Effects of Sintering on the Properties of La0.6Sr0.4Co0.2Fe0.8O3-δ Perovskite HollowFiber Membranes

Xiaoyao Tan,*,†,‡ Zhigang Wang,†,‡ and Kang Li§

School of EnVironmental and Chemical Engineering, Tianjin Polytechnic UniVersity, Tianjin 300160, China,School of Chemical Engineering, Shandong UniVersity of Technology, Zibo 255049, China, and Department ofChemical Engineering and Technology, Imperial College London, South Kensington,London SW7 2AZ, United Kingdom

La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) hollow fiber membrane precursors were prepared by spinning a startingsuspension containing 68.75 wt % LSCF powders, 6.25 wt % polyethersulfone (PESf), and 25.0 wt % N-methyl-2-pyrrolidinone (NMP) at room temperature using deionized water and tape water as the internal and externalcoagulants, respectively. High temperature sintering was carried out in a range of 1200-1500 °C to study theinfluences of the sintering process on the properties of the LSCF hollow fiber membranes including themicrostructure, crystalline phase, mechanical strength, as well as the oxygen permeability. Mechanical strengthof the LSCF hollow fibers increased with increasing sintering temperature and reached a maximum of 115MPa at 1500 °C sintering temperature. To obtain gastight LSCF hollow fiber membranes, the sinteringtemperature must be higher than 1250 °C, and the sintering time must be longer than 2 h. However, higherthan 1350 °C sintering temperature would facilitate the formation of sulfate impurity phases, resulting innoticeable reduction of oxygen permeation flux. The optimum sintering temperature should be around 1300°C, and the sintering time should be within the range of 2-4 h to obtain the gastight and high flux LSCFhollow fiber membranes.

1. Introduction

Since Teraoka et al.1 first reported the SrCo0.8Fe0.2O3-δoxygen permeable membrane, the mixed ionic and electronicconducting oxides have attracted much interest due to theirpotential applications in air separation, oxy-fuel combustion,and partial oxidation of hydrocarbons into value-addedproducts.1-5 Most research was focused on the developmentof new perovskite materials with higher oxygen permeabilityand better stability.5,6 However, for the practical applications, themixed conducting membrane must also have high mechanicalstrength, which is closely related to the sintering process, so as toassemble membrane modules. Among the various types of mixedconducting membranes, La1-xSrxCo1-yFeyO3-δ (LSCF) perovskiteis one of the most important oxygen selective membrane materialsbecause of its outstanding mechanical and chemical stability,although it exhibits a relatively low oxygen permeability.5-8

It is well-known that the microstructure of the membranesplays a predominant role in their transport properties. Forceramic membranes, the microstructure depends not only onthe preparation method but also on the sintering procedures.Several groups worldwide have investigated the effects ofmicrostructure on the oxygen permeation properties of mixedconducting membranes.9-17 They have obtained very differentconclusions for different membranes or even for the samecompositions. For example, for the LaCoO3-δ,9 La0.3Sr0.7-CoO3-δ,10 Ba0.5Sr0.5Fe0.8Zn0.2O3-δ,11 La0.6Sr0.4Co0.2Fe0.8O3-δ,12

and Ba0.5Sr0.5Co0.8Fe0.2O3-δ13 membranes, the increase of grain

size by increasing sintering temperature leads to an enhancedpermeation, but for the SrFe0.2Co0.8O3-δ,14 La0.5Sr0.5FeO3-δ,15

La0.1Sr0.9Co0.9Fe0.1O3-δ,16 and La0.6Sr0.4Fe0.9Ga0.1O3-δ17 mem-

branes, the oxygen permeation flux decreased noticeably with

increasing grain size induced by the increase of sinteringtemperature. This indicated whether the grain boundaries actas high diffusivity paths or barriers for oxygen transport dependson the type of crystalline solids that adjoin the interfaces.Therefore, the influence of sintering on the permeation propertiesof mixed conducting membranes is complicated and cannot begeneralized. It depends not only on the chemical nature ofmembrane materials and microstructures but also on the rate-controlling step of the oxygen permeation process (surfaceexchange kinetics or bulk diffusion). For instance, in the surfaceexchange-controlled processes, an increase in oxygen permeationis expected with decreasing grain size because the oxygenexchange coefficient increases significantly when the averagegrain size on the membrane surface decreases.11,18

In recent years, the combined phase inversion/sinteringtechnique has been extensively applied to fabricate ceramichollow fiber membranes.19-23 The hollow fiber configurationexhibits many advantages over the planar or tubular membranesin particular such as high surface area/volume ratio and facilehigh-temperature sealing. More importantly, because the mem-brane cross section of the hollow fiber membranes preparedthrough the phase inversion process is asymmetric (i.e., a thinseparating layer integrated with a substrate), the resistance topermeation is therefore low. As a result, the hollow fibermembranes have more potential than other configurations tomeet commercial targets in air separation units. As comparedto the relatively homogeneous ceramics manufactured using thenormal pressing or casting method, the sintering process of theceramic hollow fiber membranes prepared by a phase inversionmethod is more complex because of their special asymmetricstructure.24,25 However, up to now, all of the studies on theinfluence of sintering on the microstructure and the permeationproperties of mixed conducting membranes were carried outon the basis of the disk membranes prepared by the pressingmethod.12 In this work, we presented a detailed investigationof the influence of sintering on the microstructure and the

* To whom correspondence should be addressed. Tel.: 86-533-2786292. Fax: 86-533-2786292. E-mail: cestanxy@yahoo.com.cn.

† Tianjin Polytechnic University.‡ Shandong University of Technology.§ Imperial College London.

Ind. Eng. Chem. Res. 2010, 49, 2895–2901 2895

10.1021/ie901403u 2010 American Chemical SocietyPublished on Web 02/18/2010

properties of the phase inversion-induced LSCF hollow fibermembranes. Such a study is helpful to identify the optimumsintering conditions of the production of LSCF hollow fibermembranes for oxygen separation by the phase inversion/sintering technique.

2. Experimental Section

2.1. Materials. Sr(NO3)2 (AR), La(NO3)3 · 6H2O (AR),Co(NO3)3 ·6H2O (>99%), and Fe(NO3)3 ·9H2O (AR) werepurchased from Kermel Chem Inc., Tianjin, China, and wereused as the metallic precursors for the preparation ofLa0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) powders. Citric acid (>99%,Ajax) and ethylene glycol (AR, Longjili, Tianjin) were used asthe complexing agents. Nitric acid and ammonium hydroxidewere used to adjust the pH of the starting solution. Polyether-sulfone (PESf) [(Radel A-300), Ameco Performance, USA] andN-methyl-2-pyrrolidone (NMP) [AR grade, >99.8%, KermelChem Inc., Tianjin, China] were used to prepare the spinningsuspension.

2.2. Preparation of LSCF Powders and Hollow FiberMembranes. LSCF powders were prepared through a sol-gel-combustion process.26 Stoichiometric amounts of Sr(NO3)2,La(NO3)3 ·6H2O, Co(NO3)3 ·6H2O, and Fe(NO3)3 ·9H2O weredissolved in distilled water. Citric acid and ethylene glycol inquantities of 3 times the desired LSCF product were then addedunder magnetic stirring until they were dissolved completely.The pH of the mixture was adjusted to be 3-4 using nitric acidand ammonium hydroxide to avoid precipitation. Subsequentlythe solution was stirred at 70 °C on a hot plate for 5-10 h toform a transparent brown sol. Further heating was conductedunder continuous stirring until a viscous gel was formed. Asthe temperature was increased to around 300 °C, autocombustiontook place to form a fluffy black powder (LSCF powderprecursor). Under an air flow, the powder precursor was calcinedat 800 °C for 3 h to remove the residual carbon and to form thedesired structure.

Prior to the preparation of spinning suspensions, the resultantpowders were ball-milled for 48 h, followed by sieving througha sieve of 200-mesh or 24 µm sieve-pore diameter. LSCF hollowfiber membranes were prepared by the phase inversion andsintering technique at room temperature.27 The spinning suspen-sion consisted of 68.75 wt % LSCF powders, 6.25 wt % PESf,and 25.0 wt % NMP. A spinneret with the orifice diameter/inner diameter of 3.0/1.2 mm was applied to form the hollowfiber precursors. Deionized water and tap water were used asthe internal and external coagulants, respectively. The hollowfiber precursors were preserved in a water bath for 48 h tocomplete solidification.

Sintering was conducted in a high temperature furnace afterthe hollow fiber precursors were dry-treated at ambient tem-perature. The sintering temperature range studied in this workwas 1200-1500 °C within which the hollow fibers can besintered into its dense form. The heating rate was of 1-3 °Cmin-1 applied during the entire sintering process.

2.3. Characterization. The gas-tightness of the sinteredhollow fiber membranes was examined through a gas permeationmeasurement that was described elsewhere.28 Nitrogen was usedas the test gas. The densities of the hollow fibers were measuredby the Archimedes method in water.

Morphology and microstructures of the hollow fiber mem-branes were observed with scanning electron microscopy (SEM)(FEI Sirion200, The Netherlands). Gold sputter coating wasperformed on the samples under vacuum before the measure-ments. Crystalline phases of the membranes were determined

by X-ray diffraction (BRUKER D8 Advance, Germany) usingCu-KR radiation (λ ) 0.15404 nm). The fibers were groundinto fine powders prior to the XRD measurements. Continuousscan mode was used to collect 2θ data from 20° to 80° with a0.02° sampling pitch and a 2° min-1 scan rate. The X-ray tubevoltage and current were set at 40 kV and 30 mA, respectively.

The mechanical strength of the hollow fibers was measuredon a three-point bending instrument (Instron model 5544) witha crosshead speed of 0.5 mm min-1. Hollow fiber samples werefixed on the sample holder at a distance of 32 mm. The bendingstrength, σF, was calculated from the following equation:

where F is the measured force at which fracture takes place; L,D, and d are the length (32 mm), the outer diameter, and theinner diameter of the fiber sample, respectively. The values ofouter diameter (D) and inner diameter (d) were obtained fromthe SEM graphs. Three or four fiber samples were taken foreach sintering temperature to measure mechanical strength. Thedifference between the measurements was within 11%, and theaveraged value was used to present the mechanical strength ofthe hollow fiber corresponding to the sintering temperature.

Oxygen permeation properties of the LSCF membranes weremeasured using a single fiber permeation cell with detailsdescribed elsewhere.29 Only those fibers confirmed to be gastightat room temperature in advance were used for the permeationmeasurement. Air was fed in the shell side and helium passedthrough the fiber lumen to collect the permeate gas. Thecomposition of the permeate gas was measured online using agas chromatograph (Agilent 6890N) fitted with a 5 Å molecularsieve column (Φ3 mm × 3 m) and a TCD detector. The overalloxygen permeation flux of the hollow fiber was calculated by:

or

where Am is the effective membrane area for oxygen permeation,Am ) πDmL, in which Dm is the average logarithmic diameterof the hollow fiber, Dm ) (D - d)/ln(D/d); Ft and FHe are thevolumetric flow rates of the effluent stream and of the sweepgas, respectively; and yO2 is the oxygen fraction in permeatestream. All of the values of oxygen permeation fluxes or othergas flow rates in this study were calculated at the conditions ofthe standard temperature and pressure (STP). The resultsobtained are average values of three measurements, and theirdifferences are less than 5%.

3. Results and Discussion

To acquire the exact knowledge of relationships betweensintering, microstructure, and the properties of LSCF hollowfiber membranes, all of the hollow fibers discussed in this workwere made from the same dope composition. In addition,because the gastight hollow fiber membranes can only beobtained by sintering at higher than 1250 °C, the sinteringtemperatures studied in this study ranged within 1200-1500°C.

σF ) 8FLD

π(D4 - d4)(1)

JO2)

FtyO2

Am

JO2) 1

Am·

FHeyO2

1 - yO2

(2)

2896 Ind. Eng. Chem. Res., Vol. 49, No. 6, 2010

3.1. Microstructure. The overall microstructure of thegastight LSCF hollow fiber membranes that were sintered at1300 °C for 4 h can be shown in Figure 1. As can be seen, theresultant hollow fiber membrane has an asymmetric structureformed during the phase inversion process, which is attributedto the rapid precipitation occurring at the outer-layer resultingin thin and dense skin layer and to the slow precipitation givingthe porous sublayer structure. As compared to the membranesprepared by the extrusion or pressing method, the phaseinversion-derived hollow fiber membranes possess a largenumber of macro voids inside the membrane wall. Therefore,the sintering temperature to achieve gas-tightness for the hollowfiber membranes would be much higher than those derived byother methods. For example, to obtain gastight hollow fibermembranes, the sintering temperature must be higher than 1250°C, but for the pressing-derived disk membranes, the sinteringtemperature to achieve gas-tightness could be as low as 1000°C.12 Although there are some pores still present on both theinner and the outer surfaces, as can be seen in Figure 1c and d,they did not affect the gas-tightness property of the membrane.This implies the gas-tightness of the hollow fibers was mainlyachieved due to the central dense layer of the fiber wall markedby the rectangle shape in Figure 1b.

Figure 2 displays the surface morphology of the LSCF hollowfiber membranes sintered at the temperatures between 1200 and1500 °C for duration of 4 h. As can be seen from Figure 2a,the membrane surface calcined at 1200 °C is composed ofgranular ceramic particles of 0.28-0.91 µm that have beenpartially calcined. Smaller particles have disappeared, and thedominated grain size (>90%) is within 0.41-0.74 µm. However,the membrane surface at this stage is still very porous. The gaspermeation test also showed that such hollow fibers were notgastight. When the sintering temperature was increased to 1250°C (Figure 2b), obvious coalescence of granular ceramicparticles takes place, and the number of pores on the membranesurfaces was reduced noticeably. Although there were still somepores present on the membrane surface, the gas permeation testindicated that these hollow fibers were gastight. This indicatedthat the middle dense layer takes more effect on the gas-tightnessof the membranes. Further increasing the sintering temperature

Figure 1. SEM photographs of the LSCF hollow fiber membranes sintered at 1300 °C for 4 h: (a) cross section; (b) fiber wall; (c) outer surface; and (d) innersurface.

Figure 2. Surface morphology of the LSCF hollow fiber membranes sinteredat (a) 1200 °C; (b) 1250 °C; (c) 1275 °C; (d) 1300 °C; (e) 1350 °C; (f)1400 °C; (g) 1450 °C; and (h) 1500 °C for 4 h.

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to 1275 °C led to the increase of grain size to 0.51-2.15 µmwith the majority ranging within 0.63-1.72 µm (Figure 2c).Some large pores on the membrane surfaces were preservedeven if the sintering temperature was increased to 1300 °C, asshown in Figure 2d. Under this temperature the membranesurfaces became smooth with distinct grain boundaries, indicat-ing noticeable fusion has taken place. The grain size wasincreased to 0.57-2.67 µm with most of the grains within therange of 0.83-2.13 µm. When the sintering temperature wasincreased to 1350 °C (Figure 2e), all of the pores on themembrane surfaces have disappeared, and the grain sizeincreased to 0.96-4.26 µm with the majority at 1.40-3.01 µm.Interestingly, a closing pore was observed on the membranesurface, as marked by the red circle in Figure 2e. With thesintering temperature being further increased to 1400, 1450, and1500 °C (Figure 2f-h), the grain size was developed remarkablyto 1.62-6.53, 3.71-15.38, and 4.57-25.67 µm with themajority at 2.9-5.1, 7.0-13.0, and 13.0-20.0 µm, respectively.Figure 3 plots the average grain size, which was calculated fromthe average grain area, as a function of sintering temperaturein the form of Arrhenius equation. As the sintering temperaturewas increased from 1200 to 1500 °C, the average grain sizeincreased from 0.22 to 8.51 µm. The activation energy for thegrain growth obtained from the slope of the line is 270.2 kJmol-1.

Figure 4 shows the apparent density and shrinkage of thehollow fibers as a function of sintering temperature. As can beseen, both the apparent density and the shrinkage of the hollowfibers changed little within the whole sintering temperature rangeexcept at 1200 °C. This implies that the macrostructure of thehollow fibers would not be changed noticeably with the increaseof sintering temperature after the hollow fiber was sintered intogastight.

Figure 5 shows the SEM pictures of surface morphology ofthe LSCF hollow fiber membranes sintered at 1300 °C fordifferent dwelling times. Noteworthy is that the membraneswould not be gastight if the sintering time was less than 1.0 h.It can be seen from Figure 5a that the membrane surface has

been sintered with isolated pores even if the sintering time wasonly 2 h. As the sintering time was prolonged, the grains wereenlarged, but the pores on the membrane surfaces would notdisappear completely (Figure 5b-d). As compared to thesintering temperature, the sintering time has less influence onthe microstructure of the LSCF hollow fiber membranes.

3.2. Crystalline Structure. Figure 6 depicts the XRDpatterns of the LSCF hollow fiber membranes sintered underair for 4 h at different temperatures. It can be seen that thehollow fiber samples sintered at temperatures lower than 1350°C possess the cubic perovskite crystalline structure (indicatedby “p”), and no intermediate phases were identified. This impliesthat no phase transition occurred from 1200 to 1350 °C duringthe sintering process. However, after the sintering temperaturewas increased to 1400 °C, some additional peaks (indicated by“)”) to the perovskite phase appeared on the XRD patterns.

Figure 3. Average grain size area of the LSCF hollow fiber membranes asa function of sintering temperature (sintering time ) 4 h).

Figure 4. Effect of sintering temperature on the density, shrinkage, andmechanical strength of the LSCF hollow fiber membranes (sintering time) 4 h).

Figure 5. SEM photographs of the LSCF hollow fiber membrane surfacessintered at 1300 °C for (a) 2 h; (b) 4 h; (c) 6 h; and (d) 8 h.

Figure 6. XRD patterns of the LSCF hollow fiber membranes sintered at(a) 1200 °C; (b) 1250 °C; (c) 1300 °C; (d) 1350 °C; (e) 1400 °C; (f) 1450°C; and (g) 1500 °C for 4 h (p, perovskite; ), SrSO4).

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This indicated that there are some new phases, one of whichwas identified to be SrSO4 (PDF reference code 73-0529), thathave formed at higher temperatures. Moreover, the fact that theintensity of the additional peaks increased with sinteringtemperature indicated that the amount of the impurity phasesincreased with sintering temperature. The formation of strontiumsulfate phase may be attributed to the interaction of the minoramount of SO2 with the surface of the perovskite membrane.30,31

In addition to the trace amount of SO2 presented in atmosphericair, the origin of sulfur to form strontium sulfate phase mightalso come from the decomposition of polyethersulfone, whichwas used as the polymer binder in spinning the fiber precursors.At lower temperatures (<1300 °C), the formation rate and thusthe concentration of sulfur dioxide due to the decomposition ofpolymers are low. Consequently, the amount of strontium sulfateat low temperatures is too little to be detectable by EDS. Assintering temperature is increased, the formation rate of sulfurdioxide will be improved and the concentration of SO2 will beincreased, facilitating the formation of the stable strontiumsulfate phase. Obviously, the higher is the sintering temperature,the more the amount of strontium sulfate can be produced. Itsuggests that the LSCF hollow fiber membranes cannot besintered at too high temperatures to retain the pure perovskitephase. Figure 7 shows the XRD patterns of the LSCF hollowfiber membranes sintered at 1300 °C for 2, 4, 6, and 8 h,respectively. As can be seen, all of the hollow fiber membranesamples retained the pure perovskite structure. It implies thatthe formation of impurity phases is mainly due to the highsintering temperature other than the prolonged sintering time.

3.3. Mechanical Strength. The mechanical strength ofperovskite hollow fiber membranes is a very important param-eter because they have to be assembled into membrane modulesin practical applications. As is shown in Figure 8, the mechanicalstrength of the LSCF hollow fibers increased with increasingthe sintering temperature. At a lower sintering temperature, thatis, 1200 °C, the mechanical strength was only 87.7 MPa. Whenthe sintering temperature was increased to 1250 °C, themechanical strength was increased to 100.5 MPa. However, withfurther increasing the sintering temperature to 1500 °C, themechanical strength was only reached to 115.5 MPa. Themechanical strength of perovskite membrane depends not onlyon the inherent properties of the membrane material, but alsoon both the macrostructure (i.e., the enclosed pores) and themicrostructure (i.e., grain size) of the hollow fibers. In general,the higher sintering temperature can facilitate the densificationof perovskite, and hence the grain size in membrane increasedexponentially with temperature as shown in Figure 2. It usually

favors increasing the mechanical strength of the hollow fibers.However, the mechanical strength of the hollow fiber is mainlycontrolled by the binding force between the grain boundariesbut not by the grain bulk. Furthermore, the formation the newsulfate phases, as indicated in Figures 6 and 7, especially onthe grain boundaries would also reduce the mechanical strengthof the hollow fiber membranes. Therefore, the mechanicalstrength did not increase linearly with the grain size butapproached a maximum value.

3.4. Oxygen Permeation. As described above, the sinteringhad significant influences on the microstructure and crystallinestructure of the resultant hollow fiber membranes, and wouldfinally influence their oxygen permeation properties. Figure 9depicts the oxygen permeation flux through the LSCF hollowfiber membranes sintered at different temperatures for 4 h, wherethe helium sweep rate and the air feed flow rate were 147.3and 200 mL min-1, respectively. It can be seen that themembranes sintered at lower temperatures exhibited higheroxygen permeation fluxes. For example, the permeation flux at1000 °C through the 1250 °C-sintered membrane was 2.24 mLcm-2 min-1, but it was only 0.22 mL cm-2 min-1 through the1500 °C-sintered membrane under the same permeation condi-tions. This phenomenon was quite different form the observationusing the disk LSCF membranes prepared by pressing, whichshowed the permeation flux increased with increasing sinteringtemperature.12 The reason for this result may be explained asfollows. It is well-known that the oxygen permeation throughthe perovskite membranes from the high oxygen partial pressureside to the low oxygen partial pressure side undergoes in seriessurface exchange reactions and membrane bulk diffusion. Whichstep predominates the permeation process depends on themembrane surface properties and the membrane thickness. Inthe diffusion-controlled process, the oxygen permeation rate isdetermined by the diffusion rate of ions. It is generally

Figure 7. XRD patterns of the LSCF hollow fiber membranes sintered at1300 °C for (a) 2 h; (b) 4 h; (c) 6 h; and (d) 8 h.

Figure 8. Mechanical strength of the LSCF hollow fiber membranes as afunction of sintering temperature (sintering time: 4 h).

Figure 9. Temperature dependence of oxygen permeation flux through thedifferent LSCF hollow fiber membranes sintered at 1250-1500 °C for 4 h(He flow rate ) 147.3 mL(STP) min-1, air feed flow rate ) 200 mL(STP)min-1).

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considered that the grain boundary in LSCF perovskite hindersthe transfer of oxygen ions.12 Therefore, the membranes withlarger grains usually exhibit higher permeation flux because theyhave less grain boundaries. However, in the surface exchange-controlled process, an increase in oxygen permeation may beexpected with decreasing grain size because the oxygenexchange coefficient increases significantly when the averagegrain size on membrane surface decreases.11,18 For the LSCFhollow fiber membranes, the permeation is primarily controlledby the surface exchange mechanism.32,33 Consequently, theLSCF hollow fiber membranes sintered at lower temperaturesexhibited higher oxygen flux, as shown in Figure 9. On the otherhand, there were impurity phases such as SrSO4 formed duringthe sintering, and the amount of such impurities increased withincreasing sintering temperature, as can be seen from the XRDpatterns. These impurity phases would yield additional resistanceto oxygen permeation, leading to the observed decrease of theoxygen flux.31 Therefore, the hollow fiber membrane sinteredat 1500 °C still exhibited much lower oxygen fluxes than thosesintered at lower temperatures.

Figure 10 shows the oxygen permeation fluxes of the LSCFhollow fiber membranes sintered at 1300 °C for differentdwelling times. Although there were no impurity phases formedunder this sintering temperature (from XRD patterns) and thegrains increased with increasing the sintering time, the mem-branes sintered for longer time exhibited lower oxygen fluxes.This further suggested that the oxygen permeation through theLSCF hollow fiber membranes was predominantly controlledby the surface exchange kinetics at operating temperatures usedfor oxygen permeation.

4. Conclusions

Sintering is a key process in the preparation of gastight LSCFperovskite hollow fiber membranes by the phase inversiontechnique. The sintering temperature higher than 1250 °C andthe sintering time longer than 2 h are essential to achievinggastight properties for the LSCF hollow fiber membranes.However, the sintering temperature higher than 1350 °C wouldlead to the formation of impurity phases, which may reducethe oxygen permeation flux of the resultant membranes notice-ably. Mechanical strength of the LSCF hollow fibers increasedwith increasing the sintering temperature and could reach amaximum of 115.5 MPa at 1500 °C sintering temperature. Toobtain the gastight and high flux LSCF hollow fiber membranes,the optimum sintering temperature should be around 1300 °C,and the sintering time should be within the range of 2-4 h,

which also depends on the composition of the perovskite hollowfiber precursors.

Acknowledgment

We gratefully acknowledge the research fundings providedby the National High Technology Research and DevelopmentProgram of China (no. 2006AA03Z464), the National NaturalScience Foundation of China (no. 20676073), and EPSRC inthe United Kingdom (EP/E032079/1).

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Figure 10. Effect of sintering time on the permeation performance of theLSCF hollow fiber membranes (sintering temperature ) 1300 °C, He flowrate ) 147.3 mL(STP) min-1, air feed flow rate ) 200 mL(STP) min-1).

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ReceiVed for reView September 08, 2009ReVised manuscript receiVed February 1, 2010

Accepted February 10, 2010

IE901403U

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