A Study On Preparation Of Hexagonal Boron Nitride Nanosheets By Ball Milling

Boron nitride nanosheets( BNNSs) possess superior chemical and physical properties such as high oxidation resistance at high temperature, high chemical stability, high thermal conductivity, and high band gap. These superior properties render BNNSs a promising material for application in a wide variety of fields including high temperature composites, thermal conducting composites, electronic devices, and catalyst supports. Therefore, BNNSs have received extensive attention from researchers in different fields. However, there still are a number of problems in the preparation of BNNSs. For example, the preparation technology is not well developed yet and effective stripping agent is not found yet. Therefore, for the practical application of BNNSs, it has become a pressing task to develop an effective method for the large-scale production of BNNSs. To solve this problem, this thesis used a simple method of mechanical ball milling to exfoliate boron nitride, obtaining the BNNSs. Dispersion behavior of the BNNSs in the water was investigated and stable BNNSs suspension was obtained. BNNSs/polymethyl methacrylate(PMMA)composites were prepared, which shows much higher thermal stability property and thermal conducting property than PMMA. The main research contents and results are as follows:The dependence of the size of BNNSs was investigated on ball milling process parameters, including ball milling mode, ball milling agent, diameter of the milling ball, milling ball to boron nitride ratio, rotating speed, milling time, and milling agent to boron nitride ratio. Optimal conditions of ball milling were determined in this study,which were wet milling, isopropyl alcohol as ball milling agent, using mixed balls,ball to boron nitride ratio of 100:1, rotating speed of 400 r/min, milling time of 24 h,and agent to boron nitride ratio of 60:1. The BNNSs obtained under optimal conditions are about 2.5 μm in size and 50 nm in thickness averagely.The factors influencing the dispersion stability of BNNSs in the water were studied, including the effect of the type and dosage of the dispersant, the dispersion temperature and the size of the BNNSs. The concentration of the BNNSs in the suspension was determined and the time of stable dispersion to sedimentation of the BNNSs suspension was obtained. It was found that the best dispersant is sodium dodecyl sulfate(SDS) and the optimal SDS dosage is 1.5 mg. It was also found thatthe higher the temperature, the lower is the dispersion concentration. With increasing temperature the concentration of the BNNSs in suspension decreases until 250 oC.When the temperature is higher than 250 oC, the BNNSs concentration keeps constant at about 0.30 mg/ml with further increasing the temperature. Furthermore, the dispersion concentration of the BNNSs suspension increases with decreasing the size of the BNNSs. Highly stable BNNSs suspension were obtained, and the time of stable dispersion to sedimentation of the BNNSs suspension was about 15 days.Finally, the properties of BNNSs/PMMA composite were studied, including surface modification process of the BNNSs, and the effect of BNNSs surface modification on the properties of composite materials, and the effect of added BNNSs amount on the thermal stability property and thermal conducting property of the composite. Experimental results indicate that the surface-modified BNNSs could be better dispersed in PMMA and the thermal stability property and thermal conducting property of BNNSs/PMMA composites were greatly improved comparing with PMMA. The BNNSs/PMMA composite containing 70 wt% BNNSs shows the best thermal stability property and thermal conducting property, T0.1 is about 364 oC and the thermal conductivity is about 7.22 W/(m·K).