Synthesis of Ultrafine γ-Al2O3 Powders by Pulsed Laser Ablation

Ultrafine γ-alumina (γ-Al2O3) powders have been successfully produced by laser ablation (aluminum target, Nd:YAG laser, 1.06 μm, 7 ns, filter-target distance 6 cm, oxygen flow rate 1 l/min, pressure 200 Torr, fluence ∼16 J/cm2). The structural properties of the Al2O3 powders have been studied by X-ray diffraction, transmission electron microscopy and Brunauer, Emmet, Teller analysis. The behavior of luminous plume in background oxygen has been investigated by streak and fast photography, and emission spectroscopy. Stoichiometric γ-Al2O3 powder has been obtained at an oxygen pressure of 200 Torr, with a diameter of 11.8 nm and a specific surface area of 160 m2/g. At lower oxygen pressures the average diameter decreased to 6.1 nm, and the surface particle area increased to over 300 m2/g, but the powder composition was altered by the presence of some unreacted aluminum. The measurements on plume expansion revealed initial plume velocities of 0.04–0.18 cm/μs, slowing down with pressure and distance. Emission spectroscopy indicates the presence of both neutral and ionized species, the most prominent lines being Al(I) 394.4 and 396.1 nm, Al(I) 358.7 nm, Al(III) 360.1 and 361.2 nm and Al(II) 466.7 nm; in time, only Al(I) 394.4 and 396.1 nm lines persisted for more than 2 μs. Much weaker lines could be observed for O(II) ions. γ-Al2O3 (0.9 g) powders have been obtained for 1 kWh (laser output energy), with a powder collection efficiency of ∼75%.