Study on Controllable Preparation And Thermal Conductivity of Aligned Boron Nitride/Polymer Composites

Author:Yu Cui Ping

Supervisor:fan xiao dong tian wei yao ya gang


Degree Year:2018





With the miniaturization,high-degree integration and multifunctionalization of modern electronic devices,considerable heat is accumulated during their operation.“Hot spots” were generated due to the excessive heat in some parts of the equipments.Long-term high temperature would seriously damage the devices’ performance,reliability and lifetime.Thus,the development of high-performance thermal management materials is crucial in addressing the hot spots that occur during heat transfer.Since polymer was ease of processing,light weight,and low cost,thermally conductive polymeric materials have strong potential as thermal management materials.In particular,because of the insulating property of polymer,designing polymer-based composites with excellent thermal conductivity could meet the demand of some thermal management materials.Since polymer exhibited a poor thermal conductivity between 0.1-0.5 W/(m·K),they could not meet the requirement of fast heat conduction for the advanced electronic products.Inorganic ceramic fillers such as aluminum nitride,alumina and boron nitride were added to the polymer matrices to improve the thermal conductivity.Usually,a high ceramic content is necessary for the polymer to gain a substantial enhancement in thermal conductivity,which inevitably led to the difficulty in the preparation process and the processing conditions.Therefore,the development of the preparation strategies for thermally conductive composites have generated great attention.Hexagonal boron nitride(h-BN),also called as “white graphite”.Attributed to its unique structure characteristic,h-BN possesses some unique properties like electrical insulation,superb anti-oxidation ability,a pure white appearance,and so on,which make it a superior candidate as the filler in composites that need thermally conductive while electric insulation.Based on this,a series of h-BN/polymer composites with high thermal conductivity were prepared via designed methods.The structure,morphologies,thermal and mechanical properties were studied.The composites were further applied in thermal management.Meanwhile,the corresponding heat transfer mechanism in this structure was explored and supported by molecular dynamics simulations.The main contents and results are summarized as follows:Firstly,polydopamine(PDA)-coated exfoliated h-BN,denoted as exfoliated h-BN@PDA,was obtained via the ball milling process.The reaction components were composed of h-BN powder,dopamine and Tris-buffer solution.Since h-BN subjected shear force introduced via the ball milling procedure,the layers of h-BN decreased to less than 20 layers.Meanwhile,dopamine self-polymerized into PDA and coated on the exfoliated h-BN.The PDA contents on the surface of exfoliated h-BN were calculated to be about 3 wt%.The exfoliated h-BN@PDA was filled in cellulose nanocrystal(CNC),followed by the vacuum-assisted filtration process.Thus,exfoliated h-BN@PDA/CNC composites with a highly aligned structure was obtained.The strong hydrogen bonding occurred between the modified h-BN and CNC.Exfoliated h-BN@PDA/CNC composites with 86 wt% h-BN showed a certain flexibility.The thermal conductivity of 94 wt% exfoliated h-BN@PDA/CNC composite was about 40 W/(m·K).The heat dissipation ability of the exfoliated h-BN@PDA/CNC composites become more efficient with higher filler loadings.Secondly,h-BN/thermoplastic polyurethane(TPU)composite films were fabricated by a combination of the ball milling process and the hot-pressing treatment.h-BN/TPU mixtures were obtained by solution ball milling of h-BN and TPU in N,N’-dimethylformamide and subsequent precipitating in ethanol.The resulted h-BN/TPU coagulated solids were then hot pressed to fabricate h-BN/TPU composite films.h-BN sheets dispersed well and oriented parallelly in the TPU matrix.The h-BN/TPU composite film showed outstanding flexibility even with 80 wt% h-BN.An ultrahigh in-plane thermal conductivity of about 50.3 W/(m·K)was achieved in 95 wt% h-BN/TPU composite film.The corresponding out-of-plane thermal conductivity reached 6.9 W/(m·K).The stable temperature of 95 wt% h-BN/TPU composite film decreased by about 40 ℃ compared with that of TPU film when they were employed as heat spreaders for light-emitting diodes.Thirdly,h-BN/SEBS composite films were fabricated via the ball milling process followed by the hot-pressing treatment.h-BN was well-dispersed and highly-aligned in the h-BN/SEBS composite films.The thermal conductivity of 95 wt% h-BN/SEBS composite film was about 45 W/(m·K).The corresponding out-of-plane thermal conductivity reached 4.4 W/(m·K).The in-plane and out-of-plane percolation threshold of h-BN/SEBS composites were over 40 wt% and 60 wt%,respectively.And these results were supported by molecular dynamics simulations.Based on the molecular dynamics simulations results,the construction of heat conductive networks was the main thermally conductive mechanism in oriented h-BN/SEBS composites.The linear coefficients of thermal expansion(CTE)values for the composites decreased with increasing the h-BN contents.The CTE of 95 wt% h-BN/SEBS composite film was 16 ppm/K.The break stress of 95 wt% h-BN/SEBS composite film was about 2.6MPa.The h-BN/SEBS composite film was also used to cool the CPU,resulting in a decrease by about 4 ℃ in the stable temperature compared with that of pure SEBS film.Finally,vacuum filtration-slicing up-infiltration was used to fabricate vertically oriented h-BN/epoxy(EP)composites.h-BN aligned in the through-plane direction in the h-BN/EP composites.The through-plane thermal conductivity of oriented h-BN/EP composites was 9 W/(m·K)at 44 vol% h-BN loading,which was more than twice higher than that of disordered h-BN/EP composites(3.5 W/(m·K))for the same h-BN loading.The tensile strength of oriented h-BN/EP composites was 25 MPa,and this value was comparable to that of disordered h-BN/EP composites(26 MPa).An infrared camera was employed to record the temperature response of h-BN/EP composites during heating.The oriented h-BN/EP composites showed more excellent thermal conductivity.