Effect of alumina addition on the microstructure and grain boundary resistance of magnesia partially-stabilized zirconia

• Al2O3 induced an increase in monoclinic and Mg-rich phases at the grain boundaries in Mg-PSZ.
• An intergranular siliceous impurity was present in conjunction with the Mg-rich phase.
• The deteriorated ionic conductivity was observed in Mg-PSZs with Al2O3.
• The grain boundary resistance increased due to the presence of intergranular phases.

The electrical properties of 9 mol% MgO–ZrO2 (Mg-PSZ) with 1 mol% Al2O3 and the mechanisms for electrical degradation were investigated using structural, morphological, and electrochemical analyses. The addition of Al2O3 caused an increase in both the monoclinic and the Mg-rich phases at the grain boundaries in the Mg-PSZ. Coarse grains larger than 20 μm and an intergranular layer composed of the Mg-rich phase were identified in a specimen sintered at 1600 °C. This specimen exhibited a minimum of ionic conductivity (4.98 × 10−4 S cm−1 at 700 °C) due to the grain boundary resistance (245 Ω cm2), which dominated the overall resistance. A similar trend was observed over the entire temperature range (600–1500 °C). An intergranular siliceous impurity (SiO2) was present in conjunction with the Mg-rich phase. This impurity and the Mg-rich phase acted as a barrier layer for oxygen ion diffusion. The presence of the intergranular phases (i.e. the monoclinic and Mg-rich phases) contributed to the degradation of the ionic conductivity in Mg-PSZ with an Al2O3 addition.