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Electrical properties of nanocrystalline Sm-doped ceria ceramics
Solid State Ionics 177 (2006) 2509 – 2512 www.elsevier.com/locate/ssi
Electrical properties of nanocrystalline Sm-doped ceria ceramics Piotr Jasinski Faculty of Electronics, Telecommunication and Informatics, Gdansk University of Technology, ul.Narutowicza 11/12, 80-952 Gdansk, Poland Received 1 September 2005; received in revised form 6 April 2006; accepted 10 April 2006
Abstract Electrical properties of nanocrystalline materials are usually different than that of microcrystalline. This phenomenon is not fully understood and therefore it is of continuous interest in the scientific community. In this paper, the results of investigation of nanocrystalline Sm-doped ceria prepared by a net shape technology are presented. The net shape technology is a relatively new ceramic preparation method, which combine powder and polymer precursor methods. The ceramics prepared in this way can be dense and nanocrystalline. Net-shaped prepared Sm-doped ceria was fabricated in the form of bulk ceramics and film on platinum foil. Their electrical properties were investigated and compared with the results obtained from microcrystalline sample. © 2006 Elsevier B.V. All rights reserved. Keywords: Ceria; Nanocrystalline; Low temperature processing; Net shape technology
1. Introduction Over the past decade, nanostructured materials have been the subject of enormous interest, with the potential for wide-ranging industrial, biomedical and electronic applications [1]. Nanocrystalline materials exhibit chemical and physical properties, which are frequently different than that of microcrystalline. Due to difficulties in preparation of nanocrystalline electroceramics, their electrical properties are not fully understood. The nanocrystalline bulk materials, which have to be prepared in low temperature, are frequently not fully dense. On the other hand, the nanocrystalline films, which are usually dense, might react with the supporting materials during the fabrication process. The most common preparation method of nanocrystalline ceramics utilizes hot die press [2]. However, in this paper, an alternative method called net shape technology is proposed and used. The net shape technology is combining powder processing and polymer precursor fabrication methods, i.e. powder is used to prepare net-shape body, while polymer precursor to densify its structure. We have been successfully developing and using this technology for fabrication of YSZ electrolyte [3,4] and composite anodes [5,6]. In this study, the net shape technology is used for fabrication of nanocrystalline ceria ceramics and films. E-mail address: [email protected]. 0167-2738/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2006.04.018
Ceria-based materials are of special interest due to their potential application in fuel cells, catalyzers, sensors, etc. [7]. Studies based on nanocrystalline undoped or lightly doped ceria showed that its electronic conductivity is increased by several orders of magnitude [8–10]. It is generally agreed that this phenomenon is caused by oxygen vacancy depletion and electron accumulation in space charge layer, which is formed at the grain boundary [11–15]. The influence of grain size on the conductivity of highly doped ceria in submicron/micron [16–18] and nano-grain size [19,20] range was studied as well. In this case, the grain boundary resistance of nanocrystalline ceria was much higher than that of microcrystalline one, which is again attributed to the space charge layer formation. The highly doped ceria (20 mol% samarium) will be investigated in this paper. 2. Experimental Nanocrystalline Sm-doped ceria was prepared in the form of bulk ceramics and film on platinum foil by net shape technology. In the case of the bulk ceramics, a commercially available Sm-doped ceria powder (Sm0.2Ce0.8Ox, Daiichi Kigenso, Co., Japan) was isostatically pressed into pellets. Obtained in this way, ceria skeleton was impregnated multiple times by Sm0.2Ce0.8Ox polymer precursor. The polymer precursor was prepared using Sm nitrate, Ce nitrate and
2510
P. Jasinski / Solid State Ionics 177 (2006) 2509–2512
A
B
400 °C. This procedure was repeated until full densification of the film was obtained. Additionally, microcrystalline bulk ceria was fabricated, so that electrical properties of nanocrystalline and microcrystalline ceria could be easily compared. For this purpose, the same powder was iso-axially pressed at 150 MPa and sintered at 1300 °C for 2 h. The microstructures of the resulting ceria ceramics were analyzed using a Hitachi S4700 field emission gun (FEG). A scanning probe microscopy Digital Instruments Nanoscope was used to obtain atomic force microscopy (AFM) images. Electrical measurements were performed using a 4-probe DC Van der Pauw method and/or a 2-probe impedance spectroscopy. The DC measurements were obtained using the Keithley 2400 current–voltage source, while the impedance spectra were collected using the Solartron set consisting of 1260 impedance analyzer, 1296 dielectric interface and 1470 battery tester. Platinum paste (ESL 5542) was used to prepare electrodes. In the case of the ceria film, the measurements were performed having electrodes across thickness of the film, i.e. one of the electrodes was platinum foil, while the other was deposited on the top of the film. 3. Results and discussion
C
In Fig. 1, SEM images of bulk ceria are presented. The skeleton of ceria, which was later used for polymeric precursor impregnation, is shown in Fig. 1A. Its structure consisted of fiber-like conglomerates of ceria powder. The density of skeleton, which was measured by the Archimedes method, was found to be 58% of theoretical. Fig. 1B and C presents bulk ceria sintered at 1000 °C after polymeric precursor impregnation. Its structure consisted of nanocrystallites (Fig. 1B), in which size was found to be about 40 nm. The bulk ceria density was about 91% theoretical. The image, which was taken in lower magnification (Fig. 1C), shows some cracks, which can be caused by rapid gasification during decomposition of o
T [ C] 0.1 Fig. 1. SEM image of the Sm-doped ceria skeleton used for soaking (A) and net shape prepared ceramics sintered at 1000 °C (B, C).
600
400
0.01
[S/cm]
ethylene glycol (all from Alfa Aesar) precursors (the details of polymer preparation can be found elsewhere [20]). After each impregnation, the pellets were annealed at 700 °C, so that the polymer precursor could decompose. In the case of the ceria film on platinum foil, the same powder was used to prepare colloidal suspension. The powder was ultrasonically dispersed in water and ethanol. The suspension, which contained about 50 wt.% of ceria powder, was spin-coated on platinum foil and then slowly dried. This procedure was repeated few times. Obtained in this way, ceria framework was used for the polymeric precursor impregnation. The polymeric precursor was spin-coated on the framework and then decomposed at
800
1E-3 Sintering temperature o
700 C - net shaped o
800 C - net shaped
1E-4
o
1100 C - net shaped o
1300 C - net shaped o
1300 C - die pressed
1E-5 1.0
1.2
1.4
1.6
1.8
1000/T [1/K] Fig. 2. The electrical conductivity of Sm-doped ceria ceramics sintered at different temperatures.
P. Jasinski / Solid State Ionics 177 (2006) 2509–2512
A
2511
dependence of the electrical conductivities of the bulk ceria sintered at different temperatures is presented in Fig. 2. The ceramics with smaller grain sizes have lower conductivity. This is pronounced in the lower temperature range, what is related with higher activation energy of the ceria with smaller grain sizes. The electrical conductivity of the ceria, which was sintered at 1100 °C and 1300 °C, is virtually the same. However, it is lower than that which was measured for die pressed ceria. Probably, the lower conductivity of net-shaped prepared ceria is related to cracks, which are visible in Fig. 1C. Therefore, special attention should be paid during decomposition procedure of polymer. A cross section of ceria film on platinum is presented in Fig. 3A. The film was about 8.2 μm think and free of any cracks. In Fig. 3B and C are presented AFM images of the film sintered at 800 °C and 1000 °C, respectively. The surface of the film shows crystallites of 40 nm and 55 nm in size for the sintering temperature of 800 °C and 1000 °C, respectively. The impedance spectra of the ceria film were collected using across
8.2 μ m
B
A
-15000
grain size 40nm grain size 55nm
Z'' [kΩ cm]
0.8 0.6 0.4 0.2
C
-10000
1kHz
-5000
0 0
5000
10000
15000
Z' [kΩ cm]
B
-1000
0.8 0.6
-800
0.4
Fig. 3. The images of Sm-doped ceria film on Pt: the cross section of the film sintered at 800 °C (A) and AFM images of the film surface sintered at 800 °C (B) and 1000 °C (C).
polymer. Those cracks were visible, only deep inside the samples, while the surfaces were shiny and free of any defects. It can be expected that those cracks can influence electrical conductivity of ceria. The bulk ceria was investigated by impedance spectroscopy. However, even from the spectra, which were collected at low temperatures, it was not possible to clearly distinguish grain and grain boundary semicircles. Therefore, Van der Pauw method was used for total conductivity measurements. The temperature
Z'' [kΩ cm]
0.2
-600
-400
30kHz -200
grain size 40nm grain size 55nm
0 0
200
400
600
800
1000
Z' [kΩ cm] Fig. 4. The impedance spectra of Sm-doped ceria film on platinum foil at 200 °C in the low (A) and high (B) frequency range.
2512
P. Jasinski / Solid State Ionics 177 (2006) 2509–2512
T [ C] 300
1E-4
250
200
is 35 [21]). Those values are comparable with the values obtained for microcrystalline ceria (4 nm in case 20% Y-doped ceria [22]), which might suggest space charge origin of the grain boundary resistance.
150
0.72eV
4. Conclusions
σ [S/cm]
1E-5 1E-6
The electrical properties of nanocrystalline Sm-doped ceria, which was prepared in the form of bulk ceramics and film on platinum foil by the net shape technology, have been evaluated. The ceria with smaller grain sizes showed lower conductivity. The grain boundary conductivity was about two times lower in the case of the ceria film with the grain size of 40 nm than of 55 nm. In the low temperature range, grain boundary conductivity is about one order magnitude lower than that of grain. The calculated grain boundary thickness might suggest space charge origin of grain boundary resistance.
0.9eV
1E-7 1E-8
grain size 40nm grain size 55nm solid symbol - grain conductivity open symbol - grain boundary conductivity
1.8
2.0
2.2
2.4
1000/T [1/K] Fig. 5. The electrical conductivity of Sm-doped ceria film on Pt sintered at different temperatures.
the film thickness electrode configuration. The typical spectra for the low and high frequency range, which were measured at 200 °C, are presented in Fig. 4A and B, respectively. The spectra consist of two distinguishable semicircles, which were attributed to grain and grain boundary resistance. The ceria with lower size of grain shows higher grain and grain boundary resistance. This phenomenon is more pronounced for grain boundary resistance. The Arrhenius plot of conductivity in the low temperature range is presented in Fig. 5. The grain boundary conductivity is about one order of magnitude lower than that of grain. The activation energy was calculated to be of 0.7 eV and 0.9 eV in case of the grain and grain boundary resistance, respectively. The grain boundary resistance for the 55 nm grain size ceria is about two times higher than that of 40 nm. In order to grasp physical origin of the grain boundary, a brick layer model was used to evaluate its thickness Lgb [16]: Lgb ¼ ee0
sdLg Cgb dl
ð1Þ
where Cgb is the grain boundary capacitance, Lg is the grain size, s is the area of electrodes, l is the thickness of film, ε0 is the permittivity of vacuum ε0 = 8.85 * 10− 14 (F/cm) and ε is the dielectric constant of the grain boundary. About 5 nm and 7 nm were obtained for the ceria with grain size of 40 nm and 55 nm, respectively (it was assumed that the dielectric constant of ceria
References [1] T. Trindade, P. O'Brien, N.L. Pickett, Chem. Mater. 13 (2001) 3843. [2] M. Winterer, Nanocrystalline Ceramics Synthesis and Structure, Springer Series in Materials Science, vol. 53, Springer Verlag, New York, 2002. [3] V. Petrovsky, T. Suzuki, P. Jasinski, T. Petrovsky, H.U. Anderson, Electrochem. Solid-State Lett. 7 (2004) A137. [4] P. Jasinski, V. Petrovsky, T. Suzuki, T. Petrovsky, H.U. Anderson, J. Electrochem. Soc. 152 (2005) A454. [5] V. Petrovsky, T. Suzuki, P. Jasiński, H.U. Anderson, Electrochem. SolidState Lett. 8 (2005) A341. [6] P. Jasinski, T. Suzuki, V. Petrovsky, H.U. Anderson, Electrochem. SolidState Lett. 8 (2005) A219. [7] M. Mogensen, N.M. Sammes, G.A. Tompsett, Solid State Ionics 129 (2000) 63. [8] S. Kim, J. Maier, J. Eur. Ceram. Soc. 24 (2004) 1919. [9] S. Kim, J. Maier, J. Electrochem. Soc. 149 (2002) J73. [10] Tschope, E. Sommer, R. Birringer, Solid State Ionics 139 (2001) 255. [11] X. Guo, Solid State Ionics 81 (1995) 235. [12] X. Guo, R. Waser, Solid State Ionics 173 (2004) 63. [13] A. Tschope, Solid State Ionics 139 (2001) 267. [14] A. Tschope, C. Bauerle, R. Birringer, J. Appl. Phys. 95 (2004) 1203. [15] A. Tschope, R. Birringer, J. Electroceramics 7 (2001) 169. [16] G.M. Christie, F.P.F. van Berkel, Solid State Ionics 83 (1996) 17. [17] C. Tian, S.-W. Chan, Solid State Ionics 134 (2000) 89. [18] A. Tschope, S. Kilassonia, R. Birringer, Solid State Ionics 173 (2004) 57. [19] Y.M. Chiang, E.B. Lavik, D.A. Blom, Nanostruct. Mater. 9 (1997) 633. [20] T. Suzuki, I. Kosacki, H.U. Anderson, Solid State Ionics 151 (2002) 111. [21] A.S. Nowick, A.V. Vaysleyb, I. Kuskovsky, Phys. Rev., B 58 (1998) 8398. [22] X. Guo, W. Sigle, J. Maier, J. Am. Ceram. Soc. 86 (2003) 77.
Electrical properties of nanocrystalline Sm-doped ceria ceramics Piotr Jasinski Faculty of Electronics, Telecommunication and Informatics, Gdansk University of Technology, ul.Narutowicza 11/12, 80-952 Gdansk, Poland Received 1 September 2005; received in revised form 6 April 2006; accepted 10 April 2006
Abstract Electrical properties of nanocrystalline materials are usually different than that of microcrystalline. This phenomenon is not fully understood and therefore it is of continuous interest in the scientific community. In this paper, the results of investigation of nanocrystalline Sm-doped ceria prepared by a net shape technology are presented. The net shape technology is a relatively new ceramic preparation method, which combine powder and polymer precursor methods. The ceramics prepared in this way can be dense and nanocrystalline. Net-shaped prepared Sm-doped ceria was fabricated in the form of bulk ceramics and film on platinum foil. Their electrical properties were investigated and compared with the results obtained from microcrystalline sample. © 2006 Elsevier B.V. All rights reserved. Keywords: Ceria; Nanocrystalline; Low temperature processing; Net shape technology
1. Introduction Over the past decade, nanostructured materials have been the subject of enormous interest, with the potential for wide-ranging industrial, biomedical and electronic applications [1]. Nanocrystalline materials exhibit chemical and physical properties, which are frequently different than that of microcrystalline. Due to difficulties in preparation of nanocrystalline electroceramics, their electrical properties are not fully understood. The nanocrystalline bulk materials, which have to be prepared in low temperature, are frequently not fully dense. On the other hand, the nanocrystalline films, which are usually dense, might react with the supporting materials during the fabrication process. The most common preparation method of nanocrystalline ceramics utilizes hot die press [2]. However, in this paper, an alternative method called net shape technology is proposed and used. The net shape technology is combining powder processing and polymer precursor fabrication methods, i.e. powder is used to prepare net-shape body, while polymer precursor to densify its structure. We have been successfully developing and using this technology for fabrication of YSZ electrolyte [3,4] and composite anodes [5,6]. In this study, the net shape technology is used for fabrication of nanocrystalline ceria ceramics and films. E-mail address: [email protected]. 0167-2738/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2006.04.018
Ceria-based materials are of special interest due to their potential application in fuel cells, catalyzers, sensors, etc. [7]. Studies based on nanocrystalline undoped or lightly doped ceria showed that its electronic conductivity is increased by several orders of magnitude [8–10]. It is generally agreed that this phenomenon is caused by oxygen vacancy depletion and electron accumulation in space charge layer, which is formed at the grain boundary [11–15]. The influence of grain size on the conductivity of highly doped ceria in submicron/micron [16–18] and nano-grain size [19,20] range was studied as well. In this case, the grain boundary resistance of nanocrystalline ceria was much higher than that of microcrystalline one, which is again attributed to the space charge layer formation. The highly doped ceria (20 mol% samarium) will be investigated in this paper. 2. Experimental Nanocrystalline Sm-doped ceria was prepared in the form of bulk ceramics and film on platinum foil by net shape technology. In the case of the bulk ceramics, a commercially available Sm-doped ceria powder (Sm0.2Ce0.8Ox, Daiichi Kigenso, Co., Japan) was isostatically pressed into pellets. Obtained in this way, ceria skeleton was impregnated multiple times by Sm0.2Ce0.8Ox polymer precursor. The polymer precursor was prepared using Sm nitrate, Ce nitrate and
2510
P. Jasinski / Solid State Ionics 177 (2006) 2509–2512
A
B
400 °C. This procedure was repeated until full densification of the film was obtained. Additionally, microcrystalline bulk ceria was fabricated, so that electrical properties of nanocrystalline and microcrystalline ceria could be easily compared. For this purpose, the same powder was iso-axially pressed at 150 MPa and sintered at 1300 °C for 2 h. The microstructures of the resulting ceria ceramics were analyzed using a Hitachi S4700 field emission gun (FEG). A scanning probe microscopy Digital Instruments Nanoscope was used to obtain atomic force microscopy (AFM) images. Electrical measurements were performed using a 4-probe DC Van der Pauw method and/or a 2-probe impedance spectroscopy. The DC measurements were obtained using the Keithley 2400 current–voltage source, while the impedance spectra were collected using the Solartron set consisting of 1260 impedance analyzer, 1296 dielectric interface and 1470 battery tester. Platinum paste (ESL 5542) was used to prepare electrodes. In the case of the ceria film, the measurements were performed having electrodes across thickness of the film, i.e. one of the electrodes was platinum foil, while the other was deposited on the top of the film. 3. Results and discussion
C
In Fig. 1, SEM images of bulk ceria are presented. The skeleton of ceria, which was later used for polymeric precursor impregnation, is shown in Fig. 1A. Its structure consisted of fiber-like conglomerates of ceria powder. The density of skeleton, which was measured by the Archimedes method, was found to be 58% of theoretical. Fig. 1B and C presents bulk ceria sintered at 1000 °C after polymeric precursor impregnation. Its structure consisted of nanocrystallites (Fig. 1B), in which size was found to be about 40 nm. The bulk ceria density was about 91% theoretical. The image, which was taken in lower magnification (Fig. 1C), shows some cracks, which can be caused by rapid gasification during decomposition of o
T [ C] 0.1 Fig. 1. SEM image of the Sm-doped ceria skeleton used for soaking (A) and net shape prepared ceramics sintered at 1000 °C (B, C).
600
400
0.01
[S/cm]
ethylene glycol (all from Alfa Aesar) precursors (the details of polymer preparation can be found elsewhere [20]). After each impregnation, the pellets were annealed at 700 °C, so that the polymer precursor could decompose. In the case of the ceria film on platinum foil, the same powder was used to prepare colloidal suspension. The powder was ultrasonically dispersed in water and ethanol. The suspension, which contained about 50 wt.% of ceria powder, was spin-coated on platinum foil and then slowly dried. This procedure was repeated few times. Obtained in this way, ceria framework was used for the polymeric precursor impregnation. The polymeric precursor was spin-coated on the framework and then decomposed at
800
1E-3 Sintering temperature o
700 C - net shaped o
800 C - net shaped
1E-4
o
1100 C - net shaped o
1300 C - net shaped o
1300 C - die pressed
1E-5 1.0
1.2
1.4
1.6
1.8
1000/T [1/K] Fig. 2. The electrical conductivity of Sm-doped ceria ceramics sintered at different temperatures.
P. Jasinski / Solid State Ionics 177 (2006) 2509–2512
A
2511
dependence of the electrical conductivities of the bulk ceria sintered at different temperatures is presented in Fig. 2. The ceramics with smaller grain sizes have lower conductivity. This is pronounced in the lower temperature range, what is related with higher activation energy of the ceria with smaller grain sizes. The electrical conductivity of the ceria, which was sintered at 1100 °C and 1300 °C, is virtually the same. However, it is lower than that which was measured for die pressed ceria. Probably, the lower conductivity of net-shaped prepared ceria is related to cracks, which are visible in Fig. 1C. Therefore, special attention should be paid during decomposition procedure of polymer. A cross section of ceria film on platinum is presented in Fig. 3A. The film was about 8.2 μm think and free of any cracks. In Fig. 3B and C are presented AFM images of the film sintered at 800 °C and 1000 °C, respectively. The surface of the film shows crystallites of 40 nm and 55 nm in size for the sintering temperature of 800 °C and 1000 °C, respectively. The impedance spectra of the ceria film were collected using across
8.2 μ m
B
A
-15000
grain size 40nm grain size 55nm
Z'' [kΩ cm]
0.8 0.6 0.4 0.2
C
-10000
1kHz
-5000
0 0
5000
10000
15000
Z' [kΩ cm]
B
-1000
0.8 0.6
-800
0.4
Fig. 3. The images of Sm-doped ceria film on Pt: the cross section of the film sintered at 800 °C (A) and AFM images of the film surface sintered at 800 °C (B) and 1000 °C (C).
polymer. Those cracks were visible, only deep inside the samples, while the surfaces were shiny and free of any defects. It can be expected that those cracks can influence electrical conductivity of ceria. The bulk ceria was investigated by impedance spectroscopy. However, even from the spectra, which were collected at low temperatures, it was not possible to clearly distinguish grain and grain boundary semicircles. Therefore, Van der Pauw method was used for total conductivity measurements. The temperature
Z'' [kΩ cm]
0.2
-600
-400
30kHz -200
grain size 40nm grain size 55nm
0 0
200
400
600
800
1000
Z' [kΩ cm] Fig. 4. The impedance spectra of Sm-doped ceria film on platinum foil at 200 °C in the low (A) and high (B) frequency range.
2512
P. Jasinski / Solid State Ionics 177 (2006) 2509–2512
T [ C] 300
1E-4
250
200
is 35 [21]). Those values are comparable with the values obtained for microcrystalline ceria (4 nm in case 20% Y-doped ceria [22]), which might suggest space charge origin of the grain boundary resistance.
150
0.72eV
4. Conclusions
σ [S/cm]
1E-5 1E-6
The electrical properties of nanocrystalline Sm-doped ceria, which was prepared in the form of bulk ceramics and film on platinum foil by the net shape technology, have been evaluated. The ceria with smaller grain sizes showed lower conductivity. The grain boundary conductivity was about two times lower in the case of the ceria film with the grain size of 40 nm than of 55 nm. In the low temperature range, grain boundary conductivity is about one order magnitude lower than that of grain. The calculated grain boundary thickness might suggest space charge origin of grain boundary resistance.
0.9eV
1E-7 1E-8
grain size 40nm grain size 55nm solid symbol - grain conductivity open symbol - grain boundary conductivity
1.8
2.0
2.2
2.4
1000/T [1/K] Fig. 5. The electrical conductivity of Sm-doped ceria film on Pt sintered at different temperatures.
the film thickness electrode configuration. The typical spectra for the low and high frequency range, which were measured at 200 °C, are presented in Fig. 4A and B, respectively. The spectra consist of two distinguishable semicircles, which were attributed to grain and grain boundary resistance. The ceria with lower size of grain shows higher grain and grain boundary resistance. This phenomenon is more pronounced for grain boundary resistance. The Arrhenius plot of conductivity in the low temperature range is presented in Fig. 5. The grain boundary conductivity is about one order of magnitude lower than that of grain. The activation energy was calculated to be of 0.7 eV and 0.9 eV in case of the grain and grain boundary resistance, respectively. The grain boundary resistance for the 55 nm grain size ceria is about two times higher than that of 40 nm. In order to grasp physical origin of the grain boundary, a brick layer model was used to evaluate its thickness Lgb [16]: Lgb ¼ ee0
sdLg Cgb dl
ð1Þ
where Cgb is the grain boundary capacitance, Lg is the grain size, s is the area of electrodes, l is the thickness of film, ε0 is the permittivity of vacuum ε0 = 8.85 * 10− 14 (F/cm) and ε is the dielectric constant of the grain boundary. About 5 nm and 7 nm were obtained for the ceria with grain size of 40 nm and 55 nm, respectively (it was assumed that the dielectric constant of ceria
References [1] T. Trindade, P. O'Brien, N.L. Pickett, Chem. Mater. 13 (2001) 3843. [2] M. Winterer, Nanocrystalline Ceramics Synthesis and Structure, Springer Series in Materials Science, vol. 53, Springer Verlag, New York, 2002. [3] V. Petrovsky, T. Suzuki, P. Jasinski, T. Petrovsky, H.U. Anderson, Electrochem. Solid-State Lett. 7 (2004) A137. [4] P. Jasinski, V. Petrovsky, T. Suzuki, T. Petrovsky, H.U. Anderson, J. Electrochem. Soc. 152 (2005) A454. [5] V. Petrovsky, T. Suzuki, P. Jasiński, H.U. Anderson, Electrochem. SolidState Lett. 8 (2005) A341. [6] P. Jasinski, T. Suzuki, V. Petrovsky, H.U. Anderson, Electrochem. SolidState Lett. 8 (2005) A219. [7] M. Mogensen, N.M. Sammes, G.A. Tompsett, Solid State Ionics 129 (2000) 63. [8] S. Kim, J. Maier, J. Eur. Ceram. Soc. 24 (2004) 1919. [9] S. Kim, J. Maier, J. Electrochem. Soc. 149 (2002) J73. [10] Tschope, E. Sommer, R. Birringer, Solid State Ionics 139 (2001) 255. [11] X. Guo, Solid State Ionics 81 (1995) 235. [12] X. Guo, R. Waser, Solid State Ionics 173 (2004) 63. [13] A. Tschope, Solid State Ionics 139 (2001) 267. [14] A. Tschope, C. Bauerle, R. Birringer, J. Appl. Phys. 95 (2004) 1203. [15] A. Tschope, R. Birringer, J. Electroceramics 7 (2001) 169. [16] G.M. Christie, F.P.F. van Berkel, Solid State Ionics 83 (1996) 17. [17] C. Tian, S.-W. Chan, Solid State Ionics 134 (2000) 89. [18] A. Tschope, S. Kilassonia, R. Birringer, Solid State Ionics 173 (2004) 57. [19] Y.M. Chiang, E.B. Lavik, D.A. Blom, Nanostruct. Mater. 9 (1997) 633. [20] T. Suzuki, I. Kosacki, H.U. Anderson, Solid State Ionics 151 (2002) 111. [21] A.S. Nowick, A.V. Vaysleyb, I. Kuskovsky, Phys. Rev., B 58 (1998) 8398. [22] X. Guo, W. Sigle, J. Maier, J. Am. Ceram. Soc. 86 (2003) 77.