| The problem of heat dissipation management of various high-power components and circuits caused by the rapid development of microelectronics and the promotion of 5G communication is now urgent to be solved.Insulated electronic packaging grade polymer materials,which are often used as protective circuits,need simple and inexpensive methods to improve their thermal conductivity because of their poor thermal conductivity.The method of preparing composites by adding thermal conductive reinforced fillers to polymer materials has this characteristic.Among different kinds of thermal conductive fillers,hexagonal boron nitride(hBN)is of great significance due to its excellent physical and chemical properties,insulation and high thermal conductivity.In order to solve the problem of poor dispersion of hexagonal boron nitride as filler due to its high chemical inertia and poor interaction with polymer materials,it is necessary for exfoliation and functionalization of hexagonal boron nitride with a simple,low-cost and large-scale method.In addition,how to enhance the thermal conductivity of composites under low filling volume in order to avoid the deterioration of processability and strength caused by excessive addition is also the focus of the research.This paper mainly studies the influence of various parameters of mechanical ball milling on the exfoliation effect in order to obtain the best ball milling conditions.Then,under the optimum conditions,the simultaneous stripping and functionalization of hBN were carried out by mechanical ball milling using urea as a ball milling agent.Finally,the obtained products were used to prepare epoxy resin composites by solution blending to improve the thermal conductivity of the composites.The specific research contents and results are as follows:(1)Starting with exploring the working law of mechanical ball milling of hBN,BNNS was prepared with different ball milling parameters and the changes of its size and thickness were analyzed.It is found that when the surface tension of the ball milling agent is similar to that of hBN,it will adsorb on the surface of hBN and buffer the impact force of ball milling,thus reducing the size change of BNNS.When the ball-to-material ratio increases from 10-1to 25-1 and 50-1,the ball milling efficiency is reduced due to the agglomeration of the powder due to the excessive impact chance between the ball materials.When the rotational speed increases from 200 r/min to 400 r/min,the ball mill changes from spilling motion to throwing motion.,which produces strong impact force and shear force.When the rotational speed exceeds the critical value of 800 r/min and reaches 1000 r/min,the throwing will be transformed into a centrifugal motion with only strong shear force.When the milling time increases from 2 h to 16 h,the size and thickness of hBN decrease gradually,but when the time increases from 16 h to 32 h,too long time will lead to the agglomeration of small size BNNS due to excessive surface energy,which will lead to the decrease of ball milling efficiency.(2)Starting with exploring the exfoliation and functionalization mechanism of ureaassisted ball milling hBN,functionalized BNNS was prepared by dry and wet milling and different urea-hBN mass ratio,and the changes of size and thickness and functional group grafting content were analyzed.It is found that both dry milling and wet milling can decompose urea and obtain BNNS grafted with amino groups in the form of covalent bond by the impact of ball milling.And because the urea molecule is more mobile with the aid of water,the size distribution of BNNS obtained by wet milling is more uniform in 150300 nm than that obtained by dry milling,and the hBN can be functionalized more efficiently under the strong impact force of ball milling,and the amount of amino group grafted by wet milling is 8.90% higher than that of dry milling 6.22% under the same amount of solvent / material ratio,and the size distribution of hBN obtained by wet milling is more uniform than that obtained by dry milling,and the functionalization of urea can be carried out more efficiently under the strong impact force of ball milling.Under the condition of wet milling,when the solvent-material ratio increases from 1-1 to 16-1,the impact will be buffered due to the adsorption of a large number of urea molecules on the surface of hBN,and the stripping effect of hBN will be weakened,but more functional groups will be provided at the same time,so that the amount of grafting will increase from 1.30% at 1-1 to 12.26% at 16-1.In addition,if the ratio of solvent to material is increased to 64-1,the crystallization of urea solution will hinder the movement of ball pellets,which will greatly weaken the impact force of ball milling,so that the peeling and functionalization effect of hBN becomes worse,and the amount of grafting is only 4.27%.(3)Starting with the construction of low-filling heat conduction network,epoxy resin composites with various forms of hBN as fillers were prepared.It mainly includes the effects of the size of hBN,the functionalization of hBN,and the proportion of mixing different sizes of hBN on the properties of the composites.The results show that the decrease of hBN size is beneficial to the formation of heat conduction network at the same filling amount.The functionalization of hBN can reduce the interfacial thermal resistance.The mixed filler can make the small size hBN fill the gap of the large size hBN to form a more complete heat conduction network and increase the utilization of the filler.The largest increase in the thermal conductivity of the sample is that when the total filling amount of the mixed filler is10 wt%,the ratio of large / small size hBN is 2.5 / 7.5,the thermal conductivity of the composite can reach 0.384 W/m K,which is 96% higher than that of pure epoxy resin,and the thermal stability is also improved. |
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