Interface Modification And Synergism Enhancement Of Boron Nitride/Polyimide Thermally Conductive Composites

In order to build an ideal three-dimensional heat conduction network in polyimide（PI）films,it is necessary to break through the technical bottleneck as a thermal management material.This work was devoted to studying the theory of interfacial modification of thermal conductivity and synergism of thermal conductive fillers,so as to enhance the thermal conductivity of PI composites.The research strategies of two-step synergism,directional synergism and multi-dimensional synergism were proposed for the four thermally conductive fillers from spherical（zero-dimensional）,rod-shaped（one-dimensional）,lamellar（two-dimensional）and four-needle（three-dimensional）,in which the surface of spherical alumina（Al₂O₃）and tetrapod-like zinc oxide（ZnO）were modified by silane coupling agent（KH550）,and the surface functional groups of boron nitride（BN）were modified by calcination method.Then boron nitride coated alumina（Al₂O₃@BN）and boron nitride coated zinc oxide（ZnO@BN）thermal conductive fillers were successfully prepared by electrostatic self-assembly.In addition,ferric oxide coated（FF）composite particles were prepared by liquid phase deposition-air oxidation method and calcination method,and graphene oxide（GO）,was prepared by improved Hummers method,and then the thermal conductive composites of GO coated FF composite particles（GF）were successfully prepared by Polydopamine（PDA）coating technology.The interface regulation mechanism of Al₂O₃@BN,ZnO@BN and GF thermal conductive composite fillers were studied respectively,and the synergistic enhancement effect of the thermal conductive composite fillers and BN and the magnetic field-induced regulation of（MF）were investigated.Finally,the composite films of Al₂O₃@BN&BN/PI,GF-BN/PI（MF）and ZnO@BN/PI were prepared by two-step method.In this work,we mainly studied and analyzed the thermal conductivity,thermal conductivity structure and thermal conductivity mechanism of PI composites formed by the synergistic action of different morphologies of thermal conductive fillers and BN.The main conclusions were as follows:（1）Preparation and study of Al₂O₃@BN&BN/PI thermal conductive composites.To improve the perfection of a three-dimensional thermally conductive network in PI composite film and with respect to the economy and simplicity of processing,a strategy of the two-step synergism of Al₂O₃ microspheres and hexagonal BN nanosheets was proposed.First,BN nanosheet-coated Al₂O₃ microspheres were prepared by electrostatic self-assembly method for the first step of the synergism.Then,the Al₂O₃@BN&BN/PI composite film containing Al₂O₃@BN and BN was fabricated by a two-step method for the second step of the synergism,and was systematically characterized.With an optimized mass ratio of 2:1 of Al₂O₃@BN to BN,the thermal conductivity of the 35 wt%Al₂O₃@BN&BN/PI composite film reach 3.35 Wm/K,and is increased by 1664%compared to that of pure PI.The enhancement mechanism of heat conduction was clearly demonstrated.（2）Preparation and study of GF-BN/PI（MF）thermal conductive composites.GO composite rod-like particles were successfully coated by PDA,which endowed GO with certain magnetism and special nanorod-like structure.at the same time,part of GO,reduced by PDA improved the order of GF.GF was oriented along the in-plane direction under the action of magnetic field,which regulates the distribution of GO,gives full play to the characteristics of high in-plane thermal conductivity of GO,and is easy to achieve directional bridging,which strengthens the synergistic effect with BN.GF and BN cooperate to form a better fixed-guide thermal network in the in-plane direction.The GF-BN/PI（MF）composite films prepared by two-step method have higher thermal conductivity.When the filling amount is 30 wt%（GF:BN=1:3）,the in-plane thermal conductivity of GF-BN/PI（MF）composite films is as high as 2.532 W/m K,which is 1233%higher than that of pure PI and 27%higher than that of BN/PI.At the same time,GF-BN/PI（MF）composite films can still maintain good dielectric and mechanical properties.（3）Preparation and study of ZnO@BN/PI thermal conductive composite films.ZnO@BN thermal conductive composite particles were successfully prepared by the principle of electrostatic self-assembly,and the four-needle morphology of ZnO was combined with the high thermal conductivity of BN lamellae.At the same time,the surface modification of ZnO increase the compatibility of ZnO and PI.The ZnO@BN/PI heat conduction composite film obtainer a special heat conduction network structure,in which BN is stripped from the ZnO surface by shearing,on the contrary,the stripping phenomenon make BN uniformly dispersed in the PI matrix,and the BN layer achieve a high orientation effect during thermal imination.In addition,the ZnO whisker penetrate through the BN layer,and the two cooperated to form a special interheat conduction network structure.The ZnO@BN/PI composite film prepared by two-step method has high thermal conductivity,and when the filling amount is 30 wt%（ZnO:BN=2:1）,the in-plane thermal conductivity of ZnO@BN/PI composite film reaches 2.072 W/m K,which was 991%higher than that of pure PI.