Synthesis And Properties Of Ultra-long SiC And Si₃N₄ Nanowires

One-dimensional nanostructures are potentially applied in electronics, optoelectronics, nanocomposites, energy resources, life science and medicine due to their special structures and remarkable physicochemical properties. Among the various one-dimensional nanostructures, SiC nanowires and Si₃N₄ nanowires have been studied extensively because they combine the properties of wide band-gap semiconducting materials and one-dimensional nanostructures, and exhibit excellent properties in high temperature, high power, high frequency, high radiation and chemical harsh environments. Ultra-long nanowires will provide novel features and advantages in electronics, nanocomposites, and basic research, etc. In this dissertation, we prepared large amount of SiC nanowires with several centimeters-long by a polymer pyrolysis chemical-vapor-deposition route (PPCVD) for the first time; and also prepared ultra-long Si₃N₄ nanowires of several millimeters by method of high temperature nitrogenization reaction. The composition, structures and physicochemical properties of the nanowires were investigated in details.Millimeters-long Si₃N₄ nanowires are prepared by method of simple sandwich arrangement of raw materials, via the nitrogenization reaction among SiO₂ powders, short carbon fibers and N₂ at high temperature. The nanowires are composed ofα-Si₃N₄ single crystals along the [001] direction and the diameters are in the range of 70-300 nm. The nanowires are grown by the vapor-solid mechanism, and the Si and N elements in the Si₃N₄ nanowires are from the gaseous SiO and N₂, respectively. The slow release of SiO at high temperature and the special arrangement of raw materials are responsible for the large length in one dimension. The relatively stable gaseous eddies formed between each two graphite wafers are favorable for the nanowires growth.For the potential applications in oxidation environment at high temperature, the anti-oxidation properties of Si₃N₄ nanowires are investigated. And the results show that the Si₃N₄ materials are stable in morphology at temperatures below 1000℃, and convert into nanowebs due to the fusion of the oxided surface at 1200℃, and finally into nanofilms at 1400℃. This is an important basic problem to be considered in the fabrication of nanocomposites and nanodevices based on Si₃N₄ nanowires for high temperature applications.A special PPCVD route is designed based on the principle of vapor-liquid-solid (VLS) mechanism, and large amount of cotton-like centimeters-long SiC nanowires are synthesized via the base-growth mode on the ceramic boats, with polysilicarbosilane as the raw materials and ferrocene as the catalyst. The best substrate for the nanowires growth is the ceramic boat for the moment. And millimeters-long SiC nanowires are prepared via the tip-growth mode on the graphite substrate by the same route. The nanowires are mainly composed ofβ-SiC single crystals along the <111> direction, with the diameters of 100-300 nm, and are sheathed with amorphous SiO₂ of about 2-3 nm thicknesses. Some stacking faults, dislocations, and polycrystals are found in the nanowires. Several SiC nanowires with diameters of 6-40 nm are prepared in the low temperature regions, and SiC nanowires with diameters of 50-100 nm are prepared in-situ on the raw materials. Several SiC nanowires with different morphology, such as nanowires with fluctuating diameters, helical nanowires, spindle-like nanowires, tower-like nanowires, nanosphere chain nanowires, and branched nanowires, are found.The VLS mechanism is employed to interpret the ultra-long SiC nanowires growth. The catalytic nanodroplets come from the decomposition of ferrocene and the precipitation of Fe in the ceramic boat. The raw silane fragments come from the decomposition of polysilicarbosilane. The H2 from the decomposition of raw materials and the base-growth mode are responsible for the large length in one dimension.The ultra-long SiC nanowires are featured with nanoscale tips, large aspect ratio, thermal and chemical stability, and are expected to have good field emitting property. Therefore, we investigate the electron emitting properties of SiC nanowires in high-voltage single-pulsed electric field and low-voltage electric field. The results show that the cathode based on SiC nanowires gives higher current intensity than the usual materials employed in engineering. The current density increases with the electric field, with distinct point effect. And it opens a fascinating prospective application for SiC nanowires in vacuum microelectronics such as high power microwave devices, betatrons, and so on. The electron emitting property of SiC nanowires in low-voltage is also characterized, and the turn-on and the threshold fields are 7.0 and 11.7 V/μm, respectively.For the potential applications in high temperature and oxidation environment, the thermal stability and the anti-oxidation properties of SiC nanowires are investigated. The results show that the morphology of the SiC nanowires are stable when the temperature is below 1800℃in inertia gas, and begin to convert into nanosphere chains at 1800℃due to the Rayleigh Instability. In oxidation environment, the SiC nanowires are stable at the temperatures below 1000℃, and the oxided surface begin to melt at the crossed sites to form nanojunctions at 1000℃, and are converted into nanowebs with the nanojunctions increase at 1300℃, and further into silica films at 1400℃.The effects of the surface on the Raman scattering spectrum and the FT-IR absorption spectrum are investigated. The results show that the amorphous sheath of SiO₂ is responsible for the suppressed LO peak because of the effective damping of phonons.We expect a perspective application in high-temperature structural microwave absorbers for SiC nanowires because of the high strength and the high dielectric loss at high temperature, and measured the electromagnetic parameters, based on which the reflectivity is calculated. The results show that the SiC nanowires have good microwave absorbed property.β-SiC nanowires are potentially applied in photocatalyst, and the SiC nanowires are prepared on the activated carbon to increase the efficiency of photocatalytic activity in our work. The SiC nanowires grown on the activated carbon show good photocatalytic activity on the degradation of methyl orange and can be recycled simply.