Synthesis And Property Research Of Boron Nitride Nanotubes And Nanosheets

In this thesis,the preparation,functionalization and applications of the boron nitride nanotubes and nanosheets are reviewed,and it is concluded that the synthesis is still the main issue hindering the practical applications of boron nitride nanomaterials.On this basis,this thesis proposed several novel methods to effectively fabricate boron nitride nanotubes and nanosheets.The structures and properties of the obtained nanomaterials were characterized,and the applications in mechanics,thermotics,and tribology were also briefly studied.The main contents and conclusions are listed as follows.(1)B/MgO is the classical precursor in synthesizing boron nitride nanotubes by chemical vapor deposition.However,this precursor only functions well in vertical tube furnace,and has a low efficiency in horizontal tube furnace.X-ray diffraction indicates that much magnesium borate is produced during reaction,which has high melting point and will lead to the inactivation of B2O3.This thesis respectively used B2O3 and ammonium tungstate as alternatives of MgO to synthesize boron nitride nanotubes on stainless steel substrates.These methods can obtain high-quality boron nitride nanotubes and efficiently avoid the formation of borate.(2)MgO is considered as a catalyst in B/MgO method for the growth of boron nitride nanotubes,but the formation of borates will lower the catalytic efficiency and lead to the unsatisfaction of vapor-liquid-solid mechanism for MgO catalyst.By carefully analyzing the formed chemicals,we found that MgB2 can be stably maintained as liquid at the growth temperature of boron nitride nanotubes,which may be the possible catalyst.Thus,MgB2 was served as catalyst in a horizontal tube furnace to grow boron nitride nanotubes,and the results indicate that this catalyst has high catalytic efficiency and good reproducibility,and is suitable to various boron sources,such as B2O3,H3BO3,and B/CaO.The molecular dynamics simulation and transmission electron microscopic characterizations prove that the high catalytic activity of MgB2 is derived from the liquid characteristic and the strong nucleation ability for boron nitride nanotubes.(3)Based on the high catalytic activity of MgB2,ball-milling and annealing B2O3/B/MgB2 at ambient pressure can produce boron nitride nanotubes at a high yield,and each time can obtain about 200-300 mg nanotubes.The synthesized nanotubes can enhance the tensile strength and thermal conductivity of pure thermoplastic polyurethane.(4)By combining the methods of ball-milling and annealing,Lewis acid base interactions,and ultrasonic exfoliation,this thesis fabricated highly dispersed thin layer boron nitride nanosheets,which has a yield of 40%.The high yield is derived from the synergistic effect of multi-step processing,and the condition tests suggest that the ball-milling and annealing makes the main contribution to the final yield.These nanosheets can also improve the thermal conductivity of pure thermoplastic polyurethane.(5)Template method is an effective route to fabricate boron nitride nanosheets,and the reported template chemicals often are urea,guanidine hydrochloride,and melamine,which have low decomposition temperature and are volatile.To obtain nanosheets with few layers,much template chemical should be added.This thesis used Zn(Ac)2 as template chemical to fabricate boron nitride nanosheets,which can obtain few layered nanosheets under a low dosage.The as-prepared nanosheets can be served as additives to enhance the tribological performances of pure water.The high efficiency of Zn(Ac)2 is caused by the multi-step template action,and the produced ZnO is gradually reduced by NH3 with the formation of boron nitride nanosheets.(6)Boron nitride nanosheets film can be deposited on silicon by the nitridation of MgB2 powder with the assistance of FeO,and this film can decrease the friction and wear of bare silicon wafer.