Synthesis and characterization of lanthanum silicate oxyapatites co-doped with A (A = Ba, Sr, and Ca) and Fe for solid oxide fuel cells

Abstract

The co-doped lanthanum silicate oxyapatites, La_9.5A_0.5Si_5.5Fe_0.5O_26.5 (A = Ba, Sr, and Ca), are synthesized by the high-temperature solid state reaction process. The phase formation and structure properties of undoped lanthanum silicate oxyapatite (La_9.67Si₆O_26.5, LSO), Fe-doped lanthanum silicate oxyapatite (La₁₀Si₅FeO_26.5, LSFO) and co-doped lanthanum silicate oxyapatites (La_9.5A_0.5Si_5.5Fe_0.5O_26.5, A = Ba, Sr, and Ca) are characterized by X-ray diffraction (XRD) and scanning electron spectroscopy (SEM). The effect of co-doping of A (A = Ba, Sr, Ca) and Fe on the microstructure, sinterability and oxide ion conductivity of lanthanum silicate oxyapatites is investigated in detail. The results show that, as compared to LSO and LSFO oxyapatites, co-doping of A (A = Ba, Sr, and Ca) and Fe significantly benefits the sintering and densification process, and enhances the oxide ion conductivity. For co-doped oxyapatites, the oxide ion conductivities are related to the dopant size, the best properties are obtained for the oxyapatite co-doped with Ca and Fe. The co-doped La_9.5Ca_0.5Si_5.5Fe_0.5O_26.5 (LCSFO) oxyapatite is a good electrolyte for SOFCs, with an oxide ion conductivity of 1.39 × 10⁻² S cm⁻¹ at 800 °C and a low activation energy of 90.71 kJ mol⁻¹. The bulk density and oxide ion conductivities of co-doped oxyapatite ceramics increase significantly with the increase of the sintering temperature. The grain bulk and grain boundary resistances of La_9.5Ba_0.5Si_5.5Fe_0.5O_26.5 (LBSFO), La_9.5Sr_0.5Si_5.5Fe_0.5O_26.5 (LSSFO) and LCSFO oxyapatite ceramics are significantly smaller than those of LSO and LSFO oxyapatite ceramics sintered under the identical conditions.