Nanostructured alumina from freeze-dried precursors

Abstract

Nanocrystalline alumina has been obtained on the 100 g scale by thermal decomposition of precursors resulting from the freeze-drying of aqueous solutions of different aluminum-containing products, namely aluminum acetate and aluminum l-lactate. Samples prepared at different temperatures (from 873 to 1573 K in steps of 100 K) were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and surface area measurements. In the acetate case, the transformation sequence involves the formation of θ-Al2O3 as an intermediate phase between γ-Al2O3 and α-Al2O3, whereas this θ phase is not observed in the lactate case. TEM and SEM images show the nanoparticulate character of the aluminas obtained at relatively low temperatures, with typical particle size in the 5–10 nm range. Progressive grain growth occurs as temperature increases. Otherwise, the precursor characteristics have a clear influence on the microstructure of the resulting aluminas, as reflected also by the measured BET surface area values. Whereas long aluminum acetate fibers results in open arrays of low aggregated alumina particles, large aluminum lactate sheets lead to comparatively compact alumina microstructures. Nanostructured alumina obtained from the lactate precursor has been reconstituted in a granulated powder with sufficient consistence and flowability to allow it to be thermal sprayed and deposited on a stainless steel substrate. XRD data show that γ-Al2O3 is the major phase in the coating, which includes also α-Al2O3 particles. SEM results offer evidences on the nanostructured character of the coating.