Catalyst-assisted synthesis of α-Si3N4 in molten salt

Abstract Silicon nitride (Si3N4) powder was synthesised under a nitrogen gas flow (150 mL/min) using molten salt media at relatively low temperatures (950–1350 °C) to investigate the effects of temperature and cobalt (Co) content on its phase assembly and micro-morphology. Analysis of purified α-Si3N4 obtained using X-ray diffraction, scanning electron microscopy, transmission electron microscopy and energy-dispersive X-ray spectroscopy revealed that the fabrication of α-Si3N4 from silicon by molten salt media is possible at temperatures as low as 1050 °C. Furthermore, Co played an important role in accelerating Si nitridation through the formation of a Co–Si liquid phase, with overall conversion increasing from 5% to 60% with the addition of 0.5 wt% Co at 1150 °C. At temperatures below 1350 °C, silicon nitride exists mainly as flaky nanoparticles, but at higher temperatures changes to α-Si3N4 nanorods that are 200–500 nm in diameter and 500–1000 nm in length. The growth of these nanorods and nanoparticles was governed by the vapour–liquid–solid and vapour–solid mechanisms.