Preparation,Modification,and Applications Of Boron Nitride Nanosheets

Materials are an important embodiment of human civilization,and it is also an essential physical foundation for the evolution of production and living in the modern society.The research of advanced materials to achieve the liberation and development of productivity is a significant theme of today’s scientific research.Boron nitride nanosheet(BNNS)has a two-dimensional structure similar to graphene,which is called"white graphene",has excellent in-plane mechanical strength,thermal conductivity,and many unique and compelling properties.It has great potential application prospects in the microelectronics industry,aviation industry,military industry,and nuclear industry.However,there are two problems in the in-depth research and industrialization of BNNS:one is the lack of existing exfoliation preparation methods,it is difficult to realize the large-scale preparation of BNNS,and the other is the difficulty of surface modification of h-BN due to its chemical inertness.In this thesis,BNNS is selected as the research object,novel methods for the exfoliation preparation and modification of BNNS were designed and developed,which combined with the basic principles of radiation chemistry and mechanochemistry from the perspective of the intercalation exfoliation preparation and mechanical exfoliation modification of BNNS.Meanwhile,according to the structure and properties of the exfoliated and modified h-BN,their diverse applications in the fields of photocatalysis,dielectric materials,radiation-resistant flexible devices,and engineering materials have been investigated.The main research results are as follows:1.A novel method of radiation-induced reduction-exfoliation of h-BN in alcohol aqueous solution was proposed.Through the combination of the volume expansion effect caused by y-ray radiation reduction and aggregation growth of the interlayer metal ions of the h-BN crystal with the synergistic exfoliation and stabilizing effect of the stabilizer molecule,this method can achieve the exfoliation of h-BN layers and obtain nickel nanoparticle-loaded boron nitride nanosheets(Ni/BNNS)simultaneously under room temperature and normal pressure.The intercalation behavior of different metal ions in h-BN crystals and the influence of factors such as absorbed dose,the concentration of Ni2+ ion,and the type and concentration of stabilizer on the morphology of the exfoliation product were investigated,which confirmed the mechanism and feasibility of radiation exfoliation.The results indicate that the minimum thickness of obtained BNNS by this method is 2.4 nm,and the proportion of BNNS less than 5 nm can reach 55%.In addition,the obtained Ni/BNNS has high catalytic activity and cycle performance in the reduction of 2-nitroaniline by sodium borohydride and the photocatalytic hydrogen evolution.2.The y-ray radiation chemical effects of h-BN and ball-milled BNNS and its photocatalytic properties were studied.The results reveal that y-ray radiation effect of h-BN and BNNS is mainly the formation of defects,which leads to changes in the band structure and improvement of carrier separation efficiency,and the visible light catalytic hydrogen evolution of h-BN and BNNS in water is realized finally.After irradiation,both h-BN powder and BNNS displayed obvious visible light catalytic activity,the hydrogen production of h-BN increased significantly with the absorbed dose,while the hydrogen production of BNNS increased first and then decreased with absorbed dose.The maximum hydrogen production can be increased by two orders of magnitude compared with h-BN before irradiation(550 times),and the irradiated samples also show excellent performance in photolysis of methanol and the selectivity of CO in the gas products.3.The exfoliation preparation and covalent modification of BNNS was achieved by tannic acid-assisted ball milling method in one step.The preparation of tannic acid modified BNNS composite hydrogel and its application in radiation-resistant pressure sensor were discussed.A tannic acid-assisted ball milling method was firstly utilized to prepare tannic acid-modified BNNS(BNNS-TA)in one-step.After that,BNNS-TA was dispersed into the acrylamide(AAm)monomer aqueous solution to fabricate PAAmbased hydrogel by polymerization of AAm,and then immersed them in a ferric chloride solution to fabricate BNNS-TA/Fe3+/PAAM composite hydrogel.The obtained BNNSTA/Fe3+/PAAm hydrogel exhibits excellent mechanical properties and pressure-sensing performance,and can be assembled into pressure-sensitive sensors for applications in control circuit and behavior monitoring.More remarkably,the composite hydrogel displays unique radiation resistance and can maintain mechanical strength and pressuresensing characteristics under y-ray radiation,which provides a new idea for the design and preparation of radiation-resistant flexible devices.4.Chitosan-modified BNNS was prepared by chitosan-assisted ball milling method,and BNNS aerogel was fabricated by chemical cross-linking and its mechanical and dielectric properties were characterized.In this section,the chitosan modified BNNS was prepared by ball milling,and then the BNNS aerogel was obtained by the reaction of the crosslinking agent with the chitosan on the surface of BNNS.With the support of the chitosan cross-linked network,the BNNS aerogel demonstrates outstanding mechanical strength and dielectric properties,which is expected to be applied as a high-strength and low-dielectric material.Moreover,the phase change material based on the aerogel also exhibits heat storage,shape stability,zero leakage,and enhanced thermal conductivity.This work provides a new strategy for the preparation of high-strength BNNS aerogels.5.The composite material was obtained by blending the modified BNNS and polyethylene terephthalate(PET),and their radiation resistance was investigated.During the experiment,the different thickness and surface properties of h-BN were selected for solution blending with PET,and the blending composites were processed by injection molding or hot pressing method.The mechanical properties,thermal conductivity,dielectric,and radiation resistance properties of BN/PET composites were discussed.The introduction of BN improved the thermal conductivity,dielectric properties,and neutron shielding performance of the PET composites,and the thinner and surface-modified BN also improved the radiation resistance performance of PET under neutron radiation and γ-ray radiation with high dose(>120 kGy).The changes of the molecular chain and crystalline structure of the BN/PET composite under neutron and y-ray radiation were expounded,which provided a practical reference for the design and preparation of radiation-resistant multifunctional polymer materials.