Electrical Properties of lanthanum silicate apatite electrolytes prepared by an Innovative Chemical Route

Apatite-type lanthanum silicates have been widely used as solid oxide fuel cell (SOFC) electrolytes due to their relatively high oxide ionic conductivity (around 2×10⁻³ Scm⁻¹ at 800 °C) and low activation energy for conduction. In the present study, we have focused on the preparation of apatite powders of composition La₁₀Si₆O₂₇, synthesized by a novel combination of sol-gel and co-precipitation methods, and the correlation between microstructure and electrical properties of sintered ceramics. The herein proposed synthetic approach is advantageous in reducing energy consumption and processing time, key elements to decrease the total manufacturing cost.

 The as-prepared powder was calcined between 500 and 900 °C, with dwell time from 2 to 4h. Phase formation of powders was determined by X-ray diffraction. The pure apatite powder was further analyzed by scanning electron microscopy (SEM). The sintering behaviour was studied by dilatometry in the temperature range of 30-1500 °C. The effects of the sintering temperature, ranging from 1350 to 1550 °C, on the microstructure and electrical properties were investigated by SEM and impedance spectroscopy. Ionic conductivity properties were measured in the 200 – 800 °C temperature range in air atmosphere.

 Structure of the calcined powders, characterized by X-ray diffraction (XRD) and Rietveld refinement, indicated the formation of single – phase crystalline lanthanum silicate at 900 °C. Fig. 1 shows a representative impedance spectrum for the La₁₀Si₆O₂₇ pellet sintered at 1400 °C and measured at 600 ºC in air. The features in this plot can be attributed to contributions of grain boundary at intermediate frequency and electrode polarization at low frequency. Fig. 2 shows the calculated total electrical conductivities, for the pellets sintered at 1400 and 1450 °C, as a function of temperature following the Arrhenius law. The lanthanum silicate electrolyte sintered at 1450 °C exhibited electrical properties comparable with samples prepared by solid state reaction and sintered at 1600 °C [1].

 The findings in this study are an indication for the potential of an innovative chemical route, based on a combination between sol-gel and co-precipitation methods, for the synthesis of lanthanum silicate powders with apatite-type structure. Nanoparticles were obtained even after calcination at temperatures as high as 900 °C. Densities above 90% of the theoretical one were achieved after sintering at 1450 °C. The total electrical conductivity at 600 °C was comparable with the best values reported in the literature. The lower activation energy of total conductivity was 80 kJ/mol.

 Acknowledgements

The authors acknowledge CAPES and FCT (Portugal) for the financial support.

 Reference

[1] E. Béchade et al. Journal of the European Ceramic Society 28, 2717 (2008).