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

This work reports the kinetics of α-alumina transformation from γ-alumina, prepared by the sol-gel method from a solution of saturated aluminum nitrate and urea. The γ-alumina phase transformed directly into α-alumina at 750, 800, 850 and 900 ^oC, without any intermediate phases, such as θ- or δ-alumina. The kinetics of γ → α-alumina phase transformation was monitored by determination of α-alumina fraction formed through quantitative X-ray diffraction (XRD), with calcium fluoride added as an internal standard. The crystallised fractions of α-alumina increased sigmoidally with time, indicating that the γ → α-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 γ → α-alumina transformation, as a consequence of the high surface area (425 m²/g) of γ-alumina. The t _0.75/t _0.25 ratio of (2.1 ± 0.1) suggested that the α-alumina growth was plate-like, which was confirmed by the SEM micrograph of α-alumina.