Synthesis And Properties Of SI₃N₄ Nanofibers Via Gas-pressure Annealing Method

In this paper, Si₃N₄ nanofibers were synthesized from silica sol and sucrose through the processing of sol-gel, thermal decomposition and gas-pressure annealing. First, the Si-O-C nanoparticles were designed and synthesized as the precursor for Si₃N₄ nanofibers; then the Si-O-C nanoparticles were annealed under N2 gas pressure atmosphere to produce Si₃N₄ nanofibers. The influences of the temperature and gas pressure on the growth of Si₃N₄ nanofibers were systematically investigated, and the optical, electrical and mechanical properties of Si₃N₄ nanofibers were also studied.The reaction temperature was a key factor for the growth of Si₃N₄ nanofibers in the experiments. When the reaction temperature was set at 1400℃, only very few Si₃N₄ nanowires, with an average diameter of 100nm, were found on the surface of the sample. When it was 1500℃, the products were belt-like Si₃N₄ nanofibers with width of 300-400nm and thickness of 70-100nm. When it was up to 1600℃, the size of belt-like Si₃N₄ fibers was more than the nanoscale. With the holding time changed from 0.5h to 4h at 1500℃, the yield of Si₃N₄ nanofibers were significantly increased, and the products were Si₃N₄ nanobelts with the bandwidth of 150³00nm and the thickness of 20 90nm.The initial N2 gas-pressure for annealing ranged from 0.2 MPa to 1.0 MPa in the experiments. The size of Si₃N₄ nanofibers grown larger with the increase of the presssure under 1500℃, and the products was nanobelts with the bandwidth of 200⁴00nm and the thickness of 30⁹0nm. When the pressure was 0.2MPa under 1600℃, there were SiC nanowires appeared due to the lack of the nitrogen pressure. With the gas-pressure increased, Si₃N₄ nanofibers were found much closer to the raw materials.The optimal synthetic parameters for the growth of Si₃N₄ nanofibers were proposed according to the experiments. When the Si-O-C nanoparticles were annealed at 1500℃for 2-4h under 0.2MPa N2 gas-pressure atmosphere, large quantities of pure a-Si₃N₄ nanobelts were obtained with the bandwidth of 150³00nm and the thickness of 20 90nm.The prepared Si₃N₄ nanofibers exhibit good luminescent properties, which can emit kinds of spectra ranging from 246nm to 595nm. The conductivity of Si₃N₄ nanofibers was 10-9 S·cm^-1. The nano-hardness and elastic modulus of Si₃N₄ nanofiber were 19.7GPa and 203.8GPa respectively, indicating that the Si₃N₄ nanofibers have excellent mechanical properties.