Thermal stability and crystallisation behaviour of amorphous alumina-yttria synthesised by co-precipitation and combustion synthesis

摘要

Amorphous alumina-yttria powders with 20, 25, 30 and 37.5 mol% yttria were synthesised by a co-precipitation method using the respective nitrates as precursors for identifying a composition with the best amorphous phase stability. Thermal analysis showed that calcination to yield amorphous oxides completed at around 800 degrees C. Crystallisation behaviour investigated by x-ray diffraction (XRD) showed that Al2O3-25 mol% Y2O3 (25Y) had the highest crystallisation temperature of 900 degrees C. Crystallisation involved the appearance and disappearance of metastable hexagonal yttrium aluminate (H-YAlO3) below 1000 degrees C. The phase evolution sequences for all the compositions were studied in detail. All the phases were nanocrystalline, establishing the efficacy of amorphous phase crystallisation as a viable route for the synthesis of nanocrystalline ceramics. Solution combustion synthesis (SCS) was selected to synthesis 25Y with enhanced amorphous phase stability using different proportions of citric acid and ethylene glycol. Thermal analysis revealed that the powder was free of organic residue after heat treating at 650 degrees C. The powder produced with 10 vol% of fuel (citric acid + ethylene glycol) yielded the highest crystallisation temperature of 925 degrees C and the appearance and disappearance of H-YAlO3 between 950 and 1100 degrees C. Fourier transform infrared (FT-IR) spectroscopy revealed that the co-precipitation synthesis might have residual NH4+ and NO3- radicals whereas SCS did not contain any residual carbonate above 700 degrees C.