Influences of NH4F on transformation and morphology of high-pure α-alumina

High pure alpha-alumina powder and disc samples were prepared from a boehmite (AlOOH) precursor with 5% NH4F. The addition of NH4F significantly decreased the transformation temperature of the transient alumina to alpha-Al2O3. Meanwhile, the morphology of alpha-Al2O3 crystal was changed with the calcinating processes. Two forms of the alpha-Al2O3 crystals were observed: one showed the particle form and the other exhibited the platelet-like shape. With the increase of the calcinating temperature, the particle-like grains were gradually transformed to the platelet-like crystals. At 1300 degrees C for 1 h or at 1200 degrees C for longer time, however, the formed platelet-like shaped crystals changed into particle-like grains again. In the disc samples, the reversible transformation was not found, in which the formed platelet-like alpha-Al2O3 crystals did not disappear with the rise of temperature or the extension of time. It has been proved that the remaining content of fluorine is a key factor to achieve and keep the platelet-like alpha-Al2O3.