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

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

Conduction in polycrystalline electroceramics x 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

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

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

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

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

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)

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

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

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••

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)

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

Effective defect concentration in the space charge zone 1 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)

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)

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  - 2.37§ eV + 0.4 eV § Tuller and Nowick, J. Electrochem. Soc., 122, 255 (1975)  - 1.97 eV

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.

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