Synthesis of equiaxed Si3N4 from Si(NH)2 by adding seeds formed by in situ nitridation of silicon

Abstract

High-quality Si3N4 powder is a prerequisite for the preparation of high-performance Si3N4 ceramics. In this paper, Si3N4 powders with regular morphology were successfully prepared by pyrolysis of Si(NH)2 using silicon as a nucleation inducing agent. The mechanism of the silicon in regulating the morphology and particle size of Si3N4 powders was investigated. The results show that when Si(NH)2 is pyrolyzed directly at 1500 °C under N2 atmosphere, the products are mainly whiskers accompanied by some particles with irregular shape. In the presence of silicon, the powder particles exhibit a regular hexagonal prismatic morphology. With the increase of silicon content, the whisker content decreases obviously, and the whisker disappears completely when the silicon content exceeds 10 %. Furthermore, the particle size of Si3N4 powder decreases with the increase of silicon content, and the particle size is the smallest when the silicon content is 15 wt%, which is about 802 nm. The nucleation and growth process of Si3N4 powder particles can be divided into three stages. Firstly, silicon reacts with N2 to form Si3N4 nuclei. Then, amorphous Si3N4 partially decomposes to form Si(g) and SiO(g) and grows on the formed Si3N4 nuclei as the temperature increased. Finally, undecomposed amorphous Si3N4 spontaneously nucleates and grows of the solid-phase diffusion mechanism. Ultimately, all the powders show equiaxial regular morphology. Preferential nitridation of silicon to form Si3N4 nuclei provides nucleation sites for vapor-phase products, which is the key to inhibiting whisker formation and regulating the particle size of Si3N4 powders.