Nanosize powders of AlN was successfully synthesized by pulsed laser ablation. The target can be either Al or AlN, with ambient gas nitrogen. With Al targets, typical powder diameters were in the range of 2.5 divided by 25 nm. In order to study the process of powder formation, we have used a high-speed camera and a time-resolved spectrum analyzing system to investigate the behavior of the ablation plasma and the ion species in the plasma. For the diagnostic experiments, the target was Al and the ambient gas was oxygen.
Fast-framing photography was used to study the effect of various parameters on the expansion of the visible plasma plume produced by KrF laser irradiation of a YBa<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>7-x</SUB> target. The ambient gas pressure has a considerable effect on the evolution and structure of the plasma. In vacuum, the visible emission is strongly forward-directed and has a relatively short duration. In the presence of an oxygen atmosphere, a long-lived, highly luminous front is formed, whose expansion may be well described by a drag model or a shock model (spherical or plane shock). Ahead of the luminous front there is weak emission, which for pressures over about 700 mTorr evolves into a highly emissive `tip'. The decrease of energy density on the target by increase of the laser spot (defocusing) leads to a more forward-directed motion of the visible species. Larger energy densities obtained by increasing the energy/pulse lead to a sharper, `V' shaped front. The effect of a biased ring electrode placed 0.5 cm to 2 cm in front of the target was also studied. The emission characteristics in the electrode's presence change greatly. The effect of ring-target distance, voltage magnitude and polarity, and pressure were observed.
Ultrafine alumina powder was produced by aluminum target ablation with a Nd:YAG laser beam (1064 nm wavelength; 340 mJ/pulse energy; 7 ns pulse duration; 10 pps repetition rate), in a 120 Torr O<SUB>2</SUB> atmosphere. A theoretical approach for the ablation process, based on laser energy absorption and energy balance in the target, is used for comparison with the experimental production rate. Three experimental irradiation conditions were chosen: (1) energy density of 5 J/cm<SUP>2</SUP>, considered reference (RDE), (2) lower energy density of 2.5 J/cm<SUP>2</SUP> (LDE) and (3) RDE with auxiliary discharge between an auxiliary electrode and target, triggered by the ablation plume (RDA). Calculated and experimental data of target weight loss show good agreement (135 ng/pulse, calculated, and 100, 110, 120 ng/pulse, experimentally, for RDE, LDE, RDA conditions, respectively). The X-Ray Diffraction spectra of alumina show formation of (gamma) -alumina particles only, especially in LDE and RDA conditions. From Scanning Electron Microscopy size analysis, a decrease of particle size, in LDE and RDA irradiating conditions, is observed. The decrease of laser beam fluence or aiding of ablation using an auxiliary discharge is both favorable for the production of high quality powder, without diminishing the production rate. This can be a good basis for future improvement of the ultrafine powder production process.