New approach to develop dense lanthanum silicate oxyapatite sintered ceramics with high conductivity

Recently, apatite-type lanthanum silicate, La9.33(SiO4)6O2, and its analogues with cationic doping in the La/Si site have been attracting significant interests due to their high oxide ion conduction. However, the well-known synthesis method is solid-state reaction, which suffers from problems such as the high sintering temperatures for dense pellets and the impurity phases in the final ceramics. In this paper apatite-type lanthanum silicates were prepared by precipitate method combining with an azeotropic-distillation process. The influences of powder calcination temperature on the relative density were investigated and the optimal calcination temperature was proved to be 1100 °C. The main phase of the powder calcined at 1100 °C was lanthanum silicate La9.33(SiO4)6O2 with trace impurities La2O3 and La2SiO5. However, dense pellet (96.63%) with pure lanthanum silicate phase was obtained at relatively low temperature (1550 °C). Electrical measurements have shown that conductivity behaviors of the sintered pellets were also calcination-temperature dependent. The highest overall conductivity values, σ500 °C = 1.58 × 10− 3 S cm− 1 and σ800 °C = 2.21 × 10− 2 S cm− 1, were obtained for pellet prepared from powder calcined at 1100 °C, which was much higher than that synthesized by other methods. The samples presented activation energies of 0.66 ± 0.01 eV. The results in the present study demonstrate that the precipitate-azeotropic-distillation approach is an alternative method to synthesize lanthanum silicate powders.