Preparation of high-purity α-Si3N4 nano-powder by precursor-carbothermal reduction and nitridation

The high-purity α-Si3N4 nano-powder without any β-Si3N4 impurities was prepared by using a precursor-carbothermal reduction and nitridation method, and the detailed technical parameters were optimized. Using HNO3 and CO(NH2)2 as reaction raw materials, C6H12O6·H2O as carbon source and SiO2 as silicon source, a porous precursor was prepared at 350 °C, in which the SiO2 particles were uniformly distributed and in full contact with C. This precursor can be transformed into high-purity α-Si3N4 nano-powder under such optimal technical conditions as the nitriding temperature of 1500 °C, the nitriding duration of 1 h and the nitrogen flow rate of 1800 ml/min. As intermediate products, SiO and Si2N2O were considered to be involved in the carbothermal process, and the presence of SiO accelerated the carbothermal reaction. Furthermore, the higher nitrogen flow rate took more heat away from the reaction area, suppressing the transformation from α-Si3N4 to β-Si3N4, which might explain why the final products are mainly composed of high-purity α-Si3N4 powder.