Sucrose synthesis of nanoparticulate alumina

The synthesis of nanoparticulate α-Al 2 O 3 at 600 • C using a sucrose-based polymer dispersion technique was reported recently by Das et al. [1]. This is of interest, as γ-Al 2 O 3 is frequently an unwanted intermediate phase in the conversion of nanoparticulate amorphous alu-mina into nanoparticulate α-Al 2 O 3. γ-Al 2 O 3 has a relatively disordered structure, but usually requires temperatures in the order of 1100 • C to transform completely to the stable α-Al 2 O 3 phase [2]. Dispersion of cations in a polymer matrix has been widely used as a " chimie douce " method for the production of oxide materials at low temperatures. The " Pechini " process, where citric acid and ethylene gly-col are co-polymerized to form a resin is a particularly common variant [3]. A highly acidic sucrose solution can self-polymerize or cross-link with polyvinyl alcohol (PVA) to form a heavier and more 3 dimensional resin [4]. The charred polymer also acts as a fuel during calcination, raising the local temperature briefly to much higher temperatures than the equilibrium temperature in the furnace. The extent of cation dispersion in a matrix has been seen to affect phase composition and crystallite size in other systems [3]. Consequently a study was under-taken to ascertain the effect of drastically increasing the dispersion of Al 3+ cations in the matrix by increasing the sucrose content. This would be expected to increase the surface area of the material, but as the sucrose : Al 3+ molar ratio is greater than 18 : 1, calcining at 600 • C should still produce α-Al 2 O 3 according to the hypothesis of Das et al. [1]. The resin was produced by adding 250 ml of 10 M su-crose solution acidified to a pH of approximately 1 with nitric acid, to 3.75 g of Al(NO 3) 3 · 9H 2 O. This formulation yielded a 250 : 1 molar ratio. The intimate mixture was caramelized on a hot plate at approximately 250 • C to reduce the aqueous solution to a thick syrup. After-wards, it was placed in an air-circulating oven at 200 • C for 18 h resulting in a dehydrated charred resin. The resulting carbon-rich precursor was calcined in a muffle furnace at 600 • C for 24 h to produce the aluminum oxide material. Samples of this material were further calcined in a …