- Email: [email protected]
Synthesis and characterization of LSGM thin film electrolyte by RF magnetron sputtering for LT-SOFCS
Available online at www.sciencedirect.com
Solid State Ionics 179 (2008) 1268 – 1272 www.elsevier.com/locate/ssi
Synthesis and characterization of LSGM thin film electrolyte by RF magnetron sputtering for LT-SOFCS K. Sasaki a,⁎, M. Muranaka a , A. Suzuki b , T. Terai a a
Department of Nuclear Engineering and Management, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-8656, Japan b Nuclear Professional School, The University of Tokyo, 2-22 Shirakatashirane, Tokai, Naka, Ibaraki 319-1188, Japan Received 18 July 2007; received in revised form 5 February 2008; accepted 13 February 2008
Abstract La0.9Sr0.1Ga0.8Mg0.2O3 (LSGM) thin film electrolytes of 1.6–5 μm thicknesses were fabricated on Ni-based anode supports by RF magnetron sputtering using a sintered LSGM target and post annealing in air. The effect of sputtering gas pressure on the LSGM thin film density was examined. The effect of flatness of the Ni-based anode supports on the gas tightness and oxygen-ion conductivity was clarified. RF magnetron sputtering was conducted in a mixed gas of Ar and O2. The Ni-based anode supports were heated to 400 °C during the deposition. The post annealing was carried out at 1100 °C in air. The microstructure of the fabricated LSGM thin films and compatibility between the fabricated LSGM thin film and the Ni-based anode support were examined using FE-SEM, EDX and XRD. The gas tightness of the fabricated LSGM thin film electrolytes was confirmed by measuring the open circuit voltage. The composition of the fabricated LSGM thin films was determined by ICP analysis. The fabricated thin film electrolytes, which were annealed at 1000-1100 °C, consisted of the LSGM single phase with the perovskite structure. There was no element migration between the LSGM thin film and the Ni-based anode support. The fabricated LSGM thin films were gas tight. The density of the LSGM thin film fabricated under the sputtering gas pressure of 1 Pa were higher than those sputtered at 2 Pa. The data clearly showed the feasibility of the RF magnetron sputtering method to directly fabricate the LSGM thin film electrolyte on the Ni-based anode. © 2008 Elsevier B.V. All rights reserved. Keywords: Solid oxide fuel cell (SOFC); RF magnetron sputtering; LSGM; Thin films
1. Introduction The Solid Oxide Fuel Cell (SOFC) is a promising candidate as a next-generation energy conversion system for stationary applications such as home co-generation systems which supply electricity and heat [1–3]. For utilization of the SOFCs, a reduction in the manufacturing cost and realization of a short start-up time are indispensable. The introduction of low cost alloy materials as the interconnector is desired for cost reduction instead of expensive ceramic materials. For the introduction of low cost alloy materials, a reduction in the operating temperature of the SOFCs is necessary. The reduction in operating temperature is also an effective way to shorten the start-up time and retard material degradation. However, the reduction in op⁎ Corresponding author. E-mail address: [email protected] (K. Sasaki). 0167-2738/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2008.02.039
erating temperature of the SOFCs leads to a rapid increase in the bulk resistance of the electrolyte. Doped lanthanum gallate (La0.9Sr0.1Ga0.8Mg0.2O3, LSGM) has a superior oxygen-ion conductivity of about 0.1 Scm- 1 at 750 °C, negligible electronic conduction and high chemical stability over a broad range of oxygen partial pressures (10- 20-1 atm) [4,5]. Therefore, LSGM is a candidate electrolyte material for the reduced temperature SOFCs. The introduction of an extremely thin film of LSGM is one of the promising approaches to overcome the issue mentioned above. Recently, various attempts have been made to fabricate LSGM thin films on Ni-based anode supports. However, almost all of the attempts required some kind of interlayer because LSGM forms a secondary phase with Ni at high temperature during fabrications of the LSGM films [6,7]. Unfortunately, it is difficult to thoroughly prevent the reaction, and the existence of this interlayer decreases the oxygen-ion conductivity [8]. Therefore, it is desired to
K. Sasaki et al. / Solid State Ionics 179 (2008) 1268–1272
directly fabricate the LSGM thin film on the Ni-based anode support at low temperature to avoid forming the secondary phase between the LSGM thin film and the Ni-based anode support. RF magnetron sputtering is an extremely reliable technique for deposition of thin films and is used in industry for large scale production. In case of RF magnetron sputtering, it is easy to uniformly deposit dense ceramic thin films only by controlling a few parameters. The deposited thin films consist of small active particles, so that it is easy to sinter at low temperature. The low sintering temperature enables the direct fabrication of LSGM thin films on Ni-based anode supports without any secondary phases. This paper examines a novel approach to directly fabricate the LSGM thin film by RF magnetron sputtering on the Nibased anode support.
1269
2.2. Deposition and annealing of LSGM thin films A custom-designed RF magnetron sputtering system with a 60 mm target diameter was used to deposit the La0.9Sr0.1Ga0.8Mg0.2O3 (LSGM). A sintered pellet made of a commercially available LSGM powder (SEIMI CHEMICAL Co., Ltd.) was used as the sputtering target. Sputtering was conducted at 7 W/cm2 RF power on the target with a 40 mm distance between the substrates and target. A mixed gas of Ar and O2 at the ratio of Ar/O2 = 80/20 was used for the sputtering. The sputtering gas pressure during the deposition was maintained at 1 or 2 Pa. The deposition rate of the LSGM at 1 and 2 Pa were 0.8 and 1 μm/h respectively. The temperature of the Ni-based anode supportive pellets as the substrate for deposition of the LSGM thin films was 400 °C. The deposited LSGM thin films were post annealed in air at 6001100 °C for 1 h.
2. Experimental 2.3. Characterization of LSGM thin films 2.1. Preparation of anode supports Porous NiO–YSZ and NiO-ScSZ substrates were prepared by conventional powder pressing and sintering. NiCO3 powder (Kojundo Chemical Laboratory) was thermally decomposed on NiO at 600 °C. The NiO powder and polymethylmethacrylate (PMMA) (Soken Chemical and Engineering) as a pore former were mixed with 8 mol% yttria-stabilized zirconia (YSZ) powder (Daiichi Kigenso Kagaku Kogyo) or scandia-doped zirconia (ScSZ) powder (Daiichi Kigenso Kagaku Kogyo) using a ball mill, pressed into pellets and sintered at 1320 °C to prepare the porous NiO-YSZ or NiO-ScSZ supportive pellets. A dense NiOScSZ functional layer was prepared on the porous NiO-YSZ or NiO-ScSZ supportive pellets. NiO powder and ScSZ powder were mixed with ethyl-cellulose and an organic solvent to prepare the NiO-ScSZ paste. The NiO-ScSZ paste was painted by a screen printing technique on the polished surface of the NiOYSZ or NiO-ScSZ support pellets, and fired at 1100 °C.
The microstructure of the LSGM films was confirmed using a field emission scanning electron microscope (FE-SEM) (JEOL Ltd., JSM7000F) and energy dispersive spectroscope (EDX) (JEOL Ltd., JED2300). The thickness of the LSGM thin films was determined using the FE-SEM on the fractured cross sections. Compatibility between the fabricated LSGM thin film and the Ni-based anode supports was examined by an EDX mapping analysis and X-ray diffraction (XRD) (MAC Science, MXP-3) using Cu-Kα radiation. The chemical composition of the deposited thin films was analyzed by the Inductively Coupled Plasma (ICP) method using a sequential plasma spectrometer (Shimadzu, ICPS-1000IV). The LSGM thin films deposited on the Si wafer by RF magnetron sputtering were dissolved in warm 10% dilute HNO3 and measured. The chemical composition of the LSGM powder used as the source material for the sputtering target was also measured as a reference.
Fig. 1. XRD patterns of LSGM thin films, which were prepared by RF magnetron sputtering and annealing in air at 600-1100 °C, and the LSGM powder which was used to make the sputtering target.
1270
K. Sasaki et al. / Solid State Ionics 179 (2008) 1268–1272
In order to prepare complete cells, Ag-LSGM composite thin films were prepared as air electrodes by RF magnetron sputtering on the LSGM thin films [9]. Pt meshes were used as current collectors for both the fuel and air electrodes. No reference electrode was used, but two additional voltage probes placed on both the fuel electrode and the air electrode were used to avoid measuring the resistance of the Pt lead wires. The
electrochemical properties of the cells were examined under constant flows of H2 (3% H2O) as the fuel and O2 as the oxidant. Glass rings (BK-7, m.p. = 740 °C) were used as the gas sealants. The open circuit voltage (OCV) was measured to confirm the gas tightness. Th electrochemical measurements were conducted using an electrochemical analyzer (Ivium, Ivium Stat type10 V/5A).
Fig. 2. EDX mapping on a fractured cross section of the LSGM thin film annealed in air at 1100 °C. (It was difficult to separate the Sr peak from the Zr peak.)
K. Sasaki et al. / Solid State Ionics 179 (2008) 1268–1272
3. Results and Discussion 3.1. Crystallization and compatibility The X-ray diffraction patterns of the LSGM thin films, which were prepared by the RF magnetron sputtering and the annealing in air at 600-1100 °C, are shown in Fig. 1 and compared with that of the LSGM powder which was used to make the sputtering target. The LSGM thin films were fabricated on the NiO-ScSZ functional layer prepared on the NiO-YSZ supportive pellets, so that peaks of NiO, ScSZ and YSZ were found for all specimens. The LSGM thin films after the annealing at 600 °C were amorphous. Phases with the perovskite structure existed in the LSGM thin films annealed at 700 °C or higher. However, trace intermediate products were also found in the thin films annealed at 700-900 °C. Thin films annealed at 10001100 °C were single phases of LSGM with the perovskite structure. The LSGM thin films had no specific crystal orientation. EDX mapping on a cross section of the LSGM thin film annealed in air at 1100 °C for 1 h is shown in Fig. 2. There was no element migration between the LSGM thin film and the Ni-based anode support. These results clearly showed that the LSGM thin film fabricated by the RF magnetron sputtering is compatible with the Ni-based anode support.
1271
Therefore, it is clear that the pores remaining in the LSGM thin films are not penetrating holes and the LSGM thin films are gas tight. Therefore, the sputtering gas pressure should be maintained low in order to fabricate the LSGM thin film with a high density. In this study, 1 Pa was better than 2 Pa as the sputtering gas pressure.
3.2. Microstructures and gas tightness Fig. 3-1 shows the surfaces and fractured cross sections of the LSGM thin films fabricated by the RF magnetron sputtering for 5 h under the sputtering gas pressure of 2 Pa ((a) as deposited, (b) after annealing at 1100 °C). The thickness of the LSGM as deposited thin film was 5 μm, therefore, the deposition rate was 1 μm/h. The deposited LSGM thin film did not shrink during the post annealing. The deposited LSGM thin film consisted of particles with diameters of 50-100 nm. During the annealing, the particles aggregated and pores with a 50-200 nm diameter formed inside the LSGM thin film. Fig. 3-2 shows the surfaces and fractured cross sections of the LSGM thin films fabricated by deposition for 2 h under the sputtering gas pressure of 1 Pa ((c) as deposited, (d) after annealing at 1100 °C). The thickness of the LSGM thin film after the annealing was 1.6 μm and the deposited LSGM thin film might not have shrunk during the annealing, so that the deposition rate decreased to 0.8 μm/h with the decrease in the sputtering gas pressure. Particles forming the LSGM thin film deposited under the sputtering gas pressure of 1 Pa were also 50-100 nm in diameter but piled closer with a higher density than at 2 Pa, therefore, the pores that remained in the annealed LSGM thin film deposited under the sputtering gas pressure of 1 Pa became smaller than at 2 Pa. This suggests that the density of the LSGM thin film fabricated at 1 Pa is higher than that at 2 Pa. The OCV of the cells with the LSGM thin films fabricated at the 2 Pa sputtering gas pressure were measured using humidified H2 (3% H2O) as the fuel and O2 as the oxidant. OCVs of both the LSGM thin film with a 2 μm thickness fabricated under 2 Pa and with a 1.6 μm thickness fabricated under 1 Pa were around 1.1 V at 600 °C, which was very close to the theoretical value.
Fig. 3. SEM images of LSGM thin films: (3-1) 2 Pa for 5 h (a) as deposited and (b) after annealing, (3-2) 1 Pa for 2 h(c) as deposited and (d) after annealing, (3-3) on swelled Ni-ScSZ (e) continuous part and (f) broken part.
1272
K. Sasaki et al. / Solid State Ionics 179 (2008) 1268–1272
Table 1 Chemical compositions of the LSGM thin film and the LSGM powder used to prepare the sputtering target
LSGM thin film (a) LSGM powder (b) ratio of (a) and (b)
La
Sr
Ga
Mg
0.878 0.903 0.97
0.070 0.100 0.70
0.862 0.801 1.08
0.138 0.199 0.69
(atm ratio).
Independent of the sputtering gas pressure, the OCVs of some cells were below 0.5 V even when their electrolyte was 5 μm thick. These low OCVs result from a gas leak through the LSGM thin films. Fig. 3-3 shows an SEM of the cross sections of this LSGM thin film which is not gas tight. This LSGM thin film was fabricated by sputtering under 2 Pa and post annealing. Some part of the Ni-ScSZ function layers swelled to a 6 μm height. Although the major part of the LSGM thin film was dense and continuous (Fig. 3-3(e)), some part of the LSGM thin film on the swelled Ni-ScSZ function layer was broken and penetrating holes occurred (Fig. 3-3(f)). This result indicates that the Ni-based anode support must be flat in order to fabricate the LSGM thin film by RF magnetron sputtering. 3.3. Chemical compositions The chemical composition of the LSGM thin film fabricated in this study is shown in Table 1 with the values for the LSGM powder used to prepare the sputtering target. The chemical composition of the LSGM thin film had shifted from the value of the LSGM powder. There was a gain in the Ga composition and deficits in Sr and Mg in the LSGM thin film. The shift of this chemical composition results from differences in the sputtering rate of each element. Countermeasures for adjusting the chemical composition of the LSGM thin film remains as a matter to be discussed further.
4. Conclusions Impervious La0.9Sr0.1Ga0.8Mg0.2O3 (LSGM) thin film electrolytes, which are compatible with the Ni-based anode support, have been successfully fabricated on Ni-based anode supports by RF magnetron sputtering and post annealing in air. Thin films annealed at 1000-1100 °C are single phases of LSGM with the perovskite structure. The LSGM thin film has no specific crystal orientation. The Ni-based anode support must be flat in order to fabricate the LSGM thin film by RF magnetron sputtering. Otherwise, some penetrating holes occur in the LSGM thin film. The sputtering gas pressure should be low in order to fabricate the LSGM thin film with a high density. The chemical composition of the LSGM thin film fabricated in this study was different from the LSGM powder used as the reference. Countermeasures for adjusting the chemical composition of the LSGM thin film remains as a matter to be discussed further. The data clearly showed the feasibility of the RF magnetron sputtering method to directly fabricate the LSGM thin film electrolyte on a Ni-based anode support. References [1] S.C. Singhal, in: O. Yamamoto, Y. Takeda, S. Noda, S. Kawatsu, N. Imanishi (Eds.), International Symposium on Fuel Cells for Vehicles, Electrochem. Soc. Jpn., Tokyo Japan, 2000, pp. 109–122. [2] N.Q. Minh, Solid State Ionics 174 (2004) 271. [3] O. Yamamoto, Electochem. Acta 45 (2000) 2423. [4] T. Ishihara, H. Matsuda, Y. Takita, Solid State Ionics 79 (1995) 147. [5] J. Kim, H. Yoo, Solid State Ionics 136/137 (2000) 91. [6] K. Huang, M. Feng, J.B. Goodenough, C. Milliken, J. Electrochem. Soc. 144 (1997) 3620. [7] K. Huang, J. Wan, J.B. Goodenough, J. Electrochem. Soc. 148 (2001) A788. [8] K.N. Kim, B.K. Kim, J.W. Son, J. Kim, H.-W. Lee, J. Moon, Solid State Ionics 177 (2006) 2155. [9] K. Sasaki, M. Muranaka, A. Suzuki, T. Terai, ECS Transactions - Solid Oxide Fuel Cells 7 (2007) 1311.
Solid State Ionics 179 (2008) 1268 – 1272 www.elsevier.com/locate/ssi
Synthesis and characterization of LSGM thin film electrolyte by RF magnetron sputtering for LT-SOFCS K. Sasaki a,⁎, M. Muranaka a , A. Suzuki b , T. Terai a a
Department of Nuclear Engineering and Management, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-8656, Japan b Nuclear Professional School, The University of Tokyo, 2-22 Shirakatashirane, Tokai, Naka, Ibaraki 319-1188, Japan Received 18 July 2007; received in revised form 5 February 2008; accepted 13 February 2008
Abstract La0.9Sr0.1Ga0.8Mg0.2O3 (LSGM) thin film electrolytes of 1.6–5 μm thicknesses were fabricated on Ni-based anode supports by RF magnetron sputtering using a sintered LSGM target and post annealing in air. The effect of sputtering gas pressure on the LSGM thin film density was examined. The effect of flatness of the Ni-based anode supports on the gas tightness and oxygen-ion conductivity was clarified. RF magnetron sputtering was conducted in a mixed gas of Ar and O2. The Ni-based anode supports were heated to 400 °C during the deposition. The post annealing was carried out at 1100 °C in air. The microstructure of the fabricated LSGM thin films and compatibility between the fabricated LSGM thin film and the Ni-based anode support were examined using FE-SEM, EDX and XRD. The gas tightness of the fabricated LSGM thin film electrolytes was confirmed by measuring the open circuit voltage. The composition of the fabricated LSGM thin films was determined by ICP analysis. The fabricated thin film electrolytes, which were annealed at 1000-1100 °C, consisted of the LSGM single phase with the perovskite structure. There was no element migration between the LSGM thin film and the Ni-based anode support. The fabricated LSGM thin films were gas tight. The density of the LSGM thin film fabricated under the sputtering gas pressure of 1 Pa were higher than those sputtered at 2 Pa. The data clearly showed the feasibility of the RF magnetron sputtering method to directly fabricate the LSGM thin film electrolyte on the Ni-based anode. © 2008 Elsevier B.V. All rights reserved. Keywords: Solid oxide fuel cell (SOFC); RF magnetron sputtering; LSGM; Thin films
1. Introduction The Solid Oxide Fuel Cell (SOFC) is a promising candidate as a next-generation energy conversion system for stationary applications such as home co-generation systems which supply electricity and heat [1–3]. For utilization of the SOFCs, a reduction in the manufacturing cost and realization of a short start-up time are indispensable. The introduction of low cost alloy materials as the interconnector is desired for cost reduction instead of expensive ceramic materials. For the introduction of low cost alloy materials, a reduction in the operating temperature of the SOFCs is necessary. The reduction in operating temperature is also an effective way to shorten the start-up time and retard material degradation. However, the reduction in op⁎ Corresponding author. E-mail address: [email protected] (K. Sasaki). 0167-2738/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2008.02.039
erating temperature of the SOFCs leads to a rapid increase in the bulk resistance of the electrolyte. Doped lanthanum gallate (La0.9Sr0.1Ga0.8Mg0.2O3, LSGM) has a superior oxygen-ion conductivity of about 0.1 Scm- 1 at 750 °C, negligible electronic conduction and high chemical stability over a broad range of oxygen partial pressures (10- 20-1 atm) [4,5]. Therefore, LSGM is a candidate electrolyte material for the reduced temperature SOFCs. The introduction of an extremely thin film of LSGM is one of the promising approaches to overcome the issue mentioned above. Recently, various attempts have been made to fabricate LSGM thin films on Ni-based anode supports. However, almost all of the attempts required some kind of interlayer because LSGM forms a secondary phase with Ni at high temperature during fabrications of the LSGM films [6,7]. Unfortunately, it is difficult to thoroughly prevent the reaction, and the existence of this interlayer decreases the oxygen-ion conductivity [8]. Therefore, it is desired to
K. Sasaki et al. / Solid State Ionics 179 (2008) 1268–1272
directly fabricate the LSGM thin film on the Ni-based anode support at low temperature to avoid forming the secondary phase between the LSGM thin film and the Ni-based anode support. RF magnetron sputtering is an extremely reliable technique for deposition of thin films and is used in industry for large scale production. In case of RF magnetron sputtering, it is easy to uniformly deposit dense ceramic thin films only by controlling a few parameters. The deposited thin films consist of small active particles, so that it is easy to sinter at low temperature. The low sintering temperature enables the direct fabrication of LSGM thin films on Ni-based anode supports without any secondary phases. This paper examines a novel approach to directly fabricate the LSGM thin film by RF magnetron sputtering on the Nibased anode support.
1269
2.2. Deposition and annealing of LSGM thin films A custom-designed RF magnetron sputtering system with a 60 mm target diameter was used to deposit the La0.9Sr0.1Ga0.8Mg0.2O3 (LSGM). A sintered pellet made of a commercially available LSGM powder (SEIMI CHEMICAL Co., Ltd.) was used as the sputtering target. Sputtering was conducted at 7 W/cm2 RF power on the target with a 40 mm distance between the substrates and target. A mixed gas of Ar and O2 at the ratio of Ar/O2 = 80/20 was used for the sputtering. The sputtering gas pressure during the deposition was maintained at 1 or 2 Pa. The deposition rate of the LSGM at 1 and 2 Pa were 0.8 and 1 μm/h respectively. The temperature of the Ni-based anode supportive pellets as the substrate for deposition of the LSGM thin films was 400 °C. The deposited LSGM thin films were post annealed in air at 6001100 °C for 1 h.
2. Experimental 2.3. Characterization of LSGM thin films 2.1. Preparation of anode supports Porous NiO–YSZ and NiO-ScSZ substrates were prepared by conventional powder pressing and sintering. NiCO3 powder (Kojundo Chemical Laboratory) was thermally decomposed on NiO at 600 °C. The NiO powder and polymethylmethacrylate (PMMA) (Soken Chemical and Engineering) as a pore former were mixed with 8 mol% yttria-stabilized zirconia (YSZ) powder (Daiichi Kigenso Kagaku Kogyo) or scandia-doped zirconia (ScSZ) powder (Daiichi Kigenso Kagaku Kogyo) using a ball mill, pressed into pellets and sintered at 1320 °C to prepare the porous NiO-YSZ or NiO-ScSZ supportive pellets. A dense NiOScSZ functional layer was prepared on the porous NiO-YSZ or NiO-ScSZ supportive pellets. NiO powder and ScSZ powder were mixed with ethyl-cellulose and an organic solvent to prepare the NiO-ScSZ paste. The NiO-ScSZ paste was painted by a screen printing technique on the polished surface of the NiOYSZ or NiO-ScSZ support pellets, and fired at 1100 °C.
The microstructure of the LSGM films was confirmed using a field emission scanning electron microscope (FE-SEM) (JEOL Ltd., JSM7000F) and energy dispersive spectroscope (EDX) (JEOL Ltd., JED2300). The thickness of the LSGM thin films was determined using the FE-SEM on the fractured cross sections. Compatibility between the fabricated LSGM thin film and the Ni-based anode supports was examined by an EDX mapping analysis and X-ray diffraction (XRD) (MAC Science, MXP-3) using Cu-Kα radiation. The chemical composition of the deposited thin films was analyzed by the Inductively Coupled Plasma (ICP) method using a sequential plasma spectrometer (Shimadzu, ICPS-1000IV). The LSGM thin films deposited on the Si wafer by RF magnetron sputtering were dissolved in warm 10% dilute HNO3 and measured. The chemical composition of the LSGM powder used as the source material for the sputtering target was also measured as a reference.
Fig. 1. XRD patterns of LSGM thin films, which were prepared by RF magnetron sputtering and annealing in air at 600-1100 °C, and the LSGM powder which was used to make the sputtering target.
1270
K. Sasaki et al. / Solid State Ionics 179 (2008) 1268–1272
In order to prepare complete cells, Ag-LSGM composite thin films were prepared as air electrodes by RF magnetron sputtering on the LSGM thin films [9]. Pt meshes were used as current collectors for both the fuel and air electrodes. No reference electrode was used, but two additional voltage probes placed on both the fuel electrode and the air electrode were used to avoid measuring the resistance of the Pt lead wires. The
electrochemical properties of the cells were examined under constant flows of H2 (3% H2O) as the fuel and O2 as the oxidant. Glass rings (BK-7, m.p. = 740 °C) were used as the gas sealants. The open circuit voltage (OCV) was measured to confirm the gas tightness. Th electrochemical measurements were conducted using an electrochemical analyzer (Ivium, Ivium Stat type10 V/5A).
Fig. 2. EDX mapping on a fractured cross section of the LSGM thin film annealed in air at 1100 °C. (It was difficult to separate the Sr peak from the Zr peak.)
K. Sasaki et al. / Solid State Ionics 179 (2008) 1268–1272
3. Results and Discussion 3.1. Crystallization and compatibility The X-ray diffraction patterns of the LSGM thin films, which were prepared by the RF magnetron sputtering and the annealing in air at 600-1100 °C, are shown in Fig. 1 and compared with that of the LSGM powder which was used to make the sputtering target. The LSGM thin films were fabricated on the NiO-ScSZ functional layer prepared on the NiO-YSZ supportive pellets, so that peaks of NiO, ScSZ and YSZ were found for all specimens. The LSGM thin films after the annealing at 600 °C were amorphous. Phases with the perovskite structure existed in the LSGM thin films annealed at 700 °C or higher. However, trace intermediate products were also found in the thin films annealed at 700-900 °C. Thin films annealed at 10001100 °C were single phases of LSGM with the perovskite structure. The LSGM thin films had no specific crystal orientation. EDX mapping on a cross section of the LSGM thin film annealed in air at 1100 °C for 1 h is shown in Fig. 2. There was no element migration between the LSGM thin film and the Ni-based anode support. These results clearly showed that the LSGM thin film fabricated by the RF magnetron sputtering is compatible with the Ni-based anode support.
1271
Therefore, it is clear that the pores remaining in the LSGM thin films are not penetrating holes and the LSGM thin films are gas tight. Therefore, the sputtering gas pressure should be maintained low in order to fabricate the LSGM thin film with a high density. In this study, 1 Pa was better than 2 Pa as the sputtering gas pressure.
3.2. Microstructures and gas tightness Fig. 3-1 shows the surfaces and fractured cross sections of the LSGM thin films fabricated by the RF magnetron sputtering for 5 h under the sputtering gas pressure of 2 Pa ((a) as deposited, (b) after annealing at 1100 °C). The thickness of the LSGM as deposited thin film was 5 μm, therefore, the deposition rate was 1 μm/h. The deposited LSGM thin film did not shrink during the post annealing. The deposited LSGM thin film consisted of particles with diameters of 50-100 nm. During the annealing, the particles aggregated and pores with a 50-200 nm diameter formed inside the LSGM thin film. Fig. 3-2 shows the surfaces and fractured cross sections of the LSGM thin films fabricated by deposition for 2 h under the sputtering gas pressure of 1 Pa ((c) as deposited, (d) after annealing at 1100 °C). The thickness of the LSGM thin film after the annealing was 1.6 μm and the deposited LSGM thin film might not have shrunk during the annealing, so that the deposition rate decreased to 0.8 μm/h with the decrease in the sputtering gas pressure. Particles forming the LSGM thin film deposited under the sputtering gas pressure of 1 Pa were also 50-100 nm in diameter but piled closer with a higher density than at 2 Pa, therefore, the pores that remained in the annealed LSGM thin film deposited under the sputtering gas pressure of 1 Pa became smaller than at 2 Pa. This suggests that the density of the LSGM thin film fabricated at 1 Pa is higher than that at 2 Pa. The OCV of the cells with the LSGM thin films fabricated at the 2 Pa sputtering gas pressure were measured using humidified H2 (3% H2O) as the fuel and O2 as the oxidant. OCVs of both the LSGM thin film with a 2 μm thickness fabricated under 2 Pa and with a 1.6 μm thickness fabricated under 1 Pa were around 1.1 V at 600 °C, which was very close to the theoretical value.
Fig. 3. SEM images of LSGM thin films: (3-1) 2 Pa for 5 h (a) as deposited and (b) after annealing, (3-2) 1 Pa for 2 h(c) as deposited and (d) after annealing, (3-3) on swelled Ni-ScSZ (e) continuous part and (f) broken part.
1272
K. Sasaki et al. / Solid State Ionics 179 (2008) 1268–1272
Table 1 Chemical compositions of the LSGM thin film and the LSGM powder used to prepare the sputtering target
LSGM thin film (a) LSGM powder (b) ratio of (a) and (b)
La
Sr
Ga
Mg
0.878 0.903 0.97
0.070 0.100 0.70
0.862 0.801 1.08
0.138 0.199 0.69
(atm ratio).
Independent of the sputtering gas pressure, the OCVs of some cells were below 0.5 V even when their electrolyte was 5 μm thick. These low OCVs result from a gas leak through the LSGM thin films. Fig. 3-3 shows an SEM of the cross sections of this LSGM thin film which is not gas tight. This LSGM thin film was fabricated by sputtering under 2 Pa and post annealing. Some part of the Ni-ScSZ function layers swelled to a 6 μm height. Although the major part of the LSGM thin film was dense and continuous (Fig. 3-3(e)), some part of the LSGM thin film on the swelled Ni-ScSZ function layer was broken and penetrating holes occurred (Fig. 3-3(f)). This result indicates that the Ni-based anode support must be flat in order to fabricate the LSGM thin film by RF magnetron sputtering. 3.3. Chemical compositions The chemical composition of the LSGM thin film fabricated in this study is shown in Table 1 with the values for the LSGM powder used to prepare the sputtering target. The chemical composition of the LSGM thin film had shifted from the value of the LSGM powder. There was a gain in the Ga composition and deficits in Sr and Mg in the LSGM thin film. The shift of this chemical composition results from differences in the sputtering rate of each element. Countermeasures for adjusting the chemical composition of the LSGM thin film remains as a matter to be discussed further.
4. Conclusions Impervious La0.9Sr0.1Ga0.8Mg0.2O3 (LSGM) thin film electrolytes, which are compatible with the Ni-based anode support, have been successfully fabricated on Ni-based anode supports by RF magnetron sputtering and post annealing in air. Thin films annealed at 1000-1100 °C are single phases of LSGM with the perovskite structure. The LSGM thin film has no specific crystal orientation. The Ni-based anode support must be flat in order to fabricate the LSGM thin film by RF magnetron sputtering. Otherwise, some penetrating holes occur in the LSGM thin film. The sputtering gas pressure should be low in order to fabricate the LSGM thin film with a high density. The chemical composition of the LSGM thin film fabricated in this study was different from the LSGM powder used as the reference. Countermeasures for adjusting the chemical composition of the LSGM thin film remains as a matter to be discussed further. The data clearly showed the feasibility of the RF magnetron sputtering method to directly fabricate the LSGM thin film electrolyte on a Ni-based anode support. References [1] S.C. Singhal, in: O. Yamamoto, Y. Takeda, S. Noda, S. Kawatsu, N. Imanishi (Eds.), International Symposium on Fuel Cells for Vehicles, Electrochem. Soc. Jpn., Tokyo Japan, 2000, pp. 109–122. [2] N.Q. Minh, Solid State Ionics 174 (2004) 271. [3] O. Yamamoto, Electochem. Acta 45 (2000) 2423. [4] T. Ishihara, H. Matsuda, Y. Takita, Solid State Ionics 79 (1995) 147. [5] J. Kim, H. Yoo, Solid State Ionics 136/137 (2000) 91. [6] K. Huang, M. Feng, J.B. Goodenough, C. Milliken, J. Electrochem. Soc. 144 (1997) 3620. [7] K. Huang, J. Wan, J.B. Goodenough, J. Electrochem. Soc. 148 (2001) A788. [8] K.N. Kim, B.K. Kim, J.W. Son, J. Kim, H.-W. Lee, J. Moon, Solid State Ionics 177 (2006) 2155. [9] K. Sasaki, M. Muranaka, A. Suzuki, T. Terai, ECS Transactions - Solid Oxide Fuel Cells 7 (2007) 1311.