Because of its high oxide-ion conduction and the chemical inactivity, Ce_1-xGd_xO_2-δ fluorites are used as a reaction protection layer between the electrode and the ZrO₂ electrolyte and addition materials to electrode elements for intermediate-temperature SOFCs. The rare-earth ores have 17 kinds of rare-earth elements including Sc and Y. However, Gd exists a low mineral content in the ores. Gadolinium element is used as magnetic devices, magnetooptic disk device, and control rod of the nuclear reactor. Under the present conditions, demand for Gd increases.

The conductivity and sintering characteristics are very important as SOFC components and it is well known that two typical compositions, Ce_0.9Gd_0.1O_1.95 and Ce_0.8Gd_0.2O_1.9, have be commercially available. Although many papers for the properties have been reported, the appropriate compositions of Gd are unclear. The reason for this is that the properties are strongly affected by synthesis methods and sintering conditions.

In the present study, the appropriate compositions of Gd-doped cerias with a Gd content of 0 ≤ x ≤ 0.2, which were prepared using the conventional co-precipitation method, have been discussed from the viewpoint of sintering characteristics, electrical conductivity and its long term stability. Sintering characteristics of the doped ceria samples have a tendency to decrease with increasing Gd content. The maximum conductivity of 2.56×10^-2 S/cm at 600ºC was observed for the sample with 10 mol% Gd dopant. During the annealing process at 600ºC, the conductivity decreased with annealing time due to the grain-boundary resistance. The synchrotron X-ray diffraction analysis showed to be Gd₂O₃ phase in the fluorite and the decrease of lattice parameter. These results suggest that the decrease of the conductivity should be related to the segregation of Gd element at the grain-boundary of the Ce_0.9Gd_0.1O_1.95 fluorite.