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Process of solid state reaction between doped ceria and zirconia
Solid State Ionics 135 (2000) 589–594 www.elsevier.com / locate / ssi
Process of solid state reaction between doped ceria and zirconia K. Eguchi*, N. Akasaka, H. Mitsuyasu, Y. Nonaka Department of Molecular and Material Science, Graduate School of Engineering Sciences, Kyushu University, Kasuga 816 -8580, Japan
Abstract Solid state reaction at an interface between doped zirconia and ceria has been analyzed by X-ray diffraction and EDX. The solid state reaction between Y-doped ceria and Y-stabilized zirconia occurred on heating at 13008C or higher temperatures to form a single cubic fluorite Y-doped ceria–zirconia phase. The reaction proceeded by preferential migration of the Ce component to the zirconia lattice. The reaction significantly lowered the electrical conductivity. The reaction between Sm-doped ceria and Y-stabilized zirconia, on the other hand, proceeded by migration of every component equally, i.e. preferential migration of a specific component was hardly observed. 2000 Elsevier Science B.V. All rights reserved. Keywords: Ceria; Zirconia; Solid state reaction; Fluorite; Electrical conductivity
1. Introduction Solid oxide electrolyte materials are used in oxygen sensors, solid oxide fuel cells (SOFC), and electrochemical oxygen pumps. As the electrolyte for SOFC, yttria-stabilized zirconia has been regarded as the most promising material with high conductivity and transference number of oxide ions which is operative at high temperatures around 10008C. Cation-doped ceria has been investigated as a substitute for stabilized zirconia for reduced temperature SOFC, though its reduction in hydrogen atmosphere and resultant electronic conduction is a problem for practical application. We have previously proposed to overcome this difficulty using a bi-phasic electrolyte consisting of cation-doped ceria and stabilized zir*Corresponding author. Tel.: 1 81-92-583-7526; fax: 1 81-92573-0342. E-mail address: [email protected] (K. Eguchi).
conia, in which reduction of ceria with a fuel can be avoided with the thin zirconia layer coated on the anode side of the electrolyte [1]. However, these two fluorite phases based on zirconia and ceria undergo solid state reaction at elevated temperatures. The reaction proceeded by dissolution of ceria into zirconia to form a new fluorite phase with a poor ionic conductivity [2,3]. Thus, it is important to avoid the interfacial reaction between these two fluorite phases for the electrochemical applications. In our previous work, the Ce component migrated into the zirconia lattice at the ceria / zirconia interface from TEM and XRD observation [4,5]. High temperature sintering for preparation and long-term SOFC operation causes cation diffusion and leads to deterioration of the cell performance. The purpose of the present study is to analyze the solid state reaction at the interface between stabilized zirconia and cation-doped ceria and to elucidate the effect of the reaction on conductivity.
0167-2738 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 00 )00416-1
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K. Eguchi et al. / Solid State Ionics 135 (2000) 589 – 594
2. Experimental
2.1. Sample preparation Yttria-doped ceria (YDC) powder with a composition of (YO 1.5 ) 0.2 (CeO 2 ) 0.8 was prepared by thermal decomposition of two kind of precursors by heating at 10008C in air. One precursor was obtained by evaporation of the solution of corresponding nitrates mixture. The other was prepared by adding malic acid to the nitrate solution and subsequent drying [5]. The samples after heat treatment at 10008C consisted of a single phase of a cubic fluorite structure as was identified by XRD. The samples were expressed as n-Y 0.2 Ce and m-Y 0.2 Ce where nand m- stand for the nitrate decomposition and malic acid precursor method, respectively, being followed by the dopant, its atomic fraction, and host cation. Commercially available yttria-stabilized zirconia (YSZ, TOSOH TZ-8Y) powder was used without further treatment. The composition of the powder was (YO 1.5 ) 0.15 (ZrO 2 ) 0.85 (abbreviated as Y 0.15 Zr(TZ8YS)) with a single cubic fluorite phase as was confirmed by XRD. The ceria and zirconia powders were sintered at 14008C for 10 h for SEM observation. The lattice constants of YDC and YSZ agreed with the reported values. For SEM observation, the powders of ceria and zirconia were uniaxially pressed at 1.8 3 10 7 Pa and isostatic pressed at 1.0 3 10 8 Pa for pellets, respectively, and they were sintered at 14008C for 15 h. The other compositions of ceria and zirconia powders and pellets were prepared by the same procedure. Samaria and gadolinia was also employed as dopants for ceria. These samples were also expressed as in the same manner such as m-Sm 0.2 Ce.
2.2. Sample characterization X-ray powder diffraction was performed using a Rigaku RINT 1400 with monochromated Cu K a radiation. Silicon and magnesia standards were used for calibration of diffraction angles. The marker method was employed to analyze the reaction between doped ceria and zirconia by inserting Pt at their interface [6]. The interface in the course of the solid state reaction between the two
pellets was observed by SEM with an EDX. The polished Y 0.2 Ce pellet with Pt and the Y 0.15 Zr pellet were attached with a weight of 50 g / cm 2 to the contacting faces and fired at 17008C to 24 h. After this sintering couple was cut and ground, the reacted interface between the two pellets was observed by SEM with an EDX detector.
3. Results and discussion
3.1. Crystalline phases determined by X-ray diffraction Fig. 1 shows XRD patterns of a powder mixture of m-Y 0.2 Ce / Y 0.15 Zr(TZ8YS) (molar ratio 5 1:1) after heating at various temperatures. The diffraction pattern from the simple mechanical mixture of the Y 0.2 Ce and Y 0.15 Zr powders without heat treatment is also shown as a reference. This pattern could be attributed to a mixture of two fluorite phases from Y 0.2 Ce and Y 0.15 Zr with different lattice constants [7]. After heating at 13008C, the lines from the YDC phase were weakened, but did not shift from the original position. On the contrary, all the lines from the zirconia phase slightly shifted to lower angles. The reason of the shift of the zirconia lines is solid state reaction between ceria and zirconia to form a yttria-doped ceria–zirconia solid solution (YCZ) phase [5]. The above mentioned change in XRD pattern strongly implies that the solid state reaction between ceria and zirconia proceeds by migration of the Ce component into zirconia particles. This behavior was previously reported in the XRD, EDX, and TEM observation of the solid state reaction between Ydoped ceria (Y 0.2 Ce) and yttria stabilized zirconia (YSZ) at different temperatures (1100–17008C) [4,5]. The Y 0.2 Ce powder used for the previous report was prepared from nitrate decomposition [4]. The powder microstructure was quite different depending on the method of preparation for the precursor. The solid state reaction was rapidly proceeded by use of the powder from the malic acid precursor as compared with the powder from nitrate decomposition. However, the reaction process was basically the same for these two source powders. Crystallite and particle sizes of Y-doped ceria pow-
K. Eguchi et al. / Solid State Ionics 135 (2000) 589 – 594
591
od. Although the rate of solid state reaction depended on the method of preparation, the reaction process was common to every Y 0.15 Zr powder. The effect of dopant was investigated using Sm and Gd as well as Y. Sm 0.2 Ce and Gd 0.2 Ce samples were prepared by the malic acid precursor method and mixed with Y 0.15 Zr(TZ8YS). The diffraction patterns after heating at 13008C are also listed in Fig. 1. Two groups of lines originating from ceria phase and zirconia phase appeared. Although only the lines from zirconia phase shifted for Y 0.2 Ce / Y 0.15 Zr, the diffraction lines from the both phases shifted slightly for the Sm 0.2 Ce / Y 0.15 Zr and Gd 0.2 Ce / Y 0.15 Zr samples after heating for 1 h, i.e. the lines from ceria and zirconia shifted to higher and lower angles, respectively. The lines are broadened after the shift due to solid state reaction. This means that the solid state reaction in this case proceeds either by dissolution of Ce in zirconia or Zr in ceria. The formation of single ceria phase was observed for the samples heating at 15008C.
3.2. Effect of solid state reaction on electrical conductivity
Fig. 1. X-ray diffraction patterns of rare earth-doped ceria and Y-stabilized zirconia during course of solid state reaction. Powder mixture samples were heated at given temperatures for 5 h.
ders were determined by XRD line broadening analysis and scanning electron microscopy, respectively, as listed in Table 1. Similarly, n-Y 0.15 Zr powder was prepared by nitrate decomposition meth-
The electrical conductivity was measured for the sample after solid state reaction between Y 0.15 Zr and Y 0.2 Ce samples. The samples were prepared by mixing the Y 0.15 Zr and Y 0.2 Ce powders in various proportions; then, the mixture was heated at 15008C. The reacted powder was again ground and sintered at 17008C into a pellet for the conductivity measurement. Every diffraction line after this heat treatment was attributed to single cubic fluorite phase of the solid solution. Thus, the solid state reaction between two phases was completed. The conductivity at various temperatures was plotted as a function of
Table 1 Crystallite and particle sizes of Y-doped ceria powder Sample name
Composition
Preparation method
Crystallite size / nm
Particle size / mm
m-Y 0.2 Ce
Y 0.2 Ce 0.8 O 1.9
Combustion of malic acid precursor
40.3
0.4
n-Y 0.2 Ce
Y 0.2 Ce 0.8 O 1.9
Thermal decomposition of nitrate
51.0
2.5
592
K. Eguchi et al. / Solid State Ionics 135 (2000) 589 – 594
Fig. 2. Electrical conductivity of Y 0.2 Ce–Y 0.15 Zr system as a function of Y 0.2 Ce fraction.
Y 0.2 Ce molar fraction (Y 0.2 Ce / Y 0.2 Ce 1 Y 0.15 Zr) in Fig. 2. The conductivity was high for the Y 0.2 Ce and Y 0.15 Zr samples, especially that of Y 0.2 Ce was the highest, whereas the solid solution of two samples led to low conductivity. The conductivity was lowest for the sample with Y 0.2 Ce molar fraction of 0.5. This result confirms that the solid state reaction between these two phases significantly lower the conductivity.
3.3. Compositional analysis at interface between ceria and zirconia The above-mentioned process for the solid state reaction was further confirmed by observing the YDC-YSZ interface by SEM with a EDX compositional analysis. The EDX intensities were calibrated by YSZ and YDC pellets prior to the analysis of the samples after reaction. After heating at 17008C for 24 h, the Pt marker could be found at the interface between Y 0.15 Zr and Y 0.2 Ce disk by SEM. Fig. 3a shows the backscattered electron image at the Y 0.15 Zr / Y 0.2 Ce interface. The image of the Pt region was distinct and the migration of Pt was hardly observable in the Y 0.15 Zr and Y 0.2 Ce regions from the SEM image with line EDX analysis near the Pt marker. The gradated compositional change was obvious from the backscattered electron image in the zirconia side from the Pt marker. This indicates that the compositional
gradient developed in the YSZ region near the interface. A compositional analysis was carried out by EDX by irradiating a narrow electron beam spot along a line across the interface and the Pt marker. Atomic contents of Zr, Ce and Y, which were measured by using EDX were calculated and plotted against distance from the interface. The Pt marker position was defined as zero in Fig. 3b. Compositional changes of the elements were also observed dominantly in zirconia side with reference to the Pt marker position, as mentioned above from the backscattered electron image. Thus, it confirms that the solid state reaction is caused by migration of the Ce component into the zirconia region. Yttrium was contained in both of the sintered pellets; thus the exact compositional distribution of this component was difficult to evaluate. It is obvious that the compositional curves for Zr and Ce were asymmetric with reference to the Pt marker. A larger amount of the Ce component was detected in the zirconia side than the amount of the Zr component detected in the ceria side. This result from the marker method also indicates that the solid state reaction between ceria and zirconia was mainly caused by diffusion of the Ce component into zirconia [4,5]. The Y 0.2 Ce fraction was also estimated from the observed value in Fig. 3b as shown in Fig. 3c. From the molar fraction of Y 0.2 Ce and the data in Fig. 2, the conductivity at each location can be estimated as plotted in Fig. 3d. The conductivity was significantly lowered in the vicinity of the interface region and minimum at d 5 1 mm position which was obviously in zirconia side of the sample. The conductivity was almost one order of magnitude smaller as compared with the unreacted Y 0.15 Zr and Y 0.2 Ce samples. Thus, the thick product layer formed by the reaction significantly deteriorates the overall electrical conductivity. The solid state reaction was also observed for the samaria-doped ceria (SDC)-YSZ interface by a compositional analysis. The Y 0.15 Zr and Sm 0.2 Ce disks were attached and heated at 17008C for 24 h before compositional analysis by EDX. Atomic contents of Zr, Ce, Sm, and Y were calculated and plotted as a function of distance from the interface in Fig. 4. Curves for compositional change of each element were symmetrical with reference to the Pt marker
K. Eguchi et al. / Solid State Ionics 135 (2000) 589 – 594
593
Fig. 3. (a) Backscattered electron image and (b) compositional analysis at the reaction interface of Y 0.2 Ce / Y 0.15 Zr. (c) Y 0.2 Ce fraction estimated from (b), and (d) estimated electrical conductivity from (c) and Fig. 2 as a function of distance from the Pt marker.
position. Thus, the compositional interface agreed with that marked by Pt. The preferential migration of a specific component was hardly observed for the solid state reaction between SDC and YSZ. This reaction process is in agreement with that expected
from the powder X-ray diffraction. The reason for the observed difference in the process of the solid state reaction between Y-doped ceria and Sm-doped ceria is unclear. However, the stability, and hence the chemical reactivity, of the doped ceria phase
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K. Eguchi et al. / Solid State Ionics 135 (2000) 589 – 594
importance for the application of this two phase system as an electrolyte. The process of the solid state reaction between doped ceria and zirconia should be an interesting example of reaction of ionic solid. The process of solid state reaction is strongly dependent on the kind of dopant for ceria. Especially, the mechanism of the reaction and diffusion process of each component in the preferential migration of Y-doped ceria into Y-stabilized zirconia should be analyzed in the future.
References
Fig. 4. Compositional analysis at the reaction interface of Sm 0.2 Ce / Y 0.15 Zr as a function of distance from the Pt marker.
appears to depend on the kind of dopant cation. The diffusion of dopant may strongly affect the diffusivity of the Ce and Zr components. Therefore, the solid state reaction between ceria and zirconia phase is strongly affected by the kind of dopant.
4. Conclusion The reaction at the interface between doped zirconia and ceria is expected to be of primary
[1] K. Eguchi, T. Setoguchi, T. Inoue, H. Arai, Solid State Ionics 52 (1992) 165. [2] B. Cales, J.F. Baumard, J. Electrochem. Soc. 131 (1984) 2407. [3] C. Leach, N. Khan, B.C.H. Steele, J. Mat. Sci. 27 (1992) 3812. [4] H. Mitsuyasu, Y. Nonaka, K. Eguchi, H. Arai, J. Solid State Chem. 129 (1997) 74–81. [5] H. Mitsuyasu, Y. Nonaka, K. Eguchi, Solid State Ionics 113–115 (1998) 279. [6] H. Schmalzried, Z. Physik. Chem., N.F. 33 (1962) 136. [7] S. Meriani, G. Spinolo, Powder Diffraction J. 2 (1987) 255.
Process of solid state reaction between doped ceria and zirconia K. Eguchi*, N. Akasaka, H. Mitsuyasu, Y. Nonaka Department of Molecular and Material Science, Graduate School of Engineering Sciences, Kyushu University, Kasuga 816 -8580, Japan
Abstract Solid state reaction at an interface between doped zirconia and ceria has been analyzed by X-ray diffraction and EDX. The solid state reaction between Y-doped ceria and Y-stabilized zirconia occurred on heating at 13008C or higher temperatures to form a single cubic fluorite Y-doped ceria–zirconia phase. The reaction proceeded by preferential migration of the Ce component to the zirconia lattice. The reaction significantly lowered the electrical conductivity. The reaction between Sm-doped ceria and Y-stabilized zirconia, on the other hand, proceeded by migration of every component equally, i.e. preferential migration of a specific component was hardly observed. 2000 Elsevier Science B.V. All rights reserved. Keywords: Ceria; Zirconia; Solid state reaction; Fluorite; Electrical conductivity
1. Introduction Solid oxide electrolyte materials are used in oxygen sensors, solid oxide fuel cells (SOFC), and electrochemical oxygen pumps. As the electrolyte for SOFC, yttria-stabilized zirconia has been regarded as the most promising material with high conductivity and transference number of oxide ions which is operative at high temperatures around 10008C. Cation-doped ceria has been investigated as a substitute for stabilized zirconia for reduced temperature SOFC, though its reduction in hydrogen atmosphere and resultant electronic conduction is a problem for practical application. We have previously proposed to overcome this difficulty using a bi-phasic electrolyte consisting of cation-doped ceria and stabilized zir*Corresponding author. Tel.: 1 81-92-583-7526; fax: 1 81-92573-0342. E-mail address: [email protected] (K. Eguchi).
conia, in which reduction of ceria with a fuel can be avoided with the thin zirconia layer coated on the anode side of the electrolyte [1]. However, these two fluorite phases based on zirconia and ceria undergo solid state reaction at elevated temperatures. The reaction proceeded by dissolution of ceria into zirconia to form a new fluorite phase with a poor ionic conductivity [2,3]. Thus, it is important to avoid the interfacial reaction between these two fluorite phases for the electrochemical applications. In our previous work, the Ce component migrated into the zirconia lattice at the ceria / zirconia interface from TEM and XRD observation [4,5]. High temperature sintering for preparation and long-term SOFC operation causes cation diffusion and leads to deterioration of the cell performance. The purpose of the present study is to analyze the solid state reaction at the interface between stabilized zirconia and cation-doped ceria and to elucidate the effect of the reaction on conductivity.
0167-2738 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 00 )00416-1
590
K. Eguchi et al. / Solid State Ionics 135 (2000) 589 – 594
2. Experimental
2.1. Sample preparation Yttria-doped ceria (YDC) powder with a composition of (YO 1.5 ) 0.2 (CeO 2 ) 0.8 was prepared by thermal decomposition of two kind of precursors by heating at 10008C in air. One precursor was obtained by evaporation of the solution of corresponding nitrates mixture. The other was prepared by adding malic acid to the nitrate solution and subsequent drying [5]. The samples after heat treatment at 10008C consisted of a single phase of a cubic fluorite structure as was identified by XRD. The samples were expressed as n-Y 0.2 Ce and m-Y 0.2 Ce where nand m- stand for the nitrate decomposition and malic acid precursor method, respectively, being followed by the dopant, its atomic fraction, and host cation. Commercially available yttria-stabilized zirconia (YSZ, TOSOH TZ-8Y) powder was used without further treatment. The composition of the powder was (YO 1.5 ) 0.15 (ZrO 2 ) 0.85 (abbreviated as Y 0.15 Zr(TZ8YS)) with a single cubic fluorite phase as was confirmed by XRD. The ceria and zirconia powders were sintered at 14008C for 10 h for SEM observation. The lattice constants of YDC and YSZ agreed with the reported values. For SEM observation, the powders of ceria and zirconia were uniaxially pressed at 1.8 3 10 7 Pa and isostatic pressed at 1.0 3 10 8 Pa for pellets, respectively, and they were sintered at 14008C for 15 h. The other compositions of ceria and zirconia powders and pellets were prepared by the same procedure. Samaria and gadolinia was also employed as dopants for ceria. These samples were also expressed as in the same manner such as m-Sm 0.2 Ce.
2.2. Sample characterization X-ray powder diffraction was performed using a Rigaku RINT 1400 with monochromated Cu K a radiation. Silicon and magnesia standards were used for calibration of diffraction angles. The marker method was employed to analyze the reaction between doped ceria and zirconia by inserting Pt at their interface [6]. The interface in the course of the solid state reaction between the two
pellets was observed by SEM with an EDX. The polished Y 0.2 Ce pellet with Pt and the Y 0.15 Zr pellet were attached with a weight of 50 g / cm 2 to the contacting faces and fired at 17008C to 24 h. After this sintering couple was cut and ground, the reacted interface between the two pellets was observed by SEM with an EDX detector.
3. Results and discussion
3.1. Crystalline phases determined by X-ray diffraction Fig. 1 shows XRD patterns of a powder mixture of m-Y 0.2 Ce / Y 0.15 Zr(TZ8YS) (molar ratio 5 1:1) after heating at various temperatures. The diffraction pattern from the simple mechanical mixture of the Y 0.2 Ce and Y 0.15 Zr powders without heat treatment is also shown as a reference. This pattern could be attributed to a mixture of two fluorite phases from Y 0.2 Ce and Y 0.15 Zr with different lattice constants [7]. After heating at 13008C, the lines from the YDC phase were weakened, but did not shift from the original position. On the contrary, all the lines from the zirconia phase slightly shifted to lower angles. The reason of the shift of the zirconia lines is solid state reaction between ceria and zirconia to form a yttria-doped ceria–zirconia solid solution (YCZ) phase [5]. The above mentioned change in XRD pattern strongly implies that the solid state reaction between ceria and zirconia proceeds by migration of the Ce component into zirconia particles. This behavior was previously reported in the XRD, EDX, and TEM observation of the solid state reaction between Ydoped ceria (Y 0.2 Ce) and yttria stabilized zirconia (YSZ) at different temperatures (1100–17008C) [4,5]. The Y 0.2 Ce powder used for the previous report was prepared from nitrate decomposition [4]. The powder microstructure was quite different depending on the method of preparation for the precursor. The solid state reaction was rapidly proceeded by use of the powder from the malic acid precursor as compared with the powder from nitrate decomposition. However, the reaction process was basically the same for these two source powders. Crystallite and particle sizes of Y-doped ceria pow-
K. Eguchi et al. / Solid State Ionics 135 (2000) 589 – 594
591
od. Although the rate of solid state reaction depended on the method of preparation, the reaction process was common to every Y 0.15 Zr powder. The effect of dopant was investigated using Sm and Gd as well as Y. Sm 0.2 Ce and Gd 0.2 Ce samples were prepared by the malic acid precursor method and mixed with Y 0.15 Zr(TZ8YS). The diffraction patterns after heating at 13008C are also listed in Fig. 1. Two groups of lines originating from ceria phase and zirconia phase appeared. Although only the lines from zirconia phase shifted for Y 0.2 Ce / Y 0.15 Zr, the diffraction lines from the both phases shifted slightly for the Sm 0.2 Ce / Y 0.15 Zr and Gd 0.2 Ce / Y 0.15 Zr samples after heating for 1 h, i.e. the lines from ceria and zirconia shifted to higher and lower angles, respectively. The lines are broadened after the shift due to solid state reaction. This means that the solid state reaction in this case proceeds either by dissolution of Ce in zirconia or Zr in ceria. The formation of single ceria phase was observed for the samples heating at 15008C.
3.2. Effect of solid state reaction on electrical conductivity
Fig. 1. X-ray diffraction patterns of rare earth-doped ceria and Y-stabilized zirconia during course of solid state reaction. Powder mixture samples were heated at given temperatures for 5 h.
ders were determined by XRD line broadening analysis and scanning electron microscopy, respectively, as listed in Table 1. Similarly, n-Y 0.15 Zr powder was prepared by nitrate decomposition meth-
The electrical conductivity was measured for the sample after solid state reaction between Y 0.15 Zr and Y 0.2 Ce samples. The samples were prepared by mixing the Y 0.15 Zr and Y 0.2 Ce powders in various proportions; then, the mixture was heated at 15008C. The reacted powder was again ground and sintered at 17008C into a pellet for the conductivity measurement. Every diffraction line after this heat treatment was attributed to single cubic fluorite phase of the solid solution. Thus, the solid state reaction between two phases was completed. The conductivity at various temperatures was plotted as a function of
Table 1 Crystallite and particle sizes of Y-doped ceria powder Sample name
Composition
Preparation method
Crystallite size / nm
Particle size / mm
m-Y 0.2 Ce
Y 0.2 Ce 0.8 O 1.9
Combustion of malic acid precursor
40.3
0.4
n-Y 0.2 Ce
Y 0.2 Ce 0.8 O 1.9
Thermal decomposition of nitrate
51.0
2.5
592
K. Eguchi et al. / Solid State Ionics 135 (2000) 589 – 594
Fig. 2. Electrical conductivity of Y 0.2 Ce–Y 0.15 Zr system as a function of Y 0.2 Ce fraction.
Y 0.2 Ce molar fraction (Y 0.2 Ce / Y 0.2 Ce 1 Y 0.15 Zr) in Fig. 2. The conductivity was high for the Y 0.2 Ce and Y 0.15 Zr samples, especially that of Y 0.2 Ce was the highest, whereas the solid solution of two samples led to low conductivity. The conductivity was lowest for the sample with Y 0.2 Ce molar fraction of 0.5. This result confirms that the solid state reaction between these two phases significantly lower the conductivity.
3.3. Compositional analysis at interface between ceria and zirconia The above-mentioned process for the solid state reaction was further confirmed by observing the YDC-YSZ interface by SEM with a EDX compositional analysis. The EDX intensities were calibrated by YSZ and YDC pellets prior to the analysis of the samples after reaction. After heating at 17008C for 24 h, the Pt marker could be found at the interface between Y 0.15 Zr and Y 0.2 Ce disk by SEM. Fig. 3a shows the backscattered electron image at the Y 0.15 Zr / Y 0.2 Ce interface. The image of the Pt region was distinct and the migration of Pt was hardly observable in the Y 0.15 Zr and Y 0.2 Ce regions from the SEM image with line EDX analysis near the Pt marker. The gradated compositional change was obvious from the backscattered electron image in the zirconia side from the Pt marker. This indicates that the compositional
gradient developed in the YSZ region near the interface. A compositional analysis was carried out by EDX by irradiating a narrow electron beam spot along a line across the interface and the Pt marker. Atomic contents of Zr, Ce and Y, which were measured by using EDX were calculated and plotted against distance from the interface. The Pt marker position was defined as zero in Fig. 3b. Compositional changes of the elements were also observed dominantly in zirconia side with reference to the Pt marker position, as mentioned above from the backscattered electron image. Thus, it confirms that the solid state reaction is caused by migration of the Ce component into the zirconia region. Yttrium was contained in both of the sintered pellets; thus the exact compositional distribution of this component was difficult to evaluate. It is obvious that the compositional curves for Zr and Ce were asymmetric with reference to the Pt marker. A larger amount of the Ce component was detected in the zirconia side than the amount of the Zr component detected in the ceria side. This result from the marker method also indicates that the solid state reaction between ceria and zirconia was mainly caused by diffusion of the Ce component into zirconia [4,5]. The Y 0.2 Ce fraction was also estimated from the observed value in Fig. 3b as shown in Fig. 3c. From the molar fraction of Y 0.2 Ce and the data in Fig. 2, the conductivity at each location can be estimated as plotted in Fig. 3d. The conductivity was significantly lowered in the vicinity of the interface region and minimum at d 5 1 mm position which was obviously in zirconia side of the sample. The conductivity was almost one order of magnitude smaller as compared with the unreacted Y 0.15 Zr and Y 0.2 Ce samples. Thus, the thick product layer formed by the reaction significantly deteriorates the overall electrical conductivity. The solid state reaction was also observed for the samaria-doped ceria (SDC)-YSZ interface by a compositional analysis. The Y 0.15 Zr and Sm 0.2 Ce disks were attached and heated at 17008C for 24 h before compositional analysis by EDX. Atomic contents of Zr, Ce, Sm, and Y were calculated and plotted as a function of distance from the interface in Fig. 4. Curves for compositional change of each element were symmetrical with reference to the Pt marker
K. Eguchi et al. / Solid State Ionics 135 (2000) 589 – 594
593
Fig. 3. (a) Backscattered electron image and (b) compositional analysis at the reaction interface of Y 0.2 Ce / Y 0.15 Zr. (c) Y 0.2 Ce fraction estimated from (b), and (d) estimated electrical conductivity from (c) and Fig. 2 as a function of distance from the Pt marker.
position. Thus, the compositional interface agreed with that marked by Pt. The preferential migration of a specific component was hardly observed for the solid state reaction between SDC and YSZ. This reaction process is in agreement with that expected
from the powder X-ray diffraction. The reason for the observed difference in the process of the solid state reaction between Y-doped ceria and Sm-doped ceria is unclear. However, the stability, and hence the chemical reactivity, of the doped ceria phase
594
K. Eguchi et al. / Solid State Ionics 135 (2000) 589 – 594
importance for the application of this two phase system as an electrolyte. The process of the solid state reaction between doped ceria and zirconia should be an interesting example of reaction of ionic solid. The process of solid state reaction is strongly dependent on the kind of dopant for ceria. Especially, the mechanism of the reaction and diffusion process of each component in the preferential migration of Y-doped ceria into Y-stabilized zirconia should be analyzed in the future.
References
Fig. 4. Compositional analysis at the reaction interface of Sm 0.2 Ce / Y 0.15 Zr as a function of distance from the Pt marker.
appears to depend on the kind of dopant cation. The diffusion of dopant may strongly affect the diffusivity of the Ce and Zr components. Therefore, the solid state reaction between ceria and zirconia phase is strongly affected by the kind of dopant.
4. Conclusion The reaction at the interface between doped zirconia and ceria is expected to be of primary
[1] K. Eguchi, T. Setoguchi, T. Inoue, H. Arai, Solid State Ionics 52 (1992) 165. [2] B. Cales, J.F. Baumard, J. Electrochem. Soc. 131 (1984) 2407. [3] C. Leach, N. Khan, B.C.H. Steele, J. Mat. Sci. 27 (1992) 3812. [4] H. Mitsuyasu, Y. Nonaka, K. Eguchi, H. Arai, J. Solid State Chem. 129 (1997) 74–81. [5] H. Mitsuyasu, Y. Nonaka, K. Eguchi, Solid State Ionics 113–115 (1998) 279. [6] H. Schmalzried, Z. Physik. Chem., N.F. 33 (1962) 136. [7] S. Meriani, G. Spinolo, Powder Diffraction J. 2 (1987) 255.