Effect of nitrogen gas pressure during heat treatment on the morphology o silicon nitride fibers synthesized by carbothermal nitridation

The effects of N-2 gas pressure on the diameter and morphology of Si3N4 fibers obtained via carbothermal nitridation were investigated. Sol-gel-derived precursors containing a silica and carbon mixture were placed in an Al2O3 crucible and heat-treated under various N-2 pressures from 0.1 to 0.5 MPa at 1500 degrees C. 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 a- and (3-phase Si3N4 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'-SiA1ON, was produced due to high N-2 pressure and doping with Al originating from the Al2O3 crucible. This oxynitride was considered to act as a template for Si3N4 fiber growth by the direct phase transformation mechanism. It was shown that the diameter of the Si3N4 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.