- Email: [email protected]
Effect of Bi2O3 additives in Sc stabilized zirconia electrolyte on a stability of crystal phase and electrolyte properties
Solid State Ionics 158 (2003) 215 – 223 www.elsevier.com/locate/ssi
Effect of Bi2O3 additives in Sc stabilized zirconia electrolyte on a stability of crystal phase and electrolyte properties Masanori Hirano a,*, Takayuki Oda a, Kenji Ukai b, Yasunobu Mizutani b a
Department of Applied Chemistry, Aichi Institute of Technology, Yachigusa, Yakusa, Toyota, Aichi, 470-0392, Japan b Technical Research Institute, Toho Gas Co., Ltd., Shinpo-machi, Tokai, Aichi, 476-8501, Japan Received 21 April 2002; received in revised form 1 September 2002; accepted 6 September 2002
Abstract Sintering properties, microstructure, crystal phase transformation, electrical conductivity, mechanical strength, and electrolyte performance of Bi2O3-doped Sc2O3-stabilized zirconia prepared by homogeneous precipitation using hydrolysis were investigated. Adding 1 mol% Bi2O3 decreased the sintering temperature of 10 mol% Sc2O3-doped zirconia (10ScSZ) above 300 jC, while accelerating the growth of zirconia grains, so dense sintered ceramics in the cubic phase were formed in a temperature range of 1000 – 1100 jC. The addition of 1 mol% Bi2O3 to the 10ScSZ inhibited the cubic – rhombohedral phase transformation. Sufficient conductivity of 0.33 S/cm at 1000 jC and 0.12 S/cm at 800 jC for an electrolyte of solid oxide fuel cells (SOFC) was attained for 1 mol% Bi2O3 – 10 mol% Sc2O3 – codoped cubic zirconia (1Bi10ScSZ) sintered at 1200 jC. A maximum power density of 1.61 W/cm2 was obtained at 1000 jC using 1Bi10ScSZ as the electrolyte in SOFC. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Zirconia; Scandia; Bismuth oxide; Solid oxide fuel cells; Electrolyte
1. Introduction Electrolytes with improved ionic conductivity for low temperature (below 800 jC) solid oxide fuel cells (SOFC) and establishment of co-firing process of the component materials, i.e., air electrode, electrolyte, and fuel electrode at low sintering temperature (1000 –1200 jC), are desired. Using scandia-doped zirconia (ScSZ; Sc2O3 –ZrO2) as solid electrolyte in SOFC allows for higher ionic conductivity compared
* Corresponding author. Tel.: +81-565-48-8121; fax: +81-56548-0076. E-mail address: [email protected] (M. Hirano).
to that of Y2O3-stabilized zirconia (YSZ) [1 – 13], since the activation energy of mobile oxygen ions is smaller due to the ionic radius of Sc3 + being very close to that of Zr4 + [1]. In planar SOFC, the ohmic resistance of electrolytes is known to contribute the most to the cell’s internal resistance. In the scope of our search for electrolyte materials with improved conductivity and mechanical strength, we prepared ScSZ powders containing less than 7 mol% Sc2O3. Electrolytes with excellent crack resistance and good electrical conductivity have been fabricated by normal sintering [14 –17] and by applying the hot isostatic press (HIP) technique [18,19]. Generally, 8– 12 mol% ScSZ allows for optimal oxide ion conductivity at increased temperatures for all zirconia-based electro-
0167-2738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-2738(02)00912-8
216
M. Hirano et al. / Solid State Ionics 158 (2003) 215–223
lytes, but disadvantages of ScSZ cubic – rhombohedral phase transformation occur at 600 – 700 jC when doped with 8 mol% Sc 2O3 or more [5,20 – 23]. Decrease in conductivity of 8 mol% ScSZ by annealing at 1000 jC [3,6,12] have also been reported. Adding Gd2O 3 [5], Y 2O3 [11], CeO 2 [11], and Al2O3 [2,8,9] suppressed the cubic – rhombohedral phase transformation. Bi2O3, on the other hand, is known to be one of the aides for sintering, effectively decreasing the sintering temperature of cubic YSZ [24 – 27] and tetragonal zirconia (TZP) [28]. However, no effect of Bi2O3 addition on the properties of ScSZ has been reported, so we reported our preliminary results [29]. The present study was concerned with the effect of the addition of Bi2O3 on sintering properties, crystalline phase, grain growth, electrical conductivity, and mechanical properties of ScSZ. We tested the SOFC using the new electrolytes. The electrolyte performance of Bi2O3 –Sc2O3 –codoped cubic zirconia (BiScSZ) in SOFC was described.
Ni was used as the fuel electrode, 80 wt.% La0.8Sr0.2MnO3 20 wt.% 8 mol% YSZ as the air electrode, and 1Bi10ScSZ as the electrolyte in the cell tests; the electrolyte had a diameter of 25 mm and was 300 Am thick. The electrode materials were screen-printed with polymer binder on the electrolyte disks and fired at 1150 jC for 2 h. The electrodes were 40 – 50 Am thick with the active electrode surface area of 0.2 cm2. 97% Hydrogen (99.9%)– 3% H2O was used as fuel and oxygen (99.9%) as oxidant.
2. Experimental procedure Bi 2 O 3 – Sc 2 O 3 – codoped zirconia and Sc 2 O 3 doped zirconia powders were prepared using hydrolysis and homogeneous precipitation techniques described previously [15,19,29]. The powders were uniaxially pressed at 49 MPa and then cold-isostatically pressed at 196 MPa to form green compacts. The green compacts were sintered at 900 –1400 jC for 1 h in air. The bulk density of the sintered bodies was measured using the Archimedes principle. The crystal phases of sintered body were examined using X-ray diffractometry (XRD) with CuKa radiation. The fracture strength was measured via three-point bending on a universal testing machine, using a span of 30 mm and a crosshead speed of 0.5 mm/min, on the basis of a Japan Industrial Standards (JIS-R1601). The specimen dimensions for the bending test were 40 4 3 mm. Scanning electron microscopy (SEM) was used to analyze the microstructure of thermally etched surfaces of the samples. The electrical conductivity was measured in a temperature range of 600 – 1000 jC by AC impedance method using a frequency response analyzer in the frequency range of 1 –104 Hz.
Fig. 1. (a) Bulk density and (b) apparent porosity for 1Bi10ScSZ and 10ScSZ sintered at 900 to 1400 jC for 1 h.
M. Hirano et al. / Solid State Ionics 158 (2003) 215–223
3. Results and discussion 3.1. Effect of Bi2O3 addition on the sintering and crystalline phase of ScSZ The effect of 1 mol% Bi2O3 co-doping on the sintering behavior of 10 mol% Sc2O3-doped zirconia (10ScSZ) is shown in Fig. 1(a) and (b). Dense sintered ScSZ bodies were fabricated by adding 1 mol% Bi2O3 at 1000 to 1100 jC. The sintering temperature over 1300 jC is necessary for commercially available powders of 10ScSZ composition. Accelerated sintering at a considerably lower temperature within the narrow temperature range of 950– 1000 jC was found to be characteristic for 1 mol% Bi2O3-doped sample (1Bi10ScSZ) as compared to the (10ScSZ) sample free of Bi2O3 having general sintering temperature range from 950 to 1300 jC as
217
shown in Fig. 1. The addition of Bi2O3 to ScSZ is thought to be effective for low-temperature co-firing of SOFC components. XRD patterns for ‘‘as-prepared’’ powder, calcined powder treated at 600 jC for 1 h, as well as for the sintered bodies fired at 1100 and 1400 jC for 1 h, are shown in Fig. 2 for the 10 mol% Sc2O3 90 mol% ZrO2 (10ScSZ) composition. ‘‘As-prepared’’ powder has been precipitated from hydrous ZrO2 and dissolved Sc2O3 in dilute nitric acid solution through homogeneous precipitation technique. Before calcination, it showed only monoclinic ZrO2 pattern. The zirconia powder was stabilized to cubic structure by calcination at 600 jC to yield solid solution with Sc2O3. However, the structure of 10ScSZ sintered at 1100 –1400 jC was rhombohedral as shown in Fig. 2. XRD patterns of 1Bi10ScSZ sintered at 1000 – 1400 jC for 1 h are shown in Fig. 3. Although a trace
Fig. 2. X-ray diffraction patterns of as-prepared powder, calcined powder at 600 jC for 1 h, and sintered bodies fired at 1100 and 1400 jC for 1 h for the composition 10 mol% Sc2O3 – 90 mol% ZrO2.
218
M. Hirano et al. / Solid State Ionics 158 (2003) 215–223
Fig. 3. X-ray diffraction patterns of 1Bi10ScSZ sintered at 1000 to 1400 jC for 1 h.
of monoclinic ZrO2 was detected in sintered bodies fired below 1200 jC, no rhombohedral phase was observed in all samples indicated in Fig. 3. ScSZ was almost perfectly stabilized in the cubic phase by 1 mol% Bi2O3. The Bi2O3 was found to effectively stabilize ScSZ in order to maintain cubic structure while aiding the low-temperature sintering. The effect of the Bi2O3 amount added on the ScSZ crystal phase was examined for sintering at 1100, as well as 1200 jC for 1 h as shown in Fig. 4(a) and (b), respectively. Inhibition of cubic – rhombohedral phase transformation was observed when 0.5 mol% Bi2O3 were added, the amount of crystals in rhombohedral phase detected in the bodies sintered at 1100 jC was very small. However, since cubic – rhombohedral phase transformation in the bodies sintered above 1200 jC were not suppressed by the adding less than 0.5 mol% Bi2O3, the con-
centration of 1 mol% Bi2O3 was needed to maintain sufficiently cubic structure. 3.2. Effect of Bi2O3 addition on the microstructure and mechanical strength of ScSZ The microstructures of 1Bi10ScSZ thermally etched surfaces sintered at 1000 – 1400 jC for 1 h are shown in Fig. 5. The micrographs reveal dense grains of polygonal shape without pores. Abrupt grain growth was confirmed in the sintered bodies as the sintering temperature increased. Occurrence of small rod-like particles, under 1Am in length, was observed at the grain boundaries of the bodies sintered at 1000 and 1100 jC (Fig. 5(a) and (b)). Occurrence of submicron particles at the boundaries of 2 Am grains was also observed for the sample sintered at 1200 jC (Fig. 5(c)). We reported a few
Fig. 4. X-ray diffraction patterns of 10ScSZ and Bi10ScSZ doped with different amounts of Bi2O3 sintered at (a) 1100 jC for 1 h and (b) 1200 jC for 1 h.
M. Hirano et al. / Solid State Ionics 158 (2003) 215–223
219
220
M. Hirano et al. / Solid State Ionics 158 (2003) 215–223
Fig. 5. SEM micrographs of the polished and thermally etched surfaces of 1Bi10ScSZ sintered at (a) 1000 jC, (b) 1100 jC, (c) 1200 jC, (d) 1300 jC, and (e) 1400 jC for 1 h.
angular particles of 1 Am to be located at the boundary corners of the 3 –6 Am bulk grains of the thermally etched surfaces of material sintered at 1300 jC (Fig. 5(d)). Rod-like particles were also found on the thermally etched surfaces of the sample sintered at 1400 jC (Fig. 5(e)) consisting of large
bulk grains f 10 Am in size. These grains were observed only in the BiScSZ microstructures, suggesting that the material have grown at grain boundaries during sintering. These grains may have the cubic phase similar to matrix because XRD profiles showed cubic phase in Fig. 3.
M. Hirano et al. / Solid State Ionics 158 (2003) 215–223
Effect of Bi2O3 on the ScSZ grain growth during sintering at 1000– 1400 jC for 1 h is shown in Fig. 6. Average grain size of dense 10ScSZ without Bi2O3 sintered at 1300 and 1400 jC for 1 h was less than 1.5 Am. Dense 1Bi10ScSZ sintered at 1000 jC for 1 h consisted of fine grains less than 1 Am in size. The grain size of the BiScSZ increased constantly at a higher rate than that of the sintered bodies free of bismuth for temperatures up to 1300 jC, but the grain growth accelerated much more above 1300 jC. The presence of 1 mol% Bi2O3 greatly affected the ScSZ grain growth during sintering. Average bending strength for 1Bi10ScSZ sintered at 1200 jC for 1 h was 220 MPa. Although the bending strength for 1Bi10ScSZ was a little lower than for the 8 mol% YSZ (279 MPa) and 11ScSZ (313 MPa) sintered at 1450 jC [13], the material appears to have adequate strength for use as an electrolyte in electrolyte-supported planar SOFC. 3.3. Effect of Bi2O3 addition on the electrical properties of ScSZ Electrical conductivity of BiScSZ sintered at 1050 and 1200 jC for 1 h is shown in Fig. 7. The electrical conductivity of 1Bi10ScSZ sintered at 1200 jC showed higher values (0.33 S/cm) at 1000 jC compared to that (0.197 S/cm) of the material sintered at
Fig. 6. Variation in measured grain size from thermally etched surface of 10ScSZ and 1Bi10ScSZ with sintering temperature.
221
Fig. 7. Electrical conductivity of 1Bi10ScSZ sintered at 1050 and 1200 jC for 1 h.
1050 jC due to both (1) decreased grain boundary resistance through grain growth with increasing sintering temperature, and (2) elimination of monoclinic ZrO2 trace in the materials sintered at lower temperature of 1000– 1100 jC, as shown in Fig. 3. The electrical conductivity increased together with temperature to 0.12 S/cm at 800 jC, and to 0.33 S/cm at 1000 jC, so the material is supposed to have sufficient electrical conductivity to be used as an electrolyte in SOFC. The 1Bi10ScSZ sintered at 1200 jC for
Fig. 8. Electrical conductivity change of 1Bi10ScSZ sintered at 1050 and 1200 jC for 1 h on annealing at 1000 jC in air.
222
M. Hirano et al. / Solid State Ionics 158 (2003) 215–223
ported planar SOFC. The power density was a little lower than that of 11 mol% ScSZ electrolyte (>2.0 W/cm2) using Ni cermet (Ni/YSZ weight ratio, 4:6) fuel electrode due to insufficient porosity and large grains caused by growth of Ni grains in the pure Ni fuel electrode used in this study. Development of low-temperature sinterable fuel electrode materials with sufficient porosity that are suitable for BiScSZ electrolyte is necessary to improve the cell performance and power density.
4. Summary
Fig. 9. The characteristics of I – V and power density of the cell using 1Bi10ScSZ sintered at 1200 jC for 1 h.
1 h (63.9 kJ/mol) had activation energy similar to that of the ScSZ system (about 60 kJ/mol) [3] (e.g., 11ScSZ + 1 wt.% Al2O3 ! 61.3 kJ/mol), although 1Bi10ScSZ sintered at 1050 jC produced 77.2 kJ/ mol. Conductivity variation in 1Bi10ScSZ at 1000 jC is shown in Fig. 8 as a function of annealing time at 1000 jC. Although a slight conductivity decrease (5%) was observed after an annealing time of about 100 h, change in conductivity after annealing from 100 to 2000 h was hardly observed, and it was stable at 1000 jC. This decrease in conductivity is thought to be negligible compared to those observed for 7.8 mol% ScSZ (0.32 ! 0.18 S/cm after aging at 1000 jC for 300 h) [6] and 8 mol% ScSZ (0.34 ! 0.14 S/ cm after aging at 1000 jC for 1000 h) [2]. 3.4. Electrolyte performance of Bi2O3 – Sc2O3 – codoped zirconia Power generation tests for 1Bi10ScSZ as potential SOFC electrolyte materials were performed using Ni fuel electrode, and 80 wt.% La0.8Sr0.2MnO3 20 wt.% 8 mol% YSZ as air electrode. The electrolyte layer was 300 Am thick and the active electrode surface was 0.2 cm2. Fig. 9 shows the initial cell performance. Maximum power density of 1.61 W/cm2 at 1000 jC was achieved at 4.0 A/cm2. The data was confirmed to be very favorable for electrolyte-sup-
Fabrication of stable cubic ScSZ having excellent conductivity without appearance of rhombohedral phase was achieved at low temperature sintering (1000 –1200 jC) by the addition of Bi2O3. A concentration of 1 mol% Bi2O3 was needed to maintain an adequate cubic structure. Electrical conductivity of 1Bi10ScSZ sintered at 1200 jC was 0.33 S/cm in 1000 jC annealing, and 0.12 S/cm in 800 jC annealing; the activation energy of the 1Bi10ScSZ sintered at 1200 jC was 63.9 kJ/mol. A maximum power density of 1.61 W/cm2 was obtained at 1000 jC during a cell test. The present technique is considered to be applicable in low-temperature co-firing of components for SOFC.
References [1] D.W. Stricker, W.G. Carlson, J. Am. Ceram. Soc. 48 (1965) 286. [2] O. Yamamoto, T. Kawahara, Y. Takeda, N. Imanishi, Y. Sakaki, in: S.P.S. Badwal, M.J. Bannister, R.H. Hannink (Eds.), Science and Technology of Zirconia V, Technomic, Lancaster, PA, USA, 1993, pp. 733 – 741. [3] O. Yamamoto, Y. Arati, Y. Takeda, N. Imanishi, Y. Mizutani, M. Kawai, Y. Nakamura, Solid State Ionics 79 (1995) 137. [4] O. Yamamoto, Y. Takeda, N. Imanishi, Y. Arati, H. Sakai, Y. Mizutani, M. Kawai, Y. Nakamura, N. Sammes, in: B.V.R. Chowdari, et al. (Eds.), Proc. of 5th Asian Conference on Solid State Ionics, World Scientific Publishing Co., Singapore, 1996, pp. 221 – 233. [5] T. Ishii, T. Iwata, Y. Tajima, Solid State Ionics 57 (1992) 153. [6] S.P.S. Badwal, J. Mater. Sci. 22 (1987) 4125. [7] S.P.S. Badwal, J. Drennan, Solid State Ionics 53 – 56 (1992) 769. [8] Y. Mizutani, M. Tamura, M. Kawai, O. Yamamoto, Solid State Ionics 72 (1994) 271.
M. Hirano et al. / Solid State Ionics 158 (2003) 215–223 [9] Y. Mizutani, M. Tamura, M. Kawai, K. Nomura, Y. Nakamura, O. Yamamoto, in: M. Dokiya, et al. (Eds.), Proc. of 4th Int. Symp. on SOFC (SOFC IV), Yokohama, Japan, Electrochem. Soc. Proc., vol. 95-1. Electrochem. Soc., New Jersey, USA, 1995, pp. 301 – 309. [10] Y. Mizutani, M. Kawai, K. Nomura, Y. Nakamura, O. Yamamoto, in: S.C. Singhal, et al. (Eds.), Proc. of 5th Int. Symp. on SOFC (SOFC V), Pennington, NJ, U.S.A., Electrochem. Soc. Proc., vol. 97-40. Electrochem. Soc., New Jersey, USA, 1997, pp. 196 – 203. [11] K. Ukai, Y. Mizutani, M. Kawai, Y. Nakamura, Proc. of 3rd Int. Fuell Cell Conf., Nagoya, Japan, Fuel Cell Development and Information Center (FCDIC), Tokyo, Japan, 1999, pp. 441 – 444. [12] K. Nomura, Y. Mizutani, M. Kawai, Y. Nakamura, O. Yamamoto, Solid State Ionics 132 (2000) 235. [13] Y. Mizutani, M. Kawai, K. Nomura, Y. Nakamura, O. Yamamoto, in: S.C. Singhal, M. Dokiya (Eds.), Proc. of 6th Int. Symp. on SOFC (SOFC VI), Honolulu, Hawaii, U.S.A., Electrochem. Soc. Proc., vol. 99-19. Electrochem. Soc., New Jersey, USA, 1999, pp. 185 – 192. [14] M. Hirano, E. Kato, J. Ceram. Soc. Jpn. 105 (1997) 37. [15] M. Hirano, S. Watanabe, E. Kato, Y. Mizutani, M. Kawai, Y. Nakamura, Solid State Ionics 111 (1998) 161. [16] M. Hirano, E. Kato, J. Mater. Sci. 34 (1999) 1399.
223
[17] M. Hirano, S. Watanabe, E. Kato, Y. Mizutani, M. Kawai, Y. Nakamura, J. Am. Ceram. Soc. 82 (1999) 2861. [18] M. Hirano, M. Inagaki, Y. Mizutani, K. Nomura, M. Kawai, Y. Nakamura, J. Am. Ceram. Soc. 83 (2000) 2619. [19] M. Hirano, M. Inagaki, Y. Mizutani, K. Nomura, M. Kawai, Y. Nakamura, Solid State Ionics 133 (2000) 1. [20] L. Lefevre, Ann. Chim. 8 (1963) 117. [21] F.M. Spiridonov, L.N. Popova, R.Y. Popilskii, J. Solid State Chem. 2 (1970) 430. [22] R. Ruh, H.J. Garrett, R.F. Domagala, V.A. Patel, J. Am. Ceram. Soc. 60 (1977) 399. [23] F.K. Moghadam, T. Yamashita, R. Sinclair, D.A. Stevenson, J. Am. Ceram. Soc. 66 (1983) 213. [24] K. Keizer, M.J. Verkerk, A.J. Burggraaf, Energy Ceram. (1980) 981. [25] K. Keizer, A.J. Burggraaf, G. De With, J. Mater. Sci. 17 (1982) 1095. [26] M.J. Verkerk, A.J.A. Winnubst, A.J. Burggraaf, J. Mater. Sci. 17 (1982) 3113. [27] A.J.A. Winnubst, A.J. Burggraaf, Mater. Res. Bull. 19 (1984) 613. [28] M.M.R. Boutz, A.J.A. Winnubst, F. Hartgers, A.J. Burggraaf, J. Mater. Sci. 29 (1994) 5374. [29] M. Hirano, T. Oda, K. Ukai, Y. Mizutani, J. Am. Ceram. Soc. 85 (2002) 1336.
Effect of Bi2O3 additives in Sc stabilized zirconia electrolyte on a stability of crystal phase and electrolyte properties Masanori Hirano a,*, Takayuki Oda a, Kenji Ukai b, Yasunobu Mizutani b a
Department of Applied Chemistry, Aichi Institute of Technology, Yachigusa, Yakusa, Toyota, Aichi, 470-0392, Japan b Technical Research Institute, Toho Gas Co., Ltd., Shinpo-machi, Tokai, Aichi, 476-8501, Japan Received 21 April 2002; received in revised form 1 September 2002; accepted 6 September 2002
Abstract Sintering properties, microstructure, crystal phase transformation, electrical conductivity, mechanical strength, and electrolyte performance of Bi2O3-doped Sc2O3-stabilized zirconia prepared by homogeneous precipitation using hydrolysis were investigated. Adding 1 mol% Bi2O3 decreased the sintering temperature of 10 mol% Sc2O3-doped zirconia (10ScSZ) above 300 jC, while accelerating the growth of zirconia grains, so dense sintered ceramics in the cubic phase were formed in a temperature range of 1000 – 1100 jC. The addition of 1 mol% Bi2O3 to the 10ScSZ inhibited the cubic – rhombohedral phase transformation. Sufficient conductivity of 0.33 S/cm at 1000 jC and 0.12 S/cm at 800 jC for an electrolyte of solid oxide fuel cells (SOFC) was attained for 1 mol% Bi2O3 – 10 mol% Sc2O3 – codoped cubic zirconia (1Bi10ScSZ) sintered at 1200 jC. A maximum power density of 1.61 W/cm2 was obtained at 1000 jC using 1Bi10ScSZ as the electrolyte in SOFC. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Zirconia; Scandia; Bismuth oxide; Solid oxide fuel cells; Electrolyte
1. Introduction Electrolytes with improved ionic conductivity for low temperature (below 800 jC) solid oxide fuel cells (SOFC) and establishment of co-firing process of the component materials, i.e., air electrode, electrolyte, and fuel electrode at low sintering temperature (1000 –1200 jC), are desired. Using scandia-doped zirconia (ScSZ; Sc2O3 –ZrO2) as solid electrolyte in SOFC allows for higher ionic conductivity compared
* Corresponding author. Tel.: +81-565-48-8121; fax: +81-56548-0076. E-mail address: [email protected] (M. Hirano).
to that of Y2O3-stabilized zirconia (YSZ) [1 – 13], since the activation energy of mobile oxygen ions is smaller due to the ionic radius of Sc3 + being very close to that of Zr4 + [1]. In planar SOFC, the ohmic resistance of electrolytes is known to contribute the most to the cell’s internal resistance. In the scope of our search for electrolyte materials with improved conductivity and mechanical strength, we prepared ScSZ powders containing less than 7 mol% Sc2O3. Electrolytes with excellent crack resistance and good electrical conductivity have been fabricated by normal sintering [14 –17] and by applying the hot isostatic press (HIP) technique [18,19]. Generally, 8– 12 mol% ScSZ allows for optimal oxide ion conductivity at increased temperatures for all zirconia-based electro-
0167-2738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-2738(02)00912-8
216
M. Hirano et al. / Solid State Ionics 158 (2003) 215–223
lytes, but disadvantages of ScSZ cubic – rhombohedral phase transformation occur at 600 – 700 jC when doped with 8 mol% Sc 2O3 or more [5,20 – 23]. Decrease in conductivity of 8 mol% ScSZ by annealing at 1000 jC [3,6,12] have also been reported. Adding Gd2O 3 [5], Y 2O3 [11], CeO 2 [11], and Al2O3 [2,8,9] suppressed the cubic – rhombohedral phase transformation. Bi2O3, on the other hand, is known to be one of the aides for sintering, effectively decreasing the sintering temperature of cubic YSZ [24 – 27] and tetragonal zirconia (TZP) [28]. However, no effect of Bi2O3 addition on the properties of ScSZ has been reported, so we reported our preliminary results [29]. The present study was concerned with the effect of the addition of Bi2O3 on sintering properties, crystalline phase, grain growth, electrical conductivity, and mechanical properties of ScSZ. We tested the SOFC using the new electrolytes. The electrolyte performance of Bi2O3 –Sc2O3 –codoped cubic zirconia (BiScSZ) in SOFC was described.
Ni was used as the fuel electrode, 80 wt.% La0.8Sr0.2MnO3 20 wt.% 8 mol% YSZ as the air electrode, and 1Bi10ScSZ as the electrolyte in the cell tests; the electrolyte had a diameter of 25 mm and was 300 Am thick. The electrode materials were screen-printed with polymer binder on the electrolyte disks and fired at 1150 jC for 2 h. The electrodes were 40 – 50 Am thick with the active electrode surface area of 0.2 cm2. 97% Hydrogen (99.9%)– 3% H2O was used as fuel and oxygen (99.9%) as oxidant.
2. Experimental procedure Bi 2 O 3 – Sc 2 O 3 – codoped zirconia and Sc 2 O 3 doped zirconia powders were prepared using hydrolysis and homogeneous precipitation techniques described previously [15,19,29]. The powders were uniaxially pressed at 49 MPa and then cold-isostatically pressed at 196 MPa to form green compacts. The green compacts were sintered at 900 –1400 jC for 1 h in air. The bulk density of the sintered bodies was measured using the Archimedes principle. The crystal phases of sintered body were examined using X-ray diffractometry (XRD) with CuKa radiation. The fracture strength was measured via three-point bending on a universal testing machine, using a span of 30 mm and a crosshead speed of 0.5 mm/min, on the basis of a Japan Industrial Standards (JIS-R1601). The specimen dimensions for the bending test were 40 4 3 mm. Scanning electron microscopy (SEM) was used to analyze the microstructure of thermally etched surfaces of the samples. The electrical conductivity was measured in a temperature range of 600 – 1000 jC by AC impedance method using a frequency response analyzer in the frequency range of 1 –104 Hz.
Fig. 1. (a) Bulk density and (b) apparent porosity for 1Bi10ScSZ and 10ScSZ sintered at 900 to 1400 jC for 1 h.
M. Hirano et al. / Solid State Ionics 158 (2003) 215–223
3. Results and discussion 3.1. Effect of Bi2O3 addition on the sintering and crystalline phase of ScSZ The effect of 1 mol% Bi2O3 co-doping on the sintering behavior of 10 mol% Sc2O3-doped zirconia (10ScSZ) is shown in Fig. 1(a) and (b). Dense sintered ScSZ bodies were fabricated by adding 1 mol% Bi2O3 at 1000 to 1100 jC. The sintering temperature over 1300 jC is necessary for commercially available powders of 10ScSZ composition. Accelerated sintering at a considerably lower temperature within the narrow temperature range of 950– 1000 jC was found to be characteristic for 1 mol% Bi2O3-doped sample (1Bi10ScSZ) as compared to the (10ScSZ) sample free of Bi2O3 having general sintering temperature range from 950 to 1300 jC as
217
shown in Fig. 1. The addition of Bi2O3 to ScSZ is thought to be effective for low-temperature co-firing of SOFC components. XRD patterns for ‘‘as-prepared’’ powder, calcined powder treated at 600 jC for 1 h, as well as for the sintered bodies fired at 1100 and 1400 jC for 1 h, are shown in Fig. 2 for the 10 mol% Sc2O3 90 mol% ZrO2 (10ScSZ) composition. ‘‘As-prepared’’ powder has been precipitated from hydrous ZrO2 and dissolved Sc2O3 in dilute nitric acid solution through homogeneous precipitation technique. Before calcination, it showed only monoclinic ZrO2 pattern. The zirconia powder was stabilized to cubic structure by calcination at 600 jC to yield solid solution with Sc2O3. However, the structure of 10ScSZ sintered at 1100 –1400 jC was rhombohedral as shown in Fig. 2. XRD patterns of 1Bi10ScSZ sintered at 1000 – 1400 jC for 1 h are shown in Fig. 3. Although a trace
Fig. 2. X-ray diffraction patterns of as-prepared powder, calcined powder at 600 jC for 1 h, and sintered bodies fired at 1100 and 1400 jC for 1 h for the composition 10 mol% Sc2O3 – 90 mol% ZrO2.
218
M. Hirano et al. / Solid State Ionics 158 (2003) 215–223
Fig. 3. X-ray diffraction patterns of 1Bi10ScSZ sintered at 1000 to 1400 jC for 1 h.
of monoclinic ZrO2 was detected in sintered bodies fired below 1200 jC, no rhombohedral phase was observed in all samples indicated in Fig. 3. ScSZ was almost perfectly stabilized in the cubic phase by 1 mol% Bi2O3. The Bi2O3 was found to effectively stabilize ScSZ in order to maintain cubic structure while aiding the low-temperature sintering. The effect of the Bi2O3 amount added on the ScSZ crystal phase was examined for sintering at 1100, as well as 1200 jC for 1 h as shown in Fig. 4(a) and (b), respectively. Inhibition of cubic – rhombohedral phase transformation was observed when 0.5 mol% Bi2O3 were added, the amount of crystals in rhombohedral phase detected in the bodies sintered at 1100 jC was very small. However, since cubic – rhombohedral phase transformation in the bodies sintered above 1200 jC were not suppressed by the adding less than 0.5 mol% Bi2O3, the con-
centration of 1 mol% Bi2O3 was needed to maintain sufficiently cubic structure. 3.2. Effect of Bi2O3 addition on the microstructure and mechanical strength of ScSZ The microstructures of 1Bi10ScSZ thermally etched surfaces sintered at 1000 – 1400 jC for 1 h are shown in Fig. 5. The micrographs reveal dense grains of polygonal shape without pores. Abrupt grain growth was confirmed in the sintered bodies as the sintering temperature increased. Occurrence of small rod-like particles, under 1Am in length, was observed at the grain boundaries of the bodies sintered at 1000 and 1100 jC (Fig. 5(a) and (b)). Occurrence of submicron particles at the boundaries of 2 Am grains was also observed for the sample sintered at 1200 jC (Fig. 5(c)). We reported a few
Fig. 4. X-ray diffraction patterns of 10ScSZ and Bi10ScSZ doped with different amounts of Bi2O3 sintered at (a) 1100 jC for 1 h and (b) 1200 jC for 1 h.
M. Hirano et al. / Solid State Ionics 158 (2003) 215–223
219
220
M. Hirano et al. / Solid State Ionics 158 (2003) 215–223
Fig. 5. SEM micrographs of the polished and thermally etched surfaces of 1Bi10ScSZ sintered at (a) 1000 jC, (b) 1100 jC, (c) 1200 jC, (d) 1300 jC, and (e) 1400 jC for 1 h.
angular particles of 1 Am to be located at the boundary corners of the 3 –6 Am bulk grains of the thermally etched surfaces of material sintered at 1300 jC (Fig. 5(d)). Rod-like particles were also found on the thermally etched surfaces of the sample sintered at 1400 jC (Fig. 5(e)) consisting of large
bulk grains f 10 Am in size. These grains were observed only in the BiScSZ microstructures, suggesting that the material have grown at grain boundaries during sintering. These grains may have the cubic phase similar to matrix because XRD profiles showed cubic phase in Fig. 3.
M. Hirano et al. / Solid State Ionics 158 (2003) 215–223
Effect of Bi2O3 on the ScSZ grain growth during sintering at 1000– 1400 jC for 1 h is shown in Fig. 6. Average grain size of dense 10ScSZ without Bi2O3 sintered at 1300 and 1400 jC for 1 h was less than 1.5 Am. Dense 1Bi10ScSZ sintered at 1000 jC for 1 h consisted of fine grains less than 1 Am in size. The grain size of the BiScSZ increased constantly at a higher rate than that of the sintered bodies free of bismuth for temperatures up to 1300 jC, but the grain growth accelerated much more above 1300 jC. The presence of 1 mol% Bi2O3 greatly affected the ScSZ grain growth during sintering. Average bending strength for 1Bi10ScSZ sintered at 1200 jC for 1 h was 220 MPa. Although the bending strength for 1Bi10ScSZ was a little lower than for the 8 mol% YSZ (279 MPa) and 11ScSZ (313 MPa) sintered at 1450 jC [13], the material appears to have adequate strength for use as an electrolyte in electrolyte-supported planar SOFC. 3.3. Effect of Bi2O3 addition on the electrical properties of ScSZ Electrical conductivity of BiScSZ sintered at 1050 and 1200 jC for 1 h is shown in Fig. 7. The electrical conductivity of 1Bi10ScSZ sintered at 1200 jC showed higher values (0.33 S/cm) at 1000 jC compared to that (0.197 S/cm) of the material sintered at
Fig. 6. Variation in measured grain size from thermally etched surface of 10ScSZ and 1Bi10ScSZ with sintering temperature.
221
Fig. 7. Electrical conductivity of 1Bi10ScSZ sintered at 1050 and 1200 jC for 1 h.
1050 jC due to both (1) decreased grain boundary resistance through grain growth with increasing sintering temperature, and (2) elimination of monoclinic ZrO2 trace in the materials sintered at lower temperature of 1000– 1100 jC, as shown in Fig. 3. The electrical conductivity increased together with temperature to 0.12 S/cm at 800 jC, and to 0.33 S/cm at 1000 jC, so the material is supposed to have sufficient electrical conductivity to be used as an electrolyte in SOFC. The 1Bi10ScSZ sintered at 1200 jC for
Fig. 8. Electrical conductivity change of 1Bi10ScSZ sintered at 1050 and 1200 jC for 1 h on annealing at 1000 jC in air.
222
M. Hirano et al. / Solid State Ionics 158 (2003) 215–223
ported planar SOFC. The power density was a little lower than that of 11 mol% ScSZ electrolyte (>2.0 W/cm2) using Ni cermet (Ni/YSZ weight ratio, 4:6) fuel electrode due to insufficient porosity and large grains caused by growth of Ni grains in the pure Ni fuel electrode used in this study. Development of low-temperature sinterable fuel electrode materials with sufficient porosity that are suitable for BiScSZ electrolyte is necessary to improve the cell performance and power density.
4. Summary
Fig. 9. The characteristics of I – V and power density of the cell using 1Bi10ScSZ sintered at 1200 jC for 1 h.
1 h (63.9 kJ/mol) had activation energy similar to that of the ScSZ system (about 60 kJ/mol) [3] (e.g., 11ScSZ + 1 wt.% Al2O3 ! 61.3 kJ/mol), although 1Bi10ScSZ sintered at 1050 jC produced 77.2 kJ/ mol. Conductivity variation in 1Bi10ScSZ at 1000 jC is shown in Fig. 8 as a function of annealing time at 1000 jC. Although a slight conductivity decrease (5%) was observed after an annealing time of about 100 h, change in conductivity after annealing from 100 to 2000 h was hardly observed, and it was stable at 1000 jC. This decrease in conductivity is thought to be negligible compared to those observed for 7.8 mol% ScSZ (0.32 ! 0.18 S/cm after aging at 1000 jC for 300 h) [6] and 8 mol% ScSZ (0.34 ! 0.14 S/ cm after aging at 1000 jC for 1000 h) [2]. 3.4. Electrolyte performance of Bi2O3 – Sc2O3 – codoped zirconia Power generation tests for 1Bi10ScSZ as potential SOFC electrolyte materials were performed using Ni fuel electrode, and 80 wt.% La0.8Sr0.2MnO3 20 wt.% 8 mol% YSZ as air electrode. The electrolyte layer was 300 Am thick and the active electrode surface was 0.2 cm2. Fig. 9 shows the initial cell performance. Maximum power density of 1.61 W/cm2 at 1000 jC was achieved at 4.0 A/cm2. The data was confirmed to be very favorable for electrolyte-sup-
Fabrication of stable cubic ScSZ having excellent conductivity without appearance of rhombohedral phase was achieved at low temperature sintering (1000 –1200 jC) by the addition of Bi2O3. A concentration of 1 mol% Bi2O3 was needed to maintain an adequate cubic structure. Electrical conductivity of 1Bi10ScSZ sintered at 1200 jC was 0.33 S/cm in 1000 jC annealing, and 0.12 S/cm in 800 jC annealing; the activation energy of the 1Bi10ScSZ sintered at 1200 jC was 63.9 kJ/mol. A maximum power density of 1.61 W/cm2 was obtained at 1000 jC during a cell test. The present technique is considered to be applicable in low-temperature co-firing of components for SOFC.
References [1] D.W. Stricker, W.G. Carlson, J. Am. Ceram. Soc. 48 (1965) 286. [2] O. Yamamoto, T. Kawahara, Y. Takeda, N. Imanishi, Y. Sakaki, in: S.P.S. Badwal, M.J. Bannister, R.H. Hannink (Eds.), Science and Technology of Zirconia V, Technomic, Lancaster, PA, USA, 1993, pp. 733 – 741. [3] O. Yamamoto, Y. Arati, Y. Takeda, N. Imanishi, Y. Mizutani, M. Kawai, Y. Nakamura, Solid State Ionics 79 (1995) 137. [4] O. Yamamoto, Y. Takeda, N. Imanishi, Y. Arati, H. Sakai, Y. Mizutani, M. Kawai, Y. Nakamura, N. Sammes, in: B.V.R. Chowdari, et al. (Eds.), Proc. of 5th Asian Conference on Solid State Ionics, World Scientific Publishing Co., Singapore, 1996, pp. 221 – 233. [5] T. Ishii, T. Iwata, Y. Tajima, Solid State Ionics 57 (1992) 153. [6] S.P.S. Badwal, J. Mater. Sci. 22 (1987) 4125. [7] S.P.S. Badwal, J. Drennan, Solid State Ionics 53 – 56 (1992) 769. [8] Y. Mizutani, M. Tamura, M. Kawai, O. Yamamoto, Solid State Ionics 72 (1994) 271.
M. Hirano et al. / Solid State Ionics 158 (2003) 215–223 [9] Y. Mizutani, M. Tamura, M. Kawai, K. Nomura, Y. Nakamura, O. Yamamoto, in: M. Dokiya, et al. (Eds.), Proc. of 4th Int. Symp. on SOFC (SOFC IV), Yokohama, Japan, Electrochem. Soc. Proc., vol. 95-1. Electrochem. Soc., New Jersey, USA, 1995, pp. 301 – 309. [10] Y. Mizutani, M. Kawai, K. Nomura, Y. Nakamura, O. Yamamoto, in: S.C. Singhal, et al. (Eds.), Proc. of 5th Int. Symp. on SOFC (SOFC V), Pennington, NJ, U.S.A., Electrochem. Soc. Proc., vol. 97-40. Electrochem. Soc., New Jersey, USA, 1997, pp. 196 – 203. [11] K. Ukai, Y. Mizutani, M. Kawai, Y. Nakamura, Proc. of 3rd Int. Fuell Cell Conf., Nagoya, Japan, Fuel Cell Development and Information Center (FCDIC), Tokyo, Japan, 1999, pp. 441 – 444. [12] K. Nomura, Y. Mizutani, M. Kawai, Y. Nakamura, O. Yamamoto, Solid State Ionics 132 (2000) 235. [13] Y. Mizutani, M. Kawai, K. Nomura, Y. Nakamura, O. Yamamoto, in: S.C. Singhal, M. Dokiya (Eds.), Proc. of 6th Int. Symp. on SOFC (SOFC VI), Honolulu, Hawaii, U.S.A., Electrochem. Soc. Proc., vol. 99-19. Electrochem. Soc., New Jersey, USA, 1999, pp. 185 – 192. [14] M. Hirano, E. Kato, J. Ceram. Soc. Jpn. 105 (1997) 37. [15] M. Hirano, S. Watanabe, E. Kato, Y. Mizutani, M. Kawai, Y. Nakamura, Solid State Ionics 111 (1998) 161. [16] M. Hirano, E. Kato, J. Mater. Sci. 34 (1999) 1399.
223
[17] M. Hirano, S. Watanabe, E. Kato, Y. Mizutani, M. Kawai, Y. Nakamura, J. Am. Ceram. Soc. 82 (1999) 2861. [18] M. Hirano, M. Inagaki, Y. Mizutani, K. Nomura, M. Kawai, Y. Nakamura, J. Am. Ceram. Soc. 83 (2000) 2619. [19] M. Hirano, M. Inagaki, Y. Mizutani, K. Nomura, M. Kawai, Y. Nakamura, Solid State Ionics 133 (2000) 1. [20] L. Lefevre, Ann. Chim. 8 (1963) 117. [21] F.M. Spiridonov, L.N. Popova, R.Y. Popilskii, J. Solid State Chem. 2 (1970) 430. [22] R. Ruh, H.J. Garrett, R.F. Domagala, V.A. Patel, J. Am. Ceram. Soc. 60 (1977) 399. [23] F.K. Moghadam, T. Yamashita, R. Sinclair, D.A. Stevenson, J. Am. Ceram. Soc. 66 (1983) 213. [24] K. Keizer, M.J. Verkerk, A.J. Burggraaf, Energy Ceram. (1980) 981. [25] K. Keizer, A.J. Burggraaf, G. De With, J. Mater. Sci. 17 (1982) 1095. [26] M.J. Verkerk, A.J.A. Winnubst, A.J. Burggraaf, J. Mater. Sci. 17 (1982) 3113. [27] A.J.A. Winnubst, A.J. Burggraaf, Mater. Res. Bull. 19 (1984) 613. [28] M.M.R. Boutz, A.J.A. Winnubst, F. Hartgers, A.J. Burggraaf, J. Mater. Sci. 29 (1994) 5374. [29] M. Hirano, T. Oda, K. Ukai, Y. Mizutani, J. Am. Ceram. Soc. 85 (2002) 1336.