Synthesis and characterization of doped La9ASi6O26.5 (A = Ca, Sr, Ba) oxyapatite electrolyte by a water-based gel-casting route

Alkaline elements doped lanthanum silicate oxyapatites, La9ASi6O26.5 (A = Ca, Sr, Ba), are synthesized by a water-based gel-casting technique as electrolytes of solid oxide fuel cells (SOFCs). The phase formation, thermal and structure properties of alkaline elements doped apatite-type silicates are characterized by X-ray diffraction (XRD), thermal expansion coefficient, scanning electron spectroscopy and Raman spectroscopy. The effect of calcination and sintering temperatures on the conductivity was investigated in detail at 300–800 °C by the complex impedance spectroscopy. It has been found that the distribution of grain and grain boundary resistances depends strongly on powder calcination temperature. However, sintering temperature has the most critical effect on the overall conductivity of the apatite silicates. The significantly enhanced conductivity with slightly increased activation energy of the alkaline elements doped lanthanum silicates implies that the broadening and shifting of the bands in the stretching region of the Raman spectra is most likely associated with the increased population of interstitial oxygen. The best conductivity is 1.18 × 10−3 S cm−1 at 500 °C and 2.54 × 10−2 S cm−1 at 800 °C for La9BaSi6O26.5 apatite sintered at 1600 °C, which is one order of magnitude higher than that of La10Si6O27 under the identical synthesis and test conditions.