Tressler Seminar - Juan Claudio Nino - "Point Defect Engineering and Dynamics in Fluorites: Lessons from Doped Cerium Oxide"

26 Hosler

"Point Defect Engineering and Dynamics in Fluorites: Lessons from Doped Cerium Oxide"

Engineering the point and extended defects within functional materials is an essential component in the optimization of their properties and performance.  Focusing on rare-earth doped ceria as a model system, two cases of defect engineering are here showcased. First, the phenomenon of bias induced mixed ionic and electronic conductivity in gadolinia doped ceria (GDC) bulk ceramics and thin films deposited using RF sputtering is demonstrated.  Electrochemical impedance spectroscopy is used to track the defect dynamics via changes in the electrical conductivity of the compounds under applied dc biases as a function of frequency and temperature.  It is found that the application of dc bias produces a reversible decrease in both the grain and grain boundary resistances for GDC, and allows for the injection of electrons, thus leading to mixed conduction.  Here we showcase this unique behavior and analyze the data through residual optimization of equivalent circuits models in order to gain insight on the underlying conduction and diffusion mechanisms at play.  As a second example, using atom probe tomography we show that in Nd-doped ceria the dopants and oxygen vacancies segregate at grain boundaries, with the compositional difference extending up to 5-6 nm from the structural center of the grain boundary.  This high concentration of dopants and defect-defect interactions at the grain boundaries makes them highly resistive and causes a drop in the overall ionic conductivity and performance. To overcome this problem, we demonstrate that in ceria sintered under a reducing atmosphere and lower temperatures compared to conventional sintering, the preferential dopant segregation is curtailed and a corresponding increase in the ionic conductivity of doped ceria is obtained.  Opportunities to apply and extend these defect engineering approaches to other functional ceramics will be discussed.