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The effects of N 2 gas pressure on the diameter and morphology of Si 3 N 4 fibers obtained via carbothermal nitridation were investigated. Sol-gel-derived precursors containing a silica and carbon mixture were placed in an Al 2 O 3 crucible and heat-treated under various N 2 pressures from 0.1 to 0.5 MPa at 1500ºC. The heat-treated samples were characterized by X-ray diffraction (XRD), a scanning electron microscope (SEM) coupled with an energy dispersion X-ray (EDX) spectrometer, and transmission electron microscopy (TEM). Various nitride fibers with αand β-phase Si 3 N 4 were formed. The fiber diameter decreased with increases in N 2 pressure, and nanometer-scale fibers of around 200 nm in diameter were obtained at a N 2 pressure of 0.5 MPa. It was found that silicon oxynitride Si 2 N 2 O or its Al-doped form, O′-SiAlON, was produced due to high N 2 pressure and doping with Al originating from the Al 2 O 3 crucible. This oxynitride was considered to act as a template for Si 3 N 4 fiber growth by the direct phase transformation mechanism. It was shown that the diameter of the Si 3 N 4 fibers decreased for two reasons: decreased partial pressure of SiO gas due to high N 2 pressure and increased formation of the silicon oxynitride templates.

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