Thermally Induced Formation of Transition Aluminas from Boehmite

Abstract

Structural changes occurring during the conversion of boehmite over the transient phases γ-, δ- and θ-Al2O3 to corundum were investigated. During the whole transition process, the specific surface area was reduced. The X-ray and electron diffraction experiments revealed that the transition from γ- to θ-Al2O3 is continuous and that it proceeds over different δ-states rather than over a distinct δ-Al2O3 phase. The reduction of the specific surface area was most pronounced prior to the onset of the phase transition from γ-Al2O3 to the first δ-state. In γ-Al2O3, approximately 45% of Al cations were found to be located on tetrahedral sites within a slightly tetragonal distorted oxygen sublattice.