Solute segregation and grain-boundary impedance in high-purity stabilized zirconia

Solute segregation at grain boundaries has been correlated with grain-boundary conductivity in high-purity 15-mol%-CaO-stabilized ZrO2. STEM measurements of solute coverage show that the segregation of impurity silicon (present at bulk levels <80 ppm) is grain-size dependent. The boundary coverage of silicon can be systematically varied by varying grain size at concentrations low enough that a discrete siliceous film does not form. The cosegregation of calcium and silicon is observed. The grain-boundary solute coverage (Gamma(Si+Ca)) has been correlated with the specific grain-boundary conductivity ((sigma(gh)(sp)) determined using impedance spectroscopy. At monolayer segregation levels, the specific boundary conductivity is less than the bulk conductivity by a factor >10(3) at 500 degrees C. At the lowest levels of segregation achieved, <0.1 monolayer, sigma(gb)(sp) remains similar to 10(2) less, and possibly represents an ''intrinsic'' limiting value for the grain boundary. Comparison with Y2O3-doped ZrO2 suggests similar behavior in this system. The control of grain-boundary segregation through purity, microstructure, and thermal history is discussed from the objective of engineering the grain-boundary impedance of polycrystalline ionic conductors.