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Sr- and Mg-doped LaGaO3 powder synthesis by carbonate coprecipitation
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Colloids and Surfaces A: Physicochem. Eng. Aspects 313–314 (2008) 154–157
Sr- and Mg-doped LaGaO3 powder synthesis by carbonate coprecipitation Nam Soo Chae a , Kil Soon Park a , Young Soo Yoon b , In Sang Yoo a , Jong Sung Kim a , Hyon Hee Yoon a,∗ a b
Department of Chemical Engineering, Kyungwon University, Sungnam 461-701, Republic of Korea Department of Advanced Technology Fusion, Konkuk University, 173-401 Seoul, Republic of Korea Received 18 November 2006; accepted 20 April 2007 Available online 2 June 2007
Abstract Sr- and Mg-doped LaGaO3 (LSGM) and Co-doped LSGM (LSGMC) powders were synthesized by coprecipitation using ammonium carbonate or ammonium bicarbonate precipitant. Precursor, calcined particles, and sintered pellets of LSGM were characterized using TGA, XRD, and SEM. The calcined powders of LSGM showed uniform microstructures with nearly spherical morphology and average particle size of 100–200 nm. The ionic conductivity of the sintered LSGM and LSGMC pellets at 1400 ◦ C was 4.5 × 10−2 S/cm and 1.13 S/cm at measuring temperature of 800 ◦ C, respectively. The internal microstructure was observed by FIB-SEM and it was found that the internal pores affected adversely on the ionic conductivity of the sintered LSGM pellets. The ionic conductivity of the as-prepared powders would be improved by optimization of the preparation process. The results of this study indicated that ammonium carbonate coprecipitation could be used as a convenient and economic method to produce LSGM electrolyte for low temperature SOFCs. © 2007 Elsevier B.V. All rights reserved. Keywords: Solid oxide fuel cell; Coprecipiation; Electrolyte; LSGM; Ion conductivity
1. Introduction Recent solid oxide fuel cell (SOFC) development has been focused on a lower temperature operation less than 800 ◦ C [1]. One of the methods to reduce the operating temperature is the use of new electrolyte materials which has higher oxide ionic conductivity. Sr- and Mg-doped lanthanum gallate (LSGM) has been found to have higher oxide ionic conductivity over a wide range of oxygen partial pressures at a low temperature (600–800 ◦ C) compared to commonly used electrolyte materials such as YSZ [2]. Further improvement of the ionic conductivity has been reported by doping cobalt into the LSGM. Many researchers had demonstrated high performance SOFCs using LSGM as the electrolyte. LSGM powders are typically prepared by solid-state reactions. The conventional techniques require high temperatures for sintering. Several types of wet-chemical methods, such
∗
Corresponding author. Tel.: +82 31 750 5356; fax: +82 31 750 5574. E-mail address: [email protected] (H.H. Yoon).
0927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2007.04.084
as sol–gel, hydrothermal treatment, and coprecipitation, have been reported for the synthesis of LSGM powders [3]. The wet chemistry-derived powders were reported to show better reactivity than those prepared by solid-state reaction. However, the required sintering temperature is still above 1400 ◦ C [4]. Recently, a carbonate coprecipitation method was developed to prepare rare-earth-doped ceria [5]. It was reported that the oxide powders synthesized by carbonate coprecipitation method could be sintered to >99.5% of the theoretical density at temperatures of 1100–1250 ◦ C. Lowering the sintering temperature of electrolyte materials will allow fine grain sizes as well as the saving of energy [6]. In this study, we applied the carbonate coprecipitation method for the synthesis of LSGM electrolytes. Cobalt-doped LSGM (LSGMC) electrolyte powder was also prepared using this method. The powders and sintered pellets obtained were characterized by XRD, scanning electron microscopy (SEM), and focused ion beam-scanning electron microscopy (FIB-SEM). An impedance analyzer was used to determine the ionic conductivity of the sintered pellets at various temperatures. The influence of precipitation reaction conditions was evaluated.
N.S. Chae et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 313–314 (2008) 154–157
Fig. 1. TGA curves of the precursors of LSGM obtained by carbonate coprecipitation.
Fig. 2. XRD patterns of LSGM powder calcined at 1100 ◦ C and LSGM pellet sintered at 1400 ◦ C.
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bicarbonate (NH4 )HCO3 as a precipitant. The starting salts are La(NO3 )3 ·6H2 O, Sr(NO3 )2 , Ga(NO3 )3 ·xH2 O, Mg(NO3 )2 · 6H2 O, and Co(NO3 )2 ·6H2 O. The stoichiometric amounts of each component of the final product La0.9 Sr0.1 Ga0.8 Mg0.2 O2.85 for LSGM and La0.9 Sr0.1 Ga0.8 Mg0.1 Co0.1 O2.85 for LSGMC were dissolved in distilled water. The concentration of stock solution was 0.2 M for La3+ . An aqueous solution of ammonium carbonate or ammonium bicarbonate with a concentration of 1.5 M was used as the precipitant. The mixed salt solution of 200 mL was dripped at a speed of 5 mL/min into the precipitant solution in a beaker kept at 70 ◦ C with mild stirring. The resulting suspension was aged at 70 ◦ C for 2 h after completion of precipitation. The precipitate was filtered and dried at room temperature over 24 h. The dried powder was ground in an agate mortar for 10 min and calcined at various temperatures. The calcined powder was pressed into several disks of 15 mm diameter and 1 mm thickness at 30 MPa. The pellets were then sintered at various temperatures for 6 h in air [7]. The crystal structures of the sintered LSGM samples were analyzed via X-ray diffractometry. Differential thermal analysis/thermogravimetry (DTA/TG) of the precursor powder was performed using a TG–DTA analyzer. The morphology of the LSGM powders and sintered pellets were observed through a high resolution scanning electron microscopy (HRSEM) (Model S-4700, Hitachi). The internal microstructure of the sintered LSGM pellets was observed by a focused ion beam-scanning electron microscopy (FIB-SEM) (Model Nova-200, FEI Company). The oxide ionic conductivity of sintered samples was measured using a two-probe ac impedance method (Solatron 1280B) as a function of temperature from 773 K to 1173 K in air.
3. Results and discussion
2. Experiment LSGM and LSGMC powders were prepared by coprecipiation using ammonium carbonate (NH4 )2 CO3 or ammonium
The thermal behavior of precursors obtained using the ammonium coprecipitation method was investigated by DSC/TG analysis. Fig. 1 shows DSC/TG curves of LSGM precursors.
Fig. 3. SEM micrographs of LSGM powders calcined at different temperatures [(a) 900 ◦ C, (b) 1000 ◦ C, and (c) 1100 ◦ C] and LSGM pellets sintered at different temperatures [(d) 1400 ◦ C and (e) 1600 ◦ C].
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N.S. Chae et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 313–314 (2008) 154–157
Table 1 Nominal and experimental stoichiometric indexes of the sintered pellet (La0.9 Sr0.1 Ga0.8 Mg0.2 O2.85 ) Element
Nominal stoichiometric index Experimental stoichiometric index
La
Sr
Ga
Mg
0.9 0.91 ± 0.01
0.1 0.09 ± 0.01
0.8 0.82 ± 0.02
0.2 0.18 ± 0.01
The experimental values were determined by EDX analyses.
The curve indicates that the thermal decomposition occurred in two steps. Weight loss between 100 ◦ C and 300 ◦ C could be attributed to the partial decomposition of the ammonium lanthanide carbonates into oxycarbonates and the weight loss between 300 ◦ C and 850 ◦ C could be attributed to the decomposition of oxycarbonates into oxides. Weight loss below 100 ◦ C was contributed to absorbed moisture. Thermal decomposition of precursors was completed at about 900 ◦ C. The XRD pattern of LSGM powders and sintered pellets was shown in Fig. 2. The LSGM powders calcined at 1100 ◦ C exhibited typical XRD patterns of LaGaO3 -based perovskite structure although it contained a small amount of secondary phases such as of LaSrGa3 O7 and LaSrGaO4 . The XRD pattern of the LSGM sintered pellet showed that the secondary phases considerably decreased after sintering for 6 h at 1400 ◦ C. This result indicated that pure LSGM electrolytes could be prepared by carbonated coprecipitation method. In addition, EDX analysis showed the chemical composition of sintered LSGM pellets were in good agreement with stoichiometric composition of La0.9 Sr0.1 Ga0.8 Mg0.2 O2.85 , as shown in Table 1. Fig. 3 shows SEM micrographs of LSGM powders obtained after calcination at different temperatures and pellets obtained after sintering at 1400 ◦ C and 1600 ◦ C. The particle size of the powders was in the range of 100–200 nm as determined from SEM micrographs. The average grain size of LSGM pellets was in the range of 1–3 m. The oxide ionic conductivity of the sintered LSGM pellets was measured at temperatures from 500 ◦ C to 900 ◦ C. Fig. 4 shows Arrhenius plots of the ionic conductivities of the LSGM and LSGMC pellets sintered at 1400 ◦ C. The ionic conductivity of the LSGM and LSGMC samples was 4.5 × 10−2 S/cm and
Fig. 4. Arrhenius plot of ionic conductivity of the LSGM and LSGMC pellets sintered at 1400 ◦ C.
1.13 S/cm at 800 ◦ C, respectively. The ionic conductivity was observed to be significantly improved by doping cobalt into the LSGM. It was found that the coprecipitation conditions such as precipitant type affected the oxide ionic conductivity of the sintered LSGM pellets. As shown in Table 2, the ionic conductivity of the sintered LSGM pellets prepared using ammonium bicarbonate precipitant was lowered to 7.2 × 10−3 S/cm at 800 ◦ C. When ammonium bicarbonate was used as a precipitant, lower density of the sintered pellet was observed, as shown in Table 2. This result can be explained by comparing the internal microstructure of each sample as appeared in FIB-SEM micrographs of the sintered pellets. As shown in Fig. 5, the sintered pellet obtained
Fig. 5. FIB-SEM micrographs showing internal microstructure of LSGM pellets sintered at 1400 ◦ C: (a) prepared using ammonium carbonate precipitant and (b) prepared using ammonium bicarbonate precipitant.
N.S. Chae et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 313–314 (2008) 154–157 Table 2 Relative density and oxide ionic conductivity of the sintered LSGM pellets prepared using different precipitants Precipitants
Relative density sintered at 1400 ◦ C (%)
Oxide ionic conductivity (σ) at 800 ◦ C (S/cm)
Ammonium bicarbonate Ammonium carbonate
88 93
7.2 × 10−3 4.5 × 10−2
using ammonium carbonate had less pores than that obtained using ammonium bicarbonate. 4. Conclusions Nanocrystalline La0.9 Sr0.1 Ga0.8 Mg0.2 O2.85 (LSGM) and La0.9 Sr0.1 Ga0.8 Mg0.1 Co0.1 O2.85 (LSGMC) powders have been synthesized by coprecipiation using ammonium carbonate and ammonium bicarbonate as the precipitant. The calcined powders of LSGM showed uniform microstructures with nearly spherical morphology. The average particle size was in the range of 100–200 nm. The chemical composition of the sintered LSGM pellets was in good agreement with stoichiometric composition of La0.9 Sr0.1 Ga0.8 Mg0.2 O2.85 . The ionic conductivity of the LSGM and LSGMC samples sintered at 1400 ◦ C was 4.5 × 10−2 S/cm and 1.13 S/cm at 800 ◦ C, respectively. Considerable improvement of ionic conductivity was observed by doping cobalt into the LSGM. The internal microstructure was observed by FIB-SEM and it was found that the internal pores affected adversely on the ionic conductivity of
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the sintered LSGM pellets. After improving the ionic conductivity by optimization of the preparation process, ammonium carbonate coprecipitation could be used as a convenient and economic method to produce LSGM electrolyte for low temperature SOFCs. Acknowledgment This work was supported by the Gyeonggi Regional Research Center and partly by the research fund from Kyungwon University. References [1] I.N. Sora, R. Pelosato, G. Dotelli, C. Schid, R. Ruffo, C.M. Mari, The system Al2 O3 and (Sr,Mg)-doped LaGaO3 : phase composition and electrical properties, Solid State Ionics 176 (2005) 81. [2] T. Ishihara, H. Matsuda, Y. Takita, Doped LaGaO3 perovskite type oxide as a new oxide ionic conductor, J. Am. Chem. Soc. 116 (1994) 3801. [3] K. Huang, J.B. Goodenough, Wet chemical synthesis of Sr- and Mg-doped LaGaO3 , a perovskite-type oxide-ion conductor, J. Solid State Chem. 136 (1998) 274. [4] R. Polini, A. Pamio, E. Traversa, Effect of synthetic route on sintering behaviour, phase purity and conductivity of Sr- and Mg-doped LaGaO3 perovskites, J. Eur. Ceram. Soc. 24 (2004) 1365. [5] J.G. Li, T. Ikegami, T. Mori, T. Wada, Reactive Ce0.8 RE0.2 O1.9 (RE = La, Sm, Gd, Dy, Y, Ho, Er, and Yb) powders via carbonate coprecipitation. 1. Synthesis and characterization, Chem. Mater. 13 (2001) 2913. [6] J.G. Li, T. Ikegami, T. Mori, T. Wada, Reactive Ce0.8 RE0.2 O1.9 (RE = La, Sm, Gd, Dy, Y, Ho, Er, and Yb) powders via carbonate coprecipitation. 2. Sintering, Chem. Mater. 13 (2001) 2921. [7] A.I.Y. Tok, L.H. Luo, F.Y.C. Boey, Carbonate co-precipitation of Gd2 O3 doped CeO2 solid solution nano-particles, Mater. Sci. Eng. A 383 (2004) 29.
Colloids and Surfaces A: Physicochem. Eng. Aspects 313–314 (2008) 154–157
Sr- and Mg-doped LaGaO3 powder synthesis by carbonate coprecipitation Nam Soo Chae a , Kil Soon Park a , Young Soo Yoon b , In Sang Yoo a , Jong Sung Kim a , Hyon Hee Yoon a,∗ a b
Department of Chemical Engineering, Kyungwon University, Sungnam 461-701, Republic of Korea Department of Advanced Technology Fusion, Konkuk University, 173-401 Seoul, Republic of Korea Received 18 November 2006; accepted 20 April 2007 Available online 2 June 2007
Abstract Sr- and Mg-doped LaGaO3 (LSGM) and Co-doped LSGM (LSGMC) powders were synthesized by coprecipitation using ammonium carbonate or ammonium bicarbonate precipitant. Precursor, calcined particles, and sintered pellets of LSGM were characterized using TGA, XRD, and SEM. The calcined powders of LSGM showed uniform microstructures with nearly spherical morphology and average particle size of 100–200 nm. The ionic conductivity of the sintered LSGM and LSGMC pellets at 1400 ◦ C was 4.5 × 10−2 S/cm and 1.13 S/cm at measuring temperature of 800 ◦ C, respectively. The internal microstructure was observed by FIB-SEM and it was found that the internal pores affected adversely on the ionic conductivity of the sintered LSGM pellets. The ionic conductivity of the as-prepared powders would be improved by optimization of the preparation process. The results of this study indicated that ammonium carbonate coprecipitation could be used as a convenient and economic method to produce LSGM electrolyte for low temperature SOFCs. © 2007 Elsevier B.V. All rights reserved. Keywords: Solid oxide fuel cell; Coprecipiation; Electrolyte; LSGM; Ion conductivity
1. Introduction Recent solid oxide fuel cell (SOFC) development has been focused on a lower temperature operation less than 800 ◦ C [1]. One of the methods to reduce the operating temperature is the use of new electrolyte materials which has higher oxide ionic conductivity. Sr- and Mg-doped lanthanum gallate (LSGM) has been found to have higher oxide ionic conductivity over a wide range of oxygen partial pressures at a low temperature (600–800 ◦ C) compared to commonly used electrolyte materials such as YSZ [2]. Further improvement of the ionic conductivity has been reported by doping cobalt into the LSGM. Many researchers had demonstrated high performance SOFCs using LSGM as the electrolyte. LSGM powders are typically prepared by solid-state reactions. The conventional techniques require high temperatures for sintering. Several types of wet-chemical methods, such
∗
Corresponding author. Tel.: +82 31 750 5356; fax: +82 31 750 5574. E-mail address: [email protected] (H.H. Yoon).
0927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2007.04.084
as sol–gel, hydrothermal treatment, and coprecipitation, have been reported for the synthesis of LSGM powders [3]. The wet chemistry-derived powders were reported to show better reactivity than those prepared by solid-state reaction. However, the required sintering temperature is still above 1400 ◦ C [4]. Recently, a carbonate coprecipitation method was developed to prepare rare-earth-doped ceria [5]. It was reported that the oxide powders synthesized by carbonate coprecipitation method could be sintered to >99.5% of the theoretical density at temperatures of 1100–1250 ◦ C. Lowering the sintering temperature of electrolyte materials will allow fine grain sizes as well as the saving of energy [6]. In this study, we applied the carbonate coprecipitation method for the synthesis of LSGM electrolytes. Cobalt-doped LSGM (LSGMC) electrolyte powder was also prepared using this method. The powders and sintered pellets obtained were characterized by XRD, scanning electron microscopy (SEM), and focused ion beam-scanning electron microscopy (FIB-SEM). An impedance analyzer was used to determine the ionic conductivity of the sintered pellets at various temperatures. The influence of precipitation reaction conditions was evaluated.
N.S. Chae et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 313–314 (2008) 154–157
Fig. 1. TGA curves of the precursors of LSGM obtained by carbonate coprecipitation.
Fig. 2. XRD patterns of LSGM powder calcined at 1100 ◦ C and LSGM pellet sintered at 1400 ◦ C.
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bicarbonate (NH4 )HCO3 as a precipitant. The starting salts are La(NO3 )3 ·6H2 O, Sr(NO3 )2 , Ga(NO3 )3 ·xH2 O, Mg(NO3 )2 · 6H2 O, and Co(NO3 )2 ·6H2 O. The stoichiometric amounts of each component of the final product La0.9 Sr0.1 Ga0.8 Mg0.2 O2.85 for LSGM and La0.9 Sr0.1 Ga0.8 Mg0.1 Co0.1 O2.85 for LSGMC were dissolved in distilled water. The concentration of stock solution was 0.2 M for La3+ . An aqueous solution of ammonium carbonate or ammonium bicarbonate with a concentration of 1.5 M was used as the precipitant. The mixed salt solution of 200 mL was dripped at a speed of 5 mL/min into the precipitant solution in a beaker kept at 70 ◦ C with mild stirring. The resulting suspension was aged at 70 ◦ C for 2 h after completion of precipitation. The precipitate was filtered and dried at room temperature over 24 h. The dried powder was ground in an agate mortar for 10 min and calcined at various temperatures. The calcined powder was pressed into several disks of 15 mm diameter and 1 mm thickness at 30 MPa. The pellets were then sintered at various temperatures for 6 h in air [7]. The crystal structures of the sintered LSGM samples were analyzed via X-ray diffractometry. Differential thermal analysis/thermogravimetry (DTA/TG) of the precursor powder was performed using a TG–DTA analyzer. The morphology of the LSGM powders and sintered pellets were observed through a high resolution scanning electron microscopy (HRSEM) (Model S-4700, Hitachi). The internal microstructure of the sintered LSGM pellets was observed by a focused ion beam-scanning electron microscopy (FIB-SEM) (Model Nova-200, FEI Company). The oxide ionic conductivity of sintered samples was measured using a two-probe ac impedance method (Solatron 1280B) as a function of temperature from 773 K to 1173 K in air.
3. Results and discussion
2. Experiment LSGM and LSGMC powders were prepared by coprecipiation using ammonium carbonate (NH4 )2 CO3 or ammonium
The thermal behavior of precursors obtained using the ammonium coprecipitation method was investigated by DSC/TG analysis. Fig. 1 shows DSC/TG curves of LSGM precursors.
Fig. 3. SEM micrographs of LSGM powders calcined at different temperatures [(a) 900 ◦ C, (b) 1000 ◦ C, and (c) 1100 ◦ C] and LSGM pellets sintered at different temperatures [(d) 1400 ◦ C and (e) 1600 ◦ C].
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N.S. Chae et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 313–314 (2008) 154–157
Table 1 Nominal and experimental stoichiometric indexes of the sintered pellet (La0.9 Sr0.1 Ga0.8 Mg0.2 O2.85 ) Element
Nominal stoichiometric index Experimental stoichiometric index
La
Sr
Ga
Mg
0.9 0.91 ± 0.01
0.1 0.09 ± 0.01
0.8 0.82 ± 0.02
0.2 0.18 ± 0.01
The experimental values were determined by EDX analyses.
The curve indicates that the thermal decomposition occurred in two steps. Weight loss between 100 ◦ C and 300 ◦ C could be attributed to the partial decomposition of the ammonium lanthanide carbonates into oxycarbonates and the weight loss between 300 ◦ C and 850 ◦ C could be attributed to the decomposition of oxycarbonates into oxides. Weight loss below 100 ◦ C was contributed to absorbed moisture. Thermal decomposition of precursors was completed at about 900 ◦ C. The XRD pattern of LSGM powders and sintered pellets was shown in Fig. 2. The LSGM powders calcined at 1100 ◦ C exhibited typical XRD patterns of LaGaO3 -based perovskite structure although it contained a small amount of secondary phases such as of LaSrGa3 O7 and LaSrGaO4 . The XRD pattern of the LSGM sintered pellet showed that the secondary phases considerably decreased after sintering for 6 h at 1400 ◦ C. This result indicated that pure LSGM electrolytes could be prepared by carbonated coprecipitation method. In addition, EDX analysis showed the chemical composition of sintered LSGM pellets were in good agreement with stoichiometric composition of La0.9 Sr0.1 Ga0.8 Mg0.2 O2.85 , as shown in Table 1. Fig. 3 shows SEM micrographs of LSGM powders obtained after calcination at different temperatures and pellets obtained after sintering at 1400 ◦ C and 1600 ◦ C. The particle size of the powders was in the range of 100–200 nm as determined from SEM micrographs. The average grain size of LSGM pellets was in the range of 1–3 m. The oxide ionic conductivity of the sintered LSGM pellets was measured at temperatures from 500 ◦ C to 900 ◦ C. Fig. 4 shows Arrhenius plots of the ionic conductivities of the LSGM and LSGMC pellets sintered at 1400 ◦ C. The ionic conductivity of the LSGM and LSGMC samples was 4.5 × 10−2 S/cm and
Fig. 4. Arrhenius plot of ionic conductivity of the LSGM and LSGMC pellets sintered at 1400 ◦ C.
1.13 S/cm at 800 ◦ C, respectively. The ionic conductivity was observed to be significantly improved by doping cobalt into the LSGM. It was found that the coprecipitation conditions such as precipitant type affected the oxide ionic conductivity of the sintered LSGM pellets. As shown in Table 2, the ionic conductivity of the sintered LSGM pellets prepared using ammonium bicarbonate precipitant was lowered to 7.2 × 10−3 S/cm at 800 ◦ C. When ammonium bicarbonate was used as a precipitant, lower density of the sintered pellet was observed, as shown in Table 2. This result can be explained by comparing the internal microstructure of each sample as appeared in FIB-SEM micrographs of the sintered pellets. As shown in Fig. 5, the sintered pellet obtained
Fig. 5. FIB-SEM micrographs showing internal microstructure of LSGM pellets sintered at 1400 ◦ C: (a) prepared using ammonium carbonate precipitant and (b) prepared using ammonium bicarbonate precipitant.
N.S. Chae et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 313–314 (2008) 154–157 Table 2 Relative density and oxide ionic conductivity of the sintered LSGM pellets prepared using different precipitants Precipitants
Relative density sintered at 1400 ◦ C (%)
Oxide ionic conductivity (σ) at 800 ◦ C (S/cm)
Ammonium bicarbonate Ammonium carbonate
88 93
7.2 × 10−3 4.5 × 10−2
using ammonium carbonate had less pores than that obtained using ammonium bicarbonate. 4. Conclusions Nanocrystalline La0.9 Sr0.1 Ga0.8 Mg0.2 O2.85 (LSGM) and La0.9 Sr0.1 Ga0.8 Mg0.1 Co0.1 O2.85 (LSGMC) powders have been synthesized by coprecipiation using ammonium carbonate and ammonium bicarbonate as the precipitant. The calcined powders of LSGM showed uniform microstructures with nearly spherical morphology. The average particle size was in the range of 100–200 nm. The chemical composition of the sintered LSGM pellets was in good agreement with stoichiometric composition of La0.9 Sr0.1 Ga0.8 Mg0.2 O2.85 . The ionic conductivity of the LSGM and LSGMC samples sintered at 1400 ◦ C was 4.5 × 10−2 S/cm and 1.13 S/cm at 800 ◦ C, respectively. Considerable improvement of ionic conductivity was observed by doping cobalt into the LSGM. The internal microstructure was observed by FIB-SEM and it was found that the internal pores affected adversely on the ionic conductivity of
157
the sintered LSGM pellets. After improving the ionic conductivity by optimization of the preparation process, ammonium carbonate coprecipitation could be used as a convenient and economic method to produce LSGM electrolyte for low temperature SOFCs. Acknowledgment This work was supported by the Gyeonggi Regional Research Center and partly by the research fund from Kyungwon University. References [1] I.N. Sora, R. Pelosato, G. Dotelli, C. Schid, R. Ruffo, C.M. Mari, The system Al2 O3 and (Sr,Mg)-doped LaGaO3 : phase composition and electrical properties, Solid State Ionics 176 (2005) 81. [2] T. Ishihara, H. Matsuda, Y. Takita, Doped LaGaO3 perovskite type oxide as a new oxide ionic conductor, J. Am. Chem. Soc. 116 (1994) 3801. [3] K. Huang, J.B. Goodenough, Wet chemical synthesis of Sr- and Mg-doped LaGaO3 , a perovskite-type oxide-ion conductor, J. Solid State Chem. 136 (1998) 274. [4] R. Polini, A. Pamio, E. Traversa, Effect of synthetic route on sintering behaviour, phase purity and conductivity of Sr- and Mg-doped LaGaO3 perovskites, J. Eur. Ceram. Soc. 24 (2004) 1365. [5] J.G. Li, T. Ikegami, T. Mori, T. Wada, Reactive Ce0.8 RE0.2 O1.9 (RE = La, Sm, Gd, Dy, Y, Ho, Er, and Yb) powders via carbonate coprecipitation. 1. Synthesis and characterization, Chem. Mater. 13 (2001) 2913. [6] J.G. Li, T. Ikegami, T. Mori, T. Wada, Reactive Ce0.8 RE0.2 O1.9 (RE = La, Sm, Gd, Dy, Y, Ho, Er, and Yb) powders via carbonate coprecipitation. 2. Sintering, Chem. Mater. 13 (2001) 2921. [7] A.I.Y. Tok, L.H. Luo, F.Y.C. Boey, Carbonate co-precipitation of Gd2 O3 doped CeO2 solid solution nano-particles, Mater. Sci. Eng. A 383 (2004) 29.