Direct synthesis of La9.33Si6O26 ultrafine powder via sol–gel self-combustion method

Single phase La9.33Si6O26 ultrafine powder, as a kind of highly activated precursor to prepare medium-to-low temperature electrolyte for solid oxide fuel cells (SOFCs), has been successfully synthesized via a non-aqueous sol–gel and self-combustion approach from the starting materials: lanthanum nitrate (La(NO3)3·6H2O), citric acid, ethylene glycol (EG), tetraethyl orthosilicate (TEOS) and ammonium nitrate. The details of gel's self-combustion were investigated by DTA–TG and the structural characterization of as-synthesized powder from self-combustion was performed by XRD and SEM. The results show that La9.33Si6O26 single phase of apatite-type crystal structure can be directly synthesized by sol–gel self-combustion method without further calcinations on the condition that the molar ratio (R) of NO3− to citric acid and ethylene glycol being 6:1. Such powders composed of well-dispersed particles with an average size of 200 nm and a specific surface area of 5.54 m2/g. It can be sintered to 90% of its theoretical density at 1500 °C for 10 h, about 200 °C lower than the sintering temperature for the powder derived from traditional solid reactions. The sintered material has a thermal expansion coefficient of 9.2 × 10−6 K−1 between room temperature and 800 °C.