Water-based tape-casting of SOFC composite 3YSZ/8YSZ electrolytes and ionic conductivity of their pellets

Electrolyte supports for SOFC applications were obtained by water-based tape-casting. Slurries of commercial 3YSZ and 8YSZ and mixtures with a mass ratio of 25/75 and 75/25, respectively, were prepared. A commercial acrylic polymer binder with dispersive properties was used. The drying conditions for obtaining crack-free green tapes were studied. Crack formation during drying was greatly affected by the carrier speed at a fixed temperature. Thermal analysis was performed on the green tapes to study the binder burn out properties. Weight loss of the green tapes occurred until 400 °C and a pre-heating step was therefore included in the sintering program. The microstructure of the green and sintered tapes were investigated by SEM. Fully densified tapes were obtained after sintering at 1450 °C. The ionic conductivity of pressed pellets were characterized by impedance spectroscopy in the temperature range of 200–1000 °C. Phase and crystal structure of pressed pellets were analyzed by XRD. The crystallinity varied from a majority of monoclinic structure (pure 3YSZ) to a majority of tetragonal structure with increasing the 8YSZ content. A cubic single phase structure was obtained by further increasing the 8YSZ content. It was shown that the ionic conductivity of the composite samples were modified with respect to the pure materials. At high temperatures the pure cubic structure gave rise to high conductivity. At low temperatures the conductivity was affected by the grain boundary resistance which was the lowest for the pure 3YSZ, the highest for the pure 8YSZ and decreased as a function of increasing 8YSZ content.