Rare earth silicates powder synthesis for use as SOFC electrolyte

Abstract

Rare-earth apatite type materials (general formula A10-xM6O26±y; where A=rare earth, M=Si, Al, Ge or P) have been investigated as alternative solid electrolyte materials for SOFC (Solid Oxide Fuel Cells) [1]. This is because of their higher ionic conductivity than conventional zirconia electrolytes, at temperatures between 600ºC and 700ºC. Among apatite rare-earth silicates, the lanthanum silicates, exhibit the highest values of oxide ion conductivity at intermediate temperatures. Lanthanum silicates with general formula La9.33+x (SiO4)6 O2+1.5x (0 ≤ x ≤ 0.67) exhibit high ionic conductivity and low activation energy values at intermediate temperatures (500–700°C). Therefore, they are considered as promising electrolytes for intermediate temperature SOFCs (IT-SOFCs) [2]. Doping into La or Si sites can enhances the ion conductivity of apatite-type lanthanum silicates (ALS). It was observed, for example, to the Mg-doped lanthanum silicate, La9.533(Si 5.7Mg0.3)O26 [3]. The main effort to the actual application of ALS as IT-SOFC electrolyte remains in the difficulty to prepare dense ceramics. High temperatures (1600–1800 °C) and prolonged time are required to obtain a relative density of 90% for ALS prepared by conventional solid-state synthesis route. Secondary phases, such as LaSi2O5 and La2Si2O7 could also remain in the system. Those phases deteriorate the conductivity of the ceramic electrolyte. To synthesize powders with higher sinterability wet-chemical routes, such as, sol–gel, modified sol-gel, molten salts, freeze-drying, co-precipitation and acid citric methods has been practiced. In his work, pure lanthanum silicate La9,56(SiO4)6O2,34 and Mg-doped lanthanum silicate, La9,8Si5,7Mg0,3O26,4 were successfully synthesized by using the sol-gel followed by precipitation method. The crystalline apatite phase of La9,56(SiO4)6O2,34 and La9,8Si5,7Mg0,3O26,4 was obtained by calcination at 900ºC. The density measurements revealed that the samples have a density of higher than 90%.