Ph.D. Graduates (2012)
Thesis title: Clustering behavior of Yttrium and scandium dopant ions in cubic stabilized zirconia electrolytes at high temperature.
This work investigates the role that dopant clustering plays in the aging phenomena observed in scandia-stabilized zirconia electrolytes. Molecular dynamics simulations have been conducted on supercells containing compositions of xSc2O3 + (11x)Y2O3 + 89 ZrO2 for x = f0; 1; 2; 11g and also on the composition 8Y2O3 + 92 ZrO2. It was discovered that individual dopant-dopant barriers have a relatively small effect on bulk ionic conductivity when the dopants are arranged in small clusters, as the vacancies rarely migration through the small clusters. However, larger clustering of ions can have a significant impact on ionic conductivity, mostly due to localized destabilization of the high-conductivity cubic phase, which forces oxygen ions to migrate through the remaining dopant-rich cubic matrix. Part of the destabilization occurs due to vacancies becoming trapped within the dopant clusters, which forces the Zr ions in the zirconia-rich regions to assume a higher coordinated state with oxygen. However, the zirconium ions are known to abhor eight-fold coordinated states, and these regions form precipitates of low-conductivity phases. These phases have been identifed as tetragonal, and their formation coincides with the simultaneous reductionin ionic conductivity. The tetragonal precipitates are found to be energetically favorable at temperatures of 1073 K, indicating that colloidal stratification of the mixture is inevitable due to cationic migration during long term annealing.
However, clustering of dopants appears to reach a limiting point as the dopants exhibit a repulsive interaction that limits the thermodynamic stability of the zirconia precipitates. Therefore, the dopant clusters will reach a certain maximum size at which point clustering is expected to terminate at an equilibrium state. Scandia stabilized zirconia was observed to provide better conductivity, lower zirconium coordination and superior stabilization of the cubic phase for identical conditions compared to yttria stabilized zirconia, but the energy barriers for dopant clustering was higher for samples containing yttrium, indicating that yttria stabilized zirconia was more resistant to clustering due to aging of the electrolyte.