Phase diagram of CeO2–CoO for nano-sized powders

The CALPHAD (Calculation of Phase Diagrams) framework to calculate the thermodynamic properties of multicomponent systems is extended to account for the change in thermodynamic properties by the size effect in small particle systems. We have applied this framework to the CeO2–CoO binary in order to elucidate the “liquid phase sintering like” fast densification at unusually low temperatures of cobalt oxide doped ceria. The phase diagram for 5 nm particles shows a significant drop of the eutectic from 1645 °C to 1327 °C. The size-dependent thermodynamic stability of intergranular films is examined using CALPHAD approach and melting temperature of 1180 °C is found for 1 mol% CoO doped CeO2 corresponding to an intergranular film thickness between 1 and 4 nm. This result comes close to the unusual low sintering temperatures of 1100 °C, with a “liquid-sintering”-like characteristic that was previously experimentally observed for CeO2 with small amounts of transition metal oxides such as CoO.