Re-examination of effects of alumina on bulk and grain boundary conductivities of CGO solid electrolytes

Identification of secondary phases and their dependence on composition and firing conditions were used as guidelines to re-examine effects of alumina on bulk and grain boundary properties of ceria-gadolinia electrolytes. Solubility of alumina may contribute to enhance bulk properties of samples with low contents of gadolinia, probably by enhancing the concentration of oxygen vacancies as prevailing charge carriers. This gain is slightly reverted for excessive addition of Al, probably combining effects of depletion of Gd in the fluorite phase, due to onset of GdAlO3, and microstructural changes. For samples with intermediate contents of Gd one found that alumina affects mainly the grain boundary properties, as a consequence of depletion of Gd, even for relatively low additions of Al, possibly due to heterogeneities induced by preferential location of secondary phases at grain boundaries. Additions of Al may exert a slight positive effect on bulk conductivity of samples with relatively high contents of Gd, i.e., above the composition with highest bulk conductivity. In this case, precipitation of GdAlO3 may contribute to optimize the final composition of the fluorite phase.