Mechanical Properties of Ceria Based Oxygen Ionic Conductors for SOFC

The fracture strength and creep rate of rear earth (Y and Gd) doped ceria are systematically studied from the viewpoints of dopant concentration, oxygen partial pressure and temperature dependences. Fracture strength and creep rate are measured by modified small punch test and four point bending method, respectively. From results of fracture test, the highest fracture strength is obtained on the samples sintered at 1600 °C for Y and Gd-doped CeO₂. The unique temperature dependence of fracture strength on doped CeO₂ is observed. It shows the local minimal value at around 600 °C and the fracture strength increases with increasing temperature. The fracture surface structure drastically changes with changing temperature observed by SEM. Since we observed the close coincidence between the fracture strength and the ratio of transcrystalline fracture surface for all samples, it is concluded that the increase of fracture strength at high temperature in doped CeO₂ can be attributed to the temperature dependence of transcrystalline fracture strength. Typical creep curves of 2, 5, 10 and 20 YDC were measured under constant load in air. The creep rate decreases with increasing the dopant concentration. From the analysis of creep properties, the creep is controlled by cerium vacancy diffusion and change of ceria vacancy concentration decreases creep rate.