MORPHOLOGY AND COMPOSITION CHANGES DURING PREPARATION OF Al_2O_3 POWDERS BY SPRAY PYROLYSIS OF AMMONIUM ALUM
Globular Al 2O 3 powders with diameter ( d 50 ) of 5.82?μm were prepared by spray-drying the solution of ammonium alum combined with pyrolysis process.SEM,XRD,DTA-TG,and Energy Spectrum were used to analyze the morphology and composition changes of powders obtained at different preparation stages.The results showed that the particles prepared by spray-drying were relatively coherent,then were dried,separated,dehydrated and shrunk after drying and pyrolysis processes.As a result,the diameter of powders decreased gradually,and the proportion of bigger particles significantly decreased.The sequence of the phase transition of ammonium alum was found to be NH 4Al(SO 4) 2·12H 2O→NH 4Al(SO 4) 2→Al 2(SO 4) 3→ θ -Al 2O 3→ α -Al 2O 3 according to DTA-TG and XRD results.Sintered at 1370?℃ for 2?h, α -Al 2O 3 powders of high purity was obtained.