Effect of impurities on silicon nitride whiskers synthesized from silica-containing natural substances

摘要

Silicon nitride (Si_3N_4) 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 Fe_2O_3 content contributed to the formation ratio of the alpha-type, was suitable as a raw material for the synthesis of Si_3N_4 whiskers. Fe_2O_3, Al_2O_3, and TiO_2 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 Fe_2O_3 content was the highest in the volcanic ash, whose Fe_2O_3 content was the highest among the natural substances.