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Sintering behavior and electrical conductivity of Ce0.9Gd0.1O1.95 powder prepared by the gel-casting process
Materials Chemistry and Physics 78 (2003) 791–795
Sintering behavior and electrical conductivity of Ce0.9Gd0.1O1.95 powder prepared by the gel-casting process Ji-Gui Cheng, Shao-Wu Zha, Jia Huang, Xing-Qin Liu, Guang-Yao Meng∗ Department of Materials Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, PR China Received 22 November 2001; received in revised form 24 July 2002; accepted 12 August 2002
Abstract Nanoscale Ce0.9 Gd0.1 O1.95 (gadolinia doped ceria, GDC) powders were prepared by a gel-casting process. Differential thermal analysis, thermogravimetry and X-ray diffraction results showed that the single-phase fluorite structure forms at a relatively low calcination temperature of 600 ◦ C. X-ray patterns of the GDC powders revealed that the crystallite size of the powders increases with increasing calcination temperature, which is consistent with transmission electron microscopy observations. The sintering behavior and the ionic conductivity of the cast tapes prepared from GDC powders calcined at 600–1000 ◦ C were also studied. At sintering temperatures ≥1450 ◦ C, more than 96% of the relative density is obtained for tapes prepared from powders calcined at three different temperatures. The average grain size increases with decreasing powder calcination temperature. The alternating-current impedance spectroscopy results showed that the GDC sample sintered at 1450 ◦ C has ionic conductivity of 0.046 S cm−1 at 700 ◦ C in air. The present work results have indicated that the gel-casting route is a relatively low-temperature preparation technique to synthesize GDC powders with a high sinterability and a good ionic conductivity. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Ce0.9 Gd0.1 O1.95 powder; Sintering behavior; Electrical conductivity; Gel-casting process
1. Introduction In order to realize a practical solid oxide fuel cell (SOFC) which operates at intermediate temperatures ≤700 ◦ C, it is essential to develop new electrolytes with higher performance than the traditional Y2 O3 –stabilized ZrO2 (YSZ) electrolyte [1]. Recently, doped ceria (DCO), especially doped with trivalent rare-earth oxides, has drawn much attention in use as electrolytes for reduced-temperature SOFCs [2–4]. DCO has an oxygen ionic conductivity about one order higher than YSZ at temperatures from 600 to 800 ◦ C [5]. Furthermore, the interfacial losses of DCO electrolyte with cathode and anode are also lower than those of YSZ [6]. The reduced SOFC operating temperature by using a DCO electrolyte has a number of benefits, including, for example, cheaper materials, lower degradation problems, and less thermal mismatch [6,7]. Dense ceramics are needed for SOFC electrolytes. However, DCO powders prepared by the traditional solid state reaction technique usually have to be calcined at more than 1000 ◦ C to form the single-phase fluorite structure. They are usually fired to only 95% of the theoretical density at fairly
high sintering temperatures around 1600 ◦ C [8,9]. Sintering at high temperatures entails high energy costs and does not allow a co-firing of the electrolyte with other materials, as in the case of an anode-supported thin electrolyte bi-layer, which is a common structure design for intermediate temperature SOFCs with DCO electrolyte [10]. Recent studies have shown that DCO powders prepared by co-precipitation and other soft chemical routes can be sintered to more than 96% of the theoretical density at relatively low temperatures (<1500 ◦ C) [11,12]. But up to now, little work has been undertaken on synthesizing DCO powders by the gel-casting route. In this paper, Ce0.9 Gd0.1 O1.95 powders were prepared, using a modified gel-casting process. A water-based tape casting process was employed to form the powders, and the sintering and electrical properties of the resulting gadolinia doped ceria (GDC) tapes were investigated.
2. Experimental 2.1. Powder preparation
∗
Corresponding author. Tel.: +86-551-360-3234; fax: +86-551-363-1760. E-mail address: [email protected] (G.-Y. Meng).
Ce0.9 Gd0.1 O1.95 powders were prepared by a modified gel-casting route. Analytically pure cerium nitride
0254-0584/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 2 ) 0 0 3 8 4 - X
792
J.-G. Cheng et al. / Materials Chemistry and Physics 78 (2003) 791–795
hexahydrate (Ce(NO3 )3 ·6H2 O) and gadolinia (Gd2 O3 ) were mixed with organic monomer (mix of acrylamide (AM) and N,N -methylenebis-acrylamide (MBAM), AM:MBAM = 20:1) in an aqueous solution by ball milling. The resulting slurry with the initiator ammonium bisulphate ((NH4 )2 S2 O8 ) was cast into a container and was gelatinated by heating. Before calcination, the wet gel was further heated to about 120 ◦ C to remove the solvent water. The dried gel was then calcined at 600, 800 and 1000 ◦ C, respectively. After being ground, the resulting GDC powders were characterized and were ready for later tests.
the experimentally observed cubic lattice parameters. The microstructures of the specimens sintered at different temperatures were observed by scanning electron microscopy (SEM). The electrical conductivity of the sintered GDC specimens was measured in the temperature range 500–800 ◦ C in air by alternating-current impedance spectroscopy. Pt paste was painted onto either side of the sintered GDC discs and fired at 800 ◦ C for 2 h to act as electrodes.
2.2. Sample preparation
3.1. Powder characterization
The green tapes were prepared from the resulting GDC powders by the water-based tape casting, using the same monomer and the other additions as in the powder preparation. After being dried, the green tapes were finished, and discs of 25 mm in diameter and 0.5 mm in thickness were punched from the tapes. The green discs were then sintered in a programmable high-temperature furnace (Nabertherm, Germany) at temperatures from 1300 to 1500 ◦ C for later measurements.
Fig. 1 shows the DTA–TG results of the dried gels. In the temperature range 190–280 ◦ C, an abrupt loss of about 45 wt.% is accompanied by a strong exothermic DTA peak at 212 ◦ C, which indicates the removal of the crystal water and the thermal decomposition of the dried gels. No apparent mass loss occurs at temperatures T > 400 ◦ C, indicating the formation of the GDC oxide products. XRD patterns of the calcined powders are shown in Fig. 2. All powders had a single-phase fluorite structure, even when calcined at 600 ◦ C. This indicates that a relatively low calcination temperature is needed to prepare GDC powders by the gel-casting route. The XRD peak broadening in Fig. 2 shows that the crystallite size of the calcined powders is small. The degree of peak broadening decreases with increasing calcination temperature, which indicates the increase of the crystallite size. Fig. 3a–c show TEM micrographs of the three powders calcined at different temperatures. The average particle size was about 20, 40 and 70 nm for powders calcined at 600, 800 and 1000 ◦ C, respectively. All powders are in the nanometer range. The particle size also increases with the calcination temperature, which is consistent with the XRD patterns.
2.3. Characterization Simultaneous differential thermal analysis and thermogravimetry (DTA–TG) were carried out on the dried gel. The samples were heated from room temperature to 1000 ◦ C at a heating rate of 1 K/min under a dynamic air flow. The resulting GDC powders were characterized by X-ray diffraction (XRD). The particle size and morphology of the powders were examined by transmission electron microscopy (TEM). The density of the GDC sintered bodies was determined by Archimedes methods and expressed as relative to the theoretical value of 7.28 g cm−3 , which was calculated from
3. Results and discussion
Fig. 1. Differential thermal analysis (DTA) and thermogravimetry (TG) curves for the dried gels.
J.-G. Cheng et al. / Materials Chemistry and Physics 78 (2003) 791–795
Fig. 2. XRD patterns of the GDC powders calcined at different temperatures: (a) 600 ◦ C, 2 h; (b) 800 ◦ C, 2 h; (c) 1000 ◦ C, 2 h.
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Fig. 4. Relative density of the GDC samples as a function of the sintering temperature.
3.2. Sinterability Fig. 4 shows the effect of the sintering temperature on the relative density of the sintered GDC samples prepared from powders calcined at different temperatures. As the sintering temperature increased, all samples became denser. More than 96% of the theoretical density was obtained when the sintering temperature exceeds 1450 ◦ C. Higher density values were also observed on samples prepared from powders calcined at lower temperatures. The powders calcined at lower temperatures may have a smaller crystallite size, and hence a higher sintering activity. It is noted that after being sintered at 1500 ◦ C for 5 h, the density of the specimens prepared from powders calcined at 600, 800 and 1000 ◦ C are 98.4, 97.5 and 96.1%, respectively. These values are much higher than those of (CeO2 )0.8 (GdO1.5 )0.2 powders prepared by a high-temperature solid state reaction, even higher than those of (CeO2 )0.8 (GdO1.5 )0.2 powders prepared by co-precipitation [8]. This indicates that the gel-casting technique may be a relatively low-temperature preparation route to synthesize GDC powders with a high sinterability. Fig. 5a–c show SEM micrographs of sintered GDC samples prepared from powders calcined at 600 ◦ C. Some small
Fig. 5. SEM micrographs of the sintered GDC samples: (a) powder calcined at 600 ◦ C, sintered at 1400 ◦ C, 5 h; (b) powder calcined at 600 ◦ C, sintered at 1450 ◦ C, 5 h; (c) powder calcined at 600 ◦ C, sintered at 1500 ◦ C, 5 h; (d) powder calcined at 1000 ◦ C, sintered at 1500 ◦ C, 5 h.
Fig. 3. TEM micrographs of GDC powders calcined at different temperatures: (a) 600 ◦ C, 2 h; (b) 800 ◦ C, 2 h; (c) 1000 ◦ C, 2 h.
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J.-G. Cheng et al. / Materials Chemistry and Physics 78 (2003) 791–795
true even if the samples had been calcined at more than 1000 ◦ C, and were doped with 20 mol% gadolinium [8]. It is quite clear from Fig. 6 and Table 1 that the sample prepared from powder calcined at 600 ◦ C has a higher ionic conductivity than those prepared from powders calcined at 1000 ◦ C. This is due to the fact that the former has a large mean grain size, resulting in a significant increase in grain boundary conductivity, but a slight decrease in bulk conductivity. This leads to an increase in the total conductivity [13]. The activation energy of the two samples prepared from powders calcined at 600 and 1000 ◦ C are 0.47 and 0.51 eV, respectively. As discussed above, the conductivity of the sintered bodies prepared from powders calcined at low temperatures is higher. Therefore, the activation energy determined from the conductivity is lower. Fig. 6. Arrhenius plot of the conductivity for sintered discs prepared from powders calcined at different temperatures.
4. Conclusions 1400 ◦ C
pores still existed in a sample sintered at (Fig. 5a). However, as the sintering temperature increased to 1450 ◦ C or higher, the sintered bodies became dense and a very little porosity was observed (Fig. 5b and c). It demonstrates the high sintering activity of GDC powders prepared by the gel-casting process. The average grain sizes shown in Fig. 5a–c are about 3, 5 and 8 m, respectively. The average grain size increases as the sintering temperature increases. A dense microstructure was also observed for a sintered body prepared from GDC powder calcined at 1000 ◦ C (Fig. 5d). Comparing Fig. 5c and d, however, it is quite clear that as the calcination temperature increases from 600 to 1000 ◦ C, the mean grain size of the sintered bodies decreases from about 8 to 5 m. This further confirmed that the finer powders calcined at the lower temperature, have a higher sintering growth rate. 3.3. Conductivity Owing to the relatively high density, the sintered GDC specimens have a high ionic conductivity. Fig. 6 presents an Arrhenius plot for the sintered tape cast samples prepared from powders calcined at 600 and 1000 ◦ C. Table 1 summarizes the conductivity of two samples calcined at different temperatures. The cast tapes were sintered at 1450 ◦ C, a relatively low temperature, but the conductivity values obtained in the present work are higher than those of samples prepared by a high-temperature solid state reaction. This is Table 1 Ionic conductivity (S cm−1 ) of the two samples sintered at 1450 ◦ C, 5 h; powder calcined at (A) 600 ◦ C and (B) 1000 ◦ C Sample
A B
Calcination temperatures (◦ C) 500
600
700
800
0.0098 0.0075
0.0244 0.0199
0.0462 0.0396
0.072 0.063
Ce0.9 Gd0.1 O1.95 powders were prepared using a gelcasting route, in which a relatively low calcination temperature was needed, compared with the conventional solid state reaction technique, to form the single-phase fluorite structure. The resulting powders exhibited a good sinterability and good electrical conductivity. For cast tapes sintered at 1450 ◦ C, more 96% of the theoretical density were obtained, and the maximum ionic conductivity of 0.0462 S cm−1 was measured at 700 ◦ C in air. This shows the great potential of GDC powders in use as electrolytes for intermediate-temperature solid oxide fuel cells. The calcination temperature during the preparation of the GDC powders also has an important influence on the sintering behavior and electrical conductivity of the cast tapes. When the powders were calcined at increasing temperature from 600 to 1000 ◦ C, the sintered samples became less dense, and the crystallite size decreased. This results in a higher electrical conductivity for a sintered body prepared from powder calcined at a lower temperature.
Acknowledgements The authors would like to thank the Chinese Natural Science Foundation (Grant No. 20071029) and the Ministry of Science and Technology of China (Grant No. G-2000026409 and Grant No. 2001AA323090) for their financial supports. References [1] R. Doshi, V.L. Richards, J.D. Carter, X. Wang, M. Krumpelt, J. Electrochem. Soc. 146 (1999) 1273. [2] D.L. Maricle, T.E. Swarr, S. Karavolis, Solid State Ionics 52 (1992) 173. [3] B.C.H. Steele, Solid State Ionics 129 (2000) 95. [4] K. Zheng, B.C.H. Steele, M. Sahibzada, I.S. Metcalfe, Solid State Ionics 86–88 (1996) 1241.
J.-G. Cheng et al. / Materials Chemistry and Physics 78 (2003) 791–795 [5] K. Eguchi, T. Setoguchi, T. Inoue, H. Arai, Solid State Ionics 52 (1992) 165. [6] H. Inabi, H. Tagawa, Solid State Ionics 83 (1996) 1. [7] N.Q. Minh, J. Am. Ceram. Soc. 76 (1993) 563. [8] R.S. Torrens, N.M. Sammes, G.A. Tompsett, Solid State Ionics 111 (1998) 9. [9] K. Huang, M. Feng, J.B. Goodenough, J. Am. Ceram. Soc. 81 (1998) 357.
795
[10] P. Charpentier, P. Fragnaud, D.M. Schleich, E. Gehain, Solid State Ionics 135 (2000) 373. [11] J. Van Herle, T. Horita, T. Kawada, N. Sakai, H. Yokokaya, M. Dokiya, Solid State Ionics 86–88 (1996) 1255. [12] J. Van Herle, T. Horita, T. Kawada, N. Sakai, H. Yokokaya, M. Dokiya, Ceram. Int. 24 (1998) 229. [13] G.M. Chritie, F.P.F. Van Berkel, Solid State Ionics 83 (1996) 297.
Sintering behavior and electrical conductivity of Ce0.9Gd0.1O1.95 powder prepared by the gel-casting process Ji-Gui Cheng, Shao-Wu Zha, Jia Huang, Xing-Qin Liu, Guang-Yao Meng∗ Department of Materials Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, PR China Received 22 November 2001; received in revised form 24 July 2002; accepted 12 August 2002
Abstract Nanoscale Ce0.9 Gd0.1 O1.95 (gadolinia doped ceria, GDC) powders were prepared by a gel-casting process. Differential thermal analysis, thermogravimetry and X-ray diffraction results showed that the single-phase fluorite structure forms at a relatively low calcination temperature of 600 ◦ C. X-ray patterns of the GDC powders revealed that the crystallite size of the powders increases with increasing calcination temperature, which is consistent with transmission electron microscopy observations. The sintering behavior and the ionic conductivity of the cast tapes prepared from GDC powders calcined at 600–1000 ◦ C were also studied. At sintering temperatures ≥1450 ◦ C, more than 96% of the relative density is obtained for tapes prepared from powders calcined at three different temperatures. The average grain size increases with decreasing powder calcination temperature. The alternating-current impedance spectroscopy results showed that the GDC sample sintered at 1450 ◦ C has ionic conductivity of 0.046 S cm−1 at 700 ◦ C in air. The present work results have indicated that the gel-casting route is a relatively low-temperature preparation technique to synthesize GDC powders with a high sinterability and a good ionic conductivity. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Ce0.9 Gd0.1 O1.95 powder; Sintering behavior; Electrical conductivity; Gel-casting process
1. Introduction In order to realize a practical solid oxide fuel cell (SOFC) which operates at intermediate temperatures ≤700 ◦ C, it is essential to develop new electrolytes with higher performance than the traditional Y2 O3 –stabilized ZrO2 (YSZ) electrolyte [1]. Recently, doped ceria (DCO), especially doped with trivalent rare-earth oxides, has drawn much attention in use as electrolytes for reduced-temperature SOFCs [2–4]. DCO has an oxygen ionic conductivity about one order higher than YSZ at temperatures from 600 to 800 ◦ C [5]. Furthermore, the interfacial losses of DCO electrolyte with cathode and anode are also lower than those of YSZ [6]. The reduced SOFC operating temperature by using a DCO electrolyte has a number of benefits, including, for example, cheaper materials, lower degradation problems, and less thermal mismatch [6,7]. Dense ceramics are needed for SOFC electrolytes. However, DCO powders prepared by the traditional solid state reaction technique usually have to be calcined at more than 1000 ◦ C to form the single-phase fluorite structure. They are usually fired to only 95% of the theoretical density at fairly
high sintering temperatures around 1600 ◦ C [8,9]. Sintering at high temperatures entails high energy costs and does not allow a co-firing of the electrolyte with other materials, as in the case of an anode-supported thin electrolyte bi-layer, which is a common structure design for intermediate temperature SOFCs with DCO electrolyte [10]. Recent studies have shown that DCO powders prepared by co-precipitation and other soft chemical routes can be sintered to more than 96% of the theoretical density at relatively low temperatures (<1500 ◦ C) [11,12]. But up to now, little work has been undertaken on synthesizing DCO powders by the gel-casting route. In this paper, Ce0.9 Gd0.1 O1.95 powders were prepared, using a modified gel-casting process. A water-based tape casting process was employed to form the powders, and the sintering and electrical properties of the resulting gadolinia doped ceria (GDC) tapes were investigated.
2. Experimental 2.1. Powder preparation
∗
Corresponding author. Tel.: +86-551-360-3234; fax: +86-551-363-1760. E-mail address: [email protected] (G.-Y. Meng).
Ce0.9 Gd0.1 O1.95 powders were prepared by a modified gel-casting route. Analytically pure cerium nitride
0254-0584/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 2 ) 0 0 3 8 4 - X
792
J.-G. Cheng et al. / Materials Chemistry and Physics 78 (2003) 791–795
hexahydrate (Ce(NO3 )3 ·6H2 O) and gadolinia (Gd2 O3 ) were mixed with organic monomer (mix of acrylamide (AM) and N,N -methylenebis-acrylamide (MBAM), AM:MBAM = 20:1) in an aqueous solution by ball milling. The resulting slurry with the initiator ammonium bisulphate ((NH4 )2 S2 O8 ) was cast into a container and was gelatinated by heating. Before calcination, the wet gel was further heated to about 120 ◦ C to remove the solvent water. The dried gel was then calcined at 600, 800 and 1000 ◦ C, respectively. After being ground, the resulting GDC powders were characterized and were ready for later tests.
the experimentally observed cubic lattice parameters. The microstructures of the specimens sintered at different temperatures were observed by scanning electron microscopy (SEM). The electrical conductivity of the sintered GDC specimens was measured in the temperature range 500–800 ◦ C in air by alternating-current impedance spectroscopy. Pt paste was painted onto either side of the sintered GDC discs and fired at 800 ◦ C for 2 h to act as electrodes.
2.2. Sample preparation
3.1. Powder characterization
The green tapes were prepared from the resulting GDC powders by the water-based tape casting, using the same monomer and the other additions as in the powder preparation. After being dried, the green tapes were finished, and discs of 25 mm in diameter and 0.5 mm in thickness were punched from the tapes. The green discs were then sintered in a programmable high-temperature furnace (Nabertherm, Germany) at temperatures from 1300 to 1500 ◦ C for later measurements.
Fig. 1 shows the DTA–TG results of the dried gels. In the temperature range 190–280 ◦ C, an abrupt loss of about 45 wt.% is accompanied by a strong exothermic DTA peak at 212 ◦ C, which indicates the removal of the crystal water and the thermal decomposition of the dried gels. No apparent mass loss occurs at temperatures T > 400 ◦ C, indicating the formation of the GDC oxide products. XRD patterns of the calcined powders are shown in Fig. 2. All powders had a single-phase fluorite structure, even when calcined at 600 ◦ C. This indicates that a relatively low calcination temperature is needed to prepare GDC powders by the gel-casting route. The XRD peak broadening in Fig. 2 shows that the crystallite size of the calcined powders is small. The degree of peak broadening decreases with increasing calcination temperature, which indicates the increase of the crystallite size. Fig. 3a–c show TEM micrographs of the three powders calcined at different temperatures. The average particle size was about 20, 40 and 70 nm for powders calcined at 600, 800 and 1000 ◦ C, respectively. All powders are in the nanometer range. The particle size also increases with the calcination temperature, which is consistent with the XRD patterns.
2.3. Characterization Simultaneous differential thermal analysis and thermogravimetry (DTA–TG) were carried out on the dried gel. The samples were heated from room temperature to 1000 ◦ C at a heating rate of 1 K/min under a dynamic air flow. The resulting GDC powders were characterized by X-ray diffraction (XRD). The particle size and morphology of the powders were examined by transmission electron microscopy (TEM). The density of the GDC sintered bodies was determined by Archimedes methods and expressed as relative to the theoretical value of 7.28 g cm−3 , which was calculated from
3. Results and discussion
Fig. 1. Differential thermal analysis (DTA) and thermogravimetry (TG) curves for the dried gels.
J.-G. Cheng et al. / Materials Chemistry and Physics 78 (2003) 791–795
Fig. 2. XRD patterns of the GDC powders calcined at different temperatures: (a) 600 ◦ C, 2 h; (b) 800 ◦ C, 2 h; (c) 1000 ◦ C, 2 h.
793
Fig. 4. Relative density of the GDC samples as a function of the sintering temperature.
3.2. Sinterability Fig. 4 shows the effect of the sintering temperature on the relative density of the sintered GDC samples prepared from powders calcined at different temperatures. As the sintering temperature increased, all samples became denser. More than 96% of the theoretical density was obtained when the sintering temperature exceeds 1450 ◦ C. Higher density values were also observed on samples prepared from powders calcined at lower temperatures. The powders calcined at lower temperatures may have a smaller crystallite size, and hence a higher sintering activity. It is noted that after being sintered at 1500 ◦ C for 5 h, the density of the specimens prepared from powders calcined at 600, 800 and 1000 ◦ C are 98.4, 97.5 and 96.1%, respectively. These values are much higher than those of (CeO2 )0.8 (GdO1.5 )0.2 powders prepared by a high-temperature solid state reaction, even higher than those of (CeO2 )0.8 (GdO1.5 )0.2 powders prepared by co-precipitation [8]. This indicates that the gel-casting technique may be a relatively low-temperature preparation route to synthesize GDC powders with a high sinterability. Fig. 5a–c show SEM micrographs of sintered GDC samples prepared from powders calcined at 600 ◦ C. Some small
Fig. 5. SEM micrographs of the sintered GDC samples: (a) powder calcined at 600 ◦ C, sintered at 1400 ◦ C, 5 h; (b) powder calcined at 600 ◦ C, sintered at 1450 ◦ C, 5 h; (c) powder calcined at 600 ◦ C, sintered at 1500 ◦ C, 5 h; (d) powder calcined at 1000 ◦ C, sintered at 1500 ◦ C, 5 h.
Fig. 3. TEM micrographs of GDC powders calcined at different temperatures: (a) 600 ◦ C, 2 h; (b) 800 ◦ C, 2 h; (c) 1000 ◦ C, 2 h.
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J.-G. Cheng et al. / Materials Chemistry and Physics 78 (2003) 791–795
true even if the samples had been calcined at more than 1000 ◦ C, and were doped with 20 mol% gadolinium [8]. It is quite clear from Fig. 6 and Table 1 that the sample prepared from powder calcined at 600 ◦ C has a higher ionic conductivity than those prepared from powders calcined at 1000 ◦ C. This is due to the fact that the former has a large mean grain size, resulting in a significant increase in grain boundary conductivity, but a slight decrease in bulk conductivity. This leads to an increase in the total conductivity [13]. The activation energy of the two samples prepared from powders calcined at 600 and 1000 ◦ C are 0.47 and 0.51 eV, respectively. As discussed above, the conductivity of the sintered bodies prepared from powders calcined at low temperatures is higher. Therefore, the activation energy determined from the conductivity is lower. Fig. 6. Arrhenius plot of the conductivity for sintered discs prepared from powders calcined at different temperatures.
4. Conclusions 1400 ◦ C
pores still existed in a sample sintered at (Fig. 5a). However, as the sintering temperature increased to 1450 ◦ C or higher, the sintered bodies became dense and a very little porosity was observed (Fig. 5b and c). It demonstrates the high sintering activity of GDC powders prepared by the gel-casting process. The average grain sizes shown in Fig. 5a–c are about 3, 5 and 8 m, respectively. The average grain size increases as the sintering temperature increases. A dense microstructure was also observed for a sintered body prepared from GDC powder calcined at 1000 ◦ C (Fig. 5d). Comparing Fig. 5c and d, however, it is quite clear that as the calcination temperature increases from 600 to 1000 ◦ C, the mean grain size of the sintered bodies decreases from about 8 to 5 m. This further confirmed that the finer powders calcined at the lower temperature, have a higher sintering growth rate. 3.3. Conductivity Owing to the relatively high density, the sintered GDC specimens have a high ionic conductivity. Fig. 6 presents an Arrhenius plot for the sintered tape cast samples prepared from powders calcined at 600 and 1000 ◦ C. Table 1 summarizes the conductivity of two samples calcined at different temperatures. The cast tapes were sintered at 1450 ◦ C, a relatively low temperature, but the conductivity values obtained in the present work are higher than those of samples prepared by a high-temperature solid state reaction. This is Table 1 Ionic conductivity (S cm−1 ) of the two samples sintered at 1450 ◦ C, 5 h; powder calcined at (A) 600 ◦ C and (B) 1000 ◦ C Sample
A B
Calcination temperatures (◦ C) 500
600
700
800
0.0098 0.0075
0.0244 0.0199
0.0462 0.0396
0.072 0.063
Ce0.9 Gd0.1 O1.95 powders were prepared using a gelcasting route, in which a relatively low calcination temperature was needed, compared with the conventional solid state reaction technique, to form the single-phase fluorite structure. The resulting powders exhibited a good sinterability and good electrical conductivity. For cast tapes sintered at 1450 ◦ C, more 96% of the theoretical density were obtained, and the maximum ionic conductivity of 0.0462 S cm−1 was measured at 700 ◦ C in air. This shows the great potential of GDC powders in use as electrolytes for intermediate-temperature solid oxide fuel cells. The calcination temperature during the preparation of the GDC powders also has an important influence on the sintering behavior and electrical conductivity of the cast tapes. When the powders were calcined at increasing temperature from 600 to 1000 ◦ C, the sintered samples became less dense, and the crystallite size decreased. This results in a higher electrical conductivity for a sintered body prepared from powder calcined at a lower temperature.
Acknowledgements The authors would like to thank the Chinese Natural Science Foundation (Grant No. 20071029) and the Ministry of Science and Technology of China (Grant No. G-2000026409 and Grant No. 2001AA323090) for their financial supports. References [1] R. Doshi, V.L. Richards, J.D. Carter, X. Wang, M. Krumpelt, J. Electrochem. Soc. 146 (1999) 1273. [2] D.L. Maricle, T.E. Swarr, S. Karavolis, Solid State Ionics 52 (1992) 173. [3] B.C.H. Steele, Solid State Ionics 129 (2000) 95. [4] K. Zheng, B.C.H. Steele, M. Sahibzada, I.S. Metcalfe, Solid State Ionics 86–88 (1996) 1241.
J.-G. Cheng et al. / Materials Chemistry and Physics 78 (2003) 791–795 [5] K. Eguchi, T. Setoguchi, T. Inoue, H. Arai, Solid State Ionics 52 (1992) 165. [6] H. Inabi, H. Tagawa, Solid State Ionics 83 (1996) 1. [7] N.Q. Minh, J. Am. Ceram. Soc. 76 (1993) 563. [8] R.S. Torrens, N.M. Sammes, G.A. Tompsett, Solid State Ionics 111 (1998) 9. [9] K. Huang, M. Feng, J.B. Goodenough, J. Am. Ceram. Soc. 81 (1998) 357.
795
[10] P. Charpentier, P. Fragnaud, D.M. Schleich, E. Gehain, Solid State Ionics 135 (2000) 373. [11] J. Van Herle, T. Horita, T. Kawada, N. Sakai, H. Yokokaya, M. Dokiya, Solid State Ionics 86–88 (1996) 1255. [12] J. Van Herle, T. Horita, T. Kawada, N. Sakai, H. Yokokaya, M. Dokiya, Ceram. Int. 24 (1998) 229. [13] G.M. Chritie, F.P.F. Van Berkel, Solid State Ionics 83 (1996) 297.