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Low temperature synthesis of BaCeO3 nano powders by the citrate process
Materials Letters 57 (2003) 3346 – 3351 www.elsevier.com/locate/matlet
Low temperature synthesis of BaCeO3 nano powders by the citrate process Dong Wook Lee, Jong Han Won, Kwang Bo Shim * Ceramic Processing Research Center, Department of Ceramic Engineering, Hanyang University, 17 Heangdang-Dong, Seongdong-Ku, Seoul 133-791, South Korea Received 20 January 2002; received in revised form 17 January 2003; accepted 20 January 2003
Abstract Nanosized BaCeO3 powders with the homogeneous composition were synthesized at the relatively low temperature of 900 jC by a citrate process based on the Pechini method. A polymeric precursor was formed by use of citric acid and ethylene glycol as a chelating agent of metal ions and reaction medium, respectively. The orthorhombic BaCeO3 powders, about 100-nm sized and uniformly shaped, were obtained through the calcination of the polymeric precursor at 900 jC for 4 h. It was found that the small quantities of the remanent carbonate ions (CO23 ) were completely decomposed at over 1100 jC. D 2003 Elsevier Science B.V. All rights reserved. Keywords: BaCeO3; Nanosized powder; Citrate process
1. Introduction Solid oxide fuel cell (SOFC) materials based on Y2O3-stabilized ZrO2 have a major influence on the selection of other components for the fuel cell because they need to be heated above 1000 jC to maintain sufficient ionic conductivity. Therefore, SOFC materials which can be used at relatively low temperature range of 600 –800 jC need to be developed. Perovskite materials such as BaCeO3 or SrCeO3 are considered as the best candidates for this demand because they possess high proton conductivity [1,2]. In addition, such materials doped with rare earth element exhibit excellent proton conduction [3,4] character* Corresponding author. Tel.: +82-2-2290-0501; fax: +82-22291-7395. E-mail address: [email protected] (K.B. Shim).
istics at high temperatures in a hydrogen atmosphere. Therefore, they have been considered as possible sensors for hydrogen or humidity and are promising materials as the electrolyte of SOFCs requiring direct energy conversion [5]. The interests in these materials have also increased gradually because of the high efficiency for recycling of the fuel gas by bursting out of the exhausting steam gas toward air electrode [6]. Until now, the BaCeO3 and SrCeO3 powders have been fabricated by the conventional solid-state reaction method of oxide materials: BaCO3 + CeO2 ! BaCeO3 + CO2z [7,8]. However, it is very difficult to obtain a homogeneous composition by this method. Furthermore, dense, fine-grained sintered bodies cannot be made because of the inhomogeneity in shape and size of the initial powders, resulting from the prolonged calcinations at a high temperature for a long time. The repetitive milling process can also degrade
0167-577X/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(03)00072-7
D.W. Lee et al. / Materials Letters 57 (2003) 3346–3351
the electric and mechanical properties of the sintered bodies through impurity contamination [9,10]. In general, nanosized ceramic powders with a stoichiometric composition, good compositional homogeneity and a high purity can be prepared from wet chemical methods such as sol –gel, co-precipitation and the Pechini method. However, in the sol – gel process, it is difficult to control the degree of hydrolysis of the desired metal alkoxides. The coprecipitation process can produce an inhomogeneous compositional distribution because of vastly different solubilities of the metal ions. The citrate process, a modified Pechini method, is a polymerized complex method using citric acid for synthesizing ceramic powders [11]. The powders are synthesized from the organic liquid and the polymeric precursor, which contains the metal salt of the final oxide. In this citrate process, the distributions of each component are homogeneous because the metal ions are completely dissolved in polymeric resin during the process, and so the resultant powders have good compositional homogeneity, and narrow size and shape distributions. As a result, sintered bodies with high density and
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homogeneity can be made. This therefore makes the process relevant for SOFC electrolyte materials with excellent proton conductivity. There have been a number of reports on growing BaCeO3 thin films through a modified Pechini method [12,13]. The aim of this study was to synthesize nanosized BaCeO3 powders with a stoichiometric composition and uniform homogeneity at low temperature using this method. In particular, the synthesizing mechanism, thermal decomposition and phase change of the polymeric precursor have been evaluated.
2. Experimental procedure For the citrate process, cerium nitrate hexahydrate (Ce(NO3)36H2O; Yakuri Pure Chemicals, Japan) and barium carbonate (BaCO3; Yakuri Pure Chemicals) were used as starting materials. Citric acid monohydrate (CA) (C6H8O7H2O; Junsei Chemical, Japan) and ethylene glycol (EG) (C2H6O2; Showa Chemical, Japan) were used as a chelating agent and a reaction medium, respectively.
Fig. 1. Flow chart for preparing BaCeO3 by the citrate process.
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Fig. 2. TG/DTA curves of the polymeric precursor pyrolyzed at 300 jC for 4 h.
The molar ratio of EG/CA/Me (Me = BaCO3 + Ce(NO3)36H2O) was fixed as 20:5:1 [10,14]. Firstly, the 0.4 mol of CA was dissolved in 1.6 mol of EG by stirring at 50 F 5 jC for 40 min in order to prepare transparent chelating solution. Then 0.04 mol of Ce(NO3)36H2O was added to this solution without any other solvent, and stirred at 50 F 5 jC for 30 min in order to obtain a clear and transparent solution. Finally, 0.04 mol of BaCO3 was added to the solution and then the solution was heated at a temperature of 80 F 5 jC. At this stage, the reaction behavior with resin was observed. About 5 h later, the solution had transformed to yellowish transparent state, without any precipitates, and the pH value of this resin was measured as 2.7. The transparent solution was then heated at 90 jC for 12 h in the drying oven to promote gelation, and heat-treated at 300 jC for 4 h to obtain the polymeric precursor. This was black in color. The
Fig. 3. XRD patterns of the calcined powder at various temperatures for 4 h.
D.W. Lee et al. / Materials Letters 57 (2003) 3346–3351
resultant polymeric precursor was heat-treated at different temperatures between 800 and 1000 jC in air from 1 to 5 h using alumina crucible in an electric furnace. After heat treatment, white and homogeneous BaCeO3 powders were obtained. The decomposition and crystallization of the precursors obtained from the heat treatment at 300 jC for 4 h in drying oven were analyzed by thermogravimetric and differential thermal analysis (TG/DTA, SDT-2960, TA Instrument) at a heating rate of 10 jC/min to 1200 jC. The crystallization behavior was studied by X-ray diffraction (XRD, D/max-2C, Rigaku denki), CuKa, Ni filter, 40 kV –30 mA, the scan rate of 5j (2h)/min. A Fourier transform infrared spectrophotometer (FT-IR, Magna-IR 760 spectrometer, Nicolet) was used for the analysis of the remained carbonate ions. The morphology and size of the synthesized powders were studied by scanning electron microscopy (SEM, JSM-5900 LV, JEOL) and transmission electron microscopy (TEM, 200CX, JEOL).
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3. Results and discussion The powder-synthesizing procedure is summarized in Fig. 1. Firstly, each metallic ion formed a polymeric network structure with the citric acid and then was captured very strongly in the network structure by the esterification [15]. Polymeric precursors were formed through the decomposition of the organic phase by pyrolysis of the resin. Finally, metallic ions formed perovskite structured crystalline BaCeO3 via intermediate phases such as BaO, BaCO3 and CeO2 through the decomposition of the remaining organic materials by heat treatment. Fig. 2 shows the DTA result of the polymeric precursor dried at 300 jC for 4 h. Two kinds of exothermic peaks are observed, a weak one at around 360 jC and a strong one at around 458 jC. This DTA behavior correlates well with the continuous weight loss seen in the TGA curves between 300 and 500 jC. The DTA and TGA results can be explained in terms of an initial dehydration reaction, and the decompo-
Fig. 4. XRD patterns of powders calcined at 900 jC as a function of soaking duration.
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D.W. Lee et al. / Materials Letters 57 (2003) 3346–3351
sition and the oxidation of inorganic materials combined with the metallic ions, the citric acid and ethylene glycol. When they were heated over 300 jC, the CeO2, BaCO3 and BaO phases were formed by the oxidation of metallic ions. The BaCeO3 started to form the crystalline perovskite phase at about 600 jC where most organic elements were decomposed. Figs. 3 and
4 show the result of the X-ray diffraction analysis of the calcined powders as a function of temperature in the range of 800– 1000 jC and holding time in 1 – 5 h. When the powder was synthesized below 850 jC at the fixed holding time of 4 h, the perovskite BaCeO3 crystalline phase formed with the unwanted phases CeO2, BaCO3 and BaO (Fig. 3). The metallic ions in the complete metal-chelating state with citric acid did
Fig. 5. Electron micrographs and diffraction patterns of BaCeO3 powders calcined at 900 jC for 4 h. (a) SEM micrograph; (b) TEM micrograph and diffraction pattern.
D.W. Lee et al. / Materials Letters 57 (2003) 3346–3351
not produce any crystalline phase in the polymeric precursor state. As the calcining temperature increases from 800 to 900 jC, the proportions of the secondary phases of CeO2, BaCO3 and BaO reduce, so that at 900 jC XRD showed pure BaCeO3. Therefore, 900 jC was considered an optimum temperature for obtaining single phase BaCeO3. The minimum holding time for the production of single phase BaCeO3 at 900 jC was then examined. CeO2, BaCO3 and BaO were found after 1 h at 900 jC in the synthesized powders, but the proportion of these phases reduced at longer times, so that after 3 h of heat treatment, only weak XRD peaks could be detected from these phases. Finally, only perovskite BaCeO3 peaks were obtained after 900 jC for 4 h. SEM and TEM show that the average size of these BaCeO3 powders is about 100 nm and exhibits homogeneous distribution of the size and the shape (Fig. 5(a) and (b)). Small amounts of the carbonate (CO32 ) present in the synthesized BaCeO3 powders, which is used to be possible during the citrate process, was completely decomposed at the temperature of 1100 jC.
4. Conclusion The nanosized BaCeO3 powders were successfully synthesized by the citrate process based on Pechini method. The BaCeO3 powders obtained are in the size of about 100 nm and show a uniform distribution of the size and shape. It was investigated that small amount of the remained CO32 could be completely decomposed at the temperature of 1100 jC.
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Acknowledgements This work was supported by the Korea Science and Engineering Foundation (KOSEF) through the Ceramic Processing Research Center (CPRC) at Hanyang University.
References [1] T. Schober, F. Krug, W. Schilling, Solid State Ionics 97 (1997) 369. [2] N. Matsunami, T. Shimura, H. Iwahara, Solid State Ionics 106 (1998) 155. [3] W. Munch, K.D. Kreuer, G. Seifert, J. Maier, Solid State Ionics 136 – 137 (2000) 183. [4] K. Kunstler, H.J. Lang, A. Maiwald, G. Tomandl, Solid State Ionics 107 (1998) 221. [5] T. Norby, Solid State Ionics 125 (1999) 1. [6] K. Joo, J. Paitz, D. Klimm, P. Reiche, K.H. Auh, W. Schroder, Cryst. Res. Technol. 35 (4) (2000) 493. [7] K.S. Knight, N. Bonanos, Mater. Res. Bull. 30 (3) (1995) 347. [8] K. Ouchetto, F. Archaimbault, A. Pineau, J. Choisnet, J. Mater. Sci. Lett. 10 (1991) 1277. [9] P.P. Phule, S.H. Risbud, J. Mater. Sci. 25 (1990) 1169. [10] M. Arina, M. Kakihana, Y. Nakamura, M. Yashima, M. Yoshimura, J. Am. Ceram. Soc. 79 (11) (1996) 2847. [11] M.P. Pechini, US Patent no. 3330676, 1967. [12] V. Agarwal, M. Liu, J. Electrochem. Soc. 143 (10) (1996) 3239. [13] V. Agarwal, M. Liu, J. Mater. Sci. 32 (1997) 619. [14] C.P. Udawatte, M. Kakihana, M. Yoshimura, Solid State Ionics 128 (2000) 217. [15] P. Duran, C. Moure, M. Villegas, J. Tartaj, A.C. Caballero, J.F. Fernandez, J. Am. Ceram. Soc. 83 (5) (2000) 1029.
Low temperature synthesis of BaCeO3 nano powders by the citrate process Dong Wook Lee, Jong Han Won, Kwang Bo Shim * Ceramic Processing Research Center, Department of Ceramic Engineering, Hanyang University, 17 Heangdang-Dong, Seongdong-Ku, Seoul 133-791, South Korea Received 20 January 2002; received in revised form 17 January 2003; accepted 20 January 2003
Abstract Nanosized BaCeO3 powders with the homogeneous composition were synthesized at the relatively low temperature of 900 jC by a citrate process based on the Pechini method. A polymeric precursor was formed by use of citric acid and ethylene glycol as a chelating agent of metal ions and reaction medium, respectively. The orthorhombic BaCeO3 powders, about 100-nm sized and uniformly shaped, were obtained through the calcination of the polymeric precursor at 900 jC for 4 h. It was found that the small quantities of the remanent carbonate ions (CO23 ) were completely decomposed at over 1100 jC. D 2003 Elsevier Science B.V. All rights reserved. Keywords: BaCeO3; Nanosized powder; Citrate process
1. Introduction Solid oxide fuel cell (SOFC) materials based on Y2O3-stabilized ZrO2 have a major influence on the selection of other components for the fuel cell because they need to be heated above 1000 jC to maintain sufficient ionic conductivity. Therefore, SOFC materials which can be used at relatively low temperature range of 600 –800 jC need to be developed. Perovskite materials such as BaCeO3 or SrCeO3 are considered as the best candidates for this demand because they possess high proton conductivity [1,2]. In addition, such materials doped with rare earth element exhibit excellent proton conduction [3,4] character* Corresponding author. Tel.: +82-2-2290-0501; fax: +82-22291-7395. E-mail address: [email protected] (K.B. Shim).
istics at high temperatures in a hydrogen atmosphere. Therefore, they have been considered as possible sensors for hydrogen or humidity and are promising materials as the electrolyte of SOFCs requiring direct energy conversion [5]. The interests in these materials have also increased gradually because of the high efficiency for recycling of the fuel gas by bursting out of the exhausting steam gas toward air electrode [6]. Until now, the BaCeO3 and SrCeO3 powders have been fabricated by the conventional solid-state reaction method of oxide materials: BaCO3 + CeO2 ! BaCeO3 + CO2z [7,8]. However, it is very difficult to obtain a homogeneous composition by this method. Furthermore, dense, fine-grained sintered bodies cannot be made because of the inhomogeneity in shape and size of the initial powders, resulting from the prolonged calcinations at a high temperature for a long time. The repetitive milling process can also degrade
0167-577X/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(03)00072-7
D.W. Lee et al. / Materials Letters 57 (2003) 3346–3351
the electric and mechanical properties of the sintered bodies through impurity contamination [9,10]. In general, nanosized ceramic powders with a stoichiometric composition, good compositional homogeneity and a high purity can be prepared from wet chemical methods such as sol –gel, co-precipitation and the Pechini method. However, in the sol – gel process, it is difficult to control the degree of hydrolysis of the desired metal alkoxides. The coprecipitation process can produce an inhomogeneous compositional distribution because of vastly different solubilities of the metal ions. The citrate process, a modified Pechini method, is a polymerized complex method using citric acid for synthesizing ceramic powders [11]. The powders are synthesized from the organic liquid and the polymeric precursor, which contains the metal salt of the final oxide. In this citrate process, the distributions of each component are homogeneous because the metal ions are completely dissolved in polymeric resin during the process, and so the resultant powders have good compositional homogeneity, and narrow size and shape distributions. As a result, sintered bodies with high density and
3347
homogeneity can be made. This therefore makes the process relevant for SOFC electrolyte materials with excellent proton conductivity. There have been a number of reports on growing BaCeO3 thin films through a modified Pechini method [12,13]. The aim of this study was to synthesize nanosized BaCeO3 powders with a stoichiometric composition and uniform homogeneity at low temperature using this method. In particular, the synthesizing mechanism, thermal decomposition and phase change of the polymeric precursor have been evaluated.
2. Experimental procedure For the citrate process, cerium nitrate hexahydrate (Ce(NO3)36H2O; Yakuri Pure Chemicals, Japan) and barium carbonate (BaCO3; Yakuri Pure Chemicals) were used as starting materials. Citric acid monohydrate (CA) (C6H8O7H2O; Junsei Chemical, Japan) and ethylene glycol (EG) (C2H6O2; Showa Chemical, Japan) were used as a chelating agent and a reaction medium, respectively.
Fig. 1. Flow chart for preparing BaCeO3 by the citrate process.
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D.W. Lee et al. / Materials Letters 57 (2003) 3346–3351
Fig. 2. TG/DTA curves of the polymeric precursor pyrolyzed at 300 jC for 4 h.
The molar ratio of EG/CA/Me (Me = BaCO3 + Ce(NO3)36H2O) was fixed as 20:5:1 [10,14]. Firstly, the 0.4 mol of CA was dissolved in 1.6 mol of EG by stirring at 50 F 5 jC for 40 min in order to prepare transparent chelating solution. Then 0.04 mol of Ce(NO3)36H2O was added to this solution without any other solvent, and stirred at 50 F 5 jC for 30 min in order to obtain a clear and transparent solution. Finally, 0.04 mol of BaCO3 was added to the solution and then the solution was heated at a temperature of 80 F 5 jC. At this stage, the reaction behavior with resin was observed. About 5 h later, the solution had transformed to yellowish transparent state, without any precipitates, and the pH value of this resin was measured as 2.7. The transparent solution was then heated at 90 jC for 12 h in the drying oven to promote gelation, and heat-treated at 300 jC for 4 h to obtain the polymeric precursor. This was black in color. The
Fig. 3. XRD patterns of the calcined powder at various temperatures for 4 h.
D.W. Lee et al. / Materials Letters 57 (2003) 3346–3351
resultant polymeric precursor was heat-treated at different temperatures between 800 and 1000 jC in air from 1 to 5 h using alumina crucible in an electric furnace. After heat treatment, white and homogeneous BaCeO3 powders were obtained. The decomposition and crystallization of the precursors obtained from the heat treatment at 300 jC for 4 h in drying oven were analyzed by thermogravimetric and differential thermal analysis (TG/DTA, SDT-2960, TA Instrument) at a heating rate of 10 jC/min to 1200 jC. The crystallization behavior was studied by X-ray diffraction (XRD, D/max-2C, Rigaku denki), CuKa, Ni filter, 40 kV –30 mA, the scan rate of 5j (2h)/min. A Fourier transform infrared spectrophotometer (FT-IR, Magna-IR 760 spectrometer, Nicolet) was used for the analysis of the remained carbonate ions. The morphology and size of the synthesized powders were studied by scanning electron microscopy (SEM, JSM-5900 LV, JEOL) and transmission electron microscopy (TEM, 200CX, JEOL).
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3. Results and discussion The powder-synthesizing procedure is summarized in Fig. 1. Firstly, each metallic ion formed a polymeric network structure with the citric acid and then was captured very strongly in the network structure by the esterification [15]. Polymeric precursors were formed through the decomposition of the organic phase by pyrolysis of the resin. Finally, metallic ions formed perovskite structured crystalline BaCeO3 via intermediate phases such as BaO, BaCO3 and CeO2 through the decomposition of the remaining organic materials by heat treatment. Fig. 2 shows the DTA result of the polymeric precursor dried at 300 jC for 4 h. Two kinds of exothermic peaks are observed, a weak one at around 360 jC and a strong one at around 458 jC. This DTA behavior correlates well with the continuous weight loss seen in the TGA curves between 300 and 500 jC. The DTA and TGA results can be explained in terms of an initial dehydration reaction, and the decompo-
Fig. 4. XRD patterns of powders calcined at 900 jC as a function of soaking duration.
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D.W. Lee et al. / Materials Letters 57 (2003) 3346–3351
sition and the oxidation of inorganic materials combined with the metallic ions, the citric acid and ethylene glycol. When they were heated over 300 jC, the CeO2, BaCO3 and BaO phases were formed by the oxidation of metallic ions. The BaCeO3 started to form the crystalline perovskite phase at about 600 jC where most organic elements were decomposed. Figs. 3 and
4 show the result of the X-ray diffraction analysis of the calcined powders as a function of temperature in the range of 800– 1000 jC and holding time in 1 – 5 h. When the powder was synthesized below 850 jC at the fixed holding time of 4 h, the perovskite BaCeO3 crystalline phase formed with the unwanted phases CeO2, BaCO3 and BaO (Fig. 3). The metallic ions in the complete metal-chelating state with citric acid did
Fig. 5. Electron micrographs and diffraction patterns of BaCeO3 powders calcined at 900 jC for 4 h. (a) SEM micrograph; (b) TEM micrograph and diffraction pattern.
D.W. Lee et al. / Materials Letters 57 (2003) 3346–3351
not produce any crystalline phase in the polymeric precursor state. As the calcining temperature increases from 800 to 900 jC, the proportions of the secondary phases of CeO2, BaCO3 and BaO reduce, so that at 900 jC XRD showed pure BaCeO3. Therefore, 900 jC was considered an optimum temperature for obtaining single phase BaCeO3. The minimum holding time for the production of single phase BaCeO3 at 900 jC was then examined. CeO2, BaCO3 and BaO were found after 1 h at 900 jC in the synthesized powders, but the proportion of these phases reduced at longer times, so that after 3 h of heat treatment, only weak XRD peaks could be detected from these phases. Finally, only perovskite BaCeO3 peaks were obtained after 900 jC for 4 h. SEM and TEM show that the average size of these BaCeO3 powders is about 100 nm and exhibits homogeneous distribution of the size and the shape (Fig. 5(a) and (b)). Small amounts of the carbonate (CO32 ) present in the synthesized BaCeO3 powders, which is used to be possible during the citrate process, was completely decomposed at the temperature of 1100 jC.
4. Conclusion The nanosized BaCeO3 powders were successfully synthesized by the citrate process based on Pechini method. The BaCeO3 powders obtained are in the size of about 100 nm and show a uniform distribution of the size and shape. It was investigated that small amount of the remained CO32 could be completely decomposed at the temperature of 1100 jC.
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Acknowledgements This work was supported by the Korea Science and Engineering Foundation (KOSEF) through the Ceramic Processing Research Center (CPRC) at Hanyang University.
References [1] T. Schober, F. Krug, W. Schilling, Solid State Ionics 97 (1997) 369. [2] N. Matsunami, T. Shimura, H. Iwahara, Solid State Ionics 106 (1998) 155. [3] W. Munch, K.D. Kreuer, G. Seifert, J. Maier, Solid State Ionics 136 – 137 (2000) 183. [4] K. Kunstler, H.J. Lang, A. Maiwald, G. Tomandl, Solid State Ionics 107 (1998) 221. [5] T. Norby, Solid State Ionics 125 (1999) 1. [6] K. Joo, J. Paitz, D. Klimm, P. Reiche, K.H. Auh, W. Schroder, Cryst. Res. Technol. 35 (4) (2000) 493. [7] K.S. Knight, N. Bonanos, Mater. Res. Bull. 30 (3) (1995) 347. [8] K. Ouchetto, F. Archaimbault, A. Pineau, J. Choisnet, J. Mater. Sci. Lett. 10 (1991) 1277. [9] P.P. Phule, S.H. Risbud, J. Mater. Sci. 25 (1990) 1169. [10] M. Arina, M. Kakihana, Y. Nakamura, M. Yashima, M. Yoshimura, J. Am. Ceram. Soc. 79 (11) (1996) 2847. [11] M.P. Pechini, US Patent no. 3330676, 1967. [12] V. Agarwal, M. Liu, J. Electrochem. Soc. 143 (10) (1996) 3239. [13] V. Agarwal, M. Liu, J. Mater. Sci. 32 (1997) 619. [14] C.P. Udawatte, M. Kakihana, M. Yoshimura, Solid State Ionics 128 (2000) 217. [15] P. Duran, C. Moure, M. Villegas, J. Tartaj, A.C. Caballero, J.F. Fernandez, J. Am. Ceram. Soc. 83 (5) (2000) 1029.