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Anomalous conductivity and microstructure in gadolinium doped ceria prepared from nano-sized powder
Solid State Ionics 177 (2006) 2503 – 2507 www.elsevier.com/locate/ssi
Anomalous conductivity and microstructure in gadolinium doped ceria prepared from nano-sized powder Natsuko Sakai a,⁎, Yue Ping Xiong a , Katsuhiko Yamaji a , Harumi Yokokawa a , Yoshitake Terashi b , Hiroaki Seno b a
Fuel Cell Group, Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology, AIST Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan b R&D Center Kagoshima, KYOCERA Corporation, 1-4 Yamashita-cho, Kokubu, Kagoshima 899-4312, Japan Received 28 June 2005; received in revised form 23 March 2006; accepted 29 March 2006
Abstract The temperature and the oxygen partial pressure dependences of the electron and hole conductivities were measured by the dc polarization method using a Hebb–Wagner's ion blocking cell for Gd0.2Ce0.8O1.9 polycrystalline bodies with grain size of 0.5 μm prepared by sintering of nano-sized powder. A significant enrichment of gadolinium was observed in the vicinity of the grain boundary by TEM/EDS analyses. The electron conductivity were comparable with those of conventional Gd0.2Ce0.8O1.9 polycrystalline body with grain size of 2 μm, and it followed p(O2)− 1/4 dependence at temperatures T = 973–1273 K. However, the observed hole conductivity was higher than that of conventional Gd0.2Ce0.8O1.9, and it did not follow p(O2)1/4 dependence. This anomalous p(O2) dependence disappeared after the sample was treated at T = 1773 K for 38 h and grain size was enlarged to 2–10 μm. © 2006 Elsevier B.V. All rights reserved. Keywords: Ceria; Electron; Hole; Conductivity; Ion blocking; Grain boundary
1. Introduction Rare earth doped ceria (RDC) have good oxide ion conductivities even at T = 500 °C, which is attractive for electrolyte material of solid oxide fuel cells which operate at intermediate temperatures [1]. In addition, we have reported that a significant amount of hydrogen component from water vapor dissolved into RDC polycrystalline bodies [2], which can be correlated with the interesting electro-catalytic activities of RDC in electrode reactions [3,4]. However, the rising of electron conductivity at reducing atmosphere may lower the efficiency, and poor mechanical strength in comparison with zirconia-based electrolytes, which are regarded as important problems to be overcome. We have determined the electron and hole conductivities of rare earth doped ceria separately from oxide ion conductivity by using dc polarization method using a modified Hebb–Wagner's ion blocking cell [4,5].
⁎ Corresponding author. Tel.: +81 29 861 4890; fax: +81 29 861 4540. E-mail address: [email protected] (N. Sakai). 0167-2738/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2006.03.049
In recent 10 years, the preparation of nano-sized ceria powder is intensively investigated mainly in anticipation of improvement in sintering behavior [6–16]. Although those nano-sized powders exhibit rapid sintering at lower temperatures, it was somewhat difficult to control the grain growth during the sintering procedure. Quite recently, the preparation of dense polycrystalline bodies with grain size less than 1 μm has been reported [17–21], and it has been revealed that the grain size may affect on mechanical property [18] or electrical conductivity [19–23], and local structures [20]. Based on these experimental facts, it is supposed that the effects of grain boundaries on the physical and chemical properties of such polycrystalline bodies with grain submicron grains may be more significant than those of normal polycrystalline bodies with grain size of several micrometers. We focused on the effect of grain boundary on the transport property of gadolinium doped ceria (GDC) polycrystalline bodies with grain size of ca. 0.5 μm which were prepared by sintering nano-sized powders. The electron and hole conductivities were measured by using a modified Hebb–Wagner ion-blocking method, and the results were compared with those of normal polycrystalline bodies with grain size of several micrometers.
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2. Experimental The dense polycrystalline pellets (diameter: 17 mm, thickness: 2 mm) of Ce0.8Gd0.2O1.9 with average grain size of 0.47 μm was prepared by sintering nano-sized powder compact at T = 1673 K in air. This sample is named as “nano-GDC” for convenience. The relative density was 96.1% and the surface was polished by diamond pastes, and the sample thickness (L) is 1–2 mm. The electron and hole conductivities were measured by the DC polarization method using a modified Hebb–Wagner's ion blocking cell. The detail of the ion blocking cell is described in our previous paper [24]. The ion-blocking cell is consists of a sample pellet with platinum electrodes and alumina spacers which shape a closed chamber. Platinum paste as electrodes was painted at the center of both (outer and inner) surface of a sample. The electrode area (A) was 0.785 cm2. Pyrex® glass seal was used to maintain the gas tightness of the chamber. The dc polarization measurements were carried out with a potentiostat (Toho Technical Research potentio/galvanostat 2020). The ion blocking cell is heated in 1% O2–99% Ar gas flow and sealed at glass melting temperature. When no electric potential is applied, the oxygen partial pressure, p(O2) is 1 kPa both inner and outer atmosphere. When a voltage (Eapp) was applied to the sample samples to pump out the oxygen gas in the cell, the current changed until a steady state was established. In the steady state, the oxygen chemical potential at the inner electrode/sample interface was reduced against the outer electrode. The oxide ion flow was blocked at the inner electrode because there was no electrochemical potential gradient of oxide ions inside the sample. Hence, the measured current at the steady state should be electronic current (Ie), governed by the electrons and holes in the sample. The electronic (hole) conductivity (σe, h) was determined by the following equation. L AIe re;h ¼ ð1Þ A AEapp The data at p(O2) = 10− 3 to 10−10 MPa were collected under 1% O2–99% Ar gas flow and applied voltage from 0.0 to 0.4 V. The data at p(O2) > 10− 3 MPa were collected by sealing the ion blocking cell in 10% O2–90%Ar, 30%O2–70%Ar, or 100% O2
Fig. 2. The total conductivity (σtot) and electron (hole) conductivity of nanoGDC at T = 973 K. The data are compared with the conductivity data of microGDC [5].
gas flow and applied voltage was limited to 10 mV, which avoided gas leakage due to the pressure difference between the outer and the inner space of the blocking cell. The total conductivity (σtot) was separately measured by using AC impedance method at T = 973 K, p(O2) = 10− 26– 0.1 MPa. Gas mixtures O2–Ar or H2–CO2 were used to control oxygen partial pressure. The microstructure, cerium/gadolinium ratio, and valence state of cerium ion in nano-GDC before and after the dc polarization measurement were examined by using a high resolution transmission electron microscope (HRTEM, HF2210, Hitachi Co. 200 kV)/energy dispersed X-ray Analyses and electron energy loss spectroscopy (EDX, EELS, NORAN VOYAGER). For comparison, a dense Y0.2Ce0.8O1.9 (YDC) polycrystalline body prepared by a conventional ceramic method from Y2O3 and CeO2 and sintered at 1923 K in air was examined in TEM/EDX analyses. 3. Results and discussion 3.1. Electron, hole and total conductivity of nano-GDC Fig. 1 shows the electronic (hole) conductivity (σe, h) of nano-GDC as a function of oxygen partial pressure at inner electrode p(O2), and was calculated by the following equation, Eapp ¼
RT pout ðO2 Þ ln 4F pðO2 Þ
ð2Þ
where R, T, F and pout(O2) are the gas constant, the sample temperature, Faraday's constant and the oxygen partial pressure Table 1 Evaluated constants, a, b and n, in Eq. (3) for nano-GDC at 873–1273 K Temperature/K
Fig. 1. Electronic and hole conductivity of nano-GDC at 873–1273 K. Lines are reproduced from the obtained constants listed in Table 1 based on Eq. (3) in the text.
1273 1173 1073 973 873
a/S cm− 1 −4
3.90 × 10 6.00 × 10− 5 5.58 × 10− 6 2.65 × 10− 7 1.48 × 10− 8
b/S cm− 1
n
0.051 0.051 0.051 0.051 0.051
1.65 1.38 1.25 1.18 1.14
N. Sakai et al. / Solid State Ionics 177 (2006) 2503–2507
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Fig. 2 shows the present results of total conductivity (σtot) and electron (hole) conductivity (σe,h) of nano-GDC at T = 973 K as a function of oxygen partial pressure. The data are compared with our previous results of a conventional Gd0.2Ce0.8O1.9 polycrystalline body (abbreviated to micro-GDC, average grain size ca. 2 μm, relative density >95%) [25]. There is no significant difference for the total conductivity between nano- and micro-GDC. It does not agree with the results reported by Wang et al. for Dy2O3 doped CeO2, i. e. the total conductivity has a minimum at the grain size 0.24 μm. However, it is clearly shown that the nano-GDC has a remarkably higher hole conductivity than micro-GDC in the p(O2) range of 10− 5–0.1 MPa. The observed electron and hole conductivities were fitted into the following equation. Fig. 3. EDS analyses results around the grain boundary of as-prepared nanoGDC (sintering temperature 1673 K) and conventional YDC sintered at T = 1923 K.
around the outer electrode, respectively. The electronic conductivity of nano-GDC exhibits p(O2)− 1/4 dependence in reducing atmosphere, which indicates that the electron is the main charge carrier in those conditions. The hole conductivity should have exhibited p(O2)1/4 dependence at higher range of oxygen partial pressure, however, the obtained conductivity did not follow the p(O2)1/4 dependence.
Fig. 4. TEM image and EELS spectra at bulk (solid line), triple point (dashed line) and grain boundary (dashed and dotted line) of as prepared nano-GDC.
re;h ¼ aðpðO2 Þ=0:1 MPaÞ1=4 þ bðpðO2 Þ=0:1 MPaÞ1=n
ð3Þ
The parameter a corresponds to the electronic conductivity at p(O2) = 0.1 MPa. The parameter b corresponds to the σh at p (O2) = 0.1 MPa, and it was fixed 0.51 S cn− 1 according to the observed conductivity data. The derived constants, a, b and n, are listed in Table 1, and the evaluated values for Eq. (3) are plotted in Fig. 1 as lines. The n obtained values ranged from 1.14 to 1.65, which were much lower than the theoretical
Fig. 5. TEM image and EELS spectra of normal grain (solid line) and inclusion (dashed line) of nano-GDC after the hole conductivity measurement (T = 973– 1273 K, p(O2) = 0.1 MPa, E = 10 mV).
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value (n = 4) and can not be explained by a simple defect model. It indicates that additional oxygen which does not contribute to the hole conduction is absorbed in the solid. 3.2. Microstructure of nano-GDC and its effect on conductivities In the TEM images of as prepared nano-GDC, no visible precipitation of secondary phase was observed in the vicinity of grain boundary (GB). However, semi-quantitative EDX analyses revealed that the relative gadolinium content is significantly higher (over 28%) on grain boundary as shown in Fig. 3. The gadolinium content is the same level as in the bulk at the point is which located 10 nm from the grain boundary. However, similar enrichment is also observed for the case of yttrium doped ceria (Ce0.8Y0.2O1.9) which was sintered at T = 1923 K as shown in the same figure. Hence it is considered that this phenomena is commonly observed for rare earth doped ceria, regardless of dopant types and preparation condition. The EELS spectra taken at the different point of the same sample are shown in Fig. 4. The peaks of CeN 4,5 (Ce4+) and GdN 4,5 (Gd3+) are observed at energy levels 132 eV and 150 eV, respectively. There are no significant differences in valence states among grain boundaries and bulk. However, after the hole conductivity measurement in p(O 2) = 0.1 MPa, E = 10 mV at T = 973–1273 K, the microstructure of nano-GDC changed as shown in Fig. 5(a). The grain contains many inclusions inside. The EELS spectrum at the inclusion revealed a significant enrichment of gadolinium (Fig. 5(b)). Furthermore, the energy level of CeN 4,5 peak shifted to 126 eV, which indicates the Ce3+ contribution. Recently Higuchi et al. reported the electronic structure of Ce1−xNdxO2−δ by using the resonant photoemission spectroscopy (RPES) [26]. They suggest that the existence of trivalent cerium ion (Ce3+) is observed for the Nd doped sample which is prepared by high temperature sintering in air. With increasing Nd3+ concentration, the hybridization effect of Ce 4f and O 2p states decreases and the effective charges of Ce4+ and O2− ions decreases. Hence it is not unusual that Ce3+ exists in oxidizing atmosphere. However, the formation of the Gd3+ and Ce3+ rich inclusions in the present nano-GDC after the hole conductivity measurement is extraordinary, because it was not observed in as-prepared sample. It is still uncertain about the correlation of the inclusion formation and anomalously high hole conductivity. In order to examine the effect of chemical homogeneity of the sample, the nano-GDC was annealed at T = 1773 K for 38 h and its grain size was enlarged to 2–10 μm. The effect of the post sintering is shown in Fig. 6. The conductivity of the post-sintered nanoGDC exhibited p(O2)1/4 dependence, which is quite normal and same as that of micro-GDC. Note that the post sintering will not suppress the enrichment of gadolinium at grain boundary, because the enrichment of yttrium is still observed for YDC which was sintered at very high temperature (1923 K) as shown in Fig. 3. However, the formation of inclusion may be suppressed by improving chemical homogeneity by high
Fig. 6. Effect of sintering on electron and hole conductivity of nano-GDC at T = 973 K.
temperature treatment. Hence the relationship between microscopic chemical homogeneity and hole conductivity should be carefully investigated. 4. Summary The anomalous high hole conductivity and p(O2) dependence were observed for Gd0.2Ce0.8O1.9 polycrystalline bodies with grain size of 0.5 μm prepared from nano-sized powder. The sample after hole conductivity measurement exhibited a significant compositional fluctuation such as the inclusion containing higher content of Gd3+ and Ce3+, although the asprepared is quite homogeneous structure. Since this anomaly disappears after the post sintering procedure at T = 1773 K, it can be correlated to some compositional and morphological change which was not eliminated by the initial sintering at T = 1673 K. It is suggested that the electron and hole conductivity measured by dc polarization method is sensitive to such changes. Further investigation will be made to examine the relationship of microstructure change and hole conductivity dependence for samples with different dopant type, concentration and preparation methods. References [1] B.C.H. Steele, Solid State Ionics 129 (2000) 95. [2] N. Sakai, K. Yamaji, T. Horita, H. Yokokawa, Y. Hirata, S. Sameshima, Y. Nigara, J. Mizusaki, Solid State Ionics 125 (1999) 325. [3] T. Horita, N. Sakai, K. Yamaji, H. Yokokawa, T. Kato, J. Power Sources 131 (2004) 299. [4] H. Yokokawa, T. Horita, N. Sakai, K. Yamaji, M.E. Brito, Y.P. Xiong, H. Kishimoto, Solid State Ionics 174 (2004) 205. [5] K. Yamaji, Y.P. Xiong, N. Sakai, T. Horita, H. Hokokawa, J. Electrochem. Soc. 149 (11) (2002) E450. [6] V. Briois, C.E. Williams, H. Dexpert, F. Villain, B. Camane, F. Deneuve, C. Magnier, J. Mater. Sci. 28 (1993) 5019. [7] P.L. Chen, I.W. Chen, J. Am. Ceram. Soc. 76 (1993) 1557. [8] T.J. Kirk, J. Winnick, J. Electrochem. Soc. 140 (1993) 3494. [9] J. Van herle, T. Horita, T. Kawada, N. Sakai, H. Yokokawa, M. Dokiya, J. Eur. Ceram. Soc. 16 (1996) 961. [10] Y.M. Chiang, E.B. Lavik, D.A. Blom, Nanostruct. Mater. 9 (1997) 633. [11] Y.M. Chiang, E.B. Lavik, I.K. Osack, H.L. Tuller, J.M. Ying, J. Electroceram. 1 (1997) 7.
N. Sakai et al. / Solid State Ionics 177 (2006) 2503–2507 [12] K. Higashi, K. Sonoda, H. Ono, S. Sameshima, J. Mater. Res. 14 (1999) 957. [13] S. Zha, Q. Fu, Y. Lang, C. Xia, G. Meng, Mater. Lett. 47 (2001) 351. [14] J.P. Hos, P.G. McCormick, Scr. Mater. 48 (2003) 85. [15] Y. Wang, T. Mori, J.-G. Li, Y. Yajima, Sci. Technol. Adv. Mater. 4 (2003) 229. [16] L.P. Meier, L. Urech, L.J. Gauckler, J. Eur. Ceram. Soc. 24 (2004) 3753. [17] Y. Wang, T. Mori, J.-G. Li, Y. Yajima, Sci. Technol. Adv. Ceram. 4 (2003) 229. [18] T.S. Zhang, L. Ma, L.B. Kong, P. Hing, J.A. Kilner, Solid State Ionics 167 (2004) 191. [19] J.-G. Li, T. Ikegami, T. Mori, Acta Mater. 52 (2004) 2221.
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[20] T. Mori, Y. Wang, J. Drennan, G. Auchterlonie, J.-G. Li, T. Ikegami, Solid State Ionics 175 (2004) 641. [21] Y. Wang, T. Mori, J.-G. Li, J. Drennan, J. Eur. Ceram. Soc. 25 (2005) 949. [22] I. Kosacki, T. Suzuki, V. Petrovsky, H.U. Anderson, Solid State Ionics 136–137 (2000) 1225. [23] T. Suzuki, I. Kosacki, H.U. Anderson, Solid State Ionics 151 (2002) 111. [24] Y. Xiong, K. Yamaji, T. Horita, N. Sakai, H. Negishi, H. Yokokawa, J. Electrochem. Soc. 148 (12) (2001) E489. [25] Y. Xiong, K. Yamaji, T. Horita, N. Sakai, H. Yokokawa, J. Electrochem. Soc. 151 (3) (2004) A407. [26] T. Higuchi, S. Yamaguchi, K. Kobayashi, T. Hattori, A. Fukushima, S. Shin, T. Tsukamoto, Jpn. J. Appl. Phys. 43 (11B) (2004) L1463.
Anomalous conductivity and microstructure in gadolinium doped ceria prepared from nano-sized powder Natsuko Sakai a,⁎, Yue Ping Xiong a , Katsuhiko Yamaji a , Harumi Yokokawa a , Yoshitake Terashi b , Hiroaki Seno b a
Fuel Cell Group, Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology, AIST Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan b R&D Center Kagoshima, KYOCERA Corporation, 1-4 Yamashita-cho, Kokubu, Kagoshima 899-4312, Japan Received 28 June 2005; received in revised form 23 March 2006; accepted 29 March 2006
Abstract The temperature and the oxygen partial pressure dependences of the electron and hole conductivities were measured by the dc polarization method using a Hebb–Wagner's ion blocking cell for Gd0.2Ce0.8O1.9 polycrystalline bodies with grain size of 0.5 μm prepared by sintering of nano-sized powder. A significant enrichment of gadolinium was observed in the vicinity of the grain boundary by TEM/EDS analyses. The electron conductivity were comparable with those of conventional Gd0.2Ce0.8O1.9 polycrystalline body with grain size of 2 μm, and it followed p(O2)− 1/4 dependence at temperatures T = 973–1273 K. However, the observed hole conductivity was higher than that of conventional Gd0.2Ce0.8O1.9, and it did not follow p(O2)1/4 dependence. This anomalous p(O2) dependence disappeared after the sample was treated at T = 1773 K for 38 h and grain size was enlarged to 2–10 μm. © 2006 Elsevier B.V. All rights reserved. Keywords: Ceria; Electron; Hole; Conductivity; Ion blocking; Grain boundary
1. Introduction Rare earth doped ceria (RDC) have good oxide ion conductivities even at T = 500 °C, which is attractive for electrolyte material of solid oxide fuel cells which operate at intermediate temperatures [1]. In addition, we have reported that a significant amount of hydrogen component from water vapor dissolved into RDC polycrystalline bodies [2], which can be correlated with the interesting electro-catalytic activities of RDC in electrode reactions [3,4]. However, the rising of electron conductivity at reducing atmosphere may lower the efficiency, and poor mechanical strength in comparison with zirconia-based electrolytes, which are regarded as important problems to be overcome. We have determined the electron and hole conductivities of rare earth doped ceria separately from oxide ion conductivity by using dc polarization method using a modified Hebb–Wagner's ion blocking cell [4,5].
⁎ Corresponding author. Tel.: +81 29 861 4890; fax: +81 29 861 4540. E-mail address: [email protected] (N. Sakai). 0167-2738/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2006.03.049
In recent 10 years, the preparation of nano-sized ceria powder is intensively investigated mainly in anticipation of improvement in sintering behavior [6–16]. Although those nano-sized powders exhibit rapid sintering at lower temperatures, it was somewhat difficult to control the grain growth during the sintering procedure. Quite recently, the preparation of dense polycrystalline bodies with grain size less than 1 μm has been reported [17–21], and it has been revealed that the grain size may affect on mechanical property [18] or electrical conductivity [19–23], and local structures [20]. Based on these experimental facts, it is supposed that the effects of grain boundaries on the physical and chemical properties of such polycrystalline bodies with grain submicron grains may be more significant than those of normal polycrystalline bodies with grain size of several micrometers. We focused on the effect of grain boundary on the transport property of gadolinium doped ceria (GDC) polycrystalline bodies with grain size of ca. 0.5 μm which were prepared by sintering nano-sized powders. The electron and hole conductivities were measured by using a modified Hebb–Wagner ion-blocking method, and the results were compared with those of normal polycrystalline bodies with grain size of several micrometers.
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2. Experimental The dense polycrystalline pellets (diameter: 17 mm, thickness: 2 mm) of Ce0.8Gd0.2O1.9 with average grain size of 0.47 μm was prepared by sintering nano-sized powder compact at T = 1673 K in air. This sample is named as “nano-GDC” for convenience. The relative density was 96.1% and the surface was polished by diamond pastes, and the sample thickness (L) is 1–2 mm. The electron and hole conductivities were measured by the DC polarization method using a modified Hebb–Wagner's ion blocking cell. The detail of the ion blocking cell is described in our previous paper [24]. The ion-blocking cell is consists of a sample pellet with platinum electrodes and alumina spacers which shape a closed chamber. Platinum paste as electrodes was painted at the center of both (outer and inner) surface of a sample. The electrode area (A) was 0.785 cm2. Pyrex® glass seal was used to maintain the gas tightness of the chamber. The dc polarization measurements were carried out with a potentiostat (Toho Technical Research potentio/galvanostat 2020). The ion blocking cell is heated in 1% O2–99% Ar gas flow and sealed at glass melting temperature. When no electric potential is applied, the oxygen partial pressure, p(O2) is 1 kPa both inner and outer atmosphere. When a voltage (Eapp) was applied to the sample samples to pump out the oxygen gas in the cell, the current changed until a steady state was established. In the steady state, the oxygen chemical potential at the inner electrode/sample interface was reduced against the outer electrode. The oxide ion flow was blocked at the inner electrode because there was no electrochemical potential gradient of oxide ions inside the sample. Hence, the measured current at the steady state should be electronic current (Ie), governed by the electrons and holes in the sample. The electronic (hole) conductivity (σe, h) was determined by the following equation. L AIe re;h ¼ ð1Þ A AEapp The data at p(O2) = 10− 3 to 10−10 MPa were collected under 1% O2–99% Ar gas flow and applied voltage from 0.0 to 0.4 V. The data at p(O2) > 10− 3 MPa were collected by sealing the ion blocking cell in 10% O2–90%Ar, 30%O2–70%Ar, or 100% O2
Fig. 2. The total conductivity (σtot) and electron (hole) conductivity of nanoGDC at T = 973 K. The data are compared with the conductivity data of microGDC [5].
gas flow and applied voltage was limited to 10 mV, which avoided gas leakage due to the pressure difference between the outer and the inner space of the blocking cell. The total conductivity (σtot) was separately measured by using AC impedance method at T = 973 K, p(O2) = 10− 26– 0.1 MPa. Gas mixtures O2–Ar or H2–CO2 were used to control oxygen partial pressure. The microstructure, cerium/gadolinium ratio, and valence state of cerium ion in nano-GDC before and after the dc polarization measurement were examined by using a high resolution transmission electron microscope (HRTEM, HF2210, Hitachi Co. 200 kV)/energy dispersed X-ray Analyses and electron energy loss spectroscopy (EDX, EELS, NORAN VOYAGER). For comparison, a dense Y0.2Ce0.8O1.9 (YDC) polycrystalline body prepared by a conventional ceramic method from Y2O3 and CeO2 and sintered at 1923 K in air was examined in TEM/EDX analyses. 3. Results and discussion 3.1. Electron, hole and total conductivity of nano-GDC Fig. 1 shows the electronic (hole) conductivity (σe, h) of nano-GDC as a function of oxygen partial pressure at inner electrode p(O2), and was calculated by the following equation, Eapp ¼
RT pout ðO2 Þ ln 4F pðO2 Þ
ð2Þ
where R, T, F and pout(O2) are the gas constant, the sample temperature, Faraday's constant and the oxygen partial pressure Table 1 Evaluated constants, a, b and n, in Eq. (3) for nano-GDC at 873–1273 K Temperature/K
Fig. 1. Electronic and hole conductivity of nano-GDC at 873–1273 K. Lines are reproduced from the obtained constants listed in Table 1 based on Eq. (3) in the text.
1273 1173 1073 973 873
a/S cm− 1 −4
3.90 × 10 6.00 × 10− 5 5.58 × 10− 6 2.65 × 10− 7 1.48 × 10− 8
b/S cm− 1
n
0.051 0.051 0.051 0.051 0.051
1.65 1.38 1.25 1.18 1.14
N. Sakai et al. / Solid State Ionics 177 (2006) 2503–2507
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Fig. 2 shows the present results of total conductivity (σtot) and electron (hole) conductivity (σe,h) of nano-GDC at T = 973 K as a function of oxygen partial pressure. The data are compared with our previous results of a conventional Gd0.2Ce0.8O1.9 polycrystalline body (abbreviated to micro-GDC, average grain size ca. 2 μm, relative density >95%) [25]. There is no significant difference for the total conductivity between nano- and micro-GDC. It does not agree with the results reported by Wang et al. for Dy2O3 doped CeO2, i. e. the total conductivity has a minimum at the grain size 0.24 μm. However, it is clearly shown that the nano-GDC has a remarkably higher hole conductivity than micro-GDC in the p(O2) range of 10− 5–0.1 MPa. The observed electron and hole conductivities were fitted into the following equation. Fig. 3. EDS analyses results around the grain boundary of as-prepared nanoGDC (sintering temperature 1673 K) and conventional YDC sintered at T = 1923 K.
around the outer electrode, respectively. The electronic conductivity of nano-GDC exhibits p(O2)− 1/4 dependence in reducing atmosphere, which indicates that the electron is the main charge carrier in those conditions. The hole conductivity should have exhibited p(O2)1/4 dependence at higher range of oxygen partial pressure, however, the obtained conductivity did not follow the p(O2)1/4 dependence.
Fig. 4. TEM image and EELS spectra at bulk (solid line), triple point (dashed line) and grain boundary (dashed and dotted line) of as prepared nano-GDC.
re;h ¼ aðpðO2 Þ=0:1 MPaÞ1=4 þ bðpðO2 Þ=0:1 MPaÞ1=n
ð3Þ
The parameter a corresponds to the electronic conductivity at p(O2) = 0.1 MPa. The parameter b corresponds to the σh at p (O2) = 0.1 MPa, and it was fixed 0.51 S cn− 1 according to the observed conductivity data. The derived constants, a, b and n, are listed in Table 1, and the evaluated values for Eq. (3) are plotted in Fig. 1 as lines. The n obtained values ranged from 1.14 to 1.65, which were much lower than the theoretical
Fig. 5. TEM image and EELS spectra of normal grain (solid line) and inclusion (dashed line) of nano-GDC after the hole conductivity measurement (T = 973– 1273 K, p(O2) = 0.1 MPa, E = 10 mV).
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value (n = 4) and can not be explained by a simple defect model. It indicates that additional oxygen which does not contribute to the hole conduction is absorbed in the solid. 3.2. Microstructure of nano-GDC and its effect on conductivities In the TEM images of as prepared nano-GDC, no visible precipitation of secondary phase was observed in the vicinity of grain boundary (GB). However, semi-quantitative EDX analyses revealed that the relative gadolinium content is significantly higher (over 28%) on grain boundary as shown in Fig. 3. The gadolinium content is the same level as in the bulk at the point is which located 10 nm from the grain boundary. However, similar enrichment is also observed for the case of yttrium doped ceria (Ce0.8Y0.2O1.9) which was sintered at T = 1923 K as shown in the same figure. Hence it is considered that this phenomena is commonly observed for rare earth doped ceria, regardless of dopant types and preparation condition. The EELS spectra taken at the different point of the same sample are shown in Fig. 4. The peaks of CeN 4,5 (Ce4+) and GdN 4,5 (Gd3+) are observed at energy levels 132 eV and 150 eV, respectively. There are no significant differences in valence states among grain boundaries and bulk. However, after the hole conductivity measurement in p(O 2) = 0.1 MPa, E = 10 mV at T = 973–1273 K, the microstructure of nano-GDC changed as shown in Fig. 5(a). The grain contains many inclusions inside. The EELS spectrum at the inclusion revealed a significant enrichment of gadolinium (Fig. 5(b)). Furthermore, the energy level of CeN 4,5 peak shifted to 126 eV, which indicates the Ce3+ contribution. Recently Higuchi et al. reported the electronic structure of Ce1−xNdxO2−δ by using the resonant photoemission spectroscopy (RPES) [26]. They suggest that the existence of trivalent cerium ion (Ce3+) is observed for the Nd doped sample which is prepared by high temperature sintering in air. With increasing Nd3+ concentration, the hybridization effect of Ce 4f and O 2p states decreases and the effective charges of Ce4+ and O2− ions decreases. Hence it is not unusual that Ce3+ exists in oxidizing atmosphere. However, the formation of the Gd3+ and Ce3+ rich inclusions in the present nano-GDC after the hole conductivity measurement is extraordinary, because it was not observed in as-prepared sample. It is still uncertain about the correlation of the inclusion formation and anomalously high hole conductivity. In order to examine the effect of chemical homogeneity of the sample, the nano-GDC was annealed at T = 1773 K for 38 h and its grain size was enlarged to 2–10 μm. The effect of the post sintering is shown in Fig. 6. The conductivity of the post-sintered nanoGDC exhibited p(O2)1/4 dependence, which is quite normal and same as that of micro-GDC. Note that the post sintering will not suppress the enrichment of gadolinium at grain boundary, because the enrichment of yttrium is still observed for YDC which was sintered at very high temperature (1923 K) as shown in Fig. 3. However, the formation of inclusion may be suppressed by improving chemical homogeneity by high
Fig. 6. Effect of sintering on electron and hole conductivity of nano-GDC at T = 973 K.
temperature treatment. Hence the relationship between microscopic chemical homogeneity and hole conductivity should be carefully investigated. 4. Summary The anomalous high hole conductivity and p(O2) dependence were observed for Gd0.2Ce0.8O1.9 polycrystalline bodies with grain size of 0.5 μm prepared from nano-sized powder. The sample after hole conductivity measurement exhibited a significant compositional fluctuation such as the inclusion containing higher content of Gd3+ and Ce3+, although the asprepared is quite homogeneous structure. Since this anomaly disappears after the post sintering procedure at T = 1773 K, it can be correlated to some compositional and morphological change which was not eliminated by the initial sintering at T = 1673 K. It is suggested that the electron and hole conductivity measured by dc polarization method is sensitive to such changes. Further investigation will be made to examine the relationship of microstructure change and hole conductivity dependence for samples with different dopant type, concentration and preparation methods. References [1] B.C.H. Steele, Solid State Ionics 129 (2000) 95. [2] N. Sakai, K. Yamaji, T. Horita, H. Yokokawa, Y. Hirata, S. Sameshima, Y. Nigara, J. Mizusaki, Solid State Ionics 125 (1999) 325. [3] T. Horita, N. Sakai, K. Yamaji, H. Yokokawa, T. Kato, J. Power Sources 131 (2004) 299. [4] H. Yokokawa, T. Horita, N. Sakai, K. Yamaji, M.E. Brito, Y.P. Xiong, H. Kishimoto, Solid State Ionics 174 (2004) 205. [5] K. Yamaji, Y.P. Xiong, N. Sakai, T. Horita, H. Hokokawa, J. Electrochem. Soc. 149 (11) (2002) E450. [6] V. Briois, C.E. Williams, H. Dexpert, F. Villain, B. Camane, F. Deneuve, C. Magnier, J. Mater. Sci. 28 (1993) 5019. [7] P.L. Chen, I.W. Chen, J. Am. Ceram. Soc. 76 (1993) 1557. [8] T.J. Kirk, J. Winnick, J. Electrochem. Soc. 140 (1993) 3494. [9] J. Van herle, T. Horita, T. Kawada, N. Sakai, H. Yokokawa, M. Dokiya, J. Eur. Ceram. Soc. 16 (1996) 961. [10] Y.M. Chiang, E.B. Lavik, D.A. Blom, Nanostruct. Mater. 9 (1997) 633. [11] Y.M. Chiang, E.B. Lavik, I.K. Osack, H.L. Tuller, J.M. Ying, J. Electroceram. 1 (1997) 7.
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