Title

High-energy milling of silicon and graphite mixtures and synthesis of nanostructured Si3N4/SiC composites

Date of Completion

January 1999

Keywords

Engineering, Materials Science

Degree

Ph.D.

Abstract

In this study, high energy milling, a modification of mechanical alloying, has been employed to synthesize nanostructured Si3N4/SiC composites. Some fundamental understandings on the processing and materials have been achieved. It was shown that high energy milling of silicon and graphite mixtures in NH3, followed by annealing in N2 is a viable approach to produce nanostructured Si3N4/SiC Composites. The characteristics of final products are associated with the defect structure and absorbed nitrogen introduced during milling, which in turn depends on milling parameters. The introduced defects and absorbed nitrogen change the energy state of reactants, minimize the kinetic barrier and alter the formation mechanism, and thus enhance the compound formation. ^ The microstructure of the milled powder and its evolution during milling have been extensively studied. It has been found that significant refinement of particles and crystallites occurs at the early stage of milling (within ~10-hours of milling), after which the average particle size and crystallite size decrease gradually and approach to a lower limit of ~100 run and 6.0 run, respectively. It is proposed that the refinement of Particle sizes is dominated by the fragmentation process, whereas the refinement of crystallite sizes is dictated by plastic deformation. Substantial amorphization does not occur until most of the crystallites have reached the average size of 6.0 nm. The crystallite-refinement-induced amorphization has been identified as the major mechanism for the nanocrystalline-to-amorphous transformation. ^ In addition, the sorption of a substantial amount of nitrogen has been found in the milled samples, although the equilibrium phase diagram indicates that the solubility of nitrogen in silicon is extremely small (2.0 × 10–19). This enhanced sorption, including adsorption and absorption, is believed to be related to the defect structure generated during milling and the mechanical excitation during ball collisions. The sorbed nitrogen is primarily dissolved in the amorphous Si phase and the concentration can reach a Si-to-N molar ratio of about 43 to 57. In this research, the enhanced sorption was employed to pump nitrogen into silicon and graphite mixtures for production of Si-N-C precursors. ^