La2Zr2O7/LaAlO3 composite prepared by mixing precipitated precursors: Evolution of its structure under sintering

For La2Zr2O7/LaAlO3 composite prepared by aging (100 degrees C/10 h) of the mixture of precipitated precursors of La-Zr-O and La-Al-O, evolution of its structural features after sintering in air in the range of 100-1300 degrees C was investigated by applying XRD, HRTEM, FTIR, Raman and UV-Vis spectroscopies, Al-27 MAS NMR and La-139 NMR. The initial composite forms a highly dispersed and disordered structure retaining considerable concentrations of residual anions (anions from the initial salts, water molecules and hydrogen bound hydroxyls) due to basic properties of La cations. Stepwise removal of these anions by calcinations results in the crystallization of La-2(Al)O2CO3 at 500 degrees C with complete disordering of the system after heating at 700 degrees C. These transitions are accompanied by appearance of AlO4 polyhedra. The La-Zr-O- containing phase appeared for the first time at 900 degrees C as fluorite-like ZrO2 stabilized by La (Al) cations, and LaAlO3 (P2) phase is also observed along with admixtures of La2O3 and La10Al4O21. At 1100 degrees C fluorite-like ZrO2 converts into the La2Zr2O7 (P1) phase, and in the range of 1100-1300 degrees C mixed P1 and P2 oxides are the main phases. For P1 phase, structure becomes more ordered at higher sintering temperatures, while for P2 phase in composite the nanodomain structure is still observed in comparison with individual mixed oxide at 1300 degrees C. The molecular -scale features of domain boundaries in nanostructured La-Zr-O/La-Al-O composite as well as applied inexpensive method of its fabrication provide required bases for a broad practical application of this material for Thermal barrier coatings (TBCs).