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Synthesis and electrical properties of Fe-doped Y0.08Sr0.92TiO3 mixed ionic–electronic conductor
Materials Letters 105 (2013) 196–198
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Synthesis and electrical properties of Fe-doped Y0.08Sr0.92TiO3 mixed ionic–electronic conductor Ke Shan, Xing-Min Guo n State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, China
art ic l e i nf o
a b s t r a c t
Article history: Received 12 November 2012 Accepted 30 March 2013 Available online 9 April 2013
A single phase perovskite Y0.08Sr0.92Ti1−xFexO3−δ (x¼0.20, 0.40 and 0.50) was fabricated at 1350 1C in air by the sol–gel method and its mixed ionic–electronic conductivity was investigated as a function of Fe content. Fe was selected as a B-site dopant with the primary aim to improve the ionic conductivity of SrTiO3-based materials. With increasing Fe-doping amount, the electrical and ionic conductivities of Y0.08Sr0.92Ti1−xFexO3−δ increased in 400–900 1C and 600–950 1C, respectively. The possible charge com3+ pensation mechanism of Y0.08Sr0.92Ti1−xFexO3−δ can be described as Y0.08Sr0.92FexTi4+ 0.92-xTi0.08O3-(δ+x/2) or 4+ 3+ Y0.08Sr0.92FexTi0.92−x1Ti0.08−x2 O3−(δ+x1/2) (x¼x1+x2). & 2013 Elsevier B.V. All rights reserved.
Keywords: Fe-doped Y0.08Sr0.92TiO3 Mixed ionic–electronic conductor Sol–gel SrTiO3-based ceramics Charge compensation mechanism
1. Introduction
2. Material and methods
Mixed ionic–electronic conductors play an important role due to the wide range of applications in modern solid state ionic devices. Typically doped strontium titanates (SrTiO3) are one of the most promising mixed ionic–electronic conductors for solid oxide electrolysis cells, solid oxide fuel cells and separation membranes due to its excellent chemical stability, thermal stability, tolerance to coking and sulfur poisoning [1,2]. However, many efforts have been focused on improving electrical conductivity of SrTiO3-based material. Doping with trivalent atoms, such as Y3+ and La3+, in place of the divalent Sr2+ allows SrTiO3 to become an n-type semiconductor by enhancing the electronic conductivity [3–6], while doping with Fe, Cr, Co, etc., in place of the B site, results in a p-type semiconductor by increasing the ionic conductivity [7–13]. Fe-doping on Y0.08Sr0.92TiO3 may change the defect formation and charge compensation mechanisms, thus altering the electrical conduction behavior. In this study, A- and B-sites co-doped in SrTiO3, Y0.08Sr0.92Ti1−xFexO3−δ (x ¼0.20, 0.40 and 0.50), was synthesized by the sol–gel method. The effects of Fe-doping content on electrical conductivity, including electronic and ionic conductivities, and the conduction behavior of Y0.08Sr0.92Ti1−xFexO3−δ with temperature were investigated with the aid of a blocking electrode. A possible charge compensation mechanism in Y0.06Sr0.94FexTi1−xO3−δ was proposed.
Y0.08Sr0.92Ti1−xFexO3−δ (x¼0.20, 0.40 and 0.50) powders were synthesized by the sol–gel method. Sr(CH3COO)2 2H2O and Ti (CH3CH2CH2CH2O)4, Fe2O3 and Y2O3 were used as starting materials. Ti(CH3CH2CH2CH2O)4 was completely dissolved in isopropanol and ethanol (volume ratio¼4:1) and Fe2O3 and Y2O3 were added to the solution; the solution of Sr(CH3COO)2 2H2O dissolved in deionized water was added to Ti(CH3CH2CH2CH2O)4 including Fe2O3 and Y2O3 at room temperature with high stirring rate on a magnetic stirrer. The reaction mixture was stirred for 30 min and was then allowed to settle down for 24 h, which was heated at 50 1C for 24 h. The obtained mixtures were calcined at 1100 1C for 10 h in air. The synthesized powders were uniaxially pressed into disks (diameter 10 mm), followed by sintering at 1350 1C for 5 h in air to achieve dense samples for the measurement of total electrical and ionic conductivities. XRD (X-ray diffraction, Rigaku D/ max-A X-ray diffractometer, using CuKα radiation) and SEM (scanning electron microscope, ZEISS Ultra 55) were employed to examine the phase purity and fracture microstructure of sintered samples. The total electrical conductivity was measured by the ac impedance within 400–900 1C in air and the ionic conductivity was determined by the electron-blocking method within 600–950 1C in air. Details of the electron-blocking method can be found in the literature[7,14].
3. Results and discussion
n
Corresponding author. Tel./fax: +86 10 62334957. E-mail address: [email protected] (X.-M. Guo).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.03.140
Phase development: Single-phase samples were observed by XRD for Y0.08Sr0.92Ti1−xFexO3−δ (x¼0.20, 0.40 and 0.50) after sintering at 1350 1C for 5 h, as shown in Fig. 1. The samples
K. Shan, X.-M. Guo / Materials Letters 105 (2013) 196–198
197
showed a single cubic perovskite structure without detectable impurity. The microstructures as shown in Fig. 2 evidently reveal that the densities of Y0.08Sr0.92Ti1−xFexO3−δ samples increase with increasing Fe-doping level, indicating that Fe-doping is favorable to the densification process of the materials. Effect of Fe-doping on the electronic and ionic conductivities of Y0.08Sr0.92Ti1−xFexO3−δ: Fig. 3 shows that the total electrical conductivity increases steadily with temperature and is further enhanced with increasing Fe-doping amount . The electrical conductivities were 0.11 S/cm for Y0.08Sr0.92Ti0.5Fe0.5O3−δ and 0.017 S/cm for Y0.08Sr0.92Ti0.8Fe0.2O3−δ at 800 1C. The value at 800 1C is approximately 6.2 times higher than that of Y0.08Sr0.92Ti0.8Fe0.2O3−δ. It appears that the higher charge carrier (electron–hole) concentration is responsible for the observed high conductivity in Y0.08Sr0.92Ti0.5Fe0.5O3−δ. Fig. 4 shows that the ionic conductivity increases with temperature and is improved with increasing of Fe-doping amount . The ionic conductivity of Y0.08Sr0.92Ti0.5Fe0.5O3−δ varies from 0.015 S/cm at 700 1C to 0.024 S/cm at 800 1C. The value at 700 1C is approximately 2.2 times higher than Y0.08Sr0.92Ti0.8Fe0.2O3−δ. The values of the most common perovskite-type mixed conductors vary from 10−3 to 10−2 S/cm at 700 1C [2]. The fact that active energy decreases with Fe-doping amount increasing indicates that Fe-dopant facilitates oxygen ion conduction (Fig. 4 (b)). As shown in Fig. 5, the ion transference number decreases with increasing Fe-doping level, which indicates Fe-doping is more favorable for the electronic conductivity. Charge compensation mechanism of Fe-doped Y0.08Sr0.92Ti1−x FexO3−δ: According to the above results, the charge compensation mechanism of Y, Fe co-doped SrTiO3 can be deduced as follows. For Y0.08Sr0.92Ti1−xFexO3−δ, there are three possible charge compensation mechanisms. The first possible doping mechanism can be expressed as 3+ 3+ Y0.08Sr0.92FexTi4+ ions take the place 0.92Ti0.08−x O3−(δ+x/2), i.e. all Fe of Ti3+ ions. The defect produced by Fe acceptor doping in Y0.08Sr0.92TiO3 can be also expressed as Fe2 O3
Yx Sr1x TiO3
!
2FeTi3þ þ 3O O
In this case, the electronic conductivity of Y0.08Sr0.92TiO3 may decrease for the reduced amount of Ti3+ and oxygen vacancy concentration will remain unchanged. This conflicts with the experimental results. The second possibility is that Fe3+ ions in place of Ti4+ ions, which 3+ can be expressed as Y0.08Sr0.92FexTi4+ 0.92−xTi0.08O3−(δ+x/2). So the defect
Fig. 2. SEM micrographs of fracture surfaces of (a) Y0.08Sr0.92Ti0.8Fe0.2O3−δ and (b) Y0.08Sr0.92Ti0.5Fe0.5O3−δ.
Fig. 3. Temperature dependences of the total electrical conductivities of Y0.08Sr0.92Ti1−xFexO3−δ(x ¼ 0.20, 0.40 and 0.50) in 400–900 1C in air.
produced in Fe acceptor doping Y0.08Sr0.92TiO3 can be expressed as Fe2 O3 Fig. 1. XRD patterns of Y0.08Sr0.92Ti1−xFexO3−δ (x ¼0.20, 0.40 and 0.50) after sintering at 1350 1C for 5 h.
Yx Sr1−x TiO3
!
0 2FeTi 4þ þ V O þ 3OO
1 V O þ 2O2 ðgÞ ¼ OO þ2h
198
K. Shan, X.-M. Guo / Materials Letters 105 (2013) 196–198
Fig. 5. Temperature dependences of ion transference numbers of Y0.08Sr0.92 Ti1−xFexO3−δ (x¼ 0.20, 0.40 and 0.50).
and 600–900 1C respectively, which results from increasing hole and oxygen ionic concentrations. The electrical and ionic conductivities were 0.11 S/cm and 0.024 S/cm for Y0.08Sr0.92Ti0.5Fe0.5O3−δ and 0.075 S/cm and 0.016 S/cm for Y0.08Sr0.92Ti0.8Fe0.2O3−δ at 800 1C. Compared with Y0.08Sr0.92Ti0.8Fe0.2O3−δ, the ionic conductivity of Y0.08Sr0.92Ti0.5Fe0.5O3−δ increased about 106% at 800 1C. The possible charge compensation mechanism of Fe-doped 3+ Y0.08Sr0.92TiO3 can be described as Y0.08Sr0.92FexTi4+ 0.92−xTi0.08 4+ 3+ O3−(δ+x/2) or Y0.08Sr0.92FexTi0.92−x1Ti0.08−x2 O3−(δ+x1/2) (x ¼x1+x2).
Acknowledgments
Fig. 4. Temperature dependences of the ionic conductivities of Y0.08Sr0.92Ti1−xFex O3−δ (x¼ 0.20, 0.40 and 0.50) in 600–900 1C in air.
In this case, the ionic and electronic conductivities of Fe-doped Y0.08Sr0.92TiO3 may increase because of the increased oxygen vacancy concentration and hole concentration with increasing Fe-doping amount which is consistent with the experimental results. The last possibility can be described as Y0.08Sr0.92FexTi4+ 0.92−x1 3+ Ti3+ (0.645 Å) ions replace both 0.08−x2 O3−(δ+x1/2) (x ¼x1+x2) and Fe Ti4+ (0.605 Å) and Ti3+ (0.67 Å)sites. In this case, Fe3+ ions tend to replace a larger proportion of Ti4+ ions with Fe-doping amount increasing, which results in the concentration of oxygen vacancy and increasing hole concentration. So, the ionic conductivities and the electronic conductivities increase. This agrees well with the experimental results. 4. Conclusions With increasing Fe-doping amount, the electronic and the ionic conductivities of Y0.08Sr0.92Ti1−xFexO3−δ increased in 400–900 1C
This research is supported by the National Natural Science Foundation of China (No. 50974012) and The Program for Changjiang Scholars and Innovative Research Team in University (No. 0708). References [1] [2] [3] [4] [5]
[6] [7] [8] [9] [10] [11] [12] [13] [14]
Moos R, Hardtl KH. J Appl Phys 1996;80:393–400. Li X, Zhao H, Luo D, Huang K. Mater Lett 2011;65:2624–7. Hui S, Petric A. J Eur Ceram Soc 2002;22:1673–81. Fu QX, Mi SB, Wessel E, Tietz F. J Eur Ceram Soc 2008;28:811–20. Kurokawa H, Yang L, Jacobson CP, Lutgard C De JongheLutgard C has been changed to Lutgard C De JonghePlease check if the name is accurate and correct if necessary, Visco SJ. J Power Sources 2007;164:510–8. Li X, Zhao H, Shen W, Gao F, Huang X, Li Y, et al. J Power Sources 2007;166:47–52. Li X, Zhao H, Xu N, Zhou X, Zhang C, Chen N. Int J Hydrogen Energy 2009;34:6407–14. Li X, Zhao H, Gao F, Chen N, Xu N. Electrochem Commun 2008;10:1567–70. Litzelman SJ, Rothschild A, Tuller HL. Sens Actuators B 2005;108:231–7. Kharton VV, Kovalevsky AV, Viskup AP, Jurado JR, Figueiredo FM, Naumovich EN, et al. J Solid State Chem 2001;156:437–44. Hui SQ, Petirc A. Mater Res Bull 2002;37:1215–31. Yoon JS, Yoon MY, Kwak C, Park HJ, Lee SM, Lee KH, et al. Mater Sci Eng B 2011;177:151–6. Fagg DP, Kharton VV, Frade JR, Ferreira AAL. Solid State Ionics 2003;156:45–57. Li X, Zhao H, Gao F, Zhu Z, Chen N, Shen W. Solid State Ionics 2008;179:1588–92.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Synthesis and electrical properties of Fe-doped Y0.08Sr0.92TiO3 mixed ionic–electronic conductor Ke Shan, Xing-Min Guo n State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, China
art ic l e i nf o
a b s t r a c t
Article history: Received 12 November 2012 Accepted 30 March 2013 Available online 9 April 2013
A single phase perovskite Y0.08Sr0.92Ti1−xFexO3−δ (x¼0.20, 0.40 and 0.50) was fabricated at 1350 1C in air by the sol–gel method and its mixed ionic–electronic conductivity was investigated as a function of Fe content. Fe was selected as a B-site dopant with the primary aim to improve the ionic conductivity of SrTiO3-based materials. With increasing Fe-doping amount, the electrical and ionic conductivities of Y0.08Sr0.92Ti1−xFexO3−δ increased in 400–900 1C and 600–950 1C, respectively. The possible charge com3+ pensation mechanism of Y0.08Sr0.92Ti1−xFexO3−δ can be described as Y0.08Sr0.92FexTi4+ 0.92-xTi0.08O3-(δ+x/2) or 4+ 3+ Y0.08Sr0.92FexTi0.92−x1Ti0.08−x2 O3−(δ+x1/2) (x¼x1+x2). & 2013 Elsevier B.V. All rights reserved.
Keywords: Fe-doped Y0.08Sr0.92TiO3 Mixed ionic–electronic conductor Sol–gel SrTiO3-based ceramics Charge compensation mechanism
1. Introduction
2. Material and methods
Mixed ionic–electronic conductors play an important role due to the wide range of applications in modern solid state ionic devices. Typically doped strontium titanates (SrTiO3) are one of the most promising mixed ionic–electronic conductors for solid oxide electrolysis cells, solid oxide fuel cells and separation membranes due to its excellent chemical stability, thermal stability, tolerance to coking and sulfur poisoning [1,2]. However, many efforts have been focused on improving electrical conductivity of SrTiO3-based material. Doping with trivalent atoms, such as Y3+ and La3+, in place of the divalent Sr2+ allows SrTiO3 to become an n-type semiconductor by enhancing the electronic conductivity [3–6], while doping with Fe, Cr, Co, etc., in place of the B site, results in a p-type semiconductor by increasing the ionic conductivity [7–13]. Fe-doping on Y0.08Sr0.92TiO3 may change the defect formation and charge compensation mechanisms, thus altering the electrical conduction behavior. In this study, A- and B-sites co-doped in SrTiO3, Y0.08Sr0.92Ti1−xFexO3−δ (x ¼0.20, 0.40 and 0.50), was synthesized by the sol–gel method. The effects of Fe-doping content on electrical conductivity, including electronic and ionic conductivities, and the conduction behavior of Y0.08Sr0.92Ti1−xFexO3−δ with temperature were investigated with the aid of a blocking electrode. A possible charge compensation mechanism in Y0.06Sr0.94FexTi1−xO3−δ was proposed.
Y0.08Sr0.92Ti1−xFexO3−δ (x¼0.20, 0.40 and 0.50) powders were synthesized by the sol–gel method. Sr(CH3COO)2 2H2O and Ti (CH3CH2CH2CH2O)4, Fe2O3 and Y2O3 were used as starting materials. Ti(CH3CH2CH2CH2O)4 was completely dissolved in isopropanol and ethanol (volume ratio¼4:1) and Fe2O3 and Y2O3 were added to the solution; the solution of Sr(CH3COO)2 2H2O dissolved in deionized water was added to Ti(CH3CH2CH2CH2O)4 including Fe2O3 and Y2O3 at room temperature with high stirring rate on a magnetic stirrer. The reaction mixture was stirred for 30 min and was then allowed to settle down for 24 h, which was heated at 50 1C for 24 h. The obtained mixtures were calcined at 1100 1C for 10 h in air. The synthesized powders were uniaxially pressed into disks (diameter 10 mm), followed by sintering at 1350 1C for 5 h in air to achieve dense samples for the measurement of total electrical and ionic conductivities. XRD (X-ray diffraction, Rigaku D/ max-A X-ray diffractometer, using CuKα radiation) and SEM (scanning electron microscope, ZEISS Ultra 55) were employed to examine the phase purity and fracture microstructure of sintered samples. The total electrical conductivity was measured by the ac impedance within 400–900 1C in air and the ionic conductivity was determined by the electron-blocking method within 600–950 1C in air. Details of the electron-blocking method can be found in the literature[7,14].
3. Results and discussion
n
Corresponding author. Tel./fax: +86 10 62334957. E-mail address: [email protected] (X.-M. Guo).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.03.140
Phase development: Single-phase samples were observed by XRD for Y0.08Sr0.92Ti1−xFexO3−δ (x¼0.20, 0.40 and 0.50) after sintering at 1350 1C for 5 h, as shown in Fig. 1. The samples
K. Shan, X.-M. Guo / Materials Letters 105 (2013) 196–198
197
showed a single cubic perovskite structure without detectable impurity. The microstructures as shown in Fig. 2 evidently reveal that the densities of Y0.08Sr0.92Ti1−xFexO3−δ samples increase with increasing Fe-doping level, indicating that Fe-doping is favorable to the densification process of the materials. Effect of Fe-doping on the electronic and ionic conductivities of Y0.08Sr0.92Ti1−xFexO3−δ: Fig. 3 shows that the total electrical conductivity increases steadily with temperature and is further enhanced with increasing Fe-doping amount . The electrical conductivities were 0.11 S/cm for Y0.08Sr0.92Ti0.5Fe0.5O3−δ and 0.017 S/cm for Y0.08Sr0.92Ti0.8Fe0.2O3−δ at 800 1C. The value at 800 1C is approximately 6.2 times higher than that of Y0.08Sr0.92Ti0.8Fe0.2O3−δ. It appears that the higher charge carrier (electron–hole) concentration is responsible for the observed high conductivity in Y0.08Sr0.92Ti0.5Fe0.5O3−δ. Fig. 4 shows that the ionic conductivity increases with temperature and is improved with increasing of Fe-doping amount . The ionic conductivity of Y0.08Sr0.92Ti0.5Fe0.5O3−δ varies from 0.015 S/cm at 700 1C to 0.024 S/cm at 800 1C. The value at 700 1C is approximately 2.2 times higher than Y0.08Sr0.92Ti0.8Fe0.2O3−δ. The values of the most common perovskite-type mixed conductors vary from 10−3 to 10−2 S/cm at 700 1C [2]. The fact that active energy decreases with Fe-doping amount increasing indicates that Fe-dopant facilitates oxygen ion conduction (Fig. 4 (b)). As shown in Fig. 5, the ion transference number decreases with increasing Fe-doping level, which indicates Fe-doping is more favorable for the electronic conductivity. Charge compensation mechanism of Fe-doped Y0.08Sr0.92Ti1−x FexO3−δ: According to the above results, the charge compensation mechanism of Y, Fe co-doped SrTiO3 can be deduced as follows. For Y0.08Sr0.92Ti1−xFexO3−δ, there are three possible charge compensation mechanisms. The first possible doping mechanism can be expressed as 3+ 3+ Y0.08Sr0.92FexTi4+ ions take the place 0.92Ti0.08−x O3−(δ+x/2), i.e. all Fe of Ti3+ ions. The defect produced by Fe acceptor doping in Y0.08Sr0.92TiO3 can be also expressed as Fe2 O3
Yx Sr1x TiO3
!
2FeTi3þ þ 3O O
In this case, the electronic conductivity of Y0.08Sr0.92TiO3 may decrease for the reduced amount of Ti3+ and oxygen vacancy concentration will remain unchanged. This conflicts with the experimental results. The second possibility is that Fe3+ ions in place of Ti4+ ions, which 3+ can be expressed as Y0.08Sr0.92FexTi4+ 0.92−xTi0.08O3−(δ+x/2). So the defect
Fig. 2. SEM micrographs of fracture surfaces of (a) Y0.08Sr0.92Ti0.8Fe0.2O3−δ and (b) Y0.08Sr0.92Ti0.5Fe0.5O3−δ.
Fig. 3. Temperature dependences of the total electrical conductivities of Y0.08Sr0.92Ti1−xFexO3−δ(x ¼ 0.20, 0.40 and 0.50) in 400–900 1C in air.
produced in Fe acceptor doping Y0.08Sr0.92TiO3 can be expressed as Fe2 O3 Fig. 1. XRD patterns of Y0.08Sr0.92Ti1−xFexO3−δ (x ¼0.20, 0.40 and 0.50) after sintering at 1350 1C for 5 h.
Yx Sr1−x TiO3
!
0 2FeTi 4þ þ V O þ 3OO
1 V O þ 2O2 ðgÞ ¼ OO þ2h
198
K. Shan, X.-M. Guo / Materials Letters 105 (2013) 196–198
Fig. 5. Temperature dependences of ion transference numbers of Y0.08Sr0.92 Ti1−xFexO3−δ (x¼ 0.20, 0.40 and 0.50).
and 600–900 1C respectively, which results from increasing hole and oxygen ionic concentrations. The electrical and ionic conductivities were 0.11 S/cm and 0.024 S/cm for Y0.08Sr0.92Ti0.5Fe0.5O3−δ and 0.075 S/cm and 0.016 S/cm for Y0.08Sr0.92Ti0.8Fe0.2O3−δ at 800 1C. Compared with Y0.08Sr0.92Ti0.8Fe0.2O3−δ, the ionic conductivity of Y0.08Sr0.92Ti0.5Fe0.5O3−δ increased about 106% at 800 1C. The possible charge compensation mechanism of Fe-doped 3+ Y0.08Sr0.92TiO3 can be described as Y0.08Sr0.92FexTi4+ 0.92−xTi0.08 4+ 3+ O3−(δ+x/2) or Y0.08Sr0.92FexTi0.92−x1Ti0.08−x2 O3−(δ+x1/2) (x ¼x1+x2).
Acknowledgments
Fig. 4. Temperature dependences of the ionic conductivities of Y0.08Sr0.92Ti1−xFex O3−δ (x¼ 0.20, 0.40 and 0.50) in 600–900 1C in air.
In this case, the ionic and electronic conductivities of Fe-doped Y0.08Sr0.92TiO3 may increase because of the increased oxygen vacancy concentration and hole concentration with increasing Fe-doping amount which is consistent with the experimental results. The last possibility can be described as Y0.08Sr0.92FexTi4+ 0.92−x1 3+ Ti3+ (0.645 Å) ions replace both 0.08−x2 O3−(δ+x1/2) (x ¼x1+x2) and Fe Ti4+ (0.605 Å) and Ti3+ (0.67 Å)sites. In this case, Fe3+ ions tend to replace a larger proportion of Ti4+ ions with Fe-doping amount increasing, which results in the concentration of oxygen vacancy and increasing hole concentration. So, the ionic conductivities and the electronic conductivities increase. This agrees well with the experimental results. 4. Conclusions With increasing Fe-doping amount, the electronic and the ionic conductivities of Y0.08Sr0.92Ti1−xFexO3−δ increased in 400–900 1C
This research is supported by the National Natural Science Foundation of China (No. 50974012) and The Program for Changjiang Scholars and Innovative Research Team in University (No. 0708). References [1] [2] [3] [4] [5]
[6] [7] [8] [9] [10] [11] [12] [13] [14]
Moos R, Hardtl KH. J Appl Phys 1996;80:393–400. Li X, Zhao H, Luo D, Huang K. Mater Lett 2011;65:2624–7. Hui S, Petric A. J Eur Ceram Soc 2002;22:1673–81. Fu QX, Mi SB, Wessel E, Tietz F. J Eur Ceram Soc 2008;28:811–20. Kurokawa H, Yang L, Jacobson CP, Lutgard C De JongheLutgard C has been changed to Lutgard C De JonghePlease check if the name is accurate and correct if necessary, Visco SJ. J Power Sources 2007;164:510–8. Li X, Zhao H, Shen W, Gao F, Huang X, Li Y, et al. J Power Sources 2007;166:47–52. Li X, Zhao H, Xu N, Zhou X, Zhang C, Chen N. Int J Hydrogen Energy 2009;34:6407–14. Li X, Zhao H, Gao F, Chen N, Xu N. Electrochem Commun 2008;10:1567–70. Litzelman SJ, Rothschild A, Tuller HL. Sens Actuators B 2005;108:231–7. Kharton VV, Kovalevsky AV, Viskup AP, Jurado JR, Figueiredo FM, Naumovich EN, et al. J Solid State Chem 2001;156:437–44. Hui SQ, Petirc A. Mater Res Bull 2002;37:1215–31. Yoon JS, Yoon MY, Kwak C, Park HJ, Lee SM, Lee KH, et al. Mater Sci Eng B 2011;177:151–6. Fagg DP, Kharton VV, Frade JR, Ferreira AAL. Solid State Ionics 2003;156:45–57. Li X, Zhao H, Gao F, Zhu Z, Chen N, Shen W. Solid State Ionics 2008;179:1588–92.