Grain Boundary Segregation and Conductivity in Yttria‐Stabilized Zirconia
Grain boundary segregation of Y, Mn, La and Fe in doped and undoped 3YSZ and 8YSZ ceramics and its influence on the intra- and intergranular conductivity is studied by spatially resolved analytical transmission electron microscopy, electron energy loss spectroscopy and impedance spectroscopy. EELS profiles reveal significant enrichment of di- and trivalent ions in an 1-2nm wide grain boundary core with total concentrations of Mn 2+ , Fe 2+ , La 3+ and Y 3+ reaching up to 40cat%. The segregation factors depend on the crystallographic structure of the boundary. Enrichment of di- and trivalent dopants produces a negative grain boundary core charge. EELS scans also show the space charge layer. It extends over a few nanometers and is characterized by an increased O K absorption edge height compared to the grain core and characteristic O K ELNES features. O K edge height and fine structure directly indicate the oxygen vacancy depletion in the space charge layer and allow to track local changes in chemical bonding. Compared to 3YSZ ceramics, cation segregation is less pronounced in 8YSZ ceramics. Inter- and intragranular resistivity of doped and undoped ceramics are measured by impedance spectroscopy in the temperature range of 200-500°C. While the oxygen ion bulk conductivity of YSZ ceramics remains unaffected by the dopants, the grain boundary resistance suffers a significant increase. The behavior is explained by strong interactions of oxygen vacancies with di- and trivalent cations in the grain boundary core and by the lowered oxygen vacancy concentration in the space charge layer.