Preparation Of Boron Nitride Nanosheets And Study Of Their Thermal Conductivity

The rapid development of the electronics industry has led to an increase in the power density of electronic products,and the high power density leads to an increase in heat,which has an impact on the lifetime and performance of electronic products,thus placing higher demands on the thermal management of electronic products.Boron nitride nanosheets（BNNS）with a graphene-like two-dimensional（2D）structure have attracted much attention due to their high thermal conductivity,low thermal expansion,good chemical stability and excellent electrical insulation properties,making them ideal candidates as thermally conductive fillers for the preparation of thermally conductive composites.However,The efficient preparation of BNNS with large lateral sizes and low thicknesses is still a challenge.This paper focuses on the preparation of high quality BNNS by three different methods and its composite with polymers to prepare thermally conductive composites to investigate their thermal conductivity and heat dissipation properties.The specific work is as follows:1.Coordination polymers（CP）are capable of undergoing a phase transition to a liquid state,forming an ionic liquid analogue with multi-stage ionic fragments that can be intercalated into the hexagonal boron nitride（h-BN）layers as an aid to their exfoliation during the mechanical ball milling process.The BNNS prepared asisted by CPs has a transverse dimension of approximately 200 nm and a thickness of 2 nm,with a yield of 34%.PVA/BNNS composite was prepared by a scratch coating method.The composite exhibited excellent thermal conductivity,with an in-plane thermal conductivity of up to 11.5 W m^-1 K^-1 with 30wt%BNNS addition,at the same time,the thermal stability of composite was significantly improved.Using the composite as a thermal interface material,PVA/BNNS-30 is 4.2 ^oC lower than pure PVA and shows potential applications in thermal management materials.2.BNNS were prepared by liquid-phase sonication using N,N-dimethyl-m-toluamide（DEET）,a highly efficient non-toxic mosquito repellent,as a green solvent,with exfoliation efficiencies of up to 40%,and the resulting BNNS were 1-3μm in lateral dimensions and 4 nm in thickness,which has potential applications in the preparation of large-size BNNS.Mechanistic studies have shown that the polarity of the solvent and hydrogen bonding interactions are the key factors affecting the exfoliation efficiency and stability.The obtained BNNS was blended with polyvinylidene fluoride（PVDF）using a solution blending-hot pressing method to prepare PVDF/BNNS composite t.The results show that BNNS is highly oriented within the polymer matrix,and the in-plane thermal conductivity of the composite is as high as 11.61 W m^-1 K^-1 at 30 wt%BNNS addition.As thermal interface materials,the surface temperature of the PVDF/BNNS-30 was 78.3 ^oC after 2 min,8.7 ^oC lower than that of pure PVDF,showing excellent heat dissipation performance.3.BNNS was prepared under ultrasonication using pre-expanded h-BN via CO2 and NH3gas produced by ammonium bicarbonate（NH4HCO3）decomposition high quality BNNS was obtained with a yield of 25.9%,lateral dimensions of 1-3μm and thickness of 2.5 nm.cellulose nanofibres（CNF）/BNNS composite as thermal interface materials was prepared using vacuum filtration.BNNS has a distinctly oriented structure in the polymer matrix and shows an in-plane thermal conductivity of up to 18.86 W m^-1 K^-1 when 50 wt%BNNS is added.The maximum temperature of CNF/BNNS-30 was 67 ^oC after 2 min of power application,which is 13 ^oC lower than that of pure PVDF,when the composite was placed under an LED lamp as a thermal interface material for heat dissipation measurements.This shows excellent thermal performance.