Synthesis, Characterization And Properties Of One-dimensional Silicon Carbide Nanomaterials

Silicon carbide (SiC) possesses wide band gap, high breakdown electric field, excellent thermal stability, high thermal conductivity and high electron saturation drafting velocity, and these characteristics make SiC excellent properties in high temperature, high frequency, high radiation and high power environment. For their unique morphologies and structure, one-dimensional silicon carbide nanomaterials have special physical and chemical properties, and have wide range of applications in electronic nanodevices, field-emission nanodevices, nanocomposites, catalysis, and so on. Therefore, the preparation and properties research of one-dimensional silicon carbide nanomaterials has considerable significance.In this paper, SiC nanowires have been prepared via carbothermal reduction method using different carbon source and silicon source, and the large-scale preparation technology of SiC nanowires also has been studied. The compositions, morphologies and microstructure of the products were characterized by X-ray powder diffraction, scanning electron microsopy, transmission electron microsopy and selected area electronic diffraction; the optical properties, energy bandgap structure and oxidation resistance were characterized by Fourier transform infrared spectroscopy, fluorescence photometer, UV-Vis spectroscopy and thermal analysis instruments. On the basis of above investigation, the growth mechanism and the factors that affected the growth of SiC nanowires have been analyzed; the thermal stability and relationships between the structure and properties also have been studied. The main conclusions are listed as follows:Firstly, SiC nanowires have been synthesized via carbothermal reduction method by using tetraethoxysilane and carbon black as main raw materials, and then analyzed the growth mechanism and the factors that affected the growth of SiC nanowires and achieved effective morphologies control of SiC nanowires. The morphologies of SiC nanowires were affected by Si/C, temperature and the holding time: Linear nanowires, hierarchical nanowires and nanorods with diameters of about 100-240 nm have been prepared at 1500°C, 1550°C and 1600°C, respectively; the diameters of nanowires are relatively uniform and hierarchical structure is clear when Si/C=1:1; short holding time is not conducive to the formation of hierarchical nanowires. Transmission electron microsopy and selected area electronic diffraction show that the growth direction of SiC nanowires is (111) direction and a high density of stacking faults and microtwins can be observed in the nanowire. Vapor-solid (VS) mechanism with two stages growth process was proposed to explain the growth of hierarchical SiC nanowires. The first stage was a fast growth of core SiC nanowires, and the second stage was the alternate formation of SiCxOy aggregations onto the surface of core SiC nanowires and the epitaxial growth of SiC along the surface of core nanowires to form branched nanoplatelets for the reaction of SiO/CO vapor adsorbed on the aggregations.The photoluminescence spectrum exhibits two photoluminescence peaks located at 2.87 eV and 3.06 eV. The emission peak centered at 2.87 eV, which shows blue-shifted compared with bulk single-crystallineβ-SiC (Eg≈2.39 eV), while the emission peak at 3.06 eV may be attributed to 6H-SiC segments (Eg≈3.02 eV) formed by stacking faults in 3C-SiC nanowires. UV-Vis spectroscopy shows that SiC nanowires is direct transition semiconductor and the Eg value is 2.60 eV, which is close to the photoluminescence peak (2.87 eV).SiC nanowires were prepared by carbothermal reduction method using expandable graphite as carbon source and silicon powders as silicon source, and the large-scale preparation technology of SiC nanowires also has been investigated. It is found that as the reaction temperature and Si/C increase, the residual graphite decrease, and catalyst and low degree of mixing of raw materials are not conducive to the growth of SiC nanowires. Transmission electron microsopy and selected area electronic diffraction show that the morphologies of SiC nanowires could be branched and linear, and the growth direction of SiC nanowires is (111) direction with a high density of stacking faults in the nanowires. Vapor-solid (VS) mechanism is proposed to explain the growth of SiC nanowires, and the loose and porous structures of expandable graphite can provide plenty of growth space for SiC nanowires, which is the key factor for the preparation of nanowires in large-scale. Thermal analysis shows that SiC nanowires begin to be oxidized at 400°C. The photoluminescence spectrum shows that the photoluminescence peak blueshifts after oxidation at 700°C for the dimension decrease of core SiC nanowires.The thermal stability of SiC nanowires was investigated in air. Thermal analysis of two kinds of SiC nanowires with different diameters prepared by carbothermal reduction method shows that SiC nanowires begin to be oxidized at 450°C. X-ray diffraction analysis displays that SiC nanowires are oxidized constantly to form silica as temperature increasing. A pronounced morphology change of SiC nanowires begins at 1100°C, the surface of SiC nanowires melts and sticks together, and the nanowires with smaller diameters (20-50 nm) become porous membrane at 1300°C while few nanowires also can be observed for the nanowires with large diameters (100-150 nm). The thermal oxidation mechanism of SiC nanowires shows that the oxidation of SiC and the melting of silica are the key reasons for the morphology change.