Two typical electrolytes, i.e., 8YSZ (8 mol% yttria-stabilized zirconia) and CGO10 (10 mol% Gd-doped ceria), with Si contents of similar to 30 ppm and >= 500 ppm were prepared, whose grain-boundary (GB) conductivities should be controlled by intrinsic (space-charge layer) and extrinsic (resistive siliceous films) effects, respectively. 1 at% FeO1.5 was loaded into these materials via a conventional mixed-oxide method. A comparative study was carried out to demonstrate how 1 at% Fe addition affected these materials with different levels of SiO2 impurities with respect to sintering, GB and GI (grain interior) conductivities. FeO1.5 was found to be a sintering promoter for both 8YSZ and CGO10 ceramics, but it is more effective to enhance the densification of ceria-based electrolytes. A reduction in sintering temperature of similar to 200 degrees C for 1 at% Fe-doped CGO10 was achieved compared with similar to 110 degrees C reduction for the 8YSZ with the same amount of Fe loading. The effect of FeO1.5 loading on the electrical conduction was found to be different, depending significantly on the impurity level and the types of electrolytes. In general, the loading of FeO1.5 is Positive for ceria-based ceramics since FeO1.5 has a scavenging effect on SiO2 impurity with little effect on the GI conduction. Although the scavenging behavior of FeO1.5. was also found in the impure 8YSZ, it led to a significant reduction in the GI conductivity. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.