Parameters influencing thermal shock resistance and ionic conductivity of 8 mol% yttria-stabilized zirconia (8YSZ) with dispersed second phases of alumina or mullite

Improved thermal shock resistance for cubic 8 mol% yttria-stabilized zirconia (8YSZ) used in fuel cells and oxygen sensors can be achieved by the addition of higher thermal conductivity second phases. This work compares 10–20 vol% alumina (α-Al2O3) and mullite (3Al2O3·2SiO2) additions that increase thermal conductivity, reduce grain size, and increase strength and fracture toughness of 8YSZ. Improvements in thermal shock behavior correlate best with increased thermal conductivity. Second phase additions result in a smaller grain size that reduces the ionic conductivity, measured by electrochemical impedance spectroscopy, primarily through the creation of a higher density of blocking grain boundaries. The blocking effect correlates with decreasing grain size in 8YSZ but also is strongly influenced by the wetting behavior and distribution of intergranular phases. The addition of an appropriate dilute second phase of higher thermal conductivity, however, may compensate for a slightly lower ionic conductivity in certain applications such as oxygen sensors.