Synthesis of lanthanum silicate oxyapatite materials as a solid oxide fuel cell electrolyte

摘要

Lanthanum silicate oxyapatites of structural formula La_(9.33+x)(SiO_4)_6O_(2+1.5x) (0 < x < 0.27) are currently investigated for their high ionic conductivity. We have studied the synthesis of pure apatite powders, especially La_(9.33)(SiO_4)_6O_2 and La_(9.56)(SiO_4)_6O_(2.34), by solid state reactions. We have finalized a synthesis process which limits the formation of the often encountered secondary phases La_2SiO_5 and La_2Si_2O_7 by using an appropriate thermal treatment of the starting mixture. Lanthanum oxyapatite powders were synthesized at a temperature much lower (1200 deg C) than that used in the conventional powder solid-state synthesis routes (T> 1600 deg C). X-ray diffraction, scanning electron microscopy and specific surface area measurements were used to analyze the structural and microstructural changes of the as prepared powders heated at different temperatures. Electrical characterization of pure sintered materials was conducted and showed that the incorporation of extra oxide ions, corresponding to the La_(9.56)(SiO_4)_6O_(2.34) composition, enhanced the ionic conductivity by one order of magnitude when comparing to that of the La_(9.33)(SiO_4)_6O_2 sample.