Synthesis And Preparation Of Precursors Of Hexagonal Boron Nitride

In this paper,the molecular structure of hexagonal boron nitride precursor was studied.Three precursors were prepared by three different routes:Diethanolamineborate?DEAB?,trichloroborazine?TCB?,tris?diethylamino?borazine.The effects of different process conditions on the synthesis of precursors were studied respectively to determine the optimum synthesis conditions.The molecular structures of the precursors were characterized by IR,¹HNMR and X-ray diffraction.The advantages and disadvantages of different precursors were also discussed.At the same time,DEAB was sintered at low temperature and then refined by high temperature to obtain hexagonal boron nitride particles.The crystallinity and morphology of hexagonal boron nitride under different sintering conditions were investigated.The precursor DEAB was prepared by using boric acid and diethanolamine as raw materials.The optimum synthesis conditions were as follows:the molar ratio of raw materials n?H₃BO₃?:n(C₄H₁₁NO₂)=1:2.2,reaction temperature 130?,reaction time 5h,the final yield was 85.3%.Under low temperature sintering in ammonia atmosphere,the optimum sintering conditions are:sintering temperature 1200?,sintering time 4h.The hexagonal boron nitride crystals obtained after the high-temperature refining have high crystallinity and the crystal grains are lamellar structures with a diameter of about 10-15?m and a thickness of about 1?m,and the surface is dense and smooth without pores.The precursor of TCB was prepared by boron trichloride and ammonium chloride,and the utilization of boron trichloride in the reaction was improved by changing the way of injecting BCl₃ into the solution.The optimum synthesis conditions were:the molar ratio of raw materials n?BCl₃?:n?NH₄Cl?=1.2:1,reaction temperature 100?,reaction time 15h,toluene as solvent,BCl₃ gas was injected into three time periods in the reaction process,the final yield was 91.3%.The monomer of Dichloro?diethylamino?borane was prepared by boron trichloride and diethylamine as raw materials followed by reaction with ammonia to synthesize tris?diethylamino?borazine as precursor.The optimum synthesis conditions were:the molar ratio of raw materials n?BCl₃?:n(C₄H₁₀NH)=1:1.1.toluene as solvent,After 2h reaction at low temperature and then increased to 30?for 5h.the yield was 80.6%.Then the monomer was carried out with ammonia at 0?for 1.5h,then heated to 25?for 3h and then heated to 50?for 3h to obtain the precursor tris?diethylamino?borazine.The yield was 84.7%.