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

Abstract

Silicated rare earth apatite with formula La 9.33(SiO 4) 6O 2 has been prepared by solid state reaction at high temperature. The reagents (La 2O 3 and SiO 2) 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.