In situ X-ray diffraction of the transformation of gibbsite to α-alumina through calcination: effect of particle size and heating rate

A study was conducted on the gibbsite to α-alumina (α-Al₂O₃) transformation dynamics with particular reference to the influence of particle size and heating rate. Coarse- and fine-grained gibbsites were used to examine the transformation paths for the two materials, with particular reference to the upper and lower branch sequences described by Wefers & Misra [Oxides and Hydroxides of Aluminium, (1987), Alcoa Technical Paper No. 19 Revised, Aluminium Company of America]. The main techniques used to assess gibbsite calcination were thermogravimetric differential scanning calorimetry and in situ X-ray diffraction, which provided complementary information on transformation behaviour. Whilst the quantitative phase analysis results indicated that the coarse- and fine-grained gibbsites followed both the upper and the lower branch sequences, detailed analysis of the results highlighted specific differences in the transformation behaviour for the two materials. With the loss of three molecules of water as gibbsite transformed to transition aluminas and finally α-Al₂O₃, there was a high degree of disorder in the crystal structure, resulting in broad and diffuse reflections in the diffraction patterns.