Sol-gel derived alumina glass: Mechanistic study of its structural evolution

Single-component Al₂O₃ atomic-level glass free of some degree of crystallinity cannot be obtained by conventional melt quenching methodology. So far, glassy alumina was obtained only by far from equilibrium conditions, which limit their dimensions to thin films or nanoparticles. In 2007 we reported a sol-gel Al-lactate route to amorphous alumina (L. Zhang et al., J. Non-Cryst. Solid, 353, 1255), which recently was proven to be glassy on the atomistic level. This, to the best of our knowledge, is the first time that a real alumina glass has been obtained. Therefore there is much interest in understanding the mechanistic details of this aqueous sol-gel process which leads to homogeneous Al₂O₃ glass, as a guideline for the formation of other non-trivial ionic-oxide based glasses. Detailed ²⁷Al NMR analysis supported by several other analytical methods, indicate the formation of a metal coordination network based on aluminum cations-lactate ligand interactions, which occurs during the sol-gel transition. This water soluble metal coordination network demonstrated a facile reversible sol-gel-sol transition based on concentration-driven formation and breaking of the intramolecular non-covalent interactions between the monomers. Subsequent annealing of the xerogel by gradual heating to 500–700 °C promotes the formation of the interconnecting Al–O–Al bonds, concomitant with the removal of the lactate ligands upon heating, resulting in pure alumina glass as the final product. A glass transition temperature (T_g) of Al₂O₃ glass was observed at ∼670 °C. Similar observations are reported here also for aluminum citrate, which indicates that the Al-chelating approach to alumina glass is general.