Silicon Carbide Polycrystalline Fibers and Single-Crystal Whiskers

The origin of the high inhomogeneity of powders of nonoxide refractory compounds, related to the chemical nature of the compounds and the preparation procedure (usually, solid-state reactions) is considered. Conclusive evidence is presented that vapor transport reactions have considerable potential for the reproducible synthesis of such compounds. It is shown that carbothermic reduction of silicon dioxide can be run as a vapor transport reaction using activated carbon fibers as a reductant. The forming polycrystalline SiC fibers are similar in particle morphology to the parent carbon fibers, suggesting that the reaction proceeds through the incorporation of silicon into the lattice of amorphous carbon. Experimental data are presented for the first time which demonstrate that, under similar process conditions, silicon carbide can be obtained in the form of polycrystalline fibers, single-crystal whiskers, or fine powder. The reaction products differ not only in morphology but also in the crystal structure of SiC (α or β form). The mechanisms of α- and β-SiC nucleation and growth are discussed.