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Electrical conductivity of samarium–ytterbium zirconate ceramics
Electrochimica Acta 54 (2009) 3968–3971
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electrical conductivity of samarium–ytterbium zirconate ceramics Zhan-Guo Liu, Jia-Hu Ouyang ∗ , Yu Zhou, Xiao-Liang Xia Institute for Advanced Ceramics, Department of Materials Science, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 21 December 2008 Received in revised form 4 February 2009 Accepted 6 February 2009 Available online 13 February 2009 Keywords: (Sm1 − x Ybx )2 Zr2 O7 Electrical conductivity Impedance spectroscopy Electrolyte Order–disorder transition
a b s t r a c t (Sm1 − x Ybx )2 Zr2 O7 (0 ≤ x ≤ 1.0) ceramic powders were prepared by chemical-coprecipitation and calcination method, and were pressureless-sintered at 1973 K for 10 h to fabricate dense bulk materials. (Sm1 − x Ybx )2 Zr2 O7 has a single phase with a pyrochlore or defect fluorite structure, depending mainly upon the Yb content. They are found to be pyrochlores for 0 ≤ x ≤ 0.1, and defect fluorites for 0.3 ≤ x ≤ 1.0. The electrical conductivity of (Sm1 − x Ybx )2 Zr2 O7 was investigated by complex impedance spectroscopy over a frequency range of 200 Hz to 20 MHz from 723 to 1173 K in air. The measured electrical conductivity obeys the Arrhenius relation. The grain conductivity of (Sm1 − x Ybx )2 Zr2 O7 ceramics gradually increases with increasing temperature. A decrease of about one order of magnitude in grain conductivity is found at all temperature levels when the Yb content increases from x = 0.1 to x = 0.3. The electrical conductivities of defect fluorite-type materials are lower than those of pyrochlore-type materials in (Sm1 − x Ybx )2 Zr2 O7 system, whereas activation energies for the conduction process increase monotonically as the structure becomes disordered. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction Complex oxides with the general formula Ln2 Zr2 O7 (Ln = lanthanide) exhibit a pyrochlore-type structure or a defect fluoritetype structure, which is mainly governed by the ion size difference between Ln3+ and Zr4+ [1,2]. They have a wide variety of interesting physical and chemical properties, such as high melting point, high thermal expansion coefficient, low thermal conductivity, high thermal stability, high radiation stability and high electrical conductivity. These properties make rare-earth zirconates suitable for extensive applications such as high-temperature thermal barrier coating materials, nuclear waste forms and solid electrolytes, etc. [3–6]. Their electrical properties make them potential candidates for solid oxide fuel cells applications. The advantage of lowering the operating temperature of solid oxide fuel cells has attracted great interest worldwide. Enormous amounts of efforts were found in the literature to improve electrical conductivity of oxide electrolyte materials [7–10]. In recent years, new rareearth zirconates with various ionic radius ratios of r(Ln3+ )/r(Zr4+ ) are of considerable scientific interest [11–14]. Mandal et al. [11] found that a significant increase in electrical conductivity was obtained at 622–696 K by properly doping at the Gd site with isovalent rare-earth cations like Nd in Gd2 Zr2 O7 ceramic. Diaz-Guillen et al. [12,13] prepared (Gd1 − x Lax )Zr2 O7 (0 ≤ x ≤ 1.0) ceramics by mechanical milling and firing of the corresponding stoichio-
∗ Corresponding author. Tel.: +86 451 86414291; fax: +86 451 86414291. E-mail address: [email protected] (J.-H. Ouyang). 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.02.017
metric elemental oxides, and found that electrical conductivity of (Gd1 − x Lax )Zr2 O7 (0 ≤ x ≤ 0.4) ceramics was almost La-content independent in the temperature range of 773–1023 K. The conductivity of pyrochlore-type Sm2 Zr2 O7 was comparable to those of other good conductors such as yttria-stabilized zirconia in lowtemperature regions [3]. (Sm1 − x Gdx )2 Zr2 O7 (0 ≤ x ≤ 1.0) ceramics were synthesized by pressureless-sintering method, and the highest electrical conductivity was obtained for SmGdZr2 O7 in the temperature range of 623–873 K [14]. In the present work, electrical conductivity of (Sm1 − x Ybx )2 Zr2 O7 (0 ≤ x ≤ 1.0) ceramics was investigated to obtain a better understanding on the relationship between structure and conductivity at different temperatures. 2. Experimental The base chemicals used in this investigation were zirconium oxychloride (Zibo Huantuo Chemical Co. Ltd., China; Analytical), samarium oxide and ytterbium oxide powders (Rare-Chem HiTech Co., Ltd., Huizhou, China; purity ≥ 99.99%). Samarium oxide and ytterbium oxide powders were heat-treated at 1173 K for 2 h in air before weighing as rare-earth oxides were hygroscopic. (Sm1−x Ybx )2 Zr2 O7 (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9, 1.0) ceramic powders were synthesized by chemical-coprecipitation and calcination method. For each composition, samarium oxide and ytterbium oxide powders were weighed, and dissolved in dilute nitric acid, while zirconium oxychloride was dissolved in distilled water. These solutions were mixed, stirred, filtered and slowly added to dilute ammonium hydrate solution to obtain gel-like precipitates. These gels were washed with distilled water, and were then washed in
Z.-G. Liu et al. / Electrochimica Acta 54 (2009) 3968–3971
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Table 1 Relative densities and chemical compositions of (Sm1−x Ybx )2 Zr2 O7 ceramics sintered at 1973 K for 10 h. Ceramic materials
Sm2 Zr2 O7 (Sm0.9 Yb0.1 )2 Zr2 O7 (Sm0.7 Yb0.3 )2 Zr2 O7 (Sm0. 5 Yb0.5 )2 Zr2 O7 (Sm0.3 Yb0.7 )2 Zr2 O7 (Sm0.1 Yb0.9 )2 Zr2 O7 Yb2 Zr2 O7
Fig. 1. XRD patterns of (Sm1−x Ybx )2 Zr2 O7 (x = 0, 0.1, 0.3, 0.9, 1.0) ceramics sintered at 1973 K for 10 h. The symbol “” represents the super-lattice diffraction peaks.
absolute alcohol. The washed precipitates were dried, calcined, ground and compacted by cold isostatic pressing at 280 MPa for 5 min. Finally, the compacts were pressureless-sintered at 1973 K for 10 h in air. Crystal structures of sintered (Sm1−x Ybx )2 Zr2 O7 ceramics were identified by an X-ray diffraction technique (XRD) at room temperature. XRD analysis of the specimens was carried out using a Rigaku Diffractometer (D/Max-2200VPC, Japan) with graphite monochromator and Cu K␣ radiation at a scan rate of 4◦ /min. The bulk density of sintered (Sm1−x Ybx )2 Zr2 O7 ceramics was measured by the Archimedes principle with an immersion medium of deionized water. The microstructural analysis of each specimen was carried out with a scanning electron microscope (SEM, Hitachi S–4700, Japan) equipped with an energy-dispersive X-ray spectroscopy (EDS). For SEM observations, the specimens were first polished with 1 m diamond paste, and were then thermally etched at 1873 K for 1 h in air, and finally a thin carbon coating was evaporated onto the surface of each specimen for electrical conductivity. The electrical conductivity of sintered (Sm1−x Ybx )2 Zr2 O7 ceramics was measured using an impedance/gain-phase analyzer (SolatronTM SI 1260, UK). Cylindrical disc-shaped specimens with a diameter of 8 mm and a thickness of 1 mm were machined from the sintered ceramics, and the surfaces were ground to be flat and parallel. Platinum paste was brushed onto both sides of the cylindrical shaped pellets to serve as the electrode. Platinum wires (99.9% pure) with diameter 0.2 mm were attached to the cell using platinum paste to perform ionic conductivity measurements. Each pellet was then cofired at 1223 K for 2 h at a heating rate of 5 K min−1 in order to ensure intimate contact with the specimen surface and eliminate organic components. The complex impedance spectra of all specimens were measured using a four-probe method over a frequency range of 200 Hz–20 MHz in air. The measurements were carried out from 723 to 1173 K at a temperature interval of 50 K. The heating rate was 5 K min−1 , while the stabilisation time between consecutive measurements was 15 min. The specimen temperature was monitored by a K-type thermocouple positioned adjacent to the specimen.
Relative density
Mole ratio
(%)
Yb
Sm
Zr
97.6 96.5 95.3 96.6 96.9 96.3 96.0
0 5.0 14.8 24.9 35.2 44.7 49.7
49.6 44.8 35.2 24.8 14.7 4.9 0
50.4 50.2 50.0 50.3 50.1 50.4 50.3
tent. Sm2 Zr2 O7 and (Sm0.9 Yb0.1 )2 Zr2 O7 have a pyrochlore-type structure, which is characterized by the presence of typical super-lattice peaks at 2 values of about 14◦ (1 1 1), 28◦ (3 1 1), 37◦ (3 3 1), 45◦ (5 1 1) and 51◦ (5 3 1) using Cu K␣ radiation [15,16]. However, (Sm1−x Ybx )2 Zr2 O7 (0.3 ≤ x ≤ 1.0) exhibit a defect fluorite-type structure. Sm2 Zr2 O7 and Yb2 Zr2 O7 ceramics are completely soluble. In the Ln2 Zr2 O7 system, the crystal structure is mainly governed by the ratio of r(Ln3+ )/r(Zr4+ ). The stability of pyrochlore-type structure in zirconates is limited to the range of 1.46 ≤ r(Ln3+ )/r(Zr4+ ) ≤ 1.78 at an atmospheric pressure [1,2]. Below 1.46, the array of unoccupied anion sites disorders to produce a defect fluorite-type structure. Above 1.78, there is a transition to monoclinic phase with a La2 Ti2 O7 -type structure. The ionic radius of Sm3+ and Yb3+ are 1.079 Å and 0.985 Å in the eight-fold coordination, respectively; however, the ionic radius of Zr4+ is 0.72 Å in the six-fold coordination [17]. The values of r(Ln3+ )/r(Zr4+ ) are equal to 1.50 and 1.48 for Sm2 Zr2 O7 and (Sm0.9 Yb0.1 )2 Zr2 O7 , respectively. Therefore, Sm2 Zr2 O7 and (Sm0.9 Yb0.1 )2 Zr2 O7 exhibit a pyrochlore-type structure. As for (Sm1−x Ybx )2 Zr2 O7 (0.3 ≤ x ≤ 1.0), the values of r(Ln3+ )/r(Zr4+ ) are clearly lower than 1.46. Therefore, (Sm1−x Ybx )2 Zr2 O7 (0.3 ≤ x ≤ 1.0) ceramics exhibit a defect fluorite-type structure. The value of r(Ln3+ )/r(Zr4+ ) gradually decreases with increasing Yb content, which also represents the increase in degree of structure disordering. The relative densities of (Sm1−x Ybx )2 Zr2 O7 ceramics are in the range of 95.3–97.6%, as shown in Table 1. The microstructures of (Sm1−x Ybx )2 Zr2 O7 ceramics with various chemical compositions are very similar. Fig. 2 shows typical microstructure of (Sm0.3 Yb0.7 )2 Zr2 O7 ceramic. The average grain size of (Sm0.3 Yb0.7 )2 Zr2 O7 is several micrometers, and the grain boundaries in (Sm0.3 Yb0.7 )2 Zr2 O7 are very clean. The chemical compositions of each specimen were determined by EDS. According to the EDS results in Table 1, the mole ratios of different metallic
3. Results and discussion The X-ray diffraction patterns of (Sm1−x Ybx )2 Zr2 O7 (x = 0, 0.1, 0.3, 0.9, 1.0) ceramics sintered at 1973 K for 10 h in air are shown in Fig. 1. It can be observed that all the compositions studied are single phase, and the positions of all reflections shift gradually to the high angle side with increasing Yb con-
Fig. 2. Microstructure of (Sm0.3 Yb0.7 )2 Zr2 O7 sintered at 1973 K for 10 h in air.
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Fig. 3. Typical complex impedance and schematic equivalent electrical circuits plots for (Sm1−x Ybx )2 Zr2 O7 ceramics at 723 K: (a) x = 0, 0.1; (b) x = 0.3, 0.5,0.7, 0.9, 1.0; (c) magnification of (b); Rg , Rgb , CPEg and CPEgb represent grain resistance, grain boundary resistance, constant phase element of the grain and constant phase element of the grain boundary. The grain (G) and grain-boundary (GB) contributions are also indicated.
elements in (Sm1−x Ybx )2 Zr2 O7 ceramics have the nominal stoichiometry to within ± 2%. It is convenient to distinguish between the grain and grain boundary effects using the complex impedance plane plot (Z vs. Z ). Fig. 3 shows the typical impedance plane plots of (Sm1−x Ybx )2 Zr2 O7 ceramics measured at 723 K in air. Fig. 3(a) is impedance plane plots of Sm2 Zr2 O7 and (Sm0.9 Yb0.1 )2 Zr2 O7 . Two distinct contributions manifested in the form of semicircular arcs are clearly identified. The high frequency semicircles correspond to the grain impedance, and the low frequency semicircles represent the grain-boundary impedance. From Fig. 3 (a), capacitance values found for the high- and low-frequency arcs are 8.56 × 10−11 and 6.72 × 10−8 F/cm for Sm2 Zr2 O7 , which corresponds to the grain and grain boundary contributions, respectively. However, for (Sm0.9 Yb0.1 )2 Zr2 O7 ceramic, the values of capacitance for grain and grain boundary are 9.15 × 10−11 and 6.69 × 10−8 F/cm, respectively. Fig. 3 (b) and (c) are impedance plane plots of (Sm1−x Ybx )2 Zr2 O7 (0.3 ≤ x ≤ 1.0) ceramics. Only one distinct contribution manifested in the form of semicircular arc is identified. From Fig. 3 (b) and (c), the capacitance values of different semicircular arcs are 1.57 × 10−10 F/cm for (Sm0.7 Yb0.3 )2 Zr2 O7 , 1.22 × 10−10 F/cm for (Sm0.5 Yb0.5 )2 Zr2 O7 , 8.79 × 10−11 F/cm for (Sm0.3 Yb0.7 )2 Zr2 O7 , 6.41 × 10−11 F/cm for (Sm0.1 Yb0.9 )2 Zr2 O7 and 4.45 × 10−11 F/cm for Yb2 Zr2 O7 , respectively. These semicircular arcs in Fig. 3 (b) and (c) correspond to the grain contributions. Typical equivalent electrical circuits applied to reproduce such impedance plots consists of parallel resistance–capacitance (R–C) blocks [18,19], as inset in Fig. 3 (a) and (b). These contributions are separated by fitting semicircles to each of the arcs, and the grain resistance values, Rg , are determined from the intercepts of high frequency-range semicircles on the Z -axes [20], respectively. The electrical conductivity at different temperatures is calculated from the values of resistance and the dimensions of the specimens.
The temperature dependence of grain conductivity is analyzed using an Arrhenius equation with the following expression: g T = 0 exp
−E kB T
(1)
where pre-exponential factor 0 is a measurement of the effective number of mobile oxide-ions, E is the activation energy for the conduction process, kB is the Boltzmann constant, and T is absolute temperature. Fig. 4 shows the Arrhenius plots of the grain conductivity for each composition studied in this work, where the lines are fitted to an Arrhenius equation. This confirms that the ionic diffusion process is thermally activated. The values of activation energy (Eg ) and pre-exponential factor ( 0g ) for each composition are calculated from the slope and the intercept of the linear fits in the Arrhenius plots (Fig. 4), respectively. The calculated values of acti-
Fig. 4. Arrhenius plots of the grain (G) conductivity of (Sm1−x Ybx )2 Zr2 O7 ceramics.
Z.-G. Liu et al. / Electrochimica Acta 54 (2009) 3968–3971 Table 2 Activation energy Eg and pre-exponential factor 0g for the grain (G) contributions to conductivity. Ceramic materials
Sm2 Zr2 O7 (Sm0.9 Yb0.1 )2 Zr2 O7 (Sm0.7 Yb0.3 )2 Zr2 O7 (Sm0. 5 Yb0.5 )2 Zr2 O7 (Sm0.3 Yb0.7 )2 Zr2 O7 (Sm0.1 Yb0.9 )2 Zr2 O7 Yb2 Zr2 O7
Grain (G) contributions Activation energy Eg (eV)
Pre-exponential factor 0g (S cm−1 K)
0.69 0.85 1.16 1.32 1.42 1.53 1.56
9.98 × 103 8.94 × 104 5.63 × 105 2.04 × 106 5.65 × 106 1.40 × 107 7.08 × 106
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ity of (Sm1−x Ybx )2 Zr2 O7 (0.1 ≤ x ≤ 0.9) decreases with increasing Yb content. This indicates that the increase in g is not able to compensate for the increase in Eg , and finally causes the drop in electrical conductivity. The g for Yb2 Zr2 O7 is clearly lower than that of (Sm0.1 Yb0.9 )2 Zr2 O7 , while the Eg for Yb2 Zr2 O7 is slightly higher than that of (Sm0.1 Yb0.9 )2 Zr2 O7 . Therefore, the electrical conductivity of Yb2 Zr2 O7 should be lower than that of (Sm0.1 Yb0.9 )2 Zr2 O7 , which is also consistent with the measurement results in this work. 4. Conclusions Sm2 Zr2 O7 and (Sm0.9 Yb0.1 )2 Zr2 O7 have a pyrochlore-type structure, while (Sm1−x Ybx )2 Zr2 O7 (0.3 ≤ x ≤ 1.0) exhibit a defect fluorite-type structure. The degree of structural disordering for (Sm1−x Ybx )2 Zr2 O7 ceramics increases with increasing Yb content. The grain conductivity of (Sm1−x Ybx )2 Zr2 O7 ceramics gradually increases with increasing temperature. The electrical conductivities of defect fluorite-type materials are lower than those of pyrochlore-type materials, whereas activation energies for the conduction process increase monotonically as the structure becomes disordered. The grain conductivity reaches the highest value of 1.71 × 10−2 S cm−1 at 1173 K for (Sm0.9 Yb0.1 )2 Zr2 O7 ceramics in this work. Acknowledgements The authors would like to thank the financial support from the Program of Excellent Teams in Harbin Institute of Technology (HIT) and the Start-up Program for High-level HIT Faculty Returned from Abroad.
Fig. 5. Variations of the grain (G) conductivity of (Sm1−x Ybx )2 Zr2 O7 ceramics as a function of Yb content and temperature.
References
vation energy and pre-exponential factor are presented in Table 2. It can be seen that the activation energy Eg gradually increases with increasing Yb content. The activation energy Eg of Sm2 Zr2 O7 in this study is 0.69 eV, which is slightly higher than the results obtained by Shinozaki et al.’s [3]. From Table 2, the pre-exponential factor 0g of (Sm1−x Ybx )2 Zr2 O7 ceramics except for Yb2 Zr2 O7 gradually increases with increasing Yb content. Fig. 5 shows the variations of grain conductivity as a function of Yb content and temperature for all compositions in this work. Clearly, the grain conductivity gradually increases with increasing temperature for each composition. A decrease of about one order of magnitude in grain conductivity is found at all temperature levels when the Yb content increases from x = 0.1 to x = 0.3. With further increasing the Yb content from x = 0.3 to x = 1.0, the grain conductivity g slightly decreases, and reaches a minimum value at x = 1.0 for all temperature levels. It means that electrical conductivities of fluorite-type materials are lower than those of pyrochlore-type materials in (Sm1−x Ybx )2 Zr2 O7 ceramics. The electrical conductivity value of Sm2 Zr2 O7 is close to that of (Sm0.9 Yb0.1 )2 Zr2 O7 in this work. The highest electrical conductivity value obtained in this work is 1.71 × 10−2 S cm−1 at 1173 K for (Sm0.9 Yb0.1 )2 Zr2 O7 . The increase in g would lead to an increase in electrical conductivity; however, the increase in Eg would hinder the oxide-ion migration. Thus, these two processes are competing. As the Yb content increases from 0.1 to 0.9, both g and Eg increase as shown in Table 2. From Fig. 5, the electrical conductiv-
[1] M.A. Subramanian, G. Aravamudan, G.V. Subba Rao, Prog. Solid State Chem. 15 (1983) 55. [2] M.A. Subramanian, A.W. Sleight, in: K.A. Gschneidner Jr., L. Eyring (Eds.), Handbook on the Physics and Chemistry of Rare Earths, vol.16, Elsevier Science Publishers B.V, 2003, Chapter 107. [3] K. Shinozaki, M. Miyauchi, K. Kuroda, O. Sakurai, N. Mizutani, M. Kato, J. Am. Ceram. Soc. 62 (1979) 538. [4] X.Q. Cao, R. Vassen, D. Stoever, J. Eur. Ceram. Soc. 24 (2004) 1. [5] J. Lian, S.V. Yudintsev, S.V. Stefanovsky, L.M. Wang, R.C. Ewing, J. Alloys Compd. 444–445 (2007) 429. [6] Z.-G. Liu, J.-H. Ouyang, Y. Zhou, Li, X.-L. Xia, Int. J. Appl. Ceram. Technol. (2008), doi:10.1111/j.1744-7402.2008.02289.x. [7] J.B. Goodenough, Annu. Rev. Mater. Res. 33 (2003) 91. [8] O. Yamamoto, Electrochim. Acta 45 (2000) 2423. [9] J.W. Fergus, J. Power Sources 162 (2006) 30. [10] S. Hui, J. Roller, S. Yick, X. Zhang, C. Decès-Petit, Y. Xie, R. Maric, D. Ghosh, J. Power Sources 172 (2007) 493. [11] B.P. Mandal, S.K. Deshpande, A.K. Tyagi, J. Mater. Res. 23 (2008) 911. [12] J.A. Díaz-Guillén, M.R. Díaz-Guillén, J.M. Almanza, A.F. Fuentes, J. Santamaría, C. León, J. Phys.: Condens. Matter. 19 (2007) 356212. [13] J.A. Díaz-Guillén, M.R. Díaz-Guillén, K.P. Padmasree, A.F. Fuentes, J. Santamaría, C. León, Solid State Ionics 179 (2008) 2160. [14] Z.-G. Liu, J.-H. Ouyang, Y. Zhou, X.-L. Xia, J. Power Sources 185 (2008) 876. [15] Z.-G. Liu, J.-H. Ouyang, Y. Zhou, J. Mater. Sci. 43 (2008) 3596. [16] Z.-G. Liu, J.-H. Ouyang, Y. Zhou, Q.-C. Meng, X.-L. Xia, Phil. Mag. 89 (2009) 553. [17] G.S. Rohrer, Structure and Bonding in Crystalline Materials, Cambridge University Press, Cambridge, 2004, p. 521. [18] J.R. Macdonald, W.B. Johnson, in: E. Barsoukov, J.R. Macdonald (Eds.), Impedance Spectroscopy: Theory, Experiment and Applications, 2nd ed., John Wiley & Sons, Inc., New Jersey, 2005, Chapter 1. [19] Q.-A. Huang, R. Hui, B. Wang, J. Zhang, Electrochim. Acta 52 (2007) 8144. [20] M.H. Abdullah, A.N. Yussof, J. Mater. Sci. 32 (1997) 5817.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electrical conductivity of samarium–ytterbium zirconate ceramics Zhan-Guo Liu, Jia-Hu Ouyang ∗ , Yu Zhou, Xiao-Liang Xia Institute for Advanced Ceramics, Department of Materials Science, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 21 December 2008 Received in revised form 4 February 2009 Accepted 6 February 2009 Available online 13 February 2009 Keywords: (Sm1 − x Ybx )2 Zr2 O7 Electrical conductivity Impedance spectroscopy Electrolyte Order–disorder transition
a b s t r a c t (Sm1 − x Ybx )2 Zr2 O7 (0 ≤ x ≤ 1.0) ceramic powders were prepared by chemical-coprecipitation and calcination method, and were pressureless-sintered at 1973 K for 10 h to fabricate dense bulk materials. (Sm1 − x Ybx )2 Zr2 O7 has a single phase with a pyrochlore or defect fluorite structure, depending mainly upon the Yb content. They are found to be pyrochlores for 0 ≤ x ≤ 0.1, and defect fluorites for 0.3 ≤ x ≤ 1.0. The electrical conductivity of (Sm1 − x Ybx )2 Zr2 O7 was investigated by complex impedance spectroscopy over a frequency range of 200 Hz to 20 MHz from 723 to 1173 K in air. The measured electrical conductivity obeys the Arrhenius relation. The grain conductivity of (Sm1 − x Ybx )2 Zr2 O7 ceramics gradually increases with increasing temperature. A decrease of about one order of magnitude in grain conductivity is found at all temperature levels when the Yb content increases from x = 0.1 to x = 0.3. The electrical conductivities of defect fluorite-type materials are lower than those of pyrochlore-type materials in (Sm1 − x Ybx )2 Zr2 O7 system, whereas activation energies for the conduction process increase monotonically as the structure becomes disordered. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction Complex oxides with the general formula Ln2 Zr2 O7 (Ln = lanthanide) exhibit a pyrochlore-type structure or a defect fluoritetype structure, which is mainly governed by the ion size difference between Ln3+ and Zr4+ [1,2]. They have a wide variety of interesting physical and chemical properties, such as high melting point, high thermal expansion coefficient, low thermal conductivity, high thermal stability, high radiation stability and high electrical conductivity. These properties make rare-earth zirconates suitable for extensive applications such as high-temperature thermal barrier coating materials, nuclear waste forms and solid electrolytes, etc. [3–6]. Their electrical properties make them potential candidates for solid oxide fuel cells applications. The advantage of lowering the operating temperature of solid oxide fuel cells has attracted great interest worldwide. Enormous amounts of efforts were found in the literature to improve electrical conductivity of oxide electrolyte materials [7–10]. In recent years, new rareearth zirconates with various ionic radius ratios of r(Ln3+ )/r(Zr4+ ) are of considerable scientific interest [11–14]. Mandal et al. [11] found that a significant increase in electrical conductivity was obtained at 622–696 K by properly doping at the Gd site with isovalent rare-earth cations like Nd in Gd2 Zr2 O7 ceramic. Diaz-Guillen et al. [12,13] prepared (Gd1 − x Lax )Zr2 O7 (0 ≤ x ≤ 1.0) ceramics by mechanical milling and firing of the corresponding stoichio-
∗ Corresponding author. Tel.: +86 451 86414291; fax: +86 451 86414291. E-mail address: [email protected] (J.-H. Ouyang). 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.02.017
metric elemental oxides, and found that electrical conductivity of (Gd1 − x Lax )Zr2 O7 (0 ≤ x ≤ 0.4) ceramics was almost La-content independent in the temperature range of 773–1023 K. The conductivity of pyrochlore-type Sm2 Zr2 O7 was comparable to those of other good conductors such as yttria-stabilized zirconia in lowtemperature regions [3]. (Sm1 − x Gdx )2 Zr2 O7 (0 ≤ x ≤ 1.0) ceramics were synthesized by pressureless-sintering method, and the highest electrical conductivity was obtained for SmGdZr2 O7 in the temperature range of 623–873 K [14]. In the present work, electrical conductivity of (Sm1 − x Ybx )2 Zr2 O7 (0 ≤ x ≤ 1.0) ceramics was investigated to obtain a better understanding on the relationship between structure and conductivity at different temperatures. 2. Experimental The base chemicals used in this investigation were zirconium oxychloride (Zibo Huantuo Chemical Co. Ltd., China; Analytical), samarium oxide and ytterbium oxide powders (Rare-Chem HiTech Co., Ltd., Huizhou, China; purity ≥ 99.99%). Samarium oxide and ytterbium oxide powders were heat-treated at 1173 K for 2 h in air before weighing as rare-earth oxides were hygroscopic. (Sm1−x Ybx )2 Zr2 O7 (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9, 1.0) ceramic powders were synthesized by chemical-coprecipitation and calcination method. For each composition, samarium oxide and ytterbium oxide powders were weighed, and dissolved in dilute nitric acid, while zirconium oxychloride was dissolved in distilled water. These solutions were mixed, stirred, filtered and slowly added to dilute ammonium hydrate solution to obtain gel-like precipitates. These gels were washed with distilled water, and were then washed in
Z.-G. Liu et al. / Electrochimica Acta 54 (2009) 3968–3971
3969
Table 1 Relative densities and chemical compositions of (Sm1−x Ybx )2 Zr2 O7 ceramics sintered at 1973 K for 10 h. Ceramic materials
Sm2 Zr2 O7 (Sm0.9 Yb0.1 )2 Zr2 O7 (Sm0.7 Yb0.3 )2 Zr2 O7 (Sm0. 5 Yb0.5 )2 Zr2 O7 (Sm0.3 Yb0.7 )2 Zr2 O7 (Sm0.1 Yb0.9 )2 Zr2 O7 Yb2 Zr2 O7
Fig. 1. XRD patterns of (Sm1−x Ybx )2 Zr2 O7 (x = 0, 0.1, 0.3, 0.9, 1.0) ceramics sintered at 1973 K for 10 h. The symbol “” represents the super-lattice diffraction peaks.
absolute alcohol. The washed precipitates were dried, calcined, ground and compacted by cold isostatic pressing at 280 MPa for 5 min. Finally, the compacts were pressureless-sintered at 1973 K for 10 h in air. Crystal structures of sintered (Sm1−x Ybx )2 Zr2 O7 ceramics were identified by an X-ray diffraction technique (XRD) at room temperature. XRD analysis of the specimens was carried out using a Rigaku Diffractometer (D/Max-2200VPC, Japan) with graphite monochromator and Cu K␣ radiation at a scan rate of 4◦ /min. The bulk density of sintered (Sm1−x Ybx )2 Zr2 O7 ceramics was measured by the Archimedes principle with an immersion medium of deionized water. The microstructural analysis of each specimen was carried out with a scanning electron microscope (SEM, Hitachi S–4700, Japan) equipped with an energy-dispersive X-ray spectroscopy (EDS). For SEM observations, the specimens were first polished with 1 m diamond paste, and were then thermally etched at 1873 K for 1 h in air, and finally a thin carbon coating was evaporated onto the surface of each specimen for electrical conductivity. The electrical conductivity of sintered (Sm1−x Ybx )2 Zr2 O7 ceramics was measured using an impedance/gain-phase analyzer (SolatronTM SI 1260, UK). Cylindrical disc-shaped specimens with a diameter of 8 mm and a thickness of 1 mm were machined from the sintered ceramics, and the surfaces were ground to be flat and parallel. Platinum paste was brushed onto both sides of the cylindrical shaped pellets to serve as the electrode. Platinum wires (99.9% pure) with diameter 0.2 mm were attached to the cell using platinum paste to perform ionic conductivity measurements. Each pellet was then cofired at 1223 K for 2 h at a heating rate of 5 K min−1 in order to ensure intimate contact with the specimen surface and eliminate organic components. The complex impedance spectra of all specimens were measured using a four-probe method over a frequency range of 200 Hz–20 MHz in air. The measurements were carried out from 723 to 1173 K at a temperature interval of 50 K. The heating rate was 5 K min−1 , while the stabilisation time between consecutive measurements was 15 min. The specimen temperature was monitored by a K-type thermocouple positioned adjacent to the specimen.
Relative density
Mole ratio
(%)
Yb
Sm
Zr
97.6 96.5 95.3 96.6 96.9 96.3 96.0
0 5.0 14.8 24.9 35.2 44.7 49.7
49.6 44.8 35.2 24.8 14.7 4.9 0
50.4 50.2 50.0 50.3 50.1 50.4 50.3
tent. Sm2 Zr2 O7 and (Sm0.9 Yb0.1 )2 Zr2 O7 have a pyrochlore-type structure, which is characterized by the presence of typical super-lattice peaks at 2 values of about 14◦ (1 1 1), 28◦ (3 1 1), 37◦ (3 3 1), 45◦ (5 1 1) and 51◦ (5 3 1) using Cu K␣ radiation [15,16]. However, (Sm1−x Ybx )2 Zr2 O7 (0.3 ≤ x ≤ 1.0) exhibit a defect fluorite-type structure. Sm2 Zr2 O7 and Yb2 Zr2 O7 ceramics are completely soluble. In the Ln2 Zr2 O7 system, the crystal structure is mainly governed by the ratio of r(Ln3+ )/r(Zr4+ ). The stability of pyrochlore-type structure in zirconates is limited to the range of 1.46 ≤ r(Ln3+ )/r(Zr4+ ) ≤ 1.78 at an atmospheric pressure [1,2]. Below 1.46, the array of unoccupied anion sites disorders to produce a defect fluorite-type structure. Above 1.78, there is a transition to monoclinic phase with a La2 Ti2 O7 -type structure. The ionic radius of Sm3+ and Yb3+ are 1.079 Å and 0.985 Å in the eight-fold coordination, respectively; however, the ionic radius of Zr4+ is 0.72 Å in the six-fold coordination [17]. The values of r(Ln3+ )/r(Zr4+ ) are equal to 1.50 and 1.48 for Sm2 Zr2 O7 and (Sm0.9 Yb0.1 )2 Zr2 O7 , respectively. Therefore, Sm2 Zr2 O7 and (Sm0.9 Yb0.1 )2 Zr2 O7 exhibit a pyrochlore-type structure. As for (Sm1−x Ybx )2 Zr2 O7 (0.3 ≤ x ≤ 1.0), the values of r(Ln3+ )/r(Zr4+ ) are clearly lower than 1.46. Therefore, (Sm1−x Ybx )2 Zr2 O7 (0.3 ≤ x ≤ 1.0) ceramics exhibit a defect fluorite-type structure. The value of r(Ln3+ )/r(Zr4+ ) gradually decreases with increasing Yb content, which also represents the increase in degree of structure disordering. The relative densities of (Sm1−x Ybx )2 Zr2 O7 ceramics are in the range of 95.3–97.6%, as shown in Table 1. The microstructures of (Sm1−x Ybx )2 Zr2 O7 ceramics with various chemical compositions are very similar. Fig. 2 shows typical microstructure of (Sm0.3 Yb0.7 )2 Zr2 O7 ceramic. The average grain size of (Sm0.3 Yb0.7 )2 Zr2 O7 is several micrometers, and the grain boundaries in (Sm0.3 Yb0.7 )2 Zr2 O7 are very clean. The chemical compositions of each specimen were determined by EDS. According to the EDS results in Table 1, the mole ratios of different metallic
3. Results and discussion The X-ray diffraction patterns of (Sm1−x Ybx )2 Zr2 O7 (x = 0, 0.1, 0.3, 0.9, 1.0) ceramics sintered at 1973 K for 10 h in air are shown in Fig. 1. It can be observed that all the compositions studied are single phase, and the positions of all reflections shift gradually to the high angle side with increasing Yb con-
Fig. 2. Microstructure of (Sm0.3 Yb0.7 )2 Zr2 O7 sintered at 1973 K for 10 h in air.
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Fig. 3. Typical complex impedance and schematic equivalent electrical circuits plots for (Sm1−x Ybx )2 Zr2 O7 ceramics at 723 K: (a) x = 0, 0.1; (b) x = 0.3, 0.5,0.7, 0.9, 1.0; (c) magnification of (b); Rg , Rgb , CPEg and CPEgb represent grain resistance, grain boundary resistance, constant phase element of the grain and constant phase element of the grain boundary. The grain (G) and grain-boundary (GB) contributions are also indicated.
elements in (Sm1−x Ybx )2 Zr2 O7 ceramics have the nominal stoichiometry to within ± 2%. It is convenient to distinguish between the grain and grain boundary effects using the complex impedance plane plot (Z vs. Z ). Fig. 3 shows the typical impedance plane plots of (Sm1−x Ybx )2 Zr2 O7 ceramics measured at 723 K in air. Fig. 3(a) is impedance plane plots of Sm2 Zr2 O7 and (Sm0.9 Yb0.1 )2 Zr2 O7 . Two distinct contributions manifested in the form of semicircular arcs are clearly identified. The high frequency semicircles correspond to the grain impedance, and the low frequency semicircles represent the grain-boundary impedance. From Fig. 3 (a), capacitance values found for the high- and low-frequency arcs are 8.56 × 10−11 and 6.72 × 10−8 F/cm for Sm2 Zr2 O7 , which corresponds to the grain and grain boundary contributions, respectively. However, for (Sm0.9 Yb0.1 )2 Zr2 O7 ceramic, the values of capacitance for grain and grain boundary are 9.15 × 10−11 and 6.69 × 10−8 F/cm, respectively. Fig. 3 (b) and (c) are impedance plane plots of (Sm1−x Ybx )2 Zr2 O7 (0.3 ≤ x ≤ 1.0) ceramics. Only one distinct contribution manifested in the form of semicircular arc is identified. From Fig. 3 (b) and (c), the capacitance values of different semicircular arcs are 1.57 × 10−10 F/cm for (Sm0.7 Yb0.3 )2 Zr2 O7 , 1.22 × 10−10 F/cm for (Sm0.5 Yb0.5 )2 Zr2 O7 , 8.79 × 10−11 F/cm for (Sm0.3 Yb0.7 )2 Zr2 O7 , 6.41 × 10−11 F/cm for (Sm0.1 Yb0.9 )2 Zr2 O7 and 4.45 × 10−11 F/cm for Yb2 Zr2 O7 , respectively. These semicircular arcs in Fig. 3 (b) and (c) correspond to the grain contributions. Typical equivalent electrical circuits applied to reproduce such impedance plots consists of parallel resistance–capacitance (R–C) blocks [18,19], as inset in Fig. 3 (a) and (b). These contributions are separated by fitting semicircles to each of the arcs, and the grain resistance values, Rg , are determined from the intercepts of high frequency-range semicircles on the Z -axes [20], respectively. The electrical conductivity at different temperatures is calculated from the values of resistance and the dimensions of the specimens.
The temperature dependence of grain conductivity is analyzed using an Arrhenius equation with the following expression: g T = 0 exp
−E kB T
(1)
where pre-exponential factor 0 is a measurement of the effective number of mobile oxide-ions, E is the activation energy for the conduction process, kB is the Boltzmann constant, and T is absolute temperature. Fig. 4 shows the Arrhenius plots of the grain conductivity for each composition studied in this work, where the lines are fitted to an Arrhenius equation. This confirms that the ionic diffusion process is thermally activated. The values of activation energy (Eg ) and pre-exponential factor ( 0g ) for each composition are calculated from the slope and the intercept of the linear fits in the Arrhenius plots (Fig. 4), respectively. The calculated values of acti-
Fig. 4. Arrhenius plots of the grain (G) conductivity of (Sm1−x Ybx )2 Zr2 O7 ceramics.
Z.-G. Liu et al. / Electrochimica Acta 54 (2009) 3968–3971 Table 2 Activation energy Eg and pre-exponential factor 0g for the grain (G) contributions to conductivity. Ceramic materials
Sm2 Zr2 O7 (Sm0.9 Yb0.1 )2 Zr2 O7 (Sm0.7 Yb0.3 )2 Zr2 O7 (Sm0. 5 Yb0.5 )2 Zr2 O7 (Sm0.3 Yb0.7 )2 Zr2 O7 (Sm0.1 Yb0.9 )2 Zr2 O7 Yb2 Zr2 O7
Grain (G) contributions Activation energy Eg (eV)
Pre-exponential factor 0g (S cm−1 K)
0.69 0.85 1.16 1.32 1.42 1.53 1.56
9.98 × 103 8.94 × 104 5.63 × 105 2.04 × 106 5.65 × 106 1.40 × 107 7.08 × 106
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ity of (Sm1−x Ybx )2 Zr2 O7 (0.1 ≤ x ≤ 0.9) decreases with increasing Yb content. This indicates that the increase in g is not able to compensate for the increase in Eg , and finally causes the drop in electrical conductivity. The g for Yb2 Zr2 O7 is clearly lower than that of (Sm0.1 Yb0.9 )2 Zr2 O7 , while the Eg for Yb2 Zr2 O7 is slightly higher than that of (Sm0.1 Yb0.9 )2 Zr2 O7 . Therefore, the electrical conductivity of Yb2 Zr2 O7 should be lower than that of (Sm0.1 Yb0.9 )2 Zr2 O7 , which is also consistent with the measurement results in this work. 4. Conclusions Sm2 Zr2 O7 and (Sm0.9 Yb0.1 )2 Zr2 O7 have a pyrochlore-type structure, while (Sm1−x Ybx )2 Zr2 O7 (0.3 ≤ x ≤ 1.0) exhibit a defect fluorite-type structure. The degree of structural disordering for (Sm1−x Ybx )2 Zr2 O7 ceramics increases with increasing Yb content. The grain conductivity of (Sm1−x Ybx )2 Zr2 O7 ceramics gradually increases with increasing temperature. The electrical conductivities of defect fluorite-type materials are lower than those of pyrochlore-type materials, whereas activation energies for the conduction process increase monotonically as the structure becomes disordered. The grain conductivity reaches the highest value of 1.71 × 10−2 S cm−1 at 1173 K for (Sm0.9 Yb0.1 )2 Zr2 O7 ceramics in this work. Acknowledgements The authors would like to thank the financial support from the Program of Excellent Teams in Harbin Institute of Technology (HIT) and the Start-up Program for High-level HIT Faculty Returned from Abroad.
Fig. 5. Variations of the grain (G) conductivity of (Sm1−x Ybx )2 Zr2 O7 ceramics as a function of Yb content and temperature.
References
vation energy and pre-exponential factor are presented in Table 2. It can be seen that the activation energy Eg gradually increases with increasing Yb content. The activation energy Eg of Sm2 Zr2 O7 in this study is 0.69 eV, which is slightly higher than the results obtained by Shinozaki et al.’s [3]. From Table 2, the pre-exponential factor 0g of (Sm1−x Ybx )2 Zr2 O7 ceramics except for Yb2 Zr2 O7 gradually increases with increasing Yb content. Fig. 5 shows the variations of grain conductivity as a function of Yb content and temperature for all compositions in this work. Clearly, the grain conductivity gradually increases with increasing temperature for each composition. A decrease of about one order of magnitude in grain conductivity is found at all temperature levels when the Yb content increases from x = 0.1 to x = 0.3. With further increasing the Yb content from x = 0.3 to x = 1.0, the grain conductivity g slightly decreases, and reaches a minimum value at x = 1.0 for all temperature levels. It means that electrical conductivities of fluorite-type materials are lower than those of pyrochlore-type materials in (Sm1−x Ybx )2 Zr2 O7 ceramics. The electrical conductivity value of Sm2 Zr2 O7 is close to that of (Sm0.9 Yb0.1 )2 Zr2 O7 in this work. The highest electrical conductivity value obtained in this work is 1.71 × 10−2 S cm−1 at 1173 K for (Sm0.9 Yb0.1 )2 Zr2 O7 . The increase in g would lead to an increase in electrical conductivity; however, the increase in Eg would hinder the oxide-ion migration. Thus, these two processes are competing. As the Yb content increases from 0.1 to 0.9, both g and Eg increase as shown in Table 2. From Fig. 5, the electrical conductiv-
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