Preparation of Si3N4 Whiskers from Various Natural Resources

Silicon nitride whiskers have been prepared by reduction and nitridation of natural resources, occurred in Oita prefecture, such as Asono diatomaceous earth, Beppu Terra abla, Youra quartzite and casting sand, at 1370°C in an NH3-N2 stream on a carbon plate. Two types of Si3N4 whiskers, short needlelike whisker (inside whisker) and woollike whisker (outside whisker) were obtained from the inexpensive raw materials. The shape of these whiskers, however, varied with the intrinsic composition of the starting materials. The outside whiskers obtained from the casting sand were almost all composed of Si3N4 with a few whiskers composed mainly of Al, and which looked like abacus or connected hexagonal beads. The outside whiskers obtained from the four kinds of raw materials had droplets on one of their ends, indicating that the outside whiskers grow by the VLS mechanism. The droplets produced from diatomaceous earth and quartzite were mainly composed of Fe and Si. On the other hand, those from Terra abla were composed of Fe, Si and a small amount of Ti. In the case of the casting sand, two types of droplets, one composed of Fe and Si and the other composed of Cr, Fe and Si, were observed. These metals, which composed the droplets, were derived from metal oxides in the raw materials. The metal elements are thought to have migrated from the starting material to the outside whisker in a vapor phase.