Influence of mechanical activation on boehmite (γ-AlOOH) to γ-Al2O3 transformation: differential scanning calorimetric (DSC) analysis

The influence of mechanical activation on boehmite (γ-AlOOH) to γ-Al2O3 transformation is investigated using
differential scanning calorimetry (DSC). Boehmite prepared by the thermal decomposition of a typical Bayer
process gibbsite was mechanically activated (MA) in a planetary ball mill for different durations. The comparison
of heat flow curves of non-activated and the activated samples indicated shifting of the γ-Al2O3 transformation
endotherm towards a lower temperature. Based on peak area analysis, the enthalpy of phase transformation (ΔH)
decreased from 13.21 to 1.4 kJ/mol as the activation time increased from 0 to 240 min. The kinetic analysis
revealed a lowering of activation energy from 220 to 154 kJ/mol. A plausible explanation for the DSC results is
presented in terms of the presence of energetically different hydroxyl ions and changes in the bond character-
istics due to structural degradations occurring during MA. The nature of γ-Al2O3 formed changes with me-
chanical activation. Rietveld analysis revealed that the diffraction patterns of γ-Al2O3 are better explained if a
bimodal crystallite size distribution is assumed.