Preparation Of Various Nanostructures From C-Si-O, C-Si-O-X Systems And Investigation Of Structures And Properties

As one of the third generation wide band-gap semiconductor, silicon carbide (SiC) has wide energy band gap at room temperature, high breakdown electric field, high thermal conductivity, and high elelctron drift velocity. These excellent properties make SiC become a promising semiconductor material for the fabrication of optic and electronic devices, especially for the devices used at high temperature, high power, high irradiation and high frequency. Besides all the mentioned properties above SiC nanostructures have many other remarkable optical, electrical and mechanical properties which bulk SiC crystals don't have, therefore they have great potential applications as interconnects or functional components in nanoscale electronics, optoelectronics and mechanical devices. All researches in this aspect take the preparartion of SiC nanostructures as their starting points. In this thesis we studied the preparation of SiC nanostructures and other nanostructures in C-Si-O, C-Si-O-Fe, C-Si-O-Ni, C-Si-O-Al, C-Si-O-Zn-S systems by thermal evaporation method. We obtained some important conclusions and some innovative achievements.In C-Si-O system, we obtained SiC nanowires and SiC nanotubeson the alumina grid where multi-wall carbon nanotubes (MWCNT's) were situated, while SiC/SiO₂ nanocables on on the inner wall of the graphite crucible where Si melt was contained. It is believed that SiC nanowires were formed from those MWCNT's still with metallic catalyst particles attached by Vapor-Liquid-Solid (VLS) mechanism, and SiC nanotubes were obtained by Vapor-Solid (VS) mechanism due to the tranforamtion of other MWCNTs without catalyst particles attached (falling down). In both situations, CNTs supplied C and Si vapor due to the evaporation of Si was Si source In the cooling stage temperature in the inner wall of the graphite crucible will be always lower than the alumina grid, so the SiC nanowires on the inner wall could firstly reach the reaction temperature for the deposition of SiO₂ and then always be prior to form SiC/SiO₂ nanocables.Temperature plays an important role for the synthesis of SiC nanowires. At low temperatures the SiC nanowires produced are cylindrical shaped with smooth surface. When the temperature is high enough, Si atoms and C atoms on the surface of SiC nanowires are rearranged as the kinetic qualification could be conquered to grow hexagonal prism shaped SiC nanowires with {111} plane side facets.For the syntheses of SiC/SiO₂ composites the two different shapes of SiC nanowires will determine the geometrical of the products. For low temperature the core is cylindrical SiC nanowires with very smooth surface, and SiO₂ thus formed on the cooling process would wrap on the SiC core uniformly to produce SiC/SiO₂ nanocables. At high temperature however, because the core is hexagonal prism shaped SiC nanowires with continual side facets, SiO₂ formed on cooling do not wrap on the core but rather assemble to some spherical particles to become Tang Hum-like SiC/SiO₂ composites at last.The joining of other elements in C-Si-O system may bring about many changes in size, shape and yield etc. to the nanostructures. In C-Si-O-Fe system, we find that Fe addition favored the formation of hexagonal prism ans at the same time affected the diameter and growth rate regularly. With increasing Fe content, the diameter of the produced SiC nanoprisms increased from about 50 nm to hundreds of nanometer, and the yield of SiOx was enhanced. In C-Si-O-Ni system, cylindrical SiC nanowires were mostly obtained at the temperature where nanoprism should be formed in other systems. It demonstrates that not only temperature but also new elements can affect the shape of the synthesized nanostructures. In C-Si-O-Al system we obtained ultra-thin nanobelts.The addition of ZnS in C-Si-O-Fe system further increased the diameter of the products to about 500 nm. Furthermore, big size metal catalyst particles were often found on the top of these SiC nanowires. After these metal catalyst particles removed by erosion we can see two types of tips, one was even tip, we believe these nanowires growth under two dimensional nucleation mechanism; other was thiner needle tip, which is very close to those nanowires growth under screw dislocation growth mechanism. That is to say some of the nanowires growth controlled under screw dislocation growth mechanism, but this is not the major mechanism, the reason of such phenomena still needs further investigation.