Influence of the co-precipitation temperature on phase evolution in yttrium-aluminium oxide materials

Yttrium-aluminium garnet (YAG) powders were synthesized using a reverse-strike precipitation, by adding an aqueous solution of yttrium and aluminium chlorides to dilute ammonia while monitoring the pH to a constant value of 9. After precipitation, the gelly product was washed with dilute ammonia and absolute ethanol for avoiding hard agglomeration during drying. Precipitation and washing procedures were performed at three different temperatures, namely at 5, 25 and 60 °C. After drying, the powders were characterized by DTA/TG simultaneous analysis and then calcined at different temperatures and times. Phase evolution was investigated by X-ray analysis; the evolution of crystallites formation and growth as a function of the temperature was followed by TEM observations. In this paper, a relevant influence of the co-precipitation temperature on the phases appearance, crystallisation path and final homogeneity of these powders was demonstrated. Pure YAG was yielded starting from powder synthesized at low temperature, whereas the precipitation performed at 60 °C leaded to monoclinic Y4Al2O9 appearance near YAG.