Controlled synthesis of Bi2O3–YSZ composite powders and their sintering behavior for high-performance electrolytes

Bismuth oxide (Bi₂O₃) is a promising additive to decrease the sintering temperature of yttria-stabilized zirconia (YSZ)-based electrolyte for solid oxide fuel cell application. However, Bi₂O₃ tends to grow into large column bars (>50 µm) in a chemical coprecipitation method, which dramatically limits the mixing uniformity of Bi₂O₃ and YSZ, even much worse than that of mechanical mixing. In this study, the reaction temperature was increased from room temperature to 90°C to increase the number of nucleation during the violate reaction between Bi³⁺ solution and YSZ suspension in NaOH. On this basis, the violence of the reaction was further moderated by adding half of NaOH first, then YSZ powders and the other half of an NaOH solution. The size of Bi₂O₃ was further decreased to sub-micrometer and Bi₂O₃ was homogeneously mixed with YSZ particles, even when its addition amount was as large as 20 mol%. These composite powders effectively promoted the sintering behavior of YSZ. The sintering temperature of YSZ was decreased to 900 and 1000°C with 10 and 5 mol% Bi₂O₃ doping, respectively. Increasing the doping ratio induced severe volatilization of Bi₂O₃ and pore formation. Raising the sintering temperature (no more than 1200°C) enhanced the doping effect of Bi₂O₃ into the YSZ lattice but induced instability in the YSZ crystal structure.