Synthesis of and densification of oxygen-conducting La_0.8Sr_0.2Ga_0.8Mg_0.2O_2.8 nano powder prepared from a low temperature hydrothermal urea precipitation process

In this work, the solid electrolyte powder having a composition of La_0.8Sr_0.2Ga_0.8Mg_0.2O₃ (LSGM) was prepared by the hydrolysis of a urea-containing (CO(NH₂)₂) solution under hydrothermal conditions and followed by calcination at a temperature range between 350 and 900 °C in air. The particles precipitated after reaction between ammonia, decomposed from urea, and a stock solution containing metal nitrates. Samples characterizations were performed using X-ray diffractometry (XRD), thermogravimetry analysis (TGA), differential thermal analysis (DTA), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Thermal analysis was used to verify the decomposition of the precursor. The results show that crystalline phase of GaO(OH) was formed in the dried precursors. After heating up to ∼350 °C, the crystalline La(OH)₃ was gradually formed. Subsequently, the La(OH)₃ decomposed to La₂O₃ at temperatures ranging from 500 to 800 °C. Several reflections based on LSGM perovskite were observed at 800 °C. After the precursors were further calcined at 900 °C for 12 h, single-phase perovskite, LSGM, was formed without any traceable impurities based on XRD diffraction analysis. TEM observations show LSGM powders with an average particle size of ∼150 nm after calcination at 900 °C for 12 h. The LSGM powders agglomerated together due to the necking behavior at 1100 °C. This result also indicates that LSGM powders started to densify at relatively low temperature when the powder is synthesized using a hydrothermal urea precipitation process. The hydrothermally processed LSGM samples were finally sintered at 1400 °C for 3 h. The relative density of sintered LSGM bulk sample was as high as 98%. The electrical conductivity of hydrothermally processed LSGM was measured to be 0.056 S/cm at 800 °C.