Effect of nitrogen pressure and oxygen-containing impurities on self-propagating high temperature synthesis of Si3N4

The effects of nitrogen pressure and oxygen-containing impurities on self-propagating high-temperature synthesis (SHS) of Si3N4 were studied. The growth mechanism of β-Si3N4 columnar crystal was investigated in detail using a gas-releasing method. The growth of α-Si3N4 occurred by a vapor-phase reaction, and the columnar β-Si3N4 grew by the vapor–liquid–solid (VLS) mechanism. Increase of the nitrogen pressure promoted volatilization of Si, and formation of α-Si3N4 during combustion, but did not favor the transition of the initial α-Si3N4 to β-Si3N4 during cooling. Therefore, the α/β ratio in the product increased with increasing nitrogen. Moisture in the nitrogen was helpful to the conversion of Si to α-Si3N4. Liquid formation, required by the VLS mechanism, depended upon the impurities containing oxygen in the reactant, rather than that in the nitrogen gas. The aspect ratio of columnar β-Si3N4 depended on the oxygen content in the reactant and the distribution coefficient of oxygen in the solid and liquid. Short β-Si3N4 columnar crystal was generally achieved because oxygen cannot get a supply from nitrogen gas. Increase of the oxygen content in the reactant resulted in a great improvement of the ratio of length and diameter of β-Si3N4.