Effect of Impurities on Silicon Nitride Whiskers Synthesized from Silica-Containing Natural Substances

Silicon nitride (Si3N4) whiskers were synthesized via a carbothermal reduction and nitridation process from silica-containing natural substances, such as volcanic ash, diatomaceous earth, white sand, and rice hull, using the flux effect of cryolite. It was clarified that the volcanic ash, whose Fe2O3 content contributed to the formation ratio of the α-type, was suitable as a raw material for the synthesis of Si3N4 whiskers. Fe2O3, Al2O3, and TiO2 contributed to the growth in the whisker-axis direction by the formation of droplets inducing the vapor-liquid-solid mechanism. CaO and MgO led to an increase in the diameter of the whiskers by vapor-solid and vapor-liquid-solid mechanisms, and the Fe2O3 content was the highest in the volcanic ash, whose Fe2O3 content was the highest among the natural substances.