFeGa0.25O3-d. In-situ XRD was

used to investigate its high-temperature phases, which

strongly dependent on atmospheric oxygen activity.

2. Experimental

SrFeGa0.25O3-d powderwas synthesized by solid-state reaction.

Stoichiometric amounts of SrCO3 (99.9%, Aldrich Chemical;

the rest are the same), Fe2O3 (99.9%), and Ga2O3 (99.99%) were

mixed by wet-milling for 24 h with isopropyl alcohol and

zirconia balls. The mixed powders were dried at 110 �C and

were then calcined at either 850 or 1250 �C for 2 h in order to

confirm the phase formation. A disk type of membrane was

prepared by cold isostatic pressing at 300 MPa and then sin-

tering at 1200 �C for 5 h.

The phases of the calcined powders and sintered disk were

analyzed by X-Ray Diffractometry (XRD, Rigaku, Japan). The

sintered disk was pulverized into powder in order to investi-

gate the phase change with an increasing temperature. The

high-temperature XRD experiments were carried out under

ambient air, He, and 8%H2 (balancedwith He) at temperatures

from 500 to 1000 �C. The specimen was heated from one

measurement temperature to another by a rate of 10 �C/min.

The intensity of the diffracted X-ray was detected at angles

(2q) ranging from 20 to 80� by a scan speed of 5�/min.

Electrical conductivities of the sample were measured by

a four-probe dc method under air, He, and 4% H2 (balanced

with Ar) atmospheres. The sintered sample was cut into a bar

(0.52mm� 0.31mm� 0.1mm). The bar specimenwas painted

with Pt paste (Engelhard model# 6082, USA) at specific inter-

vals and cured at 900 �C for 30 min. Four Pt wires were wound

around the bar in order to be connected with probes from

a source-measure unit (Keithley, K2400, USA). Current-voltage

characteristics were measured in the current range of �0.5 to

0.5 mA. The measurements were carried out in the mode of

increasing the temperature under the same atmosphere. The

in-situ oxygen partial pressures were measured by using

a stabilized-zirconia oxygen sensor.

The oxygen permeation fluxes of a SrFeGa0.25O3-d sintered

disk were characterized at temperatures ranging from 850 to

1000 �C. Fig. 1 shows the configuration of oxygen permeation

measurement installation used in this study. The polished

membrane with a diameter of 20 mm and a thickness of

1.0 mm was sealed with Au ring (inner diameter ¼ 16 mm,

outer diameter ¼ 20 mm, thickness ¼ 0.24 mm) at the end of

the alumina tube. Thus the effective area of membrane

exposed to gas was 2.01 cm2. Under fixed PO2 gradient (Air:

0.21 atm, He: 3.2 � 10�4 atm), the reactor was heated to 980 �Cat 2.0 �C/min and maintained for 5 h in order to gas-tight.

Synthetic air was used as the feed side (high-PO2 side), and

high purity He (99.999%) or 8% H2 balanced with He was used

as the sweep gas on the permeate side (low-PO2 side). The gas

flow rates were kept at 30 ml/min by a gas flow controller. Gas

leakage due to sealing problem was detected by monitoring

nitrogen concentration with gas chromatograph (GC, ACME

6000, carboxen-1000 column). These values were obtained

under the assumption that the gas travels through pores or

cracks by Knudsen diffusion, and thus the ratio of leaked

oxygen and nitrogen ðjO2 ;leak=jN2 ;leakÞ would beffiffiffiffiffiffiffiffiffiffiffiffiffi28=32

p � 0:21=0:79. Accordingly, the oxygen permeation flux

was calculated by using the following equation [25]:

jO2

�molcm�2s�1

� ¼"CO � CN

0:210:79

ffiffiffiffiffiffi2832

r #FS;

where CO and CN are the concentrations of oxygen and

nitrogen, respectively, measured at the permeate side. F is the

flow rate of the sweeping gas, including permeated gas, and S

is the active area of the disk membrane. The amount of

oxygen leak was less than 0.02 ml/min, which was negligible

compared with permeated oxygen flow by ionic diffusion.

The morphology of the membrane after the oxygen

permeation test under Air/8% H2 condition was analyzed by

Scanning Electron Microscopy and Energy Dispersive X-ray

Spectroscopy (SEM/EDX, Hitachi, Japan)

3. Results and discussion

3.1. Phases of synthesized SrFeGaxO3-d powders

Fig. 2 shows the XRD spectra of SrFeGaxO3-d (x ¼ 0.25, 0.5)

powders synthesized by the solid-state reaction of carbonate

and oxide mixture. In SrFeGa0.25O3-d, a perovskite phase

(ABO3) was formed from the mixture calcined at 850 �C, butresidual SrCO3 was detected as well. Those peaks by the SrCO3

disappeared as the firing temperature was increased to

1250 �C.SrFeGa0.25O3-d powder showed a small amount of the

SrFe2O4 phase, regardless of the firing temperature. The Ga3þ

cations (0.61 A) would replace Fe3þ cations (0.63 A) in the

sweeping gas in

feed gas out

GCGC

feed gas in

sweeping gas out

GCGCHe

He + O2

Air

MembraneSealant

(Au)

2

Fig. 1 e Schematic diagram of measurement setup for oxygen permeation.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 5 ( 2 0 1 0 ) 7 5 1 2e7 5 1 87514

tetrahedral layer without significant lattice distortion.

However, the segregation of secondary phase, SrFe2O4,

occurred as overstoichiometric ratio, ([Ga] þ [Fe])/[Sr],

increased. It seems that the addition of excess Ga with fixed

valence, þ3, is less accommodable than mixed valenced (þ3

and þ4) Co into SrFeCo0.5O3-d [20]. As shown in Fig. 2, SrFe-

Ga0.5O3-d specimen showed a considerable amount of SrFe2O4.

We tried to synthesize stoichiometric and overstoichiometric

compounds such as SrFe0.5Ga0.5O3-d, SrFe0.25GaO3- d, and

SrFe0.5GaO3-d, but the specimen with the [Ga]/[Fe] ratio more

than unity did not show any perovskite-related structure. As

for the stoichiometric SrFe1-xGaxO3-d compounds [23], it has

been reported that the single perovskite phase could be

synthesized as long as Gawas added up to 25mol%. It was also

found from our experiment that the substitution over 50% Fe

Fig. 2 e X-ray diffraction patterns of SrFeGaxO3-d (x [ 0.25,

0.5) powder calcined at 850 �C and 1250 �C.

even in stoichiometric composition, i.e. SrFe0.5Ga0.5O3-d,

caused formation of non-perovskite phases. Due to these

thermodynamics limitation, this work including in-situ XRD

and oxygen permeation study is focused only on SrFeGa0.25O3-

d membrane, which is mainly composed of perovskite phase

but small amount of SrFe2O4.

SrFe2O4 is known as a metastable phase which consists of

a three-dimensional network of corner-sharing BO4-tetra-

hedra and A-site cations residing in the voids of the frame-

work [26]. An overstoichiometric addition of Al into SrFe1-

xAlxO3-d led to the segregation of secondary phase such as

SrAl2O4, which improved sinterability and reduced thermal

expansion coefficient as well [13]. Significant amount of

SrAl2O4, however, lowered the oxygen permeability of SrFe1-xAlxO3-d-SrAl2O4 composite due to its negligible ionic and

electronic conductivity [27].

The morphology of the powder, as well as the crystalline

phase, is a crucial factor in the fabrication of a dense struc-

tured membrane. Although the SrFeGa0.25O3-d powder, which

was fired at 850 �C, still contained strontium carbonate, as

shown in Fig. 2, its fine particles (�1 mm) made it easier to

compact the powder into disks and to sinter them into a dense

structured membrane than the coarse particles (w4 mm) fired

at 1250 �C. Thus, the finer SrFeGa0.25O3-d powder synthesized

at 850 �Cwas used to fabricate the disk type ofmembrane. The

relative sintered density of the membrane measured by the

Archimedes method was approximately 96%.

3.2. High-temperature XRD analysis in oxidation andreduction atmospheres

The in-situ phase transition of SrFeGa0.25O3-d was investigated

by using high-temperature XRD under oxidizing and reducing

atmospheres. The pulverized powder from the sintered disk

was heated on a quartz holder from room temperature to

1000 �C and its XRD patterns were obtained in air, He and 8%

hydrogen, respectively. In ambient air, SrFeGa0.25O3-d showed

an identical perovksite structure to that of the powder

calcined at 1250 �C regardless of any temperature variation.

Table 1 e Unit cell parameters of SrFeGa0.25O3-d at different temperatures in ambient air.

Temperature (�C) 30 500 600 700 800 900 1000

Lattice parameter (A) 3.877 3.915 3.923 3.934 3.947 3.961 3.969

a

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The unit cell parameters of SrFeGa0.25O3-d, which were calcu-

lated from peaks of the perovskite phase, are given in Table 1.

In the oxidation condition, the lattice of SrFeGa0.25O3-d was

identified as simple cubic, which is in agreement with SrFeOx

and SrFe0.8Ga0.2O3-d at room temperature [10,23]. The lattice

expansions of the perovskite phase in SrFeGa0.25O3-d were

fitted in Fig. 3. The thermal expansion coefficient of SrFe-

Ga0.25O3-d fitted by a linear regression was w29.3 � 10�6 K�1

between 500 and 1000 �C in air.

The perovskite phase of SrFeGa0.25O3-d changed with

reducing oxygen partial pressure. In the He atmosphere, the

cubic perovskite was transformed into orthorhombic brown-

millerite (Sr2Fe2O5) at 600 �C as shown in Fig. 4(a). The peaks by

the orthorhombic brownmillerite phase changed at around

1000 �C and the material seemed to return to cubic brown-

millerite. According to the phase diagram for the SrFe1-xGax-O

2.5�SrFe1-xGaxO3 (x ¼ 0, 0.1, and 0.2), cubic perovskite, cubic

brownmillerite, and orthorhombic brownmillerite structures

are shown depending on oxygen content and gallium doping.

Substitution of gallium for iron increased the stability of the

cubic brownmillerite phase with respect to variations of

temperature and oxygen content [24]. In our experiment, the

high-temperature phase of SrFeGa0.25O3-d was found as cubic

brownmillerite due to overstoichiometric addition of gallium.

The high-temperature cubic brownmillerite phase of SrFe-

Ga0.25O3-d changed to orthorhombic brownmillerite again

during cooling to room temperature.

Fig. 4(b) shows the phase transitions of SrFeGa0.25O3-d in the

8% H2 (diluted with He). It was found that the brownmillerite

began decompose into other phases at 900 �C. In order to

identify the high-temperature phase of SrFeGa0.25O3-d, this

Fig. 3 e Lattice expansion of SrFeGa0.25O3-d calculated from

the in-situ XRD spectra in ambient air.

sample was quenched to room temperature in the 8% H2

condition. By XRD analysis, the new structures were identified

to Sr3Fe2O6, metallic Fe, Sr3Ga2O6, and Sr10Ga6O19. As for

Ba0.5Sr0.5Fe1-xCoxO3-d (x ¼ 0, 0.2, 0.4, 0.6, 0.8, and 1) [28], the

thermal decomposition of the sampleswas observed under 4%

H2. The onset temperature of decomposition decreased from

675 �C to 375 �C with increasing Co content of Ba0.5Sr0.5Fe1-

xCoxO3-d. Considering the decomposition temperature of

SrFeGa0.25O3-d (900 �C), SrFeGa0.25O3-d membrane prepared in

this study might be more favorable than Ba0.5Sr0.5Fe1-xCoxO3-

d to be operated under reducing atmosphere.

b

Fig. 4 e In-situ XRD patterns of SrFeGa0.25O3-d at

temperatures from room temperature to 1000 �C in (a) He

and (b) 8% H2 (balanced with He).

Fig. 6 e Temperature dependence of theoxygenpermeation

flux of SrFeGa0.25O3-d in He/air and 8% H2/air conditions.

Data on SrFeCo0.5O3-d [20], La0.3Sr0.7Fe1-xGaxO3-d [16] and

La0.3Sr0.7Fe1-xAlxO3-d [30] are presented for comparison.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 5 ( 2 0 1 0 ) 7 5 1 2e7 5 1 87516

3.3. Electrical conductivity

The electrical conductivity of SrFeGa0.25O3-d was investigated

under the conditions similar to those of the high-temperature

XRD experiment. The oxygen partial pressures in air, He, and

4% H2 atmospheres were 0.2, 3.2 � 10�4, and 4.2 � 10�18 atm,

respectively. The results on the total electrical conductivity as

a function of atmosphere and temperature are plotted in

Fig. 5. The total conductivity of SrFeGa0.25O3-d increases with

increasing oxygen partial pressure. In the air, the electrical

conductivity decreased as the temperature increased. Since

SrFeGa0.25O3-d is a p-type conductor [6] that conducts elec-

tricity through electron holes, the reduction of material while

heating consumes the concentration of the major carrier,

electronic holes ðO�O þ 2h�/1=2 O2ðgÞ þ V��

O Þ, thereby lowering

the electrical conductivity. Such pseudo-metallic behavior at

high temperatures is a typical electrical property of strontium

ferrites which are either undoped or doped with Al and Ti

[11,14,17].

In the He atmosphere, the electrical conductivity

decreased as the temperature was increased to 700 �C, andthen increased with further increases in temperature. The

temperature dependence of electrical conductivity from 600 to

700 �C was almost identical to the changes in the crystal

structure observed in Fig. 4(a). The XRD pattern measured in

He indicated that the cubic perovskite turns into ortho-

rhombic brownmillerite between 600 and 700 �C where elec-

trical conductivity decreased. On the other hand, electrical

conductivity increased above 700 �C, where the transition

from the orthorhombic brownmillerite to the cubic brown-

millerite might be occurred upon heating.

The electrical conductivity became even reduced (w0.1 S/

cm)with the introduction of 4%hydrogen, which seemed to be

strongly dependent on the phase transition observed in Fig. 4

(b). In order to understand the electrical properties in the 4%

Fig. 5 e Temperature dependence of the electrical

conductivity of SrFeGa0.25O3-d in air, He, and 4% H2

(balanced with Ar). Dashed line represents the electronic

conductivity of Sr2(Fe1-xGax)2O5 (x [ 0.2) at 10L16 atm [29].

H2 condition, it would be helpful to compare the results with

the electrical conductivity of Sr2(Fe1-xGax)2O5 (x ¼ 0, 0.1, 0.2) of

a brownmillerite structure in low oxygen pressures (dashed

line in Fig. 5) [29]. The electrical conductivity values of SrFe-

Ga0.25O3-d sample were in good agreement with literature data

at temperatures from 750 to 900 �C. It has been reported that

a structural transition from orthorhombic brownmillerite to

more disordered cubic brownmillerite raise the electrical

conductivity. Further increase of temperature, however, made

the specimen decompose and thus electrically resistive. As

shown in Fig. 4(b), the specimen decomposed into Sr3Fe2O6,

Fe, Sr3Ga2O6, and Sr10Ga6O19 above 900 �C, which was coinci-

dent with the temperature where the electrical conductivity

was started to decrease.

Fig. 7 e The oxygen permeation flux of SrFeGa0.25O3-d in 8%

H2/Air during 100 h.

Fig. 8 e SEM micrographs of the surface and cross section of SrFeGa0.25O3-d membrane exposed to (a) air and (b) 8% H2.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 5 ( 2 0 1 0 ) 7 5 1 2e7 5 1 8 7517

3.4. Oxygen permeation flux

Fig. 6 shows the oxygen permeation fluxes of the SrFeGa0.25O3-d

membrane (thickness w1.0 mm) that measured during cooling

from 1000 to 850 �C under He/air and 8% H2/air conditions. The

oxygen permeation flux in He/air and 8% H2/air was 0.12 and

0.42 ml cm�2 min�1 at 900 �C, respectively. The oxygen

permeation flux in SrFeGa0.25O3-d membrane is inferior to

those in SrFeCo0.5O3-d and La0.3Sr0.7Fe1-xGaxO3-d, but is similar

to that in La0.3Sr0.7Fe1-xAlxO3-d under He/Air condition. As the

flux through a membrane is proportional to the chemical

potential gradient ðjO2f vlnPO2=vxÞ, the oxygen flux through

the membrane under H2/air condition would be higher than

that under He/air. The oxygen permeation flux dropped below

900 �C under 8% H2/air, which might be caused by the dis-

ordereorder transition from cubic to orthorhombic brown-

millerite structure of the SrFeGa0.25O3-d membrane. Although

the oxygen permeation flux of the SrFeGa0.25O3-d membrane in

He/air is much less than that of Ba0.5Sr0.5Fe1-xCoxO3-d

(w1.0 ml cm�2 min�1 at 900 �C) [31], it is noticeable that the

material showed considerable permeation flux under reducing

atmosphere.

In order to identify the effect of phase transition on the

microstructure of the membrane, a stability test was con-

ducted at 950 �C for 100 h in 8% H2/air. The oxygen flux

decreased within 10 h at initial stage. Despite the possible

progress of decomposition of SrFeGa0.25O3-d in hydrogen as

shown in Fig. 4(b), the average oxygen flux slightly increased

as time progressed (Fig. 7). After the stability test for 100 h, the

surfaces exposed to air and hydrogen were observed by

scanning electron microscopy as shown in Fig. 8. The porous

morphology shown in Fig. 8(b) was observed from the surface

exposed to the 8% H2 while the surface on the feed side (air)

was kept clearly dense after the operation (Fig. 8(a)). The

porous surface indicates that the surface on the permeate side

(8% H2) was decomposed to Sr3Fe2O6, metallic Fe, Sr3Ga2O6,

and Sr10Ga6O19 that were detected from the high-temperature

XRD. The porous structure on the permeate side might be the

reason of unexpected increase in oxygen permeation flux

shown in Fig. 7. The effective thickness of the membrane

would be reduced as the decomposition progressed as well as

the catalytic reduction of oxygen would be enhanced by the

increased surface area on the permeate side.

4. Conclusions

The phase transitions of SrFeGa0.25O3-d were investigated by

using high-temperature XRD in oxidizing and reducing

atmospheres. Cubic perovskite structure was stable in air

condition, regardless of any temperature variation. However,

the cubic perovsktie structure changed to orthorhombic

brownmillerite inHe condition.Another phase transformation

from orthorhombic brownmillerite to cubic brownmillerite

was also observed at 1000 �C. These phase transitions

affected the electrical conductivity of SrFeGa0.25O3-d.

Particularly in 8%H2 condition, the SrFeGa0.25O3-d decomposed

into other phases, which significantly reduced electrical

conductivity under hydrogen condition. The onset tempera-

ture of the phase decomposition of perovskite-related SrFe-

Ga0.25O3-d was relatively higher than reported values for the

perovskite Ba0.5Sr0.5Fe1-xCoxO3-d. The temperature

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 5 ( 2 0 1 0 ) 7 5 1 2e7 5 1 87518

dependence of oxygen permeation flux of SrFeGa0.25O3-

d membrane, measured in 8% H2/air condition, reflected the

phase transition. The disordereorder transition from cubic to

orthorhombic brownmillerite below 900 �C increased the

activation energy of oxygen transport. After the oxygen

permeation stability test (100 h) under the hydrogen condition,

a porous morphology on the permeate side of SrFeGa0.25O3-

d membrane was observed due to the decomposition under

reducing atmosphere.

Acknowledgements

This work was supported by the Korea Ministry of Knowledge

Economy.

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