GAMMA TO ALPHA TRANSFORMATION IN SPHERICAL ALUMINA POWDERS

The structural mechanism of the (gamma)(--->)(alpha)Al(,2)O(,3) phase transformation in spherical alumina powders precipitated from a sulfate solution was investigated using transmission electron microscopy as the primary analytical technique. Other techniques were utilized to aid in interpretation of the results. These techniques included x-ray diffraction (XRD), infrared reflection spectroscopy (IRRS), and electron spectroscopy for chemical analysis (ESCA). Data from porosity, particle size, density, and specific surface area measurements were also used to explain the transformation.;Stacking faults and twins were observed in thin (gamma)-Al(,2)O(,3) films which were prepared to aid in the understanding of the structure of (gamma)-Al(,2)O(,3). These films were heated in situ in a transmission electron microscope to 1050(DEGREES)C and transformed to polycrystalline (alpha)-Al(,2)O(,3). Evidence of twinned crystallites was also seen in both bulk and sectioned (gamma)-Al(,2)O(,3) particles. These results together with results obtained from the powder transformation suggested a possible transformation mechanism. This mechanism involves the growth of stacking faults in the gamma phase leading to (alpha)-Al(,2)O(,3).;It was found that after calcination in air to 900(DEGREES)C the powder consisted of particles of polycrystalline (gamma)-Al(,2)O(,3). Upon further heating to 1175(DEGREES)C in air the polycrystalline (gamma)-Al(,2)O(,3) particles transformed to essentially single crystal (alpha)-Al(,2)O(,3). Results from particles heated in situ in a high voltage electron microscope and from sectioned particles established that the nucleation site of (alpha)-Al(,2)O(,3) was the surface of an isolated particle or the neck region of two particles in contact.