Effect of Mn addition on the densification, grain growth and ionic conductivity of pure and SiO2-containing 8YSZ electrolytes

1 at.% Mn was introduced to highly pure (~ 30 ppm SiO2) and impure (~ 500 ppm SiO2) 8YSZ electrolytes via a wet chemical method. The densification behavior, sintering mechanism and ionic conductivity of the two groups of samples were systematically investigated. The addition of 1 at.% Mn enhanced the densification and promoted grain growth of both pure and impure samples, which is attributed to the fact that the presence of Mn significantly reduced the apparent activation energy, although it did not change the early-stage sintering mechanism (dominated by volume-diffusion). The addition of Mn decreased the grain interior (GI) conductivity, with almost little impact on the grain-boundary (GB) conduction for the pure samples. However, the combination of silica (a contaminant in impure 8YSZ) with Mn led to a significant deterioration to the GB effect compared with silica alone. Based on the estimated activation energies for GB conduction, it is found that the “constriction model” can be used to describe the GB behaviors of the samples more appropriately than other models. The GB coverage fraction decreased with increasing sintering temperature, leading to a continuous increase in the GB conduction, which is found to be associated with the dissolution of silica into zirconia lattice during sintering at high temperatures.