Effect And Mechanism Of The Three-dimensional Boron Nitride Structure On The Performance Of Thermally Conductive Polymeric Materials

The rapid advancement of technology in the fields of aerospace,defense and military industry,and microelectronics has put forward more stringent requirements for device thermal management systems.Due to the unique advantages of thermally conductive polymeric materials,they have shown good potential and obtained certain applications.At present,improving the thermal conductivity of polymer materials is usually achieved by adding thermally conductive functional fillers to the polymer matrix to construct a thermally conductive pathway,but the overall performance of the material is still difficult to fully meet the application requirements of different fields.To this end,this thesis used hexagonal boron nitride（h-BN）with electrical insulation properties as the thermally conductive filler and prepared polymer matrix composites by constructing a three-dimensional BN thermally conductive network.The effect and mechanism of the BN network structure and interaction on the thermal conductivity,mechanical properties,and electrical properties of the material were studied,and its application performance in thermal management occasions such as CPU was tested.Aiming at the problem of poor dispersibility of thermally conductive fillers and high filler content can effectively improve the thermal conductivity of polymeric materials,a polyamide-imide（PAI）composite filled with a BN interpenetrating network structure was prepared by the"cross-linking-freeze-drying-infiltration"method.Studies have shown that by regulating the hydrogen bond interaction between hydroxylated BN and polyvinyl alcohol（PVA）,the morphology and structure of the three-dimensional BN network could be regulated.When the BN loading is only 4wt%（2vol%）,the thermal conductivity of the composite material reaches 1.17W·m^-1·K^-1,and the thermal conductivity enhancement is 409%,realizing high thermal conductivity and thermal conductivity enhancement under low filler content.At the same time,the thermally conductive polymeric material maintains good electrical insulation and mechanical properties and exhibits excellent heat transfer performance and device thermal management capabilities.By designing a dual-cross-linked network with strong covalent bonds as permanent cross-linking and tunable hydrogen bonds as recoverable cross-linking,combined with a heat-sensitive phase change material polyethylene glycol（PEG）to play a synergistic effect,a thermally conductive polymeric material with thermal response properties was prepared.Attribute to the guiding effect of molecular simulation,the prepared dual-cross-linked network has excellent mechanical properties,especially elasticity.Combined with the optimized design of the thermal conductive network,the composite material exhibits enhanced thermal performance,thermal response capability and has active thermal management effects when applied to electronic devices.The shape of the composite material changes,increasing the heat dissipation area,the equilibrium temperature difference of the device can reachΔT_max=10°C,and indicating device temperature capability through the shape change.This work innovatively makes a preliminary exploration of the intelligentization of thermal management materials and provides research and development ideas for the development of smart materials.The vertically aligned three-dimensional BN network structure was prepared by the directional freezing strategy,and the thermally conductive polymeric material was prepared by PEG vacuum infusion.Studies have shown that by designing a series of functionalized BN/PVA suspensions with different BN content,the wall density of the three-dimensional BN network can be regulated so that the thermal conductivity and mechanical properties of the composite show a strong correlation with the wall density of the three-dimensional BN framework.Proper wall density would bring the greatest thermal conductivity enhancement and the best mechanical properties.The thermal conductivity reaches 1.3W·m^-1·K^-1,the thermal conductivity enhancement is 550%,and Young’s modulus reaches 23.3MPa.The Young’s modulus enhancement is 356%.The synergistic enhancement of the thermal conductivity and mechanical properties of the composite material is realized,thereby overcoming the damage to the mechanical properties of the polymer material by the addition of fillers.The thermally conductive polymeric material can effectively dissipate the heat of the LED lamp,and the maximum temperature difference of the LED lamp during the heating process isΔT_max=34°C,showing a good prospect for thermal management applications.Finally,the relationship model between thermal conductivity and Young’s modulus of composite materials was established by using the data-driven method“sure independence screening and sparsifying operator（SISSO）”.The BN/polyurethane（PU）@polydopamine（PDA）three-dimensional network was prepared by a facile water-based dip-coating and surface treating process,and then the thermally conductive polymeric material was prepared by PEG vacuum infusion.Studies have shown that due to the formation of a complete and continuous thermally conductive network by PDA@PU skeleton and the hydrogen bond interaction between PDA and functionalized BN,the thermal conductivity of the composite material reaches 2.4W·m^-1·K^-1,the thermal conductivity enhancement is1100%.The mechanical performance is improved by about 4 times,and at the same time,it has excellent shape memory properties,which realizes the improvement of the overall performance of the thermally conductive polymeric material and increases the multifunctionality.The thermally conductive polymeric material can effectively dissipate the computer CPU,showing a good application prospect for thermal management.The preparation method is simple and easy to operate and has certain guiding significance for the large-scale thermally conductive polymeric material.