Study On Preparation And Properties Of Hexagonal Boron Nitride Synthesized Via High-Energy Microwave Heating Method

As a non-metallic material with similar crystal structure to graphite,hexagonal boron nitride(h-BN)has excellent thermal conductivity,high temperature resistance,oxidation resistance,electrical insulation and chemical stability.Therefore,h-BN shows a very broad application prospect in fields of adsorption,ultraviolet laser device,sensor,anti-oxidation coating and so on.But for now,the practical application of h-BN has been still limited by two factors.On the one hand,the related theoretical research needs to be further discussed.On the other hand,the exsisting synthesis approaches have various problems to be resolved and improved,such as harsh conditions,complicated processes,high cost and so on.In this paper,h-BN was efficiently and rapidly synthesized via direct high-energy microwave heating.The structures and morphologies of the samples were characterized by various analytical test methods,and the synthesis mechanism under microwave irradiation was discussed.In addition,the adsorption performances for methylene blue and congo red were analyzed.The effects of experimental conditions containing concentration,temperature and pH on adsorption properties of the product were investigated by setting different comparative tests.The main research contents were shown as follows:(1)The whisker-like h-BN was prepared by high-energy microwave method without catalysts and protective gas.Compared with traditional preparation methods,the microwave process is simple,safe and environmental-friendly.During the experiment,the precursor was firstly obtained in the form of precipitation from the mixed solution of melamine and boric acid.Using carbon fiber as microwave absorbent,and then h-BN was synthesized by heating the mixture of dried precursor and carbon fiber through microwave irradiation.After further analysis about the structures,compositions and morphologies of samples,it was found that there was a morphological similarity between the precursor and the product.The synthesis mechanism of the product was preliminarily discussed.It was thought that the cleavage and transformation of the precursor happened quickly under microwave irradiation,and the rod-like precursor finally turned into elongated h-BN whiskers with similar morphologies.(2)During the synthesis of precursor and the heating process under microwave irradiation,the conditions of precipitation and the microwave power were adjusted,respectively.The results showed that the morphology and size were not affected by the adjustment of microwave power.However,the adjustment of experimental conditions through changing the precipitation temperature could have an influence on supersaturation,which ultimately affected the size of precursor.After microwave irradiation,the corresponding products were obtained quickly.The morphologies and chemical structures of as-obtained products were not changed,but the size distribution was effectively improved.The product derived from the precursor precipitated at80℃ exhibited regular morphology,uniform size,and good dispersion.The diameters and lengths of h-BN whiskers were around 200-500 nm and 25-50μm,respectively.The specific surface area of the product was up to 510.941 m~2/g.(3)The product derived from the precursor precipitated at 80℃ was chosen to explore the adsorption properties for methylene blue(MB)and congo red(CR)under different external conditions.The results showed that the product exhibited good adsorption performances for MB and CR.The maximum adsorption capacities of MB at different temperatures(273 K,293K,313 K)were around 228 mg/g,235 mg/g and 242 mg/g,respectively.The maximum adsorption capacities of CR at different temperatures(273 K,293 K,313 K)were around 71mg/g,150 mg/g and 179 mg/g,respectively.The superior adsorption properties might be due to theπ-πstacking occured between benzene molecules and B-N rings which benefited in the enhancement of interaction.In addition,the defects and oxygen-containing functional groups accompanied with the products might also contribute to the adsorption property of the products.Based on the results under different test conditions,it was found that the effects of initial pH varied with different dyes.With the increase of initial pH,the adsorption performance for MB was gradually enhanced,while the adsorption performance for CR was gradually weakened.Furthermore,the kinetic fitting results showed that the pseudo-second-kinetic model was more suitable to be applied in describing the adsorption process,indicating that the adsorption rate was mainly controlled by the chemical adsorption mechanism.The adsorption isotherms showed better consistency with Langmuir isotherm,which indicated that the adsorption was monalayer adsorption.Besides,the maximum adsorption capacity rised with the increase of temperature,suggesting that the increase of temperature was beneficial to the adsorption property.