Synthesis and characterization of oxide ions conductors with the apatite structure for intermediate temperature SOFC

Silicated rare earth apatite with formula La9.33(SiO4)6O2 has been prepared by solid state reaction at high temperature. The reagents (La2O3 and SiO2) have first been characterized. Then the synthesis process was studied and optimized. Samples with various densities (from 67 to 92% of the theoretical value) have been obtained by hot pressing at 1400 °C or by sintering at 1500 and 1550 °C. The presence of secondary phases leads to the formation of a liquid phase at temperatures above 1600 °C. A microstructural study has been performed on these samples. Electrical properties of all the samples have been characterized between 400 and 900 °C by the complex impedance method. Conductivity values of about 2 × 10−4 S cm−1 have been measured at 700 °C. The samples present activation energies of less than 1 eV. The influences of the densification ratio and of the microstructure on the electrical properties of the material have been underscored.