Long-Term Conductivity Stability of Metastable Tetragonal Phases in 1Yb₂O₃–xSc₂O₃–(99 – x)ZrO₂ (x = 7, 8 mol %)

The present work studies the influence of 1 mol % Yb 2 O 3 codoping on the long-term conductivity stability in the metastable tetragonal phases present in 8–9 mol % Sc 2 O 3 –ZrO 2 . Dense polycrystalline samples are synthesized via conventional solid-state reaction and pressureless sintering methods. The 8 mol % dopant containing ZrO 2 compositions contain the metastable t ′-phase, while the t ″-phase exists in the 9 mol % doped compositions. However, the grain conductivity values in all of the compositions are comparable and insensitive to the type of tetragonal phase present in the samples. The total conductivity follows a similar trend except in the 1Yb 2 O 3 –8Sc 2 O 3 –91ZrO 2 specimen, which shows a low total conductivity because of the resistive grain boundaries. Long-term thermal aging is performed on all of the tested samples at 650 °C for 2000 h in air. The in situ conductivity study suggests that the conductivity declines occur primarily in the first 650 h. The total conductivity loss at 650 °C is higher in the t ′-phase (∼10–16%) compared to the t ″-phase (∼3%). Even though substituting 1 mol % Yb 2 O 3 (for Sc 2 O 3 ) is beneficial in reducing the % conductivity degradation from 16 to 10% in 8Sc 2 O 3 –92ZrO 2, it does not affect the long-term conductivity stability in 9Sc 2 O 3 –91ZrO 2 (∼3%). Thus, the 9Sc 2 O 3 –91ZrO 2 composition exhibits excellent temporal stability of oxygen-ion conductivity with the magnitude of 20.4 mS·cm –1 after 2000 h aging at 650 °C in air.