Synthesis of Nanocrystalline Nb-Aluminides by Laser Ablation Techniques

摘要

This report describes research efforts in the synthesis by laser ablationdeposition of NbAl3 in nano-crystalline forms, as well as process characterization and modeling. Nanocrystalline powders and films were produced under a range of process conditions. Multilayer structures, alternating Al and NbAl3 films were also fabricated. Process yields ranged over 0.1-0.3 micrograms/laser pulse. Variations in yield reflect different gas-dynamics at different process conditions. Mean powder particle sizes ranged over 5-10 nm, increasing with gas pressure and decreasing with laser power. The width of size distribution exhibited a similar dependence. Stoichiometric ratio of Al:Nb varied from 2:1 to 3:1, approaching congruence at low laser power and high gas pressure. Absorption spectroscopy of the ablation plume measured Nb atomic densities corresponding to an ablation rate of a few ng/pulse, implying near-complete nucleation within 100 ns. Temperatures and expansion velocities were also measured via the optical diagnostic; axial expansion velocity correlates well to mean particle size. The numerical model predicted low ion concentration (confirmed by the optical diagnostic) and saturation in the ablation rate at high laser pulse energies due to plasma coupling (confirmed by target mass loss and optical diagnostic).