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Sol-Gel Derived Alumina Glass: Mechanistic Study of Its Structural Evolution

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Document pages: 24 pages

Abstract: Single-component Al2O3 atomic-level amorphous glass free of some degree of crystallinity cannot be obtained as a bulk by conventional glass forming methodology. So far, such glassy amorphous alumina was obtained only by far from equilibrium conditions, which limit their dimensions to thin film 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 proven. 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 (citrate) ligand interactions, which occur 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. Similar observations are reported here also for aluminum citrate, which indicates that the Al-chelating approach to alumina glass is general.

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