| Boron nitride (BN) is a covalent semiconductor material which is composed of group IIIA of boron and VA of nitrogen. Crystalline BN has four variants:hexagonal boron nitride (h-BN), rhombohedral boron nitride (r-BN), cubic boron nitride (c-BN) and wurtzite boron nitride (w-BN). As h-BN has a similar structure with graphite, they have very similar excellent physical and chemical properties such as good lubricity, high strength and high heat conductivity, etc. But, unlike graphite, the band gap energy (Eg) of h-BN is widely dispersed in the range from3.6to7.1eV and can be doped for both n-and p-type conductivity. Moreover,h-BN can stay up to900℃in oxidizing atmosphere, which can be used for high temperature, high pressure, insulation, cooling parts, etc. These make h-BN an interesting material for many applications such as chemical industry, machinery, electronics, aerospace and other high-tech fields.Along with the further understanding of the materials, the nanomaterials with good performances increasingly become the research focus. Therefore, both theoretical research and preparation technology of BN nanomaterials are rapidly developed. Unfortunately, few reliable methods were reported to produce bulk amounts of BN nanomaterials. The current synthetic methods are usually high cost and low purity and need extreme condition and complicated equipments, which seriously limit the study of their properties and the potential applications. Therefore, it is of great significance to explore simple and efficient routes to synthesize high purity, well crystallized BN nanomaterials with uniform structure in large scales.In this dissertation, we successfully prepared BN nanotubes, nanoflake-decorated BN hollow microspheres and nanoflake-decorated BN microwires with high purity and bulk production by improved chemical vapor deposition (CVD) method and solid-state reaction method. The thermal performance and luminescent properties of the related products were preliminary studied. The growth mechanism of these BN nanostructures were studied in detail based on the crystal growth theory. The main contents include:1. High purity BN nanotubes were synthesized on stainless-steel substrates through CVD method using amorphous B and iron oxide (Fe2O3) as raw materials at1300℃in ammonia atmosphere. The nanotubes have diameters of40-100nm and lengths of more than200μm. The nanotubes are usually decorated with O-Si-Mn species embedded in the walls or filled within cavities. The influence of Fe2O3on the structure and formation of BN nanotubes were studied systematically. In the case of low Fe2O3addition, the yield of BN nanotubes is low but with relatively high purity. There are nearly no B-N-O-Si-Mn particles in the product and less inclusion of O-Si-Mn species into the growing BN nanotubes. Whereas, in the case of high Fe2O3addition, the reaction between B and Fe2O3could generate more B2O2vapor at a fast rate within a short time, which led to the inclusion of more O-Si-Mn species into the growing BN nanotubes and finally resulted in the formation of BN nanotubes with high yield but more B-N-O-Si-Mn particles in the product. The reaction temperature has great influence on the morphologies and production of BN nanotubes. Lower temperature is favorable for the formation of bamboo-like nanotubes, while higher temperature enhaces the formation of quasi-cylindrical nanotubes and and the yield of nanotubes increases. In addition, the reaction atmosphere also has effect on the formation of BN nanotubes. The growth mechanism of the nanotubes was governed by vapor-liquid-solid (VLS) model.2. A new CVD method for synthesizing BN nanotubes was developed, which was assisted with gaseous oxides or oxygen. High purity BN nanotubes were deposited on the surface of stainless-steel substrates by using B powder, ammonia gas and water vapor as the starting materials. It was found that the replacement of metal oxide by water vapor could continuously generate intermediate B2O2vapor and enhance the production of the BN nanotubes. Moreover, the yield and purity of the BN nanotubes could be controlled through the tuning of water vapor amount in this new method. The formation of the BN nanotubes followed a combination of oxide-assisted VLS method. The photoluminescence spectra of the obtained BN nanotubes show three main emission bands centered at443,512and703nm, suggesting that BN nanotubes have excellent luminescent properties and could be a kind of potential luminescent materials.3. BN nanotubes were fabricated by solid state reaction method using B powder, carbon powder and iron-based catalyst as raw materials. The catalytic performance of Fe, Fe2O3and Fe(NO3)3·9H2O for the synthesis of BN naotubes was studied. The catalytic efficiency order is:Fe<Fe2O3<Fe(NO3)3·9H2O.4. Mixed B-C powder was added into the alcohol solution containing Fe(NO3)3and kept stirring to obtain the wet mixed materials. After annealed at high temperature, a lot of BN nanotubes were obtained. The dispersion behavior of catalyst was greatly improved by solvent dispersion compared with the solid mixed method, which effectively avoided the agglomeration behavior of particles. The effects of the quantity of catalyst, reaction temperature and reaction time on the formation of BN nanotubes were studied and the optimum reaction conditions were obtained:B:C:Fe(NO3)3·9H2O=1:1:0.05(molar ratio), T=1300℃, t=5h, flow ratio of ammonia=50sccm. Different molysite catalysts have similar effect on the formation of the BN nanotubes and they have similar catalytic efficiency order:Fe(NO3)3-9H2O≈FeCl3·6H2O≈FeCl2·4H2O>=Fe2(SO4)3·9H2O. The growth mechanism of the nanotubes is governed by a combination of VLS and solid-liquid-solid (SLS) model.5. BN hollow microspheres were fabricated by solid state reaction method using B powder and Fe(NO3)3·9H2O which were mixed in ethanol solution. The reaction temperature has a great influence on the yield and morphology of BN hollow microspheres. It was found that smooth BN hollow microspheres with low yield were formed at a relatively low temperature (1100℃) and the nanoflake-decorated hollow microspheres with high yield were formed at high temperature (1300℃). The BN composite structure exhibits excellent anti-oxidation performance up to900℃. VLS and vapor-solid (VS) mechanisms were suggested to be responsible for the growth of microspheres and nanoflakes, respectively. 6. A novel BN composite structure composed of nanoflake-decorated microwires were fabricated by solid state reaction method using B powder and FeCl3·6H2O which were mixed in ethanol solution. The microwires have diameters of3-4μm and the lengths of more than100μm, while the nanoflakes have thickness of2-5nm and lengths up to600nm. The nanoflakes can greatly increase the specific surface area of the BN microwires and would be a novel and effective catalyst supporting material and energy storage material. |
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