Understanding the conductivity degradation mechanism and enhancing the kinetic stability of Er2O3-stabilized Bi2O3 at 600℃

Abstract

Erbia-stabilized Bi2O3 (ESB) possesses higher ionic conductivity compared to conventional ionic conductors such as doped CeO2 and stabilized ZrO2. However, the stabilized Bi2O3 experiences conductivity degradation on extended annealing at 600°C. In order to examine this phenomenon, X-ray diffraction was carried out on ESB for over 400h, and it was found that ESB transforms from cubic to rhombohedral phase. X-ray diffraction analysis revealed that ESB transformed from cubic to rhombohedral phase during first 100h. However, discrepancy in time constant between the evolution of rhombohedral phase (65h) and conductivity degradation(52h) was observed. In order to investigate this discrepancy, the activation energy of low temperature domain after annealing was measured. The activation energy of as-sintered sample was 1.2eV, while that of annealed sample was 1.01eV. Such a drop of the activation energy is partially attributed to the order-disorder transition of the anion sublattice. Thus, it could be concluded that the conductivity degradation of ESB at 600°C resulted from the combined effect of the phase transformation and the order-disorder transition of the anion sublattice. In order to suppress such a decay of ionic conductivity in ESB, aliovalent dopants with higher oxidation state were added into ESB. On this regards, ZrO2 was added to ESB and the stability was evaluated as a function of ZrO2 contents. The solubility limit was 5% and higher concentration led to unwanted secondary phase. The stability was the highest for ESB with 5% ZrO2 and it was found that the lattice constant and bond length of Bi-O was the shortest for this composition. The reason ESB with 5% ZrO2 is the most stable might be attributed to the fact that bonding gets stronger as bond length gets short. Thus, atomic diffusion, involved in the phase transition, becomes hard. Lastly, in order to observe the effect of ionic radii on the long-term stability, the same contents of TiO2, ZrO2 and CeO2 was added to ESB. As a result, the one with the highest stability was ESB with ZrO2, and the one with the poorest stability was ESB with CeO2. ⓒ 2016 DGIST