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Synthesis and electrical properties of mixed-conducting YxSr1−xTi0.6Fe0.4O3−δ
Materials Letters 121 (2014) 251–253
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Synthesis and electrical properties of mixed-conducting YxSr1 xTi0.6Fe0.4O3 δ Ke Shan a,b, Xing-Min Guo a,b,n a b
State Key Laboratory of Advanced Metallurgy, China School of Metallurgical and Ecological Engineering,, 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 1 November 2013 Accepted 25 January 2014 Available online 2 February 2014
A single phase perovskite, YxSr1 xTi0.6Fe0.4O3 δ (x ¼0.07, 0.08, 0.09), was synthesized at 1350 1C by the sol–gel method. The effects of Y-doping on the electronic and ionic conductivities of YxSr1 x Ti0.6Fe0.4O3 δ were investigated. The ionic conductivity of SrTiO3-based materials can be significantly improved by Y-doping on the A-site and Fe-doping on the B-site. We report in this paper a remarkable enhancement of ionic conductivity of Y, Fe co-doped SrTiO3 by the increase in Y-doping amount on the Asite. The total electrical conductivity and ionic conductivity were 0.135 S/cm and 0.017 S/cm for Y0.07Sr0.93Ti0.6Fe0.4O3 δ at 800 1C, respectively, while were 0.056 S/cm and 0.02 S/cm for Y0.09Sr0.91Ti0.6Fe0.4O3 δ. & 2014 Elsevier B.V. All rights reserved.
Keywords: YxSr1 xTi0.6Fe0.4O3 δ Electrical properties Ionic conductivity Sol–gel preparation Solid oxide fuel cell
1. Introduction
2. Experimental
Perovskite-type mixed electronic–ionic conductors have been received much attention due to the wide range of application in modern solid state ionic devices. Typically doped strontium titanate (SrTiO3) is one of the most promising mixed conductors for solid oxide electrolysis cells and separation membranes due to its excellent chemical stability, thermal stability, tolerance to coking and sulfur poisoning [1,2]. However, the electrical conductivity of un-doped SrTiO3 is too low to meet the requirements for materials. Therefore, many efforts have been made to enhance the electronic and ionic conductivities of SrTiO3 material. Donor doping on the Sr site of SrTiO3 could improve the electrical conductivity, while acceptor doping on the Ti site of SrTiO3 could improve the electronic or ionic conductivity [3–8]. In our previous work, it has been shown that (Y, Fe) co-doped SrTiO3, such as Y0.08Sr0.92Ti1 xFexO3 δ and (Y0.08Sr0.92)1 xTi0.6 Fe0.4O3 δ, has high electronic and ionic conductivities [9,10]. In the present work, YxSr1 xTi0.6Fe0.4O3 δ (x¼0.07, 0.08, 0.09) was synthesized by the sol–gel method to investigate the effects of yttrium and iron on electrical properties of SrTiO3, including total electrical and ionic conductivities.
YxSr1 xTi0.6Fe0.4O3 δ (x ¼0.07, 0.08, 0.09) powders were synthesized by the sol–gel method. Sr(CH3COO)2 2H2O and Ti(CH3 CH2CH2CH2O)4, Fe2O3 and Y2O3 were used as starting materials. Details of the experiments can be found in the literature [9]. The synthesized powders were uniaxially pressed into a pellet (diameter 8 mm and thickness 1.20 mm). The green pellets were densified at 1350 1C for 5 h in air to achieve dense samples for the measurement of total electrical and ionic conductivities. The phase composition of samples was identified using an X-ray diffractometer (XRD) with CuKα radiation (Rigaku D/ max-A, Tokyo, Japan). The total electrical conductivity (s) was measured by AC impedance in range of 400–900 1C in air and the ionic conductivity (sion) was determined by the electron-blocking method within 600–950 1C in frequency range 0.01–100 kHz in air. YSTF (YxSr1 xTi0.6Fe0.4O3 δ) pellets were plastered onto dense YSZ pellets with a little Pt paste which was used to overcome the interface resistance. Then Pt paste was painted to the outside of both the connected YSTF and YSZ pellets as electrodes (Fig. 1). In the device, the electron flux is blocked by YSZ layer because YSZ is considered to be almost a pure oxygen ion conductor. Glass seal (SiO2–B2O3–BaO–Al2O3) was used to prevent oxygen leakage along the sides of the assembled samples [3] because the testing sample and the YSZ layer connect in series, the total impedance (Rtotal ) should be the sum of the oxygen ionic impedance of YSTF sample (Rsample) and YSZ layer ( RYSZ). As Eqs. ((1) and (2)) show, the ionic conductivity of sample (sion) can be worked out by its oxygen ionic impedance. The total electrical and ionic conductivities data
n
Corresponding author. Tel./fax: þ 86 10 62334957. E-mail addresses: [email protected] (K. Shan), [email protected] (X.-M. Guo). 0167-577X/$ - see front matter & 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2014.01.152
252
K. Shan, X.-M. Guo / Materials Letters 121 (2014) 251–253
were taken in an interval of 100 1C and 50 1C respectively. Rsample ¼ Rtotal RYSZ
ð1Þ
sion ¼ ðh=SÞ Rsample
ð2Þ
S and h stands for the surface area and thickness of YSTF, respectively.
3. Results and discussion Fig. 2 shows the XRD patterns of the powders of YxSr1 x Ti0.6Fe0.4O3 δ (x¼ 0.07, 0.08, 0.09, 0.10 and 0.12) obtained by the sol–gel method after sintering at 1350 1C for 5 h. As Fig. 2(a) shows, all of the peaks are assigned to a single cubic perovskite structure and no impurity peaks are detected, which means that Y, Fe co-doping does not negatively affect the formation of a solid solution, although the ionic radius of Y3 þ (0.90 Å) is much smaller than that of Sr2 þ (1.44 Å). However, for the one with x ¼0.10 and 0.12 (Fig. 2(b)), an obvious second phase Y2O3 is observed. Thus it is reasonable to state that the doping limit of Y in SrTi0.6Fe0.4O3-δ at 1350 1C is less than 10 mol%. Fig. 3 presents the temperature dependencies of the total electrical conductivity of YxSr1 xTi0.6Fe0.4O3 δ (x ¼0.07, 0.08, 0.09) perovskites. As is evident in Fig. 3, the total electrical conductivity of YxSr1 xTi0.6Fe0.4O3 δ increases as temperature increases, indicating that the p-type conductivity coming from the substitution of Fe3 þ for Ti4 þ contributes more effectively to the conductivity [3,11–13]. On the other hand, the total electrical conductivity decreases with the increase in Y-doping amount,
although their electrical conductivities are still much higher than that of SrTiO3. The electrical conductivity was 0.135 S/cm for Y0.07Sr0.93 Ti0.6Fe0.4O3 δ and 0.056 S/cm for Y0.09Sr0.91Ti0.6 Fe0.4O3 δ at 800 1C, respectively. The value at 800 1C is approximately 2.4 times higher than that of Y0.09Sr0.91Ti0.6Fe0.4O3 δ. The total electrical conductivity of the YxSr1 xTi0.6Fe0.4O3 δ perovskite increased with the temperature increasing in the low temperature range and thereafter became saturated. This conduction behavior should result from the predominant ionic compensation. The electron hole concentration in such materials decreases with increasing temperature due to progressive oxygen losses from the crystal lattice. On the other hand, a possible explanation for the decreasing electrical conductivity with the increase in Y-doping amount may refer to a larger disorder and lead to a significant decrease of the electron–hole transport [14–16]. Fig. 4(a) shows complex-plane impedance plots of the total ionic conduction for YSZ and Y0.09Sr0.91Ti0.6Fe0.4O3 δ. As Fig. 4(a) shows, with increase in temperature, the resistances of grain and grain boundary decrease, which is observed by the shrunken intercept and semicircle, respectively. Fig. 4(b) shows that the partial ionic conductivities of perovskite-type YxSr1 xTi0.6Fe0.4O3 δ (x ¼0.07, 0.08, 0.09) increase steadily with the elevated temperature and the increase in Y-doping amount, respectively. As shown in Fig. 4 (b), the ionic conductivity of Y0.09Sr0.91Ti0.6Fe0.4O3 δ sample varies from 0.0052 S/cm at 600 1C to 0.02 S/cm at 800 1C. The activation energy for ionic conduction of YxSr1 xTi0.6Fe0.4O3 δ is 0.39, 0.33 and 0.32 eV for 0.07, 0.08 and 0.09, respectively. The activation energy decreases with the increase in Y-doping amoun, which
Pt Pt mesh O
2-
glass sample Pt mesh
YSZ
Pt
Fig. 1. Schematic diagram of ionic conductivity test configuration by using YSZ as a electron blocking electrode.
Fig. 3. Temperature dependences of the total electrical conductivities of YxSr1 xTi0.6Fe0.4O3 δ measured in 400–900 1C.
Fig. 2. XRD patterns of YxSr1 xTi0.6Fe0.4O3 δ after sintering at 1350 1C for 5 h: (a) x ¼0.07, 0.08, 0.09 and (b) x ¼0.10, 0.12.
K. Shan, X.-M. Guo / Materials Letters 121 (2014) 251–253
253
Fig. 4. (a) Complex-plane impedance plots of the total ionic conduction for YSZ and Y0.09Sr0.91Ti0.6Fe0.4O3 δ (the inset is equivalent circuit) and (b) Temperature dependences of the ionic conductivities of YxSr1 xTi0.6Fe0.4O3 δ.
indicates that Y-doping is favorable for the ionic conduction in YxSr1 xTi0.6Fe0.4O3 δ. On the other hand, the ionic conductivity is primarily dependent on the oxygen vacancy concentration and the migration energy of oxygen ion at a certain temperature [17,18]. Although the energetic barrier for ion jumps is a very important factor, the key factor of governing ionic transport in such perovskites relates to oxygen vacancy concentration and ordering. Namely, Fecontaining perovskites tend to form ordered domains with brownmillerite-like structure, ordered vacancies and low ionic conductivity. Doping with Y should increase the disorder, whilst doping with Fe should simply increase the concentration of oxygen vacancies. Therefore, Y-doping facilitates the ionic conduction of the materials.
4. Conclusions The electrical conductivity of SrTiO3 without doping was very low, while it was remarkably enhanced by Y, Fe co-doping. The total electrical conductivity decreases steadily with the increase in Y-doping amount due to the decrease of the electron–hole transport. Oxygen ionic conductivity was found to increase with the increase in yttrium addition because yttrium doping increases the oxygen vacancy disorder and then results in the improvement of ionic conduction. The total electrical and ionic conductivities were 0.135 S/cm and 0.017 S/ cm for Y0.07Sr0.93Ti0.6Fe0.4O3 δ and 0.056 S/cm and 0.02 S/cm for Y0.09Sr0.91Ti0.6Fe0.4O3 δ at 800 1C, respectively.
Acknowledgments The financial support provided by the National Natural Science Foundation of China (Nos. 50974012 and 51374017) and Program Changjiang Scholars and Innovative Research Team in University (No. 0708) is gratefully acknowledged. References [1] Moos R, Hardtl KH. J Appl Phys 1996;80:393–400. [2] Li X, Zhao HL, Luo DW, Huang K. Mater Lett 2011;65:2624–7. [3] Li X, Zhao HL, Xu NS, Zhou X, Zhang CJ, et al. Int J Hydrog Energy 2009;34:6407–14. [4] Li X, Zhao HL, Gao F, Chen N, Xu NS. Electrochem Commun 2008;10:1567–70. [5] Litzelman SJ, Rothschild A, Tuller HL. Sens Actuators B 2005;108:231–7. [6] Kharton VV, Kovalevsky AV, Viskup AP, Jurado JR, Figueiredo FM, et al. J Solid State Chem 2001;156:437–44. [7] Hui SQ, Petric A. Mater Res Bull 2002;37:1215–31. [8] Yoon JS, Yoon MY, Kwak C, Park HJ, Lee SM, Lee KH, et al. Mater Sci Eng B 2011;177:151–6. [9] Shan K, Guo XM. Mater Lett 2013;105:196–8. [10] Shan K, Guo XM. Mater Lett 2013;113:126–9. [11] Steinsvik S, Bugge R, Gjonnes J, Tafto J, Norby T. J Phys Chem Solids 1997;58:969–76. [12] Ferreira AAL, Abrantes JCC, Jurado JR, Frade JR. Solid State Ionics 2000;135:761–4. [13] Stevenson JW, Armstrong TR, Carneim RD, Pederson LR, Weber WJJ. Electrochem Soc 1996;143:2722–9. [14] Rothschild A, Litzelman SJ, Tuller HL, Menesklou W, Schneider T, I-Tiffee E. Sens Actuators B 2005;108:223–30. [15] Blaise GJ. Electrostatics 2001;50:69–89. [16] Fagg DP, Kharton VV, Frade JR, Ferreira AAL. Solid State Ionics 2003;156:45–57. [17] Leng YL, Chan SH, Khor KA, Jiang SP, Int J. Hydrog Energy 2004;29:1025–33. [18] Cook RL, Sammells AF. Solid State Ionics 1991;45:311–21.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Synthesis and electrical properties of mixed-conducting YxSr1 xTi0.6Fe0.4O3 δ Ke Shan a,b, Xing-Min Guo a,b,n a b
State Key Laboratory of Advanced Metallurgy, China School of Metallurgical and Ecological Engineering,, 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 1 November 2013 Accepted 25 January 2014 Available online 2 February 2014
A single phase perovskite, YxSr1 xTi0.6Fe0.4O3 δ (x ¼0.07, 0.08, 0.09), was synthesized at 1350 1C by the sol–gel method. The effects of Y-doping on the electronic and ionic conductivities of YxSr1 x Ti0.6Fe0.4O3 δ were investigated. The ionic conductivity of SrTiO3-based materials can be significantly improved by Y-doping on the A-site and Fe-doping on the B-site. We report in this paper a remarkable enhancement of ionic conductivity of Y, Fe co-doped SrTiO3 by the increase in Y-doping amount on the Asite. The total electrical conductivity and ionic conductivity were 0.135 S/cm and 0.017 S/cm for Y0.07Sr0.93Ti0.6Fe0.4O3 δ at 800 1C, respectively, while were 0.056 S/cm and 0.02 S/cm for Y0.09Sr0.91Ti0.6Fe0.4O3 δ. & 2014 Elsevier B.V. All rights reserved.
Keywords: YxSr1 xTi0.6Fe0.4O3 δ Electrical properties Ionic conductivity Sol–gel preparation Solid oxide fuel cell
1. Introduction
2. Experimental
Perovskite-type mixed electronic–ionic conductors have been received much attention due to the wide range of application in modern solid state ionic devices. Typically doped strontium titanate (SrTiO3) is one of the most promising mixed conductors for solid oxide electrolysis cells and separation membranes due to its excellent chemical stability, thermal stability, tolerance to coking and sulfur poisoning [1,2]. However, the electrical conductivity of un-doped SrTiO3 is too low to meet the requirements for materials. Therefore, many efforts have been made to enhance the electronic and ionic conductivities of SrTiO3 material. Donor doping on the Sr site of SrTiO3 could improve the electrical conductivity, while acceptor doping on the Ti site of SrTiO3 could improve the electronic or ionic conductivity [3–8]. In our previous work, it has been shown that (Y, Fe) co-doped SrTiO3, such as Y0.08Sr0.92Ti1 xFexO3 δ and (Y0.08Sr0.92)1 xTi0.6 Fe0.4O3 δ, has high electronic and ionic conductivities [9,10]. In the present work, YxSr1 xTi0.6Fe0.4O3 δ (x¼0.07, 0.08, 0.09) was synthesized by the sol–gel method to investigate the effects of yttrium and iron on electrical properties of SrTiO3, including total electrical and ionic conductivities.
YxSr1 xTi0.6Fe0.4O3 δ (x ¼0.07, 0.08, 0.09) powders were synthesized by the sol–gel method. Sr(CH3COO)2 2H2O and Ti(CH3 CH2CH2CH2O)4, Fe2O3 and Y2O3 were used as starting materials. Details of the experiments can be found in the literature [9]. The synthesized powders were uniaxially pressed into a pellet (diameter 8 mm and thickness 1.20 mm). The green pellets were densified at 1350 1C for 5 h in air to achieve dense samples for the measurement of total electrical and ionic conductivities. The phase composition of samples was identified using an X-ray diffractometer (XRD) with CuKα radiation (Rigaku D/ max-A, Tokyo, Japan). The total electrical conductivity (s) was measured by AC impedance in range of 400–900 1C in air and the ionic conductivity (sion) was determined by the electron-blocking method within 600–950 1C in frequency range 0.01–100 kHz in air. YSTF (YxSr1 xTi0.6Fe0.4O3 δ) pellets were plastered onto dense YSZ pellets with a little Pt paste which was used to overcome the interface resistance. Then Pt paste was painted to the outside of both the connected YSTF and YSZ pellets as electrodes (Fig. 1). In the device, the electron flux is blocked by YSZ layer because YSZ is considered to be almost a pure oxygen ion conductor. Glass seal (SiO2–B2O3–BaO–Al2O3) was used to prevent oxygen leakage along the sides of the assembled samples [3] because the testing sample and the YSZ layer connect in series, the total impedance (Rtotal ) should be the sum of the oxygen ionic impedance of YSTF sample (Rsample) and YSZ layer ( RYSZ). As Eqs. ((1) and (2)) show, the ionic conductivity of sample (sion) can be worked out by its oxygen ionic impedance. The total electrical and ionic conductivities data
n
Corresponding author. Tel./fax: þ 86 10 62334957. E-mail addresses: [email protected] (K. Shan), [email protected] (X.-M. Guo). 0167-577X/$ - see front matter & 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2014.01.152
252
K. Shan, X.-M. Guo / Materials Letters 121 (2014) 251–253
were taken in an interval of 100 1C and 50 1C respectively. Rsample ¼ Rtotal RYSZ
ð1Þ
sion ¼ ðh=SÞ Rsample
ð2Þ
S and h stands for the surface area and thickness of YSTF, respectively.
3. Results and discussion Fig. 2 shows the XRD patterns of the powders of YxSr1 x Ti0.6Fe0.4O3 δ (x¼ 0.07, 0.08, 0.09, 0.10 and 0.12) obtained by the sol–gel method after sintering at 1350 1C for 5 h. As Fig. 2(a) shows, all of the peaks are assigned to a single cubic perovskite structure and no impurity peaks are detected, which means that Y, Fe co-doping does not negatively affect the formation of a solid solution, although the ionic radius of Y3 þ (0.90 Å) is much smaller than that of Sr2 þ (1.44 Å). However, for the one with x ¼0.10 and 0.12 (Fig. 2(b)), an obvious second phase Y2O3 is observed. Thus it is reasonable to state that the doping limit of Y in SrTi0.6Fe0.4O3-δ at 1350 1C is less than 10 mol%. Fig. 3 presents the temperature dependencies of the total electrical conductivity of YxSr1 xTi0.6Fe0.4O3 δ (x ¼0.07, 0.08, 0.09) perovskites. As is evident in Fig. 3, the total electrical conductivity of YxSr1 xTi0.6Fe0.4O3 δ increases as temperature increases, indicating that the p-type conductivity coming from the substitution of Fe3 þ for Ti4 þ contributes more effectively to the conductivity [3,11–13]. On the other hand, the total electrical conductivity decreases with the increase in Y-doping amount,
although their electrical conductivities are still much higher than that of SrTiO3. The electrical conductivity was 0.135 S/cm for Y0.07Sr0.93 Ti0.6Fe0.4O3 δ and 0.056 S/cm for Y0.09Sr0.91Ti0.6 Fe0.4O3 δ at 800 1C, respectively. The value at 800 1C is approximately 2.4 times higher than that of Y0.09Sr0.91Ti0.6Fe0.4O3 δ. The total electrical conductivity of the YxSr1 xTi0.6Fe0.4O3 δ perovskite increased with the temperature increasing in the low temperature range and thereafter became saturated. This conduction behavior should result from the predominant ionic compensation. The electron hole concentration in such materials decreases with increasing temperature due to progressive oxygen losses from the crystal lattice. On the other hand, a possible explanation for the decreasing electrical conductivity with the increase in Y-doping amount may refer to a larger disorder and lead to a significant decrease of the electron–hole transport [14–16]. Fig. 4(a) shows complex-plane impedance plots of the total ionic conduction for YSZ and Y0.09Sr0.91Ti0.6Fe0.4O3 δ. As Fig. 4(a) shows, with increase in temperature, the resistances of grain and grain boundary decrease, which is observed by the shrunken intercept and semicircle, respectively. Fig. 4(b) shows that the partial ionic conductivities of perovskite-type YxSr1 xTi0.6Fe0.4O3 δ (x ¼0.07, 0.08, 0.09) increase steadily with the elevated temperature and the increase in Y-doping amount, respectively. As shown in Fig. 4 (b), the ionic conductivity of Y0.09Sr0.91Ti0.6Fe0.4O3 δ sample varies from 0.0052 S/cm at 600 1C to 0.02 S/cm at 800 1C. The activation energy for ionic conduction of YxSr1 xTi0.6Fe0.4O3 δ is 0.39, 0.33 and 0.32 eV for 0.07, 0.08 and 0.09, respectively. The activation energy decreases with the increase in Y-doping amoun, which
Pt Pt mesh O
2-
glass sample Pt mesh
YSZ
Pt
Fig. 1. Schematic diagram of ionic conductivity test configuration by using YSZ as a electron blocking electrode.
Fig. 3. Temperature dependences of the total electrical conductivities of YxSr1 xTi0.6Fe0.4O3 δ measured in 400–900 1C.
Fig. 2. XRD patterns of YxSr1 xTi0.6Fe0.4O3 δ after sintering at 1350 1C for 5 h: (a) x ¼0.07, 0.08, 0.09 and (b) x ¼0.10, 0.12.
K. Shan, X.-M. Guo / Materials Letters 121 (2014) 251–253
253
Fig. 4. (a) Complex-plane impedance plots of the total ionic conduction for YSZ and Y0.09Sr0.91Ti0.6Fe0.4O3 δ (the inset is equivalent circuit) and (b) Temperature dependences of the ionic conductivities of YxSr1 xTi0.6Fe0.4O3 δ.
indicates that Y-doping is favorable for the ionic conduction in YxSr1 xTi0.6Fe0.4O3 δ. On the other hand, the ionic conductivity is primarily dependent on the oxygen vacancy concentration and the migration energy of oxygen ion at a certain temperature [17,18]. Although the energetic barrier for ion jumps is a very important factor, the key factor of governing ionic transport in such perovskites relates to oxygen vacancy concentration and ordering. Namely, Fecontaining perovskites tend to form ordered domains with brownmillerite-like structure, ordered vacancies and low ionic conductivity. Doping with Y should increase the disorder, whilst doping with Fe should simply increase the concentration of oxygen vacancies. Therefore, Y-doping facilitates the ionic conduction of the materials.
4. Conclusions The electrical conductivity of SrTiO3 without doping was very low, while it was remarkably enhanced by Y, Fe co-doping. The total electrical conductivity decreases steadily with the increase in Y-doping amount due to the decrease of the electron–hole transport. Oxygen ionic conductivity was found to increase with the increase in yttrium addition because yttrium doping increases the oxygen vacancy disorder and then results in the improvement of ionic conduction. The total electrical and ionic conductivities were 0.135 S/cm and 0.017 S/ cm for Y0.07Sr0.93Ti0.6Fe0.4O3 δ and 0.056 S/cm and 0.02 S/cm for Y0.09Sr0.91Ti0.6Fe0.4O3 δ at 800 1C, respectively.
Acknowledgments The financial support provided by the National Natural Science Foundation of China (Nos. 50974012 and 51374017) and Program Changjiang Scholars and Innovative Research Team in University (No. 0708) is gratefully acknowledged. References [1] Moos R, Hardtl KH. J Appl Phys 1996;80:393–400. [2] Li X, Zhao HL, Luo DW, Huang K. Mater Lett 2011;65:2624–7. [3] Li X, Zhao HL, Xu NS, Zhou X, Zhang CJ, et al. Int J Hydrog Energy 2009;34:6407–14. [4] Li X, Zhao HL, Gao F, Chen N, Xu NS. Electrochem Commun 2008;10:1567–70. [5] Litzelman SJ, Rothschild A, Tuller HL. Sens Actuators B 2005;108:231–7. [6] Kharton VV, Kovalevsky AV, Viskup AP, Jurado JR, Figueiredo FM, et al. J Solid State Chem 2001;156:437–44. [7] Hui SQ, Petric A. Mater Res Bull 2002;37:1215–31. [8] Yoon JS, Yoon MY, Kwak C, Park HJ, Lee SM, Lee KH, et al. Mater Sci Eng B 2011;177:151–6. [9] Shan K, Guo XM. Mater Lett 2013;105:196–8. [10] Shan K, Guo XM. Mater Lett 2013;113:126–9. [11] Steinsvik S, Bugge R, Gjonnes J, Tafto J, Norby T. J Phys Chem Solids 1997;58:969–76. [12] Ferreira AAL, Abrantes JCC, Jurado JR, Frade JR. Solid State Ionics 2000;135:761–4. [13] Stevenson JW, Armstrong TR, Carneim RD, Pederson LR, Weber WJJ. Electrochem Soc 1996;143:2722–9. [14] Rothschild A, Litzelman SJ, Tuller HL, Menesklou W, Schneider T, I-Tiffee E. Sens Actuators B 2005;108:223–30. [15] Blaise GJ. Electrostatics 2001;50:69–89. [16] Fagg DP, Kharton VV, Frade JR, Ferreira AAL. Solid State Ionics 2003;156:45–57. [17] Leng YL, Chan SH, Khor KA, Jiang SP, Int J. Hydrog Energy 2004;29:1025–33. [18] Cook RL, Sammells AF. Solid State Ionics 1991;45:311–21.