Preparation And Properties Of BN Nano Materials

Hexagonal boron nitride (h-BN) has received considerable attention because of its advantageous properties, such as anti-oxidation at high temperature, excellent chemical inertness, outstanding anti-wear, high thermal conductivity, absorbency of neutron and optical transparency. These make BN an interesting material for many applications such as mechanism, metallurgy, electron, aviation and spaceflight. In order to get h-BN nanomaterial, many synthesis methods have been developed, for example, chemical vapor deposition (CVD) method, solvothermal method, high temperature and high pressure (HTHP) method and so on. In this thesis, on the basis of investigation on the preparation of boron nitride from trimethylborate (TMOB) by CVD, systematical research to some key factors influencing preparation of BN nanosphere was performed. Fluffy-like BN nanoparticle and BN nano fluffy-film/Cf composite fiber had also been prepared successfully and its growth mechanism was investigated by various charaterization methods. The major results are as follows:(1) It was shown that preheat of boron sources, slight vacuum in production system, product-capture by cold trap could change the morphology and the yield of BN samples. Uniform and least BN sample could be synthesized at appropriate CVD temperature by the introduction of slight vacuum into the production system and adopting cold trap to capture product, and simultaneously the yield increased.(2) The influence of CVD temperature was investigated. It was confirmed that the product could be synthesized at 950℃with morphology of smooth sphere and uniform least diameter of 60 nm. With CVD temperature increasing, the mol rate of B and N approachs to 1:1, BN samples become irregular, but the level of O content always keeps no less than 5%. In the range of 800-1300℃, no matter what CVD temperature is, the main crystal lattice of samples is hexagonal boron nitride.(3) The spherical BNO nanoparticles could be synthesized at 900℃. It were provided with favorable anti-wear property which may be attributed to the "effect of tiny bearing" and oxygen element existing in BNO nanosphere with 16% level. Boron oxidation compound in samples could contribute to the anti-wear property.(4) Fluffy-like BN nanosphere could be synthesized by second epitaxial growth method and using the as-prepared BN ones as CVD substrates which becomes core of the samples finally. The surface layer is fluffy nanofilm with thickness of 50nm which is made up of wrapped leaf-like BN piece. It is proved that the morphology of surface layer was controlled by the epitaxial growth temperature. At lower temperature, only sparse grainules with diameter of 10-20nm grow on the surface. Along with the increase of reaction temperature, the surface layer was covered with wrapped leaf-like BN piece with lengh of 30-50nm until fluffy nanofilm formed finally. In the process of the whole epitaxial growth, the samples keep morphology of sphere.(5) The second epitaxial growth method attributes to agglomerate growth mechanism. The synthetic route induced to a novel BN morphology with the fluffy structure on the smooth substrate. The intermediate product coagulating on the surface was affected by space inducement of as-prepared BN core. With temperature increasing, granular outshoot prefer to grow along with vertical direction of surface of BN core, as well as the wrapped leaf-like shape form from granular ones. And the productions always kept complete morphology of sphere, which was due to the uniform growth of fluffy film on the BN core. Wrapped leaf-like structure of BN piece in fluffy film favored increasement of measured surface area and elimination of oxygen element.(6) Fluffy BN nanofilm/Cf composite fiber could be synthsized by in situ epitaxial growth method which has higher measured surface area than carbon fiber(Cf). The outer fluffy BN nanofilm with uniform thickness of 150-200nm is tightly combined to Cf. The fluffy film may fall off from Cf if the pretreat method of Cf suface was not appropriate. Boron sources was ahead loaded on the surface of Cf by in situ chemical reaction to protect the fiber from corrasion of vapor such as NH₃ and increase the combination between BN film and Cf. The measured surface area of the composite fiber increased greatly to 96 m²/g.