Preparation of zinc and alkaline earth metal co-doped apatite-type lanthanum silicate electrolyte ceramics by combustion method and mechanism of co-doping enhanced conductivity

Abstract

The amorphous and high purity apatite-type lanthanum silicate La9.33A0.2Si5ZnO25.2 (LASZO, A=Ca, Sr and Ba) co-doped with zinc and alkaline earth metal has been successfully prepared at 600 °C in 7-9 min by employing the combustion method, along with revealing the combustion thermochemical mechanism. The LASZO ceramics were sintered at 1400-1500 °C after pressing molding. The XRD, IR, SEM and EIS analyses confirm that Zn2+ replaces Si4+ and A2+ replaces LaⅡ3+, along with the lattice volume increasing with the A2+ radius. The conductivity of co-doped La9.33Sr0.2(Si5ZnO24)OiOi*0.2VO·· at 1073 K is noted as 5.43 × 10-3 S·cm-1, which is higher than undoped, Zn2+ single-doped and A2+ single-doped ceramics. It is due to the generation of the oxygen vacancy VO·· by the Zn2+ doping, formation of an oxygen vacancy VO·· and increased lattice volume leading to a reduced space drag of the oxygen ion transport and A2+ doping increasing the concentration of the interstitial oxygen ions by introducing the additional interstitial oxygen ions Oi*. Therefore, the mechanism of co-doping driven increment in the conductivity is the oxygen vacancy defect-interstitial oxygen concentration-lattice volume composite enhancement.