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Effect of Ni doping on the phase stability and conductivity of scandia-stabilized zirconia
Solid State Ionics 180 (2009) 252–256
Contents lists available at ScienceDirect
Solid State Ionics j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / s s i
Effect of Ni doping on the phase stability and conductivity of scandia-stabilized zirconia Jong Hoon Joo, Gyeong Man Choi ⁎ Fuel Cell Research Center and Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang, Gyeongbuk 790-784, Republic of Korea
a r t i c l e
i n f o
Article history: Received 19 June 2008 Received in revised form 2 December 2008 Accepted 3 December 2008 Keywords: Scandia-stabilized zirconia Ni Phase transition Electrical conductivity Thick film electrolyte Solid oxide fuel cell
a b s t r a c t The effect of Ni doping on the phase stability and conductivity of scandia-stabilized zirconia (SSZ) thick film was studied. A free-standing 10SSZ thick-film (10 mol% Sc2O3-stabilized zirconia, ~ 10 μm thick) that was previously in contact with a Ni layer during co-firing was fabricated. The 10SSZ thick-film showed a cubic phase in contrast to the rhombohedral phase shown for a bulk 10SSZ sample. The Ni content in the SSZ thick film was ~ 1.7 mol%. The effect of Ni on the cubic phase formation was also confirmed by the similar observation of the cubic phase in the Ni-doped bulk 10SSZ sample. The observed conductivity behavior also supported the XRD observation. Ni was found to hinder the transformation of the cubic phase to the rhombohedral on cooling in 10SSZ samples after a reduction treatment. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Rare-earth ion doped zirconia is widely used in SOFCs (solid oxide fuel cells), oxygen sensors and structural applications due to its advantages in electrical and mechanical properties. In particular, yttria-stabilized zirconia (YSZ) has been widely studied as a choice for practical SOFCs because of its high ionic and low electronic conductivity as well as its excellent chemical stability under both reducing and oxidizing atmospheres at high temperature. The scandiastabilized zirconia (SSZ) has been intensely investigated in recent years due to its possible use as an electrolyte for intermediate temperature SOFCs [1,2] since SSZ shows the highest ionic conductivity among many zirconia-based electrolytes. The phenomenon may be explained by the low association enthalpy of the defects and the similarity between the ionic radii of Sc3+ and Zr4+ ions. However, the cubic to rhombohedral phase transformation near 600 °C upon cooling for a zirconia containing more than 10 mol% Sc2O3 is unfavorable for an electrolyte due to the irreversible thermal expansion and the abrupt decrease in the ionic conductivity [3]. It has been reported that the phase transformation can be prevented by adding 1–2 mol% of additives such as Y2O3, CeO2, or Ga2O3 [4,5]. In the fabrication of SOFCs, the interaction of transition metals with the zirconia electrolyte is in most cases unavoidable since transition metals are frequent constituents of the electrodes. For example, Mn, Fe,
⁎ Corresponding author. E-mail address: [email protected] (G.M. Choi). 0167-2738/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2008.12.003
or Co is present in almost all SOFC cathodes, and Ni in the anodes [6]. In fact, the state-of-the-art anode in SOFCs is Ni/electrolyte composite. The anode is typically fired in the temperature range of 1000–1600 °C. Thus, Ni diffusion into the electrolytes is unavoidable. A number of reports have already been made about the effect of Ni diffusion on the YSZ [6–9]. Linderoth et al. reported that Ni dissolves in the cubic zirconia phase, decreasing the conductivity in air [6]. However, when Nicontaining YSZ is reduced, some portion of the metallic Ni comes out from the solid solution and induces a cubic to tetragonal phase change in the Ni-containing YSZ [6]. The resulting mixture of tetragonal and cubic phase is responsible for the reduced conductivity. Although Ni-SSZ composites have been used as anodes in SOFC [10–12], to the best of our knowledge, no reports have been made on the effect of Ni on the SSZ. The effect of Ni doping on the SSZ thick-film has also not been reported, although the conductivity and the phase stability of SSZ in film form are important in the performance of anode-supported cells. This is in part because the film is always coated on conductive support materials such as Ni-electrolyte composite, and thus the deconvolution of the electrolyte film and substrate is difficult. During SOFC operation, NiO in composite NiO-electrolyte is reduced to metallic Ni and is expected to react with the electrolyte. However, it is not clear what happens when Ni is dissolved in SSZ electrolyte film. In this study, the effect of the reaction between Ni and SSZ film on the phase stability and the electrical conductivity of SSZ were studied. After co-firing of SSZ film on the NiO-coated NiO-SSZ composite substrate, the SSZ film was successfully detached from the support. Thus, a free-standing SSZ film which has possibly reacted with Ni was fabricated. 10SSZ (10 mol% Sc2O3-doped ZrO2) was selected as a test composition since the electrical conductivity of 10SSZ shows a maximum at this composition. The phase and
J.H. Joo, G.M. Choi / Solid State Ionics 180 (2009) 252–256
conductivity values of free-standing 10SSZ film were compared to those of the 10SSZ bulk specimens, with or without the addition of Ni. 2. Experimental procedures NiO and 10SSZ thick-films were sequentially coated on NiO-10SSZ composite substrate. In order to make NiO-10SSZ composite substrates, 10SSZ (99.99%, Daiichi Kigenso Kagaku Kogyo, Japan) and NiO (99.97%, High Purity Chemicals, Japan) powders were ball-milled for 12 h using zirconia balls in ethyl alcohol. The substrate was uniaxially pressed to form a disk-shaped sample of ~ 13 mm (ø) × ~ 1 mm (t) and pre-sintered at 900 °C for 1 h in air. NiO and 10SSZ slurries were screen-printed sequentially on the 10SSZ-NiO substrate with the thickness of ~20 μm. The screen printing slurries were prepared by mixing the powders with the appropriate amount of alpha-terpineol, methyl cellulose and ethylene glycol. The 3-layered 10SSZ(film)/NiO (film)/ 10SSZ-NiO(bulk) sample was co-fired at 1600 °C for 3 h in air and subsequently reduced in 97% H2/3% H2O gas mixture at 800 °C for 5 h. The pellet was immersed in a HCl solution and Ni was dissolved. Thus, the sintered 10SSZ thick-film was successfully detached from the Ni-10SSZ substrate with this method. In order to measure the in-plane (parallel to the film surface) conductivity of the freestanding thick-film, Pt paste (No.6082, Engelhard, USA) was painted on two ends of the film and fired at 800 °C for 1 h. Pt meshes (52 mesh, Alpha Aesar, USA) were used as a current collector. The fabrication process of free-standing 10SSZ film is schematically shown in Fig. 1.
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For comparison, the bulk samples (both undoped and Ni-doped 10SSZ) were also fabricated by pressing 10SSZ or 10SSZ ([(ZrO2)0.9 (Sc2O3)0.1]0.98·(NiO)0.02) powders doped with 2 mol% NiO, followed by cold-isostatic pressing and sintering at 1600 °C for 3 h in air and subsequently reduced in 97% H2/3% H2O gas mixture at 800 °C for 5 h. Pt paste electrodes were similarly painted on both sides of the disc-shaped specimen. The electrical conductivities of the film and the bulk were determined as a function of temperature from impedance spectra that were obtained by using an Impedance GainPhase Analyzer (Solartron SI 1260, UK) in the frequency region of 0.01 Hz to 5MHz. The temperature dependence of conductivity was measured while cooling the samples from 800 °C to 300 °C with a 3 °C/min rate in dry air with 50 °C steps. To assure equilibrium, the sample was maintained for 2 h at each temperature before a measurement was made. The phase of thick film was characterized by X-ray diffraction (XRD) using Cu-Kα radiation (MAC Science, M18XCE, Japan). XRD patterns of bulk samples were obtained from the diffraction for the sample surface. The XRD patterns of the detached films were taken from both the surface previously in contact with Ni and the free surface. The analysis of the Ni in SSZ thick film was performed with inductively-coupled plasma atomicemission spectroscopy (ICP-AES IRIS Advantage, TJA Solution, USA). Microstructure observation was performed by a field-emission scanning-electron-microscope (JEOL, model 3330F, Japan). In order to analyze the impedance patterns, the spectra were fitted using analysis software (ZSimpWin, PerkinElmer Instruments, USA).
Fig. 1. Procedure for fabricating free-standing 10SSZ thick-film sample was described. (a) NiO and 10SSZ pastes were sequentially screen-printed on a NiO-10SSZ pellet and co-fired at 1600 °C in air. (b) The cell was reduced at 800 °C for 5 h in 97%H2 + 3%H2O gas. (c) The Ni film was etched in hydrochloric (HCl) acid and 10SSZ thick-film was separated from Ni-10SSZ substrate. (d) Free-standing 10SSZ thick-film sample was obtained. (e) Pt paste electrodes were painted on the two ends of the film for the in-plane conductivity measurement.
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3. Results and discussion 3.1. Characterization of crystalline structure Fig. 2 shows the cross-sectional SEM micrograph of a free-standing 10SSZ thick film. The thickness of the film was ~ 10 μm, and the average grain size was ~ 8 μm due to the high sintering temperature (1600 °C). The error in the conductivity due to the rough surface is estimated as less than ~10% under the assumption that the error comes from the estimation of film thickness. The relative density of the sintered freestanding 10SSZ film was more than 95%, calculated from the weight and the dimension of the sample. Fig. 3 shows the XRD patterns for the 10SSZ bulk and the thick film. The XRD patterns from both surfaces of the detached 10SSZ film are shown. As expected, the 10SSZ bulk sample showed a rhombohedral phase at room temperature. It has been reported that samples with Sc2O3 content higher than 10% (10–15 mol%) show a rhombohedral phase which transforms to a cubic phase at ~ 600 °C [3]. However, the free-standing 10SSZ thick film, detached from the Ni/Ni-10SSZ substrate, showed a cubic phase on both surfaces with a trace of monoclinic phase. Thus, different phases were observed for the same composition depending upon the sample form or the fabrication process. The bulk 10SSZ and the detached 10SSZ film were made from the same powders and were heat treated at the same condition. The only difference is that the 10SSZ film was previously being in contact with the Ni layer. The Ni doping in the SSZ film was determined by ICP-AES. The SSZ thick film contained ~ 1.7 mol% Ni. It is thus reasonable to assume that Ni in SSZ film hinder the transformation of the cubic phase to the rhombohedral on cooling. The stability of the cubic phase in the thick film is attributed to the Ni-doping in 10SSZ sample after the reaction with Ni. We may also estimate the diffusion distance of Ni into the 10SSZ film. A rough estimation shows that the diffusion distance, s = (Dt)1/2, becomes ~ 104 µm using D = ~ 10− 8 cm2/s at 1600 °C after 3 h using the value of the bulk diffusion coefficient (D) of Ni in 9.5 mol% Y2O3-doped zirconia [13]. If we estimate the diffusion profile of Ni in 10SSZ using Fick's second law of diffusion and semi-infinite medium [14], the depth at which the Ni concentration reaches 2 mol% is ~ 13 μm. Thus, Ni doping due to Ni diffusion is a reasonable assumption for the present film. In order to confirm the effect of Ni on the cubic-phase stability of the 10SSZ thick film, a 10SSZ bulk specimen doped with 2 mol% NiO was fabricated and the phase was determined. We may assume that both 10SSZ and YSZ compositions have a similar solubility limit of NiO. The solubility limit of NiO in YSZ was reported as ~ 2% [15].
Fig. 2. Cross-sectional SEM micrographs of 10SSZ thick-film detached from the Ni/Ni10SSZ substrate.
Fig. 3. XRD patterns of 10SSZ (a) bulk and (b, c) thick-film samples were compared between 20° and 70° (2Θ). The XRD patterns of the detached film were taken from the surfaces (b) with and (c) without previously in contact with Ni layer. The 10SSZ bulk sample (a) showed rhombohedral phase. However, the co-fired, free-standing 10SSZ thick-film (b, c) showed a cubic phase.
Fig. 4 shows XRD patterns (48o to 53o of 2Θ) of 10SSZ bulk sample doped with 2 mol% NiO; (a) as-sintered, (b) after reduction, and (c) after re-oxidation in air at 800 °C for 5 h. The (220) peaks are typically used to distinguish the cubic from the rhombohedral phases in SSZ [16]. The as-sintered 10SSZ bulk sample doped with 2 mol% NiO showed the rhombohedral phase as expected. The phase transformation of the Ni-containing 10SSZ bulk sample after the reduction can be seen in the XRD patterns (Fig. 4). Although the as-sintered sample contains NiO, it showed a rhombohedral phase. The sample after reduction exhibited strong cubic peaks showing the dominance of cubic phase. The rhombohedral phase which remains in the bulk sample may be due to the kinetics problem. The sample in contact with a higher Ni content or after longer contact time than the present sample may readily transform to the cubic phase. When the reduced sample was re-oxidized in air at 800 °C for 5 h, the dominance of cubic phase did not change. For comparison, the 10SSZ bulk without Ni addition showed no phase change after reduction as shown in
Fig. 4. XRD patterns of 10SSZ bulk sample with 2 mol% NiO doping; (a) as-sintered, (b) after subsequent reduction, and (c) after re-oxidation in air at 800 °C, for a selected range of 2Θ (48° to 53°).
J.H. Joo, G.M. Choi / Solid State Ionics 180 (2009) 252–256
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Fig. 6. Impedance spectra of 10SSZ thick film at 700 °C were compared to that of bulk 10SSZ sample by taking the geometrical factors into account.
Fig. 5. XRD patterns of 10SSZ bulk sample; (a) as-sintered and (b) after subsequent reduction at 800 °C, for a selected range of 2Θ (48° to 53°).
Fig. 5. Thus, the comparison of Figs. 4 and 5 shows that the presence of Ni makes a big difference in the final phase of the 10SSZ bulk sample. The phases observed at room temperature in this study are summarized in Table 1. The prevention of the transformation of the cubic phase to the rhombohedral on cooling is apparently induced by both Ni and reduction treatment. Although Ni dissolves in the 10SSZ sample, it does not prevent the transformation of the cubic phase to the rhombohedral in air. Ni was found to hinder the transformation of the cubic phase to the rhombohedral on cooling in 10SSZ samples only in reducing atmosphere. In the case of YSZ, it has been reported that the NiO dissolves in the cubic zirconia phase, causing a decrease in the lattice parameter and also of the conductivity in air. When a NiOcontaining YSZ was reduced, the solubility of Ni in YSZ decreased. Thus, excess Ni comes out from the YSZ lattice. The precipitation of Ni is accompanied by the formation of a tetragonal phase. Ni facilitates the transformation of a cubic phase into a tetragonal phase for YSZ in reducing atmosphere [6]. 3.2. Conductivity of the 10SSZ thick film
interpretation of impedance spectra. The capacitance (C) can be written as C = εrεo A/l (area/length) where εo is the permittivity of vacuum (8.845 × 10− 14 F cm− 1), and εr is the relative dielectric permittivity of sample. For the 10SSZ film, the observed peak capacitance (~ 10− 12 F) of the impedance semicircle is very close to the experimental limit due to the internal capacitance of impedance meter (5 × 10− 12 F) [17]. The impedance spectrum cannot distinguish between the effects of grain and the grain boundary due to the small capacitance of 10SSZ thick-film sample which originated from the small geometrical factor, A/l ~1.3 × 10− 3 cm. Only the total (grain + grain boundary) resistance value can be obtained by an impedance spectrum analysis. Since the impedance spectrum analysis for the inplane conductivity cannot separate the grain from the grain boundary contributions, we may only deduce the effect of grain boundary by considering other properties. The activation energy (Ea) of the thick film (1.08 ± 0.012 eV, 600–700 °C) was similar to that of the grain (1.02 ± 0.008 eV, 600–700 °C) of 10SSZ bulk sample doped with 2 mol% NiO. In addition, the film sample showed a very large grain size (~8 μm). We thus conclude that the total conductivity of the thick film was mostly determined by the grain conductivity. On the other hand, due to the relatively large geometrical factor (A/l ~1.72 cm) and thus small resistance of bulk 10SSZ sample, the semicircle responsible for the electrode resistance was only shown in an impedance plane.
Fig. 6 compares the impedance spectra at 700 °C obtained for the thick film and the bulk 10SSZ samples by taking the geometrical factor into consideration. The impedance spectrum of thick film shows two depressed semicircles: a high-frequency semicircle caused by a bulk (grain and/or grain boundary) process and a low-frequency curve attributed to the interface between the 10SSZ film and the Pt electrode. It is well-known that the influence of the electrode on impedance pattern depends upon oxygen partial-pressure (PO2). Thus, a low frequency semicircle, which varies with PO2, is clearly due to the electrode impedance. The spectra were analyzed using an equivalent circuit that consists of series connection of R1Q1 and R2Q2 where Ri is the resistance and Qi is the constant-phase element as shown in Fig. 6. A fitting line was drawn for the film in air, for example. The capacitance value at the peak of semicircle is often useful in the
Table 1 The XRD phase of the co-fired 10SSZ thick-film sample was compared with those of 10SSZ bulk samples with and without 2 mol% Ni doping. R = Rhombohedral, C = Cubic 10SSZ
As-sintered (1600 °C/3 h)
After reduction (800 °C/5 h)
Re-oxidation (800 °C/5 h)
Bulk Co-fired thick film 2 mol%-NiO doped bulk
R – R
R C C (major) + R (minor)
R C C (major) + R (minor)
Fig. 7. The conductivity values of 10SSZ thick-film and bulk samples measured in air were shown as a function of temperature. For comparison, the conductivities of 10SSZ bulk sample doped with 2 mol% NiO and reduced in hydrogen atmosphere, and those of the 10SSZ bulk sample with 1 mol% CeO2 doping [19] were also shown.
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conductivity than the 10SSZ thick-film sample, a 10SSZ thick-film may be used as an electrolyte in an anode-supported cell in view of the difficulty in doping a small amount of Ce in the film fabrication process. The conductivity values of 10SSZ bulk samples doped with 2 mol% Ni and free-standing thick-film are nearly the same at temperatures above 550 °C. A single-continuous curve was shown for 10SSZ thick-film sample, corresponding to the nearly cubic-nature of the sample. The 10SSZ bulk and the detached 10SSZ film samples were made from the same powders and were heat treated using the same condition. The only difference is that the 10SSZ film was previously in contact with Ni layer and the film contains Ni in the layer. In addition, when the 10SSZ bulk sample doped with 2 mol% NiO was heat treated in reducing atmosphere, the cubic phase became dominant. Thus it was concluded that Ni hinders the transformation of the cubic phase to the rhombohedral on cooling in 10SSZ samples only in reducing atmosphere. Fig. 8. The conductivity values of 10SSZ bulk samples (a–c); (a) undoped, (b) 1 mol% CeO2-doped [19], (c) 2 mol% Ni-doped, and (d) 10SSZ thick-film were compared at 700 °C in air.
The Ea of the 10SSZ bulk sample (0.88 ± 0.027 eV, 600–700 °C) in this study was similar to that of the reported 10SSZ bulk data (~0.84 eV, 600–700 °C) in the same temperature range [18]. However, it is larger than that (~ 0.65 eV) reported in the high temperature region (800–1000 °C) [4]. The addition of 2 mol% yttria to 11SSZ increased the activation energy from 0.65 to 0.69 eV (800–1000 °C) [4]. The large Ea (~0.88 eV) of 10SSZ bulk sample is due to the fact that our temperature range is narrow and close to the phase-transition temperature (~ 550 °C) where the gradual increase in the fraction of rhombohedral phase and the accompanying increase in the Ea is expected. However, the large Ea (1.02 eV) of the 10SSZ sample due to the Ni doping in this study is notable with the lack of phase transition. The addition of Ce or Y in SSZ is known to decrease the conductivity and increase the Ea value. The Ea value of 10SSZ doped with 1 mol% CeO2 (0.82 ± 0.052 eV, 600–700 °C) [19] is slightly lower than that (~0.88 eV) of 10SSZ bulk in this study. Thus, further study is necessary to understand the origin of large Ea of Ni-doped thick film and bulk samples in this study. However, the lower conductivity of Ni-doped sample than that of Ce- or Y-doped sample is consistent with the higher Ea value. Fig. 7 compares the conductivity of free-standing 10SSZ thick-film and 10SSZ bulk samples determined from the impedance measurements in air. For comparison, the conductivities of 10SSZ bulk sample doped with 2 mol% NiO and reduced in hydrogen atmosphere and those of the 10SSZ bulk sample doped with 1 mol% CeO2 were also shown. It has been reported that 1 mol% CeO2 addition into 10SSZ sample stabilizes the cubic phase at room temperature [19]. Since bulk samples show negligible grain-boundary contributions in this study, Fig. 7 shows the temperature dependence of the grain conductivity of 10SSZ bulk in air. The curve consisting of two linear parts with a jump at ~550 °C was observed for the 10SSZ bulk sample due to the phase transition from the cubic to the rhombohedral phase [4,20]. However, for the 10SSZ bulk sample doped with 2 mol% NiO after reduction at 800 °C, the conductivity drop below 500 °C was much smaller than that of the undoped 10SSZ sample. This coincides with the XRD observation of the same sample which shows cubic (major) and rhombohedral (minor) phases. It was also shown that 2 mol% NiO addition leads to a lower conductivity than that of 10SSZ above 500 °C. Fig. 8 shows that the conductivity of Ni-doped 10SSZ bulk sample is about two times lower than that of undoped 10SSZ bulk sample at 700 °C. Thus, when 10SSZ is used as a thick-film electrolyte supported on a Ni-electrolyte composite, the ohmic resistance at 700 °C is expected to be about twice the value of 10SSZ bulk sample. Although a 10SSZ sample doped with 1 mol% CeO2 [19] still shows a higher
4. Conclusions The free-standing 10SSZ (10 mol% Sc2O3-stabilized zirconia) thickfilm, detached from the Ni substrate, showed the cubic phase after reduction in contrast to the rhombohedral phase of a bulk 10SSZ sample. The Ni doping in the SSZ thick film was confirmed and the film contained ~ 1.7 mol% Ni. The Ni doping effect on the phase formation of 10SSZ thick-film was further confirmed by fabricating an Ni-doped 10SSZ bulk sample and comparing the phases. When the 10SSZ bulk sample doped with 2 mol% NiO was reduced, the cubic phase became dominant. Thus Ni may have dissolved in the 10SSZ bulk sample and decreased the electrical conductivity of 10SSZ bulk sample in air. Ni was thus found to hinder the transformation of the cubic phase to the rhombohedral on cooling in 10SSZ sample when reduced. From the present results, it is suggested that additives such as Ce that stabilize the cubic phase of 10SSZ are not necessary when fabricating anode-supported SOFC using 10SSZ electrolyte. Acknowledgement This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MEST) (No. R01-2008-000-10636-0). References [1] S.P.S. Badwal, J. Mater. Sci. 22 (1987) 4125. [2] S.P.S. Badwal, F.T. Ciacchi, D. Milosevic, Solid State Ionics 136–137 (2000) 91. [3] Y. Arachi, H. Sakai, O. Yamamoto, Y. Takeda, N. Imanishai, Solid State Ionics 121 (1999) 133. [4] T.I. Politova, J.T.S. Irvine, Solid State Ionics 168 (2004) 153. [5] Y. Arachi, T. Asai, O. Yamamoto, Y. Takeda, N. Imanishi, K. Kawate, C. Tamakoshi, J. Electrochem. Soc. 148 (2001) A520. [6] S. Linderoth, N. Bonanos, K.V. Jensen, J.B. Bilde-Sørensen, J. Am. Ceram. Soc. 84 (2001) 2652. [7] A. Kuzjukevics, S. Linderoth, Solid State Ionics 93 (1997) 255. [8] O.H. Kwon, G.M. Choi, Solid State Ionics 177 (2006) 3057. [9] P.M. Delaforce, J.A. Yeomans, N.C. Filkin, G.J. Wright, R.C. Thomson, J. Am. Ceram. Soc. 90 (2007) 918. [10] H. Sumi, K. Ukai, Y. Mizutani, H. Mori, C.-J. Wen, H. Takahashi, O. Yamamoto, Solid State Ionics 174 (2004) 151. [11] A. Gunji, C. Wen, J. Otomo, T. Kobayashi, K. Ukai, Y. Mizutani, H. Takahashi, J. Power Source 131 (2004) 285. [12] K. Yamaji, H. Kishimoto, Y. Xiong, T. Horita, N. Sakai, M.E. Brito, H. Yokokawa, J. Power Source 159 (2006) 885. [13] C. Argirusis, M.A. Taylor, M. Kilo, G. Borchardt, F. Jomard, B. Lesage, O. Kaïtasov, Phys. Chem. Chem. Phys. 6 (2004) 3650. [14] D.V. Ragone, Thermodynamics of Materials, vol. 2, John Wiley & Sons Inc., 1995, p. 127. [15] Y.M. Park, G.M. Choi, Solid State Ionics 120 (1999) 265. [16] S. Sarat, N. Sammes, A. Smirnova, J. Power Sources 160 (2006) 892. [17] I. Kosacki, H.U. Anderson, Y. Mizutani, K. Ukai, Solid State Ionics 152–153 (2002) 431. [18] B. Bai, N.M. Sammes, A.L. Smirnova, J. Power Sources 176 (2008) 76. [19] J.H. Joo, G.M. Choi, Solid State Ionics 179 (2008) 1209. [20] T. Ishii, T. Iwata, Y. Tajima, Solid State Ionics 57 (1992) 153.
Contents lists available at ScienceDirect
Solid State Ionics j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / s s i
Effect of Ni doping on the phase stability and conductivity of scandia-stabilized zirconia Jong Hoon Joo, Gyeong Man Choi ⁎ Fuel Cell Research Center and Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang, Gyeongbuk 790-784, Republic of Korea
a r t i c l e
i n f o
Article history: Received 19 June 2008 Received in revised form 2 December 2008 Accepted 3 December 2008 Keywords: Scandia-stabilized zirconia Ni Phase transition Electrical conductivity Thick film electrolyte Solid oxide fuel cell
a b s t r a c t The effect of Ni doping on the phase stability and conductivity of scandia-stabilized zirconia (SSZ) thick film was studied. A free-standing 10SSZ thick-film (10 mol% Sc2O3-stabilized zirconia, ~ 10 μm thick) that was previously in contact with a Ni layer during co-firing was fabricated. The 10SSZ thick-film showed a cubic phase in contrast to the rhombohedral phase shown for a bulk 10SSZ sample. The Ni content in the SSZ thick film was ~ 1.7 mol%. The effect of Ni on the cubic phase formation was also confirmed by the similar observation of the cubic phase in the Ni-doped bulk 10SSZ sample. The observed conductivity behavior also supported the XRD observation. Ni was found to hinder the transformation of the cubic phase to the rhombohedral on cooling in 10SSZ samples after a reduction treatment. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Rare-earth ion doped zirconia is widely used in SOFCs (solid oxide fuel cells), oxygen sensors and structural applications due to its advantages in electrical and mechanical properties. In particular, yttria-stabilized zirconia (YSZ) has been widely studied as a choice for practical SOFCs because of its high ionic and low electronic conductivity as well as its excellent chemical stability under both reducing and oxidizing atmospheres at high temperature. The scandiastabilized zirconia (SSZ) has been intensely investigated in recent years due to its possible use as an electrolyte for intermediate temperature SOFCs [1,2] since SSZ shows the highest ionic conductivity among many zirconia-based electrolytes. The phenomenon may be explained by the low association enthalpy of the defects and the similarity between the ionic radii of Sc3+ and Zr4+ ions. However, the cubic to rhombohedral phase transformation near 600 °C upon cooling for a zirconia containing more than 10 mol% Sc2O3 is unfavorable for an electrolyte due to the irreversible thermal expansion and the abrupt decrease in the ionic conductivity [3]. It has been reported that the phase transformation can be prevented by adding 1–2 mol% of additives such as Y2O3, CeO2, or Ga2O3 [4,5]. In the fabrication of SOFCs, the interaction of transition metals with the zirconia electrolyte is in most cases unavoidable since transition metals are frequent constituents of the electrodes. For example, Mn, Fe,
⁎ Corresponding author. E-mail address: [email protected] (G.M. Choi). 0167-2738/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2008.12.003
or Co is present in almost all SOFC cathodes, and Ni in the anodes [6]. In fact, the state-of-the-art anode in SOFCs is Ni/electrolyte composite. The anode is typically fired in the temperature range of 1000–1600 °C. Thus, Ni diffusion into the electrolytes is unavoidable. A number of reports have already been made about the effect of Ni diffusion on the YSZ [6–9]. Linderoth et al. reported that Ni dissolves in the cubic zirconia phase, decreasing the conductivity in air [6]. However, when Nicontaining YSZ is reduced, some portion of the metallic Ni comes out from the solid solution and induces a cubic to tetragonal phase change in the Ni-containing YSZ [6]. The resulting mixture of tetragonal and cubic phase is responsible for the reduced conductivity. Although Ni-SSZ composites have been used as anodes in SOFC [10–12], to the best of our knowledge, no reports have been made on the effect of Ni on the SSZ. The effect of Ni doping on the SSZ thick-film has also not been reported, although the conductivity and the phase stability of SSZ in film form are important in the performance of anode-supported cells. This is in part because the film is always coated on conductive support materials such as Ni-electrolyte composite, and thus the deconvolution of the electrolyte film and substrate is difficult. During SOFC operation, NiO in composite NiO-electrolyte is reduced to metallic Ni and is expected to react with the electrolyte. However, it is not clear what happens when Ni is dissolved in SSZ electrolyte film. In this study, the effect of the reaction between Ni and SSZ film on the phase stability and the electrical conductivity of SSZ were studied. After co-firing of SSZ film on the NiO-coated NiO-SSZ composite substrate, the SSZ film was successfully detached from the support. Thus, a free-standing SSZ film which has possibly reacted with Ni was fabricated. 10SSZ (10 mol% Sc2O3-doped ZrO2) was selected as a test composition since the electrical conductivity of 10SSZ shows a maximum at this composition. The phase and
J.H. Joo, G.M. Choi / Solid State Ionics 180 (2009) 252–256
conductivity values of free-standing 10SSZ film were compared to those of the 10SSZ bulk specimens, with or without the addition of Ni. 2. Experimental procedures NiO and 10SSZ thick-films were sequentially coated on NiO-10SSZ composite substrate. In order to make NiO-10SSZ composite substrates, 10SSZ (99.99%, Daiichi Kigenso Kagaku Kogyo, Japan) and NiO (99.97%, High Purity Chemicals, Japan) powders were ball-milled for 12 h using zirconia balls in ethyl alcohol. The substrate was uniaxially pressed to form a disk-shaped sample of ~ 13 mm (ø) × ~ 1 mm (t) and pre-sintered at 900 °C for 1 h in air. NiO and 10SSZ slurries were screen-printed sequentially on the 10SSZ-NiO substrate with the thickness of ~20 μm. The screen printing slurries were prepared by mixing the powders with the appropriate amount of alpha-terpineol, methyl cellulose and ethylene glycol. The 3-layered 10SSZ(film)/NiO (film)/ 10SSZ-NiO(bulk) sample was co-fired at 1600 °C for 3 h in air and subsequently reduced in 97% H2/3% H2O gas mixture at 800 °C for 5 h. The pellet was immersed in a HCl solution and Ni was dissolved. Thus, the sintered 10SSZ thick-film was successfully detached from the Ni-10SSZ substrate with this method. In order to measure the in-plane (parallel to the film surface) conductivity of the freestanding thick-film, Pt paste (No.6082, Engelhard, USA) was painted on two ends of the film and fired at 800 °C for 1 h. Pt meshes (52 mesh, Alpha Aesar, USA) were used as a current collector. The fabrication process of free-standing 10SSZ film is schematically shown in Fig. 1.
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For comparison, the bulk samples (both undoped and Ni-doped 10SSZ) were also fabricated by pressing 10SSZ or 10SSZ ([(ZrO2)0.9 (Sc2O3)0.1]0.98·(NiO)0.02) powders doped with 2 mol% NiO, followed by cold-isostatic pressing and sintering at 1600 °C for 3 h in air and subsequently reduced in 97% H2/3% H2O gas mixture at 800 °C for 5 h. Pt paste electrodes were similarly painted on both sides of the disc-shaped specimen. The electrical conductivities of the film and the bulk were determined as a function of temperature from impedance spectra that were obtained by using an Impedance GainPhase Analyzer (Solartron SI 1260, UK) in the frequency region of 0.01 Hz to 5MHz. The temperature dependence of conductivity was measured while cooling the samples from 800 °C to 300 °C with a 3 °C/min rate in dry air with 50 °C steps. To assure equilibrium, the sample was maintained for 2 h at each temperature before a measurement was made. The phase of thick film was characterized by X-ray diffraction (XRD) using Cu-Kα radiation (MAC Science, M18XCE, Japan). XRD patterns of bulk samples were obtained from the diffraction for the sample surface. The XRD patterns of the detached films were taken from both the surface previously in contact with Ni and the free surface. The analysis of the Ni in SSZ thick film was performed with inductively-coupled plasma atomicemission spectroscopy (ICP-AES IRIS Advantage, TJA Solution, USA). Microstructure observation was performed by a field-emission scanning-electron-microscope (JEOL, model 3330F, Japan). In order to analyze the impedance patterns, the spectra were fitted using analysis software (ZSimpWin, PerkinElmer Instruments, USA).
Fig. 1. Procedure for fabricating free-standing 10SSZ thick-film sample was described. (a) NiO and 10SSZ pastes were sequentially screen-printed on a NiO-10SSZ pellet and co-fired at 1600 °C in air. (b) The cell was reduced at 800 °C for 5 h in 97%H2 + 3%H2O gas. (c) The Ni film was etched in hydrochloric (HCl) acid and 10SSZ thick-film was separated from Ni-10SSZ substrate. (d) Free-standing 10SSZ thick-film sample was obtained. (e) Pt paste electrodes were painted on the two ends of the film for the in-plane conductivity measurement.
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3. Results and discussion 3.1. Characterization of crystalline structure Fig. 2 shows the cross-sectional SEM micrograph of a free-standing 10SSZ thick film. The thickness of the film was ~ 10 μm, and the average grain size was ~ 8 μm due to the high sintering temperature (1600 °C). The error in the conductivity due to the rough surface is estimated as less than ~10% under the assumption that the error comes from the estimation of film thickness. The relative density of the sintered freestanding 10SSZ film was more than 95%, calculated from the weight and the dimension of the sample. Fig. 3 shows the XRD patterns for the 10SSZ bulk and the thick film. The XRD patterns from both surfaces of the detached 10SSZ film are shown. As expected, the 10SSZ bulk sample showed a rhombohedral phase at room temperature. It has been reported that samples with Sc2O3 content higher than 10% (10–15 mol%) show a rhombohedral phase which transforms to a cubic phase at ~ 600 °C [3]. However, the free-standing 10SSZ thick film, detached from the Ni/Ni-10SSZ substrate, showed a cubic phase on both surfaces with a trace of monoclinic phase. Thus, different phases were observed for the same composition depending upon the sample form or the fabrication process. The bulk 10SSZ and the detached 10SSZ film were made from the same powders and were heat treated at the same condition. The only difference is that the 10SSZ film was previously being in contact with the Ni layer. The Ni doping in the SSZ film was determined by ICP-AES. The SSZ thick film contained ~ 1.7 mol% Ni. It is thus reasonable to assume that Ni in SSZ film hinder the transformation of the cubic phase to the rhombohedral on cooling. The stability of the cubic phase in the thick film is attributed to the Ni-doping in 10SSZ sample after the reaction with Ni. We may also estimate the diffusion distance of Ni into the 10SSZ film. A rough estimation shows that the diffusion distance, s = (Dt)1/2, becomes ~ 104 µm using D = ~ 10− 8 cm2/s at 1600 °C after 3 h using the value of the bulk diffusion coefficient (D) of Ni in 9.5 mol% Y2O3-doped zirconia [13]. If we estimate the diffusion profile of Ni in 10SSZ using Fick's second law of diffusion and semi-infinite medium [14], the depth at which the Ni concentration reaches 2 mol% is ~ 13 μm. Thus, Ni doping due to Ni diffusion is a reasonable assumption for the present film. In order to confirm the effect of Ni on the cubic-phase stability of the 10SSZ thick film, a 10SSZ bulk specimen doped with 2 mol% NiO was fabricated and the phase was determined. We may assume that both 10SSZ and YSZ compositions have a similar solubility limit of NiO. The solubility limit of NiO in YSZ was reported as ~ 2% [15].
Fig. 2. Cross-sectional SEM micrographs of 10SSZ thick-film detached from the Ni/Ni10SSZ substrate.
Fig. 3. XRD patterns of 10SSZ (a) bulk and (b, c) thick-film samples were compared between 20° and 70° (2Θ). The XRD patterns of the detached film were taken from the surfaces (b) with and (c) without previously in contact with Ni layer. The 10SSZ bulk sample (a) showed rhombohedral phase. However, the co-fired, free-standing 10SSZ thick-film (b, c) showed a cubic phase.
Fig. 4 shows XRD patterns (48o to 53o of 2Θ) of 10SSZ bulk sample doped with 2 mol% NiO; (a) as-sintered, (b) after reduction, and (c) after re-oxidation in air at 800 °C for 5 h. The (220) peaks are typically used to distinguish the cubic from the rhombohedral phases in SSZ [16]. The as-sintered 10SSZ bulk sample doped with 2 mol% NiO showed the rhombohedral phase as expected. The phase transformation of the Ni-containing 10SSZ bulk sample after the reduction can be seen in the XRD patterns (Fig. 4). Although the as-sintered sample contains NiO, it showed a rhombohedral phase. The sample after reduction exhibited strong cubic peaks showing the dominance of cubic phase. The rhombohedral phase which remains in the bulk sample may be due to the kinetics problem. The sample in contact with a higher Ni content or after longer contact time than the present sample may readily transform to the cubic phase. When the reduced sample was re-oxidized in air at 800 °C for 5 h, the dominance of cubic phase did not change. For comparison, the 10SSZ bulk without Ni addition showed no phase change after reduction as shown in
Fig. 4. XRD patterns of 10SSZ bulk sample with 2 mol% NiO doping; (a) as-sintered, (b) after subsequent reduction, and (c) after re-oxidation in air at 800 °C, for a selected range of 2Θ (48° to 53°).
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Fig. 6. Impedance spectra of 10SSZ thick film at 700 °C were compared to that of bulk 10SSZ sample by taking the geometrical factors into account.
Fig. 5. XRD patterns of 10SSZ bulk sample; (a) as-sintered and (b) after subsequent reduction at 800 °C, for a selected range of 2Θ (48° to 53°).
Fig. 5. Thus, the comparison of Figs. 4 and 5 shows that the presence of Ni makes a big difference in the final phase of the 10SSZ bulk sample. The phases observed at room temperature in this study are summarized in Table 1. The prevention of the transformation of the cubic phase to the rhombohedral on cooling is apparently induced by both Ni and reduction treatment. Although Ni dissolves in the 10SSZ sample, it does not prevent the transformation of the cubic phase to the rhombohedral in air. Ni was found to hinder the transformation of the cubic phase to the rhombohedral on cooling in 10SSZ samples only in reducing atmosphere. In the case of YSZ, it has been reported that the NiO dissolves in the cubic zirconia phase, causing a decrease in the lattice parameter and also of the conductivity in air. When a NiOcontaining YSZ was reduced, the solubility of Ni in YSZ decreased. Thus, excess Ni comes out from the YSZ lattice. The precipitation of Ni is accompanied by the formation of a tetragonal phase. Ni facilitates the transformation of a cubic phase into a tetragonal phase for YSZ in reducing atmosphere [6]. 3.2. Conductivity of the 10SSZ thick film
interpretation of impedance spectra. The capacitance (C) can be written as C = εrεo A/l (area/length) where εo is the permittivity of vacuum (8.845 × 10− 14 F cm− 1), and εr is the relative dielectric permittivity of sample. For the 10SSZ film, the observed peak capacitance (~ 10− 12 F) of the impedance semicircle is very close to the experimental limit due to the internal capacitance of impedance meter (5 × 10− 12 F) [17]. The impedance spectrum cannot distinguish between the effects of grain and the grain boundary due to the small capacitance of 10SSZ thick-film sample which originated from the small geometrical factor, A/l ~1.3 × 10− 3 cm. Only the total (grain + grain boundary) resistance value can be obtained by an impedance spectrum analysis. Since the impedance spectrum analysis for the inplane conductivity cannot separate the grain from the grain boundary contributions, we may only deduce the effect of grain boundary by considering other properties. The activation energy (Ea) of the thick film (1.08 ± 0.012 eV, 600–700 °C) was similar to that of the grain (1.02 ± 0.008 eV, 600–700 °C) of 10SSZ bulk sample doped with 2 mol% NiO. In addition, the film sample showed a very large grain size (~8 μm). We thus conclude that the total conductivity of the thick film was mostly determined by the grain conductivity. On the other hand, due to the relatively large geometrical factor (A/l ~1.72 cm) and thus small resistance of bulk 10SSZ sample, the semicircle responsible for the electrode resistance was only shown in an impedance plane.
Fig. 6 compares the impedance spectra at 700 °C obtained for the thick film and the bulk 10SSZ samples by taking the geometrical factor into consideration. The impedance spectrum of thick film shows two depressed semicircles: a high-frequency semicircle caused by a bulk (grain and/or grain boundary) process and a low-frequency curve attributed to the interface between the 10SSZ film and the Pt electrode. It is well-known that the influence of the electrode on impedance pattern depends upon oxygen partial-pressure (PO2). Thus, a low frequency semicircle, which varies with PO2, is clearly due to the electrode impedance. The spectra were analyzed using an equivalent circuit that consists of series connection of R1Q1 and R2Q2 where Ri is the resistance and Qi is the constant-phase element as shown in Fig. 6. A fitting line was drawn for the film in air, for example. The capacitance value at the peak of semicircle is often useful in the
Table 1 The XRD phase of the co-fired 10SSZ thick-film sample was compared with those of 10SSZ bulk samples with and without 2 mol% Ni doping. R = Rhombohedral, C = Cubic 10SSZ
As-sintered (1600 °C/3 h)
After reduction (800 °C/5 h)
Re-oxidation (800 °C/5 h)
Bulk Co-fired thick film 2 mol%-NiO doped bulk
R – R
R C C (major) + R (minor)
R C C (major) + R (minor)
Fig. 7. The conductivity values of 10SSZ thick-film and bulk samples measured in air were shown as a function of temperature. For comparison, the conductivities of 10SSZ bulk sample doped with 2 mol% NiO and reduced in hydrogen atmosphere, and those of the 10SSZ bulk sample with 1 mol% CeO2 doping [19] were also shown.
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conductivity than the 10SSZ thick-film sample, a 10SSZ thick-film may be used as an electrolyte in an anode-supported cell in view of the difficulty in doping a small amount of Ce in the film fabrication process. The conductivity values of 10SSZ bulk samples doped with 2 mol% Ni and free-standing thick-film are nearly the same at temperatures above 550 °C. A single-continuous curve was shown for 10SSZ thick-film sample, corresponding to the nearly cubic-nature of the sample. The 10SSZ bulk and the detached 10SSZ film samples were made from the same powders and were heat treated using the same condition. The only difference is that the 10SSZ film was previously in contact with Ni layer and the film contains Ni in the layer. In addition, when the 10SSZ bulk sample doped with 2 mol% NiO was heat treated in reducing atmosphere, the cubic phase became dominant. Thus it was concluded that Ni hinders the transformation of the cubic phase to the rhombohedral on cooling in 10SSZ samples only in reducing atmosphere. Fig. 8. The conductivity values of 10SSZ bulk samples (a–c); (a) undoped, (b) 1 mol% CeO2-doped [19], (c) 2 mol% Ni-doped, and (d) 10SSZ thick-film were compared at 700 °C in air.
The Ea of the 10SSZ bulk sample (0.88 ± 0.027 eV, 600–700 °C) in this study was similar to that of the reported 10SSZ bulk data (~0.84 eV, 600–700 °C) in the same temperature range [18]. However, it is larger than that (~ 0.65 eV) reported in the high temperature region (800–1000 °C) [4]. The addition of 2 mol% yttria to 11SSZ increased the activation energy from 0.65 to 0.69 eV (800–1000 °C) [4]. The large Ea (~0.88 eV) of 10SSZ bulk sample is due to the fact that our temperature range is narrow and close to the phase-transition temperature (~ 550 °C) where the gradual increase in the fraction of rhombohedral phase and the accompanying increase in the Ea is expected. However, the large Ea (1.02 eV) of the 10SSZ sample due to the Ni doping in this study is notable with the lack of phase transition. The addition of Ce or Y in SSZ is known to decrease the conductivity and increase the Ea value. The Ea value of 10SSZ doped with 1 mol% CeO2 (0.82 ± 0.052 eV, 600–700 °C) [19] is slightly lower than that (~0.88 eV) of 10SSZ bulk in this study. Thus, further study is necessary to understand the origin of large Ea of Ni-doped thick film and bulk samples in this study. However, the lower conductivity of Ni-doped sample than that of Ce- or Y-doped sample is consistent with the higher Ea value. Fig. 7 compares the conductivity of free-standing 10SSZ thick-film and 10SSZ bulk samples determined from the impedance measurements in air. For comparison, the conductivities of 10SSZ bulk sample doped with 2 mol% NiO and reduced in hydrogen atmosphere and those of the 10SSZ bulk sample doped with 1 mol% CeO2 were also shown. It has been reported that 1 mol% CeO2 addition into 10SSZ sample stabilizes the cubic phase at room temperature [19]. Since bulk samples show negligible grain-boundary contributions in this study, Fig. 7 shows the temperature dependence of the grain conductivity of 10SSZ bulk in air. The curve consisting of two linear parts with a jump at ~550 °C was observed for the 10SSZ bulk sample due to the phase transition from the cubic to the rhombohedral phase [4,20]. However, for the 10SSZ bulk sample doped with 2 mol% NiO after reduction at 800 °C, the conductivity drop below 500 °C was much smaller than that of the undoped 10SSZ sample. This coincides with the XRD observation of the same sample which shows cubic (major) and rhombohedral (minor) phases. It was also shown that 2 mol% NiO addition leads to a lower conductivity than that of 10SSZ above 500 °C. Fig. 8 shows that the conductivity of Ni-doped 10SSZ bulk sample is about two times lower than that of undoped 10SSZ bulk sample at 700 °C. Thus, when 10SSZ is used as a thick-film electrolyte supported on a Ni-electrolyte composite, the ohmic resistance at 700 °C is expected to be about twice the value of 10SSZ bulk sample. Although a 10SSZ sample doped with 1 mol% CeO2 [19] still shows a higher
4. Conclusions The free-standing 10SSZ (10 mol% Sc2O3-stabilized zirconia) thickfilm, detached from the Ni substrate, showed the cubic phase after reduction in contrast to the rhombohedral phase of a bulk 10SSZ sample. The Ni doping in the SSZ thick film was confirmed and the film contained ~ 1.7 mol% Ni. The Ni doping effect on the phase formation of 10SSZ thick-film was further confirmed by fabricating an Ni-doped 10SSZ bulk sample and comparing the phases. When the 10SSZ bulk sample doped with 2 mol% NiO was reduced, the cubic phase became dominant. Thus Ni may have dissolved in the 10SSZ bulk sample and decreased the electrical conductivity of 10SSZ bulk sample in air. Ni was thus found to hinder the transformation of the cubic phase to the rhombohedral on cooling in 10SSZ sample when reduced. From the present results, it is suggested that additives such as Ce that stabilize the cubic phase of 10SSZ are not necessary when fabricating anode-supported SOFC using 10SSZ electrolyte. Acknowledgement This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MEST) (No. R01-2008-000-10636-0). References [1] S.P.S. Badwal, J. Mater. Sci. 22 (1987) 4125. [2] S.P.S. Badwal, F.T. Ciacchi, D. Milosevic, Solid State Ionics 136–137 (2000) 91. [3] Y. Arachi, H. Sakai, O. Yamamoto, Y. Takeda, N. Imanishai, Solid State Ionics 121 (1999) 133. [4] T.I. Politova, J.T.S. Irvine, Solid State Ionics 168 (2004) 153. [5] Y. Arachi, T. Asai, O. Yamamoto, Y. Takeda, N. Imanishi, K. Kawate, C. Tamakoshi, J. Electrochem. Soc. 148 (2001) A520. [6] S. Linderoth, N. Bonanos, K.V. Jensen, J.B. Bilde-Sørensen, J. Am. Ceram. Soc. 84 (2001) 2652. [7] A. Kuzjukevics, S. Linderoth, Solid State Ionics 93 (1997) 255. [8] O.H. Kwon, G.M. Choi, Solid State Ionics 177 (2006) 3057. [9] P.M. Delaforce, J.A. Yeomans, N.C. Filkin, G.J. Wright, R.C. Thomson, J. Am. Ceram. Soc. 90 (2007) 918. [10] H. Sumi, K. Ukai, Y. Mizutani, H. Mori, C.-J. Wen, H. Takahashi, O. Yamamoto, Solid State Ionics 174 (2004) 151. [11] A. Gunji, C. Wen, J. Otomo, T. Kobayashi, K. Ukai, Y. Mizutani, H. Takahashi, J. Power Source 131 (2004) 285. [12] K. Yamaji, H. Kishimoto, Y. Xiong, T. Horita, N. Sakai, M.E. Brito, H. Yokokawa, J. Power Source 159 (2006) 885. [13] C. Argirusis, M.A. Taylor, M. Kilo, G. Borchardt, F. Jomard, B. Lesage, O. Kaïtasov, Phys. Chem. Chem. Phys. 6 (2004) 3650. [14] D.V. Ragone, Thermodynamics of Materials, vol. 2, John Wiley & Sons Inc., 1995, p. 127. [15] Y.M. Park, G.M. Choi, Solid State Ionics 120 (1999) 265. [16] S. Sarat, N. Sammes, A. Smirnova, J. Power Sources 160 (2006) 892. [17] I. Kosacki, H.U. Anderson, Y. Mizutani, K. Ukai, Solid State Ionics 152–153 (2002) 431. [18] B. Bai, N.M. Sammes, A.L. Smirnova, J. Power Sources 176 (2008) 76. [19] J.H. Joo, G.M. Choi, Solid State Ionics 179 (2008) 1209. [20] T. Ishii, T. Iwata, Y. Tajima, Solid State Ionics 57 (1992) 153.