| With the development of microelectronic devices towards miniaturization,high integration and high power,the power density per unit area(up to 200 W/cm~2)is rising rapidly,which will lead to a sharp increase in the heat generated and even thermal barrier problems.Therefore,efficient heat dissipation has become one of the most pressing issues in determining the working performance and long-term reliability of microelectronic products.Usually,the insulation properties of composites ensure the safety and stability of electronic devices when operating under electric fields,so the preparation of composites with efficient insulation and thermal conductivity pathways is crucial for the development of electronic power and other fields.In the past decades,the excellent electrical insulation properties,good flexibility,light weight,easy processing,low cost,and corrosion resistance of polymers have led to their wide application in the field of thermal conductivity.However,the low thermal conductivity(<0.5 W/(m·K))of the polymer cannot meet the needs of practical applications.To improve the insulating thermal conductivity of polymer composites,introducing insulating thermally conductive fillers is the simplest and easiest way.However,in order to achieve good thermal conductivity,it is often necessary to add high content of filler,which inevitably impacts the processability,flexibility and mechanical properties of the composite.Therefore,preparation of insulating and thermally conductive composites with excellent thermal conductivity,good flexibility and mechanical properties is meaningful to broaden the scope of their applications.Therefore,selective distribution of the thermally conductive fillers was applied to construct dense 3D filler network with fiber-reinforced segregated structure and double filler network segregated-like structure;moreover,insulating thermally conductive network with in-plane orientated alternating multilayer structures and through-plane oriented multilayer tubular structures were constructed through multilayer co-extrusion technology and convergent cold-pressing extrusion technology respectively.Then,thermal conductivity,anisotropic electrical resistivities of the composite with excellent overall performance were prepared.The major results are as follows.1.A long-range ordered dense 3D electrical insulating thermally conductive pathway was constructed by introducing PP fiber covered with thermally conductive filler to enhance the thermal conductivity and mechanical properties simultaneously.Compared with the traditional segregated structure,an enhancement of 1.03 times and1.33 times in the tensile strength and elongation at break respectively;The thermal conductivity increased by 2.4 times to 3.85 W/(m·K).The improvement of mechanical properties is attributed to the increased continuity and compactness of the thermally conductive filler network by the fiber.The changing trend of thermal conductivity is ascribed to the local flow of the fiber,which can improve the flowability of the boron nitride at the fiber surface to construct a long-range ordered continuous thermal conductivity pathway under high pressure.This work provides new ideas for the simultaneous enhancement of thermal and mechanical properties of the thermally conductive composites with segregated structure.2.A dense 3D segregated-like structure thermally conductive pathway was constructed through thermally conductive,electrically insulating filler(Boron nitride,BN)and thermally,electrically conductive filler(Graphite,Gt)which were selectively distributed in the continuous phase and isolated phase of the segregated structure respectively.The denseness of the thermally conductive network was regulated by changing content of boron nitride,which increased the heat transfer path and reduced phonon scattering.This work reduces interfacial scattering between the continuous phase and isolated phase of the segregated structure.It shows that the thermal conductivity of the composite increases by 412%reaches 6.04 W/(m·K)compared with conventional segregated structure when the boron nitride content was 35 wt%.On the basis of achieving excellent thermal conductivity,the resistivity of the composites is maintained at 10~9Ω·cm,which meets the insulation demand of thermally conductive composites in practical applications.At the same time,the segregated-like structure composite also has low hardness and good mechanical properties.Therefore,a 3D segregated-like structure thermally conductive filler network was constructed,which provides a basis for its application in insulating thermal conductive composites and a new method for the preparation of thermal interfacial materials with excellent comprehensive performance.3.A multilayer structure thermally conductive pathway with alternating in-plane oriented thermally conductive,electrically insulating filler and thermally,electrically conductive filler which was constructed through the stretch force field and the space confinement effect of the multilayer co-extrusion process.Such thermally conductive and electrically insulating composites achieved continuous in-plane oriented filler network,solved the conflict between insulating coating and efficiency thermal conductivity.Furthermore,through introducing silicon carbide(Si C),an alternating multilayer thermally conductive filler network with in-plane orientation was constructed by the space confinement effect of multilayer co-extrusion processing.As expected,the thermal conductivity of the composites with alternating multilayer structure prepared by multilayer co-extrusion process is increased by 95%to 7.79W/(m·K)compared with the simple laminated multilayer structure,and the mechanism of thermal conductivity enhancement is elucidated by Ansys fluent.At the same time,the composites have excellent insulating and mechanical properties as well as low hardness.Thus,it achieved high efficiency of thermally,electrically conductive filler in the insulating polymer composite,which provides a new way to prepare thermally conductive,electrically insulating composites with excellent overall performance.4.A multilayered tubular structure with alternating through-plane oriented thermally conductive,electrically insulating filler and thermally,electrically conductive filler was constructed through a shear-tensile force field of multilayer convergent extrusion processing.As expected,the vertical thermal conductivity of the composite with multilayer concentric structure reaches 14.45 W/(m·K),which is 277%higher than that of the conventional composite structure,and the mechanism of thermal conductivity enhancement is elucidated by finite element simulation.Moreover,the composite has good compression cycling and insulation properties as well as low hardness,thus meeting the comprehensive performance requirements of thermal interfacial materials.This research work provides a new idea and method for the preparation of thermal interface materials with highly vertically oriented structures. |
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