Effects of Various Reducing Agents on the Synthesis of Si<sub>3</sub>N<sub>4</sub> Whiskers from Diatomaceous Earth

Abstract

Carbothermal reduction and nitridation of diatomaceous earth were carried out in a flow of nitrogen containing various reducing gases (NH3, H2, CO, and CH4) at 1350°C. Addition of 75mol% NH3 to N2 brought about a marked effect upon the inside and outside whisker formation. The ability of the various reducing agents in the reduction of SiO2 was investigated, and found that a carbon plate or methane, can reduce SiO2 in diatomaceous earth, but that neither H2 nor CO can reduce SiO2 at 1350°C. The reduction rate in the presence of both the carbon plate and H2 was higher than that for either one of them. The formation of CH4 in a blank experiment using no diatomaceous earth suggests that the CH4 formed from NH3 and the carbon plate plays an important role in the reduction of SiO2 into SiO (g). Next, the effect of addition of powdered carbon to diatomaceous earth was studied. Since an excess amount of powdered carbon accelerates the reduction and nitridation of SiO2 and SiO (g), the yields of inside and outside whiskers decreased with increasing amount of powdered carbon added. On the other hand, the yield of Si3N4 powder increased with increasing carbon content in the starting materials up to 20wt%. SiC was also obtained in the Si3N4 powder at carbon contents >20wt%. When only methane was used as a reducing agent, the maximum yield of inside whiskers was about 28% at 1.0vol% CH4 in N2. On the other hand, the yield of the outside whiskers was lower over the CH4 concentration examined.