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Synthesis, conductivity and oxygen diffusivity of Sr2Fe3Ox
June 1998
Materials Letters 35 Ž1998. 303–308
Synthesis, conductivity and oxygen diffusivity of Sr2 Fe 3 O x B. Ma
a,)
, U. Balachandran a , J.P. Hodges b, J.D. Jorgensen b, D.J. Miller b, J.W. Richardson Jr. c a
Energy Technology DiÕision, Argonne National Laboratory, Argonne, IL 60439, USA Materials Science DiÕision, Argonne National Laboratory, Argonne, IL 60439, USA Intense Pulsed Neutron Source, Argonne National Laboratory, Argonne, IL 60439, USA b
c
Received 28 August 1997; revised 10 October 1997; accepted 7 November 1997
Abstract Sr2 Fe 3 O x samples were synthesized by solid-state reaction at 12008C in atmospheres with various oxygen partial pressures. X-ray and neutron powder diffraction experiments were conducted to determine the crystal structure. Electrical conductivity was measured by the four-probe method at elevated temperatures in atmospheres with various oxygen partial pressures. The oxygen chemical diffusion coefficient was determined by the conductivity relaxation method. Neutron powder diffraction showed that the Sr2 Fe 3 O x material has a layered structure with an orthorhombic unit cell with lattice ˚ b s 18.98 A, ˚ c s 5.585 A, ˚ and V s 1178.97 A˚ 3. The total conductivity of Sr2 Fe 3O x increases parameters a s 11.122 A, with increasing temperature and oxygen partial pressure; at 8008C in air, the conductivity is f 0.5 Vy1 cmy1. Oxygen chemical diffusion coefficient increases with increasing temperature. The coefficient at 9008C f 1.2 = 10y5 cm2 sy1, is more than one order of magnitude higher than that of the mixed-conducting Sr2 Fe 2 CoO x material. The activation energy for oxygen diffusion in Sr2 Fe 3 O x , derived from the oxygen chemical diffusion coefficient, is f 0.9 eV in air. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Mixed-conducting oxides; Sr2 Fe 3 O x ; Synthesis; Crystal structure; Electrical conductivity; Oxygen diffusivity
1. Introduction In recent years, mixed-conducting oxides, which contain both oxygen ionic and electronic charge carriers, have received increased attention because of their technological importance for high-temperature electrochemical applications w1–4x. Teraoka et al. showed that the perovskite oxides La 1y x Sr x Fe1yy Co yO 3y d exhibit mixed electronic–ionic ) Corresponding author. Tel.: q1-630-2529961; fax: q1-6302523604; e-mail: [email protected].
conductivity with appreciable oxygen permeability w2,3x. A non-perovskite Sr2 Fe 2 CoO x material was recently reported by Balachandran et al. w5,6x; this material exhibits mixed conductivities Želectronic and ionic. with unusually high oxygen permeability w7,8x. Its electronic and ionic conductivities are 10 and 7 S cmy1 , respectively, at 8008C in air. Oxygen ionic conductivity at 8008C is about two orders of magnitude higher than that of yttria stabilized zirconia. Interstitial oxygen ions and holes were found to be the dominant charge carriers in air w9x. Unlike most of the known mixed conductors, which generally
00167-577Xr98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 9 7 . 0 0 2 7 0 - X
304
B. Ma et al.r Materials Letters 35 (1998) 303–308
have much higher electronic conduction than ionic conduction, this material has comparable conductivities, with an ionic transference number of 0.4 obtained at 8008C in air. The combined electronic and oxygen ionic conductivities have made the Sr2 Fe 2 CoO x material attractive for use as an oxygen separation membrane. It has been shown to be a promising material for partial oxidation of methane to syngas ŽCO q H 2 . at an efficiency of ) 98% w5x. Oxygen can transport from the oxygen-rich side to the oxygen-lean side of the membrane at elevated temperature under the driving force of electrochemical potential and without the need for external electrical circuitry. In order to understand the crystal chemistry of the Sr2 Fe 2 CoO x system, many efforts have been made to determine its crystal structure w10–13x by X-ray and neutron diffraction experiments. The matrix phase of the Sr2 Fe 2 CoO x material was found to have the isostructure of Sr4 Fe 6 O 12q d w14x. To understand the relationship between transport properties and crystal structure, single-phase Sr2 Fe 3 O x samples were prepared, and their transport properties were determined at elevated temperature in atmospheres with various oxygen partial pressures Ž pO 2 .. The oxygen chemical diffusion coefficient was determined from conductivity relaxation experiments.
2. Experimental 2.1. Sample preparation Sr2 Fe 3 O x powder was made by a solid-state reaction method with appropriate amounts of SrCO 3 and Fe 2 O 3 ; mixing and grinding was done in isopropanol with zirconia media for 15 h. After drying, the mixture was calcined in air at 8508C for 16 h, with intermittent grinding. After the final calcination, the powder was ground with an agate mortar and pestle to an average size of f 5 m m, and the resulting powder was pressed with a 120 MPa load into rectangular bars 45 mm long, 5 mm wide, and f 5 mm thick. The bars were covered by powder of the same composition to eliminate contamination and then sintered at f 12008C in air for 5 h. Subsequently, some of the bars were cut with a diamond saw into thin strips of f 1 = 2 = 15 mm3 for four-
probe conductivity experiments. Other bars were used for neutron diffraction experiments. 2.2. Sample characterization The theoretical density of Sr2 Fe 3 O x was measured on the powder with an AccuPyc 1330 pycnometer and confirmed by X-ray powder diffraction. The bulk density of the samples used in our experiments was f 95% of the theoretical value. Scanning electron microscopy ŽSEM. observations were conducted with a JEOL LSM-5400 scanning microscope at an accelerating voltage of 20 keV. Neutron powder diffraction data were collected for the bar specimen at the Argonne’s Intense Pulsed Neutron Source ŽIPNS. and analyzed by the Rietveld structure refinement technique w15,16x. 2.3. ConductiÕity measurement The resistance of Sr2 Fe 3 O x was measured by the four-probe method w17x with an HP 4192A LF impedance analyzer at 23 Hz. Four platinum wires of 0.2 mm diameter were wound around the specimen to serve as probes for the total conductivity. At this low frequency, the resistance measured with an impedance analyzer was the same as that obtained with the conventional four-probe DC method. The conductivity of the specimen was calculated from its measured resistance R Žin V . and geometry parameters of the specimen by
ss
d vv RPS
Ž 1.
where d vv is the separation of the potential probes Žin cm., and S is the cross-sectional area of the specimen Žin cm2 ., respectively. The experimental arrangement used for conductivity measurements was reported in Ref. w7x. Atmospheres with various pO 2 were achieved by using dry premixed gases. 2.4. Determination of oxygen diffusion coefficient The oxygen chemical diffusion coefficient D can be derived from time-dependent conductivity relax-
B. Ma et al.r Materials Letters 35 (1998) 303–308
ation data and Eq. Ž2. shown below w9x Ds Ž t. D s Ž `.
`
s1y
Ý ns0
the same oxygen diffusion coefficient was obtained for samples with different thicknesses, thus indicating that the surface reaction coefficient Žsometimes also called the surface exchange coefficient. of the Sr2 Fe 3 O x was sufficiently high.
8 2 Ž 2p q 1 . p 2
=exp y
2 Ž 2p q 1 . p 2 Dt
4l2
305
Ž 2.
where l is the half-thickness of the specimen Žin cm., t is relaxation time Žin s., D s Ž t . s s Ž t . y s Ž0. and D s Ž`. s s Ž`. y s Ž0. with s Ž0., s Ž t . and s Ž`. represent the conductivities of the specimen at the starting time, at time t, and at time infinite, respectively. Time to flush the system with a new gas atmosphere was several seconds, which is sufficiently short to affect the initial diffusion kinetics. To investigate the surface effects on the oxygen chemical diffusion coefficient, comparison experiments were conducted on a specimen with a different thickness. If diffusion of oxygen is limited by the surface reaction, then the bulk diffusion coefficient obtained would be different for specimens with different thicknesses. However, this was not the case in our comparison experiment. At a constant temperature,
3. Results and discussion 3.1. Micrographic and crystal structures An SEM image of the polished surface of Sr2 Fe 3 O x is shown in Fig. 1. The SEM observations indicated that the Sr2 Fe 3 O x sample has a dense homogeneous structure and an absence of cracks or large pores. Energy-dispersive X-ray ŽEDX. elemental analysis revealed phase integrity. The room-temperature X-ray powder diffraction ŽXRD. pattern is plotted in Fig. 2. The obtained XRD pattern is consistent with that reported by Fjellvag et al. w13x. At room temperature, the Sr2 Fe 3 O x sample was found to have a layered structure with an orthorhombic unit cell. The cell parame˚ b s 18.98 A, ˚ c s 5.585 A, ˚ ters are: a s 11.122 A, 3 ˚ and V s 1178.97 A , as determined by a Rietveld
Fig. 1. SEM morphology of polished surface of sintered Sr2 Fe 3 O x sample.
306
B. Ma et al.r Materials Letters 35 (1998) 303–308
Fig. 2. Room-temperature X-ray diffraction pattern of sintered Sr2 Fe 3 O x .
analysis. Neutron diffraction experiments were also conducted on the Sr2 Fe 3 O x sample at Argonne’s IPNS. The detailed structure of the Sr2 Fe 3 O x , solved from the neutron diffraction data, will be reported elsewhere w12x. If all the oxygen sublattices were occupied, the x value would be 6.5. 3.2. ConductiÕity Fig. 3 shows the temperature-dependent conductivities of sintered Sr2 Fe 3 O x sample in three atmospheres of various pO 2 . Conductivity increases with increasing temperature in all three atmospheres. At the same temperature, conductivity increases with increasing pO 2 . At 8008C in air, the total conductivity of Sr2 Fe 3 O x is f 0.5 S cmy1 . Arrhenius plots
Fig. 3. Temperature-dependent conductivity of sintered Sr2 Fe 3 O x .
Fig. 4. Arrhenius plot of conductivity of Sr2 Fe 3 O x .
for the total conductivities of Sr2 Fe 3 O x in the three atmospheres are shown in Fig. 4. Activation energies were calculated from the slopes and indicated in the plot. Activation energy increases with decreasing pO 2 and has lower values than those of the Sr2 Fe 2 CoO x system w8x. 3.3. Oxygen diffusion coefficients The conductivity relaxation data of Sr2 Fe 3 O x are shown in Fig. 5 as a function of relaxation time after the surrounding atmosphere is suddenly switched. Conductivity relaxation data were analyzed by leastsquares fitting to Eq. Ž2. with the corresponding
Fig. 5. Time-dependent conductivity transient behavior of Sr2 Fe 3 O x at 7508C after switching surrounding atmosphere from air to 2% oxygen and then back to air.
B. Ma et al.r Materials Letters 35 (1998) 303–308
Fig. 6. Conductivity relaxation data along with its fitting curve at 7508C after switching surrounding atmosphere from air to pure oxygen.
geometric parameters of the specimens. Fig. 6 shows the conductivity relaxation data, along with the fitting curve for Sr2 Fe 3 O x at 7508C after switching from air to pure oxygen. The data appear to be in good agreement. The oxygen chemical diffusion coefficient derived from the conductivity relaxation data for Sr2 Fe 3 O x are shown in Fig. 7 as a function of temperature. The oxygen chemical diffusion coefficient increases with increasing temperature, as expected. At 9008C in air, the oxygen chemical diffusion coefficient is f 1.2 = 10y5 cm2 sy1 , more than one order of magnitude higher than that of the Sr2 Fe 2 CoO x system w9x.
Fig. 7. Temperature-dependent oxygen diffusion coefficient of Sr2 Fe 3 O x .
307
Fig. 8. Arrhenius plot of oxygen diffusion coefficient of Sr2 Fe 3 O x .
An Arrhenius plot of the oxygen chemical diffusion coefficient of Sr2 Fe 3 O x is shown in Fig. 8. Activation energy of Sr2 Fe 3 O x derived from oxygen diffusion coefficient f 0.9 eV in air, within our experimental temperature region of 750 to 9508C.
4. Conclusions Sr2 Fe 3 O x sample was successfully synthesized by a solid-state reaction method with SrCO 3 and Fe 2 O 3 as starting chemicals. X-ray diffraction showed that our polycrystalline Sr2 Fe 3 O x material has the same crystal structure as that reported by Fjellvag et al. w13x. At room temperature, Sr2 Fe 3 O x has a layered structure with an orthorhombic unit cell. The cell ˚ b s 18.98 A, ˚ cs parameters are: a s 11.122 A, 3 ˚ ˚ 5.585 A, and V s 1178.97 A , as determined by Rietveld analysis. Scanning electron microscopy showed that the air-sintered Sr2 Fe 3 O x sample has a dense homogeneous structure without cracks or large pores. Energy-dispersive X-ray elemental analysis confirmed the phase integrity. Conductivity of Sr2 Fe 3 O x increases with increasing temperature and pO 2 . At 8008C in air, the total conductivity of Sr2 Fe 3 O x is f 0.5 S cmy1 . The activation energy of Sr2 Fe 3 O x , as derived from conductivity data, is low and decreases with increasing pO 2 . The oxygen chemical diffusion coefficient obtained from conductivity relaxation data increases
308
B. Ma et al.r Materials Letters 35 (1998) 303–308
with increasing temperature. At 9008C in air, the oxygen chemical diffusion coefficient of Sr2 Fe 3 O x is f 1.2 = 10y5 cm2 sy1 , which is more than one order of magnitude higher than that of Sr2 Fe 2 CoO x . The activation energy of Sr2 Fe 3 O x derived from the oxygen chemical diffusion coefficient is f 0.9 eV in air.
Acknowledgements We are grateful to Mr. B.L. Fisher of Argonne National Laboratory and to Mr. J.L. Newman of Loyola University for their assistance in the conductivity relaxation experiments. Work at Argonne National Laboratory is supported by the U.S. Department of Energy, under Contract W-31-109-Eng-38.
References w1x H.L. Tuller, Solid State Ionics 94 Ž1997. 63. w2x Y. Teraoka, H.M. Zhang, S. Furnkawa, N. Yamozoe, Chem. Lett. 1985 Ž1985. 1743. w3x Y. Teraoka, T. Nobunaga, N. Yamazoe, Chem. Lett. 1988 Ž1988. 503.
w4x U. Balachandran, S.L. Morissette, J.T. Dusek, R.L. Mieville, R.B. Poeppel, M.S. Kleefisch, S. Pei, T.P. Kobylinski, C.A. Udovich, Proc. Coal Liquefaction and Gas Conversion Contractor Review Conf., S. Rogers et al. ŽEds.., vol. 1, U.S. Dept. of Energy, Pittsburgh Energy Technology Center, 1993, pp. 138–160. w5x U. Balachandran, M.S. Kleefisch, T.P. Kobylinski, S.L. Morissette, S. Pei, U.S. Patent No. 5,580,497, Dec. 3, 1996. w6x U. Balachandran, T.J. Dusek, S.M. Sweeney, R.B. Poeppel, R.L. Mieville, P.S. Maiya, M.S. Kleefisch, S. Pei, T.P. Kobylinski, C.A. Udovich, A.C. Bose, Am. Ceram. Soc. Bull. 74 Ž1995. 71. w7x B. Ma, J.-H. Park, C.U. Segre, U. Balachandran, Mater. Res. Soc. Symp. Proc. 393 Ž1995. 49. w8x B. Ma, U. Balachandran, J.-H. Park, C.U. Segre, J. Electrochem. Soc. 143 Ž1996. 1736. w9x B. Ma, U. Balachandran, J.-H. Park, C.U. Segre, Solid State Ionics 83 Ž1996. 65. w10x B. Ma, U. Balachandran, Solid State Ionics Ž1997., in press. w11x S. Guggilla, A. Manthiram, J. Electrochem. Soc. 144 Ž1997. L120. w12x J.P. Hodges et al., to be published. w13x H. Fjellvag, B.C. Hauback, R. Bredesen, J. Mater. Chem. Ž1997., to be published. w14x A. Yoshiasa, K. Ueno, F. Kanamaru, H. Horiuchi, Mater. Res. Bull. 21 Ž1986. 175. w15x H.M. Rietveld, J. Appl. Cryst. 2 Ž1969. 65. w16x R.B. Von Dreele, J.D. Jorgensen, C.G. Windsor, J. Appl. Cryst. 15 Ž1982. 581. w17x H.C. Montgomery, J. Appl. Phys. 42 Ž1971. 2971.
Materials Letters 35 Ž1998. 303–308
Synthesis, conductivity and oxygen diffusivity of Sr2 Fe 3 O x B. Ma
a,)
, U. Balachandran a , J.P. Hodges b, J.D. Jorgensen b, D.J. Miller b, J.W. Richardson Jr. c a
Energy Technology DiÕision, Argonne National Laboratory, Argonne, IL 60439, USA Materials Science DiÕision, Argonne National Laboratory, Argonne, IL 60439, USA Intense Pulsed Neutron Source, Argonne National Laboratory, Argonne, IL 60439, USA b
c
Received 28 August 1997; revised 10 October 1997; accepted 7 November 1997
Abstract Sr2 Fe 3 O x samples were synthesized by solid-state reaction at 12008C in atmospheres with various oxygen partial pressures. X-ray and neutron powder diffraction experiments were conducted to determine the crystal structure. Electrical conductivity was measured by the four-probe method at elevated temperatures in atmospheres with various oxygen partial pressures. The oxygen chemical diffusion coefficient was determined by the conductivity relaxation method. Neutron powder diffraction showed that the Sr2 Fe 3 O x material has a layered structure with an orthorhombic unit cell with lattice ˚ b s 18.98 A, ˚ c s 5.585 A, ˚ and V s 1178.97 A˚ 3. The total conductivity of Sr2 Fe 3O x increases parameters a s 11.122 A, with increasing temperature and oxygen partial pressure; at 8008C in air, the conductivity is f 0.5 Vy1 cmy1. Oxygen chemical diffusion coefficient increases with increasing temperature. The coefficient at 9008C f 1.2 = 10y5 cm2 sy1, is more than one order of magnitude higher than that of the mixed-conducting Sr2 Fe 2 CoO x material. The activation energy for oxygen diffusion in Sr2 Fe 3 O x , derived from the oxygen chemical diffusion coefficient, is f 0.9 eV in air. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Mixed-conducting oxides; Sr2 Fe 3 O x ; Synthesis; Crystal structure; Electrical conductivity; Oxygen diffusivity
1. Introduction In recent years, mixed-conducting oxides, which contain both oxygen ionic and electronic charge carriers, have received increased attention because of their technological importance for high-temperature electrochemical applications w1–4x. Teraoka et al. showed that the perovskite oxides La 1y x Sr x Fe1yy Co yO 3y d exhibit mixed electronic–ionic ) Corresponding author. Tel.: q1-630-2529961; fax: q1-6302523604; e-mail: [email protected].
conductivity with appreciable oxygen permeability w2,3x. A non-perovskite Sr2 Fe 2 CoO x material was recently reported by Balachandran et al. w5,6x; this material exhibits mixed conductivities Želectronic and ionic. with unusually high oxygen permeability w7,8x. Its electronic and ionic conductivities are 10 and 7 S cmy1 , respectively, at 8008C in air. Oxygen ionic conductivity at 8008C is about two orders of magnitude higher than that of yttria stabilized zirconia. Interstitial oxygen ions and holes were found to be the dominant charge carriers in air w9x. Unlike most of the known mixed conductors, which generally
00167-577Xr98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 9 7 . 0 0 2 7 0 - X
304
B. Ma et al.r Materials Letters 35 (1998) 303–308
have much higher electronic conduction than ionic conduction, this material has comparable conductivities, with an ionic transference number of 0.4 obtained at 8008C in air. The combined electronic and oxygen ionic conductivities have made the Sr2 Fe 2 CoO x material attractive for use as an oxygen separation membrane. It has been shown to be a promising material for partial oxidation of methane to syngas ŽCO q H 2 . at an efficiency of ) 98% w5x. Oxygen can transport from the oxygen-rich side to the oxygen-lean side of the membrane at elevated temperature under the driving force of electrochemical potential and without the need for external electrical circuitry. In order to understand the crystal chemistry of the Sr2 Fe 2 CoO x system, many efforts have been made to determine its crystal structure w10–13x by X-ray and neutron diffraction experiments. The matrix phase of the Sr2 Fe 2 CoO x material was found to have the isostructure of Sr4 Fe 6 O 12q d w14x. To understand the relationship between transport properties and crystal structure, single-phase Sr2 Fe 3 O x samples were prepared, and their transport properties were determined at elevated temperature in atmospheres with various oxygen partial pressures Ž pO 2 .. The oxygen chemical diffusion coefficient was determined from conductivity relaxation experiments.
2. Experimental 2.1. Sample preparation Sr2 Fe 3 O x powder was made by a solid-state reaction method with appropriate amounts of SrCO 3 and Fe 2 O 3 ; mixing and grinding was done in isopropanol with zirconia media for 15 h. After drying, the mixture was calcined in air at 8508C for 16 h, with intermittent grinding. After the final calcination, the powder was ground with an agate mortar and pestle to an average size of f 5 m m, and the resulting powder was pressed with a 120 MPa load into rectangular bars 45 mm long, 5 mm wide, and f 5 mm thick. The bars were covered by powder of the same composition to eliminate contamination and then sintered at f 12008C in air for 5 h. Subsequently, some of the bars were cut with a diamond saw into thin strips of f 1 = 2 = 15 mm3 for four-
probe conductivity experiments. Other bars were used for neutron diffraction experiments. 2.2. Sample characterization The theoretical density of Sr2 Fe 3 O x was measured on the powder with an AccuPyc 1330 pycnometer and confirmed by X-ray powder diffraction. The bulk density of the samples used in our experiments was f 95% of the theoretical value. Scanning electron microscopy ŽSEM. observations were conducted with a JEOL LSM-5400 scanning microscope at an accelerating voltage of 20 keV. Neutron powder diffraction data were collected for the bar specimen at the Argonne’s Intense Pulsed Neutron Source ŽIPNS. and analyzed by the Rietveld structure refinement technique w15,16x. 2.3. ConductiÕity measurement The resistance of Sr2 Fe 3 O x was measured by the four-probe method w17x with an HP 4192A LF impedance analyzer at 23 Hz. Four platinum wires of 0.2 mm diameter were wound around the specimen to serve as probes for the total conductivity. At this low frequency, the resistance measured with an impedance analyzer was the same as that obtained with the conventional four-probe DC method. The conductivity of the specimen was calculated from its measured resistance R Žin V . and geometry parameters of the specimen by
ss
d vv RPS
Ž 1.
where d vv is the separation of the potential probes Žin cm., and S is the cross-sectional area of the specimen Žin cm2 ., respectively. The experimental arrangement used for conductivity measurements was reported in Ref. w7x. Atmospheres with various pO 2 were achieved by using dry premixed gases. 2.4. Determination of oxygen diffusion coefficient The oxygen chemical diffusion coefficient D can be derived from time-dependent conductivity relax-
B. Ma et al.r Materials Letters 35 (1998) 303–308
ation data and Eq. Ž2. shown below w9x Ds Ž t. D s Ž `.
`
s1y
Ý ns0
the same oxygen diffusion coefficient was obtained for samples with different thicknesses, thus indicating that the surface reaction coefficient Žsometimes also called the surface exchange coefficient. of the Sr2 Fe 3 O x was sufficiently high.
8 2 Ž 2p q 1 . p 2
=exp y
2 Ž 2p q 1 . p 2 Dt
4l2
305
Ž 2.
where l is the half-thickness of the specimen Žin cm., t is relaxation time Žin s., D s Ž t . s s Ž t . y s Ž0. and D s Ž`. s s Ž`. y s Ž0. with s Ž0., s Ž t . and s Ž`. represent the conductivities of the specimen at the starting time, at time t, and at time infinite, respectively. Time to flush the system with a new gas atmosphere was several seconds, which is sufficiently short to affect the initial diffusion kinetics. To investigate the surface effects on the oxygen chemical diffusion coefficient, comparison experiments were conducted on a specimen with a different thickness. If diffusion of oxygen is limited by the surface reaction, then the bulk diffusion coefficient obtained would be different for specimens with different thicknesses. However, this was not the case in our comparison experiment. At a constant temperature,
3. Results and discussion 3.1. Micrographic and crystal structures An SEM image of the polished surface of Sr2 Fe 3 O x is shown in Fig. 1. The SEM observations indicated that the Sr2 Fe 3 O x sample has a dense homogeneous structure and an absence of cracks or large pores. Energy-dispersive X-ray ŽEDX. elemental analysis revealed phase integrity. The room-temperature X-ray powder diffraction ŽXRD. pattern is plotted in Fig. 2. The obtained XRD pattern is consistent with that reported by Fjellvag et al. w13x. At room temperature, the Sr2 Fe 3 O x sample was found to have a layered structure with an orthorhombic unit cell. The cell parame˚ b s 18.98 A, ˚ c s 5.585 A, ˚ ters are: a s 11.122 A, 3 ˚ and V s 1178.97 A , as determined by a Rietveld
Fig. 1. SEM morphology of polished surface of sintered Sr2 Fe 3 O x sample.
306
B. Ma et al.r Materials Letters 35 (1998) 303–308
Fig. 2. Room-temperature X-ray diffraction pattern of sintered Sr2 Fe 3 O x .
analysis. Neutron diffraction experiments were also conducted on the Sr2 Fe 3 O x sample at Argonne’s IPNS. The detailed structure of the Sr2 Fe 3 O x , solved from the neutron diffraction data, will be reported elsewhere w12x. If all the oxygen sublattices were occupied, the x value would be 6.5. 3.2. ConductiÕity Fig. 3 shows the temperature-dependent conductivities of sintered Sr2 Fe 3 O x sample in three atmospheres of various pO 2 . Conductivity increases with increasing temperature in all three atmospheres. At the same temperature, conductivity increases with increasing pO 2 . At 8008C in air, the total conductivity of Sr2 Fe 3 O x is f 0.5 S cmy1 . Arrhenius plots
Fig. 3. Temperature-dependent conductivity of sintered Sr2 Fe 3 O x .
Fig. 4. Arrhenius plot of conductivity of Sr2 Fe 3 O x .
for the total conductivities of Sr2 Fe 3 O x in the three atmospheres are shown in Fig. 4. Activation energies were calculated from the slopes and indicated in the plot. Activation energy increases with decreasing pO 2 and has lower values than those of the Sr2 Fe 2 CoO x system w8x. 3.3. Oxygen diffusion coefficients The conductivity relaxation data of Sr2 Fe 3 O x are shown in Fig. 5 as a function of relaxation time after the surrounding atmosphere is suddenly switched. Conductivity relaxation data were analyzed by leastsquares fitting to Eq. Ž2. with the corresponding
Fig. 5. Time-dependent conductivity transient behavior of Sr2 Fe 3 O x at 7508C after switching surrounding atmosphere from air to 2% oxygen and then back to air.
B. Ma et al.r Materials Letters 35 (1998) 303–308
Fig. 6. Conductivity relaxation data along with its fitting curve at 7508C after switching surrounding atmosphere from air to pure oxygen.
geometric parameters of the specimens. Fig. 6 shows the conductivity relaxation data, along with the fitting curve for Sr2 Fe 3 O x at 7508C after switching from air to pure oxygen. The data appear to be in good agreement. The oxygen chemical diffusion coefficient derived from the conductivity relaxation data for Sr2 Fe 3 O x are shown in Fig. 7 as a function of temperature. The oxygen chemical diffusion coefficient increases with increasing temperature, as expected. At 9008C in air, the oxygen chemical diffusion coefficient is f 1.2 = 10y5 cm2 sy1 , more than one order of magnitude higher than that of the Sr2 Fe 2 CoO x system w9x.
Fig. 7. Temperature-dependent oxygen diffusion coefficient of Sr2 Fe 3 O x .
307
Fig. 8. Arrhenius plot of oxygen diffusion coefficient of Sr2 Fe 3 O x .
An Arrhenius plot of the oxygen chemical diffusion coefficient of Sr2 Fe 3 O x is shown in Fig. 8. Activation energy of Sr2 Fe 3 O x derived from oxygen diffusion coefficient f 0.9 eV in air, within our experimental temperature region of 750 to 9508C.
4. Conclusions Sr2 Fe 3 O x sample was successfully synthesized by a solid-state reaction method with SrCO 3 and Fe 2 O 3 as starting chemicals. X-ray diffraction showed that our polycrystalline Sr2 Fe 3 O x material has the same crystal structure as that reported by Fjellvag et al. w13x. At room temperature, Sr2 Fe 3 O x has a layered structure with an orthorhombic unit cell. The cell ˚ b s 18.98 A, ˚ cs parameters are: a s 11.122 A, 3 ˚ ˚ 5.585 A, and V s 1178.97 A , as determined by Rietveld analysis. Scanning electron microscopy showed that the air-sintered Sr2 Fe 3 O x sample has a dense homogeneous structure without cracks or large pores. Energy-dispersive X-ray elemental analysis confirmed the phase integrity. Conductivity of Sr2 Fe 3 O x increases with increasing temperature and pO 2 . At 8008C in air, the total conductivity of Sr2 Fe 3 O x is f 0.5 S cmy1 . The activation energy of Sr2 Fe 3 O x , as derived from conductivity data, is low and decreases with increasing pO 2 . The oxygen chemical diffusion coefficient obtained from conductivity relaxation data increases
308
B. Ma et al.r Materials Letters 35 (1998) 303–308
with increasing temperature. At 9008C in air, the oxygen chemical diffusion coefficient of Sr2 Fe 3 O x is f 1.2 = 10y5 cm2 sy1 , which is more than one order of magnitude higher than that of Sr2 Fe 2 CoO x . The activation energy of Sr2 Fe 3 O x derived from the oxygen chemical diffusion coefficient is f 0.9 eV in air.
Acknowledgements We are grateful to Mr. B.L. Fisher of Argonne National Laboratory and to Mr. J.L. Newman of Loyola University for their assistance in the conductivity relaxation experiments. Work at Argonne National Laboratory is supported by the U.S. Department of Energy, under Contract W-31-109-Eng-38.
References w1x H.L. Tuller, Solid State Ionics 94 Ž1997. 63. w2x Y. Teraoka, H.M. Zhang, S. Furnkawa, N. Yamozoe, Chem. Lett. 1985 Ž1985. 1743. w3x Y. Teraoka, T. Nobunaga, N. Yamazoe, Chem. Lett. 1988 Ž1988. 503.
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