Preparation And Thermal Conductivity Of Modified Carbon Fiber/Epoxy Composites

With the increasing power density and high integration of electronic devices and components,heat dissipation has become a burning issue since it can result in some rise in work temperature and thus harm the service life and reliability of the devices.Compared with ceramic and metal materials,polymer composites have been widely studied and used because of their low cost,corrosion resistance,easy processing,good insulation and other characteristics.Adding high thermal conductivity filler to polymer matrix to improve the thermal properties of composites is a hot research topic at present such as pitch-based carbon fiber.It is found that pitch-based carbon fibers（CFs）have higher thermal conductivity（>1000 W/m·K）than polyacrylonitrile carbon fibers,but the high degree of graphitization,low surface activity,poor wettability with resin matrix and weak interface bonding ability limit the application of CFs in the field of thermal conductivity.Therefore,it is necessary to modify the surface of CFs to increase the interface bonding force between CFs and resin matrix,and then improve the thermal conductivity of composites.This paper mainly studies the influence of carbon fiber modification on the thermal conductivity of epoxy resin（EP）composites,with high thermal conductivity CFs as the main filler,by physical modification of CFs,chemical grafting modification of CFs and synergistic lapping with silanized expanded graphite,mixed with EP to prepare high thermal conductivity composites.The main research contents and results of this paper are as follows:（1）The effects of CFs with length of 50μm,150μm,250μm,3 mm and diameter of 10μm on thermal conductivity of composites were investigated.The results showed that the longer the length of CFs,the better the thermal conductivity of the composite,that is,the carbon fiber with a length of 3 mm and a diameter of 10μm had the best contribution to the thermal conductivity of the composite.Then,a carbon fiber with a length of 3 mm and a diameter of 10μm was selected as the modification object.The surface of the carbon fiber was modified with 1-aminopyrene,and the modified carbon fiber（CFs-NH₂）and EP were prepared by vacuum assisted curing process to prepare the CFs-NH₂/EP composite with high thermal conductivity.The SEM,Raman spectroscopy and AFM qualitatively proved that the surface morphology and structure of carbon fiber changed and the surface roughness increased.XPS analysis showed that the N atom content on the surface of the modified carbon fiber increased from 0.48%to 5.15%.The thermal conductivity test results showed that the thermal conductivity of CFs-NH₂/EP composite increases from 1.38 W/m·K to 1.8 W/m·K when the additive amount was 10 wt%,which was 9 times higher than that of pure EP（0.2W/m·K）.Infrared thermal imaging results showed that CF_S-NH₂/EP composites had good thermal conductivity.In addition,the tensile strength of CF_S-NH₂/EP composites was increased.（2）The carbon fibers were modified by K₂S₂O₈/Ag NO₃ oxidation system（m-CFs）in advance,and maleic anhydride（MA）was chemically grafted onto the surface of carbon fibers（m-CFs-MA）through cycloaddition reaction,so as to improve the surface oxygen content and polarity of carbon fibers.Finally,the m-CF_S/EP and m-CFs-MA/EP composites were prepared by compounding with epoxy resin respectively.The results of FTIR spectra,Raman spectra and SEM showed that the surface structure of CFs changed.XPS showed that after K₂S₂O₈/Ag NO₃ oxidation treatment,the oxygen content on the surface of m-CFs increased from 1.43%to 4.31%,and the oxygen content on the surface of m-CFs-MA increased to 7.68%.The thermal conductivity test results showed that the thermal conductivity of m-CFs-MA/EP and m-CFs/EP composites was significantly improved than that of CFs/EP composites,especially when the content of filler was 20 wt%,the thermal conductivity of m-CFs-MA/EP composites reached 4.43 W/m·K.It was 2.34 times that of CFs/EP composites with the same content（1.89 W/m·K）.This was because the chemical bond between the anhydride group and the epoxy group was formed at the interface of the composite material,so the thermal conductivity of the modified carbon fiber thermal conductive composite had been improved.The results of thermal analysis showed that the modified carbon fiber can improve the thermal stability of the composites.（3）A 3D thermal conductivity network was constructed by mixing silanized expanded graphite（m-EG）and carbon fiber（m-CFs-MA）,and the low content and high thermal conductivity m-CFs-MA/m-EG/EP composites were prepared by mixing them with epoxy resin.Raman spectrum,SEM and XPS characterization showed that expanded graphite was successfully modified by KH550.When the content of m-EG was 4 wt%,the thermal conductivity of m-CFs-MA/m-EG/EP composite was 3.2W/m·K,while the unmodified m-CFs-MA/EG/EP was only 2.5W/m·K.Infrared thermal imaging showed that the m-CFs-MA/m-EG/EP composite had good heat dissipation performance.The mechanical properties test showed that the tensile strength of m-CFs-MA/m-EG/EP composites increased by 25%when the content of m-EG was 4 wt%.Thermogravimetric analysis also showed that the modified carbon fiber had a positive effect on the improvement of thermal stability of the composites.