Preparation,Characterization And Performance Simulation Of BN

Boron nitride(BN)is a new type of inorganic non-metallic material,which has the advantages of good heat resistance,low thermal expansion coefficient,high thermal conductivity,excellent lubricating properties and high chemical stability.It is widely used in ceramic materials,solid lubricating materials,adsorbent materials,hydrogen storage materials,electronic devices,cutting tools and optical devices.Therefore,the preparation of boron nitride has become a research hotspot.At present,the methods for preparing boron nitride include carbon thermal reduction method,chemical vapor deposition method,self-propagating combustion method,solvothermal synthesis method,microwave heating method and molten salt method.Among them,the microwave heating method can not only reduce the reaction time,but also improve the reaction efficiency;while the molten salt method can reduce the synthesis temperature of the reaction,decrease the formation of impurities in the product,and drop the particle size of the product.Therefore,in this thesis,hexagonal boron nitride(h-BN)was prepared by microwave heating method and molten salt method,and the products were characterized by XRD,FTIR,SEM and laser particle size distribution analyzer;In addition,the changes of mechanical,electronic structure and optical properties of cubic boron nitride(c-BN)at 0～100 GPa were investigated by using Materials Studio simulation software.The main research contents and conclusions are as follows:(1)The h-BN was prepared using microwave heating method with silicon carbide as the microwave absorber and Na Cl-KCl as the molten salt system.Firstly,melamine and boric acid were used as reaction raw materials,and different influencing factors were studied to determine the optimal reaction conditions,the h-BN obtained under this condition has the highest purity and no impurity phase,and the morphology consists of uniformly dispersed lamellar h-BN;In addition,urea and anhydrous borax were used as reaction raw materials,and different influencing factors were studied to determine the optimal reaction conditions,when the resulting product had no impurity phase and the highest crystallinity.(2)Using the molten salt method to prepare h-BN,urea and anhydrous borax as reaction raw materials,Na Cl-KCl as molten salt system,and tube furnace as reaction apparatus,different influencing factors were studied to determine the optimal reaction conditions.In this case,the product is pure phase h-BN and tends to form uniform particle size and lamellar h-BN.(3)The changes in the mechanical,electronic structure and optical properties of c-BN in the range 0～100GPa were investigated using Material Studio software.It is found that c-BN remains mechanically stable with increasing pressure,but its elastic constant gradually increases and the compressibility of the material becomes progressively worse;The band gap of c-BN at zero pressure is 4.367 e V,so c-BN is an indirect wide band gap semiconductor.With the increase of pressure,the band gap of c-BN gradually increases,but the density of states spectrum does not change significantly;Analysis of the Mulliken Bourget distribution of c-BN shows that the covalency of the B-N bond increases with increasing pressure;In addition,the complex dielectric function,refractive index,and reflectivity of c-BN under different pressures are also analyzed,it was found that they all produce a certain blue shift with increasing pressure and are transparent in the entire visible spectral range as well as a large range of the infrared and ultraviolet(starting from about 204 nm)spectra.