Reinforcement Of Thermally Conductive Network And Performance Of Boron Nitride Nanosheet/PET Nonwoven Fabric Multilayer Composite Films

With the development of electronic devices toward miniaturization,high power and integration,the problem of heat accumulation is becoming more and more serious,and the development of new thermal interface materials（TIMs）that combine excellent thermal conductivity,insulation,and mechanical properties with low cost and facilitate large-scale production has become an important research area.In this work,a three-dimensional BNNS thermal conductivity skeleton（BNNS@NWF）was formed by using PET nonwoven fabric（NWF）as a template with the unique fiber structure of NWF and constructing a continuous BNNS thermal conductivity pathway on the surface of NWF through the dispersion and interfacial binding of carboxylated nanocellulose（CNFs）to boron nitride nanosheets（BNNS）.By combining the filler interlayer synergy strategy,filler hybridization synergy strategy,Spatial Confining Forced Network Assembly（SCFNA）strategy,and the design of sandwich and multilayer structures,a series of composite films with high thermal conductivity and comprehensive performance were prepared by utilizing the bridging effect of alumina（Al₂O₃）.the modification effect of silver nanoparticles（AgNPs）and densification effect of SCFNA with further enhancement of three-dimensional thermal conductive network.The mechanism of thermal conductivity enhancement and the insulation,mechanical and thermal management properties under different strategies were studied and analyzed,and the following main conclusions were drawn:（1）Preparation and properties of Al₂O₃/PDMS-BNNS@NWF-Al₂O₃/PDMS sandwich structured thermally conductive composite films.In order to improve the in-plane thermal conductivity of the composite films and to ensure the insulation performance and flexibility of the composite films,BNNS thermally conductive pathway was constructed by cyclic impregnation adsorption layered assembly based on the filler layer synergistic strategy to obtain BNNS@NWF thermally conductive skeleton.Al₂O₃/PDMS layers were introduced on the surface of BNNS@NWF thermal conductivity skeleton and sandwich structure was constructed.The enhanced three-dimensional thermal conductivity network was constructed by the bridging effect of Al₂O₃ and the interlayer synergistic effect of Al₂O₃ and BNNS.The results show that when the Al₂O₃/PDMS layer Al₂O₃ content is 40 wt%and the number of cycle adsorption of the intermediate layer is 5 times,the in-plane thermal conductivity of the composite film reaches up to 6.42 W/m K,the volume resistivity reaches 1.06×10¹⁵Ω·cm,the tensile strength reaches 17.5 MPa,and also has good flexibility and thermal management properties.（2）Preparation and properties of BNNS-AgNPs/BNNS-BNNS sandwich structured thermally conductive composite films.In order to further enhance the thermal conductivity at low filling rate while ensuring its comprehensive performance,a synergistic strategy of thermal conductive filler hybridization was combined with the construction of sandwich structure in this study.The surface modification of NWF fibers was successfully performed by Polydopamine（PDA）and the surface modification of NWF by in-situ reduction of AgNPs,combined with cyclic impregnation adsorption layered assembly technique.Hence,AgNPs/BNNS hybridized thermally conductive network（AgNPs/BNNS@NWF）was obtained,and enhanced thermal conductive network was constructed,which reduces phonon scattering and improves the thermal conductivity by dual lapping effect of AgNPs and BNNS.The BNNS-AgNPs/BNNS-BNNS composite films were prepared by hot pressing AgNPs/BNNS@NWF as an intermediate high thermal conductivity layer and BNNS@NWF as a thermally conductive insulating layer to construct a sandwich structure.The results show that the composite film achieves an in-plane thermal conductivity of 7.56 W/m K at 34.8 wt%BNNS and 3.3 wt%AgNPs content,along with a volume resistivity of 3.54×10¹³Ω·cm and a tensile strength of 19.45 MPa,as well as excellent thermal management properties.The purpose of integrating thermal conductivity,insulation and mechanical properties is achieved.（3）Preparation and properties of spatially confining reinforced BNNS@NWF multilayer structured thermally conductive composite films.In order to obtain higher thermal conductivity at a certain filling rate,this study combined SCFNA strategy with multilayer structure through the structural design of composite materials,and uses needle template to strengthen BNNS@NWF thermal conductivity skeleton by spatial confining,so that the fibers covered with BNNS were in close contact with each other under the effect of spatial repulsion and fit closely under the action of CNFs to reduce the interfacial thermal resistance.The reinforced BNNS@NWF thermal conductivity network（E-BNNS@NWF）was constructed by the densification and local orientation effect of SCFNA on the fibers.Further,the ME-BNNS@NWF composite film was prepared by multi-layer hot pressing to enable the formed fiber bundles to bridge each other outside the layers to build a complete three-dimensional thermal conductivity network.The results show that when the number of layers is 5,the thermal conductivity of the composite film reaches 3.27 W/m K within 21 wt%filling rate,and the overall TCE reaches 140.4%compared to BNNS@NWF,volume resistivity reaches 5.05×10¹¹Ωcm,and tensile strength reaches 12.9 MPa.The purpose of enhancing the thermal conductivity of the composite film at a certain filling rate through structural strengthening is achieved.