Kinetics of the γ→α-alumina phase transformation by quantitative X-ray diffraction

This work reports the kinetics of alpha-alumina transformation from gamma-alumina, prepared by the sol-gel method from a solution of saturated aluminum nitrate and urea. The gamma-alumina phase transformed directly into alpha-alumina at 750, 800, 850 and 900 degrees C, without any intermediate phases, such as theta- or delta-alumina. The kinetics of gamma -> alpha-alumina phase transformation was monitored by determination of alpha-alumina fraction formed through quantitative X-ray diffraction (XRD), with calcium fluoride added as an internal standard. The crystallised fractions of alpha-alumina increased sigmoidally with time, indicating that the gamma -> alpha-alumina phase transformation had a nucleation and growth mechanism. The kinetic parameters for this transformation were determined through the Kolmogorov-Johnson-Mehl-Avrami (KJMA) model and the Arrhenius' law. The apparent activation energy, the Avrami exponent, n, and the t(0.75)/t(0.25) ratio determined for the transformation were, respectively, of (201 +/- 4) kJ mol(-1) (2.1 +/- 0.2) and (2.1 +/- 0.1). The apparent activation energy is lower than the values previously reported for the gamma -> alpha-alumina transformation, as a consequence of the high surface area (425 m(2)/g) of gamma-alumina. The t(0.75)/t(0.25) ratio of (2.1 +/- 0.1) suggested that the alpha-alumina growth was plate-like, which was confirmed by the SEM micrograph of alpha-alumina.