Eric Wachsman, Director
Professor, Materials Science

Fundamentals of Ionic Transport

Ion conducting ceramics are at the forefront of ceramic bsasic science. They are used in, and being investigated for, a wide variety of technological applications. Their required functionality goes far beyond mechanical strength and toughness, to include such properties as ionic and electronic conductivity, thermochemical stability, and catalytic activity. These latter properties arise out of their defect chemistry and resultant wide range in stoichiometry. Depending on their defect chemistry and environment, these materials can exhibit exclusively ionic (electrolyte) or electronic conduction, or mixed (both ionic and electronic) conduction.

The majority of solid oxide electrolytes investigated have been those based on one of the quadravalent-cation oxides ZrO₂, HfO₂, CeO₂ or ThO₂ doped with either a divalent-cation oxide, such as CaO, or a trivalent-cation oxide, such as Sc₂O₃, Y₂O₃ or any of the rare earthoxides. These solid oxide electrolytes have very high oxygen ion conductivities, on the order of 1.0 ohm -1 cm -1 at temperatures in the range of 1000°C. Other solid oxide electrolytes, such as those based on Bi₂O₃ doped with similar rare earth or alkaline earth oxides, havecomparable oxygen ion conductivities to the group IVB oxides but at much lower temperatures.

The fluorite structure is common to all of these systems. It is the structure stable at high temperatures for the pure oxides and it is the phase associated with high conductivity. The presence of the dopant oxide serves two purposes: stabilization of this high temperature phase down to room temperature; and in the case of the ZrO₂ based electrolytes, the formation of anion vacancies in order to preserve electroneutrality. It is these oxygen vacancies that are the mobile species in solid oxide electrolytes.

The most common use of solid-oxide electrolytes are in oxygen sensors, for automobile exhaust systems and industrial processes. They are being developed for use in fuel cells, and O₂ generators, and have been investigated for the electrolytic reduction of NO_x and oxidation of various hydrocarbons.

Electronic or mixed conducting oxides exist in numerous structures, with variations on the perovskite ABO₃ lattice as one of the more common group of structures (e.g., LaMnO₃, LaFeO₃, LaCoO₃, etc.). These oxides are typically p-type semiconductors in which substitution of lower valent cations produces additional mobile anion vacancies. These materials are active oxidation catalysts due to the mobility and thermodynamic activity of their oxygen-ions/vacancies. They are being used as electrode materials in the devices described above and are under investigation for O₂ separation and hydrocarbon partial-oxidation membranes. In addition, both p and n-type semiconducting oxides are used as gas sensing elements as well as advanced battery electrodes.

Selected Relevant Publications:

"High Temperature Ion Conducting Ceramics," T.A. Ramanarayanan, S.C. Singhal, and E. D. Wachsman, in Interface, The Electrochemical Society, 10-2, 22-27, (2001).

“Direct Current Bias Studies on (Bi₂O₃)_0.8(Er₂O₃)_0.2 Electrolyte and Ag-(Bi₂O₃)_0.8(Er₂O₃)_0.2 Cermet Electrode,” A. Jaiswall and E.D. Wachsman, Solid State Ionics, submitted.

“Effect of Oxygen Sublattice Order on Conductivity in Highly Defective Fluorite Oxides,” E. D. Wachsman, Journal of the European Ceramic Society, 24, 1281-1285 (2004).

"Numerical Modeling of Hydrogen Permeation in Chemical Potential Gradients," S. J. Song, E. D. Wachsman, J. Rhodes, S. E. Dorris, and U. Balachandran, Solid State Ionics, 164, 107-116 (2003).

"Defect Structure and n-Type Electrical Properties of SrCe_0.95 Eu_0.05 O_{3- d} ," S. J. Song, E. D. Wachsman, S. E. Dorris, and U. Balachandran, Journal of the Electrochemical Society, 150, A1484 (2003).

“Defect Structure and n-Type Electrical Properties of SrCe_0.95 Eu_0.05O_3-d ,” S. Song, E.D. Wachsman, S.E. Dorris and U. Balachandran, Solid State Ionic Devices III, Electrochem. Soc., E.D. Wachsman, K.S. Lyons, M. Carolyn, F. Garzon, M. Liu, and J. Stetter, Ed., 2002-26, 456-470 (2003).

"Electrical Properties of p-Type Electronic Defects in the Protonic Conductor SrCe_0.95 Eu_0.05 O_3-d ," S. J. Song, E. D. Wachsman, S. E. Dorris, and U. Balachandran, Journal of the Electrochemical Society, 150, A790 (2003).

"A Higher Conductivity Bi₂O₃ -Based Electrolyte," N. Jiang, E. D. Wachsman, and S. H. Jung, Solid State Ionics, 150, 347-353 (2002).

"Structural Stability and Conductivity of (WO₃)_x (Dy₂O₃)_y (Bi₂O₃ )_1-x-y ," S. H. Jung, E. D. Wachsman, and N. Jiang, Ionics, 8, 210-214 (2002).

"Defect Chemistry Modeling of High-Temperature Proton-Conducting Cerates," S. J. Song, E. D. Wachsman, S. E. Dorris, and U. Balachandran, Solid State Ionics, 149, 1-10 (2002).

"Effect of Dopant Polarizability on Oxygen Sublattice Order in Phase-Stabilized Cubic Bismuth Oxides," E. D. Wachsman, S. Boyapati, and N. Jiang, Ionics, 7, 1-6 (2001).

"Effect of Oxygen Sublattice Ordering on Interstitial Transport Mechanism and Conductivity Activation Energies in Phase-Stabilized Cubic Bismuth Oxide," S. Boyapati, E. D. Wachsman, and N. Jiang, Solid State Ionics, 140, 149-160 (2001).

"Neutron Diffraction Study of Occupancy and Positional Order of Oxygen Ions in Phase-Stabilized Cubic Bismuth Oxides," S. Boyapati, E. D. Wachsman, and B. C. Chakoumakos, Solid State Ionics, 138, 293-304 (2001).

"A Model for the Spatial Distribution and Transport Properties of Defects in Mixed Ionic-Electronic Conductors: Part I Defect Concentration - Pressure Relationship," K. L. Duncan and E. D. Wachsman, Ionics,6, 145-155 (2000).

"Modeling of Ordered Structures in Phase-Stabilized Cubic Bismuth Oxide," E. D. Wachsman, S. Boyapati, M. J. Kaufman, and N. Jiang, Journal of the American Ceramic Society, 83 [8], 1964-68 (2000).

"Structural Stability and Conductivity of Phase-Stabilized Cubic Bismuth Oxides," N. Jiang and E. D. Wachsman, Journal of the American Ceramic Society, 82 [11], 3057-64 (1999).

"Order-Disorder Transformation in Stabilized Bismuth Oxides and its Effect on Ionic Conductivity," E. D. Wachsman, S. Boyapati, M. J. Kaufman, and N. Jiang, Solid State Ionic Devices, Electrochem. Soc., E.D. Wachsman, J. Akridge, M. Liu, and N. Yamazoe, Ed., 99-13, 42-51 (1999).

"The Spatial Distribution and Transport of Defects in Mixed Ionic-Electronic Conductors: A Rigorous Model," K. L. Duncan and E. D. Wachsman, Solid State Ionic Devices, Electrochem. Soc., E.D. Wachsman, J. Akridge, M. Liu, and N. Yamazoe, Ed., 99-13, 159-171 (1999).

"Aging Phenomenon of Stabilized Bismuth Oxides," N. Jiang, R. M. Buchanan, F. E. G. Henn, D. A. Stevenson, and E. D. Wachsman, Materials Research Bulletin, 29-3, 247-254 (1994).

"Luminescence of Anion Vacancies and Dopant-Vacancy Associates in Stabilized Zirconia," E. D. Wachsman, F. E. G. Henn, N. Jiang, P. B. Leezenberg, R. M. Buchanan, C. W. Frank, D. A. Stevenson, and J. F. Wenckus, in Science and Technology of Zirconia V, S. P. S. Badwal, M. J. Bannister, and R. H. J. Hannink, Eds., Technomic Publishing Co., Penn. 584-592 (1993).

"Structural and Defect Studies in Solid Oxide Electrolytes," E. D. Wachsman, G. R. Ball, N. Jiang, and D. A. Stevenson, Solid State Ionics, 52, 213-218 (1992).

"Spectroscopic Investigation of Oxygen Vacancies in Solid Oxide Electrolytes," E. D. Wachsman, N. Jiang, C. W. Frank, D. M. Mason, and D. A. Stevenson, Applied Physics A 50, 545-549 (1990).

"Solid State Oxygen Kinetics in Er₂O₃ Stabilized Bi₂O₃," E. D. Wachsman, N. Jiang, D. M. Mason, and D. A. Stevenson, in Proceedings of the First International Symposium on Solid Oxide Fuel Cells, Electrochemical Society 89-11, 15-29 (1989).