Carbothermal reduction and nitridation of diatomaceous earth were carried out in a flow of nitrogen containing various reducing gases (NH
3, H
2, CO, and CH
4) at 1350°C. Addition of 75mol% NH
3 to N
2 brought about a marked effect upon the inside and outside whisker formation. The ability of the various reducing agents in the reduction of SiO
2 was investigated, and found that a carbon plate or methane, can reduce SiO
2 in diatomaceous earth, but that neither H
2 nor CO can reduce SiO
2 at 1350°C. The reduction rate in the presence of both the carbon plate and H
2 was higher than that for either one of them. The formation of CH
4 in a blank experiment using no diatomaceous earth suggests that the CH
4 formed from NH
3 and the carbon plate plays an important role in the reduction of SiO
2 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 SiO
2 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 Si
3N
4 powder increased with increasing carbon content in the starting materials up to 20wt%. SiC was also obtained in the Si
3N
4 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% CH
4 in N
2. On the other hand, the yield of the outside whiskers was lower over the CH
4 concentration examined.
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