Synthesis and characterization of nanocrystalline niobium aluminides by laser ablation technique.

摘要

This thesis presents a study of the synthesis and characterization of nanocrystalline Nb-aluminides by laser ablation in an ambient gas atmosphere. The goal of this work is to establish the science base for the nanocrystalline synthesis by this method by studying the relationships between processing conditions and physical characteristics of products. Using a newly developed ablation system, nanocrystalline Nb-aluminides have been synthesized by varying the processing variables such as laser energy density and He backing gas pressure. Ablation rate is relatively insensitive to the variation of He gas pressure. However, we observe a threshold laser energy density above which the ablation rate does not increase. Collection rate of powders depends strongly on the He backing gas pressure. We observe that no nanocrystalline material is formed at 0.1 Torr and the collection rate peaks near 1.0 Torr at all laser energy densities. By conventional transmission electron microscopy (TEM), we observe that the mean diameter of the nanocrystalline particles is 5 to 13 nm depending on the processing condition. Analysis of electron diffraction patterns suggests that the crystal structure of the majority of particles is of the NbAl 3 (DO22) type. Through energy dispersive spectroscopy (EDS), we observe that the atomic ratio of Al/Nb decreases from the stoichiometric composition of NbAl3 with increasing laser energy density and decreasing backing gas pressure. High-resolution electron microscopy (HREM) reveals the atomic structure of the NbAl3 particle of the DO22 type at all the processing conditions. Nb3Al particles of the A15-type crystal structure are identified at lower gas pressure and higher laser energy. The twinning structure in a NbAl3 particle is revealed. X-ray diffraction reveals that particles of small- and large-modes are synthesized at the same time and the volume ratio of the two distributions is between 0.7 and 1.1. Lattice constants of nanocrystalline NbAl3 intermetallic compounds are in agreement with the lattice constants of the ordinary NbAl3 despite the change of the processing condition.