Grain size dependent grain boundary defect structure: case of doped zirconia Original Research Article

The electrical properties of 3 mol% Y2O3 doped ZrO2 were measured by impedance spectroscopy as a function of grain size, and the microstructure studied by SEM and HREM. In spite of the very clean grain boundaries, the grain boundary conductivity was still found to be ∼2 orders of magnitude lower than the bulk conductivity, while it increased with decreasing grain size. The low grain boundary conductivity, according to the Schottky barrier model, is due to the depletion of oxygen vacancies in the grain boundary space charge layers. Within this framework, the grain boundary space charge potential and the concentration of oxygen vacancies in the space charge layers were calculated; it was found that the space charge potential decreased, but the oxygen vacancy concentration increased with decreasing grain size. Analyses of literature results for 8.2 mol% Y2O3 and 15 mol% CaO doped ZrO2, respectively, revealed similar phenomena.