Oxygen Transport Properties and Stability of Mixed-Conducting ZrO

weight relaxation technique. A simple transport equation correlating oxygen flux to the oxygen diffusion coefficient was deduced. Model calculations, ...
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Ind. Eng. Chem. Res. 2002, 41, 4273-4280

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Oxygen Transport Properties and Stability of Mixed-Conducting ZrO2-Promoted SrCo0.4Fe0.6O3-δ Oxides Li Yang, Xuehong Gu, Liang Tan, Wanqin Jin,† Lixiong Zhang, and Nanping Xu* Membrane Science & Technology Research Center, Nanjing University of Technology, Nanjing 210009, P. R. China

The oxygen nonstoichiometry, transport properties, and structure stability of ZrO2-promoted SrCo0.4Fe0.6O3-δ (SCFZ) were investigated. The nonstoichiometry of SCFZ increased with increasing temperatures and decreasing oxygen partial pressures. The results of X-ray diffraction and oxygen desorption experiments showed that the addition of ZrO2 in SrCo0.4Fe0.6O3-δ (SCF) stabilized the phase structure under low oxygen partial pressure. The chemical diffusion coefficient of oxygen in SCFZ in a temperature range of 928-1178 K was obtained using the weight relaxation technique. A simple transport equation correlating oxygen flux to the oxygen diffusion coefficient was deduced. Model calculations, based on the transport equation in conjunction with data of oxygen permeation and oxygen nonstoichiometry, were performed. The rates of oxygen permeation fluxes measured at various temperatures and time showed that the long-term operation stability of the SCF membrane with the addition of ZrO2 was greatly improved. The oxygen permeation flux, however, was slightly reduced. 1. Introduction Mixed-conducting oxides have received considerable attention as possible candidate materials for application in oxygen separation membranes, solid oxide fuel cell electrodes, and catalytic membrane reactors.1-9 For practical applications, these materials were required to have high oxygen permeability and sustainable structure stability to withstand harsh conditions. Perovskite-like oxides are among the most promising groups of mixed conductors because of their high oxygen ionic and electronic conductivities at elevated temperatures.7,10 Teraoka et al.11 were the first to report very high oxygen fluxes through cobalt-rich compositions La1-xSrxCo1-yFeyO3-δ, which were 2-4 orders of magnitude higher than those of stabilized zirconia at the same temperature. Since then, increasing studies have been carried out on the synthesis and properties of these perovskite-type ceramics.4,6,10 Although La1-xSrxCo1-yFeyO3-δ oxides with high Sr and Co contents showed sufficiently high oxygen fluxes, the practical application of these membranes faced difficulties because of their poor structure stability in reducing atmosphere.12,13 One of the typical examples was SrCo0.8Fe0.2O3-δ, which had a high oxygen flux of 3.1 cm3 (STP) min-1 cm-2 (temperature, 1123 K; membrane thickness, 1 mm)11 but had both an abrupt orderdisorder phase structure transition and a susceptibility to reducing atmosphere.14-16 Using SrCo0.8Fe0.2O3-δ tubular membranes for partial oxidation of methane to syngas (POM), Pei et al.13 observed that the fracture of the tubular membrane occurred shortly after the initiation of the reaction. They concluded that the fracture was due to the lattice mismatch resulting from the phase change caused by the oxygen gradient across the membrane. Therefore, it is important to develop a * Corresponding author. Tel.: +86-25-331-9580. Fax: +8625-330-0345. E-mail: [email protected]. † Present address: Institut fu¨r Physikalische Chemie, Universita¨t zu Ko¨ln, Luxemburger Strasse 116, 50939 Ko¨ln, Germany. E-mail: [email protected].

suitable material possessing sustainable structure stability in reducing atmosphere as well as high oxygen flux. In our previous work,17,18 a novel perovskite-related ZrO2-promoted SrCo0.4Fe0.6O3-δ (SCFZ) has been reported to exhibit high oxygen permeability as well as structure stability in reducing atmosphere, which makes this oxide a promising candidate for membrane materials. Apart from oxygen permeation measurements and some information on the thermochemical stability of SCFZ membranes, there was almost a dearth of detailed study on the defect chemistry, relevant transport parameters, and problems associated with the stability of the oxide. In this paper, we attempted to address these deficiencies by reporting the oxygen nonstoichiometry and transport properties, including chemical diffusion and oxygen permeation. Results pertinent to some of the additional membrane performance criteria mentioned above, such as structure and phase stability, were also obtained. 2. Principles 2.1. Relaxation Technique. Oxygen transport in SCFZ membranes involves surface oxygen exchange and the bulk diffusion rate. Our previous study18 showed that oxygen transport through SCFZ membranes was primarily controlled by bulk diffusion when the membrane thickness was larger than 1.2 mm. The oxygen transport kinetics can be studied by a number of methods including isotopic exchange analysis,19 the potentiostatic step method,20 and the weight relaxation technique.21 The weight relaxation technique was used in the present work because it was a simple, quick, and accurate method to determine the oxygen diffusion coefficient. During the relaxation experiment, we abruptly changed the oxygen partial pressure (PO2) in the surrounding atmosphere and then monitored the weight change of the specimen as a function of time by using thermogravimetric analysis (TGA). The weight change

10.1021/ie020132w CCC: $22.00 © 2002 American Chemical Society Published on Web 07/25/2002

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corresponds to a net transport of oxygen into the membrane (weight increase) or out of the membrane (weight loss). Under the assumption of a constant diffusion coefficient in the one-dimensional case, the diffusion equation can be written as

where Vm (mol cm-3) represents the molar volume of the solid. The chemical potential of oxygen, µ0, can be written as

µ0 )

2

∂C dC )D ∂t dX2

(1)

where D is the oxygen diffusion coefficient (cm2 s-1) and C the concentration of the diffusing substance. The transient behavior in the reequilibration process was analyzed by fitting the time relaxation data to the solution of eq 1 with appropriate boundary conditions. For a thin slab specimen, the weight change during the relaxation process can be expressed by22

m(t) - m(0) m(e) - m(0)

∞

8

∑

)1-

n)0(2n

+ 1)2π2

[

exp -

]

(2)

where l (cm) is the half-thickness of the specimen and m(0), m(t), and m(e) are the weights of the specimen at the starting time, at time t, and at infinite time, respectively. 2.2. Correlation of Oxygen Flux to the Oxygen Diffusion Coefficient. In the following analysis, oxygen ions and electrons or corresponding lattice defects are considered to be mobile in a mixed-conducting oxide. When such a material is placed in an oxygen potential gradient, oxygen ions will move through the membrane from the high to the low oxygen partial pressure side. The electrons will migrate in the opposite direction. The driving force for the chemical diffusion is the gradient of the chemical potential of oxygen. When the flux is governed by bulk oxygen diffusion, it is generally described by Wagener’s equation:23

JO2 ) -

RT 4FL 2 2

∫lnlnP′P′′

O2

O2

telσi d ln PO2

(3)

where σi (S cm-1) is the ionic conductivity, te the electronic transference number, F (C mol-1) the Faraday constant, T (K) the absolute temperature, and L (mm) the thickness of the membrane and P′O2 and P′′O2 (Pa) are the partial pressures of oxygen on upstream and downstream, respectively. In the derivation of this equation it has been assumed that the fluxes of the oxygen ions and electrons or holes are related to each other by the condition of charge neutrality and that local equilibrium of the defect reactions prevails. For mixed conductors in which the electronic conduction predominates, i.e., tel ) 1, eq 3 can be simplified as

JO2 ) -

RT 4FL 2 2

∫lnlnP′P′′

O2

O2

σi d ln PO2

(4)

The ionic conductivity σi can be expressed in terms of the chemical diffusion coefficient D and the chemical potential of oxygen, µ0, by24

σi ) -

4F2D ∂δ Vm ∂µ0

(6)

where 1 atm of oxygen is taken as a standard state. By substituting eqs 5 and 6 into eq 4, we can obtain the correlation of oxygen flux to the oxygen chemical diffusion coefficient as

JO2 )

∫lnlnP′P′′

D 2VmL

∂δ d ln PO2 ∂ ln PO2

O2

O2

(7)

∂δ/(∂ ln PO2) can be determined directly from the experiment by measuring the oxygen stoichiometry as a function of oxygen partial pressure. 3. Experimental Section

(2n + 1)2π2Dt 4l2

RT ln PO2 2

(5)

3.1. Sample Preparation. SCFZ powders were prepared by the conventional solid-state reaction at a high temperature. The required amounts of SrCO3 (99.9%), Co2O3 (99.9%), Fe2O3 (99.9%; The Second Chemical Industry of Shanghai), and monoclinic ZrO2 (Shenzhen Nanbo Structure Ceramics Co. Ltd.; ZrO2 of 9 wt % in SCFZ) were mixed and milled for 24 h followed by calcination in air at 1223 K for 4 h with heating and cooling rates of 2 K min-1. The calcined powders were pressed into disks of 16 mm diameter with an oil pressure of 200 MPa and then sintered in air at 1473 K for 5 h. The densities of the sintered membranes were determined by the Archimedes method. Only those membranes that had relative densities higher than 90% were used for oxygen diffusion and oxygen permeation measurements. Rectangular SCFZ specimens with dimensions of 9.86 mm × 9.82 mm × 2.26 mm were cut from the disks and employed for the oxygen diffusion measurements. To compare the difference in properties between SrCo0.4Fe0.6O3-δ (SCF) with and without addition of ZrO2, SCF oxide powders and membranes were also prepared with the same method. 3.2. Crystal Structure Characterization. The crystal structures of the powders and structural changes occurring in the samples under the reducing conditions were characterized by X-ray diffraction (XRD; Bruker D8 Advance). 3.3. TGA. TGA was taken on a WCT-2 thermogravimetric analyzer (Beijing Optical Instrument Factory, China). The oxygen contents in SCFZ and SCF were determined by a reduction experiment in a gas mixture of 8% H2 and 92% Ar with a flow rate of 100 mL min-1. The samples were first heated in a thermobalance to 373 K for 2 h to remove absorbed water and then reduced by heating to 1373 K at 10 K min-1 and held until a constant weight was obtained. The changes of oxygen contents with temperatures in SCFZ and SCF oxides were measured using the same apparatus by heating the samples to 1173 K at 10 K min-1 in oxygen and nitrogen atmospheres, respectively. Phase transition temperatures were determined by differential scanning calorimetry (DSC; Perkin-Elmer DTA 1700 differential thermal analyzer). Measurement was conducted in pure nitrogen with a flow rate of 100 mL min-1 at a heating rate of 20 K min-1.

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Figure 1. Schematic diagram of the experimental setup for oxygen diffusion measurement.

Oxygen desorption properties of SCFZ and SCF were also studied by TGA. Each sample was pretreated in an oxygen atmosphere at 1073 K for 2 h, then cooled to room temperature in the same atmosphere, and finally heated in a pure nitrogen stream to 1223 K at a rate of 10 K min-1. The weight loss corresponding to the oxygen desorption was monitored by the thermogravimetric analyzer. The apparent oxygen diffusion coefficient of the SCFZ membrane was measured by the weight relaxation method. Figure 1 shows the experimental setup. The weight of the sample was measured by the WCT-2 thermogravimetric analyzer and was recorded by a PC through data acquisition software. The sample was put on the sample pan and heated to a desired temperature under the flow of 60% O2 and 40% N2. After the weight of the sample reached a steady-state value, the atmosphere surrounding the sample was changed to nitrogen. Simultaneously, the weight changes vs time were observed. The flow rate of the gas was controlled at 100 mL min-1. 3.4. Oxygen Permeation Measurement. Oxygen permeation measurements were performed using a permeation apparatus reported previously.17 Disks were sealed between two gold rings, and the effective area for oxygen permeation was around 0.28 cm2. One side of the membrane was exposed to air (Ph ) 0.209 atm) at a flow rate of 200 mL min-1, while the other side was exposed to a lower PO2 that was controlled by regulating the He flow rate by mass flow controllers (model D07/ZM, Beijing Jianzhong Machine Factory, China). A gas chromatograph (GC; Shimabzu model GC7A) equipped with a 5A molecule sieve column was connected to the exit of the sweep side. The amount of oxygen passing through the membrane was calculated using the measured outlet flow rate and the oxygen content.

Figure 2. XRD patterns of as-synthesized powders: (a) SCF; (b) SCFZ. P: perovskite. S: SrZrO3.

4. Results and Discussion 4.1. XRD Results. XRD patterns of the as-synthesized SCFZ and SCF powders are shown in Figure 2. It was identified that SCF had a cubic perovskite structure and SCFZ consisted of mainly a perovskite-type phase and a trace of SrZrO3, which was attributed to a solidstate reaction between ZrO2 and perovskite at elevated temperatures. A more detailed study of structure change with the addition of ZrO2 and the role of zirconium will be published elsewhere. 4.2. Nonstoichiometry and Oxygen Desorption Properties. The variations in the stoichiometry (3 δ) of SCFZ and SCF in oxygen and nitrogen atmo-

Figure 3. Temperature dependence of stoichiometry in oxygen and nitrogen atmospheres: (a) SCF; (b) SCFZ.

spheres are shown in Figure 3. The samples had a constant weight at temperatures below 673 K but exhibited a weight loss above 673 K due to the loss of oxygen. At 1173 K, the oxygen content of SCFZ changed from 2.71 in pure oxygen to 2.61 in pure nitrogen, a

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Figure 4. Oxygen desorption curves of SCFZ and SCF.

difference of 0.1. For comparison, the corresponding difference in SCF was 0.14 at the same temperature, which is higher than that in SCFZ. Although such a difference in oxygen content was required for driving oxygen permeation, it could be a potential problem for the integrity of the membrane when the difference is large. In principle, the expansion of the unit cell was closely associated with the oxygen content for these oxygen-deficient materials,13,25 and as a result, the lattice constant on the low PO2 side of the membrane would be larger than that on the air side. This difference would create a sufficiently large strain, leading to the fracture of the membrane. Therefore, the above results suggested a higher structure stability of SCFZ than SCF. We point out that the structure failures due to the thermomechanical stresses would be sufficiently minimized by depositing a thin, dense, mixed-conducting oxide layer onto a porous support.26 Figure 4 shows the oxygen desorption profiles for the SCFZ and SCF samples measured by TGA. The TGA profiles illustrated that oxygen was released from the samples as they were heated in pure nitrogen. It could easily be discerned from the DTG plot, obtained from the TGA curves, that there were two main areas of oxygen loss on heating, with onset temperatures at about 670 and 1116 K, respectively. These two regions of oxygen loss were generally referred to as being from so-called R and β oxygen, respectively.27 In the case of Co-containing perovskite materials, the oxygen desorption responses were characterized by two peaks, namely, R and β oxygen. The R oxygen desorption was suggested to be associated with the oxygen vacancies introduced by the A site substitution and formally corresponded to the reduction of Co4+ and Fe4+ to Co3+ and Fe3+, whereas β desorption was the desorption of normal lattice oxide ions corresponding to the reduction of Co3+ to Co2+.27 Zhang et al.28 have investigated the oxygen absorption and desorption properties of La1-xSrxCo1-yFeyO3-δ solids; the oxygen desorption responses of SrCo0.4Fe0.6O3-δ were similar to the ones observed in this work on SCF. It could also be seen from Figure 4 that the amount of β oxygen desorption of SCFZ was smaller than that of SCF. Because the nature of the β desorption was really a measure of the ease with which the cobalt ion was reduced, it could be assumed that the stability of Co3+ ions against the reduction to Co2+ in SCFZ oxide was enhanced with the addition of ZrO2. Thus, SCFZ is expected to be more stable than SCF in a reducing atmosphere.

Figure 5. XRD patterns of annealed SCF samples: (a) after the O2 desorption experiment; (b) annealed at 973 K in N2 for 8 h; (c) annealed at 1073 K in N2 for 8 h; (d) annealed at 1173 K in N2 for 8 h.

4.3. Structure Stability. To study the phase behavior at high temperatures and under reducing atmospheres, XRD was applied to examine the samples of SCFZ and SCF that had been used in oxygen desorption experiments and annealed from various temperatures in pure nitrogen. The results are shown in Figures 5 and 6. Comparing with the XRD patterns of the fresh powders shown in Figure 2, we could see that the cubic phase had changed to an oxygen-vacancy-ordered structure for all SCF samples tested, whereas SCFZ exhibited remarkable structure stability in reducing atmospheres. However, XRD was inadequate to examine vacancy ordering if samples contain too many microdamains.14 To study the transformation of the samples to the vacancy-ordered phase in more detail, the annealed samples were used for DSC analysis. Figure 7 shows the DSC curves, which were obtained by heating samples in nitrogen from 473 to 1273 K. A deep endothermic peak was observed at 1073 K for SCF with a heat effect, ∆H ) 56 J g-1, which was caused by the oxygen-vacancy-ordered brownmillerite phase to disordered cubic phase transition.14,16 The order-disorder phase transition of SrCo0.8Fe0.2O3-δ oxides was reported previously to occur at 1063 and 1043 K by Kruidhof et al.14 and Liu et al.,16 respectively. Although XRD patterns showed an unchanged structure for SCFZ annealed in a nitrogen atmosphere (Figure 6), DSC measurements showed a small endothermic peak with a heat effect of 14 J g-1 upon heating to about 1110 K. A tentative explanation would be that a small amount of the low-temperature phase was produced in the SCFZ sample when annealed in low oxygen partial pressure. 4.4. Oxygen Diffusion in SCFZ. As discussed above, the structure of SCF could change in the oxygen-lean atmosphere. Because the weight change of a sample in the oxygen diffusion measurement was not only related

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Figure 8. Weight change of SCFZ as a function of the relaxation time at 1178 K after a step change of PO2.

Figure 6. XRD patterns of annealed SCFZ samples: (a) after the O2 desorption experiment; (b) annealed at 973 K in N2 for 8 h; (c) annealed at 1073 K in N2 for 8 h; (d) annealed at 1173 K in N2 for 8 h.

Figure 9. Experimental data along with fitting curves in the form of fractional weight change as a function of the relaxation time for the SCFZ membrane.

Figure 7. DSC curves of SCFZ and SCF annealed at 973 K in nitrogen.

to the oxygen transport but also associated with the phase change involved, the oxygen diffusion properties of only SCFZ membranes were studied in this work. Figure 8 shows a typical example of the weight relaxation curves of SCFZ at 1178 K as a function of time after switching the surrounding atmosphere to nitrogen. Relaxation data were analyzed by a leastsquares curve fit to eq 2 with the corresponding geometric parameters of the specimens. Experimental data and their fittings at 1178, 1028, 978, and 928 K in the form of fractional weight change as a function of time are shown in Figure 9. Agreement appeared to be good between the experimental data and the theoretical curves. The oxygen diffusion coefficients derived from the weight relaxation data are shown in Figure 10. A linear relationship with an activation energy of 37.0 kJ mol-1 was observed.

Figure 10. Temperature dependence of the chemical diffusion coefficient of the SCFZ membrane: (2) from the weight relaxation experiment; (b) derived from the transport equation.

Figure 11 shows the relationship between δ and ln PO2 of SCFZ measured at three oxygen partial pressures: 100% O2, 10% O2, and 1% O2 using O2/N2 mixtures. It could be seen that the nonstoichiometry of SCFZ increased with decreasing oxygen partial pressures. Within the T and PO2 ranges of experimental conditions, an almost linear relationship was observed between δ and ln PO2. Thus, the term of ∂δ/(∂ ln PO2) in eq 7 can be regarded as a constant. Using the transport

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Figure 11. Nonstoichiometry of SCFZ in the δ vs ln PO2 plot.

Figure 12. Temperature dependence of oxygen permeation fluxes through SCFZ (b) and SCF (2) membranes [ambient air/He (50 mL min-1)].

equation in conjunction with data of oxygen nonstoichiometry (Figure 11) and oxygen permeation discussed below (Figure 12), we obtained the oxygen diffusion coefficient, which is also shown in Figure 10. The determined activation energy for diffusion was 85.0 kJ mol-1. Examination of the chemical diffusion coefficient from the weight relaxation method and oxygen permeation experiments in the temperature range of this study showed that the two results of the diffusion coefficient were in agreement within a factor of 2. The differences in the diffusion coefficient from the two measurements could be explained in two ways. First, the difference may be associated with PO2 because chemical diffusion was the function of PO2.29 Second, buoyancy would also affect the weight relaxation experiment. During the relaxation experiment, when the atmosphere was switched to nitrogen, the measured weight value of the sample would be higher than its actual value because the buoyancy of the sample in a nitrogen atmosphere was larger than that in an oxygen atmosphere. This might result in a decrease in the calculated D value. Ma et al.21 reported that the activation energy was a function of PO2 and increased with a decrease in PO2 in the intermediate- and low-PO2 range. The differences in activation energies from the two measurements could be rationalized in terms of the effect of PO2. The PO2 values from permeation experiments represented an average value over the applied oxygen pressure gradient, whereas the values from the weight relaxation

Figure 13. Unsteady oxygen fluxes of SCFZ (b) and SCF (2) membranes at 1223 K [ambient air/He (50 mL min-1)].

method were obtained by switching atmospheres from 60% O2 and 40% N2 to 100% N2. 4.5. Oxygen Permeation Flux. The membrane was tested for oxygen permeation after the temperature dropped to 1223 K after being sealed at 1308 K for 4 h. Helium was introduced to the line, and one of the membrane sides was changed from high oxygen partial pressure to low oxygen partial pressure. An oxygen content gradient was then gradually established in the membrane. The unsteady permeation flux includes two parts: permeation flux due to diffusion, which increased with an increase of the oxygen gradient in the membrane until a steady value was reached, and a timedependent contribution due to the loss of lattice oxygen. Figure 13 shows unsteady oxygen fluxes of SCFZ and SCF membranes at 1223 K. The oxygen fluxes increased with time during the transient period, similar to the results of La0.6Sr0.4Co0.2Fe0.8O3-δ reported by Xu and Thomson,5 SrCo0.8Fe0.2O3-δ by Zhang et al.,30 and Ba0.5Sr0.5Co0.8Fe0.2O3-δ by Shao et al.31 The increase in oxygen fluxes during the transient period was due to the gradual facilitation and movement of an oxygendefect structure from the oxygen-lean side to the oxygenrich side of the membrane. As the oxygen-defect zone spread toward the oxygen-rich surface, the bulk oxygen permeation resistance gradually decreased with onstream time, until a steady-state oxygen-deficient gradient was established. It took about 10 h for the SCFZ sample to reach the steady state, shorter than SCF, which needed almost 20 h. This was due to the fact that less lattice oxygen loss was experienced in the SCFZ membrane, as demonstrated by the TGA analysis. The temperature dependence of the permeation fluxes through SCFZ and SCF membranes with a thickness of 1.78 mm is shown in Figure 12. The oxygen flux was slightly reduced with the addition of ZrO2 in SCF, while the activation energy of oxygen permeation was also reduced, especially at a lower temperature range. A change in the slope of oxygen flux versus reciprocal temperature, at about 1053 K, was observed for the SCF membrane. A similar change was observed by Kruidhof et al.14 for SrCo0.8M0.2O3-δ (with M ) Cr, Fe, and Cu) and by Qiu et al.15 for SrCo0.8Fe0.2O3-δ. It was related to the order-disorder transition of oxygen vacancies verified by the XRD (Figure 5) and DSC (Figure 7) measurements. In contrast to SCF, single activation energy for oxygen permeation was observed for the SCFZ membrane, which could be interpreted by the

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All of the results indicated that SFCZ oxide could be an attractive stable material for oxygen separation. Acknowledgment This work is supported by the National Natural Science Foundation of China (NNSFC; No. 20125618) and the Key Laboratory of Chemical Engineering and Technology of Jiangsu Province. Nomenclature

Figure 14. Long-term oxygen fluxes of SCFZ and SCF membranes.

structure stability characterized by XRD (Figure 6) of the samples annealed in the nitrogen atmosphere. Meanwhile, a useful membrane material, in addition to possessing sufficiently high oxygen permeability, should be stable for a long time in the appropriate oxygen partial pressure gradient. Figure 14 shows the long-term oxygen permeation fluxes of SCFZ at 1173 and 1023 K and of SCF at 1023 K. The permeation flux of SCFZ was very stable at 1173 K during an operation of 240 h. At 1023 K, a very slow decay in the oxygen flux occurred when SCFZ was operated over a period of 240 h, while the permeation flux declined significantly with time in the SCF membrane. Although the initial oxygen flux of SCF was almost 2 times higher than that of SCFZ, the flux of SCF was lower than that of SCFZ after the 240 h operation. Shao et al.31 and Kruidhof et al.14 have studied the long-term oxygen permeation of Ba0.5Sr0.5Co0.8Fe0.2O3-δ and La0.6Sr0.4CoO3-δ membranes at 1023 K, respectively; a substantial decline of fluxes was observed as the membranes were operated for 240 h. It is suggested that SCFZ would be a promising stable material for oxygen separation. 5. Conclusions (1) The oxygen nonstoichiometry of SCFZ increased with increasing temperatures and decreasing oxygen partial pressures. The difference of oxygen contents in oxygen and nitrogen atmospheres in SCFZ was less than that in SCF at high temperatures. The addition of ZrO2 in SCF enhanced the stability of Co3+ ions against the reduction to Co2+ and therefore stabilized the phase structure under low oxygen partial pressures. (2) XRD patterns of SCF annealed in nitrogen showed a change from the cubic phase to an oxygen-vacancyordered structure, while no phase transition was found for SCFZ. DSC experiments indicated that a small amount of the low-temperature phase also existed in the nitrogen-annealed samples of SCFZ. (3) At 1178 K, the obtained oxygen diffusion coefficient for SCFZ was 3.6 × 10-5 cm2 s-1. The diffusion coefficient derived from the transport equation and weight relaxation experiment was in agreement within a factor of 2. (4) Although the oxygen permeation flux decreased a little with the addition of ZrO2 in SCF, the long-term operation stability was greatly improved. SCFZ membranes exhibited a stable oxygen permeation flux of 4.04 × 10-7 mol cm-2 s-1 at 1173 K over a period of 240 h.

D ) oxygen chemical diffusion coefficient, cm2 s-1 F ) Faraday constant, C mol-1 JO2 ) oxygen flux through membrane, mol cm-2 s-1 l ) half-thickness of the membrane, mm L ) thickness of the membrane, mm m(0) ) weight of the specimen at the starting time, g m(t) ) weight of the specimen at time t, g m(e) ) weight of the specimen at infinite time, g P′O2 ) oxygen partial pressure at high-pressure side of membrane, Pa P′′O2 ) oxygen partial pressure at low-pressure side of membrane, Pa PO2 ) oxygen partial pressure, Pa tel ) the electronic transference number T ) temperature, K Vm ) molar volume of the solid, mol cm-3 Greek Letters δ ) oxygen nonstoichiometry σi ) ionic conductivity, S cm-1

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Received for review February 11, 2002 Revised manuscript received June 25, 2002 Accepted June 27, 2002 IE020132W