Study On Preparation Of Two-dimension Boron Nitride Crystal By Chemical Vapor Deposition And Its Properties

Boron nitride(BN)is an important III-V compound,which possesses four crystal forms,i.e.hexagonal phase(h-BN),cubic phase(c-BN),rhombohedral phase(r-BN),and wurtzite phase(w-BN).The structure of hexagonal boron nitride(h-BN)is analogous to that of graphite.Hexagonal boron nitride is also called white graphite.Two-dimension(2D)boron nitride crystal has many unique properties,such as high surface area and stable chemical properties,so it can be used as non-wetting coatings,corrosion protection coatings,insulating thermal conductive filler in composite materials,field emitters,and substrate material for graphene.Mechanical cleavage,sonication-assisted direct solvent exfoliation and chemical vapor deposition are the main methods used to prepare 2D boron nitride.Most methods have their unique advantages for specific targeted applications but often there are shortcomings as well.Therefore,it is necessary to develop an easier and cheaper method for synthesis of 2D boron nitride with high crystallinity and high purity.In this dissertation,we studied the preparation of ammonia borane,which is the precursor of chemical vapor deposition for BN preparation.And then we studied the pyrolysis regularity of ammonia borane in airtight reactor.According to the pyrolysis regularity of ammonia borane,we prepared 2D boron nitride crystal successfully.Crystallinity of 2D boron nitride became better after post-annealing process.We also studied the properties 2D boron nitride crystal.The details of the study and the the results are as follows:We used sodium borohydride,ammonium formate and dioxane to prepare ammonia borane successfully and decomposition process of ammonia borane was investigated by thermogravimetric analysis.There are two stages in decomposition process of ammonia borane.Firstly,ammonia borane almost loses 15.47% of its initial mass in the temperature between 118 ? and 126 ?.Secondly,when temperature is between 126 ? and 176 ?,ammonia borane loses its another 34.9% of initial mass.The pyrolysis regularity of ammonia borane in airtight reactor was studied systemically.Heating temperature and holding time influence the pyrolysis products of ammonia borane.When the heating temperature is below 400 ?,there is no boron nitride in the products.When the heating temperature is above 600 ?,boron nitride appears.The crystallinity of 2D boron nitride became better when the heating temperature increases.2D boron nitride crystal is prepared by reaction of two stages of heating and insulation.We studied the effects of holding time and mass of ammonia borane on the structure of boron nitride.We successfully prepared 2D boron nitride.When the mass of ammonia borane is 0.05 g,the heating temperature of first stage is 400 ?,the holding time of first stage is 30 min,and the heating temperature of second stage is 1000 ?,the holding time of second stage is 60 min,the size of 2D boron nitride crystal is 15 ?m.Some of the 2D boron nitride crystals pile up.Post-annealing process has effects on the shape and crystallinity of the 2D boron nitride.It is indicated that the crystallinity of the 2D boron nitride is improved by annealing process in NH3.The TG curves show that the 2D boron nitride starts to be oxidized at 900 ?.There are three peaks at 213 nm,250 nm and 323 nm in the ultraviolet-visible absorption spectra.There are also three peaks at 332 nm,425 nm and 511 nm in the photoluminescence spectra.All of these results turn out that 2D boron nitride can be used for ultraviolet light emission.