Grain boundary ionic conductivity of yttrium stabilized zirconia as a function of silica content and grain size

Impedance spectroscopy at temperatures from 350 to 700 °C and analytical electron microscopy were used to characterize grain boundary conductivity and grain boundary segregation of SiO2 in 8 mol% yttrium stabilized zirconia (Y-CSZ). Colloidal silica in the amount of 1–10 wt.% (2.5–25 vol.%) was added as an intergranular phase. Various grain sizes were produced by sintering or annealing at temperatures of 1350–1600 °C for times from 0.1 to 100 h. The addition of intergranular SiO2 led to decreased grain size (due to grain boundary pinning), increased grain boundary volume, and reduced total ionic conductivity. An increase in grain boundary width, from approximately 4 to 8 nm, was correlated with higher temperature anneal treatments and greater amounts of SiO2. High temperature anneals of samples with silica reduced the grain boundary volume fraction by increasing the grain size and restored much of the total conductivity lost by adding silica. The grain boundary specific conductivity and the total ionic conductivity were not significantly affected by SiO2 additions less than 5 wt.% when samples of a similar grain size were compared. This led to the conclusion that limited use of intergranular additives to create superplastic Y-CSZ should not degrade the ionic conductivity for a specific engineered grain size.