Abstract
γ-Alumina is a widely used material, while its precise crystal structure and transformation mechanism derived from boehmite have remained unclear in the literature for decades. In this work, quantitative electron microscopy has been applied to study the crystalline structure of γ-alumina and its transformation mechanism from boehmite. Based on Rietveld refinement of electron diffraction patterns, a new tetragonal structure model, with a space group of I41/amd (No. 141), was proposed for the γ-alumina phase, with Al cations on 4a, 8c, 8d and 16g sites and O anions on the 16h site, which could provide better fits than current models. During the boehmite to γ-alumina transformation induced by e-beam irradiation, when the boehmite layers were oriented along the edge-on direction, a shrinkage caused by dehydration was directly observed. Two kinds of boehmite to γ-alumina transformation mechanisms, namely collapse and reaction mechanisms, were elucidated crystallographically in detail with new insights through an intermediate structure, and the reaction mechanism was demonstrated to produce much reduced changes in dimensions and volume, compared with the collapse mechanism. The experimental observations supported the reaction mechanism, which occurred through partial occupation of the dehydrated space by diffusion in the initial stage of the transformation, without the formation of voids that only appeared after the initial stage. Filling tetrahedral interstices of the intermediate structure with Al cations in different ways yields tetragonal or cubic γ-alumina structures, and the tetragonal structure is energetically favorable because of smaller lattice distortions required, compared with the cubic structure. The crystallographic orientation relationships of γ-alumina with the parent boehmite phase deduced from the proposed mechanisms are consistent with the experimental observations.
Highlights
Alumina, or aluminium oxide (Al2O3), is an important ceramic material because of its distinct physical properties, such as low density, high porosity and high specific surface area, high modulus, high melting point, and low thermal conductivity (Poco et al, 2001)
(1) Based on quantitative Rietveld refinement of electron diffraction patterns using various -alumina structural models, better fits were found for cubic Smrcok or tetragonal Paglia models, compared with other cubic or monoclinic models
Among the Smrcok or tetragonal Paglia models, a direct comparison indicated that the former one could provide slightly better or similar fits, much higher Al fraction was found on nonspinel sites than the report by Smrcok et al (2006)
Summary
Aluminium oxide (Al2O3), is an important ceramic material because of its distinct physical properties, such as low density, high porosity and high specific surface area, high modulus, high melting point, and low thermal conductivity (Poco et al, 2001). Among its polymorphs, -Al2O3 (corundum) is a thermodynamically stable phase, which is formed from aluminium oxyhydroxide AlOOH (boehmite) in a thermal transformation sequence as follows (Ingram-Jones et al, 1996; Boumaza, Favaro, Ledion et al, 2009): AlOOH ðboehmiteÞ ! -, - and -Al2O3 are metastable transitional alumina forms. Among these polymorphs, -Al2O3 is the most widely studied alumina because of its wide applications for sorbents, support for catalysts, and materials used in refining and. As recently pointed out by Prins (2020), the main reason is that it has not proved possible to prepare macrocrystalline -Al2O3, and only nanocrystalline -Al2O3 has been prepared, limiting the structure determination by X-ray (XRD) or neutron diffraction (ND) techniques because of peak broadening.
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