The growth mechanism of self-propagating high-temperature synthesized Si₃N₄

The growth mechanism of Si₃N₄ made by the self-propagating high temperature synthesis (SHS) was investigated using a gas-releasing method. The intermediate growing morphology of the columnar β-Si₃N₄ crystals by the vapor-liquid-solid (VLS) mechanism was viewed in the gas-released samples. The liquid phase required for the growth of β-Si₃N₄ depends upon the oxygen-containing impurities in the reaction product, but not the metallic impurities in it, nor the oxygen impurity and water vapor in nitrogen. In the growth process, β-Si₃N₄ is precipitated continuously from the liquid phase and the oxygen redistributes itself between the liquid phase and the solid phase so that the oxygen content in liquid phase gradually decreases and can not be compensated and thus the β-Si₃N₄ crystals grow into short columns. Increasing the oxygen content in the reaction product can obtain β-Si₃N₄ columns with high height to diameter ratios. Suitable amounts of oxygen and water vapor in nitrogen are beneficial to increase the ratio of α phase to β-Si₃N₄ phase in the product.