Microstructure characterization of a cobalt-oxide-doped cerium-gadolinium-oxide by analytical and high-resolution TEM

The microstructure and chemistry of 2 mol.% and 5 mol.% cobalt-oxide-doped Ce0.8Gd0.2O1.9 sintered at different temperatures were examined by a combination of electron energy-loss spectroscopy and energy-filtering and high-resolution transmission electron microscopy. Co grain boundary excess was evaluated. It is found that Co solubility in Ce0.8Gd0.2O1.9 is low at temperatures between 800 and 1150 °C, resulting in a large number of Co precipitates at grain boundaries. With increasing sintering temperature, precipitates grow, influencing the Co redistribution and further altering the segregation amount in the grain boundary. The Co grain boundary concentration is shown to increase with the increase of sintering temperature from 890 to 1050 °C, which is suggested to be due to grain growth. It is found that Co grain boundary segregation induces a detectable variation in the ELNES of Ce-M4,5 and O-K absorption edges, indicating a reduction of Ce atoms in the grain boundary region. The phase of the precipitates was identified as CoO at temperatures between 890 °C and 1150 °C. HRTEM reveals that grain boundaries are less disordered after prolonged sintering time at higher temperature. At a dopant level of 5 mol.% Co oxide in Ce0.8Gd0.2O1.9, the grain boundaries become more disordered, and exhibit a high amount of Co segregation.