Impact of fabrication temperature on the stability of yttria doped bismuth oxide ceramics

Abstract

Bi2O3 doped with Y2O3 in the cubic-δ structure is a promising electrolyte material to bring the operating temperatures of solid oxide fuel cells (SOFCs) down to ≤650 °C. However, even in the doped state, the stability of its fast-ionic conductor phase (cubic-δ) at this targeted operating temperature is questionable. In this work, the stability of the electrical conductivity of 28 mol% Y2O3 doped Bi2O3 (YDB) ceramics sintered at different temperatures was investigated. YDB ceramics sintered at 800 °C exhibited a fast, exponential-type conductivity decay with a concomitant cubic to rhombohedral phase transformation upon a 100-hour exposure to 650 °C, in stagnant air. On the other hand, all the YDB ceramics sintered at higher temperatures (900–1100 °C) experienced a slower, linear conductivity decay under the same conditions, but with no observable phase transformation. This difference was explained in the premise that, based on the most recent Y2O3-Bi2O3 phase diagram, YDB sintered at 800 °C had a higher amount of metastable cubic-δ phase than those sintered at higher temperatures. Practically, it was suggested that for SOFC applications, the conventional approach of lowering the sintering temperature to reduce the manufacturing costs would result in poor stability in YDB electrolytes.