Low-temperature mechanochemical–thermal synthesis of α-Al2O3 nanocrystals

The great capability of high-energy ball milled basic polyaluminium chloride gel (PACl) which consisted of the monomeric Al3+ ions and polymerized Al3+ species such as [Al13O4(OH)24(H2O)12]7+ with a Keggin like structure to enhance the phase transformation into α-Al2O3 nanocrystals at low temperature was verified. PACl gel 10 min milled and annealed at 400 °C, partially transformed to nanocrystalline α-Al2O3 with the mean XRD crystallite size in the range of 15–17 nm, embedded in an amorphous or transition alumina matrix. Further crystal growth up to ∼50 nm and phase pure α-Al2O3 powder was obtained when heat-treated at 1000 °C. In contrast to this, the non-milled PACl gel transforms to the transition θ-Al2O3 phase at 1000 °C. The evolution of α-Al2O3 nanocrystals was studied by XRD, TEM, selected area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM) methods.