1. Partial Electronic and Ionic Conductivities of
Nanocrystalline Ceria Ceramics
Sangtae Kim, Jürgen Fleig, Joachim Maier
Max Planck Institute for Solid State Research
Stuttgart, Germany
IMSPEMAS
Warsaw, Poland
September 26, 2003

2. Contents Introduction
-Different conduction pathways in polycrystalline ceramics and their superposition
-Bricklayer model
-Core-space charge model for grain boundary
Detectability in impedance spectroscopy
-Highly conductive grain boundary
-AgBr bicrystal
-AgCl polycrystalline ceramics
-AgCl polycrystalline ceramics with microelectrodes
-Highly blocking grain boundary
-SrTiO3 polycrystalline ceramics with microelectrodes
-SrTiO3 bicrystal
-Highly selective grain boundary for partial electronic and ionic conduction
-nanocrystalline CeO2 ceramics
-Quantitative analysis based on space charge models

3. Conduction in polycrystalline electroceramicsx y z
Bricklayer model
For quantitative analysis
Assumption:
cubic-shaped grains of identical size and property
identical, homogeneous grain boundaries
Bulk
Grain
boundary (gb)
dgc= 2b dg 2l
Space charge
zones (sc)
Grain boundary
core (gc)
Core-space charge model
J. Jamnik, et al.,
Solid State ionics, 75, 51 (1995) x y z
Equivalent circuit
Electroceramics for practical applications
are polycrystalline forms.
Effective complex conductivity
J. Maier, Ber. Bunsenges. Phys. Chem., 90, 26 (1986)
Importance of understanding grain boundary
effects relative to the bulk effect on total conduction
Impedance analysis

4. Highly conductive space charge zones with blocking grain boundary core:
AgBr bicrystal
96oC
190oC 1h
300oC
w/o
annealing
J. Maier,
Ber. Bunsenges. Phys. Chem., 90, 26 (1986)
annealing

5. Highly conductive space charge zones with blocking grain boundary core:
AgCl polycrystalline ceramic
annealing
J. Maier, Ber. Bunsenges. Phys. Chem., 90, 26 (1986)

6. J. Fleig, J. Maier, Solid State Ionics, 86-88, 1351 (1996)
Highly conductive space charge zones:
AgCl polycrystalline ceramic - Microelectrodes
Measurement principle
inverse resistance
in 10-9S/cm
1 Hz Z re
/ G W 25 50 75
100
125
150
175
200 - i m / G
on grain
on grain boundary
23.9
34.6
8.4
7.8
7.2
7.5
5.4
5.1
11.8
7.9
8.9
9.0
9.5
31.7
66.1
40.5
64.1
J. Fleig, J. Maier, Solid State Ionics, 86-88, 1351 (1996)
25 mm
Microelectrode
on grain
on grain boundary

7. Charge carrier accumulation in space charge zones
Gouy-Chapman Profile Ag +
gb core
space charge zone
Ag-vacancy
concentration
space charge potential = 300 mV V 2l x

8. Blocking space charge zones: Fe-doped SrTiO3 polycrystalline ceramic:
Microelectrodes
Measurement principle a b
1e+9
2e+9
3e+9
4e+9 - Z i m / W re Z
real / W
2e+9
4e+9
6e+9
8e+9
1e+10
0 V
0,2 V
0,6 V
1,0 V -Z im b
S. Rodewald, et al.,
J. Am. Ceram. Soc., 84, 521 (2001)

9. S5 tilt grain boundary: [Fe] = 21018cm-1
Blocking space charge zones: Fe-doped SrTiO3 bicrystal
S5 tilt grain boundary: [Fe] = 21018cm-1
2nm
5 10 x 3
T = 598K
P = 105 Pa O 2
zero bias
200 mV
400 mV
600 mV x
4 10 x 3 W
3 10 x 3
-Im Z /
2 10 x 3
20 Hz
1 MHz
1 10 x 3 x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x
1 10 x 3
3 10 x 3
5 10 x 3
7 10 x 3
Re Z / W

10. Charge carrier depletion in space charge zones' Fe Ti
Defect concentration h•
Vo•• x l*
2nm
mean space charge potential  650 mV
Mott-Schottky Profile

11. Defect equilibrium in CeO2
Space charge effects on the partial electronic and ionic conductivity of
nanocrystalline CeO2 ceramic
Defect equilibrium in CeO2
Mixed conductor
S. Kim, J. Maier, J. Electrochem. Soc, 149, J73 (2002) e´
Nanocrystalline CeO2
Nanocrystalline CeO2
For intrinsic
Vo••
concentration
Defect l x gc
bulk e´
Vo••

12. S. Kim, J. Maier, J. Electrochem. Soc, 149, J73 (2002)
Ionic conductivity behavior of nanocrystalline CeO2 ceramic:
0.15 mol% Gd-doped CeO2
Vo•• l* e´ x
Gd´
Vo•• l* e´ x
Gd´
R1 + R2 ~ Rdc
t ~  ion
Pt/n-CGO/YSZ/Pt
S. Kim, J. Maier, J. Electrochem. Soc, 149, J73 (2002)

13. S. Kim, J. Maier, J. Electrochem. Soc, 149, J73 (2002)
Electronic conductivity behavior of nanocrystalline CeO2 ceramic:
Nominally pure CeO2
e = 33
Pt|nano-CeO2|Pt
S. Kim, J. Maier, J. Electrochem. Soc, 149, J73 (2002)
Vo•• l* e´ x A´
Pt|nano-CeO2|YSZ|Pt
1.9 eV

14. Effective defect concentration in the space charge zone1 2 3
For z1 = -z2 a b c d e f g h i j k l m p o
Gouy-
Chapman
Mott-
Schottky
Combined
Model
For 2z1 = -z2
For z1 = -2z2 n 
1: accumulated defect
2: depleted defect
3: dopant
S. Kim et al., Phys. Chem. Chem. Phys., 5, 2268 (2003)

15. Quantitative analyses of pO2 and T dependence
for r based on Mott-Schottky model: Gd-doped nano-CeO2
Vo•• l* e´ x
Gd´
Concentration profile
in n-CGO
Mott-Schottky situation
 0
Quantitative analyses
Experimental results
Space charge potential for
J.Fleig et al., J. Appl. Phys., 87(5), 2372 (2000)

16. Lower impurity concentration
Quantitative analyses of pO2 and T dependence
for s|| based on Mott-Schottky model: Nominally pure nano-CeO2
Mott-Schottky situation
Lower impurity concentration
n-CeO2-x
Vo•• l* e´ x A´ -
- 1.9 eV
Experimental results
e = 33
 -1/4
Quantitative analyses
 § eV eV
§ Tuller and Nowick, J. Electrochem. Soc.,
122, 255 (1975)
 eV

17. Summary
Not only highly resistive but also highly conductive grain boundary effects are demonstrated with respect to detectability of impedance spectroscopy
In ceria, grain boundary becomes highly selective for electronic and ionic conduction. This can be quantitatively explained based on the space charge models.

18. Quantitative analysis ® numerical finite element calculations
microelectrode on grain
potential distribution
Most potential drops close
to microelectrode
R as for
microelectrode on single crystal
method to obtain
bulk conductivity s
8·10 -9 1/ W cm
bulk
more complicated numerical
analysis
grain boundary conductance
(w · s )
4·10
-11 1/ W
potential distribution
microelectrode on grain boundary