Structural and thermal characterisation of nanostructured alumina templates

Nanostructured anodic aluminium oxide materials containing a two dimensional array of high aspect ratio aligned pores of 200 nm diameter have been heat treated in air at temperatures up to 1340 °C. Thermal analysis shows two discrete irreversible exothermic events at 850 °C and 1020 °C. XRD and 27Al MAS NMR show the progressive development of local and long range order in the heated structures and indicate a reaction sequence of amorphous Al2O3 → θ-Al2O3 → α-Al2O3 (corundum). NMR shows the co-existence of aluminium in 4, 5 and 6-coordinated sites through most of the heating sequence until the stable (6-coordinated) corundum phase is established. Phosphorus impurities incorporated during the membrane fabrication process crystallise as an AlPO4 phase above 850 °C and play an important role in directing the chemical, physical and structural outcomes of the heat treatment.