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.

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► Polyaluminium chloride as α-Al2O3 precursor was obtained by simple dissolution of Al rods in hydrochloric acid.
► A facile low temperature synthesis of α-Al2O3 was obtained from 10 min high-energy ball milled and annealed polyaluminium chloride.
► ∼15 nm α-Al2O3 nanocrystals emended in amorphous alumina matrix were prepared at temperature as low as 400 °C.
► Gradual α-Al2O3 formation up to 1000 °C was obtained.
► 50 nm α-Al2O3 nanocrystals were obtained after annealing at 1000 °C.