Carbothermal synthesis of Si₃N₄ powders using a combustion synthesis precursor

Si₃N₄ powders were synthesized by a carbothermal reduction method using a SiO₂ + C combustion synthesis precursor derived from a mixed solution consisting of silicic acid (Si source), polyacrylamide (additive), nitric acid (oxidizer), urea (fuel), and glucose (C source). Scanning electron microscopy (SEM) micrographs showed that the obtained precursor exhibited a uniform mixture of SiO₂ + C composed of porous blocky particles up to ~20 μm. The precursor was subsequently calcined under nitrogen at 1200–1550℃ for 2 h. X-ray diffraction (XRD) analysis revealed that the initial reduction reaction started at about 1300℃, and the complete transition of SiO₂ into Si₃N₄ was found at 1550℃. The Si₃N₄ powders, synthesized at 1550℃, exhibit a mixture phase of α- and β-Si₃N₄ and consist of mainly agglomerates of fine particles of 100–300 nm, needle-like crystals and whiskers with a diameter of about 100 nm and a length up to several micrometers, and a minor amount of irregular-shaped growths.