Research On The Preparation And Performance Of High Thermal Conductivity Polymer Composites

Organic polymers have been widely used in our daily life and many manufacturing industries due to their outsatanding properties,such as excellent processability,good corrosion resistance,small specific gravity,high specific mechanical strength and low cost etc.However,most organic polymers are poor thermal conductors(thermal conductivity normally in range of 0.1-0.5 W/m·K)and are often used as thermal insulation materials.With the rapid development of electronic technology and semiconductor industry,organic polymer materials with low thermal conductivities have gradually become the main bottleneck problem for their continuous development.So far,introducing inorganic thermally conductive fillers to the polymer matrix is still the most common and effective method to improve the thermal conductivity of organic polymer materials.In recent years,low-dimensional materials with high thermal conductivity,such as graphite nano-flakes,carbon nanotubes,SiC whiskers,graphene,hexagonal boron nitride nano-platelets,etc.,have become hot research topics for thermally conductive fillers.A large number of research results show that the anisotropy of low-dimensional fillers effectively promotes the formation of thermally conductive pathways.At the same time,the anisotropic filler also causes the anisotropic property of the final composite.Unfortunately,the low-dimensional morphology of the fillers and the common polymer processing methods often lead to the orientation of the high heat conduction surface of the fillers along the parallel plane of the composites.It makes the in-plane thermal conductivity of the composite increase significantly while the increase of the cross-plane thermal conductivity which is more important for effective heat dissipation is very small or negligible.Therefore,in this research work,we will focus on the key factors affecting the thermal conductivity of composite in order to improve the cross-plane thermal conductivity of composite by controlling filler's morphology and using novel processing method.The main research results are summarized as following:(1)Three h-BN fillers with different morphologies were selected: BN nanosheets(two-dimensional),BN fibers(one-dimensional)and BN microspheres.Series of BN / PVA composites were prepared by simple casting to study the differences of the thermal conductivities of the compoaites.The results show that one-dimensional BN fibers and two-dimensional BN nanosheets both have a good effect on improving the in-plane thermal conductivity of the composite,but they have no obvious effect on increasing the cross-plane thermal conductivity of the composite.The improvement of in-plane thermal conductivity by BN microsphere as fillers is significantly lower than the improvement induced by low-dimensional BN fillers,while the improvement in crossplane value induced by BN microspheres is significantly higher than that induced by low-dimensional BN fillers.(2)3D printing technology was applied to produce the composite material.The orientation of thermal fillers in the matrix is controlled by changing the printing angles,so as to control the thermal conductivity of the composite materials.Graphite nanoflakes with diameter of 200-300 nm and thickness of about 50 nm were selected as fillers.Series of Gt/PAA and Gt/Epoxy composites were produced using a DLP 3D printer or a SLA 3D printer at printing angle from 0 to 90 °.The results show that with the increase of the printing angle,the cross-plane of the composite increases gradually as the in-plane thermal conductivity decreases.In the XRD diffraction profile,the characteristic peaks of the composites printed at 0 ° and 90 ° changed obviously as well,which further confirmed the change of the filler orientation.It appears 3D printing technology can control the thermal conductivity of the material in a specific direction.In this work,a composite material with a cross-plane thermal conductivity of 1.077 W/mK was obtained at 0.6 wt% graphite nano-flakes.(3)At present,the in-plane thermal conductivities of materials reported were mostly acquired by a so-called “in-plane mode”,which are calculated by a physical model rather than direct measurement.Experiment verification of the accuracy and reliability of this model has not been reported in the literature yet.Our experimental results of 3D printing intuitively show that the model has certain accuracy,but still needs optimization.