Studies On The Preparation And Properties Of BN-based Nanomaterials

Boron nitride(BN)materials are structrually similar to carbon,possessing excellent properties including good thermal stability,thermal conductivity,lubricity,mechanical strength and chemical stability.Different morphological BN nanostructures like multi-walled BN nanotubes,single-wall BN nanotubes,and BN nanosheets,were successfully prepared.People also successfully synthesized boron carbonitride(BCN)nanostructures on the basis of the BN nanostructures.Such BN-based nanomaterials have been one of the most promising inorganic nanomaterials in the recent 20 years.At present,BN-based nanomaterials have been successfully prepared by various methods such as chemical vapor deposition,arc discharge,and mechanical milling.But these methods normally have shortcomings,such as low-yield,low-purity,high-cost,using dangerous or toxic chemicals,and so on.In this study,BN-based nanomaterials were prepared and their properties were studied.Firstly,a novel route to coat carbon fibers with BN nanosheets was demonstrated by annealing of carbon fibers with boron and FeCl3·6H2O at elevated temperatures under ammonia atmosphere.The effects of annealing temperature,reaction time and gas flow rate on the thickness and microstructure of BN coating were investigated.The results showed that uniform BN coating could be obtained at 1200 ? with an ammonia flow rate of 50 ml/min and holding time of 1 h.A growth mechanism of vapor-solid(VS)model is proposed to be responsible for the formation of these BN nanosheets.The oxidation resistance of the carbon fibers was dramatically enhanced after BN nanosheets were coated onto the carbon fibers.Moreover,given that the low dielectric loss tangent of BN,the impedance matching of carbon fibers was improved and the microwave-absorbing property of carbon fibers was enhanced significantly.Secondly,a novel route to the synthesis of boron carbonitride(BC1.61N0.76)micro-nano structures called nanosheet-decorated BCN submicrowires was demonstrated for the first time,by annealing amorphous boron powder,natural rubber latex(NRL)and ferric chloride at elevated tempetures in flowing ammonia atmosphere.The microwires have diameters of about 200-500 nm,while the nanosheets have an average thickness of less than 20 nm.The nanosheets are mostly separated with a bending and crumpling morphology.The effects of annealing temperature,reaction time and natural rubber latex content on the micro-nano structure of BCN were investigated.The results showed that a uniform BC1.61N0.76 micro-nano structure could be obtained at a reaction temperature of 1200 ? with natural rubber latex content of B:NRL=1:50 and reaction time of 3 h.This micro-nano structure shows good visible-light emission property at 501.2 and 592.6 nm.A combination growth mechanism of vapor-liquid-solid(VLS)and vapor-solid(VS)model is proposed to be responsible for the formation of this BC1.61N0.76 micro-nano structures.