| A fundamental study of the grain growth anisotropy of silicon nitride in liquids has been conducted in this thesis. The research employs a set of model experiments with dispersed silicon nitride crystals growing in different liquids. Through a systematic variation of the liquid compositions, the general growth behavior of β-Si3N4 has been identified and investigated. The results of a statistical analysis of the size distribution of β crystals, in length and width directions, have provided direct evidence that the kinetics is closely related to the successive phase transformations in the system, from α-Si3N4 to β-Si 3N4 and to β′-SiAlON. Velocity of crystal growth has also been calculated by analyzing the dynamics of crystal populations, which confirms that large crystals grow while small crystals shrink in the system at all time. The anisotropy in growth velocity is found to correlate to the ratio of Si to metal ions (Al, Mg and Y) in the system. In addition, electron microscopy has revealed a novel shape transition in silicon nitride crystals: the end caps evolve from a convex shape to a concave shape as the crystal width and length velocity increase. This leads to a newly proposed growth mechanism, which envisions transverse flux to aid length growth due to the inability of side surfaces to accommodate matter (i.e., under interface control). The shape transition is thus a result of long relaxation time at the end caps, which arises when the width is too wide and the length growth is too fast for the transverse flux to smooth out the end caps. Mathematical analysis of this shape transition, based on coupled surface diffusion equations, has been performed analytically and numerically (using finite element method, FEM). This analysis has identified a dimensionless parameter that governs shape transition and produced simulated crystal shapes that closely resemble the observed morphologies. From the above study, an improved understanding of the kinetics of silicon nitride, concerning phase transformation, growth dynamics, and interface processes, has been obtained. |
